Shear Zones and Crustal Blocks of southern India vol 2

ISBN 978-81-923449-4-2

Shear zones Shear zones & &

crustal blocks of southern Indiacrustal blocks of southern Indiavol 2vol 2

UGC SAP DRS Phase I I (2013–18) Seminar: 2nd year (31 Mar 2015)

MineralizationFluid inclusionsPetrologyTectonics

Dept. of GeologyUniversity of Kerala,Trivandrum 695 581, India

Shear Zones and Crustal

Blocks of Southern Indiavol 2

Proceedings of 2nd UGCSAPDRS II (2013 – 18)Seminar31 March 2015

Department of GeologyUniversity of KeralaTrivandrum, India

2015

A. P. Pradeepkumar and E .Shaji (ed.s)

ISBN 9788192344942

Email: [email protected] [email protected]

Dedication

This volume is dedicated to all the faculty members of the senior generation of this department who nurtured and guided the department to its current stage of growth and strengths.

Late Prof. K.K.MenonProf. K.V.K.Nair Late Prof. R.KrishnanathProf. Raju PhilipProf. P.K.Rajendran NairProf. K.P.ThrivikramjiProf. Narayanan NairProf. Roy ChackoProf. S.AnirudhanProf. V.Prasannakumar

Table of contents

Preface

Significance of UHT granulites from the Cauvery Shear Zone, India 1K. Sajeev, D.J. Dunkley, P.M. George, C. IshwarKumar and V.J. Rajesh

Contrasting geological conditions across Palghat–Cauvery Shear Zone, Kerala 2P . Soney Kurien and M.N. Praveen

Charnockitisation of Achankovil Shear Zone 3G.Manimaran, J.Besheliya, P .T. Roy Chacko, D.Manimaran

Terrestrial anorthosites as probable lunar analogues: a study on anorthosites from 14various shear/suture zones in south India

V . J. Rajesh and K. Sajeev

Lineaments in Kerala – a holistic window for tectonic events in the Indian peninsula 16 M. P. Muraleedharan

Petrogenesis and tectonic setting of ultramafic complexes in western India and eastern 23 Madagascar: Inferences from chromian spinel chemistry

C. IshwarKumar, V .J. Rajesh, B.F. Windley, T. Razakamanana, T. Itaya,E.V.S.S.K. Babu, K. Sajeev

Significance of magma emplacement in the regional dyke swarms: a case study from 25Moyar–Bhavani Shear Zone

P. Pratheesh and C. Vikas

Mineralization associated with granitic pegmatites of Nagamalai – Pudukottai area, 34Madurai district, Tamil Nadu, India

R. Manu Raj and S. N. Kumar

Unusual metaultramafics of Wayand, southern India: a petrological report 39Shaji E, Indu G, Arungokul J, Dhanil Dev S G and Pradeepkumar A P

Cordierite gneisses in Munnar granite, southwestern India: implications for 40isothermal decompression history

S. Rajesh and A. P. Pradeepkumar

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 UGCSAPDRS (University Grants CommissionSpecial Assistance ProgramDepartmental Research Support) Phase I with thrust area of research 'Kinematics of south Indian shear zones'. The second phase of UCGSAPDRS (2013 to 2018) has been sanctioned 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 lowerlevels of the continental crust, dissected by crustalscale shear zones among which the PalghatCauvery 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 this project detailed investigations have been 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, the first UGC–SAP– DRS Phase II conference was held on 29 March 2014. It brought together experts in the thrust area leading to very vibrant presentations and discussions. In this second edition of the seminar series wellknown experts as well as budding geoscientists from various scientific organizations/universities/research centers in petrology, tectonics and geochemistry have come together 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 go forward with the phase II research program in a wellplanned 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, IISc, IIST, VOC college are greatly acknowledged. The financial support received from the University Grants Commission has helped this department aspire for excellence in research and this is gratefully acknowledged. The department is on the anvil of building up a strong petrological and fliud inclusion lab with the UGC SAP financial support. This will benefit the students and faculty of this University as well as neighbouring ones and will be open to all researchers of this country.

E. ShajiDty coordinator, UGCSAPDRS II

A. P. PradeepkumarCoordinator, UGCSAPDRS II

K Sajeev et al. Significance of UHT granulites from Cauvery Shear Zone

Significance of UHT granulites from the Cauvery Shear Zone, India

K. Sajeev1, D.J. Dunkley

2,3 , P.M. George

1, C. IshwarKumar

1 and V.J. Rajesh

4

1 Centre for Earth Sciences, Indian Institute of Science, Bangalore 560012, India

2 Department of Applied Geology, Western Australian School of Mines,

Curtin University, GPO Box U1987 Perth, Western Australia 68453

National Institute of Polar Research, 310 Midoricho, Tachikawashi, Tokyo1908518, Japan

4 Department of Earth & Space Sciences, Indian Institute of Space Science & Technology,

Thiruvananthapuram 695 547, India

Email: [email protected]

AbstractThe PTt evolution of UHT granulites from the Cauvery Shear Zone in southern India, have significant bearings in understanding the highgrade metamorphism and lower crustal processes during NeoproterozoicCambrian orogeny. Garnet kyanite rock that consist inclusions of gedritespinelquartz as well as sapphirine spinel assemblages within garnet, together mark the prograde formation of garnet from lower to higher pressure condition. Garnet surrounding gedrite reveals the later formation of garnet. The garnetcorundumstauorlitekyanite assemblage formed at highpressure represents the peak metamorphism. Garnet cores have a Ferich composition and Mgrich rim. The REE chemistry of zircon rims is comparable with that of garnet cores except for a significant absence of Eu. Staurolite in the investigated samples have moderate to high Mgcontent which again indicates highpressure stability. Thermodynamic modelling results show that the PT peak of this gedritebearing granulite was at UHT conditions (ca. 19 kbar and ca. 925°C). The petrographic and phase diagrams together demonstrate the evolution of this granulite, having a tight hairpintype anticlockwise PT path. The UPb ages and REE analyses suggest that the metamorphic rims on zircon grew at ca. 537 ± 5 Ma, i.e., in equilibrium with the garnet cores. The garnetcorezircon equilibrium was stable during the ultrahightemperature condition. The SmNd mineral data suggests a cooling age of ca. 511 Ma. The biotite in a garnetkyanite rock has a KAr cooling age of ca. 519 Ma, which can be interpreted as the time of uplift. Comparison of the present results with other published petrographical and geochronological datasets from the Cauvery Shear Zone enables to determine a realistic PTt evolution of this terrene during NeoproterozoicCambrian orogeny.

A. P. Pradeepkumar and E .Shaji (eds) Shear Zones and Crustal Blocks of Southern India vol.2 Proc of the UGCSAPDRSII seminar, Dept of Geology, University of Kerala, India, 31 March 2015, 40p ISBN 978-81-923449-4-2 1

mailto:[email protected]

P Soney Kurien and MN Praveen Contrasting geological conditions across PCSZ

Contrasting geological conditions across PalghatCauvery Shear Zone, KeralaP. Soney Kurien and M.N. Praveen

State Unit: Kerala, Geological Survey of India, Thiruvananthapuram, IndiaEmail: [email protected]

AbstractThe crustal blocks of Southern Granulite terrain are amalgamated along different shear/suture zones of Proterozoic age. PalghatCauvery shear zone (PCSZ) is one such major suture zone which runs across the Western Ghats. It is bounded on the north by Dharwar Craton and on the south by Proterozoic granulite gneisses of Madurai Block. Earlier workers identified it as a dextral shear zone and recently, the PCSZ has been identified as the trace of the Cambrian suture representing Mozambique Ocean closure during the final phase of amalgamation of the Gondwana suprercontinent. At the same time, a few workers doubted the status of PCSZ as a shear zone. The shear zones represent zones along which two crustal blocks are in juxtaposition and this can be envisaged to have developed in collisional setting with transpressional movements. These crustal blocks have different geological histories and as a result, they have different structural and metamorphic setups. Intensive field studies can bring out such differences across the shear zones.

In this paper, the structural and metamorphic conditions across the PalghatCauvery lineament are analysed. A protomylonite zone running almost parallel to the EW trend of the Bharatapuzha, near Ottapalam, separates two different terrains. The northern terrain is occupied by biotite gneiss with enclaves of metapyroxenite, amphibolite, banded iron formations (BIF) and minor bands of sillimanite and garnet bearing metapelites. These can be considered as oceanic plate components. The amphibolite and BIF bands are always seen in physical contact indicating their closely related origin in an Archean ocean floor. Minor occurrences of massive sulphide zones observed within amphibolite could represent volcanogenic exhalites similar to present day sea floor black smokers. Southern terrain is devoid of the ocean plate components, while the major rocks here are the TTG gneisses represented by augen gneiss, biotite gneiss and banded charnockite. Structural set up on either side of the Bharatapuzha River are different. The northern terrain preserves elements of at least three episodes of deformation. The first generation folds are generally noticed on BIF bands. The second generation folds are the most conspicuous structural feature which coaxially refolded the axial planes of the first generation folds under NS compressional regimes. The terrain south of the Bharatapuzha River does not seem to have undergone the above mentioned folding episodes. In this terrain, only one set of folds is noted which is having the nature of a recumbent fold with a NE vergence. This recumbent fold can be attributed to thrust movement during the collision of the two terrains. This has resulted in the subhorizontal shear and foliation planes in the southern terrain.

A. P. Pradeepkumar and E.Shaji (eds) Shear Zones and Crustal Blocks of Southern India vol.2 Proc UGCSAPDRS II seminar, Dept of Geology, University of Kerala, India, 31 March 2015, 40p ISBN 978-81-923449-4-2 2


G.Manimaran et al. Charnockitisation of Achankovil Shear Zone

Charnockitisation of Achankovil Shear Zone

G.Manimaran1, J.Besheliya1, P.T. Roy Chacko2, D.Manimaran1

1School of Tectonics, Department of Geology, V.O.Chidambaram College, Tuticorin 628008, India2Department of Geology, University of Kerala, Kariavattom, Trivandrum695581, India


AbstractAchankovil shear zone of Tirunelveli regions revisited to examine the charnockitisation processes of this region. Two periods of charnockitic magma intrusion during D2 and D3

deformations are delineated. First charnockite magmatic episodes were related to TenmalaiGatana dextral shearing and F2 upright folding of D2. The second period of charnockite magma intrusion was resulted due to D3 deformation associated with F3

coaxial folding and Achankovil sinistral shearing. Along dextral D2 shearing and sinistral D3 shearing CO2 infiltration from the mantle/deep seated source resulted in the incipient charnockites of pelitic gneisses, grey granites and pink granites of the Tirunelveli regions were also identified. Adjacent to granite dykes and also near a K2O metasomatic areas in granites were charnockitised. Fluid present metamorphism and charnockite magma intrusions play vital role in the charnockitisation of Achankovil shear zone.

