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1.
Woodlark Island (Muyuw) is located in a tectonically complex region, one of the few places on Earth where continental breakup is occurring ahead of seafloor spreading. Rifting commenced in the late Miocene (8.8–6 Ma) and is associated with the westward-propagating Woodlark Basin Spreading Centre. The island comprises approximately 850 km2 of raised Pleistocene coral reef and associated sediments with a central, moderately elevated range underlain by the middle Miocene calc-alkaline to shoshonitic Okiduse Volcanic Group (new name). It provides an exposure of upper Cenozoic geology in close proximity to the spreading centre. The Okiduse Volcanic Group is host to most of the island's historical gold and silver production and recently defined mineral resources totalling 1.75 Moz gold. This study uses facies analysis of pyroclastic deposits to develop a detailed geological map of the Okiduse Volcanic Group, with a revision and reinterpretation of the unit. Facies associations suggest that two major volcanic centres erupted synchronously during the middle Miocene (14–12 Ma), referred to as the Watou Mountain Eruptive Centre (new name) and the Uvarakoi Caldera (new name). The mafic–intermediate Watou Mountain Eruptive Centre formed during frequent small eruptions of widely varying style. Strombolian, subplinian, vulcanian and dome-related explosive eruptions occurred, alternating with extrusion of block and ash flow deposits and lava domes. Pyroclastic deposits were rapidly reworked from the steep cone, and were redeposited in a series of coalescing aprons surrounding the volcano. The felsic Uvarakoi Caldera formed during a series of violent explosive eruptions by rapid removal of magma from the underlying chamber, followed by collapse. Plinian and possibly phreatoplinian eruptions, as a result of magma–water mixing in the surface environment, resulted in widely dispersed, highly fragmented tuff deposits. The caldera was modified by widespread erosion following eruptions, resulting in fluvial, laharic and slope-wash deposits. This study highlights lithological controls (porosity and permeability) by various units within the Okiduse Volcanic Group on ore deposition.  相似文献   

2.
The 3.7 ka year-old Averno 2 eruption is one of the rare eruptions to have occurred in the northwest sector of the Phlegraean Fields caldera (PFc) over the past 5 ka. We focus here on the fallout deposits of the pyroclastic succession emplaced during this eruption. We present major and trace element data on the bulk pumices, along with major and volatile element data on clinopyroxene-hosted melt inclusions, in order to assess the conditions of storage, ascent, and eruption of the feeding trachytic magma. Crystal fractionation accounts for the evolution from trachyte to alkali-trachyte magmas; these were intimately mingled (at the micrometer scale) during the climactic phase of the eruption. The Averno 2 alkali trachyte represents one of the most evolved magmas erupted within the Phlegraean Fields area and belongs to the series of differentiated trachytic magmas erupted at different locations 5 ka ago. Melt inclusions record significant variations in H2O (from 0.4 to 5 wt%), S (from 0.01 to 0.06 wt%), Cl (from 0.75 up to 1 wt%), and F (from 0.20 to >0.50 wt%) during both magma crystallization and degassing. Unlike the eruptions occurring in the central part of the PFc, deep-derived input(s) of gas and/or magma are not required to explain the composition of melt inclusions and the mineralogy of Averno 2 pumices. Compositional data on bulk pumices, glassy matrices, and melt inclusions suggest that the Averno 2 eruption mainly resulted from successive extrusions of independent magma batches probably emplaced at depths of 2–4 km along regional fractures bordering the Neapolitan Yellow Tuff caldera.  相似文献   

