Intercept Age Research Articles

Thickening and partial melting of the Northern Qinling Orogen, China: insights from zircon U–Pb geochronology and Hf isotopic composition of migmatites

The widespread migmatites in the Precambrian basement of the North Qinling Belt are products of partial melting of the Earth's crust and record the tectono-thermal response to Late Paleozoic collision along the Shangdan suture. The morphology and trace element geochemistry of zircons from the migmatites suggest a metamorphic origin related to partial melting. The zircons from leucosome, melanosome and banded migmatites yield U–Pb weighted mean ages of 408 ± 2, 403 ± 2 and 400 ± 2 Ma, respectively, constraining the timing of migmatization. The granitic dyke intruded into the migmatites gives a zircon U–Pb weighted mean age of 396 ± 2 Ma, constraining the minimum time of migmatization. Zircons from the melanosome and banded migmatite yield upper intercept ages of 1088–1061 Ma, representing the protolith ages of the basement of the North Qinling Belt. The zircon rims of the melanosome and banded migmatite predominantly display negative and positive ε Hf ( t ) values, respectively, suggesting the variable input of mantle material. Together with the regional geology and previous geochronological data, we propose that the thickening and partial melting in the North Qinling Belt was induced by the collision along the Shangdan suture between the North and South China Blocks from c. 408 to 400 Ma. Supplementary material: LA-ICP-MS U–Pb data, Lu–Hf isotopic compositions and trace element geochemistry data of the zircons and Ti-in-zircon thermometry are available at https://doi.org/10.6084/m9.figshare.c.4521389

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.

Basement evolution of the Sulu orogenic belt: Constraints on zircon U–Pb ages and trace elements from the Weihai gneisses

Early Paleoproterozoic rocks occur sporadically in the Sulu ultrahigh‐pressure (UHP) belt. The tectonic affinity and evolution of these rocks are still disputed. Two magmatic events (Early Paleoproterozoic and Neoproterozoic) and two metamorphic events (Late Paleoproterozoic and Triassic) were revealed by the zircon U–Pb ages and trace elements from the Weihai gneisses, which provide a key clue to understand the evolution of the Sulu orogen. The Early Paleoproterozoic zircons occur as residual zircons or zircon‐inherited cores in the two gneisses. They generally show oscillation zoning and enriched HREE patterns. The weighted mean age of the upper intercept age of these zircons is ~2.46 Ga, suggesting that the protolith of the ancient gneisses formed at ~2.46 Ga. The Late Paleoproterozoic zircons occur as metamorphic zircon core or mantle domains in the two gneisses and a quartzite. They generally show planar zoning or no zoning, with relative lower Th/U ratios. These zircons yield metamorphic ages of 1.8–1.84 Ga, which are consistent with the age of the Paleoproterozoic ultrahigh‐temperature metamorphism of the northern Sulu orogenic belt. The Neoproterozoic zircons occur as magmatic cores or mantles in the zircon grains of the two gneisses and their wall rocks. They show typical oscillatory zoning and enriched HREE patterns, yielding magmatic ages of ca. 740–787 Ma. The Neoproterozoic igneous rocks are the dominant basement rocks in the Sulu orogenic belt. Triassic UHP metamorphism has been well studied in the Sulu orogenic belt. Two Triassic age groups were also recognized here: 228–229 and 208 Ma, respectively. The Triassic zircons occur as metamorphic rims of the two ancient gneisses and their wall rocks. These two age ranges correspond to the time of subduction and exhumation of the the South China Craton (SCC) during the Triassic orogeny. This study provides solid evidence that the Weihai ancient gneisses have a strong affinity with the Yangtze Block. The protoliths of them are Paleoproterozoic (ca. 2.46 Ga) magmatic rocks. They were subsequently suffered from ca. 1.8‐ to 1.84‐Ga metamorphism, new crustal materials overgrowth during ca. 740‐ to 787‐Ma magmatism, and then experienced Triassic continental deep subduction and exhumation process to build the Sulu orogenic belt.

Geochemistry, Zircon U-Pb Age and Hf Isotope of the Huilanshan Granitoids in the North Dabie Terrane: Implications for Syn-Collapse Magmatism of Orogen

