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

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) [=Fe(2+)/ (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(2)O-CaO-K(2)O-FeO-MgO-Al(2)O(3)-SiO(2)-H(2)O-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 degrees C, 17-20 kbar and 430-530 degrees C for the first retrograde stage (A) and 15-17 kbar and 470-530 degrees 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. (C) 2011 Elsevier Ltd. All rights reserved.