Extreme element mobility during transformation of Neoarchean (ca. 2.7 Ga) pillow basalts to a Paleoproterozoic (ca. 1.9 Ga) paleosol, Schreiber Beach, Ontario, Canada

ArticleinChemical Geology s 326–327:145–173 · October 2012with 95 Reads
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Abstract
This study reports petrographic, major and trace element, and oxygen isotopic data for a subaerial weathering profile at Schreiber Beach, Ontario, Canada. The weathering profile developed on Neoarchean (ca. 2.7 Ga) pillow basalts and is unconformably overlain by the Paleoproterozoic (ca. 1.88 Ga) Gunflint Chert and basal conglomerates. This stratigraphy suggests that the basalts were uplifted and subaerially weathered prior to deposition of the Gunflint Formation. Rocks at Schreiber Beach have been classified as pillow cores, pillow rims, hyaloclastites, weathered red basalts, and weathered brown to green basalts. There are gradual textural, mineralogical, and geochemical transitions from unweathered basalts to intensely weathered hematite-bearing basalts with stratigraphic height.

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    Paleosols contain information about ancient atmospheric and climate conditions, as well as the secular change in supply of nutrients to the ocean. A newly exposed, ~5 meters deep 1.85 Ga weathering profile is described. It developed on pillowed metabasalt (Flin Flon, Manitoba, Canada) and was sampled (~120 samples) and analyzed at much higher spatial resolution (>50 samples) than previous such profiles for ultra-trace, major element, and ferrous iron. Consistent with previous studies on nearby paleosol exposures [1,2], the examined weathering profile is characterized by a spectacularly preserved, coherent upward Fe enrichment, accompanied by loss of Fe 2+ and the development of a positive Ce anomaly, implying weathering in an oxic atmsophere. The new, high-precision trace element data collected reveal enrichment of several alkali (Cs, Rb, in addition to K, Na) and alkaline earth (Be, Sr, Ba) elements upward towards the paleosol contact with overlying sandstone. By contrast, the HFSE remained largely immobile in the profile which is evident through the highly consistent Zr/Hf, Th/Nb, and Nb/Ta ratios. This presentation will discuss the distinction of original chemical features preserved in the profile, for example those inherited from the separate pillow basalt flows, versus those superimposed by weathering, water table fluctuation and possible extraneous sediment input. The broad similarities in age and compositonal range found in plutonic and volcanic exposures from a given magmatic province clearly indicate that these rocks types are closely related genetically. However, to which extent plutons (particularly the ones with intermediate to silicic compositions) are former magma reservoirs that have lost melt remains controversial. Recent work using both geochemical and physical modeling provides new avenues to explore these issues that are fundamental to our understanding of crustal evolution. First, mechanical constraints on rates of crystal-liquid separation indicate that most chemical differentiation is likely to occur within an melt extraction window located between 50 and 80 vol% crystals across the whole range of magma composition (from mafic to silicic). This extraction window will lead to compositional gaps in extracted liquids, as observed in numerous volcanic series around the world. In contrast, plutonic exposures will span a range of compositions that will vary continously as a function of how much melt was lost at a given location. Trace element modelling suggest that variable degrees of crystal accumulations are common in all plutonic sequences, including in intermediate to silicic compositions (tonalites -granodiorites). We also report erupted crystal cumulates, excavated from shallow magma chambers during large explosive eruptions. Such findings, alongside with thermal models of magma-crust interaction, suggest that crystal fractionation occurring by melt extraction in mush zones, and accompanied by a limited amount of crustal assimilation, is the dominant differentiation process in the Earth's crust. In this framework, volcanic rocks mostly represent liquids extracted from different reservoirs while plutons typically correspond to the left-overs crystal mushes that have been periodically stripped from a fraction of their interstitial melts.
  • Article
    This chapter reviews some of the paleontological evidence for the effects of both intrinsic and extrinsic biotic factors and discusses how the interplay and feedbacks among these biotic and physical factors shape large-scale evolutionary patterns. Intrinsic biotic factors such as dispersal ability and environmental tolerance, or at the species or lineage levels geographic range or species richness, clearly influence the origination and extinction rates that underlie the dynamics of evolution above the species level. Such biotic factors determine the differential response of taxa to a physical perturbation but can be overwhelmed if the perturbation is sufficiently severe or extensive. This chapter necessarily addresses both origination and extinction: these are the fundamental terms of the macroevolutionary equation. Some major evolutionary events are discussed in the chapter, where the relative roles of the different factors are controversial, with at least some evidence for physical drivers as well as for either intrinsic or extrinsic biotic factors.
