Caldera formation in the Rum Central Igneous Complex, Scotland

The Northern Marginal Zone of the Rum Central Igneous Complex in NW Scotlan d represents part of the early, felsic phase of the volcano. The marginal z one is a relic of the early caldera floor and the infilling of sedimentary and igneous rocks. Its formation has been explored through field examinatio n of the ring fracture system of the Complex and its pyroclastic and epicla stic intracaldera facies. A sequence of magmatic tumescence and chamber gro wth caused initial doming, followed by the formation of a collapse structur e without accompanying volcanism. This collapse structure, circular in plan , is akin in origin to a salt basin formed by crustal stretching above a ri sing diapir. We call this the proto-caldera. Collapse breccias, which repre sent the slumping and sliding of megablocks, blocks and boulders of the Tor ridonian sandstones which form the walls of the basin, were the original in filling. Logs of these deposits reveal considerable variation in thickness of the breccias (from 80-170 m) in the Complex, indicating an uneven floor to the proto-caldera, consistent with piecemeal collapse. Following accumul ation of up to >70 m thickness of breccia, thin interbedded rhyodacitic cry stal tuffs (10-30 cm) record the earliest eruptions of felsic magma in the caldera. Caldera formation was then interrupted by a period of quiescence, recorded by the presence of an epiclastic sandstone of locally several metr es thickness, formed by washout of fines from the breccias. Subsequent resu rgence created a fracture pattern characteristic of doming, along which rhy odacite magma rose in dykes and erupted up to perhaps 10 km(3) of rhyodacit ic intracaldera ignimbrites. This major eruption caused further incremental subsidence of the caldera floor into a now partly emptied magma chamber. M afic inclusions in the ignimbrites point to the eruption being triggered by multiple injections of basic magma into a chamber occupied by felsic magma .