S-type ignimbrites with polybaric crystallisation histories: the Tolmie Igneous Complex,Central Victoria,Australia |
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| Authors: | J D Clemens W D Birch R A Dudley |
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| Institution: | (1) Department of Earth Sciences, University of Stellenbosch, Private Bag X1, Matieland, Stellenbosch, 7602, South Africa;(2) Geosciences Section, Museum Victoria, PO Box 666, Melbourne, VIC, 3001, Australia;(3) School of Earth Sciences, University of Melbourne, Melbourne, VIC, 3010, Australia |
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| Abstract: | The Late Devonian Tolmie Igneous Complex (in north-eastern Victoria, Australia) contains S-type, intracaldera, rhyolitic ignimbrites
with multiple generations of phenocrysts of biotite, garnet, cordierite and orthopyroxene; one unit also contains fayalitic
olivine. Geothermometry and calculated phase relations indicate high-T deep- to mid-crustal origins for the magmas, with crystallisation at several levels. At least four separate magma groups
make up the complex. Compositional variations within and between ignimbrites are adequately modelled by selective entrainment
of peritectic garnet, ilmenite, orthopyroxene and plagioclase into the magmas. Neither crystal fractionation nor mafic-felsic
magma mixing played a role. Chemical and isotope data suggest that the magma sources were once variably Ba-enriched arc greywackes
with different proportions of clay. The deep origin of some of the Tolmie Complex magmas means that supracrustal rocks underlie
parts of north-eastern Victoria at depths of around 35 km. This has important implications for understanding the region’s
tectonic development. |
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