Petrogenesis of garnet-bearing rhyolites from Canterbury, New Zealand

Abstract

The Cretaceous, almandine-bearing rhyolites of the Mt Somers and Mt Misery Volcanics of Canterbury, New Zealand, can be subdivided on the basis of ferro-magnesian phenocryst assemblages as garnet rhyolite, garnet-biotite rhyolite, garnet-orthopyroxene-bioitite rhyolite, and garnet-orthopyroxene rhyolite. The garnets are considered to be primary igneous phenocrysts. Garnets from Mt Somers Volcanics are less variable in composition and have consistently higher grossular and spessartine contents and almandine/pyrope ratios than garnets from Mt Misery Volcanics. There is a regional correlation between Fe/Mg ratios in coexisting garnet, biotite, and parent rock. Application of published experimental data to feldspar and ferromagnesium phenocryst assemblages allows interpretation of feldspar crystallisation histories and estimates of physical conditions of crystallisation. The rhyolite magmas were generated as primary melts whose origin and subsequent development were controlled largely by variations in water content and pressure. Suggested total pressures were 10–15 kilobar, whilst liquidus temperatures and water contents and pressures were as follows: (a) garnet rhyolite, 1000–1050°c, PH₂O about 500 bar, and water content up to 2%; (b) garnet-biotite rhyolite, 900–950°c, PH₂O about 1 kbar, and water content up to 4%; (c) garnet-orthopyroxene- (± biotite) rhyolite, 900–1000°c, PH₂O 1.5–2.5 kbar, and water content about or over 5%. It is suggested that the magmas were generated by crustal anatexis during the Rangitata Orogeny. Crustal thickening, combined with a moderate geothermal gradient, allowed under-saturated, partial-melting of granulite facies rocks under conditions of rising temperature, falling water content, and high load pressure. Possibly, the garnetiferous rhyolites are the rarely erupted equivalents of garnet migmatite or charnockitic granite.