Gold Mineralization in Izu Peninsula,Central Japan,during Crustal Extension in Response to Double Subduction

Izu Peninsula in central Japan, the northern tip of the Izu‐Bonin arc, hosts numerous epithermal Au–Ag vein deposits of low‐sulfidation style. All have similar vein textures, mineralogy, and alteration. Geochemical data from fluid inclusions in vein quartz, the mineralogy and mineral chemistry of alteration, and stable isotope data indicate that auriferous hydrothermal activity occurred under subaerial conditions. The K–Ar ages of auriferous vein minerals are <1.5 Ma, indicating that the mineralization took place after extensive submarine volcanism for the host rocks. These observations suggest that Au–Ag mineralization was synchronous with the development of an extensional regime of the Izu block after its collision with the Honshu arc after 1.5 Ma. This collision resulted in the shifting of the Izu block far from the trench to the rear position, and the subduction of the Izu block along the Suruga trough to the west and along the Sagami trough to the east. The reararc position of the Izu block and double subduction resulted in crustal extension, upwelling of asthenospheric mantle, and tholeiitic magmatism reflected by mafic dyke swarms and subsequent monogenetic volcanic activity in the Izu peninsula. The timing of the Au mineralization in the Izu Peninsula during the beginning of lithospheric extension is similar to that of the Sado Au–Ag deposit on Sado island in the Japan Sea. Two mineralization events coincide with extensive tholeiitic mafic volcanism and injections of dyke swarms related to the back‐arc opening of the Japan Sea. The geological setting of the Au–Ag mineralization in Izu and Sado is also similar to that of the epithermal Au–Ag deposits in northern Nevada, where mineralization was contemporaneous with crustal extension and tholeiitic mafic magmatism derived from the asthenospheric mantle. This study suggests that epithermal Au mineralization at shallow crustal depths is a product of large‐scale lithospheric evolution.     

岛弧火山岩成因和地球化学特征

岛弧火山岩主要为俯冲带的俯冲板片脱水形成的富大离子亲石元素流体交代地幔楔,并使其发生部分熔融,产生岛弧岩浆作用而形成的,岩石组合通常为玄武岩—安山岩—英安岩—流纹岩及相应侵入岩组合。它以Al2O3、K2O高,低Ti O2,且K2ONa2O为特征,相对富集LILE,亏损HFSE,特别是Ti、Nb、Ta等。本文主要从岛弧岩浆作用的起因着手,分析流体和熔体对地幔楔的交代作用,以及岛弧岩浆作用过程,进而分析岛弧火山岩的地球化学特征。     

初论环准噶尔斑岩铜矿带的地质构造背景与形成机制

准噶尔地区构造-岩浆-成矿带具环准噶尔地块分布的特征,这一格局是准噶尔地区古生代大地构造演化的结果。哈萨克斯坦-准噶尔板块在北侧古亚洲洋与南侧南天山洋的俯冲下不断侧向增生,并形成与岩浆作用伴生的火山岩型铜铁多金属矿带、斑岩铜钼金矿带与浅成低温金矿带。哈萨克斯坦-准噶尔板块与西伯利亚板块和塔里木板块碰撞发生了强烈挤压-剪切变形,并导致准噶尔地块发生逆时针旋转,从而造成构造-岩浆-成矿带发生位移、呈环状分布于准噶尔地块周边。环准噶尔斑岩铜矿形成于俯冲成因的大陆岛弧、大洋岛弧与弧后盆地及后碰撞阶段板内4种构造背景,晚古生代是成矿的高峰时期。     

Genesis of the Ciemas Gold Deposit and Relationship with Epithermal Deposits in West Java, Indonesia: Constraints from Fluid Inclusions and Stable Isotopes

