Tonian Age Research Articles

Tonian rift successions in Newfoundland, Canada: a window to late tectonic events in the Mesoproterozoic Laurentian margin

Basement rocks in the Humber terrane of the Appalachian Orogen record the last stages in the history of the Mesoproterozoic Laurentian margin in Canada. These stages were revealed by recent work in the East Pond Metamorphic Suite on the western Baie Verte Peninsula, Newfoundland, where two Tonian bimodal volcaniclastic-sedimentary successions were recognized (Pine Pond successions). The older, ca. 980 Ma succession, contains detrital igneous zircon and titanite (ca. 1160–1057 Ma) presumably derived from the Mesoproterozoic Laurentian margin, while the younger, ca. 950 Ma succession, contains 980 Ma detrital igneous zircon and titanite. Although metamorphosed to eclogite facies during the assembly of the Appalachian Orogen, the successions preserve protolith features and geochemical data that indicate melt likely originated in an extensional setting. The new ages integrated with geochemical and Sm–Nd isotopic data suggest that the felsic volcaniclastic units of the Pine Pond successions are related to 975–950 Ma granitic plutons in the Pinware terrane of the eastern Grenville Province in southeastern Labrador. These new data solidify a previous interpretation that the Pine Pond successions were deposited at the continental apex of the Asgard Sea and that the Pinware terrane intrusions are a part of this event. Furthermore, these new Tonian ages for rift-related strata call into question the interpretation of Ediacaran depositional ages for clastic sequences in the northern Appalachian Orogen, with youngest detrital zircons that are Tonian, and show that the tectonic evolution of the Mesoproterozoic Laurentian margin in Canada is more complex than previously known.

A complex history of extension, subduction and collision in west Gondwana: Clues from the Riacho do Pontal orogen, Borborema Province (NE Brazil)

The geodynamic understanding of the Riacho do Pontal Orogen, in the Borborema Province, NE-Brazil, has progressed substantially in the last decade, mainly from geochemical and isotopic studies. However, the role of the Neoproterozoic magmatism has not totally been unraveled and has left some crucial questions unanswered. In order to contribute to this debate, we present petrographic, geochemical and isotopic (U–Pb, Sm–Nd and Sr–Sr) data obtained for a set of granitoids and metavolcanic rocks from the Internal and Central domains of the Riacho do Pontal Orogen. The Neoproterozoic granitoids comprise a major alkaline pluton of the Serra da Aldeia suite; two syenitic plutons – Nova Olinda and Caboclo –, and high-K, sub-alkaline granitoids from the Betânia granite. The alkaline pluton from the Serra da Aldeia suite yielded a zircon U–Pb age of 578 ± 5 Ma, while the Nova Olinda and Caboclo syenites yielded ages of 613 ± 2 Ma and 620 ± 3 Ma, respectively. The Betânia granite, displaying a volcanic-arc geochemical signature, yielded the age of 627 ± 4 Ma. Tonian ages were also obtained between 956 ± 8 Ma and 965 ± 3 Ma for three unclassified orthogneisses from the Internal Domain of the Orogen. A sample of metarhyolite from the Paleoproterozoic basement yielded an age of 1631 ± 21 Ma. The isotopic data for the Nova Olinda and Caboclo syenites revealed εNd(620 Ma) values between −7.72 and −8.45 and TDM values from 1.76 to 1.97.87Sr/86Sr ratios of 0.71594 ± 2 and 0.74432 ± 1 suggest mixing of crustal and mantle materials during their evolution. In contrast, the Betânia granite presented a ratio of 0.70669 ± 1, reinforcing the correlation with mantle sources. The integrated analysis of the data indicate that the Betânia granite is part of an Ediacaran magmatic-arc system, with the Nova Olinda and Caboclo syenites characterized as products of the late-orogenic magmatism, while the alkaline rocks of the Serra da Aldeia suite represent the final of post-orogenic stages of one of the multiple collisions of the West Gondwana.

Scapolite metagabbros of the Xambica Suite: A Tonian OIB magmatism in the crustal evolution of the Araguaia Belt

