Early Paleoproterozoic Research Articles

Protracted post-collisional magmatism during plate subduction shutdown in early Paleoproterozoic: Insights from post-collisional granitoid suite in NW China

Tectonic regimes display different styles worldwide during the early Paleoproterozoic ranging from plate tectonic slowdown or shutdown to vigorous subduction-related magmatism. Here we report a suite of early Paleoproterozoic granitoids from the Quanji Massif in northwestern China that provides potential insights on the tectonic regime in this period. The granitoids formed at 2.37–2.34Ga as evidenced by LA-ICP-MS zircon U-Pb ages. These dioritic to granitic rocks with amphibole and biotite as the major mafic minerals display a wide range of SiO2 (55.8–66.2wt%), low to moderate MgO (1.4–4.1wt%), low to high K2O (1.6–5.9wt%) and Al2O3 (16.3–21.5wt%), with Mg# values in the range of 36–47 and relatively low Cr (7.7–79.2ppm) and Ni (2.3–19.3ppm) concentrations. Geochemically they correspond to high‑potassium calc-alkaline I-type granitoids, with some showing dominantly adakitic but minor A-type affinities. Both the I-type and A-type granitoids show moderately fractionated LREE to HREE and moderately to weakly negative Eu anomalies. However, the adakitic granitoids display strongly fractionated REE patterns and weak negative to positive Eu anomalies. All the rock types from this suite show enrichment in LILEs and LREEs and depletion in HFSEs (Ta, Nb). The Nd-Hf isotope data suggest that the granitoids were sourced from Mesoarchean to Neoarchean metamafic rocks in the lower crust. The coeval nature of the complex genetic types of rocks in this suite suggest that their precursor magmas formed in an extensional tectonic regime similar to those previously reported for the ~2.39–2.37Ga granitoids in central to eastern domains of the Quanji Massif. Our new results in conjunction with those from previous studies suggest protracted post-collisional magmatism lasting for >50Myr along the periphery Quanji Massif following the cessation of plate subduction in the early Paleoproterozoic.

Archean‐Paleoproterozoic Lithospheric Mantle at the Northern Margin of the North China Craton Represented by Tectonically Exhumed Peridotites

AbstractTectonically emplaced peridotites from North Hebei Province, North China Craton, have retained an original harzburgite mineral assemblage of olivine (54%–58%) + orthopyroxene (40%–46%) +minor clinopyroxene (<1%)+spinel. Samples with boninite‐like chemical compositions also coexist with these peridotites. The spinels within the peridotites have high‐Al end‐members with Al2O3 content of 30 wt%–50 wt%, typical of mantle spinels. When compared with experimentally determined melt extraction trajectories, the harzburgites display a high degree of melting and enrichment of SiO2, which is typical of cratonic mantle peridotites. The peridotites display variably enriched light rare earth elements (REEs), relatively depleted middle REEs and weakly fractionated heavy REEs, which suggest a melt extraction of over 25% in the spinel stability field. The occurrence of arc‐ and SSZ‐type chromian spinels in the peridotites suggests that melt extraction and metasomatism occurred mostly in a subduction‐related setting. This is also supported by the geochemical data of the coexisting boninite‐like samples. The peridotites have 187Os/188Os ratios ranging from 0.113–0.122, which is typical of cratonic lithospheric mantle. These 187Os/188Os ratios yield model melt extraction ages (TRD) ranging from 981 Ma to 2054 Ma, which may represent the minimum estimation of the melt extraction age. The Al2O3–187Os/188Os‐proxy isochron ages of 2.4 Ga‐2.7 Ga suggest a mantle melt depletion age between the Late Achaean and Early Paleoproterozoic. Both the peridotites and boninite‐like rocks are therefore interpreted as tectonically exhumed continental lithospheric mantle of the North China Craton, which has experienced mantle melt depletion and subduction‐related mantle metasomatism during the Neoarchean‐ Paleoproterozoic.

Late Neoarchean to early Paleoproterozoic tectonic evolution of the southern North China Craton: Evidence from geochemistry, zircon geochronology and Hf isotopes of felsic gneisses from the Taihua complex

