Late Tonian Research Articles

Geochronological Assessment of the Arabian-Nubian Shield plutonic intrusions in the arc assemblages along the Qift-Quseir transect, Central Eastern Desert of Egypt

The ophiolitic mélange and granitic intrusions along the Qift‒Quseir transect in the Central Eastern Desert (CED) of Egypt are parts of the Egyptian Nubian Shield (ENS), which is the northern segment of the East African Orogeny (EAO). The Arabian-Nubian Shield (ANS) is the longest Neoproterozoic belt on Earth, and it was formed during the East and West Gondwanaland collisions within the framework of the EAO. The ANS basement rocks were developed during three distinct phases of magmatic activity: the island arc and syn-collisional phases, identifying a compressional tectonic regime, and a post-collisional phase that identifies changing the tectonic regime into an extensional type. The geochronological assessment of these magmatic activities is essential for understanding the regional geology and tectonic development of the ANS. In our study, we dated different rock units along the Qift‒Quseir transect and revealed ages ranging from the Late Tonian (820 ± 8 Ma) to the Late Ediacaran (563 ± 4 Ma). These ages were associated with three different magmatic pulses: (1) a seafloor spreading and island arc phase (ophiolite and related rocks), represented by sample QQ05, which was dated from 820 ± 8 Ma; (2) a syn-collisional phase, represented by samples QQ08 and QQ10, dated from 733 ± 10 Ma and 729 ± 10 Ma, respectively; and (3) a post-collisional phase, represented by all the other samples, dated from the Ediacaran at 603 ± 9 Ma to 563 ± 5 Ma. These results showed that the post-collisional phase was dominant, especially in terms of the alkali-feldspar granites, relative to ophiolitic rocks, and the syn-collisional granites in the CED. Initiation of the Dokhan Volcanic eruptions at 639 ± 2 Ma gave us the date of the compressional-to-extensional tectonic transition setting, and the post-collisional tensional regime was activated at 603 ± 9 Ma. Additionally, we identified evidence of local magmatic sources by dating 11 grains of Paleo- to Meso-Proterozoic xenocrysts with ages ranging from 1876 ± 18 to 1070 ± 13 Ma (i.e., components of the pre-Arabian-Nubian Shield).

Neoproterozoic arc-magmatism of the Silet Terrane, Western Hoggar (Algeria), new constraints from zircon U-Pb-Hf isotope analyses and whole rock geochemistry

This study presents results of zircon U-Pb-Hf isotope analyses and whole-rock geochemistry applied to magmatic rocks of the Silet terrane in the Western Hoggar, forming a part of the Neoproterozoic arc exposed along the boundary between the LATEA metacraton and the Paleoproterozoic In Ouzzal terrane. According to zircon U-Pb geochronology four distinct pulses of Neoproterozoic granitoids magmatism are recognized in the Silet terrane: Tonian (c. 870–840 Ma), Late Tonian (c. 770 Ma), Latest Tonian (c. 730 Ma) and Late Cryogenian (c. 675 Ma).Geochemical and Hf isotope data indicate a progressive shift in geotectonic settings. The c. 870–840 Ma Tonian TTG pulses (Tonalite-Trondhjemite-Granodiorite) in the North Silet batholith, with superchondritic εHf values (+5.8 to + 7.2), suggest formation in a subduction zone. The subsequent c. 770 Ma Late Tonian magmatism, characterized by mafic compositions, reflects a back-arc tectonic setting, as evidenced by high Ti/V ratios, low V contents, and εHf values of + 6.12 to + 8.95. The c. 730 Ma Latest Tonian magmatism, with more evolved acidic pulses, indicates hybrid mantle–crustal involvement (εHf + 6.53 to + 8.08). The Late Cryogenian intrusions (c. 675 Ma), featuring the lowest εHf values (+1.84 to + 4.09) and inherited c. 870 Ma zircons, suggest formation from the melting of Tonian TTGs during continental collision.Integrating U-Pb dating with geochemical data provides a comprehensive view of the Silet terrane’s tectonic evolution, supporting a continental arc origin. Disparities in magmatism ages between the northern (c. 870–675 Ma) and southern (c. 740–640 Ma) segments of the Silet arc correlate with the diachronous collision/subduction of a V-shaped “Silet Ocean”, forming an introverted oceanic system (oceans that have formed by rifted continental blocks that were subsequently re‑joined in approximately the same position) within the broader West Gondwana context. Structural analysis indicates that the regional structures of the Silet arc were shaped in a WNW-ESE collisional transpressive regime during the Pan-African orogeny, offering new insights into the Neoproterozoic Silet arc’s geodynamic evolution and its paleogeographical location in the West Gondwana orogeny.

