High δ18O Research Articles

In situ zircon Hf–O isotopic analyses of late Mesozoic magmatic rocks in the Lower Yangtze River Belt, central eastern China: Implications for petrogenesis and geodynamic evolution

A combined study of whole-rock major and trace elements, Sr–Nd isotopes, zircon U–Pb dating, and in situ zircon Hf–O isotopes has been carried out for late Mesozoic magmatic rocks in the Lower Yangtze River Belt. The results provide insights into the origin of mantle sources of magma above a subduction zone, and thus into the petrogenesis of high-K calc-alkaline rocks, shoshonites, and A-type granites on continental margins, and the associated tectonic transformation from a continental arc to a back-arc extensional setting. The late Mesozoic magmatism can be subdivided into three stages: high-K calc-alkaline intrusions (148–133Ma), shoshonitic rocks (133–127Ma), and A-type granitoids (127–123Ma). All the rocks have consistent arc-like trace element characteristics with positive anomalies of Rb, Th, U, Pb, and LREE, negative anomalies of Nb, Ta, and Ti, and enriched Sr–Nd–Hf isotopic signatures. The first-stage intrusions in the Tongling area usually host dark enclaves of diorite, have high Sr/Y ratios, and low Y contents, and contain zircons with relatively low εHf(t) values (−38.6 to −6.6) and high δ18O values (5.7‰ to 10.1‰). A few inherited zircons with Neoarchean to Paleoproterozoic ages and highly enriched Hf isotopic compositions were detected in both the host intrusive rocks and the enclaves. The second-stage Ningwu volcanics contain zircons with moderate εHf(t) values (−13.3 to −3.8) and elevated δ18O values (5.4‰ to 7.6‰). The third-stage intrusions can be divided into A1- and A2-type granitoids, and their zircons have relatively high δ18O values of 6.7‰ to 10.3‰ and high εHf(t) values of 0 to −7.9. Based on these geochemical data we drew the following conclusions. Before 148Ma, following metasomatism by slab-derived fluid/melts, partial melting of the lithospheric mantle produced basaltic magma in the context of a subducting paleo-Pacific plate. This basaltic magma mixed with magma derived from the Archean lower crust, and the underplated and thickened juvenile lower crust. From 148 to 133Ma, continuous production of these mixed magma resulted in their intrusions as basic rocks at shallow levels. Meanwhile, partial melting of the thickened juvenile crust formed the intermediate-acid rocks of the first stage. As subduction continued, and the dip angle of the subducting plate increased, the continental arc tectonic setting was transformed to one of back-arc extensions. Metasomatism and decompression induced intensive partial melting of the lithospheric mantle, and these magmas, together with a limited amount of assimilated crustal materials, formed the second-stage volcanics. Roll-back of the subducted slab resulted in extension, causing disassembling of the lower lithosphere and lithospheric thinning, and the upwelling of hot asthenosphere. The A2-type granites were the result of the reworking of the Mesoproterozoic juvenile crust in this tectonic setting of extension. On the other hand, the A1-type granitoids were formed from magmas that were derived both from the matasomatized mantle and from the A2-type granitic material that had its origins in the Mesoproterozoic juvenile crust. We suggest that an integrated and comparative study of the multiple stages of development of these magmatic rocks is the key to understanding the tectonic evolution and associated magmatic activities in this continental intraplate setting.

Phanerozoic amalgamation of the Alxa Block and North China Craton: Evidence from Paleozoic granitoids, U–Pb geochronology and Sr–Nd–Pb–Hf–O isotope geochemistry

The North China Craton (NCC) has been considered to be part of the supercontinent Columbia. The nature of the NCC western boundary, however, remains strongly disputed. A key question in this regard is whether or not the Alxa Block is a part of the NCC. It is located in the vicinity of the inferred boundary, and therefore could potentially resolve the issue of the NCC's relationship to the Columbia supercontinent. Some previous studies based on the Alxa Block's geological evolution and detrital zircon ages suggested that it is likely not a part of the NCC. The lack of evidence from key igneous rock units, however, requires further constraints on the tectonic affinity of the western NCC and Alxa Block and on the timing of their amalgamation.In this study, new zircon U–Pb age and Hf–O isotopes and whole-rock geochemical and Sr–Nd–Pb isotopic data for the Paleozoic granitoids in or near the eastern Alxa Block were used to constrain the petrogenesis of these rocks and the relationship between the Alxa Block and NCC. Secondary ion mass spectrometry (SIMS) U–Pb zircon dating indicates that the Bayanbulage, Hetun, Diebusige and South Diebusige granitoids were formed at ca. 423Ma, 345Ma, 345Ma and 337Ma, respectively. The Late Silurian (Bayanbulage) quartz diorites have variable SiO2 (58.0–67.9wt.%), and low Sr/Y (20–24) values, while the Early Carboniferous (Hetun, Diebusige and South Diebusige) monzogranites have high SiO2 (71.5–76.7wt.%) and Sr/Y (40–94) values. The Late Silurian quartz diorites display relatively homogeneous and high zircon δ18O (8.5–9.1‰) and εHf(t) (−8.6 to −5.3) values, high whole-rock εNd(t) values (−9.2 to −7.6) and highly radiogenic Pb isotopes (206Pb/204Pb=18.13–18.25), whereas the Early Carboniferous monzogranites exhibit relatively low and variable zircon δ18O (5.7–7.2‰) and εHf(t) (−23.1 to −7.4) values, low whole-rock initial 87Sr/86Sr (0.7043–0.7070) and εNd(t) (−19.1 to −13.5) values and variable Pb isotopes (206Pb/204Pb=16.06–18.22). The differences in whole rock Nd model ages and Pb isotope compositions of the Paleoproterozoic–Permian rocks in either side of the west fault of the Bayanwulashan–Diebusige complexes suggest that the Alxa Block is not a part of the NCC, and that the western boundary of the NCC is probably located on this fault. Furthermore, the linear distribution of the Early Paleozoic–Early Carboniferous granitoids, the high zircon δ18O values of the Late Silurian quartz diorites, the Early Devonian metamorphism and the foreland basin system formed during the collision between the Alxa Block and the NCC indicate that a Paleozoic cryptic suture zone likely existed in this area and records the amalgamation of the Alxa Block and North China Craton. Together with detrital zircon data, the initial collision was considered to have possibly occurred in Late Ordovician.

