Paleoproterozoic Orogenic Event Research Articles

Unraveling the relationship between a tonalitic pluton and shear zones: Insights from magnetic fabrics and microstructures of the Alto Maranhão batholith, Mineiro belt, southern São Francisco craton

The Alto Maranhão batholith (SE Brazil) comprises granitic juvenile arc-related rocks within the Paleoproterozoic Mineiro Belt. The pluton is bounded by the NE-SW sinistral Jeceaba-Bom Sucesso (JBSZ) and NW-SE dextral Congonhas-Itaverava (CISZ) shear zones. Field, microstructural and magnetic fabric studies were employed in order to constrain its emplacement model and the spatial and temporal relationships between plutons and shear zones during Paleoproterozoic orogenic events. Combined meso- and microscale fabric analyses allowed us to individualize two different domains or lobes across the intrusion. The magmatic foliation, defined by biotite and hornblende, strikes NE-SW in the western lobe and rotates to NW-SE in the eastern lobe, moderately to steeply dipping, trending accordingly with the solid-state foliation exhibited by the country rocks. Field observations show that magmatic structures are preserved, such as elongated to rounded syn-magmatic enclaves and magmatic layering. The microstructures described for both domains are a direct record that deformation occurs locally and under magmatic conditions, with limited features of solid-state deformation such as undulose extinction in quartz and plagioclase on both tonalite and dioritic enclaves. AMS data exhibits dominantly steep magnetic lineations in the eastern lobe of the intrusion, except for sampling sites close to the CISZ, whose lineations are shallowly plunging. In both cases, the magnetic lineation trends NW-SE. Towards the western lobe, the magnetic lineation becomes horizontal and randomly oriented close to its westernmost tip. The magnetic and magmatic foliations follow the same orientation, therefore striking NW-SE in the eastern lobe and NE-SW in the west. Considering the batholith's heterogeneities highlighted by the AMS measurements, we suggest that the eastern lobe of the intrusion is characterized as a feeder zone in which magma was injected along the CISZ, being subsequently spread into the pluton's western lobe, until being stopped by a rheological barrier defined by JBSZ and country rocks, controlling its boomerang-like form. The proposed emplacement model suggests that the Alto Maranhão batholith was emplaced in a contractional site, by arc-normal dextral simple shear revealing that transcurrent deformation was taking place concurrently with the growth of the São Francisco paleocontinent, and addition of mantle-derived magmas to the continental crust.

Genesis and source affinities of heterogeneous ultramafic rocks in the North Kongling Complex, Yangtze craton: Architecture of a Paleoproterozoic accretionary orogen

Ultramafic massifs are minor but meaningful components of orogens because they record the mantle properties and the processes of plate generation, subduction, accretion, and collision, thus providing key information about crust-mantle interaction processes and the evolution of orogens. Here, we present an integrated study of field observations, whole-rock and mineral geochemistry, zircon U-Pb ages, and whole-rock Sm-Nd and Re-Os isotopes for serpentinized harzburgites and olivine pyroxenites from the Paleoproterozoic Shuiyuesi mélange belt, North Kongling Complex, to constrain the tectonic evolution of the Yangtze craton, South China. The serpentinized harzburgites are characterized by enrichment of light rare earth elements (LREEs) and negative εNd(t) (−4.3 to −3.9) values, but they have high olivine Mg# (91.7−93.1) and very low Re contents (<0.09 ppb) and negative γOs(t) (−5.1 to −4.0) values. The Archean Re-Os model ages (TRD = 2.82 Ga) of the harzburgites that have the most depleted Re-Os isotope composition represent estimates for the age of the initial melt extraction. Metasomatic zircons from the serpentinized harzburgites yield a weighted average age of ca. 2.04 Ga, which is interpreted to represent the age of metasomatism during a subduction event. These geochemical and isotopic features and geochronologic results suggest that the protolith of the serpentinized harzburgites in the North Kongling Complex originated as fragments of late Mesoarchean (ca. 2.82 Ga) subcontinental lithospheric mantle and experienced melt/fluid metasomatism in a subarc mantle wedge at 2.04 Ga. The olivine pyroxenites are cumulates, and they are characterized by high MgO, Ni, and Cr contents and depleted to slightly enriched LREEs but depletion in high field strength elements (HFSEs). They have positive εNd(t) (+2.0 to +6.6) values and Paleoproterozoic Nd model ages (2.41−2.05 Ga), suggesting that the olivine pyroxenites were derived from a Paleoproterozoic depleted mantle source. Metamorphic zircons from the olivine pyroxenites indicate subsequent metamorphism at 2.0 Ga, in response to a collision. The olivine pyroxenites together with the metabasites in the Shuiyuesi mélange belt represent the dismembered fragments of oceanic lithosphere that formed in a suprasubduction zone during the Paleoproterozoic. Combined with the Archean and Paleoproterozoic rocks in the North Kongling Complex, it appears that the Mesoarchean subcontinental lithospheric mantle peridotites and Paleoproterozoic ophiolitic rocks were simultaneously accreted during a Paleoproterozoic orogenic event. We further propose that a subduction channel may have played an important role in the architecture of the accretionary orogen, in which both oceanic lithospheric materials and subcontinental lithospheric mantle wedge materials would have been simultaneously scraped off and accreted during slab subduction-accretion.

