Southern Pilbara Research Articles

Reef island evolution in a turbid‐water coral reef province of the Indo‐Pacific

AbstractCoral reef islands are vulnerable landforms to environmental change. Constructed of largely unconsolidated reef‐derived sediments, they are highly sensitive to variations in metocean boundary conditions, raising global concern about their future resilience and stability in the face of increased natural hazards, sea‐level rise and anthropogenic climate change. This study examines the evolution of an inshore turbid reef island from the southern Pilbara region of Western Australia (Indo‐Pacific) using detailed analyses of island chronostratigraphy (composition, texture) and geochronology (21 in‐situ radiometric dates) from Eva Island. Downcore, composition of island‐grade (reef‐derived) sediments were homogenous, dominated by molluscan (37%–42%) and coral (32%–37%) constituents. The 14C radiometric dating of island sediments, beachrock and coral microatolls identified five stages of island formation across changing sea‐level regimes over the mid to late Holocene: (1) limestone platform accretion at ca 6,000 cal yr BP, coinciding with reef decline or ‘give‐up’ on neighbouring Exmouth Gulf reefs; (2) sand cay (i.e. core) initiation and vertical aggregation at ca 5,000 cal yr BP during the point of sea‐level regression to current levels; (3) major accretion and lateral progradation of the island between 3,500 cal yr BP and 2,500 cal yr BP including the modification of island shorelines through alongshore reworking of sediment; (4) lateral accretion and minor expansion to the north and formation of beachrock pavement between 2,500 and 650 cal yr BP; and (5) planform adjustment (erosion of the north‐west island) and backstepping under stabilised sea levels over the past 650 years. While this model is comparative to studies on island formation following incremental sea‐level fall following the mid‐Holocene highstand, it demonstrates active landform readjustment under stabilised sea levels over the past 2,000 years, probably the influence of local‐scale metocean boundary conditions within climate windows across the mid to late Holocene period (i.e. independent of sea‐level fluctuations). Importantly, while sediment production rates are predicted to be lower in turbid‐water reef systems than clear water, Eva Island shows no change in carbonate producers (i.e. proportion of mollusc and coral) over the course of island building, indicating the carbonate factory has not experienced significant adjustments in reef ecology, but has remained stable despite low water quality.

Tectonic fluid expulsion: U-Pb evidence for punctuated hydrothermal fluid flow and hydraulic fracturing during orogenesis

Tectonic fluid expulsion has been linked to fluid flow in permeable foreland basin sequences, but it is unclear how fluids migrate through underlying low-permeability successions. Here we report U-Pb data which indicate that fluids expelled during tectonic compression migrated through older compacted shales inboard of the deforming craton margin by fracture permeability and enhanced diffusion. From a ∼350 m drill-core interval of Neoarchean shale (drill-hole ABDP9), Hamersley region, Australia, we document abundant bedding-parallel veins indicating pervasive hydraulic fracturing. Thermometry of vein-filling chlorite indicates temperatures between 329-374°C, which is higher than estimates (175-280°C) from metamorphic assemblages in underlying basalts, implying the passage of hydrothermal fluids. New in situ secondary ion mass spectrometry (SIMS) U-Pb geochronology of monazite and xenotime in bedding-parallel veins verifies and expands earlier results, showing that microfracturing and hydrothermal fluid flow spanned ∼650 million years and was restricted to five short intervals: 2.30 Ga, 2.20 Ga, 2.10 Ga, 2.05 Ga and 1.65 Ga. Three of the five age populations coincide with known fabric-forming orogenic events (i.e. 2.30 Ga Sylvania orogeny; 2.22-2.15 Ga Ophthalmia orogeny; 1.68-1.65 Ga Mangaroon orogeny), which are focused along the southern Pilbara Craton margin. Our results show that the timing of veining was not random, but coeval with major orogenic events, linking fluid overpressure and hydraulic fracturing to the expulsion of heated orogenic fluids 100s of km inboard of the craton margin. The crack-seal veins indicate repeated cycles of fluid overpressure resulting in hydrofracturing and fluid drainage, followed by cavity collapse. The tectonic expulsion of pressurized hydrothermal fluids produced fracture and matrix permeability in Earth's upper crust, leading to episodic open-system behaviour with enhanced fluid-rock reactions and the large-scale transfer of heat and mass.

