Typical Island Arc Research Articles

Remnants of a Jurassic marginal sea plate in the Meso-Tethyan Ocean revealed by multi-stage magmatism in the Lagkorco ophiolite, Central Tibet

The formation of marginal sea plate, through multi-stage arc rupture and back-arc spreading, is a fundamental process in mature intra-ocean convergent plate margins. Identifying remnants of ancient marginal sea microplates within suture zones is crucial for unraveling the evolution of ancient ocean basins, although it presents significant challenges. In this study, we present new zircon and titanite UPb ages, along with Sr-Nd-Fe isotopic compositions, for a suite of well-characterized cumulates from the Lagkorco ophiolite which represents remnants of the Bangong-Nujiang Meso-Tethyan oceanic lithosphere. Our integrated dataset, combined with previous findings, reveals three distinct magmatic phases within the Lagkorco ophiolite: i) Unit A: early Jurassic (~184 Ma) amphibolite-facies mafic-felsic complex, ii) Unit B1: early Jurassic (187–192 Ma) oceanic lithosphere, and iii) Unit B2: middle Jurassic (160–170 Ma) oceanic lithosphere. Geochemical data from Unit B1 cumulates indicate a damp (~1 % H2O) but arc-like parental magma. These cumulates exhibit more enriched SrNd isotopes compared to mid-ocean ridge basalts and typical island arc basalts. Unit B1 cumulates exhibit clinopyroxene Fe isotopes (δ56Fe = −0.06 − +0.01) lighter than mid-ocean ridge cumulate clinopyroxene (δ56Fe = +0.02 − +0.05). These data indicate that the Unit B1 cumulates originated from a subduction-modified depleted mantle mixed with minor sediments. Combining our results with previously published data, we propose that the parental magmas for both Unit B1 and Unit B2 were damp but arc-like, which aligns with the repeated re-melting of previously depleted sub-arc mantle during episodic back-arc spreading. Our interpretation suggests two-stage back-arc spreading (Unit B1 and Unit B2) and at least one episode of arc magmatism (Unit A) between 192 and 160 Ma. These results imply the existence of a Jurassic marginal sea plate, akin to the present-day Philippine Sea plate, within the Meso-Tethyan Ocean, which was likely later subducted beneath the Qiangtang continent.

Early Cretaceous to Eocene magmatic evolution of Livingston Island, South Shetland Archipelago: Geochronological and isotope geochemical constraints from intrusive suites

The magmatic products exposed across Livingston Island comprise a part of the South Shetland magmatic arc built on an active continental margin. The main magmatic phases of Livingston Island include the Early to Late Cretaceous plutonic and volcanic rock associations, the Eocene plutons, and some sporadically distributed intrusions of Paleocene to Eocene dykes. The intrusions were emplaced into the strongly deformed turbiditic sedimentary rocks of the Miers Bluff Formation. 40Ar39Ar and zircon UPb geochronologic data presented here define four distinct episodes of intrusive activity represented by the emplacement of the Hesperides (125 Ma), the Cerro Mirador (104–97 Ma), the Siddons Point (80 Ma) and the Barnard Point suites (42 Ma), in addition to some dyke intrusions with emplacement ages of 105 to 40 Ma. The intrusions are represented by mafic to intermediate compositions characterized by middle potassic, calc-alkaline affinities, with only a few samples from the Eocene activity exhibiting tholeiitic character. Trace element, Sr-Nd-Pb isotope and zircon Hf isotope variations indicate magma generation in a subduction setting with extensive involvement of continental crust-derived melts for the Early to Late Cretaceous activity. The effects of crustal input on melt chemistry appear to have diminished in time, especially from the Paleocene onwards, leading to magma generation with typical oceanic island arc geochemical signatures. This change in melt chemistry is likely partly related to the roll-back induced overriding plate extension which resulted in a time-integrated increase in the rate of melt movement and a decrease in the residence time in crustal levels. Slab steepening caused by the systematic decrease in plate convergence rate led to an enhanced involvement of isotopically depleted mantle component, producing a heterogeneous source with variable subduction signature.

