Neogene volcanic activity of western Syria and its relationship with Arabian plate kinematics

摘要

The Cenozoic (mostly Neogene) volcanic activity in Syria is part of the extensive magmatism that took place in the Mashrek Region, Middle East, from upper Eocene to Holocene (～40-0.0005 Ma). Samples in western Syria are mostly high TiO_2 (TiO_2 ～1.8-3.7 wt.%) alkaline mafic rocks (basanites, hawaiites and alkali basalts) plus rare transitional/tholeiitic basalts and basaltic andesites) with within-plate-like trace element signature. On the basis of incompatible trace element content, the volcanic activity in Syria has been divided into two stages: the first lasting from ～25 to ～5 Ma and the second from ～5 to recent times. Indeed, the Syrian lavas show incompatible trace element content increasing with decreasing age from ～25 to ～5 Ma, followed by an abrupt decrease to low values roughly at the Miocene-Pliocene boundary. This temporal shift in composition is related to major tectonic re-organization occurred during upper Miocene. The proposed petrogenetic model invokes three steps: (a) passive upwelling of the shallow asthenosphere during the development of the Dead Sea transform fault system. Different degrees of partial melting were followed by variable extents of fractional crystallization and limited upper crustal contamination; (b) the Miocene-Pliocene boundary tectonic change enhanced passive decompression of the same sources and a consequent increase in degree of partial melting resulting in low incompatible trace element content of the relatively high-volume liquids; (c) after this phase, the incompatible trace element content in the basaltic magmas increased as consequence of fractional crystallization processes. Major and trace element content similarities with the rest of the circum-Mediterranean igneous rocks are consistent with a common relatively shallow origin for the Cenozoic anorogenic magmatism of the entire circum-Mediterranean area (the so-called Common Magmatic Reservoir). Because much of the igneous activity in the studied area is concentrated near the Dead Sea fault, the origin of Cenozoic magmatism in Syria (and in the rest of the circum-Mediterranean area) reflects a strong lithospheric control on the loci of partial melting. Mantle plumes from lower mantle and/or north-westward channelling of the Afar mantle plume is not needed to explain volcanic activity in Syria and the Mashrek area.