Island Arc Continental Margin Research Articles

Mesozoic–Cenozoic tectonic–palaeogeographical evolution of Bayingobi Basin: response to subduction and collision of the Mongol–Okhotsk Ocean plate

The structures, sedimentary fill and magmatic rocks of sedimentary basins are the products of regional tectonic evolution and are crucial to studying the basin's tectonic–palaeogeographical environment and regional tectonic evolution. The present study discusses the zircon U–Pb dating of volcanic rock samples, in situ Hf isotope and whole-rock geochemistry analysis, and the palaeogeographical, sedimentary filling and tectonic background of the Bayingobi basin through outcrop and core descriptions of Meso-Cenozoic strata, stratigraphic correlation and lithofacies analysis. The U–Pb age of the volcanic rocks is 132–102 Ma, and the in situ Hf isotopic values range from −20.99 to +29.48. The 87 Sr/ 86 Sr and 143 Nd/ 144 Nd isotopic ratios of the volcanic rocks are 0.707049–0.879761 and 0.511846–0.512540, respectively, and the Nd isotopic values range from −0.62 to −6.83. The volcanic rocks in the basin have the characteristics of continental margin island arc volcanic rocks (CAA) with loss of Nb and Ta and enrichment of Pb, originating mainly from dehydration melting of the subducting plate. The lower Cretaceous Bayingobi, Suhongtu and Yingen Formations developed specifically delta–lacustrine deposits. The lower and upper member of the Bayingobi Formation was deposited in the Berriasian to Valanginian and Valanginian to late Aptian, respectively. The Suhongtu Formation was deposited during the late Aptian to early Albian and was controlled by the strike of the Engeer Us fault. The Yingen Formation was deposited in the late faulted depression stage in the late Albian. Because of the subduction of the Mongol–Okhotsk Ocean plate in the Early Cretaceous, several depressions (sags) were formed in the Bayingobi basin, accompanied by the eruption of continental plate margin island arc magmas. With the closure of the Mongol–Okhotsk Ocean in the late Early Cretaceous, the basin was uplifted as a whole and the Upper Cretaceous Ulansuhai Formation was deposited. Supplementary material : Supplementary figures and tables are available at https://doi.org/10.6084/m9.figshare.c.7105807

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.

Diagenesis and metallogenetic mechanisms of the Tuanjiegou gold deposit from the Lesser Xing’an Range, NE China: Zircon U–Pb geochronology and Lu–Hf isotopic constraints

The Tuanjiegou gold deposit is a large-sized breccia-hosted epithermal gold deposit in Heilongjiang Province, eastern China, and was discovered in the late 1950s. The scale and unusual metallogenic setting of this gold deposit have made it the focus of many previous studies. We systematically analyzed ore samples, and volcanic and granitic rocks that are associated with the formation of the deposit by single zircon U–Pb dating, obtaining three distinct concordant groups of U–Pb ages from dacitic pyroclastic rocks and lavas: 119.3±3.3 (n=2), 113.2±1.1 (n=13), and 109.1±1.4Ma (n=8); four groups of concordant U–Pb ages from an ore-bearing granite porphyry: 112.5±2.2 (n=2), 107.0±1.2 (n=4), 103.0±0.70 (n=10), and 99.9±1.1Ma (n=4); two groups of concordant U–Pb ages from a pyrite-rich, quartz vein-cemented, brecciated and silicified granite porphyry: 104.23±0.51 (n=16) and 101.3±1.3Ma (n=2); and five groups of concordant U–Pb ages from pyrite-rich, quartz vein-cemented, brecciated or chunky ore samples: 139±4 to 144±3 (n=2), 127±4 (n=1), 113.8±2.1 (n=8), 101.3±3.4 (n=3), and 91±3Ma (n=1). Combining the whole-rock and Lu–Hf isotope geochemistry with the geology of the mining area and these new U–Pb dates, we have redefined the ore deposit-related temporal history of the area, with formation age of the magma associated with the Ningyuancun Stage volcanics being 119±1.4Ma and eruption of the volcanics at 113±1.1Ma. The granodiorite-porphyry and granite porphyry were emplaced at 107.0±1.2Ma and 103–102Ma, respectively, with ore deposit formation at 102–100Ma. The magmato–hydrothermal interaction that formed the deposit was closely related to Late Mesozoic crust–mantle interaction, with the early stage calc-alkaline and high-K calc-alkaline magmatism in this area potentially being derived from interaction between asthenospheric mantle material and the underplated lower crust. However, the magmas associated with late-stage intrusive calc-alkaline and high-K calc-alkaline magmatism may have been generated during interaction between adakite-like magmas derived from the subduction of an oceanic plate and lower crustal material. Here, we suggest that the fluids that formed this deposit formed by interaction with both early- and late-stage mantle-derived magmas and the lower crust, with subsequent aggregation after significant igneous differentiation. During ore formation, the area was in a back-arc initial (passive) rift and continental margin island arc tectonic environment associated with Late Mesozoic subduction of the Pacific Plate, with metallogenesis occurring during a change in the tectonic environment between the latest Early Cretaceous and the Late Cretaceous.

