湘西南兰蓉岩体锆石SHRIMP U-Pb年龄、地球化学特征及构造背景

SHRIMP U-Pb zircon dating,geochemical features and tectonic setting of the Lanrong pluton in southwestern Hunan Province

湘西南兰蓉岩体为一加里东期小侵入体,由黑云母二长花岗岩和二云母二长花岗岩组成。(443.5±8.1)Ma的锆石SHRIMP U-Pb年龄表明花岗岩形成于早志留世早期。主量元素组成表明岩体总体属钙碱性-高钾钙碱性系列强过铝质花岗岩类。该侵入体Ba、(Ta+Nb)、Sr、P、Ti强烈亏损,Rb、(Th+U+K)、(La+Ce)、Nd、(Zr+Hf+Sm)、(Y+Yb+Lu)等相对富集;稀土元素含量较高、轻稀土富集明显、Eu显著亏损;ISr值为0.71299,εSr(t)值为120,εNd(t)值为-8.11和-8.89,t2DM为1.82和1.84Ga。C/MF-A/MF图解显示其源岩为泥质岩和砂屑岩。上述地球化学特征表明兰蓉岩体为陆壳碎屑岩石部分熔融形成的S型花岗岩。基于岩石成因、构造环境判别以及区域构造演化过程,推断兰蓉岩体的具体形成机制为:奥陶纪末—志留纪初的北流运动(板内造山运动)导致地壳增厚、升温,尔后在挤压减弱、应力松弛的后碰撞-减压构造环境下,中、上地壳酸性岩石发生部分熔融并向上侵位而形成兰蓉岩体。

Located in southwestern Hunan Province, Lanrong pluton is a small Caledonian intrusive body, which is composed of biotite monzogranites and two-mica monzogranites. The U-Pb zircon dating for a biotite monzogranite sample yields the weighted average age of （443. 5±8. 1）Ma, which indicates that the Lanrong pluton was formed in early episode of Early Silurian. The major element compositions of granites indicate that the Lanrong granitic rocks belong to calc-alkaline and high-K calc-alkaline series strongly peraluminous granitoids. Ba, （Ta＋Nb）, Sr, P and Ti of the Lanrong granites are pronounced depleted, while Rb, （Th＋U＋K）, （La＋Ce）, Nd, （Zr＋Hf＋Sm） and （Y＋Yb＋Lu） are comparatively enriched. The rocks have high∑REE contents, enriched LREE values, obviously depletedδEu values, Isr value of 0. 71299, εSr（t ） value of 120, εNd（t ） values of -8. 11 and -8.89, t2DM ages of 1. 82 Ga and 1.84 Ga. The C/MF-A/MF diagram indicates that the source rock of the Lanrong granites was originated from pelites and psammites. All above geochemical characteristics indicate that the granites are S-type gra-nitiod and were generated by the partial melting of clastic rocks of the continental crust. Basing on petro-genesis, tectonic setting discrimination and regional tectonic evolution, the formation mechanism of the Lanrong granites is inferred as follows. The Beiliu Movement（ intracontinental orogenesis during late epi-sode of Ordovician to early stage of Silurian） caused crustal thickening and heating, then the weakening of compression and relaxation of stress in post-collisional tectonic setting caused the partial melting of acidic rocks of the middle-upper crust, and upward magmatic emplacement forming the Lanrong pluton.