Hydrothermal alteration and melting of the crust during the Columbia River Basalt–Snake River Plain transition and the origin of low-δ18O rhyolites of the central Snake River Plain

We present compelling isotopic evidence from ~15 Ma rhyolites that erupted coeval with the Columbia River Basalts in southwest Idaho's J-P Desert and the Jarbidge Mountains of northern Nevada at that suggests that the Yellowstone mantle plume caused hydrothermal alteration and remelting of diverse compositions of shallow crust in the area where they erupted. These rhyolites also constitute the earliest known Miocene volcanism in the vicinity of the Bruneau–Jarbidge and Twin Falls (BJTF) volcanic complexes, a major center of voluminous (103–104km3) low-δ18O rhyolitic volcanism that was previously defined as being active from 13 to 6Ma. The Jarbidge Rhyolite has above-mantle δ18O (δ18O of+7.9‰ SMOW) and extremely unradiogenic εHf (−34.7) and εNd (−24.0). By contrast, the J-P Desert units are lower in δ18O (+4.5 to 5.8‰), and have more moderately unradiogenic whole-rock εHf (−20.3 to −8.9) and εNd (−13.4 to −7.7). The J-P Desert rhyolites are geochemically and petrologically similar to the younger rhyolites of the BJTF center (the one exception being their high δ18O values), suggesting a common origin for J-P Desert and BJTF rhyolites. The presence of low-δ18O values and unradiogenic Nd and Hf isotopic compositions, both of which differ greatly from the composition of a mantle differentiate, indicate that some of these melts may be 50% or more melted crust by volume. Individual J-P Desert units have isotopically diverse zircons, with one lava containing zircons ranging from −0.6‰ to +6.5‰ in δ18O and from −29.5 to −2.8 in εHf. Despite this diversity, zircons all have Miocene U/Pb ages. The range of zircon compositions fingerprints the diversity of their source melts, which in turn allow us to determine the compositions of two crustal end-members which melted to form these rhyolites. These end-members are: 1) Archean basement with normal to high-δ18O and unradiogenic εHf and 2) hydrothermally altered, shallow, young crust with low-δ18O (0–1‰) and more radiogenic εHf. We suggest that the shallow crust's low-δ18O composition is the result of hydrothermal alteration which was driven by a combination of normal faulting and high heat fluxes from intruding Yellowstone plume-derived basalts shortly prior to the onset of silicic magmatism. Furthermore, zircon diversity in the J-P Desert units suggests rapid assembly of zircon-bearing melts of varying isotopic composition prior to eruption, creating well-mixed magmas with heterogeneous zircons. We suggest that this hydrothermal priming of the crust followed by remelting upon further heating may be a common feature of intraplate mantle plume volcanism worldwide.

Garnet geochemistry records the action of metamorphic fluids in ultrahigh-pressure dioritic gneiss from the Sulu orogen

A combined study of major and trace elements, water content and oxygen isotope composition was carried out for newly grown garnets in ultrahigh-pressure dioritic gneiss from the Sulu orogen. The results indicate the occurrence of a core–rim structure of garnet grains with growth of rims over cores via the mechanism of coupled dissolution–reprecipitation. This provides new insights into the action of geofluids on garnet growth during continental subduction-zone metamorphism. The garnet cores and rims show distinct geochemical compositions. The cores have high spessartine contents (4.2–19.4mol%), relatively low grossular contents (26.5–29.8mol%) and low δ18O values (5.0±0.2‰). The rims have relatively low spessartine contents (1.0–3.4mol%), high grossular contents (28.7–36.1mol%) and high δ18O values (6.1±0.3‰). Furthermore, the rims show much lower contents of Li and HREE, and considerably higher contents of Sc, V and Ga than the cores. In addition, the cores exhibit very low contents of structural hydroxyl in 32±10 to 48±20ppm H2O, whereas the rims show much higher contents of structural hydroxyl in 112±16ppm to 171±34ppm H2O. Although the boundary between core and rim is highly irregular in the garnet grains, the zoning in water contents, element and oxygen isotope compositions are all spatially correlated with each other. Taken all the observations together, it is suggested that the geochemical compositions of garnet cores and rims would be primarily dictated by two types of geofluids with distinctive compositions. Some garnet fragments, bounded by fractures, show the two types of domains with distinct water contents. This difference indicates insignificant exchange of structural hydroxyl by diffusion during growth of the rims over the cores. Mineral-pair O isotope thermometry yields consistent temperatures of ~550°C for the quartz-garnet rim and quartz-titanite, and titanite U–Pb dating yields a metamorphic age of 226±6Ma. Thus, the growth of garnet rims is linked to the action of geofluids (probably anatectic melts) during the exhumation of deeply subducted continental crust. Therefore, the compositions of garnet cores and rims are primarily controlled by the composition of attending fluids. As such, garnet provides a mineralogical record of the fluid action during subduction-zone metamorphism.

