Paleoproterozoic Tectonics Research Articles

The transition from ancient to modern-style tectonics: Insights from lithosphere dynamics modelling in compressional regimes

Orogens are traditionally classified according to their tectonic style. Paleoproterozoic tectonics is referred to as “ancient-style tectonics” while Proterozoic tectonics is referred to as “modern-style tectonics”. Ancient-style tectonics is characterised by distributed vertical structures and low topography gradients, often associated with diapirism and partial melting. In contrast, modern-style tectonics involve prominent strain localisation and the formation of thrusts, nappes and high topographic gradients. However, the parameters controlling the transition from ancient to modern-style tectonics are poorly understood. To quantify this transition, a combination of 1D and 2D high resolution lithospheric-scale thermo-mechanical models was conducted. The parameters controlling the strength of the lithosphere (i.e., Moho temperature, strain rate, crustal rheology, crustal radiogenic heat production and role of shear heating) were investigated in detail. Our results show that tectonic style is controlled by the maximum of crustal strength (shear stress). Modern-style tectonics is observed to occur when the maximum of crustal strength is greater than 300 MPa. At the opposite, a maximum crustal strength lower than 300 MPa leads to ancient style tectonic structures. Therefore, crustal rheology, temperature and background strain rate significantly influence the transition from ancient to modern-style tectonics. Shear heating remains a key factor in promoting strain localisation in modern-style tectonics. Crustal radiogenic heat production has a moderate influence by increasing/decreasing the tendency for faulting within the crust. This crustal strength criterion also provides an excellent fit for a second potential proxy: a localisation criterion of ca. 225°C. These two proposed proxies can be used interchangeably to predict the transition from ancient to modern-style tectonics.

Geophysical insights into Paleoproterozoic tectonics along the southern margin of the Superior Province, central Upper Peninsula, Michigan, USA

The southern margin of the Archean Superior Province in the central Upper Peninsula of Michigan was a nexus for key Paleoproterozoic tectonic events involved in the ~2.1 Ga rifting of proposed Archean supercraton Superia and subsequent assembly of Laurentia. Interpretations of the region’s tectonic history have historically been hampered by extensive Pleistocene glacial and Paleozoic sedimentary cover and a lack of appropriate geophysical data. These rifting and orogenic events formed geologic effects that are readily mappable with modern geophysical methods. New aeromagnetic and gravity data provide a critical means of mapping and interpreting the complex geological framework through cover, allowing development of significantly richer geographical and process-based perspectives on all these tectonic events. Interpretations of Precambrian contacts and structure are here, for the first time, carried >30 km eastward under Paleozoic cover. Effects of ~2.1 Ga rifting are strongly expressed geophysically, including the Dickinson Group, perhaps a unique record of the progression of rift-related sedimentation and magmatism, shown here to be a geographically extensive and largely concealed tectonic feature of the southern Superior Province. The geophysical evidence for plausible ~2.1 Ga rift-related intrusive magmatism includes a previously unrecognized swarm of northeast-striking mafic dikes cutting Archean rocks and gravity lows produced by granites. Effects of the ~1.87–1.83 Ga Penokean orogeny include gravity and magnetic gradients and pattern breaks along the Niagara fault zone suture, abundant evidence for thin-skinned thrusting and folding in the Menominee iron district, and speculative emplacement of an allochthonous sedimentary sequence in the Calumet trough. Numerous east–west trending structures imaged geophysically likely originated, or were significantly reactivated by, post-Penokean deformation. Metamorphic events at ~1.76 Ga and ~1.65 Ga may correspond to orogenies involving younger, outboard Paleoproterozoic crustal provinces recognized in southern Laurentia. For example, the previously unrecognized West Branch fault, separating the Dickinson Group from Archean rocks, is shown to be a major structure in the region, and is a proposed expression of ~1.76 Ga thick-skinned deformation. Oblique disruptions of crudely east–west striking structures have robust geophysical expressions and are speculatively connected to transpressive deformation at ~1.65 Ga. These new geophysical observations and interpretations collectively help illuminate a critical period in the tectonic evolution of Laurentia, as it transitioned from a disparate array of Archean cratons to a more coherent, growing continent.

