Magmatic Zircon Grains Research Articles

First evidence for Neoproterozoic magmatism in the Quadrilátero Ferrífero of Minas Gerais, Brazil, and geotectonic implications

A decimetre-thick, deeply weathered sill intrusive in the Itabira iron formation and exposed at the historical Veloso underground mine in Ouro Preto, Minas Gerais, contains three populations of magmatic zircon grains: (i) xenocrysts with Archaean Pb–Pb and Hf model ages; (ii) xenocrysts of Palaeoproterozoic crystallisation ages of ca. 2200 Ma, with superchondritic εHf2.2 Ga values between 1.5 and 11.6; (iii) autocrysts and overgrowths formed at ca. 620 Ma, with εHf0.62 Ga values between −17.6 and 11.5 (mean = 1.5 ± 6.9, n = 15). Furthermore, there is evidence for zircon age reset at ca. 565 Ma. The three magmatic zircon populations indicate that the sill emplaced during the Neoproterozoic and probed different magmatic sources at depth, comprising anatectically reworked Palaeoproterozoic and Archaean felsic rocks. Variable Hf-isotope compositions of zircon rims and whole-rock geochemical data additionally suggest a hybrid origin for the sill magma. Our results provide the first evidence that the southern edge of the Quadrilátero Ferrífero is underlain not only by Archaean, but also by juvenile Palaeoproterozoic rocks, similar to those exposed in the adjacent Mineiro belt, and that the latter became accreted to the São Francisco craton prior to lower crustal anatexis during an early extension at ca. 620 Ma.

A new concordia age for the 'forearc' Bay of Islands Ophiolite Complex, Western Newfoundland utilizing spatially-resolved LA-ICP-MS U-Pb analyses of zircon

Advancements of zircon U-Pb dating techniques using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) provide an opportunity to precisely constrain the age of the Bay of Islands Ophiolite Complex (BOIC) in the Newfoundland Appalachians. New LA-ICP-MS U-Pb dates are presented for a large population of relatively simple magmatic zircon grains from trondhjemites using selective domain analyses and a 3% discordance filter in five samples collected from a single ophiolite massif, with four of the samples from a single pluton. All samples were derived from plagiogranite plutons intruded just below the gabbro-sheeted dike contact of the Blow Me Down Mountain massif, the massif sampled for three of the four prior single sample ID-TIMS age studies of the ophiolite. A highly consistent series of LA-ICP-MS concordant ages from the five samples collected yield a composite concordia age of 488.3 ± 1.5 Ma, which we propose for the revised igneous age of the ophiolite massif. This new age for the BOIC is based on 127 individual LA-ICP-MS zircon analyses and is slightly older and with lower uncertainty than the previous consensus age. We review previous pioneering ID-TIMS U-Pb dates, ages, intercept assumptions, and uncertainties for the BOIC in light of this new formation age. Legacy ages and uncertainties differ somewhat when data are reduced with modern recommended techniques and without geological interpretations that fix lower intercepts. The ca. 488.3 Ma BOIC age indicates its formation along a younger supra-subduction forearc trench-orthogonal spreading center that rifted the older Coastal Complex forearc ophiolitic assemblages reported to be formed at ca. 500-508 Ma. This rifted forearc is analogous to many modern ophiolitic forearcs. The BOIC age is highly correlated with similar ages of the peri-Laurentian Notre Dame arc-proximal Betts Cove and Point Rousse ophiolites, indicating temporal BOIC spreading center linkages within the Baie Verte Oceanic Tract.

Rapid cold slab subduction of the Paleo-Tethys: Insights from lawsonite-bearing blueschist in the Changning–Menglian orogenic belt, southeastern Tibetan Plateau