U-Pb dating reveals multiple Paleoproterozoic orogenic events (Hamersley orogenic cycle) along the southern Pilbara margin (Australia) spanning the onset of atmospheric oxygenation

Abstract The early Paleoproterozoic was a time of unprecedented change in Earth's climate and surface environment. The key to resolving some of the controversies surrounding the timing and causes of these changes lies with supracrustal sequences, such as the 2.45–2.22 Ga Turee Creek Group in the southern Pilbara craton, northwestern Australia. The group preserves a predominantly siliciclastic sequence; however, its precise age, tectonic setting, and postdepositional history are disputed. Although it is interpreted to have been deposited in a foreland basin setting shortly after 2.45 Ga, the oldest well-recognized deformational event, marked by northward folding and thrusting, is the 2.20–2.15 Ga Ophthalmia orogeny. Evidence for a pre-Ophthalmia fabric-forming tectonic event north of the Archean Sylvania Inlier, southeast Pilbara craton, which is marked by tight to isoclinal folding, has been largely overlooked. In this area, we report in situ U-Pb geochronology of authigenic monazite and xenotime in shale with a well-developed tectonic cleavage from the ca. 2.63 Ga Jeerinah Formation. Monazite porphyroblasts, which are locally wrapped by strain fringes aligned in a tectonic cleavage, yielded weighted mean 207Pb/206Pb ages at 2370 ± 11 Ma and 2312 ± 8 Ma, whereas xenotime, which overprints a crenulation cleavage, gave a weighted mean 207Pb/206Pb age of 2291 ± 11 Ma, constraining fabric development to between 2.31 Ga and 2.29 Ga. Our results confirm the existence of a pre-Ophthalmia deformational event in the southeastern Pilbara craton, herein referred to as the Sylvania orogeny, which is part of an ~300 m.y. interval (2.45–2.15 Ga) of northward-directed compression (“Hamersley orogenic cycle”). This orogenic cycle is marked by east-west and northwest-southeast folding, cleavage development, veining, hydrothermal gold mineralization, and isotopic resetting across the southern Pilbara craton. Our results indicate that the syn–Great Oxidation Event Turee Creek Group was deposited in one or more foreland basins after 2.45 Ga. Our results provide a new tectonostratigraphic and geodynamic framework for understanding the timing and origin of geochemical records in a key succession deposited during an interval of global environmental change.

U-Pb dating of overpressure veins in late Archean shales reveals six episodes of Paleoproterozoic deformation and fluid flow in the Pilbara craton

Abstract Fluid flow in the upper crust not only impacts the redistribution of heat and elements, driving the formation of economic ore deposits, but it also exerts control on metamorphism, metasomatism, and deformation. However, reconstructing the history of fluid flow in ancient basins is exceedingly difficult, particularly in Archean sedimentary rocks because of extensive overprinting and recrystallization. Here, we report U-Pb ages for monazite and xenotime that grew in bedding-parallel veins in 2.63–2.5-b.y.-old shales along the southern Pilbara craton, Australia. The U-Pb ages define six discrete populations, at 2.41 Ga, 2.30 Ga, 2.20 Ga, 2.10 Ga, 2.05 Ga, and 1.66 Ga, which formed ≥200 m.y. after deposition. The abundance of bedding-parallel crack-seal and fibrous veins in banded iron formations (BIFs) and underlying shales suggests a history of episodic buildup of fluid pressure followed by microfracturing, fluid expulsion, and mineral growth. Thermometry of vein minerals indicates temperatures between 230 °C and 320 °C, implicating the migration of hydrothermal fluids. The development of bedding-parallel veins at 2.41 Ga, 2.20 Ga, and 1.66 Ga was coeval with regional orogenic events known to have affected the craton, whereas vein growth at 2.30 Ga, 2.10 Ga, and 2.05 Ga reveals new episodes of deformation and fluid flow. Our results show that well-preserved Archean shales devoid of structural fabrics and >150 km inboard of the craton margin preserve a cryptic history of fluid overpressure, crack-seal vein development, and hydrothermal fluid flow between 2.41 and 1.66 b.y. ago.

Open-holed trapdoor spiders of the genus Teyl (Mygalomorphae: Nemesiidae: Anamini) from Western Australia's Pilbara bioregion: a new species and expanded phylogenetic assessment.

