Continental basalts generally display enrichment of fluid-mobile elements and depletion of high-field-strength elements, similar to those that evolved in the subduction environment, but different ...from oceanic basalts. Based on the continental flood basalt database for six large igneous provinces, together with rift-related basalt data from the Basin and Range Province, this study aimed to test the validity of geochemical tectonic discrimination diagrams in distinguishing arc-like intra-continental basalts from arc basalts and to further investigate the role of deep-Earth water cycling in producing arc-like signatures in large-scale intra-continental basalts. Our evaluation shows that arc-like intra-continental basalts can be distinguished from arc basalts by integrating the following factors: (1) the FeO, MgO, and Al2O3 concentrations of the primary melt; (2) TiV, ZrZr/Y, ZrTi, and Ti/VZr/SmSr/Nd discrimination diagrams; (3) the coexistence of arc-like and OIB-like subtype basalts within the same province; (4) primitive mantle-normalized trace element distribution patterns. The similarity of enrichment in fluid-mobile elements (Ba, Rb, Sr, U, and K) between arc-like and true arc basalts suggests the importance of water flux melting in producing arc-like signatures in continental basalts. Experimentally determined liquid lines of descent (LLD) imply high magma water concentrations for continental flood basalts (CFBs) and the Basin and Range basalts. Furthermore, estimates based on the Al2O3–LLD method indicates 4.0–5.0wt% pre-eruptive magma H2O concentration for CFBs and the Basin and Range basalts. The tight relationships between H2O/Ce and Ba/La, Ba/Nb and Rb/Nb based on global arc basalt data were further used to estimate the primary H2O concentrations. With the exception of the Emeishan CFBs (mainly containing 4.0–5.6wt% H2O), all other CFBs investigated have similar estimated primary H2O contents, with values ranging from 1.0 to 2.0wt%. The estimated primary H2O content of the Basin and Range basalts is extremely high and up to 10.0wt%. Thus, this study demonstrates that water flux melting played an important role in the generation of many intra-continental igneous provinces. This new finding was further employed to investigate the tectonic setting of 320–270Ma basalts in Inner Mongolia, North China. Most basalts from three key rock units (i.e. Amushan, Benbatu, and Dashizhai formations) from the Central Asian Orogenic belt are classified as non-arc types. The estimated magma H2O concentrations suggest a strong link between H2O content and arc-like geochemical signatures. Together with established geological evidence, we proposed that these 320–270Ma basaltic rocks were most likely produced in a post-orogenic extensional environment facilitated by subducted slab-driven deep-Earth fluid cycling. We propose a mantle transition zone water-filtering model that links deep-Earth fluid cycling, large-scale intra-continental basaltic magmatism, and supercontinent cycles into a self-organized system.
•Arc-like continental basalts have 4.0–5.0wt% magma H2O concentration.•Primary melts of CFBs have 1.0–2.0wt% H2O concentration.•Arc-like geochemical signatures related to deep-Earth water cycling.•Arc-like continental basalts can be distinguished from arc basalts using the proposed factors.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
It has previously been postulated that the Earth's hydrous mantle transition zone may play a key role in intraplate magmatism, but no confirmatory evidence has been reported. Here we demonstrate that ...hydrothermally altered subducted oceanic crust was involved in generating the late Cenozoic Chifeng continental flood basalts of East Asia. This study combines oxygen isotopes with conventional geochemistry to provide evidence for an origin in the hydrous mantle transition zone. These observations lead us to propose an alternative thermochemical model, whereby slab-triggered wet upwelling produces large volumes of melt that may rise from the hydrous mantle transition zone. This model explains the lack of pre-magmatic lithospheric extension or a hotspot track and also the arc-like signatures observed in some large-scale intracontinental magmas. Deep-Earth water cycling, linked to cold subduction, slab stagnation, wet mantle upwelling and assembly/breakup of supercontinents, can potentially account for the chemical diversity of many continental flood basalts.
