Shear Zones and Crustal Blocks of southern India

5/28/2018 Shear Zones and Crustal Blocks of southern India

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hear zones & crustalblocks of southern India

luid InclusionsTectonicsPetrology

UGC SAP DRS Phase II Seminar

Dept. of GeologyUniversity of Kerala, March 29 2!"#


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UGC-SAP-DRS II Seminar

Shear Zones and Crustal

Blocks of Southern India

Proceedings of the UGC-SAP-DRS II seminar

29 March 2014

Department of Geology, University of KeralaTrivandrum, India

Year of publication: 2014

A. P. Pradeepkumar and E .Shaji (eds)Editors

ISBN 978-81-923449-1-1

Dept. of GeologyUniversity of Kerala,Trivandrum 695 581, [email protected]@gmail.com


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Contents

Preface

Evidence for multiple magma processes in syenite petrogenesis: Yelagiri alkaline complex, south India.M. L. Renjith 1

Role of saline brine in graphite-bearing high grade metamorphic rocks a fluid inclusion studyK.R.Baiju, C.G.Nambiar, G.N.Jadhav 8

A note on the brittle fault zones observed in the southwestern terminus of PalghatCauvery Shear ZoneBiju John, Sandeep Nelliat and Yogendra Singh 14

From arc to highlands: the story of origin and exhumation of granulite-facies rocks in the KeralaKhondalite Belt, southern IndiaC. Sreejith and G. R. Ravindra Kumar 26

Petrology and Tectonic Significance of Probable Neo-Archaean Alaskan-Type Ultramafic Rocks

in Palghat-Cauvery Suture Zone, southern IndiaV. J. Rajesh, S. Arai and M. Santosh 32

Rutile exsolution in garnet in Mg-Fe-Al granulites from Karur area, Madurai Block, southern India: anindicator of decompression P-T path

Y. Anilkumar and S.C. Patel 34

Protolith constraints of garnetiferous biotite gneiss of Kerala Khondalite Belt (KKB)E. Shaji and A.P. Pradeepkumar 37

A chemico-mineralogical study of dolerites of Perinthalmanna area, Malappuram district, Kerala, IndiaDeeju T. R. and S. N. Kumar 38

Mechanism of formation of dehydration patches in the Munnar granite, southwestern IndiaS. Rajesh and A. P. Pradeepkumar 39


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Preface

The Department of Geology, University of Kerala established in 1963, is one of the pioneering

educational institutes, imparting studies in earth system sciences, in Kerala, India. The

department has successfully completed the UGC-SAP-DRS Phase I with thrust area of research

'Kinematics of south Indian shear zones'. The second phase of UCG SAP (2013 to 2018) has beensanctioned to this department in order to strengthen the research facilities of the department

with a thrust area of research on 'Shear zones and crustal blocks of south India with special

emphases on fluid inclusions and tectonics'. The Southern Granulite Terrain is composed of a

collage of blocks exposing mid- and lower-levels of the continental crust, dissected by crustal-

scale shear zones among which the Palghat-Cauvery Shear Zone (PCSZ) in the north and the

Achankovil Shear Zone (ACSZ) in the south have been interpreted as suture zones. These

domains continue to attract the attention of geologists worldwide to get a clear understanding of

the fluid activities and crustal dynamics. Under the proposed project detailed investigations

will be carried out on the metamorphic rocks, shear zone rocks, kinematics of shear zones and

the tectonics of the crustal blocks on the basis of petrography, geochemistry, fluid inclusions.As a prelude to the research initiative, we have decided to organize a UGCSAP DRS Phase II

conference to bring geoscientists together and collate abstracts of their scientific studies carried

out so far in the thrust area. In this connection, well known experts from various scientific

organizations/universities/research centers in petrology, tectonics and geochemistry are invited

to present their research work. It is hoped that the outcome and deliberations of the conference

would give a strong foundation for the department to start the phase II research progarmme in a

well-planned and systematic manner. We are extremely happy to bring out this volume, which

contains the full papers and abstracts of the papers presented in the conference. The

contributions received from the experts from GSI, CUSAT, NIRM, MES College, CESS and IIST are

greatly acknowledged. The financial support received from UGC has helped this department toproudly stand as one of the pioneering research centers of the country. The department is going

to build up a strong petrological and fliud inclusion lab with the financial support of UGC SAP

during the tenure of the scheme between 20132018. This will benefit the students and faculty

of this University as well as neighbouring ones and will open to all researchers of this country.

E.ShajiDty coordinator, UGC-SAP-DRS II

A.P.PradeepkumarCoordinator, UGC-SAP-DRS II


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M.L.Renjith Evidence for multiple magma processes in syenite petrogenesis

Evidence for multiple magma processes

in syenite petrogenesis:Yelagiri alkaline complex, south India.

M. L. Renjith

Marine and Coastal Survey Division, Geological Survey of India, Cochin 682037, India

E-mail: [email protected]

Abstract

Yelagiri alkaline complex, southern India, provides a unique opportunity to study shallow crustalmagma chamber processes involved in syenite petrogenesis. Yelagiri syenite is a batholith-size,dome-shaped massive intrusive body, which has developed three litho-facies (inner, middle andouter) of reversely zoned character as a result of systematic modal variation of K-feldspar,plagioclase, augite, biotite, edenite and quartz. Reverse zoning indicates i) differentiation by in-situ fractional crystallization and ii) existence of chamber-scale magmatic gradient in terms ofcomposition (increasing SiO2from inner to outer facies), fO2(stabilization of quartz-magnetite-titanite assemblages at middle and outer facies) and water content (variation in biotite-edenitemodal content and K-feldspar/plagioclase ratio across the lithofacies). Dimensional preferredorientation of K-feldspar megacrysts in magmatic flow fabrics imply chamber-scale convection orshear flow at a pre-rheological critical melt percentage (30%). In addition, scattered occurrenceof crystal accumulation fabrics of K-feldspar megacrysts suggest quiescent crystal settling. Theseoutcrop-scale evidences strongly advocate a heterogeneous magma chamber in terms of adynamically active as well as a stagnant state. Other significant features in Yelagiri syenite arethe widespread occurrence of microgranular mafic enclaves (MME) and synplutonic dykes. Theyare the robust witness of magma mixing process by the incremental supply of more-mafic magmainto the crystallizing magma chamber. Efficient magma mixing at the early Newtonian conditionimparted disequilibrium growth of feldspar crystals (Ca-spike zoning, zone truncation andsynneusis) in host syenite magma. Mechanical mixing (magmatically deformed enclaves)enhanced by chaotic-advection played a significant role as the rheological state changed withprogressive crystallization. Dispersion of the invaded mafic magma was restricted to the conduitas synplutonic dykes when the host magma attained near-solid state. Various grain to outcrop-scale magma mixing features suggest that the syenite magma chamber experienced input of moremafic magma during it entire crystallization-solidification history from Newtonian to near-solidstate and evolved through multiple magma processes.

Key words: Syenite, Magma mixing, Flow fabrics, Crystal accumulation, Magma chamber process

Introduction

In any igneous system magma undergoes numerous complex physical as well as geochemicalprocesses (or changes) during its entire crystallization history. Evidences for such magmaticevolution are systematically recorded in grain-to-outcrop scale features. Particularly inplutonic alkaline complexes, the magma processes involved are more complex than in any

another rock suites. Occurrences of a diverse spectrum of igneous rocks with chaotic field


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relationships in a small volume make the alkaline complex rocks more enigmatic. In thispaper one such plutonic complex, the Yelagiri alkaline complex of south India, is investigatedto understand the shallow crustal magma chamber processes involved in syenite petrogenesis.

Geological setting

Southern Granulite Terrain (SGT), a polymetamorphic terrain of south India hosts a numberof A-type granite, syenite, ultramafic and carbonatite complexes dating between 850-450 Ma(Veevers, 2007). Spatially they are closely associated with crustal-scale shear and fault zones.The Yelagiri alkaline complex (YAC) (75732 Ma; Miyazaki et al., 2000) under investigationis one amongst them, and occurs in the northeastern part of the SGT (Fig.1a).

Fig. 1(a) Map of South India showing major tectonic blocks and shear zones. SB: Salem block; MB:Madurai block; TB: Trivandrum block. Major shear zones are numbered as 1 to 7 and a transitionzone as 8. (b) Geological map of the Yelagiri Alkaline Complex (after Renjith et al., 2014).

This unmetamorphosed and undeformed intrusive complex emplaced into late Archaeanepidote-hornblende gneiss is constituted of three intrusive rock units: dunite, pyroxenite andsyenite, as ordered by their decreasing areal extent. The present study is restricted to theyoungest intrusive phase.

Field relations and petrography

Yelagiri syenite forms a dome shaped massive hilly outcrop of ~828 m height from ground

level. Pegmatoidal to medium grained syenite is found in grey (inner facies), pink (middle


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facies) and leucocratic varieties (outer facies), distributed concentrically (Fig.1b). Salientpetrographic features of syenite lithofacies are: i) K-feldspar/plagioclase ratio and cpx contentgradually increase and decrease respectively from inner to outer facies; ii) amphibole and

biotite are absent from inner and outer facies respectively; iii) liquidus quartz stabilized onlyat the northeastern margin and iv) titanite is always associated with amphibole and present

Fig.2.Packing density of K-feldspar grains (number of grains per unit area) and direction ofmagmatic foliation. Three types of grain packing: a.wide, b.moderate and c.closely spaced.

at the middle and outer facies. Presence of megacryst-size (2-6 cm long) K-feldspars is theunique feature in Yelagiri syenite and they impart diverse meso-scale fabrics to the outcrops.Dimensional preferred orientations of K-feldspars define magmatic flow fabrics which arefound in all the syenite facies. Packing density of grains (number of grains per unit area) anddirection of magmatic foliation varies at the meter scale. Three types of grain packing areobserved: wide, moderate and closely spaced (Fig.2).

