Fractionation in a zoned monzonite pluton: Mount Dromedary, southeastern Australia

Mount Dromedary pluton is one of several predominantly monzonite plutons and smaller intrusive bodies which constitute the Dromedary igneous complex in southeastern New South Wales. The pluton exhibits a striking arrangement of petrographically, but not always chemically, distinct zones ranging from mafic monzonite at the outside to quartz monzonite in the centre. The rocks display a mineralogical and geochemical integrity which indicates a consanguineous relationship. Minor compositional discontinuities between zones, together with observed and inferred minor intrusive zone boundaries, suggest that each zone has to some extent evolved independently. Negative Eu anomalies in REE abundance patterns show that some of the zones have been affected by fractionation of feldspar, but complementary accumulates are not found at the present levels of exposure. The pattern of zoning can be explained by a process of shallow fractional crystallization in which variations within zones are the result of lateral accretion of alkali feldspar as well as settling and/or lateral accretion of mafic phases at lower levels in the intrusion and upward displacement of fractionated magma. The parental magma of the pluton probaby originated by partial melting of an alkali basalt composition with an amphibolite mineralogy at the base of the crust.