Precambrian Research最新文献

The Neoproterozoic is marked by two long-lasting global Snowball Earth glaciations—the Sturtian (ca. 715–660 Ma) and the Marinoan (ca. 639–635 Ma)—as well as the shorter-lived, potentially regional Gaskiers glaciation at ca. 580–579 Ma. The subsequent Ediacaran and early Cambrian periods are generally interpreted to be characterized by a warm climate without low-latitude, sea-level glaciations. However, a number of locally, and, sometimes regionally developed sedimentary units worldwide have been interpreted to record sea-level glaciations during this time interval. Some of these units crop-out poorly, were affected by deformation and metamorphism, lack definitive sedimentologic textures and structures, and are poorly dated. Their sedimentological characteristics, as well as age constraints, are thus often insufficient to determine whether they have a glacial origin and if their ages fit into well-defined time intervals marked by cold climate indicators. Detrital zircon geochronology may help distinguish between the heterogeneous and extensive provenances that are indicative of glaciation, and more homogenous, and local provenances that are more typical of alluvial settings. In this paper, we use trends in detrital zircon age distribution patterns from the lowermost late Ediacaran sedimentary succession of the Moldova-Podillya basin in Baltica to constrain the provenance of the Volyn Group sediments. Here, the origin of diamictites within the Hrushka Formation has long been a topic of debate, centered on either a glacial or alluvial origin. Detrital zircons from two sandstone samples and one conglomerate sample from the Bakhtyn Beds of the Hrushka Formation, and two sandstone samples from the Lomoziv Beds of the unconformably overlying Mohyliv Formation were dated using U-Pb zircon LA-ICP-MS techniques. Their age patterns are compared with published data for the overlying Cosăuți (Yampil) and Bernashivka beds as well as new dates for the immediately underlying Paleoproterozoic granites to test for a proposed glacial origin for the Bakhtyn Beds. The areal distribution of the Volyn Group sediments, inferred from drill-core data, is used to constrain the evolution of the sedimentary basin. The detrital zircon results indicate that terrigenous material in the Moldova-Podillya sedimentary basin was sourced from the Precambrian basement during deposition of the Bakhtyn to Cosăuți beds. This is consistent with an alluvial rather than a glacial origin. We infer that detrital zircon geochronology provides a largely untapped potential for testing a glacial origin for units that are otherwise poorly genetically characterized and dated.

The Southern Marginal Zone (SMZ) comprises, in part, Archean granulite facies metapelitic rocks deposited at ∼ 2733 Ma and metamorphosed to granulite facies at ∼ 2713 Ma of the Limpopo Belt in direct contact with the Kaapvaal craton. The SMZ also contains amphibolite facies metapelites within a fragment that is in direct contact with the thrust-faulted tectonic boundary with the Kaapvaal craton. These metapelites are proposed to represent former granulites that were comprehensively rehydrated under amphibolite facies. However, no previous study has directly investigated the conditions of metamorphism or timing of the proposed higher-grade events in these retrograde rocks. Neither has the timing of retrogression in the retrogressed metapelites been well constrained. Here, detailed petrographic analysis, mineral composition and in situ U-Pb dating of zircon, monazite and rutile are presented for the amphibolite facies metapelites. These rocks are characterized by assemblages consisting of garnet, orthoamphibole, biotite, quartz, plagioclase, rutile, kyanite and graphite. Although there is pervasive retrogression to orthoamphiboles (anthophyllite to gedrite series), rare relics of orthopyroxene are preserved. The compositions of the first garnet (Grt 1) generation suggests granulite facies peak metamorphic conditions of 860 ± 10 °C and 11 ± 0.4 kbar. Metamorphic zircon grains and overgrowth rims and monazite, included in Grt 1, indicate that peak metamorphic conditions occurred between 2714 ± 7 and 2713 ± 4 Ma. Dating of rutile inclusions in Grt 1 from granulite yield a Concordia age of 2677 ± 6 Ma. This indicates post-granulite facies cooling and shows that later retrograde metamorphism did not reach temperatures high enough to reset rutile inclusions in Grt 1 garnet. Orthoamphibole and second garnet with kyanite inclusions (Grt 2) compositions suggest that retrogression occurred under amphibolite facies conditions of 600 ± 10 °C and 6.0 ± 0.5 kbar. Rutile included in Grt 2 and as discrete grains equilibrated with the retrograde textures record a range of Paleoproterozoic ages (2212 ± 19, 2148 ± 12 and 2012 ± 10 Ma). Thus, the metamorphic history of the amphibolite facies metapelites of the SMZ is hotter than thought and similar to their Archean granulite facies counterparts, except for younger Paleoproterozoic ages, likely indicating a pervasive retrogression to amphibolite facies conditions.

In this contribution, we present new petrochronological results from samples collected the Shabogamo Domain, of the Grenville Province, Québec, Canada. The Shabogamo domain has a distinct geophysical signature and has been proposed to be linked with the well-studied allochthonous Manicouagan Imbricate Zone. We document a Pressure-Temperature-time (P-T-t) path for an amphibolite sample by combining several approaches including: i) an ordinary least square model able to estimate P-T conditions from amphibole composition using a previously semi-quantitative amphibole thermobarometer ii) titanite petrochronology; iii) phase equilibria modelling; iv) trace element thermometry. We show that single element thermobarometry on titanite and amphibole pairs yield the information necessary to reconstruct a decompression P-T path from peak P-T conditions of 1.8 GPa – 775 °C down to 1.2 GPa – 750 °C and are consistent with results of phase equilibria modelling. Titanite U-Pb geochronology results indicate growth between 987 ± 16 Ma and 969 ± 17 Ma. These results are consistent with U-Pb geochronology on zircon from a dyke that crosscuts the amphibolite and from a deformed leucosome sampled in the domain’s footwall. Zircon from the crosscutting dyke appear to have grown at 985 ± 18 Ma without interaction with garnet, whereas zircon from the deformed leucosomes grew at 972 ± 16 Ma during high-pressure metamorphism. Integrating the results of this study into the regional context outlines the exhumation of a second high-P allochthonous nappe, most likely by ductile extrusion, coeval with the burial of the parautochthonous footwall. This study further supports the growing body of evidence for a major tectonic event during the later phase of the Grenvillian Orogeny.