Two-stage Hf Model Ages Research Articles

The Late Ordovician granitoids in the East Kunlun Orogenic Belt, Northwestern China: petrogenesis and constraints for tectonic evolution of the Proto-Tethys Ocean

Petrology, whole-rock geochemistry, in situ zircon U–Pb ages, and Lu–Hf isotopic data are reported for granodiorite and monzonitic granite samples from the Harizha Region in the eastern section of the East Kunlun Orogenic Belt. The results of the LA-ICP-MS zircon dating indicate that the granodiorite and monzonitic granite were emplaced at 447.9 ± 3.0 Ma and 443.8 ± 3.5 Ma, respectively, i.e., in the Late Ordovician–Early Paleozoic periods. The granodiorite is characterized by relatively high MgO (4.52–5.91 wt%), Sr (541–712 ppm), Cr (218–357 ppm), Ni (56.7–80.7 ppm) and V (66.8–94.7 ppm), with low Y (10.40–13.60 ppm) and Yb (1.05–1.45) contents. This results in elevated Mg# (65–68), Sr/Y (39.78–58.84), and (La/Yb)N (10.21–18.30) ratios, common features for adakitic high-Mg andesite (HMA). The granodiorite is metaluminous to weakly peraluminous and high-K calc-alkaline in composition. In addition, the granodiorite is enriched in light rare-earth elements (LREE) and large ion lithophile elements (LILEs, Rb, K, Sr, and Pb) U, and Nd; and depleted in high-field strength elements (HFSEs, Nb, Ta, Zr, Hf, Ti) and P. The zircon grains from the granodiorites have eHf(t) values of − 5.5 to + 4.7, with two-stage Hf model ages (tDM2) ranging from 0.92 to 1.71 Ga. These features suggest that the granodiorites were formed by mixing processes of crustal melts and magma from a metasomatized mantle in a subduction setting. The monzonitic granite possesses high normative corundum values (2.34–4.10 wt%), is strongly peraluminous (A/CNK ratios of 1.16–1.32, with minor mineral muscovite), with high-K calc-alkaline affinity. Furthermore, enrichments in large ion lithophile elements (Rb, K, and Pb), U, Th, and Nd, depletions in high-field strength elements (Nb, Ta, and Ti) and Ba and P, and mainly right-inclined REE patterns with negative Eu anomalies, exhibits an S-type granite affinity. The magmatic zircons from the monzonitic granite yielded eHf(t) values between − 4.9 and − 2.7, corresponding to two-stage Hf model ages of 1.60–1.73 Ga. Therefore, this study suggests that the monzonitic granite magma was derived from crustal material (metagreywacke) in a syn-collision setting. Subsequently, by combining these findings with the previous research results, this study propose that a transverse diachronism closure model which entailed an “earlier on the sides–later in the middle” in an East–West direction in the East Kunlun Orogenic Belt for the Proto-Tethys Ocean’s evolution process.

Nature and origin of the volcanic ash beds near the Permian–Triassic boundary in South China: new data and their geological implications

AbstractPermian–Triassic boundary (PTB) volcanic ash beds are widely distributed in South China and were proposed to have a connection with the PTB mass extinction and the assemblage of Pangea. However, their source and tectonic affinity have been highly debated. We present zircon U–Pb ages, trace-element and Hf isotopic data on three new-found PTB volcanic ash beds in the western Hubei area, South China. Laser ablation inductively coupled plasma mass spectrometry U–Pb dating of zircons yields ages of 252.2 ± 3.6 Ma, 251.6 ± 4.9 Ma and 250.4 ± 2.4 Ma for these three volcanic ash beds. Zircons of age c. 240–270 Ma zircons have negative εHf(t) values (–18.17 to –3.91) and Mesoproterozoic–Palaeoproterozoic two-stage Hf model ages (THf2) (1.33–2.23 Ga). Integrated with other PTB ash beds in South China, zircon trace-element signatures and Hf isotopes indicate that they were likely sourced from intermediate to felsic volcanic centres along the Simao–Indochina convergent continental margin. The Qinling convergent continental margin might be another possible source but needs further investigation. Our data support the model that strong convergent margin volcanism took place around South China during late Permian – Early Triassic time, especially in the Simao–Indochina active continental margin and possibly the Qinling active continental margin. These volcanisms overlap temporally with the PTB biocrisis triggered by the Siberian Large Igneous Province. In addition, our data argue that the South China Craton and the Simao–Indochina block had not been amalgamated with the main body of Pangea by late Permian – Early Triassic time.