Geochemical constrains on pyroxenites from Aniyapuram Mafic–Ultramafic Complex, Cauvery Suture Zone, southern India: Suprasubduction zone origin

The Aniyapuram Mafic and Ultramafic Complex (AMUC) is a dismembered ophiolite of Neoarchean to Paleoproterozoic age, exposed in south-central part of the Cauvery Suture Zone (CSZ), Southern Granulite Terrane (SGT), India. The complex comprises ultramafic rocks like serpentinized peridotites of harzburgite variety and pyroxenites of websterite variety along with metagabbros, amphibolites, plagiogranites and metacherts. The pyroxenites in the complex occur in the form of dykes, veins and lenses in association with serpentinized peridotites and metagabbros. The chemical compositions of olivine from the peridotites show a moderate forsterite content (Fo88–87) and spinels are enriched in aluminum (Al2O3 > 60 wt%) with depletion of Cr contents (Cr# < 0.5). The pyroxenites are composed of orthopyroxenes with dominance of clinopyroxene and sulphide occurrences (up to 62 wt% S) along the grain boundaries of pyroxenes. The whole rock geochemistry of these pyoxenites shows enrichment of LILE and depletion of HFSE with negative Nb anomalies on N-MORB and primitive normalization. The mineral chemistry of clinopyroxenes from the pyroxenites shows tholeiitic in nature with high Mg# ratios [Mg/(Mg + Fe) = 0.70–0.88]. These mineral chemistry results together with whole rock chemistry reveal their origin in Island arc setting of supra-subduction zone (SSZ) tectonics possibly evolved by the interaction of subduction derived fluids of host serpentinized peridotites. The estimated two-pyroxene thermobarometry of these pyroxenites represents varied re-equilibrium temperatures of 820–980°C and medium to slightly high pressures of 10–12 kbar. With the available age relationship from the complex, the study supports as a strong evidence for the occurrence of Neoarchean to Paleoproterozoic suprasubduction tectonics and associated lithologies of formation in the terrane.

Data for: Discovery of kyanite in typical cordierite/sillimanite-bearing low- to medium-pressure pelitic granulites: constraint on Paleoproterozoic plate tectonics

Data for: Discovery of kyanite in typical cordierite/sillimanite-bearing low- to medium-pressure pelitic granulites: constraint on Paleoproterozoic plate tectonics

Orosirian magmatism in the Tapajós Mineral Province (Amazonian Craton): The missing link to understand the onset of Paleoproterozoic tectonics

The Tapajós Mineral Province (TMP) located at south-central Amazonian Craton is dominated by igneous suites that show geochemical and petrologic evidence of a long-lived subduction process accounting for continental crust generation. The process lasted for approximately 100 Ma and yielded in different intrusive and extrusive manifestations within the 1.99–1.89 Ga interval. At its initial stage, arc magmatism is characterized by a strong crustal component on the magmas possibly due to subcrustal-erosion upon the onset of subduction. After this phase, metaluminous primary melts provided by the subduction zone gradually differentiate to peraluminous compositions, the case for the ca. 1.98–1.95 Ga Creporizão Intrusive Suite (CRP) and its extrusive correspondences. Geochemical modelling reveals that the first bursts of magmatism at the TMP (at ca. 1.99 Ga) evolved through pyroxene fractionation, whereas the following pulses are amphibole-bearing. As the arc evolves, the mantle wedge gets progressively metasomatized and hydrated, eventually melting to generate the high-K metaluminous to mildly peraluminous granitoids of the ca. 1.89 Ga Parauari Intrusive Suite (PAR), accompanied by more evolved εNd(1.88). The first pulses of the Iriri Group volcanism are synchronous with PAR, nevertheless, Nd isotopes point to different sources. Around 1.87 Ga, slab break off provokes astenospheric upwelling, which is responsible for destabilizing and melting of the subcontinental-lithospheric-mantle severely metasomatized by the previous magmatic events, generating shoshonitic and metaluminous melts with variable degrees of crust assimilation, translated into mildly negative εNd(1.86) of the Maloquinha Intrusive Suite (MLQ) and Iriri volcanic rocks at an early-anorogenic tectonic setting. Geochemical modelling and the depletion in Ba, Sr and Eu of MLQ and part of Iriri are compatible with early extraction of plagioclase and support the relatively dry evolution of these units. In addition, fractional crystallization alone cannot account for the geochemistry variability of MLQ and Iriri, reinforcing crust assimilation on their petrogenesis. In terms of metallogenetic potential, CRP and PAR have petrologic evolutions consistent with amphibole and magnetite crystallization, pointing to hydrated and oxidized magmas that might be considered as favorable for the formation of magmatic-hydrothermal mineral deposits (porphyry AuCu (Mo) and epithermal systems). Lastly, the magmatic events younger than 1.89 Ga might act incorporating residual Au-rich sulfide from the mantle, explaining the Au-only characteristic of many mineral deposits described at the TMP.

