Petrology and geochronology of a Neoproterozoic dyke swarm from Marbat, South Oman

Abstract

A suite of NW–SE trending tholeiitic dykes in the Marbat area of south Oman was emplaced into calc-alkaline gneisses and intrusives during the Neoproterozoic. A three point Sm/Nd isochron from the most primitive sample and an Rb/Sr whole rock isochron together with geological evidence, suggest an intrusion age of about 700 Ma. The dykes form a co-genetic suite ranging in composition from basalt to rhyolite. The most primitive rock has an Mg # of 67.48, a Ni content of 268 ppm and Cr of 571 ppm. All rocks in the suite are quartz normative. Initial 143Nd/ 144Nd and 87Sr/ 86Sr isotope ratios suggest some interaction between the magmas and the continental crust during ascent. Magmatic evolution was controlled by AFC processes involving fractionation of hornblende, clinopyroxene, plagioclase and magnetite. Trace element variation diagrams show negative spikes for Rb, Ta, Nb, Sr and Ti. Nb depletion results in high Th/Nb (0.9) and La/Nb ratios (3.8). Comparison of the Marbat dykes with Neoproterozoic dykes from SW Egypt, Sinai, volcanic rocks from the Andean active continental margin and the Basin and Range Province, a low-Nb dykes suite from New South Wales, Australia and a suite from the Congo shows considerable compositional overlap. These comparisons suggests that the dykes were formed by melting of subduction modified lithospheric mantle and mobilised by crustal extension. The ubiquity of dykes in the ANS showing similar signatures suggests that such modified mantle is regionally developed and results from enrichment is a supra-subduction setting during the Neoproterozoic accretionary phase of the ANS. The mantle shows compositional similarities to lithospheric mantle underlying the western USA. The calc-alkaline geochemistry of the basement gneisses and intrusive suite into which the Marbat were intruded suggest analogy with the Al-Bayda island arc terrane in Yemen. Fracture pattern analysis from Marbat suggests that intrusion of the major swarm may be related to sinistral strike-slip faulting caused by regional east–west compression. A similar stress regime can also explain the orientation of the Mukeras dyke swarms of the Al-Bayda terrane. Additional geochronological data from Marbat and the Hallaniyat Islands imply the existence of a second phase of dyke intrusion at ∼600 Ma which is again similar to the situation prevailing in the Al-Bayda terrane.