Geochronology, petrogenesis and tectonic significance of Dahongliutan pluton in Western Kunlun orogenic belt, NW China

The Dahongliutan granitic pluton, in the eastern part of the West Kunlun orogenic belt, provides significant insights for studying the tectonic evolution of West Kunlun. This paper presents a systematic study of LA-ICP-MS zircon U-Pb age, major and trace elements, Sr-Nd-Hf isotopes, and the first detailed Li isotope analysis of the Dahongliutan pluton. LA-ICP-MS zircon U-Pb dating shows that the Dahongliutan granites were emplaced in the Late Triassic ((213±2.1) Ma). Geochemical data show relatively high SiO2 contents (68.45 wt%–73.62 wt%) and aluminum saturation index (A/CNK=1.11–1.21) indicates peraluminous high-K calc-alkaline granite. The Dahongliutan granites are relatively high in light rare earth elements (LREE) and large ion lithophile elements (LILEs) (e.g., Rb, K, Th), and relatively depleted in high field strength elements (HFSEs) (e.g., Nb, Ta, P, Ti). The eNd(t) values range from −8.71 to −4.73, and (87Sr/86Sr)i=0.7087–0.71574. Zircons from the pluton yield 176Hf/177Hf values of 0.2826181 to 0.2827683, and eHf(t) values are around 0; the two-stage Hf model ages range from 0.974 to 1.307 Ga. The δ7Li values are 0.76‰–3.25‰, with an average of 2.53‰. Isotopic compositions of the pluton suggest a mixed trend between the partial melting of the Middle Proterozoic ancient crustal material and a juvenile mantle-derived material. This study infers that the Dahongliutan rock mass is formed in the post-collisional extension environment, when the collision between South Kunlun and the Tianshuihai terranes results in the closure of the Palaeo-Tethys. The mantle-derived magma results in partial melting of the lower crust.

Zircon LA-ICP-MS U-Pb Ages and the Hf Isotopic Composition of the Ore-Bearing Porphyry from the Yanghuidongzi Copper Deposit, Heilongjiang, China, and Its Geological Significance

The Yanghuidongzi copper deposit is a typical porphyry copper deposit located at the eastern margin of the Xing’anling-Mongolian Orogenic Belt (XMOB). While much attention have been paid to the ore-forming age of the deposit and the magma source of the ore-bearing porphyry, this paper approaches this issue with the methods of the LA-ICP-MS zircon U-Pb dating and Lu-Hf isotopic composition of the Yanghuidongzi porphyry copper deposit. The results reveal that the Yanghuidongzi porphyry copper deposit was formed in the Early Jurassic (189.6 ± 1.0 Ma), which corresponds to the time of magmatic activity in this region. The background studies of ore-forming dynamics indicate that the formation of the Yanghuidongzi copper deposit is related to the subduction of the Paleo-Pacific plate. The Yanghuidongzi ore-bearing porphyry zircons have a positive εHf(t) value (4.4–7.0), a high 176Hf/177Hf ratio (0.282786–0.282854), and a two-stage Hf model ages (TDM2) ranging from 783 Ma to 943 Ma, all of which suggest that the Early Jurassic granodiorite porphyry of the Yanghuidongzi deposit was formed by the partial melting of newly grown crustal material from the depleted mantle in the Neoproterozoic.

Genesis of the Shangjinshan W–Mo polymetallic deposit in the Eastern Jiangnan tungsten belt: Evidences from geochemistry, geochronology and zircon Hf isotope analyses

The newly discovered Shangjinshan W–Mo polymetallic deposit, located at the southern Anhui, southeast China, is a large-scale skarn-type tungsten polymetallic deposit. According to mineral assemblages, ore textures/structures and superposition relationships of the mineralization, three ore types have been recognized: veinlet-disseminated skarn (the primary type), quartz vein type and porphyry type. The mineralized event of this deposit could be divided into four stages: (I) the prograde skarn stage, (II) retrograde skarn stage, (III) quartz-sulfide vein stage, and (IV) quartz-calcite vein stage. The intrusion in the tungsten-rich district of the Shangjinshan is mainly composed of the granodiorite with zircon U–Pb ages of 138.4 ± 1.2 Ma to 136.7 ± 1.3 Ma. Meanwhile, Re–Os isotopic dating of five molybdenite samples yields an isochron age of 141.9 ± 3.1 Ma and a weighted average age of 139.3 ± 1.7 Ma, respectively. The ore-forming ages are consistent with the rock-forming ages within errors, indicating that the W-Mo mineralization is genetically related to the Shangjinshan granodiorite. The granodiorite is I-type granites, characterized by metaluminous to slightly peraluminous and high-K cal-alkaline compositions. They are also enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depleted in high field strength elements (HFSEs) and heavy rare earth element (HREEs), with obviously negative Eu anomalies (Eu/Eu* = 0.50–0.84). The zircon εHf (t) values and two-stage Hf model ages (TDM2) fall into the range −6.18 to −2.04 and 1.5–1.3 Ga, respectively. The relatively low rhenium contents in molybdenite and high radioactive Pb isotopes of the pyrites from stage II indicate that the ore-forming material was derived from Shangjinshan granitic magmatism. Together with previously published data, we propose that the W-related granitoids in the JNB were likely generated by partial melting of the Shangxi Group with some additional mantle materials. The granitic magmatism was caused by the asthenosphere upwelling, triggered by the roll-back of subduced Paleo-Pacific plate during Late Jurassic to Early Cretaceous. These granitic stocks predominately contributed to the production of skarn alteration and mineralization, resulting in the giant tungsten polymetallic district in the Jiangnan tungsten belt.

