新疆南天山南缘库车河流域早二叠世酸性火山岩的地球化学、锆石年代学及构造意义

库车河流域小提坎里克组火山岩主要为一套流纹岩建造，富碱（Na₂O+K₂O=6.89～8.46%）、高钾（K₂O/Na₂O=1.6～2.1）、铝含量12.83～14.78%（平均为14.18%），低钙（1.06～2.51%，平均为1.50%）、低镁（MgO=0.27～0.44%，平均为0.37%），Fe₂O₃^*=1.29%～2.68%（平均为2.17%）。其铝饱和指数A/CNK为0.98～1.15，为过铝质高钾钙碱性-钾玄质花岗岩类。流纹岩的轻稀土强烈富集，重稀土亏损，明显富集Rb、Th、U、K、Pb等大离子亲石元素，亏损Sr、Nb、Ta、P、Ti等元素，具明显的铕负异常（δEu为0.46～0.54）。主微量元素特征显示其壳源成因，原岩为变质杂砂岩，源区有石榴石、角闪石、斜长石的残留。对两个流纹岩样品中的锆石进行LA-ICP-MS U-Pb年龄测定，获得288.3 Ma和290.3 Ma的形成年龄，与西部温宿和拜城地区的小提坎里克组火山岩为同期火山活动产物。该套火山岩具有同碰撞过铝质S型花岗岩特征，结合区域上广泛分布的二叠纪后造山花岗岩，认为西南天山洋至少在早二叠世已经闭合。

Geochemistry and geochronology of Early Permian acid volcanic rocks along Kuqa River and its tectonic implication in the southern margin of South Tianshan Orogen,Xinjiang

There are debates on the tectonic setting of the acid volcanic rocks of Xiaotikanlike Formation exposed along Kuqa River in Southwest Tianshan,and precise U-Pb ages of these rocks have not been reported yet. 7 geochemical samples and 2 samples for zircon U-Pb dating from this volcanic suit are analyzed. Geochemical data indicate that the Xiaotikanlike Formation volcanic rocks are mainly rhyolites. They are characterized by high alkali(Na₂O+K₂O=6.89%～8.46%),potassium(K₂O/Na₂O=1.6～2.1)and Al₂O₃(12.83%～14.78%)content,while low CaO(1.06%～2.51%),MgO(0.27%～0.44%)and Fe₂O₃(1.29%～2.68%)content. They are strongly enriched in LREE and depleted in HREE,and have medium negative Eu anomaly(δEu=0.46～0.54)in the chondrite-normalized REE-patterns diagram. They are also enriched in LILE elements like Rb,Th,U,K,Pb,and depleted in Sr,Nb,Ta,P and Ti. Based on the geochemical features and CaO/Na₂O,(La/Yb)_N ratios,we suggest that the rhyolites were derived from partial melting of metagreywackes. The residual minerals were with garnet,hornblende and plagioclase. In situ U-Pb LA-ICP-MS dating on magmatic zircons from the two samples yield ages of 288.3 Ma and 290.3 Ma respectively. Integrating previous studies,especially the extensive distributed post-orogenic granites of Permian in southwest Tianshan,we conclude that the Xiaotikanlike Formation volcanic rocks have the features of peraluminous S-type granite formed in a syn-collisional setting,and we believe that the southwest Tianshan Ocean has been closed at least in Early Permian.