Burial and exhumation history of a Lesser Himalayan schist ... · erosion, burial of thrust sheets beneath the active orogen, and overprinting of earlier records by subsequent deformation

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tters 264 (2007) 375–390www.elsevier.com/locate/epsl

Earth and Planetary Science Le

Burial and exhumation history of a Lesser Himalayan schist:Recording the formation of an inverted metamorphic

sequence in NW India

M.J. Caddick a,⁎, M.J. Bickle a, N.B.W. Harris b, T.J.B. Holland a,M.S.A. Horstwood c, R.R. Parrish c,d, T. Ahmad e

a Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UKb Department of Earth Sciences, Open University, Milton Keynes MK7 6AA, UK

c NERC Isotope Geosciences Laboratory, Kingsley Dunham Centre, Keyworth NG12 5GG, UKd Department of Geology, University of Leicester, Leicester LE1 7RH, UK

e Department of Geology, University of Delhi, Delhi 11 00 07, India

Received 18 April 2006; received in revised form 11 September 2007; accepted 17 September 2007

Available online
Editor: C.P. Jaupart
22 September 2007

Abstract

Coupled analysis of the pressure–temperature (PT) evolution and accessory phase geochronology of a single sample reveals theburial-uplift history of part of the Lesser Himalaya during the Middle Miocene. Phase-equilibria calculations indicate that a peaktemperature of 600–640 °C followed burial to approximately 25 km depth. Laser-ablation monazite geochronology yields a weightedmean 206Pb⁎/238U age of 11.1±2.0 Ma and a Tera-Wasserburg Concordia intercept age of 10.6±0.9 Ma, with no distinguishable agedifference between matrix and inclusion grains. Considerations of the likelihood of excess 206Pb further suggest that thecrystallization age lies in the range 9–10 Ma. Textural analysis suggests that monazite grew during prograde metamorphism. Peakmetamorphic conditions were followed by exhumation and cooling, forming a distinctively tight PT path closure. Both the shape ofthis path and its relatively young prograde phase distinguish Lesser Himalayan evolution from that typically inferred for the HighHimalaya, and allow exploration of the thermal mechanisms that operated in the western Himalaya during the interval ca. 23–6 Ma.The PTt history is characteristic of footwall heating due to rapid overthrusting of hot rock (the Higher Himalaya), followed byincorporation into a thrust sheet that exhumed the sequence rapidly enough to preserve an inverted metamorphic gradient.© 2007 Elsevier B.V. All rights reserved.

Keywords: PTt evolution; monazite chronometry; Lesser Himalaya; western Himalaya; inverted metamorphism

⁎ Corresponding author. Institute for Mineralogy and Petrology,ETH Zurich, NWE83.2, Clausiusstrasse 25, 8092 Zurich, Switzerland.Fax: +41 44 632 16 36.

E-mail address: [email protected] (M.J. Caddick).

0012-821X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.epsl.2007.09.011

1. Introduction

Despite the success of plate tectonics explanationsfor the surface expression of tectonic processes, themechanisms and controls of large-scale deformation ofthe continental lithosphere remain controversial. Major

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http://dx.doi.org/10.1016/j.epsl.2007.09.011

Fig. 1. Sketchmap of the geology of the Sutlej Valley, after Vannay andGrasemann (1998), with modifications after Vannay et al. (2004), Richards (2004)and with reference to field observations (Caddick, 2004). Foliation orientations are averages of bedding (Haimanta Fm, Rampur window and Chail Fm)and dominant schistosity (HHCS and LHCS) measurements. Lines A–B and C–Dmark location of cross-sections given in Fig. 2. Approximate samplinglocation of I01/59.2 marked by star. HHCS = High Himalayan Crystalline Sequence; LHCS = Lesser Himalayan Crystalline Sequence; LHS = LesserHimalayan Sequence; STDS = South Tibetan Detachment System; MCT = Main Central Thrust; MT = Munsiari Thrust; CT = Chail Thrust.

376 M.J. Caddick et al. / Earth and Planetary Science Letters 264 (2007) 375–390

uncertainties include whether strength resides mostly inthe upper part of the mantle lithosphere or in the lowercrust (e.g. Chen andMolnar, 1983; Jackson, 2002; Maggiet al., 2000), and whether deformation of thickened crustis controlled primarily by propagation of faults in itsbrittle part or by viscous flow of its deeper, hotter parts(e.g. Beaumont et al., 2004; Willett et al., 1993).Interpretation of the geological evolution of theHimalaya,as a prime example of active continental collision, iscentral to these debates. Present Himalayan kinematicsare well-known from earthquake distributions and faultplane solutions, fault geometries, and GPS geodesy (e.g.Molnar and Lyon-Caen, 1989; Bilham et al., 1997;Jouanne et al., 2004), but evidence of the orogen's historyis essential for reconstruction of large-scale deformationprocesses. Retrieval of this evidence is commonlyfrustrated by removal of large parts of the crust byerosion, burial of thrust sheets beneath the active orogen,and overprinting of earlier records by subsequent

deformation. Despite this, the thermal history recordedby metamorphic mineral assemblages has proved infor-mative for both Alpine and Himalayan tectonics (e.g.Oxburgh and Turcotte, 1974; England and Richardson,1977; Harrison et al., 1998) because, since the thermaltime constant of the continental lithosphere is comparableto the duration of orogenic evolution, thermal structurestypically reflect advection by tectonic displacementswithin the orogen. Consequently, the pressure–tempera-ture (PT) paths followed by various parts of an orogen canprovide a sensitive record of the tectonic regimes andtimescales experienced (England and Thompson, 1984).

The thermal evolution of the Himalaya has presented anumber of particularly intriguing problems. Most strikingis that isograds in the metamorphic core are commonlyinverted, with the highest grade rocks exposed structurallyabove lower grades. This disposition has been explainedin terms of both primary (i.e. syn-metamorphic inversionof isotherms) and secondary (post-metamorphic isograd

Fig. 2. Sketch cross sections of the Sutlej Valley. Locations, sources and abbreviations as Fig. 1. Approximate sampling location of I01/59.2 markedby star.

