Petrogenesis  of  eclogite  and  mafic  

granulite  xenoliths  from  South  

Australian  Jurassic  kimberlitic  

intrusions:  Tectonic  Implications  

 Thesis  submitted  in  accordance  with  the  requirements  of  the  University  of  

Adelaide  for  an  Honours  Degree  in  Geology  

 

Angus  Tod  November  2012  

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PETROGENESIS   OF   ECLOGITE   AND   MAFIC   GRANULITE   XENOLITHS   FROM  SOUTH   AUSTRALIAN   JURASSIC   KIMBERLITIC   INTRUSIONS:   TECTONIC  IMPLICATIONS  

ANGASTON,  EL  ALAMEIN  AND  PITCAIRN  ECLOGITES  AND  MAFIC  GRANULITES  

ABSTRACT  

Jurassic kimberlites in South Australia have entrained sub lithospheric mafic granulites and eclogites from the eastern margin of the Australian Craton. This thesis looks at these rocks as a unique window into the sub-lithospheric mantle beneath the south eastern margin of Gondwana. Samples collected from Angaston, El Alamein and Pitcairn included eclogites, amphibole eclogites, amphibole granulites and feldspar rich granulites. These samples were prepared for analytical work at the University of Adelaide. Whole rock geochemistry was collected from x-ray fluorescence in the Mawson Laboratories. Mineral identification and geochemistry was determined by the Cameca SX 51 microprobe at Adelaide Microscopy. Geothermobarometry showed pressures between 6-30kbar, which represent 15-90km of depth and temperatures between 620-1200oC. These rocks experience very high pressure and temperatures and show petrological evidence of isobaric cooling path from the adiabat to the stable geotherm. Magma crystallisation models using MELTS program helped to determine the protoliths that appear to represent mafic underplates. The cumulate and melts that make up these xenoliths have been shown in this thesis to most likely have been derived from a MORB source that crystallised at high pressures (up to 30kbar). Pseudosections produced with the Theriak-Domino program were used to produce a metamorphic path and show that rock type is closely linked to emplacement depth and bulk composition. Radiogenic dating using Neodymium and Samarium system created isochron’s using IsoPlot and gave ages supporting protolith emplacement during the Neoproterozoic (≈670Ma) around the breakup Rodinia.

KEYWORDS:   ECLOGITES,   GRANULITES,   EMAC,   PETEROGENESIS   AND  METAMORPHISM  

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TABLE  OF  CONTENTS    Angaston, El Alamein and Pitcairn Eclogites and Mafic Granulites ............................... 1  Abstract ............................................................................................................................. 1  Keywords: Eclogites, granulites, EMAC, Peterogenesis and metamorphism .................. 1  List of Figures and Tables ................................................................................................ 3  Introduction ...................................................................................................................... 8  Background ..................................................................................................................... 10  

Setting  ........................................................................................  Error!  Bookmark  not  defined.  

Petrology  .................................................................................................................................  14  

Methods .......................................................................................................................... 22  Observations and Results ................................................................................................ 23  Discussion ....................................................................................................................... 47  Conclusions .................................................................................................................... 58  References ...................................................................................................................... 60  Appendix A: Methods .................................................................................................... 64  Appendix B Average garnet and clinopyroxene data (see extended appendix for all data) ........................................................................................................................................ 66  appendix C Hand Sample descriptions ........................................................................... 68  appendix DThinsection Descriptions .............................................................................. 69      

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LIST  OF  FIGURES  AND  TABLES    

Figures Figure 1 Location map for the xenoliths which also shows locations of relevant locations for this thesis such as the Australian Craton, Tasman line and Kayrunnera xenoliths. Map adapted from Tappert et al (2011). .......................................................... 9  Figure 2 Modal percentages of minerals garnet (Gt), pyroxene (Px) and Feldspar (Fd) for the South Australian xenoliths. Xenoliths are distinguished by rock type is Figure 2.A and location of kimberlitic pipe withe two areas being El Alamein and Pitcairn in Figure 2.b. The rock type definitions are found in table 1 ............................................. 15  Figure 3 Photomicrographs of the three main types of textural xenoliths from Angaston, El Alamein and Pitcairn. Image A shows the common eclogitic texture such as triple points in plain polarised light (PPL) (Ai) and cross Polarised light (CPL) (Aii) in sample PA 6x2, Image B shows the common granulite texture and the main metamorphic reaction that defines theses suites of mafic rocks in PPL and CPL (Bii) in smaple Pit-M25, Image C shows gabbroic texture of xenoliths within a matrix of fine grained plagioclase and pyroxene in PPL (Ci) and CPL (Cii) in sample PA 7x9. ...................... 17  Figure 4 Photomicrographs of important textures found within the studied xenoliths. Image (A) shows the metamorphic reaction plagioclase + pyroxene ↔ garnet + quartz as seen at El Alamein. Exsolution features such as vermicular exsolution of clinopyroxene of garnet and orthopyroxene exsolution of clinopyroxene in plane polarised light (PPL) (Ai) and cross polarised light (CPL) in sample PA 7x1. Image (B) shows garnet mineral relationships within the granulites in the Pitcairn xenoliths. Garnets form as blebs around pyroxene in PPL (Bi) and CPL (Bii) in sample JS Kim. Image (C) shows the metamorphic reaction Plagioclase + pyroxene ↔ garnet + quartz as seen at El Alamein. Exsolution features such as garnet exsolution of clinopyroxene and orthopyroxene exsolution of clinopyroxene in PPL and CPL (Cii) in sample PA 7x1. ................................................................................................................................. 20  Figure 5 Photomicrograph of orthopyroxene crystal core of a complex carona structure in plane polarised light (PPL) (i) and cross polarised light (CPL) (ii) in sample PA 7x1. ........................................................................................................................................ 21  Figure 6 Modal percentages of end members for garnet of the South Australian xenoliths. The iron end member is almandine (Al), magnesium end member is pyrope (Py) and calcium end member is grossular (Go), xenoliths are distinguished by rock type in diagram 6a and xenolith location 6b ................................................................... 23  Figure 7 Modal percentages of end members for pyroxene of the South Australian xenoliths. The iron end member is Forsterite (Fs), iron calcium end member is Hedenbergite (Hd), magnesium end member is Enstatite (En) and the magnesium calcium end member is diopside (Di). Xenoliths are distinguished by rock type in diagram 6a and xenolith in location 6b. ......................................................................... 24  Figure 8 Graph of Jadeite cation % vs pressure. Pressure was calcutalted using Nimmis and Taylor (2000) clinopyroxene barometer. Data taken from table 2 and table 6. ....... 25  Figure 9 Modal percentages of end members for feldspars of the South Australian xenoliths. The Sodium end member is albite (Ab), calcium end member is anorthite (An) and potassium end member orthoclase (Or). Xenoliths are distinguished by rock type in diagram 6a and xenolith location 6b. .................................................................. 25  Figure 10 Plotted amphiboles from Pitcairn xenoliths using A-site occupancy by alkalis (Na + K) vs SiO2 to discriminate between the end members (HAWTHORNE et al.

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1997) pargasite (Pa), edenite (Ed), tschermakite (Ts), hornblende (Hb) and tremolite (Tr). ................................................................................................................................. 26  Figure 11Alkalis (Na2O + K2O) vs SiO2 whole rock geochemistry for the xenoliths plotted using IgPet (Carr 2002). Rock type boundaries described by Cox et al (1979). The South Australian xenoliths plot within the Basaltic region. This shows Angaston (red circles and blue squares) (Segui 2010), El Alamein (yellow crosses) and Pitcairn (green triangles) .............................................................................................................. 34  Figure 12 (A) Calculated CIPW Norm for the South Australian xenoliths, the proposed classification by Thompson (1984) for basalt based on their normative proportions of nepheline (Ne), olivene (Ov), albite (Ab), hypersthenes (Hy) and quartz (Qt). Red circles represent South Australian xenoliths (Segui 2010) which plot within the silica saturated and silica undersaturated portions. Green circles represent MORB (Jenner & O'Neill 2012) which plot in the silica oversaturated and silica saturated parts of the diagram. (B) Mole percent diagram (petrogenetic grid) relevant to variable precent melting (5% to the point where clinopyroxene disappears from the residue) of lherzolite over a pressure range of 0.5 to 3GPa (i.e., about 15-90km depth; pressure shown in bold). Each dashed line at a given pressure represents loci of melt compositions (molar normative) generated by progressive partial melting of lherzolite assemblage (ol + opx + cpx + melt) at that pressure (melt % increasing from left to right on each dashed curve). Each continuous line represents a fixed %melting curve. Aldo shown is the cpx out line. A lherzolitic source rock will lose cpx to the melt beyond this line. Sources of data: Takahashi and Kushiro (1983), Hirose and Kushiro (1993), and Baker and Stopler (1995). Note that it is mainly schematic and does not take into account the changing source composition that must happen as the melt in removed from the source. ............ 35  Figure 13 Plate of whole rock geochemical graphs of South Australian xenoliths (Segui 2010) with MORB (Jenner & O'Neill 2012) for comparison. Graphs A, B, C and D are MgO vs SiO2, CaO, TiO2 and Al2O3 respectively. Diamonds on Graph A and B show mineral compositions plagioclase (PLAG), clinopyroxene (CPX), orthopyroxene (OPX) and the mid ocean ridge basalt (MORB) melt composition, with two distinct trends; 1) a trend towards orthopyroxene showing orthopyroxene crystallisation driving the melt and 2) a trend clinopyroxene + plagioclase showing clinopyroxene + plagioclase driving crystalisation. Black arrow shows igneous variation trends, “M” is the direction towards melt differentiation and “C” is towards the cumulates or crystal extracts that must drive the magmatic trend. ...................................................................................... 36  Figure 14 Graph of barium (Ba) vs wt% MgO for the South Australian xenoliths (Segui 2010) (red circles) with MORB (Jenner & O'Neill 2012) (blue circle). South Australian xenoliths show a several order magnitude higher amounts. ........................................... 37  Figure 15 Isochrons calculated using IsoPlot (Ludwig 2003) graphs show 143Nd/144Nd vs 147Sm/144Nd (A) represents whole rock isotope data for the South Australian xenoliths from Angaston (Segui 2010), El Alamein and Pitcairn and gives an age 739±680Ma. (B) South Australian xenoliths (Segui 2010) (green triangles) and Neoproterozoic Cambrian and South Australian Adelaidean basalts (John Foden, per comms) (Blue diamond’s) and gives an age of 656 ± 92Ma. ......................................... 39  Figure 16 Pseudosection calculated for Pit M22 (see table 2) using THERIAK-DOMINO program (De Capitani & Petrakakis 2010), for the geologically realistic chemical system SiO2-Al2O3-FeO-Fe2O3-MgO-CaO-Na2O-K2O-H2O-TiO2 (NCKFMASHTO). The dataset used compiles the following a-x models which incorporate Fe3+ end-member minerals: garnet, biotite and melt (White et al. 2007),

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orthopyroxene and magnetite (White et al. 2002), amphibole (Diener et al. 2007), clinopyroxene (Green et al. 2007), K-feldspar and plagioclase (Holland & Powell 2003) and ilmenite (White et al. 2000). Mn is not considered for the reasons given by White et al (White et al. 2007). Blue lines represent major introduction of a mineral to the assemblage (amphibole, garnet and plagioclase), arrow represent direction on pseudosection the mineral labled is introduced. The introduction of garnet to the assemblage turns to Gabbroic rock to granulite and the loss of plagioclase turns granulite to eclogite. Blue shaded polygon represents the mineral assemblage seen for Pit M22 and the blue star represents the pressure and temperature estimations for the sample (see table 6). ....................................................................................................... 43  Figure 17 Pseudosection calculated for Pit M25 (see table 2) using THERIAK-DOMINO program (De Capitani & Petrakakis 2010), for the geologically realistic chemical system SiO2-Al2O3-FeO-Fe2O3-MgO-CaO-Na2O-K2O-H2O-TiO2 (NCKFMASHTO). The Dataused compiles the following a-x models which incorporate Fe3+ end-member minerals: garnet, biotite and melt (White et al. 2007), orthopyroxene and magnetite (White et al. 2002), amphibole (Diener et al. 2007), clinopyroxene (Green et al. 2007), K-feldspar and plagioclase (Holland & Powell 2003) and ilmenite (White et al. 2000). Mn is not considered for the reasons given by white et al (2007). Red lines represents major introductions of mineral to an assemblage (amphibole, garnet and plagioclase), arrows represent direction on pseudosection the mineral in labled is introduced. The Introduction of garnet to the assemblage turns Gabbroic rock to granulite and the loss of plagioclase turns granulite to eclogite. Red shaded polygon represents the mineral assemblage seen for Pit M25 and the red star represents the pressure and temperature estimations for the sample (see table 6). ............................... 45  Figure 18 Pressure and temperature plot of geothermobarometry estimations for the SEA (O'Relly & Griffin 1985), EMAC (Pearson & O'REILLY 1991), Monk Hill (Tappert et al. 2011), Angaston (Segui 2010) and Pitcairn and El Alamein. Pressure and temperature estimations using garnet-clinopyroxene Fe-Mg thermometer (Ellis & Green 1979, Krogh 1988) and clinopyroxene barometer (Nimis & Taylor 2000). Data for UHP metamorphic rocks Refrence) schematic subduction metamorphic path taken from Agard (2009) and subduction data points taken from numerous sources(Gao 1999, Dale 2003, Janak 2004). Arrows right of Monk Hill Geotherm (Tappert et al. 2011) show the metamorphic path for the South Australian xenoliths. ................................................... 47  Figure 19 Ni-Cr (ppm) variation of the South Australian mafic xenoliths (Segui 2010) (red circles) and MORB data (Jenner & O'Neill 2012) (green circles). Trends on this Figure show melt fractionation curves for high pyroxene/olivene (high pressure) (blue line) and lower pyroxene/olivene trends (black line with yellow triangles) and the complimentary cumulate trend for a high pressure (black line with orange circles). Trends created using MELTS (Ghiorso & Sack 1995). The specific chosen starting basalt used was an olivene tholeiite from the Adelaidean Smithon basin in N.W. Tasmania. This was chosen as it clearly had experienced no crustal contamination (John Foden, per comms) ......................................................................................................... 51  Figure 20 % of melt remaining vs Temperature showing melt evolution path (red crossed) and the complimentary solid cumulate path (black dashes) created from the results of MELTS (Ghiorso & Sack 1995) modelling on the specific chosen starting basalt, an olivene tholeiite from the Adelaidean smithton basin in N.W. Tasmania. This was chosen as it clearly had experienced no crustal contamination (John Foden, per comms). The most favourable run made at pressure 8.5 kbar, low water content and

