滇西保山地块早古生代花岗岩类的年代学、地球化学及意义

位于滇西特提斯构造带保山地块的平河岩体,岩石类型主要为花岗岩、二长花岗岩,其次有少量的花岗闪长岩。4件样品的锆石U-Pb年龄变化于480～486Ma,表明这些花岗岩类侵位于早奥陶世。平河花岗岩类的K2O/Na2O值大于1,铝饱和指数(A/CNK)为1.07～1.1,属高钾钙碱性过铝质花岗岩。岩石总体上富集大离子亲石元素和Pb,亏损高场强元素。明显富集轻稀土元素[(La/Yb)N=4.33～7.05],显示明显的负Eu异常(δEu=0.25～0.48)。4件样品58个测点的锆石εHf(t)值变化范围较大(主要集中于-12.4～-3.0之间),对应的Hf同位素地壳模式年龄集中于2.2～1.7Ga。这些地球化学特征指示平河花岗岩类为S型花岗岩,主要来源于古老地壳物质(如砂屑岩)的重熔,并不同程度地混入了幔源物质。平河花岗岩类与出露于中部拉萨地体的变质酸性火山岩存在可比性,可能代表了早古生代冈瓦纳大陆原特提斯边缘岩浆弧的一部分。

Geochronology and geochemistry of the Early Palaeozoic granitoids in Baoshan block, western Yunnan and their implications

The Pinghe granitoid pluton,located in the Baoshan block of the Tethyan belt,western Yunnan,consists mainly of granite and monzogranite with subordinate granodiorite.Zircon U-Pb ages of 480 ～ 486Ma for four samples from this pluton indicate that this pluton was emplaced during the Early Ordovician.The Pinghe granitoids are high-K calc-alkaline and peraluminous,with K2O/Na2O > 1 and A/CNK = 1.07 ～ 1.13.These rocks are enriched in large-ion lithophile elements(LILEs),light rare-earth elements(LREEs) [(La/Yb) N =4.33 ～7.05]and Pb,and depleted in high field-strength elements(HFSEs).They show similar chondritenormalized REE patterns,with negative Eu anomalies(δEu = 0.25 ～ 0.48).58 analyses from four samples have a wide range zircon εHf(t) values(-12.4 ～-3.0) and varying Hf-isotope crustal model ages(2.2 ～ 1.7Ga).Such geochemical signatures indicate that the Pinghe granitoids are S-type granite in character and were derived mainly from anatexis of ancient crustal materials(such as greywacke) with varying contributions from mantle-derived components.The Pinghe granitoids are geochemically comparable to the coeval silicic metavolcanic rocks reported in the central Lhasa Terrane.It is suggested that the Baoshan block may represent a part of an Early Paleozoic magmatic arc in the Gondwana continental margin facing the proto-Tethyan Ocean.