Amphibolite–granulite facies mid-crustal basement in Deccan Large Igneous Province and its implication on Precambrian crustal evolution: evidence from Killari borehole studies

The evolutionary nature of the Neoarchean basement, concealed below 66 Ma Deccan Large Igneous Province, remains largely unknown. We present here detailed results of integrated geoscientific investigations, carried out on 43 basement cores that cover an entire 270 m thick column, penetrated by the KLR-1 deep scientific borehole, drilled in the epicentral zone of 1993 Killari earthquake (Maharashtra, India). They dominantly contain high velocity-high density, halogen-rich, retrogressed and metasomatized amphibolite to granulite facies mid-crustal rocks, with intercalations of TTG, which were subjected to temperatures between 540 and 860 °C and pressure 5–7 kb, equating to 15–21 km emplacement depth. The granitic-gneissic layer, typical of the upper crust, appears absent. Geochemically, these iron-rich rocks are characterized by wide variation in which SiO2, MgO and FeOT contents vary from 45.12 to 69.96 wt% (avg. 58.11 wt%), 0.18–11.95 wt% (avg. 4.22 wt%), and 0.34–22.92 wt% (avg. 9.01 wt%) respectively. Measured FeOT contents are even higher than the normal lower crust. The granulite rocks exhibit tholeiite affinity, while amphibolite (which dominate the lithology) and TTG rocks show calc-alkaline character. Further, chondrite and primitive mantle normalized plots, show consistent and complimentary REE patterns (except in HREE), with negative Eu anomalies and LILE enrichment, indicating source rock fractionation. Well-marked depletion in Nb–Ta, Zr–Hf, subordinate depletion in Sr and Ba, and their distinct calc-alkaline to tholeiite character, indicate their subduction zone/arc-related affinity. In addition, large-scale crust-mantle interaction and pervasive metasomatism, have led to considerable alteration to the basic fabric of the rock thereby making it weak and characterized by reduced velocities due to biotitization and substantial Fe-enrichment. Quite likely, earthquake nucleation in intraplate stable regions may have a close link with mantle metasomatism.