Geochemistry of Volcanic Rocks in Ponelo Island, North Gorontalo, Indonesia

Ponelo Island is located in the northern part of Sulawesi, which is still an enigma regarding the genesis of the volcanic rocks found on this island. Therefore, the objective of this study is to understand the petrogenesis and tectonic implication of these volcanic rocks. Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) to obtain trace and rare earth elements is the method of this study. The volcanic rocks found on Ponelo Island consist of basalt and basaltic andesite rocks with a calc-alkaline affinity. The transition data suggested a highly fractionated cause of low transition element (Ni=17-38 ppm; Cr=13-47) compared to primary magma concentration, anomalies negative of Ba, Sr, and Ti of spider diagrams, and negative anomaly of Eu (Eu/Eu*=0.88-0.99). Relationship between low concentration between Ce/Y (0.74-0.76) and La/Yb vs Sm/Yb ratio indicated ~5% spinel-lherzolite mantle source partial melting. On the other hand, incompatible element ratios, such as Ba/Nb (39.03-45.28), Ba/Th (75.52-82.67), Rb/Nb (3.93-6.22), K/Nb (1772.22-2703.45), Ba/La=13.67-14.57, Th/La (0.17-0.18), La/Nb (2.91-3.16), depleted Nb/U (6-6.74), and also lack of xenolith or enclaves indicate cryptic crustal contamination. The slab-derived fluid indicated by ratios of Rb/Y (0/019-0/05), Nb/Y (0.10-0.11), Th/Yb (0.52-0.61), and Ba/La ratio (13.29-14.57). Ponelo volcanic rocks shows typical calc-alkaline island arc tectonic setting particularly with enrichment in ion lithophile element (LILE) and light rare earth elements (LREE) along with depletion in high field strenght elements (HFSE) and heavy rare earth elemets (HREE), as shown by spider diagrams.

Neoproterozoic Ridge Subduction Beneath South China Craton: TTG Suites of Kangding Complex in Western Yangtze Block and its Geochemistry Characteristics

ABSTRACT A multidisciplinary study involving field investigation, petrographic, geochronology, and whole-rock geochemical data is presented for TTG suites in the western Yangtze Block. In this paper, we compiled element geochemistry data that we obtained from our previous works for China Geological Survey Projects and presented the zircon U-Pb age and the whole-rock elemental compositions of the Na-rich TTG rocks to provide constraints on the origin of the Neoproterozoic magmatism in the western Yangtze Block. The studied rocks show a calc-alkaline trend. They are comparable in whole-rock chemistry to Archean TTG suites from Finland, Labrador, India, Zimbabwe, Brazil, and Ontario, as well as the modern calc-alkaline magmas such as volcanic rocks in the Kastamonu area of Northern Turkey and the Cenozoic andesitedacite-rhyolite suites from the North American Cordillera. Zircon U-Pb age shows the crystallization age of the plagioclase gneiss from Kangding Complex is 772.4 ± 6.9 Ma. The granodiorites are more enriched in LREEs but depleted in HREEs, showed strong negative Eu anomalies, and are characterized by fractionated LREE, flat HREE, and depletion of high-field-strength trace elements (HFSE) relative to normal mid-ocean basalts (N-MORB). However, most trondhjemites, all tonalites, and all quartz diorites have no Eu anomalies. Trace element distribution patterns for all samples show typical island arc signatures with relative enrichment in LILE and depletion in HFSE like Nb and Ta. We assume that the TTG suites were derived from melting of metabasalt in a subducted slab heated by a slab window, which provides further support for a Neoproterozoic intra-oceanic ridge subduction along the western margin of Yangtze Block.

The mafic–ultramafic roots of a Mesoarchean microblock in southern India: Insights from petrochemistry and isotope geochronology of the Coorg Block

The Coorg Block in southern India is an archive of Paleo-Mesoarchean arc magmatism and continental building in the early Earth. Here we investigate a suite of mafic and ultramafic rocks and associated lithologies to characterize the roots of this microcontinent. We present petrologic, geochemical, as well as U-Pb and Lu-Hf data on zircon and U-Pb data on monazite from these rocks which provide insights into the petrogenesis of the rocks as well as the timings of magmatism and subsequent metamorphism. The geochemical features of the mafic granulites indicate that the rock suite was derived from the fractionation of a sub-alkaline tholeiite magma. Most of the rocks correspond to arc setting in a subduction-related tectonic regime. The rocks exhibit relative enrichment in LILE, Pb, and LREE with depletion in HFSE such as Nb and Ta suggesting partial melting of a metasomatized mantle wedge. Their high LILE/HFSE and LREE/HFSE ratios indicate slab dehydration and mantle wedge melting in typical island arc settings. The overall trace element and REE patterns of the rocks indicate that the source magma of the ultramafic and mafic rocks might have been derived from a refractory mantle source followed by the metasomatic enrichment of lithospheric mantle wedge by incompatible elements through the influx of subduction-derived fluids/melts. We present comprehensive zircon U-Pb data and Lu-Hf data on a suite of mafic granulite, garnet-bearing granulite, tremolite actinolite schist, charnockite, and anorthositic gabbro as well as monazite U-Pb data from a charnockite sample. Most zircon grains show cores with magmatic crystallization features and many grains are mantled by metamorphic rims. The age data show a prominent peak at 3.1 Ga for most of the rocks suggesting that the major crustal building in the Coorg Block occurred during the Mesoarchean. Rocks on the margins of the block were affected by the younger (Neoarchean) tectonothermal event in the surrounding blocks. The Lu-Hf isotopic data on zircon grains with ca. 3.1 Ga ages show εHf(t) values ranging from −4.4 to + 3.7 (average + 0.3). In the εHf(t) versus 207Pb/206Pb age diagram, the plots are distributed close to the CHUR line suggesting Paleo-Eoarchean juvenile magma sources. The voluminous mafic lower crust at the root of the Coorg Block is consistent with addition of mafic magmas through slab melting induced by high mantle potential temperatures in the Archean, as well as the interaction of fluids/melts with the mantle wedge.

