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Magmas with low δ18O values, which require a contribution of isotopically light oxygen from hydrothermally altered rocks, are globally rare despite the common association of large-scale magmatism with intense hydrothermal activity. Here we consider this paradox in the central Taupō Volcanic Zone (TVZ, New Zealand), one of the most intense regions of Quaternary silicic magmatism and hydrothermal activity on Earth. We present new data on phenocryst δ18O values from 62 magma batches erupted since ∼1.6 Ma, ranging from basalt to rhyolite, and match these against existing and new Sr and Nd isotopic data. Basalt δ18O melt values (typically ≥+6.3 to +6.5‰) calculated from phenocrysts are mostly high relative to mid-ocean ridge basalts (MORB), whereas basaltic andesites extend to +7.1‰. Rhyolite δ18O melt values are mainly +7.0 to +8.5‰, which is considered high-δ18O in most settings. However, assimilation and fractional crystallisation (AFC) models show a mismatch between the amount of greywacke basement assimilation required to explain the Sr-Nd isotopic ratios and the δ18O values of many TVZ rhyolites. We attribute this mismatch to shallow assimilation of low-δ18O altered rocks after prior assimilation of unaltered greywacke at deeper levels. We infer that interactions between large silicic magmatic systems and shallow altered crustal rocks are common and widespread in the TVZ, despite the absence of silicic magmas with low δ18O values (<∼+6.0‰). This absence reflects the combined influence of high δ18O values inherited from the slab in parental basalts, and the limited isotopic leverage available in the TVZ to drive large 18O depletions via hydrothermal alteration. The TVZ example suggests that large-scale interactions between magmas and altered host rocks are likely to be important in many silicic magmatic provinces and not restricted to the sparse occurrences of low-δ18O magmas. Spatial clustering of the lowest TVZ rhyolite δ18O melt values (+6.3 to +7.0‰) occurs along a western corridor of presently weak hydrothermal activity, implying that hydrothermal activity was more intense there in the past, and may have represented a focus of heat flow similar to the present-day eastern TVZ margin. Magma δ18O values can thus be useful for tracing areas of past alteration even in settings where conventionally normal to high δ18O values are retained in erupted products. [ABSTRACT FROM AUTHOR]

The International Ocean Discovery Program (IODP) Expedition 396 to the mid-Norwegian margin recovered >1300 m of pristinely preserved, volcanic ash-rich sediments deposited during the late Paleocene and early Eocene, from close to the centre of the North Atlantic Igneous Province (NAIP). Remarkably, many of these cores contain glendonites, pseudomorphs after the purported cold-water mineral ikaite, from sediments dated to the late Paleocene, Paleocene – Eocene boundary and early Eocene. These time intervals span some of the hottest climates of the Cenozoic, including the Paleocene-Eocene Thermal Maximum (PETM). Global deep ocean temperatures are not thought to have dropped below 10 °C at any point during this time, making the occurrence of supposedly cold-water (near-freezing temperature) glendonite pseudomorphs seemingly paradoxical. This study presents a detailed sedimentological, geochemical, and microscopic study of the Exp. 396 glendonites, and presents an updated model for the ikaite-to-calcite transformation for these glendonites. Specifically, we show that early diagenesis of basaltic ashes of the NAIP appear to have chemically promoted ikaite growth in the sediments in this region. Together with existing knowledge of late Paleocene and early Eocene glendonites from Svalbard to the north, and early Eocene glendonites from Denmark to the south, these new glendonite finds possibly imply episodic, short-duration, and likely localised cooling in the Nordic Seas region, which may have been directly or indirectly linked to the emplacement of the NAIP. [ABSTRACT FROM AUTHOR]

The complex structure of the region has been formed as a result of ​kinematic reorganization of plate boundaries, accompanied by the extinction of old spreading centers, the formation of new ones, and plume magmatic activity in the South Atlantic southeastern part. These processes resulted in the formation of a system of ridges, rises, and plateaus with different morphologies and geochemical characteristics. The results of density modeling of the structure of the crust and tectonosphere along the profiles, which extend from the Falkland Plateau to the Mozambique Ridge and cross a series of rises and ridges divided by deep-water basins, showed that the rises have different crustal structures indicative of their different origins. The formation conditions of various submarine rises have been studied on the basis of physical modeling. The new experimental model of the formation of the lithosphere and submarine plateaus of the region is provided, in which the important role belongs to the breakup of the Agulhas large igneous province on the Agulhas Plateau and Northeast Georgia Rise, the accretion of the oceanic crust on the Agulhas spreading ridge, and a further jump of the Agulhas spreading ridge axis leading to the termination of spreading at this ridge and the formation of a southern segment of the Mid-Atlantic Ridge and conjugated Meteor and Islas Orcadas Rises. Spreading axis jumps, which are accompanied by periodic activity of hotspots, played an important role in the formation of the submarine rises of various genetic types that, in turn, determined the different structures of their crust. [ABSTRACT FROM AUTHOR]

The better understanding of the initial rifting would complement the knowledge of continental breakup and the oceanic opening process. Traditionally, mechanisms controlling the initial rifting have been divided into two schools of end-member models, magma-rich vs magma-poor rifting and pure-shear vs simple shear rifting. However, the linkage between these two schools has been rarely addressed. In our study, extension magnitude and ratio obtained from the balanced cross-sections were applied in magma-poor and magma-rich segments in South Atlantic margins to establish the possible linkage, as classic pure-shear and simple-shear models predict different extension magnitudes and ratios. Two pairs of structural sections Campos basin‒Kwanza basin from the central segment and Pelotas basin‒Walvis basin from the south segment, the southern segment temporally and spatially related to the (Paraná‒Etendeka Large Igneous Province) were subjected to balanced cross-section analysis to determine the extension magnitudes and ratios at the initial rifting stage predating finally opening of South Atlantic Ocean. Comparing the results in the South Atlantic margins with those obtained in classic shear models may suggest the magma-rich segment would return a larger extension ratio and possibly prefer to pure-shear rifting. Results of this study might build a likely linkage between the magma-rich and pure-shear mechanisms during the initial rifting of the oceanic opening process. [ABSTRACT FROM AUTHOR]

The Late Cretaceous Mata da Corda Formation, located in the eastern part of the Alto Paranaíba Igneous Province (APIP), Central Brazil, is one of the few places on Earth where kamafugite melts reached the surface generating large volumes of lava, pyroclastic rocks and shallow intrusions over an area of 4,500 km2. The western part of the APIP, however, is dominated by hundreds of diatreme-like kamafugites and shallow kimberlite intrusions and by the occurrence of multi-stage alkaline-carbonatite complexes. These complexes feature silica-undersaturated K-rich alkaline rocks, such as aillikite, that closely resemble the mineralogy and geochemistry of kamafugite, albeit lacking feldspathoids. The spatial and temporal distribution of kamafugite and aillikite within the APIP suggests a connection between them. In addition, on a regional scale, airborne magnetic data show three highly magnetic dipole-like structures to the south of the Mata da Corda Formation of an undisclosed nature, which bear geophysical similar responses to the neighbouring alkaline-carbonatite complexes. Links between kamafugite and aillikite are evaluated by the following chemical and isotopic evidence: (1) kamafugite and aillikite compositions plot in the kamafugite field of Foley's ultrapotassic rock classification; (2) similar CI chondrite-normalized REE distribution, with aillikite enriched up to 2 times in REE compared to kamafugite; (3) both lithologies share almost the same rock-forming minerals; and (4) similar 143Nd/144Nd(i) and 87Sr/86Sr(i) ratios for all the APIP alkaline-carbonatite rocks, indicating a common source from an enriched lithospheric mantle. Therefore, silica-undersaturated rocks from alkaline-carbonatite complexes display an evolved ultrapotassic affinity indicative of a genetic link. [ABSTRACT FROM AUTHOR]

It is generally recognized that the crustal flow in the Yunnan region is attributed to the continental collision between the Indian and Eurasian plates. Therefore, studying the crustal flow and its dynamic processes in this region can provide valuable insights for understanding the collision process and crustal thickening. The S-wave velocity can provide constraints on the lithological state, interface delineation, and geodynamic processes of the Earth's interior, which are critical for explaining the properties of the crustal flow within a particular region. In this study, we employ 2-year continuous waveforms of the vertical component of 58 seismic stations using ambient noise tomography in the Yunnan region to build a 3-D S-wave velocity crust model. On this basis, the spatial distribution and dynamic processes of regional crustal flow are discussed. Our model reveals the distribution features of the mid-lower crustal flow with different deformation mechanisms at different depths. Finally, a dynamical model is proposed to interpret the dynamic processes of the regional mid-lower crustal flow. The model reveals that a large amount of crustal flow material on the northeastern side of the Red river fault is partially blocked by the Emeishan large igneous province, resulting in the lateral escape and rotation of the mid-lower crustal flow. Another part of the crustal flow material is controlled by the strike of the Red river fault, and the high-velocity Indo-China block on the southwestern side of the Red river fault prevents the crustal flow not escape to this area. We believe that the control effect of the Red river fault on the mid-lower crustal flow may be one of the main reasons for the crustal thickening and surface uplift on the northeastern side of the Red river fault. [ABSTRACT FROM AUTHOR]

