Along‐strike variation of the sheeted dike complex in the Oman Ophiolite: Insights into subaxial ridge segment structures and the magma plumbing system

Abstract

[1] Along-strike variations of the 60-km-long sheeted dike complex in the northern Oman Ophiolite were studied in order to understand the shallow magma plumbing system beneath the fossil fast spreading ridge. The presence of numerous dikes intruding into the layered gabbro defines the northern end of the paleoridge segment at Wadi Fizh. The postulated segment center is located at Wadi Thuqbah, which has a thicker Mono transition zone than elsewhere along the fossil segment. Aphyric dikes predominate in the sheeted dikes, of which 99% are simple, while multiple and composite dikes are few. The thickness of 1511 dikes ranges from <1 cm to >13 m, with an average thickness of 71.3 cm. Restored dike trends in the 30-km-long northern half of the dike complex display the NS trending north domain and NNE-NS trending south domain bordered at the south of Wadi Bani Umar al Gharbi. In the domain boundary, dikes gradually change strikes or are mutually intrusive. Dike thickens northward with the largest peak along Wadi Fizh at the northern end of the paleoridge segment and a small peak at Wadi Bani Umar al Gharbi. Most dikes have a bulk Mg# of 55-66, which overlaps the majority of MORB. Less common, highly evolved dikes with Mg# 34-40 characterize the north domain. Thicker dikes (>3 m) tend to have high Mg#, while thinner dikes (<2 m) are highly variable in Mg#. The regional variations of the dike trends and the whole rock compositions can be explained by the secular variation in the structures of the paleoridge segment comparable to a third-order segment of the present mid-ocean ridge system spanning a few tens of thousands of years. Initially, the segment-long melt lens developed along the paleoridge axis, which fed long and thick dikes with high Mg#. With decreasing supply of magma, the melt lens split up into the north and south smaller lenses bordered by a DEVAL that fed thinner dikes intruding into the former thick dikes. Cut-off of the northern melt lens from the magma source changed the melt composition to highly evolved, low-Mg# magmas, which subsequently intruded as short evolved dikes. Meanwhile, the main melt lens in the segment center continued feeding the high-Mg# dikes which were maintained by the larger melt lens size and intermittent magma supply from deep magma chambers.

Geology and petrology of the plutonic complexes in the Wadi Fizh area: Multiple magmatic events and segment structure in the northern Oman ophiolite

“…These studies showed that opensystem fractional crystallization controlled fundamentally the magmatic processes. The gabbroic unit shows conspicuous layering similar as those in layered intrusions such as the Skaergaard Intrusion [Wager and Brown, 1968]. However, origin of layering in the Oman ophiolite is still controversial, e.g., due to ''sedimentary'' [Pallister and Hopson, 1981], intrusive , or deformation [Nicolas, 1989[Nicolas, , 1992.…”

“…The gabbro blocks in the Wadi Fizh area were produced at the tip of retreating ridge and the main crustal sequence probably generated at the advancing ridge axis, respectively. Umino et al [2003] showed that the width of dikes in the sheeted dike complex varies systematically along strikes, and claimed that the Fizh area may correspond to the segment margin, independently. Also Miyashita et al [2003] showed that the bulk rock compositions of sheeted dike complex vary systematically along strikes over 60 km and smaller degree of partial melting was detected around the Fizh area, in concordant with this study.…”

Root zone of the sheeted dike complex in the Oman ophiolite

“…They were mapped as ''late intrusive'' [Lippard et al, 1986; Ministry of Petroleum and Minerals, 1986], encompassing, wehrlites and gabbronorites, uralitic gabbros, diorites and tronjhemites, the last one making up magmatic breccias with diabase dikes. Olivine gabbros have not been reported among these intrusions These ''late intrusive'' have been related to domains where accretion was under the influence of major tectonic and magmatic activity related to ridge propagation [Juteau et al, 1988;MacLeod and Rothery, 1992;Nicolas and Boudier, 1995;Boudier et al, 2000;Adachi and Miyashita, 2003;Miyashita et al, 2003;Umino et al, 2003]. Here, they are referred to as domains of ''ridge segmentation'' and opposed to domains where the accretion should have been steady state, called here ''normal'' domains, considering their continuity and homogeneity.…”

Advanced geophysical studies of accretion of oceanic lithosphere in Mid-Ocean Ridges characterized by contrasting tectono-magmatic settings

“…We can put constrains on the volume of magma involved in the eruption. The width of the AMC near 9°50'N is 500-700 m and the thickness is [Gudmundsson, 1995;Harper, 1984;Kidd, 1977;Oliver and McAlpine, 1998;Rosencrantz, 1983;Tryggvason, 1994;Umino et al, 2003]. However, Soule et al [2007] suggested that the dike fed the eruption over the whole 18-km-long section covered by the erupted lava flows on the seafloor, and using this length the calculated width of the dike is more reasonable 0.95 m. The second scenario suggests the 2005-06 eruption was fed from the accumulated southern and northern melt-rich sections.…”

“…The low velocity just above the AMC has been interpreted to be due to a thermal anomaly, and the low velocity below the AMC is thought to represent a mush zone underlying the melt sill [Toomey et al, 1990;Vera et al, 1990]. Vera et al [Gudmundsson, 1995;Harper, 1984;Kidd, 1977;Oliver and McAlpine, 1998;Rosencrantz, 1983;Tryggvason, 1994;Umino et al, 2003]. However, Soule et al [2007] suggested that the dike fed the eruption over the whole 18-km-long section covered by the erupted lava flows on the seafloor, and using this length the calculated width of the dike is more reasonable 0.95 m. …”