Yangtze Block Research Articles

Element geochemical differences in lower Cambrian black shales with hydrothermal sedimentation in the Yangtze block, South China

Abstract Hydrothermal activities occurred in the Yangtze block, South China, and affected the process of black shale sedimentation in the early Cambrian. Their specific influences, such as the sources, sedimentary environment, and mineralization, have not yet been revealed. Fortunately, the influences are explained through the geochemistry comparison of different wells FY1, XJ1, HY1, and XA1 in northwestern Hunan. The outcomes of the tectonic setting, distinguishable by element indicators, are disorganized. This is caused by the variable element composition, sedimentary recirculation of material source, and hydrothermal materials. FY1, the closest well to Zhangjiajie where Ni–Mo ores were formed by hydrothermal sedimentation, has more different features on the elemental geochemistry, but many indexes still indicate that it is normal sediments. XA1, which is far from the other wells and deposited in the deep-water basin, has significantly more differences in geochemical properties and shows more about normal marine deposition. XJ1 and HY1 are intermediate between them. Based on these wells, the hydrothermal contribution to the black shale sedimentation is not significant. However, some contents of trace and rare earth elements change widely because hydrothermal materials can diffuse and deposit over long distances. The concentration of Ag, As, Ba, Mo, Ba, U, and V generally has dozens of times higher than that of Upper Continental Crust. The early Cambrian environment was primarily anoxic/euxinic with enough sulfur, which is beneficial for enriching metal elements and organic matter. The sequence FY1, XJ1, HY1, and XA1 in turn has the same performance on the distance with Zhangjiajie, different intervals of suboxic environment and element enrichment, and hydrothermal-fluid-addition Ni abundance. Therefore, hydrothermal processes indeed provide materials for element enrichment and support the reducing environment, but the impact of hydrothermal activity decreases on the plane.

Fluid Mixing, Organic Matter, and the Origin of Permian Carbonate-Hosted Pb-Zn Deposits in SW China: New Insights from the Fuli Deposit

The Fuli Pb-Zn deposit is situated at the southwestern margin of the Yangtze Block in Yunnan. The deposit, which is hosted in the Permian Yangxin Formation dolomite, is a recent discovery. Our study indicates a significant presence of fluid inclusions in sphalerite from the Fuli Pb-Zn deposit, with fewer inclusions observed in dolomite and calcite. We conducted comprehensive petrographic, microthermometric, and laser Raman analyses on the inclusions within sphalerite and dolomite. Additionally, six samples of dolomite from the mineralization period were selected for H-O isotope analysis. The results of our study reveal the characteristics of ore-forming fluids and explore the mechanisms of ore formation. The study results indicate that the Fuli Pb-Zn deposit is a low- to medium-temperature hydrothermal deposit with fluid inclusions mainly composed of two-phase gas-liquid inclusions. Salinity and homogenization temperature analyses affirmed that there are two types of fluids present, one with low salinity and the other with high salinity. Laser Raman tests demonstrated the presence of CH4, N2, and CO2 in the gas phase of the inclusions. Microthermometric analyses indicated that the sphalerite ore-forming fluids consist of a multicomponent system of Mg2+ and Ca2+ enriched fluids. The features of the ore-forming fluids in the Fuli deposit arise from a blend of high-temperature, low-salinity metamorphic fluids and low-temperature, high-salinity basin brines. The basin brines in question have the potential to emanate from the Youjiang Basin. The formation of the ore is ascribed to the TSR and the mixing of fluids. The combination of these processes provided the requisite materials (SO42−), catalysts (Mg2+), and reducing agents (organic matter, CH4, and H2S) required to initiate the thermochemical sulfate reduction (TSR). As the TSR proceeded, it caused a shift in the pH of the fluids, thus promoting the precipitation of metal sulfides.

Prolonged exhumation and preservation of the Yuku molybdenum ore field, East Qinling, China: Constraints from medium- to low-temperature thermochronology