3.
The Recent stratigraphy of Sao Miguel records large numbers of trachytic pyroclastic deposits produced by sub-plinian to plinian eruptions. Tephrochronological studies by Walker and Croasdale (1971) and Booth et al. (1978) have shown that in the last 5,000 years there have been five such eruptions from the caldera of Agua de Pau, one of the three active stratovolcanoes on Sao Miguel.A geochemical and electron microprobe study made on the resultant pyroclastic succession, revealed significant variations in pumice clast chemistry and mineralogy between the individual deposits; most of these variations show temporal control. For example, Sr and Eu/Eu{sr*} decrease in value up through the succession, whereas incompatible elements such as La, Zr and Nb show stepwise enrichment, attaining highest concentrations in the most recent deposit. It is proposed that the five air fall pumice deposits represent successive samples of an evolving trachytic magma body in which fractionation of alkali feldspar has largely controlled the liquid line of descent. This crystal fractionation had resulted in the development of peralkalinity in the melt by the time Fogo D, the second youngest deposit, was erupted.The presence of some mineralogical and chemical peculiarities suggest that the trachytic melt has been periodically contaminated by less evolved magmas.  相似文献   

4.
The Bandas del Sur Formation preserves a Quaternary extra-calderarecord of central phonolitic explosive volcanism of the LasCañadas volcano at Tenerife. Volcanic rocks are bimodalin composition, being predominantly phonolitic pyroclastic deposits,several eruptions of which resulted in summit caldera collapse,alkali basaltic lavas erupted from many fissures around theflanks. For the pyroclastic deposits, there is a broad rangeof pumice glass compositions from phonotephrite to phonolite.The phonolite pyroclastic deposits are also characterized bya diverse, 7–8-phase phenocryst assemblage (alkali feldspar+ biotite + sodian diopside + titanomagnetite + ilmenite + nosean–haüyne+ titanite + apatite) with alkali feldspar dominant, in contrastto interbedded phonolite lavas that typically have lower phenocrystcontents and lack hydrous phases. Petrological and geochemicaldata are consistent with fractional crystallization (involvingthe observed phenocryst assemblages) as the dominant processin the development of phonolite magmas. New stratigraphicallyconstrained data indicate that petrological and geochemicaldifferences exist between pyroclastic deposits of the last twoexplosive cycles of phonolitic volcanism. Cycle 2 (0·85–0·57Ma) pyroclastic fall deposits commonly show a cryptic compositionalzonation indicating that several eruptions tapped chemically,and probably thermally stratified magma systems. Evidence formagma mixing is most widespread in the pyroclastic depositsof Cycle 3 (0·37–0·17 Ma), which includesthe presence of reversely and normally zoned phenocrysts, quenchedmafic glass blebs in pumice, banded pumice, and bimodal to polymodalphenocryst compositional populations. Syn-eruptive mixing eventsinvolved mostly phonolite and tephriphonolite magmas, whereasa pre-eruptive mixing event involving basaltic magma is recordedin several banded pumice-bearing ignimbrites of Cycle 3. Theperiodic addition and mixing of basaltic magma ultimately mayhave triggered several eruptions. Recharge and underplatingby basaltic magma is interpreted to have elevated sulphur contents(occurring as an exsolved gas phase) in the capping phonoliticmagma reservoir. This promoted nosean–haüyne crystallizationover nepheline, elevated SO3 contents in apatite, and possiblyresulted in large, climatologically important SO2 emissions. KEY WORDS: Tenerife; phonolite; crystal fractionation; magma mixing; sulphur-rich explosive eruptions  相似文献   

5.
深源岩浆作用与江西德兴大型矿集区成矿关系   总被引：6，自引：0，他引：6  
德兴大型矿集区范围约长２０ｋｍ,宽１０ｋｍ,已发现３个大矿田１３个矿床和许多矿点。赣东北深断裂带控制了区域构造演化,岩浆活动和成矿作用。德兴矿集区的形成是构造－岩浆－成矿统一的地质作用的结果。深源岩浆作用对大规模多金属成矿有决定性的影响。中－新元古代海相火山喷发营造了双桥山群成矿建造— 矿源层。中生代I型花岗岩浆活动对大规模多金属成矿的制约主要有5个方面：(1)供给Cu、Pb、Zn、Au、Ag等成矿金属元素;(2)产生成矿热流体;(3)提供成矿热驱动力;(4)营造成矿空间;(5)激活围岩“矿源层”中的成矿物质参与成矿。  相似文献   

