Pilbara Craton Research Articles

The geochemistry, petrogenesis and age of an unusual alkaline intrusion in the western Pilbara craton, Western Australia

The recently discovered Yanyare dykes are located approximately 80 km southwest of Karratha in the north-western Pilbara craton, Western Australia. The dykes intrude intercalated sedimentary and volcanic units of the PalaeoProterozoic Fortescue Group. The Yanyare-02 dyke has an inequigranular texture and is comprised of intensely altered olivine macrocrysts set in a matrix of phlogopite (55%), sanidine (20%) and apatite (5%). Replacement of phlogopite by rutile and chlorite is common. Sanidine occurs as euhedral grains and poikilitic plates enclosing apatite and is variably altered to muscovite and calcite. Aside from fluorapatite, no accessory minerals thought to characterise lamproites were observed. Yanyare-02 is classified as an olivine–phlogopite sanidine lamproite. Average initial (1.72 Ga) ratios for Yanyare-02 are: 87Sr/ 86Sr i = 0.708; 143Nd/ 144Nd i = 0.510; and 176Hf/ 177Hf i = 0.281 ( n = 6). Trace element data reveal extreme LREE enrichment, with La N/Lu N ≈ 100. The petrogenesis of Yanyare-02 is proposed to have involved melting of depleted lithosphere veined with ancient, enriched material of MARID composition. 40Ar/ 39Ar laser step-heating analyses of phlogopite grains produced a weighted mean age of 1724 ± 8 Ma (2σ). Rb–Sr analyses of whole-rock and phlogopite separates yielded an 8-point isochron age of 1681.6 ± 8.3 Ma (95% CL). An age of ca. 1.7 Ga makes Yanyare-02 the oldest lamproite known.

Mafic-ultramafic magmatism of the Early Precambrian (from the Archean to Paleoproterozoic)

Compositional evolution of the Archean mafic-ultramafic volcanics is considered in comparison with evolution of the Paleoproterozoic volcanism using available data on the Baltic shield, Pilbara (Australia) and Superior (Canada) cratons, and the Isua greenstone belt (Greenland). The Archean volcanics of mantle origin are of two major types, represented (a) by komatiite-basaltic complexes (komatiites, komatiitic and tholeiitic basalts) and (b) by geochemical analogs of boninites (GAB) and siliceous high-Mg series (SHMS) of volcanic rocks. As is established, the komatiitic and GAB volcanism ceased in the terminal Archean, whereas the SHMS rocks prevailed in the Paleoproterozoic to become extinct about 2 Ga ago in connection with transition to the Phanerozoic type of tectonomagmatic activity. Geochemical trends of mafic-ultramafic associations occurring in the considered cratons are not uniform, being of particular character to certain extent. With transition from the Paleo- to Neoarchean, rock associations of both types reveal a minor increase in Ti and Fe contents. Comparatively high Fe2O3tot TiO2, and P2O5 concentrations (maximal ones in the Archean), which are characteristic of the Neoarchean (2.75–2.70 Ga) basalts from the Superior and Pilbara cratons or the Baltic shield, represent a result of relatively high-Ti intracratonic magmatic activity that commenced in that period practically for the first time in the Earth history. This magmatic activity of the Neoarchean was not as intense as the high-Mg basaltic volcanism, and the absolute maximum in concentrations of the above components was attained only 2.2–1.9 Ga ago, at the time of appearance in abundance of Fe-Ti picrites and basalts typical of the Phanerozoic intraplate magmatism. The Archean volcanic complexes demonstrate gradual secular increase in concentrations of incompatible elements (LREE inclusive) and growth of Nb/Th ratio that apparently reflected the progressing influence of mantle plumes. In the early Paleoproterozoic (2.5–2.35 Ga), values of that ratio considerably declined in the SHMS rocks and then quickly grew in the Middle Paleoproterozoic volcanics (2.2–1.9 Ga) to attain finally the values typical of the Phanerozoic magmas associated in origin with mantle plumes. The ɛNd(T) parameter was decreasing with time from positive values in the Paleoarchean to negative ones in the SHMS rocks of the Paleoproterozoic most likely in response to grown proportion of ancient crustal material in magmatic melts. Since the mid-Paleoproterozoic, the ɛNd(T) values turn in general into positive again reflecting change in the character of magmatic activity: the SHMS melts gave place at that time to the Fe-Ti picrite-basaltic magmas. The primary crust of the Earth was presumably of sialic composition and originated during solidification from the bottom upward of the global magma ocean a few hundreds kilometers deep, when most fusible components migrated up to the surface to form there the granitic crust. Geological history of the Earth commenced at the appearance time of granite-greenstone terranes and granulite belts separating them, the first large tectonic structures formed under influence of raising mantle superplumes.

