Isotopic disequilibrium in ultrahigh-pressure and retrograde metamorphism of eclogite and gneiss from the Chinese Continental Scientific Drilling in the Sulu orogen, China: evidence from mineral Nd–Sr–O isotopic composition

Diffusion rates of Sr and O in minerals are often comparable while Nd has a lower diffusion rate during thermal overprint(s); thus, the O isotope systems between metamorphic minerals can serve as an indicator to evaluate whether equilibrium of Rb–Sr and Sm–Nd systems has been preserved in the metamorphic minerals that experienced retrograde metamorphism. This study presents a combination of investigation on Sm–Nd, Rb–Sr, and O isotopic compositions of minerals separated from ultrahigh-pressure eclogite and gneiss that were collected from the main hole of the Chinese Continental Scientific Drilling project located in the Sulu orogen, eastern China. Oxygen isotopic compositions of minerals from gneiss and eclogite yield two temperature groups of 620–740 and 460–590°C, representing diffusion cessation of isotopic exchange during the eclogite-facies recrystallization and later amphibolite-facies retrograde overprint. Rb–Sr mineral regressions of two eclogite samples give consistent Triassic ages of 244 Ma, corresponding to eclogite-facies metamorphism, while the same minerals do not yield meaningful Sm–Nd isochron ages. This phenomenon likely suggests that Rb–Sr isotopic equilibrium was achieved during eclogite-facies metamorphism and preserved during late amphibolite-facies retrogression. In contrast, Sm–Nd isotopic equilibrium between the minerals of eclogite was not achieved under UHP metamorphic conditions. Regressions of epidote and biotite of one gneiss sample give a Triassic Sm–Nd age of 243 ± 34 Ma, corresponding to the time of the eclogite-facies metamorphism, and a Jurassic mineral Rb–Sr age of 187.5 ± 1.8 Ma. These results imply that fluids have played an important role to achievement of the Sm–Nd isotopic equilibrium during eclogite-facies metamorphism and re-equilibration of the Rb–Sr isotopic system during later retrograde overprint.     

Geochemical and Sr–Nd isotopic characteristics of granitic rocks from northern Vietnam

Five major felsic igneous suites from northern Vietnam, with ages from mid-Proterozoic to early Cenozoic, were studied. Representative granitic rocks from the Posen Complex (mid-Proterozoic) and the Dienbien Complex (late Permian to early Triassic) show geochemical characteristics similar to those of calc-alkaline to high-K calc-alkaline I-type granites. However, the former, located in the South China block, has significantly higher initial Nd isotopic ratios [ε_Nd(T)=+0.7 to +1.5] and older Nd isotopic model ages (TDM∼1.7 Ga) than the latter [ε_Nd(T)=−4.7 to −9.7; T_DM∼1.3–1.5 Ga] which were emplaced south of the Song Ma Suture and thus in the Indochina block. The generation of both complexes may be attributed to subduction-related processes that occurred in two distinct crustal provenances with different degrees of mantle inputs. On the other hand, Jurassic to Cretaceous granitic rocks from the Phusaphin Complex, contemporaneous rhyolites from the Tule Basin, and late Paleogene granitic rocks from the Yeyensun Complex, all exposed in the South China block between the Ailao Shan–Red River shear zone and the Song Ma Suture, display geochemical features similar to those of A-type granites with intermediate ε_Nd(T) values (+0.6 to −2.8) and younger T_DM ages (0.6–1.1 Ga). These magmas are suggested to have been generated as a consequence of intraplate extension in the western part of the South China block (Yunnan), and to have been transported to their present position by mid-Tertiary continental extrusion along the Ailao Shan–Red River shear zone related to the India–Asia collision. Overall, the isotopic and model age data, reported in this study indicate that in northern Vietnam, the most important crust formation episode took place in the Proterozoic. Likewise, repeated mantle inputs have played a role in the petrogenesis of Phanerozoic granitic rocks.     

