矿物结构特征对岩浆混合过程的指示:以新西兰Ruapehu全新世安山岩中的辉石为例  

Implications of Mineral Textures for Magma Mixing: A Case Study of Pyroxenes from Holocene Ruapehu Andesite, New Zealand

作　　者：洪文涛[1] 王天刚[1] 邢光福[1] 姚仲友[1] 林培军[2] 

机构地区：[1]中国地质调查局南京地质矿产研究所,南京210016 [2]中国冶金地质总局山东局测试中心,济南250100

出　　处：《高校地质学报》2015年第3期478-491,共14页Geological Journal of China Universities

基　　金：中国地质调查局:"中国及邻区构造演化过程中几个关键时段火山岩研究"(12120113014000);"西南太平洋重要成矿带矿床地质背景;成矿作用和找矿潜力研究"(12120114010501);"太平洋构造域构造格局;演化历史和1:250万构造图编制"(12120115070701)联合资助

摘　　要：根据矿物结构可以有效的识别开放性体系内的各种岩浆过程,在岩相学研究基础上,对全新世喷发的Ruapehu火山Whakapapa组安山岩内的辉石斑晶进行了系统的矿物学分析。所研究样品中辉石斑晶以普通辉石及顽火辉石为主,具多样结构与形态,根据其结构和成分特征可分为3类:(1)骸晶状的辉石(SP型),具有破碎的、筛孔状外形,核部具有异常高的Fe OT含量,可能为壳源捕掳晶;(2)具均一结构及成分的辉石(HP型),其内部未显示明显的熔融结构,形成于岩浆房的内部,基本未受基性岩浆混合的影响;(3)具核-幔-边结构的"绿核"辉石(GCP型),其幔部相比核部及边部具有更高的En、Mg#、Ca O、Mg O和Cr2O3含量以及更低的Fe OT、Ti O2含量。且GCP辉石边部相比核部更加富Ca O、Mg O而贫Fe OT、Ti O2。GCP型辉石幔部(边部)与核部形成明显的反环带结构,两者界线清晰,成分上存在突变,是偏基性岩浆注入岩浆房边界层时,岩浆发生大规模扩散、混合之前形成于脉状岩浆"通道"内;而GCP辉石边部则结晶于混合后的岩浆之中。辉石的结构及岩相学特征暗示,安山岩在喷发前经历了岩浆混合及地壳物质的混染,混合过程主要发生于地壳浅部岩浆房的边界层及粥状层内,且这种混合过程可能是诱发火山喷发的重要机制之一。Mineral textures and zoning provide a way to investigate the evolution process of an open magmatic system. In this study, petrographic and EMPA mineral analysis have been taken in the pyroxene phenocrysts from Holocene Ruapehu andesite. These pyroxenes are augite and enstatite, and can be divided into three types based on their texture and compositional features, including homogeneous pyroxenes (HP), green-core pyroxenes (GCP) and skeletal pyroxenes (SP). HP pyroxenes are crystallizing in a steady, showing homogeneous composition from core to rim, and lacking any dissolution texture. GCP exhibit the core-mantle-rim texture. Comparing with the core and the rim, the mantle zones show higher En, Mg#, CaO, MgO and Cr2O3 contents, while lower FeOT, TiO2 contents, displaying clearly reverse zoning. Broken cellular core of SP show similar compositions to the crustal meta-igneous xenolith, indicating a xenocrystic origin for the SP and crustal contamination during magma ascent. The reverse zoning of pyroxenes and other petrographic features suggest that the mixing mainly take place in the rigid and mush zone of shallow magma chamber, and just prior to the eruption.

关 键 词：岩浆混合 反环带 辉石 安山岩 Ruapehu 新西兰 

分 类 号：P588.1[天文地球—岩石学]