High δ18O Research Articles

Evolution of A-type granites during the Late Paleoproterozoic orogenic cycle of North China craton: Implications for post-collisional magmatism

“A-type” granites were originally considered anorogenic, however current consensus closely associates them with orogenic settings. The correlation between the prolific emergence of A-type granites and the assembly of the Columbia (Nuna) supercontinent during the Late Paleoproterozoic remains ambiguous possibly being indicative of a geodynamic shift from the amalgamation to the fragmentation of the supercontinent. This study identifies a Late Paleoproterozoic pluton exhibiting A2-type granite characteristics in the Shaerqin area of the Khondalite belt, North China craton. The plutonic rocks exhibit evolved whole-rock Nd isotopic (εNd(t) = −6.6 to −8.0) and zircon Hf isotopic (εHf (t) = −6.3 to −2.9) compositions, along with notably high zircon δ18O values (averaging 8.35 ± 0.29 ‰, 1SD). The Hf–Nd–O isotope systematics imply that the Shaerqin A-type granites originated from the partial melting of preexisting crustal materials under conditions of low pressure and high temperature. Incorporating the outcomes of this research with a compiled dataset of 1.9–1.5 Ga A-type granites across the entire craton, we deduce that: 1) Key geochemical ratios and indices, such as 10,000 Ga/Al, Y/Nb, and Zr/Nb, in A-type granites exhibit temporal variations, signifying a transition from A2-type to A1-type granite; 2) There is an increasingly apparent involvement of mantle-derived magmas in their genesis, coupled with a more pronounced role of fractional crystallization in the development of A1-type granites compared to their A2-type counterparts. We contend that these compositional and isotopic transitions reflect the tectonic evolution from post-orogenic extension to anorogenic rifting occurring around 1.7 Ga. The refined geochemical variations revealed in this study may offer a paradigm for post-orogenic evolutionary processes in general.

How beech provenance affects the structure of secondary xylem, leaf traits, and the ectomycorrhizal community under optimal growth conditions

Key messageProvenance controls conductive area of stem secondary xylem, leaf area, and stable isotope (C, N, and O) ratios of beech leaves, while no significant effects were observed for ectomycorrhizal community composition.Beyond growth parameters and drought tolerance, comparatively little is known about the functioning of different beech (Fagus sylvatica L.) provenances. We investigated properties of leaves, stem secondary xylem, and ectomycorrhiza (ECM), and explored their interdependencies to identify the best performing beech provenance in optimal growth conditions. The study was conducted on 23-year-old trees in a provenance trial. The investigated provenances originated from Atlantic (Belgium—BE), Alpine (Italy—IT, Slovenia—SI), and continental climates (the Czech Republic—CZ). A significant effect of provenance was observed for stem vessel diameters and conductive area, as well as for foliar %C, δ13C, δ15N, and δ18O. δ13C as a proxy of intrinsic water use efficiency (iWUE) showed that the highest iWUE was achieved in BE provenance. Individuals with a better iWUE had wider growth rings regardless of provenance. Better iWUE was associated with lower specific leaf area (SLA). ECM community composition and diversity indices did not differ significantly among the provenances. Specific ECM taxa were associated with individuals with high SLA, δ13C, δ15N, and δ18O. In optimal growth conditions with no stress events, BE is a promising provenance due to an efficient water conducting system with high vessel diameters and conductive area, and high iWUE, while Alpine provenances showed an adaptation of their water conducting system to freezing conditions at their original locations. Integrating findings from different compartments improves our understanding of functioning of different beech provenances.

Reconciling Modeled and Observed Δ17O(NO3−) in Beijing Winter Haze With Heterogeneous Chlorine Chemistry

AbstractThe fine particulate matter (PM2.5) in Chinese megacities has declined significantly after the implementation of strict mitigation strategies since 2013. However, the concentration of wintertime nitrate in PM2.5 (p‐NO3‐) has only changed slightly and became the dominant inorganic component despite considerable precursor emission reductions. Discerning chemical mechanisms leading to nitrate growth during haze events is critical to implementing effective pollution mitigation policies. The oxygen isotope anomaly of nitrate (Δ17O(NO3−)) is a powerful means to distinguish nitrate formation mechanisms. Nevertheless, the observed high Δ17O(NO3−) values were significantly underestimated by chemical transport models during extreme haze events (PM2.5 > 225 μg m−3) in Beijing, indicating an incomplete understanding of nitrate chemistry. To reconcile this model‐observation discrepancy, we compiled reported Δ17O(NO3−) data in Beijing haze along with relevant observational parameters (e.g., hydroxyl (OH) reactivity, peroxyl radical concentrations), then tested assumptions on Δ17O of key precursors (e.g., OH and nitrogen dioxide (NO2)), recalculated Δ17O(NO3−) and compared them with observations. Our results indicate that considering heterogeneous dinitrogen pentoxide reactions on chlorine‐containing aerosols (N2O5 + Cl− chemistry) with a nitryl chloride (ClNO2) yield of ∼0.75 can explain the observed high Δ17O(NO3−) during extreme haze events. According to the Δ17O(NO3−) data, on average this heterogeneous N2O5 + Cl− chemistry can explain ∼60% of nighttime nitrate production and make daytime and nocturnal pathways equally important in winter Beijing haze (PM2.5 > 75 μg m−3). Our results highlight the critical role of reactive chlorine chemistry in air pollution/chemistry in inland cities.

