Early Tonian Research Articles

Anorogenic plutonism in the West Congo Belt of the Democratic Republic of Congo further supports early Tonian continental rifting in the Congo – São Francisco palaeocontinent

Our paper presents an updated geological map and simplified lithostratigraphy of the West Congo Belt (WCB) in the Democratic Republic of Congo. The WCB holds a key position for understanding the construction of Gondwana and constitutes a larger orogenic system along with its Brazilian counterpart (i.e. Araçuaí orogen). Our current contribution focuses on a newly found and mapped early Tonian small-sized (6 × 4 km) plutonic body (so-called “Shinkakasa intrusive complex”) located in the westernmost amphibolite facies tectono-metamorphic domain of the West Congo fold-and-thrust belt. The complex is hosted by the Kimeza gneissic basement. Our fieldwork, samples and a new geological map provide a solid and updated geological background for more in-depth future work and proves essential for reinterpreting the broader tectonic framework. Our findings show that the Shinkakasa complex comprises felsic, intermediate and mafic to ultramafic rocks displaying mixing-mingling features and well-preserved igneous textures with only limited Panafrican metamorphic and tectonic overprint. According to previously published zircon U–Pb (SHRIMP) data, the Shinkakasa magmatism lasted from 911 Ma to 869 Ma and the complex was hosted by the 2069 Ma Kimeza gneissic basement.We compare the Shinkakasa complex with other small-sized intrusive complexes of the WCB, emplaced in a similar geological setting, such as the contemporaneous Mayumba bimodal complex of southwestern Gabon. This comparison is extended across the Atlantic Ocean to the recently studied early Tonian Salto da Divisa complex of southeastern Brazil. In the African counterpart we correlate the plutonic complexes to the early Tonian eastward arcuate (sub)volcanic bimodal successions (Seke-Banza Group of the DRC) largely exposed along trend of the orogen over a length of ca. 550 km in the greenschist facies domain of the WCB. Finally, we give an updated account of the early Tonian emplacement ages and lithostratigraphic terminology of the multiple bodies constituting this arcuate structure. Our data show that i) the vast majority of the anorogenic magmatism testifying for the aborted early Tonian continental rifting of the Congo - Sao Francisco palaeocontinent is exposed in the African counterpart, and ii) the extent of the (sub)volcanic successions largely predominates compared to the small-sized intrusive bodies such as the Shinkakasa complex.

Molar-Tooth Carbonate: An excellent proxy for reconstructing lithium isotopic composition of Precambrian seawater

Shallow water carbonates are extensively utilized as archives for reconstructing past seawater chemistry and understanding global geochemical cycles. However, their reliability is often compromised by diagenetic alteration, obscuring primary environmental signals. Molar-tooth carbonate (MTC) exhibits exceptional resistance to diagenesis compared to other carbonate minerals, making it a promising candidate for reliably reconstructing ancient seawater chemistry. This study investigates the preservation of primary lithium isotopic composition in Neoproterozoic (Tonian) MTC and its host carbonate from the Dalian area, northeastern China (ca. 980 Ma – 920 Ma). We assess the reliability of MTC in reconstructing paleo-seawater δ7Li evolution and provide a reconstruction of Tonian seawater δ7Li.Geochemical filter tests indicate that both MTC and host carbonate remain largely unaltered by meteoric and burial diagenesis. Carbonate δ7Li values exhibit a consistent decrease with stratigraphic height, with MTC displaying a narrower range of variation (4–12‰) compared to its host carbonate (1–15‰). Numerical modeling of early marine carbonate diagenesis suggests minimal impact on MTC δ7Li values, while host carbonates show varying degrees of diagenetic alteration. Based on the consistent Li isotopic offset observed between ‘fluid-buffered’ diagenetic host carbonate and MTC in our data, we deduce a fractionation factor of 4.8 ± 1.2‰ for seawater-MTC Li isotopic exchange.Reconstructed paleoseawater δ7Li values reveal a progressive decrease from 16.9 ± 1.2‰ to 8.8 ± 1.2‰ during the early Tonian period. This change is likely linked to the assembly of the Rodinia supercontinent and emplacement of large igneous province (LIP). Furthermore, mass balance modeling and Monte Carlo simulations suggest that this decline in seawater δ7Li can be attributed to the transition towards more congruent weathering during the early Tonian, facilitated by the assembly of Rodinia and associated large igneous province events.Overall, our findings underscore the potential of MTC as a reliable proxy for elucidating Precambrian seawater Li isotopic composition and understanding associated paleoenvironmental and paleoclimatic changes.

