Geochemistry and Petrogenesis of Late Ediacaran Rare-metal Albite Granites of the Arabian-Nubian Shield

The Abu Dabbab albite granite(ADAG),in the central Eastern Desert of Egypt,hosts the most significant rare metal ore deposit in the northern part of the Neoproterozoic Arabian-Nubian Shield.Here,we report detailed field,petrographic,mineralogical and geochemical investigation of the ADAG,an isolated stock-like granitic body with sharp intrusive contacts against metamorphic country rocks,probably emplaced at about 600 Ma.The fine-grained porphyritic upper unit is a preserved remnant of the shallowly-emplaced apex of the magma chamber,whereas the medium-grained lower unit crystallized at deeper levels under subvolcanic conditions.The peraluminous leucocratic ADAG shares common geochemical characteristics with post-collisional intraplate A-type magmas.In addition to the conspicuous enrichment in Na2O,the ADAG is remarkable for its anomalous concentrations of Ta,Nb,Li,Hf,Ga,Sn,Zn and heavy rare-earth elements.Nb-Ta minerals in the ADAG are mixed with Fe-Mn oxides,forming black patches that increase in abundance toward of the base of the intrusion.Columbite-tantalite,cassiterite and wolframite are the most important ore minerals.Pronounced negative Eu anomalies(Eu/Eu*=0.10–0.24)reflect extreme magmatic fractionation and perhaps the effects of late fluid-rock interaction.The ADAG was most likely generated by partial melting of the juvenile middle crust of the ANS as the geotherm was elevated by erosional uplift following lithospheric delamination and it was emplaced at the intersection of lineations of structural weakness.Although formation of the ADAG and its primary enrichment in rare metals are essentially due to magmatic processes,late-stage metasomatism caused limited redistribution of rare metals.Fluid-driven subsolidus modification was limited to the apex of the magma chamber and drove development of greisen,amazonite,and quartz veins along fracture systems.

Lineament Characterization and Their Tectonic Significance Using Gravity Data and Field Studies in the Al-Jufr Area,Southeastern Jordan Plateau

Lineaments in the southeastern Jordan plateau are mapped using gravity data and field studies in order to understand the tectonic origin of these lineaments, especially in relation to the Dead Sea transform （DST） and the Red Sea opening. Four sets trending E-W, NW-SE, NE-SW, and N-S are identified in gravity data. Field studies generally reveal similar orientations. Field and gravity studies indicate that most of the lineaments are extensional features that correspond to normal faults. Most of these were subsequently reactivated into strike-slip shear fractures. The NW-SE and N-S lineaments represent dilatational fractures. The N-S trending lineaments are the oldest. The E-W lineaments form conjugate shear fractures and are younger than the N-S lineaments. These conjugate shear fractures are also older than other set of conjugate shear fractures oriented NE-SW. The evolution of all these fractures is attributed to the DST and the Red Sea spreading. Kinematic and dynamic analysis of the two, older and younger, pairs of conjugate strike-slip fractures revealed, respectively, broadly NW-SE and N-S oriented transpressional stress with corresponding transtensional stress oriented NE-SW and E-W.

The Use of Ground Penetrating Radar for Mapping Rock Stratigraphy and Tectonics:Implications for Geotechnical Engineering

This paper presents results from ground penetrating radar surveys using the SIR-10 B GPR instrument（manufactured by Geophysical Survey System Inc., USA）, with 400 MHz monostatic antenna（model 5 103）. Survey was made over 3 excavation levels along the highway section at the Ras en Naqab escarpment area, Southwest Jordan. A total of 217 m along 4 profiles were covered in the winter of 2012. The objectives of the study are（i） to evaluate the resolution of the GPR technique in the field for detecting and locating anomalies caused by subsurface structures like cavities, fractures and faults, and（ii） to describe stratigraphic nomenclature of the subsurface rocks of the area. 2D interpretation of the obtained data and the geological information demonstrate a strong correlation between the GPR anomalies and the subsurface geology. Based upon the lateral and vertical velocity changes with depth, the thickness and orientation of the subsurface layers are outlined. Analysis of the exposed section shows good agreement between the estimated thicknesses of lithostratigraphic units and the quantitative assessment of the radar waves velocity inferred from GPR data.