Isotopic fractionation of nitrogen and carbon in Paleoarchean cherts from Pilbara craton, Western Australia: Origin of 15N-depleted nitrogen

Nitrogen and carbon isotopic compositions, together with mineralogy and trace element geochemistry, were studied in a few kerogen-rich Paleoarchean cherts, a barite and a dolomitic stromatolite belonging to the eastern (Dixon Island Formation) and western (Dresser and Strelley Pool Chert Formations; North Pole Dome and Marble Bar) terranes of Pilbara Craton, Western Australia. The aim of the study was to search for 15N-depleted isotopic signatures, often found in kerogens of this period, and explain the origin of these anomalies. Trace elements suggest silica precipitation by hydrothermal fluids as the main process of chert formation with a contamination from volcanoclastic detritus. This is supported by the occurrence of hydrothermal-derived minerals in the studied samples indicating precipitation temperatures up to 350 °C. Only a dolomitic stromatolite from Strelley Pool shows a superchondritic Y/Ho ratio of 72 and a positive Eu/Eu * anomaly of 1.8, characteristic of chemical precipitates from the Archean seawater. The bulk δ 13C vs. δ 15N values measured in the cherts show a roughly positive co-variation, except for one sample from the North Pole (PI-85-00). The progressive enrichment in 15N and 13C from a pristine source having δ 13C ⩽ −36‰ and δ 15N ⩽ −4‰ is correlated with a progressive depletion in N content and to variations in Ba/La and Co/As ratios. These trends have been interpreted as a progressive hydrothermal alteration of the cherts by metamorphic fluids. Isotopic exchange at 350 °C between NH 4 + (rock) and N 2(fluid) may explain the isotopic and elemental composition of N in the studied cherts. However, we need to assume isotopic exchange at 350 °C between carbonate C and graphite to explain the large 13C enrichment recorded. Only sample PI-85-00 shows a large N loss (90%) with a positive δ 15N value (+11‰), while C (up to 120 ppm and δ 13C −38‰) seems to be unaffected. This pattern has been interpreted as the result of devolatilization and alteration (oxidation) of graphite by low-temperature fluids. The 15N- 13C-depleted pristine source has δ 15N values from −7‰ to −4‰ and 40Ar/ 36Ar ratios from 30,000 to 60,000, compatible with an inorganic mantle N source, although the elemental abundance ratios N/C and 40Ar/C are not exactly the same with the mantle source. The component alternatively could be explained by elemental fractionation from metabolic activity of chemolithoautotrophs and methanogens at the proximity to the hydrothermal vents. However, ambiguities between mantle vs organic sources of N subsist and need further experimental work to be fully elucidated.

