England Orogen Research Articles

Disputed tectonic setting for the late Silurian–early Carboniferous development of the northern New England Orogen: detrital zircon ages suggest a continental margin arc association

Devonian successions in the Calliope and Yarrol provinces of the northern New England Orogen in central to northern Queensland have a lithological assemblage and depositional environments indicative of a close association with a subduction-related volcanic arc. These rocks have been interpreted as related to island arc and backarc settings based on geochemical characteristics of magmatic rocks. We present new detrital zircon ages from the Devonian to lower Carboniferous successions of the Calliope and Yarrol province successions. Most samples have no or relatively few zircon grains extracted except for samples of quartz-rich sandstone in the Middle Devonian Erebus beds of Hunter Island, located 160 km north-northwest of Rockhampton and another sample in the Rockhampton district. The samples with abundant zircon, and some with less common zircon, have prominent peaks of mid Cambrian to Ordovician ages and are consistent with derivation from the northern Thomson Orogen where igneous rocks of this age are documented in the Charters Towers Province. These data indicate a connection between the Devonian arc-flank deposits of the northern New England Orogen and the Thomson Orogen, which had previously been well established for Carboniferous deep-marine sandstones of the subduction complex (e.g. Shoalwater Formation). These results favour an east-facing continental margin arc setting in the Devonian contra to intra-oceanic island arc settings previously proposed. Detrital zircon age spectra from sampled early Carboniferous units of northern New England Orogen association indicate a pulse of felsic magmatism of that age.

Provenance of the Copi North heavy mineral sands deposit, Murray Basin, Australia, based on the geochronology and geochemistry of detrital zircon, rutile and monazite

The Murray Basin hosts one of the world’s largest strandplain systems and has enormous potential for heavy mineral sand resources of titanium, zircon and rare earth elements. Further discovery of heavy mineral sand deposits can be aided via improved understanding of the source and transport of detrital heavy minerals of the basin. In this study, the provenance of two heavy mineral sand units of the Copi North deposit (western NSW) was determined using U–Pb geochronology and geochemistry of detrital zircon, rutile and monazite. Results from the two units are similar, indicating they derive from the same sources. Approximately 42% of the zircon population is aged <470 Ma and is interpreted to have been derived from the Tasmanides. Older zircon populations, up to ca 2.5 Ga, were largely derived from the Adelaide Superbasin and Delamerian Orogen, or from metasedimentary rocks of the Lachlan Orogen. Over 90% of rutile U–Pb ages are between 570 and 450 Ma, correlating either to Delamerian metamorphism or to detrital rutile from the Darling Basin; both sources are supported by rutile geochemistry that is indicative of a metasedimentary source. Most monazites have U–Pb ages of ca 600 Ma, which is consistent with an ultimate source from the Mt Arrowsmith Volcanics and/or Antarctica. A minor proportion of the monazite was sourced from igneous rocks of the New England Orogen. Temporal changes in sediment source regions can be inferred when considering the detrital mineral record for Copi North with other mineral sand deposits of the basin. Older (>ca 6.7 Ma) deposits such as Copi North feature a significant proportion of post-350 Ma detritus, reflecting the important role of the ancestral Darling River in supplying sediment from the north at the earliest stages of Murray Basin development. Post-6.7 Ma deposits lack a major component of detritus for far northern sources, and instead consist of sediment derived dominantly from western (Adelaide Superbasin) and eastern (Lachlan Orogen) sources, the latter delivered via the Murray River.

Multivariate examination of the sediment-deficient southeast Australian continental shelf

Continental shelves are the most morphologically variable element within the source-to-sink system owing to the numerous processes that influence their formation. A recent multivariate analysis of a global compilation of modern continental shelf data showed that much of the variability is related to tectonic setting, the degree to which the shelf has been glaciated, and carbonate production. While these factors play first-order roles in determining the morphology of shelves, other controlling mechanisms such as siliciclastic sediment supply, wave and tidal energy, bedrock lithology, and sea-level fluctuations are not as well understood. Here, we report findings from a detailed investigation of the southeast Australian shelf that explored how sediment distribution, wave energy, and bedrock lithology influence shelf morphology. The high-resolution analysis suggests that the southeast Australian shelf has 11 distinct shelf types. No strong relationships exist between the shelf attributes or shelf type with their onshore catchments. However, a substantial section boundary correlates with a bedrock contact between the Sydney Basin in the south and the New England Orogen to the north. South of this boundary, we propose that the shelf morphology reflects transgression with low sediment supply, whereas to the north, the morphology reflects transgression with higher sediment input. Although several factors contributed to this difference in shelf morphology, we suggest that sediment distribution and retention due to the active wave climate during the most recent transgression likely played a vital role.