Molybdenum (Mo) is an important globally strategic metal and mostly occurs as molybdenite (MoS2) in diverse Mo deposits. The Yuku ore field includes multiple porphyry-skarn Mo deposits and forms a significant world-class Mo cluster in the Qinling molybdenum belt, central China. Previous studies on the ore field were largely related to its genesis, and did not highlight the post-mineralization modifications that are important to formulate prospecting strategies. The exhumation and preservation processes of the ore field through multiple tectonic events also remain poorly understood. Here, we conducted multi-method apatite U-Pb, fission-track and (U-Th)/He and zircon (U-Th)/He medium- to low-temperature thermochronology and related thermal history modelling on granitoids from the ore-hosting Shibaogou and Yuku plutons in the Yuku ore field, and integrated published studies to unravel the post-mineralization exhumation and preservation of Mo deposits. The newly determined apatite U-Pb (158.3–104.9 Ma), zircon (U-Th)/He (158.2–115.0 Ma), apatite fission-track (90.8–63.1 Ma), and apatite (U-Th)/He (67.8–35.3 Ma) ages in this study document multiple cooling pulses during post-magma emplacement. The low-temperature thermochronological ages of 158.2–35.3 Ma also record the post-mineralization cooling and exhumation history. The thermal history modelling indicates an Early Cretaceous (132–125 Ma) rapid cooling pulse, two enhanced (or accelerated) cooling pulses at ca. 125–59 Ma and post-10 Ma, together with a slow cooling or reheating pulse at ca. 59–10 Ma in the Yuku ore field. The Early Cretaceous rapid cooling is related to the coeval collisional tectonics of the East Qinling Orogen. The enhanced cooling at ca. 125–59 Ma is triggered by the post-collisional assembly of the North China and Yangtze Blocks, the subduction of Paleo-Pacific Plate, and the sinistral strike-slip motion of the nearby Tan-Lu Fault Zone. The slow cooling during ca. 59–10 Ma and the post-10 Ma enhanced cooling are correlated with the extensional tectonics derived by the subduction of Pacific Plate and the far-field effects from India-Eurasia collision. The Yuku area underwent lower exhumation in comparison to the nearby Shibaoguo and Huangbeiling areas in the Yuku ore field, which thus is helpful to the preservation of Mo deposits. The un-exhumated areas within 1.0 km at the inner and outer contact zones between the Yuku pluton and its wall rocks are postulated to be favorable sites for Mo prospecting. In addition, the un-exhumated areas surrounding other Late Mesozoic plutons in the Luanchuan region are also suggested to have high potential for Mo exploration.

The enrichment mechanism of indium in Fe-enriched sphalerite from the Bainiuchang Zn-Sn polymetallic deposit, SW China

Dispersed metals, such as indium and cadmium, are widely used in certain high-tech applications and are important resources for world economies. Sphalerite is an important host mineral for indium, cadmium and other trace elements which can record genetic information in hydrothermal systems. The Bainiuchang Zn-Sn polymetallic deposit, located in the southwest margin of the Yangtze Block, has substantial zinc concentrates with indium and cadmium mineralization. In this study, we examined different sphalerite grains from this deposit with elemental mapping, in-situ trace element geochemistry and S-Pb isotopic composition. The discrepancy of indium and Zn/Cd ratio suggest that there are at least two generations of sphalerite. The mineralization of indium is mainly enriched in marmatite [(Zn,Fe)S)] from cassiterite-bearing ore, rather than in that from cassiterite-poor ore. The mineralization of cadmium occurred in all types of ore. The relevance of trace elements suggests that indium can be incorporated into sphalerite by a couple substitution of (In, Ga)3+ + (Cu, Ag)+ ↔ 3(Zn, Fe)2+, whereas Fe, Cd can be incorporated into sphalerite by substitutions of Fe2+ ↔ Zn2+ and Cd2+ ↔ Fe2+. Sulfur (δ34SV-CDT =+2.43 to +3.88 ‰) and lead (206Pb/204Pb = 18.346–18.396, 207Pb/204Pb = 15.675–15.691) isotopic compositions indirectly suggest that indium and cadmium in the Bainiuchang Zn-Sn polymetallic deposit may be sourced from magmatic-hydrothermal fluids associated with Late-Cretaceous magmatic activities in this district.

Controls on the formation of Mn carbonates in mudrocks of the Datangpo Formation, Northern Margin Rift Basin, Yangtze Block

Deglaciation following the Sturtian Ice Age led to widespread manganese carbonate precipitates within marine mudrocks, reflecting unique climatic and oceanographic changes. The Mn carbonates hold economic importance in the Datangpo Formation, Yangtze Block, South China. Although their genesis in the Nanhua Rift Basin is well understood, little is known regarding their formation mechanism in the North Margin Rift Basin (NMRB). With growing interest in Mn ore exploration in this region, this study examines the weathering trend, climate patterns and watermass conditions using a variety of geochemical proxies. The results reveal two phases of climate variation during the deglaciation. Climate during the early phase (Phase I) was unstable, characterized by fluctuating values of Chemical Index of Alteration (CIA = 65 ± 4) and elevated Li isotope (8.8 ± 3.0‰) patterns. The degree of chemical weathering during the later stage (Phase II) was relatively stable (CIA = 65 ± 1, δ7Li = 5.4 ± 2.7‰), but the weathering intensity increased due to the reduced denudation rate, possibly related to a shift in the basin development. Fe speciation data indicate euxinia prevailing much of the section. However, Mo and U enrichment (> 1) and fluctuating ratios of Mo versus total organic content (0–40) suggest that pulses of freshwater influx associated with climate variation during Phase I caused the fluctuation of the chemocline along the continental shelf, facilitating the formation of Mn carbonates through the particulate Fe-Mn-hydroxide shuttle. With the establishment of stabilized climate, basin tectonics, and watermass circulation during Phase II, the accumulation of Mn carbonates in the sediments became less significant. The accumulation of Mn carbonates in the NMRB conforms to the “bathtub ring model”, distinct from the metallogenic model in the NRB, providing valuable guidance for Mn ore prospecting within this area and elsewhere with similar settings.

