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The provenance of sediments in western Taiwan has been the subject of debate in recent years. Previous investigators, solely based on the U-Pb zircon geochronology for the detrital grains, all advocated the important roles, but with varying emphasis, of drainage systems in the South China Block. Influence of the Changjiang (Yangtze) River from the far-source Yangtze block versus nearby systems like Zhujiang, Minjiang and Jiulongjiang Rivers in the Cathaysia block has been the main issue. However, the fundamental question of whether the provenance is really the South China Block has never been evaluated. Here we report both the U-Pb zircon and Electron MicroProbe (EMP) monazite age data for seven sediment samples from two sides of the Taiwan Strait and synthesize the published data for all these rivers and sedimentary strata in western Taiwan to tackle this problem. The lack of, or insufficient, ~1.8 Ga monazites in the sediments of river mouths for all drainage systems in the South China Block makes them unlikely to be the source of Miocene sediments in western Taiwan where ~1.8 Ga monazite grains occupy 14.5–33.3% of the total age population. With reference to the tectonic evolution of the coastal southeast China-Taiwan region during Early Cretaceous to recent time, we propose a new explanation that invokes a now-concealed microcontinent along the Manila Trench. This microcontinent, probably related to a Paleoproterozoic orogen in Gondwana, eventually collided with the South China Block causing uplift of the easternmost structural element—the Pingtan-Dongshan Metamorphic Belt (PDMB) at ca. 130–120 Ma. It then drifted away sometime after 100 Ma and split into two parts with one becoming today's Palawan-Mindoro terrain of the Philippines. As indicated by the geophysical data near southern Taiwan, the remainder of this split microcontinent had subducted underneath the Philippine Sea plate along Manila Trench as a result of the opening of the South China Sea at ~33 Ma. Both the Peikang and Kuanyin basement highs in the offshore western Taiwan represent the unsubducted remnants of this microcontinent and also the major suppliers of Late Oligocene-Miocene sediments in western Taiwan. On the other hand, the Eocene-Early Oligocene strata of the Hsuehshan Range in central Taiwan show both zircon and monazite age patterns resembling the Eocene-Early Oligocene and recent sediments of the Zhujiang drainage system, indicating the southwestern origin of the protolith Taiwan. This scenario also matches the recent proposition that part of the Central Range belongs to the deep-level accretionary prism of a Miocene subduction system.

The origins of the Mesozoic low-δ18O A-type granites in eastern China are quite controversial. A new “low-δ18O” granite, the Taohuadao alkaline (A1-type) granite from the Zhoushan archipelago, offshore from Zhejiang, is reported in this study. Integrated in situ analyses of Hf-O-Li isotopes in zircon are used to provide insights on the petrogenesis and evolution of this granite, and low-δ18O granites in general. Three types of zircon have been recognized in Taohuadao samples: zircon with sector/oscillatary zoning (Type 1), disturbed zoning (Type 2) and extremely high-U (Type 3) grains. Data are carefully screened for radiation damage and post-magmatic alteration and only Type 1 zircon represents a magmatic origin. In Type 1 zircons, Type 1a with normal mantle δ18Ozrn value of 5.23‰ has a positive δ7Li value (2.0 ± 3.3‰), while Type 1b has a δ18O value of ∼4.81‰ and highly variable negative δ7Li values (−31.9‰ to −0.3‰). In contrast to the O-Li isotopes, zircon is quite homogeneous in terms of Hf isotopes (eHf(t) = −3.5 ± 1.7), suggesting that the Taohuadao A1-type granites were derived from a simple source. Our results show that the magmatic zircon δ18Ozrn values are negatively correlated with Hf content but positively correlated with Zr/Hf ratio in zircon, which indicates that fractional crystallization plays an important role in producing the slightly low δ18O signature. The anhydrous nature of the A1-type granites suppressed the crystallization of biotite and hornblende, while low oxygen fugacity is responsible for insufficient precipitation of magnetite, both of which drive the later differentiates towards slight depletion in 18O. Therefore, we suggest a ferrogabbro-type fractional crystallization may have played a major role in producing the chemical and oxygen isotopic variations of the Taohuadao A1-type granite and accounts for ∼0.5‰ 18O-depletion. The extremely depleted δ7Li in Type 1b zircons is most likely a result of diffusion-driven fractionation. Mantle-like O-Li isotopes and very low Li abundance in zircon suggest a predominantly mantle origin with little, if any, involvement of continental crustal materials in the granite. Thus the Taohuadao A1-type granite was most likely produced by intensive fractionation of reduced, anhydrous basaltic melts, rather than by mixing of mantle- and crustally-derived melts as previously thought.