The Changning–Menglian orogenic belt (CMOB) in the southeastern Tibetan Plateau is considered as the main suture of the Paleozoic Paleo-Tethys that separates Gondwana-derived continental fragments from Eurasia-derived ones. Understanding the evolutionary history of this orogenic belt is of critical importance in the reconstruction of the tectonic history of Paleo-Tethys. The CMOB preserves well-exposed blueschist facies rocks, albeit their tectonometamorphic history and protolith signatures remain poorly constrained. Here we present, for the first time, results from a detailed investigation of lawsonite-bearing blueschist rocks, including epidote-magnesioriebeckite schist and garnet-ferroglaucophane schist from the CMOB and involving petrological, mineralogical, thermodynamic modeling, whole-rock geochemical, and geochronological studies. The epidote-magnesioriebeckite schist samples and garnet-ferroglaucophane schist samples display OIB- and E-MORB-like geochemical affinities, respectively, and have whole-rock εNd (t) values in the range of −5.4–+4.4, suggesting that their protoliths were mainly oceanic crust with limited degree of crustal assimilation. Magmatic zircon grains from the garnet-ferroglaucophane schist samples yield protolith ages of 253–250 Ma. Combined with previous data, our data represent the youngest ages of rock formation in the Paleo-Tethyan Ocean reported to date. The epidote-magnesioriebeckite schist samples show a peak assemblage of magnesioriebeckite + lawsonite + augite-aegirine + phengite + titanite + allanite, with peak P–T conditions of 12.4–16.8 kbar and 350–406 °C. In contrast, the garnet-ferroglaucophane schist samples preserve a peak assemblage of garnet + ferroglaucophane + omphacite + lawsonite + phengite + titanite/ilmenite/rutile ± allanite and yield peak P–T conditions of 19.5–22.6 kbar and 490–510 °C. The epidote-magnesioriebeckite schists and garnet-ferroglaucophane schists record a nearly complete clockwise P–T loop characterized by a steep prograde P–T path with a low thermal gradient of ~5–8 °C/km followed by cooling or overprinting by isothermal decompression. Reconstruction of the metamorphic P–T evolution, together an evaluation of previous age data, allows us to propose rapid subduction of cold oceanic lithosphere to depths of 50–75 km at a rate of ~6.3–9.3 km/Myr during Early-Middle Triassic (248–240 Ma), followed by exhumation in the Late Triassic (231–214 Ma). The short time lag (<10 Ma) between the protolith generation and the high-pressure metamorphic peak further supports a rapid subduction process. Our results suggest that the young oceanic slab was dragged down by the thicker, long-lived, and cold downgoing Paleo-Tethyan Ocean plate, where it experienced blueschist-facies metamorphism. The co-occurrence of lawsonite-bearing epidote-magnesioriebeckite schists and garnet-ferroglaucophane schists and eclogites further points to a cold thermal structure at the convergent plate interface, leading to the interpretation that the CMOB represents a typical oceanic subduction-accretion belt. The results presented in this study provide important insights into the geodynamic evolution of the Paleo-Tethys.

Neoarchean magmatism and Palaeoproterozoic metamorphism along the margin of the Qianhuai microblock in the North China Craton

The tectonic framework of the North China Craton (NCC) involved the building of microblocks and their incorporation into larger crustal blocks during the Late Archean followed by final cratonization through collisional assembly of the crustal blocks during the Late Palaeoproterozoic. Here we investigate a newly identified gabbro‐diorite‐anorthosite suite, metamorphosed under granulite facies, along the margin of the Qianhuai microblock based on field, petrological, geochemical and zircon U–Pb and Lu–Hf studies, with a view to understand the petrogenetic and metamorphic history. Mineral phase equilibria modelling of metagabbros from the suite reveals relatively high temperatures and pressures of 800–820°C and 13–14 kbar, with a typical isobaric cooling pressure–temperature path characterized by initial heating followed by moderate to high‐pressure metamorphism and cooling, consistent with a thickened arc setting. The gabbro–diorite–anorthosite association is considered to represent the fractionated sequence from a common mafic magma. Magmatic zircon grains from the metagabbro yield upper intercept ages of 2522 and 2540 Ma, corresponding to the timing of formation of the mafic protolith. The lower intercept ages of 1785 and 1833 Ma mark the timing of metamorphism associated with the collisional event between the Western and Eastern blocks of the NCC. Zircon εHf(t) values are mostly positive for the magmatic domains suggesting that the melts were derived from depleted mantle sources. The Hf model ages indicate that the magma source involved Neoarchean–Mesoarchean juvenile components and the continental crust was reworked during metamorphism in the Late Palaeoproterozoic. The lithological association and geochemical as well as isotopic features are consistent with a convergent margin setting along a continental arc, where basaltic melts were underplated beneath the overriding arc, accompanied by lower crustal melting. Our study confirms subduction‐related crust building events during the late Neoarchean followed by Late Palaeoproterozoic high‐grade metamorphism associated with the final cratonization of the NCC.

Comparison of Magma Oxygen Fugacity and Zircon Hf Isotopes between Xianglushan Tungsten-Bearing Granite and Late Yanshanian Granites in Jiangxi Province, South China

Jiangxi Province (South China) is one of the world’s top tungsten (W) mineral provinces. In this paper, we present a new LA-ICP-MS zircon U-Pb age and Hf isotope data on the W ore-related Xianglushan granite in northern Jiangxi Province. The magmatic zircon grains (with high Th/U values) yielded an early Cretaceous weighted mean U-Pb age of 125 ± 1 Ma (MSWD = 2.5, 2σ). Zircon εHf(t) values of the Xianglushan granite are higher (−6.9 to −4.1, avg. −5.4 ± 0.7) than those of the W ore-related Xihuanshan granite in southern Jiangxi Province (−14.9 to −11.2, avg. −12.5 ± 0.9), implying different sources between the W ore-forming magmas in the northern and southern Jiangxi Province. Compiling published zircon geochemical data, the oxygen fugacity (fO2) of the late Yanshanian granitic magmas in Jiangxi Province (the Xianglushan, Ehu, Dahutang, and Xihuashan plutons) were calculated by different interpolation methods. As opposed to the W ore-barren Ehu granitic magma, the low fO2 of the Xianglushan granitic magma may have caused W enrichment and mineralization, whilst high fO2 may have led to the coexistence of Cu and W mineralization in the Dahutang pluton. Additionally, our study suggests that the absence of late Mesozoic Cu-Mo mineralization in the Zhejiang, Jiangxi, and Anhui Provinces (Zhe-Gan-Wan region) was probably related to low fO2 magmatism in the Cretaceous.

