Facies in an Upper Permian volcanic succession,Emmaville Volcanics,Deepwater, northeastern New South Wales     

A. L. Stewart 《Australian Journal of Earth Sciences》2013,60(6):929-942

The Upper Permian Emmaville Volcanics at Deepwater, northeastern New South Wales, consist of a diverse succession of calc‐alkaline silicic‐intermediate ignimbrites, volcaniclastics and minor lavas. This 2.5 km‐thick sequence underlies and outcrops extensively along the northern margin of the Dundee Rhyodacite Outlier at Dundee. Detailed mapping and facies analysis have revealed eight locally mappable units namely; Magistrate Volcanic Member (rhyolitic ignimbrites), Wollundi Mudstone Member, Dellwood Ignimbrite Member, Marrawarra Rhyolite Member, Top‐Crossing Sandstone Member, Arranmor Ignimbrite Member, Yarramundi Andesite Member (lava, breccia) and Welcome Volcanic Member (rhyolitic ignimbrites). All volcanic units are contained in two fault‐bounded blocks of different lithology and structure. The volcanic succession ranges in composition from andesite to high‐silica rhyolite (58.6–78% SiO₂). Chemical characteristics include enrichment in K₂O (>3.5%), Al₂O₃ and large‐ion lithophile elements (LILE: Rb, K and light rare‐earth elements (LREE)), and depletion in high field strength elements (HFSE: Ti, Nb and Zr). These geochemical attributes reflect a continental subduction‐related signature. The facies architecture indicates that the principal volcanic features of the Late Permian palaeogeography in northeastern New South Wales was a topographically subdued depression flanked by low‐angle ignimbrite sheets with rhyolitic‐intermediate volcanic centres rising gently from the sloping terrain. The succession demonstrates that during the Late Permian andesitic volcanism was present, although localised. A modern analogue for the setting of the Emmaville Volcanics is the Quaternary Taupo Volcanic Zone (New Zealand).  相似文献   

东沙海区海丘发现多底栖方式深水珊瑚     

阎贫  王彦林  靳永斌  赵美霞  钟广见 《地学前缘》2022,29(4):202-210

东沙岛周围海区发育众多深水(200~2 000 m)海丘,其中一些显示了明显的泥火山活动特征。近年的取样调查在多个底质差异明显的海丘都发现了非常丰富的深水珊瑚。为什么这些海丘上深水珊瑚兴盛尚不清楚。根据样品形态分析,初步识别的珊瑚既有以基座紧密固着于碳酸盐岩结核硬底的Madrepora oculate(多眼筛孔珊瑚)、Lochmaeotrochus(灌丛珊瑚)、Enallopsammia(突出海沙珊瑚)、Solenosmilia variabilis、Dendrophyllia(树珊瑚)、 Bamboo Coral (竹节珊瑚)、Golden Gorgonians (金柳珊瑚),还新发现有以鸭蹼状凹凸不平形底壳贴附于海底砂泥级生物碎屑的Desmophyllum dianthus(葵珊瑚)和尖底浅植于碎屑沉积中的Flabellum(扇珊瑚)、Crispatotrochus(卷轮珊瑚)、Balanophyllia(栎珊瑚),还有饼形、杯形能在软质海底缓慢自由移动的Fungiacyathus(蕈杯珊瑚)、Deltocyathusi(角杯珊瑚),指示在底质硬度及海流强度迥异的东沙海丘环境中有多种属珊瑚生长。大量具有机动性的深水珊瑚栖居于弱水动力海底,应更多依赖于栖居地本地而非海流带来的丰富食物,意味着东沙海区的泥火山活动泄漏的烃类流体可能是深水珊瑚食物的主要来源。泥火山的流体活动与珊瑚的兴盛可能具有相互指示意义。  相似文献   

A dated volcano-tectonic deformation event in Jan Mayen causing landlocking of Arctic charr     

Eiliv Larsen  Astrid Lyså  Ármann Höskuldsson  Jan G. Davidsen  Marie J. Nadeau  Michael Power  Georgios Tassis  Stefan Wastegård 《第四纪科学杂志》2021,36(2):180-190