A Late Paleoproterozoic (1.80 Ga) subduction‐related mafic igneous suite from Lomza, NE Poland

AbstractThe Lomza orthoamphibolites in the crystalline basement of NE Poland, dated at 1802 ± 9 Ma by SHRIMP zircon U–Pb, are characterized by high incompatible and REE element contents. These features, the low Nb, and the position of the rock compositions on La–Y–Nb, Zr–Nb/Zr and Zr–Nb–Y tectonomagmatic discrimination diagrams, suggest that the metavolcanics formed in a subduction setting, possibly within a continental margin island arc. The likely extension of the belt of Paleoproterozoic mafic magmatism from Latvia and Lithuania into NE Poland provides convincing evidence for the existence of an active plate margin in that region at that time.

Alpine and Pacific styles of Phanerozoic mountain building: subduction‐zone petrogenesis of continental crust

AbstractA broad continuum exists between two distinct end‐member types of mountain building. Alpine‐type orogenic belts develop during subduction of an ocean basin between two continental blocks, resulting in collision. They are characterized by an imbricate sequence of oceanward verging nappes; some Alpine belts exhibit superimposed late‐stage backthrusting. Sediments are chiefly platform carbonates and siliciclastics, in some cases associated with minor amounts of bimodal volcanics; pre‐existing granitic gneisses and related continental rocks constitute an autochthonous–parautochthonous basement. Metamorphism of deeply subducted portions of the orogen ranges from relatively high‐pressure (HP) to ultrahigh‐pressure (UHP). Calcalkaline volcanic–plutonic rocks are rare, and have peraluminous, S‐type bulk compositions. In contrast, Pacific‐type orogens develop within and landward from long‐sustained oceanic subduction zones. They consist of an outboard oceanic trench–accretionary prism, and an inboard continental margin–island arc. The oceanic assemblage consists of first‐cycle, in‐part mélanged volcaniclastics, and minor but widespread cherts ± deep‐water carbonates, intimately mixed with disaggregated ophiolites. The section recrystallized under HP conditions. Recumbent fold vergence is oceanward. A massive, slightly older to coeval calcalkaline arc is sited landward from the trench complex on the stable, non‐subducted plate. It consists of abundant, dominantly intermediate, metaluminous, I‐type volcanics resting on old crust; both assemblages are thrown into open folds, intruded by comagmatic I‐type granitoids, and metamorphosed locally to regionally under high‐T, low‐P conditions. In the subduction channel of collisional and outboard Circumpacific terranes, combined extension above and subduction below allows buoyancy‐driven ascent of ductile, thin‐aspect ratio slices of HP–UHP complexes to midcrustal levels, where most closely approached neutral buoyancy; exposure of rising sheets caused by erosion and gravitational collapse results in moderate amounts of sedimentary debris because exhumed sialic slivers are of modest volume. At massive sialic buildups associated with convergent plate cuSPS (syntaxes), tectonic aneurysms may help transport HP–UHP complexes from mid‐ to upper‐crustal levels. The closure of relatively small ocean basins that typify many intracratonic suture zones provides only limited production of intermediate and silicic melts, so volcanic–plutonic belts are poorly developed in Alpine orogens compared with Circumpacific convergent plate junctions. Generation of a calcalkaline arc mainly depends on volatile evolution at the depth of magma generation. Phase equilibrium studies show that, under typical subduction‐zone P–T trajectories, clinoamphibole ± Ca–Al hydrous silicates constitute the major hydroxyl‐bearing phases in deep‐seated metamorphic rocks of MORB composition; other hydrous minerals are of minor abundance. Ca and Na clinoamphiboles dehydrate at pressures of above approximately 2 GPa, but low‐temperature