Northwestern Yangtze Block Research Articles

Climatic-hydrologic influence on redox condition in the Cryogenian interglacial Nanhua Basin: Insights from the Datangpo Formation in the northwestern Yangtze Block, South China

Tracking climatic and oceanic redox changes throughout the Neoproterozoic Cryogenian is crucial to a better understanding of the coevolution of life and environment in geological history. However, the processes and driven mechanisms of redox evolution in the Cryogenian interglacial ocean still remain uncertain. Here, we present a combined study of geochemical proxies, such as paleoclimatic proxies (CIA and Zr/Al) and paleosalinity proxies (B/Ga and Sr/Ba), from the interglacial Datangpo Formation in the shallow-water Zuojiawan section exposed in the northwestern Yangtze Block, to constrain climatic and hydrologic changes on redox conditions. Our data indicate that two apparently climatic cycles from warming to cooling occurred during the deposition of the Datangpo Formation, supplementing the monotonous climatic warming in previous research during the Cryogenian interglaciation. The low B/Ga and Sr/Ba ratios throughout Datangpo Formation suggest a continuous freshwater condition, which is in contrast to a transition of marine condition to brackish condition in relatively deep-water sections. Compiled salinity reconstructions at different paleo-depths imply the salinity gradient in the Nanhua Basin with a density stratification like modern Baltic Sea, at least during the early Cryogenian interglacial period. We find that the climate-driven local riverine freshwater input may be a significant driver for maintaining long-term freshwater condition in the Zuojiawan section. Accompanied by synchronously climatic warming, the progressive desalinization with declining B/Ga and Sr/Ba ratios in the middle to upper deposition of the Datangpo Formation are probably attributed to the persistent freshwater input to a nearshore shallow-water paleogeographic setting. Combined the improving oceanic oxygenation backdrop with the elevating seawater sulfate concentration in the Nanhua Basin in this period, we conclude that the continuous dilution of seawater in the restrict (semi-) Nanhua Basin could attenuate the density stratification and enhance the vertical mixing of watermass, favoring the ventilation of deepwater and penetration of sulfate. This contribution provides a new insight for the study of salinity changes on redox conditions in the Yangtze Block during the Cryogenian interglacial period, and provides new data for supporting a systematic understanding of the evolutional mechanism of oceanic redox state.

Neoproterozoic tectonic evolution of the northwestern Yangtze Block: Constraints from the Bijigou intermediate-acid magmatism in the Hannan massif

Neoproterozoic tectonic evolution of the northwestern Yangtze Block: Constraints from the Bijigou intermediate-acid magmatism in the Hannan massif

Late Mesoproterozoic passive continental margin in the northwestern Yangtze Block, South China: Insights from the Huodiya Group in the Hannan‐Micangshan Massif

Whether the Yangtze Block was involved in the global late Mesoproterozoic orogeny remains contentious. The Mesoproterozoic Huodiya Group exposed in the Hannan‐Micangshan Massif on the northwestern margin of the Yangtze Block could provide sufficient evidence to further clarify the regional tectonic affinity. However, quantitative insights into the depositional age, provenance and basin tectonic setting of the Huodiya Group still lack comprehensively constraining. This study presents a combined analysis of zircon U–Pb dating for meta‐sedimentary rocks of the Shangliang Formation of the upper Huodiya Group and intruded gabbroic dykes. Intergraded dating results constrain the late Mesoproterozoic maximum depositional age at ca. 1050 Ma. Ca. 900 Ma formation age for the intruded gabbroic dyke, plus the minimum formation age of the overlying Xixiang Group at ca. 950 Ma, further suggests its deposition should be prior to ca. 950–900 Ma. Concordant detrital zircon ages define pronounced age clusters of ca. 2229–1741 and 2950–2388 Ma and few age populations of ca. 3232–3082 and 1069–1033 Ma, consistent with the tectonothermal events in proximal domains at the northwestern and northern Yangtze Block. Detrital zircon age distribution patterns and cumulative curves, coupled with the stable carbonate platform and shallow‐marine facies sedimentation, suggest a passive continental margin setting for the Huodiya Group on the Yangtze Block margin at the late Mesoproterozoic. Comparative insights into the contemporaneous passive continental margin from north to southwest argue against the existence of a late Mesoproterozoic orogeny along the exterior periphery of the Yangtze Block.

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.

