Phase equilibria constraints on the stability of garnet in mafic granulite: An example from Karimnagar granulite terrain, Eastern Dharwar Craton, India

Abstract

Phase equilibria modeling becomes a widely accepted tool to constrain the P-T conditions experienced by the metamorphic terrain because of its advantage over the earlier methods e.g., conventional methods and petrogenetic grids. In this study, garnet stability in the mafic granulite has been examined in response to bulk composition and P-T conditions. Karimnagar granulite terrain (KGT) consists of garnet-free mafic granulite along with garnet-bearing metapelite and silica deficient Mg-Al granulite as an enclave within granite gneiss. Mafic granulite consists of amphibole, orthopyroxene, clinopyroxene, and plagioclase with a minor modal amount of biotite and quartz. Mafic granulite has experienced three distinct metamorphic stages: (a) prograde stage defined by inclusions of minerals like amphibole and quartz within the orthopyroxene and augite, (b) peak stage characterized by coarse grain association of amphibole, orthopyroxene, augite, plagioclase, and minor quartz, (c) the retrograde stage illustrated with coronal growth of amphibole over orthopyroxene and augite. The phase equilibria diagrams were calculated in the Na₂O-CaO-K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O-TiO₂-O₂ (NCKFMASHTO) model system using bulk rock compositions. Our results show that mafic granulite of the study area has achieved the highest P-T condition of 7 kbar and 800 °C and followed a clock-wise metamorphic trajectory. The calculated P-T pseudosection for two samples indicates the stability of garnet varies between 7.5 and 9.5 kbar at 800 °C. The calculated P-Mo pseudosection (at 800 °C) illustrates the lower stability limit of garnet further goes down with the increase in Al₂O₃ and FeO in the bulk composition.