Zircon U–Pb ages and O–Nd isotopic composition of basement rocks in the North Qinling Terrain, central China: evidence for provenance and evolution

The Qinling Group was previously interpreted as the oldest Precambrian basement unit of the North Qinling Terrain, recording its formation and early crustal evolution. The Qinling Group consists predominantly of gneisses, amphibolites, and marbles, which underwent multi-phase deformation and metamorphism. In order to better constrain the provenance and tectonic setting of this group and the evolution of the North Qinling orogenic belt, in situ U–Pb dating and oxygen isotopic analysis of zircons in combination with whole-rock geochemistry and Sr–Nd isotope analysis was performed on the two dominant rock types, amphibolite and felsic gneiss. Felsic gneisses exhibit enrichment of LREEs and LILEs (Rb, Ba, Th, K, Pb), negative Eu anomalies and depletion of HFSEs (Nb, Ta, P, Ti). The rocks have slightly elevated δ¹⁸O values (6.5–9.3 ‰) and initial ε _Nd values of ?4.6 corresponding to two-stage Nd model age of 1.99 Ga. Amphibolites are also enriched in LILEs and LREEs and depleted in Nb and Ta and have homogeneous δ¹⁸O values (5.0–6.0 ‰), but higher initial ε _Nd values (2.8–3.3) and younger two-stage Nd model ages (1.29–1.24 Ga) compared to the gneisses. The zircon age record indicates that the gneisses and amphibolites were formed in a ~960 Ma volcanic arc environment rather than in a rift setting as previously suggested. A major metamorphic event took place during the Early Paleozoic. Based on the age spectrum of detrital zircons, the Qinling Group is interpreted as an autonomous geological unit, which was mainly derived mostly from 1,000 to 900 Ma old granitoid rocks. The North Qinling Terrain can be regarded as a remnant of the Grenville orogenic belt with an early Neoproterozoic evolution different from that of the North and South China blocks.     

Variations of Sr–Nd–Hf isotopic systematics in basalt during intensive weathering

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 ε_Sr 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 ε_Sr 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 ε_Nd and ε_Hf of the saprolites drift only 0–2.6 and 0–3.7 away from the parent rocks, respectively. The negative drifts of the ε_Nd and ε_Hf are coupled with Nd and Hf losses in the saprolites; i.e., larger proportions of Nd and Hf loss correspond to lower ε_Nd and ε_Hf. 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 ε_Nd and ε_Hf 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 ε_Nd and ε_Hf 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 ε_Hf values at a given ε_Nd value, and the regression line between these ε_Nd and ε_Hf 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 ε_Nd and ε_Hf correlation. Our results also indicate that caution is needed when using ε_Sr, ε_Nd, and ε_Hf to trace provenances for sediments and soils.     

Recycling oceanic crust for continental crustal growth: Sr–Nd–Hf isotope evidence from granitoids in the western Junggar region,NW China

The juvenile component of accretionary orogenic belts has been declining since the Archean. As a result, there is often controversy regarding the contribution of oceanic basalts to Phanerozoic crustal growth, as in the case of the Central Asian Orogenic Belt (CAOB). Here we report on three groups of Late Carboniferous (316–305 Ma) granitoids in the western Junggar region of northern Xinjiang, NW China, which is part of the southwestern CAOB. They consist of adakites and I and A-type granites, and as a whole have the most depleted isotopic compositions (ε_Nd(t) = + 6–+9, (⁸⁷Sr/⁸⁶Sr)_i = 0.7030–0.7045, and ε_Hf(t) = + 12–+16) among the granitoids of the CAOB. These features are nearly identical to those of pre-Permian ophiolites in northern Xinjiang, and are clearly different from those of Carboniferous basalts in the western Junggar region. These relationships indicate that the granitoids were mainly derived from recycled oceanic crust by melting of subducted oceanic crust (e.g., adakites), and of the middle–lower crust of intra-oceanic arc that mainly consisted of oceanic crust (e.g., I and A-type granites). Based on evidence from the CAOB, we suggest that recycling of oceanic crust has made a significant contribution to continental crustal growth and evolution during the Phanerozoic.     

Sr and Nd isotopic geochemistry of the early ultramafic–mafic rocks of the Mako bimodal volcanic belt of the Kedougou–Kenieba inlier (Senegal)

The Mako bimodal volcanic belt of the Kédougou–Kéniéba inlier is composed of volcanic basalts and peridotites interbedded by quartzites and limestones intruded by different generations of granitoïds. The early volcanic episode of the belt is constituted of submarine basalts with peridotite similar to those of the oceanic abyssal plains. It is intruded by the Badon Kakadian TTG?granitic batholite dated around 2200 Ma. The second volcanic phase is constituted of basaltic, andesitic, and felsitic flows exhibit structures of aerial volcanic rocks. It is intruded by granites dated between 2160 and 2070 Ma. The general pattern of trace element variation of submarine volcanic rocks is consistent with those of basalts from oceanic plateaus which are the modern equivalent of the Archean greenstones belts. The Nd and Sr isotopic systematics typical of juvenile material indicates that the source of these igneous rocks is derived from a depleted mantle source. These results are consistent with the idea of a major accretion within the West African Craton occurring at about 2.1 Ga and corresponding to an important process of mantle–oceanic lithosphere differentiation.     

