Western Qinling Orogen Research Articles

Mineral Chemistry and In Situ LA-ICP-MS Titanite U-Pb Geochronology of the Changba-Lijiagou Giant Pb-Zn Deposit, Western Qinling Orogen: Implications for a Distal Skarn Ore Formation

The giant Changba-Lijiagou (Ch-L) Pb-Zn deposit is in the northeast part of the Xicheng ore cluster, Western Qinling Orogen. The ore genesis remains controversial; it could be either a sedimentary exhalative genetic type or an epigenetic hydrothermal genetic type. Here, in situ titanite U-Pb dating for the two kinds of titanite is presented, yielding ages of 212.8 ± 3.0 Ma in the mineralized skarn ore and 214.6 ± 5.1 Ma in the host rock. These ages conform to the previously reported magmatic zircon age (229–211 Ma) based on the in situ zircon U-Pb dating of plutons in this district and the time of large-scale magmatic–hydrothermal activities in Western Qinling Orogen (229–209 Ma). Titanites occurring in mineralized skarn and those that are calcite-hosted are similar to hydrothermal-origin titanites in major element characteristics. The Eu anomalies in the two types of titanite record oxidizing conditions during the mineralization process. A mineral assemblage of garnet, pyroxene, riebeckite, biotite, and potash feldspar, replacing the albite, is well-developed in the deposit. The mineralogical and geochronological characteristics indicate that the Ch-L Pb-Zn deposit is a distal skarn deposit and the result of intensive tectonomagmatic processes in the Xicheng ore cluster during the process of the Late Triassic orogeny.

Gold Mineralization at the Syenite-Hosted Anwangshan Gold Deposit, Western Qinling Orogen, Central China

The Anwangshan gold deposit is located in the northwestern part of the Fengtai Basin, Western Qinling Orogen (WQO). The gold ore is hosted within quartz syenite and its contact zone. The U–Pb weighted mean age of the quartz syenite is 231 ± 1.8 Ma. It is characterized by high potassium (K2O = 10.13%, K2O/Na2O > 1) and high magnesium (Mg# = 55.31 to 72.78) content, enriched in large ion lithophile elements (Th, U, and Ba) and light rare earth elements (LREE), with a typical “TNT” (Ti, Nb, and Ta) deficiency. The geochemical features and Hf isotope compositions (εHf(t) = −6.68 to +2.25) suggest that the quartz syenite would form from partial melting of an enriched lithospheric mantle under an extensional setting. Three generations of gold mineralization have been identified, including the quartz–sericite–pyrite (Py1) stage I, the quartz–pyrite (Py2)–polymetallic sulfide–early calcite stage II, and the epidote–late calcite stage III. In situ sulfur isotope analysis of pyrite shows that Py1 (δ34S = −1.1 to +3.8‰) possesses mantle sulfur characteristics. However, Py2 has totally different δ34S (+5.1 to +6.7‰), which lies between the typical orogenic gold deposits in the WQO (δ34S = +8 to +12‰) and mantle sulfur. This suggests a mixed source of metamorphosed sediments and magmatic sulfur during stage II gold mineralization. The fluid inclusions in auriferous quartz have three different types, including the liquid-rich phase type, pure (gas or liquid)-phase type, and daughter-minerals-bearing phase type. Multiple-stage fluid inclusions indicate that the ore fluids are medium-temperature (concentrated at 220 to 270 °C), medium-salinity (7.85 to 13.80% NaCleq) CO2–H2O–NaCl systems. The salinity is quite different from typical orogenic gold deposits in WQO and worldwide, and this is more likely to be a mixture of magmatic and metamorphic fluids as well. In summary, the quartz syenite should have not only a spatio-temporal but also a genetical relationship with the Anwangshan gold deposit. It could provide most of the gold and ore fluids at the first stage, with metamorphic fluids and/or gold joining in during the later stages.

High-resolution 3-D lithospheric structure beneath the Qinling-Dabie orogenic belt from joint inversion of receiver functions and ambient noise