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oxygen fugacity of QFM + 1as seen in the MELTS list. Minerals crystallised orthopyroxene (OPX), clinopyroxene (CPX), spinel (SP) and plagioclase (PLAG) ..... 53  Figure 21 MELTS (Ghiorso & Sack 1995) modelling of the south Australian xenoliths on Wt% MgO vs Wt% CaO (A) and SiO2 (B). Melt evolution path (blue crosses) and the complimentary solid cumulate path (black dashes) are shown on the diagram. Starting compostion is shown to be the orthopyroxenite (sample EA08 6). .................. 54  

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Tables Table 1 Distinguishing characteristics for xenoliths different rock types. ..................... 14  Table 2Whole rock major element geochemistry collected using XRF ......................... 29  Table 3Whole rock trace element geochemistry collected using XRF .......................... 31  Table 4 Radiogenic Isotope data .................................................................................... 38  Table 5 Equations used for Geothermobarometry .......................................................... 41  Table 6 Presure and temperature estimates where TEG79  (Ellis  &  Green  1979),  TK88(Krogh  1988)  and  PNT95(Nimis  &  Taylor  2000) ............................................................................... 44    

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INTRODUCTION  

Jurassic kimberlites occur at a number of locations in the mid north of South Australia.

They have transported a diverse range of lower crustal and upper mantle xenoliths.

Studies of these will help to explain the thermal and compositional evolution of the

lithosphere of South Eastern Gondwana. These xenoliths include mafic eclogite and

granulite, garnet peridotite (Tappert et al. 2011) and xenocrysts including diamonds

(Tappert et al. 2009), Cr diopside, garnet, picroilmenite and spinel. The mineral

assemblages of the eclogites range from garnet + clinopyroxene eclogite to amphibole +

garnet + clinopyroxene eclogite. The mineral assemblages of the granulites range from

Feldspar rich (Fd rich) to amphibole rich granulites but both granulite and eclogite

generally have a garnet and clinopyroxene dominated mineralogy. This project aims to

identify the protolith of the kimberlite transported eclogites and the pressure,

temperature and timing of their metamorphic evolution. This will lead to a better

understanding of the paleo-geotherm and structure along the south eastern margin of

Australian Craton. These xenoliths are unique samples of the lithospheric mantle

beneath the important transition zone between Precambrian craton and the Paleozoic

fold belt of Eastern Australia (Figure 1). We will test the hypothesis that these eclogites

and granulites are mafic under plates recording mantle derived magmatism supplied to

the evolving crust. It is possible they originated during Late Proterozoic rifting and

early Proterozoic subduction.

This Thesis focuses on xenoliths from kimberlitic intrusions at three localities; El

Alamein, Pitcairn and Angaston (Figure 1). In this study we used a number of

techniques to help constrain the P-T-t path and the possible protoliths of the mafic

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Figure 1 Location map for the xenoliths which also shows locations of relevant locations for this thesis such as the Australian Craton, Tasman line and Kayrunnera xenoliths. Map adapted from Tappert et al (2011).

xenoliths. Bulk geochemistry analysis were used to constrain the pressure, temperature

and time paths through the production of a pseudosection using the program

THERIAK-DOMINO program (De Capitani & Petrakakis 2010). Bulk geochemistry

was also used to make comparisons with other data sets. Mineral comparison analyses

were done using the electron microprobe to provide mineral geochemistry and

geothermobarometry determination. Geothermometry determination were based on the

exchange of Mg-Fe2+ between garnet (gt) and clinopyroxene (cpx) (Ellis & Green 1979,

Krogh 1988) and geobarometry estimations were made using the clinopyroxene site

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occupancy barometer formulated by Nimis and Taylor (2000). The most common

mineral assemblage was Gt-cpx-rutile±kyanite±plag and made up the bulk of the mafic

xenolith mineral assemblage’s prograde and retrograde metamorphic reactions and

metamorphic/igneous textures were used to show the infered metamorphic path through

the pseudosections (Pearson et al. 1991, Jacob 2004).

BACKGROUND    

Kimberlites are volatile rich, highly potasic, ultra mafic rock that ascend from the deep

mantle/lithospheric mantle (Sparks et al. 2006). Rocks from the mantle lithosphere and

lower crust, such as peridotite, granulite and eclogite, are commonly sampled as

xenoliths by the ascending kimberlites. Kimberlites have been studied due to their

association with diamonds for their economic importance; but also to provide rare

sampling windows into the lower crust and upper mantle. Eclogitic and granulitic

xenoliths are believed to form in three different ways (Jacob 2004, Griffin & O'Reilly

2007).

1) As a high pressure cumulate from ascending mafic magma.

2) Whole-sale under plating of ascending mafic magma at the Moho; forming

Gabbro, which eventually cools first into garnet granulite field then into the

eclogite facies.

3) Oceanic crust subducted at a convergent margin and converted to eclogite during

subduction, which is then tectonically accreted to the base of the lithosphere.

Many eclogites show MORB like chemistry and P-T values like the upper subducted

slab. This has led to many papers linking eclogites to kimberlite subduction processes

(Jacob 2004). Kimberlitic eclogites are thought to experience exceptionally high

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pressures and temperatures when compared to eclogites that occur in glaucophane schist

terrain and those within crustal migmatite gneissic terrains (Coleman et al. 1965) that

are commonly associated with subduction zones. Eclogites found entrained within

kimberlites can be referred to as high Temperature (HT) and ultra high pressure

eclogites due to the forming temperatures of T>900oC and pressures P> 36Kbar.

Geological setting

The Precambrian Australian Craton is made up of a mosaic of Archean and Proterozoic

crustal elements on the western side of Australia. The Eastern side of Australia is made

up of the Paleozoic to Mesozoic Tasman fold belt, which shows a thinner crustal

package. The Tasman Line defines the boundary between these two building blocks that

make up Australia (Veevers & Conaghan 1984) (Figure 1). Xenoliths from kimberlite

pipes and dykes have been found in a number of places in south and south-eastern

Australia (O'Relly & Griffin 1985, Pearson . 1991, Song 1994, Segui 2010, Tappert.

2011). South Australian Kimberlites occur in the Mid North of South Australia at Port

Augusta, Eurelia, Terowie, Orroroo and in the Adelaide Hills (Figure 1). The Eurelia

and Adelaide Hills kimberlites have diamonds as xenocrysts and have, therefore, been

well studied. South Australian kimberlites intrude the Burra and Umbertana sequences

of the Adelaidean sediments (Colchester 1972, McCulloch 1982). They occur as pipes

with N.W.-S.E. trending dykes that are deeply weathered and have been dated to the

Jurassic (~180Ma) by Stracke et al (1979) and Tappert et al (2011). Xenoliths found in

South Australia are a mixture of country rock and nodules of ultra mafic to felsic rocks

derived from upper and lower lithosphere.

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Paleogeothermal gradients have been determined at various locations on both sides of

the Tasman line in Australia where data is available. Xenolith suites from East of the

Tasman Line have been extensively studied by a number of previous authors

(Sutherland & Hollis 1982, O'Relly & Griffin 1985, Pearson & O'REILLY 1991,

Pearson et al. 1991, Pearson et al. 1995).These studies have led to the definition of the

South-eastern Australian geotherm (SEA). They concluded that the protoliths for these

mafic xenoliths were a number of mafic underplates from magmatic plumes that rose

during the opening of the Tasman sea (O'Relly & Griffin 1985). Pearson et al (1991)

studied xenoliths from the Pine Creek and Calcutteroo kimberlites and from the New

South Wales Kayrunnera alkali basalt (Tappert et al. 2011). These lie on the eastern

margin of the Australian Craton (EMAC) just to the west of the Tasman Line. They

used this data to define the EMAC geotherm, which appeared slightly cooler than the

SEA geotherm (REFRENCE). They concluded that the EMAC defined geotherm started

out with a similar crust-mantle boundary as seen in the SEA geotherm but have now

cooled due to equilibrations under different pressure and temperature conditions closer

to the Australian Craton (Pearson & O'REILLY 1991).

Song (1994) collected a number of kimberlitic xenoliths from Culcutteroo, Pine Creek

and Port Augusta (Figure 1) for geochemical and pressure temperature analysis for his

study on the evolution of the lithospheric mantle below S.E. South Australia. His work

concluded that the South Australian xenoliths reflected several periods of underplating

by basaltic liquids derived from deep lithosphere or athenosphere, subsequent

metamorphism, and metasomatism of the original melt (Song 1994). He also concluded

that his calculated P-T estimates illustrate that the lithosphere beneath the Paleozoic

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Adelaide geosyncline is cooler than that of the Phanerozoic eastern Australia; indicating

of different petrology or geochemical composition, which is also shown between the

EMAC and SEA geotherms. Tappert et al (2011) in addition conducted research into the

Monk Hill garnet peridotites xenoliths (Figure 1) and xenocrysts. This locality is on the

eastern margin of the Australian Craton. Monk Hill P-T estimations also defined a

significantly lower geothermal gradient (40mW/m2) than that under south-eastern

Australia. However the estimates of Song (1994) and Tappert et al (2011) show a much

lower geotherm than the EMAC model of Pearson et al (1991). This may indicate that

the EMAC and SEA estimations are both still re-equilibrating to the stable geotherm in

the Jurassic, shown by the Monk Hill geotherm.

Tappert et al (2011) have showed the differences in goetherms are due to the

Kayrunnera volcanic rocks used in the Pearson et al (1991) P-T estimations. The

volcanic hosts for these xenoliths have Permian emplacement age, compared to the

Jurassic kimberlites in South Australia (Gleadow & Edwards 1978, Stracke et al. 1979),

and their geochemistry indicates they are alkaline basalts rather than kimberlites

(Ferguson & Sheraton 1979). Therefore, the earlier transport of the Kayrunnera volcanic

rocks reveals a different, and in this case, a hotter geothermal gradient compared to the

South Australian kimberlitic xenoliths.

The El Alamein and Pitcairn kimberlites are both located along the EMAC and are

assumed to be Jurassic in age (Stracke et al. 1979, Tappert et al. 2011). El Alamein is

the most northern and western kimberlite locality studied within the central Adelaide

fold belt of South Australia and was chosen to investigate any possible regional trends

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in locality in mantle P-T conditions and mafic magma composition. Only limited prior

studies have been made at Pitcairn. The geochemistry and petrography is hoped to help

better constrain the paleogeotherm along the EMAC and to define the age, source and

tectonic signatures of their mafic protoliths. The inferred depth to the Moho in the Mid

North of South Australia is 35km (Branson et al. 1968)

Petrology  Nineteen samples were collected for petrological descriptions from the Pitcairn and El

Alamein kimberlites (Figure 1) and were combined with 18 petrographical descriptions

of Angaston xenoliths from Segui (2010). The xenoliths were divided into five

lithological groups based on their mineral assemblage, mineral chemistry and textures.

The ratio Al(6)/Al(4) is a ratio used to distinguish eclogites from granulites based on the

clinopyroxene chemical composition and represents the ratio of Tschermaks to Jadeite

molecule within the mineral (Jacob 2004).

Table 1 Distinguishing characteristics for xenoliths different rock types.

Lithology Mineralogy Chemistry Texture Eclogitic rock

Gt+ CPX±Qt±OPX±Am

Al(6)/Al(4) ≥2 (CPX)

Triple point equi- to sub granular euhedral texture

Amphibole Eclogite

CPX+Gt+Am±Qt±OPX

Al(6)/Al(4) <2 (CPX) Modal Am≥10%

Triple point equi- to sub granular euhedral texture

Orthopyroxenite

OPX+CPX±Am±Gt

Modal OPX>70%

_

Feldspar rich granulite

Pl+CPX+Kfd±Opx±Gt±Am±Qt

Modal Pl+K+Fd 40-50%

Diverse stages of exsolution textures, rutile inclusions

within garnet and or clinopyroxene

Amphibole granulite

Pl+Am+CPX+Kfd±Opx±Gt±Qt

Modal Pl+K+Fd 40-50%

and Am >10%

Diverse stages of exsolution textures, rutile inclusions

within garnet and or clinopyroxene

Gabbroic Rock

Pl+CPX+OPX+Am±Il±Ru

Doleritic microstructure, lath shaped plagioclase,

clinopyroxenes showing an intergrowth with amphibole

and minor ilmenite

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The El Alamein (Figure 1) xenoliths were collected from weathered kimberlitic sills

near Port Augusta in the South Australian Mid North. Ten samples were examined and

used for petrological description and mineral geochemistry. The Pitcairn xenoliths

(Figure 1) were collected from a location 160km East S.E. of Port Augusta. Nine

samples were examined and used for peterological descriptions and mineral

geochemistry and whole rock geochemistry.