NEW EVIDENCE OF THE INDEPENDENT PRECAMBRIAN TECTONIC HISTORY OF THE FADDEY TERRANE OF THE CENTRAL TAIMYR ACCRETIONARY BELT

New geostructural, petro-geochemical, U-Pb geochronological and paleomagnetic data for sills of metagabbro-dolerites of the Severnobyrranga and Yasnenskiy complex specify the paleotectonic position of the Faddey terrane of the Central Taimyr accretionary belt. The structural position of the sills in the rocks of the Oktyabr and Zhdanov formations is indicative of their intrusion before deformations caused by the accretion of the Central Taimyr belt. The U-Th-Pb age of crystallization for the metagabbro-dolerites has been established as 1357 ± 9 Ma. The primary paleomagnetic record reflecting the time of intrusion has not been preserved, but two metachronous components have been identified. The first one corresponds to crust formation processes at ca. 840 Ma due to collision, probably of continent – island arc type involving the cratonic block that included the Faddey and Mamont-Shrenk terranes. These collisional events took place several hundreds of kilometers from the Siberian margin. The second component corresponds to thermal events on the Paleozoic-Mesozoic boundary and reflects the strike-slip deformation in the Kara orogen.

New Evidence of the Independent Precambrian Tectonic History of the Faddey Terrane of the Central Taimyr Accretionary Belt

New geostructural, petro-geochemical, U–Pb geochronological and paleomagnetic data for sills of metagabbro-dolerites of the Severobyrranga and Yasnenskiy complex specify the paleotectonic position of the Faddey terrane of the Central Taimyr accretionary belt. The structural position of the sills in the rocks of the Oktyabr and Zhdanov formations is indicative of their intrusion before deformations caused by the accretion of the Central Taimyr belt. The U–Th–Pb age of crystallization for the metagabbro-dolerites has been established as 1357 ± 9 Ma. The primary paleomagnetic record reflecting the time of intrusion has not been preserved, but two metachronous components have been identified. The first one corresponds to crust formation processes at ca. 840 Ma due to collision, probably of continent – island arc type involving the cratonic block that included the Faddey and Mamont-Shrenk terranes. These collisional events took place several hundreds of kilometers from the Siberian margin. The second component corresponds to thermal events on the Paleozoic-Mesozoic boundary and reflects the strike-slip deformation in the Kara orogen.

Volcanism of the Initial Stage of Subduction of the Northern Part of the Pacific Plate (Kamchatka Peninsula, Kumroch Ridge)

We studied whole rocks compositions of the Baydara and Semkorok Mountains, which are located at the north-western part of Kumroch Ridge. Rocks are represented by Amf-Px basaltic andesites and andesites, and have microelements distribution typical for island-arc type of rocks. Some mineralogical and geochemical characteristics of the studied lavas of the Baydara Mt. (low concentrations of K2O, all REE, LILE, Th and U) and the Semkorok Mt. (low LREE concentrations) make them principally different from the rocks of the located nearby Late Pleistocene-Holocene Shiveluch volcanic massif. Isotopic K-Ar age of lavas (0.7 Ma for Baydara and 1.3 Ma for Semkorok) allow us to propose that their eruptions might be caused by the initial phase of the northern segment of the Pacific plate subduction. The Early Paleocene age (~62 Ma) of the Khapitsa series rocks, which compose north-western part of Kumroch Ridge, is confirmed for the first time by the isotopic-geological methods.

Geochemistry and geochronology of basic igneous rocks in Bairin Right banner, southeastern inner Mongolia, China: Implications for the final closure of the Paleo—Asian Ocean along the Xar Moron suture zone

This paper reports the first discovery of basic igneous rocks in Bairin Right Banner in southeastern Inner Mongolia, China. To understand the genesis and tectonic evolution of these rocks, we have carried out geochronology and geochemistry research on them. The results show that LA-ICP-MS zircon U-Pb concordant mean ages of massive basalt, diabase, and pillow basalt are 272.0 ± 2.8 Ma, 261.5 ± 3.2 Ma, and 256.5 ± 2.5 Ma, respectively, suggesting their formation during the Late Permian. The massive basalt, diabase, and pillow basalt are tholeiitic with low Mg# values in the ranges of 46.2–66.8, 54.4–71.0, and 53.5–58.2, respectively. They have similar chondrite-normalized rare earth element (REE) patterns. The intensity of REE differentiation is similar to that of typical enriched mid-ocean ridge basalts (E-MORB). The pillow basalts are relatively enriched in Ba and U, while slightly depleted in Rb and Th, and show geochemical affinity to E-MORB. The massive basalts and diabases have the same primitive mantle-normalized patterns and are relatively enriched in Rb, Ba, and U, while depleted in Nb and Ta, similar to the typical island arc magmas. Our new findings indicate that the intra-oceanic subduction of the Paleo-Asian Oceanic slab was still ongoing during the late Permian in the Bairin Right Banner area and, combined with previous studies, it might have been closed during the Late Permian to Early Triassic along the Xar Moron suture zone.

