East Pilbara Research Articles

Reply to comments on “Subsurface distribution of granites and greenstones in the Paleoarchean East Pilbara Terrane from 3-D gravity inversion” of Hayward et al. (2024)

Reply to comments on “Subsurface distribution of granites and greenstones in the Paleoarchean East Pilbara Terrane from 3-D gravity inversion” of Hayward et al. (2024)

Subsurface distribution of granites and greenstones in the Paleoarchean East Pilbara Terrane from 3-D gravity inversion

The Paleoarchean East Pilbara Terrane contains some of the oldest rocks on Earth, and has a distinctive surface architecture of granite domes surrounded by arcuate greenstone belts. Explanations for creation of the domes include fold interference, core complex formation and the subsidence of a dense, mafic crust between relatively buoyant granitic complexes. In the latter “sagduction” process, greenstones are often viewed as descending to the base of the crust. Here we present a 3-D inversion of Bouguer gravity data for density contrast constrained by shear wave velocities and a seismic reflection profile. We show that the East Pilbara Terrane is predominantly characterized by a two-layer crust with high-density greenstone rocks largely confined to the upper crust, which is inconsistent with vertical tectonic models requiring large-scale greenstone preservation in the lower crust. The eastern margin of the terrane is marked by a west-to-east transition from low-density ovoid bodies to linear northwest-trending sources associated with the Proterozoic Paterson Orogen. We identify the previously unknown 10–15 km wide Tumbinna Pool Dome, between the Mount Edgar and Yilgalong domes. Higher density sources related to the surface distribution of greenstone rocks largely occur above 10–15 km depth, though they may locally reach >15 km depth on the north side of the Mount Edgar Dome. Some granitic domes have higher density cores related to distinct granite suites, while the Split Rock Supersuite appears to exhibit systematically lower density at depths of 5–10 km. Low-density sources due to small intrusions such as the North Pole Dome only extend to depths up to 10 km, while the large Mount Edgar Dome can be identified at depths as great as 15–20 km. Some low-density bodies in the lower crust, one of which underlies the Yilgalong Dome, correlate with regions of reduced seismic reflectivity, which we interpret to be possible relict migmatisation zones or melt pathways. A weakly constrained ESE-trending high-density region immediately below the Moho correlates with isolated sub-Moho reflectors, and may be a relict zone of rifting and delamination.

Growth faults and avalanches: Reconstructing Paleoarchean basins in the Pilbara and Kaapvaal cratons

Four new volcano-sedimentary complexes (VSC's), respectively c. 3510, 3460, 3430, and 3320 Ma old, are identified in the East Strelley, Coongan, and Kelly belts of the East Pilbara craton and compared with the extraordinarily complete c. 3450 Ma Buck Reef-VSC in the southern African Kaapvaal craton. The VSC’s reveal an intricate relationship between volcanic deposition, sedimentation, development of syndepositional fault arrays, both extensional and contractional, and magmatism. The geometry and kinematics of these fault systems were analyzed after restoring the tilt of the stratigraphic sequences back to the depositional horizontal. The growth fault arrays are interpreted to have been generated by lack of lateral support of depositional basin margin prisms as known from present-day deltas and passive margins. This ‘basin margin collapse’ scenario for deformation and kinematics of the topmost section of the crust relies on topographic relief combined with vertical crustal oscillation. The relationship between supracrustal collapse and coeval deformation on deeper-seated detachments within the basement of the Pilbara remains as yet unsolved.The restoration corroborates the (semi-)circular basin architecture we proposed in 2017, which preceded, and is unrelated to, the present-day configuration of granitoid complexes and greenstone belts. The new findings also assess an early, syndepositional presence of the east–west Warrawoona Lineament as a major dividing line of as yet unknown structural character between two areas of basin superposition.For the Coonterunah Subgroup a multistorey architecture is established: the Table Top/Coucal-VSC with its near-water level chert top is overlain by regularly bedded Double Bar Basalt. The ensemble is truncated by the newly introduced tectono-stratigraphic Bergamina Unit, interpreted as a mega-avalanche emplaced somewhere between 3496 and 3466 Ma, possibly time-equivalent to the Duffer Fm.The crustal depths of the detachments below the (water-level) tops of all VSC’s identified, in other words the thicknesses of the VSC’s, are used as proxy for determining minimum basin centre depths ranging from 1000 to 3800 m. The greatest depth was reached in the North and South Coongan Basins, where voluminous bimodal volcanism of the Duffer Fm, maximum subsidence and deposition rates resulted in maximum basin margin instability.

