Ga Magmatic Rocks Research Articles

An isolated mildly depleted mantle source for the north atlantic craton

Generation of continental crust during early Earth was a vital step towards stabilizing the cratonic lithosphere. How and when this happened is, however, still poorly understood. Radiogenic isotope data from the oldest crust, and minerals therein, provide a unique insight into the timing and processes that operated during Earth’s earliest evolution. Using combined zircon U-Pb, O and Lu-Hf isotope data from a glacial stream-sediment within the Isukasia (Isua) terrane of southern West Greenland, we argue for an isolated mantle source for the ca. 1.3 Ga of initial stages of crustal growth in the North Atlantic Craton. Zircon Hf isotope data between 3939 and 2950 Ma yield strikingly homogeneous, near chondritic signatures, with the few analyses that deviate slightly being indicative of ancient Pb-loss. Combined with published data from carefully selected and well-characterized ca. 3.9–2.6 Ga magmatic rocks of the NAC, almost no indications of reworking of older crust can be detected, but all strongly sub-chondritic data rather fall on Pb-loss trends tracing back to chondritic sources. This long-lived and near-chondritic signature with negligible influence from a strongly depleted mantle, or from un-radiogenic sources, argues for protracted crust separation from an isolated mantle source. We suggest a scenario whereby primary melting to produce basaltic and komatiitic crust leaves an infertile harzburgitic residue, unlikely to re-melt, that becomes sequestered as mantle lithosphere. Hence, domains of primordial fertile mantle are continuously sourced from an undepleted mantle reservoir. Combined with small wavelength mantle convection, a near-chondritic to mildly depleted regional mantle is retained into at least the Mesoarchean.

Tectonomagmatic evolution of southwestern Laurentia: Insights from zircon U-Pb geochronology and hafnium isotopic composition of the Red Bluff Granite Suite, west Texas, USA

AbstractWe provide laser ablation–multicollector–inductively coupled plasma–mass spectrometry (LA-MC-ICP-MS) and high-precision chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb ages and Hf isotopic compositions of zircons from the Red Bluff Granite Suite and mafic dikes in the Franklin Mountains of El Paso, Texas, USA. Granitoids exposed in the Franklin Mountains were previously divided into five magmatic stages based on cross-cutting relationships. Major and trace element compositions showed that these granitoids are ferroan, alkaline, and A2 type. Homogeneity in the whole-rock geochemistry suggests that the granite stages are genetically related and share similar petrogenetic histories. Weighted mean zircon 206Pb/238U dates from the older magmatic stage 1 alkali-feldspar quartz syenite and stage 2 alkali-feldspar granite are 1112.36 ± 0.35 and 1112.46 ± 0.37 Ma, respectively. The weighted mean εHf(t) values varying from +5.3 to +7.2 are similar to those of other regional ca. 1.1 Ga magmatic rocks throughout southwestern Laurentia. Geochemical characteristics, petrological modeling, and enriched Hf isotopic composition suggest fractional crystallization of a basaltic magma that was produced by melting of an enriched mantle reservoir. However, zircon inheritance ages of ca. 1.3 Ga and 1.26–1.15 Ga are consistent with a minor contribution from felsic crustal basement. Our data and regional geology are consistent with a post-collisional slab break-off that facilitated asthenospheric upwelling and partial melting of the enriched mantle, possibly subcontinental lithospheric mantle, extending from Llano Uplift, Texas, in the southeast to California to the northwest. Magma thus generated upon differentiation produced ferroan and A-type granitoids.

Paleoproterozoic Plate Tectonics Recorded in the Northern Margin Orogen, North China Craton

AbstractThe occurrence of plate tectonic processes on Earth during the Paleoproterozoic is supported by ca. 2.2–1.8 Ga subduction‐collision orogens associated with the assembly of the Columbia‐Nuna supercontinent. Subsequent supercontinent breakup is evidence by global ca. 1.8–1.6 Ga large igneous provinces. The North China craton is notable for containing Paleoproterozoic orogens along its margins, herein named the Northern Margin orogen, yet the nature and timing of orogenic and extensional processes of these orogens and their role in the supercontinent cycle remain unclear. In this contribution, we present new field observations, U‐Pb zircon and baddeleyite geochronology dates, and major/trace‐element and isotope geochemical analyses from the northern margin of the North China craton that detail its Paleoproterozoic tectonic and magmatic history. Specifically, we record the occurrence of ca. 2.2–2.0 Ga magmatic arc rocks, ca. 1.9–1.88 Ga tectonic mélange and mylonitic shear zones, and folded lower Paleoproterozoic strata. These rocks were affected by ca. 1.9–1.8 Ga granulite‐facies metamorphism and ca. 1.87–1.78 Ga post‐collisional, extension‐related magmatism along the cratonal northern margin. We interpret that the generation and emplacement of these rocks, and the coupled metamorphic and magmatic processes, were related to oceanic subduction and subsequent continent‐continent collision during the Paleoproterozoic. The occurrence of ca. 1.77–1.73 Ga mafic dykes and ca. 1.75 Ga mylonitic shear zones along the northern margin of the North China craton may have been related to a regional mantle plume event. Our results are consistent with modern style plate tectonics, including oceanic subduction‐related plate convergence and continent‐continent collision, operating in the Paleoproterozoic.

