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Relict clinopyroxenes from within-plate metadolerites of the Petroi Metabasalt, the New England Fold Belt, Australia

Published online by Cambridge University Press:  05 July 2018

Deepanker Asthana*
Affiliation:
Department of Geology and Geophysics, The University of Sydney, N.S.W. 2006, Australia

Abstract

Relict clinopyroxenes from metadolerites of the Early Permian Petroi Metabasalt formation, studied by electron microprobe, show a limited compositional range near the diopside-augite boundary in the pyroxene quadrilateral. Clinopyroxene analyses from three metadolerites, grouped in approximately equal Fs contents, define an overall smooth trend between Fs10 and Fs16. This is typical of clinopyroxenes from mildly alkaline basic magmas. Pyroxene stoichiometry suggests high Fe3+ contents (0.04 to 0.20 a.f.u.), which with high Al (0.10 to 0.31 a.f.u.) and Ti (0.03 to 0.08 a.f.u.) implies that CaTiAl2O6 and CaFe3+AlSiO6 are important ‘other components’. Relative Alz in CaFe3+AlSiO6 decreases and consequently CaTiAl2O6 increases with progressive fractionation. This, with the Fe2+:Fe3+ ratios in the Petroi clinopyroxenes, suggests falling ƒO2 in the magma with fractionation. The ƒO2 controlled entry of Alz, Tiy and Nax into the clinopyroxenes and hence the Petroi clinopyroxene trend.

Type
Mineralogy
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1991

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Footnotes

*

Present address: Dept. of Applied Geology, Indian School of Mines, Dhanbad-826004, India.

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