Late Triassic successive amalgamation between the South China and North China blocks: Insights from structural analysis and magnetic fabrics study of the Bikou Terrane and its adjacent area, northwestern Yangtze block, central China

ABSTRACT The Bikou Terrane, located at the conjunction of the Longmenshan fold-thrust belt and the west Qinling orogenic belt in centeral China, was involved in the Late Triassic collision between the South China and North China blocks. The Bikou Terrane has preserved crucial information on structural geometry and kinematics of Triassic tectonics, and is therefore of great importance for reconstructing the Paleo-Tethyan evolutionary history. However, multi-phase tectonic events of the Bikou Terrane are unsettled. This work presents detailed structural analysis based on both the field and laboratory works, which reveals three phases of deformation events in Bikou and its adjacent areas, including top-to-the-SW shearing related to SW-ward thrusting (DI) mainly to the north of the Bikou Terrane, top-to-the-NNW shearing related to NNW-ward thrusting (DII) in the Bikou Terrane, and strike-slip faulting (DIII) locally developed in the northern Bikou Terrane. Anisotropy of magnetic susceptibility (AMS) study and related structural analysis not only support the multiphase deformation but also reveal a gradual transition from the DII-related magnetic fabrics to the DIII-related magnetic fabrics in the Bikou Terrane. Integrating published geochronological data, it is constrained that DI occurred at ca. 237–225 Ma, DII occurred at ca. 224–219 Ma, and DIII possibly occurred during the Early Cretaceous. Based on regional tectonics, the DI event corresponds to the collision between the South Qinling block and the Bikou Terrane, and the DII event reflects the intracontinental amalgamation between the Bikou Terrane and the Yangtze block, which indicates a Late Triassic successive amalgamation from the North China block to the South China block. Intracontinental adjustment represented by the strike-slip (DIII event) occurred after the final amalgamation between the North China and South China blocks. By applying AMS on deciphering structural geometry and multi-phase deformation, our study suggests that AMS is a useful tool for structural analysis.

Neoproterozoic metavolcanic suites in the Micangshan terrane and their implications for the tectonic evolution of the NW Yangtze block, South China

The Micangshan terrane is located along the NW margin of the Yangtze block and contains the second oldest Precambrian basement in South China. It comprises the Beiba and the Wangcang-Nanjiang segments, while the oldest suites (∼2.1 Ga Houhe complex) occur in the Beiba. The Hekou and the Wangjiaping metamorphic volcano-sedimentary sequences in the Wangcang are characterized by intensive metamorphism and deformation, long being regarded as the equivalent of the Houhe suite. This contribution presents an integrated study of zircon U-Pb geochronology and Hf isotope, whole rock elemental and Sr-Nd isotope geochemistry on volcanic rocks of the metastrata. It reveals that they formed at ∼ 865–860 Ma and underwent upper amphibolite facies metamorphism at ∼ 815 Ma with lower limits in P-T conditions of ∼ 5.0 kbar and ∼ 650–660 °C. The Hekou metavolcanics comprise dacitic-rhyolitic and alkaline basaltic rocks with minor tholeiitic mafics, whereas those of the Wangjiaping are composed of dacitic and tholeiitic mafic rocks. The Hekou alkaline metamafics show affinity to high-Nb to Nb-enriched basalts (HNB-NEB) with εNd(t) values of + 5.80 to + 6.25, whereas tholeiitic metamafics of the both sequences display characteristics typifying high-Al basalt (HAB) with εNd(t) value ∼+2.66. The Hekou and the Wangjiaping metafelsics have εNd(t) ranges of −2.33 to + 5.60 and −0.82 to + 0.99 (except one + 2.29), respectively, and the dacitic rocks show features akin to the high Mg-number andesites. The ∼ 865–860 Ma magmatism is suggested to have occurred in an extensional basin with a post-subduction affinity. It was induced by upwelling of convective asthenosphere succeed the ceased slab subduction: the Wangjiaping metafelsics were derived from partial melting of juvenile lower crust, the Hekou dacitic rocks were produced by partial melts of the relict of subducted oceanic crust interacted with mantle wedge; the Hekou HNB-BEB were sourced by metasomatized mantle wedge, whereas the HAB lavas were generated by anhydrous partial melts of the upwelling asthenosphere. The ∼ 815 Ma metamorphism was caused by an amalgamation between the South Qinling belt and the proto margin of the northwestern Yangtze block. This event was followed by an expanding rift from the Micangshan successively to the South Qinling.

Timing and nature of intracontinental deformation at the northwestern margin of the Sichuan Basin, China: evidence from structural analysis and detrital zircon U–Pb ages