The initiation and duration of magmatism in the Tatun Volcano Group (TVG), likely resulting from extensional collapse of the northern Taiwan mountain belt, remains an important issue of debates in the Taiwan orogeny. Here we report a detailed study of U-Pb ages determined by SIMS and LA-ICPMS, together with Hf isotopic data, of zircon separates from a total of 22 extrusive rocks from the TVG lava flow and additional two andesites from Keelungyu (KLY), a volcanic islet off NE Taiwan. Magmatic zircons from the TVG are characterized by uniform and high Hf isotopic ratios, with eHf(T) mostly from +20 to +10. Their overall 206Pb/238U ages suggest that the TVG magmatism was initiated ∼3 Ma but more active ≤0.8 Ma with a flare up ≤0.35 Ma. Magmatic zircons from the two KLY samples reveal identical 206Pb/238U ages (0.41 Ma), coupled with Hf isotopes [eHf(T): +25 to +9] comparable to those of the TVG. Thus, KLY is proposed as part of the TVG. Consequently, we argue that the main body of the TVG formed ≤0.8 Ma by post-collisional volcanism, but the mechanism resulting in the magmatic episodes ≥1.0 Ma remains obscure due to the scarcity of rock exposures.

The complete volcanic sequences restored in the Coastal Range of Taiwan are key archives for better understanding the magmatic and tectonic evolution of the Northern Luzon Arc. This paper reports (1) new zircon U-Pb ages and Hf isotopic data of fourteen volcanic samples from different sequences of four major volcanoes in the Coastal Range, (2) Hf isotopic data of dated magmatic and detrital zircons from two offshore volcanic islands, Lutao and Lanyu. These data indicate that the arc magmatism in the Coastal Range started at ~15 Ma, most active at ~9 Ma, and ceased at ~4.2 Ma. Magmatic zircons from the arc rocks show a significant variation in Hf isotopic composition, with εHf(T) values varying from +24.9 to +4.8. As pointed out by our previous studies, old continental zircons that show Cathaysian-type ages and Hf isotope features are common in samples from the Yuemei, Chimei, and Lanyu volcanoes, supporting the notion for the influence of the existence of an accreted micro-continent or continental fragment plays a role in the petrogenesis. Such inherited zircons are not observed in the Chengkuang’ao and Tuluanshan volcanoes and uncommon in Lutao, implying the discontinuity or a limited extent of the accreted continental fragment. The εHf(T) values are high and positive from ~15 - 8 Ma (+25 to +15; ±5ε-unit variation), and became lower from ~6 to 4.2 Ma (+20 to +8; ±6ε units) and the lowest from ~1.3 Ma (+19 to +5; ±7ε units). Such a temporal variation in zircon Hf isotopic ratios can be also identified in whole-rock Hf and Nd isotopic compositions, which decrease from ~6 Ma when the Northern Luzon Arc may have started colliding with the Eurasian continental margin.

