Discovery of deerite from the Aksu Precambrian blueschist terrane and its geological significance

Deerite [F-12(2+) Fe-6(3+) Si-12 O-40 (OH)(10)] was the first discovered in the metamorphic magnetite-bearing quartzites which are interlayered with b lueschists in Aksu Precambrian blueschist terrane. The deerite-bearing mine ral association includes (1) deerite + nebeckite + stilpnomelane + ilvaite + magnetite + quartz and (2) deerite + stilpnomelane + magnetite + quartz b ased on the investigation under microscope. The study of mineral chemistry shows that the deerites from Aksu Precambrian blueschist are Mn-poor deerit e similar to that from Alps. But the deerite from Aksu is the nearest to th e end member of deerite. According to the equilibrium P-T region of deerite determined by Lattard and Breton (1994), the deerites in metamorphic magne tite-bearing quartzites from Aksu Precambrian blueschists were formed under the conditions of pressure lower than 1.0 Gpa, temperature ranging from 30 0 to 400 degrees C and about the 10 degrees C/km geothermal gradient. The deerite in metamorphic magnetite-bearing quartizes from Aksu blueschist s is the only one Precambrian deerite reported now. This suggests that the earth began to cool, and the modern cooling subduction regime between plate s started at Late Proterozoic. The Late Proterozoic may be the important pe riod during the evolution of the earth.