Mineral chemistry, whole-rock geochemical and Sr–Nd isotopic data are reported for the Abu-Diab granitoids in the northern Arabian-Nubian Shield (ANS) of Egypt, to investigate their petrogenesis and ...geodynamic significance. Gabal Abu-Diab constitute a multiphase pluton, consisting largely of two-mica granites (TMGs) enclosing microgranular enclaves and intruded by garnet bearing muscovite granites (GMGs) and muscovite granites (MGs). The granitoids are weakly peraluminous (A/CNK = 1.01–1.12) and show high SiO2 (>72.9 wt%) and alkali (K2O + Na2O = 8.60–9.13) contents. The geochemical features show that they are post-collisional and highly fractionated A-type granitoids. Compared to their host TMGs, the microgranular enclaves are strongly peraluminous (A/CNK = 1.18–1.24) with lower SiO2 and higher abundances of trace elements. The TMGs are depleted in Ba, Nb, P and Ti and are enriched in LREEs relative to HREEs with weakly negative Eu anomalies (Eu/Eu* = 0.45–0.64). In contrast, the GMGs and MGs are extremely depleted in Ba, Sr and Ti and have tetrad-type REE patterns (TE1–3 = 1.1–1.3) with strongly pronounced negative Eu anomalies (Eu/Eu* = 0.03–0.26), similar to rare metals bearing granites. The Ediacaran (585 ± 24 Ma) TMGs, are characterized by restricted and relatively low initial 87Sr/86Sr ratios (0.70337–0.70382) that suggests their derivation from a depleted mantle source, with little contamination from the older continental crust. In contrast, the GMGs and MGs have extremely high 87Rb/86Sr and 87Sr/86Sr ratios that reflect the disturbance of the Rb-Sr isotopic system and may give an indication for magmatic-fluid interaction. However, all the granitoids display positive εNd(t) (4.41–6.57) and depleted mantle model ages TDM2 between 777 and 956 Ma, which indicate their derivation from a Neoproterozoic juvenile magma sources and preclude the occurrence of pre-Neoproterozoic crustal rocks in the ANS. The microgranular enclaves represent globules of hot mafic magma that have injected and partly mixed with the colder and more felsic TMGs magma. Geochemical and isotopic data along with petrogenetic modelling, suggest that the TMGs were formed by low degrees of partial melting of the pre-existing I-type granodiorites, followed by extensive fractional crystallization and fluid fractionation to produce the geochemically specialized rare metals GMGs and MGs in the margin of Abu-Diab pluton. During the post-collisional stage of ANS and due to lithospheric delamination processes, the underplated fluid/volatile rich mantle magma had interplated and migrated upward to shallow crustal levels, through extensional faults/shear zones, and enhanced the partial melting and fractionation of granodiorites to eventually form Abu-Diab A-type granitoids.
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•The Abu-Diab pluton contains three highly fractionated A-type granitoids.•The Ediacaran age (585 ± 24 Ma) was confirmed using Rb-Sr method.•The young TDM2 model ages reflect the juvenile crustal nature of the granitoids.•The granitoids were formed by both partial melting and fractional crystallization.•Tectono-magmatic evolution model of the Abu-Diab granitoids was proposed.
The central Eastern Desert (CED) of Egypt is well-known for its granite-related Nb–Ta mineralization. The garnet-bearing muscovite granite (GMG) of the Abu-Diab composite pluton in the CED consists ...mainly of quartz, K-feldspar (Or88–98), albite (An0-4) and muscovite, with accessory minerals including garnet, zircon, columbite, ilmenite, Ti-rich hematite, rutile, ilmenorutile, thorite, apatite, xenotime and chlorite. The GMG is weakly peraluminous and has low Nb/Ta (9.6–15.4) and Zr/Hf (16–31) with discernible REEs tetrad effect (TE1-3 = 1.11–1.35), typical of highly evolved granites. Zircon contains high concentrations of U and Th typical of late-magmatic zircon and similar to zircon type from highly evolved granite. The homogenous and weak zoned columbites are classified as manganocolumbite. The formation of Ta-rich rim in the columbite may indicate that later fluids were from the GMG granite itself at advanced fractionation into exsolving fluids, and not from an external source. Ilmenite is greatly enriched in MnO, which indicates the significant pyrophanite (up to 29 mol %) molecules in ilmenite through simple substitution of Mn for Fe2+ with increasing oxygen fugacity under magmatic-hydrothermal conditions. Xenotimes show low analytical totals, suggesting probably hydration during their post-magmatic alteration, while apatite is small-grained associated with tiny zircons, suggesting late-crystalized phases. During late magmatic differentiation stage, the interaction of granitic melt with F-rich late magmatic fluids could be resulted in the formation of Ti–Nb–Ta–Zr minerals. Overall, the Abu Diab GMG possesses mineralogical and geochemical features that make it a potential target for Ti–Nb–Ta–Zr minerals.
