Phase equilibrium of low-pressure metamorphism in the Altaides, Xinjiang

Abstract：Study of the petrography and phase equilibria of typical metapelites from the low-pressure metamorphic sequence in the Altaides indicates that the P-T conditions are 445-550℃ and 0.2-0.6 GPa for the biotite zone, 480-566℃ and 0.54±0.22 GPa for the garnet zone, 601±20℃ and 0.8±0.25 GPa for the staurolite zone, 540±20℃ and 0.32±0.05 GPa for the staurolite-andalusite zone, 640℃ and 0.43 GPa for the sillimanite zone and 740-800℃ and 0.4-0.7 GPa for the cordierite-sillimanite zone. However, 632.4℃ and 0.785 GPa are not in the stable range of the andalusite but probably the medium-pressure metamorphic conditions of the orogenic belt in the early stage. For the 640℃ and 0.43 GPa of the sillimanite zone, as the garnet in the zone contains kyanite inclusions, it may also have undergone medium-pressure metamorphism. The low-pressure metamorphic sequence of the Altaides is not a normal metamorphic sequence and its field metamorphic gradient shows the “Z”-shaped character. The low-pressure metamorphism in the Altaides may have formed by compressional uplift of the medium-pressure metamorphic rocks at the early stage in a tectonic environment of extensive intrusion of collision-related granites.