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Significance of clinopyroxene compositions from the Cudgegong Volcanics and Toolamanang Volcanics: Cudgegong-Mudgee district, NSW, Australia

Published online by Cambridge University Press:  05 July 2018

J. W. Pemberton  and
R. Offler
J. W. Pemberton
Affiliation:
Department of Geology, University of Wollongong, Wollongong, NSW, Australia, 2500
R. Offler
Affiliation:
Department of Geology, University of Newcastle, Newcastle, NSW, Australia, 2308
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Synopis

Clinopyroxene phenocrysts and groundmass crystals are relict phases in altered basalt and basaltic andesite lavas, and arenites of the Cudgegong Volcanics and Toolamanang Volcanics, Cudgegong-Mudgee district, New South Wales. Petrography, field relationships and clinopyroxene compositions indicate that basaltic blocks in the latter unit are reworked from the Cudgegong Volcanics. Clinopyroxene phenocrysts show a restricted compositional range and minor Feenrichment from core to rim, features considered indicative of a calc-alkaline parent magma. It is proposed that the Cudgegong Volcanics crystallized under hydrous conditions, at least in the later stages, with rising fO2 resulting in a Fe-Ti oxide crystallizing as a primary phase. The clinopyroxenes are considered to have crystallized at moderate (5–6 kbar) and falling pressures and at minimum temperatures in the range 900 to 1000°C. Coupled substitutions affecting the “other” components in the clinopyroxene structural formula indicate that the ivAl-viFe3+, ivAl-viAl and ivAl-viTi4+ couples are important. The Sofala Volcanics, south of the study area, and the Cudgegong Volcanics are similar in age, petrography and stratigraphic position, and contain relict clinopyroxenes which are chemically similar. This suggests that the units are laterally equivalent and adds further evidence to the proposal that an oceanic island arc system was active in central western New South Wales during the Late Ordovician.

Keywords

clinopyroxenebasaltsarenitesCudgegong VolcanicsToolamanang VolcanicsNew South WalesAustralia
Type
Research Article
Information
Mineralogical Magazine , Volume 49 , Issue 353 , September 1985 , pp. 591 - 599
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1985

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