ABSTRACT

Harra El-Jabban is the Jordanian part of the large intra-continental basalt terrain of Harra Ash Shamah. It covers an area of about 11, 400 sq. km and lies in NE-Jordan. It consists of six successive thick basalt flows, dotted with tephra volcanoes of Oligocene-Holocene age. This Harra is one part of a large number of very poorly studied Cenozoic basalt fields, that are known to exist from Yemen through Saudi Arabia, Jordan, Syria, and Turkey. Detailed investigations of two volcanic cones, namely, Jebal Aritain and Jebal Fahem, aligned along a fissure system trending NNW-SSE in the middle part of the Harra, indicate that they consist of scoria air-fall deposits, dominated by lapilli. They are reasonably classified as “cinder cones” of Strombolian type of volcanicity. The petrographical and geochemical data show that they are of scoriaceous glassy olivinephyric basalts, with average modal olivine (∼23 vol%) derived from primary magmas giving magnesian chrysolite (Fo₈₉) with NiO contents of 0.33–0.47 wt% and CaO > 0.1 wt%. The chemical composition of 20 fresh pyroclastic samples taken from the two cones is similar in contents of the major oxides and trace elements with limited variations. Their chemical trend suggests that the parental magmas ascended rapidly, and underwent only limited fractional crystallization, without significant crustal contamination. The primary nature of this magma is reflected by the high MgO content (>8 wt%), the high Mg-number (0.69–0.75), the low silica content (<50 wt%) and the high Cr and Ni contents of 147–353 ppm and 180–341 ppm, respectively. This conclusion is further supported by the occurrence of mantle-derived ultramafic xenoliths. The similarity in major and trace element composition of the two suites suggests that the magmas were derived from a single homogeneous mantle source. It is estimated that the magmas were generated by low degrees of partial melting (3–8%) of garnet peridotite at >100 km depth.