Fe and O isotope composition of meteorite fusion crusts: Possible natural analogues to chondrule formation?

AbstractMeteorite fusion crust formation is a brief event in a high‐temperature (2000–12,000 K) and high‐pressure (2–5 MPa) regime. We studied fusion crusts and bulk samples of 10 ordinary chondrite falls and 10 ordinary chondrite finds. The fusion crusts show a typical layering and most contain vesicles. All fusion crusts are enriched in heavy Fe isotopes, with δ56Fe values up to +0.35‰ relative to the solar system mean. On average, the δ56Fe of fusion crusts from finds is +0.23‰, which is 0.08‰ higher than the average from falls (+0.15‰). Higher δ56Fe in fusion crusts of finds correlate with bulk chondrite enrichments in mobile elements such as Ba and Sr. The δ56Fe signature of meteorite fusion crusts was produced by two processes (1) evaporation during atmospheric entry and (2) terrestrial weathering. Fusion crusts have either the same or higher δ18O (0.9–1.5‰) than their host chondrites, and the same is true for Δ17O. The differences in bulk chondrite and fusion crust oxygen isotope composition are explained by exchange of oxygen between the molten surface of the meteorites with the atmosphere and weathering. Meteorite fusion crust formation is qualitatively similar to conditions of chondrule formation. Therefore, fusion crusts may, at least to some extent, serve as a natural analogue to chondrule formation processes. Meteorite fusion crust and chondrules exhibit a similar extent of Fe isotope fractionation, supporting the idea that the Fe isotope signature of chondrules was established in a high‐pressure environment that prevented large isotope fractionations. The exchange of O between a chondrule melt and an 16O‐poor nebula as the cause for the observed nonmass dependent O isotope compositions in chondrules is supported by the same process, although to a much lower extent, in meteorite fusion crusts.

A high-resolution fluid inclusion δ18O record from a stalagmite in SW France: modern calibration and comparison with multiple proxies

The δ18O of speleothem fluid inclusions can provide information about local climate and the atmospheric circulation if the fate of oxygen isotopes from precipitation to the stalagmite is well understood. At Villars cave, SW France, the present-day relationship between the δ18O of precipitation and the δ18O of cave drip water has been determined by previous monitoring studies. In this article, we examine the link between drip water and fluid inclusions in speleothems from the same cave, and investigate whether the fossil water in the stalagmites provides a record of the isotopic composition of precipitation.Replicated fluid inclusion isotope measurements (δD and δ18O) from two modern calcite samples, which had precipitated under the monitored stalactites, are in agreement with the corresponding contemporaneous drip waters. The drip water, in turn, represents average pluri-annual precipitation. This gives strong evidence that the isotopic composition of the infiltrating water is preserved in the pores of speleothem calcite, thus providing a record of average precipitation δ18O at the site.Furthermore, we measured isotope ratios in fluid inclusions in the stalagmite vil-stm1, a 2300 year old sample with a high growth rate (up to 0.6 mm yr−1) and a high water content, which allows a high-resolution reconstruction of the drip water isotopic composition covering the last two millennia. The stalagmite is well dated by 13 U–Th ages and the counting of annual growth layers. The fluid inclusion δ18O varies by 2‰, with high δ18O values in Medieval times, and low values during the Little Ice Age, indicating a temperature influence on precipitation δ18O. However, temperature change alone cannot account for the full range of variability observed.Other proxies measured in the same stalagmite indicate that the cave environment has undergone substantial changes during this period: Stable isotopes and trace element concentrations in calcite show large variations, with amplitudes similar to those found in other Villars speleothems during large climatic shifts such as Dansgaard–Oeschger events or glacial–interglacial transitions. In the context of the last 2000 years, the proxy variability cannot be attributed to climate alone, but is likely influenced by anthropogenic deforestation and the associated changes in carbon dynamics and hydrology.These changes might also have an influence on the isotope ratios in drip water relative to precipitation, e.g. through an altered evaporation–transpiration ratio. Nevertheless, despite some common trends in all proxy records, the marked shifts in the other proxies are not seen in the fluid inclusion stable isotopes, which gives evidence that the relationship between precipitation and drip water δ18O in the past was similar to present-day conditions, and not much affected by the anthropogenic impact. A replication of the fluid inclusion isotope record using stalagmites from the same region will be necessary to determine whether local cave processes or large-scale temperature and atmospheric circulation dominate the fluid inclusion δ18O signal, and in how far the fluid inclusions can provide a direct record of precipitation δ18O.