The open-holed trapdoor spiders of the genus Teyl Main, 1975 from the Pilbara bioregion of Western Australia are investigated. A single endemic species from the southern Pilbara, T. heuretes sp. nov., is newly described, representing the northern-most occurrence of the genus in Australia. Legacy molecular data for Australian Nemesiidae, along with newly generated sequences for all described species of Teyl known from Western Australia, are analysed using Maximum Likelihood methods, providing molecular data for T. heuretes and an expanded phylogenetic assessment of the genus.

Conspicuously concealed: revision of the arid clade of the Gehyra variegata (Gekkonidae) group in Western Australia using an integrative molecular and morphological approach, with the description of five cryptic species.

The methods used to detect and describe morphologically cryptic species have advanced in recent years, owing to the integrative nature of molecular and morphological techniques required to elucidate them. Here we integrate recent phylogenomic work that sequenced many genes but few individuals, with new data from mtDNA and morphology from hundreds of gecko specimens of the Gehyra variegata group from the Australian arid zone. To better understand morphological and geographical boundaries among cryptic forms, we generated new sequences from 656 Gehyra individuals, largely assigned to G. variegata group members over a wide area in Western Australia, with especially dense sampling in the Pilbara region, and combined them with 566 Gehyra sequences from GenBank, resulting in a dataset of 1,222 specimens. Results indicated the existence of several cryptic species, from new species with diagnostic morphological characters, to cases when there were no useful characters to discriminate among genetically distinctive species. In addition, the cryptic species often showed counter-intuitive distributions, including broad sympatry among some forms and short range endemism in other cases. Two new species were on long branches in the phylogram and restricted to the northern Pilbara region: most records of the moderately sized G. incognita sp. nov. are near the coast with isolated inland records, whereas the small-bodied saxicoline G. unguiculata sp. nov. is only known from a small area in the extreme north of the Pilbara. Three new species were on shorter branches in the phylogram and allied to G. montium. The moderately sized G. crypta sp. nov. occurs in the western and southern Pilbara and extends south through the Murchison region; this species was distinctive genetically, but with wide overlap of characters with its sister species, G. montium. Accordingly, we provide a table of diagnostic nucleotides for this species as well as for all other species treated here. Two small-bodied species occur in isolated coastal regions: G. capensis sp. nov. is restricted to the North West Cape and G. ocellata sp. nov. occurs on Barrow Island and other neighbouring islands. The latter species showed evidence of introgression with the mtDNA of G. crypta sp. nov., possibly due to recent connectivity with the mainland owing to fluctuating sea levels. However, G. ocellata sp. nov. was more closely related to G. capensis sp. nov. in the phylogenomic data and in morphology. Our study illustrates the benefits of combining phylogenomic data with extensive screens of mtDNA to identify large numbers of individuals to the correct cryptic species. This approach was able to provide sufficient samples with which to assess morphological variation. Furthermore, determination of geographic distributions of the new cryptic species should greatly assist with identification in the field, demonstrating the utility of sampling large numbers of specimens across wide areas.

Making magnetite late again: Evidence for widespread magnetite growth by thermal decomposition of siderite in Hamersley banded iron formations

Banded iron formations (BIFs) are marine chemical sedimentary rocks that were widely deposited before the start of the Great Oxidation Event (2.45–2.32 Ga). They represent an important archive into the chemistry of the Precambrian oceans and atmosphere, and the evolution of the marine biosphere. It is believed that BIFs were derived from ferric oxides/hydroxides that were deposited by iron-oxidizing bacteria or by reaction with oxygen released by Cyanobacteria. Hematite is traditionally interpreted to represent the dehydrated product of the ferric oxide/hydroxide precipitate, whereas magnetite is thought to have formed from ferric oxides/hydroxides via bacterial iron reduction and oxidation of microbial biomass. However, studies are equivocal about when and how magnetite formed in BIFs. We present the results of a detailed petrographic study of magnetite in BIFs of the Hamersley Group, Western Australia. We find no evidence that magnetite was precipitated from seawater or was a product of bacterial iron reduction. Instead, most of the magnetite formed by replacement of siderite and other ferrous-rich minerals after burial. Our observations suggest that magnetite growth was linked to the breakdown of siderite during metamorphism. Published experimental data indicate that in the presence of water and under the metamorphic conditions experienced in the southern Pilbara Craton (200–350 °C), the thermal decomposition of siderite could account for much of the magnetite in the Hamersley BIFs (3FeCO3 + H2O → Fe3O4 + 3CO2 + H2).The relationship between magnetite and the primary precipitates, which are preserved as greenalite nanoparticles and microgranules in chert bands, suggests that much of the magnetite formed in siderite-rich, iron-silicate bands that developed by compaction of non-silicified iron-silicate muds. These bands acted as reactive pathways for fluid flow and the formation of magnetite. If most of the magnetite formed after burial then it is necessary to reassess assumptions about the chemistry of the original precipitates and the role of microbial life in the redox cycling of iron. Our results are consistent with models for the deposition of BIFs that center around a precursor sediment comprising greenalite precipitates and diagenetic siderite that was partially replaced by magnetite and hematite after burial.

