陆内“滞后”弧岩浆岩特征及其地质意义:来自西准噶尔乌尔禾北早二叠世岩浆作用的证据

Intracontinental“lagged arc volcanic rocks”and its geological significance:Evidence from early Permian lagged arc magmatism in northern Urho area of Western Junggar

产于西准噶尔哈拉阿拉特山南麓的"滞后"弧岩浆岩,由乌尔禾中基性岩墙群和白杨河组玄武岩组成,二者构成一完整陆相火山机构,并呈角度不整合覆于下二叠统佳木河组陆相磨拉石建造之上。中基性岩墙群岩性主要为辉绿岩、辉绿玢岩和辉长闪长岩,其LA-ICP-MS锆石U-Pb年龄为(283.6±2)Ma(MSWD=0.57,95%置信度)、(286.3±4.0)Ma(95%置信度,MSWD=0.32)、(286.5±5.6)Ma(MSWD=0.3,95%置信度)。白杨河组玄武岩LA-ICP-MS锆石U-Pb年龄为(283.3±3.1)Ma(MSWD=0.45,95%置信度),均属早二叠世。二者具有相似的与岛弧俯冲相关的地球化学性质,属钙碱性中低钾系列,富集轻稀土,δ_(Eu)异常不明显,富集K、Ba、Sr等大离子亲石元素,而高场强元素Nb、Ta强烈亏损,Zr、Hf适度亏损,正的ε_(Nd)(t)值和相对低的初始^(87)Sr/^(86)Sr比值表明源区为亏损地幔。该岩浆岩形成于洋盆关闭,即沟弧盆体系消失之后,时代"滞后"于造山后沉积记录,表明西准噶尔晚古生代俯冲作用结束的最终时限应为早二叠世晚期。根据准噶尔地区滞后型岩浆岩区域性分布特点,推断其产地的连线区为天山—塔里木板块与西伯利亚板块最终深部俯冲的接合部位。

The＂Lagged arc volcanic rocks＂distributed in Urho area in the south of Hala＇alat mountain of West Junggar.It is composed of Urho dyke swarms and Baiyanghe basalt,which constitute a complete continental volcanic apparatus and is angular unconformably overlying on the Jiamuhe molasse formation of Lower Permian system.The dyke swarms are composed of diabase,diabase porphyrite and gabbro-diorite.The LA-ICP-MS zircon U-Pb ages of them are 283.6±2 Ma（MSWD=0.57,95% conf.）,286.3±4.0 Ma（MSWD=0.32,95% conf.）and 286.5±5.6Ma（MSWD=0.3,95% conf.）.The LA-ICP-MS zircon U-Pb age of Baiyanghe basalt is 283.7±3.1Ma（MSWD=0.45,95%conf.）.All of these are in the late stage of the Early Permian.Urho dyke swarms and Baiyanghe basalt both have similar geochemical properties which are related to subduction of island arc,showing the features of middle-low K calc-alkaline basalt to andesite series and enrichment of LREE.The anomaly ofδEu is not obvious.All samples are characterized by the enrichment of LILE（K,Ba,Sr）,the depletion of HFSE（Nb,Ta,Zr,Hf）,positiveεNd（t）values and relatively low initial Sr isotopic ratios,indicating that the source of the volcanic rocks is the enriched mantle.＂Lagged arc volcanic rocks＂occurred after the closure of ocean basin and the disappearance of trench arc basin system in surface,and later than post-orogenic sedimentary record,suggesting that the final completion of the subduction in Western Junggar was in the late Early Permian.The outcrops of＂lagged arc volcanic rocks＂distributed along a belt,which may be the joint position of Siberia plate and Tarim-Tianshan plate.