摘要
达拉布特蛇绿岩位于中亚造山带西南缘,是古亚洲洋的扩张、俯冲、消减和闭合过程的产物,保留了洋盆形成及构造演化信息。前人对达拉布特蛇绿岩的形成大地构造背景始终未取得统一的认识。为探讨蛇绿岩所代表的构造演化过程,笔者等以达拉布特蛇绿岩中的地幔橄榄岩为研究对象,通过详细的矿物地球化学及其氧同位素研究,对达拉布特地幔橄榄岩成因及构造背景提出新的制约。达拉布特方辉橄榄岩中橄榄石Ni/Co值为21~22,Ni/Mn值为3.0~7.8具有部分熔融残余的特征,此外,橄榄石中不相容元素相对于正常地幔橄榄石亏损,表明方辉橄榄岩为部分熔融的残余组分。方辉橄榄岩中尖晶石Cr^(#)为47~52、TiO_(2)含量0.01%~0.04%,橄榄石Fo为90.34%~90.98%指示方辉橄榄岩经历>20%的部分熔融。方辉橄榄岩中橄榄石δ^(18) O_(olivine)值+5.1~+6.2‰、单斜辉石δ^(18)O_(cpx)值+5.6~+6.9‰,其变化范围较大且整体高于正常地幔中橄榄石和单斜辉石的δ^(18) O值,矿物间氧同位素分馏系数Δ^(18)_(Opx—olivine)平均-0.3‰,Δ^(18)O_(px—cpx)平均-0.7‰,显著区别于正常地幔中平衡的氧同位素分馏系数,具有明显的交代作用特征。结合方辉橄榄岩橄榄石中亏损的微量元素特征,认为方辉橄榄岩可能受到流体交代作用的影响,俯冲壳源物质脱水形成的高δ^(18)O流体交代地幔橄榄岩导致了矿物与矿物间不平衡的氧同位素特征。通过尖晶石与单斜辉石成分判别,尖晶石与单斜辉石的主量元素具有介于弧前与深海地幔之间的过渡型特征,与俯冲初始阶段形成的地幔橄榄岩相似。综合矿物地球化学与氧同位素特征,笔者等认为达拉布特地幔橄榄岩为形成于弧前初始俯冲环境。板块俯冲导致弧前扩张形成新洋壳,上涌的软流圈MORB-like熔体与俯冲壳源物质熔融形成熔/流体与地幔橄榄岩相互作用,形成达拉布特地幔橄榄岩。
Objectives:The Darbute ophiolites in the southwestern CAOB(Central Asia Orogenic Belt)are the products of the expansion,subduction,and closure of the ancient Asian ocean basin in the CAOB and record the historical formation and the tectonic evolution of the ocean basin.Scholars are yet to reach a consensus on the understanding of the tectonic background resulting in the formation of the Darbute ophiolites.To explore the tectonic evolution process represented by the ophiolite,this paper presents a new constraint on the genesis and tectonic setting of the Darbute mantle peridotite through comprehensive mineral geochemistry and the study of its oxygen isotope using the mantle peridotite in the Darbute ophiolite as the object of study.Methods:The author conducted a comprehensive mineral geochemical and oxygen isotopic study of mantle peridotite in the Darbute ophiolite.Results:The olivine Ni/Co values in the Darbute mantle peridotite are 21~22,with the Ni/Mn ratios being3.0~7.8,which represent the residual components of partial melting.Furthermore,the incompatible elements in the peridotite are deficient relative to the normal mantle peridotite,which is an indication that the harzburgite is the residual components after partial melting.The spinel Crof 47~52,TiOcontent of 0.01%~0.04%,and olivine Fo of 90.34%~90.98%suggest that the harzburgite experienced>20%partial melting.The olivine values of δO+5.1‰~+6.2‰and clinopyro xeneδO+5.6‰~+6.9‰in the harzburgite,which vary extensively and are generally higher than theδO values of the olivine and the clinopyroxene in the normal mantle.The oxygen isotope fractionation coefficients between mineralsΔOaverage-0.3‰andΔOaverage-0.7‰,differ significantly from the oxygen isotope fractionation coefficients in the normal mantle equilibrium and have obvious metasomatism characteristics.When combined with the characteristics of trace elements depleted in peridotite olivine,there is evidence to infer that peridotite may be affected by fluid metasomatism.Comparing the compositions of the spinel and the clinopyroxene in the mantle peridotite produced in different tectonic environments,the major elements in this paper which are the spinel and clinopyroxene have transitional characteristics between the fore-arc and abyssal mantle peridotite,similar to the mantle peridotite formed at the initial suhduction stage.Conclusions:Combining the mineral geochemistry and the oxygen isotope characteristics gives evidence to suggest that the Darbute mantle peridotite was formed in the initial subduction environment in the fore-arc region.The initial subduction of the plate leads to the dissemination of the for-are region to form a new oceanic crust,and the upwelling of asthenosphere material due to decompression melting to form MORB-like melts.With the subduction of the plate,the melting of the crustal source material results in the intrusion of the melted fluid into the mantle wedge,which enhances the partial melting of the depleted mantle significantly and leads to the formation of the refractory harzburgite.Furthermore,highδO fluids formed by the dehydration of the crustal sourced material during the subduction interacted with the mantle peridotites resulting in an unbalanced oxygen isotopic signature between the minerals.The Darbute peridotites were formed through the interaction between the peridotites and the MORB-like melting of the upwelling asthenosphere,the melt/fluid from subductedürustal material.
作者
钱丰
田亚洲
武勇
杨经绥
QIAN Feng;TIAN Yazhou;WU Yong;YANG Jingsui(College of Resources and Environmental Engineering,Key Laboratory of Karst Georesources and Environment,Ministry of Education,Guizhou University,Guiyang,550025;Key Laboratory of Uranium Resources Exploration and Evaluation Technology,Beijing Research Institute of Uranium Geology,Beijing,100029;School of Earth Sciences and Engineering,Nanjing University,Nanjing,210023;Center for Advanced Research on Mantle(CARMA),Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources,Institute of Geology,Chinese Academy of Geological Sciences,Beijing,100037)
出处
《地质论评》
CAS
CSCD
北大核心
2022年第4期1216-1235,共20页
Geological Review
基金
国家自然科学基金资助项目(编号:41703036,41720104009,91955203)的成果。