Provenance of a Late Permian retroarc foreland basin along the eastern Gondwanan margin: northern Sydney Basin, eastern Australia

Abstract The Upper Permian sedimentary successions in the northern Sydney Basin have been the subject of several stratigraphic, sedimentological and coal petrographic studies, and recently, extensive U-Pb zircon dating has been carried out on tuffs in the Newcastle Coal Measures. However, detailed petrographic and geochemical studies of these successions are lacking. These are important because a major change in tectonic setting occurred prior to the Late Permian because of the Hunter-Bowen Orogeny that caused the uplift of the Carboniferous and Devonian successions in the Tamworth Group and Tablelands Complex adjacent to the Sydney Basin. This should be reflected in the detrital makeup of the Upper Permian rocks. This study provides data that confirms major changes did take place at this time. Petrographic analysis indicates that the source area is composed of sedimentary, felsic volcanic and plutonic and low-grade metamorphic rocks. Conglomerate clast composition analysis confirms these results, revealing a source region that is composed of felsic volcanics, cherts, mudstones and sandstones. Geochemical analysis suggests that the sediments are geochemically mature and have undergone a moderate degree of weathering. The provenance data presented in this paper indicate that the southern New England Orogen is the principal source of detritus in the basin. Discrimination diagrams confirm that the source rocks derive from an arc-related, contractional setting and agree with the provenance analyses that indicate sediment deposition in a retroarc foreland basin. This study offers new insights on the provenance and tectonic setting of the Northern Sydney Basin, eastern Australia.

Cross‐Orogen Granite Migration as an Indicator of Slab Rollback Along Eastern Gondwana

AbstractSlab rollback during subduction plays a key role in controlling continental growth at convergent plate boundaries. The dynamics of currently subducting slabs can be precisely constrained using geophysical techniques. In contrast, ancient episodes of slab rollback can be difficult to constrain, yet are critical to unlocking the tectonic evolution of long‐lived orogens such as the Phanerozoic Australian Tasmanides of eastern Gondwana. Recognition of ancient slab rollback relies on the identification of the progressive migration of magmatic arcs. Here, we investigate the timing and isotopic variation of ∼90 km of the trans‐orogen migration of the Carboniferous Bathurst Batholith as a potential indicator of slab rollback. U–Pb–Hf isotopes, combined with a regional zircon Hf isotope data set, suggest that the eastward migration of the batholith over ∼18 Myr, from 340.1 to 322.4 Ma, records a maximum slab steepening rate of ∼0.6–1.2° Myr−1 in a relatively stable trench setting. These results provide a magmatic record of Carboniferous slab rollback and establish a missing link between the long‐lived Lachlan and New England orogens of the eastern Gondwanan Tasmanides.

Precambrian history of the Pacific Mantle Domain: New constraints from Woodsreef and Port Macquarie serpentinized spinel harzburgites of the New England Orogen, Australia

Abstract The present-day mantle is divided into the African and Pacific domains by the circum-Pacific subduction girdle. Very little is known about the mantle composition of the Pacific Domain before 120 Ma due to the scarcity of the oceanic record, having mostly been destroyed by subduction processes. Accreted oceanic lithosphere (ophiolites) in orogens along the Paleo-Pacific margins provide rare opportunities to partially fill this knowledge gap. The early Cambrian (530–505 Ma) Weraerai ophiolite mélange in the New England Orogen in Eastern Australia represents fragments of the now-consumed Paleo-Pacific oceanic plate, predecessor of the Panthalassa and Pacific oceanic plates, accreted to east Gondwana during ca. 410–277 Ma. Early work revealed the presence of an accreted volcanic island(s) of possible mantle plume origin. However, due to their heavy alteration and weathering, the geochemical signature of the mafic rocks in the Weraerai ophiolite cannot be used to certify their plume origin with confidence. Therefore, mantle rocks found in the ophiolitic belt offer an alternative way to decipher the origin of the oceanic lithospheric fragments in the Weraerai ophiolite mélange. Here we report the petrographic, major, and trace elements composition, including highly siderophile elements (HSE), and Re-Os isotope composition for 16 serpentinized spinel harzburgite samples from Woodsreef and Port Macquarie. The observed spinel-orthopyroxene symplectite intergrowth textures are interpreted as garnet-breakdown textures due to cooling from an initial high temperature of &amp;gt;1200°C. Silicon and Al contents and Mg# of serpentinized spinel harzburgite, as well as heavy rare earth element modelling results, suggest a high-degree of melt extraction of 20–30% in the garnet stability field and in an anhydrous environment, probably in ocean island or oceanic plateau environments. The samples in this study have HSE concentrations interpreted to have resulted from post-melting processes rather than a melt extraction feature, indicating that their Re-Os model ages need to be interpreted with caution. Nevertheless, the melt depletion ages obtained from the Woodsreef and Port Macquaries samples range between 1.4 and 1.1 Ga, consistent with previous studies on other Pacific-rim ophiolitic rocks. We argue that these ages might be related to a major depletion event during the transition between supercontinents Nuna and Rodinia. Such depletion events affected a large proportion of this section of the mantle before the incorporation of the peridotites into the oceanic lithosphere in the Paleo-Pacific associated with rapid ascent of mantle plumes. This interpretation is consistent with the occurrence of accreted volcanic islands in the Werearai ophiolite mélange, as shown by the OIB chemical signatures of some of the mafic rocks, and their association with shallow water limestones that formed in near-equatorial latitudes. The proposed ca. 530–510 Ma Paleo-Pacific Ocean mantle plume event coincides with a global peak of oceanic mantle plume events that may record the legacy of a circum-Rodinia subduction girdle driving antipodal mantle superplume episodes.

