A geochemical and petrological study of the tertiary minor intrusions of Rhum, northwest Scotland

Abstract

The Rhum minor intrusion suite is petrographically and compositionally variable, the majority of the basic rocks are aphyric dolerites but olivine phyric (picritic) and plagioclase phyric (big feldspar) dolerites are also present. In addition, several medium to coarse grained gabbro and peridotite plugs occur. Intermediate rocks are not abundant and acid dykes are rare. The basic and intermediate intrusions occur in a variety of structural environments all over the island, the gabbros and peridotites are generally emplaced as plugs in the northern and southern Torridonian tracts, while the few acid dykes are usually closely associated with the plutonic acid rocks. Twelve classes of dolerite (Groups 1-12) and five classes of gabbro have been distinguished on geochemical and petrographic grounds. The suite as a whole is characterised by high MgO levels when compared with rocks of analogous differentiation in other Tertiary suites of North-West Britain, Several of the group 1 picrites are believed to have existed as mafic liquids with 15-20 wt% MgO at high crustal levels. In addition the basic members (dolerite groups 2 to 4) and several of the gabbros are characterised by higher Ni and Cr and lower Al(_2)O(_3) than comparable British Tertiary rocks. Discriminant Function analysis of the compositional variation within the basic members of the Rhum minor intrusion suite has shown that the majority of these rocks are completely transitional between alkali olivine basalt and tholeiite basalt and contrast with the mildly alkaline Skye Main Lava Series and the mildly tholeiitic Ardnamurchan cone sheets. In contrast, the dolerite groups 5 and 6 are very similar to some of the Skye magma types and consequently are believed to be outlying members of the Skye regional dyke swarm. Of the intermediate rocks, the dolerite groups 8 and 9 and the gabbroic differentiates are tholeiitic in affinity and the dolerite groups 10 to 12 are alkalic in affinity. Detailed field and geochemical investigation has shown there is a progression from tholeiitic basic magmas to high MgO more alkalic composition with time. Despite the known presence of a large upper crustal magma chamber on Rhum, the geochemical variation of the majority of the basic rocks is cottatible with processes operating below 15 km within the lower crust, or in the upper mantle at depths up to 80 km. In contrast the big feldspar (group 7) dolerites and the magmas which gave rise to the peridotite plugs are believed to have been tapped from a high level crustal magma chamber. In addition the divergence towards more extreme alkalic and tholeiitic compositions in the intermediate rocks coupled with their general porphyritic nature and their major oxide and trace element variation suggests that they are the product of low pressure fractionation processes.