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1.
金星薄饼状火山(陡边火山)是一类形貌非常独特的火山,有别于太阳系中其他的火山:近圆的底面轮廓、平坦的顶部和较大的顶底直径比.薄饼状火山被认为是高黏度岩浆喷发形成的,但是这一成因并不被广泛接受.火山的形貌特征主要取决于岩浆成分、喷发机制、重力和喷发环境(如大气压)等.因此,通过对比地球火山的形貌特征及形成机制可以为金星薄饼状火山的成因提供指示.我们利用底面不规则度(ii )、高径比(H/W B )、顶底直径比(W S/W B )、侧边最大坡度处的标准化高度(HS m a x )、顶部高径比(H S/W S )和喷发量(V )这些形貌参数定量地将地球表面的复式火山、盾形火山、火山锥、熔岩穹丘(Peleean 型和 Coulees 型)、海底平顶火山以及泥火山的形貌特征与金星薄饼状火山对比.Coulees型熔岩穹丘、海底平顶火山和泥火山的形貌特征与金星薄饼状火山有一定的相似性.Coulees型熔岩穹丘剖面形态与薄饼状火山最为相似,可能指示了薄饼状火山具有类似的成因机制,而海底平顶火山侧边的上凹特征可能指示了其与薄饼状火山完全不同的成因机制.因此,薄饼状火山可能是相对高黏度的岩浆以较高速率持续性喷发形成. 相似文献
2.
松辽盆地改造残留的古火山机构与现代火山机构的类比分析 总被引：20，自引：3，他引：17
现代火山机构形态有盾状、锥状和穹状,可按喷发样式进一步划分为7种类型。据此分类,在松辽盆地周缘剖面及其北部徐家围子断陷区可识别出4类火山机构：盾状火山机构,由喷溢相熔岩组成,可夹有薄层爆发相火山碎屑岩;层火山机构,由互层的熔岩与火山碎屑岩组成,喷溢相与爆发相交替的序列明显;火山碎屑锥,几乎全部由火山碎屑（熔）岩组成,爆发相为主;熔岩穹丘由高粘度的流纹质、英安质熔岩堵塞火山口后缓慢挤出形成,喷溢相和侵出相发育,兼有火山通道相。盆地内埋藏火山机构最小坡度为3°,最大坡度为25°,底部直径为2～14 km,分布面积为4～50 km2,火山岩厚度为100～600 m;总体上呈现出数目多、个体规模小、受区域大断裂控制、具裂隙式-多中心喷发、彼此相互叠置的特征。火山岩岩性和岩相是控制松辽盆地古火山机构类型及形态的主要因素。 相似文献
3.
Ioan Seghedi Alexandru Szakács Emilian Roşu Zoltán Pécskay Katalin Gméling 《Central European Journal of Geosciences》2010,2(3):321-328
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
4.
Petrological, geochemical, and isotope geochronological aspects of the evolution of calc-alkaline magmatism were investigated
in the Western Okhotsk flank zone, the Okhotsk segment, and the Eastern Chukchi flank zone of the Okhotsk-Chukotka volcanic
belt (OCVB). The OCVB is a tectonotype of continental margin volcanic belts comprising much greater volumes of felsic ignimbritic
volcanics compared with mature island arcs (MIA, Kuril-Kamchatka and Aleutian) and the Andean continental margin. The volcanic
rocks of continental margin volcanic belts (OCVB and Andean belt) are enriched in K, Ti, and P compared with the rocks of
MIA and show a trend toward the field of high-potassium calc-alkaline series. Primitive andesite varieties (Mg# > 0.6) were
not yet found in the OCVB, but there are relatively calcic varieties unknown in Andean-type structures and a significant fraction
of moderately alkaline rocks, which are not typical of MIA. Variations in trace and major element characteristics in the basalts
and andesites of the OCVB were interpreted as reflecting the competing processes of assimilation/mixing and fractional crystallization
during the evolution of the parental basaltic magma. Significant lateral variations were established in the composition of
the mantle sources of calc-alkaline magmas along the OCVB over more than 2500 km. The initial isotopic ratios of Sr, Nd, and
Pb in the volcanics of the Okhotsk segment are relatively depleted and fall near the mixing line between PREMA and BSE. The
magma source of the Western Okhotsk flank zone is most enriched and approaches EMI, whereas that of the central and eastern
Chukchi zones contains an admixture of the EMII component. The geochronological characteristics of all the main stages of
OCVB magmatism were comprehensively studied by U-Pb SHRIMP and ID-TIMS zircon dating (86 samples) and 40Ar/39Ar analysis (73 samples). In general, a discontinuous character was established for the OCVB magmatism from the middle Albian
to the early Campanian (106–77 Ma). The volcanism is laterally asynchronous. There are several peaks of volcanism with modes
at approximately 105, 100, 96, 92.5, 87, 82, and 77 Ma. The Coniacian-Santonian peaks correspond to the most extensive stages
of the middle and late cycles of felsic volcanism. A decreases and a hiatus in magmatic activity were reconstructed for the
end of the Cenomanian and the beginning of the Turonian. The volcanism was terminated by plateau basalts with ages of 76–78
