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.     

U-Pb geochronology of Paraná volcanics combined with trace element geochemistry of the zircon crystals and zircon Hf isotope data

The Paraná volcanic province is a window into mantle and crustal processes in the Cretaceous. The variability and complexity of this province can be determined through the study of minerals. An integrated study of zircon from Paraná lavas (one high-Ti basalt, one low-Ti andesite, one high-Ti rhyodacite and one high-Ti andesite sill) was achieved using backscattered electron imaging, sensitive high resolution ion microprobe (SHRIMP-IIe) for U-Pb geochronology, and laser ablation inductively coupled mass spectrometer (LA-ICPMS) to determine the Lu-Hf isotopes and the trace-element compositions. U-Pb-Hf and trace-element data indicate that zircon crystallized from the magma at approximately 134 Ma. This South American large igneous province originated from the mantle and was contaminated by crust either in the mantle or during ascent and crystallization of magma. Contaminant continental crust had Precambrian age. Trace elements point to a new compositional field for zircon, different from other types of provinces. Examination of volcanic zircon improves our understanding of age and contamination of the Paraná volcanics.     

Evidences from zircon U-Pb geochronology,whole -rock geochemistry and Hf isotope of Early Cretaceous magmatic rocks in Baishan area,southern Jilin Province: Constraints on destruction of the North China CratonSCIEI北大核心CSCD

This paper reports new geochemical data including zircon U-Pb ages with Hf isotopic data, whole -rock major and trace isotopic compositions for the Early Cretaceous magmatic rocks in the northeastern North China Craton ( NCC), with the purpose of constraining its petrogenesis and tectonic setting. It also provides a basis for further discussion on the formation and evolution of the lithosphere in the northeastern NCC during the Late Mesozoic. Zircons in the Xinglin granite porphyry, Hongtuya andesitic crystal tuff and Jiangyuan rhyolitic breccia-bearing crystal-lithic tuff all show magmatic origin. Zircon U-Pb dating yield age are 130 +/- 1Ma, 128 +/- 1Ma and 120 +/- 1Ma, respectively. The Xinglin granite porphyry and the Hongtuya andesitic crystal tuff have high content of SiO2, Al2O3 Sr, high Na2O/K2O, Sr/Y and ( La/Yb)(N) ratios. In addition, they exhibit low Y and Yb. They are enriched in LILE ( e. g., Ba, Th and U), depleted in HFSE ( e. g., Nb, Ti and P) with weak negative Eu anomalies (delta Eu =0. 82 similar to 0. 94), showing they are typical of adakitic rocks. The Xinglin granite porphyry has high content of total -alkali, low content of TiO2, characterized by peraluminous-weakly peraluminous granite A/CNK = 1. 06 similar to 1. 14, the zircon saturated temperature values of 798 similar to 815 degrees C and a distribution pattern of LREE enrichment with incline to the right, which indicates it is I -type granite. The zircon ( t) values in the Xinglin granite porphyry ranged 17. 96 0. 19, and the two -stage model ages yield t(DM2) = 1197 similar to 2313Ma, which imply that the Xinglin granite porphyry is derived from partial melting of the thickened mafic lower crust of Paleoproterozoic to Mesoproterozoic. The zircon epsilon(Hf)( t) values in the Hongtuya andesitic crystal tuff ranged 3. 37 similar to 5. 47, and the two -stage model ages yield t(DM2) = 830 similar to 967Ma, indicating that the Hongtuya andesitic crystal tuff is a metasomatism product of the Neoproterozoic delaminated eclogite lower crust partial melting and mantle peridotite. Combined with the epsilon(Hf)( t) values in the Jiangyuan rhyolitic breccia-bearing crystal-lithic tuff ranged 2. 69 1. 00, and the two -stage model ages yield t(DM2) = 1115 similar to 1349Ma, we conclude that the epsilon(Hf)( t) values of magmatic rocks are heterogeneous in the southern Jilin Province, and the magmatic source area is characterized by multi -component origin, including both ancient crust and juvenile crust. We supposed that there were two accretion evolution events in the continental crust of the northeastern NCC during Proterozoic. Comprehensive studies have shown that subduction and slab roll -back of the Paleo-Pacific plate ( Izanagi) led to the thickened the northeastern NCC being in an extensional tectonic setting during the Early Cretaceous, and the destruction of the northeastern NCC by delamination occurred spatially from west to east. The partial melting of delaminated lower continental crust resulted in a wide distribution of adakitic rocks.     

