Geochemistry and petrology of metapyroxenite and metagabbro associated with Neoproterozoic serpentinites in the Arabian-Nubian Shield: fragments of a fore-arc ophiolite

Abstract

The dismembered ophiolites in the Wadi Al-Barramiya area, Central Eastern Desert of Egypt, are one of a series of Neoproterozoic ophiolites present in the Arabian-Nubian Shield. Here we present new fieldwork, whole-rock geochemical data, and mineral chemistry of metapyroxenite and metagabbro associated with the Al-Barramiya ophiolite in order to constrain its nature and tectonic setting; in particular whether the ophiolite was formed in a subduction or non-subduction setting. The rocks selected were obtained from the mantle section (serpentinized peridotite), an ultramafic (pyroxenite) and the crustal section (metagabbro). The serpentinized peridotite is altered to talc carbonate and listvenite, and associated with magnesite. Pyroxenite occurs as irregular coarse-grained lenses of websterite and olivine websterite within the serpentinite and occurs adjacent to the ophiolitic metagabbro. The metagabbros form scattered allochthonous masses of various sizes that are distributed across the area. The ophiolitic rocks are metamorphosed from greenschist to lower amphibolite facies. Locally, fresh relicts of olivine and Cr-spinel can be found in the serpentinite, whereas pyroxenite has fresh relicts of olivine, clinopyroxene and Cr-spinel. Cr-spinel in the metapyroxenite is zoned, with Al₂O₃, Cr₂O₃ and MgO decreasing and FeO_t increasing from cores to rims, reflecting the effects of metamorphism that selectively removed the now-depleted components. The metagabbros are characterized by enrichment in large-ion lithophile elements (LILE) over high field strength elements (HFSE) and are tholeiitic with a calc-alkaline affinity. The high Cr# (0.63–0.75) of fresh Cr-spinel relicts in the metapyroxenite, together with their low TiO₂ contents (0.04–0.24 wt%), indicate that this rock is similar to highly refractory ultramafic rocks that evolved in a fore-arc setting. This is supported by the high forsterite content (Fo = 0.91–0.93) of fresh olivine and high Mg# (0.93–0.95) of fresh clinopyroxene. Clinopyroxenes in the metapyroxenite and metagabbro have the chemical characteristics of boninite, confirming the fore-arc setting.

Graphical Abstract