•O isotopes of zircons from the Baekdusan Millennium Eruption were analyzed.•The zircons yielded δ18O values consistently lower than the mantle range.•These low values indicate an involvement of hydrothermally altered intracaldera rocks.•Low-δ18O magmas may have been produced commonly in large-volume silicic eruptions.The silicic volcanism of Baekdusan (Changbaishan), which is on the border between North Korea and China, was initiated in the Late Pleistocene and culminated in the 10th century with a powerful (volcanic explosivity index = 7) commendite–trachyte eruption commonly referred to as the “Millennium Eruption.” This study presents oxygen isotope data of zircon in trachydacitic pumices ejected during the Millennium Eruption, together with whole-rock geochemical and Sr–Nd–Pb isotopic data that manifest once again the A-type and EM1 affinities of the Millennium Eruption magma. The zircon crystals, dated by previous studies at ca. 12–9 ka, show a moderate inter-grain variation in δ18O from 3.69‰ to 5.03‰. These values are consistently lower than the normal mantle range, and interpreted to have resulted from the digestion of meteoric-hydrothermally altered intracaldera rocks in the shallow magma chamber beneath Baekdusan just prior to the crystallization of the zircons, rather than from derivation from low-δ18O sources deep in the mantle. The whole-rock geochemical/isotopic considerations suggest that the magma mainly self-cannibalized the earlier erupted volcanic carapace around the magma chamber. This study highlights the usefulness of zircon oxygen isotopes for characterizing past volcanic activity that has now been commonly eroded away and implies that the generation of Yellowstone-type low-δ18O magma is not a rare phenomenon in large-volume silicic eruptions.Download high-res image (272KB)Download full-size image

The surface condition of a sample mount is an important factor influencing the precision of SIMS isotope analysis. The phenomenon that the sample topography affects the analytical precision is called the topography effect. We carried out a systematic experiment of O-isotope analyses using a Cameca IMS-1280 SIMS to quantitatively characterize the topography effect with the aim of better understanding its physical mechanism underlying such an artifact and ultimately improving the analytical precision. Our results indicate that within a mineral grain, the topography effect is obvious in the X-direction (horizontal direction) of the sample stage but insignificant in the Y-direction (vertical direction). In addition, within a single mineral grain, the topography effect creates analytical spots on the left rim (lower X-coordinates) yielding higher measured δ18O values than those on the right rim (higher X-coordinates) in our instrument. The physical reason that the topography effect compromises the analytical reproducibility is attributed to lateral energy dispersion of secondary ions caused by surface topography changing the ion position in the entrance slit plane. By increasing the transfer optics magnification, the topography effect could be significantly reduced. Beam centering parameters could be used to quantitatively assess the topography effect and improve the data quality.

Zircon is the most useful mineral for studies in U-Pb geochronology and Hf and O isotope geochemistry. Matrix effect is a major problem of the microbeam techniques such as SIMS and LA-(MC)-ICPMS. Therefore, external standardization using well-characterized natural zircon standards is fundamental for accurate microbeam measurements. While the isotopic geochronology and geochemistry laboratories equipped with microbeam analytical facilities have been increasingly established in China during the past decade, applications of the isotopic microanalysis are still limited due to shortage of available standards. We report here the Qinghu zircon as a potential new working reference for microbeam analysis of zircon U-Pb age and O-Hf isotopes. This zircon was separated from the Qinghu quartz monzonite from the western Nanling Range, Southeast China. It is fairly homogeneous in U-Pb age and Hf and O isotopes in terms of large amounts of mircobeam measurements by LA-MC-ICPMS and SIMS at the scales of 20–60 μm. SIMS measurements yield consistent 206Pb/238U age within analytical uncertainties with that obtained by ID-TIMS. Precise determinations of O isotopes by IRMS and Hf isotopes by solution MC-ICPMS are in good agreement with the statistical mean of microbeam measurements. We recommend U-Pb age of = 159.5 ± 0.2 Ma (2SE), δ18O = 5.4‰± 0.2‰ (2SD) and 176Hf/177Hf = 0.283002 ± 0.000004 (2SD) as the best reference values for the Qinghu zircon.

A single-step separation scheme is presented for Sm–Nd radiogenic isotope system on very small samples (1–3 mg) of silicate rock. This method is based on Eichrom® LN Spec chromatographic material and affords a straightforward separation of Sm–Nd from complex matrix with good purity and satisfactory blank levels, suitable for thermal ionization mass spectrometry (TIMS).This technique, characterized by high efficiency (single-step Sm–Nd separation) and high sensitivity (TIMS on NdO+ ion beam), is able to process rapidly (3–4 h), with low procedure blanks (<10 pg) and very small sample (1–3 mg). Replicate measurements by TIMS on 143Nd/144Nd ratios and Sm–Nd concentrations are presented for eleven international silicate rock reference materials, spanning a wide range of Sm–Nd contents and bulk compositions. The analytical results show a good agreement with recommended values within ±0.004% for the 143Nd/144Nd isotopic ratio and ±2% for Sm–Nd quantification at the 95% confidence level. It is noted that the uncertainty of this method is about 3 times larger than typical precision achievable with two-stage full separation followed by state-of-the-art conventional TIMS using Nd+ ion beams which require much larger amounts of Nd. Hence, our single-step separation followed by NdO+ ion beam technique is preferred to the analysis for microsamples.Graphical abstractDistribution curve of all eluting fractions for a BCR-2 (1–2–3.5–7 mg) on LN column using HCl and HF as eluting reagent.Highlights► This analytical protocol affords a simple and rapid analysis for Sm and Nd isotope in minor rock samples. ► The single-step separation method exhibits satisfactory separation effect for complex silicate samples. ► Corrected 143Nd/144Nd data show excellent accuracy even if the 140Ce16O+/144Nd16O+ ratio reached to 0.03.

Zircon is arguably the best, certainly the most commonly used mineral for U–Pb geochronology. Modern large-geometry secondary ion mass spectrometry (SIMS) has been routinely utilized for precise U–Pb zircon age determination at a lateral resolution of 10–30 μm. However, in situ U–Pb dating at a scale of ca. 5 μm scale or less for fine-grained zircons and/or zircon crystals with complex structural and chemical features is still a challenge to the geoscience community. Here we describe a method of precise U–Pb dating for zircons as young as the Jurassic age at a scale of up to <5 μm using the CAMECA ims-1280 SIMS. Gaussian mode primary O2− and O−probes of ca. 5.2 μm and ca. 4.5 μm in diameter with beam intensities of ∼100 pA were obtained, respectively, by optimizing the primary column. Secondary ion optics was optimized to ensure a high Pb+ sensitivity in zircons, producing ∼21 cps/ppm/nA using O2− and ∼13 cps/ppm/nA using O− (with oxygen flooding technique). As a demonstration of this method, three well-characterized zircon standards with a range of ages, AS3 (1099 Ma), Plešovice (377 Ma) and Qinghu (159.5 Ma), were analyzed. We demonstrate with these zircon standards that their ages could be determined with precision and accuracy of 1–2% using a spot <5 micron. The O− primary beam is preferred over the O2− beam for small-spot U–Pb zircon geochronology, because it has higher density and produces smaller craters on the target surface, with insignificant trade off in precision and accuracy of the final U–Pb ages. For U-poor minerals of younger ages, O2− might be preferred in order to generate sufficient Pb+ ions for measurement with minimal loss of spatial resolution.

