Active Island Arc Research Articles

Middle Jurassic deposits of Ulakhan-Sis Ridge (Sakha Republic): biostratigraphy, lithological features and depositional environments

The age and sedimentation conditions of the Middle Jurassic terrigenous deposits crowning the Mesozoic section of the northeastern flank of the Nagondzha terrane within the Ulakhan-Sis Ridge are discussed. Based on our own finds of retrocerams and analysis of biostratigraphic data of predecessors, we accept the age of the deposits as Bajocian–Middle Bathonian. Three lithologically different strata are identified in the section. Sandstones from the lower unit belong to feldspathic litharenites, and from the upper strata – to litharenites with a predominance of extrabasinal carbonate clastics (calklithites). Analysis of textural and structural features, mineralogical, petrographic composition and distribution of trace elements in litharenites suggests that all these strata had been accumulated in the shelf part of the paleobasin associated with an active volcanic island arc, which presumably existed from the Early Jurassic on the Omulevka craton terrane until the completion of the formation of Kolyma-Omolon microcontinent.

Geochemical and geochronological evidence of a Neoproterozoic island-arc on the easternmost Sierras Pampeanas. New U-Pb LA-ICP-MS data from amphibolites and metacarbonate depositional sequences

The first basement outcrops west of the Río de La Plata craton are metabasaltic and metasedimentary units of the Sierra Chica belt, part of the Eastern Sierras Pampeanas. These are orthoamphibolites and subordinate garnet-biotite gneisses and metacarbonate sequences that are intruded by Cambrian calc-alkaline and peraluminous rocks of the Pampean Orogeny. In accordance with one of the leading tectonic models for the region, new provenance data from this research suggest that the metabasaltic-metasedimentary association represents the proximal deposits of a Late Neoproterozoic island-arc. This contribution brings new geochemical and geochronological evidence from orthoamphibolites located on different sites along the Sierra Chica belt, which are interbedded with metacarbonate and gneisses outcrops, showing tholeiitic compositions, island-arc related tectonic settings, and Cryogenian to Ediacaran crystallization ages. Moreover, silicate-rich metacarbonates interbedded within the same sequences yielded a maximum depositional age of 569 Ma, reinforcing the assumed age for the sequence. All things considered, these rocks are interpreted as old basic lava flows and gabbro dikes linked to an island-arc activity, interspersed within the carbonatic and siliciclastic shelf sediments, together constituting the Neoproterozoic Córdoba Arc from the Sierra Chica belt. A depositional analogy with the present-day Caribbean island-arcs deposits suggests that these sequences could have been part of Oceanic Arc Depositional Systems (OADS) during the diachronous closure of the Goiás ocean and the final assembly of West Gondwana. Overlapping thermo-tectonic processes associated with the Pampean Orogeny are ubiquitous throughout all the studied outcrops, as evidenced by the petrography, the zircon morphology, and the U-Pb results, indicating a complex scenario towards a Late Cambrian slab break-off event.

Petrogenesis and Tectonic Implications of the Qulihai Pluton in the Northern Margin of the Yili Block, NW China

Abstract Despite extensive research work that has been done, whether the northern margin of the Yili Block (YB) is still an active continental margin during the early Carboniferous period is still in debate. Herein, we conducted zircon U–Pb geochronology, geochemistry, and zircon Lu–Hf isotope studies on the Qulihai pluton in the northern margin of the YB to constrain the petrogenesis and dynamic process. The Qulihai pluton is a granitoid complex that is composed of quartz diorite, quartz monzonite, and syenogranite. The zircon U–Pb dating results revealed that the pluton was formed and emplaced between 346 and 342 Ma. The three different Qulihai pluton rock types had comparable εHf(t) values, ranging from +3 to +8. The corresponding two-stage model ages of 817–1182 Ma indicated their potential derivation from the Meso-Neoproterozoic juvenile crust. The Qulihai pluton typically features medium-to-high SiO2, K2O, and Al2O3 contents and low MgO and Fe2O3T contents. The quartz diorite and quartz monzonite samples had Mg# values of 43–47, indicating the input of mantle-derived melts in the source area. In contrast, the source region of syenogranite was purely crustal material. The Qulihai pluton is mainly characterized as strong metaluminous moderate-to-high-K cal-alkaline rocks of the I-type granite series, which are enriched in large-ion lithophile elements and depleted in high field strength elements while exhibiting active continental margin island arc magmatism. Our findings, combined with the comprehensive analysis of previous studies, suggest that the Qulihai pluton formation resulted from the subduction of the North Tianshan oceanic crust beneath the YB during the early Carboniferous period, contemporary with the tectonic regime transition from subduction advance to subduction retreat.