There is a temporal correlation between the peak activity of the North Atlantic Igneous Province (NAIP) and the Paleocene–Eocene Thermal Maximum (PETM), suggesting that the NAIP may have initiated and/or prolonged this extreme warming event. However, corroborating a causal relationship is hampered by a scarcity of expanded sedimentary records that contain both climatic and volcanic proxies. One locality hosting such a record is the island of Fur in Denmark, where an expanded pre- to post-PETM succession containing hundreds of NAIP ash layers is exceptionally well preserved. We compiled a range of environmental proxies, including mercury (Hg) anomalies, paleotemperature proxies, and lithium (Li) and osmium (Os) isotopes, to trace NAIP activity, hydrological changes, weathering, and seawater connectivity across this interval. Volcanic proxies suggest that NAIP activity was elevated before the PETM and appears to have peaked during the body of the δ13 C excursion but decreased considerably during the PETM recovery. This suggests that the acme in NAIP activity, dominated by flood basalt volcanism and thermogenic degassing from contact metamorphism, was likely confined to just ∼ 200 kyr (ca. 56.0–55.8 Ma). The hundreds of thick (> 1 cm) basaltic ashes in the post-PETM strata likely represent a change from effusive to explosive activity, rather than an increase in NAIP activity. Detrital δ7 Li values and clay abundances suggest that volcanic ash production increased the basaltic reactive surface area, likely enhancing silicate weathering and atmospheric carbon sequestration in the early Eocene. Signals in lipid biomarkers and Os isotopes, traditionally used to trace paleotemperature and weathering changes, are used here to track seaway connectivity. These proxies indicate that the North Sea was rapidly cut off from the North Atlantic in under 12 kyr during the PETM recovery due to NAIP thermal uplift. Our findings reinforce the hypothesis that the emplacement of the NAIP had a profound and complex impact on Paleocene–Eocene climate, both directly through volcanic and thermogenic degassing and indirectly by driving regional uplift and changing seaway connectivity. [ABSTRACT FROM AUTHOR]

Resolving the detail of crustal deformation of the seismically and technically active Sichuan-Yunnan block located in the southeast margin of the Tibetan Plateau is crucial for understanding the lateral extrusion of the Tibetan Plateau. Using seismic data from two regional seismograph stations of the China Earthquake Networks Center and twenty recently deployed broad-band temporary stations from the MCD Array (Middle ChuanDian block seismic array), we constrained crustal thickness and seismic anisotropy at the west boundary of the Sichuan-Yunnan block, near the east Himalayan syntaxis by receiver function techniques including CCP stacking and sinusoidal moveout of PmS conversions. Crustal thickness varies significantly across major faults inside the block which increases by >20 km from west to east. East of the Chenghai Fault, a clear positive phase lies above the Moho and is interpreted to be caused by the magma intrusion related to a late Permian Emeishan large igneous province (ELIP). The crustal anisotropy shows strong zoning features and clockwise rotation with respect to due north. To the west of the Lijiang-Xiaojinhe and the Honghe Faults, anisotropic fast orientations are near NS, indicating strong crustal deformation and movement of southward crustal flow along the Nujiang-Lancangjiang Fault; to the east of this fault, anisotropic fast orientations are near NE. The variation of the anisotropic fast orientations indicates that the southeastward crustal flow from the Tibetan Plateau may be prevented by fast velocity anomalies beneath the ELIP. These observations provided new evidence for strong crustal deformation along the tectonic boundary in the southeastern Tibetan Plateau with the expansion of the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

Pure marine chemical sediments are archives of geochemical proxies for the composition of seawater and may provide information about the ancient hydrosphere–atmosphere system. The early stage of the Emeishan large igneous province (ELIP) was characterized by the subaqueous eruption of mafic igneous rocks around the J. altudaensis zone of the Capitanian Stage that has been proposed to have contributed to the Guadalupian mass extinction. However, detailed mechanisms and the impact of the eruption on the Guadalupian marine environment have yet to be assessed. Here, to examine the Guadalupian marine environment, we studied major and trace element concentrations, particularly rare earth element and yttrium data, along with high-precision Sr-Nd isotope ratios, of three types of Mn ores (i.e., clastic, massive, and oolitic) and siliceous limestones from the Zunyi Mn deposit in South China formed following the early-stage eruption of the ELIP. Our results indicate that the clastic Mn ores contain notable detrital mafic aluminosilicates. In contrast, the massive and oolitic Mn ores and siliceous limestones preserved the pristine geochemical signatures of the Middle–Late Permian seawater characterized by distinctly low (87Sr/86Sr)i and high εNd(t) values. These data indicate a strong impact of the early-stage submarine eruptions of the ELIP on the marine environment in South China and worldwide, likely through intensive seawater–rock interaction. [ABSTRACT FROM AUTHOR]

The Soarinho Alkaline Intrusion, southeastern Brazil, makes up part of the Serra do Mar Igneous Province, and it is composed of alkali feldspar syenite, alkali feldspar trachyte, quartz syenite, and monzonite. Geochemical and geochronological analyses of the Soarinho were compared with data from its neighbors, Rio Bonito and Tanguá. Zircon U-Pb Laser Ablation ages show that Soarinho is younger than Rio Bonito and Tanguá. Ages obtained from the monzonite (60 ± 2 Ma), alkali feldspar syenite (58 ± 2 Ma), and quartz syenite (58 ± 2 Ma) are indistinguishable within error. One nepheline syenite sample from Tanguá gives an age equal to those of Rio Bonito already published (65 ± 1 Ma). Petrographic and geochemical data show a clear contrast among Soarinho, Rio Bonito, and Tanguá. Lu-Hf isotopic data for the three alkaline intrusions (all negative) point to some similarities in their sources. The data suggest an enriched mantle source for Soarinho; however, an older continental crust contribution cannot be discarded. [ABSTRACT FROM AUTHOR]

Throughout the Flinders Ranges of South Australia, the red bed Brachina sequence comprises the lower of two regressive, coarsening-upward successions that together constitute the Ediacaran-aged Wilpena Group. Detailed examination of the Brachina sequence has identified evidence of ice-rafting, supporting the contention that frigid climatic conditions continued well after cessation of the Marinoan Glaciation, as well as features postulated to be harbingers of the macroscopic metazoans that occur in abundance higher in the local stratigraphy. Although tuffaceous and lying stratigraphically between deposits of the Marinoan Glaciation and the Acraman Impact/Gaskiers Glaciation, no absolute age dates exist for the sequence. However, through correlation to the global carbon isotope profile, the timespan of Brachina sequence deposition can be constrained to between 635 and 586 Ma, when the regression climaxed with terrestrial deposition upon a local disconformity. The tuffaceous input is timed to have begun at 620 Ma, essentially coincident with initial formation of the passive volcanic continental margin of southeastern Australia and initiation of both the Volyn Large Igneous Province and activity within the Central Iapetus Magmatic Province, all components marking the demise of Rodinia and initial amalgamation of Gondwana. Correlation to the global carbon isotope profile dates the early Ediacaran Brachina sequence of the Flinders Ranges to between 635 and 586 Ma. The ABC Range Quartzite in the central Flinders Ranges is derived from, and hence younger than, the deltaic ABC Range Quartzite in the southwestern Flinders Ranges. Several volcanic outpourings across the Ediacaran globe during the final breakup of Rodinia and initial assembly of Gondwana, including that associated with the development of the eastern Australian passive margin, produced an abundance of atmospheric ash that settled during and beyond deposition of the Brachina sequence. [ABSTRACT FROM AUTHOR]

Sedimentary pyrite trace-element composition is an established proxy for determining paleo-ocean geochemistry and atmospheric oxygen concentrations through deep time. However, its applicability over shorter time-scales (i.e. <20 Ma) is not well known. To test this, we targeted fine-grained pyrite in the Hovea Member of the Kockatea Shale (Perth Basin, Western Australia), which encompasses the late Permian inertinitic interval and the end-Permian to Early Triassic sapropel, and spans approximately 10 million years. The end-Permian mass extinction (EPME) was the largest extinction event in Earth history, and its greatest effect is documented in the marine environment. Samples were collected from two oil exploration wells—Redback-2 and Hovea-3—spaced ∼20 km apart. In the two boreholes, a change in depositional facies (i.e. between the inertinite and sapropel) occurs below the Permian–Triassic boundary and records the transition from a marginal marine to a shelf environment. This transition is highlighted by several lithogeochemical indicators (e.g. negative shift δ13C values and Corg reduction; increases in Ca, Fe and P), which are themselves tied to fundamental changes in modal mineralogy between the two zones. Importantly, the sapropel also records a major increase in iron sulfide burial over that in the inertinite. LA-ICPMS analyses of pyrite demonstrate that trace-element abundance is highest in samples below the facies transition, and in places reaches a few percent, particularly of Ni (4 wt%), Co (1.5 wt%) and As (2.8 wt%). Moreover, these and other trace elements decrease by an order of magnitude in concert with the negative shift in δ13C values in the sapropel zone. Various whole-rock based paleosalinity indicator ratios (e.g. B/Ga) indicate that the areas of the Perth Basin intersected by Redback-2 and Hovea-3 were not fully connected to the open ocean at the time of the EPME, which leads us to conclude that the very high trace-element values in the sedimentary sulfides are reflective of regional environmental shifts rather than a global signal. Nonetheless, a geochemical contribution from a distant igneous province, such as the Siberian Traps Large Igneous Province, cannot be ruled out. Our work underscores the strength of sedimentary pyrite as a robust paleoenvironmental proxy in the marine environment and highlights the need for further investigation of pyrite trace-element profiles across the mass extinction interval in other sedimentary sequences around the globe. LA-ICPMS-based geochemistry of sedimentary pyrite from the Hovea Member of the Kockatea Shale is considered within a lithochemostratigraphic context. The overall interpretation of the results involves a change in depositional setting from the marginal in the late Permian brackish waters to shelfal marine and loss of oxygen in the Early Triassic Perth Basin. [ABSTRACT FROM AUTHOR]