Final suture of the Palaeo-Asian Ocean domain: Insights from the geochemistry and geochronology of the early to Middle Permian Elitu Formation volcanics

The consumption of the Palaeo-Asian Ocean domain triggered substantial resources and environmental effects, but the final suture timing remains debated. This study presents the zircon U-Pb age and Hf-O isotope, whole–rock element and Sr–Nd–Pb isotope data from the Elitu Formation trachyandesite and rhyolitic tuff in the Bainaimiao arc belt. The zircon U-Pb ages (260–280 Ma) indicate that the Elitu Formation volcanism erupted at the Kungurian to Capitanian. Trachyandesites exhibit an arc-like whole–rock trace element composition and have enriched Sr–Nd–Pb isotopic composition (ISr(t) = 0.7062–0.7065; εNd(t) = −8.8 to −9.4; (206Pb/204Pb)t = 17.14–17.31) and high zircon δ18O (6.08‰–7.59‰) values, which are derived from the partial melting of the lithosphere mantle metasomatised by ancient sediment-derived melts. Rhyolitic tuffs have variable εHf(t) (−7.15 to +10.47) and low δ18O (4.99‰–6.21‰) values and exhibit affinities with I-type felsic rocks. They mainly originate from the partial melting of the basement of the Bainaimiao arc belt. Our compiled geochemical and geochronological data indicate that there were significant differences between the Early to Middle Permian Elitu Formation volcanic rocks and intrusive counterparts and the Early to Middle Triassic magmatic rocks from the Bainaimiao arc belt, and the Late Permian was the key tectonic transition period. The Late Permian transition was compatible with the regional crustal-shortening deformation, greenschist facies metamorphism and shift of deposition environment. This study argues that the Elitu Formation volcanism should be subduction-related and predate the final suture of the Palaeo-Asian Ocean domain at the Changhsingian. The suture of the Palaeo-Asian Ocean domain was not the original trigger of the Kungurian climate change in the North China Craton.

From Triassic metamorphism to Early Cretaceous anatexis in the Dabie orogen, central China: Constraints from in-situ U-Pb age and Hf and O isotopes of zircon from migmatites

The Dabie orogen underwent a ‘complete’ orogenic cycle from Triassic collisional orogeny to Early Cretaceous tectonic collapse. Zircons of the migmatite in the North Dabie terrane recorded the processes from Triassic granulite-facies metamorphism to Early Cretaceous anatexis. Three types of zircon are recognized in the migmatite: the Neoproterozoic inherited magmatic zircon, the Triassic metamorphic zircon, and the Late Jurassic–Early Cretaceous anatectic zircon. The Neoproterozoic inherited magmatic zircons suggest the protolith of the migmatite has a tectonic affinity with the Yangtze Craton (YC). The Triassic metamorphic zircon domains have variable εHf(t) values from −4.1 to 3.6 and low δ18O compositions ranging from −4.36‰ to −0.30‰. The anatectic zircon domains have clearly negative εHf(t) values of −19.8 to 0.8 (mostly less than −10.0) and relatively high δ18O values from −4.24‰ to 3.49‰. Based on the chronological and isotopic data of migmatite and the results of previous studies, the Triassic granulite-facies metamorphism was closely related to the asthenosphere upwelling after the slab break-off during the orogenic stage, and the Early Cretaceous anatexis respond to widely heat event during the collapse stage of orogen.

Geochemical diversity of continental arc basaltic mushy reservoirs driven by reactive melt infiltration

The reactive melt flow emerges as an important factor for diversification of basaltic magmatic reservoirs, but whether and how it influences continental arc basaltic mushes are enigmatic. Here, we used mineral and whole-rock geochemistry to examine the petrogenesis of a suit of mafic and intermediate plutons in western Yangtze Block, which were emplaced at continental arc crust and primarily had plagioclase and clinopyroxene as early cumulate mineral phases. We found the crystal mushes were infiltrated by externally-derived reactive melt with high δ18O and fertile crustal signatures, resulting in the changes of mineral phases (e.g., clinopyroxene transformed to hornblende) and bulk-rock geochemistry (including isotopes). Then, the reacted granitic melt was prone to either be extracted from or stall in the crystal mushes, generating quartz-poor (mafic) or quartz-rich (intermediate) plutons, respectively. This study supports the reactive melt infiltration may serve as an important engine for compositional diversity of basaltic mush system within continental arc settings.

