Yangtze Block Research Articles

Insights into ore genesis of the Beishan Pb–Zn deposit in Guangxi, South China: Evidence from Rb–Sr dating and in-situ S–Pb isotopes

The Beishan Pb–Zn deposit, situated in the transitional zone between the Yangtze Block and Cathaysia Block, is a significant source of non-ferrous metal ore in Guangxi, China. The orebodies are hosted within distinct fine- and coarse-grained dolomites, exhibiting layered and lenticular shapes controlled by stratigraphy and structural factors, demonstrating good continuity. No evidence of magmatic activity has been observed in the Beishan deposit. To ascertain the sources of ore-forming materials, age of formation, and genesis of the Beishan deposit, this study employed a systematic approach involving trace element analysis, Rb–Sr dating, and in-situ S–Pb isotope analysis of representative sulfide minerals, namely, sphalerite, pyrite, and galena. The sphalerite within the Beishan deposit displayed enrichment of Cd, Ga, and Ge, while exhibiting depletion of Fe, Mn, and In, indicating a medium–low formation temperature. An in situ S isotope study suggests that sulfur primarily originates from seawater sulfate, with bacterial sulfate reduction (BSR) and thermochemical sulfate reduction (TSR) processes playing a crucial role in mineral deposition. In-situ Pb isotope analysis indicated that Pb was derived from a combination of crustal and mantle sources. Multiple geochemical findings indicate that the Beishan deposit is a typical Mississippi Valley Type (MVT) Pb–Zn deposit. Furthermore, the obtained Rb–Sr isochron age of 330 ± 12 Ma (1σ) indicates that the Beishan deposit was formed during the Hercynian orogeny, aligning with the formation timing of the Siding Pb–Zn deposit in northwestern Guangxi.

Spatial distribution of the critical mineral resource element titanium in China and its influencing factors

In China, titanium is found in multi-metal symbiotic ore deposits that present sorting difficulties, low recovery rates, high production costs, and low overall resource utilization rates. The concentration and distribution of titanium in the pedosphere must be better understood to assess the potential titanium resources. The China Geochemical Baseline project obtained harmonized soil and rock geochemical data covering mainland China and provided reliable data for 76 elements (including titanium) from 3382 top (0–25 cm) and 3380 deep (>100 cm) sediment or alluvial soil samples and 12,201 rock samples. In this study, titanium concentrations were determined by X-ray fluorescence spectrometry under strict quality control. The median titanium concentration values in the top and deep samples, which were considered geochemical baseline values, were 3498 mg/kg and 3406 mg/kg, respectively, with mean values of 3661 mg/kg and 3585 mg/kg, respectively. The titanium concentrations ranged from 266 to 24,674 mg/kg and 324 to 25,504 mg/kg in the top and deep soil samples, respectively. The 75 % range (Q25–Q95) of titanium concentrations in the rock ranged from 978 to 8299 mg/kg. The median titanium concentration was 2555 mg/kg in the rock samples, while the mean concentration was 3089 mg/kg. The geochemical map of titanium indicated that low titanium concentrations below Q25 (<3498 and 3406 mg/kg in the top and deep samples, respectively) are primarily distributed in southern Xinjiang, Tibet, Qinghai, Gansu, and Inner Mongolia. The distribution of low titanium content in the soils is likely attributed to these regions being covered by deserts (e.g., Gobi) and glaciers. Soils with high titanium concentrations are widely distributed in the Emeishan large igneous province at the southwest margin of the Yangzi block, which is related to the large distribution of intermediate-basic rock in the area, as well as along the northeast-southwest direction of the Yangtze block, which extends for nearly 1600 km. Moreover, these soils are mainly associated with the black rock system and intermediate-basic dikes. Soil titanium concentrations are affected by various factors, including the occurrence of tectonic activity and distribution of the black rock system, intermediate-basic rock (especially basalt), and VTi magnetite deposits. In particular, the large igneous province area generated by a mantle column represents the main location of VTi magnetite deposit formation and the area with the highest titanium content. In addition to providing detailed information about the titanium distribution in the pedosphere and rock of mainland China, the project can also provide basic geochemical data for the development and utilization of titanium resources and the formulation of critical mineral industry policies.

Deep geodynamics and metallogenic mechanism of the South China block: New insight from mantle tomography

The South China block (SCB) is located at the southeastern margin of the Eurasian plate, where intense Mesozoic magmatism and metallogenic belts occurred. The mechanism of mineralization is still unclear and debated. To clarify this issue, we determine a robust 3-D model of P-wave tomography of the upper mantle beneath the SCB by using 77,969 high-quality travel-time data of 934 teleseismic events recorded at 315 permanent and 72 portable stations in the SCB. Our results show that high-velocity (high-V) zones exist in the upper mantle under the Yangtze block and in the mantle transition zone under the eastern segment of the Jiangnan orogen. The former reflects thicker cratonic lithosphere, whereas the latter may reflect the deeply subducted paleo-Pacific plate. Low-velocity (low-V) anomalies are revealed in the upper mantle beneath the Cathaysia block, which are located above the high-V anomaly in the mantle transition zone. These low-V anomalies may reflect hot upwelling flows in the upper mantle, which might be related to the large-scale magmatic activities in the late Mesozoic. Our results suggest that the low-V hot mantle materials beneath the Cathaysia block have risen to the shallow lithosphere along magmatic channels such as fault zones, which eroded the nearby cratonic root of the Yangtze block and the overlying lithosphere and destroyed the lithosphere beneath the eastern SCB. The hot upwelling materials mineralized in the shallow part and so formed many metallogenic belts in the SCB.

