Pressure-temperature-deformation paths from P-T pseudosectionsand zoned garnets: implications for metamrophic evolution of the

Eastern Fold Belt of the Mt Isa Inlier

Mohammad Sayab and Mike RubenachJames Cook University, QLD

450 475 500 525 550 575 600 625 650

2

3

4

5

6

7

8

9

43

44

0.0870.057

0.0300.045

0.023

0.020

X Garnet Mn

Isopleths

450 475 500 525 550 575 600 625 650

2

3

4

5

6

7

8

9

91

0.01

0.05

0.10

0.15

0.20

0.25

0.30

0.35

Modal proportion isopleth for garnet

450 475 500 525 550 575 600 625 650

2

3

4

5

6

7

8

9

3

41

0.72

0.764

0.788

0.81

0.827

0.832

X Garnet Fe

Isopleths

450 475 500 525 550 575 600 625 650

2

3

4

5

6

7

8

9

0.12

50.

103

0.07

60.

067

0.05

7

0.04

7

0.036

X Garnet Ca

Isopleths

oTemperature ( C)

Pre

ssu

re (

kb

ars)

Sample No. SC-192 Snake Creek Anticline

400 425 450 475 500 525 550 575 600 625 650 675 700

0

1

2

3

4

5

6

7

8

9

10

1

4

7

44

101

pl-g

bi-pl-gchl-g-pl

zo-chl-g-pl

zo-chl-pl

bi-pl-g-sill

bi-pl-sill

cd-bi-pl-sill

cd-bi-pl-and

bi-pl-g-and bi-pl-and

chl-pl

chl-bi-g-pl

chl-bi-pl

bi-st-g-pl

bi-st-pl-g-sill

chl-bi-st-g-pl

cd-bi-pl-g-and

chl-bi-st-pl

chl-bi-st-pl-g-andchl-bi-pl-g-and

chl-bi-pl-and-g-cd

MnNCKFMASH

Fe: 0.764

Mn: 0.057C

a: 0

.125

Garnet Core

Garnet Rim

Garnet rim (Average P-T mode; Thermocalc-3.2)o

T: 602± 27 CP: 6.5 ±1.1kbarssigfit: 0.9Garnet core (intersection isopleths)

oT: 526-532 CP: 5.6-6.1kbars

Al O =17.23; CaO= 1.40; MgO=2.18; FeO=7.892 3

MnO=0.14; Na O=2.33; K O=3.86 (mol%)2 2

0

0.5

1

1.5

2

2.5

3

Mn

O(w

t%)

0

0 . 5

1

1 . 5

2

2 . 5

3

3 . 5

4

4 . 5

5

CaO

(wt%

)

3 3

3 3 . 5

3 4

3 4 . 5

3 5

3 5 . 5

3 6

3 6 . 5

3 7

3 7 . 5

3 8

3 8 . 5

FeO

(Wt%

)

Plag

Garnet core Garnet Rim

Chlorite

Grt

1mm

1mm 0.2mm

100

100

H

P

Eastern Succession, Mt Isa Inlier

Yellow waterhole granite

Kuridala

Marimo-staveleyBlock

Squirrel HillGranite

Mt DoreGranite

Selwyn

Canningtion

MaramungeeGranite

Snake Creek Anticline

Weatherly Creen Syncline

Middle Creek Anticline

Eloise

Maronan

Fairmile

Mitakoodi Blook

Duc

k C

reek

Ant

iclin

e

Tommy Creek Block

Mt AngelayGranite

SaxbyGranite

Osborne

Earnest Henry

Pil

gram

F

ault

Z

one

Cloncurry

Fault

1550-1490Ma Granite

Tommy Creek Block

Wimberu Granite

Mario-staveley BlockMarimo SlateAnswer SlateStavely Formation

Roxmere QuartziteSoldiers Cap Group

Corella / Doherty Formation

Undifferentiated Cover Sequence-II

Late Fault

Major Folds TN

NarakuBatholith

Cloncurry

140 141

21

22

0 10 20

Duchess

Mary Kathleen

Dugald River

20

Mt Isa

50 km

o141 00’o139 00’

o19 00’

o21 00’