The Re–Os isotopic heterogeneity of mantle-derived rocks has been well documented. However, it is unclear whether and how the Os isotopic heterogeneity of a mantle source can influence Re–Os isotopic ...dating and source tracing of mantle-derived melts. Here we report that replicate analyses of gram aliquots of single basaltic powders (one of reference material BHVO-2 and three of the Hatu basalts from the western Junggar region, China) show large variations in both Os concentrations and isotopic ratios. More importantly, these replicate data define good apparent Re–Os isochron correlations, yielding ages significantly older than their formation ages, and display remarkable correlations between the 187Os/188Os ratio and the reciprocal of the common Os concentration (i.e., 1/192Os). This indicates that the initial Os isotopic composition in a basaltic magmatic system may be heterogeneous and cannot be homogenized by pulverizing. Theoretical and mathematical deduction demonstrates that the observed apparent Re–Os isochron correlations are the result of binary mixing without complete isotopic equilibrium at the time of formation, primarily due to limited diffusional exchange of Os isotopes between refractory Os-bearing inclusions and host minerals under mantle conditions. The regressed initial Os isotopic composition may bias the true value of the mantle source. Pulsed mantle melting and magma mixing during ascent are potential processes responsible for such initial Os isotopic heterogeneity in basaltic rock systems. Thus, when using the Re–Os isotopic system to date and to trace the source of basaltic rocks or other rocks with relatively low Os concentrations, it is necessary to consider whether the initial Os isotopic composition was heterogeneous or whether it had reached complete isotopic equilibrium.
•Replicate Re–Os isotope analyses were carried out for gram-aliquots of basaltic rocks.•Replicate analyses define good apparent Re–Os isochron correlations.•They all yield ages significantly older than their formation ages.•They can be accounted for by binary mixing without Os isotopic equilibrium.•Caution must be taken in using Re–Os isotope system for dating and source tracing.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Distinct signatures are present in the circum‐Pacific continental margins (e.g., kinematics, magmatism, and basin evolution), possibly influenced by the input of mid‐ocean ridge. It remains enigmatic ...why the circum‐Pacific continental margins that have experienced trench‐parallel mid‐ocean ridge subduction show diverse geological evolution. Here we present geodynamic modeling results investigating trench‐parallel mid‐ocean ridge subduction and demonstrate two distinct types of model evolution. Type‐Ⅰ model includes a two‐stage steep subduction and is featured by slab detachment preceding the arrival of ridge at the trench. Type‐Ⅱ model is marked by a continuous flat subduction of mid‐ocean ridge with the opening of a slab window beneath intracontinental lithosphere. These two subduction styles produce diverse tectono‐magmatic responses. Our results could explain the magmatic gap and forearc uplift during the Izanagi‐Pacific ridge subduction and the intraplate magmatic flare‐ups and tectonic uplift during the Nazca‐Antarctic ridge subduction, respectively.
Plain Language Summary
Mid‐ocean ridge subduction potentially occurs during the evolution of all subduction zones and has significant influence on subduction dynamics and the related geological evolution. The circum‐Pacific domain that has experienced trench‐parallel mid‐ocean ridge subduction shows contrasting tectono‐magmatic evolution. However, it remains poorly understood how the input of mid‐ocean ridge in subduction zones affects the geological evolution of these continental margins. Here, we conduct a series of numerical experiments and reveal two distinct subduction styles. The modeling results show that an interruption of ridge subduction can induce magmatic quiescence, forearc mid‐ocean ridge basalts, and forearc tectonic uplift, whereas the continuous ridge subduction can induce magmatic flare‐ups and tectonic uplift in intraplate region. These two contrasting models are strongly dependent on subducting slab dips. This study provides a new simple model to explain the complex Cenozoic geological evolution observed in the northwest and southeast Pacific continental margins.