Diverse magmatism along the northern margin of Tarim during the Ediacaran: Transition from Rodinian dispersing to Gondwana assemblage

The Ediacaran igneous rocks and sedimentary sequences play a key role for our understanding the transition from dispersing of the Rodinian continents to the assemblage of the Gondwana. On the other hand, some continental massifs at the marginal area of the Rodinia and Gondwana could preserve solid evidences for deciphering the geodynamic background from the break-up of Rodinia to the assemblage of Gondwana. In this contribution, we report field observations, petrography, ages and systematic geochemistry of the diverse Ediacaran igneous activities in the northern margin of the Tarim Block, which was favored as a marginal continental massif in the Rodinia configuration by most geologists. The Ediacaran igneous rocks along the northern margin of Tarim include the Baicheng and Yangxia granites, Tiemenguan syenite, Kurle mafic dykes and the basalt layers in the Zamoketi Formation (northeastern Tarim) and the Sugetbrak Formation (northwestern Tarim). Precise zircon UPb dating revealed that these distinct rocks were broadly coeval with crystallization/eruption ages of 630–600 Ma. Bulk-rock elemental and SrNd isotope compositions as well as zircon LuHf isotope compositions demonstrate that the Baicheng and Yangxia granites were derived from Neoarchean mafic lower crust and underwent intensive fractionation before their emplacements. The Tiemenguan syenites have potassic andesitic compositions, in combination with their pronounced enriched NdHf isotopes and extremely high Zr saturation temperatures (∼900 °C), we argue that their possible shoshonitic primary magmas were derived from previously metasomatized subcontinental lithospheric mantle source (amphibole-bearing) and then, coupling with assimilation and fractional crystallization (AFC) effects in magma chamber and/or en route to the emplacement space. This can account for their intriguing geochemical features. The ca. 615 Ma Zamoketi and Sugetbrak basalts and the slightly younger porphyrite dykes (ca. 580 Ma) cutting the Zamoketi basalt layer, show typical OIB-like geochemical signatures, they were most likely derived from a depleted asthenospheric mantle source with variable fractionation of olivine and pyroxene and negligible crustal contamination before eruption. On the whole, the geochemistry of the diverse Ediacaran igneous rocks unambiguously demonstrates that they were genetically related to an extensional environment. In combination with the significant passive continental features of the Ediacaran-Cambrian sedimentary sequences, we consider that these Ediacaran igneous rocks were the latest phase of igneous activity related to the dispersing of the Tarim at the margin of the Rodinia supercontinent.The dispersing of the Rodinian supra-continent was diachronic with the assemblage of the Gondwana. Late Tonian to Ediacaran dispersing of the Tarim from northern margin of Australia induced the opening of the Proto-Tethys Ocean. Then the initiation of the southward subduction of the Proto-Tethys Ocean at ca. 530 Ma (Fortunian) led to the Tarim assemblage to Gondwana at ca. 440 Ma.

Late Tonian (ca. 770-720 Ma) arc activity in the Wuyi domain of the West Cathaysia terrane, South China

The late Tonian tectonic setting of the West Cathaysia terrane in South China is highly controversial and is key to better understanding the timing of amalgamation between the Yangtze Block and the West Cathaysia terrane and the positions of these two terranes in Rodinia and Gondwana. A detailed field-based study was conducted on the petrology, zircon U-Pb geochronology and Lu-Hf isotopes, and whole-rock geochemistry of a series of Neoproterozoic meta-volcanic and meta-sedimentary rocks in the Wuyi domain in the northeastern West Cathaysia terrane. The results indicate that the meta-volcanic rocks formed in an arc setting at 756-740 Ma, and the meta-sedimentary rocks were developed in an arc-related basin with single zircon age peaks of 765-728 Ma. We propose that the late Tonian arc in the Wuyi domain was part of a major arc system in the West Cathaysia terrane. An arc setting for the West Cathaysia terrane is distinct from the continental rift setting for the southeastern margin of the Yangtze Block at the time, supporting an interpretation that the two terranes were not amalgamated until after the late Tonian and occupied separate positions in Rodinia.

The passive margin of the southern São Francisco paleocontinent, metamorphic record and implications for the assembly of West Gondwana: Evidence from the Lima Duarte Nappe, Ribeira Orogen (SE Brazil)

