Temperature of prograde metamorphism, decompressional partial melting and subsequent melt fractional crystallization in the Weihai migmatitic gneisses, Sulu UHP terrane: Constraints from Ti-in-zircon thermometer

Abstract

In order to constrain temperature during subduction and subsequent exhumation of felsic continental crust, we carried out a Ti-in-zircon thermometer coupled with zircon internal structure and U-Pb age on migmatitic gneisses from the Weihai (威海) region in the Sulu (苏鲁) ultra-high pressure (UHP) metamorphic terrane, eastern China. The Weihai migmatitic gneisses are composed of intercalated compositional layers of melanosome and plagioclase (Pl)-rich leucosome and K-feldspar (Kfs)-rich pegmatite veins. Four stages of zircon growth were recognized in the Weihai migmatitic gneisses. They successively recorded informations of protolith, prograde metamorphism, decompressional partial melting during early stage exhumation and subsequent fractional crystallization of primary melt during later stage cooling exhumation. The inherited cores in zircon from the melanosome and the Pl-rich leucosome suggest that the protolith of the migmatitic gneiss is Mid-Neoproterozoic (∼780 Ma) magmatic rock. Metamorphic zircons with concordant ages ranging from 243 to 256 Ma occur as overgrowth mantles on the protolith magmatic zircon cores. The estimated growth temperatures (625–717 °C) of the metamorphic zircons have a negative correlation with their ages, indicating a progressive metamorphism in HP eclogite-facies condition during subduction. Zircon recrystallized rims (228±2 Ma) in the Pl-rich leucosome layers provide the lower limit of the decompresssional partial melting time during exhumation. The ages from 228±2 to 219±2 Ma recorded in the Pl-rich leucosome and the Kfs-rich pegmatite vein, respectively, suggest the duration of the fractional crystallization of primary melt during exhumation. The calculated growth temperatures of the zircon rims from the Pl-rich leucosome range from 858 to 739 °C, and the temperatures of new growth zircon grains (219±2 Ma) in Kfs-rich vein are between 769 and 529 °C. The estimated temper atures have a positive correlation with ages from the Pl-rich leucosome to the Kfs-rich pegmatite vein, strongly indicating that a process of fractional crystallization of the partial melt during exhumation.