New insights into Paleoproterozoic tectonics of the Yangtze Block in the context of early Nuna assembly: Possible collisional granitic magmatism in the Zhongxiang Complex, South China

The newly discovered Zhongxiang Complex preserving extensive Paleoproterozoic granitoids plays a pivotal role in understanding the early tectonic processes of the Yangtze Block, South China, in the context of the Nuna supercontinent cycle. New zircon U-Pb age and Hf-isotope coupled with whole-rock chemical and Nd-isotope results from three granitic intrusions in the southern Zhongxiang Complex document a magmatic record of significant late Paleoproterozoic tectonic convergence in the region. Geochronological data show that granites from these intrusions were emplaced in a relatively long period at 2.0–1.93 Ga. The granites have variably high SiO2 contents of 74.01–77.19 wt%, and are weakly to strongly peraluminous with high ASI (alumina saturation index) values (>1.05). They are generally magnesian, showing fractionated S-type granite affinities. They have negative zircon εHf(t) of −9.69 to −17.11 and negative whole-rock εNd(t) of −7.43 to −11.69, corresponding to two-stage Hf model ages of 3.24–3.67 Ga and two-stage Nd model ages of 3.0–3.37 Ga, respectively, indicating a derivation of their parental magmas from remelting of ancient crustal components (i.e., the Yangpo “Group” paragneisses). The moderate to strong LREE/HREE fractionation ((La/Yb)N > 22, average 73.15) and the relatively low Sr/Y and negative Eu anomalies of the least felsic samples further imply a residual assemblage probably involving both garnet and plagioclase, corresponding to middle-lower crustal levels. This magmatic event most likely occurred at a convergent continental margin related to continental collision, which, in conjunction with broadly coeval metamorphism and the succeeding A-type granitic magmatism in the region, delineates a possible late Paleoproterozoic tectonic transition from compression to extension, signifying a cycle from collision (2.0–1.93 Ga) to post-collision (1.85 Ga) in the Zhongxiang Complex. Such a collisional orogenesis was potentially linked to the Paleoproterozoic internal assembly of the eastern part of the Yangtze Block, presumably as part of the early assembly of the Nuna supercontinent.

Late Paleoproterozoic tectonic evolution of the Olongbuluke Terrane, northern Qaidam, China: Constraints from stratigraphy and detrital zircon geochronology

The late Paleoproterozoic tectonic processes of the Olongbuluke Terrane in the northern Qaidam of northwestern China remain unclear due to rare contemporaneous metamorphic and magmatic records. The sedimentary succession of the Quanji Group preserves detailed geological information of erosion, transportation and deposition, which is pivotal for constraining the late Paleoproterozoic tectonics of the Olongbuluke Terrane. The Quanji Group comprises conglomerate-sandstone of nearshore facies and mudstone-dolomite of tidal flat facies in the lower and upper sections, respectively, displaying an upward fining and transgression sequence. Together with featured sedimentary structures, the facies assemblage and vertical sequence suggest that the Olongbuluke Terrane has experienced a basin evolution of initial rifting, rifting climax and passive continental margin during ∼1780–1640 Ma. Four samples collected from the low to top positions within the profiles yield quite similar detrital zircon age patterns with two distinct clusters of 2510–2216 Ma and 2062–1728 Ma. These detrital zircon ages coincide with those of magmatism–metamorphism in the basement of Olongbuluke Terrane, suggesting the detritus were sourced from the Olongbuluke Terrane itself. The zircons with ages of 2510–2216 Ma display εHf (t) ratios from −2.29 to +6.81 and corresponding TDM2 ages of ∼3.1 to 2.5 Ga, suggesting they were originated from reworking of the Meso- to Neoarchean juvenile crust with mantle inputs or mantle derived rocks. In comparison, the zircons with ages of 2062–1728 Ma exhibit εHf (t) ratios ranging from −17.4 to −0.1 and TDM2 ages of ∼3.6 to 2.5 Ga, indicating their origin in partial melting of the Paleo- to Neoarchean crust. The two sets of zircons display TDM2 ages with three peaks of ∼2.9 Ga, ∼2.7 Ga and ∼2.5 Ga. Comparing with those of the other main blocks all over the world, all the above detrital zircon U-Pb ages and TDM2 ages together with the late Paleoproterozoic tectono-stratigraphic processes suggest that the Olongbuluke Terrane has a Precambrian tectonic affinity close to the western North China Craton.

Extending the realm of Archean crust in the Great Falls tectonic zone: Evidence from the Little Rocky Mountains, Montana