AbstractThe theory of plate tectonics suggests that deformation occurs mainly along plate boundaries; however, compression can result in the formation of orogens and basins within intracontinental settings. During these two tectonic processes, the sedimentation and environmental changes occur in response to marginal and intracontinental deformation. Early Jurassic – Early Cretaceous deformation and basin formation along the Qinling orogenic belt and the northwestern Sichuan Basin in central–SW China are ideal for investigating a reactivated tectonic belt and basin formation. We studied the Lower Jurassic – Lower Cretaceous sedimentary sequences and structures along the northwestern margin of the Sichuan Basin, and obtained detrital zircon U–Pb ages for these rocks. The structures show that deformation migrated SE-wards and S-wards into the Sichuan Basin along the Longmen Shan, Micang Shan and Daba Shan tectonic belts during middle–late Mesozoic time. The Lower Jurassic oligomictic conglomerates have a smaller grain size and thicken towards the south, indicating protracted transport from a northern source. The conglomerates deposited near-source record post-orogenic south-vergent thrusting during the Late Triassic – Early Jurassic epochs. The Lower Cretaceous conglomerates and sandstones have multiple sources, which indicate that they were rapidly deposited near their source, synchronous with thrusting that occurred in response to coeval SE-wards and S-wards thrusting in the Longmen Shan and Daba Shan tectonic belts during the Late Jurassic – Early Cretaceous epochs. Detrital zircon grains from the Lower Jurassic – Lower Cretaceous sedimentary rocks yielded age peaks of 2600–2200, 1850–1600, 850–700, 540–400, 250–180 and 180–140 Ma. A comparison of these ages with those of surrounding exposed rocks indicates that the sediments in the northwestern Sichuan Basin were supplied from the Qinling orogenic belt, the northwestern Yangtze Block, the south margin of the North China Block and the Songpan–Garzê Terrane. The youngest peaks of detrital zircon U–Pb ages at 207 and 159 Ma constrain the two stages of intracontinental shortening and highlight the link between intracontinental deformation and sedimentation.

Neoproterozoic active margin in the northwestern Yangtze Block, South China: new clues from detrital zircon U–Pb geochronology and geochemistry of sedimentary rocks from the Hengdan Group

AbstractThe Yangtze Block in South China constitutes an important Precambrian landmass in the present East Asian continent. The Neoproterozoic sedimentary successions of the Hengdan Group in the NW Yangtze Block record essential information for deciphering the Neoproterozoic tectonics along the NW margin. However, its depositional age, provenance and tectonic properties remain uncertain. Here, a combined analysis of detrital zircon U–Pb dating and geochemistry is performed on representative samples from the Hengdan Group. Concordant dating results of samples from the bottom and upper parts constrain the maximum depositional age atc.720 Ma. Detrital zircon age patterns of samples reveal a uniformly pronounced age peak atc.915–720 Ma, which is consistent with the magmatic pulses in domains at the NW end of the Yangtze Block. In addition, these samples display left-sloping post-Archaean Australian shale (PAAS)-normalized rare-earth element patterns and variable trace element patterns, resembling sediments accumulated in a basin related to an active continental margin geodynamic setting. Provenance analysis reveals that the main sources featured intermediate to felsic components, which experienced rapid erosion and sedimentation. These integrated new investigations, along with previous compilations, indicate that the Hengdan Group might have been deposited in a fore-arc basin controlled by subduction beneath the Bikou Terrane. Thus, such interpretation further supports proposals for subduction-related tectonics along the western margin of the Yangtze Block during the early Neoproterozoic.

Origin of mafic intrusions in the Micangshan Massif, Central China: Implications for the Neoproterozoic tectonic evolution of the northwestern Yangtze Block

The mafic intrusions exposed in the Micangshan Massif preserve important information pertaining to the Neoproterozoic tectonic evolution along the northwestern margin of the Yangtze Block. In this paper, we report new LA-ICP-MS zircon U–Pb ages, Lu–Hf isotopes and whole-rock compositions for mafic sills in the Micangshan Massif to understand their petrogenesis and supplement the regional tectonic evolution. Xenocrystic zircon grains from two mafic sill samples yield weighted mean ages of 860 ± 4 Ma and 858 ± 5 Ma, respectively, suggesting the maximum emplaced age of the mafic sills was at ca. 860 Ma. A granitic dyke sample which intruded into the mafic sills, yields a crystallization age of 799 ± 4 Ma, implying the minimum emplacement of the mafic sills was at ca. 800 Ma. These results indicate that the mafic sills were formed within the span of ca. 860–800 Ma. Geochemical results show that most of these mafic sills are tholeiitic in composition and are characterized by low to moderate FeOtotal and TiO2 contents, as well as moderate to high MgO contents and Mg# values. They display distinct enriched light rare earth element (LREE) patterns, variable remarkable enrichment in large-ion lithophile elements (LILEs; e.g., Cs and Ba), and evident depletion of high-field strength elements (HFSEs; e.g., Nb, Ta and P), resembling signatures of typical subduction-related basaltic rocks. In addition, these sills exhibit comparatively high Nb/Ta, moderate Nb/Yb, as well as low Nb/La, Th/Yb, and Nb/Y ratios, indicating that they probably originated from partial melting of a previously metasomatized mantle wedge source. These observations, together with regional tectonics, suggest that these mafic sills in the Micangshan Massif might be formed in a subduction-related continental arc setting. Therefore, combining our new data with available results from previous studies in the adjacent regions, our study supports the dominant subduction-related tectonics along the northwestern margin of the Yangtze Block during the Early Neoproterozoic time.