The Ciemas gold deposit is located in West Java of Indonesia,which is a Cenozoic magmatism belt resulting from the Indo-Australian plate subducting under the Eurasian plate.Two different volcanic rock belts and associated epithermal deposits are distributed in West Java:the younger late Miocene-Pliocene magmatic belt generated the Pliocene-Pleistocene epithermal deposits,while the older late Eocene-early Miocene magmatic belt generated the Miocene epithermal deposits.To constrain the physico-chemical conditions and the origin of the ore fluid in Ciemas,a detailed study of ore petrography,fluid inclusions,laser Raman spectroscopy,oxygen-hydrogen isotopes for quartz was conducted.The results show that hydrothermal pyrite and quartz are widespread,hydrothermal alteration is well developed,and that leaching structures such as vuggy rocks and extension structures such as comb quartz are common.Fluid inclusions in quartz are mainly liquid-rich two phase inclusions,with fluid compositions in the NaCl-H20 fluid system,and contain no or little CO_2.Their homogenization temperatures cluster around 240℃-320℃,the salinities lie in the range of 14-17 wt.%NaCl equiv,and the calculated fluid densities are 0.65-1.00 g/cm~3.The values of δ~(18)O_(H2O-VSMOW)for quartz range from +5.5‰ to +7.7‰,the δD_(VSMOW) of fluid inclusions in quartz ranges from-70‰ to-115‰.All of these data indicate that mixing of magmatic fluid with meteoric water resulted in the formation of the Ciemas deposit.A comparison among gold deposits of West Java suggests that Miocene epithermal ore deposits in the southernmost part of West Java were more affected by magmatic fluids and exhibit a higher degree of sulfldation than those of Pliocene-Pleistocene.     

Potash–rich Magmatism and Associated Gold-Copper Mineralization in the Yishu Deep Fault Zone and Its Vicinity, Eastern China

Abstract: The Mesozoic potash‐rich volcanic rocks which hosted several gold or gold (copper) deposits are widely distributed around the Yishu deep fault zone, eastern China. Lithologically, these rocks include basaltic trachyandesite, trachyandesite, latite and trachyte, of which the trachyandesite and latite are the predominant rock types. Whole‐rock Rb‐Sr isochron ages and ⁴⁰Ar‐³⁹Ar plateau dates of them are 108.2 ? 119.6 Ma and 114.7 ? 124.3 Ma, respectively. Chemically, they are characterized by high and variable Al₂O₃ contents, high K₂O+Na₂O values, and high K₂O/Na₂O and Fe₂O₃/FeO ratios. The rocks also have enriched LILE and LREE concentrations, low HFSE abundance, and display extraordinary Sr‐Nd isotope signatures (I_Sr = 0.7084 ? 0.7125, ε_Nd(t) = ‐9.43 ? ?18.07). Integrated geological and geochemical data suggest that they were formed in a continental‐arc setting and most likely originated from the partial melting of enriched mantle which was induced by source contamination of subducted continental crustal materials. Gold (copper) deposits in this district are closely related to Mesozoic volcanic‐subvolcanic magmatism. They are frequently located either at the margin or adjacent to the volcanic basins. Most of them are spatially associated with maar‐diatreme systems and/or flow‐dome complexes. The formations of two gold (copper) deposits, the Qibaoshan breccia pipe‐porphyry type Au‐Cu deposit and the Guilaizhuang tellurium‐gold type epithermal Au deposit, have been proved to be in close relation with potash‐rich magmatism. The genetic relations between potash‐rich magmatism and Cu‐Au mineralization is still quite unclear. Detailed review of the previous works demonstrates that the high contents of volatiles (such as H₂O, CO₂, S, F and Cl, especially F and Cl) and the high oxidation state of the potash‐rich magmas may be the main favorable factors for the formation of the Cu‐Au deposits.     

Porphyry-Type Mineralization at Selogiri Area, Wonogiri Regency, Central Java, Indonesia

The Selogiri area, situated in Wonogiri regency, Central Java, is one of several gold prospecting areas in the Southern areas Mountain Range in Java, Indonesia. Three types of dioritic–andesitic intrusive rocks occur in the Selogiri area, namely, hornblende andesite porphyry, hornblende diorite porphyry and hornblende diorite, exposed in a half‐circular depression where volcanic breccia and tuff are widely distributed. The occurrence of stockwork quartz veinlets and associated with magnetite and malachite coating along the cracks in the diorite porphyry suggests porphyry type mineralization. This is also supported by the occurrence of polyphase hypersaline fluid inclusions in the stockwork veinlet quartz. Small‐scale miners are mining NS‐trending quartz veins for gold associated with base metal sulfides. These veins are probably epithermal‐type mineralization that overprinted porphyry‐type mineralization. The Neogene intermediate to silicic hydrous magmatism in Java could have formed the porphyry‐type mineralization in Selogiri, as in the rest of the Sunda–Banda arc.     