Quaternary caldera-forming eruptions at the Sanzugawa caldera, NE Japan, revealed by zircon U-Pb geochronology

Recent application of zircon U-Pb geochronology has contributed to deciphering the evolution of caldera systems worldwide. However, in the Tohoku region, NE Japan, this is not the case, although it is well studied as a typical island arc subduction system. Here, U-Pb dating of zircon suggests that the Sanzugawa caldera, the largest caldera in Tohoku since late Miocene, initiated its caldera-forming volcanic activity at ∼7 Ma and culminated its activity in late Pliocene to Quaternary (3.0–1.5 Ma). This is contrary to a previous notion that the caldera developed in late Miocene to Pliocene (6–3 Ma) based on mainly whole-rock K-Ar dating results. This finding may also question a current hypothesis of relatively subdued volcanism at 3.5–1.5 Ma in NE Japan and thus shows the need to date other calderas in the Tohoku region with zircon U-Pb and/or Ar-Ar methods to better understand the magmatic history of this region.

Petrogenesis of Neoproterozoic mafic dykes in western Yangtze Block, South China: implications for the assembly and break-up of Rodinia

ABSTRACT Neoproterozoic mafic dyke swarms are an important probe for reconstructing the evolutionary history of the Rodinia supercontinent. Here, we present detailed geochronological, geochemical, and Sr–Nd–Hf isotopic data to constrain the petrogenesis and geodynamic setting of Neoproterozoic mafic dyke swarms in the western Yangtze Block, South China. Petrological and zircon U–Pb dating studies reveal that the studied dykes mainly comprise subophitic-textured dolerite and intergranular dolerite with crystallization ages of ca. 823–817 Ma and ca. 795–794 Ma, respectively. The two groups of dykes exhibit varied geochemical and Sr–Nd–Hf isotopic compositions. Among them, the older group has moderate rare earth elements (∑REE = 71.0–94.9 ppm) with flat REE patterns ((La/Yb)N = 1.38–1.81) and exhibits enriched large ion lithophile elements and Sr–Nd–Hf isotopes ((87Sr/86Sr)i = 0.704865–0.707641, εNd(t) = −6.15 to 1.26, εHf(t) = −2.59 to 2.11), resembling typical island arc basalts. Whereas, the younger group has high Nb contents (4–10 ppm) with high Nb/U values (6.93–29.3), high REE contents (∑REE = 123.3–180.8 ppm) with moderate (La/Yb)N values (1.84–4.51), and slightly depleted but variable Sr–Nd–Hf isotopes ((87Sr/86Sr)i = 0.702280–0.706761, εNd(t) = −2.67 to 4.07, εHf(t) = −0.74 to 5.91), akin to Nb-enriched basalts in the subduction zone. Petrogenesis studies indicate that the ca. 823–817 Ma and ca. 795–794 Ma dykes may be derived from different degrees of melting of the enriched mantle wedge that was metasomatized by subducted slab-related fluids and melts, respectively. Varying degrees of the interaction of the slab-derived fluid/melts with mantle peridotite account for their elemental–isotopic heterogeneity. The geochemical database compilation of Neoproterozoic igneous rocks suggests that the Yangtze Block may be located on the periphery of the Rodinia supercontinent and slab tearing or break-off, rather than mantle plume, may account for the Rodinia break-up and associated transition of the mantle metasomatic mechanism at ca. 830–820 Ma.

Compositional variation of mafic calc-alkaline lavas at the submarine Pausanias Volcanic Field, western South Aegean Volcanic Arc: Implications for magma formation and ascent

