M.J. Van Kranendonk

Age and significance of voluminous mafic–ultramafic magmatic events in the Murchison Domain, Yilgarn Craton

Mafic–ultramafic rocks in structurally dismembered layered intrusions comprise approximately 40% by volume of greenstones in the Murchison Domain of the Youanmi Terrane, Yilgarn Craton. Mafic–ultramafic rocks in the Murchison Domain may be divided into five components: (i) the ∼2810 Ma Meeline Suite, which includes the large Windimurra Igneous Complex; (ii) the 2800 ± 6 Ma Boodanoo Suite, which includes the Narndee Igneous Complex; (iii) the 2792 ± 5 Ma Little Gap Suite; (iv) the ∼2750 Ma Gnanagooragoo Igneous Complex; and (v) the 2735–2710 Ma Yalgowra Suite of layered gabbroic sills. The intrusions are typically layered, tabular bodies of gabbroic rock with ultramafic basal units which, in places, are more than 6 km thick and up to 2500 km2 in areal extent. However, these are minimum dimensions as the intrusions have been dismembered by younger deformation. In the Windimurra and Narndee Igneous Complexes, discordant features and geochemical fractionation trends indicate multiple pulses of magma. These pulses produced several megacyclic units, each ∼200 m thick. The suites are anhydrous except for the Boodanoo Suite, which contains a large volume of hornblende gabbro. They also host significant vanadium mineralisation, and at least minor Ni–Cu–PGE mineralisation. Collectively, the areal distribution, thickness and volume of mafic–ultramafic magma in these complexes is similar to that in the 2.06 Ga Bushveld Igneous Complex, and represents a major addition of mantle-derived magma to Murchison Domain crust over a 100 Ma period. All suites are demonstrably contemporaneous with packages of high-Mg tholeiitic lavas and/or felsic volcanic rocks in greenstone belts. The distribution, ages and compositions of the earlier mafic–ultramafic rocks are most consistent with genesis in a mantle plume setting.

Review of hydrothermal processes and systems on Earth and implications for Martian analogues

A review of hydrothermal processes and systems on Earth shows how they provide the environment for both the formation of ore deposits and for life on the modern through to the ancient Earth. We discuss submarine hydrothermal systems, subaerial volcanic and subvolcanic systems (low and high sulfidation), hydrothermal systems in the East African rifts and those of the Red Sea. Geothermal springs in both modern and ancient Earth exemplify the links between hydrothermal systems and life. Examples are discussed from the Sulphur Springs Group and Dresser Formation in the Pilbara Craton, Western Australia. These hydrothermal systems illustrate the dynamic, continuously evolving link between the lithosphere, hydrosphere and biosphere. A summary of the geology of Mars is used to develop ideas as to what type of hydrothermal systems may occur there and where to search for signs of ancient and possibly extant life, most notably around Tharsis volcanoes, caldera floors, in fractures and in rift valleys. The interaction of hot magmas with the cryosphere is proposed as a mechanism to explain many of the surface features of Mars. A model is developed to explain the widespread deposition of sulfates on Mars as hydrothermal precipitates, generated through the interaction of magmatic H2S in hydrothermal solutions with water in the cryosphere.

Geochemistry and tectonic setting of basalts from the Eastern Goldfields Superterrane