377M.J. Caddick et al. / Earth and Planetary Science Letters 264 (2007) 375–390

deformation) processes (Le Fort, 1975; Brunel andKienast, 1986; Searle and Rex, 1989; Stephenson et al.,2000). Shear-heating, conduction from a hot thrust sheetand heat refraction due to conductivity contrasts have allbeen invoked as possible explanations for a primaryinversion (Le Fort, 1975; England and Molnar, 1993;Jaupart and Provost, 1985). A number of studies haverecovered PT histories of rocks from the main metamor-phic unit, the High Himalayan Crystalline Sequence(HHCS), but the generally low metamorphic grade of theunderlying Lesser Himalayan Sequence (LHS) hasrendered determination of its evolution difficult. Further-more, deciphering the geometry of PT paths from peakPT estimates and quantifying the effects of bothdisequilibrium at peak PT and partial retrogression duringcooling are well-known problems (e.g. Florence andSpear, 1993). However, both the orogenic core and itslower-grade envelope are sensitive to the controllingtectonics, so understanding the history of the LHS isrequired to fully illustrate these controls. Accordingly, thispaper presents a detailed assessment of the PT history of asingle LHS rock by coupling novel pseudosectionanalysis that traces the evolution of metamorphicassemblage for a specific bulk-rock composition (e.g.Powell et al., 1998) with in-situ (in thin section)measurements of monazite (U/Pb) ages. The chief benefitof such an approach is that chemical and texturalconsiderations allow the geochronology to be closelyassociated with segments of the PT path (e.g. Foster et al.,2000; Catlos et al., 2002). ThePT path is used to elucidatethe timing, extent and significance of heat flow in theLesser Himalaya, permitting direct comparison with

predictions from thermal and mechanical models oforogenic evolution.

1.1. Geological framework

The Himalaya consist of a thrust stack of tectono-stratigraphic units overlain by marine sediments to thenorth and emplaced upon younger foreland basinsediments to the south or south-west (Figs. 1 and 2).The HHCS is commonly exposed as an 8–15 km thicknorthward dipping sequence of schists, paragneisses,calc-silicates and migmatites, with variable amounts ofleucogranitic intrusion into the uppermost parts. This isoverlain by low-grade to un-metamorphosed sedimentsalong the South Tibetan Detachment System (STDS)(Burg et al., 1983). Metamorphism of the HHCS isgenerally interpreted as a response to burial following theIndia–Asia collision at ca. 54–50 Ma (Searle et al.,1997), with maximum temperatures obtained at 30–18 Ma. Exhumation and cooling since approximately23 Ma (Vance and Harris, 1999; Hubbard and Harrison,1989; Coleman, 1998; Harris et al., 2004) was probablydue to a combination of focused denudation, extensionon the overlying STDS and shortening along the basalMain Central Thrust (MCT), the latter accommodatingup to 250 km of movement (see discussion by Hodges(2000)).

Miocene uplift of the HHCS resulted in burial andmetamorphism of part of the LHS as the footwall of theMCT. Subsequent LHS exhumation was facilitated bypropagation of shortening onto foreland thrusts, althoughevidence of Mid-to-Late Miocene LHS metamorphism


has controversially been attributed to sporadic re-activation of the MCT (Catlos et al., 2001; Harrisonet al., 1997a,b). The LHS is now usually exposed asweakly to moderately metamorphosed sediments, gran-ites and minor basic enclaves, with metamorphic gradegenerally increasing with proximity to the MCT (Pêcher,1989; Bollinger et al., 2004).

Numerous criteria have been used to map the MCTsince its original description as a juxtaposition ofcrystalline and weakly metamorphosed rocks (Heim andGansser, 1939) and there is currently debate regarding itsposition and significance. While there is general agree-ment that the MCT must be represented by intensedeformation, it has previously been identified fromcriteria including the recognition of a major shear zone,a metamorphic discontinuity, specific lithological mar-kers, a discontinuity in isotopic characteristics, or acombination of any of these. These contrasting definitionshave led to conflicting opinions regarding whether theMCT lies within or below the crystalline sequence,although there is growing acceptance that the ‘MCT-zone’evolved spatially over time and does not represent a singleshear-plane (e.g. Harrison et al., 1998; Stephenson et al.,2001; Robinson et al., 2006). Difficulty in identifying theMCT if the footwall is metamorphosed to amphibolite-grade was well illustrated by Gansser (1964), who arguedthat the Almora sheet described in his classic earlier work(Heim and Gansser, 1939) should be reinterpreted as partof the MCT footwall. Subsequent deformational andlithological correlations showed this to be consistent withother parts of the central and western Himalaya (e.g.Brunel, 1986; Valdiya, 1980).

1.2. Geological setting of the Sutlej Valley, NW India

The Sutlej Valley section exposes a metamorphicgrade increase from garnet-in to muscovite-out withincreasing structural height (Figs. 1 and 2) (Bhargavaand Bassi, 1994; Jain et al., 2000; Vannay andGrasemann, 1998, 2001; Vannay et al., 2004). Bothupper and lower margins of the crystalline zone aremajor tectonic boundaries, as evidenced by deformationindicators (Vannay and Grasemann, 2001; Vannay et al.,2004) and contrasting prograde (Vannay and Grase-mann, 1998; Caddick, 2004; Vannay et al., 1999) andcooling (Vannay et al., 2004) histories. In addition, thecrystalline core contains at least one major shearstructure (the MCT, Fig. 1), which separates a crystallinepart of the LHS (the Lesser Himalayan CrystallineSequence, LHCS) from the overlying HHCS. Thisstructure, a km-scale mylonite zone locally cross-cut bya secondary extensional overprint (Vannay et al., 2004),

juxtaposes two sheets with similar metamorphic gradesand is, thus, more difficult to distinguish than the MCTas originally described by Heim and Gansser (1939).Indeed, based upon their chief criterion of metamorphicgrade, the MCT would be situated at our MunsiariThrust (Fig. 1), which lithological and geochemicalcriteria suggest is a juxtaposition of two LesserHimalayan bodies. Formation of an extensive mylonitezone and the shape and timing of partial PT paths(Caddick, 2004), however, suggest that the mappedMCT (Fig. 1) represents a major tectono-metamorphicbreak which is also coincident with changes indeformation style (Vannay et al., 2004), a characteristicchange in garnet zoning and deformation (Caddick,2004), and a juxtaposition of isotopic characteristics(Richards et al., 2005). Hanging wall rocks consistentlyyield whole-rock 87Sr/86Srb0.8 and ɛNdN−13 (cor-rected for a 500 Ma reference), whilst those associatedwith the footwall Lesser Himalaya typically yield87Sr/86Sr of 0.75–1.1 and ɛNdb−16 (Richards, 2004).In common with other areas (De Celles et al., 2000;Parrish and Hodges, 1996), detrital zircons from theHigh Himalaya characteristically yield either Archaean-to-Palaeoproterozoic or Neoproterozoic ages, but onlyArchaean-to-Palaeoproterozoic zircons have beenrecorded in the Lesser Himalaya (Richards et al., 2005).