Selenium enrichment in the Dingtoushan carbonate-hosted epigenetic Pb-Zn deposit in the western Yangtze Block, SW China

Selenium (Se) is a typical dispersed element (crust Clarke value only 0.05 × 10-6) and is mainly enriched in black shales (>20 × 10-6). Selenium can also be associated with Pb-Zn and U deposits. In carbonate-hosted epigenetic hydrothermal Pb-Zn deposits, the selenium contents in sulfides are commonly < 50 × 10-6, and the selenium is mainly enriched in galena, whereas the selenium contents in sphalerite are often < 10 × 10-6. There are few studies on Se-rich carbonate-hosted epigenetic hydrothermal Pb-Zn deposits, and the origin of such deposits needs more in-depth exploration. In this study, we selected the Dingtoushan Se-rich Pb-Zn deposit in the western Yangtze Block, SW China, and utilized in-situ trace elements and S-Pb isotopes of Se-rich sphalerite to investigate this issue. There are two textures of sphalerite formed in the Dingtoushan deposit, of which subhedral sphalerite (Sp-I) precipitated during the early stage, whereas euhedral sphalerite (Sp-II) generated during the late stage. The results of trace elements show that the Dingtoushan deposit is significantly enriched in selenium (>22 × 10-6) compared with other carbonate-hosted Pb-Zn deposits (<22 × 10-6, generally < 10 × 10-6) in the Sichuan-Yunnan-Guizhou (SYG) Pb-Zn metallogenic province. In detail, a positive correlation between Se, Cd, In and Cu contents and sulfur isotopes of sphalerite is identified at the deposit scale, but no relationship is observed at the grain scale. In addition, the late stage Sp-II with δ34S values of −7‰ to −3‰ is rich in selenium and other trace elements, whereas the early stage Sp-I with isotopically light sulfur (δ34S = -29 ‰ to −8‰) is depleted of them. This indicates that in the early stage, insufficient addition of hydrothermal fluid resulted in relatively low temperature, and sulfur formed by bacterial sulfate reduction (BSR) was mainly involved in mineralization. With the addition of a large amount of hydrothermal fluid, thermochemical sulfate reduction (TSR) was induced, which leads to the formation of sphalerite in the late stage with relatively high δ34S values. Furthermore, the Pb isotopes reveal that the ore-forming metals were likely derived from the Cambrian black shales. Hence, we infer that fluid mixing is the main sulfide precipitation mechanism and may cause the positive correlation of selenium concentrations and δ34S values. On the other hand, at the grain scale of sphalerite, the variation of selenium contents shows a negative correlation with temperature, suggesting that temperature may play a critical role in selenium enrichment in sphalerite. This study highlights that selenium enrichment in sphalerite is jointly controlled by fluid mixing and temperature. The degree of fluid mixing dominated the selenium concentration in hydrothermal fluid, but high temperature may not be conducive to selenium incorporation into sphalerite. Our new results indicate that sphalerite in the carbonate-hosted epigenetic hydrothermal deposits is also a major host for selenium and such type deposits may have a great economic potential for selenium resource.

Petrogenesis of Neoproterozoic high-K intrusion in the southwestern Yangtze Block, South China: Implication for the recycled subducted-sediment in the mantle source

The study of K-enriched intrusive rocks is essential for deciphering mantle metasomatism beneath active continental arcs. In this contribution, high-precision zircon U–Pb–Hf isotope, whole-rock geochemistry, Sr–Nd isotope, and mineral chemistry analyses were performed to evaluate the petrogenesis and geodynamic system of the Yunnongfeng intrusion on the southwestern margin of the Yangtze Block. The Yunnongfeng intrusion consists of a high-K to shoshonitic rock assemblage with variable lithology from gabbro-diorite to granite. Zircon U–Pb dating gives concordant crystallization ages of ca. 782.5 ± 3.8 Ma for gabbro-diorite, ca. 774 ± 4.1 and 776 ± 4.1 Ma for diorite, ca. 770 ± 4.7 Ma for quartz monzonite, ca. 763 ± 3.4 Ma for quartz syenite, and ca. 764 ± 16 Ma for granite. These samples also show similar Sr–Nd, and Lu–Hf isotopic compositions, implying a common magma source. The similar crystallization age and regular variation of major and trace element contents suggest that these rocks were formed through fractional crystallization of cogenetic primitive mantle magmas. The enriched εNd(t) (−5.7 to − 5.1) and εHf(t) (−6.7 to − 1.2) values, high Rb/Y and Th/La ratios, slight Nd–Hf decoupling, and high-K and Th contents demonstrate that their lithospheric mantle source was enriched by slab-related fluid and sediment-related melt. The samples also exhibit remarkable enrichment in large-ion lithophile elements and depletion in high-field-strength elements, indicative of subduction-related arc magmatism. Taking into account previous studies, we suggest that the western margin of the Yangtze Block experienced a long-term subduction process during the Neoproterozoic, and the Yunnongfeng intrusion formed in an extensional back-arc basin. Based on the K-enriched mafic–intermediate rocks from the western margin of the Yangtze Block commonly show high K2O/Na2O, Rb/Sr, low Ba/Rb ratios, and enriched εNd(t) values, our study, coupled with numerous previous reports, proposes that the K-enrichment resulted from the breakdown of phlogopite, owing to subduction-related sediment melt reacting with peridotite in the mantle source area.

