Crustal growth at ～2.5 Ga in the North China Craton：evidence from whole-rock Nd and zircon Hf isotopes in the Huai＇an gneiss terrane

The Huai’an gneiss terrane mainly consists of TTG gneisses and dioritic gneisses. Laser in situ U-Pb dating of magmatic zircon cores indicates that protolith of these gneisses was formed at ~2.5 Ga. The TTG gneisses have positive εNd（t） values of 2.7 to 4.3, and most of the magmatic zircons have positive εHf（t） values of 2.0 to 8.3. These positive εNd（t） and εHf（t） values are both similar to those of the contemporaneous depleted mantle at 2.5 Ga. Moreover, the young Hf model ages of 2.44 to 2.73 Ga for the magmatic zircons are close to the timing of the zircons growth. The whole-rock εNd（t） values are lower in the dioritic gneisses （0.8 to 1.7） than in the TTG gneisses due to the involvement of ancient crust in its source. However, many magmatic zircons from the dioritic gneisses have similar εHf（t） values （2.0 to 7.9） to that of the coeval depleted mantle; their Hf model ages of 2.49 to 2.75 Ga are close to the U-Pb ages of zircons. The highest εHf（t） values are close to the value of the depleted mantle, and the relatively high εHf（t） values corresponds to the relatively young Hf model age. These Nd and Hf isotope features suggest that these two types of gneisses of the Huai’an gneiss terrane originated from the juvenile crust at ca. 2.5 Ga.     

Stable carbon and oxygen isotopic compositions of carbonates in middle Mesoproterozoic Wumishan Formation and sea-level change

δ13C of dolostones from the Wumishan Formation (1 310-1 207 Ma) in the Ming Tombs area near Beijing ranges from - 1.5 to 1.5‰ (PDB), and generally, the δ18O ranges from - 4 to - 5‰ (PDB). The secular variation of stable carbon isotopic compositions is cyclic. δ13C displays positive shifts from a sequence boundary to 1.5 per mil (PDB), then a negative shifts to - 1.5 per mil (PDB). In some unit, where δ13C values of the rocks are negative, they have relatively high δ18O values close to -4.0‰ (PDB). These characteristics of the carbon and oxygen isotopic compositions may be associated with sea-level changes.     

The possible contribution of a mantle-derived fluid to the Ningwu porphyry iron deposits-Evidence from carbon and strontium isotopes of apatites

Apatite is a characteristic mineral for the iron deposits in the Ningwu region. Apatite shows a range of δ^13CPDB values from -9.6‰ to -0.6‰, and most of them with the δ^13CPDB values distributed in a range of mantle carbon （i.e., -2‰to -9‰）, except for samples T-06-1 and J-06-3. Apatites in the Ningwu porphyry iron deposits yield a narrow variation range of ^87Sr/^86Sr values from 0.706326 to 0.707577, similar to those of the volcanic and subvolcanic rocks and higher than that of a typical mantle-derived magmatic apatite （i.e., about 0.7040）, indicating that formation of the apatites was involved with crust material contamination.δ^13CPDBand ^87Sr/^86Sr values of the apatite indicate a possible contribution of mantle-derived fluid and material to formation of the Ningwu porphyry iron deposits, supported by regional geology, regional gravities and geochemistry of the volcanic and subvolcanic rocks in the Ningwu region.     

Caledonian reworking of Paleoproterozoic basement in the Cathaysia Block: Constraints from zircon U-Pb dating,Hf isotopes and trace elements

A combined study of zircon U-Pb dating, Hf isotopes and trace elements has been carried out for granodioritic neosomes of migmatites from the Tianjingping area in northwestern Fujian Province. Zircons are characterized by zoning, higher Th/U ratios （mostly≥0.1）, HREE enrichment, and positive Ce and negative Eu anomalies, and show features similar to magmatic or anatectic zircons. Apparent ^206Pb/^238U ages for the zircons are 447±2 Ma （95 % conf., MSWD=0.88）, corresponding to a Caledonian event. εHf（t） values are -13.3 to -9.7, indicating a crustal source. Two-stage Hf model ages are 1.7 to 1.9 Ga, suggesting that protolith of the migmates was probably formed in the Paleoproterozoic. The granodioritic neosomes have the characteristics of peraluminous calc-alkaline granite, and their REE patterns and trace elements spidergrams show features of middle to upper crustal rocks. Together with previous studies, we conclude that the protolith of the Cathaysia basement in the Tianjingping area was likely formed in the middle-late Paleoproterozoic and experienced partial melting during the Caledonian period. The recognition of Caledonian reworking of the Paleoproterozoic basement in the Cathaysia Block provides a new insight into the tectonic evolution of the Cathaysia Block in the Caledonian period and the interaction between the Cathaysia Block and the Yangtze Block.     