Titanium and iron isotopic records of granitoid crust production in diverse Archean cratons

Archean granitoid provides critical clues on how Earth's felsic crust was established and its geodynamics evolved. In this study, we present Fe-Ti isotope and trace-element data for granitoids (mostly TTGs) from the ∼3.8-3.6 Ga Itsaq Gneiss Complex and ∼3.3 Ga East Pilbara Terrane, Pilbara Craton. TTGs from those localities and several other cratons follow the same Ti isotopic fractionation trend as modern calc-alkaline rocks. This similarity hides important petrogenetic differences as partial melting could have played a much more important role in establishing the felsic character of TTGs compared to modern granites of calc-alkaline affinity. Our thermodynamics-based modeling of isotopic fractionation shows that partial melting of hydrated metabasite can explain the Fe and Ti isotopic compositions of many TTGs but reworking of tonalite crust is needed to explain the most extreme compositions. The same Ti isotopic fractionation trends have been found now in several cratons (West Greenland, Slave, Pilbara, and Kaapvaal cratons) in granitoids formed in different geothermal gradients, meaning that it was likely a global signature of the Archean crust. Sediment provenance studies that use those TTG compositions and point to a primarily felsic crust in the Archean are therefore valid.

The unroofing of Archean crustal domes as recorded by detrital zircon and apatite

This study presents in-situ U–Pb, Lu–Hf, and Sm–Nd isotopic data for detrital zircon and apatite collected from ephemeral streams of the East Pilbara Terrane, Western Australia. Given their disparate abundances in felsic versus mafic lithologies, a tandem apatite-zircon approach may offer more holistic insights into crust formation. Apatite U–Pb data define a single age peak at c. 2.9 Ga, consistent with labile, proximal, and first-cycle detritus from the Pilbara Craton. Conversely, zircon, a more refractory and durable mineral, records a more diverse geological history with U–Pb ages spanning from 3.6 to 0.2 Ga. The apatite age of c. 2.9 Ga records the timing through the Pb closure temperature during regional cooling following prograde metamorphism, while the Lu–Hf and Sm–Nd isotopic systems in the same grains yield c. 3.2 Ga isochrons, consistent with magmatic crystallisation at that time. Crystallisation age, initial 143Nd/144Nd and trace element geochemistry (Eu/Eu*) imply a chondritic or mixed (more radiogenic plus a less radiogenic) source for the apatite grains locally derived from the East Pilbara Terrane. Conversely, zircon ɛHf data reveal a broadly chondritic Paleoarchean proto-crust undergoing continual isotopic evolution punctuated by the input of juvenile, more radiogenic material on a quasiperiodic basis. Previous workers have invoked a crust-mantle overturn model triggered by stagnant-lid cooling and the episodic (re)fertilisation of the upper mantle to account for the periodic nature of crust formation in the East Pilbara Terrane. Detrital zircon grains track this process from a c. 3.8 Ga component that may have acted as a nucleus for subsequent crust formation. The oldest detrital zircon, on average, encompass less radiogenic (−ve ɛHf) components suggesting that the oldest grains preserve the unroofing of an ancient reworked crustal nucleus. Thus, the detrital zircon load arguably provides a more holistic record of the older crust in the region than the crystalline domes alone. Specifically, the less radiogenic dome cores are preferentially eroded due to their structural position and their mineral cargo lost into the detrital archive. We demonstrate that the apatite-zircon approach can be limited by the ability of apatite to be retained through crustal denudation.

Constraints on the age of dome-and-keel structures in the Pilbara Craton through integrated garnet geochronology and microstructural analyses

The dome-and-keel granite-greenstone structures in the East Pilbara Terrane are commonly interpreted to result from a crustal overturn process that underpinned the growth and stabilization of continental crust on early Earth. We investigate the timing and timescales of this overturn process using integration of garnet and zircon geochronology with microstructural analyses. We use this integrated approach to provide insight into the evolution of dome-and-keel structures in the East Pilbara Terrane. Garnet Lu-Hf and Sm-Nd geochronology shows that two discrete periods of fabric development related to dome formation occurred between ∼ 3.42 to 3.39 Ga and 3.34 to 3.30 Ga. The older population of garnet is mechanically disaggregated, and we interpret that their growth is related to an early metamorphic event in the East Pilbara that occurred prior to the main phase of dome-and-keel formation. The younger population of garnet, between 3.34 and 3.30 Ga, include garnet with post-kinematic growth structures. By coupling these microstructures and garnet Lu-Hf ages, we show that the main phase of dome formation is dated by this younger event of garnet growth at about 3.34 and 3.30 Ga. The Sm-Nd ages of these garnets are systemically younger than the Lu-Hf ages, consistent with protracted cooling of the system after dome formation. Together, these data show that dome-and-keel structures in the East Pilbara formed quickly, perhaps in a few tens of million years.

