NEOPROTEROZOIC EGYPTIAN CENTRAL EASTERN DESERT ARC-BACK-ARC SYSTEM AS EVIDENCED BY SUPRA-SUBDUCTION OPHIOLITES

Ophiolites are broadly distributed in the Central Eastern Desert (CED) of Egypt, occurring as clusters in its northern (NCEDO) and southern (SCEDO) segments. They show supra-subduction zone geochemical signature with prevalent island-arc and minor boninitic affinities in the NCEDO and MORB/island arc association in the SCEDO. Mineralogical and geochemical data on the volcanic sections of Wizer (WZO) and Abu Meriewa (AMO) ophiolites as representative of the NCEDO and SCEDO, respectively, are presented.

The WZO volcanic sequence comprises massive lower part of more MORB-like compositions intruded by minor boninitic dykes and thrust over island-arc metavolcanic blocks in the mélange matrix, pointing most likely to formation in a protoarc-forearc setting. Chemical compositions of primary clinopyroxene and Cr-spinel relicts from the WZO volcanic section further confirm this interpretation. Such compositional variability in the WZO crustal sequence is comparable with its mantle section that varies from slightly depleted harzburgites of MORB/IAT affinities to highly depleted harzburgites of IAT affinity containing small dunite bodies of boninitic affinity. Source characteristics of the WZO different lava groups indicate generation via partial melting of MORB source progressively depleted by melt extraction and variably enriched by subduction zone fluids. MORB-like magma may have derived from ~20% partial melting of an undepleted lherzolite source, leaving slightly depleted harzburgite as a residuum. The generation of island-arc magma can be accounted for by partial melting (~15%) of the latter mantle source, whereas boninites may have derived from partial melting (~20%) of a more refractory mantle source previously depleted by melt extraction of MORB and IAT melts, leaving ultra-refractory dunite bodies as residuum.

On the other hand, the AMO volcanic unit occurs as highly deformed pillowed metavolcanic rocks in mélange matrix. They can geochemically be categorized into LREE-depleted (La/Yb_CN= 0.41-0.50) and LREE-enriched (La/Yb_CN= 4.7-4.9) lava types that show a MORB/within plate to island arc geochemical signature, signifying back-arc basin setting, consistent, as well, with their mantle section. Source characteristics indicate depleted to slightly enriched mantle sources with overall slight subduction-zone effect as compared to WZO.

Such polarity in geologic and geochemical characteristics of the NCEDO and SCEDO, along with the abundance of mature island-arc metavolcanics which are close in ages (~750 Ma) to ophiolitic rocks, systematic enrichment in HFSE of ophiolites from north to south, and lack of crustal break and major shear zones, is best explained by a geotectonic model whereby the CED represents an arc-back-arc system above a southeast-dipping subduction zone.