Geochronology, geochemistry and petrogenesis of the Dengfuxian lamprophyres: Implications for the early Cretaceous tectonic evolution of the South China Block

The middle–late Mesozoic tectonic evolution of the South China Block is not well constrained due to limited studies of synchronous mafic rocks in this area. The Dengfuxian area is located at the central to northern parts of the Nanling Range in the South China Block with many lamprophyre dikes seen cross-cutting the late Jurassic granites. This contribution is the first to report the whole-rock geochemistry, zircon U–Pb age and Sr–Nd–Hf isotope signature of these lamprophyres; provide a geochronological and petrogenetic studies that sheds new light on regional geodynamic evolution. The Dengfuxian lamprophyre dikes have variable contents of major oxides like SiO2 (58.45–65.19 %), Al2O3 (12.18–15.73 %), Fe2O3T (6.31–6.93 %), MgO (2.48–4.14 %), CaO (4.21–5.19 %), Na2O (1.08–3.02 %), K2O (2.78–4.00 %), TiO2 (0.59–1.00 %), MnO (0.15–0.33 %) and P2O5 (0.28–1.22 %). Identified as high-K calc-alkaline rock series, the lamprophyres are enriched in large ion lithophile elements (e.g., Rb, U, K and Pb) but depleted in Ba, Sr and high field strength elements (e.g., Nb, Ta, Zr and Hf). The HREEs are relative depleted compared to LREEs, with weak Ce anomalies (Ce/Ce* = 0.99–1.06) but marked negative Eu anomalies (Eu/Eu* = 0.61–0.79). A total of 30 magmatic zircon grains from the studied lamprophyre yields weighted mean U–Pb ages of 141–143 Ma, suggestive of an early Cretaceous emplacement. The Negative εNd(t) (–4.3 to –6.0) and εHf(t) (–4.1 to –10.9) values, old Nd depleted mantle model ages (TDM = 1231–1628 Ma) and Hf single stage model ages (TDM = 998–1243 Ma), with high initial 87Sr/86Sr (0.71057–0.71322) indicate that the parent magma was sourced from a subduction-modified enriched mantle. Together with integrated major and trace elements studies, it can be inferred that the parent magma of the Dengfuxian lamprophyres originated from low degree (1–3 %) partial melting of an enriched mantle source (garnet peridotite) that was altered by subduction sediments in a progressive intra-plate extension environment. The magma experienced significant fractional crystallization of olivine, clinopyroxene, plagioclase, Fe-Ti oxides and apatite with little crustal contamination. A two-stage geodynamic model is presented to constrain the tectonic evolution of the South China Block during the early Cretaceous.

Neoproterozoic felsic magmatism in southern Kerala, India: The building blocks of Gondwana

The assembly of Gondwana supercontinent involved multiple stages of continent convergence during the Neoproterozoic including arc magmatism and final collisional metamorphism. Peninsular India occupies a key position in the Gondwana supercontinent assembly where widespread Neoproterozoic magmatic records are preserved. Here, we investigate representative felsic magmatic intrusions from the Achankovil Suture Zone that marks the boundary between the Madurai and Trivandrum crustal blocks. We present petrological, geochemical, and zircon U‐Pb and Lu‐Hf data from the Chengannur (CH) and Pathanapuram (PT) plutons. Petrographic studies on syenogranites from the CH pluton indicate I‐type features, and feldspar thermometry shows ultrahigh temperatures (~1,000°C) corresponding to similar temperatures reported for the surrounding metamorphic rocks. Geochemically, the PT suite displays island arc affinity, whereas the CH suite shows adakitic, oxidized A‐2 type features. In tectonic discrimination plots, the CH samples correspond to volcanic arc granite, whereas the PT samples traverse syn‐collisional granite and volcanic arc granite zones, indicating a transitional feature. Trace element relationships suggest magma fractionation dominantly controlled by plagioclase, K‐feldspar, amphibole, and biotite. Magmatic zircon grains from the CH rocks show upper intercept ages in the range of 783 to 819 Ma and lower intercept ages in the range of 519 to 545 Ma, with cumulative age peaks at 766 and 529 Ma. The combined age population of monzogabbro, granodiorite, and granite from the PT pluton shows distinct peaks at 1,021, 950, and 537 Ma. Zircon grains from the CH pluton show dominantly negative εHf (t) values in the range of −5.8 to −13.4 and yield Hf depleted model age (TDM) ranging from 1,541 to 1857 Ma and Hf crustal model ages ( ) of 2,029 to 2510 Ma. In contrast, zircon grains from the PT pluton show dominantly positive εHf (t) values (2.9 to 9.8) with depleted model ages (TDM) in the range of 1,145 to 1434 Ma and Hf crustal model ages ( ) in 1,164 to1,686 Ma. The zircon U‐Pb and Lu‐Hf data clearly indicate that the timing of magmatism and magma sources is different for the two plutons, although both witnessed a common metamorphic event during late Neoproterozoic–Cambrian. The PT pluton was emplaced during the early Neoproterozoic with magma source involving mostly juvenile or depleted mantle components, as also attested to by the geochemical signature of arc affinity. In contrast, the CH pluton formed later during mid‐Neoproterozoic with magma from deeper source and dominated by recycled Paleoproterozoic material, which possibly involved depleted mantle components, both associated with an earlier subduction regime. The results reported in our study correlate with those from the arc magmatic rocks in the southern part of the Madurai Block as well as from the crustal blocks in the adjacent terrane of Sri Lanka. The extensive Neoproterozoic arc magmatism in a subduction‐related setting involving both juvenile additions and recycled older components represents the building blocks of Gondwana.