Exceptional preservation of organic matter and iron-organic colloidal mineralization in hydrothermal black smoker-type sulfide mineralization from the Paleoarchean seafloor

Fossil organic matter (OM) in Paleoarchean rocks is an invaluable tracer of ancient life, yet it is often contentious due to low preservation potentials of its original organic molecular characteristics under generally high metamorphic grades. This study reports on exceptionally preserved OM within black smoker-type sulfide mineralizations from the 3.24 billion-years-old Sulphur Springs volcanic-hosted massive sulfide deposit, East Pilbara Terrane, northwestern Australia. Fine scale mineralogical and organic molecular variations – documented by SEM and TEM techniques, Raman and FTIR Spectroscopy, as well as ToF-SIMS analysis – trace the formation of millimetre-scale pyritic hydrothermal orifices. Overmature OM (≳200–250 °C) enriched in aromatic hydrocarbons occurs within earliest formed colloform-banded pyrite that could have precipitated at high temperature upon mixing of hot hydrothermal fluids with cold seawater. In contrast, mature OM (within the catagenesis window; ∼100–150 °C) with lower proportions of aromatic hydrocarbons occasionally occurs alongside pyrite within barite near the inner paleofluid conduits of the hydrothermal orifices. A deposition of this OM generation from cooler fluids enriched in sulfate during waning hydrothermal activity is corroborated by its association with original mineralogy, particularly the occurrence of isolated nanoinclusions of OM within hydrothermal barite away from grain boundaries or fissures and cracks, which renders impossible an origin from post-depositional hydrocarbon migration. When compared to the overmature OM within colloform-banded pyrite, the better-preserved inner OM not only contains less abundant, smaller, and less ordered aromatic hydrocarbons, but also higher amounts of aliphatic molecules with longer chain lengths and higher proportions of carbonyl and amide functional groups. Although the latter characteristics are consistent with contributions by microbial biomass, abiotic origins are equally plausible because hydrothermal processes can produce OM with similar organic molecular attributes. Irrespective of these uncertainties on the ultimate sourcing of OM, common intergrowths of the cooler OM with nanoscopic iron oxides – hematite and lesser magnetite – hint at its hydrothermal deposition from colloidal particles that have been stabilized by iron-organic complexation. Further research is required to ascertain the significance of this process for the hydrothermal cycling and release of organic carbons and bound iron into the Paleoarchean oceans. Collectively, our results show that ancient marine hydrothermal systems can preserve OM with differing degrees of thermal degradation, which allows for insights into its sourcing, cycling, and deposition.

Earth's Earliest Phaneritic Ultramafic Rocks: Mantle Slices or Crustal Cumulates?

AbstractWhen plate tectonics initiated remains uncertain, partly because many signals interpreted as diagnostic of plate tectonics can be alternatively explained via hot stagnant‐lid tectonics. One such signal involves the petrogenesis of early Archean phaneritic ultramafic rocks. In the Eoarchean Isua supracrustal belt (Greenland), some phaneritic ultramafic rocks have been dominantly interpreted as subduction‐related, tectonically‐exhumed mantle slices or cumulates. Here, we compared Eoarchean phaneritic ultramafic rocks from the Isua supracrustal belt with mantle peridotites, cumulates, and phaneritic ultramafic samples from the Paleoarchean East Pilbara Terrane (Australia), which is widely interpreted to have formed in non‐plate tectonic settings. Our findings show that Pilbara samples have cumulate and polygonal textures, melt‐enriched trace element patterns, relative enrichment of Os, Ir, and Ru versus Pt and Pd, and chromite‐spinel with variable TiO2 and Mg#, and relatively consistent Cr#. Both, new and existing data show that cumulates and mantle rocks potentially have similar whole‐rock geochemical characteristics, deformation fabrics, and alteration features. Geochemical modeling results indicate that Isua and Pilbara ultramafic rocks have interacted with low‐Pt and Pd melts generated by sequestration of Pd and Pt into sulphide and/or alloy during magmatism. Such melts cannot have interacted with a mantle wedge. Correspondingly, geochemical compositions and rock textures suggest that Isua and Pilbara ultramafic rocks are not tectonically‐exhumed mantle peridotites, but are cumulates that experienced metasomatism by fluids and co‐genetic melts. Because such rocks could have formed in either plate or non‐plate tectonic settings, they cannot be used to differentiate early Earth tectonic settings.