Two prominent features separate the Archean Wyoming and Hearne cratons: the Paleoproterozoic Great Falls tectonic zone (GFTZ) and the Medicine Hat block (MHB), neither of which is well defined spatially because of Phanerozoic sedimentary cover. Based on limited data, the MHB is thought to be a structurally complex mix of Archean (2.6–3.1 Ga) and Proterozoic (1.75 Ga) crust, but is recognized primarily by its geophysical signature, and its influence on the geochemistry of younger igneous rocks. Similarly, the GFTZ was recognized on the basis of broad differences in geophysical patterns, isopachs of Paleozoic sedimentary sections, and lineaments; however, juvenile arc rocks in the Little Belt Mountains (LBM) and strongly overprinted Archean rocks in southwestern Montana show it to be a dominantly Paleoproterozoic feature. The Little Rocky Mountains (LRM) of Montana provide access to exposures of the northeastern-most Precambrian crust in the MHB-GFTZ region. U/Pb ages of zircons from Precambrian rocks of the LRM range from 2.4 to 3.3 Ga, with most ages between 2.6 and 2.8 Ga. Whole-rock analyses yield Sm-Nd TDM from 3.1 to 4.0 Ga and initial εNd(T) values calculated at U-Pb zircon crystallization ages range from −0.9 to −10.5, indicating significant contributions from older Archean crust. The high proportion of 2.6–2.8 Ga U/Pb ages differentiates LRM crust from arc-related Paleoproterozoic magmatic rocks exposed in the LBM to the southwest. The age and isotopic composition of the LRM gneisses are similar to crust in the northern Wyoming Province (2.8–2.9 Ga), but Paleoproterozoic K-Ar cooling ages suggest crust in the LRM experienced the Paleoproterozoic metamorphism and deformation that characterizes the GFTZ. Consequently, its history differs markedly from the adjacent Beartooth-Bighorn magmatic zone of the northern Wyoming Province, which does not record Paleoproterozoic tectonism, but has a strong correlation with the Montana metasedimentary terrane that was strongly overprinted during the Paleoproterozoic Great Falls orogeny that defines the GFTZ. The LRM, therefore, likely provides a unique, and perhaps the only, opportunity to characterize Archean crust of the MHB.

Multi-stage zircon and monazite growth revealed by depth profiling and in situ U–Pb geochronology: Resolving the Paleoproterozoic tectonics of the Trans-Hudson Orogen on southeastern Baffin Island, Canada

Understanding the growth and recrystallization of zircon and monazite during regional metamorphism is important for identifying the nature and timescales of tectonic processes in continental crust. We present extensive in situ U–Pb monazite geochronology and U–Pb zircon depth profiling analyses from the mid-crust exposed on southeastern Baffin Island, Canada, in the internal zone of the Paleoproterozoic Trans-Hudson Orogen. In situ LA-ICP-MS U–Pb monazite dating on ten metasedimentary rocks (totalling >300 analyses) from different paleodepths reveals a dominant ca. 1860–1820Ma monazite population that records regional mid-amphibolite- to granulite-facies metamorphism with coeval fabric development. Subordinate monazite with ages between ca. 1800–1750Ma are interpreted to have formed through dissolution-reprecipitation. The SIMS U–Pb depth-profiling technique allowed dating of ca. 1861–1833Ma zircon rims and less abundant ca. 1800–1764Ma zircon rims that would be unresolvable using conventional spot dating. Trace element depth profiling via LA-ICP-MS shows that zircon rims typically have lower Th/U values, higher Hf contents and different REE compositions than zircon interiors. Zircon rim compositions are highly variable, with no apparent link to U–Pb age, and are interpreted as the combined result of metamorphic zircon growth and dissolution-reprecipitation. Our data demonstrate that the timing of peak metamorphism is recorded by both monazite and zircon, and that they are both sensitive to U–Pb age resetting at lower temperatures during late-orogenic tectonism. Regional mid-amphibolite- to granulite-facies metamorphism and east-directed crustal shortening are bracketed between ca. 1860–1820Ma. These events are interpreted to result from the accretion of cratonic blocks in the internal zone of the Trans-Hudson Orogen. The integrated geochronological approach on regionally-distributed samples enabled us to document ca. 1800–1750Ma monazite domains and zircon rims, which we interpret to result from the terminal collision with the lower-plate Superior craton followed by late- to post-orogenic tectonothermal activity that continued for at least 70 Myr after thermal peak. This prolonged history is characteristic of the northeastern Trans-Hudson Orogen, and implies that orogenic collapse may not be a hallmark of all hot, thickened orogens.

Geochronological evidence for an extension of the Northern Lobe of the Bushveld Complex, Limpopo Province, South Africa

We have conducted the first geochronological study of the ultramafic–mafic succession (the Waterberg Project) located in the Southern Marginal Zone of the Limpopo Belt north of the Northern Lobe of the Bushveld Complex using U/Pb dating on zircons extracted from both mineralized reefs as well as on detrital zircons of the Waterberg Group unconformably overlying the mafics. This contribution is the first in a series of publications describing the well-mineralized succession and will therefore give an overview of the local geology. U/Pb baddeleyite ages for the main mafic sill intruding the area supplement the data. The succession is extremely well mineralized and therefore of interest to the wider geological community.The intrusion ages for the Lobe are 2059±3 and 2053±5Ma and within error coeval with the intrusion ages of the Bushveld Complex. Detrital zircon age probability density distributions indicate a maximum age of deposition of c. 2045Ma for the Waterberg Group and show abundant peaks of Archean age. Paleoproterozoic zircons are interpreted to represent zircon from the detritus of the now eroded roof rocks of the lobe, while the Archean zircons are most likely sourced from the Limpopo Complex to the northeast. The major dolerite sill intruding the project area has been confirmed to belong the Umkondo Igneous Province with a baddeleyite age of 1125±5Ma.The combination of these new age dates, local geology and existing literature is used to argue that (i) the ultramafic–mafic succession is related to the Bushveld Complex; (ii) a significant unconformity is present between igneous rocks and the overlying sediments, and (iii) the area was exposed to a level of Paleoproterozoic tectonism not previously observed in the Bushveld Complex.