Genesis of the Guangshigou pegmatite-type uranium deposit in the North Qinling Orogenic Belt, China

The Guangshigou uranium deposit is an important pegmatite-type uranium deposit located in the North Qinling Orogenic Belt (NQB), China. The characteristics pertaining to its chronology, petrogenesis, and geodynamic setting remain unclear till date. In this study, we present systematic zircon U–Pb ages, Lu–Hf isotopic data, and mineral chemistry of uranium-rich biotite pegmatite dikes. In situ zircon U–Pb dating of the biotite pegmatite dikes yielded early Caledonian ages (422–417 Ma). The initial 176Hf/177Hf ratios of the zircons from the biotite pegmatites showed a narrow range (0.282378–0.282590), with εHf values ranging from −5.1 to 2.8 and Paleoproterozoic to Mesoproterozoic two-stage Hf model ages between 1723 and 1227 Ma. The bulk and mineral chemistries of the pegmatites also indicate that the pegmatite dikes were derived from low-degree partial melting of Qinling Group rocks with monazite as the residual mineral. Differences between the normal biotite pegmatite and the mineralized biotite pegmatite dikes with respect to mineralogy, texture, structure, and mineral chemistry indicate that the assimilation between the pegmatite dikes and the wall rock, may be the primary mechanism for the mineralization of the uranium, with the fractional crystallization process being another important mechanism. Amphibolite facies metamorphism, which was most likely caused by the exhumation of the subducted slab in the NQB, may have been a key factor in the genesis of the uranium-rich pegmatitic magma.

Tectonic setting and metallogenic chronology of the Ashele Cu–Zn deposit in Xinjiang, NW China: Constraints from Re-Os dating of pyrite, U-Pb dating of zircon and Hf isotopes

The Ashele Cu-Zn deposit, an important volcanogenic massive sulfide (VMS) deposit, is located in the centre of the Ashele volcano-sedimentary basin of the Altay metallogenic belt, North Xinjiang, NW China. The No. 1 orebody is located between the roof basalt and the base tuff. The roof basalt yielded a zircon U-Pb age of 342 ± 4.6 Ma (MSWD = 1.5). Four pyrite samples from massive ores sulfides yielded a Re-Os isochron age of 342 ± 11 Ma (MSWD = 9.2), while the vein sulfide has a model age of 315 Ma. This shows that massive sulfide orebodies developed contemporaneously with the emplacement of the roof basalt. The basalts have geochemical characteristics of tholeiitic basalts with relatively high Mg# values (62.7–63.9), Cr and Ni, a slightly negative Eu anomaly (δEu = 0.75–0.87), and relatively low concentrations of Sr, Nb/Ta and (La/Yb)N. The dacite has characteristics similar to those of sanukite, in terms of high concentrations of Mg# values (52.7–53.0), a weakly negative Eu anomaly (δEu = 0.90–1.04), and low concentrations of Cr, Ni, Sr and (La/Yb)N. Hf isotope analysis in the basalt exhibits the εHf(t) value of (+9.68 to + 17.03). The two-stage Hf model age (TDM2) shows 259–727 Ma with an average of 500 Ma. These geochemical characteristics indicate that the deposit formed in the magmatic arc setting, whereas the basalt originated from young (regenerated) mantle material. Re/Os and common Os imply that crust-derived and S-rich sediments also contributed to the ore formation. Accordingly, based on the results of this study and review of the regional magmatic framework, we can conclude that the Ashele deposit is diplogenetic, with VMS epigenetic and hydrothermal mineralization expressed.

Geochronology, Oxidization State and Source of the Daocheng Batholith, Yidun Arc: Implications for Regional Metallogenesis

The Daocheng batholith consists of granite, granodiorite and K-feldspar megacrystic granite, which is located in the north Yidun Arc. It is a barren batholith in contrast to plutons of the same age that contain major copper deposits, such as Pulang to the south. In the Daocheng, abundant mafic microgranular enclaves (MMEs) mainly developed within granodiorite and K-feldspar megacrystic granite, which are characterized by quenched apatite, quartz eyes and plagioclase phenocrysts. LA-ICP-MS zircon U–Pb dating of host granodiorite yielded ages ranging from 223 Ma to 210 Ma, with a weighted mean of 215.3 ± 1.8 Ma. Zircons from MMEs yielded ages ranging from 218 Ma to 209 Ma, with a weighted mean of 214.2 ± 1.4 Ma. Geochemical analyses show that granodiorite is high-K, calc-alkaline and I-type, with SiO2 contents ranging from 67.90% to 70.54%. These rocks are metaluminous to marginally peraluminous (A/CNK = 0.98–1.00) and moderately rich in alkalis with K2O ranging from 3.28% to 4.59% and Na2O ranging from 3.18% to 3.20%, with low MgO (1.08%–1.29%), Cr (12.7 ppm–16.8 ppm), Ni (5.19 ppm–6.16 ppm) and Mg# (35–49). The MMEs have relatively low SiO2 contents (56.34%–60.91%), higher Al2O3 contents (16.06%–17.98%), higher MgO and FeO abundances and are metaluminous (A/CNK = 0.82–0.83). The MMEs and host granodiorite are enriched in light rare-earth elements (LREEs) relative to heavy rare-earth elements (HREEs), with slightly negative Eu anomalies, and enriched in Th, U and large ion lithophile elements (LILEs; e.g., K, Rb and Pb), and depleted in high field strength elements (HFSEs; e.g., Nb, Ta, P and Ti), showing affinities typical of arc magmas. The zircon εHf(t) values (−6.28 to −2.33) and ancient two-stage Hf model ages of 1.92 to 1.25 Ga, indicating that the magmas are generally melts that incorporated significant portions of Precambrian crust. The relatively low silica contents and high Mg# values of the MMEs, and the linear patterns of MgO, Al2O3 and Fe2O3 with SiO2 between the MMEs and host granodiorite, showing the formation of MMEs are genetically related to magma mixing. The Daocheng granodiorite is characterized by much lower zircon Ce4+/Ce3+ (average of 3.53) and low fO2 value (average of ∆FMQ = –10.84), whereas the ore-bearing quartz monzonite porphyries in the Pulang copper deposit are characterized by much higher zircon Ce4+/Ce3+ (average of 52.10) and high fO2 value (average of ∆FMQ = 2.8), indicating the ore-bearing porphyry intrusions had much higher fO2 of magma than the ore-barren intrusions considering that the high oxygen fugacity of the magma is conducive to mineralization.

