Mantle sources and magma‐continental crust interactions during early Red Sea‐Gulf of Aden rifting in southern Yemen: Elemental and Sr, Nd, Pb isotope evidence
Authors: Chazot Gilles , Bertrand Herve 
Published: February 2008

Large‐scale magmatic activity, ranging from late Oligocene to Quaternary, is associated with the Red Sea‐Gulf of Aden rifting throughout the Arabian passive margin. The Southern Yemen area represents the southernmost extremity of this magmatic range, facing the Afar area, and provides a means of studying the magmatic records of early stages of rifting (30–16 Ma) in a plume‐related context. We investigate major and trace elements, and Nd, Sr and Pb isotopes of a bimodal series of transitional affinity consisting of (1) thick olivine‐basalt traps overlain by ignimbritic rhyolites, (2) basaltic, rhyolitic, trachytic and peralkaline dykes with a prevailing N120–140° E orientation, and (3) gabbroic, syenitic and granitic plutons. Major and some trace element variations from basalts to felsic rocks are consistent with low‐pressure fractional crystallization. Mass balance calculations using major elements suggest the fractionation of clinopyroxene (6–9%), olivine (∼6%), plagioclase (42–43%), magnetite (∼12%), apatite (1–2%) ± alkali feldspar (16%). However, LILE (large ion lithophile element) enrichment and high initial 87Sr/86Sr ratios (up to 0.7074 and 0.710 in rhyolites and pantellerites, respectively) require the felsic rocks to be generated through significant crustal assimilation. Nd and Sr isotopic ratios of the rhyolitic traps can be reproduced by bulk mixing between magmas similar to the underlying basaltic unit and the Arabian Proterozoic basement. On the other hand, an assimilation‐fractional crystallization process is required to account for the isotopic diversity of the rhyolitic and peralkaline dykes. Rhyolites can be derived from a basaltic liquid by a moderate fractionation rate (F = 0.47) and a high crustal assimilation rate (R = 0.45), whereas the pantellerites require more significant fractionation rate (F = 0.07) and a very low assimilation rate (R = 0.05). Elemental and isotopic signatures of the basalts do not support a significant crustal contribution in their formation and their isotopic diversity (87Sr/86Sr from 0.7034 to 0.7051, 143Nd/144Nd from 0.512676 to 0.513045 and 206Pb/204Pb from 17.96 to 18.66) mainly reflects mantle source heterogeneities. Sr‐Nd‐Pb isotopic data are consistent with a binary mixing between depleted and enriched source regions. The depleted end‐member corresponds to an asthenospheric reservoir approaching that producing mid‐ocean ridge basalts at Gulf of Aden/Red Sea spreading centers. The enriched reservoir, intermediate between enriched mantle I and II end‐members, is supposed to be located within old subcontinental lithosphere related to the Pan‐African orogenic events. Unlike the modern volcanics from Afar, no HIMU (high U/Pb ratio) signature has been recognized in our sampling. This rules out any significant chemical influence of the Afar plume upon early rift‐related volcanism in Southern Yemen and suggests a continental rift initiation of passive type. However, one can suspect that the Afar plume may have supplied the excess heat required to produce so voluminous traps and to trigger melting in the lithospheric mantle, making the distinction between passive and active rifting more ambiguous.