The association of certain types of low-potassium acid igneous rocks known as plagiogranites with oceanic crust and ophiolite complexes is well known. Both this association and experimental studies have suggested that oceanic plagiogranites could have originated by differentiation of magmas of mid-ocean ridge basalt (MORB) type. However, no actual example appears to have been recorded in which the progressive differentiation of MORB-type magma can be demonstrated to generate integrally associated plagiogranite. The occurrence in the Faeroe–Shetland sill complex of intrusions containing plagiogranite within tholeiitic olivine dolerite of MORB-type composition provides direct evidence of this process for the first time.

The occurrence and composition of oceanic plagiogranites have been reviewed by Coleman & Donato (1979). They describe plagiogranites as leucocratic rocks which occur within ophiolite complexes (in the upper parts of cumulate gabbros and within the sheeted dyke complexes) and which have also been occasionally recovered by sea-floor dredging and drilling (see also Hekinian 1982). Plagiogranites characteristically differ from most continental granitic rocks by having very low K2O and Rb contents, having Rare Earth element (REE) distributions that resemble those of MORB (light REE-depleted), and typically exhibiting interstitial vermicular and/or micrographic intergrowths of quartz and plagioclase.

The main models proposed for the origin of plagiogranites are (1) fractional crystallization of MORB-type magma, (2) liquid immiscibility, and (3) anatexis of basic rocks. Some workers (e.g. Saunders et al. 1979; Ashley et al. 1983) have concluded that although fractionation of basic liquids was the dominant process by which plagiogranites originated in the ophiolites

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