Multimethod provenance analysis using detrital zircon and rutile U-Pb geochronology across Devonian basin systems in the Tasmanides of eastern Australia

The Devonian represents a period of transition for the accretionary orogens of the Australian Tasmanides where the largest and inboard Thomson Orogen became stabilised and orogenic processes became focused in the outboard and newly developing New England Orogen. The end of tectonic activity in the Thomson and the adjacent Delamerian and Lachlan orogens culminated with the development of several overlying intracratonic sedimentary basins. We examine the two largest cover basins, the Adavale and Darling, using stratigraphic logging, sandstone petrography, detrital zircon and rutile U-Pb geochronology to fingerprint sediment sources to test whether the basins were once connected and part of a much larger cover basin system. Sediment provenance in the Adavale Basin is characterised by (i) continuous input from a basement-derived Ordovician (∼480 Ma) igneous zircon source, (ii) reworking of metasedimentary basement rocks with a ‘Pacific-Gondwana’ age signature (iii) reworking of detrital rutile from Cambro-Ordovician sedimentary rocks in the Thomson Orogen ultimately sourced from the Musgrave Block and (iv) an addition of syn-depositional volcanic zircon from contemporary volcanism between ∼380 and 360 Ma. Sediment provenance of the Darling Basin is dominated by reworking of (meta)sedimentary basement, evident from large proportions of rounded zircons exhibiting a ‘Pacific-Gondwana’ age signature and detrital rutile with Peterman Orogeny ages. A much less significant age population of syn-depositional volcanic detrital zircons suggests input from contemporary volcanic sources which were more distal and extra-basinal.The comparison of sandstone compositions and detrital age information indicate both basins record similar provenance signals in terms of reworking of their respective hinterlands and receiving contributions from relatively distal syn-depositional volcanism. However, the comparison of sediment provenance proxies, suggests that the Adavale and Darling basins were not connected during the Devonian and that the basins are intracratonic or cover basins, recording the stabilisation of the Thomson and Lachlan orogens, respectively.

U-Pb detrital zircon geochronology of the Middle to Upper Jurassic strata in the Surat Basin: New insights into provenance, paleogeography, and source-sink processes in eastern Australia

