Geology, petrochemistry and tectonics of the Lowder-Mudiah area, Abyan Province, Yemen Republic.

The Precambrian basement rocks of the Lowder-Mudiah area are divided into three main belts separated by two northeast-striking ductile shear (thrust) zones. The central belt consists of highly metamorphosed granitic gneisses intruded by granites. The eastern belt comprises greenschist-grade northeast-trending metavolcanics; these rocks are deformed in open antiforms and synforms. The western belt is composed of granite (740 22 Ma), diorite and gabbro traversed by two extensive dyke swarms, forming 75-90% of the total rock volume. The major swarm (709 21 Ma) trends NE-SW and the minor (587 18 Ma) E-W to NW-SE. Three deformational episodes are recognized. Northeast trending recumbent isoclinal folds were followed by open folds with vertical axial planes and northwest trending crossfolds and kink bands. The thrust-lineation relationships suggest that there was an early phase of thrusting which produced down-dip lineations, and that these thrust surfaces were later reactivated to become dextral trans current ductile shear zones which gave rise to the present predominant shallow-plunging lineations. Tertiary block faulting occurred along reactivated Precambrian thrust planes. Peraluminous, high silica, "A-type" granites formed from LILE-and LREE-enriched crustal melts in an extensional tectonic regime. These granitoids are similar to the felsic Mudiah metavolcanics, the felsic Mukeras dyke swarm and the "A-type" young granitoids (686 - 517 Ma) of Central Hijaz, Saudi Arabia. The Mudiah metavolcanics and the NE-SW Mukeras dyke swarm are bimodal formed from LILE- and LREE-enriched source(s), and they exhibit Andean-type convergent margin characteristics. However, a large degree of compositional overlap may be expected between lavas in highly magmatic, rapidly distending rifts, and Andean-type convergent margins. Moreover, the Mudiah metavolcanics and the Mukeras dyke swarms are chemically similar to the dyke swarms in the Northeastern Desert of Egypt and the "sequence" A volcanics of Central Arabian Shield. It is concluded that the late Precambrian accretion of arc systems resulted in a thickening of the crust, which led to deep crustal melting, which was probably triggered by upper mantle diapirism, caused by adiabatic decompression, updoming, stretching and crustal thinning and the injection of mantle-derived basaltic magma, that gave rise to the basaltic Mudiah metavolcanics and the basaltic Mukeras dyke swarms. In contrast, crustal melts' are represented by the Lowder-Mudiah "A-type" granitoids, the felsic Mudiah metavolcanics, and the felsic Mukeras dyke swarms. The host rocks of the dykes (granite (740 22 Ma), quartzdiorite, diorite and gabbro) have a calc-alkaline subduction zone signature, and they were probably emplaced in the roots of contemporary island arcs, similar to the old calc-alkaline granodiorite association (720 Ma) of Central Hijaz of Saudi Arabia. Carbonatite bodies, mainly those which occur as dykes, which are either apatite-bearing or RE-mineral-bearing (group III and group IV) are confirmed as carbonatites. However, petrochemistry suggests that other carbonate sheets are sedimentary marbles enveloped by granitoid gneisses.