湘西南猫儿界花岗斑岩锆石U-Pb年代学、地球化学特征及其地质意义

Zircon U-Pb Geochronology,Geochemical Characteristics of the Mao’erjie Granite Porphyry in Southwestern Hunan Province,and Its Geological Significance

文章对苗儿山—越城岭地区西北侧猫儿界花岗斑岩开展了锆石U-Pb年代学、岩石地球化学和Nd同位素研究,探究了岩体的形成时代、成因类型及源区性质。三件花岗斑岩样品LA-ICP-MS锆石U-Pb定年给出的206Pb/238U加权年龄分别为824.3±4.5 Ma、825.4±4.1 Ma和826.8±4.8 Ma,表明猫儿界岩体属新元古代岩浆活动的产物。猫儿界花岗斑岩具有高硅、铝饱和特征,富集Cs、Rb、U、K等大离子亲石元素和Th、Hf等高场强元素,明显亏损Ba、Sr、Ti等元素。球粒陨石标准化稀土配分曲线呈右倾形,并具明显的Eu负异常(δEu=0.37~0.43)。岩体的εNd(t)值为-6.3~-8.7,TDM2(Nd)值为2.00~2.20 Ga,表明岩体由老的地壳物质部分熔融形成。在猫儿界岩体三个定年样品中发现了10颗年龄在852~861 Ma的继承锆石颗粒,加权平均年龄为856.3±7.4 Ma,说明在苗儿山—越城岭地区可能存在~856 Ma的岩浆岩,推测猫儿界岩体在形成过程中可能有~856 Ma的岩浆物质的卷入。

Based on detailed studies on zircon U-Pb geochronology,whole-rock geochemical and Nd isotope compositions of the Mao’erjie granite porphyry in the northwestern Miao’ershan-Yuechengling area,the emplacement age,genetic type and source of the granite porphyry are discussed.LA-ICP-MS U-Pb dating of the zircons from three granite porphyry samples yielded weighted mean 206Pb/238U ages of 824.3±4.5 Ma,825.4±4.1 Ma,and 826.8±4.8 Ma,respectively,indicating that the Mao’erjie granite porphyry was emplaced in the Neoproterozoic.It is peraluminous with high silica contents and is enriched in large ion lithophile elements(LILE)such as Cs,Rb,U and K,and high field-strength elements(HFSE)such as Th and Hf,but is relatively depleted in Ba,Sr and Ti.Its chondrite-normalized REE pattern is right-inclined and has obvious Eu negative anomaly(δEu=0.37-0.43).TheεNd(t)values of the granite porphyry are from-6.3 to-8.7,and the TDM2(Nd)values are from 2.00 to 2.20 Ga,indicating that the granite porphyry was formed by partial melting of old crust-derived basement material.Ten inherited zircon grains with U-Pb ages of 852-861 Ma were found in the three dated granite porphyry samples and yield a weighted average age of 856.3±7.4 Ma,implying the existence of~856 Ma magmatic activity in the Miao’ershan-Yuechengling district.It can be speculated that~856 Ma magmatic rocks was involved during the formation of the Mao’erjie granite porphyry.