New insights into Paleoproterozoic tectonics of the Yangtze Block in the context of early Nuna assembly: Possible collisional granitic magmatism in the Zhongxiang Complex, South China

The newly discovered Zhongxiang Complex preserving extensive Paleoproterozoic granitoids plays a pivotal role in understanding the early tectonic processes of the Yangtze Block, South China, in the context of the Nuna supercontinent cycle. New zircon U-Pb age and Hf-isotope coupled with whole-rock chemical and Nd-isotope results from three granitic intrusions in the southern Zhongxiang Complex document a magmatic record of significant late Paleoproterozoic tectonic convergence in the region. Geochronological data show that granites from these intrusions were emplaced in a relatively long period at 2.0–1.93 Ga. The granites have variably high SiO2 contents of 74.01–77.19 wt%, and are weakly to strongly peraluminous with high ASI (alumina saturation index) values (>1.05). They are generally magnesian, showing fractionated S-type granite affinities. They have negative zircon εHf(t) of −9.69 to −17.11 and negative whole-rock εNd(t) of −7.43 to −11.69, corresponding to two-stage Hf model ages of 3.24–3.67 Ga and two-stage Nd model ages of 3.0–3.37 Ga, respectively, indicating a derivation of their parental magmas from remelting of ancient crustal components (i.e., the Yangpo “Group” paragneisses). The moderate to strong LREE/HREE fractionation ((La/Yb)N > 22, average 73.15) and the relatively low Sr/Y and negative Eu anomalies of the least felsic samples further imply a residual assemblage probably involving both garnet and plagioclase, corresponding to middle-lower crustal levels. This magmatic event most likely occurred at a convergent continental margin related to continental collision, which, in conjunction with broadly coeval metamorphism and the succeeding A-type granitic magmatism in the region, delineates a possible late Paleoproterozoic tectonic transition from compression to extension, signifying a cycle from collision (2.0–1.93 Ga) to post-collision (1.85 Ga) in the Zhongxiang Complex. Such a collisional orogenesis was potentially linked to the Paleoproterozoic internal assembly of the eastern part of the Yangtze Block, presumably as part of the early assembly of the Nuna supercontinent.

The Middle–Late Silurian granitoids in the Eastern Kunlun Orogenic Belt, NW China: petrogenesis and implications for tectonic evolution

The Eastern Kunlun Orogenic Belt lay on a significant tectonic boundary between Gondwana and Eurasia, produced widespread granitoids, and recorded at least two orogenic episodes since the Phanerozoic eon. In this paper, we provided new petrography, geochemistry, zircon U–Pb dating, and Lu–Hf isotopic research on the Middle–Late Silurian granitoids from the eastern segment of The Eastern Kunlun Orogenic Belt. These granitoids comprised of Zhongzhigou granodiorites and Xintuo alkali-feldspar granites, formed in the Middle–Late Silurian (420–425 Ma). Geochemically, the granodiorites exhibit high Sr/Y (31.2–35.2) as adakitic affinities, and alkali-feldspar granites belong to highly fractionated I-type granites. Additionally, granodiorites have zircon eHf(t) values show limited positive variations from − 0.5 to + 3.2, with two-stage Hf model ages (TDM2) of 1105–1306 Ma, while alkali-feldspar granites show negative zircon eHf(t) values ranged from − 4.6 to − 0.1 with two-stage Hf model ages (TDM2) of 1289–1541 Ma, implying that their primary magma originated from different sources. Taken together, we conclude that granodiorites had been derived from partial melting of the juvenile newly underplated mafic lower crust, while alkali-feldspar granites were derived from partial melting of the shallower ancient crustal source. Combined with temporal and spatial distribution of magmatic activities, sedimentation, and metamorphism, it can be concluded that the whole EKOB underwent a transition from syn-collision compression to post-collision extension and extensive crustal reworking occurred during the Middle–Late Silurian.