The evolution of the majiang paleoreservoir, west margin of xuefeng uplift: Insights from in situ calcite U–Pb dating and bitumen and shale trace element analysis

The Majiang Reservoir, situated within the Yangtze Block, is the largest paleoreservoir in the region. Employing a comprehensive methodology involving the analysis of trace and rare earth elements in bitumen and shale, in situ U–Pb dating of hydrocarbon-associated calcite, and incorporating previously published data on oil-source correlation and significant stages of reservoir development, we successfully reconstructed the complete evolution of the Majiang Reservoir quantitatively. The bitumen trace elements data show the ratio of V/(V + Ni) ranging from 0.64 to 0.82, Ni/Co exceeding 7.0 and Zr/Cr below 1.0. PAAS-normalized rare earth element distribution patterns of both bitumen and Cambrian shales present zig-zags distributions. In addition, biomarker distribution acquired by catalytic hydropyrolysis and bulk δ13C values of both bitumen and Cambrian shales (−29.1‰ to −34.7‰ vs −30.2‰ to −32.3‰) also show similarity. All the geochemical parameters together indicate that the primary source of bitumen stems from Early Cambrian shales. Integrating fluid inclusion analysis, specifically examining two Th groups with temperatures of 97.2 °C and 142.6 °C, and U–Pb dating on hydrocarbon-bearing calcite (∼300 Ma), referencing prior Re–Os dates on bitumen (∼430 Ma) and pyrobitumen (∼80 Ma) and quartz fluid inclusion Ar–Ar dating (∼225 Ma), we successfully reconstructed the entire hydrocarbon evolution. The findings reveal that initial oil generation occurred during the Caledonian event, followed by secondary continuous oil and gas generation and migration, influenced by the Indosinian event. The combination of fluid inclusion analysis and isotopic geochronology data exemplifies immense potential in precisely determining the progression of petroleum evolution.

Late Paleoproterozoic collision-related granitic magmatism in the Cuoke Complex, SW China: New evidence for the early evolution of the Yangtze Block

The Paleoproterozoic tectonic evolution of the southwestern Yangtze Block has been ambiguous due to the scarce preservation of geological records, confusing the history of the entire Yangtze Block in the Nuna supercontinent. We here report an integrated study involving petrology, whole-rock geochemistry, zircon U-Pb geochronology and Hf-Nd isotopes of the recently identified late Paleoproterozoic granitoids from the Cuoke Complex in the southwestern Yangtze Block. LA-ICP-MS zircon U-Pb dating reveals three episodes of granitic magmatism at ca. 1.94 Ga, 1.89 Ga, and 1.85 Ga. Geochemically, the ca. 1.94 Ga albite granites (AGs) and ca. 1.89 Ga and 1.85 Ga monzogranites (MGs) exhibit features of I-type granite, while the ca. 1.85 Ga AGs display characteristics of A-type granite. The zircon εHf(t) values range from −13.0 to −8.1 for the 1.94–1.89 Ga granitoids and from −8.9 to −7.2 for the ca. 1.85 Ga granitoids, with two-stage model ages of 3.3–3.1 Ga and ca. 3.0 Ga, respectively, indicative of their derivation from remelting of Mesoarchean crustal materials. The relatively low heavy rare earth elements (HREEs) (e.g., Yb < 1.9 ppm) and Y (<18 ppm) contents, and moderately negative Eu anomalies of the ca. 1.94–1.89 Ga granitiods suggest they may have been produced in a late collisional or earliest post-collisional setting. The ca. 1.85 Ga AGs and MGs were probably formed in a post-collisional setting indicated by their high HREE (e.g., Yb > 1.9 ppm) and Y (>18 ppm) contents and strongly negative Eu anomalies. Combined with available data, we provisionally divided the tectonic evolution of the southwestern Yangtze Block into four stages: 1) ca. 2.01 Ga subduction, 2) ca. 1.97–1.95 Ga metamorphism associated with collision events, 3) ca. 1.94–1.89 Ga late collisional or earliest post-collisional stage, and 4) ca. 1.85 Ga post-collisional extensional stage. The subduction and collision of the southwestern Yangtze Block initiated slightly later than the northern part, but their near-synchronous tectonic process since ca. 1.97 Ga suggest they may have collaged together at that time. The ca. 1.94–1.89 Ga and ca. 1.85 Ga late collisional to post-collisional granitic magmatism likely recorded the convergence of discrete Archean rock provinces of the Yangtze Block during the Nuna supercontinent assembly.