New U-Pb detrital zircon ages from sandstones have been determined for the Walloon Coal Measures-Springbok Sandstone section of the Surat Basin. In the late Jurassic Surat Basin, there is a shift from relatively thick coal measures – the Walloon Coal Measures – consisting of laterally discontinuous coal-seam horizons intercalated with lithic sandstones and mudstones to an overlying coal-poor sandstone-dominated unit – the Springbok Sandstone – that is more laterally continuous. Across the basin, the contact between the two units varies from transitional to unconformable, and the sandstone becomes less lithic. The nature of the contact and the compositional variability of the sandstone across the basin have implications for predicting communication between gas production fields and potential aquifers in the Walloon Coal Measures and overlying Springbok Sandstone. This study tests whether detrital zircon age (LA-ICP-MS) populations from sandstones within these two units can identify provenance shifts and potential tectonic drivers for basin infill at this time. Detrital zircon ages of both units, informed by paleocurrent measurements showing a southerly flowing depositional system, suggest sediment was sourced from three distinct terranes: the Thomson Orogen in northeastern Queensland; Permian-Triassic granitic intrusions emplaced within the northeast New England Orogen during the Hunter-Bowen Orogeny accretion event; and an inferred but now eroded Jurassic magmatic-arc located along the eastern margin of Gondwana. Two major source-delivery systems, named here Western and Eastern Source systems, are recognized by distinct detrital zircon age populations in both formations in the northern Surat Basin. The Western Source of the Walloon Coal Measures delivered substantial Jurassic age zircons, with minor Pre-Cambrian input; the Eastern Source delivered substantial reworked Precambrian and lesser Jurassic zircons. In contrast, the Western Source of the Springbok Sandstone delivered substantial Jurassic and distinct Triassic zircons dated between ca 234 Ma and 224 Ma in the north-west, while the Eastern Source delivered more pronounced Pre-Cambrian input, with minor Jurassic and absent or minor age zircons in the northeast. Maximum depositional ages for the Springbok Sandstone are approximately 2 Ma younger in the northwestern than in the northeastern Surat Basin. The westward younging could be due to migration of the system and/or differential movement on the bounding faults.

Jurassic tectonics of northeastern Gondwana: evidence from the detrital zircon record of the Nambour Basin

The Nambour Basin provides the easternmost record of Jurassic sedimentation for Australia. U–Pb detrital zircon age spectra constrain basin infill as Early–Middle Jurassic ( c. 195–163 Ma). Early Jurassic samples are dominated by 650–500 Ma detrital zircon sourced from the Lachlan Orogen of southeastern Australia. Distal Jurassic upland associated with this orogen was generated by epeiric uplift. Sediment sourced from it was transported northwards and eastwards into the Nambour Basin as corroborated by basinal paleocurrent data. In contrast, Devonian to Triassic detrital zircon from the New England Orogen, which abuts the Nambour Basin margin, are scarce. Topographic relief across this orogen generated by the Permo-Triassic ( c. 260–230 Ma) Hunter Bowen Orogeny, had been largely eliminated by the Early Jurassic. Dominant Jurassic detrital zircon for one sample of this age, combined with related paleocurrent data and the absence of a Jurassic zircon source in eastern Australia, reflects an easterly Jurassic igneous assemblage now located on submerged northern Zealandia. The detrital zircon data presented herein constrains tectonic interpretations proposed for Jurassic east Gondwana, continental margin sedimentary assemblages in New Caledonia and New Zealand. Documentation of continental epeiric uplift from detrital zircon age data obtained from basin fill has potential yet to be explored. Supplementary material: Tables S1 and S2 are available at https://doi.org/10.6084/m9.figshare.c.5988017

Geochronology and sediment provenance of the Precipice Sandstone and Evergreen Formation in the Surat Basin, Australia: Implications for the palaeogeography of eastern Gondwana

A new view of the palaeogeography and tectonic evolution of eastern Gondwana during the late Mesozoic is emerging, driven largely by the detrital zircon record of sedimentary basins comprising the Great Australian Superbasin (GAS). The updated model suggests that a long-lived magmatic arc was present on the eastern margin of Australia, which is in stark contrast to some previous views that eastern Australia was situated well-inland of a plate boundary. However, the active magmatic arc model is derived from Late Jurassic-Cretaceous strata, with tectonic processes and sediment pathways operating in the Early Jurassic still poorly understood. The Lower Jurassic Precipice Sandstone in the Surat Basin – an important stratigraphic component of the GAS – affords an opportunity to fill such a knowledge gap. The Precipice Sandstone is also economically important due to its aquifer capacity and prospectivity for CO2 sequestration. Previous work has hypothesized that the formation contains deposits sourced from multiple terranes, yet this has not been rigorously tested. In this study, we used detrital zircon geochronology, CA-TIMS dating, petrography, and palaeocurrent analysis to characterise the sedimentary provenance of the Precipice Sandstone and the overlying Evergreen Formation. Our analysis revealed mixed provenance with multiple source terranes within the Tasmanides. The bulk of sediment was derived from the Thomson Orogen, with a lesser contribution from the New England Orogen. We found no evidence for sediment derived from the Lachlan Orogen, despite a distinct northward palaeocurrent component. Syn-depositional volcanic material within the interval corroborates the notion of continued arc magmatism along the eastern Gondwana margin, which contributed sediment to the GAS throughout the Early Jurassic. Radiometric ages place the Precipice-Evergreen succession in the Sinemurian-Toarcian which falls within the broader Hettangian-Aalenian age range suggested by recent palynostratigraphic data and suggest diachronous deposition across the basin. Despite pronounced variations in formation thickness, the Precipice Sandstone shows relatively uniform provenance across the basin, showing little evidence for multiple depocenters.