The Archean-Paleoproterozoic Taihua Complex, extending from east to west along the southern margin of the North China Craton (NCC), is an ideal window to understand the tectonic evolution of the early Earth. Here we present new data on petrology, zircon geochronology, whole-rock geochemistry and isotopes of TTGs, granitic gneisses and felsic leucosomes of migmatites from the Taihua complex in the Xiaoqinling area. Zircon LA-ICP-MS analyses of rocks yield the formation ages of 2564–2503 Ma for TTGs, 2481–2476 Ma for felsic leucosomes and 2359–2334 Ma for granitic gneisses, respectively. The zircon Lu-Hf isotopes suggest that crustal growth occurred in the late Neoarchean and crustal reworking occurred in the end of Neoarchean and the early Paleoproterozoic. The TTG gneisses are characterized by high Na2O/K2O, La/Yb, Sr/Y and low Nb/Ta ratios, and they were likely generated by partial melting of subducted juvenile oceanic crust with garnet or amphibole in residuals, a mechanism similar to the slab melting model. Both the felsic leucosomes and granitic gneisses have high SiO2 contents and various Hf isotopic compositions, and they probably were formed in an extensional setting, attributing to partial melting as well as mixing of the preexisting TTG suite.Statistically, we report a major transitional phase in the tectonic milieu of the southern NCC from compression to extension during the late Neoarchean (ca. 2.57–2.42 Ga). The compressional tectonics can be attributed to amalgamation of micro-continental blocks during the late Neoarchean (ca. 2.57–2.5 Ga) as the leading period of crustal growth and associated multistage TTG magmatism. In contrast, the extensional phase occurred during the end of Neoarchean (ca. 2.5–2.42 Ga) that may represent the end of cratonic stabilization period and formation of large amounts of K-rich granitic rocks, widespread high-grade metamorphism and migmatites. Additionally, the formation of a large amount of crust-derived granites and mafic dykes during the early Paleoproterozoic in southern NCC reflect another stage (ca. 2.36–2.24 Ga) of crustal extension, which represents an episode of initial worldwide rifting events probably linking to the great oxygen event (GOE). Therefore, the present study proposes a new profile on multi-stage crustal growth/reworking and craton stabilization during Neoarchean-Paleoproterozoic period along the southern NCC, and provides useful insights on complex and heterogeneous evolutionary processes of early Earth.

Paleomagnetic age of ferruginous weathering beneath the Hamersley Surface, Pilbara, Western Australia, and the Cenozoic apparent polar wander path

ABSTRACTDuring the Mesozoic and Paleogene, the Precambrian rocks in the Pilbara, Western Australia, underwent erosion and deep weathering that produced an undulating landform now represented by the duricrusted and partly eroded Hamersley Surface. A reddened, ferruginous weathering zone occurs immediately beneath this duricrusted surface. Oriented block samples of ferruginised strata of the Neoarchean–early Paleoproterozoic Hamersley Group exposed within approximately 15 m below the duricrust were collected at 20 sites in roadcuts along the Great Northern Highway between Munjina and Newman and exposures along the adjoining Karijini Drive. Stepwise thermal demagnetisation of cored specimens revealed a stable, high-temperature (680°C) component carried by hematite, with a mean direction (n = 55 specimens) of declination D = 182.0°, inclination I = 52.9° (α95 = 3.6°), indicating a pole position at latitude λp = 77.6°S, longitude ϕp = 113.2°E (A95 = 4.3°) and a paleolatitude λ = 33.5 +3.6/–3.3°S. Both normal and reversed polarities are present, indicating that the remanent magnetism was acquired over an interval of at least two polarity chrons (say 105–106 years). Chi-square tests on the determined pole position and three different sets of Cenozoic poles, namely those for dated volcanic rocks in eastern Australia supplemented by poles for Australian Cenozoic weathering horizons, and inferred poles from Pacific Ocean and Indian Ocean hotspot analyses and North American Cenozoic poles rotated to Australian coordinates, yielded a mean age of ca 24 ± 3 Ma, i.e. late Oligocene to early Miocene, interpreted as the time of formation of hematite in the sampled ferruginous zone. The ferruginous weathering occurred under globally warm conditions and was followed during the early to middle Miocene climatic optimum by the deposition of channel iron deposits, which incorporated detrital hematitic material derived from erosion of the ferruginous weathering zone beneath the Hamersley Surface.

Origin of red beds in the Paleoproterozoic Franceville Basin, Gabon, and implications for sandstone-hosted uranium mineralization

Red beds are extensively used as evidence for an oxygenated atmosphere in the Paleoproterozoic. Red beds in the unmetamorphosed, *ca*. 2.15 Ga FA Formation of the Franceville Basin, Gabon were geochemically, petrographically, and mineralogically characterized to constrain the process of their formation and their relationship to atmospheric oxygenation in the early Paleoproterozoic. Petrographic observations indicate that ferric oxides are dispersed in clay filling intergranular pores and along platy cleavage in altered phyllosilicates. Grain-coating hematite is generally rare to absent in most samples suggesting hematite precipitation after sediment deposition and during burial diagenesis. Textures and geochemical data suggest that iron was likely sourced internally by alteration of iron-bearing minerals such as biotite and chlorite in sediments and redistributed in rocks during late diagenesis. Positive correlation between Fe/Mg ratio and δ^56^Fe values of bulk samples suggests mixing relationship with end members being authigenic hematite and iron-bearing silicates (biotite and chlorite). The lack of relationship between Fe^3+^/Fe~T~ ratios and iron isotope compositions suggest that the isotopically heavy iron oxide was already present in the sediments during early diagenesis, and was incorporated into green (reduced) facies that likely replaced red facies during diagenesis and burial. Large range of δ^56^Fe values extending towards positive values, up to +1 permil, is similar to that observed in Archean and Paleoproterozoic iron formations or modern groundwater-derived Fe-oxyhydroxides, suggesting partial oxidation of Fe under mildly oxidizing conditions during early diagenesis. In addition, positive correlation between Cr/Fe ratios and iron isotope values, especially in the fine-grained sandstones and mudstones, is evidence for authigenic Cr enrichment under locally mildly-reducing conditions in a fluvial setting and strongly oxidizing conditions during weathering and riverine transport. Uranium released from the lower, fluvial oxidized sandstones and added to the reduced sandstones and silty mudstones in the upper tidal-deltaic sediments potentially resulted in a uranium mineralization of a sandstone-type in the FA Formation of the Franceville Basin.

