Tonian Age Research Articles

The Neoproterozoic basement of the Sauce Chico Inlier (Ventania System): Geochemistry and U–Pb geochronology of igneous rocks with African lineage in central-eastern Argentina

This study describes the geology, geochemistry, and LA-ICP-MS U–Pb geochronology of igneous rocks that crop out in the Sauce Chico Inlier (SCI) and constitute the Neoproterozoic basement of the Ventania System, Argentina. Magmatism registered in the SCI has developed in two phases. The first phase, of Tonian age, is represented by rift-related calc-alkaline and alkaline granites with ages of 783.8 ± 3.7 Ma (εNd(t) = −7.20) and 776.5 ± 4.7 Ma (εNd(t) = +1.65), respectively. This phase would be related to the break-up of the Rodinia supercontinent and subsequent opening of the Adamastor Ocean. In the Dom Feliciano Belt of southern Brazil and eastern Uruguay, the Tonian magmatism is represented in the Cerro Olivo Complex of the Punta del Este Terrane and in basement inliers of the Pelotas Batholith. The second phase, of Ediacaran age, started with the intrusion of syn-orogenic calc-alkaline granites (620.8 ± 5.8 Ma and 620.3 ± 2.5 Ma; εNd(t) = −9.94/−9.18) and continued with the extrusion of post-orogenic alkaline (577.3 ± 3.9 Ma; εNd(t) = −6.29) and calc-alkaline (543.6 ± 4.0 Ma; εNd(t) = −3.38) acid volcanic rocks. This phase would be related to the closure of the Adamastor Ocean, generation of a magmatic arc along the western margin of the Kalahari Craton, and collision between this and the Río de la Plata Craton. A post-collisional magmatism would have developed due to orogenic collapse (< 580 Ma). With the exception of the Tonian alkaline granite, in the remaining SCI igneous rocks, the juvenile component becomes more important as the crystallization age decreases. The available Nd model ages are between 1.80 and 1.14 Ga and suggest a mixing of older crust with juvenile material. The tectonic evolution of the Ventania System basement is consistent with that observed in the Dom Feliciano Belt and its African counterparts. We present here the first evidence of Tonian magmatism in the Ventania System basement. The results presented here also confirm those obtained previously by other authors. Zircon cores from the SCI igneous rocks have U–Pb inherited ages between ca. 1200 and 900 Ma that could be indicative of a lineage with the Gariep Belt and its Namaqua basement in southwestern Africa.

Organic-walled microfossils from the Tonian Tongjiazhuang Formation of the Tumen Group in western Shandong, North China Craton and their biostratigraphic significance

Neoproterozoic successions in western Shandong of North China Craton contain fine-grained siliciclastic rocks which offer a unique opportunity to explore the Neoproterozoic biosphere. In this study, we have revealed a well-preserved and diversified microfossil assemblage from the Tongjiazhuang Formation, Tumen Group in western Shandong, North China Craton, which is characterized by abundant smooth-walled sphaeromorphic acritarchs and cyanobacterium-like filamentous forms and relatively low abundance of more complex acritarchs. A total of 40 microfossil taxa belonging to 20 morphological genera have been identified, most of which are described for the first time from the Tongjiazhuang Formation of the Tumen Group in western Shandong, including Simia sp., Trachyhystrichosphaera aimika, T. botula, Germinosphaera bispinosa, Unnamed sp. A, Jacutianema solubila, and Pololeptus rugosus. The Tongjiazhuang assemblage revealed in this study is much more diverse than previously reported and well comparable to the Tonian Liulaobei and Gouhou assemblages in the Xuhuai region. Particularly, the occurrence of T. aimika, T. botula, G. bispinosa, J. solubila, and P. rugosus indicates a late Mesoproterozoic to Tonian age for the Tongjiazhuang Formation. In addition, the occurrences of macroscopic carbonaceous compressions, including Chuaria-Tawuia assemblage from the Tongjiazhuang Formation and Tawuia and Sinosabellidites from the Shiwangzhuang Formation, are consistent with geochronological data, suggesting a Tonian age for the Tongjiazhuang, Fulaishan, and Shiwangzhuang formations of the middle and upper Tumen Group, while the Heishanguan and Erqingshan formations of the lower Tumen Group are likely late Mesoproterozoic to Tonian in age. Thus, available biostratigraphic data, lithostratigraphic correlation, and geochronological data suggest that the whole Tumen Group is late Mesoproterozoic to Tonian in age, rather than Cryogenian or Ediacaran. This study significantly improves our knowledge about the diversity of the Tonian biosphere and implies a ∼200–300 million years unconformity (the ‘Great Unconformity’) between the late Mesoprotoerozoic to Tonian Tumen Group and the early Cambrian Liguan Formation in western Shandong.