Various crystal accumulation fabrics are also observed from many out crops. Suchfabrics show cryptic layering (Fig.3a,c), random orientation (Fig.3b), and graded fining

upward variations (Fig.3b). K-feldspar megacrysts show various primary dissolution-regrowthtextures. Compositional growth zoning (Ca-spike zone) is a common feature. Growth zonesshow synneusis plagioclase inclusions, curved corners and zone truncation. Centimeter scale,rounded to irregular shaped, fine-grained melanocratic microgranular mafic enclaves arescattered over the syenite (Fig.4a, b). They show sharp to diffusive boundaries with hostsyenite and magmatic deformations (Fig.4a). They frequently carry feldspar xenocrystsincorporated from the host syenite. Their texture varies from hypidiomorphic toallotriomorphic, and is dominantly constituted of edenite, plagioclase and K-feldspar.Presence of acicular apatite and amphibole are characteristic. Synplutonic dykes are anotherimportant feature in syenite (Fig.4c). Some of them carry MME and show diffusive to sharpboundary.


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Fig.3Field photographs of meso-scale crystal accumulation fabrics in syenite developed by K-feldsparmegacrysts. (a) cryptic crystal accumulation layering ; (b) randomly oriented crystal in a mush zone;(c) gradual settling of crystals ; (d) cryptic graded variation of K-feldspar crystals. White triangle isprovided for correlating gradational variation of grain size.

Fig.4 Field photograph of MME with sharp to diffusive boundary (a) and magmatic deformation(b) K-feldspar rich synplutonic dyke. (c)Synplutonic dykes in syenite


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Discussion and conclusion

Yelagiri syenite is a batholith-size reversely zoned plutonic body. The three lithofaciesdemarcated show systematic increase in SiO2 content from the inner to the outer facies

syenite (Fig.5). More evolved as well as quartz-bearing rock units occurring at the plutonmargins imply a reversely zoned character for the syenite body. Systematic variation in Fe-Mgminerals and CaO in the Harker diagram (Fig.5) suggests that in situ fractional crystallizationplayed a significant role in the development of reverse zonation. Early crystallization ofanhydrous phases such as cpx and feldspars stabilize biotite in the grey inner facies syenite.

Liquidus edenite-amphibole was stabilized in the middle and outer facies syenitesuggesting that progressive crystallization increases the water content. Intense crystallizationof silica-poor amphibole (edenite) stabilized quartz in the more evolved composition. Biotiteand amphibole have markedly different phase boundaries in the TcH2O space (Conrad et al.,1988; DallAgnol et al., 1999). Amphibole requires a relatively high water content in the meltof at least 4 to 5 wt.% H2O whereas stabilization of biotite requires a stable water content of

as low as 2-2.5 wt.% (DallAgnol et al., 1999). Biotite-rich inner facies and amphibole-richmiddle and outer facies syenite imply the existence of a chamber-scale magmatic gradient interms of water content i.e., more hydrous outer margin and less hydrous inner region of thechamber (Fig.6). Systematic variation of K-feldspar/plagioclase ratio also substantiates thisfractional crystallization and increase in water content (e.g., Whitney, 1975, 1988; Day andFenn, 1982). Stabilization of titanite-magnetite-quartz assemblage in the more evolved

Fig. 5Harker variation diagram for CaO content in syenite lithofacies.

composition suggests high oxygen fugacity conditions during the crystallization (e.g., Wones,1989). Estimated P-T conditions suggest that syenite crystallized at a 2-4 kb pressure range(Al-in Amphibole and Amphibole-Plagioclase pair) and 660-740 C temperature (Renjith,2010). Estimated zircon saturation temperature yielded a temperature between 900 and 850C for the grey inner facies syenite whereas middle and outer facies syenite yielded less than775 C (Renjith, 2010). Zircon crystallization is well correlated with the reverse zoning andfractional crystallization.

Various crystal accumulation fabrics in scattered outcrops suggest localized crystalsettling in a quiescent magmatic environment. Cryptic layering and crystal-rich mush zones

indicate that crystal settling was intense at the early stages of the melt-supported state of the


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magma chamber (e.g., Vernon and Paterson, 2008). When the crystals were suspendedenough to move freely in a liquid, mechanical accumulation by gravity settling was thesignificant process responsible for the accumulation fabrics in Yelagiri syenite.

In contrast to this settling process, the magma flow fabrics developed by K-feldsparmegacrysts imply a shape preferred orientation (SPO) due to magmatic flow (a pre-rheological critical melt percentage, 30%; Arzi, 1978) in the magma chamber. The earlyformed tabular crystals in a more melt supported system and suspended state were flowingalong with the magma turbulence or shear flow as the logs do in a river (e.g., Vernon andPaterson, 2008). Variation in packing density of the grains indicates that magma flow wasactive at different stages of crystallization.

Features like fine grained nature, sharp contact, presence of acicular amphibole andapatite in microgranular enclaves indicate that they were formed by thermal quenching ofmafic magma globules upon injection into relatively cold host syenite magma (e.g., Vernon,1983; Vernon et al., 1988). Hybridization between enclave and host magmas occurred

Fig. 6Cartoon illustrate a proposed magma chamber model for the Yelagiri syenite.

through different mechanisms. At the Newtonia stage, homogenization of invaded magmawas efficient and caused fluctuating phase equilibrium in the crystallizing melt.Disequilibrium micro-textures in K-feldspars indicate magma- mixing-driven growth textures.Deformed MMEs and schleiren-like structures indicate that mafic magma blobs weremechanically diluted through chaotic dynamic advection (Renjith et al., 2014). Synplutonicdykes in syenite indicate injection of mafic magma at the nearly-solid state crystallization ofthe host syenite magma (Fig.4c) Due to the high thermal rheological contrast with host


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syenite the invaded magma remained in the conduit of injection and solidified as dykes.MMEs and synplutonic dykes are witnesses of magma mixing features in syenite.

From various grain to outcrop scale evidences a magma chamber model is envisaged for the

Yelagiri syenite (Fig.6). Multiple magma chamber processes were involved in thecrystallization-solidification history of the syenite. Crystal accumulation and mechanicalsettling was the major physical differentiation of the syenite magma in the early stage ofcrystallization.In situ fractional crystallization promoted the development of chamber-scalehydrous and less hydrous magma. This resulted in the formation of syenite lithofacies. Inputof more mafic magma into the crystallizing chamber was another significant process thatcontributed to the evolution of Yelagiri syenite.

References

Arzi, A. A. 1978. Critical phenomena in the rheology of partially melted rocks. Tectonophysics 44, 173-

184.Conrad, W.K., Nicholls, I.A., Wall, V.J. 1988. Water satuarted and under saturated melting of

metaluminous and peraluminous crustal compositions at 10kb: evidence for the origin of silicicmagmas in the Taupo volcanic zone, New Zealand, and other occurrences. J. Petrol., 29, 765-803.

DallAgnol, R., Scaillet, B., Pichavant, M. 1999b. An experimental study of a lower Proterozoic A-typegranite from the Eastern Amazonian Craton, Brazil. J. Petrol., 40, 16731698.

Day, H.W., Fenn, P.W., 1982. Estimating the P-T-XH2O conditions during crystallization of low calciumgranites. J.Geol., 90, 485-508

Miyazaki, T., Kagami, H., Shuto, K., Morikiyo, T., Ram Mohan, V., Rajasekaran, K.C., 2000. Rb-Srgeochronology, Nd-Sr isotopes and whole-rock geochemistry of Yelagiri and Sevattur syenites, TamilNadu, South India. Gondwana Research 3, 39-53.

Renjith M.L., S.N. Charan, D.V. Subbarao, Babu, E.V.S.S.K., Rajashekhar, V.B. 2014. Grain to outcrop-

scale frozen moments of dynamic magma mixing in the syenite magma chamber, Yelagiri alkalinecomplex, south India, Geoscience Frontiers, http://dx.doi.org/10.1016 /j.gsf.2013.08.006

Renjith, M.L., 2010. Mineralogy, geochemistry and genesis of the Yelagiri alkaline complex, TamilNadu. Unpublished Ph.D. Thesis. Osmania University, India.

Veevers, J.J., 2007. Pan-Gondwanaland post-collisional extension marked by 650-500 Ma alkalinerocks and carbonatites and related detrital zircons: a review. Earth Science Reviews 83, 1-47.

Vernon, R. H., Paterson, S. R. 2006. Mesoscopic structures resulting from crystal accumulation andmelt movement in granites. Transactions of the Royal Society of Edinburgh: Earth Sciences, 97 (04),369-381.

Vernon, R.H. 1983. Restite, xenoliths and microgranitoid enclaves in granites. J. R. Sot. New SouthWales 116, 77-103.

Vernon, R.H., Etheridge, M.A., Wall, V.J., 1988. Shape and microstructure of microgranitoid enclaves:indicators of magmatic mingling and flow. Lithos 22, 1-12.

Whitney, J.A. 1975. The effects of pressure, temperature, and XH2O on phase assemblages in foursynthetic rock compositions. J.Geol., 83, 1-31.

Whitney, J.A.1988. The origin of granite: The role and source of water in the evolution of graniticmagmas. Geol. Soc. Am. Bull., 100, 1886-1897.