Crustal and thermal structure of the Thomson Orogen: constraints from the geochemistry,zircon U–Pb age,and Hf and O isotopes of subsurface granitic rocks

Abstract

The origin of elevated geothermal gradients in the subsurface Thomson Orogen and the nature of the crustal basement beneath it, whether oceanic or continental, remain enigmatic. Previous studies have demonstrated that a higher crustal radiogenic input is required to explain these anomalous thermal gradients. In this study, we have investigated the nature and age of this crustal input by undertaking geochemical, geochronological and Hf and O isotope analyses of buried granitic rocks as well as evaluating the heat-producing potential of metasedimentary rocks. The mineralogy, composition and Neoproterozoic/Cambrian to Devonian age of the low to moderate heat-producing I- and S-type granitic rocks strongly contrast with the Carboniferous A-type high-heat-producing granites of the Big Lake Suite, which have been suggested to be an important contributor to the elevated geothermal gradients, near the southwest corner of the Thomson Orogen. These differences suggest the Big Lake Suite rocks do not extend into the Queensland part of the temperature anomaly. Heat production of the metasedimentary rocks is also low to moderate. Based on Hf isotope compositions of zircons characterised by mantle-like oxygen signature (?_Hf(t) = –12 to +2), we propose the temperature anomaly results from the occurrence of Mesoproterozoic and/or Paleoproterozoic high-heat-producing rocks beneath the Thomson Orogen. Precambrian crust, therefore, lies well east of the Tasman line. The results do not support a Neoproterozoic to Cambrian oceanic crust, as previously suggested, but instead point to a continental substrate for the Thomson Orogen. Hf isotopes indicate an overall trend towards more isotopically juvenile compositions with a progressive reduction in the contribution of older crustal sources to granitic magmas towards the present time. Different Hf isotopic signatures for the Lachlan (?_Hf(t) = –13 to +15), Thomson (?_Hf(t) = –14 to +5) and Delamerian (?_Hf(t) = –7 to +4) orogens highlight lateral variations in the age structures of crustal basement beneath these orogens.     

Origin of the Late Cretaceous syenite from Yandangshan,SE China,constrained by zircon U–Pb and Hf isotopes and geochemical data

The Yandangshan syenite is a representative Late Cretaceous igneous pluton cropping out in SE China. U–Pb zircon dating using LA‐ICP‐MS yielded a crystallization age of 98±1 Ma for the syenite. Petrographically and geochemically of shoshonitic affinity, it is enriched in LREE and LILE, and has a pronounced Nb–Ta trough in the primitive mantle‐normalized trace element spider diagram. Zircon ?_Hf(t) values vary from ?3.04 to ?7.71, displaying a unimodal distribution. The syenite also has high Sr [(⁸⁷Sr/⁸⁶Sr) _i  = 0.7086–0.7089], low Nd [?_Nd(t) = ?6.57 to ?7.64] isotopic ratios, plotting in the enriched mantle field on an ?_Nd(t) versus (⁸⁷Sr/⁸⁶Sr) _i diagram. We propose that the Yandangshan syenite was generated by pyroxene‐dominated high‐pressure fractional crystallization from basaltic magma that was derived from an enriched mantle source. Although coexisting Yandangshan rhyolites have Sr–Nd isotopic compositions similar to the Yandangshan syenite, they were not derived from the same source. Instead, the rhyolitic magma was produced by partial melting of crustal materials as a result of the underplating of basaltic magma. The crust‐like Sr–Nd–Hf isotopic signature of the Yandangshan syenite is ascribed to mantle sources that were enriched by subducted sediments. Formation of Yandangshan syenite may represent roll‐back of the subducting palaeo‐Pacific plate and migration of the arc front to the Yandangshan area at ～98 Ma.     

Detrital zircon U–Pb geochronology,trace-element and Hf isotope geochemistry of the metasedimentary rocks in the Eastern Himalayan syntaxis: Tectonic and paleogeographic implications