Variations in zircon Hf isotopes support earliest Proterozoic Wilson cycle tectonics on the Canadian Shield

Sedimentary basins provide an archive of magmatic and tectonic events in their detrital zircon record, which can be dated and coupled with Hf isotopic data to reveal secular changes in the production of juvenile and evolved magmatism and track the history of orogenic events. The Rae craton, which forms a substantial portion of the northern Canadian Shield, experienced successive orogenic events along its margins during most of the Paleoproterozoic Era. Yet, some of these orogenic events are poorly defined and their details remain controversial. We present coupled detrital zircon U–Pb age and εHf data from Paleoproterozoic metasedimentary successions of the eastern Rae craton. The zircon preserve an archive of ∼600million yr of magmatic and tectonic history on the Rae craton and its periphery. The U–Pb and Hf isotopic data indicate that zircon provenance was dominantly from the northern and western Rae craton, where magmatic and tectonic activity was focused during the ∼2.5 to 2.3Ga Arrowsmith orogeny and the ∼2.0 to ∼1.93Ga Taltson and Thelon events. Our detrital zircon dataset holds a more complete record of these Proterozoic orogenic events than is currently defined from known and dated crustal rocks on the Rae craton and shows that juvenile magmatism is more common than current models for either event suggest. Notably, an inverted U or horseshoe-shaped εHf array (−15.5 to +5.0) in our zircon dataset reflects a Wilson cycle that began in the earliest Paleoproterozoic on the Rae craton.

Neoarchean–Early Paleoproterozoic and Early Neoproterozoic arc magmatism in the Lützow–Holm Complex, East Antarctica: Insights from petrology, geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes

The Lützow–Holm Complex (LHC) of East Antarctica forms part of the Neoproterozoic–Cambrian high-grade metamorphic segment of the East African–Antarctic Orogen. Here we present new petrological, geochemical, and zircon U–Pb and Lu–Hf isotopic data for meta-igneous rocks including charnockite, felsic gneiss, metagabbro, and mafic granulite from the LHC and evaluate the Neoarchean to Early Paleoproterozoic (ca. 2.5Ga) and Early Neoproterozoic (ca. 1.0Ga) arc magmatic events. The trace element geochemical signatures reveal a volcanic arc affinity for the charnockites from Sudare Rocks and Vesleknausen and felsic gneiss from Rundvågshetta, suggesting that the protoliths of these rocks were derived from felsic arc magmas. In contrast, metagabbros from Skallevikshalsen and Austhovde, occurring as boudins in metasediments, show non-arc signatures (within-plate basalt or mid-oceanic ridge basalt). The upper intercept ages of magmatic zircons in charnockite plotted on concordia diagrams yielded 2508±14Ma (Sudare Rocks) and 2490±18Ma (Vesleknausen), clearly suggesting a Neoarchean to Early Paleoproterozoic arc magmatic event. A subsequent thermal event during Early Neoproterozoic traced by 206Pb/238U age of oscillatory-zoned core of zircon in mafic granulite from Langhovde (973±10Ma) is consistent with a similar Early Neoproterozoic magmatic event reported from the LHC, suggesting a second stage of arc magmatism. The timing of peak metamorphism has been inferred from 206Pb/238U mean ages of structureless zircons in metagabbros from Skallevikshalsen and Austhovde, mafic granulite from Langhovde, and felsic gneiss from Rundvågshetta in the range of 551±5.4 to 584±5.0Ma. Zircon Lu–Hf data of Neoarchean charnockites from Sudare Rocks and Vesleknausen indicate that the protolith magma was sourced from Paleo- to Neoarchean juvenile components mixed with reworked ancient crustal materials. Protolith magmatic rock of the felsic gneiss from Rundvågshetta might have been sourced from reworked Paleoarchean crust. In contrast, metagabbros from Skallevikshalsen and Austhovde were derived from the Paleoproterozoic reworked crust. The results of this study suggest the occurrence of several discrete crustal fragments of ca. 2.5Ga and ca. 1.0Ga juvenile magmatic arcs in the southwestern part of the LHC, which amalgamated during the latest Neoproterozoic collisional event related to the final stage of assembly of the Gondwana supercontinent.

