Active Magmatic Arc Research Articles

High-Latitude Depositional Systems, Provenance, and Basinal Setting of the Late Cretaceous Cantwell Basin, Denali National Park and Preserve, Alaska: A Stratigraphic Framework for Paleontological and Paleoclimatic Studies

The Cantwell Formation of the central Alaska Range provides a robust archive of high-latitude, Late Cretaceous depositional systems and paleo-floral/faunal assemblages. Our stratigraphic analysis defines two mappable members. The lower member (1500–2000 m thick) represents vegetated alluvial fan and braided fluvial systems that transition up-section to fluvial–estuarine systems that drained into an inland continental seaway. The upper member (~2000 m thick) represents estuarine–marginal marine and lacustrine systems. Previous paleontological studies demonstrate that the Cantwell basin was populated by various dinosaurs, fishes, bivalves, birds, and marginal marine micro-organisms. Integration of new and published geologic mapping allows for reconstruction of depositional systems at the basin scale and provides additional paleogeographic context. The northern basin margin was defined by a previously unrecognized south-verging thrust belt, whereas the southern margin of the basin was defined by a north-verging thrust belt inboard of an active magmatic arc. Sediment sources interpreted from U-Pb detrital zircon geochronology included the coeval magmatic arc and older Cretaceous plutons, and Proterozoic–Mesozoic strata exhumed along the basin margins. Results of our study provide a depositional, stratigraphic, and structural framework that may serve as a guide for future paleontological and paleoclimatic investigations of Late Cretaceous Arctic environments of the Cantwell basin.

Late Palaeozoic final subduction records of the southwestern Paleo-Asian Ocean revealed by ca. 286–283 Ma basaltic rocks in the West Junggar, NW China

ABSTRACT The late Early Permian basaltic rocks in the West Junggar of northwest China provide a critical geological record for revealing the termination of the Late Palaeozoic final-stage subduction history of the southwestern Palaeo-Asian Ocean. With the aim of providing constraints on the late stage of the Late Palaeozoic geodynamic evolution of the southwestern Altaids, an integrated study of detailed field observation, geochronology and geochemistry has been carried out on the Early Permian basaltic rocks from the Hala’alate Mountain of the southern West Junggar. Zircon LA-ICP-MS U-Pb dating of four basaltic andesites suggests that they were erupted during the late Early Permian with ages of ca. 286–283 Ma, coeval with the neighbouring mafic diabase stock and gabbroic dyke swarms. The basaltic rocks have low SiO2 and moderate MgO contents, with low (87Sr/86Sr)i ratios (0.70397–0.70447) and high positive εNd(t) values (+5.94–+7.19), and also show remarkable enrichment in large ion lithophile elements (Ba, U, K, Pb, Sr) and prominent depletion in high field strength elements (Nb, Ta, Zr, Hf, Ti), indicating that they were originated from the partial melting of a garnet-dominant with minor spinel lherzolitic depleted mantle source previously metasomatized by slab-derived fluids and subducted sediments. These basaltic rocks were most likely formed in a fore-arc extensional setting in response to the slab retreat of the Junggar oceanic slab during the mid-late stage of Early Permian. Combined with previously published regional achievements, our new results support that the down-going of the remnant oceanic slab and the resultant arc magmatic activity of the southern West Junggar probably continued until the ca. 283 Ma, and the closure of the West Junggar branch of the Palaeo-Asian Ocean should be later than the late Early Permian.

Potassium silicate alteration in porphyry copper‑gold deposits: a case study at the giant maar-diatreme hosted Grasberg deposit, Indonesia

