Garnet Websterites and Associated Ultramafic Inclusions from a Nepheline Mugearite in the Walcha Area, New South Wales, Australia

A. J. Stolz
Department of Geology and Geophysics, University of New England, Armidale, NSW, 2351, Australia

Abstract: A nepheline mugearite dyke in the Walcha area, New South Wales, contains xenoliths of Cr-diopside lherzolite, garnet- and spinel websterites, and two- pyroxene-garnet granulites. The Cr-diopside lherzolite xenoliths are regarded as refractory upper mantle which finally equilibrated at c. 950–980°C. The garnet websterites are considered to represent spinel websterite segregations from an Mg-rich olivine tholeiite magma at 14–16 kbar which on isobaric cooling to 1000–1050°C passed into the garnet pyroxenite stability field. Spinel-amphibole websterites which probably last equilibrated at c. 950°C and pressures of 9–12 kbar have dose mineralogical affinites with the Cr-diopside lherzolite xenoliths. The mafic granulites may represent metamorphosed crystal segregates resulting from relatively large degrees of crystallization of tholeiitic basaltic magma which finally equilibrated at c. 14 kbar and 1000°C Subcalcic clinopyroxene and spinel megacrysts crystallized from the host nepheline mugearite at c. 13–20 kbar.

Mineralogical Magazine; June 1984 v. 48; no. 347; p. 167-179; DOI: 10.1180/minmag.1984.048.347.02
© 1984, The Mineralogical Society
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