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The calcium isotope composition of seawater (δ44/40Ca sw) records important information on the evolution of the calcium and carbon cycles over geologic time. Over the past two decades, many attempts have been made to reconstruct the Neogene δ44/40Ca sw variation using various calcium isotope records of seawater or seawater-like pore fluids. Nonetheless, large discrepancies still exist among these Neogene δ44/40Ca sw records. Here we report δ44/40Ca values in a suite of Neogene island dolomites (20.8–3.4 Ma, n = 18) from the South China Sea. Combining multiple geochemical proxies (δ44/40Ca, δ13C, and δ18O, this study; Sr/Ca, ∑REY, δ7Li, δ26Mg, 87Sr/86Sr, and Δ 47 , previous studies) and evidence from petrography, fluid inclusions, and magnetostratigraphy, we suggest that the dolomitization for these dolomites was buffered by seawater-like fluids in the shallow marine burial domain or even marine diagenetic domain (<180 m) over a considerable period of time (<2 Myr). Then, we apply the equilibrium calcium isotope fractionation factor between dolomite and seawater of −0.52 ± 0.20 ‰ (2SD, n = 13) estimated from the literature to these fluid-buffered dolomites and reconstruct a new Neogene δ44/40Ca sw record. After using the Locally Weighted Regression (LOWESS) curve fitting method, our dolomite-based δ44/40Ca sw record is consistent with the reconciled seawater record from other calcium isotope archives, including shark teeth, corals, foraminifera, brachiopods, barites, phosphates, and bulk pelagic carbonates. Our results support the use of fabric-retentive early marine diagenetic dolomites to trace the evolution of δ44/40Ca sw through Earth's history. [ABSTRACT FROM AUTHOR]

High-precision potassium (K) isotope measurements in marine carbonates allow using this novel geochemical proxy to constrain seawater chemistry through geologic time. However, these measurements are still scarce due to the challenges of low-K contents in carbonates during K ion chromatography, such as insufficient sample purification, non-quantitative yield, and high accumulative blank. Here we optimize a dual-column K purification method that addresses these challenges, achieving a satisfactory K separation using 100–150 mg carbonates for routine high-precision K isotope analysis on the Sapphire™ MC-ICP-MS. We then report K isotope compositions in multiple certified reference materials, including limestone, dolostone, coral, and basalt for future inter-laboratory comparisons. The optimized K purification method provides great potential for future K isotope studies of marine carbonates. [ABSTRACT FROM AUTHOR]

We present new geochronological and geochemical data for granites and volcanic rocks of the Erguna massif, NE China. These data are integrated with previous findings to better constrain the nature of the massif basement and to provide new insights into the subduction history of Mongol–Okhotsk oceanic crust and its closure. U–Pb dating of zircons from 12 granites previously mapped as Palaeoproterozoic and from three granites reported as Neoproterozoic yield exclusively Phanerozoic ages. These new ages, together with recently reported isotopic dates for the metamorphic and igneous basement rocks, as well as Nd–Hf crustal-residence ages, suggest that it is unlikely that pre-Mesoproterozoic basement exists in the Erguna massif. The geochronological and geochemical results are consistent with a three-stage subduction history of Mongol–Okhotsk oceanic crust beneath the Erguna massif, as follows. (1) The Erguna massif records a transition from Late Devonian A-type magmatism to Carboniferous adakitic magmatism. This indicates that southward subduction of the Mongol–Okhotsk oceanic crust along the northern margin of the Erguna massif began in the Carboniferous. (2) Late Permian–Middle Triassic granitoids in the Erguna massif are distributed along the Mongol–Okhotsk suture zone and coeval magmatic rocks in the Xing’an terrane are scarce, suggesting that they are unlikely to have formed in association with the collision between the North China Craton and the Jiamusi–Mongolia block along the Solonker–Xra Moron–Changchun–Yanji suture zone. Instead, the apparent subduction-related signature of the granites and their proximity to the Mongol–Okhotsk suture zone suggest that they are related to southward subduction of Mongol–Okhotsk oceanic crust. (3) A conspicuous lack of magmatic activity during the Middle Jurassic marks an abrupt shift in magmatic style from Late Triassic–Early Jurassic normal and adakite-like calc-alkaline magmatism (pre-quiescent episode) to Late Jurassic–Early Cretaceous A-type felsic magmatism (post-quiescent episode). Evidently a significant change in geodynamic processes took place during the Middle Jurassic. Late Triassic–Early Jurassic subduction-related signatures and adakitic affinities confirm the existence of subduction during this time. Late Jurassic–Early Cretaceous post-collision magmatism constrains the timing of the final closure of the Mongol–Okhotsk Ocean involving collision between the Jiamusi–Mongolia block and the Siberian Craton to the Middle Jurassic. [ABSTRACT FROM AUTHOR]

