王仙岭岩体地质地球化学特征及其对湘东南印支晚期构造环境的制约

Geochemical characteristics of Wangxianling granitic pluton and its constraints on Late Indosinian tectonic setting of Southeast Hunan

王仙岭岩体位于湘东南,形成于晚三叠世,主要由黑云母二长花岗岩和二云母二长花岗岩组成。岩石高硅、高钾、富碱,SiO2含量为70.99%～75.93%;K2O含量平均为4.32%;Na2O+K2O为5.10%～8.67%;K2O/Na2O平均为2.09。A/CNK比值为1.07～2.44,KN/A比值为0.37～0.86,属高钾钙碱性系列强过铝质花岗岩类。REE含量低,平均为90.19μg/g;δEu值0.23～0.38;(La/Yb)N值为6.17～14.1;具Ba、Nb、Sr和Ti负异常。ISr值为0.72074,εSr(t)值为30,εNd(t)值为-12.0,t2DM为1.97Ga。源岩成分的A/MF-C/MF图解判别显示为碎屑岩。结合区域岩石圈结构,上述特征表明花岗岩源岩应为中地壳结晶片岩、片麻岩。根据多种氧化物与微量元素构造环境判别图解,结合地质特征、构造演化背景等,认为包括王仙岭岩体在内的湘东南印支期花岗岩形成于陆内同造山阶段的后碰撞构造环境。印支期花岗岩的具体形成机制为:在峰期变形之后挤压应力相对松弛,深部压力降低,因地壳增厚而升温的中地壳岩石因熔点降低而得以熔融,并在相对开放的环境下侵位成岩。

Wangxianling granitic pluton is a Late Triassic （Late and dimicaceous monzogranite in Southeast Hunan. The rocks ha Indosinian） body composed of biotite monzogranite Lve high silicon （SiO2 = 70.99% - 75.93% ）, high potassium （K2O = 4.32% on the average） and are rich in alkali （Na2O ＋ K2O = 5.10% - 8.67% ）, and have an average of K2O/Na2O value of 2.09, A/CNK value of 1.07 - 2. 44 and KN/A value of 0.37 - 0. 86, and belong to high-K calc-alkaline series strong peraluminous granitoids. The rocks have low ∑REE values with an average of 90. 19 μg/g, 3Eu values of 0.23 -0.38, （La/Yb）N values of 6. 17 - 14. 1, and evident negative anomalies of Ba, Nb, Sr, Ti; have ISr values of 0. 720 74, εNd（ t ） values of - 12. 0 and t2DM age of 1.97 Ga. The A/MF-C/ MF diagram indicates that the granites came from clastic rocks. According to all above characteristics and regional lithospheric structure, the granites should come from schist and gneiss of middle crust. Based on multiple oxideand trace element-diagrams for discrimination of structural environment, and geological characteristics and tectonic evolutional setting, the Indosinian granitoids （including Wangxianling pluton） formed in post-collisional tectonic setting of syn-orogenic stage. The formation mechanism of the Indosinian granitoids is as follows： the thickening of crust in primary-deformation stage caused the rise of the temperature of crust, and later weakening of stress caused the melting of middle crust, and the magma emplaced in relative open environment.