U–Pb zircon dating, geochemical and Sr–Nd–Hf isotopic compositions of Early Indosinian intrusive rocks in West Qinling, central China: petrogenesis and tectonic implications

The Qinling–Dabie–Sulu orogenic belt is the junction between the North and South China blocks, which resulted from the final amalgamation of China continents during the Indosinian. Indosinian granitoids are widespread in the Qinling orogen, and their geneses can thus constrain the evolution of China continent. We carried out a combined U–Pb zircon dating and geochemical study for the Shuangpengxi granodiorite pluton and the Xiekeng diorite–granodiorite pluton in the middle part of the West Qinling orogen. U–Pb zircon dating shows that the magma crystallization ages of 242 ± 3 Ma for the Shuangpengxi pluton and ~244–242 Ma for the Xiekeng pluton. Geochemical and Sr–Nd–Hf isotopic compositions reveal that the magma of the Shuangpengxi granodiorite was derived from partial melting of crustal materials. The Xiekeng diorites can be divided into high-Al diorite and high-Mg diorite. Both of them resulted from partial melting of enriched lithospheric mantle, but their mantle source had been modified by previous slab-derived melt. The high-Al diorite was formed by fractional crystallization of olivine, pyroxene and/or preferential accumulation of plagioclase, and the high-Mg diorite was formed by fractional crystallization of olivine and/or preferential accumulation of pyroxene. The Xiekeng granodioritic porphyry was formed by mixing of crust-derived and mantle-derived melts. We propose that the Early Indosinian magmatism resulted from break-off of subducted oceanic slab after collision. The slab break-off model can well explain the linear distribution of the Early Indosinian plutons and rapid crustal uplift during the Middle Triassic in the West Qinling.     

Geochemistry and Sr–Nd–Pb–Hf isotopic composition of the Donggou Mo-bearing granite porphyry,Qinling orogenic belt,central China

The eastern Qinling belt is characterized by widespread Mesozoic post-orogenic magmatism and abundant Mo–(Au–Ag) polymetallic mineralization. Most Mo deposits in this belt are genetically related to Mesozoic granitoids. The tectonic context of this close spatial and temporal relationship is still debated. This study reports U–Pb ages and Hf isotopic composition of zircons, major and trace element and Sr–Nd–Pb isotopic composition of the Donggou granite porphyry, host rock to one of the important Mesozoic Mo deposits in this orogen. Based on geochemical results, the Donggou granite porphyry is a silica-supersaturated, high-K metaluminous A-type granite showing enrichment in light REEs, depletion in middle REEs and significant negative Eu, Ba, Nb, Sr, P, and Ti anomalies. Negative initial ?_Nd values of??17.0 to??13.2 for whole-rock and negative initial ?_Hf values of??19.9 to??7.8 for zircon suggest that the magma was derived from a mixture of Archaean/Proterozoic crustal rocks and mantle-derived or newly added crust. Its Pb isotopic composition is similar to the lower crust of the North China block, but different from superjacent country rocks (Xiong'er and Taihua Groups). Zircon U–Pb dating yields a late Mesozoic emplacement age of 118–117 Ma, identical with the third episode of Mo mineralization in the eastern Qinling–Dabie belt. We postulate that the Donggou Mo-related porphyry granite formed by reworking of North China lower crust with significant input of juvenile material. The magmas formed in an extensional tectonic setting, induced by lithospheric thinning and asthenospheric upwelling beneath eastern China during Cretaceous time.     

Geochemistry and Sm–Nd geochronology of the metasomatised mafic rocks in the Khetri complex,Rajasthan, NW India: Evidence of an Early Cryogenian metasomatic event in the northern Aravalli orogen