Syn-collapse magmatism is a critical issue for evolution of the continental orogen. The Dabie Orogen is a typical orogen which was suffered from a complete collapse. Two kinds of granitoids, namely, the coarse-grained diorite and the fine-grained granite, are recognized at the center of the Luotian extensional dome, providing an opportunity to decipher the syn-collapse magmatism in the Dabie Orogen. The diorites (125±3 Ma) are high K calc-alkaline rocks, with low SiO2 (51.9 wt.%–56.6 wt.%) and high MgO (3.5 wt.%–4.0 wt.%) contents. They are enriched in LREE and LILEs (e.g., Ba, K, Rb) and depleted in HFSEs (e.g., Ta, Nb, and Hf) with low ratio of Sr/Y (30.82–46.89). The granites (118±2 Ma) are shoshonite series rocks, with relatively high SiO2 (68.9 wt.%–72.6 wt.%) and low MgO (0.32 wt.%–0.66 wt.%) contents. They are also enriched in LREE and LILEs with weakly negative Eu anomalies (δEu=0.81–0.85), and are depleted in HFSEs with low Sr contents (338 ppm–477 ppm) and Sr/Y ratios (23.80–33.13). Therefore, the two kinds of granitoids have no geochemical characteristics of adakitic rocks, suggesting that they were generated from a normal or thinned crust level. The diorites have quite negative zircon eHf(t) values (-18.4 to -21.1), suggesting they were from partial melting of the mafic lower continental crust. The granites have relatively higher zircon eHf(t) values (-14.4 to -18.1). The granites also contains a series of old inherited zircon cores, such as two upper intercept ages of 2 628±41 and 1 840±37 Ma, and a concordant age of 807±9 Ma. All these features suggest that the granites were generated from partial melting of the felsic middle-lower continental crust. Thus, the Huilanshan Early Cretaceous granitoids coupled with the Luotian extensional dome revealed the collapsed process of the Dabie Orogen.

Perda de Pb por Difusão Contínua em Zircão Ígneo: Estudo da Espectroscopia BSE, ID-TIMS e Raman em Granito Mesoproterozoico do SW do Cráton Amazônico

This work presents a detailed investigation on zircon crystals of a Mesoproterozoic granite from the SW Amazonian Craton using a combination of back scattered electron (BSE) imaging, thermal ionisation mass spectrometry (TIMS) and Raman scattering spectroscopy. Six zircon grains were analysed. The results plotted in the Concordia diagram yielded an upper intercept age of 1423.0 ± 3.8 Ma, which is interpreted as the crystallization age of the rock. The scattering of the analyses in such diagram indicates that U-Pb isotopes are volume-dependent in the studied zircon grains. BSE reveals zonation in the zircon crystals, and Hf is the main element causing the variability in the BSE intensity and U has a secondary effect. Both elements have much higher atomic number than the principal constituents of zircon (Zr, Si and O), so the substitution of Zr by U and Hf results in increasing image brightness. BSE images from the zircon grains exhibit a euhedral external shape and fine-scale euhedral oscillatory zoning. The predominance of magmatic zoned crystals points out that the granite has not suffered metamorphic heating or any other processes that induce the formation of new phases. The results of the Raman scattering technique indicate that a SiO 4 tetrahedral internal vibrational structure is sufficiently sensitive to determine the increasing degree of metamictization. Furthermore, evidence for metamictization is given by the heterogeneity of the half-width values of the Raman scattering peaks, which exhibit a decreasing trend from the core to the rims of the analysed grains. We concluded that the combination of the three techniques provides information on the Pb loss and that the degree of U-Pb isotopic discordance correlates closely with the volume of the zircon grain. The data, in addition to the lateral degree of metamictization detected by the Raman scattering technique, indicate that the loss of radiogenic Pb may be linked to the continuous diffusion of this element. The correlation of the U-Pb discordance and metamictization emphasizes the importance of the Raman scattering spectroscopy analysis to performing zircon dating. Indeed, it is a helpful tool for geochronologists.

New insights into Neoarchean–Paleoproterozoic crustal evolution in the North China Craton: Evidence from zircon U–Pb geochronology, Lu–Hf isotopes and geochemistry of TTGs and greenstones from the Luxi Terrane

Tonalite–trondhjemite–granodiorite (TTG) suites constitute dominant components of Precambrian continental crust and provide important insights into the crustal evolution history of the early Earth. Here we investigate a suite of TTGs and associated greenstones from the Luxi Terrane in the south–eastern part of the North China Craton (NCC). We present data from petrology, geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes to characterize these rocks and understand their genesis. Mineral assemblage in the greenstones and TTGs suggests that the rocks have undergone amphibolite–facies metamorphism with regression under greenschist–facies. Zircon U–Pb data from the magmatic grains yield upper intercept ages of 2753 ± 37 and 2582 ± 40 Ma for greenstone, 2552 ± 13 Ma for TTG gneiss and 2476 ± 18 Ma for tonalite, and weighted mean 207Pb/206Pb age of 2507 ± 22 Ma for granodiorite. Lu–Hf isotope systematics show εHf(t) values ranging from −2.13 to 7.29 and crustal model ages (TCDM) in the range of 2603–3125 Ma for the TTGs, whereas the εHf(t) values are up to 8.53 with TCDM ages of 2644–3006 Ma for the greenstone. The results suggest that the protolith for the greenstone was derived from a Meso- to Neoarchean mantle contaminated by the crust source, whereas the TTGs were mostly sourced from juvenile crustal sources with reworking of older basement. The geochemical features of greenstone are similar to island arc basalt, with low Na2O, K2O and TiO2, low total rare earth elements (REEs) values, relative enrichment in light REEs and depletion in heavy REEs absence of obvious Ce anomaly, and negative Nb, Ta and Ti anomalies. The TTGs show similar features, except for the high K2O content and moderate total REEs values. We propose that the greenstone likely represent a part of the early and initial crustal growth at ca. 2750–2600 Ma, and the TTGs were generated subsequently at ca. 2550–2450 Ma. Our results are consistent with the previous models on the crustal growth models in the NCC and provide further insights into the Neoarchean to Paleoproterozoic tectonic history of the craton.