A Detailed Geochemical Study of Island Arc Crust: the Talkeetna Arc Section, South-Central Alaska

The Early to Middle Jurassic Talkeetna Arc section exposed inthe Chugach Mountains of south–central Alaska is 5–18km wide and extends for over 150 km. This accreted island arcincludes exposures of upper mantle to volcanic upper crust.The section comprises six lithological units, in order of decreasingdepth: (1) residual upper mantle harzburgite (with lesser proportionsof dunite); (2) pyroxenite; (3) basal gabbronorite; (4) lowercrustal gabbronorite; (5) mid-crustal plutonic rocks; (6) volcanicrocks. The pyroxenites overlie residual mantle peridotite, withsome interfingering of the two along the contact. The basalgabbronorite overlies pyroxenite, again with some interfingeringof the two units along their contact. Lower crustal gabbronorite(10 km thick) includes abundant rocks with well-developed modallayering. The mid-crustal plutonic rocks include a heterogeneousassemblage of gabbroic rocks, dioritic to tonalitic rocks (30–40%area), and concentrations of mafic dikes and chilled mafic inclusions.The volcanic rocks (7 km thick) range from basalt to rhyolite.Many of the evolved volcanic compositions are a result of fractionalcrystallization processes whose cumulate products are directlyobservable in the lower crustal gabbronorites. For example,Ti and Eu enrichments in lower crustal gabbronorites are mirroredby Ti and Eu depletions in evolved volcanic rocks. In addition,calculated parental liquids from ion microprobe analyses ofclinopyroxene in lower crustal gabbronorites indicate that theclinopyroxenes crystallized in equilibrium with liquids whosecompositions were the same as those of the volcanic rocks. Thecompositional variation of the main series of volcanic and chilledmafic rocks can be modeled through fractionation of observedphase compositions and phase proportions in lower crustal gabbronorite(i.e. cumulates). Primary, mantle-derived melts in the TalkeetnaArc underwent fractionation of pyroxenite at the base of thecrust. Our calculations suggest that more than 25 wt % of theprimary melts crystallized as pyroxenites at the base of thecrust. The discrepancy between the observed proportion of pyroxenites(less than 5% of the arc section) and the proportion requiredby crystal fractionation modeling (more than 25%) may be bestunderstood as the result of gravitational instability, withdense ultramafic cumulates, probably together with dense garnetgranulites, foundering into the underlying mantle during thetime when the Talkeetna Arc was magmatically active, or in theinitial phases of slow cooling (and sub-solidus garnet growth)immediately after the cessation of arc activity. KEY WORDS: island arc crust; layered gabbro; Alaska geology; island arc magmatism; lower crust     

Late Cenozoic evolution of epithermal gold metallogenic provinces in Kyushu, Japan