Clockwise P-T-t path of Paleoproterozoic metamorphism from the Dengfeng Complex, southern North China Craton

Garnet-bearing metapelites are widely distributed in the Dengfeng Complex situated in the southern segment of the North China Carton (NCC). Petrological studies indicate that garnet-bearing metapelites mainly comprise quartz, muscovite, chlorite, plagioclase, garnet and biotite, with minor ilmenite, zircon, and apatite. Garnet porphyroblasts show obvious growth zoning from the core to inner rim and they are separated by a prominent chemical discontinuity narrow edge. By pseudosection modeling calculations, the P-T conditions of sample 20DF17-C determined by the isopleths of XGrs and XPrp of the garnet core and inner rim are ∼ 530 °C/6.7 kbar and ∼ 560 °C/5.1 kbar, respectively; while the P-T conditions of sample 20DF24 determined by the isopleths of the garnet core and inner rim are ∼ 530 °C/5.2 kbar and ∼ 550 °C/4.5 kbar, respectively. LA-ICP-MS zircon U-Pb dating from the Dengfeng metapelites suggest that the maximum depositional age of their protoliths was ∼ 2.52 Ga. These metapelites were affected by the hydrothermal event immediately after diagenesis. The age of ∼ 1.92 Ga contributed to clockwise P-T paths containing decompression with slight heating segments, which occurred at the Paleoproterozoic and likely were related to a subduction-collision event in the NCC. Combined with the occurrence features of the amphibolites, TTG gneisses, metapelites and quartzites, we inferred that all the Dengfeng metapelites and Songshan Group involved a Paleoproterozoic orogenic event along the Trans-North China Orogen (TNCO)/Jinyu Mobile Belt (JMB).

Decoupling of metamorphic zircon U-Pb ages and P-T paths in the Dunhuang metamorphic complex, northwestern China

U-Pb dating of metamorphic zircon is commonly used to determine the timing of high-grade metamorphism, but linking zircon U-Pb ages to metamorphic P-T paths can be challenging, especially in metamorphic terranes where multiple metamorphic events result in various degrees of re-equilibration and complex metamorphic zircon growth/recrystallization. Here we carried out a detailed P-T path study, as well as zircon/titanite/apatite U-Pb dating and mineral Sm-Nd isochron dating, for garnet amphibolites from the Dunhuang Block, northwestern China, where late Paleoproterozoic metamorphism was extensively overprinted by a Paleozoic tectonothermal event and the P-T conditions of the earlier event remain elusive. Petrographic observation, phase equilibria modeling and geothermobarometry show that the garnet amphibolites share a clockwise P-T path, from 675 – 689 °C/10.0 – 11.6 kbar to 780 – 807 °C/9.9 – 11.8 kbar for the prograde to peak metamorphism, followed by significant cooling and decompression to 612 – 689 °C/4.1 – 6.0 kbar, indicating an orogenic setting. U-Pb dating of metamorphic zircons yields two groups of upper intercept ages of ca. 1.82 – 1.85 and 1.95 Ga. The ca. 1.82 – 1.85 Ga metamorphic zircons have flat HREE patterns and weak Eu* anomalies, suggesting zircon growth when abundant garnet but little plagioclase was present, consistent with high-pressure metamorphism. In contrast, the ca. 1.95 Ga zircons have steep HREE and negative Eu anomalies, suggesting formation at relatively low-pressures. These zircon results provide evidence for 1.8 – 1.9 Ga metamorphism, possibly due to variable crustal thickening during a late Paleoproterozoic orogenic event. However, garnet porphyroblasts and the matrix minerals yield a Sm-Nd isochron age of 333.5 ± 4.0 Ma, whereas metamorphic titanites record a U-Pb age of 345.4 ± 2.7 Ma. Considering the high closure temperatures of the garnet Sm-Nd (650 – 720℃) and titanite U-Pb (710 – 780℃) systems, our data suggest that the late Paleoproterozoic metamorphic mineral assemblages were likely completely re-equilibrated by a Paleozoic tectonothermal event and that the P-T path documented above is a result of Paleozoic orogeny. In addition, apatite U-Pb dating yields 236.6 ± 4.3 and 300 ± 10 Ma for samples from the southern and central Dunhuang area, respectively, indicating different cooling histories. Combining with previous results, we propose that the Paleozoic metamorphism occurred in a long-lived hot continental arc with slow cooling rates (0.3 – 5℃/Myr), rather than an subduction–accretionary complex. Our study highlights the decoupling of metamorphic zircon U-Pb ages and P-T paths, which has significant implications for P-T-t path studies in polyphase metamorphic terranes.