6.
Major and trace element data for a sequence of peralkaline silicic lavas and pyroclastic flows, exposed in the caldera wall of the Paisano volcano, west Texas, document systematic fractional crystallization during magmatic evolution and an open system, magma mixing event in the upper parts of the sequence. Stratigraphically lowest flows are comendite and comenditic quartz trachyte lavas and ash flow tufts. Overlying these units is a trachyte with compositional, textural and mineralogical features indicating that it is the product of magma-mixing; similar flows occur in other parts of the volcano at the same stratigraphic level. This composite trachyte is considered to be a mixture of mugearitic or mafic trachytic magma, derived from a similar source region which yielded the earlier caldera wall flows. Trace element concentrations of the post-trachyte comenditic quartz trachyte lavas suggest they were erupted from a chamber whose magma was diluted by an influx of mugearitic or mafic trachytic magma during a magma mixing event.Rayleigh fractionation calculations show that the comendites and comenditic quartz trachytes can be derived from a parental mugearite magma by 88% to 93% fractionation of dominantly plagioclase and alkali feldspar, with lesser amounts of clinopyroxene, magnetite and apatite. Zircon was not a significant fractionating phase. The composition, mineralogy and depth of the source region(s) which generated these magmas cannot be constrained from the present data set.  相似文献   

7.
Mount Pinatubo in the Philippines, known for its cataclysmic eruption in 1991, hosts several porphyry copper deposits and active geothermal systems. An underlying mafic melt supplied much of the sulphur for the dacitic magma and its injection into the dacitic magma chamber triggered the eruption. The eruption caused purging of the sulphur-rich fluid from the dacite to the atmosphere and extensive fracturing. Similar events took place at Bingham Canyon, Utah, site of the largest copper and gold deposit in North America at 38 Ma. The Bingham Canyon mineralization took place beneath an active stratovolcano and pyroclastic flows contemporaneous with the mineralization show evidence for magma mingling. Ascent of mafic melt supplied sulphur and chalcophile elements to the felsic magma, which consolidated to form the Bingham stock and its underlying magma chamber. Injections of the mafic melt caused periodic eruptions of felsic magma to form the stratovolcano and deposition of sulphide minerals in highly fractured rocks in and around the stock.  相似文献   

8.
The Glaramara tuff presents extensive exposures of the medial and distal deposits of a large tuff ring (original area >800 km2) that grew within an alluvial to lacustrine caldera basin. Detailed analysis and correlation of 21 sections through the tuff show that the eruption involved phreatomagmatic to magmatic explosions resulting from the interaction of dacitic magma and shallow-aquifer water. As the eruption developed to peak intensity, numerous, powerful single-surge pyroclastic density currents reached beyond 8 km from the vent, probably >12 km. The currents were strongly depletive and deposited coarse lapilli (>5 cm in diameter) up to 5 km from source, with only fine ash and accretionary lapilli deposited beyond this. As the eruption intensity waned, currents deposited fine ash and accretionary lapilli across both distal and medial regions. The simple wax–wane cycle of the eruption produced an overall upward coarsening to fining sequence of the vertical lithofacies succession together with a corresponding progradational to retrogradational succession of lithofacies relative to the vent. Various downcurrent facies transitions record transformations of the depositional flow-boundary zones as the depletive currents evolved with distance, in some cases transforming from granular fluid-based to fully dilute currents primarily as a result of loss of granular fluid by deposition. The tuff-ring deposits share several characteristics with (larger) ignimbrite sheets formed during Plinian eruptions and this underscores some overall similarities between pyroclastic density currents that form tuff rings and those that deposit large-volume ignimbrites. Tuff-ring explosive activity with such a wide area of impact is not commonly recognized, but it records the possibility of such currents and this should be factored into hazard assessments.  相似文献   

