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1.
Geodynamic drivers for the supercontinent cycle are generally attributed to either top-down(subduction-related)or bottom-up(mantle-related)processes.Compiled geochemical data and U-Pb ages and Hf isotopic signatures for magmatic and detrital zircons from the Tarim Craton reveal a distinct change in subduction style during the Neoproterozoic.The subduction cycle is recorded in increasing and decreasing intensity of subduction-related magmatic rocks and time-equivalent sedimentary successions,and converse trends ofεHf(t)values and corresponding changes in crustal incubation time.These trends are consistent with a switch from advancing to retreating subduction.The switch likely occurred at ca.760 Ma when zirconεHf(t)values increase and crustal incubation times decrease following a transitional shift between 800 Ma and 760 Ma.A switch at this time is consistent with Rodinia breakup and may have resulted in the late Neoproterozoic Tarim rift basin.The long-lived(ca.500 Ma)subduction recorded in the Tarim Craton suggests the predominance of a top-down process for Rodinia breakup on this part of its margin.  相似文献   

2.
In this study, a combined study of zircon U–Pb and Hf–O isotopes, as well as whole-rock major and trace elements and Nd isotopes has been conducted for Yangjia gneissic granite from the southern Wuyishan terrane, Southeast China, to constrain its petrogenesis and provide a new window for investigating the tectonic evolution of the Cathaysia basement. U–Pb dating for magmatic zircons yields a 207Pb/206Pb age of ca. 1.80 Ga, interpreted as the emplacement age of the Yangjia granite. The granites have relatively high K2O, Rb, Ga, Zr, Nb, Y, and Ce contents and show low Al2O3, CaO, and Ba concentrations. Their 10,000*Ga/Al ratios range between 2.8 and 3.2. Zircons from the granite have εHf(t) values ranging from ?13.2 to ?7.2, corresponding to THfDM2 model ages of 2.99 Ga to 2.72 Ga. The zircon δ18O values range between 6.7‰ and 9.1‰ with an average of 7.7‰. In addition, the whole-rock εNd(t) values of the granites range from ?6.5 to ?5.4 and the TNdDM2 model ages from 2.73 Ga to 2.82 Ga. All these geochemical and Nd–Hf–O isotopic signatures suggest an A-type affinity for the Yangjia granites, and they were likely generated by partial melting of Palaeoproterozoic parametamorphic rocks of the Wuyishan terrane in a post-collisional extensional setting. When our data is combined with existing geochronological data, it provides further evidence for the Palaeoproterozoic basement in the southern Wuyishan terrane, which records a rapid tectonic transition from post-collision to intraplate extension (1.80–1.77 Ga) related to the break-up of the supercontinent Columbia.  相似文献   

3.
The Sr–Nd–Hf isotopic compositions of both saprolites and parent rocks of a profile of intensively weathered Neogene basalt in Hainan, South China are reported in this paper to investigate changes of isotopic systematics with high masses. The results indicate that all these isotopic systematics show significant changes in saprolites compared to those in corresponding parent rocks. The 87Sr/86Sr system was more seriously affected by weathering processes than other isotope systems, with εSr drifts 30 to 70 away from those of the parent rocks. In the upper profile (> 2.2 m), the Sr isotopes of the saprolites show an upward increasing trend with εSr changing from ~ 50 at 2.2 m to ~ 70 at 0.5 m, accompanying a upward increasing of Sr concentrations, from ~ 10 μg/g to ~ 25 μg/g. As nearly all the Sr of the parent rock has been removed during intensive weathering in this profile, the upward increasing of Sr concentrations in the upper profile suggests import of extraneous Sr. Rainwater in this region, which enriches in Sr (up to 139 μg/L) from seawater, may be the important extraneous source. Thus, the Sr isotopes of the saprolites in the upper profile may be mainly influenced by import of extraneous materials, and the Sr isotopic characteristics may not be retained. In contrast, the εNd and εHf of the saprolites drift only 0–2.6 and 0–3.7 away from the parent rocks, respectively. The negative drifts of the εNd and εHf are coupled with Nd and Hf losses in the saprolites; i.e., larger proportions of Nd and Hf loss correspond to lower εNd and εHf. Compared with the relative high Nd and Hf concentrations of the saprolites, the contributions of extraneous Nd and Hf both from wet and dry deposits of aeolian input are negligible. Thus, the εNd and εHf changes in the profile are mainly resulted from consecutive removal of the Nd and Hf. Calculation indicates that the 143Nd/144Nd and 176Hf/177Hf ratios in saprolites are all significantly lower than their initial values in the parent rock. Simply removing part of the Nd and Hf by incongruent decomposing some of the minerals may not account for this. Fractionation should be happen, which 143Nd and 176Hf may be preferentially removed from the profile relative to 144Nd and 177Hf during intensive chemical weathering, resulting in lower 143Nd/144Nd and 176Hf/177Hf ratios in saprolites relative to the parent rock, even though details for this process is not known. A positive correlation is observed between the εNd and εHf of the saprolites. Interestingly, the saprolites with a net loss of Nd and Hf in the upper profile show good positive correlation, and the regression line parallels the terrestrial array. By contrast, saprolites with a net gain of Nd and Hf in the lower profile generally show higher εHf values at a given εNd value, and the regression line between these εNd and εHf appears to parallel the seawater array. This supports the hypothesis that the contribution of continental Hf from chemical weathering release is the key to the obliquity of the seawater array away from the terrestrial array of the global εNd and εHf correlation. Our results also indicate that caution is needed when using εSr, εNd, and εHf to trace provenances for sediments and soils.  相似文献   