Key Points
Geodynamic modeling reveals the dynamics of trench‐parallel mid‐ocean ridge subduction affected by the dip angles of the subducting slab
Steep subduction promotes slab detachment along the ridge axis, but flat subduction facilitates continuous subduction of the ridge
Model results could explain the contrasting geological features observed in the northwest and southeast Pacific continental margins
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Although deep carbon recycling plays an important role in the atmospheric CO2 budget and climate changes through geological time, the precise mechanisms remain poorly understood. Since recycled ...sedimentary carbonate through plate subduction is the main light-δ26Mg reservoir within deep-Earth, Mg isotope variation in mantle-derived melts provides a novel perspective when investigating deep carbon cycling. Here, we show that the Late Cretaceous and Cenozoic continental basalts from 13 regions covering the whole of eastern China have low δ26Mg isotopic compositions, while the Early Cretaceous basalts from the same area and the island arc basalts from circum-Pacific subduction zones have mantle-like or heavy Mg isotopic characteristics. Thus, a large-scale mantle low 8 26Mg anomaly in eastern China has been delineated, suggesting the contribution of sedimentary carbonates recycled into the upper mantle, but limited into the lower mantle. This large-scale spatial and temporal variation of Mg isotopes in the mantle places severe constraints on deep carbon recycling via oceanic subduction.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
The North China Craton (NCC) has been considered to be part of the supercontinent Columbia. The nature of the NCC western boundary, however, remains strongly disputed. A key question in this regard ...is whether or not the Alxa Block is a part of the NCC. It is located in the vicinity of the inferred boundary, and therefore could potentially resolve the issue of the NCC's relationship to the Columbia supercontinent. Some previous studies based on the Alxa Block's geological evolution and detrital zircon ages suggested that it is likely not a part of the NCC. The lack of evidence from key igneous rock units, however, requires further constraints on the tectonic affinity of the western NCC and Alxa Block and on the timing of their amalgamation.
In this study, new zircon U–Pb age and Hf–O isotopes and whole-rock geochemical and Sr–Nd–Pb isotopic data for the Paleozoic granitoids in or near the eastern Alxa Block were used to constrain the petrogenesis of these rocks and the relationship between the Alxa Block and NCC. Secondary ion mass spectrometry (SIMS) U–Pb zircon dating indicates that the Bayanbulage, Hetun, Diebusige and South Diebusige granitoids were formed at ca. 423Ma, 345Ma, 345Ma and 337Ma, respectively. The Late Silurian (Bayanbulage) quartz diorites have variable SiO2 (58.0–67.9wt.%), and low Sr/Y (20–24) values, while the Early Carboniferous (Hetun, Diebusige and South Diebusige) monzogranites have high SiO2 (71.5–76.7wt.%) and Sr/Y (40–94) values. The Late Silurian quartz diorites display relatively homogeneous and high zircon δ18O (8.5–9.1‰) and εHf(t) (−8.6 to −5.3) values, high whole-rock εNd(t) values (−9.2 to −7.6) and highly radiogenic Pb isotopes (206Pb/204Pb=18.13–18.25), whereas the Early Carboniferous monzogranites exhibit relatively low and variable zircon δ18O (5.7–7.2‰) and εHf(t) (−23.1 to −7.4) values, low whole-rock initial 87Sr/86Sr (0.7043–0.7070) and εNd(t) (−19.1 to −13.5) values and variable Pb isotopes (206Pb/204Pb=16.06–18.22). The differences in whole rock Nd model ages and Pb isotope compositions of the Paleoproterozoic–Permian rocks in either side of the west fault of the Bayanwulashan–Diebusige complexes suggest that the Alxa Block is not a part of the NCC, and that the western boundary of the NCC is probably located on this fault. Furthermore, the linear distribution of the Early Paleozoic–Early Carboniferous granitoids, the high zircon δ18O values of the Late Silurian quartz diorites, the Early Devonian metamorphism and the foreland basin system formed during the collision between the Alxa Block and the NCC indicate that a Paleozoic cryptic suture zone likely existed in this area and records the amalgamation of the Alxa Block and North China Craton. Together with detrital zircon data, the initial collision was considered to have possibly occurred in Late Ordovician.
