柴达木盆地北缘小赛什腾山二叠纪花岗岩的发现及其构造意义

Discovery of the permian granite in saishiteng mountain of the northern qaidam basin and its tectonic significance

赛什腾山位于柴达木地块北缘构造带(柴北缘构造带)与阿尔金走滑断裂带斜交部位,属于柴北缘残山带的一部分,带内零星出露的花岗岩体一直被认为形成于早古生代,属于早古生代洋陆碰撞期的花岗岩。我们对小赛什腾山两个花岗岩体重新进行了LA-ICP-MS锆石U-Pb年代学、Lu-Hf同位素和微量元素分析,结果表明,两个岩体三个花岗岩样品的锆石U-Pb年龄分别为260±3Ma, 268±8Ma和263±8Ma,表明其形成于中二叠世,而非原先认为的早古生代。所有锆石的^(176)Hf/^(177)Hf比值分布在0.28294～0.28320之间,并具有极高的正ε_(Hf)(t)值(11～20),其平均二阶段Hf模式年龄为350Ma,稍高于其U-Pb年龄,反应其源区可能为具有亏损地幔特征的新生镁铁质物质。结合区域地质特征和已有研究成果的分析认为,宗务隆小洋盆向西可能延伸至小赛什腾山地区,中二叠世俯冲的洋壳发生部分熔融形成了具有极高的正ε_(Hf)值的花岗岩。该发现表明柴北缘地区存在二叠纪时期的洋壳俯冲,为深入理解柴北缘晚古生代-中生代的构造演化提供了新的佐证。

The Saishiteng Mountain,located in the convergent part of North Qaidam tectonic belt and Altyn Tagh fault zone,belongs to one part of the North Qaidam tectonic belts.Granitic pluton outcropping in this area has long been regarded to form in early Mesozoic and should be the granitic rock resulting from the early Mesozoic ocean-continent collision.This study investigated the two grantic plutons in Saishiteng Mountain using LA-ICP-MS zircon U-Pb dating,Lu-Hf isotopic and trace element analysis.The results show that three granite samples from the two plutons have zircon U-Pb ages of 260±3 Ma,268±8 Ma and 263±8 Ma,respectively,indicating that they formed in the Middle Permian,but not previously suggested early Mesozoic.176 Hf/177 Hf values of all zircons range from 0.28294 to 0.28320,with extremely high positiveεHf value of 11 to 20,and average two-stage model age of 350 Ma,higher than U-Pb age,suggesting that source region might be characterized by new-born mafic material with feature of depleted mantle.Combined with regional geological characteristics and existing research results,this study concludes that the Middle Permian granite with high positiveεHf values might form from patial melting of Middle Permian subduction of oceanic crust due to the extension of the Zongwulong oceanic basin westward to Saishiteng Mountain area.The discovery indicates that oceanic plate subduction occurred in the Permian,which provides new evidence for understanding the Late Paleozoic-Mesozoic tectonic evolution in the northern Qaidam Basin.