R. H. Flood

Geochemical character and tectonic significance of Early Devonian keratophyres in the New England Fold Belt, eastern Australia

Extrusive and high level intrusive Early Devonian keratophyres are the oldest in situ igneous rocks in the Tamworth Block of the New England Fold Belt of eastern Australia. They show extensive evidence of degradation, including the destruction of magmatic phases, the growth of low grade metamorphic minerals, and changes in composition involving the dilution of elemental abundances in response to silica addition. Relations between the less mobile minor and trace elements, and limited data on clinopyroxene compositions, lead to the conclusion that these Early Devonian volcanic rocks are mostly calc‐alkaline volcanic arc andesites with minor dacite. These rocks unconformably overlie a sequence of Early Palaeozoic forearc basin deposits, indicating that the Early Devonian marks a period of readjustment of tectonic elements within the New England Fold Belt, associated with a marked east‐directed stepping out of the magmatic arc. Generation of the keratophyres in a subduction zone environment limits the positio...

Eclogite from serpentinite near Attunga, New South Wales

Abstract Eclogite of high‐pressure low‐temperature origin occurs within the Great Serpentine belt of New South Wales. The presence of glaucophane‐bearing rocks and other medium to high‐pressure assemblages associated with the belt is similar in many respects to the Californian and Oregon occurrences. The chemical composition of the eclogite is characterized by low K2O values comparable to many oceanic tholeiites, although one analysis is nepheline‐normative. Ti‐Zr‐Y ratios also show affinities to ocean‐floor basalts. The garnet contains approximately 30% grossular and is strongly zoned from almandine (Alm 56%, Py 9%) at the core towards pyrope (Alm 44%, Py 27%) at the margin. Sodic augite contains 30–33% Jd, 4–7% Ac, and 72–74% Di+He. Distribution of Fe and Mg between co‐existing garnet and pyroxene would suggest an increasing temperature during eclogite crystallization with a possible range from 290°C to 600°C and a minimum pressure of 7–12 kb.

The Wologorong Batholith, New South Wales, and the extension of the I‐S line of the Siluro‐Devonian granitoids

Abstract New data for the Wologorong Batholith, northwest of Goulburn, New South Wales, indicate that it is S‐type and probably similar in age to the major batholith in the southern part of the Lachlan Fold Belt. Its component rocks are predominantly biotite and biotite‐muscovite adamellite, and have a strong N‐S metamorphic foliation. An isochron, defined by fourteen rocks, gives a Rb/Sr age of 405 ±11 Ma and an initial 87Sr/86Sr ratio of 0.7110 ± 0.0012. Five Rb/Sr biotite ages range between 339 and 381 Ma, the older ages being obtained from the least deformed parts of the Batholith. Whereas uncertainties inherent in dating a deformed S‐type batholith make 405 Ma a reconnaissance value only, the minimum emplacement age for the Batholith, as determined from two biotites, is around 381 Ma. The younger biotite ages, around 339 to 348 Ma, are considered to date a major period of regional metamorphism in the northeastern part of the Lachlan Fold Belt, similar to that recorded from K/Ar ages of metamorphic bi...

Low‐K tholeiites and high‐K igneous rocks from Woodlark Island, Papua New Guinea

Abstract Woodlark Island, the largest above‐sea portion of the Woodlark Rise, has an exposed basement of pre‐Miocene (?Cretaceous‐Eocene) low‐K tholeiitic basalt and dolerite, and minor sediments. The basement is unconformably overlain by Early Miocene limestone and volcaniclastic sediments and later Miocene high‐K volcanics and comagmatic intrusives. Pleistocene to Recent sediments partly blanket the Tertiary sequence. Basement low‐K tholeiites vary only slightly in composition and are interpreted as ocean floor or possible marginal basin material. The high‐K suite appears to be chemically similar to late Tertiary to Recent high‐K igneous rocks of mainland Papua New Guinea. It includes porphyritic hornblende‐, clinopyroxene‐, biotite‐ and magnetite‐bearing shoshonite, latite and toscanite, and intrusive equivalents that range from olivine normative to strongly quartz normative compositions (S1Q2 46% to 75%). Computer mixing models indicate that separation of the pheno‐crysts in the shoshonites, particula...