We provide the first documentation of tectonic deformation resulting from a volcanic eruption on the island of Jan Mayen. Vertical displacement of about 14 m southwest of the stratovolcano Beerenberg is associated with an eruption in ad 1732 on its southeastern flank. The age of the uplift event is bracketed by radiocarbon-dated driftwood buried by material deposited due to uplift, and by tephra from this eruption. Constraints, inferred from radiocarbon ages alone, allow for the possibility that uplift was completed prior to the ad 1732 eruption. However, the occurrence of tephra in the sediment column indicates that some displacement was ongoing during the eruption but ceased before the eruption terminated. We attribute the tectonic deformation to intrusion of shallow magma associated with the volcanic eruption. Our results complement previous studies of volcanic activity on Jan Mayan by providing precise age constraints for past volcanic activity. Also, it raises new hypotheses regarding the nature, timing and prevalence of precursor tectonic events to Jan Mayan eruptions. The uplift caused the complete isolation of a coastal lake by closing its outlet to the sea, thus landlocking the facultative migratory fish species Arctic charr (Salvelinus alpinus).  相似文献   

Zircon U-Pb ages,geochemistry, and Sr-Nd isotope ratios for early cretaceous magmatic rocks,southern Saqqez,northwestern Iran     

《Chemie der Erde / Geochemistry》2021,81(1):125687

Sanandaj-Sirjan Zone (SaSZ) is one of the most dynamic structural zones of Iran, which is divided into three main parts: Northern, Central and Southern. The northern SaSZ has been affected by deformation due to fault activities near the Zagros suture zone, and mylonitic structures have overprinted these rocks and was affected by three episodes of magma injection during the Permian-Carboniferous, Early Cretaceous and Cenozoic. In this study, the rock units investigated that have been considered Precambrian-Paleozoic basement on geological maps. This paper considers zircon U-Pb dating, whole-rock chemistry and Sr-Nd isotope ratios of Cretaceous magmatic rocks in the N-SaSZ to develop a new geodynamic model for the evolution of these magmatic rocks. The new zircon U-Pb ages obtained in this study show that the magmatic rocks crystallized at 115–107 Ma in the Early Cretaceous (Aptian-Albian) and are much younger than the supposed ages presented on geological maps. This complex classified into two main groups of basic-intermediate and acidic rocks based on SiO₂ contents. The whole-rock chemistry of the basaltic and andesitic rocks, which are interbedded with marine shallow-water sedimentary deposits, shows their typical calc-alkaline affinity and subordinate tholeiitic series on an active margin. The positive ε_Nd(t) of approximately +4 for some undifferentiated basalts with negative Ti and Nb anomalies shows the relation of these rocks to calc-alkaline magmatism and was generated by the partial melting of subcontinental lithospheric mantle (SCLM). Granitoid rocks with some affinity to the peraluminous group with a negative ε_Nd(t) value (-3.2) mainly and negative Ti and Nb anomalies plot in an active margin tectonic setting. Simultaneous mafic calc-alkaline volcanism and the generation of granitic intrusions in the Early Cretaceous could have occurred on an active margin. Due to the absence of Jurassic arc related magmatic rocks in northern SaSZ and presence of Cretaceous calc alkaline magmatic activity, which are not observed in the central SaSZ, support the idea that the subduction of the Neotethys beneath the northern SaSZ started in the Early Cretaceous.  相似文献   

论新疆东准噶尔陆相火山成矿作用   总被引：6，自引：0，他引：6  

刘家远 《大地构造与成矿学》2001,25(4):434-438

早石炭世晚期至早二叠世时期，东准噶尔地区陆相火山作用广为发育。近年来的研究证明，这一时期的陆相火山作用形成了两套重要的成矿岩浆建造，即安山－英安质喷溢、喷发岩建造和中性－中酸性潜火山杂岩建造，它们具有同一的成矿专属性，与金、铜、铁成矿密切相关。前者主要形成特征的浅成低温热液陆相火山岩型金矿。后者与金、铜成矿关系极为密切，其突出特征表现为受同－成矿岩浆作用的制约，在不同的外部条件下，形成不同类型的多种矿床，分别构成自成体系的金矿床系列和铜矿床系列，充分揭示了东准噶尔与陆相火山作用有关金、铜成矿的广阔前景和深远意义。  相似文献   