Turonian highly differentiated pyroclastic xenoliths and Cretaceous sedimentary record challenge mantle-plume view of Costa Rican forearc basement successions

The Mesozoic basaltic basement of southern Central America is believed to consist of proxy sections of mantle-plume events leading to the Caribbean igneous province. A significant xenolith assemblage of radiolarite, tuffite, and Turonian (∼93 Ma) highly differentiated pyroclastic rock discovered in basalt of the Costa Rican forearc basement fundamentally challenge this long-held view, revealing bimodal Cretaceous island-arc activity and forearc basin development instead. The pyroclastic materials originally accumulated with monotonous radiolarite deposits in a deep-marine forearc basin slope environment to build part of the Mesozoic sedimentary cover of the evolving arc. Xenolith formation was related to widespread Turonian to Campanian intrusive basaltic activity on the Nicoya Peninsula that affected Albian-Turonian and Berriasian-Barremian radiolarite and pyroclastic deposits as well as Jurassic siliceous strata. Strong correlation with Cretaceous biotite- and hornblende-bearing pyroclastic beds reveals explosive calc-alkaline volcanic arc episodes alongside basaltic activity linked to marine forearc basin development. These materials provide decisive evidence of explosive calc-alkaline volcanic activity of the Cretaceous Costa Rican arc as well as the probable intraoceanic-arc nature of its basaltic forearc basement and a ∼90 Ma aged komatiite suite found at Tortugal. The resulting tectonic scenario shows the Caribbean Plate was separated in southern Central America from any potential Pacific or Galápagos hotspot sources by a subduction zone. This favours in-place origin of the Caribbean Plate and its igneous province between the Americas. The findings also reveal the unsuitability of basaltic forearc basement successions to deduce with certainty mantle-plume events and hotspot evolutions in adjoining oceanic domains.

Genesis and interaction of magmas at Nishinoshima volcano in the Ogasawara arc, western Pacific: new insights from submarine deposits of the 2020 explosive eruptions

Sudden changes of eruption styles and magma compositions at arc volcanoes are enigmatic processes. Nishinoshima volcano, western Pacific, has had historical eruptions in 1973–1974 and from 2013 on and off to the present day. These eruptions were characterized by effusive Strombolian eruptions of andesite magmas until mid-June 2020, when they suddenly transitioned to violent explosive Strombolian eruptions that produced tephra fallout over a wide area. To understand this transition, we conducted marine surveys and sampling of the extensive submarine deposits of the tephra fallout. Our new data demonstrate that the full compositional range of the 2020 eruptions spans from basalt to dacite. We present evidence for magma mixing of newly injected basalt with andesite magmas. Nishinoshima consists of an andesitic main edifice surrounded by basaltic knolls: previous studies have shown that Nishinoshima andesite compositions can be generated by olivine fractionation of primary andesitic magmas that result from partial melting of hydrous mantle at relatively low pressures under the thin crust of the Ogasawara arc; knoll basalt compositions can be generated by partial melting of mantle at greater depths and were interpreted as older events of the volcano. We show that basalt magmas could have been generated throughout the entire history of Nishinoshima. In addition, we show that andesites from Nishinoshima and nearby Nishinoshima-Minami Knoll, which are only ∼8 km apart, have distinct subduction components. Together, these data improve our understanding of the diverse primary magmas responsible for the construction and continuing eruptive activity of an active island arc volcano.

Study on the Provenance and Tectonic Setting of Mudstone in the Lower Silurian Longmaxi Formation of the Yanyuan Basin on the Western Margin of the Yangtze Platform

This study investigates the provenance and tectonic background of the lower Silurian Longmaxi Formation black shale of the Yanyuan Basin in the western Kangdian ancient land and provides guidance for shale gas exploration and development in the area. The mineral petrological and geochemical characteristics of the Longmaxi Formation black shale in this area have been studied in detail. The study area is mainly a passive continental margin environment, but also has the attributes of an active continental margin and island arc environment due to the collision between the western oceanic crust and the Yangtze continental crust. The source rocks are mainly felsic igneous rocks, with a small contribution of intermediate–basic rocks. It is inferred that the Kangdian ancient land in the eastern part of the area could be the main provenance area, but with the contribution of sediments derived from oceanic island arc located in the west. During the whole Longmaxi period, the tectonic movement was intense, the climate was cold, the degree of chemical weathering was low, and it was a highly volatile geological sedimentary basin. Therefore, the highly siliceous organic-rich mudstone, which was different from the Sichuan Basin, was deposited.

