Gabbros versus granites of the subduction regime of South Sinai, Egypt: discrimination and geochemical modelling

Abstract

The Arabian–Nubian Shield of South Sinai, Egypt, comprises older granite (OG) and metagabbro–diorite complex (MG) outcrops pertaining to the island arc regime. The study aims to throw some light on the role played by these rocks during the tectonic evolution of the Sinai, Egypt. Three selected areas via Nesyrin, El-Fringa-Minader and Shahera outcrops are selected as case example. Field, petrography, geochemistry and thermobarometric studies are considered to elucidate discriminations and genetic variations. The OG compositions are tonalite and granodiorite, whereas MG constitutes hornblende–metagabbros, hornblende–leucogabbro and diorite. Opaque mineral contents are relatively enriched in the MG and represented by magnetite, hematite and ilmenite with few sulphides. Geochemically, the OG rocks are peraluminous, calc-alkaline and I-type and belong to syncollision volcanic arc. The MG varieties exhibit transitional calc-alkaline/tholeiite magma types and are comparable to rocks of island arc setting. Amphiboles of MG are calcic-type (actinolite, actinolite hornblende and subordinate of magnesia-hornblende). Rocks are crystallised under medium (El-Fringa-Minader and Shahera) to low (Nesyrin) pressure. Amphiboles from the OG have the composition of actinolitic hornblende (Shahera area), ferroedenite to ferroedenitic hornblende (Nesyrin area). They are formed under low pressure. Plagioclase ranges in composition from oligoclase to andesine. Biotite of the OG is Mg-rich (Shahera area), Fe-rich (Nesyrin area) and lepidomelane. OG rocks exhibit calc-alkaline magma affinity. The thermobarometric study indicates that MG rocks are formed at a temperature between 600 and 730 °C and the pressure of emplacement varies from 2 to 4 kbar. OG rocks gave a temperature of 648–717 °C and an emplacement pressure of 1–4 kbar. Both modelling and thermobarometric calculations prove that magmas are generated at a depth of about 30 km for OG and from 15 to 35 km for MG. Geochemical modelling favours that MG is derived from 40% partial melting of the lower crust followed by 15% to 35% fractional crystallisation. Granodiorite is derived through the 25% to 45% fractional crystallisation of MG.