1.2.1. The Lesser Himalayan crystalline sequenceThe LHCS consists of a granitic gneiss and a sequence

of medium to high-grade metasediments. The Wangtugneiss is a 6–7 km thick Palaeoproterozoic (Richards etal., 2005; Miller et al., 2000) peraluminous granite sheetwith highly deformed northern and eastern margins.Asymmetrically-mantled feldspar blasts imply top-to-the-SW shear (Grasemann et al., 1999). The Jutogh Group isan ca. 9 km thick sheet of Lesser Himalayan (Valdiya,1980) micaschists and quartzites, interbedded with minormarbles, calc-silicates and graphitic schists. Metamorphicgrade increases stratigraphically upwards from garnet- tosillimanite-grade (Vannay and Grasemann, 1998; Vannayet al., 1999) and a basal mylonite zone marks a faultedcontact with underlying chlorite-grade rocks of theRampurWindow (Figs. 1 and 2). Zircons from the JutoghGroup and the RampurWindow yielded 207Pb/206Pb agesof 2.26–1.89 and 1.95–1.87 Ga, respectively (Richardset al., 2005), and meta-volcanic layers within the windowyielded zircon evaporation ages of 1.80±0.01 Ga(Miller et al., 2000). Six Jutogh Group samples yieldedɛNdb−17.2 and two from the underlying window yieldedvalues of −18.5 and −18.6 (Richards, 2004). These areconsistent with other Lesser Himalayan isotopic data, andthe window may represent a weakly metamorphosed

Fig. 3. Photomicrographs of textures in sample I01/59.2. a), b) and c) ca. 3 mm diameter garnet porphyroblast with poikiloblastic staurolite in aquartz+biotite+muscovite matrix. Garnet overgrew a pre-existing foliation defined by quartz and ilmenite inclusions. A sub-parallel foliation ispreserved as inclusions in staurolite. Garnet has subsequently been fractured, with quartz and biotite infilling the crack (see Fig. 4). d) small staurolitecrystal included in biotite and a larger staurolite crystal partially mantled by biotite+muscovite and undergoing replacement by chlorite. Chlorite alsolocally replaces biotite.


equivalent of the overlying amphibolite-grade sheet(Caddick, 2004). Palaeoproterozoic anatectic graniteswithin the Jutogh Group conclusively correlate it withother Lesser Himalayan lithologies, considerably pre-dating the deposition of High Himalayan protoliths(Chambers et al., 2006).

2. Analyses of sample I01/59.2

Sample I01/59.2 is a staurolite, garnet, biotite,muscovite, quartz and chlorite bearing pelitic schistfrom the Jutogh Group of the LHCS (Fig. 1). Ilmenite,zircon, apatite and monazite are present as accessoryphases. Analysis yielded an ɛNd(500) value of −17.7(Richards, 2004).

2.1. Petrography and mineral compositions

The dominant ferro-magnesian phase is staurolite(modal proportion ca. 23%) of which 1–3 mm poikilo-blasts overgrew a foliation preserved as aligned

inclusions (Fig. 3). Cross-cutting relationships (e.g.Fig. 3d) suggest that staurolite pre-dated biotite growth.Garnet (modal proportion ca. 4.5%) appears to havegrown with staurolite and occurs as 1–3 mm porphyr-oblasts containing aligned inclusion trails with an open,ca. 1 mm wavelength folding. Both garnet and stauroliteare locally replaced by chlorite, although chloritisationis spatially patchy, presumably reflecting local fluidavailability during retrogression.

Unlike staurolite, which preserves no majorelement zoning, the decreasing Mn content and Fe/Mg ratio from core to rim of garnet porphyroblastsindicate growth during heating (e.g. Spear et al.,1990). Absolute Fe and Mg contents both increasefrom core to rim (Fig. 4a), Ca is relatively unzoned,and retrogressive Mn-rich rims are absent (see Table 1for average rim composition). Crystal cores areunzoned in Y, but can be mantled by a distinct Y-depleted rim which also extends as a thin layer alongfracture walls in one porphyroblast (Fig. 4b). Thisprobably records either a hiatus in crystal growth (the

Fig. 4. a) Fetotal, Mg, Mn and Ca X-ray maps of a garnet crystal sectioned perpendicular to the primary foliation. In each case, lighter grey implieshigher concentration, as shown by quantitative spot analyses (recalculated and shown as cations per formula unit, with all Fe assumed to equal Fe2+).b) YLα X-ray map showing a homogenous garnet core mantled by a lower-Y rim (relative abundance scalebar given in c). c)–f) show a proposedgarnet growth and cracking history, assuming extension parallel to the plane of the section: c) garnet with an homogenous Y profile overgrew acrenulated foliation; d) continued garnet growth followed a phase of Y-depletion, resulting in formation of a low Y rim; e) rim growth continued as thecrystal was sheared, fracturing along an inclusion-rich plane of weakness. A thin, low Y rim to the fracture wall formed; f) subsequent biotite andquartz growth infilled the fracture.


rim growing under significantly different PTX condi-tions to the core) or continual growth during which anadditional LREE-bearing phase sequestered availableY. Later Y-loss from the crystal rim cannot explain theprofile since the sharp step separates two relativelyhomogenous domains and is thus not indicative ofdiffusion following Y-depletion at the garnet–matrixinterface. Back-rotation of fractured garnet segmentsproduces a good fit if the low Y domain is removed(Fig. 4), suggesting that the outermost parts of thecrystal grew during or after extensional cracking.Biotite and quartz subsequently grew within thisfracture.

