Abstract:

 The study of ultrahightemperature (UHT) metamorphism has become a key topic in the field of earth sciences recently because the occurrence of such as orthopyroxenesillimanitequartz, sapphirinequartz, spinelquartz and osumilitequartz assemblages and high Al2O3bearing orthopyroxene could indicate the ultrahightemperature metamorphism (>900 ℃). We have studied the spinel+quartz associations, ZnO contents, XZn［Zn/(Mg+Fe2++Zn)］ and XMg of the spinels from the UHT rocks worldwide for the first time, and the study shows that the ZnO contents, XZn and XMg have some differences due to the different modes of spinels, and tectonic setting of the study areas; but ZnO contents with less than 25wt% from the spinels coexisting with quartz of the UHT rocks are generally common, indicating a UHT metamorphic condition. The study of ZnO contents, XZn and XMg in the spinels of the UHT granulite from the South Altay orogenic belt indicates that the ZnO contents of ca. 250 wt% and ca. 185 wt% from the spinels coexisting with quartz and ilmenite respectively can be obtained, and they are similar to those from the UHT rocks in the world. Combined with other specific mineral evidence, the coexisting spinelquartz association in the UHT rock from the South Altay orogenic belt can be  an evidence of the occurrence of UHT metamorphism. Based on the preliminary study of paragenesis of spinel+quartz and reactions among specific minerals, we propose that the metamorphism can be divided into two episodes: the peak metamorphism with the temperature condition of 1000 ℃ deduced and the retrograde metamorphism, and suppose a clockwise pT path for the formation of the UHT rock from the South Altay orogenic belt. The formation of the UHT rock should be genetic link with the subduction of the PaleoAsian Ocean and collision and orogeny of the Siberian plate and KazakhstanJunggar plate probably during the Paleozoic.

Key words: Key words: spinel composition, spinelquartz paragenesis, reaction episode of metamorphic minerals, ultrahightemperature granulite, Altay orogenic belt