Provenance of metasiliciclastic rocks at the northwestern margin of the East Gabonian Block: Implications for deposition of BIFs and crustal evolution in southwestern Cameroon

The Congo craton hosts banded iron formations (BIFs) in southern Cameroon. The Gouap iron deposit is in the central part of the Nyong Complex, next to the Ntem Complex in the northwestern part of the East Gabonian Block of the Congo craton. The Gouap BIFs are interbedded with metasiliciclastic rocks consisting of gneisses, schists, and quartzites. We present the first integrated study combining whole-rock and Sm-Nd isotope geochemistry with SIMS and LA-ICP-MS U-Pb-Hf-O isotope analyses of detrital zircons from metasiliciclastic rocks from the Gouap iron deposit to assess the provenance and tectonic setting during deposition of BIFs as well as the crustal evolution at the northwestern margin of the East Gabonian Block. Geochemical data for the metasiliciclastic rocks indicate that their protolith comprised a sequence of greywacke. Along with regional geology, our U-Pb-Hf-O detrital zircon and Sm-Nd whole-rock isotope data suggest that these rocks were predominantly derived from Neoarchean charnockite and TTG suites of the Ntem Complex with a variable and episodic contribution from ca. 2.2–2.0 Ga charnockites of the Nyong and Ogooué complexes. Metasiliciclastic rocks and BIFs were deposited between ca. 2.1 and 2.0 Ga in a foreland basin developed along the northwestern continental margin of the East Gabonian Block. So far unrecognized ca. 2.50–2.45 and 2.45–2.22 Ga magmatic rocks were exposed in their provenance. The Gouap succession experienced amphibolite-facies metamorphism when the Congo and São Francisco cratons collided during the Eburnean-Transamazonian orogeny to form the core of the Nuna/Columbia supercontinent. With the subduction dipping to the west, under the West Gabonian Block, slices of Paleoproterozoic oceanic crust and sediments scraped off the margin of the East Gabonian Block formed an accretionary prism. BIFs of the Nyong Complex likely represent a deep-water temporal equivalent of shallow-marine Mn-deposits of the Francevillian basin, Gabon, and together they belong to the same redox-fractionated deep-water upwelling system that was charged with iron and manganese via hydrothermal alteration of oceanic crust.

1.79–1.75 Ga mafic magmatism of the Siberian craton and late Paleoproterozoic paleogeography

The paper presents new geological, geochronological, geochemical, and Nd isotopic data on the late Paleoproterozoic dolerites of the Siberian craton. U-Pb baddeleyite ages of the Doros dolerites (Aldan shield, south-eastern Siberia) and East Anabar dolerites (Anabar shield, northern Siberia) are 1757 ± 4 and 1774 ± 6 Ma, respectively. This new geochronological data together with published 1.76–1.75 Ga ages for Timpton-Algamay dolerites of the Aldan shield and Chaya dolerites of the Baikal uplift (southern Siberia) revealed a 20 my difference between this magmatism in the north and south Siberia. The Doros dolerites in their chemical compositions correspond to sub-alkaline basaltic andesites, but the East Anabar dolerite is chemically close to basaltic trachyandesite. The Doros dolerites demonstrate negative and close to zero ɛNd(t) values (from −7.0 to +0.1), which correlate well with SiO2 and Mg#. All Doros dolerites have Nb–Ta and Ti anomalies in multielement spectra. The geochemical and Nd isotopic data suggest that the Doros dolerites have been formed by the mixing of mantle and crustal material. The East Anabar dolerites are characterized by a negative ɛNd(t) value of −3.7, negative Nb–Ta anomaly in multielement spectra, high concentrations of TiO2 and P2O5. The 1775 Ma East Anabar dolerites could be generated from a subcontinental lithospheric mantle source, possibly with some mantle plume interaction (possibly centred at the convergence with the 1.76–1.75 Ga fanning Aldan swarm). Geochemistry and Nd isotope systematics of all 1.78–1.75 Ga mafic dykes and intrusions of the Siberian craton indicate the subcontinental lithospheric mantle source or mantle source contaminated by crustal material. Geochronological data from the 1.79–1.75 Ga magmatic rocks of Siberia and other continents suggest continuous magmatism over this interval in some continents, but a series of short magmatic events/pulses (from one to four) separated by intervals of quiescence in other continents. We locate the analysed dykes and sills on new 1750 Ma and 1790 Ma global paleogeographic reconstructions. Analysis of 1.79–1.75 Ga geochemical data on mafic intrusions from Late Paleoproterozoic continents suggests the prevalence of subcontinental lithospheric mantle source for the mafic intrusions over the pure mantle plume source.

