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

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.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.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.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.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 εNd (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.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%)

Geochemistry and Nd-isotope systematics of chemical and terrigenous sediments from the Dun Mountain Ophiolite, New Zealand

Two sedimentary associations closely related to temporally discrete ophiolitic magma suites occur within the Early Permian Dun Mountain Ophiolite Belt (DMOB), New Zealand. These are: (1) a suite of diverse chemical sediments and turbidite argillites (TA) that bear an intimate depositional relationship to early-formed pillow lavas (back-arc basin basalts); and (2) a younger, lithic-dominated, bimodal, coarse sandstone–ophiolitic rudite assemblage of proximal turbidite/mass-flow origin, rich in clasts of infant-arc magmas which comprise the bulk of the ophiolite. There are four groups of DMOB chemical sediments. Red hematitic chert (group 1) fills interstices between basalt pillows, and black nodular Fe-Mn deposits (group 2) occur along pillow lava/sediment interfaces. These facies are overlain by red mudstones (group 3), and mottled orange-olive brown mudstones (altered hyaloclastites; group 4). Geochemical features (including REE contents, Fe/Ti and transition metal ratios) indicate that the cherts reflect silica and metalliferous contributions to sea water, promoted by low-temperature hydrothermal alteration of glassy basalt, while group 4 muds represent residual components in halmyrolytically altered volcanic glass. Transition metal and REE enrichments in group 2 nodules (with high Ce/Ce*, Ni/Fe, and Cu/Fe ratios) reflect hydrogenous chemisorption to a hydrothermal component (with high Ba and Sr). The nodules possess ε[Nd](T) values (c. 0) identical to those calculated for Permian sea water. Group 3 red muds have lower ε[Nd](T) = –1 to –2, and Nd model ages (T[Nd]DM) that indicate contributions from continentally derived fluvial particulate fallout of mean Proterozoic age. For the nodules and red muds, strong negative correlations between Mn/Fe, Nd, Ce*/Ce, and ε[Nd](T) are attributed to increasing diagenetic influence in the muds. ε[Nd](T) values (c. +2) in group 4 muds are transitional toward higher values in their (hyaloclastite) basalt glass precursors. Metalliferous contributions to red and green TA in the overlying terrigenous sedimentary sequence also link these facies to early DMOB back-arc eruptives. The red and green TA pass upwards into ungraded Atomodesmid-bearing grey TA, implying turbidite deposition in a shallowing marine environment. The (mainly andesitic) TA show systematic trends of decreasing Eu/Eu* and ε[Nd](T) with increasing ΣREE, La/Y, Th/Sc, and SiO₂. Th abundances and Th/Sc ratios (up to 1.6) in the TA are akin to those of calc-alkaline magmas in continental arcs, and are significantly greater than island-arc or ophiolitic volcanics. A narrow range of positive ε[Nd](T) values (+0.5 to +2.0) for the TA suggests a young differentiated continental arc source, less dissected than the quartzofeldspathic plutonic provenance for the Torlesse (ε[Nd](T

Mineral chemistry, petrogenesis, and tectonic setting of the Wateranga layered intrusion, Southeast Queensland, Australia

The Wateranga layered mafic intrusion (28 km² in area, > 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. Fractionation followed a tholeiitic trend with iron enrichment in the liquid. Petrographic, mineral chemical, and whole-rock geochemical data have been used to divide the intrusion into Lower, Middle, and Upper zones, which are interpreted as reflecting magma chamber replenishment. The observed changes in the crystallization order between the zones reveal that a single parental magma is 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), all vary widely with stratigraphic depth and display abrupt shifts at the zone boundaries, indicating open system addition of new mafic magma. Temperatures estimated from two-pyroxene geothermometer vary from 1057 to 927°C. During the course of crystallization, pressure probably was > 2 and 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. Fractionation followed a tholeiitic trend with iron enrichment in the liquid. Petrographic, mineral chemical, and whole-rock geochemical data have been used to divide the intrusion into Lower, Middle, and Upper zones, which are interpreted as reflecting magma chamber replenishment. The observed changes in the crystallization order between the zones reveal that a single parental magma is 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), all vary widely with stratigraphic depth and display abrupt shifts at the zone boundaries, indicating open system addition of new mafic magma. Temperatures estimated from two-pyroxene geothermometer vary from 1057 to 927°C. During the course of crystallization, pressure probably was > 2 and 2 an

