Sapphire occurrences in connemara: Field and mineralogical descriptions from an erratic, and from bedrock pelitic xenoliths in the grampian metagabbro-gneiss suite

The discovery of a sapphire (blue corundum) bearing erratic on the granite island of Lettermore, south Connemara, has led to further discoveries of generally non-gem-grade sapphire in hornfelsed desilicated pelitic xenoliths within the 470Ma metagabbros of Connemara’s Grampian Metagabbro-Gneiss Suite. The petrology and mineralogy of the erratic and the desilicated pelitic xenoliths are described. Well-established constraints on the timing of magmatism and on metamorphic conditions show that colourless corundum and a hercynite-magnetite spinel formed 470Ma ago at >900°C and the spinel later (~468Ma) unmixed to corundum, which was sometimes sapphire, and magnetite at temperatures of ~750°C and 3.5-6kb. Consideration of global sapphire occurrences indicates that sapphire can form over a wide range of P and T, i.e. greenschist to granulite facies. The Connemara sapphire formed during contact granulite facies metamorphism.