Paleoproterozoic Sediments Research Articles

Paleoproterozoic Mineralization of the Lijiapuzi Gold Deposit in the Liaodong Peninsula, NE China: Constraints from 40Ar-39Ar Age, S-Pb Isotopes, and In Situ Analyses

A Paleoproterozoic 40Ar/39Ar age, interpreted as the gold metallogenic epoch of the Lijiapuzi deposit in NE China, is reported in this paper. The ore body of this deposit is hosted in the marbles and schists of the Paleoproterozoic Dashiqiao Formation and exhibits parallel development to the foliation of the country rocks. Coexisting auriferous pyrite and tremolite can be observed both in the field and under a microscope, revealing the presence of hydrothermal activity in the deposit. The δ34S composition of the auriferous pyrite varies from +8‰ to +12.3‰, indicating a sedimentary origin. The auriferous pyrite and host marble have similar Pb isotopic ratios, indicating the contribution of Paleoproterozoic sediments to gold mineralization. Furthermore, in situ S and trace element analyses of the auriferous pyrite reveal that the main gold mineralization stage was Py2, which coexists with radial tremolite. 40Ar/39Ar dating of these tremolite samples yields a plateau age of ~1851.23 Ma, with similar isochronal and inverse isochronal ages. The Paleoproterozoic gold metallogenesis, together with the contemporary Pb-Zn, boron, graphite, and magnetite mineralization, constitute an important mineralogenetic series during the evolution of the Jiao-Liao-Ji orogeny in the Liaodong Peninsula.

New chronological and paleontological evidence for Paleoproterozoic eukaryote distribution and stratigraphic correlation between the Yanliao and Xiong’er basins, North China Craton

North China Craton is one of the old cratons preserved continues recording of the early evolution of eukaryotes. Organic wall microfossils (OWMs) of eukaryotic affinity have been found in the late Paleoproterozoic sediments from the Yanliao and Xiong’er basins that located in the northern and southern margins of the North China Craton. However, accurate Paleoproterozoic stratigraphic correlation of these two basins still lacks precise chronological and convincing paleontological evidence. Here, we report a SIMS zircon U-Pb age (1647.8 ± 4.3 Ma) from a tuffite layer at the bottom of the Cuizhuang Formation of the Xiong’er Basin, which provides direct chronological evidence for its isochronous deposition with the Chuanlinggou Formation in the Yanliao Basin. Besides, abundant OWMs were recovered from red shales of the Chuanlinggou Formation from the present Taihang Mountains, which is located in the southern branch of the Yanliao Basin and close to the Xiong’er Basin. They are primarily spheroidal microfossils and contain at least one unambiguous unicellular eukaryotic taxa (Valeria lophostriata) and three probable eukaryotic taxa (Schizofusa sinica, Dongyesphaera tenuispina?, large Leiosphaeridia spp.). Our finding further confirms the appearance of unicellular eukaryotes in the Paleoproterozoic ocean and the expanding of their living scopes from the cratonic marginal basin to the intracratonic basin. However, the absence of eukaryotic OWMs in black shales and green shales with more hypoxic depositional waters than those of red shales, indicates that the oxygenated water is probably a key factor to the survival of eukaryotes and even their preservation as OWMs.

Giant Quartz Veins of the Bundelkhand Craton, Indian Shield: New Geological Data and U-Th-Pb Age

Giant quartz veins are widespread on the Bundelkhand Craton of the Indian Shield which precise ages with a SHRIMP-II, U-Th-Pb isotope are quantified in this article. Their relative geological age is well-documented: they cut the Paleoproterozoic (2150–1800 Ma) sediments of the Bijawar Group and are overlain by Proterozoic (1670–1030 Ma) sediments at the base of the Upper Vindhyan Supergroup. U-Th-Pb dating of zircon grains from a quartz vein was carried out to assess major event of their formation as 1866 ± 12 Ma. This data is consistent with the relative geological age of the veins. In addition, the quartz veins were shown to contain 2.86, 2.7, and 2.54 Ga xenocrystic zircon grains. Rocks with these ages are abundant in the craton. The formation of a giant quartz vein swarm is associated with the deformation of the Bundelkhand Craton lithosphere during 1.9–1.8 Ga ago triggered by compression caused by collision processes at the western flank of the Columbia Supercontinent on one side and plume activity on the other.