The mafic magmatic rocks associated with 1720–1700 Ma albitised A-type granites in the northern segment of the Aravalli orogen, NW India show evidence of metasomatism. It is, however, not clear whether the metasomatism of mafic rocks is related to the cooling of these associated granites or whether it took place much later after the emplacement of the granites on a regional scale. For this, we have investigated the mafic magmatic rocks, which occur in close association with these granites. In the Biharipur intrusive, the mafic rocks are intensely commingled with the A-type granites, whereas in the vicinity of the Dosi intrusive, the mafic rocks (clinopyroxenite) do not show any evidence of granite mingling. The commingled and metasomatised Biharipur mafics occur in contact with the albitised granites instead of original granite, indicating that the mafics were metasomatised along with the granites. This is supported by the similarity in REE and spider patterns of the intermixed mafic rocks and the albite granites. On the other hand, the Dosi mafic rocks, free from granite commingling, are scapolitised where the original diopside has been partly transformed to chlorine-rich marialites with a meionite component ranging from Me14.0 to Me16.0. The scapolite, occurring as anastomosing veins, within these rocks is also of similar composition, and the undeformed nature of these veins suggests that the scapolitisation postdates regional metamorphism in the region. Mineralogical, geochemical and Nd isotopic characteristics of the mafic rocks indicate that originally, these were clinopyroxenites, which have been altered to a monomineralic actinolite-bearing rock. The immobile incompatible trace element ratios indicate a continental tholeiite affinity for the mafics, which is in consonance with the A-type nature of the associated granites. During this metasomatic event, the mafic magmatic rocks experienced albitisation and scapolitisation, although the dominance of these processes varied on a local scale depending on the fluid composition.A whole-rock-mineral (clinopyroxene and scapolite) Sm–Nd isochron of the scapolitised clinopyroxenite at Dosi yields an age of 831 ± 15 Ma. Synthesis of this age data along with previously published geochronological data indicate an important Early Cryogenian (850–830 Ma) metasomatic event in the northern Aravalli orogen, which is also synchronous with the Erinpura granite event in the southern Aravalli orogen.     

Uplift-denudation history of the Qinling orogen: Constrained from the detrital-zircon U–Pb geochronology

This paper presents a great number of detrital zircon U–Pb ages from the Middle Triassic to the Middle Jurassic sediments in the Jiyuan basin, southern North China. The results represent age spectra from 2.9 Ga to 216 Ma, with five peaks at 2.5 Ga, 1.9 Ga, 840 Ma, 440 Ma, and 270 Ma and two grains of ∼220 Ma. The ages of 2.5 Ga and 1.9 Ga are mainly derived from the Precambrian basement of the North China Block, whereas the others are typical characteristics of the Qinling orogenic belt. An important observation is that the Qinling-sourced detrital zircons become older as the strata get younger. Samples from the Middle Triassic to early Late Triassic strata are characterized by the age peak at 270 Ma, whereas the Late Late Triassic to Early Middle Jurassic samples are dominated by age peaks at 840 Ma and 440 Ma and minor grains within 800–650 Ma. Two grains of ∼220 Ma are preserved in the Late Middle Jurassic sample, which may be contributed by the Carnian deep plutons. These signatures indicate that the unroofing pattern of the Qinling orogenic belt developed by the denudation of materials from young covers to old basements and the Carnian deep plutons. Integrated with the data reported from the Hefei Basin, it is well-established that the intensity of unroofing increased from the Qinling to the Dabie orogen in the Early Jurassic, and the denudation timing of the ultra-high pressure (UHP) and high pressure (HP) rocks or Carnian plutons changed successively from the Early Jurassic in the Dabie to the Late Middle Jurassic in the Qinling orogen.     

Sr–Nd isotope geochemistry of the early Precambrian sub-alkaline mafic igneous rocks from the southern Bastar craton, Central India

Sr–Nd isotope data are reported for the early Precambrian sub-alkaline mafic igneous rocks of the southern Bastar craton, central India. These mafic rocks are mostly dykes but there are a few volcanic exposures. Field relationships together with the petrological and geochemical characteristics of these mafic dykes divide them into two groups; Meso-Neoarchaean sub-alkaline mafic dykes (BD1) and Paleoproterozoic (1.88 Ga) sub-alkaline mafic dykes (BD2). The mafic volcanics are Neoarchaean in age and have very close geochemical relationships with the BD1 type. The two groups have distinctly different concentrations of high-field strength (HFSE) and rare earth elements (REE). The BD2 dykes have higher concentrations of HFSE and REE than the BD1 dykes and associated volcanics and both groups have very distinctive petrogenetic histories. These rocks display a limited range of initial ¹⁴³Nd/¹⁴⁴Nd but a wide range of apparent initial ⁸⁷Sr/⁸⁶Sr. Initial ¹⁴³Nd/¹⁴⁴Nd values in the BD1 dykes and associated volcanics vary between 0.509149 and 0.509466 and in the BD2 dykes the variation is between 0.510303 and 0.510511. All samples have positive ? _Nd values; the BD1 dykes and associated volcanics have ? _Nd values between +0.3 and +6.5 and the BD2 dykes between +1.9 to +6.0. Trace element and Nd isotope data do not suggest severe crustal contamination during the emplacement of the studied rocks. The positive ? _Nd values suggest their derivation from a depleted mantle source. Overlapping positive ? _Nd values suggest that a similar mantle source tapped by variable melt fractions at different times was responsible for the genesis of BD1 (and associated volcanics) and BD2 mafic dykes. The Rb–Sr system is susceptible to alteration and resetting during post-magmatic alteration and metamorphism. Many of the samples studied have anomalous apparent initial ⁸⁷Sr/⁸⁶Sr suggesting post-magmatic changes of the Rb–Sr system which severely restricts the use of Rb–Sr for petrogenetic interpretation.     

