Search

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]

[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]

In order to better understand the nature and formation of the lower continental crust beneath northern Arabia, we studied lower crustal xenoliths brought up by Neogene basalts in NE Jordan. Most of these xenoliths are comprised of primary phases plagioclase + two-pyroxenes with magnetite and ilmenite. Most clinopyroxene are augite whereas orthopyroxene mostly are hypersthene (Mg# = 50–80). Plagioclase feldspar is dominantly andesine–labradorite; pyrope-rich garnet and Fe-rich olivine (Fo 75 to Fo 62 ) are rare. These xenoliths represent cumulates formed from intermediate magmas that pooled in the lower crust. Many xenoliths also contain small, fine-grained K-rich zones interpreted as melt pockets reflecting late magmatic infiltration of the lower crust. The xenoliths display a wide range in major element compositions (37–51 wt.% SiO 2 , 4–15 wt.% MgO and 0.1–6.3 wt.% TiO 2 ), enrichment in Ba, K, Sr, Pb and Eu, and some trace element ratios atypical of bulk continental crust (e.g., K/Rb = 1265 ± 565 , K/U = 63 000 ± 60 080 and Th/U = 0.96 ± 0.56 ); these extreme ratios reflect widespread K-metasomatism associated with melt pockets. The magmas from which these cumulates formed may have been generated at a reararc convergent margin setting. Four U–Pb zircon populations yield indistinguishable ages of 554 ± 4 Ma ; 559 ± 5 Ma ; 559 ± 6 Ma , and 563 ± 5 Ma . Initial 87 Sr/ 86 Sr values (0.70260–0.70352) and positive ε Nd ( 560 ) (with the exception of a single, more radiogenic sample (+9.6), range = + 1.3 to +4.8) indicate that the lower crust sampled by the xenoliths originated in the asthenospheric mantle, with little or no interaction with older crust, although Pb isotopic compositions allow for some interaction with older or subducted crustal materials. We interpret the geochemistry and mineralogy of these xenoliths to indicate that the lower crust beneath NE Jordan is mafic and comprised of plagioclase-rich 2-pyroxene igneous rocks. The lower crust of this area formed by magmatic underplating over less than 18 Ma. The crust of NE Jordan is significantly younger than the crust of the northernmost Arabian–Nubian Shield and represents a fragment of Cadomian (600–520 Ma) crust that may make up the NE margin of the Arabian Plate. [ABSTRACT FROM AUTHOR]

Continental crust beneath northern Arabia is deeply buried and poorly known. To advance our knowledge of this crust, we studied 8 xenoliths brought to the surface by Neogene eruptions of Tell Thannoun, S. Syria. The xenolith suite consists of two peridotites, one pyroxenite, four mafic granulites, and one charnockite. The four mafic granulites and charnockite are probably samples of the lower crust, and two mafic granulites gave 2-pyroxene equilibration temperatures of 780–800 °C, which we take to reflect temperatures at the time of formation. Peridotite and pyroxenite gave significantly higher temperatures of ∼900 °C, consistent with derivation from the underlying lithospheric mantle. Fe-rich peridotite yielded , perhaps representing a cumulate layer in the crust. Three samples spanning the lithologic range of the suite (pyroxenite, mafic granulite, and charnockite) yielded indistinguishable concordant U–Pb zircon ages of ∼357 Ma, interpreted to approximate when these magmas crystallized. These igneous rocks are mostly juvenile additions from the mantle, as indicated by low initial 87Sr/86Sr (0.70312 to 0.70510) and strongly positive initial εNd(357 Ma) (+4 to +9.5). Nd model ages range from 0.55 to 0.71 Ga. We were unable to unequivocally infer a tectonic setting where these melts formed: convergent margin, rift, or hotspot. These xenoliths differ from those of Jordan and Saudi Arabia to the south in four principal ways: 1) age, being least 200 Ma younger than the presumed Neoproterozoic (533–1000 Ma) crust beneath Jordan and Saudi Arabia; 2) the presence of charnockite; 3) abundance of Fe-rich mafic and ultramafic lithologies; and 4) the presence of sapphirine. Our studies indicate that northern Arabian plate lithosphere contains a significant proportion of juvenile Late Paleozoic crust, the extent of which remains to be elucidated. This discovery helps explain fission track resetting documented for rocks from Israel and provides insights into the nature of Late Paleozoic (Hercynian) deformation that affected Arabia near the Persian Gulf. [ABSTRACT FROM AUTHOR]

Continental-arc igneous rock compositions change in response to the transition from subduction to collision and these changes can reveal how the crust, lithosphere and magma sources evolved. Neotethys-related Late Cretaceous to Pleistocene subduction- and collision-related magmatic rocks from the ~350 km long southeast Urumieh-Dokhtar Magmatic Belt (UDMB) of Iran provide an excellent natural laboratory to better understand these changes. These igneous rocks are well-exposed and moderately eroded to reveal a nearly complete record since subduction initiation at ~95 Ma. We analyzed new samples for major and trace elements (83 samples), Sr Nd isotopic compositions (47 samples), and U Pb zircon ages (26 samples) and compiled geochemical and geochronological data on the southeast segment of the UDMB. The geochronological data reveal two magmatic pulses at ~80–70 Ma and ~50–0 Ma. Important changes in magmatic compositions reflect initial collision with Arabia at ~32 Ma, changing from normal calc-alkaline to increasingly adakitic immediately after collision began. Five stages can be identified: 1) normal continental-arc magmatism during the Late Cretaceous; 2) arc quiescence in Paleocene and Early Eocene time; 3) Middle-Late Eocene extensional arc magmatism related to slab rollback; 4) early collision and crustal thickening during the Early Oligocene; and 5) slab breakoff, asthenospheric upwelling, and associated adakitic magmatism from Middle Miocene onward. Temporal changes in UDMB magmas reflect the response of the overriding plate to changes in the geometry of the subducting Neotethyan lithosphere and to collision between Arabia and Iran. Crustal thickening and arc narrowing during Miocene to Pleistocene post-collisional magmatism caused adakitic magmatism and associated Cu mineralization. Zircon O Hf and apatite O isotopes as well as bulk-rock Nd isotopes of Cu-bearing adakitic rocks are similar to other barren rocks, but nearly all fertile rocks have higher Hf/Y, Eu/Eu⁎ (n) in zircon and higher Sr/Y, V/Y, Eu/Eu⁎ (n) in apatite than barren rocks. • Magmatism in southeast Iran occurred during neotethyan subduction and collision. • Magmatism changed from pre-collision calc-alkaline to Syn -collisional adakitic. • Syn-collisional magmatism at ca 32 Ma was characterized by perturbation of isotopic systems. [ABSTRACT FROM AUTHOR]