The tectonic evolution of the Southern Granulite Terrane of India and its role in the amalgamation of Gondwana.

The southernmost portion of the Indian subcontinent, the Southern Granulite Terrane of India, holds a pivotal role in the reconstructions of the Ediacaran-Cambrian supercontinent Gondwana. Bound to the west by Madagascar and the East African continental fragments and to the east by Sri Lanka and Antarctica, this terrane offers a breadth of information regarding across terrane correlations and palaeotectonic settings, Arguably, within the Southern Granulite Tenane, the most debated issue of all is the existence of a late Neoproterozoic (ca. 550 - 500 Ma) suture zone between the Salem and Madurai Blocks, termed the Palghat Cauvery ShearZone (or alternatively, the Cauvery Shear Zone). The U-Pb zircon geochronology of the orthopyroxene-bearing orthogneisses (charnockites) in the northern part of Madurai Block yielded Neoarchaean and Cryogenian (ca.2.7 - 2.5 Ga and 795 ± 17 Ma) crystallization ages with metamorphic overprinting at ca. 535 Ma. Their Nd and Sr isotopic ratios (εNd₍τ₎ = -16.9 and +4.55, ⁸⁷Sr/⁸⁶Sr = 0.7017 - 0.7567) suggest they were generated by variable mixing of mantle-derived magmas with the older Archaean crust. The charnockites and garnet-biotite bearing granitoids in the southern part of the Madurai Block yielded much younger crystallization (1007 ± 23 Ma and 784 ± 18Ma) and Nd depleted mantle model ages ranging between 1.69 Ga and 1.38 Ga with low initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7054 - 0.7084, suggesting that the Madurai Block is not a uniform crustal domain, The age of metamorphism in this southern part of the Madurai Block is similar to that further north and indicates isotopic disturbance and zircon growth during the Ediacaran-Cambrian metamorphism. The coupled U-Pb and Lu-Hf isotopic study of the metasedimentary rock packages from the Salem Block mainly yielded late Archaean to early Palaeoproterozoic (ca. 2.7 - 2.45 Ga) detrital zircon ages with εHf values between +0.3 and +8.8, suggesting derivation from largely juvenile sources. ln contrast, the metasedimentary rocks from the Madurai Block are dominated by Mesoarchean to Palaeoproterozoic detrital zircon ages (ca. 3.2 Ga and 1.7 Ga) to the north and Mesoproterozoic to Neoproterozoic (ca. 1.5 - 0.65 Ga) detrital zircon ages in the south. Collectively, the Hf isotopic signatures of detrital zircons from the Madurai Block suggest they were derived from variably mixed reworked Archaean and Palaeoproterozoic sources, with additional input from the juvenile Palaeoproterozoic, late Mesoproterozoic and evolved Neoproterozoic sources. When compared to the metasedimentary rock packages from adjacent tenanes in Gondwana, the Madurai Block metasedimentary rock units correlate best with the basement sources derived from East Africa. The disparity between the delrital signatures from the Salem and Madurai Block suggest they had dissimilar depositional histories until the latest Neoproterozoic, The Palghat Cauvery Shear System (PCSS) at the southernmost margin of the Salem Block, represents a network of anastomosing crustal-scale shear zones with a largely dextral offset. The careful structural field observations coupled with U-Pb zircon geochronology of cross-cutting pegmatites, leucosomes and deformed basement gneisses show that the deformation in the Palghat Cauvery Shear System (southern Salem Block) is not a result of a single deformational event. Rather, the E-W and NE-SW trending fabrics that characterize much of the northern portion of the PCSS formed during an early Palaeoproierozoic (ca. 2.5 - 2.48 Ga) metamorphic event associated with largely dextral strike-slip motion. This deformation is overprinted by the E-W and ESE-WSW trending fabrics in the southern part of the PCSS (the Cauvery Shear zone) associated largely with dip-slip and minor dextral strike-slip contractional deformation suggesting dextral transpression. This latest deformation is loosely constrained to ca. 740 - 550 Ma. The Madurai Block deformational events (D₁– D₄) are different in character to those further north and are constrained to ca. 550 - 500 Ma. The latest Neoproterozoic event (ca. 550 - 500 Ma) appears to have affected both the southern portion of the Salem Block and all of the Madurai Block and is most likely linked to the final stages of Gondwana amalgamation. This study also suggest that care should be taken when regarding the regional-scale structural fabrics of the PCSS as being associated with the latest Neoproterozoic metamorphism. The Southern Granulite Terrane records two major episodes of magmatism at ca. 2.7 - 2.43 Ga and 0.83 - 0.75 Ga. ln the Salem Block, the ca. 2.7 – 2.5 Ga magmatism is associated with production of tonalitic-trondhjemitic-granitic (TTG) magmas during partial melting of basaltic/amphibolitic sources with limited contributions directly from the mantle, similar to the "typical" Archaean TTG provinces worldwide. Somewhat younger (ca. 2.54 - 2.43 Ga) basement rocks in the Madurai Block also show some similarities to the Archean TTG rocks further north. However, a large portion of the gneisses also show higher K₂O/Na₂O ratios, Ni and Cr concentrations and Mg numbers (molar ratio of MgO/(MgO + FeO)) suggesting higher inputs from the mantle during their genesis. The Cryogenian aged magmatism (ca. 0.8 - 0.75 Ga) in the Salem Block is associated with production of highly alkaline magmas (syenites/carbonitites/ultramafics), while in the Madurai Block similarly aged magmas are calc-alkaline, largely granitic to granodioritic, with high LILE/HFSE and LREE/HFSE ratios, low Ni and Cr concentrations typical of magmas generated above a subduction-zone. The differences in the structural character, age and geochemistry of the basement gneissic rocks as well as the differences in the depositional histories of the metasedimentary rock units between the Salem and Madurai Blocks suggests these two domains underwent widely different tectonothermal histories prior to the latest Neoproterozoic orogeny associated with the amalgamation of Gondwana. This study strongly supports a presence of a terrane boundary (a suture zone) between the Salem and Madurai Blocks.Thesis (Ph.D.) -- University of Adelaide, School of Earth and Environmental Sciences, 201