42. Umino, S., Miyashita, S., Hotta, F. and Adachi, Y.，2003. Along-Strike Variation of the Sheeted Dike Complex in the Oman Ophiolite -- Insights into Subaxial Ridge Segment Structures and Magma Plumbing System. Geochiem. Geophys. Geosyst. (in press).

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 thinner 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 1511dikes ranges from < 1 cm to > 13 m, with the 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 > 3m tend to have high Mg#s, while thinner dikes (< 2m) 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 thousand years. Initially, the segment-long melt lens developed along the paleoridge axis, which fed long and thick dikes with high-Mg#s. 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.

Keywords: Oman Ophiolite, sheeted dike complex, along-axis variation, segmentation, magma plumbing system