Tholeiitic andesite of high-pressure origin from the tweed shield volcano,northeastern New South Wales

A high-alumina tholeiitic andesite from the southern portion of the Tweed Shield Volcano in northeastern New South Wales contains abundant megacrysts of plagioclase (Ab₅₀An₄₆Or₄) and megacrysts of aluminian bronzite (Ca₄Mg₇₁Fe₂₅) and relatively Ca-poor aluminian augite (Ca₃₆Mg₄₁Fe₂₀). The pyroxenes commonly occur as inclusions in the plagioclase megacrysts. Electron probe microanalyses of the pyroxene megacrysts indicate that they differ in character and composition from the groundmass ferromagnesian phases, namely a more Al- and Na-poor augite (Ca₄₁Mg₄₂Fe₁₇) and olivine (Fa₅₃). The bulk composition of the plagioclase megacrysts is slightly more Ab-rich than that of the groundmass plagioclase, but differences in the two compositions are extended by microanalyses of groundmass plagioclases. Evaluation of the megacryst compositions in the light of experimental data and analogous occurrences in alkaline volcanics leads to the interpretation that the megacrysts represent cognate precipitates formed at pressures broadly equivalent to the crust-mantle boundary. More important, they provide strong evidence for the high pressure origin of tholeiitic andesites, customarily interpreted as the products of low pressure fractional crystallization of tholeiitic magma.