Using in situ SHRIMP U-Pb Monazite and Xenotime Geochronology to Determine the Age of Orogenic Gold Mineralization: An Example from the Paulsens Mine, Southern Pilbara Craton

Paulsens is a mesothermal orogenic gold deposit located in the Wyloo Inlier on the southern margin of the Pilbara craton of Western Australia. Gold occurs in quartz-sulfide veins hosted within a folded and faulted gabbro dike, from which baddeleyite yields a U-Pb crystallization age of 2701 ± 11 Ma. Monazite and xenotime in the veins and from hydrothermally altered country rocks yield three distinct U-Pb dates of ca. 2400, 1730, and 1680 Ma. Textural relationships between euhedral xenotime and pyrite with rounded native gold inclusions from within the quartz-sulfide veins show that the primary gold mineralization was synchronous with xenotime crystallization at 2403 ± 5 Ma, and coeval with pervasive alteration of the host rocks, which yield monazite ages of 2398 ± 37 and 2403 ± 38 Ma. Regional-scale hydrothermal events at ca. 1730 and 1680 Ma are linked to the growth of monazite within phyllitic rocks at 1730 ± 28 and 1721 ± 32 Ma, carbonate veining at 1655 ± 37 Ma, and gold remobilization or introduction of new gold at 1680 ± 9 Ma. The ca. 2400 Ma age for mineralization and hydrothermal alteration does not correspond with any known deformation event in the region, indicating a significantly different and more complicated low-temperature tectonothermal evolution for the southern Pilbara region than previously recognized. The in situ secondary ion mass spectrometry dating of monazite and xenotime employed here will lead to better targeting of orogenic gold deposits in the northern Capricorn Orogen, and these techniques can be utilized for orogenic gold exploration worldwide.

Discovery of a pupping site and nursery for critically endangered green sawfish Pristis zijsron.

A pilot study targeting sawfishes in the southern Pilbara region of Western Australia, which is undergoing a major expansion in human activity, was conducted using gillnets during April and October 2011 in the Ashburton Estuary and adjacent mangrove creeks. Catch per unit effort was greatest in the Ashburton Estuary in October, due to an influx of green sawfish Pristis zijsron pups, and was orders of magnitude higher than previously reported for any Pristidae; the study sites contained P. zijsron up to almost 3 m total length. This study identified the first pupping site for P. zijsron in Western Australia, and the most southerly known nursery area for the species in Australian waters, and is potentially the most important globally.

Speciation on the Rocks: Integrated Systematics of the Heteronotia spelea Species Complex (Gekkota; Reptilia) from Western and Central Australia

The isolated uplands of the Australian arid zone are known to provide mesic refuges in an otherwise xeric landscape, and divergent lineages of largely arid zone taxa have persisted in these regions following the onset of Miocene aridification. Geckos of the genus Heteronotia are one such group, and have been the subject of many genetic studies, including H. spelea, a strongly banded form that occurs in the uplands of the Pilbara and Central Ranges regions of the Australian arid zone. Here we assess the systematics of these geckos based on detailed examination of morphological and genetic variation. The H. spelea species complex is a monophyletic lineage to the exclusion of the H. binoei and H. planiceps species complexes. Within the H. spelea complex, our previous studies based on mtDNA and nine nDNA loci found populations from the Central Ranges to be genetically divergent from Pilbara populations. Here we supplement our published molecular data with additional data gathered from central Australian samples. In the spirit of integrative species delimitation, we combine multi-locus, coalescent-based lineage delimitation with extensive morphological analyses to test species boundaries, and we describe the central populations as a new species, H. fasciolatus sp. nov. In addition, within the Pilbara there is strong genetic evidence for three lineages corresponding to northeastern (type), southern, and a large-bodied melanic population isolated in the northwest. Due to its genetic distinctiveness and extreme morphological divergence from all other Heteronotia, we describe the melanic form as a new species, H. atra sp. nov. The northeastern and southern Pilbara populations are morphologically indistinguishable with the exception of a morpho-type in the southeast that has a banding pattern resembling H. planiceps from the northern monsoonal tropics. Pending more extensive analyses, we therefore treat Pilbara H. spelea as a single species with phylogenetic structure and morphological heterogeneity.

