Geochemistry and petrogenesis of Pan-African I-type granitoids at Gabal Igla Ahmar, Eastern Desert, Egypt

Abstract

Younger granites (post tectonic) are common throughout the Precambrian igneous/metamorphic terrain of Egypt and they played a significant role in the evolution of the Pan-African crust. The Gabal Igla Ahmar pluton comprises two magmatic suites: a calc-alkaline diorite/quartz diorite-granodiorite suite and an aluminous monzogranite-granophyre suite. The calc-alkaline rocks have low K 2O, relatively low LREE and display fractionated HREE ( Tb Yb n = 1.3–2.2 ). They appear to represent a suite of andean-type intrusives emplaced in an active continental margin. The monzogranites are metalummous to slightly peralummous, highly differentiated I-type granitoids apparently representing a post-collision phase of intrusion. Three distinct petrogenetic models for magma genesis are suggested to explain the petrological, major, trace and REE element variations in these magmatic suites: 1. i) The calc-alkaline quartz diorite was derived by partial melting of garnet amphibolite leaving a homblende-rich residue. 2. ii) The monzogranites evolved by 75–85% crystal fractionation of the quartz diorite melt. The crystallization took place at depth from a water saturated magma of minimum melt composition. After a further interval, the granitic melt was emplaced at shallow crustal levels at pressures of 1–3 kbar. 3. iii) Simple mixing of quartz dioritic and granitic melts as two end-member components could explain the origin of the granodiorite. This model is consistent with the field petrographical and chemical characteristics of the granodiorite. At a late stage of monzogranite crystallization, the water contents in residual, intercrystalline melt became sufficiently high to promote the development of eutectoid intergrowths of quartz and feldspar to form the granophyre.