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 the 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 modeling 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.