Records of Nanhua island arc activity between the north and south Wuyi terranes in the Cathaysia Block

AbstractThe characteristics and pre‐Devonian tectonic evolution between the North and South Wuyi terranes are important for understanding the tectonic framework and aggregation of the micro‐terrenes in the Cathaysia Block, as well as the evolution of the South China Block. This paper provides systematic studies on the petrologic features, zircon U–Pb geochronology and geochemical characteristics of the low‐grade meta‐volcanic and metasedimentary rocks between the North and South Wuyi terranes, and proposes an alternative model for their tectonic evolution. Geochronological data show that the metasedimentary rocks have detrital zircon U–Pb ages ranging from 583 to 3284 Ma with a record of the Nanhua period (764–722 Ma). The meta‐volcanic rocks have centralized ages ranging from 720 to 760 Ma and yield weighted average ages of 746–733 Ma. Geochemical studies of the meta‐volcanic rocks show similar characteristics to island arc magmatic rocks, with relatively high SiO2, K2O and Al2O3, low TFeO and MgO contents, and enrichment of Rb, Ba, Th, Pb and Hf elements, and depletion in Ti, Nb, Sr and Ta. These features indicate that the North and South Wuyi terranes may have collided at about 746–733 Ma and eventually pieced together at 583 Ma, with the protoliths of the meta‐volcanic rocks and the metasedimentary rocks formed in the same tectonic setting of an island arc during Nanhua island arc magmatism related to the collision. The occurrence of Nanhua island arc activity between the North and South Wuyi terranes suggests that multi‐micro‐terranes and multi‐stage collisions may be major characteristics of the tectonic evolution process in the Cathaysia Block.

Tectonic setting, provenance, depositional, and paleo-climatic conditions of the late quaternary subcrop sediments of the southeastern coastal region of the Bengal basin

This is a systematic attempt to depict the genetic evolution of the Late Quaternary sediments of the southeastern (SE) coastal region of the Bengal basin regarding paleotectonic settings, sedimentation, provenance, paleo-climatic conditions, weathering condition and age. The study has considered multiple attributes such as, lithology/lithofacies, sedimentary features/records, major oxides, clay minerals, foraminifera, and radiocarbon dating. The lithological characters along with associated clay minerals confirmed that a Pleistocene paleosol horizon (over-bank deposits) of warm-humid nature is commonly encountered immediately on top of the sub-crop bed-rock in the area overlain by Holocene fluvio-marine sediments of the same nature. The lithofacies, foraminiferal assemblages, and sedimentary structures of the analyzed samples suggest that the Holocene sediments have been presumably deposited in a fluvio-marine condition after the Last Glacial Maximum (LGM) due to the transgression of the sea. Geochemically, the sediments are classified as Fe-rich shale, shale, and wake and primarily intermediate to felsic orogen provenance. These are possibly derived from intense weathered sources from the upheaval of Himalayan ranges of both active continental margin and Island Arc paleotectonic setting. The plot of the Index of Compositional Variability versus the Chemical Index of Alteration indicates that the sediments seemingly experienced intense weathering associated with warm and humid climatic conditions. The sedimentation rates of the area vary from place to place and layer to layer due to the complex delta-building process. The reconstructed Relative Sea Level Curve reveals that presumably, the sea level has reached its current position after the LGM. The deduction possibly will facilitate the (1) reconstruction of Late Quaternary coastal evolution after LGM, (2) support for future urbanization, land use plans, etc., and (3) also be helpful for international researchers to understand the possible sources of sediment input in the area from the complex interplay of the Indian-, Eurasian- and Myanmar-plates.

