Zircon δ18O Research Articles

Initiation of Mesoarchean nascent subduction zone in the North China Craton

Exploring the nature of Archean plate tectonics is crucial for understanding the geodynamic evolution of early Earth. While previous studies have focused on (ultra-)mafic metavolcanic and tonalite−trondhjemite−granodiorite (TTG) rocks, the crust-mantle interactions of Archean intermediate magmatism have received much less attention. Mesoarchean (ca. 2936−2855 Ma) dioritic and tonalitic-trondhjemitic magmatism occurred in the Jiaobei terrane, North China Craton. While low-silica (65.67−68.82 wt%) TTG magmatism persisted throughout the entire magmatic period, both the high-silica (69.79−74.54 wt%) TTG and biotite dioritic (MgO of 1.52−2.45 wt%) magmatism were transient (ca. 2922−2901 Ma). They were mainly sourced from metabasaltic crust, though the biotite dioritic and low-silica TTG rocks show variable mantle input. In comparison, the hornblende dioritic magmatism (MgO of 2.07−5.00 wt%) occurred later (ca. 2912−2865 Ma) and was derived from melt-metasomatized mantle sources. Spatially, the magmatism became younger from northeast to southwest. Combined with increasing crustal melting pressures after ca. 2920 Ma and mildly elevated zircon δ18O (up to +6.8‰) of TTG rocks, a Mesoarchean nascent subduction zone is proposed for the Jiaobei terrane that is featured by the recycling of supracrustal basaltic rocks to thickened lower crust (forming high-pressure and high-δ18O TTG) and subsequent melt-related mantle metasomatism (forming relatively high-Mg hornblende dioritic rocks). The recycled supracrustal materials may have lubricated the basal part of the thickened crust and then triggered the opening of a nascent mantle wedge. Further compilation of data from global Archean dioritic and TTG rocks reveals chemical discontinuities at ca. 3.6−3.5 Ga and after ca. 3.0 Ga; these periods are marked by the occurrence of both high-Mg (median MgO >3 wt%) dioritic magmatism and high-(La/Yb)N (median >30) and high-δ18O (median >5.9‰) TTG magmatism. These data, along with independent geological evidence, suggest that the nascent subduction processes may have initiated locally at ca. 3.6−3.5 Ga but globally after ca. 3.0 Ga.

Reconstructing the evolution of an ancient continental arc: Implications from the temporally-spatially related shoshonitic to calc-alkaline intrusive magmatism in the North China Craton

Coeval shoshonitic and calc-alkaline to high-K calc-alkaline rock series were discovered in the eastern North China Craton (NCC). Here, we report zircon and titanite UPb dating, zircon oxygen isotopic, whole-rock major and trace element and SmNd isotopic data for these rocks. Zircon and titanite UPb geochronology suggests that the coeval shoshonitic to calc-alkaline mantle-derived magmas from the Jinan terrane were emplaced at ∼2.5 Ga and recorded multi-phases of metamorphic imprints. The shoshonitic series in Group I show higher K2O, but in contrast, the calc-alkaline to high-K calc-alkaline series (Group II) are characterized by higher Na2O. The two groups have enriched light rare earth elements (LREE) and depleted/nearly flat heavy rare earth elements (HREE). They also show negative anomalies in some HFSE and positive anomalies in LILE. Therefore, the geochemical data, SmNd and O isotopes included, show that the Jinan shoshonitic to calc-alkaline magmas were generated by melting of the metasomatized mantle at an ancient continental arc. Distinct geochemical features possibly suggest different mantle source features (metasomatic agents). The elevated zircon δ18O values imply that the weathering was significant during the end of Archean. In addition, the crustal reworking or intra-crustal recycling process played an important role in the continental crust evolution. The low-δ18O magmas documented here possibly indicate high-temperature water-rock interaction that occurs between continental crust and meteoric water. This study represents an example of co-existed shoshonitic and calc-alkaline mantle-derived magma derived from the melting of metasomatized mantle above an ancient subduction zone. Similar coeval associations of shoshonitic to calc-alkaline mantle-derived rocks, in combination with high-P granulites and crust-derived granites, may be used to constrain the ancient subduction-collisional belts.

Secular change in the nature of mantle and tectonic evolution of northwestern margin of the Yangtze Block during Neoproterozoic: Constraints from the mafic intrusions and associated granitoids of the Hannan and Xiaomoling complexes