N

EFB

WFB

KL

B

Argylla Formation

Mount Albert Group

Undifferentiated Cover Sequence-I

Basement

Km

1750 - 1720Ma Wonga Granite

Sc189Sc192Sc112

Tc186Wb45

Sample locations

Sample No. SC-189Snake Creek Anticline

450 475 500 525 550 575 600 625 650

2

3

4

5

6

7

8

9

0.089

0.062

0.0390.033

0.027

0.016

0.011

0.013

X Garnet Mn

Isopleths

400 425 450 475 500 525 550 575 600 625 650 675 700

0

1

2

3

4

5

6

7

8

9

10

0.01

0.05

0.100.15

0.20

0.25

Modal proportion isopleth for garnet

450 475 500 525 550 575 600 625 650

2

3

4

5

6

7

8

9

0.61

0.64

0.67

0.710.73

0.76

0.80

20.

818

X Garnet Fe

Isopleths

450 475 500 525 550 575 600 625 650

2

3

4

5

6

7

8

9

0.25

0.18

0.12

5

0.09

5

0.06

5

X Garnet Ca

Isopleths

oTemperature ( C)

Pre

ssu

re (

kb

ars)

400 425 450 475 500 525 550 575 600 625 650 675 700

0

1

2

3

4

5

6

7

8

9

10

chl-pl-zo

zo-chl-g-pl

chl-bi-pl-zo

chl-

g-pl

chl-bi-g-pl

chl-bi-pl

bi-g-pl-sill

bi-g-pl

bi-pl-sill-cd

bi-st-g-pl

bi-pl-and-cd

bi-pl-sill

Chl-bi-pl-and-cd

bi-pl-and-chl

chl-bi-st-pl

chl-bi-st-g-pl

bi-st-pl-and-chl

bi-st-pl-and

bi-st-pl-sill

bi-st-pl-g-sill

zo-chl-bi- g-pl

st-bi-pl

MnNCKFMASHAl O =14.88; CaO= 0.85; MgO=2.39; FeO=7.102 3

MnO=0.07; Na O=0.95; K O=4.33 (mol%)2 2

H

1mm

060

30

31

32

33

34

35

36

37

38

FeO

(Wt%

)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Mn

O(W

t%)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

CaO

(Wt%

)

Garnet CoreGarnet Rim

Garnet rim (Average P-T mode; Thermocalc-3.2)o

T: 581± 26 CP: 5.5 ±1.2kbarssigfit: 0.6Garnet core (intersection isopleths)

oT: 526-530 CP: 4.4-4.7kbars

P

BSE

PPL

PPLPPL

0.2mmPPL

Chlorite

Introduction

The Eastern Fold Belt of the Mt Isa Inlier hosts polymetamorphic signatures of pro- and retrograde mineral assemblages. Examples include And-Crd-St-Bt-Ms in the Snake Creek Anticline (Rubenach and Lewthwaite, 2002) and Rosebud Syncline (Reinhardt, 1992). The occurrence of such polymetamorphic mineral assemblage implies that the Eastern Succession of the Mt Isa inlier has undergone discrete phases of metamorphism and associated deformation. This study seeks to understand the tectono-thermal evolution of the Eastern Fold Belt. Detailed structural mapping and systematic sampling has been carried out in the key areas of the Eastern Fold Belt to answer some of the fundamental and most critical questions, e.g., What is the link between the polymetamorphic nature of the Eastern Fold Belt with crustal-scale deformation episodes? We present our preliminary P-T results based on microstructural relationships and thermodynamic modeling from the Snake Creek Anticline (samples Sc192, Sc189, Sc112), Tommy Creek Block (Tc186) and White Blow Formation (Wb45) of the Mary Kathleen Fold Belt.

