Volcanic-sedimentary Succession Research Articles

Testing alternative tectonic models for the Permian-Pleistocene tectonic development of the Kyrenia Range, N Cyprus: Implications for E Mediterranean Tethyan palaeogeography

Three published alternative tectonic models of the Permian-Pleistocene development of the Kyrenia Range, N Cyprus are tested, supported by new field, geochemical and micropalaeontological evidence: 1. The Kyrenia Range represents the northern continental margin of the S Neotethys, close to its present relative position. The range initiated as a Permian-Cretaceous rift/passive margin, switching to a N-facing active margin during Late Cretaceous-Neogene; 2. The Range was located along the N African continental margin until the Neogene when northward subduction transferred it to the southern margin of the Eurasian plate; 3. The Range is a far-travelled allochthon that was emplaced to near its present position, probably during the Eocene.In the light of regional comparisons, especially with southern Turkey, the combined evidence mainly supports tectonic model 1. Sedimentary and palaeontological data show that the restored stratigraphy of the Kyrenia Range indicates Late Permian initial rifting and Early-Middle Triassic advanced rifting, followed by Jurassic-Early Cretaceous passive margin subsidence. Small exposures of ophiolite-related melange located between the Mesozoic carbonate platform and the overlying latest Cretaceous-Palaeogene deep-water volcanic-sedimentary succession include evidence of HP/LT metamorphism, pointing to Late Cretaceous subduction. MORB/boninites, diabase-gabbro and extensive harzburgitic serpentinite originated as a SSZ ophiolite, together with a possible high-grade metamorphic sole (garnet amphibolite) and an accretionary prism (E-MORB/OIB; metachert). Microfossil evidence indicates exhumation of the melange and the underlying platform prior to Late Maastrichtian. A mass-transport complex formed within a compression-related foredeep during the Middle Eocene. Associated southward thrusting and folding culminated in emergence and subaerial erosion, generating a major unconformity, that was followed by subaerial and then marine deltaic deposits (Late Eocene-Oligocene). Following major Late Miocene southward thrusting, uplift of the Kyrenia Range took place during the Pleistocene, related to collision of the leading edge of the North African plate (Eratosthenes Seamount) with the Eurasian (Anatolian) plate.

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

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

Large-scale Mesoproterozoic iron formations in northwestern China

Iron formations (IFs) represent an economically important iron-rich rock type in the Precambrian geologic record. It is generally assumed that after ca. 1.85 Ga, large-scale IFs were not deposited for approximately 1.1 Gy. However, substantial IFs, including over 15 large- to small-scale iron deposits, have been identified recently in Northwest China, and they would constitute the largest Mesoproterozoic IFs globally if their Mesoproterozoic ages can be confirmed. Field observations reveal that these iron deposits are dominantly stratiform and stratoid within the thick clastic-carbonate and volcanic-sedimentary succession. The ores mainly comprise Fe-rich (specularite, hematite, and siderite) and Si-rich (jasper and quartz) minerals with banded or massive structures. Geochemical investigations of typical iron ores show high SiO2 and TFe2O3 and relatively low Al2O3 and TiO2 contents, indicating that they are primary chemical sediments with high hydrothermal and low detrital contributions. These features suggest that these iron deposits in the NQOB are typical IFs, which resemble the Archean and Paleoproterozoic IFs worldwide. New zircon U-Pb dating and lithostratigraphic correlations indicate that these IFs were deposited at ca. 1.1 Ga. The sizable ore reserves and Mesoproterozoic ages confirm that these IFs are the largest reported IFs for the billion years between 1.8 and 0.8 Ga. The large-scale Mesoproterozoic-aged IFs help fill the longest gap in Earth history where the IFs were thought absent from the geologic record and provide an unusual perspective to understand the reappearance mechanism of Precambrian IFs.

Lower Cretaceous volcanic-sedimentary successions of the continental rift basin in the Songliao Basin, northeast China: Implication in high-quality reservoir prediction and hydrocarbon potential