Wones, D.R. 1989. Significance of assemblage titanite+magnetite+quartz in granitic rocks. Am.Mineral., 74, 744-749.
http://dx.doi.org/10.1016%20/j.gsf.2013.08.006http://dx.doi.org/10.1016%20/j.gsf.2013.08.006


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K R Baiju et al., Saline brine in graphite bearing high grade metamorphic rocks a fluid inclusion study

Role of saline brine in graphite-bearing high

grade metamorphic rocks a fluid inclusion study

K.R.Baiju1*, C.G.Nambiar1, G.N.Jadhav2

1Department of Marine Geology and Geophysics, Cochin University of Science and Technology,Lakeshore Avenue, Kochi, India

2Department of Earth Sciences, Indian Institute of Technology Bombay, Mumbai, India

*Corresponding author

Email:[email protected]

Abstract

Saline brine occurrences have been reported from many granulite terrains which are believed tobe generated from the lower crust and upper mantle. They may be concentrated in deep fluidsystem from vivid sources like saline connate fluid, dissolution from sedimentary salts, and H2Oloss by preferential partitioning into hydrous minerals during retrograde metamorphism or intohydrous silicate liquids during melting and infiltration of externally derived magmatic fluids. Inthe Madurai Granulite Block (MGB), a prominent tectonic block of the central region of theSouthern Granulite Terrain, the role of CO2-rich fluidsin charnockitization process was proposedby many early workers derived support from the common occurrence of high-density CO2inclusions in various minerals (e.g. Srikantappa et al., 1992; Mohan et al., 1996). The presentstudy reveals the presence of high saline brines inclusions in graphite-bearing charnockite andassociated gneisses from the MGB and its implication in the metamorphic history of the terrain.The calculated density of the primary inclusions falls in a nominal range of 0.77-0.87 g/cc whichdenotes slightly lesser pressure than the peak metamorphic condition of the terrain. The totalhomogenisation temperature of the youngest generation biphase/polyphase inclusions of vapour+ liquid + halite ranges between 150-500 OC has inclusions low CO2densities ranging between0.58 -0.75 g/cc. The salinity of the halite bearing inclusions were calculated from halite meltingtemperature and it ranges between 32-52. wt% NaCl equivalent, which is comparatively high.This indicates a late stage saline-hydrothermal activity at a lower temperature has affected theterrain, which might have initiated a retrogressive stage. The immiscible nature of brine andcarbonic fluids, which can persist together, even at very high magmatic temperatures, providesthe explanation for the coexistence of CO2 inclusions and NaCl-H2O high salinity inclusions insame trails.

IntroductionThe thermodynamically isolated micro geochemical systems entrapped in various mineralsduring their growth are popularly known as fluid inclusions. Fluid and melt inclusionsprovide a wealth of information on the genetic and evolutionary history of the mineralassemblages in a rock or ore formation as they furnish valid clues for the physico-chemicalinterpretations. They represent ideal samples of the fluid or melt, which were in equilibriumwith host mineral either during its genesis or at later stages.


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Fig. 1. Sample locations. Inset shows the general geological setting of the MGB

By far the most abundant type of fluid inclusion is that which contains a low viscosity liquidand a smaller volume of gas or vapour bubble. The liquid is generally aqueous has a pHwithin one unit of neutral, and contains a total salt concentration between 0 and 40 weightpercent. The salts contain major amounts of Na+, K+, Ca2+, Mg2+, CI- and SO42- ions, with

minor amount of Li+

, Al3+

, BO33-

, H4SiO4, and bicarbonate and carbonate ions. Na and Cl ionsare usually dominant. CO2 in both liquid and gas form and liquid hydrocarbon are fairlycommon. Liquid hydrogen sulphide has also been observed, but is rare. Carbon dioxideoccurs as a supercritical fluid above 31C, its critical point. Daughter minerals, usually cubesof halite or sylvite form when nearly saturated fluids cool from the initial temperature ofentrapment. Other crystals that are observed but are not simple precipitates of asupersaturated solution include sulphides, quartz, anhydrite, calcite, hematite and gypsum.Such crystals either formbefore the inclusion was finally sealed, as a result of secondarilyintroduced fluids or even through oxidation resulting from hydrogen diffusion (Roedder,1979).

Studies on fluid inclusion in rocks of different metamorphic grade shows that there is

a consistent relation of composition of fluids found in inclusion to the metamorphicconditions and the variation will be only according to the difference in grade (Crawford and


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Hollister, 1986). In a host mineral, the inclusions behave as closed volumes and during itspath to exhumation in a definite P-T trajectory it maintains isochoric behaviour. So in high-grade metamorphic rocks all most all the generation fluids are well preserved rather than in

low-grade rocks which makes the studies much easier. (Frezzotti et. al., 2004). The southernIndian granulite terrain (SGT) has been a key area in understanding the nature and role offluids in deep crustal processes in the past three decades (e.g. Newton et al., 1989; Santosh etal., 1991). The Madurai Granulite Block (MGB), which is a prominent tectonic block of thecentral region of the SGT (Fig. 1), the role of CO2-rich fluidsin charnockitization process wasproposed by many early workers derived support from the common occurrence of high-density CO2inclusions in various minerals (e.g. Srikantappa et al., 1992; Mohan et al., 1996).The present study reveals the presence of high saline brines inclusions in graphite-bearingcharnockite from the MGB and its implication in the metamorphic history of the terrain.

Geological Setting

The MGB is the largest single granulite block among the different tectonic units in the SGTand is bounded by the Palghat-Cauvery Shear system (PCS) in the north and the AchankovilShear (ACS) in the south. The block is predominantly constituted of high-grade metamorphicrocks of the granulite facies (Fig 1). The major rock types include charnockites, maficgranulites, hornblende-biotite gneisses, migmatitic gneisses, garnet-cordierite-sillimanitegneisses, garnet-biotite gneiss, marbles, sapphirine-bearing granulites, and other quartzo-feldspathic gneisses (eg. Naqvi and Rogers, 1987; Sivasubramanian et al., 1991; Mohan,1996; Satish-Kumar et al., 2002; Sajeev et al., 2004 etc.). Some of the previous studies pointout a polymetamorphic and multistage evolution for the MGB.

Analytical ProceduresThe microthermometric investigations of the samples were carried out on a Linkam TMSG-600 heating-freezing stage (-196C to +600C) housed in the Dept. of Earth Sciences, IITBombay. The precision of measurements is 1.1C. The data obtained frommicrothermometry, and occasionally volume fraction estimates of the fluid inclusions wereevaluated using computer program package FLUIDS-1 (Bakker, 2003) in order to transformmelting temperatures, homogenization temperatures and optical volume fraction estimatesinto bulk compositions and densities.

Results

Fluid inclusion petrography

In metamorphic rocks, fluid inclusions are studied in various rock forming minerals,

particularly quartz or garnet which have no cleavage and therefore suffer less fluid leakageafter trapping (Touret, 2001). The absence of cleavage and the ability of easy re-crystallization make quartz an ideal mineral to preserve the fluid inclusions (Van den Kerkhofand Hein, 2001; Touret, 2001). Thus quartz-rich samples were selected in this study. Thefluid inclusion petrography was observed at room temperature (26-28C). The most commontype of fluid inclusions in the studied samples is a monophase gas rich type. These inclusionsare found as solitary negative crystal type inclusions or in isolated groups, which can beconsidered to be of primary origin, compared to those occurring as arrays along healed orfresh fractures (pseudosecondary and secondary). Biphase liquid/gas rich and polyphaseinclusions of vapour + liquid + halite are also noticed as pseudosecondary or secondary type(Table1).

Fluid inclusion chronology


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Chronologically the fluid inclusions can be grouped into three major generations. They are:1. Monophase carbonic inclusions with well-developed negative crystal shape seen

isolated or in isolated groups in the samples. Based on fluid inclusion petrography this

can be considered to be of primary origin (Fig. 2a).2. Monophase carbonic inclusions of pseudosecondary type (occurring in arrays mostly

along healed fractures) (Fig. 2b). This can be considered to be the modified firstgeneration fluid inclusions. Sometimes biphase (CO2 + H2O) inclusions are also foundas pseudosecondary type, which may be of the same generation due to someheterogeneous trapping, or of two different generations. Their relative chronology isnot clear from the fluid inclusion petrography.

3. Secondary type (occurring along fractures) biphase (CO2+ H2O) as well as polyphase(CO2liquid + CO2gas+ H2O+ NaCl) inclusions that can be considered to be the lastgeneration fluid inclusions (Fig. 2c).

Table 1. Type of inclusions present in the samples studied

Sample No. Types of fluid inclusions presentG4B Monophase CO2

Biphase (CO2 + H2O)CK108 Monophase CO2

Biphase (CO2 + H2O)G5 Monophase CO2

Biphase (CO2 + H2O)Polyphase (CO2

gas+ CO2liquid +H2O)G6 Monophase CO2

Biphase (CO2 + H2O)CK6B Monophase CO2


gas+ CO2liquid + H2O)Polyphase (CO2

+ H2O+ Halite)CK42 Monophase CO2


+ H2O+ Halite)CK57G Monophase CO2

Biphase (CO2 + H2O)CK73 Monophase CO2

Biphase (CO2 + H2O)

CK109 Biphase (CO2 + H2O)Polyphase (CO2 + H2O+ Halite)

MicrothermometryThe freezing stage experiments conducted in all samples confirms the presence of CO2inclusions from its melting temperature (TM) ranging between (-55.7

C) to (58.5C). Thecalculated density of the primary inclusions falls in a nominal range of 0.77-0.87 g/cc whichdenotes slightly lesser pressure than the peak metamorphic condition of the terrain isexplained in Baiju et al 2009. The total homogenisation temperature of the youngestgeneration biphase/polyphase inclusions ranges between 150-500 OC and has inclusions low


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0 0.1mm

a

0 0.1mm

0 0.1mm

c

Fig. 2. a)Primary monophase carbonic inclusion, b)pseudosecondary monophase inclusion,c) secondary biphase inclusions

CO2densities (0.58 -0.75 g/cc). The inclusions of vapour + liquid + halite are prominent insamples G4B, CK6B, CK42, showing halite dissolution temperature at 210.6C , 400-412C

and 443 C (Fig. 3) respectively from which the salinity is calculated as 32.4 wt%, 48.5 wt%and 52.0 wt% NaCl equivalent. The presence of clathrates are identified in samples G6, CK42,CK57G, CK108 and CK109.

Fig.3.Dissolution of halite in CK42.

