首页|西藏班公湖-怒江缝合带中段东巧豆荚状铬铁矿地球化学特征及构造意义

西藏班公湖-怒江缝合带中段东巧豆荚状铬铁矿地球化学特征及构造意义

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东巧蛇绿岩位于班公湖-怒江缝合带中段北部,岩石组合为方辉橄榄岩、纯橄岩、铬铁岩、辉长岩、辉石岩、玄武岩和少量的硅质岩.东巧铬铁矿石的铬尖晶石化学成分表明岩体内同时存在高铬型铬铁矿(Cr#值为60.7~79.8,Mg#值为57.2~68.1)和高铝型铬铁矿(Cr#值为56.4~59.6,Mg#值为67.2~72.7).高铬型铬铁矿体多呈稠密浸染状,部分为致密块状,而高铝型铬铁矿体规模较小,兼具稠密浸染状和稀疏浸染状.两种铬铁矿矿体的围岩都是纯橄岩,并且矿体以透镜状分布于方辉橄榄岩中.从纯橄岩包壳到条带浸染状铬铁矿石的过渡带中,铬尖晶石呈Cr#值增加(67.2~68.5),Mg#值降低(57.4~74.6)的特点,表明东巧地幔橄榄岩及其铬铁矿可能经历了一定程度的熔-岩反应.方辉橄榄岩中橄榄石Fo值为91.2~92.3,铬尖晶石Cr#值为70.2~75.1;纯橄岩橄榄石Fo值为92.7~92.9,铬尖晶石Cr#值为75.4~76.6,与班公湖-怒江缝合带上丁青、切里湖、依拉山等岩体较为相似,表明东巧地幔橄榄岩形成条件可能与其相近.东巧地幔橄榄岩具有弧前地幔橄榄岩稀土元素特征,微量元素配分模式与Izu-Bonin-Mariana具有极高相似性,指示东巧蛇绿岩可能形成于板块俯冲消减带上盘海底扩张环境.铬铁矿的铂族元素(PGE)配分模式与罗布莎高铬铬铁矿相似,具有富集IPGE、亏损PPGE的特点.对东巧地幔橄榄岩及其铬铁矿平衡熔体进行分析,高铬型铬铁矿形成于俯冲带上岛弧环境(SSZ型)的弧前盆地,高铝型铬铁矿形成于洋中脊型环境(MOR型),并对比班-怒带毗邻岩体,指示东巧地幔橄榄岩及其铬铁矿经历了 MOR→SSZ的多期次演化过程.
Geochemical characteristics and tectonic significance of Dongqiao podiform chromite in the middle segment of the Bangong-Nujiang suture zone(Tibet)
The Dongqiao ophiolite,located in the northern part of the middle segment of the Bangong-Nujiang suture zone,comprises a suite of lithologies,including harzburgites,dunites,chromitites,gabbros,pyroxenites,basalts,and minor silicitic units.The high-chromium chromitites are mostly densely impregnated,with some occurrences as dense blocks,while the high-aluminum chromitites are smaller in size and exhibit both dense and sparsely impregnated.Both chromitite types are enveloped by dunites and are distributed in lenticular forms within the harzburgites.Chemical analyses of chromium spinel from the Dongqiao chromitite reveal the presence of both high-chromium(Cr# values ranging from 60.7 to 79.8,Mg# values from 57.2 to 68.1)and high-aluminum varieties(Cr# values from 56.4 to 59.6,Mg# values from 67.2 to 72.7).In the transition zone between dunite cladding and banded disseminated chromitite,an increase in Cr# values(67.2~68.5)and a decrease in Mg# values(57.4~74.6)are observed.This suggests that the mantle peridotite and its associated chromitite have undergone varying degrees of melt-rock interaction.The olivine Fo values in harzburgites range from 91.2 to 92.3,with corresponding chromium spinel Cr# values of 70.2 to 75.1.In dunites,the olivine Fo values range from 92.7 to 92.9 and the chromium spinel Cr# values range from 75.4 to 76.6.These values are broadly consistent with those observed in the Dingqing,Qielihu,and Yilashan massifs in the Banong-Nujiang suture zone,indicating similar formation conditions for the Dongqiao mantle peridotite.The Dongqiao mantle peridotite exhibits rare earth element characteristics typical of pre-arc mantle peridotite,with trace element compositions closely resemblingthose found in the Izu-Bonin-Mariana.This suggests that the Dongqiao ophiolite may have formed in an ocean floor-spreading environment above a subduction zone.The platinum group element(PGE)patterns of the chromite display a strong affinity to those observed in Luobusa chromitites,characterized by enrichment of IPGE and depletion of PPGE.Equilibrium melt analysis of the Dongqiao mantle peridotite and chromite suggests that the high-chromium chromitite formed in a subduction zone island arc(SSZ)forearc basin environment,while the high-aluminum chromitite may have formed in a mid-ocean ridge(MOR)environment.The juxtaposition of these lithologies within the Bangong-Nujiang suture zone provides compelling evidence for a multi-stage MOR-to-SSZ evolutionary process that affected the Dongqiao mantle peridotite and its associated chromitites.

Bangong-Nujiang suture zonemantle peridotitewhole rock/mineral geochemistryDongqiao chromitite

张博扬、熊发挥、徐向珍、邱添、张承杰、何兰芳、王天泽、杨经绥

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自然资源部地幔研究中心,自然资源部深地动力学重点实验室,中国地质科学院地质研究所,北京,100037

中国冶金地质总局第二地质勘查院,福建 福州,350108

中国科学院地质与地球物理研究所,矿产资源研究重点实验室,北京,100029

西藏自治区地质矿产勘查开发局第五地质大队,青海省格尔木市,816099

南京大学地球科学与工程学院,江苏南京,210023

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班公湖-怒江缝合带 地幔橄榄岩 全岩/矿物地球化学 东巧铬铁矿

2024

地质学报
中国地质学会

地质学报

CSTPCD北大核心
影响因子:2.17
ISSN:0001-5717
年,卷(期):2024.98(11)