The Mesoproterozoic Husky Creek Formation is an interlayered redbed and basalt package that overlies a ca. 2.5 km thick, regionally extensive basaltic plateau linked to the 1.27 Ga Mackenzie Large Igneous Province. This succession provides an opportunity to study the interaction between contemporaneous fluvial sedimentation and flood basalt volcanism. The Husky Creek Formation is approximately 1900 m thick, consisting predominantly of fluvial channel and subordinate floodplain and eolian strata dominated by lithic detritus intercalated with basalt flows. U–Pb dating of detrital zircon grains collected from four stratigraphic levels in the Husky Creek Formation reveals two main age groupings: (1) a 1270 Ma peak attributed to the Mackenzie Large Igneous Province; and (2) Pre-1.6 Ga detrital zircon ages, which reflect sources from a broad region of northwestern Laurentia and are interpreted to be recycled from underlying sedimentary rocks of the Hornby Bay Basin. An upsection decrease in the proportion of ca. 1270 Ma zircon grains, coupled with increasing sandstone compositional maturity, reflects the expansion of the drainage basin during burial of a volcanic pile, with recycling becoming more dominant as the Husky Creek Formation accumulated. The Husky Creek Formation was deposited in the waning stages of the Mackenzie Igneous Event by west-flowing rivers into a geographically restricted basin above an extensive mafic volcanic plateau. This paper provides insight into fluvial basin development during the aftermath of one of Earth's largest igneous events. [ABSTRACT FROM AUTHOR]

Breakup of the proposed greater Ontong Java Nui large igneous province during the Cretaceous Normal Superchron can be constrained by the opening of the Ellice Basin (EB) separating the Ontong Java and Manihiki Plateaus and the Osbourn Basin separating the Manihiki and Hikurangi Plateaus. Dating of recovered dredged samples using plagioclase 40Ar/39Ar and zircon U/Pb geochronology methods indicates that spreading was well underway in the EB by 118 Ma with full spreading rates up to 3X faster than any observed today of 30–45 cm/yr and spreading likely continued until 112‐108 Ma. Ellice Basin samples show diverse geochemical affinities ranging from mid‐ocean ridge basalt (MORB) or Ontong Java‐like to more enriched OIB‐like. Pb and Nd isotopes from six samples contain varying influences from Pacific MORB and possibly Ontong Java. The geochemistry shows a lack of a clear mantle plume influence despite EB's close temporal and spatial relationship to Ontong Java, while some data resemble the Louisville Seamounts. This compositional diversity complements morphological differences among dredge sites and shows that both in situ MORB and younger overprinted features related to the nearby Tuvalu Seamounts were sampled. 40Ar/39Ar geochronology confirms the age of International Ocean Discovery Program Site U1365 near the Osbourn Trough (OT) to be 102.60 ± 0.26 Ma (2σ, n = 18). This age constrains the timing of a spreading reorientation event observed in the OB to coincide with a global plate reorganization event around 105 Ma and estimates the cessation of spreading at the OT to 96 Ma. Plain Language Summary: Ontong Java Nui is proposed to have been the largest volcanic system on Earth when it erupted about 120 million years ago on the sea floor in the Pacific Ocean. Shortly after it erupted, plate tectonics supposedly split this massive volcanic feature into five fragments that drifted away from each other. We present new age and geochemistry results from samples collected in two ocean basins that separate the three main fragments of this feature still located in the western Pacific Ocean. We suggest that all three fragments separated extremely rapidly, possibly as much as 3X faster than any plate tectonics today and stopped in roughly their current configuration only 20 million years after they began to spread. We also show that the source of magma in one of the basins, the Ellice Basin (EB), differed slightly from the initial volcanism that made up Ontong Java Nui as a whole. These are the first ages and geochemistry data for the EB, and they, along with our new ages from the Osbourn Basin, are critical to further understanding the eruption and breakup history of Ontong Java Nui. Key Points: New geochronology from the Ellice Basin (EB) suggests it formed between ∼123 and 112–108 Ma with ultra‐fast spreading rates up to 45 cm/yrWe confirm that the Osbourn Basin continued opening later than the EB and finished by 96 MaThe EB mantle source appears mostly like Pacific mid‐ocean ridge basalt with a possible influence from Ontong Java and the Louisville hotspot [ABSTRACT FROM AUTHOR]

Volcanism is a dominant natural source of mercury (Hg) to the atmosphere, biosphere, ocean and sediments. In recent years, sedimentary Hg contents have emerged as a tool to reconstruct volcanic activity, and particularly activity of (subaerially emplaced) large igneous provinces in geological deep time. More specifically, Hg has shown potential as a useful proxy to illuminate the previously elusive impact of such large-scale volcanism on marine and terrestrial paleo-environments. While Hg is now widely applied as volcanism tracer, non-volcanic factors controlling sedimentary Hg content are generally not well constrained. Part of this uncertainty stems from our inability to directly observe a natural unperturbed "steady-state" environment as a baseline, as the modern Hg cycle is heavily influenced by anthropogenic activity. Here we focus on the effects of ambient redox conditions in the water column and shallow sediments (early diagenesis), quantify their influence on the geological Hg record and thereby contribute to constraining their potential impact on the use of Hg as a proxy for deep-time volcanic activity. Constraining these factors is of critical importance for the application of Hg as such a proxy. Many periods in the geological past for which records have been generated, such as the Mesozoic Oceanic Anoxic Events, are marked by a variety of high-amplitude environmental perturbations, including widespread deoxygenation and deposition of organic-rich sediments. We estimate the impact of redox changes and early diagenesis on the geological Hg record using a suite of (sub)recent–Pleistocene and Upper Cretaceous sediments representing oxic to euxinic marine conditions. Our sample set includes a transect through an oxygen minimum zone and cores that record transient shifts in oxygenation state, as well as post-depositional effects – all unrelated to volcanism, to the best of our knowledge. We find substantial alterations to the Hg record and the total organic carbon and total sulfur content, which are typically assumed to be the most common carrier phases of Hg in marine sediments. Moreover, these biases can lead to signal alteration on a par with those interpreted to result from volcanic activity. Geochemical modifications are ubiquitous and their potential magnitude implies that the factors leading to biases in the geological record warrant careful consideration before interpretation. Factors of particular concern to proxy application are (1) the disproportionate loss of organic carbon and sulfur relative to Hg during oxidation that strongly modulates normalized Hg records, (2) the evasion of Hg in anoxic and mildly euxinic sediments and (3) sharp focusing of Hg during post-depositional oxidation of organic matter. We suggest that paired analyses of additional redox-sensitive trace elements such as molybdenum, and organic-matter characteristics, particularly the type of organic matter, could provide first-order constraints on the role that redox and diagenetic changes played in shaping the Hg record as part of checking the attribution of enrichments to volcanic activity. [ABSTRACT FROM AUTHOR]

The submarine volcanic emplacement of the Ontong Java Plateau (OJP) is the suggested cause of Ocean Anoxic Event 1a (OAE 1a). However, no precise timing and duration exists for the formation of OJP, and its connection to OAE1a relies mainly on proxies in the sedimentary record. We provide high-precision 40Ar/39Ar data from OJP drill and dredge sites that considerably improve OJPÕs eruptive history. The ages determined from this work are as much as 10 million years younger than previous dates and indicate a protracted formation over at least 6 million years. OJP now appears too young to have caused OAE1a, but we suggest that it may have had a role in the later OAE1b. The protracted eruptive sequence has implications for the emplacement dynamics of OJP and other large igneous provinces. [ABSTRACT FROM AUTHOR]

Textural and compositional zoning within plagioclase phenocrysts records the magma chamber processes, such as magma differentiation, magma recharge and mixing, and crustal contamination. The plagioclase phenocrysts in the Daqiao and Qiaojia plagioclase-phyric basalts from the Emeishan Large Igneous Province (LIP) show complex textural and compositional zoning patterns, e.g., normal, reverse, oscillatory, and patchy zoning patterns. Most plagioclase phenocrysts exhibit a core–rim normal zoning pattern (Pl-A) with euhedral high-An cores (An = 76–78%, in mole fraction) and low-An rims (An = 68–72%), indicative of the crystal regrowth processes caused by recharge of relatively evolved magmas after the formation of high-An cores. Some phenocrysts have a core–rim reverse zoning pattern (Pl-B) with irregular ovaloid cores, characterized by extremely low An (60–61 mol%) and Ba (84–88 ppm) contents and extremely high 87Sr/86Sr ratios (0.7120–0.7130). The rims of the Pl-B have relatively high An (69–72 %), Ba (~160 ppm) contents, and low 87Sr/86Sri (~0.7056). These Pl-B plagioclase phenocrysts preserve the information about the interaction between the crustal xenocrysts and the transporting magmas. Some plagioclase phenocrysts show a core–mantle–rim oscillatory zoning pattern (Pl-C) with multiple oscillations of An (70–80 %), Ba (88–147ppm) from core to rim, revealing replenishment and mixing of multiple batches of basaltic melts with diverse compositions. 87Sr/86Sr ratios of the Pl-C do not vary significantly (0.7050–0.7054). A small portion of phenocrysts has patchy patterns in the cores (Pl-D), where the low-An patches (72–75 %) in form of elliptical or irregular elongated shapes were enclosed by the high-An domains (80–87 %). These features can be attributed to crystal dissolution and regrowth processes during the reaction between early-formed low-Cumulates and recharged hot primitive melts. The cores, mantles, and rims of different types of plagioclase phenocrysts (except the core of Pl-B) commonly display nearly constant Sr isotopic compositions, implying insignificant wall-rock assimilation at shallow-level magma reservoir(s) during the growth of these plagioclase phenocrysts. In conclusion, the massive crystallization of plagioclase in the late stage was an important controlling factor for the formation of iron-rich basalts in the Emeishan LIP. [ABSTRACT FROM AUTHOR]

