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1.
A useful tool to elucidate past tectonic environments is the geochemistry of volcanic and sedimentary rocks when used together.The regional structural setting of the Oman Mountains indicates that deep-water sediments and volcanic rocks formed adjacent to the rifted Arabian margin in the Late Triassic near the axis of a narrow ocean basin of Red Sea-type. Tholeiitic to trachytic extrusives formed seamounts associated with Late Triassic reefal build-ups. “Immobile” trace element compositions point to a within-plate origin. The interbedded and overlying Late Triassic deep-sea sedimentary cover comprises ribbon radiolarites and both distal siliclastic and calcareous turbidites that accumulated on an abyssal plain at least ca. 180 km northeast of the Arabian continent. Associated ferromanganiferous oxide-sediments are interpreted as chemical precipitates derived from high-temperature vents in the spreading axis of the young ocean basin. Pervasive regional subsidence took place during end Triassic/Early Jurassic time.Later, in the Cretaceous, oceanic crust was consumed in a northeast-dipping subduction zone. MORB-type crust was subducted while Late Triassic volcanic edifices and sedimentary cover were accreted. During eventual trench-margin collision the Semail ophiolite split into blocks allowing sub-ophiolite melange rocks to be expelled upwards through corridors, creating the Batinah Melange. As the ophiolite nappe ploughed inboard over already thrust-assembled abyssal plain sediments (Hawasina Complex), some duplexes were uplifted, oversteepened, overturned and then slid backwards onto the ophiolite to form the Batinah Sheets. 相似文献
2.
Bedded tuffs in diatremes and related volcanic vents have been reported from a wide range of localities. Most of the stratified pyroclastic material is interpreted as subaerial in origin, although local, thin deposits are clearly subaqueous and apparently were laid down in crater lakes. Bedded tuff fillings of pipes commonly attain thicknesses of 300–500 m, and some estimates in excess of 1200 m appear to be reliable. Most explanations for the presence of bedded tuffs at considerable depths in pipes involve a cauldron or caldera-like subsidence mechanism that relies heavily upon withdrawal of magma from lower pipe regions. Compaction and marginal slumping of ejecta may be important factors in subsidence, but are probably significant as a « mechanism » only near the surface. Based on experimental studies, a fluidization mechanism of subsidence is proposed as a viable explanation for the development of at least some bedded pipe fillings, particularly those that occur at greatest depths. Bedded materials may subside in a quiescent bed fluidized state when upward gas flow is less than free fall velocity of pyroclastic particles. A similar volume of material is simultaneously carried upward by higher velocity gases through the central portion of the vent and erupted at the surface to contribute more air-fall ash to developing surface beds. 相似文献
3.
Antoni Camprubí Carles Canet Augusto A. Rodríguez-Díaz Rosa M. Prol-Ledesma David Blanco-Florido Ruth E. Villanueva Abigail López-Sánchez 《Island Arc》2008,17(1):6-25
Abstract Bahía Concepción is located in the eastern coast of the Baja California peninsula and it is shaped by northwestern–southeastern normal faults. These are associated with a 12–6 Ma rifting episode, although some have been reactivated since the Pliocene. The most abundant rocks correspond to the arc related Comondú Group, Oligocene to Miocene, which forms a mainly calc‐alkaline volcanic and volcaniclastic sequence. There are less extensive outcrops of sedimentary rocks, lava flows, domes and pyroclastic rocks of Pliocene to Quaternary ages. The Neogene volcanism in the area indicates a shift from a subduction regime to an intraplate volcanism related to continental extension and the opening of an oceanic basin. The Bahía Concepción area contains numerous Mn ore deposits, being the biggest at El Gavilán and Guadalupe. The Mn deposits occur as veins, breccias and stockworks, and are composed by Mn oxides (pyrolusite, coronadite, romanechite), dolomite, quartz and barite. The deposits are hosted in volcanic rocks of the Comondú Group and, locally, in Pliocene sedimentary rocks. Thus, the Mn deposits formed between the Middle Miocene and the Pliocene. The mineralized structures are associated with Miocene northwestern–southeastern fault systems, which are analogous to those associated with the Cu‐Co‐Zn‐Mn deposits of El Boleo. The Bahía Concepción area also bears subaerial and submarine hot springs, which are associated with the same fault systems and host rocks. The submarine and subaerial geothermal manifestations south of the bay are possibly related with recent volcanism. The geothermal manifestations within the bay are intertidal hot springs and shallow submarine diffuse venting areas. Around the submarine vents (5–15 m deep, 87°C), Fe‐oxyhydroxide crusts with pyrite and cinnabar precipitate. In the intertidal vents (62°C), aggregates of opal, calcite, barite and Ba‐rich Mn oxides occur covered by silica‐carbonate stromatolitic sinters. Some 10–30 cm thick crustiform veins formed by chalcedony, calcite and barite are also found close to the vents. The hydrothermal fluids exhibit mixed isotopic compositions between δ18O‐enriched meteoric and local marine water. The precipitation of Ba‐rich Mn oxides around the vent sites could be an active analog for the processes that produced Miocene to Pliocene hydrothermal Mn‐deposits. 相似文献
4.