U-Pb dating reveals multiple Paleoproterozoic orogenic events (Hamersley orogenic cycle) along the southern Pilbara margin (Australia) spanning the onset of atmospheric oxygenation

Abstract The early Paleoproterozoic was a time of unprecedented change in Earth's climate and surface environment. The key to resolving some of the controversies surrounding the timing and causes of these changes lies with supracrustal sequences, such as the 2.45–2.22 Ga Turee Creek Group in the southern Pilbara craton, northwestern Australia. The group preserves a predominantly siliciclastic sequence; however, its precise age, tectonic setting, and postdepositional history are disputed. Although it is interpreted to have been deposited in a foreland basin setting shortly after 2.45 Ga, the oldest well-recognized deformational event, marked by northward folding and thrusting, is the 2.20–2.15 Ga Ophthalmia orogeny. Evidence for a pre-Ophthalmia fabric-forming tectonic event north of the Archean Sylvania Inlier, southeast Pilbara craton, which is marked by tight to isoclinal folding, has been largely overlooked. In this area, we report in situ U-Pb geochronology of authigenic monazite and xenotime in shale with a well-developed tectonic cleavage from the ca. 2.63 Ga Jeerinah Formation. Monazite porphyroblasts, which are locally wrapped by strain fringes aligned in a tectonic cleavage, yielded weighted mean 207Pb/206Pb ages at 2370 ± 11 Ma and 2312 ± 8 Ma, whereas xenotime, which overprints a crenulation cleavage, gave a weighted mean 207Pb/206Pb age of 2291 ± 11 Ma, constraining fabric development to between 2.31 Ga and 2.29 Ga. Our results confirm the existence of a pre-Ophthalmia deformational event in the southeastern Pilbara craton, herein referred to as the Sylvania orogeny, which is part of an ~300 m.y. interval (2.45–2.15 Ga) of northward-directed compression (“Hamersley orogenic cycle”). This orogenic cycle is marked by east-west and northwest-southeast folding, cleavage development, veining, hydrothermal gold mineralization, and isotopic resetting across the southern Pilbara craton. Our results indicate that the syn–Great Oxidation Event Turee Creek Group was deposited in one or more foreland basins after 2.45 Ga. Our results provide a new tectonostratigraphic and geodynamic framework for understanding the timing and origin of geochemical records in a key succession deposited during an interval of global environmental change.

Disclosing Rhyacian/Orosirian orogenic magmatism within the Guanhães basement inlier, Araçuaí Orogen, Brazil: A new piece on the assembly of the São Francisco-Congo paleocontinent

The Guanhães block is one of the basement units present in the context of the Neoproterozoic West Gondwana assembly. It was reworked as a basement inlier within the Araçuaí orogen, in the margins of the São Francisco-Congo (SF-C) paleocontinent. The Guanhães block’s Rhyacian-Orosirian record is revealed for the first time by the Felício and the Mercês granitic plutons. Geophysical, petrographic, lithogeochemical and U-Pb (LA-ICPMS) analyses define and characterize these plutons, and link their genesis to the major Paleoproterozoic orogenic event that is thought to have amalgamated the SF-C paleocontinent. The Felício pluton is composed of mesocratic fine-grained epidote-biotite-granodiorite, the Mercês pluton comprises biotite-monzogranite and hornblende-syenogranite. These two plutons show different geochemical features; the Felício rocks are low-K, magnesian, calcic to calc-alkalic, metaluminous, and the Mercês rocks are rich in K, ferroan, alkali-calcic to alkalic, peraluminous, their shared common features are enrichment in LILE, LREE and HFSE and negative anomalies of Ti, Ta, Nb and P. Zircons collected from the Felício pluton granodiorite yielded a U-Pb concordia age of 2151 ± 12 Ma, interpreted as the pluton’s crystallization age. The Mercês pluton monzogranite yielded a discordia with upper intercept at 2014 ± 11 Ma and lower intercept at 539 ± 13 Ma, interpreted as the Mercês pluton’s cristallization age and the Brasiliano metamorphic overprint, respectively. Trace elements and REE signatures suggest that the magma that originated both plutons shares a common source, compatible with the metasomatization of a mantle wedge by dehydration of a subducted slab. Based on the petrographic, geochemical and geochronological data, the following tectonic scenario is suggested: subduction and accretion of the Mantiqueira arc and emplacement of the Felício pluton at c. 2.15 Ga, followed by collision, gravitational collapse of the orogen, post-collisional magmatism and emplacement of the Mercês pluton at c. 2.01 Ga. This scenario is reinforced by the regional geochemical correlations among the many coeval terranes in the eastern margin of the São Francisco craton, along the Minas-Bahia orogen. At a broader scale, these correlations can be expanded to the African counterpart, giving insights into the SF-C paleocontinent framework in the Precambrian.

Extent and age of Mesoarchean components in the Nagssugtoqidian orogen, West Greenland: Implications for tectonic environments and crust building in cratonic orogenic belts

The Nagssugtoqidian orogen (NO), which forms the northern margin of the North Atlantic craton in West Greenland, is largely comprised of ~2870 to 2720 Ma juvenile continental crust reworked during Paleoproterozoic orogenic events. Sparse evidence of components older than 3100 Ma in the area (U–Pb zircon ages and bulk-rock Sm–Nd isotopes) have hinted at a unit called the Qorlortoq gneiss but were not substantiated by regional studies. Here we report the “rediscovery” of the Qorlortoq gneiss, documented through a new geochemical and geochronological dataset of this orthogneiss and related components, which include ultramafic cumulate enclaves and late basaltic dykes. Laser Ablation Split Stream (LASS) U-Pb/Lu-Hf isotopic analyses of zircon from the gneiss give a U–Pb concordia upper-intercept age of 3177 ± 12 Ma and weighted mean εHf(t) of 1.7 ± 0.5, indicating a juvenile protolith extracted from a depleted mantle source.Ultramafic enclaves of cumulate origin make up a key component of the Qorlortoq gneiss. Their trace element systematics indicate a parental melt with tholeiitic affinities, sourced from depleted mantle beneath relatively thin lithosphere (<80 km), that was subsequently contaminated by crustal assimilation. Re–Os isotope systematics of the cumulates and their relationship with the younger Qorlortoq dykes place the age of these cumulates close to that of the Qorlortoq gneiss.The age of the Qorlortoq gneiss makes it the oldest known component in the NO by ~300 Ma. The extended timescale between episodes of crustal production in this area, could indicate a geodynamic environment characterized by stagnant-lid or episodic mobile-lid tectonics, at least on a regional scale. These constraints provide important information about the diversity and nature of Archean geodynamic environments.

