SHRIMP U-Pb zircon chronology of ultrahigh-temperature spinel-orthopyroxene-garnet granulite from South Altay orogenic belt, northwestern China

Diagnostic mineral assemblages, mineral compositions and zircon SHRIMP U-Pb ages are reported from an ultrahigh-temperature (UHT) spinel-orthopyroxene-garnet granulite (UHT rock) from the South Altay orogenic belt of northwestern China. This Altay orogenic belt defines an accretionary belt between the Siberian and Kazakhstan-Junggar Plates that formed during the Paleozoic. The UHT rock examined in this study preserves both peak and retrograde metamorphic assemblages and microstructures including equilibrium spinel + quartz, and intergrowth of orthopyroxene, spinel, sillimanite, and cordierite formed during decompression. Mineral chemistry shows that the spinel coexisting with quartz has low ZnO contents, and the orthopyroxene is of high alumina type with Al2O3 contents up to 9.3 wt%. The peak temperatures of metamorphism were > 950 degrees C, consistent with UHT conditions, and the rocks were exhumed along a clockwise P-T path. The zircons in this UHT rock display a zonal structure with a relict core and metamorphic rim. The cores yield bimodal ages of 499 +/- 8 Ma (7 spots), and 855 Ma (2 spots), with the rounded clastic zircons having ages with 490-500 Ma. Since the granulite was metamorphosed at temperatures > 900 degrees C, exceeding the closure temperature of U-Pb system in zircon, a possible interpretation is that the 499 +/- 8 Ma age obtained from the largest population of zircons in the rock marks the timing of formation of the protolith of the rock, with the zircons sourced from a similar to 500 Ma magmatic provenance, in a continental margin setting. We correlate the UHT metamorphism with the northward subduction of the Paleo-Asian Ocean and associated accretion-collision tectonics of the Siberian and Kazakhstan-Junggar Plates followed by rapid exhumation leading to decompression.