Prograde P–T path of kyanite eclogites from Junan in the Sulu ultrahigh‐pressure province, eastern China

Abstract

AbstractKyanite‐bearing ultrahigh‐pressure (UHP) eclogites occur as blocks in orthogneisses at Yangzhuang, in the Junan area of the southwestern Sulu province, eastern China. Eclogites have variable bulk rock compositions, with Al2O3 = 16–27 wt%, FeO* + MgO = 6–22 wt% and CaO = 9–13 wt%. Major minerals are garnet, omphacite, phengitic white mica, zoisite, kyanite, rutile and an SiO2 phase. Fe‐rich staurolite (Mg ⇒ Mg# = 0.24 ± 0.01) and paragonite–margarite aggregates are rarely included in the cores of prograde zoned garnet. Metamorphic conditions ranged from 520 to 650°C and <1.4 GPa at an early prograde stage, and mostly reached 660–830°C and 2.7–3.5 GPa at the peak UHP stage. The estimated dP/dT of the prograde P–T path is less than 0.25 GPa/100°C at earlier stages and increases to 0.7–1.4 GPa/100°C just before the UHP stage. The kink of the prograde P–T path closely resembles the steady‐state P–T paths proposed, assuming a two‐parameter brittle‐plastic shear stress model. The estimated P–T path adequately explains the absence of prograde lawsonite and sodic amphibole and the common occurrence of coexisting zoisite, kyanite and sodic‐calcic amphibole in the UHP eclogites throughout the Sulu province. Simple clockwise prograde P–T paths for Sulu UHP eclogites proposed in earlier studies should be carefully re‐examined.