2.2. PT evolution

Sample I01/59.2's PT evolution has been assessedwith themethod outlined byPowell et al. (1998), using theThermocalc 3.25 update of the Holland & Powell (1998)thermodynamic data set. Equilibria were calculated in aNa2O–CaO–MnO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 bearing system, where Fe2O3 calcula-tion followed White et al. (2000). Inclusion of TiO2 andFe2O3 facilitates modelling of oxide stability and Tiexchange into phases such as biotite (which typicallycontains N1.7 wt.% TiO2, Table 1). Biotite and stauroliteare both partly retrogressed to chlorite, so H2O was

Table 1Representative electron-microprobe analyses (EPMA) of major minerals in sample I01/59.2

SiO2 TiO2 Al2O3 Cr2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O

Si Ti Al Cr Fe3+ Fe2+ Mn Mg Ca Na K

Garnet (rims)wt.% ox 37.16 0.01 21.26 0.01 1.43 34.39 2.54 2.92 1.56 0.05 0.01CPFU 2.96 0.00 2.00 0.00 0.09 2.29 0.17 0.35 0.13 0.01 0.00

Biotitewt.% ox 35.48 1.75 19.76 0.02 0.35 18.55 0.06 9.69 0.03 0.36 9.09CPFU 2.69 0.10 1.76 0.00 0.02 1.18 0.00 1.10 0.00 0.05 0.88

Muscovitewt.% ox 45.71 0.51 35.94 0.02 0.05 1.38 0.02 0.72 0.01 1.02 10.09CPFU 3.04 0.03 2.82 0.00 0.00 0.08 0.00 0.07 0.00 0.13 0.86

Staurolitewt.% ox 27.44 0.62 54.46 0.04 0.00 14.25 0.17 1.61 0.01 0.04 0.02CPFU 7.57 0.13 17.72 0.01 0.00 3.29 0.04 0.66 0.00 0.02 0.01

Chloritewt.% ox 25.19 0.07 20.78 0.02 0.00 27.48 0.20 12.01 0.09 0.13 0.15CPFU 2.75 0.01 2.66 0.00 0.00 2.51 0.02 1.94 0.01 0.03 0.02

Results (recast with the AX program—available for download from http://www.esc.cam.ac.uk/astaff/holland/ax.html) given in weight % oxide andcations per formula unit (CPFU).


modelled in excess. Separate pseudosections werecalculated for the whole-rock bulk composition and arecalculated ‘effective’ bulk composition representingconditions following element sequestration into the coresof zoned garnet porphyroblasts. Staurolite is unzoned sosequestration of components within it was not considered.Activity-composition models were modified from Zeh &Holness (2003), White et al. (2000), Coggon & Holland(2002), and references therein.

2.2.1. Prograde PT historyFig. 5 is a pseudosection for sample I01/59.2′s

whole-rock composition (calculated by point countingand from XRF). Heating from upper greenschisttemperatures at pressures above ca. 4 kbars wouldresult in the isograd reaction sequence:

(chloritoid + staurolite + garnet + chlorite + muscovite)Y chloritoid-outY biotite-inY chlorite-outYAl2SiO5�in Y staurolite-out Y melt-in.

This reversal in the Barrovian sequence (garnet+staurolite-bearing assemblages occurring before biotitestability) is also evident texturally (Fig. 3) and results fromhigh bulk-rock FeO/FeO+MgO andAl2O3/Al2O3+K2O,above the garnet–chlorite tie line in AFM-space. Thestaurolite-in reaction is calculated at temperatures 10–15 °C cooler than chloritoid-out, with rapid growth of upto 18% staurolite predicted during the first few degrees

(°C) of its stability (Fig. 5). Garnet-in occurs at asimilar temperature, replacing plagioclase (at lowpressures) or epidote (above ca. 5 kbars). The increasein garnet modal proportion is, however, significantlyless rapid than that of staurolite, with garnet comprisingless than 2 % of the assemblage at chloritoid-out (ca.560 °C at 7 kbars).

Isopleths representing the measured composition ofgarnet crystal cores from sample I01/59.2 intersect inthe garnet + staurolite + biotite + chlorite+muscovite(+quartz, ilmenite and water) field, at approximately6.8–7.6 kbars, 580–590 °C (Fig. 5c). Within this narrowfield the predicted modal proportion of biotite increasesrapidly with temperature (from 0% to 11%), chlorite andmuscovite contents decrease (from 9% to 0% and 22%to 17%, respectively), staurolite content reaches 24%,and garnet content reaches 4%. No evidence remains toilluminate the PT path prior to these conditions, largelydue to the absence of chloritoid or epidote whichpresumably reflects either sluggish growth rates at lowtemperature or subsequent resorption during heating.Furthermore, although diffusional modification of thegarnet-core composition was probably extremely limit-ed for 1–3 mm diameter porphyroblasts (given that theydid not reach temperatures exceeding ca. 650 °C), anysuch modification would move crystal-core compositionisopleth intersections towards higher temperature than

http://www.esc.cam.ac.uk/astaff/holland/ax.html

Fig. 5. a) Pseudosection for sample I01/59.2. Mineral abbreviations after Kretz (1983). Supra-solidus equilibria have not been calculated (melt-bearing fields are stippled). Diagram b) highlights several important (isograd) equilibria. c) isopleths corresponding to garnet-core almandine andgrossular contents determined by EPMA.


experienced during initial garnet-growth. Microprobeanalysis of an un-centered garnet section would have anidentical effect.

2.2.2. Peak PT conditionsSequestration of components into porphyroblasts

resulted in a modified reactive bulk composition,modelled in Fig. 6. Many stabilities are almost identicalto those in Fig. 5 (e.g. the field across which biotitegrows at the expense of chlorite above ca. 4.5 kbars),but all low pressure assemblages, most low temperatureassemblages, and the compositions of all co-existingphases differ. Note in particular that garnet stability isreduced and muscovite stability increased at lowpressures and moderate-to-high temperatures, and thatplagioclase stability extends to higher pressures in theregion of the Al2SiO5 triple point.