IntroductionThe Southern Granulite Terrain (SGT) of southern India comprises a collage of Archaean and Neoproterozoic high grade metamorphic terrains. The protoliths of the late Archaean charnockites were ascribed to calcalkaline felsic magmas produced by subduction process during the latest Archaean with variable extent of incorporation of older crustal components. Incipient charnockites were formed in orthopyroxene bearing dehydration zones and mechanism of dehydration related to charnockitisation is under debate. The fluid absent dehydration melting reaction in which hydrous minerals such as biotite and hornblende melt incongruently to form orthopyroxene and a H2O bearing melt (Fyfe, 1973; Powell, 1983; Lamb and Valley, 1984; Waters, 1988; Thompson, 1990; Clemens, 1990). The fluid present dehydration melting by an immiscible low – a H2O fluid (Janardhan et al., 1979; Santosh, 1986; Newton, 1992) that is either CO2 rich or a Na and K concentrated Cl rich brine or both as coexisting immiscible fluids (Perchuk and Gerya, 1992; Touret and Huizenga, 2012) resulted in conversion of amphibolite facies rocks to granulite facies rocks from Söndrum in southwestern Sweden; Kigluaik Mountain in Alaska; IvreaVerbano zone, Italy and in Dharwar craton, south India (Harlov, 2012).




Tectonic setting of charnockites of Achankovil shear zone Achankovil shear zone encloses rocks of amphibolitegranulite facies. In southern India the gneissgranulite transformation are reported (Yoshida et al., 1991; Tomson et al., 2006, 2013; Manimaran 2014). In the field khondalites (pelitic gneisses), quartzites, calcsilicate rocks, basic granulites, charnockites, granites, hornblendites

Figure 1. Study area – Achankovil Shear Zone

and pyroxenites are exposed. The area was deformed with a NWSE dextral Tenmalai shearing (TS) of D2 of ASZ at ca.670570 Ma was followed by a WNWESE sinistral Achankovil shearing (AS) of D3 of ASZ at ca.560500 Ma.

Two periods of incipient and massive charnockitisation of pelitic gneisses were delineated from associated shear setting of NWSE striking dextral TenmalaiGatana shearing of D2 (ca. 670 Ma) and WNWESE sinistral Achankovil Tambraparni shearing of D3 (ca. 550 Ma) from Ambasamudram Tenkasi region of Tamil Nadu (Manimaran and Roy Chacko, 1996) (Fig.1, 2, 3). CO2 infiltrations from deep seated source are highlighted for both incipient and massive charnockitisation of pelitic gneisses (Manimaran et al., 2014).

Ctype magmaCharnockite of igneous origin and massive bands occur at Ambasamudram and Kadayam area of ASZ. A linear, mega lineament, massive belt of charnockite from running from Ambasamudram to Ravanasamudram and beyond is a Ctype charnockite (Figure 4). It is green glistening charnockite. It is acidic, high temperature antiperthitic charnockite. Euhedral zircon and monazite bearing



Figure 3. Patchy incipient charnockites found in garnetiferous biotite gneiss, in Kallikulam near Valliyur.


Figure 2. Incipient charnockite formation along conjugate shear fractures in the host of grey granites of Achankovil shear zone southern boundary. Location: Malaiyadipudur, near Kalakkad, Tamil Nadu, India


charnockite. Disseminated, assimilated cordierite and garnets are observed in the thin section of the Ambasamudram charnockite and also in field exposures. The above features suggest that the charnockites are of igneous origin which are intruded the cordierite bearing khondalites and leptinites of the ASZ.

Figure 4. Massive charnockite exposure at Ambasamudram. The intruded grey granite is also seen.

Folded charnockiteFolded igneous charnockite is exposed at Kalyaniamman kovil near Kadayam. It is an isoclinal F2 fold of massive charnockite dipping steeply towards NE. It is also cordierite bearing and antiperthitic charnockite of high temperature (Figure 5). At Tenkasi, Kuttalam and Valliyur area both massive and incipient charnockites are observed in the field. The above charnockitisation process are related to Achankovil sinistral shearing episodes due to CO2 related dehydration of earlier gneisses and granites of the area due to carbonic metamorphism (Figure 6).

Incipient charnockitesIncipient charnockites associated with shear planes of NWSE dextral shears of Tenmalai Gatana shear (figure 7) and associated with NWSE sinistral shears of Achankovil Tambraparani shear are studied at a quarry near IIPE School of palayamkotti from Achankovil shear zone. The decimeter to 6 meter scale incipient



charnockite suggest that they are formed due to CO2infiltration during Tenmalai Gatana shearing D2 and Achankovil Tambraparani shearing of D3.

Figure 5. Folded igneous charnockite at Kadayam


Figure 6. Incipient charnockite folded isoclinally along N70°W 60° SW in pelitic gneiss, formed during D3. Location –

Samiyarpothai, near Valliyur.


Figure 7. Incipient charnockite formation along NWSE dextral shearing of D2 near IIPE School, Tirunelveli

K2O metasomatic charnockitesIn the same exposure of special kind of incipient charnockite formation formed in granite due to invasion of pink orthoclase feldspars deposited by metasomatic fluids as suggested by Friend 1981 and Janardhan et al., 1982 from Kabbaldurga quarry of Bangalore. The above features of the quarry pointed out that the thermal impetus were gained from high temperature K2O metasomatic fluids as well as CO2 infiltration from the deep seated source mantle (figure 8).

Decompression charnockitesA special kind decompression charnockite of incipient nature fromed near the pink granite and grey granite small bands are also seen in ASZ. i.e. at Tenkasi, Ambasamudram Sivasailam areas. The thermal impetus for the following reaction might be evolved from granite intrusions (figure 9).Garnet + calcic plagioclase = Hypersthene + quartz ± sodic plagioclase (Gneiss) (Charnockite) Hence four different kinds of episodic charnockites are formed during the two periods of Tenmalai Gatana shearing of D2 deformation and Achankovil Tambraparani shearing of D3 deformation.



Figure 8. Incipient charnockitisation of granites formed due to sinistral Achankovil shears at Paraikulam exposure near IIPE School, Tirunelveli

Figure 9. Decompression incipient charnockitisation near granite dyke near Sivasailam, Tirunelveli



Shear localized charnockitizationVeins and bands of incipient charnockites evolved from dehydration of pelitic gneisses due to CO2 infiltrations from deep crustal sources were formed and associated with NWSE dextral principal shears of D2 and WNWESE sinistral principal shears of D3.

Figure 10. Dextrally sheared pelitic gneiss with veins and bands of incipient charnockite developed along and across the foliation. Location – near INS, Vijayanagaram

At places charnockite magma evolved from melting of underplating of slab of Madurai block under Kerala Khondalite Block. CO2 liberated due to decarbonation reaction of subducted calcsilicate sediments dehydrated gneisses into incipient charnockite bands and resulted in charnockitic and gneissic mixed migmatitic terrain of ASZ and KKB. In the field the igneous intermediate charnockites (Ambasamudram, Kadaiyam and Thalaiyuthu charnockites) and incipient charnockites formed during D2

dextral shearing are always associated with NWSE dextral shears (figure 10) and F2

upright/isoclinal folds with NWSE axial planes.ASZ and KKB were subjected to sinistral shearing and pink granites,

pyroxenites, hornblendites are vertically intruded into ASZ and KKB along WNWESE to NWSE sinistral shears. The second stage of charnockitic magma intrusion and incipient charnockitisation of pelitic gneisses of ASZ and KKB are due to CO2

infiltration during sinistral shearing of D3. Igneous charnockite and incipient charnockites pink granites, pyroxenites and hornblendites are associated with WNWESE to NWSE sinistral shears are observed in the field and are usually associated with F3 coaxial/isoclinal folds with NWSE to WNWESE axial planes.



NWSE dextrally and sinistrally sheared incipient charnockites, mixed migmatic terrain of charnockite and pelitic gneisses are seen near to the surface. Incipient charnockite of D2 again displaced by WNWESE sinistral Achankovil shearing observed in Nanguneri (Figure 11).

Sinistrally sheared pink granites (ca. 550 Ma) and incipient charnockites (ca. 550 Ma) are associated with sinistral Achankovil shear zone. CO2 dehydrated gneisses evolved as a common incipient charnockites of D2 (dextrally affected) and of D3

(sinistrally affected) are observed.

ConclusionA preferential pervasive CO2 vapour phase from the mantle dehydrated the precursor gneisses into charnockite. Later intrusion of granites and their complementary pegmatites in both the gneisses and charnockite were brought about regressive changes. The charnockitisation process of Achankovil shear zone involves CO2

infillteration from deep crustal source, crustal melting, and igneous intrusive chanockites also. The incipient charnockites are evolved during the D2 Tenmalai shearing and D3 Achankovil shearing. The intrusive charnockite magmatic episodes were coinciding with F2 of D2 and F3 of D3 folding of the area. A multi episodic charnockitisations were observed in ASZ.

Figure 11. Incipient charnockite formed in pelitic gneiss along and across the shear planes, possibly due to CO2 infiltration from lower crust. Location: south of Nanguneri. Achankovil shear zone, southern India. The incipient charnockite and pelitic gneiss are sinistrally displaced



AcknowledgementsThe first author thanks Department of Science and Technology, Government of India, New Delhi for DST Project (No.SR/S4/ES498/2010). The second author thanks the Department of Science and Technology, Government of India, New Delhi for awarding DST INSPIRE Fellowship (IF1302332013). The authors are also grateful to Shri A.P.C.V.Chockalingam, Secretary and Dr.C.Veerabahu, Principal, V.O.C College, Tuticorin for their encouragements.

References

Clemens, J.D. (1990) The granulitegranite connexion. In: Granulites and Crustal Evolution. Vieleuf, D. and Vidal, Ph., (Eds.) Kluwer Academic Publishers, Dordrecht., pp. 2536.