The Fox Tor Diorite, a newly recognised intrusion within the New England Batholith, northern New South Wales

The Fox Tor Diorite is a newly recognised intrusion with an I‑type intermediate composition. It is totally enclosed by the S‑type Pringles Monzogranite, a member of the Bundarra Supersuite of the New England Batholith. The intrusion was initially detected by its strong aeromagnetic signature and subsequent mapping showed that there are six separate outcrops covering 84 hectares within an area of 2.6 km north–south and 1.2 km east–west. Field relationships imply intrusive contacts against the Pringles Monzogranite, with localised contamination of the diorite at the contact. Modelling of magnetic data indicates that the intrusive mass is steep‑sided and extends to considerable depth. K–Ar geochronology on the Fox Tor Diorite gave an age of 239.7±6.7 Ma, a result that overlaps with age determinations for the Uralla and Moonbi Supersuites in the southern New England Orogen, but is approximately 50 Ma younger than granitoids of the Bundarra Supersuite. Rocks of the Fox Tor Diorite are medium‑grained, with early crystallised orthopyroxene, clinopyroxene and plagioclase, and subsequent crystallisation of minor hornblende, biotite, K‑feldspar and quartz. The intrusion is characteristically magnetic, with susceptibilities of 900×10‑5 to 2800×10‑5 SI. Although ilmenite is present, magnetite is more abundant and the magnetic (and oxidation state) characteristics are more typical of the Moonbi Supersuite granitoids, rather than those of the Uralla Supersuite. On the other hand, geochemical characteristics of the Fox Tor Diorite, such as contents of K2O, P2O5, Rb, Sr, Nb and Zr, accord better with mafic members of the Uralla Supersuite. Geochemical and mineralogical criteria indicate that the Fox Tor Diorite magma had a significant mantle‑derived component and that it has undergone fractionation, perhaps largely by precipitation of pyroxenes and possibly plagioclase

Cainozoic igneous rocks in the Bingara to Inverell area, northeastern New South Wales

Three Cainozoic intraplate volcanic suites in the Bingara to Inverell area, northeastern New South Wales, have been discriminated on the basis of differing geophysical responses and contrasting K–Ar ages. Major isotopic and chemical characteristics can also be used to distinguish the three suites. These newly defined suites are the Middle Eocene–Early Oligocene Maybole Volcanic Suite; the Late Oligocene–Early Miocene Delungra Volcanic Suite; and the Middle Miocene Langari Hill Volcanic Suite. Four basaltic volcanic units within the Delungra Volcanic Suite have also been distinguished: Mount Russell Volcanics; Derra Derra Volcanics; Inverell Volcanics; and Bingara Volcanics. The Maybole Volcanic Suite is dominated by mafic volcanic rocks of alkaline affinity. These rocks include hawaiite, transitional basalt, basanite and rare phonolite (not included in this study). Volcanogenic and non-volcanogenic sedimentary units are minor but significant components, hosting world-class concentrations of sapphires in the Inverell–Glen Innes region. The Maybole Volcanic Suite occupies the eastern portion of the study area, forming ridges that outline the radial drainage pattern of the deeply eroded Eocene–Oligocene Maybole shield volcano. The Delungra Volcanic Suite is geochemically diverse and consists of alkaline members (Inverell and Bingara Volcanics) and tholeiitic members (Mount Russell and Derra Derra Volcanics). These are dominated by mafic lava flows with minor interflow sedimentary horizons. The Delungra Volcanic Suite forms broad elevated plains and prominent plugs in the central and western portions of the study area. Diamond occurrences in the Bingara district are spatially associated with the Bingara and Derra Derra Volcanics. The Langari Hill Volcanic Suite consists of a mafic tholeiitic lava flow that is spatially restricted to a prominent east–west ridge east of Inverell overlying the Maybole Volcanic Suite. The Langari Hill Volcanic Suite is significantly younger than the Maybole and Delungra Volcanic suites and represents the youngest recognised volcanic episode in the Bingara–Inverell area