The geological evolution of the Araguaia Belt (AB), an important Neoproterozoic orogen located in central-north Brazil, is marked by different stages in its tectonic evolution. These stages are characterized by various magmatic events represented by felsic alkaline plutons, ophiolitic suites, mafic-ultramafic intrusions, S-type granites, diabase dykes and tholeiitic basalts distributed along its 1200 km N–S long. One of these events was identified in the Xambioá region, in the NW of Tocantins/SE of Pará state, in which dozens of bodies of scapolite metagabbros and amphibolites were identified through mapping and fieldwork, which were gathered in the Xambica Gabbroic Suite (XGS). Field relations and structural observations reveal that they were intrusive bodies in the sedimentary successions of the Estrondo Group before metamorphism. We present complementary studies involving detailed petrographic, lithochemical and geochronological analyses to characterize these rocks and contextualize this magmatic event in AB evolution. The petrographic study allowed to classify the rocks of the XGS into metagabbros with plagioclase and clinopyroxenes relicts, scapolite-hornblende metagabbro, garnet-scapolite metagabbro, amphibolites, and rarely listwanite and steatite. Geochronological dating of zircon crystals from this suite of rocks was performed using two different techniques. Single zircon Pb-evaporation dating of a metagabbro (XL-02) yielded the age of 819 ± 5 Ma. U–Pb zircon dating by LA-MC-ICP-MS of a metagabbro (BV-01) and an amphibolite (XL-09) gave 822 ± 2 Ma and 826 ± 26 Ma, respectively. Two other metagabbro samples (XL-02 and XB-08) yielded concordant ages of 861 ± 21 Ma and 873 ± 15 Ma. The younger concordant ages are interpreted as the crystallization of the gabbroic protoliths in the Tonian, while the older concordant ages are related to the presence of inherited zircon. The geochemical signature of metagabbros and amphibolites is similar to that of ocean island basalts (OIB) related to an intraplate tectonic environment, characterizing an extensional tectonic phase in AB evolution, which differs from other mafic events already recognized in AB of a MORB or tholeiitic signatures. Similar Tonian age are reported in basic to intermediate volcanic secession in the Goiás Magmatic Arc representing a contemporary collisional tectonic event, which reveals a link in the evolution between these two geotectonic units.

Anticlockwise metamorphic paths at ca. 890–790 Ma from the NE Baidrag block, Mongolia, indicate back-arc compression at the Rodinia periphery

The processes leading to the assembly of the Rodinia supercontinent through Grenvillian collisional orogeny are relatively well known. In contrast, accretionary orogenic processes occurring at the supercontinent periphery following Rodinia assembly are poorly understood. To fill this gap, we have identified metamorphic rocks in the Mongolia collage of the Central Asian Orogenic Belt, where numerous data testify for Meso- to Neoproterozoic magmatic reworking. The tectono-metamorphic evolution of the peri-Siberian tract of the Central Asian Orogenic Belt is mainly characterized by the late Proterozoic–early Cambrian (Baikalian) cycle. However, we document here a Tonian age metamorphism at the northern part of the Precambrian Baidrag block, previously considered as a typical example of the Baikalian metamorphic belt. This study incorporates zircon and in-situ monazite geochronology linked to P-T modelling of Grt-Sil-Ky migmatite gneiss and Grt-St micaschist. Grt-Sil-Ky gneiss records initial burial to the sillimanite stability field at ∼720 °C and 6.0 kbar followed by further burial to the kyanite stability field at ∼750 °C and ∼9 kbar and decompression to ∼650 °C and ∼8 kbar. The Grt-St schist records initial burial to the staurolite stability field at ∼620 °C and 6 kbar, followed by further burial to ∼590 °C and 8.5 kbar. The monazite data yield a continuum of 207Pb-corrected 238U/206Pb dates of ca. 926–768 Ma in the Grt-Sil-Ky gneiss, and ca. 937–754 Ma in the Grt-St schist. Based on monazite textural positon, internal zoning, and REE patterns, the time of prograde burial to 6.0 kbar under a thermal gradient of 27–32 °C/km is estimated at ca. 890–853 Ma. It is not clear whether such high-grade conditions prevailed until a phase of further burial under a geothermal gradient of 18–22 °C/km dated at ca. 835–815 Ma. The late monazite recrystallization at ca. 790 Ma is related to decompression. Additionally, monazite with dates of ca. 568–515 Ma occur as whole grains or as rims with sharp boundaries on Tonian monazite in Grt-St schist suggesting a minor Baikalian overprint. Metamorphic zircon rims with Th/U ratios of ∼ 0.01–0.06 in Grt-Sil-Ky gneiss with 877 ± 7 Ma age, together with lower intercepts of detrital zircon discordia lines in both Grt-Sil-Ky gneiss and Grt-St schist further support the Tonian age of high-grade metamorphism. The anticlockwise P-T evolution is interpreted as a result of thickening of a supra-subduction extensional and hot edifice – probably of back-arc or arc type. This kind of prograde metamorphism has so far only been described on the northern part of the Tarim block and was interpreted to be a result of initiation of peri-Rodinian subduction of the Mirovoi Ocean. The geodynamic consequences of a unique discovery of Tonian metamorphism are discussed in terms of tectonic switch related to initiation of peri-Rodinian oceanic subduction during supercontinent assembly, followed by strong mechanical coupling potentially related to onset of Rodinia dispersal.