The origin of the Greater Himalayan Sequence in the Himalaya and the paleogeographic position of the Lhasa terrane within Gondwanaland remain controversial. In the Eastern Himalayan syntaxis, the basement complexes of the northeastern Indian plate (Namche Barwa Complex) and the South Lhasa terrane (Nyingchi Complex) can be studied to explore these issues. Detrital zircons from the metasedimentary rocks in the Namche Barwa Complex and Nyingchi Complex yield similar U–Pb age spectra, with major age populations of 1.00–1.20 Ga, 1.30–1.45 Ga, 1.50–1.65 Ga and 1.70–1.80 Ga. The maximum depositional ages for their sedimentary protoliths are ~ 1.0 Ga based on the mean ages of the youngest three detrital zircons. Their minimum depositional ages are ~ 477 Ma for the Namche Barwa Complex and ~ 499 Ma for the Nyingchi Complex. Detrital zircons from the Namche Barwa Complex and Nyingchi Complex also display similar trace-element signatures and Hf isotopic composition, indicating that they were derived from common provenance. The trace-element signatures of 1.30–1.45 Ga detrital zircons indicate that the 1.3–1.5 Ga alkalic and mafic rocks belt in the southeastern India is a potential provenance. Most 1.50–1.65 Ga zircons have positive ε_Hf(t) values (+ 1.2 to + 9.0), and most 1.70–1.80 Ga zircons have negative ε_Hf(t) values (− 7.1 to − 1.9), which are compatible with those of the Paleo- to Mesoproterozoic orthogneisses in the Namche Barwa Complex. Provenance analysis indicates that the southern Indian Shield, South Lhasa terrane and probably Eastern Antarctica were the potential detrital sources. Combined with previous studies, our results suggest that: (1) the Namche Barwa Complex is the northeastern extension of the Greater Himalaya Sequence; (2) the metasedimentary rocks in the Namche Barwa Complex represent distal deposits of the northern Indian margin relative to the Lesser Himalaya; (3) the South Lhasa terrane was tectonically linked to northern India before the Cambrian.     

Zircon U–Pb geochronology and geochemistry of the post-collisional volcanic rocks in eastern Xinjiang Province,NW China: implications for the tectonic evolution of the Junggar terrane

We report zircon U–Pb geochronologic and geochemical data for the post-collisional volcanic rocks from the Batamayineishan (BS) Formation in the Shuangjingzi area, northwestern China. The zircon U–Pb ages of seven volcanic samples from the BS Formation show that the magmatic activity in the study area occurred during 342–304 Ma in the Carboniferous. The ages also indicate that the Palaeo-Karamaili Ocean had already closed by 342 Ma. Moreover, the volcanic rocks also contained 10 inherited zircons with ages ranging from 565 to 2626 Ma, indicating that Precambrian continental crust or microcontinents with accretionary arcs are two possible interpretations for the basement underlying the East Junggar terrane. The sampled mafic-intermediate rocks belong to the medium-K to high-K calc-alkaline and shoshonitic series, and the formation of these rocks involved fractional crystallization with little crustal contamination. These Carboniferous mafic-intermediate rocks show depletions in Nb and Ta and enrichments in large ion lithophile elements (e.g. Rb, Ba, U, and Th) and light rare earth elements. The low initial ⁸⁷Sr/⁸⁶Sr values (0.7034–0.7042) and positive ε_Nd(t) values (+2.63 to +6.46) of these rocks suggest that they formed from depleted mantle material. The mafic-intermediate rocks were most likely generated by 5–10% partial melting of a mantle source composed primarily of spinel lherzolite with minor garnet lherzolite that had been metasomatized by slab-derived fluids and minor slab melts. In contrast, the felsic rocks in the BS Formation are A-type rhyolites with positive ε_Nd(t) values and young model ages. These rocks are interpreted to be derived from the partial melting of juvenile basaltic lower crustal material. Taken together, the mafic-intermediate rocks formed in a post-collisional extensional setting generated by slap breakoff in the early Carboniferous (342–330 Ma) and the A-type rhyolites formed in a post-collisional extensional setting triggered by the upwelling asthenosphere in the late Carboniferous (330–304 Ma).     

Provenance of metasedimentary rocks from the Beishan orogenic collage,southern Altaids: Constraints from detrital zircon U–Pb and Hf isotopic data

The origin and tectonic settings of metasedimentary sequences in the Central Asian Orogenic Belt have been a matter of debate regarding their contributions with some proposals of being microcontinents or accreted material, largely due to a lack of high resolution geochronological data. This paper reports detrital zircon U–Pb age and Hf isotopic data for the previously mapped Precambrian metasedimentary rocks from the Beishan orogenic collage, southern Altaids. Our data show that Precambrian ages dominate all the analyzed samples, but two samples yield Paleozoic zircons which suggest that they were not deposited in the Precambrian. The late Paleoproterozoic–early Mesoproterozoic group (~ 2000–1300 Ma) is the largest age population among the six samples analyzed. This age population (~ 2000–1300 Ma) corresponds to the assembly and subsequent break-up of the Columbia supercontinent. Only one sample (11SYS01) yields Neoproterozoic ages (with two peaks at 930 and 785 Ma), which shows a possible affinity with the Tarim Craton. Hence, the age spectra presented here are generally different from that of the Tarim Craton and the metasedimentary rocks from the Central Tienshan. Our data show that the Tarim Craton is not the main source area for the metasedimentary rocks from the Beishan orogenic collage, but instead multiple source areas may have contributed to the Beishan collage. Combining our new results with published data, we favor an allochthonous origin for the metasedimentary sequences which may be associated with major thrust tectonics. Therefore, a long-lived arc accretionary model is proposed for the tectonic evolution of the Beishan orogenic collage.     