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•The highly evolved granitic phase in the Abu-Diab composite pluton contains significant Ti-Nb-Ta-Zr bearing minerals.•Zircon shows a feature of late-magmatic type with high U and Th contents.•The columbite-Mn is homogenous to weakly zoned of magmatic origin.•The F-rich late magmatic fluids play a vital role in the formation of the Abu-Diab Ti–Nb–Ta–Zr minerals.
The partly to pervasively metasomatized doleritic and gabbroic dykes or small to medium sized bodies found in East Othris, within Mid to Late Jurassic serpentinized peridotites of ophiolitic units ...and ophiolitic mélange formations are classified as rodingites and can be divided into two types. Type 1 rodingites are mainly characterized by the frequent occurrence of prehnite, while Type 2 rodingites include mostly garnets and vesuvianite. Isocon analysis showed that rodingitization essentially occurred with mass and volume preservation. Desilification, depletion of alkalies, as well as Ca enrichment was more intense for the Type 2 rodingites. Al, Fe and Mg remained rather immobile, while Ti, Y, Zr and REE were variably depleted.
Rodingitization took place in an intraoceanic subduction system. It occurred in three successive stages during the exhumation of the mafic–ultramafic mantle wedge rocks in a fore-arc setting within a serpentinitic subduction channel, which developed close to the slab. The incorporation of the mafic rocks to the subduction channel probably resulted after entraining a directed mantle flow towards the slab. The first stage of rodingitization formed mainly grossular, hydrogrossular, Ti- and Cr-bearing hydrogarnets and calcite under relatively acidic and mildly oxidizing physicochemical conditions, with increased CO2/H2O ratio. During the second and more extensive rodingitization stage, alkaline and reducing conditions prevailed and CO2/H2O ratio was decreased. The modeling of the mineral reactions of this stage, using the software winTWQ v. 2.34 in the CFMASH system, reveals that in Type 1 rocks prehnite replaced most of the initial garnet, while Type 2 rocks continued to be rodingitized, mostly forming grossular and/or hydrogrossular and chlorite. Hydrogrossular, instead of grossular, was crystallized from hydrous fluids under high silica activity. Type 2 rodingites underwent further rodingitization during the third stage, due to infiltration of Ca-rich hydrothermal fluids of oceanic and/or subducted slab origin, at lower temperatures and depths. This stage is characterized by the appearance of hydroandradite and vesuvianite, under alkaline and oxidizing conditions, due to very low CO2/H2O ratio and relatively high fO2. All three rodingitization stages are estimated to have occurred under relatively moderate temperature and pressure (~300 to 400°C; ~3–6kbar respectively). Locally, Type 2 rodingites show derodingitization of variable extent, forming high-variance assemblages mostly consisting of chlorite±pumpellyite. Some chlorite marginal zones in rodingite dykes may also have been developed by Mg-rich diffusional fluid flow, during this derodingitization process.
•We distinguished two different types of rodingites in East Othris ophiolites.•Fluids were alkaline with relatively high pH.•Rodingites were incorporated into the exhumed materials of the subduction channel.•Late-stage derodingitization processes have been identified.
The origin and evolution of subcontinental lithospheric mantle (SCLM) are important issues of Earth’s chemical and physical evolution. Here, we report detailed textural and chemical analyses on a ...mantle xenolith suite from Befang (Oku Volcanic Group, Cameroon Volcanic Line), which represents a major tectono-magmatic structure of the African plate. The samples are sourced from spinel-facies mantle and are dominated by lherzolites. Their texture is cataclastic to porphyroclastic, and foliation defined by grain-size variation and alignment of spinel occurs in part of peridotites. Spinel is interstitial and has amoeboidal shape. Clinopyroxene REE patterns are similar to those of Depleted MORB Mantle (DMM) except LREEs, which vary from depleted to enriched. The A-type olivine fabric occurs in the subset of one harzburgite and 7 lherzolites studied by EBSD. Orthopyroxene shows deformation consistent with olivine. The fabric of LREE-enriched clinopyroxene is equivalent to those of orthopyroxene and olivine, whereas spinel and LREE-depleted clinopyroxene are oriented independently of host rock fabric. The textural, chemical and thermobarometric constraints indicate that the Befang mantle section was refertilised by MORB-like melt at pressures of 1.0–1.4 GPa and temperatures slightly above 1200–1275 °C. The olivine-orthopyroxene framework and LREE-enriched clinopyroxene preserve the protolith fabric. In contrast, the LREE-depleted clinopyroxene, showing discordant deformation relative to the olivine-orthopyroxene protolith framework, and amoeboidal spinel crystallized from the infiltrating melt. The major element and REEs composition of minerals forming the Befang peridotites indicate subsequent reequilibration at temperatures 930–1000 °C. This was followed by the formation of websterite veins in the lithospheric mantle, which can be linked to Cenozoic volcanism in the Cameroon Volcanic Line that also brought the xenoliths to the surface. This study therefore supports the origin of fertile SCLM via refertilization rather than by extraction of small melt fractions, and further emphasizes the involvement of depleted melts in this process.