Oxygen isotope geochemistry of phosphorite and dolomite and palaeo-ocean temperature estimation: A case study from the Neoproterozoic Doushantuo Formation, Guizhou Province,South China

The Neoproterozoic Doushantuo Formation at Weng'an in Guizhou Province of South China consists of phosphorites and dolomites.Three types of samples are recongnized for the phosphate-bearing rocks.In Type I samples, interstitial phosphates occur within the dolomite rocks, whereas in Type III samples, interstitial dolomites occur within the phosphorites; both of them may have diagenetic origin.The Type II samples are interbedded phosphorites and dolomites.Oxygen isotopic compositions of phosphate and coeval carbonate were analyzed.Phosphates of Type I and II samples show similar low δ18Ophos (SMOW) values of 10.9‰～12.1‰ and 10.7‰～12.8‰, respectively, whereas Type III phosphates show higher δ18Ophos (SMOW) values of 13.2‰～15.0‰.Dolomites from Type I and II samples have δ18Ocarb (PDB) and δ13Ccarb (PDB) values of -6.2‰～-3.4‰ and -2.2‰～-0.7‰, respectively, whereas interstitial dolomites from Type III samples show lower δ18Ocarb (PDB) and δ13Ccarb (PDB) values of -12.9‰～-8.6‰ and -4.4‰～-2.6‰.Petrology and oxygen isotope data suggest that diagenetic alteration may have partially affected the studied rocks, but the dolomites from Type I and phosphates from Type III samples represent the most pristine isotopic compositions of primary carbonates and phosphates precipitated from seawater.Calculations of palaeocean temperatures from the most 18O enriched dolomite and phosphate indicate a temperature range of 32.2 ℃ to 34.0 ℃ for the seawater of the Doushantuo Formation period.Ce anomaly also suggests more oxic conditions and possibly shallower water depth in the upper strata of the Doushantuo Formation at Weng'an, Guizhou Province.     

Transmission of oxygen isotope signals of precipitation-soil water-drip water and its implications in Liangfeng Cave of Guizhou,China

According to systemically monitoring results of oxygen （hydrogen） isotope compositions of precipitation, soil waters, soil CO2, cave drip waters and their corresponding speleothems in Liangfeng Cave （LFC） in Guizhou Province, Southwest China, it is found that local precipitation is the main source of soil waters and drip waters, and that the amplitudes of those δ18O values of three waters （precipitation, soil water and drip water） decrease in turn in the observed year, which are 0‰ to -10‰, -2‰ to -9‰ and -6‰ to -8‰, respectively. Moreover, the δ18O values for three waters show a roughly simultaneous variation, namely, that those values are lighter in the rainy seasons, weightier in the dry seasons, and that the average δ18O value of drip waters is about 0.3‰ weightier than that of precipitation, which is modified by surface evaporation processes. We also find that oxygen isotope equilibrium is reached or neared in the formation processes of speleothems in LFC system, and that it is feasible to reconstruct paleotemperature and paleoprecipitation by using δ18O values of speleothems. However, it should be noted that surface evaporation would affect the oxygen isotope values in the study area.     