Thermodynamic model of REE leaching from monazite by hydrothermal fluids

Monazite is the most abundant REE mineral that is formed in a wide range of geochemical conditions. This mineral is the most widespread in the felsic magmatic rocks, and it occurs in alkaline rocks, their pegmatites, and definite metamorphic complexes. Since, besides REEs, it contains significant amounts of Th and U cations, an increasing number of works in recent decades have focused on the use of this mineral for geochronological dating. Moreover, monazite and, more precisely, its compositional specifics in different phases of granitoid intrusions are employed not only for dating, but also for understanding the physicochemical evolution of magmatic complexes. In particular, significant differences were found between the compositions of predominant primary monazite in the two-mica Ireteba granites, southern Nevada (with an age of ~64 Ma), and late (secondary) monazite, which replaces it and is restricted to the influence zones of Late Miocene granite plutons (~14‐18 Ma). The replacement occurred at temperatures below 400 ° C and was expressed in a significant decrease in the content of the huttonite (ThSiO 4 ) end member with a simultaneous increase in the La/Sm and Sm/Y ratios. There is an article [2] dedicated to a detailed SHRIMP study of monazite from the Middle Archean greenschist metamorphic rocks of the Pilbara craton, Western Australia. It was shown that metamorphic monazite is represented by three generations corresponding to different “growth stages.” The La, Sm, Y, and Th distribution mapping combined with U‐Pb dating revealed the presence of distinct cores and rims in some crystals and provided insight into a hidden spatiotemporal pattern of distribution of fluid flows related to tectonic processes. Additionally, the authors of [3] demonstrated that a similar technique can be applied to determine the main trends in monazite evolution during formation of highgrade granulite-facies rocks in gneisses contained in the Challenger Au deposit (South Australia). The determination of Ce, Nd, Sm, Eu, and Gd contents in this mineral made it possible to trace the formation of its compositional domains towards the core: growth during partial melting in the closed system.

Identifying the lithospheric structure of a Precambrian orogen using magnetotellurics: The Capricorn Orogen, Western Australia

A 300-km long magnetotelluric (MT) survey was carried out across the Capricorn Orogen in Western Australia. The Capricorn Orogen includes reworked crust of the Archaean Yilgarn and Pilbara Cratons, and the allochthonous latest Archaean to Palaeoproterozoic Glenburgh Terrane in the southern Gascoyne Complex and the Palaeoproterozoic northern Gascoyne Complex. The survey aimed to increase our understanding of the juxtaposition of these crustal elements by obtaining information about their electrical structure with depth. Phase tensor analysis of the data showed that 2D MT inversion could only be carried out for stations in the centre of the profile, crossing the Gascoyne Complex. The resulting model showed that there is no electrical distinction between the Glenburgh Terrane and the northern Gascoyne Complex, suggesting that the Glenburgh Terrane may form basement to the whole of the Gascoyne Complex. 3D forward modelling was carried out for the entire survey line, incorporating the basement blocks as well as the surrounding ocean and sedimentary basins. Station data are most closely reproduced by a model in which the margin between the Glenburgh Terrane and the Yilgarn Craton (the Errabiddy Shear Zone) dips south, with crust of the Glenburgh Terrane wedged beneath the northern Yilgarn Craton. Forward modelling results also suggest that the main boundary between the Gascoyne Complex and the Pilbara Craton to the north is a steeply dipping structure beneath the Edmund and Collier Basins that may correlate with the Talga Fault.

Assembling Australia: Proterozoic building of a continent

The western two-thirds of Australia is underlain by Precambrian rocks that are divisible into three Archean to Paleoproterozoic cratons, the West Australian, North Australian and South Australian cratons, separated by Paleoproterozoic to Mesoproterozoic orogens. Prior to about 1500 Ma, the North Australian and South Australian (along with extensions into Antarctica) cratons show a similar geological history and are herein assumed to have evolved as a single entity, termed the Diamantina Craton. The temporal and spatial record of Proterozoic rock units and orogenic events documents accretion and assembly of Precambrian proto-Australia. The Archean Yilgarn and Pilbara cratons were assembled into the West Australian Craton along the Capricorn Orogen during the late Paleoproterozoic (2000 Ma) Glenburgh Orogeny, which then combined with the North Australian segment of the Diamantina Craton along the Paterson Orogen during the 1800–1765 Ma Yapungku Orogeny to form proto-Australia. It was bounded throughout most of the late Paleoproterozoic to earliest Mesoproterozoic along its south western and probably its north eastern margins by subduction zones such that much of the craton occupied an upper plate, backarc basin environment. After ∼1500 Ma, proto-Australia differentiated into two cratonic masses, the combined North Australian and West Australian cratons and the Mawson Craton, consisting of the South Australian Craton and extensions into Antarctica. This breakup, through rotation and lateral translation of the Mawson Craton, resulted in convergence and collisional suturing with the West Australian Craton along the 1345–1140 Ma Albany–Fraser Orogen. The integrated continental assemblage of West Australian, North Australian and South Australian (including parts of Antarctica) cratons are herein referred to as the Great Southern Continent, which persisted until final breakup of Pangea associated with the current northward drift of Australia. The Pinjarra Orogen developed along the margin of the West Australian Craton and records late Mesoproterozoic to Neoproterozoic strike-slip juxtaposition of India within an assembling Gondwana. The Neoproterozoic record of the Terra Australis Orogen, which extends along the eastern side of Precambrian Australia, records rifting and continental breakup within the supercontinent of Rodinia. Australian Proterozoic rocks host significant mineral resources, including world class banded iron-formations in the West Australian Craton (Hamersley), and iron oxide copper gold deposits (Olympic Dam), Pb–Zn–Ag systems (Mount Isa and Broken Hill) and uranium deposits in the Diamantina Craton.