In this contribution, we report new P-T (pressure–temperature) and U-Pb zircon geochronological data from the Lima Duarte Nappe and basement rocks from the Mantiqueira Complex, Central Ribeira Orogen, southeastern Brazil, highlighting their relevance to the understanding of the West Gondwana assembly. The lower metasedimentary sequence of the Lima Duarte Nappe records a passive margin environment, with a majority of Paleoproterozoic detrital zircon grains, with probable main source rocks located in the São Francisco paleocontinent. The upper foreland sequence of the Lima Duarte Nappe has a prevalence of Tonian-Cryogenian detrital zircon grains, absent in the lower passive margin sequence and source rocks were most likely in a magmatic arc developed during the convergence between the São Francisco and the Paranapanema paleoplates. Zircon rims from the Lima Duarte passive margin and foreland sequences, and zircon grains from an amphibolite of the Lima Duarte Nappe record granulite facies metamorphism at ca. 560–590 Ma (Th/U ratio < 0.1). Similar ages are recorded by CL-dark zircon rims and overgrowths in orthogneisses of the Mantiqueira Complex and are interpreted to reflect basement reworking. Metapelitic metatexites from the foreland sequence record granulite facies peak metamorphism at ∼ 780–800 °C and 8–9.5 kbar in the sillimanite-rutile stability field, constrained by phase equilibrium modeling. Rutile inclusions in garnet record minimum peak temperatures of ∼ 750 °C. The retrograde mineral assemblage in the matrix is marked by the disappearance of rutile and the presence of ilmenite and mineral compositions re-equilibrated at upper amphibolite facies. Late Tonian (ca. 850–750 Ma) metamafic rocks within the Mantiqueira and Juiz de Fora complexes might record an extensional intraplate mafic magmatism episode that culminated with the establishment of the passive margin at the south of the São Francisco paleocontinent. The Lima Duarte metasedimentary sequence along with the Canastra and Carrancas groups (western and southern portions of the Brasília Orogen, respectively) represent a Tonian proximal passive margin environment, developed at the margin of the São Francisco paleocontinent. The Lima Duarte Nappe metasedimentary units and the orthogneisses of the Mantiqueira Complex record Ediacaran syn-collisional metamorphism related to the Araçuaí-Ribeira Orogenic System.

Late Tonian (ca. 780–750 Ma) bimodal magmatism in the Ogcheon Metamorphic Belt, Korea, at the eastern margin of North China Craton

Neoproterozoic rift-related rocks of the North China Craton (NCC), including the Korean Peninsula, provide crucial information for global supercontinent reconstruction during the Rodinia breakup and Gondwana assembly. Here, we report not only zircon and baddeleyite U-Pb ages but also zircon Hf isotopic and bulk rock compositions of amphibolites in the Ogcheon Metamorphic Belt (OMB), Korea. Zircon occurs as either primary phase or secondary one replacing igneous baddeleyite. Primary zircons analyzed from three amphibolites yielded the weighted mean 206Pb/238U ages of 755 ± 5 Ma (n = 19), 772 ± 3 Ma (n = 9), and 779 ± 13 Ma (n = 12), respectively; this result is corroborated by the baddeleyite age of ∼760–745 Ma. Secondary zircons define discordia lines with lower intercepts corresponding to the Jurassic or Cretaceous age of granites emplaced adjacent to the amphibolite. In conjunction with the occurrence of coeval (ca. 762–745 Ma) metatrachytes, our result suggests that late Tonian bimodal magmatism has lasted for ∼30 m.yr. in the OMB. Moreover, positive εHf(t) values (4.9–21.4) of zircon in amphibolites indicate a depleted mantle source of mafic magma, whereas trace element compositions attest to an intraplate setting for the crystallization. A compilation of Nb-Th and Zr–Zr/Y data suggests that the intraplate setting is also pertinent to ca. 950–750 Ma mafic rocks from the Gyeonggi Marginal Belt, Korea, as well as the NCC. Such a tectonomagmatic correlation is corroborated by detrital zircon age spectra of metasedimentary rocks in the OMB equivalent to those of early Tonian rocks in the NCC. In marked contrast to tectonic scenarios conjecturing a linkage between the OMB and the Nanhua rift, South China Craton (SCC), we conclude that the Ogcheon Belt provides rare geologic-geochronologic evidence for late Tonian breakup of the Rodinia supercontinent at the eastern NCC margin.

Tracing the crustal evolution of the Precambrian Southern Granulite terrane in East Gondwana: New insights from zircon U-Pb/Hf geochronology

The Precambrian Southern Granulite terrane of south India has a crustal evolution history broadly bracketed between the late Archean and Cambrian with records of polyphase deformation, metamorphism, and magmatism. The Southern Granulite terrane comprises distinct crustal blocks bounded by shear/suture zones that have been variably correlated with supercontinent fragments including Madagascar, Sri Lanka, Africa, Eastern Ghats, and Antarctica. However, the timing and mechanism of assembly of different crustal blocks within the Southern Granulite terrane and its linkages with counterparts in East Gondwana are highly debated. This study aimed to unravel the complex crustal evolutionary pattern of the terrane by generating robust zircon U-Pb/Hf isotopic data from basement charnockites, gneisses, granitoids, and alkaline intrusive units from the central part of Southern Granulite terrane and comparing these results with similar data from different East Gondwanan terranes. The study identified four distinct crustal growth episodes in the Madurai block: (1) Neoarchean−early Paleoproterozoic, (2) Rhyacian−Orosirian, (3) late Tonian, and (4) Ediacaran−Cambrian. Analysis of zircon Hf isotope data revealed that the first two events are marked by juvenile magmatic signatures, whereas the latter two are distinctly associated with intense reworking and remelting of older crust with no significant juvenile input. Our new results combined with existing data from other Gondwanan terranes suggest a common Paleoproterozoic ancestry for the Southern Granulite terrane and its corresponding Gondwanan fragments, proposing a revision to the existing geodynamic models.