Fig. 6b shows calculated contours for modal propor-tion biotite and XGrossular which imply that sample I01/59.2 equilibrated at peak conditions of ca. 7.0–7.3 kbars,600–640 °C. The upper temperature limit is constrainedby the lack of kyanite in the sample. At these PTconditions, the predicted assemblage contains approxi-

mately 39% quartz, 24% staurolite, 17% muscovite, 12%biotite, 5% garnet, and small quantities of ilmenite andwater. This agrees well with observed proportions, butcontains no chlorite. Biotite content is strongly pressuredependent within this field, decreasing rapidly towardshigher pressure as both garnet and muscovite contentsincrease (garnet proportion isopleths would be sub-parallel to the XGrossular isopleths shown in Fig. 6b).Equilibration at greater depth would, therefore, haveresulted in a lower biotite content than observed.Calculated XAlmandine (not shown) closely matches theanalysed value (Table 1) at 7.3 kbars, 630 °C.

2.2.3. RetrogressionLack of plagioclase growth at the expense of garnet,

and growth of chlorite around biotite and staurolite(Fig. 3d) suggest a PT trajectory which cooled to thegarnet+chlorite+staurolite+biotite+muscovite bearingfield instead of decompressing into plagioclase-bearingfields (for example, field ‘3’ in Fig. 6). This necessitates a‘tight’PT loop closure (i.e. little decompression precedingcooling). Retention of over 18% biotite in sample I01/59.2 is consistent with a limited free-water content during

Fig. 6. a) Pseudosection representing sample I01/59.2 following sequestration of components into garnet porphyroblasts (see text). Supra-solidusequilibria have not been calculated. Black star represents an average PT estimate (calculated with Thermocalc), with 2σ uncertainty ellipse (dashedline) indicating the large pressure uncertainty (±3.3 kbars) associated with this plagioclase-absent sample. b) predicted garnet composition isopleths(X Grossular contour frequency=0.002, average garnet-rim composition from EPMA marked with a bolder contour) and a 12 % predicted modalproportion biotite contour (dashed).


retrogression, with chlorite growth arrested wherever anH2O-rich fluid was absent.

2.3. Monazite geochronology

Monazite has been dated with an in-situ technique,allowing interpretation of data in light of textural and

Table 2EPMA monazite compositions (wt.% oxide)

I01/59.2 SiO2 P2O5 ThO2 CaO Y2O3 La2O3

Matrix mon 1 a1 0.6 30.0 5.9 1.1 0.9 14.9Matrix mon 1 a2 0.5 29.6 4.3 0.8 1.4 15.3Matrix mon 3 a1 0.6 29.7 6.5 1.2 0.7 14.6Matrix mon 3 a2 0.6 29.7 5.7 1.0 1.4 14.1Matrix mon 3 a3 0.6 31.1 5.2 1.0 1.5 15.3Matrix mon 3 a4 0.5 29.3 5.2 1.0 1.7 14.5Mon in staur 1 a1⁎ 0.4 30.3 4.4 0.8 1.5 15.1Mon in staur 1 a2 0.4 30.6 4.6 0.9 1.2 15.3Mon in staur 1 a3 0.4 30.5 4.6 0.9 1.6 14.9Mon in staur 1 a5 0.4 30.5 4.2 0.8 1.8 14.9Mon in staur 3 a1 0.5 30.1 4.5 0.8 1.5 15.1Mon in staur 3 a2 0.4 30.7 4.1 0.8 1.5 15.3Mon in staur 3 a3 0.5 29.4 4.0 0.8 1.3 15.2Detection limit 29 172 497 64 618 768

Typical detection limits (given in ppm) calculated for analysis ‘Mon in staconsistently high (100–104%). Pb was consistently below detection limit (1

chemical factors (e.g. Foster et al, 2000; Catlos et al.,2002) and close integration with PT evolution estimates.

2.3.1. Sample selection and analysisMonazite grains were identified and characterized

using a back-scattered electron detector and energy-dispersive spectrometer. X-ray maps for YLα, ThLα and

Ce2O3 Nd2O3 Pr2O3 SmO Gd2O3 U2O3 Total

31.0 11.8 2.8 1.5 1.6 0.2 102.331.3 12.1 2.8 1.6 1.8 0.2 101.731.4 11.7 2.9 1.6 1.5 0.3 102.831.0 11.8 2.8 1.6 1.8 0.3 101.730.8 12.2 2.8 1.6 1.9 0.2 104.130.2 11.9 2.8 1.6 1.9 0.3 100.931.0 12.0 2.8 1.5 1.8 0.2 101.931.6 11.7 2.7 1.5 1.7 0.3 102.631.0 12.0 2.8 1.6 1.7 0.3 102.230.7 12.0 2.9 1.6 2.0 0.3 101.931.4 12.0 2.8 1.5 1.8 0.2 102.330.9 12.2 2.9 1.6 1.8 0.3 102.531.8 11.9 2.9 1.6 1.7 0.2 101.3795 733 634 519 571 406

ur 1 a1’ with Cameca system software. Notice that total counts are300 ppm) and is not shown.

Fig. 7. Back scattered electron and YLα maps of monazite crystals from I01/59.2. a) and b) = matrix grain 1; c) and d) = matrix grain 3; e) and f) =grain 1 included in staurolite; g) and h) = grain 3 included in staurolite. Numbered spots represent Y2O3 content (in wt %, see Table 2). Element mapscollected with probe current of 100 nA, accelerating voltage of 15 kV, and dwell time of 30 ms/pixel.


CaLα were then collected for larger grains and spotanalyses were made following the procedure outlined byScherrer et al. (2000). Compositions are given in Table 2and Fig. 7.

Laser-ablation multi-collector ICPMS (LA-MC-ICPMS) analyses followed the method outlined byHorstwood et al. (2003), using a 266 nm laser rasteredover the ca. 50×50 μm sample and penetrating to ca.15–20 μm depth. Five grains were successfullyanalysed and were accompanied by analyses of amatrix-matched standard (Manangotry monazite: ca.555 Ma). Dating of additional grains was limited by thenumber and small size of crystals present, and numerousadditional analyses were discarded because the grain ofinterest was destroyed before satisfactory count timeswere reached. Separate analyses of surrounding phases(garnet, biotite, staurolite, muscovite and quartz)liberated barely detectable U and 206Pb concentrations,ensuring that the vast majority of the measured U and Pbions originated from monazite ablation, and thatunavoidable ablation of these other phases did notsignificantly contribute to U or radiogenic Pb measuredin monazite.

