Metamorphic phase relations in orthopyroxene-bearing granitoids: implication for high-pressure metamorphism and prograde melting in the continental crust

Abstract

In this work, the factors controlling the formation and preservation of high‐pressure mineral assemblages in the metamorphosed orthopyroxene‐bearing metagranitoids of the Sandmata Complex, Aravalli‐Delhi Mobile Belt (ADMB), northwestern India have been modelled. The rocks range in composition from farsundite through quartz mangerite to opdalite, and with varying K2O, Ca/(Ca + Na)rock and FeOtot + MgO contents. A two stage metamorphic evolution has been recorded in these rocks. An early hydration event stabilized biotite with or without epidote at the expense of magmatic orthopyroxene and plagioclase. Subsequent high‐pressure granulite facies metamorphism (∼15 kbar, ∼800 °C) of these hydrated rocks produced two rock types with contrasting mineralogy and textures. In the non‐migmatitic metagranitoids, spectacular garnet ± K‐feldspar ± quartz corona was formed around reacting biotite, plagioclase, quartz and/or pyroxene. In contrast, biotite ± epidote melting produced migmatites, containing porphyroblastic garnet incongruent solids and leucosomes. Applying NCKFMASHTO T–M(H2O) and P–T pseudosection modelling techniques, it is demonstrated that the differential response of these magmatic rocks to high‐pressure metamorphism is primarily controlled by the scale of initial hydration. Rocks, which were pervasively hydrated, produced garnetiferous migmatites, while for limited hydration, the same metamorphism formed sub‐solidus garnet‐bearing coronae. Based on the sequence of mineral assemblage evolution and the mineral compositional zoning features in the two metagranitoids, a clockwise metamorphic P–T path is constrained for the high‐pressure metamorphic event. The finding has major implications in formulating geodynamic model of crustal amalgamation in the ADMB.