Carbonate chemostratigraphy of the Vazante Group, Brazil: A probable Tonian age

The thick carbonate-siliciclastic succession of the Vazante Group was deposited on the western border of the São Francisco craton, central Brazil, which was deformed during the Brasiliano Orogeny that ended at 540 Ma. In this research, we studied five drill cores and outcrops to obtain integrated isotope data (δ13Ccarb, δ18Ocarb, and 87Sr/86Sr) as well as U-Pb ages of detrital zircons to understand the chronostratigraphic relationships between the Tonian and Cryogenian periods. The Vazante Group forms a succession of more than 3 km thick of limestones, dolostones, shales and rare sandstones. These rocks were deposited on a marine shelf that shows distinct δ13C variations throughout the stratigraphic succession. The lower limestone of this group (Lagamar Formation) has low positive δ13C values (0.3–1.0‰) and 87Sr/86Sr ratios of 0.70646–0.70692 (with 423–1,719 ppm Sr), and possible vase-shaped microfossils (VSMs) that may be equivalent to those found in Huaibei Group in northern China (lower Tonian) or has stratigraphic correlations to the VSMs-bearing units of the upper Tonian. The Serra do Poço Verde Formation has a plateau of moderate δ13C positive values (0.0–4‰) followed by dolostones of the Morro do Calcário Formation, which is a 50-m-thick interval with negative values (as low as −4‰). The upper unit of the Vazante Group (Serra da Lapa Formation) begins with a sharp negative δ13C excursion locally overlying a few layers of diamictite, which may be the equivalent of the Sturtian glacial age. Restrictions on establishing the depositional age of the Vazante Group continue; however, the combination of biostratigraphic and chemostratigraphic data and the detrital zircons spectra suggest a Tonian age for the Vazante Group.

Geochemical and isotope evidence for mantle-derived source rock of high-K calc-alkaline I-type granites, Pernambuco–Alagoas Domain, northeastern Brazil

We report whole-rock major, trace, mineral, and isotope chemistries of the Jacare dos Homens, and Santo Antonio granitic rocks from the Aguas Belas–Caninde batholith, Pernambuco–Alagoas Domain of the Borborema Province, northeastern Brazil. These rocks exhibit low-angle foliation, suggesting emplacement under a regional strain field, associated with transpressive deformation tied to the onset of the Brasiliano orogeny. Both rocks are dominantly made up of K-feldspar, quartz, and plagioclase with biotite as the main mafic phase and minor hornblende. The Jacare dos Homens orthogneiss displays magnesian, slightly peraluminous, and alkali-calcic character, while the Santo Antonio granite is magnesian, metaluminous to slightly moderately peraluminous and alkali-calcic to alkalic. These granitic rocks are enriched in alkalis (up to 10.28 wt%), mg#, Rb, Ba, Sr, Th, and LREE, and depleted in HREE and HFSE, with distinct Nb, Ta, and Ti depletion. Decreasing MgO, CaO, TiO2, Na2O, Fe2O3T, P2O5, Al2O3, Sr, Zr, and Hf with increasing SiO2 are consistent with fractional crystallization of biotite, amphibole, plagioclase, apatite, zircon, titanite, and Fe–Ti oxides. U–Pb zircon dating of the Jacare dos Homens and Santo Antonio rocks yielded concordia ages of 642.4 ± 3 Ma and 636 ± 4 Ma, respectively. The isotope data for the Jacare dos Homens orthogneiss yielded an Nd-model age of 1.21 Ga with slightly negative eNd(642 Ma) of − 1.58, initial 87Sr/86Sr ratios of 0.7068, and δ18O (zircon) value of + 7.0‰ VSMOW. The isotope data for Santo Antonio pluton yielded Nd-model ages between 0.96 and 1.07 Ga, with eNd(636 Ma) from + 1.17 to − 0.67, initial 87Sr/86Sr ratios from 0.7048 to 0.7056, and δ18O (zircon) value of + 5.0 to 5.9‰ VSMOW. Altogether, the petrological, geochemical, and isotopic data are typical of I-type granites. We propose that the studied granitic rocks were derived from mantle-derived medium- to high-K basaltic lower crust of Stenian/Tonian (Jacare dos Homens) or Tonian (Santo Antonio) ages and that partial melting was triggered by the uplift of asthenosphere and underplating of lithospheric mantle. The migration and buoyancy-driven ascent of the magma to their present level were probably favored by the Jacare dos Homens shear zone.