New advances of seismic tomography and its applications to subduction zones and earthquake fault zones: A review   总被引：9，自引：0，他引：9  

Dapeng Zhao 《Island Arc》2001,10(1):68-84

Abstract There have been significant advances in the theory and applications of seismic tomography in the last decade. These include the refinements in the model parameterization, 3-D ray tracing, inversion algorithm, resolution and error analyses, joint use of local, regional and teleseismic data, and the addition of converted and reflected waves in the tomographic inversion. Applications of the new generation tomographic methods to subduction zones have resulted in unprecedentedly clear images of the subducting oceanic lithosphere and magma chambers in the mantle wedge beneath active arc volcanoes, indicating that geodynamic systems associated with the arc magmatism and back-arc spreading are related to deep processes, such as the convective circulation in the mantle wedge and deep dehydration reactions in the subducting slab. High-resolution tomographic imagings of earthquake fault zones in Japan and California show that rupture nucleation and earthquake generating processes are closely related to the heterogeneities of crustal materials and inelastic processes in the fault zones, such as the migration of fluids. Evidence also shows that arc magmatism and slab dehydration may also contribute to the generation of large crustal earthquakes in subduction regions.  相似文献   

Tephrochronology of the Kamchatka–Kurile and Aleutian arcs: evidence for volcanic episodicity     

L. M. Prueher  D. K. Rea   《Journal of Volcanology and Geothermal Research》2001,106(1-2)

Volcanic ash layers in North Pacific deep-sea sediment provide a record of episodic explosive volcanism in the Kamchatka–Kurile and Aleutian arcs over the past five million years. We counted 450 ash layers, determined layer thickness and cumulative ash thickness to quantify the flux of ash with time. We use this record to investigate the eruptive history of these arcs, test the reliability of the marine ash record, and inquire into the regional episodicity of North Pacific explosive volcanic history. Episodes of explosive volcanism occurred at approximately 0.2–0.5, 0.7–0.9, 1.5–1.7, and 2.5–2.65 Ma in the Kamchatka arc and 0.15–0.4, 1.7–1.8, 2.55–2.65, and at 3.0–3.1 Ma in the eastern Aleutian arc. These generally coeval eruptive episodes suggest that the pulses in explosive volcanism in the North Pacific enumerated here and recognized by others are regionally episodic over a wide portion of the Pacific rim and not just a response to local volcanogenic processes.  相似文献   

Sulfur dioxide emissions from Popocatépetl volcano (Mexico): case study of a high-emission rate, passively degassing erupting volcano     

H. Delgado-Granados  L. Crdenas Gonzlez  N. Piedad Snchez 《Journal of Volcanology and Geothermal Research》2001,108(1-4)

Popocatépetl volcano in central Mexico has been erupting explosively and effusively for almost 4 years. SO₂ emission rates from this volcano have been the largest ever measured using a COSPEC. Pre-eruptive average SO₂ emission rates (2–3 kt/d) were similar to the emission rates measured during the first part of the eruption (up to August 1995) in contrast with the effusive–explosive periods (March 1996–January 1998) during which SO₂ emission rates were higher by a factor of four (9–13 kt/d). Based on a chronology of the eruption and the average SO₂ emission rates per period, the total SO₂ emissions (up to 1 January 1998) are estimated to be about 9 Mt, roughly half as much as the SO₂ emissions from Mount Pinatubo in a shorter period. Popocatépetl volcano is thus considered as a high-emission rate, passively degassing eruptive volcano. SO₂ emission rates and SO₂ emissions are used here to make a mass balance of the erupted magma and related gases. Identified excess SO₂ is explained in terms of continuous degassing of unerupted magma and magma mixing. Fluctuations in SO₂ emission rate may be a result of convection and crystallization in the chamber or the conduits, cleaning and sealing of the plumbing system, and/or SO₂ scrubbing by the hydrothermal system.  相似文献   

Crystal retention,fractionation and crustal assimilation in a convecting magma chamber,Nisyros Volcano,Greece   总被引：2，自引：0，他引：2  

