Ultrahigh-temperature decompression of sapphirine-granulites from the southern Madurai block (South India): Insights from geothermobarometry, pseudosection modelling and U-Pb geochronology

Abstract

Madurai block in South India stands as an excellent example of ultrahigh temperature metamorphic terranes being studied globally to understand the extreme thermal capacities of crustal rocks and the processes that cause these extreme thermal environments. The granulites of the Madurai block were subjected to extreme crustal metamorphism during the Late Neoproterozoic, as evidenced by the texture of the sapphirine + quartz association registered as peak metamorphic assemblage. The metamorphic zircon from sapphirine-granulites produces a U-Pb age of 546.5 ± 7.8 Ma which is interpreted as a global Pan-African crustal evolution event and the dominant zircon population indicates multiple sources of Neoarchean to Paleoproterozoic age in the southern Madurai block. Decompression textures other than orthopyroxene-cordierite ± sapphirine symplectites are indicative of retrograde / exhumation in the granulites metamorphic cycle. P–T pseudosections have been constructed for sapphirine-granulites in relation to the NCKFMASHTO system (Na2O-CaO-K2O-FeO-MgO-Al2O3–SiO2-H2O-TiO2-O2) using Perple_X software (version 6). The limitations in the interpretation of mineral reactions and the metamorphic history provided by pseudosection modelling for mineral proportions suggest that the peak metamorphic temperature is ~960 °C and pressure is ~9.2 kbar.