Figure 2 Modal percentages of minerals garnet (Gt), pyroxene (Px) and Feldspar (Fd) for the South Australian xenoliths. Xenoliths are distinguished by rock type is Figure 2.A and location of kimberlitic pipe withe two areas being El Alamein and Pitcairn in Figure 2.b. The rock type

definitions are found in table 1

Eclogitic Rock

The Eclogitic rocks are dominantly composed of garnet (30-60%), clinopyroxene (20-

40%, with essential rutile (1-10%) ± ilmenite ± spinel (Figure 2). They have

equigranular textures with well-equilibrated triple point grain boundary textures

indicative of equilibrated rocks (Figure 3A). These rocks show unstrained textures with

clinopyroxene and garnet making up 90% of the rocks composition. Rutile is in

abundance within this rock type and occurs at the junction between clinopyroxene and

garnet. The little amount of plagioclase that is present (0-3%) exists as small crystals

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Figure 3 Photomicrographs of the three main types of textural xenoliths from Angaston, El Alamein and Pitcairn. Image A shows the common eclogitic texture such as triple points in plain polarised light (PPL) (Ai) and cross Polarised light (CPL) (Aii) in sample PA 6x2, Image B shows the common granulite texture and the main metamorphic reaction that defines theses suites of mafic rocks in PPL and CPL (Bii) in smaple Pit-M25, Image C shows gabbroic texture of xenoliths within a matrix of fine grained plagioclase and pyroxene in PPL (Ci) and CPL (Cii) in sample PA 7x9.

with their shape dictated by the other surrounding minerals. Alteration within the

eclogite rock type is readily seen along fractures and mineral boundaries. The Alteration

minerals are carbonates and micas.

Amphibole Eclogite

The amphibole eclogite rocks are dominantly composed of garnet (30-50%),

clinopyroxene (20-40%,), Amphibole (>10%) with essential ruitle (1-10%) ± ilmenite ±

spinel (Figure 2). These rocks show triple point textures indicative of equilibrated rocks

(Figure 3A). They also show layering textures defined by garnet-rich layers and then

clinopyroxene rich layers. Amphibole demonstrates a strong relationship to rutile

forming as inclusions within amphibole. The low amounts of plagioclase (0-3%) within

this rock type is rimmed by garnet.

Feldspar (Fd) rich Granulite and Amphibole Granulite

These Fd-rich granulite and amphibole granullite rocks are mafic rocks. The Fd-rich

granulites are composed of clinopyroxene (20-45%), garnet (15-40%), plagioclase (20-

55%) ± ilmenite ± rutile (Figure 2). The amphibole granulites are composed of

clinopyroxene (20-40%), garnet (15-35%), plagioclase (20-55%), Amphibole (>10%) ±

ilmenite ± rutile (Figure 2). They are characterised by fine to medium grained

interlocking sub- to equiangular textures (Figure 3B). The diagnostic mineral for these

rock types is large elongate plagioclase which display simple and multiple twinning.

18

 

Reddish-pyrope garnet occurs as sub to euhedral grains that form rings around the

clinopyroxenes, llmenite and plagioclase. Clinopyroxene are subhedral and dark green,

their boundaries with plagioclase are straight and curved with garnet. Ilmenite,

pyroxene, amphiboles and garnets all show exsolution textures implying the breakdown

of this mineral assemblage. Within this rock type we see three types of exsolution: 1)

garnet and amphibole exsolution from clinopyroxenes (Figure 4C and 5); 2)

clinopyroxene as vermicular exsolution in garnet (Figure 3A), and; 3) clinopyroxene

exsolution from orthopyroxene (Figure 5). These textures are believed to show the

transition from primary igneous (gabbroic rock) textures towards granulite metamorphic

textures under static stress.

Orthopyroxenite1

There was one orthopyroxenite sample found from El Alamein (EA 08 6) with 70%

orthopyroxne (OPX) and 10% clinopyroxene (CPX) and minor Garnet. Orthopyroxenite

are formed as early high pressure cumulates from fractionated magmas

Gabbroic rock

These xenoliths show plagioclase laths and prismatic clinopyroxenes that are

intergrown with amphibole and minor illmenite and rutile (Figure 3C). Garnet rings

encompass the clinopyroxenes in some of these rock type.

1 No thin section was created for this sample so the petrography was concluded from the hand sample only.

19

 

20

 

Figure 4 Photomicrographs of important textures found within the studied xenoliths. Image (A) shows the metamorphic reaction plagioclase + pyroxene ↔ garnet + quartz as seen at El Alamein. Exsolution features such as vermicular exsolution of clinopyroxene of garnet and orthopyroxene exsolution of clinopyroxene in plane polarised light (PPL) (Ai) and cross polarised light (CPL) in

sample PA 7x1. Image (B) shows garnet mineral relationships within the granulites in the Pitcairn xenoliths. Garnets form as blebs around pyroxene in PPL (Bi) and CPL (Bii) in sample JS Kim.

Image (C) shows the metamorphic reaction Plagioclase + pyroxene ↔ garnet + quartz as seen at El Alamein. Exsolution features such as garnet exsolution of clinopyroxene and orthopyroxene

exsolution of clinopyroxene in PPL and CPL (Cii) in sample PA 7x1.

Mineral Relationships

Several of the xenoliths studied from El Alamein contain microstructures that show

evidence of arrested local equilibrium and re-equilibrium in different locations

throughout the slide. Evidence for this includes: 1) Double and single corona of

garnet/garnet + CPX around OPX on the boarder of plagioclase that is readily seen in

the garnet granulites from El Alamein (Figure 3B, 4A and 4B) and within the Pitcairn

xenoliths. 2) Garnet and pyroxene exsolution of relict pyroxenes’ (Figure 4A and 4B).

3) The replacement of plagioclase by garnet and quartz assemblage (Figure 3A, 4A and

4B).

Pearson and O’Reilly (1991) also noted these microstructures and deduced that they

were the result of the breakdown of the primary assemblages, mainly olivine +

plagioclase and pyroxene + plagioclase as a response to cooling from igneous

temperatures. Garnet coronas can be explained by Equation (1):

𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 + 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 = 𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔 + 𝑞𝑞𝑞𝑞𝑞𝑞𝑞𝑞𝑞𝑞𝑞𝑞 (1)

The garnets form as blebs or rims around pyroxenes or as lamellae or blebs in

pyroxenes (Figure 4B). Garnet-clinopyroxene symplectites also form at the interface of

plagioclase OPX relict boundaries and can be expressed in Equation 2 that depicts the

transition from granulite to eclogite.

21

 

𝑜𝑜𝑜𝑜𝑜𝑜ℎ𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜 + 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 = 𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔 + 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 + 𝑞𝑞𝑞𝑞𝑞𝑞𝑞𝑞𝑞𝑞𝑞𝑞   (2)

This reaction also shows the compositional change from a more jadeite-rich pyroxene to

a more omphacite-rich pyroxene due to the breakdown of albite.

Figure 5. Photomicrograph of orthopyroxene crystal core of a complex carona structure in plane polarised light (PPL) (i) and cross polarised light (CPL) (ii) in sample PA 7x1.

Slide PA78X shows an OPX crystal at the core of a complex corona structure and may

illustrate either re-equilibration of primary Al-rich orthopyroxene with garnet, or an

intermediate stage in the transformation of olivene + plagioclase to garnet +

clinopyroxene (Pearson et al. 1991) (Figure 5).

Segui (2010) discussed a similar group of rocks located within the Angaston kimberlite

dykes and diatremes which also lies on the eastern margin of the Australian Craton

(Figure 1). The Angaston mafic xenoliths however had an important addition to their

suite, the inclusion of kyanite. This inclusion of kyanite to the assemblage was in the

form of fine grained crystals that make up a matrix and symplectites with clinopyroxene

22

 

that aid in the breakdown of plagioclase with garnet (Segui 2010). Re-equilibration

textures mainly that of garnet corona and garnet and pyroxene exsolution were also

discussed.

METHODS    

Samples were collected in the field from locations labelled (Angaston, El Alamein and

Pitcairn) and transported in clear bags to Mawson Laboratories. Sample descriptions

were made at location sites.Sample preparation was conducted at Adelaide University

Mawson Laboratories. A diamond saw, stainless steel jaw crusher, tungsten mill and

Franz separator were used to preparethe samples for major element geochemistry, trace

element geochemistry and radiogenic isotope geochemistry.

Major mineral geochemistry was conducted on in-situ mineral grains using the Cameca

SX 51 microprobe at Adelaide Microscopy. Whole rock major element analysis was

conducted using fused glass discs and x-ray fluorescence (XRF). Total FeO was

determined by the digestion in HCl in the presence of CO2 with the solution then being

titrated using potassium di-chromate with BADS as an indicator. Whole rock trace

element geochemistry was obtained by using pressed pellets and XRF.

Phase diagram calculations are based on whole-rock compositional data. Water content

is based of weight loss on ignition. Analysis via titration methods provides the

concentration of ferric iron. Due to the inevitable hydration and oxidation of the sample

during retrogression and/or weathering at the surface, the H2O and Fe2O3 values of the

composition used for phase diagram calculations should be considered a maximum

estimate.

23

 

Radiogenic Isotope work was carried out on seven whole rock samples which were

prepared at the Mawson Laboratories. Neodymium and Samarium analysises were

carried out on a Finigan MAT 262 TIMS at Adelaide University. The Nd and Sm values

were calculated using depleted mantle values (from Goldstein et al 1984).

OBSERVATIONS  AND  RESULTS    

Mineral  Geochemistry  

Mineral geochemistry was collected using the SX 51 microprobe at Adelaide

Microscopy and used to help with geothermobarometry, mineral identification and end

members.

Figure 6. Modal percentages of end members for garnet of the South Australian xenoliths. The iron end member is almandine (Al), magnesium end member is pyrope (Py) and calcium end member is

grossular (Go), xenoliths are distinguished by rock type in diagram 6a and xenolith location 6b

Garnet

The garnets analysed from both localities were homogenous throughout with no

zonation along the rims. Figure 6 shows garnets plotted within a compositional end

member triangle with almandine (Fe), pyrope (Mg) and grossular (Ca) end members.

24

 

The Almandine composition of all garnets is roughly <50% with most differences

occurring within the pyrope and gossular end member for garnet. Pitcairn displays a

more pyrope-rich end member for garnet and El Alamein towards the Almandine end

member. The rock types ternary diagram also depicts subtle differences in garnet end

member compositions of garnet. The Fd-rich granulites display a higher amount of

pyrope and gossular than the other rock types. The rock types that have >10%

amphibole also display a trend towards the almandine end member, away from the

samples with <10% amphibole.

Figure 7. Modal percentages of end members for pyroxene of the South Australian xenoliths. The iron end member is Forsterite (Fs), iron calcium end member is Hedenbergite (Hd), magnesium end member is Enstatite (En) and the magnesium calcium end member is diopside (Di). Xenoliths are distinguished by rock type in diagram 6a and xenolith in location 6b.

Pyroxene

The pyroxenes found in all samples were homogenous, showing no zoning. The

dominant clinopyroxene throughout all samples was a type of clinopyroxene with a high

diopside (Ca) (Figure 7). No systematic difference can be seen between the

compositions of clinopyroxene through location of xenolith or type of xenoliths.

Orthopyroxene is only present in two samples from El Alamein and show enstatite as

the dominating end member. Figure 8 shows a plot of pressure (Nimis & Taylor 2000)

vs Jadeite (table 6) which illustrates a positive correlation, where omphacite is

25

 

consumed by jadeite during increased metamorphism (Fleet & Zussman 2003). The

percentage of Jadeite was calculated as the sodium cation amount.

Figure 8 Graph of Jadeite cation % vs pressure. Pressure was calcutalted using Nimmis and Taylor (2000) clinopyroxene barometer. Data taken from table 2 and table 6.

Figure 9 Modal percentages of end members for feldspars of the South Australian xenoliths. The Sodium end member is albite (Ab), calcium end member is anorthite (An) and potassium end member orthoclase (Or). Xenoliths are distinguished by rock type in diagram 6a and xenolith location 6b.

Feldspar

The composition of feldspars in the Pitcairn and El Alamein xenoliths lay within the

plagioclase feldspar series. Feldspars were not identified within the eclogitic rock type

0  

0.05  

0.1  

0.15  

0.2  

0.25  

0.3  

0.35  

0.4  

0.45  

0   5   10   15   20   25   30   35  

Jade

ite  %  

Pressure  (kbar)  

26

 

group. The majority of plagioclase plot towards the albite (Na) end member (Figure 9).

Regional differences can be seen between El Alamein and Pitcairn feldspars. Pitcairn

feldspars display a broader range of plagioclase compositions (30-90%albite), with

some plotting towards the orthoclase (K) end member. El Alamein Feldspars all plot

along the plagioclase series and have a much tighter range of composition (55-75%).

Figure 10 Plotted amphiboles from Pitcairn xenoliths using A-site occupancy by alkalis (Na + K) vs SiO2 to discriminate between the end members (HAWTHORNE et al. 1997) pargasite (Pa), edenite (Ed), tschermakite (Ts), hornblende (Hb) and tremolite (Tr).

