滇西澜沧江构造带南段沙乐花岗岩的锆石U-Pb年龄、地球化学特征及其地质意义

The Shale granites from the southern part of the Lancangjiang tectonic belt,western Yunnan:Zircon U-Pb age,geochemistry and geological implications

沙乐花岗岩位于滇西思茅地块南涧县南西沙乐乡一带,主要由黑云母二长花岗岩和花岗闪长岩组成,被少量正长斑岩侵入。LA-ICP-MS锆石U-Pb测年得出黑云母二长花岗岩的形成年龄为246.4±2.6Ma,花岗闪长岩的形成年龄为245.7±3.6Ma,表明该花岗岩体的形成时代属于早三叠世。岩石Si O2含量71.44%~76.39%,Al2O3含量12.72%~16.15%,K2O/Na2O=0.1~1.30,均属高钾钙碱性过铝-强过铝花岗岩。根据地球化学特征和微量元素构造判别图解,样品点主要落入"火山弧花岗岩"、"板内花岗岩"区域,少量落入"碰撞花岗岩"区域,表明其形成于俯冲-碰撞岩浆弧转换环境。在Sr-Yb花岗岩分类图解中,主要属常见的低Sr高Yb花岗岩,少数样品属低Sr低Yb花岗岩,其物质来源为含砂质的变质泥质岩。结合锆石定年结果及岩体产出的区域地质背景,认为沙乐花岗岩形成于缅泰马陆块与思茅地块大陆碰撞造山过程的初始阶段,同时表明昌宁-孟连洋碰撞最早时限为早三叠世。

The Shale granites from Shale,Nanjian,western Yunnan consist of biotite monzogranite,granodiorite and a small amount of orthophyre. The LA-ICP-MS zircon U-Pb dating gives an age of 246. 4 ± 2. 6 Ma for biotite monzogranite,and 245. 7 ± 3. 6 Ma for granodiorite,indicating that the Shale granites were formed during the Early Triassic. The Shale granites are characterized by Si O2 contents ranging from 71. 44% to 76. 39 %,Al2 O3 contents ranging from 12. 72% to 16. 15%,and K2 O/Na2 O ratios ranging from 1 to 1. 30,suggesting the high-K calcalkaline peraluminous and strongly peraluminous granites. The samples of the Shale granites mostly projected into the fields of volcanic arc granites,within plate granites and collision-type granites indicated by the geochemical signatures and trace element discriminantion diagram for tectonic interpretation,implying that the granites were formed in the subduction-collision magmatic arc transitional environments. The Sr-Yb diagram for the granite classification has disclosed that most of the Shale granite samples are assigned to the low-Sr and high-Yb granites,and a few of them belong to the low-Sr and low-Yb granites,both of which are derived from the sandy metamorphic mudstones. It can be seen from the zircon U-Pb age determinations and regional tectonic setting that the Shale granites were originated during the initial stages of the collision and mountain building processes of the MyanmarThailand-Malaysia continent and Simao landmass. The initial collision of the Changning-Menglian ocean may be traced back to the Early Triassic.