Reply to comment on “How diverse is the source? Age, provenance, reworking, and overprint of Precambrian meta-sedimentary rocks of West Gondwana, Cameroon, from zircon U-Pb geochronology’’ by Hubert Mvondo and Thierry Bineli Betsi

The Congo craton hosts banded iron formations (BIFs) in southern Cameroon. The Gouap iron deposit is in the central part of the Nyong Complex, next to the Ntem Complex in the northwestern part of the East Gabonian Block of the Congo craton. The Gouap BIFs are interbedded with metasiliciclastic rocks consisting of gneisses, schists, and quartzites. We present the first integrated study combining whole-rock and Sm-Nd isotope geochemistry with SIMS and LA-ICP-MS U-Pb-Hf-O isotope analyses of detrital zircons from metasiliciclastic rocks from the Gouap iron deposit to assess the provenance and tectonic setting during deposition of BIFs as well as the crustal evolution at the northwestern margin of the East Gabonian Block. Geochemical data for the metasiliciclastic rocks indicate that their protolith comprised a sequence of greywacke. Along with regional geology, our U-Pb-Hf-O detrital zircon and Sm-Nd whole-rock isotope data suggest that these rocks were predominantly derived from Neoarchean charnockite and TTG suites of the Ntem Complex with a variable and episodic contribution from ca. 2.2–2.0 Ga charnockites of the Nyong and Ogooué complexes. Metasiliciclastic rocks and BIFs were deposited between ca. 2.1 and 2.0 Ga in a foreland basin developed along the northwestern continental margin of the East Gabonian Block. So far unrecognized ca. 2.50–2.45 and 2.45–2.22 Ga magmatic rocks were exposed in their provenance. The Gouap succession experienced amphibolite-facies metamorphism when the Congo and São Francisco cratons collided during the Eburnean-Transamazonian orogeny to form the core of the Nuna/Columbia supercontinent. With the subduction dipping to the west, under the West Gabonian Block, slices of Paleoproterozoic oceanic crust and sediments scraped off the margin of the East Gabonian Block formed an accretionary prism. BIFs of the Nyong Complex likely represent a deep-water temporal equivalent of shallow-marine Mn-deposits of the Francevillian basin, Gabon, and together they belong to the same redox-fractionated deep-water upwelling system that was charged with iron and manganese via hydrothermal alteration of oceanic crust.

Isotopic modelling of Archean crustal evolution from comagmatic zircon–apatite pairs

The composition of Earth's early crust is challenging to assess as only a fragmented record remains. As a consequence, deciphering the composition of early crust as a means to understand early processes on our planet often relies on the isotopic composition of resistive minerals. Here we present a new tool for investigating igneous petrogenesis and crustal evolution by combining 87Sr/86Sr measurements of apatite inclusions with U–Pb and Hf isotope analysis of their host zircon crystals. This approach takes advantage of the complementary inverse fractionation behaviour of Rb/Sr and Lu/Hf, and the cogenetic host/inclusion relationship, to link the three isotopic systems and construct an evolution model in triple isotope space. By applying this triple isotope system modelling to Mesoarchean igneous rocks from the Akia terrane in SW Greenland, we reconstruct a bulk SiO2 composition of 63-68 wt% for the precursor source crust and infer an average crustal residence time of 300–350 Ma. Our modelling implicates involvement of intermediate to felsic Paleo-Eoarchean crust in the genesis of voluminous 3.0 Ga magmatic rocks of the Akia terrane, one of the largest components of the Archean North Atlantic Craton. A further outcome of this approach is the ability to place more robust empirical constraints on the Lu/Hf ratio of the precursor crust by comparison of the modelling output to a global whole-rock dataset. The approach in this work provides a template for further application to ancient rocks in order to better understand the evolution of early Earth.

