Significance of clinopyroxene compositions

CLINOPYROXENE phenocrysts and groundmass crystals are relict phases in altered basalt and basaltic andesitc lavas, and arcnites of the Cudge-gong Volcanics and Toolamanang Volcanics, Cudgcgong-Mudgee district, New South Wales. Petrography, field relationships and clinopyroxcnc compositions indicate that basaltic blocks in the latter unit are reworked from the Cudgegong Volcanics. Clinopyroxene phenocrysts show a restricted compositional range and minor Fe-enrichment from core to rim, features considered indicative of a calc-alkaline parent magma. It is proposed that the Cudgegong Volcanics crystal-lized under hydrous conditions, at least in the later stages, with rising fo ~ resulting in a Fe-Ti oxide crystallizing as a primary phase. The clino-pyroxenes are considered to have crystallized at moderate (5-6 kbar) and falling pressures and at minimum temperatures in the range 900 to 1000 ~ Coupled substitutions affecting the 'other ' com-ponents in the clinopyroxene structural formula indicate that the i~Al-~iFe3+, iVAl-ViA1 and ivAl-~Ti 4 + couples are important. The Sofala Volcanics, south of the study area, and the Cudgegong Volcanics are similar in age, petrography and stratigraphic position, and contain relict clino-pyroxenes which are chemically similar. This suggests that the units are laterally equivalent and adds further evidence to the proposal that an oceanic island arc system was active in centra