Ultrabasic rocks of Rhum and Skye: the nature of the parent magma

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Abstract
Rocks from the peridotite dykes of southern Skye ( Gibb 1968 ) are virtually identical to those found in the large layered intrusions of Rhum and Skye, suggesting that they may have been formed in a similar manner. The rocks of the layered intrusions are generally regarded as having formed by the gravitative accumulation of the early precipitating phases from a basaltic magma, usually with subsequent extensive ad-cumulus growth. Such a mechanism is totally inapplicable to the dykes and it can be established that they were formed by flow differentiation of a suspension of olivine in an eucritic liquid which subsequently underwent near-equilibrium crystallization. The possibility therefore arises that the layered intrusions originated from an ultrabasic magma similar to that which formed the dykes. Consideration of this possibility suggests that the petrogenetic models proposed for the layered ultrabasic intrusions of Rhum and Sgurr Dubh (Skye) ( Brown 1956 , Wadsworth 1961 , Weedon 1965 , Wager & Brown 1968) be slightly modified: in particular, that the parental magma was not a basaltic liquid but rather a suspension of olivine in a eucritic liquid. Such a modification is consistent with the features of the layered intrusions and appears to overcome several problems posed by the basaltic liquid hypothesis.

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