Detrital zircons in Triassic–Cretaceous sandstones, Clarence-Moreton Basin, eastern Australia: speculations upon Australia and Zealandia provenances

Detrital zircon U–Pb age patterns in sandstones from the Triassic–Cretaceous Clarence-Moreton Basin, eastern New South Wales and Queensland, unexpectedly reveal sediment sources overwhelmingly dominated (>70%) by Precambrian–Cambrian zircons, with virtually no input, as was anticipated, from the nearby Carboniferous, Permian or Triassic magmatic arcs and contemporary accretionary wedge associated with the New England Orogen. Some rare, youngest zircons (<5%) are mostly contemporaneous with the estimated depositional age. The older, reworked zircon populations can only be acquired by postulating either (1) an Australian provenance by long overland sediment pathways from the interior to the west, during one or more transport cycles, perhaps from within the Thomson Orogen, and then bypassing or crossing the New England Orogen without any local contribution from that sector or (2) a North Zealandia provenance with a shorter transport distance from a postulated basement block to the east, within the Northern Lord Howe Rise and Kenn Plateau. This would comprise late Mesoproterozoic (1200–1000 Ma) and late Neoproterozoic–Cambrian (700–500 Ma) igneous and metamorphic complexes, similar to that proposed in South Zealandia. KEY POINTS Triassic to Jurassic Clarence-Moreton Basin sedimentary rocks have provenances unlike their hinterland time-correlates in New South Wales and Queensland. The latter better share similarities with those in Murihiku Terrane, North Island, New Zealand. The provenance of Clarence-Moreton sediments is similar to that of time-correlates in New Caledonia, and it is postulated that they share a source region on the northern Lord Howe Rise or Kenn Plateau.

The Onset of Gondwanide Orogeny in Eastern Australia: Insight From the Provenance of Syn‐Orogenic Strata in the New England Orogen (Australia)

AbstractThe last major episode of cordilleran‐style tectonism in eastern Australia was the late Paleozoic‐early Mesozoic Gondwanide Orogeny. When exactly this deformation commenced and what caused this phase of orogenesis is still debated. Using previous stratigraphic and sedimentological data from pre‐orogenic to syn‐orogenic strata, integrated with new detrital zircon U‐Pb geochronology, we investigated the onset of the Gondwanide Orogeny in the northern New England Orogen (Gogango Overfolded Zone, eastern Australia). The lowermost syn‐orogenic strata display evidence of rapid base‐level change coeval with increasingly abundant and more proximal mass‐wasting deposits, which reflect a shift to static loading‐induced subsidence and the establishment of a ∼1,600‐km long foreland basin system. The syn‐orogenic strata contain abundant detritus sourced from syn‐depositional magmatism, alongside an up‐section increase in detritus derived from uplifted older rocks of the New England Orogen. Detrital zircons show maximum depositional age of ∼276 Ma from immediately above the sequence boundary, and ∼269 Ma from an overlying formation. These results confirm previous suggestions that tectonic forcing of the orogenic hinterland affected subsidence and sedimentation patterns in the easternmost part of the basin during the middle Permian, &gt;11 Myr prior to deformation in the western part of the foreland basin. The onset of the Gondwanide Orogeny in eastern Australia, and in other sectors of the Gondwanan margin, likely resulted from a tectonic switch from crustal extension to contraction, which was driven by increased convergence rates due to a plate reorganization event following the final assembly of Pangea.

Provenance of sandstones in the Hastings Block, southern New England Orogen: implications for tectonic setting