Kyushu Island, Japan, is located at the junction of the Southwest Japan arc and the Ryukyu arc. There are two major late Cenozoic epithermal gold-silver provinces in Kyushu, which are termed the Northern and Southern provinces. The provinces are characterized by: 1) Pliocene volcanism dominated by calc-alkaline andesite, followed by Quaternary volcanism including extrusion of both calc-alkaline and tholeiitic magmas; 2) formation of extensional grabens; 3) Pliocene to Pleistocene mineralization, which was dominated by abundant low sulfidation (LS) epithermal deposits with a few high sulfidation (HS) examples. The two epithermal gold-silver provinces have evolved differently since about 5 Ma; the Northern province has exhibited diminished hydrothermal activity from the Pliocene to Pleistocene, whereas the Southern province has witnessed increased hydrothermal activity mainly in easterly and northerly directions. Changes of tectonic setting from the Pliocene to Pleistocene account for the variable trends in epithermal gold deposit formation. Westward oblique subduction of the Philippine Sea plate beneath the Southwest Japan arc caused development of the Hohi graben and arc-related volcanism at about 6 Ma. This was associated with widespread LS mineralization in and surrounding the Hohi graben, as is represented by the Bajo and Taio deposits. The subduction of the relatively buoyant Kyushu-Palau ridge during the early Pliocene strengthened the coupling between the slab and overriding Ryukyu arc, leading to polygenetic andesite volcanism with associated HS (Kasuga, Iwato, and Akeshi) and LS (Kushikino) mineral deposits forming in the Southern province. A change of the subduction direction of the Philippine Sea plate, from west to north-northwest in the early Pliocene, increased the orthogonal convergence rate between the Southwest Japan arc and the Philippine Sea plate, resulting in a decrease of volcanic and hydrothermal activity in the Hohi graben of the Northern province. The more northerly subduction of the Philippine Sea plate shifted the locus of the Kyushu-Palau ridge subduction northward, resulting in underplating of the older (85–60 Ma), negatively buoyant Amami basin oceanic slab in the Southern province, rather than continued subduction of the young (27–15 Ma), buoyant Shikoku basin slab. This replacement caused steepening of the slab angle and slab-rollback in the Southern province, which was associated with regional extension, an eastward shift of the Ryukyu volcanic front, and development of the Kagoshima and Shimabara grabens, as well as the Okinawa trough. Rhyolite and basalt volcanism, in addition to andesite volcanism, have occurred since 2 Ma in the area of the Ryukyu back arc; coincident LS mineralization at Hishikari and Ohkuchi was affiliated with the rhyolite volcanism. Another change of the subduction direction of the Philippine Sea plate to the northwest occurred at 2–1 Ma. The forearc sliver of the Southwest Japan arc shifted westward, in association with right-lateral strike-slip faulting along the Median tectonic line, due to the increase of the westward convergence rate. This shift resulted in shortening and cessation of graben development in the Hohi area, restricting the subsequent volcanism and related hydrothermal activity to the central part of the graben.     

中国浅成低温热液金矿床

浅成低温热液金矿床在中国传统上称为陆相火山岩型金矿床,主要发现在中国东部,后来在北疆地区也有新的发现.根据产出的大地构造背景,它们集中分布在3个带,并分属于3个成矿时期.它们包括:(1)新生代台湾东部岛弧带;(2)晚古生代北疆岛弧带;(3)中生代沿中朝克拉通北界的大陆边缘带;(4)中生代中国东南沿海地区的大陆边缘带.绝大多数矿床是低硫化型的,只有3个是高硫化型的,另有1个是与碱性岩系有关的Au-Te型矿床.除了中国最大的金矿床金瓜石矿床外,迄今为止中国大陆上的浅成低温热液金矿床总的来说只有较小的经济重要性.在中国东部发现的浅成低温热液金矿床的总储量,与区内广泛分布的中生代陆相火山岩十分巨大的体积极不相称.较古老的成矿年龄,中国东部的中生代和北疆的晚古生代,是中国大陆浅成低温热液金矿床的一个鲜明的特点.根据中国的成矿条件和保存条件的分析,以及与美国西部和俄国东部的对比,提出了中国浅成低温热液金矿床成矿潜力的一个初步评估.北疆可能有较大的寻找浅成低温热液金矿床的潜在重要性.     

Volatiles in submarine volcanic rocks from the Mariana Island arc and trough

High temperature mass spectrometric analyses of glasses from quenched pillow rims of andesites dredged from 1170 m water depth in the northern portion of the Mariana Island arc indicate substantially less H₂O (~ 1 wt.%) and more CO₂ (~ 0.24 wt.%) than previously reported for volcanic arc rocks. Glass-vapor inclusions within plagioclase phenocrysts from quenched rims have $\text{CO}{}_2\text{H}{}_2\text{O}$ ratios of 1:1. These results are similar to analyses of basaltic samples from the Mariana Trough (a back-arc basin). Generally, F and Cl contents are higher and S lower in the arc rocks compared to the samples from the back-arc basin. These results favor models for the production of island arc magmas which involve melting of the subducted slab, rather than just melting of the overlying mantle wedge because of the high volatile content needed to produce island arc magmas from peridotite (10–15 wt.%). The trough samples, although similar in non-volatile composition to mid-ocean ridge rocks, have much higher H₂O. somewhat higher CO₂ and lower S contents. Either near surface addition of voiatiles has enriched the magmas or H₂O must be a more important component in the generation and evolution of back-arc basin lavas than in the genesis of mid-ocean ridge basalts.     