Tectonics and metallogeny of the junction zone between the North Asian craton and Pacific tectonic belt

The tectonics and metallogeny of the junction zone between the North Asian craton and Pacific tectonic belt are considered. This zone is characterized by a wide variety of structures superposed on the metamorphic basement, which was formed in the course of a multistage geologic development of the craton from the Precambrian to the Cenozoic. They are related to the craton evolution and its response to the collision and subduction processes in the adjacent orogenic belt, processes in the passive and active continental margins, and plume magmatism. The geological structure of the region includes blocks of metamorphic rocks of the Aldan–Stanovoi shield, Paleoproterozoic volcanogenic troughs, Mesoproterozoic–Neoproterozoic and Early Paleozoic structures of the platform cover, Late Paleozoic volcanic and terrigenous troughs, structures of the Late Mesozoic Okhotsk–Chukotka volcanic belt of the active continental margin, and Late Cretaceous riftogenic structures formed in response to plume magmatism. In total, six metallogenic epochs are recognized in the development of ore mineralization: Archean–Early Paleoproterozoic, Late Paleoproterozoic, Mesoproterozoic, Neoproterozoic, Late Paleozoic, and Late Mesozoic. The minerageny of the junction zone between the craton and Pacific belt is highly diversified, being characterized by distinct evolution in time and space. Each development stage features its own set of mineral resources.

Euxinic conditions recorded in the ca. 1.93 Ga Bravo Lake Formation, Nunavut (Canada): Implications for oceanic redox evolution

The composition of seawater changed dramatically during the initial rise of atmospheric oxygen in the earliest Paleoproterozoic, but the emerging view is that atmosphere–ocean system did not experience an irreversible transition to a well-oxygenated state. Instead, it has been suggested that the oxygen content of the atmosphere–ocean system decreased considerably after ca. 2.06billionyearsago (Ga), which resulted in a crash in marine sulfate concentrations. The end of the deposition of major granular iron formations at ca. 1.85Ga has been linked either to the development of extensive euxinic conditions along continental shelves or a decrease in hydrothermal flux. The record of oceanic redox state is not well constrained for the period between ca. 2.06Ga, the end of the Lomagundi positive carbon isotope excursion, and ca. 1.88Ga when major granular iron formations appeared. We address this gap by presenting new iron-speciation, major and trace element data, as well as sulfur, organic carbon, and molybdenum isotopic data for greenschist facies organic matter-rich mudrocks (ORMs) of the ca. 1.93Ga Bravo Lake Formation, Piling Group, Baffin Island. The iron speciation data suggest deposition of the Bravo Lake Formation under a euxinic (anoxic and sulfidic) water column. Trace metal enrichments and Mo isotope data suggest extensive marine euxinia ca. 90millionyears before the disappearance of large-scale, economic granular iron formations. The addition of new Mo data in this time interval is important, as it contributes to filling in the sparse Proterozoic record. Lastly, this work provides further support for the idea that there was widespread anoxia shortly after the end of the Lomagundi Event.

An iodine record of Paleoproterozoic surface ocean oxygenation

Constraining oxygen levels in the early Precambrian surface ocean has been a longstanding goal, but efforts have been challenged by the availability of suitable proxies. Here we present a novel approach, iodine geochemistry, which broadens our perspective by providing constraints on shallow, carbonate-dominated marine settings. Iodate (IO 3 – ) persists exclusively in oxic waters and is the sole iodine species incorporated into carbonate minerals, allowing iodine-to-calcium ratios (I/Ca) in shallow carbonates to be used as a paleoredox indicator. Our data from a series of Mesoarchean through Paleoproterozoic carbonates deposited under shallow-marine conditions reveal a progressive surface ocean oxygenation in the early Paleoproterozoic. These data seem to indicate that a largely anoxic surface ocean extended throughout the Archean until the Great Oxidation Event (GOE) at ca. 2.4 Ga, implying that previous inferences of pre-GOE oxygen production may reflect oxygen oases, transient oxidation events, or oxygen levels below those required for IO 3 – accumulation. The data suggest formation and persistence of IO 3 – and, consequently, surface ocean oxygen concentrations of at least 1 μM during the GOE. Following the initial rise of oxygen, carbonate-associated iodine in globally extensive carbonate units deposited during the Lomagundi positive carbon isotope excursion at ca. 2.22–2.1 Ga suggests a widespread aerobic iodine cycle beyond that operating prior to the event, synchronous with high relative rates of organic carbon burial and apparent expansion of oxidative conditions.

Geochemistry and isotopic evolution of the central African Domes, Bangweulu and Irumide regions: Evidence for cryptic Archean sources and a Paleoproterozoic continental arc

The interregional cratonic relations between the Paleo- and Mesoproterozoic basement units surrounding the Neoproterozoic Central African Copperbelt are still largely unresolved, although they are regarded as major potential metal sources. This study focuses on the Domes region basement at depth below the Copperbelt and its relationship to the neighboring Bangweulu Block and its destabilized margin, the Irumide Belt. We applied an integrated whole rock petrochemical and Sm–Nd isotopic approach to major lithological units to assess the proposed mid-Proterozoic arc setting for the Domes basement inliers along with their relationship to the neighboring areas. The available petrochemical and isotopic data for the Paleoproterozoic eastern Domes granitoids and magmatic units in the SW Bangweulu Block is consistent with a continental arc setting. Moreover, the mid-Paleoproterozoic Nd isotope ratios preclude an island arc because they are significantly less radiogenic than the depleted mantle. Predominantly Archean and Early Paleoproterozoic depleted mantle model ages in all terranes indicate limited juvenile input during Paleo- and Mesoproterozoic magmatic phases. Finally, broadly similar model ages in the Domes inliers and the Bangweulu–Irumide region suggest a relationship between these terranes.