Deformation history and U–Pb zircon geochronology of the high grade metamorphic rocks within the Klaeng fault zone,eastern Thailand

In eastern Thailand the Klaeng fault zone includes a high-grade metamorphic rock assemblage, named Nong Yai Gneiss, which extends about 30 km in a NW–SE direction along the fault zone. The rocks of this brittle-fault strand consist of amphibolite to granulite grade gneissic rocks. Structural analysis indicates that the rocks in this area experienced three distinct episodes of deformation (D₁–D₃). The first (D₁) formed large-scale NW–SE-trending isoclinal folds (F₁) that were reworked by small-scale tight to open folds (F₂) during the second deformation (D₂). D₁ and D₂ resulted from NE–SW shortening during the Triassic Indosinian orogeny before being cross-cut by leucogranites. D₁ and D₂ fabrics were then reworked by D₃ sinistral shearing, including shear planes (S₃) and mineral stretching lineations (L₃). LA–MC–ICP–MS U–Pb zircon dating suggested that the leucogranite intrusion and the magmatic crystallization took place at 78.6 ± 0.7 Ma followed by a second crystallization at 67 ± 1 to 72.1 ± 0.6 Ma. Both crystallizations occurred in the Late Cretaceous and, it is suggested, were tectonically influenced by SE Asian region effects of the West Burma and Shan-Thai/Sibumasu collision or development of an Andean-type margin. The sinistral ductile movement of D₃ was coeval with the peak metamorphism that occurred in the Eocene during the early phases of the India–Asia collision.     

U–Pb zircon,zircon Hf and whole-rock Sm–Nd isotopic constraints on the evolution of Paleoproterozoic rocks in the northern Gawler Craton

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 ε_Hf 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 ε_Nd 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 ε_Hf zircon values of ca +5.2 to +8.2, and initial ε_Nd 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.     

Magma mixing origin for the post-collisional adakitic monzogranite of the Triassic Yangba pluton, Northwestern margin of the South China block: geochemistry, Sr–Nd isotopic, zircon U–Pb dating and Hf isotopic evidences

Petrogenesis of high Mg# adakitic rocks in intracontinental settings is still a matter of debate. This paper reports major and trace element, whole-rock Sr–Nd isotope, zircon U–Pb and Hf isotope data for a suite of adakitic monzogranite and its mafic microgranular enclaves (MMEs) at Yangba in the northwestern margin of the South China Block. These geochemical data suggest that magma mixing between felsic adakitic magma derived from thickened lower continental crust and mafic magma derived from subcontinental lithospheric mantle (SCLM) may account for the origin of high Mg# adakitic rocks in the intracontinental setting. The host monzogranite and MMEs from the Yangba pluton have zircon U–Pb ages of 207 ± 2 and 208 ± 2 Ma, respectively. The MMEs show igneous textures and contain abundant acicular apatite that suggests quenching process. Their trace element and evolved Sr–Nd isotopic compositions [(⁸⁷Sr/⁸⁶Sr)_i = 0.707069–0.707138, and ε_Nd(t) = −6.5] indicate an origin from SCLM. Some zircon grains from the MMEs have positive ε_Hf(t) values of 2.3–8.2 with single-stage Hf model ages of 531–764 Ma. Thus, the MMEs would be derived from partial melts of the Neoproterozoic SCLM that formed during rift magmatism in response to breakup of supercontinent Rodinia, and experience subsequent fractional crystallization and magma mixing process. The host monzogranite exhibits typical geochemical characteristics of adakite, i.e., high La/Yb and Sr/Y ratios, low contents of Y (9.5–14.5 ppm) and Yb, no significant Eu anomalies (Eu/Eu* = 0.81–0.90), suggesting that garnet was stable in their source during partial melting. Its evolved Sr–Nd isotopic compositions [(⁸⁷Sr/⁸⁶Sr)_i = 0.7041–0.7061, and ε_Nd(t) = −3.1 to −4.3] and high contents of K₂O (3.22–3.84%) and Th (13.7–19.0 ppm) clearly indicate an origin from the continental crust. In addition, its high Mg# (51–55), Cr and Ni contents may result from mixing with the SCLM-derived mafic magma. Most of the zircon grains from the adakitic monzogranite show negative ε_Hf(t) values of −9.4 to −0.1 with two-stage Hf model ages of 1,043–1,517 Ma; some zircon grains display positive ε_Hf(t) of 0.1–3.9 with single-stage Hf ages of 704–856 Ma. These indicate that the source region of adakitic monzogranite contains the Neoproterozoic juvenile crust that has the positive ε_Hf(t) values in the Triassic. Thus, the high-Mg adakitic granites in the intracontinental setting would form by mixing between the crustal-derived adakitic magma and the SCLM-derived mafic magma. The mafic and adakitic magmas were generated coevally at Late Triassic, temporally consistent with the exhumation of deeply subducted continental crust in the northern margin of the South China Block. This bimodal magmatism postdates slab breakoff at mantle depths and therefore is suggested as a geodynamic response to lithospheric extension subsequent to the continental collision between the South China and North China Blocks.     

