U–Pb dating and Sm–Nd isotopic analysis of granitic rocks from the Tiris Complex : new constaints on key events in the evolution of the Reguibat Shield, Mauritania

The Reguibat Shield of N Mauritania and W Algeria represents the northern exposure of the West African Craton. As with its counterpart in equatorial West Africa, the Leo Shield, it comprises a western Archaean Domain and an eastern Palaeoproterozoic Domain. Much of the southern part of the Archaean Domain is underlain by the Tasiast-Tijirit Terrane and Amsaga Complex which, along with the Ghallaman Complex in the northeast, preserve a history of Mesoarchaean crustal growth, reworking and terrane assembly. This study presents new U–Pb and Sm–Nd data from the Tiris Complex, a granite–migmatite–supracrustal belt, that intervenes between these units and the Palaeoproterozoic Domain to the northeast. New U–Pb geochronology indicates that the main intrusive events, broadly associated with formation of dome-shaped structures, occurred at around 2.95–2.87 Ga and 2.69–2.65 Ga. This study also recognises younger regional metamorphism and intrusion of syn-tectonic granites located within major shear zones at around 2.56–2.48 Ga. Sm–Nd depleted mantle model ages indicate that magmatism involved recycling of crustal source components older than at least 3.25 Ga in age. Comparison with other Archaean units in the Reguibat Shield and in the Leo Shield illustrate the importance of deformation and tectonism of a regional greenstone-sedimentary province prior to around 3.00 Ga as well as subsequent magmatic episodes broadly equivalent in age to those in the Tiris Complex

Primary crustal melt compositions: Insights into the controls, mechanisms and timing of generation from kinetics experiments and melt inclusions

We explore the controls, mechanisms and timing of generation of primary melts and their compositions, and show that the novel studies of melt inclusions in migmatites can provide important insights into the processes of crustal anatexis of a particular rock. Partial melting in the source region of granites is dependent on five main processes: (i) supply of heat; (ii) mineral–melt interface reactions associated with the detachment and supply of mineral components to the melt, (iii) diffusion in the melt, (iv) diffusion in minerals, and (v) recrystallization of minerals. As the kinetics of these several processes vary over several orders of magnitude, it is essential to evaluate in Nature which of these processes control the rate of melting, the composition of melts, and the extent to which residue–melt chemical equilibrium is attained under different circumstances. To shed light on these issues, we combine data from experimental and melt inclusion studies. First, data from an extensive experimental program on the kinetics of melting of crustal protoliths and diffusion in granite melt are used to set up the necessary framework that describes how primary melt compositions are established during crustal anatexis. Then, we use this reference frame and compare compositional trends from experiments with the composition of melt inclusions analyzed in particular migmatites. We show that, for the case of El Hoyazo anatectic enclaves in lavas, the composition of glassy melt inclusions provides important information on the nature and mechanisms of anatexis during the prograde suprasolidus history of these rocks, including melting temperatures and reactions, and extent of melt interconnection, melt homogenization and melt–residue equilibrium. Compositional trends in several of the rehomogenized melt inclusions in garnet from migmatites/granulites in anatectic terranes are consistent with diffusion in melt-controlled melting, though trace element compositions of melt inclusions and coexisting minerals are necessary to provide further clues on the nature of anatexis in these particular rocks.This work was supported by the National Science Foundation [grants EAR-9603199, EAR-9618867, EAR-9625517 and EAR-9404658], the Italian Consiglio Nazionale delle Ricerche, the European Commission (grant 01-LECEMA22F through contract No. ERAS-CT-2003-980409; and a H2020 Marie Skłodowska-Curie Actions under grant agreement No. 654606), the Italian Ministry of Education, University and Research (grants PRIN 2007278A22, 2010TT22SC and SIR RBSI14Y7PF), the Università degli Studi di Padova [Progetto di Ateneo CPDA107188/10 and a Piscopia—Marie Curie Fellowship under grant agreement No. 600376], the Australian Research Council (Australian Professorial Fellowship and Discovery Grants Nos. DP0342473 and DP0556700), and the National Research Foundation (South Africa; Incentives For Rated Researchers Program)

Geophysical assessment of subsurface conditions at proposed building sites: implications for foundation failure and building collapse