4.
The Central Hebei Basin （CHB） is one of the largest sedimentary basins in the North China Craton, extending in a northeast-southwest direction with an area of 〉350 km2. We carried out SHRIMP zircon dating, Hf-in-zircon isotopic analysis and a whole-rock geochemical study on igneous and metasedi- mentary rocks recovered from drill holes that penetrated into the basement of the CHB, Two samples of gneissic granodiorite （XG1-1） and gneissic quartz diorite 048-1） have magmatic ages of 2500 and 2496 Ma, respectively. Their zircons also record metamorphic ages of 2.41-2.51 and ～2.5 Ga, respec- tively. Compared with the gneissic granodiorite, the gneissic quartz diorite has higher REE contents and lower Eu/Eu＊ and （La/Yb）n values. Two metasedimentary samples （MG1, H5） mainly contain ～2,5 Ga detrital zircons as well as late Paleoproterozoic metamorphic grains. The zircons of different origins have eHf （2.5 Ga） values and Hf crustal model ages ranging from 0 to 5 and 2.7 to 2,9 Ga, respectively, Therefore, ～2.5 Ga magmatic and Paleoproterozoic metasedimentary rocks and late Neoarchean to early Paleoproterozoic and late Paleoproterozoic tectono-thermal events have been identified in the basement beneath the CHB. Based on regional comparisons, we conclude that the early Precambrian basement beneath the CHB is part of the North China Craton.  相似文献   

5.
We conducted in situ U–Pb and Lu–Hf isotope analyses of 401 detrital zircons collected from the Songpan–Ganzi Triassic turbidite complex in an attempt to understand the provenance variations of the siliciclastic rocks and the crustal growth history of central China. These detrital zircons exhibit a wide age spectrum with three major peaks at 1.7–2.0 Ga, 750–1050 Ma, and 210–500 Ma. They are dominated by negative ?Hf(t) values with a large range. Synthesis of the zircon U–Pb and Lu–Hf isotopic data indicate that the Triassic Songpan–Ganzi turbiditic succession could have been derived dominantly from the Tibetan terrains + the Kunlun and Qinling orogens. Our samples are characterized by a common, prominent group of Hf crust formation model ages at 0.8–4.1 Ga with a peak at 2.7–3.4 Ga. This fact indicates that (1) Phanerozoic magmatism in central China could have been predominantly products of crustal reworking with insignificant formation of juvenile crust and (2) the Neoarchaean was an important period of continental growth in central China. In addition, our data set also reveal that three widespread tectonothermal events could have occurred in the region during the late Mesoproterzoic, Palaeozoic, and early Mesozoic, respectively.  相似文献   

6.
The East Kunlun Orogenic Belt(EKOB) provides an important link to reconstruct the evolution of the Proto-Tethys and Paleo-Tethys realm. The EKOB is marked by widespread Early Paleozoic magmatism.Here we report the petrology, bulk geochemistry, zircon Ue Pb dating and, Lue Hf and SreN d isotopic data of the Early Paleozoic granitic rocks in Zhiyu area of the southern EKOB. Based on the zircon U-Pb dating, these granitoids, consisting of diorite, granodiorite and monzogranite, were formed during 450 -430 Ma the Late Ordovician to Middle Silurian. The diorite and granodiorite are high Sr/Y ratio as adakitic affinities, and the monzogranite belongs to highly fractionated I-type. Their(~(87)Sr/~(86)Sr)ivalues range from 0.7059 to 0.7085, εNd(t) values from -1.6 to -6.0 and the zircon εHf(t) values show large variations from +9.1 to -8.6 with Hf model ages(T_(DM2)) about 848 Ma and 1970 Ma. The large variations of whole-rock Nd and zircon Hf isotopes demonstrate strong isotopic heterogeneity of the source regions which probably resulted from multi-phase underplating of mantle-derived magmas. Geochemical and isotopic studies proved that the diorite and granodiorite had been derived from partial melting of heterogeneous crustal source with variable contributions from ancient continental crust and juvenile components, and the monzogranites were representing fractional crystallization and crustal contamination for arc magma. The Early Paleozoic adakitic rocks and high-K calc-alkaline granitoids in the southern EKOB were likely emplaced in a continental marginal arc setting possibly linked to the southwards subduction of the Paleo Kunlun Ocean and the magma generation is linked to partial melting of thickened continental crust induced by underplating of mantle-derived magmas.  相似文献   

