Lawsonite-bearing chloritoid–glaucophane schist from SW Tianshan, China: Phase equilibria and P– T path

Abstract

Lawsonite-bearing chloritoid–glaucophane schist has been discovered firstly in the high to ultra-high pressure (HP–UHP) metamorphic belt in Southwestern Tianshan, China. It is characterized by the mineral assemblage of lawsonite + chloritoid + glaucophane + phengite + paragonite + epidote/allanite + chlorite + quartz + dolomite/calcite + rutile + tourmaline. Lawsonite and chloritoid exclusively occur as micro-inclusions in the cores and mantles of porphyroblastic glaucophane. The glaucophane is coarse-grained (about 1–20 mm long), and forms euhedral to subhedral crystals with distinct growth zonation. The X(gln) [=Fe2 +/(Fe 2+ + Mg)] increases outwards from 0.40 to 0.63, indicating growth under decreasing temperature and pressure. Phase equilibrium modeling in the system of NCKFMASHO (Na 2O–CaO–K 2O–FeO–MgO–Al 2O 3–SiO 2–H 2O–O) by THERMOCALC 3.30 reveals that glaucophane was formed during retrograde metamorphism. Based on the compositional zonation of glaucophane and its mineral inclusions, the peak P– T conditions were 23.2–25.3 kbar and 499–519 °C, 17–20 kbar and 430–530 °C for the first retrograde stage (A) and 15–17 kbar and 470–530 °C for the second retrograde stage (B). The obtained retrograde P– T path indicates a rapid uplift involving an early isothermal decompressional stage similar to that of Alpine-type blueschists.