We have undertaken major and trace element analyses of volcanic rocks in Northeast China, as well as U–Pb dating and Hf isotopic analysis of their zircons, in order to determine the petrogenesis and tectonic setting of the volcanics. Mesozoic volcanism in the southern Manzhouli area occurred in two stages: Middle to Late Jurassic (164–147 Ma) and Early Cretaceous (142–123 Ma). The first stage is represented by the Tamulangou, Jixiangfeng, and Qiyimuchang formations. The Jixiangfeng Formation (162–156 Ma) is a rhyolite–trachyte dominated unit that lies between two basalt units, namely the underlying Tamulangou (164–160 Ma) and overlying Qiyimuchang (151–147 Ma) formations. The second igneous stage is dominated by rhyolitic lavas and tuffs of the Shangkuli Formation and basaltic rocks of the Yiliekede Formation, and they yield zircon U–Pb ages of 142–125 and 135–123 Ma, respectively. Basaltic rocks of the Tamulangou and Yiliekede formations have a wide range of MgO contents (1.64–9.59 wt%), but are consistently depleted of Nb and Ta and enriched with incompatible trace elements such as large ion lithophile elements (LILEs) and light rare earth elements (LREEs). Trachytes and rhyolites of the Jixiangfeng and Shangkuli formations are characterized by enrichment in LILEs and LREEs relative to HFSEs and HREEs, and with negative Nb, Ta, P, and Ti anomalies and positiveϵHf(t) values (3.49–9.98). These data suggest that basaltic volcanic rocks in southern Manzhouli were generated by fractional crystallization of a common parental magma, which was derived by partial melting of metasomatized (enriched) lithospheric mantle, whereas the trachytic and rhyolitic magmas were produced by the melting of lower crustal mafic and felsic granulites, respectively. Geochronological data indicate that Mesozoic volcanism in southern Manzhouli was initiated in the Middle to Late Jurassic and continued into the Early Cretaceous. It was mainly induced by lithospheric extension after the closure of the Mongol–Okhotsk Ocean. [ABSTRACT FROM PUBLISHER]