Age and geochemistry of the Naxiguole banded iron formation (BIF), NW China: recurrence of superior-type BIF in the Neoproterozoic

ABSTRACT The Naxiguole Banded Iron Formation (BIF), located in the Eastern Kunlun Orogenic Belt, Northwest China, is hosted by the Jinshuikou Group and has a Fe of 75 Mt at a grade of 15–28.83 wt.% Fe. In this paper, we use integrated petrological, geochemical, and geochronological data to limit the age and origin of the Naxiguole BIF. Country rocks mainly comprise marble, felsic paragneiss, plagioclase amphibolite schist, and biotite plagioclase gneiss. The features of sedimentary structure (blastopsammitic texture and blastobedding structure) and trace elements (e.g. La, Ni, Zr) from the country rock indicate that their protoliths are sedimentary rock. The 141 detrital zircons from the Jinshuikou Group yield a minimum age of 908 Ma, which can be considered the upper limit for the Naxiguole BIF. According to the age (602 Ma) of the mafic rocks intruding into the Jinshuikou Group, it is reasonable to assume that the formation age of BIF can be limited at 908–602 Ma (i.e. Neoproterozoic). The high contents of Al2O3 + TiO2 (3–6 wt.%), ΣREE (36–135 ppm) for BIF samples suggest that terrestrial clastic material contributed to BIF formation. The BIF samples show seawater-like features, such as depletion in LREEs, positive Y anomaly and high Y/Ho ratios (30.7–57.5, average = 42.5), implying that the Naxiguole BIF precipitated from seawater. The positive Eu/Eu*PAAS anomalies (average 1.52) suggest the influence of high-T hydrothermal fluids. Based on positive Eu anomalies and high terrestrial clastic components and no obvious Ce anomalies, the Naxiguole BIF was formed in an anoxic or suboxic sedimentary basin within active continental island arc, and is similar to Superior-type BIFs worldwide.

Magmatism and tectonic setting of proto-Japan during the Early Carboniferous: Constraints from the geochemical characteristics of mafic volcanic rocks in the Hida Gaien Belt, SW Japan

• Early Carboniferous subduction-related basalts–andesites are found in SW Japan. • High-Al basalts were generated by arc tholeiitic magma near crust–mantle boundary. • Basaltic andesites were generated by fractional crystallization of the high-Al magma. • Early Carboniferous tectonic setting of proto-Japan was a primitive island arc . The Hida Gaien Belt in southwest Japan consists mainly of Paleozoic to Mesozoic shelf-facies rocks and records the tectonic evolution of East Asia. Here we report a study of the geochemistry of mafic volcanic rocks from the Early Carboniferous Arakigawa Formation of this belt. Two types of mafic volcanic rock are identified: (1) high-Al basaltic lavas, which were generated by island arc tholeiitic magma that formed under high-pressure conditions near the crust–mantle boundary and in which the nucleation of plagioclase was suppressed and delayed; and (2) basaltic andesitic lavas, which were generated by fractional crystallization of the high-Al tholeiitic magma. Combining our new results with previous geological findings, we consider that the tectonic setting of proto-Japan during the Early Carboniferous was a primitive island arc characterized by active island arc tholeiitic magmatism and the absence of a granitic batholith.

Geochemical and organic petrographic analyses on the Late Eocene bentonitic shale bed in Kom Aushim vicinity, Fayum, Egypt; Paleoenvironmental implications

Bentonitic shale bed of the Qasr El Sagha Formation (Late Eocene), located in the Kom Aushim area, Fayum, composes of 5 m thickness of dark massive stratum intercalated with veins of iron bands. In this study, twelve fresh shale samples from this area were investigated concerning their geochemical and organic petrography analyses. A total organic carbon (TOC) of the shale samples is very low (0.07–0.39 wt%). Petrographic analysis of samples shows a dominance of the vitrinite maceral followed by either inertinite and/or liptinite macerals. Theoretically derived vitrinite reflectance (Rocal) reveals that the bentonitic shale bed was subjected to an abnormal increase in maturity toward the invasion of Fe, Zn, Cu rich hydrothermal sulfates and oxides veins. The composition and origin of these veins were investigated by XRD and XRF analyses. The results suggested that the Fe brought from these veins affected the main composition of the shale stratum to exhibit Fe-shale facies. Moreover, rich iron in the depositional medium leads to form siderite minerals (≈10%) and the replacement of lime associated clay to Ferron dolomite (≈25%). The dominant paleoclimate synchronized shale deposition was semi-arid and arid climate, as confirmed by the domination of evaporite (≈44% gypsum). The source area of shale components was found to be sedimentary and mafic igneous (basaltic trend). CIA-ICV and A–CN–K ternary diagrams showed that the shale components were subjected to a variety of intensive to moderate weathering, and they were probably coming from gabbro and tonalite igneous rock. An active sedimentary margin and island arc are the main provinces, as the shale was deposited at the marine anoxic condition of a clay-lime platform of a pro-delta sedimentary setting. The organic matters (OM) are of type III and IIIC that refer to terrestrial OM of marginal aquatic environments. Organic and inorganic studies confirm a prevailing depositional environment of a shallow shelf, shoreline mudflats and evaporitic lagoon that reflects the general trend of sea-level regression in the Fayum area during the Late Eocene.

