The rare earth element geochemistry of granite,gneiss, and migmatite from the Western Metamorphic Belt of South-eastern Australia

Four muscovite-biotite granites from the Western Metamorphic Belt of South-eastern Australia have rare earth element patterns characterized by: (i) light rare earth element enrichment; (ii) slight Eu depletion; (iii) varying degrees of heavy rare earth element depletion. The rare earth element and major element chemistry of three of these muscovite-biotite granites (the Koetong, Lockharts and Yabba Granites) can be approximated very closely by a model involving 20% partial melting of an ultrametamorphosed pelitic sediment and contamination of this minimum melt by the residual material left after melting, in the ratio 60% melt: 40% residue. Granitoids can be very largely solid material at the time of emplacement.The other muscovite-biotite granite studied (the Hawksview Granite) has major and trace element characteristics which distinguish if from the other three granitoids and these differences are attributed to variations in source material at the site of melt generation.The rare earth element and major element chemistry of a garnet-cordierite gneiss from the Western Metamorphic Belt can be modelled assuming 5% partial melting of a pelitic metamorphic rock and contamination of the minimum melt by the residue in the ratio 30% melt: 70% residue.Separated granitic and biotitic portions of a migmatite from the Western Metamorphic Belt have rare earth element characteristics which are inconsistent with a simple partial-melting model, but it is suggested that re-equilibration following, or during, separation of the vein material could obscure the process by which the vein of the migmatite developed. It is however certain that the vein developed in situ from a pelitic meta-sediment leaving the biotite rich selvage, without the introduction of material from an external source.Leucogranites which crop out to the east of the Western Metamorphic Belt are high level intrusions of highly fractionated granitic melt. Their Sr isotopic characteristics and features of their major and trace element chemistry suggest that they derive from an igneous source and are not directly related to the granites and gneisses to the west.