Abstract: U–Pb detrital zircon geochronology from Lower Devonian quartz arenites of the northwestern margin of the Yangtze block yields dominant early Neoproterozoic (0.85–1.0Ga), Pan-African (0.5–0.65Ga) and middle Neoproterozoic (0.68–0.8Ga) age populations and minor Mesoproterozoic to middle Mesoarchean (1.0–3.0Ga) ages. Middle Mesoarchean to Mesoproterozoic rocks, however, are widespread in the South China block. Although Hf isotopic compositions show both juvenile crustal growth and crustal reworking for all the age groupings, the crust growth, essentially mantle-derived, occurred mainly around 3.1Ga, 1.9Ga and 1.0Ga, respectively. Zircon typology and youngest grain ages indicate that this suite of quartz arenites was the product of multiphase reworking. Abundant magmatic zircon detritus with concordant U–Pb Grenvillian and Pan-African ages, together with accompanying various ε Hf(t) values, indicate an exotic provenance for the quartz arenite external to the South China block. Qualitative comparisons of age spectra for the late Neoproterozoic sediments of the Cathaysian Block, early Paleozoic sediments of pre-rift Tethyan Himalaya sequence in North India and lower Paleozoic sandstone from the Perth Basin in West Australia, show that they all have two the largest age clusters representing Grenvillian and Pan-African orogenic episodes. The resemblance of these age spectra and zircon typology suggests that the most likely source for the Lower Devonian quartz arenites of the South China block was the East African Orogen and Kuunga Orogen for their early Grenvillian and Pan-African populations, whereas the Hannan–Panxi arc, Jiangnan orogen, and the Yangtze block basements might have contributed to the detrital zircon grains of the Neoproterozoic and Pre-Grenvillian ages. Hf isotopic data indicate that the crustal evolution of the drainage area matches well with the episodic crust generation of Gondwana. These results imply that the previously suggested position of the SCB in Gondwana should be re-evaluated, and the South China block should be linked with North India and West Australia as a part of East Gondwana during the assembly of Gondwana, rather than a discrete continent block in the paleo-Pacific. [Copyright &y& Elsevier]

U–Pb dating and Hf isotopic analyses of zircons from various granitoids, combined with major and trace element analyses, were undertaken to determine the petrogenesis and geodynamic setting of Neoproterozoic and Late Paleozoic magmatism in the Manzhouli–Erguna area of Inner Mongolia, China. The Neoproterozoic granitoids are mainly biotite monzogranites with zircon U–Pb ages of 894±13Ma and 880±10Ma, and they are characterised by enrichment in large ion lithophile elements (LILEs; e.g., Rb, Ba, K) and light rare earth elements (LREEs), depletion in high field strength elements (HFSEs; e.g., Nb, Ta, Ti) and heavy rare earth elements (HREEs). The Late Devonian granitoids are dominantly syenogranites and mylonitised syenogranites with zircon U–Pb ages of 360±4Ma, and they form a bimodal magmatic association with subordinate gabbroic rocks of the same age. The Late Devonian syenogranites have A-type characteristics including high total alkalis, Zr, Nb, Ce and Y contents, and high FeOt/MgO, Ga/Al and Rb/Sr ratios. The Carboniferous granitoids are mainly tonalites, granodiorites and monzogranites with U–Pb ages varying from 319 to 306Ma, and they show very strong adakitic characteristics such as high La/Yb and Sr/Y ratios but low Y and Yb contents. The Late Permian granitoids are dominated by monzogranites and syenogranites with zircon U–Pb ages ranging between 257 and 251Ma. Isotopically, the ε Hf(t) values of the Neoproterozoic granitoids range from +4.3 to +8.3, and the two-stage model ages (T DM2) from 1.2 to 1.5Ga. The Late Devonian granitoids are less radiogenic [ε Hf(t) from +12.0 to +12.8 and T DM2 from 545 to 598Ma] than the Carboniferous [ε Hf(t) from +6.8 to +9.5 and T DM2 from 722 to 894Ma] and Late Permian granitoids [ε Hf(t) from +6.1 to +9.4 and T DM2 in the range of 680–895Ma]. These data indicate (1) the Neoproterozoic granitoids may have been generated by melting of a juvenile crust extracted from the mantle during the Mesoproterozoic, probably during or following the final stages of assembly of Rodinia as a result of the collision and amalgamation of Australia and the Tarim Craton; (2) the Late Devonian granitoids may have formed by partial melting of a new mantle-derived juvenile crust in a post-orogenic extensional setting; (3) the Carboniferous granitoids appear to have been produced by melting of garnet-bearing amphibolites within a thickened continental crust during and following the collision of the Songnen and Erguna–Xing’an terranes; and (4) the Late Permian granitoids may have been generated by melting of garnet-free amphibolites within the Neoproterozoic juvenile continental crust, probably in the post-collisional tectonic setting that followed the collision of the North China and Siberian cratons. [ABSTRACT FROM AUTHOR]