U-Th-Pb Shrimp dating of hydrothermal monazite from the Trairão Gold Deposit - Alta Floresta Gold Province (Amazon Craton)

Abstract Alta Floresta Gold Province occurs in the center-south portion of the Amazon Craton. Trairao gold deposit, which is located in the Alta Floresta Gold Province, is hosted by 1878 to 1854 Ma arc-related granites. Two important Au deposits take place in the region, the Trairao and Chumbo Grosso, which are structurally controlled by N80°W/S80°E trending lineament and associated with quartz veins and disseminated sulfides in a strong phyllic alteration zone of the host granite. Hydrothermal monazite grains formed during the Au mineralization event occur as fine anhedral crystals filling fractures or as isolated grains associated with Ag, Au, molybdenite, barite, pyrite, galena, and sphalerite. The hydrothermal monazite grains contain very low U, relatively low Th, and moderate Nd and La contents. SHRIMP U-Th-Pb dating of these crystals yielded an age of 1798 ± 12 Ma for Trairao and 1805 ± 22 Ma for Chumbo Grosso Au-deposits, whereas magmatic zircon grains of the granitic host rocks yielded an age of 1854 ± 8 Ma. The ages obtained in this paper are similar to those reported by Assis (2015), who studied Pe Quente (Re-Os in molybdenite, 1792 ± 9 Ma and 1784 ± 11) and Francisco deposits (40Ar-39Ar ages from sericitic halo: 1779 ± 6.2 and 1777 ± 6.3 Ma). Serrato et al. (2014) acquired Re-Os dating in pyrite and molybdenite from Juruena gold deposit. The results show isochronous ages at 1786 ± 1 Ma, with a model age at 1787 ± 3.2 Ma, suggesting a major Statherian gold metallogenic event at the Alta Floresta Gold Province region. Regional fluids-flow has a close relationship with the generation and concentration of several important economic deposits in the Eastern border of Alta Floresta Gold Province, including Pe Quente, Francisco, Juruena, Chumbo Grosso, and Trairao Au-deposits.

Hafnium Isotopic Composition of the Bushveld Complex Requires Mantle Melt–Upper Crust Mixing: New Evidence from Zirconology of Mafic, Felsic and Metasedimentary Rocks

Abstract The origin of magmas that formed the Bushveld Complex remains highly debated in spite of many decades of intense research. Previous geochemical–petrological studies have shown a strong mantle derivation resulting ultimately in highly economic ore bodies of platinum group elements and chromium. However, geochemistry also points to the contribution of a significant crustal component, which may have been derived singly or in combination from a number of different sources. These include subcontinental lithospheric mantle that was enriched prior to Bushveld magma formation, possibly by subduction, assimilation of lower and upper crust during magma ascent, and contamination during magma chamber accretion within sedimentary rocks of the enclosing Transvaal Supergroup. In this study, the contributions of these various reservoirs will be evaluated by employing Hf isotopic data of well-characterized zircon grains in mafic, felsic and metasedimentary rocks, together with Zr–Hf bulk-rock compositions. The results reveal that magmatic zircon grains in mafic cumulate rocks from the floor to the roof of the c. 9 km thick Rustenburg Layered Suite (RLS) show essentially the same variations in εHf2·055 Ga from −7·5 to −10·2 as those of metamorphic zircon grains and overgrowths in the immediate surrounding quartzite and metapelitic rocks, as well as in granitic melt batches, granophyres, and the upper Rooiberg volcanics. The same values are also obtained by estimating the average Hf isotopic compositions of detrital zircon grains in many quartzite and metapelitic rocks from the surrounding Magaliesberg (εHf2·055 Ga = −6·2 to −10·8, six samples, maximum deposition age at 2080 Ma) and Houtenbeck formations (εHf2·055 Ga = −7·1 to −8·9, three samples, maximum deposition age at 2070 Ma), and by a six-point isochron of a garnet-schist from the Silverton Formation (εHft = −6·6 ± 0·7; age = 2059·4 ± 2·7 Ma). Zircon morphologies, zoning patterns, Hf isotopic data and petrological constraints furthermore reveal that metamorphic zircon was precipitated from aqueous fluids and/or felsic melts at temperatures between 550 and 900 °C, and that the Hf isotopic composition became homogenized during fluid transport in the contact aureole. However, results of numerical modelling indicate that fluid infiltration had only a minor effect on the Zr–Hf budget and Hf isotopic composition of the RLS, and that these parameters were mainly controlled by the mixing of melts derived from three major sources: (1) the asthenospheric mantle (&amp;gt;20 %); (2) enriched subcontinental lithospheric mantle (&amp;lt;80 %); (3) assimilation of significant amounts of crust (up to 40 %). The modelling furthermore suggests that assimilation of lower Kaapvaal Craton crust was minor (&amp;lt;15 %) during B1 (high-Mg andesite) magma formation, but up to 40 % during B3 (tholeiite) magma formation. The minor variation in εHft of zircon throughout the entire stratigraphy of the RLS resulted from the interplay of three dominant contributing factors: (1) intrusion of hot (&amp;gt;1200 °C) mantle-derived magmas with relatively low Zr–Hf concentrations having a similar εHf2·055 Ga of −8·5 ± 1·9 to that of upper crust rocks surrounding the RLS; (2) significant assimilation of volcanic and metasedimentary rocks with high Zr–Hf concentration; (3) mingling, mixing and/or diffusive exchange of Zr and Hf between crust and mantle-derived melts and aqueous fluids prior to late-magmatic crystallization of zircon at temperatures between 700 and 900 °C. This study shows that the combination of Zr–Hf bulk-rock data with Hf isotopic data of well-characterized zircon grains provides a powerful tool to quantify various mantle and crustal reservoirs of mafic layered intrusions, and allows new insights into magma chamber and related contact metamorphic processes.