A database of more than 650 whole-rock analyses on basaltic rocks from the Eastern Goldfields Superterrane has been compiled from the literature and from public-domain datasets. The data fall into three distinct geochemical categories: a High-Th Siliceous Basalt group, a Low-Th Basalt group and an Intermediate-Th Basalt group. The Low-Th Basalt group shows SiO2 values between 50 and 53 wt%; Al2O3 around 15 wt%; elevated Cr and Ni; MgO mostly between 5.5 and 8 wt%; and flat REE–HFSE patterns with slight depletion in Th and minor positive and negative Nb anomalies. The High-Th Siliceous Basalt group has high SiO2, commonly >54 wt%; MgO between 6 and 9 wt% with higher values reflecting presence of accumulated olivine or pyroxene; low Fe and Ti compared with Low-Th Basalt group; depleted Ni for given Mg#; enriched LREE and Th, combined with strongly negative Nb anomalies and mantle-like Zr/Nb, Nb/Y, Al/Ti and HREE ratios. The Intermediate-Th Basalt group is intermediate between these two end-members in almost all respects. All three groups are represented across the entire Eastern Goldfields Superterrane. The most widespread group, the Low-Th Basalt, is remarkably homogeneous across terranes and domains. It is interpreted as the Archean analogue of plume-head related Large Igneous Province basalts, showing a close match to flood basalts associated with late-stage continental rifting. The High-Th Siliceous Basalt group displays a distinctive geochemical signature, which is evidently unique to Archean greenstone terranes. Derivation by contamination during fractionation of komatiites, probably deep in the crust, and followed by mid-crustal homogenisation in magma chambers, is the most likely hypothesis. Basalts with characteristic island arc signatures such as Nb depletion and low Ni and Cr contents, have not been recorded in the Eastern Goldfields Superterrane. This poses a significant challenge to uniformitarian models, which attempt to explain the evolution of the entire east Yilgarn craton in terms of modern arc accretion tectonics. Distribution of mafic and ultramafic mafic magmatism across the superterrane at ca 2700 Ma can be explained by emplacement of a major driving plume under the ‘lid’ of the Youanmi Craton. The keel of thickened, buoyant lithosphere under the archon diverted the plume head towards the craton margin, where it induced continental rifting. Voluminous eruption of plume-tail komatiite was concentrated and focused through this zone of rifting along the eastern margin of the Youanmi craton in the Kalgoorlie Terrane, while plume-head basalts and less voluminous komatiites were erupted over a much wider area.

Comment on “Evidence for multiphase deformation in the Archaean basal Warrawoona group in the Marble Bar area, East Pilbara, Western Australia” by van Haaften, W.M., White, S.H., 1998: Precambrian Research 88, 53–66

van Haaften and White (1998) presented kine-matic data from shear zones within the TalgaTalga Anticline of the Marble Bar belt to in-voke a regional thrust-accretion tectonic originfor the East Pilbara Craton. By examination ofsmall-scale, local faults, they inferred five phasesof regional deformation, of which an early ESE-directed thrusting event which reactivated ca.3450 Ma normal faults, and a NE-directedtransport event at Further, van Haaften and White (1998) use3300 Ma, were interpretedto be the most important. van Haaften andWhite (1998) state that a lack of radially plung-ing lineations in the Talga Talga Anticline andthe presence of thrust-sense kinematics in bed-ding-parallel shear zones indicate that the TalgaTalga Anticline could not have formed throughthe solid-state diapiric rise of the Mount EdgarBatholith as suggested by Hickman (1984) andCollins (1989). Rather, they interpret the TalgaTalga Anticline to be a thrust culminationformed as a result of three phases of east- tonorth-eastern directed crustal shortening. Theearly thrusting was interpreted to have occurredduring the accretion of a so-called ‘East Pilbaraterrain’, as originally suggested by Krapez(1993).the presence of thrusts, and a re-interpretationof the previous geochronology, to suggest thatthe volcanic stratigraphy of the Marble Bar beltrepresent an imbricate lithotectonic sequence.They conclude that ‘the Talga Talga Anticline isan unsuitable area for the type section of thelower Warrawoona Group’, that the con-

Origin of fine-scale sheeted granites by incremental injection of magma into active shear zones: examples from the Pilbara Craton, NW Australia

In the Archaean Pilbara Craton of Western Australia, three zones of heterogeneous centimetre- to metre-scale sheeted granites are interpreted to represent high-level, syn-magmatic shear zones. Evidence for the syn-magmatic nature of the shear zones include imbricated and asymmetrically rotated metre-scale orthogneiss xenoliths that are enveloped by leucogranite sheets that show no significant internal strain. At another locality, granite sheets have a strong shape-preferred alignment of K-feldspar, suggesting magmatic flow, while the asymmetric recrystallisation of the grain boundaries indicates that non-coaxial deformation continued acting upon the sheets under sub-solidus conditions. Elsewhere, randomly oriented centimetre-wide leucogranite dykes are realigned at a shear zone boundary to form semi-continuous, layer-parallel sheets within a magma-dominated, dextral shear zone. It is proposed that the granite sheets formed by the incremental injection of magmas into active shear zones. Magma was sheared during laminar flow to produce the sheets that are aligned sub-parallel to the shear zone boundary. Individual sheets are fed by individual dykes, with up to 1000s of discrete injections in an individual shear zone. The sheets often lack microstructural evidence for magmatic flow, either because the crystal content of the magma was too low to record internal strain, or because of later recrystallisation.

