南秦岭构造带十堰—武当地区岩石变质作用

Metamorphism of Rocks in Shiyan-Wudang Area of South Qinling Tectonic Belt, China

十堰—武当地区位于南秦岭构造带和扬子板块的结合部位,内部发育的变质岩可为秦岭造山带印支期造山过程提供重要信息。在该地区含石榴子石片岩的岩石学、矿物学研究基础上,利用石榴子石-黑云母地质温度计和石榴子石-白云母地质温度计计算变质作用条件。结果表明:石榴子石-黑云母地质温度计计算得到方滩—王家山剖面南、北两侧变质岩的主期变质温度分别为344℃~410℃和368℃~431℃,属于低绿片岩相;由石榴子石-白云母地质温度计计算得到十堰—大木剖面南、北两侧变质岩主期变质温度分别为322℃~440℃和535℃~540℃,属于低—高绿片岩相。结合区域变质变形研究成果,认为十堰—武当地区中低级变质岩的变质作用演化与南秦岭构造带印支期洋盆闭合伴生的俯冲-增生过程有关,它使得不同变质级别的岩石发生并置然后一起抬升。

Shiyan-Wudang area is located at the junction of South Qinling tectonic belt and Yangtze plate. Its metamorphism can provide important information for Indosinian orogenic process of Qinling orogenic belt. Based on the study of petrology and mineralogy of garnet-bearing schist in Shiyan-Wudang area, the metamorphic conditions were calculated by using garnet-biotite geothermometer and garnet-muscovite geothermometer. The results show that main metamorphism temperatures of metamorphic rocks from south and north parts of Fangtan-Wangjiashan section calculated by garnet-biotite geothermometer are 344 ℃-410 ℃ and 368 ℃-431 ℃, which are in agreement with lower greenschist facies;meanwhile, main metamorphism temperatures of metamorphic rocks from south and north parts of Shiyan-Damu section calculated by garnet-muscovite geothermometer are 322 ℃-440 ℃ and 535 ℃-540 ℃, agreeing with lower to higher greenschist facies condition. Combining with regional metamorphism and deformation data, it is suggested that metamorphic evolution of middle- and low-grade metamorphic rocks in Shiyan-Wudang area is related to the subduction-accretion process caused by the closure of ocean in South Qinling tectonic belt during Indosinian, which juxtaposes different grades of metamorphic rocks and makes them uplift together.