Post-collisional ultramafic complex in the northern North China Craton: Implications for crust-mantle interaction

The post-collisional ultramafic intrusions within the northern margin of the North China Craton (NCC) preserve important imprints of crust-mantle interaction. Here we investigate ultramafic intrusions from the Luojianggou complex composed of pyroxenite and hornblendite associated with serpentine, with a view to gain insights into the nature of orogenic lithospheric mantle in this major continental collision zone. Zircon UPb data from the ultramafic suite define different age populations, with Paleoproterozoic ages (>2.2 Ga and 1.82 Ga) representing xenocrystic grains captured from the basement. The magmatic zircon grains range in age from 872 Ma to 458 Ma, and are possibly related to multiple magma emplacement associated with the Paleo-Asian Ocean closure. The youngest age of ca. 230 Ma is related to a period of post-collisional extension in the northern margin of the NCC, and this inference is further supported by apatite UPb ages of ∼207 Ma. Phase equilibrium modeling suggests temperatures of 700–800 °C and pressures of 11–13 Kbar. The pyroxenite and hornblendite show similar geochemical features and REE patterns, indicating the same magma source and formation through differentiation and accumulation. The arc-like geochemical features of the rocks with enrichment of LILE (Rb, Th and La) and LREE, but depletion of HFSE (Nb, Zr and Hf), possibly formed at the boundary of sub-continental lithospheric mantle (SCLM) and crust through metasomatic reaction of peridotite with felsic melts derived from subduction-related components. The arc-like features, zircon rare earth element patterns and obvious positive Pb anomaly in primitive mantle-normalized trace element spidergrams also indicate the mixing of continental materials in the magma source. The post-collisional extensional setting is correlated to the tectonics associated with the assembly of the Mongolian arc terranes within the northern NCC during the Triassic.

Shallow reworking of magmatic zircon grains of latest Neoproterozoic (Timanian) age in serpentinite of the Voykar Massif, Polar Urals: new constraints from U-Pb isotopic data, and first trace elements and Lu-Hf isotopic data

ABSTRACTThe Voykar Massif of the Polar Urals in Russia consists of an ultramafic complex (mantle section) in the northwest, followed by a late Cambrian to Silurian mafic complex (intra-oceanic primitive island arc) and early Devonian intrusive rocks of an evolved island arc to the southeast. These complexes represent tectonic nappes thrust over the East European continental margin during the late Palaeozoic Uralian Orogeny. LA-ICP-MS U-Pb dating of zircon grains (n = 42) from an antigorite-serpentinite lens within the mafic complex yielded a Concordia age of 542 ± 2 Ma with an age range of 549–527 Ma. Additionally, few grains contain inherited domains with ages between ~990 and 3277 Ma. Hafnium isotopic data of the main age group show 176Hf/177Hft from 0.28242 to 028249 and εHft ranging from +1.9 to −6.5. The evolved Hf isotope data and the trace-element composition of the zircon grains point to an involvement of a continental crustal component in the parental magma. The zircon grains originate from igneous rocks formed during the Timanian Orogeny that affected the East European margin in the latest Neoproterozoic. During the Timanian or Uralian Orogeny, the magmatic zircons were eroded and shallowly recycled into the serpentinised mantle above the subduction zone. Finally, Uralian thrusting led to juxtaposition and imbrication of the zircon-bearing serpentinite and intra-oceanic volcanic rocks of the mafic complex.

