HYBRIDIZATION OF SHOSHONITIC LAMPROPHYRE AND CALC-ALKALINE GRANITE MAGMA IN THE EARLY PROTEROZOIC MT BUNDEY IGNEOUS SUITE, NORTHERN-TERRITORY

The Mt Bundey igneous suite (1831 +/- 6Ma) intrudes deformed and low-g rade metasedimentary rocks of the Early Proterozoic Pine Creek Inlier. The suite consists of a composite pluton of calc-alkaline granite (Mt Bundey Granite) and shoshonitic syenite (Mt Goyder Syenite), and a sw arm of spatially, temporally and probably genetically associated, K-ri ch shoshonitic lamprophyre dykes. Grey, ovoid to tabular lamprophyric enclaves, up to about Im in length, are widespread within the syenite. The enclaves have the same mineralogy as, and similar mineral and who le-rock compositions to, the lamprophyre dykes. These features, in con junction with textural studies, imply that the enclaves represent lamp rophyre magma injected into a partly crystalline syenite magma. Lampro phyre dykes, lamprophyric enclaves and syenite contain numerous, widel y dispersed uralitized clinopyroxenite enclaves 0.5-5.0 mm in diameter , suggesting that magma mingling was preceded by hybridization of lamp rophyric and granitic magmas. The syenite, granite and lamprophyre dyk es have near-identical trace element abundance patterns on MORB- and c hondrite-normalized plots, and define linear arrays on homologous elem ent plots. Least-squares mixing calculations indicate that the syenite could have been generated by hybridization of lamprophyric and granit ic magmas, with some superimposed accumulation of K-feldspar. Hybridiz ation probably occurred at or near the source region of the granite ma gma precursor to the Mt Goyder Syenite, with further injection of lamp rophyre magma at depth of emplacement.