The origin and geodynamic evolution of peak Early Cretaceous magmatism in the southern Great Xing'an Range, North China, have long been controversial. Here we report new U-Pb zircon ages (141–129 Ma) of a suite of dioritic-granitic rocks from central Inner Mongolia, far from the sutures or plate boundaries of the Paleo-Pacific and Mongol-Okhotsk oceans, thus delineating an Early Cretaceous intracontinental magmatic province, which had a peak activity at 130–120 Ma. Dioritic suite including diorite, tonalite, and granodiorite shows variable zircon eHf(t) of +1.4 to + 11.8 and δ18O values of +5.7 to +6.9‰, while granitic suite consisting of monzogranite, syenogranite, and granite porphyry also records variable zircon eHf(t) of −0.9 to +15.0 and δ18O values of +6.3 to +8.1‰, suggesting crustal melting by preexisting crustal source with important recycled supracrustal components including fluids. Furthermore, these rocks show variable whole-rock δ7Li values (−0.6 to +12.1‰), indicating fluids played an important role in magma source. We propose a deep-sourced water-fluxed melting scenario by ancient hydrous slabs inherited from the Paleo-Asian Ocean that were trapped in the deep interior, thus releasing aqueous fluids to melt the lithospheric mantle and produce water-rich mafic magmas. These mafic magmas were underplated into crust where they promoted water-fluxed partial melting to generate the large-scale Early Cretaceous magmatism in the southern Great Xing'an Range. Such melting due to fluxing of aqueous fluids was probably operating as a widespread process responsible for the Early Cretaceous dramatically tectonomagmatic events and evolution of continental crust in NE Asia.

This paper reports the first systematic analysis of age and geochemical variations in volcanic rocks from the Coastal Range of Taiwan, the Northern Luzon Arc. The rocks, recovered from four main volcanoes, vary from low-K tholeiitic to medium-K calc-alkaline basalts to dacites. The rocks are typical of arc magmatic products, exhibiting enrichment in the large ion lithophile elements and depletion in the high field strength elements. Our new 40Ar/39Ar age data constrain the youngest eruption time in each of the four volcanoes, i.e., from north to south, at ~ 7.2 Ma (Yuemei), ~ 4.2 Ma (Chimei), ~ 6.2 Ma (Chengkuang'ao) and ~ 8.5 Ma (Tuluanshan), respectively. These data indicate that volcanism in the Northern Luzon Arc did not cease progressively from north to south, as previously alleged. The high and broadly uniform Nd isotope ratios [eNd = + 10.1 to + 8.8] and trace element characteristics of the rocks suggest a principal magma source from the depleted mantle wedge. Their overall geochemical variations are ascribed to magma chamber processes. The effects of magmatic differentiation and crustal contamination differ among each volcano, most likely owing to the discrepancy of residence time in individual magma chambers. Consequently, we propose a binary mixing model for the magma generation that involves arc magmas sourced from the depleted mantle wedge and up to ~ 5% crustal contamination with a continental fragment split off from the Eurasian margin.

The stable isotope systems of Li and B are useful tools for the understanding of ocean island basalt genesis. We have applied both systems to a comprehensive set of well-characterised samples across the Azores islands in order to further evaluate the nature of the mantle source. These ocean islands represent the surface expression of a low-buoyancy mantle plume. The variability of Li and B concentrations and their isotopic compositions across the archipelago builds on recent radiogenic isotope studies from the islands to the west of the Mid-Atlantic Ridge (Flores and Corvo). The results for both systems yield some values that are typical for ocean island basalts; however, the variability of both δ7Li and δ11B observed in primitive lavas is most extreme on the western islands of Flores and Corvo (δ7Li = + 3.5 to + 8.2‰ and δ11B = − 3.5 to + 11.8‰). The large spread in isotopic composition is most likely due to contamination of the magmas during ascent through hydrothermally altered oceanic crust. Models of assimilation fractional crystallisation best explain the variability observed on each island and allow for robust estimates of the mantle source. The implication of these observations is that the interpretation of the radiogenic isotopes and trace elements may have to be treated with care if the stable isotopes record contamination by assimilation of hydrothermally altered material and that the use of Li and B as mantle source tracers may be obscured by shallow level processes. Further, it seems unlikely that the large range observed in Sr and Nd isotopes can reflect variable contributions from recycled low-T materials such as sediment or altered MORB, since there is no correlation between the radiogenic and stable isotope data.

