2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Petrological Characteristics of Subduction-related Ultramafic Cumulates from

the Salatit Ophiolite, Central Eastern Desert, Egypt

 Gahlan H.*, Abu El-Ela F.^* , Arai S.^**

^* Department of Geology, Faculty of Science, Assiut University, Assiut 71516, Egypt

^** Department of Earth Sciences, Kanazawa University , Kanazawa 920-1192, Japan

hjhlan@yahoo.com

Crustal wehrlite cumulates were discovered and described for the first time in the mantle-crust transition zone (MTZ) and the extremely lower layered gabbro sequence of the Salatit ophiolite, Central Eastern Desert, Egypt. They form lensoidal tabular bodies. The wehrlite cumulates show interlayering and concordant relations with the host MTZ dunite and/or layered gabbro. The contact between wehrlites and the host MTZ dunite or layered gabbro is razor sharp, lobate and/or sinuous. Neither chilled margins, xenoliths from the host nor any visible deformations have been observed along the wehrlite contacts. The Salatit wehrlites are orthopyroxene-free, composed mainly of olivine and clinopyroxene, texturally equilibrated and show a characteristic poikilitic texture.

The Salatit wehrlites show the following crystallization order: olivine/spinel followed by clinopyroxene with the absence of plagioclase. Olivine and clinopyroxene of the Salatit wehrlites are compositionally uniform and conspicuously high in Mg#, 0.93 and 0.92 on average, respectively. The high Mg#s of Salatit wehrlites' olivine and clinopyroxene, and changing the order of minerals crystallization (ol → cpx with the absence of plag) are probably due to prevailing higher oxidizing conditions. The clinopyroxene shows low Ti and Al contents (corresponding to slow-cooling rates), and marked depletion in LILE. The calculated melt in equilibrium with clinopyroxene from the Salatit wehrlites is largely similar to lavas from the Izu-Bonin fore-arc.

Given the above scenario, the Salatit wehrlites were produced by simple accumulation of fractionating crystals from a hydrous depleted basaltic/tholeiitic melt corresponding to temperatures >1000 and <1100°C at the oceanic crustal pressure (~2 kbar). The involved hydrous (high-Mg, and low Ti, Al and LILE) tholeiitic melt has been probably formed by fluid-enhanced partial melting of a refractory mantle source (similar to the underlying harzburgites) in a somewhat shallow sub-arc environment.