The Habutengsu metapelites and metagreywackes in western Tianshan, China: metamorphic evolution and tectonic implications

The HP-UHP metamorphic belt of western Tianshan in northwestern China is a rarely preserved oceanic UHP terrane which consists predominantly of meta-siliciclastic rocks, occasionally accompanied by lens-shaped metabasites. The metapelites and metagreywackes from the Habutengsu Valley and adjacent area within this belt contain quartz, albite, garnet, white mica, chlorite and rutile/titanite, with or without minor amounts of barroisite, glaucophane, clinozoisite, allanite, graphite, carbonate and tourmaline. Included in coarse-grained garnet, pseudomorphs of clinozoisite + paragonite after lawsonite are common, seldom also together with inclusions of chloritoid, jadeite and glaucophane. In the northern Habutengsu area, garnet is compositionally characterized by similar cores with consistently low-Ca content. Similar garnet armouring coesite has been reported in UHP schists from the same area. Deduced P-T conditions during formation of these Ca-poor garnet cores are 25-31 kbar and 430-510 degrees C, which are consistent with the computed stability of the observed assemblage Grt + Gln + Lws +/- Jd +/- Cld in the coesite stability field. Thus, the occurrences of the UHP metapelites and metagreywackes define an internally coherent UHP unit in the north of the Habutengsu area, the spatial extension of which is much larger than previously known. Compared with the northern ones, the southern metapelites and metagreywackes in the Habutengsu area consist of similar minerals and have similar bulk rock compositions but significantly different garnet chemistry, indicating an abrupt variation in P-T conditions during garnet growth. The derived conditions initiating the garnet growth for the southern rocks in a similar range (18-21 kbar and 450-500 degrees C) and thus constrain a coherent HP unit in the south of the Habutengsu area. The juxtaposition of two exhumed slices of contrasting metamorphic grades probably indicates the change of subduction dynamics of the palaeo-Tianshan oceanic crust, the subduction polarity (from south to north) of which accounts for the spatial relationship between these two units.