The calc-alkaline lavas of the submarine Pausanias Volcanic Field and the neighboring Methana peninsula represent the largest volcanic edifices of the western South Aegean Volcanic Arc (SAVA) in the Mediterranean Sea and erupted on 30–35 km thick continental crust. We report the first whole rock major and trace elements together with Sr-Nd-Pb isotope ratios for basaltic to andesitic lavas from the submarine Pausanias Volcanic Field consisting of six smaller, volcanic structures. The mafic lavas have the most primitive compositions of the SAVA with MgO of up to 9 wt% and Ni concentrations >100 ppm. The incompatible trace element abundance patterns of the Pausanias and Methana lavas overlap and have typical island arc patterns, but Pausanias lavas display a larger geochemical heterogeneity on a smaller spatial scale compared to those from Methana. The Pausanias lavas resemble those from Methana in Sr, Nd, and Pb isotopes reflecting higher sediment subduction in the western SAVA than in the central arc at Santorini. The variability of incompatible element concentrations at relatively constant isotope compositions suggests variable degrees of partial melting of a peridotite-sediment mélange diapir in the mantle wedge. The Pausanias magmas formed from a less depleted mantle source compared to the central SAVA and their ascent in an extensional basin supported relatively rapid ascent without the extensive stagnation in the crust observed in the Methana magmas. The compositional differences between Pausanias and Methana imply differences in their composition and/or transport in the mantle wedge and the crust, allowing highly variable melts to reach the surface within a spatial distance of few kilometers. • Small scale mantle heterogeneity recorded at the Pausanias Volcanic Field. • Melting of sediment-rich mélange diapirs contribute to Pausanias lavas. • At Pausanias, extension allows melts to ascend more direct through the crust compared to Methana.

Geodetic and Geological Deformation of the Island Arc in Northeast Japan Revealed by the 2011 Tohoku Earthquake

Northeast Japan is a typical island arc related to the Pacific plate subduction. The 2011 Mw9.0 Tohoku-oki earthquake provided a unique opportunity to analyze crustal deformation with different boundary conditions, similar to a gigantic rock deformation experiment. We review findings obtained through various observations and data analyses in Northeast Japan, focusing on the crustal deformation in different timescales. The occurrence of the M9 earthquake solved the ongoing paradox that the geodetic strain rate is an order of magnitude larger than the geologic estimate, showing that the centennial geodetic observation had mainly captured the elastic strain accumulation. Along the localized contraction zone along the Japan Sea coast, a comparison of postseismic and interseismic deformation patterns revealed a significant contribution of inelastic deformation, which plays an essential role in long-term deformation. Along the Pacific coast, rapid interseismic subsidence and unexpected coseismic subsidence were followed by a rapid postseismic uplift, indicating that viscous relaxation in the mantle is of essential importance. These findings advance our understanding of plate interactions and the tectonic evolution of the island arc. ▪ The 2011 Tohoku-oki earthquake provided the most complete crustal deformation data set ever for interseismic, coseismic, and postseismic periods. ▪ The discrepancy between the geologic and geodetic deformation rates in Northeast Japan is attributed to an elastic strain due to interplate locking. ▪ A significant contribution of inelastic deformation in the island arc crust is identified through a comparison of interseismic and postseismic deformations.

Discovery of late Early Cretaceous diorite porphyrite from the Shamuluo Formation in the Gaize area, Tibet: Response to the northward subduction plate rollback event of Bangongco-Nujiang Tethys Ocean

