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.     

Age and origin of sillimanite schist from the Chinese Altai metamorphic belt: implications for late Palaeozoic tectonic evolution of the Central Asian Orogenic Belt

Reconstructing late Palaeozoic metamorphism of the Central Asian Orogenic Belt (CAOB) can provide a better understanding of how the CAOB formed. The petrology of sillimanite-bearing metapelitic schists from high-grade portions of the Permian Chinese Altai metamorphic belt (andalusite-type) reflects the effects of poorly understood high-T, low-P metamorphism. Phase equilibria modelling in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–TiO₂–O (NCKFMASHTO) system restricts P–T conditions of the sillimanite schists to approximately 635–670°C at approximately 5.8–6.8 kbar. SHRIMP U–Pb analyses of zircon from the rocks yield a concordant age of 299.2 ± 3.4 Ma. Combined with the slightly younger (292.8 ± 2.3 Ma) areally restricted pelitic granulite with peak P?T conditions of approximately 780–800°C at approximately 5–6 kbar and high-T granulite with P?T conditions of approximately 860°C at approximately 6 kbar, these metamorphic rocks reflect prograde heating at relatively low pressure in early Permian time. Together with contemporary and widespread magmatic activities, they are best explained in the context of a post-orogenic extensional environment related to a mantle plume.     

Timing of the initiation of the Jurassic Yanshan movement on the North China Craton: evidence from sedimentary cycles,heavy minerals,geochemistry, and zircon U–Pb geochronology

A series of significant geological changes indicated by deformation, magmatic–metallogenic systems, and the climate and environment occurred in East Asia during Late Jurassic to Early Cretaceous time, but the timing and development of the ‘Yanshan movement’ on the north margin of the North China Craton has not been well-established. Based on the evidence of tectonic deformation and magmatic activity, previous studies resulted in two views of the beginning of the Yanshan movement: Early Jurassic vs. late Middle Jurassic. In this work, the timing of the initial Yanshan movement was investigated by examining the Jurassic Chenjiabangou section in the Ningwu–Jingle basin overlying the north-central part of the North China Craton. The timing of the initial Yanshan movement was constrained by restoration of stream flow directions, determination of boundaries of sedimentary cycles, identification of heavy mineral assemblages in clastic rocks, quantification of changes in chemical compositions, and zircon U–Pb isotope dating. The results indicate that the basal conglomerates of the Middle Jurassic Yungang Formation (Bathonian) mark the beginning of the Yanshan movements. Evidence supporting this conclusion includes the following. (1) The switch from transgressive lacustrine deposition to regressive lacustrine deposition in the Yungang Formation sedimentary succession indicates a change from extension to compression, possibly reflecting uplift. (2) Early-stage clastic rocks rich in quartz and feldspar are replaced by feldspar detritus in late-stage clastic rocks; the heavy mineral assemblage dominated by zircon at the early stages changed to garnet-dominated assemblage upsection. Moreover, the concentrations of CaO, MgO, CO_2, and Fe₂O₃ + FeO and the Fe₂O₃/FeO ratio changed abruptly near the basal conglomerates of the Middle Jurassic Yungang Formation, suggesting increased denudation. (3) Conglomerates at the bottom of the Middle Jurassic Yungang Formation were deposited approximately 168 million years ago, as inferred from the age of zircons in tuffaceous micrite (160.6 ± 0.55 Ma) at the bottom of the Upper Jurassic Tianchihe Formation (Oxfordian) and the age of zircons in pyroclastic rocks (179.2 ± 0.79 Ma) in the Lower Jurassic Yongdingzhuang Formation (Toarcian). These lines of evidence indicate that initial Jurassic Yanshan movement began 168 million years ago during Middle Jurassic time.     

