慢速扩张脊下的复杂熔-岩相互作用:来自大西洋北纬14°97′区段洋中脊含铬铁矿纯橄岩的记录  

Melt-rock reaction beneath the slow spreading ridge revealed by chromite bearing dunite from the 14°97′N area in the Mid-Atlantic Ridge

作　　者：周翔[1] 郑建平[1] 熊庆[1] 刘文龙[1] Henry J B DICK ZHOU Xiang;ZHENG Jianping;XIONG Qing;LIU Wenlong;Henry J B DICK(School of Earth Sciences,State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences,Wuhan,Hubei 430074,China;Department.of Geology Geophysics,Woods Hole Oceanographic Institution,Woods Hole,MA 02540,USA)

机构地区：[1]中国地质大学(武汉)地球科学学院,地质过程与矿产资源国家重点实验室,湖北武汉430074 [2]美国伍兹霍尔海洋研究所,地质与地球物理系,美国马萨诸塞伍兹霍尔,02540

出　　处：《地质学报》2022年第12期4242-4256,共15页Acta Geologica Sinica

基　　金：国家自然科学基金项目(编号42003027,42073030,41930215)资助的成果。

摘　　要：在大西洋洋中脊北纬14°97′附近贫岩浆型慢速扩张脊区段的洋底岩石调查中,拖网取样及洋底钻探均发现蛇纹石化地幔岩石中有脉状纯橄岩。纯橄岩中的铬铁矿粒度以毫米级为主,个别颗粒直径接近厘米级别,成分上显示中等程度的铬值与镁值(Cr#=52.2~56.1;Mg#=44.6~51.5)较高的TiO_(2)含量(0.60%~0.91%)。铬铁矿内部包裹体中大量出现韭闪石+金云母组合,并发现自形磷灰石包裹体。这些特征与“俯冲带型铬铁矿”的成分特征及矿物包裹体组合非常类似。本文的样品观察说明在洋中脊下方的熔体通道内,高度演化的熔体可以形成小体积的富水岩浆并参与铬铁矿的形成。拖网调查与钻探研究证实,研究区的纯橄岩、异剥橄榄岩、富橄榄石橄长岩、辉石岩、辉长岩及橄榄辉长岩等均以脉体形式产于难熔的大洋岩石圈橄榄岩质地幔中。这些脉状岩石共同记录了洋中脊玄武质熔体向上迁移过程中与围岩方辉橄榄岩间的熔-岩反应,属于“交代橄榄岩”的范畴,揭示了慢速扩张洋脊之下熔体迁移诱发的复杂熔-岩相互作用。Area south of the 15°20′N Fracture Zone in Mid-Atlantic Ridge(MAR)is characterized by slow spreading rate,typical ocean core complex system and low magma supply condition,where abundant mantle peridotite is exposed on the seafloor along with localized pillow basalts and gabbro.Reassessment of abyssal peridotites obtained by a large mantle dredge haul(MM-81)from a NOAA Researcher Cruise taken along the western wall of the rift valley of the MAR at 14°97′N area and cored sample by IODP Leg 209in the same region revealed the occurrence of dunites containing chromitites closely associated with harzburgites,which substantiates chromitite formation at mid-oceanic ridges.The harzburgites which represent the wall rock hosting these veined dunites with chromitites showed refractory characters,including olivine Mg#of 90to 92,elevated spinel Cr#of 45to 68,and clinopyroxene with highly depleted rare earth elements contents.The size of the chromitite pods varies from a few mm to 5cm in width.Chromite grains with nodular textures in dunite have medium Cr#(52~56),Mg#(45~52)and high TiO_(2)(=0.60~0.91)contents.EBSD orientation patterns did not recognize sub-grain fabrics within chromite cluster as those commonly appeared in ophiolitic chromite such as Oman samples(Boudier et al.,2021).Pargasite and aspidolite are widely observed in multiphase solid inclusions within chromite,together with occurrence of diopside,enstatite,serpentine(transformed from olivine),few sulfides and rare apatite,indicating a highly evolved and water riched parental melt for chromite growth in mid-ocean ridge settings.This observation suggests that the occurrence of hydrous mineral assemblages within ophiolitic chromitite cannot be used as robust evidence for their subduction origin.More importantly,the same dredge also obtained high percentage of‘impregnated peridotites’,include plagioclase-bearing harzburgite,olivinerich troctolite,plagioclase-bearing dunite,amphibole-bearing dunite,clinopyroxene-rich dunite and veined pyroxenites.All these li

关 键 词：洋中脊 铬铁矿 大西洋 熔-岩作用 

分 类 号：P736.3[天文地球—海洋地质]