Chemical and isotopic studies of the Wateranga layered mafic intrusion, southeast Queensland, Australia: magma sources and petrogenesis

Abstract

This research was undertaken in order to thoroughly investigate the geochemistry of the Wateranga mafic layered intrusion in southeast Queensland, Australia. Nd-Sr-0 isotopic, whole-rock major and trace element and mineral chemical studies of the Wateranga intrusion were used to understand the nature of mantle sources and mafic magma genesis during a post-compressional extensional regime.<br/>The Wateranga layered mafic intrusion (28 km²) (>500 m thick) is a tholeiitic, undeformed, unmetamorphosed, Permo-Triassic layered gabbroic pluton intruded into the Late Carboniferous Goodnight beds of the Goodnight Block in southeast Queensland. The intrusion mainly consists of gabbro and norite, associated with subordinate amounts of troctolite, anorthosite and orthopyroxenite, and rare picrite. Olivine gabbro is the dominant rock type of the intrusion. The whole-rock Sm-Nd isochron yields an age of 261±21 Ma. Petrographic, mineral chemical, whole-rock geochemical and Nd-Sr-0 isotopic data have been used to divide the intrusion into Lower, Middle and Upper Zones, which are interpreted as reflecting magma chamber replenishment. Although the general crystallization order of minerals is olivine, plagioclase, clinopyroxene, orthopyroxene, Fe-Ti oxides, hornblende and biotite, considerable differences exist between the Zones. The observed changes in the crystallization order between the Zones reveal that a single parental magma was inadequate to explain the data. The common differentiation indices, such as An content of plagioclase, Mg#s of olivine, clinopyroxene, orthopyroxene and whole-rocks, and the whole-rock concentrations of various incompatible trace elements (Zr, Y, Nb, La, Ba, Rb, Sr, and Nd) and Nd-Sr-0 isotopic compositions, all vary widely with stratigraphic depth and display abrupt shifts at the Zone boundaries, indicating open-system addition of new mafic magma. Litho- and chemo- stratigraphic analyses of the Wateranga intrusion show that it is the product of at least three major magma pulses.<br/>Mineral chemical and whole-rock geochemical data indicate that fractional crystallization played an important role in the magmatic processes. However, sharp discontinuities and contrasting fractionation trends between the individual stratigraphic Zones of the intrusion suggest polycyclic fractionation of three major batches of magma. Fractionation followed tholeiitic trends with iron enrichment in the liquids. Rare earth element distributions indicate more than 5% partial melting of the mantle source with limited amounts of residual garnet. Finely disseminated sulphides occur throughout the intrusion. Textural and compositional evidence indicate that the disseminated Fe-Ni-Cu sulphides and platinum group elements are of magmatic origin.<br/>Microprobe analyses of coexisting clinopyroxene and orthopyroxene in different rocks of the intrusion provide consistent P-T data defining the magmatic crystallization condition as 1057 - 927 °C. During the course of crystallization pressure probably was greater than 2 and less than 4 kbar. Whole-rock initial ε_Nd (3.26 - 6.44) and initial ⁸⁷Sr/⁸⁶Sr (0.7026 - 0.7049) compositions, chondrite normalized REE patterns and the variation trend of anorthite content of plagioclase versus the forsterite content of olivine precludes an arc-related magma source. The composition and geological setting of the intrusion are consistent with emplacement in a post-subduction extensional tectonic environment.<br/>The parental magmas to the Wateranga intrusion are olivine tholeiitic, derived from an asthenospheric mantle source in response to lithospheric extension during the Permo-Triassic (245±8Ma). Olivine tholeiitic magma, already contaminated by lower continental crust, was initially pooled in late Carboniferous sediments where the magma chamber evolved by fractional crystallization and periodic replenishment. Crustal assimilation was limited (2 to 8%).