Abstract: The Tianshan rodingite was formed from eclogite tectonically included in ultramafic rocks of the Changawuzi ophiolite in the southwestern Tianshan, China. Rodingites derived from eclogite contain a mineral assemblage of prehnite, clinozoisite, hydrogrossular, diopside, vesuvianite, chlorite and minor zircon. Pervasive rodingitization took place during exhumation of the subducted slab under conditions of 250–350°C and 2–10kbar. Cathodoluminescence imaging and geochronological characteristics allow the identification of two types of zircons in the rodingite. Type I zircon consists of sector-zoned grains, commonly with distinct rims and cores. A well-defined weighted mean 206Pb/238U age of 422±10Ma for the rims records an early metamorphic event that affected the Changawuzi ophiolite during the Silurian, and represents the emplacement of the ophiolite complexes along the South Tianshan Paleo-ocean coast. The type I zircon cores reflect variable degrees of alteration during subsequent serpentinization and rodingitization at ∼290Ma. The U–Pb ages of the cores vary from 416Ma to 301Ma, implying patchy and incomplete alteration. The type I zircon shows rather uniform, flat rare earth element (REE) patterns, but with a marked increase in total REE in the cores. Type II zircon has a weighted mean 206Pb/238U age at 291±15Ma, within error of the youngest ages obtained from the cores, and is therefore considered to define the time of pervasive rodingitization. This age is consistent with known regional geological events recorded from the magmatic arc along the southern margin of the Tarim–Yili suture zone and with low P–high T metamorphism to the north, attributed to dehydration of the subducted oceanic slab when the combined Tarim–North China block collided with the Yili–Central Tianshan micro-plate. [Copyright &y& Elsevier]

Abstract: The Qingshan volcanic sequences occur in the Mengyin and Jiaozhou basins, west and east of the Tan-Lu fault zone, respectively, were formed at 128±2 and 106–98Ma. Lithologically the Mengyin succession comprises unimodal andesite (53–65wt.% SiO2), whereas the Jiaozhou succession consists of bimodal andesite (56–59wt.% SiO2) and rhyolite (69–77wt.% SiO2). High-Mg (Mg# >60) and low-Mg (Mg# <60) andesitic suites are recognized in the Mengyin volcanic rocks. They show tholeiitic and calc-alkaline trends, respectively. Though the two suites share common trace element features of LILE enrichment and HFSE depletion, higher incompatible element contents in the high-Mg andesite relative to the low-Mg andesite rule out their genetic connection by magmatic differentiation. Similarly, the Jiaozhou andesites also possess higher incompatible element contents compared to their interbedded rhyolites, suggesting their distinct source rocks. The Qingshan mafic volcanic suites are characterized by radiogenic Sr and unradiogenic Nd isotopic compositions, of which the Mengyin high-Mg andesitic rocks display the lowest εNd(t) and highest initial Sr ratio. By contrast, the Jiaozhou rhyolites possess a positive correlation between Sr–Nd isotopes and relatively unradiogenic Sr isotopic ratios. The Qingshan mafic rocks exhibit geochemical characteristics distinct from those of mantle peridotite- or pyroxenite-derived melt as well as from Fangcheng basalts, the only basalts documented in the Cretaceous volcanic suites in Shandong province. They are inferred to be crust-derived melts. However, a clear decreasing trend in ACNK with Mg# increasing and the elevated MgO, Cr and Ni contents relative to basalt-derived melts indicate assimilation with mantle peridotite during their pristine magmatic evolution. Such a two-step process can be best explained by the Archean lower crust delamination of the North China craton with a ~35% partial melting followed by a variable extent of metasomatic interaction with convecting mantle peridotite, which thus resulted in the low-Mg and high-Mg andesitic suites. Nd–Sr isotopic and HFSE features imply that the convecting mantle was mixed with the foundered lithospheric peridotite, which previously experienced subduction-related melt metasomatism during the collision of the Yangtze-North China cratons in the early Triassic. The Qingshan volcanic suites in western Shandong are synchronous with the Tan-Lu strike-slip fault, inferring that the lower crust delamination was initially triggered by sinistral motion of the Tan-Lu fault at ~130Ma. Due to an eastward development of regional lithospheric delamination, the Jiaozhou bimodal volcanic succession was formed owing to the regional geothermal gradient increase and lithosphere thinning. The recognition of contrasting andesitic suites in the Qingshan volcanic succession and their temporal trend provide additional evidence for lower crust delamination during the early Cretaceous in the North China craton and new constraints on its lithosphere thinning model. [Copyright &y& Elsevier]