Geology, geochronology, geochemistry and Hf-Sr-Nd isotopes of the ‘green-bean rocks’ (felsic tuffs) in the western Yangtze Block, China: Implications for genesis and enrichment mechanism of Li

The widespread ‘green-bean rocks’ (GBRs) in the western Yangtze Block (YZB) are traditionally considered to be originally felsic volcanic tuff. These greenish or yellowish green GBRs are characterized by the presence of bean-like grains. Although the GBRs are enriched in Li in addition to K, the enrichment mechanism of Li during the formation process of GBRs remains controversial. Detailed petrography, scanning electron microscopy, X-ray diffraction, whole-rock geochemistry, U-Pb dating and Hf-Sr-Nd isotopes have been conducted on the GBRs from the Longmendong section in the western YZB to shed light on the genesis of GBRs, Li enrichment mechanism, and further exploration.The zircon and apatite from Longmendong GBRs yielded U-Pb ages of 247.2 ± 0.6 Ma and 247.0 ± 11.0 Ma, respectively. Longmendong GBRs show bull-rock (87Sr/86Sr)i ratios ranging from 0.72602 to 0.72948 and εNd(t) values between −9.8 and −10.4. Zircons have low εHf(t) values and old two-stage Hf model ages (TDM2) ranging from −16.1 to −7.4 and 1741 to 2285 Ma, respectively. Apatite grains display high (87Sr/86Sr)i ratios and low εNd(t) values in the range of 0.72225–0.72461 and −9.93–−11.18, respectively. The combined evidence from textures, mineral compositions, whole-rock geochemistry and Sr-Nd-Hf isotopic compositions indicates that the GBRs originated from felsic volcanic ashes that erupted along the Sanjiang Orogen. The magma of volcanic ashes was sourced from the remelting of ancient crust in response to the initial collision of Indochina-Yangtze Blocks during the closure of the eastern Paleo-Tethys Ocean. The apatite grains separated from GBRs show significantly low Sr and Eu contents (136–223 ppm and 4.23–6.53 ppm, respectively) and low (La/Sm)N (0.97–1.11) and (Sm/Yb)N (3.49–4.56) ratios, indicating that apatite crystallized earlier than the plagioclase and the original magma are highly evolved. The magma of GBRs originated from ancient crust with significant differentiation, facilitating the initial enrichment of Li during the magma evolution. On the other hand, the GBRs have elevated levels of K2O, MgO and clay minerals (e.g., illite and illite/smectite mixed layer minerals), which are proportional to the Li concentrations. This suggests that Li in the GBRs is adsorbed by clay minerals in ionic form and experience additional enrichment during the transformation from volcanic ashes into clay minerals. Furthermore, the GBRs in the northeastern Sichuan basin under a limited sedimentary environment have relatively higher Li concentrations, suggesting that continuous evaporation also positively contributed to Li enrichment in GBRs. Geographically, GBRs in the Sichuan basin are closely related to underground Li-rich brines hosted within the adjacent strata. Therefore, the widespread Li-rich GBRs in the western YZB not only hold promise as a potential Li resource, but also serve as a marker layer for the prospecting Li brines in the Sichuan basin.

The REY geochemistry of phosphorites during metamorphism of the Haizhou Group, NW Yangtze Block, China

Rare earth elements (REE including Y—REY) in phosphorites have been used widely as a proxy for seawater chemistry at the time of their formation. However, it is unclear whether metamorphism can lead to REE remobilization and alteration of primary seawater signatures recorded in phosphorites. In this study, we examine similarly aged metamorphosed phosphorite ore deposits from the northeastern margin of the Yangtze Block (Jinping and Xinpu ore sections), with contemporaneously deposited unmetamorphosed equivalents in the southwestern margin of the Yangtze Block (Weng'an ore section). New insights have been provided into the migration of rare earth elements and the redistribution of Zn and O isotopes in metamorphosed phosphorite, which suggests that the varying degrees of metamorphic fluid–rock reaction during the metamorphism process significantly reduced the δ18O value of phosphate (13.7‰ ± 2.2‰ of Jinping phosphate and 7.3‰ ± 1.8‰ of Xinpu phosphate) and added light Zn isotope. These metamorphic fluids were likely generated by the metamorphic dehydration of upper slab uplift during the plate subduction in the Triassic. The negative correlation between the O (and Zn) isotope compositions and REY contents in P-rich rocks suggests that REY were likely re-enriched during the recrystallization process of apatite. Preferential migration of LREE led to significant LREE and MREE enrichment in Haizhou metamorphosed P-rich rocks compared to Weng'an unmetamorphosed phosphorites.