Carbonate metasomatism in sub-continental lithospheric mantle: Insights from decoupled Nd[sbnd]Hf isotopes of Mesozoic gabbros in Jinan, eastern North China Craton

The ancient sub-continental lithospheric mantle (SCLM) of the eastern North China Craton (NCC) underwent metasomatism in the Triassic due to fluids derived from subducted Yangtze continental crust, followed by further metasomatism during the Jurassic–Cretaceous caused by fluids originating from subducted Paleo-Pacific oceanic slabs. However, the occurrence of carbonate metasomatism during these two geological events is a subject of debate, and the origin of carbonate-bearing metasomatic agents remains ambiguous. Here, we present an integrated study of petrography, whole-rock trace element data and Sr–Nd–Hf–Pb isotope data for the Jinan gabbros in the Luxi block of the eastern NCC. The Jinan gabbros show arc-like trace element patterns and enriched Sr–Nd–Hf–Pb isotopic compositions, suggesting that they originated from partial melting of the enriched ancient SCLM. In addition, there are remarkable similarities in Sr–Nd–Hf–Pb isotopes between the Jinan gabbros and HP–UHP metamorphic rocks in the Dabie–Sulu orogenic belt, indicating the involvement of continental crustal components from the subducted Yangtze block in the mantle source of the Jinan gabbros. The gabbro samples show decoupled NdHf isotopes with positive ∆εHf values (∆εHf = εHf – 1.55 × εNd – 1.21; Vervoort et al., 2011), which were mainly caused by a metasomatic agent with high Nd/Hf ratios. The increase in εNd values with increasing Hf/Hf* (Hf/Hf* = HfPM/[(NdPM × SmPM)0.5]) and Nb/La further indicates that the metasomatic agent in the mantle source was depleted in high field strength elements (HFSEs) but enriched in rare earth elements (REEs), resembling the mantle-derived carbonatites in the Luxi area. The Jinan gabbros show significant variations in εNd and εHf, which cannot be explained by the involvement of C–O–H fluids and pure carbonatitic melts in their mantle source. Instead, carbonate-bearing felsic melts from the deeply subducted Yangtze continental crust are proposed to have acted as metasomatic agents. Carbonate metasomatism can significantly weaken the strength and decrease the melting temperature of the overlying ancient SCLM. Therefore, carbonate metasomatism that occurred in the lithospheric mantle of the eastern NCC during the Triassic, likely associated with deep subduction of the Yangtze block, might have played a significant role in promoting lithospheric thinning and mafic magmatism in the late Mesozoic.

Synglacial carbonate records of snowball Earth ocean composition—Evidence from the Nantuo Formation, South China

Abstract The Cryogenian (ca. 717–635 Ma) snowball Earth glaciations ended with the precipitation of “cap” carbonate successions with negative carbon isotope (δ13Ccarb) values, which have been explained by the addition of various 13C-depleted carbon sources in the deglacial process. These arguments assumed that marine dissolved inorganic carbon (DIC) was enriched in 13C in the synglacial ocean. However, this assumption has not yet been tested, because the synglacial ocean chemistry is unknown. In this study, we carried out detailed analyses of the petrology, carbonate carbon (δ13Ccarb) and oxygen (δ18Ocarb) isotopes, organic carbon (δ13Corg) isotopes, major and minor elemental compositions (Ca, Mg, Mn, Fe, Sr), and iron speciation (total Fe, highly reactive Fe, pyrite Fe) of the carbonate layers (also called synglacial carbonate layers) from the Nantuo Formation (ca. 650–635 Ma) on the Yangtze block, South China. Petrographic observations indicated that the synglacial carbonate comprises dolomicrite, mud-crystal powder dolomite, lime dolomite, and dolomitic limestone, supporting an authigenic carbonate origin, and thus, it potentially recorded the ocean chemistry during the Marinoan ice age. The synglacial carbonate is characterized by extreme Mn enrichment, low Fe/Mn ratios, and low δ13Ccarb (−7‰) values. High Mn contents and low Fe/Mn ratios imply marine redox conditions favoring Mn2+ accumulation and Fe2+ oxidation, while low δ13Ccarb values might be attributed to CO2 degassing of submarine volcanoes as well as low primary burial during the glaciation. Since the δ13CDIC value of the synglacial ocean was lower than the δ13Ccarb values of most cap carbonates, we infer the addition of 13C-enriched DIC or removal of 12C during cap carbonate precipitation, such as through carbonate weathering or organic carbon burial. These findings provide new insights into the nature of Cryogenian glaciation, the origin of cap carbonates, and the aftermath of global glaciation.