P-T pseudosections and geothermobarometric constraints

P-T pseudosections, that is, maps of stable mineral assemblages in P-T-X space were constructed using bulk XRF analysis (mol%) of each sample using Thermocalc-3.2 using the most recent upgrades in the chemical system (MnO-Na O-CaO-K O-FeO-MgO-Al O -SiO -H O). The pseudosections were contoured for the X , X and X in garnet with respect to increasing pressure and temperature to2 2 2 3 2 2 Fe Mn Ca

provide as means for determining P-T points for the start of the garnet growth that can be tested by microprobe analyses. These compositional isopleths should intersect at a single pressure and temper-ature for the onset of garnet growth and diverge as growth continues due to the fractionation (e.g., Vance and Mahar, 1998). P-T estimates were also carried out using the average P-T mode of the Therm-ocalc-3.21 via microprobe analysis of garnet rim along with matrix mineral phases (e.g., plagioclase rim, biotite, muscovite) and the mineral end-member activities were initially calculated with program- AX (http://www.esc.cam.ac.uk/astaff/holland/index.html).

The chlorite and plagioclase inclusions in the core and median region of the garnet porphyroblasts in samples Sc192 and Sc189 and lack of chlorite in the matrix is predicted by the pseudosections by X ,Mn

, X and X compositional isopleths for the garnet core intersection in Chl-Bt-Pl-Grt stability field. Garnet rim, matrix plagioclase, biotite and muscovite geothermobarometric estimates using the average Fe Cao oP-T feature of Thermocalc-3.21 lie within Grt-Pl-Bt field. In Sc192, the garnet core grew at 5.6-6.1kb/526 C, whereas the rim with matrix phases grew at 6.5±1.1kb/602±27 C. For sample Sc189, the garnet-

o o core grew at 4.4-4.7kb/526-530 C, whereas the rim grew at 5.5±1.2kb/581±26 C. These are the highest pressure estimates so far reported from any part of the Eastern Fold Belt of the Mt Isa Inlier. Texturally,the inclusion trails within the garnets are fine- to median-grained as compared to the matrix minerals and are not continuous, i.e., the matrix foliation is strongly deflected around the garnets, suggests that- the garnet growth occurred early in the deformation history (e.g., Passchier and Trouw, 1998).

o oThe garnet core compositional isopleths in sample Sc112 and Wb45 intersect at 3.7-3.9kb/548-550 C and 3.7-4.5kb/544-547 C, respectively. For sample Sc122, garnet rim plus matrix mineral phases indicateo 4.8±1.3kb/592±32 C. No average P-T calculations were been made for sample Wb45 as no plagioclase was found in the matrix. However, the pseudosection geometry and textures of Wb45 are very similar to

o o that of Sc112, it is assumed that Wb45 had a similar path to Sc112. For sample Tc189, the garnet core grew at 2.1-2.2kb/510-515 C, whereas the rim plus matrix phases have formed at 4.8±1.1kb/600±29 C. All three of these samples (Sc112, Wb45 and Tc189) contain inclusions that are continuous with the matrix foliation.

Tectonic implications

P-T paths from sample Sc112, Wb45 and Tc186 appear to be the part of the classic P-T path proposed by Rubenach and Lewthwaite (2002). However, the P-T paths from sample Sc192 and Sc189 show near-isobaric heating at elevated pressure. Significantly, the garnets displaying elevated P-T growth conditions appear to be older based on textural evidence (i.e., truncated inclusion trails) than the relatively low-pressure samples (i.e., continuous inclusion trails). As these two rocks were collected near the vicinity of Camel Hill and Cloncurry faults, which are interpreted as thrust/reverse faults, it is suggested that the garnets from sample Sc189 and Sc192 began to grow at elevated P-T conditions at middle crustal levels followed by rapid exhumation and cooling along the faults, or these garnets formed at early high pressure phase followed by near-isothermal decompression.

