鲁西地区新太古代晚期正长-二长花岗岩成因及地质意义

Genesis and Geological Significance of Late Archean Syeno-monzogranite in Western Shandong Province

鲁西地区位于华北克拉通东部,是研究区域太古宙演化的重要地区。以鲁西地区西南部出露的正长-二长花岗岩为研究对象,系统开展了岩石学、地球化学、锆石U-Pb年龄与Hf同位素组成研究。结果表明,该正长-二长花岗岩为钙碱性、过铝质。锆石U-Pb年龄揭示正长-二长花岗岩侵位于~2540~2530 Ma;锆石Hf同位素组成表明其主体由~2.70 Ga的TTG类岩石熔融形成,但再循环的古老地壳组分也参与了岩浆产生过程。微量元素和同位素组成模拟计算结果表明,正长-二长花岗岩是由97%的新生TTG地壳组分部分熔融和3%的变质表壳岩部分熔融物混合形成。结合区域特征,我们认为正长-二长花岗岩形成于伸展构造环境中,该区新太古代晚期岩浆作用与板片俯冲后回撤作用有关,板片回撤过程所致的软流圈上涌为此类岩石的岩浆产生提供了热源。

The Western Shandong is located in the east of North China Craton, which is an important area to study the Archean evolution.Petrology, geochemistry and zircon U-Pb dating and Hf isotopic composition were studied on the syeno-monzogranite in the southwest of Western Shandong Province.The syeno-monzogranite is characterized by calc-alkaline and peraluminous affinities.Zircon U-Pb dating shows that the syeno-monzogra-nite emplaced at~2 540 Ma to~2 530 Ma.Zircon Hf isotopic compositions show that they were primarily molten by TTG rocks of~2.70 Ga, and there is recycled ancient crustal component. The mixing calculation results under constraints of isotopic and trace element compositions confirm that the primary magma of syeno-monzogranite is composed of the mixing of 97% of the juvenile TTG crustal material after partial melting and 3% of metamorphic supracrustal rocks after partial melting.Combined with previous studies, we propose that the syeno-monzogranite was formed in an extensional tectonic environment.The late Neoarchean magmatism in Western Shandong was related to slab rollback.The upwelling of asthenosphere caused by the rollback provided heat source for the formation of this group of rocks.