Amphibole

There is a large difference in abundance of amphibole between the Pitcairn and El

Alamein xenoliths, as El Alamein displays significantly lower amounts of amphibole if

any at all. The chemistry of Amphibole is very important due to its component of water,

which may indicate that Pitcarin mantle may have been more volatile enriched. The

composition of the amphiboles found at Pitcairn display a trend towards pargasite (Pr),

which is the silica-poor Alkali (Na+KA-site) high end member (Figure 10). Pargasite is

known to form in high temperature metamorphic regions such as in contact aureoles

27

 

with igneous intrusions (HAWTHORNE et al. 1997). The samples also seem to show

trends towards the pargasite end member as you move from the core of the mineral to

the rim of the mineral.

Whole  Rock  Geochemistry   Whole-rock geochemical analyses were conducted on selected xenoliths from the

Pitcairn, El Alamein and Angaston localities. These added to the existing data set of

whole rock analyses of Angaston xenoliths produced by Segui (2010). Candidate

samples for these analyses were restricted by the availability of sufficiently large and

homogeneous samples (usually those larger than ~ 300 gms). Such candidates are far

more abundant at Angaston than at other localities. These data are reported in Table 2

Although most of these xenoliths are now dominated by metamorphic

mineralogies and textures (though as discussed earlier there are good examples where

some xenoliths and some textures are in transition from igneous to metamorphic), a key

issue is the nature and age of their protoliths. It seems likely they have mafic igneous

precursors and if this is the case then key questions include: what is the age and tectonic

significance of the igneous event? Do they have equivalent volcanic suites recognizable

in the geological record of S.E. Gondwana?

Amongst the samples analysed were a few examples of the host kimberlite (Angaston

Kim, EA08 2 and EA08 4). These are ultramafic rocks characterized by having very low

SiO2 and Al2O3 and very high MgO and moderate to high TiO2. They have very high

ignition loss, dominated by CO2. Their trace elements are characterised by high Ni and

Cr and high light rare earth element (LREE) and high LREE/ heavy rare earth elements

28

 

(HREE) and very high Ba and Sr. One sample analysed is a lower crustal garnet gneiss

(CAX-M10), with a psamo-pelitic composition.

29

 

Table 2. Whole rock major element geochemistry collected using XRF

Sample   Mg#   SiO2  %  

Al2O3  %  

TiO2  

%  Fe2O3T  

%  FeO  %  

Fe2O3  

%  MnO  %  

MgO  %  

CaO  %  

Na2O  %  

K2O  %  

P2O5  %  

SO3  %  

LOI  %  

Total  %  

Metapelite                                                                  CAX-­‐M10   0.53   63.47   19.48   0.83   6.14   2.27   3.62   0.08   2.55   0.59   0.81   5.30   0.03   0.01   0.70   99.99  DS012-­‐3   0.51   45.62   12.61   1.48   14.70   8.70   5.03   0.25   9.20   11.21   2.63   0.89   0.08   0.03   1.21   99.90  Orthopyroxenite                                                                EA08-­‐6   0.83   52.52   3.09   0.11   9.47   6.36 2.41 0.17   30.06   1.35   0.16   0.04   0.00   0.03   1.32   98.31  Eclogitic  rock                                                                  EA08-­‐1   0.59   42.78   13.56   1.46   13.06   9.24   2.79   0.21   13.45   10.83   1.30   0.21   0.18   0.13   2.93   100.1  DS012-­‐5   0.43   42.05   12.42   2.79   18.01   11.32   5.43   0.18   8.64   11.82   1.36   0.17   0.07   0.29   1.15   98.95  Mafic  Gneiss                                                                  RT-­‐CAL  2X1   0.74   49.75   10.85   0.14   10.12   6.57   2.82   0.18   18.74   8.05   0.72   0.14   0.04   0.06   1.01   99.80  RT-­‐CAL  2XL10   0.54   53.42   17.35   0.45   8.62   3.90   4.29   0.05   4.63   1.81   3.70   5.82   0.07   0.09   2.31   98.30  EA08-­‐9   0.52   50.78   19.19   0.97   7.15   4.32 2.35 0.05   4.65   3.42   4.72   3.58   0.08   0.20   3.57   98.36  Amphibole  Eclogite                                                              PIT-­‐M10   -­‐   51.27   21.49   0.31   5.30   -­‐   -­‐   0.08   5.00   9.65   3.78   1.45   0.06   0.10   0.44   98.92  PIT-­‐M22   0.49   49.53   11.50   1.24   12.93   11.18   0.51   0.20   10.85   11.45   1.37   0.47   0.14   0.02   0.47   100.1  Amphibole  granulite                                                          JS  KIM  PITR   0.44   48.14   16.50   1.52   11.76   8.22 2.63 0.13   6.43   10.14   3.37   0.63   0.07   0.24   0.83   99.76  PIT-­‐M26   0.46   48.98   13.97   1.65   13.45   9.41   3.00   0.19   8.04   10.96   1.99   0.72   0.18   0.02   0.60   100.7  DS012-­‐4   0.39   37.21   11.91   5.42   17.49   12.75   3.32   0.19   8.31   12.02   2.09   0.53   3.11   0.11   0.85   99.24  

30

 

Table 2 continued

Sample     Mg#  

SiO2  %  

Al2O3  %  

TiO2  

%  Fe2O3T  

%  FeO%  

Fe2O3

%  MnO  %  

MgO  %  

CaO  %  

Na2O  %  

K2O  %  

P2O5  %  

SO3  %  

LOI  %  

Total  %  

Fd  rich  Granulite                                                              DS012-­‐1   0.81   49.39   11.89   1.21   12.63   2.43   9.93   0.20   10.64   11.69   1.54   0.50   0.14   0.03   0.90   100.7  DS012-­‐2   -­‐   45.47   16.68   0.79   10.41   -­‐   -­‐   0.15   9.41   12.05   2.38   0.20   0.10   0.47   1.20   99.32  DS012-­‐6   0.61   46.77   13.81   1.41   10.27   6.63   2.91   0.17   10.49   12.38   2.61   0.50   0.24   0.03   1.37   100.0  DS012-­‐7   0.48   44.99   13.88   1.63   14.66   9.51   4.09   0.22   8.93   13.09   2.04   0.17   0.08   0.12   0.58   100.3  EA08-­‐3   0.63   42.99   13.35   1.40   12.76   7.94   3.93   0.21   13.31   10.60   1.56   0.23   0.16   0.12   3.01   99.71  EA08-­‐5   0.60   42.70   13.75   1.43   13.06   8.61   3.49   0.21   13.02   10.83   1.46   0.19   0.14   0.14   2.70   99.64  EA08-­‐8   0.58   42.81   13.74   1.44   13.17   9.53 2.58 0.21   13.30   10.61   1.42   0.19   0.14   0.15   2.75   99.94  PIT-­‐M9   0.69   48.73   17.79   0.84   8.31   3.27 4.68 0.12   7.13   10.36   3.33   0.71   0.10   0.32   1.13   98.86  PIT-­‐M20   0.60   49.31   14.61   0.48   9.06   6.74   1.57   0.16   10.20   13.46   1.75   0.46   0.10   0.05   0.71   100.3  PIT-­‐M21   0.53   50.10   14.39   0.17   10.75   8.33   1.49   0.19   9.50   11.90   1.65   0.34   0.06   0.26   0.29   99.59  PIT-­‐M23   0.51   43.71   23.53   0.86   7.29   5.98   0.64   0.07   6.32   10.61   2.87   0.82   0.11   0.19   1.20   97.58  PIT-­‐M24   0.61   49.97   16.34   0.33   8.01   5.56   1.83   0.12   8.65   9.43   3.28   1.29   0.06   0.25   0.88   98.61  PIT-­‐M25   0.57   51.00   21.19   0.31   5.21   3.80   0.99   0.08   5.01   9.99   3.79   1.35   0.06   0.09   1.01   99.10  Kimberlite                                                                  ANGAS  KIM   0.94   17.30   3.09   1.51   5.08   0.66   4.34   0.08   10.60   30.59   0.37   0.95   0.63   0.09   17.3   87.61  EA08-­‐2   0.78   20.43   4.38   3.20   11.90   6.05   5.17   0.20   21.53   13.66   0.19   0.31   0.20   0.21   19.9   96.14  EA08-­‐4   0.79   26.75   6.53   1.54   12.70   6.10   5.92   0.20   22.89   9.60   0.18   0.23   0.19   0.03   17.3   98.18  EA08-­‐7   0.71   28.86   8.41   1.71   13.98   8.67 4.35 0.23   20.88   9.36   0.15   0.23   0.21   0.03   15.3   99.35  

31

 

Table 3. Whole rock trace element geochemistry collected using XRF

Xenolith  Type   OPXenite   Psamo  

pelite  Psamo-­‐pelite  

Eclogitic  rock  

Eclogitic  rock  

Amph  Eclogite  

Amph  Granulite  

Amph  Granulite  

Amph  Granulite  

Amph  Eclogite   Kimberlite   Kimberlite  

Location   El  Alamein   Pitcairn   Angaston   Angaston   El  Alamein   Pitcairn   Pitcairn   Angaston   Pitcairn   Pitcairn   El  

Alamein   El  Alamein  

Sample   EA08-­‐6   CAX-­‐M10   DS012-­‐3   DS012-­‐5   EA08-­‐1   PIT-­‐M22   PIT-­‐M26   DS012-­‐4   JS  KIM  PIT  

PIT-­‐M10   EA08-­‐4   EA08-­‐7  

Zr   3.7   192.8   52.4   30.3   84.6   81.9   100.3   43.7   33.5   71.1   86.9   92.0  Nb   0.5   16.4   45.9   2   12.1   4.7   7.3   4.4   22.1   5.4   31.4   23.7  Y   0.8   29.3   28.3   18.9   24.2   24.4   27.4   53.6   17.7   21.9   23.1   26.1  Sr   20.2   175.8   197.6   381.8   140.8   103.2   188.8   527.0   508.5   144.9   376.7   284.8  Rb   3.3   172.4   16.3   12.7   15.1   9.3   14.8   16.2   9.8   11.1   12.8   11.8  U   0.1   1.0   1.6   -­‐0.7   -­‐1.3   0.5   -­‐0.5   0.2   2.3   -­‐0.6   1.4   -­‐0.2  Th   -­‐0.2   6.8   0.8   -­‐3.1   1.0   -­‐0.7   2.3   -­‐1.1   -­‐0.8   2.5   4.8   2.9  Pb   2.1   75.2   -­‐0.9   -­‐8.3   -­‐3.7   2.7   3.2   0.4   -­‐0.6   1.5   2.6   3.2  Ga   4.7   26.1   17.5   12.3   16.4   21.4   21.1   15.6   18.3   17.7   4.9   7.9  Zn   49   62   81   92   100   84   92   89   45   74   61   72  Ni   802   68   347   430   392   216   133   161   89   152   362   419  Cu   87   57   35   220   169   62   96   19   55   36   220   205  Ba   533   1171   502   9492   2469   128   1131   2281   3344   227   389   497  Sc   33.5   9.9   60.9   59.7   40.1   44.9   41.5   39.9   33.1   42.8   53.3   49.8  Co   81   95   105   108   57   152   69   91   64   142   54   61  V   138   84   507   894   376   361   359   577   342   339   513   490  Ce   -­‐10   48   43   22   39   21   24   80   17   23   58   63  Nd   8   16   27   14   23   17   14   70   8   18   24   28  La   -­‐1   24   26   10   17   8   5   31   3   8   31   36  Cr   5602   73   17   77   325   434   244   -­‐18   117   405   252   217  

32

 

Table 3 continued                      Xenolith  

Type   Kimberlite   Kimberlite   Fd  Rich  Granulite  

Fd  Rich  Granulite  

Fd  Rich  Granulite  

Fd  Rich  Granulite  

Fd  Rich  Granulite  

Fd  Rich  Granulite  

Fd  Rich  Granulite  

Fd  Rich  Granulite  

Fd  Rich  Granulite  

Fd  Rich  Granulite  

Location   Angaston   El  Alamein  

El  Alamein  

El  Alamein  

El  Alamein  

El  Alamein   Pitcairn   Pitcairn   Pitcairn   Angaston   Angaston   Angaston  

Sample   Angas  Kim   EA08-­‐2   EA08-­‐5   EA08-­‐8   EA08-­‐9   EA08-­‐3   PIT-­‐M9   PIT-­‐M20   PIT-­‐M21   DS012-­‐6   DS012-­‐7   DS012-­‐1  Zr   161.6   47.2   79.4   79.5   224.0   82.0   21.1   25.2   2.8   54.7   25.0   33.2  Nb   109.7   18   7   7.6   16.9   15.5   0.1   1.9   1   8.4   2.8   6.1  Y   40.1   17.9   22.6   23.1   20.0   24.2   5.4   12.5   5.6   22.6   27.4   17.5  Sr   1016.7   476.3   159.7   328.2   674.6   146.5   624.7   130.3   181.7   188.5   269.6   758.0  Rb   73.7   15.9   11.9   12.2   45.0   15.6   18.2   5.1   4.3   12.5   8.3   10.2  U   4.0   2.2   0.6   0.9   0.8   2.2   -­‐1.0   0.3   0.8   0.3   0.8   1.3  Th   16.5   -­‐1.1   -­‐0.5   -­‐0.1   -­‐0.3   -­‐1.3   -­‐3.2   1.6   -­‐5.2   -­‐0.2   -­‐2.6   -­‐2.8  Pb   4.7   -­‐0.4   0.1   2.4   5.9   -­‐0.9   -­‐4.0   2.6   -­‐4.2   -­‐0.5   -­‐4.7   -­‐4.4  Ga   3.8   5.5   17.0   13.7   22.7   13.2   17.3   12.8   13.1   15.9   16.7   15.8  Zn   42   44   94   94   30   89   29   40   43   70   86   65  Ni   288   394   327   473   58   369   99   150   164   298   122   159  Cu   34   22   166   160   84   187   7   35   2   34   85   10  Ba   1901   3466   2155   1876   3350   2229   4947   1220   5573   630   3518   4693  Sc   9.1   38.8   42.0   44.7   22.9   41.9   17.5   45.5   49.9   40.4   57.0   33.8  Co   35   40   100   98   53   81   59   88   105   120   96   91  V   112   345   391   390   179   390   90   229   252   335   529   244  Ce   130   44   27   25   105   44   10   11   3   29   10   21  Nd   59   17   17   18   29   24   2   6   2   19   9   11  La   107   34   10   8   70   24   -­‐1   1   0   11   1   7  Cr   317   38   248   521   140   239   164   437   247   531   239   324  