Iron from continental weathering dictated soft‐part preservation during the Early Ordovician

AbstractThe Fezouata Shale in Morocco is the most diverse Lower Ordovician unit yielding soft‐tissue preservation. Iron played a crucial role in the preservation of soft parts in this formation through the damage of bacterial membranes under oxic conditions and the pyritization of soft parts under the activity of bacterial sulphate reduction. However, the origin of Fe in this formation remains largely speculative. Herein, trace and rare earth elements were investigated in drilled‐core sediments from the Fezouata Shale. It is shown that a correlation exists between Fe and Al suggesting that most Fe has a detrital source. Elemental concentrations in the Fezouata Shale are most comparable to rivers and are the least similar to loess and sediments deposited near active island arcs. In this sense, continental weathering and its related Fe in river fluxes dictated occurrences of exceptional fossil preservation in the Fezouata Shale.

Provenance and tectonic settings of the Eze-Aku Sandstone (Turonian) in Awajir and adjoining areas, Southern Benue Trough, Nigeria: evidence from petrography and geochemistry

Eighteen representative samples for petrographic (6) and geochemical (12) studies were collected from the Turonian (Eze-Aku Formation) sandstones outcropping within Awajir and adjoining areas in the northern section of the Southern Benue Trough. Sandstones of different textural variety underlie the study area: fine-, medium- and coarse-grain sandstones facies. Field study revealed an overall coarsening upward succession, ripple bedding surfaces, cross-stratification and bioturbation, which suggest prevalence of a shallow shelf with relict shoreline environmental settings. Petrographic study show that the sandstones are subarkosic to arkosic and deposited in a humid to semi-humid paleoclimatic condition. Polygenic (mixed) provenance source is inferred for the sandstones and the insignificant presence of sedimentary rock fragments in the framework constituents suggests the Precambrian Basement complex of southern Nigeria as the most likely source area. This result is consistent with trace element ratios such as Cr/Zr (< 0.6) and Cr/Th (0.3–15.7) as well as La/YbN versus CeN diagram which points all the sandstone facies to an intermediate Igneous provenance, unambiguously ruling out the chances of mafic igneous rocks being present in the source terrain. The most probable provenance for the sandstones is the continental Basement complex rocks of Obudu plateau and Oban massifs. The study showed that there was a mild influence of diagenetic alteration on the sandstone facies after deposition in an active continental margin-continental island arc setting.

The Galápagos Islands: biogeographic patterns and geology.

In the traditional biogeographic model, the Galápagos Islands appeared a few million years ago in a sea where no other islands existed and were colonized from areas outside the region. However, recent work has shown that the Galápagos hotspot is 139 million years old (Early Cretaceous), and so groups are likely to have survived at the hotspot by dispersal of populations onto new islands from older ones. This process of metapopulation dynamics means that species can persist indefinitely in an oceanic region, as long as new islands are being produced. Metapopulations can also undergo vicariance into two metapopulations, for example at active island arcs that are rifted by transform faults. We reviewed the geographic relationships of Galápagos groups and found 10 biogeographic patterns that are shared by at least two groups. Each of the patterns coincides spatially with a major tectonic structure; these structures include: the East Pacific Rise; west Pacific and American subduction zones; large igneous plateaus in the Pacific; Alisitos terrane (Baja California), Guerrero terrane (western Mexico); rifting of North and South America; formation of the Caribbean Plateau by the Galápagos hotspot, and its eastward movement; accretion of Galápagos hotspot tracks; Andean uplift; and displacement on the Romeral fault system. All these geological features were active in the Cretaceous, suggesting that geological change at that time caused vicariance in widespread ancestors. The present distributions are explicable if ancestors survived as metapopulations occupying both the Galápagos hotspot and other regions before differentiating, more or less in situ.