Records of the accretionary, collisional and post-collisional evolution of western Gondwana in the high-grade core of the Araçuaí-Ribeira orogenic system, SE Brazil

High-grade metamorphism, extensive partial melting, and polyphase deformation often obliterate records of pre-collisional stages in ancient orogens, making it hard to unravel Precambrian accretionary events. In western Gondwana, the high-grade core of the Araçuaí-Ribeira Orogenic System (AROS) comprises pre-collisional rock-assemblages largely disrupted, overprinted, and remelted by collisional and post-collisional tectono-metamorphic and igneous events, hampering reconstructions of its accretionary history. Detailed studies reveled remnants of an Early Tonian (860–840 Ma) island arc, forming km-size roof-pendants and mega-xenoliths within the composite Caxixe batholith, in the boundary zone between the Araçuaí and Ribeira orogens. We further characterize the Caxixe batholith and surrounding plutons, showing that the main stage of magmatism is composed of pre-collisional orthogneisses, partially migmatized and crystallized between ca. 607–580 Ma, thus, much later than the Tonian island arc records. Those Ediacaran granitic rocks represent intrusions of calc-alkaline magmas with elemental geochemistry and isotopic (εHf(t): −6 to −10, whole-rock εNd(t): −4.9 to −9.5, and 87Sr/86Sr(i): 0.706–0.711) data correlated with the continental-margin Rio Doce arc. This contrasts with signatures indicative of a mantle source εHf(t) = +10 to + 14, εNd(t) = +0.9 to + 6.4, and 87Sr/86Sr(i) = 0.698–0.704) of the Early Tonian arc remnants. Intruding the pre-collisional granitic plutons, garnet-bearing leucogranites dated at ca. 575 Ma and a mafic intrusion of ca. 560 Ma mark the transition from the syn-collisional to late-collisional stages. Post-collisional, non-deformed, A-type granites with the most evolved isotopic signatures (εHf(t): −22; εNd(t): −5.6 to −13.2) intruded at ca. 500 Ma, ending the long-lasting orogenic history documented in the region. Our results demonstrate that a basement composed of an early Tonian juvenile arc docked with a continental margin and hosted several intrusions of an early Ediacaran continental-margin magmatic arc that, in turn, were intruded by collisional and post-collisional magmas from the Late Ediacaran to Cambrian, finally forming the main plutonic masses of the Araçuaí-Ribeira Orogenic System (AROS) core, during the complex history of western Gondwana assembly.

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.

Nitrate limitation in early Neoproterozoic oceans delayed the ecological rise of eukaryotes

The early Neoproterozoic Era witnessed the initial ecological rise of eukaryotes at ca. 800 Ma. To assess whether nitrate availability played an important role in this evolutionary event, we measured nitrogen isotope compositions (δ15N) of marine carbonates from the early Tonian (ca. 1000 Ma to ca. 800 Ma) Huaibei Group in North China. The data reported here fill a critical gap in the δ15N record and indicate nitrate limitation in early Neoproterozoic oceans. A compilation of Proterozoic sedimentary δ15N data reveals a stepwise increase in δ15N values at ~800 Ma. Box model simulations indicate that this stepwise increase likely represents a~50% increase in marine nitrate availability. Limited nitrate availability in early Neoproterozoic oceans may have delayed the ecological rise of eukaryotes until ~800 Ma when increased nitrate supply, together with other environmental and ecological factors, may have contributed to the transition from prokaryote-dominant to eukaryote-dominant marine ecosystems.

