Subduction-channel model of prism accretion,melange formation,sediment subduction,and subduction erosion at convergent plate margins: 2. Implications and discussion

The subduction-channel model predicts five geologically and geophysically distinctive types of convergent plate margin. They differ primarily in the proportions of incoming sediment and upflowing melange offscraped and underplated near the toe of the overriding block. The model predicts complex patterns of uplift or subsidence of the forearc region due to differential underplating or subduction erosion and to initiation or cessation of melange upflow. It shows how subduction speed, sediment input, and pressure gradient along the channel govern the general distribution and magnitude of subduction-zone earthquakes. It can predict the upward trajectories of exotic blueschist or serpentinized peridotite blocks that become entrained in the upwelling melange; and it suggests mechanisms by which much larger tracts of coherent blueschist can be raised to shallower levels. It predicts characteristic structural and metamorphic histories for geologic units accreted during steady-state subduction, showing for example, that offscraped materials undergo subhorizontally-directed compression during final dewatering and accretion, whereas underplated ones undergo large simple shear. It gives the maximum depths reached by upwelled melange, which bear significantly on the metamorphic changes observed in them. It predicts large-scale melange diapirism in certain rare cases, in which the normally adverse pressure gradient is reversed in a short reach of the channel, as at Barbados Island. Finally, it explains why pelagic sediments are seldom found in accretionary complexes and gives insight into the factors controlling what portion of the incoming sediment, and the¹⁰Be and other tracers it carries, will reach the region of volcanic-arc magmagenesis.     

内蒙锡盟新生代玄武岩的岩石学研究

依照地层顺序和火山机构的完好程度,内蒙锡盟新生代玄武岩可粗略划分为三期。各期玄武岩又可分出若干层(次)。玄武岩由含量不等的橄榄石、单斜辉石和斜长石斑晶及基质组成。部分玄武岩含有二辉橄榄岩等深源包体。斑晶矿物均具有成分环带。矿物化学、岩石化学资料揭示出本区玄武岩从原生岩浆到进化岩浆的主要演化机制是以橄榄石为主的分离结晶作用,同时在部分进化岩浆中还存在岩浆混合作用。稀土元素资料还指出了一个形成本区原生岩浆以及派生岩浆的原生母岩浆的重要机制:上地幔渐进的部分熔融作用。     

The Liquid Line of Descent of Anhydrous, Mantle-Derived, Tholeiitic Liquids by Fractional and Equilibrium Crystallization--an Experimental Study at 1{middle dot}0 GPa

Two series of anhydrous experiments have been performed in anend-loaded piston cylinder apparatus on a primitive, mantle-derivedtholeiitic basalt at 1·0 GPa pressure and temperaturesin the range 1060–1330°C. The experimental data provideconstraints on phase equilibria, and solid and liquid compositionsalong the liquid line of descent of primary basaltic magmasdifferentiating in storage reservoirs located at the base ofthe continental crust. The first series are equilibrium crystallizationexperiments on a single basaltic bulk composition; the secondseries are fractionation experiments where near-perfect fractionalcrystallization was approached in a stepwise manner using 30°Ctemperature steps and starting compositions corresponding tothe liquid composition of the previous, higher-temperature glasscomposition. Liquids in the fractional crystallization experimentsevolve with progressive SiO₂ increase from basalts to dacites,whereas the liquids in the equilibrium crystallization experimentsremain basaltic and display only a moderate SiO₂ increase accompaniedby more pronounced Al₂O₃ enrichment. The principal phase equilibriacontrols responsible for these contrasting trends are suppressionof the peritectic olivine + liquid = opx reaction and earlierplagioclase saturation in the fractionation experiments comparedwith the equilibrium experiments. Both crystallization processeslead to the formation of large volumes of ultramafic cumulatesrelated to the suppression of plagioclase crystallization relativeto pyroxenes at high pressures. This is in contrast to low-pressurefractionation of tholeiitic liquids, where early plagioclasesaturation leads to the production of troctolites followed by(olivine-) gabbros at an early stage of differentiation. KEY WORDS: liquid line of descent; tholeiitic magmas; equilibrium crystallization; fractional crystallization     

Petrogenesis of the Swaziland and Northern Natal Rhyolites of the Lebombo Rifted Volcanic Margin, South East Africa