Formation of the Granodiorite-Hosting Magushan Cu–Mo Polymetallic Deposit in Southern Anhui, Eastern China: Evidences from Geochronology and Geochemistry

The newly discovered Magushan Cu-Mo polymetallic deposit, located in southeastern Anhui, eastern China, is a middle-scale skarn-type polymetallic deposit with different ore types of veinlets-disseminated skarn (the primary type), quartz veins, and porphyry. LA-ICP-MS zircon U–Pb analyses yielded a crystallization age of 135.7 ± 1.5 Ma for the ore-related granodiorite in Magushan. The granodiorites are I-type granites in nature, characterized by metaluminous and high-K calc-alkaline characteristics. They are enriched in large ion lithophile elements (LILEs, e.g., Ba, Th, and U) and light rare earth elements (LREEs), and depleted in high field strength elements (NFSEs, e.g., Nb, Ta, and Ti) and heavy rare earth element (HREEs), with slightly negative Eu anomalies (Eu/Eu* = 0.81–0.86). These granodiorites show high Mg# (mainly > 40) values, high MgO (1.73–1.96 wt. %) and low Na2O (<4.21 wt. %) contents, with whole-rock (87Sr/86Sr)i ratios (0.708877 to 0.710398), negative εNd(t) values of −5.4 to −5.2, and negative zircon εHf(t) values of −4.60 to −1.37, with old two-stage Hf model ages (TDM2) between 1.2‒1.5 Ga. Besides, they are characterized by high radiogenic Pb isotopic compositions with (206Pb/204Pb)i = 18.44–18.56, (207Pb/204Pb)i = 15.66–15.67, and (208Pb/204Pb)i = 38.77–38.87. These granodiorites are characterized by high zircon Ce4+/Ce3+ ratios (average 893) and Eu/Eu* ratios (average 0.51), indicating high magmatic oxygen fugacities. The distinct geochemical and isotopic features suggest that the Magushan granodiorites could be formed by metasomatized mantle-derived magmas, mixing with materials from Neoproterozoic crust that is widely distributed in the Southern Anhui. This study concludes that the formation of the Magushan Cu-Mo polymetallic deposits may largely depend on an oxidizing environment and multi-sources mixed of mantle- and crust-derived materials.

Petrogenesis and tectonic implications of late Oligocene highly fractionated leucogranites in the Ailao Shan-Red River shear zone, SW China

We report for the first time the occurrence of leucogranite bodies that intruded the Ailaoshan metamorphic complex along the Ailao Shan-Red River (ASRR) shear zone. Leucogranites in Yuanyang, Southeast Yunnan province, are composed of quartz (~30%), alkali feldspar (~45%), plagioclase (~5%), muscovite (~13%) and biotite (~3%) and other minor silicate minerals (~3%, garnet, tourmaline). U–Pb dating of zircon from leucogranites yields a weighted mean age of 27.4 ± 0.5 Ma, suggesting a late Oligocene anatexis event. These leucogranites are strongly peraluminous with A/CNK values of 1.01–1.19 and have high SiO2 (73.3–77.0 wt%) and K2O + Na2O (7.87–10.08 wt%), and low CaO (0.41–0.99 wt%), MgO (0.04–0.21 wt%), TiO2 (0.01–0.06 wt%) and Fe2O3T (0.44–0.86 wt%) contents. They are geochemically similar to Himalayan leucogranites and are enriched in large-ion lithophile elements (e.g. Rb), and depleted in high-field-strength elements (e.g. Nb, Zr, and Hf). They have low Zr/Hf (12.3–22.5) and Nb/Ta (2.54–12.73) with negative Eu anomalies (Eu/Eu* = 0.05–0.45), indicative of extensive fractional crystallization of plagioclase and zircon. In addition, these rocks have low 87Sr/86Sr (t) (0.7072–0.7111), high εNd(t) (−1.53 to −3.61). They contain zircon grains with εHf (−5 to +1.8) and old two-stage Hf model ages of 1.24–1.02 Ga, implying that they were probably derived from partial melting of amphibolite (old crustal material) within the lower crust. In combination with previous studies of metamorphism of the ASRR shear zone, our study demonstrates that the leucogranites may have been generated in a highly dynamic setting involving both extrusion and extension that defined the beginning of left-lateral slip-strike movement of the ASRR shear zone at 27 Ma.

The 2.73 Ga I-type granites in the Lengshui Complex and implications for the Neoarchean tectonic evolution of the Yangtze Craton

ABSTRACT We report the oldest I-type granites in the Lengshui Complex of the Yangtze Craton, providing new insights for its tectonic evolution during the Neoarchean. An approach-combined study of zircon U-Pb dating and Lu-Hf isotopes, as well as whole-rock element geochemistry and Nd isotopes, were employed. LA-ICP-MS zircon U-Pb dating for the monzogranite sample LSG03 and LSG16 yielded ages of 2732 ± 13 Ma and 2738 ± 25 Ma, respectively. The more precise age of 2732 ± 13 Ma for the sample LSG03 was taken as the crystallization age of the monzogranite. These rocks have high SiO2 (73.11–74.01 wt%), K2O (3.93–5.48 wt%), Na2O (3.93–4.86 wt%) and low CaO (0.30–0.69 wt%), MgO (0.17–0.30 wt%), TiO2 (0.14–0.17 wt%), P2O5 (0.01–0.06 wt%), Al2O3 (14.11–14.37 wt%) content with weakly peraluminous affinity (A/CNK = 1.04–1.11). Geochemically, they belong to I-type granites, indicating partial melting of a thickened lower crust. Their relatively high Nb/Ta (15.2–34.8) ratios further suggest they formed under eclogite-facies conditions. The consistent whole-rock Nd and zircon Hf isotopic compositions indicate a homogeneous source. According to their εHf(t) values (−2.0 to 0.8), two-stage Hf model ages (3.1 to 3.2 Ga) and positive εNd(t) (1.4 to 2.1), we argue that they were probably generated by partial melting of a juvenile lower crust with little ancient materials. Monzogranites formed in a late-orogenic or collisional compressive tectonic regime, whereas subsequent ca. 2.73 Ga and 2.67–2.62 Ga A-type granites in the Zhongxiang Uplift (including the Lengshui Complex) may represent a prolonged extensional setting. Thus, Archean subduction (probably unlike modern subduction) likely occurred prior to ca. 2.73 Ga. Similar magmatism in the Kongling Complex implies that the Zhongxiang Uplift may have accrete to the Kongling Complex during the early Neoarchean. The transition from I-type to A-type magmatism may have resulted from a change in the geodynamic regime from the late-orogenic or collisional compressive environment to an extensional environment caused by the subsequent lithospheric collapse and mantle upwelling, suggesting an early Neoarchean orogenic event in the eastern Yangtze Craton.

