Origin of multilayer corona textures in mafic granulites from the Sandmata Complex, Aravalli Craton (northwestern India): petrological characteristics and tectonic implications

Abstract

In Late Proterozoic mafic granulites of the Sandmata Complex in Rajasthan, northwestern India, multilayer corona textures were formed along the interface between orthopyroxene and plagioclase. We examined the metamorphic conditions and processes of formation of these coronae, which provide an insight into the interplay between steady state and sequential diffusion-controlled mineral growth mechanisms. The individual corona–symplectite layers consist of clinopyroxene + quartz|garnet + clinopyroxene|garnet + quartz|K-feldspar, from the inner to the outer margins of the coronae. The single-value decomposition models suggest that the multilayered coronae were formed in a locally closed system, via sequential diffusion of Mg, Fe and Ca into the reaction zone, which has acted as the main driving force for the growth of corona textures. The relict orthopyroxene with exsolved clinopyroxene yields primary crystallization conditions of 8.4 ± 1.5 kbar and ~ 1100–1000 °C. Clinopyroxene in the innermost corona layer grew at ~ 9 kbar and 850–800 °C, whereas clinopyroxene + garnet grew outward at ~ 8 kbar and 700–600 °C. Subsequent hydrous retrogression (~ 6 kbar and 600–550 °C) resulted in the development of rimward zoning in garnet and the growth of amphibole. On combining textural relations and the above conventional P–T estimates, a near-isobaric cooling P–T path was reconstructed using phase equilibria modeling. Further, the near-isobaric cooling path is consistent with the magmatic underplating hypothesis in the Sandmata Complex, where the intrusion of magmatic bodies (i.e., Gyangarh–Asind igneous complex and Anjana granite) favored the development of granulite facies assemblage in norite and gabbronorite protoliths.