Emeishan Flood Basalts Research Articles

Hydrothermal silicification in Ediacaran Dengying Formation fourth member deep dolomite reservoir, Central Sichuan Basin, China: Implications for reservoir quality

The Ediacaran Dengying Formation fourth member in Central Sichuan Basin is the deep dolomite gas reservoir with the largest natural gas reserves in China, providing an excellent example for understanding the effect of hydrothermal silicification on deep dolomite reservoir quality. By integrating petrology, geochemistry and physical property measurement, this study aims to reveal the genesis of different quartz phase and their corresponding diagenetic events, and to discuss the temporal relationship of diagenetic events and their effects on the quality of deep dolomite reservoir in Dengying fourth member. Three phases of quartz cement are identified in Ediacaran Dengying Formation deep dolomite reservoirs of Central Sichuan Basin as follows: (1) first‐stage quartz filled in primary reservoir spaces, (2) second‐stage quartz mainly filled in secondary reservoir spaces, and (3) third‐stage quartz filled in secondary reservoir spaces with Mississippi Valley‐Type minerals. By analysing petrographic and temporal relationships between these fabrics, it is suggested that the first‐stage quartz precipitated during the syndiagenetic stage, the second‐stage quartz precipitated during middle diagenetic stage, and the third‐stage quartz precipitated during middle diagenetic stage as well. Silicification during syndiagenetic stage is characterized by replacement of cryptocrystalline quartz and cementation of first‐stage quartz within matrix dolomite (MD). After syndiagenetic silicification, the mineral composition of the MD changed from magnesium calcium carbonate to silica, and the later constructive diagenetic alterations were almost ineffective. The distribution of syndiagenetic silicification during syndiagenetic stage gradually decreases from the platform‐interior to the platform‐margin. The second‐stage and third‐stage quartz cements, derived from two‐stage hydrothermal silicification in the middle stage, are both destructive for reservoir preservation. This study, combined with coeval vapour–liquid inclusions Th data and thermal‐burial models, constrains the time of the second‐stage quartz to ca. 403–408 Ma, suggesting an hydrothermal silicification related to the Caledonian movement. The time of the third‐stage quartz cement is constrained to ca. 259.4 Ma, associated with Late Permian eruption of Emeishan flood basalts.

Origin and metal-enrichment mechanism of sedimentary rare earth element deposits in Yunnan and Guizhou provinces, western Yangtze Block, China

In southwest China, claystone layers occur in the lower part of the late Permian Xuanwei Formation, immediately overlying the Permian Emeishan flood basalts. The lowermost claystone layers are characterised by high contents of rare earth elements (REEs; 1100–16,000 ppm, average 2700 ppm), indicating a new type of REE deposit. However, the origin of this deposit and the enrichment mechanisms are unclear. We collected samples from 10 sections of the Xuanwei Formation, including REE-rich claystones, siltstones, iron claystones, and tuff. Detrital zircons gave U–Pb ages of 275–245 Ma, εHf(t) values of –16.7 to 8.9, and TDM2 model ages of 0.70–2.32 Ga. The field, petrological, geochemical, and isotopic data suggest: (i) the REE-rich claystones were deposited over 2.6 Myr in the period 257.22–254.62 Ma at a very low average rate of deposition (0.076–0.61 cm/ka), and (ii) the REE-rich claystones were derived from a region of denudation in the inner zone of the Emeishan Large Igneous Province, with the ore metals sourced mainly from syenite but with minor contributions from high-Ti basalts and felsic igneous rocks. The siltstones near the base of the Xuanwei Formation were derived mainly from felsic volcanic rocks with minor contributions from basalt. The iron-bearing claystones resulted from weathering of the Emeishan flood basalts. Our study shows a surface REE migration pattern of “weathering of REE-rich matrix → fluid migration → physical or chemical precipitation”, which is consistent with the origin of the REE-rich claystones.

Geochemistry and petrogenesis of Late Permian basalts from the Sichuan Basin, SW China: Implications for the geodynamics of the Emeishan mantle plume

Plume-lithosphere interactions are significant in the formation of Large Igneous Provinces (LIPs). The Permian Emeishan Large Igneous Province (ELIP) is considered to be the result of a mantle plume. The Emeishan flood basalts comprise a major part of the ELIP and they define three zones: the inner, intermediate and outer zones. Both high-Ti and low-Ti basalts are present in the inner zone, whereas only high-Ti basalts are found in the intermediate zone and outer zone. However, there are only sparse outcrops in the outer zone, and so geochemical data on basalts from the outer zone are rare and the role of plume-lithosphere interaction in the petrogenesis of volcanic rocks in the outer zone remains poorly understood. In the Sichuan basin, the Basalt Formation is found between the Permian Maokou Formation limestone and the Longtan Formation marl in some drill cores as well as in outcrops in the basin. This relationship demonstrates that the basaltic layer in the basin is part of the Emeishan flood basalts. These basalts have TiO2 contents of 3.7–4.2 wt% and Ti/Y ratios of 604–720, being high-Ti sub-alkaline basalts. They display chondrite-normalized rare earth elements (REE) patterns enriched in light rare earth elements (LREE) relative to heavy rare earth elements (HREE) and have elevated large ion lithophile elements (LILE) and high field strength elements (HFSE). Lead isotope ratios are high (206Pb/204Pb(t) = 18.102–18.392, 207Pb/204Pb(t) = 15.578–15.606, 208Pb/204Pb(t) = 38.410–38.850), and εNd(t) values are −0.38 ∼ 1.17. Detailed petrology and geochemistry suggest that the high-Ti basalts from the Sichuan Basin did not experience significant contamination of crustal and lithospheric mantle material during the ascent of magma. We infer that these basalts resulted from low-degree melting of the plume mantle source and underwent fractional crystallization of clinopyroxene. The distribution and petrogenesis of the Sichuan Basin basalts in the outer zone are different from those of the basalts in the inner zone and there are clearly different plume-lithosphere interactions in different parts of the ELIP. In the inner zone, the temperature of the lithosphere mantle was markedly elevated due to underplating of the mantle plume, causing a substantial quantity of lithosphere mantle melting and the initial formation of low-Ti basalts. This was followed by melting of the mantle plume and the formation of high-Ti basalts. In the outer zone, lower temperatures further from the plume centre were insufficient to generate extensive melting of the lithospheric mantle. Consequently, only the mantle plume melted in the outer zone, resulting in the formation of high-Ti basalts with minimal lithospheric input.