Sedimentary evolution from greenhouse to icehouse of Neoproterozoic and age constraints in the northern Yangtze Craton

The late Tonian is a critical period of the global climate transition from greenhouse to icehouse. Systematic analysis of its sedimentary evolution is critical to properly understand the initiation of snowball Earth and the nature of Cryogenian glaciers. However, it is difficult to infer this continuous response to climate change from the perspective of studying the continuous sedimentary sequence and sedimentary evolution, because of the depositional hiatus between the Tonian and Snowball Earth strata in almost all cratons. A unique and continuous Neoproterozoic clastic rock succession developed well in the northern Yangtze Craton. This study analyzes the sedimentology, paleocurrent, major and trace elements, volcanic tuff geochronology, sedimentary sequences, and depositional environment of the Tonian Liantuo Formation to the Cryogenian Gucheng Formation in the Shennongjia area of the northern Yangtze Craton with a focus on sedimentary evolution. UPb zircon radiometric ages from interbedded tuffs obtained by LA-ICP–MS method allow constraining the age of the sedimentary evolution in the northern Yangtze Craton to the late Tonian-early Crygenian. The northern Yangtze Craton began deposition at 763.1 ± 6.2 Ma and the sedimentary environment was dominated by terrestrial fluvial action with a provenance from the continental magmatic arc on the South Qinling Belt. At 752.1 ± 6.5 Ma, a transition from the fluvial-dominated terrestrial system to the transgressive tidal-influenced marine system was coeval with the peak rifting related to the Rodinia breakup. After 722.5 ± 2.1 Ma, the northern Yangtze Craton entered the glaciomarine environment under the influence of a global cooling trend. Our analysis of the sedimentary evolution from the late Tonian to the Sturtian glaciation in the northern Yangtze Craton forced by tectonic- climate deepens our understanding of the paleoenvironment at the beginning of snowball Earth.

From Rodinian passive margin to peri-Siberian continental arc: Evidence from the multiphase Neoproterozoic–early Paleozoic magmatic record of the Zavkhan Block in the Mongolian Collage

Geochronological and geochemical study of the Neoproterozoic–early Paleozoic magmatic record was carried out in two sections of the Zavkhan Block, one of the largest continental fragments within the Mongolian Collage of the Central Asian Orogenic Belt. Our results show that two distinct magmatic events occurred in both studied domains, the Late Tonian (c. 840–800 Ma) and the Cambro–Ordovician (c. 510–470 Ma). Based on the variable geochemical signatures and two-stage Nd model ages (TNd.2stgDM=1.4–3.0 Ga), the late Tonian magmatism included both mantle-derived magmas that suffered a crustal contamination as well as partial melts of a heterogeneous continental crust that ranged from Paleo- to Mesoproterozoic (felsic metaigneous rocks or immature metagreywackes) to Neoarchean–Paleoproterozoic (metabasalts or metatonalites). The Cambro–Ordovician basic–acidic magmatism (TNd.2stgDM=1.3–2.2 Ga) reflected interplay between two components, mafic juvenile (depleted-mantle-derived) and felsic, generated by anatexis of Tonian, mainly metabasic crust. The late Tonian (c. 840–800 Ma) magmatic activity is interpreted as result of intracontinental rifting, whereas the Cambro–Ordovician (c. 510–470 Ma) magmatism is linked to a continental magmatic-arc system.The comparison of the data from the Zavkhan Block with other microcontinents of the Mongolian Collage shows that most of them experienced an identical two-stage magmatic evolution. The first stage is interpreted as a result of the Rodinia break-up, whereas the second one was associated with inception of the Paleo-Pacific Plate subduction. Based on similarities in the magmatic record, we conclude that the Mongolian Collage microcontinents were located at the Rodinia periphery, as a part of either the Siberian or Tarim cratons. They were separated in the late Tonian, established a passive margin and drifted towards southern Siberia, where they recorded arc magmatism related to the Cambro–Ordovician subduction of the Paleo-Pacific Plate.

Mesoproterozoic–Neoproterozoic chronostratigraphic framework and provenance analysis in the southeastern North China Craton and its tectonic significance

The Sishilichangshan area of southwestern Xuhuai Basin, located in southeastern North China Craton (NCC) and to the north of the Qinling–Dabie Orogenic belts, is a key, optimal area to study the regional evolution of the NCC because of its stratigraphic differences within the NCC. An integrated approach of the field investigation, element geochemistry, and detrital zircon U–Pb–Hf isotope analysis reveals essential information on the Mesoproterozoic–Neoproterozoic tectonic‐sedimentary evolution of the Xuhuai Basin. The youngest graphical peak ages of detrital zircons from the Shouxian Formation at the top of the Bagongshan Group, the Jiuliqiao Formation at the bottom of the Huainan Group, and the Anyangshan Formation are 1003 ± 25 Ma, 991 ± 80 Ma, and 832 ± 23 Ma, respectively. New age constraints and stratigraphic correlations show that the traditional Neoproterozoic strata in the southern Xuhuai Basin consist of Late Stenian to Early Tonian Bagongshan and Huainan groups, and the Late Tonian Anyangshan Formation, which overlapped unconformably with mid‐late Ediacaran or Cambrian Series 2 strata. The Anyangshan Formation is first termed in this study, and its equivalent strata occur sporadically in the southeastern, southwestern and northwestern NCC. The Tonian Shouxian, Jiuliqiao, and Anyangshan formations are characterized by detrital zircons of multi‐stage Mesoproterozoic ages and predominantly positive εHf(t) values, with differences in mineral composition, whole‐rock geochemistry, and the Early Neoproterozoic detrital zircon age population. Their possible provenance was a Grenvillian orogen extending from the North Qinling Terrane (NQT) to the central Korean Peninsula, supporting Late Mesoproterozoic to Early Neoproterozoic initial accretion related to the continental arc between the NCC and NQT. Tectonic transition from Late Stenian to Early Tonian (~1.18–0.94 Ga) collisional basin to mid‐late Tonian (0.94–0.74 Ga) rift basin is interpreted as the response of the NCC to the assembly and extension of the Rodinia Supercontinent.

