East Pilbara Research Articles

Paleomagnetic evidence for modern-like plate motion velocities at 3.2 Ga.

The mode and rates of tectonic processes and lithospheric growth during the Archean [4.0 to 2.5 billion years (Ga) ago] are subjects of considerable debate. Paleomagnetism may contribute to the discussion by quantifying past plate velocities. We report a paleomagnetic pole for the ~3180 million year (Ma) old Honeyeater Basalt of the East Pilbara Craton, Western Australia, supported by a positive fold test and micromagnetic imaging. Comparison of the 44°±15° Honeyeater Basalt paleolatitude with previously reported paleolatitudes requires that the average latitudinal drift rate of the East Pilbara was ≥2.5 cm/year during the ~170 Ma preceding 3180 Ma ago, a velocity comparable with those of modern plates. This result is the earliest unambiguous evidence yet uncovered for long-range lithospheric motion. Assuming this motion is due primarily to plate motion instead of true polar wander, the result is consistent with uniformitarian or episodic tectonic processes in place by 3.2 Ga ago.

Pilgangoora Lithium-Tantalum Project: deposit geology and new constraints on rare-metal pegmatite genesis

The Pilgangoora lithium-tantalum project encompasses a globally significant Li-Ta pegmatite district which formed in the late-Mesoarchaean, along the western margin of the East Pilbara Terrane in W...

A new 3.59 Ga magmatic suite and a chondritic source to the east Pilbara Craton

The Pilbara Craton, Western Australia hosts one of the best-preserved Paleoarchean granite-greenstone terrains on Earth, and is inferred to have developed on an older (>3.8 Ga), possibly Hadean, continental substrate. Such ancient crust has, however, never been identified in outcrop. Here, we show that metamorphosed gabbroic, leucogabbroic and anorthositic rocks of the South Daltons area, in the western part of the Shaw Granitic Complex, formed at 3.59–3.58 Ga and were intruded by granitic magma at 3.44 Ga. The 3.59–3.58 Ga gabbroic rocks, here named the Mount Webber Gabbro, represent the oldest, unambiguous igneous rock emplacement in the Pilbara Craton and significantly predate the oldest volcanic activity of the 3.53–3.23 Ga Pilbara Supergroup within the East Pilbara Terrane. We interpret the Mount Webber Gabbro samples to represent fragments of a dismembered layered mafic intrusion. Mantle-like zircon δ18O and Hf isotope signatures indicate derivation from a chondritic to near chondritic mantle at ~3.59 Ga, and do not support the existence of a >3.8 Ga basement to the East Pilbara Terrane. These results strengthen the notion of an approximately chondritic >3.5 Ga mantle beneath the Pilbara Craton, and provide further evidence that recent estimates of Archean stabilised continental volumes, based on the assumption of crust extraction from a global, convecting depleted mantle reservoir, may be overestimated.

The Pilgangoora Lithium-Tantalum Deposit - Geological Overview and Evolution of Discovery

The Pilgangoora Lithium-tantalum pegmatite deposit with a total resource of 156.3 Mt grading 1.25% Li2O and 128ppm Ta2O5, is a globally significant hard-rock lithium-tantalum deposit. The deposit is located in the East Pilbara Terrane of the northwest Pilbara Craton in Western Australia. The Northwest Pilbara Craton is one of the world’s major lithium-tanatlum provinces with large scale lithium-caesium-tantlum bearing pegmatites located at Mt Francisco, Wodgina, Pilgangoora and Strelley. Pegmatites bearing columbite-tantalite at Mt. York were first described in government geological surveys in 1906. Subsequent interest in the pegmatites focussed on their tin and tantalum mineral potential, with small scale hardrock, eluvial and alluvial mining, chiefly in the period 1947-1978. Larger scale alluvial and eluvial mining of tin-tantalum was carried out over 1978-1982 and 1992-1996 by a number of junior companies. In May 2014, Pilbara Minerals acquired the Pilgangoora Project for its lithium potential and has since drilled over 1450 holes for approximately 120,000 metres. The Pilgangoora pegmatite intrusions crop out in a well exposed greenstone belt, with little weathering at surface. Exploration drilling programs along with detailed geological mapping of the Pilgangoora tenement group has provided a better understanding of the geological setting of the fractionated pegmatite intrusions within the East Strelley greenstone belt. This work has led to the recognition of some valuable exploration criteria that may be applied locally to locate additional resources and, longer term, may be used more strategically to review other pegmatite fields across the Pilbara region.