To reveal the Neoproterozoic tectonic evolution of the northwestern margin of the Yangtze Block, a petrological and geochemical study was carried out for the mafic intrusions and associated granitoids from the Hannan and Xiaomoling Complexes. The ca. 900–880 Ma Xiaomoling diorite and granodiorite have high K2O contents (2.29–3.57 wt%) and belong to the high-K calc-alkaline series. They display depleted whole-rock Sr–Nd–Hf isotope compositions (0.70151–0.70398, +0.6 to +2.5 and +4.4 to +11.2, respectively) and positive zircon εHf(t) (+3.6 to +9.9) with slightly elevated bulk zircon δ18O values (6.4–6.8 ‰) and minor Paleoproterozoic (∼2.2 Ga) inherited zircons, indicative of derivation from remelting of juvenile mafic crust with crustal contamination in a continental arc setting. The ca. 866 Ma Hannan gabbro-diorites show enrichments in LILEs and LREEs and depletions in HFSEs and display depleted Sr–Nd–Hf isotope compositions (0.70267, +5.4 and +17.8, respectively), positive zircon εHf(t) values (+8.6 to +12.0) and mantle-like zircon δ18O values (5.0–6.2 ‰), suggesting that they were generated through flux melting of a depleted mantle wedge. The ca. 850–840 Ma Xiaomoling hornblende gabbro and diabase display depleted Sr–Nd–Hf isotope compositions (0.70279–0.70288, +2.7 to +3.2 and +9.8 to +10.5, respectively) and are characterized by high Nb/U ratios and less enrichment in LREEs and LILEs, indicating that they were formed by decompression melting of a depleted mantle wedge in a back-arc setting. However, the coeval high-Mg diorite displays enriched whole-rock Sr–Nd–Hf isotopes (0.70477–0.70701, −4.0 to −6.0 and −7.2 to −7.4, respectively), negative zircon εHf(t) values (−1.6 to −9.2) and elevated zircon δ18O values (5.3–7.5 ‰), reflecting that the mantle-derived melts have underwent various degrees of crustal contamination. The ca. 780 Ma Hannan gabbro is characterized by earlier crystallization of plagioclase than clinopyroxene, implying that their parental magma is anhydrous. Additionally, the relatively shallow emplacement depth and tholeiitic affinities imply that they were generated by mantle decompression melting probably associated with a continental rift environment in response to Rodinia breakup. Taken together, the secular changes in the geochemical and isotopic compositions of these mafic rocks in the northwestern margin of the Yangtze Block record the evolution from a continental arc at ca. 900–880 Ma to continental rifting at ca. 780 Ma.

Low-δ18O and negative-Δ199Hg felsic igneous rocks in NE China: Implications for Early Cretaceous orogenic thinning

Magmatic rocks carry key information to understand the processes of material cycling in the Earth's interior and record crucial clues to reconstruct the important tectonic evolution of the Earth. Therefore, it is of great importance to constrain the magma source (such as origin from crust or mantle, oceanic crust or continental crust, lower or upper continental crust) accurately, which however remains to be unraveled because of the variety magmatic sources and complex magmatic processes. High-precision stable isotopic analyses provide new tools to decipher the genesis of igneous rocks and to reconstruct their associated tectonic settings. Here, we report a pioneering work using integrated O-Hg-Hf-Nd isotopes to understand the genesis of the Early Cretaceous A-type granites and rhyolites from the Paleozoic-Mesozoic accretionary orogen in NE China. Low δ18O zircon values (3.2–5.7‰) of the A-type rocks reveal a magma source affected by meteoric fluids. Their whole-rock negative Δ199Hg values (−0.24 to −0.01‰), which are similar with the terrestrial systems but clearly different from the marine systems, indicate that hydrothermal alteration took place in the continental setting. Positive zircon εHf(t) (4.2–10.7) and mainly positive whole-rock εNd(t) (−0.5 to 2.4, mean = 1.2 ± 1.0, 1SD) values indicate that the magma source was dominated by juvenile crustal materials. This study suggests an elevated geothermal gradient due to the lithospheric thinning triggered by rollback and foundering of a Paleo-oceanic slab. The specific thermal anomaly caused the remelting of the altered upper continental crust, resulting in the formation of the A-type granites and rhyolites, with the characters of low-δ18O and negative-Δ199Hg values. Our study shows that Hg isotope, coupled with O and NdHf isotopes, is a promising tool for petrogenetic studies.

First identification of Early Cretaceous mafic dikes in the Baingoin area, central Tibet: Implications for crust-mantle interactions and magmatic flare-up

Mafic dikes are generally emplaced in extensional tectonic settings and provide key information regarding deep mantle processes and sources. The Bangong-Nujiang suture zone was formed by the collision of the Qiangtang and Lhasa terranes and experienced intense magmatism during the Early Cretaceous. However, the deep mantle processes and mechanisms involved in this magmatic flare-up (ca. 115 Ma) in the collisional belt remain controversial because of the lack of evidence for coeval mafic magmatism. Here, we present detailed petrological, geochronological, geochemical, and Sr-Nd-Hf-O isotope data for the newly discovered hypersthene-bearing mafic dikes in the Baingoin area in the middle-eastern parts of the Bangong-Nujiang suture zone, central Tibet. Secondary ion mass spectroscopy (SIMS) zircon U-Pb dating showed that the mafic dikes were emplaced during 120−115 Ma. These mafic rocks are characterized by variable MgO contents (2.7−5.2 wt%) and Mg# values (38.5−52.8), slight enrichment in light rare earth elements (REEs; [La/Yb]N = 7.5−8.1), relatively flat heavy REE patterns ([Gd/Yb]N = 1.75−1.84), and negative Eu, Ta, Nb, and Ti anomalies. The dikes also have relatively low initial 87Sr/86Sr ratios (0.7060−0.7062) and negative εNd(t) (−2.2 to −1.6) and positive εHf(t) (+2.5 to +3.6) values, and variable zircon εHf(t) (−2.2 to +7.2) and slightly elevated zircon δ18O (5.6‰−7.0‰) values. These geochemical characteristics indicate that the mafic dikes were derived from an enriched lithospheric mantle source. However, compared with coeval magmatic rocks, the mafic dikes have relatively high εNd(t) and εHf(t) values, indicating that they contain a depleted mantle component. The mafic dikes contain clinopyroxene and orthopyroxene (i.e., hypersthene), indicative of derivation from a high-temperature magma source. Clinopyroxene-melt thermobarometry yielded a temperature range of 1167−1213 °C, further supporting the involvement of a high-temperature asthenospheric component. Therefore, we suggest that the parental magmas of the Nakoulai mafic dikes were probably generated by the interaction between the asthenospheric mantle and overlying metasomatized lithospheric mantle. Combined with data from nearby Cretaceous magmatic rocks and sedimentary rocks, we suggest that the mafic dikes were generated in a postcollisional setting caused by upwelling of asthenospheric mantle owing to slab breakoff beneath the Bangong-Nujiang suture zone. Slab breakoff played a key role in the crust-mantle interactions and the onset of the magmatic flare-up in the middle-eastern parts of the Bangong-Nujiang suture zone.