7.
Neoproterozoic sedimentary sequences in the South China Block provide great opportunity to examine the tectonic evolution and crustal accretion during this period. This study presents U–Pb ages and Hf isotope composition of detrital zircons and Nd isotope composition of whole rocks of the Neoproterozoic sequences from the Yangtze Block, part of the South China Block. Age patterns of detrital zircons imply that the source area experienced three major periods of magmatic activity at 2,300–2,560, 1,900–2,100 and 770–1,000?Ma and two major episodes of juvenile crust accretion at 2,600–3,400 and 770–1,000?Ma. The maximum age of the Gucheng glaciation can be restricted at?~768?Ma from the youngest detrital zircon ages, probably corresponding to the Kaigas glaciation rather than to the Sturtian glaciation. High La/Sc ratio and low Cr/Th, Sc/Th and Co/Th ratios of the sedimentary rocks point to a derivation from dominantly felsic upper continental crustal sources, whereas large variation of εNd(t) and εHf(t) values indicates that mantle-derived magmatic rocks also provided material to the sedimentary sequences in different degrees. The shift in εNd(t) values of whole rocks and U–Pb age spectra of detrital zircons records the evolution from a back-arc to retro-arc foreland to a rift basin. Age distribution of detrital zircons from the Neoproterozoic sequences, compared with those of the major crustal blocks of Rodinia, implies that the position of the Yangtze Block was probably adjacent to northern India rather than between Australia and Laurentia before the breakup of the Rodinia supercontinent.  相似文献   

8.
The covariant behavior of Lu-Hf and Sm-Nd isotopes during most magmatic processes has long been recognized, but the details of this behavior in the depleted mantle reservoir have not been adequately examined. We report new whole-rock Hf and Nd isotope data for 1) juvenile, mantle-derived rocks, mid-Archean to Mesozoic in age, and 2) early Archean gneisses from West Greenland. Hf and Nd isotopic compositions of the juvenile rocks are well correlated, with the best fit corresponding to the equation εHf = 1.40 εNd + 2.1, and is similar to the collective Hf-Nd correlation for terrestrial samples of εHf = 1.36 εNd + 3.0. The early Archean Greenland gneisses, in contrast, have an extreme range in εNd values (−4.4 to +4.2; Bennett et al., 1993) that is not mirrored by the Hf isotopic system. The εHf values for these rocks are consistently positive and have much less variation (0 to +3.4) than their εNd counterparts.The information from the Hf isotopic compositions of the West Greenland gneisses portrays an early Archean mantle that is relatively isotopically homogeneous at 3.8 to 3.6 Ga and moderately depleted in incompatible elements. There is no evidence that any of these gneisses have been derived from an enriched reservoir. The Hf isotopic data are in stark contrast to the Nd isotopic record and strongly imply that the picture of extreme initial isotopic heterogeneity indicated by Nd isotopes is not a real feature of the West Greenland gneisses but is rather an artifact produced by disturbances in the Sm-Nd isotope system of these rocks.Although Hf and Nd isotopic data do not uniquely constrain either the nature of the earliest crust or the timing of crustal growth, the most probable candidate for the enriched reservoir complementary to the depleted mantle in the pre-4.0 Ga Earth is a mafic, oceanic-type crust. In order to explain the predominantly positive εHf and εNd values for the early Archean rocks, this crust must have had a short residence time at the surface of the Earth before returning to the mantle where it was isolated from mixing with the depleted mantle for several hundred million years. The following period from 3.5 to 2.7 Ga may mark a transition during which this early formed mafic crust was mixed progressively back into the depleted mantle reservoir. While a present-day volume of continental crust at 4.0 Ga cannot be excluded on isotopic grounds, we find such a scenario unlikely based on the lack of direct isotopic and physical evidence for its existence. An important aspect of crustal growth and evolution, therefore, may be the transformation of the enriched reservoir from being predominantly mafic in the early Earth to becoming progressively more sialic through time.  相似文献   

