PETROGENESIS OF PLUTONIC ROCKS IN A PROTEROZOIC GRANULITE-FACIES TERRANE - THE BUNGER HILLS, EAST ANTARCTICA

Several compositionally varied suites of plutonic rocks (gabbro-quartz monzogabbro, granite, quartz monzodiorite) in the Bunger Hills were emplaced at depths of approximately 20 km over a relatively short time interval (1170-1150 Ma) during the waning stages of granulite-facies metamorphism, the peak of which occurred approximately 1190 Ma ago. Alkaline (syenite to granite) rocks were emplaced west of the Denman Glacier at David Island approximately 515 Ma ago. Most of these rocks crystallised from magmas derived by melting of heterogeneous mantle comprising at least two components-Nb-poor, long-term enriched lithosphere, and a Nb-rich OIB (ocean island basalt) -type within-plate component. Metasomatic enrichment of the source of the plutonic rocks, as well as that of 500-Ma alkaline dykes, may have been contemporaneous with continental crust formation in the area during the late Archaean or Early Proterozoic. Much of the chemical variation of the different suites' can be explained by fractional crystallisation, but other processes, particularly variations in the extent and/or P-T conditions of melting were also important. Crustal contamination does not appear to have been a major factor in the petrogenesis of the Bunger Hills magmas, but cannot be entirely discounted, particularly for some of the granitic rocks; the more evolved David Island syenitic intrusives may have been derived by melting of lower-crustal rocks.