A paleosol is exposed along the north bank of the Sturgeon River, some 25 km SW of Baraga, Michigan. The paleosol was developed on hydrothermally altered Keweenawan basalt and is overlain by the Jacobsville sandstone. Textures, mineralogy, and chemical composition change gradually upwards from unweathered metabasalt, through the paleosol, to the contact of the paleosol with the Jacobsville sandstone. Many of these changes are similar to those in modern soils developed on basaltic rocks. However, K has clearly been added to the paleosol, probably by solutions which had equilibrated with K-feldspar in the Jacobsville sandstone. The Keweenawan basalt was oxidized quite extensively during its conversion to greenstone. During weathering, the remaining Fe2+ was oxidized to Fe3+ and was retained in the paleosol. The composition of the parent greenstone and its change during weathering can be used to define an approximate lower limit to the ratio of the O2 pressure to the CO2 pressure in the atmosphere during the formation of the paleosol [formula: see text]. Free O2 must have been present in the atmosphere 1.1 Ga ago, but its partial pressure could have been 10(3) times lower than in the atmosphere today.
The Godthåbsfjord region of West Greenland contains the most extensive, best exposed and most intensely studied early Archean rocks on Earth. A geological record has been described of numerous magmatic events between ~3.9 and 3.6 Ga, and evidence of life at >3.85 Ga and ~3.8-3.7 Ga has been proposed from two widely-separated localities. Some of these claims have recently been questioned, and the nature of the best preserved remnants of the oldest known terrestrial volcanic and sedimentary rocks in the Isua greenstone belt are being reinvestigated and substantially reinterpreted. The first part of this article reviews the evolution of geological research and interpretations, outlining the techniques by which the geological history has been determined and the ensuing controversies. The second part re-examines crucial field evidence upon which the antiquity of the oldest terrestrial life is claimed from the island of Akilia.
The 3.22-3.10 Ga old Moodies Group, uppermost unit of the Swaziland Supergroup in the Barberton Greenstone Belt (BGB), is the oldest exposed, well-preserved quartz-rich sedimentary sequence on earth. It is preserved in structurally separate blocks in a heavily deformed fold-and-thrust belt. North of the Inyoka Fault, Moodies strata reach up to 3700 m in thickness. Detailed mapping, correlation of measured sections, and systematic analysis of paleocurrents show that the lower Moodies Group north of the Inyoka Fault forms a deepening- and fining-upward sequence from a basal alluvial conglomerate through braided fluvial, tidal, and deltaic sandstones to offshore sandy shelf deposits. The basal conglomerate and overlying fluvial facies were derived from the north and include abundant detritus eroded from underlying Fig Tree Group dacitic volcanic rocks. Shoreline-parallel transport and extensive reworking dominate overlying deltaic, tidal, and marine facies. The lithologies and arrangement of Moodies Group facies, sandstone petrology, the unconformable relationship between Moodies strata and older deformed rocks, presence of at least one syndepositional normal fault, and presence of basaltic flow rocks and airfall tuffs interbedded with the terrestrial strata collectively suggest that the lower Moodies Group was deposited in one or more intramontane basins in an extensional setting. Thinner Moodies sections south of the Inyoka Fault, generally less than 1000 m thick, may be correlative with the basal Moodies Group north of the Inyoka Fault and were probably deposited in separate basins.
There is widespread textural evidence for microbial activity in the cherts of the Early Archean Onverwacht Group. Layers with fine carbonaceous laminations resembling fossil microbial mats are abundant in the cherty metasediments of the predominantly basaltic Hooggenoeg and Kromberg Formations. In rare cases, filamentous microfossils are associated with the laminae. The morphologies of the fossils, as well as the texture of the encompassing laminae suggest an affinity to modern mat-dwelling cyanobacteria or bacteria. A variety of spheroidal and ellipsoidal structures present in cherts of the Hooggenoeg and Kromberg Formations resemble modern coccoidal bacteria and bacterial structures, including spores. The development of spores may have enabled early microorganisms to survive the relatively harsh surficial conditions, including the effects of very large meteorite impacts on the young Earth.
In the Olenek Uplift of northeastern Siberia, the Khorbusuonka Group and overlying Kessyusa and Erkeket formations preserve a significant record of terminal Proterozoic and basal Cambrian Earth history. A composite section more than 350 m thick is reconstructed from numerous exposures along the Khorbusuonka River. The Khorbusuonka Group comprises three principal sedimentary sequences: peritidal dolomites of the Mastakh Formation, which are bounded above and below by red beds; the Khatyspyt and most of the overlying Turkut formations, which shallow upward from relatively deep-water carbonaceous micrites to cross-bedded dolomitic grainstones and stromatolites; and a thin upper Turkut sequence bounded by karst surfaces. The overlying Kessyusa Formation is bounded above and below by erosional surfaces and contains additional parasequence boundaries internally. Ediacaran metazoans, simple trace fossils, and vendotaenids occur in the Khatyspyt Formation; small shelly fossils, more complex trace fossils, and acritarchs all appear near the base of the Kessyusa Formation and diversify upward. The carbon-isotopic composition of carbonates varies stratigraphically in a pattern comparable to that determined for other terminal Proterozoic and basal Cambrian successions. In concert, litho-, bio-, and chemostratigraphic data indicate the importance of the Khorbusuonka Group in the global correlation of terminal Proterozoic sedimentary rocks. Stratigraphic data and a recently determined radiometric date on basal Kessyusa volcanic breccias further underscore the significance of the Olenek region in investigations of the Proterozoic-cambrian boundary.
Simple (one-dimensional) climate models suggest that carbon dioxide concentrations during the Archean must have been at least 100-1000 times the present level to keep the Earth's surface temperature above freezing in the face of decreased solar luminosity. Such models provide only lower bounds on CO2, so it is possible that CO2 levels were substantially higher than this and that the Archean climate was much warmer than today. Periods of extensive glaciation during the early and late Proterozoic, on the other hand, indicate that the climate at these times was relatively cool. To be consistent with climate models CO2 partial pressures must have declined from approximately 0.03 to 0.3 bar around 2.5 Ga ago to between 10(-3) and 10(-2) bar at 0.8 Ga ago. This steep decrease in carbon dioxide concentrations may be inconsistent with paleosol data, which implies that pCO2 did not change appreciably during that time. Oxygen was essentially absent from the Earth's atmosphere and oceans prior to the emergence of a photosynthetic source, probably during the late Archean. During the early Proterozoic the atmosphere and surface ocean were apparently oxidizing, while the deep ocean remained reducing. An upper limit of 6 x 10(-3) bar for pO2 at this time can be derived by balancing the burial rate of organic carbon with the rate of oxidation of ferrous iron in the deep ocean. The establishment of oxidizing conditions in the deep ocean, marked by the disappearance of banded iron formations approximately 1.7 Ga ago, permitted atmospheric oxygen to climb to its present level. O2 concentrations may have remained substantially lower than today, however, until well into the Phanerozoic.
Two distinct generations of microfossils occur in silicified carbonates from a previously undescribed locality of the Lower Proterozoic Duck Creek Dolomite, Western Australia. The earlier generation occurs in discrete organic-rich clasts and clots characterized by microquartz anhedra; it contains a variety of filamentous and coccoidal fossils in varying states of preservation. Second generation microfossils consist almost exclusively of well-preserved Gunflintia minuta filaments that drape clasts or appear to float in clear chalcedony. These filaments appear to represent an ecologically distinct assemblage that colonized a substrate containing the partially degraded remains of the first generation community. The two assemblages differ significantly in taxonomic frequency distribution from previously described Duck Creek florules. Taken together, Duck Creek microfossils exhibit a range of assemblage variability comparable to that found in other Lower Proterozoic iron formations and ferruginous carbonates. With increasing severity of post-mortem alteration, Duck Creek microfossils appear to converge morphologically on assemblages of simple microstructures described from early Archean cherts. Two new species are described: Oscillatoriopsis majuscula and O. cuboides; the former is among the largest septate filamentous fossils described from any Proterozoic formation.
A diverse assemblage of well-preserved microorganisms has been detected in black cherts from the approximately 1200 Ma-old Avzyan Formation (Suite) of the southern Ural Mountains, Russian Federation. The lower Kataskin Member contains a diverse, abundant microbiota dominated by mat-forming filamentous cyanobacteria, several types of colonial unicells, and morphologically distinctive stalked cyanobacteria. The upper Revet Member contains a less diverse biota dominated by unicellular cyanobacteria. Palaeoecological evidence indicates that the microbial community of the Kataskin Member inhabited a shallow water, presumably marine, carbonate environment. Revet microorganisms possibly lived in restricted peritidal environments. The biostratigraphic significance of the Avzyan microbiota is limited. Many of the taxa are long-ranging; they were already abundant in Palaeoproterozoic successions and continue into the Neoproterozoic. Nevertheless, in many respects, the Kataskin assemblage is comparable to those reported from the Middle-Late Riphean deposits of Northern America, Australia and Eurasia. The following taxa are here described: Chroococcaceae-Eogloeocapsa avzyanica Sergeev, Gloeodiniopsis lamellosa Schopf emend. Knoll et Golubic; Entophysalidaceae-Eoentophysalis belcherensis Hofmann; Dermocarpaceae-Polybessurus bipartitus Fairchild ex Green et al.; Nostocaceae-Eosphaeronostoc kataskinicum Sergeev; Nostocaceae or Oscillatoriaceae-Siphonophycus robustum (Schopf) emend. Knoll et Golubic emend. Knoll et al., Siphonophycus sp.; Incertae sedis-Eosynechococcus amadeus Knoll et Golubic.