Magma-mixing origin for the Early Jurassic intrusive rocks in the eastern Songnen–Zhangguangcai Range Massif, NE China: Evidence from geochronology, geochemistry and Hf–O–Sr–Nd isotopes

Granites containing abundant dioritic enclaves are widespread in the Songnen–Zhangguangcai Range Massif (SZRM). However, the role of mantle- and crust-derived magmas in the petrogenesis of the dioritic enclaves is debated. We present new zircon U–Pb–Hf–O isotopic compositions, whole-rock geochemistry, mineral geochemistry, and Sr–Nd isotopic compositions of hornblendites and dioritic enclaves and their host granites to elucidate their origins and investigate crust–mantle hybridization. Zircon U–Pb dating suggests that these intrusive rocks were emplaced during the Early Jurassic (197–186 Ma). The geochemical characteristics of hornblendites are similar to those of the Early Jurassic mafic–ultramafic rocks in the Lesser Xing’an–Zhangguangcai Range. They yield low (87Sr/86Sr)i ratios (0.704266–0.704324), high εNd(t) values (3.87–3.91), and are enriched in Ba, Sr, and K and depleted in Nb, Ta, and Ti, suggesting that they were derived from the partial melting of mantle wedge metasomatized by sediment melt. The host granites have high SiO2 (>65.12 wt%) and low MgO (<1.92 wt%) and Al2O3 (<16.23 wt%) contents and are to metaluminous–peraluminous high-K calc-alkaline rocks. In addition, they yield positive zircon εHf(t) values (3.42–7.58), zircon δ18O values (4.87‰–5.85‰) and whole-rock εNd(t) values (0.10–0.44), suggesting that the granitic magma originated from the partial melting of continental crust with the involvement of mantle-derived melts. The field and petrographic observations and geochemical analyses indicate that the enclaves were derived from magma mixing and mingling. The dioritic enclaves contain K-feldspar megacrysts and back-veins, and plagioclase crystals within them are reversely zoned and have ocellar textures. The weighted mean ages of the host granites (186 ± 3 and 188 ± 2.3 Ma) are similar to those of the dioritic enclaves (186 ± 1.8 and 188 ± 1.1 Ma). The major element contents of the intrusive rock are linearly correlated with the SiO2 contents, and the whole-rock geochemical and isotopic characteristics of the hornblendites and host granites show that they are the mafic and felsic end-members, respectively, involved in the magma mixing. Therefore, we propose that underplating by mantle-derived magma likely triggered partial melting of the continental crust to produce felsic magma, and the dioritic enclaves were formed by the mixing and mingling of mantle- and crust-derived magma alongside melting, assimilation, storage, and homogenization. This suggest that the Mudanjiang Oceanic crust was subducted westward beneath the SZRM during the Early Jurassic.

Niobium-enriched basalts: Partial melting of a sediment-metasomatised mantle source in subduction zones?

Crust–mantle interaction and arc magmatism in subduction zones record the global recycling of elements from Earth's surface to its interior. Nb-enriched basalts are specific to arc settings and are generated through the partial melting of heterogeneous mantle sources or mantle wedge metasomatised by subducted slab-derived melts. However, it is unclear what role subducted sediments play in the formation of Nb-enriched basalts. We present the first report of Cambrian Nb-enriched basalts at A'qiang and Pulu in the Western Kunlun Orogen (NW China), which might have formed above a subducted ridge-related slab window. The A'qiang basalts (Nb = 6.7–7.7 ppm) exhibit E-MORB-like geochemical characteristics, with positive Nb and Ta anomalies, high Mo/Ce ratios (∼0.02), and positive εNd(t) (4.9–5.0) and low δ98/95Mo (−0.74‰ to −0.21‰) values. These basalts were generated through the partial melting of sub-slab asthenosphere modified by minor altered ocean crust and/or marly sediments. In contrast, the Pulu basalts (Nb = 10.9–13.7 ppm) are characterised by enrichment in light rare earth elements (LREEs), high Th/Ce (>0.1) and low Mo/Ce (∼0.001) ratios, negative εNd(t) (−1.5 to −1.1) and mantle-like zircon δ18O values (4.25‰–5.69‰), and high δ98/95Mo values (−0.03‰ to +1.17‰). The Pulu Nb-enriched basalts were derived from the partial melting of sub-slab asthenosphere modified by moderate siliciclastic sediments. Therefore, we propose that subducted sediments play a vital role in the transport of high field strength elements back into the mantle and their recycling into the crust, for example through the formation of Nb-enriched basalts.