Ma, which mark a change in the geodynamic setting from frontal subduction to the regime of a transform margin with local extension
in zones normal to the slip direction. A catastrophic character of eruptions with rather narrow ranges of volcanism (<2 Myr)
were established taking into account new reliable age estimates for some individual large calderas. The accumulation rate
of volcanic materials in such structures was up to 0.15–0.36 km3/yr and even higher. 相似文献
5.
Neogene-Quaternary post-collisional volcanism in Central Anatolian Volcanic Province (CAVP) is mainly characterized by calc-alkaline
andesites-dacites, with subordinate tholeiitic-transitional-mildly alkaline basaltic volcanism of the monogenetic cones. Tepekoy
Volcanic Complex (TVC) in Nigde area consists of base surge deposits, and medium to high-K andesitic-dacitic lava flows and
basaltic andesitic flows associated with monogenetic cones. Tepekoy lava flows petrographically exhibit disequilibrium textures
indicative of magma mixing/mingling and a geochemisty characterized by high LILE and low HFSE abundances, negative Nb–Ta,
Ba, P and Ti anomalies in mantle-normalized patterns. In this respect, they are similar to the other calc-alkaline volcanics
of the CAVP. However, TVC lava flows have higher and variable Ba/Ta, Ba/Nb, Nb/Zr, Ba/TiO2 ratios, indicating a heterogeneous, variably fluid-rich source. All the geochemical features of the TVC are comparable to
orogenic andesites elsewhere and point to a sub-continental lithospheric mantle source enriched in incompatible elements due
to previous subduction processes. Basaltic monogenetic volcanoes of CAVP display similar patterns, and HFS anomalies on mantle-normalized
diagrams, and have incompatible element ratios intermediate between orogenic andesites and within-plate basalts (e.g. OIB).
Accordingly, the calc-alkaline and transitional-mildly alkaline basaltic magmas may have a common source region. Variable
degrees of partial melting of a heterogeneous source, enriched in incompatible elements due to previous subduction processes
followed by fractionation, crustal contamination, and magma mixing in shallow magma chambers produced the calc-alkaline volcanism
in the CAVP. Magma generation in the TVC, and CAVP in general is via decompression melting facilitated by a transtensional
tectonic regime. Acceleration of the extensional regime, and transcurrent fault systems extending deep into the lithosphere
favoured asthenospheric upwelling at the base of the lithosphere, and as a consequence, an increase in temperature. This created
fluid-present melting of a fluid-enriched upper lithospheric mantle or lower crustal source, but also mixing with asthenosphere-derived
melts. These magmas with hybrid source characteristics produced the tholeiitic-transitional-mildly alkaline basalts depending
on the residence times within the crust. Hybrid magmas transported to the surface rapidly, favored by extensional post-collision
regime, and produced mildly alkaline monogenetic volcanoes. Hybrid magmas interacted with the calc-alkaline magma chambers
during the ascent to the surface suffered slight fractionation and crustal contamination due to relatively longer residence
time compared to rapidly rising magmas. In this way they produced the mildly alkaline, transitional, and tholeiitic basaltic
magmas. This model can explain the coexistence of a complete spectrum of q-normative, ol-hy-normative, and ne-normative monogenetic
basalts with both subduction and within-plate signatures in the CAVP. 相似文献
6.