33

 

Table 3 conitinued Xenolith  type  

Fd  Rich  Granulite  

Fd  Rich  Granulite  

Fd  Rich  Granulite  

Fd  Rich  Granulite  

Fd  Rich  Granulite  

Fd  Rich  Granulite  

Location   Angaston   Pitcairn   Pitcairn   Pitcairn   Pitcairn   Pitcairn  

Sample   DS012-­‐2   PIT-­‐M23   PIT-­‐M24   PIT-­‐M25   RT-­‐CAL  2X1   RT-­‐CAL  2XL10  

Zr   34.7   28.4   11.5   19.6   4.6   303.8  Nb   -­‐0.2   4.1   1.5   1   0.5   6.7  Y   13.3   14.1   9.2   5.7   3.8   10.2  Sr   1122.2   572.2   364.6   616.7   50.2   656.1  Rb   16.9   10.7   19.8   14.9   3.5   121.0  U   0.8   0.8   -­‐0.1   -­‐1.0   -­‐1.1   0.6  Th   0.8   -­‐3.0   -­‐2.3   -­‐3.6   1.7   1.9  Pb   -­‐3.2   -­‐5.0   -­‐1.1   0.0   -­‐2.5   4.7  Ga   17.5   14.5   12.6   16.7   9.9   22.8  Zn   39   16   53   24   72   29  Ni   101   163   145   89   358   31  Cu   78   6   93   14   156   21  Ba   3517   9691   6751   3867   558   4579  Sc   25.5   16.2   28.9   17.9   41.2   12.9  Co   87   53   55   46   119   21  V   177   172   136   87   146   166  Ce   26   18   13   10   -­‐1   57  Nd   11   4   5   2   7   16  La   7   2   2   1   -­‐2   32  Cr   175   316   400   189   1174   95  

34

 

The bulk of the samples, the main subject of this study, are mafic rocks with SiO2 in the

range of 40 – 53%. They plot in the “basalt” and “picritic basalt” fields on the Total

Figure. 11Alkalis (Na2O + K2O) vs SiO2 whole rock geochemistry for the xenoliths plotted using IgPet (Carr 2002). Rock type boundaries described by Cox et al (1979). The South Australian xenoliths plot within the Basaltic region. This shows Angaston (red circles and blue squares) (Segui 2010), El Alamein (yellow crosses) and Pitcairn (green triangles)

Alkalis v SiO2 plot (Figure 11). The suites from the three localities are geochemically

similar. They have high Mg#, mostly >0.6. They are mostly critically undersaturated

and some are Ol-Hy normative (Figure 12). They show positive correlations of MgO

with CaO, FeO (T), Ni and Cr, and show negative correlations of MgO with SiO2 with

TiO2. They have high Ni, Cr and Sc, and relatively low Zr (mostly between 50 and 100

ppm). On a MORB-normalised incompatible trace element diagram (“spider plot”), they

have show flat middle to heavy REE patterns, with associated elements including the

HFSE (Zr, Nb), these elements are MORB-like in relative concentration. They have

somewhat elevated LREE, U, Th and alkalis. Their most distinctive feature is the

presence of very significant Ba enrichments and somewhat lesser Sr.

35

 

Figure. 12 (A) Calculated CIPW Norm for the South Australian xenoliths, the proposed classification by Thompson (1984) for basalt based on their normative proportions of nepheline (Ne), olivene (Ov), albite (Ab), hypersthenes (Hy) and quartz (Qt). Red circles represent South Australian xenoliths (Segui 2010) which plot within the silica saturated and silica undersaturated portions. Green circles represent MORB (Jenner & O'Neill 2012) which plot in the silica oversaturated and silica saturated parts of the diagram. (B) Mole percent diagram (petrogenetic grid) relevant to variable precent melting (5% to the point where clinopyroxene disappears from the residue) of lherzolite over a pressure range of 0.5 to 3GPa (i.e., about 15-90km depth; pressure shown in bold). Each dashed line at a given pressure represents loci of melt compositions (molar normative) generated by progressive partial melting of lherzolite assemblage (ol + opx + cpx + melt) at that pressure (melt % increasing from left to right on each dashed curve). Each continuous line represents a fixed %melting curve. Aldo shown is the cpx out line. A lherzolitic source rock will lose cpx to the melt beyond this line. Sources of data: Takahashi and Kushiro (1983), Hirose and Kushiro (1993), and Baker and Stopler (1995). Note that it is mainly schematic and does not take into account the changing source composition that must happen as the melt in removed from the source.

In order to demonstrate that the mafic xenoliths do indeed have igneous geochemical

trends, Figure 13 shows the variation of these samples compared with a large global

data set of MORB compositions (Jenner & O'Neill 2012). The Figure 13 plots show

MgO plotted against SiO2, CaO, TiO2 and Al2O3. In these Figures the igneous variation

trend of the MORB suite is indicated with an arrow. The direction towards ‘M’ is that of

melt differentiation, that towards ‘C’ is cumulates or crystal extracts that must drive this

magmatic trend. As can be seen, the xenolith suite shows significant overlap with the

MORB field, leading us to conclude that: 1) these are indeed metamorphosed

36

 

Figure 13 Plate of whole rock geochemical graphs of South Australian xenoliths (Segui 2010) with MORB (Jenner & O'Neill 2012) for comparison. Graphs A, B, C and D are MgO vs SiO2, CaO, TiO2 and Al2O3 respectively. Diamonds on Graph A and B show mineral compositions plagioclase (PLAG), clinopyroxene (CPX), orthopyroxene (OPX) and the mid ocean ridge basalt (MORB) melt composition, with two distinct trends; 1) a trend towards orthopyroxene showing orthopyroxene crystallisation driving the melt and 2) a trend clinopyroxene + plagioclase showing clinopyroxene + plagioclase driving crystalisation. Black arrow shows igneous variation trends, “M” is the direction towards melt differentiation and “C” is towards the cumulates or crystal extracts that must drive the magmatic trend.

mafic igneous rocks, and 2) that their parent magmas were MORB-like. It is also

obvious that many of the xenolith samples are scattered outside the MORB field,

generally towards higher MgO and lower SiO2, in the ‘C’ direction of the indicated

trend. This is most likely because the xenoliths have experienced some crystal – melt

sorting, many being melt-depleted cumulate-enriched samples (originally gabbros). The

polygons on Figures 13 A and B illustrate the compositional space created by the

mixture of melt and the probable primary igneous phases (orthopyroxene,

clinopyroxene, plagioclase ± spinel). Using the compositional space there appears to be

37

 

no apparent control on garnet; inferring it was not crystallised as a primary phase but

rather metamorphic.

Figure 14 Graph of barium (Ba) vs wt% MgO for the South Australian xenoliths (Segui 2010) (red circles) with MORB (Jenner & O'Neill 2012) (blue circle). South Australian xenoliths show a several order magnitude higher amounts.

As we mentioned earlier, one very strikingly ubiquitous feature of these rocks’ trace

elements is their extraordinary Ba content. Figure 14 illustrates that these have Ba

several orders of magnitude higher than MORB. We attribute this feature to

contamination by the kimberlite that transported the xenoliths to the surface.

Interestingly, this is a very selective contamination because other trace elements (e.g.

Nb, HREE and Zr) are not affected.

Radiogenic  Isotopes  

The Finigan MAT 262 TIMS at the University of Adelaide was used to obtain

Samarium-Neodymium radiogenic isotopic data. These analyses were carried out to aid

in identifying the xenoliths’ protoliths and their age, and to compare with the

Neoproterozoic-Cambrian basalt data (Foden et al. 2002) (John Foden per Comms,

38

 

2012). Seven samples were selected to best represent the three xenolith suites. Three

samples were chosen from each El Alamein (EA08 #) and Pitcairn (Pit M#), and

represent the granulites and eclogites, and one was chosen from Angaston (DS012 #) to

add to Segui’s (2010) isotope data. All data is recorded in Table 2. These data were

plotted using isoplot (Ludwig 2003) with those of Segui (2010), and for comparison

data, Cambrian-Neoproterozoic basalts from S.E. Australia (Foden et al. 2002) (John

Foden per Comms, 2012). The xenoliths isotopic data shows initial εNd0 between -

22.51 (EA08 9) and +8.09 (34.3.2) and Neoproterozoic data εNd700 between -12.02

(EA08 9) and +7.32 (34.4.9).

Table 4 Radiogenic Isotope data

Sample   Sm(ppm)   Nd(ppm)   147Sm/144Nd   5%  error   144Nd/143Nd   2  σ   εNd0   εNd700  

EA08  3   4.87   21.83   0.1349456   6.75E-­‐04   0.512624   8.20E-­‐06   0.282851   5.2595917  

EA08  5   4.64   17.26   0.1626150   8.13E-­‐04   0.512608   1.28E-­‐05   0.593011   2.4536666  

EA08  9   3.97   30.31   0.0792297   3.96E-­‐04   0.511484   8.40E-­‐06   22.51296   12.024218  

Pit  M25   0.66   2.49   0.1603349   8.02E-­‐04   0.512565   1.08E-­‐05   1.433760   1.7837394  

Pit  M24   1.14   4.57   0.1508940   7.54E-­‐04   0.512249   2.87E-­‐05   7.588201   3.4835258  

Pit  M22   2.22   14.2   0.0945688   4.73E-­‐04   0.512481   1.33E-­‐05   3.064541   6.0865036  

DS012  #1   3.81   8.97   0.2569307   1.28E-­‐03   0.512566   1.46E-­‐05   1.408401   6.8313195  

The Nd-Sm isotopic ratios have been compiled with Segui’s (2010) isotope values for

whole rock samples (Figure 15A). An isochron calculated using the whole rock data

from Pitcairn, Angaston and El Alamein gives an age of 739±680Ma (Figure 15A). This

age is believed to be approximated to the age of the protoliths of the metamorphosed

xenoliths. It is possible that the hint of a steeper trend in the 143/144Nd vs 147Sm/144Nd

isochron diagram is due to inclusion of sample EA08 9, which have some crustal

contamination of the original magmatic protolith by entrained crustal material. This

process has been documented and studied by Rudnick et al (1986)

39

 

Figure 15. Isochrons calculated using IsoPlot (Ludwig 2003) graphs show 143Nd/144Nd vs 147Sm/144Nd (A) represents whole rock isotope data for the South Australian xenoliths from Angaston (Segui 2010), El Alamein and Pitcairn and gives an age 739±680Ma. (B) South Australian xenoliths (Segui 2010) (green triangles) and Neoproterozoic Cambrian and South Australian Adelaidean basalts (John Foden, per comms) (Blue diamond’s) and gives an age of 656 ± 92Ma.

40

 

The Late Neoproterozoic age of the mantle xenoliths indicate they probably correlate

well with the suites of Neoproterozoic to Early Cambrian mafic magmas that were

intruded into the Australian passive margin during Rodinia breakup and Gondwana

assembly. These suites include Cambrian and South Australian Adelaidean basalts, such

as the Wooltana and Depot creek basalts and the Gairdner and Broken Hill dykes and

equivalent aged suites on King Island and in Western Tasmania (Foden et al. 2006).

The Nd isotopic composition of these suites show broad correlation with those of the

xenoliths studied (Figure 15B). The El Alamein, Pitcairn, Angaston, the Cambrian and

South Australian Adelaidean data shows positive εNd value at 700Ma but not the

contaminated (EA08 9). When the Cambrian and South Australian Adelaidean basalts

and the El Alamein, Pitcairn and Angaston data is collated the isochron gives the age

656±92Ma, which indicates they both originated in the Late Proterozoic.

Segui (2010) recorded Early Jurassic ages for the Angaston garnet and clinopyroxene

separates. His conclusion was that this age represents the closure temperature (Tc) of the

Sm-Nd system for garnet and clinopyroxene. The clolsure temperature is debated to be

between 600-800oC (Mezger et al. 1992) and the associated age is likely to rep[resent

the timing of the emplacement by the kimberlite, when the minerals (clinopyroxene and

garnet) cooled below their Tc.