Low δ18O zircon grains in the Neoarchean Rum Jungle Complex, northern Australia: An indicator of emergent continental crust

The timing of widespread continental emergence is generally considered to have had a dramatic effect on the hydrological cycle, atmospheric conditions, and climate. New secondary ion mass spectrometry (SIMS) oxygen and laser-ablation–multicollector–inductively coupled plasma–mass spectrometry (LA-MC-ICP-MS) Lu-Hf isotopic results from dated zircon grains in the granitic Neoarchean Rum Jungle Complex provide a minimum time constraint on the emergence of continental crust above sea level for the North Australian craton. A 2535 ± 7 Ma monzogranite is characterized by magmatic zircon with slightly elevated δ18O (6.0‰–7.5‰ relative to Vienna standard mean ocean water [VSMOW]), consistent with some contribution to the magma from reworked supracrustal material. A supracrustal contribution to magma genesis is supported by the presence of metasedimentary rock enclaves, a large population of inherited zircon grains, and subchondritic zircon Hf (eHf = −6.6 to −4.1). A separate, distinct crustal source to the same magma is indicated by inherited zircon grains that are dominated by low δ18O values (2.5‰–4.8‰, n = 9 of 15) across a range of ages (3536–2598 Ma; eHf = −18.2 to +0.4). The low δ18O grains may be the product of one of two processes: (1) grain-scale diffusion of oxygen in zircon by exchange with a low δ18O magma or (2) several episodes of magmatic reworking of a Mesoarchean or older low δ18O source. Both scenarios require shallow crustal magmatism in emergent crust, to allow interaction with rocks altered by hydrothermal meteoric water in order to generate the low δ18O zircon. In the first scenario, assimilation of these altered rocks during Neoarchean magmatism generated low δ18O magma with which residual detrital zircons were able to exchange oxygen, while preserving their U-Pb systematics. In the second scenario, wholesale melting of the altered rocks occurred in several distinct events through the Mesoarchean, generating low δ18O magma from which zircon crystallized. Ultimately, in either scenario, the low δ18O zircons were entrained as inherited grains in a Neoarchean granite. The data suggest operation of a modern hydrological cycle by the Neoarchean and add to evidence for the increased emergence of continents by this time.

The ca 2.74 Ga Mopoke Member, Kylena Formation: a marine incursion into the northern Fortescue Group?

The northern part of the Fortescue Group consists of interbedded flood basalts and sedimentary rocks that were deposited on the southern margin of the Pilbara Craton, Western Australia, during one or more periods of continental rifting between ca 2.78 and ca 2.63 Ga. Well-preserved sedimentary intervals within the group have yielded stable carbon and sulfur isotope data that have been used to infer changes in geobiological processes in the Neoarchean. However, the Fortescue Group is notable for being a predominantly subaerial succession, and it remains unclear whether data obtained from these intervals should be interpreted in the context of deposition in marine environments, possibly recording changes in the global ocean/atmosphere system, or in local and restricted lacustrine settings. Here, we describe the sedimentology, stratigraphy, stromatolites and stable carbon isotope geochemistry of the ca 2.74 Ga Mopoke Member, Kylena Formation, the oldest stromatolitic horizon in the Fortescue Group. This unit differs in terms of internal stratigraphic relationships, sedimentology, carbonate mineralogy and stable isotope geochemistry when compared with intervals of probable lacustrine origin in the overlying Tumbiana and Maddina formations. In contrast, we suggest that parts of the Mopoke Member may have been deposited under open marine conditions, or alternatively, in a lacustrine environment characterised by differing water chemistry and basement topography. Stromatolitic microfabrics of the Mopoke Member are dominated by spar, dolospar and vertically aligned calcitic crusts, rather than the micritic microfabrics described from other Fortescue Group stromatolites. Mud-draped ripples are common sedimentary features in the Mopoke Member, suggesting a tidal influence. Mopoke Member δ13Ccarb values are generally slightly positive, but also include some significantly depleted values, which may relate to the reoxidation of 13C-depleted organic matter. δ13Corg values average –36.7‰, consistent with Neoarchean marine units reported from elsewhere, but significantly less 13C-depleted than values reported from overlying lacustrine intervals in the Fortescue Group. We conclude that some features of Fortescue Group datasets relevant to the field of geobiology may be facies dependent, and that more work focusing on the overall depositional environments of the Fortescue Group is needed in order to appropriately interpret geobiological data reported from that group.

