Geology and geochronology of the Amram Massif, southern Negev Desert, Israel

This study describes the geology and geochronology of the Neoproterozoic Amram Massif, southern Negev Desert, Israel. The Amram Massif consists of a well-exposed alkaline rock suite comprised of alkali granites, rhyolites, monzonites, quartz syenites, and various types of mafic to felsic dikes, overlain unconformably by Cambrian sandstones. The field relations, geochronology, and chemical composition of the rock units indicate that the Amram magmatic sequence evolved through two magmatic cycles, in which mafic magmas evolved into felsic magmas by fractional crystallization. The Neoproterozoic magmatic activity in the massif was terminated by the emplacement of doleritic dikes, which closely resemble the Phanerozoic alkali basalts of this region. Rocks of the second magmatic cycle in Amram were dated by the Rb-Sr isochron method to 526 ± 22 Ma. The magmatic activity in the Amram Massif was accompanied by E-W normal faulting and graben formation. The Amram Massif displays petrological and structural similarities to the nearby Neshef Massif. Together, they comprise one geological unit, broken apart by young Cenozoic faults. The alkaline affinity, magmatic stratigraphy, and internal structure of the Amram and Neshef massifs resemble Neoproterozoic alkaline ring complexes in Sinai and the Eastern Desert of Egypt. All these alkaline magmatic bodies belong to the final episode of the Pan-African orogeny in the Arabian-Nubian Shield (Phase IV of Bentor, 1985), which lasted from ∼600 to 530 Ma. They mark the transition to intraplate alkaline magmatism, which characterized magmatic activity in this region during the Phanerozoic.