Dating the Oldest Greenstone in India: A 3.51-Ga Precise U-Pb SHRIMP Zircon Age for Dacitic Lava of the Southern Iron Ore Group, Singhbhum Craton

Abstract This article reports a precise 3506.8 ± 2.3-Ma U-Pb SHRIMP zircon age for dacitic lava in a well-preserved low-grade metamorphic and low-strained greenstone belt succession of the southern Iron Ore Group, Singhbhum craton, India. This age makes the succession the oldest-known greenstone belt succession in India and one of the oldest low-strain greenstone successions in the world after the 3.51-Ga Coonterunah Group of the Pilbara craton, Western Australia, and the moderately deformed 3.54-Ga Theespruit Formation of the Barberton Greenstone Belt, Kaapvaal craton, South Africa. The geochemical composition of the dacitic lava and related volcanic rocks suggests that they formed in a volcanic arc setting. The succession also contains a major ∼120-m-thick oxide facies banded iron formation that distinguishes it from the slightly older successions of the Pilbara and Kaapvaal cratons. This banded iron formation may well be one of the oldest and most well preserved, and together with associated volcanics,...

Geochronology of the Archean of Australia

The Archean eon is a chronometrically defined subdivision of the Precambrian that extends from 3800 to 2500 Ma. In the Australian continent, Archean rocks (i.e. those with sedimentary deposition or igneous crystallisation ages confidently interpreted to be older than 2500 Ma) occur in the Pilbara and Yilgarn Cratons of Western Australia, the Mulgathing and Sleaford Complexes in the Gawler Craton of South Australia and as minor inliers in the Pine Creek Orogen of the Northern Territory. Metamorphosed lithological components with igneous or sedimentary precursors inferred to be of Archean age also occur in the Browns Range Dome and Billabong Complexes of the Tanami Inlier, Northern Territory, and possibly also in the Broken Hill Inlier. This contribution outlines aspects of the historical development of geochronological methods applied to the Archean of Australia, the respective strengths and weaknesses of these methods and how our understanding of the continent's Archean geology and tectonics was related to this development. The geological formation of the Yilgarn and Pilbara Cratons is investigated using a new statistical approach based on the distribution of SHRIMP U–Pb zircon and monazite dates obtained from rock samples that may have complex mineral age populations. Spatial and temporal characteristics of the magmatic episodes identified in the Yilgarn and Pilbara granite–greenstone terranes are atypical of rift/collision processes attributable to tectonic-plate interactions, but are consistent with their interpretation as a response to major mantle convection events. The strengths of this geochronology-based approach are demonstrated by its application to the Narryer Gneiss Complex of the northwestern part of the Yilgarn Craton. It is shown that this approach offers unique insights into geological events that may be under-represented at the outcrop scale due to later crustal re-working.