Petrogenesis and In Situ U‐Pb Geochronology of a Strongly Shocked L‐Melt Rock Northwest Africa 11042

AbstractNorthwest Africa (NWA) 11042, originally classified as a primitive achondrite with no chondritic relicts, is rather a unique L‐melt rock. It is a severely shocked, igneous‐textured ultramafic rock composed of euhedral to subhedral olivine (Fa25.1 ± 0.5) and pyroxenes (low‐Ca pyroxene Fs20.7 ± 0.8Wo4.2 ± 1.0 and Ca‐rich pyroxene Fs11.5 ± 0.5Wo37.6 ± 1.2) with interstitial albitic plagioclase (Ab80.7 ± 1.7Or5.0 ± 0.7) that has been completely converted to maskelynite. Mineral compositions are similar to those of equilibrated L‐chondrites. Melt pockets are scattered throughout the sample, containing high‐pressure minerals including ringwoodite, wadsleyite, jadeite, and lingunite. Merrillite and apatite in NWA 11042 contain significantly higher REE abundances than those of ordinary chondrites, indicative of igneous fractional crystallization. In situ U‐Pb dating of apatite in NWA 11042 reveals an upper intercept age of 4,479 ± 43 Myr and a lower intercept age of 465 ± 47 Myr on the normal U‐Pb concordia diagram. The upper intercept age recorded the time when NWA 11042 initially crystallized. This age is much younger than when the decay of short‐lived nuclides (e.g., 26Al) would act as a major heat source, suggesting that melting and crystallization of NWA 11042 could be otherwise triggered by an impact event. The lower intercept age represents a reset age due to a later impact event, which is in coincidence with the disruption event of L‐chondrite parent body at ~470 Myr. NWA 11042 is an excellent example to link igneous‐textured meteorites with a chondritic parent body through shock‐induced melting.

Fabrics and geochronology of the Taibai ductile shear zone: Implications for tectonic evolution of the Qinling Orogenic Belt, central China

The Taibai ductile shear zone affecting the northern margin of the Taibai granitic pluton which emplaced in the North Qinling Orogenic Belt, is crucial to understand the amalgamation and post-collision history between the North China Block and the South China Block. The shear sense indicators and the kinematic vorticity numbers (0.42–0.59) suggest that the Taibai shear zone is an elongating general shear zone involving both sinistral shear and pure shear components. The quartz lattice preferred orientations mainly indicate the activation of prism 〈a〉 slip system combined with rhomb 〈a〉 slip to prism 〈a〉 slip system. The dynamic recrystallization of quartz is accommodated by sub-grain rotation and bulging recrystallization while plagioclase mainly displays bulging recrystallization. These characteristics suggest that the mylonites have experienced ductile shear deformation under greenschist facies conditions at temperatures of ∼400–550 °C. Zircons from granitic mylonite yield upper and lower intercept ages of 425 ± 4 Ma and 144 ± 1 Ma, respectively, suggesting the protolith of the mylonite crystallized at ca. 425 Ma and was involved into a hydrothermal activity at ca. 144 Ma. Muscovite 40Ar/39Ar dating of the mylonite sample yield a plateau age of 346 ± 5 Ma and an inverse isochron age of 344 ± 4 Ma, constraining the ductile shear deformation occurring during the Early Carboniferous time. We suggest that the Taibai shear zone was formed by NE-SW oblique compression with top-to-the-NW shear sense during the Early Carboniferous time, following the N-S shortening caused by collision between the North China Block and South China Block along the Shangdan suture at ca. 400 Ma.