Stenian arc-magmatism and early Tonian metamorphism and anatexis along the northern border of Amazonia during the Rodinia assembly: The Pochotepec suite in southern Mexico

The eastern and southern Precambrian basement of Mexico is composed of crustal blocks linked to northern Amazonia before and during the assembly of the Rodinia supercontinent. In this paper, we examine an extensive basement block known as the Sierra de Juárez Complex (SJC) in southern Mexico by integrating fieldwork, in-situ zircon U–Pb geochronology by LA-ICPMS and whole-rock geochemistry by X-ray fluorescence. The results provide evidence for a continuous low-to high-grade metamorphic succession here re-defined as the Pochotepec suite (PS), which comprises an upper lithodeme with volcanosedimentary precursors and a lower lithodeme that includes mafic to intermediate orthogneisses and migmatites. The analytical results suggest that the PS protoliths originated in a continental-arc tectonic setting ca. 1080-1050 Ma along the northern edge of Amazonia before the consolidation of Rodinia. In addition, xenocrystic zircon U–Pb analyses indicate the presence of a recycled older crust of ca. 1200–1300 Ma. Finally, U–Pb data of recrystallized zircon rims and younger metamorphic zircon grains recorded the timing of metamorphism and anatexis of the PS during the early Tonian (954-933 Ma). The crystallization ages of two trondhjemitic leucosomes of 954 and 933 Ma strongly support the migmatization age. The Tonian metamorphic/anatectic episode of the PS differs in grade and age from the typical metamorphism related to the amalgamation of Rodinia reported in Mexico and Colombia (granulite-facies at ca. 990 Ma). Therefore, we interpret this event as produced by either post-orogenic crustal collapse or contractive tectonics.

Discovery of Early Tonian Calc‐alkaline and Shoshonitic Metamafic Rocks from the North Purulia Shear Zone, Chhotanagpur Gneissic Complex, Eastern India: Implications of Proterozoic Sub‐continental Lithospheric Mantle

AbstractReports of shoshonitic rocks in Precambrian terrains are relatively rare. Pl‐Grt amphibolites and Hbl‐Bt mafic granulites occurring in the migmatitic gneisses of the Chhotanagpur Gneissic Complex (CGC) show calc‐alkaline and shoshonitic characteristics. Relict porphyritic, sub‐ophitic and poikilitic textures are noted in these rocks. Their parent magma was emplaced during the waning phase of the regional metamorphism. Geochemically, these metamafics are similar to the Group III potassic and ultrapotassic rocks of Foley et al. (1987). The magma was derived from the metasomatized subcontinental lithospheric mantle (SCLM). Subduction‐related sediment melts metasomatized the SCLM. Compositionally, the SCLM is a metasomatized phlogopite‐amphibole‐spinel‐bearing harzburgite. 1%–5% batch melting of the SCLM could produce the parental magma of the mafic granulites. Pressures and temperatures of metamorphic equilibration were carried out by pseudosection modeling. Peak metamorphic assemblage (M1: Grt‐Cpx‐Pl‐Qz) in garnetiferous amphibolite equilibrated at 740°C and 8.7 kbar. The Cpx‐Pl corona appeared around the garnet during decompression (M2: 655°C, 6 kbar). The Hbl‐Pl symplectites around garnet formed during isobaric cooling (M3: 580°C and 5.9 kbar). The emplacement of shoshonitic magma and subsequent decompression happened at the slab break‐off stage of continental collision (∼990 Ma).

Meso- to Neoproterozoic terrane accretion: Insights from juvenile mafic magmatism from the Votuverava Group and Embu Complex, southern Ribeira Belt, Brazil

Terranes of the Neoproterozoic Ribeira Belt record processes related to West Gondwana assembly. However, the evolutionary history of the belt in the preceding Rodinia supercontinent cycle and its role in the transition to West Gondwana assembly is poorly known. New geochemical and geochronological data of mafic magmatism from Meso- to early Neoproterozoic rock units (1500–900 Ma) integrated with published data provides new constraints to unravel the tectonic setting of the Ribeira Belt. Geochemical and isotopic data from the Votuverava Group and Embu Complex from the southern Ribeira Belt record three distinct periods of juvenile magmatism. Mafic rocks from the Votuverava Group define the oldest magmatism as Calymmian (1490–1475 Ma) and Ectasian (1300–1260 Ma). Both age groups display Th/Yb ratios greater than N-MORB, low Ta/Yb and Nb/La ratios, low Nb and Zr concentrations, negative Nb anomaly, whole-rock ɛNd(T) from +2.25 to +4.01, an Nd two-stage TDM model age spanning 1629–1471 Ma, and zircon εHfT mainly between +5 and +10, characteristics of juvenile mantle source arc-related magmatism. The third event occurred during early Tonian (900 Ma) and is recorded in the mafic rocks from the Embu Complex. Geochemical data indicate that these mafic rocks belong to the tholeiitic series with low LREE enrichment, negative Nb and Ta anomalies, low Nb and Zr contents and low Nb/La and Zr/Y ratios, consistent with arc-related basalts. Whole-rock Sm–Nd isotopic data yielded ɛNd(T) of +0.72 and Nd two-stage TDM model age of 1921 Ma, with zircon Hf isotopes indicating juvenile mantle sources to the mafic magmatism with ɛHfT between +4.5 and +17.2. Thus, mafic rocks from the southern Ribeira Belt record multiple periods of accretion and convergence settings associated with assembly of Rodinia and Gondwana.