Crustal deformation in the vicinity of the Tan-Lu Fault Zone in East China constrained by receiver function analyses

To investigate the spatial distribution of crustal deformation and related geodynamic processes associated with the Tan-Lu Fault Zone (TLFZ) in East China, we measured crustal anisotropy based on the sinusoidal moveout of the P-to-S conversions from the Moho observed on receiver functions. Totally 142 well-defined crustal anisotropy measurements were obtained from the China National Seismic Network and other broadband dense seismic stations, including three stations exhibiting the pattern of a double layered anisotropy structure. The fast orientations observed at stations in the vicinity of the TLFZ were dominantly fault-parallel except for the southernmost part of the study area, where the fast orientations were approximately normal to the strike of the fault zone and may primarily be attributed to the NW-SE oriented Dabie Orogenic Belt. In the North China Block, NW-SE fast orientations were found to be consistent with the dominant orientation of the secondary faults. The Lower Yangtze Block showed two dominant fast orientations. The fault-parallel measurements in the west of the Jiangshao Fault Zone were controlled by the NE-trending faults in the Lower Yangtze Block, while the fast orientations in the east and south of the Jiangshao Fault Zone were dominantly E-W, which could be mainly influenced by the regional principal compressive stress. The average delay time of the entire study area was 0.27 ± 0.07 s. In the vicinity of the TLFZ where fault-parallel fast orientations were observed, an approximately 44% increase in delay time was observed, suggesting a well-developed fault zone.

Widespread Thalassinoides facies from the upper Silurian of South China: A case study from the northwestern Hunan Province, middle to upper Yangtze Block

The upper Silurian of South China records the first, and perhaps the only occurrence of widespread (spanning over 1400 km), thick to very thick bedded Thalassinoides facies (TVTFs) after the Late Ordovician mass extinction. However, the palaeobiological, palaeoecological features and potential significance of these facies are yet to be demonstrated. A case study at northwestern Hunan Province, middle to upper Yangtze Block, reports the occurrence of TVTFs in a storm dominated open marine succession of the upper Silurian Xiaoxi Formation, which can be further assigned to lower offshore, upper offshore, offshore transition to distal lower shoreface, and proximal lower to middle shoreface settings. The regular changes of bioturbation intensity and maximum penetrating depth from various settings suggest that local energy levels, especially frequency of storms could have controlled the development of Thalassinoides, while their burrow sizes were generally confined by population densities. The Thalassinoides facies could have been constructed by unknown crustaceans, which generally conducted a deposit-feeding strategy. The return of widespread TVTFs in the tropical oceans of South China during the late Silurian could reflect a restored primary productivity, an elevated oxygen level, and a final recovery of benthic ecosystem to pre-extinction level after the Late Ordovician mass extinction. TVTFs could have remarkably shaped the late Silurian benthic environment in view of their high abundance, large burrow sizes, nature of deep tiered burrow systems and a deposit-feeding strategy.

Geochemical diversity of continental arc basaltic mushy reservoirs driven by reactive melt infiltration

The reactive melt flow emerges as an important factor for diversification of basaltic magmatic reservoirs, but whether and how it influences continental arc basaltic mushes are enigmatic. Here, we used mineral and whole-rock geochemistry to examine the petrogenesis of a suit of mafic and intermediate plutons in western Yangtze Block, which were emplaced at continental arc crust and primarily had plagioclase and clinopyroxene as early cumulate mineral phases. We found the crystal mushes were infiltrated by externally-derived reactive melt with high δ18O and fertile crustal signatures, resulting in the changes of mineral phases (e.g., clinopyroxene transformed to hornblende) and bulk-rock geochemistry (including isotopes). Then, the reacted granitic melt was prone to either be extracted from or stall in the crystal mushes, generating quartz-poor (mafic) or quartz-rich (intermediate) plutons, respectively. This study supports the reactive melt infiltration may serve as an important engine for compositional diversity of basaltic mush system within continental arc settings.