9.
La Palma is the northwestern island in the Canary archipelago. The island is volcanically active and in recent years there have been two eruptions (1949 and 1971). The oldest rocks that crop out on the island are altered spilitic lavas and they are intruded by numerous dykes and a number of mafic and ultramafic plutonic rocks. These rocks form part of the floor of the huge Caldera de Taburiente. In the walls of the Caldera one finds a 1000 metres thick unit that is composed of lavas and tephra. The rocks are called the El Time formation; and they together with the rocks of the Cumbre Vieja ridge are mainly composed of accumulative mafic rocks, basanites, hawaiites, benmoreites and phonolitic trachytes. These sodarich alkalic rocks are considered to have evolved by fractional crystallization of a basanitic magma in a near surface environment.It is proposed that La Palma developed on a relatively cold section of a plate of oceanic lithosphere that was over 100 m.y. old. During the Alpine-Atlas orogeny this section of the African plate was disrupted and a basanitic magma was extruded onto the sea floor. The volcanic focus of this eruption became established as a local centre for the degassing of the mantle and as a result this area has experienced a long series of eruptions.During the first phase in the evolution of La Palma the submarine edifice of the island was constructed and the rocks of the Caldera Floor formation were emplaced. After a period of marine erosion the huge old volcano Taburiente developed in the northern half of the island. During the final phase in the evolution of the island the southern section of the volcanic edifice subsided and formed the El Paso tectonic basin and the volcanic rift zone along which the many smaller volcanoes of the Cumbre Vieja ridge have developed.  相似文献   

10.
Although Barren Island (Andaman Sea, Indian Ocean) witnessed several volcanic eruptions during historic times, the eruptions that led to the formation of this volcanic island occurred mainly during prehistoric times. It is still active and currently in the fumarolic stage. Its volcanic evolution appears to be characterized by a constructive phase with the piling up of lava flows and scoria deposits and Strombolian activities, followed by a sudden collapse of the main cone. Deposits of a possible caldera-forming eruption were not recognized earlier. After a period of peri-calderic hydromagmatic activity, whose deposits presently mantle inner and outer caldera walls, a new phase of intracalderic Vulcanian activities took place. A prominent dyke in the SE inner side of the caldera wall was recognized. Petrographically the lava flows and dyke are similar but they differ in their chemical composition (viz., SiO2, MgO, Ni, Cr) significantly. Similarity in major, minor and trace element composition (viz., K/La, K/Nb, K/Rb, K/Ti ratios) of these rocks together with Chondrite normalized trace element (Rb, Ba, Sr, P, Zr, Ti and Nb) and REE (La, Ce, Nd and Y) patterns of the Barren Island prehistoric lava flows and dyke and low-K lavas of Sunda Arc indicates that Barren Island must have evolved from a source similar to that of Sunda Arc lavas during the Quaternary Period.  相似文献   

11.
Mid-Miocene volcanic rocks are rare in the Aegean region, although early Miocene and late Miocene-Quaternary volcanism is widespread. At Oxylithos (island of Evia), 14 Ma dacites form a dome or sub-volcanic complex. Phreatomagmatic eruptions formed nearby rhyolitic pyroclastic surge deposits. The calc-alkaline dacites contain bronzite, Mg-rich clinopyroxene, phlogopite and plagioclase phenocrysts. The magma results from mixing of a Mg-rich andesitic magma, similar to that in the nearby island of Skyros, with more felsic magmas represented by the rhyolitic pyroclastics. The dacites are geochemically similar to adakites, which are derived by partial melting of eclogitic subducted oceanic crust and have low Y and Yb and high Sr/Y ratio. 87Sr/86Sr≈0.7095 is found in both dacite and rhyolite. Lead isotopic composition from the high-Mg andesite from Skyros, with 207Pb/204Pb= 15.70 and 208Pb/204Pb= 38.90, forms a linear trend with Evia dacite and rhyolite with 207Pb/204Pb= 15.71 and 208Pb/204Pb= 39.05. The high temperatures required to produce such magma resulted from decompression due to extension of the Aegean basin at the same time as the initial intrusion of the detached subducted slab in the western Aegean that has been imaged by seismic tomography. The Oxylithos rocks extend the known occurrences of adakite series rocks: this series is not restricted to sites with subduction of young oceanic crust.  相似文献   

12.
Volcanism associated with the middle Proterozoic Gawler Range acid volcano‐plutonic province was initiated in the Kokatha area by the construction on Archaean basement of a large stratovolcano composed mainly of tholeiitic basalt and potassic basaltic‐andesite erupted possibly from a mantle‐derived ultramafic diapir.