Plate motion and a dipolar geomagnetic field at 3.25 Ga

The paleomagnetic record is an archive of Earth's geophysical history, informing reconstructions of ancient plate motions and probing the core via the geodynamo. We report a robust 3.25-billion-year-old (Ga) paleomagnetic pole from the East Pilbara Craton, Western Australia. Together with previous results from the East Pilbara between 3.34 and 3.18 Ga, this pole enables the oldest reconstruction of time-resolved lithospheric motions, documenting 160 My of both latitudinal drift and rotation at rates of at least 0.55°/My. Motions of this style, rate, and duration are difficult to reconcile with true polar wander or stagnant-lid geodynamics, arguing strongly for mobile-lid geodynamics by 3.25 Ga. Additionally, this pole includes the oldest documented geomagnetic reversal, reflecting a stably dipolar, core-generated Archean dynamo.

The Kelly Dyke swarm, Pilbara Craton: a 3317 Ma large igneous province?

While the onset of plate tectonics has been proposed as far back in time as the Hadean, there is no evidence for rigid crust that can accommodate linear dyke swarms until ca 2700 Ma in the Pilbara Craton, and at ca 2500 Ma in most other Archean cratons. Here, we provide a precise 3317.0 ± 1.1 Ma baddeleyite U–Pb ID-TIMS age determination for a dolerite from a regionally extensive dyke swarm underlying the Euro Basalt in the East Pilbara Terrane, with links to the Kaapvaal Craton that would establish it as part of what may be the oldest large igneous provinces on Earth. We link this event to the deposition of large volcano-sedimentary basins, reflecting the brittle fracturing that enables emplacement of the dyke swarm and the overlying thick succession of mafic-to-ultramafic lavas. This is the first time that a dyke swarm has been used as evidence for rigid crust prior to 3000 Ma, suggesting that lithospheric conditions appropriate for some form of plate tectonics existed at least as far back as the Paleoarchean. KEY POINTS We provide a precise 3317.0 ± 1.1 Ma baddeleyite U–Pb ID-TIMS age determination for a dolerite in the Pilbara Craton. The dated dyke is part of the potentially oldest large igneous province on Earth. The dolerite dyke swarm was emplaced into rigid crust at ca 3.32 Ga. Lithospheric conditions that accommodated plate tectonics existed as least as far back as the Paleoarchean.

Greenstone‐Up Shear Sense at the Margin of the Mt Edgar Dome, East Pilbara Terrane: Implications for Dome and Keel Formation in the Early Earth

AbstractThe Paleoarchean Mt Edgar dome in the East Pilbara Terrane has long been studied as an archetypal dome within Archean dome‐and‐keel terranes, but the history of its formation is debated. Kinematic data presented in this study provide new insights into the late‐stage development of the Mt Edgar dome and East Pilbara Terrane. Quartz crystallographic preferred orientation (CPO), optical microstructures, and field structures all indicate that the granite‐greenstone contact of the Mt Edgar dome experienced reverse (greenstone‐up, dome‐down) sense of shear after the formation of the dominant schistosity. This reverse sense of shear is observed at localities along the entire extent of the sheared margin that rings most of the Mt Edgar dome, but is best documented along the southwest margin in the Warrawoona Greenstone Belt. Additionally, quartz CPO data from a dome triple junction outside of the sheared margin dominantly indicate a constrictional strain geometry, consistent with the previous interpretation that this area represents a zone of vertical foundering in a buoyancy‐instability driven system. However, buoyancy‐instability models do not necessarily predict the occurrence of greenstone‐up sense of shear preserved in solid‐state fabrics along the dome margin. Several geologic explanations are considered, including dome expansion or post‐doming deformation. The data are most consistent with explanations that directly relate to dome formation, especially when considered in tandem with recently published structural data from within the Mt Edgar dome. These kinematic data suggest that late dome development occurred in a near‐static crustal environment rather than an extensional or contractional setting.

Seismic evidence of two cryptic sutures in Northwestern Australia: Implications for the style of subduction during the Paleoproterozoic assembly of Columbia