Oxygen and hydrogen isotope signatures of Northeast Atlantic water masses

Only a few studies have examined the variation of oxygen and hydrogen isotopes of seawater in NE Atlantic water masses, and data are especially sparse for intermediate and deep-water masses. The current study greatly expands this record with 527 δ18O values from 47 stations located throughout the mid- to low-latitude NE Atlantic. In addition, δD was analyzed in the 192 samples collected along the GEOTRACES North Atlantic Transect GA03 (GA03_e=KN199-4) and the 115 Iberia-Forams cruise samples from the western and southern Iberian margin. An intercomparison study between the two stable isotope measurement techniques (cavity ring-down laser spectroscopy and magnetic-sector isotope ratio mass spectrometry) used to analyze GA03_e samples reveals relatively good agreement for both hydrogen and oxygen isotope ratios. The surface (0–100m) and central (100–500m) water isotope data show the typical, evaporation related trend of increasing values equatorward with the exception for the zonal transect off Cape Blanc, NW Africa. Off Cape Blanc, surface water isotope signatures are modified by the upwelling of fresher Antarctic Intermediate Water (AAIW) that generally has isotopic values of 0.0 to 0.5‰ for δ18O and 0 to 2‰ for δD. Along the Iberian margin the Mediterranean Outflow Water (MOW) is clearly distinguished by its high δ18O (0.5–1.1‰) and δD (3–6‰) values that can be traced into the open Atlantic. Isotopic values in the NE Atlantic Deep Water (NEADW) are relatively low (δ18O: −0.1 to 0.5‰; δD: −1 to 4‰) and show a broader range than observed previously in the northern and southern convection areas. The NEADW is best observed at GA03_e Stations 5 and 7 in the central NE Atlantic basin. Antarctic Bottom Water isotope values are relatively high indicating modification of the original Antarctic source water along the flow path. The reconstructed δ18O–salinity relationship for the complete data set has a slope of 0.51, i.e., slightly steeper than the 0.46 described previously by Pierre et al. (1994, J. Mar. Syst. 5 (2), 159–170.) for the tropical to subtropical Northeast Atlantic. This slope decreases to 0.46 for the subtropical North Atlantic Central Water (NACW) and the MOW and to 0.32 for the surface waters of the upper 50m. The δD–salinity mixing lines have estimated slopes of 3.01 for the complete data, 1.26 for the MOW, 3.47 for the NACW, and 2.63 for the surface waters. The slopes of the δ18O–δD relationship are significantly lower than the one for the Global Meteoric Water Line with 5.6 for the complete data set, 2.30 for the MOW, 4.79 for the NACW, and 3.99 for the surface waters. The lower slopes in all the relationships clearly reflect the impact of the evaporation surplus in the subtropics.

Zircon U–Pb ages and Hf–O isotopes, and whole-rock Sr–Nd isotopes of the Bozhushan granite, Yunnan province, SW China: Constraints on petrogenesis and tectonic setting

The Bainiuchang silver-polymetallic ore deposit is a super-large deposit in the western part of the South China tungsten–tin province (or the Nanling tungsten–tin province). The deposit is spatially and temporally associated with the Bozhushan granite pluton. Our new data indicate that the Bozhushan granitoids formed at 86–87Ma. The granitoids are geochemically consistent with A-type granite. The Bozhushan pluton consists predominantly of biotite granite that is characterized by weakly peraluminous to metaluminous compositions and high alkali contents (Na2O+K2O=7.51–9.06wt.%). The granitic rocks are enriched in large-ion lithophile elements (LILE) Rb, Th, U, and K, but relatively depleted in Ba and Sr. In addition, they have high Zr+Nb+Ce+Y contents (310–478ppm) and high 10,000× Ga/Al ratios (2.7–3.1). The temperatures of the parental magmas for the Bozhushan granites are estimated to be 790–842°C based on the zircon saturation thermometer. Isotopically, the Bozhushan granites are characterized by elevated initial 87Sr/86Sr ratios (0.7126–0.7257) and low εNd values (−11.2 to −12.4), and high δ18O values (7.91–9.58‰) and low εHf values (−9.5 to −6.1) for zircon crystals, which indicate a dominant continental crustal source. The two-stage Hf model ages vary from 1.53 to 1.86Ga. The isotopic compositions support the interpretation that the granitic rocks formed by melting of the Meso- and Neoproterozoic metasedimentary basements of the Cathaysia block. These results, together with geological records in the other parts of the western Cathaysia block, suggest that the formation of the Bozhushan A-type granites is related to lithospheric extension and asthenospheric upwelling that are associated with the change of plate motion in Late-Cretaceous.

The case of the carbonatite-like dyke of the Madenska River complex at the Kriva Lakavica section in the Republic of Macedonia: oxygen and carbon isotopic constraints

The oxygen and carbon isotope compositions of carbonate samples from the carbonatite-like dyke of the Madenska River complex at the Kriva Lakavica section, samples of calcite skarns from the Damjan Fe-ore deposit and the Sasa Pb-Zn ore deposit, and samples of marbles from the Pohorje Mountains were analyzed to provide critical evidence in favor of the magmatic or sedimentary origin of this carbonatite-like dyke. We suggest that the carbonatite-like dyke is not a normal carbonatite, but instead represents melted carbonates, which can be associated with an unexposed, deep-seated, causative magmatic body. This dyke has a fluidal texture and carries xenoliths of ultramafic rocks that can be up to 35 cm in size. Its isotopic composition plots between primary unaltered carbonatites and marine carbonates, and ranges between 13.79? and 18.89? for Δ18OVSMOW and between -1.22? and 1.31? for Δ13CVPDB. These values are significantly lower than those observed in carbonatites analyzed during this study and range between 6.53? and 8.10? for δ18OVSMOW and between -5.82? and -4.32? for Δ13CVPDB, which is the primary isotope signature of most magmatic carbonatites. Similarly high δ18O and Δ13C values were found in skarns of the Damjan Fe deposit close to the Madanska River complex and in the Sasa Pb-Zn deposit, as well as in high-grade regional metamorphic calcitic marbles of the Pohorje massive. The emplacement levels of the carbonatite-like dyke, due to several tectonic processes, are uncertain. The type of country rocks (sedimentary carbonates, ultramafic, mafic, and granitic rocks), hydrothermal alternation, and metasomatic and regional metamorphic processes seem to be the most important parameters that affect the O and C isotopic patterns found in the Kriva Lakavica carbonatite-like dyke and in the investigated samples.

