内蒙古阿拉善右旗大山口地区黑云母花岗岩地球化学特征及成因

GEOCHEMICAL CHARACTERISTICS AND ORIGIN OF DASHANKOU BIOTITE GRANITE IN ALASHANYOUQI OF INNER MONGOLIA

内蒙古自治区阿拉善右旗大山口地区发育有大面积侵入岩,主要为华力西中晚期和印支期酸性侵入岩体及各类脉岩。本次运用LA-ICP-MS锆石U-Pb测年方法获得黑云母花岗岩的成岩年龄为229.5±5.6 Ma,属晚三叠世,大山口黑云母花岗岩SiO_2含量68.89%～71.04%,相对富钾贫钠;A/NKC介于0.89～1.05,强烈富集强不相容元素,亏损高场强元素Na、Ta、Ti、P,Eu异常不明显,HREE呈近平坦型分布。通过黑云母花岗岩的成因研究和构造环境判别分析,认为大山口黑云母花岗岩为加厚的中下地壳物质拆沉进入地幔加热并发生部分熔融,且熔体可能与地幔基性熔体发生一定程度的混合作用后上升形成的花岗岩,形成于晚三叠世后碰撞伸展垮塌的构造环境。

There is a large area of intrusive rocks in Inner Mongolia Alashanyouqi Dashankou region, which mainly are middle to later Variscan and Indosinian epoch acid intrusive rocks and all kinds of vein rocks.according to LA-ICP-MS zircon U-Pb dating method, it got the diagenetic age of biotite granite is 229.5 ＋ 5.6 Ma, which is the late Triassic. The content of SiO_2 in the biotite granite is 68.89% ～ 71.04%, which is relatively rich in potassium and poor in sodium. A/NKC is between 0.89 and 1.05, which strongly enriched with strong incompatible elements and loss of high field strength elements Na, Ta, Ti, P. Eu anomaly is not obvious, and HREE is nearly flat distribution.Through the causes study and tectonic environment discriminant analysis of the biotite granite, it thinks Dashankou biotite granite is formed by the thickened crust-derived material sinking into the mantle, then heating and partial melting, and it might be mixing with the mantle basic melt in a certain degree, which formed in tectonic environment of collision, stretching and collapse after late Triassic.