Petrogenesis and tectonic implications of granitoids from western North Altun, Northwest China

The Altun orogenic belt in northwest China is part of the northern margin of the Tibetan Plateau. The North Altun ophiolitic mélange belt is an important tectonic unit within the Altun orogenic belt that contains voluminous early Paleozoic granitoids. In this study, we report the petrological features, geochemical compositions, and zircon UPb and Hf isotope data of three granitic plutons from the western segment of the North Altun ophiolitic mélange belt. Zircon UPb dating yields magmatic crystallization ages of 499, 493 and 496 Ma for samples of granodiorite, quartz diorite and syenogranite, respectively. The granitoids have metaluminous to weakly peraluminous and medium-K to high-K calc–alkaline characteristics and display relative enrichments in large ion lithophile elements (Rb, Th, U, K) and light rare earth elements (LREE) and relative depletions in Nb, Ta, Sr, P and Ti, suggesting an arc-related origin. The granodiorites (499 Ma) have positive zircon εHf(t) values ranging from +1.87 to +6.59 with two-stage Hf model ages (TDMC) of 1.05 to 1.35 Ga, implying that the granodiorites were derived from juvenile crust. The quartz diorites (493 Ma) have similar Hf isotopic characteristics to the granodiorites (εHf(t) = +2.59 to +6.04, TDMC = 1.08 to 1.30 Ga), indicating derivation from juvenile crust. The syenogranites (496 Ma) have high total REE and K2O contents, and low zircon εHf(t) values (−1.69 to +1.54), suggesting that they were derived mainly from juvenile crust mixed with ancient crustal materials. Combined with data from previous studies, we conclude that magmatism in the North Altun ophiolitic mélange belt can be subdivided into three episodes: Episode 1 (520–470 Ma) granitoids are related to subduction; Episode 2 (460–425 Ma) granitoids formed in a continent–continent collisional setting; and Episode 3 (<420 Ma) granitoids are post-collisional granites. Our results are consistent with south-directed subduction of the North Altun oceanic lithosphere beneath the Central Altun Block during the early Paleozoic (520–460 Ma), which was followed by collision with the Dunhuang Block.

Neoproterozoic S-type granites in the western Jiangnan Orogenic Belt, South China: Implications for petrogenesis and geodynamic significance

ABSTRACT The Neoproterozoic Jiangnan Orogenic Belt in south China was formed by the amalgamation of the Yangtze and Cathaysia blocks, however, the timing of its formation is still debated. In this study, we present whole rock geochemistry, U-Pb zircon ages, Lu-Hf and Nd-isotope systematics on the Neoproterozoic Yexijiang and Baomuping granites occurring in the western segment of the Jiangnan Orogenic Belt. These granites mark the initiation of post-collision extension environment. Zircon U-Pb age data identify 807–805 Ma emplacement age for these granites. They have low and variable ΣREE concentrations (47–115 ppm) and show moderate to highly fractionated REE patterns ([La/Sm]N = 2.83–3.78) with low [Gd/Yb]N (1.34–2.31) and negative Eu anomalies (Eu* = 0.34–0.57). Geochemical features and presence of peraluminous minerals classify them as S-type granites and P-T parameters indicate low to moderate temperature (730–791 °C) and pressure (middle crustal level) conditions. These granites show consistent negative whole rock ɛNd(t) (−5.7 to −6.6) and magmatic zircon ɛHf(t) values from −0.3 to −3.8 while inherited zircon grains show variable ɛHf(t) values (+2.1 to −13.0) and two-stage Hf model ages (1.80 to 2.88 Ga). In particular, one Neoproterozoic (899 Ma) inherited zircon grain displays depleted Hf [ɛHf(t) = +5.4 and TDM2 = 1.34 Ga] isotopic characteristic. These lines of evidence indicate a sedimentary source for Yexijiang and Baomuping granites, possibly the Archean to Neoproterozoic crustal rocks, and emplacement in a post-collision tectonic environment triggered by mafic magma underplating during the amalgamation of the Yangtze and Cathaysia blocks. We infer that convergence in the Jiangnan Orogenic Belt occurred during 830–800 Ma therefore, unrelated to the global Grenvillian Orogeny.