Resulting from the convergence of the Yangtze and North China Cratons, the Qinling-Dabie orogenic zone (QD) represents an important element in the central China orogenic system. To fully comprehend the craton evolution and lower crustal flow from the Tibetan Plateau, it is important to understand the crust and mantle structure of the QD. We reconstructed the three-dimensional lithospheric structure beneath the QD with high resolution using the joint inversion of receiver functions and ambient noise. Observations reveal that a high-velocity anomaly in the middle to lower crust beneath the western Qinling (WQL) orogen obstructs the eastward extension of a crustal low-velocity anomaly originating from the Tibetan Plateau. This finding provides unambiguous evidence that the WQL orogen is not crossed by eastward lower crustal flow from the Tibetan Plateau. The lithospheric mantle beneath the Weihe Rift and East Qinling orogen exhibits low-velocity characteristics, indicating that eastward asthenospheric flow from the Tibetan Plateau has caused substantial thermal-chemical erosion in the uppermost mantle beneath these regions. The results additionally indicate that the uppermost mantle high-velocity anomalies beneath the Dabie orogen is confined in a limited area and extends only to a depth of 70 km. We propose that during the Triassic, deeply subducted continental lithosphere returned into the uppermost mantle, forming the high-velocity anomalies beneath the Dabie orogen.

Mineral Assemblages and Ore-Forming Physicochemical Conditions of the La’erma and Qiongmo Au–Se Deposits in the Western Qinling Orogen, Central China

Mineral Assemblages and Ore-Forming Physicochemical Conditions of the La’erma and Qiongmo Au–Se Deposits in the Western Qinling Orogen, Central China

Enrichment of Coal-Hosted Graphite Deposits Caused by Magmatic Heat Transfer and Tectonic Stress at Feng County, Western Qinling Orogen, China

China has ranked first worldwide in graphite imports in recent years, facing a graphite supply risk. Coal-hosted graphite is the focus of future graphite deposit exploration. The current research on the enrichment and mineralization mechanism of coal-hosted graphite is superficial, and the identification standard of coal-hosted graphite is incomprehensive, restricting the exploration of coal-hosted graphite mineral resources and the development of coal metamorphic evolution theory. In this study, the Caotangou–Meigoucoal-hosted graphite deposit in western Qinling Mountain was taken as a case study for dissection. Based on the data from 1/50,000 and 1/200,000 regional geological mapping and the data of graphite mines in the study area, the samples were systematically collected and analyzed to explore the mechanism of coal graphitization through a 1:5000 geological profile survey, 1/10,000 geological mapping in key areas, and the investigation and cataloguing of abandoned coal-hosted graphite adit. The result was that there were two main coal-hosted graphite ore bodies, striking from nearly east to west. The Rmax values of the samples were 7.23–8.15%, the average values of Vdaf were around 5.0%, the d002 value of the II ore body was 0.3433–0.3389 nm, the d002 value of the I ore body was mainly 0.3418–0.3429 nm, the graphitization degree G value of the II ore body was 8.14–59.30%, the graphitization degree G value of the II ore body was 12.79–25.58%. The II ore body was coal-hosted graphite, while some samples of the I ore body were coal-hosted graphite, and some samples were coal. The magmatic heat controls the thermal metamorphism of coal seams to form graphite. The closer the distance to the magma body, the larger the crystals, and the higher the euhedral degree, indicating the higher degree of coal seam metamorphism. The nearly north–south compressive structures mainly provided effective tectonic stress for the evolution of coal graphitization during the Yanshan period; the basic structural units (BSUs) rotated and rearranged, eventually forming a straight graphite structure, and tectonic stress catalyzed the graphitization process. The coal-hosted graphite deposits formed under the dual effects of magmatic heat transfer and tectonic stress.

Different gravity anomaly, crustal structure, and uplift mechanism of the eastern and western Qinling Orogen revealed by full coverage gravimetry

Different gravity anomaly, crustal structure, and uplift mechanism of the eastern and western Qinling Orogen revealed by full coverage gravimetry

Magnesium and Calcium Isotopic Geochemistry of Silica‐Undersaturated Alkaline Basalts: Applications for Tracing Recycled Carbon

AbstractCarbon plays important roles in the evolution of the atmosphere and biosphere, and geochemical differentiation in Earth's interior. Most subducted C is recycled to the deep mantle and returns to the surface by degassing from erupted basalts and associated fluids. The Mg–Ca isotopic systems have been widely used in tracing recycled C. However, the idea that these geochemical proxies truly reflect the deep C cycle has been challenged. Here we present whole‐rock geochemical and Mg–Ca isotopic compositions of Miocene silica‐undersaturated alkaline basalts of the Western Qinling orogen, China. These alkaline basalts are associated with carbonatites and are characterized by low SiO2 (39.0–43.2 wt.%) and Al2O3 (6.98–9.15 wt.%) contents, high CaO/Al2O3 ratios (1.4–1.8), and positive Nb–Ta and negative Pb–Zr–Hf–Ti anomalies, suggesting they were derived by partial melting of carbonatite‐metasomatized asthenospheric mantle. The studied samples have δ26Mg values of −0.24‰ to −0.44‰, ranging from mantle‐like values (δ26Mg = −0.25‰ ± 0.07‰) to lower values. This implies that carbonatite metasomatism does not always produce low‐δ26Mg anomalies. The samples have δ44/40Ca values (0.59‰–0.77‰; relative to the standard SRM915a) that are lower than Bulk Silicate Earth (0.94‰ ± 0.05‰), which are attributed to the involvement of low δ44/40Ca recycled carbonate in the mantle source. We suggest that the shift in mantle δ26Mg values during carbonatite metasomatism is controlled by the type and amount of carbonatite involved, while Ca isotope variations depend largely on the δ44/40Ca values of subducted carbonates. Mg or Ca isotopes alone, however, may not be sufficient to track the deep carbon cycle.