The Macururé Complex (Sergipano Belt, NE Brazil) in southern Alagoas state: Geology and geochronology

The Sergipano Belt is part of the Southern Subprovince of the Borborema Province (NE Brazil) and separates the Pernambuco-Alagoas Domain from the Sao Francisco Craton. The Macurure Domain is the largest unit of the Sergipano Belt and comprises a basement of Paleoproterozoic age (Jirau do Ponciano Dome) and a supracrustal succession (Macurure Complex). Here we present the results of field, petrographic and structural work and of geochronological dating conducted in an area in the southern part of Alagoas state, northeast of the city of Sao Bras. The lithological associations of the Macurure Complex include quartzite, schist and banded metasedimentary rocks (metarhythmites). The main foliation shows a gentle dip to the southwest and is associated with a SW-plunging mineral lineation. The mineral assemblage muscovite+biotite+garnet±kyanite±staurolite in schist and metarhythmite indicates medium pressure and temperature conditions (amphibolite facies) typical of Barrovian metamorphism. U-Pb dating of detrital zircon grains from a quartzite sample yielded dominantly Tonian ages, showing a large age peak at c. 985 Ma. These zircon grains are thus derived from sources related to the Cariris Velhos event. The youngest grain has a 206Pb/238U age of 939±36 Ma (2s), which is considered as the maximum age of deposition. Two smaller age populations of c. 2.0 Ga and 1.1 Ga are also present. The most probable provenance of c. 2.0 Ga zircon grains is the nearby Jirau do Ponciano Dome. The other age group is more enigmatic and could be related to erosion of A-type rocks formed during local extensional events preceding the Cariris Velhos event.

Repositioning the Great Unconformity at the southeastern margin of the North China Craton

The Great Unconformity between the Precambrian basement rock and Cambrian sedimentary cover has been extensively documented in Laurentia and may provide insights into the environmental context of the Cambrian explosion. A similar unconformity has been known in the North China Craton (NCC) and it offers an opportunity to constrain the geographic extent and geochronological magnitude of the Great Unconformity. However, the placement and magnitude of the Great Unconformity in the NCC, particularly at the southeastern margin of the NCC, is uncertain. For example, in the Huaibei region of the southeastern margin of the NCC, this unconformity has been variously placed beneath or above the Gouhou Formation, which preserves critical micro- and macrofossil assemblages with great paleobiological and biostratigraphic significance. This uncertainty significantly hampers our ability to fully appreciate the geological history of the NCC. To resolve this issue, we carried out an integrated biostratigraphic, sedimentary petrological and detrital zircon geochronological investigation on the Gouhou Formation and the overlying Houjiashan Formation in the Huaibei region. Our data suggest that the Great Unconformity lies within the Gouhou Formation and represents a ca. 200–300 Myr depositional gap: the Lower Member of the Gouhou Formation is of Tonian age whereas the Middle-Upper members of the Gouhou Formation are of early Cambrian. Therefore, we propose that the Gouhou Formation be redefined to contain only the Lower Member, and the Middle-Upper members be included in the Houjiashan Formation. Our analysis also confirms that the Great Unconformity can be widely recognized along the southeastern margin of the NCC. The recognition of the Great Unconformity in the Huaibei region helps to clarify the tectonic history, basin development, stratigraphic correlation, and paleogeographic reconstruction of the southeastern margin of the NCC during the Tonian–Cambrian periods.

Tonian Fe-Ti-P ferronorite and alkali anorthosite in the northern Appalachian orogen, southern New Brunswick, Canada: Amazonian basement in Ganderia?

The Lower Coverdale plutonic suite is located in the Ganderian Brookville terrane of southern New Brunswick, Canada. It does not outcrop at surface but has been intersected in 11 drill holes at depths of 100–200 m below unconformably overlying Carboniferous sedimentary rocks, and extends to a depth of at least 1100 m. No host rocks are present in the drill core, which consists entirely of plutonic and meta-plutonic rocks. The dominant rock types are anorthosite, mela-anorthosite, and ferronorite with layers of ilmenite and ilmenite-apatite rocks. They are intruded by abundant undated mafic dykes and minor ca. 390 Ma felsic dykes. A ferronorite sample yielded a Tonian age of 975.5 ± 2.3 Ma, interpreted to be the igneous crystallization age of the plutonic suite. A mela-anorthosite sample yielded a similar but less precise age of 979 ± 16 Ma, likely also the igneous crystallization age. The anorthosite has characteristics of alkali anorthosite massifs, including andesine (An30-35) commonly with exsolution lamellae of K-feldspar. The Lower Coverdale suite is interpreted to have been emplaced during orogen-scale extension following crustal thickening associated with the Grenvillian orogeny. The Lower Coverdale anorthosite and ferronorite have more isotopically evolved signatures (higher 206Pb/204Pb and 207Pb/204Pb and lower ∊Nd) compared to typical Laurentian Grenville-age plutons. We infer from these data that the Lower Coverdale plutonic suite originated in the Amazonian part of the Grenville orogen during the early stages of Tonian breakup of Rodinia. It was then incorporated into Ganderia before accreting to Laurentia as part of the Brookville terrane during the Pangean phase of supercontinent assembly.