Review of post-collisional volcanism in the Central Anatolian Volcanic Province (Turkey), with special reference to the Tepekoy Volcanic Complex 总被引：2，自引：0，他引：2
Neogene-Quaternary post-collisional volcanism in Central Anatolian Volcanic Province (CAVP) is mainly characterized by calc-alkaline andesites-dacites, with subordinate tholeiitic-transitional-mildly alkaline basaltic volcanism of the monogenetic cones. Tepekoy Volcanic Complex (TVC) in Nigde area consists of base surge deposits, and medium to high-K andesitic-dacitic lava flows and basaltic andesitic flows associated with monogenetic cones. Tepekoy lava flows petrographically exhibit disequilibrium textures indicative of magma mixing/mingling and a geochemisty characterized by high LILE and low HFSE abundances, negative Nb–Ta, Ba, P and Ti anomalies in mantle-normalized patterns. In this respect, they are similar to the other calc-alkaline volcanics of the CAVP. However, TVC lava flows have higher and variable Ba/Ta, Ba/Nb, Nb/Zr, Ba/TiO2 ratios, indicating a heterogeneous, variably fluid-rich source. All the geochemical features of the TVC are comparable to orogenic andesites elsewhere and point to a sub-continental lithospheric mantle source enriched in incompatible elements due to previous subduction processes. Basaltic monogenetic volcanoes of CAVP display similar patterns, and HFS anomalies on mantle-normalized diagrams, and have incompatible element ratios intermediate between orogenic andesites and within-plate basalts (e.g. OIB). Accordingly, the calc-alkaline and transitional-mildly alkaline basaltic magmas may have a common source region. Variable degrees of partial melting of a heterogeneous source, enriched in incompatible elements due to previous subduction processes followed by fractionation, crustal contamination, and magma mixing in shallow magma chambers produced the calc-alkaline volcanism in the CAVP. Magma generation in the TVC, and CAVP in general is via decompression melting facilitated by a transtensional tectonic regime. Acceleration of the extensional regime, and transcurrent fault systems extending deep into the lithosphere favoured asthenospheric upwelling at the base of the lithosphere, and as a consequence, an increase in temperature. This created fluid-present melting of a fluid-enriched upper lithospheric mantle or lower crustal source, but also mixing with asthenosphere-derived melts. These magmas with hybrid source characteristics produced the tholeiitic-transitional-mildly alkaline basalts depending on the residence times within the crust. Hybrid magmas transported to the surface rapidly, favored by extensional post-collision regime, and produced mildly alkaline monogenetic volcanoes. Hybrid magmas interacted with the calc-alkaline magma chambers during the ascent to the surface suffered slight fractionation and crustal contamination due to relatively longer residence time compared to rapidly rising magmas. In this way they produced the mildly alkaline, transitional, and tholeiitic basaltic magmas. This model can explain the coexistence of a complete spectrum of q-normative, ol-hy-normative, and ne-normative monogenetic basalts with both subduction and within-plate signatures in the CAVP. 相似文献
7.
Hetu C. Sheth George Mathew Kanchan Pande Soumen Mallick Balaram Jena 《Journal of Earth System Science》2004,113(4):831-838
Rootless cones, also (erroneously) called pseudocraters, form due to explosions that ensue when a lava flow enters a surface
water body, ice, or wet ground. They do not represent primary vents connected by vertical conduits to a subsurface magma source.
Rootless cones in Iceland are well studied. Cones on Mars, morphologically very similar to Icelandic rootless cones, have
also been suggested to be rootless cones formed by explosive interaction between surface lava flows and ground ice. We report
here a group of gentle cones containing nearly circular craters from Mount Pavagadh, Deccan volcanic province, and suggest
that they are rootless cones. They are very similar morphologically to the rootless cones of the type locality of Myvatn in
northeastern Iceland. A group of three phreatomagmatic craters was reported in 1998 from near Jabalpur in the northeastern
Deccan, and these were suggested to be eroded cinder cones. A recent geophysical study of the Jabalpur craters does not support
the possibility that they are located over volcanic vents. They could also be rootless cones. Many more probably exist in
the Deccan, and volcanological studies of the Deccan are clearly of value in understanding planetary basaltic volcanism. 相似文献
8.