Late Neoarchean to Paleoproterozoic arc magmatism in the Shandong Peninsula, North China Craton and its tectonic implications

The transition in tectonic regime of our planet during the late Archean with the initiation broadly modern style subduction system is one of the major topics of interest in earth science. The North China Craton (NCC), one of the oldest cratonic nuclei in the world, preserves important records of Archean and Paleoproterozoic crustal evolution, cratonization and related geodynamic history that are important in understanding the continental growth of the early Earth. Here we investigate a suite of Archean to Paleoproterozoic rocks including hornblendite, amphibolite, gabbro, diorite, basalt, plagiogranite, granite porphyry and greenschist from Shandong Peninsula within the southwestern Jiaoliao block of the eastern NCC using zircon U-Pb geochronology, Lu-Hf isotopes, and whole-rock geochemistry. We aim to understand the timing and petrogenesis of the rock suite and its implications on the geodynamic history of the NCC. Zircon U-Pb data show 207Pb/206Pb ages in the range of 2.68–1.83 Ga, and is divided into three groups of 2.68–2.43, 2.38–1.96 and 1.95–1.83 Ga. Among these, the 2.68–2.43 Ga group confirms with the major magmatic phase during Neoarchean-Paleoproterozoic transition in the eastern NCC, whilst the other groups of 2.38–1.96 and 1.95–1.83 Ga ages might represent the Paleoproterozoic orogenic events in the NCC. Zircon Lu-Hf isotopic data display εHf(t) values ranging from −0.4 to 10.0 and TDMC of 3074–2080 Ma, which are subdivided into one group with U-Pb ages of 2672–2445 Ma characterized by εHf(t) values from −0.4 to 7.0 and TDMC of 3074–2588 Ma and another group (2134–2009 Ma) with εHf(t) values in the range of 2.8–10.0 and TDMC ranging from 2458 to 2080 Ma. These results indicate that the magmas (2672–2445 Ma group) were sourced mainly from Late Mesoarchean to Neoarchean juvenile components with minor input of reworked Mesoarchean crustal components, whereas the 2134–2009 Ma group was derived from Paleoproterozoic juvenile components. Whole-rock geochemical data of the granite porphyry and plagiogranites show typical subduction-related arc signature with the enrichment of LREEs and LILEs and depletion of HREEs and HFSEs (e.g. Nb, Ta, Ti), and mainly belong to I-type granites generated at arc-related setting. The ultramafic–mafic magmatic suite corresponds with tholeiitic to calc-alkaline series which were formed by partial melting of enriched mantle within an island arc setting in an active continental margin. In conjunction with the results from previous studies in the NCC, we correlate the 2.68–2.43 Ga ages with Late Neoarchean amalgamation of microblocks and cratonization of the NCC driven by late Neoarchean oceanic plate subduction, and the 2.38–1.83 Ga ages represent Paleoproterozoic orogenic events including the assembly of major crustal blocks in the NCC within the Paleoproterozoic supercontinent Columbia, and the final collision of these crustal blocks and cratonization.

Late Neoproterozoic intracontinental rifting of the Tarim carton, NW China: An integrated geochemical, geochronological and Sr–Nd–Hf isotopic study of siliciclastic rocks and basalts from deep drilling cores

Late Neoproterozoic extensive rifting in Tarim has been intensely debated regarding an intracontinental or a back-arc environment, which forms a key aspect for elucidating the Precambrian basement architecture and evolution of the Tarim craton. Recently, a deep well called Tarim Geological Survey Well (X1) was drilled from the northwest Tarim basin. Sixty-six samples were collected from the Sugetbrak Formation of the X1 well, including 6 sandstone samples, 29 mudstone samples and 31 basalt samples, for integrated geochemical, geochronological, and Sr–Nd–Hf isotopic analyses. The results indicate that the basalts geochemically show distinct affinity with ocean-island basalts, characterized by enrichments in light rare-earth elements (REEs) and no depletions in Nb and Ta and indicating an intracontinental rifting environment. The presence of 572-Ma zircon in the Sugetbrak basalts reveal the rifting mainly developed in an interval of 541–572 Ma. Geochemical and petrographic data show that the coeval basaltic tuffs or clasts contributed 5–20% to the deposition of the Sugetbrak Formation. The zircon age cluster between 1.8 and 2.0 Ga and abundant Paleoproterozoic detrital zircons with ultrahigh-temperature (UHT) granulate REE signature in the Sugetbrak sandstones indicate a Paleoproterozoic orogenic event occurred in the Tarim craton that could be related to the assembly of the Columbia supercontinent. In contrast, the Neoproterozoic zircons from the Sugetbrak sandstones display positive εHf(t) values, and some have hydrothermal alteration REE signature, indicating Tarim could be a locus for early mantle plume activities that broke the Rodina supercontinent.