Does Neoproterozoic Nam Co formation in Northwest Vietnam belong to South China or Indochina?

The Nam Co belt is located to the northeast of Song Ma fault in Northwestern Vietnam. The nature of the Precambrian basement rocks in this area and its tectonic attribute have still been controversial. In this study, zircon U-Pb-Hf isotopic compositions and whole rock geochemical compositions of meta-sedimentary rocks from the Nam Co Formation in this belt have been integrated to reveal the components and tectonic setting of the formation of the basement rocks in the Nam Co belt. Zircon U-Pb dating results show that the Nam Co Formation is of middle-late Neoproterozoic age. The lower Nam Co Formation probably formed between 821 and 726 Ma, and the upper Formation was deposited later than 726–721 Ma. The sedimentary rocks from the lower and upper Nam Co formations are characterized by numerous Neoproterozoic (900–800 Ma) detrital materials, but the upper part contains more Meso- to Paleoproterozoic sediments than the lower part. Detrital zircon age distribution and Hf-isotope compositions of the Nam Co Formation are similar to those of Neoproterozoic sedimentary rocks in the southern Yangtze Block. Both zircon U-Pb-Hf isotopic and whole rock geochemical data suggest that the source materials of the Neoproterozoic Nam Co Formation were dominated by felsic rocks with minor mafic materials, and the detrital materials were mainly derived from the Jiangnan Orogenic Belt in the southern Yangtze Block, with minor contribution from the Panxi belt in the western Yangtze Block, Ailaoshan-Song Hong and Phan Si Pan belts. The lower Nam Co Formation contains more sediments derived from the Panxi belt than the upper part. The middle-late Neoproterozoic Nam Co Formation probably was deposited in a continental extensional basin, and the sedimentation environment in later period (<726–721 Ma) was more stable. Integration of our data with other lines of evidence suggests that the Nam Co Formation belongs to the basement of the Yangtze Block, and the Nam Co belt was originally located in the southwest corner of the Yangtze Block. This further suggests that the Song Ma fault probably is the boundary between the South China and Indochina blocks.

VS30 mapping and site characterization in the seismically active intraplate region of Western India: implications for risk mitigation

ABSTRACTWe carried out shallow subsurface investigations at 600 sites covering all types of geological units in the Northwestern Deccan Volcanic Province of India, using the multichannel analysis of surface waves and suspension PS‐logging methods. The results reveal that VS30 values are in the range of 760–1500 m/s for granites and Deccan traps, thus enabling their classification as B‐class, as per the National Earthquake Hazard Reduction Program recommendations. The Tertiary, Cretaceous, Jurassic and Paleoproterozoic sediments show VS30 values between 360 and 760 m/s, and hence, are assigned C‐class. Further, the Quaternary soils are categorized as D‐class, since they show VS30 in the range of 180–360 m/s. Also, the Holocene tidal flat and Rann sediments are classified as E‐class, since the VS30 values are less than 180 m/s. The observed site response reveals that the D‐ and E‐type soils have significantly higher seismic amplification than that in the B‐ and C‐category soils. We noticed that the buildings on D‐ and E‐classes soils experienced higher damage than those on the B‐ and C‐classes, during the past large earthquakes in the Northwestern Deccan Volcanic Province. Our study suggests that VS30 is a good proxy for soil classifications in the Northwestern Deccan Volcanic Province. We validated our results using available geological, geophysical and geotechnical data. For the first time, a regional site characterization map for the Northwestern Deccan Volcanic Province of India is prepared based on geology and VS30.