Evolution of the preserved European continental crust, constrained by U-Pb, O and Hf isotopic analyses of river detrital zircons

Integrated U-Pb, O and Lu-Hf isotopic analyses of detrital zircons, collected from fourteen European river sands and one beach sand from northern Germany, were carried out to study the evolution of the preserved European continental crust. The drainage area of these sand samples covers large areas in eastern and western Europe, which evolved independently until their amalgamation during the Paleozoic. U-Pb ages of detrital zircons from eastern and western Europe generally match the ages of the major magmatic events in the two tectonic regions. The combined histogram of 2403 zircon U-Pb ages, obtained for this study, shows five major periods of zircon crystallisation: 2.80–2.65, 1.95–1.35, 1.25–0.85, 0.65–0.40, and 0.40–0.25 Ga, which correspond broadly with the timing of supercontinent assembly. The correlation could be due to enhanced granitoid magmatism and zircon growth during the supercontinent amalgamation, or the high preservation potential of igneous rocks generated during continental collisions. The O and Hf isotopic data of the analysed European river zircons show wide spreads in δ18O and εHf(t) values at ca. 1.9 Ga and 650–250 Ma, contrasting with the limited range for the Mesoproterozoic zircons. This indicates that extensive reworking of pre-existing crustal rocks occurred during the late Paleoproterozoic and Paleozoic in the evolution of the European continental crust, whereas the formation of new crust played an important role during the Mesoproterozoic.Arc mantle Hf model ages for the detrital zircons were calculated to estimate when European juvenile crust was extracted from the mantle. Model age uncertainties were estimated using the bootstrap method, considering three sources of uncertainty: (i) uncertainty in the Hf isotopic composition of the mantle reservoir, (ii) uncertainty in the 176Lu/177Hf for the crustal source region, and (iii) analytical uncertainties. Two types of model ages are calculated based on constant crustal 176Lu/177Hf (TAMC), and on variable176Lu/177Hf (TAMV), the latter being constrained by zircon δ18O values. Histograms of the calculated TAMC and TAMV model ages both show major juvenile extraction events between the late Paleo- and Mesoproterozoic. Although the principal periods in the European crustal growth contrast with those identified in the African, Russian and Mississippi river basins, the overall growth curves for the four continents show comparable results: (i) the formation of little or no continental crust before 4.0 Ga, and (ii) the formation of 50 % of the present crustal volume by ca. 2.0–1.6 Ga.

Quantitative record of the Neoarchean water cycle from a 2.67 Ga magmatic-hydrothermal system, Fennoscandian Shield

Given the scarcity of reliable paleoclimate record, the surface temperatures of the first half of Earth’s history remain poorly constrained. Here we show how the climate-sensitive δ18O value of surface precipitation recorded in Archean igneous and hydrothermal formations can help to resolve the state of early Earth climate. The Keivy complex, Kola craton (Fennoscandian Shield), formed via the intrusion of granitic and mafic magmas in the shallow crust at 2.67 Ga, where circulation of meteoric water created a distinct archive of the contemporaneous water cycle. Using whole rock data, mineral separates, and in situ zircon δ18O measurements, we disentangle the reaction mechanisms between the shallow magma and local precipitation. Syn-emplacement hydrothermal alteration produced near-contact lithologies with δ18O values as low as –8‰ recorded in amphiboles, while igneous zircons from granites crystallized from melts with δ18O from +1‰ to +4.5‰. High-precision U-Pb geochronology constrains the granite intrusion at 2673.5 ± 0.3 Ma. Using the Δ17O approach, these rocks reveal that the precipitation had a δ18O value 18‰ lower than the hydrosphere, providing one of the earliest quantitative records of continental precipitation generally compatible with a cold climate at high latitudes.

Quantitative record of the Neoarchean water cycle from a 2.67 Ga magmatic-hydrothermal system, Fennoscandian Shield

Abstract Given the scarcity of reliable paleoclimate record, the surface temperatures of the first half of Earth’s history remain poorly constrained. Here we show how the climate-sensitive δ18O value of surface precipitation recorded in Archean igneous and hydrothermal formations can help to resolve the state of early Earth climate. The Keivy complex, Kola craton (Fennoscandian Shield), formed via the intrusion of granitic and mafic magmas in the shallow crust at 2.67 Ga, where circulation of meteoric water created a distinct archive of the contemporaneous water cycle. Using whole rock data, mineral separates, and in situ zircon δ18O measurements, we disentangle the reaction mechanisms between the shallow magma and local precipitation. Syn-emplacement hydrothermal alteration produced near-contact lithologies with δ18O values as low as −8‰ recorded in amphiboles, while igneous zircons from granites crystallized from melts with δ18O from +1‰ to +4.5‰. High-precision U-Pb geochronology constrains the granite intrusion at 2673.5 ± 0.3 Ma. Using the Δ17O approach, these rocks reveal that the precipitation had a δ18O value 18‰ lower than the hydrosphere, providing one of the earliest quantitative records of continental precipitation generally compatible with a cold climate at high latitudes.