Zircon is the most useful mineral for studies in U–Pb geochronology, Hf and O isotope geochemistry, trace element geochemistry, and increasingly geothermometry. In situSIMS zircon Li isotope analysis shows potential for studying the genesis of crustal magmas and evolution of the continental crust, but its application has been hampered due to lack of well-characterized zircon Li isotope standards. Reconnaissance SIMS measurement of Li isotope ratio and concentration for several commonly-used zircon U–Pb age standards, including M257, BR266, Plešovice, 91500 and TEMORA 2 zircons are reported here. Of these, the M257 zircon is demonstrated to be homogeneous in Li isotopic composition, with δ7Li = 2.1 ± 1.0‰ (2SD). It is also relatively homogeneous in Li concentration, with Li concentration = 0.86 ± 0.18 ppm (2SD). Therefore, we recommend the new M257 zircon standard as a working reference for SIMS Li isotope and concentration measurements in zircons.

Baddeleyite has long been recognized as one of the most important U-bearing minerals for dating silica undersaturated igneous rocks. Age determination of baddeleyite calls for analysis within small volumes using high-resolution secondary ion mass spectrometry (SIMS) because of its minuscule grain size as well as potential altered domains or micro-inclusions. However, precise SIMS U–Pb dating has been hampered for baddeleyite owing to crystal orientation effects that bias Pb/U ratio measured in baddeleyite. In this study we carried out a series of tests of U–Pb and Pb–Pb measurements on Phanerozoic baddeleyite using a multi-collector Cameca 1280 IMS with oxygen flooding technique. Our results demonstrate that the oxygen flooding can not only enhance secondary Pb+ ion yield by a fact of 7 for baddeleyite, but also depress the baddeleyite U/Pb orientation effect down to ∼2% (1 RSD). Therefore, Phanerozoic (as young as Cenozoic) baddeleyite can be precisely dated by SIMS Pb–Pb and/or U–Pb measurements with precision of 1–3% (2 RSE).

Early Yanshanian (Jurassic) granitoids are widespread in the Nanling Range of South China, and are associated with numerous non-ferrous and rare metal mineral deposits. These granitoids consist mainly of slightly peraluminous biotite monzogranites and K-feldspar granites that are closely associated in time and space with subordinate amphibole-bearing granites and muscovite-and garnet-bearing granites. In most previous studies, the biotite-bearing granites were classified as crustal transformation-type (corresponding to the S-type) granites which were interpreted to be derived from the regional Paleoproterozoic meta-sedimentary rocks. In this paper, we re-analyze the geochemical characteristics of a number of representative Early Yanshanian Nanling granitoids. There exists a clear negative correlation between SiO2 and P2O5 for the studied granitoids. The Early Yanshanian Nanling granitoid suites (including amphibole-bearing granodiorites→biotite monzogranites→K-feldspar granites→two-mica (muscovite) granites) are metaluminous to slightly peraluminous I-type or fractionated I-type granites. They were derived predominantly from Proterozoic igneous protoliths. The juvenile crust and/or newly mantle-derived materials might also have been involved in some of these granites. In addition, the I-and fractionated I-type granites are closely associated in time and space with minor amount of A-type felsic and mafic volcanic and intrusive rocks as well as alkaline rocks. All these rocks constitute a typical assemblage of anorogenic, intraplate magmatism, suggesting a dominant lithospheric extensional regime for the Nanling Range and neighboring region during the Early Yanshanian period.

•Track the evolution of Central Iranian terrane via U–Pb age, Hf and O isotope data on detrital zircons.•Detrital zircon grains range in age from 0.52 and 3.2 Ga with four major peaks.•The provenances of pre-Neoproterozoic zircons are mainly from northern Gondwana.U–Pb dating and oxygen and Lu–Hf isotope analyses are applied to ~ 400 detrital zircon grains from the Neoproterozoic–Cambrian Kahar, Bayandor and Zaigun sandstones. The results reveal the evolutionary history of the Central Iranian continental crust in the northern margin of Gondwana during the Neoproterozoic–Cambrian. The U–Pb dating produces major peaks of crystallization ages at 0.5–0.7 Ga and minor peaks around the Tonian, Paleoproterozoic and Neoarchean. The zircon population in the Zaigun sandstone is dominated by long-transported grains and exhibits slightly different zircon distribution patterns than those from the older Kahar and Bayandor units. The zircon population ages and Hf isotopes of the Zaigun sample are very similar to the Neoproterozoic–Early Palaeozoic siliciclastic units in the Arabian Nubian shield (ANS) and Turkey, which suggests the late to post–Pan-African unroofing of the Afro–Arabia realm as the main process for detritus accumulation in Central Iran during the early Palaeozoic. A significant proportion of the Tonian-aged zircons (~ 64%) in the Kahar and Bayandor samples show positive εHf(t) values, whereas those with late Cryogenian–Ediacaran ages have high δ18O and variable εHf(t) values (~− 30‰ to + 17‰), suggesting that the crustal evolution of provenance of the Tonian-aged zircons commenced in an island arc setting and continued in an active continental margin. All the samples contain pre-Neoproterozoic zircons that are ca 1.9–2.3 Ga or 2.5–3.2 Ga, which are much older than the known Neoproterozoic igneous rocks in Iran and are more consistent with pre-Neoproterozoic igneous-metamorphic rocks in the eastern ANS and northern Africa. These ages support the eastern sector of the Afro–Arabia margin as a provenance for the detrital zircons in the oldest sedimentary sequences of Iran during the late Neoproterozoic–Cambrian. The Hf model ages of zircons with mantle-like δ18O values suggest that a significant amount of continental crust in the provenance of the detrital zircons was generated at around 1.0–2.0 and 3.0–3.5 Ga, likely by mantle-derived mafic magmas, and subsequently reworked during crustal differentiation into younger, more felsic crust with varying crustal residence times.Download high-res image (298KB)Download full-size image

South China was formed through the amalgamation of the Yangtze Block with the Cathaysia Block, but the timing of this amalgamation is controversial, ranging from Mesoproterozoic to Mesozoic. We report here SHRIMP U–Pb zircon ages, geochemistry and Nd–Hf isotopes of the Shuangxiwu Group volcanic rocks from the southeastern Yangtze Block. These rocks were strongly deformed, metamorphosed to greenschist-facies, intruded by 849 ± 7 Ma dolerites, and unconformably overlain by Neoproterozoic rift successions of no older than ca. 820 Ma. The Beiwu and Zhangcun volcanic rocks from the middle and uppermost Shuangxiwu Group were dated at 926 ± 15 Ma and 891 ± 12 Ma, respectively. All the studied rocks are characterized by highly positive ɛNd(T) (5.4–8.7) and ɛHf(T) (11.0–15.3) values. The Pingshui basaltic and andesitic rocks from the lower Shuangxiwu Group, which were previously dated at ca. 970 Ma, are high in Al2O3 (15–20%) but low in MgO (<8%), and are characterized by enrichments in Th and LREE but depletions in Nb, Ta, Zr, Hf and Ti, broadly similar to high-Al basaltic rocks in many volcanic arcs. The Beiwu andesitic to rhyolitic rocks have higher MgO than the experimental melts of basaltic rocks, and their Al2O3 content decreases with increasing SiO2, similar to the regional coeval tonalites and granodiorites, suggesting their formation by crystal fractionation of basaltic parent magma. The Zhangcun volcanic rocks are high in SiO2 (mostly >69%), low in MgO (0.35–1.2%), and have nearly constant Al2O3 contents of 14–15% and relatively uniform trace element concentrations. They were generated by remelting of juvenile mafic to intermediate arc rocks. Overall, the Shuangxiwu Group volcanic rocks and associated intrusive tonalites and granodiorites constitute a typical calc-alkaline magmatic assemblage of a 970–890 Ma active continental margin. These results and the 849 ± 7 Ma zircon U–Pb age for the undeformed doleritic dikes intruding the Shuangxiwu Group suggest that the tectonic regime of the study region transformed from plate convergence to intracontinental rifting in the time period between ca. 890 Ma and ca. 850 Ma. Previously reported 1.04–0.94 Ga metamorphic and deformation ages from the nearby Tianli Schists and evidence for the final closure of the back-arc basin at ca. 880 Ma (ophilitic obduction at Xiwan), further suggest that the amalgamation between the Yangtze and Cathaysia Blocks, likely through “soft docking” at the eastern segment of the Sibao orogen, was completed at ca. 880 Ma or soon after.

