江西云山岩体的成因:年代学、地球化学及Nd-Hf同位素制约

Petrogenesis of the Yunshan pluton in Jiangxi Province:constrains from geochronology,geochemistry and Nd-Hf isotopes

云山岩体位于赣北的江南造山带东段九岭隆起带东北端,主要由二云母二长花岗岩组成。本文对该岩体进行了详细的锆石U-Pb年代学、主量元素、微量元素以及Nd-Hf同位素研究。LA-ICP-MS锆石U-Pb定年表明云山二云母二长花岗岩的形成年龄为125.6±1.1Ma,为燕山晚期早阶段岩浆活动的产物。岩相学和岩石地球化学研究表明云山岩体属于高分异的S型花岗岩,具高硅、富碱、过铝质,锆饱和温度低、轻重稀土分馏明显、富集Rb、Th、U、K、Pb等元素而亏损Ba、Nb、Sr和Ti等元素、铕负异常显著(Eu/Eu~*=0.13～0.20)的特点。云山岩体的全岩ε_(Nd)(t)值与锆石ε_(Hf)(t)值分别变化于-3.9～-5.1和-1.0～-8.8,两阶段Nd和Hf同位素模式年龄分别为T_(DM2)(Nd)=1.35～1.44 Ga和T_(DM2)(Hf)=1.25～1.75 Ga,Nd同位素的模式年龄重叠于Hf同位素模式年龄。结合其CaO/Na_2O值均小于0.3,本次研究认为云山岩体的源区很可能是来自于双桥山群中的富泥质变质沉积岩及少量火成岩,形成于早白垩世古太平洋板块俯冲之后的弧后伸展的构造环境。

The Yunshan pluton, located in the northeast of Jiuling uplift in the Jiangnan orogen, northern Jiangxi Province, consists lithologically of two-mica monzogranite.In this paper, we reported detailed studies on the LA-ICP-MS zircon U-Pb dating, major elements, trace elements, whole-rock Nd isotopic compositions and zircon Hf isotopic compositions of the two-mica monzogranite. LA-ICPMS zircon U-Pb ages show that the two-mica monzogranite was emplaced at 125.6±1.1 Ma and the product of early Late Yanshanian magmatic activity. The studies of petrography and geochemistry of this rock indicate that it is characterized by silica-rich, high alkalinity, peraluminous, low zircon saturation temperatures, enrichment in Rb, Th, U, K, Pb and depletion in Ba, Nb, Sr, Ti and significant negative Eu anomalies(Eu/Eu*=0.13~0.20). Hence, this rock belongs to highly fractionated S-type granite.εNd(t) and εHf(t) values of the Yunshan pluton vary from-3.9 to-5.1 and from-1.0 to-8.8, respectively, and the calculated two-stage model ages(TDM2) of Nd and Hf isotopes are 1.35~1.44 Ga and 1.25~1.75 Ga, respectively. Coupled with CaO/Na2O of<0.3,this study suggests that pelitic metasediments and minor amount of igneous rocks of Proterozoic Shuangqiaoshan Group may be the source rocks for the two-mica monzogranite. We propose that the ca. 126 Ma Yunshan granite was formed in a back-arc extensional setting in SE China during Early Cretaceous.