devolatilization may be delayed by pressure overstepping; thus metabasaltic blueschists and amphibolites expel H2O at melt‐generation depths, and commonly achieve stable eclogitic assemblages. Partly serpentinized mantle beneath the oceanic crust dehydrates at roughly comparable conditions. For reasonable subduction‐zone geothermal gradients however, white micas ± biotites remain stable to pressures >3 GPa. Accordingly, attending descent to depths of >100 km, mica‐rich quartzofeldspathic lithologies that constitute much of the continental crust fail to evolve substantial amounts of H2O, and transform incompletely to stable eclogite‐facies assemblages. Underflow of amphibolitized oceanic lithosphere thus generates most of the deep‐seated volatile flux, and the consequent partial melting to produce the calcalkaline suite, along and above a subduction zone; where large volumes of micaceous intermediate and felsic crustal materials are carried down to great depths, volatile flux severely diminishes. Thus, continental collision in general does not produce a volcanic–plutonic arc whereas in contrast, the long‐continued contemporaneous underflow of oceanic lithosphere does.

Provenance and Depositional Setting of Paleozoic Chert and Argillite, Sierra Nevada, California

ABSTRACT The Shoo Fly Complex, the remnants of an early to middle Paleozoic subduction system, contains the Culbertson Lake allochthon. Included in the Culbertson Lake allochthon are the post-Cambrian and pre-Upper Devonian Quartz Hill, Toms Creek, and McMurray Lake cherts. Geochemical data derived from these three units plot on the Fe2O3/TiO2 versus Al2O3/(Al2O3 + Fe2O3) and Lan/Cen versus Al2O3/(Al2O3 + Fe2O3) discrimination diagrams within either the continental margin-island arc field, or that part of the pelagic field overlapping the continental margin-island arc field. These relationships a e consistent with the presence of argillaceous turbidites interstratified with radiolarite, and suggest a relatively distal continental-island arc setting where muddy turbidity currents episodically interrupted pelagic deposition. Specimens from the Quartz Hill and Toms Creek cherts with Al2O3/TiO2 values > 20 make up Group I samples, whereas specimens with Al2O3/TiO2 values < 10 are classified as Group II samples. The Al2O3/TiO2 ratios of Group II specimens suggest derivation from mafic rock. Mean Th/Sc and Th/U values, as well as LREE-enriched patterns, are consistent with this interpretation, and indicate a source area dominated by alkalic seamount or ocean island material. The Al2O3/TiO2 ratios of Group I samples indicate a source area with an average andesitic to rhyodacitic composition. Chondrite-normalized REE distribution patterns, as well as mean Th/Sc and Th/U value , support such an interpretation. Samples from the McMurray Lake chert have Al2O3/TiO2 values ranging between 19 and 28, and REE patterns characterized by LREE enrichment, negative Eu anomalies, and slightly fractionated HREE patterns. These features, along with mean Th/Sc and Th/U values, indicate a source area dominated by old upper continental crust. The location of the source area(s) supplying material to cherts and argillites of the Culbertson Lake allochthon is unknown. However, existing tectonic models commonly portray the source of old upper continental crustal material in the Shoo Fly Complex as being located somewhere along the western North American continental margin, whereas the source of island-arc debris is portrayed as being located in a fringing arc system. Our data suggest that a relatively high-standing seamount or ocean island probably resided on the subducting oceanic plate. Thus, our work supports the idea that Al, Fe, Ti, Th, Sc, and the REEs in chert and argillite deposited in proximal continental-island arc settings can be used to assess the characteristics of sources supplying particulate matter to ancient t ench systems.