Zircon U–Pb, Lu–Hf and O isotopes from the 3414 Ma Strelley Pool Formation, East Pilbara Terrane, and the Palaeoarchaean emergence of a cryptic cratonic core

The detrital zircon record yields important information on crustal evolution that may be missing from extant magmatic rocks. The Palaeoarchaean to Neoarchaean East Pilbara Terrane (EPT), Western Australia, is the ancient core of the Pilbara Craton, and the archetypal granite-greenstone terrane. Magmatic zircon U–Pb crystallization ages from the EPT record crustal magmatism spanning 3.53–3.22 Ga. However, detrital zircons with ages as old as 3.65 Ga have been identified in EPT supracrustal sequences, which may provide key insights into the EPT’s early Archaean history. The Strelley Pool Formation (SPF), one of the earliest EPT siliciclastic sedimentary successions, is a critical unit in the field of Precambrian palaeobiology, containing multiple lines of evidence for some of Earth’s earliest life forms. The SPF was deposited on a continental shelf in a shallow water environment, upon perhaps the oldest preserved terrestrial erosion surface on Earth; it thus provides a record of a newly-emergent Palaeoarchaean continent. We report U–Pb, Lu–Hf and O isotope data from a suite of detrital zircon crystals sourced from the type locality of the SPF. Two hundred and five U–Pb detrital zircon analyses were undertaken via SIMS and LA–ICPMS. Four analyses define a young age peak of 3414 ± 34 Ma (2σ), which we interpret as a maximum depositional age. Zircon Hf isotope analyses yield εHfi of −3.5 to +5.2, clustering around chondritic values, with two-stage Hf crustal model ages of 3.9–3.5 Ga. Zircon O isotopes give a range in δ18O values from mantle-like (5.3‰) to more elevated (∼6.4‰). Taken together, the Hf and O isotope record, from new and published detrital and magmatic zircon crystals, implies a juvenile (mafic) source for evolved rocks of the EPT until ca. 3.3 Ga, after which time it experienced a period dominated by reworking of existing crust, possibly reflective of a change in geodynamics. The relationship between the cryptic core of the EPT, which is perhaps as old as 3.8–3.7 Ga, and the extant magmatic rocks that comprise the exposed Palaeoarchaean granite domes, remains uncertain.

Genesis of the Archean–Paleoproterozoic Tabletop Domain, Rudall Province, and its endemic relationship to the West Australian Craton

ABSTRACTThe Tabletop Domain of the Rudall Province has been long thought an exotic entity to the West Australian Craton. Recent re-evaluation of this interpretation suggests otherwise, but is founded on limited data. This study presents the first comprehensive, integrated U–Pb geochronology and Hf-isotope analysis of igneous and metasedimentary rocks from the Tabletop Domain of the eastern Rudall Province. Field observations, geochronology and isotope results confirm an endemic relationship between the Tabletop Domain and the West Australian Craton (WAC), and show that the Tabletop Domain underwent a similar Archean–Paleoproterozoic history to the western Rudall Province. The central Tabletop Domain comprises Archean–Paleoproterozoic gneissic rocks with three main age components. Paleo–Neoarchean (ca 3400–2800 Ma) detritus is observed in metasedimentary rocks and was likely sourced from the East Pilbara Craton. Protoliths to mafic gneiss and metasedimentary rocks are interpreted to have been emplaced and deposited during the early Paleoproterozoic (ca 2400–2300 Ma), and exhibit age and isotopic affinities to the Capricorn Orogen basement (Glenburgh Terrane). Mid–late Paleoproterozoic mafic and felsic magmatism (ca 1880–1750 Ma) is assigned to the Kalkan Supersuite, which is exposed in the western Rudall Province. The Kalkan Supersuite provided the main source of detritus for mid–late Paleoproterozoic metasedimentary rocks in the Tabletop Domain. Similarities in the age and Hf-isotope compositions of detrital zircon from these metasedimentary rocks and Capricorn Orogeny basin sediments suggests that a regionally extensive, linked basin system may have spanned the northern WAC at this time. The Tabletop Domain records evidence for two metamorphic events. Mid–late Paleoproterozoic deformation (ca 1770–1750 Ma) was high-grade, regional and involved the development of gneissic fabrics. In contrast, early Mesoproterozoic (ca 1580 Ma) high-grade deformation was localised and associated with more widespread, late-stage, greenschist facies alteration. These new findings highlight that the Tabletop Domain experienced a much higher grade of deformation than previously assumed, with a Paleoproterozoic metamorphic history similar to that of the western Rudall Province.