Plate tectonics, including rifting, subduction, and collision processes, was likely to have been different in the past due to the secular cooling of the Earth. The northeastern part of the West Australian Craton (WAC) has a complex Archean and Paleoproterozoic tectonic history; therefore, it provides an opportunity to study how subduction and collision processes evolved during the emergence of plate tectonics, particularly regarding the assembly of Earth's first supercontinent, Columbia. Because the northeastern boundary of the WAC and the southwestern boundary of the North Australian Craton (NAC) are covered by the Phanerozoic Canning Basin, the regional tectonic evolution has remained enigmatic, including how many tectonic elements were assembled and what may have driven rifting and subsequent collision events. Here, we use new passive-source seismic modeling to identify a seismically distinct segment of the lithosphere, the Percival Lakes Province, which lies east of the Pilbara Craton and is separated by two previously unknown southeast-trending lithosphere scale Paleoproterozoic sutures. We interpret that the northeastern suture, separates the Percival Lakes Province from the NAC, records the amalgamation of the WAC with the NAC. The southwestern suture separates the PLP from the reworked northeastern margin of the Pilbara Craton, including the East Pilbara Terrane and the Rudall Province. A significant upper mantle dipping structure was identified in the southwestern suture, and we interpret it to be a relic of subduction that records a previously unknown Paleoproterozoic collision that pre-dated the amalgamation of the WAC and NAC. By comparing our findings with previously documented dipping features, we show that the Paleoproterozoic collisions are seismically distinguishable from their Phanerozoic counterparts.

A Pilbara perspective on the generation of Archaean continental crust

Models for the generation and evolution of continental crust are informed by a detailed understanding of regional geology and geochemistry. This contribution therefore evaluates the increasing amounts of high quality geochemical data, on well-characterized samples, now available from the magmatic rocks of the Pilbara Craton, Western Australia. The East Pilbara Terrane is characterized by vertical dome and basin tectonics, and the younger (Mesoarchean) rocks to the west are associated with lateral tectonics. The craton stabilized at ~2.95 Ga (Hickman, 2012), and it was overlain by voluminous, dominantly mafic lavas of the Fortescue Group in an intra-cratonic setting at ~2.7 Ga. The magmatic rocks of the Pilbara Craton are bimodal in SiO2, with the volcanic rocks typically having <60% SiO2, and the plutonic rocks >60% SiO2, and such differences are in part linked to different depths of magma emplacement.The volcanic rocks are divided into two groups, with Dy/La > and < 0.6. The geochemistry of the former group is consistent with broadly closed system differentiation, whereas the low Dy/La rocks largely reflect open system magmatic processes, and contributions from high silica magmas. The high Dy/La rocks have restricted Th/Nb ratios similar to those of primitive mantle, whereas the low Dy/La rocks range to higher Th/Nb values. In most cases these higher Th/Nb values reflect open system magma differentiation, but in some of the younger volcanic suites (the Bookingarra Group and Cooneeina Basalt) these values appear to be associated with subduction. There is no evidence for garnet or amphibole in the generation of the volcanic rocks, however the composition of the plutonic rocks reflects the involvement of both garnet and plagioclase, during melt generation and differentiation.The high Dy/La rocks have Sm/Nd ratios similar to CHUR, but their Lu/Hf ratios are significantly sub-chondritic. Many of the Pilbara rocks from 3.5–3.0 Ga were generated from mantle sources with broadly chondritic Hf isotope ratios, and so in the preferred model the mean composition of the high Dy/La group was generated by ~20% melting of chondritic mantle. Apart from the high Th/Nb magmas at 3.05–2.97 Ga (Smithies et al., 2005), there are surprisingly few changes in the minor and trace element features of the magmas associated with the generation of the craton (3.5–3.0 Ga), and after it stabilized at ~2.95 Ga. The 3.05–2.97 Ga plutonic rocks in the granitic complexes also have elevated Th/Nb, suggesting that such ratios were inherited from their mafic source rocks. This observation implies that the mafic sources and the granitic rocks were of similar age, and that there was rapid conversion of young mantle-derived basaltic crust into felsic continental crust, with little contribution from older basement material. The volcanic rocks are characterized by relatively low water contents (~<2%) on the basis of their low maximum Al2O3 values, a mean Fe3+/ΣFe of 0.19 ± 0.01, and models in which the high Dy/La rocks were derived from primitive mantle. Such low water contents relative to modern subduction-related magmas are a feature of the magmatic rocks of the Pilbara Craton, and the development of ‘granitic crust’ appears to be linked to the development of higher water contents associated with subduction, locally commencing at ca. 3.1 Ga, and to globally dominant plate tectonics.