Variation of soil Δδ13C values in Xifeng loess-paleosol sequence and its paleoenvironmental implication

The carbon isotopic compositions of soil organic matter （SOM） and total carbonate （TC） in Xifeng Ioess-paleosol sequence have been studied. The δ^13CsoM values vary from -23.8‰ to -20.2‰, which are higher in interglacial than in glacial stages. Contrary to δ^13Csoi values, the δ^13CTc values vary from -8.5‰ to -3.6‰ and are lower in interglacial than in glacial stages. The differences （△δ^13C） between the δ^13CsoM and δ^13CTc values vary from 14.1‰ to 19.4‰. Our results from the Xifeng loesspaleosol sequence indicate that the △δ^13C values represent the ratio of primary carbonate （PC） to secondary carbonate （SC）. The △δ^13C values were high in the loess stages, and the maximal PC-to-SC ratio can reach 6：4. But in the paleosol stages, the △δ^13C values were low, with a small proportion of PC. The △δ^13C values in Ioess-paleosol sequence also indicate the contributions of the dust to the loess sediment in the Chinese Loess Plateau because the dust contains the PC.     

Oxygen isotope variation in the water cycle of the Yamzho lake Basin in southern Tibetan Plateau

Given the potential use of stable isotope in the paleoclimate reconstruction from lacustrine records as well as in the local hydrology cycle, it is crucial to understand the processes of stable isotope evolution in catchment in the Tibetan Plateau region. Here we present a detailed study on the water oxygen isotope based on 2 years observation including precipitation, river water and lake water in the Yamzho Lake, south of the Tibetan Plateau. Temporal variation of local precipitation 5180 shows an apparent ＂monsoon cycle＂. In monsoon season, 5180 in waters is lower. In non-monsoon season, δ^18O in precipitation and lake water is higher and higher river δ^18O exists in spring, probably reflecting the effect of land surface evaporation, together with the higher δ^18O values in spring precipitation. It is also found that the surface lake water δ^18O varies seasonally and annually. The lower lake water δ^18O in the late summer is apparently related to the summer monsoon precipitation. The mean δ^18O value of lake water in 2007 is 1.2‰ higher than that in 2004, probably due to the less monsoon precipitation in summer of 2007, as can be confirmed from the precipitation data at the Langkazi meteorological data. It is also found that an obvious shift of vertical lake water δ^18O reflects the fast mixture of lake water. δ^18O values of lake water are over 10‰ higher than those of precipitation and river water in this region due to the evaporation fractionation. The modeled results show that the evaporation process of the lake water is sensitive to relative humidity, and the present lake water δ^18O reflects a relative humidity of 51% in the Yamzho Lake. It shows that the lake will take 30.5 years to reach present lake water δ^18O given a large shift in the input water δ^18O. The modeled results also reveal that surface lake water temperature and inflow δ^18O have slight effect on the isotopic balance process of lake water in the Yamzho Lake.     

An in situ zircon Hf isotopic,U-Pb age and trace element study of banded granulite xenolith from Hannuoba basalt： Tracking the early evolution of the lower crust in the North China craton

Backscattered electron images, in situ Hf isotopes, U-Pb ages and trace elements of zircons in a banded granulite xenolith from Hannuoba basalt have been studied. The results show that the banded granulite is a sample derived from the early lower crust of the North China craton. It is difficult to explain the petrogenesis of the xenolith with a single process. Abundant information on several processes, however, is contained in the granulite. These processes in-clude the addition of mantle material, crustal remelting, metamorphic differentiation and the delamination of early lower crust. About 80% of zircons studied yield ages of 1842 ±40 Ma, except few ages of 3097-2824 Ma and 2489-2447 Ma. The zircons with ages older than 2447 Ma have high εHf (up to +18.3) and high Hf model age (2.5-2.6 Ga), indicating that the primitive materials of the granulite were derived mainly from a depleted mantle source in late Archean. Most εhf of the zircons with early Proterozoic U-Pb age vary around zero, but two have     

U-Pb ages and Hf isotopes for detrital zircons from quartzite in the Paleoproterozoic Songshan Group on the southwestern margin of the North China Craton