The mineralization of Daxiao carbonate-hosted Pb-Zn deposit, northeast Yunnan province, SW China: Constraints from Rb-Sr isotopic dating and H[sbnd]O[sbnd]S-Pb isotopes

The Sichuan-Yunnan-Guizhou (SYG) Pb-Zn metallogenic province located in western Yangtze Block contains more than 400 carbonate-hosted Pb-Zn deposits. Uncertain mineralization age and ore-forming material source causing debates on the genesis of these carbonate-hosted Pb-Zn deposits. To address this issue, the Daxiao Pb-Zn deposit was chosen as a case study. The orebodies of Daxiao deposit are hosted in Mesoproterozoic Heishan formation argillaceous dolomite interbedded with slate. Three main mineralization stages have been identified, including syn-sedimentary stage, hydrothermal stage and supergene stage. The ore minerals consist of galena, sphalerite, chalcopyrite, argentite, smithsonite, anglesite and cerussite in the deposit. The early hydrothermal stage sphalerite yielded a Rb-Sr isochron age of 223.5 ± 2.9 Ma, consistent with the spike of Pb-Zn mineralization in SYG metallogenic province. Quartz in hydrothermal stage yields δDSMOW values of −69.6 to −65.8 ‰ and calculated δ18OFluid values of + 3.4 – +5.6 ‰, showing an affinity of metamorphic water. While the quartz in supergene stage yields δDSMOW values of −68.3 to −65.8 ‰ and calculated δ18OFluid values of −4.1 to −1.7 ‰, indicating abundant basinal brine or meteoric water had mixed into ore-forming fluid during its evolution from hydrothermal stage to supergene stage. Sulfur isotopic composition (δ34S = +6.3 – +19.1 ‰) of different stage sulfides reveal that the ore-forming sulfur originated from sulfates within the wall rocks, and mainly reduced by Thermal-chemical reduction (TSR). Moreover, the 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios of early hydrothermal stage sphalerite are 18.282 – 18.312, 15.603 – 15.681and 38.287 – 38.311, respectively. Those of late hydrothermal stage galena are 18.315 – 18.491, 15.676 – 15.721 and 38.314 – 38.326, respectively. All of them plot in the overlapping area of Emeishan flood basalts, Kunyang and Huili groups and Sinian to Triassic strata, indicating that these wall rocks jointly contributed the ore-forming metals for these Pb-Zn deposits in SYG metallogenic province. Hence, a metallogenic model for Daxiao and other carbonate-hosted Pb-Zn deposit in SYG metallogenic province was proposed: In the thrustfold systems caused by the subduction of Paleo-Pacific and the closure of Jinshajiang-Ailaoshan Paleo-Tethys ocean in Late Triassic to Early Jurassic. The fluids originated from metamorphic water migrated along deep faults and extracted ore-forming materials from wall rocks (including Proterozoic basement rocks, Sinian to Triassic sedimentary wall rocks and Permian Emeishan flood basalts), and then precipitated ores in the secondary thrust faults or interlayered fractures within these thrustfold systems.

Geochronology and petrogenesis of the Shuizi pyroxenite at Danba, western Yangtze Craton