The Devonian–Carboniferous sandstones in the Hastings Block, southern New England Orogen have a predominantly arc-like provenance based on ternary QFL or QmFLt ternary diagrams and have a lower quartz content than Permian sandstones. Stratigraphically correlated formations show similar QFL components but not the same patterns of sandstone compositional variability for formations that outcrop over a wide area such as the Mingaletta and Pappinbarra formations. It is possible to correlate most of the petrological intervals in the Tamworth Belt with the Hastings Block. The trend of petrofacies from Devonian to Permian for the southern Tamworth Belt, Tamworth Belt overall and Hastings Block, is towards sandstone with more monocrystalline quartz and less feldspar and lithics. The areal pattern of sandstone compositions within individual formations shows similarities in the percentage of quartz, feldspar or lithics. This suggests areal continuity of sandstone compositions within a formation during deposition and that the faulting did not substantially change the formations continuity at the time of deposition. The arc-like provenance of Devonian to Permian sandstones within the Hastings Block indicates that the block was likely an along-strike continuation of the Tamworth Belt. KEY POINTS The Devonian–Carboniferous sandstones have predominantly arc-like provenance on QFL or QmFLt ternary plots with a higher quartz content in younger formations. The Hastings Block and Tamworth Belt sandstones of comparable age are similar but not the same on ternary QFL and QmFLt plots. The areal pattern of the composition of sandstones within formations suggests that faulting was not active during deposition.

Terminal tectono-magmatic phase of the New England Orogen driven by lithospheric delamination

The recognition of lithospheric delamination as a mechanism for magmatism and uplift is under-recognized in the geological record. A pertinent example is the terminal phase of the New England Orogen in eastern Australia, where current explanations of slab roll-back-driven extension are incompatible with plate motions in the Late Triassic. Although abundant mafic rocks are present, almost all Late Triassic temporal information is from felsic rocks. To investigate potential Late Triassic mafic magmatism in the New England Orogen, we date a series of tholeiitic and alkaline mafic products in its back-arc (Sydney Basin) using plagioclase 40Ar/39Ar and apatite U-Pb geochronology. We obtained a plagioclase 40Ar/39Ar plateau age of 202.77 ± 0.68 Ma (2σ) from tholeiitic magmatic products and an apatite U-Pb age of 202 ± 7 Ma (2σ) from a proximal alkaline sill some 70 m deeper, both of which overlap within uncertainty. Complementary trace element geochemistry shows that the tholeiitic and alkaline magmatic products were derived from a similar deep, garnet-bearing source, which we attribute to upwelling asthenosphere underneath a thickened lithosphere. Our data suggest that extension occurred ∼10 m.y. later in the back-arc basin than along its arc, further supporting the notion that slab roll-back could not have caused this terminal phase of Late Triassic extension in the New England Orogen. The Late Triassic magmatism in the New England Orogen is best explained by lithospheric delamination as it accounts for the orogenic architecture, chemical signature of the ca. 200 Ma products and spatio-temporal distribution of Late Triassic magmatic products.

Paleomagnetism of the Carboniferous–Permian Myall blocks, Tamworth Belt, southern New England Orogen: Permian counterclockwise rotations and Triassic clockwise rotation

This paleomagnetic study on the Myall blocks of the southern New England Orogen complements studies on four other Tamworth Belt blocks aiming to unravel evolution of the Tamworth Belt and the Manning Orocline in particular. Focus is on ignimbritic successions that have partly interlinked stratigraphies across the five blocks and can retain primary magnetisations despite prevalent overprinting. This makes the Tamworth Belt prospective to oroclinal testing by comparing poles and pole paths for individual blocks across its curved structure. Paleomagnetic, rock magnetic and magnetic fabric results are detailed from an upper lower Carboniferous to lower Permian succession, sampled across forearc basin and arc sites (64) of the western Myall Block and forearc basin sites (16) of the eastern Myall Block. Predominantly thermal demagnetisations show a widely present low-temperature component, attributed to late Oligocene weathering, intermediate-temperature primary and overprint components confined to the Nerong Volcanics, and high-temperature primary and overprint components. Overprinting proved more prevalent in the Myall blocks than in other Tamworth Belt blocks, with five phases dated from mid-Carboniferous to mid-Triassic. Comparison of primary and overprint poles against Carboniferous, Permian and Triassic reference paths shows counterclockwise rotations ranging from 20–45° for the western Myall Block forearc basin, to 30° to 90° west to east across the southwestern Myall Block arc complex, to 85–110° for the eastern Myall Block forearc basin, occurring from the latest Carboniferous–early Permian Tablelands phase probably to the mid- to late Permian initial pulse of the Hunter-Bowen phase. The southwestern Myall Block also shows a mid-Triassic, or later, clockwise rotation ranging west to east from 50° to 110°. KEY POINTS Paleomagnetism of upper lower Carboniferous to lower Permian successions of the Myall blocks constrains evolution of the Manning Orocline. The Myall blocks are heavily affected by overprints, with five phases dated from mid-Carboniferous to mid-Triassic. Primary and overprint magnetisations show various counterclockwise and clockwise rotations of the western, eastern and southwestern parts of the Myall blocks. Counterclockwise rotations occurred from latest Carboniferous–earliest Permian probably to mid- to late Permian and clockwise rotation during or after mid-Triassic.