Magmatic diversity in continental rifts: A case study on the Early Tonian, plutono-volcanic Salto da Divisa Complex, Araçuaí Orogen, Eastern Brazil

Exposures from plutonic roots to volcanic roofs in deeply eroded regions may disclose the architecture and igneous processes in ancient metamagmatic edifices, like it is the Early Tonian Salto da Divisa complex (SADICO) of Eastern Brazil. Field, petrographic, lithochemical and isotopic (in-zircon U-Pb and Lu-Hf, and whole-rock Sm-Nd) studies were conducted on the SADICO, a plutono-volcanic complex located in the northeast Araçuaí orogen. The SADICO magmatic record comprises (with U-Pb crystallization ages, and isotopic Hf and Nd data): i) pyroxenite (ε Nd(t) : +2.2 to −5.3; Nd T DM : 1.2–1.7 Ga); ii) mafic and intermediate dykes (ε Nd(t) :+2.1 to −5.9; Nd T DM : 1.2–1.8 Ga) and enclaves with OIB-like signature; ii) ferroan, A-type granitoid with mafic-felsic mingling-mixing features (885 ± 9 Ma; ε Hf(t) : −5 to −7, Hf T DM : 2.0–2.1 Ga; ε Nd(t) : −3.2 to −4.3, Nd T DM : 1.5–1.7 Ga); iii) fluorite-bearing, metaluminous to peraluminous, ferroan A-type granites, including biotite-amphibole granite (ca. 915 to 875 Ma; ε Nd(t) : −2.8 to −5.8, Nd T DM : 1.6–1.9 Ga), biotite granite (894 ± 10, ε Hf(t) : −4 to −11, Hf T DM : 1.9–2.3 Ga; ε Nd(t) : −1.6 to −8.0, Nd T DM : 1.4–2.2 Ga), and amazonite-bearing two-mica granite (ε Nd(t) : −4.0; Nd T DM : 1.7 Ga); iv) rhyolite (905 ± 24 Ma; ε Hf(t) : −1 to −8.7, Hf T DM : 1.8–2.2 Ga; ε Nd(t) : +3.1, Nd T DM : 1.1 Ga) and subvolcanic silica-undersaturated trachyte (912 ± 13 Ma, ε Hf(t) : −14 to −18, Hf T DM : 2.5–2.7 Ga; ε Nd(t) : +0.3, Nd T DM : 1.2 Ga), also with ferroan A-type signature. Our integrated petrogenetic model envisages mantle-derived magmas (ultramafic and mafic rocks) evolved by assimilation-fractional crystallization to felsic subvolcanic and volcanic rocks, interacting with granitic magmas produced by crustal anatexis (biotite-amphibole granite) and subsequently fractionated (biotite granite) and highly fractionated (two-mica granite) with the involvement of F-rich fluids up to subvolcanic levels. • Dissecting an Early Tonian, anorogenic, plutono-volcanic complex of Eastern Brazil. • Rift's igneous diversity: ultramafic to mafic rocks, granites, felsic volcanic rocks. • From plutonic root to volcanic roof of an ancient anorogenic igneous complex. • Field, lithochemical and isotopic data tracking rift-related igneous processes. • Integrated model for Early Tonian anorogenic igneous processes in the Araçuaí orogen.

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.

