New models for the genesis of plagiogranites in the Oman ophiolite

Abstract

The formation of tonalites, trondhjemites and granodiorites in oceanic crust, here grouped together as plagiogranites, offers an important opportunity to examine the process whereby silicic melts are produced from mafic crust. Differences in field relationships and geochemistry show that there are three types of plagiogranite in the Oman ophiolite. These plagiogranites formed in different ways and at different times within the geological history of the ophiolite. Early-formed plagiogranites, documented from Wadi Rajmi in northern Oman, were produced by the partial melting of hornblende gabbros in the roof zone of an axial magma chamber, facilitated by the influx of seawater. Late gabbro-plagiogranites of the Jebel Fayyad intrusive complex form rounded intrusions several km in diameter, which are normally emplaced into the sheeted dyke unit and/or the pillow lavas of the ophiolite. These plagiogranites formed from a mafic magma by the remelting of previously depleted mantle harzburgite followed by fractional crystallisation. Plagiogranites intrusive into the mantle sequence in Wadi Hamaliya are very different from the early and late plagiogranites. They are strongly enriched in light REE and fluid-mobile elements and contain components from both crustal and mantle sources. These findings imply firstly that the spread of ages, previously recorded for the Oman plagiogranites might be real and secondly that the geology of the Oman ophiolite cannot be explained in its totality through ocean-ridge processes. When compared to Archaean TTGs and average continental crust the Oman plagiogranites indicate that, whereas oceanic crust may be an important component in the process of crust generation, other components are also required.