Pseudosection  and  Geothermobarometry  P–T pseudosections were calculated for sample Pit –M22 (Amphibole eclogite) (Fig

16,Table 2) and Pit-M25 (Fd rich- eclogite) (Fig 17, Table 2) using the THERIAK-

41

 

DOMINO software program (De Capitani & Petrakakis 2010), for the geologically

realistic chemical system SiO2–Al2O3–FeO–Fe2O3–MgO–CaO–Na2O–K2O–H2O–TiO2

(NCKFMASHTO). The dataset used compiles the follwing a–x models, which

incorporate Fe3+ end-member minerals: garnet, biotite and melt (White et al. 2007),

orthopyroxene and magnetite (White et al. 2002), amphibole (Diener et al. 2007),

cordierite (Holland & Powell 1998), Clinopyroxene (Green et al. 2007), K-feldspar and

plagioclase (Holland & Powell 2003) and ilmenite (White et al. 2000). Mn is not

considered for the reasons given by White et al. (2007). Mineral abbreviations are as

follows: opx – orthopyroxene; g – garnet; sp – spinel; bi – biotite; ksp – K-feldspar;

ANAB – plagioclase; ilm– ilmenite; mt – magnetite; q – quartz; liq – silicate liquid ⁄

melt. The THERIAK–DOMINO software calculates equilibrium mineral assemblages

for specific bulk-rock compositions that minimises Gibbs-free energy at a given point in

P-T space. Sample Pit M22 (Table 2, Figure 16) petrography defines a peak assemblage

of garnet + clinopyroxene + amphibole. Sample Pit M25 (Table 2, Figure 17)

petrography defines a peak assemblage of garnet + clinopyroxene + plagioclase ±

amphibole

Table 5 Equations used for Geothermobarometry

Thermometer Equation

(Ellis & Green 1979,

Krogh 1988)

→⅓Mg3Al2Si2O12  (pyrope)  +  CaFeSi2O6  (hedenbergite)  ↔  

⅓Fe3Al2Si3O12  (almandine)  +  CaMgSi2O6  (diopsode)

Barometer Equation

(Nimis & Taylor 2000)

CaMgSi2O6  (diopside)  +  CaCrAlSiO6  (Ca  Cr  tschermak’s)  ↔  

½(Ca2Mg)Cr2Si3O12   (uvarovite   and   knorringite)   +  

½(Ca2Mg)Al2Si3O12  (grossular  and  pyrope)

42

 

The xenoliths thermobarometry calculations have been compared to a number of

different data sets to better understand the geotherm under South Australia (Pearson &

O'reilly 1991, Pearson et al. 1991, Pearson et al. 1995, Segui 2010, Tappert et al. 2011).

Individual spot Temperature estimations were produced using the Ellis and Green

(1979) and Krogh’s (1998) Fe2+-Mg garnet-clinopyroxene exchange thermometers.

These estimations can be seen in Table 5, pressure estimations were calculated with

Nimmis and Taylor (2000) CPX barometer. In addition, these pressures were used for

the temperature calculations. Fe microprobe data used in the geothermobarometry

calculations has been assumed as Fe2+ as the microprobe does not distinguish between

ferric (Fe3+) and ferrous (Fe2+) iron. This assumption of all ferrous iron gives a

minimum temperature for both thermometers. Averaged microprobe data of garnets and

single values of adjacent clinopyroxene spot values were used to create temperature

estimations. The garnet values were averaged to give more confidence they were in

equilibrium with the clinopyroxene. The Ellis and Green (1979) thermometer showed

slightly higher temperatures than Krogh (1988) throughout the slides. Except for rocks

with high pressures (≈ >15kbar) and higher Temperatures (≈ >1035oC), Krogh’s (1988)

thermometer estimated higher temperatures (Table 5).

43

 

Figure 16. Pseudosection calculated for Pit M22 (see table 2) using THERIAK-DOMINO program (De Capitani & Petrakakis 2010), for the geologically realistic chemical system SiO2-Al2O3-FeO-Fe2O3-MgO-CaO-Na2O-K2O-H2O-TiO2 (NCKFMASHTO). The dataset used compiles the following a-x models which incorporate Fe3+ end-member minerals: garnet, biotite and melt (White et al. 2007), orthopyroxene and magnetite (White et al. 2002), amphibole (Diener et al. 2007), clinopyroxene (Green et al. 2007), K-feldspar and plagioclase (Holland & Powell 2003) and ilmenite (White et al. 2000). Mn is not considered for the reasons given by White et al (White et al. 2007). Blue lines represent major introduction of a mineral to the assemblage (amphibole, garnet and plagioclase), arrow represent direction on pseudosection the mineral labled is introduced. The introduction of garnet to the assemblage turns to Gabbroic rock to granulite and the loss of plagioclase turns granulite to eclogite. Blue shaded polygon represents the mineral assemblage seen for Pit M22 and the blue star represents the pressure and temperature estimations for the sample (see table 6).

44

 

Table 6 Presure and temperature estimates where TEG79  (Ellis  &  Green  1979),  TK88(Krogh  1988)  and  PNT95(Nimis  &  Taylor  2000)

Sample     TEG79   TK88   PNT95  Min   Max   Min   Max   Min   Max  

Fd  rich  Granulite              Pit  M9   933   972   925   971   14.7   15.3  

Pit  M20   857   895   836   879   13.1   14.1  Pit  M23   1035   1144   1031   1164   23.4   24.8  Pit  M24   783   851   715   790   12.4   14.5  Pit  M25   893   969   878   967   14.8   15.5  PA  7x2   737   889   659   823   7.1   9.4  PA  78x   733   871   630   818   6.5   8.8  PA7x1   751   896   705   877   6.3   9.6  Eclogitic  Rock  

           PA  5x1   946   1054   931   1060   13   14  PA  5x2   941   977   930   971   29.1   29.8  PA  6x2   857   903   832   883   21   22  PA  6x12   955   1048   946   1056   21.8   28.1  Amphibolite  Granulite  

           Pit  M26   812   840   779   810   5.9   7.2  JS  Kim     897   944   864   922   15.4   15.9  Amphibolite  Eclogite  

           Pit  M10   785   825   724   768   9.1   9.5  Pit  M22   724   799   629   739   8.9   11.5  

The P- T estimations for the El Alamein xenoliths show a minimum temperature range

between 620°C-1200°C and pressures between 5 and 30kbar (Figure 18). This equates to

a geotherm. Pitcairn xenoliths show a minimum temperature range between 620-1120°C

and pressures between 6-24kbar. The Angaston xenoliths (Segui, 2010) displayed very

similar temperature but slightly more xenoliths with an overall range between 11-

30kbar and between 800-1130°C. The Monk Hill estimated Geotherm (Tappert et al.

2011) will be referred to as the Jurassic paleo-geotherm as it represents a steady state,

whereas the xenoliths of Angaston, Pitcairn, Angaston, EMAC (Pearson & O'REILLY

1991) and SEA (O'Relly & Griffin 1985) show variable equilibration towards this

geotherm. The Monk Hill geotherm is calculated to 40mW/m2

45

 

Figure 17 Pseudosection calculated for Pit M25 (see table 2) using THERIAK-DOMINO program (De Capitani & Petrakakis 2010), for the geologically realistic chemical system SiO2-Al2O3-FeO-Fe2O3-MgO-CaO-Na2O-K2O-H2O-TiO2 (NCKFMASHTO). The Dataused compiles the following a-x models which incorporate Fe3+ end-member minerals: garnet, biotite and melt (White et al. 2007), orthopyroxene and magnetite (White et al. 2002), amphibole (Diener et al. 2007), clinopyroxene (Green et al. 2007), K-feldspar and plagioclase (Holland & Powell 2003) and ilmenite (White et al. 2000). Mn is not considered for the reasons given by white et al (2007). Red lines represents major introductions of mineral to an assemblage (amphibole, garnet and plagioclase), arrows represent direction on pseudosection the mineral in labled is introduced. The Introduction of garnet to the assemblage turns Gabbroic rock to granulite and the loss of plagioclase turns granulite to eclogite. Red shaded polygon represents the mineral assemblage seen for Pit M25 and the red star represents the pressure and temperature estimations for the sample (see table 6).

46

 

47

 

Figure 18 Pressure and temperature plot of geothermobarometry estimations for the SEA (O'Relly & Griffin 1985), EMAC (Pearson & O'REILLY 1991), Monk Hill (Tappert et al. 2011), Angaston (Segui 2010) and Pitcairn and El Alamein. Pressure and temperature estimations using garnet-clinopyroxene Fe-Mg thermometer (Ellis & Green 1979, Krogh 1988) and clinopyroxene barometer (Nimis & Taylor 2000). Data for UHP metamorphic rocks Refrence) schematic subduction metamorphic path taken from Agard (2009) and subduction data points taken from numerous sources(Gao 1999, Dale 2003, Janak 2004). Arrows right of Monk Hill Geotherm (Tappert et al. 2011) show the metamorphic path for the South Australian xenoliths.

and is equivalent to a steady-state continental geotherm (Tappert et al. 2011). Pressure

temperature data for all three regions Pitcairn, El Alamein and Angaston plot at

significantly higher T-values than the subduction P-T related eclogites (Gao 1999, Dale

& Holland 2003, Janak 2004) (Figure 18). This emphasises that the eclogite and

granulite were probably unrelated to subduction. The El Alamein sample derived P-T

array geotherm plot as two distinct groups: 1) a shallow group that plot a wide range of

temperatures for pressures, and ;2) a deep group that plot on the Jurassic paleogeotherm

(Tappert et al. 2011). The Pitcairn geotherm plots very similar to the EMAC (Pearson et

al. 1991) pressure and temperature data.

DISCUSSION    

The aim for this study was to better understand the protoliths and metamorphic history

of these unique and distinct upper mantle rocks from the eastern margin of the

Australian Craton, close to the Tasman Line (Veevers & Conaghan 1984). This

knowledge has helped to tell us about paleo tectonics and geothermal dynamics near the

eastern margin of the Australian Craton before the Jurassic (~180ma), the emplacement

age of the xenoliths from kimberlite intrusions (Stracke et al. 1979, Tappert et al. 2011).

The core to this study is the interpretation of the origin of the mafic granulite and

eclogite xenoliths transported from the mantle by Jurassic kimberlite dykes that intrude

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Adelaidean rocks in the Adelaide Fold Belt. The Adelaide Fold Belt is a composed of

rocks from the Neoproterozoic to Early Cambrian passive margin/rift sequences that

were deformed and metamorphosed during the Mid– to Late Cambrian Delamerian

Orogeny. This orogeny resulted from the onset of subduction in this part of the west

Pacific Margin of Gondwana (Foden et al. 2002, 2006). These mafic xenoliths are,

therefore, potentially very important as they may present us with a unique sample set

from the mantle beneath this rifted-orogen.

These are clearly metamorphic rocks, with strong indications that metamorphism

occurred at upper mantle depths (i.e. within the sub-continental lithospheric mantle).

Their equant and well-equilibrated textures indicate passive metamorphism,

unconnected with strain. As we will discuss, their garnet-cpx-rich metamorphic

assemblages in some samples clearly replace prior plagioclase bearing igneous

assemblages and textures. All the indications are that they have undergone

metamorphism during cooling at depth.

Based on their major element geochemical characteristics (see section Whole rock

geochemistry), there are good grounds for concluding that the origins of the mafic

xenoliths are an igneous suite with geochemical affiliations to rift –related parent

magmas of generally MORB –like geochemical character. This is also the conclusion if

their HFSEs (Nb, Zr and Ti) and HREE and Y concentrations and ratios are considered.

However, they are systematically more silica undersaturated then MORB (Figure 12)

and it is clear that many have bulk compositions that fall outside the field of mafic

igneous melts (Figure 13). These samples may result from crystal-liquid sorting (i.e. the

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bulk compositions of some of the xenoliths is biased towards their crystallizing

minerals, specifically pyroxenes and plagioclase ± spinel). Some preferential

contamination of the xenoliths has been observed and is likely a result of being

transported by kimberlites, particularly affecting Ba and perhaps other LIL and LREE

elements.

It seems increasingly likely that these eclogites and mafic granulites are mafic magmas

emplaced in the upper mantle below the moho. Their Nd-isotope compositions make

them like some of the Late Neoproterozoic rift tholeiites emplaced during the passive

margin stage in this part of S.E. Gondwana in Adelaidean sequences in Tasmania, South

Australia, W. New South Wales and W. Victoria, (Foden et al., 2002). This origin is

similar to that proposed for some petrologically similar suites of xenoliths transported

by Cenozoic alkali basalts in eastern Australia (O'Relly & Griffin 1985, Rudnick et al.

1986). However, it is supposed that the eastern Australian xenolith suite represents

mafic sub-crustal underplating of Cenozoic age (Rudnick et al. 1986) whereas these

South Australian xenoliths probably preserve Late Neoproterozoic to Early Cambrian

mantle events.

If the xenoliths represent magmas that crystallized at high pressures, then they may

have distinctive characteristics that distinguish them from erupted or shallowly intruded

mafic magmas, such as MORB. We have already observed that though there are

suggestions that the suite may have a MORB-like mantle source (based on HFSE and

HREE geochemistry), they are systematically more silica undersaturated than MORB

(Figure 12A). It is a systematic feature that melts derived from increasingly high

50

 

pressure partial melting of mantle peridotite are increasingly more silica undersaturated

(Takahashi 1983, Hirose 1993, Baker 1995) (Figure 12B). The liquidus phase of mafic

melts typically shifts from being olivine (± plagioclase) at low pressures (< 5 kbar) to

pyroxene (CPX and/ or OPX) + olivine at pressures > 5 kbar, to be joined by garnet

above ~ 11-12 kbar (see fig 2 in Rudnick et al. 1986).

Under these circumstances, the fractionation trends of mafic melts crystallizing at >

Moho depths should be dominated much more by pyroxene than those of shallowly

crystallizing mafic magmas, where early olivine will dominate. As pyroxene has a

crystal-melt distribution Cr >> Ni (but both > 1) , whereas olivine has Ni>>Cr (Ni >>1),

this should mean that high pressure fractionation should lead to more rapid Cr depletion

than in lower pressure fractionation. Figure 19 shows the Ni-Cr variation of the South

Australian mafic xenoliths and the field of MORB melts. This shows that the most melt-

like xenoliths have relatively lower Cr/Ni ratios than MORB. Figure 19 shows melt

fractionation curves for high pyroxene/olivine (high pressure) and lower

pyroxene/olivine trends and indicate that the cumulate compliment to the high pressure

trend falls across the more cumulative xenolith compositions.