Isotopic signatures of snow, sea ice, and surface seawater in the central Arctic Ocean during the MOSAiC expedition

The Arctic Ocean is an exceptional environment where hydrosphere, cryosphere, and atmosphere are closely interconnected. Changes in sea-ice extent and thickness affect ocean currents, as well as moisture and heat exchange with the atmosphere. Energy and water fluxes impact the formation and melting of sea ice and snow cover. Here, we present a comprehensive statistical analysis of the stable water isotopes of various hydrological components in the central Arctic obtained during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition in 2019–2020, including the understudied Arctic winter. Our dataset comprises >2200 water, snow, and ice samples. Snow had the most depleted and variable isotopic composition, with δ18O (–16.3‰) increasing consistently from surface (–22.5‰) to bottom (–9.7‰) of the snowpack, suggesting that snow metamorphism and wind-induced transport may overprint the original precipitation isotope values. In the Arctic Ocean, isotopes also help to distinguish between different sea-ice types, and whether there is a meteoric contribution. The isotopic composition and salinity of surface seawater indicated relative contributions from different freshwater sources: lower δ18O (approximately –3.0‰) and salinities were observed near the eastern Siberian shelves and towards the center of the Transpolar Drift due to river discharge. Higher δ18O (approximately –1.5‰) and salinities were associated with an Atlantic source when the RV Polarstern crossed the Gakkel Ridge into the Nansen Basin. These changes were driven mainly by the shifts within the Transpolar Drift that carried the Polarstern across the Arctic Ocean. Our isotopic analysis highlights the importance of investigating isotope fractionation effects, for example, during sea-ice formation and melting. A systematic full-year sampling for water isotopes from different components strengthens our understanding of the Arctic water cycle and provides crucial insights into the interaction between atmosphere, sea ice, and ocean and their spatio-temporal variations during MOSAiC.

Mid‐Holocene rainfall seasonality and ENSO dynamics over the south‐western Pacific

AbstractEl Niño–Southern Oscillation dynamics affect global weather patterns, with regionally diverse hydrological responses posing critical societal challenges. The lack of seasonally resolved hydrological proxy reconstructions beyond the observational era limits our understanding of boundary conditions that drive and/or adjust El Niño–Southern Oscillation variability. Detailed reconstructions of past El Niño–Southern Oscillation dynamics can help modelling efforts, highlight impacts on disparate ecosystems and link to extreme events that affect populations from the tropics to high latitudes. Here, mid‐Holocene El Niño–Southern Oscillation and hydrological changes are reconstructed in the south‐west Pacific using a stalagmite from Niue Island, which represents the period 6.4–5.4 ka BP. Stable oxygen and carbon isotope ratios, trace elements and greyscale data from a U/Th‐dated and layer counted stalagmite profile are combined to infer changes in local hydrology at sub‐annual to multi‐decadal timescales. Principal component analysis reveals seasonal‐scale hydrological changes expressed as variations in stalagmite growth patterns and geochemical characteristics. Higher levels of host rock‐derived elements (Sr/Ca and U/Ca) and higher δ18O and δ13C values are observed in dark, dense calcite laminae deposited during the dry season, whereas during the wet season, higher concentrations of soil‐derived elements (Zn/Ca and Mn/Ca) and lower δ18O and δ13C values are recorded in pale, porous calcite laminae. The multi‐proxy record from Niue shows seasonal cycles associated with hydrological changes controlled by the positioning and strength of the South Pacific Convergence Zone. Wavelet analysis of the greyscale record reveals that El Niño–Southern Oscillation was continuously active during the mid‐Holocene, with two weaker intervals at 6–5.9 and 5.6–5.5 ka BP. El Niño–Southern Oscillation especially affects dry season rainfall dynamics, with increased cyclone activity that reduces hydrological seasonality during El Niño years.

Calcareous Nannofossils and Paleoclimatic Evolution Across the Eocene‐Oligocene Transition at IODP Site U1509, Tasman Sea, Southwest Pacific Ocean

AbstractThe Eocene‐Oligocene transition (EOT; ∼34 Ma) was one of the most prominent global cooling events of the Cenozoic, coincident with the emergence of continental‐scale ice‐sheets on Antarctica. Calcareous nannoplankton experienced significant assemblage turnover at a time of long‐term surface ocean cooling and trophic conditions, suggesting cause‐effect relationships between Antarctic glaciation, broader climate changes, and the response of phytoplankton communities. To better evaluate the timing and nature of these relationships, we generated calcareous nannofossil and geochemical data sets (δ18O, δ13C and %CaCO3) over a ∼5 Myr stratigraphic interval recovered across the EOT from IODP Site U1509 in the Tasman Sea, South Pacific Ocean. Based on trends observed in the calcareous nannofossil assemblages, there was an overall decline of warm‐oligotrophic communities, with a shift toward taxa better adapted to cooler more eutrophic conditions. Assemblage changes indicate four distinct phases caused by temperature decrease and variations in paleocurrents: late Eocene warm‐oligotrophic phase, precursor diversity‐decrease phase, early Oligocene cold‐eutrophic phase, and a steady‐state cosmopolitan phase. The most prominent shift in the assemblages occurred during the ∼550 kyr‐long precursor diversity‐decrease phase, which has relatively high bulk δ18O and %CaCO3 values, and predates the phase of maximum glacial expansion (Earliest Oligocene Glacial Maximum–EOGM).

