Cambrian magmatic flare-up, central Tibet: Magma mixing in proto-Tethyan arc along north Gondwanan margin

Accompanying Gondwana assembly, widespread but diachronous Ediacaran−early Paleozoic magmatism of uncertain origin occurred along the supercontinent’s proto-Tethyan margin. We report new geochemical, isotopic, and geochronological data for Cambrian magmatic rocks (ca. 500 Ma) from the Gondwana-derived North Lhasa terrane, located in the present-day central Tibetan Plateau. The magmatic rocks are composed of basalts, gabbros, quartz monzonites, granitoids (with mafic microgranular enclaves), and rhyolites. Nd-Hf isotopic and whole-rock geochemical data indicate that these rocks were probably generated by mixing of mantle-derived mafic and crust-derived felsic melts. The mantle end-member volumes of mafic, intermediate, and felsic rocks are ∼75%−100%, 50%−60%, and 0−30%, respectively. Integration of our new data with previous studies suggests that the North Lhasa terrane experienced long-term magmatism through the Ediacaran to Ordovician (ca. 572−483 Ma), with a magmatic flare-up at ca. 500 Ma. This magmatism, in combination with other Ediacaran−early Paleozoic magmatism along the proto-Tethyan margin, was related to an Andean-type arc, with the magmatic flare-up event related to detachment of the oceanic slab following collisional accretion of Asian microcontinental fragments to northern Gondwana. Diachroneity of the proto-Tethyan arc system along the northern Gondwanan margin (ca. 581−531 Ma along the Arabian margin and ca. 512−429 Ma along the Indian-Australian margin) may have been linked to orogenesis within Gondwana. The North Lhasa terrane was probably involved in both Arabian and Indian-Australian proto-Tethyan Andean-type orogens, based on its paleogeographic location at the northern end of the East African orogen.

Petrogenesis of a rare Ediacaran tonalite–trondhjemite–granodiorite suite, Egypt, and implications for Neoproterozoic Gondwana assembly

Most tonalite–trondhjemite–granodiorite (TTG) suites are Archean–Palaeoproterozoic in age, but those of Neoproterozoic–Phanerozoic age are scarce. A rare Ediacaran high-Al TTG suite has been identified at the Fannani Igneous Complex (FIC) in the northern Arabian–Nubian Shield, which is essentially composed of amalgamated Neoproterozoic island-arc Pan-African composite terranes that contain several ophiolitic sutures. The FIC exhibits a wide range of SiO2, Al2O3, Sr and Zr, shows moderate rare earth element (REE) enrichment, and K, Ti, Nb, Y and heavy REE depletion. It is a subsolvus suite with clear orogenic affinities and strong arc-geochemical signatures. The precise U–Pb zircon thermal ionization mass spectrometry age obtained (607.4 ± 1.95 Ma) indicates oceanic subduction extended to late stages of the East African Orogeny. The FIC exhibits 87Sr/86Sr compositions of 0.70346–0.71091 (Sr(i) ratio, 0.70284), and 143Nd/144Nd of 0.51254–0.51270 (ϵNd(t) = +5.12 to +7.16), typical of modern oceanic-arc rocks (as Japan-arc basalts), and suggestive of mantle sources and island-arc settings. The FIC possesses low values of Yb (1.55 ppm), Nb (14 ppm) and Y (24 ppm), and high ratios of Sr/Y (27), Zr/Sm (46) and Nb/Ta (11.8), typical of magmas produced by anatexis of a basaltic slab. Partial melting models show that the FIC magma was generated by melting (F = 0.25–0.50) of a subducted oceanic crust transformed into eclogite, leaving 10–25% garnet in the residue. The FIC and similar complexes produced via slab melting during the closure of the Mozambique Ocean formed large juvenile belts along the East African Orogen that sutured East and West Gondwana together into a united supercontinent.

Detrital zircon provenance of north Gondwana Palaeozoic sandstones from Saudi Arabia

We present the first comprehensive detrital zircon U–Pb age dataset from Palaeozoic sandstones of Saudi Arabia, which provides new insights into the erosion history of the East African Orogen and sediment recycling in northern Gondwana. Five main age populations are present in varying amounts in the zircon age spectra, with age peaks at ~625 Ma, ~775 Ma, ~980 Ma, ~1840 Ma and ~2480 Ma. Mainly igneous rocks of the Arabian–Nubian Shield are suggested to be the most prominent sources for the Ediacaran to middle Tonian zircon grains. Palaeoproterozoic and Archaean grains may be xenocrystic zircons or they have been recycled from older terrigenous sediment. A primary derivation from Palaeoproterozoic and Archaean basement is also possible, as rocks of such age occur in the vicinity. Approximately 4 % of the detrital zircons show Palaeozoic (340–541 Ma) ages. These grains are likely derived from Palaeozoic post-orogenic and anorogenic igneous rocks of NE Africa and Arabia. A few single grains gave up to Eoarchaean (3.6–4.0 Ga) ages, which are the oldest zircons yet described from Arabia and its vicinity. Their origin, however, is yet unknown. Detrital zircons with U–Pb ages of ~1.0 Ga are present in varying amounts in all of the samples and are a feature of terrigenous sediment belonging to the Gondwana super-fan system with an East African – Arabian zircon province.

Pannotia didn’t exist, but the “Pannotian geodynamic cell” formed as the Mozambique Ocean closed and Gondwana amalgamated—the view from Arabia and the East African Orogen

<p>There is a view that a supercontinent, called Pannotia, existed for a short time at the end of the Neoproterozoic. This hypothetical continent requires collision between Neoproterozoic India, Australia-Mawson and the African and South American continents to occur before formation of Iapetus as Laurentia rifted off Amazonia.</p><p>Data from the last decade demonstrate the complexity of consumption of the Mozambique Ocean that separated Neoproterozoic India from the African Neoproterozoic continents (Congo-Tanzania-Bangweulu, the Sahara Metacraton and Kalahari). In particular, the presence of pre-Neoproterozoic terranes that lie within the East African Orogen of Arabia, east Africa, Madagascar and South India demonstrate the multi-phase collision of the this ocean closure?. Here we examine the Cryogenian to Cambrian tectonic geography of the closure of the Mozambique Ocean from a full-plate perspective. We focus on the northern East African Orogen, where Gondwana-formation shortening and crustal thickening has been considerably less than seen in East Africa/Madagascar/South India. We focus on the Neoproterozoic India–Azania–Sahara Metacraton collision represented by the northernmost part of Madagascar (the Bemarivo Domain), and throughout Arabia. We conclude that final ocean closure and formation of central Gondwana occurred in the latest Ediacaran and into the Cambrian, along a suture that passes under the Rub' al Khali region of Arabia and through the northeast of Madagascar. It separates the extended Neoproterozoic India margin (now in Oman, The Seychelles and the northern Bemarivo Domain), from the growing kernel of Gondwana (the east-most parts preserved in Saudi Arabia, Yemen and Central Madagascar).</p><p>Considering the early Ediacaran formation of Iapetus, there is growing evidence that Pannotia never existed as connected continental crust, yet the ‘Pannotian geodynamic cell’ with lithosphere divided into continental and oceanic hemispheres had formed. The closure of the Mozambique Ocean represented the termination of >500 million years of subduction at this locale. The termination of this subduction with the formation of Gondwana, and the initiation of the Terra Australis Orogen led to the present geodynamic configuration.</p>