Late Tonian (ca. 785 Ma) subduction-related mafic-ultramafic cumulates in the West Cathaysia terrane, South China

The Neoproterozoic tectonic setting of the West Cathaysia terrane and its relationship with the Yangtze Block are critical to understanding the formation and tectonic evolution of South China. A detailed study of late Tonian mafic-ultramafic cumulate slivers embedded in the Shenshan tectonic mélange along the northwest margin of West Cathaysia is presented here with the aim of elucidating its Neoproterozoic tectonic history. These partly-altered mafic-ultramafic rocks include olivine-bearing cumulates (olivine pyroxene hornblendite or olivine hornblende pyroxenite), pyroxene hornblendite, and gabbroic cumulates. The early crystallization of clinopyroxene and the abundance of hornblende indicate that they formed from hydrous magmas. The rocks have similar geochemical distribution patterns and show variable initial 87Sr/86Sr ratios of 0.702679 to 0.709742 and positive εNd(t) values of +2.10 to +4.52. The mineral compositions of the clinopyroxene and hornblende and their whole-rock extended normalized trace element patterns show features (e.g., negative Nb anomalies, LILE enrichment) typical of rocks with an arc affinity. The high calculated Mg# values (58–77, average 71) of the equilibrium magmas are close to those of primary mantle-derived basalts. We conclude that these mafic and ultramafic cumulates were derived from a subduction-related, hydrous, magnesian basalt in a subduction setting. The estimated crystallization temperature and pressure, based on the composition of the amphiboles, suggest that the minerals accumulated in magma chambers at middle to lower crustal depths. LA-ICP-MS U-Pb dating of zircon from the gabbroic cumulate indicates crystallization at ca. 785 Ma. A previous interpretation for West Cathaysia at this time involved a continental rift basin (the Nanhua basin) that straddled the southeastern margin of the Yangtze Block and the northwestern margin of West Cathaysia. In contrast, our results suggest that West Cathaysia was an arc at ca. 785 Ma, which supports the interpretation that the amalgamation between Yangtze and West Cathaysia did not occur in the Tonian, but more likely in the early Paleozoic.

Organic preservation of vase-shaped microfossils from the late Tonian Chuar Group, Grand Canyon, Arizona, USA.

Vase-shaped microfossils (VSMs) are found globally in middle Neoproterozoic (800-730 Ma) marine strata and represent the earliest evidence for testate (shell-forming) amoebozoans. VSM tests are hypothesized to have been originally organic in life but are most commonly preserved as secondary mineralized casts and molds. A few reports, however, suggest possible organic preservation. Here, we test the hypothesis that VSMs from shales of the lower Walcott Member of the Chuar Group, Grand Canyon, Arizona, contain original organic material, as reported by B. Bloeser in her pioneering studies of Chuar VSMs. We identified VSMs from two thin section samples of Walcott Member black shales in transmitted light microscopy and used scanning electron microscopy to image VSMs. Carbonaceous material is found within the internal cavity of all VSM tests from both samples and is interpreted as bitumen mobilized from Walcott shales likely during the Cretaceous. Energy dispersive X-ray spectroscopy (EDS) and wavelength dispersive X-ray spectroscopy (WDS) reveal that VSM test walls contain mostly carbon, iron, and sulfur, while silica is present only in the surrounding matrix. Raman spectroscopy was used to compare the thermal maturity of carbonaceous material within the samples and indicated the presence of pyrite and jarosite within fossil material. X-ray absorption spectroscopy revealed the presence of reduced organic sulfur species within the carbonaceous test walls, the carbonaceous material found within test cavities, and in the sedimentary matrix, suggesting that organic matter sulfurization occurred within the Walcott shales. Our suite of spectroscopic analyses reveals that Walcott VSM test walls are organic and sometimes secondarily pyritized (with the pyrite variably oxidized to jarosite). Both preservation modes can occur at a millimeter spatial scale within sample material, and at times even within a single specimen. We propose that sulfurization within the Walcott Shales promoted organic preservation, and furthermore, the ratio of iron to labile VSM organic material controlled the extent of pyrite replacement. Based on our evidence, we conclude that the VSMs are preserved with original organic test material, and speculate that organic VSMs may often go unrecognized, given their light-colored, translucent appearance in transmitted light.