扬子克拉通西缘在~260Ma发生短期内大规模峨眉山玄武岩溢流喷发。攀西地区发育的镁铁-超镁铁质岩被广泛认为是峨眉山大火成岩省的产物，但在北端松潘-甘孜岩区一直缺乏该类岩石的报道。本文首次报道扬子西缘丹巴水子乡单斜辉石岩的准确年龄，其锆石LA-ICP-MS U-Pb加权平均年龄为260.7±3.3Ma，表明其为峨眉山大火成岩省北端松潘-甘孜岩区镁铁-超镁铁质岩的组成部分。通过与攀枝花钒钛磁铁矿含矿岩体边缘相带苦橄岩和上部相带浅色辉长岩进行锆石微量元素对比显示，水子乡单斜辉石岩具有相近的高氧逸度，其ΔQFM为0~3，Ce_N/Ce_N^*平均为~30，该性质可能同样源自扬子西缘洋壳板片俯冲交代形成的较高氧逸度地幔源区。尽管如此，水子乡辉石岩体并未因高氧逸度而有明显的含钛磁铁矿饱和结晶，可能由其较低结晶分异程度造成。相比之下，攀枝花岩体经历了更高程度的含钛磁铁矿和斜长石分离结晶作用，伴随大规模的钒钛磁铁矿成矿。;Large-scale eruption of Emeishan flood basalt occurred in the western margin of the Yangtze Craton in a short period during~260Ma. Mafic-ultramafic rocks found in the Panxi area are widely considered to be products of the Emeishan Large Igneous Province (ELIP). However, there is a lack of reports on this kind of rocks in the Songpan-Ganzi area in the northern end of the ELIP. This paper first documents a precise age of the Shuizi pyroxenite in the Danba County. The zircon LA-ICP-MS U-Pb weighted mean age is 260.7±3.3Ma, indicating a representative mafic-ultramafic constituent of the northern end of the ELIP in the Songpan-Ganzi area. Comparing zircon trace elements of the Shuizi pyroxenite with those of picrite in the marginal zone and gabbro in the upper zone of the Panzhihua Fe-Ti-V oxide deposit were conducted. It is found that the Shuizi pyroxenite has similarly high magmatic oxygen fugacity, ΔQFM of 0~3 and average Ce_N/Ce_N^* of~30, to the Panzhihua mafic rocks. The high oxygen fugacity for both is considered to be derived from a mantle lithosphere source which had been metasomatized by subducted oceanic slab materials in the western margin of the Yangtze Craton. Although the Shuizi pyroxenite has high oxygen fugacity, there is less obvious Fe-Ti oxide saturation and precipitation here. This could be attributed to its relatively low degree of crystallization and differentiation. In contrast, the Panzhihua mafic-ultramafic intrusion experienced higher degree of titanomagnetite and plagioclase fractional crystallization, accompanied by a massive Fe-Ti-V oxide mineralization.

Geochemistry of Late Permian basalts from boreholes in the Sichuan Basin, SW China: Implications for an extension of the Emeishan large igneous province

The emplacement of the Late Permian Emeishan large igneous province (ELIP) in the Upper Yangtze craton, SW China, is considered to be a crucial factor that triggered the end-Guadalupian biodiversity crisis. However, the contribution of ELIP remains debated because of its relatively modest known size. The restricted assessment of its areal extent is, in part, due to the lack of study of the potential volcanic rocks concealed beneath younger strata. Here we present the first geochemical and isotopic variations and detailed spatial distribution of Late Permian basaltic rocks penetrated by deep exploratory boreholes in the Sichuan Basin. The Late Permian basaltic rocks are mainly distributed in the western and northeastern parts of the Sichuan Basin and are positioned at the same stratigraphic level as the Emeishan flood basalts. Petrography, whole-rock trace elements and Sr-Nd-Pb isotopic compositions demonstrate that these rocks are alkaline basalts and have oceanic island basalt - like characteristics, indicating a similar origin as the Emeishan flood basalts. The trace element and isotope ratios suggest that the basaltic magmas in the western Sichuan Basin were derived by low degrees of partial melting of a garnet-bearing source. In contrast, the basalts in the northeastern part of the basin were generated by higher degrees of melting of a shallower mantle source in the spinel-garnet transition zone. Varying amounts of contamination by lower continental crustal melts are suggested by rough correlations between isotope and immobile trace element ratios. The spatial changes in composition and origin of the basaltic rocks in the Sichuan Basin can be interpreted as the result of varying extents of Emeishan mantle plume-lithosphere interaction under different lithospheric stress backgrounds, such that the reactivation of pre-existing basement faults controlled the emplacement of basaltic magma in the periphery of the ELIP. Our work reveals a substantially broader extent of the ELIP in buried sedimentary strata, which might have contributed to the abrupt end-Guadalupian climatic change.

Rapid eruption of the Emeishan continental flood basalts: New paleomagnetic and geochronologic constraints

The duration of the eruption of the Emeishan large igneous province is hotly debated. We conducted a magnetostratigraphic and geochronological study of the core area of the large igneous province in the Binchuan area of Yunnan Province, southwestern China, in order to constrain the duration of the eruption. The results of detailed thermal demagnetization experiments revealed two remanent magnetic components from the volcanic rocks of 11 composite sections. A low-temperature component separated below 300 °C is interpreted as a recent viscous remanence. Additionally, reliable characteristic remanent magnetizations were revealed above 400 °C, with unblocking temperatures up to 580−680 °C, which passed the fold test and record three magnetozones. Zircons from the felsic ignimbrites exposed in the final stage of the mafic volcanism are dated to 258.2 ± 0.7 Ma (n = 15; mean square of weighted deviates = 1.3) by sensitive high-resolution ion microprobe. Stratigraphic and magnetostratigraphic correlations of the Emeishan basalts in the Binchuan sections indicate that the eruption of the mafic rocks of the Emeishan large igneous province can be clearly divided into early (reverse polarity subzone), middle (normal polarity subzone), and late (reverse polarity subzone) stages, with a total duration of less than 1.7 m.y. (260.8−259.1 Ma). However, by combining this chronology with previously reported conodont biostratigraphic results from locations around the Emeishan large igneous province, and comparing the dominant normal-reverse polarity sequence in the Emeishan large igneous province with the geomagnetic polarity time scale, we obtain a much shorter duration of the main eruptive stage of <0.8 m.y. (260.4−259.6 Ma). About three quarters of the basalts of the Emeishan large igneous province record have a normal polarity and erupted within 0.4 m.y., while the other quarter, mainly distributed in the central zone, shows a reverse polarity and much shorter duration. Given the short duration of the eruption, gas volatiles would have been released into the atmosphere at high rates, which might provide a causal link between the rapid eruption and the end-Guadalupian mass extinction. Before the mantle plume eruption, localized eruptions probably occurred. After eruption of the mafic Emeishan flood basalts, an acid volcanic eruption occurred in the early Wuchiapingian, which was sporadically distributed in the Emeishan large igneous province.

