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

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.

阿拉伯-努比亚地盾（Arabian-Nubian Shield）最北端延伸带基底之下的南西奈半岛，出露了碰撞后钙碱性与碱性花岗岩，其代表了新元古代晚期岩浆活动的最晚阶段。本文报道了碱性（部分为过碱性）花岗岩碰撞后侵入体的岩相学、矿物学与地球化学研究成果。该类花岗岩侵入于变质围岩以及同碰撞、碰撞后钙碱性花岗质岩石之中。西奈半岛南端的碱性与过碱性花岗岩可划分为三个亚类：正长花岗岩、碱长花岗岩与钠闪石花岗岩。各亚类岩石的主要矿物组成为碱性长石与石英，辅以含量不等的斜长石与铁镁矿物。正长花岗岩与碱长花岗岩含少量钙闪石与黑云母，含量通常低于3%；而钠闪石花岗岩则以含钠质闪石（5%~10%）为典型特征。该类侵入体具有碰撞后A型花岗岩的典型地球化学特征，其形成很可能处于造山环境向非造山环境的过渡阶段。其原始铁镁质母岩浆可能与岩石圈拆沉以及软流圈地幔物质上涌相关。底侵玄武质岩浆经分异演化，并混入新生地壳物质，最终形成了本次研究的碰撞后碱性花岗岩。对本次研究的花岗岩组合进行岩石成因模拟后发现，单纯的分离结晶无法定量解释观测到的地球化学变化：主量氧化物与部分微量元素的变化趋势与平衡矿物组合所要求的趋势相悖。与此相反，本次研究表明，从正长花岗岩、碱长花岗岩到钠闪石花岗岩的成分变化，主要由两种端元岩浆混合所主导：一种是SiO₂含量较低的原始（或比SiO₂含量最低的正长花岗岩更原始）岩浆，另一种是演化程度较高、SiO₂含量较高的岩浆，后者可能是下地壳高度部分熔融的产物。