The youngest Neoproterozoic mafic dyke suite in the Arabian Shield: mildlyalkaline dolerites from South Jordan - their geochemistry and petrogenesis

The Arabian-Nubian Shield evolved through a sequence of tectonomagmatic cyc les, which took place during Neoproterozoic time (1000-540 Ma). Dyke emplac ement constitutes one of the conspicuous features of the Arabian-Nubian Shi eld, with mafic dykes being the most abundant. The investigated dykes repre sent the youngest Neoproterozoic mafic dykes and have been dated in Jordan at 545 +/- 13 Ma. Geochemically the studied dykes are mildly alkaline, are enriched in large ion lithophile elements (LILE) and high field strength ca tions (HFSC), show moderate enrichment of REE, and lack Nb anomaly. These f eatures are consistent with a predominantly extensional continental tectoni c setting. Crystallization temperatures of the suite fall between 1050 and 800 degreesC to as low as 650 degreesC as deduced from pyroxene thermometry . The investigated dykes were derived from a metasomatized lithospheric man tle by 5% modal batch partial melting of phlogopite-bearing spinel Iherzoli te, according to geochemical modelling. The intra-suite geochemical feature s are explicable by 64% fractional crystallization of olivine, pyroxene, pl agioclase and titanomagnetite and possibly other accessories like apatite a t a later stage. The cumulate produced from this fractionation of the inves tigated dyke suite contributed to the formation of the mafic lower crust of the Arabian-Nubian Shield. Elemental ratios and petrographic evidence indi cate possible minor crustal contamination of the suite. The youngest mafic dykes show striking geochemical similarities to the same generation of dole rite dykes in the adjacent countries, to transitional young basalt suites o f the Main East African Rift, and to Quaternary Jordanian basalts. The youn gest mafic dyke suite, the rhyolites of the Aheimir suite, and St Katherina rhyolites of Sinai represent the last igneous activity in the Arabian-Nubi an Shield before the onset of the Cambrian at about 545 Ma ago.