Megacrysts from the Newer Basalts and Other Basaltic Rocks of Southeastern Australia

摘要

Megacrysts of aluminous clinopyroxene, aluminous orthopyroxene, anorthoclase, kaersutite, ferrokaersutite, titanbiotite, titanphlogopite, and apatite are present at some of the explosive alkaline basalt eruptive centers within the Newer Basalts province of Victoria and South Australia, and also in other basaltic rocks of southeastern Australia. Clinopyroxenes from particular Newer Basalts centers display a systematic chemical variation involving increasing Al, Fe, Ca, Ti, Na, and Fe3+ with decreasing Si, Cr, Mg, and Mg/Mg+Fe ratio, which may reflect magmatic fractionation processes. Field, chemical, and experimental evidence suggest that clinopyroxene and orthopyroxene megacrysts represent disaggregated parts of pegmatitic polycrystal-line aggregates, precipitated as near-liquidus phases from basaltic (particularly basanitic) magmas in association with ultramafic xenoliths of the Al-augite series, at pressures of 10 to 20 kb. Magnesian kaersutite and titanphlogopite from The Anakies, Victoria, are possibly near-liquidus phases of their associated nepheline mugearite, but physical conditions of formation of such megacrysts cannot be uniquely fixed. The assemblage ferrokaersutite + titanbiotite + anorthoclase + apatite from The Anakies is considered to represent a near-solidus precipitate at pressures less than 12 kb. On the basis of a recurring association at four New South Wales localities, it is suggested that aluminous clinopyroxene and titanphlogopite are near-liquidus phases of wet olivine nephelinite or olivine analcimite magmas at high pressures. In the Newer Basalts province, the observed inconsistencies between megacryst assemblage and host basalt composition, as well as the varied compositions of the megacrysts, imply a random sampling of phases precipitated from possibly the host magmas and (or) derivative and parental magmas, and thus megacryst suites are probably of limited utility in deducing detailed fractionation trends in basaltic magmas.