9.
We present results of combined in situ U–Pb dating of detrital zircons and zircon Hf and whole-rock Nd isotopic compositions for high-grade clastic metasedimentary rocks of the Slyudyansky Complex in eastern Siberia. This complex is located southwest of Lake Baikal and is part of an early Paleozoic metamorphic terrane in the eastern part of the Central Asian Orogenic Belt (CAOB). Our new zircon ages and Hf isotopic data as well as whole-rock Nd isotopic compositions provide important constraints on the time of deposition and provenance of early Paleozoic high-grade metasedimentary rocks as well as models of crustal growth in Central Asia. Ages of 0.49–0.90 Ga for detrital zircons from early Paleozoic high-grade clastic sediments indicate that deposition occurred in the late Neoproterozoic and early Paleozoic, between ca. 0.62–0.69 and 0.49–0.54 Ga. Hf isotopic data of 0.82–0.69 Ga zircons suggest Archean and Paleoproterozoic (ca. 2.7–2.8 and 2.2–2.3 Ga; Hfc = 2.5–3.9 Ga) sources that were affected by juvenile 0.69–0.82 Ga Neoproterozoic magmatism. An additional protolith was also identified. Its zircons yielded ages of 2.6–2.7 Ga, and showed high positive εHf(t) values of +4.1 to +8.0, and Hf model ages tHf(DM) = tHfc = 2.6–2.8 Ga, which is nearly identical to the crystallization ages. These isotopic characteristics suggest that the protolith was quite juvenile. The whole-rock Nd isotopic data indicate that at least part of the Slyudyansky Complex metasediments was derived from “non-Siberian” provenances. The crustal development in the eastern CAOB was characterized by reworking of the early Precambrian continental crust in the early Neoproterozoic and the late Neoproterozoic–early Paleozoic juvenile crust formation.  相似文献   

10.
The neodymium (Nd) isotopic composition (?Nd) of seawater is a quasi-conservative tracer of water mass mixing and is assumed to hold great potential for paleoceanographic studies. Here we present a comprehensive approach for the simulation of the two neodymium isotopes 143Nd, and 144Nd using the Bern3D model, a low resolution ocean model. The high computational efficiency of the Bern3D model in conjunction with our comprehensive approach allows us to systematically and extensively explore the sensitivity of Nd concentrations and ?Nd to the parametrisation of sources and sinks. Previous studies have been restricted in doing so either by the chosen approach or by computational costs. Our study thus presents the most comprehensive survey of the marine Nd cycle to date.Our model simulates both Nd concentrations as well as ?Nd in good agreement with observations. ?Nd covaries with salinity, thus underlining its potential as a water mass proxy. Results confirm that the continental margins are required as a Nd source to simulate Nd concentrations and ?Nd consistent with observations. We estimate this source to be slightly smaller than reported in previous studies and find that above a certain magnitude its magnitude affects ?Nd only to a small extent. On the other hand, the parametrisation of the reversible scavenging considerably affects the ability of the model to simulate both, Nd concentrations and ?Nd. Furthermore, despite their small contribution, we find dust and rivers to be important components of the Nd cycle. In additional experiments, we systematically varied the diapycnal diffusivity as well as the Atlantic-to-Pacific freshwater flux to explore the sensitivity of Nd concentrations and its isotopic signature to the strength and geometry of the overturning circulation. These experiments reveal that Nd concentrations and ?Nd are comparatively little affected by variations in diapycnal diffusivity and the Atlantic-to-Pacific freshwater flux. In contrast, an adequate representation of Nd sources and sinks is crucial to simulate Nd concentrations and ?Nd consistent with observations. The good agreement of our results with observations paves the way for the evaluation of the paleoceanographic potential of ?Nd in further model studies.  相似文献   