Sedimentary Provenance Analysis of Palaeocene in the X Sag, East China Sea Shelf Basin

The analysis of sedimentary provenance characteristics is the premise of sedimentary system analysis. At present, the understanding of sedimentary provenance is not unified in the exploration and development of Lishui Sag. To solve this problem, in this paper, we used the clastic components composition and the major element oxides to analyze the tectonic setting in provenance area, the analysis of paleogeomorphology, heavy minerals, mudstone color index and paleocurrent indicated by seismic reflection characteristics were used to analyze the provenance direction. Through the analysis of clastic components composition, it is considered that the provenance of Paleocene sedimentary system in the X sag mainly comes from the areas of Recycled Orogenic, Dissected Arc and Transitional Arc, and the analysis of major element oxides shows that the provenance tectonic setting and type of Paleocene sedimentary system in the X sag is mainly active continental margin and island arc, these two conclusions are basically consistent. The paleogeomorphology, heavy minerals, mudstone color index and seismic reflection characteristics analysis shows that the provenance direction in the sedimentary period mainly comes from the west, southwest of Minzhe Uplift and East of Yandang Uplift. Different provenance areas provide substantial sediments basis for the development of sedimentary facies in the study area.

Changes in particle sizes and geochemistry of Siyah Keshim lagoon sediment of Gilan province to determine origin and tectonic position of sediment

Useful information was obtained about the environmental condition of this region such as energy, sediment location, origin, sediment carrying path, pond evolutionary process, and tectonic conditions and origins of sediments by studying physical and geochemical sediment changes processes in place and time, distribution of sediments and elements in lagoon bed, and also identification the effective factors on sediment distribution model. In this regard, 59 sediment samples were taken from Siyah Keshim lagoon and were analyzed chemically and aggregation by XRF and ICP-MS technic. Adaptation of data by Folk diagrams showed that this region is placed in a range of sand, muddy sand, sand with a little gravel, muddy sand with a little gravel, and silty sand. Weak to medium sorting and negative tilting shows sediment in a coastal area. Geochemical evidence shows that SiO2/Al2O3 ratio in these sediments is 2.6-3.7. In addition, Na2O value shows relatively low sediment investigation for sediments of this lagoon. Moreover, determination of the weathering effect in origin place by the chemical index of alteration (CIA) and chemical weathering index (CWI) shows intensive chemical alternation on sediments. Index of combined variety was averagely 1.54 in the studied sediments and show that sediments resulted from the first cycle sediment. Using tectonic separating diagrams based on the primary and secondary oxidants percentage show the sediment in the active continent margin (ACM), continent-island arc (CIA), and oceanic island arc (OIA) and shows that the studied sediments are related to subduction margin.

Famennian crinoids and blastoids (Echinodermata) from Mongolia

Herein we report on the most abundant and diverse fauna of Palaeozoic crinoids and blastoids collected from Mongolia to date. The fauna is from the Late Devonian (Famennian) Samnuuruul Formation in western Mongolia. The fauna consists of two genera of blastoids and twelve genera of crinoids—four genera of camerates, three genera of flexibles, one disparid genus, and four genera of cladids. The crinoids and blastoids were living on an active island arc complex in the Central Asian Orogenic Belt (CAOB) in a high physical stress environment with frequent and often voluminous pyroclastic eruptions. The Mongolian fauna is similar to coeval faunas collected from the Hongguleleng Formation in western China and supports the hypothesis that the CAOB was a biodiversity hotspot for Famennian echinoderms and a precursor to the very successful echinoderm communities that dominated Mississippian shallow-marine ecosystems globally. Three new taxa are described. Mongoliacrinus minjini, new genus and species, is the oldest member of the Acrocrinidae, previously known from the Mississippian and Pennsylvanian and the first occurrence of the family outside North America. Eutaxocrinus ariunai and Eutaxocrinus sersmaai are new species of the flexible crinoid Eutaxocrinus, a genus with a widespread distribution during the Early and Middle Devonian, which survived into the Lower Mississippian. It is restricted to the CAOB in the Late Devonian.

