Yoshiko Adachi

Evolution of volcanism and magmatism during initial arc stage: constraints on the tectonic setting of the Oman Ophiolite

Abstract Based on detailed stratigraphy, petrology and geochemistry, the initial arc magmatism of the Oman Ophiolite consisting of tholeiitic lavas followed by boninite flows and tephras is studied in the Wadi Bidi area, northern Oman Mountains. An 1110-m-thick V2 sequence is divided into the lower 970 m (LV2) and upper 140 m (UV2) thick subsequences by a 1.0-m-thick sedimentary layer. Pahoehoe flows dominate in the lower part of the LV2, while the upper part consists mainly of sheet flows with sparse interbedded pelagic sediments and a cylindrical plug. In addition to the presence of a feeder conduit, the flow-dominant lithofacies with a few thin sedimentary interbeds in the LV2 indicates that the study area was the centre of a volcano grown in a short period. The UV2 is composed of boninite sheet flows overlain by a 2.0-m-thick pyroclastic fall deposit. A small amount of boninite lavas at the end of the V2 sequence overlain by thick pelagic sediments suggests that the subduction-related arc volcanism was short lived and terminated long before the ophiolite obduction. Supplementary material: Locations, mode of occurrence, phenocryst assemblages and bulk-rock major and trace element compositions of lavas in the Wadi Bidi area are available at http://www.geolsoc.org.uk/SUP18684.

Origin of large wehrlitic intrusions from the Wadi Barghah to Salahi area in the northern Oman Ophiolite

Abstract Wehrlitic intrusions constitute an important element of oceanic lower crust of the Oman Ophiolite, and numerous such dunite and plagioclase wehrlitic intrusions cut gabbro units in the northern Salahi Block of this ophiolite. For the first time, we describe a large wehrlitic intrusion (the Barghah complex) that has disturbed and folded surrounding gabbro units around this complex. The Barghah complex contains many gabbro blocks that record evidence of magma mingling. The crystallization sequence of the wehrlitic intrusions is olivine followed by co-crystallization of plagioclase and clinopyroxene. The forsterite content of olivine and the Mg# of clinopyroxene are more evolved than in rocks from the Moho Transition Zone (MTZ). TiO2 and Na2O contents of clinopyroxene are similar to those of the MTZ and are characterized by wide compositional variability over a narrow range of Mg#, which is indicative of melt–mantle interaction. Compositional co-variation of plagioclase and olivine in the wehrlitic intrusions has a similar signature to that of V1 magmatism (ocean ridge stage). In light of these observations, we conclude that the wehrlitic intrusions are the product of off-axis magmatism. Inversion tectonics that led to a transition from spreading to compression may have accelerated the emplacement of the wehrlitic intrusions.