Petrogenesis of Mesoproterozoic K-rich granitoids, southern Mt Angelay igneous complex, Cloncurry district, northwest Queensland

The K-rich granitoids of the southern Mt Angelay igneous complex belong to the younger phases of the Williams and Naraku Batholiths (<1540 Ma) in the Cloncurry district. Granitoids of the complex form a series of I-type, K-ri ch, metaluminous monzodiorite to subaluminous syenogranite. These intrusion s have geochemical affinities akin to 'A-type' granites and contain plagioc lase, alkali feldspar, quartz, biotite, hornblende and typically accessory magnetite, titanite, apatite and zircon. With increasing SiO2 the granitoid s vary from alkaline to subalkaline, and exhibit a decrease in TiO2, Al2O3, Fe2O3*, MnO, MgO, CaO, P2O5, Cu, Sr Zr, LREE and Eu, with an increase in N a2O, K2O, Rb, Pb, Th, U, Y and HREE. This suite of relatively oxidised gran itoids (<1.0 log units above NNO) were emplaced after the peak of metamorph ism and pre- to post-D-3, a major east-west horizontal-shortening event. Th e synchronous emplacement of high-temperature mafic (>960 degrees C) and fo liated felsic (>900 degrees C) granitoids formed zones of mingled and mixed monzonite and quartz monzonite to monzogranite containing abundant rapakiv i K-feldspar. These intrusions are interpreted to have been derived from so urce rocks of different compositions, and probably by different degrees of partial melting. The unfoliated felsic granitoids are considered to represe nt the fractionated equivalents of older foliated felsic granitoids. All gr anitoids possess a Sr-depleted and Y-undepleted signature, which suggests t hat the source material probably contained plagioclase and no garnet, restr icting magma production to (800-1000 MPa (similar to 24-30 km). Underplatin g of mantle-derived mafic material into mid-crustal levels is considered th e most viable mechanism to produce these high-temperature K-rich granitoids at these pressures. The composition of the felsic granitoids is consistent with derivation from a crustal source with a tonalitic to granodioritic co mposition. However, the mafic granitoids require a more mafic, possibly gab broic source, which may have been supplemented with minor mantle-derived ma terial. These granitoids are also enriched in Th, U, LREE and depleted in B a, Ti, Nb and Sr and compare closely to the Mesoproterozoic granitoids of t he Gawler Craton. The economic significance of these styles of granitoids m ay also be highlighted by the close spatial relationship of hematitic K-fel dspar, magnetite, fluorite and pyrite-rich veins, alteration and filled mia rolitic cavities with the least-evolved felsic intrusions. This style of ve ining has a probable magmatic origin and is similar to the gangue assemblag e associated with Ernest Henry-style Fe-oxide-(Cu-Au) mineralisation, which suggests that these granitoids represent prospective sources of fluids ass ociated with Cu-Au mineralisation in the district.