Geochemistry and Petrogenesis of Basaltic Rocks and Enclosed Xenoliths from the Ghab Pliocene Volcanic Field in Northwestern Syria

Abstract

Basaltic rocks and their content of ultramaf i c xenoliths are common in the Ghab Pliocene Volcanic Field in northwestern Syria in the form of lava flows, cinder cones and pyroclastic deposits. The rocks occur within the Ghab pull- apart graben that formed by sinister strike slip faults within the zone that defines the boundary between the African and the Arabian plates. Three petrographic types occur: basanite, olivine basalt and more commonly alkali olivine basalt. The peridotite xenoliths are spinel lherzolite and harzburgit e. Geochemical analysis indicates that the basalts are mostly alkaline to subalkaline. A distinctive feature of these rocks is the narrow compositional var iations in the content of most major oxides and minor elements, SiO 2 (44.33 - 46.43 wt%) and MgO (4.01 - 8.28 wt%). Some of the refractory and high field strength elements and incompatible minor elements in the basalts are relatively high (Cr average = 303 ppm and Ni average = 185 ppm) compared with their content in average basalts. These geochemical characteristics reflect cr ystallization of the Ghab basalts from pristine and primary magmas that have experienced minimal fractional crystallization and crustal contamination. Similarly, chemical compositions of the ultramafic xenoliths vary within a restricted geochemical range. They are compatible with the generation of these rocks from partial melting of a primitive mantle pyrolite to yield the xeno liths . These magmatic processes operated during the Pliocene in a regional transtensional stress environment attending the development of the Dead Sea Rift.