  • Article
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    The Hemlo deposit is one of Canada's largest gold camps. Gold is both structurally and lithologically controlled, but shows unusual relationships relative to many other Archean lode deposits. Field evidence suggests that gold was emplaced within mechanical and chemical traps along a jog in a major sinistral shear zone, most likely during early formation of second-generation (G(2)) structures, which represent the most intense phase of regional deformation. Mineralization was followed by amphibolite-facies regional metamorphism. U-Pb geochronology has been carried out on zircons from rocks associated with the deposit, in order to determine the precise chronology of geologic events and their relationship to mineralization. Zircon populations are commonly affected by both inheritance and hydrothermal/metamorphic crystal growth. Meaningful and precise age information can only be determined by precise single zircon dating on rocks from a well constrained, structural, and stratigraphic context. Gold is mostly found near the deformed contact between the Moose Lake quartz porphyry volcanic complex and stratigraphically underlying metasedimentary rocks. These rocks have been folded by a camp-scale, noncylindrical F, fold. Zircon from the Moose Lake quartz porphyry is dominated by inheritance and gives ages that extend from similar to2800 to similar to2690 Ma. Metasedimentary rocks that underlie and overlie the Moose Lake porphyry show a much narrower range of provenance ages, and are dominated by 2,690- to 2,693-million-year-old zircon. Two detrital zircons from a metasedimentary rock directly beneath the porphyry define an age of 2685 +/- 4 Ma. This should be an older age limit on volcanism, deformation, and gold mineralization. The four youngest near-concordant zircons from the Cedar Lake pluton define a magmatic age of 2680 +/- 1 Ma. This pluton was emplaced prior to or early during G(2) deformation, while emplacement of gold was most likely controlled by G(2) structures. Therefore, 2680+/- 1 Ma probably also represents an older age limit on gold mineralization. A feldspar porphyry dike that cuts gold ore gives a younger age bracket of 2677 +/- 1 Ma. The results of zircon dating suggest that a volcanosedimentary complex was constructed in the area over the period of 2693 to 2685 Ma. This developed on earlier crust, ca. 2720 Ma in age, but there is evidence from detritus and xenocrysts for older rocks dating back to over 2800 Ma. Deposition of supracrustal rocks was followed by granodiorite plutonism at 2680 +/- 1 Ma, then major deformation (G2) and gold mineralization, overprinted by amphibolite-facies metamorphism (previously dated at 2676 Ma from titanite ages). Late-tectonic "sanuktoid suite" plutons were emplaced at 2677 +/- 1 Ma. Thus, the primary gold-forming event occurred near the beginning of a period of plutonism and crustal reworking that lasted, at most, a few million years. This time association supports the view that granitoid magmas were the source of auriferous fluids. Plutons intruded into an actively deforming crust and were probably also the heat source for regional metamorphism. The most likely environment was a tectonically active, Timiskaming-type sedimentary basin. Similarly aged basins are found in the Timmins area to the east and the Shebandowan greenstone belt to the west.
  • Article
    An extensive database, including Th–;U–Nb–REE systematics, for diverse magmatic and sedimentary lithologies of 2.7 Ga Wawa greenstone belts provide new constraints on the mechanism of crustal growth in the southern Superior Province, and controls on its composition. The greenstone belts are characterized by collages of oceanic plateaus, oceanic island arcs, and trench turbidites; these lithotectonic fragments were tectonically assembled in a large subduction–accretion complex. Following juxtaposition, these diverse lithologies were collectively intruded by syn-kinematic TTG (tonalite–trondhjemite–granodiorite) plutons and ultramafic to felsic dykes and sills, with subduction zone geochemical signatures. Intra-oceanic basalts are characterized by near-flat REE patterns, and Nb/U and Nb/Th ratios generally greater than primitive mantle values, consistent with positive ϵNd values. They are associated with komatiites, the association being interpreted as an ocean plateau sequence erupted from a mantle plume. Bimodal arc volcanic sequences, trench turbidites, and contemporaneous TTG suites are characterized by fractionated REE, with Nb/U and Nb/Th ratios less than primitive mantle values. Mixing hyperbolae between oceanic plateau and magmatic arc sequences pass through the estimated composition of bulk continental crust, suggesting that crustal growth in the late Archean was by tectonic, sedimentary, and chemical mixing of oceanic plateau and arc sequences at convergent plate boundaries. Mixing calculations suggest that oceanic plateau and subduction zone components in the Wawa continental crust are represented by 6–12% and 88–94%, respectively. High Nb/U and Nb/Th ratios of plateau tholeiitic basalts are interpreted as a complementary reservoir to arc magmatism (low Nb/U and Nb/Th), hundreds of millions of years prior to recycling of oceanic lithosphere through a subduction zone (high Nb/U, Nb/Th), and its incorporation into a mantle plume from which 2.7 Ga plateau tholeiites erupted. The variably high Nb/U ratios of the plateau basalts are consistent with early extraction of large quantities of the protoliths (magmatic precursor) of continental crust from the southern Superior Province asthenospheric mantle.
  • Article
    All of the analyzed igneous rocks from DSDP (Deep-Sea Drilling Project) legs 34 and 37 are altered. Their primary oxygen isotope composition, which can be estimated from the analysis of separated minerals, was indistinguishable from that of fresh unaltered basalts dredged from midocean ridges (5.7 ± 0.2‰ SMOW), but the alteration of the rocks has either enriched or depleted them of 18O. Low-temperature weathering of basalts by seawater has increased their 18O content by 1 to 3‰. Basalts recovered from 600 m within the oceanic crust in site 332B have a δ18O value of 8.3‰. On the other hand, the 18O contents of intrusive rocks from site 334 have been lowered 1 to 3‰ by high-temperature, postsolidus exchange with seawater. Both kinds of altered rocks are already known from the study of dredged materials, but only the ocean drilling program can determine the depth to which seawater penetrates in the oceanic crust and the prevalence of altered rock. Implicit in current tectonic models is the hypothesis that sufficiently large volumes of oceanic crust are produced each year, so that any appreciable exchange between the crust and the oceans will influence the chemical composition of seawater. If site 332B has sampled typical oceanic crust, then weathering of the crust is indeed a major sink for 18O. However, if intrusives are prevalent in the crust, then their alteration is a major source of 18O to the ocean. Estimates of the volumes of materials involved in the formation and alteration of the oceanic crust (1 km of hydrothermally altered rock, 600 m of weathered rock), cycling of water through the mantle (2% of the subducted crust), and weathering of the continents indicate that the isotopic composition of the ocean may be held at its present value as a consequence of the 18O enrichments balancing the 18O depletions. Approach to the present isotope ratio is governed by a time constant of 200 or 300 m.y.
