The petrology and geochemistry of ophiolites from the Khawr Fakkan region. Northern Oman range, United Arab Emirates

Abstract

The field relationships, petrography, geochemistry and phase chemistry of the peridotite-gabbro-basalt sequence of the Khawr Fakkan - Wadi Shi - Wadi Madha area suggest that the various components are genetically related. They result from partial melting of the upper mantle, followed by fractional crystallisation of the basic magma generated by this process. The rocks comprise a typical ophiolite sequence and are similar to assemblages from other Alpine-type ultramafic complexes. The area studied may be broadly subdivided into two major zones. In the west the rocks are principally tectonised ultramafics, while to the east of these a series of cumulate gabbros, with dolerite dykes particularly prevelant in the east, passes upwards into basaltic lavas. The tectonised ultramafics are separated from the gabbroic and basaltic rocks by an easterly-dipping thrust- fault zone, which strikes approximately north-south. The rock types exposed in the area to the west of the major thrust fault are harzburgites, dunites and wehrlites. They are all, to some extent, serpentinised. The harzburgites have an intensely tectonised fabric, and consist of an assemblage of highly refractory chemistry. The dunites also show a strong tectonic fabric and total absence of cumulus textures. Textural evidence shows that most of the wehrlites are strongly deformed. A set of criteria to distinguish the refractory residua of partial melting from the cumulates are developed from analytical data and textural evidence. The rock types exposed to the east of the major thrust fault are gabbyo cumulates. The cumulus types recognised include olivine pyroxenites, olivine gabbros, norites and hypersthene gabbros, gabbros and anorthosites and leucocratic gabbros. Where transected by faults gabbro mylonites are developed. Cumulus textures include heteradcumulates, adcumulates and mesocumulates. The basaltic rocks occur both as intrusive dolerite dykes and as lava flows. The basalts are invariably altered, with ferroraagnesian minerals being represented by amphibole in the most part. They are also silicified. variation in major and trace elements reveal that the crustal members of the ophiolite suite (gabbros and basaltic rocks) are tholeiitic. The parent magma being a low-Ti and low-K tholeiitic type. Whole-rock analyses and mineral chemistry are used in an attempt to estimate the nature and composition of the partial-melt magma from which both the cumulate gabbros and basaltic rocks have evolved. A variety of methods have been used, and compared, in an attempt to estimate the equilibration temperatures and pressures of the ultramafic rocks and cumulate gabbros. The application of several geothermometers gave values for dunite of 872 to 1075ºC, for harzburgites of 1099 to 1167ºC, for a wehrlite of 1104ºC, and for the rare rock- type plagioclase peridotite, a somewhat higher value of 1263ºC. Temperatures recorded for olivine pyroxenite cumulates range from 879 to 974ºC, and for gabbros and norites from 839 to 999ºC. Lower temperatures of 909 to 989ºC using an orthopyroxene-clinopyroxene geothermometer, as compared to 1099 to 1167ºC, for the harzburgites may reflect the late, lower temperature, crystallisation of clinopyroxene from trapped liquid. In all cases pressures were less than 7 Kb and correspond to a maximum depth of about 22 Km in the oceanic setting.