ABSTRACT

This paper deals with the mineral chemistry and geochemistry of ophiolitic peridotites from three localities in the Eastern Desert of Egypt, namely; Wadi Muweilha, Wadi Arayis and Gabal Kurbiyay. The present peridotites show variable degrees of alteration, including; serpentinized peridotites, serpentinites and talc-carbonate rocks with small-scale outcrops of chromitites, serpentinized pyroxenites and chloritites. Cr-spinel together with olivine and pyroxene relics represent the most common primary phases in the studied rock types. The investigated rocks represent mantle residues that experienced high degrees of partial melting. The Cr numbers of Cr-spinels reveal that serpentinized pyroxenites exhibit lower degrees of partial melting (~35%) than serpentinites and chromitites (> 35%). The whole-rock trace and rare earth elements (REE) data together with the elevated Cr# of Cr-spinels (>0.6), high Mg# of olivines (Fo = 90.82–92.61) and depleted composition of clinopyroxene are consistent with a fore-arc supra-subduction zone environment. The enrichment of light rare earth elements (LREEs) relative to heavy ones (HREE) in samples of W. Muweilha and W. Arayis could be attributed to the induction of serpentinization-related hydrothermal fluids. In contrast, the studied samples of G. Kurbiyay are depleted in LREE compared with HREE in common with a previously formed abyssal peridotite that later modified in a fore-arc region. The serpentinized pyroxenites and chloritites are speculated a genetic relationship with the rest of the mantle suite. The former is produced by the assimilation of subducted sediments in arc-related magmas. The latter is considered to be products of retrograde metamorphism simultaneously with the obduction and thrusting of the ophiolitic ultramafics. The investigated Cr-rich chromitites were formed from boninitic melts resulting from the high-degree melting of the sub-arc depleted mantle in the presence of slab-derived fluids at a mature-arc stage.

摘要

本文研究了埃及东部沙漠三个地区的脂橄榄石橄榄岩的矿物化学和地球化学。瓦迪·穆韦拉（Wadi Muweilha），瓦迪·阿赖伊斯（Wadi Arayis）和加巴尔·库比耶（Gabal Kurbiyay）。目前的橄榄岩显示出不同程度的变化，包括：蛇纹岩橄榄岩，蛇纹岩和滑石碳酸盐岩，小范围露有铬铁矿，蛇纹石辉石岩和绿泥石。Cr-尖晶石以及橄榄石和辉石遗迹代表了所研究岩石类型中最常见的初级相。所研究的岩石代表经历高度局部熔融的地幔残留物。Cr-尖晶石的Cr值表明，蛇纹石化的辉石矿的部分熔融度（〜35％）低于蛇纹石和铬铁矿（> 35％）。整个岩石中的痕量和稀土元素（REE）数据，以及Cr-spinels的Cr＃升高（> 0.6），橄榄石的Mg＃高（Fo = 90.82–92.61）和斜长石的耗尽组成与前-弧超俯冲带环境。W. Muweilha和W. Arayis样品中轻稀土元素（LREE）相对于重稀土元素（HREE）的富集可归因于蛇纹石化相关的热液。相反，与先前形成的深渊橄榄岩相比，H。Kurbiyay对G. Kurbiyay的研究样品的LREE减少了，而后来在前弧区域进行了深槽橄榄岩的修饰。蛇纹石化的辉石岩和绿泥石被推测与其余的地幔套有遗传关系。前者是由于弧相关岩浆中俯冲沉积物的同化而产生的。后者被认为是逆行变质作用的产物，同时还引起了脂溶性超镁铁质的形成和推进。所研究的富铬铬铁矿是由板状熔体形成的，该板状熔体是在存在板坯衍生流体的情况下，在成熟弧段阶段，亚弧贫化地幔高度熔融而形成的。