Potassium silicate alteration is a hallmark of the porphyry copper deposits that supply two thirds of the world's annual copper demand. These deposits formed in the cores of calc-alkaline to alkaline volcanic systems from the flux of magmatic gas that transported copper and other metals from source to the surface. The giant 3.2 Ma Grasberg CuAu deposit formed within a maar-diatreme complex following a resurgence in magmatism. The defined resources of this deposit occur from a few hundred metres depth to 1.7 km below the paleosurface which, uniquely in this deposit, is partially preserved as a section of maar tuffs.Potassium silicate alteration has commonly been interpreted as being the result of the addition of potassium to the porphyry copper host rocks via pervasive interaction with a large volume of a potassium-rich brine that is commonly presumed to be of magmatic origin. However, the data reported here show that alteration at the deposit scale is essentially isochemical with respect to the major rock-forming components and that only sulphur and the economic metals (Cu, Mo, Au, etc.) are added by flux of reactive magmatic gas containing SO2 and HCl from source intrusions at depth. Silicate solubilities are very low so that only a minor fraction of the total alkalis in the host rock are extracted by alteration reactions and then discharged at the paleo-surface. Reaction of the magmatic gas phase with plagioclase results in the coupled deposition of anhydrite (CaSO4) and disproportionation of SO2 to release H2S. The in-situ release of H2S immediately scavenges Cu and other chalcophile metals from the continuing magmatic gas flux to form the Cu-, Fe- and other sulphides that, in sufficient concentration, make up the economic reserve available to mining.The sequestration of Ca into anhydrite, along with deposition of silica into early quartz veins, increases the concentration of the other major components (K2O, Na2O, MgO, etc) in the remaining silicate assemblage within the porous host rock. The result is the development of intermingled potassium-enriched silicate and sulphur-rich (anhydrite-sulphide) sub-assemblages that constitute the mineralised phyllic or potassic alteration zones. These crystallise according to their pressure and depth into alteration assemblages dominated by potassic phyllosilicates, quartz and pyrite in the phyllic alteration zone, and alkali feldspar and phlogopitic-biotite plus minor andalusite and corundum in the central potassic zone. Dissolution and recrystallisation of primary magmatic biotite in the host rock releases K as well as Fe, the latter (along with amphibole and feldspar) providing iron for the formation of chalcopyrite, bornite and pyrite. The in-situ release of H2S through anhydrite formation, immediately scavenges Cu and other chalcophile metals from the continuing magmatic gas flux to form the CuFe sulphides that, in sufficient concentration, make up the economic reserve available to mining. Understanding of the alteration processes during porphyry copper formation also provides insights into gas-solid reactions processes inside active magmatic arc volcanoes but the magnitude of copper mineralisation is dependent on the original metal content of the source of the magmatic gas phase.

PLATE TECTONICS AS A TOOL FOR GLOBAL SCREENING OF MAGMATIC ARCS AND PREDICTIONS FOR RELATED PORPHYRY DEPOSITS

Abstract The formation of most mineral deposits is closely linked to a geodynamic context—for example, the association of porphyry Cu-Au-Mo deposits with subduction and development of volcanic arcs. This paper proposes a new approach to the global screening of volcanic arcs and their duration, as a predictive method for a mineral systems-based approach (e.g., discovery of potential porphyry occurrences). The method utilizes geospatial and temporal analytics run on a combination of large global data sets and a global plate tectonic model (covering the time period 600 Ma to present) containing plate boundaries. The method involves (1) calculating present-day arc volcano-to-trench distances and obtaining average volcanic-arc widths in both continental and intraoceanic settings, (2) applying these values to the paleotrenches contained in the plate tectonic model on 53 time intervals spread throughout the Phanerozoic, (3) unreconstructing the results back to present day, and (4) summing up all magmatic arc occurrences using their cumulative durations. This results in a spatiotemporal model of the total cumulative duration of magmatic arc activity at the global scale, back to 600 Ma, that is updatable and can serve as a proxy to predict porphyry deposit likelihood. The model output is tested against a porphyry copper occurrence data set to validate the approach as a predictive proxy for arc-related porphyry deposits. The alignment of the model results with data control is high for most geologic time periods throughout the Phanerozoic—up to 90% in the case of buffered (1σ) magmatic arcs and up to 100% in the case of buffered magmatic arcs with an additional search distance (2σ). Recent advances in plate tectonic model quality and detail now offer a higher level of precision and confidence than ever before and enable tools for the prediction and screening of porphyry deposit locations, as well as opening the potential to screen for other geodynamic context-dependent commodities (e.g., orogenic gold, volcanogenic massive sulfide, or Ni and platinum group element-sulfide deposits), particularly in the search for poorly exposed or subsurface orebodies.

Geochronology and sediment provenance of the Precipice Sandstone and Evergreen Formation in the Surat Basin, Australia: Implications for the palaeogeography of eastern Gondwana

A new view of the palaeogeography and tectonic evolution of eastern Gondwana during the late Mesozoic is emerging, driven largely by the detrital zircon record of sedimentary basins comprising the Great Australian Superbasin (GAS). The updated model suggests that a long-lived magmatic arc was present on the eastern margin of Australia, which is in stark contrast to some previous views that eastern Australia was situated well-inland of a plate boundary. However, the active magmatic arc model is derived from Late Jurassic-Cretaceous strata, with tectonic processes and sediment pathways operating in the Early Jurassic still poorly understood. The Lower Jurassic Precipice Sandstone in the Surat Basin – an important stratigraphic component of the GAS – affords an opportunity to fill such a knowledge gap. The Precipice Sandstone is also economically important due to its aquifer capacity and prospectivity for CO2 sequestration. Previous work has hypothesized that the formation contains deposits sourced from multiple terranes, yet this has not been rigorously tested. In this study, we used detrital zircon geochronology, CA-TIMS dating, petrography, and palaeocurrent analysis to characterise the sedimentary provenance of the Precipice Sandstone and the overlying Evergreen Formation. Our analysis revealed mixed provenance with multiple source terranes within the Tasmanides. The bulk of sediment was derived from the Thomson Orogen, with a lesser contribution from the New England Orogen. We found no evidence for sediment derived from the Lachlan Orogen, despite a distinct northward palaeocurrent component. Syn-depositional volcanic material within the interval corroborates the notion of continued arc magmatism along the eastern Gondwana margin, which contributed sediment to the GAS throughout the Early Jurassic. Radiometric ages place the Precipice-Evergreen succession in the Sinemurian-Toarcian which falls within the broader Hettangian-Aalenian age range suggested by recent palynostratigraphic data and suggest diachronous deposition across the basin. Despite pronounced variations in formation thickness, the Precipice Sandstone shows relatively uniform provenance across the basin, showing little evidence for multiple depocenters.