New species of Mouse Spiders (Araneae: Mygalomorphae: Actinopodidae: <i>Missulena</i>) from the Pilbara region, Western Australia

Two new species of Mouse Spiders, genus Missulena, from the Pilbara region in Western Australia are described based on morphological features of males. Missulena faulderi sp. nov. and Missulena langlandsi sp. nov. are currently known from a small area in the southern Pilbara only. Mitochondrial cytochrome c oxidase subunit I (COI) sequence divergence failed in clearly delimiting species in Missulena, but provided a useful, independent line of evidence for taxonomic work in addition to morphology.

Taxonomic assessment of Diporiphora (Reptilia: Agamidae) dragon lizards from the western arid zone of Australia

Members of the genus Diporiphora are slender perching agamid lizards from Australasia, with a conservative morphologyand some outstanding taxonomic issues. Here we assess morphological variation in the morphologically similar D. pin-dan, D. valens, and D. winneckei from the western deserts of Australia. A reassessment of morphological differences thatincluded the presence or absence of a gular fold, revealed D. pindan to be much more widely distributed than previouslythought, occurring as far south as the northern Pilbara and east to the Tanami Desert. Examination of D. valens specimensrevealed a north-south split within the Pilbara, with specimens conspecific with the types from the Hamersley Range inthe southern Pilbara, whereas recently collected specimens from the Chichester and Roebourne regions in the northernPilbara differ morphologically, and are described as a new species. Examination of the type of D. winneckei and topotypicmaterial indicates that populations referable to this species are confined to the eastern arid zone. The isolated far westernpopulation of ‘D. winneckei’ from the Carnarvon Basin differs in morphology from the eastern arid zone D. winneckei andis described as a new species. The western arid zone ‘D. winneckei’ is also morphologically distinctive from the easternarid zone D. winneckei and is described as a new species. We also redescribe D. pindan, D. valens, and D. winneckei, and return Caimanops amphiboluroides to Diporiphora based on the results of previous genetic studies.

Early Earth evolution: evidence from the 3.5–1.8 Ga geological history of the Pilbara region of Western Australia

The Pilbara region of Western Australia is one of only two areas on Earth - the other being the Kaapvaal Craton of southern Africa - that contain well preserved, near-continuous geological records of crustal evolution from the Paleoarchean into the late Paleoproterozoic. The Pilbara is famous for hosting fossil evidence of early life (stromatolites and microfossils), and for containing a record of the early Archean atmosphere. The geological record extends from granite-greenstone terranes and overlying clastic basins of the 3.53-2.83 Ga Pilbara Craton, across a major unconformity, to a series of 2.78-1.79 Ga volcanic and sedimentary successions. Between 3.53-3.23 Ga, a succession of mantle plume events formed a thick volcanic plateau on older continental crust, remnants of which include enclaves of c. 3.6 Ga granitic gneiss and abundant 3.8-3.6 Ga inherited and detrital zircons. During each of the plume events, the volcanic plateau was intruded by crustally-derived granitic rocks, leading to vertical deformation by partial convective overturn. By 3.23 Ga, these processes had established thick continental crust that was then rifted into three microplates separated by c. 3.2 Ga basins of oceanic crust. Subsequent plate tectonic processes to 2.90 Ga included subduction, terrane accretion, and orogeny. From 2.78-2.63 Ga the northern Pilbara Craton was affected by minor rifting, followed by deposition of thick basaltic formations separated by felsic volcanic and sedimentary rocks (Fortescue Basin). Rifting in the southern Pilbara resulted in progressively deepening marginal basin sedimentation, including thick units of banded iron formation (Hamersley Basin: 2.63-2.45 Ga). At c. 2.45 Ga, sedimentation in the southern Pilbara changed to a mixed assemblage of clastic and carbonate sedimentary rocks of the Turee Creek Basin, including one unit of glacial diamictites. Deposition of the unconformably overlying 2.21-1.79 Ga Wyloo Group in the Ashburton Basin followed the Ophthalmian Orogeny, and all of these rocks were deformed by the Panhandle (c. 2 Ga) and Capricorn (c. 1.78 Ga) orogenies.

