Ages are used to constrain the temporal evolution of the Meatiq Gneiss Dome, Eastern Desert, Egypt, by dating (ID-TIMS) pre-, syn-, and post-tectonic igneous rocks in and around the dome. The Um Ba'anib Orthogneiss, comprising the deepest exposed structural levels of the dome, has a crystallization age of 630.8 ± 2 Ma. The overlying mylonites are interpreted to be a thrust sheet/ complex (Abu Fannani Thrust Sheet) of highly mylonitized metasediments (?), migmatitic amphibolites, and orthogneisses with large and small tectonic lenses of lessdeformed intrusives. Two syn-tectonic diorite lenses in this complex have crystallization ages of 609.0 ± 1.0 and 605.8 ± 0.9 Ma, respectively. The syn-tectonic Abu Ziran diorite, cutting across the tectonic contact between mylonite gneisses of the Abu Fannani Thrust Sheet and a structurally overlying thrust sheet of eugeoclinal rocks (''Pan-African nappe''), has a magmatic emplacement age of 606.4 ± 1.0 Ma. Zircons from a gabbro (Fawakhir ophiolite) within the eugeoclinal thrust sheet yielded a crystallization age of 736.5 ± 1.2 Ma. The post-tectonic Fawakhir monzodiorite intrudes the ophiolitic rocks and has an emplacement age of 597.8 ± 2.9 Ma. Two other post-tectonic granites, the Arieki granite that intrudes the foliated Um Ba'anib Orthogneiss, and the Um Had granite that cuts the deformed Hammamat sediments, have emplacement ages of 590 ± 3.1 and 596.3 ± 1.7 Ma, respectively. We consider formation of the Meatiq Gneiss Dome to be a young structural feature (\631 Ma), and our preferred tectonic interpretation is that it formed as a result of NE-SW shortening contemporaneous with folding of the nearby Hammamat sediments around 605-600 Ma, during oblique collision of East and West Gondwana.

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Meatiq and Hafafit core complexes are large swells in the Eastern Desert of Egypt, comprising two major tectono-stratigraphic units or tiers. The lower (infrastructure) unit is composed of variably cataclased gneissose granites and high-grade gneisses and schists. It is structurally overlain by Pan-African ophiolitic mélange nappes (the higher unit). The two units are separated by a low-angle sole thrust, along which mylonites are developed. Major and trace element data indicate formation of the gneissose granites in both volcanic arc and within-plate settings. Nevertheless, all analyzed gneissose granites and other infrastructural rocks, exhibit low initial ratios (Sr i ) (<0.7027), positive e Nd (t) (+4.9 to +10.3) and Neoproterozoic Nd model age (T DM ) (592-831 Ma for the gneissose granite samples). Although these values are compatible with other parts of the Arabian-Nubian Shield considered to be juvenile, the e Nd (t) values and several incompatible element ratios of the gneissose granites are too low to be derived from a mantle source without contribution from an older continental crust. Our geological, Sr-Nd isotopic and chemical data combined with the published zircon ages indicate the existence of a pre-Neoproterozoic continent in the Eastern Desert that started to break up at ca 800 Ma. Rifting and subsequent events caused the formation of oceanic crust and emplacement within-plate alkali basalts in the hinterland domains of the old continent. The emplacement of basaltic magma might have triggered melting of lower crust in the old continent and resulted in emplacement of the within-plate granite masses between 700 Ma and 626 Ma. The granite masses and other rocks in the old continent have been subjected to deformation during the over-thrusting of Pan-African nappes, probably because of the oblique convergence between East and West Gondwanaland. Rb-Sr isotopes of the gneissose granites in both Meatiq and Hafafit core complexes defines an isochron age of 619 Ϯ 25 Ma with Sr i of 0.7009 Ϯ 0.0017 and mean squares of weighted deviates = 2.0. We interpret this age as the date of thrusting of the Pan-African nappes in the Eastern Desert. Continued oblique convergence between East and West Gondwanaland could have resulted in the formation northwest-southeast-trending Meatiq and Hafafit anticlinoriums.

New Caledonia is one of the world’s best-exposed subduction/obduction complexes and is central to understanding the geodynamic evolution of the southwest Pacific region. We present new geochemical and Ar/Ar age dates from the in situ eruptive sequences of the La Conception basaltic-andesite lavas (ca. 29.12 Ma) and correlate the generation of these lavas with the generation of the Saint Louis and Koum/Borindi Massifs (ca. 24 Ma) to provide information on the magmatic processes operating within the mantle wedge over time and to indicate the direction of the subducted slab during the Oligocene. The La Conception basaltic-andesite lavas were emplaced in an arc-to-trench direction to the southwest of New Caledonia due to the partial melting of metasomatized, amphibole-bearing garnet peridotites at the base of mantle wedge (∼112−118 km). However, both the Saint Louis and Koum/Borindi granodiorite massifs were derived from melting within the lower crust of an island arc overlying a mantle wedge. Such temporal and spatial variations of magmatism in New Caledonia are consistent with a northeast subduction zone during the Oligocene. The absence of voluminous arc magmatism related to the Oligocene subduction is a consequence of the low Tp temperatures of the mantle wedge, the old age and the high sinking velocity of the downgoing slab, and the absence of stress within the overriding plate. The Oligocene subduction beneath New Caledonia, therefore, is an ideal example of Ampferer-type subduction, which is an amagmatic closure area due to the inefficient subduction of hydrated lithologies into the convective upper mantle. Once the Paleocene to Miocene contraction in the southwest Pacific region ceased, the subduction west of New Caledonia shut down, the system relaxed, and no more melting occurred during the Oligocene.

Subalkaline basalts from NE Egypt represent an episode of magmatism at c. 24 Ma, coincident with widespread eruptive activity in northern Africa. New geochemical data provide insight into the mineralogical and isotopic characteristics of the underlying mantle. The basalts show little geochemical variation, with incompatible trace element abundances similar to those of ocean island basalts. They display fairly smooth primitive mantle-normalized incompatible trace element patterns. Trace element abundances and Sr–Nd–Pb–Hf isotopic signatures are consistent with contributions from two distinct source regions, one similar to the Afar plume and the other located within the metasomatized spinel-facies subcontinental lithosphere. Mixing of melts from these two domains was followed by minor crustal contamination during prolonged ascent or emplacement. Integrating the geochemical data with available tomographic information allows us to develop a framework for understanding mid-Tertiary magmatic activity throughout northern Africa. A model for this widespread volcanism involves ascent of upwelling mantle derived from the margins of the South African Superplume rooted at the core–mantle boundary and/or through small-scale convection at the 660 km discontinuity. Ascent of magmas to the surface was facilitated by pre-existing structures within the lithosphere, including those associated with incipient rifting of the Red Sea.Supplementary material:Mineral chemistry data are available at http://www.geolsoc.org.uk/SUP18483.