Geochemical Constraints on the Magmatism Along the Rift Axis of the Northern Red Sea

Richard Mühe Geologisch-Paläontologisches Institut der Universität Kiel, Olshausenstr. 40, 24118 Kiel, Germany

Karsten Haase Max-Planck-Institut für Chemie, Abt. Geochemie, Postfach 3060, 55020 Mainz, Germany

Peter Stoffers Geologisch-Paläontologisches Institut der Universität Kiel, Olshausenstr. 40, 24118 Kiel, Germany

The northernmost Red Sea Rift (~24°-26°30'N) consists of several deeps which are the site of tholeiitic and transitional basaltic volcanism. In contrast, the central Red Sea (~21°-23°N) is characterised by oceanic crust of depleted MORB-composition. The change in chemical composition of the magmas reflects the variation from initial rifting in the north to normal spreading in the central region. We present new geochemical and isotopic data from one of the northernmost deeps, the Shaban Deep at ~26°15'N, and discuss them in the context of previously published data.

The Shaban Deep has a diameter of about 8 km and a maximum water depth of 1550 m. An elongated SE-striking volcanic ridge rises to a depth of ~950 m and is surrounded by a heavily sedimented plain. During the Meteor 31/2 cruise in January 1995 the volcanic edifice and the surrounding regions were sampled. Volcanics with the freshest appearance were sampled on the northwestern edge of the Shaban Deep. All lavas have glass rims, and most contain phenocrysts of olivine and plagioclase and sometimes clinopyroxene. The Shaban volcanics are tholeiites but with high contents of alkalies, and most are light rare earth element-enriched [(La/Sm)Chondrite-Normalized 0.9-1.3]. 87Sr/86Sr range from 0.70305-0.70315 suggesting a heterogenous source for the small Shaban volcano.

Comparing the new data with existing data of the Mabahiss Deep (25°30'N) and the Atlantis II Deep (21°N) (Altherr et al., 1988, 1990) we note several significant variations. 87Sr/86Sr rise from 0.7024-0.7030 in the Atlantis II Deep to 0.7031-0.7032 in the Shaban Deep. This implies a more enriched mantle source in the northern Red Sea. This source appears to reside in the asthenosphere rather than the continental lithosphere, as Sr-Nd isotopic compositions of mantle xenoliths from the Arabian lithosphere (Henjes-Kunst et al., 1990) differ from those of the Shaban lavas. The Shaban lavas have the lowest Fe8.0 and the highest Na8.0 of the northern Red Sea volcanics implying the lowest degrees of partial melting at relatively shallow levels. Melting at low pressures is reflected by the high SiO2 contents although the heavy rare earths are fractionated both in light rare earth-enriched and -depleted lavas. Thus melting probably occurs mainly in the spinel peridotite field with some contributions of melts from garnet peridotite.

The shallow melting implies that the asthenosphere rises to high levels beneath the Shaban Deep and replaces the continental lithosphere very effectively, even though the volcanically active zone is very narrow (< 5 km). As melts from garnet peridotite seem to play a minor rôle in the Shaban magmas melting appears to start at shallow levels which in turn would imply a relatively cold mantle underneath the northern Red Sea. Thus, although the Shaban lavas are radiogenic in Sr isotopes compared to the central Red Sea MORB, there is no evidence for a mantle plume beneath the northern Red Sea. The Shaban magmas appear to be low degree melts (< 10%) sampling preferentially enriched parts of the asthenosphere.


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