Detrital zircon U-Pb and Hf constraints on provenance and timing of deposition of the Mesoproterozoic to Cambrian sedimentary cover of the East European Craton, part II: Ukraine
Mariusz PaszkowskiBartosz BudzyńStanisław MazurJiří SlámaJan ŚrodońIan MillarLeonid ShumlyanskyyArtur KędziorSirle Liivamägi
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We present the U-Pb geochronology and Hf isotope analysis of detrital zircons from the Ediacaran/Cambrian sediments of Podillya and south Volyn in western Ukraine, supplemented by the bulk rock XRD mineralogy of the host rocks. Such a combined analytical approach allows for identifying the source areas supplying detritus to sediments and for constraining an age of deposition. Our provenance analysis is based on fourteen samples collected from six exposures, mostly in the valley of the Dniester river. 84 mudstone samples were also examined by the XRD method. U-Pb dating of detrital zircons yielded two sets of maximum depositional ages: 578–546 Ma and 547–523 Ma, for the Mohyliv-Podilsky and Kanyliv Series, respectively. This suggests that the Ediacaran-Cambrian boundary in Podillya coincides with a major erosional gap, with a major change in provenance, and the disappearance of the Ediacaran fauna at the base of the Kanyliv Series, with implications for the stratigraphy and paleogeography of the entire East European Platform. Zircon U-Pb age spectra from the lower part of the Mohyliv-Podilsky Series include a large quantity of 2.2 to 1.9 Ga grains that reveal predominantly negative to nearly chondritic ɛHf values, jointly suggesting detritus supply from the crystalline basement of Sarmatia. Both U-Pb and mineralogical data also indicate a major contribution of volcanic detritus from the Volyn flood basalts. The younger Nagoryany rocks yielded zircon age spectra with peaks at c. 1.80 and 1.49 Ga, implying a shift of the catchment area to Fennoscandia. Above an erosional gap, the zircon age spectra in the Kanyliv and Baltic Series are dominated by peaks at 560–535 Ma. These data and ɛHf values ranging from negative to chondritic and juvenile suggest, in line with the mineralogical data, detritus supply from a continental magmatic arc and collisional orogen. Thus, we interpret the Kanyliv Series as infill of an early Cambrian foreland basin that was established in front of the Scythides and Santacrucides orogens, overriding the SW margin of Baltica.Keywords:
Detritus
Geochronology
Basement
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Constraints on models of the evolution of the Zimbabwe Archaean Craton are derived primarily from geological mapping of the exposed and well-preserved supracrustal successions in the granite-greenstone terrain of the south-central part of the Craton (Fig. 1; Wilson et al., 1978). The gross geology of the area also has important implications for the development of the Archaean trust in general. We analyse gravity and aeromagnetic data from the region to study the granite-greenstone relationship and for crustal structure and tectonic interpretation.
Greenstone belt
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The original connections of Archean cratons are becoming traceable due to an increasing amount of paleomagnetic data and refined magmatic barcodes. The Uauá block of the northern São Francisco craton may represent a fragment of a major Archean craton. Here, we report new paleomagnetic data from the 2.62 Ga Uauá tholeiitic mafic dyke swarm of the Uauá block in the northern São Francisco craton, Eastern Brazil. Our paleomagnetic results confirm the earlier results for these units, but our interpretation differs. We suggest that the obtained characteristic remanent magnetization for the 2.62 Ga swarm is of primary origin, supported by a provisionally-positive baked contact test. The corresponding paleomagnetic pole (25.2°N, 330.5°E, A95 = 8.1°, N = 20) takes the present northern part of the São Francisco craton to moderate latitudes. Based on the comparison of the paleolatitudes of cratons with high-quality paleomagnetic data and magmatic barcodes, we suggest that the northern part of the São Francisco craton could have been part of the proposed Supervaalbara supercraton during the Archean. Supervaalbara is proposed as including (but not limited to) the part of the São Francisco craton as well as the Superior, Wyoming, Kola + Karelia, Zimbabwe, Kaapvaal, Tanzania, Yilgarn, and Pilbara cratons.
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Southern Superior Province of the Canadian Shield contains cratonic and basinal elements arranged in high- to low-grade metamorphic terranes such that higher grade gneissic cratons are interpreted to represent primary infrastructure to lower grade volcanic-rich (greenstone) basins. Ensimatic accumulation of volcanic components is favoured with derivation of gneissic (granitic) components by ensuing metamorphic differentiation and granitization processes. Such vertically reconstructed basin–craton complexes which are tentatively ascribed to initial Archean mantle convection, are viewed as building units of growing Precambrian shields.Globally, twenty-seven identified Archean cratons belong to three main age groups based on maximum recorded ages as follows: 3.5–3.8 Ga, 2.9–3.1 Ga, and 2.6–2.7 Ga. The three age groups that correspond to major periods of Archean orogeny may represent accretion superevents (after Moorbath).Most cratons as presently exposed display lithologic characteristics of lower superstructure–upper infrastructure of typical basin–craton complexes thereby suggesting a common degree of crustal buoyancy, hence level of erosion. Archean belts of southern Superior Province provide unique opportunity for reconstruction of the typical basin–craton complex.
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Basement
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Seismic anisotropy from the southern African mantle has been inferred from shear‐wave splitting measured at 79 sites of the Southern African Seismic Experiment. These data provide the most dramatic support to date that Archean mantle deformation is preserved as fossil mantle anisotropy. Fast polarization directions systematically follow the trend of Archean structures and splitting delay times exhibit geologic control. The most anisotropic regions are Late‐Archean in age (Zimbabwe craton, Limpopo belt, western Kaapvaal craton), with delay times reduced dramatically in off‐craton regions to the southwest and Early‐Archean regions to the southeast. While thin lithosphere can account for weak off‐craton splitting, small or vertically incoherent anisotropy is a more likely explanation for the Early‐Archean region. We speculate that this difference in on‐craton anisotropic structure is the result of two different continent‐forming processes operating.
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Shear wave splitting
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