The Alxa Block is the westernmost part of the North China Craton (NCC). In the past, it has been considered to be part of the Archean NCC. However, formation and evolution of this block remain poorly understood, and this has hampered a broader understanding of the NCC. In this study we analyzed the in situ zircon U–Pb age and Hf–O isotopes for the two oldest rock units in the eastern Alxa Block, namely the Bayanwulashan and Diebusige Complexes. The Bayanwulashan Complex consists mainly of metamorphic rocks with mafic and felsic protoliths. SIMS U–Pb zircon dating results indicate that the primary magmatic ages of the mafic and felsic igneous rocks are ca. 2.34 Ga and ca. 2.32–2.30 Ga, respectively, and both sets of rocks were overprinted by two metamorphic events at ca. 1.89 Ga and ca. 1.79 Ga. Geochemical and zircon Hf–O isotopic data suggest that the ca. 2.34 Ga amphibolites within the Bayanwulashan Complex are characteristically high in TiO2 (up to 3.2%), Zr (up to 394 ppm) and Ti/V (> 30), resembling the basaltic rocks formed within continental rifts. The ca. 2.32–2.30 Ga felsic rocks were generated by remelting of dominant meta-igneous rocks that have zircon Hf model ages of 2.92–2.81 Ga. The Diebusige Complex is composed of amphibolites, mafic gneisses and paragneisses. It was intruded by granites dated at ca. 1.97–1.98 Ga, and subjected to high grade metamorphic events at ca. 1.89 Ga and ca. 1.79 Ga. Re-examination of U–Pb ages for detrital and metamorphic zircons indicate that the depositional ages of protoliths of the Diebusige paragneisses may be considered to be between ca. 2.45 and 2.0 Ga. These U–Pb zircon age data do not support the existence of exposed Archean rocks in the eastern Alxa Block, although the possibility of Archean rocks at deeper crustal levels cannot be ruled out entirely.The eastern Alxa Block contrasts to the neighboring Yinshan Block and Khondlite Belt, as well as the Trans-North China Orogen, in terms of the timing and evolution of magmatism and metamorphism. Thus, we conclude that the Alxa Block is a separated Paleoproterozoic terrane from the Western Block of the NCC, rather than the western extension of the Yinshan Block or part of the Khondalite Belt, as previously proposed.Download full-size imageHighlights► In situ U-Pb age and Hf-O isotope analyses for the Bayanwulashan and Diebusige Complexes in the eastern Alxa Block. ► No evidence to support existence of exposed Archean rocks in the eastern Alxa Block. ► Alxa Block was most likely an independent terrane to other parts of the North China Craton in the Paleoproterozoic.

•In situ U-Pb and Hf-O isotopes were analyzed for orthogneiss, leucosome and granite rocks from NFBC.•Syn-D2 magamtism and metamorphism took place at 620–610 Ma.•Syn- and post-D4 granites are dated respectively at 561 and 557 Ma.•≥620 Ma, 620–600 Ma, 590–580 Ma and 560 Ma correspond to respectively D1, D2, D3 and D4 phases.High-precision geochronological framework is crucial to understanding of the evolution of orogenesis. As part of the Pan-African North Equatorial Fold Belt, the Neoproterozoic Fold Belt of Cameroon (NFBC) involved polyphase deformation (D1 to D4 phases), metamorphism (up to granulite facies) and widespread crustal anatexis and magmatism. While a general geochronological framework has been established, controversy still exists on the temporal evolution of the metamorphic and magmatic events associated with polyphase deformation, in particular, the accurate age and geodynamic manifestations underlining the end of the orogeny are yet to be well constrained. We conduct in this work integrated analyses of SIMS U-Pb zircon and monazite dating and in-situ zircon Hf-O isotopes on the orthogneiss, leucosome and granite samples from Wum and Yaoundé of southwestern Cameroon in order to provide further constraints on temporal evolution of the NFBC. Our high-precision SIMS U-Pb age data demonstrate that the magmatic protoliths of orthogneisses from Wum and Yaoundé were emplaced synchronously at ca. 620 Ma. They were generated by partial melting of the thickened Paleoproterozoic crustal basement rocks with zircon Hf TDMc model age of ca. 1.9–2.0 Ga. Metamorphic zircons and two generations of monazites from an in-situ leucosome within the granulitic paragneiss of the Yaoundé Group yield indistinguishable ages of ca. 610 Ma. These new results, in combination with previous isotopic data, suggest that the Yaoundé Group supracrustal rocks with zircon Hf TDMc model age of ca. 1.7 Ga experienced a very rapid evolution from sedimentation at <626 Ma, burial to deformation and granulite facies metamorphism at ca. 610 Ma. The deformed two-mica granite and undeformed biotite granite in Wum were precisely dated at 561 ± 3 Ma and 557 ± 4 Ma, respectively. They both were derived from similar supracrustal sources with zircon Hf TDMc model age of ca. 2.56–2.40 Ga. On the basis of our new SIMS U-Pb ages, along with existing dates from the literature, an improved geochronological framework is proposed for the NFBC, with ages of ≥620 Ma, ca. 620–600 Ma, ca. 590–580 Ma, and ca. 580–560 Ma for the D1, D2, D3, and D4 phase events, respectively.

