Constraints on the timing of Marinoan “Snowball Earth” glaciation by 187Re–187Os dating of a Neoproterozoic, post-glacial black shale in Western Canada (original) (raw)
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Chemical Geology, 1993
Granitic plutons in the Meguma Terrane may be broadly divided into two suites: (1) an areally predominant, cogenetic suite of peraluminous granitoid rocks (tonalite-leucogranite); and (2) an areally subordinate, paraintrusive suite specialized in Sn-W-Mo-Cu-F-UP -Zn-As which appears to have been produced by fluid interaction with either residual magma or crystallized rock. Granite-related mineralization is generally considered to have developed during terminal stages of magmatic crystallization that lead to metasomatism and hydrothermal alteration. Previous geochronological data have suggested that the main, cogenetic suite was emplaced at ~ 350-375 Ma whereas some of the paraintrusive suite is markedly younger (~ 250-350 Ma). The apparently large time interval between the intrusion of the cogenetic suite and some of the mineralization suggests that they are not coeval, and has led to the conclusions that the younger mineralization may be related to later magmatism and/or tectonothermal events. Rocks associated with two mineral occurrences in the South Mountain Batholith that had previously yielded anomalously young isotopic ages (Millet Brook and Westfield) have been analyzed for U, Pb and Ar isotopes in an attempt to resolve this dilemma. New geochronological data from the Millet Brook area indicate that intrusion of granodiorite at 377+_~ Ma (nearly concordant monazite age) was closely followed by intrusion of most other phases all of which had cooled through ~ 375 ° C (closure temperature for Ar in muscovite) by ~ (377-373) + 6 Ma (4°Ar/3gAr plateau ages). New data from Westfield indicate that a paraintrusive porphyry was intruded at ~ 385 +_ 2 Ma (upper intercept of a chord through four, nearly concordant zircon fractions) and that greisen muscovite cooled through the closure temperature by 368.2 + 6 Ma (4°Ar/ 39Ar plateau age). These data indicate that a previously reported Rb-Sr whole-rock isochron age of ~ 270 Ma at Westfield reflect isotopic resetting that occurred below the blocking temperature for 4°Ar/39Ar in muscovite (~ 375 °C). Clearly, further Rb-Sr analysis is required to document this process. Other ~ 250-300 Ma ages in the Meguma Terrane are recorded along the Tobeatic Fault zone. Away from the fault, ages increase upto ~ 370 Ma, suggesting that the fault may have acted as an important pathway for fluids and heat that facilitated isotopic resetting. Rb-Sr whole-rock and mineral systems may be affected after formation of an alteration assemblage associated with mineralization by the migration of Rb and its concentration in secondary minerals.
Canadian Journal of Earth Sciences, 2011
Radiocarbon dates, rhythmite occurrences, mineralogy, grain size, and stable isotopic compositions of detrital calcite obtained for four sediment cores from Lake Superior have been used to produce a chronology for these fine-grained deposits over the last ∼10 500 cal BP (calibrated years before A.D. 1950). Most of the rhythmites have been interpreted as annual laminations (varves) based on systematic variations in the carbonate content of rhythmite couplets and correlation of the rhythmite packages with varve sequences reported previously for Lake Superior sediments. Glacial sediments in these cores, which consist of red and grey clay, range in age from ∼10 500 to 8800-8700 cal BP. Distinctive carbon and oxygen isotopic compositions of allochthonous calcite in the red versus grey glacial clays point to different sources for this detritus. Final termination of glacial meltwater input into the Lake Superior Basin occurred between 9000 and 8700 cal BP. The significant decrease in sediment carbonate content over this time is a convenient marker for the start of postglacial sedimentation across the Lake Superior Basin.