Petrogenesis of the late Cenozoic potassium-rich volcanism in the NW Tibetan Plateau: Constraints on the mantle evolution through post-orogenic events

The late Cenozoic Heishi Beihu volcanic rocks represent potassium-rich magmatic events at the northwesternmost margin of the Tibetan Plateau following the Indian–Asian continental collision. The petrogenesis of this juvenile igneous activity plays a crucial role in revealing the geochemical features of the deep mantle and contributes to our understanding of orogenic geodynamic evolution. This study, identified three eruption episodes at Heishi Beihu volcano (7.1–6.2, 4.3–1.4, and 1.3–1.0 Ma) using SHRIMP zircon U-Pb dating. Geochemically, the three episodes are characterized by shoshonitic affinities (K2O = 2.96–4.36 wt%) and crust-like trace-element signatures (negative Ta, Nb, and Ti anomalies), with enrichment in light rare earth (LREE) and large-ion lithophile elements (LILE), depletions in high-field-strength elements (HFSE), and small negative Eu anomalies (0.74–0.86). Their enriched whole-rock Sr ([87Sr/86Sr]i = 0.7072–0.7090), Nd (εNd(t) = −6.91 to −6.24), and zircon Hf (εHf(t) –5.64 to −9.13) isotopic compositions, and high zircon δ18O values (+7.4‰ to +10.6‰), are indicative of an enriched and heterogeneous mantle source (EMII-type) that contained recycled continental crust. The Heishi Beihu volcanic rocks were derived by low-degree (2%) partial melting of a mantle source with dominant pyroxenite and minor peridotite under conditions of the garnet-eclogite-facies. Results obtained from clinopyroxene–liquid geothermobarometry are indicative of diverse magma reservoirs, ranging from deep crustal levels (41–56 km) during Episode I to shallower depths for Episodes II (26–35 km) and III (17–29 km). In combination with the geological development of the area, we infer that magmatism at Heishi Beihu was genetically related to asthenospheric upwelling and induced by underthrusting of the Indian lithosphere beneath Western Kunlun.

Petrogenesis and Tectonic Implication of Jurassic Granites in Central Guangdong, SE China: Constraints from Zircon U-Pb-Hf-O and Whole-Rock Geochemical and Sr-Nd Isotopic Data

Abstract The origin and tectonic regime responsible for the inland Jurassic granites in Southeast (SE) China remain controversial. This study presents zircon secondary ion mass spectrometry (SIMS) U-Pb ages, in situ zircon Hf-O isotopes, and whole-rock geochemical and Sr-Nd isotopic data for the Fogang and Xinxing Batholiths in central Guangdong. Mineralogical and geochemical features indicate that these granites are high-K (>4.8 wt% K2O at 72 wt% SiO2), calc-alkaline I-type granites. SIMS U-Pb analyses on magmatic zircons yield consistent ages ranging from 158 to 163 Ma, suggesting that the Fogang and Xinxing granites were emplaced in the period of 163–158 Ma. In addition, these granites have whole-rock initial Sr87/Sr86 ratios of 0.6802–0.7072 and negative εNd(t) values of −9.5 to −8.2, zircon negative εHf(t) values of −12.34 to −0.56, and high δ18O values of 7.64‰–10.08‰. The above features imply that the granites were most likely generated through the mixture of supracrustal sedimentary components with minor addition of mantle-derived magmas. Granites from the Fogang and Xinxing Batholiths in SE China should be derived from the Proterozoic crustal reworking due to asthenosphere upwelling or underplating and intrusion of mafic magmas. These Jurassic granites reflect anorogenic magmatism probably formed in an intraplate extensional setting resulted from the foundering of the flat slab beneath SE China.