Pyrite multiple-sulfur isotope evidence for rapid expansion and contraction of the early Paleoproterozoic seawater sulfate reservoir

Earthʼs oxygenation is often described in terms of two unidirectional steps at the beginning and end of the Proterozoic Eon, separated by a long-lived intermediate redox state. Recent work defines a more complicated path to oxygenation, exemplified by an apparent drop in oxidation state following the early Paleoproterozoic Lomagundi carbon isotope excursion. The timing of this proposed drop in oxidation state is not well constrained, and it is not clear how it relates to redox conditions during the remainder of the Proterozoic. Here we present a study of pyrite multiple-sulfur isotopes, supported by Fe speciation and organic carbon isotopes, from early Paleoproterozoic black shales. We find evidence for the rapid expansion of the seawater sulfate reservoir during the Great Oxidation Event at ca. 2.3 Ga followed by a subsequent contraction in the size of the seawater sulfate reservoir at ca. 2.05 Ga. This scenario is consistent with the emerging view of a rise and fall in surface oxidation state during the early Paleoproterozoic. Comparison of our new data to other records of the seawater sulfate reservoir suggests that the elevated sulfate concentrations that characterize the early Paleoproterozoic did not return until the late Neoproterozoic.

Episodic crustal growth in the southern segment of the Trans-North China Orogen across the Archean-Proterozoic boundary

The Dengfeng and Taihua Complexes are well-exposed Neoarchean to Paleoproterozoic units in the southern segment of the Trans-North China Orogen (TNCO). Zircon U–Pb dating shows that the Dengfeng Complex records two episodes (2568±11Ma and 2306±6Ma) of tonalite–trondhjemite–granodiorite (TTG) magmatism. All of the TTG rocks are characterized by high SiO2 (66.7–75.4wt%), Na2O (3.20–5.06wt%) and relatively low MgO (0.40–1.78wt%). The Late Neoarchean TTG gneisses have very low contents of HREE (YbN=0.69–2.75) and Y (1.73–7.07ppm), with moderate [La/Yb]N (24.1–53.8) and high Sr/Y (65.1–291.3) ratios. The Early Paleoproterozoic TTG gneisses have low contents of HREE (YbN=2.93–6.37) and Y (6.7–11.0ppm), with moderate [La/Yb]N (10.1–27.0) and Sr/Y (10.6–52.1) ratios. Both suites show pronounced negative Nb–Ta, P and Ti anomalies but positive Sr and Pb anomalies. The Late Neoarchean TTG gneisses all have similar bulk-rock Nd and zircon Hf model ages with mainly positive ɛNd(t), and are interpreted as resulting from the melting of dominantly juvenile thickened lower crust with residual garnet and amphibole. The Early Paleoproterozoic TTG gneisses have extremely variable ɛNd(t) (−6.23 to +4.23) and heterogeneous zircon ɛHf(t) (−3.3 to +3.1), which are also best interpreted as resulting from the partial melting of thickened lower crust with residual amphibole and garnet. The Taihua Complex in the Xiaoqinling area records three episodes of Early Paleoproterozoic TTG magmatism (2.48Ga at Caotan, 2.31Ga at Houjiacun and 2.16Ga at Bayuan), younger than the Taihua Complex in the Lushan area (2.85–2.72Ga). All rocks have relatively low contents of HREE (YbN=1.03–8.32) and Y (2.84–24ppm), with moderate [La/Yb]N (8.7–88.4) and Sr/Y (19.8–125.8) ratios, and show negative Ta–Nb and Ti anomalies and positive Sr and Pb anomalies. The Caotan gneisses at 2.48Ga and the Houjiacun TTG gneisses at 2.31Ga have low Mg# (0.14–0.45), low Cr (<42ppm) and Ni contents (1–21ppm), with variable but overall positive ɛNd(t) and ɛHf(t) values, and were derived from the partial melting of thickened lower crust with residual garnet and amphibole. The younger Bayuan TTG gneisses at 2.16Ga have low SiO2 (57.11–64.89wt%), high MgO (2.64–4.62wt%), Cr (100–247ppm) and Ni (32–80ppm), with negative whole rock ɛNd(t) and zircon ɛHf(t) values, resulted from the partial melting of delaminated lower crust that interacted with peridotitic mantle.The geochronology of the Dengfeng Complex (in the Dengfeng area) and the Taihua Complex (in the Lushan, Xiong’er and Xiaoqinling areas) reveals at least four magmatic episodes in the southern segment of the TNCO from the Late Mesoarchean to Early Paleoproterozoic (2.85–2.72Ga, 2.57–2.48Ga, 2.34–2.30Ga and 2.20–2.07Ga). The rocks of the two early episodes are dominantly of juvenile compositions with mostly positive whole rock ɛNd(t) and zircon ɛHf(t) values, suggesting two episodes of crustal growth formed in a subduction tectonic setting. The magmatic rocks of the third episode consist of both the juvenile and pre-existing crustal materials with variable whole rock ɛNd(t) and zircon ɛHf(t) values, which were generated in a subduction zone during the initial assembly of the NCC within the Columbia supercontinent cycle. The final episode of magmatism lacks juvenile materials with whole rock ɛNd(t) and zircon ɛHf(t) values being consistently negative. These may have resulted from the orogenic collapse. The episodic continental growth recorded in the southern segment of the TNCO was caused by subduction and consequent orogeny, consistent with global supercontinent cycles within the Late Archean and Early Paleoproterozoic.