5.
The Calipuy Formation is a primarily volcanic sequence deposited during the period 33 to 10 m.y. ago when basaltic and andesitic volcanoes developed concomitant with a N60°E horizontal foreshortening of the Peruvian Andes. The axis of compression is inferred from both fault tectonics in and near the Calipuy sequence and from N30°W-striking fold axes within it. Dacitic domes younger than 10 m.y. unconformably overlie the Calipuy sequence.Basaltic and andesitic effusive rocks concurrently filled a basin which developed during volcanism. During subsidence 80% of the fill was provided by primary extrusive igneous material, whereas sedimentary rocks associated with the igneous assemblage account for only 20%.Limited chemical data indicate that Calipuy andesitic rocks are slightly richer in alkalies than the average Cenozoic andesite, but petrographic data show that they are similar to other andesites of this age found in similar environments in the Andes. However, the analyses are too few to make any real generalization concerning petrogenesis. 相似文献
6.
At Gross Brukkaros a central depression has developed within domed Nama Group sediments and has functioned as a local depocenter, with a primary fill deposited during the Cretaceous and a small secondary fill by alluvial fans during the Tertiary and Quaternary. The diameter of the entire structure is about 10 km and that of the central depression is about 3 km. Within this depocenter the sedimentary sequence consists mainly of debris-flow and mudflow deposits, with minor intercalations of fluviatile (braided channel) sediments and fossiliferous lacustrine deposits. The sedimentary system represents a set of coalesced subaerial fans which formed a fringing sedimentary apron along the margin of the depocenter. This sedimentary apron passed distally and centrally into a permanent lake, which was characterized by a fluctuating water level. Facies transitions observed are typical of those described from modern and ancient fan delta systems. Contact relationships show the Gross Brukkaros sediments to be about the same age (Upper Cretaceous) as the surrounding carbonatitic volcanism. An Upper Cretaceous age is also consistent with the plant fossil association recently recognized within the lacustrine beds of Gross Brukkaros. We attribute the genesis of the dome structure to the shallow intrusion of a laccolith-shaped, strongly alkaline to carbonatitic magma body. Subsequent depletion of the reservoir due to volcanic activity around and in(?) Gross Brukkaros led to subsidence resulting in the development of the Gross Brukkaros depocenter. Differences between Gross Brukkaros and the general caldera model consist of a radially oriented dike pattern and the formation of the caldera by downsagging rather than cauldron subsidence, as derived from the absence of ring faults and ring dikes. The first (radial dikes) may be attributed to comparatively strong initial doming; the latter (lack of ring faults) to the small size of the caldera, its incremental subsidence, and finally the sedimentary wall rocks instead of a rigid crystalline crust. 相似文献
7.
Characteristics of gravity anomalies and prediction of volcanosedimentary boron deposit distribution in the Xiongba area,Tibet 总被引：1，自引：0，他引：1
Volcanosedimentary boron deposits are present within Tertiary lacustrine sediments and volcanic rocks in Xiongba, Tibet. Boron deposits are characterized by low density relative to country rocks; thus, it is possible to locate them by gravity measurements. We conducted a 1:50000 high-precision gravity survey in the Xiongba area, Tibet, and obtained the Bouguer and residual gravity anomalies. We analyzed fault systems and the distribution of sedimentary and volcanic rocks and their relation to the volcanosedimentary boron deposits. The processing of the gravity data revealed local gravity variations and fault structures. We applied preferential downward continuation and wavelet transform to the gravity data, and in conjunction with geological data, we predicted the distribution of volcanosedimentary boron deposits. 相似文献
8.