Building collapse has been a recurrent environmental hazard in Nigeria in the last two decades. This is a corollary of inadequate foundation investigation prior to construction, poor government policies, and general lack of awareness on the importance of geophysical and geotechnical investigations. In this study, geological mapping and detailed geophysical investigation using Electrical Resistivity Imaging (ERI) and Vertical Electrical Sounding (VES) were carried out to understand the suitability of proposed building sites at the main campus of the Olabisi Onabanjo University (OOU), Ago-Iwoye, Nigeria for construction. Both Wenner array and dipole-dipole were used for profiling and Schlumberger for sounding. Four transverses and VES were used in each of the three areas investigated. Our results show that the subsurface of the study areas is underlain by Precambrian basement rock of Nigeria. Rocks in the study area include banded gneiss, porphyroblastic gneiss, biotite-hornblende granite and quartzite schist. The sounding stations across the three areas and 2D resistivity imaging revealed three principal geoelectric layers, the topsoil, the weathered layer and the fractured/fresh basement with varied resistivity values for each layers. At the VES stations, the three geoelectric layers have resistivity values of 62 to 1182 Ωm, 3.2 to 1360Ωm and 87 to 4680 Ωm. On the 2D resistivity imaging profiles, the resistivity of the three layers varies from 2 to 1182 Ωm, 30to 1360 Ωm, and 40 to 2904 Ωm for the topsoil, the weathered basement, and fractured/fresh bedrock. Our work demonstrates that some of the proposed sites are structurally incompetent for engineering or foundation purposes. Excavation of the topsoil and reinforcement are required to sustain the proposed structures

The role of boron and fluids in high temperature, shallow level metamorphism of the Chugach Metamorphic Complex, Alaska

The possible role of boron (B) involvement in granite equilibria and generation of melts during crustal metamorphism has been a focus of speculation in recent literature. Most of the evidence for such involvement derives from experimental data which implies that the addition of B will lower the temperature of the granite solidus. Also the presence of tourmaline has a minor effect on the temperature of the solidus. Further indirect evidence that B may be involved in partial melting processes is the observation that granulites are commonly depleted in B, whereas the B content of low grade metapelites can be high (up to 2000 ppm). Researchers' measurements of the whole-rock B contents of granulites from the Madras region, India are low, ranging from 0.4 to 2.6 ppm. Ahmad and Wilson suggest that B was mobilized in the fluid phase during granulite facies metamorphism of the Broken Hill Complex, Australia. Thus, it appears that during the amphibolite to granulite transition, B is systematically lost from metasediments. The B that is released will probably partition into the vapor phase and/or melt phase. Preliminary measurements imply that the boron content of rocks in the Chugach Metamorphic Complex is not sufficient to influence the processes of partial melting at low pressures

Exhumation history of the Higher Himalayan Crystalline along Dhauliganga-Goriganga river valleys, NW India: new constraints from fission track analysis

New apatite and zircon fission track data collected from two transects along the Dhauliganga and Goriganga rivers in the NW Himalaya document exhumation of the Higher Himalayan Crystalline units. Despite sharing the same structural configuration and rock types and being separated by only 60 km, the two study areas show very different patterns of exhumation. Fission track (FT) data from the Dhauliganga section show systematic changes in age (individual apatite FT ages range from 0.9 ± 0.3 to 3.6 ± 0.5 Ma, r 2 = 0.82) that record faster exhumation across a zone that extends from the Main Central Thrust to north of the Vaikrita thrust. By contrast, FT results from the Goriganga Valley show a stepwise change in ages across the Vaikrita thrust that suggests Quaternary thrust sense displacement. Footwall samples yield a weighted mean apatite age of 1.6 ± 0.1 Ma compared to 0.7 ± 0.04 Ma in the hanging wall. A constant zircon fission track age of 1.8 ± 0.4 Ma across both the footwall and hanging wall shows the 0.9 Ma difference in apatite ages is due to movement on the Vaikrita thrust that initiated soon after ∼1.8 Ma. The Goriganga section provides clear evidence for >1 Ma of tectonic deformation in the brittle crust that contrasts with previous exhumation studies in other areas of the high Himalaya ranges; these studies have been unable to decouple the role of climate erosion from tectonics. One possibility why there is a clear tectonic signal in the Goriganga Valley is that climate erosion has not yet fully adjusted to the tectonic perturbation