Early Paleoproterozoic magmatism in the Quanji Massif, northeastern margin of the Qinghai–Tibet Plateau and its tectonic significance: LA-ICPMS U–Pb zircon geochronology and geochemistry

The Quanji Massif is located on the north side of the Qaidam Block and is interpreted as an ancient cratonic remnant that was detached from the Tarim Craton. There are regionally exposed granitic gneisses in the basement of the Quanji Massif whose protoliths were granitic intrusive rocks. Previous studies obtained intrusion ages for some of these granitic gneiss protoliths. The intrusion ages span a wide range from ~ 2.2 Ga to ~ 2.47 Ga. This study has determined the U–Pb zircon age of four granitic gneiss samples from the eastern, central and western parts of the Quanji Massif. CL images and trace elements show that the zircons from these four granitic gneisses have typical magmatic origins, and experienced different degrees of Pb loss due to strong metamorphism and deformation. LA-ICPMS zircon dating yields an upper intercept age of 2381 ± 41 (2σ) Ma from monzo-granitic gneiss in the Hudesheng area and 2392 ± 25 (2σ) Ma from granodioritic gneiss in the Mohe area, eastern Quanji Massif, and 2367 ± 12 (2σ) Ma from monzo-granitic gneiss in the Delingha area, central Quanji Massif, and 2372 ± 22 (2σ) Ma from monzo-granitic gneiss in the Quanjishan area, western Quanji Massif. These results reveal that the intrusive age of the protoliths of the widespread granitic gneisses in the Quanji Massif basement was restricted between 2.37 and 2.39 Ga, indicating regional granitic magmatism in the early Paleoproterozoic, perhaps related to the fragmentation stage of the Kenorland supercontinent. Geochemical results from the granodioritic gneiss from the Mohe area indicate that the protolith of this gneiss is characterized by adakitic rocks derived from partial melting of garnet-amphibolite beneath a thickened lower crust in a rifting regime after continent–continent collision and crustal thickening, genetically similar to the TTG gneisses in the North China Craton. This suggests that the Quanji Massif had a tectonic history similar to the Archean Central Orogenic Belt of North China Craton during the early Paleoproterozoic. We tentatively suggest that the Quanji Massif and the parental Tarim Craton and the North China Craton experienced rifting in the early Paleoproterozoic, after amalgamation at the end of the Archean. The Tarim Craton and North China Craton might have had close interaction from the late Neoarchean to the early Paleoproterozoic.

Rare Earth Element and yttrium compositions of Archean and Paleoproterozoic Fe formations revisited: New perspectives on the significance and mechanisms of deposition

The ocean and atmosphere were largely anoxic in the early Precambrian, resulting in an Fe cycle that was dramatically different than today’s. Extremely Fe-rich sedimentary deposits—i.e., Fe formations—are the most conspicuous manifestation of this distinct Fe cycle. Rare Earth Element (REE) systematics have long been used as a tool to understand the origin of Fe formations and the corresponding chemistry of the ancient ocean. However, many earlier REE studies of Fe formations have drawn ambiguous conclusions, partially due to analytical limitations and sampling from severely altered units. Here, we present new chemical analyses of Fe formation samples from 18 units, ranging in age from ca. 3.0 to 1.8 billion years old (Ga), which allow a reevaluation of the depositional mechanisms and significance of Precambrian Fe formations. There are several temporal trends in our REE and Y dataset that reflect shifts in marine redox conditions. In general, Archean Fe formations do not display significant shale-normalized negative Ce anomalies, and only Fe formations younger than 1.9 Ga display prominent positive Ce anomalies. Low Y/Ho ratios and high shale-normalized light to heavy REE (LREE/HREE) ratios are also present in ca. 1.9 Ga and younger Fe formations but are essentially absent in their Archean counterparts. These marked differences in Paleoproterozoic versus Archean REE + Y patterns can be explained in terms of varying REE cycling in the water column. Similar to modern redox-stratified basins, the REE + Y patterns in late Paleoproterozoic Fe formations record evidence of a shuttle of metal and Ce oxides across the redoxcline from oxic shallow seawater to deeper anoxic waters. Oxide dissolution—mainly of Mn oxides—in an anoxic water column lowers the dissolved Y/Ho ratio, raises the light to heavy REE ratio, and increases the concentration of Ce relative to the neighboring REE (La and Pr). Fe oxides precipitating at or near the chemocline will capture these REE anomalies and thus evidence for this oxide shuttle. In contrast, Archean Fe formations do not display REE + Y patterns indicative of an oxide shuttle, which implies an absence of a distinct Mn redoxcline prior to the rise of atmospheric oxygen in the early Paleoproterozoic. As further evidence for reducing conditions in shallow-water environments of the Archean ocean, REE data for carbonates deposited on shallow-water Archean carbonate platforms that stratigraphically underlie Fe formations also lack negative Ce anomalies. These results question classical models for deposition of Archean Fe formations that invoke oxidation by free oxygen at or above a redoxcline. In contrast, we add to growing evidence that metabolic Fe oxidation is a more likely oxidative mechanism for these Fe formations, implying that the Fe distribution in Archean oceans could have been controlled by microbial Fe uptake rather than the oxidative potential of shallow-marine environments.