Devonian volcanism in the Minusa basin in the Altai–Sayan area: geological,geochemical, and Sr–Nd isotopic characteristics of rocks

Based on geological data and the geochemical and isotopic (Sr, Nd) parameters of the Devonian volcanic associations of the Minusa basin, the main regularities of volcanism development are considered, the composition of magmatic sources is studied, and the geodynamic mechanisms of their involvement in rifting are reconstructed. The early stage of formation of the Minusa basin was characterized by intense volcanism, which resulted in differentiated and, more seldom, bimodal volcanic complexes composed of pyroclastic rocks and dolerite sills. At the late stage, only terrigenous deposits accumulated in the basin. It has been established that the basites are similar in composition and are intermediate in geochemical characteristics between intraplate rocks (OIB) and continent-marginal ones (IAB). The basites, like OIB, have high contents of all lithophile elements, which is typical of enriched mantle sources, and, like IAB, show negative anomalies of Nb, Ta, Ti, and, to a smaller extent, Rb, Th, Zr, and Hf, selective enrichment in Pb and Ba (and, sometimes, Sr), and a weak REE differentiation (7 < (La/Yb)N < 17). In contrast to the basins in other segments of the Devonian Altai–Sayan rift area, the igneous associations in the Minusa basin are characterized by a worse expressed geochemical inhomogeneity of rocks and lack of high-Ti (> 2 wt.% TiO2) basites. The Sr and Nd isotope compositions of the Minusa basites deviate from the mantle rock series toward the compositions with high radiogenic-strontium and low REE contents.This points to the melting of a mantle substratum (PREMA-type) and carbonate-rich sedimentary rocks, which were probably assimilated by basaltic magma. The correlations between the contents of trace incompatible elements in rocks with SiO2 = 53–77 wt.% testify to the assimilation of crustal substrata by parental basaltic melts and the subsequent differentiation of contaminated magmas (AFC model). We propose a model for the formation of primary melts with the simultaneous participation of magmatic sources of two types: plume and fluid-saturated suprasubductional, localized beneath the active continental margin.     

Petrogenesis of gold-mineralized magmatic rocks of the Taerbieke area,northwestern Tianshan (western China): Constraints from geochronology,geochemistry and Sr–Nd–Pb–Hf isotopic compositions

