南秦岭勉略构造带中堂沟火山岩地球化学特征及其地质意义

Geochemical characteristics of the volcanic rocks from Zhongtanggou area of Mianlue suture zone in South Qinling Mountains and their geological significance

通过对分布于勉略构造带中段三岔子西侧中堂沟地区的火山岩进行详实的野外地质剖面调查,发现其为一套绿片岩相浅变质火山岩,岩石组合以变安山岩为主,含有少量的变玄武岩和变英安岩,主体为钙碱性系列。岩石地球化学研究表明3类岩石均具有轻稀土富集型稀土配分曲线,并显示弱的Eu负异常;微量元素方面,岩石普遍富集大离子亲石元素Cs、Rb、Ba,亏损高场强元素Nb、Ta,相对亏损P、Ti,不活动微量元素Zr、Hf、Tb、Y等既无明显的相对亏损也无显著的相对富集。在La-La/Nb、Nb-Nb/Th和La/Yb-Sc/Ni等环境判别图解中,变玄武岩和变安山岩样品点均落在岛弧火山岩范围内,并结合其岩石组合特征指示该套火山岩应形成于大陆边缘弧(活动陆缘)环境。其中变玄武岩和变安山岩岩浆起源于楔形地幔的部分熔融,且有陆壳物质的参与;变英安岩的形成则与壳源物质的部分熔融有关。

A detailed geological cross-sectional survey was conducted for the volcanic rocks in Zhongtanggou area on the western side of Sanchazi along Mianlue suture zone.The volcanic rocks are mainly greenschist facies metamorphic rocks,which comprise mainly metamorphic andesite rocks and subordinately metamorphic basalt and metamorphic dacite rocks,belonging mainly to the calc-alkaline series.Their REE distribution patterns are characterized by LREE enrichment,right-oblique shape and depletion of Eu.Their trace elements generally have features of enrichment of Cs,Rb and Ba,obvious depletion of Nb and Ta,and slight depletion of P and Ti.Trace elements such as Zr and Hf are relatively stable.In the La-La/Nb,Nb-Nb/Th and La/Yb-Sc/Ni discrimination diagrams,the metamorphic basalt and metamorphic andesite samples mainly fall in the region of island arc volcanic rocks.These characteristics,in combination with features of volcanic rocks,indicate that the volcanic rocks in Zhongtanggou area were formed in a typical tectonic setting of island arc.The authors hold that the material source of metamorphic andesite and metamorphic basalt rocks might have been closely related to a locally depleted area in the mantle wedge with the addition of continental crustal material.The material source of metamorphic dacite was probably related to the partial melting of the crustal granitic magma.