Origin and mechanisms of K–Si-metasomatism of ca. 3.4–3.3 Ga volcaniclastic deposits and implications for Archean seawater evolution: Examples from cherts of Kittys Gap (Pilbara craton, Australia) and Msauli (Barberton Greenstone Belt, South Africa)

The effects of K–Si-metasomatism during the formation of Early Archean replacement cherts have been quantified in this study by the investigation of two well-known stratigraphic sections: the Msauli chert (MC, Barberton greenstone belt, South Africa) and the Kittys Gap chert (KGC, Pilbara craton, Western Australia). The KGCs have a dacitic precursor similar to Duffer Formation dacites (Pilbara craton), while the MCs are derived from Al-depleted komatiites similar to those from the Weltevreden Formation (Barberton greenstone belt). Mass balance calculations reveal that the volcaniclastic deposits had initial porosities of up to 85 vol.% for the KGC and of 65 vol.% for the MC. Secondary porosities (27 vol.%: MC, 8 vol.%: KGC) produced during K-metasomatism are proportional to the dissolution of Fe, Ca, Mg-rich glass and precursor minerals. Komatiites have a higher chemical exchange potential than dacites, each gram releasing 1.2 mmol Fe 2+, 2.8 mmol Mg 2+, 1.4 mmol Ca 2+ and 1.1 mmol Na + to seawater, together with 4.4 mmol O 2−. K-metasomatism of 1 g of komatiite further implies an uptake of 0.67 mmol of K + and 2.7 mmol of H +. The highest silica uptake is achieved for the KGC (82 mmol/g of precursor). This silica enrichment most likely operated in the water column and at the sediment–water interface by sorption mechanisms on the surface of detrital particles and particulate organic matter, as a result of seawater silica-saturation. Acidic conditions (pH 5.5–6.5) and hot temperatures (>70 °C) favored the formation of K-rich phyllosilicates by interaction with seawater during the early diagenetic alteration of the volcaniclastic particles. The widespread occurrence of K–Si-metasomatism in volcanic and sedimentary rocks can be regarded as a general alteration process of the Early Archean seafloor, with a major influence on seawater composition. The highly K-selective metasomatism confirms previous studies suggesting that the Archean ocean was acidic and probably in equilibrium with a CO 2-rich atmosphere.

NanoSIMS analysis of Archean fossils and biomarkers

The study of fossils and biomarkers from Archean rocks is of vital importance to reveal how life arose on Earth and what we might expect to find on other planets such as Mars. The Cameca NanoSIMS 50 has the unique ability to measure stable isotopes and map biologically relevant elements at the micron-scale, in situ. This makes it the perfect tool for testing the biogenicity of a range of putative biomarkers from early Archean rocks (∼3.50 billion-year-old). NanoSIMS has been used to investigate ambient inclusion trails (AITs) in a 3.43 Ga beach sand deposit from the Pilbara craton, Western Australia. Chemical maps of the light elements necessary for life (C, N and O) and several transition metals commonly associated with biological processing (Ni, Zn and Fe), coupled with 13C/ 12C isotope ratios from carbonaceous linings, strongly suggest a biological component in the formation of AITs.

Surface heat flow and the mantle contribution on the margins of Australia

We present thermal data from 473 oil exploration wells in Australia and New Zealand. Approximately 2300 bottom‐hole temperatures are corrected to form a homogeneous set along with 86 temperatures from reservoir tests. Thermal conductivity profiles are estimated from a set of geophysical well logs using a recently developed neural network approach. Retaining wells in which temperature and thermal conductivity data overlap over an interval greater than 1000 m, we estimate 10 heat flow values in the Taranaki basin of New Zealand and 270 values in the northwestern, western, and southern margins and in the intracontinental Canning basin of Australia. The values are in the range 30–80 mW m−2. As a result of several differences in the data and methods, our heat flow values are 10–20 mW m−2 lower compared to previously published estimates for the same wells in New Zealand. For Australia, our values are consistent with previously measured values and trends in the continental and marine regions. On the northwestern and southeastern margins, we interpret the variations as reflecting changes in the nature of the underlying basement. Consistent with onshore data, it is inferred that the Archean crust is depleted in radiogenic elements compared to Proterozoic regions and that recent volcanism affects the eastern Paleozoic area. After removing from surface heat flow the sediment contributions, including a permanent radiogenic heat component and a transient sedimentation effect, a simple crustal model suggests that mantle heat flow on the continental margin bordering the Pilbara craton is higher than below the craton itself. Moreover, heat flow corrected for the sediment contributions is markedly lower in the Petrel intracontinental basin than in the adjacent margin, although the crust is thinner below this latter region. As both are underlaid by the same basement, this observation may indicate that the mantle contribution is also higher below that margin. Such a higher mantle heat flow on old continental margins is consistent with experiments of fluid convection below an insulating lid and suggests that the thermal regime of the continental lithosphere never returns to its prerift state, as usually assumed by several thermomechanical models of evolution of continental margins.