The high-quality reservoirs of volcanic-sedimentary successions in faulted lake basins are important enrichment sites for deep natural gas reservoirs. However, complex volcanic-sedimentary lithology and reservoir characteristics impede the efficient prediction of hydrocarbon in these successions. Therefore, we study the volcanic-sedimentary succession of the Huoshiling Formation (northern Dehui depression, Songliao Basin), focusing on the volcanic-sedimentary stratigraphic cycle (VSC), facies models and the distribution of high-quality reservoirs. Based on the log, core, casting thin-section, seismic and other data, fissure-type and central-type volcanic eruptions are identified. Three VSCs (1–3) are recognised, and six volcanic-sedimentary facies are determined based on lithological characteristics: effusive, explosive, volcanic conduit, volcanic-sedimentary transition, fan-delta and lacustrine. Based on the flow regimes and lithological associations, we develop three models: a fissure-central volcanic effusive model, an intermittent volcanic activity model and a fissure-central explosive model. Three types of pores occur in the volcanic-sedimentary reservoir space: primary pores, dissolution pores and micro-fractures. Lithology and dissolution are critical factors controlling reservoir porosity and connectivity. The distribution of high-quality reservoirs of volcanic-sedimentary successions can be effectively predicted using geostatistical seismic-inversion based on porosity constraints. High-quality reservoir characteristics, source rock and structural characteristics show that crystal debris and breccia-bearing tuff in VSC3 can form structural-lithologic, fault-block and self-generating and -storing hydrocarbon reservoirs. This study's results and methods of high-quality volcanic-sedimentary reservoirs can provide a reference for the efficient exploration and development of oil and gas in similar basins.

Genesis and depositional environment of the Carboniferous Baishanquan iron deposit in Eastern Tianshan, Northwestern China

Submarine volcanic-hosted iron deposits are the most important iron deposit in Northwestern China. However, their origin remains controversial. Here, we present detailed studies of the petrology, geochronology, geochemistry and FeSi isotopes of iron ores from Baishanquan deposit in Eastern Tianshan, Northwestern China. Similar to Precambrian banded iron formations (BIFs), the Baishanquan deposit is hosted in a volcanic-sedimentary succession, the iron ores are characterized by banding with alternate iron-rich and silica-rich layers, and the mineral assemblage is dominant by magnetite, quartz and Fe-silicates. The iron ores show distinct light rare earth elements (REE) depletion [(La/Yb)PAAS = 0.24–0.47], no significant Eu anomalies (Eu/Eu⁎ = 0.88–1.16), relatively high Y/Ho ratios (30–34), and δ30Si values in quartz from the iron ores ranging from −0.80 to 1.67‰. These data indicate mixed seawater and low-temperature submarine hydrothermal source for both the iron and silica. The absence of negative Ce anomalies suggests that the depositional environment was characterized by low oxygen conditions similar to modern oxygen minimum zones. Positive δ56Fe values in most magnetite (−0.45 to +1.02‰) suggest partial Fe(II) oxidation of the iron ores. Interestingly, the iron ores also contain relatively high Al2O3, CaO, MgO, and Zr concentrations, indicating that the sedimentary basin was subject to periodic clastic input. Indeed, zircon LA-ICP-MS UPb dating of interlayered dacite indicates that the Baishanquan volcanic-sedimentary rocks and associated iron ores were deposited at 327.3 ± 2.8 Ma (N = 25, MSWD = 0.52). This is significant because the appearance of these iron deposit provides direct evidence that marine ferruginous conditions – at least locally - may have still been present at Carboniferous period.

Neotethyan Ankara Melange, central Turkey: Formation by accretion of seamounts and supra-subduction zone ophiolites in an oceanic fore-arc setting

The Neotethyan Ankara Melange is a global reference for oceanic accretionary processes preserved in a collisional orogen. Here we show, based on eight representative outcrops around Ankara, that the melange encompasses stratigraphically coherent volcanic-sedimentary successions of mainly Late Jurassic-Early Cretaceous age. Our results indicate that the melange has a relatively organised tectonostratigraphy, in contrast to previous interpretations that emphasised its chaotic nature. Coherent volcanic-sedimentary successions of Late Jurassic-Early Cretaceous age are interpreted as flank facies of a large oceanic seamount (probably plume related) and its capping carbonate platform (atoll), including reef-and back-reef facies. Remnants of other seamounts are dated as Late Triassic-Early Jurassic and Late Cretaceous. Variably dismembered supra-subduction zone ophiolites of latest Early Jurassic-Middle Jurassic age, including boninites, formed in an oceanic fore-arc setting, similar to that of the Cenozoic Izu-Bonin fore arc, NW Pacific. The melange in the Ankara region partly accreted during the Early Cretaceous (pre-Albian) in response to collision of the inferred large seamount with the oceanic (ophiolitic) fore arc. During the collision which involved subduction-erosion slices of proximal–distal seamount flank lithologies accreted sequentially, whereas the seamount core subducted. Also during the collision, the distal outer edge of the ophiolitic fore arc, mainly mantle harzburgite, detached from the over-riding lithosphere and was incorporated into the melange. Structural evidence emphasises the importance of tectonic processes (rather than olistostromes) in the melange accretion. The accretionary prism in the outcrops studied was covered by arc-related sediments and volcanics during the Late Cretaceous and deformed by collision-related processes during latest Cretaceous-Paleogene.

