斜长石结构和成分对开放体系下岩浆演化的指示:以浙江拔茅白垩纪英安岩为例

Implications of Textures and Compositions of Plagioclases for the Magmatic Evolution in an Open Magmatic System:A Case Study of Plagioclases in Cretaceous Dacite of the Bamao Volcano,Zhejiang,China

矿物的结构和成分可用来反演岩浆演化过程。为探讨酸性岩浆房内岩浆演化过程,以浙江拔茅火山主体英安岩中的斜长石为例,对其开展了矿物学、地球化学研究。结果显示,斜长石普遍具有核-幔-边结构,暗示其在喷发前处于开放性岩浆房内,并经历了复杂的岩浆过程。对代表性斜长石的电子探针成分及背散射图像分析表明,斜长石核部、幔部及边部分别具有不同的成因,反映了英安岩岩浆从深部聚集并在浅部岩浆房内周期性补给的过程。岩浆的这种周期性补给过程抑制了结晶分异作用对熔体成分的改变,全岩地球化学特征主要反映其源区特征及熔融过程。

Mineral texture and composition can be used to invert the magma evolution process.In this article,a case study of mineralogy and geochemistry of plagioclases in the dacite from the Bamao volcano in Zhejiang Province have been specifically undertaken in order to discuss the evolution process of magma in the acidic magma chamber.The results show that plagioclases of the dacite commonly have the core-mantle-rim complex zonal texture,suggesting that the magma experienced a complex magmatic process in an open magmatic system before its eruption.Chemical compositions and BSE images of the representative plagioclase crystal grains analyzed by using microprobe indicate that the cores,mantles,and rims of plagioclases have different genesis,implying an evolved process of dacitic magma from the aggregation in the deep magma chamber to the periodic replenishment in the shallow reservoir.This periodic magmatic replenishment process inhibits the compositional variation of melt due to the crystallization differentiation.The whole-rock composition of dacite could mainly reflect characteristics of the source and its melting process.