Tracing arclogites in the Paleoproterozoic Era – A shift from 1.88 Ga calc-alkaline to 1.86 Ga high-Nb and adakite-like magmatism in central Fennoscandian Shield

Arclogites, i.e., lower crustal garnet-pyroxenite cumulates, are suggested to play an important role in controlling magma differentiation in modern continental arcs. Until now, arclogite-related magmatism has only been described from the Phanerozoic Era. The Svecofennian orogen in the central Fennoscandian Shield hosts a rare association of 1.86 Ga igneous rocks geochemically distinct from the surrounding and much more abundant 1.90–1.87 Ga subduction-related calc-alkaline magmatism. The 1.86 Ga magmatic rocks are divided into three groups: 1) high-Nb gabbros (HNB) which are enriched in Fe2O3T, TiO2, P2O5, F, LILE, and HFSE (especially Nb; 18.9–44 ppm), show positive initial εNd value, and near-chondritic but variable initial zircon εHf values; 2) high-Mg gabbros (HMG) which are characterised by high MgO, CaO, Cr and Ni contents, slight enrichment in LILE, positive εNd, and positive but variable zircon εHf values; 3) adakite-like rocks showing high Al2O3 and Na2O contents, slight enrichment in LILE, relative depletion in some HFSE, positive εNd value, and chondritic to negative zircon εHf values. The three groups yield zircon U-Pb ages of ~1.86 Ga and exhibit undeformed textures in contrast to the surrounding supracrustal rocks metamorphosed at ~1.88 Ga. The ages and compositions are clearly different from the adjacent 1.90–1.87 Ga arc-related igneous rocks suggesting a distinct origin. Despite similar ages and close spatial relationship, separate sources are required for each of the different 1.86 Ga rock groups. Trace element modelling of partial melting suggests that arclogites, with compositions similar to pyroxenite xenoliths found in the kimberlite pipes of eastern Finland, are the source for the HNB rocks. In contrast, subduction-modified mantle peridotite is the source for the HMG rocks, and a mafic lower crustal source is suggested for the adakite-like rocks. The following geodynamic model is suggested: (rutile-bearing) arclogite formation at 1.90–1.87 Ga followed by arclogite delamination and partial melting during extension of the thickened Svecofennian crust at 1.86 Ga.

Depositional history of the Mesoproterozoic Chhattisgarh Basin, central India: insights from geochemical provenance of siliciclastic sediments

ABSTRACT The Chhattisgarh Basin is one of the most important Proterozoic basins of peninsular India. Owing to its deposition in the Mesoproterozoic, the largely undeformed and unmetamorphosed sedimentary sequence of this basin is believed to hold vital clues to our understanding of the evolution of the Indian craton subsequent to the disintegration of the supercontinent Columbia. Despite its importance, only limited studies have been carried out to decipher the depositional history of the basin and to correlate various litho-units in it. Here, we present results of quantitative provenance analysis of the sediments in the basin using trace element and Nd isotopic ratios and discuss their implications for the evolution of the basin in the context of regional tectonics. The sediment provenance analysis reveals that the spatially extensive ~2.5 Ga old granitoids of the Bastar craton were the major contributors (45% to 65%) of the total sediment budget into the basin during its initial stage of evolution. As sedimentation progressed the contribution from the Bastar granitoids was restricted to less than 30% of the total budget with the remaining supplied by younger 1.6–1.7 Ga magmatic rocks. The overall provenance of sediments appears to have remained constant throughout the evolutionary history of the basin; however, the relative contributions of various sources did change with time. The latter is highly conspicuous across the stratigraphic boundary between the Singhora and Chandarpur groups, in the lower part of the Chhattisgarh Supergroup. The geochemical data suggest that the basin developed as a result of either rift or sag into which the sea incursion occurred through the adjacent grabens during the Mesoproterozoic marine transgression.

Chapter 11 Reworking of older (1.8 Ga) continental crust by Mesoproterozoic (1.5–1.4 Ga) orogeny, Blekinge–Bornholm orogen, southeastern Sweden

Abstract The Blekinge–Bornholm orogen in southeastern Sweden consists of calc-alkaline to alkali–calcic intrusive rocks, rhyolites and dacites (1.8 Ga) that were structurally reworked under amphibolite facies conditions, affected by migmatization at mid-crustal levels at c. 1.44 Ga and intruded at c. 1.47–1.43 Ga by ferroan alkali–calcic plutons. This Mesoproterozoic orogen is bordered westwards by the Sveconorwegian orogen and northwards, along the boundary with well-preserved 1.8 Ga magmatic rocks in the Svecokarelian orogen, by a stitching c. 1.45 Ga pluton and steeply dipping ductile zones with a south-side-up, dip-slip shear component. A variably developed gneissic fabric (S 1 ) dips gently to moderately northwards and is affected by asymmetrical F 2 folds with a southerly vergence. Ductile high-strain zones with top-to-the south shear sense are suggested to correspond at depth to anomalously reflective zones along seismic profile BABEL line A. Open folding of the gneissosity around gently, north-plunging fold axes (F 3 ) completed the ductile deformational evolution. Uncertainty remains about the timing of the amphibolite facies ductile fabric and the D 2 folding, which is either late-stage Svecokarelian ( c. 1.77–1.75 Ga) or Hallandian ( c. 1.47–1.43 Ga). Non-collisional, accretionary orogenic systems are suggested to have operated during both time periods, radical reorganization of the subduction trend accompanying the Mesoproterozoic event.

