Felsites and breccias in the Northern Marginal Zone of the Rum Central Complex: changing views, c.1900-2000

As in several other parts of the British Tertiary Igneous Province, breccia s and felsite sheets are closely associated on the Isle of Rum. This associ ation has been described and interpreted by several workers over the last 1 25 years. Opinion has divided into an intrusive origin for both rock types, as explosion breccias and felsite intrusions, versus a sedimentary origin for the breccias and an extrusive origin for the felsite. Evidence is revie wed for both opinions and it is concluded that the latter is substantially correct, as indicated by the presence of sedimentary structures and interbe dded tuffs in the breccias and eutaxitic textures in the felsites. The brec cias formed by inwards slumping of rocks from the oversteepened walls of a caldera, whereas the felsites formed by eruption of pyroclastic flows which were thick and hot enough to weld. It is inferred that the caldera formed initially and subsided progressively without any accompanying eruptions, an d this is attributed to growth of the underlying magma chamber. The breccia s accumulated during this stage. There followed a resurgent stage in which caldera collapse occurred in response to repeated ignimbrite eruptions part ially emptying the magma chamber. The chamber is inferred to have been chem ically and mineralogically zoned.