Zircon U–Pb geochronology,major-trace elements and Sr–Nd isotope geochemistry of Mashhad granodiorites (NE Iran) and their mafic microgranular enclaves: evidence for magma mixing and mingling

ABSTRACT

Mashhad granitoids and associated mafic microgranular enclaves (MMEs), in NE Iran record late early Mesozoic magmatism, was related to the Palaeo-Tethys closure and Iran-Eurasia collision. These represent ideal rocks to explore magmatic processes associated with Late Triassic closure of the Palaeo-Tethyan ocean and post-collisional magmatism. In this study, new geochronological data, whole-rock geochemistry, and Sr–Nd isotope data are presented for Mashhad granitoids and MMEs. LA–ICP–MS U–Pb dating of zircon yields crystallization ages of 205.0 ± 1.3 Ma for the MMEs, indicating their formation during the Late Triassic. This age is similar to the host granitoids. Our results including the major and trace elements discrimination diagrams, in combination with field and petrographic observations (such as ellipsoidal MMEs with feldspar megacrysts, disequilibrium textures of plagioclase), as well as mineral chemistry, suggest that MMEs formed by mixing of mafic and felsic magmas. The host granodiorite is a felsic, high K calc-alkaline I-type granitoid, with SiO₂ = 67.5–69.4 wt%, high K₂O (2.4–4.2 wt%), and low Mg# (42.5–50.5). Normalized abundances of LREEs and LILEs are enriched relative to HREEs and HFSEs (e.g. Nb, Ti). Negative values of whole-rock εNd_(t) (?3 to ?2.3) from granitoids indicate that the precursor magma was generated by partial melting of enriched lithospheric mantle with some contributions from old lower continental crust. In the MMEs, SiO₂ (53.4–58.2 wt%) is lower and Ni (3.9–49.7 ppm), Cr (0.8–93.9 ppm), Mg# (42.81–62.84), and εNd_(t) (?2.3 to +1.4) are higher than those in the host granodiorite, suggesting a greater contribution of mantle-derived mafic melts in the genesis of MMEs.     

The genesis of the ores and granitic rocks at the Hongshi Au deposit in Eastern Tianshan,China: Constraints from zircon U–Pb geochronology,geochemistry and isotope systematics

The Hongshi gold deposit is located in the southwestern margin of the Kanggur–Huangshan ductile shear zone in Eastern Tianshan, Northwest China. The gold ore bodies are predominantly hosted in the volcanogenic metasedimentary rocks of the Lower Carboniferous Gandun Formation and the Carboniferous syenogranite and alkali-feldspar granite. The syenogranite and the alkali-feldspar granite yield SHRIMP zircon U–Pb ages of 337.6 ± 4.5 Ma (2σ, MSWD = 1.3) and 334.0 ± 3.7 Ma (2σ, MSWD = 1.1), respectively, indicating that the Hongshi gold deposit is younger than 334 Ma. The granitoids belong to shoshonitic series and are relatively enriched in large ion lithophile elements (Rb, K, Ba, and Pb) and depleted in high field-strength elements (Nb, Ta, P, and Ti). Moreover, these granitoids have high SiO₂, Al₂O₃, and K₂O contents, low Na₂O, MgO, and TiO₂ contents, low Nb/Ta ratios, and slightly positive Eu anomalies. The ε_Hf(t) values of the zircons from a syenogranite sample vary from + 1.5 to + 8.8 with an average of + 5.6; the ε_Hf(t) values of the zircons from an alkali-feldspar granite sample vary from + 5.0 and + 10.1 with an average of + 7.9. The δ³⁴S values of 10 sulfide samples ranged from − 11.5‰ to + 4.2‰, with peaks in the range of + 1‰ to + 4‰. The above-mentioned data suggest that the Hongshi granitoids were derived from the melting of juvenile lower crust mixed with mantle components formed by the southward subduction of the paleo-Tianshan ocean plate beneath the Aqishan–Yamansu island arc during the Early Carboniferous. The Hongshi gold deposit was formed by post-collisional tectonism during the Permian. The granitoids most likely acted as impermeable barriers that prevented the leakage and runoff of ore-bearing fluids. Thus, the granitoids probably played an important role in controlling gold mineralization.     