Ca. 825–720 Ma global continental intraplate magmatism is generally linked to mantle plumes or a mantle superplume that caused rifting and fragmentation of the supercontinent Rodinia. Widespread Neoproterozoic igneous rocks in South China are dated at ca. 825–760 Ma. There is a hot debate on their petrogenesis and tectonic affiliations, i.e., mantle plume/rift settings or collision/arc settings. Such competing interpretations have contrasting implications to the position of South China in the supercontinent Rodinia and in Rodinia reconstruction models.Variations in the bulk-rock compositions of primary basaltic melts can provide first order constraints on the mantle thermal–chemical structure, and thus distinguish between the plume/rift and arc/collision models. Whole-rock geochemical data of 14 mid-Neoproterozoic (825–760 Ma) basaltic successions are reviewed here in order to (1) estimate the primary melts compositions; (2) calculate the melting conditions and mantle potential temperature; and (3) identify the contributions of subcontinental lithosphere mantle (SCLM) and asenthospheric mantles to the generation of these basaltic rocks.In order to quantify the mantle potential temperatures and percentages of decompression melting, the primary MgO, FeO, and SiO2 contents of basalts are calculated through carefully selecting less-evolved samples using a melting model based on the partitioning of FeO and MgO in olivine. The mid-Neoproterozoic (825–760 Ma) potential temperatures predicted from the primary melts range from 1390 °C to 1630 °C (mostly > 1480 °C), suggesting that most 825–760 Ma basaltic rocks in South China were generated by melting of anomalously hot mantle sources with potential temperatures 80–200 °C higher than the ambient Middle Ocean Ridge Basalt (MORB)-source mantle.The mantle source regions of these Neoproterozoic basaltic rocks have complex histories and heterogeneous compositions. Enriched mantle sources (e.g., pyroxenite and eclogite) are recognized as an important source for the Bikou and Suxiong basalts, suggesting that their generations may have involved recycled components. Trace elements variations show that interactions between asthenospheric mantle (OIB-type mantle) and SCLM played a very important role in generation of the 825–760 Ma basalts. Our results indicate that the SCLM metasomatized by subduction-induced melts/fluids during the 1.0–0.9 Ga orogenesis as a distinct geochemical reservoir that contributed significantly to the trace-elements and isotope inventory of these basalts.The continental intraplate geochemical signatures (e.g., OIB-type), high mantle potential temperatures and recycled components suggest the presence of a mantle plume beneath the Neoproterozoic South China block. We use the available data to develop an integrated plume-lithosphere interaction model for the ca. 825–760 Ma basalts. The early phases of basaltic rocks (825–810 Ma) were most likely formed by melting within the metasomatized SCLM heated by the rising mantle plume. The subsequent continental rift allowed adiabatic decompression partial melting of an upwelling mantle plumes at relatively shallow depth to form the widespread syn-rifting basaltic rocks at ca. 810–800 Ma and 790–760 Ma.

The Mesozoic geology of SE China is characterized by widespread Jurassic to Cretaceous igneous rocks consisting predominantly of granites and rhyolites and subordinate mafic lithologies. However, the tectonic regime responsible for the inland Jurassic granites remains controversial. We report here U–Pb zircon ages, geochemical and Sr–Nd–Hf isotopic data for the Nankunshan alkaline granite and the Fogang granitic batholith in central Guangdong. Mineralogical and geochemical features suggest that the Fogang and Nankunshan rocks are I- and aluminous A-type granites, respectively. SHRIMP U–Pb zircon analyses yield consistent ages ranging from 159 ± 3 Ma to 165 ± 2 Ma for four samples from the Fogang Batholith, and an age of 158 ± 5 Ma for the Nankunshan Granite. The Fogang granites, having ISr = 0.7098–0.7136, εNd(T) = − 4.3 to − 12.2 and εHf(T) = − 11.5 to − 3.1 for the magmatic zircons, were derived from Paleoproterozoic mafic-intermediate igneous protolith with minor addition of mantle-derived magmas. The Nankunshan rocks have relatively lower ISr ≈ 0.706–0.708, higher εNd(T) = 0.3 to − 2.4 and εHf(T) = − 5.7 to 1.1 for the magmatic zircons, and some OIB-like trace element ratios. They were likely generated through extensive fractional crystallization of mantle-derived alkaline parental magma associated with crustal assimilation. These ∼ 160 Ma I- and A-type granites in central Guangdong were emplaced coeval with the widespread 165–155 Ma I- and A-type granites and syenites following the initiation of intraplate basaltic and/or bimodal igneous magmatism at 180–170 Ma in the adjacent regions. We interpret these Jurassic igneous rocks as anorogenic magmatism formed during a major igneous event in response to foundering of an early Mesozoic subducted flat-slab beneath SE China continent.

The Chenaillet Ophiolite is one of the best-preserved remnants of the Piemont-Liguria oceanic basin, a branch of the Central Atlantic that opened during the separation of Adria/Africa from Europe. Despite numerous studies of structure, petrology, geochemistry and isotope geochronology, the timing and genesis of various magmatic rocks within the Chenaillet Ophiolite are still controversial. We provide in this study integrated in situ analyses of zircon U–Pb age and O–Hf isotopes for the troctolite and albitite within the Chenaillet Ophiolite. Our new results indicate that the troctolite and albitite crystallized synchronously at ~ 165 Ma. Zircons from the troctolite have homogeneous Hf and O isotopic compositions, with εHf(T) = + 13.5 ± 1.0 (2SD) and δ18O = 5.4 ± 0.4‰ (2SD), indicating crystallization from magmas that were derived from a depleted, MORB-like mantle. The albitite zircons give consistent εHf(T) values (+ 13.0 to + 13.5) within errors with those of troctolite zircons, but variable δ18O values. The altered zircon domains have relatively low δ18O values of 4.7 ± 0.6‰ (2SD) due to subsolidus hydrothermal alteration, whilst the least-altered zircon domains give δ18Ozir values of 5.1 ± 0.4‰ (2SD), indistinguishable within errors with the troctolite zircons and the igneous zircons from the Mid-Atlantic and Southwest Indian Ridges gabbros, norites, and plagiogranites of modern oceanic crust. In situ zircon O–Hf isotopic data suggest that the troctolites and albitites are most likely cogenetic, with the albitites being formed by extreme fractional crystallization from the basaltic magma.Based on our new age results and compilation of the reliable literature U–Pb age data, the ophiolitic gabbros from Eastern, Central and, Western Alps, Liguria and Corsica crystallized nearly synchronously at 158–166 Ma, suggesting a short life span of < 11 m.y. for the formation of the Piemont-Liguria oceanic domain, rather than a ~ 30 m.y. life span as previously thought. The Chenaillet ophiolite is likely a remnant of embryonic oceanic crust, rather than a piece of “mature” oceanic crust. Provided the spreading velocities of < 3 cm/yr full rate for magma-poor MOR sequences, the maximum width to the Piemont-Liguria oceanic floor would have been in the order of 300 km.Highlights► The troctolite and albitite from the Chenaillet Ophiolite crystallized at ~ 165 Ma. ► Albitite was formed by extreme fractional crystallization from MORB-like magma. ► There is a short life span of < 11 m.y. for the Piemont-Liguria ocean formation. ► The Piemont-Liguria oceanic floor might have a maximum width of 300 km. ► The Chenaillet ophiolite is likely a piece of embryonic oceanic crust.

•Bangangte syenomonzonites in West Cameroon are emplaced at ∼584 Ma.•They are typical shoshonitic rocks generated by partial melting of an enriched mantle source.•These rocks display a transitional feature of subduction-related and within-plate shoshonites.•They emplaced in a post-collisional setting at the waning stage of Pan-African collisional orogeny.The Bangangte pluton is a SW-NE elongated (5 × 20 km) massif located in the southeastern part of the Pan-African North Equatorial Fold Belt in Cameroon, consisting of two units with dominant monzonites in the south and syenites in the north. SIMS U-Pb zircon dating yields consistent emplacement ages of 585 ± 4 Ma and 583 ± 4 Ma for the southern unit and the northern unit, respectively. The Bangangte rocks display typical shoshonitic compositions characterized by Na2O + K2O > 5 wt%, K2O/Na2O ∼2, enrichment in LILE and LREE, but depletion in HFSE. Rocks from both units have similar O-Hf isotopes, with the monzonite zircons from the southern unit showing slightly higher δ18O (7.0 ± 0.4‰) but lower εHf(t) (−15.3 ± 1.4) value than the syenite zircons from southern unit (δ18O = 6.0 ± 0.4‰; εHf(t) = −14.0 ± 2.0). They were generated by partial melting of an enriched mantle source metasomatized by previous subduction processes, accompanied by crystal fractionation of pyroxene, Ti-Fe oxides and apatite, as well as crustal contamination to varying degrees. These rocks display a transitional geochemical feature of the subduction-related and within-plate shoshonites, suggesting that they were most likely emplaced in a post-collisional setting at the waning stage of the Pan-African orogeny.