Petrochronological evolution of Mg-Al granulites and associated metapelites from the contact zone of the Archean Bastar Craton and Proterozoic Eastern Ghats Province, and its implications

• Mg-Al granulites and khondalites in the Bastar Craton-Eastern Ghats Belt interface. • Ultrahigh-temperature peak metamorphic conditions along a clockwise P-T path. • Mg-Al granulites record decompression-cooling from 8.5kbar, 925°C to 5.5kbar, 770°C. • Texturally controlled in-situ monazite dating reveal two distinct age populations. • Tonian age amalgamation of the Bastar craton with the EGP and subsequent reworking. Mg-Al granulites and associated metapelites occurring in a shear zone at the contact of the Archean Bastar Craton and the Proterozoic Eastern Ghats Province (EGP) have been studied here. Reaction textures, geothermobarometry and P-T pseudosection studies reveal that the rocks record ultrahigh peak temperatures (>900°C) at pressures of 8-8.5 kbar and were exhumed along a retrograde decompression-cooling trajectory reaching ∼770°C and 5-6 kbar following an overall clockwise P-T path. Texturally constrained in-situ monazite chemical dates establish the age of peak metamorphism at 920 ± 11 Ma. Another prominent age cluster is at ∼500 Ma, which is ascribed, on the basis of petrographic characteristics, to tectonothermal reworking in the presence of reactive fluid. UHT metamorphism at ∼920 Ma along a clockwise path and extensive amphibolite facies reworking accompanying fluid infiltration at ∼500 Ma for the studied rocks contrast with the known petrological history of the EGP. We tentatively correlate these features in the contact shear zone to Tonian age amalgamation of the Bastar Craton with the EGP and subsequent tectonothermal reworking, aided by fluid infiltration, during the Kuunga orogeny.

Metamorphic evolution of the juvenile Serrinha forearc basin in the southern Brasiliano Orogen

Understanding the accretion of an oceanic island arc to a craton is a major issue in unravelling the evolution of the continents. P-T-t conditions registered in metamorphic rocks, the boron isotopic compositions of tourmaline provide essential information to delimit the accretionary processes. The Tonian-Cryogenian São Gabriel Terrane is a prominent juvenile structure in the southern Brasiliano Orogen that records key information about accretionary processes. Limited previous studies have investigated the São Gabriel Terrane, with little presently known about the metamorphic conditions. To pursue the objective, we combined multiple methods, including pseudosection modeling, U-Pb garnet geochronology and δ11B isotopes of tourmaline applied to one key sample from the Serrinha Formation of the São Gabriel Terrane. The recorded peak P-T conditions occurred at 4.5–5.1 kbar and 536–555 °C. U-Pb dating of low-U (<1 ppm) garnet yielded a lower intercept age of 721 ± 14 Ma for the bulk garnet growth. Dravite isotopic δ11B (+0.51 to + 3.39) compositions indicate oceanic crust components present in the island arc rocks. The calculated P-T conditions correspond to the metamorphic peak from heat pulses during burial events. Collision and thrusting of the São Gabriel arc against and over the Rio de La Plata Craton occurred at 721 ± 14 Ma. The evaluation of the parameters obtained in this study indicate a Barrovian-type evolution and a Late Tonian age of metamorphism for the Serrinha Basin. The sedimentation of the Serrinha Basin occurred in a forearc basin of the São Gabriel island arc. Collision of the oceanic arc with the craton is thus better constrained. Our study contains one point in the evolution of the protracted, complex amalgamation of the supercontinent Gondwana during the Brasiliano Orogeny.

High pressure granulite facies metamorphism at the interface of the Archean Bastar craton and the Proterozoic Eastern Ghats Belt, India

Nd model ages show that the suture between the Archean Bastar craton and Proterozoic Eastern Ghats Province (a part of the Eastern Ghats Belt), India is marked by a broad shear zone that contains a mixture of rocks from both the blocks. In this suture zone, amphibolite facies rocks of the Bastar craton were subjected to high pressure granulite facies metamorphism possibly at ~500 Ma (published isotopic data). New petrological, geothermobarometric and P-T pseudosection studies show that dehydration-melting of amphibole (in mafic protolith) and of biotite (in quartzofeldspathic protolith) along a clockwise P-T path led to peak metamorphic conditions of 9.75 ± 0.5 kbar; 875 ± 30 °C. Mafic granulites from the Eastern Ghats Province had an earlier metamorphic history of isobaric cooling from unknown peak metamorphic conditions, but currently record P-T maximum values of 9.25 ± 0.25 kbar; 825 ± 25 °C. Followed by cooling through ~100 °C from the peak metamorphic conditions, mafic granulites from the Bastar craton were exhumed to mid-crustal levels by decompression-cooling, probably as a tectonic wedge. This decompression-cooling is also shared by charnockitic rocks of the Bastar craton and mafic granulites of the Eastern Ghats Province. We interpret further burial of the Bastar craton rocks as a result of underthrusting beneath the Eastern Ghats Province, consistent with available seismic data. Total absence of any evidence of ocean opening or closure between the Bastar craton and the Eastern Ghats Province at around 500 Ma, coupled with interpretation of deduced P-T paths, suggest the shear zone resulted from far-field stress related to the Kuunga orogeny in an intraplate setting. This would support a model of Tonian age amalgamation of the Bastar craton and the Eastern Ghats Province as a part of the Greater Indian Landmass.