Role of backarc processes in the origin of across-arc geochemical zoning in volcanics of early evolutionary stages in Kunashir Island

Geochemical data obtained on volcanic rocks produced during the early evolutionary stages in Kunashir Island provide insight into certain important aspect of the evolution of the subduction system. The mafic lavas of all age intervals exhibit clearly pronounced across-arc geochemical zoning, which implies that these rocks were produced in the environment of a subducted oceanic slab. The high Ba/Th and U/Th ratios of basalts from the frontal zones suggest that an important role in magma generation was played by a low-temperature aqueous fluid. The arc lavas of the Early Miocene, Pliocene, and Pliocene-Pleistocene episodes in the evolution of the island arc system provide evidence of the melting of subducted sediments, which testifies, when considered together with the calculated P-T conditions under which the high-Mg basalts were derived, that backarc tectono-magmatic processes affected subduction-related magmatic generation. Active mantle diapirism and volcanic activity in the opening Kurile Basin resulted in the heating of the suprasubduction mantle in the rear zone, the involvement of the upper sedimentary layer of the oceanic slab in the process of melting, and the eventual generation of basaltic magmas with unusual geochemical characteristics.     

Andesites from northeastern Kanaga Island,Aleutians

Kanaga island is located in the central Aleutian island arc. Northeastern Kanaga is a currently active late Tertiary to Recent calc-alkaline volcanic complex. Basaltic andesite to andesite lavas record three episodes (series) of volcanic activity. Series I and Series II lavas are all andesite while Series III lavas are basaltic andesite to andesite. Four Series II andesites contain abundant quenched magmatic inclusions ranging in composition from high-MgO low-alumina basalt to low-MgO highalumina basalt. The spectrum of lava compositions is due primarily to fractional crystallization of a parental low-MgO high-alumina basalt but with variable degrees of crustal contamination and magma mixing. The earliest Series I lavas represent mixing between high-alumina basalt and silicic andesite with maximum SiO₂ contents of 65–67 wt %. Later Series I and all Series II lavas are due to mixing of andesite magmas of similar composition. The maximum SiO₂ content of the pre-mixed andesites magmas is estimated at 60–63 wt %. The youngest lavas (Series III) are all non-mixed and have maximum estimated SiO₂ contents of 59 wt %. The earliest Series I lavas contain a significant crustal component while all later lavas do not. It is concluded that the maximum SiO₂ contents of silicic magmas, the contribution of crustal material to silicic magma generation, and the role of magma mixing all decrease with time. Furthermore, silicic magmas generated by fractional crystallization at this volcanic center have a maximum SiO₂ content of 63 wt %. All of these features have also been documented at the central Aleutian Cold Bay Volcanic Center (Brophy 1987). Based on data from these two centers a model of Aleutian calc-alkaline magma chamber development is proposed. The main features are: (1) a single low pressure magma chamber is continuously supplied by primitive low-alumina basalt; (2) non-primary high-alumina basalt is formed along the chamber margins by selective gravitational settling of olivine and clinopyroxene and retention of plagioclase; (3) sidewall crystallization accompanied by crustal melting produces buoyant silicic (>63 wt % SiO₂) liquids that pond at the top of the chamber, and; (4) continued sidewall crystallization, now isolated from the chamber wall, produces silicic liquids with 63 wt % SiO₂ that increase the thickness and lowers the overall SiO₂ content of the upper silicic zone. It is suggested that the maximum SiO₂ content of 63% imposed on fractionation-generated magmas is due to a rheological barrier that prohibits the extraction of more silicic liquids from a crystal-liquid mush along the chamber wall.     

Variation of Cl and SO3 Contents of Microphenocrystic Apatite in Intermediate to Silicic Igneous Rocks of Cenozoic Japanese Island Arcs: Implications for Porphyry Cu Metallogenesis in the Western Pacific Island Arcs