Zircon U–Pb geochronology and Hf isotope geochemistry of magmatic and metamorphic rocks from the Hida Belt,southwest Japan

Zircon U–Pb and Hf isotope data integrated in this study for magmatic and metamorphic rocks from the Hida Belt,southwest Japan,lead to a new understanding of the evolution of the Cordilleran arc system along the ancestral margins of present-day Northeast Asia.Ion microprobe data for magmatic zircon domains from eight mafic to intermediate orthogneisses in the Tateyama and Tsunogawa areas yielded weighted mean ~(206)Pb/~(238)U ages spanning the entire Permian period(302–254 Ma).Under cathodoluminescence,primary magmatic growth zones in the zircon crystals were observed to be partially or completely replaced by inward-penetrating,irregularly curved featureless or weakly zoned secondary domains that mostly yielded U–Pb ages of 250–240 Ma and relatively high Th/U ratios( 0.2).These secondary domains are considered to have been formed by solid-state recrystallization during thermal overprints associated with intrusions of Hida granitoids.Available whole-rock geochemical and Sr–Nd isotope data as well as zircon age spectra corroborate that the Hida Belt comprises the Paleozoic–Mesozoic Cordilleran arc system built upon the margin of the North China Craton,together with the Yeongnam Massif in southern Korea.The arc magmatism along this system was commenced in the Carboniferous and culminated in the Permian–Triassic transition period.Highly positive εHf(t) values( +12) of late Carboniferous to early Permian detrital zircons in the Hida paragneisses indicate that there was significant input from the depleted asthenospheric mantle and/or its crustal derivatives in the early stage of arc magmatism.On the other hand,near-chondritic εHf(t) values(+5 to-2) of magmatic zircons from late Permian Hida orthogneisses suggest a lithospheric mantle origin.Hf isotopic differences between magmatic zircon cores and the secondary rims observed in some orthogneiss samples clearly indicate that the zircons were chemically open to fluids or melts during thermal overprints.Resumed highly positive zircon εHf(t) values(+9) shared by Early Jurassic granitoids in the Hida Belt and Yeongnam Massif may reflect reworking of the Paleozoic arc crust.     

Zircon U–Pb and Hf isotopic study of Mesozoic felsic rocks from eastern Zhejiang,South China: Geochemical contrast between the Yangtze and Cathaysia blocks

The Jiangshan–Shaoxing Fault Zone (JSFZ) in Zhejiang Province has been proposed to represent a suture between the Yangtze and Cathaysia blocks in South China. In this study, in-situ zircon U–Pb and Hf isotopic analysis and whole-rock major- and trace-element measurement of early to middle Cretaceous felsic rocks across the fault zone were conducted to constrain the nature of the fault zone. Twelve Cretaceous granitoid bodies were sampled from the NW and SE sides of the fault zone, respectively, with composition ranging from diorite to granite (SiO₂ = 56.2–76.6 wt.%). These granitoids yielded U–Pb ages ranging from 135–100 Ma, with a systematic variation in zircon Hf isotopic compositions (ε_Hf(t) = + 6.9 to –7.0 in the NW side vs. + 1.9 to ? 12.9 in the SE side). The T_DM2 values for the granitoids from the NW side are 0.34 to 1.33 Ga, with two peaks at ca. 876 and 1170 Ma respectively, whereas those from the SE side are 0.70 to 1.62 Ga, with a single peak at ca. 1126 Ma. The Hf isotopic disparity for the two sides may indicate a fundamental difference in the lower crustal compositions of the Yangtze and Cathaysia blocks, supporting that the JSFZ is possibly a suture zone between the two blocks. Our results together with the available geological data suggest that the Mesoproterozoic materials are important for both the Yangtze and Cathaysia basement and the Neoproterozoic magmatic activities were important in the Yangtze Block, possibly related to the break-up of the Rodinia supercontinent, but less significant in the Cathaysia Block. This may imply that the two blocks have not completely juxtaposed in the Neoproterozoic.     