Triassic granites in the West Qinling Orogen, China: implications for the Early Mesozoic tectonic evolution of the Paleo-Tethys ocean

ABSTRACT The Western Qinling Orogen is marked by secular Triassic magmatism, a thorough understanding of the mechanism of which could provide critical insights into the reconstruction of the Palaeo-Tethys tectono-magmatic activity. Here, we present new geochemical, geochronological and Lu-Hf isotopic data for three batholiths and review regional data. These batholiths have similar rock assemblages (mainly granodiorite and monzogranite), which are characterized by dominantly plagioclase, quartz, and K-feldspar, subsequently amphibole and biotite. Zircon U-Pb data reveal that they represent the long-duration magmatism during the Triassic (ca. 251–223 Ma). Mineralogical characteristics and geochemical affinities manifest that these rocks in the Zeku district can be explicitly delineated as high-K, calc-alkaline, weakly fractionated I-type granites. They yield significantly negative εHf(t) values ranging from −12.81 to −1.26 with old two-stage mantle depleted model ages between 2054 and 1319 Ma. In tandem with Th/Nb (0.98), Th/La (0.39) and La/Nb (2.40) ratios and moderate pressure conditions (ca. 7–10 kbar), the studied granites were derived from partial melting of the Middle Paleoproterozoic to Middle Mesoproterozoic lower crust source region with minor mafic hydrous magma addition. Our new understandings, in conjunction with the temporal-spatial distribution characteristics of magmatism in the West Qinling Orogen, as well as the regional tectonic evolution, suggest that the superimposed orogeny evolved from the northward subduction of the Palaeo-Tethys Ocean (264–225 Ma) through syn-collision (225–215 Ma) to post-collision (beginning at ca.215 Ma) between the North China Craton and South China Block. In this scenario, at the convergent continental margin, the oceanic plate may have undergone melting producing the melts parental to the Zeku granites.

Syn-Tectonic Dolomite U-Pb Geochronology Constraining Intracontinental Deformation: A Case Study from the Gelouang Gold Deposit in the Qinling Orogen, China

Determining absolute ages of orogenic faults is critical to understanding the deformation process in the upper crust, but obtaining age remains a problem due to the lack of readily available techniques. Carbonates occur as veins in faults in a range of geological settings, and thus it is a suitable mineral for U-Pb geochronology. Here, we apply the new approach of U-Pb dating on syn-tectonic dolomite veins from the Gelouang gold deposit in the western Qinling Orogen to unravel the absolute timing of the fault formation shedding new light on the regional upper crustal deformation archive. In situ LA-ICP-MS U-Pb dating of dolomite yielding a successful age of 115–112 Ma demonstrates that the dolomite precipitated coeval with tectonic events ascribed to the post-orogenic deformation phase in the Qinling Orogen. This event is possibly correlated with broader intracontinental processes and might be an inevitable response to the extensional deformation of the Qinling Orogen. The presented LA-ICP-MS dolomite U-Pb age successfully represents the age of a specific structure that encompasses the intracontinental process in the Qinling Orogen. Moreover, it demonstrates the utility of the method to decipher a response to complex deformation histories on a regional scale.