Many Late Paleozoic Cu–Au–Mo deposits occur in the Central Asian Orogenic Belt (CAOB). However, their tectonic settings and associated geodynamic processes have been disputed. This study provides age, petrologic and geochemical data for andesites and granitic porphyries of the Taerbieke gold deposit from the Tulasu Basin, in the northwestern Tianshan Orogenic Belt (western China). LA-ICP-MS zircon U–Pb dating indicates that the granitic porphyries have an Early Carboniferous crystallization age (349 ± 2 Ma) that is broadly contemporaneous with the eruption age (347 ± 2 Ma) of the andesites. The andesites have a restricted range of SiO₂ (58.94–63.85 wt.%) contents, but relatively high Al₂O₃ (15.39–16.65 wt.%) and MgO (2.51–6.59 wt.%) contents, coupled with high Mg^# (57–69) values. Geochemically, they are comparable to Cenozoic sanukites in the Setouchi Volcanic Belt, SW Japan. Compared with the andesites, the granitic porphyries have relatively high SiO₂ (72.68–75.32 wt.%) contents, but lower Al₂O₃ (12.94–13.84 wt.%) and MgO (0.10–0.33 wt.%) contents, coupled with lower Mg^# (9–21) values. The andesites and granitic porphyries are enriched in both large ion lithophile and light rare earth elements, but depleted in high field strength elements, similar to those of typical arc magmatic rocks. They also have similar Nd–Hf–Pb isotope compositions: ε_Nd(t) (+0.48 to +4.06 and −0.27 to +2.97) and zircons ε_Hf(t) (+3.4 to +8.0 and −1.7 to +8.2) values and high (²⁰⁶Pb/²⁰⁴Pb)_i (18.066–18.158 and 17.998–18.055). We suggest that the Taerbieke high-Mg andesitic magmas were generated by the interaction between mantle wedge peridotites and subducted oceanic sediment-derived melts with minor basaltic oceanic crust-derived melts, and that the magmas then fractionated to produce the more felsic members (i.e., the Taerbieke granitic porphyries) during late-stage evolution. Taking into account the Carboniferous magmatic record from the western Tianshan Orogenic Belt, we suggest that the formation of the Early Carboniferous andesites and granitic porphyries in the Taerbieke area were related to the Paleo-Junggar Oceanic plate southward subduction under the Yili–Central Tianshan plate. The close association of the Early Carboniferous magmatic rocks and Au mineralization in the Taerbieke area suggests that the arc magmatic rocks in the Tulasu basin may have a high potential for Au mineralization.     

Petrogenesis of adamellites from eastern Shandong Province: geochronological,geochemical, and Sr–Nd–Pb isotopic evidence

Geochronology, geochemistry, and whole-rock Sr–Nd–Pb isotopes were studied on a suite of Mesozoic adamellites from eastern China to characterize their ages and petrogenesis. Sensitive high-resolution ion microprobe U–Pb zircon analyses were done, yielding consistent ages of 123.2 ± 1.8 to 122.1 ± 2.1 Ma for the samples. These rocks belong to the alkaline magma series in terms of K₂O + Na₂O contents (8.45–9.58 wt.%) and to the shoshonitic series based on their high K₂O contents (5.23–5.79 wt.%). The adamellites are further characterized by high light rare earth element contents [(La/Yb)_N = 14.96–45.99]; negative Eu anomalies (δEu = 0.46–0.75); positive anomalies in Rb, Th, Pb, and U; and negative anomalies in Sr, Ba, and high field-strength elements (i.e. Nb, Ta, P, and Ti). In addition, all of the adamellites in this study display relatively low radiogenic Sr [(⁸⁷Sr/⁸⁶Sr)_i = 0.7081–0.7089] and negative ?_Nd(t) values from –16.70 to –17.80. These results suggest that the adamellites were derived from low-degree partial melting of an enriched lithospheric mantle below the North China Craton (NCC). The parent magmas likely experienced fractional crystallization of potassium feldspar, plagioclase and Fe–Ti oxides (e.g. rutile, ilmenite, and titanite), apatite, and zircon during the ascent of alkaline rocks without crustal contamination.     

Sub-arc mantle heterogeneity in oxygen isotopes: evidence from Permian mafic–ultramafic intrusions in the Central Asian Orogenic Belt

Integrated zircon–olivine O–Hf isotope data have been successfully used to unravel the nature of the source mantle for the early Permian post-collisional mafic–ultramafic intrusive rocks in the southern margin of the Central Asian Orogenic Belt in NW China. Olivine crystals with forsterite (Fo) contents varying from 91 to 87 mol% from the Permian Pobei mafic–ultramafic complex in the region yield highly elevated δ¹⁸O from 6.0 to 7.2‰. These values are much higher than typical mantle values (~?5.3‰) and are apparently at odds with the mantle-like ε_Nd(t) values of whole rocks (4.9–5.4). Magmatic zircon crystals from troctolite and gabbroic rocks show divergent oxygen and hafnium isotopic compositions: mantle-like ε_Hf(t) values from 5.1 to 11.9 and crust-like δ¹⁸O values from 7.6 to 10.1‰. The observed increase of δ¹⁸O values from olivine (an early crystallizing phase) to zircon (a late crystallizing phase) in the mafic–ultramafic rocks is generally consistent with an AFC process. However, this process cannot fully explain the highly elevated δ¹⁸O values (6–7‰) for the most primitive olivine containing Fo as high as mantle olivine (>?90 mol%) and the mantle-like Hf isotope composition of zircon. Mixing calculation indicates that such highly unusual isotope compositions can be explained by the previous source mantle contamination with subducted sediment-derived melts and slab-derived fluids. Our results show that the combination of zircon O–Hf isotopes and olivine oxygen isotopes is more effective than the data of zircon or olivine alone to distinguish the effect of AFC process from source contamination. The results from this study provide a new line of evidence that the sub-arc mantle is not homogeneous in oxygen isotopes.     