Archean crustal evolution of the northern Tarim craton, NW China: Zircon U–Pb and Hf isotopic constraints

Tarim craton is one of the major cratons in Central Asia. Basement rocks of the craton, such as TTG gneisses, are dominantly exposed in the northern (Kuluketage area) and the Eastern (Altyn Tagh Mountain). Recent years, only a few reliable Archean ages have been obtained for the basement rocks in the Altyn Tagh Mountain. Geochronological and geochemical studies have been conducted on the orthogneisses exposed in the Kuluketage area in order to unravel complex history of the Tarim craton. Zircon U–Pb dating of the rocks yielded three weighted mean 207Pb/ 206Pb ages of 2516 ± 6, 2575 ± 13 and 2460 ± 3 Ma, indicating that their protoliths were formed in the late Neoarchean to early Paleoproterozoic. The late Neoarchean orthogneisses exhibit low Sr/Y ratios (4–19) and Mg # values (35–44), with depleted Sr contents, negative Nb, Ta and Ti anomalies and strongly fractionated REE patterns ((La/Yb) N = 13–62), displaying typical geochemical features of arc igneous rocks. In contrast, the early Paleoproterozoic orthogneisses are characterized by high-SiO 2 contents, Sr/Y ratios (37–67) and Mg # values (46–67), with variably fractionated REE patterns ((La/Yb) N = 9–89), slight Sr enrichment and positive Eu anomalies, resembling high-SiO 2 adakites derived from subducted basaltic slab-melts. The late Neoarchean TTGs have low ɛ Hf( t) values (–5 to +1) and initial Hf compositions (0.280987–0.281160) with Mesoarchean two-stage model ages ( T DM2 = 2.9–3.3 Ga), suggesting that the crustal materials of the basement rocks in this area were initially extracted from a depleted mantle in the late Paleo- to Mesoarchean and were reworked in the late Neoarchean. The history of crustal growth is different from that of the North China and Yangtze cratons and thus implies relatively younger cratonization than the North China and Yangtze cratons. Zircons in the early Paleoproterozoic orthogneisses have high ɛ Hf( t) values (+4 to +10) and young Neoarchean two-stage model ages (2.5–2.7 Ga), revealing a juvenile crustal growth event in the late Neoarchean. Because there is no record of coeval juvenile crustal growth in the other two cratons, we suggest a separate crustal evolution for the northern Tarim craton which was stayed far from the other two cratons before the early Paleoproterozoic.

Within-plate (intracontinental) and postorogenic magmatism of the East European Craton as reflection of the evolution of continental lithosphere

A comparative analysis of within-plate (intracontinental) and orogenic magmatic series formed during various evolution stages of the East European Craton (EEC) was performed using geological-petrological, geochemical, and isotopic data. The example of Baltic shield indicates that the compositions and tectonic settings of mantle melts in the Early Precambrian (Archean and Early Paleoproterozoic) significantly differed from those in the Phanerozoic. The Early Precambrian magmas were dominated by high-Mg low-Ti melts of the komatiite-basaltic and boninite-like series; this tectonomagmatic activity was determined by the ascent of mantle superplumes of the first generation, which originated in the depleted mantle. In the interval of 2.3–2.0 Ga, high-Mg mantle melts gradually gave place to the Fe-Ti picrites and basalts that are typical of within-plate Phanerozoic magmatism; at ∼2 Ga, plume tectonics of the Early Precambrian gave way to plate tectonics. This is considered to be linked to the activity of mantle superplumes of the second generation (thermochemical), which originated from the liquid metallic core/mantle interface. Owing to the presence of fluid components, these superplumes reached much higher levels, where spreading of their head portions led to the active interaction with overlaying thinned rigid lithosphere. Sm-Nd isotopic studies showed that orogenic Neoarchean and Middle Paleoproterozoic magmatism of the Baltic shield was connected to the melting of the lithospheric mantle and crust; the melting of crustal sources gave rise to felsic members of the considered complexes. The systematic geochemical variations observed in these rocks with time presumably reflect a general trend toward an increase of the thickness of the continental crust serving as the basement for orogens. Beginning at ∼2 Ga, the Meso, Neoproterozoic, and Phanerozoic including, no systematic variations were observed in the isotopic-geochemical characteristics of within-plate magmatism. All considered age sections demonstrate that isotopic-geochemical characteristics of parental mantle melts were strongly modified by crustal contamination. Mesoproterozoic magmatism of EEC was unique in the development of giant anorthosite-rapakivi granite complexes. Kimberlites and lamproites were repeatedly formed within EEC in the time interval from 1.8 to 0.36 Ga; their maximal development was noted in the Late Devonian. It was shown that only kimberlites derived from weakly enriched mantle are diamondiferous in the Arkhangelsk province; in the classic diamond provinces (Africa and Yakutia), diamondiferous kimberlites were derived from both depleted and enriched mantle.