The Draken Formation (120-250 m) of northeast Spitsbergen (Svalbard) forms part of a thick Upper Proterozoic carbonate platform succession. It consists predominantly of intraformational dolomitic conglomerates, with excellent textural preservation. Six main lithofacies were recognized in the field: quartz sandstones, stromatolitic mats, conglomerates with silicified intraclasts, dolostone conglomerates with desiccated mudrocks, oolitic/pisolitic grainstones and fenestral dolostones. A series of five main gradational biofacies were recognized from silicified (and rare calcified) microfossils. Biofacies 1 represents low-energy subtidal benthos (erect filaments) and plankton (acritarchs and vase-shaped microfossils) whereas biofacies 2 to 5 are microbial mat assemblages (with filamentous mat-builders, and associated dwellers and washed-in plankton) ranging from basal intertidal to high intertidal/supratidal. Colour values (a measure of the lightness of the colour shade) of sawn rock samples were quantified using a Munsell chart, and exhibit a pronounced variation (means of major groups varying from 4.0 to 5.95) across the spectrum of subtidal to supratidal sediments as inferred from other criteria. The lightening in progressively more exposed sediments is related to lowering of organic carbon contents, probably mainly by oxidation. Six types of early cement have been recognized. Calcite microspar (type 1) is common as a subtidal cement in many Proterozoic formations, whereas types 2 (subtidal isopachous fringes), 3 (subtidal hardground dolomicrite) and 4 (intertidal meniscus dolomicrite) are very similar to Phanerozoic examples except for their dolomitic mineralogy. Types 5 and 6 are complex and variable dolomite growths associated with expansion and replacive phenomena. They characterize the fenestral lithofacies and compare with modern supratidal cements. Consideration of diagenetic fabrics and truncation textures of intraclasts indicates that leaching, dolomitization, silicification were all significant syndepositional processes altering the original metastable carbonates. The data set provides evidence for a spectrum of peritidal environments including ooid shoals, protected subtidal, tidal sandflats and protected carbonate mudflats. Different sections show a preponderance of particular facies. The coastal lithofacies continuum was completely dolomitized, unlike offshore to ooid shoal facies of adjacent formations. Dolomitization thus bears a relationship to depositional bathymetry. Although hydrodynamics clearly have a role, the potential importance of whiting precipitation in raising Mg/Ca in marginal marine environments is also stressed.
The recent proliferation of stratigraphic studies of delta 13C variation in carbonates and organic C in later Neoproterozoic and basal Cambrian successions (approximately 850-530 Ma) indicates a strong oscillating trend in the C-isotopic composition of surface seawater. Alone, this trend does not adequately characterize discrete intervals in Neoproterozoic time. However, integrated with the vectorial signals provided by fossils and Sr-isotopic variations, C isotope chemostratigraphy facilitates the interbasinal correlation of later Neoproterozoic successions. Results of these studies are evaluated in terms of four stratigraphic intervals: (1) the Precambrian/Cambrian boundary, (2) the post-Varanger terminal Proterozoic, (3) the late Cryogenian, and (4) the early Cryogenian. Where biostratigraphic or radiometric data constrain the age of Neoproterozoic sedimentary sequences, secular variations in C and Sr isotopes can provide a level of stratigraphic resolution exceeding that provided by fossils alone. Isotopic data place strong constraints on the chemical evolution of seawater, linking it to major tectonic and paleoclimatic events. They also provide a biogeochemical framework for the understanding of the initial radiation of macroscopic metazoans, which is associated stratigraphically, and perhaps causally, with a global increase in the burial of organic C and a concomitant rise of atmospheric O2.
Tentative geochemical cycles for the pre-biologic Earth are developed by comparing the relative fluxes of oxygen, dissolved iron, and sulfide to the atmosphere and ocean. The flux of iron is found to exceed both the oxygen and the sulfide fluxes. Because of the insolubility of iron oxides and sulfides the implication is that dissolved iron was fairly abundant and that oxygen and sulfide were rare in the atmosphere and ocean. Sulfate, produced by the oxidation of volcanogenic sulfur gases, was the most abundant sulfur species in the ocean, but its concentration was low by modern standards because of the absence of the river-borne flux of dissolved sulfate produced by oxidative weathering of the continents. These findings are consistent with the geologic record of the isotopic composition of sedimentary sulfates and sulfides. Except in restricted environments, the sulfur metabolism of the earliest organisms probably involved oxidized sulfur species not sulfide.
The oldest filament- and colonial coccoid-containing microbial fossil assemblage now known is described here from drill core samples of stromatolitic cherty limestones of the Neoarchean, approximately 2600-Ma-old Campbell Group (Ghaap Plateau Dolomite, Lime Acres Member) obtained at Lime Acres, northern Cape Province, South Africa. The assemblage is biologically diverse, including entophysalidacean (Eoentophysalis sp.), probable chroococcacean (unnamed colonial coccoids), and oscillatoriacean cyanobacteria (Eomycetopsis cf. filiformis, and Siphonophycus transvaalensis), as well as filamentous fossil bacteria (Archaeotrichion sp.); filamentous possible microfossils (unnamed hematitic filaments) also occur. The Campbell Group microorganisms contributed to the formation of stratiform and domical to columnar stromatolitic reefs in shallow subtidal to intertidal environments of the Transvaal intracratonic sea. Although only moderately to poorly preserved, they provide new evidence regarding the paleoenvironmental setting of the Campbell Group sediments, extend the known time-range of entophysalidacean cyanobacteria by more than 400 million years, substantiate the antiquity and role in stromatolite formation of Archean oscillatoriacean cyanobacteria, and document the exceedingly slow (hypobradytelic) evolutionary rate characteristic of this early evolving prokaryotic lineage.
The carbon isotope geochemistry of carbonates and organic carbon in the late Proterozoic Damara Supergroup of Namibia, including the Nama, Witvlei, and Gariep groups on the Kalahari Craton and the Mulden and Otavi groups on the Congo Craton, has been investigated as an extension of previous studies of secular variations in the isotopic composition of late Proterozoic seawater. Subsamples of microspar and dolomicrospar were determined, through petrographic and cathodoluminescence examination, to represent the “least-altered” portions of the rock. Carbon-isotopic abundances in these phases are nearly equal to those in total carbonate, suggesting that 13C abundances of late Proterozoic fine-grained carbonates have not been significantly altered by meteoric diagenesis, although 18O abundances often differ significantly. Reduced and variable carbon-isotopic differences between carbonates and organic carbon in these sediments indicate that isotopic compositions of organic carbon have been altered significantly by thermal and deformational processes, likely associated with the Pan-African Orogeny.
The semidiurnal atmospheric thermal tide would have been resonant with free oscillations of the atmosphere when the day was approximately 21 h long, c. 600 Ma ago. Very large atmospheric tides would have resulted, with associated surface pressure oscillations in excess of 10 mbar in the tropics. Near resonance the Sun's gravitational torque on the atmospheric tide--accelerating Earth's rotation--would have been comparable in magnitude to the decelerating lunar torque upon the oceanic tides. The balance of the opposing torques may have long maintained a resonant approximately 21 h day, perhaps for much of the Precambrian. Because the timescale of lunar orbital evolution is not directly affected, a constant daylength would result in fewer days/month. The hypothesis is shown not to conflict with the available (stromatolitic) evidence. Escape from the resonance could have followed a relatively abrupt global warming, such as that occurring at the end of the Precambrian. Alternatively, escape may simply have followed a major increase in the rate of oceanic tidal dissipation, brought about by the changing topography of the world's oceans. We integrate the history of the lunar orbit with and without a sustained resonance, finding that the impact of a sustained resonance on the other orbital parameters of the Earth-Moon system would have not been large.
In 1989, the International Commission on Stratigraphy established a Working Group on the Terminal Proterozoic Period. Nine years of intensive, multidisciplinary research by scientists from some two dozen countries have markedly improved the framework for the correlation and calibration of latest Proterozoic events. Three principal phenomena--the Marinoan ice age, Ediacaran animal diversification, and the beginning of the Cambrian Period--specify the limits and character of this interval, but chemostratigraphy and biostratigraphy based on single-celled microfossils (acritarchs), integrated with high-resolution radiometric dates, provide the temporal framework necessary to order and evaluate terminal Proterozoic tectonic, biogeochemical, climatic, and biological events. These data also provide a rational basis for choosing the Global Stratotype Section and Point (GSSP) that will define the beginning of this period. A comparable level of stratigraphic resolution may be achievable for the preceding Cryogenian Period, providing an opportunity to define this interval, as well, in chronostratigraphic terms--perhaps bounded at beginning and end by the onset of Sturtian glaciation and the decay of Marinoan ice sheets, respectively. Limited paleontological, isotopic, and radiometric data additionally suggest a real but more distant prospect of lower Neoproterozoic correlation and stratigraphic subdivision.
Silicified carbonates of the latest Mesoproterozoic Sukhaya Tunguska Formation, northwestern Siberia, contain abundant and diverse permineralized microfossils. Peritidal environments are dominated by microbial mats built by filamentous cyanobacteria comparable to modern species of Lyngbya and Phormidium. In subtidal to lower intertidal settings, mat-dwelling microbenthos and possible coastal microplankton are abundant. In contrast, densely woven mat populations with few associated taxa characterize more restricted parts of tidal flats; the preservation of vertically oriented sheath bundles and primary fenestrae indicates that in these mats carbonate cementation was commonly penecontemporaneous with mat growth. Eoentophysalis mats are limited to restricted environments where microlaminated carbonate precipitates formed on or just beneath the sediment surface. Most microbenthic populations are cyanobacterial, although eukaryotic microfossils may occur among the simple spheroidal cells interpreted as coastal plankton. Protists are more securely represented by large (up to 320 micrometers in diameter) but poorly preserved acritarchs in basinal facies. The Sukhaya Tunguska assemblage contains 27 species in 18 genera. By virtue of their stratigraphic longevity and their close and predictable association with specific paleoenvironmental conditions, including substrates, Proterozoic cyanobacteria support a model of bacterial evolution in which populations adapt rapidly to novel environments and, thereafter, resist competitive replacement. The resulting evolutionary pattern is one of accumulation and stasis rather than the turnover and replacement characteristic of Phanerozoic plants and animals.