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•Alxa Block is not a part of the North China Craton.•Paleozoic granites were (423–337Ma) discovered in eastern Alxa Block.•A Paleozoic cryptic suture zone was proposed in the eastern Alxa Block.•I-type quartz diorite has homogeneous and high zircon δ18O value of 8.9‰.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
We introduce a potential new working reference material – natural zircon megacrysts from an Early Pliocene alkaline basalt (from Penglai, northern Hainan Island, southern China) – for the microbeam ...determination of O and Hf isotopes, and U–Pb age dating. The Penglai zircon megacrysts were found to be fairly homogeneous in Hf and O isotopes based on large numbers of measurements by LA‐multiple collector (MC)‐ICP‐MS and SIMS, respectively. Precise determinations of O isotopes by isotope ratio mass spectrometry (IRMS) and Hf isotopes by solution MC‐ICP‐MS were in good agreement with the statistical mean of microbeam measurements. The mean δ18O value of 5.31 ± 0.10‰ (2s) by IRMS and the mean 176Hf/177Hf value of 0.282906 ± 0.0000010 (2s) by solution MC‐ICP‐MS are the best reference values for the Penglai zircons. SIMS and isotope dilution‐TIMS measurements yielded consistent 206Pb/238U ages within analytical uncertainties, and the preferred 206Pb/238U age was found to be 4.4 ± 0.1 Ma (95% confidence interval). The young age and variably high common Pb content make the Penglai zircons unsuitable as a primary U–Pb age reference material for calibration of unknown samples by microbeam analysis; however, they can be used as a secondary working reference material for quality control of U–Pb age determination for young (particularly < 10 Ma) zircon samples.
Nous proposons un nouveau matériel de référence potentiel – des mégacristaux de zircon naturel provenant d’un basalte alcalin du début du pliocène (région de Penglai, nord de l’île de Hainan, Chine méridionale) destinéà l’analyse microfaisceaux des isotopes de l’O et de l’Hf, et la datation U–Pb. Les mégacristaux de zircon de Penglai se sont révélés, après un grand nombre de mesures par LA‐MC‐ICP‐MS et SIMS, assez homogènes en terme d’isotopes de l’O et de l’Hf. Les déterminations précises des isotopes de l’O par IRMS et des isotopes de l’Hf par MC‐ICP‐MS solution sont en bon accord avec la moyenne statistique des analyses microfaisceaux. La valeur moyenne δ18O de 5.31 ± 0.10‰ (2s) par IRMS et la valeur moyenne 176Hf/177Hf de 0.282906 ± 0.0000010 (2s) par solution MC‐ICP‐MS sont les meilleures valeurs de référence pour les zircons de Penglai. Les mesures SIMS ainsi que celles par dilution isotopique‐TIMS donnent, compte tenu des incertitudes analytiques, des âges 206Pb/238U conformes, et l’âge préféré206Pb/238U est estiméà 4.4 ± 0.10 Ma (intervalle de confiance de 95%). Leur jeune âge et leurs teneurs variables et élevées en Pb commun font que les zircons de Penglai ne sont pas utilisables comme matériel de référence primaire pour l’étalonnage d’échantillons inconnus dont les âges U–Pb doivent être déterminés par analyses microfaisceaux; Toutefois, ils peuvent être utilisés comme matériel de référence secondaire pour le contrôle qualité de la détermination de l’âge U–Pb des zircons jeunes (en particulier pour ceux < 10 Ma).