11.
We have carried out a comparative Rb-Sr, Sm-Nd and Lu-Hf isotopic study of a progressively deformed hercynian leucogranite from the French Massif Central, belonging to the La Marche ductile shear zone, in order to investigate the respective perturbation of these geochronometers with fluid induced deformation. The one-meter wide outcrop presents a strongly deformed and mylonitized zone at the center, and an asymmetric deformation pattern with a higher deformation gradient on the northern side of the zone. Ten samples have been carefully collected every 10 cm North and South away from the strongest deformed mylonitic zone. They have been analyzed for a complete major, trace element data set, oxygen isotopes, Rb-Sr, Sm-Nd and Lu-Hf isotopic systematics.We show that most of major and trace elements except SiO2, alkaline elements (K2O, Rb), and some metal transition elements (Cu), are progressively depleted with increasing deformation. This depletion includes REE + Y, but also HFS elements (Ti, Hf, Zr, Nb) which are commonly considered as immobile elements during upper level processes. Variations in elemental ratios with deformation, e.g. decrease in LREE/MREE- HREE, Nd/Hf, Th/Sr, increase in Rb/Sr, U/Th and constant Sr/Nd, lead to propose the following order of element mobility: U ? Th > Sr = Nd ? Hf + HREE. We conclude in agreement with previous tectonic and metallogenic studies that trace element patterns across the shear zone result from circulation of oxidizing F-rich hydrothermal fluids associated with deformation. A temperature of the fluid of 470-480 °C can be deduced from the δ18O equilibrium between quartz-muscovite pairs.Elemental fractionation induces perturbation of the Rb-Sr geochronometer. The well-defined 87Rb/86Sr-87Sr/86Sr correlation gives an apparent age of 294 ± 19 Ma, slightly younger than the 323 ± 4 Ma age of leucogranites in this area. This apparent age is interpreted as dating event of intense deformation and fluid circulation associated with mass transfer, and exhumation of the ductile crust shortly after the leucogranite emplacement. Sm-Nd and Lu-Hf isochron-type diagrams do not define any correlation, because of the low fractionated Sm/Nd and Lu/Hf ratios. Isotopic data demonstrate that only the Lu-Hf geochronometer system is not affected by fluid circulation and gives reliable TDM age (1.29 ± 0.03 Ga) and εHf signatures. By contrast, the Sm-Nd geochronometer system gives erroneous old TDM ages of 2.84-4 Ga. There is no positive εNd-εHf correlation, because of decreasing εNd values with deformation at constant εHf values. However, εNd-εHf values remain in the broad εNd-εHf terrestrial array, which strongly indicates that fluid-induced fractionation can contribute to the width of the terrestrial array. The strong εHf negative values of the leucogranite are similar to metasedimentary granulitic xenoliths from the French Massif Central and confirm the generation of the leucogranite by several episodes of reworking of the lower crust.  相似文献   

12.
ABSTRACT

In this article we present zircon U–Pb ages, Hf isotopes, and whole-rock geochemistry of the Longzhu rhyolite porphyry from the Cathaysia Block, Southeast China to constrain its petrogenesis and provide insights into the early Precambrian tectonic evolution of the Cathaysia Block. LA-ICP-MS zircon U–Pb dating of a representative sample yields a weighted mean 206Pb/207Pb age of 1819 ± 16 Ma, interpreted as the crystallization age of the Longzhu rhyolite porphyry. Zircons from this sample have εHf(t) values ranging from – 8.4 to – 2.2 and THfDM2 model ages from 2.76 to 2.46 Ga. The whole-rock Nd isotopic data from the Longzhu rhyolite porphyries yield εNd(t) values spanning – 6.3 to – 4.7 and TNdDM2 model ages from 2.81 to 2.69 Ga. The rhyolite porphyries have geochemical features similar to those of the typical A-type granites (rhyolites), with high SiO2, total alkali contents and FeOt/MgO ratios, and low CaO and MgO contents. Additionally, the rhyolite porphyries have high total rare earth element concentrations (627 ~ 760 ppm), high (La/Yb)N values (14.5 ~ 26.9), strongly negative Eu anomalies (δEu = 0.28 ~ 0.41), and display enrichments of Rb, Ga, Th, and U and depletions of Sr, Nb, Ta, Eu, and Ti. The geochemical and Nd-Hf isotopic features suggest that the Palaeoproterozoic Longzhu rhyolite porphyries were generated by partial melting of source rocks similar to those of the Badu Complex in an intra-plate extensional setting. The results from this study, when combined with existing geochronological data, further demonstrate that the Palaeoproterozoic rocks of Wuyishan terrane probably represent a remnant of the Columbia supercontinent.  相似文献   

13.
U–Pb zircon analyses from a series of orthogneisses sampled in drill core in the northern Gawler Craton provide crystallisation ages at ca 1775–1750 Ma, which is an uncommon age in the Gawler Craton. Metamorphic zircon and monazite give ages of ca 1730–1710 Ma indicating that the igneous protoliths underwent metamorphism during the craton-wide Kimban Orogeny. Isotopic Hf zircon data show that 1780–1750 Ma zircons are somewhat evolved with initial εHf values –4 to +0.9, and model ages of ca 2.3 to 2.2 Ga. Isotopic whole rock Sm–Nd values from most samples have relatively evolved initial εNd values of –3.7 to –1.4. In contrast, a mafic unit from drill hole Middle Bore 1 has a juvenile isotopic signature with initial εHf zircon values of ca +5.2 to +8.2, and initial εNd values of ～+3.5 to +3.8. The presence of 1775–1750 Ma zircon forming magmatic rocks in the northern Gawler Craton provides a possible source for similarly aged detrital zircons in Paleoproterozoic basin systems of the Gawler Craton and adjacent Curnamona Province. Previous provenance studies on these Paleoproterozoic basins have appealed to the Arunta Region of the North Australian Craton to provide 1780–1750 Ma detrital zircons, and isotopically and geochemically similar basin fill. The orthogneisses in the northern Gawler Craton also match the source criteria and display geochemical similarities between coeval magmatism in the Arunta Region of the North Australian Craton, providing further support for paleogeographic reconstructions that link the Gawler Craton and North Australian Craton during the Paleoproterozoic.  相似文献   

