江西九瑞矿集区郎君山第三纪玄武岩的成因与岩浆演化:来自辉石和长石的矿物学证据

Petrogenesis and magma evolution of Tertiary basalts from the Langjunshan area in the Jiurui mineralization district,Jiangxi Province:Evidence from pyroxene and feldspar

近期在江西九瑞矿集区朗君山地区通过钻探获取了新鲜的第三纪火山岩样品,本文对这些样品进行了系统的矿物学研究。结果表明：这些火山岩样品均为玄武岩,可明显分为两个喷发旋回,主要组成矿物为单斜辉石和斜长石,并有少量的磷灰石和铁钛氧化物等矿物。根据其矿物成分特征判断,早期旋回的玄武岩属于钾玄岩系列,晚期旋回的玄武岩属于拉斑玄武岩系列。对比两套玄武岩中单斜辉石的成分可以发现,早期玄武岩岩浆朝着富钙富锰和贫铝贫碱的方向演化,而晚期玄武岩岩浆则朝着富钛富锰和贫铝贫钙贫铬的方向演化。早期旋回玄武岩中斜长石斑晶发育反环带和韵律环带,辉石斑晶中发育正环带、反环带和韵律环带,且基质中斜长石普遍被碱性长石所交代,推测早期喷发的岩浆遭受过多次岩浆混合作用。早期旋回玄武岩中单斜辉石形成的温度为1124～1157℃,压力为1.23～1.42GPa,相应深度为41～47km,表明这套玄武岩可能是由相对原始岩浆上升到较浅部位分异形成的。根据单斜辉石F1-F2图解和Ti O2-Al Z图解判断,该区玄武岩具有明显的与弧有关的构造环境特征,推测应为弧后拉张盆地环境。

Tertiary volcanic rocks were found recently during deep drilling in Jiurui district. A detailed mineralogical study has been carried out in this study for these rocks. The results show that the volcanic rocks are all basalts,which can be divided into two eruption cycles. They are mainly composed of clinopyroxene and feldspar,and also contain a small amount of other minerals like apatite and irontitanium oxides. The mineralogical characteristics indicate that the nature of the magma in the early cycle（ the lower layer） belongs to shoshonite series,while the nature of the magma in the late cycle（ the upper layer） belongs to tholeiite series. Comparison of clinopyroxene composition in the two cycles of volcanic rocks indicates that the magma evolution in the early cycle is towards calcium rich,manganese rich and aluminium poor and alkali poor direction,while the magma evolution in the late cycle is towards titanium rich,manganese rich and aluminium poor,calcium poor and chromium poor direction. The volcanic rocks in the early cycle contain abundant plagioclase phenocrysts which show reverse and oscillatory zoning patterns,and abundant clinopyroxene phenocrysts which show normal,reverse and oscillatory zoning patterns. The plagioclase were commonly replaced by alkaline feldspar in feldspar matrix in the rocks. It can be infered that the magma in the early cycle has gone through multi-stage magmatic mixing process. The estimated temperature and pressure of clinopyroxene in the early cycle volcanic rocks are 1124 ～ 1157℃ and 1. 23 ～ 1. 42 GPa,at depth of 41 ～ 47 km. It can be inferred that these rocks were formed during the crystallization differentiation of relatively primitive magma at a relatively shallow level. According to the F1-F2 discrimination diagram and Ti O2-AlZdiagram of the clinopyroxene,the tectonic environment of the volcanic rocks shows an obvious association of arc affinity. It can be speculated that the volcanic rocks were likely generated in a back-arc extensional basin environment.