The features of the structure formation of the Earth's crust at the early stage of formation of the Southeast Indian Ridge were studied. The process of formation was associated with the separation of Australia from Antarctica and the advance of the rift zone westward towards the ancient oceanic lithosphere and, then, towards the large igneous province of Kerguelen, formed by the activity of the plume of the same name. The separation of Australia and Antarctica spanned a long period of continental rifting (~160‒80 Ma), which was followed by ultraslow spreading (~80‒45 Ma), slow spreading (~45‒40 Ma), and, finally, by recent stationary spreading at average rates (after 40 Ma). The advance of the rift zone towards the ancient oceanic lithosphere gave way to the accretion of young oceanic crust on the emerging Southeast Indian Mid-Oceanic Ridge. The early stages of development of the young spreading ridge are reflected in the modern structural plan of the study region. The advance of the rift zone from the continent into the boundaries of the ancient oceanic lithosphere led to the formation of the Naturalist Plateau and the Bruce Bank near the Antarctic margin. The break-up of the ancient oceanic lithosphere and the formation of the young crust on the Southeast Indian Ridge led to the formation of conjugated Diamantina and Labuan suture zones, fixing the position of the initial rifting. The transition from ultraslow spreading at the initial stage of oceanic crust formation to stationary spreading at average rates is clearly recorded in the change in the ruggedness of the accretionary relief. The Southeast Indian Mid-Oceanic Ridge collided as a result of its propagation westward with a large igneous province during the formation of the Kerguelen Plateau and separated the Broken Range from the plateau. The authors carried out physical modeling of the development conditions of rifting and spreading processes, as well as structure formation within the Southeast Indian Ridge. [ABSTRACT FROM AUTHOR]

In many alkaline complexes, large amounts of ultramafic rocks occur together with carbonatites, melilitolites and other alkaline silicate rocks. There is an ongoing debate if and how these contrasting lithologies were formed by differentiation of a common, mantle-derived silicate magma or rather by metasomatic processes between carbonatite and country rocks. In order to find petrological evidence for one or the other, two key examples, the Gardiner (E Greenland) and Kovdor (Russia) complexes are compared in this study. Despite their similar tectonic setting and succession of rock types, they show significant differences in the texture and mineral composition of ultramafic rocks. Ultramafic rocks from Kovdor include calcite- and biotite-rich dunites and pyroxenites without typical cumulate textures. They consist of Ni-poor forsterite, Cr-poor diopside and Ni-Cr-poor spinel and are possibly metasomatic reaction products between mantle-derived carbonatite melts and silicic host rocks. Similar ultramafic rocks are associated with carbonatites e. g. at Palabora (South Africa), Afrikanda (Russia), and Salitre (Brazil). In contrast, the ultramafic rocks from Gardiner show well-preserved cumulate textures and consist of Ni-rich forsterite, Cr-rich diopside as well as Cr-Ni-Ti-rich spinel and also contain F-Cl-rich apatite. They record an increase in aSiO2 from dunite to pyroxenite at similar f O2 (ΔFMQ ~ +1.2, with FMQ = fayalite-magnetite-quartz buffer), indicating that these rocks represent cumulates of an evolving, moderately oxidized mafic melt derived from a Ti-rich mantle source, similar to other rocks of the North Atlantic igneous province. In contrast to systems like Kovdor where carbonatite metasomatism is likely dominant, Ti-rich parental silicate magmas can abundantly crystallize Ti phases, as recorded by massive perovskite cumulates in Gardiner melilitolites. This can effectively scavenge HFSE from the magmatic system early in its evolution and likely explains HFSE-barren carbonatites at Gardiner, while those from Kovdor are highly HFSE-enriched. In summary, the results of our study provide strong textural and mineral chemical evidence that ultramafic rocks in alkaline complexes can be of both cumulate and metasomatic origin; the specific type has an important bearing on their HFSE enrichment and on the types of ores present in such complexes. [ABSTRACT FROM AUTHOR]

The study provides whole-rock geochemical and Rb–Sr and Sm–Nd isotope examinations of Permian–Triassic volcanics (basalts, basaltic andesites) and sills (trachydolerites, monzodiorites) from the Kuznetsk Basin, which is part of the Siberian Large Igneous Province. It is shown that the volcanic section of the Kuznetsk Basin consists of the Lower (I and II units: TiO2 = 1.6–1.9 wt %, Gd/Yb = 2.0–2.1) and Upper (III–VIII units: TiO2 = 1.4–1.8 wt %, Gd/Yb = 1.7–1.9) sequences. The Mayzass sill is geochemically correlated with the Unit II basaltic andesite of the Lower Sequence, and the Syrkashev sill is geochemically akin to the Upper Sequence. The Kuznetsk traps originated from the partial melting of a Sr–Nd isotopically heterogeneous subduction-modified lithospheric mantle. They are geochemically comparable to low-Ti basalts of the low–middle Nadezhdinsky suite from the Noril'sk region, but differ in Sr–Nd isotope composition. The Sr–Nd isotope signatures of non-contaminated lavas and rocks of the Syrkashev sill (εNd(T) = (+4.6)–(+2.4), (87Sr/86Sr)T = 0.7047–0.7054) are inherited from the ancient lithospheric mantle and do not represent the contribution of the Siberian plume. The Mayzass sill rocks (εNd(T) = (+2.2)–(+2.1); (87Sr/86Sr)T = 0.7055–0.7063) are derived from a parental melt contaminated with crustal carbonates. [ABSTRACT FROM AUTHOR]

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LA-ICPMS U-Pb isotope analyses are presented for zircons from a thin granite sheet intersected in a borehole drilled into the upper Transvaal Supergroup wall rocks to the Molopo Farms Complex in southern Botswana. Many of the zircons have irregular or angular grain margins, and some have rounded cores. Approximately half of the analysed grains yielded concordant 207Pb/206Pb ages ranging between 2 282 ± 29 and 2 113 ± 16 Ma. Assuming that these grains were inherited from the surrounding upper Transvaal Supergroup sedimentary strata, the youngest zircon age provides a maximum depositional age for these sediments. Importantly, these zircon ages coincide with dates from detrital zircons in upper Transvaal Supergroup strata in the eastern Kaapvaal Craton and in the Magondi Supergroup in western Zimbabwe. Hf isotope analyses of the rounded zircons suggest that these grains were sourced from a variable mixture of an old basement and more juvenile material. A younger age of 2 060 ± 12 Ma was obtained from a single subhedral zircon grain from the granite sheet. This is interpreted as the granite’s emplacement age, making it coeval with the adjacent Molopo Farms Complex that forms part of the Bushveld Large Igneous Province. [ABSTRACT FROM AUTHOR]

In Large Igneous Provinces (LIPs), sills are a key component of the plumbing system responsible for magma transport through the crust where magma fractionation takes place. In this work, we investigate four dolerite sills related to the Paraná-Etendeka LIP from Cerro do Coronel region, southern Brazil. They are basaltic andesites composed of plagioclase, augite+pigenonite, Ti-magnetite, and rare olivine, divided into a high- and a low-MgO group. The former is more depleted in SiO2, alkalis, and trace elements, with lower FeOt/MgO ratios, higher Eu/Eu*(N), less radiogenic 87Sr/86Sr and more radiogenic 143Nd/144Nd. The composition of Cerro do Coronel dolerites resemble those of evolved rocks from Morungava Intrusions, the silica saturated tholeiite dykes in southern Brazil, and the Cuaró Dyke Swarm dolerites from Uruguay. They evolved by fractional crystallization from primitive Gramado-type basalts under FMQ-2 fO2 conditions at low pressures (<3 kbar) and underwent assimilation of variable upper and lower crust, representing shallow-depth equilibration of magmas sourced from deeper crustal levels. The studied sills share some geochemical features with the Vale do Sol Fm. lavas, although we cannot assume they have the same age, and are part of a major plumbing system related to the low-Ti magmas of the Paraná-Etendeka LIP. [ABSTRACT FROM AUTHOR]

Evidence of early oceanization during transition from large igneous provinces (LIPs) to new ocean crust is mostly missing in the geological record. Mafic dykes geochemically akin to mid‐ocean ridge basalts (MORBs) occur in late stages of LIPs, but their tectonic role during margin evolution remains poorly constrained. Here we use whole‐rock and high‐resolution clinopyroxene chemistry of two distinct types of mafic dykes from the southeastern Brazilian margin to show that a MORB‐like plumbing system became magmatically active in the central South Atlantic ∼2 Ma after the volcanic peak of the Paraná‐Etendeka Magmatic Province. The LIP basaltic dyke compositions resulted from the mixing between a highly metasomatized lithospheric mantle comprising enriched DMM and Archean contributions. The lack of elemental oscillatory zoning in clinopyroxene crystals suggests that they were produced by a single magma pulse, consistent with the strong magma fluxes in LIPs. In contrast, the MORB‐like dykes, which exhibit strong similarities with South Atlantic MORBs, are interpreted to represent early stages of oceanization deriving from the mixing of enriched asthenospheric melts either with ancient SCLM contributions or small crustal assimilation. The MORB‐like intrusions ascended and cooled rapidly along faults and were fed by numerous magma injections that suggest episodic decompression of the asthenospheric source ∼20 Ma before the birth of central South Atlantic. The combination of subduction‐influenced and MORB‐like dykes is recurrent in ancient and modern rifts globally, suggesting that thinned lithospheric blocks affected by ancient subduction events may become more susceptible to early oceanization in rifted margins. Plain Language Summary: The processes that promote initial formation of new ocean floor, whenever a continent breaks, are still poorly described. In the South Atlantic Ocean case, for example, magnetic registers of oceanic rocks indicate that seafloor boomed ∼20 Ma after the ancient Gondwana continent broke up, creating distinct South American and African plates. However, we know very little about the preliminary steps of "oceanization" of the continental margins during the transition period. In this research, we studied two groups of magmatic rocks located in the Brazilian southeast margin. One of them is geochemically similar to the abundant volcanism that affected Gondwana prior to breakup; the other is akin to South Atlantic seafloor. The later was formed only ∼2 Ma after initial stages of Gondwana breakup. Geochemical signatures and mineralogical textures allowed us to describe how the two types of magmas were formed and how they ascended through the crust. The results and comparisons suggest that ∼20 Ma before the South Atlantic seafloor developed there were localized areas of the continental margins affected by early "oceanization," where ocean floor‐like rocks started forming along weakened continental structures. Such areas were probably more susceptible to breakup because of their more complex previous geological histories. Key Points: Oceanization of the South Atlantic started as early as ∼132 Ma through lithospheric thinning and focused mid‐ocean ridge basalt (MORB)‐like intrusive magmatismMagma dynamics of Paraná‐Etendeka and MORB dykes in the Brazilian margin suggest an intermittent shift toward a passive regime at ∼132 MaThinned lithospheric blocks affected by ancient subduction potentially allow early asthenospheric magma ascent and crustal oceanization [ABSTRACT FROM AUTHOR]