Chapter 7 Småland lithotectonic unit dominated by Paleoproterozoic (1.8 Ga) syn-orogenic magmatism, Svecokarelian orogen

Abstract The Småland lithotectonic unit in the 2.0−1.8 Ga Svecokarelian orogen, southeastern Sweden, is dominated by a c. 1.81−1.77 Ga alkali–calcic magmatic suite (the Transscandinavian Igneous Belt or TIB-1). At least in its central part, the TIB-1 suite was deposited on, or emplaced into, c. 1.83–1.82 Ga calc-alkaline magmatic rocks with base metal sulphide mineralization and siliciclastic sedimentary rocks (the Oskarshamn–Jönköping Belt). Ductile deformation and metamorphism under low- to medium-grade conditions affected the Oskarshamn–Jönköping Belt prior to c. 1.81 Ga. Both suites were subsequently affected by low-grade ductile deformation, mainly along steeply dipping, east–west to NW–SE shear zones with dip-slip and dextral strike-slip displacement. Sinistral strike-slip NE–SW zones are also present. In the northern part of the lithotectonic unit, 1.9 Ga magmatic rocks, c. 1.87–1.81 Ga siliciclastic sedimentary rocks and basalt, and c. 1.86–1.85 Ga granite show fabric development, folding along steep NW–SE axial surfaces and medium- or high-grade metamorphism prior to c. 1.81 Ga and, at least partly, at c. 1.86–1.85 Ga; base metal sulphide, Fe oxide and U or U–REE mineralizations also occur. Magmatism and siliciclastic sedimentation along an active continental margin associated with subduction-related, accretionary tectonic processes is inferred over about 100 million years.

Mesoproterozoic juvenile crust in microcontinents of the Central Asian Orogenic Belt: evidence from oxygen and hafnium isotopes in zircon

We report in situ O and Hf isotope data of zircon grains from coeval Mesoproterozoic (ca. 1.4 Ga) igneous metamafic (amphibolite) and granitic rocks of the Chinese Central Tianshan microcontinent (CTM) in the southern Central Asian Orogenic Belt (CAOB). Zircon grains from amphibolite have mantle-like δ18OVSMOW values of 4.7–5.6‰ and juvenile Hf isotopic compositions (εHf(t) = 8.4–15.3; TDMC = 1.57–1.22 Ga), whereas those from granitic rocks have δ18OVSMOW values of 5.6–7.0‰ and evolved Hf isotopic compositions (εHf(t) = −1.0–8.2; TDMC = 2.09–1.62 Ga). Zircon O–Hf isotopic compositions of the metamafic and granitic rocks provide evidence for Mesoproterozoic (ca. 1.4 Ga) crustal growth and a substantial Palaeoproterozoic supracrustal component in the CTM. These findings and previous studies, reporting ca. 1.4 Ga magmatic rocks from other microcontinents of the CAOB, suggest that a large belt of Mesoproterozoic (ca. 1.4 Ga) juvenile continental crust formed in a continental terrane, fragments of which now occur over a distance of more than a thousand kilometres in the southern CAOB.

2090–2070 Ma A-type granitoids in Zanhuang Complex: Further evidence on a Paleoproterozoic rift-related tectonic regime in the Trans-North China Orogen