Provenance of sandstones in the Parnaíba Basin through detrital zircon geochronology

Abstract This paper obtains numerous U–Pb detrital zircon ages obtained from the Paleozoic–Triassic sandstones of the Parnaíba Basin. The resulting spectra enable us to distinguish three common age populations: Paleoproterozoic, Stenian–Tonian (mainly between 1100 and 900 Ma) and Late Neoproterozoic. Minor contributions are observed from Archean, Paleozoic (mainly Cambrian) and Mesoproterozoic sources. Differences are only highlighted by variances in the relative abundances of each age population within the stratigraphic units. The presence of metamorphic phases, schists clasts and some diagnostic heavy minerals suggest that the Neoproterozoic mobile belts to the east and west of the basin are potential source areas. The erosion of metasedimentary rocks exposed in these provinces could explain the Paleoproterozoic and Stenian–Tonian age populations found in the sandstones. The Late Neoproterozoic ages, in turn, are not relevant in the metasedimentary sequences, requiring that the voluminous granitic magmatism mainly exposed in the Borborema Province had directly participated as a primary source of the detritus. The maintenance of common age spectra leads to two (not exclusive) hypotheses: the long-term denudation of the source areas and/or intrabasinal recycling, which can be considered in future investigations on the sedimentary and tectonic evolution of the Parnaíba Basin.

Vase-shaped microfossil biostratigraphy with new data from Tasmania, Svalbard, Greenland, Sweden and the Yukon

The recent revision of the Cryogenian lower boundary from 850 Ma to 720 Ma has nearly doubled the duration of the Tonian Period to 280 Myrs, an interval longer than the Mesozoic and Cenozoic eras combined. Although subdivision of the Tonian will likely emphasize chemostratigraphy, the relatively rich fossil record of this time period may also be useful. In particular, vase-shaped microfossils (VSMs) occur globally and often abundantly in a variety of lithofacies, exhibit a variety of preservational modes, and are stratigraphically constrained by the two major carbon isotopic events of the Tonian Period, the Bitter Springs and Islay anomalies. VSMs have been used as informal, grade-level indicators of late Tonian age (e.g. ca. 800–700 Ma) for several decades but here, based on new data from an assemblage in northwestern Tasmania and restudy of samples from Svalbard, Greenland, Sweden and the Yukon, we report a species-level faunal assemblage indicative of the interval ca. 789 to 729 Ma. Species-level VSM biostratigraphy presents an opportunity for potential biostratigraphic subdivision of the lengthy Tonian Period and is particularly useful in successions where chemostratigraphy and radiometric age constraints are absent.Eight taxa are described from shales and early diagenetic cherts of the Black River Dolomite, Togari Group, Tasmania, including seven previously named species (Bombycion micron, Bonniea dacruchares, Bonniea pytinaia, Cycliocyrillium simplex, Cycliocyrillium torquata, Melanocyrillium hexodiadema and Palaeoarcella athanata), and one newly described genus and species Pakupaku kabin n. gen et n. sp. Study of additional units resulted in new recognition of B. pytinaia, C. simplex and C. torquata in the Draken Formation, Spitsbergen, Svalbard, B. dacruchares and P. athanata in the Ryssö Formation, Nordaustlandet, Svalbard, B. dacruchares, B. pytinaia, C. torquata and P. athanata in Bed 18 of the Eleonore Bay Group, Greenland, and B. dacruchares, B. pytinaia and Bombycion micron in the Visingsö Group, Sweden.

Tonian (Neoproterozoic) eukaryotic and prokaryotic organic-walled microfossils from the upper Visingsö Group, Sweden

ABSTRACTThe diversification of protists and multicellular microorganisms is recorded in numerous worldwide Tonian age successions, including the Visingsö Group in Sweden. The Visingsö Group contains a taxonomically rich assemblage of cyanobacteria, stromatolites, algal phytoplankton and vase-shaped microfossils. A new record of organic-walled microfossils from the Visingsö 1 drillcore reveals the high taxonomic diversity. Several species are reported for the first time from the Visingsö Group, and one new species Leiosphaeridia gorda n. sp. is described. They are in gross phycoma-like cysts of the prasinophycean algae Pterospermopsimorpha, Pterospermella, Simia, Macroptycha and Dictyotidium. Morphologically similar to zygotic cysts of chlorophycean algae are Leiosphaeridia gorda n. sp., Cerebrosphaera, Culcitulisphaera and Lanulatisphaera. Schizofusa may represent the earliest yellow-green algae of the Eustigmatiphyte among Stramenopiles. The recorded biodiversity documents the global trend in the evolution of eukaryotic protists during the Tonian Period and the increased radiation of numerous, presumably photoautotrophic biotas, representing various algal lineages.