Zircon U‐Pb ages and Hf isotopes of the Askot klippe, Kumaun, northwest India: Implications for Paleoproterozoic tectonics, basin evolution and associated metallogeny of the northern Indian cratonic margin

Zircon U‐Pb ages and Hf isotopes of the Askot klippe, Kumaun, northwest India: Implications for Paleoproterozoic tectonics, basin evolution and associated metallogeny of the northern Indian cratonic margin

Neoarchean to Paleoproterozoic continental growth in the southeastern margin of the North China Craton: Geochemical, zircon U–Pb and Hf isotope evidence from the Huoqiu complex

The Huoqiu complex in the southeastern margin of the North China Craton (NCC) is dominated by Neoarchean gray gneisses, amphibolites and voluminous metasediments. Here we report the occurrence of Neoarchean to Paleoproterozoic rocks from drill-core samples. The gneisses are similar to TTG (tonalite–trondhjemite–granodiorite) in composition and show close spatial association with amphibolites. Geochemical characteristics such as high Sr/Y and (La/Yb)N with steep REE patterns and trace element modeling suggest that these rocks were generated by partial melting of hydrous meta-basalts (amphibolites) at the base of a thickened mafic continental crust, leaving a rutile-bearing eclogite residue. LA-MC-ICP-MS U–Pb age data from magmatic zircon grains show protolith emplacement ages of 2.76–2.71Ga. Subsequently, widespread migmatization took place at 1.91–1.82Ga, generating voluminous migmatites and high-K granites. Hf isotopic compositions of zircon grains from the amphibolite and gneiss show εHf(t) values of 2.4–15.5 and −3.0–1.5, respectively. The tDM2(Hf) model ages of the gneisses range from 2.87 to 3.14Ga, and are identical to the tDM1(Hf) ages of amphibolites (2.84–3.16Ga) within analytical uncertainty, suggesting that the gneisses formed by partial melting of amphibolite, and attest to large-scale reworking of the ancient continental crust during Neoarchean. The zircon grains from the granites define two groups with regard to their Hf isotopic composition. The older group (1916±42Ma) has εHf(t) values and tDM2(Hf) ages of −10.5–2.4 and 2.40–3.20Ga, respectively, whereas the younger one (1823±41Ma) shows a large variation in εHf(t) values ranging from −18.1 to 12.5, with tDM2(Hf) model ages of 1.70–3.59Ga. A couple of zircon grains from the younger group display consistent U–Pb ages and tDM2, indicating accretion of juvenile crust from depleted mantle sources during 1.82 to 1.91Ga. However, the dominant Hf isotope features are consistent with the reworking of preexisting continental crust. We therefore infer that only limited accretion of juvenile crust occurred during this time, and that the Paleoproterozoic (1.82 to 1.91Ga) tectonics in the southeastern margin of the NCC witnessed extensive reworking of older continental crust.

Isotopic geochronological evidence for the Paleoproterozoic age of gold mineralization in Archean greenstone belts of Karelia, the Baltic Shield

The Rb-Sr age of metasomatic rocks from four gold deposits and occurrences localized in Archean granite-greenstone belts of the western, central, and southern Karelian Craton of the Baltic Shield has been determined. At the Pedrolampi deposit in central Karelia, the dated Au-bearing beresite and quartz-carbonate veins are located in the shear zone and replace Mesoarchean (∼2.9 Ga) mafic and felsic metavolcanic rocks of the Koikar-Kobozero greenstone belt. At the Taloveis ore occurrence in the Kostomuksha greenstone belt of western Karelia, the dated beresite replaces Neoarchean (∼2.7 Ga) granitoids and is conjugated with quartz veins in the shear zone. At the Faddeinkelja occurrence of southern Karelia, Aubearing beresite in the large tectonic zone, which transects Archean granite and Paleoproterozoic mafic dikes, has been studied. At the Hatunoja occurrence in the Jalonvaara greenstone belt of southwestern Karelia, the studied quartz veins and related gold mineralization are localized in Archean granitoids. The Rb-Sr isochrons based on whole-rock samples and minerals from ore-bearing and metasomatic wall rocks and veins yielded ∼1.7 Ga for all studied objects. This age is interpreted as the time of development of ore-bearing tectonic zones and ore-forming hydrothermal metasomatic alteration. New isotopic data in combination with the results obtained by our precursors allow us to recognize the Paleoproterozoic stage of gold mineralization in the Karelian Craton. This stage was unrelated to the Archean crust formation in the Karelian Block and is a repercussion of the Paleoproterozoic (2.0–1.7 Ga) crust-forming tectonic cycle, which gave rise to the formation of the Svecofennian and Lapland-Kola foldbelts in the framework of the Karelain Craton. The oreforming capability of Paleoproterozoic tectonics in the Archean complexes of the Karelian Craton was probably not great, and its main role consisted in reworking of the Archean gold mineralization of various genetic types, including the inferred orogenic mesothermal gold concentrations.