Identification of ca. 2.65 Ga TTGs in the Yudongzi complex and its implications for the early evolution of the Yangtze Block

In this contribution, we carry out an integrated study of petrology, whole-rock geochemistry, as well as zircon morphology, trace element, U–Pb and Lu–Hf isotopes for granitic gneisses and leucogranites from the Yudongzi complex, a poorly studied Archean outcrop in the northwestern Yangtze Block. The granitic gneisses have the TTG affinity characterized by relatively high Na2O/K2O and Sr/Y ratios, and fractionated REE pattern with relatively high LREE but low HREE concentrations. Zircon grains in the granitic gneisses show magmatic cores and metamorphic rims. U–Pb dating on zircon core of four granitic gneiss samples yielded their formation ages as 2660–2632 Ma with a weighted average of 2650 ± 21 Ma (MSWD = 1.6; n = 4). Their average εHf (t) values range from −0.6 to 0.5 and two-stage model ages from 3.20 to 3.10 Ga, suggesting that the granitic gneisses were likely generated by remelting of thickened Mesoarchean proto-mafic crust. The metamorphic zircon rims have relatively high Th/U ratios and identical Hf isotope compositions to their corresponding cores, indicating that the rims were formed via solid-state recrystallization process. The metamorphic zircons recorded a metamorphic event at ca. 2.5 Ga. U-Pb dating and Lu-Hf isotopes on magmatic zircons from a leucogranite sample revealed a magmatic event at 2477 ± 18 Ma. The recognization of the 2.65 Ga TTG magmatism and the 2.48 Ga intercrustal remelting event in the Yudongzi complex demonstrates the widespread Neoarchean magmatism in the Yangtze Block. Combined with the age spectra of previous detrital zircons, we show that the Yangtze Block has experienced a major tectonothermal event in the late Archean. The Yudongzi complex has a distinct crustal evolution history compare with the Paleoarchean continent nucleus in the eastern part of the Yangtze Block, suggesting that the continent nucleus might have subjected to lateral growth by accretion processes.

Neoproterozoic continental back-arc rift development in the Northwestern Yangtze Block: Evidence from the Hannan intrusive magmatism

Neoproterozoic intrusions are widespread in the northwestern margin of the Yangtze Block, and their petrogenesis was closely related to the assembly and breakup of Rodinia supercontinent. A combined study of zircon U–Pb dating, geochemistry and Sr–Nd–Hf isotopes was carried out for three types of intrusive rocks from the Hannan region, including the Beiba gabbros, the Tianpinghe and Taojiaba I-type granites and the Tiechuanshan and Huangguan A-type granites. LA-ICP-MS zircon U–Pb dating results show that the studied intrusions have crystallization ages ranging from ca. 880 to 770Ma. The ca. 880Ma Beiba gabbros have variable contents of MgO, Al2O3, Cr and Ni and extremely low high field strength elements (HFSE) concentrations that indicate a cumulate origin. These gabbros display subduction-related geochemical signatures with whole-rock initial 87Sr/86Sr (ISr)=0.704–0.705, εNd(t)=−0.4 to +0.6 and zircon εHf(t)=+6.9 to +8.4, suggesting that they were derived from the partial melting of an enriched lithospheric mantle that had been modified by slab-derived materials. The ca. 860Ma Tianpinghe I-type granites have high ISr (> 0.704) and negative εNd(t) (−5.6 to −3.5) values and zircon εHf(t)=−1.9 to +1.6, indicating their sources were mixtures of Neoproterozoic juvenile mafic crust and minor Paleoproterozoic crustal components, whereas the ca. 770Ma Taojiaba I-type granites have relatively low ISr (<0.704) and positive εNd(t) (+1.5 to +2.4) with zircon εHf(t)=+6.2 to +9.5, implying that they were dominantly sourced from the Neoproterozoic juvenile crustal materials. The geochemical and Sr–Nd–Hf isotopic data imply that the ca. 780Ma Tiechuanshan A-type granites were crystallized from a reduced and peralkaline magma, which was generated by partial melting of a heterogeneous source involving Neoproterozoic juvenile and ancient mafic crust under dry and reducing conditions. In contrast, the parental magma of the ca. 774Ma Huangguan A-type granites was oxidized, aluminous and calc-alkalic and resulted from the dehydration melting of tonalitic to granodioritic rocks under water-subsaturated and oxidizing environments. In combination with previous work, the ca. 880–860Ma Beiba gabbros and Tianpinghe I-type granites are interpreted to be formed during the stage of initial back-arc extension that was caused by subduction-induced convection in the mantle wedge. Subsequently, the ca. 780–770Ma Tiechuanshan and Huangguan A-type granites and Taojiaba granodiorites were formed in a continental back-arc rift setting related to the rollback of subducted oceanic slab. Our new results support that an arc-back-arc system developed in the northwestern Yangtze Block during the Neoproterozoic.

