Origin and evolution of Mid-Cretaceous, garnet-bearing, intermediate and silicic volcanics from Canterbury, New Zealand

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doi: 10.1016/0377-0273(87)90047-3
Authors:Barley, M. E.
Author Affiliations:Primary:
Univ. Canterbury, Dep. Geol., Christchurch, New Zealand
Other:
Bur. Miner. Resour., Australia
Volume Title:Tectonic controls on magma chemistry
Volume Authors:Weaver, S. D., editor; Johnson, R. W.
Source:Journal of Volcanology and Geothermal Research, 32(1-3), p.247-267; Geochemistry of eruptive magmas tectonic controls on petrogenesis, Auckland, New Zealand, Feb. 1-9, 1986, edited by S. D. Weaver and R. W. Johnson. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0377-0273
Publication Date:1987
Note:In English. 53 refs.; illus. incl. 6 tables, geol. sketch map, sketch map
Summary:The Mt. Somers Volcanics are part of a suite of mid-Cretaceous (89±2 Ma) intermediate to silicic volcanics, erupted onto an eroded surface of Torlesse sediments. Rock types vary from basaltic andesite to high-silica rhyolite. Andesites are medium- to high-K with phenocrysts of plagioclase, orthopyroxene and pigeonite. Dacites are peraluminous and commonly contain granulite facies xenoliths and garnet xenocrysts. Equilibrium mineral assemblages indicate metamorphic pressures of close to 6 kbar at 800°C. Rhyolites are peraluminous with phenocrysts of quartz, sanidine, plagioclase, biotite, garnet and orthopyroxene. The ferromagnesian phases show textural evidence of magmatic crystallization and are chemically distinct from xenocryst phases in dacites. Equilibrium assemblages indicate that early magmatic crystallization occurred at close to 7 kbar (greater than or equal to 20 km depth) at above 850°C, with melt-water contents of less than 3.5%. Major-element contents, trace-element contents and an initial 87Sr/86Sr ratio of 0.7085 indicate that the rhyolites formed by partial melting of dominantly quartzo-feldspathic Torlesse sediments, leaving a granulite-facies residue. The chemical variation displayed by the rhyolites is best explained by fractional crystallization of the observed high-pressure phenocryst assemblage. Most elements show a compositional gap between rhyolite and dacite. The major-element, trace-element and Sr isotope compositions of the intermediate lavas are best explained by assimilation of lower crustal material combined with fractional crystallization in mantle-derived tholeiitic magmas. Magmatism was the result of heat the magma flux from the mantle, during the change from compressive to extensional tectonics after the culmination of the Rangitata Orogeny. [R.E.S.]
Subjects:Alkaline earth metals; Andesites; Basalts; Cretaceous; Dacites; Differentiation; Extension tectonics; Facies; Fractional crystallization; Garnet group; Genesis; Granulite facies; Igneous rocks; Inclusions; Interpretation; Isotopes; Magmas; Major elements; Mesozoic; Metals; Metamorphism; Middle Cretaceous; Mineral assemblages; Minerals; Nesosilicates; Orthosilicates; P-T conditions; Petrology; Phase equilibria; Rhyolites; Silicates; Sr-87/Sr-86; Stable isotopes; Strontium; Tectonics; Tholeiite; Trace elements; Volcanic rocks; Xenoliths; Australasia; Canterbury New Zealand; New Zealand; South Island; Mount Somers Volcanics; Rangitata Orogeny
Abstract Numbers:88M/0686
Record ID:1987083195
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data supplied by Institute of Geological and Nuclear Sciences Limited (GNS Science), Lower Hutt, New Zealand
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