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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.

Weight loss effects of KTP were significant at 1 month, walking steps increase was maintained for 7 months.

Multiple magmatic events are recorded in the gabbroic unit in the Fizh area of the northern Oman ophiolite. Gabbroic blocks intruded by sheeted dike complex and upper gabbros of the main crustal sequence show the oldest event. Gabbronorite sills in the gabbroic blocks are nearly coeval with the host gabbro. Wehrlitic intrusions (wehrlite I) mark the third event of magmatism. These three magmatic events occurred at the retreating (dying) ridge axis because all these rocks are intruded by dolerite dike swarm, which is generally regarded as a precursor of advancing ridge axis. The next stage of magmatism is a main phase of oceanic crust generation in this area. Wehrlite II and then gabbronorite dikes intrude the still hot main gabbro unit. All of these above rocks have similar signatures with respect to clinopyroxene compositions and covariations between plagioclase and mafic minerals, though slight differences are present in the compositional ranges and clinopyroxene compositions of each unit. After considerable cooling of the main gabbro unit, primitive basalt dikes intrude the main gabbro unit, which may correspond to the Lasail unit. Finally, the Fizh‐South complex intrudes into considerably cooled crustal sequence, being below the brittle‐plastic transition temperatures. The Fizh‐South complex, which was regarded as a common wehrlitic intrusion, is significantly different from all of the above mentioned rocks, with respect to the covariation between plagioclase and associating mafic minerals, crystallization order, and clinopyroxene compositions. The clinopyroxenes are characterized by extremely low Ti and Na contents, comparable with those of the V2 unit (Alley volcanics), suggesting that the Fizh‐South complex correlates with the plutonic facies of the V2 unit during arc stage. Layered gabbros in the Wadi Zabin area, about 10 km north of the Fizh area, may be a northern extension of the gabbro blocks of the Fizh area, because they are intruded by numerous dolerite dikes. On the other hand, basin‐like structure of the main gabbroic unit in the northern end of the Fizh area may demonstrate a fossilized magma chamber beneath the advancing ridge axis due to the ceasing of magmatic crustal accretion. On the basis of these lines of evidence, we propose that the Fizh area indicates the northward propagation tip of the ridge axis.

The Samail (Oman-UAE) ophiolite is the largest and best studied ophiolite in the world, and observations from the ophiolite have had a major impact on models for the structure and petrogenetic history of fastspread oceanic lithosphere (e.g.,

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[1] Along-axis variations for 70 km from the northern propagating tip at Wadi Fizh to the southern margin of the Hilti block in the northern Oman ophiolite are studied on the basis of bulk rock and pyroxene compositions of the sheeted dike complex. Less evolved dikes with tight compositions predominate at the segment center (Wadi Thuqbah-Ays area), while dikes with much broader compositions occur at both northern and southern ends. The appearance of highly evolved rocks at the segment margin suggests more intensive fractional crystallization at the margin. Paradoxically, primitive rocks occur at both ends, indicating processes other than simple crystallization. These features are interpreted as follows: a more steady state melt lens at the segment center would result in less evolved melts with tight compositions, and a smaller melt lens at the segment margin due to cooler conditions would result in two melts, evolved melts ascribed to a more closed system fractional crystallization, and primitive melts emplaced from depths without stagnancy in the melt lens. Zr/Nb and Y/Nb ratios of the bulk analyses are constant over the whole segment, indicating a comparatively uniform source along the segment. For quantitative considerations, 26 analyses were selected from 125 analyses. Na 8.0 and Ti 8.0 values are lower at the segment center than at the north and south parts. Partial melting degrees are estimated to be about 20% at the center and 16% at the segment boundary, respectively. This is consistent with clinopyroxene compositions of the sheeted dike complex and layered gabbros. The pyroxenes are lower in Na 2 O at the segment center than those at the segment margin as compared with the same Mg#, suggesting independently a higher degree of partial melting at the segment center. The trace element contents of the sheeted dike complex show MORB signatures. However, the Ti 8.0 and Si 8.0 of the sheeted dike complex are lower and slightly higher than modern N-MORBs, respectively. This implies that the source mantle beneath paleo-ridges which produced the Oman ophiolite was different from those of modern mid-ocean ridges.

Detailed lithological study combined with geochemical variations of lavas reveals the across‐axis accretionary process at Wadi Fizh in the northern Oman ophiolite. The >900 m thick V1 sequence is divided into the lower V1 (LV1), middle V1 (MV1) and upper V1 (UV1) sequence by 0.4 m and 0.8 m thick umbers at 410 mab (meters above the base of the extrusive rocks) and 670 mab, respectively. The lowest part of the LV1 (LV1a) consists of lobate sheet and pillow lava flows extruded on the relatively flat ridge crest. Elongate pillows at 230 mab are flows draping downslope from the ridge crest and characterize the lithofacies on the ridge flank. Just above a jasper layer at 270 mab, 130 m thick evolved lavas were transported from the crest and emplaced on the ridge flank (LV1b). Off‐axial accretionary processes recorded in the MV1 resulted in alternating flows of less evolved, depleted lava and evolved lava, suggesting that the MV1 off‐axial lava sequence comprises flows emanated from both on‐ and off‐axis source vents. The less evolved and depleted UV1 flows suggest independent sources distinct from the axial lavas. The Lasail Unit is regarded as a subunit of the V1 because it is comparable to the UV1 in the geological, petrological, and geochemical characteristics. The broad compositional range of the V1 sequence endorses a view that the Wadi Fizh area corresponds to a segment end of the Oman paleospreading system accompanied by off‐axis volcanism as in segment boundaries of the present East Pacific Rise.