松辽盆地白垩系营城组古火山机构特征 总被引：9，自引：1，他引：8
在系统总结Hawaiian等7种喷发方式、互层状火山等3种火山机构的岩性岩相和垂向序列特征与识别标志基础上,通过剖面火山机构剖析及其与盆内埋藏火山机构对比,总结出营城组2类古火山机构特征。营城组玄武岩火山机构自下而上为枕状、渣状集块熔岩（占总厚度30%）,气孔杏仁和致密块状熔岩（70%）,喷溢相为主,火山口附近隐爆角砾岩发育,为夏威夷式喷发。火山机构厚度以200~500 m居多,顶面盾状,相对高差100~250 m,以坡角小于10°为特征,属于盾状火山。营城组流纹岩火山机构的纵向序列300~700 m,内部结构呈现上中下三段式：下部火山碎屑（熔）岩为主（30%）,爆发相为主,以基浪（base surge）沉积为标志,喷发方式主要表现为高粘度岩浆强烈气射作用的培雷式喷发;中部主要为气孔、石泡和流纹构造流纹岩（60%）,构成火山机构的主体,喷溢相为主,火山口附近常见侵出相珍珠岩穹隆,喷发类型接近于斯通博利式;上部主要为细粒（层）凝灰岩（火山灰湖相沉积,10%）,爆发相为主,以普林尼式喷发为主。流纹岩火山机构顶面呈丘状,相对高差200~300 m,以坡角多大于15°为特征,属于互层状火山。 相似文献
9.
大兴安岭哈拉哈河-绰尔河第四纪火山分期: K-Ar年代学与火山地质特征 总被引：9，自引：6，他引：3
大兴安岭中部哈拉哈河-绰尔河第四纪火山区分布有34座火山,这些火山总体呈北东向带状分布,火山岩分布面积约400 km2,岩性主要为碱性橄榄玄武岩.根据火山地质特征,结合火山岩K-Ar测年结果,哈拉哈河-绰尔河第四纪火山可进一步划分为早、中、晚更新世和全新世4期.早更新世火山岩,由于被后期火山岩覆盖,主要分布于火山区周边和出露在河谷中.中更新世火山活动最强,不论火山数量(27座)还是熔岩流规模都超过该区第四纪火山的一半以上.晚更新世时期火山活动趋弱,火山活动范围缩小,只局限于小范围区域.全新世火山活动又进入新的高峰期,强爆破式喷发和规模宏大的熔岩流,以及保存完好的熔岩流地貌是全新世火山之特点. 相似文献
10.
11.
《Chemie der Erde / Geochemistry》2022,82(4):125886
Much has been discovered about volcanism on Mars over the past fifty years of space exploration. Previous reviews of these discoveries have generally focused on the volcanic constructs (e.g., Olympus Mons and the other volcanoes within the Tharsis and Elysium regions), the analysis of individual lava flows, and how volcanic activity on Mars has evolved over time. Here we focus on attributes of volcanology that have received less attention and build upon characteristics of terrestrial volcanoes to pose new questions to guide future analyses of their Martian equivalents either with existing data sets or with new types of measurements that need to be made. The remarkable lack of exposed dikes at eroded ancient volcanoes attests to an internal structure that is different from terrestrial equivalents. Enigmatic aspects of the origin of the ridged plains (commonly accepted to be volcanic but with few identifiable flow fronts and only rare vents), the style(s) of volcanism during the earliest period of Martian history (the Noachian), and the possible mode(s) of formation of the Medusae Fossae Formation are considered here. Martian meteorites have been dated and are volcanic, but they cannot be correlated with specific geographic areas, or the chronology of Mars derived from the number of superimposed impact craters. Some of these questions about Martian volcanism can be addressed with existing instrumentation, but further progress will most likely rely on the acquisition of new data sets such as high-resolution gravity data, the return of samples from known localities, the flight of a synthetic aperture imaging radar, penetrators sent to the Medusae Fossae Formation, and detailed in situ field observations of selected volcanic sites. 相似文献
12.
13.