Late Permian High-Ti Basalt in Western Guangxi, SW China and Its Link With the Emeishan Large Igneous Province: Geochronological and Geochemical Perspectives

High-Ti (Ti/Y) flood basalts are widely distributed in the Late Permian Emeishan large igneous province (LIP), SW China, and their spatial distribution and genetic mechanism are important to reveal the role of plume-lithosphere interactions in the LIP origin. Western Guangxi is located on the eastern edge of Emeishan LIP. To explore the genesis of the high-Ti basalt in western Guangxi and any genetic link with the Emeishan LIP, we performed whole-rock geochemical and Sr-Nd isotope and zircon U-Pb-Hf isotope analyses on the Longlin basalts from western Guangxi. The results indicate that the Longlin basalt from Tongdeng area has relatively high SiO2 but low MgO and TFe2O3 contents. The rocks have zircon εHf(t) = −0.42 to 6.41, whole-rock (87Sr/86Sr)i = 0.707167–0.707345, and εNd(t) = −2.5 to −2.14. In contrast, the Longlin basalt from Zhoudong area has relatively low SiO2 but high MgO and TFe2O3 contents. The rocks have whole-rock (87Sr/86Sr)i = 0.706181–0.706191 and εNd(t) = −0.57 to 0.69. Four Longlin basalt samples display LREE enrichments and HREE depletions, and with indistinct δEu and δCe anomalies. LA-ICP-MS zircon U-Pb dating on three Longlin basalt samples (from different localities) yielded consistent weighted average age of 257.9 ± 2.6 Ma (MSWD = 0.55), 259.5 ± 0.75 Ma (MSWD = 3.0), and 256.7 ± 2.0 Ma (MSWD = 0.68), indicating a Late Permian emplacement. Considering the similar age and geochemical features between the Longlin basalt and Emeishan flood basalts, we interpret that the former is spatially, and temporally associated with the Emeishan LIP. Geochemical features show that the high-Ti basalts in western Guangxi resemble Deccan-type continental flood basalts (CFBs), which were derived by decompression melting of the mantle plume. Combined with previous geochemical studies, we suggest that the difference in Ti content and Ti/Y ratio in CFBs are related to the depth and melting degree of mantle source, in which high-Ti features may have been linked to low degree of partial melting in the deep mantle.

Lateral Seismic Anisotropy Variations Record Interaction Between Tibetan Mantle Flow and Plume‐Strengthened Yangtze Craton

AbstractThe southeastern Tibetan Plateau experienced intense Cenozoic shortening and extrusion along the western Yangtze Craton, SW China, where the Permian Emeishan flood basalts were emplaced. Seismic anisotropy along a dense E–W‐trending linear seismic array across the west Yunnan is determined by shear‐wave splitting analysis to characterize the deformation associated with these processes. Interestingly, a strong N–S‐oriented anisotropic zone collocated with the Eocene–Oligocene potassic magmatism is observed between two weak anisotropic zones in the Three‐River Orogenic Zone and the western Yangtze Craton, respectively. The lateral anisotropy variation and distinct upper mantle structures collectively suggest that the western most Yangtze Craton, strengthened by the Permian mantle plume, acted as a rigid barrier to divert the Cenozoic Tibetan mantle flow into the Three‐River Orogenic Zone. Consequently, the western edge of the Yangtze Craton has been partially eroded and its anisotropic orientation was realigned to the direction of the mantle flow. This observation indicates that both the remodeled anisotropy in the lithosphere and the dynamic flow in the asthenosphere contribute to the mantle anisotropy at the lateral lithosphere‐asthenosphere transition, which is usually hard to seismologically resolve from the vertical lithosphere‐asthenosphere boundary.

Late Permian–Early Triassic mafic dikes in the southwestern margin of the South China block: Evidence for Paleo-Pacific subduction