Isotopic and geochemical constraints on the Paleoproterozoic Hutchison Group, southern Australia: Implications for Paleoproterozoic continental reconstructions

Geochemical, Sm–Nd isotopic, and U–Pb–Lu–Hf zircon analyses reveal the sedimentary succession formerly recognised as the Hutchison Group consists of three temporally and isotopically distinct groups, divided by a crustal-scale shear zone (Kalinjala Mylonite Zone). The BIF endowed, newly recognised Middleback Group lies to the east of the Kalinjala Mylonite Zone and has a provenance consistent with uniquely Archean source rocks, including highly evolved ɛ Nd(1790 Ma) values, positive Eu anomalies, flat REE patterns and no detrital zircon younger than 2570 Ma. The depositional age of this Group is likely to be c. 2500 Ma and is a possible temporal equivalent to Late Archean successions in the Gawler Craton and the BIF-endowed Hamersley Group in the Pilbara Craton, Western Australia. Two groups are recognised to the west of the Kalinjala Mylonite Zone. The older of the metasedimentary successions, here termed the Darke Peak Group, consists of felsic volcanic rocks, quartzite, pelite and carbonate rocks deposited at c. 1865 Ma. The volcanic rocks have a geochemical signature of enriched, alkaline, intracontinental magmatism with A-type affinities, and mark the initiation of Paleoproterozoic rifting. Sources to the metasedimentary component of this Group were dominated by highly evolved, intra-crustally reworked, c. 2500 Ma and c. 2000 Ma felsic rocks, consistent with a provenance dominated by local basement complexes of the Late Archean to Early Proterozoic Gawler Craton. A succession of dominantly psammitic and pelitic units, here termed the Cleve Group, disconformably overlies the Darke Peak Group and was deposited between 1780 Ma and 1730 Ma. Provenance to the Cleve Group included abundant 1850 Ma and 1790 Ma zircons, and had a geochemical composition consistent with felsic, upper crustal sources which were significantly less evolved than the Darke Peak Group. The more radiogenic initial ɛ Nd values recorded in the Cleve Group require the input of juvenile felsic material not easily reconcilable with sources within the Southern Australian Proterozoic. Provenance data strengthen Proterozoic assembly models which suggest that the Southern Australian Proterozoic was contiguous with components of either the North Australian Craton or the southern Pilbara Craton by the time of deposition of the Cleve Succession ( c. 1780 Ma). The data are also internally consistent with reconstructions of Nuna which place southern Laurentia proximal to the southeast margin of Proterozoic Australia at c. 1800 Ma.

Evolution of Neoarchaean and Proterozoic basins of Australia

This study correlates basin initiation, sedimentation and major tectonic events on an Australia-wide scale, over the Neoarchaean and Proterozoic. An interpretation of a depth-to-basement surface and faults, based on magnetic and gravity data, was calibrated with limited well data, magnetic depth models, geological maps and depth data from cross-sections and literature. Tectonic events are correlated to basin formation and deposition of basin units. Event maps show the interpreted active faults. A principal stress direction has been assigned to basin-forming tectonic events. The results of this study can be used as a kinematic, stratigraphic and temporal framework to examine the development of individual basins and the regional influence of tectonic events. Tectonic events are grouped into four major time intervals: the Neoarchaean and early Palaeoproterozoic (∼2775–2000 Ma), the mid-late Palaeoproterozoic to early Mesoproterozoic (∼2000–1400 Ma), the mid-late Mesoproterozoic (∼1400–1000 Ma) and the Neoproterozoic (∼1000–570 Ma). During the Neoarchaean and early Palaeoproterozoic, basins formed along the southern Pilbara and the northern Yilgarn margins, mainly in response to differential movements between these and other Archaean Cratons. Events in the mid-late Palaeoproterozoic to early Mesoproterozoic were related to the assembly and subsequent intracontinental extension of the Australian continent. Mid-late Mesoproterozoic events were due to interactions between Australia and other continents during assembly of the supercontinent Rodinia. Preservation of sediments from this time period is limited. Neoproterozoic basin-forming events mainly related to the break-up of Rodinia, and led to the formation of the Centralian Superbasin. Sedimentation in this basin was terminated at the end of the Neoproterozoic due to compression associated with the amalgamation of the Gondwana Supercontinent.