Zircon U‐Pb geochronology and geochemistry of the metabasite and gabbro: Implications for the Neoproterozoic and Paleozoic tectonic settings of the Qinzhou Bay‐Hangzhou Bay suture zone, South China

The Qinzhou Bay‐Hangzhou Bay Suture Zone (QHSZ) lies between the northwestern Yangtze Block and the southeastern Cathaysia Block in South China. An integrated zircon U‐Pb age, whole‐rock and mineral geochemical analyses have been enabled for understanding the Neoproterozoic and early Paleozoic geological evolution of the QHSZ. Zircon U‐Pb dating reveals that the Guizi metabasite and Zhuya gabbro were formed at 948 ± 11 Ma and 450.8 ± 2.9 Ma, respectively. The Guizi metabasites are characterized by Nb enrichments (8.18–16.11 ppm) with high Nb/La (0.76–0.98) and Nb/U (17.86–25.65) ratios. The samples are also LREE/HREE ([La/Yb]N = 2.66–3.29) and LILEs (Rb, Th, U and Pb) enriched with weakly depleted HFSEs (Nb, P and Ti), resembling the Nb‐enriched basalts. These geochemical signatures were likely to be originated from a newly subducted slab. Elemental contrasts illustrate that the Guizi metabasites are geochemically consistent with the Neoproterozoic Nb‐enriched basalts along the Wuyi‐Yunkai Domain, South China. Age and elemental characteristics of the Guizi samples suggest an earliest Neoproterozoic subduction within the QHSZ. The Zhuya gabbro samples have high contents of MgO (11.23–14.48%), Cr (520.8–782.7 ppm) and Ni (126.1–203.7 ppm), (La/Yb)N = 1.89–2.41, Eu/Eu* = 0.84–0.90, positive LILEs (Rb, Th, Sr and Pb) anomalies and negative HFSEs (Nb, P and Ti) anomalies. Clinopyroxene geochemical data show a significant arc cumulates trend. They were considered to be derived from an E‐MORB‐like source modified by subduction‐related fluids. Geochemical modelling indicates that a mixture of the Neoproterozoic mafic magma and SCB sediments (20–40%) can match the elemental and Sm‐Nd isotopic compositions of the Zhuya gabbros, which suggests the involvement of Neoproterozoic subduction‐modified mantle. Combined with regional geological and geochemical data, it is concluded that the Yangtze and Cathaysia blocks were likely sutured along the QHSZ forming an earliest Neoproterozoic arc‐back‐arc system, and the early Paleozoic mafic rocks are dominantly remelting products of the Neoproterozoic subduction modified mantle.

The Shimian ophiolite in the western Yangtze Block, SW China: Zircon SHRIMP U-Pb ages, geochemical and Hf-O isotopic characteristics, and tectonic implications

The Shimian mafic-ultramafic rocks in the western Yangtze Block is a structurally dismembered complex which is intruded by the Neoproterozoic granite and faulted against the Neoproterozoic Suxiong Group. In this study, we divide the Shimian mafic-ultramafic rocks into two suites including the Late Mesoproterozoic Shimian ophiolite and Early Neoproterozoic gabbros, based on differences in the field occurrence, age, and geochemical characteristics. The Late Mesoproterozoic Shimian ophiolite mainly consist of serpentinites, gabbros, basalts, and rodingites. Zircon U–Pb dating using a sensitive high-resolution ion microprobe (SHRIMP) on a gabbro sample yielded an age of 1066±11Ma, representing the formation age of the Shimian ophiolite. All basalt and gabbro samples are tholeiitic and have normal mid-ocean ridge basalt (N-MORB)-like immobile element patterns with negative Nb and Ti anomalies. The low zircon δ18O (4.6–5.6 ‰) and high positive zircon εHf(t) (+10.8 to +18.3) values indicate that these rocks were derived from a depleted mantle source. These features are comparable with those of the basaltic rocks in SSZ-type ophiolite. The Early Neoproterozoic gabbros intruded into the Shimian ophiolite at ca. 937Ma. The geochemical and isotopic characteristics of these gabbros are similar to those of the island arc basalts. Integrating previous studies with the data presented in this contribution, we suggest that the northwestern Yangtze Block has probably witnessed the Late Mesoproterozoic–Early Neoproterozoic diachronous subduction.

