Petrology and geochemistry of igneous rocks in the Comboyne Plateau - Lorne Basin area, New South Wales

Mineralogical and geochemical data indicate that the Igneous rocks in the Lome Basin - Ccmboyne Plateau area on the midnorth coast of New South Wales consist primarily of alkaline and calc-alkaline types, with minor occurrences of tholeiitic andesite. Alkaline rocks are centred on the Comboyne Plateau, and include hawaiite, mugearite, benmoreite, mafic trachyte, anorthoclase trachyte and comendite. The rocks form domes, plugs and flows and there is a dominance of intermediate and silicic types. There are also rare occurrences of tholeiitic andesite on the Comboyne Plateau. The calc-alkaline rocks are predominantly high level intrusive granodiorites and granites and extrusive rhyolites. More basic types include basaltic andesite and diorite. Data presented in this study indicate that the Comboyne alkaline rocks represent a mildly alkaline-transitional association in which strong fractionation has resulted in the formation of peralkaline rhyolites. Geochemically the Comboyne association has tholeiitic affinities with normative hypersthene occurring in all rock types. Mineralogically they are alkaline, although analyses and normative components for clinopyroxenes in the more basic rocks confirm the mildly alkaline-transitional character of the host rocks. Textural and chemical criteria suggest that megacrysts and/or cumulates of orthopyroxene, olivine, clinopyroxene, plagioclase, ilmenite, titenomagnetite and apatite in the hawaiites, and less commonly, in the mugearites, crystallised at elevated pressures. Similarly, crystal aggregates of anorthoclase, olivine, edenitic amphibole, Fe-Ti oxides and apatite in the benmoreites indicate that these also formed at depth and that fractionation in nearsurface magma chambers was not significant, at least in obtaining compositions as evolved as benmoreite. lOOMg/Mg + Fe²⁺ valuss of less than 50 in the hawaiites indicate that these rocks do not represent primitive mantle melts, and that fractionation of magnesium-rich phases occurred prior to the crystallisation of the present megacryst/cumulate assemblage. The derivation of the hawaiites at elevated pressures is supported by experimental work undertaken in this study. Ths near-duplication of the natural megacryst/cumulate assemblage in runs at 6.5 kb with 2% H₂O indicates that the hawaiite megacryst/ cumulate assemblages precipitated at a depth of 20 - 24 km before rapid ascent to higher crustal levels. It is suggested that megacrysts of orthopyroxene and olivine, and gabbroic aggregates of plagioclase, clinopyroxene, olivine and ilmenite, in the Comboyne tholeiitic andesite precipitated at pressures up to 6 kb. This is supported by mineralogical and experimental data on similar compositions from other basaltic associations. Both major and trace element data indicate that the Lorne Basin granodiorites, granites and rhyolites are not genetically related to the more basic diorites and basaltic andesite. Partial melting of andesitic material in the crust best explains the origin of the silicic rucks, but the relatively high MgO, FeO and CaO in the diorites and basaltic andesite points to their origin from the mantle by a two-stage melting process.