The Ampferer-Type Subduction: A Case of Missing Arc Magmatism

Ampferer-type subduction is a term that refers to the foundering of hyper-extended continental or embryonic oceanic basins (i.e., ocean-continent transitions) at passive continental margins. The lithospheric mantle underlying these rift basins is mechanically weaker, less dense, and more fertile than the lithospheric mantle underlying bounded continents. Therefore, orogens resulting from the closure of a narrow, immature extensional system are essentially controlled by mechanical processes without significant thermal and lithologic changes. Self-consistent, spontaneous subduction initiation (SI) due to the density contrast between the lithosphere and the crust of ocean-continent transitions is unlikely to occur. Additional far-field external horizontal forces are generally required for the SI. When the lithosphere subducts, the upper crust or serpentinized mantle and sediments separate from the lower crust, which becomes accreted to the orogen, while the lower crust subducts into the asthenosphere. Subduction of the lower crust, which typically consists of dry lithologies, does not allow significant flux-melting within the mantle wedge, so arc magmatism does not occur. As a result of melting inhibition within the mantle wedge during Ampferer-type subduction zones, the mantle beneath the resulting orogenic belts is fertile and thus has a high potential for magma generation during a subsequent breakup (i.e., magma-rich collapse)

Mueilha rare metals granite, Eastern Desert of Egypt : An example of a magmatic-hydrothermal system in the Arabian-Nubian Shield

The Mueilha granite pluton is one of the rare-metals bearing peraluminous granitic plutons in the Arabian-Nubian Shield. It represents the apical part of a highly evolved magma chamber emplaced at a shallow level subsequent to the post Pan-African orogeny. The pluton can be seen as a highly leucocratic medium-grained albite/oligoclase framework infilled with quartz, K-feldspar and muscovite that are variably overgrown by K-feldspar, muscovite, quartz and topaz megacrysts. The increasing number and size of the K-feldspar megacrysts at the expense of the whitened albite/oligoclase framework imparts variably red color to the Mueilha granite. Contacts between the milky white and red granites are usually gradational, but may be locally sharp or may form narrow transition zones resulting from abrupt variations in texture and lithology. Textural relations indicate an initial stage of hydrothermal albitization of magmatic plagioclase and crystallization of topaz megacrysts resulting from infiltration of Na-rich fluorine bearing fluids. A subsequent stage of metasomatic enrichment is characterized by extensive growth of large K-feldspar, quartz and muscovite megacrysts at the expense of the albite/oligoclase crystals as a result of infiltration of K-Si rich hydrous fluids. Post-magmatic infiltration of hydrous fluids along the fault planes is shown by the intense replacement of alkali feldspar megacrysts by quartz, development of myrmekitic intergrowth pockets along the K-feldspar megacrysts and sealing of the micro-fractures by cryptocrystalline mixtures of clay minerals, iron oxides, sericite and chlorite. Compositionally, the red granitic rocks have higher SiO2, Fe2O3total, K2O/Na2O, Σ REE, Y, Th, U, Zr and Zn and lower Al2O3, Ga, Ta, Nb and Mo compared to the milky white granites. LILE and Sn do not show clear variation trends throughout the Mueilha granite pluton, suggesting their immobility during hydrothermal alteration. Microthermometric measurements indicate that the interactions with the hydrothermal fluids started at a minimum temperature > 400°C, most likely during the late-stage crystallization of the Mueilha granite and continued after their complete solidification (i.e. subsolidus conditions) at a temperature as low as 120 °C. The high fertility of Mueilha granite is most plausibly the result of partial melting within the undepleted juvenile crust of the Arabian–Nubian Shield that has formed during the Pan-African orogeny

Tracing the HIMU component within Pan-African lithosphere beneath northeast Africa: Evidence from Late Cretaceous Natash alkaline volcanics, Egypt