The ultramafic Eocene Missouri River Breaks volcanic field (MRBVF, Montana, USA) includes over 50 diatremes emplaced in a mostly soft substrate. The current erosion level is 1.3–1.5 km below the pre-eruptive surface, exposing the deep part of the diatreme structures and some dikes. Five representative diatremes are described here; they are 200-375 m across and have sub-vertical walls. Their infill consists mostly of 55-90 % bedded pyroclastic rocks (fine tuffs to coarse lapilli tuffs) with concave-upward bedding, and 45–10 % non-bedded pyroclastic rocks (medium lapilli tuffs to tuff breccias). The latter zones form steep columns 15–135 m in horizontal dimension, which cross-cut the bedded pyroclastic rocks. Megablocks of the host sedimentary formations are also present in the diatremes, some being found 1 km or more below their sources. The diatreme infill contains abundant lithic clasts and ash-sized particles, indicating efficient fragmentation of magma and country rocks. The spherical to sub-spherical juvenile clasts are non-vesicular. They are accompanied by minor accretionary lapilli and armored lapilli. The deposits of dilute pyroclastic density currents are locally observed. Our main interpretations are as follows: (1) the observations strongly support phreatomagmatic explosions as the energy source for fragmentation and diatreme excavation; (2) the bedded pyroclastic rocks were deposited on the crater floor, and subsided by 1.0–1.3 km to their current location, with subsidence taking place mostly during the eruption; (3) the observed non-bedded pyroclastic columns were created by debris jets that punched through the bedded pyroclastic material; the debris jets did not empty the mature diatreme, occupying only a fraction of its width, and some debris jets probably did not reach the crater floor; (4) the mature diatreme was nearly always filled and buttressed by pyroclastic debris at depth – there was never a 1.3–1.5-km-deep empty hole with sub-vertical walls, otherwise the soft substrate would have collapsed inward, which it only did near the surface, to create the megablocks. We infer that syn-eruptive subsidence shifted down bedded pyroclastic material and shallow sedimentary megablocks by 0.8–1.1 km or more, after which limited post-eruptive subsidence occurred. This makes the MRBVF diatremes an extreme end-member case of syn-eruptive subsidence in the spectrum of possibilities for maar-diatreme volcanoes worldwide. 相似文献
9.
Yuji Nishi Steven Sherburn Bradley J. Scott Mituhiko Sugihara 《Journal of Volcanology and Geothermal Research》1996,72(3-4)
Volcano-tectonic earthquakes at White Island are concentrated in a single seismically active zone, southeast of the active vents and at depths of less than 1 km. A few deeper earthquakes also occur beneath the active vents. A composite focal mechanism indicates that the stress regime in the shallow seismic zone is N-S extensional. Shallow seismicity occurs within the main volume of the volcano-hydrothermal system that underlies the Main Crater floor, and we interpret this as a region where the rocks have been weakened by past magmatic intrusions, elevated pore fluid pressure and physico-chemical effects of acid volcanic fluids, thereby allowing preferential seismic failure. Brittle seismic failure within this region requires a temperature less than about 400 °C, and implies high horizontal temperature gradients close to the active craters and fumaroles. Spasmodic bursts events are also a result of brittle failure, but occur close to zones of significant permeability in response to changes in local fluid pressure. 相似文献
10.
M. J. Le Bas 《Bulletin of Volcanology》1970,34(2):518-536
The Nyamaji volcano is a small eruptive complex of late Miocene age associated with the nearby Usaki ijolite and Sokolo carbonatite intrusion of Homa Bay in the Kavirondo Rift valley of Kenya. It is probably a satellite volcano to the major volcanic structure of Kisingiri - Rangwa which lies 25 km to the west. The Nyamaji volcanic complex is composed of agglomerates, breccias and tuffs erupted from a central vent, whilst at much the same time lavas were extruded from fissures which are now occupied by dykes. These two contemporaneous events gave rise to an interdigitated sequence of pyroclastic deposits and effusive lavas. The pyroclastic rocks of Vulcanian origin cover an area at least 30 km2 in extent, are poorly bedded, and usually are about 25 m (80ft.) thick though they often thin to zero over topographic highs in the pre-existing landscape. At Nyamaji itself, the Strombolian style pyroclastic pile exceeds 330 m (1100 ft.) in thickness over an area of 1 km2, and this marks the position of the original central vent. The fragmental material in the pyroclastic rocks includes ijolite, phonolite, nephelinite, trachyte, carbonatite, granite, and feldspathic and aegirine-bearing fenites; the matrix is sometimes calcareous, sometimes feldspathic. Nephelinitic lavas occur amongst the lowest lavas, but the lavas above are nearly all phonolitic. The oldest dykes are nephelinitic and are rare; the youngest dykes are phonolitic and are abundantly exposed. Both lavas and dykes contain xenoliths similar to those in the pyroclastic rocks. A series of volcanic plugs pierce the lavas. These plugs, mostly non-conduit type, average 200–500 m diameter, are mainly composed of glassy to very fine-grained phonolites, and show good flow structures. The plugs, especially those near the Ruri hills, tend to lie along N - S and E - W lines. The majority of the dykes also lie along these directions. The dominant structural directions within the nearby Usaki ijolite complex and the Wasaki carbonatite are also N - S and E - W, respectively. These directions are quite different from the axis of the Kavirondo rift valley which here is NE - SW, and from the strike of the Precambrian basement. The Nyamaji volcanic structure differs from nearly all the other East African volcanoes by its dominant phonolitic petrochemistry. 相似文献
11.