The maximum quantity of 206Pb measured in eachgrain did not exceed 50–100 fg, and given the time ofablation and sensitivity, uncertainties on individual206Pb⁎/238U measurements were 2–6 % (1σ). Sincemonazite incorporates Th, it is probable that excess206Pb originating from 230Th decay was present in themonazites analysed (Table 3) (Schärer et al., 1984;Parrish, 1990). Against this background, the 206Pb/238U

ages reported here are likely to be maximum ages ofcrystallization, with the most likely age lying nearer theyounger range of the reported uncertainty bands.

2.3.2. LA-MC-ICPMS resultsFour analysed monazite crystals are shown in Fig. 7.

An additional (unpictured) matrix grain was alsoanalysed. All crystals were euhedral-to-subhedral, 30–60 μm long and 20–40 μm wide, releasing 20–80 ng ofablated material during analysis. The three matrix grainswere at the intersection of muscovite and quartz crystalsand the inclusion grains were completely surrounded bystaurolite. All monazite grains (including additional,undated crystals) were zoned in Y, with sub-to-anhedralhigh-Y cores overgrown by low Y rims. Corescontained 1.4–1.8 wt.% Y2O3, with rim overgrowthscontaining 0.7–0.9 wt.% Y2O3 in matrix crystals and1.1–1.2 wt.% Y2O3 in inclusion crystals.

LA-MC-ICPMS results are shown in Table 3.Regression through all five analyses yields a Tera-Wasserburg concordia intercept age of 10.7±3.1 Ma,with a mean square of the weighted deviates (MSWD) of6.1. The analysis of matrix grain 10 appears not to fit thearray of the other four analyses, which yield an interceptage of 10.6±0.9 Ma with a MSWD of 1.4 (Fig. 8a).Regression through just matrix grain 10 and the mostconcordant point yields an intersection at 10.4±0.9 Ma,and regression through the two inclusion grains yields10.3±0.9 Ma. Application of a common-Pb correctionwith a 10 Ma Stacey-Kramers Pb model (Stacey andKramers, 1975) results in 206Pb⁎/238U ages ranging from

Table3

LA-M

C-ICPM

Sresults

Ratiosno

tcorrectedforcommon-Pb

Corrected

forcommon

-Pbat

10Ma

Analysis

206Pb⁎

238U

Ua

f206

b208Pb/206Pb

1σ207Pb/206Pb

1σ206Pb/238U

1σ207Pb/235U

1σRho

c206Pb⁎/238U

1σ206Pb⁎/238U

age

2σ(m

V)

(mV)

(ppm

)(%

)(%

)(%

)(%

)(%

)(%

)(M

a)(M

a)

I01/59

.2Mon

instaur1

0.302

169

1065

6.5

3.58

71.51

0.08

2816

.22

0.0016

93.69

0.0193

16.63

0.22

20.00

147

12.36

9.49

2.35

Mon

instaur3

0.409

181

1139

48.3

2.96

81.69

0.40

115.92

0.0035

76.42

0.1975

8.73

0.73

50.00

202

7.51

12.99

1.95

Matrix1

0.292

132

833

28.7

3.24

81.30

0.27

824.64

0.0027

93.76

0.1071

5.97

0.63

00.00

191

17.46

12.28

4.29

Matrix3

0.229

121

761

61.8

3.05

91.57

0.43

564.42

0.0041

35.32

0.2482

6.92

0.76

90.00

155

15.51

9.99

3.10

Matrix10

0.431

383

2409

53.3

1.73

30.76

0.49

202.20

0.0034

27.24

0.2320

7.57

0.95

70.00

160

10.73

10.28

2.21

aU

contentaccurate

toca.10

%(H

orstwoo

det

al.,20

03).

bPercentageof

206Pbwhich

iscommon.

cError

correlationbetween

206Pb/238U

and

207Pb/235U.


9.5–13.0 Ma, with a weighted mean age of 11.1±2.0 Ma(2σ) and aMSWDof 1.8 (Fig. 8b). This is consistent withthe more precise Tera-Wasserburg regression. Analysis ofsmall and young crystals (containing little total Pb andproportionately less radiogenic Pb, respectively) results incomparatively large analytical uncertainty, as reflected inthe 2.0–4.3 Ma 2σ uncertainties on individual analyses(Fig. 8b).

Given that there is no appreciable age differencebetween matrix and inclusion grains we take the precisefour grain Tera-Wasserburg age (10.6±0.9 Ma, Fig. 8a)to be a reasonable estimate of the growth age. Th–Pbages were not measured so bias arising from excess206Pb cannot be estimated, but comparison withliterature data (e.g. Parrish, 1990) suggests that this isunlikely to exceed 1–2 Ma. The most likely monazitegrowth age, therefore, probably lies towards the youngerlimit of the reported uncertainty range (i.e. 9–10 Ma).

2.3.3. Geochronology in a metamorphic contextGrowth ages of matrix and inclusion monazite grains

are indistinguishable within error, implying either thatall grains grew simultaneously and some grains weresubsequently incorporated into a quartz+muscovitematrix whilst others were occluded by a porphyroblast;that ‘inclusion’ grain growth preceded ‘matrix’ graingrowth but that the difference between ages is notresolved by the dating technique; or that all ages werereset by, or grew from, a late low T fluid (e.g. Searleet al., 2002). We discount resetting by late fluids becauseinclusion grains appear to be completely shielded andthere is little evidence of retrogression of the hostpoikiloblasts. This is consistent with recent Th/Pb datafrom the Lesser Himalaya of western Nepal whichreveals prograde monazite growth at 11.4–9.3 Ma andretrogressive growth at the expense of allanite at 5.8–3.3 Ma (Bollinger and Janots, 2006). Alignment ofadditional monazite inclusion grains parallel to afoliation preserved by quartz and ilmenite inclusionssupports growth before occlusion by staurolite.

