内蒙古中部苏尼特左旗早三叠世枕状玄武岩地球化学特征及地质意义

Geochemistry of Early Triassic Pillow Basalts from Sonid Zuoqi,Central Inner Mongolia and Their Geological Significance

内蒙古中部苏尼特左旗地区最新发现的枕状玄武岩,对研究兴蒙造山带的构造演化将提供重要线索。本文首次报道了苏尼特左旗枕状玄武岩的地球化学组成以及锆石U-Pb年代学数据。锆石U-Pb测年结果显示其形成于早三叠世。该枕状玄武岩具有较低的SiO_(2)(43.44%~52.84%)含量,以及较高的Fe_(2)O_(3)(8.02%~13.65%)和MgO(3.68%~10.34%)含量,属于拉斑玄武岩系列。微量元素及同位素地球化学组成揭示苏尼特左旗枕状玄武岩并未受到明显的地壳混染作用,其岩浆演化主要受铁镁矿物的分离结晶作用控制。该枕状玄武岩中的锆石组成复杂,不同于大洋中脊产出的玄武岩,且其微量元素地球化学特征和全岩Sr-Nd同位素组成也明显不同于正常洋脊玄武岩。同时,该枕状玄武岩显示近平坦的稀土元素配分模式,与典型的岛弧玄武岩存在明显差别。总体来看,本文研究的苏尼特左旗枕状玄武岩地球化学特征及同位素组成与大陆裂谷玄武岩相近。因此,该枕状玄武岩可能起源于交代地幔物质的减压部分熔融。结合前人研究成果,我们认为苏尼特左旗枕状玄武岩可能产自古亚洲洋闭合后由于岩石圈拆沉导致的伸展环境,与新疆北山地区早二叠世枕状玄武岩产出环境类似。

The Early Triassic pillow basalts discovered at Sonid Zuoqi are vital to understand the tectonic evolution of the Xingmeng Orogenic Belt.In this contribution,we present geochemical and geochronological results of those pillow basalts and provide constraints on the tectonic setting and mantle source of the Xingmeng Orogenic Belt.Zircon U-Pb dating result and whole-rock geochemical compositions reveal that the pillow basalts formed in Early Triassic or Early to Middle Triassic,which is consistent with the Early to Middle Triassic pillow basalts in the Baiyinnuoer area.The basalts show relatively low SiO_(2)(43.44%to 52.84%),and high Fe_(2)O_(3)(8.02%to 13.65%)and MgO(3.68%to 10.34%)contents,and belong to sub-alkaline tholeiite.Trace element modelling suggests that the magma evolution of the Sonid Zuoqi pillow basalts was dominated by fractional crystallization of ferromagnesian,whereas crust contamination is negligible.Their geochronological and geochemical data reveal that the Sonid Zuoqi pillow basalts are different from the Normal Middle Ocean Ridge Basalts,and the flattened rare earth element patterns also differ from typical arc-related basalts.Overall,the Sonid Zuoqi pillow basalts show bulk-rock Sr-Nd isotopic compositions and trace element geochemical features similar to those of continental flood basalts.The trace element signatures suggest the Sonid Zuoqi pillow basalts are derived from decompression melting of the metasomatized mantle materials.These data,in incorporation with previous results,suggest the Sonid Zuoqi pillow basalts formed in an intraplate extension setting after the final closure of the ocean basin,which is similar to the scenario of the Early Permian pillow basalts in the Beishan area of Xinjiang.