Gravity and magnetic signatures of volcanic plugs related to Deccan volcanism in Saurashtra, India and their physical and geochemical properties 总被引：1，自引：0，他引：1
D.V. ChandrasekharD.C. Mishra G.V.S. Poornachandra RaoJ. Mallikharjuna Rao 《Earth and Planetary Science Letters》2002,201(2):277-292
The Bouguer anomaly and the total intensity magnetic maps of Saurashtra have delineated six circular gravity highs and magnetic anomalies of 40-60 mGal (10−5m/s2) and 800-1000 nT, respectively. Three of them in western Saurashtra coincide with known volcanic plugs associated with Deccan Volcanic Province (DVP), while the other three in SE Saurashtra coincide with rather concealed plugs exposed partially. The DVP represents different phases of eruption during 65.5±2.5 Ma from the Reunion plume. The geochemical data of the exposed rock samples from these plugs exhibit a wide variation in source composition, which varies from ultramafic/mafic to felsic composition of volcanic plugs in western Saurashtra and an alkaline composition for those in SE Saurashtra. Detailed studies of granophyres and alkaline rocks from these volcanic plugs reveal a calc-alkaline differentiation trend and a continental tectonic setting of emplacement. The alkaline plugs of SE Saurashtra are associated with NE-SW oriented structural trends, related to the Gulf of Cambay and the Cambay rift basin along the track of the Reunion plume. This indicates a deeper source for these plugs compared to those in the western part and may represent the primary source magma. The Junagadh plug with well differentiated ring complexes in western Saurashtra shows well defined centers of magnetic anomaly while the magnetic anomalies due to other plugs are diffused though of the same amplitude. This implies that other plugs are also associated with mafic/ultramafic components, which may not be differentiated and may be present at subsurface levels. Paleomagnetic measurements on surface rock samples from DVP in Saurashtra suggest a susceptibility of 5.5×10−2 SI units with an average Koenigsberger ratio (Qn) of almost one and average direction of remanent magnetization of D=147.4° and I=+56.1°. The virtual geomagnetic pole (VGP) position computed from the mean direction of magnetization for the volcanic plugs and Deccan basalt of Saurashtra is 30°N and 74°W, which is close to the VGP position corresponding to the early phases of Deccan eruption. Modeling of gravity and magnetic anomalies along two representative profiles across Junagadh and Barda volcanic plugs suggest a bulk density of 2900 and 2880 kg/m3, respectively and susceptibility of 3.14×10−2 SI units with a Qn ratio of 0.56 which are within the range of their values obtained from laboratory measurements on exposed rock samples. The same order of gravity and magnetic anomalies observed over the volcanic plugs of Saurashtra indicates almost similar bulk physical properties for them. The inferred directions of magnetization from magnetic anomalies, however, are D=337° and 340° and I=−38° and −50° which represent the bulk direction of magnetization and also indicate a reversal of the magnetic field during the eruption of these plugs. Some of these plugs are associated with seismic activities of magnitude ≤4 at their contacts. Based on this analysis, other circular/semi-circular gravity highs of NW India can be qualitatively attributed to similar subsurface volcanic plugs. 相似文献
12.
13.