The existence of a 0.3 Ga "great unconformity" between the Mesoproterozoic "Undefined System" Xiamaling Formation and the "Neoproterozoic" Longshan Formation has long been controversial. In this study, stratigraphy, sedimentology, detrital zircon dating, lithium isotope, and major and trace elements were applied to systematically analyze their relationship. Results demonstrate that coarse to fine sandstone-mudstone normal cycles with different grades and thicknesses exist from the Xiamaling to the Longshan formations. Sedimentary succession formed from a restricted platform with lagoon-shallow marine shelf-tidal flat-beach and tidal shoal developing in ascending order. The pebbly sandstone at the bottom of the Longshan Formation does not have the properties of basal conglomerate with tectonic significance. The youngest detrital zircon is older than 1.6 Ga for the Longshan Formation and overlying Jing'eryu Formation sandstones. In addition, no (zircon) provenance has been found from the large igneous province of 1.32–1.2 Ga in Xiamaling Formation. The δ7Li isotope values exhibit a gradually increasing trend. The Li content and CIA (Chemical index of alteration) gradually decreases from the top of the Xiamaling Formation to the lower part of the Longshan Formation. Simultaneously, as K2O/Al2O3 and FeO/Fe2O3 decreases, Na2O/Al2O3 increases, and Li/Al, V/Cr, and V/(V+Ni) fluctuates slightly. At the boundary, the Ce/Ce* and Eu/Eu* show positive and negative anomalies, respectively. Further, the Rb/Sr and Al/Si values decrease, and U/Th values increase. This indicates that the geochemistry of the weathering crust at the interface of the Xiamaling Formation-Longshan Formation is not characteristic. However, it indicates a weakening of continental weathering intensity from the bottom to the top of the interface. These results do not support the existence of a large unconformity between the Xiamaling and the Longshan formations. Therefore, it is suggested that the Longshan Formation-Jing'eryu Formation should be moved to the lower level in the stratigraphic chart, where it should form part of the Mesoproterozoic "Undefined System" along with the Xiamaling Formation. On this basis, the paleogeography of the Xiamaling Formation-Longshan Formation-Jing'eryu Formation sedimentary period in the Yanliao area of North China has been reclassified. [ABSTRACT FROM AUTHOR]

Recent studies indicate that volcanism may be a potentially important factor in the formation of karstic bauxite, but more evidence is needed. Six billion tons of bauxite formed upon paleo-karstic terrain within a short time in the Late Carboniferous in the North China Basin (NCB) and in the Late Permian in the Youjiang Basin (YB) of China. The factors that trigger their apace formation remain unclear. Herein, we proposed that extensive volcanic eruptions have accounted for this large-scale bauxite formation based on the mineralogy, Hg isotopes, and Hg concentration enrichment proxies of the samples of bauxitic sequence in the NCB and YB. NCB bauxite generally has three layers, namely bottom Fe-bearing claystone, intermediate bauxite ore, and top claystone, while bottom Fe-bearing claystone is usually absent in YB bauxite, which directly covers carbonate rocks. The mineral assemblages of NCB and YB bauxite confirm that they were deposited in a superficial alkaline and reducing karstic environment. Strong Hg enrichment peaks with corresponding nearzero Δ199Hg, a signal of excess volcanogenic Hg, were discovered in the upper claystone of NCB bauxitic sequences, which overlaid the Ordovician carbonate substrate. It denotes volcanism occurred immediately subsequent to deposition of terrestrial weathered Al-rich remnants in paleo-karstic depressions during the Late Carboniferous. This volcanism, occurring in the northern margin of the NCB, is considered to have triggered the apace bauxitization in the NCB after its long exposure and weathering. In the YB, the remarkable Hg enrichment and nearzero Δ199Hg were observed in the entire Late Permian bauxitic sequences with Late Permian carbonate as the substrate. It denotes enhanced volcanogenic Hg inputs throughout the whole deposition process of bauxite during the Late Permian. This episode of volcanism associated with the Emeishan large igneous province and contemporaneous arc system resulted in the short term weathering of source materials and the apace bauxitization in YB. Volcanism is suggested to aggravate surface acidic and oxidizing conditions, in which rapid decomposition of source materials occurred to release Al3+ to precipitate diaspore above the physiochemical barrier of underlying carbonate. [ABSTRACT FROM AUTHOR]

The Llano Uplift in central Texas, USA, exposes the southernmost expanse of Laurentian crystalline basement in North America and the overlying lower Paleozoic strata deposited on the Great Unconformity. Systematic detrital zircon (DZ) U-Pb provenance analysis of the Hickory Sandstone--the basal unit of the Cambrian Riley Formation that onlaps the Mesoproterozoic core of the Llano Uplift--yielded locally variable DZ U-Pb signatures with ages ranging from 1800 Ma to 485 Ma (n >1700). The Hickory Sandstone zircons are dominated by 1550-1300 Ma (50%) and 1300-1000 Ma (46%) ages. These two dominant Mesoproterozoic DZ age components likely were sourced from local Grenville metamorphic and igneous basement of the Llano Province (1300-1000 Ma) and from the extra-regional Granite-Rhyolite Province basement to the north and northwest of the Grenville Front, which suggests both local sourcing and a regional drainage system supplying sediment to the southern Laurentian margin during the Cambrian. This interpretation is supported by a minor component of 1800-1600 Ma zircons, sourced from the Yavapai-Mazatzal Province, and zircons with Early Cambrian ages that were likely sourced from the Wichita Igneous Province, which is located to the northwest and to the north of the Llano Uplift, respectively. Documented NNW-trending topographic ridges in the Mesoproterozoic basement surface of the Llano Uplift could have funneled aeolian and fluvial sand supply to the southern Laurentian coast that was reworked in the marginal marine environment. Samples from the western Llano Uplift are dominated by regionally sourced, early Mesoproterozoic Granite-Rhyolite Province DZ ages, whereas samples from the eastern Llano Uplift exhibit a dominant locally sourced Grenville signature. Hickory Sandstone samples also contain a small number of Neoproterozoic (850-600 Ma) and Cambrian (541-487 Ma) zircons. Sources for Neoproterozoic zircons likely were located along the Laurentian continental margins and derived from extension-related magmatism associated with the breakup of Rodinia. Cambrian zircons are most common in Hickory Sandstone samples on the southern and western flanks of the Llano Uplift. Some are reasonably sourced from the Wichita Igneous Province to the north, but the younger Cambrian zircons suggest sources to the west. Upper Hickory strata are dominated (>60%) by 1550-1300 Ma grains with up to 10% >1600 Ma Paleoproterozoic grains. Upper Cambrian sandstones of the Wilberns Formation also contain a significant contribution of >1600 Ma grains, which suggests a possible enlargement of the fluvial headwaters over time extending into older Laurentian provinces to the north and northwest. Differences among DZ populations over relatively limited distances may have been related to NW-oriented ridge and corridor landforms that developed on the Great Unconformity surface, which were products of the Proterozoic tectonic and lithologic architecture affected by aeolian and fluvial processes at this unusual stage in Earth history. Paleocurrent data and atypical detrital grain characteristics for a Hickory Sandstone locality in the southeastern Llano Uplift and its DZ age similarities with Cambrian sandstones of the Argentine Precordillera suggest a common provenance prior to or during Rodinia breakup. [ABSTRACT FROM AUTHOR]

The origin of the Cambrian--Ordovician tectono-magmatic events affecting NE Gondwana and the adjacent peri-Gondwanan terranes (e.g., Himalaya, Lhasa, Southern Qiangtang, Baoshan, Tengchong, Sibumasu, Helmand, and Karakorum) is controversial. Although its volume is poorly constrained, we propose that an extensive belt of granitic rocks that formed in various pulses between ca. 510 Ma and 460 Ma may represent the remains of a potential silicic large igneous province (LIP), which is referred to here as the Pinghe silicic LIP, with an areal extent of ~2.5 Mkm². The putative Pinghe silicic LIP is composed predominantly of S-type granites with subordinate A-type granites and minor intraplate mafic rocks. The recognition of this belt of granitic rocks aids in the refinement of tectonic reconstructions of Gondwana and of models for the rifting of terranes from its northern margin. The Pinghe silicic LIP broadly coincides with the adjacent 511 Ma Kalkarindji LIP in northern Australia, and the plume or mantle upwelling that triggered the Kalkarindji LIP may have been responsible for driving crustal melting that generated the granitic rocks, in a manner analogous to the Karoo--Chon Aike association. [ABSTRACT FROM AUTHOR]

