Along-axis magmatic system in the northern Oman ophiolite: Implications of compositional variation of the sheeted dike complex


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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. Na8.0 and Ti8.0 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 Na2O 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 Ti8.0 and Si8.0 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.