The Nilgiri enderbites, South India: nature and age constraints on protolith formation, high-grade metamorphism and cooling history

Abstract

The trident-shaped Nilgiri highland massif represents the deepest level exposure (9–10 kbar; c. 35 km palaeo-depth) of granulite grade lower crust in southern Peninsular India. The crustal section is essentially composed of garnetiferous enderbitic granulites with minor kyanite-bearing gneisses, quartzites and banded magnetite quartzites. Its deeper, northern part is intruded by intermediate and basic magmatic rocks now represented by non-garnetiferous enderbites, garnetiferous basic granulites, gabbroic to anorthositic two-pyroxene granulites and pyroxenites. The garnetiferous enderbitic granulites are tonalitic in composition, displaying a typical calc–alkaline trend, and exhibit chemical features of both I-type as well as S-type granitoids. They show moderate enrichment of LREE (La N 35–70), no to weakly negative Eu anomalies, and fairly constant levels of HREE (Yb N 6.5–13.0), and resemble the REE spectra of tonalitic–trondhjemitic granitoids and greywackes of late Archaean to Palaeoproterozoic age. Sedimentary protoliths are indicated by negative DF values ranging from −1 to −4, Al 2O 3/(CaO+Na 2O+K 2O) molar ratios >1.1, high δ 18O whole-rock values (10–12‰) which match the values reported for greywackes. Whole-rock isotope data for garnetiferous enderbites define linear arrays in the Rb–Sr and Sm–Nd evolution diagrams (Rb–Sr system: 2460±81 Ma with Sr i 0.7030±5; Sm–Nd system: 2506±70 Ma with Nd i 0.509363±60), implying a provenance of the sedimentary precursors that was dominated by felsic volcanic rocks or tonalitic–trondhjemitic granitoids of uniform isotopic composition and early Proterozoic age. The U–Pb systematics of composite zircon grains with detrital cores and metamorphic overgrowths, in accord with the Sm–Nd and Rb–Sr errorchron ages and Sm–Nd garnet–whole-rock data, suggest that formation of the provenance, deposition of the sedimentary precursors and subsequent high-grade tectono-metamorphic processes must have occurred in the short time span between 2600 and 2500 Ma. Based on the regional geological framework, the petrological, geochemical and isotopic data, the Nilgiri granulite terrain is interpreted as a metasedimentary allochthonous unit that was thrust onto the Dharwar Craton during Palaeoproterozoic collisional tectonics.