Geochemistry and detrital zircon records of the Ruyang-Luoyu groups, southern North China Craton: Provenance, crustal evolution and Paleo–Mesoproterozoic tectonic implications

Paleo- to Mesoproterozoic sedimentary rocks in the southern margin of the North China Craton (NCC) are represented by the Ruyang and Luoyu groups. We studied the sedimentary rocks from the Yunmengshan and Beidajian formations of the Ruyang Group and the Cuizhuang and Sanjiaotang formations of the Luoyu Group. Detrital zircon grains from these formations have U–Pb age populations of 3.64–3.31 ​Ga, 2.96–2.86 ​Ga, 2.72–2.59 ​Ga, 2.56–2.47 ​Ga, 2.45–2.0 ​Ga, 1.99–1.85 ​Ga and 1.84–1.65 ​Ga. The geochemical features of the sedimentary rocks suggest that some of the sediments were sourced from intermediate to felsic magmatic rocks. The age groups of the detrital zircon are roughly consistent with the tectono-thermal events in the southern margin of the NCC. The Hf isotopic compositions of detrital zircon from the sedimentary rocks in Ruyang and Luoyu groups suggest that significant crustal growth and reworking of the NCC took place during the Neoarchean and early- to mid-Paleoproterozoic, while crustal reworking at the Paleoarchean and late-Paleoproterozoic, and crustal growth at the Mesoarchean. We suggest the depositional times of the Ruyang Group and Luoyu Group are constrained to no older than 1.75–1.7 ​Ga and 1.7–1.65 ​Ga, respectively. Formation of late-Paleoproterozoic basins related to the strike slip and extrusion tectonics that cross-cut the NCC during the late Paleoproterozoic (<1.75 ​Ga), and the late Paleoproterozoic sedimentation once isochronous developed in the southern margin of the NCC through the Taihang region of the interior NCC and linked the Yanshan–Liaoxi regions of the northern NCC.

Discussion on the southeastern boundary location of the Liao-Ji orogenic belt

The basement of the North China Craton formed through collision of the Eastern and Western blocks along the Trans- orth China Orogen at ca. 1.85 Ga. The Liao-Ji orogenic belt in the Liaodong Peninsula and southern Jilin represents a Paleoproterozoic convergent belt between the Longgang and Nangrim blocks in the Eastern Block, and has been considered as the northern segment of the entire NNE- to NE-trending Jiao-Liao-Ji Orogen across the whole Eastern Block from south to north. The Liao-Ji orogenic belt is characterized by the presence of sedimentary and volcanic successions, metamorphism, deformation and magmatism of the Paleoproterozoic ages whereas the Archean blocks show the dominance of Archean TTG gneisses and the absence of the Paleoproterozoic sedimentation, metamorphism and magmatism. However, the southeastern boundary location of the Liao-Ji orogenic belt remains uncertain and controversial, and the southeastern Liaodong Peninsula was considered as an Archean block by many authors. Two samples of meta-sediments and seven samples of meta-igneous rocks were collected from the Dandong area for zircon U-Pb dating in this study. Detrital zircon dating for the two meta-sediment samples, muscovite schist and meta-sandstone, constrains their depositional ages between 2013 (youngest magmatic zircon age) and 1864 Ma (metamorphic age), 2064 and 1910 Ma respectively, indicating Paleoproterozoic ages for their deposition. The Paleoproterozoic meta-sediments, also called the Liaohe Group, are scattered in many places of the Dandong area, and show similar lithology and metamorphic grade to those in the Liao-Ji orogenic belt. These show that the Paleoproterozoic meta-sediments in the Dandong area can be correlated with the Liaohe Group in the Liao-Ji orogenic belt. The basement rocks in the Dandong area, including meta-sediments and meta-plutons, exhibit extensive medium-grade metamorphism. Our zircon dating results suggest that the regional metamorphism experienced by the basement rocks in the Dandong area took place at 1910–1864 Ma (Paleoproterozoic). The obtained metamorphic ages in the Dandong area are synchronous with those recognized in the Liao-Ji orogenic belt, which have been interpreted as time of the collision-related metamorphism. The zircon U-Pb dating results for seven igneous samples in the basement, including granitic pegmatite, weakly deformed granite, monzonitic granitic gneiss and granitic gneiss, give late Paleoproterozoic emplacement time (1882–1827 Ma) for six samples and a 2470 Ma emplacement age for one sample, indicating extensive magmatism of the late Paleoproterozoic. Age spectra of magmatic zircons obtained in this study exhibit that the basement in the Dandong area experienced multi-stage magmatic activities, with age peaks at ca. 2665, 2500, 2200, 2095, 2030, and 1865 Ma respectively. They are similar to those obtained from Liao-Ji orogenic belt, but different from those from the Longgang Block. Previous detrital zircon dating also proved the presence of the Paleoproterozoic sediments and metamorphism in the Dandong area. These facts demonstrate that component and evolution of the basement rocks in the Dandong area show obvious similarity to those in the Liao-Ji orogenic belt, and the basement in the Dandong area belongs to a part of the Liao-Ji orogenic belt, rather than an Archean block as previously considered. Previous zircon dating results have shown that the Paleoproterozoic sediments and metamorphism occur extensively in the Changhai area, just east of the Liaodong Peninsula. The absence of the Paleoproterozoic sediments in the southern Liaodong Peninsula might be related to exhumation and erosion caused by the Liaonan and Wanfu metamorphic core complexes of the Early Cretaceous. It is proposed therefore that the Liao-Ji orogenic belt extends across the entire Liaodong Peninsula, and its southeastern boundary should reach to the NE-striking Yalujiang Fault Zone at least.