Detrital zircon U–Pb and Lu–Hf constraints on the age, provenance and tectonic setting of arc-related high-grade units of the transition zone of the Araçuaí and Ribeira orogens (SE Brazil)

Sedimentary basins with epiclastic and volcanoclastic components are important geotectonic components for understanding the complex evolution of orogenic systems. The age, provenance and tectonic setting of ancient basins are, however, often hard to constrain, especially in the high-grade core of ancient mountain belts where original sedimentary and stratigraphic relations have been largely obliterated by deformation and metamorphism. In this case, analytical tools such as whole-rock geochemistry and U–Pb and Lu–Hf analyses on detrital zircon grains can be deployed to unravel the provenance and tectonic setting of high-grade metasedimentary and metavolcaniclastic units. Here, we apply these analytical tools on samples from gneissic paleosomes of migmatites found in the eastern transition zone between the Neoproterozoic–Eopaleozoic Araçuaí and Ribeira orogens (southeastern Brazil). The studied paleosomes comprise peraluminous paragneiss rich in biotite, garnet and sillimanite, metaluminous hornblende-bearing gneiss free of peraluminous silicates, and intermediate terms between these compositions, all of them with geochemical characteristics pointing to sources located in magmatic arc systems. Overall, U–Pb data from detrital zircon grains bracket a maximum sedimentation age for all samples between 600 and 627 Ma, while εHf(t): +5.7 to −18.7 and Hf TDM model ages of 1.0–2.2 Ga suggest moderately juvenile to evolved sources for the gneiss protoliths. However, despite the similarity in age range, the analyzed samples show contrasting isotope data indicating significant variability in the source areas. Sample 7 only has grains with negative εHf (−5.9 to −18.7), most of them ranging in age from 708 Ma to 758 Ma, with very few older (1021–1143 Ma) and younger (642–651 Ma) zircons grains, indicating evolved crustal sources of distinct ages (e.g., the Late Tonian – Early Cryogenian South Bahia Alkaline Province, the Early Cryogenian magmatism found in the West Congo Belt, the Stenian to Early Tonian Espinhaço and Matadi-Noqui igneous rocks, and magmatic rocks of the early Rio Doce arc). Conversely, sample 13 has twenty grains with positive εHf values and ages between 602 and 758 Ma, suggesting sourcing from (meta)igneous rocks of the Rio Negro arc and their remelting products, and forty grains with negative εHf values with ages between 591 and 646 Ma suggesting magmatic sources in the Rio Doce arc. Additionally, the two youngest spot ages (553 Ma and 571 Ma) have highly negative εHf values (−11.1 and −16.4) suggesting their origin from collisional anatexis. Similarly, sample 12 shows detrital grains within the consistent time interval (ca. 596 Ma – 798 Ma) of the Rio Negro arc and Rio Doce arcs, reinforcing that the fragments of these arcs, or zircons that were inherited from these terranes, are potential sources to the sedimentary protoliths of the paragneisses. The regional scenario together with our field and analytical data, compared with data available in the literature, suggest the sedimentary protoliths of the studied samples filled orogenic basins related to the Rio Doce magmatic arc. Most detrital zircon ages are close to the maximum depositional age, a classical signature of orogenic basins related to magmatic arcs. The studied paragneiss unit correlates with the Nova Venécia Complex, located on the intra-arc to back-arc zones of the Araçuaí orogen, and with the orogenic succession of the distal Andrelândia Group, shared by both the Araçuaí and Ribeira orogens. Furthermore, our data also suggest the juvenile Rio Negro - Serra da Prata arc system was either amalgamated with the Rio Doce arc or that fragments of these arcs were incorporated as unmelted xenoliths and xenocrystals within the Rio Doce arc rocks before ca. 614 Ma, providing key hints to unravel the evolution of the Araçuaí - Ribeira orogenic system (AROS) and its role in the Gondwana assembly.

A Bayesian Approach to Inferring Depositional Ages Applied to a Late Tonian Reference Section in Svalbard

Increasing application of high precision uranium-lead (U-Pb) and rhenium-osmium (Re-Os) geochronology to the ancient geological record has resulted in massive improvement in age control and calibration of key Proterozoic stratigraphic successions and events. Nevertheless, some successions and time intervals remain poorly dated. Insufficient age constraints are particularly problematic for successions that are otherwise rich in geochemical, fossil, or other data with high potential to illuminate our understanding of Proterozoic Earth history. The latter Tonian succession in northeastern Svalbard is one such example. The ca. 820–740 Ma Akademikerbreen and lowermost Polarisbreen groups contain important microfossils and well-established carbon- and strontium-isotopic records, but they remain poorly dated. Here we use radioisotopic dates correlated from other Tonian successions across the globe using carbon isotope chemostratigraphy to calibrate a Tonian composite section in Svalbard by integrating Bayesian inference with a simple 1D thermal subsidence model. This approach allows us to assign realistic ages and uncertainties to all stratigraphic heights in a Akademikerbreen-lower Polarisbreen composite reference section. For example, the Bayesian age-height model yields ages for the onset and end of the Bitter Springs negative carbon isotope anomaly of 808.7 +3.3/−3.5 Ma and 801.9 +3.2/−3.3 Ma, respectively, and a total duration of 6.9 ± 0.2 Ma. These age and duration estimates can be applied to calibrate other Tonian successions that capture the Bitter Springs anomaly assuming that this anomaly is globally correlative.