Abstract. Determinations of SO₃ and Cl contents of igneous accessory apatite were carried out on Late Cenozoic intermediate to silicic intrusive and volcanic rocks in the Japanese island arcs of the western Pacific rim including the southwestern Kuril arc (eastern Hokkaido), Northeast Japan arc (southwestern Hokkaido through northeastern Honshu to central Honshu), Izu‐Bonin arc, Kyushu‐Palau ridge, Southwest Japan arc (northern Kyushu) and northern Ryukyu arc (southern Kyushu). These were compared to those from the Western Luzon arc, Philippines, to better understand the metallogenesis of porphyry Cu deposits in the western Pacific island arcs. In addition, SO₃ and Cl contents of accessory apatite in the Cretaceous magnetite‐series granitic rocks in the Kitakami belt (northeastern Honshu) and the Miocene ilmenite‐series granitic rocks in the Outer Zone of Southwest Japan (southern Kyushu) were also examined. Microphenocrystic apatites in shallow intrusions associated with porphyry Cu deposits in the Western Luzon arc contain >0.1 wt% S as SO₃. Such high SO₃ contents of microphenocrystic apatite are a common characteristic of hydrous mag‐matism in the Western Luzon arc, from 15 Ma old tonalitic plutonic rocks of the Luzon Central Cordillera to present‐day volcanism at Mount Pinatubo. The accessory apatite in intrusive rocks associated with porphyry Cu deposits, especially those at the Santo Tomas II deposit, show significantly high Cl contents (>2 wt%). The SO₃ contents of microphenocrystic apatite in most of the hydrous silicic rocks along the volcanic front, in andesites related to native sulfur deposits, and in Miocene and younger shallow granitic intrusions in northeastern Honshu, are generally <0.1 wt%. On the other hand, the SO₃ contents of apatite in such rocks from eastern Hokkaido, southwestern Hokkaido, Izu, northern Kyushu and southern Kyushu are similar to those from the Western Luzon arc. The SO₃ contents of accessory apatite in the Cretaceous magnetite‐series granitic rocks in the Kitakami belt are variable, whereas those of the Miocene ilmenite‐series granitic rocks in southern Kyushu are extremely low. The Cl contents of accessory apatite in some rocks of the Northeast Japan arc, Izu‐Bonin arc and Southwest Japan arc are significantly high. In terms of the Cl and SO₃ contents of microphenocrystic apatite, Cenozoic Japanese arc magmatism show similarities with arc magmatism associated elsewhere with porphyry Cu mineralization, except for the most of northeastern Honshu of the Northeast Japan arc. Apatite commonly occurs as inclusions in other phenocrystic phases. Thus the variation in SO₃ contents of apatite is a feature of early stage magmatic differentiation. The SO₃ contents of microphenocrystic apatite are considered to reflect the redox state of the magma source region or fluids encountered during magma generation.     

Lateral flow of African mantle below the nearby Tyrrhenian plate: geochemical evidence

The Quaternary magmatism of the Southern Tyrrhenian basin represents a rare example of an active volcanic arc system where ocean island basalt (OIB) and island arc basalt (IAB) magmas coexist. Although there is general agreement on the importance of the Ionian oceanic lithosphere subduction in the genesis of the IAB magmatism, the tectono‐magmatic processes producing the coexisting OIB magmas are still poorly understood. Here we show that the geochemistry of the Quaternary Southern Tyrrhenian OIB‐type magmatism (i.e. Ustica island and Prometeo, a previously unknown submarine lava field) is very similar to that of OIB‐type volcanoes situated on the nearby African plate (i.e. Etna and Hyblean Plateau). Among the possible geodynamic scenarios proposed to explain the coexistence of OIB and IAB magmas in arc settings, we consider the development of a tear at the edge of the Ionian plate as the more likely mechanism to favour the flow of African asthenospheric mantle below the Tyrrhenian plate.     

Zircon U-Pb Geochronology and Geochemical Characteristics of the Volcanic Host Rocks from the Tongyu VHMS Copper Deposit in the Western North Qinling Orogen and Their Geological Significance