Temporal Evolution of Island Arc Magmatism and Its Influence on Long-Term Climate: Insights From the Izu Intra-Oceanic Arc

Continental arcs have an episodic magmatic activity over long-time periods, which is believed to modulate long-term climate. Island arcs have also the potential to release large amount of CO2 into the atmosphere, but whether they display an episodic magmatic history throughout their lifespan that contributes to the long-term (>10 Ma) climate changes remains an open question. To set additional constraints on the magmatic history of island arcs, here we examine fresh basalts and mineral-hosted melt inclusions from the Izu intra-oceanic arc, shortly after the eruption of boninites (∼45 Ma ago). Using chemical markers, we show that the long-term magmatic activity of the mature Izu arc has been relatively continuous over its lifespan, except during opening of the Shikoku back-arc Basin (∼23–20 Ma). Because slab dehydration and slab melting trigger decarbonation and carbonate dissolution of the subducted plate, we use slab-fluid markers (Ba/Th, Cs/Th, Cs/Ba, Rb/Th, Th/Nb) to examine the variations of slab-derived CO2 captured by the arc magmas. The long-term steadiness in the arc magmatic activity and in the slab-fluid contribution suggests that the CO2 outgassed during mature arc volcanism may have remained relatively homogeneous for the past 40 Ma in Izu. If worldwide mature island arcs also maintain a relatively steady-state magmatic activity over their lifespan, the long-term CO2 outgassed by these arc volcanoes may be rapidly balanced by chemical weathering and tectonic erosion, which rapidly draw down the atmospheric CO2 (within 200–300 kyr). This rapid negative feedback to long-term volcanic degassing permits to sustain a viable atmospheric CO2 for millions of years. The lack of co-variations between the markers of climate changes (δ13O, δ18C) and the long-term averages of the markers of slab fluids further implies that long-term volcanic degassing of CO2 from mature island arcs might play a minor role in the slide into icehouse climatic conditions. This long-term degassing stability may be, instead, a contributor to maintaining a broadly stable climate over long timescales.

Petrogenesis of the Tampanchi Ultramafic–Mafic Complex (Ecuador): Geodynamic implications for the northwestern margin of South America during the late Cretaceous

The Tampanchi Ultramafic–Mafic Complex (TUMC), located in the central segment of the Cordillera Real (Ecuador), is an oval-shaped intrusive body of approximately 18 km2 emplaced within a Cretaceous metavolcano-sedimentary sequence. Field investigations, zircon geochronology, mineral and whole-rock elemental and isotopic compositions constrain the nature of the parental magma as well as the physical conditions of emplacement, age of crystallization and tectonic setting of formation of the TUMC. The Complex consist of wehrlite and olivine-hornblende clinopyroxenite crosscut by hornblende gabbros and minor dikes/veins of diorite and granite. Hornblendites formed mainly at the contact between the olivine-hornblende clinopyroxenite and the intrusive hornblende gabbros by reaction-replacement processes. Geochemical data for both the wehrlites and pyroxenites define a trend with dominantly olivine and clinopyroxene accumulation, whereas a second trend is formed by hornblende gabbros that differentiated to leucocratic rocks, with amphibole as the dominant fractionating mafic phase. The trace elements show enrichment in large-ion lithophile elements, depletion in high field strength elements (Nb, Zr, Ti) and P, and enrichment in Pb, Sr and Ba relative to primitive mantle, indicating subduction-related parental liquids. The Nd and Sr isotopic composition suggests a uniform mantle source metasomatized by subducted crustal components. Thermobarometric estimations constrain emplacement and crystallization at intermediate to shallow continental crust levels (∼12.5 km depth) of hydrated basaltic melt under oxygen fugacity conditions above the nickel-nickel oxide buffer (ΔNNO = 0 to 2). Zircon SHRIMP U-Th-Pb age data constrain crystallization at 75.1–76.0 Ma, with scarce inherited zircons that record limited recycling of the Cordillera Real basement. These results confirm magmatic arc activity at the continental margin of South America during the late Cretaceous as a result of an E-dipping subduction zone prior to the accretion of the Ecuadorian-Colombian-Caribbean oceanic plateau, and support simultaneous double subduction at the northwest margin of South America during the Cretaceous.