The O and H isotope characteristics of water from major rivers in China

Isotopic compositions of the water sources and evapotranspiration in large rivers of China are not well explored. In this investigation, δ18O and δD values of water from major large rivers of China are integrated to understand the spatial and temporal variations in the isotopes of water and d-excess. River water samples were collected from the channels and tributaries of the Changjiang, Yellow, Songhua, and Liao Rivers in China. Water from upstream was generally enriched with light isotopes relative to that from downstream areas. Water from the Songhua River and upper reach of the Changjiang River has relatively negative isotopic values, impacted by latitude and altitude effects, respectively. The spatial pattern showed that δ18O-H2O values in rivers mainly ranged from −9 to −7 ‰ in most of China based on roughly mapped contours, and rivers enriched in light isotopes were primarily located in the Tibetan Plateau and northeast China. The slopes of linear regression from the Yellow and Liao Rivers suggested that the loss of water by direct evaporation shifted the isotopes of water, especially in some small tributaries. Additionally, there was a narrow range of low δ18O-H2O values from the main channel of rivers during the low-flow season relative to the high-flow season due to influence of the monsoon climate. No distinct spatial variation of d-excess values was found in the rivers of China. However, water samples in some rivers with low d-excess values had high δ18O values, indicating that evaporation in the tributaries has led to significant water loss, especially in tributaries of the Yellow and Liao Rivers.

The source of Mesozoic granitoids in South China: Integrated geochemical constraints from the Taoshan batholith in the Nanling Range

A combined study of zircon U–Pb ages and Lu–Hf isotopes, whole-rock major-trace elements and Sr–Nd isotopes as well as whole-rock and mineral O isotopes was carried out for Mesozoic granitic intrusions from the Taoshan batholith in the Nanling Range, South China. The results not only place constraints on the origin of granitoids but also provide insights into the effect of melting temperature on granitoid compositions. LA-ICPMS zircon U–Pb dating yields weighted 206Pb/238U ages of 230±2Ma for the Caijiang intrusion, 167±2Ma for the Huangpi intrusion, 152±3Ma for the Daguzhai intrusion, and 146±3Ma for the Luobuli intrusion. Relict zircon cores with older Mesozoic to Paleozoic U–Pb ages are present in some samples. These Mesozoic granitoids are weakly to strongly peraluminous with A/CNK ratios of 1.05 to 1.23. They exhibit arc-like trace element distribution patterns, with enrichment of LREE and LILE (e.g., Rb, K, Pb) but depletion of HFSE (e.g., Nb, Ta, Ti). They show high whole-rock initial 87Sr/86Sr ratios of 0.7101 to 0.7156 and low εNd(t) values of −11.2 to −8.8 as well as negative zircon εHf(t) values of −14.3 to −4.8 and Paleoproterozoic Nd-Hf model ages. These results suggest their derivation from partial melting of ancient continental crust. Furthermore, they have high zircon δ18O values of 8.6 to 10.6‰ and calculated whole-rock δ18O values of 10.8 to 12.5‰. Along with their peraluminous features, high K2O/Na2O ratios, arc-like trace element distribution patterns and enriched Sr–Nd–Hf isotope compositions, these Mesozoic granitoids are unambiguously derived from partial melting of metasedimentary rocks and thus are of S-type affinity. There are significant differences in both major and trace elements between the Caijiang intrusion of Triassic age and the Huangpi, Daguzhai and Luobuli intrusions of Jurassic age. While the Triassic granites exhibit high (Fe2O3)T+MgO, P2O5, Th, LREE and Zr+Nb+Ce+Y contents but low Al2O3/TiO2, Rb/Ba and Rb/Sr ratios in association with high zircon saturation temperatures, the Jurassic granites have low (Fe2O3)T+MgO, P2O5, Th, LREE and Zr+Nb+Ce+Y contents but high Al2O3/TiO2, Rb/Ba and Rb/Sr ratios in association with low zircon saturation temperatures. These correlations indicate that melting temperature and thus extent of partial melting have played an important role in dictating the compositions of granitoids. Therefore, the geochemical compositions of S-type granitoids are dictated not only by the composition of source rocks but also by the temperature of partial melting.

Latest Early Permian granitic magmatism in southern Inner Mongolia, China: Implications for the tectonic evolution of the southeastern Central Asian Orogenic Belt

Latest Early Permian (277–275Ma) magmatic episode is one of the most important tectono-magmatic events in southeastern Central Asian Orogenic Belt (CAOB), recording a tectonic transition from accretionary to post-accretionary evolution of the Paleo-Asian Ocean. The latest Early Permian granitoids along the Solonker suture zone in southern Inner Mongolia were emplaced between 275Ma and 277Ma as determined by SHRIMP zircon U–Pb dating. They are characterized by variable alkali contents, a range from calcic to slightly alkalic compositions, and are magnesian I-type granitoids. Their εNd(t) values from −0.4 to +3.1 record predominantly juvenile crustal sources with moderate involvement of old crustal components. Furthermore, these granitoids show positive zircon εHf(t) values of +7.6 to +10.7 that are also indicative of predominantly juvenile nature. Their high zircon δ18O values of 6.29‰ to 8.13‰, suggest significant involvement of supracrustal materials in their generation. These characteristics imply they were derived from an already hybrid andesitic magma in active continental margin. The resultant composite magmas were finally attached (relaminated) to the lower crust, where they produced segregated melt, to rise with minor assimilation of supracrustal materials, and to form these granitic plutons. The outboard migration of continental arc in response to slab roll-back, from the Late Carboniferous to the latest Early Permian, and tectonic switching from this prolonged extension to the short-lived contraction resulting from slab break-off and final amalgamation of the southeastern CAOB in the Middle-Late Permian lead to efficient continental growth in the southeastern CAOB.

