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

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doi: 10.1016/S0377-0273(96)00040-6
Authors:Smith, T. R.; Cole, J. W.
Author Affiliations:Primary:
University of Canterbury, Department of Geological Sciences, Christchurch, New Zealand
Volume Title:Journal of Volcanology and Geothermal Research
Source:Journal of Volcanology and Geothermal Research, 75(1-2), p.39-57. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0377-0273
Publication Date:1997
Note:In English. 39 refs.; illus., incl. sect., strat. cols., 1 table, geol. sketch maps
Summary: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. Abstract Copyright (1997) Elsevier, B.V.
Subjects:Boudinage; Breccia; Calc-alkalic composition; Clastic dikes; Clastic rocks; Clasts; Cooling; Cretaceous; Devitrification; Emplacement; Folds; Igneous rocks; Ignimbrite; Magma chambers; Magmas; Mesozoic; Petrography; Phenocrysts; Pumice; Pyroclastic flows; Pyroclastics; Rheomorphism; Sedimentary rocks; Sedimentary structures; Soft sediment deformation; Textures; Thickness; Tuff; Upper Cretaceous; Volcanic breccia; Volcanic rocks; Zoning; Australasia; Canterbury New Zealand; New Zealand; South Island; Mount Somers; Somers Ignimbrite Formation
Coordinates:S440000 S430000 E1730000 E1720000
Abstract Numbers:97M/4402
Record ID:1997020044
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands
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