Combined Sm-Nd, Lu-Hf, and 142Nd study of Paleoarchean basalts from the East Pilbara Terrane, Western Australia

The chemistry of the major reservoirs in the silicate Earth reflects a long history of differentiation into- and interaction between- incompatible element enriched crust and depleted mantle. Evidence from 142Nd variability in ancient mantle-derived rocks imply such differentiation began very early in Earth's history, while the short-lived 146Sm (half-life 103 Ma) was extant, but the size and composition of primitive reservoirs in the Hadean and early Archean is debated. Therefore, pivotal to the study of Earth evolution is the recognition of well-preserved ancient magmatic rocks that can provide robust constraints on source compositions to test geodynamic models. We present the first combined petrological, geochemical and isotope (146-147Sm-142-143Nd and 176Lu–176Hf) study of basaltic rocks from the East Pilbara Terrane, Pilbara Craton, Western Australia. The basaltic pillow lavas, from the ca. 3470 Ma Mount Ada Basalt of the Pilbara Supergroup, include quartz-phyric quartz-normative tholeiites, plagioclase-phyric olivine-normative tholeiites and clinopyroxene spinifex textured olivine-normative tholeiites and include samples that classify as siliceous high Mg basalts. Their major and trace-element compositions suggest that the basalts are high-degree partial melts (20–30%) produced at shallow mantle depths of 0.5–1 GPa, which is consistent with formation of both quartz-normative and olivine-normative parental magmas. Isotopic data from this sample suite defines Sm–Nd and Lu–Hf isochrons with dates of 3484 ± 113 Ma and 3463 ± 50 Ma, respectively. The samples yield coupled initial εNd = +1.1 and εHf = +2.1, and μ142Nd values indistinguishable from the modern mantle. The uniform initial εNd and εHf in samples from this study and other Mount Ada Basalt basaltic samples across four greenstone belts indicates that this portion of the Paleoarchean mantle was well mixed at the scale of melt generation and evolved with slightly suprachondritic Sm/Nd and Lu/Hf. Absence of 142Nd variations suggests the portion of the Paleoarchean Pilbara mantle sampled by these basalts retains no signature of Hadean Sm/Nd fractionation.

Internal structure of the Paleoarchean Mt Edgar dome, Pilbara Craton, Western Australia

The Paleoarchean East Pilbara Terrane of Western Australia is a dome-and-keel terrane that is often highlighted as recording a vertically convective tectonic regime in the early Earth. In this model, termed 'partial convective overturn', granitic domes diapirically rose through a dense, foundering mafic supracrustal sequence. The applicability of partial convective overturn to the East Pilbara Terrane and to other Archean dome-and-keel terranes is widely debated and has significant implications for early Earth geodynamics. A critical data gap in the East Pilbara Terrane is the internal structure of the granitic domes. We present field-based, microstructural, and anisotropy of magnetic susceptibility (AMS) data collected within the Mt Edgar dome to understand its internal structure and assess its compatibility with existing dome formation models. Field and microstructural observations suggest that most fabric development occurred under submagmatic and high-temperature solid-state conditions. The AMS results reveal a coherent, dome-wide structural pattern: 1) Sub-vertical lineations plunge radially inward towards the center of the dome and foliations across much of the dome consistently strike northwest; 2) Shallowly plunging lineations define an arch that extends from the center of the dome to the southwest margin; and 3) Migmatitic gneisses, which represent the oldest granitic component of the dome, are folded and flattened against the margin of the dome in two distinct lobes. The structural relationships between rocks of different ages indicate that units of different crystallization ages deformed synchronously during the last major pulse of granitic magmatism. These data are broadly consistent with a vertical tectonics model, and we synthesize our structural results to propose a three-stage diapiric evolution of the Mt Edgar dome. The critical stage of dome development was between 3.3 and 3.2 Ga, when widespread, melt-assisted flow of the deep crust led to the formation of a steep-walled, composite dome. These data suggest that diapiric processes were important for the formation of dome-and-keel terranes in the Paleoarchean.

Paleoarchean variole-bearing metabasalts from the East Pilbara Terrane formed by hydrous fluid phase exsolution and implications for Archean greenstone belt magmatic processes

