内蒙古镶黄旗乌兰哈达中二叠世碱长花岗岩LA-ICP-MS锆石U-Pb年龄和地球化学特征

La-ICP-MS zircon U-Pb dating of Wulanhada middle Permian alkali-feldspar granites in Xianghuang Banner, central Inner Mongolia, and its geochemical characteristics

内蒙古镶黄旗乌兰哈达岩体位于华北克拉通北缘中段，岩性以中细一中粗粒碱长花岗岩为主。LA—ICP—MS锆石U—Pb定年结果表明．该花岗岩的侵位年龄为265．1Ma_＋1．5Ma（MSWD=I．5），形成于中二叠世。该花岗岩富硅（SiO2为73．63％-75．47％），富碱（Na2O＋K2O介于9．25％-9．49％）之间。在SiO2-K2O图中，样品多数落入高钾钙碱性系列。A／CNK值在1．00-1．03之间，属弱过铝质花岗岩。稀土元素含量高（∑REE=94×10^-6-383×10-6），轻重稀土元素分异明显，（La／Yb）N=6．59～13．20，具有明显的负Eu异常（δEu=0.22-38）。相对亏损Ba、sr、Nb、P、Ti元素，而高场强元素含量高（Zr＋Nb＋Ce＋Y〉350×10^-6），且具有较高的10000XGa／M比值（236-2.79）。地球化学特征暗示了乌兰哈达碱长花岗岩具有典型的A型花岗岩特征。相对低的Nb的含量进一步暗示了该岩体具有A，型属性。构造环境R，一R：判别图和构造环境（Y＋Nb）一Rb判别图解显示，该岩体可能形成于后碰撞的拉张环境。结合鸟兰哈达岩体的大地构造位置、构造环境及侵位时代．初步推断研究区内华北板块北缘与西伯利亚板块南缘碰撞缝合时代应早于265．1Ma。

Wulanhada pluton, located in Xianghuang Banner of Inner Mongolia, belongs to the middle section of the northern edge of theNorth China Craton （NCC）. It is mainly composed of middle-fine to middle-coarse grained alkali-feldspar granites. LA-ICP-MS zircon U-Pb dating indicates that the pluton was emplaced in the middle Permian （265.1Ma±1.5Ma, MSWD=1.5）. These granites have high SiO2 （73.63%-75.47%） and are enriched in alkali （Na20＋K20=9.25%-9.49%）, belonging to high-K calc-alkaline rocks as shown in the plot of SiO2-KzO. They are weakly peraluminous with ACNK of 1.00-1.03. The Wulanhada granites show high REE content （94×10^-6-383×10^-6） and are relatively enriched in LILEE （（La/Yb）y=6.59-13.20）, with evident negative Eu anomalies （δEu-0.22-0.38）. Their trace elements are characterized by distinct negative Ba, Sr, Nb, Ti and P anomalies, with high HFSE content （Zr＋Nb＋Ce＋Y〉 350×10^-6） and relatively high 10000×Ga/Al ratios （2.36-2.79）. In conclusion, the Wulanhada alkali-feldspar granites show typical A-type geochemical affinity. Furthermore, the relatively low Nb content implies that the pluton is A2 subtype granite. Based on the plots of R1- R2 and Y＋Nb-Rb, the authors hold that the pluton was probably emplaced in an extensional post-collision setting. The results obtained by the authors suggest that the collision between NCC and Siberian Craton took place as early as 265.1Ma.