This paper systematically describes the molecular and isotopic characteristics of Meso- to Neoproterozoic (1.73–0.85 Ga) sediments from the Jixian section, Yanshan Basin, North China. A correlation is made between molecular fingerprints and paleoenvironmental and paleontologic data acquired from these strata. The agreement between these parameters suggest that the biomarkers preserved in these Proterozoic sediments are good indicators of the evolution of the paleoenvironment and early life in the Yanshan Basin. The fact that extended regular isoprenoids (>C20) were detected in most of the studied samples suggest a sustained lipid input to sedimentary organic matter from halophiles and other possible archaebacteria through Meso- to Neoproterozoic. Two dehydroabietin isomers (i.e. 18- and 19-norabieta-8,11,13-trienes) were unexpectedly present in these samples, which provides evidence suggesting that the compounds with abieta structure could be derived from primitive organisms such as algae. The stable carbon isotope characteristics of kerogen and coexisting carbonate show a rapid decline in atmospheric CO2 concentration from 1.73 to1.58 Ga, and a stable concentration afterwards. The difference (ΔC) in isotopic composition between carbonate and associated kerogen lies in zonal variations along the stratigraphic sequence, which can be attributed to the alternations of marine transgressions and regressions that occurred in the Yanshan Basin during the Proterozoic. The carbon isotope relationships between kerogen and alkanes from these strata are consistent with the hypothesis that before 1.4 Ga both n-alkanes and isoprenoids had been derived mainly from heterotrophic halophiles and other archaebacteria, but after that only isoprenoids originated from archaebacteria whereas n-alkanes were possibly derived from photosynthetic organisms. Changes in the source of n-alkanes around 1.4 Ga may suggest a decline of heterotrophic reworking to primary lipids, which can be related to the formation of a quiet, shallow-water environment of an epicontinental sea at this stage in the Yanshan Basin.
Between 1.8 and 1.0 Ga (Grenville-age), a series of subparallel accretionary orogens were added progressively to the southern edge of Laurentia. These belts now extend from Greenland/Labrador to southern California and are truncated at late Precambrian passive margins, suggesting that they once extended farther. We propose that Australia and Baltica contain their continuations. Together they comprise a long-lived orogenic system, >10 000 km long, that preserves a record of 800 million years of convergent margin tectonism. This tectonism culminated during Grenvillian continent–continent collisions in the assembly of the supercontinent Rodinia. Our reconstruction of the Australia–western US part of this assembly (AUSWUS) differs from the SWEAT reconstruction in that Australia is adjacent to the southwestern US rather than to northern Canada. The AUSWUS reconstruction is supported by a distinctive ‘fingerprint’ of geologic similarities between Australia and the southwestern US from 1.8 to 1.0 Ga, by numerous possible piercing points, and by an arguably better agreement between 1.45 and 1.0 Ga paleomagnetic poles between Australia and Laurentia. Geologic and paleomagnetic data suggest that separation between Laurentia and Australia took place ∼800–755 Ma and between Laurentia and Baltica ∼610 Ma. The proposed association of Australia, Laurentia, and Baltica, and the long-lived convergent margin they expose, provide a set of testable implications for the tectonic evolution of these cratons, and an important constraint for Proterozoic plate reconstructions.
Intermittent, widespread and often bimodal magmatism characterized the Mesoproterozoic development in both western Baltica and eastern Laurentia. Interorogenic intrusions representing early episodes of post-Gothian and pre-Sveconorwegian/Grenvillian magmatism in SW Sweden, have yielded U–Pb zircon ages at 1502±2, 1503+3/-2 and 1457±6 Ma. Integration of these new ages with well-constrained U–Pb data for other 1.53–1.13 Ga interorogenic intrusions in western Baltica implies that crustal addition occurred in discrete magmatic episodes. The occurrence of temporally and petrologically similar rocks in the North Atlantic region supports models of a coherent Laurentia–Baltica supercontinent during the Mesoproterozoic. The prolonged interorogenic magmatism in Baltica east of the Oslo Rift was typically manifested by mafic dyke swarms and gabbro–dolerite–granite complexes. This lithological association, absence of attendant regional deformation and lack of evidence of continental-margin processes, collectively imply an intracratonic position for this segment between 1.50 and 1.20 Ga. It further implies that some segment of Baltica west of the Oslo Rift was attached prior to 1.50 Ga. These observations also imply that large-scale rifting, now dated at 1.46 Ga in western Baltica, did not lead to full-scale crustal separation and development of a new continental margin. During the same period, the evolution in eastern Laurentia included orogenic conditions at 1.51–1.45 Ga, and continental margin events between 1.45 and 1.19 Ga. This discrepancy in tectonic settings for eastern Laurentia and the area in Baltica east of the Oslo Rift indicates fundamental geodynamic differences along the southern margin of Mesoproterozoic Laurentia–Baltica or that the lesser-known segment west of the Oslo Rift may have been close enough to the proto-margin to experience an evolution more akin to that of eastern Laurentia.
Rb-Sr and K-Ar investigations of rocks from the Garzón Massif in the northern Andes indicate the presence of three chronological units: (1) an ∼ 1.6 Ga old basement of augen gneisses showing ages and lithology similar to the adjacent parts of the Guiana Shield, (2) a sequence of supracrustals metamorphosed to the granulite facies around 1.2 Ga ago, and (3) a set of pegmatite dykes intruded into the other units ∼ 850 Ma ago. It is suggested that the supracrustal protoliths of the granulites were deposited in a Cordillera-type continental margin of the Guiana Shield, cratonized in the Parguazan tectonomagmatic event, and that the 1.2 Ga orogenic event in the Andes was of the continental collision type. The latter event is tentatively correlated with the Grenville Orogeny in North America.
A 4‰ positive shift in the carbon isotopic composition of the oceans, recorded globally in marine carbonate rocks at ∼1.3 Ga, suggests a significant change in Mesoproterozoic carbon cycling. Enhanced burial fluxes of organic carbon, relative to inorganic carbon, implied by this isotopic shift may have resulted in increased oxygenation of the Earth's biosphere, as has been suggested for similar Paleoproterozoic and Neoproterozoic carbon isotope events. This hypothesized Mesoproterozoic oxygenation event may be recorded in the geologic record by the appearance of the oldest preserved, laterally extensive, bedded marine CaSO4 evaporites in the ∼1.2 Ga Grenville and Bylot supergroups. Speculation that the appearance of extensively preserved marine gypsum and/or anhydrite reflects increased biospheric oxygenation has been challenged, however, by the hypothesis that CaSO4 precipitation prior to the Mesoproterozoic may have been inhibited by significantly higher marine carbonate saturation, which would have facilitated carbonate precipitation and effectively limited Ca2+ availability during seawater evaporation (Grotzinger, J.P., 1989. Controls on Carbonate Platform and Basin Development, vol. 44, SEPM, Tulsa, OK, pp. 79–106), regardless of O2 levels. The 1.2 Ga Society Cliffs Formation (Bylot Supergroup, northern Baffin Island) consists of ∼720 m of peritidal carbonates, evaporites, and minor siliciclastic rocks. Evaporites occur predominantly in the lowermost 300 m of the Society Cliffs Formation, where gypsum beds (1–250 cm thick) constitute up to 15% of the exposed strata. Stratigraphic and sedimentologic constraints, as well as isotopic (C, O, Sr) and elemental (Ca, Sr, Na, K, Ba) compositions of evaporites and associated carbonates, indicate a marine origin for Society Cliffs gypsum. An upsection increase in δ34S of Society Cliffs gypsum (from +22‰ to +32‰ VCDT) is therefore interpreted to reflect primary variation in Mesoproterozoic marine sulfate compositions, although the inferred rapidity of isotopic change requires a marine sulfate reservoir significantly smaller than that of the modern ocean. Examination of the maximum fractionation between coeval sulfide and sulfate reservoirs, however, indicates that Mesoproterozoic oceans were not sulfate-limited with respect to bacterial sulfate reduction either before or after the hypothesized 1.3 Ga oxygenation event. Although increased ocean-atmosphere oxygenation may have increased marine sulfate concentrations at this time, the exact role of a Mesoproterozoic oxygenation event cannot be ascertained. Furthermore, high Mg/Ca ratios measured in Society Cliffs gypsum suggest that elevated Mg2+ concentrations in Proterozoic marine systems may have helped sustain carbonate hypersaturation, and that Ca2+-limitation may have played a significant role in the Proterozoic record of evaporite deposition.
Acritarchs are a group of organic-walled vesicular microfossils interpreted as protists, and are among the first eukaryotes preserved in the fossil record. Taxonomic inconsistencies amongst acritarch workers have made it difficult to address the evolutionary history of this group through more traditional methods (i.e., biodiversity through species counts). We have constructed an empirical morphospace to examine the first 1.3 billion years of acritarch evolution. We show that protist morphologic evolution is broadly correlated with major environmental and biologic revolutions in Earth history such as late Neoproterozoic global glaciations, the first appearance of the Ediacaran metazoans and the Cambrian explosion. Our results also show that protist morphologic expansion precedes their taxonomic diversification; this pattern, similar to that seen in Phanerozoic animal clades, suggests that early morphospace saturation and convergence are common occurrences in eukaryote macroevolution. In addition, our data do not support a monotonic increase in maximum diameter of acritarch vesicles through the Proterozoic; instead, maximum vesicle diameter appears to fluctuate in the Proterozoic before decreasing significantly in the early Cambrian.