Limited flexural control of fold-thrust belts on the Jurassic Sichuan Basin, South China

The northern part of the Jurassic Sichuan Basin has long been thought of as a foreland basin in relation to the post-collisional compression along the northern margin of the Yangtze block. However, the exact coupling mechanism between mountain building and basin formation remains unclear. Here, we integrate stratigraphic correlation, basin subsidence analysis and flexural simulation to quantitatively assess the extent to which the fold-thrust belts have controlled basin subsidence. Flexural backstripping of the stratigraphic record, spanning from 201 to 149 Ma, along two cross sections that are perpendicular to the Micangshan fold-thrust belt and the Dabashan fold-thrust belt, respectively, reveals a limited flexural control of mountain loading on basin subsidence. Owing to the short-wavelength nature of plate flexure, the basin-adjacent thrust belts exerted dominant control on basin subsidence only along its margin, with the width of the foredeeps not exceeding ca. 120 km, failing to drive subsidence in the forebulge and backbulge regions. Flexural modeling results suggest that crustal thickening was relatively weak during the Early to Middle Jurassic. This was followed by a more rapid and intense phase of crustal growth in the Late Jurassic, possibly extending into the earliest Early Cretaceous. Compared to the Micangshan region, the Dabashan region has experienced more intense compression during the Late Jurassic. Additionally, our results reveal spatial variations in plate rigidity along the northern margin of the Yangtze block, with greater plate stiffness in the Dabashan region. The presence of residual subsidence, an anomalous long-wavelength subsidence component corrected for both basin-adjacent thrust loading and associated sediment loading, highlights the necessity for an additional driving mechanism for basin subsidence. This residual subsidence was likely dynamic subsidence induced by the flat subduction of the Paleo-Pacific plate (the Izanagi plate) beneath East Asia as the flat slab progressively migrated inland.

Seismic evidence for collisional tectonics of the North China and Yangtze blocks in the Tongbai-Dabie orogenic belt

The Tongbai-Dabie orogenic belt was formed in the Middle to Late Triassic through continental collision between the North China Block and the Yangtze Block. To reveal the collisional tectonics of the two blocks, we apply teleseismic tomography to study the 3-D P-wave velocity structure down to 400 km depth beneath the Tongbai-Dabie orogenic belt using 37,251 first P-wave arrival times of 1423 teleseismic events recorded at 99 seismic stations in the study region. Our tomography reveals three obvious high-velocity anomalies. One is dipping southward beneath the Tongbai-Dabie orogenic belt, reflecting the southward subduction of the North China Block. The second is visible down to 100 km depth beneath the eastern Tongbai-Dabie orogenic belt, which reflects the Yangtze Block. The third appears at depths of 300–400 km, which is interpreted as the retained broke-off paleo-Tethyan oceanic slab. On the basis of our tomographic results and previous findings, we propose a two-stage subduction model showing that the Yangtze Block subducted northward in the Triassic, and then the North China Block subducted southward in the Jurassic beneath the Tongbai-Dabie orogenic belt.

Fluctuation in redox conditions and the evolution of early Cambrian life constrained by nitrogen isotopes in the middle Yangtze Block, South China

AbstractThe Ediacaran–Cambrian (E-C) transition (∼542–517 Ma) witnessed the rapid evolution of Cambrian animals, which was accompanied by carbon cycling anomalies and a significant increase in the concentration of oxygen in Earth’s atmosphere. The mechanisms stimulating the evolution of complex eukaryotes, however, remain problematic, especially concerning the link between biological evolution and contemporaneous changes in the oceanic environment. In this study, integrated δ13Ccarb–δ13Corg–δ15N compositions were analysed from the YD-4 core samples to understand redox fluctuations and nitrogen cycling of the middle Yangtze Block across the E-C transition. Two negative δ13Ccarb excursions (N1 and N2) and a positive δ13Ccarb excursion (P1) are identified from the studied samples and are supposedly of primary origin. Constrained by of the U-Pb age, biolithology and pattern of isotopic variation, N1, P1 and N2 are comparable to the Basal Cambrian Carbon Isotope Excursion (BACE), Zhujiaqing Carbon Isotope Excursion (ZHUCE) and Shiyantou Carbon Isotope Excursion (SHICE). We interpreted the decreased δ15N values in this study as resulting from intensified atmospheric nitrogen fixation driven by enhanced denitrification associated with expanded marine anoxia, as well as partial ammonium assimilation, while increased δ15N values suggest weakened denitrification associated with an amplified oxic water mass. The temporal coincidence of N1 and N2, with two episodes of negative δ15N excursions, and of P1, with a positive δ15N excursion, suggests that variable oceanic redox conditions and nitrogen bioavailability may have influenced the evolution of the Cambrian eukaryote-dominated community.

