The petrogenesis of late Neoproterozoic gabbro/diorite intrusion at Sheikh El-Arab area, central Sinai, Egypt

Abstract

The late Neoproterozoic gabbro/diorite intrusion (~11.7 km2) at Sheikh El-Arab area represents the only mafic exposure in the basement rocks of Katharina province of central Sinai largely occupied by granitoids and their volcanic equivalents. The field relations indicate that the intrusion is younger than the lower unit (630–615 Ma) of the volcanics and clastics of the Rutig Formation, and older than the surrounding granodiorites of Sheikh El-Arab and Rahaba (~610 Ma) plutons. It is not affected by regional metamorphism or ductile deformation, but recorded petrographic uralitization/amphibolitization signature. It is composed chiefly of pyroxene-hornblende gabbro, diorite, and quartz diorite. The chemical composition of the mafic minerals indicated that the suite was derived from calc-alkaline magma. Geochemically, the studied rocks are characterized by enrichment of LILE relative to HFSE and LREE relative to HREE. The gabbros are notably low in total REE (38–56 ppm) with (La/Yb) n = 2.7–4.8, while the dioritic rocks are high in ΣREE (142–161 ppm) with high (La/Yb) n values (12.5–15.8); both are characterized by the absence of Eu anomaly, their Eu/Eu* ratios are close to unity (0.93–1.11). The studied intrusion evolved from mafic mantle magma into different types by assimilation fractional crystallization process (AFC) and/or gradual decrease in oxygen fugacity. The initial magma corresponds, most probably, to pyroxene-hornblende gabbro and the crystallization of hornblende was ascribed by slight H2O increase in magma after crystallization of near-liquidus clinopyroxene with high Ca content (Wo42–52) and Ca-rich plagioclase (An80–83). The intrusion is related to the later calc-alkaline substage of post-collisional tectonic setting.