Xie-Yan Song

A temporal link between the Emeishan large igneous province (SW China) and the end-Guadalupian mass extinction

Abstract Previous studies have suggested that there were two mass extinction events in the Late Permian: one that occurred at the Permo-Triassic (P/T) boundary (251 Ma) and a second, smaller mass extinction that occurred 5–8 Myr earlier at the end of the Guadalupian. Many workers have argued that there is a causal relationship between large-scale volcanic activity and mass extinctions. The major mass extinction event at the P/T boundary coincides with the outpouring of huge quantities of lava that formed the Siberian flood basalt province in Russia. Courtillot et al. [Earth Planet. Sci. Lett. 166 (1999) 177–195] and Wignall [Earth Sci. Rev. 53 (2001) 1–33] suggested that the earlier Late Permian mass extinction coincided with the eruption of the lavas that formed the Emeishan flood basalt (EFB) province in SW China. However, the age of eruption of the EFB lavas is poorly constrained. Using the Sensitive High-Resolution Ion Microprobe to analyze zircons, we have established the age of the Xinjie intrusion, believed to be a feeder to the main phase of EFB volcanism, to be 259±3 Ma. Hence, the formation of the EFB is coincident with a proposed extinction event at 256–259 Ma. This result supports a temporal link between the Emeishan large igneous province and the end-Guadalupian mass extinction.

Geochemical Constraints on the Mantle Source of the Upper Permian Emeishan Continental Flood Basalts, Southwestern China

The widespread Emeishan igneous province in southwestern China comprises the Emeishan continental flood basalts (ECFB) and associated mafie-ultramafic intrusions. The ECFB have variable SiO2, ranging from 43.6 to 52.1 wt%, Al2O3 from 5.0 to 12.6 wt%, and total alkali (K2O + Na2O) from 0.7 to 6.5 wt%. These oxides exhibit negative correlations with MgO (5.4 - 23.1 wt%), implying fractional crystallization of olivine and clinopyroxene, which occur as phenocrysts in the rocks. Linear correlations between Zr, Nb, and La suggest that crustal contamination is not important. The primitive-mantle-normalized trace-element patterns show that the ECFB are enriched in high-field-strength trace elements, large-ion-lithophile elements, and light-rare-earth elements, similar to ocean-island basalt. Incompatible element ratios of the ECFB, such as Zr/Nb (7-10), Th/La (0.1-0.15), and Rb/Nb (0.9-1.7), differ from those of primitive mantle, N-MORB, and continental crust, but are similar to ocean-island basalts from an enriched mantle source (EM-1). However, the ECFB have isotopic ratios (143Nd/144Nd = 0.51229 -0.51276 and 87Sr/86Sr = 0.70480-0.70647) that imply that the ECFB were derived from a homogeneous, primitive lower mantle carried upward by a mantle plume. We propose that the original melts derived from the mantle plume were contaminated through interaction at shallower depth with an enriched lithospheric mantle. This model suggests that the lithospheric mantle beneath the ECFB was modified by subduction of an oceanic slab.

Late Permian rifting of the South China Craton caused by the Emeishan mantle plume

Stratigraphic relationships and bulk-rock geochemical data indicate that Upper Permian metabasalts in the Songpan–Ganzi and Yidun terranes, on the eastern margin of the Tibetan Plateau, are part of the Emeishan large igneous province, which is believed to have formed from the Emeishan mantle plume. Eruption of the Emeishan basalts at 260 Ma was coincident with rifting of the western margin of the South China Craton to form the Songpan–Ganzi ocean basin. The spatial and temporal coincidence between basalt eruption and continental rifting, as well as regional doming prior to eruption, suggest that continental break-up was a response to the Late Permian Emeishan plume. The Songpan–Ganzi ocean basin was rapidly filled with Triassic flysch deposits, then deformed and uplifted during Mesozoic collision between the North China and South China Blocks and the Tertiary collision of India and Eurasia.

Emeishan Basalts (SW China) and the 'end-Guadalupian' crisis: magnetobiostratigraphic constraints

A magnetostratigraphic investigation of the Permian Emeishan LIP (large igneous province) was carried out on a composite section in Ebian County, close to the type area in Sichuan, SW China. The main succession of twelve flows (175u2009m thick) carries a normal polarity whilst the one reliable site from the overlying 30u2009m thick volcanic waning sequence is marked by a reverse polarity. The data enable a correlation to be proposed with an Emeishan Basalt sequence in western Guizhou, c.u2008400u2009km to the SE. From our knowledge of the geomagnetic fields reversal behaviour during the Permian, it suggests that the main lava pile along the eastern half of the Emeishan Basalt outcrop belt was erupted within a half to one million years. Using magnetobiostratigraphic data from the adjacent South China platform, the normal polarity magnetozone is correlated with the normal polarity chron associated with the upper part of the Maokou Limestones. Constrained by conodont data, the main Emei Basalts appear to be at least two biozones older (1–1.5u2009Ma) than the Mid–Late Permian boundary. It is possible, however, that the Emei Basalt waning zone sequence, which represents an explosive volcanic phase, might be coeval with the ‘end-Guadalupian’ biotic crisis. Thus arguments implicating Emei Basalt volcanism as the causal mechanism behind this major global event have to accommodate the new relative-age constraints.