Crustal melting above the diapir generated acid magma, rich in silica and potassium, which rose by major block‐stoping to form a subvolcanic magma chamber. Leakage from this chamber during the premonitory caldera phase gave rise to small explosive and effusive eruptions around an incipient ring‐fracture zone. In the caldera phase, the eruption of voluminous rhyodacite to dacite ignimbrite from the subvolcanic magma chamber resulted in collapse of the roof partway through the eruption to form the Chandabooka caldera, 15 x 10 km across: the ignimbrite comprises a thick compound cooling unit, the Chandabooka Dacite, of which both the caldera and outflow facies are preserved. Resurgent doming and subsequent uplift of the caldera block by 1 km followed in the post‐caldera phase, accompanied by minor acidic volcanism. Flat‐roofed stocks of the primitive S‐type Hiltaba Granite and a major dyke swarm intruded the volcanic pile to complete the volcano‐plutonic episode.  相似文献   

13.
Gold bearing metavolcanics of Gadag Gold Field (GGF) are represented by mafic (metabasalt, metabasaltic andesite), intermediate (metaandesite) and felsic (metadacite, metarhyolite) rocks. Mafic metavolcanic rocks are low-K Fe-rich tholeiites and were derived by partial melting of the upper mantle sources with high Fe/Mg ratios and low M values. Intermediate and felsic metavolcanics were formed by remelting of these tholeiites mainly in crustal regimes. Although a complete sequence of metavolcanic rocks from mafic to intermediate to felsic fractions occurs, these products were not the result of differentiation from a single magma, crustal contamination was involved in the formation of intermediate and felsic rocks. A clear gap in the chemical composition as well as index of differentiation among the mafic, intermediate and felsic fractions indicate that these metavolcanics constitute a typical bimodal character. It is suggested that these metavolcanics were emplaced in an active continental margin or a continental island arc setting. The petrogenetic processes of formation of Fe-rich tholeiites that evolved in an active continental margin or a continental island arc setting could have provided a favourable geochemical environment for gold mineralisation under the conditions of deformation and metamorphism.  相似文献   

14.
Bontâu is a major eroded composite volcano filling the Miocene Zârand extensional basin, near the junction between the Codru-Moma and Highi?-Drocea Mountains, at the tectonic boundary between the South and North Apuseni Mountains. It is a quasi-symmetric structure (16–18 km in diameter) centered on an eroded vent area (9×4 km), buttressed to the south against Mesozoic ophiolites and sedimentary deposits of the South Apuseni Mountains. The volcano was built up in two sub-aerial phases (14–12.5 Ma and 11–10 Ma) from successive eruptions of andesite lava and pyroclastic rocks with a time-increasing volatile budget. The initial phase was dominated by emplacement of pyroxene andesite and resulted in scattered individual volcanic lava domes associated marginally with lava flows and/or pyroclastic block-and-ash flows. The second phase is characterized by amphibole-pyroxene andesite as a succession of pyroclastic eruptions (varying from strombolian to subplinian type) and extrusion of volcanic domes that resulted in the formation of a central vent area. Numerous debris flow deposits accumulated at the periphery of primary pyroclastic deposits. Several intrusive andesitic-dioritic bodies and associated hydrothermal and mineralization processes are known in the volcano vent complex area. Distal epiclastic deposits initially as gravity mass flows and then as alluvial volcaniclastic and terrestrial detritic and coal filled the basin around the volcano in its western and eastern part. Chemical analyses show that lavas are calc-alkaline andesites with SiO2 ranging from 56–61%. The petrographical differences between the two stages are an increase in amphibole content at the expense of two pyroxenes (augite and hypersthene) in the second stage of eruption; CaO and MgO contents decrease with increasing SiO2. In spite of a ～4 Ma evolution, the compositions of calc-alkaline lavas suggest similar fractionation processes. The extensional setting favored two pulses of short-lived magma chamber processes.  相似文献   

