内蒙古锡林浩特东部地区早白垩世花岗岩地球化学、锆石UPb年龄及地质意义
Geochemistry and Zircon UPb Dating of Granites in Early Cretaceous in Eastern Xilin Hot, Inner Mongolia and Its Geological Implications

摘要： 通过对锡林浩特东部地区早白垩世花岗岩体进行SHRIMP锆石UPb测年、地球化学测试，讨论其形成构造环境。花岗岩测年结果为：正长花岗岩(DS214)(1391±17) Ma，花岗岩(DS220)(1347±17) Ma，表明研究区花岗岩形成于早白垩世早期。花岗岩地球化学具有高硅、富碱、相对低铝的特征，A/CNK平均值106，为弱过铝质花岗岩。微量元素相对富集大离子亲石元素(Th、U、K)，明显亏损Nb、Ba、Sr、P、Ti等高场强元素；稀土总量高，为12290×10-6～36877×10-6，LREE/HREE值为571～1436，呈右倾模式，负Eu异常显著(010～050)，表现为A型花岗岩特征。K2O-Na2O构造环境判别图表明样品为A型花岗岩，Y/NbCe/Nb图解显示花岗岩为A2型。主量元素、微量元素特征指示花岗岩形成于造山后岩石圈伸展作用阶段，在壳源岩浆演化过程中存在幔源物质混染作用。花岗岩成因可能是晚古生代末—中生代初期间古亚洲洋闭合引起的一系列板块碰撞作用(包括蒙古—鄂霍次克洋闭合)，使造山后期地壳逐渐增厚并发生重力垮塌，导致构造环境由挤压转变为伸展，同时受古太平洋板块西向俯冲的影响。
Abstract: In this paper, a series of analyses including SHRIMP zircon UPb isotopic dating, major elements and trace elements, have been performed on the granite in Early Cretaceous in eastern Xilin Hot in order to make the discussion of tectonic environment in the study area. The results of the SHRIMP zircon UPb dating are (1391±17) Ma for syenogranite(DS214) and (1347±17) Ma for granite(DS220), which suggests that the granite formed in the early period of Early Cretaceous. The geochemical feature of major elements in the granites is found containing high SiO2 and K2O +Na2O contents, low Al2O3 content relatively. The average A/CNK value is 106, which belongs to weak peraluminous series. The trace element geochemistry shows evident enrichment of LILE(Th, U and K), deficit of HFSE (Nb, Ba, Sr, P and Ti). The REE patterns are the richment type of LREE/HREE value (571-1436) with high total REE contents(12290×10-6-36877×10-6) and strong negative Eu anomalies(010-050), which implies that the samples are Atype granites. The K2ONa2O diagram for tectonic environments also shows that they are Atype granite. The Y/NbCe/Nb diagram of Atype granites of different tectonic environments displays the feature of A2type granite. These geochemical characteristics of the granites reveal that the granites formed in the extending phase of the postorogenic by the evolution of crustal magma with mantle mixing. The cause of the granite may be related to the procession from the extruding to extending environment which is the result of the lithosphere thinning after the plates collision (including MongoliaOkhotsk ocean) with the close of PaleoAsian Ocean during the Late Paleozoic to the Early Mesozoic. It was also effected by the westward subduction of the PaleoPacific Ocean