Origin of the Dexing Cu-bearing porphyries,SE China: elemental and Sr–Nd–Pb–Hf isotopic constraints

The Dexing porphyry copper deposit, part of the circum-Pacific porphyry copper ore belt, is the largest porphyry copper deposit in China. We present new LA–ICP–MS zircon U–Pb and molybdenite Re–Os dating, bulk-rock elemental and Sr–Nd–Pb isotopic as well as in situ zircon Hf isotopic geochemistry for these ore-bearing porphyries, in an attempt to better constrain their petrogenesis. LA–ICP–MS zircon U–Pb dating shows that the Dexing porphyries were emplaced in the early Middle Jurassic (～171 Ma); molybdenite Re–Os dating indicates that the associated Cu–Mo mineralization was contemporaneous (～171 Ma) with the igneous intrusion. The rocks are mainly high-K calc-alkaline and show adakitic affinities, including high Sr and low Y and Yb contents, high Sr/Y and La/Yb ratios, and high Mg# (higher than pure crustal melts). These porphyries have initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7044?0.7047, ?_Nd(T) values of –1.5 to?+0.6, and ?_Hf(T) (in situ zircon) values of?+2.6 to?+4.6. They show unusually radiogenic Pb isotopic compositions with initial ²⁰⁶Pb/²⁰⁴Pb ratios up to 18.41 and ²⁰⁷Pb/²⁰⁴Pb up to 15.61. These isotopic compositions are distinctly different from either Pacific MORB or Yangtze lower crust but are similar to the subducting sediments in the western Pacific trenches. Detailed elemental and isotopic data suggest that the Dexing porphyries were emplaced in a continental arc setting coupled with westward subduction of the palaeo-Pacific plate. Partial melting involved the subducted slab (mainly the overlying sediments), with generated melts interacting with the lithospheric mantle wedge, thereby forming the investigated high-K calc-alkaline porphyry magmas.     

Crust–mantle interaction triggered by oblique subduction of the Pacific plate: geochronological,geochemical, and Hf isotopic evidence from the Early Cretaceous volcanic rocks of Zhejiang Province,southeast China

Large-scale volcanism in the late Mesozoic was a prominent geological event in southeast China. The late Mesozoic volcanic sequences, named the Moshishan Group, are exposed in Zhejiang Province and are predominantly felsic in composition with subordinate mafic magma and rare andesites. To understand the late Mesozoic tectonic evolution of southeast China, we present zircon U–Pb dating, major and trace element analyses, and Hf isotopic compositions from felsic volcanic rocks of the Moshishan Group. Zircon U–Pb dating shows that the Moshishan Group formed between 145 and 129 Ma. The ε_Hf(t) of the analysed zircons ranges from ?16.58 to +6.89, and the T_DM2 age ranges from 753 to 2238 Ma with a major peak at ca. 1870 Ma. Hf isotopic compositions of zircons in Early Cretaceous volcanic rocks are more radiogenic than that of the metamorphic basement rocks, indicating a juvenile component in these magmas. Major element concentrations show that the volcanic rocks mainly belong to the high-K calc-alkaline series. Both zircon saturation temperatures and the ε_Hf(t) values of zircons gradually increased with the evolution of the magma. Trace element data indicates that neither magmatic differentiation of mantle-derived magma nor mixing of magmas from different sources were the predominant magmagenetic processes. Earlier studies suggest that contemporaneous underplating contributed to the heat source that induced crustal melting and to the material origin that inconsistently mixed with the local crustal melts. Magmatic underplating is likely to have occurred because of the southwestward subduction of the Pacific plate with episodic slab rollback. The data obtained in this study suggest that the crust–mantle interaction under the influence of slab rollback played a progressive role in the formation of Early Cretaceous felsic volcanic rocks in southeast China.     