The unregenerate São Rafael pluton, Borborema Province, Northeastern Brazil

The late Neoproterozoic São Rafael pluton, located in the Seridó Belt of the Borborema geologic province, northeastern Brazil, is an unfoliated, metaluminous, epidote-bearing, high-K calc-alkalic granitoid. Three facies are present in its northern lobe: (i) a dominant massive granite to quartz monzonite porphyritic facies containing microcline phenocrysts, (ii) subordinate fine-grained facies, similar to the porphyritic facies except lacking amphibole, that crop out as inselbergs (“Serras”), and (iii) rare pillows of microgranular mafic enclaves (MMEs) that are present within the porphyritic facies.Chemical analyses include 19 bulk-rock data, and 33 microprobe data of epidote and amphibole. Beginning with bulk chemistry and postulated mineral compositions, an iterative procedure of solving matrices characterizes a rock sample. Derived information includes: (i) identifying what minerals are present, (ii) their relative abundances, and (iii) their chemical compositions, even for minerals (such as biotite or amphibole) with complexly variable compositions. Calculated mineral compositions are compatible with microprobe data. The matrix solutions further indicate alternative mineral assemblages, and the compositions of “phantom” minerals that could have existed in the magma source, although absent in the pluton. They establish that olivine, but not clinopyroxene, was a possible phantom mineral and that MMEs are of unrelated magmatic origin.According to matrix analysis, amphibole in an outer zone of the porphyritic facies is solely of neoformed origin, whereas amphibole in an inner zone of the porphyritic facies is of mixed origin—partly neoformed and partly residual (i.e., xenocrystic) “patchy” amphibole derived from MMEs. A qualified geothermometer provides Tsolidus = 730 ± 6 °C for the outer subfacies, and a qualified geobarometer provides P = 0.78 ± 0.06 kbar. Amphibole in MMEs crystallized at depth, at P~3.3 kbar. Corroded epidote attests to the pressure-sensitive instability of that mineral during rapid ascent decompression.Cathodoluminescence reveals complexly structured zircons with magmatic overgrowths and inherited cores. 207Pb/206Pb ages of inherited components cluster at ~2.0 Ga and ~2.4 Ga, these ages being prominent in the local basement. Oscillatory-zoned magmatic components yield a concordia upper intercept age of 580 ± 9 Ma, interpreted as the age of igneous crystallization. Values of δ18O in zircon are only slightly higher than δ18O in mantle-like zircon. At 0.58 Ga, εHf in magmatic zircon was notably unradiogenic (−22 to −26), indicating a magma source in old continental crust as adduced by ages of inherited zircon. A U-Pb titanite date is around 0.55 Ga.For the Rb-Sr and Sm-Nd whole-rock systems, isotopic inheritance from the magma protolith is so extreme and variable that isochrons are actually “scatterchrons” that provide no useable age information. Rb-Sr cooling ages of biotite are 498 ± 1 Ma. Most Sm-Nd depleted mantle ages lie in the range of 2.6 ± 0.13 Ga. São Rafael magma was “unregenerate,” not isotopically homogenized when emplaced.

Post-crystallization alteration of natural uraninites: Implications for dating, tracing, and nuclear forensics

Abstract This study presents systematic chemical (U, Pb, Ca, Si, Fe) mapping coupled with in situ analyses of major, minor and trace elements, U/Pb, 207Pb/206Pb, and O isotopic compositions of natural uraninites (UO2) from two samples of the high-grade uranium ore from the Cigar Lake unconformity-related uranium deposit (Athabasca Basin, Saskatchewan, Canada). The studied uraninites are characterized by major chemical and isotopic heterogeneities expressed at small scale (µm to tens of µm), from almost pristine zones to strongly altered material. The 206Pb/238U and 207Pb/235U ratios of the different areas are widely spread and depict two similar and well-defined Discordia, providing an upper intercept age of crystallization at ca. 1300 Ma (1299 ± 4 and 1308 ± 14 Ma, respectively) and lower intercepts at 38 ± 13 and 72 ± 22 Ma, respectively. The freshest areas are characterized by sub-concordant 206Pb/238U and 207Pb/235U ratios, identical chemical compositions and similar very low δ18O values (−39.3 to −31.4‰). These data indicate that the two uraninites both crystallized at ca. 1300 Ma, from the same fluid and under identical physico-chemical conditions. Alteration is characterized by (i) the progressive incorporation of Ca, Si, and Fe, reaching several wt.%, which substitute to the radiogenic Pb and cause a progressive decrease in the Pb/U isotopic ratios. The radiogenic Pb is also substituted by water during the alteration, (ii) concomitant variations in trace element contents (As, Mn, V, LREEs, Sr, Th, B, Ba, Nb, for example) and (iii) heavier δ18O signatures (−22.5 to −8.91‰), typical of meteoric waters, in the altered zones. This combined approach demonstrates that fluid-driven post-crystallization exchanges affected each uraninite during recent fluid flow events (ca. 40 Ma and 70 Ma respectively). The relatively high dispersion of the Pb/U ratios in relation to the Discordia for both samples is considered as linked to a local (nm to µm-scale) differential mobility between lead and uranium within the uranium oxides. The chemical changes affecting elements previously considered as immobile in uraninite, such as REEs, indicate that these elements are not preserved during the post-crystallization alteration process studied here. Alteration processes may therefore have a major impact on the classical geochemical tracers, such as REE patterns or LREE/HREE ratios, currently used in nuclear forensic studies. The isotopic and chemical tracers currently used to track back the origin, age and history of natural uraninites should therefore be considered with a high degree of caution to avoid misleading and erroneous conclusions. Moreover, the comparison of calculated U-Th-Pb chemical, 207Pb/206Pb, and Pb/U isotopic ages shows that the use of age clustering for determining U-Th-Pb chemical ages and 206Pb/207Pb ages is not appropriate for constraining crystallization stage(s) of altered uraninites and for deciphering the different fluid events that potentially altered or recrystallized the uraninites over time. This study also indicates that estimation of the crystallization age of uraninite from substitution trends of Pb to Ca is not applicable to unconformity-related U deposits and results in overestimated ages because of an initial integration of calcium in the uraninite lattice at the time of crystallization.

