Petrogenesis of the post-collisional rare-metal-bearing Ad-Dayheen granite intrusion, Central Arabian Shield

At Hadhb't Ad-Dayheen, in the central Arabian Shield, a post-collisional igneous complex called the Ad-Dayheen intrusion is exposed. It was emplaced in the Early Ediacaran (613–625 Ma), during the final tectono-magmatic stage of Arabian Shield development. Despite limited and discontinuous ring-shaped outcrops due to alluvial cover and later faulting, three pulses of intrusion can be recognized in the field: an early pulse of monzogranite; a second pulse of syenogranite and alkali feldspar granite; and a final pulse of alkaline and peralkaline granite, mineralized microgranite, and pegmatite. Samples show distinctively low contents of CaO, MgO, and Sr in contrast to elevated concentrations of alkalis, Rb, Nb, Y, Ta, Hf, Ga, Zr and rare-earth elements (REE); these are common characteristics of post-collisional rare-metal-bearing A-type granites. The suite displays positive Nb–Ta anomalies and pronounced negative Eu anomalies (Eu/Eu* = 0.11–0.35). The alkaline/peralkaline granites and microgranite of the Ad-Dayheen intrusion feature disseminated mineralization, whereas mineralization is localized in the pegmatite. The primary magma feeding the Ad-Dayheen intrusion was mostly generated by partial melting of the juvenile crust of the Arabian Shield, with a minor mantle contribution. We argue that an episode of lithospheric delamination led to crustal uplift, erosional decompression, and generation of mantle melts that supplied heat to drive crustal melting. The anatectic deep crustal melts assimilated a F-bearing component that also added rare metals to the magma. Each pulse can be described by a fractional crystallization model, but the parental liquid of each subsequent pulse was first modified by further addition of fluorine and rare metals and loss of CaO, Sr, Ba, and Eu due to fluorite fractionation. Texture and morphology of the ore minerals indicate that mineralization (U, Th, Zr, Nb, Ta, Y, Hf and REE) took place in two stages: a magmatic stage coinciding with emplacement of the intrusion, followed by a hydrothermal stage. The magmatic process enriched the residual melt in high field strength elements (HFSE) and REE. The later hydrothermal stage further localized these elements and increased their concentrations to economic grades. The pegmatite is highly mineralized and contains high concentrations of U (81–179 μg/g), Th (244–600 μg/g), Zr (2397–14,927 μg/g), Nb (1352–2047 μg/g), Ta (96–156 μg/g), Y (828–2238 μg/g), Hf (131–377 μg/g) and ∑REE (1969–4761 μg/g)

Suprasubduction-zone origin of the podiform chromitites of the Bir Tuluhah ophiolite, Saudi Arabia, during Neoproterozoic assembly of the Arabian Shield

The ultramafic section of a dismembered ophiolite is exposed at Bir Tuluhah, in the north-central part of the Arabian Shield. It is penetratively serpentinized and locally carbonate-altered to talc‑carbonate and quartz‑carbonate rocks (listvenite) along shear zones and fault planes. Despite the high degree of mineral replacement, preserved mesh and bastite textures and fresh relics of primary Cr-spinel and olivine show that the protoliths were mainly harzburgite with minor dunite, with sparse massive chromitite bodies of various forms and sizes. Olivine inclusions in the chromitite lenses have higher forsterite content and NiO concentrations than fresh olivine relics in the host harzburgites and dunites, due to subsolidus re-equilibration. Cr-spinels in the chromitites have higher Cr# (0.74–0.82) than those hosted in dunite (0.72–0.76) or harzburgite (0.55–0.66). The scarce Cr-spinel crystals in harzburgite that have Cr# < 0.6 are interpreted to represent the population least affected by melt-rock interaction. The chromitite bodies are interpreted to have formed just below the contact between the oceanic crust and mantle sections (i.e., the petrologic Moho). The primary olivine (high Fo and Ni content) and Cr-spinel core compositions (high Cr# and low TiO2 content) of the Bir Tuluhah serpentinized peridotite are typical of modern supra-subduction zone (SSZ) fore-arc peridotites and consistent with crystallization from boninitic magma. The multistage petrogenesis leading to the chromitite bodies begins with moderate to high degrees of melt extraction from the protoliths of the serpentinized harzburgites, followed by reaction with melt compositions that evolved from tholeiite to boninite and left dunite residues. The massive Cr-rich chromitites in the Bir Tuluhah ophiolite are most probably the residues of such interaction between depleted harzburgite and ascending melts; mixtures of the reacted melts formed boninites, which became saturated with chrome-rich spinel and crystallized chromite pods before ascending past the Moho. We offer a novel thermodynamic model of this mixing and reaction process that quantifies the yield of Cr-spinel

