PETROLOGY OF THE STRATHBOGIE BATHOLITH: A CORDIERITE-BEARING GRANITE

The Strathbogie batholith is a high-level. discordant, composite granitoid intrusion in southeastern Australia. Granitic rocks containing magmatic garnet, cordierite and biotite predominate in the batholith. Intrusion involved marginal faulting. roof lifting and "shouldering aside" of the country rocks. which include the Violet Town volcanic complex of similar Late Devonian age. Textural evidence indicates early crystallization of biotite together with garnet, plagioclase (similar to An(45)) and some quartz. The bulk of the quartz, cordierite and K-feldspar crystallized later during emplacement. Early magmatic cfystallization occurred with P >= 4 kbar, T >= 800 degrees C and water contents in the magma of about 4 wt, %. Thus the Strathbogie magma was relatively hot and water-undersaturated. Emplacement occurred at P similar to 0.5 kbar with initial temperatures well in excess of the wet granite solidus. Oxygen fugacity was < QFM throughout the magma's history. The Strathbogie rocks have SiO, contents in the range 70-76 wt. % with high K/ (K + Na + Ca), normative corundum around 2 wt, % and Mg/(Mg + Fe) = 0.4-0.5. CaO. MgO, TiO2 and P2O5 decrease linearly as SiO2 increases. These chemical trends cannot he fully explained by removal or variable incorporation of restite materials, as inclusions of any kind are scarce in all variants. Differentiation mechanisms involving separation and accumulation of early magmatic biotite. plagioclase and accessory phases are favored over batch partial-melting processes to model the chemical variation. Magma chemistry, together with inferred early magmatic conditions, and the mineralogy of inclusions in the granites suggest initial formation of the Strathbogie magma by partial fusion of biotite-bearing quartzofeldspathic and pelitic rocks under granulite-facies conditions. These source rocks are apparently below the presently exposed lower Paleozoic sequences of southeastern Australia. The transitional orogenic-anorogenic setting of the batholith is compatible with the partial-fusion event having resulted from intrusion of mafic magma into the deep crust.