Geochronology, Petrogenesis, and Tectonic Setting of the Late Triassic Banpo Pluton from the Western Part of the North Qinling Orogenic Belt

Precise determination of the tectonothermal events at the Qinling–Qilian junction is significant for understanding the tectonic evolution of the eastern branch of the Paleo-Tethys. The Banpo pluton outcrops in the northern margin of the North Qinling were chosen as the research subject and their formation and tectonic environment were investigated using zircon U-Pb age and geochemical compositions. The weighted average values of 206Pb/238U ages of samples from three sites of the Banpo pluton corresponded to 213.4 ± 2.1 Ma (MSWD = 0.56), 213.0 ± 1.6 Ma (MSWD = 0.92), and 216.3 ± 2.3 Ma (MSWD = 2.0). All the samples are rich in light rare earth elements, exhibiting a seagull-type right-leaning partitioning curve, with obviously negative Eu anomalies. In addition, based on the regional geological data, it can be concluded that the Banpo pluton was formed during the transition period from a post-Orogenic environment to an anorogenic environment. The results indicated the final stage of the Triassic orogeny and the closure phase of the Paleo-Tethys. Also, it shows closure of the Paleo-Tethys-Mianxian-Lueyang Ocean by the Late Triassic period as well as the completion of the collision between the North China and Yangtze Blocks along the Qinling Orogenic Belt.

Discussion of the W-Sn-REE Metallogenic Background in the Nanling Region of South China: Evidence from Satellite Gravity and Magnetic Data

The Nanling region is located at the intersection of the Yangtze Block and Cathaysia Block and is characterized by complex geological and tectonic processes, as well as distinct W-Sn-REE mineralization. Despite extensive research on the mineralization of W-Sn and REE deposits in the Nanling region, the factors impacting the distribution pattern of eastern tungsten and western tin deposits, as well as the mechanism of REE enrichment in the parent rocks, remain uncertain. Deep structural and tectonic variability plays a crucial role in the formation of mineral deposits in the upper crust. Information on deep structural and tectonic variability is contained in the Moho depth, Curie depth, effective elastic thickness, lithospheric density, and thermal structure derived from the processing and inversion of satellite gravity and magnetic data. In this paper, we comprehensively analyse satellite gravity and magnetic data from the Nanling region, integrating the processing and inversion results with the tectonic evolution of this region and relevant geological information. It is hypothesized that the Chenzhou–Linwu fault serves as a channel for mineral and thermal transfer in the Sn ore aggregation zone, facilitating the material transport from the deep mantle to the surface and ultimately leading to the formation of Sn-enriched granite. The collection area of tungsten ore is more weakly associated with the Chenzhou–Linwu fault, and through deep heat transfer, tungsten components are primarily concentrated in the Earth’s crust to produce W-enriched granite. The primary source of REE enrichment in the parent rocks associated with REE mineralization is predominantly derived from the felsic crust, and the rapid intrusion of deep magma resulting from the subduction and retraction of the Palaeo-Pacific Plate is a contributing factor to the contrasting enrichment of light and heavy rare-earth elements. Mineral crystalline differentiation is relatively high, leading to the formation of ore-forming parent rocks with high heavy rare-earth element contents.

Northwestward migration of siliciclastic sedimentation during late Tonian to Cambrian: Implications for tectonic evolution of the South China Block

The late Tonian to Cambrian sedimentary sequences in the Xiang-Gui-Yue (XGY) Basin across the potential suture zone between the Yangtze and Cathaysia blocks preserves invaluable information on the geological evolution of the South China Block, while the sedimentary provenance is not well constrained and tectonic affinity of the basin is still highly controversial. This study presents systematic provenance analysis of late Tonian to Cambrian clastic rocks to reconstruct the formation and evolution of sedimentary basin. A significant change from dominant 900–700 Ma to dominant 1300–900 Ma zircons in sediments has been observed along with the basin evolution. It has been widely considered that the 900–700 Ma detritus are largely from the Yangtze Block in the northwest, while the 1300–900 Ma detritus are from the Cathaysia Block in the southeast or terranes linked to the Cathyasia Block at that time. More remarkably, the provenance transition occurred at Cryogenian, Ediacaran and Cambrian in the southeastern, central and northwestern parts of the XGY Basin, respectively. All the above observation indicates the Yangtze-derived detritus arrived at the southeast end of the basin as early as late Tonian and gradually faded away until at least Ordovician, as probably caused by the denudation of the Jiangnan Orogenic Belt after the amalgamation of the Yangtze and Cathaysia blocks. On the other hand, the Cathaysia-derived detritus became dominant due to the uplift of the Cathaysia Block. The XGY Basin was a rift basin until ca. 660 Ma, evolved into a passive margin setting during Ediacaran-early Cambrian, and became a foreland basin after the middie-late Cambrian. Collectively, the provenance variation from late Tonian to Cambrian basin suggests that the Yangtze and Cathaysia blocks may have been amalgamated before late Tonian and an intra-plate basin has been developed along the orogenic belt, allowing the formation and evolution of the XGY Basin.