Isotopic insight into the Proterozoic crustal evolution of the Rudall Province, Western Australia

The Proterozoic assembly of Australia involved the convergence of three main Archean cratonic entities: the North, West and South Australian Cratons, and is recorded in the Proterozoic orogenic belts surrounding these continental nuclei. The Rudall Province of northern Western Australia is the sole exposure of a Paleo- to Mesoproterozoic orogen lying between the North and West Australian Cratons, and may record the effects of their amalgamation. We present new zircon O, U–Pb and Lu–Hf isotope data from magmatic rocks across the Rudall Province, to which we add existing isotope data to yield a crustal evolution overview. Hf evolution trends for the ca. 1804–1762 Ma Kalkan Supersuite, the ca. 1589–1549 Ma Krackatinny Supersuite and the ca. 1310–1286 Ma Camel Suite, indicate a significant input of Archean East Pilbara Basement material, albeit as a mix with more juvenile material, including a possible ca. 1900 Ma component. Zircon δ18O data suggest a contribution from supracrustal material into the magmatic source of the Kalkan Supersuite, which may have been emplaced in an extensional setting. In contrast, the Krackatinny Supersuite and Camel Suite have mantle-like δ18O which may reflect partial melting of deeper Archean sources. Geochemical data for the Krackatinny Supersuite shows geochemical trends implying that melting of thickened mafic crust progressed from deeper to shallower levels, possibly in a rift setting. Camel Suite K-rich leucogranites may also have been emplaced in an extensional setting towards the end of high-P metamorphism. All terranes of the Rudall Province are para-autochthonous with respect to the Pilbara Craton, with no requirement for arc-related magmatism. We outline two potential scenarios for the Paleo- to Mesoproterozoic geodynamic evolution of the Rudall Province: an early cratonic amalgamation between the West and North Australian Cratons ca. 1680 Ma followed by Mesoproterozoic intraplate events; or a later assembly ca. 1377–1275 Ma. We lean towards this later amalgamation scenario.

Earth's oldest stable crust in the Pilbara Craton formed by cyclic gravitational overturns

During the early Archaean, the Earth was too hot to sustain rigid lithospheric plates subject to Wilson Cycle-style plate tectonics. Yet by that time, up to 50% of the present-day continental crust was generated. Preserved continental fragments from the early Archaean have distinct granite-dome/greenstone-keel crust that is interpreted to be the result of a gravitationally unstable stratification of felsic proto-crust overlain by denser mafic volcanic rocks, subject to reorganization by Rayleigh–Taylor flow. Here we provide age constraints on the duration of gravitational overturn in the East Pilbara Terrane. Our U–Pb ages indicate the emplacement of ~3,600–3,460-million-year-old granitoid rocks, and their uplift during an overturn event ceasing about 3,413 million years ago. Exhumation and erosion of this felsic proto-crust accompanied crustal reorganization. Petrology and thermodynamic modelling suggest that the early felsic magmas were derived from the base of thick (~43 km) basaltic proto-crust. Combining our data with regional geochronological studies unveils characteristic growth cycles on the order of 100 million years. We propose that maturation of the early crust over three of these cycles was required before a stable, differentiated continent emerged with sufficient rigidity for plate-like behaviour.