Earth and Planetary Science Letters, 2009
Rhenium-osmium (Re-Os) dating of carbonaceous rocks using H 2 SO 4 -CrO 3 dissolution provides a powerful new geochronologic tool for Archean terranes. Here we document the oldest Re-Os depositional ages yet reported for carbonaceous sedimentary rocks. Carbonaceous slates are intercalated with graywackes and mafic tuffs of the Joy Lake sequence in the western Wawa subprovince of the Superior Province. Welldeveloped D 2 cleavage is sub-parallel to bedding. Pyrite occurs in nodules, disseminations, and beddingparallel stringers. Samples from two drill holes (DH26503 and DH26506) yield isochron ages of 2695 ± 14 Ma and 2684 ± 16 Ma and initial 187 Os/ 188 Os ratios (Os i ) of 0.15 ± 0.16 and − 0.29 ± 0.20, respectively. The ages, identical within uncertainty, provide the first temporal constraints for the westernmost Wawa subprovince greenstone sequence. The ages are consistent with cessation of mafic/ultramafic volcanism before ca. 2695 Ma in the western Wawa subprovince and D 2 deformation before 2685 Ma farther east. The Joy Lake sequence is likely the temporal equivalent of turbiditic graywackes deposited further east, both sourced from arc volcanoes during collision of the Wawa and Wabigoon arcs along the southern margin of the Superior Province. A chondritic Os i (0.109 at 2.69 Ga) is expected for Archean shales, but the negative Os i for DH26506 requires explanation. Re, which is significantly more mobile than Os, may have been released from intercalated high-Re/Os mafic tuffs by local oxidation during recent or Cretaceous exposure to meteoric waters. The adjacent carbonaceous rocks are a ready sink for mobile Re. A small Re addition can shift isochron data points to higher 187 Re/ 188 Os ratios with minimal slope change. This increases scatter and uncertainty for the Os i , shifts the isochron to the right, and produces a spuriously low Os i intercept. If Re addition is recent, the age is not significantly affected. Notably, the slates in DH26506 which yield the negative Os i are intercalated with relatively permeable and reactive lithic wackes and mafic tuffs, whereas the slates in DH26503 are in contact with less permeable massive intrusive rocks. This process can be detected only if the true Os i is approximately chondritic, and is more likely where reactive mafic rocks are prevalent-both the case for Archean greenstone belts.
Quaternary Science Reviews, 2020
The North American Ice Sheet Complex (NAISC; consisting of the Laurentide, Cordilleran and Innuitian ice sheets) was the largest ice mass to repeatedly grow and decay in the Northern Hemisphere during the Quaternary. Understanding its pattern of retreat following the Last Glacial Maximum is critical for studying many facets of the Late Quaternary, including ice sheet behaviour, the evolution of Holocene landscapes, sea level, atmospheric circulation, and the peopling of the Americas. Currently, the most upto-date and authoritative margin chronology for the entire ice sheet complex is featured in two publications (Geological Survey of Canada Open File 1574 [Dyke et al., 2003]; 'Quaternary Glaciations e Extent and Chronology, Part II' [Dyke, 2004]). These often-cited datasets track ice margin recession in 36 time slices spanning 18 ka to 1 ka (all ages in uncalibrated radiocarbon years) using a combination of geomorphology, stratigraphy and radiocarbon dating. However, by virtue of being over 15 years old, the ice margin chronology requires updating to reflect new work and important revisions. This paper updates the aforementioned 36 ice margin maps to reflect new data from regional studies. We also update the original radiocarbon dataset from the 2003/2004 papers with 1541 new ages to reflect work up to and including 2018. A major revision is made to the 18 ka ice margin, where Banks and Eglinton islands (once considered to be glacial refugia) are now shown to be fully glaciated. Our updated 18 ka ice sheet increased in areal extent from 17.81 to 18.37 million km 2 , which is an increase of 3.1% in spatial coverage of the NAISC at that time. Elsewhere, we also summarize, region-by-region, significant changes to the deglaciation sequence. This paper integrates new information provided by regional experts and radiocarbon data into the deglaciation sequence while maintaining consistency with the original ice margin positions of Dyke et al. (2003) and Dyke (2004) where new information is lacking; this is a pragmatic solution to satisfy the needs of a Quaternary research community that requires up-to-date knowledge of the pattern of ice margin recession of what was once the world's largest ice mass. The 36 updated isochrones are available in PDF and shapefile format, together with a spreadsheet of the expanded radiocarbon dataset (n ¼ 5195 ages) and estimates of uncertainty for each interval.