Evaluation of metals that are potentially toxic to agricultural surface soils, using statistical analysis, in northwestern Saudi Arabia

© 2015, Springer-Verlag Berlin Heidelberg. Heavy metals in agricultural soils enter the food chain when taken up by plants. The main purpose of this work is to determine metal contamination in agricultural farms in northwestern Saudi Arabia. Fifty surface soil samples were collected from agricultural areas. The study focuses on the geochemical behavior of As, Cd, Co, Cr, Cu, Hg, Pb and Zn, and determines the enrichment factor and geoaccumulation index. Multivariate statistical analysis, including principle component analysis and cluster analysis, is also applied to the acquired data. The study shows considerable variation in the concentrations of the analyzed metals in the studied soil samples. This variation in concentration is attributed to the intensity of agricultural activities and, possibly, to nearby fossil fuel combustion activities, as well as to traffic flows from highways and local roads. Multivariate analysis suggests that As, Cd, Hg and Pb are associated with anthropogenic activities, whereas Co, Cr, Cu and Zn are mainly controlled by geogenic activities. Hg and Pb show the maximum concentration in the analyzed samples as compared to the background concentration

Ambient noise tomography in the upper crust of North Harrat Rahat, Saudi Arabia

Images of Rayleigh-wave group velocity were derived for the 1–15 s period range that is sensitive to shear wave velocities in the crust of north Harat Rahat, Saudi Arabia. We used a continuous dataset of ambient noise that was processed by applying the technique depending on the Rayleigh-wave group velocity tomography. The dataset comprises six months of recordings extracted from 12 broadband seismic stations in north Harrat Rahat, Saudi Arabia. Using a cross-correlation technique, the Rayleigh waves have been generated and stacked along the whole time series. Dispersion curves representing fundamental mode Rayleigh waves were derived by a multiple filter technique. Dispersion curves for each station pair were inverted to obtain the one-dimensional model of corresponding shear wave velocities. The spatial distribution of shear wave velocities was depicted from the inverted one-dimensional models at 138 nodes and a reliable resolution was spatially resolved using grid cells of 0.2° expanded over both latitude and longitude. Images obtained from this study improve our understanding of the tectonic development as influenced by magma upwelling beneath the Arabian Shield. Results obtained from this study revealed two low-velocity zones in the upper crust at depths of 15 km west and shallow 10 km east, respectively. This suggests that high conductivity is present as a result of upwelling magma that flows into the upper crust of Arabian Shield coming from plausible deep sources such as the Red Sea Rift or the Afar plume, which are located to the west of the studied area

Loveringite from the Khamal Layered Mafic Intrusion: The First Occurrence in the Arabian Shield, Northwest Saudi Arabia

Loveringite, a rare member of the crichtonite group with nominal formula (Ca,Ce)(Ti,Fe,Cr,Mg)21O38, was found in the Khamal layered mafic intrusion, the first known locality for this mineral in the Arabian Shield. The Khamal intrusion, a large post-collisional mafic complex, is lithologically zoned, bottom to top, from olivine gabbro through gabbronorite, hornblende gabbro, anorthosite, and diorite to quartz diorite. Loveringite is found near the base of the complex, as an intercumulus phase in olivine gabbro. Most loveringite grains are homogeneous, although a few grains are zoned from cores rich in TiO2, Al2O3, Cr2O3, and CaO towards rims rich in FeO*, ZrO2, V2O3, Y2O3, and rare earth elements (REE). Petrographic relations indicate that loveringite formed after crystallization of cumulus olivine, pyroxenes, and plagioclase. Anhedral and corroded crystals of loveringite are surrounded by reaction rims of Mn-bearing ilmenite and baddeleyite, suggesting that the residual liquid evolved into and subsequently out of the stability field of loveringite. The budget of incompatible elements (Zr, Hf, REE, U, and Th) hosted in loveringite is anomalous for a primitive mafic liquid. Saturation in loveringite is likely the result of early contamination of the primary melt by anatexis of country rock, followed by isolation of evolving liquid in intercumulus space that restricted communication with the overlying magma chamber. The zoned crystals likely reflect diffusive equilibration between residual loveringite grains and their reaction rims of ilmenite