Petrogenesis of Early Mesozoic volcanic rocks in southeastern NE China: Geochemical and Sr–Nd–Pb–O isotopic evidences

This paper reports a study of whole-rock geochemical, Sr–Nd–Pb isotopic compositions and zircon UPb ages and O isotopic compositions of Late Triassic to Early Jurassic volcanic rocks of the Yanbian–Dongning area, northeastern China. The obtained data are used to constrain the petrogenesis of these rocks and to provide new insights into the timing of transition from the Paleo-Asian to the Paleo-Pacific tectonic regime. The studied Late Triassic to Early Jurassic volcanic rocks can be divided into two periods: Late Triassic–early Early Jurassic (212–198 Ma) and late Early Jurassic (ca. 181 Ma). The former rocks display high SiO2 and total alkalis contents, high Ga/Al ratios, depleted SrNd and highly radiogenic Pb isotopic signatures, and low zircon δ18O values. The former rocks have a genetic relationship with coeval intrusions, and their primitive melt was derived from juvenile crustal materials and underwent subsequent differentiation and assimilation of low-δ18O rocks. In contrast, the latter rocks have SiO2 contents of 49.00–63.30 wt%, are classified as calc-alkaline, and display arc-type trace-element features. Their geochemical and Sr–Nd–Pb–O isotopic compositions imply derivation from the partial melting of depleted mantle wedge metasomatized by oceanic slab-derived fluid and sediment melt. Combining these new results with published data, the Late Triassic–early Early Jurassic volcanic rocks are inferred to have formed in a rift-like extensional setting, whereas the late Early Jurassic volcanic rocks were generated in a subduction-related setting. The transition from the Paleo-Asian to the Paleo-Pacific tectonic regime occurred during the Early Jurassic.

Emplacement and evolution of zoned plutons: Multiproxy isotopic and geochemical evidence from the peraluminous Laojunshan leucogranite suite, southwestern China, and implications on the regional geodynamic and metallogenic history

The petrogenesis and geodynamic setting of peraluminous leucogranites are important topics for reconstructing regional geodynamic and metallogenic models. Here we present the results of an integrated study of zircon U-Pb ages and Hf-O isotopic compositions, monazite U-Th-Pb-Nd isotopes, tourmaline boron isotope, and bulk-rock elemental and Sr-Nd isotope geochemistry of an unusual suite of leucogranite, the Laojunshan leucogranite (LJSL), from the southwestern margin of the Yangtze Block in southwest China. The LJSL samples are strongly peraluminous with their zircon δ18O (8.8–10.2 ‰) and tourmaline δ11B (−14.32 to −12.75 ‰) values suggesting S-type origin. The LJSL rocks show high SiO2, Al2O3, K2O and Rb/Sr, low P2O5, CaO, FeOt, MgO, low REE abundances with moderate LREE enrichment and strong negative Eu anomalies, high (87Sr/86Sr)i (0.7127–0.7421) and low εNd(t) (−10.36 to −12.67). These features suggest protracted fractional crystallization from S-type parental granitoid magmas with Ti-in-zircon thermometry giving a mean temperature of ∼760 °C. The bulk-rock Sr-Nd, monazite Nd and zircon Hf-O isotope data suggest that the S-type magmas parental to the LJSL formed through partial melting involving muscovite-breakdown in the Paleoproterozoic middle-upper crustal metasedimentary rocks. Zircon and monazite U-(Th)-Pb dating reveals three intrusive phases for the LJSL (91–89 Ma, 87–85 Ma and 83–82 Ma), correlating with the compression-to-extension tectonic regime transition in response to the subduction polarity change from northwestward subduction of the Okhotomorsk block to northward subduction of the Neotethys oceanic lithosphere. Our findings contribute to the understanding of Mesozoic plate reconstruction associated with the India-Asia collision in the Cenozoic.

Geochemistry and Sr–Nd–Hf–O isotopes of ore-bearing plutons from the Yundukala Au–Cu–Co deposit, East Junggar, Xinjiang, Northwest China: Implications for petrogenesis and tectonic setting

Ongoing exploration in East Junggar, Northwest China, has led to the discovery of the intrusion-related Yundukala Au–Cu–Co deposit. The alteration and mineralization in the Yundukala deposit are closely related to diorite and gabbro-diorite, whereas the geochemical features of these ore-related plutons are still unclear. In this study, in order to unlock the nature of the ore-forming magmas, zircon U–Pb dating, whole-rock major and trace elemental and Sr–Nd isotope analysis, as well as zircon O–Hf isotope analysis, were conducted. Our results show that diorite and gabbro-diorite were both generated in the Early Devonian, with ages being 401.3 ± 3.8 Ma and 401.6 ± 4.5 Ma, respectively. Geochemically, they are all characterized by LREE-enriched patterns in chondrite-normalized REE diagrams, and evident enrichment of Rb, U and K and depletion of Nb and Ta contents in primitive mantle-normalized patterns, although the degrees of enrichment and depletion are variable. Isotopically, the diorite and gabbro-diorite display relatively high initial 87Sr/86Sr (0.704966–0.707395) and positive εNd(t) (1.29–3.84) of whole-rock, as well as positive zircon δ18O (5.3–8.9 ‰) and εHf(t) (2.4–18.8). These lines of evidence collectively indicate that these magmatic rocks were generated by mixing between mantle- and crustal-derived melts, and were emplacement in an island arc environment. The magma mixing during the formation of ore-bearing diorite and gabbro-diorite might bring abundant sulfur and metals, thus favoring the formation of Au–Cu–Co mineralization. This work emphasizes that further mineral exploration in East Junggar needs to pay attention to the Early Devonian granitoids.