Geochemical transition from Cretaceous island arc basalt-like to oceanic island basalt-like basaltic rocks in the Jiurui district of the Yangtze Block, South China

The Cretaceous volcanic-sedimentary succession in the Jiurui district of the Yangtze Block contains basalts that are interlayered with fine clastic rocks, conglomerate and breccia strata. Here, we present data of whole-rock40Ar–39Ar dating, major and trace elements, Sr–Nd–Pb isotopes, zircon U–Pb dating and Hf isotopes, and plagioclasetrace element analyses to constrain the petrogenesis and tectonic background of these basaltic rocks,. Two episodiceruptions (ca. 126 Ma and ca. 91 Ma) are recognized, and the early basaltsdisplay evident enriched source characteristics such as high incompatible and low compatible elements, negative Nb, Ta, and Ti, slightly positive Pb anomaly, negative εNd(t) and εHf(t) values, higher (87Sr/86Sr)i, andhigh incompatible elements in plagioclase,indicating anisland arc basalt-like nature. The younger basalts show low incompatible and high compatible elements, negative Rb, Pb, Nb, and Ta, positive Sr anomaly, positive εNd(t) values, lower (87Sr/86Sr)i, and low incompatible elements in the plagioclase, indicating an oceanic island basalt-like characteristic. The early ca. 126 Ma basalts were probably derived from a deeper and older enriched lithospheric mantleconsisting of rutile-bearing garnet pyroxenite or eclogite, whereas the younger ca. 91 Ma basalts were derived from a shallower and younger depletedasthenospheric mantle constituting peridotite, with limited amounts of enriched lithospheric mantlecomponent. The retreat and rollback of the paleo-Pacific slab and related lithospheric thinning and mantle composition transition processes probably occurred at ca. 126 Maand ca. 91 Ma in this inland region of the Yangtze Block in northwestern Jiangxi province.

Submarine hydrothermal fluids facilitated the accumulation of cadmium in Lower Cambrian black shales, South China

The effects of Cd-rich venting fluids on the Cd contents and isotopic compositions of continental margin sediments remain poorly understood. Characterization of the Cd isotopic composition of sediment overprinted by venting fluids is critical, as this may provide insights into how mixing between the venting fluids and bottom waters affects the Cd isotopic composition. Our study determined the Cd isotopic compositions of Lower Cambrian metalliferous sediment deposited on the passive continental margin of the southeastern Yangtze Block, South China. The Cd is predominantly hosted in sphalerite of sulfide-bearing barite sediments, V deposits, and Ni-Mo−platinum group element (PGE) sulfide layers. The Cd isotopic compositions of samples from these three units indicate that the distribution of Cd at this type of continental margin was controlled by submarine hydrothermal activity. Following the venting of hydrothermal fluids onto the seafloor, Cd2+ reacted with a limited amount of H2S generated mainly by thermochemical sulfate reduction near the vents. This resulted in isotopically lighter Cd in the sulfide-bearing barite sediments and V deposits, which have δ114/110Cd values of 0.00‰ ± 0.13‰ and 0.00‰ ± 0.14‰, respectively. The residual dissolved Cd in the fluids migrated with the upwelling seawater and ultimately precipitated almost entirely as sphalerite in the sulfidic wedge, which resulted in Ni-Mo−PGE sulfide layers with a narrow range of high δ114/110Cd values (0.35‰ ± 0.03‰). The presence of Cd-rich venting fluids can alter the Cd contents and isotopic compositions of passive margin sediments, particularly those located within long-term stratified seawater and fault depressions. Previous studies may have underestimated the migration distance and affected areas of hydrothermal activity involving Cd-rich fluids.