DiscussionIn the present study all the samples selected is graphite bearing and the inclusions arecompositionally CO2, CO2+H2O, CO2+H2O+ NaCl, with minor amounts of CH4. The youngestgeneration biphase/polyphase inclusions identified with low CO2 densities, totalhomogenisation temperature below 500 OC and the presence of halite illustrate a late stagehydrothermal activity at a lower temperature. In the field it is noted that the charnockites ofthe present study are retrogressed to biotite/hornblende-bearing gneisses at places controlled

by fractures associated with pegmatitic veins and/or quartz veins. The presence of haliteswithin inclusions associated with this stage suggests an influx of saline brine at the

-78.8C -58.5C 400C

443C 512C

H H H

0 0.1mm


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retrogressive stage. The presence of clathrates identified in samples may indicate aninteraction of brine fluids and graphite to produce CO2 -CH4rich fluids (Baiju et al 2009). Theimmiscible nature of brine and carbonic fluids, which can persist together, even at very high

magmatic temperatures, provides the explanation for the coexistence of CO2inclusions andNaCl-H2O high salinity inclusions in same trails.

ConclusionThe younger generation low density CO2 inclusions along with high saline brine noticed allover the study area points towards a universal injection of the fluids at shallow depth. TheCO2density pertains to a low pressure condition and the fluid regime might have occurredduring the exhumation of the terrain together with a retrogressive mechanism in themetamorphic history of the MGB.

References

Baiju, K.R., Nambiar, C.G., Jadhav, G. N., Kagi, H., Satish-Kumar, M., 2009. Low-density CO2-rich fluidinclusions from charnockites of southwestern Madurai Granulite Block, southern India; implicationson graphite mineralization. J. Asian Earth Sci. 36, 332-340.

Bakker, R.J., 2003. Package FLUIDS 1. Computer programs for analysis of fluid inclusion data and formodelling bulk fluid properties. Chemical Geology 194, 3-23

Crawford, M.L. and Hollister, L.S. 1986 Metamorphic fluids: the evidence from fluid inclusions. In: J.V.Walther and B.J. Wood (eds), Fluid-rock interactions during metamorphism. Adv. Phys. Geochem., 5,1-35.

Frezzotti, M.L., Cezare, B. and Scambelluri, M. 2004. Fluids at extreme P-T metamorphic conditions:the message from high-grade rocks. Per. Mineral., 73, 209-219.

Mohan, A., Prakash, D., Sachan, H.K., 1996. Fluid inclusions from charnockites from Kodaikanal massif(South India): P-T record and implication for crustal uplift history. Mineral. Petrol. 57, 167-184.

Naqvi, S.M., Rogers, J.J.W., 1987. Precambrian geology of India. Oxford University Press. 223.Newton, R.C., 1989. Metamorphic fluids in the deep crust. Ann. Rev. Earth and Planet. Sci. 17, 385-

412.Newton, R. C., Aranovich, L.Y., Hansen, E. C., Vandenheuvel, B. A., 1998. Hypersaline fluids in

Precambrian deep -crustal metamorphism. Precamb. Res. 91, 41-63.Newton, R. C., Manning C. E., 2010. Role of saline fluids in deep-crustal and upper-mantle

metasomatism: insights from experimental studies. Geofluids. 10, 58-72.Roedder, E., 1979. Origin and significance of magmatic inclusions. Bull. Mineral., 102, 487-510.Sajeev, K., Osanai, Y., Santosh, M., 2004. Ultrahigh-temperature metamorphism followed by two-stage

decompression of garnet-orthopyroxene-sillimanite granulites from Ganguvarpatti, Madurai Block,southern India. Contribution to Mineralogy and Petrology 148, 29-46.

Santosh, M., Jackson, D.H., Harris, N.B.W., Mattey, D.P., 1991. Carbonic fluid inclusions in South India

granulites: evidence for entrapment during charnockite formation. Contrib. Mineral. Petrol. 108, 318-330.

Sivasubramanian, P., Natarajan, R. and Janardhan, A.S. 1991. Sapphirine-bearing assemblages fromPerumalmalai, Palani hills, Tamil Nadu. J. Geol. Soc. India, 38, 532-537.

Srikantappa, C., Raith, M., Touret, J.L.R., 1992. Synmetamorphic high density carbonic fluids in thelower crust: Evidence from the Nilgiri granulites, southern India. J. Petrol. 33, 733-760.

Touret, J.L.R., 2001. Fluids in metamorphic rocks. Lithos, 55, 1-25.Van den Kerkhof, A.M., Hein, U.H., 2001. Fluid inclusion petrography. Lithos 55, 27-47.


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B John et al. Brittle fault zones in the southwestern terminus of PalghatCauvery Shear Zone

A note on the brittle fault zones observed

in the southwestern terminus ofPalghatCauvery Shear Zone

Biju John, Sandeep Nelliat and Yogendra Singh

Seismotectonic Group, Engineering Seismology Department,

National Institute of Rock MechanicsChampion Reef P.O., Kolar Gold Fields

Karnataka 563117 IndiaE-mail: [email protected]

Abstract

Geological mapping in the western end of Palghat-Cauvery Shear Zone (PCSZ) identifieddeformation ranging from ductile to semi-ductile to brittle. Previous studies suggest that theductile and semi-ductile deformation associated with this shear zone corresponds to Pan Africanorogeny or older. Even though the thermo-tectonic events and associated ductile deformations inthis region are well constrained, the younger tectonic deformation and associated brittle faultingwere not given due importance. The present study concentrates on the brittle deformationsobserved in the southwestern terminus of PCSZ. The present work is limited to rock exposuresfrom the quarry sections south of Bharathapuzha River. These brittle faults are generated from

both tension and compression. Compressional movements manifested as strike-slip and reversefaults and have fluid activity indicating their deep seated continuity. The strike-slip faultsgenerally trend in NNE-SSW direction with a steep dip whereas the reverse faults trend in NW-SEdirections with a southward dip. The major fault in the area, Desamangalam Fault, showsmultiple gouge generation and fluid activity identified through cross cutting relationshipindicating episodic nature of deformation. The normal fault zones are generally wider at thesurface and narrowing towards depth. Though the youngest deformation is dated using ESRtechnique, no date could be derived for older events. The presence of fluids associated withfaulting indicate the scope for secondary fluid inclusion studies which can provide vital clues tothe timing of faulting especially those happened in the present stress regime.

Key words: Precambrian shear zones, brittle deformation, fault core, fluid activity

IntroductionSouthern Peninsular India consists of high grade Precambrian crystalline terrains which areseparated by shear zones. These terrains were well studied earlier to understand the variousthermo-tectonic events that occurred (Santosh et al., 2003; Ghosh et al., 2004). The youngestof the events was occurred around 550 Ma which were associated with the pegmatiteintrusions (Soman et al., 1990). All these thermo-tectonic events occurred well before thesplit of Gondwana around 120 Ma. The entire Peninsular India experienced tension till itcollided with Eurasian plate (around 40 Ma; Molnar and Tapponnier, 1977) to form theHimalayas. The Indian plate witnessed volcanic activity 68-63 Ma (Radhakrishna, et al.,

1994) when it passed over the Kerguelen hotspot. Related dyke formation occurred alongsome of the deep tension fractures even in southern Peninsula. The continued northward


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push of the Indian continent towards the Eurasian plate changed the tensional regime ofIndian plate into compression (Gowd et al., 1992). Under the present compressional stressregime, the favorably oriented preexisting weak zones show reactivation (Rajendran et al.,

1992; Gowd et al., 1996).

Fig 1.Study area and the earthquake events

Palghat-Cauvery Shear Zone (PCSZ) is considered as one of the weak structures in PeninsularIndia, in terms of seismicity (Rajendran and Rajendran, 1996). The studies consequent to theoccurrence of 1994 Wadakkancheri (M=4.3) earthquake (Fig. 1) identified a south dippingreverse fault (John and Rajendran, 2009). This fault, named Desamangalam fault, influencedthe drainage network of the area (John and Rajendran, 2008). The low level seismicityobserved in the area mainly confined to the hanging wall side of the fault (John andRajendran, 2009). Deformational features ranging from ductile to brittle are observed in thearea. However, ductile and semi ductile deformations are rare in the southern part of theDesamangalam fault. Instead we identified brittle deformation showing normal, reverse andstrike-slip movements and associated secondary mineralization as well as gouge formation. Inthis paper, we present our observations on brittle faults near Wadakkancheri area at thewestern end of the PCSZ.Geologic and structural setupThe study area lies on the southern flank of the Palghat gap (Fig. 1), a conspicuous EWtrending linear valley developed within the Proterozoic granulite terrain in South India(Arogyaswami, 1962; Drury et al., 1984; Subramaniam and Muraleedharan, 1985; D'Cruz etal., 2000), which is a part of the Proterozoic Palghat-Cauvery Shear Zone (PCSZ) (BhaskarRao et al., 1996). The region essentially consists of charnockite and khondalite group of rocks(Fig 2). The higher levels of the crystalline basement are occasionally covered by lateriticregolith and the terraces adjoining the southern riverbank are made up of older alluvium


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Fig. 2 Geology (adopted from Geological Survey of India, 1992) and lineament map of the area

(after John and Rajendran, 2008). The folded rock units are part of Palghat-Cauvery shear zone.


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Fig. 3One of the semi-ductile faults observed in the area; please see Fig. 2 for location.

Observations on brittle faultsBrittle fault growth commonly produces a fault core composed of slip surfaces andcomminuted rock material, and also a broader volume of distributed deformation called thedamage zone (McGrath and Davison, 1995; Caine et al., 1996; Vermilye and Scholz, 1998).In this study we define the fault zone into Fault Core, Damage Zone and Protolith, as per themodel put forward by Caine et al. (1996). Fault core is the portion of a fault zone wheremuch of the displacement is accommodated. The damage zone consists of a network of faultrelated subsidiary structures that bound the fault core. Protolith is the undeformed host rocksurrounding the fault rock and the damaged zone.