15.
火山是唯一能够直接反映地球深部存在岩浆的地球动力学现象,对于地球气候演变和宜居性具有重要影响。相比年轻火山,古老火山因其活动过程完整、且可能剥蚀出露多阶段火山喷发产物及岩浆通道、岩浆房等,从而为揭示火山岩浆系统演化和火山活动过程提供重要的研究窗口。本文选择中国东南沿海晚白垩世长屿破火山为研究对象,对其开展火山地质、岩石学、年代学及地球化学研究,以期揭示其火山活动历史及其对中国东南沿海白垩纪古环境的启示意义。长屿火山出露面积约100km^(2),呈近圆形分布,具有较厚的火山地层厚度(总厚度约570m),类似于破火山内的火山堆积特点。由早至晚三个喷发阶段形成的流纹质角砾凝灰岩都显示了典型的火山碎屑流相的特点,发育典型的条纹斑状结构,但表现出不同的晶屑、玻屑及岩屑含量以及熔结程度等岩相学特征,反映火山喷发从初始到高峰、再到减弱的过程,最后岩浆沿火山通道侵出形成流纹斑岩穹隆,标志着火山活动的结束。长屿火山的火山碎屑流式喷发伴随着快速的岩浆房塌陷,以及缺少普林尼式空落堆积,反映了火山活动发生在伸展的构造背景。系统的LA-ICP-MS锆石U-Pb年代学研究获得了不同阶段火山岩一致的形成年龄(97~96Ma),暗示它们具有较短的形成时限,是由同期火山岩浆活动先后喷发形成的。长屿火山岩高的SiO_(2)含量(67%~76%)以及分异的锆石微量元素地球化学特征,表明喷发岩浆来自晶粥提取的熔体,并有晶粥来源晶体的混入。此外,我们在长屿火山岩中发现了南洋杉型丝炭化木(贝壳杉型木属),暗示晚白垩世中国东南沿海地区可能为温暖湿润的亚热带山地气候环境,表明中国东南沿海白垩纪大规模火山活动,在古武夷山脉以东地区形成了地形高耸的海岸山脉。  相似文献   

16.
Kīlauea is the youngest of five basaltic shield volcanoes on the island of Hawai’i. It is located to the south‐east of the much larger Mauna Loa volcano, and rose above sea level about 100 ka ago. Kīlauea is one of the most monitored, and arguably the best understood volcanoes on Earth, providing scientists with a good understanding of its current eruption, in which magma rises from depth and is stored beneath its 4 × 3.2 km summit caldera in an underground reservoir. The reservoir is connected to a lava lake within a crater called Halema’uma’u, which is situated on the floor of the caldera. When magma drains from the summit area it travels in underground conduits and emerges on the flanks of the volcano at a rift zone, where it erupts through fissures. The magma is sometimes stored in other reservoirs along the way. This link between summit magma storage and fissure eruptions on the flanks has occurred thousands of times at many Hawai’ian volcanoes. The current eruptive episode is, however, a ‘once‐in‐a‐century’ show, because it is the first time since 1924 that fissure‐fed lava flow eruptions have been accompanied by significant explosive eruptions within Halema’uma’u Crater. This gives scientists a unique opportunity to use modern methods to understand exactly how such hazardous explosions happen at Kīlauea, a volcano that receives about 2 million visitors a year.  相似文献   

17.
The Rodalquilar caldera complex is located in the western part of the Cabo de Gata volcanic field in southeastern Spain and is the first documented example of epithermal gold-alunite mineralization within a caldera in Europe. The Rodalquilar caldera is an oval collapse structure having a maximum diameter of 8 km and formed at 11 Ma from eruption of the Cinto ash-flow tuff. The oval Lomilla caldera, with a diameter of 2 km, is nested within the central resurgent dome of the older Rodalquilar caldera. The Lomilla caldera resulted from the eruption of the Lazaras ash-flow tuff which was ponded within the moat of the Rodalquilar caldera. The last phase of volcanic activity in the caldera complex was the emplacement of hornblende andesite flows and intrusions. This magmatic event resulted in structural doming of the caldera, opening of fractures and faults, and provided the heat source for the large hydrothermal systems which deposited quartz-alunite type gold deposits and base metal vein systems. The gold-alunite deposits are enclosed in areas of intense acid sulfate alteration and localized in ring and radial faults and fractures present in the east wall of the Lomilla caldera. Like other acid-sulfate type deposits, the Rodalquilar gold-alunite deposits are closely related in time and space to porphyritic, intermediate composition magma emplaced along caldera structures but unrelated to the caldera forming magmatic system.  相似文献   