Petrogenesis and geochronology of Paleoproterozoic magmatic rocks in the Kongling complex: Evidence for a collisional orogenic event in the Yangtze craton

The early Precambrian crustal evolution of the Yangtze craton, especially the Paleoproterozoic orogenic event, has drawn great attention. The early Precambrian magmatism in the Archean-Palaeoproterozoic basement Kongling complex is critical for understanding the early Precambrian tectonic evolution of the Yangtze craton. We report the presence of two distinct groups of Paleoproterozoic felsic rocks in the Kongling complex, providing new insights into the Paleoproterozoic orogenic event in the northern Yangtze craton. Group 1 is composed mainly of deformed or weakly-deformed monzogranite, syenite, and quartz monzonite, and formed at 2.0–1.95 Ga. They have high Sr/Y (60–71) and La/Ybcn (32–464) ratios and are inferred to have equilibrated with garnet-rich residues, suggesting that they have been generated by partial melting of thickened continental crust. Group 2 is represented mainly by undeformed to weakly-deformed K-feldspar granitic rocks, with minor rhyolites, and was emplaced at 1.87–1.84 Ga. In contrast to Group 1, Group 2 rocks display flat HREE patterns and large negative Eu, Sr and Ti anomalies, with low Sr/Y (<10) and La/Ybcn (16–21) ratios. Group 2 rocks have a typical A-type affinity with high Ga/Al ratios (2.83–3.82), and high crystallization temperatures (850–950 °C). Group 2 rocks are interpreted to have been derived from relatively shallow partial melting of thin continental crust during post-orogenic collapse, which marked lithospheric extension of the Yangtze craton in the late Paleoproterozoic (ca. 1.85 Ga).Zircons from Group 1 rocks have εHf (t) values ranging from −9.4 to −14.9, whereas zircons from Group 2 rocks have lower εHf (t) values ranging from −14.2 to −20.6. Both groups have similar two-stage Hf model ages ranging between 3.2 and 3.6 Ga. The production of the two groups of granitoids in the Huangling dome is ascribed to the reworking (dehydration melting) of Archean rocks in the Paleoproterozoic. Widespread Paleoproterozoic magmatism and metamorphism indicate that the Yangtze craton experienced an orogenic event at ca. 2.0 Ga and subsequent orogenic collapse caused by lithospheric extension at ca. 1.85 Ga.

The multistage tectonic evolution of the northeastern Carajás Province, Amazonian Craton, Brazil: Revealing complex structural patterns

Structural data collected in the Carajás Province region led to a new interpretation of the southeastern Amazonian Craton geotectonic evolution. The purpose of this article is to present a new evolutionary proposal for the region. A detailed analysis of the several extensional-compressional cycles that overprinted each other from the Archean to the Neoproterozoic-Cambrian is presented. At about 2.87–2.83 Ga, collisional processes led to the formation of a stable crustal substrate that supported the installation of an extensional basin at 2.76–2.70 Ga and the deposition of the Itacaiúnas Supergroup shallow marine volcanosedimentary sequences, together with contemporary bimodal plutonism. Paleoproterozoic arc magmatism in the Bacajá Domain was followed by collision with the Carajás Province between 2.09 and 2.06 Ga, resulting in expressive tectonic thickening and low to high grade regional metamorphism, and in the deposition of the Águas Claras Formation. A second Paleoproterozoic orogenic event affected the Carajás Province, which resulted in oblique tectonism and regional counterclockwise rotation of previous associations, followed by late to post-orogenic sedimentation and 1.88 Ga anorogenic alkaline A-type magmatism. The eastern Carajás Province margin was extensionally reactivated during the Neoproterozoic, in a rifting event, followed by tectonic inversion during the Ediacaran/Cambrian.

Early Proterozoic granitoids of the Olenek complex (northern Siberian craton): petrogenesis and geodynamic setting

The paper deals with geological and geochemical studies of granitoids of the Olenek complex in the Olenek uplift of the basement of the northern Siberian craton. The age of these granitoids was earlier estimated at 2036 ± 11 Ma. The granitoids of the Olenek complex correspond in composition to high-alumina quartz diorites, granites, and leucogranites of the normal petrochemical series. According to geochemical and mineralogical characteristics, the quartz diorites can be assigned to granites of the transitional I-S type, and the granites and leucogranites, to S-type granites. The 8Nd(T values in the granites of the Olenek complex vary from -0.2 to +1.4, and the Nd model age is 2.4-2.5 Ga. The quartz diorite is characterized by 8Nd(T) = +3.0 and a Nd model age T(DM) = 2.2 Ga. The geochemical characteristics of the granites and leucogranites indicate their formation through the melting of a source of graywacke composition, whereas the quartz diorites resulted, most likely, from the mixing of granitic and basaltic melts. The fact that the granitoids of the Olenek complex intruded the folded rocks of the Eekit Formation but stay virtually undeformed massive bodies suggests that they formed at the postdeformation stage of the regional evolution after the completion of the Paleoproterozoic orogenic events. The intrusion of granitoids marks the completion of the formation of the Early Proterozoic Eekit fold belt on the western (in the recent coordinates) margin of the Birekta terrane of the Olenek superterraine and the final formation of the superterrane structure. At the next stage of magmatism (1.98-1.96 Ga), best pronounced in the uplifts of the basement of the northern Siberian craton, all terranes forming the Anabar and Olenek superterranes assembled into a single structure.