The link of the widespread Late Permian–Triassic magmatism in the southwestern corner of the South China block to a subduction zone or the Emeishan mantle plume is debated. We have carried out an integrated geochronological-petrological-geochemical study of four mafic dikes in the Gulinjing–Nanxi area of this region to shed new light on the issue. Zircon grains from the Gulinjing–Nanxi mafic dikes yield UPb ages from 254 ± 2 to 250 ± 2 Ma, indicating that they are coeval with abundant felsic intrusive-extrusive rocks in the region, but are 6–7 myr younger than the nearby Emeishan flood basalts and associated mafic-ultramafic intrusions. The Gulinjing–Nanxi mafic dikes are characterized by light rare-earth element enrichments and pronounced negative NbTa anomalies, similar to typical arc basalts worldwide, plus enriched Sr-Nd-Hf isotope compositions (initial 87Sr/86Sr from 0.7088 to 0.7103, εNd(t) from −6.6 to −4.6, and zircon εHf(t) from −3.7 to −0.3). The isotopes and incompatible trace element ratios indicate that the parental magmas for the Gulinjing–Nanxi mafic dikes are consistent with contaminated arc mafic magmas, but not contaminated Emeishan flood basalts in the region. This, together with a significant age difference between the mafic dikes and the Emeishan flood basalts and associated mafic-ultramafic intrusions in the nearby areas, support the interpretation that the Gulinjing–Nanxi mafic dikes are unlikely to be parts of the Emeishan mantle plume. Based on regional igneous association and the distribution of known paleo subduction zones in the surrounding regions, we conclude that the Gulinjing–Nanxi mafic dikes are linked to Paleo-Pacific subduction to the south, and not Paleo-Tethys subduction to the west.

LA-ICP-MS analyses of trace elements in base metal sulfides from carbonate-hosted Zn-Pb deposits, South China: A case study of the Maoping deposit

The Sichuan-Yunnan-Guizhou metallogenic province (SYGMP) contains over 400 different size Zn-Pb deposits hosted in late Ediacaran to early Permian carbonate sequences and accounts for approximately 27% of the total Zn-Pb production in China. The Maoping deposit is a typical example that is not only mainly hosted in two different strata i.e., Devonian Zaige Formation (Fm.) and Carboniferous Baizuo Fm. but also shows a close spatial relationship with the Emeishan flood basalt. However, its ore genesis is poorly constrained. In this study, LA-ICP-MS trace element maps and spot analyses were generated on the sphalerite and galena selected from Maoping, aiming to establish tarce element partitioning behavior in co-crystallized sphalerite–galena, constrain the possible controls on the variation of trace element, and provide new insights into the ore genesis. The results show that Mn, Cd, Fe, Ge, Cu, In and Sn concentrations in sphalerite are higher than coexisting galena, yet galena is the preferential host for As, Ag, Sb, Tl, and Bi. The sphalerite from Carboniferous Baizuo Fm. is more concentrated in Cu, Ge, As, Ag and Sb than that of Devonian Zaige Fm., but has comparable trace element compositions to many other SYGMP Zn-Pb deposits hosted within the same sediment strata such as Huize and Tianqiao. This difference may be thought to have been controlled by host rock chemical composition with limited effect by physicochemical conditions and/or source(s) of metal elements. Sphalerite from the Maoping deposit is characterized by enrichments in Cd, Ge but depletions in Mn, Fe, In and Co, which are markedly distinct from that of the magma-related hydrothermal system (e.g., high-temperature (high-T) magmatic-hydrothermal, VMS and skarn deposits) but consistent with that of MVT deposits. Based on these results, together with the geological and geochemical features of the deposit, we considered that the Maoping Zn-Pb deposit belong to a MVT deposit.

Provenance discrimination of upper Yangtze River basin sediments: New insights from heavy mineral signatures and detrital magnetite geochemistry

Magnetite is typomorphic, and its crystal textures and chemical fingerprint form unique signatures that can be used to determine sediment provenance. Determination of the provenance of sediments in the upper Yangtze River is critical for understanding its evolution and the uplift of the Tibetan Plateau. In this study, petrographic analysis and electron-probe microanalysis (EPMA) of 800 detrital magnetite grains were performed to differentiate the provenance of sediments within the mainstream and major tributaries of the upper Yangtze River catchment. Chemical analyses show that the elemental composition of magnetite grains shows pronounced variations among the different sections of the mainstream and tributaries. Based on the observed chemical fingerprints, we infer that the source rocks control the elemental composition of magnetite grains. The magnetite contribution is closely related to the tectonics, climate, and erosion of parent rocks within different drainage basins. Magnetite from the trunk stream of the Jinsha River is found to be a significant contributor to the upper Yangtze River. Magnetite grains of the upper Jinsha River are primarily derived from source areas composed of silicic plutonic rocks and ophiolite lithologies that crop out on the SE Tibetan Plateau. Magnetite grains within the sands of the lower Jinshajiang sediments are characterized by high concentrations of Ti, Mg, Cr, Al, and Mn, and are derived from the widely distributed Emeishan flood basalts and Pan-Xi layered basic–ultrabasic igneous rocks. This source is also a major contributor of magnetite to the upper Yangtze River, and the parent rock distributed in this region is of typomorphic significance to the development of the Yangtze River. The origin of magnetite grains in the tributaries depends on the parent rocks distributed in their catchments. In summary, the presence of detrital magnetite grains provides valuable information for discriminating the provenance of sediments in the upper Yangtze River drainage basins.