Analysed grains were simply zoned, with high-Ycores overgrown by lower-Y rims (Fig. 7). Zoning wasmore prevalent in matrix crystals, where the rims werelarger and less Y-rich than in inclusion grains. This isindicative of either two phases of monazite growth or ofcontinual growth during which an additional Y-seques-tering reaction occurred. Since the Y budget of a peliticrock is mainly governed by garnet and xenotime,monazite Y content is likely to reflect the local presenceor absence of these phases during its growth, and Yreleased during prograde xenotime breakdown willfractionate strongly into growing monazite and/or garnet

Fig. 8. a) Tera-Wasserburg plot of data from matrix monazites 1 and 3 and monazite inclusions 1 and 3 in staurolite (see text for details). Concordia(un-dashed line) labelled with ages (Ma). b) Weighted mean of all 206Pb⁎/238U data (calculated using ISOPLOT, Ludwig, 2003).


(Bea and Montero, 1999; Pyle and Spear, 1999). Fosteret al. (2002) observed that monazite grains recordingseveral age components (e.g. crystal rims overgrowingdemonstrably younger cores) may also preserve dis-continuities in major or trace element chemistry,particularly in Y content. Our data may, therefore,record mixing of older (Y-rich crystal core) and younger(rim) components, although no grains were sufficiently

Fig. 9. a) Postulated PTt evolution based upon pseudosection constraints andmuscovite cooling age from Vannay et al. (2004), PT position of monazite grComparison of results with paths derived from garnet zoning in the LHCSBeaumont et al. (2004) and Jamieson et al. (2004) numerical model. Starscollision occurred 54 Ma. Rocks experiencing paths L3 and L4 are predicte

large to allow separate analysis of core and rim segmentsand there is no systematic relationship between texturalsetting of a grain and its measured age. Our conservativeinterpretation of the data in accord with texturalindicators, therefore, is that the Tera-Wasserburg agebrackets staurolite growth at approximately 10.6±0.9 Ma (or 9–10 Ma assuming 206Pb excess as discussedabove).

monazite ages. Mineral equilibria and bold arrows from Figs. 5 and 7,owth bracket as discussed in text. Dashed arrows are assumed paths. b)of central Nepal (Kohn et al., 2001) and with paths predicted by theon the latter are 6 Myr markers, calibrated assuming that continentald to occur at approximately the same position as I01/59.2.


3. Discussion

3.1. A PTt path for the Lesser Himalaya

Combining PT results with monazite geochronologyand inferences of the early burial and late exumationhistory leads to a postulated PTt path of sample I01/59.2(Fig. 9). This implies burial to ca. 7.5 kbars, withresultant heating through reactions producing stauroliteand garnet at ca. 10 Ma. Biotite then grew at theexpense of chlorite at ca. 580 °C, with garnet zoning,modal proportion biotite, the loss of prograde chloriteand the absence of kyanite growth all implying amaximum temperature of 600–640 °C at ca. 7 kbars.Absence of an appreciable 206Pb⁎/238U age differencebetween matrix and inclusion monazite grains impliesrapid heating between staurolite-in and the attainment ofpeak T. The Y-rich garnet rim shown in Fig. 4 must havegrown during the final stage of heating and wassubsequently wrapped by muscovite bundles (Fig. 3)that also surround staurolite and monazite. This suggeststhat localized extension (resulting in garnet cracking,Fig. 4) must have pre-dated cooling, and is consistentwith biotite growth infilling the fracture. Low Ymonazite rims may indicate a late growth componentrelated to the Y-depleted garnet rim shown in Fig. 4, butthis was not resolved analytically. Cooling accompa-nying decompression from ca. 7 kbars resulted inchlorite growth without garnet breakdown to plagiocase.

3.2. Geochronology

Prograde LHCS metamorphism at ca. 10Ma contrastswith earlier heating in the HHCS of the western Himalayaand supports a foreland-directed progression of metamor-phism during the Miocene (e.g. Stephenson et al., 2000;Robinson et al., 2006). HHCS garnet Sm–Nd ages of 33–28 Ma (Vance and Harris, 1999) and 44–36 Ma (Fosteret al., 2000) indicate Oligocene HHCS burial, and 22.4–21.9Mamonazites in leucogranite (Harrison et al., 1997a,b; Searle et al., 1999) record anatexis that has been linkedwith exhumation (Harris and Massey, 1994). Given thatdetrital mica ages imply very rapid (ca. 5 mm yr−1)HHCS exhumation at 21–19 Ma (White et al., 2002),it seems probable that shortening on the MCT initiatedbefore 20 Ma, burying the LHS from that time.Sample I01/59.2 and a published LHCS garnet Sm–Ndage of 11±1.1Ma (Vannay et al., 2004) suggest that burialceased at or before ca. 10 Ma with a near isobaric finalstage of heating followed by rapid exhumation. Exhuma-tion is consistent with the appearance of low-grade LHSdetritus in the sediment record at ca. 11 Ma followed by

erosion and deposition of increasingly highmetamorphic-grades until ca. 6 Ma (Najman, 2006). 40Ar/39Armuscovite ages from Sutlej Valley LHS rocks (Vannayet al., 2004) confirm that average exhumation ratesbetween 2 and 3 mm yr− 1 persisted from peakmetamorphism (ca. 25 km depth) until ca. 5 Ma, whenmuscovite blocking temperatures of ca. 450 °C (Vannayet al., 2004) were reached (assuming from Fig. 9 that450 °Cwas reached at ca. 11 km depth). This is consistentwith LHCS zircon and apatite fission track ages in theranges 4.8±0.8–1.7±0.3 Ma and 1.7±0.3–0.7±0.6 Ma,respectively (Vannay et al., 2004). It is clear, however, thatour understanding of the movement history of the majorHimalayan faults is still incomplete, a problem highlight-ed by detrital records which document major changes insource provenance and metamorphic grade that aredifficult to reconcile with simple models of thrustevolution (e.g. White et al., 2002).