Abstract: The Great Xing''an Range in northeastern China is characterized by large-scale Mesozoic magmatism, and forms a key part of the NE-trending Mesozoic magmatic belt in East China. However, only limited precise age data for the volcanic rocks in this belt were previously available, which significantly hampers understanding of the petrogenesis and geodynamic setting of these rocks. Systematic dating of volcanic rocks in the Great Xing''an Range was undertaken in order to rectify this situation. The Mesozoic volcanic rocks in the area have been sub-divided into the Tamulangou, Shangkuli and Yilieke formations. The Tamulangou and Yiliekede formations are composed of basalt and basaltic andesite, whereas the Shangkuli Formation consists of rhyolite and dacite. Zircon U–Pb and whole rock 40Ar/39Ar dating, combined with published data, indicate that the Tamulangou Formation was formed over a large time interval from the Early Jurassic to Early Cretaceous; it should therefore not be regarded as a single stratigraphic unit. In contrast, volcanic rocks of the Shangkuli and Yiliekede formations were erupted in the Early Cretaceous, between 125–115 Ma. Overall, most Mesozoic volcanic rocks in the Great Xing''an Range were erupted during the Early Cretaceous with an age peak at ∼125 Ma, coeval with the time of lithospheric thinning in the eastern part of the North China Craton. Therefore, the Great Xing''an Range constitutes an important area that records a significant Early Cretaceous giant igneous event in eastern China. It is possible that this activity was related to subduction of the Pacific plate during the Late Mesozoic, as has been suggested to explain crustal thinning of the North China Craton. [Copyright &y& Elsevier]

Abstract: In the Jiangnan orogen, a clear angular unconformity between the Precambrian basement sequences and the overlying Neoproterozoic sedimentary strata (e.g. the Danzhou/Banxi Group, younger than ca. 800Ma) marks the collisional orogenesis (the Jinning orogeny) between the Yangtze and Cathaysia Blocks. In contrast to the upright, open folds in the Danzhou/Banxi Group, the basement sequences were deformed into high-angle tight linear and isoclinal overturned folds. It has been previously accepted that the basement sequences are of Mesoproterozoic age. However, LA-ICP–MS U–Pb dating of detrital zircons suggests that the maximum depositional age of the basement sedimentary rocks in the western part of the Jiangnan orogen (i.e. the Sibao/Lengjiaxi Group) is ca. 860Ma. This provides a lower limit for the assembly of the Yangtze and Cathaysia Blocks. Consequently, there may be no significant (ca. 200Ma) early Neoproterozoic sedimentary hiatus in South China. These data, combined with published dates on orogeny-related igneous rocks in the Jiangnan orogen, indicate that the Jinning orogeny took place at 860–800Ma, significantly younger than the typical Grenvillian orogeny at 1.3–1.0Ga. The Sibao/Lengjiaxi Group may have been deposited in a foreland basin. The Yangtze Block and the arc terrains that resulted from the early subduction along the Jiangnan orogen might be the two main source regions for the sedimentary rocks. [Copyright &y& Elsevier]