What drives metamorphism in early Archean greenstone belts? Insights from numerical modeling

Geodynamic regimes responsible for the formation of Eoarchean-to-Mesoarchean continental crust may be investigated using numerical modeling in which deeper mantle processes are coupled with shallower processes responsible for the formation and modification of the crust. In an earlier study using a 2D coupled petrological–thermomechanical tectono-magmatic numerical model constrained by information from the geological record, experiments with initial conditions appropriate to the Eoarchean–Mesoarchean revealed multiple tectonic processes by which mostly felsic continental crust could have been formed from an initial primary mafic crust in a non-uniformitarian geodynamic regime. We use the model pressure–temperature–time (P–T–t) paths for material particles from our previous study to test the hypothesis that the range of metamorphic P–T conditions recorded in Archean granite–greenstone domains could have been generated without subduction and plate tectonics. We compare the experimental results with P–T and age data recovered from two early Archean domains, the East Pilbara Terrane (EPT), part of the Pilbara craton in Western Australia, and the Barberton greenstone belt (BGB), part of the Kapvaal craton in South Africa. The dome-and-keel structures that develop in the crust in the numerical experiment and the associated P–T–t paths at various locations within these structures are similar to the structures and P–T and age data recovered from the early Archean crust in the EPT and BGB. This study shows that metamorphism in granite–greenstone-like crust could have been the result of both vertical and horizontal tectonic processes (coupled with crustal convective overturns) that occurred over linked sites of crustal delamination and mantle upwelling. The range of thermal gradients derived from peak P–T values for markers in the experiment matches the range of apparent thermal gradients retrieved from the EPT and BGB, demonstrating that even the lowest apparent thermal gradients associated with dome-and-keel structures in greenstone belts can be achieved in the absence of subduction.

Pilbara rock art: laser scanning, photogrammetry and 3D photographic reconstruction as heritage management tools

Recording techniques such as laser scanning, photogrammetry and photographic reconstruction are not new to archaeology. However as technology evolves and becomes more readily available such methods are being more regularly employed within a cultural heritage management context, often by people with little experience in using these technologies for heritage applications. For most cultural heritage management practitioners, the awe and lure of technology and the ease with which it can bring archaeology to life can distract from the end game of managing the site on the ground. This paper examines the advantages and disadvantages of laser scanning, photogrammetry and photographic reconstruction in recording, managing and interpreting rock art sites with an emphasis on its practical applications to the field of heritage management. Using a case study from West Angelas in the East Pilbara region of Western Australia, we will examine how these technologies assist in the practical management of heritage sites, and the significant outputs achieved for Aboriginal stakeholder groups in remote access to, and the interpretation of indigenous heritage sites.

Statistical model selection between elastic and Newtonian viscous matrix models for the microboudin palaeopiezometer

We carried out statistical evaluations of two probability density functions for the microboudin palaeopiezometer using the Akaike information criterion (AIC) and the cross-validation (CV) technique. In terms of the relevant stress-transfer model, these functions are defined as the elastic matrix model and the Newtonian viscous matrix model, respectively. The AIC and CV techniques enable us to evaluate the relative quality of both models when applied to nine data sets collected from metachert samples containing tourmaline grains in a quartz matrix, collected from the East Pilbara Terrane, Western Australia. The results show that the elastic matrix model is the more appropriate probability density function for analysis of fracturing of tourmaline grains in a quartz matrix. This statistical evaluation shows the validity of the elastic matrix model for the microboudin palaeopiezometer when analysing such data sets.Graphical abstract.

