U–Th–Pb monazite geochronometry of the Eastern Ghats Belt, India: Timing and spatial disposition of poly-metamorphism

Abstract

The Eastern Ghats Belt of peninsular India exposes the deeply eroded high-grade parts of a composite orogenic belt that, including the Rayner Complex in Antarctica, once formed an important crustal component of Proterozoic East Gondwana. Recent work established four major crustal units separated by tectonic boundaries: the Late Archaean Jeypore and Rengali Provinces (northwest and north), the Late Palaeoproterozoic Ongole Domain (southwest) and the Meso-Neoproterozoic Eastern Ghats Province (east). To identify the timing and regional distribution of tectonothermal events within the composite belt, monazites in 66 pelitic and felsic granulite samples from 26 localities were studied with the electron microprobe chemical dating technique. In most cases the monazites are polygenetic with rock- and grain-scale chemical and chronological heterogeneities that document growth during episodes of partial melting and significant modifications through strain- and fluid-induced recrystallisation and replacement. In the Ongole Domain, monazite apparent age populations show that the fabric-defining high- to ultrahigh-grade metamorphism occurred between 1650 and 1590 Ma, after the emplacement of basic and felsic plutonic complexes into the supracrustal granulites at ca. 1.7 Ga. The monazites do not record pre-intrusion tectonothermal events, but give evidence of later imprints of ductile to brittle deformation accompanied by hydration between 1450 and 1350 Ma that are correlated with Mesoproterozoic crustal extension, rifting and alkaline plutonism along the eastern cratonic margin. The Ongole Domain was not involved in Grenvillian crustal reworking, and Pan-African tectonism (∼520 Ma) was focused in shear zones, especially at tectonic boundaries. In the Eastern Ghats Province, monazite apparent age populations in the time span between 1260 and 500 Ma witness a prolonged and polyphase tectono-metamorphic evolution during Mesoproterozoic and Neoproterozoic–Early Phanerozoic times. Apparent ages of high-Y monazite grains armoured in garnet of khondalites and high-MgAl granulites bracket the timing of early ultra-high temperature metamorphism between 1250 and 1100 Ma. Dominant and comparable apparent age populations of matrix monazites in metapelitic and felsic granulites indicate that the fabric-defining granulite facies metamorphism and anatexis of the entire terrane occurred between 980 and 940 Ma, synchronous with the intrusion of voluminous megacrystic granitoid plutons. The easternmost Chilka Lake domain experienced renewed pervasive deformation and high-grade metamorphism and melting at ca. 800 Ma coeval with emplacement of massif-type anorthosite, and further high-grade reworking during Neoproterozoic tectonism (680–650 Ma). Apparent ages of 530–470 Ma recorded in matrix monazites of strongly sheared mylonitic granulites indicate that the internal segmentation and structural configuration of the Eastern Ghats Belt and its cratonic forelands occurred during late Neoproterozoic–Early Phanerozoic times through W- to NW-directed intracratonic deformation focussed on major shear zones and terrane boundaries, when the Grenvillian Eastern Ghats Province–Rayner Complex terrane collided with India.