To better understand Tibetan orogenesis, which involves complex tectonic processes, here we report the first integrated analysis of Hf isotopes of zircon separates and their host rocks from the Transhimalayan batholiths. The rocks studied include 19 granitoids from different parts of the Gangdese batholith, the largest Transhimalayan intrusive complex, and five postcollisional adakites that occurred as plugs or dikes cutting the Gangdese batholith, in the southern Lhasa terrane, southern Tibet. Published zircon Hf isotope data from other Transhimalayan granitoids and the Linzizong volcanic rocks are also synthesized. The magmatic zircons, crystallizing between ca. 200 and 15 Ma, show depleted mantle-type Hf isotopic characteristics throughout the Mesozoic but shift markedly in the Paleogene. The Hf isotopic shift is interpreted as tracking the evolving progress of Himalayan sediment subduction driven by the approaching Indian continent, and thus signals the initiation of the India–Asia collision that we infer to have occurred by 55 Ma. Our data furthermore indicate that southern Tibet underwent significant crustal thickening during ca. 45 and 30 Ma, before emplacement of the postcollisional adakites that exhibit Hf–Nd isotopic systematics suggesting binary mixing of melts from the juvenile Gangdese mafic crust and incorporated Himalayan sediments in the petrogenesis.

Before the Indian collision with Asia, northward subduction of the Neo-Tethyan oceanic lithosphere produced an Andean-type convergent margin in South Asia characterized by arc magmatism starting from the early Jurassic and lasting until the Eocene. The magmatic arc is now represented by widespread intrusive bodies or the so-called Transhimalayan batholiths in the Lhasa terrane of southern Tibet that have been divided into two main magmatic suites, i.e., the northern plutonic belt and the southern Gangdese Batholith. Their temporal distribution, concerning how exactly the magmatic suites correlate eastwards and then southeastwards around the eastern Himalayan syntaxis, however, remains poorly constrained. Here we report the first combined zircon U–Pb and Hf isotopic study of the Transhimalayan batholiths from the eastern part of the Lhasa terrane (∼ 95–97.5°E and ∼ 28.5–30°N), SE Tibet. Zircon U–Pb dating results of 24 granitoids indicate that the rocks were emplaced principally in the Early Cretaceous (∼ 133–110 Ma) and subordinately in the Paleocene (∼ 66–57 Ma), long after an older granite intrusion in the earliest Jurassic (∼ 198 Ma). The zircon eHf(T) values range from + 5 to − 20, yielding Hf crustal model ages (TDMC) between 0.8 and 2.4 Ga that peak at ∼ 1.7 Ga, suggesting a major episode of crustal growth in the Proterozoic and a predominantly, or even exclusively, crustal source for the granitoid petrogenesis. These zircon U–Pb and Hf isotopic constraints, together with whole-rock geochemical characteristics, allow us to correlate them to S-type granitoids in the northern plutonic belt, rather than to the I-type Gangdese Batholith from the central part of the southern Lhasa terrane. In contrast to the Gangdese magmatism that formed with a significant juvenile mantle input related to the Neo-Tethyan subduction, these S-type granitoids show geochemical similarities to the North American Cordilleran Interior batholiths that did not form directly from a subduction setting. Thus, we attribute the petrogenesis of the widespread Early Cretaceous granitoids in the northern belt and eastern Himalayan batholiths to a postcollisional regime due to the Late Jurassic–Early Cretaceous continental collision between the Lhasa and Qiangtang terranes. Under this framework, the S-type magmatism was generated in response to collision-induced crustal thickening, which may have capabilities of causing crustal anatexis by itself in the region. However, continued interplay with the Neo-Tethyan subduction zone processes such as back-arc extension could also have played a crucial, and long-lasting, role in the magma generation.