High-pressure granulites in the Fuping Complex of the central North China Craton: Metamorphic P–T–t evolution and tectonic implications

Mafic granulites in the Fuping Complex occur as lenses or boudins within high-grade TTG (Trondhjemite–Tonalite–Granodiorite) gneisses. Petrographic observations reveal four generations of mineral assemblage in the granulites: an inclusion assemblage of hornblende + plagioclase + ilmenite + quartz within garnet core; an inferred peak assemblage composed of garnet ± hornblende + plagioclase + clinopyroxene + rutile/ilmenite + quartz; a decompression assemblage characterized by symplectites of clinopyroxene ± orthopyroxene + plagioclase, coronae of plagioclase ± clinopyroxene ± hornblende around embayed garnet porphyroblasts or a two-pyroxene association; and a late amphibolite-facies retrogressive assemblage. Two representative samples were used for pseudosection modeling in NCFMASHTO model system to determine their metamorphic evolution. The results show that these granulites experienced a high-pressure stage of metamorphism with peak P–T conditions of 12–13 kbar and 760–800 °C (Pmax) and a post-peak history under P–T conditions of ∼9.0 kbar and 805–835 °C (Tmax), indicating a nearly isothermal decompression process (ITD) with a slight heating. Metamorphic evolution from the Pmax to the Tmax is predicted to be dominated by garnet breakdown through continuous metamorphic reactions of garnet + quartz ± diopside = hornblende + plagioclase + liquid and garnet + quartz + hornblende = plagioclase + diopside + liquid + orthopyroxene. Further metamorphic evolution after the Tmax is dominated by cooling, suggesting that high-pressure (HP) granulites may also exist in the Fuping Complex. Metamorphic zircons in the Fuping HP mafic granulites have left inclined REE patterns, Ti contents of 1.68–6.88 ppm and crystallization temperatures of 602–712 °C. SIMS zircon U–Pb dating on these zircons yields 207Pb/206Pb ages of 1891 ± 14 Ma and 1849 ± 6 Ma, interpreted to represent the cooling stage of metamorphism. The P–T–t evolution of the Fuping HP mafic granulites records well the protracted Paleoproterozoic orogenic event occurred in the central North China Craton.

Geochemical and Sr‐Nd isotopic records of Paleoproterozoic metavolcanics and mafic intrusive rocks from the West African Craton: Evidence for petrogenesis and tectonic setting

Metavolcanics and mafic intrusive rocks of the Paleoproterozoic Birimian terrane in the southeastern part of the West African Craton, Ghana, were analyzed for major and trace elements and Nd and Sr isotopic data to constrain the geodynamic evolution of the Birimian Supergroup. The metavolcanic rocks consist of metabasalts, meta‐andesites, and amphibolites, whereas the mafic intrusions are mainly gabbros, hornblendites, and dolerites. The rocks are tholeiitic in composition and show the classic features of arc magmatism. The metavolcanics display significant enrichments in large ion lithophile elements (LILE) and light rare earth elements, relative to high field strength elements (HFSE) and heavy rare earth elements. The multielement patterns of the rocks also show positive Pb, Ba, Th, and Sr and negative Nb, Ta, and Ce anomalies that are typical characteristics of subduction‐related magmas. They also have La/Nb ratios &lt;3 and La/Ta ratios &lt;43 that are similar to other Archean and Birimian greenstone belts in West Africa. The rocks have εNd (2.1 Ga) values of −0.96 to +2.60, and Nd model ages of 2.24–2.51 Ga (TDM1) and 2.16–2.45 Ga (TDM2), indicating their juvenile character with possible contributions from pre‐Birimian crustal materials in their sources. The εNd values suggest a depleted source and further indicate that they were probably produced in an almost entirely oceanic environment with minor influence from the continental crust. The Nd isotopic results are consistent with the island arc model, which views Paleoproterozoic terranes of the West African Craton in the context of subduction–accretion processes. Accordingly, these processes may have played a role in the formation of the Columbia supercontinent during the Paleoproterozoic (2.1–1.8 Ga) orogenic events.

Mineralogical and chemical evolution of tantalum–(niobium–tin) mineralisation in pegmatites and granites. Part 2: Worldwide examples (excluding Africa) and an overview of global metallogenetic patterns

Mineralogical and chemical evolution of tantalum–(niobium–tin) mineralisation in pegmatites and granites. Part 2: Worldwide examples (excluding Africa) and an overview of global metallogenetic patterns

The Archean-Paleoproterozoic crustal evolution in the Dunhuang region, NW China: Constraints from zircon U–Pb geochronology and in situ Hf isotopes