Mn

Fe

Mn Fe

Low

High

LowHigh

475 500 525 550 575 600 625 650 675

2

3

4

5

6

7

8

9

0.130

0.107

0.074

0.038

Mn isoplethsin garnet

Grt in

450 475 500 525 550 575 600 625 650

2

3

4

5

6

7

8

0.20

0.15

0.10

0.05

0.01

Modal proportionisopleths forgarnet

450 475 500 525 550 575 600 625 650

2

3

4

5

6

7

8

303132

Fe isoplethsin garnet

Grt in 0.7760.7970.827 0.846

475 500 525 550 575 600 625

2

3

4

5

6

7

8

0.04

90.

039

0.03

20.

026

0.02

1

0.01

9

0.01

7

Ca isoplethsin garnet

Pre

ssu

re (

kb

ars)

oTemperature ( C)

400 425 450 475 500 525 550 575 600 625 650 675 700

0

1

2

3

4

5

6

7

8

9

1

7

40

chl-g-pl-zo

chl-pl-zo

chl-g-pl

Chl-pl

ky-bi-g-plky-st-bi-pl-g

st-bi-g-pl

sill-bi-g-pl

st-bi-g-pl-sill

bi-pl-sill

cd-bi-pl-sill

cd-bi-pl-and

chl-

g-bi

-pl

st-bi-pl

st-c

hl-b

i-pl

chl-b

i-pl

st-bi

-pl-s

ill

bi-pl-and

bi-pl-and-g

chl-bi-st-pl-g-andchl-bi-st-pl-and

chl-bi-pl-and

chl-bi-pl-and-cd

MnNCKFMASH

0.776

0.130

0.02

1

st-chl-g-bi-pl

Garnet rim (Average P-T mode; Thermocalc-3.2)o

T: 592±32 CP: 4.8 ±1.3kbarssigfit: 1.0Garnet core (intersection isopleths)

oT: 548-550 CP: 3.7-3.9kbars

Al O =13.43; CaO= 0.22; MgO=2.83; FeO=5.982 3

MnO=0.10; Na O=1.61; K O=2.94 (mol%)2 2

Sample No. SC-112Snake Creek Anticline

0

1

2

3

4

5

6

Mn

O(w

t%)

33

33.5

34

34.5

35

35.5

36

36.5

37

37.5

FeO

(wt%

)

0

0.5

1

1.5

2

CaO

(wt%

)

Garnet core Garnet Rim

100

1mmPPL

Grt

475 500 525 550 575 600 625 650 675 700

1.5

2.5

3.5

4.5

5.5

6.5

7.5

8.5

42

0.148

0.109

0.065

0.040

Mn isoplethsin garnet

475 500 525 550 575 600 625

1.5

2.5

3.5

4.5

5.5

6.5

7.5

8.5

0.01

0.05

0.10

0.15

0.20

Modal proportionisopleths for garnet

475 500 525 550 575 600 625

1.5

2.5

3.5

4.5

5.5

6.5

7.5

8.5

0.686

0.750

0.778

Fe isoplethsin garnet

475 500 525 550 575 600 625

1.5

2.5

3.5

4.5

5.5

6.5

7.5

8.5

42

0.06

5

0.10

0.15

0.05

9

0.049

Ca isoplethsin garnet

oTemperature ( C)

Pre

ssu

re (

kb

ars)

Sample No. WB-45White Blow Formation

450 475 500 525 550 575 600 625 650 675 700

0

1

2

3

4

5

6

7

8

9

10

chl-pl

chl-pl zo

chl-g pl zo

chl-g-pl

chl-g-bi-pl

chl-bi-pl

st-c

hl-g

-bi-

pl

st-g-bi-plst-bi-g-pl-sill

ky-g-pl-st-bi

ky-g-pl-bi

st-g-bi-pl

g-pl-bi

g-bi-pl-sill

bi-pl-sill

cd-bi-pl-sill

cd-bi-pl-sill

st-bi-pl

g-bi-pl-and

0=g

0=bi

zo=0

cd=

0

chl=0

0=st

0=sill

0=ky

chl-bi-st-pl

chl-bi-st-pl-and

chl-bi-pl-and

cd-chl-bi-pl-and

cd-g-bi-pl-and

chl-bi-g-pl-and

bi-pl-and

MnNCKFMASH

Garnet core (intersection isopleths) o

T: 544-547 CP: 3.7-4.5kbars

2

3

4

5

6

7

Mn

O(w

t%)