•Geochemical and Sr–Nd–Hf–O–C isotopic analyses are conducted on the Qieganbulake complex.•Carbonatites and silicate ultramafic rocks were cognetic and formed synchronously at ~ 810 Ma.•Primary carbonatitic magma was derived from an enriched subcontinental lithospheric mantle.•The Qieganbulake complex was formed by liquid immiscibility of carbonated silicate melts.•The complex is likely related to the Rodinian mantle plume.The Qieganbulake ultramafic–carbonatite complex located within the northeastern margin of the Tarim Block of Northwest China hosts the world's second largest vermiculite deposit. Field observations, radiometric dating results and Sr–Nd–Hf isotopes reveal that the parental magmas of the carbonatite and ultramafic rocks are cogenetic and formed synchronously at ~ 810 Ma. They are characterized by unusually enriched Sr–Nd–Hf isotopic compositions (ISr = 0.70570–0.70762, εNd(t) = − 7.7 to − 12.5, and εHf(t) = − 6.7 to − 12.9), indicating that the parent magmas were derived mainly from a subcontinental mantle source that had been metasomatised by subduction processes. Higher δ13C (− 3.65 to − 4.11‰) values compared to primary magmatic carbonate (− 8 to − 4‰) argue for incorporation of recycled inorganic carbon derived from subducted oceanic crusts. The carbonatites and clinopyroxenties define two distinct differentiation trends, which suggest that liquid immiscibility rather than crystal fractionation controlled the petrogenetic process. Pyroxenities have clearly higher apatite ISr and δ18O values than coexisting carbonatites, indicating involvement of crustal components during their emplacement. The Qieganbulake complex is closely associated in time and space with the mid-Neoproterozoic Rodinia breakup event triggered by mantle plume activities in the Tarim Block. Therefore, the mantle plume likely induced partial malting of, and likely mixed with, the metasomatized subcontinental lithospheric mantle, to form the Qieganbulake ultramafic–carbonatite complex.

It has been generally accepted that the South China Block was formed through amalgamation of the Yangtze and Cathaysia Blocks during the Proterozoic Sibaoan orogenesis, but the timing and kinematics of the Sibao orogeny are still not well constrained. We report here SHRIMP U–Pb zircon geochronological and geochemical data for the Taohong and Xiqiu tonalite–granodiorite stocks from northeastern Zhejiang, southeastern margin of the Yangtze Block. Our data demonstrate that these rocks, dated at 913 ± 15 Ma and 905 ± 14 Ma, are typical amphibole-rich calc-alkaline granitoids formed in an active continental margin. Combined with previously reported isotopic dates for the ∼ 1.0 Ga ophiolites and ∼ 0.97 Ga adakitic rocks from northeastern Jiangxi, the timing of the Sibao orogenesis is thus believed to be between ∼ 1.0 and ∼ 0.9 Ga in its eastern segment. It is noted that the Sibao orogeny in South China is in general contemporaneous with some other early Neoproterozoic (1.0–0.9 Ga) orogenic belts such as the Eastern Ghats Belt of India and the Rayner Province in East Antarctica, indicating that the assembly of Rodinia was not finally completed until ∼ 0.9 Ga.

•Ophiolitic gabbros and plagiogranites in Schistes Lustrés nappe formed at ~ 159 Ma.•These ophiolitic rocks were cogenetic and derived from N-MORB type mantle.•They represent the youngest remnants of the western Liguria–Piemonte ocean basin.Knowledge of the age and timing of ophiolite sequences is essential for understanding the mechanisms of plate tectonics. The ophiolites in the Schistes Lustrés and the Upper nappes of Alpine Corsica represent remnants of the Liguria–Piemonte ocean basin that formed as a branch of the Central Atlantic basin during the opening of the Mesozoic Western Alpine Tethys. Despite numerous isotopic and paleontological studies, the age and timing of the ophiolites in the Schistes Lustrés nappe are still controversial. This study presents integrated in situ analyses of zircon U–Pb age and O–Hf isotopic data for ophiolitic gabbros and plagiogranites from three localities in the Schistes Lustrés nappe of Eastern Corsica. Our new results demonstrate that these rocks crystallized synchronously at ~ 159 Ma, approximately 10 m.y. younger than the ophiolites in the Balagne Upper nappe. Zircons from the gabbros and plagiogranites are characterized by highly positive εHf(t) (+ 15.0 to + 15.9) and mantle-like δ18O (5.2–5.4‰) values. Thus, these ophiolitic rocks were cogenetic, and crystallized from magmas produced by partial melting of a depleted, N-MORB type mantle. By contrast, in the Balagne Upper nappe, the ~ 169 Ma ophiolites contain numerous xenocrystic zircons inherited from a continental crust. Our current knowledge of isotopic geochronology and geochemistry supports a paleogeographic reconstruction, in which the earliest ophiolites in the Balagne nappe were emplaced close to a continental margin at ~ 169 Ma, while the N-MORB type ophiolites in the Schistes Lustrés nappe were likely formed approximately 10 m.y. later in the central part of the Liguria–Piemonte oceanic basin. The relative location of the Schistes Lustrés and Balagne Upper nappes with respect to continental margins is discussed.

•The nephrite from Luanchuan, China is formed in a dolomite marble metasomatism condition.•In-Situ SIMS U–Pb dating on the syngenetic titanite revealed the formation age of nephrite.•The Luanchuan nephrite formation is not related to the intrusion of the intruded gabbro.Nephrite is a kind of valuable jade which forms during metamorphism under greenschist facies conditions. Most investigations focus on the genesis of nephrite, however, the formation age of nephrite remains poorly constrained due to the lack of suitable dating methods. In this paper, the petrological, chemical characteristics, and mineral inclusions of nephrite collected from Luanchuan, Henan, China have been studied by optical and electron microscopy and wavelength-dispersive electron microprobe (EMP). The petrological observations show that nephrite consists mainly of tremolite, minor calcite and titanite, occasionally with rutile, quartz, serpentine, chlorite, pyrite, and apatite. The titanite and tremolite which are intergrown with each other share low-energy grain boundaries. It indicates that the two phases are equilibrated and can be considered as cogenetic. The chemical composition of tremolite is high in Si, Mg, Ca, but low in Fe, Cr, and Ni, which indicates that the nephrite from Luanchuan belongs to the dolomite type rather than the serpentinite type. The petrography of different nephrite structures suggests that the grain size of tremolite was determined by the stress intensity undergone during the nephrite formation progress. In-situ Secondary Ion Mass Spectrometer (SIMS) U–Pb dating of titanite from nephrite yielded an age of 361 ± 4 Ma, which is the first estimate for the time of formation of the Luanchuan nephrite deposit. This investigation provides a powerful in-situ dating method to unravel the age of nephrite, which could be served as a tool for future research on other nephrite deposits.