Pyrite paragenesis and multiple sulfur isotope distribution in late Archean and early Paleoproterozoic Hamersley Basin sediments

The sulfur isotope record in late Archean and early Paleoproterozoic rocks is of considerable importance because it provides evidence for changes in early Earth atmospheric oxygen levels and potentially constrains the origin and relative impact of various microbial metabolisms during the transition from an anoxic to oxic atmosphere. Mass independently fractionated (MIF) sulfur isotopes reveal late Archean and early Paleoproterozoic sulfur sources in different pyrite morphologies in Western Australia's Hamersley Basin. Multiple sulfur isotope values in late Archean pyrite vary according to morphology. Fine grained pyrite has positive sulfur MIF, indicating a reduced elemental sulfur source, whereas pyrite nodules have negative sulfur MIF, potentially derived from soluble sulfate via microbial sulfate reduction. The Hamersley Basin δ 34S–Δ 33S record suggests that the extent of oxygenation of the surface ocean fluctuated through the Late Archean from at least 2.6 Ga, more than 150 million yr before the Great Oxidation Event. In the early Paleoproterozoic, there is less distinction between pyrite morphologies with respect to sulfur isotope fractionation, and pyrite from the Brockman Iron Formation trends toward modern sulfur isotope values. An important exception to this is the strong negative MIF recorded in layer parallel pyrite in Paleoproterozoic carbonate facies iron formation. This may suggest that deeper water hydrothermal environments remained anoxic while shallower water environments became more oxidised by the early Paleoproterozoic. The results of the current study indicate that sulfide paragenesis is highly significant when investigating Archean and early Paleoproterozoic multiple sulfur isotope compositions and sulfur sources.

Mineralization and deformation of the Malanjkhand terrane (2,490–2,440 Ma) along the southern margin of the Central Indian Tectonic Zone

The Malanjkhand Cu–Mo–Au deposit, located near the northwest margin of the Malanjkhand batholith (terrane), is a strategic and significant porphyry-style deposit that experienced a protracted 50 m.y. deformational history shortly after its formation at 2,490±8 Ma (Stein et al. 2004). In a recent study, Panigrahi et al. (2004) averaged U–Pb SHRIMP zircon data from a pooled set of samples from the Malanjkhand batholith to advocate a meaningless intermediate age of ~2,476 Ma for the Malanjkhand granitoid and its Cu–Mo–Au deposit. In the northwest part of the Malanjkhand batholith, Re–Os dating of occurrence-specific molybdenite captures not only the age of porphyry-style mineralization and associated magmatism, but also elucidates a complex deformational history that extends to ~2,450 Ma. In the central part of the Malanjkhand batholith, Re–Os dating of delicate spindles of accessory molybdenite occurring with pristine muscovite in miarolitic cavities within the undeformed microgranitoid at the Devgaon Mo prospect unequivocally shows that deformation ceased at this location no later than 2,470–2,465 Ma. The deformational history recorded at the Malanjkhand deposit in the northwest most likely reflects prolonged transpressive convergence and docking of the Malanjkhand terrane with units in the poorly understood (proto) Central Indian Tectonic Zone (CITZ) along its southern margin, the Central Indian shear zone. The timing for this convergence is Late Archean–Early Paleoproterozoic.

Giant iron-ore deposits of the Hamersley province related to the breakup of Paleoproterozoic Australia: New insights from in situ SHRIMP dating of baddeleyite from mafic intrusions

Banded iron formations of the ca. 2770–2405 Ma Hamersley province of Western Australia were locally upgraded to high-grade hematite ores during the Early Paleoproterozoic by a combination of hypogene and supergene processes after the initial rise of atmospheric oxygen. Ore genesis was associated with the stratigraphic break between the Lower and Upper Wyloo Groups of the Ashburton province, and has been variously linked to the Ophthalmian orogeny, late-orogenic extensional collapse, and anorogenic continental extension. Small-spot in situ Pb/Pb dating of baddeleyite by sensitive high-resolution ion microprobe (SHRIMP) has resolved the ages of two key suites of mafic intrusions, constraining for the first time the tectonic evolution of the Ashburton province and the age and setting of iron-ore formation. Mafic sills dated as ca. 2208 Ma were folded during the Ophthalmian orogeny and then cut by the unconformity at the base of the Lower Wyloo Group. A mafic dike swarm that intrudes the Lower Wyloo Group and has a close genetic relationship to iron ore is ca. 2008 Ma, slightly younger than a new syneruptive 2031 ± 6 Ma zircon age for the Lower Wyloo Group. These new ages constrain the Ophthalmian orogeny to the period between ca. 2208 and 2031 Ma, before Lower Wyloo Group extension, sedimentation, and flood-basalt volcanism. The ca. 2008 Ma dikes pre s ent a new maximum age for iron-ore genesis and deposition of the Upper Wyloo Group, thereby linking ore genesis to a ca. 2050–2000 Ma period of continental extension similarly recorded by Paleoproterozoic terrains worldwide well after the initial oxidation of the atmosphere by ca. 2320 Ma.