Manganese oxide shuttling in pre-GOE oceans – evidence from molybdenum and iron isotopes

The local occurrence of oxygen-rich shallow marine water environments has been suggested to significantly predate atmospheric oxygenation, which occurred during the Great Oxidation Event (GOE) ca. 2.4 billion years ago. However, the potential influence of such ‘oxygen oases’ on the mobility, distribution and isotopic composition of redox sensitive elements remains poorly understood. Here, we provide new molybdenum and iron isotopic data from shallow marine carbonate and silicate iron formations of the Koegas Subgroup, South Africa, that confirm local ocean redox stratification prior to the GOE.Mn concentrations correlate negatively with both δ98Mo and δ56Fe values, which highlights the substantial role of particulate manganese for the cycling of Mo and Fe in the Paleoproterozoic oceans. Based on these trends we propose that pore water molybdate was recharged (1) by the diffusional transport of seawater molybdate with high δ98Mo and (2) by the re-liberation of adsorbed molybdate with low δ98Mo during Mn oxide dissolution within the sediment. The relative contribution of isotopically light Mo is highest close to a Mn chemocline, where the flux of Mn oxides is largest, causing the negative correlation of Mn concentrations and δ98Mo values in the Koegas sediments. The negative correlation between δ56Fe values and Mn concentrations is likely related to Fe isotope fractionation during Fe(II) oxidation by Mn oxides, resulting in lower δ56Fe values in the uppermost water column close to a Mn chemocline. We argue that the preservation of these signals within Paleoproterozoic sediments implies the existence of vertically extended chemoclines with a smoother gradient, probably as a result of low atmospheric oxygen concentrations. Furthermore, we suggest that abiotic oxidation of Fe(II) by a Mn oxide particle shuttle might have promoted the deposition of the Koegas iron formations.

Late Paleoproterozoic volcanic associations in the southwestern Siberian craton (Angara-Kan block)