International audienceA large late Cretaceous (~ 90 Ma) volcanic field (the Natash volcanic province) crops out in southeast Egypt at the northwestern boundary of the Arabian-Nubian shield. The lavas are mainly of alkaline affinity and exhibit a continuous compositional range from alkali olivine basalt (AOB) to trachyte and rhyolite. All basaltic lavas in the province record various extents of fractional crystallization of olivine, clinopyroxene, plagioclase and spinel. The basaltic lavas show variations in Sr-Nd-Pb-Hf isotopic ratios [(87Sr/86Sr)i = 0.7030–0.70286; (143Nd/144Nd)i = 0.512653–0.512761; (206Pb/204Pb)i = 19.28–19.94; (177Hf-176Hf)i = 0.28274–0.28285], that correlate markedly with the major and trace element ratios and abundances. Assimilation of crustal material cannot explain these correlations, and we invoke instead melting of a multicomponent mantle source. We infer the existence of High-μ (HIMU), Enriched mantle type-I (EM-I) and Depleted mantle (DM) domains in the melting source, with a predominant contribution from the HIMU-type. We suggests further that the basaltic lavas originate from low degrees of partial melting (F < 5%) at moderate potential temperatures (TP) 1391–1425 °C and pressures of 2.0–2.6 GPa. The melting pressure estimations imply that melting entirely occurred within lithospheric mantle, most likely in the presence of residual amphibole as presence negative K-anomalies in the primitive mantle-normalized patterns of the fractionation-corrected melts. The presence of amphibole within the lithosphere is a strong evidence that the lithospheric mantle underwent metasomatic enrichment prior to melting in Late Cretaceous. This metasomatic event affected on the Pb isotopic compositions of the Natash volcanics by adding Th and U to the melting source. Time-integrated calculations to remove the decoupling between 206Pb and 207Pb isotopes that most probably resulted from the metasomatic event indicate a tentative link between the metasomatism occurring in the Pan-African lithospheric mantle and the formation of juvenile crust during the Pan-African Orogeny. A two stage evolution model is therefore proposed for volcanism in the Natash area: fluxing of the lithosphere by hydrous fluids during Pan-African Orogeny forming a hybrid lithospheric mantle that in Late Cretaceous underwent thermal erosion and melting in response to upwelling asthenosphere, possibly at the onset of the extensional fracturing preceded the doming of the Afro-Arabian Shield

Cold plutonism in the Arabian–Nubian Shield: evidence from the Abu Diab garnet-bearing leucogranite, central Eastern Desert, Egypt

Appendix A: Mean and standard deviations for chosen element ratios. Appendix B: Distribution of the near-eutectic, paraluminous leucogranites listed in the Table

Bimodal zircon ages from Natash volcanics (southeast Egypt) and the link between eruption mechanisms and Late Cretaceous tectonics

International audienceTrachyte plugs from the Natash volcanic field have been precisely dated using in situ U-Pb geochronology on zircon grains at~93 Ma. Available zircon ages thus suggest that the overall volcanic history at Natash was probably shorter than previouslyestimated based on Rb-Sr and K-Ar ages, which are more sensitive to element mobility. Basaltic flows and trachyte plugs wereproduced by a largely synchronous, Upper Cretaceous volcanic activity that occurred in the Natash area at the onset of theextensional fracturing that preceded and accompanied the doming of the Afro-Arabian Shield. Extraction and eruption ofvolumetrically dominant basalts from deeper lithospheric levels were ruled by major NW-SE strike-slip faults, whereas bothmajor and subordinate fault systems activated by successive tectonic pulses favoured the ascent of more evolved melts (i.e.trachytes and rhyolites) from shallow-level magma chambers. The study revealed also the presence of zircon xenocrysts in sometrachyte plugs associated or not to magmatic zircon. These xenocrysts have pre-Cambrian ages (~681 Ma) matching those ofzircons from basement rocks in the area (~700 Ma) and were most probably incorporated during the emplacement of trachyteplugs at shallow depths. The strictly bimodal ages recorded by zircons suggest a very large time gap (> 500 Ma) between the endof the Pan-African Orogeny and the onset of new tectono-magmatic activity in the Natash area

Serpentinization and deserpentinization reactions in the upper mantle beneath fuerteventura revealed by peridotite xenoliths with fibrous orthopyroxene and mottled olivine

Mantle xenoliths collected from Fuerteventura, one of the easternmost Canary Islands, exhibit a complex evolutionary history comprising events of depletion, serpentinization, dehydration and melt metasomatism. Each of these events left imprints on both the texture and chemistry of the xenoliths. Extensive partial melting is shown by complete lack of primary clinopyroxene, the ultra-refractory trace element composition of orthopyroxene porphyroclasts, and low heavy rare earth element contents as compared with abyssal peridotites sampled along mid-ocean ridges and oceanic fracture zones, in the xenoliths least affected by later metasomatism. In many xenoliths the original orthopyroxene porphyroclasts and some olivines are replaced by fibrous aggregates of orthopyroxene and/or large, deformed olivine porphyroclasts with mottled rims with stringy glass and fluid inclusions. Such features are very rare in ocean island xenoliths. Unusually high H2O and Cl concentrations, together with very high H2O/Ce and Cl/K ratios in interstitial glasses, suggest that the fibrous orthopyroxene formed by local serpentinization by hot seawater. The volume increase accompanying the serpentinization caused extensive fracturing of adjacent olivine porphyroclasts. The most likely scenario for local mantle invasion by hydrous fluids is along deep faults and fractures caused by tectonic movements along the continent-ocean transition during the early phases of the opening of the Atlantic Ocean. The peridotites were later (probably during the Canary Islands magmatism) dehydrated, causing the serpentine minerals to be replaced by porous domains of fibrous orthopyroxene. Hydrous fluids released by the deserpentinization escaped into neighbouring and overlying rocks leaving trails of fluid inclusions along fractures and grain boundaries causing mottled rims and zones in olivine porphyroclasts. During the Canary Islands magmatism the upper mantle beneath Fuerteventura was also infiltrated by enriched silicate magmas that caused different degrees of Fe-Ti-metasomatism. A higher degree of melt metasomatism in rocks with fibrous orthopyroxene and mottled olivine than in the massive harzburgites strongly suggests that the sublithospheric Canarian magmas reused serpentinized extensional faults during their rise to the surface. The strongest degree of melt metasomatism appears to have resulted in the formation of lherzolites, wehrlites, and dunites