Transition from non-adakitic rocks to adakitic rocks in the southern Lhasa subterrane: Implications for progressive crustal thickening and tectonic evolution during the Early Cenozoic

Continental collision belts are commonly characterized by substantial granitoid formations and significant crustal thickening. However, the relationship between the record of granitoid magmatism and the change of continental crust thickness remains equivocal. To better understand this complex relationship, this study integrated new zircon U-Pb-Hf-O isotopes, whole-rock elements and Sr-Nd-Pb isotopic data of the Early Eocene collision-related monzonite, quartz-monzonite and granite in the Namling area of the southern Lhasa subterrane, South Tibet, China. These magmatic rocks are divided into two distinct episodes with contrasting geochemical characteristics. The early-episode quartz-monzonites and granites with zircon U-Pb ages of 51.4–52.3 Ma have high SiO2 (64.7–72.31 wt%) and K2O (3.69–5.68 wt%), low MgO (0.29–1.68 wt%) and Sr (151–367 ppm) contents, resulting in low Sr/Y (4.79–15.52) and (La/Yb)N (6.52–11.08) ratios. Compared with the early-episode granitoids, the late-episode monzonites and quartz-monzonites yield younger zircon U-Pb ages of 47.5–47.7 Ma and display lower SiO2 (62.34–63.62 wt%) and K2O (3.09–4.02 wt%), higher MgO (2.11–2.40 wt%) and Sr (551–620 ppm) contents, and higher Sr/Y (29.75–57.84) and (La/Yb)N (8.49–13.50) ratios, indicative of adakitic rocks. Although the two episodes of granitoids show distinct major and trace element contents, they are both characterized by positive bulk-rock ƐNd(t) (+0.09 to + 1.95; +2.14 to + 2.63) and zircon ƐHf(t) (+3.8 to + 8.5; +8.3 to + 11.5), high Pb isotopic ratios (207Pb/204Pb = 15.624–15.673 and 15.636–15.682, 208Pb/204Pb = 38.753–39.112 and 38.810–39.113), as well as intermediate δ18O ratios (+5.94 to + 6.91 ‰; +5.01 to + 5.93 ‰). These geochemical features suggest that these Namling granitoids most likely originated from the Lhasa juvenile mafic lower crust. However, the early-episode quartz-monzonites and granites may also include some ancient Indian crustal materials that caused their low ƐNd(t) (0.09–1.95) and ƐHf(t) (3.8–8.5) values and high δ18O (5.94–6.91 ‰) isotopic values. Temporal changes in Sr/Y, (La/Yb)N, Dy/Yb, and Ho/Yb ratios of the Early Cenozoic granitoid rocks (SiO2 ≥ 56 wt%) in the southern Lhasa subterrane indicate that the continental crust underwent progressive thickening, which was caused by basaltic magmas underplating and Indian indentation. The formation of the two episodes of the Early Eocene granitoids was most likely triggered by the break-off of the subducted Neo-Tethyan oceanic slab.

Pulses of South Atlantic water into the tropical North Atlantic since 1825 from coral isotopes.

Decadal and multidecadal changes in the meridional overturning circulation may originate from either the subpolar North Atlantic or the Southern Hemisphere. New records of carbon and oxygen isotopes from an eastern Martinique Island (Lesser Antilles) coral reveal irregular, decadal, double-step events of low ∆14C and enhanced vertical mixing, high δ18O and high δ13C values starting in 1885. Comparison of the new and published ∆14C records indicates that the last event (1956-1969) coincides with a widespread, double-step ∆14C low of South Atlantic origin from 32°N to 18°S, associated with a major slowdown of the Caribbean Current transport between 1963 and 1969. This event and the past Martinique ∆14C lows are attributed to pulses of northward advection of low ∆14C Sub-Antarctic Mode Waters into the tropical Atlantic. They are coeval with changes of the tropical freshwater budget and likely driven by meridional overturning circulation changes since ~1880.

Constraints of boron and oxygen stable isotopes on dehydration fluids, sediment-derived melts, and crustal assimilation of the Toba volcanic system (Indonesia)

Abstract Arc magmas are produced from the mantle wedge, with possible addition of fluids and melts derived from serpentinites and sediments in the subducting slab. Identification of various sources and their relevant contributions to such magmas is challenging; in particular, at continental arcs where crustal assimilation may overprint initial geochemical signatures. This study presents oxygen isotopic compositions of zoned olivine grains from post-caldera basalts and boron contents and isotopes of these basalts and glassy melt inclusions hosted in quartz and clinopyroxene of silicic tuffs in the Toba volcanic system, Indonesia. High-magnesian (≥87 mol% Fo [forsterite]) cores of olivine in the basalts have δ18O values ranging from 5.12‰ to 6.14‰, indicating that the mantle source underneath Toba is variably enriched in 18O. Olivine with <87 mol% Fo has highly variable (4.8–7.2‰), but overall increased, δ18O values, interpreted to reflect assimilation of high δ18O crustal materials during fractional crystallization. Mass balance calculations constrain the overall volume of crustal assimilation for the basalts as ≤13%. The processes responsible for the 18O-enriched basaltic melts are further constrained by boron data that indicate the addition of <0.1 wt% fluids to the mantle, >40% of the fluids being derived from serpentinites and others from altered oceanic crust and sediments. This amount of fluids can increase δ18O of the magma by only ~0.02‰. Approximately 6–9% sediment-derived melt hybridization in the mantle wedge is further needed to yield basaltic melts with δ18O values in equilibrium with those of the high-Fo olivine cores. The cogenetic silicic tuffs, on the other hand, seem to record a higher proportion of fluid addition dominated by sediment-derived fluids to the mantle source, in addition to crustal assimilation. Our reconnaissance study therefore demonstrates the application of combined B and O isotopes to differentiate between melts and fluids derived from serpentinites and sediments in the subducted slab—an application that can be applied to arc magmas worldwide.

