New evidence for Palaeoproterozoic high grade metamorphism in the Trivandrum Block, Southern India

• Published in: Precambrian research 2016-07, Vol.280, p.120-138
• Main Authors: Harley, Simon L., Nandakumar, V.
• Format: Article
• Language: English
• Subjects: Geochronology; Gondwana; Monazite; Trivandrum Block; Zircon
• ISSN: 0301-9268; 1872-7433
• DOI: 10.1016/j.precamres.2016.04.018

Harley&Nandakumar Precamb Res 2016. [Display omitted] •Zircon demonstrates 1.92Ga high-T metamorphism in the Trivandrum Block.•Monazites preserve a vestige of this event.•The high-T event is older than some of the basement orthogneisses in the Block.•The overprinting Neoproterozoic-Cambrian granulite was long-lived (45Myr).•At least some of the paragneisses in the Trivandrum Block are polymetamorphic. In-situ zircon U–Pb isotopic and monazite Th–U–Pb chemical ages from leucosome in migmatitic paragneiss at Kanjampara in the Trivandrum Block, India, demonstrate the occurrence of high-T metamorphism and anatexis at 1.92Ga. This Palaeoproterozoic tectonothermal event was strongly overprinted by the main Neoproterozoic-Cambrian granulite facies tectonism which itself involved significant partial melting and formation of garnet-bearing assemblages at 0.65GPa and >820–860°C. Monazite grains, which enclose zircon grains and locally preserve Palaeoproterozoic chemical age domains, were extensively reset and recrystallized at 565±6Ma. These monazite grains were further modified to form high-Th cuspate rims at 517±15Ma, equivalent to the lower intercept age for extensive Pb-loss from the highly discordant 1.92Ga zircon, 528±18Ma. These results confirm that at least some of the metasedimentary paragneisses in the Trivandrum Block are polymetamorphic, initially metamorphosed in the Palaeoproterozoic in an event older than the 1.89–1.85Ga granitic orthogneisses recognised from the area. The nature of the relationship between the paragneisses and older (2.0–2.1Ga) granitic and charnockitic orthogneisses of the Trivandrum and Nagercoil Blocks, in particular whether they share the same Palaeoproterozoic history or were interleaved after 1.92Ga, requires further investigation focussed on the presence or absence of the isotopic imprint of this event in both rock suites throughout the area. The growth or recrystallisation of monazite at ca. 565Ma and its further modification at ca. 520Ma indicates that the high-T metamorphism in the Pan-African was long-lived, with a duration of at least 45Myr. This adds further weight to recent proposals, based on zircon-monazite isotopic studies from other localities in the Trivandrum Block, that the region formed the mid- to deep-crust of a long-lived collisional hot orogenic belt that welded Gondwana from ca. 580–510Ma.