Rising as “the roof of the world” the Tibetan plateau is now underlain with the thickest continental crust on Earth. How and when was this crust formed, which would have exerted pivotal controls to the formation of the plateau, has long been an issue of hot debates. This paper reports zircon U–Pb ages and Hf isotope data for postcollisional (~ 30–9 Ma) adakites in the southern Lhasa terrane, southern Tibet. A comparative analysis of whole-rock rare earth element geochemistry and zircon Hf isotopes between the adakites and associated Gangdese igneous rocks suggests that the Tibetan crust underwent a major phase of tectonic thickening between ca. 45 and 30 Ma in the region. The lower part of the thickened crust consisted prevailingly of mafic lithologies, which we argue to have resulted from intense basaltic underplating and subsequent remelting that took place during the Late Cretaceous and Eocene time related to the Neotethyan subduction processes including breakoff of the subducted slab at ca. 50 Ma in the early stage of the India–Asia collision. These processes were responsible for not only the juvenile crust formation but also for the creation of a thermally softened lithosphere in the southern Lhasa terrane. The indentation of India, consequently, caused distributed lithospheric thickening with formation of an orogenic root beneath southern Tibet. Root foundering during the Oligocene gave rise to the adakitic magmatism, regional topographic uplift, and onset of northward underthrusting of the Indian plate that has since played a key role in forming the entire Tibetan plateau.

Apatites crystallized from different types of igneous rocks show significant variations in the abundances of some minor and trace elements. In this study, electron probe microanalysis and laser ablation inductively coupled plasma mass spectrometry were used to determine the concentrations of 25 minor and trace elements in apatite separated from three principal rock types of theTranshimalayan igneous plutonic suite: S-type granites, the I-type Gangdese batholith and postcollisional adakites. F, Mn, Sr and rare earth elements (REE) in apatite vary systematically with the composition of the host magma and thus have high potential as petrogenetic tracers. More specifically, the F and Mn contents of apatite can be used as an indicator of magma aluminosity or differentiation index. Combined with Sr and REE data, which show significant variations in apatite from different rock types, these elements are useful for constructing ‘discrimination diagrams’. This study also reveals that apatite has the capacity to retain geochemical information about the host magma through the course of magmatic evolution. Systematic variations of Sr and REE in apatite with bulk-rock aluminosity are the results of partition competition with pre-existing and coexisting major and accessory minerals in silicate melts, and thus are useful for more detailed investigations of petrogenetic processes such as fractional crystallization and magma mixing, which is signaled by inconsistent Eu anomalies, Sr abundances and REE patterns relative to bulkrock compositions.

The Trans-Himalayan magmatism, which occurred extensively in the Lhasa terrane of southern Tibet, has long been related to the Neotethyan subduction before the India–Asia collision. To better delineate the magmatic duration, we report a geochronological study with 25 SHRIMP zircon U–Pb ages from the Gangdese Batholith that represents the largest Trans-Himalayan plutonic complex. The results suggest two distinct stages of plutonism in the Late Cretaceous (ca. 103–80 Ma) and early Paleogene (ca. 65–46 Ma), respectively. Our new data confirm if not refine the notion that a Gangdese magmatic gap or quiescent period existed between ca. 80 and 70 Ma. It is furthermore identified that the early stage ended with adakitic intrusion and the latter stage is marked by a peak activity at ca. 50 Ma. We attribute the cessation of the early stage, and following magmatic gap, to a flattening of the northward Neotethyan subduction, and the initiation of the latter stage to rollback of the subducted slab. The proposed scenarios can also account for the southward migration and intensification of Cretaceous to Paleogene volcanism in the Lhasa terrane that demonstrates a coeval, eruptive “flare-up” event around 50 Ma, interpreted as the result of detaching the Neotethyan oceanic slab from the adherent, more buoyant Indian continental lithosphere owing to the India–Asia collision. Our model is, moreover, in general accord with sedimentary and structural geologic records from southern Tibet where subduction-related orogenesis appears to have evolved through time before India started colliding Asia.