O–Hf isotope ratios of Alvand S-type granite, western Iran, reveal crustal melting in an extensional regime

There are a few intrusive bodies in the central Sanandaj-Sirjan Zone (C-SaSZ), which is parallel to the Zagros suture zone, in western Iran. The Alvand batholith in the Hamedan area is one of these bodies and consists of rock types with large variation, in particular, two-mica garnet-bearing granite with an S-type signature. Zircon UPb dating gives crystallization ages of 170–163 Ma for the S-type granite. The 176Hf/177Hf ratios in the zircon grains vary from 0.282001 to 0.282780 (rim), with low 176Lu/177Hf ratios (0.002 to 0.018). The εHf(t) values (rim) are clustered into two main groups: a negative εHf(t) group (−6.9 to 0.0, n = 39), with some highly negative εHf(t) values (−25.8 to −21.9, n = 3), and slightly positive εHf(t) values (+0.3 to +2.9, n = 14) of inherited grains. The δ18O values for the zircon grains are positive and range from +10.4‰ to +11.8‰. The high δ18O (positive) and mostly negative εHf(t) values are more consistent with continental crustal sources for these rocks, and the Hf model ages (TDM2) show a wide range from 2.7 to 0.8 Ga, suggesting the heterogeneity of the crustal components. The long incubation time (2089 to 544 Ma) indicates a long history of crustal residence and crustal reworking. The incubation age of the zircon grains, >300 Ma, confirms the recycling of crustal material by partial melting. The simultaneous occurrence of S-type granites with mafic bodies, such as gabbro and diorite members with tholeiitic signatures, suggests that the injection of mafic magma increased the geothermal gradient from the bottom and sides along the major faults, which was the trajectory for the injection of hot mafic melts to shallow depths. These processes abnormally increased the geothermal gradients locally, and the partial melting of crustal and supracrustal materials produced S-type granites and migmatization among the hot blade-shaped mafic bodies. Therefore, the partial melting of crustal material in an extensional tectonic regime and/or in a thin continental crust occurred in a continental rift and/or in the Neo-Tethys passive margin. The present work concludes that the passive margin can be suggested as the tectonic setting for the generation of S-type granites, without the collisional regime that has been widely considered for the sources of this type of granite in the world.

Apatite trace elements and O-Sr isotopes reveal different magmatic sources of Fe-Ti oxide deposits in the eastern Tianshan, NW China

Magmatic Fe-Ti oxide deposits are one of the most important iron deposit in eastern Tianshan, NW China, and their ore-forming mechanism remains obscure. Here, we use trace elements and O-Sr isotopes of apatite from two representative magmatic Fe-Ti oxide deposits (Niumaoquan and Weiya) in the eastern Tianshan to constrain the genesis of ore formation. The apatites from the Niumaoquan Fe-Ti oxide ores exhibit higher Eu/Eu* values, lower Mn contents, and larger REE variations than those from the Weiya Fe-Ti oxide ores, indicating a protracted evolution process of the Niumaoquan ore-forming magmas under a relatively reduced condition. Specifically, the apatites from the Niumaoquan Fe-Ti oxide ores can be divided into two groups: Group 1 with relatively low REE contents, low 87Sr/86Sr ratios (0.70574–0.70641) and high δ18O values (5.1–5.6‰), and Group 2 with relatively high REE contents, high 87Sr/86Sr ratios (0.70703–0.70792) and low δ18O values (2.8–3.3‰). The Group 1 apatites formed by early crystallization in the mantle, while the Group 2 apatites formed by assimilation of low δ18O-high 87Sr/86Sr altered rocks during magmas emplacement en route to the continental crust. Unlike the Niumaoquan apatites, the apatites from the Weiya Fe-Ti oxide ores have relatively homogeneous, high δ18O values (8.3‰-9.1‰) and 87Sr/86Sr ratios (0.70798–0.70848), indicating involvement of continental crustal materials in their mantle sources. Combined with previous studies, we suggest that the Niumaoquan and the Weiya Fe-Ti oxide deposits formed in a non-plume tectonic setting but they originated from different magmatic sources. Our study highlights that trace elements and O-Sr isotopes of apatite can be used as powerful tools to constrain the source characteristics and ore-forming processes of magmatic Fe-Ti oxide deposits.

