SOMERS IGNIMBRITE FORMATION - CRETACEOUS HIGH-GRADE IGNIMBRITES FROM SOUTH ISLAND, NEW-ZEALAND

The Somers Ignimbrite Formation, part of an Upper Cretaceous calc-alka line volcanic association dated at 89 +/- 2 Ma, forms the summit area of Mt. Somers, mid-Canterbury, New Zealand. It is the eroded remnant o f a succession of 12 high-grade to extremely high-grade ignimbrite she ets each of which are designated as members. Each member comprises num erous 'sub-units' (< 100 mm-12 m thick) which exhibit variable degrees of welding and may represent discrete flow-units. On a finer scale, c ompositional 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, particularl y in the felsitic bands. Clastic dikes and clasts of tuff are common i n 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-un its within these two members. Welding occurred during (rather than fol lowing) initial flow emplacement while rheomorphism continued during l ate stages of emplacement and autobrecciation into the post-deposition al phase, largely in response to local variation in underlying topogra phy. Vertical chemical zonation within the Somers Ignimbrite Formation suggests progressive extraction from a compositionally zoned magma ch amber.