Petrogenesis and geochronology of Paleoproterozoic magmatic rocks in the Kongling complex: Evidence for a collisional orogenic event in the Yangtze craton

The early Precambrian crustal evolution of the Yangtze craton, especially the Paleoproterozoic orogenic event, has drawn great attention. The early Precambrian magmatism in the Archean-Palaeoproterozoic basement Kongling complex is critical for understanding the early Precambrian tectonic evolution of the Yangtze craton. We report the presence of two distinct groups of Paleoproterozoic felsic rocks in the Kongling complex, providing new insights into the Paleoproterozoic orogenic event in the northern Yangtze craton. Group 1 is composed mainly of deformed or weakly-deformed monzogranite, syenite, and quartz monzonite, and formed at 2.0–1.95 Ga. They have high Sr/Y (60–71) and La/Ybcn (32–464) ratios and are inferred to have equilibrated with garnet-rich residues, suggesting that they have been generated by partial melting of thickened continental crust. Group 2 is represented mainly by undeformed to weakly-deformed K-feldspar granitic rocks, with minor rhyolites, and was emplaced at 1.87–1.84 Ga. In contrast to Group 1, Group 2 rocks display flat HREE patterns and large negative Eu, Sr and Ti anomalies, with low Sr/Y (<10) and La/Ybcn (16–21) ratios. Group 2 rocks have a typical A-type affinity with high Ga/Al ratios (2.83–3.82), and high crystallization temperatures (850–950 °C). Group 2 rocks are interpreted to have been derived from relatively shallow partial melting of thin continental crust during post-orogenic collapse, which marked lithospheric extension of the Yangtze craton in the late Paleoproterozoic (ca. 1.85 Ga).Zircons from Group 1 rocks have εHf (t) values ranging from −9.4 to −14.9, whereas zircons from Group 2 rocks have lower εHf (t) values ranging from −14.2 to −20.6. Both groups have similar two-stage Hf model ages ranging between 3.2 and 3.6 Ga. The production of the two groups of granitoids in the Huangling dome is ascribed to the reworking (dehydration melting) of Archean rocks in the Paleoproterozoic. Widespread Paleoproterozoic magmatism and metamorphism indicate that the Yangtze craton experienced an orogenic event at ca. 2.0 Ga and subsequent orogenic collapse caused by lithospheric extension at ca. 1.85 Ga.

Switch of geodynamic setting from the Paleo-Asian Ocean to the Mongol-Okhotsk Ocean: Evidence from granitoids in the Duobaoshan ore field, Heilongjiang Province, Northeast China

This paper presents new zircon U-Pb ages and whole-rock geochemical data for Triassic granitoids in the Duobaoshan ore field, in the eastern Central Asian Orogenic Belt (CAOB). Dating results reveal that the Triassic granitoids in the Duobaoshan ore field were emplaced ca. 244–223 Ma. These samples are typical subduction-related arc magmatic rocks that are enriched with large-ion lithophile elements (LILEs), depleted in high field strength elements (HFSEs), and have relatively high Sr (average 1036 ppm) and low Y (average 10 ppm) and Yb (average 1 ppm) contents that exhibit adakite-like geochemical properties. The samples feature a juvenile whole rock average initial 87Sr/86Sr ratio of 0.70394 and an average ɛNd(t), zircon ɛHf(t), and two-stage Hf model age of 4.62, 10.29, and 766 Ma respectively. These geochemical features reveal a thickened lower crust source. Comprehensive analyses of the early Paleozoic, Triassic, and early Jurassic granitoids in the Duobaoshan ore field indicate that the magmatism of these three stages was related to the melting of the juvenile lower continental crust materials. The early Paleozoic was the primary period of vertical crust growth, during which time the juvenile lower continental crust was formed. The Triassic granitoids derived from magma were generated by partial melting of the thickened juvenile lower crust and the Jurassic granitoids derived from the partial melting of the delaminated juvenile lower crust. Geological observation and tectonic setting analyses reveal that the Paleo-Asian Ocean tectonic regime in the Duobaoshan ore field switched to the Mongol-Okhotsk Ocean in the Mesozoic after its closure. The early Paleozoic granitoids were formed in a subduction setting related to the northwestward subducted Paleo-Asian Oceanic slab underlying the Erguna-Xing'an composite block. The Triassic granitoids were formed under a supra-subduction extensional regime related to the southward subduction of the Mongol-Okhotsk Ocean, and the Jurassic granitoids were generated under a slab rollback setting related to the late stage subduction of the Mongol-Okhotsk Ocean.

Implication of Mesoproterozoic (∼1.4 Ga) magmatism within microcontinents along the southern Central Asian Orogenic Belt

