Multistage orthopyroxene formation in ultrahigh-temperature granulites of Ganguvarpatti, southern India: implications for complex metamorphic evolution during Gondwana assembly

Abstract

Orthopyroxene-sillimanite-quartz granulites of Ganguvarpatti (GVP) from the northern Madurai Block in southern India record ultrahigh-temperature (UHT) metamorphism and complex exhumation history. Orthopyroxenes in different textural setting in the GVP rocks shows marked variation in Al₂O₃ and X_Mg contents. Inclusions, cores of porphyroblasts and some of the discrete grains in the matrix possess the highest Al₂O₃ contents (8.5-9.4 wt%). Orthopyroxenes occurring as fine to medium grained symplectitic intergrowths with cordierite show Al₂O₃ values ranging from 7.6-8.2 wt%. Those which form symplectites with sapphirine have intermediate Al₂O₃ values (6.9-7.5 wt%) while orthopyroxenes associated with spinel possess values between 7.3-7.5 wt%. Late orthopyroxene rim on biotite shows the lowest Al₂O₃ range between 5.9-6.5 wt%. Orthopyroxenes from various textural associations define distinct P-T slots in the Al in orthopyroxene thermometry based on the X_Mg-X_Al isopleths. Thus, orthopyroxene porphyroblasts and inclusions yield temperatures around 1050°C and pressure above 10 kbar. Orthopyroxene composition in orthopyroxene-cordierite symplectite yields temperatures of 950-990°C at pressure above 8 kbar and those associated with sapphirine and spinel symplectites show 850-900°C at a pressure around 8 kbar. The late orthopyroxene rim around biotite yields temperatures around 850°C at 6 kbar. The UHT conditions attained by these rocks are also corroborated by garnet-orthopyroxene thermometry, which provides an upper estimate of 1050 ± 50°C. The GVP rocks preserve a series of mineral reactions indicating ultrahigh-temperature decompression. The reaction textures when evaluated in KFMASH petrogenetic grid indicate two stages of isobaric cooling and decompression, suggesting a complex exhumation history. The textures and mineral reactions observed in this study in conjunction with the available age data suggest that the multistage exhumation of the UHT rocks in Ganguvarpatti could be related to the orogenic events involving continent collision, crustal thickening and extensional collapse accompanying the step-wise suturing of Gondwana supercontinent during the late Neoproterozoic.