The Main Anorthosite Layer of the West-Pana Intrusion, Kola Region: Geology and U-Pb Age Dating

The West-Pana intrusion belongs to the Paleoproterozoic Fedorova-Pana Complex of the Kola Region in NW Russia, which represents one of Europe’s most significant layered complexes in terms of total platinum group element (PGE) endowment. Numerous studies on the age of the West-Pana intrusion have been carried out in the past; however, all published U-Pb isotope ages were determined using multi-grain ID-TIMS. In this study, the mineralized Main Anorthosite Layer from the upper portion of the intrusion was dated using SHRIMP-II for the first time. High Th/U (0.9–3.7) zircons gave an upper intercept age of 2509.4 ± 6.2 Ma (2σ), whereas the lower portion of the intrusion was previously dated at 2501.5 ± 1.7 Ma, which suggests an out-of-sequence emplacement of the West-Pana intrusion. Furthermore, high-grade PGE mineralization hosted by the anorthosite layer, known as “South Reef”, can be attributed to (1) downward percolation of PGE-enriched sulfide liquid from the overlying gabbronoritic magma or (2) secondary redistribution of PGEs, which may coincide with a post-magmatic alteration event recorded by low Th/U (0.1–0.9) zircon and baddeleyite at 2476 ± 13 Ma (upper intercept).

Magmatic and metamorphic imprints from the root of an Archean continental arc: Evidence from the Qianhuai microblock in the North China Craton

The Archean tectonic framework of the North China Craton (NCC) involved the assembly of several micro-continents along zones of ocean closure during the late Neoarchean, followed by high-grade metamorphism associated with the final assembly and cratonization of continental blocks during Late Paleoproterozoic. Here we investigate a metamorphosed igneous suite comprising pyroxenite, gabbro and trondhjemite in association with charnockite from the margin of the Qianhuai micro-block with a view to understand the magmatic and metamorphic history and crustal evolution in the NCC during late Neoarchean. We present new data from field investigations, petrology, geochemistry and zircon U-Pb, REE and Lu-Hf analyses. Mineral phase equilibria modeling shows that the pyroxenite formed at P-T conditions of 1.5–1.6 GPa and 900–1000 °C, possibly representing underplating of high temperature basaltic magma at the base of the continental arc, with the subsequent metamorphic conditions estimated as 1.2–1.4 GPa and 840–870 °C from the metagabbros. Both metagabbro and pyroxenite show an anticlockwise P-T path characterized by initial heating followed by moderate to high-pressure metamorphism and cooling. The base of a thickened arc is a viable tectonic settings to account for this scenario. Geochemical data suggest that the pyroxenite-gabbro-trondhjemite association represent the fractionated sequence from a common mafic magma, whereas the charnockite corresponds to anhydrous lower crustal melts. The high degree of hydration of the mafic and ultramafic units suggest retrograde metamorphism under hydrous conditions associated with slab subduction. The mafic-ultramafic rocks fall within the field of island arc tholeiite and the trondhjemite falls in the field of oceanic arc basalt. Geochemical data indicate arc signature of the rocks and the subduction processes involved in the generation of precursor basaltic melts that underplated the overriding arc crust and triggered lower crustal melting.The U-Pb data on magmatic zircon grains from the trondhjemite, metagabbro, pyroxenite and charnockite yield emplacement ages of 2481 Ma, 2449 Ma and 2464 and 2480 Ma and suggest subduction-related magmatism during Neoarchean-early Paleoproterozoic transition, followed by high grade metamorphism at ca. 1.94 Ga as indicated by the lower intercept ages. Zircon REE data confirm the magmatic crystallization history. The zircon Lu-Hf data show consistently positive εHf(t) values (0.5–11.7) and plot between CHUR and new crust line suggesting that depleted mantle sources contributed significantly to arc building and crustal growth. The computed model ages suggest that the magma source involved Neoarchean-Mesoarchean juvenile components. Our study provides important insights into magmatic and metamorphic processes at the base of an Archean continental arc and contributes to the understanding of the formation and growth of micro-continents within the NCC.

