Brittle deformation in Southern Granulite Terrane (SGT) : A study of Pseudotachylyte bearing fractures along Gangavalli shear zone (GSZ), , . Behera, B.M.(1), Thirukumaran, V.(2), Biswal T.K.(1), (1) Department of Earth Sciences, Indian Institute of Technology Bombay, Mumbai, India, 400076 ([email protected]) (2) Government Arts College, Salem, Tamil Nadu, India, 1. Introduction: 3. Microstructural Observation: High grade charnockite rocks of Southern Granulite Terrane Fig.III These ten fresh (SGT) are reported to be exhumed by the low angle ductile samples are selected thrust (i.e. Salem- shear zone) running E-W direction for the major oxide that produces high standing hills in the southern India analysis through ICP- (Biswal et al., 2010). Later stage of brittle deformation is AES technique. also noticed to the east of the ductile shear zone where numerous pseudotachylyte vein markers are exposed at some Silica concentration outcrops near Gangavalli area. These veins are the product of 61.72% to 71.17% high speed friction of blocks during a seismic event that generate ultrahigh temperature. No further evidences of Minerals presents: ductile shear zone are found continuing after the brittle Quartz, Plagioclase shear zone. So we suggest here that the truncation of (Ab>An), K-feldspar, mylonite zone is due to the later stage of brittle deformation Hypersthene,Diopside, of high grade rocks. This produces extensive fractures Apatite and some dark through which pseudotachylyte melt are implaced and giving Magnetite. rise an anastomose appearance to the pseudotachylyte vein. Fig.e Fig.f These veins thickness varies from mm to tens of cm. Opdalite-Enderbite Hence this work is mainly focused on the late stage brittle host rock. deformation and its association with the generation of pseudotachylyte veins. . (Geological map of Gangavalli area, modified after Sundarlingam K., 2013) 2. Field Observation:

5. Conclusion: Regional compression along NNW direction produces the brittle shear zone in Gangavalli area, further evidence of brittle deformation is supported by the generation of pseudotachylyte melt through high speed friction of adjacent blocks. Decrepitating along pure β-Quartz boundary suggest that peak temperature of 15500 C reached during the formation of the melt. The high frictional melt product has some heterogeneity in its composition which shows two distinct component rich with Albitic (Na, Ca) and Ferromagnesian (Fe, Mg). This variation occurs due to localized melting of plagioclase and pyroxene minerals. Acicular microlites wrapping around clast indicates onset of some crystallization process before the final coagulation of melt. Albitic (Na, Ca) composition is more preferable for microlites crystallization because ferromagnesian melt is not sufficiently available in the vein. So it is often noticed that the pseudotachylyte vein which is more thick and intensely dark has more chance to crystallize a microlites irrespective of composition.

