Petrogenesis and U-Pb zircon age of the Anorthosite-Gabbronorite association near Barabar Hill, Chotanagpur Granite Gneiss Complex (CGGC), eastern Indian shield

Massif-type anorthosite intrusions represent critical archives of Proterozoic mantle-crust interactions, crustal accretion and reworking processes. These voluminous, plagioclase-dominated cumulates were emplaced predominantly during the Statherian-Tonian period (1.8–0.9 Ga), and temporally associated with the assembly of Columbia and Rodinia supercontinents. Despite their widespread occurrences during the restricted time interval, the petrogenesis of massif-type anorthosite remains contentious. Competing models invoke derivation from upwelling mantle melts, partial melting of lower crust and/or magmatism within subduction-related arc settings. Their enigmatic origin and temporal confinement highlight their significance in deciphering Proterozoic tectono-magmatic evolution. The Barabar Hill anorthosite-gabbronorite association, located in the northern Chotanagpur Granite Gneissic Complex (CGGC) of Bihar (India), represents a previously undocumented Paleoproterozoic anorthosite magmatism that sheds new light on the genesis of anorthosites and the overall crustal evolution of the eastern Indian Shield. This study integrates field observations, petrography, bulk-rock major and trace element geochemistry, mineral chemistry, and U–Pb zircon geochronology to constrain the (1) source, (2) parental melt composition, (3) role of crustal contamination, if any, and (4) emplacement age of the Barabar Hill anorthosite − gabbronorite association. Thermobarometric calculations suggest that clinopyroxenes in the gabbronorites crystallized at 1100–1175 °C and 0.04–2.30 kbar, and zircons in the anorthosites crystallized at 600–765 °C under FMQ buffer conditions. Equilibrium Distribution Method (EDM) constrain parental melt compositions at TMF = 0–20 % for the anorthosites and 15–30 % for the gabbronorites. Assimilation–Fractional Crystallization (AFC) modelling suggests derivation of the anorthosites from a depleted mantle source with significant contributions from upper crustal materials.

U-Pb zircon geochronology constrains the emplacement age of the Barabar Hill suite to 1760.8 ± 7.3 to 1762.0 ± 3.8 Ma (Statherian period), thus representing the first record of Paleoproterozoic anorthosite magmatism in the eastern Indian Shield. The Barabar Hill anorthosite represents a globally significant example of subduction-related magmatism, distinct from Archean megacrystic and Proterozoic massif-type anorthosites. The findings of this study suggests that anorthosites massifs were formed from basaltic magmas derived from the depleted mantle which was contaminated by upper crustal granitic materials. A hybrid mantle-crust origin for anorthosites expand our current understanding of Proterozoic crustal evolution.