The Maravilha Orthogneiss: High-K calc-alkaline Cryogenian evidence of the pre- to early- collisional magmatism in the southern PEAL domain, SE Borborema Province

The study area is in the southern East Pernambuco–Alagoas Domain, Borborema Province, Southern sub-province, northeastern Brazil. The Maravilha Orthogneiss comprises an elongated intrusion of ~200 km2, with an ESE-WNW major axis, ranging in composition from tonalite to sienogranite, emplaced into metasedimentary rocks of the Inhapi Sequence, in the southern part of the Pernambuco-Alagoas Domain. Diorite sheets, amphibolite, leucocratic dikes and quatz-feldspar pegmatite veins are common. The Maravilha Orthogneiss show SiO2 contents ranging from 55.9 to 65.55 wt%, and Mg# (MgO/FeO + MgO) of 6.28–9.12. It has magnesian, metaluminous, and medium to high-K calc-alkaline composition. Chondrite-normalized spidergrams are characterized by troughs at Ba, Rb, Sr, Th, Nb and Ta. Chondrite-normalized REE patterns are moderately fractionated, with (Ce/Yb)N ratios ranging from 2.28 to 17.29 and display discrete negative Eu anomalies. This rock shows volcanic-arc geochemical signature. U–Pb zircon SHRIMP data yielded an age of 646 ± 5 Ma, interpreted as the crystallization age of the protolith. The Geochemical and isotopic data suggest that the source of the Maravilha orthogneiss protolith involved mixing of crustal and mantle melts. The resulted magma evolved by amphibole, plagioclase and apatite fractionation. A sample of migmatitic orthogneisses of the Inhapi Sequence basement show U–Pb zircon crystallization age of 983 ± 7.2 Ma. The lack of zircon grains with Tonian age in the studied orthogneiss indicates no contribution of Tonian crust in their source. The Maravilha Orthogneiss is a record of a pre-to early-collisional calc-alkaline magmatism with arc-related geochemical signature emplaced during the beginning of the Brasiliano Orogeny compressive stage.

Origin and age of the Shenshan tectonic mélange in the Jiangshan-Shaoxing-Pingxiang Fault and late Early Paleozoic juxtaposition of the Yangtze Block and the West Cathaysia terrane, South China

AbstractWhen and how the Yangtze Block (Yangtze) and the West Cathaysia terrane (West Cathaysia) in South China were amalgamated are critical to a better understanding of the Neoproterozoic to early Paleozoic tectonic evolution of South China and remain highly debatable. A key to this debate is the tectonic significance of the Jiangshan-Shaoxing-Pingxiang (JSP) Fault, the boundary between Yangtze and West Cathaysia. The Shenshan mélange along the JSP Fault has the typical block-in-matrix structure and is composed of numerous shear zone-bounded slivers/lenses of rocks of different types and ages that formed in different tectonic environments, including middle to late Tonian volcanic and volcanogenic sedimentary rocks (turbidite) of arc/back-arc affinity, a series of middle Tonian ultramafic to mafic plutonic rocks of oceanic island basalt affinity, a carbonaceous shale that was deposited in a deep marine environment, and a red mudstone. U-Pb zircon ages and acritarch assemblages (Leiosphaeridia-Brocholaminaria association) found in the turbidite confirm its Tonian age, and fossils from the carbonaceous shale (Asteridium-Comasphaeridium and Skiagia-Celtiberium-Leiofusa) constrains its age to the Early to Middle Cambrian. Field relationships and available age data leave no doubt that the ultramafic-mafic rocks are exotic blocks (rather than intrusions) in the younger metasedimentary rocks. We conclude that the Shenshan mélange is not an ophiolitic mélange, but rather a tectonic mélange that formed as a result of movement along the JSP Fault in the early Paleozoic. We suggest that Yangtze and West Cathaysia were two separate microcontinents, were accreted to two different parts of the northern margin of Gondwana in the early Early Paleozoic, and juxtaposed in the late Early Paleozoic through strike-slip movement along the JSP Fault. We further suggest that the ca. 820 Ma collision in the Jiangnan Orogen took place between Yangtze and a (micro)continent that is now partly preserved as the Huaiyu terrane and was not related to West Cathaysia. We compare our model for South China with the accretion of terranes in the North American Cordillera and propose a similar model for the relationship between the Avalon and Meguma terranes in the Canadian Appalachians, i.e., the two terranes were accreted to two different parts of the Laurentian margin and were later juxtaposed through margin-parallel strike slip faulting.