Unradiogenic strontium and moderate-amplitude carbon isotope variations in early Tonian seawater after the assembly of Rodinia and before the Bitter Springs Excursion

Neoproterozoic sedimentary records reveal a poorly documented, ∼150m.y. time interval, between ∼1000 and 850Ma, that limit our understanding of C and Sr isotope variations in seawater and thus operation of the biogeochemical carbon cycle, changes in surface redox state, and fluctuations in continental vs. hydrothermal fluxes to the oceans following assembly of the supercontinent Rodinia. Carbon and Sr isotope data for carbonates from the Karatau Group of the Southern Ural Mountains in Russia provides a record partially covering the younger portion of this time interval. The lower part of the Karatau Group (LKG) contains well-preserved carbonate strata of the Katav, Inzer, and Min’yar formations that are focus of this study. Pb-Pb isochron ages for carbonates from the Inzer and Min’yar formations are 844±24 and 820±77Ma, respectively, establishing an early Tonian age. Carbon isotope data for unaltered carbonates in the LKG show a range of moderately positive to negative values from −2.8 to +5.9‰, with the majority below +3.0‰. 87Sr/86Sr values range from 0.70522 to 0.70534 in the Lower Inzer Member, increasing in the overlying Upper Inzer Member and Min’yar Formation, from 0.70555 to 0.70600. The Sr isotope range of the Lower Inzer Member is similar to that typical for the 1.03–0.95Ga seawater (0.70519–0.70554), suggesting that the Sr isotope composition of seawater in the aftermath of Rodinian assembly was unradiogenic for 150m.y. This pattern indicates that supercontinent-scale orogenic events do not result in an enhanced, long-term flux of radiogenic Sr from continents once the supercontinent assembled. To account for the prolonged period characterized by unradiogenic Sr seawater composition and moderate-amplitude C isotope variations, we suggest a supercontinent configuration ringed by continental arcs and with predominantly internal runoff. In this model, accompanying supercontinent blanketing resulted in thermal perturbation in the mantle, emplacement of multiple Large Igneous Provinces, and high continental freeboard. Weathering of juvenile arcs would have provided unradiogenic Sr and a high flux of sediments to the oceans, enhancing organic carbon burial and progressive oxygenation of surface environments and contributing to instability in the biogeochemical carbon cycle. Mass anomalies induced by protracted mantle plume activity led to True Polar Wander and brought the supercontinent, covered with juvenile mafic volcanics and segmented by failed rift systems, to low latitudes thus enhancing chemical weathering, atmospheric CO2 consumption, and eventually ushering in Snowball Earth conditions. Comparison with the records for early Paleoproterozoic strata indicates that a similar sequence of events bracketed both ends of the Proterozoic, highlighting fundamental relationships among plate tectonics, mantle dynamics, biogeochemical carbon cycling, surface oxygenation, and climate change.

Cryptic sub-ice geology revealed by a U-Pb zircon study of glacial till in Dronning Maud Land, East Antarctica

We have targeted the southern side of the Dronning Maud Land (DML) mountains in search of moraine material that might reveal the geology in the ice-covered region of the East Antarctic polar plateau. Nine samples of unconsolidated moraine material, carried by the northward flowing East Antarctic ice sheet to the southern side of the DML escarpment, were collected and processed for U-Pb zircon analyses. The analyses resulted in ca. 1100 new U-Pb zircon spot ages between ca. 2000 and 500Ma. The oldest, Palaeoproterozoic rocks come from the easternmost localities with a probable source region between the Sør Rondane Mountains and Dome Fuji. The samples have major Stenian and early Tonian age peaks. Early Tonian rocks are well known from the SW Terrane in the Sør Rondane Mountains and characterise the major Tonian Oceanic Arc Super Terrane (TOAST). Stenian ages are far less common in the outcropping region. Although Stenian ages are common in both the Maud Province of western-central DML as well as in the Rayner Complex, our samples differ with respect to composition and/or isotope geochemistry; they are juvenile, subduction-related and resemble an early phase of oceanic arcs that was so far unknown in this region. In the westernmost sample, the oldest age peak is ca. 800–720Ma with possible counterparts in the Schirmacher Oasis. All samples show a protracted Late Neoproterozoic/Early Palaeozoic metamorphic overprint, accompanied by igneous addition, most likely related to the East African-Antarctic Orogen. This overprint appears most intense in clasts collected from the westernmost locality, lasted for ca. 150Ma and an eastward younging of metamorphic ages is observed. The new moraine samples together with previous outcrop studies reveal that this region has undergone two major phases of oceanic arc/terrane accretion; the first one from ca. 1100 to 900Ma is probably related to accretion tectonics outboard of Rodinia, the second one from ca. 850 to 580Ma occurred as a result of ocean closure and finally Gondwana amalgamation. No significant old components are found, but the high proportion of Late Neoproterozoic/Early Palaeozoic ages confirms that the East African-Antarctic Orogen transects DML and East Antarctica.