Desamangalam fault

Studies subsequent to 1994 Wadakkancheri earthquake identified a NW-SE trending reversefault (John, 2003). The trace of the fault is identified near Desamangalam in a charnockitequarry (Fig. 4). This fault zone can be divisible into core and damage zone. Weakly developedfoliation planes were observed within the outcrops. There are some quartzo-feldspathic veinsobserved in the section. Some of the veins are parallel to the fault trend. The present faultzone seems to have been formed along a quartzo-feldspathic vein. The relative movementbetween the hanging wall and foot wall is reflected on the vertical offsets exhibited by thequartzo-feldspathic veins.

The fault core, labeled as F2 in Fig. 4, shows clear-cut evidence of offsets and gougeformation. Within the principal slip plane, the host rock appears to have been pulverized, andthe original bulk fabrics completely disrupted. The faulting has created a 3 to 6 cm thick

gouge zone, consisting of both consolidated and unconsolidated forms, where a thin layer(0.6 cm) of loose gouge is developed all along the slip plane. Similar observations of narrow

N


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zone accommodating large slip are reported from San Gabriel Fault (Chester et al., 1993) andPunchbowl Fault (Chester and Chester, 1998) of USA and in Nojima fault of Japan (Bouller etal., 2004).

Fig. 4 View of Desamangalam fault; F2 is the main fault described in the text.

Detailed petrological studies of exhumed fault zones generally provide insight into thedeformation processes associated with fault slip and also the role of pore fluid in earthquakecycle (Sleep and Blanpied, 1992). Petrologicaland X-ray diffraction studies conducted on thefault rocks from Desamangalam fault zone identified different secondary minerals namely,chlorite, muscovite, clinoptilolite and montmorillonite (John and Rajendran 2009). It ispossible that each of these minerals might have formed during the faulting events at differenttemperature conditions. For example, chlorite has a temperature of formation between 322 C

and 150 C (De Caritat et al., 1993), whereas zeolites form below 100 C (Karlsson, 2001).Ray et al. (2003), based on the heat flow studies, calculated the present day geothermalgradient of southern granulateterrain as ca. 20 C/km. ESR dating of the loose fault gougeindicates a major movement along the fault around 430 43 ka BP (Rao et al., 2002). Basedon the mineralogical changes in different gouge zones (distinguished through magascopicand microscopic observations) and based on the cross cutting relations exhibited by differentfracture fills, John and Rajendran (2009) identified four episodes of brittle deformation inthis fault zone (Figs. 5 & 6).Thin section studies also identified inclusions of primary andsecondary nature.


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Fig. 5Photomicrograph showing clinoptilolite bearing fracture cutting both host rock and gougezone (G1). The contact zone between consolidated gouge (G1) and host rock is shown by blue dashedline.

Fig. 6Photomicrograph showing two generations of fractures (Fr2 and Fr3) in plane polarized light.Fr2 is filled with chlorite and in hand specimen it shows a dark green colour. Fr2 is filled with a whitemineral clinoptilolite. It should be noted that the fracture bearing chlorite further opened up duringthe development of the next set of fractures facilitating the deposition of clinoptilolite.

Strike slip fault near Parlikad

Geomorphic studies in this area identified a drainage divide at the centre of a wide channel

south of Bharathapuzha (John and Rajendran 2008). Spatially it aligns with the N-S trendingabrupt turn of the Bharathapuzha at Desamangalam. Further, south the lineament is

Fr2

Fr3

1mm


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associated with two narrow valleys. The continuity of this lineament further north of theBharathapuzha is marked by a third order stream. Along this lineament, zone brittle fault isidentified in a charnockite quarry near Parlikad (Fig.7). The fault plane is observed dipping

steep and showing a dextral strike slip movement indicated by slickensides. The fault coreand damage zone are very thin (15-10 cm). Upward branching of fractures is observed in thefoot wall at the tip of the fault. Secondary minerals are observed in the fault zone indicatingits association with fluid activity.

Fig. 7A steeply dipping strike-slip fault observed near Parlikad.

Normal fault near TayyoorAn E-W trending normal fault is observed near Tayyoor (Fig. 8). Studies did not find anyregional association of this fault with identified lineaments. The main fault is dipping towardsNorth. The fault zone appears as a wedge shaped feature due to a south dipping fracture thatjoins the main fault below 10 m from the surface. Closely spaced fractures (Fig. 9) parallel tothe main fault mark the damage zone (2 m). The slip is accommodated in the narrow zone of20 cm. Chlorite is present in the fault gouge. At the surface there is no laterite cover in thefoot wall block whereas the hanging wall shows thick laterite.


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Fig. 8Normal fault and associated wedge shaped structure observed near Tayyoor. Note the absenceof laterite cap in the foot wall. Close up view of the area marked in rectangle is shown in figure 9.

Fig. 9The damage zone at the Normal fault near Tayyoor (Fig. 8) showing closely spaced fractures


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GPR survey was carried out across this fault using 100 Hz antenna to study theextension of the fault in the subsurface. GPR survey clearly delineated the laterite rockcontact and also showed the extension of the fault up to a depth of 5-6 m. However,

reflections from the deeper part of the fault could not be differentiated. Near the surface thefault could be picked up from the strong reflections from the moisture in the fault core. Poorreflection from the fault below the depth of 6 m may be due to the near vertical orientation ofthe fault or due to the tightness of the contact plane. In the present case the study revealsthat the normal faults observed are confined to the upper few meters of the surface (Fig 10).

Fig. 10 GPR profile across the normal fault at Tayyoor.

Conclusions

It is well known that the deformations in Precambrian rocks are also associated with fluidactivity. The brittle deformations at depth are invariably filled with pegmatite veins or quartzveins. At shallower condition where the deformation creates fine grained material from thecrystalline rocks, the fluids alter the composition of the product or leach out the componentsfrom the host rock to produce new secondary mineral.

Ongoing studies in the Precambrian rocks of Peninsular India in the vicinity of PalghatGap identified deformations ranging from ductile to semi-ductile to brittle. Fault zones areidentified which bear consolidated and unconsolidated gouge zones. The reverse and strikeslip fault identified in the area appears to be mimicking the present day regional stressconditions. Multiple gouge generation and fluid activity were identified in these fault zonesthrough cross cutting relationship. The studies identified presence of the Chlorite, Muscovite

Montmorillonite and Clinoptilolite in the gouge zone and fracture fills with a deformationtime clocked at 430 43 ka BP (ESR Dating; Rao et al., 2002). However, with the present


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knowledge, earlier events cannot be dated and a dedicated work is required to understandthe exhumation rate from the host rock as well as the nature of fluids present at variousdepths of deformation from the fault rocks. The sealed nature of earlier deformation observed

in the fault zone indicates the role of fluids in sealing the faults even at the shallower level.These fluids are trapped in the minerals as secondary inclusions within the fault rocks.Occurrence of such inclusions are reported elsewhere in the fault zones (e.g. Boullier, andRobert, 1992). These inclusions may give vital information on the temperature andcomposition of fluids present during the faulting. Such studies are expected to provide insightinto the recurrence of faulting in crystalline rocks of shield areas.

Acknowledgements

We thank the Director NIRM for his support, encouragement and the facility provided for thestudy. We also thank the financial assistance from DST SERC project award no SR/S4/ES-

434/2009.

References

Arogyaswami, R.N.P., 1962. The origin of the Palghat Gap. Rec. Geol. Sur. India 93, 129-134.Bhaskar Rao, Y.J., Chetty, T.R.K., Janardhan, A.S., Gopalan, K., 1996. Sm-Nd and Rb-Sr ages and P-T

history of the Archaean Sittampundi and Bhavani layered meta-anorthosite complexes in Cauveryshear zone system, South India. Evidence for Neoproterozoic reworking of Archaean crust.Contributions to Mineralogy and Petrology 125, 237-250

Bouller, A-M., Fujimoto, K., Ohtani, T., Roman-Ross, G., Lewin, E., Ito, H., Pezard, P., Ildefonse, 2004.Textural evidence for recent co-seismic circulation of fluids in the Nojima fault zone, Awaji Island,Japan. Tectonophysics 378, 165-181.

Boullier, A.-M., and Robert, F., 1992 Paleoseismice vents recorded in Archean gold-quartz vernnetworks, Val d'Or, Abitibi, Quebec, Canada. Journal of Structural Geology, 14, l6L-179.Caine, J.S., Evans, J.P., Forster, C.B., 1996. Fault zone architecture and permeability structure. Geology

24, 1025-1028.Chester, F.M., Chester, J.S., 1998. Ultracataclastic structure and friction processes of the Punchbowl

fault, San Andreas System, California. Tectonophysics 295, 199-221.Chester, F.M., Evans, J.P., Biegel, R.L., 1993. Internal structure and weakening mechanisms of the SanAndreas Fault. J. Geophys. Res. 98, 771-786.

DCruz, E., Nair, P.K.R., Prasannakumar, V., 2000. Palghat gap- A Dextural shear zone from the southIndian granulite terrain. Gondwana Research 3, 21-31.

De Caritat, Hutcheon, I., Walshe, J.L., 1993. Chlorite geothermometry: a review. Clays and ClayMinerals, 41, 219-239.

Drury, S.A, Harris, N.B.W., Holt, R.W., Reeves-Smith, G.J., Wightman, R.T., 1984. Precambrian tectonicsand crustal evolution of south India. J. Geology 92, 3-20.

Ghosh, J.G., De Wit, M.J., Zartman, R.E., 2004. Age and tectonic evolution of Neoproterozoic ductileshear zone in the Southern Granulite Terrain of India, with implications for Gondwana studies.Tectonics 23, TC3600 138.

Gowd, T.N., Srirama Rao, S. V., Chary K. B., 1996. Stress field and seismicity in the Indian shield:Effects of the collision between India and Eurasia. PAGEOPH 146, 503-531.

Gowd, T.N., Srirama Rao, S.V., Gaur, V.K., 1992. Tectonic stress field in the Indian subcontinent. J.Geophys. Res. 97, 11,87911,888.