  • Article
    Detailed examination of two new profiles through the Denison Series paleosol, and one new profile through a Pronto Series paleosol in the Elliot Lake area, lend fresh insight into atmospheric and hydrogeologic environmental conditions of Precambrian Earth. These are reassessed, with reference to the results of previous studies, as true indigenous paleosols of polygenetic character. Virtually all of the criteria (Grandstaff et al., 1986) for identification of paleosols are met. The paleosols bear strong mineralogical and chemical similarities despite derivation from different parent materials, and sampling at locations distant from each other. There has evidently been a common reaction to weathering processes peculiar to the Early Proterozoic.
  • Article
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    Reddened, clay-rich horizons between basalt flows have historically been identi- fied as ''baked zones'' or as zones of lateral groundwater movement. The reddening has been attributed to thermal or chemical al- teration of the permeable portion of the ba- salt flow. Many of these red horizons are instead paleosols that record significant hi- atuses in basalt flow emplacement. Paleo- sols were identified in the field on the basis of features such as horizonation, ped mor- phology, and root traces. Terrestrial gas- tropods, fossil plants, and peats also occur locally. Plots of weathering indices, such as base loss, are consistent with a pedogenic origin for these horizons. Ca, Na, Mg, Fe 21 , and P have been extensively lost from the A and Bt horizons of the paleosols, consistent with modern weathering patterns. K and Rb have been added to the upper portions of the profiles and lost in the lower portions of the profiles relative to the parent mate- rial, with Rb added disproportionately. Historically, this pattern of enrichment has been cited as evidence of K metasomatism. The apparent metasomatism can be ex- plained using a new model in which there was extensive felsic volcanism contempo- raneous with pedogenesis, and K and Rb were likely added by windborne ash. The apparent discordance between K and Rb contents of the paleosols can be explained by the presence of land plants because K is an important electrolyte and Rb does not serve any major biological role. This model could account for apparent K metasoma- tism in other paleosols following the advent of land plants and may be applicable to old- er paleosols as well.
  • Article
    Although mineralogical changes are complex, bulk compositional changes to weathering profiles, resulting from chemical weathering, are simple and predictable from kinetic, thermodynamic, and mass balance considerations. Predicted bulk compositional changes are corroborated by studies of Recent weathering profiles developed on a variety of plutonic and volcanic rocks under different climatic regimes. Unlike the mineralogical compositions of profiles, the bulk compositional trends are not noticeably modified by climate. -from Authors
  • Article
    Many interpretations of oxygen levels in the Precambrian atmosphere use interpretations of Fe distributions measured in paleosols. This article addresses the current lack of knowledge concerning "baseline" Fe translocations in modern soil analogs for the 2.25-Ga Hekpoort paleosol at Waterval Onder, South Africa, as well as the lack of understanding of diagnostic petrographic features related to Fe translocation preserved in the Precambrian paleosols. Petrographic features related to Fe translocations include redox-related Fe depletions and enrichments, and possibly Fe-Mn nodules, which compare favorably with similar pedogenic features in Paleozoic paleo-Vertisols and modern analog Vertisols. Additionally, preservation of sepic-plasmic microfabrics in the Hekpoort paleosol, which are characteristic of clay soils experiencing shrink-swell, supports previous paleo-Vertisol interpretations. Total Fe losses in Vertisols are 10%-50%; losses increase with increasing soil age and with increasing mean annual precipitation but appear to be independent of parent material differences (unconsolidated sediments, sedimentary rocks, mafic rocks). Total Fe losses in Paleozoic paleo-Vertisols are comparable to those of Vertisols. Nearly complete (75%-99%) loss of total Fe characterizes the upper 220 cm of the Hekpoort paleosol, if the underlying basalt is assumed to be the parent material. The apparent greater mobility of Fe in the Hekpoort paleosol most likely reflects the lower (but not anoxic) PO2 conditions postulated for this time by some researchers but could also indicate oxic conditions and the presence of a significant terrestrial biomass as a source of organic ligands that enhanced Fe mobility, which has also been recently proposed.
  • Article
    This chapter highlights the reciprocal influences of evolution on oxygen and of oxygen on evolution. Without life there would be very little free oxygen at the earth's surface. Oxygen has risen from negligible concentrations to 21 % of the present atmosphere. Atmospheric oxygen is the product of oxygenic photosynthesis. The capacity to detoxify reactive oxygen species and the potential for aerobic respiration arose before oxygenic photosynthesis. Oxygen remained below 0.0008 atm until 2.2 Gyr ago, but probably reached significant dissolved concentrations in localized oxygen oases, including microbial mats. Localized accumulation of oxygen triggered an increase in defenses against oxygen toxicity, the development of metabolic pathways utilizing oxygen, and facilitated the appearance of eukaryotic cells with mitochondria. Between 2.2 and 2.0 Gyr ago, the Earth underwent a great oxidation event in which atmospheric oxygen rose above 0.002 atm. The rise of oxygen generated an ozone layer providing effective ultraviolet (UV) protection and increased the availability of nitrogen in the ocean but reduced the availability of many bioessential metals. Eukaryotes with chloroplasts appeared at this time possibly as a result of increased nitrogen availability. By 0.3 Gyr ago, oxygen had reached at least 0.21 atm, driven upwards by vascular plants colonizing the land surface and increasing organic carbon burial. Since then, the atmospheric fraction of oxygen has been stabilized close to 21% by feedback mechanisms involving terrestrial and marine biota.