Neoproterozoic pre-collisional events recorded in the Sergipano belt, Southern Borborema Province, West Gondwana

ABSTRACT The Canindé Domain is crucial for understanding the pre-collisional events in Sergipano Belt, Borborema Province, especially during the Late Stenian/Early Tonian (740–680 Ma). We present thermobarometric, geochronology, and isotopic studies of the metavolcanosedimentary rocks of the Novo Gosto unit. The calculated metamorphic conditions resulted in a range of T: 600–740°C and P: 3.5–10.0 kbar for non-mylonitic amphibolites and values of T: 636–808°C and P: 7.0–12.0 kbar for mylonites, both of amphibolite facies. The minimum metamorphic conditions imprint in these rocks are T: 390°C and P: 2.5 kbar, indicating retrometamorphism in greenschist facies. The probable age of the peak metamorphic event was obtained in zircon rims from a quartzite at 692 ± 8.5 Ma. U-Pb detrital zircon and Nd model ages of the metasedimentary rocks point out Early Tonian as primary source rocks. Subordinate contributions of Mesoproterozoic, Paleoproterozoic, and Neoarchean sources were also recognized. These primary sources are related to Cariris Velhos event rocks, whereas the subordinate sources are from the Pernambuco-Alagoas superterrane. Early Tonian and Late Stenian ages were recognized for the first time in the Canindé Domain, with U-Pb zircon ages of 1005 ± 3 and 989 ± 6 Ma in amphibolites and mylonitized granites, mostly outcropping at the southwestern limit of the area. New ages were also obtained in the Novo Gosto surrounding intrusions: Canindé Layered Gabbroic Intrusion (~718 Ma), Garrote (~715 Ma) and Curralinho/Boa Esperança (~708 Ma) metagranites, and Gentileza metavolcanic rock (~700), which integrate the early pre-collisional magmatism of the Sergipano Belt (~740-680 Ma). Finally, U-Pb zircon ages were obtained in the Poço Redondo Domain, potential source area of the Novo Gosto metasedimentary rocks, resulting in ages of 957 ± 11 and 988 ± 15 Ma. The new data bring perspective to future correlations with adjacent belts and the understanding of the structuration of West Gondwana during the Brasiliano-Pan-African Orogeny.

Early Diagenetic Dolomite as a Potential Archive of Paleo‐redox Fluctuations in an Early Tonian Marine Basin?

AbstractThe Tonian Period witnessed important environmental changes and critical evolutionary innovations. Published iron speciation data suggest a global redox transition of mid‐depth seawaters from euxinic to ferruginous in early Tonian, but details of this transition remain unknown. This study explores Tonian stromatolitic carbonates as a possible archive of paleoenvironmental changes, through the investigation of dolomitic limestones and dolostones associated with stromatolites of the Weiji Formation in the Huaibei region of North China. Three types of dolomitization are recognized on the basis of petrographic and geochemical data. Type I and II dolomitization resulted in dolomitic limestones characterized by LREE depletions, MREE enrichments, positive yttrium anomalies, and a lack of europium anomalies, indicating early diagenetic dolomitization, possibly in the iron reduction zone and under the influence of bottom seawater. The lack of cerium anomalies in these carbonates suggests anoxia in shallow marine environments. The coexistence of ferroan/non‐ferroan dolomite crystals and overgrowth bands is interpreted as possible evidence for rapid fluctuations between iron‐rich and iron‐depleted conditions in pore‐waters or seawaters. In contrast, type III dolomitization resulted in pervasively dolomitized stromatolitic carbonates and likely represents late diagenetic processes. This study highlights the potential of early diagenetic dolomite as an archive for paleoenvironmental investigations.

New multicellular marine macroalgae from the early Tonian of northwestern Canada

AbstractMolecular phylogenetic data suggest that photosynthetic eukaryotes first evolved in freshwater environments in the early Proterozoic and diversified into marine environments by the Tonian Period, but early algal evolution is poorly reflected in the fossil record. Here, we report newly discovered, millimeter- to centimeter-scale macrofossils from outer-shelf marine facies of the ca. 950–900 Ma (Re-Os minimum age constraint = 898 ± 68 Ma) Dolores Creek Formation in the Wernecke Mountains, northwestern Canada. These fossils, variably preserved by iron oxides and clay minerals, represent two size classes. The larger forms feature unbranching thalli with uniform cells, differentiated cell walls, longitudinal striations, and probable holdfasts, whereas the smaller specimens display branching but no other diagnostic features. While the smaller population remains unresolved phylogenetically and may represent cyanobacteria, we interpret the larger fossils as multicellular eukaryotic macroalgae with a plausible green algal affinity based on their large size and presence of rib-like wall ornamentation. Considered as such, the latter are among the few green algae and some of the largest macroscopic eukaryotes yet recognized in the early Neoproterozoic. Together with other Tonian fossils, the Dolores Creek fossils indicate that eukaryotic algae, including green algae, colonized marine environments by the early Neoproterozoic Era.