Archaean basin margin geology and crustal evolution: an East Pilbara traverse

A palinspastic reconstruction of a 100 km long traverse through Archaean rocks of the East Pilbara, Western Australia, includes new observations of the deformation preceding the now visible greenstone belt pattern. The restoration is time-calibrated with all available U–Pb datings. Between incompletely preserved basin sequences, two superposed Palaeoarchaean volcano-sedimentary basins (the Coongan and Salgash Basins) are separated by an eastwards time-transgressive interface tentatively interpreted as an onlap surface. For over 140 Ma, the basin margin architecture was structurally controlled by superposed extensional growth fault arrays (D 1 ) with associated dyke swarms in a curved pattern spatially not related to that of the actual distribution of granite domes and greenstone belts. The basins are interpreted to have formed by collapse after arching above hotspots due to phase transitions by mini-subduction of slabs of cooled water-saturated basalt towards the base of an originally c. 45 km mafic crust. At c. 3.31 Ga, the extension was replaced by plate-driven regional NW–SE compression (D 2 ) inferred from NW-over-SE shear and ramp-and-flat thrusts, partly reversing offsets of the D 1 extension. The recognition of widespread D 2 pre-doming compression is important because it triggered the c. 3.18 Ga start of formation of the dome-and-keel pattern (D 3 ) visible today, which culminated at c. 2.9 Ga. Supplementary material: Eight figures, numbered ‘Supplementary material fig. a–h’, and a GPS list of observation sites are available at: https://doi.org/10.6084/m9.figshare.c.3808243

Processes of crust formation in the early Earth imaged through Hf isotopes from the East Pilbara Terrane

The Pilbara Craton, Western Australia, is one of the best preserved Palaeo- to Mesoarchaean terrains on Earth. The East Pilbara Terrane is the archetypical granite-greenstone belt, the dome-like complexes of which were formed through three major magmatic events. These granite domes are comprised of metamorphosed granitic igneous rocks that exhibit a magmatic evolution from early tonalite-trondhjemite-granodiorite (TTG) rocks towards K-rich granites over the period 3.53–2.83Ga. Accordingly, East Pilbara has been a focus for workers seeking to constrain early Archaean geodynamic processes. One way to inform on this debate is to interrogate successive igneous supersuites using tools sensitive to magmatic source. We present new zircon Hf and whole-rock Nd isotope data from four major supersuites of the Mount Edgar Dome, of the East Pilbara Terrane. Early ca. 3.45Ga TTGs exhibit isotopic signatures that imply their partial derivation from existing crust, with addition of some juvenile material. Subsequent Palaeoarchaean magmatic events show a secular trend towards more evolved isotopic signatures, interpreted as a dominance of increasing reworking of existing crust, with only minor addition of new juvenile crust. The implication of this is that these later Palaeoarchaean supersuites were largely derived from the melting of older granitic crust, with mass balance modelling suggesting an input of ca. 20% juvenile (depleted mantle) material. The limited addition of juvenile material and increased reworking of existing crust with time, does not support a model of modern-style subduction, but is consistent with vertical tectonic processes in a volcanic plateau-type setting for the East Pilbara Terrane >3.2Ga. All Palaeoarchaean Mount Edgar samples resolve to two-stage Hf model ages of ca. 3.7Ga and Nd model ages of ca. 3.6Ga. These Eoarchaean model ages support the existence of a cryptic pre-3.5Ga protocrust, albeit of unknown extent. Analysis of late Mesoarchaean granites yields highly evolved Hf isotope signatures of K-rich monzogranites. This is consistent with a trend in the East Pilbara Terrane from sodic TTG type magmatism in the early Palaeoarchaean to K-rich magmas in the late Mesoarchaean.

Earth's first stable continents did not form by subduction.

The geodynamic environment in which Earth's first continents formed and were stabilized remains controversial. Most exposed continental crust that can be dated back to the Archaean eon (4 billion to 2.5 billion years ago) comprises tonalite-trondhjemite-granodiorite rocks (TTGs) that were formed through partial melting of hydrated low-magnesium basaltic rocks; notably, these TTGs have 'arc-like' signatures of trace elements and thus resemble the continental crust produced in modern subduction settings. In the East Pilbara Terrane, Western Australia, low-magnesium basalts of the Coucal Formation at the base of the Pilbara Supergroup have trace-element compositions that are consistent with these being source rocks for TTGs. These basalts may be the remnants of a thick (more than 35 kilometres thick), ancient (more than 3.5 billion years old) basaltic crust that is predicted to have existed if Archaean mantle temperatures were much hotter than today's. Here, using phase equilibria modelling of the Coucal basalts, we confirm their suitability as TTG 'parents', and suggest that TTGs were produced by around 20 per cent to 30 per cent melting of the Coucal basalts along high geothermal gradients (of more than 700 degrees Celsius per gigapascal). We also analyse the trace-element composition of the Coucal basalts, and propose that these rocks were themselves derived from an earlier generation of high-magnesium basaltic rocks, suggesting that the arc-like signature in Archaean TTGs was inherited from an ancestral source lineage. This protracted, multistage process for the production and stabilization of the first continents-coupled with the high geothermal gradients-is incompatible with modern-style plate tectonics, and favours instead the formation of TTGs near the base of thick, plateau-like basaltic crust. Thus subduction was not required to produce TTGs in the early Archaean eon.