Phosphatic scales in vase-shaped microfossil assemblages from Death Valley, Grand Canyon, Tasmania, and Svalbard.

Although biomineralized skeletal elements dominate the Phanerozoic fossil record, they did not become common until ~550-520Ma when independent acquisitions of biomineralization appeared in multiple lineages of animals and a few protists (single-celled eukaryotes). Evidence of biomineralization preceding the late Ediacaran is spotty aside from the apatitic scale microfossils of the ~811Ma Fifteenmile Group, northwestern Canada. Here, we describe scale-shaped microfossils from four vase-shaped microfossil (VSM)-bearing units of later Tonian age: the Togari Group of Tasmania, Chuar and Pahrump groups of southwestern United States, and the Roaldtoppen Group of Svalbard. These scale-shaped microfossils consist of thin, ~13 micron-long plates typically surrounded by a 1-3 micron-thick colorless envelope; they are found singly and in heterotypic and monotypic clusters of a few to >20 specimens. Raman spectroscopy and confocal laser scanning microscopy indicate these microfossils are composed of apatite and kerogen, just as is seen in the Fifteenmile Group scale microfossils. Despite compositional similarity, however, these scales are probably not homologous, representing instead, an independent acquisition of apatite mineralization. We propose that these apatite-kerogen scale-shaped microfossils are skeletal elements of a protistan cell. In particular, their consistent co-occurrence with VSMs, and similarities with scales of arcellinid testate amoebae, a group to which the VSMs are thought to belong, suggest the possibility that these microfossils may be test-forming scales of ancient arcellinid testate amoebae. The apparent apatite biomineralization in both these microfossils and the Fifteenmile scales is unexpected given its exceedingly rare use in skeletons of modern protists. This modern absence is attributed to the extravagance of using a limiting nutrient in a structural element, but multiple occurrences of apatite biomineralization in the Tonian suggest that phosphorus was not a limiting nutrient for these organisms, a suggestion consistent with the idea that dissolved seawater phosphate concentrations may have been higher at this time.

Oldest-known Neoproterozoic carbon isotope excursion: Earlier onset of Neoproterozoic carbon cycle volatility

The Neoproterozoic Era (1000–541 Ma) is characterized by the largest negative carbon isotope excursions in Earth history. Younger Neoproterozoic excursions are well-documented on multiple continental margins and associated with major events including snowball Earth glaciations, ocean-atmosphere oxygenation, and the evolution of animal multicellularity. However, due to a large gap in carbon isotopes before 900 Ma, the onset of early Neoproterozoic carbon cycle volatility has heretofore been obscure. Here, we present a mid-amplitude, negative carbon isotope excursion with δ13C values reaching a nadir of ~ − 6‰ from the Majiatun Formation in the Dalian Basin of the North China Craton dated between 950 and 920 Ma. The whole-rock 87Sr/86Sr ratios of limestone and dolostone are as low as ~0.7055, which is compatible with the Tonian age of the global carbonate curve. Identification of the Majiatun anomaly thus fills the critical gap in carbon isotopes and implies the onset of Neoproterozoic carbon cycle volatility ~130 m.y. earlier than previously thought. Now 5 negative carbon isotope excursions occur with increasing amplitude throughout Neoproterozoic time, implying deeper roots for the biogeochemical processes that may have causally led to late Neoproterozoic snowball glaciation, oxygenation, and biological innovation.

Lu-Hf Isotopic Data of the Mb&amp;#233;-Sassa-Mbersi Tonalite (Central Cameroon Domain): Indicator of ca. 1.0 Ga Juvenile Tonian Magmatism in the Region

The Pan-African/Brasiliano orogenic belts are a part of numerous Neoproterozoic orogenic belts that belong to the long-lived orgenic cycle that distancing phase started at the Tonian around 1.0 Ga. The Tonian magmatism fairly documented in the Neoproterozoic belts of Borborema Province (NE-Brazil), seemed so far inexistent in the Central African Orogenic Belt (CAOB) although these two belts were geologically correlated. Through the Lu-Hf geochronological analysis on zircon of tonalite, the present work, coupled with the previous data, suggests the existence of a Tonian age magmatism in the Central Cameroon Domain of the CAOB although the latter is much reworked. The Nguesseck tonalite outcrops in the northern part of the Mbe-Sassa-Mbersi region, in the northern edge of Central Cameroon Domain of the CAOB and in the Tchollire-Banyo shear zone (TBSZ). The Lu-Hf data obtain on the zircon grains of this tonalite reveal juvenile HfTDM age of ca. 1.0 Ga. This age, combined to the previous geochemical data suggests that the protholites of this tonalite would have extracted from the source during the distancing phase (rifting and oceanization) of the Pan-african/Brasiliano orogeny at the early Neoproterozoic.