Abstract: The western end of the Proterozoic Jiangnan orogen is located at the Northern Guangxi, South China. Neoproterozoic granitoids are dominant (>90%) in the area, with ca. 8% being the mafic-ultramafic rocks. The generation of these igneous rocks was previously considered to be related to a mantle plume (or superplume) event that led to the breakup of the Rodinia supercontinent. In this work, we present new laser ablation-ICP-MS U-Pb zircon data for the igneous rocks from Northern Guangxi. The ages for the Zhaigun, Bendong, Dongma, Sanfang and Tianpeng granitic plutons are 835.8±2.5, 822.7±3.8, 824±13, 804.3±5.2 and 794.2±8.1Ma, respectively, and the Hejiawan layered diabases are 811.5±4.8Ma. These ages indicate a broad duration of magmatic activities (ca. 35million years), inconsistent with plume models that predict widespread magmatic eruption and emplacement within period of 1–5million years. The granitoids in Northern Guangxi are typical S-type granites with high ACNK values (1.10–1.87), and are generally plotted in the collision-related areas in the tectonic discrimination diagrams. They should not be the products of the mantle plume activity. On the contrary, they might be related with the continent–continent collisional orogeny between the Yangtze and Cathysia blocks. A total of eight spot analyses of zircon cores and two from single zircon xenoliths gave early Neoproterozoic ages ranging from ca. 870–950Ma. These ages might record subduction or collision-related magmatic events during 950–870Ma in Northern Guangxi. Combined with previous geochronological and geochemical data, our new dating results support post-collisional extension, instead of mantle plume or superplume model, for the genesis of 835–800Ma granites and mafic rocks in Northern Guangxi. The upwelling of deep mantle due to the detachment of subducted slab and the delamination of the lithosphere might cause partial melting of the continental crust to generate S-type granites. The continent–continent collisional orogenic event along the Jiangnan orogen may have spanned ca. 70million years from 870 to 800Ma, and the early Neoproterozoic subduction might last for ca. 130million years. It is proposed that South China might have been located at the western margin in the Rodinia supercontinent during the period of ca. 870–800Ma. [Copyright &y& Elsevier]

• The quartz monzodiorites are contemporaneous with the UHT pelitic granulites. • The quartz monzodiorites formed by differentiation of mantle-derived magma. • Crust–mantle interaction at ca. 1.93 Ga in the Helanshan Complex. Insights into crust–mantle interaction in continental collisional orogens is key to understanding high- and ultrahigh-temperature (HT–UHT) metamorphism, continental crustal growth, and the tectonic evolution of such orogens. Here, we present whole-rock major- and trace-element and Nd isotope geochemistry, as well as zircon U–Pb geochronology and Hf isotope data, for quartz monzodiorites from the Helanshan Complex to constrain their petrogenesis and draw implications regarding crust–mantle interaction during formation of the Khondalite Belt in the North China Craton. LA–ICP–MS zircon U–Pb dating yields crystallization ages of 1938–1931 Ma for three samples, contemporaneous with spinel-bearing UHT pelitic granulites from the Helanshan Complex. The quartz monzodiorite samples have variable whole-rock compositions, with relatively low SiO 2 (low to 51.41 wt.%) but high MgO (up to 5.05 wt.%) and Cr (up to 80.9 ppm) contents, as well as high Mg# values (up to 0.50), which suggest their derivation from the mantle. These samples exhibit a narrow range of ε Nd (t) values of −0.3 to + 1.0, with no decrease as decreasing whole-rock MgO content, and variable zircon ε Hf (t) values of −2.4 to + 5.8. In addition, Mg# values of hornblende and biotite, and X an values of plagioclase decrease gradually with increasing whole-rock SiO 2 content. All of these geochemical data indicate that the studied quartz monzodiorites were produced by fractional crystallization of mantle-derived magma, with assimilation of crustal material. On the basis of previously published data in the Khondalite Belt, the Helanshan quartz monzodiorites formed in the post-collisional setting, and record a tectonic transition from compression to extension during the period ca. 1.95–1.93 Ga. The emplacement of mantle-derived magma that formed the quartz monzodiorites through differentiation at ca. 1.93 Ga, was possibly responsible for the formation of ca. 1.93 Ga UHT rocks in the Helanshan Complex. [ABSTRACT FROM AUTHOR]