内蒙古中部四子王旗地区晚二叠-早三叠世过铝花岗岩定年及成因

Dating and Petrogenesis of Late Permian-Early Triassic Peraluminous Granites in the Siziwangqi Area,Inner Mongolia

研究区位于华北板块北缘中段,紧邻兴蒙造山带,是研究古生代—中生代地壳增生演化的重要地区。本文对区内三个花岗岩体进行了锆石LA-ICP-MSU-Pb定年及岩石地球化学研究。结果表明阿玛乌苏花岗闪长岩形成于267±9Ma(MSWD=9.7);布龙二长花岗岩和格尔图白云母正长花岗岩分别侵位于239±4Ma和238±6Ma(MSWD=0.91);而侵入格尔图白云母正长花岗岩的白云母二长花岗岩脉则获得了224±8Ma(MSWD=5.7)的侵位年龄。岩石地球化学特征显示阿玛乌苏花岗闪长岩为岩浆混合成因,暗示大规模岩浆底侵作用的存在;格尔图白云母正长花岗岩具明显的稀土元素"四分组"效应,是分异岩浆与流体作用的结果,可能经历一定程度的地壳混染,非真正意义上的S型花岗岩,属于造山后岩浆热事件,应代表造山的结束。格尔图白云母正长花岗岩及白云母二长花岗岩脉均受到热液流体作用,说明在中三叠世(224～238Ma),区内一直存在有大量的热液流体活动,且热液流体可能富含F和Cl,区内产出的大规模的萤石矿是否为该期流体活动的产物尚需进一步工作。

Study area is located in the central Segment of the northern margin of the North China Craton and adjacent to the Xing-Meng orogenic belt.LA-ICP-MS U-Pb dating and petrochemical analysis of three zircon samples from the Amawusu granodiorite yielded U-Pb ages of 267±9Ma（MSWD=9.7）.The Bulong monzogranite and the Geertu muscovite granite formed at 239±4Ma and 238±6Ma（MSWD =0.91）,respectively.Zircons collected from muscovite monzogranite vein which intruded into the Geertu pluton,gave a mean 235U/206Pb age of 224±8Ma（MSWD=5.7）.Petrochemical data indicate that the Amawusu granodiorite were generated by magma mixing,triggered by extensive underplating;The Geertu muscovite granites show a spectacular tetrad effect in their REE distribution patterns as well as non-CHARAC（charge-and radius-controlled）trace element behavior,caused by intense interaction of the residual melts with aqueous hydrothermal fluids.Those characters indicates that the Geertu muscovite granites are not typical S-type granites,belong to post-orogenic setting,represented the ending of orogeny.Both the Geertu muscovite granite and muscovite monzogranite vein had suffered intense effect by hydrothermal fluids.So in mid-Trias,there are extensive hydrothermal（probably rich in F and Cl）activity in this area.More work need to do for make clear that the fluorite in study area are the results of the hydrothermal activity.