オーストラリア・ピルバラクラトンのMt. Roe玄武岩の熱水変質過程解明のためのU-Pb同位体研究

オーストラリア・ピルバラクラトンのMt. Roe玄武岩の熱水変質過程解明のためのU-Pb同位体研究

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

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

Archaean tectonics in the Pilbara and Yilgarn Cratons

The Pilbara and Yilgarn Cratons in Western Australia record the Earth?s early history from ca. 3.7 Ga to ca. 2.5 Ga. This paper highlights recent scientific work, and advances in understanding the similarity and differences in granitic rocks, greenstone stratigraphy, structural styles, and tectonic settings of the major granite?greenstone terranes within the cratons, and then discusses the crustal evolution trends from the Paleo- to Neoarchaean. We conclude that in the Pilbara and Yilgarn Cratons, the Paleoarchaean areas are characterized by vertical tectonics, whereas the Meso- to Neoarchaean areas are dominated by horizontal tectonics ? plate tectonic processes may have started in the Mesoarchaean.

Paleomagnetism of the Marble Bar Chert Member, Western Australia: Implications for apparent polar wander path for Pilbara craton during Archean time

The Archean Biosphere Drilling Project (ABDP) drilled a continuous 270 m long oriented core from the Towers Formation, which includes the Marble Bar Chert Member (3456.1–3476.0 Ma) in the Pilbara craton, northwestern Australia. A paleomagnetic study of 261 discrete specimens, collected from a 158.5 to 182.0 m section of the Marble Bar Chert Member, revealed two distinct magnetic components (LT and MT). The MT component yields seven different mean paleomagnetic directions clustered as MB1 to MB7. These, together with the published paleomagnetic poles of early Archean rocks from the Pilbara craton, draw a continuous paleomagnetic pole path, which likely to be regarded as the early to late Archean apparent polar wander path (APWP) for the Pilbara craton. The APWP implies that the Pilbara craton underwent a latitudinal drift of about 21° during the interval when the magnetization of the Marble Bar Chert Member was acquired. The estimated speed of the lateral drift is 12–112 cm/yr (120–1120 km/Myr), which is large compared with current plate motion velocities, suggesting that continents might have moved during the Archean faster than in the Phanerozoic.

Petrology, geochemistry and tectonic significance of Palaeoproterozoic alkaline lamprophyres from the Jungel Valley, Mahakoshal supracrustal belt, Central India

Palaeoproterozoic (∼1.6 Ga) lamprophyres occur as dykes, plugs and lava flows within the Mahakoshal supracrustal belt of the Jungel Valley, Central India. Although metamorphosed under greenschist facies conditions, the lamprophyres still retain remnants of the diagnostic igneous textures viz., porphyritic-panidiomorphic habit of mafic phenocrysts, leucocratic (carbonate-rich) ocelli and volatile-rich mineralogy. The studied lamprophyres are mineralogically uniform but texturally variable (porphyritic to aphanitic). Chemically they are silica undersaturated, ultrabasic, dominantly alkaline (some of them have normative nepheline) through mildly alkaline to sub-alkaline, sodic to mildly potassic and their magmas are primitive as well as evolved. The lamprophyres are characterized by Ni and Cr abundances and MgO sufficiently high to indicate a mantle source. The chondrite-normalized REE patterns confirm crystallization from a LREE enriched magma. Combined petrological and geochemical studies classify them as belonging to alkaline lamprophyre category in general and camptonites in particular. Geochemistry also suggests that their magmas are not affected by any crustal contamination. This study establishes the occurrence of lamprophyres in the Jungel area and brings to end the speculations about their mistaken identity. The presence of a wide spectrum of undersaturated alkaline magmatism comprising lamprophyres, syenites, tinguites, and possibly kimberlties, highly carbonated rocks and ultrabasic rocks as intrusives in the Mahakoshal supracrustal belt undoubtedly highlights the presence of a carbonate-rich palaeo- to early Proterozoic alkaline province. The occurrence of greenschist facies metamorphosed lamprophyres of Palaeoproteorozoic age in the eastern central Indian shield is also significant in the context of re-construction of ancient continental fits as similar rocks of comparable age have been reported from the Yilgarn and Pilbara cratons of Western Australia.