The Jozini and Mbuluzi rhyolites and Oribi Beds of the southernLebombo Monocline, southeastern Africa, have geochemical characteristicsthat indicate they were derived by partial melting of a mixtureof high-Ti/Zr and low-Ti/Zr Sabie River Basalt Formation types.Compositional variations within the different rhyolite typescan largely be explained by subsequent fractional crystallization.The Sr- and Nd-isotope composition of the rhyolites is uniqueamongst Gondwana silicic large igneous provinces, having _Ndvalues close to Bulk Earth (–0·94 to 0·35)and low, but more variable, initial ⁸⁷Sr/⁸⁶Sr ratios (0·7034–0·7080).Quartz phenocryst ¹⁸O values indicate that the rhyolite magmashad ¹⁸O values between 5·3 and 6·7, consistentwith derivation from a basaltic protolith with ¹⁸O values between4·8 and 6·2. The low-¹⁸O rhyolites (< 6·0)come from the same stratigraphic horizon and are overlain andunderlain by rhyolites with more ‘normal’ ¹⁸O magmavalues. These low-¹⁸O rhyolites cannot have been produced byfractional crystallization or partial melting of mantle-derivedbasaltic material. The rhyolites have low water contents, makingit unlikely that the low ¹⁸O values are the result of post-emplacementalteration. Modification of the source by fluid–rock interactionat elevated temperatures is the most plausible mechanism forlowering the ¹⁸O magma value. It is proposed that the low-¹⁸Orhyolites were derived by melting of earlier altered rhyolitein calderas situated to the east, which were not preserved afterGondwana break-up. KEY WORDS: rhyolite; Lebombo; stable and radiogenic isotopes; low-¹⁸O magmas; partial melting     

Erratum to Compositional variations and magma mixing in the 1991 eruptions of Hudson volcano,Chile

The August 1991 eruptions of Hudson volcano produced ~2.7 km³ (dense rock equivalent, DRE) of basaltic to trachyandesitic pyroclastic deposits, making it one of the largest historical eruptions in South America. Phase 1 of the eruption (P1, April 8) involved both lava flows and a phreatomagmatic eruption from a fissure located in the NW corner of the caldera. The paroxysmal phase (P2) began several days later (April 12) with a Plinian-style eruption from a different vent 4 km to the south-southeast. Tephra from the 1991 eruption ranges in composition from basalt (phase 1) to trachyandesite (phase 2), with a distinct gap between the two erupted phases from 54–60 wt% SiO₂. A trend of decreasing SiO₂ is evident from the earliest part of the phase 2 eruption (unit A, 63–65 wt% SiO₂) to the end (unit D, 60–63 wt% SiO₂). Melt inclusion data and textures suggest that mixing occurred in magmas from both eruptive phases. The basaltic and trachyandesitic magmas can be genetically related through both magma mixing and fractional crystallization processes. A combination of observed phase assemblages, inferred water content, crystallinity, and geothermometry estimates suggest pre-eruptive storage of the phase 2 trachyandesite at pressures between ~50–100 megapascal (MPa) at 972 ± 26°C under water-saturated conditions (log fO₂ –10.33 (±0.2)). It is proposed that rising P1 basaltic magma intersected the lower part of the P2 magma storage region between 2 and 3 km depth. Subsequent mixing between the two magmas preferentially hybridized the lower part of the chamber. Basaltic magma continued advancing towards the surface as a dyke to eventually be erupted in the northwestern part of the Hudson caldera. The presence of tachylite in the P1 products suggests that some of the magma was stalled close to the surface (<0.5 km) prior to eruption. Seismicity related to magma movement and the P1 eruption, combined with chamber overpressure associated with basalt injection, may have created a pathway to the surface for the trachyandesite magma and subsequent P2 eruption at a different vent 4 km to the south-southeast. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The plutonic igneous complex at Sorel Point,Jersey, Channel Islands: a high level multi-magma assemblage

Sorel Point lies on the north coast of Jersey, Channel Islands, and comprises a variety of lithologies within the compositional spectrum gabbro–diorite–granodiorite–granite. The rocks of the complex can be separated into four groups, each representing a separate intrusive episode. Each group consists of at least two lithologies which were present as coexisting magmas. The groups are separated by the time required for the preceding intrusive episode to crystallize completely. The rocks thus display a variety of contact relationships. Certain of the rocks present were produced by physical and chemical interactions between coexisting magmas. The rocks have calc-alkaline chemistry and were formed at high crustal levels during the latter stages of the Cadomian orogeny. The Sorel Point complex demonstrates the availability of a wide variety of magmas in a relatively condensed space and time and allows an insight into the formation of a constantly evolving subvolcanic magma chamber. © 1998 John Wiley & Sons, Ltd.     