A microcosm of modern crust formation: Evidence from zircon ages, Hf[sbnd]O and Nd[sbnd]Sr isotopes and bulk geochemistry of the Menglian Batholith, SE Tibet

Modern continental crust has evolved to a more potassic, granitic composition than early continental crust, which comprises largely sodic TTG-suite magmas. The present paradigm holds that the latter are largely “juvenile” (in the sense that the time from mantle extraction to felsic crust production is comparatively short, of the order 10–100 Ma) while the former represent recycled older crust of igneous or sedimentary composition. The data from high-Mg diorites, tonalites, granodiorites and potassic granites of the 125–115 Ma Menglian Batholith (SE Tibet) exemplify the modern situation and can therefore be used to constrain current crust formation processes. These rocks have higher concentrations of incompatible elements than magmatic rocks from typical continental arc settings, with a continuum of increasing K2O/Na2O ratios, SiO2, K2O, Rb, and Th concentrations juxtaposed with decreasing MgO, CaO, and Sr. They consistently record both higher zircon δ18O values than mantle values and decoupled NdHf isotope systems caused by the interaction of subducted sediments with the mantle wedge. Petrogenetic mechanisms that connect the suite include crystal fractionation within the diorites, melting of the lower crust induced by advection of heat and water by the diorites, and high-level fractionation of the tonalite-granodiorite suite to produce the high-silica granites. Therefore, this example of modern fertile continental crust had a five-stage evolution: (1) subduction-enrichment of the mantle source, (2) mantle melting to produce mafic magmas that pooled in or below the lower crust, (3) mafic magma differentiation to produce the tonalite-granodioritic magmas controlled by crystal-liquid equilibria, (4) crustal melting and admixture to the evolving felsic magmas and (5) final high-level fractionation and melt extraction to produce the silicic extreme, enriched in incompatible elements such as Rb, Th and K. This model could be a general mechanism for how modern mature continental crust evolves. Importantly, it indicates a significant role for mafic magmas and thence a more important role for juvenile additions than is generally accepted.

Crustal evolution and tectonomagmatic history of the Indian Shield at the periphery of supercontinents

River sand detrital zircons from several rivers flowing through the Peninsular Indian cratons were analyzed for U-Pb, Lu-Hf, and O isotopes to characterize the Precambrian crustal evolution of the Indian Shield and to constrain its role in the early supercontinent cycles. Analyzed river sand zircon samples exhibit a prominent age grouping at 2.7–2.4 Ga and additional peaks at 1.8–1.7 Ga, 1.0–0.8 Ga, and 0.6–0.5 Ga. The time-related Hf and O isotope trends of the Indian zircons display a slight offset from the global trends. The age peaks and distinct Hf-O isotopic compositions of the Indian zircons also lack coherence with the global patterns associated with supercontinent cycles, implying a discrete crustal evolutionary history for the Indian Shield. Their contrasting εHf-age trajectories indicate that the Indian Shield was accreted to the Columbia/Nuna supercontinental framework through a collision event that postdated the 2.1–1.8 Ga global-scale orogeny, and was part of a long-lived subduction system along the margin of Rodinia. This implies that the Indian Shield occupied a peripheral paleo-position during the assembly of the two Precambrian supercontinents. The ca. 2.6–2.4 Ga Indian river sand zircons have remarkably low δ18O values that are distinct from the global zircon O isotopic record. This period coincides with the fragmentation of the Archean supercraton and the flaring-up of a subaerial Large Igneous Province that facilitated the generation of 18O-depleted magmas. Depending on whether the 2.6–2.4 Ga zircons with low δ18O values (<4.7 ‰) are considered or not, approximately 2.77 ‰ or 1.78 ‰ rise in the zircon δ18O values occurred between ca. 2.4 Ga and ca. 1.87 Ga (decoupled from zircon Lu-Hf isotopes), further suggesting that the fine-grained sediments were enriched in 18O after the Great Oxidation Event. These high δ18O sediments were subsequently incorporated into the magmatic systems resulting in elevated δ18O in the zircons crystallizing from such melts. Nevertheless, a ca. 0.9 Ga peak of >10 ‰ in δ18O system was followed by a pronounced δ18O drop at ca. 0.759 Ga, suggesting that δ18O of recycled sedimentary reservoirs was not the only controlling factor for zircon δ18O characteristics. Variations in the Hf-O data for Indian and global zircons could be attributed to the initiation of the supercontinent cycle at ca. 2.0 Ga. A significant volume of the juvenile crust was added during the 3.0–2.7 Ga mafic magmatism in the Indian Shield, as underlined by the Hf model ages of all the zircon grains and those with mantle-like δ18O signatures. A gradual addition of continental crust during ca. 3.6 Ga to 3.1 Ga can also be deduced, with the oldest crust being derived from the mantle at ca. 4.4 Ga.

