Mineral chemistry of the ultramafic and mafic cumulates in the eastern part of the Sabzevar ophiolite (NE Iran): evidence for formation of high pressure cumulates in thickened arc crust

摘要

The Sabzevar ophiolite is the largest and most complete ophiolitic sequence in northeastern Iran. Its eastern part contains a well-developed ultramafic and mafic cumulates section, comprising plagioclase bearing wehrlite, olivine gabbronorite, gabbronorite, amphibole gabbronorite and quartz gabbronorite with layers of clinopyroxenite and thin layers of anorthosite. These rocks have adcumulate, mesocumulate, heteradcumulate and orthocumulate textures and consist of olivine, clinopyroxene, orthopyroxene, plagioclase and amphibole. The crystallization order in these rocks is olivine (Fo(89.8-80.0)) +/- chromian spinel, clinopyroxene [100* Mg/(Mg + Fe+2) = 98-77], plagioclase (An(93.1-87.6)), orthopyroxene [100* Mg/(Mg + Fe+2) = 93-75] and amphibole. The whole-rock geochemistry of the cumulative rocks suggests that they can be classified as low-Ti ophiolite and originated from an island arc tholeiitic magma source. The crystallization order and existence of orthopyroxene and clinopyroxene with very high Mg numbers, compared with those of coexisting olivines, and plagioclase with extremely high anorthite contents in the ultramafic and mafic cumulate samples, except for quartz gabbronorite samples, are in good agreement with products of high-pressure crystal fractionation of primary basaltic melts beneath an intra-oceanic island arc/supra-subduction zone setting. In contrast, the quartz gabbronorite samples in the upper crustal level show evidence of a low-pressure crystal fractionation within the same tectonic setting. We suggest that the arc crust of the Sabzevar ophiolite was thick, with the active magma chamber situated at the base of the crust at a pressure of 8-13 kbar for the high-pressure cumulates, and a related active shallower level magma chamber for the low-pressure rocks.