The Caribbean Island of Hispaniola resulted from plate convergence that brought together distinct terranes and may include one or more active microplates. These processes may have brought seismically anisotropic materials to the island or created anisotropic structures through deformation. Our goal is to identify and model interfaces that bound seismically anisotropic media and/or dipping interfaces between isotropic layers beneath Hispaniola and assess the implications of our results for the region's tectonics. We perform harmonic decomposition of receiver function gathers for 17 broadband seismic stations deployed on the island. Our results reveal (a) a layer bounded by interfaces that produce opposite polarity on the transverse receiver functions at 25 and 41 km depth in the northern and eastern Dominican Republic that corresponds to the subducting North American oceanic lithosphere, (b) a layer at ∼8–17 km depth (also bounded by interfaces that produce opposite polarity) beneath the Cordillera Central in the central and eastern parts of the island that may indicate the crystalline, well‐differentiated lower crust of this long‐lived terrane, and (c) an interface that bounds anisotropic material at 22 km depth beneath southwestern Hispaniola. This last interface may correspond to the underthrusting Caribbean Large Igneous Province (CLIP), in which case it would mark the CLIP's upper boundary and the lower boundary is too gradual a transition to produce conversions. Key Points: We modeled seismic anisotropy and dipping interfaces beneath Hispaniola with receiver functions computed for 17 seismic stationsWe found a layer in the north at 25–41 km depth that may mark the subducting North American oceanic lithosphereOur results indicate that an interface at ∼22 km depth beneath SW Hispaniola corresponds to the underthrusting Caribbean Plate [ABSTRACT FROM AUTHOR]

The Karoo large igneous province has been divided into rift zone and basin-related groups, with picrites from the Luenha river, Mozambique, representing an end-member of the latter. New O isotope, major and trace element data for olivine have been combined with MELTS crystallisation modelling to deconvolve compositional diversity associated with magma differentiation from source-derived heterogeneity. Three olivines populations have been discerned as follows: (1) the 'main trend', which records crystallisation from a variety of magma compositions; (2) the 'low Cu trend', which is inferred to represent xenocrysts or antecrysts; and (3) the 'high CaO' olivines, which record polybaric crystallisation of a primitive, little fractionated magma. The trace element variability in olivine phenocrysts relates partially to sampling of different parts of the same overall magma transport and storage systems, and partly to heterogeneity of parental magmas and their mantle sources. When the measured δ18Oolivine values have been converted into δ18Omelt values, the mean δ18Omelt values for the 'main trend' and 'low Cu' groups are indistinguishable from each other (5.7 ± 0.1‰, 2σ); however, the mean δ18Omelt value of 6.1 ± 0.1‰ for the 'high CaO' group is distinctly enriched. These data record source heterogeneity and suggest contributions from two mantle sources, one with elevated δ18O, and another with more 'typical' mantle δ18O. Combining these data with previously reported trace element and Nd and Sr isotope data support derivation from a mantle source similar to non-chondritic bulk silicate earth, but with minor contributions (1–2% for the enriched magmas) from a recycled sedimentary component. This points to the importance of a primitive mantle source for the basin-related successions in Karoo. [ABSTRACT FROM AUTHOR]

The Ethiopia‐Yemen flood basalts are spatially zoned with progressively lower TiO2 lavas from near the Afar depression toward the margins. The timing and rate of emplacement of low TiO2 (LT) lavas are poorly known compared with the ultra‐high TiO2 (HT2) lavas. We measured two high‐precision 40Ar/39Ar ages of 29.63 ± 0.14 and 30.02 ± 0.22 Ma (2σ) from basalts of the 2‐km‐thick LT lava sequence at the Afar plume head margin. Using our eruption age model constructed from our and previous 40Ar/39Ar ages with the paleomagnetic directions, we estimate that the LT lava eruption continued over Chrons C12r‐C12n‐C11r. The eruption of the plume head margin started earlier than the plume head axis emplacement in C12n. Also, the eruption rate was low at the margin, high at the axis. We estimate that the LT lavas are induced by the edge‐driven convection, the result of a plume‐lithosphere interaction, not a plume head. Plain Language Summary: The Ethiopia‐Yemen Flood Basalts are thought to be the expression of a mantle plume erupting millions of km3 of basaltic lava in a geologically short period (1–3 million years [Myr]). Titanium concentrations in the flood basalts are zoned and named HT2, HT1, and LT basalts (from high to low Ti). The eruption timing and rate of the HT2 basalts are constrained with high precision, but those of the LT basalts remain ambiguous. Therefore, we measured two high‐precision 40Ar/39Ar ages from LT basalts in the 2‐km‐thick Lima‐Limo section, which erupted northwest of the Afar area. Based on our eruption age model constructed from 40Ar/39Ar ages and paleomagnetic directions, we estimate that the eruption of the LT basalts started earlier than the HT2 basalts that erupted in Chron C12n and lasted over at most ∼2 Myr. The eruptive rate of the LT basalts in the earliest interval was lower than that of the HT2 basalts. This feature of the eruption may be explained by edge‐driven convection caused by the interaction of a gradient of the lithospheric thickness with the Afar mantle plume. Key Points: We obtained two new 40Ar/39Ar ages for the low‐Ti basalt from the Lima‐Limo section of the Ethiopia–Yemen flood basaltsBased on our eruptive age model, we correlated the Lima‐Limo section to Chrons C12r to C11rWe concluded that the non‐plume low‐Ti basalts erupted before the Afar plume high‐Ti basalts because of the plume–lithosphere interaction [ABSTRACT FROM AUTHOR]

The end-Triassic extinction (ETE) event represents one of the 'big five' episodes of mass extinction. The leading hypothesis for the cause of the ETE is the intrusion of voluminous magmas of the Central Atlantic Magmatic Province (CAMP) into carbon-rich sediments of two South American sedimentary basins, around 201.5 Ma. The timing of dikes and sills emplacement, however, must be considered in light of age models from CAMP rocks occurring in North America. In this work, we present new high-precision ages for critical samples in NE Brazil (201.579 ± 0.057 Ma) and Canada (201.464 ± 0.017 Ma), in order to evaluate how the South and North American magmatic events compare at the 100-ka level, and to the ETE timing. We also discuss inter-laboratory reproducibility of high-precision CAMP ages, including the 230Th disequilibrium corrections that are made to zircon U–Pb dates. Our findings in this newly discovered extension of the CAMP large igneous province in NE Brazil support the hypothesis that the CAMP may be responsible for the ETE through the triggering of greenhouse gas release from magma-evaporite interactions (contact metamorphism) in the South American basins. [ABSTRACT FROM AUTHOR]

The rock record from the late Early Jurassic in southern Africa encompasses the history of voluminous continental flood basalt outpourings associated with the magmatic events in the Karoo–Ferrar Large Igneous Province (LIP) in southern and eastern Gondwana. This multiphase magmatism produced one of Earth's largest continental flood basalt successions volumetrically and is assumed to have been a main driving mechanism in late Early Jurassic global environmental perturbations, including mass extinctions and changes in climate. In southern Africa, these Lower Jurassic flood basalts are interbedded with fossiliferous sedimentary rocks, which in turn host the last signs of 'Karoo life' in the form of fossil plants, invertebrates and vertebrates, including the trackways of hopping mammals and the ultimate Karoo dinosaurs. The sedimentology and palaeontology of the interbeds archived depositional and biotic processes in running water as well as in and around shallow, up to ∼10 m deep freshwater lakes and ponds in the late Early Jurassic. This study explains how a complex freshwater palaeo-habitat prevailed – albeit temporarily – in this extremely stressful environment, which was unlike any modern volcanic system. The evidence collectively points to seasonally wet, warm temperate climatic conditions during the early phases of Karoo volcanism. Moreover, the evidence in the rocks also suggests that the dynamic volcanic conditions resulted in shifting habitats that likely facilitated the migration of the ultimate Karoo biota towards the north and west, away from the main Karoo land of fire, just before Gondwana started to disassemble. This refinement of the environmental dynamics in southern Gondwana presented herein lays the groundwork for future high-resolution volcanological, geochronological and chemostratigraphical studies aimed at the nuanced understanding of the global environmental effect of the Karoo–Ferrar LIP. [ABSTRACT FROM AUTHOR]

The intrusion of igneous sills into organic-rich sediments accompanies the emplacement of igneous provinces, continental rifting, and sedimented seafloor spreading. Heat from intruding sills in these settings alters sedimentary organic carbon, releasing methane and other gasses. Recent studies hypothesize that carbon released by this mechanism impacts global climate, particularly during large igneous province emplacements. However, the direct impacts of sill intrusion, including carbon release, remain insufficiently quantified. Here, we present results from International Ocean Discovery Program (IODP) Expedition 385 comparing drill-core and wireline measurements from correlative sedimentary strata at adjacent sites cored in Guaymas Basin, Gulf of California, one altered by a recently intruded sill and one unaffected. We estimate 3.30 Mt of carbon were released due to this sill intrusion, representing an order of magnitude less carbon than inferences from outcrops and modeling would predict. This attenuated carbon release can be attributed to shallow intrusion and the high heat capacity of young, high-porosity sediments. Shallow intrusion also impacts sub-seafloor carbon cycling by disrupting advective fluxes, and it compacts underlying sediments, increasing potential carbon release in response to subsequent intrusions. [ABSTRACT FROM AUTHOR]