Sedimentology and Stratigraphy of an Upper Permian Sedimentary Succession: Northern Sydney Basin, Southeastern Australia

This study integrates sedimentological and stratigraphic insights into the Upper Permian sedimentary rocks of the Wittingham, Tomago and Newcastle Coal Measures in the Northern Sydney Basin, Australia. Facies analysis documented fifteen facies that belong to seven facies associations. These facies associations correspond to different depositional environments and sub-environments including prodelta, delta-front, upper, lower delta-plain and fluvial. The stratigraphic development points to a shallowing upward trend and is reflected with fluvial deposits sitting on top of the deltaic deposits. The fluvio-deltaic contact is represented by an unconformity and displays an upward increase in sediment caliber. The delta front is mainly controlled by wave, storms- and/or river currents, even though the contribution of tides also occurs in the form of sedimentary structures that suggest tidal influence. In contrast, prodelta and delta-plain are significantly modulated by tidal currents. The impact of tides in the delta plain is fading away upward and therefore, the upper delta plain is much less impacted compared to the lower delta plain. The low abundance of wave ripples suggests that the wave action was not very important in the delta plain. Steep topographic gradients and increased sediment input are suggested, based on the limited or absent evidence of tides in the fluvial realm, related to the growing New England Orogen. In sequence stratigraphic terms, the deltaic system accumulated during highstand normal regression, while the deposition of the overlying fluvial system occurred during lowstand normal regression. The two systems are separated by a subaerial unconformity developed during an intervening forced regression. Short periods of transgression are inferred from the presence of higher frequency cycles in the delta-front.

Evidence for highly refractory, heat producing element-depleted lower continental crust: Some implications for the formation and evolution of the continents

The composition of the lower continental crust is estimated via the analysis of granulite xenoliths, granulite terrains and geophysical properties. All three proxies generally agree on the lower crust's refractory nature, dominated by mafic granulites. Estimates weighted using seismic velocity reference models yield lower Th and U concentrations and higher K/U ratios than granulite xenolith averages, while terrain granulites are often much less refractory than xenoliths.Here we present new data for lower crustal xenoliths from central Queensland, an understudied part of the xenolith-bearing eastern Australian basalt provinces. The granulite chemistry was estimated using the reconstitution approach in which the modal mineralogy and in situ chemical analyses are combined. High-resolution energy-dispersive X-ray spectroscopy mapping revealed that K is enriched in anastomosing grain boundary networks and fractures in granulite and co-occurring mantle peridotite xenoliths. Laser-ablation inductively-coupled-plasma mass-spectrometry 2-D traverses show that the same networks are also significantly enriched in many highly incompatible elements. There are sharp concentration contrasts with neighbouring phases for elements with very different diffusivities (Li and U), suggesting that the networks formed during entrainment, decompression and heating within the host basalt. We propose that undetected inclusion of such late enrichment skews xenolith chemistry estimates to non-representative, overly fertile compositions. In the case of the studied xenoliths, the carrier basalts are not very strongly enriched in highly incompatible elements, and even when the K-rich networks are included in the reconstitution, the resulting granulite chemistry is very refractory with 0.52 wt% K2O, 0.07 ppm Th and 0.03 ppm U.The locally dominant lithology of lower crustal xenoliths is simple two-pyroxene, plagioclase, ilmenite granulite with few accessory phases. The granulite mineralogy and chemistry were compared with results from thermodynamic models of prograde anatexis of different metabasites, variably hydrated. The comparison shows that the granulites could be restitic calc-alkaline basalts or diorites that experienced episodic melt extraction accumulating up to 50–60% total melt loss at very high temperatures (950–1050 °C), implying that the temperatures recorded by two-pyroxene thermometry (750–830 °C) do not capture the thermal maximum. The corresponding upper crustal section of the northern New England Orogen exposes a range of Devonian to Cretaceous granitoids, some of which have complementary features to the granulites, including the low modal abundance of plagioclase; low relative abundance of Ti, very high Rb/Ba ratios, and high Th/U ratios. Together, the data suggest extensive and protracted melting of the original lower crust upon lithospheric thinning and concomitant magmatic underplating. The required high temperatures favour picritic over basaltic underplates. In such a setting, the gravity-driven delamination of more mafic garnet-rich restites and olivine-rich mafic-ultramafic underplating material is physically plausible. This delamination could help explain the long-established mass balance issue for the formation of continental crust in general.