Softening of sub-continental lithosphere prior rifting: Evidence from clinopyroxene chemistry in peridotite xenoliths from Natash volcanic province, SE Egypt

International audienceMajor and trace element compositions were determined for well-preserved diopside relics in highly altered mantle xenoliths from Natash volcanic province, south Eastern Desert of Egypt, to unravel the major magmatic processes that occurred within the lithospheric mantle long time before the Red Sea rift. The diopside shows a limited compositional range as for mg# (0.89–0.92), Al2O3 (3.52–5.60 wt%), andTiO2 (0.15–0.35 wt%), whereas it is characterised by a larger variability as for Na2O (0.23–1.83 wt%) and, in particular the trace elements. The latter identify two main diopside types: 1) CPX-I has low abundances of incompatible elements, spoon-like REE patterns, small negative anomalies in Ti and Zr and a positive anomaly in Sr; and 2) CPX-II has high abundances in incompatible elements, REE patterns with steady enrichment from HREE to LREE patterns and marked negative anomalies in Ti and Zr. The range of REE patterns in the mantle section can be explained by 7–22% batch melting of the primitive mantle followed by varying degrees of trace element chromatographic exchange. CPX-I underwent only small-scale reactive porous flow metasomatism at the percolation front, whereas CPX-II resulted from large-scale rock–melt interaction close to the melt source. Trace element abundances of CPX-II suggest equilibration with carbonatite-like melts that bear close similarities with the carbonatites that enriched the lithosphere in the southern part of the Arabian plate. The similarity of the P-T gradients recorded by the Natash and southern part of Arabian lithospheres, as well as their re-fertilisation by similar, carbonatite-like agents, is consistent with the presence of a mantle plume at the base of the lithosphere after accretion of the Arabian-Nubian Shield in Late Precambrian. The plume material was fossilized due to secular cooling and became part of the lithospheric mantle before the eruption of the Natash volcanic in Late Cretaceous

A model for granite evolution based on non‑equilibrium magmaseparation: evidence from the Gharib and Qattar fluorite‑bearing granites, Eastern Desert, Egypt

We present 77 new granite whole-rock analyses from the Qattar and Gharib areas, Eastern Desert, Egypt. Both areas includea “normal” granite and either a hypersolvus (Gharib) or an almost plagioclase-free granite (Qattar) enriched in fluorite. According to earlier results, F influences element distribution in granitic melts forming complexes with specific elementsas Nb, Ta, Ga, Hf, Th, Zn, Sn, whereas F excludes Ba and Sr. We use principal component analyses to split the granite into chemical groups allowing an unbiased study of the inter-group element distribution. This adds the heavy REEs and Y to the earlier lists of elements with an affinity for F. The light REEs show a decreasing affinity with decreasing atomic mass; fluorine separates Sm from Nd, whereas Zr follows La. Opposite to some, but in accordance with other earlier results, the ratio Nb/Ta is higher in the fluorite-enriched than in the other granite. Weak tetrad effects are present. Zircon in the hypersolvus granite is high in common lead. We suggest F to be instrumental for separating Pb2+from Pb4+.Two hypotheses may explain the occurrence of the two contrasting granites: they have either different sources, or they are co-magmatic, but the magma was split into two discrete types. We apply the second hypothesis as our working hypothesis. The liquidus has a gentler slopewith pressure than the diapir requiring crystallisation to be most important in the lower part of the magma chamber. Our hypothesis suggests that globules of magma, enriched in volatile components, form during crystallisation due to slow diffusionrates in the crystallizing magma. Elements accompanying F are distributed into this magma batch, which has a lowered density and viscosity than the rest of the magma due to its increased contents of volatile components. A mushroom-formed diapir rises, forming the hypersolvus (or almost plagioclase-free) granite. Due to an edge effect, it is concentrated close to the wall of the magma chamber. The size and form of the outcropping granite depend on the intersection of the diapir with the erosion surface. Fluorine only makes it possible to follow the process. The model may be generalised to explain the diversification of non-F enriched granite, since the buoyancy of a magma batch several thousand m3 in size has a much larger impact on the system than the small negative buoyancy of crystals or small crystal aggregates. A-type granite classified merely from its trace element content may form from separated F-enriched magma batches. This may be the reason fo rtheir high frequency in the Eastern Desert

Geochemistry of 24 Ma basalts from NE Egypt: source components and fractionation history

<p>Subalkaline basalts from NE Egypt represent an episode of magmatism at <em>c</em>. 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. </p