18.
望江山层状岩体位于扬子地块北缘新元古代汉南杂岩带中,岩体从底部到顶部由超镁铁质岩过渡为中性岩:底部主要由辉石岩和橄长岩组成;中部为辉长苏长岩和辉长岩;上部为辉长岩和闪长岩。研究以中部岩相带橄榄辉长苏长岩、辉长苏长岩和辉长岩为对象,通过主要矿物的主微量元素和全岩主微量元素的分析,查明望江山岩体来源于尖晶石二辉橄榄岩组成的大陆下岩石圈地幔,并且地幔源区受到了来自俯冲板片流体的交代,岩体中部带的母岩浆为拉斑玄武质岩浆。钛铁矿—磁铁矿矿物对成分计算表明,母岩浆在形成时具有较高氧逸度。通过单斜辉石压力计得到岩体的侵位深度约为12.9~18 km。对岩体母岩浆橄榄石分离结晶过程的模拟计算表明,中部带橄榄石为母岩浆经过~28%分离结晶的产物。此外,铂族元素（PGE）组成暗示岩体并未经历过大规模的硫化物熔离,可能与缺乏地壳物质混染有关。岩体中单斜辉石与岛弧环境堆晶岩中单斜辉石成分相似,不同于裂谷环境中堆晶单斜辉石的成分;同时,全岩Th/Yb和Nb/Yb比值也与岛弧玄武岩比值相似,因此矿物和全岩成分均说明望江山层状岩体应形成于岛弧环境。研究认为扬子北缘在新元古代长期的俯冲过程中,大洋板片断离导致软流圈上涌,提供热源使交代大陆下岩石圈地幔部分熔融形成具有岛弧特征的镁铁质岩浆,在局部伸展环境中上升侵位形成汉南杂岩带中镁铁—超镁铁质层状岩体。   相似文献   

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20.
A detailed 90,000-year tephrostratigraphic framework of Aso Volcano, southwestern Japan, has been constructed to understand the post-caldera eruptive history of the volcano. Post-caldera central cones were initiated soon after the last caldera-forming pyroclastic-flow eruption (90 ka), and have produced voluminous tephra and lava flows. The tephrostratigraphic sequence preserved above the caldera-forming stage deposits reaches a total thickness of 100 m near the eastern caldera rim. The sequence is composed mainly of mafic scoria-fall and ash-fall deposits but 36 silicic pumice-fall deposits are very useful key beds for correlation of the stratigraphic sequence. Explosive, silicic pumice-fall deposits that fell far beyond the caldera have occurred at intervals of about 2500 years in the post-caldera activity. Three pumice-fall deposits could be correlated with lava flows or an edifice in the western part of the central cones, although the other silicic tephra beds were erupted at unknown vents, which are probably buried by the younger products from the present central cones. Most of silicic eruptions produced deposits smaller than 0.1 km3, but bulk volumes of two silicic eruptions producing the Nojiri pumice (84 ka) and Kusasenrigahama pumice (Kpfa; 30 ka) were on the order of 1 km3 (VEI 5). The largest pyroclastic eruption occurred at the Kusasenrigahama crater about 30 ka. This catastrophic eruption began with a dacitic lava flow and thereafter produced Kpfa (2.2 km3). Total tephra volume in the past 90,000 years is estimated at about 18.1 km3 (dense rock equivalent: DRE), whereas total volume for edifices of the post-caldera central cones is calculated at about 112 km3, which is six times greater than the former. Therefore, the average magma discharge rate during the post-caldera stage of Aso Volcano is estimated at about 1.5 km3/ky, which is similar to the rates of other Quaternary volcanoes in Japan.  相似文献   

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