Eukaryotic red and green algae populated the tropical ocean 1400 million years ago

Both molecular clocks and the fossil record indicate that eukaryotic algae evolved by the late Paleoproterozoic Era (2500 to 1600 million years ago, Ma). However, the biomarker record of steranes does not reveal their presence until the early Tonian Period (1000 to 720 Ma), suggesting that eukaryotic algae were insignificant ecosystem members for some one billion years after they evolved. Here, we conducted high-temperature pyrolysis and fixed-bed catalytic hydropyrolysis of kerogen from the ca. 1400 Ma Xiamaling Formation, liberating a host of lipid molecules, including C27 to C29 steranes. With sterane to hopane ratios ranging up to 0.17, our results indicate the presence of significant populations of both red and green algae at 1400 Ma, some 600 million years earlier than previously recognized. These algae actively contributed to primary production in what may have been a nitrogen-starved marine basin.

870-850 Ma-old magmatic event in eastern Borborema Province, NE Brazil: Another Tonian failed attempt to break up the São Francisco Paleoplate?

The Central and Southern subprovinces of the Borborema Province, northeastern Brazil, were the locus of an important early Tonian (c. 1000-920 Ma) magmatic event called Cariris Velhos. Here we report results of a geochronological study conducted in eastern Borborema showing that another magmatic event took place c. 50 Ma after this event had ceased. We dated three small orthogneiss units: one in the Rio Capibaribe Domain (RCD, Central Subprovince) and two in eastern Pernambuco-Alagoas Domain (PEALD, Southern Subprovince). We obtained LA-ICP-MS U–Pb zircon ages of 863 ± 7 Ma (RCD), and 851 ± 8 Ma and 869 ± 4 Ma (PEALD), which are interpreted as crystallization ages. The results indicate that the RCD and PEALD were part of the same crustal block at c. 870-850 Ma and thus that the East Pernambuco shear zone, which separates them, is not a suture. Rocks with ages corresponding to the Cariris Velhos event and those reported in this study also occur in the São Francisco Craton. These similarities suggest that the Borborema Province and the São Francisco Craton were part of the same plate, which was subject to several extensional episodes before the convergence that led to the Brasiliano Orogeny.

Tonian island arc remnants in the northern Ribeira orogen of Western Gondwana: The Caxixe batholith (Espírito Santo, SE Brazil)

The Tonian Serra da Prata magmatic arc is part of an association of several magmatic arcs of the Ribeira orogenic system in Southeastern Brazil. Along with the Cryogenian Rio Negro arc, the Serra da Prata arc represents an intra-oceanic arc system while the Ediacaran Rio Doce arc represents an active continental margin arc. Here, we characterize the northernmost segment of the Serra da Prata arc, based on lithochemical and isotopic (whole-rock Sm-Nd and Rb-Sr, and zircon U-Pb and Lu-Hf) data from the Caxixe batholith, located in Espírito Santo State. The Caxixe batholith mostly consists of granodioritic to granitic and minor tonalitic to gabbroic rocks, with calc-alkaline, metaluminous to slightly peraluminous, magnesian, I-type signature and magmatic arc affinity. The rocks display LREE enrichment, positive to slightly negative Eu and negative Nb-Ta anomalies. Zircon U-Pb (LA-ICP-MS and SHRIMP) data reveal magmatic crystallization ages from 859 ± 7 Ma to 847 ± 8 Ma (for zircon cores) and a probably metamorphic age of 834 ± 9 Ma (for zircon rims). Hf Isotopic data yield positive εHf(t) values between +10 and +14, and TDM Hf model ages from 1.01 Ga to 0.84 Ga, in agreement with whole-rockNd and Sr data with εNd(t) values from +0.9 to +6.4 and TDM Nd model ages between 1.2 Ga and 0.8 Ga, and 87Sr/86Sr(i) of 0.6979–0.7035. Our data characterize a juvenile magmatic arc with striking mantle contribution, formed on a supra-subduction intra-oceanic setting similar to modern island arcs. As a whole, the Serra da Prata juvenile arc points to a large ocean that underwent intra-oceanic subduction in the Early Tonian. As a corollary, the Angola, Congo – São Francisco and Paranapanema paleocontinental blocks should be far from each other during the Early Tonian, separated by that large ocean. In a broad scenario, involving ophiolites and juvenile arcs of southern and central Brazil, several cratons of Western Gondwana once had been parts of paleocontinental pieces separated by very extensive Tonian-Cryogenian oceanic realms, including the Neoproterozoic Goiás–Pharusian and Adamastor oceans.