Geochemistry and zircon geochronology of the Late Triassic volcanic–sedimentary successions in northern Tibet: Implications for the provenance and tectonic evolution of the Mesozoic Qiangtang Basin

The Late Triassic volcanic–sedimentary successions deposited in the Qiangtang Basin are linked with the initiation of the Mesozoic Qiangtang Basin. However, the nature of this basin is hotly debated. Here, we synthesize petrographic, mineralogical, geochemical, and detrital zircon U–Pb age data of these volcanic–sedimentary successions to constrain their chemical weathering history, provenance, and tectonic setting and to reconstruct the tectonic evolution of the Mesozoic Qiangtang Basin. The chemical weathering indices, especially the chemical index of alteration (CIA), are controlled by grain size and hydraulic sorting rather than sediment recycling. Low compositional and textural maturities, high index of compositional variation (ICV) values, and low CIA and plagioclase index of alteration (PIA) values in most samples suggest that the sediments from the Fanghu and north Woruoshan sections are predominantly from low-maturity sources that may have experienced a weak chemical weathering history. Based on provenance-sensitive geochemical indices (La/Th versus Hf, La/Sc versus Co/Th, and Sc/Th versus Cr/Th) and petrographic features, the detrital contributions to the Nadi Kangri Formation siliciclastic rocks are predominantly proximal felsic rocks. Detrital zircon age spectra suggest that the Nadi Kangri Formation siliciclastic rocks exhibit a provenance change from the Fanghu to the well QK-1 area, although all of the rocks were derived from the Qiangtang terrane itself. The Fanghu section was primarily sourced from synchronous magmatic rocks and pre-Triassic strata from the North Qiangtang and Central uplift belt, while well QK-1 was primarily sourced from recycled sediments from the South Qiangtang and Central uplift belt. Therefore, we propose that the Nadi Kangri Formation volcanic–sedimentary successions were deposited in a rift basin and that its evolution was likely related to the interaction of the rapid expansion of the Meso-Tethys Oceanic crust, collapse of the collisional orogen, and rollback of the Jinsha Paleo-Tethys subducting slab during the Late Triassic.

Metasomatism by Boron-Rich Fluids along Permian Low-Angle Normal Faults (Central Southern Alps, N Italy)

Low-Angle Normal Faults (LANFs) represent in the central Southern Alps area (N Italy) the main structures along which the Variscan basement is in contact with the Upper Carboniferous-Permian volcanic-sedimentary succession. Tourmalinites frequently occur along LANFs, usually replacing former cataclasites. The mineralogy and chemical composition of tourmalinites point to a metasomatic origin. LANFs, together with high-angle faults, controlled the opening of the Permian Orobic Basin and likely acted as a preferred pathway for hydrothermal fluids that triggered the Boron-metasomatism. Along the Aga-Vedello LANF, tourmalinites appear to have formed after the cessation of fault activity, as no brittle post-metasomatism deformation overprint has been observed. These relationships suggest that the circulation of B-rich fluids occurred after the opening of the Orobic Basin that is broadly constrained to the Early Permian. At the same time, ca. 285–270 Ma, a strong magmatic activity affected all the Southern Alps, ranging in composition from mafic to acidic rocks and from intrusions at deep crustal levels to effusive volcanic products. The Early Permian magmatism was likely the source of the late-stage hydrothermal fluids that formed the tourmalinites. The same fluids could also have played a significant role in the formation of the Uranium ore deposit of the Novazza-Vedello mining district, as the ore bodies in the Vedello valley are concentrated along the basement-cover contact.

What is the age of the Udzha paleorift?: U-Pb age of detrital zircons from Udzha basin terrigenous succession, northern Siberia

The Udzha paleorift is located between the Anabar and Olenek rivers and is a key structure indicative of the breakup of the Nuna supercontinent. However, the age of initiation and duration of paleorift activity is not defined nowadays. Here we present new U-Pb data for detrital zircons from two terrigenous and volcanic-sedimentary successions of the Udzha sedimentary basin (Unguokhtakh and Udzha Fm), from terrigenous rocks overlying the Udzha basin (Tomtor Fm), and from the sandstone of the lower Mesoproterozoic Mukun Group in the northwest part of Anabar region. The dating results show that sedimentation in the Udzha rift basin began later than ca 1459 Ma, and the duration of the rift activity is estimated as not longer than 73 My. The Udzha rift basin was an isolated basin in the northern part of Siberia, and detrital material came from local sources. A previously unknown source for tuff-sandstone of Unguokhtah Formation with an age of 1850 Ma has been identified, which corresponds by age to the Paleoproterozoic post-orogenic magmatism of the Siberian Craton. In the Neoproterozoic, detrital material of the Tomtor Fm was supplied from the northeast, and the sources were igneous suites of active margin or collision settings. The maximum depositional age of Tomtor Fm is estimated as 565 Ma on the youngest zircon population, which suggests an over 800 Ma gap in sedimentation in northern Siberia in Meso- Neoproterozoic.

