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Geological Magazine; September 2008; v. 145; no. 5; p. 685-701; DOI: 10.1017/S0016756808004901
© 2008 Cambridge University Press (CUP)
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The Euphrates volcanic field, northeastern Syria: petrogenesis of Cenozoic basanites and alkali basalts

NANCY A. LEASE and ABDEL-FATTAH M. ABDEL-RAHMAN*

Ministère de l’Agriculture, des Pêcheries et de l’Alimentation, 200 chemin Sainte-Foy, Québec (Québec), G1R 4X6, Canada, and Department of Geology, American University of Beirut, P.O. Box 11-0236, Beirut, Lebanon

* Author for correspondence: arahman{at}aub.edu.lb

The Plio-Quaternary Euphrates volcanic field of NE Syria includes large discontinuous exposures of basanitic and basaltic lava flows (1200 km2 in area). It represents the northern segment of the Cenozoic volcanic province of the Middle East and is located near the Bitlis collision suture. The rocks consist of olivine (15–20 %), clinopyroxene (30–35 %), plagioclase (45–55 %) and opaque phases. Chemically, the rocks are largely ultrabasic (SiO2 38.2–45.5 wt %, MgO 8.7–13.0 wt % and average Mg number of 0.65). They are enriched in incompatible trace elements such as Zr (133–276 ppm), Nb (25–71 ppm) and Y (17–28 ppm). The REE patterns are strongly fractionated ((La/Yb)N = 19.6), indicative of a garnet-bearing source. The 143Nd/144Nd isotopic compositions range from 0.512868 to 0.512940 ({varepsilon}Nd = 4.5 to 5.9), and 87Sr/86Sr from 0.70309 to 0.70352. These chemical and isotopic compositions reflect strong affinities to OIB. Elemental ratios such as K/P (3.4), La/Ta (13) and La/Nb (0.77), and the low SiO2 values, suggest that the Euphrates magma was subjected to minimal crustal contamination. Petrogenetic modelling has been carried out using a variety of mantle source materials, different degrees of partial melting (0.1 to 10 %), and a number of scenarios including metasomatized sources. Modelling suggests that the magma could have been produced as a result of a small degree of partial melting of either (1) a garnet-bearing depleted source enriched with a small addition of metasomatizing fluids, or (2) a garnet-bearing fertile source. The overall chemical and petrological characteristics are more consistent with the generation of the Euphrates magma by a small degree of partial melting (F = 1 %) of a primitive, garnet-lherzolite mantle source, possibly containing a minor spinel component. The Neogene collision of the Arabian plate with Eurasia along the Bitlis suture resulted in reactivation (beneath the Euphrates basin) of deep-seated fractures, along which lavas may have penetrated the crust.

Key Words: basanite • northeastern Syria • olivine • Nd–Sr isotopes • petrogenesis • Euphrates graben • Bitlis suture






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