Origin of ignimbrites of the North Island, New Zealand : a new petrogenetic concept (print copy)

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Steiner, A. 1960 Origin of ignimbrites of the North Island, New Zealand : a new petrogenetic concept. Wellington: Government Printer. New Zealand Geological Survey bulletin 68 41 p.

Abstract: Diagnostic petrographic features of ignimbrites are listed, and Marshall’s classification is modified. The proposed classification into two groups – pulverulites and lenticulites – is based on the primary texture of the glassy constituents. Each group is subdivided into three phases – vitreous, pectinate to axiolitic, and radial to spherulitic – according to the state and type of secondary crystallisation. The nomenclature for ignimbrite-like rocks is reviewed, and it is suggested that the term “ignimbrite” be retained as a descriptive, non-generic name. Physico-chemical conditions of ignimbrite magma ascending along fissures, and the factors controlling liquid immiscibility in an ignimbrite magma, are discussed, and contrasted with those controlling ash and nuee ardente eruptions. The inter-relationship of four principal types of eruption of acid magma is shown, and an analogy between ignimbrites and plateau basalt drawn. A brief historical review of the status of liquid immiscibility in geology over the last three decades is offered, and a plea is made for further experimental investigation. (auth)

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