The PTt path calculation of blueschists on the compositional zonings of sodic amphiboles: An example from Aksu Precambrian blueschists of Xinjiang.

The occurrence of zoned sodic amphiboles is common in Aksu Precambrian Blue schists of Xinjiang, with the typical mineral assemblage: Na-Amp + EP + Ab + Chl + Qz + Stil. The study on mineralogy revealed that zoned amphiboles a re mainly crossites with growing zones. This paper calculated the PTt path of Aksu blueschists, using P-T grid method with crossite zones. The result is a retrograde path in which the temperature changed from 460 degreesC to 180 degreesC while the pressure changed from 0.85GPa to 0.39GPa. It is assu med that the prograde path parallels to the retrograde path. The path refle cts a rapid lifting process and is of Franciscan type. The discovery of zon ed crossite in Aksu Precambrian blueschists indicates that the geothermal g radient decreased and the cold plate mechanism occurred in late Proterozoic .