Precise in situ zircon U-Pb dating and Lu–Hf isotopic measurement using an LA-ICP-MS system, whole-rock major and trace element geochemistry and Sr–Nd isotope geochemistry were conducted on the volcanic host rocks of the Tongyu copper deposit on the basis of further understanding of its geological characteristics. Three zircon samples from the volcanic host rocks yielded 206Pb/238 U weighted average ages ranging from 436±4 Ma to 440±5 Ma, which are statistically indistinguishable and coeval with the ca. 440 Ma northward subduction event of the Paleo-Qinling oceanic slab. The volcanic host rocks were products of magmatic differentiation that evolved from basalt to andesite to dacite to rhyolite, forming an integrated tholeiitic island arc volcanic rock suite. The primitive mantle-normalized trace element patterns for most samples show characteristics of island arc volcanic rocks, such as relative enrichment of LILE(e.g. Th, U, Pb and La) and depletion of HFSE(e.g. Nb, Ta, Ti, Zr and Hf). Discrimination diagrams of Ta/Yb vs Th/Yb, Ta vs Th, Yb vs Th/Ta, Ta/Hf vs Th/Hf, Hf/3 vs Th vs Nb/16, La vs La/Nb and Nb vs Nb/Th all suggest that both the volcanic host rocks from the Tongyu copper deposit and the volcanic rocks from the regional Xieyuguan Group were formed in an island arc environment related to subduction of an oceanic slab. Values of ISr(0.703457 to 0.708218) and εNd(t)(-2 to 5.8) indicate that the source materials of volcanic rocks from the Tongyu copper deposit and the Xieyuguan Group originated from the metasomatised mantle wedge with possible crustal material assimilation. Most of the volcanic rock samples show good agreement with the values of typical island arc volcanic rocks in the ISr-εNd(t) diagram. The involvement of crustal-derived material in the magma of the volcanic rocks from the Tongyu copper deposit was also reflected in the zircon εHf(t) values, which range from-3.08 to 10.7, and the existence of inherited ancient xenocrystic zircon cores(2616±39 Ma and 1297±22 Ma). The mineralization of the Tongyu copper deposit shows syn-volcanic characteristics such as layered orebodies interbedded with the volcanic rock strata, thus, the zircon U-Pb age of the volcanic host rocks can approximately represent the mineralization age of the Tongyu copper deposit. Both the Meigou pluton and the volcanic host rocks were formed during the ca. 440 Ma northward subduction of the Paleo-Qinling Ocean when high oxygen fugacity aqueous hydrothermal fluid released by dehydration of the slab and the overlying sediments fluxed into the mantle wedge, triggered partial melting of the mantle wedge, and activated and extracted Cu and other ore-forming elements. The magma and ore-bearing fluid upwelled and erupted, and consequently formed the island arc volcanic rock suite and the Tongyu VHMS-type copper deposit.     

新疆西天山吐拉苏火山岩盆地浅成低温热液-斑岩型金多金属成矿系统的形成与演化

新疆西天山吐拉苏地区发育的与中酸性火山-次火山岩有关的浅成低温热液-斑岩型金多金属成矿系统,是在晚古生代北天山洋向南部伊犁-中天山板块之下俯冲消减的活动大陆边缘背景下形成的。赋矿的大哈拉军山组火山岩及相关的次火山岩形成于晚泥盆世-早石炭世,岩石总体显示钾质-高钾质、准铝质-过铝质的钙碱性-高钾钙碱性特征,其轻稀土富集、Eu负异常显著、大离子亲石元素富集和高场强元素亏损等,均显示出俯冲带岛弧岩浆作用的特点。阿希(低硫型)和京希-伊尔曼得(高硫型)浅成低温热液金矿床以及塔北、吐拉苏铅锌矿床,受大哈拉军山组火山岩中的断裂破碎带以及具高孔隙度和渗透率的岩性控制;塔吾尔别克斑岩型金矿化主要受斑岩体及火山岩中的断裂和裂隙系统控制,并很可能存在浅成低温热液型金矿化的套合或叠加。硫、铅、碳、氧同位素特征显示,成矿物质主要来自岩浆所分泌的热液和/或赋矿的火山-次火山岩。根据成矿系统形成后的保存和变化情况,认为在吐拉苏盆地内剥蚀程度较低的地区,浅成低温热液型金铅锌矿床具备良好的保存条件,同时在其深部还应注意寻找斑岩型或矽卡岩型铜金矿床。     

Evolution of an intra‐oceanic island arc during the Late Silurian to Late Devonian,New England Fold Belt

