Permian-Triassic magmatic and thermal events in the Dunhuang orogenic belt: implications for subduction records of the Paleo-Asian Ocean

The final subduction and suture of the Paleo-Asia Ocean (PAO) are controversial, and the key to this question is to locate the latest subduction/final suture of the accretionary type orogenic belt. Previous studies mainly focused on the Tianshan, Beishan, Alxa, and Solonker Orogens for this issue. As a Phanerozoic orogenic belt located further south of the Beishan-Alxa Tectonic Belts, the Dunhuang Orogenic Belt (DOB) is becoming a research hotspot for revealing the evolutionary history of the southernmost Central Asia Orogenic Belt (CAOB). The youngest subduction-related arc magmatism is pivotal for constraining the lower limit of subduction of a lost ocean. This study presents integrated investigations of field observation, whole-rock geochemistry, and geochronology, targeting diabase dykes, intermediate-acid magmatic rocks, and garnet-bearing metabasite samples in the middle part of the DOB. LA-ICP-MS U-Pb dating of zircon yielded episodic magmatism of ca. 274–273 Ma for Xiaohongshan granite and Dongbatu syenogranite, ca. 250 Ma for Keziletage syenite, ca. 243–239 Ma for diabase and granitic dykes in the Kalatashitage area and Mogutai syenogranite, and maximum deposition age of ca. 262 Ma for pyroclastic rock in the Kalatashitage area. Most of those Permian-Triassic magmatic rocks show continental arc-related geochemical features during the subduction stage. The negative εHf(t) values and ancient TDM2 model ages of zircon indicate they were mainly derived from the partial melting of ancient crustal materials. The hornblende 40Ar/39Ar ages of ca. 258–250 Ma of the garnet-bearing metabasite samples also reflect a high heat flow setting during this period possibly triggered by arc activities. These Permian-Triassic magmatic and thermal records in the DOB are similar to contemporaneous records in surrounding South Tianshan-Alxa-Beishan orogens. We suggest that the final subduction of the PAO in the DOB possibly at least lasted to the Middle Triassic.

古亚洲洋（Paleo-Asia Ocean, PAO）的最终俯冲与缝合长期存在争议，该科学问题的核心在于定位增生型造山带（accretionary type orogenic belt）的最新俯冲界面或最终缝合带位置。以往相关研究多聚焦于天山造山带、北山造山带、阿拉善造山带以及索伦造山带。敦煌造山带（Dunhuang Orogenic Belt, DOB）作为位于北山-阿拉善构造带以南的显生宙造山带，正逐步成为揭示最南端中亚造山带（Central Asia Orogenic Belt, CAOB）演化历史的研究热点。与俯冲过程相关的最年轻弧岩浆作用，对约束已消亡大洋的俯冲下限具有关键意义。本研究针对敦煌造山带中部的辉绿岩脉、中酸性岩浆岩及含石榴石变基性岩样品，开展了野外地质调查、全岩地球化学与年代学的综合研究。对锆石开展LA-ICP-MS U-Pb定年结果显示：红山花岗岩与东巴图正长花岗岩存在约274~273 Ma的幕式岩浆活动，克孜勒塔格正长岩的岩浆活动时限为约250 Ma，卡拉塔什塔格地区的辉绿岩、花岗质岩脉与莫古台正长花岗岩的岩浆活动时限为约243~239 Ma，卡拉塔什塔格地区火山碎屑岩的最大沉积年龄约为262 Ma。上述二叠纪-三叠纪岩浆岩中，多数在俯冲阶段表现出大陆弧相关的地球化学特征。锆石的负εHf(t)值与古老TDM2模式年龄表明，这些岩浆岩主要源自古老地壳物质的部分熔融。含石榴石变基性岩样品的角闪石40Ar/39Ar定年结果为约258~250 Ma，同样反映了该时期存在由弧活动可能诱发的高热流构造背景。敦煌造山带内的这套二叠纪-三叠纪岩浆与热演化记录，与周边南天山-阿拉善-北山造山带的同期记录高度相似。据此我们提出，古亚洲洋在敦煌造山带内的最终俯冲作用至少持续至中三叠世。