Origins and evolution of two types of Late Triassic granitic magmas in the Caolong-Xiangkariwa area of central-eastern Songpan-Ganze terrane, northern Tibet: Implications for pegmatite lithium mineralization

Pegmatite-type lithium (Li) deposits are an important source of the low-carbon energy metal, which is generally considered to be formed by the high degrees of differentiation of strongly peraluminous granitic magma. However, the question of whether the parental rocks of these peraluminous magmas are igneous or sedimentary has been widely debated. Recent studies have identified a world-class pegmatite-type Li deposit belt (up to 2800 km long) closely associated with the Late Triassic granitoids and (meta)sedimentary rocks along the West Kunlun and Hoh Xil-Songpan-Ganze terranes in northern Tibet. This study presents a comprehensive petrological, geochronological, and geochemical study on the two types of Late Triassic granitoids in the Caolong-Xiangkariwa area of the Yushu region of central-eastern Hoh Xil-Songpan-Ganze terrane in northern Tibet: type A includes the Tongtianhe-Zhenqin-Zhaduo-Zhaya-Xiangkariwa (TZX) diorites and granodiorites (218−212 Ma), and type B includes the Caolong two-mica and muscovite granites (213−205 Ma), as well as ore-barren, beryl-bearing, and spodumene-bearing pegmatite dikes (209−196 Ma). The TZX diorites and granodiorites contain variable amounts of amphibolite and biotite. They are metaluminous to slightly peraluminous with variable SiO2 (54.4−71.3 wt%), MgO (0.72−7.26 wt%) contents, and Mg# values (40−70). Some samples with low SiO2 (54.4−58.5 wt%) contents have geochemical features similar to high-Mg andesites: e.g., high MgO contents (5.24−7.26 wt%) and Mg# values (61−70). The TZX diorites and granodiorites exhibit less enriched (86Sr/87Sr)i (0.7080−0.7118) and εNd(t) (−4.8 to −8.0). By contrast, the Caolong two-mica and muscovite granites are characterized by the occurrence of abundant muscovite (plus tourmaline and garnet) and are strongly peraluminous. The Caolong granites and pegmatites are geochemically characterized by high SiO2 contents (68.8−79.8 wt%) and low MgO contents (0.01−0.10 wt%) and Mg# values (4−23). They have εNd(t) values (−8.8 to −11.4; granites: −8.8 to −10.8; pegmatites: −9.2 to −11.4) that are more enriched than the TZX diorites and granodiorites but similar to those of Songpan-Ganze (meta)sedimentary rocks (−7.4 to −12.9), and the Caolong two-mica granites have high zircon δ18O values (11.6−12.1). In addition, decreasing K/Rb ratios correspond to increasing Cs contents in K-feldspar and muscovite from the Caolong granites and pegmatites. Taking into account the Late Triassic granitoids and (meta)sedimentary rocks in the Hoh Xil-Songpan-Ganze terrane, we suggest that the TZX diorites and granodiorites were most probably formed by the partial melting of metasomatized lithospheric mantle that subsequently underwent extensive fractional crystallization. Conversely, the Caolong two-mica and muscovite granites were likely generated purely by the partial melting of (meta)sedimentary rocks rather than via the evolution of dioritic-granodioritic magmas. In summary, there were two kinds of granitic magmas with different sources in the Caolong-Xiangkariwa area. Finally, the Caolong pegmatites were most likely formed by the extreme differentiation of two-mica granitic magmas rather than the dioritic-granodioritic magmas. Therefore, the evolution of magmas derived from (meta)sedimentary rocks were most likely to have formed the Li-rich pegmatites. Moreover, these (meta)sedimentary rocks representing the ultimate Li source must have undergone strong chemical weathering, resulting in significant Li enrichment.

The ca. 349 Ma metamorphic enrichment of the Tianhu metamorphosed sedimentary Fe deposit in the eastern Tianshan: Evidence from apatite trace elements, oxygen isotopes, and U-Pb chronology

Apatite is a common accessory mineral in iron oxide-apatite (IOA), magmatic Fe-Ti oxide, and metamorphosed sedimentary Fe deposits. Apatite chronology and oxygen isotopes in the IOA deposits have been extensively studied, however, such information in the metamorphosed sedimentary Fe deposits remains poorly understood. Here, we report detailed trace elements, oxygen isotopes, and U-Pb chronology of apatite from the Tianhu metamorphosed sedimentary Fe deposit in the eastern Tianshan, NW China. To our knowledge, this is the first report on the in situ oxygen isotopes of apatite from the metamorphosed sedimentary Fe deposits. Cathodoluminescence imaging of the apatite grains from the Tianhu massive Fe ores show a homogeneous texture, suggesting that they were formed by a single event rather than multiple superimposed events. These apatite grains contain abundant fluid inclusions, and show relatively high δ18O values of 7.9 ‰-14.0 ‰ and low Th and U contents, which are consistent with their metamorphic hydrothermal origin. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) dating of the studied apatite grains yielded a U-Pb age of 348.5 ± 18.5 Ma, indicating that the metamorphic enrichment of the Tianhu metamorphosed sedimentary Fe deposit occurred considerably later than the Ordovician ore-hosting rocks. Combined with previous studies, we suggest that the metamorphic massive ores of the Tianhu metamorphosed sedimentary Fe deposit formed in an orogenic setting. Our study highlights that a combination of trace elements, oxygen isotopes, and U-Pb chronology of apatite can be used as powerful tools to constrain the genesis of metamorphosed sedimentary Fe deposits.