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Large-volume silicic eruptions in large igneous provinces are unique in the geological record because there are no observed analogues. Their facies architecture do not strictly follow the diagnostic features proposed in the literature for rheomorphic ignimbrites and lava flows. The models proposed for their emplacement depend on conditions such as high temperature (> 950 °C) and low viscosity (< 106 Pa s) coupled with unusually high effusion rates. Low-Ti dacitic lavas from the Paraná-Etendeka Large Igneous Province (LIP) have a wide variety of morphologies and lithofacies on both sides of the Atlantic Ocean. A common facies association framework for the Caxias do Sul/Grootberg dacites (outcropping in Namibia and southern Brazil) include the presence of platy and thinly jointed facies with a columnar jointed massive core and an amygdaloidal upper facies, with an absence of basal breccia. In this work, we describe and interpret features observed in the basal portion of these silicic units, focusing on the thinly jointed facies. This basal facies includes what has been termed as "zebra-like banding." Zebra-like banding is an apparent structure that originates from a fracture network which involves horizontal and long first-order fractures, oblique and short second-order fractures, oblique short to long third-order fractures, and intersection zones of fourth-order structures. First-order fractures are related to pure shear, the second- and third-order fractures are Riedel shears, and the fourth-order structures represent angular contacts between different sets containing first-, second-, and third-order fractures. The combination of first-, second-, and third-order fractures evolves to shear lenses. The zebra-banding" is caused by oxidation/reduction halos developing in the host rock during post-emplacement fluid circulation along the fractures, culminating in precipitation of silica polymorphs and zeolite. The fracture system, including all orders of fracture, formed during late to post emplacement stages, and accommodated pure and simple shear in a ductile–brittle basal zone during oscillations in effusion rate. Fracturing of the basal zone, rather than the formation of basal breccia, demonstrates stress accumulation in this part of the flow and helps to explain why the presence of basal breccia is not a diagnostic feature in distinguishing large-volume silicic lava flows from rheomorphic ignimbrites. [ABSTRACT FROM AUTHOR]

Patterns of magma transport during the emplacement of Large Igneous Provinces (LIPs) are extremely important for the understanding of their formation. The Permian-Triassic Siberian Traps LIP is considered to be one of the largest in the Phanerozoic; however, mechanisms of magma transfer within and under the crust are still poorly studied. This problem is vital for the reconstruction of the dynamics of magmatic activity and eruption styles, ascertaining the position of magmatic centers and feeding zones, and conception of ore deposits genesis. Here, we present the detailed results of anisotropy of magnetic susceptibility measurements for lava flows and intrusions from the Noril'sk and Kulumbe regions (the northwestern Siberian platform). We reconstructed patterns of magma flow based on the magnetic fabric analysis of more than 100 sites. Distribution of the magnetic lineation in the studied intrusions and flows points out that the lateral magma flow of NW-SE directions was predominant. Our results support the idea of a magma-controlling role of Noril'sk-Kharaelakh and Imangda-Letninskiy regional fault zones. Furthermore, the reconstructed geometry of magma transport in intrusions is contrasting with that in the Angara-Taseeva depression (the southern part of the LIP) due to the presence of the long-lived mobile zones in the northwestern Siberian platform. [ABSTRACT FROM AUTHOR]

Cratons are the most mature parts of the continents on Earth and can remain stable for billions of years. Cratonic keels may be destroyed, for which a prime example is the eastern North China craton (NCC), which was removed during the Mesozoic. However, the critical stable state of the keel and the destroying mechanism of the rigid and buoyant cratonic roots remain enigmatic. Here, rhenium-osmium and neodymium isotope datasets from the eastern NCC open up a possible approach to reveal its evolution prior to the destruction. Two Re (rhenium) -depletion model age peaks of the peridotite xenoliths entrained by Phanerozoic basalts reconcile the ages of the large igneous provinces (LIPs) of Yanliao (ca. 1.3 Ga) and Xuhuai (ca. 0.92 Ga), respectively. This indicates that the mantle lithosphere of the eastern NCC has undergone metasomatism and growth during the emplacement of the two LIPs. The Proterozoic LIPs probably produced buoyant mantle residues that were then captured beneath thinned lithosphere regions. Such igneous processes occur at a depth of ~100 km and may induce the formation of weak zones (e.g. palaeo-mid-lithosphere discontinuity) partially within the rigid mantle lithosphere. The palaeo-weak zones might be rheologically unstable structures and probably reduced the stability of the craton, presenting as foci for potential later keel destruction. This provides evidence that LIP event may be another key factor for the intra-cratonic mantle lithosphere destruction. [ABSTRACT FROM AUTHOR]

The Cenomanian/Turonian boundary interval is associated with an oceanic anoxic event (OAE 2, 94.0 Ma) during one of the warmest episodes in the Mesozoic. To date, plant responses to these climatic conditions are known only from the northern mid-latitudinal succession in Cassis, France. There, conifer-dominated and angiosperm-dominated vegetation types alternate. However, whether the exceptional environmental conditions had an impact on plant reproduction is unknown to date. We applied a new environmental proxy based on spore and pollen teratology on palynological samples from the Cassis succession, to explore if this phenomenon also occurs across the OAE 2. The observed frequencies of<1% malformed spores and pollen grains suggest that plant reproduction was not affected during the Cenomanian/Turonian boundary interval. While the effects of continental Large Igneous Province(s) on plant reproduction have shown to produce abnormal spore or pollen morphologies as evidence for severe environmental pollution, by contrast the effects of oceanic LIP(s) seems to be inconsequential. [ABSTRACT FROM AUTHOR]

This paper presents zircon U-Pb, REE and Hf data for Paleoproterozoic felsic volcanic rocks from the Colíder Group with outcrops in the southwestern Amazonian Craton. It is estimated that these rocks cover an area of 85,000 km2 with a volume of 2.7 × 106 km3. The Colíder volcanics are comprised of basalts, andesites and a variety of felsic rocks. The felsic rocks are characterized by rhyolite, andesite and dacite lavas with frequent intercalated pyroclastic and epiclastic deposits. Effusive and pyroclastic rocks are intruded in places by hypabyssal porphyritic rocks. The U-Pb ages obtained for the northeastern domain (1782–1776 Ma) are younger than the ages obtained for the southern domain (1800–1787 Ma). The NE volcanic facies show Hf TDM ages between 2.3 and 2.5 Ga (εHf from +1 to −3) and greater fractionation (Ce/Yb – 0.03–0.06) than the younger facies that has TDM from 2.4 to 2.7 Ga (εHf from −3 to −6) with Ce/Yb values of 0.025–0.030. The Hf isotope compositions, REE contents and U-Pb analyses suggest geographic (north-south) variations in composition, crystallization ages, and TDM age zonation during the generation of the Colíder large igneous province. [ABSTRACT FROM AUTHOR]

The remnant Comei igneous province in SE Tibet has been recognized as a product of early Kerguelen mantle plume activity. In this paper, we investigate mafic dykes and sills intruded into Lakang Formation in the southeastern Tethyan Himalaya. Based on new whole-rock geochemical, Sr-Nd-Pb isotopic and SHRIMP zircon U-Pb geochronological data, these rocks can be divided into three groups. Group 1 is composed of OIB-like mafic dykes and sills with lower εNd(t) (+1.78-+2.55) and dated at 124.8 ± 1.7 and 133.3 ± 1.5 Ma. Group 2 consists of N-MORB-like mafic dykes and sills with lower TiO2 and P2O5 contents. Group 3 has geochemical compositions transitional between Group 1 and Group 2, with εNd(t) ranging from +1.98 to +2.24 and an age of 125.1 ± 2.1 Ma. We interpret Group 1 as being derived from an OIB-like source related to a mantle plume. Groups 2 and 3 are considered to have been generated from melts of the asthenosphere and the lower crust, with some involvement of melts from the upper crust. Considering the oldest (ca. 147 Ma) and the youngest (ca. 125 Ma) ages of the OIB-like mafic dykes and sills related to the Kerguelen mantle plume and their palaeogeographic location, we suggest that the Kerguelen mantle plume head had reached the lithosphere during the late Jurassic and was later directly underneath the Cona area. We propose a new model to describe the early activity of Kerguelen mantle plume, which put the mantle plume below the mid-ocean ridge and can well explain the long-lived magmatism related to Kerguelen mantle plume. [ABSTRACT FROM AUTHOR]

Mesozoic oceanic anoxic events are recognized as widespread deposits of marine organic-rich mudrocks temporally associated with mass extinctions and large igneous province emplacement. The Toarcian Oceanic Anoxic Event is one example during which expanded ocean anoxia is hypothesized in response to environmental perturbations associated with emplacement of the Karoo–Ferrar igneous province. However, the global extent of total seafloor anoxia and the relative extent of euxinic (anoxic and sulfide-rich) and non-euxinic anoxic conditions during the Toarcian Oceanic Anoxic Event are poorly constrained. Here we present estimates of the global total anoxic and euxinic seafloor areas before and during the Toarcian Oceanic Anoxic Event based on rhenium and molybdenum enrichments, respectively, in organic-rich mudrocks of the Fernie Formation (British Columbia, Canada). We find that mass balance models depict an expansion of up to ~7% total seafloor anoxia, which was dominated by euxinia, at the onset of the Toarcian Oceanic Anoxic Event, followed by a contraction before the end of the event. The global ocean redox trends revealed by the rhenium data mirrors the collapse and recovery patterns of global ammonite and foraminiferal biodiversity. Geochemical modeling shows that just a several percent expansion of O2-free areas with toxic sulfide build-up likely contributed to biodiversity loss or reorganization during the Toarcian mass extinction 183 million years ago. [ABSTRACT FROM AUTHOR]

