Somers Ignimbrite Formation: Cretaceous high-grade ignimbrites from South Island, New Zealand

Abstract

The Somers Ignimbrite Formation, part of an Upper Cretaceous calc–alkaline volcanic association dated at 89±2 Ma, forms the summit area of Mt. Somers, mid-Canterbury, New Zealand. It is the eroded remnant of a succession of 12 high-grade to extremely high-grade ignimbrite sheets each of which are designated as members. Each member comprises numerous `sub-units' (<100 mm–12 m thick) which exhibit variable degrees of welding and may represent discrete flow-units. On a finer scale, compositional layering (1–30 mm) occurs with alternations of darker and lighter coloured poikilomosaic and felsitic bands which are commonly folded due to rheomorphism. Broken phenocrysts are common, particularly in the felsitic bands. Clastic dikes and clasts of tuff are common in Member 1 and at the base of Member 2, and fine grained tuff layers, interpreted as co-ignimbrite deposits, occur at the top of some sub-units within these two members. Welding occurred during (rather than following) initial flow emplacement while rheomorphism continued during late stages of emplacement and autobrecciation into the post-depositional phase, largely in response to local variation in underlying topography. Vertical chemical zonation within the Somers Ignimbrite Formation suggests progressive extraction from a compositionally zoned magma chamber.