The Cuillin layered igneous complex—evidence for multiple intrusion and former presence of a picritic liquid

Abstract

Synopsis Much of the western Cuillin is formed by part of a funnel-shaped intrusion comprising layered ultrabasic rocks which range upwards from dunite to peridotite to allivalite (feldspar-olivine cumulate). The funnel-shaped intrusion has a marginal border group on which allivalites are banked. Recent mapping indicates that a peridotite mass previously regarded as a sill, is part of this layered sequence, and that inner, ultrabasic cumulates to the E are merely a downthrown portion of the same funnel-shaped mass. The eastern rocks were cut by a later pyroxene-rich magma from which feldspar-olivine-pyroxene cumulates were formed. In the lower dunite-peridotite-allivalite series (Unit 1), cumulus olivine and feldspar range from Fo 90–86 and An 91–85 , respectively. In the upper unit (Unit 2) of feldspar-olivine-pyroxene cumulates cryptic variation is more complex; feldspar cores are An 84–83 , and olivines range from Fo 77–67 . The high level of the Cuillin complex suggests that there was insufficient room for a layered basic intrusion some 10,000 metres thick. Evidence is presented for successive pulses of ultrabasic magma, indicating that the complex as a whole comprises intrusions of tholeiitic affinity ranging broadly with time from ultrabasic to gabbroic, rather than a tectonically disrupted single, layered basic intrusion.