U-Pb geochronology and structural geology across major shear zones of the Southern Granulite Terrain of India

ABSTRACT This thesis is divided into 2 parts. The first part (Chapters 1-5) concentrates on unravelling the . Precambrian geological history of the high-grade poly-metamorphic and poly-deformed basement of southernmost India, using field geology and U-Pb geochronology. Because Peninsular India occupied a central position in Gondwana, this work bears on reconstruction models of central Gondwana between circa 600 Ma and 200 :f
,a. The second part of the thesis (Chapter 6) tests the use of organic carbon isotopes as a chemostratigraphic tool to correlate between the lower part of terrestrial Gondwana (Upper Carboniferous to Triassic) sequences in India. This work contributes to (i) intra-basinal and inter-basinal stratigraphic correlation and carbon isotope variations during the Pennian-Triassic extinction event of Gondwana; and (ii) refines the inter-continental correlations of the Gondwana type sequences in India with those in South Africa and Madagascar. This century much attention has been focused on the reconstruction of Gondwana, and as a result, the broad relative positions of different Gondwana fragments and their later dispersal histories is now well understood. Finer-scale reconstructions, based on modern geochronological and isotope studies integrated with field studies of continental rocks, are still needed to resolve the detailed kinematic histories of continental break-up and the evolution of continental lithosphere in general. Shear zones that cross the boundaries of continental fragments provide 'piercing points' in fitting the fragments back together. To realize this, the kinematics and the timing of major shear zones need to be detennined. The first part of this thesis, therefore, primarily focuses on understanding the kinematics and geochronology of a number of subvertical continental scale shear zones in the Southern Granulite Terrain ofindia (SGT). Peninsular India is a complex Precambrian terrain .in which the Archean granite-greenstone basement of the Dharwar craton is flanked by high-grade metamorphic terrains of the SGT in the south and the Eastern Ghats in the east. These two high-grade terrains have suffered a Proterozoic polyphase history of deformation and charnockitization. The boundaries between the Dharwar craton and the two high-grade terrains intersect the eastern coast of India. Whether these high-grade terrains are parts of a single orogenic belt, however, remains an outstanding question. Also it is not known if the two high-grade terrains comprise remobilised Dharwar basement or not. A number of models propose that terrane boundaries exists (i.e., the SGT comprises a number of terranes) along some major shear zones/lineaments in the Peninsular India. However, these models are rarely backed up by field data and precise geochronology. To test these models, a number of corridors across major shear zones in the SGT were mapped in detail and 35 rock samples were collected and dated precisely using UPb geochronological techniques. The findings of this study are: (i) Circa 2.5 Ga rocks of the Dharwar craton extend at least -250 km into the SGT at least up to a newly defined shear zone, the Karur-Kambam-Painavu-Trichur Shear Zone (KKPTSZ) (ii) The KKPT Shear Zone is the best candidate for a terrane boundary in the SGT. Highgrade equivalents of the Dharwar craton occur in the north of this shear zone and rocks of the · Eastern Ghats extend to the south of this shear zone. (iii) The SGT has experienced many magmatic and high-grade metamorphic and tectonic events spanning from Mid-Archean to Early Paleozoic. Events at circa 2.9 Ga, 2.5 Ga, 1.6 Ga, 1.0 Ga, 800 Ma, 720 Ma, 600 Ma, 570 Ma, 550 Ma and at 525 Ma are recorded from different part of the SGT using U/Pb zircon and monazite dating. (iv) Two new major shear zones in the SGT have been proposed and the positions of two other shear zones have been re-defined. All these shear zones date between circa 600 Ma and circa 560 Ma. None of them, excepting the KKPTSZ, represents a terrane boundary. (v) Charnockitization is not all of the same age. In the southern part of the SGT, it post-dates the shear zones, and has occurred between circa 550 Ma and 525 Ma. Much older -2.5 Ga charnockitization occurs in the northern part of the SGT, but in places these rocks are also affected by the late Neoproterozoic - early Paleozoic charnockitization. (vi) There is a marked similarity in the ages of charnockites, shear zones and other thermal events of southern India with those in Madagascar and Antarctica. The new data suggest that the KKPT Shear Zone in India is possibly continuous to the east with the boundary zone between the Napier Complex and the Rayner Complex in Antarctica, and to the west, with the Ranotsara Shear Zone in Madagascar_ The second part of this thesis examines the organic carbon isotope variations in terrestrial Gondwana sequences. To correctly understand the larger context of Gondwana evolution during its life span between about 500 Ma and 200 Ma, the terrestrial stratigraphic records preserved in the type localities, now separated as far as South Africa and India, need to be correlated with confidence. The plant and vertebrate fossil records cannot be correlated with sufficient resolution. Therefore, the use of organic carbon ( C0rg ) isotopes as a high-resolution chemical stratigraphic tool is examined. More than 500 samples from different Gondwana sections in Peninsular India, Madagascar and South Africa, spanning - l 00 million years of Middle to Late Paleozoic history, have been analysed for C0 rg isotope ratios (expressed as o13C0 rg). A number of samples across the palynologically determined Permian-Triassic boundary came from different boreholes in India. The results of this pilot study demonstrate exciting new possibilities of using this technique for stratigraphic correlation between Gondwana basins. The findings of this study are: (i) Sharp negative o13C0 rg spikes of >5-6%0 across the Permian-Triassic boundary are present in most but not all of the terrestrial Gondwana sections. The largest rapid 013C0rg variation records a change of l 9o/oo, between -18o/oo to -3 7o/oo. Instead of single sharp negative spike · commonly present at the Permian-Triassic boundary of marine carbonates, 2 or 3 sharp negative spikes, spanning a lithologic thickness of 40-60m, have been discovered across the 11 Permian-Triassic boundary in the terrestrial deposits of Gondwana oflndia. (ii) In the Permian section of the type Gondwana sequences, a cyclic variation in the organic carbon isotope ratios is evident These variations broadly correspond to established lithostratigraphic boundaries and/or well defined palynologic zones, suggesting that these patterns may be useful for high-resolution stratigraphic correlations. (iii) Prelimanary inspection of the isotopic signatures suggests that the P-T boundary in Madagascar has been incorrectly placed. In summary, both the radiogenic and light stable isotope studies of this thesis have improved correlation of both the basement and cover rocks between fragments of the former Gondwana supercontinent.