δ18O characteristics of meteoric precipitation and its water vapor sources in the Guilin area of China

The stable isotopic composition of meteoric precipitation is an important component of global and regional water cycle research. Stable isotope data can help reconstruct paleoclimate related to ice cores, lake sediments, and stalagmites. We investigated the daily variation in isotopic composition of meteoric precipitation from 2008 to 2012 in the Guilin region of China. δ18O of meteoric precipitation ranged from −14.21 to +2.38 ‰, with an average value of −5.78 ‰ (Vienna Standard Mean Ocean Water; VSMOW). Meteoric precipitation in the summer half-year (May through October) increased, with a relatively low δ18O value of −8.04 ‰ (VSMOW; average of 261 groups) and −56.03 ‰ (VSMOW) of average δD, accounting for 67.6 % of the total annual meteoric precipitation. Meteoric precipitation in the winter half-year (November through April) decreased, with a relatively high δ18O value of −2.89 ‰ (average of 210 groups) and −9.23 ‰ of average δD, accounting for 32.4 % of total meteoric precipitation. The local meteoric water line (LMWL) equation in Guilin area has local climate characteristics. By combining environmental isotope data of precipitation using a backwards trajectory, water vapor sources of the meteoric precipitation in Guilin area are inferred and traced. Our results show that the isotopic composition of meteoric water vapor sources in Guilin area are related to monsoon type, source of precipitation cloud masses, and precipitation properties. The isotopic composition of meteoric precipitation in the summer half-year (May through October) was mainly affected by the summer monsoon or summer typhoons, namely controlled by water vapor source from the Bay of Bengal and the South China sea, and the second the West Pacific, and the δ18O value of meteoric precipitation was strongly negative. There was a significant negative correlation between the δ18O values of meteoric precipitation and the amount of precipitation and temperature in the summer half-year. The amount effect of the meteoric precipitation often concealed the temperature effect. The isotopic composition of meteoric precipitation in the winter season or winter half-year was affected by the water vapor source of the warm moist air masses from the West Pacific and continental cold air masses from the Siberia–Mongolia/winter monsoon or local evaporation vapor circulation, and the δ18O value of meteoric precipitation was relatively positive, and indicated that the water vapor of meteoric precipitation along the water vapor trajectory was affected by the evaporation as well as the local water vapor evaporation. The research results have showed that different sources of water vapor has a significant influence on the δ18O variation of meteoric precipitation, therefore, analysis of δ18O in the meteoric precipitation, especially its seasonal variation characteristics of analysis, can conversely reveal the water vapor sources of local meteoric precipitation.

Lake level and climate records of the last 90 ka from the Northern Basin of Lake Van, eastern Turkey

Sedimentary, geochemical and mineralogical analyses of the ICDP cores recovered from the Northern Basin (NB) of Lake Van provide evidence of lake level and climatic changes related to orbital and North Atlantic climate system over the last 90ka. High lake levels are generally observed during the interglacial and interstadial periods, which are marked by deposition of varved sediments with high total organic carbon (TOC), total inorganic carbon (TIC), low detrital influx (high Ca/F) and high δ18O and δ13C values of authigenic carbonate. During the glacial and stadial periods of 71–58kaBP (Marine Isotope Stage 4, MIS4) and end of last glaciation–deglaciation (30–14.5kaBP; MIS3) relatively low lake levels prevailed, and grey homogeneous to faintly laminated clayey silts were deposited at high sedimentation and low organic productivity rates.Millennial-scale variability of the proxies during 60–30kaBP (MIS3 is correlated with the Dansgaard–Oeschger (D–O)) and Holocene abrupt climate events in the Atlantic. These events are characterized by laminated sediments, with high TOC, TIC, Ca/Fe, δ18O and δ13C values. The Lake Van NB records correlate well in the region with the climate records from the lakes Zeribar and Urmia in Iran and the Sofular Cave in NW Anatolia, but are in general in anti-phase to those from the Dead Sea Basin (Lake Lisan) in the Levant.The relatively higher δ18O values (0 to −0.4‰) for the interglacial and interstadial periods in the Lake Van NB section are due to the higher temperature and seasonality of precipitation and higher evaporation, whereas the lower values (−0.8 to −2‰) during the glacial and stadial periods are caused mainly by relative decrease in both temperature and seasonality of precipitation. The high δ18O values (up to 4.2‰) during the Younger Dryas, together with the presence of dolomite and low TOC contents, supports evaporative conditions and low lake level. A gradual decrease in the δ18O values from an average of −0.4‰ during the humid early Holocene to an average of −3.5‰ during the more arid late Holocene suggests an increasing contribution of winter precipitation. The changes in the seasonality of precipitation in eastern Anatolia are probably caused by changes in the temperatures of North Atlantic and Mediterranean and in the strength of Siberian High.

