哀牢山中段镇沅金矿晚二叠世花岗斑岩U-Pb年代学、岩石地球化学及其构造意义

U-Pb chronology,Lithogeochemistry and tectonic significance of Late Permain granite porphyry in Zhenyuan gold deposit,the middle section of Ailaoshan

云南镇沅地区位于哀牢山构造带中段,是古特提斯哀牢山洋关闭及碰撞造山的关键部位。本文对该地区出露的花岗斑岩进行了年代学、岩石地球化学与同位素地球化学研究。锆石U-Pb年代学给出加权平均年龄为253.3±2.0Ma,表明花岗斑岩形成于晚二叠世。3个花岗斑岩脉共5个样品分析结果表明,其SiO2含量65.30%~70.79%,全碱含量(K2O+Na2O)3.53%~4.39%,Al2O3含量13.42%~16.40%,为高钾钙碱性亚碱性系列岩石,成分上与花岗闪长岩类似,为S型花岗岩。稀土元素含量较低,轻重稀土分异程度较高,稀土配分曲线为右倾,无明显Ce异常,具弱负Eu异常,微量元素配分曲线为右倾,大离子亲石元素相对富集,高场强元素相对亏损,与典型地壳相一致。锆石Hf同位素εHf(t)为-1.0~4.1,显示岩浆源区为亏损地幔和古老地壳物质部分熔融。岩石大地构造环境分析表明,该花岗斑岩形成于晚二叠世同碰撞构造环境。

Zhenyuan area,located in the middle of Ailaoshan,is a key area for investigating and understanding closure and collision orogenic of Ailaoshan suture of the Paleotethys.This paper presents a set of new chronology,geochemistry,and isotope geochemistry data for the granite porphyry in the middle of Ailaoshan.These rocks gave a zircon U-Pb age of 253.3±2 Ma,which formed in the Late Permian.The five rhyolite samples from three granite porphyry dikes have relatively high SiO2(65.30%~70.79%),low K2O+Na2O(3.53%~4.39%)and low Al2O3(13.42%~16.40%),which are K-calc-alkaline-subalkaline series rocks and belongs to the Stype granite.The samples exhibit lower LREE and lower ratios of LREE/HREE.The REE patterns are right-dipping,with no obvious Ce anomalies and slightly negative Eu anomaly.The trace element patterns are right-dipping,which are enriched in LILE and depleted in HFSE.It is like typical crust.The zircon Hf isotopic data exhibit variableεHf(t)values ranging from-1.0 to 4.1,suggesting from depleted mantle and the partial melting of ancient crustal materials.The petro-tectonic environment shows that the granite porphyry formed in the Late Permian syn-collisional tectonic environment.