Geochronology and geochemistry ofca. 2.48 Ga granitoid gneisses from theYudongzi Complexin the north‐westernYangtze Block,China

The Early Precambrian magmatism in the Archean–Palaeoproterozoic basement Yudongzi Complex is critical for understanding the early crustal formation and evolutionary history of the Yangtze Block in the South China Craton. In this paper, combined zircon UPb geochronological, zircon Hf isotopic, and whole‐rock geochemical studies are carried out on the granitoid gneisses from the Yudongzi Complex, north‐western Yangtze Block. The magmatic zircons from the granitoid gneiss yield a UPb upper intercept age of 2,477±18 Ma, interpreted as the crystallization age of the granitoid gneiss. The granitoid gneiss samples are characterized by high Al2O3contents, moderate to high Sr/Y, (La/Yb)Nand Na2O/K2O ratios, no obvious Eu and Sr anomalies, and depleted Nb, Ta, and Ti values that are similar to those of Archean trondhjemite–tonalite–granodiorite (TTG) suites, indicating that the investigated rocks belonging to TTG‐series rocks. They display pronounced enriched‐LREE patterns with high La/Yb values and relatively high Zr/Sm and Eu/Eu* but low Nb/Ta and Dy/Yb ratios, implying that these rocks were formed by partial melting within amphibolite‐facies conditions with mainly residual amphibole, garnet, and accessory rutile in the magma source. These rocks also display apparently low MgO, Mg#, Ni, and Cr contents, suggesting derivation from partial melting of thickened ancient lower crust. Zircon εHf(t) values from the granitoid gneiss sample vary from −10.1 to −6.9 with TDM2ofca. 3.59–3.40 Ga, what demonstrates their crustal source nature as well. Thus, taking all these factors together, we propose that theca. 2.48 Ga granitoid gneisses in the Yudongzi Complex might have been produced by partial melting of ancient crust material under amphibolite‐facies conditions, with primarily residual assemblages of amphibole, garnet, and attached rutile. Taking together our new data and such from previous studies, the Yangtze Block has been probably involved into a significant reworking of ancient crust event during the Early Palaeoproterozoic time.

Pb age of inherited zircons from leucosomes in grey gneisses of the Salma and Gridino study areas, Belomorian province

Inherited zircons in leucosomes from the Salma and Gridino study areas have been studied. The leucosomes were formed in Palaeoproterozoic shear zones crossw-cutting Archaean tonalite-trondhjemite-granodiorite (grey) gneisses that host amphibolites with eclogite relics . Zircon crystals contain rounded inherited cores whose proportion is usually ≥ 90% of a grain volume. The cores display a fine oscillatory zonation typical for zircon from granitoids and are enveloped by rims that show a coarse zonation and define the highly euhedral prismatic morphology. The cores have been analyzed by the U-Pb SIMS (SHRIMP II) technique. Cores of the zircon from a leucosome from the Salma study area have yielded an upper intercept age of 2740 ± 8 Ma, with most of them having concordant ages. Cores of the zircon from the Gridino leucosome have given a Concordia age of 2831 ± 8 Ma. Both these ages are referred to as the time of crystallization of igneous protoliths of grey gneisses in which the leucosomes occur. In the Gridino study area the grey gneisses seem to have been reworked 2.76–2.73 and 2.71–2.68 Ga ago. All the obtained dates are consistent with ages of the extensive formation of the grey gneisses in the Belomorian region at ca. 2.8 Ga, the emplacement of a new generation of granitoids at 2.74 Ga and a reworking at ca. 2.7 Ga. The oldest ages of individual crystals inherited by igneous protoliths of the Gridino grey gneisses are ~ 3.15 Ga.

Downward rejuvenation of the continental lower crust beneath the southeastern North China Craton

Felsic to mafic granulite xenoliths and zircon xenocrysts from the Cenozoic (~1 Ma) Nushan basalts were analyzed to construct a lower-crustal column illustrating the fine structure and evolution of the deep crust beneath the southeastern North China Craton. Felsic granulites, sampled from the lowermost middle to the upper lower crust have a calculated Vp of 6.32 km/s. Relict magmatic zircons from them yield an upper intercept age of 2716 ± 32 Ma and depleted-mantle Hf model ages (TDM) of 3.1–2.9 Ga. Metamorphic zircons with ages of ca. 2100 Ma have initial 176Hf/177Hf ratios similar to the magmatic ones, implying in situ recrystallization of the older zircons. The intermediate granulites with Paleoproterozoic ages (2.1–1.9 Ga; whole-rock Nd TCHUR = 3.4 Ga and zircon Hf TDM = 3.1–2.8 Ga; Huang et al., 2004; Zheng et al., 2012) have a calculated Vp of 6.53 km/s, being considered that they underlie the felsic granulites. The lowermost crust is represented by mafic granulites (Vp = 6.86 km/s). With the exception of three inherited zircons from the mafic granulites, which yielded an upper intercept age of 2455 Ma and TDM of ca. 2.7 Ga, most zircons formed in the late Mesozoic (ca. 115 Ma). Zircon xenocrysts have concordant ages, including the Neoproterozoic, early Paleozoic, late Paleozoic, early Mesozoic and Cenozoic. Our data, integrated with published data, suggest that the lower crust is younger at the bottom and dominantly composed of felsic, intermediate and mafic granulites. This trend reflects the vertical accretion of the crust, including the addition of juvenile material during the Mesoarchean and Neoarchean, and repeated reworking during the Neoarchean, Paleoproterozoic and late Mesozoic. The Neoproterozoic, Paleozoic and early Mesozoic zircon xenocrysts may record the subduction events between the craton and the Yangtze Block, which also affected the nature of the lower crust beneath the southeastern part of the craton.