Collapsed upwelling and intensified euxinia in response to climate warming during the Capitanian (Middle Permian) mass extinction

The Middle Permian was marked by a major biotic crisis, in the Capitanian, and the termination of the long-lasting Gondwanan glaciation, which began in the Carboniferous. Eruption of the Emeishan large igneous province (ELIP) in South China may have been linked with this event and contemporary changes of ocean circulation. However, causal mechanisms linking the extinction with potential ELIP-climate-ocean co-variations remain poorly constrained. Here we have examined these themes by studying a deep-water section on the margin of the South China continent (lower Yangtze platform margin) region. Our results show chemical weathering increased from the Roadian to the Capitanian suggesting a change from a warm-humid to a hot-humid climate. This trend was enhanced at the time of ELIP eruption in the middle Capitanian. Using the Cd/Mo and Co x Mn proxies reveals that a strong coastal upwelling system with anoxic (but not euxinic) conditions and moderate-high productivity persisted for at least 10 m.y. on the Yangtze Platform margin. However, it abruptly collapsed in the middle Capitanian and then shifted to a relatively restricted but nutrient-enriched system with increased productivity and intensified anoxia. Climate warming slightly precedes these changes, making it likely that it was a primary trigger. Not only does warming slow down ocean circulation, it also promotes the delivery nutrients to the ocean through accelerated continental weathering. All contributed to the intensification and expansion of ocean anoxia-euxinia which occurred immediately prior to the Capitanian biotic crisis. Large-scale volcanic activity of the ELIP likely triggered these decoupled climate-ocean changes. An upwelling regime was re-established in the late Capitanian flowing a cooling that may have been caused by chemical weathering of the Emeishan flood basalts.

Dating and characterizing primary gas accumulation in Precambrian dolomite reservoirs, Central Sichuan Basin, China: Insights from pyrobitumen Re-Os and dolomite U-Pb geochronology

Primary gas accumulation with trillions of cubic meters of gas volumes was discovered recently in the Precambrian Dengying Formation of the Gaoshiti-Moxi block, Central Sichuan Basin, China. In this study we investigated the pervasive saddle dolomite cements and solid bitumens within karst vugs in the Dengying dolomite reservoirs. Using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) in situ U-Pb dating, we obtained a high-precision absolute age of 259.4 ± 3.0 Ma for the saddle dolomite cementation, consistent in timing with the eruption of Emeishan flood basalts (262–251 Ma). Combined with high homogenization temperatures (Th) and salinities of primary fluid inclusion, as well as rare earth element (REE) compositions, we consider that the saddle dolomite precipitation was likely related to hydrothermal activity driven by the Emeishan mantle plume.The identification of rare surviving oil inclusions and extensive solid bitumens within the dolomite vugs is indicative of the presence of paleo-oil reservoirs. These bitumens are characterized by high equivalent vitrinite reflectance (Requ), no fluorescence, and deformed fibrous textures, typical of highly mature pyrobitumens. Gas geochemistry data indicates that only minor thermochemical sulfate reduction (TSR) has occurred. Thus the pyrobitumen was interpreted to result mainly from the thermal cracking of paleo-oils, and its associated Re-Os isochron age of 154 ± 21 Ma, representing the timing of dry gas formation. Petrographical evidence suggests that the paleo-oil charge must have taken place after sparry dolomite cementation but prior to pyrobitumen generation. This, combined with coeval aqueous inclusion Th data and published thermal-burial models, constrains the time of paleo-oil emplacement to ca. 222–205 Ma, suggesting an oil-filling event related to the Indosinian Orogeny. This study documented the first chronological data constraining the complex evolution history of a petroleum system in one of the oldest primary gas reservoirs in the world.

Sources of the Laoxiongdong carbonate-hosted Pb–Zn deposit in Southwest China: constraints from S–Pb–Zn isotopic compositions

The Sichuan–Yunnan–Guizhou (SYG) Pb–Zn mineral province, which has significant Pb–Zn repositories in China, is situated in the western Yangtze Block. Despite decades of research, the Pb–Zn source of deposits is still disputed between the basement rocks, sedimentary wall rocks, or the Emeishan flood basalts. The newly-discovered Laoxiongdong Pb–Zn deposit is hosted by the Late Ediacaran Dengying Formation in the SYG province. Moreover, the Laoxiongdong orebodies lie between regional deep faults and close to the Proterozoic basement and Emeishan basalts. Hence, this deposit represents a good case study on the ore-forming material source for the regional Pb–Zn mineralization. The Laoxiongdong Pb–Zn ores are massive, vein, or disseminated and have mainly sphalerite, galena, pyrite, quartz, and calcite. In this paper, we present new sulfide S–Pb–Zn isotope data of the deposit. The sulfide δ34SCDT values (+ 11.8 to + 16.5 ‰) suggest that the reduced sulfur was mainly sourced from evaporites in the Late Ediacaran–Cambrian sequences. Pb isotopic ratios (206Pb/204Pb = 18.004–18.107, 207Pb/204Pb = 15.652–15.667, and 208Pb/204Pb = 38.037–38.248) suggest that the lead metal was primarily originated from the basement rocks. The sphalerite δ66Zn values (+ 0.16 to + 0.37 ‰) are also highly similar (within error) to those of basement rocks (+ 0.10 to + 0.34 ‰), suggesting a basement-rocks zinc source with minor contributions from the carbonate host rocks and Emeishan flood basalts. The narrow sphalerite Zn isotopic range (0.21 ‰) also indicates that the Zn isotopic fractionation between the sphalerite and initial fluid was limited during the sphalerite ore precipitation. Therefore, we propose that both the Late Ediacaran Dengying Formation rocks and Proterozoic basement rocks were important ore-forming material source for the Laoxiongdong deposit, whereas the Emeishan flood basalts represent only a minor ore-material source.