We present data on the geologic setting, mineral composition, and conditions of metamorphism of volcanics of andesite-dacite-trachyrhyo-dacite, leucobasalt-basalt, basalt-andesite-basalt-trachyandesite, and basalt associations in the Yenisei amphibolite-gneiss complex of the Angara-Kan terrane. We have determined the age of zircon (SHRIMP II U-Pb dating) from two samples of volcanics of the andesite-dacite-trachyrhyodacite association. The volcanics formed in the Late Paleoproterozoic (~1.74 Ga) and were metamorphosed at 750 Ma. Inherited zircon and the Sm-Nd model age (2.4-2.5 Ga) of intermediate-felsic volcanics testify to their formation predominantly from the ancient crustal source. The Late Paleoproterozoic (1.78-1.74 Ga) sedimentation and volcanism in the Angara-Kan terrane followed the main collision events: high-temperature metamorphism (1.89-1.87 Ga) and formation of the first-phase granitoids of the Taraka massif (1.84 Ga). The volcanics have a predominantly bimodal character and belong to the tholeiitic and subalkalic series; this testifies to their formation in an extension setting. The formation of the volcanics of the Yenisei complex correlates with the intrusion of the intraplate granites of the Taraka massif, whereas the terrigenous sediments correlate with the sediments of the middle Subluk Group in the Urik-Iya graben. The rocks of the Yenisei complex are characterized by inhomogeneous metamorphism, which corresponds to the conditions of transition from epidote-amphibolite to amphibolite facies and to the whole PT-range of the amphibolite facies.

Hadean to Neoarchean episodic crustal growth: Detrital zircon records in Paleoproterozoic quartzites from the southern North China Craton

The Archean terranes exposed in several regions of the North China Craton (NCC) provide important windows to evaluate the Hadean-Archean continental crustal evolution history. Here we report results from SIMS zircon geochronology and oxygen isotopes, as well as LA-ICPMS zircon Hf isotopic analyses on detrital zircons from Paleoproterozoic quartzites to probe the early crustal evolution in the southern NCC. Our data show episodic magmatism during the Eoarchean, Paleoarchean, Mesoarchean, Neoarchean and Paleoproterozoic as inferred from the concordant ages of 3.6Ga and 2.2Ga and the well-defined upper intercept ages of 3404±30Ma, 2919±31Ma, 2772±9.5Ma, 2698±4.7Ma, 2652±6.7Ma, and 2532±5.2Ma. The Hf and oxygen isotopic compositions of Eoarchean–Paleoarchean zircons demonstrate that the cratonic nucleus of the NCC was built in the Hadean (∼4.0Ga), similar to the timing of formation of the nuclei of old cratons elsewhere on the globe such as the Yilgarn Craton, Western Australia, and the Slave Craton, Canada, with the Eoarchean–Paleoarchean (3.8Ga, 3.6Ga and 3.4Ga) marking the first phase of crustal reworking of the Hadean crust. The salient aspect of our data is that the Meoarchean to early Neoarchean (3.0–2.6Ga with a peak at 2.7Ga) was a significant period for crustal growth from the mantle throughout the NCC, comparable with the event of major Archean crustal growth worldwide. The latest Neoarchean (∼2.5Ga) marks a period of significant crustal reworking that led to remelting of the newly-formed juvenile mafic crust (∼2.7Ga). Integration with the data obtained from the TTG (tonalite–trondhjemite–granodiorite) and granitoid rocks of the Archean terranes and the lower crustal granulite xenoliths from various parts of the NCC suggests that the episodic Eoarchean via Paleoarchean and Mesoarchean to Neoarchean crustal growth as well as crustal reworking intensely destroyed the ancient cratonic nucleus, resulting in the scarce preservation of the Hadean and Eoarchean remnants in both the upper and lower crust of the NCC. Following the Paleoproterozoic sedimentation of the Archean zircons, apart from Pb loss during the subsequent thermal events, there was no major alteration in the Hf and oxygen isotopic compositions. Thus, our detrital zircon data provide convincing evidence for the repeated crustal growth and reworking of the NCC in the Archean.