The Dunhuang region is located on the eastern margin of the Tarim Craton and eastward to the Alxa block. The Archean-Paleoproterozoic TTG gneisses, potassic granites and supracrustal rocks (the Dunhuang Complex), and the Paleozoic magmatic and metamorphic complexes are exposed in this region. Here we report U–Pb geochronology and Hf isotope analyses on zircons from TTG gneisses in the Dunhuang Complex to probe the Archean-Paleoproterozoic tectono-thermal events and crustal formation and evolution in the Dunhuang region and to provide new insights in understanding the affinity of the Dunhuang Complex. LA-ICP-MS zircon U–Pb dating results in this study, together with previous data reveal that the Dunhuang Complex document six stages magmatic events at ∼3.06Ga, ∼2.7–2.6Ga, ∼2.56–2.45Ga, ∼2.3Ga, ∼2.0–1.9Ga and 1.9–1.8Ga. However, the regional metamorphism only occurred at ∼2.5Ga, ∼2.0–1.9Ga and 1.9–1.8Ga. Thus, the Archean-Paleoproterozoic tectono-thermal events in the Dunhuang region can be divided into three stages at 3.1–2.5Ga, ∼2.0–1.9Ga and 1.9–1.8Ga. Zircon Hf isotope compositions reveal that continental crust in the Dunhuang region was mainly formed by ∼2.7–2.6Ga and ∼2.56–2.45Ga magmatic events and reworked at ∼2.0–1.9Ga and 1.9–1.8Ga. In addition, small volume of continental crust was formed before ∼2.7Ga. The Archean TTG rocks in the region extensively underwent the global Paleoproterozoic (ca. 2.0–1.8Ga) orogenic event associated with the assembly of the Columbia supercontinent, and the Dunhuang Complex generally underwent the Paleozoic (ca. 440–400Ma) orogeny-related metamorphism. TTG rocks emplaced at ∼3.06Ga and ∼2.7–2.6Ga in the central part of the Dunhuang region underwent regional metamorphism at ∼2.0–1.9Ga. Whereas, TTG rocks formed at ∼2.56–2.45Ga and ∼2.3Ga in the northeast of the central part (the Yulinhe area) and the southern part of the Dunhuang region underwent regional metamorphism at 1.9–1.8Ga. Thus, the Precambrian basement rocks in the region underwent different geological evolutionary history. Compare with other cratons in China, they exhibit similarities not only with those in the Tarim Craton, but also with those in the North China Craton. Therefore, we infer that the Tarim Craton and North China Craton might have been a whole during Archean-Paleoproterozoic, and the Precambrian basement rocks in the Dunhuang region may be different parts of Tarim-North China Craton aggregated together by the Paleozoic (ca. 440–400Ma) orogenic event.

Hallandian 1.45 Ga high-temperature metamorphism in Baltica: P–T evolution and SIMS U–Pb zircon ages of aluminous gneisses, SW Sweden

The southernmost Baltic Shield exposes polymetamorphic continental crust that was largely formed and accreted during a series of 1.92–1.66Ga Paleoproterozoic orogenic events and later reworked during the 1.14–0.90Ga Sveconorwegian orogeny. An intermediate period of metamorphism, deformation and magmatism at 1.47–1.38Ga has been attributed to the Hallandian orogeny, but due to overprinting by Sveconorwegian high-grade metamorphism and deformation, the P–T-t evolution and deformation of the Hallandian event have remained obscure. This study presents the first quantitative P–T model of the Hallandian event using high-temperature aluminous gneisses in the south-easternmost marginal part of the Sveconorwegian orogen. The high-grade metamorphism and spatially associated granite magmatism are dated using U–Pb SIMS analysis of zircon. Petrography, bulk and mineral geochemistry, and pseudosection models demonstrate prograde staurolite-sillimanite-grade metamorphism reaching granulite-facies temperatures (700–750°C) at low pressures (4–5kbar), with the formation of Crd+Sil+Grt+K-fsp+Ilm+Melt±Bt. The rocks followed a clockwise P–T path. Later stages involved the formation of sillimanite+biotite at the expense of garnet and cordierite. Local low-temperature and fluid-assisted retrogression also caused the formation of chlorite and muscovite at the expense of cordierite. Both granite and aluminous gneisses contain complex zircon with inherited 1.70Ga igneous cores and high-U, secondary zircon, mainly formed by reworking of protolith cores. The latter date the Hallandian high-grade metamorphism at 1451±6Ma and the granite magmatism at 1445±8Ma. The presence of 1.70Ga igneous zircon cores in both metamorphic and magmatic rocks suggests that they formed from similar protoliths. The protolith ages correlate with the youngest generation of magmatic rocks of the Transscandinavian Igneous Belt. The aluminous gneisses are of supracrustal origin, and may have formed by chemical alteration of magmatic rocks. Hallandian regional metamorphism took place under a strongly elevated geotherm and was associated with granitic magmatism, suggesting an accretionary orogenic setting. The Hallandian event may demonstrate an 1.47–1.38Ga Andean-type continental margin at the SW margin of Baltica.

Metamorphic P–T–t paths of the Zanhuang metamorphic complex: Implications for the Paleoproterozoic evolution of the Trans-North China Orogen