14.
Here we first present samarium (Sm)–neodymium (Nd) isotopic data for the ∼2.5 Ga Wangjiazhuang BIF and associated lithologies from the Wutai greenstone belt (WGB) in the North China Craton. Previous geochemical data of the BIF indicate that there are three decoupled end members controlling REE compositions: high-T hydrothermal fluids, ambient seawater and terrigenous contaminants. Clastic meta-sediment samples were collected for major and trace elements studies in an attempt to well constrain the nature of detrital components of the BIF. Fractionated light rare earth elements patterns and mild negative Eu anomalies in the majority of these meta-sedimentary samples point toward felsic source rocks. Moreover, the relatively low Th/Sc ratios and positive εNd(t) values are similar to those of the ∼2.5 Ga granitoids, TTG gneisses and felsic volcanics in the WGB, further indicating that they are derived from less differentiated terranes. Low Chemical Index of Weathering (CIW) values and features in the A-CN-K diagrams for these meta-sediments imply a low degree of source weathering. Sm–Nd isotopes of the chemically pure BIF samples are characterized by negative εNd(t) values, whereas Al-rich BIF samples possess consistently positive εNd(t) features. Significantly, the associated supracrustal rocks in the study area have positive εNd(t) values. Taken together, these isotopic data also point to three REE sources controlling the back-arc basin depositional environment of the BIF, the first being seafloor-vented hydrothermal fluids (εNd(t) < −2.5) derived from interaction with the underlying old continental crust, the second being ambient seawater which reached its composition by erosion of parts of the depleted landmass (likely the arc) (εNd(t) > 0), the third being syndepositional detritus that received their features by weathering of a nearby depleted source (likely the arc) (εNd(t) > 0).  相似文献   

15.
The Gaoligong belt is located in the southeastern margin of the Tibetan plateau, and is bound by the Tengchong and Baoshan blocks. This paper presents new data from zircon geochronology, geochemistry, and whole-rock Sr–Nd–Pb–Hf isotopes to evaluate the tectonic evolution of the Gaoligong belt. The major rock types analysed in the present study are granitic gneiss, granodiorite, and granite. They are metaluminous to peraluminous and belong to high-K, calc-alkaline series. Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) analyses of zircons from nine granitic rocks yielded crystallization ages of 495–487 Ma, 121 Ma, 89 Ma, and 70–63 Ma. The granitoids can be subdivided into the following four groups. (1) Early Paleozoic granitic gneisses with high εNd(t) and εHf(t) values of − 1.06 to − 3.45 and − 1.16 to 2.09, and model ages of 1.16 Ga to 1.33 Ga and 1.47 Ga to 1.63 Ga, respectively. Their variable 87Sr/86Sr and Pb values resemble the characteristics of the Early Paleozoic Pinghe granite in the Baoshan block. Our data suggest that the rocks were derived from the break-off of the Proto-Tethyan oceanic slab between the outboard continent and the Baoshan block, which induced the partial melting of Mesoproterozoic pelitic sources mixed with depleted mantle materials. (2) Early Cretaceous granodiorites with low εNd(t) and εHf(t) values of − 8.92 and − 4.91 with Nd and Hf model ages of 1.41 Ga and 1.49 Ga, respectively. These rocks have high initial 87Sr/86Sr (0.711992) and lower crustal Pb values, suggesting that they were derived from Mesoproterozoic amphibolites with tholeiitic signature, leaving behind granulite residue at the lower crust. (3) Early Late Cretaceous granites with low εNd(t) and εHf(t) values of − 9.58 and − 4.61 with Nd and Hf model ages of 1.43 Ga and 1.57 Ga, respectively. These rocks have high initial 87Sr/86Sr (0.713045) and lower crustal Pb isotopic values. These rocks were generated from the partial melting of Mesoproterozoic metapelitic sources resulting from the delamination of thickened lithosphere, following the closure of the Bangong–Nujiang Ocean and collision of the Lhasa–Qiangtang blocks. (4) Late Cretaceous to Paleogene granitic gneisses with low εNd(t) and εHf(t) values of − 4.41 to − 10 and − 5.95 to − 8.71, Nd model ages ranging from 1.08 Ga to 1.43 Ga, and Hf model ages from 1.53 Ga to 1.67 Ga, respectively. These rocks show high initial 87Sr/86Sr (0.713201 and 714662) and lower crustal Pb values. The data suggest that these rocks are likely related to the eastward subduction of the Neo-Tethyan Oceanic slab, which induced partial melting of Mesoproterozoic lower crustal metagreywacke. The results presented in this study from the Gaoligong belt offer important insights on the evolution of the Proto-Tethyan, Bangong–Nujiang, and Neo-Tethyan oceans in the southeastern margin of the Tibetan Plateau.  相似文献   

