Genesis of silicic magmas during tertiary continental rifting in Yemen

Abstract

The magmatism in Yemen associated with Oligo-Miocene continental rifting in the Red Sea and the Gulf of Aden comprises a bimodal mafic-felsic suite represented by flows, dykes and plutons. This paper reports the chemical and Nd-Sr isotopic compositions of the felsic rocks (trachytes, syenites, rhyolites and granites) associated with the Yemen Trap Series and deals with the controversy in regard to the crustal or mantle origin of A-type magmas generated in tensional contexts. The overall age of the studied rocks ranges from 26.5 to 16 Ma. The major and trace element compositions of the felsic rocks can partly be explained by fractional crystallization from some of the basic magmas. This process was more extensive in the case of the rhyolites and some of the granites than that of the trachytes and the syenite. In addition, the trachytes differ from the rhyolites by featuring a negative Nb anomaly that suggests contamination by continental crust. The trachytes have much lower 143Nd 144Nd ratios, and higher 87Sr 86Sr ratios than the associated basalts, whereas the rhyolites have much higher 87Sr 86Sr ratios, but 143Nd 144Nd ratios identical to those of the basalts. The plutons span the whole compositional range of isotopic compositions displayed by the trachytes and the rhyolites. The isotopic compositions of the felsic rocks do not support their genesis by direct melting of continental crust. Rather, these rocks were generated in an open magma system that evolved by a combination of fractional crystallization and crustal contamination. Mixing and AFC processes involving heterogeneous crust are both required to explain the trachytic and rhyolitic compositions where various amounts of crystal fractionation and crustal contamination were involved. The genetic differences between the trachytes and the rhyolites are related to their respective ages. The highest degree of contamination observed in the trachytes occurred contemporaneously with the emplacement of the traps and could have been related to a high thermal gradient in the crust due to massive magma transfer at that time. This stage presumably corresponds to the impingement of the Afar plume head against the lithosphere. The less contaminated and more fractionated rhyolites, which formed later during continental rifting, are consistent with a lower thermal gradient.