LITHOLOGY AND TECTONOTHERMAL HISTORY OF THE PRECAMBRIAN ROCKS OF ORISSA ALONG THE EASTERN COAST OF INDIA

Orissa possesses an excellent record of geological history spanning mo st of the geologic time from Archaean to the Quaternary. It has most o f the typical lithologies and many tectonothermal events preserved in the various rock groups. Three distinct crustal blocks could be identi fied: two cratonic blocks and a mobile belt separated from each other by deep-seated regional fault boundaries. These are the north Orissa c raton (NOC), the west Orissa craton (WOC) and the Eastern Ghats granul ite belt (EGB). The fault boundaries separating them are identified as the north Orissa boundary fault (NOBF) and the west Orissa boundary f ault (WOBF). The NOBF fault running along Mahanadi Valley could be ter med the 'Mahanadi rift'. The NOC contains extensive occurrences of low -grade folded banded iron formations (BIFs), granite intrusives and un deformed volcano-sedimentary assemblages belonging to the Archaean to early Proterozoic times. They are succeeded by medium-grade folded Pro terozoic limestone-bearing sequences. The WOC craton is underlain by e xtensive occurrences of Archaean granites and undeformed Proterozoic l imestone-bearing platform sediments. Small occurrences of Archaean BIF s and greenstones have also been noticed. The EGB consists of high-gra de granulite-facies rocks such as the khondalites, charnockites, basic granulites, migmatites and augen gneisses. It has been considered as a 'mobile belt' during the middle Proterozoic Era. A distinct early Ne oproterozoic (ca. 1000 Ma) charnockite event has been recorded in this belt. Close to the faulted boundaries of the crustal blocks are seen occurrences of anorthosites, alkaline rocks and chromite-bearing ultra mafics. The rocks in each crustal block show evidence of multiphase te ctonothermal history. Similarity of lithology, tectonothermal events a nd major rift features, e.g. the Mahanadi rift, place Orissa close to Eastern Antarctica. Copyright (C) 1996 Elsevier Science Ltd