U-Pb zircon and Sm—Nd geochronology of mafic and ultramafic rocks from the central part of the Tauern Window (eastern Alps)

Geochronological and geochemical analyses were carried out in order to identify the pre-Variscan basement of the Tauern Window (eastern Alps). Maficultramafic rocks from the central part of the Tauern window have been studied by REE-analysis and U-Pb and Sm-Nd isotopic analyses on whole rock, zircons, garnets and sphene. U-Pb and Sm—Nd zircon dating define both magmatic Pan-African and Cambro-Ordovician events from 650 Ma to 486 Ma within the Alpine fold belt. This indicates a time span of 150 Ma for magmatic activities in the Tauern Window of the eastern Alps. The ages of 657 Ma (U-Pb zircon) and 644 (Sm—Nd zireon) obtained from an amphibolite are the oldest dates of the Eastern Alps; they may be related to the Pan-African orogeny, and imply an early cycle of magmatic intrusion before major activity started at around 500 Ma. Sm-Nd whole rock analyses of the Precambrian rocks do not define an isochron, reflecting heterogenities within the mantle source. The initial _Nd values (+1.2 to +4.7) are very low, implying an enrichment of the magma source. The second main phase of magmatic activity (539 486 Ma) is characterized by the emplacement of mafic/ultramafic rock sequences. As no ophiolitic relies are observed in these domains, the Early Paleozoic magmatism was likely associated with extensional tectonics. Obtained ages of 301±3 and 314+4/-3 Ma point to a Variscan metamorphism. The first combined U-Pb zircon/Sm—Nd zircon data for an amphibolite from the Basal Amphibolite Formation (BAF) favoured the Sm-Nd zircon isochron age as a magmatic age, whereas the low initial _Nd value point to an enriched magma source as well as to heterogenities within the magma source. The obtained ages suggest that parts of the pre-Variscan basement within the Alpine fold belt were formed during the Pan-Africa cycle. The detection of Pre-Variscan ages within the Alpine basement must reffect a complex history involving significant pre-Variscan activity.     

Geochronology,geochemistry and Sr–Nd–Hf–S–Pb isotopes of the Early Cretaceous Taoxihu Sn deposit and related granitoids,SE China

The Taoxihu deposit (eastern Guangdong, SE China) is a newly discovered Sn polymetallic deposit. Zircon U-Pb dating yielded 141.8 ± 1.0 Ma for the Sn-bearing granite porphyry and 145.5 ± 1.6 Ma for the biotite granite batholith it intruded. The age of the granite porphyry is consistent (within error) with the molybdenite Re–Os isochron age (139.0 ± 1.1 Ma) of the Sn mineralization, indicating a temporal link between the two. Geochemical data show that the granite porphyry is weakly peraluminous, contain high Si, Na and K, low Fe, Mg, Ca and P, and relatively high Rb/Sr and low K/Rb values. The rocks are enriched in Rb, Th, U, K, and Pb and depleted in Ba, Sr, Ti and Eu, resembling highly fractionated I-type granites. They contain bulk rock initial ⁸⁷Sr/⁸⁷Sr of 0.707371–0.707730 and εNd(t) of −5.17 to −4.67, and zircon εHf(t) values from −6.67 to −2.32, with late Mesoproterozoic T_DM2 ages for both Nd and Hf isotopes. This suggests that the granite porphyry was likely formed by the partial melting of the crustal basement of Mesoproterozoic overall residence age with minor mantle input.δ³⁴S_CDT values of the Taoxihu chalcopyrite and pyrite range from 0.1 to 2.1‰ (average: 0.9‰), implying a dominantly magmatic sulfur source. The ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios of the Taoxihu sulfide ores are 18.497–18.669, 15.642–15.673 and 38.764–38.934, respectively, indicating a mainly upper continental crustal lead source with minor mantle contribution. The highly fractionated and reduced (low calculated zircon Ce⁴⁺/Ce³⁺ and Eu_N/Eu_N¹ values) nature of the ore-forming granitic magma may have facilitated the Sn enrichment and played a key role in the Sn mineralization. We propose that the ore-forming fluids at Taoxihu were of magmatic-hydrothermal origin derived from the granite porphyry, and that both the granite porphyry and the Sn mineralization were likely formed in an extensional setting, possibly related to the subduction slab rollback of the Paleo-Pacific Plate.     