Major and trace element, whole rock Sr and Nd isotope and zircon Hf isotope data are reported for a suite of A-type granites and mafic microgranular enclaves from the Early Cretaceous (126 ± 2 Ma) Qianshan pluton, Liaodong Peninsula, northeast China, with the aim of investigating the sources and petrogenesis of A-type granites. The Qianshan pluton includes hornblende alkali-feldspar granite, graphic biotite granite and mafic microgranular enclaves. The hornblende alkali-feldspar granites have high SiO 2 , Fe 2 O 3T / MgO, K 2 O + Na 2 O, Rb, Zr and LREE contents and low Ba and Sr concentrations with strongly negative Eu anomalies. Their high Rb / Sr ( 87 Rb / 86 Sr = 16.76–24.15) and initial 87 Sr / 86 Sr ratios (0.7215 to 0.7283), negative e Nd ( t ) values (− 14.1 to − 16.5) and zircon e Hf ( t ) values (− 18.9 to − 11.5) indicate they were mainly derived from a crustal source, but with involvement of high e Nd ( t ) and e Hf ( t ) materials. Graphic biotite granites have similar geochemical features and Sr–Nd–Hf isotopic compositions to enclaves, indicating they were the result of crystal fractionation of evolved mafic magmas, but with involvement of low e Nd ( t ) and e Hf ( t ) materials. The mafic enclaves have an igneous texture and contain acicular apatite, suggesting quenching of mafic magmas that have co-mingled with the host granites. They have low initial 87 Sr / 86 Sr ratios (0.7097–0.7148), negative e Nd ( t ) (− 14.5 to − 11.9) and zircon e Hf ( t ) (− 17.1 to − 6.9) values, and are enriched in LILEs and LREEs and depleted in HFSEs. When coupled with the high MgO (Mg# up to 54), this indicates derivation from an enriched lithospheric mantle source, but contaminated by crustal materials. Geochemical and Sr-, Nd- and zircon Hf-isotopic compositions rule out simple crystal–liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks. Instead, magma mixing of mantle-derived mafic and crustal-derived magmas, coupled with crystal fractionation, is compatible with the data. This example shows that at least some A-type granites formed through a complex process involving mantle- and crustal-derived magma mixing, crystal fractionation and infracrustal melting.

Geochronological, geochemical, whole-rock Sr–Nd and zircon Hf isotopic analyses have been carried out on two suites of Late Mesozoic mafic to felsic magmatic rocks in the Sulu orogenic belt (east-central China) with the aim of characterizing their petrogenesis and tectonic implications. The Shijiusuo monzogranite has a SHRIMP zircon 206Pb / 238U age of 127 ± 2 Ma and an 40Ar / 39Ar age on hornblende of 123.5 ± 0.4 Ma. A mafic enclave from this pluton has a SHRIMP zircon 206Pb / 238U age of 124 ± 3 Ma and a hornblende 40Ar / 39Ar age of 124.2 ± 0.4 Ma, indicating coeval crystallization of the mafic enclaves and host monzogranite. Whole rock 40Ar / 39Ar dating gives an emplacement age of 111.2 ± 0.1 Ma for the mafic dikes. The monzogranites have low MgO and Cr, high Na2O, and Sr–Nd–Hf isotopic data (87Sr / 86Sr > 0.7085, ɛNd(t) = − 20.5 and ɛHf(t) = − 22.5 to − 56.6) consistent with derivation from Late Archean to Paleoproterozoic lower crust with involvement of mantle materials. The presence of coeval mafic enclaves with high ɛNd(t) and ɛHf(t) values indicates magma mixing and involvement of mantle-derived materials in the generation of the Shijiusuo pluton. The mafic dikes that intrude the monzogranite have characteristics of ultrapotassic rocks. Their geochemical features, such as high MgO (Mg# up to 75) and Cr (up to 1233 ppm), low TiO2 (1.11–1.24 wt.%) and total Fe2O3 (8.33–9.09 wt.%), enrichment in LILEs (e.g., Rb, Ba, Sr) and LREEs, and depletion in HFSE (e.g., Nb, Ta and Ti), together with initial 87Sr / 86Sr ratios of 0.7076–0.7078 and negative ɛNd(t) values (− 17.6 to − 18.2), indicate they were derived from an amphibole-bearing, refractory lithospheric mantle. The Shichang–Fangzi monzodioritic to monzonitic rocks have zircon SHRIMP U–Pb ages of ∼122 Ma. These rocks have Sr, Nd and Hf isotopic compositions (initial 87Sr / 86Sr = 0.7083–0.7088, ɛNd(t) = − 16.5 to − 17.7 and ɛHf(t) = − 20.4) similar to the mafic dikes in the nearby Shijiusuo pluton, indicating they were derived from a common source. High Rb and HREE, low Sr and Ba and strongly negative Eu anomalies indicate that the monzodioritic magmas resulted from pyroxene- and plagioclase-dominated fractionation of magma derived from an enriched mantle source. Taken together, these features indicate that Early Cretaceous magmatism in the Sulu orogenic belt was not related to Late Triassic subduction or exhumation of the ultrahigh-pressure metamorphic rocks that characterize the Sulu region; instead they resulted from mantle–crust interaction in an extensional setting, most likely induced by widespread removal of lithospheric mantle in the eastern North China Craton during the Early Cretaceous.