Emeishan Basalt Ar-Ar overprint ages define several tectonic events that affected the western Yangtze platform in the Mesozoic and Cenozoic

Abstract Ar–Ar whole-rock dating was carried out as part of a detailed stratigraphical investigation of the Emeishan Basalt large igneous province (LIP) in the stratotype area, Sichuan, China. Thirteen (from twenty-one) specimens from three sections yielded reliable reversed isochron and plateau ages (maximum 1σ errors of ≤2.1 and 1.6 m.y., respectively), and form two clusters centered on the Late Jurassic–Early Cretaceous and Early–Late Cretaceous, with a tail spanning 82–40 Ma. However, all are appreciably younger than the magnetobiostratigraphically-constrained late Middle Permian (255–260 Ma) age of the basalts, reflecting varying degrees of thermal resetting during the Mesozoic and Cenozoic. Recently, Middle Jurassic–Early Cretaceous ages reported from elsewhere in the LIP have been used to infer a single 40 m.y.-long tectonic episode that affected the western Yangtze Platform. Developing this idea, the new information have been combined with data we hold for other parts of the terrain, and results from three Emeishan LIP Ar–Ar dating studies published during 2002, to give 32 reliable age dates. Three 10–12 m.y. events appear to be recorded in different parts of the province: Middle Jurassic, Late Jurassic–Early Cretaceous and Early–Late Cretaceous, with a fourth shorter middle Eocene episode (the 40 m.y. Mesozoic episode appears to be a sampling artifact). Following a review of the regional deformation/tectonic features, it is argued that activity related to various phases of deformation in the Longmen Shan Thrust Belt is the most likely cause of resetting. The final suturing of the North and South China blocks may have also been responsible for the Middle Jurassic event. However, the Oligocene-present indentation of India into Asia appears not to have had an impact, possibly due to the large-scale related strike-slip faults that have effectively shielded the LIP/Sichuan Basin.

Emeishan Basalts, SW China: reappraisal of the formation’s type area stratigraphy and a discussion of its significance as a large igneous province

The late Permian Emeishan Basalt Formation of SW China is one of Earth’s LIPs (large igneous provinces), yet its basic geology remains poorly documented. Recent work on sections close to the type area in Sichuan Province enable us in part to rectify this. Descriptions of the formation and associated units at two areas, one on the lower flanks of Mt Emei and another from a series of outcrops in Ebian County, 50–70u2009km to the SW, are presented. The basalt pile is 180–270u2009m thick and in both areas comprises 12 flows that were erupted in relatively quick succession. It rests conformably upon shallow-marine limestones/lignites suggesting emplacement close to sea level. The upper half of the youngest basalt was intensively weathered, but not eroded, prior to it being conformably succeeded by complex body of rocks c.u200930u2009m thick, that includes thin basalts, pyroclastic rocks, tuffs and organic-rich terrestrial sediments. This unit, which has previously been described as a sedimentary package, presumably because intense weathering has obscured the primary lithological fabric in key outcrops, is considered to mark the volcanic waning phase. Uppermost Permian and Triassic terrestrial sediments conformably overlie the terminal volcanic rocks. The sub-regional stratigraphy is compared, as best it can be, with that described from two sections 400u2009km to the SE; one section matches reasonably well, the other does not, indicating that regional correlations need to be developed carefully. The information is discussed in the context of LIP generator models; several key features of the Emeishan Basalt terrain are at odds with those commonly encountered in LIP’s. The most important conclusion is that the unit marks a prematurely terminated system in which full bloodied rifting leading to the development of an ocean basin never started.

Syncollisional tholeiitic magmatism induced by asthenosphere upwelling owing to slab detachment at the southern margin of the Central Asian Orogenic Belt

The mafic–ultramafic complexes containing magmatic sulphides at the southern margins of the Central Asian Orogenic Belt have been recently proposed to result from an Early Permian mantle plume. However, in this study we show that the plume model cannot account for the observed geological characteristics of the Huangshan–Jingerquan mafic–ultramafic belt in the Northern Tianshan. Low K2O contents and positive correlation between TiO2 and (Fe2O3)T/MgO of the mafic–ultramafic complexes of this belt demonstrate a tholeiitic affinity. Enrichment of large ion lithophile elements and depletion of high field strength elements (in particular Nb and Ta) relative to mid-ocean ridge basalt indicate a subduction-modified mantle source. Lead isotope values and compositions of chromite indicate a significant contribution from the melting of asthenosphere. The absence of Late Carboniferous strata in the Huangshan–Jingerquan belt and Early Permian exhumation of blueschist and eclogite along the Aqikkuduk suture at the southern boundary of the belt indicate that an arc–continent collision occurred in the Late Carboniferous to Early Permian. We propose that the detachment of oceanic lithosphere from continental lithosphere during the collision induced asthenospheric upwelling, which resulted in melting of both the asthenosphere and the overlying metasomatized mantle wedge, and the formation of the mafic–ultramafic complexes with ages of 270–285 Ma along the Huangshan–Jingerquan belt. Supplementary materials: Zircon SHRIMP U–Pb ages of the Tudun and Huangshannan intrusions, major oxide and trace element compositions of the discussed intrusions, and clinopyroxene and chromite compositions of the selected intrusions are available at www.geolsoc.org.uk/SUP18656.

