Sodicgedrite in ultrahigh-temperature Mg-Al-rich rocks from the Palghat-Cauvery Shear Zone system, southern India

Abstract

We report high-Na gedrite (sodicgedrite) with up to 3.1 wt% Na2O (∼ 0.81 NaA pfu) in ultrahigh-temperature (UHT) Mg-Al-rich rocks from the northern margin of the Pan-African Madurai Block along the Palghat-Cauvery Shear Zone system, southern India. An amount of NaA of the gedrite increases with increasing AlIV. The sum of NaA and AlIV also increases linearly with decreasing Si suggesting the substitution of NaAAlIV ↔ Si. The negative correlation between Fe + Mg + Si and Al is probably due to the tschermak substitution ((Fe,Mg)VISiIV ↔ AlVIAlIV). The relatively coarse-grained nature and porphyroblastic occurrence of the sodicgedrite suggest that the mineral recrystallized at the peak UHT metamorphism. In contrast, retrograde gedrite with cordierite is less sodic with 1.1-1.6 wt% Na2O (NaA = 0.24-0.35 pfu). Our data suggest that high-Na content in gedrite is a possible indicator of UHT metamorphism.