The origin of manganese formations at Çağalali Hill and Kevri̇kevr Hill related to the late Triassic rifting of the South Neotethys, SE Turkey: The geological, mineralogical and geochemical components

The Koçali Complex includes continental rifting and subduction-related units that make up the southern Neotethys. This complex is a folded, tectonic-segmented succession with lavas, volcaniclastic sediments, pelagic carbonates, radiolarites and manganese deposits (the Tarasa and Konak formations) and ophiolitic units (the Kale formation). Manganese mineralizations in the host formation are found in banded, lenticular and massive forms within red to brown radiolarian cherts. Pyrolusite and polianite are the main manganese minerals, while psilomelane, manganite and minor hausmannite are other manganese minerals. Gangue minerals include quartz and carbonate, with minor barite being observed. The geochemical proofs (MnO: 8.98–73.64%, SiO2: 3.26–86%) originating from the main oxides indicates a hydrothermal rooted source for the manganese mineralization. This is confirmed by anomalies and trace element patterns including very low Fe/Mn (<0.1), low Co/Zn ratios (mean 0.86), very low ∑REE contents (mean 34.67), slightly negative Eu* value (mean 0.74) and low Ce* value (mean −0.23). Manganese-oxide mineralizations observed in the volcanic sedimentary succession of the Koçali Complex have been interpreted as hydrothermal deposits associated with volcanism during the post-rifting seafloor extension of the southern Neotethys during the Late Triassic period.

A putative Tournaisian and Visean volcanic-sedimentary succession in the Lublin Basin, SE Poland: depositional processes, petrological characteristics and sequence stratigraphy

The aim of this study was to establish the stratigraphic extent of a putative Tournaisian Stage within the Carboniferous succession in the Lublin Basin. The oldest part of the succession, known as the Huczwa Formation and comprising depositional sequences 1–4, was investigated based on lithofacies analysis, sequence stratigraphy and petrographic studies. The article provides descriptions of depositional sequences, parasequences (cyclothems) and lithofacies that were formed in a range of environments (elements of depositional architecture) and as a result of volcanic processes – lava and pyroclastic eruptions and chemical weathering of their products. Correlation of the sequence stratigraphy to the West European and global Carboniferous chronostratigraphic divisions, as well as to the Khoriv suite in the Lviv-Volyn Basin in adjacent Ukraine, indicates a putative late Tournaisian age for sequence 1, and a late Visean age for sequences 2–4. There is a stratigraphic gap between sequences 1 and 2, spanning probably the uppermost Tournaisian and the lower and middle Visean. The upper Tournaisian is represented by the FRST-LST deposits of sequence 1, comprising mainly volcaniclastic conglomerates and sandstones developed in braided-river channels and incised valleys with hyperconcentrated flow processes. These deposits are represented by polymictic paraconglomerate and lithic/sublithic/subarkose arenites or sublithic wackes, and contain predominantly grains of acidic and alkaline volcanic and igneous rocks. This material probably came from the Łuków-Wisznice Elevation and the Volynian Polesia region, located to the NE and E of the Lublin Basin. In the uppermost part of sequence 1, volcanic rocks and tuffs appear which developed during the activity of at least three volcanic cones in the Lublin Basin. The volcanoes were the source of alkaline lavas in the central and SW areas of the basin, and of acidic lavas in the SE area, previously undescribed. The Visean sequences 2–4 consist of the FRST-LST sediments deposited within incised valleys. The TST and HST deposits accumulated mainly in a shallow ramp-type carbonate shelf, shallow clayey shelf and deltaic environments.