2006
Layered gneisses in the Winnipeg River subprovince contain magmatic zircon with U-Pb ages of 3317 ± 9 and 3055 ± 4 Ma at Tannis Lake, and-3170 and 3255 ± 5 Ma at Cedar Lake, indicating widespread occurrence of Mesoarchean crust. This is in contrast to the well-documented Neoarchean age of the western Wabigoon subprovince. Further geochronology using both SHRIMP (sensitive high resolution ion microprobe) and ID-TIMS (isotope dilution thermal ionization mass spectrometry), combined with structural observations, in the Kenora area and Lake of the Woods greenstone belt show the effects of juxtaposition of these two terranes. Isoclinally folded gneiss north of the subprovince boundary zone near Kenora gives a magmatic age of 2882 ± 2 Ma with 3051 ± 6 Ma inheritance. Ages of syntectonic dykes show that asymmetric refolding of these gneisses occurred between 2717 ± 2 and about 2713 ± 1 Ma. Subsequent regional vertical flattening and horizontal extension are dated at 2708 ± 2 Ma by syntectonic tonalite sheets. These events are broadly coeval with deposition of orogenic sediments in the Warclub Group and a first stage of regional folding (age brackets of 2716-2709 Ma) in the Lake of the Woods greenstone belt to the south. A second stage of folding and regional faulting in the greenstone belt occurred about 2695 ± 4 Ma and is approximately coeval with open upright folding in the Winnipeg River subprovince. These observations are consistent with overthrusting and collapse of a Mesoarchean continental terrane by a juvenile Neoarchean arc terrane over the time span 2717-2695 Ma. Résumé : Des gneiss stratifiés dans la sous-province de Winnipeg River contiennent des zircons magmatiques avec des âges U-Pb de 3317 ± 9 et 3055 ± 4 Ma au lac Tannis et d'environ 3170 et 3255 ± 5 Ma au lac Cedar, indiquant une grande étendue de croûte mésoarchéenne. Cela contraste fortement avec les âges néoarchéens bien documentés pour la parte ouest de la sous-province de Wabigoon. D'autres essais de géochronologie utilisant une microsonde ionique à haute résolution et à haut niveau de sensibilité (SHRIMP) et par spectrométrie de masse à thermo-ionisation avec dilution isotopique (ID-TIMS), combinés à des observations de la structure, dans le secteur de Kenora et la ceinture de roches vertes de Lake of the Woods, montrent les effets de la juxtaposition de ces deux terranes. Des plis isoclinaux dans le gneiss au nord de la zone limite de la sous-province, à proximité de Kenora, donnent un âge magmatique de 2882 ± 2 Ma avec un héritage de 3051 ± 6 Ma. Les âges des dykes syntectoniques montrent que les plissements de ces gneiss ont été repris de manière asymétrique entre 2717 ± 2 Ma et vers 2713 ± 1 Ma. Un aplatissement vertical régional subséquent et une extension horizontale sont datés à 2708 ± 2 Ma par des feuillets syntectoniques de tonalite. Ces événements sont généralement contemporains de la déposition des sédiments orogéniques du Groupe de Warclub et d'un premier étage de plissement régional (groupe d'âges 2716-2709 Ma) dans la ceinture de roches vertes de Lake of the Woods, au sud. Un second étage de plissement et de failles régionales dans la ceinture de roches vertes a eu lieu vers 2695 ± 4 Ma et il est presque contemporain d'un plissement vertical ouvert dans la sous-province de Winnipeg River. Ces observations concordent avec le chevauchement et l'effondrement d'un terrane continental mésoarchéen par un terrane d'arc néoarchéen juvénile au cours de l'intervalle 2717-2695 Ma.