Six large Late Miocene to Quaternary calderas, > 10 km in diameter, cluster together with several medium to small calderas and stratovolcanoes in a 60 × 30 km area of the Aizu volcanic field, southern NE Japan arc. These caldera volcanoes were built on a WNW–ESE trending highland coincident with a local uplifted swell since Late Miocene. The flare-up of felsic volcanism occurred synchronously along the NE Japan arc. Pyroclastic flow sheets from the calderas spread over the surrounding intra-arc basins and are interstratified with various sediments. Geochronological data indicates that the large-caldera eruptions have occurred six times since 8 Ma, at intervals of 1 to 2 million years. Late Miocene to Early Pliocene extra-caldera successions in the basin consist of nine sedimentary facies associations: (1) primary pyroclastics, (2) lahars, (3) gravelly fluvial channels, (4) sandy fluvial channels, (5) floodplains, (6) tidal flats, (7) delta fronts, (8) pro-delta slopes, and (9) pro-delta turbidites. The distribution of facies associations show westward prograding of volcaniclastic aprons, made up of braid delta, braidplain, pyroclastic flow sheet, and incised braided river deposits. The extra-caldera successions record: 1) an increase in felsic volcanism with an associated high rate of volcaniclastic sediment supply at about 10 Ma, prior to catastrophic caldera-forming eruptions; and 2) progradation of volcaniclastic aprons toward the back-arc side in response to the succeeding caldera-forming eruptions and sea-level changes, until about 3 Ma. 相似文献
14.
Dredged samples from the Geophysicist seamount volcano in the northeastern part of the Kurile Basin include volcanic and volcanoclastic rocks ranging from basalt to andesite. The rocks have geochemical features typical of high-K island-arc calc-alkaline volcanism. They are enriched in LILE and depleted in Zr, Ti, Nb, Ta and Y. The chondrite-normalized REE patterns are characterized by enrichment of LREE similar to those of island-arc lava from the submarine volcanoes of rear-arc zone of the Kurile Island Arc. The volcanic rocks have a wide range of 87Sr/86Sr ratios (0.70287-0.70652), varying 143Nd/144Nd and Pb isotopic ratios. Their trace-element compositions and Sr-Nd-Pb isotope signatures may be explained by a small addition of crustal continental component to mantle-derived magmas that suggest the existence of thinned continental basement under the eastern part of the Kurile Basin. 相似文献
15.
Tertiary collision-related volcanic rocks of the Eastern Rhodopes (37–25.5 Ma) display calc-alkaline and shoshonitic affinities, with (A) intermediate to basic and (B) acid compositions. (A) Latites, andesites, also shoshonites and basaltic andesites and scarce basalts, absarokites and ultrapotassic latites were emitted through different eruptive styles: lava flows often autobrecciated, domes, ash and scarce pumice falls and flows. Lahars are frequent. K2 O contents of intermediate volcanics decrease from North to South towards the collision suture. (B) Rhyolites, trachyrhyolites and trachydacites show explosivity progressively decreasing with time. Several eruptive types can be distinguished: pyroclastic flows (weakly and strongly welded ignimbrite deposits), ash and lapilli falls, domes and lava flows. The large (30×10 km) Borovitza caldera is the result of a paroxysmic explosive phase.
All rocks are characterized by high contents of Rb, Th and Y. Conversely, negative Ba and Ta–Nb anomalies are typical of collision-related magmatism.
Intense hydrothermal episodes, contemporaneous with the volcanic activity, have converted large amounts of explosive products into bentonite and zeolites deposits. Typical metallogeny is associated with this collision-related volcanism: large Pb, Zn with Cu and Ag deposits and small U or Au deposits are exposed. 相似文献
All rocks are characterized by high contents of Rb, Th and Y. Conversely, negative Ba and Ta–Nb anomalies are typical of collision-related magmatism.
Intense hydrothermal episodes, contemporaneous with the volcanic activity, have converted large amounts of explosive products into bentonite and zeolites deposits. Typical metallogeny is associated with this collision-related volcanism: large Pb, Zn with Cu and Ag deposits and small U or Au deposits are exposed. 相似文献
16.