29.5

30

30.5

31

31.5

32

32.5

33

33.5

Fe

O(w

t%)

1

1.5

2

2.5

3

CaO

(wt%

)

Garnet core Garnet Rim

90

0.5mmBSE

Sample No. TC-186Tommy Creek Block

400 425 450 475 500 525 550 575 600 625 650

0

1

2

3

4

5

6

7

34

35

Mn isoplethsin garnet

0.04

0.07

0.10

0.17

0.190.23

400 425 450 475 500 525 550 575 600 625 650

0.5

1.5

2.5

3.5

4.5

5.5

6.5

7.5

0.200.15

0.10

0.05

Modal proportionisopleths for garnet

400 425 450 475 500 525 550 575 600 625 650

0

1

2

3

4

5

6

7

0.81

0.75

0.690.66

Fe isoplethsin garnet

400 425 450 475 500 525 550 575 600 625 650

0.5

1.5

2.5

3.5

4.5

5.5

6.5

0.13

0.10 0.09

0.08

Ca isoplethsin garnet

oTemperature ( C)

Pre

ssu

re (

kb

ars)

400 425 450 475 500 525 550 575 600 625 650 675 700

0

1

2

3

4

5

6

7

8

9

10

34

78

1722

34

38

bi-g

bi-pl-g

+pl-pl

bi-pl sill-g

-sill

+sill

bi-pl-sill-g-cd-cd

+cd

bi-pl-and-g-cd

bi-pl-and-gchl-bi-st-pl-and-g

chl-bi-pl-and-g

bi-st-pl-and-g

bi-st-pl-gbi-st-pl-sill-g

-st

-st

chl-bi-st-pl-g

chl-bi-pl-g

-and

-chl

bi-st-gchl-bi-st-g

chl-bi-g

+chl -chl

-pl

+pl

chl-bi-pl

chl-b

i-pl-z

o

chl-bi-pl-g-zo +zo -zo

-g +g

chl-bi-g-zo

chl-g-zo

chl-g

-bi +bi

chl-pl-g-zo

0.10

0.190.66

MnNCKFMASH

Al O =9.94; CaO= 0.25; MgO=3.46; FeO=5.902 3

MnO=0.11; Na O=0.54; K O=2.55 (mol%)2 2

Garnet rim (Average P-T mode; Thermocalc-3.2)oT: 600±29 C

P: 4.8 ±1.1kbarssigfit: 0.8Garnet core (intersection isopleths)

oT: 510-515 CP: 2.1-2.3kbars

Al O =14.61; CaO= 0.85; MgO=2.16; FeO=15.822 3

MnO=0.40; Na O=0.20; K O=4.05 (mol%)2 2

120

BSE1.5mm

Pressure-temperature-deformation paths from P-T pseudosectionsand zoned garnets: implications for metamrophic evolution of the

Eastern Fold Belt of the Mt Isa InlierMohammad Sayab and Mike Rubenach

James Cook University, QLD

0

1

2

3

4

5

6

7

8

9

Mn

O(w

t%)

25

27

29

31

33

35

37

39

FeO

(wt%

)

1

1.5

2

2.5

3

3.5

4

4.5

CaO

(wt%

)

Garnet core Garnet Rim

High

Low

Mn

Fe

Compositional X-ray maps

Ca

Mn

Fe

Ca

Compositional X-ray maps

High

Low

Mn

Compositional X-ray maps

Fe

Ca

H

P ?

H

P

H

High

Low

P