The Sr,Nd, and Hf isotopic geochemistry of rocks of the gabbro–diorite–tonalite association from the Eastern Segment of the Middle Urals as an indicator of the age of the continental crust in this area

According to isotopic analysis of rocks of the Reft gabbro–diorite–tonalite complex (Middle Urals), gabbro and related diorite and dikes and vein-shaped bodies of plagiogranitoids, crosscutting gabbro, are similar to the depleted mantle substance in ε_Nd(T) = 8.6–9.7 and ε_Hf(T) = 15.9–17.9. Their model Hf ages are correlated with the time of crystallization. Here, the tonalites and quartz diorites constituting most of the Reft massif are characterized by lower values: ε_Nd(T) = 3.7–6.0, ε_Hf(T) = 11.1–12.7, and T _DM values significantly exceeding the age datings. This is evidence that Neoproterozoic crustal rocks were a source of parental magma for these rocks. The primary ⁸⁷Sr/⁸⁶Sr ratio in rocks of both groups is highly variable (0.70348–0.70495). The data obtained allow us to reach the conclusion that the Reft gabbro–diorite–tonalite complex was formed as a result of nearly synchronous processes occurring in the crust and the mantle within a limited area.     

Sr–Nd–Hf–Pb isotope geochemistry of basaltic rocks from the Cretaceous Gyeongsang Basin,South Korea: Implications for basin formation

To better understand the formative mechanism of the Cretaceous Gyeongsang Basin in South Korea, we determined the geochemical compositions of Early Cretaceous syntectonic basaltic rocks intercalated with basin sedimentary assemblages. Two distinct compositional groups appeared: tholeiitic to calc-alkaline basalts from the Yeongyang sub-basin and high-K to shoshonitic basaltic trachyandesites from the Jinju and Uiseong sub-basins. All collected samples exhibit patterns of light rare earth element enrichment and chondrite-normalized (La/Yb)_N ratios ranging from 2.4 to 23.6. In a primitive-mantle-normalized spidergram, the samples show distinctive negative anomalies in Nb, Ta, and Ti and a positive anomaly in Pb. The basalts exhibit no or a weak positive U anomaly in a spidergram, but the basaltic trachyandesites show a negative U anomaly. The basalts have highly radiogenic Sr [(⁸⁷Sr/⁸⁶Sr)_i = 0.70722–0.71145], slightly negative ε_Nd, positive ε_Hf [(ε_Nd)_i = −2.7 to 0.0; (ε_Hf)_i = +2.9 to +6.4], and radiogenic Pb isotopic compositions [(²⁰⁶Pb/²⁰⁴Pb)_i = 18.20–19.19; (²⁰⁷Pb/²⁰⁴Pb)_i = 15.60–15.77; (²⁰⁸Pb/²⁰⁴Pb)_i = 38.38–39.11]. The basaltic trachyandesites are characterized by radiogenic Sr [(⁸⁷Sr/⁸⁶Sr)_i = 0.70576–0.71119] and unradiogenic Nd, Hf, and Pb isotopic compositions [(ε_Nd)_i = −14.0 to −1.4; (ε_Hf)_i = −17.9 to +3.7; (²⁰⁶Pb/²⁰⁴Pb)_i = 17.83–18.25; (²⁰⁷Pb/²⁰⁴Pb)_i = 15.57–15.63; (²⁰⁸Pb/²⁰⁴Pb)_i = 38.20–38.70]. The “crust-like” signatures, such as negative Nb–Ta anomalies, elevated Sr isotopic compositions, and negative ε_Nd(t) and ε_Hf(t) values, of the basaltic trachyandesites resemble the geochemistry of Early Cretaceous mafic volcanic rocks from the southern portion of the eastern North China Craton. Considering the lower-crust-like low U/Pb and high Th/U ratios and the unradiogenic Pb isotopic compositions, the basaltic trachyandesites are considered to be derived from lithospheric mantle modified by interaction with melts that originated from foundered eclogite. Basaltic volcanism in the Yeongyang sub-basin is coeval with the basaltic trachyandesite magmatism, but it exhibits an elevated ⁸⁷Sr/⁸⁶Sr ratio at a given ¹⁴³Nd/¹⁴⁴Nd and highly radiogenic Pb isotopic compositions, which imply an origin from an enriched but heterogeneous lithospheric mantle source. Melts from subducted altered oceanic basalt and pelagic sediments are considered to be the most likely source for the metasomatism. An extensional tectonic regime induced by highly oblique subduction of the Izanagi Plate beneath the eastern Asian margin during the Early Cretaceous might have triggered the opening of the Gyeongsang Basin. Lithospheric thinning and the resultant thermal effect of asthenospheric upwelling could have caused melting of the metasomatized lithospheric mantle, producing the Early Cretaceous basaltic volcanism in the Gyeongsang Basin.