The Xuting pluton, located at the north section of the Zanhuang Complex, contains potassic and sodic granite in both of which shallow level emplacement is shown by graphic texture feldspar and quartz intergrowth. Two potassic granite and one sodic granite samples yielded zircon 207Pb/206Pb ages of 2066±17Ma, 2092±14Ma, and 2071±14Ma, respectively, suggesting that the Xuting pluton emplaced between 2070Ma and 2090Ma, which is coeval with the Paleoproterozoic Gantaohe Group dominated by mafic volcanic and sedimentary rocks. The potassic granite is enriched in SiO2, total FeO, REEs (except Eu), Zr, Nb, Ga and Y, and depleted in MgO, CaO, Al2O3, Sr, V, Cr and Ni, with high TFeO/MgO ratios, consistent with the features of A-type granite. Except for the high MgO and low K2O contents, the sodic granite also presents most of the features of A-type granites. The zircon saturation temperatures from the potassic and sodic granites range from 855 to 998°C and 853 to 933°C respectively, indicating that they are from the high temperature magma. The potassic granites have the εHf(t) ranging from −5.9 to +1.33 and Hf model ages of 2.5–2.8Ga, which are identical with the ages of Neoarchean TTG gneisses in the Zanhuang Complex. We propose that the potassic granite in the Xuting pluton derived from the partial melting of Neoarchean TTG gneisses. The sodic granites also present negative εHf(t) values from −4.81 to −0.76 and Hf model ages of 2.5–2.8Ga, suggesting that they come from the partial melting of the Neoarchean crust. However, with relative enrichment in MgO and total FeO, and lower K2O and Rb, the sodic granite was derived from a mafic source in the lower crust. The likely heat source was high heatflow associated with mantle upwelling. The A-type Xuting pluton and mafic volcanic rocks in the Gantaohe Group constitute a bimodal lithologic association from a Paleoproterozoic continental rift environment. 2.2–2.0Ga magmas are widely distributed along the Trans-North China Orogen (TNCO), and also present a bimodal character. We propose that 2.2–2.0Ga bimodal magmas throughout the TNCO were likely formed in a Paleoproterozoic rifting setting. Considering the widespread 2.2–2.0Ga magmatic rocks in the North China Craton (NCC), we infer that the magmatism likely occurs in tensional environment after the initial amalgamation of the NCC in the late Archean.

New U–Pb baddeleyite age, and AMS and paleomagnetic data for dolerites in the Lake Onega region belonging to the 1.98–1.95 Ga regional Pechenga–Onega Large Igneous Province

A new U–Pb baddeleyite age of 1970 ± 3 Ma for the Unoi dolerite sill of the Onega structure of Karelia craton matches other 1.98–1.95 Ga units across the Kola craton (Pechenga) and widely separated parts of the Karelian craton, including the Lake Onega sill area and a extensive NW-trending dolerite dyke swarm. Herein these coeval units are combined into the Pechenga–Onega Large Igneous Province. The sills in the Lake Onega area exhibit similar geochemical patterns, although the Unoi dolerite sills appear less contaminated and less differentiated than the Pudozhgora intrusion, Gabnev sill and Koikary-Svatnavolok and Palieyeozero sills but are similar to other doleritic sills in the northern part of the Onega structure. New AMS data from sills are consistent with emplacement along the same NW–SE trend as the dykes, consistent with the dykes acting as their feeder system. Paleomagnetic data obtained on 1.98–1.95 Ga magmatic rocks result in a new robust paleopole for the Karelian craton and accentuate its variable paleolatitude and paleoorientation during the Paleoproterozoic.

Early Paleoproterozoic (2.45–2.20 Ga) magmatic activity during the period of global magmatic shutdown: Implications for the crustal evolution of the southern North China Craton

A global database of zircon ages from both granitoids and detrital sediments shows an exceptional and robust age gap between 2.45 and 2.20Ga. The early Paleoproterozoic magmatism dramatically decreased on the Earth, which was proposed to relate to the global plate tectonic shutdown. However, the available data indicate that 2.45–2.20Ga magmatic rocks are widespread in the Taihua Complex during the quiet interval. The Taihua Complex is located in both the Lushan and Xiaoqinling areas along the southern segment of North China Craton (NCC). The Complex is composed mainly of a gneiss series and the Khondalite-dominated supracrustal rocks. There was an original unconformable relationship between the two rock associations. The latter occurs as a linear structural belt and was named the Khondalite Belt. The gneiss series in the Lushan area has an age of 2.85–2.72Ga, whereas the Xiaoqinling region contains widespread tonalitic–trondhjemitic–granodioritic gneisses with ages of 2.45–2.20Ga. Significant variations of Hf isotopes in zircons and whole-rock ɛNd(t) values suggest that these rocks were produced by variable mixing of a juvenile materials with older crust in an Andean-type continental margin arc or island arc setting. The depositional age of the sedimentary protoliths of the khondalite series can be roughly constrained in the period between 2300 and 1970Ma. It has been considered that they were deposited in a stable continental margin environment. The U–Pb dating of metamorphic zircons from the Taihua Complex suggest that the southern NCC underwent a polyphase tectonic evolution during the period 1.97–1.82Ga, and the peak metamorphism coeval with crustal thickening has occurred at ∼1.94Ga.