Tectonic Settings and Geological Implications of Neoproterozoic Liujiaping VMS Deposit, Northwestern Yangtze Block, China

AbstractLiujiaping VMS (volcanic massive sulfide) deposit contains mainly copper and zinc, which is located at the Longmenshan orogenic belt of the northwestern margin of Yangtze block. The deposit is hosted in Neoproterozoic Datan terrane (composed of Datan granitoids and Liujiaping group) and is a typical, and the biggest, VMS deposit in this area. The Datan granitoids and Liujiaping group are contemporary and both parental magmas have the same genesis. The tectonic evolution history of Northwestern Yangtze is complicated. Chronology, isotope and geochemistry of the Liujiaping VMS ores and wall rocks (especially the Datan granitoids) are analyzed to restrict the tectonic progress. High‐precision secondary ion mass spectrometry (SIMS) analysis of the Datan granitoids resulted in two concordant ages, 815.5 ± 3.2 Ma and 835.5 ± 2.6 Ma, which are contemporary with the Liujiaping Cu–Zn ore and volcanics. The wall rocks are characterized by enrichment in LREE and with a weak negative anomaly of Eu. The Pb isotope data of sulfide and volcanics from the Liujiaping deposit indicate that the material source is lower crust. Together with variable negative anomalies of high strength field elements HFSE (Th, Nb, Ta, Zr, Hf, P and Ti), positive εNd (825 Ma) values (+1.8 to +3.1) and the Nd model age T2DM = 1.2–1.3 Ga, it shows that the Liujiaping deposit and wall rocks were formed by partial melting of Mesoproterozoic lower crust. Geological and geochemical characteristics of Liujiaping deposit indicate that this deposit was formed during subduction of the oceanic crust. This study clarified that that the Liujiaping deposit and the northwestern margin of the Yangtze block were part of an arc setting at ~820 Ma rather than intra‐continental rift.

Neoproterozoic alkaline intrusive complex in the northwestern Yangtze Block, Micang Mountains region, South China: petrogenesis and tectonic significance

ABSTRACTVoluminous Neoproterozoic mafic–ultramafic, felsic, and alkaline intrusions are found in the northern Yangtze Block, South China. Here, we present whole-rock major and trace element, and Sr–Nd isotopic compositions, together with zircon U–Pb ages, for syenite and gabbro samples from the Shuimo–Zhongziyuan alkaline intrusive complex in the Micang Mountains region at the northwestern margin of the Yangtze Block. Zircon U–Pb dating yields crystallization ages for the Na- and K-rich Shuimo syenites of 869 ± 4 (MSWD = 0.85, 2σ) and 860 ± 5 Ma (MSWD = 0.47, 2σ), respectively, and for the Zhongziyuan gabbros of 753 ± 4 Ma (MSWD = 0.23, 2σ), indicating that the syenites and gabbros represent different stages of magmatism. The syenites include both Na- and K-rich types and have high values of the Rittman index (σ), and high SiO2 and Na2O + K2O contents. These syenites are enriched in light rare earth elements (LREE) and large-ion lithophile elements (LILE), but depleted in high-field-strength elements (HFSE), with high (La/Yb)N values and small negative and positive Eu anomalies (Eu/Eu* = 0.74–1.17). In contrast, the gabbros have lower SiO2 and Na2O + K2O contents, are only slightly enriched in LREEs, are enriched in LILE but depleted in HFSEs, and have small negative and positive Eu anomalies (Eu/Eu* = 0.86–1.37). The syenites have low initial 87Sr/86Sr (0.703340) and ɛNd(t) values (+1.9 to +7.7). The gabbros have relatively high initial 87Sr/86Sr (0.703562–0.704933) and positive ɛNd(t) values (+1.6 to +4.5). These data suggest that the syenites and gabbros are isotopically similar and were largely derived from melts of depleted mantle. The syenites underwent significant fractional crystallization and small amounts of crustal contamination during magma evolution. In contrast, the gabbros were formed by partial melting (>15%) of a garnet lherzolite source and might also have experienced crustal assimilation. Taking into account the geochemical signatures and magmatic events, we propose that the Shuimo syenites formed in an intra-arc rifting setting, however, the Zhongziyuan gabbros were most likely produced in a subduction-related, continental margin arc setting during the Neoproterozoic, thus suggesting that the alkaline intrusive complex were formed by the arc-related magmatism in the Micang Mountains.

U–Pb Zircon Geochronology and Geochemistry of the Neoproterozoic Liujiaping Group Volcanics in the Northwest Margin of the Yangtze Block: Implications For the Breakup of the Rodinia Supercontinent

AbstractInvestigation of the petrogenesis and the origin of zircons from the volcanic rocks of the Liujiaping Group of the back‐Longmenshan tectonic belt in the northwest margin of the Yangtze Block is conducted by analysis of U–Pb geochronology and geochemistry. Results show that selected zircons are characterized by internal oscillatory zonings and high Th/U ratios (0.43–1.18), indicating an igneous origin. Geochronological results of LA–ICP–MS U–Pb dating of the Liujiaping Group zircons yield an age of 809 ± 11 Ma (MSWD = 2.2), implying that the volcanic rocks were formed in the Late Neoproterozoic. Geochemical analysis shows that the rocks are calc‐alkaline, supersaturated in Al, and metaluminous to weakly peraluminous. Rare‐earth elements are present at high concentrations (96.04–265.48 ppm) and show a rightward incline and a moderately negative Eu anomaly, similar to that of continental rift rhyolite. Trace element geochemistry is characterized by evident negative anomalies of Nb, Ta, P, Th, Ti, inter alia, and strong negative anomalies of K, Rb, Sr, et al. We conclude that the Liujiaping Group volcanic rocks resulted from typical continental crust source petrogenesis and were formed in a continental margin setting, which had no relation to subduction, and thus, were the products of partial melting of the lower crust due to crustal thickening caused by active continental margin subduction and arc‐continent collision orogeny in the northwestern Yangtze Block and were triggered by the breakup of the Rodinia supercontinent during the Neoproterozoic.