Crustal structure beneath the northeastern margin of the Tibetan plateau and its surrounding regions revealed by direct surface wave radial anisotropy tomography

We assemble the continuous waveform data recorded by 571 broadband temporary seismic stations from 2014 to 2015 in the northeastern (NE) margin of the Tibetan plateau and its surrounding regions. Rayleigh and Love wave phase velocity dispersion curves (T = 5–50 s) are obtained by the image transformation technique. Then the phase velocity dispersion data are used to jointly invert the 3-D shear wave velocity (Vs) and radial anisotropy (ξ) models in the study region with the direct surface wave radial anisotropy tomography (DRadiSurfTomo) method. The ξ is negative and positive in the upper crust and mid-to-lower crust, respectively, indicating that the crustal deformation pattern is depth-dependent. The middle crust beneath the NE margin of the Tibetan plateau generally exhibits positive ξ. Two low-velocity zones (LVZs) are detected beneath the Songpan-Garzê block and the western Qinling orogen as well as the northern Qilian orogen. High-velocity zone (HVZ) and positive ξ are characterized in the mid-to-lower crust beneath both the Alxa block and the Ordos basin. Combining with the previous results, we propose these two blocks, as a whole belonging to a part of the North China Craton (NCC), effectively restrict the northeastward growth of the Tibetan plateau. The uneven crustal deformation along the boundary between the NE margin of the Tibetan plateau and its surrounding regions is likely to be affected by the far-field stress originating from the collision between the Indian and Eurasian plates.

Petrological and geochemical constraints on the petrogenesis of granitoids in the Gonghe geothermal basin, western Qinling (China)

The Gonghe geothermal basin is situated in the westernmost part of the western Qinling orogen and possesses the supreme potentials to explore and develop hot dry rock (HDR) geothermal resources in China. The basal rock of HDR reservoir in Gonghe basin is composed of granitoids. In this contribution, in order to constrain the formation mechanism of the Gonghe HDR, granitic samples from outcropping have been collected and drilling core samples data from the archives also been compiled. The studied results show that the obtained zircon U–Pb ages of 255.1 ± 1.9 Ma and 253.9 ± 2.6 Ma for granodiorite and monzogranite, respectively. All granitoids from both outcropping and drilling core have the similar rock associations and their major and trace elements possess the consistent evolved patterns, such as relative enrichment of LILEs (e.g., K, Rb), depletion of HFSEs (e.g., Nb, Ti), and negative Eu anomalies (0.32–0.66), indicating that they share the same magma sources and evolutional processes. And they have the affinity of I-type granites. They were derived from the mixture of partial melting of metabasaltic rocks in the lower crust and a small amount of mantle-derived magma and formed in a subduction-related setting relative to the southward subduction of the Zongwulong oceanic crust beneath the west Qinling terrane. The heat production values of rocks around the Gonghe basin were calculated and all basal granitic rocks ranges from 0.35 to 8.51 μW/m3, yielding an arithmetic mean value of 1.97 μW/m3, indicating that the radioactive heat generation capacity of granitoids in the Gonghe basin is slightly lower. The HDR reservoir in the Gonghe basin was probably due to an allied thermal effect of the radiogenic heat production in the thickening continental crust and the heat contributed by a deep magma chamber below the basin. The early Triassic granitoids with a higher thermal conductivity serve as the conductive medium of the geothermal heat; whilst the sedimentary rocks marked by lower thermal conductivity above the granitic batholiths act as the reservoir cap. This contribution provides a relatively rational understanding for the mechanism of the HDR reservoir in Gonghe geothermal basin from the petrological and geochemical perspectives, which is conducive to the geothermal potential assessment and estimation, and to-be implementation of the Enhanced Geothermal System demonstration project in China.

Mineral paragenesis of the Anfangba gold deposit, western Qinling Orogen, China: Implication for coupled dissolution-reprecipitation reactions and the liquid bismuth collector model

The newly discovered Anfangba gold deposit is hosted in the Middle Triassic pumpellyite-actinolite facies metamorphosed turbidites in the western Qinling Orogen, Central China. It is characterized by a pyrite-quartz vein type, a medium-high temperature, and the development of visible gold and abundant bismuthides. It is the first reported Au-Bi-associated deposit in this area and remains poorly understood. In this study, the mineral paragenesis has been studied to further understand ore formation, including sulfides (pyrite, chalcopyrite, pyrrhotite, bornite, bismuthinite, tetrahedrite and arsenopyrite), Cu-Bi(Sb) sulfosalts (emplectite, wittichenite, and skinnerite), sulfoselenide (unnamed sulpho-tellurian nevskite), tetradymite group minerals (joséite-A, joséite-B and tetradymite), native bismuth and electrum. The physicochemical conditions of the ore-forming process can be estimated based on the mineral paragenetic sequence. The calculated logαH2S and logfO2 of the main mineralization stage are higher than −2.25 and lower than −37.96, respectively. EPMA mapping indicated that ore-forming fluid with low sulfur and oxygen fugacities may have played an important role in provoking the activity of the semimetals such as Bi, Sb, Se and Te. This may have result in the formation of accessory minerals, including native bismuth, skinnerite and an unnamed sulpho-tellurian nevskite via coupled dissolution-reprecipitation (CDR) reactions. Mineralography, EPMA and LA-ICP-MS analysis have revealed a close relationship between Au mineralization and Bi-blebs. It suggests that the “liquid bismuth collector model” contributed to the Au enrichment of the Anfangba deposit. As the coexistence of native bismuth and bismuthinite is commonly seen in different types of hydrothermal gold deposits, the presence of Bi-melts and the influence of the “liquid bismuth collector model” in these gold deposits may have been underestimated.