In situ U-Pb dating and Lu-Hf isotopic analysis were carried out for detrital zircons from quartzite in the Paleoproterozoic Songshan Group on the southern margin of the North China Craton （NCC）. The results provide further constraints on the crustal formation and evolution history of NCC. Four ^207Pb/^206Pb age populations were obtained from 99 analyses, with clusters at -3.40 Ga, 2.77-2.80 Ga, -2.50 Ga and 2.34 Ga, respectively. The 3.40 Ga old zircons have similar Hf isotopic compositions to those from Archean rocks in the Jidong and Anshan areas of NCC. However, crustal remnants older than 3.6 Ga have been identified in the southern margin of NCC, the South China Craton, the northwestern part of the Qinling Orogen and its adjacent area. Thus, it is not easy to trace the source rock from which the 3.40 Ge detrital zircons were derived. It can be inferred that the crustal remnants older than 3.40 Ga might have been widely distributed in the North China Craton. The 2.77-2.80 Ga zircons make up a relatively small proportion and have the highest εHf（t） values （up to 6.1±1.6）, consistent with the Hf isotopic composition of the depleted mantle at 2.83 Ga. Their single-stage Hf model age of 2.83 Ga is close to their crystallized age, suggesting that their source rocks were extracted from the contemporaneous depleted mantle. The -2.50 Ga zircon grains constitute about 85% of the total grain population and their Hf isotopic compositions indicate major growth of juvenile crust at -2.50 Ga but minor reworking of ancient crust. The youngest zircon dated in this study gave an U-Pb age of 2337±2.3 Ma, which can be considered the maximum depositional age of the formation of the Songshan Group.     

Oxygen isotopic compositions of zircons from pyroxenite of Daoshichong,Dabieshan: Implications for crust-mantle interaction

Oxygen isotopic compositions of zircons from pyroxenite (～145 Ma) of Daoshichong, Dabieshan have been measured by an ion microprobe. Both within the single grain and among different grains, oxygen isotopic ratios are homogeneous, δ 18O = (7.66‰±0.46)‰ (1 SD, 1 σ = 0.10, n = 22). High δ 18O values indicate that the mantle-derived parent magma of Daoshichong pyroxenite have undergone interaction with crustal materials. Combing with other geochemical constraints, the way of crust-mantle interaction is suggested to be source mixing other than crustal contamination. The time interval between crust-mantle interaction and formation of the parent magma of Daoshichong pyroxenite is less than several million years. The crustal component involving in crust-mantle interaction is mafic lower crust, and the parent magma of pyroxenite possibly contain large proportion (>37%) of such lower crust.     

Calcium isotope fractionation and its controlling factors over authigenic carbonates in the cold seeps of the northern South China Sea

In this study, we analyzed stable calcium isotope results of authigenic carbonates from two cold seep areas of the Dongsha area and the Baiyun Sag in the northern South China Sea. The stable isotopes of carbon and oxygen as well as the mineral composition of authigenic carbonates were used to investigate control calcium isotope fractionation. The δ44/40Ca ratios of the southwestern Dongsha area samples ranged from 1.21‰ to 1.52‰ and the ratio of the Baiyun Sag sample was 1.55‰ of the SRM915a isotope standard. X-ray diffraction analysis showed that the carbonate samples consisted of dolomite, calcite and aragonite, with small amounts of high-Mg calcite and siderite. The δ13C values of the carbonates of the southwestern Dongsha area varied between δ49.21‰ and δ16.86‰ of the Vienna PeeDee Belemnite (VPDB) standard and the δ18O values ranged from 2.25‰ to 3.72‰ VPDB. The δ13C value of the Baiyun Sag sample was 2.36‰ VPDB and the δ18O value was 0.44‰ VPDB. The δ13C values of the carbonates of the southwestern Dongsha area revealed there is methane seeping into this area, with a variable contribution of methane-derived carbon. The sampled carbonates covered a range of δ13C values suggesting a dominant methane carbon source for the light samples and mixtures of δ13C values for the heavier samples, with possibly an organic or seawater carbon source. The δ18O values indicated that there is enrichment in 18O, which is related to the larger oxygen isotope fractionation in dolomite compared to calcite. The results of the Baiyun Sag sample exhibited normal seawater carbon and oxygen isotopic values, indicating that this sample is not related to methane seepage but instead to precipitation from seawater. The relatively high δ44/40Ca values indicated either precipitation at comparatively high rates in pore-water regimes with high alkalinity, or precipitation from an evolved heavy fluid with high degrees of Ca consumption (Raleigh type fractionation). The dolomite samples from the Dongsha area revealed a clear correlation between the carbon and calcium isotope composition, indicating a link between the amount and/or rate of carbonate precipitation and methane contribution to the bicarbonate source. The results of the three stable isotope systems, mineralogy and petrography, show that mineral composition, the geochemical environment of authigenic carbonates and carbon source can control the calcium isotope fractionation.     