Upper crustal structure of the Tamworth Belt, New South Wales: Constraints from new gravity data

New gravity data along five profiles across the western side of the southern New England Fold Belt and the adjoining Gunnedah Basin show the Namoi Gravity High over the Tamworth Belt and the Meandarra Gravity Ridge over the Gunnedah Basin. Forward modelling of gravity anomalies, combined with previous geological mapping and a seismic-reflection transect acquired by Geoscience Australia, has led to iterative testing of models of the crustal structure of the southern New England Fold Belt, which indicates that the gravity anomalies can generally be explained using the densities of the presently exposed rock units. The Namoi Gravity High over the Tamworth Belt results from the high density of the rocks of this belt that reflects the mafic volcanic source of the older sedimentary rocks in the Tamworth Belt, the burial metamorphism of the pre-Permian units and the presence of some mafic volcanic units. Modelling shows that the Woolomin Association, present immediately east of the Peel Fault and constituting the most western part of the Tablelands Complex, also has a relatively high density of 2.72 – 2.75 t/m3, and this unit also contributes to the Namoi Gravity High. The Tamworth Belt can be modelled with a configuration where the Tablelands Complex has been thrust over the Tamworth Belt along the Peel Fault that dips steeply to the east. The Tamworth Belt is thrust westward over the Sydney – Gunnedah Basin for 15 – 30 km on the Mooki Fault, which has a shallow dip (∼25°) to the east. The Meandarra Gravity Ridge in the Gunnedah Basin was modelled as a high-density volcanic rock unit with a density contrast of 0.25 t/m3 relative to the underlying rocks of the Lachlan Fold Belt. The modelled volcanic rock unit has a steep western margin, a gently tapering eastern margin and a thickness range of 4.5 – 6 km. These volcanic rocks are assumed to be Lower Permian and to be the western extension of the Permian Werrie Basalts that outcrop on the western edge of the Tamworth Belt and which have been argued to have formed in an extensional basin. Blind granitic plutons are inferred to occur near the Peel Fault along the central and the southern profiles.

Flow‐field and palaeogeographic reconstruction of volcanic activity in the Permian Gerringong Volcanic Complex, southern Sydney Basin, Australia

The Broughton Formation, the basal part of the lower Upper Permian (ca 264–263 Ma) Gerringong Volcanics of the Kiama area of the southern Sydney Basin, comprises intercalated shoshonitic lava flows and shallow‐marine, predominantly volcaniclastic, sediments emplaced under periglacial climatic conditions at high palaeolatitude. Four lower members of the Gerringong Volcanics (the Westley Park Sandstone, Blow Hole Latite, Kiama Sandstone and Bumbo Latite Members) were examined to elucidate the lava‐flow directions and coeval palaeophysiography. Field evidence indicates that the individual flows that comprise the compound‐flow‐units of the Blow Hole and Bumbo Latites advanced toward the north‐northwest and north, respectively, from an emergent island volcano or volcanic archipelago that developed offshore of the present‐day coastline, some tens of kilometres south‐southeast of Kiama. The field evidence incorporates descriptions and interpretations of features including vesicles and amygdales, lava tubes, pillow lavas and contact relationships between coherent lavas and sediments and brecciated lava. The similarity of the coherent lavas and volcaniclastic components in the intercalated sediments and in the underlying uppermost Berry Siltstone indicate that the lavas and the volcaniclastic material shared a common source. Lava flows and epiclastic deposits contributed to the formation of the volcanic edifice, as probably did hyaloclastic and pyroclastic and reworked pyroclastic deposits. Much fragmental volcanic material was dispersed into the local shallow‐marine environment by mass flows that were generated by a variety of mechanisms on the volcanos subaerial and subaqueous slopes. Redistribution of the volcanogenic sediment and erosion of the lava flows was influenced by longshore marine currents that flowed predominantly northwards across the basin floor. The Gerringong Volcanics constitute the oldest onshore‐preserved record of the regional onset of volcanolithic sediment influx into the Sydney Basin from the embryonic New England Orogen and herald the commencement of its foreland‐basin phase of development.

Two occurrences of ultramafic hornfels in the Biggenden Beds, southeastern Queensland

Abstract Ultramafic hornfelses containing the assemblages hornblende + olivine + spinel + magnetite, and clinopyroxene + olivine + spinel + magnetite, are reported from two localities in the Biggenden Beds in southeastern Queensland. They are associated with mafic hornfelses in the contact metamorphic aureoles of the Mungore Adamellite and the Wateranga Gabbro. Chemical composition and minerology of the olivine + amphibole + spinel + magnetite hornfelses suggest that they represent metamorphosed picritic rocks, or possibly, altered serpentinites (blackwall rocks), whereas the clinopyroxene + olivine + spinel + magnetite hornfelses are interpreted as metamorphosed altered clinopyroxene‐rich picritic rocks. Cr‐Fe spinel relations in the hornfelses indicate partial homogenisation of primary chromian spinel with secondary magnetite ± ferrichromite during contact metamorphism.