Origin of Archean late potassic granites: Evidence from the Yilgarn and Pilbara Cratons

Origin of Archean late potassic granites: Evidence from the Yilgarn and Pilbara Cratons

Biomarkers in Archaean banded iron formations from Pilbara and Dhawar Craton

The origin of Archeaen banded iron formations (BIF) and the role of biosphere in Fe precipitation is still highly debated. In order to elucidate these processes, detailed mineralogical and textural analyses combined with δ15N, δ56Fe and δ13C data were obtained on Fe-oxide bands from Marble Bar chert Unit (MB, 3.46 Ga, Pilbara craton, W. Australia) and a BIF from the Bababudan Group (BG, 2.7-2.9 Ga, Dhawar Craton, Southern India). Both samples are composed of alternating quartz and Fe-oxide bands with wavy microtextures. CI- normalized REE patterns show that MB reflects hydrothermal fluid/basalt interactions, while BG precipitated from a hydrothermal fluid/seawater mixture. In MB, nano-cristalline hematite replaced magnetite, Mg-calcite and Fe-sulfides producing a matlike surface, preserving nanometric N-bearing amorphous carbon nodules. Measured C/N ratios (2.3 to 52) are typical of Precambrian organic matter. The δ56Fe of -0.40±0.02‰ suggests MOR-hydrothermal fluids as a Fe-source, while a δ15N of +7.4±0.4‰ is compatible with nitrification-denitrification processes. BG is composed of intergrown magnetite and hematite. Disseminated grunerite and magnetite grew during low T metamorphism. Fe-oxide spherules compose vermicular-filaments that nucleated perpendicular to quartz surfaces. Fe-oxide spherule bunches are perfectly preserved in the silica bands forming micrometric mats, which contain heterogeneously distributed N (~0.09at.%) and C (0.51 at.%, C/N=5.73). Bulk δ13C of -15.35‰±0.10 points to an organic origin for C. The δ56Fe in Fe and Si layers (0.75‰ to 2.16‰) is compatible with a chemical precipitation for BIF. A negative correlation between δ56Fe and the Th/U ratio suggests that Fe isotopic variations are related to fluid circulation and re-precipitation of Fe-oxides. High δ15N, on one Fe-oxide layer, of +21.8±0.7‰, corresponds to that observed for Archeaen BIFs and may be related to nitrate-dependent microbial oxidation of Fe.

Origin of carbonaceous matter in 3525 Myr-old hydrothermally altered subseafloor sediments from the dresser formation, pilbara craton

Origin of carbonaceous matter in 3525 Myr-old hydrothermally altered subseafloor sediments from the dresser formation, pilbara craton

Evidence from fluid inclusions for microbial methanogenesis in the early Archaean era

Methanogenic microbes may be one of the most primitive organisms, although it is uncertain when methanogens first appeared on Earth. During the Archaean era (before 2.5 Gyr ago), methanogens may have been important in regulating climate, because they could have provided sufficient amounts of the greenhouse gas methane to mitigate a severely frozen condition that could have resulted from lower solar luminosity during these times. Nevertheless, no direct geological evidence has hitherto been available in support of the existence of methanogens in the Archaean period, although circumstantial evidence is available in the form of approximately 2.8-Gyr-old carbon-isotope-depleted kerogen. Here we report crushing extraction and carbon isotope analysis of methane-bearing fluid inclusions in approximately 3.5-Gyr-old hydrothermal precipitates from Pilbara craton, Australia. Our results indicate that the extracted fluids contain microbial methane with carbon isotopic compositions of less than -56 per thousand included within original precipitates. This provides the oldest evidence of methanogen (> 3.46 Gyr ago), pre-dating previous geochemical evidence by about 700 million years.