Multiple Stages of Carbonation and Element Redistribution during Formation of Ultramafic-Hosted Magnesite in Neoproterozoic Ophiolites of the Arabian-Nubian Shield, Egypt
Multiple Stages of Carbonation and Element Redistribution during Formation of Ultramafic-Hosted Magnesite in Neoproterozoic Ophiolites of the Arabian-Nubian Shield, Egypt
复制标题
埃及阿拉伯-努比亚地盾新元古代蛇绿岩中超镁铁质菱镁矿形成过程中的多阶段碳化和元素再分布
DOI：
10.1086/700652
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发表时间：
2019
期刊：
影响因子：
--
通讯作者：
K. Al
中科院分区：
文献类型：
--
作者：
M. Azer;H. Gahlan;P. Asimow;Heba S. Mubarak;K. Al
我们对埃及加迪尔 - 莫哈加尔 - 安博地区的蛇纹石化橄榄岩进行了研究。它们代表了一个肢解的蛇绿岩的地幔部分，构造上侵位于岛弧组合的火山 - 沉积层序之上。蛇纹岩从绿片岩相到低角闪岩相发生了不同程度的变质，发生了交代作用和蚀变，包括滑石 - 碳酸盐岩和石英 - 碳酸盐岩的发育，特别是沿剪切带和断层面。然而，一些样品含有原生铬尖晶石、橄榄石和辉石的残余。残余结构和全岩成分（Mg#[摩尔Mg/(Mg + Fe2+)] = 0.92 - 0.93，具有低的Al2O3和CaO含量）都表明原岩为方辉橄榄岩。残余橄榄石的高Mg#和Ni含量以及新鲜铬尖晶石核的高Cr#（摩尔Cr/(Cr + Al)）表明原岩经历了高度的部分熔融提取（约34% - 39%），远远超过了残余物中单斜辉石耗尽的极限，并且与弧前俯冲带上部环境中的形成过程一致。研究区的蛇纹石化超镁铁岩分为块状蛇纹岩、蛇纹岩中的菱镁矿矿体以及菱镁矿填充的矿脉。菱镁矿矿石的碳酸化和形成经历了两个交代阶段；第一个阶段以菱镁矿矿体为代表，与蛇纹石化过程中的深部交代作用和蚀变有关，而第二个成脉阶段发生在蛇纹石化之后，在蛇绿岩仰冲过程中。块状蛇纹岩和蛇纹岩中的菱镁矿矿体之间化学成分的差异表明，在碳酸化过程中一些元素被淋滤，而另一些元素富集；相对于寄主块状蛇纹岩，蛇纹岩中的菱镁矿矿体中MgO、Cr和Ni贫化，而Fe2O3、CaO、MnO、Nb、Ba、Cu、Pb、Sr和Zn富集。
We present a study of the serpentinized peridotites of the Ghadir-Mohagar-Ambaut area, Egypt. They represent the mantle section of a dismembered ophiolite, tectonically emplaced over a volcanosedimentary succession of island arc assemblages. The serpentinites are variably metamorphosed from greenschist to lower-amphibolite facies, metasomatized, and altered, including development of talc-carbonate and quartz-carbonate rocks, especially along shear zones and fault planes. Nevertheless, some samples contain relics of primary chromian spinel, olivine, and pyroxenes. Relict textures and whole-rock compositions (Mg#[molar Mg/(Mg+Fe2+)]=0.92–0.93, with low Al2O3 and CaO contents) both suggest harzburgite protoliths. The high Mg# and Ni contents of relict olivine and the high Cr# (molar Cr/(Cr+Al)) of fresh chromian spinel cores indicate that the protoliths experienced high degrees of partial melt extraction (∼34%–39%), well beyond the limit for exhaustion of clinopyroxene from the residue and consistent with formation in a forearc suprasubduction zone environment. The serpentinized ultramafic rocks in the study area are divided into massive serpentinite, serpentinite-hosted magnesite masses, and magnesite-filled veins. The carbonation and formation of magnesite ores took place through two metasomatic stages; the first is represented by the magnesite masses and associated with deep-seated metasomatism and alteration during serpentinization, whereas the second, vein-forming stage postdates serpentinization and occurred during obduction of the ophiolite. The differences in chemical composition between massive serpentinite and serpentinite-hosted magnesite masses suggest leaching of some elements and enrichment of others during carbonation; MgO, Cr, and Ni are depleted, whereas Fe2O3, CaO, MnO, Nb, Ba, Cu, Pb, Sr, and Zn are enriched in the serpentinite-hosted magnesite masses, relative to the host massive serpentinite.