Friend, C.R.L. (1981) The timing of charnockite and granite formation in relation to influx of CO2 at Kabbadurga, Karnataka, South India. Nature, V. 294, pp. 550552.Fyfe, W.S. (1973) The granulite facies, partial melting and the Archaean crust. Transactions of the

Royal Society of London., A273, pp. 457461.Harlov, D.E. (2012) The Potential role of fluids during regional granulitefacies dehydration in the

lower crust. Geoscience Frontiers., v. 3(6), pp. 813827.Janardhan A.S., Newton R.C. and Smith J.V. (1979) Ancient crustal metamorphism at low P (H2O)

charnockite formation at Kabbaldurga, South India. Nature V. 278.pp 511514. Janardhan, A.S., Newton, R.C. and Hansen, E.C. (1982) The transformation of amphibolite facies

gneiss to charnockite in southern Karnataka and northern Tamil Nadu, India. Contributions to Mineralogy and Petrology., v. 79, pp. 130149.

Lamb, W. and Valley, J.W. (1984) Metamorphism of reduced granulites in low CO2 vaporfree environment. Nature., v. 312, pp. 5658.

Manimaran G. and Roy Chacko P.T., (1996) Shear lineaments and tectonic setting of massive and incipient charnockites of Tambraparani shear zone, southern India., International symposium on charnockite and granulite facies rocks, Aug 1996, held at Uni. Of Madras, Madras, India, edited by Ram Mohan V., Abstract, pp. 1213.

Manimaran, G. (2014) Finite Strain Patterns and Transpression Regime of Achankovil Shear Zone, South India and Their Implications in Gondwana Reconstructions. Frontiers in Geosciences., v. 2, pp. 2329.

Manimaran, G., Besheliya, J., Manimaran, D., Antony Ravindran, A., Selvam, S. and Sugan, M. (2014) Multiphase shear features of PanAfrican shearing events of Achankovil shear Zone. Outreach., v. 7, pp. 128135.

Newton, R.C. (1992) Charnockite alteration: evidence for CO2 infiltration in granulite facies metamorphism. J. Metamorphic Geology., v. 10, pp. 382400.

Perchuk, L.L. and Gerya, T.V. (1992) The fluid regime of metamorphism and the charnockite reaction in granulites: a review. International Geology Review., v. 34, pp. 158.

Powell, R. (1983) Fluids and melting under upper amphibolite to facies condition. Jou. Geo. Soci. London 140. 629633.

Santosh, M. (1986) Carbonic metamorphism of charnockites in the south western Indian sheild: a fluid inclusion study. Lithos., v. 19, pp. 110.

Thompson, A.B. (1990) Heat, fluids and melting in the granulite facies. In: Vielzeuf, D., Vidal, Ph., (Eds) Granulites and crustal evolution. Klumer, Dordrecht, NATO ASI Series C., pp. 3758.

Tomson, J.K., Bhaskar Rao, Y.J., Vijaya Kumar, T. and Mallikharjuna Rao, J. (2006) Charnockite genesis across the ArchaeanProterozoic terrane boundary in the South Indian Granulite Terrain: constraiints from majortrace element geochemistry and SrN d isotopic systematic. Gondwana Res., v. 10, pp.115127.



Tomson, J.K., Bhaskar Rao, Y.J., Vijaya Kumar, T. and Choudhary, A.K. (2013) Geochemistry and neodymium model ages of Precambrian charnockites, Southern Granulite, India: Constraints on terrain assembly. Precambrian Research, V 227, pp 295315.

Touret, J. and Huizenga, J.M. (2012) Charnockite microstructures from magmatic to metamorphic. Geoscience frontiers., v. 3, pp. 745753.

Waters, D.J. (1988) Partial melting and the formation of granulite facies assemblages in Namaqualand, South Africa. J. Meta. Geo. 6, 387404.

Yoshida, M., Santosh, M. and Shirahata, H. (1991) Geochemistry of GneissGranulite transformation in the “incipient charnockite” zones of southern India. Mineralogy and Petrology., 45(1) 6983.


VJ Rajesh and K Sajeev Terrestrial analogues of lunar anorthosites

Terrestrial anorthosites as probable lunar analogues: a study on anorthosites from various shear/suture zones in south India

V. J. Rajesh1* and K. Sajeev2

1Department of Earth and Space Sciences, Indian Institute of Space Science and Technology, Thiruvananthapuram 695 547, Kerala2Centre for Earth Sciences, Indian Institute of Science, Bengaluru 560 012, Karnataka

*Correspondence Email: [email protected], [email protected]

AbstractPlanetary analogue research is an important topic in planetary geoscience both in terms of planetary origin and evolution and in the preparation and result interpretation of robotic exploration to support future manned missions. The analogue sites on Earth provide us with resources and data that can be used as ground truth for satellites and other training purposes. To understand the processes on or the evolution of one solar system object, frequently the Earth's surface and objects (like rocks and minerals) are to be compared and studied. Several analogue sites have been established on the Earth surface which gives insight into the mineralogy and mineral chemistry of the lunar surfaces. India has certain landforms having resemblances with lunar landforms and is being studied by scientists and researchers, in order to get a better understanding of lunar geological process. Further, through this studies we can propose a a probable test bed site for ISRO’s Moon mission so that variety of planetary investigations can be conducted.

The highland (terrae) region of our earth’s moon is dominated by a light coloured rock, rich in anorthite feldspar, known as anorthosite. The anorthosite on moon is interpreted by many geoscientists as probable remnants of a floating crust from a basic magma ocean. Evidence suggests that the early Moon was covered by a magma ocean which differentiated as it crystallized, forming a plagioclase flotation crust and a cumulate pile of denser mafic minerals.Subsequent bombardment of the lunar surface has disrupted the original flotation crust, and most of the remnants have been obscured by more mafic deposits, but the distribution of the outcrops of pure anorthosite that have been identified holds important implications for the evolution of the lunar crust. The lunar anorthosites, at places, are closely associated with many magnesium rich (hereafter Mgrich) rocks such as dunite, gabbro, norite and troctolite. The rock association indicates deeper mantle source regions for the formation of lunar anorthosites, probably as fractionated products. However, the relationship of lunar anorthosites with Mgrich rocks and their genetic mechanisms are not well understood due to the complexity in accessing the insitu samples.




VJ Rajesh and K Sajeev Terrestrial analogues of lunar anorthosites

The early Archean crust of the terrestrial planet Earth (after consolidation of the magma ocean in the Hadean times) was dominated by anorthosite, similar to the dominant anorthositic terrae on the moon, though the volume has been significantly reduced with the course of geological time. Therefore, the petrologic studies on anorthosites and related rocks in our Earth will provide vital clues to have a better understanding on the chemical nature and evolution of the lunar mantle and crust. Therefore, the petrologic studies on anorthosites and related rocks in the earth will provide vital clues to have a better understanding on the chemical nature and evolution of the lunar mantle and crust. The origin and petrogenesis of such rocks related to the plagioclase buoyancy and flotation accumulation was well studied worldwide in versions tectonic environments. As anorthosites are considered to be the first terrestrial rock with marked similarity to that of lunar rocks, it is important to study these rocks with a multidisciplinary approach using geochemical and hyperspectral characteristics. Such detailed research on anorthosites and associated rocks are rare on a southern Indian perspective. Southern India has several different occurrences of anorthosites, mainly in the states of Andhra Pradesh, Odisha, Tamil Nadu and Karnataka. Many places anorthosites occur as a layered anorthosite complex in close contact with deep rooted Mgrich rocks such as dunite, websterite and pyroxenite with gabbro and norite. The anorthosites here are characterized by high concentration of anorthite and pyroxene with minor hydrous minerals like amphiboles. We will discuss several geological aspects and hyperspectral features of anorthosites exposed in South India and check for its appropriateness as probable lunar analogues.


MP Muraleedharan Lineaments as holistic windows for tectonic events

Lineaments in Kerala—a holistic window for tectonic events in the Indian peninsula

Muraleedharan M.P.

Dty. Director General (Retd), Geological Survey of India Email: [email protected]

AbstractThe Kerala region in union with its peripheral zones has to be reckoned as an important segment to understand the evolution of the lithology and structure of the whole gamut of crystalline rocks that constitute the basement of the Peninsular Shield. The tectonic events in the Indian Peninsular Shield have left their imprints to their basic grains in the green schist pyroxenegranulitecharnockiteacid and basic intrusive assemblages of Kerala. These assemblages spanning from the Precambrian to Eocene serve as an interesting key that can unfold event after event of magmatism, metamorphism and tectonism involved in the transformation of a simatic protocrust into an ensialic supracrust which is the basic skeletal framework of lithology in the region. The regional, macro, meso and micro structural evidences discernible all through the basement rocks of the region help in deciphering the events with chronologic precision.

IntroductionThe primary scenario is that of a primordial crust with komatiitic affiliations, which on subduction to great depths of high temperature and pressure gave rise to granulite facies of rocks on recrystallization. The rocks which immediately succeeded this primordial crust were sedimentogenic, which continued evolving over a considerable period of the Archaean Era, and which, together with the primordial crust underwent high grade metamorphism in their deeper depths and hightemperature—low pressure metamorphism in their shallower zones. The process just mentioned account for the rocks designated under the Wayanad Schist complex, the pyroxene granulites, Khodalitic suite of rocks and Peninsular Gneissic Complex. These rock assemblages apparently underwent compressional forces from the NNE and SSW directions which folded them into folds having near East West axes. Migmatization remodeled the lithology of these assemblages in a syntectonic environment during the Proterozoic Era which as its offshoots gave rise to charnockitelooking rocks in the cases where pyroxene granulites were the source rocks and leptynitic rocks in the cases when khondalites were the source rocks for migmatization. The final exodus of mantle material is related to the Deccan Trap activity.

The skeletal frameworkFour major F2 axial conduits define the Archaean crystalline skeletal framework of the entire Peninsular India (Fig 1). From north to south they are,




1. The Aravalli axial conduit with sinistral closure and dextral opening2. The Mysore – Lucknow axial conduit with dextral closure and sinistral opening3. The Ponnani – Singhbhum axial conduit (widely known as the Palghat suture,

saddle, shear zone etc) with sinistral closure and dextral opening and4. The Kodaikkanal – Tuticorin axial conduit with dextral closure and sinistral

opening.The openings of these conduits contain a host of acid plutons, alkaline rocks, ring complexes and sediment basins. The continuous movement of the Indian Plate towards north since the Jurassic times has rendered the closures of these folds more and more appressed and the development of the F3 fold system is contemporaneous with this. Rifting have taken place parallel to the axial system of these F3 folds (roughly NS) and repeated tectonic activity has been experienced along the EW Proterozoic faults which have been sealed by Paleozoic acid intrusives. Measurable seismicity along these zones bear testimony to the view that neotectonic activity along these linears is a consequence of the gradual abutting of the Indian Plate at the margin of the Tibetan Plate.