Petrogenesis of the Wudaogou intermediate–mafic complex,NW China: zircon U–Pb dating,whole-rock and isotope geochemistry,and geological implications

The Wudaogou plutonic complex is located in the eastern Yanbian area of Jilin Province and consists of hornblende gabbros, gabbroic diorites, and quartz diorites that contain abundant dioritic microgranular xenoliths. Zircon U–Pb dating of gabbroic and quartz diorites yielded weighted mean ²⁰⁶Pb/²³⁸Pb ages of 263.5 ± 5.1 Ma (N = 12, mean squared weighted deviation (MSWD) = 0.78, probability = 0.66) and 262.0 ± 5.6 Ma (N = 10, MSWD = 0.50, probability = 0.87), respectively. These units are characterized by high Na₂O/K₂O (0.33–0.77) ratios and Al₂O₃ (15.05–18.91 wt%) concentrations and are large ion lithophile element (LILE) (light rare earth element (LREE), Rb, Ba, K, etc.) enriched and high field strength element (HFSE) (Nb, Ta, P, Ti) depleted. They also have initial ⁸⁷Sr/⁸⁶Sr values of 0.70192–0.70420 and ?_Nd(t) values of +1.9 to +4.7 with two-stage model ages (T_DM2) of 653–878 Ma. These characteristics indicate that these rocks formed from calc–alkaline magmas derived from partial melting of a mixture of juvenile crust formed attending the Neoproterozoic subduction of the Palaeo-Asian oceanic crust and lower crustal material. The dioritic xenoliths have whole-rock compositions that are similar to their host rocks, but with negative ?_Nd(t) values (?1.6 to ?4.3) and older T_DM2 ages (1166–1382 Ma), further indicating that this magmatic event involved older crustal material. Combining these data with existing knowledge of the crustal evolution of this area, we conclude that this complex formed in a post-collisional extensional setting during closure of the Palaeo-Asian Ocean.     

Ore genesis of the Tianbaoshan carbonate-hosted Pb–Zn deposit,Southwest China: geologic and isotopic (C–H–O–S–Pb) evidence

The Tianbaoshan Pb–Zn deposit, part of the Sichuan–Yunnan–Guizhou (SYG) Pb–Zn metallogenic province, is located in the western Yangtze Block and contains 2.6 million tonnes of 10–15 wt.% Pb + Zn metals. Ore bodies occur as vein or tubular types and are hosted in Sinian (late Proterozoic) carbonate rocks and are structurally controlled by the SN-trending Anninghe tectonic belt and NW-trending concealed fractures. The deposits are simple in mineralogy, with sphalerite, galena, pyrite, chalcopyrite, arsenopyrite, freibergite, and pyrargyrite as ore minerals and dolomite, calcite, and quartz as gangue minerals. These phases occur as massive, brecciated, veinlet, and dissemination in dolostone of the upper Sinian Dengying Formation. Hydrogen and oxygen isotope compositions of hydrothermal fluids range from –47.6 to –51.2‰ and –1.7 to +3.7‰, respectively. These data suggest that H₂O in hydrothermal fluids had a mixed origin of metamorphic and meteoric waters. Carbon and oxygen isotope compositions range from –6.5 to –4.9‰ and +19.3 to +20.2‰, respectively. These compositions plot in the field between mantle and marine carbonate rocks with a negative correlation, suggesting that CO₂ in the ore-forming fluids had multiple sources, including the Permian Emeishan flood basalts, Sinian-to-Permian marine carbonate rocks, and organic matters in Cambrian-to-Permian sedimentary rocks. Sulphur isotope compositions range from –0.4 to +9.6‰, significantly lower than Cambrian-to-Permian seawater sulphate (+15 to +35‰) and sulphate (+15 to +28‰) from evaporates in Cambrian-to-Permian strata, implicating that the S was derived from host-strata evaporates by thermal–chemical sulphate reduction. ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios range from 18.110 to 18.596, 15.514 to 15.878, and 38.032 to 39.221, respectively, which plot in field of the upper crust Pb evolution curve, unlike those of Proterozoic basement rocks, Sinian dolostone, Devonian-to-Permian carbonate rocks, and the Permian Emeishan flood basalts, implying complex derivation of Pb metal in the ore-forming fluids. Geological and isotopic studies of the Tianbaoshan Pb–Zn deposit reveal that constituents in the hydrothermal fluids were derived from multiple sources and that fluid mixing was a possible metallogenic mechanism. The studied deposit is not distal magmatic–hydrothermal, sedimentary exhalative (SEDEX), or Mississippi Valley (MVT) types, rather, it represents a unique ore deposit type, named in this article the SYG type.     

