埃及开罗大学Mokhtar Hesham团队近日取得一项新成果。他们报道了埃及东部沙漠的南部Wadi El Gemal地区花岗岩的地球化学和矿物化学特征，并揭示了高度分馏的同碰撞到后碰撞新元古代长英质岩浆作用的标志。相关论文于2024年7月5日发表在《地球化学学报》杂志上。

本研究探究了埃及东部沙漠南部的Wadi El Gemal地区的浅色花岗岩、伟晶岩和细晶岩的地球化学成因。这些岩石代表了晚新元古代阿拉伯-努比亚地盾(ANS)演化中，泛非造山晚期长英质岩浆作用的重要事件。

岩相学显示，浅色花岗岩有时含石榴子石花岗岩，可分为二长花岗岩、正长花岗岩和碱性长石花岗岩。对白云母、黑云母、石榴子石和磷灰石的分析揭示了所研究的浅色花岗岩的岩浆性质。所研究的浅色花岗岩、伟晶岩和细晶岩为碱钙型、钙碱性和过铝型。研究人员通过对黑云母的化学分析，证明了这些岩石的过铝质性质。这些样品显示出轻微富集轻稀土元素(LREEs)和大离子亲石元素(LILE，尤其是Rb和Th)，轻微亏损重稀土元素(HREEs)的特点。从地球化学角度来看，研究区内的浅色花岗岩体、伟晶岩和细晶岩，是由镁铁质、变质灰流和泥质等多重熔体结晶而成的。

Zr饱和地温计证明了它们来自于熔体，产生于结晶温度为568-900°C的浅色花岗岩、553-781°C的伟晶岩和639-779°C的细晶岩，压力约为0.39-0.48 GPa。研究的长英质岩石具有强烈的负Eu异常，这与上地壳成分相一致，表明长石堆积体的分馏。此外，La/Sm比值适中，表明岩浆过程以部分熔融和分离结晶为代表。全岩石化学成分和矿物显微分析表明，西Wadi El Gemal地区的长英质岩浆活动产生了大量的同碰撞到后碰撞花岗岩、伟晶岩和细晶岩。从地球动力学的角度出发，本文提出了一个演化的三阶段岩浆活动模型。该模型揭示了在同碰撞阶段到碰撞后阶段长英质岩浆活动在ANS中的传播过程。

附：英文原文

Title: Geochemistry and mineral chemistry of granitic rocks from west Wadi El Gemal area, southern Eastern Desert of Egypt: Indicators for highly fractionated syn- to post-collisional Neoproterozoic felsic magmatism

Author: Mokhtar, Hesham, Surour, Adel A., Azer, Mokhles K., Ren, Minghua, Said, Amir

Issue&Volume: 2024-07-05

Abstract: Leucogranite, pegmatite, and aplite from selected areas in the Wadi El Gemal area in the southern Eastern Desert of Egypt were investigated geochemically for their petrogenesis. These rocks represent a significant episode of felsic magmatism during the late stage of the Pan-African orogeny in the evolution of the Arabian–Nubian Shield (ANS) during the Late Neoproterozoic. On a petrographic basis, the leucogranite is sometimes garnetiferous and can be distinguished into monzogranite, syenogranite, and alkali feldspar granite. The analyses of muscovite, biotite, garnet, and apatite reveal the magmatic nature of the studied leucogranite. The investigated leucogranite, pegmatite, and aplite are alkali-calcic, calc-alkaline, and peraluminous. The peraluminous nature of these rocks is evidenced by using the chemical analyses of biotite. These studied rocks show a slight enrichment in light rare-earth elements (LREEs) and large-ion lithophile elements (LILE, especially Rb and Th), with an insignificant depletion of heavy rare-earth elements (HREEs). On a geochemical basis, the leucogranite, pegmatite, and aplite in the study area crystallized from multiple-sourced melts that include mafic, metagraywake, and pelitic. They were derived from melts generated at crystallization temperatures around 568–900 °C for leucogranite, 553–781 °C for pegmatite, and 639–779 °C for aplite based on the Zr saturation geothermometers, and at a pressure around 0.39–0.48 GPa, i.e. shallow depth intrusions. The studied felsic rocks have strong negative Eu anomalies, which are very consistent with an upper crust composition, indicating fractionation of feldspar cumulates. Also, they show a moderate La/Sm ratio indicating combined magmatic processes represented by partial melting and fractional crystallization. Integration of whole-rock chemical composition and mineral microanalysis suggests that felsic magmatism in the west Wadi El Gemal area produced voluminous masses of syn- to post-collisional granite, pegmatite, and aplite. An evolutionary three-stage model is presented to understand late magmatism in the ANS in terms of a geodynamic model. Such a model discusses the propagation of felsic magmatism in the ANS during syn-collisional to post-collisional stages.

DOI: 10.1007/s11631-024-00714-1

Source: https://link.springer.com/article/10.1007/s11631-024-00714-1