THE AHEIMIR VOLCANIC SUITE AN EDIACARAN POST-COLLISIONAL SEQUENCE FROM THE NORTHERNMOST ARABIAN NUBIAN SHIELD, WADI ARABA, SW JORDAN: AGE, GEOCHEMISTRY AND PETROGENESIS

The investigated suite forms a NNE a narrow belt (2 to 4 km wide) that extends for a distance of about 70 km along the eastern shoulder of Wadi Araba-Dead Sea Transform from Gharandal in the south to Feinan in the north . It is comprised of lava flows, pyroclastics dominated by lithic tuffs and ignimbrites; agglomerates are also present. Furthermore, a variety of dikes ranging from simple to composite are widespread. On the TAS classification diagram, the investigated volcanics fall exclusively in the fields of basaltic trachyandesite, trachyandesite, trachyte/trachydacite and rhyolites. The studied rocks range from medium- through high-K to shoshonites. The intermediate volcanics are typical latites; while the felsic are alkali feldspar rhyolites and peralkaline rhyolites, namely, commendites.

Old Rb/Sr age determinations on rhyolites and their pyroclastics yielded ages between 530 to 553 Ma. However, new zircon U-Pb dating by SIMS of rhyolite flows and of latites and rhyolite of a composite dike yielded ages between 595 and 600 Ma. This coincides with the intrusion of shallow level A-type granites and alkaline mafic magmas.

The devitrified rhyolites are characterized by strong enrichment of potassium at the expense of sodium where the K₂O/Na₂O ranges from about 1 to 100; a phenomenon that have been already reported in these volcanics and their equivalent rocks in Sinai Peninsula . This K-metasomatism has been attributed by Wachendorf et al (1985) as due to interaction of the glassy rhyolites with hydrothermal fluids enriched in K₂O under pressure release.

The petrogenesis of these volcanics, which are contemporaneous with a switch from calc-alkaline to alkaline igneous activity in northernmost Arabian Nubian Shield, is discussed. This change in chemical affinity of magmas could have been triggered by mantle delamination and a consequent rise of hot asthenosphere (Avigad and Gvirtzman 2009). This rise of hot asthenosphere resulted in decompresional melting of upper mantle and the generation of mafic magmas.

Emplacement and crystallization of these magmas at the mantle crust boundary might have been responsible for the partial melting lower continental crust and the generation of the rhyolites. On the other hand, latites could have been formed by mixing of the fractionated mafic melt with crustal melts.