Geology and Genesis of the Giant Gorevskoe Pb-Zn-Ag Deposit, Krasnoyarsk Territory, Russia

Abstract The Gorevskoe Pb-Zn-Ag mine is currently the largest producer of Pb and Zn in Russia, exploiting one of the largest sediment-hosted Pb-Zn deposits worldwide. Despite its size and economic importance, the Gorevskoe deposit remains poorly understood. It is located on the western margin of the Siberian craton within the Yenisei Ridge, a Neoproterozoic orogenic belt. Mineralization consists of three tabular orebodies that are in turn composed of multiple stacked stratiform to strata-bound lenses of galena-pyrrhotite-sphalerite-rich massive sulfide ore, hosted in organic-rich marine metalimestones and calcareous slates of Stenian to Tonian age (1,020 ± 70 Ma). Extensive Fe-Mg-Mn-carbonate alteration haloes surround the ore lenses. The Pb isotope signature of the deposit is consistent with derivation of Pb, and probably all associated metals, from an evolved crustal source at the time of formation of the host rocks. The sulfur isotope compositions of pyrrhotite, sphalerite, galena, arsenopyrite, and pyrite (δ34S = 16.0–20.4‰) do not vary considerably across the deposit and are within the range reported for contemporaneous seawater, indicating complete reduction of marine sulfate as the main source of sulfur. The available geologic and geochemical data indicate that the Gorevskoe deposit belongs to the sediment-hosted massive sulfide (SHMS) class of Zn-Pb deposits, with an affinity to Selwyn-type deposits. Hydrothermal mineralization appears to be temporally related to rifting and distal mafic volcanism in a passive margin setting. Geologic relationships suggest that the orebodies formed in a diagenetic environment. Furthermore, the predominance of primary pyrrhotite over pyrite as the major iron sulfide, the presence of abundant siderite, and the relatively homogeneous sulfur isotope signature of the ores indicate highly reducing conditions during ore formation. They also constrain the character of the metal-bearing fluid to be similarly reducing, and of moderate temperature (200°–300°C). Gorevskoe may thus be regarded as one of the world’s largest Selwyn-type SHMS deposits.

Detrital zircon U-Pb and Lu-Hf data for a kinzigitic gneiss (Jequitinhonha Complex, Araçuaí Orogen, SE Brazil) constrain the age of a huge storage of Ediacaran carbon

The precursor basin of the Araçuaí – West Congo Orogenic System (AWCO) formed a large gulf within the São Francisco – Congo Paleocontinent (SFCP), where sedimentary successions ascribed to the Jequitinhonha Complex and Macaúbas Group were deposited in the Neoproterozoic. This precursor basin system comprises an aborted continental rift of Early Tonian age and a Cryogenian rift that evolved to ocean-floor spreading up to the Early Ediacaran. The Jequitinhonha Complex is an extensive metasedimentary unit located in the northeastern Araçuaí Orogen, consisting of Al-rich paragneiss (kinzigite), sillimanite-graphite gneiss and associated economic deposits of flake graphite, quartzite and lenses of calc-silicate rock, metamorphosed to upper amphibolite or granulite facies around 570–550 Ma. U-Pb (LA-MC-ICP-MS) data of 110 detrital zircon grains from the paleosome of a migmatitic paragneiss yield five main age peaks at 2564 ± 35 Ma (5% of the grains), 2012 ± 21 Ma (7%), 1807 ± 19 Ma (12%), 960 ± 8 Ma (29%) and 662 ± 6 Ma (48%), with the youngest grains around 630–609 Ma, and a major age gap from the Statherian to Stenian. The U-Pb age spectrum coupled with Lu-Hf in zircon (LA-MC-ICP-MS) data for 106 zircon grains suggest the following sources: Neoarchean to Orosirian grains were sourced from the SFCP basement, Early Tonian grains from the anorogenic magmatic province, and Cryogenian to Early Ediacaran grains were provided by the Southern Bahia Alkaline Province and Rio Doce magmatic arc. The εHf(t) values show a predominant contribution from evolved crust, with minor involvement of moderately juvenile crust. The sedimentation of Jequitinhonha Complex protoliths developed from the Cryogenian to Early Ediacaran, partially within anoxic marine environments favorable to the preservation of organic matter that formed the huge graphite deposits and lots of smaller occurrences found in the region, accumulating probably more than one billion tons of raw graphite. The youngest detrital zircon population suggests that the deposition record of the Jequitinhonha Complex covers the Cryogenian continental rifting event followed by the development of oceanic crust to the onset of subduction and building of the Rio Doce magmatic arc in the Early Ediacaran. The regional accumulation and preservation of carbon-rich sediments (now graphite-rich gneiss) most probably represent one of the largest storages of Ediacaran carbon around the world.

