辽东岫岩地区古元古代盘道岭岩体地球化学特征及构造背景探讨

Geochemistry of the Paleoproterozoic Pandaoling pluton in Xiuyan area of eastern part of Liaoning Province and its implications for tectonic setting

【研究目的】位于华北克拉通东部陆块辽吉古元古代活动带内的盘道岭花岗岩形成于古元古代,岩性为黑云母二长花岗岩。目前关于盘道岭岩体的研究资料较少,制约了对区域大地构造背景演化的认识。【研究方法】本文基于盘道岭岩体的野外地质特征,开展了岩相学和岩石地球化学研究。【研究结果】岩石高硅、高钾、富碱,SiO2含量为74.04%~75.84%,K2O含量平均为3.81%,全碱(Na2O+K2O)含量为7.29%~7.87%,K2O/Na2O平均值为1.09,A/CNK为1.187~1.394,属于高钾钙碱性过铝质I型花岗岩类。ΣREE总量偏低,平均值为89.78×10-6,δEu值0.19~0.93,(La/Yb)N值为2.95~50.38,轻稀土相对富集,重稀土相对亏损,稀土元素球粒陨石标准化曲线呈Eu负异常的右倾“V”型特征。岩体总体上富集大离子亲石元素(LILEs)Rb、K、U和高场强元素(HFSEs)Hf,亏损Ba、Zr、Nb、Ta、P、Ti等元素,花岗岩熔体的锆石饱和温度(745℃~774℃)明显低于A型花岗岩(868℃~928℃),显示出高分异I型花岗岩的特征。【结论】岩石学及地球化学特征研究表明,盘道岭岩体形成与幔源岩浆的底侵提供的大量热量使下地壳物质发生部分熔融有关。结合地质特征、构造演化背景、元素判别图解等,认为该地区花岗岩形成于辽吉古元古代活动带晚期大陆边缘碰撞造山环境。

This paper is the result of geological survey engineering.[Objective]The Paleo-Proterozoic Pandaoling pluton is a granitic body in the Liaoji active belt,Eastern Block of the North China Craton,which is mainly composed by biotite monzogranite.It has been seldom researched so far,which restricts the recognition of evolution of tectonic setting in North China.[Methods]Based on the field geological characteristics of the Pandaoling pluton,it has been studied in terms of petrography and geochemistry in this paper.[Results]The rocks contain high silicon(74.04%-75.84%),high potassium(3.81%in average)and are rich in alkali(Na2O+K2O=7.29%-7.87%),and the average value of K2O/Na2O and A/CNK are 1.09 and 1.187-1.394,respectively.The rocks belong to high-K,calc-alkaline series peraluminous I-type granite.The granites have lowΣREE contents(with average of 89.78×10-6)and obvious negative Eu anomalies(0.19-0.93).They has variable(La/Yb)N ratios of 2.95 to 50.38,and display enriched LREE patterns,with relatively enrichment of LILEs(Rb,K,U)and HFSEs(Hf),and low contents of Ba,Zr,Nb,Ta,P and Ti.Zircon saturation temperatures(745-774℃)of granite melt are significantly lower than the A-type granites(868℃-928℃).These characteristics suggest that the Pandaoling monzogranite is characteristic of high fractionated I-type granite.[Conclusions]The petrological and geochemical features of the Pandaoling pluton indicate that they were originated from partial melting of the deep crust due to mantle magma intraplating.Based on tectonic history and the diagram of structure environment,the Pandaoling pluton was possibly formed in the continental borderland collision tectonic setting of Liaoji Paleoproterozoic active blet.