Boulder of Quartz-Feldspar-Porphyry, from the Drumadoon Sill, south-west Arran, Buteshire, with xenoliths of darker dolerite


The rock can be easily examined in boulders on the foreshore - the large crystals (phenocrysts) of quartz and feldspar are visible within a much more fine-grained groundmass (porphyry is a rock that contains phenocrysts). The boulders of dolerite in the rock are 'exotic' (i.e. not from the lava that fomed the quartz-porphyry) and are described as xenoliths. In this case we have a mixture of two very different rock types, the acidic quartz-porphyry, and the basic dolerite. the molten quartz porphyry lava must have ripped up parts of a separate, and chemically very different igneous intrusion as it was injected. The Drumadoon Sill does have some dolerite in it, so it is clear that this is a composite sill, presumably with first dolerite (because it cooled first and has been incorporated into the quartz porphyry)and them quartz-porphyry being intruded. How this came about it the subject of much debate.

DSC01967; GBCurry