The ca. 230 Ma gold mineralization in the Fengtai Basin, Western Qinling orogen, and its implications for ore genesis and geodynamic setting: A case study of the Matigou gold deposit

The Matigou gold deposit, located in the Fengtai Basin, is newly discovered and contains more than 30 t Au. The gold ore bodies are mainly hosted in shear structures as auriferous quartz veins and in the upper Devonian Luohansi Formation as disseminated ores. Granite porphyry and dacite porphyry dykes have been recognized in the ore zone. The granite porphyry dykes normally share the same structure as gold ore bodies and are locally mineralized. Dacite porphyry dykes likely intruded later, as they occasionally cut gold ore bodies. Pyrite is common in all kinds of rocks in the deposit. Multiple isotope (H-O-S) analyses of different minerals (quartz and pyrite), together with the geological characteristics, suggest that the Matigou deposit should be classified as an orogenic gold deposit like others in the Fengtai Basin. Zircon U-Pb dating of magmatic dykes and 40Ar/39Ar dating of hydrothermal ore-stage sericite both indicate that the Matigou deposit formed at ~ 234 Ma. It is coeval with the Pangjiahe deposit (231.2 Ma) 20 km to the west but much older than previously published orogenic gold ages (216 to 200 Ma). The ca. 230 Ma gold mineralization occurred between two episodes of magmatism (245–235 Ma and 227–205 Ma) in the Qinling orogen. The later orogenic gold mineralization is consistent with the later episode of magmatism, which is widely believed to have formed in a postcollisional extensional regime. The early episode of magmatism is still debated as related to either subduction or an early postcollisional stage. Because the ca. 230 Ma gold mineralization displays no strong deformation or metamorphism after formation, and the orogenic gold deposits imply a relatively extensional background. Hence, the early episode of magmatism may have intruded in a compressional regime after continental collision. The ca. 230 Ma gold mineralization occurred at the beginning of the transition in a tectonic setting from compression to transpression. Otherwise, the gold ore bodies would have been destroyed to some extent during the late collision. Due to the continuous extension background, more orogenic gold deposits with ages of 216 to 230 Ma are expected to be discovered in the Qinling orogen.

Genesis of the Maanqiao gold deposit in the western Qinling orogen, central China: Constraints from fluid inclusions and in-situ sulfur isotopes

The western Qinling orogen (WQO) is one of the most important prospective gold provinces in China. The Maanqiao gold deposit, located on the southern margin of the Shangdan suture, is a representative gold deposit in the WQO. The Maanqiao deposit is hosted by the metasedimentary rocks of the Upper Devonian Tongyusi Formation. The EW-trending brittle-ductile shear zone controls the orebodies; they occur as disseminated, and auriferous quartz–sulfide vein. The ore-related hydrothermal alteration comprises silicification, sulfidation, sericitization, chloritization, and carbonatization. Native gold is visible and mainly associated with pyrite and pyrrhotite. Mineralization can be classified into the following three stages: bedding-parallel barren quartz–pyrite–(pyrrhotite) (early-stage), auriferous quartz–polymetallic (middle-stage), and carbonate–(quartz)–sulfide (late-stage).Detailed fluid inclusion (FI) studies revealed three types of inclusions in quartz and calcite: aqueous (W-type), CO2–H2O (C-type), and pure carbonic (PC-type) FIs. The primary FIs in the early-stage quartz are C- and PC-type, in the middle-stage quartz are mainly W- and C-type, and in the late-stage calcite are only W-type. During gold mineralization, the total FI homogeneous temperatures evolved from 189–375 °C (mostly 260–300 °C) to 132–295 °C (mostly 180–240 °C) to 123–231 °C (mostly 130–150 °C), and the salinities varied among 2.2–9.1 wt.% NaCl equiv. (mostly 5–8 wt.%) to 0.2–9.0 wt.% NaCl equiv. (mostly 3–6 wt.%) to 0.3–3.6 wt.% NaCl equiv. (mostly 2–4 wt.%). The ore-forming fluid was characterized as an H2O–NaCl−CO2−CH4–(N2) system with medium-low temperature and low salinity. The fluid immiscibility and fluid-rock interaction may be responsible for the precipitation of the sulfides and gold at the Maanqiao gold deposit. Three types of pyrite corresponding to the three mineralization stages, as well as pyrrhotite and arsenopyrite in the middle stage, are micro-analyzed for in-situ sulfur isotopic composition by LA-ICP-MS. Py1 yield near-zero δ34S values of −2.5‰ to 3.0‰, which are somewhat lower than that of the granite hosted pyrites (Py-g, 4.8‰ to 6.6‰). The result suggests a mixed sulfur source from magmatic-hydrothermal fluids and the metamorphism of diagenetic pyrite. Pyrite + pyrrhotite + arsenopyrite assemblages in the middle-stage have relatively higher δ34S values (6.6‰ to 12.3‰) and are mainly developed due to the metamorphism of the ore-host and underlying Devonian sedimentary sequences. The low δ34S values of the late-stage fracture-filled Py3 (−21.9‰ to −17.0‰) resulted from an increasing oxygen fugacity, which was caused by the inflow of oxidized meteoric waters.Based on our studies, the Maanqiao gold deposit is considered to be an orogenic type and closely related to the Indosinian Qinling orogeny.