A Tonian age for the Visingsö Group in Sweden constrained by detrital zircon dating and biochronology: implications for evolutionary events

AbstractDetrital zircon U–Pb ages from samples of the Neoproterozoic Visingsö Group, Sweden, yield a maximum depositional age of ≤ 886±9 Ma (2σ). A minimum depositional age is established biochronologically using organic-walled and vase-shaped microfossils present in the upper formation of the Visingsö Group; the upper formation correlates with the Kwagunt Formation of the 780–740 Ma Chuar Group in Arizona, USA, and the lower Mount Harper Group, Yukon, Canada, that is older than 740 Ma. Mineralized scale microfossils of the type recorded from the upper Fifteenmile Group, Yukon, Canada, where they occur in a narrow stratigraphic range and are younger than 788 Ma, are recognized for the first time outside Laurentia. The mineralized scale microfossils in the upper formation of the Visingsö Group seem to have a wider stratigraphic range, and are older thanc. 740 Ma. The inferred age range of mineralized scale microfossils is 788–740 Ma. This time interval coincides with the vase-shaped microfossil range because both microfossil groups co-occur. The combined isotopic and biochronologic ages constrain the Visingsö Group to between ≤ 886 and 740 Ma, thus Tonian in age. This is the first robust age determination for the Visingsö Group, which preserves a rich microfossil assemblage of worldwide distribution. The organic and mineralized microorganisms preserved in the Visingsö Group and coeval successions elsewhere document global evolutionary events of auto- and heterotrophic protist radiations that are crucial to the reconstruction of eukaryotic phylogeny based on the fossil record and are useful for the Neoproterozoic chronostratigraphic subdivision.

Reconstructing the Cryogenian–Ediacaran evolution of the Porongos fold and thrust belt, Southern Brasiliano Orogen, based on Zircon U–Pb–Hf–O isotopes

ABSTRACTThe Neoproterozoic geotectonic triad of the Brasiliano Orogen is reconstructed in southern Brazil from studies focused on the Porongos fold and thrust belt. We integrate field geology with isotopic studies of zircon U–Pb SHRIMP and Lu–Hf–O laser determinations in seven metasedimentary and three metavolcanic rock samples. The results indicate that the Porongos palaeo-basin was derived from mixed sources (3200–550 Ma), with major contributions from Rhyacian (2170 Ma) and Ediacaran (608 Ma) sources. Minor contributions from Archaean to Tonian sources are also registered. The maximum depositional age of the Porongos palaeo-basin is established by the age range of 650–550 Ma with TDM model ages between 2.5 and 1.3 Ga. The reworked signature (εHf values = −34 to −4) and the characteristic crustal magma reservoirs (δ18O ≥5.3 ‰) indicate that these sediments are equivalent to Neoproterozoic granites of the Dom Feliciano Belt. The episodic depositional history started in the Cryogenian (650 Ma) and lasted until the Ediacaran (most likely 570 Ma). A magmatic event of Tonian age is recorded in rhyodacite samples interleaved with the metasedimentary rocks and dated at 773, 801, and 809 Ma. The crustal evolution of the Sul-Riograndense Shield included mountain building, folding and thrusting and flexural subsidence in the foreland. An orogenic triad is revealed as the Pelotas Batholith, the Porongos fold and thrust belt and the Camaquã Basin, all part of the Dom Feliciano Belt.

Measuring the ‘Great Unconformity’ on the North China Craton using new detrital zircon age data

Abstract New detrital zircon ages confirm that the Neoproterozoic strata of the southeastern North China Craton (NCC) are mostly of early Tonian age, but that the Gouhou Formation, previously assigned to the Tonian, is Cambrian in age. A discordant hiatus of &gt;150–300 myr occurs across the NCC, spanning most of the late Tonian, Cryogenian, Ediacaran and early Cambrian periods. This widespread unconformable surface is akin to the Great Unconformity seen elsewhere in the world and highlights a major shift in depositional style from largely erosional, marked by low rates of net deposition, during the mid- to late Neoproterozoic to high rates of transgressive deposition during the mid- to late Cambrian. The age spectra for the southeastern NCC and northern India are consistent with a provenance affinity linking the NCC and East Gondwana by c. 510 Ma. Supplementary material: Sample descriptions, sampling GPS locations and a compiled dataset of detrital zircon U–Pb LA-ICP-MS dating results are available at https://doi.org/10.6084/m9.figshare.c.3571119

A diverse and exquisitely preserved organic-walled microfossil assemblage from the Meso–Neoproterozoic Mbuji-Mayi Supergroup (Democratic Republic of Congo) and implications for Proterozoic biostratigraphy