Ben Bullen plutons, New South Wales: A Carboniferous gabbro-trondhjemite suite

The Ben Bullen plutons constitute a small medium-K gabbro-tonalite-trondhjemite suite spatially and temporally associated with extensive adamellite and granite of the Bathurst Batholith. The mafic composition of the bulk of the Ben Bullen rocks suggests a mantles source for the parent magma. Chemical and petrographic studies indicate that the rocks are closely comparable with medium-K calc-alkaline volcanic suites in island arcs. The Ben Bullen series are M-type granites in the sense of being formed by crystal fractionation of a gabbroic parental magma. Initial crystallization and separation of olivine and pyroxene followed by marked fractionation of hornblende probably drove the differentiating magma along a typical calc-alkaline trend of strong alkali-enrichment. It is possible that the rise of these mantle-derived melts into the crust initiated the crustal melting that produced the associated felsic K-rich granitoids of the Bathurst Batholith.

Geophysical characterisation of a blind I-type pluton emplaced within the Bundarra Suite S-type granites of the New England Batholith

Geophysical data are presented that characterise a blind pluton, the Mountain Home Pluton (MHP), which intrudes the southern portion of the Bundarra Suite (BS), 30 km northeast of Bendemeer, New South Wales. A positive magnetic anomaly within the non-magnetic granites of the BS (Banalasta and Pringles Monzogranites) was previously identified as a sub-surface intrusion. Interpretation of new gravity data and analysis of aeromagnetic data are used to infer the depth, size, density, magnetic susceptibility and likely petrology of the pluton. The best-fit model indicates that the MHP is very similar to the Looanga Monzogranite, a felsic member of the Moonbi Suite of the New England Batholith (NEB) that intrudes the BS 5–7 km southeast of the MHP. The top of the MHP is inferred to lie about 1 km beneath the surface and the pluton extends to a depth of at least 6 km. Our model furthermore suggests that the southwestern margin of the MHP is subvertical, whereas a shallower dip (<45°) towards the north is proposed for the northeastern surface of the pluton. A north-trending dyke swarm, identified on the basis of linear positive magnetic anomalies, may be related to the MHP. This swarm of more than 20 relatively magnetic dykes extends out to about 10 km north from the pluton. Magnetic modelling of the dykes indicates that susceptibility values of the dykes are probably very similar to the range of the MHP, and also suggests the width of individual dykes (also not known to be exposed at the surface) to be at most a few tens of metres. A petrographic examination of the intruded BS granites at the surface suggests that metamorphic zoning as seen in mineralogical characteristics may be related to the underlying pluton.

Trace-element correlation of Carboniferous ignimbrites, southern New England Orogen, Australia, by factor and discrimination analysis

Factor and discrimination analysis of trace-element abundances in Carboniferous (Visean to possibly Westphalian) ignimbrites and minor lavas from a forearc basin succession of the southern New England Orogen, are examined to test the validity of proposed stratigraphic correlations between ignimbrites in three adjacent thrust sheets in the Rocky Creek region of the northern Tamworth Belt, northwestern New South Wales. Analyses of the ignimbrites within individual fault blocks indicate that each has a signature that can be distinguished with a high level of statistical confidence from those of the other ignimbrites. Elements that are generally the most distinctive are Nb, Y, Zr and V. A comparison of units from the Boomi and Kathrose blocks and the southern part of the Rocky Creek Syncline indicates that an ignimbrite in the lower Clifden Formation, previously informally named the ‘Eastons Arm Rhyolite Member’, belongs to the Peri Rhyolite Member. The Peri Rhyolite Member marks the base of the Kiaman reversal. In the Kathrose block, an ignimbrite, previously mapped as Peri Rhyolite Member, is removed from the latter unit and assigned to unnamed dacitic ignimbrite Cl6. In terms of trace-element content, all of the sampled ignimbrites from the Rocky Creek Conglomerate have individual signatures and are unlikely to belong to the same unit. These include: the Plagyan Ignimbrite Member from the Boomi block, Rocky Creek Syncline and southern Nandewar region, the Mt Hook Rhyolite, Darthula Rhyodacite and Pound Rock Rhyodacite Members from the Darthula block, and unnamed red ignimbrite members (including R8 and R12) from the southern part of the Maules Creek Anticline. The Wanganui Andesite Member and unnamed dacitic ignimbrite Cl13 in the lowermost Clifden Formation, and the Ermelo Pyroclastics are distinctive in terms of trace-element signatures. The Wanganui Andesite Member is the youngest, normally polarised ignimbrite beneath the base of the Kiaman reversal.