腾冲地块内早印支期构造事件的厘定:来自那邦闪长岩锆石U-Pb定年和Lu-Hf同位素证据

摘要

The Tengchong block is an important part of the southeastern Tibetan Plateau,corresponding to the Lhasa block.The discovery of the Songduo eclogite belt in recent years proves the existence of the subduction and collision event in Lhasa block,and the problem has cropped up whether the same event existed or not in Tengchong block.The Nabang diorite in Tengchong block is a metaluminous,K-rich and calc-alkalic magmatic rock body.Zircon LA-ICP-MS U-Pb dating shows the diorite was emplaced at 245.0±2.9 Ma.The zircon ε Hf(t) values range from +7.8 to +14.9,with corresponding single-stage model ages ranging from 298 to 590 Ma.All samples fall in the region between meteoritic Hf evolution (CHUR) and depleted mantle (DM) lines in the plot of ε Hf (t) values versus U-Pb ages and in the subduction-related fluid field in the Th/Yb versus Ba/La discriminant diagram.These data,in combination with high Mg# values,low Yb/Hf ratios (＜1.2) and negative anomalies of Th and U,indicate that the materials were derived from mantle-derived magma mixed with a little crust-derived magma.The strong enrichment of large-ion lithophile elements relative to the primordial mantle,the negative anomalies of Th,U,Nd,Ta,and Ti in the primitive mantle-normalized trace element spider diagram,and the La/Yb versus Sc/Ni and La/Yb versus Th/Yb tectonic discriminant diagrams suggest that the diorite was formed in a tectonic setting related to subduction and collision.The emplacement age and tectonic setting of the diorite in Tengchong block is identical with the data of eclogite in the Lhasa block,and regional angular unconformity with the lack of P2 to T1,suggests that the subduction-collision in Tengchong block did happen during early Indosinian,like the event that happened between Lhasa and northern Australia blocks from P2 to T1.%腾冲地块是青藏高原东南缘的重要组成部分,在大地构造位置上与拉萨地块相对应.近年来,松多榴辉岩带的发现揭示拉萨地块内存在晚二叠-早三叠世的俯冲-碰撞事件,与其相对应的腾冲地块是否也经历了相应的构造事件随之成为人们关注的热点.早印支期那邦闪长岩为偏铝质、富钾、钙碱性岩浆岩.锆石LA-ICP-MS U-Pb定年结果表明闪长岩侵位于(245.0±2.9)Ma.锆石εHf(t)值变化于+7.8～+14.9,在εHf(t)-U-Pb年龄图解上所有样品都落在球粒陨石线和亏损地幔线之间、较小的单阶段模式年龄和地壳模式年龄,在Th/Yb-Ba/La图解上分布于俯冲板片流体区和Yb/Hf(＜1.2)比值,以及较高的Mg#值和Th、U负异常,说明其来源于幔源物质部分熔融,并有部分壳源物质加入的混合岩浆.LREE和LILE强烈富集、Th、U、Nb、Ta、Ti明显负异常,以及La/Yb-Sc/Ni和La/Yb-Th/Yb构造环境判别图解揭示其岛弧型岩浆岩的性质.形成时代与松多榴辉岩带相近、形成于俯冲-碰撞构造环境及腾冲地块内存在类似于拉萨地块内缺失晚二叠-早三叠世地层的区域性角度不整合,说明腾冲地块内也存在与拉萨地块和澳大利亚陆块北部边缘发生俯冲-碰撞相对应的构造事件,早印支期那邦闪长岩是腾冲地块内对这一事件的响应.