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BFBNIB, FZAB, GIS, IJS, IZUM, KILJ, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK
Adakites, or adakitic rocks, in a broad sense, have been used to describe a large range of igneous rocks with the common feature of high Sr/Y and La/Yb ratios that can be achieved though different ...mechanisms. Among them, the continental, or C-type, adakitic rocks are particularly controversial in terms of their sources and genesis. In this study we revisit both Cu–Au ore-bearing and barren “C-type adakitic rocks” in the Lower Yangtze River Belt (LYRB) of central eastern China, including comprehensive analyses of their in-situ zircon Hf–O isotopes, whole-rock geochemistry and Sr–Nd isotopes. These “C-type adakitic rocks” consist of monzodiorite, granodiorite and quartz monzonite that are classified as shoshonitic to high-K calc-alkaline series according to their chemical compositions. They are characteristically high in potassium (K2O=2.4–4.5%, K2O/Na2O=0.6–1.3), with continental crust-like isotopic compositions, i.e., whole-rock εNd(T)=−3.9 to −7.7, initial 87Sr/86Sr=0.7054–0.7085, zircon εHf(T)=0 to −11, and δ18O=6‰ to 9‰. The ore-bearing and barren rocks are cogenetic. Fractional crystallization of hornblende, titanite, magnetite and apatite played a major role in their chemical variations, with the ore-bearing rocks being more felsic (SiO2=63.3–69.6%) and higher in Sr/Y (41.2–75.6) than the barren rocks (SiO2=57.3–65.0%, Sr/Y=30.4–51.8). All these geochemical and isotopic features, in combination with regional geological data, suggest that the LYRB “C-type adakitic rocks” were unlikely to have been formed by melting of either a thickened and/or delaminated lower continental crust, or an altered oceanic crust as previously thought. These rocks are in general akin in geochemistry and isotopes to the Archean sanukitoids and the Setouchi high-Mg andesites in Japan, and are thus interpreted as being formed by melting of an enriched mantle source metasomatized by dewatering from a delaminated flat-slab. The flat subduction of an oceanic plateau and its subsequent delamination and foundering since the early Mesozoic beneath southeastern China (Li and Li, 2007) thus not only explain the temporal and spatial propagation of widespread Yanshanian igneous rocks regionally since ca. 195Ma, but also the formation of a series of enigmatic “adakitic” rocks in the region, including the LYRB potassium-rich rocks that were inappropriately called the “C-type adakitic rock” by previous workers.
•We revisit China's LYRB “C-type adakites” by Hf–O–Sr–Nd isotopes and geochemistry.•These rocks are potassium-rich monzodiorite, granodiorite and quartz monzonite.•They are derived from an enriched mantle source, not lower crust and oceanic crust.•“C-type adakites” are an inappropriate petrological classification for these rocks.•Delamination of subducted flat-slab accounts for widespread 195–85Ma igneous rocks.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
► In situ U–Pb age and Hf–O isotope analyses for detrital zircons from the Helanshan khondalite complex in the North China Craton. ► The Helanshan khondalite protoliths consisting of both juvenile ...and reworked crustal materials were sourced from a ca. 2.18–2.00Ga continental arc. ► The khondalite protoliths were most likely deposited in an active continental margin at ca. 2.00–1.95Ga.
