湖南阳明山岩体的La-ICP-MS锆石U-Pb定年及成因研究

Single-zircon La-ICP-MS U-Pb Dating of the Yangmingshan Granitic Pluton in Hunan,South China and Its Petrogenetic Study

湖南阳明山岩体主要由二云母二长花岗岩和电气石白云母花岗岩构成,两类花岗岩的锆石La-ICP-MS定年结果分别为218.0±10.0Ma和218.9±3.4Ma,属印支晚期花岗岩。该岩体具有富SiO2(74.33%~76.03%)、Al2O3(13.34%~14.47%)和P2O5(0.21%~0.34%),高A/CNK(=1.15~1.32)的特征,为强过铝花岗岩。微量元素中富集Rb、Th、U、Pb、Ta、Ce、Sm、Y,明显亏损Ba、Sr、Nb、Ti、Eu,稀土总量低(ΣREE=17.49~90.43μg/g),轻稀土富集(LREE/HREE=6.43~15.64,LaN/YbN=5.26~10.94),中稀土轻度富集(GdN/YbN=1.76~2.26),具显著的负Eu异常(δEu=0.07~0.23)。岩体具高86Sr/87Sri(0.773980~0.742118)和低εNd(t)(-11.3^-12.1)的同位素特征,Nd模式年龄(1931~1992Ma)及残留锆石年龄(581~2492Ma),都指示阳明山花岗岩为典型的壳源型花岗岩,是在地壳伸展—减薄的构造背景下,古元古代变泥质岩减压熔融的产物。此外,获得多组继承/捕虏锆石年龄值(581~2492Ma),为本区存在早前寒武纪变质结晶基底及华南陆壳具多阶段生长演化特征提供了佐证。

The Yangmingshan granitic pluton is mainly composed of two-mica monzonitic granites and tourmaline-- muscovite granites. Single-Zircon La-ICP-MS U-Pb dating indicates that it was formed during the late Indosinian, as two zircon ages of 218.0±10.0 Ma and 218.9±3.4 Ma are obtained. In geochemistry, it is a strongly peraluminous granitic pluton and characterized by higher SiO2 （74.33%-76.03% ）, Al2O3（13.34%-14.47%） and P2Os（0.21%-0. 34%） contents, as well as higher A/CNK （1. 15-1. 32）. The granite is enriched in Rb, Th, U, Pb, Ta, Ce, Sm and Y but depleted in Ba, Sr, Nb, Ti and Eu. It displays slightly lower total REE content （EREE= 17.49-90.43μg/g）, enrichment of LREE [（LREE/HREE= 6.43± 15. 64, （La/Yb）N= 5.26=10.94）-1, slight enrichment of MREE [（Gd/Yb）n= 1.76-2.26-1 and strong negative Eu anomaly （δEu=0.07=0.23）. It also has higher,（86Sr/87Sr）i（0. 773980-0. 742118） but lower δNd（t）（- 11.3 to -12.1） with Nd model ages （1931-1992 Ma） accordant with ages of inherited zircons （581-2492 Ma）. These consistently indicate that the Yangmingshan granite belonged to crust-sourced granite and was derived from decompressional partial melting of Paleoproterozoic metamorphic pelite during the thinning and extension of the curst. Moreover, inherited zircon ages （581-2492 Ma） in this pluton provide evidence for the exiting of an early Precambrian crystalline basement and multistage evolution of crust in this region.