الفهرس 
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Abstract
The present study addresses the geologic context and structural evolution of an important part of the basement complex of Yemen within the Al-Bayda district. The study aims at preparation of a detailed geological map, based on field observations, aerial photographs and Landsat (ETM+) images of appropriate scales. In this approach, investigations including petrography, ore microscopy, structural synthesis and geochemistry of the country rocks are carried out.
The study area is underlain mainly by Precambrian basement complex, classified as orthogneisses and migmatites in the central and western parts, whereas, ophiolitic rocks and island arc metavolcanic assemblage occur as disrupted fault-bound elongate belts overlying the gneisses and migmatites distributed over the entire area. Based on the field observation and interpretation of aerial photographs and landsat (ETM+) images, seven broad lithological units were distinguished, namely: gneisses and migmatites, ophiolites, metavolcanics, gabbros, sheared granite, granite and dykes.
The area is a part of a highly deformed NE-belt, displaying deformational fabrics at different scales. The prominent planar and linear elements are dealt in detail to elucidate the precise deformational history of the area. The basement complex underlying the area includes a continuous, although highly deformed, tectono-stratigraphic sequence. Thrusts and other early transcurrent faults locally repeat the section.
The area has undergone a structural history involves, at least, three phases of deformation. DI: Eat deformation phase is manifested by tight to very tight oppressed folds and intrafolial isoclinal folds. These folds are recorded within the Si penetrative foliation and cleavage. The SI is considered the most common minor structures in the gneisses and is distinguished by a strong preferred orientation of the mineral constituents. The degree of SI
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cleavage development is a sensitive indicator of lithology and tectonic strain. SI foliation is manifested by a coarse to medium grained gneissosity in gneiss and migmatites rocks. L1 lineation is mainly a mineral lineation, defined by the preferred orientation of the deformed crystals on the Si foliation planes. Dl: Regional tectonic transport (thrusting) represents a phase of regional tectonic transport of the ophiolitic blocks and the island arc sequences to ride over the gneisses and migmatites unit, most probably coeval with the regional fold initiation and through propagation of F2 folding. This transposition caused strong shearing and deformation of the pre-existing fabrics and the formation of map-scale thrust-ramp structures. These thrusts are confined mainly to the accretionary subduction complex units (ophiolitic rocks) and clearly truncate the early deformation DI domains. Tectonically, this phase coincides with a regional tectonic thickening and crustal shortening in the NW-SE direction. During this deformation phase, various deformational fabrics have developed; including major folding along NE-SW thrust faults, mylonitization cleavage and stretching lineation. D3: NE-SW weak ductile deformation is the third phase of deformation led to the formation of open to very open folds overprinting both F1 and F2 earlier formed folds. This phase was superposed by the formation of kilometer-scale strike slip faults, trending in a NE-SW direction.
Worthy noted is that the combination of instability induced by vertical density inversion (dense ophiolites and mafic metavolcanics overlying granitic gneiss) and formation of doubly plunging antiforms through thrust-duplex development is interpreted as the responsible of formation of a large gneiss dome in the central part of the study area (detailed investigation is recommended for further work).
These tectono-metamorphic events have altered the primary igneous mineralogy and obliterated most igneous textures. Metabasaltic lithologies have been altered to epidote-
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bearing amphibolites; ultramafics to tremolite- and talc-bearing serpentinites and the intermediate island-arc lithologies to biotite- and homblende-bearing quartzofeldspathic schists. All lithologies accommodate a tectonic fabric, both foliation and lineation, of which the foliation is parallel/sub-parallel to the boundaries of the differing lithologies. This deformation history has dismembered both the ophiolite and Island-arc stratigraphy such that both lithological and mapping group contacts are tectonic. These fabrics share a similar orientation to Pan African aged fabrics westwards the study area (Sakran, 1993) and are interpreted in the present work to be related to the same Pan African tectonothermal cycle.
Geochemical ”fingerprinting” data, in combination with detailed lithological and structural studies, have been a major factor in the various tectonic hypotheses of the study area. Due to the fact that metamorphism severely limits the discriminatory power of the more mobile and compatible major and trace elements, litho/geochemical assessment of the encountered lithologies was done in conjunction with independent lines of evidence and focus on generally immobile and incompatible elements such as Ti, Ni, Cr, high/field strength elements (FIFSE) and the rare earth elements (REEs).
The study area is underlain by Neoproterozoic rocks include metamorphites of different lithologies and tectonic settings, traversed by syn-and post-orogenic plutonites. The metamorphite pile represents arc/back arc rock suites, later deformed and napped through the Pan-African Orogeny. Thereafter, the younger plutonites affected the pre-existing metamorphites and do not display any response to deformation.
Based on the geochemical data, it is interpreted that the studied ophiolites have likely been formed in a back-arc basin, which was deformed later in a fracture zone. The ultramafites and metagabbros represent oceanic lithosphere imbricated with metamorphosed
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volcanic and sedimentary rocks that constitute part of an island arc. Arc/back arc collision is emphasized by crustal thickness fabrics and various geochemical features.
Magmatism in the study area was syn- to post-dating tectonism. Comparatively minor event of mafic to intermediate plutonism is related to the earlier arc-plutonism. The sheared (syn-orogenic) granitic rocks taper along mylonitic foliation and retain to volcanic arc intrusions emplaced during collision episodes. The younger (post-orogenic) gabbros has chemistry similar to the within plate tholeiites depleted in high field strength elements (HFSE) when compared to large-ion lithophile element (LILE), an intrinsic feature in the upper mantle as a result of partial melting events (Gill 1981). Lastly, the post-tectonic granites are probably resulted from intra-crustal melting after arc/continent collision. Taken together, the peraluminosity, the strong negative Eu anomalies of these younger granites support a within-plate crustal source.