Detrital Zircon Geochronology and Provenance of Metasedimentary Rocks from the Susong Complex Zone in the Dabie Orogen

AbstractThe Susong complex zone is a relatively low‐grade metamorphic unit located in the southern part of the Dabie orogen and preserves a variety of metasedimentary rocks, mostly with epidote–amphibolite facies. However, their depositional age, provenance and tectonic setting of sedimentary protolith remain controversial due to the lack of fossils, precise dating and integrated geochemical investigation. This study has conducted whole‐rock elemental, and zircon U‐Pb SHRIMP dating and Lu‐Hf isotope analyses on three types of representative metasedimentary rocks including garnet‐bearing mica‐quartz schist, graphite‐muscovite‐quartz schist and dolomitic marble from the Susong complex zone. The U‐Pb SHRIMP dating data indicate that the sedimentary protolith of these rocks have the maximum depositional ages of less than 840–750 Ma and have various sedimentary provenances. In combination with zircon Lu‐Hf isotope compositions, the provenance of the metasedimentary rocks in the region is for the first time documented to be mainly derived from four groups of magmatic rocks formed at ∼2.5 Ga, ∼2.0 Ga, ∼1.4 Ga and ∼0.8 Ga in response to four episodes of igneous activity in the northeastern margin of the Yangtze Block. The conventional geothermobarometry combined with rare metamorphic zircon ages indicate that the studied rocks underwent the Late Triassic continental subduction‐related metamorphism with peak epidote–amphibolite facies conditions at P = 0.34–0.91 GPa and T = 427–532°C. In addition, results of the elemental contents (i.e., La, Ce, Th and ΣREE) and ratios (i.e., Eu/Eu* and LaN/YbN) suggest that the protoliths of the metasedimentary rocks were mainly shales, wackes and limestones, most probably related to the Rodinia supercontinent rifting along the northern margin of the Yangtze Block during the Neoproterozoic.

Age, provenance and tectonic setting of the Tonian–Cryogenian clastic successions in the northwest Bikou terrane, NW Yangtze Block, Central China

Studies on the widely-exposed Neoproterozoic geological records in the Bikou terrane (northwestern Yangtze Block, Central China) have produced different tectonic models (e.g., arc, rift and ridge settings), confusing the relationships between the Bikou and Yangtze blocks in the Neoproterozoic. In addition to the Neoproterozoic Bikou Group igneous rocks and Hengdan Group sedimentary strata which have been well investigated, clastic sedimentary records in the westernmost regions are also crucial for understanding the Neoproterozoic tectonic evolution of the terrane. Here, we target a 3000 m-thick siliciclastic outcrop section along the Bailongjiang River and conduct sedimentological, petrographic, elemental geochemical and zircon U-Pb geochronological analyses to constrain depositional time, sediment provenance and tectonic settings. The outcrop indicates an overall coarsening-upward sedimentary succession, from deep-water sequences to ice-water sequences with dropstone textures. Two volcanic tuffaceous rocks from the basal and middle parts of the succession were dated as 773.2 ± 5.9 Ma and 758.0 ± 8.7 Ma, respectively. The upper parts are characterized by thick, continuous, poorly-sorted glacial diamictites and are conformably overlain by Ediacaran strata. We contend that these diamictites are most likely equivalents to the widespread Cryogenian strata in the Yangtze Block. While provenance analysis results demonstrate felsic bedrock-dominated source terranes with minor mafic contributions, the dropstone gravel components, sandstone petrography and heavy mineral compositions and fine-grained sedimentary rock element concentrations are evidently variable vertically, revealing temporal variations in sediment source-to-sink systems in this region during the late Tonian–Cryogenian. We suggest that the late Tonian deposits were most likely fed by proximal mixed sources with continental arc-related and recycled orogenic terranes, whereas the Cryogenian deposits were dominated by detritus from recycled orogenic terranes. Climate change during the Neoproterozoic glacial-interglacial cycles also played a crucial role in the variable deposit compositions by regulating sediment transport agents and abilities. We favor that the Bikou terrane connected with the Yangtze Block under convergent settings during the early-middle Neoproterozoic and was subsequently under extensional settings. Such findings are a significant step in our understanding of the role of the Yangtze Block in global Neoproterozoic tectonic and climatic frameworks.

Locating the Yangtze Block in Nuna: Constraints from age and isotopic data from Paleoproterozoic sedimentary rocks in the Phan Si Pan Zone, northwest Vietnam