Neoproterozoic granitoids of northwest Vietnam and their tectonic implications

ABSTRACT A combined study of whole-rock geochemical and Sr-Nd isotopic data, and zircon U-Pb geochronological and Hf isotopes has been carried out for several Neoproterozoic intrusions, including the Lung Thang, Posen and Sin Quyen in the Phan Si Pan Zone, northwest Vietnam to constrain their age, petrogenesis and tectonic implications. The Lung Thang and Posen intrusions, mainly composed of granodiorite, were formed at 803–777 Ma and are characterized by moderate SiO2 (64.43–66.65 wt.%), and K2O (4.05–4.89 wt.%), with A/CNK and A/NK values of (0.94–1.03) and (1.72–2.11), respectively. They have negative whole-rock εNd(t) (−6.16 to −3.73) and zircon εHf(t) values (−7.9 to −4.1), which suggest that the Lung Thang and Posen intrusions were generated by partial melting of ancient, K-rich crustal rocks. The Sin Quyen intrusion, occurring as dykes, is composed of monzodiorite and was emplaced at 742 ± 3 Ma. The Sin Quyen intrusion has high alkalies (K2O+Na2O = 7.42–7.47 wt.%), and low MgO (<1.31 wt.%) and Ni (6.55–6.93 ppm), with A/CNK and A/NK values of (0.67–0.68) and (1.64–1.68), respectively. Their whole-rock εNd(t) and zircon εHf(t) values are −6.15 to −5.92 and −5.8 to + 8, respectively. These geochemical characteristics suggest that the Sin Quyen intrusion was produced by the partial melting of ancient crustal sources mingled with mantle-derived components. Geochemically, the Lung Thang, Posen and Sin Quyen intrusions are medium- to high-K, calc-alkaline in nature and show enrichment in LILE (Th, U, K, Rb) and LREE, and strong negative anomalies of Nb, Ta, and Ti. Such geochemical characteristics suggest that they formed in a subduction-related tectonic environment. The geochronological and geochemical correlation of these intrusions with those along the southwestern margin of the Yangtze Block in South China suggests that the Phan Si Pan zone in northwestern Vietnam is a constituent of the SW Yangtze Block. Additionally, these intrusions show a significant correlation with other contemporaneous magmatic rocks in the northeast Indochina Block, Lhasa Block, the northwestern margin of Greater India as well as those in Seychelles and northern Madagascar. This correlation suggests a similar history and synchronous episode of crustal growth/recycling in an Andean-type arc system along the western and northern margin of the Rodinia supercontinent during the Neoproterozoic.

Cenozoic crustal thickness variations and surface uplift in the southeastern Tibetan Plateau: Insights from detrital zircon trace elements in the Gongga Shan region

The timing and mechanism of uplift in the southeastern Tibet Plateau have long been subjects of debate. The Gongga Shan region, positioned at the convergence of the Songpan-Ganzi, Chuandian, and Yangtze blocks, serves as a crucial vantage point for unraveling the evolutionary history of the southeastern Tibet Plateau. Nevertheless, the uplift history of the Gongga Shan region remains shrouded in uncertainty. By employing multivariate trace element ratios and reconstructing crustal thickness using zircon Eu/Eu*, our research has unveiled distinct periods of crustal thickening and thinning events. Within the time span of ∼240–160 Ma, a discernible thickening trend emerged from ∼35 to ∼45 km between ∼240 and ∼ 215 Ma, followed by a relatively stable phase from ∼215 to ∼170 Ma, and subsequently a decline from ∼170 to ∼160 Ma, reducing the thickness from ∼45 to ∼30 km. In the period from ∼55 to ∼5 Ma, an initial increase in thickness occurred from ∼30 to ∼60 km between ∼55 and ∼ 30 Ma, succeeded by an overall thinning from ∼60 to ∼35 km between ∼30 and ∼ 10 Ma. Moreover, based on our reconstructed crustal thickness data, we have traced the paleo-elevation changes in the Gongga Shan region from the Cenozoic to the present, revealing a fluctuation from ∼0.5 to ∼4.5 km. We attribute the Triassic-Jurassic crustal thickening and subsequent thinning to the Triassic orogeny and post-collision extension following the closure of the Paleo-Tethys Ocean. Our findings further suggest that during the Eocene-Oligocene period, the Gongga Shan region experienced significant crustal thickening and surface uplift, indicative of intense compression. This phenomenon likely resulted from the northeast-directed compression of the Indian-Eurasian plate, causing substantial crustal shortening and thickening in specific areas of the southeastern Tibet Plateau, leading to rapid surface uplift. Subsequently, during the Oligocene-Miocene, the detachment and subsidence of the lithospheric mantle triggered rapid crustal thinning in the Gongga Shan region. This phase was accompanied by relatively stable and slight surface uplift, signifying a shift in the plateau uplift mode of the southeastern Tibet Plateau from compression to lateral extrusion.

Bulldoze and rebuild: Modifying cratonic lithosphere via removal and replacement induced by continental subduction

Abstract Establishing the mechanisms for craton modification is critical for understanding cratonic stability and architecture. It is well known that oceanic subduction and mantle plumes can destroy and destabilize the overriding cratonic lithosphere, but the role of continental subduction in craton modification remains unclear. In the North China craton (NCC), which experienced deep continental subduction of the Yangtze block (YB) during the early Mesozoic, we report the presence of the YB crust embedded in the overriding craton. Combining our new results with previous observations, we argue that during 250–220 Ma, a 200-km-long tract of the NCC lithosphere was bulldozed by the subducted YB, resulting in a lithospheric suture far from the suture zone at the surface. This lithospheric removal occurred at middle-lower crustal levels (16–20 km depth)—much shallower than previously thought possible. The bulldozed NCC lithosphere was replenished by the subducted YB continental lithosphere rather than the asthenosphere, thus terminating the lithosphere modification. With essentially no net loss of lithosphere during deep continental subduction, the NCC maintained its stability until Early Cretaceous paleo-Pacific oceanic subduction. This “bulldoze and rebuild” model can thus account for how a craton maintains stability during a collision with another continental plate.

