Browsing by Author "Feng, X."
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Item Effect of pre-existing faults on the distribution of lower crust exhumation under extension: numerical modelling and implications for NW Ghana(Geosciences Journal, 2019-04-29) Amponsah, P.O.; Feng, X.; Wang, E.; Ganne, J.; Martin, R.; Jessell, M.W.3D thermo-mechanical models have been constructed to explore the influence of pre-existing faults on the temporal-spatial distribution of high-grade amphibolite-granulite facies metamorphic rocks during extension. The different dip amounts of three pre-existing faults (refer to the Bole-Nangodi shear zones, the Jirapa shear zone and the Bulenga shear zone in the study area) are studied in models. The results show the lower crust exhumes occur at a relatively low rate prior to attaining a Stretching Factor = 4.2%. The partially molten lower crustal rocks tend to move (up to 20 km) towards the center of the model and focus exhumation in regions where pre-existing faults intersect. The high-strain corridors in models are used to understand the loci of exhumation in the Bole-Bulenga domain of NW Ghana. Accordingly, in the eastern and western parts of the high-grade rock corridors in NW Ghana, partially molten rocks exhumed from the lower into middle-upper crustal levels are interpreted to have been dominantly facilitated by the km-scale high-strain corridors. In the central part of the Bole-Bulenga domain, the high-grade rocks are interpreted to have been exhumed because of a coupling between two mechanisms: (1) The exhumation of partially molten rocks between the Jirapa and Bole-Nangodi faults increases in spatially due to the reduction in space from north to south. (2) The exhumation of lower partially molten rocks in the central part, as a result of inherited orthogonal (E-W) structures.Item Role of Volcano‐Sedimentary Basins in the Formation of Greenstone‐Granitoid Belts in the West African Craton: A Numerical Model(Minerals, 2018-02) Feng, X.; Wang, E.; Ganne, J.; Amponsah, P.; Martin, R.Greenstone belts in the West African Craton (WAC) are separated by several generations of granitoids intruded at ca. 2.18–1.98 Ga. Simultaneous folding and exhumation play an important role in the formation of greenstone-granitoid belts. However, the overall tectonic regime and origin of granitoids remain controversial. In this study, we present the estimates of the mantle potential temperature (Tp) for the WAC, which yields values of about 1500–1600 °C, pressure estimates of initial and final melting yield values of about 3.7–5.2 GPa and 1–1.3 GPa, respectively. Subsequently, 2D thermo-mechanical models have been constructed to explore the width of volcano-sedimentary basin on spatial-temporal evolution of diapirs that emplaced in the lower-middle crust during compression. The models show that the width of the volcano-sediment layer plays an important role in the formation mechanisms of greenstone-granitoid belts. The lower crust beneath sedimentary sequences is deformed into a buckle fold during the first compressional stage, through which relief uplifts slowly. Subsequently, the buckle fold is further deformed into several individual folds. Diapirs made of lower crust rocks ascend and emplace in the middle-upper crust resulting from instability. Benefitting from the mantle temperature, the pressure estimates and the numerical modelling results, a new geodynamic model was constructed. This model indicates that a series of sheet-like granitoids possibly derived from either subducted mélanges, lower crust and/or mantle melting that are accumulated at depths of the subcontinental mantle would channel along diapirs before feeding the upper crust. When the granitoids arrive at the solidified lids of the diapirs, they would favour migrating horizontally and intrude into the upper crust through weakening zones between the diapirs. Our geodynamic model also suggests an asymmetry of structures between the upper and middle-lower crust, with the dome-like granitoids overlying high-grade sedimentary synforms and high-grade diapirs underlying low-grade greenstone belts.Item Selenium characterization in the global rice supply chain(American Chemical Society, 2009) Williams, P.N.; Enzo, L.; Sun, G.; Scheckel, K.; Zhu, Y.-G.; Feng, X.; Zhu, J.; Carey, A.-M.; Adomako, E.E.; Lawgali, Y.; Deacon, C.; Meharg, A.A.For up to 1 billion people worldwide, insufficient dietary intake of selenium (Se) is a serious health constraint. Cereals are the dominant Se source for those on low protein diets, as typified by the global malnourished population. With crop Se content constrained largely by underlying geology, regional soil Se variations are often mirrored by their locally grown staples. Despite this, the Se concentrations of much of the world’s rice, the mainstay of so many, is poorly characterized, for both total Se content and Se speciation. In this study, 1092 samples of market sourced polished rice were obtained. The sampled rice encompassed dominant rice producing and exporting countries. Rice from the U.S. and India were found to be the most enriched, while mean average levels were lowest in Egyptian rice: ∼32-fold less than their North American equivalents. By weighting country averages by contribution to either global production or export, modelled baseline values for both were produced. Based on a daily rice consumption of 300 g day-1, around 75% of the grains from the production and export pools would fail to provide 70% of daily recommended Se intakes. Furthermore, Se localization and speciation characterization using X-ray fluorescence (µ-XRF) and X-ray absorption near edge structure (µ-XANES) techniques were investigated in a Se-rich sample. The results revealed that the large majority of Se in the endosperm was present in organic forms.