Chronology of eventsRadiometric age data in the Kerala region gives clear indication of metamorphic events around 2700 Ma and 2500 Ma marking Archean events, 1100 Ma marking the Proterozoic event and ages of acid intrusives ranging from 700 ma to 500 Ma, the latter corresponding to the PanAfrican event. Forceful exodus of mantle material culminating into forceful emplacements of granitic differentiates took place during the late Proterozoic to early and middle Paleozoic times. These obviously postdated the second phase of regional folding and orogeny which left deep structural conduits conducive for emplacement of large plutons, massifs and ring complexes. The second folding episode was coaxial to the first, with more of a plastic flow nature, controlled by compressional and dialational forces. The third episode of folding which is along a NNWSSE to NS axis in the southern portion of the Peninsula shield appears directly related to the breaking up of Gondwana land and the separation of the Indian shield from the Antarctican shield and the commencement of its northward journey some 200 Ma years ago and the formation of the first oceanic crust around the spreading centres. A fourth folding episode with a NWSE axis under the influence of active compressional forces from NE and SW is the latest imprint of the rocks of the Peninsula. The subsequent rifting, basic volcanism of the vent type and extrusion of differentiates of basaltic magma during the Cretaceous mark the second major exodus of mantle material since the early Paleozoic. The early Paleozoic era had witnessed the culmination of the first major exodus of mantle material since the formation, rearrangement and modification of the primordial simatic crust.

The lineament windowStudies of satellite imagery of Kerala region validated with field checks over the years have brought to fore, linears/ lineaments which could be categorized as follows: (Fig. 2).



(i) Tectonic grain linears equivalent to (a) L0 lineament= F1 =F2 (Coaxial), fold axes equivalent to fold axis shear.(ii) L1 lineament = F2 fold conduits = zones of Proterozoic/ PanAfrican Migmatite and basic/ acid intrusive activity.(iii) Fault lineaments L2 = Locales of massive acid intrusive activity of PanAfrican ( 700450 Ma) Age/ Zones of neotectonic activity.(iv) L3 lineaments = F3 Fold axes= coastal and offshore rifting= Baic igneous intrusives of Cretaceous age.(v) L4 lineaments = Rift linears = F1= F2 = F3 fold axes= Regional tectonic grain in the west coast= zones of basic intrusive activity of upper Cretaceous age and later.The linears/ lineaments recognized in Kerala (NomenclatureLocality A to Locality B in the case of less known lineaments) are listed below, from north to south of the State: L0= L1 1. Kasaragod—Mercara

2. The Bavali lineament3. The Kabani Lineament4. The Nilambur Lineament5. The Bhavani Lineament6. Malappuram—Walayar (The Palghat Gap lineament)7. Shoranur—Anaimalai8. Chalakkudi—Munnar9. Achankovil—Tambraparni10. Ponmudi—Nanguneri

L2 1. Pallikkere—Sulya 2. Bekal—Bandadka 3. Ezhimala—Virarajendrapet 4. Kannur—Nanjangod 5. Koilandi—Vythiri (The Moyar shear) 6. Ponnani—Coimbatore (The Palakkad Gap) 7. Malayattur—Periyar 8. Kochi—Devikolam 9. Karumalai—Mattuppatti10. Mundakkayam—Upper Periyar11. Thanikkudi—Kambam12. Alappuzha—Periyar13. Kollam—Kadayamala14. Kovalam—Nanguneri

L3 lineaments 1. Mangalore—Bekal 2. Bekal—Pallikkere Coastal



3. Pallikkere—Ezhimala 4. Payyannur—Ottappalam—Edamalayar (The vent of Edamalayar dyke) 5. EzhimalaKannur L3 = L1 = L0

6. Kannur—Koilandi 7. Koilandi—Ponnani

8. Kilur—Monnangeri 9. Nattavaram—Kayanna—Balusseri

10. Kozhikode—Vembanad 11. Ponnani—Kochi

12. Alathur—Muvattupuzha (Gabbro intrusion of first generation) 13. Periyar Kambam 14. Kochi—Alappuzha L3 = L1

15. Alappuzha—Kollam 16. Kollam—Kovalam L3 = L1

17. Kovalam—KanyakumariL4 lineaments (manifested as rift systems along which dolerite dykes of latest generation have been emplaced)1. The Nadapuram system2. The Nanminda System3. The Kunnamangalam System4. The Attappadi System5. The Kothamangalam System6. The Perumbavoor System7. The Koothattukulam System8. The Thodupuzha System9. The Ettumanoor System 10. The Vadasserikkara system

DiscussionThe discovery of Komatiites by Viljoen and Viljoen (1969) in greenstone belts (Barberton greenstone Belt) that are 3000 4000 Ma old has led to the realization that the pyroxene granulite – norite association of rocks occurring in the shield areas have komatiitic chemistry and thereby represent uncontaminated primordial flows. Viswanathan and Sankaran (1973) have compared the evolution of the Archaean granulite terrains with the greenstone belts and treat the former as Archaean representatives of the greenstone belts. Pyroxene granulites have comparable chemistry with komatiites in terms of Ca/Al ratio which in the latter ought to be between 14 and 34.In the pyroxene granulites of the region, the said ratio ranges from 2 to 4.6 (Sukumaran, 1978). The green schists of the Wayanad Schist Complex have to be considered remnants that have escaped subduction and resultant high grade metamorphism. Archaean migmatization needs to be viewed as a preamble to



Fig. 1 Schematic Diagram Showing F2 Axial Conduits in the Crystalline Framework of the Indian Peninsular Shield.

the large exodus of later acid magmatism which together gave rise to the vast cover of the Peninsular Gneissic Complex (PGC) in the shield area. The intermediate and acidic charnockite are the resultants of these acidic activity in Proterozoic over the preexisting pyroxene granulites. During Archaean period when the sialic crust was under a process of evolution, the simatic substratum underwent partial melting giving rise to basaltic magmas some of which might have differentiated at depth and some erupted to the surface. The relatively quiescent periods might have been dominated by sedimentation leading to the formation of Khondalites which again got subducted due to orogenic movements and got metamorphosed. In the subsequent stage, orogeny and regional metamorphism of an anhydrous nature gave rise to the granulites. Oxygen isotope studies by various workers have indicated the original basic (basaltic) nature of pyroxene granulites.

The third stage witnessed the profuse migmatization of the green schists, khondalites, granulites and massive exodus of acidic magma, giving rise to Peninsular gneisses, leptynites and charnockites respectively. The next stage in the evolution of the crystallines is apparently marked by a period of retrogression and hydrous metamorphism resulting in the superimposition of the amphibolite facies.



Fig. 2 Lineament map of Kerala.



Conversion of hypersthene into hornblende and almandine garnet bears testimony to this. However, there are instances of progradation resulting in the formation of incipient charnockites at a much later stage of geological history. The formation of orthopyroxenes is selective, tectonically controlled, is of meso scale and are the resultants of differential CO2 metamorphism unaccompanied by subduction of the litho types during the PanAfrican event. The Leptynites and Peninsular gneisses were rendered more acidic with the influx of quartzofelspathic material at late Proterozoic to early Paleozoic times. The hydrous metamorphic events which predominated the Proterozoic times culminated into forceful intrusions of granitic plutons and associated rocks. The second exodus of mantle material is dated between ages of 125 Ma to 75 Ma related to the Deccan Trap basalts. The available age data for the basic dykes of south Kerala suggest that the emplacements have a protracted history. These can be considered as deeper level penecontemporaneous emplacements related to the culmination of the Deccan Trap volcanism that had numerous active effucive centres at higher levels elsewhere till the dawn of Cenozoic (Muraleedharan 2006). To sum up, it is not difficult to see that an understanding of the tectonics and metamorphism in the crystallines of the Kerala region along with the structural imprints that are verifiable in the field serves as a key for unfolding the intricacies and complexities of contemporaneous lithology all through the Peninsular Shield. Its evolutionary history from a primordial simatic protocrust to an ensialic supracrust through distinct events of transformation in space and time is available to any holistic viewer.

ReferencesMahadevan, T.M (1994): Deep Continental structure of India A Review. Geol. Soc. Ind. Mem. No.28,

pp. 24182Muraleedharan M.P. (2006): Dykes of the west coast protracted history of emplacement related to

Deccan VolcanismExtended abstracts, DST Group Discussions on Deccan Volcanism, GSI TI.Nair M.M and Subramanian K.S (1989): Transform faults of the Carlsberg Ridge—their implication in

neotectonic activity along the Kerala coast. Geol Surv. Ind. Spl. Pub no. 24, pp 327—332Nambiar, C.G (1996): Recrystallization reactions inferred from Petrography of charnockites from

northern Kerala, india. Int. sym on charnockite and granulite facies rocks, Madras. Abstt. Pp 43—44.Naqvi, S.M (1974): The Protocontinental growth of the Indian shield and the antiquity of its rift

valleys Precambrian Research v. 1, pp.345—389.Pichamuthu, C.S (1954): The charnockites of South India. Proc. Pan Indian Ocean Sci. cong., Perth,

Sec. CC pp 5051.Rao, P.S (1978): Some aspects of Structure and Tectonics of Kerala region, India and related

mineralization. Geol. Surv. Ind. Misc. Pub. 34, pp 5166.Subramanian K. S and Muraleedharan M.P (1985): Origin of the Palghat Gap in South India—A

Synthesis. Jour. Geol. Soc. Ind Vol. 26 pp 28—37.Sukumaran P.V (1978): Origin and evolution of the charnockite pyroxene granulite group of rocks of

the shield areas—(Personal communication notes).Viljoen, M.J and Viljoen, R.P (1969) : the Geology and geochemistry of the lower ultramafic unit of the

Overwacht Group and a proposed new class of igneous rocks, Spl. Publ. geol. Soc. S. Afr. V.2, p.85.Viswanathan, S and Sankaran, A.V (1973) : Discovery of Komatiite in the Precambrian of india and its

significance in the nature of Archaean Volcanism and of the early crust in the Indian Shield, Curr. Sci. V 42, pp 266269.