Geochemistry and detrital zircon U–Pb and Hf isotopes of the paragneiss suite from the Quanji massif, SE Tarim Craton: Implications for Paleoproterozoic tectonics in NW China

The Delingha paragneiss suite in the Quanji massif, southeastern Tarim Craton, is composed of mica schist, paragneiss, leptynite and quartzite, similar to the ‘khondalite suites’ described from elsewhere in the world. The mica schist is rich in Al2O3 (up to ∼26wt%) and contains graphite and diagnostic minerals including sillimanite and garnet, with metamorphism under amphibolite-facies to locally granulite-facies conditions as manifested by association with amphibolite and granulite. The detrital zircon U–Pb ages and geochemical data indicate that the protolith materials of the Delingha paragneiss suite were mainly sourced from 2.20 to 2.45Ga granites, felsic volcanic rocks and TTG, and were deposited at 2.17–1.92Ga. The detrital zircon Hf and whole-rock Nd isotopes document important crustal growth at ∼2.5–2.7Ga. The detrital zircon age spectra, the whole rock Nd and zircon Hf model ages, the low-maturity of the protolith, and short-distance transportation suggest that the detritus were derived from the underlying Delingha Complex and the lower Dakendaban sub-Group. The timing of magmatic activities in the source region, the depositional age and metamorphic histories of the Delingha paragneiss suite are all comparable to those recorded in the khondalite belt along northern margin of the Ordos Block in the North China Craton. Our study shows that the 2.2–2.45Ga magmatic rocks were generated in arc or active continental margin settings, suggesting a prolonged subduction and accretion history prior to final amalgamation (∼2.5–1.8Ga) to form the unified North China Craton and the assembly of the Tarim Craton in NW China.

Basement nature and origin of the Junggar terrane: New zircon U–Pb–Hf isotope evidence from Paleozoic rocks and their enclaves

The question of whether the Junggar terrane in the central area of the Central Asian Orogenic Belt (CAOB) has a Precambrian basement is one of the hottest topics surrounding the tectonic evolution of the CAOB. Recent work shows that the Central Tianshan range has a Mesoproterozoic–Neoproterozoic basement. Here, we reported the SIMS U–Pb age and Hf isotopes of zircons from newly discovered dioritic gneiss and magnetite quartzite enclaves and their host diorite dike in the Taheir area, deformed Ordovician conglomerate in the Dazigou area, and mica gneiss relicts and their host granite in the Shuangchagou area in the Junggar terrane. The results reveal the following evidence: 1) the dioritic gneiss was metamorphosed from 2.52Ga diorite at approximately 1.88Ga, the magnetite quartzite interlayered with hornblende–plagioclase gneiss layers has an age of approximately 1.92Ga, and the inherited zircons from the diorite TH4-6 contain three zircon populations with upper intercept ages of approximately 3.06Ga, 1.98Ga and 1.89Ga; 2) the sienna and colorless zircon populations from the conglomerate reveal two magmatic rocks with ages of approximately 1.86Ga and 0.94Ga; and 3) the magmatic zircons that form mica gneiss have TDMC and TDM2 ages ranging from 1.2Ga to 2.08Ga, and the inherited zircons from the mica gneiss and granite in the Shuangchagou area contain five zircon populations with upper intercept ages of approximately 2.48Ga, 1.80Ga, 1.47Ga,1.14Ga and 0.96Ga.By combining the published U–Pb ages and Hf isotope data of zircons from the Junggar terrane, it is suggested that the Junggar terrane has three Precambrian basement units, including the 2.1–1.7Ga Taheir–Kalamaili–Dazigou reworked Archean crust, the 2.5–1.8Ga Shuangchagou–Luliang crust and the 1.86–1.7Ga Tianshan crust between the Aermantai subduction zone and south Tianshan suture. There is also the possibility of a 1.92Ga quartzite formation in the Taheir area, 1.9–1.8Ga orogen along the Taheir–Dazigou–Shuangchagou line, 1.47–1.40Ga granitoids in the Shuangchagou–Xingxingxia area, 1.2–1.0Ga magmatic rocks in the Dazigou–Shuangchagou area, a massive 1.0–0.9Ga magmatic–metamorphic rock belt along the Tianshan range and in the Shuangchagou–Dazigou area, and 0.8–0.6Ga magmatic rocks in the Taheir, Dazigou, Shuangchagou and Baluntai area. Regular arrangements of the three basement units and various sedimentary, magmatic and metamorphic belts indicate that the Precambrian Junggar terrane was possibly split from southern Laurentia near the Lake Superior region at 1.1Ga and adjacent to the Caborca region at 1.0–0.6Ga.

