Geochemistry and Sr-Nd-Pb isotopic composition of the Harrat Al-Madinah Volcanic Field, Saudi Arabia

Whole-rock geochemical and Sr, Nd and Pb isotope data are presented for the Harrat Al-Madinah volcanic field, in the north western part of the Arabian plate, aiming to understand their origin and the composition of their mantle source. This area is an active volcanic field characterized by the occurrence of two historic eruptions approximately in 641 and 1256 A.D. Field investigation of the main volcanic landforms indicates dominantly monogenetic strombolian eruptions, in addition to local phreatomagmatic eruption style. The lavas consist mainly of alkali olivine basalt, olivine transitional basalt, and hawaiite with ocean island basalt (OIB)-like characteristics. Evolved rocks, represented by mugearites, benmoreites, and trachytes, occur mainly as domes, tuff cones and occasionally as lava flows. Chemical variations in the evolved rocks indicated their evolution by low pressure crystal fractionation of olivine, plagioclase, clinopyroxene, and Fe-Ti oxides from the relatively primitive basalts. The isotopic compositions of Nd-143/Nd-144 (0.512954-0.512995), Sr-87/Sr-86 (0.702899 to-0.702977) and Pb (Pb-206/Pb-204 = 18.5515-18.7446, Pb-207/Pb-204 = 15.5120-15.5222, Pb-208/Pb-204 = 38.1347-38.4468), show restricted variations suggesting only minor crustal contamination. They defined an array consistent with mixing of two geochemically distinct components of depleted MORB-mantle (DMM) and high U-238/Pb-204 ratio (HIMU). The variations in Tb/Yb, La/Yb and Sm/Yb ratios in the relatively primitive basalts (MgO > 6 wt.%) indicated garnet peridotite source. However, the positive Nb, Sr, Ba and Ti anomalies in the primitive mantle-normalized incompatible element patterns and the significant variation between Zr/Nb vs. Ce/Y and La/Yb vs. Yb suggest contribution of an amphibole-bearing spinel lherzolite source. Moreover, the negative correlations between SiO2 vs. Sr-87/Sr-86 and Th vs. Nd-143/Nd-144 are interpreted as an indication of mixing melts derived from two end-members; one is garnet bearing asthenospheric source with OIB characteristic and the other is amphibole-bearing spinet lherzolite. The Harrat Al-Madinah volcanic field occurs near the Red Sea Rift System and its origin reflects a strong lithospheric control on the loci of partial melting. The dominantly NNW alignment patterns of the volcanoes, which is similar to the regional Red Sea trend, may suggest that the magmas were produced by decompression partial melting triggered by lithospheric extension related to the Red Rift. (C) 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.