16.
In situ U–Pb dating and Hf isotopic of detrital zircons from beach sediments of Yalong Bay were analyzed to trace sedimentary provenance and reveal the crustal evolution of Hainan Island in South China. The grain size distribution of the sediments displays a clear single-peak feature, indicating the sediments were formed under the same condition of hydrodynamic force. The detrital zircons had Th/U ratios of greater than 0.1, and REE pattern displayed a positive Ce anomaly and a negative Eu anomaly, indicating that these zircons are predominantly of magmatic origin. The U–Pb spectrum of detrital zircons mainly peaked at the Yanshanian (96–185 Ma), Hercynian–Indosinian (222–345 Ma) and Caledonian (421–477 Ma). A portion of the detrital zircons were of Neoproterozoic origin (728–1,003 Ma), which revealed that the basement in the eastern region of Hainan Island was mainly of Neoproterozoic, with rare Archean materials. The positive ε Hf(t) values (0 to +10.1) of the Neoproterozoic detrital zircons indicated that the juvenile crust grew in the southeastern Hainan Island mainly during the Neoproterozoic period. The Neoproterozoic orogeny in the southeastern part of the island (0.7–1.0 Ga) occurred later than in the northwestern region of the island (1.0–1.4 Ga). Importantly, the Grenvillian orogeny in the southeastern area of Hainan Island shared the same timing with that of the western Cathaysia Block; i.e., both areas concurrently underwent this orogenic event, thereby forming a part of the Rodinia supercontinent. Afterwards, the crust experienced remelting and reworking during the Caledonian Hercynian–Indosinianand Yanshanian accompanied by the growth of a small amount of juvenile crust.  相似文献   

17.
Late Neoproterozoic extensive rifting in Tarim has been intensely debated regarding an intracontinental or a back-arc environment, which forms a key aspect for elucidating the Precambrian basement architecture and evolution of the Tarim craton. Recently, a deep well called Tarim Geological Survey Well (X1) was drilled from the northwest Tarim basin. Sixty-six samples were collected from the Sugetbrak Formation of the X1 well, including 6 sandstone samples, 29 mudstone samples and 31 basalt samples, for integrated geochemical, geochronological, and Sr–Nd–Hf isotopic analyses. The results indicate that the basalts geochemically show distinct affinity with ocean-island basalts, characterized by enrichments in light rare-earth elements (REEs) and no depletions in Nb and Ta and indicating an intracontinental rifting environment. The presence of 572-Ma zircon in the Sugetbrak basalts reveal the rifting mainly developed in an interval of 541–572 Ma. Geochemical and petrographic data show that the coeval basaltic tuffs or clasts contributed 5–20% to the deposition of the Sugetbrak Formation. The zircon age cluster between 1.8 and 2.0 Ga and abundant Paleoproterozoic detrital zircons with ultrahigh-temperature (UHT) granulate REE signature in the Sugetbrak sandstones indicate a Paleoproterozoic orogenic event occurred in the Tarim craton that could be related to the assembly of the Columbia supercontinent. In contrast, the Neoproterozoic zircons from the Sugetbrak sandstones display positive εHf(t) values, and some have hydrothermal alteration REE signature, indicating Tarim could be a locus for early mantle plume activities that broke the Rodina supercontinent.  相似文献   

18.
Graptolite‐bearing Middle and Upper Ordovician siliciclastic facies of the Argentine Precordillera fold‐thrust belt record the disintegration of a long‐lived Cambro‐Mid Ordovician carbonate platform into a series of tectonically partitioned basins. A combination of stratigraphic, petrographic, U‐Pb detrital zircon, and Nd‐Pb whole‐rock isotopic data provide evidence for a variety of clastic sediment sources. Four Upper Ordovician quartzo‐lithic sandstones collected in the eastern and central Precordillera yield complex U‐Pb zircon age spectra dominated by 1·05–1·10 Ga zircons, secondary populations of 1·22, 1·30, and 1·46 Ga, rare 2·2 and 1·8 Ga zircons, and a minor population (<2%) of concordant zircons in the 600–700 Ma range. Archaean‐age grains comprise <1% of all zircons analysed from these rocks. In contrast, a feldspathic arenite from the Middle Ordovician Estancia San Isidro Formation of the central Precordillera has two well‐defined peaks at 1·41 and 1·43 Ga, with no grains in the 600–1200 Ma range and none older than 1·70 Ga. The zircon age spectrum in this unit is similar to that of a Middle Cambrian quartz arenite from the La Laja Formation, suggesting that local basement rocks were a regional source of ca 1·4 Ga detrital zircons in the Precordillera Terrane from the Cambrian onwards. The lack of grains younger than 600 Ma in Upper Ordovician units reinforces petrographic data indicating that Ordovician volcanic arc sources did not supply significant material directly to these sedimentary basins. Nd isotopic data (n = 32) for Middle and Upper Ordovician graptolitic shales from six localities define a poorly mixed signal [ɛNd(450 Ma) = −9·6 to −4·5] that becomes more regionally homogenized in Upper Ordovician rocks (−6·2 ± 1·0; TDM = 1·51 ± 0·15 Ga; n = 17), a trend reinforced by the U‐Pb detrital zircon data. It is concluded that proximal, recycled orogenic sources dominated the siliciclastic sediment supply for these basins, consistent with rapid unroofing of the Precordillera Terrane platform succession and basement starting in Mid Ordovician time. Common Pb data for Middle and Upper Ordovician shales from the western and eastern Precordillera (n = 15) provide evidence for a minor (<30%) component that was likely derived from a high‐μ (U/Pb) terrane.  相似文献   

