甘肃北山新元古代侵入岩的厘定及其对北山盆山演化格局的制约

Confirmation of the granite during Neoproterozoic in Beishan of Gansu Province,and its constraints on the tectonic-evolutional pattern of Beishan orogen

北山地区是中亚造山带的组成部分,记录了重要的新元古代构造演化信息。对甘肃北山咸水沟原1∶25万区域地质调查所划的泥盆纪侵入岩开展研究,新解体出新元古代二长花岗岩体,对该二长花岗岩进行LA-ICP-MS锆石U-Pb测年,其^(206)Pb/^(238)U加权年龄为(787±15)Ma。岩石富SiO_(2)、低TiO_(2)、高Al_(2)O_(3)、富碱,里特曼指数σ和铝饱和指数A/CNK均大于1.05,轻稀土相对于重稀土富集,(La/Yb)N值为15.20~23.28,表明轻重稀土高度分异,(La/Sm)N比值为3.14~4.22,指示轻稀土分馏较大,具明显的负Eu异常,富集Rb、Th、La,亏损Nb、P、Ti、Sr,表现出S型花岗岩特征。北山地区的新元古代二长花岗岩形成于大陆碰撞构造环境,位于Rodinia超大陆边缘,该期岩浆事件反映了新元古代北山地区发生了板块碰撞。

Beishan is located in Central Asia Belt,and records sighificant information of the Neoproterozoic tectonic evolution.The Neoproterozoic monzogranite was newly confirmed by studying the 1∶250000 Devonian intrusive pluton in Xianshuigou area,Beishan,Gansu Province.LA-ICP-MS zircon U-Pb dating of the monzonitic granite was carried out,the weighted age of ^(206)Pb/^(238)U is(787±15)Ma.The monzogranite is characterized by high SiO_(2),low TiO_(2),high Al_(2)O_(3),rich alkali,andσand A/CNK is higher than 1.05.LREE is high relative to HREE.(La/Yb)N value is 15.20-23.28,which indicates LREE and HREE are highly differentiated,(La/Sm)N value is 3.14-4.22,which indicates LREE fractionation is large,δEu is negative.Rb,Th,La are high,Nb,P,Ti,Sr are depleted,showing the characteristics of S-type granite.The Neoproterozoic monzonitic granite was formed in the continental collision tectonic environment and located on the edge of Rodinia supercontinent.This magmatic event reflects the continental collision during Neoproterozoic in Beishan.