Evidence for Early Tonian (Ca. 1000-940 Ma) continental rifting in southern Borborema Province (NE Brazil): tectonic inheritance and shear zone nucleation during assembly of West Gondwana

ABSTRACT The factors controlling the inception of continental-scale transcurrent shear zones are still under debate. In order to a better understanding of the origin and evolution of the 400 km long West Pernambuco shear zone (WPSZ), we undertook a structural, geochronological and geochemical study in an area around its eastern termination. The WPSZ is one of the most expressive strike-slip shear zones in the Borborema Province of NE Brazil, which resulted from orogenic processes associated with the assembly of western Gondwana in the Late Neoproterozoic. Previous studies have shown that 1000–940 Ma-old granitic orthogneisses are located immediately to the south of the western and central portions of the WPSZ. Based on LA-ICP-MS U-Pb zircon dating, we show that rocks with similar ages (995–956 Ma) also occur to the East. The geochemical characteristics of this magmatism and the alignment of the bodies along a narrow, less than 40 km-wide belt point to emplacement in a continental rift-related setting. The WPSZ thus follows the trend of a pre-existing rheological discontinuity, which induced strain localization during the post-collisional stage of the Brasiliano-Pan-African orogeny.

Evolution of geodynamics since the Archean: Significant change at the dawn of the Phanerozoic

Abstract A time-series analysis of thermobaric ratios (temperature/pressure [T/P]) for Paleoarchean to Cenozoic metamorphic rocks identified significant shifts in mean T/P that may be related to secular change in the geodynamics on Earth. Thermobaric ratios showed significant (>95% confidence) change points at 1910, 902, 540, and 515 Ma, recording drops in mean T/P, and at 1830, 604, and 525 Ma, recording rises in mean T/P. Highest mean T/P occurred during the Mesoproterozoic, and lowest mean T/P occurred from the Cambrian to the Oligocene. Correlated changes were seen between T/P and global data sets of time-constrained hafnium (Hf) and oxygen (O) isotope compositions in zircon. The range of correlated variation in T/P, Hf, and O was larger during the formation of Rodinia than Columbia. Large changes and a wide range for these variables continued through the Phanerozoic, during which a statistically significant 83 m.y. frequency of T/P excursions recorded the high tempo of orogenic activity associated with the separation, migration, and accretion of continental terranes during the formation of Pangea. Since the early Tonian, the decreasing mean T/P of metamorphism, widespread appearance of blueschist and ultrahigh-pressure metamorphism, and wide fluctuations in Hf and O isotope compositions document a change to the modern plate-tectonic regime, characterized by widespread continental subduction and deeper slab breakoff than in the Proterozoic.

Paleoenvironment, sediment provenance and tectonic setting of Tonian basal deposits of the Macaúbas basin system, Araçuaí orogen, southeast Brazil