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Figure 19 Ni-Cr (ppm) variation of the South Australian mafic xenoliths (Segui 2010) (red circles) and MORB data (Jenner & O'Neill 2012) (green circles). Trends on this Figure show melt fractionation curves for high pyroxene/olivene (high pressure) (blue line) and lower pyroxene/olivene trends (black line with yellow triangles) and the complimentary cumulate trend for a high pressure (black line with orange circles). Trends created using MELTS (Ghiorso & Sack 1995). The specific chosen starting basalt used was an olivene tholeiite from the Adelaidean Smithon basin in N.W. Tasmania. This was chosen as it clearly had experienced no crustal contamination (John Foden, per comms)

Another piece of very good evidence for high pressure crystallization of the xenolith

pre-cursor magmas is provided by the orthopyroxenite xenolith from El Alamein (EA

08 6). This plots as a relatively high MgO, low CaO point on the MgO-CaO variation

diagram (Figure 13) and aligns with some other samples at higher Cao and lower MgO,

inferring a phase of orthopyroxene crystallization. To produce a mono-mineralic

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cumulate figure 13 shows that the P-T conditions must be on the mafic melt liquidus

and implies high-pressure crystallization.

To test if the compositions of the xenoliths can be modelled by high pressure

crystallization of an olivine tholeiite using one of the specific Neoproterozoic basalts

from this part of S.E Gondwana, calculations were made using the MELTS2 software.

The specific chosen starting basalt used was an olivine tholeiite from the Adelaidean

Smithton basin in N.W. Tasmania. This was chosen as it had clearly experienced no

crustal contamination.

This composition was run with low water content and an oxygen fugacity of QFM +1.

Figure 20 plots the temperature versus melt percentage for the most favourable run

made at a pressure of 8.5 kbar, and indicates that this composition has orthopyroxene on

its liquidus at this pressure. At pressures less than 5kbar it does, indeed, have olivine on

its liquids, while at pressures > 10 kbar it crystallizes significant amounts of garnet later

in its cooling history (note that there is no geochemical evidence that the xenoliths had a

magmatic history of garnet crystallization and accumulation).

2 MELTS is a software package designed to facilitate thermodynamic modeling of phase equilibria in magmatic systems. It provides the ability to compute equilibrium phase relations for igneous systems over the temperature range 500-2000 °C and the pressure range 0-2 GPa. Ghiorso, Mark S., and Sack, Richard O. (1995) Chemical Mass Transfer in Magmatic Processes. IV. A Revised and Internally Consistent Thermodynamic Model for the Interpolation and Extrapolation of Liquid-Solid Equilibria in Magmatic Systems at Elevated Temperatures and Pressures. Contributions to Mineralogy and Petrology, 119, 197-212 Gualda G.A.R., Ghiorso M.S., Lemons R.V., Carley T.L. (submitted) Rhyolite-MELTS: A modified calibration of MELTS optimized

53

 

Figure 20. percentage of melt remaining vs Temperature showing melt evolution path (red crossed) and the complimentary solid cumulate path (black dashes) created from the results of MELTS (Ghiorso & Sack 1995) modelling on the specific chosen starting basalt, an olivene tholeiite from the Adelaidean smithton basin in N.W. Tasmania. This was chosen as it clearly had experienced no crustal contamination (John Foden, per comms). The most favourable run made at pressure 8.5 kbar, low water content and oxygen fugacity of QFM + 1as seen in the MELTS list. Minerals crystallised orthopyroxene (OPX), clinopyroxene (CPX), spinel (SP) and plagioclase (PLAG)

The results of the MELTS modelling showing the melt evolution path (blue crosses) and

the complimentary solid cumulate path (black dashes), is shown for MgO v CaO and

SiO2 v MgO on Figure 21A and B. The model maps the early OPX-driven trend of the

melt with its prominent inflection in the MgO-CaO variation brought about by the onset

of CPX and plagioclase crystallization. It also predicts an initial liquidus OPX that is

virtually identical to that in El Alamein xenolith EA 08 6.

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In summary, there seems excellent evidence that the mafic xenolith suite formed as a

sub-Moho depth, mafic underplate during Neoproterozoic rifting.

Figure 21 MELTS (Ghiorso & Sack 1995) modelling of the south Australian xenoliths on Wt% MgO vs Wt% CaO (A) and SiO2 (B). Melt evolution path (blue crosses) and the complimentary solid cumulate path (black dashes) are shown on the diagram. Starting compostion is shown to be the orthopyroxenite (sample EA08 6).

The Sm-Nd isochron calculated from whole rock geochemistry gives the protoliths for

the xenoliths an age of 656±92Ma (Figure 15B). This age corresponds to the

Neoproterozoic to Early Cambrian passive margin/rift sequences found at the south

eastern margin of Gondwana. Magmatic underplating has been explained as the

plausible origin for these rocks, this crystallisation saw the magmatic melts rise through

the mantle and then along the adiabat (Figure 18) to crystallise at high pressures. The

rocks then proceeded to be metamorphosed and now show re-equilibration textures, as

noted in the “Petrology” section before.

A B

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Metamorphism of the xenoliths saw them evolve from plagioclase + pyroxene ± olivine

cumulates/melts to the garnet granulites and eclogites that make up the samples (Figure

3A). As observed in the “whole rock geochemistry” section and above in Figure 20, the

starting rock assemblage is assumed to be an igneous plagioclase + olivine + pyroxene

cumulate, possibly similar to the gabbroic rock seen in Figure 3C. These samples

showed no primary garnet crystallisation. I suggest that these rock cooled near

isobarically as the EMAC xenoliths did (Pearson & O'reilly 1991), to temperatures of

620-1200°C from ~1300°C at pressures between 5-30kbar (geothermobarometry

estimations). Once cooling crystallised the xenoliths, they continued to cool to the

geothermal gradient. This initiated the metamorphism and activated the plagioclase

pyroxene reaction to form garnet + quartz carona rims (Equations 1 and 2). The

Angaston xenoliths experienced a very similar event except these xenoliths

recrystallised Kyanite in their metamorphic reactions much like the EMAC suite

(Pearson & O'reilly 1991, Segui 2010). These reactions went on the colder the rocks

became, until the plagioclase was removed from the assemblage to create eclogites or

they reached the geotherm.

Figure 18 shows the pressure and temperature estimations for the mafic eclogites and

granulites along with the adiabat and subduction metamorphic paths. Rocks that have

metamorphosed through subduction are characterised by ultra high pressures and low

temperature gradient. They also display a clockwise metamorphic path due to thermal

lag. If the xenoliths were metamorphosed due to subduction you would expect there to

be some evidence of this within the rocks, such as up temperature and up pressure

metamorphic textures. However I believe these rocks however I believe have evidence

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for a metamorphic path that reflects isobaric cooling from the adiabat towards the stable

cratonic geotherm gradient, which in the Jurassic was defined by the Monk Hill

Geotherm (Tappert et al. 2011) as mentioned above.

Two groups of pressure and temperature estimations were noted in the xenoliths data; a

low pressure group with varied temperatures and a high pressure group (Figure 18). The

high pressure group has estimations that reach the Jurassic geotherm (Monk Hill

geotherm) (Tappert et al. 2011), while the low pressure group is much more diverse in

temperature and no samples have reached the geotherm. A simple answer for this

difference in location due to depth (30-90km) is the amount of cooling needed to reach

the geotherm and to equilibrate according to pressure and temperature conditions.

Pseudosections created using the THERIAK-DOMINO program (De Capitani &

Petrakakis 2010) for samples Pit M22 (Amphibole Eclogite) (Figure 16) and Pit M25

(Fd-rich Granulite) (Figure 17) give further evidence to isobarically cooling

metamorphic reactions that were observed in the “mineral relationship” section by

Equation 1 and 2 and Figure 3 A, B and C. Tracing metamorphic paths under isobaric

cooling, the pseudosections of Pit M22 and Pit M25 (Figure 16 and 17) show that the

xenoliths, depending on their original pressure will produce the assemblages and rock

types we have sampled from Angaston (Segui 2010), El Alamein and Pitcairn. Segui

(2010) created a pseudosection for a granulite xenolith from Angaston and found

similar mineral relationships, mainly that an isobarically cooled magma will form

granulite and eclogite rock types.

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The comparison of the two pseudosections (Figure 16 and 17) gives evidence for the

transition of granulite to eclogite, to not just represent differences in pressure and

therefore depth, but also differences in bulk composition. Wood (1987) showed that a

quartz-tholeiite composition, which experienced Isobaric cooling (IBC) at lower crustal

pressures, would not produce eclogites; even on a steady state geotherm (40mW/m2),

but would remain a granulite. The rock type transition zones, granulite → eclogite (loss

of plagioclase from the assemblage) and gabbroic rock → granulite (introduction of

metamorphic garnet) depicted by blue lines in Pit M22 and red lines in Pit M25 are at

much lower temperatures for Pit M22. For a given temperature of 600oC the granulite to

eclogite transition (loss of plagioclase from the mineral assemblage) is at 14.7kbars for

Pit M25 and 11.6kbars for Pit M22 this shows that it is more difficult for Pit M25 to

reach the eclogite field (no Plagioclase). This difference in transition temperatures at the

same pressures, is possibly due to the high amount of aluminium needed to form

Plagioclase. Once in the eclogite P-T field, the granulites are at a much higher

temperature; thus a lot more energy, in the form of heat or pressure, is required to from

the cpx and garnet mineralogy of eclogites.

The geochemistry section describes that garnet was not a primary mineral in

crystallisation from the original melt. The MELTS software was used to calculate the

melt and cumulate trends associated with these xenoliths (Figure 20 and 21) and it

illustrated that garnet was a part of the primary mineral trends. Therefore, I suggest the

original emplacement onto the bottom of the Moho was no deeper than 9kbar or 30km.

Using the geothermobarometry estimations we see that some of the estimates reach

pressures of 25-30kbar, which represents depths of 80 to 90km (Table 6, Figure 18).

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The cessation of the Delamerian Orogeny has been inferred to have occurred due to a

buoyancy control (i.e. delamination) (Foden et al. 2006). Modelled delamination of

eclogite has shown that some of the delaminated material can be left behind (Percival &

Pysklywec 2007) and not fully delaminated into the mantle. This delamination is

inferred to take ~12myr but the pressure increase is instantaneous and may result in the

higher pressure rocks observed in the suite (Figure 18).

In conclusion these xenoliths show metamorphic signatures indicative of near isobaric

cooling towards the stable geotherm and converted gabbroic rocks to granulites and

eclogites due to their original emplacement depth (30km) and their original bulk

composition higher pressures were then reached by some xenoliths possibly due to

delamination. There is also evidence that the metamorphic paths were not controlled by

subduction.

CONCLUSIONS    

The major aim of this thesis was to further constrain the origin and metamorphic

processes under the eastern margin of the Australian craton. Kimberlitic xenoliths from

Pitcairn, Angaston and El Alamein suites are dominated by mafic granulite and eclogite

xenoliths. Modal proportions of the minerals garnet, clinopyroxene and plagioclase

were used, along with Al(6)/Al(4) ratios of clinopyroxenes, to distinguish between the

different rock types. Geochemically all three regions of xenoliths were very similar.

However, modally the Pitcairn xenoliths showed a larger percent of feldspar and a more

varied composition of feldspar. Whole rock geochemistry gave evidence that the mafic

xenolith suite formed as a sub-Moho depth, mafic underplate during Neoproterozoic

rifting with a more than likely MORB style melt high pressure cumulates and melts

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Geothermobarometry estimates were varied but all fell between the EMAC geotherm

(Pearson et al. 1991) and the Monk Hill garnet peridotite geotherm (Tappert et al.

2011). This has been identified as the signature of the sub lithospheric mantle cooling

from underplating mafic intrusions from the adiabat towards the stable geotherm (Monk

Hill geotherm) and not subduction related.

The end of the Delamerian Orogeny has been suggested to have occurred due to a

delamination event around the middle Ordovician under the Adelaidean Fold Belt

(Foden et al. 2006). The xenoliths of this study may, therefore, represent the

delaminated remains of the eclogite that caused the cessation of the Delamerian

Orogeny. Further work could be done to justify this hypothesis through dating the

metamorphic age of these rocks to further constrain the process.

Acknowledgments.

I would like to thank Kevin Wills for his extensive knowledge of the kimberlites located

in South Australia and for his help while conducting the field work. Thanks must go to

Phd student, Alec Walsh for helping me to make my two pseudosections, which would

not have been completed without his help. I would also like to thank David Bruce, John

Stanley and Katie Howard for introductions on the sample handling and preparing

equipment in the Mawson Laboratories. I would also like to thank Angus Netting, Ben

Wad and Adelaide microscopy for the use and help with their instruments. But my

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biggest thanks must go to my supervisor John Foden for his countless talks and hours he

put into to help me understand and finish this thesis.

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APPENDIX  A:  METHODS  

Sample Preparation

Whole rock and mineral separate preparation for radiogenic isotope and XRF work was carried

out in the Mawson Laboratories at Adelaide University. Sample preparation involved the removal

of weathered surfaces from samples using a diamond saw to produce 5-7 cm3 blocks to be

crushed. The crushing was completed in a stainless steel jaw crusher which was cleaned in-

between each sample. The fine and coarse material produced from crushing was then divided

into thirds with care taken to ensure each fraction contained representative amounts. Two of

these fractions were further processes for whole rock analysis and isotope work while the other

fraction was set aside for future work. Cleaning was very extensive using all machines and

implements between samples.