改则地区闪长玢岩位于藏西北班公湖-怒江成矿带西段北缘，呈岩脉状侵位在成岩于浅海环境的沙木罗组地层中，是班公湖-怒江特提斯洋向北俯冲造山消减过程的产物，而其岩石成因、地质意义和深部动力学过程等尚欠缺详细的研究。本文研究了发育于改则地区的闪长玢岩，对其开展了岩相学、锆石LA-ICP-MS U-Pb年代学以及元素和同位素地球化学的研究，对其岩石成因、成岩构造地质背景、深部地球动力学过程等进行了分析和讨论，并对其含矿性进行了评价，为该地区下一步找矿方向提供了初步的设想。锆石U-Pb同位素年代测试显示闪长玢岩具有较多锆石结晶年龄段，相较于研究区出露的沙木罗组碎屑锆石年代学特征，其最新年龄段（117~126Ma）加权平均年龄为120.4Ma，可代表其结晶年龄。元素地球化学结果显示闪长玢岩具有弧岩浆的典型特征，属于高钾钙碱性和准铝质系列岩石，结合锆石微量元素等指标综合判断其属于I型花岗岩类岩石。本文综合全岩Sr-Nd-Pb同位素组成特征等分析认为，闪长玢岩形成于新生玄武质下地壳的部分熔融，岩浆源区残留相以角闪岩相为主，源区物质有少量沉积物组份的参与，侵位过程中分离结晶和地壳混染作用不明显。通过多种方法判别并结合区域构造背景，我们认为闪长玢岩成岩于班公湖-怒江特提斯洋北向俯冲板片折返的地球动力学背景。由于地处俯冲板片后端根部，改则地区闪长玢岩俯冲角度小于成岩于俯冲板片前端同时代的多龙矿集区中酸性岩浆岩，亦小于成岩于相同板片构造位置的103.5Ma的革吉闪长玢岩，暗示120.4~103.5Ma时班公湖-怒江特提斯洋北向俯冲板片仍处于持续折返阶段。由于其俯冲角度较小以及岩浆源区低氧逸度等因素导致其无法成矿。因此建议在多龙矿集区和改则县中间位置寻找可能的与俯冲背景有关的成矿靶区，但是否见矿还应考虑构造、围岩、后期保存等条件的影响。;The diorite porphyrite in Gaize area, located in the northern margin of the western section of the Bangongco-Nujiang metallogenic belt, emplaced into the Shamuluo Formation formed from shallow sea environment. It was the product of the northward subduction of Bangongco-Nujiang Tethys Ocean (BNTO). Taking diorite porphyrite in Gaize area as the research object, this paper studied petrography, zircon U-Pb chronology, elements, and isotope geochemistry, analyzed its petrogenesis and source characteristics, discussed its diagenetic tectonic background, described its deep dynamic process in detail, explained the possible reasons for its non-mineralization, and provided a preliminary idea for the next prospecting direction in this area. Zircon U-Pb isotopic dating shows that diorite porphyrite has many stages of age. Compared with the detrital zircon chronological characteristics of Shamulo Formation in the study area, the weighted average age of the latest age group (117~126Ma) is 120.4Ma, which can represent its crystallizing age. The element geochemical results show that diorite porphyrite is a typical island arc magma with high potassium calc-alkaline characteristics, belonging to quasi-aluminous series. Combined with the characteristics of zircon trace elements, the diorite porphyrite has the characteristics of I-type granitoids. Based on the analysis of the Sr-Nd-Pb isotopic composition of the whole rock, it is considered that the diorite porphyrite was formed in the partial melting of the juvenile basaltic lower crust, the residual facies in the magmatic source are mainly amphibolite facies, with the addition of a small amount of sediment components, and the separation, crystallization and crustal contamination in the emplacement process is not obvious. Through a variety of methods and combined with the regional tectonic background, we present that the diorite porphyrite was formed in the geodynamic background of rollback within the process of northward subduction of the BNTO. Because it is located at the root of the back end of the subducted plate, the subduction angle of the diorite porphyrite in Gaize area is smaller than that of the intermediate acid magmatic rock in the Duolong ore district, which formed from the front of the subducted plate at same time, and is also smaller than that of Geji diorite porphyrite, which is located at the same tectonic location of the subducted plate and formed at 103.5Ma. It is suggested that the northward subduction plate of the BNTO is still in the stage of continuous rollback during 120.4~103.5Ma. It cannot be mineralized because of its small subduction angle and low oxygen fugacity in the magmatic source area. Therefore, it is suggested to look for possible ore-forming targets related to the subduction background between the Duolong ore concentration area and Gaize County, but the influence of structure, surrounding rocks and late preservation should also be considered.

ВУЛКАН БОЛЬШОЙ ПАЯЛПАН (СРЕДИННЫЙ ХРЕБЕТ, КАМЧАТКА). К ПРОБЛЕМЕ КОНВЕРГЕНТНОСТИ «ОСТРОВОДУЖНЫХ» И «ВНУТРИПЛИТНЫХ» ПЕТРОЛОГО-ГЕОХИМИЧЕСКИХ ПРИЗНАКОВ В МАГМАТИЧЕСКОЙ СИСТЕМЕ

The paper presents the data on age, mineralogy, geochemistry, and isotope composition of rocks from the Bolshoi Payalpan Volcano (Sredinny Range, Kamchatka). We compared these data with those on the Nosichan and Belogolovsky volcanoes, located within the Belogolovsky volcanic center. The basalts of the neck and the upper lava complex of Bolshoi Payalpan are compositionally similar to the intraplate-type trachybasalts of the Belogolovsky Volcano, and the basaltic andesites of the lower lava and the cone complex are similar to the island arc rocks of the Nosichan Volcano. Analysis of the data obtained evidences that spatial and temporal manifestations of intraplate and island-arc volcanism at the Bolshoi Payalpan Volcano are not accidental, but may be a consequence of a change in the degree and depth of melting of the same deep source with the involvement of a mantle diapir. The Belogolovsky volcanic center formed in a setting of the Late Miocene-Early Pliocene rifting. Its evolution, right up to its extinction, proceeded in the same geodynamic setting with an increase in depth of the mantle source and a decrease in the degree of its melting. Rock compositions of the Lower-Middle Pliocene Nosichan Volcano remain of the island-arc type under conditions of rifting, since they are associated with the mantle reservoir located at a shallower depth, which has experienced a higher degree of melting. There is good reason for considering large volcanic centers as spontaneously-developing geological entities. As the endogenous activity dies down, the degree of melting decreases and the depth of melting increases with the replacement of island-arc volcanism by intraplate volcanism. The volcanic center becomes extinct.