Gold accumulation in the metavolcanic-hosted orogenic gold deposit constrained by pyrite paragenesis coupled with in-situ trace elements and sulfur isotope: The Sarekuobu example in the Chinese Altay Orogen

Origin and accumulation process of gold in the metavolcanic-hosted orogenic gold deposits is still open to debate. Approximately fifteen gold-bearing deposits are hosted in the metamorphosed volcanics of the Chinese Altay Orogen, NW China. Among them, the representative Sarekuobu gold deposit, occurs as lodes in a Devonian volcanic-sedimentary succession that was deformed into a series of multiple overturned synclines and metamorphosed to (garnet)-biotite-chlorite schists. In previous studies, little attention has been paid to pyrite varieties and their in situ geochemistry to constrain their genetic understanding. Herein, five types of pyrite with distinct textures and mineral assemblages at Sarekuobu are divided into four generations based on the crosscutting and replacing relationship. The earliest pyrite (PyI) occurs as magnetite-pyrite parallel laminated layers comparable to those in VMS deposits, and some of its beddings are crosscut by PyⅡ that is scattered in foliation of the host greenschists and likely formed during syn-deformation. The colloidal PyIIIa and inclusion-rich PyIIIb are present in the same auriferous quartz veins continuously cutting across the PyⅡ-rich schists. It is notable that PyIIIb acts as the most important gold-bearing pyrite containing some micro-mineral inclusions such as visible gold and chalcopyrite. The final inclusion-poor PyⅣ with coarse and euhedral grains replaces or envelops all above three pyrite generation. The highest gold concentrations are presented in PyIIIa and PyIIIb by LA-ICP-MS spot analysis, with the maximum values up to 16 ppm and 426 ppm, respectively. In contrast to the substituted invisible Au nano-particles in lattice structures with time-resolved LA-ICP-MS signal parallel to Fe, the visible Au occurs as sizeable micro-inclusions in PyIIIb revealed by highly variable LA-ICP-MS signal spectrum. Further LA-ICP-MS trace element mapping show Au and other trace elements (e.g., Cu, Pb, Zn, Bi, Sb and Ag) are unevenly distributed in PyIIIb but barren in coexisting PyⅣ, in agreement with the microscopic textures of inclusion-rich PyIIIb and inclusion-poor PyⅣ. Another mapping displays these trace elements are homogeneously distributed along colloidal layers in PyIIIa but depleted in the coexisting PyI. Collectively, PyIIIb is believed to experience relatively longer-time crystallization co-precipitating with native gold and chalcopyrite, whereas PyIIIa is rapidly deposited from the same auriferous ore fluids without sufficient time for crystallization and captures some clay minerals and organic matters. In-situ δ34SV-CDT values of four pyrite generations by fsLA-MC-ICP-MS range from +2.34 to + 10.14‰, and gradually decrease from PyI (mean, +9.59‰), through PyⅡ (+8.67‰), PyIIIa (+6.16‰) and PyIIIb (+4.97‰), to final PyⅣ (+3.66‰). In combination with published sulfur isotope composition of host rocks (maximum, +18.7‰), the formation of the auriferous fluids and deposition of Au in pyrite at Sarekuobu might be attributed to the sulfur pathway of progressively enhanced thermal sulfate reduction (TSR) in the Devonian marine strata. Thus, two process including the Devonian seafloor sedimentation and Triassic orogeny-related metamorphism are critical for formation of the Sarekuobu gold deposit.

U-Pb Age Dating and Geochemistry of Soft-Sediment Deformation Structure-Bearing Late Cretaceous Volcano-Sedimentary Basins in the SW Korean Peninsula and Their Tectonic Implications

Cretaceous volcano-sedimentary basins and successions in the Korean Peninsula are located along NE-SW- and NNE-SSW-trending sinistral strike–slip fault systems. Soft-sediment deformation structures (SSDS) of lacustrine sedimentary strata occur in the Wido, Buan, and Haenam areas of the southwestern Korean Peninsula. In this study, systematic geological, geochronological, and geochemical investigations of the volcanic-sedimentary successions were conducted to constrain the origin and timing of SSDS-bearing lacustrine strata. The SSDS-bearing strata is conformably underlain and overlain by volcanic rocks, and it contains much volcaniclastic sediment and is interbedded with tuffs. The studied SSDSs were interpreted to have formed by ground shaking during syndepositional earthquakes. U-Pb zircon ages of volcanic and volcaniclastic rocks within the studied volcano-sedimentary successions were ca. 87–84 Ma, indicating that active volcanism was concurrent with lacustrine sedimentation. Geochemical characteristics indicate that these mostly rhyolitic rocks are similar to subduction-related calc-alkaline volcanic rocks from an active continental margin. This suggests that the SSDSs in the study area were formed by earthquakes related to proximal volcanic activity due to the oblique subduction of the Paleo-Pacific Plate during the Late Cretaceous.