Rb–Sr geochronology and geochemistry of the Xiaotongchang basalt-hosted copper deposit in the Jinping area, SW China

The Xiaotongchang basalt-hosted copper deposit in Jinping, southern Yunnan Province, SW China, is located in the southern part of the Emeishan large igneous province. Here, we use Rb–Sr, Re–Os and S–Pb–C–O isotope data to constrain the age and genesis of the deposit. Petrological investigations reveal two main stages of mineralization. The early-stage is characterized by disseminated or massive sulfides in an assemblage of pyrite + chalcopyrite + quartz + albite + calcite. Rb-Sr isotopic dating of early-stage chalcopyrite yields an age of 256 ± 9.4 Ma (MSWD = 5.4), which is slightly younger than the eruption age of the Jinping basalts. The δ34SCDT values of the sulfides range from +5.8‰ to +7.7‰, suggesting that the metals of the early-stage copper mineralization originated from the Emeishan basalts and that the ore-forming fluids were contaminated by crustal rocks. These units may indicate that the early-stage orebodies represents syngenetic hydrothermal mineralization related to the eruption of Emeishan flood basalts. The late-stage mineralization is characterized by a pyrite + chalcopyrite + bornite + chalcocite + quartz + calcite assemblage that is developed in veins or breccia. The Re-Os isotopic dating of late-stage chalcopyrite samples reveals that they formed at 240–231 Ma. The Pb isotopic compositions ((206Pb/204Pb)t = 17.521–19.227, (207Pb/204Pb)t = 15.480–15.760, and (208Pb/204Pb)t = 37.778–38.731; t = 235 Ma) show that the late-stage copper orebodies have a heterogeneous source and originated mainly from the crust. The late-stage sulfides have higher δ34SCDT values (+7.9‰ to +10.7‰) than the early-stage sulfides. The narrow range of sulfur isotopic compositions of the sulfides associated with both stages and the Pb isotopic compositions of late-stage chalcopyrite indicate that some of the metals associated with late-stage mineralization originated from the Jinping basalts. The C–O isotopic compositions of calcite samples show that the magmatic hydrothermal fluids in the late stage flowed through and were contaminated by the Maokou limestone. Therefore, we suggest that the late-stage orebodies represent magmatic hydrothermal mineralization associated with the Late Triassic rhyolite porphyry and that the tectono-magmatic hydrothermal fluid remobilized copper from the basalts. The copper deposits formed at favourable interfaces within the basalt stratigraphy or in favorable structural locations.

Olivine O isotope and trace element constraints on source variation of picrites in the Emeishan flood basalt province, SW China

Like many continental flood basalt (CFB) provinces in the world, the source mantle compositions (peridotites or pyroxenites) of the Emeishan CFB province in SW China are still debated. We have used the combination of olivine O isotopes determined in situ using SIMS and trace element abundances measured in situ using LA-ICP-MS to address this fundamental problem. The newly discovered picrites in the Emeishan CFB province at Yumen are used in this study. In addition, we have also analyzed primitive olivine phenocrysts with known O isotope ratios from four other picrite occurrences (Maoniuping, Tanglanghe, Wuguijing and Wulongba) in the Emeishan CFB province for trace element abundances. Based on whole rock compositions the picrite samples from Wulongba belong to the low-Ti group (Ti/Y < 500) and those from the other locations belong to the high-Ti group (Ti/Y > 500). Mineral chemical data reveal that the two different groups of picrites can be distinguished by using Ti/Al of olivine phenocrysts and Cr-spinel inclusions enclosed in olivine. Specifically, the high-Ti picrite group is characterized by 10Ti/Al (molar) >0.9 and >0.5 for olivine phenocrysts and Cr-spinel inclusions, respectively. In the mantle source discrimination diagrams based on olivine 100Mn/Fe versus 100Ni/Mg or 100Ca/Fe, both groups of picrites plot between the suggested fields of peridotite and pyroxenite mantle sources. In the same type of diagrams based on olivine Mn/Zn and 10000Zn/Fe, both groups of picrites plot exclusively in the fields for a peridotite mantle source. The mean δ18O value of olivine phenocrysts from the Yumen high-Ti picrites is 5.19 ± 0.25‰ (1σ, n = 35), which is significantly lower than that for the Wulongba low-Ti picrites (5.6 ± 0.15‰) but is similar to values (5.1 ± 0.15 to 5.3 ± 0.15‰) for the high-Ti picrites from elsewhere (Maoniuping, Talanghe and Wuguijing) in the Emeishan CFB province. The mean δ18O values of olivine phenocrysts from the Emeishan high-Ti picrites at different locations are all within the range of typical mantle olivine values (5.1 ± 0.3‰). Based on these results, we conclude that the high-Ti picrites in the Emeishan CFB province were primarily derived from a modally enriched peridotite mantle source. Our new olivine Zn-Mn-Fe data are consistent with the view that the Wulongba low-Ti picrites were derived from a lithospheric peridotite mantle source.