Archean greenstone belts contain low grade metamorphic volcanic sequences with localised examples of preserved original volcanic textures and relict magmatic minerals, which are invaluable for inferring volcanic processes and by extension magma genesis. A feature observed in many Archean greenstone belts but rare in modern volcanic rocks is variolitic texture with millimetre- to centimetre-sized leucocratic globular structures (varioles), in mafic lavas. Models proposed to explain this texture include rapid undercooling, magma mingling and liquid immiscibility. Here, we investigate samples of Paleoarchean variole-bearing pillow lavas from the ca. 3469 Ma Mount Ada Basalt of the Pilbara Supergroup, Western Australia. The samples contain relict magmatic clinopyroxene and spinel and preserve sharp contacts between varioles and host. No plagioclase is observed in the samples. The clinopyroxene mineral composition is indistinguishable between in the varioles and host. This is despite the variole material having lower MgO and Fe2O3(total) and higher SiO2 and Al2O3 concentrations. However, the variole and host materials have indistinguishable TiO2, Cr2O3, NiO2 and Sm-Nd isotope systematics. These observations are consistent with an immiscibility reaction from a parental melt to a hydrous fluid phase and conjugate mafic liquid, where minerals that formed before the reaction become incorporated into both endmembers where they continued to grow by diffusive exchange between the hydrous fluid and silicate melt. Two lines of evidence indicate that hydrous flux exsolution occurred due to H2O saturation in the precursor magmas, including: (1) the lack of plagioclase, for which crystallisation is supressed at high H2O content; and (2) Nb/Th < 7, which indicates a hydrated mantle source. The variole-bearing pillow lavas are part of the 10-Myr-duration Coongan Subgroup volcanic cycle and occur interspersed with tholeiite basalts and minor komatiites of the Mount Ada Basalt and mafic to felsic volcanic rocks of the Duffer Formation. These distinctly different volcanic rock types, which have εNdi −0.3 ± 0.4, ~1ε unit lower than contemporary tholeiites, indicate that there was variation in magma genesis, which included hydrous fluxed melting likely genetically related to delamination, during the 10 Myr formation of the Coongan Subgroup volcanic cycle, as part of the Paleoarchean East Pilbara Terrane formation.

A southern African perspective on the co-evolution of early life and environments

AbstractThe Kaapvaal and Zimbabwe cratons host some of the earliest evidence for life. When compared to the contemporaneous East Pilbara craton, cherts and other metasedimentary horizons in southern Africa preserve traces of life with far greater morphological and geochemical fidelity. In spite of this, most fossiliferous horizons of southern Africa have received relatively limited attention. This review summarises current knowledge regarding the nature of early life and its distribution with respect to environments and ecosystems in the Archaean (&amp;gt;2.5 Ga) of the region, correlating stratigraphic, sedimentological, geochemical and palaeontological understanding. There is abundant and compelling evidence for both anoxygenic photosynthetic and chemosynthetic biomes dominating Palaeoarchaean-Mesoarchaean strata dating back to around 3.5 Ga, and the prevalence of each is tied to palaeoenvironmental parameters deducible from the rock record. Well-developed, large stromatolites characteristic of younger Mesoarchaean-Neoarchaean sequences were probably constructed by oxygenic photosynthesisers. Isotopic evidence from the Belingwe greenstone belt and the Transvaal Supergroup indicates that both a full sulphur cycle and complex nitrogen cycling were in operation by the Mesoarchaean-Neoarchaean. The Archaean geological record of southern Africa is thus a rich repository of information regarding the co-evolving geosphere and biosphere in deep time.

Palaeoarchaean TTGs of the Pilbara and Kaapvaal cratons compared; an early Vaalbara supercraton evaluated

AbstractThe continental crust that dominates Earth’s oldest cratons comprises Eoarchaean to Palaeoarchaean (4.0 to 3.2 Ga) felsic intrusive rocks of the tonalite-trondhjemite-granodiorite (TTG) series. These are found either within high-grade gneiss terranes, which represent Archaean mid-continental crust, or low-grade granite-greenstone belts, which represent relic Archaean upper continental crust. The Palaeoarchaean East Pilbara Terrane (EPT), Pilbara Craton, Western Australia, and the Barberton Granite-Greenstone Belt (BGGB), Kaapvaal Craton, southern Africa, are two of the best exposed granite-greenstone belts. Their striking geological similarities has led to the postulated existence of Vaalbara, a Neoarchaean-Palaeoproterozoic supercraton. Although their respective TTG domes have been compared in terms of a common petrogenetic origin reflecting a volcanic plateau setting, there are important differences in their age, geochemistry, and isotopic profiles. We present new zircon Hf isotope data from five granite domes of the EPT and compare the geochemical and isotopic record of the Palaeoarchaean TTGs from both cratons. Rare &amp;gt;3.5 Ga EPT evolved rocks have juvenile εHf(t) requiring a chondritic source. In contrast, younger TTG domes developed via 3.5 to 3.4 and 3.3 to 3.2 Ga magmatic supersuites with a greater range of εHf(t) towards more depleted and enriched values, trace element signatures requiring an enriched source, and xenocrystic zircons that reflects a mixed source to the TTGs, which variously assimilates packages of older felsic crust and a more juvenile mafic source. EPT TTG domes are composite and record multiple pulses of magmatism. In comparison, BGGB TTGs are less geochemically enriched than those of the EPT and have different age profiles, hosting coeval magmatic units. Hafnium isotopes suggest a predominantly juvenile source to 3.2 Ga northern Barberton TTGs, limited assimilation of older evolved crust in 3.4 Ga southern Barberton TTGs, but significant assimilation of older (Hadean-Eoarchaean) crust in the ca. 3.6 Ga TTGs of the Ancient Gneiss Complex. The foundation of the EPT is younger than that for the oldest components of the Eastern Kaapvaal. Although the broader prevailing Palaeoarchaean geologic framework in which these two cratons formed may reflect similar a geodynamic regime, the superficial similarities in dome structures and stratigraphy of both cratonic terranes is not reflected in their geochemical and age profiles. Both the similarities and the differences between the crustal histories of the two cratons highlights that they are formed from distinct terranes with different ages and individual evolutionary histories. Vaalbara sensu lato represents typical Palaeoarchaean cratonic crust, not in the sense of a single homogeneous craton, but one as diverse as the continents are today.