A well preserved and diversified microfossil assemblage is reported from the Meso–Neoproterozoic Mbuji-Mayi Supergroup in the Kasai oriental Province, central part of Democratic Republic of Congo. A total of 49 taxa belonging to 27 genera were identified, including 11 species of unambiguous eukaryotes, 10 species of possible eukaryotes or prokaryotes and 28 species of probable bacteria. This assemblage is more diverse than previously reported but includes taxa reported in coeval worldwide assemblages. It is characterized by abundant sphaeromorphs, filamentous colonial aggregates and filamentous forms, as well as a relatively low diversity of acanthomorphs including the Late Mesoproterozoic and Early Neoproterozoic index fossil – Trachyhystrichosphaera aimika – reported for the first time in Central Africa. This species co-occurs with other taxa also reported for the first time in Africa: Trachyhystrichosphaera botula, Jacutianema solubila, cf. Tappania sp., Valeria elongata and numerous other taxa. Correlation with other geochronologically constrained successions that contain Trachyhystrichosphaera confirms T. aimika as promising index fossil to define the Late Mesoproterozoic–Early Neoproterozoic interval. The available biostratigraphic data enable to suggest a minimum Tonian age for the Mbuji-Mayi Supergroup. This age is consistent with the published and new geochronological data. Comparison with worldwide Proterozoic assemblages permits to define microfossil assemblages useful for biostratigraphy. This study significantly improves our understanding of the diversity of the Late Mesoproterozoic–Early Neoproterozoic biosphere, and in particular the diversification of early eukaryotes, preserved in the Democratic Republic of Congo rock record and more broadly in Africa where micropaleontological investigations are sparse.

Geochemistry, U–Pb geochronology, Sm–Nd and O isotopes of ca. 50 Ma long Ediacaran High-K Syn-Collisional Magmatism in the Pernambuco Alagoas Domain, Borborema Province, NE Brazil

The Pernambuco Alagoas (PEAL) domain shows the major occurrence of granitic batholiths of the Borborema Province, NE Brazil, with Archean to Neoproterozoic range of Nd TDM model ages, giving clues on the role of granites during the Brasiliano orogeny. SHRIMP U/Pb zircon geochronological data for seven granitic intrusions of the PEAL domain divide the studied granitoids into three groups: 1) early-to syn-collision granitoids with crystallization ages ca. 635 Ma (Serra do Catú pluton), 2) syn-collision granitoids with crystallization ages 610–618 Ma (Santana do Ipanema, Água Branca, Mata Grande and Correntes plutons) and 3) late-to post-collision granitoids with ages of ca. 590 Ma (Águas Belas, and Cachoeirinha plutons). The intrusions of group 1 and 2, except the Mata Grande and Correntes plutons, show Nd TDM model ages ranging from 1.2 to 1.5 Ga, while the granitoids from group 3, and Mata Grande Pluton and Correntes plutons have Nd TDM model ages ranging from 1.7 to 2.2 Ga. The studied granitoids with ages <600 Ma are high-K, calc-alkaline, shoshonitic and those with ages <600 Ma are transitional high-K calc-alkaline to alkaline. The volcanic arc signatures associated with the Paleoproterozoic Nd TDM model ages are interpreted as inherited from the source rocks. The oldest ages and lower Nd TDM model ages are recorded from granitoids intruded in the southwest part of the PEAL domain, suggesting that these intrusions are associated with slab-tearing during convergence between the PEAL and the Sergipano domains. Zircon oxygen isotopic data in some of the studied plutons, together with the available Nd isotopic data suggest that the Brasiliano orogeny strongly reworked older crust, of either Paleoproterozoic or Tonian ages. The studied granitoids are coeval with calc-alkaline granitoids of the Transversal Zone and Sergipano domains and rare high-K calc-alkaline granitoids from the Transversal Zone domain. Such large volumes of high-K granitoids with crystallization ages older than 600 Ma are not recorded in the Transversal Zone domains, suggesting that at least between 600 and 650 Ma, the granitic magmatism of these two areas were distinct. However, the studied granitoids (630–580 Ma) located in the north part of the PEAL domain, north of the Palmares shear zone are coeval with granitoids of similar geochemical compositions in the Transversal Zone domain. It suggests that the southeastern part of the Transversal Zone and the northern part of the PEAL domains belonged to the same crustal block during the Brasiliano/Pan-African orogeny.

Tectonic inversion of compressional structures in the Southern portion of the Paramirim Corridor, Bahia, Brazil

The Paramirim Corridor represents the maximum inversion zone of the Paramirim Aulacogen. Reverse-to-reverse dextral shear zones and various types of folds dominate such corridor. These structures reflect a stress field that is WSW-ENE oriented, developed in units of Aulacogen basement, as well as in the Lagoa Real Intrusive Suite, of Statherian age, in Espinhaço and São Francisco supergroups, of Statherian-Tonian and Cryogenian ages, respectively, and in the Macaúbas-Santo Onofre Group, of Tonian age at the most. A rich collection of extensional structures truncate compressional structures of the Paramirim Corridor, characterized by normal shear zones and foliation, which is sometimes mylonitic, down-dip stretching lineation, drag folds, traction fractures and S/C structures. In these shear zones, quartz occurs truncated by the foliation, while feldspars are fractured and altered to white mica. Distribution of the quartz c-axes is at a maximum of 14° from the Z-axis. Thus, it suggests that the deformation activated mainly the basal glide planes in the &lt;a&gt; direction. The paleostress study using the Win-Tensor software demonstrated that the regimen ranged between radial and pure distention. The S1 direction oscillated around a vertical trend, while s3 was sub-horizontal, with a predominant N230-050° direction. Ar-Ar ages in biotite obtained from the extensional shear zones ranged from 480 and 490 Ma. Together, data obtained for the structures associated with the late extensional regimen described in the present study suggest that its nucleation is associated with distal and brittle-ductile sectors of the gravitational collapse zone of Araçuaí-West Congo Orogen.