Evolution of Jurassic intertrap deposits in the Parnaíba Basin, northern Brazil: The last sediment-lava interaction linked to the CAMP in West Gondwana

The opening of the Central Atlantic Ocean was accompanied by ascending plume zones that generated large igneous provinces (LIPs), such as the Central Atlantic Magmatic Province (CAMP), with voluminous lava flows and outpourings. One of the best records of the CAMP in the West Gondwana is the Jurassic volcanic-sedimentary succession exposed in the western Parnaíba Basin, northern Brazil. Diagenetic and hydrothermal processes are discussed, focusing on the lava-sediment interaction. The siliciclastic sediments called “intertrap deposits” record a wet desert system characterized by fluvial-aeolian and shallow lake deposits engulfed by lavas of the CAMP at approximately 200–202 Ma. The facies and petrographic studies, based on a drill core and outcrops, led to the interpretation of the palaeoenvironmental and diagenetic evolution of the intertrap deposits. The intertrap deposits are sandwiched by basalts and may be organized in seven sedimentary facies, grouped into two facies associations: 1) ephemeral braided river deposits (FA1), which consist of medium- to coarse-grained pebbly sandstone with poorly sorted angular to sub-rounded grains (basaltic fragments and quartz) and low-angle cross-bedding, even-parallel stratification, and trough cross-stratification, and 2) pond/shallow lake deposits (FA2) composed of tabular beds of sandstone/siltstone rhythmite and laminated mudstone. The main architectural elements of intertrap deposits are 1) channels on volcanic substrate, 2) sand sheets deposited over pre-existent lava topography, 3) tabular beds with laminated rhythmites, and 4) sand-filled fissures/fractures and clastic dykes. The intensification of magmatic processes due to the disruption of Pangaea promoted the formation of extensive volcanic plains, whereas wet desert settings were developed during the quiescent intervals between lava flow episodes. Fluvial channels with subaqueous dunes and sand sheets were incised into the basaltic substrate. The abandoned fluvial plains were affected by flash floods that promoted mechanical infiltration of clays, forming coatings over grains. The heat flow and hydrothermal activity of volcanic rocks increased the devitrification of glassy clasts, releasing silica and precipitating low-temperature authigenic mineral assemblages. The most common pore-filling process was the massive precipitation of early diagenetic-hydrothermal minerals, including smectite, chalcedony, poikilotopic zeolite, megaquartz, and haematite, which partially obliterated the primary porosity and permeability. These exceptional conditions of cementation triggered by basalt eruptions precluded the modification of a primary framework by late stages (post-Jurassic) burial diagenesis. The intertrap sandstones represent the last lava-sediment interaction related to the CAMP recorded in West Gondwana.

Is there a Grenvillian orogen in the southwestern Tarim Craton?

The Rodinia Supercontinent assembled through global-scale Grenvillian (1300–900 Ma) orogeny, so identification and correlation of Grenvillian orogens is fundamental to the geological reconstruction of Rodinia. The Tarim Craton, NW China, has been considered as a building block of Rodinia, but its Neoproterozoic tectonic evolution and relationship with Rodinia remain enigmatic. It was proposed that a Grenvillian orogenic event occurred in the southwestern Tarim, possibly related to its amalgamation with northern Australia. Key evidence for this model came from the ~1.0 Ga 40Ar/39Ar ages of the Kalakashi Group in the Tiekelike belt, southwestern Tarim. In this contribution, we report zircon U-Pb ages and Lu-Hf isotopic data, as well as whole-rock geochemical data for the Kalakashi Group from the Buqiong section where a ~1.0 Ga 40Ar/39Ar age was reported. Our field and petrographic observations indicate that the primary rock assemblage of the Kalakashi Group is orthogneiss, garnet-bearing mica schist, marble and minor amphibolite. Zircon U-Pb dating of five orthogneisses and an amphibolite yielded crystallization ages of 900–850 Ma and a thermal/fluid overprinting event at ~420 Ma. Lu-Hf isotopic analyses of the dated zircon grains from two orthogneisses yielded εHf(t) values from −4 to +7, indicating a mixture of mantle- and crust-derived materials. Geochemical data show that the orthogneisses have relatively high FeOT/MgO and 10,000*Ga/Al ratios, elevated contents of high-field-strength elements (HFSE) and negative Ba, Sr and Eu anomalies, characteristic of A1-type granite, indicating that the parental magma erupted in an intra-plate rifting setting. Our results indicate that the Kalakashi Group is a volcanic-sedimentary succession that deposited in the Neoproterozoic, rather than the Paleoproterozoic or Mesoproterozoic as previously thought. We found no evidence for a Grenvillian orogenic event from the Kalakashi Group in the Buqiong section. Instead, the age and geochemistry of the studied orthogneisses are remarkably similar to those of the Sailajiazitage Group, although the latter was affected by weaker metamorphism and deformation. Reliable geochronological data from igneous and metamorphic rocks, as well as detrital zircons from Neoproterozoic strata, indicate that no typical record of Grenvillian orogeny occurred in either the north or the south of the Tarim Craton. Instead, we suggest that the northern Tarim was probably dominated by a long-term subduction-accretion, whereas the southern Tarim was likely in a long-term rifting environment in the early Neoproterozoic.