Sr Isotope Chemostratigraphy and Pb–Pb Age of the Riphean Carbonate Deposits of the Kharaulakh Uplift (Northeastern Margin of the Siberian Platform)

Abstract —The least altered limestones of the Ukta and Eselekh formations in the Precambrian section of the Kharaulakh uplift have a minimum 87Sr/86Sr ratio of 0.70673–0.70715. The lowest 87Sr/86Sr ratio of the overlying Neleger and Sietachan formations is 0.70791–0.70817. Based on these data, along with the earlier obtained positive δ13С values (up to 8‰) for the Kharaulakh section, we have estimated the age of the Ukta and Eselekh formations at 800–670 Ma and the age of the Neleger and Sietachan formations at ~640–580 Ma. The Pb–Pb isochron age of the least altered limestones of the Eselekh Formation calculated from eight samples is 720 ± 30 Ma. This age permits us to define the lower part of the Kharaulakh section of the Ukta and Eselekh formations to be the late Tonian of the International Chronostratigraphic Chart or to the Upper Riphean of the General Stratigraphic Scale of Russia. The presence of reliably dated Upper Riphean sediments in the Kharaulakh uplift indicates a more complex structure of the Precambrian sedimentary cover on the Arctic margin of the Siberian Platform than assumed earlier.

Linking South China to North India from the late Tonian to Ediacaran: Constraints from the Cathaysia Block

Neoproterozoic paleogeography of the Cathaysia Block of South China is directly linked to the formation of Rodinia, its break-up, and the subsequent amalgamation of Gondwana. Two diamictite units occur in the Neoproterozoic sedimentary successions on the western margin of Cathaysia Block. In combination with our detrital zircon U-Pb dating results and corrected chemical weathering index data (CIAcorr), these units coincide with the global Sturtian and Marinoan glaciations. Late Tonian to early Ediacaran sedimentary rocks from western Cathaysia contain abundant late Mesoproterozoic to early Neoproterozoic (1150–870 Ma) and some mid-Neoproterozoic (800–720) detrital zircons. Paleocurrent data indicate sediment transport to the northwest. The timing and character of the ages of detrital zircons match the records of North India and East Antarctica, indicating that the Cathaysia Block received detritus from Gondwana. Thus, South China lay adjacent to northern Gondwana since at least 720 Ma, and possibly considerably earlier.

Neoproterozoic geochronology and provenance of the Adelaide Superbasin

The Adelaide Superbasin (Adelaide Rift Complex, Stuart Shelf, Torrens Hinge Zone, Coombalarnie Platform, and Cambrian Stansbury and Arrowie Basins) is a vast sedimentary basin in southern Australia that initiated due to the break-up of central Rodinia and, evolved into the Australian passive margin on edge of the Pacific Basin. Rocks within it contain evidence for the evolving earth system through the Neoproterozoic, including type sections of the Ediacaran fauna, Sturtian and Marinoan glaciations, and the GSSP for the base of the Ediacaran period. Much research over the last century has unravelled the lithostratigraphy and sedimentology of the basin. Despite this, the rocks are poorly dated, and their sedimentary provenance and link with tectonic geography is poorly known. This poor chronology hampers global and local efforts to gain a detailed understanding and chronological framework of the interplay between tectonics and momentous changes to the earth system during this time. This paper presents a comprehensive database of detrital zircon geochronology and review of geochronology for the Neoproterozoic of the Adelaide Superbasin, highlighting the stratigraphic, and spatial locations of available data.In the north of the basin, zircons were sourced locally in the initial stages of rifting, ca. 830 Ma—from the adjacent Gawler Craton and Curnamona Province. During the late Tonian, detritus was transported along graben from the north-west, from the Musgrave Orogen, as the rift basin developed during the opening of the nascent Pacific Ocean. Cryogenian icesheets punctuate the detrital record with an ephemeral return to more localised rift shoulder sources. In the Ediacaran, there is an increasing influence of younger (<740 Ma) detrital zircon from an enigmatic source that we interpret to be from southern (i.e. Antarctic) sources, with a corresponding shift in the late Mesoproterozoic age peaks, from ca. 1180 Ma to ca. 1090 Ma, and corresponding decrease in older, ca. 1600 Ma, detritus. These changes in sediment source reflect the changing tectonic geography and large-scale environmental influence of the Cryogenian glaciations as the basin evolved from a local rift, to a larger rift basin and finally to a continental margin, with sedimentary input becoming increasingly restricted over time.

