利用钠质角闪石成分环带计算蓝片岩的PTt轨迹——以新疆阿克苏前寒武纪蓝片岩为例

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

在新疆阿克苏前寒武纪蓝片岩中 ,普遍存在着具有环带结构的钠质角闪石 ,其典型的矿物组合为 :钠质角闪石 -绿帘石 -斜长石 -绿泥石 -石英 -黑硬绿泥石。矿物学研究表明 ,环带闪石主要为青铝闪石 ,其环带为生长环带。本文用 P- T格子的方法 ,利用青铝闪石环带 ,对阿克苏蓝片岩的变质 PTt轨迹进行了计算。结果得到一条退变轨迹 ,其温度变化于 4 6 0→ 180℃ ,压力变化于 0 .85→ 0 .39GPa,代表快速抬升过程 ;推测其进变轨迹与退变轨迹近似平行 ,属于 Franciscan型。青铝闪石环带的发现表明 ,在晚元古代地热梯度就已经开始降低 ,开始出现类似于现今的这种冷板块机制。

The occurrence of zoned sodic amphiboles is common in Aksu Precambrian Blueschists of Xinjiang, with the typical mineral assemblage: Na-Amp + EP + Ab + Chl + Qz + Stil. The study on mineralogy revealed that zoned amphiboles are 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 assumed that the prograde path parallels to the retrograde path. The path reflects a rapid lifting process and is of Franciscan type. The discovery of zoned crossite in Aksu Precambrian blueschists indicates that the geothermal gradient decreased and the cold plate mechanism occurred in late Proterozoic.