19.
Analyses of trace elements and Lu/Hf isotopes have been carried out in already U–Pb dated detrital zircons from the Upper Ordovician sandstones of the southern part of the Bashkir Uplift. The concentrations of trace elements in the zircons suggest that they were derived from rocks of intermediate (62%), basic (24%), and felsic (9%) compositions as well as alkaline rocks (2%). The Lu/Hf systematics of the zircons demonstrated a wide variation of both εHf (from +9.5 to–8.7) and model ages of the parental rocks (1.60 Ga < T DM C < 3.28 Ga). Comparison of the isotopic–geochemical characteristics of the detrital zircons from different levels of the Riphean–Paleozoic sequence of the Bashkir Uplift against those from the Early Cambrian Brusov Formation of the Mezen Basin and Early Neoproterozoic Dzhezhim sandstones of Timan Ridge suggests that at the end of the Late Precambrian near the Uralian margin of Baltica a large block of Late Mesoproterozoic–Early Neoproterozoic crust existed, comprising a significant proportion of melanocratic rocks.  相似文献   

20.
《地学前缘(英文版)》2020,11(6):2221-2242
We present the first evidence of Archean oceanic crust submitted to Proterozoic high-pressure (HP) metamorphism in the South American Platform. Sm–Nd and Lu–Hf isotopic data combined with U–Pb geochronological data from the Campo Grande area, Rio Grande do Norte domain, in the Northern Borborema Province, reflect a complex Archean (2.9 ​Ga and 2.6 ​Ga) and Paleoproterozoic (2.0 ​Ga) evolution, culminating in the Neoproterozoic Brasiliano/Pan-African orogeny (ca. 600 Ma). The preserved mafic rocks contain massive poikiloblastic garnet and granoblastic amphibole with variable proportions of plagioclase ​+ ​diopside in symplectitic texture, typical of high-pressure rocks. These clinopyroxene-garnet amphibolites and the more common garnet amphibolites from the Campo Grande area are exposed as rare lenses within an Archean migmatite complex. The amphibolite lenses represent 2.65 ​Ga juvenile tholeiitic magmatism derived from depleted mantle sources (positive εHf(t) values of +3.81 to +30.66) later enriched by mantle metasomatism (negative εNd(t) values of –7.97). Chondrite and Primitive Mantle-normalized REE of analyzed samples and discriminant diagrams define two different oceanic affinities, with E-MORB and OIB signature. Negative Eu anomalies (Eu/Eu1 ​= ​0.75–0.95) indicate depletion of plagioclase in the source. Inherited zircon cores of 3.0–2.9 ​Ga in analyzed samples indicate that the Neoarchean tholeiitic magmatism was emplaced into 2923 ​± ​14 ​Ma old Mesoarchean crust (εNd(t) ​= ​–2.58 and Nd TDM ​= ​3.2 ​Ga) of the Rio Grande do Norte domain. The age of retro-eclogite facies metamorphism is not yet completely understood. We suggest that two high-grade metamorphic events are recognized in the mafic rocks: the first at 2.0 ​Ga, recorded in some samples, and the second, at ca. 600 Ma, stronger and more pervasive and recorded in several of the mafic rock samples. The Neoproterozoic zircon grains are found in symplectite texture as inclusions in the garnet grains and represent the age of HP conditions in the area. These zircon grains show a younger cluster of concordant analyses between 623 ​± ​3 ​Ma and 592 ​± ​5 ​Ma with εHf(t) values of +0.74 to –65.88. Thus, the Campo Grande rock assemblage is composed of Archean units that were amalgamated to West Gondwana during Neoproterozoic Brasiliano orogeny continent-continent collision and crustal reworking.  相似文献   

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