Genesis of the Hengling magmatic belt in the North China Craton: Implications for Paleoproterozoic tectonics

The 2200–1880Ma igneous rocks in the central and eastern parts of the North China Craton (NCC) constitute a new Hengling magmatic belt (HMB), which includes the ~2147Ma Hengling mafic sill/dyke swarm, the ~2060Ma Yixingzhai mafic dyke swarm, and the ~1973Ma Xiwangshan mafic dyke swarm. The three swarms are contiguous and have experienced variable degrees of metamorphism from greenschist to low amphibolite facies (Hengling), medium granulite facies (Yixingzhai), and medium/high-pressure granulite facies (Xiwangshan). They are all tholeiitic in composition typically with 47–52wt.% SiO2 and 4–10wt.% MgO, and all show light rare earth element enrichments and Nb- and Ta-depletion. Their Nd TDM ages are in the range of 2.5–3.0Ga. Specifically, the Hengling and Yixingzhai dykes/sills are depleted in Th, U, Zr, Hf and Ti, whereas the Xiwangshan dykes are enriched in U and weakly depleted in other elements. Variable Sr-anomalies indicate significant feldspar accumulation (positive anomalies) or fractionation. The εNd(t) values of the three swarms are: −3.2–+3.0 (Hengling), −1.7–+1.8 (Yixingzhai) and −1.4–+1.0 (Xiwangshan).These mafic representatives of the HMB originated from the >2.5Ga sub-continental lithospheric mantle of the NCC, and with A-type granites and other igneous associations in this belt they likely evolved in an intra-continental rift. The progressive changing compositions of the three swarms are interpreted in terms of their source regions at different depths, i.e., shallower and shallower through time. And the decrease in scale and size of the intrusions and their magma volumes indicate the progressive weakening of magmatism in this rift. The rocks in this belt are different chronologically, petrologically and chemically from those in the Xuwujia magmatic belt (XMB). We propose that the two magmatic belts represent two different magmatic systems in different blocks of the NCC, i.e., an eastern block (with the HMB) and a western block (with the XMB). Terminal collision was possibly a result of ridge subduction between the two blocks, which led to exhumation of the igneous rocks in the two belts from different crustal levels, distinguishable by their different grades of metamorphism.

Revisiting thrusting, reverse faulting and transpression in the southern Sudbury Basin, Ontario

The synformal Sudbury Igneous Complex (SIC) is the deformed relic of a large impact melt sheet. Understanding the geometry and kinematics of deformation of the SIC is paramount for mineral resource deposit exploration of this complex, constraining its pre-impact configuration and understanding Paleoproterozoic tectonism at the southern margin of the Archean Superior Province. In particular, it is uncertain to what extent thrusting on the South Range Shear Zone (SRSZ), a ductile shear zone transecting the southern portion of the SIC, is coeval with folding of the SIC. Geometric properties of 1090 discontinuities mapped from air photos and geometry of metamorphic L–S fabrics revealed the presence of two spatially and kinematically distinct deformation regimes in the southern SIC, the so-called South Range. NW-directed thrusting on the SRSZ dominates in the western South Range, whereas dextral transpression prevails in the eastern South Range. The two deformation regimes are geometrically and kinematically related to the SRSZ. We propose that the SRSZ transects the South Range south of the SE-lobe, a tight fold of the SIC. This is at variance with previous hypotheses in which the SRSZ continues into the hinge zone of the SE-lobe and in which dextral transpression is spatially confined largely to this lobe. The curvature of the SRSZ and its mechanical coupling to its footwall rocks imparted dextral transpressive deformation to the eastern South Range. Dextral component shear of this deformation was associated with shortening of the SIC oblique to its contacts. Collectively, this led to map-view corrugation of SIC contacts and thickness variations of the SIC that culminated in the formation of the SE-lobe. Our study suggests that thrusting on the SRSZ is kinematically linked and, thus, coeval with folding of the SIC.

