中国东部及西秦岭地区新生代岩石圈地幔中的相转变带及其温压条件  

Spinel-garnet phase transition zone of Cenozoic lithospheric mantle beneath the eastern China and western Qinling and its T-P condition

作　　者：苏本勋[1] 张宏福[1] 王巧云[2] 孙赫[1] 肖燕[1] 英基丰[1] 

机构地区：[1]中国科学院地质与地球物理研究所岩石圈演化国家重点实验室,北京100029 [2]中国科学院地球化学研究所矿床地球化学国家重点实验室,贵阳550002

出　　处：《岩石学报》2007年第6期1313-1320,共8页Acta Petrologica Sinica

基　　金：中国科学院知识创新工程重要方向项目(批准号:KZCX2-YW-103);\国家自然科学基金委大陆动力学重点项目(批准号:40534022)资助成果.

摘　　要：五相(橄榄石+斜方辉石+单斜辉石+石榴石+尖晶石)共存的地幔橄榄岩捕虏体是来自岩石圈地幔相转变带的直接样品。中国东部及西秦岭地区晚第三至第四纪碱性火山岩携带的少量五相共存的地幔橄榄岩捕虏体为探讨这些地区新生代岩石圈地幔中相转变带提供了宝贵的样品。本文根据地幔橄榄岩捕虏体中石榴石和尖晶石的产出状况,将这些橄榄岩捕虏体分为三类:第一类橄榄岩中尖晶石为粒状残核,尖晶石外缘被石榴石的反应边包围。这种橄榄岩捕虏体代表尖晶石一石榴石相转变带的上限,故称为尖晶石带橄榄岩;第二类橄榄岩中尖晶石和石榴石以单颗粒零散分布为特征,二者共存但未见明显的相转变关系。这类橄榄岩多位于相转变带中部,拟称为尖晶石-石榴石过渡带橄榄岩;第三类橄榄岩中以石榴石为主,尖晶石和辉石等微晶构成石榴石反应边。这类橄榄岩代表尖晶石-石榴石相转变带的下限,故称为石榴石带橄榄岩。因此,根据不同类型橄榄岩捕虏体中矿物的组成,结合温度压力估算即可确定岩石圈地幔中相转变带的深度和厚度。本文通过对中国东部及西秦岭地区晚第三至第四纪碱性火山岩携带的尖晶石-石榴石二辉橄榄岩捕虏体的温度压力估算来进一步厘定中国东部新生代岩石圈地幔中的相转变带深度和厚度。Coexistent five-phase （ Ol ＋ Opx ＋ Cpx ＋ Gt ＋ Sp） mantle peridotitic xenoliths axe direct samples from the spinel/garnet phase transition zone of the lithospheric mantle. Late Tertiary and Quaternary alkaline volcanic rocks, eastern China and western Qinling, contain few coexistent five-phase mantle peridotitic xenoliths, which provide an opportunity to directly study the phase transition zone of the lithospheric mantle beneath the regions. According to the spinel-garnet relationship, coexistent five-phase peridotitic xenoliths can be divided into three typos. Type 1 is characterized by the granular spinel relicts surrounded with the garnet reaction rim. This type peridotite represents the upper limit of the phase transition zone, and can be called spinel-zone peridotites. Type 2 poridotite, in which garnet and spinel coexist without obvious phase transition relationship, represents the middle of the phase transition zone and can be called spinel-garnet transitional zone peridotite. Type 3 is typified by the garnet mantled with the reaction rim of microerystal pyroxene and spinel. This type peridotite represents the bottom of the phase transition zone and can be named garnet- zone poridotite. Therefore, the depth and thickness of the phase transition zone can be determined through mineral compositions of peridotitic xenoliths and estimated T and P conditions. This paper, based on T and P estimates of spinel-garnet lherzolites in Late Tertiary and Quaternary alkaline volcanic rocks from eastern China and western Qinling, further constrains the depth and thickness of the phase transition zone in the Cenozoic lithospheric mantle beneath the eastern China.

关 键 词：中国东部 西秦岭 尖晶石/石榴石 相转变带 岩石圈地幔 

分 类 号：P588.125[天文地球—岩石学]