Mineralogy of the Nandewar Volcano, Northeastern New South Wales, Australia

A. J. Stolz*
Department of Geology and Geophysics, University of New England, Armidale, NSW, 2351, Australia
*Present address: Department of Geology, University of Tasmania, Hobart, Tasmania, Australia 7001.

Abstract: The paper discusses the mineralogy of eruptives from the Nandewar Volcano, which range in composition from hawaiite and trachyandesite to comendite via tristanite and mafic and peralkaline trachyte. Olivine, Ca-rich pyroxene, and amphibole display marked decreases in 100 Mg/(Mg + Fe) ratios in the sequence trachyandesite to comendite, reflecting variation in host rock compositions. The presence of tscher-makitic subcalcic pyroxene and aluminous bronzite megacrysts in several trachyandesites indicates that these experienced intratelluric crystallization at elevated pressures (6–8 kbar). Some titanomagnetite and plagioclase phenocrysts in trachyandesites may also be moderate pressure cognate precipitates. Groundmass pyroxenes of some trachytes and comendites are strongly acmitic. The presence or absence of coexisting alkali amphiboles and aenigmatite appears to reflect stability over a relatively broad range of fO2 conditions. Aenigmatite rims on titanomagnetite and ilmenite microphenocrysts in several peralkaline eruptives provides support for a ‘no-oxide’ field in T-fO2 space. The Fe-Ti oxide compositional data indicate that magmas spanning the spectrum trachy-andesite-comendite crystallized under conditions of decreasing T and fO2 which broadly coincided with the FMQ synthetic buffer curve. However, a voluminous group of slightly older associated rhyolites appear to have crystallized under significantly more oxidizing conditions.

Keywords: olivine • pyroxene • amphibole • feldspar • titanomagnetite • aenigmatite • ilmenite • Nandewar Volcano • New South Wales • Australia

Mineralogical Magazine; June 1986 v. 50; no. 356; p. 241-255; DOI: 10.1180/minmag.1986.050.356.07
© 1986, The Mineralogical Society
Mineralogical Society (www.minersoc.org)