Variability of snail growing season at the Chinese Loess Plateau during the last 75 ka

Knowledge of seasonal climate change is one of the key issues facing Quaternary paleoclimatic studies and estimating seasonal climate change is difficult,especially changes such as seasonal length on glacial-interglacial timescales.The stable isotope composition from seasonal land snail shells provides the potential to reveal seasonal climatic features.Two modern land snail species,cold-aridiphilous Pupilla aeoli and thermo-humidiphilous Punctum orphana,were collected from different climatic zones in 18 localities across the Chinese Loess Plateau,spanning 11 degrees of longitude and covering a range of 1000 km2.The duration of the snail growing season(temperature ≥10℃) was shorter(202 ± 6 d) in the eastern Loess Plateau compared with in the western Loess Plateau(162 ±7 d).The δ13C of P.aeoli shells was ?9.1‰ to ?4.7‰ and ?5.0‰ to 0.3‰ for δ18O.For P.orphana,the δ13C ranged from ?9.1‰ to ?1.9‰ and ?8.9‰ to ?2.9‰ for δ18O.Both the δ13C and δ18O differences between the two snail species were reduced from the east to the western Loess Plateau(2.8‰ to 0.2 ± 1.1‰ for δ13C and 4.7‰ to 2.9 ± 1.3‰ for δ18O).These isotopic differences roughly reflect the difference in the growing season lengths between the east and west Loess Plateau indicating that the duration of the snail growing season shortens by 15 d or 19 d if the difference decreases by 1‰ in δ13C or δ18O,respectively.Thus,the difference in δ13C and δ18O between both snail species can be used to reveal the length of the snail growing season in the past.Based on our investigation,the length of the snail growing seasons from the Xifeng region during the last 75 ka was reconstructed.During the mid-Holocene(8-3 ka),the mean isotopic difference from both snail species reached maximum values of 2.6 ± 0.7‰ and 2.1 ± 1.4‰ for δ13C and δ18O,respectively.This was followed by MIS 3 that ranged from 2.5 ± 0.4‰ for δ13C and 1.6 ± 0.8‰ for δ18O.The Last Glacial Maximum changed by only 0.2‰ and 0.4‰ for δ13C and δ18O,respectively.Therefore,we estimate that the duration of the snail growing seasons to be ～200 ± 10 d during the mid-Holocene,190 ± 6 d in MIS 3 and 160 ± 3 d during the last glacial period.     

Discovery of ∼4.0 Ga detrital zircons in the Changdu Block,North Qiangtang,Tibetan Plateau

Using in situ zircon dating by LA-ICP-MS and MC-ICP-MS, detrital zircon of 3981±9 Ma age was found in metamorphic rocks of the Ningduo Rock Group, Changdu Block of Northern Qiangtang. This is the oldest age record that has been found in the Qiangtang area. This finding also constitutes the third zircon locality in China with an age older than 3.9 Ga. Thus, the discovery provides new information for the study of Hadean crust. In addition, we found 3.51–3.13 Ga, ∼2440 Ma, ∼1532 Ma, ∼982 Ma and ∼618 Ma age peaks from 100 test spots. The younger ages of ∼982 Ma and ∼618 Ma correspond to the formation of the Rodinian super-continent and the Pan-African event, respectively. These findings suggest a close relationship between these zircons and the Gondwanan super-continent. The age of ∼618 Ma defines the lower limit on the deposit time of the protolith for the garnet-mica-quartz schist in the Ningduo Rock Group. Zircons with an age of ∼982 Ma generally display a negative ɛ_Hf(t) and a two-stage Hf model with concentrated ages around 1933–2553 Ma. This pattern indicates that the source area of the Ningduo Rock Group underwent a significant separation of depleted mantle into the crust during the Paleoproterozoic Era. However, zircons with ages of 2854–3505 Ma also show a negative ɛ_Hf(t) and a two-stage Hf model with a concentration of ages around 3784–4316 Ma. These results demonstrate that the source area of the Ningduo Rock Group contains a residual amount of ancient (Hadean) crustal materials. This paper provides new information on the relationship between the basement of the Qiangtang area and the Paleoproterozoic basements of the Gangdese and Himalayan regions, which constrains the northern boundary of Gondwana.     

