MAGMATIC AND METAMORPHIC EVENTS FROM WESTERN AND EASTERN MARGIN BASINS OF INDIA

Abstract

The continental passive margins of India extend on either side of the Peninsular Shield of India and comprise the Western– and Eastern Continental Margins of India toward the Arabian Sea and the Bay of Bengal, respectively. These passive continental margins, characterized by vast sedimentary cover with a thick pile of sediments both onland and offshore, covet some of the geologically lesser–understood basement rocks of the Indian subcontinent. The basements to the passive margin basins on either side of the Peninsular Shield comprise volcanics, as well as highly complex exhumed metamorphic and igneous rocks, that came into existence through multitudes of polyphase magmatic and metamorphic events. The thesis discusses geochemical, genetic, and geochronological attributes of volcanic–basaltic basement rocks from the Western Continental Margin and Precambrian metamorphic basement rocks from the Eastern Continental Margin of India (ECMI), which were sampled in recovered core samples from deep basement wells. Basements of Kutch, Mumbai, and Kerala–Konkan basins from Western Margin, and Cauvery basin from Eastern Margin, were investigated using major, trace, and rare–earth elements geochemistry and radiometric dating techniques such as 40Ar–39Ar, Rb–Sr and Sm–Nd systematics, and Sr–Nd isotopic attributes. In the Kutch basin of the Western Margin of India, the dataset reports an early Paleozoic basement diorite from the Banni Half–Graben in onland region of the basin. The core sample from drilled well Nirona–A provided 40Ar–39Ar ages of 441.84 ± 2.66 Ma, and 441.28 ± 5.82 Ma to 388.08 ± 16.65 Ma for the basement diorite. These ages have constrained the basement formation age to the late Ordovician–early Silurian period, and are correlatable with the later part of Cambro–Ordovician alkaline magmatism reported from the Huqf area in Central Oman. Their lithological and petrographic characteristics are also comparable with basement diorites occurring in the Dinsi Body of Nagar Parkar igneous complex in Pakistan. The geochemical studies characterized the basement diorite with enrichment of LIL (Rb, Ba, and K) and LRE (La, Ce, Nd) elements, strong depletion of HFS elements (Nb, Sr, P, and Ti), as well as weakly negative Eu anomalies. These geochemical signatures indicate their petrogenetic affiliation with mantle–derived magmas, as well as their tectonic setting to be arc–related, having post–collisional continental–arc type affinity. The study therefore indicates that the ~440 Ma basement of Kutch appears to represent the later thermal event associated with a reworked Neoproterozoic subduction–related suite from Greater India's northwest edge, which has implications for Gondwana assembly in the northwest Indian subcontinent. In the offshore basins of the Western Continental Margin of India, namely Kutch, Mumbai, and Kerala–Konkan basins, 40Ar–39Ar geochronology, elemental geochemistry, and Sr–Nd isotopic compositions of thirty core samples from twenty–four offshore drill wells were used to characterize the genetics of the volcanic basement from the Kutch, Mumbai, and Kerala–Konkan offshore basins. The findings from these volcanic basement rocks have demonstrated extremely varied isotopic and geochemical fingerprints, which are suggestive of significantly diverse parent magma compositions and emplacement processes. Basaltic–tholeiitic basement from the Kutch Offshore basin have provided 40Ar–39Ar ages that range between 60 and 62 Ma. This basement is characterized by a within–plate basalt signature, depleted isotopic signatures similar to least contaminated Deccan Traps basalts, and a component of subducted crustal material. Basaltic basement from the Mumbai Offshore basin have yielded 40Ar–39Ar eruption ages between 63 and 65 Ma and show a strong within–plate OIB affinity. Geochemical and isotopic signatures from these samples are consistent with Renuion lavas and an enriched–end member of Deccan Traps basalts. However, the basement volcanics from Kerala–Konkan Offshore Basin show significant E–MORB affinity and are characterized by primitive–mantle signatures and the least contamination from the upper continental crust, with ages of eruption between 60 and 61 Ma based on obtained 40Ar–39Ar ages. The study on volcanic basement from Western offshore basins has suggested that the Mumbai Offshore Basalts' mode of eruption was comparable with onshore Deccan volcanism. In contrast, the basaltic basement in Kutch Offshore was formed after the primary phase of Deccan eruption and may have occurred as an offshoot of a "tail" of the main Deccan volcanism. The parent magma for the volcanic E–MORB basement in the Kerala–Konkan Offshore basin is envisaged to be generated from mixing along the Carlsberg Mid–Oceanic Ridge and the material from the Réunion plume with the northward movement of the Indian Plate during the Early Paleocene. This appears to have occurred concurrently with the emplacement of the Chagos–Laccadive Ridge. In the Cauvery basin of Eastern Continental Margin of India, the Precambrian basement that form the easternmost extremity of the Madurai Block in the Southern Granulite Terrain, reports Rb– Sr and Sm–Nd ages between 2173–2307 Ma from northern onshore and Rb–Sr ages between 1223–983 Ma from southern offshore parts of the basin. The studied samples include basement core samples of hornblende–gneisses, charnockites, granites, and metapelites (chlorite–biotite– and garnet–biotite schists). These ages represent at least two episodes of tectonothermal events during the early Paleoproterozoic and late Neoproterozoic, suggesting a polymetamorphic evolution history of the basement and are correlatable with the reported events in the Southern Granulite Terrain. The study also yielded early Paleozoic Cambro–Ordovician whole–rock–biotite mineral Rb–Sr ages of 443–487 Ma, coinciding with the cooling stages of the Pan–African tectonothermal event, post-thermal resetting in the studied basement of the Cauvery Basin. In the basement of the Cauvery basin, the Sm-Nd systematics have also yielded two groups of model ages (2.1–3.4 Ga and 1.5 Ga), on the basis of which two distinct crustal domains have been identified in the basement, viz., a Paleoarchean to early Paleoproterozoic northern domain and an early Mesoproterozoic southern domain, respectively. This has also been supported by distinct radiometric ages obtained from these domains.