C IshwarKumar et al. Spinel chemistry of ultramafic complexes of W India and E Madagascar

Petrogenesis and tectonic setting of ultramafic complexes in western India and

eastern Madagascar: Inferences from chromian spinel chemistry

C. IshwarKumar a, V.J. Rajesh b, B.F. Windley c, T. Razakamanana d, T. Itaya e, E.V.S.S.K. Babu f, K. Sajeev a*

a Centre for Earth Sciences, Indian Institute of Science, Bangalore 560012, Indiab Department of Earth and Space Sciences, Indian Institute of Space Science and Technology,

Thiruvananthapuram 695547, Indiac Department of Geology, The University of Leicester, Leicester LE1 7RH, UK

d Départment de Sciences Naturelles, Université de Toliara, BP.185, Toliara 601, Madagascare Research Institute of Natural Sciences, Okayama University of Science, 11 Ridaicho, Kitaku,

Okayama 7000005, Japanf National Geophysical Research Institute, Hyderabad 500007, India


India and Madagascar occupied the central position in the eastern Gondwana assembly, and the correlation between them is important to understand the tectonics of eastern Gondwana. The tectonic linkage between India and Madagascar is one of the most debated problems in the field of paleogeography of eastern Gondwana. Palaeomagnetic studies reveal that Madagascar and India were contiguous prior to the breakup of eastern Gondwana. Before the breakup of Gondwana supercontinent, India and Madagascar had several continuous deep crustal shear zones. These shear/ suture zones are used as one of the major criteria for the continental correlation. Based on structural, geological, geochronological and geophysical data, it has been proposed that the Kumta suture is an extension of the Betsimisaraka suture. The Bondla complex is located northwestern part of the Kumta suture zone of western India, and the Ranomena complex is located within the Betsimisaraka suture zone of eastern Madagascar. The Bondla complex is situated within metagreywacke/quartzchloritebiotite schist surrounded by quartzchloritebiotite schist and chlorite phyllite interbedded with iron formation and manganiferous chert. The Bondla complex comprises of serpentinised wehrlite, dunite, pyroxenite, chromitite and layered gabbro. The Ranomena ultramafic complex is situated within garnetsillimanite and other metasedimentary gneisses and consists of harzburgite, orthopyroxenite, clinopyroxenite, chromititelayered peridotite, two pyroxenehornblende gabbro. Both the ultramafic complexes contain rocks such as chromitite and serpentinite bearing chromian spinels. Chromian spinel is one of the first minerals to crystallize from a maficultramafic magma, and it is highly resistant to weathering, alteration and



C IshwarKumar et al. Spinel chemistry of ultramafic complexes of W India and E Madagascar

metamorphism. It is a sensitive indicator of primary magma/melt composition that has been broadly used to understand the petrogenesis of its host rocks. In the present study chromian spinel from ultramafic complexes in India and Madagascar are correlated. The chemistry of Chromian spinel in chromitite and serpentinite from the Bondla ultramaficgabbro complex in western India, and in chromitite from the Ranomena ultramafic complex from eastern Madagascar are studied to understand the petrogenesis of ultramafic complexes and paleotectonic relationships. The Chromian spinel of chromitite from the Bondla complex has Cr# [Cr/ (Cr+Al)] that ranges from 0.54 to 0.58 and Mg# [Mg/ (Mg+Fe)] that ranges from 0.56 to 0.67. The Cr# of Chromian spinel of serpentinite from Bondla complex varies from 0.56 to 0.64 and Mg# from 0.41 to 0.63, whereas in the Chromian spinel in Ranomena complex chromitites Cr# varies from 0.59 to 0.69 and Mg# (XMg) from 0.37 to 0.44. Chromian spinels in Bondla complex serpentinites have strong chemical zoning with distinctive ferrian chromite rims (Mg# [Mg/ (Mg+Fe)] = 0.41 to 0.63); the spinelcore crystallization temperature was estimated to be above 600 C (the spinel stability field was calculated for equilibrium with Fo90 olivine), which suggests the core composition is chemically unaltered. The Chromian spinels in Bondla and Ranomena complex chromitites are generally homogeneous with occasional weak zoning. The Chromian spinel grains in all the studied samples have lowTiO2 (00.6 wt.%), lowAl2O3 (1523 wt.%) contents, and moderate to high#Cr values (0.540.69), suggesting derivation from a suprasubduction zone setting. The parental melt composition calculation using unaltered Chromian spinel core chemistry suggest that the parental magma compositions of the Bondla and Ranomena complexes were similar to that of primitive tholeiitic basalts formed by highdegrees of mantle melting. Clinopyroxene from the Bondla complex serpentinite has XMg= 0.870.90 and orthopyroxene from Ranomena complex chromitite has XMg= 0.900.91. The chromian spinel chemistry in chromitite and serpentinite from the Bondla complex, and in chromitite from the Ranomena complex indicates that both complexes formed in a suprasubduction zone arc tectonic setting.


P Pratheesh and C Vikas Magma emplacement in regional dyke swarms

Significance of magma emplacement in the regional dyke swarms: a case study from

MoyarBhavani Shear Zone

P. Pratheesh1 and C. Vikas2

1 Centre for GeoInformation Science and Technology, University of Kerala, Kariavattom Campus, Thiruvananthapuram695 581, India

2 Forward Base, ONGC, Karaikal, Pondicherry609604, IndiaEmail: [email protected]

AbstractGeometric interpretations from the morphological parameters of mafic dyke swarms are attempted in this paper. Study focuses mainly on the MoyarBhavani Shear Zone (MBSZ) area of south Indian Granulite Terrain (SIGT), where three distinct groups of dykes with contrasting characteristics are observed. Study of minimum principal compressive stress ( 3) in terms of dyke trend and joint patterns indicate multiple tectonic imprints in theσ MBSZ area. Source depth calculation using aspect ratio and magmatic overpressure were also attempted. The calculated source depth indicates shallow magma chamber characteristics for MBSZ dykes. Estimated crustal dilation from the aggregate is also minimal in MBSZ dyke swarm.

Key words: Mafic dykes, principal compressive stress, Aspect ratio, Crustal dilation

IntroductionMafic dyke swarms are groups of vertical dykes with same orientation representing a system of preexisting tensional crustal fracture swarms along which mafic magmas got emplaced (Halls and Fahrig, 1987; Ernst et al., 1995; Hou et al., 2006). These dyke swarms represent conspicuous extensional structures and are widespread in cratons through out the world, especially in the Archaean shields such as the Canadian shield, the north China craton and the south Indian shield (Halls and Fahrig, 1987). Most of the giant mafic dyke swarms were developed in Proterozoic time but younger swarms are also reported. Each of the mafic dyke swarm is related to the local stress field and are excellent time marker and paleostress indicator and can be used to reconstruct the paleostress fields of cratons. In general, dyke swarms exhibit trends parallel to the contemporaneous regional horizontal maximum compressive stress orientations and in turn are perpendicular to the extension direction (Pollard, 1987).




Mafic dyke swarms in the MBSZ areaThe MoyarBhavani shear zone (MBSZ), hosts two major dyke swarms with contrasting petrogenetic characteristics by way of the mode of occurrence and composition. Among these, one is in the Bhavani shear zone (BSZ) of the Attappady valley and the other belongs to the Moyar shear zone (MSZ) in the Kannur district, Kerala (Fig. 1). The BSZ swarm shows significant variation in the physical characteristics of the individual dykes, including mode of occurrence, structure, alteration and deformation. These swarms show highly deformed Proterozoic dykes to younger ones that are fresh in nature. Based on the mode of emplacement, these dykes show various magmatic episodes through out the geological time. Since it belongs to the shear zone area, high strain fabrics are good indicators of temporal variation, especially in the absence of precise isotope age data. Major tectonometamorphic imprints available in the granulitic country rocks of MBSZ area belong to the PanAfrican orogenic period, which is of ~550700 Ma age (Ghosh et. al., 2004).

Fig. 1 Map of the MBSZ area showing the intensity of dyke swarms.

Based on these deformation fabrics, the dykes of BSZ swarm are classified in to two categories which are pretectonic and posttectonic dykes. The pretectonic dykes which witnessed the major reactivation phases and bearing deformational fabrics are considered as Group I dykes (Fig 2.a). The posttectonic dykes which are relatively younger and devoid of any kind of foliation are the Group II dykes (Fig.2.b). Similarly the MSZ swarms which show very fresh dykes with one ore two sets of emplacement directions are categorized as Group III dykes Fig. 2.c). These three groups show significant variations in their mode of emplacement, physical properties and deformation patterns and hence the classification has been followed while carrying out the different types of analysis for the present study.

Present studyDykes are wall like masses emplaced in to the preexisting rocks and hence are significant in the analysis of regional structural framework. The mode of emplacement and occurrence of the dykes vary with respect to the prevailing tectonic regime during the emplacement. To delineate the emplacement framework and the



structural control, various parameters were collected from the field in a detailed study. MBSZ dykes show significant variation in their trend, thickness, lateral extension and development of joints. Therefore detailed examinations of these morphological parameters are attempted to bring out the nature of emplacement.

Fig. 2 Field occurrence (a Group I,b Group II, c Group III) and photomicrograph (d Group I,e Group II, f Group III) of the dykes from MBSZ area.

Result and discussion Petrography : Detailed study of thin sections from the three classified groups under plane polarized light provided useful clues regarding the degree of alteration and the mineral assemblages. Group I mainly consists of amphibolite, metagabbro and metadolerite in megascopic composition. Metamorphism and deformation have erased many of the primary features, especially the cooling textures and grain contacts (Fig. 2.d). Group II dykes are mainly younger dolerite or basalts showing cooling textures with well developed grain boundaries, the dolerite shows a typical (sub)ophitic texture, made up of elongated feldspar laths and anhedral pyroxene and olivine (Fig. 2.e). Group III dykes are typical doleritic dykes, contain clinopyroxene + plagioclase ± olivine ± orthopyroxene ± spinel assemblage with typical diabasic textures (Fig. 2.f). In certen samples plagioclase shows zoning, which is an indication of in situ crystallization after emplacement of the magma or sudden pressure decrease in the magmatic system



Trend, Joint pattern and Palaeostress: Mafic dykes in the MBSZ area show distinct trends with in each studied groups. The trend of the dykes belonging to the three groups are presented separately in rose diagrams (Fig. 3). The diagram indicates that majority of the dykes of Group I trend ENEWSW with two distinct maxima of N50ES50W and N70ES70W. However the Group II and III dykes have wide variation in trends. The Group II dykes though exhibits dominant trends of N40ES40W, N65WS65E and N50WS50E, there are other significant trends also. The Group III has a prominent NWSE trend and a number of subsidiary trends.