2.9–1.9 Ga paleoalterations of Archean granitic basement of the Franceville basin (Gabon)

The Archean granitoids in the Kiéné area, Gabon, are overlained by the Paleoproterozoic sediments of the Franceville basin (2.1Ga). The basin is known for its high-grade uranium deposits among which some have been forming natural nuclear fission reactors. Most of the studies were dedicated to the FA-FB Paleoproterozoic sediments hosting these uranium deposits. Little is known on the Archean basement itself and specifically on the hydrous alteration events it experienced before and after the sediment deposition. The present work is focused on their petrographical, mineralogical and geochemical characterization. Dating the successive alteration events has been attempted on altered monazite crystals.Rocks in different alteration states have been sampled from eight drill cores crosscutting the Archean – Paleoproterozoic unconformity. The Archean granitoids observed in the deepest levels exhibit typical petrographical features of a propylitic alteration while they are intensely illitized up to the unconformity. The propylitic alteration is mainly pervasive but the original texture of the granitoïds is conserved in spite of the formation of new minerals: Mg-chlorite, allanite and epidote forming a typical paragenesis. The illitic alteration is much more invasive near the unconformity. The illitization process leads to the replacement of feldspars and the corrosion of quartz crysals by an illitic matrix while the ferromagnesian minerals are pseudomorphosed by a Fe-chlorite+phengite+hematite assemblage. The final fluid-rock interaction step is marked by fissural deposits of calcite and anhydrite. The δ13C isotopic data show that the fissural carbonates precipitated from diagenetic fluids enriched carbon products deriving from the maturation of organic matter. The U–Pb isotopic analyzes performed on monazite crystals have dated three distinct events: 3.0–2.9Ga (magmatic), 2.6Ga (propylitic alteration) and 1.9Ga (diagenetic illitization). The calculation of geochemical mass balances suggests that the water–rock ratio during the propylitic alteration event was weak. On the contrary, it was much higher during the overprinted illitization which is characterized by an intense leaching of Na, Ca, Mg, Sr, REE and an enrichment in K, Rb,Cs.Neither the petrographic features nor the geochemical data militate for an Archean weathering event (paleosol). In the present case, diagenetic fluids have percolated from the unconformity into the basement where they overprinted the illitization processes upon the previously propylitized rocks. These fluids were probably oxidant as they are also responsible of the U mobilization which led to the formation of the ore deposits close to the FA-FB interface.

Proterozoic evolution of Eastern Dharwar and Bastar cratons, India – An overview of the intracratonic basins, craton margins and mobile belts

Based on critical analysis of stratigraphic succesions in the Proterozoic intracratonic basins, geochronological data and possible connections or disconnections with adjacent fold belts and craton margin mobile belts, an overview of the Proterozoic evolution of the Eastern Dharwar and Bastar cratons in India is presented. The Chattisgarh and Cuddapah basins in these cratons with Archean nuclei host sedimentary successions ranging in age from the Paleoproterozoic to the Ediacaran period, and representing several silicilastic and carbonate cycles. The Proterozoic successions of the Pranhita–Godavari valley (PGV) basin along the Dharwar–Bastar join, record multiple rifting since late Neoarchean suturing of the two cratons. We postulate a first order stratigraphic correlation across the cratons and their join and discuss how major sea level fluctuations coupled with regional tectonic events shaped the development of unconformity bound successions, adjoining fold belts and the mobile belts with supposed global tectonic connections. The oldest Paleoproterozoic intracratonic sedimentation is preserved in the Cuddapah and PGV basins, while sedimentation in the Bastar craton is largely Mesoproterozoic. The fluviodeltaic to shallow marine Neoproterozoic sedimentation followed emplacement of end-Mesoproterozoic kimberlites and lamproites indicating thermal rejuvenation of the Indian continental crust. The allochthonus Nallamalai fold belt in eastern Cuddapah basin and the early Mesoproterozoic fold belt in PGV basin point to craton margin crustal convergences which punctuated the Paleoproterozoic sedimentation. Widespread development of carbonate bearing sequences in the Mesoproterozoic to Neoproterozoic successions point to peneplanation of the cratons and stable platformal sedimentation. The Neoproterozoic sedimentation was terminated following the docking of the Eastern Ghats belt to the eastern margin of the Dharwar–Bastar ensemble and possible final tectonic hiccups in the Central Indian Tectonic zone bordering the Bastar craton. The available data and their analysis rule out any foreland basin type connection between the Eastern Ghats belt and cratonic sedimentation in Bastar and Eastern Dharwar.