Integrated biostratigraphy and age assignment for Carboniferous successions in the Qoltag tectonic belt in eastern Tianshan, NW China

• For the first time an early Gzhelian fusulinid fauna is reported in the Qoltag tectonic belt, NW China. • An integrated biostratigraphy is recognized in the Carboniferous of the Qoltag tectonic belt. • Carboniferous volcanic-sedimentary sequences in the region are assigned ages from Tournaisian to early Gzhelian. In the Qoltag tectonic belt of eastern Tianshan, southwestern Central Asian Orogenic Belt, there are widespread Carboniferous marine volcanic-sedimentary successions. These successions record critical information about the development of the North Tianshan Ocean. To date, ages of them are uncertain, due to the lack of index fossils. In this study, abundant rugose corals, fusulines, and some conodonts were collected that allow us to assign precise ages for the successions. In ascending order, we recognized four rugose coral assemblages, including the Zaphrentites - Meniscophyllum Assemblage, Gangamophyllum-Palaeosmilia - Kueichouphyllum Assemblage, Petalaxis - Koninckophyllum Assemblage and the Ivanovia Assemblage. Four fusuline zones, including the Profusulinella parva - Profusulinella staffellaeformis Zone, Profusulinella wangyui - Eofusulina trianguliformis Zone, Fusulinella - Fusulinella Zone and the Rauserites rossicus Zone were also established. Based on these new fossil assemblages and biozones, age of the sedimentary units in the Qoltag tectonic belt was constrained as following. The Nanbeidagou Formation ranges from Tournaisian to early Viséan in age, the Yamansu Formation from early Viséan to late Bashkirian, the Shaquanzi Formation from late Bashkirian to middle Moscovian, the Dikaner Formation from Bashkirian to Moscovian, and the Qi’eshan Formation from Bashkirian to as late as early Gzhelian. The updated Carboniferous framework enables the establishment of regional standard and improves the precision of correlation with others. It further provides a sound basis for the future interpretation of tectonic evolution in the southwestern Central Asian Orogenic Belt.

Low-grade Sandow Group metasediments of the Denman Glacier area (East Antarctica): Chemical composition, age and provenance from U–Pb detrital zircon data, with some palaeotectonic implications

The Sandow Group in the Denman Glacier area consists of low-grade supracrustal (post-cratonic) strata. Such rocks are scarce and poorly studied in East Antarctica, but are significant for a better understanding of the geological history and tectonic evolution of the Precambrian supercontinents. We report U–Pb (LA-ICP-MS) detrital zircon ages which mostly fall into two groups of ca 1350−900 Ma and ca 1800−1500 Ma, which correspond to those of crystalline rocks exposed in the western Australo-Antarctica. The youngest zircons with magmatic zoning yield ages of ca 950–900 Ma which define the maximum age of sedimentation. Chemical and neodymium isotopic compositions of the Sandow Group rocks indicate derivation from a Proterozoic largely granitic source region. High-Ti mafic volcanic activity accompanied sedimentation, and therefore the Sandow Group should be considered a volcanic-sedimentary succession. The Sandow Group was presumably accumulated in a continental (pull-apart?) basin formed in the Neoproterozoic in relation to proto-Darling Fault system activity with its continuation into Antarctica. The Sandow Group may have its correlatives in Western Australia where sedimentary successions (e.g., Moora and Badgeradda Groups) occur along the proto-Darling Fault system.