The Jiazishan alkaline complex in the eastern Sulu ultrahigh pressure (UHP) metamorphic orogenic belt of eastern China is composed of potassic to ultrapotassic pyroxene syenite, quartz syenite and associated mafic dikes. A SHRIMP zircon 206Pb/238U age of 215±5 Ma was obtained for the quartz syenite and mineral 40Ar/39Ar dating gave emplacement ages of 214.4±0.3 and 214.6±0.6 Ma for the pyroxene syenite and 200.6±0.2 Ma for the mafic dike. These dates establish that the Jiazishan Complex was emplaced shortly after the UHP metamorphic event at 240 to 220 Ma due to the continental collision between the North China and Yangtze cratons. The ultrapotassic mafic dikes, with K2O≈4.4–6.4 wt.% and K2O/Na2O≈3.5, have high MgO (8.06–12.44 wt.%), Ni (119–319 ppm) and Cr (477–873 ppm) and moderately low CaO/Al2O3 (∼0.76) and TiO2 (∼1.12 wt.%). They also have high Sr (87Sr/86Sr∼0.7073), low Nd (ɛNd=∼−16.5) isotopic ratios, enriched LILE (Ba/La=66–74), LREE [(La/Yb)N=28–33] and depleted HFSE (La/Nb=4–6). It appears that the mafic dikes were derived from a refractory, re-enriched lithospheric mantle source. The syenites have Sr and Nd isotopic compositions similar to the mafic dikes, implying a common origin. Geochemical and isotopic modeling suggests that the pyroxene syenites may have been generated by early fractionation of clinopyroxene and olivine, coupled with minor amounts of crustal contamination, of a mafic magma that had a similar composition to the mafic dikes. Subsequent fractionation of feldspar-dominated assemblages, with minor or no contamination, would result in the quartz syenites. This post-orogenic magmatism, resulting most likely in an extensional setting, provides time constraints on the major geodynamic transition from convergence to extension at the eastern margin of the North China craton. The Jiazishan potassic magmatism and geodynamic transition from convergence to extension can be explained by convective removal of the lower lithospheric mantle.