Geology and genesis of the giant Beiya porphyry–skarn gold deposit, northwestern Yangtze Block, China

The Beiya ore deposit is located in the northwestern Yangtze Block, to the southeast of the Tibetan Plateau, SW China. The deposit is hosted by a porphyritic monzogranitic stock that is cross-cut by a porphyritic granite and later lamprophyre dikes. The whole-rock geochemistry of the porphyritic monzogranite and granite intrusions is both potassic and adakite-like, as evidenced by high K2O/Na2O (2.2 to 24.8), Sr/Y (53.2 to 143.2), and (La/Yb)N (4.9 to 28.9) ratios. Both intrusions have comparable zircon U–Pb ages of ca. 36Ma and εHf(t) values of −6.8 to +2.7. Zircons within these intrusions have Hf isotope crustal model ages with a prominent peak at ca. 840Ma, and both of the intrusions have similar Sr–Nd–Pb isotopic compositions that are comparable to the compositions of amphibolite xenoliths hosted by potassic felsic intrusions in western Yunnan. The contemporaneous lamprophyre dikes show Nb–Ta depletion, enriched (87Sr/86Sr)i and εNd(t), and extremely low Nb/U ratios (1.6–3.6), suggesting that these dikes were formed from magmas generated by partial melting of a metasomatized subcontinental lithospheric mantle (SCLM). The geochemistry of the porphyritic intrusions and the lamprophyre dikes suggests that the Beiya porphyries formed as a result of partial melting of a thickened and K-rich region of the lower crust, triggered by melting of metasomatized SCLM. The ca. 840Ma U–Pb ages and εHf(t) values (−6.8 to +2.7) of xenocrystic zircons within the porphyritic intrusions suggest that these zircons were produced in a continental arc setting at ca. 840Ma. The peak Hf model age of the zircons crystallized from the intrusions and the U–Pb ages of the xenocrystic zircons within the intrusions suggest that these porphyritic intrusions formed from magmas sourced from a juvenile crust that formed at ca. 840Ma. This juvenile crust is most likely the source for the metals within the porphyry–skarn deposits in the study area, as the SCLM-derived lamprophyre dikes in this area are barren.Massive Fe–Au orebodies (~99 million metric tons at an average grade of 2.61g/t Au) within the study area are generally located within the skarn-altered boundary of the porphyritic monzogranite stock and along the faults in the surrounding Triassic carbonates. The Fe–Au orebodies are spatially and genetically associated with skarn comprising garnet and diopside. Petrographic observations show that the massive Fe–Au orebodies mainly consist of hematite and magnetite with disseminated pyrite that hosts native gold and electrum.The porphyritic granite contains porphyry-style mineralization in the form of disseminated and veinlet-hosted pyrite and chalcopyrite. Pyrite-hosted lattice-bound gold is present within both the massive Fe–Au and the porphyry-type mineralization in the study area, and is present at concentrations up to 10ppm Au (as determined by in situ LA-ICP-MS analysis). Subsequent weathering altered the primary magnetite–hematite–sulfide assemblage in the Fe–Au orebody into a magnetite–limonite assemblage, and generated laterite-type mineralization in which gold is hosted by limonite.

Continental flood basalts of the Huashan Group, northern margin of the Yangtze block – implications for the breakup of Rodinia

It is generally accepted that Neoproterozoic extension and dispersal of the supercontinent Rodinia was associated with mantle plume or superplume activities. However, plume-generated contemporaneous continental flood basalts (CFBs) have rarely been identified. In this study, we present geochronological and geochemical evidence for the basalts from the Liufangzui Formation of the Huashan Group in the Dahongshan region of east-central China. A representative sample yields a SHRIMP U–Pb zircon age of 824 ± 9 Ma, interpreted as the crystallization age of the rocks. Geochemically, these basalts belong to the subalkaline tholeiite series and display slight enrichments in light rare earth elements (LREE) and varying degrees of deficiency of high field strength elements (HFSE) such as Nb, Ta, and Ti. This pattern is very similar to that of CFBs from the Bikou Group and Tiechuanshan Formation in the northwestern Yangtze block in China and Siberia in Russia. The basaltic magmas underwent partially-fractional crystallization during ascent, but were not intensely influenced by crustal contamination. The characteristic element ratios and negative Hf isotopic analyses (ϵHf(t) = −6.6–2.6) in zircons indicate that the parental magmas of the basalts might have been derived from an enriched lithospheric mantle rather than from the depleted mantle such as normal mid-ocean ridge basalts (N-MORBs). The geochemical signatures and regional geological characteristics show that these basalts were formed along intraplate continental rifts rather than in island arcs or ocean basins. Considering the coeval basic volcanic rocks in South China, we propose that these Huashan Group basalts represent the remnants of plume-generated CFBs and have close spatiotemporal ties with a coeval basic igneous province in Australia. Our results support the Neoproterozoic location of the South China block adjacent to southeastern Australia in the reconstruction model of the supercontinent Rodinia.