Geochemistry and provenance of lower Permian sedimentary rocks, Nambucca Block, southern New England Orogen

It is well documented that sediments record geochemical signatures that reflect the composition and tectonic setting of the rocks providing the detritus for them. This is true of the turbidites in the Nambucca Block that record compositions that vary from basaltic to rhyodacitic but are dominantly andesitic to dacitic. They show chondrite and N-MORB patterns, characteristic of calc-alkaline, continental arc rocks, namely light REE (av. La/Yb) N = 6.3) and Th enrichment, Ti, Nb and Ta depletion, and substantial within plate components (Nb/Yb; 0.65–5.16; x = 3.15). Rare occurrences of rocks with intra-oceanic island arc affinities and felsic volcanic rocks showing tectonic fabrics occur, indicating more distal sources contributed detritus. Most detritus in the turbidites has been derived from the calc-alkaline, continental rocks of the Keepit arc, some from the subduction–accretion sequences of the Tablelands Complex. CIA [Al2O3/(Al2O3+Na2O + K2O + CaO)×100] values (48–83; x = 68) indicate that erosion of these arc sequences occurred during glacial, temperate and humid conditions at different times. Further, index of chemical variability values (0.3–1.85; x = 0.85) reveal a compositional maturity and a high percentage of clays in the source rocks suggesting a passive tectonic setting in which minimal uplift and extensive weathering took place. KEY POINTS Metasediments in the Nambucca Block record geochemical signatures typical of calc-alkaline, continental, arc rocks. Detritus in metasediments was derived from the Keepit arc and Tablelands Complex, southern New England Orogen. Source rocks were subjected to glacial, temperate and humid conditions at different times.

Middle Ordovician Conodont Biostratigraphy of Australasia

Seven conodont biozones are recognized in the carbonate-dominated shelf-marine Middle Ordovician developed in the intracratonic sedimentary basins (Canning, Amadeus and Georgina) of central and north-western Australia, in the Lachlan and New England orogens of New South Wales, and in the Takaka Terrane of New Zealand. A separate scheme identifying seven conodont biozones spanning the Middle Ordovician has also been developed for siliciclastic sequences deposited in slope-basinal environments in the Lachlan Orogen in New South Wales and Victoria. This biozonal classification consisting of two parallel biostratigraphic schemes for the shelf-marine and deep-marine successions respectively has significantly increased precision in regional and global biostratigraphic correlation and laid a solid foundation for the Middle Ordovician chronostratigraphy of Australia and New Zealand. Recognition of short-ranging pandemic species as the eponymous species of the biozones also supports direct correlation with the classical conodont successions established in Baltoscandia and the North American Midcontinent, and with those of the major Chinese terranes (South and North China and Tarim). The Lachlan Orogen appears to be globally unique in enabling correlation of contemporaneous conodont faunas over a considerable spectrum of water depths and biofacies ranging from carbonate shelves, slopes to deep-water basins.

Source of Detritus in Subducted Turbidites, Tectonic Mélange, Port Macquarie Block, Southern New England Orogen, Australia—A Geochemical Perspective

A tectonic mélange containing blueschists and eclogites, Middle Ordovician mid-ocean ridge basalt, cherts, and clastic sediments occurs at Port Macquarie in the southern New England Orogen, Australia. The clastics are quartz-poor and are dominated by mafic volcanic and fragmented plagioclase clasts; felsic volcanic clasts are less common. They exhibit erosional bases, graded bedding, load structures, and lithologies ranging from laminated mudstones to pebbly sandstones. Based on these features, they are interpreted as turbidites. During subduction, these turbidites were deformed and metamorphosed under prehnite–pumpellyite and lower greenschist facies conditions. Geochemically, they have a calc-alkaline, intra-oceanic arc signature; show no recycling; and have been derived from a provenance dominated by mafic volcanic rocks of basaltic-andesite composition. Further, chemical index of alteration (44–69) and Index of Compositional Variability (0.8–2.4) data reveal they show little weathering and are immature. The lack of weathering of rocks in a location where tropical climatic conditions existed is attributed to extreme erosion associated with a dynamic setting resulting in rapid transportation of the sediments to the fore arc basin and subsequently to the trench. As a consequence, little time was available for weathering to take place. The detritus in the turbidites is thought to have been derived from Late Ordovician volcanics in the Macquarie Arc and fore arc basin sequences of the Murrawong Formation. The cherts with which they are associated record both a continental and oceanic arc geochemical signature.