During the evolution of Laurentia, a Mesoproterozoic felsic igneous belt extended from Fennoscandia through Canada to the southwestern United States. This belt, referred to as the granite–rhyolite province in North America, forms much of the west Texas and eastern New Mexico basement. We present data from 41 closely spaced wells in west Texas that penetrated several hundred meters into the southern granite–rhyolite province and provide the first opportunity to develop a three-dimensional view of the basement from subsurface samples. The felsic rock types include hornblende-bearing quartz monzonite, ignimbritic rhyolite, and comagmatic granite with eutectic textures. These rocks plot in the high-K to ultra-high-K field. Quartz syenite, which plots exclusively in the ultra-high-K field, is the final felsic phase of magmatism. All of the felsic magmas were variably oxidizing based on biotite compositions. UPb chronology supports the intrusive relations established by petrography: the quartz monzonite is oldest (∼1380 Ma) and is followed by the granite–rhyolite sequence (∼1360 Ma) and the quartz syenite (∼1340 Ma). Nd model ages range from 1520 to 1740 Ma, indicating involvement of older Proterozoic crust. In many wells, thick mafic sills intrude the granite–rhyolite sequence. Nd model ages for the mafic rocks range from 1560 to 1440 Ma, implying that they are Mesoproterozoic in age. The mafic rocks include an alkaline, OIB-like suite, which is not supportive of a subduction origin. UPb ages are also reported for the Mescalero Well #1 in eastern New Mexico. In this well, metasedimentary and metavolcanic rocks of the Debaca sequence unconformably overlie a quartz syenite similar to the quartz syenite from the west Texas wells. Detrital zircons from the basal meta-arkose of the sequence are ∼1690 and ∼1320 Ma, indicating a maximum age of ∼1300 Ma for this sequence.
New data for the lead isotopic composition of galena from ore deposits related to 1.8-1.4 Ga granitoids in the southeastern part of the Fennoscandian Shield are presented and discussed together with earlier published information on granitoids and associated ore deposits. Two types of sulphide deposits genetically related to the granites can be recognized based on general metallogenetic features and isotopic composition: granite-hosted deposits (including Sn-bearing greisen veins in rapakivi granites) and peri-intrusive deposits (including contact-metasomatic skarn deposits like Pitkäranta and Gruvåsen). In addition, epigenetic deposits with a spatial but not a genetic link to the granites occur at various sites. An important metallogenetic belt related to 1.7-1.5 Ga granitoids can be followed in an east-west direction across the shield from lake Siljan in Sweden to Salmi at lake Ladoga. Within this belt, a major difference in metal derivation can be recognized: ore deposits southwest of the Raahe-Ladoga line, i.e. in areas without Archaean basement, indicate metal derivation from remelted 1.9 Ga Svecofennian crust whereas ore deposits in areas underlain by Archaean rock units indicate metal derivation from highly metamorphosed Archaean sources.
The 1.4 Ga Bondy gneiss complex (BGC) is one of a series of gneiss complexes that outcrops as tectonic domes structurally below Grenvillian, 1.3–1.25 Ga, marble and quartzite assemblages in the Central Metasedimentary Belt (CMB) of Québec, western Grenville Province. The complex comprises a variety of tonalitic to granitic orthogneiss intercalated with thin units of layered metabasite and laminated felsic gneiss that host a Cu–Au–Fe oxides hydrothermal system. A metatonalite pluton outcrops at the southern end of the complex. These rocks were metamorphosed at granulite-facies at 1.20 Ga. In spite of this metamorphism and variable Rb, U and/or Th loss, LILE depletion in the mafic and felsic gneisses is subordinate, and was mainly controlled by the mineralogy of the protoliths during percolation of metamorphic fluids. In most cases, trace element signatures of the granulites do not show evidence for REE and HFSE mobility. Their major, trace element and Nd-isotope geochemistry displays systematic and highly reproducible signatures attributable to volcanic and plutonic rocks formed in a mature island-arc setting. The arc event is inferred from the association of mafic granulite, intermediate gneiss, and tonalite with similar geochemistry to that of modern mature calc-alkaline island-arc rocks. Some of the mafic granulites and laminated quartzofeldspathic gneisses display geochemical signatures characteristic of modern tholeiitic basalts and high-silica rhyolites related to back-arc rifting. The NdT values suggest a depleted mantle source with the addition of a crustal component to form the felsic and intermediate magmas. Taking as an example the Sunda, Lesser Antilles and New Zealand island-arcs, we advocate that 1.4 Ga magmatism occurred in mature island-arc, back-arc system along the Laurentian margin.
The Jack Hills greenstone belt is situated within the Narryer Terrane, Western Australia, and is an approximately 70 km long greenstone and metasedimentary belt surrounded by granitic gneisses and various granitic rocks. Detailed fieldwork, supported by previous geochronological data shows that the Jack Hills greenstone belt has undergone a long-lived, complex deformation history, spanning at least 1.5 billion years. This includes, from oldest to youngest: (1) an early stage of recumbent and chevron folding probably associated with thrust faulting; (2) east-trending, major transpressional shearing including jog formation; (3) followed by kink and conjugate-style folding, brittle faulting, and fault reactivation. Transpressional shearing is interpreted to have taken place during the Paleoproterozoic Capricorn Orogeny. Strong overprinting and reworking with coaxial geometry gives the belt an apparent simplistic structural style that in places appears to be dominated by a single foliation. Pronounced static recrystallisation and fluid flow also mask structural complexity. Satellite and aeromagnetic imagery illustrate the large-scale geometry and show that the belt has a pronounced sigmoidal curvature, suggestive of dextral kinematics associated with transpression along major, east-trending structures. The majority of kinematic indicators in the field support this interpretation. The various structures are not developed uniformly across all lithological units within and outside the belt. Structural analysis, and an increased understanding of the lithological relationships between the various units, have allowed distinctions to be made between successive tectonic events. This has provided key criteria towards understanding the depositional timing and character of the units, and understanding and relating formation of structural elements such as mineral lineations and dominant foliations to specific events in time and space. On a regional scale, the Jack Hills greenstone belt shares some structural similarities with the nearby Errabiddy Shear Zone, which marks the northern margin of the Yilgarn Craton. This, and relationships in the Jack Hills greenstone belt shows that the Paleoproterozoic Capricorn Orogeny has affected a greater portion of the Yilgarn Craton than previously thought. Unravelling these complex structural relationships is a fundamental step towards understanding granite-greenstone formation, and the early evolution of one of Earth's oldest crustal fragments.
Mafic garnet granulite xenoliths recovered from ∼500 to 600 Ma old eastern Finland kimberlites provide direct information on the petrology and physical properties of the lower cust below the Archean Karelian craton. Mineral thermobarometry, together with isotopic, petrological and seismic velocity constraints, imply that the xenolith suite is derived from a geophysically-determined, dense, high-velocity layer at the base of the crust (40–58 km depth). Single grain zircon U–Pb dates and Nd model ages (TDM) imply that this is a hybrid layer consisting of both Archean and Proterozoic mafic granulites. Zircon ages of up to ∼3.5 Ga and Nd TDM model ages ∼3.7 Ga of the xenoliths are equivalent to those of the oldest upper crustal assemblages. During the Proterozoic, transient heating of these Archean granulites by voluminous basic magmatic intrusions resulted in multiple younger zircon generations within individual xenoliths. The most important post-Archean lower crust growth took place during ∼1.9 Ga accretion of the Svecofennian arc complex to the craton margin, when underplating Proterozoic basaltic magmas became mingled with pre-existing Archean mafic granulites. Later, post-orogenic (∼1.80–1.73 Ga) transient heating of the lower crust occurred as a response to magmas ponded at the uppermost lithospheric mantle. In conclusion, Karelian lower crust records the geological evolution of the craton margin environment through a period of nearly 4 billion years. Our data add a new piece to the emerging picture that emphasises the global importance of the 3.5 Ga crustal growth episode. We suggest that it represents a major mantle-plume event – more widespread than previously recognized – when a significant fraction of Paleoarchean continental crust was formed.
Initial Nd isotopic ratios are reported for 57 samples representing all crustal components in the central and southern parts of the 1.9-1.7 Ga old Svecofennian orogenic terrains of Sweden and the 1.8-1.7 Ga old Transscandinavian Granite-Porphyry Belt (Transscandinavian Igneous Belt). U-Pb zircon ages are determined where the magmatic ages were not sufficiently constrained.The Nd isotopic results show that there are no Archaean crustal blocks within the studied 1.9-1.7 Ga old crustal segment. This is consistent with results from Finland. The main input of recycled Archaean crust to the Svecofennian was in the form of sediments, which are concentrated in the Central Svecofennian Subprovince of north central Sweden. This is consistent with the vicinity of this subprovince to the Archaean craton of the Baltic Shield. These turbidite sediments contain up to 40% Archaean material, but S-type granitoids associated with them have a smaller Archaean component, around 20%. Most of the quantitatively predominant I-type granitoids and volcanics, as well as all the basaltic rocks studied, have ϵNd from −1 to +3, and consist of 90% material newly derived from the mantle 1.9-1.7 Ga ago.We argue that the Archaean material was introduced in the form of sediments, but we cannot distinguish between pelagic sediments delivered on subducting oceanic crust and turbidite accumulations like the ones seen in north central Sweden. In either case, an overall 15% Archaean contribution for the Swedish Svecofennian was reasonably well homogenized with mantle-derived magmatic products. For this to occur, an active near-surface tectonic environment, such as a complex of subduction zones, must have been necessary.
The Pacific margin of East Antarctica records a long tectonic history of crustal growth and breakup, culminating in the early Paleozoic Ross Orogeny associated with Gondwanaland amalgamation. Periods of older tectonism have been proposed (e.g. Precambrian Nimrod and Beardmore Orogenies), but the veracity of these events is difficult to document because of poor petrologic preservation, geochronologic uncertainty due to isotopic resetting, and debated geological field relationships. Of these, the Nimrod Orogeny was originally proposed as a period of Neoproterozoic metamorphism and deformation within crystalline basement rocks of the Nimrod Group, based on ∼1000 Ma K–Ar mineral ages. Later structural and thermochronologic study attributed major deformation features in the Nimrod Group to Ross-age basement reactivation. Yet, new SHRIMP ion microprobe U–Pb zircon age data for gneissic and metaigneous rocks of the Nimrod Group indicate a period of deep-crustal metamorphism and magmatism between ∼1730–1720 Ma. Igneous zircons from gneissic Archean protoliths show metamorphic overgrowths of ∼1730–1720 Ma, and an eclogitic block preserved within the gneisses contains zircons yielding an average metamorphic crystallization age of ∼1720 Ma. Deformed granodiorite that intrudes the gneisses and associated metasedimentary rocks yields a concordant zircon crystallization age of ∼1730 Ma. Despite scant petrologic evidence for these metamorphic and igneous events, the zircon ages from these diverse rock types indicate major crustal thickening, possibly due to collision, in the late Paleoproterozoic. We therefore recommend revival of the term Nimrod Orogeny to describe Paleoproterozoic tectonic events in rocks of the East Antarctic shield. Similarities in the ages of igneous and metamorphic events in the Nimrod Group and geologic units elsewhere in present-day East Antarctica, southern Australia and southwestern North America suggest they may have played a role in early supercontinent assembly. In particular, similarity with the Laurentian Mojave province is consistent with Proterozoic plate reconstructions joining ancestral East Antarctica with western Laurentia.