Archaean Greenstone Belts in China

Abstract Chinese Achaean greenstone belts are mainly distributed along the northern and southwestern margins of the North China platform. In terms of their geological characteristics, the greenstone belts in China are comparable to those in other countries but at the same time have unique features of their own. In view of their geochemistry, the Chinese greenstone belts may be grouped into three types: the Jiapigou type, Qingyuan type and Xiaoqinling type. The greenstone belts were formed possibly in a rift‐type palaeo ‐ tectonic setting, similar to that of the modern island arc‐continental margin mobile belts.

Geometry and Style of Post-Obduction Thrusting in a Paleozoic Orogen: The Taconic Frontal Thrust System

The Taconic Frontal Thrust System, associated with the later stages of the medial Ordovician Taconic orogeny, a continental margin-volcanic arc collision, defines the present western boundary of the Taconic Allochthon in eastern New York and west-central Vermont. The Frontal Thrust emplaces regionally folded and foliated Taconic rocks over deformed, but essentially unmetamorphosed, flysch. The Frontal Thrust is temporally and kinematically related to the Champlain and Orwell faults to the north, that emplace variably deformed sections of Early Paleozoic shelf rocks over partially coeval sections of the Early Paleozoic platform. Major movement on these faults occurred after initial obduction of the Allochthon onto the Early Paleozoic North American continental shelf and prior to being unconformably overlapped by the Early Devonian. This fault system is considered to be part of the final phase of a continuum of thrusts that evolved during the Taconic Orogeny, and therefore records only the final emplacement geometry of the Allochthon. Detailed mapping has revealed a complex, fine-scale imbricate structure at and near the front of the Allochthon. The imbricate structure in front of the Allochthon is defined by slivers and small thrust sheets of Lower and Middle Ordovician shelf carbonate and overlying Middle Ordovician shales, flysch and wildflysch (debris flow deposits), and melange. These imbricates have dimensions of hundreds of meters to kilometers parallel with strike and a few to several hundred meters structural thickness across strike. Ramps and tear faults within the imbricate stack control the final geometry of the otherwise gently east-dipping Frontal Thrust surface. Complex superposed fold patterns occur locally within the overlying regionally folded Taconic sequence at these frontal structures. At Bald Mtn. and other localities to the north, the carbonate slivers have an internal duplex structure, with the Frontal Thrust forming the roof fault to the duplex. In some areas the Frontal Thrust appears to follow the earlier, pre-cleavage thrusts (e.g., Basal Taconic Thrust) along which initial obduction occurred. Microscopic and mesoscopic fault-related deformation associated with the Frontal Thrust appear to have developed in fully lithified sedimentary sequences. Deformation involving partially lithified rock is limited to the local incorporation of olistostromes into tectonic melange at the leading edge of the Allochthon. The pattern of essentially coeval thrusting, lithification, cleavage development, low-grade metamorphism deduced here may be generally representative of the final stages of continental margin-island arc collision.

The Haweswater complex

Summary The largest basic intrusive complex of the Lake District cuts Borrowdale Volcanic Group rocks around Haweswater. It is composed mainly of gabbros, dolerites, andesites and microdiorites. These have been altered in places by hydrothermal and tectonic activity as well as by later igneous intrusions. Petrology and geochemistry have established a consanguineous relationship between the Haweswater complex and the coeval Borrowdale volcanic rocks. It is suggested that the complex was intruded into an active continental margin island arc.