SHRIMP U–Pb zircon geochronology establishes that banded iron formations are not chronostratigraphic markers across Archean greenstone belts of the Pilbara Craton

Banded iron formation (BIF) of the c. 3020Ma Cleaverville Formation is correlated across the Pilbara Craton and is considered to be the first stratigraphic unit common to the West Pilbara Superterrane and the East Pilbara Terrane. New field mapping and geochronology from the Shay Gap and Goldsworthy belts on the northeastern margin of the East Pilbara Terrane test that correlation. Along that margin, the Farrel Quartzite at the base of the Gorge Creek Group nonconformably overlies granite and unconformably overlies metavolcanic rocks. The Farrel Quartzite is conformably overlain by what is interpreted to be Cleaverville Formation, which is divided into a lower BIF member, a middle mudstone member and an upper BIF member. We have dated a tuff from the middle mudstone member by the Sensitive High-Resolution Ion Microprobe (SHRIMP) U-Pb zircon method at 3104±16Ma. The underlying Farrel Quartzite has a maximum depositional age of 3295±5Ma and no zircon population at c. 3105Ma. Therefore, the SHRIMP date for the tuff does not represent reworking of underlying sedimentary rocks and is interpreted to be a depositional age. This new date refutes correlation of the Cleaverville Formation across the craton, and it implies that BIFs should not be used as chronostratigraphic markers across Archean greenstone terrains. We suggest reinstating the name Nimingarra Iron Formation for BIFs that crop out along the northeastern margin of the Pilbara Craton, to distinguish them from the Cleaverville Formation. If the Cleaverville Formation, as defined in the West Pilbara Superterrane, is not the oldest stratigraphic unit common to the craton, then our understanding of the Mesoarchean evolution of the Pilbara Craton needs to be revised.

Geology and geochemistry of fault-hosted hydrothermal and sedimentary manganese deposits in the Oakover Basin, east Pilbara, Western Australia

ABSTRACTManganese mineralisation in the Oakover Basin is associated with Mesoproterozoic extension, basin formation and deposition of the Manganese Group. The underlying basement architecture of the Oakover Basin (a local half-graben geometry), inherited from the Neoarchean rifting event, plays an important role on the distribution, style and timing of manganese deposits. Fault-hosted manganese deposits are dominant along the ‘active’ faulted eastern margin, whereas flat-lying sedimentary deposits are dominant along the western ‘passive’ margin reflecting differences in ore-forming processes. The large number of significant manganese deposits in the Oakover Basin, previously thought to reflect a spatial association with Carawine Dolomite, more likely reflects the restricted nature of the Mesoproterozoic basin and development of a large reservoir of Mn2+ and Fe2+ in an anoxic zone of a stratified basin. Low O2 conditions in the basin were caused by a paleotopographic high forming a barrier to open ocean circulation. The western margin sedimentary deposits formed later than the fault-hosted hydrothermal deposits along the eastern margin, once a significant reservoir of Mn2+ and Fe2+ had developed, and when there was sufficient subsidence to allow migration of the redox front onto the shallow shelf, with Mn precipitation on and within the seafloor sediments. The sedimentary manganese deposits are not uniformly distributed along the western edge of the basin; instead they are concentrated into discrete areas (e.g. Mt Cooke–Utah–Mt Rove, Bee Hill, Skull Springs and the Ripon Hills districts), suggesting a degree of structural control on their distribution. Fault-hosted manganese is observed beneath and adjacent to many of the sedimentary deposits. Marked geochemical differences are observed between the Woodie Woodie hydrothermal deposits and the sedimentary deposits. Woodie Woodie deposits display higher Ba, U, Mo, As, Sn, Bi, Pb, S and Cu than the sedimentary deposits, reflecting the composition of the hydrothermal fluids. The Al2O3 values of the Ripon Hills and Mt Cooke deposits are much higher than the Woodie Woodie deposits, reflecting the composition of the dominant host rock, as Al2O3 is typically <5 wt% in the Carawine Dolomite, but is >10 wt% in basal shale units of the Manganese Group. Highly variable Mn:Fe ratios (≫5:1) in the hydrothermal manganese at Woodie Woodie reflects rapid deposition of Mn in and around fault zones. In contrast, slower accumulation of Mn oxides on and within the seafloor to form the large sedimentary deposits results in Mn:Fe ratios closer to 1:1 and elevated Co + Ni and REE values.