Grain-scale zircon Hf isotope heterogeneity inherited from sediment-metasomatized mantle: Geochemical and Nd-Hf-Pb-O isotopic constraints on Early Cretaceous intrusions in central Lhasa Terrane, Tibetan Plateau

AbstractClarifying the mechanism of recycling of pre-existing continental crustal materials into the source of mantle-derived magma is a challenging effort that can be of great value to improving our understanding of mantle processes and continental crust growth. This study presents an integrated investigation of whole-rock and mineral geochemical and Nd-Hf-O-Pb isotopic data for dolerites and diorites intruded in the central Lhasa Terrane of Tibetan Plateau at ca. 120 Ma (zircon U-Pb ages). These intrusions have similar distributions of trace elements that are characterized by depletion in Nb-Ta relative to Th, Ba, and U, and moderately negative whole-rock εNd(t) (–5.0 to –1.7) values. Magmatic zircon shows dramatically variable εHf(t) values (from –5.0 to +13.7 in the same rock, including up to 12 epsilon unit variability in single grains). On the other hand, the zircon δ18O values are relatively uniform (+6.0‰ to +7.7‰). The constant 208Pb/206Pb values of clinopyroxene crystallized at ca. 500–900 MPa suggest no contamination with lower continental crust. The lack of covariation between Hf and O isotopes from the same grains, and the lack of relationship between Hf isotopes and trace elements (e.g., Hf, Th/U, and Yb/Gd) in the magmatic zircons, together with the absence of ancient zircon xenocrysts, imply limited upper crustal contamination. In combination with high-whole-rock Th/La (&amp;gt;0.29) ratios, we interpret the zircon Hf isotope heterogeneity as inherited from a depleted asthenospheric mantle with the addition of 1–4% Hf from isotopically heterogeneous sediments. Our study therefore emphasizes the need for caution when using complex Hf isotopic zonation in zircon as an argument for intracrustal hybridization of two end-member magmas derived from distinct reservoirs. In addition, the high-Zr/Y ratios and no negative Zr-Hf anomalies of the Aruo intrusions imply a high surface temperature of the down going slab that was able to fully dissolve zircons in the subducted sediments. This requires a special geodynamic condition that was most likely related to the steepening of flatly subducted Neo-Tethyan lithosphere at ca. 120 Ma according to a synthesis of regional tectonic-magmatic-sedimentary records.

Ancient crust-derived syenitic and A-type granitic intrusions in the Emeishan large igneous province, SW China

Within the inner zone of the Emeishan large igneous province (ELIP), SW China, there are several small syenitic and A-type granitic intrusions not spatially related to giant Fe–Ti–V-deposit-bearing mafic–ultramafic layered intrusions. The representative Heime syenitic and Huili granitic intrusions as well as their spatially associated mafic dykes were developed at ca. 260 Ma, contemporaneous with the ELIP. The Heime metaluminous syenites are K-rich and have moderate SiO2 (60.1–61.4 wt%) and high total-alkali (Na2O + K2O = 9.68–10.7 wt%) contents with high K2O/Na2O ratios (1.18 to 1.74). The Huili metaluminous granites are K-rich A-type granites with moderate–high SiO2 (62.7–68.9 wt%), high total-alkali (7.96–8.62 wt%) and Zr (324–853 ppm) contents, and high 10,000 × Ga/Al ratios (2.68–3.22). Both of them are enriched in high-field-strength elements and light rare-earth elements, with significantly negative Nb–Ta–Sr–P–Ti anomalies. They also have highly evolved whole-rock SrNd isotopic compositions (Heime syenites, (87Sr/86Sr)i = 0.7092–0.7105, εNd(t) = −7.77 to −6.09; Huili granites, (87Sr/86Sr)i = 0.7116–0.7146, εNd(t) = −6.34 to −5.32). Zircon δ18O values of the Heime syenites (5.73‰–6.40‰) are slightly higher than those of the mantle zircons, and lower than those of the Huili granites (5.98‰–7.52‰). Zircon Hf 8isotopic compositions of the Huili granites (εHf(t) = −3.30 to +0.73) are higher than those of the Heime syenites (εHf(t) = −7.08 to −2.97), possibly indicating a more contribution of mantle component. In addition, the spatially associated mafic dykes display considerably higher εNd(t) values (Heime mafic dykes, −3.95 to −3.01; Heime mafic dykes, −0.58 to −0.51) than those of the Heime syenites and Huili granites. The parental magmas of both the Heime syenites and the Huili A-type granites show a close affinity to the K-rich, metaluminous syenitic magmas. Based on the Sr–Nd–O–Hf isotopic compositions of the Heime syenites, Huili granites, and spatially associated mafic dykes, we suggest that the K-rich syenitic magmas parental to the Heime syenites were produced mainly by remelting of Paleo–Mesoproterozoic mafic meta-igneous rocks, whereas those parental to the Huili A-type granites were generated by the remelting of Paleo–Mesoproterozoic basement rocks and mixing with small amounts of coeval mantle-derived basaltic melts.