Fig. 3 Rose diagram of dyke trends in MBSZ area (a) Group I dykes (b) Group II dykes (c) Group III dykes.

Undeformed dykes like Group II and III are very useful palaeostress indicators, because most dykes propagate as magmadriven extension fractures (mode I cracks) akin to hydraulic fractures that form perpendicular to the minimum principal compressive stress (e.g., Pollard, 1987; Valko and Economides, 1995; Ida, 1999; Ernst et al., 2001; Gudmundsson and Marinoni, 2002) whereas the deformed ones like Group I are less significant due to the possibility of rotation in their tectonic or shearing phases. In the undeformed dyke walls, the maximum ( 1) and intermediateσ ( 2) principal compressive stress directions lie normal to one another within theσ plane of the dyke, whereas the minimum principal compressive stress ( 3) isσ perpendicular to the plane of the dyke. Present data suggest that the regional minimum principal compressive stress (i.e., maximum tensile stress, 3) in the MBSZσ area was aligned approximately NESW during dyke swarm emplacement. The spread in dyke strike may reflect fluctuations in the direction of 3 about a timeaveragedσ mean direction. Gudmundsson (1995a) and Ray et al. (2007) has made essentially similar observations in northern Iceland and Deccan area also.

Many NW–SE trending dykes may, for example, be emplaced under a situation involving a NE–SW oriented 3 direction and NWσ –SE oriented 1, but theσ pressurisation associated with the emplacement of these dykes may temporarily



increase the compressive stress in the crust enough to make the 1 direction NEσ –SW and the 3 direction NWσ –SE. New dykes emplaced under these conditions acquire a NE–SW preferred orientation. With continued crustal extension and rifting, and compressive stresses generated by the NE–SW oriented dykes, the temporary stress field can rotate to the original with a NE–SW trending 3. New dykes emplaced in itσ would again acquire NWSE preferred orientations. Thus, large numbers of parallel dykes that largely define the average direction of the swarm indicate the timeaveraged, regional 3. σ

Joint patterns in dykes are also important in terms of dyke trend and local stress field orientation. Regional stress analysis based on geological data incorporates structures of all scales. Specifically, joints are openingmode fractures that propagate in the plane of 1 and 2 and normal to 3, and thus are sensitive indicators of theσ σ σ local stress field orientation (Dyer, 1988; Pollard and Aydin, 1988). Because vertical joints, dyke patterns (Muller and Pollard, 1977) and systematic joints (Engelder and Geiser, 1980) align parallel to the trend of the maximum horizontal stress (SH), they are used to construct regional paleostress trajectories. Furthermore, lateformed joints are often aligned parallel to the regional trend of the contemporary tectonic stress (modernday SH), and are thus used for mapping the orientation of neotectonic stress fields (Engelder, 1982; Bevan and Hancock, 1986; Hancock and Engelder, 1989; Hancock, 1991; Gross and Engelder, 1991; Eidelman and Reches, 1992; Eyal et al., 2001).

The regional county rock joints in the MBSZ area (Fig. 4) are also significant to interpret the palaeostress condition prevailed during the dyke emplacement. The BSZ sector, where the Group I and II dykes were emplaced, shows prominent trends of ENEWSW, NNWSSE, NESW and NWSE. In the MSZ, joint pattern shows EW, NNWSSE and NWSE trends. Among which, the EW trend can be the older set contemporaneous with the shearing phase and the NWSE trend formed simultaneously with dyking phase. The NNWSSE trend which cross cuts both dykes and host rocks can be attributed to a later event of stress.

Aspect ratio, magmatic overpressures, and source depths: The dyke length and thickness are two important parameters in the calculation of dyke geometry. The dykes in MBSZ area show a wide range in exposed length, from <100 m to >21 km. Most of the dykes are exposed less than 1km, while only a few are extending more than that in both the swarms. The thicknesses of dykes could be measured at their sampling sites without any ambiguity. The measured thicknesses range from 40 cm to >20 m in Group I dykes, 25 cm to 4.5 m in Group II and 50 cm to >25 m in Group III dykes. Most of the Group I dykes are <4 m and their margins show fabric development. In Group II, large number of dykes show a <1 m nature and also show cross cutting relation with the host rock.

The aspect ratios (length/thickness) of the five major dykes from the BSZ and MSZ sectors are given in Table 1. Typically, longer dykes in regional dyke swarms are also thicker, as in the Tertiary dyke swarms of Iceland (Gudmundsson, 1983, 1984). However, thickness and length shows a good correlation, longer ones have larger



width and shorter ones with smaller width. The longer dykes are significant in terms of magmatic pressure and regional stress. Aspect ratio calculation was carried out on five undeformed dykes from the Group II and III categories. In which NW 1, EW 1 and NW 2 belongs the Group III dykes and BH 17 and BH 34 belong to Group II dykes. Group II dykes show relatively higher aspect ratio than Group III dykes.

Fig. 4 Rose diagram of joint trends in associated rocks

Table 1. Aspect ratio, magmatic overpressure and possible source depth calculated for 5 MBSZ dykes, in which two belongs to the BSZ area and three belongs the MSZ area.

Sl. No

Dyke No Length Width Aspect ratio

P0 (MPa) Source Depth z (km)

1 NW1 21 25 8404.46 4.6

2 EW1 5 5 10003.75 3.8

3 NW2 18 20 9004.17 4.3

4 BH17 6.5 4.5 14442.60 2.6

5 BH34 4 2.4 16662.25 2.3

Gudmundsson (1983) used dyke aspect ratios to calculate magmatic overpressures and thereby the depths of dyke origin in eastern Iceland, the latter consistent with seismic and electrical resistivity data. If the strike dimension L of the dyke is less than



its dip dimension, and its maximum thickness is bmax, then the magmatic overpressure P0 can be estimated crudely from the following equation (Gudmundsson, 2000):

Thus, the aspect ratio (L/bmax) is a crude measure of the magmatic overpressure at the time of dyke emplacement, provided the appropriate elastic moduli of the host rock are known. In the study of regional Tertiary basaltic dykes of Iceland, Gudmundsson (2000) calculated the magmatic overpressure of a dyke swarm having a length 322 km from the aspect ratios of 16 dykes with an estimated Young’s modulus of 20 GPa and Poisson’s ratio of 0.25, which are appropriate for the shallow crustal depth (Gudmundsson, 2006). The magmatic overpressure of MBSZ dykes are also attempted in a similar way in the present study. Since the crustal thickness of the shear zone area is very less (<11km) in comparison to the regional crust of south Indian shield, Gudmundsson’s (2000) assumption (E 20GPa and v 0.25) is followed for the calculation of overpressure. Calculated values are represented in Table.1, in which MSZ dykes show high P0 values (4.46, 3.75 and 4.17 MPa) while BSZ dykes show low P0 values (2.60 and 2.25 MPa). The results, although indirect, indicate that during regional basaltic dyke emplacement in a tectonic zone system, the magmatic overpressure associated with the dyke may be from several megapascals to several tens of megapascals.

The magmatic overpressures are used to calculate the depths to source magma chambers as done by Gudmundsson (1983) for some regional Icelandic dykes, using the equation:

where ρr is the average crustal density (assumed to be 2,800 kg/m3), ρm is magma density (2,700 kg/m3, Pinel and Jaupart, 2004), and g the acceleration due to gravity. The calculated depths to magma chambers (Table 1) are low (a few kilometres) for five of the dykes and is consistent with the petrological and gravity modelling.

Crustal dilation: Marinoni (2001) has reviewed algebraic and trigonometric methods of calculating the crustal dilation produced or accommodated by sheet intrusions such as dykes and inclined sheets. A commonly used formula for percentage dilation (Walker, 1959; Gautneb et al., 1989) is to divide the aggregate thickness of dykes or sheets (×100) by the length of the strip measured approximately perpendicular to the trend of the dyke swarm. However, this underestimates the dilation, and a more realistic formula (Marinoni, 2001) is:

% dilation = (aggregate dyke thickness) / (length of traverse – aggregate dyke thickness)This simple formula is valid when the traverse along which dilation is measured is perpendicular to the dyke swarm and the dykes are vertical or nearly vertical (so that the width of outcrop equals thickness). Where the traverse is at a low angle to the



trend of the swarm, or when the dykes or sheets are shallowdipping, corrections indicated by Marinoni (2001) should be applied. Crustal dilation of MBSZ dyke swarm was also attempted in a similar way and was calculated as the local dilation percentage in a small 2kmlong strip between the dykes M5a (23 m), M8a (20 m) and M9a (25 m) in MSZ area and a 5kmlong strip between the dykes BH47 to BH17 in the BSZ area. The dilation is 3.4% and 0.2% respectively. Crustal dilations of regional swarms some times show high variations (Gudmundsson, 1995a).

Conclusions Field relations and nature of occurrence of MBSZ dykes, indicate three distinct groups of dykes with contrasting character. Group I dykes are the oldest set of emplacement and they vary in composition (ie; pyroxene granulite, metagabbro/ metadolerite and amphibolite). Amphibolitised dykes are the highly deformed ones and show various deformational fabrics and structures. Metagabbros/ metadolerite are the weakly deformed members in this group. Pyroxene granulite dykes are massive and display the complete rearrangement/remelting of early formed mafic intrusion. Group II and III dykes are fresh and are more or less similar in composition. Cross cutting relationships with different suites of rocks and other dyke groups or other dykes with in the same group indicate that magmatism was not a single episode, but represents successive emplacement of mafic materials.

MBSZ dykes show variations in their regional trend and also among the individual members. Among the three groups, Group I dykes show a prominent ENEWSW direction where as the trend of Group II dykes, show wide variation from a dominant NWSE to NESW. The Group III dykes show a prominent NWSE trend along with minor NESW and EW trends. Palaeostress direction estimated from the dyke trend suggests that the regional minimum principal compressive stress (i.e., maximum tensile stress, 3) in the MBSZ area was aligned σ approximately NE–SW during dyke swarm emplacement and 3 was more or less horizontal, as required forσ the vertical or steep dipping (and rarely inclined) dykes. The spread in dyke strike may reflect fluctuations in the direction of 3 which indicates diverse generationσ tectonic events previled in the area. Thus the study of trend and joint patterns in the area support polyphase deformation in the area. Current paper also demonstrates the application of dyke length/thickness ratio in the calculation of source depth. The estimated source depth was very shallow (2.3 to 4.6) for the MBSZ, which can easily be correlated with the subduction/suture hypothesis in the MBSZ area. Present study demonstrates the application of geometrical parameters in the tectonic investigation of mafic dykes.

