湖南蓝山县天鹅寨火山杂岩体岩石地球化学特征与成因探讨

Geochemical Characteristics of Rocks and Petrogenesis of the Volcanic Complexes,Tian’ezhai,Hunan

天鹅寨火山杂岩体位于钦—杭(钦州—杭州)成矿带湖南段南部。各类岩石主量元素富Si、富Alk、Al饱和、贫Ca、贫Mg等,属高钾钙碱性岩系;稀土元素为轻稀土富集分配模式;微量元素富Rb、Pb、Yb、Zr、Th、U等,贫Ba、Sr、Nb、Ta、Hf、Y等。具有相似主量元素与微量元素特征,指示为同源岩浆分异演化形成。根据富K、Rb、Th、Zr,贫Na、Ta、P等元素特征结合(87Sr/86Sr)初始值与成岩年龄(t)图解,表明岩浆源区以上地壳物质为主,下地壳物质混染熔融形成。结合区域构造背景分析,推测由于太平洋板块自东向西俯冲,造成华南陆块内扬子板块与华夏板块碰撞,碰撞产生热效益使区内上地壳物质及部分下地壳物质熔融形成岩浆,至燕山晚期,区内演化成碰撞后伸展阶段,岩浆喷发—侵入形成此火山杂岩体。

The Tian’ezhai volcanic complexes is located in the southern part of Hunan setion in Qinzhou-Hangzhou metallogenic belt.Main elements of all kinds of rocks are silicon-rich,alkali-rich,aluminum-saturated,ca-poor,Mg poor,belongs to high-K calc-alkaline rock series.It is relative enriched in LREEs.Trace elements are rich in Rb,Pb,Yb,Zr,Th,U,and poor in Ba,Sr,Nb,Ta,Hf,Y,etc.All have similar characteristics of major elements and trace elements,indicates that it is a homologous magmatic differentiation.According to the characteristics of rich K,Rb,Th,Zr,poor Na,Ta,P,combining the initial value of(87Sr/86Sr)with diagenetic age(t)diagram,it is indicated that upper crust above the magmatic source area is dominant,and lower crust is co-formed by blending melting.Combined with regional tectonic background analysis,speculated that because of the subduction of the Pacific plate from east to west,the Yangtze plate collided with the Cathaysia plate in the south China continental block,collision generates thermal benefits in the region,causing the upper crust and part of the lower crust to melt and form magma,to the late Yanshan period,evolved into a post-collision extension stage,during which magmatic eruption and intrusion formed the volcanic complexes.