Crustal structure underneath central China across the Tibetan Plateau, the North China Craton, the South China Block and the Qinling-Dabie Orogen constrained by multifrequency receiver function and surface wave data

Abstract Investigating the crustal structure at the juncture region of the Tibetan Plateau, the North China Craton, the South China Block and the Qinling-Dabie Orogen is important to understand processes of uplift of the Tibetan Plateau and the craton destruction and preservation. In this study, we dropped the H-k stacking of receiver functions (RFs), RF nonlinear inversion, and joint inversion of RF and surface wave dispersion with multiple Gaussian factors successively to invert the crustal thickness (H), average Vp/Vs ratio (k) and S-wave velocity in the juncture region. The imaging results and the synthetic tests of multifrequency H-k stacking were further used to study the crustal structure and related tectonic evolution. Our results show that the H and k values generally increase and decrease, respectively, as the Gaussian factor increases in the study region, which is consistent with synthetic tests of H-k stacking. The cores of the Ordos Block and the Sichuan Basin exhibit a typical cratonic crust, whereas the Zhongtiao Mountains and adjacent Qinling-Dabie Orogen to the south may have undergone delamination of the lower crust. Local low-velocity anomalies in the middle-lower crust were observed in the Western Qinling Orogen and Qilian Orogen, whereas high S-wave velocities in the middle-lower crust were observed at the juncture region between the Western Qinling Orogen and the Qinling-Dabie Orogen. Our results suggest that the lower crustal flow is probably restricted to the central area in the Western Qinling Orogen and does not flow into the Qilian Orogen or the Qinling-Dabie Orogen.

Petrogenesis and Geochronology of Tianshui Granites from Western Qinling Orogen, Central China: Implications for Caledonian and Indosinian Orogenies on the Asian Plate

The precise timing, petrogenesis, and geodynamic significance of three granitoid bodies (Beidao granite, Caochuanpu granite, Yuanlongzhen granite, and the Roche type rock) of the Tianshui area in the Western Qinling Orogen, central China, are poorly constrained. We performed an integrated study of petrology, geochemistry, and zircon U-Pb dating to constrain their genesis and tectonic implication. Petrographic investigation of the granites shows that the rocks are mainly monzogranites. The Al saturation index (A/CNK versus SiO2) of the granitoid samples indicates meta-aluminous to peraluminous I-type granites. Their magmas were likely generated by the partial melting of igneous protoliths during the syn-collisional tectonic regime. Rare-earth-elements data further support their origin from a magma that was formed by the partial melting of lower continental crust. The Beidao, Caochuanpu, and Yuanlongzhen granites yielded U-Pb zircon weighted mean ages of 417 ± 5 Ma, 216 ± 3 Ma, and 219 ± 3 Ma, respectively. This study shows that the Beidao granite possibly formed in syn- to post-collision tectonic settings due to the subduction of the Proto-Tethys under the North China Block, and can be linked to the generally reported Caledonian orogeny (440–400 Ma) in the western segment of the North Qinling belt, whereas Yuanlongzhen and Caochuanpu granites can be linked to the widely known Indosinian orogeny (255–210 Ma). These granitoids formed due to the subduction of the oceanic lithospheres of the Proto-Tethyan Qinling and Paleo-Tethyan Qinling. The Roche type rock, tourmaline-rich, was possibly formed from the hydrothermal fluids as indicated by the higher concentrations of boron leftover during the late-stages of magmatic crystallization of the granites.