GSI, 1992. Quadrangle Geological Map 58B.John, B. and Rajendran, C.P. 2009. Evidence of episodic brittle faulting in the cratonic part of the

Peninsular India and its implications for seismic hazard in slow deforming regions Tectonophysics,Vol.471. 240-252.

John, B., 2003. Characteristics of near surface crustal deformation associated with shield seismicity:two examples from peninsular India. Unpublished PhD thesis.


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John, B., Rajendran, C.P., 2008. Geomorphic indicators of Neotectonism from the Precambrian terrainof Peninsular India: A study from the Bharathapuzha Basin, Kerala. J. Geol. Soc. India 71, 827-840.

Karlsson, H.R., 2001. Isotope geochemistry of zeolites. In: Natural zeolites: occurance, properties,

application. Rev. in Minerlogic and Geochemistry 45, 163-205.McGrath A.G. and Davison, I., 1995. Damage zone geometry around fault tips. J. Struct. Geol. Vol 17,1011-1024.

Molnar, P and Tapponnier, P., 1977. Collision between India and Eurasia. Scientific American, Vol 236,30-41.

Radhakrishna, T., Dallmeyer, R.D. and Joseph,. M., 1994 Palaeomagnetism and 36Ar/40Ar vs.39Ar/40Ar isotope correlation ages of dyke swarms in central Kerala, India: Tectonic implications.Earth and Planetary Science Letters Vol. 121, 213-226.

Rajendran, C. P., Rajendran, K., 1996. Low- moderate seismicity in the vicinity of Palghat Gap, SouthIndia and its implications. Current Science 70, 304-307.

Rajendran,K., Talwani, P., Gupta, H.K., 1992. State of stress in the Indian subcontinent: A review.Current Science 62, 86-93.

Rao, T.K.G.,Rajendran, C.P., Mathew, G.,John, B., 2002. Electron spin resonance dating of fault gouge

from Desamangalam, Kerala: Evidence for Quaternary movement in Palghat gap shear zone. Proc. IndianAcad. of Sciences (Earth and Planetary Sciences), 111, 103-113.

Ray, L., Senthil Kumar, P., Reddy, G. K., Sukanta Roy, Rao, G. V., Srinivasan, R., Rao, R. U. M., 2003.High mantle heat flow in a Precambrian granulite province: Evidence from southern India. J.Geophys. Res. 108 (B2) 2084 Doi:10.1029/2001JB000688.

Santosh, M., Yokoyama, S., Biju-Sekhar, S., Rogers, J.J.W., 2003. Multiple tectonothermal vents in thegranulite blocks of Southern India revealed from EPMA dating: implications on the history ofsupercontinents. Gondwana Research 6, 2963.

Sleep, N.H., Blanpied, M.L., 1992. Creep compaction and week rheology of major faults. Nature 359,687-692.

Soman, K., Tara, K.G., Arakelyants, M.M., Golubyev, V.N., 1990. Mineral ages of pegmatites from thePalghat Gap region in Kerala and their tectonic significance. J. Geol. Soc. India 35, 82-86.

Subramaniam, K.S., Muraleedharan, M.P., 1985. Origin of the Palghat Gap in South India A synthesis.J. Geol. Soc. India. 26, 28-37.

Vermilye, J.M., Scholz, C.H., 1998. The process zone; a microstructural view of fault growth. Journal ofGeophysical Research. 103, 12,22312,237.


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C Sreejith and G R Ravindra Kumar Origin and exhumation of granulite-facies rocks in the Kerala Khondalite Belt, southern India

From arc to highlands: the story of origin and

exhumation of granulite-facies rocks in theKerala Khondalite Belt, southern India

C. Sreejithand G. R. Ravindra Kumar

Geosciences Division, National Centre for Earth Science Studies, Akkulam, Thiruvananthapuram,Kerala 695011, India

Present address:Department of Studies and Research in Geology, MES Ponnani College,Ponnani South PO, Malappuram., Kerala 679586, India

E-mail: [email protected], [email protected]

Introduction

Precambrian shields of the most continental fragments comprise high-grade terranes representedby granulite-facies mineral assemblages (see, Harley, 1989). The ubiquitous presence ofgranulites suggests that high-grade metamorphism as one of the key processes in the origin andevolution of Precambrian deep crust. The formation of regional granulite-facies terranesrepresents major crustal formation or tectonic episodes in varying extents on all continents.Therefore, comprehensive studies on such rarely preserved, regional granulite-facies terrainsprovide an opportunity to understand the processes of origin and evolution of continental crust,that otherwise remain as a fundamental paradox in intriguing areas of Earth science research.The granulite blocks of southern India (Fig. 1), well known as the southern granulite terrain

(SGT), are classic examples of such a deeply eroded continental basement. In this paper, weevaluate the geochemical and thermo-mechanical characteristics of magmatic pattern preservedin the Kerala Khondalite Belt (KKB), a Proterozoic orogenic component and address the problemof magma differentiation, high-grade metamorphism and exhumation of the lower crust in theKKB.

Geology of the Kerala Khondalite BeltThe KKB (Fig. 1) occupies southernmost part of the southern Indian granulite terrain. It isbounded on the north by charnockite massifs of the Cardamom Hills and on the south byNagercoil massive charnockite unit (Ravindra Kumar and Chacko, 1986). The lithologicalarchitecture shows three distinct domains in the KKB. The central and northern parts of the

KKB are mainly composed of migmatitic garnet-biotite gneisses and pelitic (garnet + biotite+ sillimanite cordierite) granulites (Chacko et al., 1987). Later studies (e.g.,Cenki et al.,2004) substantiated this observation, identifying varying Nd model ages across the KKB, andrecognised three distinct lithotectonic units; viz., north-eastern Achankovil (AU), southernNagerkovil (NU) and central Ponmudi (PU) units.

Compositional and structural signatures of polymetamorphism are established inmany lithological units of the KKB, which was accompanied by intense migmatizationattaining granulite-facies (Chacko et al., 1987; Cenki et al., 2002). The key metamorphicminerals in the KKB gneisses include garnet, orthopyroxene cordierite and sillimanite, andare products of the high- to ultra-high temperature metamorphism during Pan-African (c. 550Ma) orogeny (Chacko et al., 1987).


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Fig. 1.Simplified geological and tectonic framework of south India showing crustal blocks and majorshear zones: (Ia) Western Dharwar Craton (WDC); (Ib) Eastern Dharwar Craton (EDC); (II) SalemBlock; (III) Madras Block; (IV) Nilgiri Block; (V) Madurai Block; and (VI) Kerala Khondalite Belt(KKB). MBSZ= Moyar (MSZ)-Bhavani (BSZ) shear zones; PCSZ= Palghat-Cauvery shear zone;KKPT= Karur-Kambam-Painavu-Trichur shear zone. EGMB= Eastern Ghats mobile belt. The portionwithin the box, comprising the KKB is the area of interest in the present study.

Very recently, subductionaccretioncollision associated geochemical signatures are inferredfrom granitic rocks of the KKB (Sreejith and Ravindra Kumar, 2013). These authors presumedthis as a product of terrane assembly of the Proterozoic supercontinent fragments associatedwith demise of the Mozambique Ocean.

High-grade metamorphism andPTtpaths

The metamorphic evolution of the KKB is characterised by a clockwisePTpath with post-peak isobaric cooling followed by isothermal decompression (Cenki et al., 2002; Sreejith andRavindra Kumar, 2012). Estimated peak pressure are lower (67 kbar; Chacko et al., 1996;Cenki et al., 2002) compared to the northerly lying Madurai Block of SGT, where maximumpressures of ca. 1012 kbar have been reported (Raith et al., 1997). However, two feldsparthermometry (Braun et al., 1996) in rocks from the central and northern KKB and calc-silicateassemblages (Chacko et al., 1996) have documented imprints of UHT metamorphism (>900


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C). Charnockites and migmatitic gneisses in the NU have recordedPTestimates of 670720C and 51 kbar (Srikantappa et al., 1985). Thermobarometric calculations on arrestedcharnockites and associated gneisses have shown that the incipient charnockitization in the

PU occurred at 5.5 kbar and 700750 C (Chacko et al., 1987; Raith et al., 1997).Multiple episodes of biotite dehydration melting, inferred in various lithologies, have

caused migmatisation and melt extraction of the KKB rocks (Chacko et al., 1987, 1996; Braunet al., 1996). Recent studies (Sreejith and Ravindra Kumar, 2012, 2013) suggested the crustalanatexis within KKB as a consequence of crustal thickening producing high geothermalgradient during a collisional orogenic event. Applications of MnNCKFMASH phase relationmodelling on cordierite-orthopyroxene migmatitic gneisses of the AU substantiated theclockwise isothermal decompressionPTpath and yielded a low-pressure (3.2 kbar) regimefor the equilibration of melt-bearing assemblages (Sreejith and Ravindra Kumar, 2012).Further, these authors advocated a model of change in lower crustal rheology due toextensive melt production leading to rapid erosional exhumation and corresponding

clockwise isothermal decompressionPTpath. The inferredPTpath for migmatitic cordieritegneisses follow two-fold post-peak PTevolution, characterized by an initial cooling stagewith limited decompression followed by a strong decompression with further drop intemperature.

The timing of tectono-metamorphic evolution of the KKB is well constrained only forthe Neoproterozoic, largely because most of the previous metamorphic records have beenobliterated during the Pan-African orogeny. Available UPb zircon and monazite, SmNdmineral and whole rock, and EPMA monazite dating clearly point towards the prevalence ofPan-African (560516 Ma) metamorphism (Braun and Kriegsman, 2003). Braun et al. (1996)reported protracted events of melt generation and crystallization in the KKB based on PbPbisochron ages from fluorapatite and monazite of leucogranites. This is further substantiated

by UPb monazite dating of granitic gneisses (Braun and Brcker, 2004). Apart from this, fewstudies were able to document an early thermal event in the early Proterozoic (~1800 Ma),reaching upper amphibolite- to granulite-facies (Braun et al., 1998). All these studies aresuggestive of multiple stages of thermal evolution within the KKB.