《Gondwana Research》2000,3(1):65-77
The late Proterozoic Malani bimodal volcanics constitute the largest suite of anorogenic acid volcanics in India. The volcanism took place during 745±10 Ma ago, succeeding the granitic activity of Abu pluton and ceased before the onset of Marwar sedimentation.On the basis of field evidences, three stages of igneous activity have been recognised. Volcanics of the first stage are mostly basalt with occasional andesite or trachybasalts. These are subsequently covered by the voluminous outpouring of peralkaline and peraluminous rhyolite, basalt, dacite and trachyte flows. The third stage ceased with the outburst of ash flow deposits.The dominant felsic volcanics are rhyolites and rhyodacites spread over an area of about 31, 000 km2. The other rock types associated with rhyolite are trachytes, dacites, pitchstone, welded tuff, vitric, lithic and crystal ash, ignimbrite, obsidian, pyroclastic slates, agglomerate, volcanic breccia and volcanic conglomerates. Majority of the acid volcanics are high potassic and a few are calcalkaline or low potassic in composition.Feldspar geothermometry suggests the temperature of equilibrium to be above 650°C. Similar results were obtained by magnetite-ulvospinel geothermometry. Oxygen fugacity is estimated to be about 10−18 under FMQ-Ni-NiO buffer conditions.Malani volcanism was essentially under terrestrial conditions, although deposition by aqueous conditions are also indicated. The volcanic eruptions have been through fissures, shield volcanoes and central cones. The volcanism was triggered in an extensional tectonic regime of continental crust, where geotherm was raised by the repeated influx of basic magma. The initial basaltic magma was possibly generated at deeper depth by ‘hot spot’ activity. This magma while migrating upwards supplied additional heat for the partial melting of lower sialic crust resulting in the generation of felsic magma. The crustal extension has helped in the upward advancement of the felsic magma. 相似文献
17.
火山活动是月球最主要的内动力地质作用之一,是研究月球地质历史和热演化的重要窗口,也是月球科学及探测的重点目标.本文概要总结了月球火山作用的基本原理,并重点介绍了"岩墙扩展"模型.基于此模型,列举了由于岩墙在月壳内部上升程度的不同,导致的不同形式的喷发活动,并在月表产生了一系列火山地貌特征:① 当岩墙仅扩展到浅月表、未能穿透月壳并引起喷发活动时,可能会在月表产生坑链构造、地堑或底部断裂型撞击坑;② 当岩墙穿透了整个月壳并引起爆裂式喷发活动时,会在月表产生小型火山锥、区域性火山碎屑堆积物、全月分布的微小火山玻璃、暗晕凹陷构造及环形火山碎屑堆积物;③ 当岩墙穿透了整个月壳并引起溢流式喷发活动时,随着岩浆喷发通量的逐步增高,会在月表产生小型熔岩流、月海穹窿、复合熔岩流、蜿蜒型月溪、巨型熔岩流及火山高原复合体.本文也简要介绍了在月表观测到的若干非典型火山地貌特征,包括不规则月海斑块、环形凹陷穹丘及非月海富硅质穹窿.近年来新的探月数据加深了对这些特殊火山地貌特征的认识,但是更多的地质特征及成因模型细节仍有待未来月球研究及探测去解决. 相似文献
18.
Kodjopa Attoh 《Precambrian Research》1982,18(3):275-290
The structure of an early Proterozoic volcanic—sedimentary belt in northeastern Ghana is inferred from the distribution of lithologic units and interpretation of Bouguer gravity anomaly associated with the belt. It is shown from gravity modelling that the vertical thickness (depth) of the volcanic—sedimentary succession is ca. 3 km and that the structure of the western part of the belt is an overturned anticline, an interpretation consistent with facing data. This structure provides the basis from which the stratigraphic order of the mapped lithic units is deduced: (1) fine-grained epiclastic sediments interbedded with minor felsic tuffs, followed by (2) tholeiitic basaltic lavas, which are overlain by (3) calc-alkaline andesitic and dacitic lavas and tuffs; the youngest volcanic unit belonging to the sequence is a calc-alkaline mafic tuff (4). A manganese-rich chemical sediment is preserved at the boundary between the tholeiitic mafic lava and calc-alkaline intermediate volcanic rock units. The early Proterozoic sequence, which is unconformably overlain by coarse fluviatile sediments, is estimated to be ca. 8500 m thick. The stratigraphic sequence in the study area contrasts strongly with the conceptual stratigraphic schemes which are currently held to be valid for similar lithologic associations of early Proterozoic age (Birimian) in the West African shield. 相似文献
19.
