YEARS OF SCHIZOPHRENIA: THE MADRAS LONGITUDINAL STUDY

Abstract

Recent geochronologic data allow us to propose a new characterization of Precambrian granulite terranes in Peninsular India. The Archean granulite terranes, including the Godavari Granulite Belt (GGB) along the Godavari Valley and the Nilgiri-Madras Granulite Belt (NMGB) along the southern fringe of the Dharwar Craton and north of the Palghat-Cauvery Shear Zone, are characterized by metamorphic events at ca. 2.8–3.0 Ga and intense granitic activity at ca. 2.5 Ga associated with middle- to high-grade metamorphism. The Proterozoic granulite terranes include the Eastern Ghats Granulite Belt (EGGB) along the Bengal Bay Coast of India, the Periyar-Madurai Granulite Belt (PMGB) south of the Palghat-Cauvery Shear Zone and north of the Achankovil Lineament, and the Trivandrum Granulite Belt (TGB) south of the Achankovil Lineament. The Proterozoic granulite belts are characterized by model Nd ages (TDM) of mostly ca. 2.0–3.0 Ga with local 1.3–1.8 Ga. Intense deformation and regional high-grade metamorphism predates emplacement of A-type granitic plutons at ca. 550–740 Ma and the last granulite metamorphism took place at ca. 550 Ma. Both GGB and NMGB have similar geochronologic characteristics to the Napier Complex of East Antarctica in that they suffered ca. 2.8–3.0 Ga and ca. 2.5 Ga tectonothermal events. EGGB, PMGB and TGB have similar ranges in Nd TDM ages as the Rayner and the Lützow-Holm complexes in East Antarctica. They are also similar to the Sri Lankan Precambrian, when the three Precambrian units in Sri Lanka are not differentiated and mixed altogether. In this assembled East Gondwana, late Archean granulite terranes form a continuous belt from NMGB to GGB through the Napier Complex, forming a horseshoe-shaped belt surrounding the Dharwar Craton. The Paleoproterozoic-Mesoproterozoic terranes form a broad belt continuing from EGGB to PMGB through the Rayner and the Lützow-Holm complexes. This belt forms a major part of the Mesoproterozoic Circum East Antarctic Mobile Belt surrounding East Antarctica, which is important for the assembly of East Gondwana. Within the Proterozoic terrains, signatures in the distribution of TDM ages facilitate the differentiation of the Paleoproterozoic and Mesoproterozoic terranes which have distinct distributional characteristics.