3.3. Tectonic controls on PTt path geometry

A tight LHCS PT path closure around the point ofmaximum temperature (Fig. 9) is consistent with heatingdue to overthrusting of the hot HHCS, followed bytectonically-driven exhumation (Harrison et al., 1998;Catlos et al., 2001; Kohn et al., 2001). Similar ‘hairpin’PT paths have been deduced from the Lesser Himalayaof central Nepal (Catlos et al., 2001; Kohn et al., 2001),although lower maximum temperatures were attainedthere (Fig. 9b). Rocks from stratigraphically beneathsample I01/59.2 typically display similar deformationcharacteristics but reached lower maximum tempera-tures (e.g. Vannay et al., 1999), whilst overlying samplespreserve evidence of kyanite and/or sillimanite stability.

The interpretation that preserved inverted metamor-phic gradients result from a temperature inversion duringthrusting of hot over cold rock (e.g. Le Fort, 1975) iscontroversial. England and Molnar (1993) examinedsimple physical models and concluded that the develop-ment and preservation of inverted temperature gradientsrequires significant shear heating on faults (requiringshear stresses N100 MPa). Deformation experiments onwet quartzite (e.g. Gleason and Tullis, 1995) and feldspar(e.g. Rybacki and Dresen, 2004), however, suggest thatsuch strength is unlikely in hot rocks deforming underrelevant strain rates. England andMolnar's (1993) modelconsidered underthrusting of an initially cold lowerblock (e.g. the LHS) beneath a stationary upper block(the HHCS) and showed that thermal evolution wasbounded by two critical time constants; the time taken formaterial initially at the surface to reach the depth ofinterest, and the time constant for diffusive cooling


through the overlying unit. However, in the case of theHimalayas during the early Miocene, the upper block(the HHCS) was being rapidly eroded, with ca. 5 mmyr−1 exhumation throughout the early evolution of theMCT (White et al., 2002) translating into a thrustvelocity of 30–60 mm yr−1 for plausible fault dips. Evenif exhumation rates drop to 2 mm yr−1, advection of heatby exhumation of the upper block is sufficient tocompensate for heat lost to the surface from the depthof interest (ca. 25 km), rendering the second timeconstant immaterial. Underthrusting at characteristicHimalayan rates of 20 mm yr−1 yields a first timeconstant of ca. 10Ma, and the resulting thermal diffusiondistance (

ffiffiffiffiffi

jtp

) is ca. 15 km. This burial and heatingduration is comparable to the time difference betweeninitiation of the MCT (N21 Ma) and peak temperature insample I01/59.2 (ca. 10 Ma), and the diffusion distanceis sufficient to cause a substantial thermal perturbation ata distance ca. 5 km below the fault (i.e. sample I01/59.2).Subsequent incorporation of this zone into the overlyingsheet, and its transport to the surface at between 2 and3mm yr−1, would preserve an inverted isograd sequencesince the appropriate time constant for cooling in thereference frame moving with the upper block is ca. 2Ma(England and Molnar, 1993). Propagation of shorteningonto the Munsiari Thrust at this time may have aided thisexhumation (Vannay et al., 2004; Caddick, 2004).

Metamorphism of the Lesser Himalaya due toincorporation into the base of a low-viscosity, south-ward extruding flow (i.e. channel flow Beaumont et al.,2004; Grujic et al., 2002; Jamieson et al., 2004) wouldimply thickening of the flow body, presumably reflect-ing softening of its edges following continued advectionof hot material in the channel core. Direct comparisonbetween model and thermobarometric data is difficultbut for the parameter values outlined by Jamieson et al.(2004), maximum burial depths predicted numerically(Jamieson et al., 2004) agree well with constraints fromour data (within ca. 1 kbar for samples at approximatelythe same position in the section, Fig. 9b). Highermaximum temperatures are, however, predicted by themodel (Jamieson et al., 2004). Given that footwallheating occurs in all models involving advection in thehanging wall (as a simple overthrust or as a channelisedflow), the spatial distribution of metamorphosedsamples and their apparent post-thermal-peak histories(whether they remained at depth for a prolonged period,were incorporated into an exhuming flow, or wereuplifted by a foreland thrust) is most diagnostic of thedynamic regime. In the case of sample I01/59.2, thetight PTt path is indicative simply of rapid uplift, but theabsence of contemporaneous (post 20 Ma) prograde

metamorphism along the northern flank of the HHCSargues against symmetrical widening of a hot channel.Furthermore, the juxtaposition of metamorphic gradesacross the Munsiari Thrust is indicative of substantialshortening along this structure, highlighting the impor-tance of focused overthrusting during at least the laterstages of emplacement (e.g. Robinson et al., 2006).

4. Conclusions

Phase-equilibria constraints, mineral textures andcompositions, and in-situ monazite geochronologyreveal the metamorphic history of a Lesser Himalayansample from the western Himalaya. The results indicatethat early Miocene burial beneath the exhuming HHCSresulted in temperatures of 600–640 °C approximately5 km beneath the overthrusting sheet at ca. 10 Ma.Simple thermal models suggest that conduction from therapidly exhuming overlying sheet was probably suffi-cient to generate these temperatures, forming a tran-siently inverted local thermal gradient. Exhumation ofthe Lesser Himalaya along a footwall structure subse-quently cooled the unit sufficiently quickly to preservethe inverted metamorphic gradient, with average upliftrates of between 2 and 3 mm yr−1 during the period 10–6 Ma. The base of the extruding orogenic core clearlymigrated down-section through time as shorteningpropagated from the MCT to the Munsiari Thrust.Depending on definition, therefore, several mappablestructures could be regarded as preserving the MCT atdifferent stages of its evolution.

Acknowledgements

Tom Argles and Andy Richards are thanked for helpand advice during fieldwork and for subsequentdiscussions which helped immensely with understand-ing the geology of the Sutlej area. Marian Holness andAlan Thompson read and gave helpful comments onearly versions of this work and Stephen Reed and ChrisHaywood advised the collection of microprobe data. Weare grateful for insightful reviews by M. Searle and M.Brunel, which helped us to improve this manuscript, andfor the editorial handling of C. Jaupart. MJC was inreceipt of a NERC studentship.

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Burial and exhumation history of a Lesser Himalayan schist ... · erosion, burial of thrust sheets beneath the active orogen, and overprinting of earlier records by subsequent deformation