First record of organic-walled microfossils from the Tonian Shiwangzhuang Formation of the Tumen Group in western Shandong, North China

Organic-walled microfossils offer important information on the biospheric evolution in pre-Cryogenian and provide biostratigraphic implications for many Proterozoic fossiliferous sequences that are poorly age constrained for the lack of reliable radiometric date. Recently, macroscopic carbonaceous compression fossils have been reported for the first time from the Tonian Shiwangzhuang Formation of the Tumen Group in western Shandong, North China. However, organic-walled microfossils have never been discovered from this formation up till now. To improve our knowledge about Proterozoic biodiversity in North China, we conducted a micropaleontological survey on the argillaceous limestone samples of the Shiwangzhuang Formation, which also contain macroscopic carbonaceous compression fossils, from the Baishicun section in Anqiu, western Shandong, North China. Our investigation shows that the Shiwangzhuang microfossil assemblage is dominated by smooth-walled sphaeromorphic acritarchs and cyanobacterium-like filaments and relatively low abundance of other acritarchs, including 16 taxa, such as Polysphaeroides filliformis, Ostiana microcystis, Simia annulare, ?Jacutianema sp., Arctacellularia tetragonala, Pellicularia tenera, Polythrichoides lineatus, and Navifusa actinomorpha. The Shiwangzhuang organic-walled microfossil assemblage, although consisting of long-ranging and not age diagnostic taxa, is consistent with a Tonian age suggested by macroscopic carbonaceous compression fossils, including the Chuaria-Tawuia and Sinosabellidites-Protoarenicola-Pararenicola assemblages, revealed from the same fossiliferous horizon of the Shiwangzhuang Formation and by organic-walled microfossil assemblage, including the late Mesoproterozoic to Tonian index fossil Trachyhystrichosphaera aimika, from the underlying Tongjiazhuang Formation. However, it is also worth noting that a Cryogenian or Ediacaran age cannot be completely excluded based just on the Shiwangzhuang microfossils because of their limited biostratigraphic utility.

An assemblage of macroscopic and diversified carbonaceous compression fossils from the Tonian Shiwangzhuang Formation in western Shandong, North China

Macroalgae play a crucial ecological and biogeochemical role in modern marine ecosystems. Although eukaryotic algae have a deep history within the Precambrian, most pre-Cryogenian alga fossils are microscopic and macroalga fossils are poorly documented during this period. Here we report a well-preserved assemblage of macroscopic and diversified carbonaceous compression fossils from the Tonian Shiwangzhuang Formation in western Shandong, North China. This assemblage consists of seventeen distinct morphotaxa, including eleven macroalgae, showing remarkable taxonomic diversity that is more diverse than many other Tonian macrofossil assemblages all over the world. Three taxa from the Shiwangzhuang assemblage show evidence of simple multicellularity with two of them consisting of giant cells and comparable to giant-celled green algae, and seven taxa with diagnostic transverse annulations are likely coenocytic algae. The Shiwangzhuang assemblage indicates that some eukaryotic clades had achieved macroscopic growth through simple multicellularity or coenocytism, paving the way, either ecologically or phylogenetically, for the eventual appearance of complex multicellularity. In addition, there are some biostratigraphically significant genera, including the Chuaria-Tawuia and Sinosabellidites-Protoarenicola-Pararenicola assemblages, indicating a Tonian age for the Shiwangzhuang Formation. This study also suggests that the Tonian diversity of multicellular or coenocytic macroalgae is probably underestimated, pending for a systematic and thorough investigation of Tonian macroscopic carbonaceous compressions. Macroalgae may have an initial ecological expansion, at least locally if not globally, in the Tonian Period.

Tonian microfossils from subsurface shales in Botswana

Shale samples recovered from the Masetlheng Pan-1 well, drilled in western Botswana, contain abundant, moderately well preserved, and modestly diverse microfossils. The fossils are assigned to eleven taxa, several reported for the first time from Africa. Distinctive acritarch taxa suggest a late Tonian age for the thick siliciclastic succession that lies unconformably beneath sandstones of the upper Ediacaran-lower Cambrian Nama Group. Masetlheng Pan-1 rocks, therefore, record the major events of Damara sedimentation and tectonics, hidden beneath surficial sands of western Botswana.