Quantifying the effects of clear-cutting and strip-cutting on nitrate dynamics in a forested watershed using triple oxygen isotopes as tracers

Abstract. Temporal variations in the stable isotopic compositions of nitrate dissolved in stream water eluted from a cool–temperate forested watershed (8 ha) were measured to quantify the biogeochemical effects of clear-cutting of trees and subsequent strip-cutting of the understory vegetation, dwarf bamboo (Sasa senanensis), with special emphasis on changes in the fate of atmospheric nitrate that had been deposited onto the watershed based on Δ17O values of nitrate. A significant increase in stream nitrate concentration to 15 μmol L−1 in spring of 2004 was correlated with a significant increase in the Δ17O values of nitrate. Additionally, the high Δ17O values of +14.3‰ suggest that the direct drainage of atmospheric nitrate accounted for more than 50% of total nitrate exported from the forested watershed peaking in spring. Similar increases in both concentrations and Δ17O values were also found in spring of 2005. Conversely, low Δ17O values less than +1.5‰ were observed in other seasons, regardless of increases in stream nitrate concentration, indicating that the majority of nitrate exported from the forested watershed during seasons other than spring was remineralized nitrate: those retained in the forested ecosystem as either organic N or ammonium and then been converted to nitrate via microbial nitrification. When compared with the values prior to strip-cutting, the annual export of atmospheric nitrate and remineralized nitrate increased more than 16-fold and fourfold, respectively, in 2004, and more than 13-fold and fivefold, respectively, in 2005. The understory vegetation (Sasa) was particularly important to enhancing biological consumption of atmospheric nitrate.

Petrogenesis of Triassic granites from the Nanling Range in South China: Implications for geochemical diversity in granites

Abstract A combined study of whole-rock major-trace elements and Sr–Nd isotopes, zircon U–Pb ages, Hf and O isotopes as well as biotite geochemistry was carried out for Triassic granite intrusions from the Nanling Range in South China. The results provide insights into the effects of source composition and melting conditions on the geochemical diversity of granites. The granites of interest are peraluminous, and contain primary muscovite and tourmaline. They are characterized by high zircon δ18O values of > 9.0‰, high initial 87Sr/86Sr values of ~ 0.7200, and homogeneous eNd(t) values of − 11.3 to − 9.8, as well as variable zircon eHf(t) values of − 12.2 to − 5.8. Biotite geochemistry is similar to that of common peraluminous granites. An integrated interpretation of these petrological, mineralogical and geochemical data indicates that these granites were derived from partial melting of metasedimentary rocks under variable physicochemical conditions. The differences in whole-rock and biotite geochemistry between the intrusions are ascribed to the variable effects of source heterogeneity and melting temperature. The Luxi intrusion exhibits higher contents of MgO, FeOT, TiO2 and CaO than common melts derived from metasedimentary rocks, tight variations in major-trace elements and homogeneous Sr–Nd isotopic compositions, and homogeneous biotite composition with high Mg# [= Mg / (Mg + Fe) in molar] and lower whole-rock A/CNK values [= Al2O3 / (CaO + Na2O + K2O) in molar]. These can be explained by originating from a relatively mafic metasedimentary source. On the other hand, the geochemical diversity of granites can be caused by the difference in melting temperature in addition to the source heterogeneity. This is suggested by the Xiazhuang and Fucheng intrusions which exhibit similar range of SiO2. Nevertheless, the Fucheng intrusion is ferroan, and high in TiO2, (Na2O + K2O)/CaO, TiO2/MgO, Ga/Al and Zr + Nb + Ce + Y, but low in CaO, MgO and Mg#. Most of its major elements and their ratios are similar to those of A-type granites that were generated from F-rich rocks at high temperatures, consistent with the enrichment of Fe and F in its biotite. Thus, partial melting of a metasedimentary restite at reduced conditions after previous melt extraction is suggested to produce the geochemical feature of A-type granites in the Fucheng intrusion. In contrast, partial melting of fertile metasedimentary rocks would produce normal S-type granites like the Xiazhuang intrusion, with its geochemical variations ascrible to source heterogeneity and differential melting. Therefore, the geochemical composition of granites is primarily dictated by the composition of source rocks in addition to the physicochemical conditions of partial melting.

Recycling of sediment into the mantle source of K-rich mafic rocks: Sr–Nd–Hf–O isotopic evidence from the Fushui complex in the Qinling orogen

Potassium (K)-rich mafic rocks are viewed as being derived from partial melting of an enriched mantle source, but it is controversial about which processes cause the mantle enrichment. The Fushui intrusive complex is the largest early Paleozoic K-rich intrusive complex in the eastern Qinling orogen. Therefore, detailed studies on the Fushui complex can contribute not only to understanding of the petrogenesis of K-rich mafic rocks, but also to unraveling the Paleozoic evolution of the Qinling orogen. In this study, we provide an integrated investigation of in situ zircon U–Pb dating and Hf–O isotopes, in combination with whole-rock major and trace elements, as well as Sr–Nd–Hf isotopes, for the Fushui intrusive complex. In situ zircon secondary ion mass spectrometry (SIMS) / laser ablation induction coupled plasma mass spectrometry (LA-ICPMS) U–Pb dating reveals that different rock types of the Fushui complex have identical formation ages of 488–484 Ma. The Fushui complex belongs to the shoshonitic series, and is characterized by extreme large ion lithophile element (LILE, e.g., Ba, U, Th and Sr) and Pb enrichment and depletion of high field-strength elements (HFSEs, e.g. Nb, Ta, Zr, Hf, P and Ti). It shows high initial Sr isotopic ratios (0.7100–0.7151), negative whole-rock e Nd(t) (−3.97 to −5.68) and negative to slight positive whole-rock (−2.24 to 2.38) and zircon (−2.85 to 0.34) e Hf(t) values, as well as high zircon δ18O values (6.86 ± 0.13 ‰). The Hf–Nd isotopic systems are decoupled with positive Δe Hf values (3.85–5.37). These geochemical features indicate that the mantle source has incorporated subducted zircon–barren oceanic sediments. A simple two-end-members mixing model constrains the amount of subducted sediments in the Fushui mantle source to 5–8 %. The Fushui complex originated from 1 to 6 % equilibrium melting of a phlogopite-bearing garnet lherzolite by non-modal melting. As shoshonitic magmas have been discovered in modern nascent arcs, we suggest that the generation of the Fushui complex was induced by the subduction of the Paleotethyan Ocean, when it jumped from the northern to the southern boundary of the North Qinling microcontinent.