Petrogenesis of Triassic granitoids in the Fengxian–Taibai ore cluster, Western Qinling Orogen, central China: Implications for tectonic evolution and polymetallic mineralization

The petrogenesis and tectonic setting of the Triassic granitoids, and their relationship with the coeval Au and Pb–Zn polymetallic mineralization in the Western Qinling Orogen (WQO) in central China remain debated. The Fengxian–Taibai Ore Cluster (FTOC) in the eastern part of the WQO hosts large-scale Triassic granitoids and numerous polymetallic deposits, providing an excellent opportunity to constrain the controversies mentioned above. In this work, we choose the Huahongshuping granodiorite pluton, Guji biotite monzogranite pluton, Taoerling biotite monzogranite dike, Dongtangzi granite porphyry dike and Dagou dioritic dikes in the FTOC for a combined study on the zircon U–Pb geochronology, whole-rock major, trace elements and Sr–Nd isotopes. LA–ICP–MS zircon U–Pb dating yields ages of ~225 Ma, ~224 Ma and 227–217 Ma for the Huahongshuping pluton, Guji pluton and granitoid dikes, respectively. All these granitoids have metaluminous and high-K calc-alkaline series affinities, and are enriched in large ion lithophile elements (LILE), light rare earth elements (LREE) and depleted in high field-strength elements (HFSE) with a marked LREE/HREE fractionation ((La/Yb)N = 10.66–32.42), fractionation of LREE ((La/Sm)N = 2.72–6.60), and negative Eu anomalies (δEu = 0.45–0.99). They have initial Sr isotopic ratios of 0.7062–0.7082, and negative εNd(t) values of –9.42 to –8.09. The above features indicate that the FTOC granitoids were mainly derived from the partial melting of the Neoproterozoic meta-volcanic rocks induced by the decrease in pressure and the addition of water dehydrated from biotite, with direct contribution of mantle materials. Combined with previous data on sedimentation, deformation and metamorphism, and tectonics, we suggest that the WQO underwent the Early Mesozoic tectonic evolution from the subduction at 248–235 Ma, through syn-collision at 235–225 Ma, to post-collision at 225–195 Ma. Integrating with the ore geology, geochronology, mineralogy, and quartz and carbonate H–O, and in-situ S isotope data of sulfides from different mineralization stages at representative Au and Pb–Zn deposits in literatures, we propose a conceptive model that the post-collision process and granitic magmatic activity played an important role in the polymetallic mineralization within the FTOC.

Petrogenesis and tectonic implications of Indosinian granitoids from Western Qinling Orogen, China: Products of magma-mixing and fractionation

The Western Qinling Orogen (WQO) is characterized by voluminous distribution of Indosinian granitoids, the formation of which provides an important window to unravel the geochemical and geodynamic evolution and associated metallogeny. Here we investigate a group of intrusions termed “Five Golden Flowers” based on petrological, geochemical, zircon U–Pb geochronological and Lu–Hf isotopic studies on the granitoids and their mafic microgranular enclaves (MMEs). Our results show that these intrusions are genetically divided into two types, namely, magma-mixing and highly fractionated. The Jiaochangba, Lujing, Zhongchuan, and Luchuba granitoids are biotite monzogranites (220 ​± ​0.8 ​Ma to 217 ​± ​2.6 ​Ma) with abundant coeval MMEs (220 ​± ​1.1 ​Ma to 217 ​± ​2.7 ​Ma). The rocks contain moderate to high SiO2, high MgO, Rb, Sr, Ba, and Th contents, but low TiO2, P2O5, and Sc values, A/CNK of <1.1, and a range of εHf(t) values of −11.7 to +2.23 with corresponding TDM2 values of 1967–1228 ​Ma. The MMEs possess K-feldspar megacrysts, abundant acicular apatites, and show lopsided textures. They have lower SiO2, Al2O3, and Th contents, but higher MgO, TiO2, and Sc, with εHf(t) values of −18.0 to +3.18 and TDM1 of 849–720 ​Ma. The data indicate that the MMEs were derived from a magma sourced from the enriched lithospheric mantle. We suggest that these host granitoids were produced by partial melting of late-Paleoproterozoic to early-Mesoproterozoic lower crust with the involvement of Neoproterozoic SCLM-derived mafic magmas. The Baijiazhuang pluton is dominantly composed of leucogranite (muscovite granite and two-mica monzogranite, 216 ​± ​1.5 ​Ma) without MMEs. The rocks are peraluminous with high A/CNK (1.06–1.27). Compared with the other four granitoids, the Baijiazhuang leucogranite shows higher SiO2 content, markedly lower concentrations of TiO2, MgO, Al2O3, CaO, and Fe2O3T, and lower LREE/HREE and (La/Yb)N values. These leucogranites are also rich in Rb, Th, and U, and display marked depletions in Ba, Sr, Ti, and Eu, indicating that they experienced significant fractionation. Zircon εHf(t) values (−10.2 to −3.27) and TDM2 (1868–1424 ​Ma), as well as the Nb/Ta and K2O/Na2O values are similar to the other four granitoids, indicating that they are likely to have been derived from a similar source; with sediments playing only a minor role in the magma generation. The low contents of Yb and Y suggest that their partial melting was controlled by garnets and micrographic texture of K-feldspar reflects high-temperature melting through undercooling. Based on the above features, we infer that the Baijiazhuang leucogranite likely represents the product of high degree fractionation of the I-type biotite monzogranite magma which generated the other four granitoids at relatively high temperatures, within magma chambers at mid-crust depths. We propose that the granitoid suite was formed in the transitional setting from syn- to post-collision during the collisional orogeny between the SCB and NCB, following break-off of the subducted South China Block lithosphere during 220–216 ​Ma.

