Identifying Ultrahigh-Temperature Metamorphism in Basic Granulites

Abstract

Identifying ultrahigh-temperature (UHT) metamorphism in basic granulites is challenged by the uncertainty of peak temperatures. Consequently, available temperature indicators are desiderated all along. In this study, we investigated low-pressure basic granulites from Daqingshan, North China Craton, and determined a UHT peak condition around 1000–1050 °C and 0.8–0.9 GPa. This peak condition was constrained by combining the AlT of 0.10–0.12 in clinopyroxene (Cpx) cores, maximum Ti of 0.36–0.38 in amphibole (Amp) cores and anorthite proportion (XAn) of 0.67–0.70 and 0.61–0.63 in plagioclase (Pl) cores/mantles analyzed in two selected samples. The P–T records obtained from several thermobarometers related to amphibole, plagioclase and/or clinopyroxene were also verified to be in agreement with the phase equilibria modelling results. Notably, AlT-in-Cpx is assessed to be a reliable temperature indicator in basic granulites with the Opx–Cpx–Pl–Amp assemblage, albeit could be influenced by bulk-rock compositions in some extent and has a temperature standard error of ~70 °C. Besides, the post-peak evolution was dominated by near isobaric cooling at 1.94–1.93 Ga, to a fluid-absent solidus as high as ~940 °C. During the cooling process, plagioclase and amphibole grew together at the expense of Al-rich clinopyroxene and unsegregated melts, accompanied by the (rimward) decrease of AlT-in-Cpx, Ti-in-Amp and XAn-in-Pl. A pre-peak process is inferred to be decompressional heating. The UHT metamorphism in Daqingshan is interpreted to result from asthenosphere upwelling and thermal advection from regional mantle-derived magmatism during post-orogenic extension, in a newly developed back-arc-related environment.