Triassic magmatism and Mo mineralization in Northeast China: geochronological and isotopic constraints from the Laojiagou porphyry Mo deposit

The Laojiagou Mo deposit is a newly discovered porphyry Mo deposit located in the Xilamulun Mo metallogenic belt, Northeast China. Mo mineralization mainly occurred within the monzogranite and monzogranite porphyry. Re–Os isochron dating of molybdenites indicate a mineralization age of 234.9 ± 3.1 Ma. Zircon LA–ICP–MS U–Pb analysis for monzogranite porphyry and monzogranite yield ²⁰⁶Pb/²³⁸U ages of 238.6 ± 1.8 and 241.3 ± 1.5 Ma, respectively, indicating that Laojiagou Mo mineralization is related to Middle Triassic magmatism. Hf isotopic compositions of zircons from both monzogranite porphyry and monzogranite are characterized by positive ε_Hf(t) values [ε_Hf(t) = 2.9–7.3 and 1.5–7.9, respectively] and young T_DM2 model ages, which implies that the magma was derived from juvenile crust created during accretion of the Central Asian Orogenic Belt (CAOB). Identification of the Laojiagou Mo deposit adds another important example of Triassic Mo mineralization in the Xilamulun Mo metallogenic belt where most Triassic Mo deposits in northeast China cluster around the northern margin of North China Craton. Based on the regional geological setting and geochronological and Hf isotope characteristics, we propose that Triassic Mo deposits and related magmatic rocks in northeast China formed during the last stages of evolution of the CAOB. These deposits formed during post-collisional extension after the closure of the Palaeo-Asian Ocean and amalgamation of the North China–Mongolian Block with the Siberian Craton.     

Late Palaeozoic and early Mesozoic tectonic and palaeogeographic evolution of central China: evidence from U–Pb and Lu–Hf isotope systematics of detrital zircons from the western Qinling region

The western Qinling region of central China is situated centrally in the Kunlun, Qilian, Qinling, Longmenshan, and Songpan–Ganzi orogens. Late Palaeozoic and Early Mesozoic sediments deposited here may provide keys to understanding the tectonic evolution of the Palaeo-Tethys and collision of the North China and Yangtze Cratons. We conducted in situ U–Pb and Lu–Hf isotope analyses of 568 detrital zircons collected from Upper Palaeozoic to Mesozoic sandstones in the central Qinling block, Taohe depression, and Bailongjiang block in western Qinling to constrain the sources of these sandstones. Our results reveal that the Bailongjiang block has affinities with the Yangtze Craton, from which it may have been rifted. Therefore, the Palaeo-Tethyan Animaqen suture between the two cratons lies north of the Bailongjiang block. We identified the North China Craton as the main source for Triassic flysch in central China. It is possible that the Bailongjiang block could have blocked detritus shed from the North China Craton into the main depositional basins in the Songpan–Ganzi area. The dominance of 300–200 Ma detrital zircons of metamorphic origin in Lower Jurassic sandstones indicates that the Dabie–Qinling orogen was elevated during Early Jurassic time. In addition, our Lu–Hf isotopic results also reveal that Phanerozoic igneous rocks in central China were mostly products of crustal reworking with insignificant formation of juvenile crust.     

Palaeoproterozoic A-type magmatism in northern Wuyishan terrane,Southeast China: petrogenesis and tectonic implications

ABSTRACT

In this article we present zircon U–Pb ages, Hf isotopes, and whole-rock geochemistry of the Longzhu rhyolite porphyry from the Cathaysia Block, Southeast China to constrain its petrogenesis and provide insights into the early Precambrian tectonic evolution of the Cathaysia Block. LA-ICP-MS zircon U–Pb dating of a representative sample yields a weighted mean ²⁰⁶Pb/²⁰⁷Pb age of 1819 ± 16 Ma, interpreted as the crystallization age of the Longzhu rhyolite porphyry. Zircons from this sample have ε_Hf(t) values ranging from – 8.4 to – 2.2 and T^Hf_DM2 model ages from 2.76 to 2.46 Ga. The whole-rock Nd isotopic data from the Longzhu rhyolite porphyries yield ε_Nd(t) values spanning – 6.3 to – 4.7 and T^Nd_DM2 model ages from 2.81 to 2.69 Ga. The rhyolite porphyries have geochemical features similar to those of the typical A-type granites (rhyolites), with high SiO₂, total alkali contents and FeO^t/MgO ratios, and low CaO and MgO contents. Additionally, the rhyolite porphyries have high total rare earth element concentrations (627 ~ 760 ppm), high (La/Yb)_N values (14.5 ~ 26.9), strongly negative Eu anomalies (δEu = 0.28 ~ 0.41), and display enrichments of Rb, Ga, Th, and U and depletions of Sr, Nb, Ta, Eu, and Ti. The geochemical and Nd-Hf isotopic features suggest that the Palaeoproterozoic Longzhu rhyolite porphyries were generated by partial melting of source rocks similar to those of the Badu Complex in an intra-plate extensional setting. The results from this study, when combined with existing geochronological data, further demonstrate that the Palaeoproterozoic rocks of Wuyishan terrane probably represent a remnant of the Columbia supercontinent.     