New U-Pb ages and Hf isotopic data for detrital zircons from the late Paleoproterozoic sedimentary rocks (Suoi Chieng Formation) in the Phan Si Pan Zone of northwest Vietnam constrain their depositional ages, provenance, and tectonic setting. The weighted average age of the youngest coherent zircon population is 1769 ± 31 Ma (MSWD = 1.3, n = 8), which provides a maximum depositional age for the Suoi Chieng Formation. The detrital zircons obtained from six Paleoproterozoic samples exhibit various ages, showing multiple magmatic and metamorphic events. The detrital zircon age pattern shows that magmatic activities in the source areas occurred at 2650 Ma, 2350–2150 Ma, and 2050 Ma, as well as minor activities at 2850 Ma, 1850 Ma, and 1780 Ma. Additionally, a prominent metamorphic age peak is observed around ca. 1850 Ma, while a few metamorphic zircons exhibit ages ranging from 2600 to 2500 Ma and 2100 Ma. This pattern indicates that the primary source of these zircons is the local basement rocks of the Phan Si Pan Zone and SW Yangtze. However, contributions from other adjoining cratonic blocks are also possible and these likely included the Rae craton of North Laurentia. Our new data verifies and expands earlier results, showing that the SW Yangtze and north Yangtze have distinct Archean-Paleoproterozoic crustal evolutionary histories prior to their assembly in the late Paleoproterozoic. Regional correlations based on detrital zircon spectra, stratigraphic and tectono-thermal histories show strong similarities between the SW Yangtze and the Rae craton of north Laurentia throughout the Paleoproterozoic, suggesting the proximity of the two blocks before or during the formation of the Nuna supercontinent. We further suggest that the north Yangtze adjoined southern Siberia in the Archean to late Paleoproterozoic as supported by their identical tectono-thermal histories and detrital zircon age spectra. In combination with published data, we propose that the Yangtze Block was located between north Laurentia and south Siberia during the Nuna assembly.

Neoproterozoic slab window in the western Yangtze Block, South China: Evidence from adakitic granodiorites, gabbro-diorites and high-K granites in the Panxi arc belt

Neoproterozoic igneous rocks are widespread along the margins of the Yangtze Block and record key information on the tectonic evolution of the South China Craton. In this paper, we present a comprehensive study of zircon U-Pb ages, whole-rock geochemistry and Sr-Nd-Hf isotopes on the Neoproterozoic granodiorites, gabbro-diorites, and high-K granites in the western Yangtze Block. The granodiorites, gabbro-diorites and high-K granites were dated at 848 ± 5.3 Ma, 824 ± 5.1 Ma and 819 ± 1.5 Ma, respectively. The granodiorites have high Sr (567 ∼ 607 ppm) contents and Sr/Y (84.4 ∼ 88.0) ratios and low alkalis (Na2O + K2O = 4.3 ∼ 4.9 wt%) and Y (6.6 ∼ 7.1 ppm) and Yb (0.7 ∼ 0.9 ppm) contents, similar to typical adakitic rocks. They show low MgO (0.9 ∼ 1.3 wt%), Cr (15.2 ∼ 26.7 ppm) and Ni (3.6 ∼ 4.4 ppm) contents and positive εNd(t) (+1.7 to + 4.2) and εHf(t) (+3.0 to + 8.2) values. The parental magmas for granodiorites were generated by partial melting of thickened juvenile mafic lower crust. The gabbro-diorites are characterized by the enrichment in LREEs, significantly negative Nb, Ta and Ti anomalies, and positive εNd(t) (+2.2 to + 4.2) and εHf(t) (+5.8 to + 10.4) values. They are inferred to be derived from partial melting of lithospheric mantle modified by subduction-related fluids. The high-K granites display high Zr + Nb + Ce + Y (208 ∼ 366 ppm) and alkali (Na2O + K2O = 7.6 ∼ 8.4 wt%) contents, high 10000*Ga/Al (2.31 ∼ 2.64) ratios and zircon saturation temperatures, similar to A-type granites. We suggest that these granites were formed by the partial melting of felsic crustal materials under low pressure and high temperature conditions. The association of granodiorites, gabbro-diorites and high-K granites indicates Neoproterozoic oceanic slab subduction in the western Yangtze Block. We suggest that a subducting oceanic slab window can account for Neoproterozoic magmatism in the western Yangtze Block.

Precambrian Tectonic Affinity of Hainan and Its Evolution from Columbia to Rodinia

The assembly and break-up of supercontinents have been hot research topics in international earth sciences because they represent a breakthrough in reconstructing the history of continental evolution and deepening the theory of plate tectonics, which is of indispensable importance to the development of earth sciences. With the continuous enrichment of paleomagnetic, paleontological, chronological, and geochemical data in the last two decades, the evolution of the supercontinent from Columbia to Rodinia has gradually gained unified understanding, and the reconstruction of the major plates within the supercontinent has basically been constrained. In contrast, the reconstruction of microplates, such as South China, Tarim, and Kabul, is controversial and has now become a popular topic and frontier area of supercontinent reconstruction. Hainan lies at the southern tip of South China, and a few Proterozoic units are exposed on the island. At present, Hainan is often taken as a part of the Cathaysia Block. However, due to the lack of exposed Mesoproterozoic igneous and supercrustal rocks in Cathaysia, the reconstruction model of the Cathaysia Block and even the South China Craton based solely on Mesoproterozoic units in Hainan are distinct from those based on units in the Yangtze Block and younger Proterozoic units within the Cathaysia Block, which makes the paleoposition of the South China Craton controversial. In this paper, we provide new detrital zircon U–Pb age data for the Baoban Complex, Hainan, together with the available data to comment on the affinities between Hainan and the Yangtze and Cathaysia Blocks in the Proterozoic, and on this basis, we can reconstruct the South China Craton within the Proterozoic supercontinents.