关于岩浆上升速度计算的新探索

本文应用流体力学中L.Rayleigh分析法和Buckingham的π定律来估算岩浆活动中一个重要的动力参数——岩浆上升速度,以期避开岩浆究竟属牛顿流体还是非牛顿流体这一尚存争议的问题。作为初步尝试,此法在研究岩浆活动的各种动力参数时是值得重视的。     

The Sora porphyry Cu-Mo deposit (Kuznetsk Alatau): magmatism and effect of mantle plume on the development of ore-magmatic system

Several complexes are recognized within the Sora porphyry Cu-Mo deposit: plutonogenic, porphyry (ore-bearing), and dike. They formed since Ordovician till Devonian at the collision, postcollisional, and rift stages of the regional evolution, respectively. Magmatism was manifested at the deposit synchronously with intraplate magmatism, which was widespread within Kuznetsk Alatau and was initiated by the Altai–Sayan mantle plume. In structural position and geochemical characteristics the dike complex is similar to the intraplate complexes in adjacent regions. It formed after the development of the Sora ore-magmatic system including the plutonogenic and porphyry complexes with similar geochemistry and metallogeny. According to the models for the relationship of mantle plumes with ore-magmatic systems, the plutonogenic and porphyry complexes of the Sora deposit developed at the stage of the thermal plume effect on lithosphere, which caused its melting and, as a result, calc-alkalic magmatism. A change of the collision and postcollisional geodynamic regime by the rift one favored the ascent of plume melts, which then participated in the formation of intraplate structures, in particular, the dike complex of the Sora deposit.     

The Ore-forming Mechanism of the Jiajika Pegmatite-Type Rare Metal Deposit in Western Sichuan Province:Evidence from Isotope Dating

Granitic pegmatites are commonly thought to form by fractional crystallization or by liquid immiscibility of granitic magma;however,these proposals are based mainly on analyses of fluid and melt inclusions.Here,we use the Jiajika pegmatite deposit,the largest spodumene deposit in Asia,as a case study to investigate ore forming processes using isotope dating.Dating of a single granite sample from the Jiajika deposit using multiple methods gave a zircon U-Pb SHRIMP age of 208.4±3.9 Ma, an ⁴⁰Ar/³⁹Ar age for muscovite of 182.9±1.7 Ma,and an ⁴⁰Ar/³⁹Ar age for biotite of 169.9±1.6 Ma. Based on these dating results and the ⁴⁰Ar/³⁹Ar age of muscovite from the Jiajika pegmatite,a temperature-time cooling track for the Jiajika granite was constructed using closure temperatures of the different isotope systems.This track indicates that the granite cooled over～40 m.y.,with segregation of the pegmatite fluid from the granitic magma at a temperature of～700℃.This result suggests that the Jiajika pegmatite formed not by fractional crystallization,but by segregation of an immiscible liquid from the granitic magma.When compared with fractional crystallization,the relatively early timing of segregation of an immiscible liquid from a granitic magma can prevent the precipitation of ore-forming elements during crystallization,and suggests that liquid immiscibility could be an important ore-forming process for rare metal pegmatities.We also conclude that isotope dating is a method that can potentially be used to determine the dominant ore-forming processes that occurred during the formation of granite-related ore deposits,and suggest that this method can be employed to determine the formation history of the W-Sn ore deposits found elsewhere within the Nanling Metallogenic Belt.     

Modeling of subduction components in the Genesis of the Meso-Cenozoic igneous rocks from the South Shetland Arc, Antarctica

Isotope data and trace elements concentrations are presented for volcanic and plutonic rocks from the Livingston, Greenwich, Robert, King George and Ardley islands (South Shetland arc, Antarctica). These islands were formed during subduction of the Phoenix Plate under the Antarctica Plate from Cretaceous to Tertiary. Isotopically (⁸⁷Sr/⁸⁶Sr)_o ratios vary from 0.7033 to 0.7046 and (¹⁴³Nd/¹⁴⁴Nd)_o ratios from 0.5127 to 0.5129. εNd values vary from +2.71 to +7.30 that indicate asthenospheric mantle source for the analysed samples. ²⁰⁸Pb/²⁰⁴Pb ratios vary from 38.12 to 38.70, ²⁰⁷Pb/²⁰⁴Pb ratios are between 15.49 and 15.68, and ²⁰⁶Pb/²⁰⁴Pb from 18.28 to 18.81. The South Shetland rocks are thought to be derived from a depleted MORB mantle source (DMM) modified by mixtures of two enriched mantle components such as slab-derived melts and/or fluids and small fractions of oceanic sediment (EM I and EM II). The isotopic compositions of the subduction component can be explained by mixing between at least 4 wt.% of sediment and 96 wt.% of melts and/or fluids derived from altered MORB.