ReferencesBevan, T.G. and Hancock, P.L. (1986) A late Cenozoic regional mesofracture system in southern

England and northern France. Jour. Geol. Soc. London, 143, 355362Dyer, R. (1988) Using joint interactions to estimate paleostress ratios. Jour. Struc. Geol., 10, 685699Eidelman, A. and Reches, Z. (1992) Fractured pebblesnew stress indicator. Geology, 20, 307310Engelder, T. and Geiser, P.A. (1980) On the use of regional joint sets as trajectories of paleostress fields

during the development of the Appalachian Plateau, New York. Jour. Geophys. Res., 85, 63196341



Engelder, T. (1982) Is there a genetic relationship between selected regional joints and contemporary stress within the lithosphere of North America?. Tectonics, 1, 161177

Ernst, R.E., Buchan, K.L., and Palmer, H.C. (1995) Giant dyke swarms: Characteristics, distribution and geotectonic applications. In Physics and chemistry of dykes (eds) G Baer and A Heimann, A A Balkema, Rotterdam, 321

Ernst, R.E., Grosfils, E.B. and Mege, D. (2001) Giant dyke swarms on Earth, Venus, and Mars. Ann. Rev. Earth Planet. Sci., 29, 489–534

Eyal, Y., Gross, M., Engelder. T. and Backer, A. (2001) Joint development during fluctuation of the region stress field in southern Israel. Jour. Struc. Geol., 23, 279296

Gautneb, H., Gudmundsson, A. and Oskarsson, N. (1989) Structure, petrochemistry and evolution of a sheet swarm in an Icelandic central volcano. Geol. Mag. 126, 659–673

Ghosh, J.G., De Wit, M.J., Zartman, R.E. (2004) Age and tectonic evolution of Neoproterozoic ductile shear zones in the Southern Granulite Terrain of India, with implications for Gondwana studies. Tectonics, 23, 30063043

Gross, M.R. and Engelder, T. (1991) A case for neotectonic joints along the Niagara Escarpment. Tectonics, 10, 631641

Gudmundsson, A. (1983) Form and dimensions of dykes in eastern Iceland. Tectonophysics, 95, 295–307

Gudmundsson, A. and Marinoni, L.B. (2002) Geometry, emplacement and arrest of dykes. Annal. Tecto., 13, 7192

Gudmundsson, A. (1984) Tectonic aspects of dykes in northwestern Iceland. Jökull, 34, 81–96Gudmundsson, A. (1995a) The geometry and growth of dykes. In: Physics and chemistry of dykes

(eds) G Baer, A Heimann, A A Balkema, Rotterdam, 23–34Gudmundsson, A. (2006) How local stresses control magma chamber ruptures, dyke injections, and

eruptions in composite volcanoes. Earth Sci. Rev., 79, 131Gudmundsson, A. (2000) Dynamics of volcanic systems in Iceland: example of tectonism and

volcanism at juxtaposed hot spot and midocean ridge system. Annu. Rev. Earth Planet. Sci., 28, 107–140

Halls, H.C. and Fahrig, W.F. (1987) Mafic dyke swarms. Geolo. Asso. Canada Spl. Pap., 34, 503pHancock, P.L. and Engelder, T. (1989) Neotectonic joints. Geol. Soc. America Bull., 101, 11971208Hancock, P.L. (1991) Determining contemporary stress directions from neotectonic joint systems. Phil.

Trans. Roy. Soc. London, 337, 2940Hou, G.T., Wang, C.C., Li, J.H. and Qian, X.L. (2006) Late Palaeoproterozoic extension and a

paleostress field reconstruction of the North China Craton. Tectonophysics, 422, 8998Ida, Y. (1999) Effects of the crustal stress on the growth of dykes: conditions of intrusion and extrusion

of magma. Jour. Geophys. Res., 104, 1789717909Marinoni, L.B. (2001) Crustal extension from exposed sheet intrusions: review and method proposal.

Jour. Volcano. Geother. Res., 107, 2746Muller, O.H. and Pollard, D.D. (1977) The stress state near Spanish Peaks, Colorado, determined from

a dike pattern. Pure. Appl. Geophys., 115, 6986Pollard, D.D. (1987) Elementary fracture mechanics applied to the structural interpretation of dykes.

In: Mafic dyke swarms (eds) H C Halls, W F Fahrig, Geol. Asso. Canada Spl. Pap., 34, 5–24Pollard, D.D. and Aydin, A. (1988) Progress in understanding jointing over the past century. Geol Soc.

America Bull., 100, 11811204Ray, R., Sheth, H.C. and Mallik, J. (2007) Structure and emplacement of the Nandurbar–Dhule mafic

dyke swarm, Deccan Traps, and the tectonomagmatic evolution of flood basalts. Bull. Volcano., 69, 537551

Valko, P. and Economides, M.J. (1995) Hydraulic fracture mechanics. John Wiley, New York, 298pWalker, G.P.L. (1959) Geology of the Reydarfjördur area, eastern Iceland. Quat. Jour. Geol. Soc., 114,

367393


R Manu Raj and SN Kumar Pegmatite mineralization of Nagamalai, Madurai

Mineralization associated with granitic pegmatites of Nagamalai – Pudukottai area, Madurai district, Tamil Nadu, India

Manu Raj R* and S. N. Kumar

Department of Geology, University Of Kerala, Kariavattom 695 581, Trivandrum, India*Corresponding author. Email: [email protected]

AbstractThe study area is located in Madurai district of Tamil Nadu, and forms part of the Southern Granulite Terrain of Peninsular India. The area is covered mainly with quartzite, garnetiferrous biotite sillimanite gneiss, biotite gneiss, granite, dolerite, charnockite and granitic pegmatites. The gneisses belong to the khondalite group. The granites, seen as small hillocks, boulders and monadnocks, intrude concordantly the gneisses, which are well foliated. Mainly two types of granites are found viz. pink and grey. The field relations, mineralogy, texture and structure point to their Atype character. Migmatized zones are associated with these granites. The granitic pegmatites, which have close association with the granites, are seen as veins having different trends. In the present study, the size, shape, structure, texture and mineralogy of different granite pegmatites are attempted. There are mainly two generations of such pegmatites. Although they are generally very coarse grained, grain size variation is noticed at places. The pegmatite bands vary in width from 25 to 30 cm. These pegmatites are parallel to the foliation in gneisses and crude gneissosity in granites. Quartz, feldspar (pink or gray) and biotite are the major constituents in pegmatites. Gray feldspar pegmatite veins, intruding the granites, show good sulphide mineralization. The sulphides present include pyrite, chalcopyrite, bornite and molybdenite.

Key words: Southern Granulite Terrain, Khondalite, granites, granitic pegmatites

IntroductionThe Precambrian shield of Southern India is a classic example of Archean continental crust where different crustal levels are exposed .The triangular tract of southern India comprises of Dharwar craton, the Southern Granulite Terrain and Eastern Ghat Belt (Naqvi and Rogers, 1987). The Madurai region in the Southern Granulite Terrain comes under the Madurai block (Ramakrishnan, 1993) which is located between the Palaghat Cauvery shear zone in the north and Achankovil shear zone in the south (Fig.1). The study area is located in Madurai district, Tamil Nadu, South India. An area of 125 sq. km was mapped for the present study.

A. P. Pradeepkumar and E .Shaji (eds) Shear Zones and Crustal Blocks of Southern India vol.2 Proc of UGCSAPDRSII seminar, Dept of Geology, University of Kerala, India, 31 March 2015, 40p ISBN 978-81-923449-4-2 34



Fig.1 Simplified geological map of South India (after Prakash, 2010)

The quartzite, garnetiferrous biotite sillimanite gneiss (GBSG), biotite gneiss and calcgneiss of the area show geosynclinal association of pelitic sediments. The GBSG, quartzite and calcgneiss, characterized by deep penetrative foliation and well developed gneissosity, reflect intense deformation. In addition granites, dolerite, pegmatites and charnockite are present in the area.

The granites, found as hillocks and monadnocks, have a concordant relationship with gneisses and quartzite. Mainly two types of granites are found viz. pink and grey. They are mainly made up of alkali feldspar, quartz and biotite. Accessory amounts of zircon, apatite and opaques are noticed. Migmatized zones are seen associated with granite. The interlocking coarse grains and sequence of mineral formation reveal the igneous nature of the granites. The texture, mineralogy and field relations of granites in the area reflect their A type nature. Geochronological studies point to their Late Proterozoic ages (Pandey, et.al, 1994). The geochemical signatures of these granites characterise them as Atype granites emplaced in a “within plate” tectonic setting (Nathan et al., 2001). Different generations of granite pegmatites are seen as veins or bands intruding the granites. These pegmatites are parallel to the foliation in gneiss and crude gneissosity in granites which strike NWSE. Feldspar (pink or gray) and quartz are the major minerals present in pegmatites. Accordingly they can be classified as pink feldspar pegmatite and gray feldspar pegmatite. Minerals like biotite, calcite and magnetite are also present in these pegmatite veins. Although both types of pegmatite veins bear sulphides, the grey feldspar type is more enriched in sulphides.

Materials and methodsDetailed geological mapping on a scale of 1:25000 was carried out to identify and study the field relations and structural relationship of the major rock types. GPS was extensively used to know the precise locations of outcrops. Fresh representative samples (chips and hand specimens) were collected from working and abandoned quarries. Photographs depicting field relation, geomorphologic and structural features were recorded. Petrographic studies of thin sections of rocks were carried out in the



Department of Geology, University of Kerala. Individual minerals were separated by conventional methods.