Provenance history of the Bangemall Supergroup and implications for the Mesoproterozoic paleogeography of the West Australian Craton

The 4–10 km-thick Bangemall Supergroup, comprising the Edmund and Collier Groups, was deposited between 1620 and 1070 Ma in response to intracratonic extensional reactivation of the Paleoproterozoic compressional Capricorn Orogen. The supergroup can be further divided into six depositional packages bounded by unconformities or major marine flooding surfaces. U–Pb dating of over 1200 detrital zircon grains from 19 samples representative of each of the major sandstone units within these packages has failed to identify any zircon populations attributable to syndepositional magmatism. However, this extensive dataset provides a provenance history of the Bangemall Supergroup, which is here integrated with paleocurrent data which indicates that all source areas were located within the Mesoproterozoic West Australian Craton, with the main source area for the northern Bangemall Supergroup being the Gascoyne Complex and southern Pilbara Craton. All samples have prominent age modes in the 1850–1600 Ma range, indicating significant contribution from the northern Gascoyne Complex and coeval sedimentary basins. Some samples also display prominent modes in the 2780–2450 Ma range, consistent with derivation from the Fortescue and Hamersley Groups of the southern Pilbara Craton. The Edmund Group has age-spectra in which the dominant modes become older upwards, recording unroofing of the underlying basement from the Gascoyne Complex to the Archean granites and greenstones of the Pilbara Craton. In contrast, the Collier Group records unroofing of the underlying Edmund Group, with possible additional contribution from the Pilbara Craton and Paterson Orogen, and is characterized by age-spectra in which the dominant modes become younger upwards. These data imply that the West Australian Craton remained intact throughout the Mesoproterozoic assembly of Rodinia, and was the only source of detritus for the Bangemall Supergroup.

Prolonged history of episodic fluid flow in giant hematite ore bodies: Evidence from in situ U–Pb geochronology of hydrothermal xenotime

Absolute ages for hydrothermal mineralization and fluid flow are critical for understanding the geological processes that concentrate metals in the Earth's crust, yet many ore deposits remain undated because suitable mineral chronometers have not been found. The origin of giant hematite ore deposits, which are hosted in Precambrian banded-iron formations (BIFs), remains contentious. Several models have been formulated based on different sources and timing for the mineralizing fluids; supergene-metamorphic, syn-orogenic, late-orogenic extensional collapse and syn-extensional. Precise geochronology of the ore offers a means of discriminating between these models. In this study, two U–Pb chronometers, xenotime and monazite, have been identified in high-grade hematite ore bodies from the Mount Tom Price mine in the Hamersley Province, northwestern Australia. Both phosphate minerals occur as inclusions within the hematite ore and as coarser crystals intergrown with martite (hematite pseudomorph after magnetite) and microplaty hematite, indicating that the xenotime and monazite precipitated during mineralization. In situ U–Pb dating by ion microprobe indicates that both phosphate minerals grew during multiple discrete events. Our results suggest that ore genesis may have commenced as early as ∼ 2.15 Ga, with subsequent hydrothermal remobilization and/or mineralization at ∼ 2.05 Ga, ∼ 1.84 Ga, ∼ 1.67 Ga, ∼ 1.59 Ga, ∼ 1.54 Ga, ∼ 1.48 Ga and ∼ 0.85 Ga. The location of the ore bodies along ancient fault systems, and the coincidence of at least some of the U–Pb phosphate dates with episodes of tectonothermal activity in the adjacent Proterozoic Capricorn Orogen, implies that fluids were channelled through major structures in the southern Pilbara Craton during discrete phases of tectonic compression and extension. Our results show that the hematite ore bodies formed at sites of repeated focussed hydrothermal fluid flow. In contrast to the aforementioned models, our results imply that iron-ore formation was probably a long-lived, multi-stage process spanning more than one billion years.