•The Suzhou zircon has mantle δ18O values of 5.34 ± 0.46‰.•SIMS apparent U–Pb age and δ18O value for high-U zircon are negatively correlated.•The lower δ18O values of high-U zircon reflect effects of radiation damage.•Quartz preserves the original O-isotopes of the magma better than high-U zircon.•Evaluating metamictization effect on zircon O-isotopes helps data interpretations.Zircon is widely used in geochronology and as an isotopic tracer in igneous processes. However, the use of isotopic data obtained from zircon requires evaluating of processes that might compromise the robustness of its isotopic systems. In this study we have assessed how different U contents and degrees of metamictization have affected the SIMS U–Pb and oxygen-isotope signatures of zircon–quartz pairs from the U-rich A-type granites from Suzhou (southern China). Our SIMS results show that zircons with low U concentrations define a U–Pb age of 126.1 ± 0.5 Ma and homogeneous mantle δ18O values of 5.34 ± 0.46‰ (2SD). In contrast, zircons with high U contents show a positive correlation between U concentration and U–Pb ages (123–157 Ma) and a negative correlation between U concentration and δ18O values (range from 5.3 to 3.7‰). These observations are integrated with Raman spectra and oxygen-isotope analyses of coexisting quartz to reveal that the anomalously high U–Pb ages and low δ18O values reflect the degree of radiation damage to the zircons (quantified by radiation dose in displacements-per-atom, Ddpa). The older SIMS U–Pb ages are a direct result of the matrix effect caused by metamictization, whereas the low δ18O values result from interaction with OH-bearing fluids, the infiltration and diffusion of which was facilitated in the radiation-damaged areas.Coexisting quartz preserves primary O-isotope signatures of the magma better than high-U zircon, and thus provides a constraint for zircon oxygen-isotope values. This study demonstrates that it is essential to evaluate the degree of radiation damage before/after carrying out dating or oxygen-isotope analysis on zircons with high U contents. The results indicate that for the Suzhou pluton, Ddpa < 0.03 is a robust discriminant threshold to identify zircons with primary oxygen-isotope ratios, and Ddpa < 0.08 is a robust discriminant to screen for reliable U–Pb dating. These different screening values reflect the evidence that oxygen-isotope compositions can be affected at a lower level of post-crystallization disturbance of the zircon lattice than the U–Pb system.

•Cathaysia Block formed at ~ 1.9-1.8 Ga, rather than in Archean.•Archean detrital zircons found in Cathaysia were likely sourced from East Antarctica.•Cathaysia locates between E. Antarctica, Laurentia and Australia in Columbia/Nuna.•South China most likely locates in the interior, but not on fringe, of Rodinia.•South China connects with Northern India by a “Pan-African” collisional orogeny.The South China Block, consisting of the Yangtze and the Cathaysia blocks, is one of the largest Precambrian blocks in eastern Asia. However, the early history of the Cathaysia Block is poorly understood due largely to intensive and extensive reworking by Phanerozoic polyphase orogenesis and magmatism which strongly overprinted and obscured much of the Precambrian geological record. In this paper, we use the detrital zircon U–Pb age and Hf isotope datasets as an alternative approach to delineate the early history of the Cathaysia Block. Compilation of published 4041 Precambrian detrital zircon ages from a number of (meta)sedimentary samples and river sands exhibits a broad age spectrum, with three major peaks at ~ 2485 Ma, ~ 1853 Ma and ~ 970 Ma (counting for ~ 10%, ~ 16% and ~ 24% of all analyses, respectively), and four subordinate peaks at ~ 1426 Ma, ~ 1074 Ma, ~ 780 Ma and ~ 588 Ma. Five of seven detrital zircon age peaks are broadly coincident with the crystallisation ages of ~ 1.89–1.83 Ga, ~ 1.43 Ga, ~ 1.0–0.98 Ga and ~ 0.82–0.72 Ga for known igneous rocks exposed in Cathaysia, whereas, igneous rocks with ages of ~ 2.49 Ga and ~ 0.59 Ga have not yet been found. The Hf isotopic data from 1085 detrital zircons yield Hf model ages (TDMC) between ~ 4.19 Ga and ~ 0.81 Ga, and the calculated εHf(t) values between − 40.2 and 14.4. The Archean detrital zircons are exclusively oval in shape with complicated internal textures, indicating that they were sourced by long distance transportations and strong abrasion from an exotic Archean continent. In contrast, the majority of detrital zircons in age between ~ 1.9 and ~ 0.8 Ga are euhedral to subhedral crystals, indicative of local derivation by short distance transportations from their sources. The oldest crustal basement rocks in Cathaysia were most likely formed by generation of juvenile crust and reworking of recycled Archean components in Late Paleoproterozoic at ~ 1.9–1.8 Ga, rather than in the Archean as previously speculated. Reworking and recycling of the continental crust are likely the dominant processes for the crustal evolution of Cathaysia during the Mesoproterozoic to Neoproterozoic time, with an intervenient period of significant generation of juvenile crust at ~ 1.0 Ga.Precambrian crustal evolutions of the Cathaysia Block are genetically related to the supercontinent cycles. By comparing detrital zircon data from Cathaysia with those for other continents, and integrating multiple lines of geological evidence, we interpret the Cathaysia Block as an orogenic belt located between East Antarctica, Laurentia and Australia during the assembly of supercontinent Columbia/Nuna at ~ 1.9–1.8 Ga. The Cathaysia Block amalgamated with the Yangtze Block to form the united South China Block during the Sibao Orogeny at ~ 1.0–0.89 Ga. The Laurentia–Cathaysia–Yangtze–Australia–East Antarctica connection gives the best solution to the paleo-position of Cathaysia in supercontinent Rodinia. The significant amount of ~ 0.6–0.55 Ga detrital zircons in Cathaysia and West Yangtze have exclusively high crustal incubation time of > 300 Ma, indicating crystallisation from magmas generated dominantly by crustal reworking. This detrital zircon population compares well with the similar-aged zircon populations from a number of Gondwana-derived terranes including Tethyan Himalaya, High Himalaya, Qiangtang and Indochina. The united South China–Indochina continent was likely once an integral part of Gondwanaland, connected to northern India by a “Pan-African” collisional orogen.Download full-size image

Leucogranitic lenses are found within the Xiwan ophiolitic mélange in northeastern Jiangxi Province, South China. The leucogranites occur exclusively within the serpentinized peridotite unit of the ophiolite suite. SHRIMP U–Pb zircon dating results indicate that these granites were formed at 880 ± 19 Ma, and were overprinted by an Indosinian tectono-thermal event at ~ 230 Ma. The leucogranites are peraluminous (A/CNK = 1.0–1.24), characterized by high Al2O3 (14–18.33%) and Na2O (6.5–10%) and clearly low εNd(T) values of 0.8 to − 3.9 compared with the other rock units of the ophiolite suite. On the basis of their REE characters, the leucogranites can be divided into three groups. Group I leucogranites show the most fractionated LREE-enrichment patterns (with LaN/YbN and LaN/SmN ratios of 30.1–75.0 and 2.3–3.9, respectively). Group II leucogranites have moderately fractionated LREE-enrichment patterns (with LaN/YbN and LaN/SmN ratios of 13.1–26.5 and 0.8–1.9, respectively). Group III leucogranites are characterized by obviously low total REE contents and flat REE patterns with significant positive Eu anomalies, probably due to small degrees of partial melting. All these leucogranites were likely formed by partial melting of sedimentary rocks from a marginal basin at the Yangtze side of the orogen, beneath a major thrust fault during the obduction of the ophiolite onto the continental crust. They are broadly similar to obduction-related granites within ophiolites identified in many places worldwide. Identification of the ca. 880 Ma obduction-type granites in the NE Jiangxi ophiolite provides a petrological constraint on the timing of the ophiolite obduction onto the continental crust. In combination with the termination of the Shuangxiwu arc magmatism at ca. 890 Ma, we interpret that the close of the Neoproterozoic back-arc basin and the termination of the continental amalgamation between the Yangtze and Cathaysia Blocks occurred at ca. 880 Ma.

