哀牢山古特提斯洋盆闭合和印支造山开启的时限:来自中-上三叠统碎屑锆石U-Pb年代学和Hf同位素约束

由于哀牢山古特提斯洋盆精确闭合时间一直存在争议,从而制约了我们对该区古特提斯洋演化及印支造山运动过程的完整认识。碎屑岩作为造山作用在地壳浅表响应的产物,保存了其物源区深部岩浆作用的重要信息,可有效地约束洋盆演化和造山过程的精细时空格架。本文选择对哀牢山构造带及其东侧地区中-上三叠统碎屑锆石年代学和Hf同位素开展了系统的研究,结果显示:构造带内部三叠统样品除了缺少240~325 Ma年龄群外,与东侧地区同时代碎屑岩样品相似,均具有480~560 Ma和900~1200 Ma两个主要年龄群,对应的εHf(t)值分别为?16.75~+17.00和?15.39~+19.20;而两地区上三叠统样品具有基本相同的年龄频谱特征,均显示250~330 Ma、480~580 Ma和920~1240 Ma三个主要年龄群,对应的εHf(t)值分别为?10.67~+12.15、?10.06~+9.57和?12.25~+15.62。综合本次研究结果与前人数据,表明哀牢山构造带内中-上三叠统及其东侧地区三叠系碎屑物质主要来源于构造带内的岩浆岩,有少量老地层再循环的贡献。进一步的源区分析指出,哀牢山古特提斯洋在早三叠世已闭合。此外,基于哀牢山构造带及两侧地区普遍缺失下三叠统地层和大量发育早-中三叠世碰撞有关的岩浆岩的特征,显示我国哀牢山地区与越北地区印支造山运动在二叠纪末-早三叠世同时开启,中-晚三叠世,哀牢山构造带进入碰撞后伸展阶段。

Timing of the Ailaoshan Paleo-Tethys Ocean Closure and the Indosinian Orogeny Initiation:Constraints from the Detrital Zircon U-Pb Geochronology and Hf Isotopes of the Middle-Upper Triassic Strata

Constraining the accurate closure timing of the Paleo-Tethys Ocean along the Ailaoshan tectonic zone(ATZ)is crucial to understand the tectonic evolution of the Paleo-Tethys Ocean and the geodynamic process of the Indosinian Orogeny within this region.Detrital zircons from the terrigenous detritus,one of the products of the orogeny on the earth’s surface,can record important information of the eroded igneous rocks on the orogeny and provide an independent insight into the spatial-temporal evolution of the orogenesis.In this paper,we present zircon U-Pb dating and Lu-Hf isotope results of the Middle-Upper Triassic detrital sedimentary rocks in the ATZ and its eastern region.Our data show that the Middle Triassic samples from the ATZ and its eastern region share similar detrital zircon U-Pb age pattern with the exception of the absence of 240–325 Ma population in the ATZ samples.They possess two common age populations at ca.480–560 Ma and ca.900–1200 Ma,with correspondingεHf(t)values of?16.75 to+17.00 and?15.39 to+19.20,respectively.The Upper Triassic samples from these regions share three similar major age populations at 250–330 Ma,480–580 Ma and 920–1240 Ma with correspondingεHf(t)values of?10.67 to+12.15,?10.06 to+9.57 and?12.25 to+15.62,respectively.In addition,an integration of our data with previously published detrital zircon geochronology data of the Ailaoshan region suggests that the Triassic sediments of the ATZ and its eastern region were mainly sourced from the igneous rocks of the ATZ,with minor contributions from the recycling of the Paleozoic strata.Provenance analysis indicates that the Ailaoshan Paleo-Tethys ocean basin had been closed during the Early Triassic.Considering the absence of Lower Triassic strata on both sides of the ATZ and the distribution patterns of Early-Middle Triassic collision-related igneous rocks across the whole ATZ,we proposed that the Indosinian Orogeny in the ATZ and NE Vietnam region commenced concurrently in the latest Permian-Early Triassic.Consequently,voluminous Middle-Late Triassic sedimentation and magmatism within the ATZ and its ambient regions developed in a post-collision extension regime during the Indosinian Orogeny.