Oxygen isotopes in detrital zircons: Insight into crustal recycling during the evolution of the Greenland Shield

Insight into the interactions between crust and hydrosphere, through the protracted evolution of the Greenland Shield, can be provided by oxygen isotopes in the mineral remnants of its denuded crust. Detrital zircons with ages of 3900 Ma to 900 Ma found within an arkosic sandstone dike of the Neoproterozoic (?Marinoan) Moraeneso Formation, North Greenland, provide a time-integrated record of the evolution of part of the Greenland Shield. These zircon grains are derived from a wide variety of sources in northeastern Laurentia, including Paleoproterozoic and older detritus from the Committee-Melville orogen, the Ellesmere-Inglefield mobile belt, and the subice continuation of the Victoria Fjord complex. Archean zircon crystals have a more restricted range of δ 18 O SMOW values (between 7.2‰ and 9.0‰ relative to standard mean ocean water [SMOW]) in comparison to Paleoproterozoic 1800–2100 Ma grains, which display significant variation in δ 18 O SMOW (6.8‰–10.4‰). These data reflect differences in crustal evolution between the Archean and Proterozoic Earth. Through time, remelting or reworking of high δ 18 O materials has become more important, consistent with the progressive emergence of buoyant, cratonized continental lithosphere. A secular increase in the rate of crustal recycling is implied across the Archean-Proterozoic boundary. This rate change may have been a response to differences in the composition of sediments and/or the stabilization of continental crust. One Eoarchean oscillatory-zoned zircon grain, free of cracks and with concordant U-Pb systematics, has an elevated δ 18 O SMOW value of 7.8‰. This is interpreted to reflect a primary magmatic signature, supporting the presence of heavy oxygen that may be compatible with a hydrosphere on early Earth, as previously determined only from Jack Hills zircons.

Phreatomagmatic boulder conglomerates at the tip of the ca 2772 Ma Black Range dolerite dyke, Pilbara Craton, Western Australia

Unusual volcanic conglomerates with a mixture of well-rounded granitic boulders (to 1.2 m diameter) derived from adjacent basement rocks, and smaller (1 – 10 cm) subspherical basaltic droplets with chilled margins occupy a linear zone along strike of the northern end of the Late Archaean Black Range dolerite dyke in the Pilbara Craton, Western Australia. The matrix of the volcanic conglomerates becomes more angular with decreasing grainsize and grades to rock flour, a trend opposite to that in sedimentary conglomerates. In other places, the matrix consists of chlorite that cuts through, and resorbs, granitic clasts, indicating an origin as volcanic melt. The volcanic conglomerates have peperitic contacts with immediately adjacent flows of the Mt Roe Basalt of the Fortescue Group. A welded volcanic tuff at the peperitic contact is dated at 2767 ± 3 Ma, within error of the 2772 ± 2 Ma Black Range dolerite dyke and the Mt Roe Basalt (2775 ± 10 Ma), confirming the contemporaneity of formation of these geological elements. Subsequent normal faulting has juxtaposed the higher level conglomerates down into their present exposure level along strike of the Black Range dolerite dyke. The linear zone of volcanic conglomerates is interpreted to represent a phreatomagmatic pebble dyke that formed immediately above, and as a result of intrusion of, the Black Range dolerite dyke. Interaction of magma with groundwater caused phreatomagmatic brecciation of the country rock, in situ milling of granitic boulders, incorporation of basaltic melt droplets, and the formation of a mixed matrix of devitrified volcanic glass and granitic material. This process was accompanied by along-strike epithermal Cu – Hg – Au mineralisation.

A crystallographic study of crystalline casts and pseudomorphs from the 3.5 Ga dresser formation, Pilbara Craton (Australia)

Crystallographic methods are used to identify the primary mineral phase of pseudomorphs of crystals embedded in 3.48 Ga bedded carbonate-chert rocks from the Dresser Formation, Pilbara Craton, Australia. This identification provides valuable information on the chemical environments at the onset of life on Earth.