The Southern Granulite Terrane, India: The saga of over 2 billion years of Earth's history

Abstract

Lying to the south of the Dharwar Craton, the expansive granulite massif, known as the Granulite Terrane of South India, preserves protracted history of the earth from ca. 3400–500 Ma. It has been divided into Northern and Southern Granulite Terranes, separated by the Moyar-Bhawani Shear Zone. Unlike the Archean Northern Granulite Terrane, the rocks of the Southern Granulite Terrane bear imprints of magmatism, sedimentation and metamorphism for over 2 billion years from Neoarchean to Cambrian. The Southern Granulite Terrane is subdivided into three geographic divisions, namely Nilgiri-Namakkal Block, Madurai Block and Trivandrum Block, separated from each other by the Palghat-Cauvery Shear Zone and Achankovil Shear Zone. The Nilgiri-Namakkal Block preserves ca. 2900–2550 Ma ensemble of mafic-ultramafic rocks, interpreted as the remnants of Archean oceanic crust formed in a suprasubduction zone. Several phases of felsic magmatism between ca. 2840–2500 Ma and vestigial records of ca. 2600–2530 Ma sedimentation and ultrahigh-temperature metamorphism are found in the Nilgiri-Namakkal and Madurai Blocks. These felsic rocks, showing distinct TTG-type chemistry, rarely record the late Archean ultrahigh-temperature metamorphism, but underwent extensive high-pressure metamorphism (>11 kbar) along 43–65 °C/kbar geothermal gradients during early Siderian (2490–2440 Ma). The late Archean felsic magmatism and the superimposed early Siderian high-pressure metamorphism has been explained by ‘peel-back’ convergence mechanism that takes in account higher mantle temperature during this time. The Siderian crust of the Nilgiri-Namakkal, Madurai and possibly Trivandrum Blocks received extensive multi-phase sedimentation, presumably in a stable continental shelf, spanning from ca. 1800–700 Ma, partially sourced from the Northern Granulite Terrane and Western Dharwar Craton. This together with the unbroken magmatic and metamorphic episodes across the Palghat-Cauvery Shear Zone do not support the view that Palghat-Cauvery Shear Zone represents a Neoproterozoic suture along which the ‘Mozambique Ocean’ was closed. The remarkable similarity of geological events across the Moyar-Bhawani Shear Zone supports that the Northern and Southern Granulite Terranes formed a coherent block at least from ca. 2600 Ma. The late Paleoproterozoic (ca. 1740–1620 Ma) subduction related granitoid magmatism signifies crustal growth during Columbia supercontinent formation. The (Northern + Southern) Granulite Terrane coherent block was extended (without opening of an ocean basin) and received magmatism ranging from alkaline–carbonatite to A-type granitoids during ca. 850–600 Ma, which is tentatively related to the breakdown of Rodinia. Subsequent Ediacaran-Cambrian high- to ultrahigh-temperature metamorphism and associated magmatism, preponderant throughout the Southern Granulite Terrane, marks the last phase of tectonothermal event in the region and is widely correlated to the amalgamation of Gondwana supercontinent.The geological history of the Northern and Southern Granulite Terranes shows striking resemblance with the Antongil-Masora-Bemarivo and Antananarivo-Ikalamavony–Itremo domains of Madagascar, respectively. Formation of a unified Indo-Madagascar landmass since ca. 2600 Ma or earlier is consistent with the extant geological evidence. Precambrian rocks of Sri Lanka share the Ediacaran-Cambrian metamorphism with the Southern Granulite Terrane and the basements rocks of Madagascar. However, inadequate data from Sri Lanka does not allow us to infer if Sri Lanka was part of the coherent Indo-Madagascar landmass since late Archean.