Homogeneous Zn isotopic compositions in the Maozu Zn-Pb ore deposit in Yunnan Province, southwestern China

The Sichuan-Yunnan-Guizhou (SYG) metallogenic province, southwestern Yangtze Block, is one of the most important Zn-Pb repositories in China, with more than 200 million tons (Mt) of Zn-Pb ores. The source regions for the regional Zn-Pb mineralization in the SYG are still controversial, mostly in regard to the Emeishan flood basalts, basement rocks and host rocks. The Maozu Zn-Pb deposit is geographically close to the SYG center, which could also indicate that it is closer to the regional Zn-bearing fluid center than those deposits along the SYG margin. However, the origin of the ore-forming Zn in the Maozu deposit has not been constrained until now. In this study, the Zn and S isotopic compositions and Fe and Cd concentrations of the sphalerite from the Maozu deposit are investigated to evaluate the Zn isotopic fractionation during sphalerite precipitation, the source regions of the ore-forming zinc and sulfur, and the center of the regional Zn-bearing fluid system. It is suggested that the Zn isotopic fractionation between the sphalerite and original fluid during sphalerite precipitation could be limited due to the striking decrease in the temperature of the incoming fluid. The Zn isotopic variation of the sphalerite from four mining levels in the Maozu deposit is really small (−0.06 to +0.23‰), which could be attributed to heterogeneous Zn isotopic compositions in original incoming fluid rather than Zn isotopic fractionation during sphalerite deposition. As such, in the Maozu deposit, the Zn isotopic compositions of original hydrothermal fluid could approximately be represented by the Zn isotopic signals documented in sphalerite (−0.06 to +0.23‰). We found that the Zn isotopic signatures of incoming fluid are largely overlapped with those of the basement rocks (+0.10 to +0.34‰), signifying that the ore-forming Zn could be predominantly sourced from the basement rocks. However, in the Maozu deposit, the ore-forming sulfur is most likely derived from the sulfur-bearing evaporates hosted in the country rocks via thermochemical sulfate reduction (TSR). The contrasting sources of ore-forming zinc and sulfur suggest the mixing of Zn-bearing fluid from the basement rocks and sulfur-bearing fluid from the host rocks, which could be responsible for the sharp decrease in the temperature of the incoming fluid in the Maozu deposit. Combined with the results of previous studies, the integrally lower Zn isotopic compositions of sphalerite in the Maozu deposit compared to those deposits along the SYG margin may demonstrate that the Maozu deposit is closer to the regional Zn-bearing fluid center. If this is the case, Zn isotopes could be an effective proxy to trace the Zn source of Zn-enriched ore deposits and to identify the regional Zn-bearing fluid center of Zn-enriched metallogenic provinces.

Crustal P wave velocity structure beneath the SE margin of the Tibetan Plateau from Deep Seismic Sounding results

The crust and uppermost mantle beneath the southeastern (SE) margin of Tibetan Plateau record the lateral expansion of Tibet and far-field effects from the ongoing continental collision and convergence. In order to obtain constraints on the deep structure and the eastward expansion of the plateau, we synthesized Deep Seismic Sounding (DSS) results in this region and constructed the P wave velocity models for each terrane. The principal characteristics of the crustal velocity model and tectonic significance are: (1) Crustal thickness decreases from 53 km beneath the Songpan-Ganzi terrane (SGT) to 34 km in the south direction beneath Simao terrane (SMT), and 42 km in the east direction beneath Chengdu area. The crust is dominantly felsic in upper crust, with a small percentage of mafic composition in the lower crust. (2) The crustal structure is very heterogeneous in this region. The Indochina Block is characterized by low seismic velocities in the upper crust, while the west Yangtze terrane (WYZT) and SGT show higher velocities in the upper crust. The lower crust beneath WYZT is relatively thicker than other terranes. (3) We deduce that the high velocity in the upper crust and relatively thick lower crust beneath the SGT and WYZT can be associated with magmatic processes that generated the Emeishan flood basalts which may have been triggered by rifting or mantle plume. The lateral variations of the crustal thickness in SE Tibet may be due to lower crustal flow from the Tibetan Plateau to the SE direction after blocked by the cold and rigid Yangtze craton in the east direction.

A positive C-isotope excursion induced by sea-level fall in the middle Capitanian of South China

A new carbon isotope excursion was recovered from the Capitanian marine carbonates at the Rencunping (RCP) section of South China. Significantly, a pronounced excursion with elevated δ13Ccarb values over +5‰ was coeval with the conodont Jinogondolella prexuanhanensis Zone and resembles the Kamura event recorded in Tethys. The excursions in δ13Ccarb, constrained by conodont biostratigraphy, however, present inconsistent carbon cycle behaviors, especially between the separated basins in South China, and evidently reflect regional litho-facies controls. In addition, a transitional environment in association with a fall in sea level, was recovered from deposits of anoxic cherty carbonates subsequent to shallow-water carbonates around the positive excursion in δ13Ccarb. Accordingly, instead of this being a signal of global-scale climatic cooling, we suggest that the positive excursion in δ13Ccarb can be attributed to eutrophication effects regionally along a continental shelf. In this scenario, the increasing dissolved O2 level in the mixing zone that induced by the initial sea-level fall will efficiently impede denitrification and increase the bio-available N in N:P ratio to satisfy the demands of primary producers in surface waters. Subsequently, deposits of shallow-water carbonates comprising calcareous algae and massively-bedded lime muds accumulated widely around South China. These deposits appear to represent an unusual environment and ecosystem fertilized, perhaps, by the weathering products from the earliest stages of volcanism prior to the main Emeishan flood basalt eruptions at the Guadalupian-Lopingian boundary (GLB).