The Role of Fractional Crystallization and Assimilationin the Evolution of the Zoned Mukhattata Pluton, Eastern Desert, Egypt

Abstract

Abstract The zoned Mukhattata pluton, covering an area of about 25 km 2, is composed of three zones becoming more felsic towards the centre of the pluton (normal zoning). The most outer zone consists of gabbro/diorite composition changing into tonalite (second zone) and granodiorite in the core (third zone). The tonalite is characterized by abundant gabbroid xenoliths, their number and size decreases towards the granodiorite in the core. The Mukhattata pluton exhibits geochemical features of I-type calc-alkaline rocks emplaced in a volcanic arc environment. The geochemical variations of major, trace and REE of gabbro/diorite, tonalite and granodiorite provide evidence that the different intrusions result from a progressive fractional crystallization of a single parental magma. Low and variable Mg#(33–48) of the gabbro/diorite outer zone and transitional contacts between them suggest that they were derived by fractional crystallization. A tonalitic magma intruded the gabbro/diorite producing the outer zone of gabbro/diorite composition. The presence of gabbroid xenoliths in the tonalite suggests that the assimilation process played some role during the formation of tonalitic rocks. Finally, the tonalite was intruded by granodioritic magma producing the core of the Mukhattata pluton. The petrological and geochemical results for the Mukhattata pluton reveal a transition from an early passive style of emplacement involving stoping of the tonalite by gabbro/diorite, to forceful emplacement of later granodiorite in the core.