Convective isolation of Hadean mantle reservoirs through Archean time

Although Earth has a convecting mantle, ancient mantle reservoirs that formed within the first 100 Ma of Earth's history (Hadean Eon) appear to have been preserved through geologic time. Evidence for this is based on small anomalies of isotopes such as 182W, 142Nd, and 129Xe that are decay products of short-lived nuclide systems. Studies of such short-lived isotopes have typically focused on geological units with a limited age range and therefore only provide snapshots of regional mantle heterogeneities. Here we present a dataset for short-lived 182Hf-182W (half-life 9 Ma) in a comprehensive rock suite from the Pilbara Craton, Western Australia. The samples analyzed preserve a unique geological archive covering 800 Ma of Archean history. Pristine 182W signatures that directly reflect the W isotopic composition of parental sources are only preserved in unaltered mafic samples with near canonical W/Th (0.07 to 0.26). Early Paleoarchean, mafic igneous rocks from the East Pilbara Terrane display a uniform pristine µ182W excess of 12.6 ± 1.4 ppm. From ca 3.3Ga onward, the pristine 182W signatures progressively vanish and are only preserved in younger rocks of the craton that tap stabilized ancient lithosphere. Given that the anomalous 182W signature must have formed by ca 4.5 Ga, the mantle domain that was tapped by magmatism in the Pilbara Craton must have been convectively isolated for nearly 1.2 Ga. This finding puts lower bounds on timescale estimates for localized convective homogenization in early Earth's interior and on the widespread emergence of plate tectonics that are both important input parameters in many physical models.

Compositional heterogeneity of Archean mantle estimated from Sr and Nd isotopic systematics of basaltic rocks from North Pole, Australia, and the Isua supracrustal belt, Greenland

Compositional variability found in modern mid-ocean ridge basalt (MORB) and ocean island basalt (OIB) reflects differentiation processes associated with material recycling in the mantle-crust system. To investigate the timing at which this recycling system was established and how it transformed into the present-day system, we present geochemical analyses of the Archean basalts from North Pole (NP) in the East Pilbara Craton, Western Australia, and the Isua supracrustal belt (ISB), southern West Greenland. These rocks represent Archean accretionary complexes with ages of ~3.5 Ga and 3.7–3.8 Ga, respectively. We analyzed the trace element contents including rare earth elements (REEs), and Sr and Nd isotopic compositions of the basalts, which may represent MORBs and OIBs, from NP and ISB.Their trace-element compositions are broadly similar, but show distinct geochemical characteristics particularly with respect to REEs that probably reflect differences in both the source mantle and degree of melting. Such differences are also evident in their initial Nd isotopic compositions, which were estimated based on equilibrium partitioning of REEs and well-defined isochron ages. In contrast, the Sr isotopic compositions of the NP and ISB basalts are highly variable and their isochron ages are inconsistent with previous studies. Furthermore, the partitioning of Rb and Sr in the NP basalts indicates disequilibrium, suggesting that the Rb-Sr system has been disturbed by post-igneous alteration and metamorphism.Based on these observations, we propose the following model to explain the temporal variations in the geochemical composition of the Archean mantle: (i) ~3800 Ma: recycling of plate material and melting occurred quite readily and, therefore, MORBs and OIBs were produced from differentiated mantle sources; (ii) 3460 Ma to ~3800 Ma: mantle-crust mixing occurred as the result of an extreme event, such as mantle overturning, reducing the compositional variation of the mantle; and (iii) after ~3460 Ma: mantle heterogeneity gradually developed in the material-recycling system, re-establishing the compositional differences between MORBs and OIBs. This model requires an extreme event to drive the homogenization during stage (ii), which may provide new insights into the evolution of the crust-mantle system.