Integration of magnetic residuals,derivates and located euler deconvolution for structural and geologic mapping of parts of the precambrian gneisses of Ago-Iwoye, Southwestern Nigeria

Ground based magnetic survey conducted between longitude 06O 55I 51IIN –06O 55I 54IIN and latitude 03O 52I 06IIE –03O 52I 4.8IIE (Olabisi Onabanjo University) remarkably revealed a consistent subsurface NW -SE structural azimuth of localized discontinuities within the shallowly buried heterogeneous basement rocks, which at exposed locations are composed of strongly foliated granite gneiss and migmatite-gneiss with veins and veinlets principally orientated in NNW –SSE direction.Magnetic survey of the area was preceded by site inspection to avoid metallic objects interferences. Field procedure in the area involved Cartesian gridding, base station establishment, data acquisition at gridded points, and repeated bihourly diurnal checksat the base station. At the processing stage, diurnal variation effect was aptly removed before subjection to Kriging (gridding). The gridded data was then prepared as input for Forward Fourier Filter Transform (FFT), which upon definition and implementation enabled Butterworth filtering of isolated ringing effects, reduction to the equator (RTE) for geomagnetic correction, and the use of Gaussian and Upward Continuation filtering for regional magnetic intensity trend determination. Removal of the regional magnetic intensity (RMI) from the total magnetic intensity (TMI) resulted in the derivation of the residual anomaly. Enhancement filters adopted for better resolution of the residual magnetic gradient include analytical signal (AS), tilt-angle derivative (TDR), vertical derivative deconvolution (VDD), and the first vertical derivatives (FVD).TMI and RMI values range between 32925nT –33050nT and 32935nT –333050nT respectively, while the residual gradient ranges between 15nT/m and10nT/m; AS ranges between 0.28nT/m and4.1nT/m; and TDR ranges from-1.4nT/m to 1.4nT/m. Source depth calculation estimated from power spectrum analysis and Euler deconvolution ranges between 1m and15m. Composite overlay of magnetic maps revealed jointed and faulted zones within the area; exhibiting a NW-SE principal azimuth of Liberian orogenic impress, which are in consistence with the foliation direction of the jagged foliated bedrock with an estimated maximum overburden of about 15m.The structural significance of this area as a prospective hydro-geological centre, and as an undesirable spot for high-rise building has been accurately evaluated from research findings. Application of integrated geophysical approach, complemented by detailed geological studies may furnish greater information about the subsurface structural architecture.Keywords:Gneisses; Ground Magnetic Surveying;RTE;Structural discontinuities;TDR.1INTRODUCTIONStructuralmapping is an integral part of geologic surveys. It involves measurements, analyses, interpretation and recognition of geometrical features (structures) generated by rock deformations [1]. These structures often serve as fountains of environmental challenges or unparalleled opportunities depending on their modesof occurrences, which in most cases are imminently controlled by the dynamic interplay of differential stress distributions within the earth interior. In line with the principle of uniformitarianism, a broad understanding about Earth’s paleo processes and internal workingsare deductible from the various deformation types for diverse applications. Deductible inferences from brittle deformationsinclude the kinematics of crustal blocks, orientation of principal axes of regional and local stresses, and geometry. Deeper insights indeep seated stresses, regional movements and block motions are obtainable from ductile deformations

Resetting and closing codition of Rb-Sr whole-rock isochron system: some samples of metamorphic and granitic rocks from the Gondwana super-continent and Japan Arc

The closure temperature of the Rb-Sr whole-rock system for felsic rocks has been generally considered to be ca. 700゜C, but it falls to ca. 400゜C, as a result of efficient action of fluids,especially H_2O. Thus,the Rb-Sr whole-rock ages obtained by rocks collected from a single outcropto specimen-size are not always coincident with those of emplacement of granitic magma or partial melting under high grade metamorphism.Although the Rb-Sr whole-rock isochron ages of metamorphic rocks collected from a wide area have been sometimes interpreted as those of metamorphism,careful consideration for these ages is needed