The Khondalite Belt, one of the three major Paleoproterozoic mobile belts in the North China Craton, consists mainly of khondalite suite and S-type granites. It is thought to be a continental collision belt along which the Yinshan and Ordos Blocks amalgamated to form the Western Block of the North China Craton at ca. 1.95Ga. While many resent studies revealed that the khondalite protoliths were sourced from juvenile crustal materials and the time interval between their deposition and subsequent metamorphism is less than 50 million years, the nature and depositional setting of the khondalite protoliths are still controversial. We carried out in this study an integrated analysis of U–Pb age and Hf–O isotopes for detrital zircons from the Helanshan Complex in the westernmost part of the Khondalite Belt. Precise SIMS U–Pb zircon results suggest that the timing of the deposition of the Helanshan khondalite protoliths can be constrained between ca. 2.00 and ca. 1.95Ga by the minimum age of the detrital zircons from the khondalites and the maximum crystallization age of ca. 1.95Ga for the intruded granites. The ca. 1.95Ga granites, the oldest granites identified within the Khondalite Belt, were coeval with the regional high-grade metamorphism. The Helanshan khondalite protoliths were sourced mainly from a provenance with prolonged, episodic magmatism of ca. 2.18Ga, 2.14Ga, 2.09Ga, 2.06Ga, 2.03Ga and 2.00Ga, respectively. Detrital zircons of ca. 2.18–2.00Ga from the khondalites have ɛHf(t) values ranging from +8.9 to −2.9 and Hf TDMC model ages between 2.8 and 2.1Ga, with two major peaks at 2.6Ga and 2.3Ga. Their δ18O values show two major peaks at 6.6‰ and 8.2‰. Zircon Hf–O isotopic data indicate that both the juvenile and ancient crustal components were involved in their source rocks, and the Helanshan khondalite protoliths were most likely sourced from a ca. 2.18–2.00Ga continental arc. The juvenile continental arc materials were uplifted, eroded, transported, deposited and metamorphosed within a short time interval of <50 million years. With the predominantly euhedral crystals of the detrital zircons and the immature clastic sediments for the Helanshan khondalite protoliths, we suggest that the khondalite protoliths were likely deposited in an active continental margin, rather than a passive margin as previously thought.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The Alxa Block is the westernmost part of the North China Craton (NCC). In the past, it has been considered to be part of the Archean NCC. However, formation and evolution of this block remain poorly ...understood, and this has hampered a broader understanding of the NCC. In this study we analyzed the in situ zircon U–Pb age and Hf–O isotopes for the two oldest rock units in the eastern Alxa Block, namely the Bayanwulashan and Diebusige Complexes. The Bayanwulashan Complex consists mainly of metamorphic rocks with mafic and felsic protoliths. SIMS U–Pb zircon dating results indicate that the primary magmatic ages of the mafic and felsic igneous rocks are ca. 2.34Ga and ca. 2.32–2.30Ga, respectively, and both sets of rocks were overprinted by two metamorphic events at ca. 1.89Ga and ca. 1.79Ga. Geochemical and zircon Hf–O isotopic data suggest that the ca. 2.34Ga amphibolites within the Bayanwulashan Complex are characteristically high in TiO2 (up to 3.2%), Zr (up to 394ppm) and Ti/V (>30), resembling the basaltic rocks formed within continental rifts. The ca. 2.32–2.30Ga felsic rocks were generated by remelting of dominant meta-igneous rocks that have zircon Hf model ages of 2.92–2.81Ga. The Diebusige Complex is composed of amphibolites, mafic gneisses and paragneisses. It was intruded by granites dated at ca. 1.97–1.98Ga, and subjected to high grade metamorphic events at ca. 1.89Ga and ca. 1.79Ga. Re-examination of U–Pb ages for detrital and metamorphic zircons indicate that the depositional ages of protoliths of the Diebusige paragneisses may be considered to be between ca. 2.45 and 2.0Ga. These U–Pb zircon age data do not support the existence of exposed Archean rocks in the eastern Alxa Block, although the possibility of Archean rocks at deeper crustal levels cannot be ruled out entirely.
The eastern Alxa Block contrasts to the neighboring Yinshan Block and Khondlite Belt, as well as the Trans-North China Orogen, in terms of the timing and evolution of magmatism and metamorphism. Thus, we conclude that the Alxa Block is a separated Paleoproterozoic terrane from the Western Block of the NCC, rather than the western extension of the Yinshan Block or part of the Khondalite Belt, as previously proposed.
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► In situ U-Pb age and Hf-O isotope analyses for the Bayanwulashan and Diebusige Complexes in the eastern Alxa Block. ► No evidence to support existence of exposed Archean rocks in the eastern Alxa Block. ► Alxa Block was most likely an independent terrane to other parts of the North China Craton in the Paleoproterozoic.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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