Lithostratigraphy and structure of the early Archaean Doolena Gap greenstone belt, East Pilbara Terrane, Western Australia

The early Archaean East Pilbara Terrane of the Pilbara Craton represents the archetypical granite dome – greenstone keel terrain and preserves much of the primary geological features of dome-and-keel formation. Here we present the first detailed lithostratigraphic and structural transect from marginal gneisses of the Muccan Granitic Complex through 6km of steeply dipping strata of the western Doolena Gap greenstone belt.Based on overprinting relationships, we identified five deformational events (D1–D5) that can be correlated between four structural domains: (i) marginal orthogneisses of the polyphase Muccan Granitic Complex, (ii) a dominantly mylonitic shear zone (South Muccan Shear Zone) representing a granite dome – greenstone keel detachment zone, (iii) a Central Fold Belt of dominantly mafic greenschists, and (iv) a lower greenschist- to sub-greenschist facies southern Low-Strain Belt.The D1 event in the Low-Strain Belt is recorded in syn-depositional normal faulting (f1) during eruption of deep water low vesicular pillow basalts of the ca. 3.47Ga Mount Ada Basalt, followed by a conformable transition towards shallow water stromatolites in the Duffer Formation. We interpret the D1 event as the result of upper-crustal extension and subsequent uplift above the Muccan Granitic Complex.The D2 event shows the most intense deformation with syn-anatectic tight folds within the Muccan Granitic Complex and tight to isoclinal transposed folds (F2) and fluid-induced normal faults (f2) within the Central Fold Belt. The D2 event resulted in a regional EW composite S1/2 foliation, and in the development of the mylonitic South Muccan Shear Zone along the dome-keel interface and within the Central Fold Belt, mineral lineations (L2) in conjunction with increasing southward D2l-tectonite. This D2 event is not observed in a ∼1km-thick sandstone package (ascribed to the <3.43Ga Strelley Pool Formation) in the Low-Strain Belt that was deposited over an angular unconformity indicating 45° SSW tilting of the underlying Mount Ada Basalt and Duffer Formation. This constrains the D2 event to between 3.47 and 3.43Ga.We propose that the D1 and D2 events record locally significant initiation of Muccan Granitic Complex doming and associated sagduction of greenstone keel rocks, suggesting that partial convective overturn initiated early in the evolution of the East Pilbara Terrane.

A new ∼3.46 Ga asteroid impact ejecta unit at Marble Bar, Pilbara Craton, Western Australia: A petrological, microprobe and laser ablation ICPMS study