•Regional migmatization under amphibolite-facies condition occurred at ~ 345 Ma.•Mg–K magmatic rocks of Calvi-IIe Rousse pluton were emplaced at ~ 330 Ma.•Slab breakoff triggers post-collisional crustal anatexis and mantle melting.High-precision SIMS U–Pb zircon age determinations are conducted in this study on migmatites and Mg–K magmatic suites from the Variscan orogen of Corsica (France). Zircons from leucosomes of four migmatites yield consistent crystallization ages of ca. 345 Ma. Four Mg–K granitoid rocks and one monzogabbro enclave from the Calvi-IIe Rousse pluton of NW Corsica yield indistinguishable U–Pb zircon ages of ca. 330 Ma. These new SIMS zircon U–Pb dating results indicate that the regional crustal anatexis under amphibolite-facies condition occurred synchronously at ca. 345 Ma throughout the Corsican Variscan chain. There is a ca. 15 m.y. time interval between anatexis of the thickened crust and partial melting of the metasomatized mantle and overlying crust to form the Mg–K rock suites. In combination with the previous dating results for the high-temperature and high-pressure granulite-facies metamorphism, three major discrete tectonothermal events at ca. 360 Ma, ca. 345 Ma, and ca. 330 Ma are acknowledged. We argue for a slab break-off model accounting for thermal and mechanical evolution of the crust within the Variscan orogen of Corsica. A tearing of the subducting oceanic lithosphere initiates the asthenosphere rise to fill the void and causes the granulite-facies metamorphism of the overlying continental crust at ca. 360 Ma. Final breaking of the subducting oceanic lithosphere results in exhumation of the subducted crust to the thickened lower to middle crustal-level. Internal radiogenic heating causes crust-scale migmatization at ca. 345 Ma. Slab break-off and foundering enhance the rise of hot asthenosphere, resulting in conducted thermal perturbation that leads to melting of the metasomatised enriched mantle lithosphere to form Mg–K mafic–granitoid magmatism at ca. 330 Ma.

Long-term couplings between the subducting and overlying plates are very important to understanding plate tectonics, in particular intraplate evolutions. Geological records of this coupling however, are usually not well preserved. Here we show a good example in eastern China where Cretaceous tectonic evolution matches remarkably well with the drifting history of the Pacific plate. The most pronounced phenomenon is that the eastern China large-scale orogenic lode gold (Au) mineralization occurred contemporaneously with an abrupt change of ~ 80° in the drifting direction of the subducting Pacific plate, concurrent with the formation of the Ontong Java Plateau. Given that lode Au deposits usually form at the onset of compressional or transpressional deformations, the Au deposits dated the major tectonic change from extension to transpression in eastern China, coherent with the subduction regime. The Cretaceous drifting history of the Pacific plate also tallies with other major geological events in eastern China, e.g., the evolution of the Tan-Lu fault and magmatic activities, suggesting that the major geological events in eastern China in the Cretaceous were mainly controlled by the subduction of the Pacific plate, and that plate interactions during subduction are important driving forces for geological evolution in eastern China and intraplate tectonics in general.

The origin and tectonic significance of high-K granites (>3 wt% K2O at 70 wt% SiO2), calc-alkaline I-type granites in particular, remain controversial. This paper takes granitic plutons distributed in the coastal region of the Guangdong Province of southeastern China as examples to explore the genesis of such rocks. SIMS zircon U–Pb geochronological data show that the granites were emplaced at 166–159 Ma. These granites can be subdivided generally into two groups on the basis of integrated mineralogical, geochemical, whole-rock Sr–Nd isotopic and in situ zircon Hf–O isotopic studies. The group A granites (SiO2 = 64–72 wt%) are characterised by their common occurrence of amphibole (±titanite) and dominantly metaluminous feature (A/CNK = 0.85–1.03). They are high in K2O (3.5–7.0 wt%) and K2O/Na2O (>1), and have trace element concentrations (e.g., Nb, Y, Zr and Ga) similar to typical I-type granites in the Lachlan Fold Belt, southeastern Australia. Their whole-rock ISr (0.7057–0.7077) and εNd(t) (−6.46 to −3.13) are less evolved than many coeval granites in this region. As in situ zircon Hf–O isotopes show little evidence of magma mixing, these granites with low zircon δ18O (6.3−7.9‰) and high εHf(t) (−5.9 to −0.2) could have been generated from melting of oxidised high-K basaltic rocks. The group B granites, emplaced to the east of group A granites, are dominantly weakly peraluminous (A/CNK = 1.00–1.05). They have higher SiO2 (70–76 wt%), less common or absence of amphibole, higher zircon δ18O (6.6−9.0‰) and lower εHf(t) (−11.4 to −5.9) than the group A granites. Zircon Hf–O isotope data reveal that the group B granites contain higher percentage of supracrustal materials than those of the group A, but the variations of major and trace elements do not support an assimilation and fractional crystallization (AFC) model. Instead, the group B granites, with features transitional between typical I-type and S-type granites, were most likely formed in a region where there was physical juxtaposition between infracrustal metaluminous and supracrustal peraluminous source rocks. Thus, granites of both groups represent products of crustal reworking likely due to asthenosphere upwelling and/or underplating and intrusion of mafic magmas. The close association in time and space of these granites with OIB-like basaltic rocks and the secular compositional change of Jurassic basaltic rocks in the region suggest that these rocks probably formed in an intraplate extensional setting resulted from the delamination of a flat-subducted oceanic slab.Highlights► In-situ zircon U–Pb–Hf–O isotopes and whole-rock geochemistry and Sr–Nd isotopes for high-K granites from SE China. ► These high-K granites are dated at 166–159 Ma. ► Some granites show features of typical high-K I-type, whereas the others’ are transitional between I- and S-type. ► These rocks are products of crustal reworking by mantle-derived magmas in an intraplate extension regime.