U-Pb ages on detrital zircons and geochemistry of Lula paragneiss from Variscan belt, NE Sardinia, Italy: implications for source rocks and early Paleozoic paleogeography

Chemical analyses of garnet-bearing metasediments and EMP and U/Pb analyses of detrital zircons of the Lula paragneisses in the Axial Zone of the Variscan belt (NE Sardinia) give significant contribution to the reconstruction of the early Paleozoic evolution of the northern Gondwana margin. The youngest middle Ordovician (465 Ma) age of detrital zircons indicate a derivation from a fore- arc sedimentary basin along an early Paleozoic convergent margin, which collected sediments from nearby emerged lands consisting of early Paleozoic volcanic arcs and pre-Paleozoic sequences. The chemical composition of metasediments is characterized by negative Sr anomaly and depletion in heavy rare earth elements (HREE) and Y as compared to the upper crust. Normalized to chondrite values, the paragneiss shows a steep REE pattern with light-REE enrichment, negative Eu anomaly and flat heavy-REE pattern. All these features suggest a derivation form older active continental margins or island arcs. Detrital zircon ages cover a very large time span (3151 + 97 Ma to 465 + 8 Ma) and reveal a complex history of inheritance and recycling. The oldest ages obtained on relic cores and/or magmatic stages, mostly enriched in Hf (Zr / Hf ratios: 45-24) and devoid of Y, indicate a contribution from granitoid rocks of mainly crustal origin. The ages of detrital zircons highlight a derivation of the sedimentary supply from the Sahara craton and/or the Arabian-Numidian shield along the northeastern margin of Gondwana. This fit well with derivation of early Paleozoic metasedimentary formations in southern Sardinia and northern Apennine, pointing out a common origin and location of these sedimentary basins along the northern margin of Gondwana at the early Paleozoic. As regards the Variscan orogeny, the common middle Ordovician age of Lula paragneiss and nearby Lodé orthogneiss highlight the role of Variscan tectonics in the assembly of different blocks of early-middle Paleozoic margin.

Provenance, palaeoweathering and tectonic setting of the Ediacaran Bayandor Formation in NW Iran: Implications for the northern Gondwana continental margin during the late Neoproterozoic

The tectonic evolution of Iran during the late Neoproterozoic and early Paleozoic is usually expressed in two models including passive (conventional) and active (alternative) margin tectonic models. The passive margin model proposes rifting of peri-Gondwanan terranes and strike-slip tectonics in a non-magmatic setting while the active margin model suggests the existence of a subduction zone and the formation of a continental arc (known as “Cadomian” arc) along the northern Gondwana margin. The Ediacaran siliciclastic Bayandor Formation from the Soltanieh Mountains in western Alborz records the geological history of Iran during the late Neoproterozoic through its provenance, as aims of our studies, which allows palaeoweathering and tectonic setting reconstructions. Thin-section petrographic analysis of the Bayandor Formation sandstones reveal that the feldspar-rich sandstones including minor values of chert rock fragments have characteristics of both active continental margin settings and recycled orogen provenances. The chemical index of alteration (CIA) indicates a moderate degree of chemical weathering in the source area. Most of the Bayandor Formation sandstones were derived from a dissected arc and a minor number of the samples seem to be derived from a basement uplift and recycled orogen provenance. Geochemical discrimination diagrams using major and trace elements indicate both active continental margin and island arc tectonic settings for the Bayandor Formation sandstones. The results indicate that it is more likely that deposition of this formation took place along an active continental margin and the Cadomian basement of NW Iran comprises a series of metagranites and metamorphic rocks and may have contributed some sediment to the localized Neoproterozoic–Cambrian basins. Other sources were likely located in the basement provinces particularly the Arabian–Nubian Shield that is consistent with the results of this study.

Cryogenian magmatic activity and early life evolution

Data from the Qinling Orogenic Belt in China indicate that a strong magmatic-volcanic event on the Snowball Earth during the Cryogenian age (approximately 720–635 million years ago) was followed by a dynamic period of accelerated evolution of early life through the Ediacaran period. The studied volcanics of the Cryogenian Yaolinghe group are mainly represented by andesite, dacite and rhyolite, with minor amounts of basalt, trachy andesite and trachyte towards the top, which formed in the environment of an active island arc related to a continental margin. Compared with average felsic volcanics, the studied Cryogenian marine volcanic strata are enriched (1.5–30.6 times) in Co, Cr, Bi, Ni, Se, Ga, As, Cu, Ba, V, and Zn. Elemental concentrations (P, Cd, Co, Ni, and Se) of the studied volcanics are more than 5–26.4 times those in the contemporaneous Liantuo tillite. We propose that Cryogenian magmatic and volcanic activity increased the flux of some trace nutritional elements into the oceans which possibly provided essential nutrients for the development of early life.