Paleoproterozoic igneous rocks in the Archean hinterland of the Paleoproterozoic Trans-Hudson orogen (THO) consist of voluminous late syn-orogenic to post-orogenic monzonite to granite (Hudson granitoids; ≈1850–1810 Ma), and contemporaneous ultrapotassic lamprophyre dykes and volcanic rocks (Dubawnt minettes) that are interbedded with alluvial fan and fluvial deposits (Baker Lake Group, lower Dubawnt Supergroup). They were followed at approximately 1750 Ma by rapakivi granite (Nueltin granite) and porphyritic rhyolite associated with aeolian sandstone (Pitz Formation, middle Dubawnt Supergroup). The tectonic cycle ended with the deposition of conglomerates and sandstones in a large sag basin (Thelon Formation, upper Dubawnt Supergroup, ≈1.72 Ga). The Hudson granitoids, which are strongly concentrated northwest of the THO, were broadly synchronous with terminal collision between the Archean Churchill and Superior cratons and the development of NE-trending ductile structures in the Western Churchill Province (WCP) that may be related to tectonic escape to the northeast. They were emplaced at mid-crustal level and no volcanic equivalents are preserved. Fault-bounded basins containing the minette volcanic rocks are located farther west in a domain dominated more by brittle faulting. The Nueltin granites, emplaced during a period of active extensional faulting, are present in a band extending southwest from the minette basins toward a preserved remnant of the sag basin (the Athabasca basin). Hudson granitoids are largely absent from this band but reappear west of it, indicating a higher crustal level of exposure in a downdropped Nueltin ‘corridor’. The Nd isotope composition of the three suites is similar (minettes: εNd,1830 Ma=−5 to −11; Hudson granitoids: εNd,1830 Ma=−7 to −13.5; Nueltin suite: εNd,1750 Ma=−7 to −10.5), and they have late Archean model ages that match those of average Archean WCP rocks. The Hudson granitoids are rich in inherited Archean zircon, and both granitoid suites are interpreted as crustal melts. Some Nueltin granites and Pitz rhyolites are mingled with basalt, and the Nueltin suite fits a commonly cited model for rapakivi granite production, which postulates injection of basalt into extending, brittly faulted crust. The Hudson granitoids are similar to late syn- to post-orogenic plutons in numerous other collisional hinterlands, which are typically associated with ultrapotassic lamprophyres. The minettes, which have high mg# and bear mantle xenocrysts, must have a mantle source component, and their source region could have been subduction-enriched lithospheric mantle. However, their source had only slightly lower time-integrated LREE enrichment than did that of the granitoids, and the incompatible element signatures of the two suites are strikingly similar. The minette source region may have been in a zone of mixed crust and upper mantle, formed during a shortening event which resulted in crustal thickening and subsequent melting at mid-crustal layers to form the Hudson granitoid plutons. The generation and emplacement of minette melts may have been promoted by extension related to a combination of slab breakoff, gravitational collapse of thickened crust, and strike-slip faulting in the deforming hinterland. Subsequent anorogenic rapakivi granite-basalt activity may have been triggered by lithospheric mantle delamination. The hinterland tectonic cycle of the WCP was repeated in other large Archean terranes that were deformed during the early Proterozoic, but the igneous and sedimentary record is unusually complete in the WCP.
Paleoproterozoic felsic and mafic volcanic rocks, extrusive and hypabyssal quartz porphyry, and high-K, A-type epizonal granites dominate the Aripuanã region in the southwestern Amazonian craton in the Mato Grosso state of Brazil. The extrusive and intrusive felsic rocks display similar major and trace elements abundance's and equivalent SHRIMP U–Pb ages of 1762–1755 Ma; contacts between them in the field occur gradually. The felsic volcanic activity occurred predominantly as explosive subaqueous volcanism. Various features such as absence of plate collision evidences, lack of any sign of oceanic crust, lack of complete trends of magmatic differentiation, and bimodal characteristics suggest that the magmatism was intracratonic. The characteristically undeformed felsic igneous rocks were formed by extensional events, probably triggered by mantle activation in an intracratonic ‘anorogenic’ or post-collisional environment. Throughout the Amazonian craton there are wide areas displaying similar magmatism responsible for the generation of distinctive A-type granites. These A-type granites and associated volcanic rocks yield ages of ∼1.76 Ga, suggesting that they are genetically linked and that there was a unique tectonic mechanism of widespread action in this craton.
The central part of the Svecofennian domain in Finland is characterized by thick lithosphere with crustal thickness up to 60–65 km. Major collisional events at 1.91-1.90 Ga and 1.89 Ga and a thrusting event at 1.86-1.84 Ga, the latter due either to continuation of shortening or to a separate intracrustal collision, are considered the major causes of crustal thickening. The origin of the Svecofennian domain has been formerly attributed to mixing of depleted mantle melts with variable amounts of Archaean crustal component via subduction, but the new data and reinterpretation of older data imply that, besides juvenile crust, the Svecofennian domain also contains older Palaeoproterozoic crustal components.
Continental siliciclastic and volcanogenic deposits of the Baker Lake Group accumulated in numerous sub-basins in the interior of the western Churchill Province between 1.84 and 1.79 Ga. In the Angikuni sub-basin, on the southeast flank of greater Baker Lake Basin, Baker Lake Group rocks outcrop in two segments that extend northeast from Angikuni Lake. They are also exposed in scattered outliers throughout the region. At northern Angikuni Lake in the northern segment, conglomerates, pebbly sandstones and mudrocks of the Angikuni Formation unconformably overlie Archean basement, and are unconformably overlain by ultrapotassic volcanic and siliciclastic rocks of the Christopher Island Formation. These rocks record alluvial fan-fluvial and sand flat-playa deposition in a fault-bounded trough formed adjacent to a wedge-shaped basement uplift. Although the Angikuni Formation was tilted before principal Christopher Island Formation volcanism at northern Angikuni Lake, geochemical and Nd isotopic data from mudrocks indicate derivation from earlier or coeval Christopher Island volcanic-like sources. The outliers demonstrate that faulting produced significant changes in the structural level of Archean basement before Christopher Island Formation volcanism.
Three major episodes of felsic plutonism can be identified in the Svecofennian of the Fennoscandian Shield. Each of these episodes is associated with coeval (but not comagmatic) mafic mantle-derived plutonism.The first episode comprises the early Svecofennian (∼1.90-1.86 Ga) granitoids, which formed the first continental crust, together with associated supracrustals, in most of the Svecofennian by a separation from the mantle at that time. They are dominated by tonalitic-granodioritic compositions and contain frequent examples of mafic-felsic magmatic interaction.The second episode (∼1.83-1.77 Ga), which is of main concern here, includes the Transscandinavian Igneous Belt (TIB), a huge N-S trending batholith of granitoids, mafics and some associated volcanics, separating the Svecofennian from the Southwest Scandinavian Gneiss Domain, and the smaller massifs and plutons of the Within Svecofennian Episode 2 (WSE2) granites. The TIB contains several examples of extensive homogenized hybrids of mafic-felsic magma-mixing, related to large aeromagnetic anomalies. The hybrids generally grade into the coarse porphyritic TIB-granites, and have intermediate mineralogy and chemistry. The geochemistry of the hybrid rocks is consistent with mixing trends in the range between ≈55% SiO2 and 65–70% SiO2, which represent the approximate end members. Mixing and mingling on a local scale is also commonly observed. The WSE2-granites have a more limited felsic composition, higher contents of incompatible trace elements, and usually lack evidence of mafic-felsic interaction, consistent with a generation by a lower degree of partial melting of the same composite, early Svecofennian crust compared to the TIB-granites.The third episode corresponds to the rapakivi granites (∼1.65-1.56 Ga). These are intimately associated with coeval mafic magmatism. Interaction between magmas has been identified in composite dikes, but is also proposed to have formed the dark, olivine- and pyroxene-bearing, more mafic rapakivi varieties, which are very similar to the homogenized hybrids of the TIB.
Early Proterozoic orogenic rocks of the Halls Creek orogen are characterized by widespread, ensialic orogeny and magmatism, and linear, post-tectonic igneous complexes several hundred kilometres long. Conventional UPb zircon techniques are applied to erect a precise chronological framework quantifying the tectonic evolution of this fold belt.Volcanism associated with early rifting (Ding Dong Downs Volcanics, lowermost Halls Creek Group) whose age has not yet been determined, is followed by a quartz-rich, arenaceous formation, and in turn by fine-grained, variably volcanogenic, phyllitic deposits with abundant carbonates. Stratabound, high-level ‘rhyolitic’ sills are emplaced into younger parts of this Biscay Formation (∼ middle Halls Creek Group). These have distinct incompatible trace element compositions, with large enrichments in Zr, Nb and Y, and are regarded as second stage A-type melts derived from a Rb-depleted granulite source. They are possibly coeval with tuffs in the same unit and, if so, the sill's UPb zircon age of 1856 ± 5 Ma closely dates this sequence, specifically the onset of turbidite deposition and commencement of orogenic activity in inferred provenance zones to the immediate west. However, confirmation of such an age interpretation depends on substantiation of this inferred coeval link between the dated sill and the Biscay Formation tuffs, and until such the 1856 ± 5 Ma result remains a minimum for the age of the Halls Creek Group.Subsequent compressional orogeny of the basinal trough sediments was accompanied by deformation, high-temperature polymetamorphism, and syn-tectonic plutonism. Anatectic pegmatite, believed to have formed as a melt product of granulite-facies metamorphism, has a UPb zircon age of 1854 ± 6 Ma. This age for high-grade metamorphism is in agreement with reassessed RbSr whole-rock ages measured previously. It closely controls the timing of the Barramundi orogeny as being somewhat younger in this subprovince than elsewhere in northern Australia, and may imply a relatively short interval, of a few to 13 million years, between supracrustal deposition and deep crustal orogenesis. A better constrained and even more rapid tectonic transition is evident from the Whitewater Volcanics, late tectonic, felsic volcanism dated at 1850 ± 5 Ma. The onset of this unconformably younger volcanism is marked by a concomitant change in tectonic style. This changeover from a pre-cratonic to cratonic setting took place in a few million years or less.