Geochemical studies of volcanic rocks in the Gamilaroi terrane and Calliope Volcanic Assemblage, New England Fold Belt, eastern Australia, indicate that the setting in which these rocks formed changed in both space and time. The Upper Silurian to Middle Devonian basalts of the Gamilaroi terrane show flat to slightly light rare‐earth element (LREE) depleted chondrite normalised patterns, depletion of high field strength elements (HFSE) relative to N‐MORB, low Ti/V and high Ti/Zr ratios, high Ni, Cr and large‐ion lithophile element (LILE) contents, features characteristic of intra‐oceanic island arc basaltic magmas. They are associated with low‐K, less mafic volcanics, showing moderate LREE enrichment, low Nb and Y contents and Rb/Zr ratios. The depletion of HFSE in the basalts indicates that the magmas were derived from a refractory source in a supra‐subduction zone setting. The presence of such a zone implies that the arc was associated with a backarc basin, the location of which was to the west where a wide backarc region existed from the Middle Silurian. This polarity of arc and backarc basin suggests that the subduction zone dipped to the west. In contrast to their older counterparts, Middle to Upper Devonian basalts of the Gamilaroi terrane have MORB‐like chondrite normalised patterns and higher Ti and lower LILE contents. Moreover, they have low Ti/Zr ratios and MORB‐like Ti/V ratios and HFSE contents, features typical of backarc basins. Dolerites of the Gamilaroi terrane also have predominantly backarc basin signatures. These features suggest that both the basalts and dolerites have been emplaced in an extensional environment produced during the rifting of the intra‐oceanic island arc lithosphere. A progressive increase in Ti/V ratios, and TiO₂ and Fe₂O₃ contents at constant MgO, of stratigraphically equivalent basalts, towards the north‐northwest part of the belt, is consistent with either greater extension to the north or melting of a more fertile magma source. By contrast, basalts in the southeast part of the terrane have moderately high Ti/Zr and low Ti/V ratios and in some samples, exhibit depletion of HFSE, compositional features transitional between island arc and backarc basin basalts. The Lower to Middle Devonian mafic rocks in the Calliope Volcanic Assemblage show both LREE enriched and depleted chondrite normalised REE patterns. Further, the majority have high Ti/Zr ratios and low Zr contents as well as relatively high Th contents relative to MORB. These features are common to rocks of Middle Devonian age as well as those of Early Devonian age and are suggestive of eruption in an arc setting. Thus, the data from this study provide new evidence for the evolution of the New England Fold Belt from the Late Silurian to the Late Devonian and reveal a history more complicated than previously reported.     

Role of back-arc tectonics in the origin of subduction magmas: new Sr,Nd, and Pb isotope data from Middle Miocene lavas of Kunashir Island (Kurile Island Arc)

Sr, Nd, and Pb isotope data for basaltic rocks of different ages from Kunashir Island (southern Kurile island arc) provide clues to investigate the subduction magmatic history. Signatures of a high-temperature slab component (melt and/or supercritical liquid produced by melting of slab sediments) involved in Early Miocene–Pleistocene back-arc basaltic magmatism indicate a relatively hot (> 800 °C) slab surface. Depleted isotope characteristics of Holocene basaltic lavas in both volcanic front and back arc indicate their origin with the participation of a cold aqueous fluid produced by dehydration of altered oceanic crust of the Pacific MORB type. The difference in geological, geochemical and isotope patterns in the Pleistocene and the Holocene lavas may be a response to stress change from extension to compression in the Kurile back-arc basin and the Kurile arc.     

The Alaska Wrangell Arc: ~30 Ma of subduction‐related magmatism along a still active arc‐transform junction

The Oligocene to Present Wrangell Volcanic Belt (WVB) extends for ~500 km across south‐central Alaska (USA) into Canada at a volcanic arc‐transform junction. Previously, geochemistry documented mantle wedge and slab‐edge melting in <12 Ma WVB volcanic rocks; new geochemistry shows that the same processes characterized ~18–30 Ma WVB magmatism in Alaska. New ⁴⁰Ar/³⁹Ar ages demonstrate that WVB magmatism in Alaska initiated at ~30 Ma due to flat‐slab subduction of the Yakutat microplate and that the dextral Totschunda fault was active at this time. Our results, together with prior studies, show that Alaskan WVB magmatism occurred chiefly due to subduction and should be considered a volcanic arc (e.g. the Wrangell Arc). The WVB provides a long‐term geological record of subduction, strike‐slip and magmatism. Slab‐edge upwelling, flat‐slab defocused fluid‐flux and faults acting as magma conduits are likely responsible for the exceptionally large volcanoes and high eruption rates of the Wrangell Arc.