Age and geochemistry of the Paleoproterozoic Bhatwari Gneiss of Garhwal Lesser Himalaya, NW India: implications for the pre-Himalayan magmatic history of the Lesser Himalayan basement rocks

Abstract The Bhatwari Gneiss of Bhagirathi Valley in the Garhwal Himalaya is a Paleoproterozoic crystalline rock from the Inner Lesser Himalayan Sequence. On the basis of field and petrographic analyses, we have classified the Bhatwari Gneiss into two parts: the Lower Bhatwari Gneiss (LBG) and the Upper Bhatwari Gneiss (UBG). The geochemical signatures of these rocks suggest a monzonitic protolith for the LBG and a granitic protolith for the UBG. The UBG has a calc-alkaline S-type granitoid protolith, whereas the LBG has an alkaline I-type granitoid protolith; the UBG is more fractionated. The trace element concentrations suggest a volcanic arc setting for the LBG and a within-plate setting for the UBG. The U–Pb geochronology of one sample from the LBG gives an upper intercept age of 1988 ± 12 Ma ( n = 10, MSWD = 2.5). One sample from the UBG gives an upper intercept age of 1895 ± 22 Ma ( n = 15, MSWD = 0.82), whereas another sample does not give any upper intercept age, but indicates magmatism from c. 1940 to 1840 Ma. Based on these ages, we infer that the Bhatwari Gneiss has evolved due to arc magmatism and related back-arc rifting over a time period of c. 100 Ma during the Proterozoic. This arc magmatism is related to the formation of the Columbia supercontinent. Supplementary material: LA–ICP-MS calibration and adjusted major element data used for computing the isometric log-ratio transformations are available at https://doi.org/10.6084/m9.figshare.c.4272158

In situ LA-ICP-MS apatite and zircon U–Pb geochronology of the Nicholson Lake impact structure, Canada: Shock and related thermal effects

In situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been deployed to determine the U–Pb ages of impact metamorphosed apatite and zircon associated with the ∼12.5 km diameter Nicholson Lake impact structure, Northwest Territories, Canada. The dated phases occur within both impact melt-bearing breccias and clast-laden impact melts. A total of 84 laser ablation analyses from 57 apatite grains within seven rock samples yield a minimum refined lower intercept at 387 ± 5 Ma (MSWD = 0.87, 2σ, n=26), and maximum age with lower intercept of ∼1740 Ma. A total of 90 laser ablation analyses on 52 zircon grains from two rock samples yield a minimum intercept age of 384 ± 8 Ma (MSWD = 1.3, 2σ, n=22), with a maximum upper intercept age of 2679 ± 14 Ma, and a second discordia with a ∼1740 Ma upper intercept age. The results are consistent with the target rocks comprising Archean Snow Island Suite (∼2.7 Ga) and Paleoproterozoic Nueltin plutonic suite (∼1.74 Ga), with the impact event occurring at approximately 385 Ma. The degree of resetting of inherited apatite can be related to its proximity to impact melt (now partly devitrified glass) within the host impact melt-bearing breccias and clast-laden impact melt rocks. Those grains in direct contact with melt bodies are reset, while those occurring as inclusions in lithic or mineral clasts partly or wholly retain their original isotopic compositions. The youngest (impact-reset) apatite ages are most closely associated with the highest shock levels (i.e., granular textured and thermally dissociated zircons), which we relate to juxtaposition of apatite with superheated melt. Due to accumulated radiation damage prior to impact, many of the relic zircons are metamict, which facilitated enhanced Pb diffusion and their resetting to the impact age. Granular zircons record impact ages while those exhibiting planar fractures or experiencing negligible shock record basement ages. We link apatite and zircon geochronology to the textural and structural states of ZrSiO4 and its associated shock levels via field emission scanning electron microscopy and micro-Raman spectrometry.

Composition and U—Pb ages of apatite in the Amba Dongar carbonatite–alkaline complex, India

