Relict Clinopyroxenes from Within-Plate Metadolerites of the Petroi Metabasalt, the New England Fold Belt, Australia

Deepanker Asthana*
Department of Geology and Geophysics, The University of Sydney, N.S.W. 2006, Australia
*Present address: Dept. of Applied Geology, Indian School of Mines, Dhanbad-826004, India.

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.

Keywords: clinopyroxenes • metadolerite • New England fold belt • Australia

Mineralogical Magazine; December 1991 v. 55; no. 381; p. 549-561; DOI: 10.1180/minmag.1991.055.381.08
© 1991, The Mineralogical Society
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