Abstract. Kuroko deposits are a representative volcanic‐hosted massive sulfide deposit and the Hokuroku district is economically the most important Kuroko containing province in Japan. There are two cycles of the bimodal volcanic sequence in the Hokuroku district. The pre‐ore volcanism started with basaltic activity and was followed by intensive felsic hyaloclas‐tic activity under bathyal conditions. The post‐ore sequence also began with basaltic activity intercalated with mudstone and was followed by alternating beds of pumice tuff with several lava flows and mudstone. Kuroko deposits are situated in the final period of the pre‐ore felsic volcanic sequence of the first bimodal volcanic cycle. Based on a detailed investigation of existing age data, it was concluded that the felsic volcanic sequences in the pre‐and post‐Kuroko formation can be divided into a pre‐ore dacite group (16–13.5 Ma), a D2 dacite group (lower unit of the post‐ore volcanic sequence, 12.7±0.6～ ll Ma) and a Dl dacite group (upper unit of the post‐ore sequence including quartz‐porphyry and granitoid, 11sim;10 Ma) in ascending order. Field and microscopic observations show that the pre‐ore dacite is characterized by aphyric to plagioclase‐phyric lava and the post‐ore dacitic rocks are characterized by quartz‐plagioclase‐phyric aphanitic lava and dome. These three dacite groups are petrochemically discriminated by SiO2‐Al2O3 and CaO‐TiO2 diagrams, excluding altered specimens. The distribution of the normative compositions on the Q‐An‐Ab‐Or diagram suggests that the pre‐ore dacites trend on the 5 kb cotectic line (equilibrated to 10—15 km deep) and those of the post‐ore trend along the 1 kb line (a few km deep). The secular variation of the major elements indicates that the rhyolitic members genetically related to the Kuroko formation could be the most differentiated products in the pre‐ore felsic volcanism. The distribution of Nb against SiO2 content in the pre‐ and post‐ore bimodal volcanic cycles indicates that these two volcanisms could have been generated by different magmatic origins. The difference would have been caused by the tectonic conversion from a back‐arc to an island‐arc setting. 相似文献
20.
New 40Ar/39Ar ages, based on incremental heating techniques for groundmass separates of 25 samples, are presented for the Harrat Al-Madinah volcanic field, part of Harrat Rahat in the north western part of the Arabian plate. This area is an active volcanic field characterized by the occurrence of two historical eruptions approximately in 641 and 1256 AD. Field investigations of the main volcanic landforms indicate dominantly monogenetic strombolian eruptions, in addition to local more explosive eruptions. The lavas consist mainly of olivine basalt and hawaiite flows with minor evolved rocks of mugearite, benmoreite, and trachyte that occur mainly as domes, tuff cones and occasionally as lava flows. Previous K/Ar dating shows that the Harrat Al-Madinah lava flows and associated domes comprise seven units spanning an age range of ca. 1.7 Ma–Recent. The new 40Ar/39Ar age determinations confirm, to a great extent, the previously obtained K/Ar ages in the sense that no major systematic biases were found in the general stratigraphy of the different flow units. However, the 40Ar/39Ar plateau ages show that volcanism in this area began in the Neogene (∼10 Ma) and continued to Recent, with the most voluminous eruptions occurring in the Quaternary. Neogene volcanism occurred in at least three pulses around 10, 5 and 2 Ma, whereas Quaternary volcanism produced at least seven units reflecting lava flow emplacement in the time period of 1.90 Ma–Recent. Thus, the whole duration of volcanic activity in the Harrat Al-Madinah (10 Ma–Recent) appears much longer than that previously identified. The longevity of volcanism in the same part of the moving Arabian plate and absence of evidence for uni-directional migration of volcanic activity indicate that there is no fixed plume beneath this region. The NNW-trending distribution of the volcanic vents is parallel to the Red Sea, and suggests their origin is related to periodic extensional episodes along the reactivated Red Sea fault system. 相似文献