Earliest Paleoproterozoic supracrustal rocks in the North China Craton recognized from the Daqingshan area of the Khondalite Belt: Constraints on craton evolution

The Upper Wulashan “Subgroup” at Daqingshan in the Khondalite Belt of the Western Block of the North China Craton contains an Early Paleoproterozoic (2.5–2.45Ga) supracrustal sequence, incorporating banded iron formations, which we refer to as the Daqingshan Supracrustal Rocks. They contain rounded to elliptical zircon grains that commonly show core–rim or core–mantle–rim structures and precise SHRIMP U–Pb dating of these domains allows the discrimination of a series of tectono-thermal events that straddle the Archean/Proterozoic boundary. Detrital zircon cores with oscillatory zoning have formation ages ranging from 2.55 to 2.50Ga, indicating derivation from Late Neoarchean magmatic rocks. As a result of exhumation and erosion, these became incorporated in earliest Paleoproterozoic sediments that were deposited between 2.50 and 2.45Ga and then underwent high-grade metamorphism at 2.45–2.40Ga. This event variously recrystallized the cores into two main domains: a dark inner domain and a gray outer domain, as imaged in cathodoluminescence. The gray domains commonly show sector zoning and have Th/U ratios of 0.1–0.5, a feature commonly noted in zircons recrystallized under high-grade metamorphic conditions. Metamorphic mantles are generally more homogeneous in structure and have lower Th/U ratios (commonly <0.1) than the recrystallized domains, although they show similar age distributions. However, they are difficult to distinguish when only two domains are present in the zircon and they represent a continuum of recrystallization. The rocks then underwent a second episode of high-grade metamorphism in the Late Paleoproterozoic, as revealed by thin overgrowth rims that are homogeneous or show only weak zoning, have low Th/U ratios (commonly <0.1), and record ages of 1.95–1.90Ga. This is the first time that two tectono-thermal events of Early and Late Paleoproterozoic age have been identified in single rock samples from the Western Block of the North China Craton. The conclusion that unequivocal earliest Paleoproterozoic sediments are present in the North China Craton is also supported by evidence that the Daqingshan Supracrustal Rocks are cut by anatectic garnet granite, previously dated at 2.39Ga. Combined with evidence obtained here and elsewhere in the North China Craton, we conclude that the 2.45–2.40Ga metamorphism was not an extension of the well-established, craton-wide Late Neoarchean tectono-thermal event, but a previously unrecognized episode that followed a short quiescent period that allowed sedimentation of the Daqingshan Supracrustal Rocks.

A new interpretation for the origin of the Norilsk type PGE–Cu–Ni sulfide deposits

The origin of PGE–Cu–Ni sulfide deposits of Norilsk and Talnakh located in the northwest flank of the Triassic basalt trap formation of Siberia is considered. It is shown that ore elements of these deposits (probably, except Fe) are derived from the crust rather than from the mantle. They entered the basalts owing to a remobilization (recycling) of ore elements from the Paleoproterozoic sediments and from the rocks of the Siberian platform’s basement. Prospecting criteria for similar deposits are as follows: (1) a presence of a large Paleoproterozoic aulacogen and a related magmatic sulfide Cu–Ni mineralization; (2) a confinement of perspective areas to troughs associated with long-lived deep fault zones; (3) association with mobile orogenic belts, island-arc systems and tectonomagmatic activation zones; (4) temporal association with boundaries of global periods characterized by active processes of continental breakup and large-scale trap magmatism. A combination of several factors (the first one is obligatory) is favorable for the discovery of a large ore body.