Neoarchean arc magmatism and Paleoproterozoic granulite‐facies metamorphism in the Bhavani Suture Zone, South India

The Bhavani Suture Zone in the Southern Granulite Terrane marks the zone of amalgamation of the Neoarchean Nilgiri Block and the northwestern Madurai Block in southern India. Here, we report detailed petrological, geochemical, and geochronological data on the Mettupalayam mafic–ultramafic complex within this suture zone with a view to evaluate the tectonothermal history of the Bhavani Suture Zone and adjoining crustal blocks. The metamorphosed complex includes charnockite, hornblende‐biotite gneiss, mafic granulite, amphibolite, garnet‐bearing mafic granulite, and dioritic gneiss along with metamorphosed banded iron formation. The mafic granulite and the dioritic gneiss occur as concordant layers of varying thickness within the hornblende‐biotite gneiss. The salient geochemical features of the mafic granulite and the dioritic gneiss including the enrichment of large‐ion lithophile elements and depletion of high‐field‐strength elements suggest a subduction‐related arc magmatic setting. However, the amphibolites show MORB‐related affinity, suggesting its formation from a N‐MORB‐related source and their accretion together with the overlying banded iron formation. The peak metamorphic conditions of the garnet‐bearing mafic granulite were estimated using conventional geothermobarometers as 800–820 °C/8.5–9.2 kbar, which we further confirm through phase equilibrium modelling in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO) system. Magmatic zircons with high Th/U ratio from the amphibolite display a well‐defined discordia with an upper intercept age of 2,600 ± 38 Ma. Zircon grains from the dioritic gneiss show weighted mean 206Pb/207Pb age of 2,524 ± 6 Ma from concordant zircon spots and a comparable upper intercept age of 2,562 ± 34 Ma from discordant zircon spots, indicating protolith emplacement related to Neoarchean arc magmatism as inferred from our geochemical data. The thin overgrowth rims around the magmatic zircon grains in the amphibolite yielded an upper intercept age of 2,520 ± 20 Ma, which is comparable with the protolith crystallization age of the dioritic gneiss. Slightly younger weighted mean 207Pb/208Pb ages of 2,463 ± 27 Ma (from dioritic gneiss) and 2,422 ± 30 Ma (from amphibolite) are also obtained from the metamorphic zircon rims. These ages are correlated to the timing of high‐grade metamorphism associated with final collision of the Nilgiri Block and the northwestern Madurai Block. Similar Neoarchean–Paleoproterozoic magmatism and high‐grade metamorphism were reported from many localities south of the Dharwar Craton. Our study further confirms the previous tectonic model that envisages multiple subduction and collision of magmatic arcs and continental fragments towards the Dharwar Craton during the Archean–Paleoproterozoic transition.

Bulk rock and zircon geochemistry of granitoids from the Chotanagpur Granite Gneissic Complex (CGGC): implications for the late Paleoproterozoic continental arc magmatism in the East Indian Shield

The north-eastern part of the Chotanagpur Granite Gneissic Complex (CGGC) exposes a vast expanse of felsic orthogneisses (~ 1450 Ma magmatic crystallization age) containing a variety of metamorphosed supracrustal rocks and augen gneiss. However, origin and timing of emplacement of the protolith of the augen gneiss are not well constrained. Owing to lack of detailed study and paucity of exposures, the antiquity of the basements of the CGGC also remains poorly understood. To address this issue, we present bulk rock geochemical data and the U–Pb and Lu–Hf isotope composition of zircon from augen gneisses from two adjoining areas within the CGGC. Our obtained data suggest that the augen gneiss have been derived from hybrid source involving older crustal rocks and metasomatised mantle. The bulk rock and zircon geochemistry also suggest that the protolith of the augen gneiss was emplaced in an early Paleoproterozoic basement in a continental arc setting. U–Pb ages of the magmatic zircon grains constrain the age of intrusion at ca. 1710 Ma and ca. 1635 Ma. The timing of this late Paleoproterozoic felsic magmatism overlaps with the timing of the regional high-to-ultrahigh-temperature granulite facies metamorphism in the CGGC.

Mesoarchaean to Neoproterozoic (3.2–0.8 Ga) crustal growth and reworking in the Aravalli Craton, northwestern India: Insights from the Pur-Banera supracrustal belt

The Banded Gneissic Complex (BGC) of the Aravalli Craton in Rajasthan, northwestern India, forms the Archaean basement to the major, extensively studied, Proterozoic-age metasedimentary sequences of the Aravalli and Delhi Supergroups. However, the poorly documented deformation history of the basement and other supracrustal sequences, and insufficient geochronological work, limit a holistic understanding of the evolution of the Aravalli Craton. Here we present structural analysis and zircon and monazite geochronology of the relatively poorly studied Pur-Banera supracrustal belt in the Bhilwara region of south-central Rajasthan. The belt comprises paragneisses, carbonates, calc-gneisses, banded iron formations, garnetiferous schists, and amphibolites, deposited over Aravalli Supergroup rocks and locally also granite. A single detrital zircon core from the Pur-Banera paragneisses yields a 207Pb/206Pb age of 3159 ± 8 Ma (1σ), indicating a Mesoarchaean protolith. Magmatic zircon grains from the granite, here termed the Malola Granite, provide a 207Pb/206Pb crystallisation age of 2538 ± 11 Ma (95% confidence). The Pur-Banera supracrustals have experienced three phases of deformation, D1–D3, each of which produced a tectonic fabric. Zircon and monazite geochronology provides complementary ages. The detrital zircons, derived from both magmatic and metamorphic protoliths, provide ages of 3.2–1.7 Ga. Monazite ages of 1.3–0.8 Ga record multiple post-depositional tectonothermal events. Textural and compositional evidence and U–Th–total Pb monazite ages indicate that D1 deformation occurred at ca. 1.30 Ga as a result of NW–SE-directed shortening, forming upright folds on the primary foliation that resulted in a penetrative fabric in the gneisses and schists. Subsequent NNE–SSW-directed deformation (D2) between 1.13 and 1.05 Ga produced WNW–ESE-trending vertical folds in the gneisses and crenulations in the schists. Asymmetrical folds developed due to N–S dextral shearing at 0.85–0.80 Ga constitute the last phase of deformation (D3). The Pur-Banera supracrustal rocks probably represent a rift-fill sequence, of Mesoproterozoic age (thus temporally equivalent to the Delhi Supergroup), deposited on an Archaean and Palaeoproterozoic basement, and repeatedly reworked during 1.30–0.80 Ga.

