The geology of the northern tip of the Arabian–Nubian Shield

Abstract

Recently, a detailed (1:50,000) geological map of the Elat area, southern Israel was published. Attached to this map is a stratigraphic table of the Neoproterozoic metamorphic–magmatic complex of the study area. The Neoproterozoic basement in the Elat area encapsulates the Arabian Nubian Shield (ANS) geologic evolution. Uranium–Lead and Lead–Lead zircon ages, included in previous studies and referred to in this paper, reveal that these rocks were formed during more than 300 million years of Neoproterozoic time. The major process controlling the formation of the ANS as part of the East African Orogen is the closure of the Mozambique Ocean. The first orogenic phase in the Elat area, represented by the metamorphic rocks, includes the development of an island arc, erosion of the islands and deposition, and metamorphism. This event took place between ∼950Ma and 780–790Ma. Elat Schist, the oldest metamorphic rock in the area, was deformed and then intruded by quartz dioritic and granitic plutons that were later deformed and metamorphosed. The amphibolite metamorphic rock facies indicate metamorphic conditions of up to 650°C and between 4 and 5kbar. The peak of the metamorphic event was most probably before 750Ma. A gradual change from compressional to extensional stress regime is evidenced by emplacement andesitic magnesium-rich dykes dated to 705Ma that were later metamorphosed to schistose dykes at a greenschist metamorphic facies. The second orogenic phase (terrane amalgamation, main shaping of crust) was associated with the emplacement of large volumes (>50% of area) of calc-alkaline intrusions in a post-collision setting. These very last stages of metamorphism and deformation are characterized by intrusion of ∼630Ma granitoids exhibiting some foliation. Pluton emplacement continued also after the end of deformation. Exhumation and transition to an extensional regime is recorded by the intrusion of shallow alkaline granites in ∼608Ma which were accompanied in ∼609Ma by rhyolite, andesite and composite dykes. The last magmatic event in the Elat area is represented by the volcano-conglomeratic series comprising rhyolites, basalts, andesites, hypabyssal intrusions of monzonite and syenite and conglomerates. The conglomerates, dated to about 590Ma, are the products of a major erosion phase in which about 12,000m of the section were removed. These conglomerates were intruded by 585Ma rhyolite, andesite and composite dykes. The Neoproterozoic basement is truncated by a peneplain whose age, post 532Ma, is constrained by the age of the youngest eroded dolerite dykes. This Early Cambrian peneplain was associated with erosion of 2000m of the section and by chemical weathering. Three major breaks in Neoproterozoic magmatic activity are recognized: the first, occurred in Cryogenian time, lasted ∼60 million years after the amphibolite facies metamorphism and before emplacement of the calc alkaline plutons, separating the first and the second orogenic phases; the second break between the orogenic and the extensional phases occurred in early Ediacaran time, encompassed ∼20 million years between the emplacement of the calc-alkaline and alkaline plutonic rocks and rhyolite, andesite and the composite dykes; and the third, ∼50Ma break, occurred between the emplacement of the last felsic intrusions at ∼585Ma and intrusion of the dolerite dykes in 532Ma, before the Early Cambrian peneplain developed.The great lateral extension of the Cambrian to Eocene sedimentary rocks and their slow facies and thickness changes suggest a stable flat platform area at the northern tip of the ANS. Early Cambrian sedimentation began with fluviatile subarkoses of the Amudei Shlomo Formation. It was overlain by an Early to Middle Cambrian transgressive–regressive lagoonal cycle of dolostones, sandstones, and siltstones of the Timna Formation. Then Middle Cambrian subarkoses and siltstones of the Shehoret Formation and the quartz arenite of the Netafim Formation were deposited in a coastal, intertidal environment representing the southern transgression of a Cambrian ocean.