AbstractThe Amba Dongar carbonatite‐alkaline complex is regarded as a late phase intrusion (ca. 65 Ma) associated with Deccan volcanism. Existing age constraints on the Amba Dongar carbonatite are based on phlogopite 40Ar–39Ar plateau ages derived from carbonatite and associated alkaline rocks within the complex. We explore the utility of apatite, a common accessory mineral in Amba Dongar carbonatites, as a petrogenetic and geochronological tool to constrain further the evolution of the carbonatites. Apatite exhibits different crystal forms—from hexagonal stubby prisms to ovoid and elongate grains. Apatite occurs as cumulate and disseminated grains that are either primary, fractured, or recrystallized. SrO content is high in primary apatite (3.17 wt%) but less in recrystallized (1.90 wt%) and disseminated grains (0.47 wt%). MnO and Cl are negligible while FeO is typically low in all samples. SiO2 content is low except for a few primary grains which measure up to 1.35 wt%, correlating with high REE content. LREEs dominate the total REE budget of Amba Dongar apatite, with HREEs below EPMA detection limit in most samples. Total REE content is generally low, though some primary apatite contains up to 3.19 wt% REEs. We also report UPb apatite ages from the carbonatites analysed in situ by LA‐ICP‐MS. The data from all samples combined on a UPb Tera–Wasserburg concordia yield a lower intercept age of 65.4 ± 2.5 Ma (MSWD = 2.8, 207Pb/206Pbi = 0.8272 ± 0.0028), and the Amba Dongar carbonatites are thus interpreted to represent a phase of coeval magmatic activity with Deccan volcanism.

Cordierite-bearing granulites from Ihosy, southern Madagascar: Petrology, geochronology and regional correlation of suture zones in Madagascar and India

Madagascar, a major fragment of Gondwana, is mainly composed of Precambrian basement rocks formed by Mesoarchean to Neoproterozoic tectono-thermal events and recording a Pan-African metamorphic overprint. The Ranotsara Shear Zone in southern Madagascar has been correlated with shear zones in southern India and eastern Africa in the reconstruction of the Gondwana supercontinent. Here we present detailed petrology, mineral chemistry, metamorphic P–T constraints using phase equilibrium modelling and zircon U-Pb geochronological data on high-grade metamorphic rocks from Ihosy within the Ranotsara Shear Zone. Garnet-cordierite gneiss from Ihosy experienced two stages of metamorphism. The peak mineral assemblage is interpreted as garnet + sillimanite + cordierite + quartz + plagioclase + K-feldspar + magnetite + spinel + ilmenite, which is overprinted by a retrograde mineral assemblage of biotite + garnet + cordierite + quartz + plagioclase + K-feldspar + magnetite + spinel + ilmenite. Phase equilibria modelling in the system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO) indicates peak metamorphic conditions of 850–960 °C and 6.9–7.7 kbar, and retrograde P–T conditions of <740 °C and <4.8 kbar, that define a clockwise P–T path. Near-concordant ages of detrital zircon grains in the garnet-cordierite gneiss dominantly exhibit ages between 2030 Ma and 1784 Ma, indicating dominantly Paleoproterozoic sources. The lower intercept age of 514 ± 33 Ma probably indicates the timing of high-grade metamorphism, which coincides with the assembly of the Gondwana supercontinent. The comparable rock types, zircon ages and metamorphic P–T paths between the Ranotsara Shear Zone and the Achankovil Suture Zone in southern India support an interpretation that the Ranotsara Shear Zone is a continuation of the Achankovil Suture Zone.

Zircon U–Pb geochronology and Hf isotope of granitoids in East Kunlun: Implications for the Neoproterozoic magmatism of Qaidam Block, Northern Tibetan Plateau

The Neoproterozoic granitoids in East Kunlun Orogen (EKO) record critical information on the composition and evolution of the early crustal materials, as well as the Precambrian tectonic evolutionary history of the Qaidam Block, northern Tibetan Plateau. Zircon U–Pb geochronology and Hf isotopic compositions of three newly discriminated gneissic granitoids in the Hongshuihe, Dangennaomuru and south Bokalike areas are employed in this study to elucidate the Neoproterozoic tectonics of EKO. Petrological studies reveal that these granitoids are S–type granite with abundant Al–rich minerals like muscovite and garnet in their mineral assemblages. Zircon U–Pb geochronological results show that the Hongshuihe and Dangennaomuru plutons yield weighted mean 206Pb/238U ages of 914 ± 5 Ma (MSWD = 0.25) and 935 ± 6 Ma (MSWD = 0.56), respectively, indicating their magmatic crystallization ages. And the south Bokalike pluton yields upper and lower intercept ages of 1002 ± 31 Ma and 388 ± 39 Ma (MSWD = 0.63), disclosing its magmatic crystallization age and the time of subsequent Paleozoic metamorphic event, respectively. The εHf(t) values of zircons in this study are dominated by negative with minor positive values ranging from −7.18 to +2.76. The two–stage Hf model ages of the zircons having negative εHf(t) values are ca. 1.7–2.0 Ga, whereas those with positive εHf(t) values are ca. 1.5 Ga. Both the εHf(t) values and the two–stage Hf model ages indicate that these granitoids were probably derived from the reworking of ancient continental crustal materials with minor mantle contribution. Combined with available data of the granitoids in EKO, central and north Qaidam Block, we propose that the Neoproterozoic granitoids in Qaidam Block have similar crystallization ages of ca. 0.9–1.0 Ga, as well as same or similar geochemical and zircon Hf isotopic compositions revealing the Qaidam Block was involved into the amalgamation of the supercontinent Rodinia.