Ca. 800 Ma I-type granites from the Hong’an Terrane, central China: New constraints on the mid-Neoproterozoic tectonic transition from convergence to extension in the northern margin of the Yangtze Block

The northern margin of the Yangtze Block was thought to have witnessed the tectonic shift from a compressional setting to an extensional setting at ca. 820 Ma, or to have remained as a subduction-related setting until ca. 700 Ma. Our new geochronological, isotopic, and geochemical data for the Daleishan granitic Pluton, which is the largest batholith in the Hong’an Terrane of the Qinling-Tongbai-Dabie Orogen, central China, provide new constraints on the Neoproterozoic magmatic and tectonic evolution of the Yangtze Block. Four representative samples from the Daleishan Pluton yield consistent zircon U–Pb ages within analytical errors of ca. 800 Ma. Samples from the pluton exhibit evolved compositions, including high SiO2 (71.53–77.05 wt%) and K2O (3.93–5.00 wt%) contents, low Zr/Hf (22–37) and Nb/Ta (7.9–16.4) ratios, enrichment in Rb, Th and U, and depletion in Ba, Sr, Eu, and Ti, with rare earth element tetrad effects in the chondrite-normalized element patterns. The studied samples have slightly elevated zircon δ18O values (+5.64 ‰ to + 7.11 ‰) relative to the mantle value but negative zircon εHf(t) (mostly of −12.0 to −5.5) and bulk rock εNd(t) (−9.6 to −7.9) values. Our new geochemical data classify the rocks as moderately to highly-fractionated I-type granite, rather than A-type granite as previously thought. These rocks may have been derived from the partial melting of mafic rocks in the region and the magma underwent fractional crystallization. Integrating the new and published data, we propose that the emplacement of the Daleishan Pluton was associated with the final stage of early Neoproterozoic oceanic-crust subduction along the northern margin of the Yangtze Block. Accordingly, a mid-Neoproterozoic tectonic transition from convergence to extension in response to the breakup of the Rodinia supercontinent may have occurred at 800–780 Ma.

Temperature and Hf-O isotope correlations of young erupted zircons from Tengchong (SE Tibet): Assimilation fractional crystallization during monotonic cooling

Young zircons from crystal-poor volcanic rocks provide the best samples for the investigations of pre-eruption magmatic processes and for testing a possible relationship between zircon Eu anomalies and crustal thickness. We report trace element chemistry and Hf-O isotope compositions of young zircons from 3 Holocene volcanoes in the Tengchong volcanic field, SE Tibet, in order to provide insights into magma evolution processes and conditions for high-K calc-alkaline volcanic rocks in a post-collisional setting. As decreasing zircon Ti content and falling temperature, zircon Hf content and Yb/Sm increase whereas zircon Eu anomaly and Th/U decrease, indicating fractional crystallization of plagioclase and zircon during magma cooling. More importantly, zircon Hf isotope ratio (εHf values) increases with decreasing zircon Ti content and falling temperature (T), suggesting gradually increasing incorporation of relatively high εHf juvenile materials in the crystallizing zircons during magma evolution. Negative correlations between zircon εHf and zircon δ18O also support open-system magma evolution. Our data suggest fractional crystallization of a magma with simultaneous contamination by high εHf and low δ 18O juvenile (immature) crustal materials during monotonic cooling after zircon saturation. The low-T, high-εHf and low- δ 18O zircons may indicate the involvement of the early Cretaceous juvenile granitic country rocks during shallow magma evolution. Average Eu anomalies in zircons from young Tengchong lavas yield crustal thickness of 40.7 ± 6.8 km, consistent with present crustal thickness (42.5 km) determined by geophysical methods.

Subducted Front of the Indian Continental Crust Beneath the Tibetan Plateau in the Early Eocene

AbstractThe immense Tibetan Plateau is a masterpiece of the ongoing India–Asia collision. The timing of the collision is a critical parameter to reconstruct the plateau evolution, but has been not well understood yet. Here, we report a newly discovered Eocene (53.8 Ma) shoshonitic intrusion with the input of Indian continental material in the Yangbajing area of the Lhasa Terrane, south Tibet. The reverse 207Pb/206Pb zoning exhibited by feldspars, the linear isotopic arrays between whole‐rock 187Os/188Os and 1/188Os, the in‐situ feldspar 208Pb/204Pb and 206Pb/204Pb values, and the in‐situ zircon εHf(t) values and δ18O values strongly demonstrate a binary interaction process for the generation of the Yangbajing shoshonitic intrusion. Our Os isotopes further illustrate that the interaction process was dominated by recycled crustal melt assimilating minor mantle melt. In addition, different isotopic systems consistently point to an end‐member represented by the underthrust crust slice of Indian continent, which indicates that the Indian lithosphere must have already subducted beneath the Lhasa Terrane at 53.8 Ma. This is also supported by the abrupt isotopic shift toward the enriched values and the composition variation toward the high potassium contents in the subsequent Tibetan magmatism, corresponding to an increasing involvement of the Indian continental crust into the upper plate magmatism. Therefore, the Yangbajing shoshonitic intrusion is one of the earliest known magmatic evidence for the onset of the India–Asia collision.