A. Pseudotachylyte vein attitude: a.Flow direction: Towards the tapering end.(Fig.a) b.Sharp cutting boundary: Intrusive nature of vein.(Fig.a) c.Differential flow: Due to heterogeneity composition.(Fig.b) d.Flow folding: Due to injection, not due to compressional stress.(Fig.c) II. X-Ray Diffraction of Incorporated Clasts: β-tridymite does not form from pure β-quartz, need trace amount of material to be added. B. Country rock fragments: - Most of the clasts within the veins are pure β-Quartz (Hexagonal system) and survived from Hence step skipped. (Heaney, 1994.) a. Sub-round to round: Grain boundary decrepitation. complete melting. b. Oriented clasts: Elongated clast orinted along the direction of flow.(Fig.d) - Melting temperature of pure β-Quartz is 15500 C. C. Deformation: a. Brittleness of country rock: High intensity of fractures are produced during brittle deformation. b. Grain size reduction: Maximum stress is exerted near the pseudotachylyte melt.(Fig.e) c. Micro fault: Some forces are still active after the formation of the vein. It causes further reduction of size within it. (Fig.f) If quick change in temperature occurs then α-quar = Trigonal β-cristobalite is skipped. Hence β-quartz D. Microlites: β-quar = Hexagonal directly melt at 15500C. a. Acicular Microlites: Wrap around clasts, directive texture, crystallize during the flow of the melt, simple structure. (Fig.g) b. Spherulitic Microlites: Radiating from the core, complex structure, found at the center of the vein. (fig.h) c. Sheaf Microlites: Consists a central bar and plumose fibre at the both end. complex structure. (Fig.i) 4. Methods and Analysis: Pseudotachylyte: In out crop pseudotachylyte vein looks like a black colour glassy and sharp cutting vein wall showing its intrusive I. Electron Probe Micro-Analysis (EPMA) of Pseudotachylyte Matrix and Microlites: nature (Fig.1). Thick veins are normally straight and thin veins shows different attitude like anastomose (Fig.2), bleb (Fig.3), dendritic ‐ The host rock of pseudotachylyte is a plagioclase dominating charnockite (i.e. Enderbite rock) (Fig.III). There are many microfaults present within the pseudotachylyte vein itself that causes further breaking of clast (Fig.4), lens (Fig.5), bifurcation (Fig.6) and injected type (Fig.7). Another distinct feature of pseudotachylyte vein is the presence of and also developed many sets of fractures. So it can be concluded here that the brittle deformation which country rock fragments that are mostly sub rounded to round known as “Quasi-conglomerate (Fig.8). - Careful point analysis is needed through EPMA technique to know the composition of the matrix, microlites and clasts (Fig.I). Due to the presence of enormous clasts varies from ultrafine to caurse size may produce an unwanted results. responsible for creating pseudotachylyte veins is still active today. It may causes reactivation of slip surface. Numerous fractures produced in Gangavalli shear zone - The vein shows heterogeneity of two chemical composition of (Na, Ca)(point 23,24) rich and (Fe, Mg) (point 20,21,22) rich that gives a fragile and brittle appearance to the rock type melts (Fig. ). But the favorable composition for microlites to be crystallized is essentially Albitic rich melt(point 6,7,8,9). (Fig. 9). Displacement of quartz vein along the fracture II References: plane indicates the governing stress condition of the area Biswal, T.K. Thirukumaran, V. Ratre, Kamaleshwar. Bandyapadhaya, Krishanu. Sundarlingam, K. Mondal, A. K. 2010. A study of mylonites from the parts of the Salem-Attur shear zone(Tamil Nadu) and its tectonics implications. Jou.Geol.Soci.of India, Vol.75,pp.128-136. (Fig.10). Thick and straight pseudotachylyte veins are Heaney P.J., 1994. Structure and chemistry of the low-pressure silica polymorphs: in reviews in Mineralogy, vol.29. mostly oriented along these two dominant directions. This Sundaralingam, K., 2013. Strain Analysis, Kinematic Interpretation and tectonic Evolution of Salem-Attur Shear Zone Around Salem, Tamil Nadu,India. suggests that subsequently conjugate brittle strike slip Ph.D. Thesis, 2013. 0 0 shear zone along 15 and 300 directions provides a regional Acknowledgement: compression of NNW. . III. Zircon Dating Through SHRIMP: The authors are greatly thankful to Prof. G. Mohan (Head of the Department of Earth Sciences, Indian Institute of Technology, and Bombay) for the permission and funding to carry out field work in Gangavalli area, Salem, Tamil Nadu. We are also grateful to staffs of Department of Earth Sciences - Some euhedral broken zircons have been discovered from pseudotachylyte vein and show for helping me to operate the instruments for XRD,EPMA, ICP-AES. We also convey our gratitude to B. Sivalingam (M.Sc. and Ph.D. dual degree student of IIT Bombay) and Sridharan, Thiyaga Ranjan and Madan Raj (students of Department of Geology, Govt. Arts College, Salem, Tamil Nadu) oscillating zoning in cathode luminescence that indicates a magmatic origin. for their extensive support and help during the field visit.

- These zircons have no relation with the formation of pseudotachylyte as it is a quick generated melt. This reveals two different age groups; older one shows 2550±16 and 2508±14 Ma & Presenting author: younger one shows 1848±24 and 1875 ±22 Ma.(Sundarlingam K., 2013) BHUBAN MOHAN BEHERA Email: [email protected] Cell: +91 7738448557 INDIA