内蒙古苏尼特左旗中元古代片麻状花岗岩的成因及大地构造意义

Petrogenesis of the Mesoproterozoic Gneissic Granite in the Sonid Left Banner Area,Inner Mongolia,and Its Tectonic Implications

内蒙古锡林浩特地块位于兴蒙造山带中部,地块前寒武纪岩系由宝音图群和片麻状花岗岩等组成。锡林浩特地块～1.4Ga片麻状花岗质岩石的发现为古老地块提供了佐证,但其岩石成因及大地构造意义不清。苏尼特左旗地区中元古代花岗岩主要由片麻状正长花岗岩和片麻状二长花岗岩组成。4件样品的LA-ICP-MS锆石U-Pb年龄为1373～1399Ma,属中元古代中期。片麻状花岗岩具有高硅、富碱、K2O/Na2O比值>1和过铝质等特征,并且稀土总量高、Eu负异常明显、轻稀土富集且重稀土亏损,属于A2-型花岗岩。总体具有弱亏损的同位素组成,全岩εNd(t)除一个样品为负值(-0.2)以外,均为较小正值(+0.3～+1.0),两阶段Nd模式年龄tDM2为1.88～1.97Ga,锆石εHf(t)值均为正值(+0.8～+11.1),tDM2大多介于1.45～2.17Ga之间,表明主要源于古元古代末期新生地壳的部分熔融。区域上～1.4Ga钾质花岗岩类的广泛出现表明为一次陆壳伸展作用引发构造岩浆事件,它是对全球哥伦比亚超大陆裂解事件的响应,锡林浩特地块中元古代(～1.4Ga)A-型花岗岩的发现表明中元古代中期华北克拉通和锡林浩特地块同处于大陆伸展的构造环境。

Xilinhot Block is located in the middle segment of the Xing-Meng Orogen in Inner Mongolia.Precambrian rocks in the block consists of Baoyintu Group and gneissic granite.Discovery of～1.4Ga Mesoproterozoic granitic rocks provides evidence for existence of the Xilinhot Block,but its petrogenesis and tectonic implication are still unclear.Mesoproterozoic granitic rocks in the Sonid Left Banner area consist mainly of yenogranite and monzogranite.And zircon LA-ICP-MS U-Pb dating for four samples yields ages of 1373±7Ma and 1399±11Ma,respectively,suggesting middle Mesoproterozoic in age.Thetotal REE content with enrichment of LREE rather than HREE,and distinct negative Eu anomaly.All＋0.8tothe gneissic granitoids were the product of partial melting of accretion crust in the end of the Paleoproterzoic（1.88～1.97Ga）.Regionally extensive occurrence of 1.4Ga potassic granitoids displays occurrence of a tectonic-magmatic event triggered by crustal extension,which was response to breakup of Colombia supercontinent.1.4Ga A-type granitoids in the Xilinhot block indicate that the Xilinhot Block and North China Craton in the Mesoproterozoic both were in a tectonic setting of continental extension.