U-Th-He Geochronology of Pyrite from Alteration of the Au-Fe-Skarn Novogodnee-Monto Deposit (Polar Urals, Russia)—The Next Step in the Development of a New Approach for Direct Dating of Ore-Forming Processes

We report on the application of the U-Th-He method for the direct dating of pyrite from the alteration halo of the Novogodnee-Monto Au-Fe-skarn deposit, Polar Urals. The deposit is genetically related to the formation of volcanogenic complexes of the Ural Paleozoic belt. A modification of the original methodology for measuring U, Th and He isotopes in a single grain allowed us to determine a U-Th-He age of 382 ± 8 Ma (2σ) based on six pyrite samples from the altered rocks of the deposit (U mass fraction ~0.2 mg/kg; Th/U ~3.5; 4He specific volume ~10−5 cm3·STP·g−1). This age is consistent with estimates of the age of ore formation and coeval with the end of the period of island arc magmatic activity. Our results indicate that U-Th-He dating for pyrite samples of ~1 mg in weight from the hydrothermal-metasomatic halo of ore bodies is possible, providing a crucial next step in the development of U-Th-He pyrite geochronology.

Lithium and Sr isotopic composition of salar deposits in the Central Andes across space and time: the Salar de Pozuelos, Argentina

The Central Andes of South America host the largest known lithium resources in a confined area, but the primary lithium sources of the salar deposits and the mobilisation process of lithium are still a matter of speculation. Chemical weathering at or near the surface and leaching in hydrothermal systems of the active magmatic arc are considered the two main mechanisms of Li extraction from the source rock. The lithium and strontium isotope composition of typical salar deposits offer insights into the processes on how Li brine deposits in Andean evaporites are formed. Data from the Salar de Pozuelos indicate near-surface chemical weathering in a cold and dry climate as the dominant mobilisation process of Li, with evaporation being responsible for the enrichment. The Cenozoic ignimbrites are the favoured source rock for the Li, with subordinate additions from the Palaeozoic basement. The identification of the source rocks is supported by radiogenic Nd and Pb and stable B isotope data from salar deposits. A comparison with other Li brine and salt deposits in the Altiplano-Puna Plateau and its western foothills places the Salar de Pozuelos as an endmember of Li solubilisation by chemical weathering with only minor hydrothermal mobilisation of Li.

Migration of Arc Magmatism Above Mantle Wedge Diapirs With Variable Sediment Contribution in the Aegean

AbstractCompiled data show the age progression of magmatic centers along the two approximately linear profiles from NE Greece and NW Turkey to the South Aegean Volcanic Arc. The age progression reveals the southwestward migration of arc magmatic activity from Oligocene to present, perpendicular to the Hellenic Trench. This is in accordance with the migration of the Aegean subduction zone due to the collision of oceanic and continental blocks, trench retreat, mantle flow, and coeval extension. We suggest that the subduction of large volumes of sediments and their contribution to the sub‐arc magma source controlled the composition of calc‐alkaline to high‐K calc‐alkaline and shoshonitic arc magmas during the past 30 Ma. The magma geochemistry and the approximately linear age‐progressive migration of magmatic activity suggest focused ascent of mixed material from the subducted slab into the mantle wedge, most likely in the form of mélange diapirs. Geochemical data along the profile reveals increasing Sr and decreasing Nd isotopes during Upper Miocene in agreement with the ongoing subduction of continental blocks, low subduction rates, and development of an accretionary wedge. The different K‐rich arc magmas reflect the variable subduction of sediments, whereas crustal assimilation often plays a minor role. Magmas with variable 87Sr/86Sr, P/Nd, and Ba/La indicate a variable contribution of clastic, phosphate‐bearing, and barite‐bearing sediments. Low‐degree partial melting in sediment‐dominated mélange diapirs causes the formation of shoshonitic magmas with high Sr and P2O5 contents and high La/Yb in the northern Aegean.

U-Pb ZIRCON DATING OF MIDDLE EOCENE CLASTIC ROCKS FROM THE GERCUS MOLASSE, NE IRAQ: NEW CONSTRAINTS ON THEIR PROVENANCE, AND TECTONIC EVOLUTION