  • Article
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    A Paleoproterozoic lateritic paleosol overlain by ultra-mature/mature quartzite is reported for the first time from three localities in the Svecofennian province in southern Finland. These are important indicators for warm paleoclimate and continental paleoenvironment. Ultra-mature quartzites above the lateritic paleosols are overlain by matrix-supported conglomerates followed by mature quartzites. These latter rocks are related to incipient rifting followed by main rift stage starting with increasingly more immature meta-arkose.
  • Article
    The Hekpoort paleosol is a regional paleoweathering horizon developed on the approximately 2.25 Ga Hekpoort basalt of the Transvaal Supergroup, South Africa. A core (Strata 1) through the Hekpoort paleosol near Gaborone, Botswana, is the most complete section through this paleosol. An uppermost ferricrete layer (∼40 cm) is followed by a mottled zone (∼2.0 m), an iron-depleted sericite-rich zone (∼2.3 m), and a FeO-enriched, chlorite-quartz zone (∼13 m), which grades downward into the Hekpoort basalt. The near-constancy of the mole ratios of Al, Zr, Nb, Hf and Th to Ti indicates that these elements were essentially immobile during the formation of the paleosol. Mg, Zn and Mn were largely removed from the upper portion of the soil and were partly reprecipitated in the chlorite zone. Fe2+, Co, Ni, V, Cr and U were depleted in the mottled and sericite zones, and were enriched in the ferricrete layer and in the chlorite zone. The vertical distribution of the major and trace redox-sensitive elements in Strata 1 indicates that the Hekpoort paleosol developed during the Great Oxidation Event (G.O.E.). The atmosphere must have contained some oxygen, but apparently not enough to oxidize all of the Fe2+ released during weathering to Fe3+. A mass balance calculation suggests a value of PO2 between 2.5 × 10-4 and 9 × 10-3 atm during the formation of the paleosol. Interpretations of the paleosol as a ground water laterite developed under a highly oxygenated atmosphere are difficult to reconcile with the distribution of the major and trace elements in Strata 1.
  • Article
    During the formation of the Jurassic Pindos ophiolite a convective seawater-fed hydrothermal system was initiated at or near the spreading centre. As a result, typical ophiolitic background metamorphic assemblages developed during the downward circulation of seawater, and epidosites and stockworks formed in the upwelling part of the system. The epidosites were analysed for major and trace elements including the REE. Substantial chemical changes occur during the evolution of the epidosites from their protoliths. While some elements (Ti, Zr, Y, Nb) appear immobile during this transformation, when a bulk quartz addition is taken into account, the REEs are mobile under these alteration conditions. Evidence for the mobility of the REE is also provided by the analyses of hydrothermal solutions from black smokers on the East Pacific Rise. REE can be very good indicators of the degree of water/rock interaction at high water/rock ratios. -from Authors
  • Article
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    Five discrete accretionary events assembled fragments of continental and oceanic crust into a coherent Superior craton by 2.60 Ga. They exhibit similar sequences of events at ~10 million year intervals: cessation of arc magmatism, early deformation, synorogenic sedimentation, sanukitoid magmatism, bulk shortening, regional metamorphism, late transpression, orogenic gold localization, emplacement of crust-derived granites, and postorogenic cooling. The Northern Superior superterrane recorded 3.7–2.75 Ga events prior to 2.72 Ga collision with the 3.0 Ga North Caribou superterrane. Following 2.98 Ga rifting, the Uchi margin of the North Caribou superterrane evolved in an upper plate setting before 2.72–2.70 Ga collision of the <3.4 Ga Winnipeg River terrane, which trapped synorogenic English River turbidites in the collision zone. The Winnipeg River terrane was reworked in 2.75–2.68 Ga magmatic and tectonic events, including the central Superior orogeny (2.71–2.70 Ga) that marks accretion of the juvenile western Wabigoon terrane. In the south, the Wawa–Abitibi terrane evolved in a mainly oceanic setting until Shebandowanian collision with the composite Superior superterrane at 2.695 Ga. Synorogenic Quetico turbidites were trapped in the collision zone. The final accretionary event involved addition of the Minnesota River Valley terrane (MRVT) from the south, and deposition and metamorphism of synorogenic turbidites of the Pontiac terrane during the ~2.68 Ga Minnesotan orogeny. Seismic reflection and refraction images indicate north-dipping structures, interpreted as a stack of discrete 10–15 km thick terranes. A slab of high-velocity material, possibly representing subcreted oceanic lithosphere, as well as Moho offsets, support a model of progressive accretion through plate-tectonic-like processes.
  • Article
    Considerable mobilization of Fe without mobilization of Al in Precambrian paleosols has been documented and attributed to either anoxic- or ligand-promoted dissolution. To elucidate these mechanisms, basalt was dissolved under oxic and anoxic conditions with and without citrate, and the mobility of several elements was analyzed. The extent of release of Fe and P was minor (in citrate-free conditions) or considerable (with citrate) regardless of oxygen pressure. Release of Al was minor in all cases, whereas release of Cu was minor (in anoxic conditions) or considerable (in oxic conditions). Release of Cu was enhanced by citrate. In comparison, in the weathered surface of two of the oldest-known basalt-derived paleosols---the Mount Roe (2.76 Ga) and the Hekpoort (2.25 Ga)--- Fe and P were considerably depleted and Al retained, consistent with the presence of organic ligands. Cu, retained in the Mount Roe paleosol but considerably mobilized in the Hekpoort paleosol, documents formation under an anoxic atmosphere and an oxic atmosphere, respectively, as inferred by others on the basis of Fe mobility. The immobility of Al in both paleosols is consistent with formation under conditions in which the annual volume of rainwater was lower than the topsoil pore volume. Mobilization of P in such paleosols developed under low-rainfall conditions provides a new proxy for identification of ligands secreted by terrestrial organisms on early Earth.
