阿拉善北大山岩带海西期中酸性岩^(40)Ar-^(39)Ar年龄及其地球化学特征

^(40)Ar-^(39)Ar Age and Geochemical Features of Hercynian Intermediate Acidity Rock in Beidashan Rock Belt, Alxa

阿拉善北大山岩带位于阿拉善地块南缘,恩格尔乌苏缝合带以南。该岩带产出大量的海西期岩浆岩,包括基性-超基性岩、石英闪长岩、石英二长岩、石英正长岩及钾长花岗岩等一系列岩体。本文对该岩带的石英闪长岩及石英二长岩进行^(40)Ar-^(39)Ar测年,获得的坪年龄分别为277.0±3.8Ma、275.0±4Ma,系早二叠世侵入产物。元素地球化学研究表明,石英闪长岩具有高Al2O3、CaO、Mg#,低K2O、(La/Yb)N、ΣREE,表明其偏向于幔源的特点。石英二长岩具有高K2O、(La/Yb)N、ΣREE、Al2O3,低CaO、Mg#,表明其偏向于壳源的特点。结合微量元素分析与大地构造环境条件,得出了该期石英闪长岩与石英二长岩产出环境为二叠纪活动大陆边缘。

Beidashan rock belt in Alxa located at southern of Alxa block, at south of enger-wsu suture zone. Abundance of Hercynian igneous rock, including mafic-ultramafic rock, quartz diorite, quartz monzonite, quartz syenite and myoite, are yielded. The ^40Ar-^39Ar dating on biotite from the Gashuntatale quartz monzonite yields a plateau age of 275.0±4Ma and hornblende from quartz diorite yields a plateau age of 277.0±3.8Ma, both of which are product of Hercynian magma activity. According to element geochemistry, quartz diorite is characterized by high Al2O3, CaO, Mg# and low KzO, （La/Yb）N, ∑REE value, indicate its mantle source feature; and the quartz monzonite of study area with a character of high K2O, （La/Yb）y, ∑REE, Al2O3, and low CaO, Mg# value, indicate its crust affinity. Synthesize trace element geochemistry and tectonic setting, the quartz diorite and quartz monzonite maybe a product of active continental margin.