Neoarchean growth and Paleoproterozoic metamorphism of an Archean ophiolite mélange in the North China Craton

The North China Craton (NCC) preserves important records of Neoarchean crustal growth and Paleoproterozoic metamorphism. Here we investigate a suite of metamorphic rocks belonging to the Hongqiyingzi Group from the northern part of the Trans-North China Orogen (TNCO). These rocks are dominantly composed of amphibole plagioclase gneisses with minor retrograded eclogites and serpentinited peridotites. We employ zircon U-Pb geochronology and Hf-O isotopes on the amphibole plagioclase gneisses to unravel the formation age and metamorphic history. The magmatic zircon cores yield upper intercept ages of 2664 Ma and 2518 Ma, suggesting that the protoliths of the Hongqiyingzi gneisses formed in the Neoarchean. The metamorphic rims with lower U and Th concentration yield upper intercept ages of 1840 Ma and 1847 Ma, constraining the timing of metamorphism as ca. 1.85 Ga. The εHf(t) values of the magmatic zircon grains are dominantly positive, whereas those of the metamorphic grains are negative. Both magmatic and metamorphic zircon grains display a restricted range of δ18O (6.11–7.44‰), slightly higher than the normal mantle value. These isotopic features indicate that the younger metamorphic event did not modify the Hf-O isotopic compositions. The P-T computations yield temperatures of 700–760 °C and pressures to be 5–7 Kbar, suggesting the rocks have been retrograded to amphibolites facies. The retrograded eclogites and serpentinized peridotites of the Hongqiyingzi Group, exposed as tectonic lenses within the amphibole plagioclase gneisses, are interpreted as parts of an ancient oceanic lithosphere, whereas the amphibole plagioclase gneisses represent meta-volcano-sedimentary units. We thus interpret that the Hongqiyingzi Group represents a tectonically dismembered Neoarchean ophiolitic mélange that formed on the continental margin and metamorphosed to eclogite facies during deep subduction in Paleoproterozoic and exhumed during Phanerozoic.

Late Neoarchean slab rollback in the Jiaoliao microblock, North China Craton: Constraints from zircon U-Pb geochronology and geochemistry of the Yishui Complex, Western Shandong Province

Earth's plate tectonic system was initiated between 3.0 and 2.5 Ga and became the dominant regime for crustal growth and differentiation. However, it is unclear whether subduction–accretion processes, including slab rollback, occurred regionally in the area of the western Shandong Province Granite–Greenstone Terrane of the North China Craton (NCC) during the early development of plate tectonics. We quantified petrology, whole-rock geochemistry, and zircon U–Pb–Hf isotopes for felsic, mafic, and ultramafic rocks from the Yishui Complex (YC) in the NCC to constrain the origin of these rocks and the associated tectonic setting(s). The granites yield 207Pb/206Pb ages of 2509 ± 20 to 2484 ± 26 Ma, and magmatic zircon grains from these rocks have εHf(t) values of 0.6–6.4. The coeval mafic–dioritic rocks represent the fractional crystallization products of a primary magma that was derived from low-degree (<7%) partial melting of a metasomatized mantle source. The granodiorites were derived from the partial melting of oceanic slab material, with slab melts being contaminated by mantle wedge material during their ascent. The high-K granites represent the crustal reworking products of regional tonalite–trondhjemite–granodiorite and sedimentary rocks. Late Neoarchean magmatism in the YC was related to slab subduction, and slab rollback at ~2.50 Ga led to the generation of potassic rocks, coeval mafic–ultramafic xenoliths, and granulite metamorphism with a counterclockwise P–T–t path in the Yishui area.

Petrogenesis of the southern Qiangtang mafic dykes, Tibet: Link to a late Paleozoic mantle plume on the northern margin of Gondwana?