  • Article
    The ca. 2.2 Ga Hekpoort paleosol of the Transvaal Supergroup in southern Africa has been considered a type example and the youngest iron-depleted paleosol formed under a reducing atmosphere in the early Precambrian. However, the mineralogical and geochemical data on recently acquired deep drill core intersections indicate that the Hekpoort paleosol represents part of an ancient lateritic weathering profile with an iron-depleted pallid lower zone and an iron-enriched lateritic upper zone. Previous studies of the paleosol took place in areas where only the lower pallid zone was preserved from erosion prior to deposition of cover beds. The laterite profile is comparable to that of modern tropical laterites formed under an oxic atmosphere in the presence of abundant terrestrial biomass. Revised stratigraphic correlation indicates that the Hekpoort laterite profile is a correlative to highly ferruginous laterite profiles of Wolhaarkop in Griqualand West. This information indicates that the oxygen-evolution curve, based on loss or retention of iron in paleosols, should be reexamined.
  • Article
    The late Archean (circa 2750-2670 Ma) Schreiber-Hemlo greenstone belt, Superior Province, Canada, is composed of tectonically juxtaposed fragments of oceanic plateaus (circa 2750-2700 Ma), oceanic island arcs (circa 2720-2695 Ma), and siliciclastic trench turbidites (circa 2705-2697 Ma). Following juxtaposition, these lithotectonic assemblages were collectively intruded by synkinematic tonalite-trondhjemite-granodiorite (TTG) plutons (circa 2720-2690 Ma) and ultramafic to felsic dikes and sills (circa 2690-2680 Ma), with subduction zone geochemical signatures. Overprinting relations between different sequences of structures suggest that the belt underwent at least three phases of deformation. During D1 (circa 2695-2685 Ma), oceanic plateau basalts and associated komatiites, arc-derived trench turbidites, and oceanic island arc sequences were all tectonically juxtaposed as they were incorporated into an accretionary complex. Fragmentation of these sequences resulted in broken formations and a tectonic mélange in the Schreiber assemblage of the belt. D2 (circa 2685-2680 Ma) is consistent with an intra-arc, right-lateral transpressional deformation. Fragmentation and mixing of D2 synkinematic dikes and sills suggest that mélange formation continued during D2. The D1 to D2 transition is interpreted in terms of a trenchward migration of the magmatic arc axis due to continued accretion and underplating. The D2 intra-arc strike-slip faults may have provided conduits for uprising melts from the descending slab, and they may have induced decompressional partial melting in the subarc mantle wedge, to yield synkinematic ultramafic to felsic intrusions. A similar close relationship between orogen-parallel strike-slip faulting and magmatism has recently been recognized in several Phanerozoic transpressional orogenic belts, suggesting that as in Phanerozoic counterparts, orogen-parallel strike-slip faulting in the Schreiber-Hemlo greenstone belt played an important role in magma emplacement.
  • Article
    Plutonic rocks from the Hemlo area in the southern Superior Province span a period from ca. 2720 to 2677Ma. Early TTG plutons intruded into lower basaltic volcanic sequences and were probably derived from melting of basaltic crust at upper mantle depths. They predate major regional deformation and span ages from 2719 to 2697Ma. Felsic to intermediate volcanism (2697–2688Ma) partially overlaps TTG plutonism but is mostly slightly younger and lacking plutonic equivalents. Regional deformation and metamorphism of the greenstone belt and TTG plutons progressed rapidly following deposition of the youngest supracrustal rocks. Syn- to late-tectonic plutons emplaced between 2684 and 2677Ma have a distinctive dual primitive-evolved character that is interpreted to reflect their derivation by direct melting of long-term depleted mantle that was metasomatically enriched in a wide range of LILE and HFSE
  • Article
    An episodic continental growth model, by lateral spread of subduction–accretion complexes, is proposed for the late Archean southern Superior Province, based on structural relationships and high precision trace element data for the 2.7 Ga Wawa and Abitibi subprovinces. Two principal volcanic associations are recognized: (1) tholeiitic basalt–komatiite; and (2) tholeiitic to calc-alkaline bimodal basalt–rhyolite. Tholeiitic basalts of the former association are characterized by near-flat REE patterns and complex Th–U–Nb–LREE systematics; rare transitional to alkaline basalts and Al-depleted komatiites have fractionated REE patterns and OIB-like trace element signatures. This is interpreted as representing an oceanic plateau derived from a heterogeneous mantle plume. The latter bimodal association has fractionated REE patterns and negative Nb, Ta, P, and Ti anomalies typical of arc magmas. Turbidites plot on mixing hyperbolae between mafic and felsic end members. This association is interpreted as an arc–trench system. Tonalite plutons derived from partial melting of subducted oceanic slabs intrude the subduction–accretion complex as the magmatic arc axis migrated towards the trench. In the Abitibi belt, boninite-series low-Ti tholeiite flows stratigraphically intercalated with the basalt–komatiite association signifying coeval eruption of plume and arc lavas. Plateau tholeiites are characterized by Nb/U, Th/U, Nb/Th, Nb/La, Th/La, and La/Smpm ratios that span the primitive mantle values. Arc associations have Nb/U, Nb/Th, and Nb/La ratios that are systematically lower than the primitive mantle ratios, but have La/Smpm ratios that are systematically higher than the primitive mantle ratio. The variability of these ratios in oceanic plateau basalts is interpreted in terms of recycling variable quantities of oceanic crust processed through a subduction zone (high Nb/U, Nb/Th), and complementary arc crust (low Nb/U, Nb/Th) into the source of the plumes. In the Wawa and Abitibi greenstone belts of the southern Superior Province, eruption of voluminous ocean plateau and island arc volcanic sequences, deposition of kilometer-thick siliciclastic trench turbidites, and intrusion of the supracrustal units by syn- to post-kinematic TTG plutons occurred in the relatively short period of time of 2.75–2.65 Ga. All these events were accompanied by contemporaneous intense poly-phase deformation, regional greenschist to amphibolite facies metamorphism, and terrane accretion along a north–northwest-dipping subduction zone. Collectively, these events are interpreted as a part of a late Archean supercontinent cycle, involving accretion of oceanic plateaus, island arcs, continental fragments, closure of ocean basins, rifting of magmatic arcs, and plume arc interactions.