Garnet-bearing metapelites are cropped out in the Zanhuang metamorphic complex located in the central section of the N–S trending Trans-North China Orogen. Petrological studies indicate that these garnet-bearing metapelites preserve three distinct metamorphic assemblages (M1–M3). The cores of garnet porphyroblasts and fine-grained inclusions of plagioclase+quartz±biotite±apatite±magnetite±rutile±ilmenite define the prograde metamorphic stage (M1). The assemblage of the peak metamorphic stage (M2) consists of biotite+plagioclase+quartz±kyanite±hornblende±magnetite±ilmenite in equilibrium with the lowest-XMn garnet rims. The peak metamorphism was followed by a near-isothermal decompressional stage (M3) and the development of symplectitic coronas of biotite+plagioclase+quartz±hornblende±magnetite surrounding garnet porphyroblasts. Quantitative phase equilibria modeling and traditional thermobarometric estimations were applied to yield P–T conditions of 4.5–5.9kbar and 551–596°C for stage M1, 9.6–12.3kbar and 770–830°C for stage M2, and ∼4.7kbar and ∼635°C for stage M3. The combination of the mineral inclusions, mineral compositions, and metamorphic reactions in the Zanhuang metapelites defines a clockwise P–T path with nearly isothermal decompression, which is considered to have been related to the amalgamation of the Eastern and Western Blocks to form the North China Craton. Abundant U–Pb spot analyses of metamorphic zircons from the gneiss and amphibolite reveal two discrete, meaningful ages of metamorphism within the Zanhuang complex: the first age dates to ∼2507Ma, and the second age dates to ∼1839Ma. The first age group (∼2507Ma) was most likely associated with a regional Neoarchean granulite-facies metamorphic event within the North China Craton, whereas the second age group (∼1839Ma) is interpreted as the result of a Paleoproterozoic orogenic event that occurred in the Trans-North China Orogen (TNCO).

Is the Precambrian basement of the Tarim Craton in NW China composed of discrete terranes?

The Precambrian evolution of the Tarim Craton in NE China, in particular during the early Precambrian stage, remains enigmatic. In this contribution, we report field observation, petrology, geochemistry, zircon Lu–Hf isotopes and U–Pb ages of the major rock formations of the Aketage area in the southeastern section of the Tarim Craton. The Milan Group in Aketage is dominantly composed of 2.7–2.5Ga gneissic amphibolite–TTG complex with minor paragneiss. Both the mafic and silicic rocks exhibit geochemical features consistent with an arc affinity. The arc-signature of the 2.01–2.03Ga gneissic granites and gabbros which intrude the Archean basement, as well as the major 2.0Ga metamorphic event revealed by zircon U–Pb dating, suggest an important subduction-collision event possibly related to the assembly of the Paleoproterozoic Columbia supercontinent. The ca. 1848–1856Ma massive potassic granites, 1867Ma mafic dyke swarm and 1844Ma massive leucogranite dykes reveal magmatism in a post-collisional extensional setting.A comprehensive synthesis of the major orogenic events and continental crust growth process from the different Precambrian terranes in Tarim Craton show significant discrepancy in time related to the late Neoarchean crust formation ages and the Paleoproterozoic orogenic events. For example, the major orogenic event took place at ∼1.85Ga in Quruqtagh-Dunhuang terrane, at ∼1.90Ga in the southwest Tarim Craton and at ∼2.0Ga in the Aketage-Qaidam terrane. These different terranes exhibit distinct periods of continental crust growth in the early Precambrian. Continental growth in the Aketage area took place during 2.7–4.3Ga. The 3.6Ga xenocrystic zircons as well as the peak of 4.2Ga zircon Hf model ages, indicate the possible existence of Paleoarchean and even Hadean crust in the Aketage area. In the Quruqtagh-Dunhuang terrane, the growth of early Precambrian continental crust took place at 2.6–3.3Ga with peaks at ca. 2.6–2.7Ga and 3.0Ga. The diachronous late Paleoproterozoic orogenic events and the significant difference in continental growth process suggest that the Precambrian basement of the Tarim Craton is composed by independent continental terranes possibly detached from the cores of discrete ancient cratonic nuclei, which were not unified until the early Neoproterozoic during assembly of the Rodinia supercontinent.

Neoarchean and Paleoproterozoic crust formation in the Ubendian Belt of Tanzania: Insights from zircon geochronology and geochemistry

LA-ICP-MS U–Pb zircon geochronological and geochemical data of meta-igneous and metasedimentary rock types of the Katuma Block of the Paleoproterozoic Ubendian Belt in Tanzania are used to unravel the crustal evolution of this metalliferous terrain. The protoliths of the metabasites and orthogneisses previously considered to be Paleoproterozoic are in fact mostly Neoarchean in age (2713±11Ma to 2638±5Ma), from which the oldest rocks experienced their first metamorphism during the same Neoarchean orogenic cycle at ca. 2650Ma. A second event of mafic magmatism (2021±11Ma) was concomitant with the migmatization of the Neoarchean orthogneisses and was succeeded by granitic intrusions at 1990–1940Ma. All rocks of the Katuma Block experienced their main metamorphic reworking during several Paleoproterozoic orogenic events, which were recognized by dating of various metamorphic zircon growth zones and the age of magmatic events dated at ca. 2050, 1960 and 1880Ma. The detritus of the high-grade metasedimentary rocks derived from Neoarchean (Katuma Block or Tanzania Craton?) and Paleoproterozoic provenances and the minimum age for the deposition is constrained by its first metamorphism at ca. 1960Ma. The Neoarchean and Paleoproterozoic metabasites, gabbronorites and orthogneisses are sub-alkaline in composition displaying a REE and trace element geochemistry akin to those of rocks formed in modern-arc settings. On the basis of the geochemical data, the presence of eclogites, deformation and metamorphic ages, we suggest that in Paleoproterozoic time the Katuma Block was again at an active continental margin, below which a Paleoproterozoic oceanic lithosphere was subducting.