Jurassic detrital zircon U–Pb and Hf isotopic geochronology of Luxi Uplift,eastern North China,and its provenance implications for tectonic–paleogeographic reconstruction

Relatively successive sequences of Late Mesozoic are preserved and exposed in Luxi Uplift (LU), eastern North China block (NCB), which is an important region to study the late Mesozoic tectonic evolution of the eastern NCB. In this study, in situ U–Pb ages and Hf isotopic analyses on detrital zircons from the sandstones of Jurassic Fangzi and Santai Formations in LU combining the analysis of sandstone detrital modes were performed, with an aim to trace the Jurassic sediment provenances and the tectonic–paleogeographic configuration of eastern NCB. Three sandstone samples (one from Fangzi Formation and two from Santai Formation) have very similar U–Pb age spectrums which can be divided into three major groups: Phanerozoic (I), Paleoproterozoic (II), and Neoarchean (III). Detrital zircons of Group II and Group III broadly match the age spectra of the basement of NCC which exposed extensively in the northern part. No middle Neoproterozoic magmatic zircons or Triassic metamorphic zircons were found in this study, ruling out the clastic provenance transported from the Sulu orogen to LU. Dominant zircon populations of Group Iare Late Paleozoic (250–393 Ma) recording the corresponding magmatic activities which are not found both in LU and its peripheral tectonic terranes, but can be well compared with that of the northern NCB (NNCB) and the Xing-Meng Orogenic Belt (XMOB). Furthermore, Hf isotope compositions of the Phanerozoic detrital zircons can be distinctly divided into two clusters with ε_Hf(t) values ranging from −1.0 to +12.7 and −21.9 to −3.0, respectively resemble those from the XMOB and NCB (mainly from NNCB). Sandstone detrital modes analysis indicates the provenance came from the areas that have been eroded deeply to expose the basement rocks which accords with the tectonic setting of the NNCB. This research proposes that an evident mountain or provenance region once increasingly developed along NNCB during Early to Late Jurassic (182–155 Ma) due to the continuous collision of the Siberia and North China–Mongolian plates, easily shed mass clastic materials southward into the inner NCB and became the major provenance of Jurassic sediments in LU.     

Origin of the Alxa Block,western China: New evidence from zircon U–Pb geochronology and Hf isotopes of the Longshoushan Complex

The NW–SE trending Longshoushan is in the southwestern margin of the Alxa Block, which was traditionally considered the westernmost part of the North China Craton (NCC). Precambrian crystalline basement exposed in the Longshoushan area was termed the “Longshoushan Complex”. This complex's formation and metamorphism are significant to understand the geotectonics and early Precambrian crustal evolution of the western NCC. In this study, field geology, petrology, and zircon U–Pb and Lu–Hf isotopes of representative orthogneisses and paragneisses in the Longshoushan Complex were investigated. U–Pb datings reveal three Paleoproterozoic magmatic episodes (ca. 2.33, ca. 2.17 and ca. 2.04 Ga) and two subsequent regional metamorphic events (ca. 1.95–1.90 Ga and ca. 1.85 Ga) for metamorphic granitic rocks in the Longshoushan Complex. U–Pb dating of the detrital magmatic zircons from two paragneisses yields concordant ²⁰⁷Pb/²⁰⁶Pb ages between 2.2 Ga and 2.0 Ga, and a small number of metamorphic zircon rims provide a ca. 1.95 Ga metamorphic age, suggesting that the depositional time of the protolith was between 2.0 and 1.95 Ga and that the sedimentary detritus was most likely derived from the granitic rocks in the Longshoushan Complex itself. Zircon Lu–Hf isotopic analyses indicate that nearly all magmatic zircons from ca. 2.0 Ga to ca. 2.17 Ga orthogneisses have positive εHf(t) values with two-stage Hf model ages (T_DMC) ranging from 2.45 to 2.65 Ga (peak at ca. 2.5 Ga), indicating that these Paleoproterozoic granitic rocks were derived from the reworking of the latest Neoarchean–early Paleoproterozoic juvenile crust. Detrital magmatic zircons from two paragneisses yield scattered ¹⁷⁶Hf/¹⁷⁷Hf ratios, εHf(t) and T_DMC values, further indicating that the sedimentary detritus was not only derived from these plutonic rocks but also from other unreported or denuded Paleoproterozoic igneous rocks. The ca. 2.15 Ga detrital magmatic zircons from one paragneiss have negative εHf(t) values with T_DMC ranging from 2.76 to 3.04 Ga, indicating another important crustal growth period in the Longshoushan region. These data indicate that the Longshoushan Complex experienced Neoarchean–Early Paleoproterozoic crustal growth, approximately ca. 2.3–2.0 Ga experienced multiphase magmatic events, and approximately ca. 1.95–1.90 Ga and ca. 1.85 Ga experienced high-grade metamorphic events. The sequence of tectonothermal events is notably similar to that of the main NCC. Together with the datasets from an adjacent area, we suggest that the western Alxa Block was most likely an integrated component of the NCC from the Neoarchean to the Paleoproterozoic.