Spatio-temporal evolution of the Satpura Mountain Belt of India: A comparison with the Capricorn Orogen of Western Australia and implication for evolution of the supercontinent Columbia

Reconstruction of the Neoproterozoic supercontinent Rodinia shows near neighbour positions of the South Indian Cratons and Western Australian Cratons. These cratonic areas are characterized by extensive Paleoproterozoic tectonism. Detailed analysis of the spatio-temporal data of the Satpura Mountains of India indicates presence of at least three episodes of Proterozoic orogeny at ∼2100–1900Ma, ∼1850Ma and ∼1650Ma, and associated basin development and closing. A subdued imprint of the Grenville orogeny (∼950Ma) is also found in rock records of this Mountain Belt. The Capricorn Orogen of Western Australia also shows three episodes of orogeny: Opthalmian–Glenburgh Orogeny (2100–1950Ma), Capricorn Orogeny (∼1800Ma) and Mangaroon Orogeny (∼1650Ma), and basin opening and closing related to these tectonic movements. These broad similarities suggest their joint evolution possibly in a near neighbour position during Paleoproterozoic Era. In view of juxtaposition of the Western Australia along the east coast of India, at the position of the Eastern Ghats, during Archean, it is suggested that the breaking of this Archean megacraton at ∼2400Ma led to northward movement of the broken components and formation of the Satpura–Capricorn Orogen (at ∼2100 and ∼1800Ma) due to the collision of cratonic blocks with the pre-existing northern cratonic nuclei of India and Western Australia. This is also the time of formation of the supercontinent Columbia. A phase of basin opening followed the ∼1800Ma event, followed by another phase of collisional event at ∼1600Ma at the site of the Satpura–Capricorn Orogen. Subsequent evolutions of the Satpura and the Capricorn Orogens differ slightly, indicating separate evolutional history.

Paleoproterozoic accretionary orogenesis in the North China Craton: A SHRIMP zircon study

The Inner Mongolia Suture Zone (IMSZ) and the Trans-North China Orogen (TNCO) incorporate the major Paleoproterozoic accretionary orogens in the North China Craton (NCC), with the Jiao-Liao-Ji Belt (JLJB) representing the third one. Here we investigate the Paleoproterozoic tectonics of the IMSZ and TNCO through zircon SHRIMP geochronology on a representative suite of rocks comprising metasediments and arc magmatic rocks. SHRIMP analysis of zircons with textures indicating extreme recrystallization under ultrahigh-temperature (UHT) conditions from the metapelites at Heling’er in the southern domain of the IMSZ reveals a single population with a weighted mean 207Pb/206Pb age of 1913±17Ma. The zircons in another UHT granulite from this locality yield a weighted mean 207Pb/206Pb age of 1910±18Ma. These data correlate with the ca. 1.92Ga age reported from zircons in sapphirine-bearing UHT granulites further north and confirm the regional extent of the Paleoproterozoic UHT metamorphism within the IMSZ. Zircons in a charnockite from the southern margin of the Khondalite Belt fringing the UHT granulites in the IMSZ show two distinct age groups: an older population with a magmatic paragenesis and a weighted mean 207Pb/206Pb age of 1932±24Ma, and a younger group of metamorphic zircons with an age of 1858±26Ma. We also report zircon ages from charnockites in two localities around Xing’he in the Huangtuyao belt belonging to the Huai’an Complex within the westernmost domain of the TNCO at the junction with the IMSZ. The charnockite from first locality carries two distinct zircon populations with the older group yielding a weighted mean 207Pb/206Pb age of 2477±2Ma and the younger population showing an age of 1807±38Ma. The internal structure as revealed from CL images and the overall high Th/U values (up to 2.42) of the older zircons suggest their magmatic affinity, whereas the younger group with extremely low Th/U (0.02–0.09) is of metamorphic origin. Zircons from the charnockite in the second locality also define two distinct age clusters with a dominant older (magmatic) group having a weighted mean 207Pb/206Pb age of 2147±11Ma and a minor younger group with an age of 1958±25Ma. The range of ages from 2477 to 2147Ma from magmatic zircons in the charnockites from the eastern periphery of the IMSZ, within the western margin of the TNCO, in combination with similar ages reported in recent studies from zircons in magmatic complexes within the IMSZ suggest a prolonged history of subduction-related arc magmatism and accretionary tectonics analogous to those in some of the Phanerozoic belts such as the Central Asian Orogenic Belt and the Western Pacific. Subsequent progressive collision and suturing of the continental blocks were accompanied by the exhumation of high-pressure (HP) and UHT metamorphic rocks. The available data from the IMSZ and TNCO suggest long-lived convergent margins associated with the southward subduction of the Yinshan Block and westward subduction of the Eastern Block in a double-sided subduction realm prior to the final amalgamation of the NCC and its incorporation within the Columbia supercontinent in the late Paleoproterozoic.