The Perşani volcanic field is a low-volume flux monogenetic volcanic field in the Carpathian–Pannonian region, eastern-central Europe. Volcanic activity occurred intermittently from 1200ka to 600ka, ...forming lava flow fields, scoria cones and maars. Selected basalts from the initial and younger active phases were investigated for major and trace element contents and mineral compositions. Bulk compositions are close to those of the primitive magmas; only 5–12% olivine and minor spinel fractionation occurred at 1300–1350°C, followed by clinopyroxenes at about 1250°C and 0.8–1.2GPa. Melt generation occurred in the depth range from 85–90km to 60km. The estimated mantle potential temperature, 1350–1420°C, is the lowest in the Pannonian Basin. It suggests that no thermal anomaly exists in the upper mantle beneath the Perşani area and that the mafic magmas were formed by decompression melting under relatively thin continental lithosphere. The mantle source of the magmas could be slightly heterogeneous, but is dominantly variously depleted MORB-source peridotite, as suggested by the olivine and spinel composition. Based on the Cr-numbers of the spinels, two coherent compositional groups (0.38–0.45 and 0.23–0.32, respectively) can be distinguished that correspond to the older and younger volcanic products. This indicates a change in the mantle source region during the volcanic activity as also inferred from the bulk rock major and trace element data. The younger basaltic magmas were generated by lower degree of melting, from a deeper and compositionally slightly different mantle source compared to the older ones. The mantle source character of the Perşani magmas is akin to that of many other alkaline basalt volcanic fields in the Mediterranean close to orogenic areas. The magma ascent rate is estimated based on compositional traverses across olivine xenocrysts using variations of Ca content. Two heating events are recognized; the first one lasted about 1.3years implying heating of the lower lithosphere by the uprising magma, whereas the second one lasted only 4–5days, which corresponds to the time of magma ascent through the continental crust. The alkaline mafic volcanism in the Perşani volcanic field could have occurred as a response to the formation of a narrow rupture in the lower lithosphere, possibly as a far-field effect of the dripping of dense continental lithospheric material beneath the Vrancea zone. Upper crustal extensional stress-field with reactivation of normal faults at the eastern margin of the Transylvanian basin could enhance the rapid ascent of the mafic magmas.
•Integrated investigation of bulk rock and mineral compositional data of mafic rocks•Origin of basaltic magmas in a low-volume flux monogenetic volcanic field•Constrain on the mantle potential temperature and depth of melting column•Subtle differences in the source region as shown by the composition of Cr-spinels•Rapid magma ascent rate calculated from the Ca profile across olivine xenocrysts
At the end of the syn-extensional phase in the Carpathian–Pannonian Region (CPR), alkaline mafic magmas were erupted during the Late Miocene in Burgenland.
The majority of these lavas (Pauliberg and ...Oberpullendorf) are basalts and tephrite basanites. They have been slightly affected by high-pressure (>1.3GPa) clinopyroxene fractionation. In the Burgenland basalts, olivine and clinopyroxene phenocrysts, as well as groundmass olivine, clinopyroxene and plagioclase grew at lower pressures.
The geochemistry of the Pauliberg lavas (alkali basalts and basanites) indicates that they originated by low, but variable, degrees of melting from the same source mantle. The Pauliberg basaltic rocks have significantly higher TiO2 and lower Al2O3 contents, compared to those of Late Miocene to Recent alkaline basalts from the Pannonian Basin. The Ti enrichment could be attributed to low degrees of melting (beneath thick lithosphere) of a peridotite source that had been affected by Ti-rich recycled ancient oceanic crust, whereas the low Al and high (La/Yb)N suggest that there was residual garnet after partial melting. The depletion in K, Rb and Ba relative to Nb and the high Nb/La and Ce/Pb ratios (OIB-like) in the Pauliberg basalts, rule out any interaction with subduction-related melts/fluids and/or contamination with crustal material.