Well-dated paleorecords from periods prior to the Last Glacial Maximum (LGM) are important for validating models of ice sheet build-up and growth. However, owing to glacial erosion, most Late Pleistocene records lie outside of the previously glaciated region, which limits their ability to inform about the dynamics of paleo-ice sheets. Here, we evaluate new and previously published chronology data from the Missinaibi Formation, a Pleistocene-aged deposit in the Hudson Bay Lowlands (HBL), Canada, located near the geographic center of the Laurentide Ice Sheet (LIS). Available radiocarbon (AMS ¼ 44, conventional ¼ 36), amino acid (n ¼ 13), uranium-thorium (U-Th, n ¼ 14), thermoluminescence (TL, n ¼ 15) and optically stimulated luminescence (OSL, n ¼ 5) data suggest that an ice-free HBL may have been possible during parts of Marine Isotope Stage 7 (MIS 7; ca. 243,000 to ca. 190,000 yr BP), MIS 5 (ca. 130,000 to ca. 71,000 yr BP) and MIS 3 (ca. 29,000 to ca. 57,000). While MIS 7 and MIS 5 are well-documented interglacial periods, the development of peat, forest bed and fluvial deposits dating to MIS 3 (n ¼ 20 radiocarbon dates; 4 TL dates, 3 OSL dates), suggests that the LIS retreated and remained beyond, or somewhere within, the boundaries of the HBL during this interstadial. Ice sheet models approximate the margin of the LIS to Southern Ontario during this time, which is 700 km south of the HBL. Therefore, if correct, our data help constrain a significantly different configuration and dynamicity for the LIS than previously modelled. We can find no chronological basis to discount the MIS 3 age assignments. However, since most data originate from radiocarbon dates lying close to the reliable limit of this geochronometer, future work on dating the Missinaibi Formation using other geochronological methods (e.g. U-Th, OSL) is necessary in order to confirm the age estimates and strengthen the boundaries of the LIS during this period.
Earth and Planetary Science Letters, 2009
Boreal and Tethyan faunas differ throughout the Triassic, limiting the use of biostratigraphy for global correlation. Rhenium-osmium (Re-Os) dating of organic-rich black shales, calibrated with existing biostratigraphy, provides a new means to establish correlations of global fauna using absolute time. Here we present Re-Os radiometric ages for Middle Triassic organic-rich shales from two biostratigraphically defined sections at Svalbard and the Svalis Dome in the Barents Sea. Mature black shales from Svalbard, inferred to be Middle to Late Anisian, define a 241.2 ± 2.2 Ma isochron with an initial 187 Os/ 188 Os ratio of 0.831 ± 0.025, the highest seawater Os isotope ratio yet recorded between the Cambrian and Lower Jurassic. Svalis Dome shales in the uppermost Anisian yield a 239.3 ± 2.7 Ma age and initial 187 Os/ 188 Os ratio of 0.679 ± 0.020. The higher initial 187 Os/ 188 Os ratio for Svalbard shales most likely reflects global seawater at the time of deposition, whereas the lower initial 187 Os/ 188 Os ratio for the Svalis Dome may represent seawater with limited communication to the open ocean, as suggested by the ratio of trace metal abundances and total organic carbon contents. These age data indicate a maximum age of 239.3 ± 2.7 Ma for the Anisian-Ladinian boundary in the Arctic Boreal realm. This age agrees with the proposed boundary in the Tethyan realm, 240 to 243 Ma, and thus confirms the correlations between Boreal and Tethys based on biostratigraphy and magnetostratigraphy. Our study affirms that Re-Os geochronology of black shale is a reliable method for obtaining depositional ages and establishing biostratigraphic correlations between paleogeographically separated regions.