The multiple mineralizations and geodynamic settings of the Laozuoshan Cu–Au deposit in the Jiamusi Massif, NE China: Zircon U–Pb geochronological, elemental and Hf isotopic geochemical evidence

Situated in the centre of the Jiamusi Block, the Laozuoshan deposit is a large-sized deposit with composite of vein and lenticular Au orebodies and veinlet-disseminated Cu-Au orebodies. It is characterized by two types of mineralization, skarn Au mineralization and porphyry Cu-Au mineralization. The former occurs along the contact zone between granodiorite and Proterozoic metamorphic rocks in the eastern and central ore zones, while the latter is hosted in granodiorite porphyry and diorite in the western ore zone. In this study, we present the results of geochronological and geochemical analyses of these rocks which show the following features. (1) Zircon U–Pb dating results show that the ages of granodiorite, diorite and granodiorite porphyry are 264.6±2.6 Ma, 103.2±1 Ma and 104.6±1.8 Ma, respectively. (2) Whole-rock elemental compositions indicate that the granodiorite shows potassium-rich adakitic rock properties, and the diorite has typical island arc calc-alkaline magmatic properties. They are all characterized by enrichment in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) and depletion in heavy rare earth elements (HREEs) and high field strength elements (HFSEs). (3) The in-situ Hf isotope analysis results show that the granodiorite and diorite have εHf(t) values of −3.7 to −8.7 and −0.4 to 10.7, respectively, corresponding to TDMC ages of 2040–2500 Ma and 630–1650 Ma, respectively. These results indicate two periods of mineralization in the Laozuoshan deposit. Skarn mineralization occurred in the late Permian and was a result of metasomatism between partial melting magma that originated from thickened Paleoproterozoic lower crust and Proterozoic marble. Porphyry mineralization took place in the Early Cretaceous and was a result of the shallow emplacement of mid-Neoproterozoic mantle-derived magma with the addition of crustal materials. In terms of geodynamic settings, skarn Au and porphyry Cu-Au mineralization occurred in a compressional setting formed during the gradual closure of the Paleo-Asian Ocean, and a crustal-thinning setting formed from Eurasian Plate-ward subduction, retreating, and rollback of the Paleo-Pacific Plate, respectively.

Volcanism and intrusive magmatism of the Magnitogorsk paleoarc in the epoch of its “soft” collision with a margin of the East European continent

Research subject. The article sets out to investigate the change of the geodynamic regime from the island-arc type to the accretionary-collisional type in the Late Devonian–Early Carboniferous, which occurred as a result of 1) a collision between the Western part of the Magnitogorsk island arc and the Eastern margin of the East European continent and 2) its later coupling with the heterogeneous composite East Uralian terrain.Materials and methods. The content of petrogenic elements and microelements in the rocks of the Late Paleozoic island-arc complexes of the Magnitogorsk island arc were determined using XRF and ICP MS methods at the Laboratory of Physicochemical Research Methods of the Institute of Geology and Geochemistry, Ural Branch of the Russian Academy of Sciences. In addition, available publications on the composition and formation conditions of these complexes were reviewed.Results. It was found that, in the Late Devonian–Early Carboniferous period, the process of island-arc magmatism of the Magnitogorsk paleoarc was substituted with the formation of intraplate volcano-intrusive complexes. The island-arc magmageneration and its manifestations were controlled by a latitudinal linear zoning and different depths of formation of magmatic cameras, reflecting the self-consistency and spatial isolation of these events.Conclusion. Due to the intensifying collision, melts from different mantle sources were mixing, thus contaminating the island-arc rocks by intraplate (plume-dependent) magmas. According to the composition and concentrations of high-field strength and fluid-mobile chemical elements, suprasubductional fluids played an important role in the evolution of late-island arc magmatic series.

Evolution of geodynamic processes in Neoarchean Kadiri greenstone belt, eastern Dharwar Craton, India: Implications on the migrating arc magmatism

Dharwar Craton of southern peninsular India is one of the significant Archean cratons in the world with well developed greenstone belt sequences representing plume-arc accretionary tectonics. The eastern Dharwar Craton (EDC) is endowed with excellent gold mineralization which is being mined and the greenstone belt lithounits among which majority of them represent the convergent margin processes. The Neoarchean Kadiri greenstone belt of EDC consists of an array of mafic, intermediate and felsic volcanic rocks among which the mafic rocks exhibit a wide range of silica (49.74–65.19 wt.%), high LILE, HFSE including the Nb contents (7−15 ppm) that are classified as Nb-enriched basaltic andesites (NEBA). Moderate composition of SiO2 (57−63 wt.%) along with higher MgO (3.96–7.58 wt.%), Cr (44−890 ppm) and Ni (22−491 ppm) contents distinguish some of the intermediate rocks as Mg-andesites. The petrographic characteristics along with higher silica (63−75 wt.%), Na2O (2.21–4.01 wt.%) and low K2O (0.7–3.35) classify the associated felsic volcanic rocks as sodic adakites. Abundance of K-feldspar phenocrysts and higher K2O content (1.19–7.0 wt.%) in some adakites identify them as potassic adakites. The geochemical characteristics of the calc-alkaline andesite-dacite-rhyolites (ADR) reflect on typical island arc environment involving the processes of fractional crystallization and crustal contamination in an active continental margin environment. The NEBA and Mg-andesites are the products of hybridized mantle wedge under the influence of slab fluids/melts whereas the Na-adakites are the slab melts erupted within the oceanic domain and the K-adakites were emplaced under the influence of continental crust during the migration of the island arc towards the continental margin. Beside the generation of various types of volcanic rocks, these convergent margin processes have played a significant role in the generation and mobilization of gold mineralized fluids, which are being mined from some greenstone belts of EDC.

