Muscovite Dehydration Melting in Silica-Undersaturated Systems: A Case Study from Corundum-Bearing Anatectic Rocks in the Dabie Orogen

Abstract

Corundum-bearing anatectic aluminous rocks are exposed in the deeply subducted North Dabie complex zone (NDZ), of Central China. The rocks consist of corundum, biotite, K-feldspar and plagioclase, and show clear macro- and micro-structural evidence of anatexis by dehydration melting of muscovite in the absence of quartz. Mineral textures and chemical data integrated with phase equilibria modeling, indicate that coarse-grained corundum in leucosome domains is a peritectic phase, reflecting dehydration melting of muscovite through the reaction: Muscovite = Corundum + K-feldspar + Melt. Aggregates of fine-grained, oriented, corundum grains intergrown with alkali feldspar in the mesosome domains are, instead, formed by the dehydration melting of muscovite with aluminosilicate, through the reaction: Muscovite + Al-silicate = Corundum + K-feldspar + Melt. P-T pseudosections modeling in the Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2 system constrains peak pressure-temperature (P-T) conditions at 900–950 °C, 9–14 kbar. The formation of peritectic corundum in the studied rocks is a robust petrographic evidence of white mica decompression melting that has occurred during the near-isothermal exhumation of the NDZ. Combined with P-T estimates for the other metamorphic rocks in the area, these new results further confirm that the NDZ experienced a long-lived high-T evolution with a near-isothermal decompression path from mantle depths to lower-crustal levels. Furthermore, our new data suggest that white mica decompression melting during exhumation of the NDZ was a long-lasting process occurring on a depth interval of more than 30 km.