Episodic water-fluxed anatexis recorded by migmatites from the Paleozoic Wuyi-Yunkai Orogeny in South China

The Wuyi-Yunkai Orogeny (WYO) in South China occurred during the assembly of Gondwana, and has abundant granites and migmatites which have been mostly interpreted as products of dehydration anatexis. However, the early Paleozoic migmatites in the Cathaysia Block, investigated in this study, instead document water-fluxed anatexis during the orogeny. The quartzo-feldspathic migmatites are moderately peraluminous, and have ca. 700 Ma and ca. 1800 Ma inherited zircon sub-populations, indicating that they were derived from Middle Neoproterozoic paragneisses. Titanite is common, while other anhydrous peritectic minerals such as garnet and pyroxene are absent in the migmatites and their leucosomes. Both mineral assemblages and microstructures indicate that the rocks were melted through reactions that include quartz + plagioclase + biotite + H2O = titanite + opaque + melt, K-feldspar + plagioclase + quartz + H2O = melt, and K-feldspar + biotite + quartz + H2O = melt. U-Pb ages of the newly-formed zircon and titanite display at least four age populations, recording episodic melting mainly at ca. 466, 437, 425 and 412 Ma, with evidence of local anatexis extending back to at least 480 Ma. Multiple stages of anatexis with short time intervals may have resulted from water-present melting due to episodic fluid influx. Zircon εHf(t) values are negative (-5 to -15) in the 700 Ma and 1800 Ma inherited zircons to nearly zero (0 to -5) in 470–400 Ma newly-formed zircons, and may have resulted from dissolution of the Lu-rich apatite during water-fluxed anatexis. The newly-formed zircons also have high δ18O values (9–12 ‰). These lines of evidence suggest that fluid flux-influenced melting was induced by metamorphic fluids that were released from polyphase metamorphism during crustal thickening and extension in the WYO, and channelized through regional shear zones. This highlights the extent and significance of water-fluxed anatexis and its role in the differentiation of the continental crust during orogeny.

Modification of Archean lower crust of the North China Craton by magma underplating during the Mesozoic: Evidence from zircon U-Pb-Hf-O isotopes of granulite xenoliths

Previous studies of felsic granulite and pyroxenite xenoliths have confirmed the existence of the Archean lower crust beneath the Fuxin region which experienced magma underplating in the Phanerozoic. U-Pb-Hf-O isotopes of zircons in felsic granulite xenoliths from the Fuxin late Cretaceous basalts in the North China Craton were analyzed to evaluate the role of magma underplating in the modification of the Archean lower crust during the Phanerozoic. The felsic granulites recorded a uniform upper intercept age of 2.5 Ga and a spread of concordant Mesozoic zircons from 177 to 105 Ma, with a peak at 118 Ma. Most Precambrian zircons show discordant U-Pb ages and obvious core-rim structure. The Mesozoic zircons from the granulite xenoliths can be divided into two groups: zircons with an age of >106 and those with an age of ≤106 Ma. Zircons with ages older than 106 Ma are characterized by an unzoned or core-rim structure and enriched Hf-O isotopic compositions, whereas zircons with ages of ≤106 Ma mainly exhibit oscillatory zoning and have depleted Hf-O isotopic compositions. Some >106 Ma zircons with strongly negative εHf(t) values of −40.4 to −24.9 are considered to be age resets of ancient zircons. The relatively higher Hf-O isotopic values for the rest of the >106 Ma zircons indicate that various proportions of underplated magmas with high εHf(t) and δ18O values contributed to their formation. The underplated magmas may have originated from the lithospheric mantle, metasomatized by melts/fluids from subducted Paleo-Asian and Paleo-Pacific oceanic crusts. The ≤106 Ma zircons probably crystallized in a melt from the depleted mantle. These observations suggest that long-lived magma underplating occurred beneath the Fuxin region during the Mesozoic, and the derivation of underplated magmas changed from an enriched lithospheric mantle to a depleted asthenospheric mantle at ∼106 Ma. Combined with our previous studies, we conclude that magma underplating not only provided heat for remelting, but also added exotic material in the modification of ancient lower crust beneath the Fuxin region during the Mesozoic. Furthermore, continuous modifications of cratonic lower crust by magma underplating weaken its refractory and made it more susceptible to chemical erosion and partial melting, which played an important role in the lower crust destruction of the craton.