Structural, lithological, and geochemical constraints on the dynamic magma plumbing system of the Jinchuan Ni–Cu sulfide deposit, NW China

Eastern and western portions of the Jinchuan ultramafic intrusion have previously been interpreted as dismembered segments of a single elongate intrusion by late faults. However, the different stratigraphic sequences of the two portions indicate that they are originally two separate intrusions, referred to as Eastern and Western intrusions in this study. The Eastern intrusion is characterized by a concentric distribution of rock types with a core of sulfide dunite enveloped by lherzolite, whereas the Western intrusion is composed of the Upper and Lower units, interpreted as magmatic mega cycles with regular variations in lithology and chemistry. In the Western intrusion, the Upper unit consists of fine-grained dunite, lherzolite, and pyroxenite from its base to its top. The MgO contents decrease upward from the dunites (42–45xa0wt.%) to the lherzolites (36–41xa0wt.%), while Al2O3 and incompatible elements increase upward. In contrast, the Lower unit consists of coarse-grained dunites and lherzolites containing 37–40 and 28–35xa0wt.% MgO, respectively. Sharp contacts between the Upper and Lower units and fine-grained dunite xenoliths at the top of the Lower unit indicate that the Lower unit intruded along the base of the Upper unit. Disseminated and net-textured sulfides primarily occur in the Lower unit and comprise the no. 24 ore body. Very low S contents (<100xa0ppm) of the wall rocks at Jinchuan indicate that they were not the source of S causing sulfide immiscibility. Sulfide segregation more likely occurred in deep-seated magma chambers, and sulfides were deposited in the Western intrusion when sulfide-bearing magmas passed through the intrusion. In contrast, the Eastern intrusion was formed by injections of sulfide-free and sulfide-bearing olivine-crystal mushes, respectively, from another deep-seated staging magma chamber. The Eastern and Western intrusions and the deep-seated magma chambers comprise a complicated magma plumbing system at Jinchuan. Normal faults played a significant role in the formation of the magma plumbing system and provided pathways for the magmas.

Role of Crustal Contamination in Formation of the Jinchuan Intrusion and Its World-Class Ni-Cu-(PGE) Sulfide Deposit, Northwest China

The Jinchuan ultramafic body on the southwestern margin of the North China craton hosts a giant Ni-Cu-(PGE) deposit. This dike-like intrusion consists mainly of lherzolite, dunite, and minor pyroxenite. Fortly-seven vol% of the intrusion is composed of disseminated sulfide ore and minor massive and net-textured ores. Ore bodies occur commonly in the lower part of the intrusion; the largest contains about 50% of the total metallic resources in Jinchuan, and has a flame-like shape in vertical cross-section. The ultramafic rocks have high MgO (19-45 wt%), low CaO (<6.5 wt%) and Al2O3 (<7 wt%), and right-inclined chondrite-normalized REE patterns. These rocks contain variable Ir (0.4-17 ppb), Ru (0.6-21 ppb), Rh (0.05-8.4 ppb), Pt (0.6-196 ppb), and Pd (1.2-135 ppb), in general lower than those in the sulfide ores (2.9-110 ppb Ir, 3.3-260 ppb Ru, 1.5-237 ppb Rh, 6.9-3972 ppb Pt, and 15-532 ppb Pd). They have high Th/Nb (0.15-0.5) ratios and show primitive mantle-normalized trace element patterns with Nb-Ta negative anomalies, consistent with derivation from mantle magmas variably contaminated by crustal materials. Compositional heterogeneity of the disseminated sulfides cannot be accounted for by in-situ fractional crystallization, but is consistent with differentiation and sulfide segregation in a staging magma chamber at depth. We propose a compressive tectonic model in which injection of sulfide-poor, crystal-rich magmas from the upper part of the staging magma chamber was followed by injection of a sulfide-rich crystal-mush from the lower part of the staging magma chamber to form the Jinchuan body.

Magmetic sulfide deposits in the Permian Emeishan large igneous province, SW China

Four genetic types of magmatic sulfide mineralization have been identified in the Emeishan large igneous province. These are: 1). Ni-Cu-(PGE) sulfide deposits produced by in-situ sulfide segregation, 2). PGE-enrich layers within layered intrusion, 3). Ni-Cu sulfide mineralization related to sulfide-bearing mush, and 4). PGE sulfide mineralization distributed throughout ultramafic rock. The distribution of the sulfide mineralization is associated with dynamic of the mantle plume.