Stages of Volcanic Activity on the Southeastern Flank of the Sredinny Range (Kamchatka): Age, Geochemistry, and Isotopic Characteristics of Volcanic Rocks of the Akhtang and Kostina Mountain Massifs

Abstract —We report the chemical and isotopic compositions of volcanic rocks of the Akhtang and Kostina mountain massifs in the Sredinny Range, Kamchatka. The analyzed rocks are similar in composition to the earlier studied volcanics of the eastern flank of the southern part of the Sredinny Range. Results of K–Ar isotope dating reveal three stages of volcanic activity in the two massifs. These stages are divided by long (1.4 and 2.4 Ma) periods of quiescence. In the Akhtang massif, the eruptive activity was at 4.9–4.0, 1.9–1.7, and 0.3–0.2 Ma, and in the Mt. Kostina massif, at ~8.0, 5.6–4.9, and ~3.5 Ma. Two early stages of both massifs are characterized by the eruption of island arc type rocks, and the late stage, by the eruption of rocks of hybrid geochemical type. The Mio-Pliocene (N1–N21) rocks of the Mt. Kostina massif are similar in geochemical features to the early Pliocene (N21) rocks of the Akhtang massif, and the late Pliocene (N22) lavas of the former massif are similar to the middle Quaternary (Q2) rocks of the superimposed monogenetic volcanism zone of the latter massif. For the Akhtang massif it has been first discovered that the volcanic reactivation after the long quiescence periods was accompanied by a change in the composition of rocks and in the type of eruptive activity (from the eruption of plateau-effusives rocks to the formation of stratovolcanoes and monogenetic volcanism zones). The obtained data on the age and composition of rocks as well as some morphological features of the studied massifs suggest that the plateau-effusive rocks of the Sredinny Range might be related to central-type eruptions.

Early to Middle Jurassic San Andrés-Cedros plutonic suite, western coast of Baja California, Mexico: Geochemical and isotopic evidence for an island arc extending to the central peninsula

In the Vizcaíno peninsula and Cedros Island region, west-central part of the Baja California peninsula, México, Upper Triassic ophiolitic sequences are intruded by arc-related Mid-Jurassic plutons. We analyze the ~150 Ma old San Roque pluton (SRp) in the Vizcaíno peninsula and plutonic rocks (~165 Ma) of the Punta Norte zone (PN) in the Cedros Island. The modal composition of the almost 10 km2 extending San Roque pluton varies from biotite-hornblende quartz diorite to biotite-hornblende tonalite intruded by granodioritic dykes. Intrusive rocks are subalkaline, calcic/calc-alkaline and metaluminous, akin to island arc granite setting. The primitive initial εNd values of +8.9 and + 9 of SRp quartz diorite and tonalite are indicative of a depleted mantle source without crustal assimilation. The sequence intrudes amphibolites that are interpreted as metavolcanic remnants of the oceanic-type crust of the Vizcaíno peninsula ophiolite. One amphibolite sample is characterized as magmatically poorly evolved, with flat REE patterns, and MORB-type initial εNd values of +10.1. The PN plutonic rocks of Cedros Island crop out at its northern edge. Their modal compositions are hornblende quartz diorite, hornblende leucotonalite, and microtonalitic dykes. These rocks are classified as subalkaline, magnesian, and calcic, typical island arc granites. The REE patterns tend to be subparallel, with progressive enrichments in REE abundances, suggesting fractional crystallization resulting from the evolution of a single magmatic system. Microtonalite dykes and leucotonalite yield initial εNd values of +7.8 to +8, indicating that the PN rocks were produced from magmas associated with a depleted mantle source. These values are in agreement with the correlation of εSr(165 Ma) vs. εNd(165 Ma), where all samples plot in the depleted mantle field. The PN plutonic rocks intruded pillow basalts with basaltic dykes. Their initial εNd and εSr plot in the depleted mantle field indicative of island arc tholeiites.The SRp and PN plutonic rocks are correlated with the coeval El Arco system (~160 Ma) in the central part of the Baja California peninsula. They show the same magmatic evolution in a primitive oceanic island arc environment derived from a depleted mantle with no crustal assimilation. We suggest that all the Jurassic sequences of the southern Peninsular Ranges batholith belong to the same arc.