Crustal evolution and tectonic processes of the Madras Block, India

Abstract

The Neoarchaean (2.8–2.5 Ga) era in the earth’s history witnessed significant crustal growth related to the amalgamation and dispersion of ancient continents. The processes leading to the formation and destruction of the crust remain enigmatic. Thus, it becomes critical to investigate the Neoarchaean continental crust for understanding these crustal and tectonic processes operated at that time. The crustal blocks which are situated as a transitional zone in between the Southern Granulite Terrane (SGT) and the Dharwar Craton in India, are the well-preserved example of Neoarchaean terranes. The Madras Block is one of the least studied among different crustal blocks in this region. It consists mainly of charnockite, felsic-orthogneiss and meta-monzo-diorite, with a minor occurrence of amphibolite, meta-volcanic, and meta-pelitic rocks. The detailed petrographical, geochemical, and geochronological investigations on these basement rocks pointed out three petrogenetically distinct types (Type-A, -B, and -C) of charnockite from the Madras Block. The Type-A charnockite comprises felsic minerals, minor clinopyroxene and orthopyroxene, abundant biotite, and amphibole. Relative to Type-A charnockite, Type-B, and Type-C, charnockite is rich in garnet and anhydrous ferromagnesium minerals, respectively. Type-A shows a highly depleted HREE pattern ((La/Yb)N = ~36) with a higher degree of chemical fractionation compared to the Type-B ((La/Yb)N ~11) and Type-C ((La/Yb)N ~5). The present results demonstrate that the Type-A charnockite was formed by melting of the subducting oceanic slab and overriding Mesoarchaean proto-crust with a minor input of mantle wedge during 2.79–2.54 Ga. The combined melting of the underplated mantle and overriding Mesoarchean proto-crust along with mixing and homogenisation of Type-A charnockite, during 2.57–2.50 Ga, led to the formation of Type-B and -C charnockite. All the charnockites were subjected to regional metamorphism at ~2.50 Ga, as the prolonged upwelling and magmatism raised the temperature of the entire crust. The felsic-orthogneiss has a highly fractionated REE profile (La/YbN = ~42) resembling Archaean TTG suite of rocks. The meta-monzo-diorite shows a weakly fractionated REE pattern (La/Yb)N = ~17.6) with relatively higher HREE (∑HREE = ~88 ppm) similar to that of Type-B charnockite. The amphibolite which occurs as enclaves, have relatively flat REE pattern, while those occurring as larger volumes (outcrops) have comparatively high LREE. The similar initial 143Nd/144Nd (0.5091–0.5094) and initial 87Sr/86Sr (0.7016–0.7033), along with field and geochemical evidence, suggest that the felsic-orthogneiss and meta-monzo-diorite were formed from the same parental magma (basaltic in affinity) during 2.57–2.51 Ga. The REE geochemical modelling demonstrates a multi-stage magma generation starting from partial melting of a mantle-derived underplated basaltic source, followed by mixing and homogenisation with crustal felsic melt and different rate of continuous fractionation (25-50%) formed ~2.54 Ga felsic and mafic rock in the Madras Block. Thus, the basement rocks of the Madras Block were formed as the result of the repetitive intrusions of basaltic magma into the lower-crust over a long period of time. Consequently, the proto-underplated basaltic magma (~2.62 Ga) be thermally rejuvenated by another pulse of hotter magma and generated ~2.54 Ga felsic-mafic rocks. Synchronous late Neoarchaean magmatic events and associated metamorphism are also identified from the eastern block of the North China Craton (Yishui terrane, Shandong Peninsula), and the north-central Korean Peninsula (Daeijak Island, NW-Gyeonggi Massif). These two terranes and the southern India (Madras Block) demonstrate their close similarities in geological setting, age, petrochemistry, isotopic composition and metamorphic history. Hence, this study proposes that these terranes were once contiguous as part of a Neoarchaean supercontinent.