The petrology and petrogenesis of a suite of minor alkaline intrusions in the Assynt District, Sutherland
The Assynt District on the North West Highlands of Scotland is composed of a thrusted sequence of Cambrian limestones and quartzites, originally unconformably laid down on Archaean Lewisian gneisses and Pre-Cambrian Torridonian arkoses. Igneous activity associated with the development of the Caldedonian orogeny occurs as a series of genetically related hornblende ± pyroxene lamprophyre sills. High level fractionation of the lamprophyric magmas has resulted in the development of a suite of tinguaitic, peralkaline, oversaturated, aegirine felsites which cut the earlier lamprophyric intrusions. Plutonic activity occurs in the Assynt District, as an early syenitic intrusion at Loch Ailsh dated at 439 ± 24 my and a late, in part ultrapotassic, carbonatite bearing, undersaturated intrusion at Loch Borralan, intruded after thrusting activity had ceased at 430 ± 4 my. The lamprophyric intrusions are relatively primitive magmas having high MgO and Ni, Cr abundances, they are enriched in light rare earths. Their trace element chemistry shows that they are derived from a lithospheric mantle, metasomatically enriched with incompatible elements and volatiles from subducted crust. Textural study shows that the primitive magmas were probably intruded as a volatile rich suspension of mafic phenocrysts. Subsequent evolution initially by the fractionation of amphibole pyroxene and plagioclase has resulted in the formation of a suite of increasingly silicic andesitic to rhyolitic sills. The later stages of evolution were due to the metasomatic effects of a volatile phase. The simultaneous removal of hornblende and plagioclase has resulted in the formation of oversaturated peralkaline felsites without the REE patterns normally associated with evolved felsitic rocks. Thrusting activity has resulted in the apparent close association between the lamprophyric/felsitic sill swarm and the loch Borralan intrusion. Pre thrusting reconstruction of the Assynt district show that the sills were intruded at least 30km to the south west and are not genetically related to the Loch Borralan intrusion.