Geochemistry and petrogenesis of post-collisional alkaline and peralkaline granites of the Arabian-Nubian Shield: a case study from the southern tip of Sinai Peninsula, Egypt
Geochemistry and petrogenesis of post-collisional alkaline and peralkaline granites of the Arabian-Nubian Shield: a case study from the southern tip of Sinai Peninsula, Egypt
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阿拉伯-努比亚地盾碰撞后碱性和过碱性花岗岩的地球化学和岩石成因：埃及西奈半岛南端的案例研究
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发表时间：
2018
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通讯作者：
P. Asimow
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作者：
A. Khalil;M. Obeid;M. Azer;P. Asimow
摘要
西奈半岛南部位于阿拉伯 - 努比亚地盾最北端的延伸部分之下，出露有碰撞后钙碱性和碱性花岗岩，它们代表了新元古代晚期火成活动的最年轻阶段。我们报道了对碱性以及在某些情况下过碱性花岗岩的碰撞后深成岩体进行的岩相学、矿物学和地球化学研究。这些花岗岩侵入变质的围岩以及同碰撞和碰撞后钙碱性花岗岩类。西奈半岛南端的碱性和过碱性花岗岩分为三个亚群：正长花岗岩、碱长花岗岩和钠闪石花岗岩。这些亚群的岩石主要由碱长石和石英组成，含有不同数量的斜长石和镁铁质矿物。正长花岗岩和碱长花岗岩含有少量（通常少于3%）的钙质角闪石和黑云母，而钠闪石花岗岩以钠质角闪石（5 - 10%）为特征。这些深成岩体具有碰撞后A型花岗岩的典型地球化学特征，很可能是在造山环境和非造山环境之间的过渡时期侵位的。母源镁铁质岩浆可能与岩石圈拆沉以及软流圈地幔物质的上涌有关。底侵玄武质岩浆在新生地壳的作用下发生分异，最终形成了碰撞后碱性花岗岩。对所研究的花岗岩套的岩石成因模拟表明，单纯的分离结晶作用无法定量解释该套岩石的化学变化，主要氧化物和几种微量元素显示出与平衡相组合所要求的相反的趋势。相反，我们表明从正长花岗岩经碱长花岗岩到钠闪石花岗岩的成分变化主要是由一种比最低二氧化硅含量的正长花岗岩更原始或同样原始的低二氧化硅液体与一种可能是下地壳高度部分熔融形成的演化的高二氧化硅液体之间的混合所主导。
ABSTRACT The southern Sinai Peninsula, underlain by the northernmost extension of the Arabian-Nubian Shield, exposes post-collisional calc-alkaline and alkaline granites that represent the youngest phase of late Neoproterozoic igneous activity. We report a petrographic, mineralogical and geochemical investigation of post-collisional plutons of alkaline and, in some cases, peralkaline granite. These granites intrude metamorphosed country rocks as well as syn- and post-collisional calc-alkaline granitoids. The alkaline and peralkaline granites of the southern tip of Sinai divide into three subgroups: syenogranite, alkali feldspar granite and riebeckite granite. The rocks of these subgroups essentially consist of alkali feldspar and quartz with variable amounts of plagioclase and mafic minerals. The syenogranite and alkali feldspar granite contain small amounts of calcic amphibole and biotite, often less than 3%, while the riebeckite granite is distinguished by sodic amphibole (5–10%). These plutons have geochemical signatures typical of post-collisional A-type granites and were most likely emplaced during a transition between orogenic and anorogenic settings. The parental mafic magma may be linked to lithospheric delamination and upwelling of asthenospheric mantle material. Differentiation of the underplated basaltic magma with contributions from the juvenile crust eventually yielded the post-collisional alkaline granites. Petrogenetic modelling of the studied granitic suite shows that pure fractional crystallization cannot quantitatively explain chemical variations with the observed suite, with both major oxides and several trace elements displaying trends opposite to those required by the equilibrium phase assemblage. Instead, we show that compositional variation from syenogranite through alkali feldspar granite to riebeckite granite is dominated by mixing between a low-SiO2 liquid as primitive or more primitive than the lowest-SiO2 syenogranite and an evolved, high-SiO2 liquid that might be a high-degree partial melt of lower crust.