西藏羌塘达尔应强过铝花岗岩地球化学特征及地球动力学意义

Geochemistry and geodynamics of the strongly peraluminous granites in Darying,Qiangtang,Xizang

关键词:  羌塘  强过铝花岗岩  地球化学  西藏

Key words:  Qiangtang  strongly peraluminous granite  geochemistry  Xizang

摘要:

羌塘地块中部达尔应首次发现强过铝质的二云母碱长花岗岩、白云母碱长花岗岩、电气石白云母碱长花岗岩,通过地球化学研究显示其SiO₂、Al₂O₃和K₂O质量分数均很高,以高铝、低镁铁为特征,A/CNK比值大于1.1,属典型的含白云母强过铝质花岗岩。稀土元素总量为(10.65~100.02)×10^-6,配分曲线显示铕负异常明显。Nb、P、Ti等高场强元素具有负异常,而Rb、Th、Y等大离子亲石元素具有明显的正异常。岩浆源区可能以粘土岩为主,是成熟陆块部分熔融作用的结果。微量元素标准化蛛网图、岩石组合R₁-R₂图解、Rb-Hf-Ta判别图、Rb-(Yb+Nb)和Rb-(Yb+Ta)图解均指示达尔应岩体是产生于同碰撞环境的花岗岩,它是冈玛错构造带在碰撞阶段过程中的岩浆岩记录。

Abstract:

Strongly peraluminous two-mica alkali-feldspar granite, muscovite alkali-feldspar granite and tourmaline muscovite alkali-feldspar granite recognized, for the first time, in Darying, central Qiangtang are characterized by high fractions of SiO₂, Al₂O₃ and K₂O, high Al, low Mg, low Fe, A CNK ratios >1.1, indicating the typical muscovitebearing strongly peraluminous granites.ΣREE values range between (10.65-100.02)×10^-6 and The REE distribution patterns exhibit marked negative Eu anomalies.The high field strength elements Nb, P and Ti display negative anomalies, while the macro-ionic lithphile elements Rb, Th and Y display positive anomalies. The magma sources may be originated from clay-rich sources, representing the products of partial melting of mature continental masses. The plot of chondrite-normalized trace element patterns, R₁-R₂ diagram, Rb-Hf-Ta discriminant diagram, Rb-(Yb+Nb)and Rb-(Yb+Ta)diagrams indicate that the Darying granites are assigned to the syn-collisional granites, and have recorded the magmatic history of the Gangmar Lake structural zone during the collisional stages. The discovery of the strongly peraluminous granites in Darying has contributed to important geological information for the exploration of geological and tectonic evolution of the Qiangtang block, Xizang.