L. Francalanci  J. C. Varekamp  G. Vougioukalakis  M. J. Delant  F. Innocenti  P. Manetti 《Bulletin of Volcanology》1995,56(8):601-620

Nisyros island is a calc-alkaline volcano, built up during the last 100 ka. The first cycle of its subaerial history includes the cone-building activity with three phases, each characterized by a similar sequence: (1) effusive and explosive activity fed by basaltic andesitic and andesitic magmas; and (2) effusive andextrusive activity fed by dacitic and rhyolitic magmas. The second eruptive cycle includes the caldera-forming explosive activity with two phases, each consisting of the sequence: (1) rhyolitic phreatomagmatic eruptions triggering a central caldera collapse; and (2) extrusion of dacitic-rhyolitic domes and lava flows. The rocks of this cycle are characteized by the presence of mafic enclaves with different petrographic and chemical features which testify to mixing-mingling processes between variously evolved magmas. Jumps in the degree of evolution are present in the stratigraphic series, accompanied by changes in the porphyritic index. This index ranges from 60% to about 5% and correlates with several teochemical parameters, including a negative correlation with Sr isotope ratios (0.703384–0.705120). The latter increase from basaltic andesites to intermediate rocks, but then slightly decrease in the most evolved volcanic rocks. The petrographic, geochemical and isotopic characteristics can be largely explained by processes occurring in a convecting, crystallizing and assimilating magma chamber, where crystal sorting, retention, resorption and accumulation take place. A group of crystal-rich basaltic andesites with high Sr and compatible element contents and low incompatible elements and Sr isotope ratios probably resulted from the accumulation of plagioclase and pyroxene in an andesitic liquid. Re-entrainment of plagioclase crystals in the crystallizing magma may have been responsible for the lower ⁸⁷Sr/⁸⁶Sr in the most evolved rocks. The gaps in the degree of evolution with time are interpreted as due to liquid segregation from a crystal mush once critical crystallinity was reached. At that stage convection halted, and a less dense, less porphyritic, more evolved magma separated from a denser crystal-rich magma portion. The differences in incompatible element enrichment of pre-and post-caldera dacites and the chemical variation in the post-caldera dome sequence are the result of hybridization of post-caldera dome magmas with more mafic magmas, as represented by the enclave compositions. The occurrence of the quenched, more mafic magmas in the two post-caldera units suggests that renewed intrusion of mafic magma took place after each collapse event.  相似文献   

Experimental constraints on pre-eruptive water contents and changing magma storage prior to explosive eruptions of Mount St Helens volcano   总被引：2，自引：0，他引：2  

J. E. Gardner  M. Rutherford  S. Carey  H. Sigurdsson 《Bulletin of Volcanology》1995,57(1):1-17

Compositionally diverse dacitic magmas have erupted from Mount St Helens over the last 4000 years. Phase assemblages and their compositions in these dacites provide information about the composition of the pre-eruptive melt, the phases in equilibrium with that melt and the magmatic temperature. From this information pre-eruptive pressures and water fugacities of many of the dacites have been inferred. This was done by conducting hydrothermal experiments at 850°C and a range of pressures and water fugacities and combining the results with those from experiments at temperatures of 780 and 920°C, to cover the likely range in equilibration conditions of the dacites. Natural phase assemblages and compositions were compared with the experimental results to infer the most likely conditions for the magmas prior to eruption. Water contents disolved in the melts of the dacites were then estimated from the inferred conditions. Water contents in the dacites have varied greatly, from 3.7 to 6.5 wt.%, in the last 4000 years. Between 4000 and about 3000 years ago the dacites tended to be water saturated and contained 5.5 to 6.5 wt.% water. Since then, however, the dacites have been significantly water-undersaturated and contained less than 5.0 wt.% water. These dacites have tended to be hotter and more mafic, and andesitic and basaltic magmas have erupted. These changes can be explained by variable amounts of mixing between felsic dacite and basalt, to produce hotter, drier and more mafic dacites and andesites. The magma storage region of the dacitic magmas has also varied significantly during the 4000 years, with shifts to shallower levels in the crust occurring within very short time periods, possibly even two years. These shifts may be related to fracturing of overlying roof rock as a result of magma with-drawal during larger volume eruptions.  相似文献