Geochemical fingerprints of seawater in the Late Mesoproterozoic Midcontinent Rift, North America: Life at the marine-land divide

The 1.1 Ga Midcontinent Rift (MCR) is a thick volcanic-sedimentary succession that forms a curvilinear belt through central North America and crops out along its northern apex around Lake Superior. Sedimentary units of the MCR have been long interpreted as fluvial-lacustrine and invited a number of studies on the early evolution of life in non-marine habitats. One of the key units is the siliciclastic Nonesuch Formation, thought to record deposition in a large lake. However, recent sedimentological observations indicate the presence of marine incursions. To further test this interpretation, we analysed trace element abundances in a broad suite of samples from multiple drill cores through the Nonesuch Formation. We aimed to differentiate geochemical influences of sediment provenance from post-depositional hydrothermal overprint and thereby identify authigenic enrichments in fluid-mobile elements that are indicators of primary environmental conditions. Our results reveal discrete enrichments in Mo and U in organic- and sulphide-rich horizons, which are most parsimoniously interpreted as marine signatures. This conclusion is supported by Sr/Ba ratios, which suggest mixing between freshwater and saltwater, and by rare cm-thick gypsum in the upper Copper Harbor Formation immediately below the Nonesuch rocks. The gypsum displays δ34S values of +25.9 ± 0.6‰, consistent with input of marine sulphate at least during parts of the basin's history. Collectively, our geochemical data support the sedimentological interpretation that this portion of the MCR archives a marine-influenced estuarine system. Although this conclusion rules out that microbial life documented from the MCR was living in exclusively freshwater habitats, the Nonesuch Fm and associated rocks still hold important clues about organisms that were capable of withstanding salinity gradients and bridging the gap between the marine and non-marine environments of the mid-Proterozoic.

Neoproterozoic glaciations and rift evolution in the northwest Tarim Craton, China: new constraints from geochronological, geochemical, and geophysical data

ABSTRACT The Tarim Craton, China, records several significant events in the history of the Rodinia supercontinent, including Neoproterozoic glaciations and rift evolution. In this study, we use geochronological, geochemical, and geophysical data from the Aksu region, northwest Tarim Craton, to investigate the rifting history and three discrete Cryogenian glaciations in the Tarim Craton. A total of 268 concordant zircon U–Pb ages were obtained from three samples in the lowermost unit of the Cryogenian Xifangshan Formation. These ages define four Neoproterozoic populations of ca. 975–900, 885–810, 795–755, and 740–720 Ma, which reflect magmatic events in the Rodinia supercontinent. The age of the Xifangshan Formation is constrained by two maximum depositional ages of 739 ± 9 Ma and 719 ± 7 Ma, suggesting a stratigraphic correlation with the Cryogenian Baiyisi Formation in the Kuruktag region, northeast Tarim Craton. Field and geochemical evidence identify three discrete Cryogenian glaciations in the Aksu region, which are the Xifangshan, Qiaoenbrak, and Youermeinark glaciations that can be chronologically correlated with the Baiyisi, Altungal, and Tereeken glaciations in the Kuruktag region, respectively. The northern rift basin in the Aksu region formed at ca. 740 Ma and is filled by a thick Cryogenian volcanic–sedimentary succession. This rift basin gradually expanded to entire northern Tarim Craton, with sediments being unconformably deposited on underlying formations. Although large-scale deformation occurred in the late Ediacaran, corresponding to the Pan-African orogeny, the Aksu rift depocentre was covered with a series of deep-water rocks in the Early Cambrian and Ordovician.

The Upper Permian volcanic-sedimentary succession in northern Qamdo Block, central Qinghai-Tibet Plateau and its sedimentary, paleogeographic and tectonic significance

A combined sedimentological and geochemical study has been carried out on a volcanic-sedimentary succession from a borehole newly drilled in the northern Qamdo Block (QMB, in central Qinghai-Tibet Plateau) in order to improve the knowledge of the sedimentary and tectonic environment. The lithologic data allow to separate four main lithofacies, including massive volcanic lithofacies (LF1), tuffaceous lithofacies (LF2), sandstone lithofacies (LF3) and mudstone lithofacies (LF4). LF1 likely reflects the products of felsic lava flows that were formed during the process of magma ascent onto the seafloor at an earlier stage. LF2 contains two sublifhofacies that respectively represents the proximal and distal accumulation of tuffaceous turbidity flows. LF3 could be the arc-sourced submarine fan deposits, and the sublithofacies LF3a and LF3b represent mid-fan distributaries and interchannel deposits in fan lobes. LF4 is mainly composed of suspension deposits, which represents the sedimentation of post-volcanic activities. Furthermore, the lithofacy associations reveal the evolution of a volcano-sourced submarine fan in the back-arc basin. Geochemically, LF1 and LF2 are characterized by LREE-enriched patterns with negative Eu anomalies, enrichment in large ion lithophile elements (LILEs) and depletion in high-field-strength elements (HFSEs), which correspond with the previously published island arc environment. However, LF2 shows more distinctive patterns, which represents a later evolution stage than that of LF1. This volcanic-sedimentary succession reveals that the sedimentary-tectonic setting could be consistent with the subduction of Jinshajiang oceanic slab in the late Permian period.