The provenance of Middle Eocene clastic rock from the Gercus Molasse, NE Iraq was determined by detrital zircon (DZ) U-Pb geochronology. The Gercus Molasse in the Iraqi segment of the north-eastern Zagros Thrust Zone provides an ideal example of foreland system evolution with respect to the transition from passive margin to the accretionary complex terrene-flexural foreland basins. The DZ U-Pb age spectra from the Gercus Molasse suggest that the foreland sediments either influx from multiple provenances or are the result of recycling from the accretionary complex terrane. During pre-accretion, however, the radiolarite basin (Qulqula Radiolarite, 221 Ma) located along Arabian passive margin likely acted as an intermediate sediment repository for most or all of the DZ. Representative DZ U-Pb measurements revealed that the Gercus clastic rocks fall into several separable age population ranges of 92-102 (Albian-Cenomanian), 221 (Upper Triassic), 395-511 (Cambrian), 570- 645 (Neoproterozoic), 1111 (Mesoproterozoic), and lesser numbers of Paleoproterozoic (1622-1991 Ma) ages. The source of Proterozoic detrital Zircons is enigmatic; the age peaks at 1.1, 1.5, 1.6, and 1.9 Ga (Proterozoic) does not correspond to any known outcrops of Precambrian rocks in Iraq, and it may be useful to continue to search for such basement. The detrital zircons with age populations at 0.63–0.86 Ga probably originated from the Arabian-Nubian Shield. The age peak at 0.55 Ga correlates with Cadomian Magmatism reported from north Gondwana. The age peaks at ~0.4 Ga is interpreted to represent Gondwana rifting and the opening of Paleotethys. The youngest ages populations at 93 Ma indicate that fraction of DZ were transported directly from the contemporaneously active magmatic arc (Zagros Ophiolite segments). The paleogeography and tectonic evolution of the Neogene Zagros foreland basin were reconstructed and divided into two tectonic stages. The early stage is defined by the Campanian accreted terranes (i.e. orogenic wedge) form loads sufficient to produce flexural basin with a deepest part is situated next to the tip of the loads. This flexural basin is filled by the flysch clastics of the Maastrichtian– Early Eocene (i.e. referred to by the Tanjero-Kolosh flysch sequence). The late stage is marked by a synchronized modification of the clastics fill of the basin and changes in dip directions to compensate for the reduction of the load by both erosion and extension and the basin, therefore, was sealed by a shallowing upwards depositional sequence ending with the terrestrial Gercus Formation.

The Carboniferous Arc of the North Pamir

Abstract In this study, we investigate the age and geochemical variability of volcanic arc rocks found in the Chinese, Kyrgyz, and Tajik North Pamir in Central Asia. New geochemical and geochronological data together with compiled data from the literature give a holistic view of an early to mid-Carboniferous intraoceanic arc preserved in the northeastern Pamir. This North Pamir volcanic arc complex involves continental slivers in its western reaches and transforms into a Cordilleran-style collision zone with arc-magmatic rocks. These are hosted in part by Devonian to Carboniferous oceanic crust and the metamorphic Kurguvad basement block of Ediacaran age (maximum deposition age) in Tajikistan. We discuss whether a sliver of Carboniferous subduction-related basalts and intruded tonalites close to the Chinese town of Mazar was part of the same arc. LA-ICP-MS U-Pb dating of zircons, together with whole rock geochemistry derived from tonalitic to granodioritic intrusions, reveals a major Visean to Bashkirian intrusive phase between 340 and 320 Ma ago. This clearly postdates Paleozoic arc-magmatic activity in the West Kunlun by ~100 Ma. This observation, along with geochemical evidence for a more pronounced mantle component in the Carboniferous arc-magmatic rocks of the North Pamir, disagrees with the common model of a continuous Kunlun belt from the West Kunlun into the North Pamir. Moreover, Paleozoic oceanic units younger than and west of the Tarim cratonic crust challenge the idea of a continuous cratonic Tarim-Tajik continent beneath the Pamir.

Jurassic - Early Cretaceous paleogeography and paleoenvironments of the north-eastern margin of Gondwana: Insights from the Carpentaria Basin, Australia

Although Jurassic-Early Cretaceous sedimentary systems were extensively developed on northeastern Gondwana, deciphering their paleogeography has been complicated by poor exposure and the lack of a robust chronostratigraphic framework. The southeastern margin of the Carpentaria Basin, northeastern Australia is one of the few regions where these sedimentary systems are extensively exposed. Employing a combination of facies analysis and new data from paleontology and detrital zircon geochronology, we present a temporally and environmentally refined paleogeographic framework for this region. A Late Jurassic, southeasterly directed marine incursion invaded northeastern Gondwana, extending inland across the Carpentaria Basin, as demonstrated by a thin (~30 m), marine influenced (fluvio-estuarine) stratigraphic succession capped by a sequence bounding ~30 myr paraconformity. The depositional hiatus marked the Late Jurassic-Early Cretaceous uplift of the Euroka Arch, with loss of sedimentary and fluvial connectivity between the Carpentaria Basin and adjoining Eromanga Basin. Subsequent deposition by low-accommodation fluvial systems resulted in a thin, fluviatile depositional package developing during the Early Cretaceous. Paleocurrent and provenance data indicate that the Middle to Late Jurassic (c. 170–160 Ma) fluvial systems predating the paraconformity extended from the Eromanga Basin to the south across the southeastern Carpentaria Basin, transporting sediment from distal sources in the Lachlan Orogen of southeastern Australia. Fluvial systems of the southeastern Carpentaria Basin post-dating the paraconformity and Euroka Arch uplift show a provenance shift to easterly sources in the Mossman Orogen and Kennedy Igneous Association. Previously unrecognised Jurassic-Early Cretaceous igneous activity provided a persistent source of sediment to the southeastern Carpentaria Basin succession due to reworking of air fall tuff from an active magmatic arc located on the continental margin of northeastern Gondwana.