The Archean rock record contains seventeen asteroid impact ejecta units that represent the terrestrial vestiges of an extended late heavy bombardment (LHB). Correlated impact ejecta units include 3472–3470Ma impact spherule layers in the Barberton Greenstone Belt, Kaapvaal Craton, South Africa, and the Pilbara Craton, Western Australia, and several ejecta units dated between 3250 and 3220Ma and between 2630 and 2480Ma (Lowe and Byerly, 2010; Lowe et al., 2003, 2014). This paper reports the discovery and investigation of a new impact ejecta unit within the Marble Bar Chert Member (MBCM) of the felsic volcanic Duffer Formation, east Pilbara Craton, Western Australia. The age of the MBCM is constrained by a 3459±2Ma U–Pb zircon date from the uppermost volcanic unit of the Duffer Formation and by a 3449±3Ma U–Pb zircon date from the overlying felsic volcanic Panorama Formation, stratigraphically above the intervening un-dated Apex Basalt. The ejecta unit, observed in a drill core (ABDP 1) ∼4km south-southwest of Marble Bar, consists of multiple lenses and bands of almost totally silicified impact spherules 1–2mm in diameter. All internal primary textures of the spherules have been destroyed. Nonetheless, Fe-rich spherule rims, largely composed of secondary siderite, are well preserved. Chemical analyses of the rims reveal iron-magnesium carbonate displaying high Fe, Mg, Ni, Co and Zn. Whole-rock and in-situ analyses (X-ray fluorescence, Inductively Coupled Plasma Mass Spectrometry (ICPMS), electron-microprobe (EMP) and EMP-calibrated laser ICPMS) reveal that the rims contain high Ni abundances and high Ni/Cr ratios (<50). The spherules are separated by an arenite matrix and spherule lenses also occur within bedded chert. The spherules are particularly visible over some ∼14m of true stratigraphic thickness in which chert breccia is interpreted to represent a tsunami-generated diamictite affected by hydrothermal fragmentation and veining. Despite the almost total silicification of the MBCM whole-rock analysis by NIS Fire Assay and ICPMS indicates high Ir (2ppb) and a low Pd/Ir ratio (2.0), consistent with geochemical features of impact ejecta units. Dense concentrations of spherules at the 57–58m level and the 77m level of the core, separated by banded chert, raise the possibility of two distinct impact events. Stratigraphic and isotopic age data distinguish between the 3459 and 3449Ma age of the MBCM ejecta unit and ∼3470.1±1.9Ma impact ejecta units in the Antarctic Creek Member, Mount Ada Basalt, about 40km to the west of Marble Bar. In combination with a 3472±2.3Ma impact unit in the Barberton greenstone belt, these impact ejecta units record large Paleoarchean asteroid impacts, significant for understanding early bombardment rates on Earth and early crustal evolution.

Relationship between shape preferred orientation of tourmaline grains and stress field in a metachert collected from East Pilbara terrane, Western Australia: pure shear dominated deformation in the early Archean

Relationship between shape preferred orientation of tourmaline grains and stress field in a metachert collected from East Pilbara terrane, Western Australia: pure shear dominated deformation in the early Archean

Early evolution of large micro-organisms with cytological complexity revealed by microanalyses of 3.4 Ga organic-walled microfossils.

The Strelley Pool Formation (SPF) is widely distributed in the East Pilbara Terrane (EPT) of the Pilbara Craton, Western Australia, and represents a Paleoarchean shallow-water to subaerial environment. It was deposited ~3.4 billion years ago and displays well-documented carbonate stromatolites. Diverse putative microfossils (SPF microfossils) were recently reported from several localities in the East Strelley, Panorama, Warralong, and Goldsworthy greenstone belts. Thus, the SPF provides unparalleled opportunities to gain insights into a shallow-water to subaerial ecosystem on the early Earth. Our new micro- to nanoscale ultrastructural and microchemical studies of the SPF microfossils show that large (20-70 μm) lenticular organic-walled flanged microfossils retain their structural integrity, morphology, and chain-like arrangements after acid (HF-HCl) extraction (palynology). Scanning and transmitted electron microscopy of extracted microfossils revealed that the central lenticular body is either alveolar or hollow, and the wall is continuous with the surrounding smooth to reticulated discoidal flange. These features demonstrate the evolution of large micro-organisms able to form an acid-resistant recalcitrant envelope or cell wall with complex morphology and to form colonial chains in the Paleoarchean era. This study provides evidence of the evolution of very early and remarkable biological innovations, well before the presumed late emergence of complex cells.