Two episodes of mid-Neoproterozoic mafic magmatism in the Hong'an Terrane, Central China: Records of continental rifting during the breakup of the Rodinia supercontinent

The mid-Neoproterozoic (800–635 Ma) tectonic regime along the northern margin of the Yangtze Block is still debated, particularly the timing and mode of transition from a subduction-related setting to a rifting-related setting. The new identification of two episodes of mafic magmatism, represented by (I) 736–710 Ma Daleishan and (II) 635 Ma Hong'an intrusions in the Hong'an Terrane, central China, provides new constraints on the nature of their mantle source and tectonic evolution of the northern Yangtze Block. (I) Daleishan mafic rocks are tholeiitic in composition and exhibit N-MORB- to arc-like trace element patterns. They have variable whole-rock initial 87Sr/86Sr (0.7041 to 0.7092), εNd(t) (−2.98 to +2.17), and εHf(t) (+3.37 to +10.25) values. Their magmatic zircons preserve relatively homogeneous δ18O (+4.50 to +6.01 ‰) and highly variable εHf(t) (+1.3 to +13.1) values. Chemical compositions and modeling results suggest that Daleishan rocks were derived from a depleted asthenospheric mantle source with involvement of recycled sediment component, and the magma subsequently underwent 5–15% contamination by the wall-rocks during its emplacement. (II) Hong'an mafic rocks mainly show E-MORB- to arc-like geochemical features with more enriched HfNd isotopes [zircon εHf(t) = −3.0 to +3.6; whole-rock εNd(t) = −6.87 to −0.75, and εHf(t) = −0.50 to +2.00], suggesting they were derived from the interactions between the asthenosphere and the previously-metasomatized lithospheric mantle. Integrating the new data with previous results for the Neoproterozoic rocks in the northern margin of the Yangtze Block (including the Qinling-Dabie orogen), we emphasize that episodic extension-related asthenosphere upwelling in a continental rifting setting, coupled with interactions between the asthenosphere and the lithosphere, played a prominent role in the formation of the 780–635 Ma bimodal-type magmatic rocks and diverse geochemical features in the mafic counterparts.

Study on the Hydrothermal Superposition Period: Mineralization–Alteration Zoning Model and Zoning Mechanism of the Dahongshan Fe-Cu Deposit in Yunnan Province

The Dahongshan large-scale iron (Fe)–copper (Cu) polymetallic deposit is in the Proterozoic metallogenic domain on the western margin of the Yangtze Block. It is a typical representative of Fe-Cu polymetallic composite mineralization in the Kangdian area. The deposit comprises a group of layered orebodies formed by volcanic exhalation sedimentation and metamorphism, and a group of vein-like orebodies formed by hydrothermal superposition. The large-scale mapping of altered lithofacies in the deposit has resolved issues of weak links and unclear mineralization and alteration zoning of hydrothermal superimposed deposits within the study area. The mineralization type, hydrothermal alteration type and intensity, mineral assemblage, and mineral structure of the vein-type Cu polymetallic deposits during the hydrothermal superposition period are meticulously analyzed and studied. Finally, the zoning relationships of vein orebodies (mineralization) are summarized. On the basis of the results of the study of the distribution pattern of this mineral body, a mineralization alteration zoning model of the hydrothermal superposition period is constructed. The results show that the alteration is primarily silicification, carbonation, and chloritization, and the mineralization is chalcopyrite, bornite, chalcocite, and pyrite. The Dibadu anticline and the cutting layer faults and fractures strictly control the hydrothermal alteration zoning. The mineralization alteration zoning from the core to the flank is divided into coarse vein zone (I) → stockwork zone (II) → veinlet zone (III). The corresponding mineral assemblages are quartz–calcite–chalcocite–bornite–(native copper) (I) → calcite–dolomite–quartz–bornite–chalcopyrite–chlorite (II) → dolomite–quartz–chalcopyrite–(pyrite) (III), where the stockwork zone has the most substantial mineralization. The mineral assemblages of each alteration zone, the characteristics of rare earth elements of typical samples, and the test results on the fluid inclusions confirm that pH and Eh primarily control the zoning mechanism. This study has significance for deepening the understanding of the composite metallogenic system, guiding deep and peripheral prospecting, and providing significant enlightenment for the study of this type of deposit.