Tonian granitic magmatism of the Borborema Province, NE Brazil: A review

Tonian granitoids, today augen-gneisses and migmatites, showing crystallization ages ranging from 870 Ma to 1000 Ma occur in the Borborema Province, NE Brazil. The majority of them have ages within the 925–970 Ma interval. Few intrusions with ages of ∼1.0 Ga and <900 Ma occur in the Transversal and South subprovinces. The Tonian granitoids constitute the most expressive magmatic rocks of the Cariris Velhos event. The studied granitoids (herein CVG -Cariris Velhos granitoids) intrude slightly older bimodal (but mostly felsic) volcanic successions and metasedimentary sequences in the Transversal and South subprovinces. Tonian granitoids are unknown in the North subprovince. The CVG comprise mainly coarse-grained augen-gneisses of granite to granodiorite composition. Fe-rich biotite (annite) is the main mafic mineral phase, constituting up to 15% of the modal composition. Garnet, muscovite and tourmaline occur as accessory phases in many plutons.The CVG augen–gneisses have high SiO2 (>71%) and alkali contents, they vary from slightly peraluminous to slightly metaluminous, and from slightly magnesian to typical ferrroan rocks. In the migmatized orthogneisses the SiO2 contents are usually <70%. Trace element variations in the CVG are extensive, reflecting the migmatization recorded in some plutons and/or distinct sources. They are Ca-, Sr- and Nb-poor, showing variable Ba (100–1260 ppm), Rb (164–400 ppm) and Zr (144–408 ppm) contents, and high abundances of Y (>40 ppm). The chondrite normalized REE patterns are characterized by strong to moderate negative Eu anomalies (Eu/Eu* = 0.23–0.70). In general, the spidergram patterns show deep troughs at Ti, P, Ba and Sr and less pronounced Nb–Ta troughs. These patterns are similar to those reported for anorogenic granites evolved from mixtures of magmas from both crustal and mantle sources. The CVG exhibit TDM model ages ranging from 1.9 to 1.1Ga, with slightly negative to slightly positive ƐNd(t) values, suggesting the involvement of distinct proportions of mantle and crustal components in the source of their protoliths.There is no consensus in the literature about the tectonic setting of the CVG ie they have been related to either continental margin magmatic arc, with possible back–arc association, or extention-related setting, with generation of A-type granites. However, all the available geochemical data suggest that the CVG represent extension related magmatism. The geochemical signature associated to bimodal volcanism, including pyroclastic rocks, with similar ages, and absence, up to now, of evidence for metamorphism of Tonian age, support the hypothesis of extension - related magmatism.

Organic-walled microfossils from the Tonian Gouhou Formation, Huaibei region, North China Craton, and their biostratigraphic implications

The Meso-(?) and Neoproterozoic Huaibei Group in the Huaibei region, North China Craton (NCC), is emerging as a target for geobiological and tectonic studies due to its thick unmetamorphosed sediments with well-preserved organic-walled fossils. However, the lack of accurate age constraints for this sedimentary sequence dramatically hampers our ability to take full advantage of the rich geological and paleontological history recorded in the Huaibei Group. Particularly, the depositional age of the uppermost unit of the Huaibei Group, the Gouhou Formation, is a controversial issue. Although it is widely accepted that there is an unconformity between the Gouhou Formation and the overlying early Cambrian Houjiashan Formation, the magnitude of this unconformity and hence the scale of the underlying tectonic event are unconstrained due to the uncertainty of the sedimentary age for the Gouhou Formation which has been variously interpreted as Cambrian, Cryogenian–Ediacaran (∼720Ma to ∼541Ma), and Tonian (1000Ma to ∼720Ma). To improve our knowledge about this sedimentary gap, we carried out a biostratigraphic study of the Gouhou Formation. Our investigation using a low manipulation maceration technique revealed a diverse organic-walled microfossil assemblage. A total of 22 taxa have been revealed, including a new species—Dictyosphaera tacita n. sp. Importantly, the co-occurrence of Trachyhystrichosphaera, Valeria, and Dictyosphaera in the Gouhou Formation suggests a likely Tonian age. The biostratigraphic data indicate an infra-Cambrian depositional gap of >200myr between the Tonian Gouhou Formation and Cambrian Houjiashan Formation, perhaps driven by a major tectonostratigraphic event.