The southern Central Asian Orogenic Belt contains numerous microcontinents including the Kazakhstan-Yili block, Central Tianshan Arc and Junggar block in the west, the Erguna, Xing'an, Songliao and Jiamusi-Khanka blocks in the east, bridging by the Tuva-Mongolia block, Beishan Orogenic Belt and Alax block in the central region. New geochemical and geochronological data from the microcontinental blocks within the southern Beishan Orogenic Belt provide new insight into the Precambrian crustal evolution of this region. Five granitoids from the Jiujing region yielded igneous zircon 206Pb/238U ages of 1450–1401 Ma, and zircon xenocrysts from late Paleozoic plutons in the Jiujing, Qiaowan, and Shibandun regions have 206Pb/238U ages that also cluster at ∼1.4 Ga. Those data suggest that crust of this age is widespread beneath the southern Beishan Orogenic Belt. The ∼1.4 Ga zircons display depleted Hf isotopic signatures (εHf(t) = 2.0–11.1) and show relatively young two-stage Hf model ages (1.9–1.4 Ga), indicating rapid reworking of juvenile mantle-derived materials. Integrating these results with similar geochronological data from the Kyrgyz North Tianshan, Yili block, Central Tianshan Arc, northern Alxa block and Xilinhot block, we propose that the CAOB incorporates an extensive assemblage of Mesoproterozoic (1450–1360 Ma) magmatism that can be traced over ca. 3000-km long a west-east trending belt. These Mesoproterozoic granitoids display distinct geochemical signatures (e.g., Ga/Al*10,000 ranging from 1.46 to 3.88 and Zr + Nb + Ce + Y varying largely from 101 to 1066 ppm) and show a temporal progression from orogenic to anorogenic over a relatively short period. This suggests a tectonic setting that likely evolved from compressional to extensional environments during 1450–1360 Ma, perhaps resulting from slab roll back. We suggest that the 1450–1360 Ma magmatism along the southern CAOB corresponds with a transition from a compressional to extensional environment on the margin of the Nuna supercontinent.

The Laqiong Sb-Au deposit: Implications for polymetallic mineral systems in the Tethys-Himalayan zone of southern Tibet, China

The Himalayan mineral field includes over 50 quartz-vein type Sb-Au deposits, and placer Au deposits. The poorly documented Laqiong deposit is a typical example of quartz-vein type Sb-Au mineralisation in Tethys Himalayan sequence. The orebody are controlled by shallow north-dipping normal faults and north–south trending faults. Magmatic zircons extracted from muscovitic leucocratic granite from the southern part of the Laqiong mine area yield a Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry U-Pb age of 14 ± 1 Ma (n = 12, MSWD = 0.9) that is similar to the 40Ar/39Ar age of ca. 14 Ma from hydrothermal sericite in auriferous sulphide-quartz veins. The εHf(t) values for the magmatic zircon rims range from −5.4 to −1.9, corresponding to two-stage Hf model ages of 1403–1214 Ma. Quartz from the mineralised veins has δ18OH2O-SMOW values varying from +4.97 to +9.59‰ and δDH2O-SMOW values ranging from −119.7 to −108.1‰. The δ13CV-PDB values for calcite from the ore Stage III range from −6.9 to −5.3‰, and calcite from Stage IV are −3.5 to −1.7‰. The δ18OV-SMOW values for calcite from Stage III are +20.3 to +20.6‰ and for Stage IV are −6.3 to −4.9‰. The stibnite and pyrite samples have 208Pb/204Pb ratios of 38.158 to 39.02, 207Pb/204Pb ratios of 15.554 to 15.698, and 206Pb/204Pb ratios of 17.819 to 18.681, and bulk and in-situ δ34SV-CDT values for stibnite, arsenopyrite and pyrite range from −1.1 to +2.3‰. The calcite from the orebodies are enriched in MREE and depleted in LREE and HREE. Fieldwork, petrological, and geochemical data collected during our study leads to the following salient findings: the mineralising fluid is a mix of magmatic and meteoric fluids; and the deposit is closely related to the emplacement of Miocene granites originating from a thickened continental crust.

Geochemistry of Late Mesozoic volcanic rocks in the central Great Xing’an Range, NE China: petrogenesis and crustal growth in comparison with adjacent areas

ABSTRACTThe central Great Xing’an Range contains widespread exposures of Mesozoic igneous rocks, yet the origin of its Late Mesozoic volcanic rocks has long been controversial. They are composed of andesite, trachyte, trachyandesite, trachydacite, rhyolite, and minor basaltic andesite. The intermediate rocks are enriched in large ion lithophiles and light rare earth elements (LILEs and LREEs) and are depleted in high field strength elements (HFSEs) such as Nb, Ta, and Ti, with positive εHf(t) values (+6.2 to +13.4) and Hf single-stage model ages (TDM1 = 556–227 Ma). This indicates that they were derived from the partial melting of a mantle wedge that was modified by previously subducted slab-derived fluids was likely introduced with limited crustal contamination. The felsic rocks show positive εHf(t) values (+4.3 to +10.2) and Hf two-stage model ages (TDM2 = 910–544 Ma), with more enrichment of LILEs and LREEs and more depletion of HFSEs and negative Eu anomalies. This indicates that felsic rocks were mainly derived from partial melting of Neoproterozoic accreted quartz-amphibolite facies mafic crust. We summarise the geochronological and Hf-Nd isotope data for Mesozoic igneous rocks within the central Great Xing’an Range (CGXR), the northern Xing’an block (NXB), and the Erguna block (EB), with the aim of constraining the geochronological framework for Mesozoic volcanism and comparing the growth of continental crust. The evolution of Late Mesozoic magmatism in the three areas reached its climax ~125 Ma ago. Our data, together with regional isotope evidence, suggest that crustal accretion of the CGXR occurred during the Neoproterozoic–Phanerozoic. However, that of the EB occurred during the Meso- to Neoproterozoic. The EB exhibits more lateral and vertical heterogeneities and ancient continental crust than the CGXR and NXB in the northern magma source region.