The origin of lamprophyres associated with large igneous provinces (LIPs) remains controversial, particularly whether they are derived by direct melting of mantle plumes, or from previously metasomatized domains in thermally perturbed subcontinental lithosphere. Here, we report the petrological and geochemical characteristics of a recently identified suite of alkaline lamprophyres (sannaites) that represent the final pulse of magmatism in the Permian Tarim LIP in NW China. The sannaites display porphyritic texture with phenocrysts of olivine, clinopyroxene, hornblende, phlogopite, and titanomagnetite in a groundmass of plagioclase, clinopyroxene, nepheline, hornblende, biotite, and titanomagnetite with minor pyrite and apatite. Carbonate ocelli and almost pure albite in the groundmass are interpreted to have crystallized from immiscible carbonate and hydrous fluids, respectively, produced by late-stage magmatic segregation. The rocks show low to moderate SiO2 (37.7–49.3 wt.%) and MgO (2.74–9.91 wt.%), together with high Fe2O3T (up to 22.7 wt.%) and alkali contents (up to 9.02 wt.% Na2O + K2O). They are characterized by high incompatible element abundances, especially a marked enrichment in large-ion lithophile elements (Rb and Ba) and light rare-earth elements (e.g. La and Ce) relative to P and high-field-strength elements (e.g. Ti). They show a relatively restricted range of δ66Zn values between 0.22‰ and 0.46‰ with an average of 0.37 ± 0.04‰ (2SE, n  = 10), which is marginally heavier than that of MORBs (0.27 ± 0.05‰). Their (87Sr/86Sr)t values range from 0.7035 to 0.7061, εNd(t) from −0.97 to +5.62, and δ26Mg from −0.36‰ to −0.17‰ (n  = 8), the latter being consistent with those of global MORBs. Based on the correlation between Zn isotopes and TiO2–FeO concentrations, we infer that the heavy Zn isotopes in some of the sannaites resulted from fractional crystallization of Fe–Ti oxide minerals. The whole rock geochemical features of these rocks (negative K anomalies and enrichment in large-ion lithophile elements) and rhyolite–MELTS simulations suggest that the primary magmas of the sannaites were derived from an amphibole-bearing enriched lithospheric mantle source. Metasomatism and related formation of amphibole-bearing metasomatized mantle may be linked to sublithospheric melts/fluids derived from the Tarim plume in the earlier stages of plume activity, rather than slab-derived fluids or carbonate melts as suggested in previous studies for other alkaline mantle-derived magmas. Partial melting may have been triggered by the thermal input from the Tarim plume during a later stage. This study suggests that exotic, alkali-rich magmas can be produced during the multi-stage evolution of large mantle plumes, involving complex cycles of lithospheric mantle metasomatism and later melting of previously enriched domains. [ABSTRACT FROM AUTHOR]

Understanding the petrogenesis of alkaline syenites is important for constraining the mechanisms of rare earth element (REE) and rare-metal mineralization. Here we report a detailed petrological and geochemical study of early Permian syenitic rocks from the southwestern Tarim large igneous province (TLIP) in the Wajilitag area (China). We use these data to investigate the complex magmatic processes responsible for the enrichment of incompatible elements (e.g. REEs and rare metals) in these rocks. The Wajilitag syenitic rocks comprise early hornblende syenite (281 Ma) and later nepheline syenite (278 Ma), both of which are spatially and temporally associated with mafic intrusions (i.e. gabbro and diabase). These syenitic rocks show continuous variations in major elements with the mafic rocks. They also have similar Sr–Nd–Hf isotopic compositions with the nearby mafic rocks, denoting that the Wajilitag syenitic rocks should be derived from the melts represented by these mafic rocks. However, these syenitic rocks, particularly the nepheline syenites, are typically characterized by extreme enrichment of incompatible elements, which is hard to be accounted by simple fractional crystallization of mafic magmas. Some clinopyroxene phenocrysts in the Wajilitag nepheline syenites show oscillatory zoning with strikingly increasing and then gradually decreasing MgO contents, which recorded replenishment of mafic magma at the interval. Low-MgO clinopyroxene mantles and/or rims have much higher incompatible element contents (e.g. Nb, Ce, and Zr) and ratios (e.g. Ce/Y) than their cores, which could reflect melt injection controlled by reactive porous flow in a mush reservoir of a crustal magma chamber. Such processes may also cause the high Ce and/or Nb contents of the mantles and/or rims of zoned titanite and apatite phenocrysts. The injection of reactive porous flow melts is the key process that produces the extreme enrichment of incompatible elements in the alkaline syenites from the western TLIP, as well as other plume-related alkaline syenites that host world-class REE and rare-metal deposits. [ABSTRACT FROM AUTHOR]

The problem of the world-class PGE-Cu-Ni Norilsk deposits' origin has attracted geologists for several decades. The main goal of this study is to determine the specific features of ore-bearing intrusions in comparison with thousands of similar barren intrusions widespread within the Siberian igneous province, and to establish their genesis. As a result of statistical processing of previously published isotope-geochemical data and obtained by the authors, systematic differences were found in the distribution of the isotopic ratio of Nd in ore-bearing and barren intrusions, as well as in volcanic rocks at the Norilsk region. Thus, ore-bearing rocks in ten deposits (Talnakh, Kharayelakh, Norilsk 1, South-Maslovsky, North-Maslovsky, Norilsk 2, Chernogorsky, Zub-Mrksheydersky, Pyasino-Vologochansky, Imangdinsky), different in Ni and PGE reserves, show a very narrow range of Nd isotopic ratio, ԐNd(T) = 1.0 ± 1.0 (2σ, N = 139), whereas barren and volcanic rocks are characterized by a rather wide ԐNd(T) range, from −10 to +7 units (N = 256). Furthermore, ore-bearing intrusions are characterized by reduced and compact variations of the La/Lu ratio due to lower concentrations of light lanthanides. For the first time the authors studied two new intrusions penetrated by MD-48 and MD-60 boreholes drilled by Norislkgeologia LLT at the eastern part of the Mikchangda area. Their economic values are still unclear and should be estimated using geochemical methods. Both intrusions lie in the Devonian rocks, have similar thickness and mineral composition, but differ in textural and structural features, which indicate a rapid crystallization of the MD-48 intrusion. According to the contents of the major oxides, the rocks in MD-48 and MD-60 are identical, but they differ in U/Nb, La/Sm, and Gd/Yb ratios. It is important that the rocks in the MD-60 borehole are characterized by ԐNd(T) = 1.0 ± 0.6 (2σ) and fall into the range of ore-bearing intrusions, whereas the rocks in MD-48 have ԐNd(T) 2.4 ± 0.9, and, thus, are outside of ore-bearing intrusions. Therefore, ԐNd(T) values can be used as a local criterion for the estimation of economic potential of mafic intrusions, which is demonstrated for the Mikachangda area. [ABSTRACT FROM AUTHOR]

The coal-bearing sequences of the Late Permian in southwest China are enriched in critical elements Sc, V, Co, Ni, Cu, Zn, Nb, Ta, Zr and Hf. Ascertaining the conditions and basis for the enrichment of critical elements in this area is very important to support the critical metal demands of China. In this study, we analyzed the concentrations of elemental compositions of coal samples collected from the Late Permian Guishan coalfield in the eastern Yunnan, China. The results show that the C4-Upper coal seam of the Feilongma mine and C5 + 10 coal seam of the Shipeng mine are indeed rich in critical elements. The average concentration coefficients (CC) of transition metal elements Sc, V, Co, Ni, Cu, and Zn are 3.23 and 2.93, respectively, in the two coal seams. The average CCs of high-field-strength elements Nb, Ta, Zr, and Hf and non-variable valence chalcophile elements Ga and In are 2.21 and 2.53, respectively, in the two coal seams. The C4-Upper coal seam of the Feilongma mine can be divided into two sections based on the different ash contents, and the CCs of the critical elements in the two sections are almost equal. The main minerals in the two coal seams are kaolinite, siderite, quartz, gypsum, rozenite and marcasite. Multiple indicators of provenance show that the enrichment of critical elements in the Guishan coalfield is controlled by clastic terrigenous material. The source area of the Guishan coalfield is Kangdian Oldland in the northwest, and the main clastic materials are related with high-Ti basalts from the Emeishan Large Igneous Province. [ABSTRACT FROM AUTHOR]

The 290–280 Ma mafic Liuyuan Complex, embedded in the terranes constituting the southern edge of the Central Asian Orogenic Belt, has been interpreted to be either an ophiolite or an intracontinental large igneous province. Our detailed mapping shows that the Liuyuan Complex preserves an almost complete oceanic crustal section, with basal troctolite cumulates, varitextured gabbro affected by intense sub‐seafloor hydrothermal circulation, a newly discovered, and laterally‐continuous sheeted dyke complex, capped by basaltic pillows. This architecture is inconsistent with a continental rift‐layered intrusion model for the Liuyuan Complex, which we interpret as oceanic crust formed by seafloor spreading. The trace element ratios (Th/Yb, V/Ti, La/Sm, and La/Nb) of the lavas is consistent with a back‐arc basin environment. We suggest that the slab subducting to the north and forming the Ganquan arc system rolled back around 292 Ma, opening a back‐arc basin (the Liuyuan Complex). This oceanic basin started to close from ca. 281 Ma, with north‐dipping subduction under the accreted terranes constituting the southern active margin of Composite Siberia. The Liuyuan Complex was eventually obducted southwards over the Ganquan arc as part of the upper plate during the final closure of the back‐arc basin. [ABSTRACT FROM AUTHOR]