Early‐Middle Paleozoic Andes‐type Continental Margin in the Chifeng Area, Inner Mongolia: Framework, Geochronology and Geochemistry and Implications for Tectonic Evolution of the Central Asian Orogenic Belt

AbstractThree tectonic units have been recognized in the Chifeng area, Inner Mongolia, from north to south, including the Qiganmiao accretionary prism, Jiefangyingzi arc belt and Sidaozhangpeng molasse basin, which formed an Andean‐type active continent margin during the early to middle Paleozoic. The Qiganmiao accretionary prism is characterized by a mélange that consists of gabbro, two‐mica quartz schist and basic volcanic rock blocks and heterogeneously deformed marble matrix. Two zircon U‐Pb ages of 446.0±6.3 Ma and 1104±27 Ma have been acquired and been interpreted as the metamorphic and forming ages for the gabbro and two‐mica quartz schist, respectively. The prism formed during the early to middle Paleozoic southward subduction of the Paleo Asian Ocean (PAO) and represents a suture between the North China craton (NCC) and Central Asian Orogenic Belt (CAOB). The Jiefangyingzi arc belt consists of pluton complex and volcanic rocks of the Xibiehe and Badangshan Formations, and Geochronology analysis indicates that the development of it can be divided into two stages. The first stage is represented by the Xibiehe Formation volcanic rocks, which belong to the subalkaline series, enriched LREE and LILE and depleted HFSE, with negative Eu anomalies, and plot in the volcanic arc field in discrimination diagrams. These characters indicate that the Xibiehe Formation results from to the continental arc magmatic activity related to the subduction of the PAO during 400‐420 Ma. Magmatism of the second stage in 380‐390 Ma consists of the Badangshan Formation volcanic rocks. Geochemistry analysis reveals that rhyolite, basaltic andesite and basalt of the Badangshan Formation were developed in continental margin arc setting. Moreover, the basaltic andesite and basalt display positive Sr anomalies, and the basalt have very low Nb/La values, suggesting that fluid is involved in magma evolution and the basalts were contaminated by continental crust. The sequence of Sidaozhangpeng molasse basin is characterized by proximity, coarseness and large thickness, similar to the proximity molasses basin. According to our field investigation, geochronological and geochemical data, combined with previous research in this area, a tectonic evolutionary model for Andes‐type active continental margin of the CAOB has been proposed, including a development of the subduction‐free PAO before 446 Ma, a subduction of the PAO and arc‐related magmatism during 446‐380 Ma, and formation of a molasse basin during 380‐360 Ma.

Early central American forearc follows the subduction initiation rule

The “subduction initiation rule” (SIR) (Whattam and Stern, 2011) advocates that proto-arc and forearc complexes preserved in ophiolites and forearcs follow a predictable chemotemporal and/or chemostratigraphic vertical progression. This chemotemporal evolution is defined by a progression from bottom to top, from less to more depleted and slab-metasomatized sources. This progression has been recently documented for other igneous suites associated with subduction initiation. The Sona-Azuero forearc complex of southern Panama represents the earliest magmatic arc activity at the Central American Volcanic Arc system. Comparison of new and existing geochemical data for the circa 82-40 Ma Sona-Azuero Proto-Arc/Arc, its underlying 89-85 Ma “oceanic plateau” of SW Panama and the 72-69 Ma Golfito Proto-Arc of southern Costa Rica with the 70-39 Ma Chagres-Bayano Arc of eastern Panama exhibits a chemotemporal progression as described above and which follows the SIR. Sona-Azuero lavas are predominantly MORB-like, whereas those of the younger Chagres-Bayano complex are mostly VAB-like; lavas of the Golfito Proto-Arc typically show characteristics intermediate to that of the Sona-Azuero and Chagres-Bayano proto-arc/arc complexes. On the basis of isotope evidence as shown in other studies, lava types of all three complexes are clearly derived from a source contaminated by the Caribbean Large Igneous Province plume; we term these “plume-contaminated” forearc basalts and volcanic arc basalts, respectively. Apart from a plume-induced subduction initiation origin for the Panamanian forearc, these insights suggest otherwise similar petrogenetic origins and tectonic setting to lavas comprising earliest-formed forearc crust, and most ophiolites, which follow the SIR.

