Petrographic study of, and trace element distribution in, high-MgO, transitional and high-Al 2O 3 basalts from the coastal region and SW-central Syria: A comparative study with similar basalts from the Aleutian Island arc

Abstract

Ten Upper Tertiary high-Al 2O 3 basalt samples, collected from Dahr Safra plateau near the eastern Mediterranean coast, and an Upper Tertiary basalt sample from southwest-central Syria, show >18 wt% Al 2O 3, and are subdivided into: high-MgO (14.43 wt% MgO), transitional (6.6 wt% MgO), and high-Al 2O 3 (<6 wt% MgO; >18 wt% Al 2O 3). The latter is dominant, and amounts to >80 vol%. Three samples are dark and non-magnetic; whereas the others are blue and magnetic. Primary hydrous mafic minerals are absent; however, olivine xenocrysts, averaging 8 vol%, are partly to wholly iddingsitized and serpentinized. Major oxides, and trace element distribution show noticeable differences in Al 2O 3, CaO, TiO 2, and Ba when compared with data measured for similar basaltic groups from the Aleutian island arc. The dark non-magnetic and blue magnetic samples represent two basaltic fractions originated from one parent magma, as supported by their field association, their equal Cr-concentration averages (262 and 266 ppmw), and the same trend for their FeO tMgONa 2O+K 2O-wt% averages. A MgO-enrichment, from olivine in the upper lithosphere, by a high-Al 2O 3 parent magma, during its rise through immature conduits, has been advocated as the possible, but not conclusive, process (lack of isotopic data and RE-analyses) for the generation of high-MgO and transitional basalts. However, a suggested alternative process is to have a high-MgO parent magma which, by deuteric alteration of olivine (iddingsitization and serpentinization are observed in all, but sample 1) and leaching from a superheated fluid phase in the upper magma chamber, loses some MgO to generate high-alumina and transitional basalts. The parent rocks, which partially melted to generate the magma, are probably wehrlite (Cr-bearing clinopyroxene-olivine peridotite) and eclogite (eclogite was found about 30 km NNE from Dahr Safra, and showed an almandine-pyrope garnet). The contribution of eclogite (rich in Al 2O 3 and TiO 2, and poor in CaO) must have been noticeable in the formation of the parent magma, in order to account for the presence of high concentration of Al 2O 3, TiO 2, and low concentration of CaO in the Syrian basalts.