AbstractThis study presents 13 new U-Pb zircon ages obtained by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) together with whole-rock geochemical, Sr-Nd isotopic and zircon Hf isotopic data for a mafic dike swarm in the southern Qiangtang area of Tibet. These data provide the basis for a new model of the late Paleozoic evolution of the Tethys. Combined with the results of previous zircon U-Pb dating, the magmatic zircon grains extracted from mafic dikes yield latest Carboniferous to Early Permian ages (317–279 Ma). The geochemistry of the southern Qiangtang mafic rocks indicates the presence of low-Ti (QLT) and high-Ti (QHT) suites. The magmas that formed the QLT suite underwent higher degrees of partial melting (&gt;5%) and display evidence of crustal contamination, whereas the QHT suite was derived from magmas generated by low-degree (1%–5%) partial melting of a garnet-bearing mantle source, with a greater extent of fractional crystallization than the QLT suite, and no evidence of crustal contamination. We propose that the QHT and QLT suites may have been derived from magmas from different parts of a single mantle plume. The formation of the southern Qiangtang mafic dikes (latest Carboniferous to Early Permian; 317–279 Ma) may have been related to the northward drift of the Cimmerian continent from the northern Gondwana margin, which resulted in the opening of the Meso-Tethys Ocean.

Paleoproterozoic (2.0–1.97 Ga) subduction-related magmatism on the north–central margin of the Yeongnam Massif, Korean Peninsula, and its tectonic implications for reconstruction of the Columbia supercontinent

The Muju area, located on the north–central margin of the Yeongnam Massif, mainly consists of Precambrian orthogneisses (granitic, leucogranitic, augen and dioritic gneisses) with minor migmatite. Zircon U–Pb dating indicates that the protoliths of the orthogneisses intruded at ca. 2.00–1.97 Ga and were metamorphosed at ca. 1.87–1.86 Ga. Magmatic zircon grains within the orthogneisses have positive to negative εHf(t) values (−7.63 to +3.3) and a Neoarchean two-stage model age (TDM2 = 2.78 Ga), indicating that the protoliths of most of the orthogneisses may have been derived from Archean crustal material. The results of geochemical analysis indicate that the protoliths of the orthogneisses formed by partial melting of metagraywacke and mafic igneous rocks in an arc-related tectonic setting. The intrusion ages and geochemical data of the Paleoproterozoic orthogneisses in the study area match well with those of Paleoproterozoic (ca. 2.00–1.97 Ga) orthogneisses in the northeastern Yeongnam Massif, indicating the presence of regional Paleoproterozoic subduction zones along the northern margin of the Yeongnam Massif at ca. 2.00–1.97 Ga. Meanwhile, ca. 2.00–1.97 Ga subduction-related magmatism has not been reported from the northern Gyeonggi and Nangrim Massifs in the Korean Peninsula or the Jiao–Liao–Ji belt in the eastern North China Craton, indicating that the Yeongnam Massif may not be correlatable with the northern Gyeonggi and Nangrim Massifs or the Jiao–Liao–Ji belt. The Yeongnam Massif may be correlated with the Cathaysia Block in the South China Craton and may have been located near Laurentia and the Siberian Craton within the Columbia supercontinent.

A gradual transition to plate tectonics on Earth between 3.2 to 2.7 billion years ago

AbstractZircon crystals precipitated from granitoid magmas contain a robust record of the age and chemistry of continental magmatism spanning some 4.375 Ga of Earth history, a record that charts initiation of plate tectonics. However, constraining when exactly plate tectonics began to dominate crustal growth processes is challenging as the geochemical signatures of individual rocks may reflect local subduction processes rather than global plate tectonics. Here we apply counting statistics to a global database of coupled U–Pb and Hf isotope analyses on magmatic zircon grains from continental igneous and sedimentary rocks to quantify changes in the compositions of their source rocks. The analysis reveals a globally significant change in the sources of granitoid magmas between 3.2 and 2.7 Ga. These secular changes in zircon chemistry are driven by a coupling of the deep (depleted mantle) and shallow (crustal) Earth reservoirs, consistent with a geodynamic regime dominated by Wilson cycle style plate tectonics.

First U–Pb LA-ICP-MS zircon ages assessed from a volcano-sedimentary complex of the mid-European Variscides (Pennsylvanian, Flöha Basin, SE Germany)

Nine radiometric ages constrained by U–Pb isotopes from magmatic zircon grains are provided for different volcanic rocks from the Floha Basin and its surroundings. For most of them, this is the first reliable data to correlate with the International Chronostratigraphic Chart. Measurements were obtained with LA-ICP-MS and revealed middle Pennsylvanian ages (311–308 Ma) allowing first consistent correlation to other Variscan volcano-sedimentary basins of the Saxo-Thuringian and Tepla Barrandian zones. The Obermuhlbach Volcano, formerly considered to be early Permian in age, is shown to be of middle Pennsylvanian age. In contrast, the dykes from Metzdorf village and Oederan have shown to be not Pennsylvanian, but rather of early Permian age. Biostratigraphic data obtained from the Floha Formation macroflora coincide with the new isotopic evidence which encompasses a likely time span from the middle to late Bolsovian (Westphalian C) up to the Bolsovian–Asturian boundary. The results not only yield a modified picture of the Floha Formation and their intercalated pyroclastics. As new insights challenge previous geological mapping they contribute to the understanding of the complex volcano-tectonic processes in the type area of the Saxo-Thuringian zone of the European Variscides.