Samples that were selected for whole rock XRF work were then milled down to a fine powder in a

tungsten carbide mill which ran for between 2 to 3 minutes.

Major Element Geochemistry

Major and minor element analysis was undertaken in situ on individual mineral grains in thin

section. The Cameca SX51 microprobe located at Adelaide University was used to collect major

and minor element results. It produced these results using a focused beam produced by a 15kV

accelerating voltage and a 20nA beam current. Calibration of the machine was carried out during

and before analysis of samples.

Whole rock major element analysis involved the production of fused glass disks for XRF work. To

remove the water from the sample 6g of the sample powder was heated at 130oC for 4 hours.

Samples were then put into ceramic crucibles and weighed in a Toledo balance. The loss of

ignition was calculated once the sample was ignited at 960oC for 3 hours.1g of this powder was

then mixed with 4 g of lithium borate flux and fused in Pt/Au crucibles and moulds producing

fused discs.

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Trace Element Geochemistry Whole rock trace element geochemistry was acquired by XRF analyses carried out on pressed

pellets created from milled sample using the same procedure as the major elements. Pressed

pellets were created by mixing 0.8mL of EtOH/PVA binder with 6g of whole rock powder and then

put into the hydraulic press found in the Mawson Laboratories.

Radiogenic Isotope work Radiogenic isotope work was carried to obtain Nd-Sm values of samples so they could be

radiometrically dated. It was undertaken for 7 samples. The standard used was a weighed and

spiked into cleaned 15mL Teflonware PFA Vials using a Mettler Toledo AT201 balance. The

samples were spiked using Nd-Sm spike F (calculated at the additions of 0.2 Sm-Nd spike F per

1µg Nd) at estimations of 4 ppm for the whole rocks and 10ppm for the mineral separates with

the standard known to be 25ppm spiked accordingly.

The samples are dissolved through a process involving 2 lots of HF, and a final dissolution in

HCL with 7M HNO3 being added at crucial stages to retard the formation of insoluble fluorides.

Samples are then centrifuged and loaded onto a Biorad Poly Prep separating columns (2mL

AG50W X8 200-400 mesh Biorad cation exchange resin) for initial REE separation and then

loaded onto the Sm-Nd separating columns (2mL Teflon powder impregnated with HDEHP)

separating the Nd and Sm. Second dissolution involved the addition of 15M HNO3 along with 2mL

.01 µg/g H3BO3 in 6M HCL with the samples being capped and boiled for 3 days after no aqua

regia was observed. Ultra sounding was also undertaken in an attempt to ensure complete

dissolution of samples.

Nd and Sm were subsequently dried and located onto double Re filaments for Thermal Ionisation

Mass Spectrometer (TIMS) analysis. Nd and Sm analysis was carried out on a Finigan Mat 262

TIMS at Adelaide University using dynamic measurement for 143Nd/144Nd and static measurement

for 150Nd/144Nd, 147Sm/149Sm and 152Sm/149Sm. The blank failed contained <200pg 150Nd/144Nd

and <150pg Sm. The international standard JNDi-1 produced 144Nd/ 143Nd measurements of

.512074 ± 27 (n=2) and the BCR2 basalt standard produced results of 144Nd/ 143Nd .512662 ± 41

(n=1). 143Nd/144Nd and 147Sm/144Nd values were calculated using depleted mantle values taken

from Goldstein et al (1984).

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APPENDIX  B  AVERAGE  GARNET  AND  CLINOPYROXENE  DATA  (SEE  EXTENDED  APPENDIX  FOR  ALL  DATA)  Garnet  

                       Sample   Rock  Type   SiO2   TiO2   Al2O3   Cr2O3   ∑FeO   MnO   MgO   CaO   Na2O   K2O   Sum  PA  5x1   ER   38.6922   0.1099   21.8583   0.0072   20.3850   0.8484   9.1744   8.3938   0.0426   0.0083   99.52  PA  5x2   ER   38.7040   0.0696   21.1053   0.0101   21.6789   0.4797   6.7896   9.5448   0.0384   0.0103   98.43  PA  6x2   ER   38.2422   0.0353   21.6609   0.0212   0.4386   22.7472   7.0507   9.2672   0.0208   0.0060   99.49  PA  6x12   ER   37.5902   0.1116   21.8501   0.0221   19.5057   0.4637   8.9194   9.4045   0.0450   0.0065   97.92  PA  78x   Fd   38.5500   0.0423   21.0820   0.0605   22.2608   0.3594   11.3838   5.7355   0.1990   0.2742   99.95  PA  7x1   Fd   38.0804   0.0318   21.1569   0.0105   24.4398   0.6274   8.1536   8.4626   0.0207   0.0323   101.02  PA  7x2   Fd   38.8425   0.0255   20.9259   0.0189   18.9980   0.3404   11.6534   5.7642   0.3288   0.6839   97.58  Pit  M9   Fd   38.8691   0.0605   22.4463   0.0226   17.1976   0.3970   9.7796   11.1657   0.0166   0.0085   99.96  Pit  M20   Fd   38.8420   0.0443   22.6527   0.0199   16.5478   0.3961   10.7405   10.2007   0.0165   0.0063   99.47  Pit  M23   Fd   38.2208   0.1030   23.5829   0.0203   14.6148   0.1915   9.0226   13.4335   0.0344   0.0174   99.24  Pit  m25   Fd   39.6510   0.0485   22.3432   0.0164   16.9428   0.4451   9.2149   11.4013   0.0810   0.0230   100.17  Pit  M24   Fd   40.2964   0.0222   22.2574   0.0387   17.6654   0.4461   12.2032   5.8706   0.0656   0.2464   99.11  JS  Kim   AM  G   37.8135   0.0478   21.8043   0.0115   22.5739   0.4605   8.5213   7.8107   0.0184   0.0075   99.07  Pit  M26   AM  G   37.6813   0.2297   21.5027   0.0270   23.2270   0.4806   7.4231   8.5601   0.0064   0.0212   99.16  Pit  M10   AM  E   39.8328   0.0000   22.3251   0.0207   21.5280   0.5797   10.0807   6.4044   0.0185   0.0116   100.80  Pit  M22   AM  E   39.1188   0.0344   22.2700   0.0275   21.5163   0.5668   10.1163   6.3541   0.0344   0.0142   100.05          

                           

                       

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CPX                          Sample   Rock  Type   SiO2   TiO2   Al2O3   Cr2O3   ∑FeO   MnO   MgO   CaO   Na2O   K2O   Sum  

PA7x2   Fd   52.7823   0.1949   3.4947   0.0472   7.3557   0.0539   17.2495   15.9123   1.5538   0.0071   98.6513  PA78x   Fd   51.4913   0.2102   4.1645   0.0666   6.2070   0.0446   13.5394   20.0805   2.0399   0.4344   98.2785  PA7x1   Fd   50.2455   0.3142   5.0639   0.0141   8.1499   0.0510   11.5260   19.3984   2.4359   0.0391   97.2380  PA6x2   ER   52.5858   0.2801   8.0752   0.0356   6.4136   0.0228   10.0207   17.0738   4.2285   0.0060   98.7421  PA5x2   ER   52.9142   0.2887   9.8585   0.0061   6.1128   0.0510   8.7463   14.9760   5.0787   0.0123   98.0445  PA5x1   ER   52.5080   0.3849   5.4640   0.0124   7.6880   0.0944   11.5366   18.1390   3.4283   0.0060   99.2615  PA6x12   ER   51.5812   0.5177   8.8141   0.0257   5.8899   0.0514   10.4128   16.1474   4.6852   0.0145   98.1399  

                         Pit  M9   Fd   50.1396   0.5055   8.6426   0.0400   4.8288   0.0397   11.8841   20.5608   2.2584   0.0118   98.9113  Pit  M20   Fd   50.7554   0.4262   7.6067   0.0452   4.0693   0.0402   12.7513   20.9767   2.0893   0.0086   98.7689  Pit  M23   Fd   45.3256   1.1680   13.3129   0.1294   4.1291   0.0293   9.6284   18.3168   2.9883   0.0695   95.0972  Pit  M24   Fd   52.9777   0.2728   5.6205   0.0405   4.6481   0.0417   13.3488   19.7133   2.2784   0.0186   98.9603  Pit  M25   Fd   51.2061   0.5082   8.6218   0.0386   4.8902   0.0398   11.7049   20.5890   2.2109   0.0062   99.8158  JS  Kim   AMG   50.8395   0.3951   7.0936   0.0217   7.5194   0.0468   10.7914   18.2690   3.2661   0.0092   98.2517  Pit  M26   AM  G   50.9573   0.3424   5.2170   0.0222   7.6806   0.0418   11.8716   20.1112   0.0042   2.2741   98.5224  Pit  M10   AM  E   53.3985   0.3110   4.8223   0.0304   6.1969   0.0601   13.0694   20.3726   2.1216   0.0081   #######  Pit  M22   AM  E   52.1200   0.3218   4.8368   0.0850   6.0273   0.0616   12.9073   20.0830   2.0993   0.0079   98.5501      

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APPENDIX  C  HAND  SAMPLE  DESCRIPTIONS  

Lithology   Mineralogy   Sample  names   Shape  and  Size   Surface  Textures  

Fd  rich  Granulite  

garnet  +  CPX  +  OPX  +plag  +quartz  

DS012-­‐1,  DS012-­‐2,  DS012-­‐6,  DS012-­‐7,  EA08-­‐3,  EA08-­‐5,  EA08-­‐8,  PIT-­‐M9,  PIT-­‐M20,  PIT-­‐M21,  PIT-­‐M23,  PIT-­‐M24,  PIT-­‐M25,  PA  7x2,  PA  78x,  PA  7x1  

small  (3cm)  to  medium  (8cm)  and  larger  (up  to  

12cm)  

coarse  grained  light  blue  green  in  colour  with  red  orange  garnets,  black  brown  Opx  and  green  CPX,  possible  fabric.weathered  plagioclase.  Some  

with  well  defined  fabrics  

Metapelite  garnet  +  qtz  +  plagioclase  +  

biotite  ±  CPX  CAX-­‐M10,  DS012-­‐3  

large(10cm)  well  rounded  xenoliths  

coarse  grained  smooth  light  in  colour  exterios  with  some  

kimberlite  still  attached.  Dark  red  large  (1-­‐3cm)  garnets  very  visible  

Pyroxenite   OPX  +  CPX  ±  Qt   DS012-­‐4,  EA08  6  5cm    rounded  

xenotliths  very  dark  OPX    and  green  cpx  

possible  foliations  

Eclogitic  Rock   garnet  +  CPX  ±  Amph  ±  Qt  DS012-­‐5,  EA08-­‐1,  PIT-­‐M10,  Pit  M22  PA  5x1,  

PA5x2,  PA  6x1,  PA  6x12  

very  small  (1cm)  and  small  (3cm)  well  

rounded  spheroidal  elipse  shaped  xenolith  

mineralogy  dominater  by  red  garnet  and  dark  CPX.  Well  weathered  outside  surface  

Mafic  Gneiss   Opx  +  Calcite  +  Plagioclase     EA08-­‐9,  RT-­‐CAL  2XL10  small  (6cm)  angular  

xenoliths  strong  fabric  defined  by  OPX  

minerals.    

Amph  Eclogite   garnet  +  CPX  +  Plag  +  Amph  +  Qt    Pit  M26,  JS  Kim  small  to  large  boulder  size  and  sub-­‐rounded  

nodules    

characterised  by  red  garnets,  green  cpx  and  Dark  amphibole.  

Some  plagioclase  also  seen.  

Kimberlite  phlogopite  +  calcite  +  glass  ±  

garnet  ±  serpentine  AngasKim,  EA02-­‐2,  

EA02-­‐4,  EA02-­‐7  

small  to  large  boulder  size  and  sub-­‐rounded  nodules  of  kimberlite  

rough  green/white  well  weathered  exterior,  visible  brecciated  clasts  

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APPENDIX  DTHINSECTION  DESCRIPTIONS  

lithology   Thin  Section  Major  

Mineralogy  Minor  

Mineralogy   Textures  

Eclogite  Rock  PA5x1,  PA5x2,  PA6x2,  

PA6x12  Grt,  Cpx,      Am,  Rt,  Qt,  llm   Triple point equi- to sub granular euhedral texture with coarse grained

rutile

Fd  rich  Granulite  PA7x1,  PA78x,  PA7x2,Pit  M9,  Pit  M20,  Pit  M23,  Pit  

M24,  Pit  M25  

Cpx,  Opx,  Grt,  Pla  

Rt,  llm,  Am   Diverse stages of exsolution textures, rutile inclusions within garnet and or clinopyroxene

Amphibole  Granulite   Pit  M26,  JS  Kim  Cpx,  Opx,  Grt,  

Pla,  Am  Rt,  llm,   Diverse stages of exsolution textures, rutile inclusions within garnet

and or clinopyroxene

Amphibole  Eclogite     Pit  M10,  Pit  M22   Grt,  Cpx,  Am   Rt,  Qt,  llm   Triple point equi- to sub granular euhedral texture with coarse grained rutile

Gabbroic  Rock   Pit  5x9   Pl,  Cpx,  Opx,  Am   llm,  Rt   Doleritic microstructure, lath shaped plagioclase, clinopyroxenes showing an intergrowth with amphibole and minor ilmenite