  • Article
    Beneath the Thelon Formation in the northwestern part of the Canadian Shield, an unmetamorphosed and undeformed paleosol is preserved on a variety of Paleoproterozoic and Archean protoliths. This paper documents the paleosol, referred to as the Thelon paleosol, in four localities where it developed on leucocratic schist, gneiss and granite. The Thelon paleosol is characterized by these ascending changes from fresh protolith, through paleosol to the unconformity: (a) saprolitic disruption of the protolith, resulting in ped and ferran formation; (b) reddening of the paleosol as a consequence of hematitization; (c) increase in dissolution of feldspar ( ), increase in the degree of iron oxidation in biotite, chlorite and garnet, and an increase in the abundance of neoform kaolinite, illite and hematite cement; (d) a loss of TiO2, Fe2O3 (total), FeO, MnO, MgO, CaO, Na2O, K2O, P2O5, U, Zr, Rb, Sr, Ba; (e) a gain of Fe2O3; and (f) an increase in CIA values. The fabric of the parent material appears to be the only soil-forming factor that locally influenced paleosol development. In comparison to thin profiles developed on massive, relatively impervious protoliths, thick profiles have developed on metamorphic protoliths in which the fabric has allowed deeper penetration of paleoweathering fluids. The behaviour of iron in the profiles, particularly Profile 2 with a granite protolith, and an increase in hematite towards the top of the profiles, suggests that the Thelon paleosol formed under an oxidizing atmosphere.
  • Article
    Two profiles from the paleosol underlying the Huronian Supergroup at Elliot Lake, Ontario were studied to determine conditions present at the time of their formation. (1) One paleosol profile (> 10.5 m thick), developed on greenstone, is exposed within the Denison Mine. It is tentatively classified as an intrazonal gley soil formed in an area of poor drainage and reducing groundwater conditions. It is characterized by extensive loss of Fe, Mn, Cu, Ni, and Mo, as well as Na, Ca, and Mg. An increase in K is attributed to diagenetic K metasomatism. Paleosol structures indicate clay translocation forming an illuvial Bg horizon and suggests a fluctuating water table to depths of 6m. (2) The second paleosol profile (> 5.5 m thick), developed on granite, is exposed near the Pronto Mine. This profile exhibits iron oxidation and enrichment. This indicates that free oxygen was present in the mid-Precambrian atmosphere, although a precise determination of pO2 is not possible from evidence from the paleosols. This paleosol exhibits many characteristics of modern spodosols.The soils probably formed in a temperate humid climate during tectonically stable conditions. The difference in oxidation and behavior of iron in these soils is inferred to result from differences in paleotopography and drainage. The preponderance of gley paleosols found underlying the Huronian Supergroup might result either from lower mid-Precambrian pO2 or from preferential preservation. Evidence suggests that by 2.3 Ga/ago, soil forming processes were capable of producing soils similar to Recent types.
  • Article
    Full-text available
    Chlorine trifluoride (ClF 3 ) is recommended as a comparable alternative to bromine pentafluoride (BrF 5 ) for the quantitative extraction of oxygen from silicate minerals.
  • Article
    ODP Hole 801C penetrates >400 m into 170-Ma oceanic basement formed at a fast-spreading ridge. Most basalts are slightly (10–20%) recrystallized to saponite, calcite, minor celadonite and iron oxyhydroxides, and trace pyrite. Temperatures estimated from oxygen isotope data for secondary minerals are 5–100 °C, increasing downward. At the earliest stage, dark celadonitic alteration halos formed along fractures and celadonite, and quartz and chalcedony formed in veins from low-temperature (<100 °C) hydrothermal fluids. Iron oxyhydroxides subsequently formed in alteration halos along fractures where seawater circulated, and saponite and pyrite developed in the host rock and in zones of restricted seawater flow under more reducing conditions. Chemical changes include variably elevated K, Rb, Cs, and H2O; local increases in FeT, Ba, Th, and U; and local losses of Mg and Ni.