Carbon isotopes in clastic rocks and the Neoproterozoic carbon cycle

It has been proposed that isotopically light inorganic carbon precipitated diagenetically in clastic sediments can explain the large carbon isotopic excursions recorded in Neoproterozoic carbonates. To date, however, the data needed to test this hypothesis have been limited. Here we report the analysis of ca. 540 clastic sedimentary rocks, including shales, siltstones, sandstones and tillites, that span the second half of the Neoproterozoic Era. A diagenetic carbon isotopic overprint does indeed occur in many of the samples; however, when we include our analyses in a carbon isotope mass balance model, they produce only a small effect on mass balance model results. Thus, clastic sedimentary rocks were not a major sink for 13C-depleted carbonate during the Neoproterozoic Era. These results do, however, produce a more accurate carbon mass balance, pointing to a high proportion of total organic carbon burial, compared to total carbon burial, during the late Tonian, Cryogenian, and late Ediacaran Periods. This result suggests a vigorous release of oxygen to the atmosphere. The clastic carbonate record also offers a chemostratigraphic tool. For example, we observe an isotope trend in clastic-hosted carbonates of the Isaac Formation, Windermere Supergroup, that strongly resembles the Shuram-Wonoka isotope anomaly, allowing us to place this previously undated section in a temporal context. We also find isotope trends in the fossiliferous and radiometrically well-dated sedimentary rocks of the Avalon Peninsula, Newfoundland, that may also reflect the Shuram-Wonoka anomaly. If correct, this constrains the timing of the Shuram event, suggesting that it began after 571 Ma and ended before 562 Ma, with the most extreme isotopic values lying well within those bounds.

Neoproterozoic geodynamic evolution of easternmost Kalahari: Constraints from U-Pb-Hf-O zircon, Sm-Nd isotope and geochemical data from the Schirmacher Oasis, East Antarctica

Late Tonian (ca. 785–760 Ma) granodioritic to granitic orthogneisses of the Schirmacher Oasis region in Dronning Maud Land (DML), East Antarctica, are interpreted as recording an active continental margin setting at the periphery of Kalahari and Rodinia. The rocks probably represent exposures of a significant tectonic province hidden beneath the ice, the erosional remnants of which are recorded as detrital zircons in late Tonian-Cryogenian metasedimentary rocks throughout central and eastern DML, as well as in ice-rafted debris from recent sediments offshore Dronning Maud Land. The orthogneisses have single-stage Sm-Nd model ages of ca. 1.3–1.5 Ga and zircon Hf-signatures (εHft = +2 – +5), indistinguishable from the adjacent Grenville-age basement rocks of easternmost Kalahari. Their geochemistry suggests that they evolved in the late stages of a continental margin magmatic arc and possibly within a roll-back tectonic framework, suggestive of subduction of relatively old oceanic lithosphere. The eastern Kalahari continental arc is one of a number of continental arcs that characterize the western part of the fragmenting Rodinia and document the supercontinent “turning inside out” after its formation at ca. 1000 Ma and a period of relative tectonic quiescence between ca. 900 and 800 Ma.The rocks show an ultra-high temperature metamorphic overprint that was accompanied by syn-tectonic magmatism from ca. 650 to 600 Ma. The high temperature metamorphism is interpreted to relate to back-arc extension that also led to major anorthosite magmatism elsewhere, prior to continental collision in the region. The rocks lack the subsequent widespread high-grade metamorphic overprint at ca. 590–500 Ma which occurs in the adjacent regions due to Himalayan-style continental collision along the East African-Antarctic Orogen during Gondwana assembly.

Stratigraphy and isotope geochemistry of the pre-Sturtian Ugab Subgroup, Otavi/Swakop Group, northwestern Namibia

The Islay carbon isotope anomaly is a late Tonian negative δ13C excursion that has been documented in marine carbonates globally. It has in the past been thought to be causally linked to the onset of Cryogenian glaciation ca. 717 Ma. However, recent work suggests that this anomaly is instead closer to ca. 739–735 Ma and that it may be one of two distinct late Tonian carbon isotope anomalies. Here we present the litho- and chemo-stratigraphy of the late Tonian Ugab Subgroup of the Otavi/Swakop Group in northwestern Namibia. Carbon isotope data from two separate outcrop belts, the Summas Mountains and the Vrede Domes, indicate that the Ugab Subgroup strata exposed in the former inlier are older than those of the latter, with only approximately 100 m of stratigraphic overlap. We use detailed measured sections to develop a sequence stratigraphic framework for the Ugab Subgroup in each outcrop belt, and correlate strata across the two exposures to construct a composite carbonate δ13C record for the late Tonian in Namibia. This carbon isotope profile of the Ugab Subgroup shows two separate pre-Sturtian negative anomalies, consistent with findings in northern Canada, Svalbard, Scotland, and Ethiopia. Our results are an important contribution to the pre-Cryogenian carbon isotope record, as these data may serve as the first definitive documentation of both anomalies within a single basin.