Southwest monsoon changes indicated by oxygen isotope of ostracode shells from sediments in Qinghai Lake since the late Glacial

The δ 18O records of ostracode shells in sediments of core QH-2000 from Qinghai Lake can be used as a better proxy to reflect monsoon changes. Low monsoon precipitation between 17.5 and 11 cal. ka BP is indicated by positive δ 18O values averaging 2.37‰. A fast shift in δ 18O from positive at 11 cal. ka BP to negative at 10 cal. ka BP indicates sharp increase of monsoon precipitation. An interval of generally high monsoon precipitation is observed between 10 and 6 cal. ka BP with δ 18O values averaging -2.15‰. Decrease of monsoon precipitation between 6 and 2.5 cal. ka BP is indicated by positive δ 18O values. δ 18O keeps positive values averaging 3.0‰ between 2.5 and 0 cal. ka BP suggesting low high monsoon precipitation. The climatic changes indicated by δ 18O records of ostracode shells in sedi- ments of core QH-2000 from Qinghai Lake and our broader regional comparison show that the climate in Qinghai Lake since the late Glacial is probably controlled by southwest monsoon other than southeast monsoon.     

<Emphasis Type="Italic">In-situ</Emphasis> trace element analyses of zircons from Dabieshan Huangzhen eclogite: Trace element characteristics of eclogite-facies metamorphic zircon

The internal structures of zircons in eclogite from Huangzhen have been studied by cathodoluminescence (CL) microscopy. Two growth stages were distinguished: protolith magmatic cores and metamorphic overgrowth rims. These different domains were analyzed for trace elements using LAM-ICP-MS. The protolith and the overgrowth zircons have different trace elements characteristics. The trace element contents of protolith zircons are high and very variable. The overgrowth zircons show a typical trace element feature of equilibrium with garnet, e.g. low contents of HREE (132.2–197.6 μg/g) and small differential degree of HREE ((Yb/Gd)_CN=8.6–11.9). The contents of Nb, Ta and the ratio of Nb/Ta are lower in the metamorphic domains (0.5–1.4 μg/g, 0.7–1.5 μg/g, 0.3–1.3, respectively) than in the protolith domains (3.8–19.7 μg/g, 2.7–12.1 μg/g, 1.0–4.6, respectively). This is the first time to give the evidence that the metamorphic zircon equilibrates with the rutile, which formed during the peak metamorphic stage. The REEs and other trace elements data demonstrate that the metamorphic zircons overgrow in the eclogite-facies conditions. The trace element composition of zircon can therefore give new ways to constrain their formation conditions.     

Oxygen and carbon isotope records of calcareous nannofossils for the past 1 Ma in the southern South China Sea

Oxygen and carbon isotopic analyses have been carried out on calcareous nannofossils from ODP Site 1143 in the southern South China Sea. The results demonstrate that the δ18O values of calcareous nannofossils for the past 1 Ma vary systematically with those of planktonic and benthic foraminifera from the same site. But the average δ18O value of nannofossils is 1.869‰ higher than that of planktonic foraminifera, and is 3.855‰ lower than that of benthic foarminifera. There are systematic differences between the δ13C values of nannofossils, planktonic foarminifera and benthic foraminifera. The average δ13C of nannofossils is 0.756‰ higher than that of benthic foraminifera, and is 0.460‰ lower than that of planktonic foraminifera. The δ13C values of nannofossils exhibit a significant positive shift beginning near isotope stage 14 and lasting until stage 8. This study also finds that there is a close ralationship between the δ13C of nannofossils and the percentage of Florisphaera profunda. This implies that the δ13C of nannofossils can be used as an indicator of sea water surface paleoproductivity.     