The Neoproterozoic Macaúbas Group represents the main precursor basin system of the Araçuaí orogen, including a Tonian volcano-sedimentary succession, at the base, overlain by a Cryogenian sequence. Belonging to the Tonian succession, the Matão – Duas Barras Formation, exposed at the southern border of the Porteirinha basement block in the proximal (western) Araçuaí orogen, represents the very first stage of rifting ever recorded in the Macaúbas basin system. Sedimentological analysis on that formation has revealed two distinct and consecutive facies associations, both indicating half-graben sedimentary filling: i) fault-sourced alluvial fans; and ii) fluvial deposits associated with braided rivers. U-Pb ages of detrital zircon grains from the Matão – Duas Barras Formation and its underlain unit (ascribed to the Espinhaço Supergroup) uniformly show a dominant Rhyacian to Orosirian peak at ca. 2182 Ma – 1863 Ma, as well as several minor Archean age peaks from ca. 3186 Ma to ca. 2655 Ma. The Matão – Duas Barras fluvial deposits also show two Mesoproterozoic age clusters at ca. 1520 Ma and ca. 1156 Ma. Together with sedimentological features, the Archean and Paleoproterozoic zircon ages indicate sediment provenance from the granite-gneiss sources located in the neighboring Porteirinha block, which would have acted as a basement high (a large horst) at least during the initial filling of the Early Tonian Macaúbas rift. The ca. 1520 Ma and ca. 1156 Ma peaks are in good agreement with the Calymmian and Stenian development stages (II and III) of the Espinhaço Supergroup, which acted as an important secondary sediment sources for the basal Macaúbas Group. This also suggests that the Mesoproterozoic sources were exposed by the Porteirinha block uplift coeval with the Matão – Duas Barras basin filling. The maximum sedimentary ages given by the youngest zircon grains, together with a thorough literature compilation, bracket a depositional interval for the basal Macaúbas deposits from 1000 Ma to 933 Ma. Changes in sedimentary style and geochronological spectra suggest variations in the tectonic activity and subsidence rates of the Early Tonian Macaúbas basin, marking its first stage as an entirely siliciclastic sedimentation.

An Early Tonian rifting event affecting the São Francisco-Congo paleocontinent recorded by the Lower Macaúbas Group, Araçuaí Orogen, SE Brazil

After it had been assembled in the very early Orosirian, the western São Francisco–Congo Paleocontinent experienced several rifting events since the Statherian (ca. 1750 Ma) to Cryogenian (ca. 700 Ma). Records of anorogenic magmatism and/or associated sedimentation from those events have been found in the Neoproterozoic Araçuaí–West Congo orogenic system (AWCO), located between the São Francisco (eastern Brazil) and Congo (central Africa) cratons. Based on detailed field studies and data from lithochemistry, zircon and titanite U-Pb dating, whole-rock Nd and Hf-in-zircon isotopic analyses, we characterize a previously poorly described Early Tonian, rift-related, volcano-sedimentary succession in order to decipher the evolution of the AWCO precursor basins. That volcano-sedimentary succession, found in the Capelinha Formation type-area, now assigned to the Lower Macaúbas Group, includes quartzites with lenses of ortho-amphibolite (metabasalt) covered by pelitic schists. Zircon grains from ortho-amphibolite samples yielded ages of 957 ± 14 Ma and 576 ± 13 Ma, constraining their magmatic crystallization and regional metamorphism, respectively. These mafic rock show ƐNd(t) from −3.64 to +0.21 and Nd TDM ages from ca. 1.4 to ca. 1.7 Ga. Positive covariation of FeOtot/MgO + FeOtot, TiO2, P2O5, V and Zr, enrichment in light rare earth elements, slightly positive Eu/Eu* anomaly and depletion of high field-strenght elements, suggest tholeiitic protolith related to a continental rifting setting. The metasedimentary rocks show broad spectra of detrital zircon ages from the Early Tonian (ca. 940 Ma) to Paleoarchean, with wide-ranging ƐHf values from predominantly negative (as low as −10.76) to positive (+9.94), evoking well-known sediment sources in the São Francisco–Congo Paleocontinent. The youngest age peak (949 ± 12 Ma) constrains a maximum sedimentation age coeval with the basaltic volcanism represented by the ortho-amphibolite. The Capelinha volcano-sedimentary succession nearly correlates in age and origin with other anorogenic units (e.g., Ilhéus and Pedro Lessa dike swarms, Salto da Divisa granitic suite, Gangila basaltic and Mayumbian felsic volcanisms) found in a large region covered by the AWCO and neighbouring cratonic region. However, the Capelinha magmatism seems to preceed by some 20–30 m.y. the main peak (930–900 Ma) of this Early Tonian anorogenic magmatism, suggesting a long-lived and complex rift system.