Provenance and palaeogeographic implications of detrital zircons from the lower Carboniferous Riwanchaka Formation of the central Tibetan Plateau

AbstractThe Longmu Co–Shuanghu suture zone, which divides the Qiangtang terrane into the northern and southern Qiangtang blocks, is regarded as a key locality in reconstructing the evolutionary history of the Palaeo-Tethys Ocean and the break-up of Gondwana. However, although low-temperature – high-pressure metamorphic rocks and ophiolites have been documented within the Longmu Co–Shuanghu suture zone, it remains unclear whether it is an in situ suture zone and represents the relic of the main Palaeo-Tethys Ocean. The uncertainty stems mainly from the limited systematic studies of the provenance, palaeontological evidence and depositional settings of strata on either side of the Longmu Co–Shuanghu suture zone (i.e. northern and southern Qiangtang blocks). Here we report new detrital zircon U–Pb ages and palaeontological data from Lower Carboniferous strata (Riwanchaka Formation) of the northern Qiangtang block, central Tibet. The Riwanchaka Formation contains warm-climate biota with Cathaysian affinities. Provenance analysis reveals that the formation has detrital zircon spectra similar to those from strata of the Yangtze Plate, and it contains a large proportion of zircons with ages (~360 Ma) similar to the timing of synsedimentary magmatic arc activity, implying an active continental margin setting associated with northward subduction of the Palaeo-Tethyan oceanic lithosphere. Conversely, the Carboniferous–Permian strata from the southern Qiangtang block contain cool-water faunas of Gondwanan affinity and exhibit minimum zircon crystallization ages that are markedly older than their depositional ages, suggesting a passive continental margin setting. The differences in provenance, palaeontological assemblages and depositional settings of the Carboniferous to Permian strata either side of the Longmu Co–Shuanghu suture zone indicate the existence of an ancient ocean between the northern and southern Qiangtang blocks. Combining the new findings with previous studies on high-pressure metamorphic rocks, arc magmatism and ophiolites, we support the interpretation that the Longmu Co–Shuanghu suture zone is an in situ suture zone that represents the main suture of the Palaeo-Tethys Ocean.

Genesis and mineralization age of the quartz‐vein‐type scheelite deposits in Eastern Yanbian, Northeast China: Constraints on the regional tectonic setting

The Eastern Yanbian area, Northeast China is tectonically located in the easternmost margin of the Xing'an Mongolian Orogenic Belt, which is a key area for providing important evidence of mineral deposits to constrain the tectonic evolution of the Paleo‐Asian Ocean. Five quartz‐vein‐type scheelite deposits have been identified in the eastern Yanbian area. These deposits are hosted within the late Palaeozoic metamorphic and granitic rocks and form under the identical hydrothermal mineralization. Zircon U–Pb ages and whole‐rock geochemistry of the ore‐hosting biotite granite and Sm–Nd ages of scheelite crystals within quartz veins are presented in this paper. Zircon U–Pb dating of the biotite granite yields a weighted mean age of 251.9 ± 2.2 Ma (mean standard weighted deviation (MSWD) = 4), and Sm–Nd dating of scheelite samples yields an isochron age of 251.7 ± 2.9 Ma (MSWD = 0.87), which constrains the scheelite mineralization in Eastern Yanbian to the late Permian. Our results, integrated with published geochronological data, indicate three periods of Phanerozoic W mineralization in Northeast China, (a) Cambrian, (b) late Permian (this work), and (c) Jurassic. Geochemical data reveal that the biotite granite is a calc‐alkaline I‐type granite, and it exhibits enrichment in large‐ion lithophile and light rare earth elements and depletion in high‐field‐strength and heavy rare earth elements (especially in Nb and Ta), consistent with the characteristics of arc magma. The subduction‐related biotite granite intrusion and quartz‐vein‐type scheelite mineralization together indicate that the Paleo‐Asian Ocean had not closed before the late Permian in the Eastern Yanbian area, and its final closure occurred in the Early‐Middle Triassic. The late Permian subduction of the Paleo‐Asian Ocean induced the partial melting of the lower crust, thus forming the magmatic arc activity and related quartz‐vein‐type scheelite mineralization, which clearly justifies the proposed tectonic–metallogenic model.

Intra‐Arc Crustal Seismicity: Seismotectonic Implications for the Southern Andes Volcanic Zone, Chile

AbstractWe examine the intra‐arc crustal seismicity of the Andean Southern Volcanic Zone. Our aim is to resolve interseismic deformation in an active magmatic arc dominated by both margin‐parallel (Liquiñe‐Ofqui fault system, LOFS) and Andean transverse faults. Crustal seismicity provides information about the schizosphere tectonic state, delineating the geometry and kinematics of high strain domains driven by oblique‐subduction. Here, we present local seismicity based on 16‐month data collected from 34 seismometers monitoring a ~200‐km‐long section of the Southern Volcanic Zone, including the Lonquimay and Villarrica volcanoes. We located 356 crustal events with magnitudes between Mw 0.6 and Mw 3.6. Local seismicity occurs at depths down to 40 km in the forearc and consistently shallower than 12 km beneath the volcanic chain, suggesting a convex shape of the crustal seismogenic layer bottom. Focal mechanisms indicate strike‐slip faulting consistent with ENE‐WSW shortening in line with the long‐term deformation history revealed by structural geology studies. However, we find regional to local‐scale variations in the shortening axes orientation as revealed by the nature and spatial distribution of microseismicity, within three distinctive latitudinal domains. In the northernmost domain, seismicity is consistent with splay faulting at the northern termination of the LOFS; in the central domain, seismicity distributes along ENE‐ and WNW‐striking discrete faults, spatially associated with, hitherto seismic Andean transverse faults. The southernmost domain, in turn, is characterized by activity focused along a N15°E striking master branch of the LOFS. These observations indicate a complex strain compartmentalization pattern within the intra‐arc crust, where variable strike‐slip faulting dominates over dip‐slip movements.