Sedimentary records of the Yangtze Block (South China) and their correlation with equivalent Neoproterozoic sequences on adjacent continents

The Neoproterozoic Danzhou Group, composed of siliciclastic sedimentary rocks interbedded with minor carbonate and volcanic rocks in the southeastern Yangtze Block, South China, is thought to be related to the breakup of Rodinia. Detrital zircon ages constrain the deposition of the Danzhou Group at ~770Ma and ~730Ma. The Danzhou Group contains dominant Neoproterozoic detrital zircon grains (~740–900Ma) with two major age groups at ~740–790Ma and ~810–830Ma, suggesting the detritus was largely sourced from the widely distributed Neoproterozoic igneous plutons within the Yangtze Block. The sedimentary rocks from the lower Danzhou Group, including sandstones, siltstone and pelitic rocks, have UCC-like chemical signatures, representing mixed products of primary sources. The upper Danzhou Group received more recycled materials because the rocks have relatively higher Zr/Sc ratios, Hf contents and a greater influx of Pre-Neoproterozoic zircons. All of the rocks have high La/Sc, low Sc/Th and Co/Th ratios, consistent with sources dominantly composed of granitic to dioritic end-members from the western and northwestern Yangtze Block. Chemical compositions do not support significant contributions of mafic components. Most Neoproterozoic zircons have positive εHf(t) (0–17) indicative of sediments derived mainly from the western and northwestern Yangtze Block. The uni-modal Neoproterozoic zircons and felsic igneous source rocks for the Danzhou Group suggest that the Yangtze Block was an independent continent in the peripheral part of Rodinia.

Geochemistry and zircon U–Pb geochronology of Paleoproterozoic arc related granitoid in the Northwestern Yangtze Block and its geological implications

The early history of the Yangtze Block has not been well constrained yet, due to the scarce outcrops of Archean to Paleoproterozoic rocks. In this study, we report an integrated study of zircon U–Pb age, major and trace element data and Sr–Nd–Hf isotope compositions for gray gneisses from the Houhe complex in the northwestern part of the Yangtze Block. Zircon U–Pb dating yields a weighted mean 207Pb/206Pb age of 2081±9Ma for a gneiss. This age is interpreted as the formation age of the Houhe complex and thus the Houhe complex represents the oldest rocks found in the western part of the Yangtze Block. The gray gneisses from the Houhe complex range in SiO2 contents from 58.63% to 68.59% and Na2O from 3.88% to 5.28%, and have relatively high Fe2O3 contents of 2.86–6.69%, Al2O3 of 16.01–18.88%, and MgO of 0.97–2.65%. These rocks show low Sr (149–390ppm), Cr (9.07–45.1ppm) and Ni (4.97–21.3ppm) contents, but high Y (12.9–32.7ppm) and Yb (0.95–2.25ppm). They are characterized by a relative enrichment in LILEs and LREEs, but a depletion in HFSEs. These features are similar to those of calc-alkaline granitoids, suggesting that their formation might be related to a subduction-related process or remelting of preexisted arc rocks. Compiled age spectra of Archean to Paleoproterozoic zircon grains reveal that the western and eastern parts of the Yangtze Block have similar Archean and early Paleoproterozoic age ranges, indicative of the occurrence of an old continental nucleus in both parts. On the other hand, there are large amounts of ca. 2050–2400Ma zircon grains revealed in the western part of the Yangtze Block, implying that there might be a microcontinent with an active-type continental margin during the Paleoproterozoic times in the western part of the Yangtze Block. Because of the poor outcrops of Archean to Paleoproterozoic rocks in the Yangtze Block, it is no possible to establish the exact nature of such an active margin yet. The occurrence of ca. 2.0Ga khondalitic and metasedimentary rocks has been documented in the eastern part of the Yangtze Block, implying that there might be a passive-type continental margin in the eastern part of the Yangtze Block. At ca. 2.0Ga, the collision between the western microcontinent and the eastern part may have resulted in the final arc-continental collision and the amalgamation of the Yangtze Block.