SHRIMP zircon dating and LA-ICPMS Hf analysis of early Precambrian rocks from drill holes into the basement beneath the Central Hebei Basin, North China Craton

The Central Hebei Basin (CHB) is one of the largest sedimentary basins in the North China Craton, extending in a northeast–southwest direction with an area of >350 km2. We carried out SHRIMP zircon dating, Hf-in-zircon isotopic analysis and a whole-rock geochemical study on igneous and metasedimentary rocks recovered from drill holes that penetrated into the basement of the CHB. Two samples of gneissic granodiorite (XG1-1) and gneissic quartz diorite (J48-1) have magmatic ages of 2500 and 2496 Ma, respectively. Their zircons also record metamorphic ages of 2.41–2.51 and ∼2.5 Ga, respectively. Compared with the gneissic granodiorite, the gneissic quartz diorite has higher ΣREE contents and lower Eu/Eu* and (La/Yb)n values. Two metasedimentary samples (MG1, H5) mainly contain ∼2.5 Ga detrital zircons as well as late Paleoproterozoic metamorphic grains. The zircons of different origins have εHf (2.5 Ga) values and Hf crustal model ages ranging from 0 to 5 and 2.7 to 2.9 Ga, respectively. Therefore, ∼2.5 Ga magmatic and Paleoproterozoic metasedimentary rocks and late Neoarchean to early Paleoproterozoic and late Paleoproterozoic tectono-thermal events have been identified in the basement beneath the CHB. Based on regional comparisons, we conclude that the early Precambrian basement beneath the CHB is part of the North China Craton.

Paleoproterozoic crustal evolution of the Tarim Craton: Constrained by zircon U–Pb and Hf isotopes of meta-igneous rocks from Korla and Dunhuang

The Tarim Craton is one of three large cratons in China. Presently, there is only scant information concerning its crustal evolutionary history because most of the existing geochronological studies have lacked a combined isotopic analysis, especially an in situ Lu–Hf isotope analysis of zircon. In this study, Precambrian basement rocks from the Kuluketage and Dunhuang Blocks in the northeastern portion of the Tarim Craton have been analyzed for combined in situ laser ablation ICP-(MC)-MS zircon U–Pb and Lu–Hf isotopic analyses, as well as whole rock elements, to constrain their protoliths, forming ages and magma sources. Two magmatic events from the Kuluketage Block at ∼2.4Ga and ∼1.85Ga are revealed, and three stages of magmatic events are detected in the Dunhuang Block, i.e., ∼2.0Ga, ∼1.85Ga and ∼1.75Ga. The ∼1.85Ga magmatic rocks from both areas were derived from an isotopically similar crustal source under the same tectonic settings, suggesting that the Kuluketage and Dunhuang Blocks are part of the uniform Precambrian basement of the Tarim Craton. Zircon Hf model ages of the ∼2.4Ga magmatism indicate that the crust of the Tarim Craton may have been formed as early as the Paleoarchean period. The ∼2.0Ga mafic rock from the Dunhuang Block was formed in an active continental margin setting, representing an important crustal growth event of the Tarim Craton in the mid-Paleoproterozoic that coincides with the global episode of crust formation during the assembly of the Columbia supercontinent. The ∼1.85Ga event in the Kuluketage and Dunhuang Blocks primarily involved the reworking of the old crust and most likely related to the collisional event associated with the assembly of the Columbia supercontinent, while the ∼1.75Ga magmatism in the Dunhuang Block resulted from a mixture of the reworked Archean crust with juvenile magmas and was most likely related to a post-collisional episode.

Extreme zircon O isotopic compositions from 3.8 to 2.5 Ga magmatic rocks from the Anshan area, North China Craton

Most zircon from Archean (3.8–2.5Ga) trondhjemitic rocks, meta-gabbro, meta-diorite and monzogranite from the Anshan area, North China Craton, has δ18O values in the range of 4.6–7.5‰, but some has extreme compositions (0.02–11.0‰, with one value as low as −11.3‰) as a result of pre-, syn- and post-magmatic processes. Nearly all zircon grains with very low δ18O (<4.0‰) are U-rich and show evidence of radiogenic Pb loss, irrespective of their origin (magmatic, metamorphic, recrystallized, inherited or trapped). The low δ18O is not a primary feature of the zircon or the magma from which it crystallized, but is a consequence of high-temperature hydrothermal alteration, probably involving meteoric water. Radiogenic Pb loss is an indicator that the zircon O isotopic composition is likely to have been modified, even if the measured δ18O falls within, or is above, the “normal” igneous range (4.6–7.5‰). Excluding those showing strong Pb loss, most magmatic zircon grains have δ18O values in the “normal” range or only slightly above the previously reported highest δ18O value for Archean igneous zircon (7.5‰), with many, including some analyzed from Eoarchean rocks, being higher than 6.5‰, evidence for crustal reworking in the Anshan area 3.8Ga ago. The 2.5Ga Qidashan syenogranite has the highest zircon δ18O (5.8–11.0‰), consistent with its derivation by melting of sedimentary materials as a result of long-term crustal evolution in the region, as indicated by its chemical and Nd isotopic compositions.