Crystallization Pressure and Cooling History of the Giles Layered Igneous Complex, Central Australia

The Giles Complex, central Australia, consists of a series of large layered gabbroic/ultramafic intrusions emplaced in acidic and intermediate granulites of the Middle Proterozoic Musgrave block. Lithologies range from well-layered dunite, wehrlite, and pyroxenite in the lower primitive series, to massive olivine gabbro, gabbronorite, and anorthosite in the main units, and ferrodiorites, vanadife-rous magnetite layers, and granophyres in the upper, most fractionated parts. Unlike many layered intrusions, the Giles Complex is tectonically dismembered to an extent that a reconstruction of the original morphology is difficult. The Complex is believed to be a type example for medium- to high-pressure differentiation. (1) Chilled margin samples (where preserved) are orthopyroxene-phyric, and liquidus olivine is replaced by liquidus orthopyroxene at an mg-number of 0.77, suggesting a pressure-related expansion of the orthopyroxene stability field (Goode & Moore, 1975). (2) Tschermaks substitution into pyroxene and plagioclase-orthoclase solid solution are extensive, indicating unusually high crystallization temperature related to high pressure; antiperthites in the Giles Complex are amongst the most calcic reported for terrestrial rocks. (3) The lower primitive cumulate units of the Complex are coronitic and feature a variety of subsolidus high-pressure reaction textures; olivine and cumulus chromite have reacted with calcic plagioclase to orthopyroxene-clinopyroxene-spinel, olivine-spinel, and clinopyroxene-spinel symplectites. The principal reaction mechanism for the symplectites was continuous mass transfer of alumina from plagioclase toward spinel, as the Complex passed from the olivine-plagioclase stability field into the pyroxene-spinel field during cooling. Geothermometers applicable to the cumulates record a wide range of equilibration temperatures from late-magmatic to granulite-metamorphic conditions. FeMg1 exchange gives closure temperatures around 600–700°C, whereas Al2Mg1Si1 net-transfer equilibria have preserved higher temperatures around 750–900 °C. Defocused beam bulk analyses of exsolved cumulus clinopyroxenes and intercumulus plagioclases recover magmatic compositions; i. e., two-pyroxene solvus CaMg-1 temperatures plot around 1120±50°C, whereas two-feldspar thermometers give 1200°C. Pressures are calculated from thermochemical data with the heterogeneous equilibria 2 fo + an = en + di + sp, fo + an = di + Mg-Ts, and fo + an = en + Ca-Ts, after