  • Article
    Burial of midocean ridges by elastic sediments, particularly at continental margins, profoundly affects the geometry and chemistry of hydrothermal circulation and mineralization in the upper crust. Middle Valley, the sediment-covered northern extension of the bare-rock Endeavour segment of the Juan de Fuca Ridge, is host to the base metal-rich (Cu-Zn) Bent Hill massive sulfide deposit. At a water depth of 2,400 m, the ∼9 Mt Bent Hill deposit is a steep-sided body ∼200 m across and ∼ 100-m-thick. Ocean Drilling Program (ODP) hole 856H penetrates through the massive sulfide and underlying feeder zone extending to a total depth of 500 m below sea floor through the base of the strongly recrystallized (quartz + chlorite) sediment pile and into the uppermost volcanic basement. The basaltic rocks beneath the Bent Hill deposit include narrow sills intruded into indurated sediments, a volcanic flow erupted on top of sediments, and pillow lavas below the lowermost sediments recovered. Similar styles of alteration are present in both the sills and flows, and alteration is dominated by the effects of large-scale hydrothermal upflow rather than hydrothermal activity associated with individual eruptions or intrusions. The basalts are slightly to completely altered to greenschist facies secondary minerals, principally quartz, chlorite, and titanite, with subsidiary epidote, Cu-Fe sulfides, and rare actinolite. There are steep mineralogical, chemical, and isotopic alteration gradients from the highly altered basalt-sediment interfaces down to the less altered flow interiors, suggesting the channeling of hydrothermal fluids along the basalt-sediment boundaries. Alteration is reflected in intense metasomatic changes in the basalts. Assuming immobile TiO 2, the most intensely altered basalts have undergone about 20 percent mass loss during recrystallization to chlorite-quartz rocks, with depletions in silica, alumina, and alkali, alkali earth, and base metals. Chloritized pillow margins with strongly light rare earth element-enriched chondrite-normalized patterns ([La/SM] N = 1.5; cf. fresh basalts, ∼0.7), that mimic profiles for midocean ridge hydrothermal fluids, require fluid-rock exchange with large quantities of hydrothermal fluid (W/R ∼27,000). Oxygen isotope compositions of chlorite-quartz rocks (δ 18O = 1.8-2.4‰) suggest that alteration occurred between ∼320° and 370°C. Strontium isotope compositions of the altered basalts and the chlorite-quartz rocks are not homogeneous and range from 87Sr/ 86Sr ratio = 0.7037 to 0.7046. There is a strong mode in 87Sr/ 86Sr ratio at ∼0.7038, suggesting that much of the alteration occurred by isotopic exchange with a hydrothermal fluid of that composition. This ratio is significantly lower than that measured for 265 °C fluids venting from the nearby ODP mound ( 87Sr/ 86Sr = 0.7044). The occurrence of epidote and isocubanite in the chloritized glassy pillow margins suggests that these rocks may retain a record of the high-temperature (>350°C) hydrothermal fluid responsible for the formation of the overlying Bent Hill massive sulfide deposit. The strontium isotope composition of the chloritized glassy pillow margins, and hence the mineralizing fluid, is slightly more radiogenic ( 87Sr/ 86Sr = 0.7046). This composition could result from the addition of ∼ 15 percent of a pelagic or sedimentary component to the 87Sr/ 86Sr ratio = 0.7038 fluid responsible for most of the Sr isotope exchange with the upper basement. The sediments beneath the Bent Hill deposit are also strongly recrystallized to quartz and chlorite. Although their strontium isotope compositions are much lower than those in pelagic or terrigenous sediments in the region ( 87Sr/ 86Sr = 0.709-0.720), the range of compositions ( 87Sr/ 86Sr = 0.7046-0.7060) has little overlap with that of the altered basalts and chlorite-quartz rocks from the sills and uppermost basement. This lack of overlap suggests that the sediments either retain some of their original sedimentary strontium or that there is a range of fluid compositions in the sediment pile beneath the Bent Hill deposit. Sediments from the margins of Middle Valley, far from zones of active black smoker venting (ODP site 855), have 87Sr/ 86Sr ratios with a significant hydrothermal component (0.7059-0.7086). These ratios indicate that the subsurface hydrology of Middle Valley is dominated by evolved fluids rather than seawater, and that recharge into this system is not through boundary faults or through the sedimentary blanket as suggested by previous models. Rather, exposed basement rocks that form the flanks of Middle Valley are the most likely zones of regional seawater recharge to the deep high-temperature hydrothermal systems.
  • Article
    The Mattagami Lake mine is one of several Archaean Zn-Cu massive sulphide deposits located in the Matagami mining district of NW Quebec, within the Abitibi greenstone belt. The present study evaluates the role of volcanic, hydrothermal and sedimentary processes in the formation of ore deposits of this type. Oxygen isotope, fluid-inclusion and major- and trace-element data are presented. The orebody apparently formed in a sea-floor depression on a vitroclastic rhyolite and was later covered by an epiclastic unit of mixed chemical sediments and volcaniclastic material. Ore mineralization was accompanied by chloritization and local potassic alteration of the host rocks - details of the various chemical changes in these rocks are summarized. Ore minerals are thought to have precipitated from sea-water-dominated hydrothermal solutions at 250-300oC and 250 bar in a dynamic sea-floor brine pool ponded in a sea-floor depression. Geochemical trends noted in the host rocks and ore minerals reflect the evolving fluid chemistry within this depositional system.-J.E.S.
  • Article
    Paleosols preserve information about the composition of the atmosphere and paleoclimatic conditions. Here we report the discovery of the first pisolitic laterites of Precambrian age. The laterites are immediately above the regional unconformity at the base of the Early Proterozoic Gamagara Formation in paleokarst depressions on the Campbellrand dolomite, Transvaal Supergroup, South Africa. The paleosol profiles are very well preserved and show zoning, relict soil textures, and chemical composition virtually identical to those of modern laterites. The pisolitic laterites provide not only evidence for a highly oxygenated atmosphere and possible hot and humid climatic conditions in Early Proterozoic time, but also indicate the presence of terrestrial life on the Kaapvaal craton ˜2.0 2.2 b.y. ago.