Zircon U-Pb and Hf isotopes of volcanic rocks from the Batamayineishan Formation in the eastern Junggar Basin

An internal structural study was conducted to investigate U-Pb age, trace elements and Hf isotopes of basaltic zircons from the Batamayineishan Formation. The basalt was obtained from drill well San-Can 1 on the eastern Luliang uplift within the Junggar Basin. Trace element data of zircons show that all samples are magmatic, with similar REE patterns, including positive Ce (δCe=5.06–134), but negative Eu (δEu=0.06⦒0.55) anomalies and enrichment in heavy rare earth elements. Among 25 grains, the concordant ages were subdivided into three groups; ages of 300.4±1.3 Ma (n=11), 339.2±2.7 Ma (n=3) and 392.0±1.7 Ma (n=8). Three remaining grains were nearly concordant, with ²⁰⁶Pb/²³⁸U ages of 510±7, 488±6 and 453±6 Ma, respectively. The youngest concordant age (i.e., 300.4±1.3 Ma) could be interpreted as the formation age of the studied basaltic rock; this is consistent with the sampling position at the upper part of the Batamayineishan Formation. On the other hand, ages such as Ordovician and early Devonian are consistent with the ages of island-arc volcanic rocks (enrichment in Pb) or ophiolites around the basin. Moreover, the positive ɛ _Hf(t) values of the early and middle Paleozoic zircons (+3.6–+10.5) may suggest that the basement traversed by the studied volcanic rocks may be Paleozoic in age, formed from the residual oceanic crust and island-arc complex. The ɛ _Hf(t) values (+4.2–+17.1) of the late Paleozoic (∼300.4 Ma) zircons suggest that the basaltic magmas were derived from partial melting of the asthenospheric mantle or depleted lithospheric mantle. These magmas were slightly contaminated by the existence of early-middle Paleozoic materials. The late Carboniferous basalts represent direct eruption of mantle-derived magmas at the upper crustal level during a post-collisional tectonic setting. We therefore consider that extensive vertical growth of the continental crust to have occurred before the late Carboniferous.     

Early Precambrian tectonothermal events of the North China Craton: Constraints from in situ detrital zircon U-Pb, Hf and O isotopic compositions in Tietonggou Formation

Clastic sediments and sedimentary rocks are widely used for understanding the formation and evolution of the continental crust. The Tietonggou Formation outcrops in the Xiaoqinling region at the southern margin of the North China Craton (NCC) and has unconformable contacts with the underlying Taihua Complex and overlying Paleoproterozoic Xiong’er Group. It mainly consists of quartzite and its protoliths are mature terrigenous clastic rocks. On the basis of the ages of the youngest detrital zircons from the quartzites and the ages of the Xiong’er Group, the depositional ages of the protoliths of the Tietonggou Formation were well constrained to 1.91–1.80 Ga. The U-Pb isotopic data of detrital zircons from the Formation show a major age peak at ～2.1 Ga, which is consistent with 2.2–2.0 Ga magmatism in the Trans-North China Orogen of the NCC. Taking into account the texctural and compositional maturity of the Tietonggou Formation quartzite, the ～2.1 Ga lithologic units in the Trans-North China Orogen are interpreted as the major source of the Tietonggou Formation. The majority of these ～2.1 Ga detrital zircons mostly have high δ ¹⁸O values (>6.5‰) and negative ? _Hf(t) values (?7.8–0.0), with corresponding Hf model ages significantly older than their crystallization ages, indicating that these zircons formed from the partial melting of ancient continental crust. The majority of the 2.8–2.7 Ga and ～2.5 Ga detrital zircons from the Tietonggou Formation had positive ? _Hf(t) values, and mantle-like δ ¹⁸O values, suggesting that the NCC has experienced two stages of significant crustal growth in the Neoarchaean at 2.7 and 2.5 Ga, respectively. The Hf isotopic data of detrital zircons from Paleoproterozoic metasedimentary rocks in the Trans-North China Orogen varied mainly toward the reduction of the radiogenic Hf isotope and gradually show a similar trend of the isotope trajectories of crustal evolution. This reveals that the NCC probably has not developed a long-lived subduction to complete the final assembly of the NCC. Alternatively, these maybe imply that the tectonic setting of the NCC substantially changed at ～2.1 Ga, the reduction of the radiogenic Hf isotope could be attributed to the rollback of the subducting slab.