New Insights into the Petrogenesis of Quaternary Peralkaline Volcanics, Jabal Al Abyad, Saudi Arabia

Jabal Al Abyad (2093 m) represents a case study of cone-type monogenetic volcano in Harrat Khaybar, Saudi Arabia., formed in subaerial intracontinental setting. Its formation involved multiple eruption styles including phreatomagmatic eruption of tuff cone truncated by spatter cone of glassy rhyolitic lava flow. The main construction of Jabal Al Abyad is a thick felsic tuff sequence interbedded in part with obsidian rocks. The felsic tuffs range from coarse to fine ash comprising mainly crystal fragments, lithic fragments and vitric shards. The sequence also exhibits changing from massive to stratified lithofacies. The geochemical signatures indicate that Jabal Al Abyad volcanics are rhyolitic peralkaline within plate volcanic rocks related to continental rifting system. Al Abyad volcanics are classified into two groups: A and B of which the former (A) is characterized by relatively higher Ti, Zr, Y, Hf, Th, La, and Ce concentrations. The peralkaline within plate signature, low La/Nb and elevated Nb/Ta ratios (11–17, OIB-like signature) as well as high concentrations of HFSEs (Zr, Y, La, Ce, Rb, Nb, Hf) support the genetic relationship and a heterogeneous mantle origin for the Al Abyad volcanics. The REE and trace element modeling interpretations indicate that a progressive closed-system fractional crystallization of an assemblage dominated by plagioclase and clinopyroxene from the alkaline mafic magma similar to that of St. Helena-SH-35 could produce range of composition from intermediate to acidic ones.