The Role of Fractional Crystallization in the Evolution of the Nandewar Volcano, North-eastern New South Wales, Australia

The Nandewar Volcano is composed of a suite of transitionalalkaline eruptives ranging in composition from mildly hy-normativehawaiites and trachyandesites through tristanites and trachytesto comendites. Field relations, mineralogy, and chemical dataare discussed and applied to evaluate those genetic controlsresponsible for the variation in this association. These dataindicate that the majority of the Nandewar volcanics were derivedfrom a common upper mantle source with specific major and traceelement characteristics. Mass-balance calculations do not favoura genetic model which relates all members of the suite by closed-systemfractional crystallization. An alternative genetic model forthe hawaiites and trachyandesites is based on differential partialmelting of discrete upper mantle peridotitic sources with differentmodal pargasitic amphibole (?mica) and apatite abundances, andwhich were relatively Fe-rich compared with a pyrolite-likesource. Lack of precise knowledge about potential source compositionsprevents quantitative evaluation of this model. Mass-balancecalculations are broadly consistent with the genesis of thetristanites and mafic trachytes by fractional crystallizationof the most ‘evolved’ trachyandesites, but the rarityof cognate cumulates as inclusions is not favourable to thismodel. Rocks spanning the compositional range trachyte-comenditecannot be related by fractional crystallization of observedphenocrysts and an alternative model based on liquid-state differentiationprocesses is tentatively proposed. Subsequent late-stage volatileloss and addition has significantly modified alkali and traceelement abundances of some comendites.