Complex Deformation of Central and North China Through Pn Velocity and Anisotropy Tomography

AbstractUsing our recently developed Pn tomography method, we obtain high‐resolution Pn velocity and anisotropy structures of uppermost mantle beneath the Central and North China, which provide useful insight concerning the mechanism of intraplate mantle dynamics. The consistency between the new Pn model and seismic reflection/refraction profiles demonstrates the high quality of our model. Based on the Pn anisotropy structure surrounding the Alxa Block, we infer that material extrusion from northeastern Tibet was impeded and turned to the west and south edges of the Alxa Block. The Pn structures beneath the Sichuan Basin and Songpan‐Ganzi Orogen suggest that the stable Sichuan Basin impeded material extrusion from the Tibetan Plateau and caused material from Tibet to be extruded separately along the west and northwest edges of the Sichuan Basin. To the north of the Sichuan Basin, we infer that the extrusion has ceased within the Songpan‐Ganzi Orogen rather than to further east. The high Pn velocities beneath the Qinling Orogen do not support the proposal of large‐scale eastward material escaping flow in the shallow mantle beneath the western Qinling Orogen. The Pn velocity distribution beneath the North China Craton indicates that hot material from the mantle is more likely to upwell beneath deep faults or active orogenic areas. Our model indicates that the northern and central parts of the Ordos Block retain stable structures at the uppermost mantle, while its southeastern has been reworked by hot material.

Typomorphic Characteristics of Pyrites from the Shuangwang Gold Deposit, Shaanxi, China: Index to Deep Ore Exploration

The large Shuangwang gold deposit (&gt;80 t gold) is located in the Western Qinling Orogen (WQO) of central China. It is an orogenic-type gold deposit hosted in an NW-extending breccia belt in the Devonian Xinghongpu Formation. Gold mineralization of the Shuangwang deposit is featured by hydrothermal breccia ores with strata fragments cemented by hydrothermal minerals dominated by ankerite, quartz, and pyrite with minor amounts of calcite and albite. Pyrite is the major gold-hosting sulfide and the most abundant ore mineral. Crystal habits, thermoelectricity, and trace-element composition of pyrites from the main ore-forming stage of the Shuangwang gold deposit were studied by microbinocular, BHTE-06 thermoelectric coefficient measuring instrument, and high-resolution inductively coupled plasma mass spectrometry (HR-ICP-MS). Spatial distribution of the above data for pyrites was delineated by contour maps of morphology index, P-type frequency, and primary halo elements (e.g., supraore halo elements Ba and Sb; near-ore halo elements Pb, Zn, and Cu; and subore halo elements Co, Mo, and Bi). Based on the above results, four target areas (areas between prospecting lines 0 and 1, between lines 14 and 18 below orebody KT9; areas between prospecting lines 30 and 34, between lines 44 and 46 below orebody KT8) were put forward for deep gold exploration in the future. These targets are consistent with the depth extrapolation of proven gold orebodies, indicating the practicality of typomorphic characterization of pyrites as vector to deep/concealed gold orebodies. The effectiveness of the pyrite typomorphic parameter for deep gold prediction seems to be chemical composition, crystal habits, and then thermoelectricity.