Thirty-nine oriented block samples of iron-formation were collected at 13 sites, including opposite limbs of major folds, from the 1.88-Ga Sokoman Formation (Knob Lake Group) in the Schefferville–Knob Lake area of the central New Québec Orogen, northern Québec. The samples assayed up to 80.24% Fe2O3T (54.08% Fe), implying Fe-enrichment of the iron-formation up to ore grade. Anisotropy of magnetic susceptibility measurements on 245 standard specimens indicate a well preserved bedding-parallel fabric in the iron-formation, suggesting minimal alteration of the magnetic mineralogy since deposition and/or a mimetic secondary magnetic mineralogy. The iron-formation has not been internally deformed since the magnetic mineralogy was established. Analyses by variable-field translation balance and X-ray diffraction showed that the predominant magnetic mineral is hematite but a small amount of magnetite also is present in most samples. Following low-temperature pre-treatment as appropriate, stepwise thermal and alternating-field demagnetization of 218 specimens revealed a low-temperature, post-folding component (maximum Tub≈400 °C, D=27.1°, I=20.1°, α95=10.9°, from seven sites; pole position of 40.6°S, 257.0°E), and components carried by magnetite (maximum Tub≈580 °C, D=35.8°, I=3.9°, α95=9.1°, from 10 sites; pole position of 29.6°S, 250.9°E) and hematite (maximum Tub≈680 °C, D=40.0°, I=1.6°, α95=18.6°, from seven sites; pole position of 26.8°S, 247.0°E). The components carried by magnetite and hematite are pre-, syn- and post-folding depending on the sampling site, indicating that the magnetization was acquired continuously with deformation in the New Québec Orogen at 1.84–1.83 Ga. No evidence was found for acquisition of magnetization during the Mesozoic, when many of the iron oxide orebodies in the Schefferville–Knob Lake area are thought to have formed. Our findings imply that an episode of Fe-enrichment of iron-formation in the Sokoman Formation involved the circulation of hydrothermal fluids related to late Paleoproterozoic orogenesis. Such orogenic circulation of fluids may have contributed to the development of hematitic orebodies in the central New Québec Orogen.
It is proposed in a companion paper that the volcanic rocks of the Skellefte Group in northern Sweden were deposited on a pre-1.9 Ga basement, rather than formed as a juvenile volcanic arc and subsequently accreted to the continent. It is suggested that the proposed basement, the Robertsfors Group to the south of Skellefteå, was already deformed during an early episode of deformation (D1), prior to the deposition of the Skellefte stratigraphic sequence during an extensional tectonic episode. A younger episode of deformation, D2, then affected both areas. This hypothesis has important implications for the evolution of the Svecofennian Province. To test it, by constraining the ages of the principal deformation episodes, we have undertaken both isotope dilution and ion microprobe studies of zircon and monazite from three localities in the Robertsfors Group. A phase of migmatisation directly related to D2 shear zones allows the dating of D2 at ≈1860 Ma. Other planar granitoid bodies, which cut the D1 structures, confirm the age of this event. In the context of earlier published data, we conclude that the correlation of D2 in the Robertsfors Group with the main folding in the Skellefte Group is valid. Another granitoid body, folded by D2 but emplaced after D1, contains a suite of zircons of apparent magmatic aspect, but with a spread of ages from 1870 to 1900 Ma. Because of the high MSWD, the suite was divided into two groups with ages of ≈1896 and 1874 Ma. Recognising the possibilities of inheritance and of updating by younger events, we have preferred to interpret the younger age as a minimum age for magmatic intrusion. This is consistent with the basement hypothesis but is our only definite constraint on the age of D1. However, we infer, using other evidence, that D1 is probably older than 1900 Ma. Xenocrysts, interpreted as detrital zircons derived from the associated metasediments, give ages between 2716 and 1941±20 Ma. The latter age provides a maximum age of deposition for part of the Robertsfors Group and a maximum age for D1. Other sequences, to the south and west, considered broadly equivalent to the Robertsfors Group, are intruded by granitoids dated between ≈1920 and 1960 Ma, providing local minimum ages of deposition for the sequences involved. Regional correlation with respect to age and deformation style can be made with the pre-1.9 Ga Kalevian rocks to the NE, deposited on Archaean basement on the SW margin of the Karelian Province. We suggest that pre-1.9 Ga complexes, equivalent to the Robertsfors Group, are widespread in the Svecofennian Province, and were deposited in a marginal basin which may have been initiated as early as 2.2 Ga ago. This hypothesis of an extensive basement crustal layer evidently requires that a substantial volume of crustal growth occurred prior to 1.9 Ga, in contrast to previous hypotheses. Erosion of the pre-1.9 Ga basin rocks after D1 is also the likely source of the 2.1–1.9 Ga detrital zircons in younger sediments associated with the overlying post-1.9 Ga volcanics. The source of these detrital zircons has previously been problematical.
The Paleoproterozoic cover sequence at the 100–150 km wide western margin of the Archean Karelian Province is dominated by deep water Lower and Upper Kaleva metasediments. We present here an interpretation of Sm–Nd isotope and geochemical data on 36 samples, TIMS multi-grain U–Pb zircon analyses on nine samples, and ca. 100 SIMS analysis of detrital zircon grains from four Upper Kaleva and one Lower Kaleva samples.The Lower Kaleva is characterized by autochthonous–parautochthonous, lithologically heterogeneous metaturbidites showing common enrichment in quartz. All the analysed detrital zircons are of a local Neoarchean source but tDM variation up to 2.4 Ga combined with geochemical data indicate abundant mixing of Paleoproterozoic mafic material, presumably from 2.1 Ga plateau lavas and dykes, in most of the Lower Kaleva samples.The Upper Kaleva is dominantly allochthonous with tectonically enclosed fragments of ophiolite bodies, and it is characterized by lithological and geochemical-isotopic homogeneity. Geochemical, isotopic and detrital zircon data favour material derived from an orogenic domain, comprising both Archean and Proterozoic units, followed by effective mixing during the transport. The Archean zircon grains (25%) are mostly Neoarchean. The Paleoproterozoic grains lack zircons at 2.5–2.2 Ga and plot dominantly (92%) between 1.92 and 2.05 Ga. The indicated maximum deposition ages vary from 1.95–1.94 Ga to 1.92 Ga. The main source area proposed is the Himalaya-type Lapland-Kola orogen (now) in the northeast, which experienced mountain building and erosion at 1.95–1.91 Ga.The western margin of the Karelian Province shows evidence of rifting and lithosphere thinning from 2.1 to 1.95 Ga but it is still under debate whether the craton breakup occurred at 2.06 Ga in a volcanic or later at 1.95 Ga in a non-volcanic margin setting. One hypothesis is that the onset of collision in the northeast changed plate motion and lead to a new spreading within the pre-existing passive margin at 1.97–1.95 Ga. Thus, both a volcanic margin at 2.06 Ga and a non-volcanic margin at ca. 1.95 Ga could have been operated at the western margin of the Karelian Province.
The δ13Ccarb record of well preserved carbonates in outcrop and core is here examined from the 2.6 to 1.9 Ga old basins of Western Australia. These data, which are constrained by a well defined stratigraphic and tectonic framework, and by U–Pb zircon ages, provide an insight into the variables coincident with the evolution of an oxidative atmosphere and the evolution of the early biosphere. In the latest Archaean (ca. 2.6 Ga) the secular δ13Ccarb curve is flat much like that seen in the later Palaeoproterozoic basins of Northern Australia (<1.8 Ga). This implies that photosynthesis was a major component of the biosphere at that time and that the carbon mass balance was stable. In the early Palaeoproterozoic, beginning after 2.5 Ga and continuing until at least 1.9 Ga, the δ13Ccarb the curve is much more dynamic, with significant positive and negative excursions, including a major positive excursion (+9‰pdb) close to 2.2 Ga. These excursions can be correlated with the Lomagundi event identified in Africa, Europe and North America. Previously published studies of the overlying Meso- to Palaeoproterozoic Bangemall basin and of 1.8–1.5 Ga old basins in northern Australia suggest that the δ13Ccarb curve became relatively monotonic again after ca. 1.8 Ga and remained so for most of the following Mesoproterozoic. Comparisons with data from other ancient cratons, especially Africa, suggest that the secular carbon curve may be even more complex than presently understood and probably comparable to the major excursions seen in the Neoproterozoic. When the Western Australian data are placed in their stratigraphic and tectonic framework we find that the monotonic latest-Archaean curve coincides with a tectonically quiescent period in which carbonates formed in an basinal setting on a craton surrounded by passive margins. The data are consistent with an earth in which the carbon mass balance was in equilibrium. The δ13Ccarb curve began to oscillate following the onset of glaciation as the Pilbara and Yilgarn Cratons began to converge during the Capricorn Orogeny suggesting periods of rapid carbon burial during continental dispersal. However, the major positive excursion is preserved in carbonates from back-arc basins formed as the ocean closed and subduction began. Since similar tectonic processes can be recognised, not only in Northern Australia but also on other early cratons, it can be argued that the carbon excursions relate to supercontinent cycles and to major periods of mantle overturn and superplume development. We explain this coincidence of carbon isotopic excursions and tectonism by the sequestration of carbon during ocean closure with organic-rich passive margin sediments containing isotopically light carbon subducted into and stored in the lower crust and mantle. The global ocean thereby became enriched in isotopically heavy carbon, releasing oxygen to the atmosphere. A second stepwise increase in atmospheric oxygen in the Neoproterozoic may also have been connected with the assembly of Rodinia. This second event has been associated with the development of multicellular life and the evolutionary ‘Big Bang’. Between the two events the carbon cycle, and to some extent the biosphere, appear to have entered a period of prolonged evolutionary stasis. This implies that the evolution of both the atmosphere, and the biosphere, may have been driven forward by planetary evolution, implying that biospheric evolution has largely been driven by the dynamo of earth's tectonism and its long term survival depends upon these endogenic (thermal) energy resources. If this is so it has fundamental implications, not only for life on earth, but for the more general problems surrounding the likelihood of life having evolved on other planetary bodies. Small planets with insufficient endogenic energy resources to sustain the crust/mantle interactions of plate tectonics (such as Mars) seem to us unlikely to have allowed evolution beyond single-celled life forms. Once a planet's energy resources are expended the biosphere would most likely enter a prolonged stasis and ultimately face extinction.
