Incipient charnockite formation in the Trivandrum Block, southern India: Evidence from melt-related reaction textures and phase equilibria modelling

Abstract

The formation of incipient charnockite is significant for understanding the deep crustal process, which, however, remains under broad controversy. Patches of incipient charnockite occur ubiquitously in garnet-biotite gneiss in southern India. Here, we present new textural observations, geochemical analyses and phase equilibria modelling on the incipient charnockite and its garnet-biotite gneiss host from the Irinrayam quarry in the Trivandrum Block, with the aim to provide some new insights into the petrogenesis of incipient charnockite. Both rock types have similar bulk compositions which are characterized by high SiO2 (c. 70 wt%) and K2O (5.5–6.2 wt%), and low K/Rb (c. 150). Both rock types share similar peak mineral assemblage of garnet, K-feldspar, plagioclase, quartz, ilmenite and biotite except for the presence of orthopyroxene in the incipient charnockite. Phase equilibria modelling and thermobarometers reveal that these rocks record similar peak P–T conditions (870–900 °C, 6.2–7.2 kbar) followed by extensive cooling. Nonetheless, temperature versus mole H2O (T–M(H2O)) pseudosections show that orthopyroxene-bearing mineral assemblage is stable at lower M(H2O) than that without orthopyroxene. The post-peak cooling stage is characterized by fine-grained secondary garnet + quartz ± biotite ± feldspar symplectite in the incipient charnockite, but by simple biotite + quartz symplectite in the garnet-biotite gneiss. The garnet-bearing symplectite overgrows around peak ferromagnesian minerals in the incipient charnockite, corresponding to the increase of garnet proportion with the consumption of anatectic melt in the phase equilibria modelling. Garnet in the symplectite is extremely depleted in heavy rare earth elements (HREEs), which might be inherited from the anatectic melt and other reactants. These phase equilibria modelling and garnet REE results suggest that the garnet-bearing symplectite is a melt-related retrograde texture. The appearance of anhydrous garnet during cooling indicates that the system of the incipient charnockite, especially the anatectic melt itself, is more anhydrous than that of the garnet-biotite gneiss. Simple process of fluid-absent melting and melt loss was likely insufficient to dehydrate the melt itself, and we propose that the infiltration of low-water-activity fluid is likely a significant process to reduce the water activity of the whole petrogenetic system, leading to the incipient charnockite formation in this area.