含刚玉变沉积岩对扬子克拉通西北缘新元古代演化史的制约

Constraints of corundum bearing metasedimentary rocks on the Neoproterozoic evolution history of the northwest margin of the Yangtze Craton

厘清扬子西北缘出露的前寒武纪变质基底对于揭示扬子克拉通新元古代的大地构造演化历史和四川盆地的深层资源形成与赋存机制具有基础性意义。本文报道了在扬子克拉通西北缘米仓山地体中识别出的黑云母斜长片麻岩与含刚玉矽线堇青云母片岩。相平衡模拟研究认为刚玉的产生与深熔过程中SiO2的丢失有关。黑云母斜长片麻岩记录的峰期变质温压条件达到759℃~770℃,0.48~0.55 GPa(地温梯度约为46℃/km~53℃/km),含刚玉矽线堇青云母片岩的典型矿物组合(斜长石+钛铁矿+黑云母+堇青石+刚玉+矽线石)所记录的温压条件为580℃~620℃,0.36~0.46 GPa(地温梯度范围为35℃/km~43℃/km)。变质锆石年代学揭示了两组变质年龄分别为814±9 Ma、736±17 Ma,但两组变质年龄的关系还有待进一步厘清。结合前人研究成果,本文认为含刚玉变沉积岩进一步精细刻画了自~814 Ma后的弧-陆碰撞过程,高地温梯度可能与碰撞过程中伴随幔源物质参与有关,相应的碰撞过程进一步证实扬子克拉通位于罗迪尼亚超大陆周缘。

Clarifying the Precambrian metamorphic basement exposed on the northwest margin of the Yangtze Craton is of fundamental significance for revealing the tectonic evolution history of the Yangtze Craton in the Neoproterozoic and the formation and occurrence mechanism of deep resources in the Sichuan Basin.This article reveals petrogenesis of biotite plagioclase gneiss and corundum bearing sillimanite cordierite schist in the Micangshan terrane on the northwest edge of the Yangtze Craton.Combined with metamorphic research,it is believed that the formation of corundum bearing mica schist is related to the loss of SiO2 in the deep melting process of the coexisting biotite plagioclase gneiss.The temperature and pressure during the metamorphic peak recorded by the migmatitic biotite plagioclase gneiss reached 759℃~770℃,0.48~0.55 GPa(with a geothermal gradient of 46℃/km~53℃/km),while the typical mineral assemblage of corundum bearing sillimanite cordierite schist records the P-T conditions of 580℃~620℃,0.36~0.46 GPa(with a geothermal gradient of 35℃/km~43℃/km).Metamorphic zircons have reported two sets of metamorphic ages,814±9 Ma and 736±17 Ma.Based on previous research results,this article believes that the sedimentary rocks containing corundum have further finely characterized the arc-continent collision process since~814 Ma.The high temperature gradient may be related to the involvement of mantle derived materials during the collision process,and the corresponding collision process further confirms that the Yangtze Craton is located on the periphery of the Rodinia supercontinent.