The mostly metasedimentary Svecofennian Western Pohjanmaa belt in Ostrobothnia, Finland, can be divided into two stratigraphic groups separated by a major unconformity that reflects deformation following regional metamorphism. The western Lappfors group, interpreted as a Svionian basement complex, has strong W-trending folding and aeromagnetic signatures that contrast with the overlying eastern Evijärvi group, interpreted as lower Bothnian, which has more open N-trending folding and magnetic patterns. Several lines of evidence date the unconformity at ∼1.92 Ga. Detrital zircons from two samples of Lappfors group metasediment, and a sample of the basal Nivala gneisses in the Eastern Pohjanmaa belt, have 1.92–1.91 Ga post-depositional low-Th/U metamorphic overgrowths. The maximum deposition age of the Lappfors sedimentary protoliths, based on detrital zircon ages, is between ∼1.99 and ∼1.95 Ga. Three samples of Bothnian sediments lack pervasive ∼1.91 Ga overgrowths, instead having a variety of detrital zircons as young as ∼1.95–1.91 Ga, reflecting recycling of the underlying basement complex. The maximum deposition age of the Bothnian sedimentary protoliths is inferred to be ∼1.91 Ga. The Niska granitoid, which intrudes the Evijärvi group and is deformed only by the younger tectonic episode affecting that sequence, has a zircon age of 1896 ± 6 Ma. That episode, which established the present relationships between basement and cover, is dated by ∼1.88 Ga metamorphic zircon overgrowths in both the Svionian and Bothnian samples, and by 1878 ± 4 Ma metamorphic monazite from a metasediment from the Savo belt, east of the Nivala district. The post-1.91 Ga volcanic sequences of the Svecofennian Province are unlikely to represent arc accretion. The Svionian metamorphic sequences are probably the remnants of a widespread marginal basin that formed between ∼1.97 and 1.92 Ga, then was accreted to the craton during an Early Svecofennian (∼1.92–1.91 Ga) orogenic phase, forming the basement on which the Bothnian volcano-sedimentary sequences were subsequently deposited.
The style of tectonic processes on the young Earth is a topic of intense debate. Palaeomagnetism provides the only robust tests with which to settle theoretical disputes based on the rock record that infer large scale horizontal motions between palaeocontinents. A possible apparent polar wander path (APWP) of the Kaapvaal Craton shows significant distances between adjacent palaeomagnetic poles that can be interpreted as evidence for such horizontal displacements during the Archaean-Palaeoproterozoic. There are, however, substantial uncertainties build into this APWP. Currently, the published APWP for the Kaapvaal Craton comprises eight palaeopoles between 3.0 and 1.9 Ga, including several between 2.8 and 2.7 Ma that indicate possible horizontal motions exceeding 6500 km. The aim of this contribution is to increase the pole density within this interval and in doing so to test the robustness of the Kaapvaal APWP within this time frame. More than 250 samples of basalt, diamictite and dolerite of the Neoarchaean Pongola (ca. 2.95–2.85 Ga) and Ventersdorp (ca. 2.71–2.70 Ga) Supergroups of the Kaapvaal Craton were sampled both at the surface and in an underground mine. Thermal demagnetisation revealed two magnetic components. A high temperature component is present in three Pongola formations and in two Ventersdorp formations; these may be recorders of the primary natural remanent magnetisation. Because of lack of precise geochronology and positive field tests, this magnetisation can only be constrained to be older than ca. 2.05 Ga. A medium temperature overprint in many samples probably records remagnetisation at ca. 180 Ma, during the emplacement of basalts and dolerites of the Mesozoic Karoo Large Igneous Province. This remagnetisation appears to have affected a vast area of the Kaapvaal Craton. Our new palaeopoles, together with those published previously, demonstrate that the Neoarchaean APWP of the Kaapvaal Craton remains poorly defined. Direct comparisons between the APWP of the Kaapvaal Craton with APWPs of other Archaean cratons to determine relative motions between Archaean continents, therefore, need more reliable data.
New ion microprobe U–Pb zircon ages, as well as some geochemical and isotopic analyses, for key igneous units within the central part of the West Congo belt are integrated with geological information to provide an updated geological map (1:1 000 000 scale) and a synthetic type cross-section of the belt, as well as an updated lithostratigraphic chart of the ‘West Congo Supergroup’. Three Neoproterozoic units are recognised, from oldest to youngest, the Zadinian, Mayumbian and West Congolian ‘Groups’. Emplacement of early Zadinian peralkaline granites (Noqui massif, 999±7 Ma) and rhyolites (Palabala) was accompanied by incipient rift sedimentation, corresponding to the onset of transtensional rifting, preferentially in a transverse mega-shear setting along the margin of the Congo craton. Subsequent upper Zadinian magmatism produced a thick (1600–2400 m) basaltic sequence (Gangila), which has geochemical characteristics typical of continental flood basalts (CFBs). The Gangila basalts, associated with major pull-apart rifting, were followed rapidly by the 3000–4000 m thick Mayumbian rhyolitic lavas, dated at 920±8 Ma at the base and 912±7 Ma at the top. The felsic lavas are intruded by coeval high-level (micro)granites, whose emplacement is dated at 924±25 Ma (Mativa body) and at 917±14 Ma (Bata Kimenga body) in the Lufu massif. This voluminous bimodal magmatic province is similar to the Paraná and Deccan provinces, and shares similar lithospheric sources. It corresponds to the initial, transtensional rifting stage along the western edge of the Congo craton before Rodinia breakup. The early Neoproterozoic rocks of the West Congo Supergroup rest unconformably on a ca. 2.1 Ga Palaeoproterozoic polycyclic basement (Kimezian Supergroup). No Mesoproterozoic events are recorded in the area. Following the initial, transtensional early Neoproterozoic (ca. 1000–910 Ma) rifting stage, Bas-Congo behaved as a passive margin of the Congo craton, as indicated by deposition of ca. 4000 m of Neoproterozoic (pre-Pan-African) platform sediments (lower part of West Congolian Group) preceding ca. 2000 m of Pan-African molasse-type sediments (upper part of West Congolian Group). In the late Neoproterozoic, during Pan-African assembly of Gondwanaland, the Bas-Congo passive margin, which was largely protected by thick lithosphere of the Congo craton, collided with a western active margin to form the Brasiliano-Araçuaı́ belt, now preserved adjacent to the São Francisco craton of Brazil. This collision, which ended in Bas-Congo at ca. 566 Ma, induced relatively limited effects in the West Congo belt, which experienced no late Neoproterozoic magmatic activity.
Possible large-scale geological correlation between Baltica (East European Craton) and Amazonia (central South America) is discussed using compilations of recent data and regional geological models. A fit with northwest Amazonia attached to southwest Baltica – the SAMBA (South America–Baltica) connection – produces a closely matching pattern of westward younging Proterozoic growth zones and is suggested to have existed from at least 1.8 Ga to at least 0.8 Ga. West Africa was probably attached to northeast Amazonia and southeast Baltica during the same time period. As in most plate tectonic reconstructions, northwest Baltica was attached to eastern Laurentia (Greenland) from 1.9 Ga to at least 1.3 Ga, and western Amazonia thus formed a continuation of the southeast Laurentia–western Baltica active margin, being characterized by intermittent crustal growth and orogenic deformation during the late Palaeoproterozoic and Mesoproterozoic, as well as periods with more inboard intraplate magmatism. In order to reach the “standard” Rodinia configuration, Baltica must have separated from Laurentia and, together with Amazonia and West Africa, rotated c. 75° clockwise relative to Laurentia. During the 1.1–0.9 Ga Grenvillian – Sveconorwegian – Sunsas orogeny, the western margin of Baltica and Amazonia collided with the southeast margin of Laurentia, as part of the formation of the Neoproterozoic supercontinent Rodinia. Final break-up of Rodinia occurred around 0.6 Ga, although separation of Amazonia and West Africa from Baltica may have occurred earlier. With ocean opening along the southwest (Tornquist) and northwest (Iapetus) margins of Baltica, and the formation of the Timanian orogen along the leading northeastern edge at 0.6–0.55 Ga, Baltica moved to the (present-day) northeast, into a 200 million year period of solitude, the only period during its history when Baltica existed as a continent of its own, before reuniting with Laurentia during the Caledonian orogeny and then merging with the rest of Eurasia.