Geodynamic evolution of KKB granulitesThe geodynamic model based on metamorphic phase relations of melt-bearing assemblagesof the KKB proposed a two-stage process for the crustal evolution initiated with thickening ofthe crust in relation to a continental-arc setting, followed by exhumation along a high-temperature stable geotherm (Sreejith and Ravindra Kumar, 2012). A very recentcontribution discussing the production of voluminous high-K metagranites within the KKBmodified and extended the early view proposing the crustal evolution as a product ofmagmatic accretion followed by intracrustal differentiation in an episodic manner (Sreejithand Ravindra Kumar, 2013).

Reported model ages (TDM and Hfc) for the central Ponmudi (PU) and southernNagerkovil (NU) units of the KKB indicate crustal accretion as a prolonged process initiatedas early in Meso-Archaean (~3 Ga.) and extended till Palaeoproterozoic (ca. 2.1 Ga.) age.This was followed by collisional orogenesis attaining high geothermal gradients leading tointracrustal melting and differentiation process generating high-K metagranites in thePalaeoproterozoic (~1800 Ma). The crystallization of high-K metagranites is highlycomparable with the doubted Palaeoproterozoic granulite-facies metamorphic age (Braun etal., 1998) and indicating intracrustal melting as a consequence of this high thermal eventformed in a collisional setting (Sreejith and Ravindra Kumar, 2013).


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Fig. 2. Cartoon showing the simplified and generalised evolution of the arc-accretion alongAchankovil suture zone.

The younger magmatic crystallization ages (ca. 1.56 Ga) of K-feldspar megacryst granites(Krner et al., 2012) are correlated in terms of subduction cessation of the convergentcontinental margin in a continent-arc setting. The latest phase of crustal addition is the felsicmagmatism occurred in the form of granites, syenites, and pegmatites throughout the east

Gondwana province during Pan-African time (Braun and Kriegsman, 2003). These areessentially alkaline in nature and are interpreted as a product of post-collisional riftingindicating a phase of orogenic collapse and extension tectonics. Though the geochronologicaldata represent a wide time span for crustal growth in the KKB from the Mesoarchaean eraand continuing to the Pan-African, each event is restricted to a distinct period representing acomplex tectono-metamorphic history of KKB. The reported magmatic and/or metamorphicages of KKB are, therefore, comparable with the major global orogenic cycles and crustalgrowth events (Condie, 2000).

Calculations based on molar volumes of major oxide concentrations using methodsand data of Bottinga and Weill (1970) indicate that the density of the original, hydrousgranitic magma would have been 2460 kg/m3 (SiO2= 63.30 wt.%) at P=0.1 Mpa andT=1200C with ~1 wt.% H2O in the melt. At these conditions the granitic magma has aneffective viscosity of 105.60 Pa s. These observations ascertain gravitational instability in thenewly accreted crust, owing to phase transformations that produce dense minerals like garnetas (eclogite or garnet amphibolite) residue. The density difference in the granite liquid mighthave induced rheological reequilibration in the continental-arc root, as melt buoyancyconsiderably decreased the bulk density of the lithosphere. Therefore, the final stages of theorogeny will lead to rapid exhumation along a high-temperature stable geotherm withsufficient pressure release associated with syn- to post-convergence transpression andtranstension. This remark supports the rheological (thermal and mechanical) reequilibrationand rapid erosional exhumation model proposed for the final stages of orogeny obtained byphase relations models (Sreejith and Ravindra Kumar, 2012). Our study demonstrates

intracrustal melting and differentiation in continental arcs as one of the possible mechanisms


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for magma diversification for the formation of granitic to granodioritic magma and rapidexhumation of rheologically modified continental crust.

SummaryThe KKB is a unique terrane representing polyphase magmatic and/or metamorphic processIn recent years, the understanding of crustal evolution within the KKB have been muchevolved from the earlier view of entirely supracrustal origin for the terrain to the discovery ofmajor magmatic crust forming events and subductionaccretioncollision tectonics (Fig. 2).The geodynamic model based on metamorphic phase relations of melt-bearing assemblagesof the KKB proposed a two-stage process for the crustal evolution initiated with thickening ofthe crust in relation to a continental-arc setting, followed by exhumation along a high-temperature stable geotherm (Sreejith and Ravindra Kumar, 2012). A very recentcontribution discussing the production of voluminous high-K metagranites within the KKBmodified and extended the early view proposing the crustal evolution as a product of

magmatic accretion followed by intracrustal differentiation in an episodic manner (Sreejithand Ravindra Kumar, 2013).

References

Bottinga, Y. and Weill D.F., 1970. Densities of liquid silicate systems calculated from partial molarvolumes of oxide components. Amer. Jour. Sci., 269, 169182.

Braun, I., Brcker, M., 2004. Monazite dating of granitic gneisses and leucogranites from the KeralaKhondalite Belt, southern India: implications for Late Proterozoic crustal evolution in EastGondwana. Int. Jour. Earth Sci., 93, 1322.

Braun, I., Kriegsman, L.M., 2003. Proterozoic crustal evolution in southernmost India and Sri Lanka.Spec. Publ., Geol. Soc., London, 206, 169202.

Braun, I., Raith, M., Ravindra Kumar, G.R., 1996. Dehydration-melting phenomena in leptyniticgneisses and the generation of leucogranites: a case study from the Kerala Khondalite Belt, southernIndia. Jour. Petrol., 37, 12851305.

Braun, I., Montel, J.M., Nicollet, C., 1998. Electron microprobe dating of monazites from high-gradegneisses and pegmatites of the Kerala Khondalite Belt, southern India. Chem. Geol., 146, 6585.

Cenki, B., Kriegsman, L.M., Braun, I., 2002. Melt-producing and melt-consuming reactions in theAchankovil cordierite gneisses, South India. Jour. Meta. Geol., 20, 543561.

Cenki, B., Braun, I., Brcker, M., 2004. Evolution of the continental crust in the Kerala Khondalite Belt,southernmost India: Evidence from Nd isotope mapping combined with U-Pb and Rb-Srgeochronology. Precam. Res., 134, 275292.

Chacko, T., Ravindra Kumar, G.R., Newton, R.C., 1987. Metamorphic PT conditions of the Kerala(South India) Khondalite Belt: a granulite facies supracrustal terrain. Jour. Geol., 96, 343358.

Chacko, T., Lamb, M., Farquhar, J., 1996. Ultra-high temperature metamorphism in the Kerala

Khondalite Belt.In: M. Santosh, M. Yoshida (Eds), The Archaean and Proterozoic Terrains in SouthernIndia within East Gondwana. Mem., Gond. Res. Group, 3, 57165.

Condie, K.C., 2000. Episodic continental growth models: afterthoughts and extensions. Tectonophy.,322, 153162.

Harley, S.L., 1989. The origin of granulites: a metamorphic perspective. Geol. Mag., 126, 215247.Krner, A., Santosh, M., Wong, J., 2012. Zircon ages and Hf isotopic systematics reveal vestiges of

Mesoproterozoic to Archaean crust within the late NeoproterozoicCambrian high-grade terrain ofsouthernmost India. Gond. Res., 21, 876886.

Raith, M., Karmakar, S., Brown, M., 1997. Ultra-high temperature metamorphism and multistagedecompressional evolution of sapphirine granulites from the Palni Hills Ranges, southern India. Jour.Metamorph. Geol., 15, 379399.

Ravindra Kumar, G.R., Chacko, T., 1986. Mechanisms of charnockite formation and breakdown in

southern Kerala: implications for the origin of the southern granulite terrain. Jour. Geol. Soc. India,28, 277288.


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Sreejith, C., Ravindra Kumar, G.R., 2012. MnNCKFMASH phase relations in cordieriteorthopyroxenemigmatitic gneisses, southern India: implications for low-pressure crustal melting under granulite-facies. Jour. Geol. Soc. India, 80, 613627.

Sreejith, C., Ravindra Kumar, G.R., 2013. Petrogenesis of high-K metagranites in the Kerala KhondaliteBelt, southern India: a possible magmatic-arc link between India, Sri Lanka, and Madagascar. Jour.Geodyn., 63, 6982.

Srikantappa, C., Raith, M., Spiering, B., 1985. Progressive charnockitization of a leptynite-khondalitesuite in southern Kerala, IndiaEvidence for formation of charnockites through decrease in fluidpressure? Jour. Geol. Soc. India, 26, 849872.


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V J Rajesh et al. Neo-Archaean Alaskan-Type Ultramafic Rocks in Palghat-Cauery !uture "one# southern $n%ia

Petrology and Tectonic Significance of Probable

Neo-Archaean Alaskan-Type Ultramafic Rocksin Palghat-Cauvery Suture Zone,southern India

V. J. Rajesh1*, S. Arai2 and M. Santosh3

1Department of Earth and Space Sciences, Indian Institute of Space Science and Technology,Thiruvananthapuram 695-547, India

2Department of Earth Sciences, Kanazawa University, Kakuma-machi, Kanazawa 920-1192,

Japan3School of Earth Science and Resources, China University of Geosciences, Xueyuan Road,Haidian District, Beijing 100-083, China

* Email: [email protected]; [email protected]

Abstract

Suture zones in high-grade metamorphic terrains exert major control over the exhumationhistory of continental deep crust, emplacement tectonics of igneous intrusives and transfer offluids from various depths. The spatial association of magmatic intrusives with suture zones isconsidered to indicate partial melting and/or melt transport favored by pre-existing structures.The proximity and alignment of the intrusions along suture zones suggest that the orientation of

these intrusions are controlled by the suture and rate of the melt or magma injection and heatloss into the country rock. The diverse emplacement styles, deformational patterns andmetamorphism may lead to the preservation of structurally dismembered ultramafic complex inmany suture zones. Detailed characterization of various lithological units within the ultramaficcomplex and the timing and geodynamic setting of the emplacement are vital as they offerwindows into processes within the upper mantle, deep crust and crust-mantle interface.