•The Longlou granite pluton dated at 73 Ma is the youngest one recognized so far in SE China.•It is generated in an active continental margin related to subduction of Paleo-Pacific plate.•Transition from Andean- to West Pacific-type continental margin of SE China occurred at 70 Ma.We report in the paper integrated analyses of in situ zircon U–Pb ages, Hf–O isotopes, whole-rock geochemistry and Sr–Nd isotopes for the Longlou granite in northern Hainan Island, southeast China. SIMS zircon U–Pb dating results yield a crystallization age of ∼73 Ma for the Longlou granite, which is the youngest granite recognized in southeast China. The granite rocks are characterized by high SiO2 and K2O, weakly peraluminous (A/CNK = 1.04–1.10), depletion in Sr, Ba and high field strength elements (HFSE) and enrichment in LREE and large ion lithophile elements (LILE). Chemical variations of the granite are dominated by fractional crystallization of feldspar, biotite, Ti–Fe oxides and apatite. Their whole-rock initial 87Sr/86Sr ratios (0.7073–0.7107) and εNd(t) (−4.6 to −6.6) and zircon εHf(t) (−5.0 to 0.8) values are broadly consistent with those of the Late Mesozoic granites in southeast China coast. Zircon δ18O values of 6.9–8.3‰ suggest insignificant involvement of supracrustal materials in the granites. These granites are likely generated by partial melting of medium- to high-K basaltic rocks in an active continental margin related to subduction of the Pacific plate. The ca. 73 Ma Longlou granite is broadly coeval with the Campanian (ca. 80–70 Ma) granitoid rocks in southwest Japan and South Korea, indicating that they might be formed along a common Andean-type active continental margin of east–southeast Asia. Tectonic transition from the Andean-type to the West Pacific-type continental margin of southeast China likely took place at ca.70 Ma, rather than ca. 90–85 Ma as previously thought.

•Ash bed from lower Liuchapo Formation is dated at 545.76 ± 0.66 Ma.•Stratigraphic correlation of E-C transitional successions in South China is present.•No significant oceanic oxygenation occurred during 551–535 Ma.The continuous late Ediacaran - early Cambrian deep-water successions of South China archive the complete evolution of seawater chemical conditions in the deep ocean during this critical time interval. However, the geochemical data from these poorly fossiliferous and condensed successions lack high-resolution stratigraphic constraints, hampering their interpretation for the spatio-temporal evolution of the sweater chemistry in this time interval. In this study, we report a new SIMS and CA-ID-TIMS zircon U-Pb age 545.76 ± 0.66 Ma (total uncertainty) for an ash bed at the lower Liuchapo Formation in the deep-water Longbizui section in western Hunan Province. The new age suggests that the lower and the middle-upper parts of the Liuchapo Formation in the deep water facies can be correlated with the lower Dengying Formation and the upper Dengying - lower Zhujiaqing formations in the shallow water facies, respectively. This correlation implies that the correlative horizon of the Ediacaran-Cambrian boundary in the deep water facies in South China is likely located near the base of a widespread negative δ13Corg excursion at the upper Liuchapo Formation. Based on our new geochronological framework, the compilations of Fe-speciation, Mo, and U data indicate that the deep ocean was characterized by widespread anoxic, ferruginous water, with intermittent euxinic water impinged on the middle-lower slope in Ediacaran-Cambrian transition, and significant oxygenation events occurred in 533–520 Ma. The compilations do not support any significant oceanic oxygenation events in 551–535 Ma.

Zircon is arguably the best, certainly the most commonly used mineral for U–Pb geochronology. Modern large-geometry secondary ion mass spectrometry (SIMS) has been routinely utilized for precise U–Pb zircon age determination at a lateral resolution of 10–30 μm. However, in situ U–Pb dating at a scale of ca. 5 μm scale or less for fine-grained zircons and/or zircon crystals with complex structural and chemical features is still a challenge to the geoscience community. Here we describe a method of precise U–Pb dating for zircons as young as the Jurassic age at a scale of up to <5 μm using the CAMECA ims-1280 SIMS. Gaussian mode primary O2− and O−probes of ca. 5.2 μm and ca. 4.5 μm in diameter with beam intensities of ∼100 pA were obtained, respectively, by optimizing the primary column. Secondary ion optics was optimized to ensure a high Pb+ sensitivity in zircons, producing ∼21 cps/ppm/nA using O2− and ∼13 cps/ppm/nA using O− (with oxygen flooding technique). As a demonstration of this method, three well-characterized zircon standards with a range of ages, AS3 (1099 Ma), Plešovice (377 Ma) and Qinghu (159.5 Ma), were analyzed. We demonstrate with these zircon standards that their ages could be determined with precision and accuracy of 1–2% using a spot <5 micron. The O− primary beam is preferred over the O2− beam for small-spot U–Pb zircon geochronology, because it has higher density and produces smaller craters on the target surface, with insignificant trade off in precision and accuracy of the final U–Pb ages. For U-poor minerals of younger ages, O2− might be preferred in order to generate sufficient Pb+ ions for measurement with minimal loss of spatial resolution.

Baddeleyite has long been recognized as one of the most important U-bearing minerals for dating silica undersaturated igneous rocks. Age determination of baddeleyite calls for analysis within small volumes using high-resolution secondary ion mass spectrometry (SIMS) because of its minuscule grain size as well as potential altered domains or micro-inclusions. However, precise SIMS U–Pb dating has been hampered for baddeleyite owing to crystal orientation effects that bias Pb/U ratio measured in baddeleyite. In this study we carried out a series of tests of U–Pb and Pb–Pb measurements on Phanerozoic baddeleyite using a multi-collector Cameca 1280 IMS with oxygen flooding technique. Our results demonstrate that the oxygen flooding can not only enhance secondary Pb+ ion yield by a fact of 7 for baddeleyite, but also depress the baddeleyite U/Pb orientation effect down to ∼2% (1 RSD). Therefore, Phanerozoic (as young as Cenozoic) baddeleyite can be precisely dated by SIMS Pb–Pb and/or U–Pb measurements with precision of 1–3% (2 RSE).

Zircon is the most useful mineral for studies in U–Pb geochronology, Hf and O isotope geochemistry, trace element geochemistry, and increasingly geothermometry. In situSIMS zircon Li isotope analysis shows potential for studying the genesis of crustal magmas and evolution of the continental crust, but its application has been hampered due to lack of well-characterized zircon Li isotope standards. Reconnaissance SIMS measurement of Li isotope ratio and concentration for several commonly-used zircon U–Pb age standards, including M257, BR266, Plešovice, 91500 and TEMORA 2 zircons are reported here. Of these, the M257 zircon is demonstrated to be homogeneous in Li isotopic composition, with δ7Li = 2.1 ± 1.0‰ (2SD). It is also relatively homogeneous in Li concentration, with Li concentration = 0.86 ± 0.18 ppm (2SD). Therefore, we recommend the new M257 zircon standard as a working reference for SIMS Li isotope and concentration measurements in zircons.

The surface condition of a sample mount is an important factor influencing the precision of SIMS isotope analysis. The phenomenon that the sample topography affects the analytical precision is called the topography effect. We carried out a systematic experiment of O-isotope analyses using a Cameca IMS-1280 SIMS to quantitatively characterize the topography effect with the aim of better understanding its physical mechanism underlying such an artifact and ultimately improving the analytical precision. Our results indicate that within a mineral grain, the topography effect is obvious in the X-direction (horizontal direction) of the sample stage but insignificant in the Y-direction (vertical direction). In addition, within a single mineral grain, the topography effect creates analytical spots on the left rim (lower X-coordinates) yielding higher measured δ18O values than those on the right rim (higher X-coordinates) in our instrument. The physical reason that the topography effect compromises the analytical reproducibility is attributed to lateral energy dispersion of secondary ions caused by surface topography changing the ion position in the entrance slit plane. By increasing the transfer optics magnification, the topography effect could be significantly reduced. Beam centering parameters could be used to quantitatively assess the topography effect and improve the data quality.