Palaeoproterozoic chert in the Bartle Member of the Killara Formation, Yerrida Group, Yerrida Basin of Western Australia, contains an anomalous association of crystal structures and rock fabrics that are difficult to interpret. The association appears to indicate an evaporitic–pyroclastic–thermal-spring environment associated with rifting at about 2.2 Ga. The chert member contains silica pseudomorphs after evaporite minerals that in places enclose relict isolated crystals and aggregates of crystals of gypsum and anhydrite. The evaporite minerals are associated with minerals such as barite and analcime. The association of these minerals, together with palaeoenvironmental evidence, invites comparison with rocks of the Afar region and Lake Magadi in the East African Rift System. The Bartle Member locally contains anomalous gold (up to twelve times the average crustal values of 4 ppb) and barium. It also contains finely disseminated kerogen together with numerous spheroids (classified here as microdubiofossils) and some curious structures showing organisation into complex petal-like structures (classified here as putative microfossils). The Bartle Member chert has many characteristics of playa lake and thermal-spring deposits, and may host epithermal precious-metal mineralisation.
This article reports the discovery of numerous subaerial exposure surfaces which occur in the ca. 2200 Ma Kuetsjärvi Sedimentary Formation (KSF) from the Pechenga Greenstone Belt, NE Fennoscandian Shield, Russia. The formation was accumulated within an intraplate rift and is composed of fluviatile-deltaic siliciclastic deposits and lacustrine dolostones. The dolostones are enriched in ( to +7.6‰ V-PDB, to 21.5‰ V-SMOW), which is, in general, assigned to a global perturbation of the carbon cycle. The lacustrine dolostone unit contains desiccated and in situ brecciated carbonate beds, subaerial erosion and dissolution surfaces, epikarst, surficial silicification (silcretes) and hot-water spring travertines. The dissolution surfaces—marked by formation of a few centimetre thick, ‘vuggy’, non-laminated, dolomicrite capped by silicified nodular zone—resemble modern calcretes which formed by capillary rise of surface waters (per-ascensum calcrete) following saturation of the regolith zone under arid or semi-arid conditions. Abundant red, iron-stained, superficial dolocrete implies the existence of an O2-rich atmosphere. Rezolith, ‘black pebbles’, alveolar septal structures, ‘Microcodium’ and other fabrics associated with plant roots and input of terrestrial organic matter, which are typical for post-Cretaceous pedogenic calcretes, have not been identified. Alfa fabrics can be applied to describe dolocretes though rhombic calcite crystals are lacking. Neither 13C-depletion nor 13C-enrichment has been documented in the dolocrete, which suggests no contribution of CO2 derived from soil or from any other Corg-rich sources enriched in 12C, and apparent lack of biological uptake of 12C during formation of the dolocrete. Both petrographic and carbon isotope data do not support earlier assertions that Palaeoproterozoic subaerial surface was colonised by microbial mats. A few samples have high values (up to 30‰) despite diagenetic and metamorphic resetting of the oxygen isotope system, suggesting evaporation during formation of the superficial dolocrete. Such documented episodes of subaerial exposure, formation of hot-water travertines, superficial dolocrete and silcrete suggest that there must have been frequent decoupling between the Kuetsjärvi depositional system and the bordering sea. This imposes an additional restriction on the use of the isotopic composition of the KSF dolostones for reconstruction of the global secular curve.
The Mesoproterozoic Telemark supracrustals in southern Norway comprise two major assemblages of bimodal volcanic and clastic metasedimentary rocks. The older Vestfjorddalen supergroup evolved from A-type, ca. 1500 Ma continental felsic volcanism, via within-plate type basaltic volcanism, into open sea siliciclastic sedimentation, and produced an at least 5 km thick, quartzite-dominated sequence, the Vindeggen group. It overlies a basement formed by just slightly older, 1550–1500 Ma mature arc rocks. The younger, 1170–1140 Ma Sveconorwegian supergroup was characterized by bimodal volcanism, associated with plutonism, and with several intervening periods of clastic sedimentation. The metadiabase dated in this study cuts the Vindeggen group at the top of the older supergroup and is itself delimited by an unconformity at the bottom of the younger supergroup. The 1347 ± 4 Ma age, obtained by ID-TIMS analysis of zircon, defines a mimimum age for deposition of the Vindeggen group. The age is unique in the regional context but in general terms it fits a pattern of episodic and locally intense magmatism that characterized the Mesoproterozoic development of the margins of Proto-Baltica and -Laurentia and has been related to the evolution of a long-lived convergent margin. The similarities between some of these terranes and distinctiveness from others, in both orogens, may indicate outboard evolution of the Telemarkia and Frontenac terranes before their aggregation within the Sveconorwegian–Grenvillian orogen.
Precambrian rocks are widely distributed in China. The Precambrian is divided into two time units, i.e., the Archaean and Proterozoic Eon, each of these is separated into three chronological intervals, also with the status of eras, with the prefixes early, middle or late. The time boundary between the Archaean and Proterozoic Eon is placed at ∼ 2500 Ma.According to the present isotopic data, the proposed subdivision for the Archaean of China is two-fold. The age of the Fuping Group is younger than 2800–2900 Ma, and that of the Qianxi Group and the corresponding stratigraphic units of eastern Liaoning are older than 2800 Ma, so that 2800+ Ma is selected as the boundary between the early—middle and late Archaean.Based on the representative stratigraphic units, the Wutai and Huto Groups, and an intervening major unconformity formed by the Wutaiian orogeny at 2200–2300 Ma, the early Proterozoic is further divided into two periods, with a time demarcation at 2200+ Ma. A major episode of orogeny known as the “Luliangian Movement” occurred at the end of the early Proterozoic at ∼ 1900 Ma. This disturbance was very extensive and is, in a way, responsible for the difference in geological conditions between the lower and middle—upper Proterozoic in China. The boundary (1900 Ma) that relates to the Luliangian Movement is more important than the boundary corresponding to the age of 1600 Ma, which is recommended as the time boundary between Proterozoic I and II, so we propose to use 1900 Ma as the boundary between the early and middle Proterozoic in China.The time boundary between the middle Proterozoic, including the Changcheng System and the Jixian System, and the late Proterozoic, which is composed of the Qingbaikou and Sinian Systems, is ∼ 1000 Ma. The age for the boundary between Cambrian and Precambrian, based upon the recent isochron data, is inferred to be 610 Ma.
Carbon and oxygen isotope measurements of 66 samples from the 60 m-thick variegated marble in the Upper Allochthon of the Norwegian Caledonides have a mean δ13Ccarb of −8.4 ± 0.9‰ (V-PDB), and a mean δ18O of 20.2 ± 2.2‰ (V-SMOW). The variegated marble is overlain by 150 m-thick pale grey marble characterised by mean δ13Ccarb of −6.5 ± 0.8‰ (n = 25) and underlain by dark grey marbles with a mean δ13Ccarb of +4.8 ± 1.1‰ (n = 61). This tripartite unit of an poorly constrained age—but between Neoproterozoic and Early Silurian—discontinuously developed over a distance of 500 km, is likely to represent one of the largest isotopically anomalous sedimentary carbonate formations yet reported. The marbles depleted in 13C beyond the canonical mantle value of −6‰ show no obvious evidence of post-sedimentary repartitioning of carbon isotopes.
Carbon and oxygen isotopic investigations have been carried out on the Archean and Paleoproterozoic carbonate rocks of the Udaipur region in the Aravalli Mountain Belt, northwestern India. The study has led to the interesting finding of 13C enrichment in the carbonate rocks of the Jhamarkotra Formation (δ13Ccarb up to 11.1‰ V-PDB) belonging to the ∼2200–1900 Ma Paleoproterozoic Aravalli Supergroup. Further, it is observed that the organic carbon from phosphorite bearing stromatolitic dolomites of the same formation are also enriched in 13C (δ13Corg up to −11.1‰). However, it must be emphasized that the 13C enriched carbonates and the organic fractions are from different stratigraphic levels although in the same formation, the former being at a lower stratigraphic level. A critical analysis of the field geological, petrological as well as the isotopic data indicates that the observed δ13C excursion in the carbonate carbon is not due to local causes such as methanogenesis or evaporitic conditions. The shallow water environment of deposition of these carbonates as well as the immediately overlying formations rules out the stratified ocean model for the observed excursion. The model involving high sedimentation rates and organic carbon burial has been preferred to explain the δ13C excursion in the carbonates of the Jhamarkotra Formation. The 13C enrichment in organic fraction occurred in localized regions of the Aravalli sedimentary basin, where high productivity was supported by phosphorous supply indicating that the diffusion-limited pathways of organic carbon fixation were operative in the Paleoproterozoic itself. The 13C enriched carbonate rocks of the Jhamarkotra Formation serve as the Indian example for the Paleoproterozoic global δ13C excursion.
In the time period since 1961, n-alkanes (straight-chain hydrocarbons) have been detected in varying amounts in many meteorites. Proposed origins for these compounds have included extraterrestrial biotic, extraterrestrial abiotic and terrestrial contamination processes. To help establish the source of these compounds, we determined the carbon isotopic compositions of individual n-alkanes in meteorites of several different classes and terrestrial histories: Orgueil (CI1), Cold Bokkeveld (CM2), Murchison (CM2), Vigarano (CV3), Allende (CV3), Ornans (CO3), and Bishunpur (LL3). The efficiency of supercritical fluid extraction (SFE) was exploited to provide extractable non-polar organic matter from the meteorites. The n-alkanes in these extracts were then identified with gas chromatography–mass spectrometry (GC–MS) and the carbon isotopic composition of individual molecules was determined using isotope ratio monitoring–gas chromatography–mass spectrometry (irm–GC–MS). n-Alkanes were found in all but one of the meteorites analysed (Allende), but carbon number distributions varied between samples. Pristane and Phytane were also detected in five of the seven meteorites. δ13C values for the individual n-alkanes occupied a range from −25.3 to −38.7‰. The δ13C values for the meteoritic n-alkanes have a similar range as those for n-alkanes measured from petroleum in the literature. Therefore, the n-alkanes in meteorites appear to be terrestrial contaminants which may have originated from fossil hydrocarbons or petroleum products. This type of contamination is analogous to that which will be threatening future meteorite falls and the samples returned from spaceflight missions over the next two decades and beyond. For falls already contaminated, irm–GC–MS appears useful in discriminating between indigenous compounds and those introduced by terrestrial contamination.