Petrochronology of the migmatization event of the Xolapa Complex,Mexico, microchemistry and equilibrium growth of zircon and garnet

ABSTRACT

The number of migmatization events in the Xolapa Complex and their absolute age are controversial. U–Pb dating by laser ablation–inductively coupled plasma–mass spectrometry was performed on zircon grains from migmatites to investigate the age of different textural domains. Rare-earth element (REE) partition coefficients between zircon and garnet were compared with those established for different temperatures in order to test for equilibrium growth. Two age domains were identified. In one sample where zircon and garnet coexist, the outer zircon overgrowths yield a mean age of 54.16 ± 0.29 Ma (mean square weighted deviation (MSWD) = 3.5), whereas intermediate zones, between the core and outer overgrowths, yield an age of 122.7 ± 1.8 Ma (MSWD = 2.5). Partition coefficients were calculated for REEs between coexisting garnet (two different populations) and zircon using (1) the composition of ca. 54 Ma zircon overgrowths and garnet rims and (2) zircon intermediate zones together with garnet cores. The cores of small garnet grains (garnet A) may have grown in equilibrium with zircon domains of ca. 122 Ma. Both garnet cores and rims of the larger porphyroblasts (garnet B) seem to be in equilibrium with ca. 54 Ma zircon overgrowths. Petrographic observations suggest that crystallization of garnet A occurred during partial melting, placing equilibrium growth and therefore a first migmatitic event during the Early Cretaceous at ca. 122 Ma. This migmatitic event may be related to the collision of the Chortís Block with western Mexico. A second migmatitic event of ca. 54 Ma is suggested by equilibrium growth of large garnets (group B) and the outer zircon overgrowths. The high geothermal gradient necessary for this second migmatitic event might be related to the exhumation of the Xolapa Complex, as a result of the transpression and tectonic transport of the Chortís Block to the southeast from the end of the Mesozoic to most of the Cenozoic.     

Ordovician volcano–sedimentary iron deposits of the Eastern Tianshan area,Northwest China: the Tianhu example

ABSTRACT

The stratabound Tianhu iron deposit, with a reserve of 104 Mt at 42% Fe, is located in the eastern part of the Central Tianshan zone in the southern part of the Central Asian Orogenic Belt. The deposit hosts schist, quartzite, marble, amphibolite, and granitic gneiss belonging to the Tianhu Group. Laser ablation inductively coupled plasma mass spectrometry was used to perform zircon U–Pb geochronology, bulk-rock geochemistry, and in situ zircon Hf isotope analyses of the metavolcanic host rocks for constraining the timing and genesis of the Tianhu iron deposit. According to the newly determined age constraints of 452 ± 3 and 477 ± 4 Ma, the iron deposit was concluded to be Ordovician in age. Geochemistry and zircon Lu–Hf isotope analyses suggested that the host rocks of the deposit represent metamorphosed arc-type volcanic rocks generated by the partial melting of a lower crustal source. Combined with geological and ore petrographic characteristics, the Tianhu iron deposit is interpreted to be of volcano–sedimentary origin with enrichment during subsequent metamorphism. The early Palaeozoic marks a critical iron mineralization epoch in the Eastern Tianshan area. The results also support the model of the Central Tianshan area as a volcanic-arc during the early Palaeozoic, associated with the subduction of the Northern Tianshan Ocean.