内蒙古索伦地区黄土达坂花岗岩形成时代、地球化学特征及构造背景

Age,geochemistry and tectonic setting of granites from Huangtudaban in Suolun area,Inner Mongolia

索伦地区黄土达坂花岗岩主要以正长花岗岩、二长花岗岩和花岗斑岩为主,LA—ICP—MS锆石U—Pb定年(129. 9±2. 3～125. 9±3. 7 Ma)显示其形成于早白垩世。岩石具有高硅富碱、贫镁钙的特征,为高钾钙碱性系列I型花岗岩。岩石大离子亲石元素K和Rb以及LREE相对富集,高场强元素Nb、Ta、P、Ti和Ba、Sr相对亏损,具明显负Eu异常(δEu=0. 18～0. 60),显示地壳来源。锆石的ε_(Hf)(t)值较高(5. 80～10. 70),Hf二阶段模式年龄在502～804 Ma之间,反映其源区物质为新元古代-显生宙期间增生的基性地壳。黄土达坂花岗岩形成于造山后伸展环境,可能与蒙古—鄂霍茨克洋闭合后的岩石圈伸展作用相关。

The granite of Huangtudaban in Suolun area is mainly composed of syenogranite,monzonitic granite and grangite-porphyry. LA-ICP-MS zircon U-Pb isotopic dating yields age of 129. 9 ± 2. 3 and 125. 9 ± 3. 7 Ma,indicating that they were formed in the Early Cretaceous. Geochemical studies show that the granites are characterized by rich silicon and alkalis,poor calcium and magnesium. Hence the granites belong to high-potassium calc-alkaline I-type rocks. he granites are enriched in K,Rb and LREE,and depleted in Nb,Ta,P,Ti,Ba and Sr,with strong negative Eu anomalies(δEu = 0. 18~0. 60). All above show that the granites were in accord with the characteristics of crust source magma. The zircons in the granites show high values of εHf( t)( 5. 80~10. 70)with two-stage Hf model ages of 502~804 Ma,suggesting that they are derived from the newly formed crust which is separated from Paleozoic-Neoproterozoic depleted mantle. The granites formed in non orogenic intraplate environment and may be closely related to the lithosphere extension after the closure of Mongol-Okhotsk Ocean.