Search

[Display omitted] • A transition from 695 Ma I-type, convergent margin magmatism in the north to 635 Ma alkalic, within magmatism in the south is recorded. • Our results thus document a chronologic and tectonic transition from synorogenic, intra-oceanic arc to post-collision within-plate magmatism in the Ediacaran. • Our results also illustrate the viability of dikes as markers of orogénesis. The tectonic evolution of the Arabian-Nubian Shield began with formation of intra-oceanic arcs followed by collisions between these in Tonian-Cryogenian time and culminated in Ediacaran continental collision to form Gondwana. We provide new insights into the middle stage of this tectonic evolution by focusing on two Neoproterozoic dike swarms of different age in northern Arabia. The 695–700 Ma Za'am dikes were emplaced about the time of collision between the Midyan and Hijaz terranes whereas the 635 Ma Bayda dikes were emplaced ∼ 60 million years after collision. These record a shift from relatively unfractionated 695 Ma arc-type magmas in the south to fractionated 635 Ma within-plate alkaline magmas in the north. Our results document mantle source and crustal evolution beneath the Midyan terrane during and after terrane accretion, revealing a tectono-magmatic transition from SSZ-type magmas accompanying arc collision to post-terrane accretion within-plate magmatism through Cryogenian time. In addition, our results confirm the potential of dike studies for reconstructing the tectono-magmatic-metallogenic evolution of orogens. [ABSTRACT FROM AUTHOR]

The Cadomian orogeny reflects S -directed subduction on the northern margin of Greater Gondwana in Ediacaran – Cambrian time (∼600-500 Ma). It is well known from the eastern seaboard of North America, western and southern Europe, Turkey and Iran but is rarely identified in what was its backarc in N. Africa and Arabia. Here I summarize evidence for the Cadomian across ∼6500 km of Arabia and N. Africa from Oman to Morocco. The beginning of the Cadomian in Afro-Arabia cuts across and overlaps in time with earlier Neoproterozoic orogenies leading to the formation of Greater Gondwana, so emphasis is on rocks <570 Ma. Three Cadomian backarc crustal tracts or segments are identified. The eastern segment (Oman, Persian Gulf, eastern Saudi Arabia) mostly consists of buried transtensional basins with little igneous activity. The central segment was characterized by lower crust magma underplating and strong uplift along a N –S axis (Jordan) accompanied by rifting leading to deposition of thick sediments reminiscent of a passive continental margin, and strong evidence for regional metasomatism (Israel). The Zenifim passive margin is 580–610 Ma, older than our arbitrary 570 Ma limit for identifying the Cadomian but clearly post-dates terminal continental collision to form the East African Orogen and so must be related to Cadomian back-arc rifting. Along with related NE-SW trending dike swarms in NE Egypt and Jordan, the orientation of the Zenifim rifted margin, orthogonal to the Cadomian convergent margin, is another indication of strong Cadomian transtension. In contrast, the western segment (Libya, Algeria, Morocco) experienced abundant igneous activity, with evidence for this found especially far south in Libya. Recognition of the Cadomian orogeny in northern Africa provides a useful first step in breaking out distinct tectonic episodes in the ∼300 m.y. long Pan-African orogeny, at least in the north. Finally, the orthogonal relationship between older N –S orogens in the interior of Greater Gondwana and the E -W Cadomian orogen illustrates how the Neoproterozoic plate mosaic grew by subduction initiation exploiting lithospheric weaknesses along its continental margin. Ten recommendations for future research are also suggested. • The ∼550 Ma Cadomian orogeny in N. Africa and Arabia is reviewed for the first time. • N. Africa-Arabia Cadomian crosscuts older Neoproterozoic structures. • N. Afro-Arabia Cadomian is mostly transtensional basins in E, magmatic rocks in W. • Rifted continental margin offshore Israel suggests Cadomian basins lie further W. • 10 suggestions for research are provided. [ABSTRACT FROM AUTHOR]