Neoproterozoic pre- and post-deformational metamorphism in the Western Domain of the Karagwe-Ankole Belt reconstructed by Lu-Hf garnet geochronology in the Kibuye-Gatumba area, Rwanda

The timing of metamorphism in the Karagwe-Ankole Belt (KAB, Central Africa) has never been investigated in detail. Although its metamorphic history and geochronology would provide a unique and crucial contribution, these aspects have largely been overlooked in most published geodynamic reconstructions of this belt. In this study, well-constrained Lu-Hf garnet dates obtained on four samples (three metasiltstones, one schist) can be directly linked to pre- and post-deformational greenschist metamorphism and broadly constrain the timing of Neoproterozoic deformation in the Karagwe-Ankole Belt. Depending on the host rock composition, garnet porphyroblasts grew over intervals of <2 Myr up to 9 Myr. Three Neoproterozoic garnet growth episodes were identified in the Western Domain of the Karagwe-Ankole Belt. In the Satinsyi Complex, just west of Gatumba, two prograde, pre-deformational garnet generations of Tonian age were identified. The first generation, mostly preserved as relict cores, grew before 920 Ma. Around 880 Ma, a second garnet generation formed new crystals, and overgrowths on first generation porphyroblasts. In the Kibuye area, garnet growth occurred between 618.9 ± 1.6 and 602.3 ± 1.1 Ma (Ediacaran period) during post-deformational greenschist metamorphism. In the western part of the Karagwe-Ankole Belt, the youngest deformation, probably of Pan-African origin, occurred after 880 Ma (2nd garnet growth episode), and had ceased by 620 Ma (3rd garnet growth episode) at the latest. The latter age is at least 70 Myr older than the Pan-African culmination and deformation observed in the northeastern part of the KAB (NW Tanzania), and the southern Ubendian Belt but corresponds to the timing of the West Gondwana Orogeny in the Congo Craton west of the study area. The Lu-Hf dates additionally indicate a limited timespan between Rodinia break-up (705–748 Ma) and Gondwana amalgamation (>620 Ma).

Dom Feliciano Belt orogenic cycle tracked by its pre-collisional magmatism: The Tonian (ca. 800 Ma) Porto Belo Complex and its correlations in southern Brazil and Uruguay

The Porto Belo Complex orthogneisses are dominantly intermediate, high-K calc-alkaline rocks of Tonian age (ca. 800 Ma) that represent the hinterland basement of northern Dom Feliciano Belt. Their significance and correlation to other pre-collisional, Tonian orthometamorphic rocks of the same belt are discussed and integrated in an Occam’s Razor model for the evolution of the Kaoko-Dom Feliciano-Gariep Orogenic System. The model proposes a single, eastward subduction with temporal variations of subduction angle leading to migration of arc-magmatism before the main collision, in the following stages: a) 925–880 Ma early subduction and forearc magmatism; b) 820–770 Ma flat-slab subduction and arc-magmatism migration towards the back-arc region, with generation of arc-related, Tonian magmatic protoliths; c) 765–700 Ma steepening subduction and arc-magmatism migration towards forearc, as the back-arc area is under extension, and rifting is triggered; d) 700–660 Ma accretionary phase, after final consumption of the oceanic plate; e) 660–640 Ma oblique collision leading to basin inversion and double-verging transpression, with hinterland high-T/low-P metamorphism and exhumation; f) 640–580 Ma post-collisional magmatism and shear belt development.

Petrological, geochemical and isotopic data of Neoproterozoic rock units from Uruguay and South Africa: Correlation of basement terranes across the South Atlantic

Felsic to intermediate igneous rocks from the Cuchilla Dionisio (or Punta del Este) Terrane (CDT) in Uruguay and the Várzea do Capivarita Complex (VCC) in southern Brazil were emplaced in the Tonian and experienced high-grade metamorphism towards the end of the Cryogenian. Geological and geochemical data indicate an S-type origin and formation in a continental within-plate setting by recycling of lower crustal material that was initially extracted from the mantle in the Palaeoproterozoic. Similar felsic igneous rocks of Tonian age occur in the Richtersveld Igneous Complex and the Vredefontein and Rosh Pinah formations in westernmost South Africa and southern Namibia and have been correlated with their supposed equivalents in Uruguay and Brazil. Geochemical and isotope data of the largely unmetamorphosed felsic igneous rocks in southwestern Africa imply a within-plate origin and formation by partial melting or fractional crystallization of mafic rocks that were extracted from the mantle in the Proterozoic. The parental melts of all of these Tonian igneous rocks from South America and southwestern Africa formed in an anorogenic continental setting at the western margin of the Kalahari Craton and were emplaced in, and/or contaminated by, Namaqua Province-type basement after separation from their source region. However, the source regions and the time of extractions thereof are different and, moreover, occurred at different palaeogeographical latitudes.New petrological data of CDT high-grade gneiss indicate a geothermal gradient of c. 20–25 °C/km, implying continental collisional tectonics following subduction and ocean basin closure at an active continental margin at the eastern edge of present-day South America in the late Cryogenian to early Ediacaran. The associated suture may be traced by the high-grade gneiss and amphibolite-facies mafic rocks in the CDT and probably continues northwards to the Arroio Grande Complex and the VCC in southern Brazil.