A new age, provenance and tectono-sedimentary setting of the middle Dengying Formation of the terminal Ediacaran in the western Yangtze Block, South China

The widespread terminal Ediacaran Dengying Formation in the western Yangtze Block, South China, is dominated by thick marine dolomite successions deposited in shallow-water platform environments. However, a distinctive succession of mixed clastic and carbonate rocks occurs widely in the middle Dengying Formation, representing sedimentary environment variations. The absolute age of the mixed sedimentary succession is poorly constrained, and tectonic and sedimentary settings remain controversial. In this study, we report a new zircon U–Pb age of 547.3 ± 4.8 Ma from a volcanic ash bed intercalated within a mudstone succession (ca. 40–100 cm thick) in the middle Dengying Formation in the western Yangtze Block. This age is consistent with previously published ages of the middle Dengying Formation or its equivalents in the Yangtze Block and in other blocks worldwide. Field investigations show that the mudstone succession overlies above an unconformity resulted from the lower Dengying Formation weathering and shows a deepening upwards depositional succession representing a period of transgression. Mineralogical analyses show that the mudstones are composed of clay minerals (ca. 68.7%, illite and illite/smectite mixed layers), quartz (24.2%) and minor K-feldspar, indicating a high compositional maturity. Redox sensitive element results (e.g., Mo and U) indicate an increasing anoxic setting upwards. Both elemental geochemical and Lu-Hf-O isotopic analyses suggest that the siliciclastic sediments were moderately weathered (average CIA = 71) and were derived from felsic-intermediate rocks, such as granites and granodiorites (or geochemically-equivalent rocks). We suggest that the mixed sedimentary succession represents as a short-lived, regionally-correlated sea level change event, rather than a tectonic uplift event. Our geochemical data reveal that the mudstone succession accumulated most likely in a passive margin setting, supporting the proposition that the Yangtze Block was located on the periphery of the Gondwana supercontinent during the Ediacaran Period.

Precambrian tectono-magmatic evolution of the western margin of the Yangtze block, South China: evidence from zircon U-Pb-Lu-Hf isotopes, REE and trace elements of Yuanmou-Miyi complexes

ABSTRACT Here new evidence of the isotope geochronology and the geochemistry from the Yuanmou-Miyi complexes reveal the Precambrian tectono-magmatism geodynamic process in the study area. The Yangtze Block basement with 1.81- ~1.77 Ga and negative ε Hf(t) values of −7.8 to −0.7 corresponded to the assembly of the Columbia-supercontinent. However, the rifting magmatic event of ~ 1.70 Ga, which caused the continental crust extension-thinning in the study area, may exert influence on the subduction-magmatism in the Grenvillian period. The Yuanmou-Miyi complexes of 1.19- ~1.00 Ga are characterized by (La/Sm)N＞1, depletion in HFSEs, negative anomalies of Nb-Ta, and enrichment of LILEs, which show a genetic link with the subduction-magmatism. The temporal tendency that the magmatism from Yuanmou to Miyi gradually became younger, may be responsible for the subducting direction. The subduction caused the reactivation of the Anninghe-Lvzhijiag fault zone, and created the rupture windows of the subduction slab, leading to upwelling of the deep magma. These granitoids were derived from the partial melting of the mantle wedge, while the small-scale gabbro dikes intruded into the granitoids were likely spillover products from the asthenosphere mantle melts through the rupture windows.

Early to Middle Jurassic sedimentation within the northern Sichuan Basin in response to exhumation of the South Qinling orogenic belt, central China

The northern Sichuan Basin of central China is an important region for investigating the continent-continent convergence that resulted from the closure of the Palaeo-Tethys Ocean. The Early-Middle Jurassic sediments of this basin record the palaeogeography, palaeoenvironments and tectonic evolution of the southern Qinling orogenic belt, especially the Dabashan thrust belt. Detailed sedimentological investigations, coupled with provenance analyses using zircon UPb geochronology, demonstrate that during Early Jurassic times, conglomerate megafan facies extended across the northern Sichuan Basin into the Micangshan area deformed by South Dabashan thrusting and reached even as far as the Mianlue suture belt. These megafan deposits were mainly reworked from the Silurian sediments of the North Dabashan thrust belt (also called the South Qinling orogenic belt), north of the Mianlue suture. These conglomerates record the initial thrust-driven uplift and exhumation of the Qinling orogenic belt that rapidly followed the diachronous closure of the Mianlue oceanic basin. Moreover, our results show that the detrital materials of meandering stream, lacustrine and braided river delta facies that accumulated in the Middle Jurassic mainly originated from the southern Qinling orogenic belt, Mianlue suture zone and northern South China block, suggesting the southward progradation of the Dabashan fold-thrust belt onto the northern Yangtze block and the southward migration of the basin centre. These findings support a model in which the Early to Middle Jurassic evolution of the northern Sichuan Basin may have been triggered by East Asian convergence.