Geophysical and geological tests of tectonic models of the North China Craton

The geometry and timing of amalgamation of the North China Craton have been controversial, with three main models offering significantly different interpretations of regional structure, geochronology, and geological relationships. One model suggests that the Eastern and Western Blocks of the NCC formed separately in the Archean, and an active margin was developed on the Eastern Block between 2.5 and 1.85 Ga, when the two blocks collided above an east-dipping subduction zone. A second presumes the Eastern Block rifted from an unknown larger continent at circa 2.7 Ga, and experienced a collision with an arc (perhaps attached to the western block) above a west-dipping subduction zone at 2.5 Ga, and the 1.85 Ga metamorphism is related to a collision along the northern margin of the craton when the NCC joined the Columbia supercontinent. A third model suggests two collisions in the Central Orogenic Belt, at 2.1 and 1.88 Ga, but recognizes an early undated deformation event. Recent seismic results reveal details of the deep crustal and lithospheric structure that support both the second and third models, showing that subduction beneath the Central Orogenic Belt was west-directed, and that there is a second, west-dipping paleosubduction zone located to the east of the COB dipping beneath the Western Block (Ordos Craton). The boundaries identified through geophysics do not correlate with the boundaries of the Trans-North China Orogen suggested in the first model, and the subduction polarity is opposite that predicted by that model. High-pressure granulite facies metamorphism at 1.85 Ga is not restricted to the “TNCO” as suggested by the first model, but is documented across the NCC, as predicted by the second model, suggesting a major continent–continent collision along the north margin of the craton at 1.85 Ga. Further, it has recently been shown that in the southern “TNCO”, there is no record of metamorphism at circa 1.85 Ga, but only at 2.7–2.5 Ga, showing that the “TNCO”, as defined as a circa 1.85 Ga orogen, does not exist. This is further confirmed by recent Re–Os isotopic studies which show that the subcontinental lithospheric mantle beneath the southern COB is late Archean in age, and that a province in the northern NCC is circa 1.8 Ga, correlating with the proposed collision belt of the NCC with the Columbia supercontinent across the entire NCC. The COB is an Archean convergent belt, re-worked in the Paleoproterozoic, and the Paleoproterozoic tectonism is widespread across the NCC, as predicted by the model whereby the previously amalgamated Eastern and Western Blocks experienced a continental collision with Columbia at circa 1.85 Ga, but uplift/exhumation rates are slow, necessitating a re-evaluation of the tectonic models of the NCC.

Precambrian plate tectonics: Seismic evidence from northern Hudson Bay, Canada

The Canadian Shield is one of the largest exposures of Precambrian rocks on Earth. It is a mosaic of several Archean terranes that were brought together during a series of Paleoproterozoic orogens culminating in the so-called Trans-Hudson orogen, which is thought to have been similar to the Himalayan orogen in scale and nature. The tectonic evolution and lithospheric subdivisions of this region are poorly understood, but new seismic networks in northern Hudson Bay provide fresh opportunity to place constraints on the Precambrian processes that formed and shaped it. We show, via a study of seismic anisotropy, that the lithosphere of the northern Hudson Bay region retains a strong signature of Archean–Paleoproterozoic tectonics. We map the boundary between the upper (Churchill) and lower (Superior) plates that collided ca. 1.8 Ga and identify back azimuth–dependent splitting parameters (ϕ, δt) on Baffin Island that indicate complex anisotropy (e.g., dipping fabric) beneath the region. Our results support the view that significant lithospheric deformation occurred during the Paleoproterozoic and that modern-day plate tectonic processes were thus in operation by at least ca. 1.8 Ga.

Superimposed Neoarchaean and Paleoproterozoic tectonics in the Terre Adélie Craton (East Antarctica): Evidence from Th–U–Pb ages on monazite and 40Ar/ 39Ar ages

In order to understand the tectonic behaviour of a stabilized Neoarchaean continental crust during subsequent tectonic activity, we investigated the composite metamorphic basement along the Terre Adélie and George Vth Land coastline, also known as the Terre Adélie Craton (East Antarctica). Two domains are recognized: (1) a Neoarchaean basement, composed of granulite rocks to the east and overlain to the west by amphibolites, and (2) two Paleoproterozoic detrital basins overly the Neoarchaean crust and extend further west. New geochronological data from the Terre Adélie Craton define a tectonic evolution with three major peaks’ activity. Th–U–Pb electron probe analyses of monazite from the Neoarchaean granulites constrain the main structuration event at ca. 2.45 Ga in agreement with zircon ages from throughout the Neoarchaean domain. Local resetting together with low temperature recrystallization of monazites occurred at ca. 1.7 Ga along hydrated anastomozing metre-scale shear zones. New 40Ar/ 39Ar ages obtained by stepwise heating techniques on amphibole, biotite and muscovite from both the Neoarchaean basement and the Paleoproterozoic basins, illustrate the differential evolutions of basement and its sedimentary cover during the major 1.7 Ga transpressive event. A final event at ca. 1.55–1.50 Ga is only recognized close to the Mertz Shear Zone (145°E) bounding the Terre Adélie Craton to the East. The new data allow us to propose detailed geological pictures of the Terre Adélie Craton geodynamic evolution, from the Neoarchaean to the Mesoproterozoic.