Magmatism during extension of the lithosphere: geochemical constraints from lavas of the Shaban Deep, northern Red Sea

doi:10.1016/S0009-2541(99)00221-1

Chemical Geology

Volume 166, Issues 3–4, 22 May 2000, Pages 225-239

https://doi.org/10.1016/S0009-2541(99)00221-1 Get rights and content

Abstract

Geochemical and isotopic data of tholeiitic lavas from the northernmost volcanic deep, the Shaban Deep in the Red Sea rift suggest the presence of small-scale heterogeneities in the mantle underneath this part of the rift. The major and rare-earth element compositions indicate less than 10% melting of a variably depleted spinel peridotite. The basalts from the Shaban Deep have significantly higher ⁸⁷Sr/⁸⁶Sr for a given Nd isotope value than basalts from the southern Red Sea rift. The magmas formed from an asthenospheric source distinct from the Arabian lithospheric mantle and have not assimilated crustal material. The mantle source is enriched in alkali and alkaline earth elements relative to Nb and the rare earth elements and in that respect resembles Indian Ocean MORB. Olivine, plagioclase and clinopyroxene were fractionated and magma mixing occurred in shallow crustal magma reservoirs. The degree of partial melting decreases towards the north along the Red Sea rift due to decreasing spreading rate and colder mantle temperatures.

Introduction

The plate tectonic cycle consists of the opening of an ocean basin by continental rifting, a few hundred million years of seafloor spreading and finally the closure of the ocean due to subduction. The Red Sea is the only region on Earth where a young oceanic basin is forming. Every tectonic setting from early rift stage, beginning of seafloor spreading to fully developed spreading can be observed along the Red Sea rift. This unique setting makes the Red Sea an important region on Earth to study this phase of the plate tectonic cycle. Models for the stretching of the lithosphere and the accompanying magmatism have been suggested from which the basic melting processes are understood Foucher et al., 1982, McKenzie and Bickle, 1988. The thinning of the lithosphere during rifting leads to ascent of the underlying mantle and adiabatic melting where the volume of melt generation depends on the temperature of the mantle and the amount of stretching (McKenzie and Bickle, 1988). In the Red Sea, spreading of basaltic crust begins in punctuated centres possibly above mantle diapirs that are evenly spaced and in that respect resemble the segments of regular oceanic spreading axes (Bonatti, 1985). Mature spreading centres exist in the central part of the Red Sea (Altherr et al., 1988). However, there is a controversy whether the ascent of the Red Sea mantle is passive (Bonatti, 1985) or active due to a mantle plume (Camp and Roobol, 1992).

In this paper we present geochemical and isotopic data on lavas from one of the northernmost deeps of the Red Sea, the Shaban (or Jean Charcot) Deep, and discuss the generation of the magmas. The objectives of the study are (1) to distinguish different magma sources along the Red Sea rift (i.e., whether it is only the asthenosphere or also the lithospheric mantle), (2) to define the depth of melting and the geometry of the thinned lithosphere, and (3) to investigate the magmatic processes during the transition from rifting to spreading in the northern Red Sea.

Section snippets

Geological setting

Extension in the Red Sea region started about 30 Ma ago Roeser, 1975, Girdler and Southren, 1987, separating the Arabian from the Nubian Shield (Fig. 1). These Proterozoic shields consist of 550 to 950 Ma old rocks and crop out on each side of the Red Sea basin Kröner et al., 1987, Stern, 1994. On the eastern Arabian side and to the north in the Sinai the shields are covered by voluminous Tertiary to Quaternary volcanics, which contain lithospheric xenoliths (Camp and Roobol, 1992). Large

Sampling and analytical methods

The samples were recovered during the Meteor 31/2 cruise in 1995 and most of them stem from the elongated volcanic edifice in the deep (Table 1, Fig. 1). Rock samples were dredged or recovered with a video-controlled grab but we analyzed also volcanic glass from sediment cores from the basins surrounding the volcano. The lava samples were crushed and clean glass was handpicked under a binocular for chemical analysis. Several samples are severely altered with palagonite rims around vesicles.

Results

Most Shaban samples contain crystals of olivine (Fo81-89) and plagioclase (An62-71) but in samples KL9-2 and 88GTV rare phenocrysts of clinopyroxene are observed. The plagioclase and olivine crystals in many samples (e.g., KL9-2, DR2-1 and DR2-2) are rounded and corroded implying disequilibrium with the surrounding magma. Some lavas also contain clusters of plagioclase and olivine crystals. The volcanic glasses from the Shaban Deep have relatively high contents of SiO₂, Na₂O and K₂O (Table 1

Shallow level crystal fractionation in the Shaban Deep

The abundance of phenocrysts and the range of MgO contents of about 8% to 6% in the glasses recovered from the Shaban Deep suggests crystal fractionation processes during the ascent of the magmas. Despite the fact that the Shaban Deep lavas formed from a source with a range of incompatible element and isotope compositions they have comparable concentrations of major elements at a given MgO (Fig. 3). Within their limited range of MgO the glasses appear to lie along a single liquid line of

Conclusions

(1) The compositional variety of the tholeiitic lavas in the Shaban Deep imply a heterogeneous mantle source on a small-scale of a few kilometres.

(2) Crystal fractionation of olivine, plagioclase and clinopyroxene similar to MORB led to relatively evolved lavas erupting together with relatively primitive lavas. Magma mixing processes occurred at crustal levels.

(3) The melts formed by 5% to 10% melting of spinel peridotite at low pressures in the mantle (1.5 to 0.4 GPa). The asthenospheric

Acknowledgements

The help of captain Kull and the crew of FS METEOR during the cruise and D. Ackermand, T. Arpe, D. Garbe-Schönberg and B. Mader during the analytical work is gratefully acknowledged. We thank R. Altherr and D. Bosch for their comments on a previous version of this paper and N. Arndt, C. Dupuy and J.-P. Eissen for constructive reviews. The Deutsche Forschungsgemeinschaft made this work possible through grant Sto 110/23 to P. Stoffers. [NA]

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Magmatism during extension of the lithosphere: geochemical constraints from lavas of the Shaban Deep, northern Red Sea

Abstract

Introduction

Section snippets

Geological setting

Sampling and analytical methods

Results

Shallow level crystal fractionation in the Shaban Deep

Conclusions

Acknowledgements

Earth Planet. Sci. Lett.

Tectonophysics

Chem. Geol.

Mar. Geol.

Tectonophysics

Geochim. Cosmochim. Acta

Tectonophysics

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Chem. Geol.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Geochim. Cosmochim. Acta

Earth Planet. Sci. Lett.

Tectonophysics

Earth Planet. Sci. Lett.

Geochim. Cosmochim. Acta

Sodium partitioning between clinopyroxene and silicate melts

J. Geophys. Res.

Punctiform initiation of seafloor spreading in the Red Sea during transition from a continental to an oceanic rift

Nature

Geology of the Red Sea transitional region (22°N–25°N)

Oceanol. Acta

Thermal consequences of lithospheric extension: pure and simple

Tectonics

Upwelling asthenosphere beneath western Arabia and its regional implications

J. Geophys. Res.

Current plate motions across the Red Sea

Geophys. J. Int.

Geochemical effects of dynamic melting beneath ridges: reconciling major and trace element variations in Kolbeinsey (and global) mid-ocean ridge basalt

J. Geophys. Res.