Formation age and genesis of the Gongchangling Neoarchean banded iron deposit in eastern Liaoning Province: Constraints from geochemistry and SHRIMP zircon U–Pb dating

The Gongchang iron deposit, a typical Algoma-type banded iron formation (BIF) deposit located in the Eastern Block of the North China Craton (NCC), is hosted in Neoarchean metamorphic rocks, including amphibolite, biotite leptynite, garnet biotite schist, plagioclase-hornblende schist, garnet chlorite schist, muscovite quartz schist and actinolite magnetite quartzite. Geochemical data suggest that protoliths of the amphibolites and biotite leptynite should be basalt and dacite, respectively. SHRIMP U–Pb dating of zircons from the plagioclase-hornblende schist shows that the Gongchangling iron deposit was formed at 2554±12Ma. Major elements and REE data of the ores indicate that the Gongchangling BIF precipitated from seawater, and the ultimate iron and silica origins may have been related to nearby volcanic activity. The δ18O values of quartz from the BIFs range between 9.1 and 12.9‰, suggesting that exotic high δ18O fluids were involved during metamorphism. The pyrite have δ34S values of range between −3.5‰ and 14.7‰, suggesting that mineralization were originally from metamorphism. Comparison of the mineralization age of the Gongchangling BIF with those of other local BIFs in North China and other cratons throughout the world indicates that the Gongchangling BIF formed coevally with the global BIF mineralization at ∼2.5Ga. Taken together, these data suggest that the Gongchangling iron deposit was formed in a volcanic submarine environment in which hydrothermal fluids associated with volcanic activity supplied both iron and silica for the banded iron formations.

Kinetic isotope effect in CO2 degassing: Insight from clumped and oxygen isotopes in laboratory precipitation experiments

Laboratory precipitation experiments provide the basis for the common calibration of both the oxygen isotope and the clumped isotope thermometers. These focus on CaCO3 crystals that form deep in the bulk of the solution, often by the bubbling of N2(g) through a saturated Ca(HCO3)2 solution, following the classic experiments on McCrea (1950). Here we examine oxygen and clumped isotopes in CaCO3 that was precipitated at the surface of a solution that undergoes passive CO2 degassing. This CaCO3 is affected by enhanced disequilibrium that is associated with degassing, and has therefore higher δ18O and lower Δ47 values than those of the accepted thermometer calibrations. These offsets from the common thermometers calibrations increase with decreasing temperature for Δ47 but decrease with decreasing temperature for δ18O. The Δ47-δ18O co-variance is therefore strongly temperature dependent, in contrast to theoretical predictions. This suggests an influence of additional fractionation in fast growing minerals, between DIC and calcite, that affects δ18O and counteracts some of the degassing related enrichment. This effect seems not to influence Δ47. The physical setup of CaCO3 minerals growing at the solution surface is analogous to the degassing and precipitation processes that occur in stalagmites, such that these laboratory experiments may help shed light on the isotopic disequilibrium that is observed in speleothems.

Paleoproterozoic S-type granites in the Helanshan Complex, Khondalite Belt, North China Craton: Implications for rapid sediment recycling during slab break-off

S-type granites, typically derived from the rapid recycling of sedimentary rocks, are sometimes accompanied by contemporary mafic magmatism and granulite metamorphism. However, the geodynamic context for such rock suites is often highly disputed, with various model proposed, including back-arc basin opening, lithospheric delamination, mantle plume and continental rifting. The Paleoproterozoic Khondalite Belt in the North China Craton provides an example of synchronous mafic and felsic magmatism that was accompanied by granulite-facies metamorphic events for which the tectonic affinities of these rocks remains unclear. This study integrates in situ zircon Hf–O isotope analyses, whole-rock geochemistry and Nd isotope results for the earliest two-mica granites (ca. 1.95Ga) in order to provide constraints on the above issues. The granites are strongly peraluminous (A/CNK value >1.1), and characterized by high zircon δ18O values of 7.3–10.6‰, corresponding to calculated magmatic δ18O values of 9.1–12.3‰, similar to those of typical S-type granites. They have relatively high and homogeneous ɛNd(t) values of −1.1 to +0.9 and highly variable zircon ɛHf(t) values ranging from −1.0 to +8.3. In situ zircon Hf–O isotopic compositions indicate that the S-type granites may contain some mantle or juvenile crustal components in addition to a sediment component. Based on the new results and published data, a slab break-off model is proposed to explain the rapid recycling of sedimentary precursors and the generation of the ca. 1.95Ga S-type granites.