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Abstract

Plagiogranites intrusive into the harzburgites of the mantle section of the Oman Ophiolite are compositionally distinct from plagiogranites in the crustal section of the ophiolite and are relatively enriched in MgO, Cr and Ni and in the incompatible trace elements K, Th, U, Ta, La and Ce. These geochemical features are used to explain the origin of the plagiogranites and it is suggested that they are the product of three-component mixing between melts from mafic, crustal and mantle sources. There is geochemical evidence to show that the plagiogranites interacted with their mantle host, although the geochemical signature of this interaction appears to be different from that produced during two-component mixing between a slab melt and the mantle wedge. This mantle interaction makes the Oman mantle plagiogranites a helpful analogue for understanding the extent to which Archaean tonalite–trondhjemite–granodiorite (TTGs) interacted with the mantle during their genesis. However, it is shown that the high Mg# in Archaean TTGs may be masked by later hornblende fractionation, obscuring the originally high Mg# of parental TTG magmas and reopening the possibility that Archaean TTGs might be slab melts.

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