Shrimp U–Pb zircon age evidence for Paleoproterozoic sedimentation and 2.05 Ga syntectonic plutonism in the Nyong Group, South-Western Cameroon: consequences for the Eburnean–Transamazonian belt of NE Brazil and Central Africa

The Nyong Group of the NW corner of the Congo craton is a metasedimentary and metaplutonic rock unit that underwent a high-grade tectono-metamorphic event at ∼2050 Ma associated with charnockite formation. However, the age of the sedimentation and associated plutonism was not known. In view of this, the unit was considered to be part of the Archean Congo craton reactivated during a Paleoproterozoic or a Pan-African orogeny. Such interpretation was widely supported by the persistence of Archean inheritance revealed by Nd isotope data on whole rocks and U–Pb on zircons. New SHRIMP analyses on detrital zircons from metasediments (BIF, orthopyroxene gneiss and garnet gneiss) yield Mesoarchean to Paleoproterozoic ages, with the youngest zircon at 2423 ± 4 Ma, thus giving the maximum deposition age for the Nyong Group. Data on a metagranodiorite at Bonguen and a metasyenite at Lolodorf yield emplacement ages of 2066 ± 4 Ma and 2055 ± 5 Ma respectively, with Archean inheritance (2836 ± 11 Ma) for the metasyenite. The syntectonic emplacement of these plutonic rocks is supported by the age of 2044 ± 9 Ma obtained on the Bienkop charnockite, associated with Eburnean high-grade metamorphism which continued probably up to 1985 ± 8 Ma. These new data permit correlation of the Nyong rocks with the Paleoproterozoic of NE Brazil and the discussion of the source provenance of detritus for the Nyong Group. Finally, it is proposed that the West Central African Belt (WCAB) in southern Cameroon, Gabon, Congo and Angola represents a segment of the Eburnean–Transamazonian orogeny that resulted from the convergence and collision between the São Francisco-Nigerian Shield block and a former Congo megacraton.

Multiple sulfur isotopes of sulfides from sediments in the aftermath of Paleoproterozoic glaciations

Geochemical evidence reported from Paleoproterozoic sediments has long been used to evaluate the transition from the anoxic Archean atmosphere to an oxygenated atmosphere. Sulfur isotopes ( 32S, 33S, 34S and 36S) in sedimentary sulfides and sulfates are an especially sensitive means to monitor this transition, such that the timing of the Paleoproterozoic “Great Oxidation Event” can be investigated using mass-independently fractionated (MIF) sulfur isotope systematics expressed as Δ 33S. Here we report data from 83 individual analyses of pyrite, pyrrhotite and chalcopyrite on a new suite of 30 different samples from Finland, South Africa, Wyoming and Ontario that span ∼600 My and follow one or several “Snowball Earth” events in the Paleoproterozoic. The samples were measured using a high-resolution secondary ion mass spectrometry technique in multicollection mode that investigates multiple sulfur isotopes in microdomains (<30 μm) within individual sulfide grains while preserving petrographic context. We focused on sediments deposited in the aftermath of the Paleoproterozoic glaciations (between 1.9 and 2.2 Ga) to trace fluctuations in atmospheric O 2 concentrations that were likely affected by an interplay of O 2 sinks in the atmosphere and the upper ocean and continental crust, and by the emergence and diversification of aerobic organisms. Our results demonstrate that MIF sulfur isotopes are absent in sediments deposited after the period of protracted global cooling in the Paleoproterozoic and independently confirm observations that MIF ceased during this time. We interpret our results by integrating Δ 33S and δ 34S data in sulfides, δ 13C data in carbonates and the estimated timing of glaciation events in the Paleoproterozoic. Data strongly hint at the presence of microbial sulfate reduction and fluctuations in the concentration of dissolved seawater sulfate and/or in δ 34S sulfate in the aftermath of glaciations and likely were affected by changing erosion rates and nutrient delivery to the oceans. These changes modulated the population of primary producers, especially oxygenic photosynthesizers, and led to fluctuations in the abundance of atmospheric O 2, CO 2 and CH 4. Our results support the interpretation that the world-wide δ 13C carb excursion observed between ∼2.25 and 2.05 Ga ( Karhu and Holland, 1996) was a period of significant accumulation of O 2 in the atmosphere.