ObservationsField study of the pegmatites points to their close association with the granitic rocks and the presence of valuable sulphide minerals. The pegmatites are composed of feldspar, quartz, biotite, magnetite, calcite and sulphides of varying composition the size of minerals vary from a few millimetres to a few centimetres. The pegmatites of the area can be broadly grouped into two; the first group is more or less barren without much sulphide minerals and the second group has rich sulphide mineralization. The former is mainly made up of pink feldspars while the latter pegmatites have gray feldspars as dominant constituents. The pink feldspar pegmatites are associated with granitic rocks which are highly migmatised. These pegmatites are thoroughly weathered. They can be traced for lengths of 2025m and the width of such pegmatites varies between 25 and 30 cm. In such areas, Gray feldspar bearing pegmatites are less found. Gray feldspar pegmatites occur as patches in both pink feldspar pegmatite and granite. These pegmatites contain mainly feldspar, quartz and biotite. Some of the migmatized portions have yellowish coating which points to the small amount of sulfide minerals present. The contact of these pegmatites with granite is sharp. Gradation in size of grains is noticed in some pegmatites from the periphery to the centre. Some feldspar grains show pinch and swell structures. Quartz veins are also associated with such pegmatites. Graphic texture is common in such pegmatites, which contain good amount of quartz and small amount of biotite. Thin sections of pink feldspar pegmatites reveal that the rock is essentially made up of orthoclase, quartz, perthite, plagioclase and biotite. Orthoclase is subhedral while quartz grains are anhedral. Euhedral grains of line, rod and string perthites are noticed. Opaques, mostly magnetite, are of various shape and size. The second group of pegmatites, bearing gray feldspars are associated with the granites of less migmatised areas and contain large quantities of sulphides. However some of these pegmatites contain ferromagnesian patches. Vug and comb structures are common in these pegmatites. In some areas, this type of pegmatites cut the migmatites and gray granite (Fig.2). In thin sections the rock is essentially made up of orthoclase, quartz, perthite, plagioclase, and biotite as dominant minerals; calcite and hornblende are minor phases. In addition apatite and magnetite grains are also noticed. Pegmatitic, hypidiomorphic and granophyric textures are well preserved in these pegmatites.

Mineralogy of gray feldspar pegmatiteThe important minerals present in gray pegmatites are feldspar (both white and gray), quartz, biotite, magnetite, calcite and different sulphides. The most dominant mineral is alkali feldspar. Some of these feldspar grains are of moonstone variety.



Fig.2. Cross cutting pegmatite veins

The second dominant mineral in gray pegmatite is quartz. Quartz of different types is present milky, transparent, smoky and amethyst varieties. Quartz crystals are of different type’s viz. trapezohedral, prismatic, skeletal (?).Twining according to the Dauphine law, Brazil law and Japan law are noticed in quartz crystals. Smoky quartz and amethyst are mainly associated with those gray pegmatite bands which bear good amount of sulphides. The quartz combs are euhedral. The vugs range in size from a few millimetrers to 1 centimeter. Biotite in granitic pegmatites of the study area is of different sizes; from few millimetres to more than fifteen centimetres in diameter. Large biotite flakes, some of them hexagonal books, are commonly associated with the massive sulphide ores. Some biotite flakes show sulphide coating also. Sulphides are seen as massive patches, and well developed crystals associated with quartz. The sulphide minerals include pyrite, chalcopyrite, Bornite and molybdnite. The dominant sulphide is pyrite. Pyrite crystals occur as typical cubes (<1 cm) or in massive form. The grains show uneven fracture, moderate to high specific gravity and metallic luster. It has a pale brassy to golden yellow colour and gives brownish black streak. Pyrites are associated with other sulphides such as chalcopyrite and bornite; also with quartz, calcite and magnetite. Chalcopyrite occurs in close association with pyrite, bornite and molybdenite. It occurs mostly as compact masses, small crystals or as leaves with golden yellow colour. It tarnishes to blue and black tints on exposure to atmosphere. Some grains are euhedral with well developed prisms.



Bornite in pegmatites is found as compact and granular masses. It has an uneven fracture, low hardness, high specific gravity and metallic lustre. It can be easily identified by its brilliant peacock colour and greenish black streak. Molybdenite is identified by its bluishgray colour, excellent basal cleavage, flexibility, very low hardness and sulphurous smell.

Fig.3. Association of sulphide minerals in pegmatite

Economic aspects of the pegmatiteThe pegmatites of the study area contain valuable minerals such as quartz, feldspar and sulphides. Quartz can be used as a raw material in the production of abrasives, refractories and in the making of glass. Feldspars are used in ceramics and glass industry. They also serve as a source of alumina. The economic significance of gray feldspar pegmatites which are rich in sulphides calls for detailed analysis so as to know about the chemical composition and possible hydrothermal/magmatic origin.

ReferencesNathan, N.P., Balasubramanian, E., Ghosh Subhasish and Roy Barman, T. (2001) Neoproterozoic acid

magmatism in Tamil Nadu, South India: Geochemcial and Geochronologic constraints, Gond. Research, Vol.4, No.4, pp.714715.

Naqvi, S. M., and J. J. W. Rogers (1987),Precambrian Geology of India, vol. 6, Oxford Univ. Press, Oxford, U. K.

Prakash,D.,Prakash,S. And Sachin, H.K.(2010) Petrological evolution of the high pressure and ultrahightemperature mafic granulites from Karur, southern India: evidence for decompressive and cooling retrogradetrajectories. Miner. Petrl.pp.3553

Pandey,U.K., Chabria,T, Krishna,V. and Krishnamurthy P.(1994) RbSr geochronology of late Proterozoic Atype granites in parts of Madurai district, Tamil Nadu: implications on uranium and rare metal exploration.Jour.Atm.Min.sci.Vol.2 pp.2943.

Ramakrishnan (1993), Tectonic evolution of granulite terrains of Southern India. Geol.soc.India, Mem.V.25,pp. 3544


E Shaji et al. Unusual metaultramafites of Wayanad

Unusual metaultramafics of Wayand, southern India: a petrological report

Shaji E, Indu G, Arungokul J, Dhanil Dev S G and Pradeepkumar A P

Dept of Geology, University of Kerala, Kariavatom campus, Trivandrum695 581, India

Email: [email protected] m

AbstractIn the Meenangadi – Pulpulli – Sulthan Bathery area of Waynad, southern India, several sequences of metamorphosed mafic and ultramafic bodies occur as rafts and enclaves within The Peninsular Gneiss Complex. The metaultramafics include talc tremolite schist, talc schists, metapyroxenites and some garnet bearing mafic gneisses, considered as retrograded eclogites. Talctremolite schist (TTS) is the dominant metaultramafic rock of the area and it shows variants such as talctremoliteactinolite schist and tremoliteactinolite schist. Another interesting rocks units reported in the study are phlogopite + cpx + opx + spinel, phlogopite + muscovite + talc + cpx + amphiboles bearing metaultramafics. These bands vary in width from 210 m. Exact width of these bodies are not clear due to the bouldery nature and weathering of the exposure in most cases. Strike of the TTS in the western part of the area is EW but it varies to EW and N80° E in the eastern part. The main sectors having maximum concentration of metaultramafics are Irulam, Ellakolli, Munnanakuzhi, Vakeri, Chethalayam and Kuppadi areas. TTS with spinifex type texture exposed at Irulam resembles komatiite but detailed studies are needed to confirm this report. Slender laths of olivine/amphibole set in a fine grained matrix suggest quenching of superheated magma, but the preservation of these laths even after the metamorphism is unusual. Geochemical studies shows that these metaultramafics are rare and shows high Mg. MgO contents vary from 24.53 to 30.45. These rocks preserve evidences of mantle metasomatism.

AcknowlegmentThe authors gratefully acknowledge the financial support through the UGC SAP DRS II, which supports the continuing field work and chemical analysis under the thrust area identified.




S Rajesh and A P Pradeepkumar Cordierite gneiss in Munnar granite

Cordierite gneisses in Munnar granite, southwestern India: implications for isothermal decompression history S. Rajesh and A. P. Pradeepkumar

Department of Geology, University of Kerala, Trivandrum, India


AbstractThe Munnar (N10°05'00";E77°05'00") granite is an EW trending irregular body emplaced within the migmatite and apophyses extend into the surrounding gneisses. The granite dated to be 740 ±30 m.y (Odom, 1982) is traversed by pegmatite, aplite and quartz veins. Gneissic layering and foliation are apparent in all but the least deformed granitic rocks in the study area. Field investigations reveal that the rocks in Munnar include granite, hornblende biotite gneiss, cordierite gneiss, granite gneiss, carbonatite, syenite and migmatite. In Munnar cordieritebearing gneisses occurring as elongate patches in a 10 to 15kmwide zone along the southeastern part of Munnar town. Cordierite (Al3(Mg,Fe)2[Si5AlO18]) which usually occurs in aluminous rocks that have been subjected to thermal or regional metamorphism. Generally cordierite in gneisses is associated with minerals like andalusite, spinel, quartz, and biotite. The textural relationship of this rock is consistent with the following main reactions: garnet + quartz = cordierite + hypersthene + biotite. Munnar granite intrusion can be noticed as the country rock. In some areas around Munnar small patches of charnockite are found within the migmatitic/cordierite gneiss which is having relict pyroxenes and appearance of newly formed hornblende. Near Devimalai (17 km from Munnar) cordierite develops at the contact between granite and calcgranulite and it has been suggested that the cordierite develops at the expense of garnet (Thampi et al. 1979), indicating the possibility of these being Type 2 Magmatic (a) Peritectic type of Clarke (1995). Numerous petrologic studies have been done on rocks of Achankovil shear zone especially in relation to the formation of cordierite (SinhaRoy et al. 1984; Srikantappa et al. 1985; Ravindra Kumar and Chacko 1986; Santosh 1987; Soman et al. 1995a, 1995b, Shaji 2009). Most of these studies conclude that granulite formation, including the development of the cordierite gneiss, occurred under conditions of isothermal decompression, possibly related to crustal thinning and extension following earlier collisionrelated thickening and also suggest a history of isothermal decompression accompanied by the flow of CO2rich fluids for the formation of the cordierite gneiss. The presence of this little studied cordierite gneiss in the Munnar area of Madurai block suggests that isothermal decompression was a component in the evolutionary history of the Munnar area and Madurai block.



http://www.encyclopedia.com/doc/1O13-biotite.html

http://www.encyclopedia.com/doc/1O13-quartz.html

http://www.encyclopedia.com/doc/1O13-spinel.html

http://www.encyclopedia.com/doc/1O13-andalusite.html

http://www.encyclopedia.com/doc/1O13-gneiss.html

http://www.encyclopedia.com/doc/1O13-metamorphism.html

Documents

Shear Zones and Crustal Blocks of southern India vol 2