Sr and Nd radiogenic isotopes range from 0.703687 to 0.704279 and from 0.512736 to 0.512774 respectively, with the basanites being the most depleted. The Oberpullendorf sample has Sr and Nd isotopic ratios (87Sr/86Sr=0.704279; 143Nd/144Nd=0.512736) similar to those of other Pannonian basalts, e.g. Saghegy. The Burgenland basalts have relatively high 206Pb/204Pb isotopic ratios (19.6–19.7) suggesting a HIMU/OIB-like character (Embey-Isztin et al., 1993).
The calculated mantle potential temperature for the Pauliberg basalts is 1386°C and a melt fraction of ~2%. Similar calculations for the Oberpullendorf basalt indicate clinopyroxene fractionation. This leads to an overestimate of the mantle potential temperature (1530°C), making it impossible to calculate the degree of partial melting involved in the genesis of the primary magma. These calculations indicate that the Burgenland basalt melted from mantle at ambient temperature. i.e. no thermal anomaly is indicated, providing an additional argument against plume activity beneath the Pannonian Basin. Consequently we propose that late Miocene lithospheric extension of the Pannonian Basin gave rise to alkaline melt generation beneath the studied area through passive upwelling and adiabatic decompressional melting of an asthenospheric mantle source.
►Burgenland alkali basalts genesis. ►Late Miocene lithospheric extension played a major role in the magma generation. ►Formation of the Pannonian basin. ►Intra-continental basalts of OIB-like signature. ►Asthenospheric source for Burgenland basalts.
A highly‐fractionated garnet‐bearing muscovite granite represents the marginal granitic facies of the Abu‐Diab multiphase pluton in the Central Eastern Desert of Egypt. New electron microprobe ...analyses (EMPA) and laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) data from garnets are reported, in order to constrain their origin and genesis. Garnet in the Abu‐Diab host granite is euhedral to subhedral, generally homogeneous and, in rare cases, it shows weak zonation. The garnet contains appreciable amounts of MnO and FeO, with lesser amounts of MgO and CaO, yielding an end‐member formula of Sps61–72Alm25–35Prp1–4Adr0–1. Moreover, it is depleted in large ion lithophile elements (LILE) with lower values of Ba, Nb and Sr relative to the primitive mantle. Additionally, it contains high concentrations of HREE and Y and their REE pattern shows strong negative Eu anomalies. The garnet was crystallized under relatively low temperature (646°C–591°C) and pressure (< 3 kbar) conditions. The textural and chemical features indicate that the garnet is magmatic in origin and is chemically similar to that from highly‐fractionated A‐type granite. It was probably formed at the expense of biotite in a highly‐evolved MnO‐rich magma and/or by hydroxyl complexing of Mn during the ascending fluid phases.
The pressure-temperature (PT) conditions and position of different groups of eclogites in the sub-cratonic lithospheric mantle (SCLM) worldwide were established using clinopyroxene Jd-Di and garnet ...thermobarometry. Beneath Siberia, Fe-eclogites found within the 3.0–4.0 GPa formed in Early Archean times. In the Middle and Late Archean, eclogites were melted during and after subduction. High-Mg eclogites (partial melts or arc cumulates) are related to low-T (LT) geotherms. Melt-metasomatized eclogites trace a high-temperature (HT) geotherm. Eclogitic diamond inclusions from Siberia mostly belong to the middle SCLM (MSCLM) part. Ca-rich eclogites from Precambrian Indian kimberlites are located in the MSCLM. In Phanerozoic time, they were located in the lithosphere base. In Proterozoic South Africa, Ca-rich eclogites and grospydites occur within 4.0–5.0 GPa and HT eclogite and diamond inclusions from the Premier pipe trace a HT geotherm at depths of 7.0–4.0 GPa, showing an increase in Fe upwards in the mantle section. Similar trends are common for eclogites worldwide. In the Wyoming craton, kimberlites captured eclogite xenoliths from the 4.0–2.5 GPa interval. Mantle eclogites have clinopyroxenes and garnet trace element patterns with high (La/Yb)n determined by KDs with melts and are magmatic. Flatter and bell-like REE patterns with Eu anomalies, HFSE troughs, and U and Pb peaks, are common for clinopyroxenes from MORB-type “basaltic” eclogites. High-Mg eclogites show less fractionated incompatible element branch in patterns. LILE-enrichments and HFSE troughs are typical for kyanite-bearing eclogites. Clinopyroxenes from diamond-bearing eclogites show lower REE, troughs in Nb and Zr, and peaks in Pb and U concentrations, compared to barren eclogites with round smooth trace element patterns and small depressions in Pb and Ba.
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