In situ U–Pb dating and element mapping of three generations of monazite: Unravelling cryptic tectonothermal events in low-grade terranes

In situ U–Pb dating of monazite and xenotime in sedimentary rocks from the mid-Archean Soanesville Group in the Pilbara Craton, yields ages for provenance, diagenesis and multiple low-grade metamorphic events. Detrital monazite and xenotime grains give dates >3250 Ma, whereas diagenetic xenotime provides a new minimum age of 3190 ± 10 Ma for deposition of the basal Soanesville Group, previously constrained between ∼3235 Ma and ∼2955 Ma. Metamorphic monazite provides evidence for three episodes of growth: at 2.88, 2.16 and 1.65 Ga. Element mapping of monazite for La, Sm, Y and Th reveals distinct cores and rims in some crystals that were used to guide the placement of analytical spots during in situ U–Pb dating by sensitive high-resolution ion microprobe (SHRIMP). Specifically, La and Sm distributions closely correlate with different generations of monazite. The presence of two generations in single monazite crystals highlights the need for characterizing mineral chemistry prior to geochronology. It also shows the importance of using in situ dating techniques rather than methods that rely on the analysis of entire, potentially multi-aged, crystals. The ages recorded by metamorphic monazite span more than one billion years and are interpreted to record cryptic tectonothermal events within the craton. The 2.88 Ga age coincides with a phase of regional deformation, metamorphism and gold mineralization along a major crustal lineament, whereas the most common monazite age population (at 2.16 Ga) corresponds with the migration of a foreland fold-and-thrust belt across the craton. The youngest age (1.65 Ga) coincides with an episode of tectonic reworking in the Capricorn Orogen along the southern Pilbara margin. The prolonged history of monazite growth may, in part, relate to channelized fluid flow during reactivation of long-lived N- to NE-trending crustal structures that transect the craton. Despite repeated episodes of metamorphism, the isotopic system in each generation of monazite remained unperturbed, yielding precise dates. The ability of monazite to record three separate events, and in some instances two events in a single crystal, distinguishes it from most other low-temperature mineral chronometers, which are readily reset during metamorphic overprinting. Low-temperature monazite geochronology can provide a detailed isotopic history of cryptic thermal events and reveal the temporal and spatial patterns of far-field fluid flow related to tectonic processes. The previously unrecognized history of crustal fluid flow in the Pilbara Craton has implications for chemical, mineralogical and isotopic studies seeking to understand conditions on the early Earth.

SHRIMP U – Pb depositional age for the lower Hardey Formation: evidence for diachronous deposition of the lower Fortescue Group in the southern Pilbara region, Western Australia

A SHRIMP zircon U – Pb age is reported for the Hardey Formation, lower Fortescue Group in the southern Pilbara region of Western Australia. Zircons extracted from an accretionary lapilli-bearing siltstone of the Hardey Formation give a 207Pb/206Pb age of 2750 ± 5 Ma (n = 36), which is regarded as an age of felsic volcanism contemporaneous with deposition of the Hardey Formation. Published zircon ages, primarily on felsic intrusives within the Hardey Formation in the northern Pilbara are statistically significantly different, and older than the age of ca 2750 Ma presented here, suggesting that a diachronous surface existed across the Pilbara Craton during deposition of the Hardey Formation.

Fish fauna in inland waters of the Pilbara (Indian Ocean) Drainage Division of Western Australia evidence for three subprovinces

This paper describes the distribution of fishes in inland waters of the Pilbara (Indian Ocean) Drainage Division of Western Australia. 48 842 fish representing 29 species (including one undescribed plotosid catfish) were recorded from 148 of the 171 sites sampled in 21 river systems throughout the Pilbara Drainage Division, i.e. from the Irwin River in the south to the DeGrey River in the north. Of these, 26 844 were from 13 native freshwater species (this total includes the catadromous Indian short-finned eel Anguilla bicolor McClelland 1844 and an undescribed plotosid catfish), 3 099 were from 12 marine/estuarine species and a further 18 899 were from four introduced species. In addition, the Pilbara Drainage Division contains two endemic cave fishes in the North West Cape (Humphreys & Adams 1991; Allen et al. 2002). The results of this study suggest that the Pilbara Drainage Division can be divided into three subprovinces, one for the westwards flowing rivers, i.e. from the Greenough to Lyndon (Southern Pilbara Subprovince), another for the northwards flowing rivers, i.e. from the Yannarie to the DeGrey (Northern Pilbara Subprovince), and a third for the subterranean waters of North West Cape (North West Cape Subprovince).