Early Palaeozoic sedimentary record and provenance of flysch sequences in the Hovd Zone (western Mongolia): Implications for the geodynamic evolution of the Altai accretionary wedge system

Sedimentological and detrital zircon provenance study of the Lower Palaeozoic Zuunnuruu, Tsetseg and Sagsai sedimentary formations was carried out in the eastern part of the Hovd Zone in western Mongolia. Sedimentological analysis has revealed two distinct and consecutive types of sedimentary environments. The Lower Ordovician–earliest Silurian sediments had dominantly volcano-sedimentary character, interpreted as reflecting deposition in a proximal part of a Pacific-type accretionary wedge. The upper, mainly Devonian part of the profile has generally siliciclastic flysch-like nature and indicates platform-type depositional setting related to the syn-extensional thinning of the accretionary wedge-system. Detrital zircon age populations of all the three studied formations uniformly show a dominant Neoproterozoic–Ordovician age group at ca. 560–460 Ma, a broad Neo- to Mesoproterozoic peak at ca. 1050–720 Ma, several minor Meso- to Palaeoproterozoic age clusters at ca. 1.4, 1.9 and 2.4 Ga and ca. 400–360 Ma peaks in the youngest Devonian formation. The early Palaeozoic part of the age spectra is interpreted as detritus mainly derived from the magmatic rocks of the Cambrian–Ordovician Ikh-Mongol Arc System within the nearby Lake Zone and the youngest ages from the neighbouring Devonian granites. The other sources were identified as more distal Tonian magmatic-arc complexes, Rodinia break-up-related volcanic rocks and basement of the Precambrian Zavkhan and Baidrag continental blocks even further east. The maximum sedimentary ages, determined by the youngest detrital zircons, shift the end of deposition of the Sagsai Formation at least to the latest Devonian. Nearly identical detrital zircon age spectra from Lower Palaeozoic sequences of the Hovd Zone and other parts of the Altai belt support an existence of a single giant accretionary complex developed along the entire outer margin of the Ikh-Mongol Arc System. The change in the sedimentary style suggests the latest Ordovician–earliest Silurian termination of magmatic-arc activity in the western Lake Zone, marking the onset of late Silurian–Devonian crustal extensional period in the Altai accretionary system.

Geochemistry and geochronology of Mississippian volcanic rocks from SW Mongolia: Implications for terrane subdivision and magmatic arc activity in the Trans-Altai Zone

Baaran and Baytag “terranes” form important part of the Trans-Altai Zone of the Central Asian Orogenic Belt in the SW Mongolia. The Trans-Altai Zone represents an oceanic domain built by Ordovician to Devonian ophiolites covered by Devonian–Carboniferous oceanic sediments. This oceanic assemblage was intruded by a spectrum of Carboniferous volcanic rocks ranging from basalts to rhyolites (325.6 ± 1.4 Ma to 351.0 ± 5.9 Ma; LA-ICP-MS U–Pb dating on zircon) that display similar whole-rock trace-element and Sr–Nd isotopic signatures regardless their silica contents and the “terrane” they belong to. They are normal-K calc-alkaline rocks whose NMORB-normalized multielement patterns show an arc-like enrichment in LILE (Large Ion Lithophile Elements) and deep negative TNT (Ta, Nb, Ti) anomalies. Based on the relatively high Nb/Yb and Th/Yb ratios, the arc was probably formed on immature continental rather than typical oceanic crust. Low (87Sr/86Sr)i (0.7043–0.7047), high εNd350(+5.2 to = +5.8) and the observed in-situ Hf isotopic variation in dated zircons (εHft = +7.4 to +13) are all attributed to a partial melting of primitive and youthful (Neoproterozoic–Ordovician) arc-type lower continental crust (metamorphosed intermediate–acid igneous rocks and/or arc-derived greywackes), augmented with, and most likely facilitated by, contemporaneous arc-related basaltic intrusions. A distinct possibility remains the previously formulated model assuming that the Carboniferous magmatism in Gobi-Altai and Trans-Altai zones sampled the juvenile Neoproterozoic to Cambrian arc crust relaminated under the Ordovician–Devonian oceanic crust. In any case, the studied arc association, together with lithological sequence of ophiolites, can be closely correlated with the Dulate arc in eastern Junggar. It is proposed that the two arc sequences form a single belt, c. 500 km long, that represents one of the main known arc structures in the Central Asian Orogenic Belt.