  • Article
    Precambrian paleosols are potentially one of the best sources of information about the composition of the atmosphere early in earth's history. To be used this way, however, an alteration profile must be shown to be an in situ product of vadose processes. Data from modern soils show that the ratios of less mobile elements such as Zr, Ti, and Al can be used as a preliminary screen to test for the in situ nature of a profile. If the Ti/Zr ratio departs from that of the host rock by more than 40%, or the Ti/Al ratio departs by more than 50% for moderate degrees of weathering, then a significant allochthonous contribution to the profile is indicated. This technique correctly separates in-place from allochthonous horizons when applied to independent data from two modern profiles. A review of published Precambrian profiles identifies two often-cited examples, Denison and Pronto, as suspect based on these criteria. In the interpretation of paleoatmospheres, it is also necessary to show that Fe is controlled not be diagenetic but by pedogenic processes. Comparison of the distribution of Fe to that of Ti and Zr shows that Fe is generally leached from confirmed paleosols older than 2.2 Ga, but is retained in modern soils and in younger paleosols. Reducing conditions developed during diagenesis can, however, produce a removal of Fe in modern alteration profiles that is very difficult to distinguish from removal by soil processes under a low-oxygen atmosphere. Diagenetic and pedogenic Fe removal can possibly be distinguished by the presence of a vertical zonation with a lower zone of Fe enrichment in the pedogenic case.
  • Article
    Micromorphologic, geochemical and isotopic analysis of an apparent Archean paleosol exposed at South Roberts Pit, Steep Rock area, Superior Province, Canada are consistent with in situ, subaerial weathering followed by potassium metasomatism and greenschist metamorphism. The profile formed on tonalite of the 3.0 Ga Marmion Complex and is unconformably overlain by the basal conglomerates of the >2.7 Ga Steep Rock greenstone series. Only the lowermost part of the soil profile is preserved. Weathering of plagioclase, biotite, ilmenite, titanite, epidote, and apatite likely produced clays such as kaolinite and smectite (now altered to sericite and paragonite) and leucoxene. Micromorphologic evidence of soil-forming processes includes intertexic and agglomeroplasmic fabric, spalling quartz grains, and clay cutans. With increased weathering, quartz and plagioclase decrease and the sericite matrix increases. Rip-up clasts are present at and above the unconformity. Base cations (Ca, Mg, Na, P), Fe and Mn are depleted relative to parent material, while K is enriched. Chondrite-normalized rare earth element (REE) plots for the paleosol show light-REE enrichment, whereas heavy-REE patterns are mostly flat. Parent-normalized patterns indicate some REE mobilization and fractionation during pedogenesis. The low K-corrected chemical index of alteration and modest changes in the overall REE budget indicate that either the profile was not well developed or that the most heavily weathered sections were eroded away. Laser ablation-ICP-MS examination shows that the REE budget is largely controlled by titanite, which is generally LREE-depleted relative to its igneous parent. Eu anomalies (Eu/Eu*) in the paleosol are more negative than that of average parent, consistent with plagioclase weathering. Significant iron loss (>40%), Fe(III)/Fe(II) ratio of >1, and lack of strong Ce anomalies suggest that the paleosol could have formed under conditions sufficient to oxidize Fe, but not Ce, and that the profile was later stripped of Fe by reductive processes such as leaching by organic acids or hydrothermal alteration. Rb-Sr systematics for the paleosol were clearly disturbed; correlation of Rb and K indicate K-metasomatism most likely occurred during the ~2.7 Ga greenschist metamorphic event related to the Kenoran Orogeny. Sm-Nd isotope data for the paleosol form an apparent isochron with an age of 3018 ± 90 Ma. This age is indistinguishable from that of the Marmion Complex on which the profile formed, and suggests either that weathering took place very soon (less than ~ 60 my) after emplacement of the Marmion complex or that the Sm-Nd systematics were only slightly affected on a cm-m scale by weathering processes.
  • Article
    Field and laboratory investigations of a 2690.83Ma (207Pb/206Pb age of Saganaga Tonalite) unconformity exposed in outcrop in northeastern Minnesota, USA, reveal evidence for development of a deep paleoweathering profile with geochemical biosignatures consistent with the presence of microbial communities and weakly oxygenated conditions. Weathering profiles are characterized by a 5–50m thick regolith that consists of saprolitized Saganaga Tonalite and Paulson Lake succession basaltic metavolcanic rocks retaining rock structure, which is cross-cut by a major unconformity surface marking development of a successor basin infilled with alluvial deposits. The regolith and unconformity are overlain by thick conglomerate deposits that contain both intrabasinal (saprock) as well as extrabasinal detritus. Thin-section microscopy and electron microprobe analyses reveal extensive hydrolysis and sericitization of feldspars, exfoliation and chloritization of biotite, and weathering of Fe-Mg silicates and Cu-Fe sulfides; weathering of Fe-Ti oxides was relatively less intense than for other minerals and evidence was found for precipitation of Fe oxides. Geochemical analyses of the tonalite, assuming immobile TiO2 during weathering (τTi,j), show depletion of SiO2, Al2O3, Na2O, CaO, MgO, and MnO, and to a lesser degree of K2O, relative to least-weathered parent materials. Significant Fe was lost from the tonalite. A paleoatmospheric pCO2 of 10–50 times PAL is estimated based on geochemical mass-balance of the tonalite profile and assuming a formation time of 50–500Kyr. Interpretations of metabasalt paleoweathering are complicated by additions of sediment to the profile and extensive diagenetic carbonate (dolomite) overprinting. Patterns of release of P and Fe and retention of Y and Cu in tonalite are consistent with recent laboratory experiments of granite weathering, and with the presence of acidic conditions in the presence of organic ligands (produced, for example, by a primitive microbial community) during weathering. Cu metal in the profile may document lower pO2 than present day at the surface. Comparison with previous studies of weathered tonalite and basalt (Denison, 2.45–2.22Ga) in Ontario, Canada, reveal general similarities in paleoweathering with our study, as well as important differences related to lower paleoatmospheric pO2 and terrestrial biosignature for the older Minnesota profile. A falling water table in the Alpine Lake locality is presumed to have promoted formation of this gossan-like deep-weathering system that extends to 50-m depth.