Mesozoic mafic rocks in the Dabie-Sulu orogen (DSO) are enriched in large-ion lithophile elements (LILE, e.g., Rb, K, Sr, Ba), relatively depleted in high-field strength elements (HFSE, e.g., Nb, Ta, P, Ti), and have strongly fractionated rare-earth elements (REE) and highly "enriched" Sr-Nd isotope ratios. These "arc-type" geochemical signatures have been generally ascribed to mantle source enrichment beneath the DSO by fluids/melts released from deeply subducted Triassic continental crust, i.e., part of the South China block (SCB). However, contemporaneous mafic rocks are widespread in the North China block (NCB) and possess similar elemental and Sr-Nd isotopic characteristics. Given that most of the NCB mafic rocks were emplaced at a considerable distance from the DSO, their arc-type geochemical features are unlikely to have been caused by Triassic continental subduction. Therefore, we argue that subduction of the SCB is a potential, but not necessarily the only, mechanism responsible for mantle enrich...

Eclogite compositions are critical parameters for under standing chemical evolution in the Earth, particularly in subduction zones. A group of eclogites from the Sulu ultra-high pres sure (UHP) metamorphic terrane in eastern China shows uncommon petrographic and compositional features. They are characterized by (1) zoisite porphyroblasts coexisting with inclusion-free, inclusion-containing, and net work kyanite, (2) high Al2O3 of 18.4 - 29.2% with high MgO of 8.59 - 11.3%, and (3) intensive HFSE-REE fractionations represented by [Zr/Sm]ch and [Ti/Gd]ch ratios of 0.1 - 3.9 and 1.1 - 9.0, respectively. High-pressure fluids played a major role on developing these features. Kyanite shows two textural varieties. KyI is inclusion-free and in apparent textural equilibrium with gar net and omphacite, implying formation from plagioclase break down. KyII includes kyanite networks and porphyroblasts; the former occurs mostly in gar net, whereas the latter encloses gar net and omphacite grains. KyII were crystallized at the expense of gar net and omphacite during eclogite-fluid interaction. The low [HREE]ch ratios of 1 - 2 indicate that the protoliths were arc cumulates. The well-de fined in verse Al2O3-SiO2 and Al2O3-CaO correlations are not the characteristics of igneous precursors; there fore, must reflect metamorphic modifications. The comparison to mafic cumulates shows that the low-Al samples are compositionally better representatives of protoliths. Mass balance calculations point to anolivine gabbronoritic protolith. The intensive HFSE-REE fractionations reflect compositional differences between two sample groups. Group I samples have superchondritic HFSE-REE ratios [(Nb/La, Zr/Sm, Ti/Gd)ch = 2 - 7] with depletions in LREE. In contrast, group II samples show HFSE depletions and LREE enrichments. These compositional differences were explained as reflecting element mobility during eclogite-fluid inter action. Released at temperatures > 750°C, the high-pres sure flu ids were enriched in HFSE, REE, and LILE. Upon migration, the flu ids first precipitated HFSE into the group I samples, then, LILE and LREE into the group II samples. This model is sup ported by the occurrence of zoisite porphyroblasts in the group II samples, and interstitial zircon and clusters of small rutilegrains along annealed fractures in the group I samples. Our interpretation for the observed compositional variations implies limited element mobility during subduction and exhumation of continental lithosphere, consistent with existing models, which proposed that the chemical flux to metasomatized mantle wedge is mainly from subducted sediments.

Adakites are geochemically distinct intermediate to felsic lavas found exclusively in subduction zones. Here we report the first example of such magmas from southern Tibet in an active continental collision environment. The Tibetan adakites were emplaced from ca. 26 to 10 Ma, and their overall geochemical characteristics suggest an origin by melting of eclogites and/or garnet amphibolites in the lower part (≥50 km) of thickened Tibetan crust. This lower-crustal melting required a significantly elevated geotherm, which we attribute to removal of the tectonically thickened lithospheric mantle in late Oligocene time. The identification of collision-type adakites from southern Tibet lends new constraints to not only the Himalayan-Tibetan orogenesis—how and when the Indian lithosphere started underthrusting Asia can be depicted—but also the growth of the early continental crust on Earth that consists dominantly of the tonalite-trondhjemite-granodiorite suites marked by adakitic geochemical affinities.