The Southern Granulite Terrain (SGT) in southern India has figured prominently inthe reconstructions of Late NeoproterozoicCambrian Gondwana supercontinent assembly.Recent geological, geochronological and geophysical studies on various crustal blocks andsuture zones in the SGT have provided important evidences for subduction and arcmagmatism which accompanied the final collision and amalgamation of the Gondwanasupercontinent. Two major zones of Mozambique oceanic closure are identified in the SGT,one in the north at the southern margin of the Dharwar Craton designated as the Palghat-Cauvery Suture Zone (PCSZ), and the other in the south termed as the Achankovil SutureZone (ACSZ). The boundary between the Archean Dharwar Craton in the north and theNeoproterozoic Southern Granulite Terrane in the south in southern Peninsular India ismarked by the Palghat-Cauvery Suture Zone (PCSZ). This zone is considered as the trace ofMozambique Ocean closure where subduction-accretion-collision processes were operatedwhen the Gondwana supercontinent was finally assembled. The PCSZ extends westwards intoMadagascar as the Betsimisaraka suture that divides the Archaean Antongil craton to the eastfrom the Antananarivo granulite-facies orogenic belt to the west and continues eastwards intoAntarctica. Close association of eclogites and ultramafic rocks were considered as one among

the critical petrological indicators to locate a suture zone. Numerous ultramafic- rocks occurin close association with eclogites in the PCSZ. The origin of most ultramafic rocks in the


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V J Rajesh et al. Neo-Archaean Alaskan-Type Ultramafic Rocks in Palghat-Cauery !uture "one# southern $n%ia

PCSZ has not been determined, despite their importance in tectonic interpretation of thissignificant suture zone. We used petrography, mineral chemistry and geochemistry tounderstand the petrogenesis and tectonic setting of ultramafic rocks having Alaskan-type

geochemical signatures in this suture zone. Alaskan-type intrusives are commonly zonedmaficultramafic complexes dominated by dunite, wehrlite, olivine clinopyroxenite andclinopyroxenite/websterite. They represent roots of island-arcs and indicate the position of asuture zone in the plate tectonic context. They also serve as host rocks for a range ofplatinum group mineralization. Therefore, their identification is vital in orogenic belts inreconstructing the paleoplates and plate boundaries, and also for economic mineralization.

The Alaskan-type intrusives in the PCSZ are represented by cumulate-textured dunitewith minor hornblende-bearing websterite intruded into the deformed mafic granulites,enderbites and hornblende-bearing gneisses. Dunite is dominated by olivine (mostly altered)with minor disseminated subhedral to anhedral chromite and interstitial primary amphibole.Websterite is characterized by an assemblage of clinopyroxene- orthopyroxene-hornblende

with magnetite as accessory phase. Alteration rims developed around chromite grains suggestpost-magmatic alteration. Chromites in dunite have high Cr# ((Cr/Cr+Al) in the range of0.77 to 0.80 towards core), high Fe3+# ((Fe3+/(Al3++Cr3++Fe3+) ~ 0.3) and low Mg#((Mg/ Mg+Fe2+)) of ~ 0.2). Major, trace and rare earth element systematics of whole rock(websterite) and silicate phases in dunite and websterite show LREE-enrichments with asmall negative Eu anomaly and prominent negative Nb, Ta and Ti anomalies. These chemicalfeatures signify the involvement of subduction components in their magma genesis. Our datafurther point towards a common parental magma of hydrous tholeiitic basalt in a subduction-related (island-arc) tectonic setting for the genesis of these intrusives in the PCSZ. Themineral compositions are similar to other Neo-Archaean Alaksan-type complexes reportedelsewhere. We infer that these Alaskan-type intrusives represent remnants of a Neo- Archaeanarc-root complex associated with the development of the PCSZ. This study has importantimplications for interpreting the subduction tectonics of the PCSZ within the Gondwanasupercontinent.

Key words:Alaskan-type ultramafics, dunite, websterite, Palghat-Cauvery Suture Zone, South India


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Y Anilkumar and S C Patel Rutile exsolution in garnet in Mg-Fe-Al granulites from Karur, Madurai Block, southern ndia

Rutile exsolution in garnet in Mg-Fe-Al

granulites from Karur area, Madurai Block,southern India: an indicator of decompression

P-T path

Y. Anilkumara, *and S.C. Patela

aDepartment of Earth Sciences, Indian Institute of Technology, Powai, Mumbai 400076, India*Present Address: Geological Survey of India, State Unit: Kerala, Thiruvananthapuram 695013,

Kerala, India, Email: [email protected]

Abstract

Exsolution of rutile in garnet is a potential indicator for high pressure metamorphism. Titanium-rich phases such as ilmenite and rutile have been reported as exsolved phases in olivine,clinopyroxene and garnet from ultra-high pressure (UHP) peridotites and eclogites (e.g.Dobrzhinetskaya et al., 1996; Zhang and Liou, 1999; Zhang et al., 2003). If rutile is exsolved, itmay occur as needles, preferentially oriented and regularly spaced in garnet (McGetchin andSilver, 1970; Hunter and Smith, 1981; Smith, 1987). Experimental studies indicate that Tisolubility in garnet increases with bothpressure and temperature, but is mainly pressuredependent (Fett, 1995).

Apart from the exsolution origin of rutile needles in garnet, there are views that the needlesform as inclusions during the growth of garnet. In this case, it is expected that thecrystallographic orientations of rutile and the host garnet would not match. OrientationImaging Microscopy (OIM) is an established technique, based on automatic indexing ofElectron Back-Scatter Diffraction (EBSD) patterns of crystalline phases, to studycrystallographic orientation in microscopic scale. In this method the electron beam in aScanning Electron Microscope (SEM) is allowed to strike the sample mounted at aninclination of 70owith respect to horizontal. Diffraction of the beam produces a pattern ofintersecting bands called Kikuchi bands or EBSD pattern. Kikuchi bands are directly related tothe crystal structure of the sample under investigation.

Mg-Fe-Al granulites are found as isolated patches within hornblende gneiss at

Panangad and Sevitturanganpatti located north of Karur (Fig. 1a & b). At Panangad, thegranulites vary from gedrite-garnet-kyanite-rich domains to kyanite-gedrite-rich domains withminor amounts of cordierite and biotite in all samples. Cordierite locally forms segregations.Garnet is reddish pink in colour and dodecahedral in form. It is commonly 12 cm in size, butcan grow as large as 8 cm. Both kyanite and gedrite form stubby prisms, which can be up to12 cm long. Few sillimanite needles growing into kyanite can be noticed. Tiny needles ofrutile within garnet are present in coarse grained Mg-Fe-Al granulites from Panangad andSevitturanganpatti of Karur area. Garnet commonly contains microscopic triangular arrays offine needles (< 1 m wide) of rutile which are apparently parallel to the isometric form{111} consistent with an origin by exsolution (Fig. 2a & b). OIM analysis of a garnet wasperformed with a FEI QUANTA 200 SEM at the Department of Metallurgical Engineering and

Material Science, IIT Bombay. The study was aimed at identifying rutile needles and thecrystallographic relationship between rutile and garnet.


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Fig. 1a.Field photographs of Mg-Fe-Al granulite at Panangad. (a) garnet-kyanite-rich domain andcordierite segregation in the left, and weathered garnet-rich rock in the right; (b) garnet-gedrite-kyanite assemblage

Fig. 2a. Photomicrographs showing fine needles of rutile in garnet (Mg-Fe-Al granulite sampleAK140C). (a) Triangular arrays of rutile needles. (b) Optical mismatch of rutile needles and garnethost evidenced by rutile gains showing interference colour in the host at extinction.

Fig. 3.False colour, orientation microscopic images showing rutile in red colour and garnet host in

green colour.


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The SEM was operated at an accelerating voltage of 20 kv and scanning step size of 0.4micron. Scan was performed with reference to known crystal structures (space group and

lattice parameter) of garnet [Ia 3d, and (a,b,c) 11.459] and rutile [P1 and (a) 4.59 (b) 4.59(c) 2.96] respectively. The resultant image of one scan is given in Fig. 3. Comparison ofseveral such images shows a parallel crystallographic pattern of needles from different partsof garnet which supports crystallographic-controlled exsolution of rutile.

Recent experiments have shown that Ti solubility in garnet depends on P-Tconditions. Zhang et al. (2003) reported increasing solubility of TiO2 (0.8 to 4.5 wt%) ingarnet with increasing P and T in the experimental conditions of 50-150 kbar and 1000-1400oC. On the other hand Kawasaki and Motoyoshi (2007) observed that TiO 2 content ofgarnet increases with temperature and decreases with pressure in the P-T range of 7-20 kbarand 850-1300oC. The results of these studies show that rutile exsolution in garnet can be theresult of decompression and/or cooling. In the Karur area, the exsolution of rutile most likely

occurred during decompression that is indicated by other rock types.

References

Dobrzhinetshaya, L., Green, H.W. and Wang (1996) Alpe Arami: a peridotite massif from depths ofmore than 300 kilometers, Science, v. 271, pp. 1841-1846.

Fett, A. (1995) Partitioning of Ti between garnet and clinopyroxene in high pressure experiments andhigh pressure rocks a comparison, Ph.D. thesis, John Gutenberg-universitat, Mainz, Germany.

Hunter, W.C. and Smith. D. (1981) Garnet peridotite from Colorado Plateau ultramafic diatremes:hydrates, carbonates, and comparative geothermometry. Contrib. Mineral. Petrol., v. 76, pp. 312-320.

Kawasaki, T. and Motoyoshi, Y. (2007) Solubility of TiO2in garnet and orthopyroxene: Ti thermometer

for ultrahigh-temperature granulites. USGS Open-File Report 2007-1047, Short Research Paper 038;doi:10.3133/of2007-1047.srp038.McGetchin, T.R. and Silver, L.T. (1970) Compositional relations in miner

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Shear Zones and Crustal Blocks of southern India