Western Anatolia, largely affected by extensional tectonics, witnessed widespread volcanic activity since the Early Miocene. The volcanic vents of the region are represented by epicontinental calderas, stratovolcanoes and monogenetic vents which are associated with small-scale intrusions as sills and dykes. The volcanic activity began with an explosive character producing a large ignimbritic plateau all over the region, indicating the initiation of the crustal extension event. These rhyolitic magmas are nearly contemporaneous with granitic intrusions in western Anatolia. The ignimbrites, emplaced approximately contemporaneous with alluvial fan and braided river deposits, flowed over the basement rocks prior to extensional basin formation. The lacustrine deposits overlie the ignimbrites. The potassic and ultrapotassic lavas with lamprophyric affinities were emplaced during the Late Miocene–Pliocene. The volcanic activities have continued with alkali basalts during the Quaternary. 相似文献
14.
G. Wadge 《Bulletin of Volcanology》1986,48(6):349-372
The orientations of dykes from many of the islands of the Lesser Antilles island arc have been mapped. Most of these dykes can be interpreted in terms of local or regional swarms derived from specific volcanoes of known age, with distinct preferred orientations. Dykes are known from all Cenozoic epochs except the Palaeocene, but are most common in Pliocene, Miocene and Oligocene rocks. A majority of the sampled dykes are basaltic, intrude volcaniclastic host rocks and show a preference for widths of 1–1.25 m. Locally, dyke swarms dilate their hosts by up to 9% over hundreds of metres and up to 2% over distances of kilometres. The azimuths of dykes of all ages show a general NE-SW preferred orientation with a second NW-SE mode particularly in the Miocene rocks of Martinique. The regional setting for these minor intrusions is a volcanic front above a subduction zone composed of three segments: Saba-Montserrat, Guadeloupe-Martinique, St. Lucia-Grenada. The spacing of volcanic centres along this front is interpreted in terms of rising plumes of basaltic magma spaced about 30 km apart. This magma is normally intercepted at crustal depths by dioritic plutons and andesitic/dacitic magma generated there. Plumes which intersect transverse fracture systems or which migrate along the front can avoid these crustal traps. Throughout its history the volcanic front as a whole has migrated, episodically, towards the backarc at an average velocity of about 1 km/Ma. The local direction of plate convergence is negatively correlated with the local preferred orientation of dykes. The dominant NE-SW azimuth mode corresponds closely to the direction of faulting in the sedimentary cover of the backarc and the inferred tectonic fabric of the oceanic crust on which the arc is founded. A generalised model of the regional stress field that controls dyke intrusion outside of the immediate vicinity of central volcanic vents is proposed, in which the maximum horizontal stress parallels the volcanic front except in the northern segment where subduction of the Barracuda Rise perturbs the stress field. There is also evidence of specific temporal changes in the stress field that are probably due to large scale plate kinematics. 相似文献
15.
《Journal of Volcanology and Geothermal Research》2007,159(1-3):4-32
The understanding of processes within the root zone of maar–diatreme volcanoes is important for the interpretation of the geology, volcanology and even hazard assessment of these volcanoes. In the phreatomagmatic model of pipe formation, the irregularly shaped root zone is the site of the phreatomagmatic explosions, and thus functions as the “engine” for pipe formation. In this model the root zone grows over a period of time in a series of many single thermohydraulic, i.e. phreatomagmatic, explosions. The explosions initially occur close to the surface and with ongoing explosive activity penetrate towards deeper levels. The ejection of country rock clasts from the root zone results in a mass deficiency in the root zone that causes the overlying tephra and the adjacent country rocks to subside passively in a sinkhole-like fashion into the root zone. Many phreatomagmatic eruptions consequently result in the formation of a cone-shaped diatreme. Thus with ongoing eruptions the cone-shaped diatreme has to grow systematically both in depth and diameter. During its growth, processes in the lower diatreme levels successively destroy the upper levels of the evolving root zone. At the surface, the maar crater in turn reacts to the underlying subsidence processes and also grows both in depth and diameter.Thermohydraulic explosions, which fragment both magma and the surrounding country rocks, mostly occur within the bottom part of the root zone. Violent explosions in small pipes may clear the overlying diatreme for a short period of time before tephra fall and collapse of the walls of the new crater refill the small initial diatreme. In larger pipes, via expansion of the mixture of highly pressurized water vapor, juvenile gas phases and explosively produced tephra, the confined and expanding eruption cloud has to pierce through the diatreme fill in a feeder conduit in order to erupt. Diatreme-clearing events in large pipes are difficult or impossible to maintain, since the explosive force in the root zone is only in exceptional instances strong enough to lift or entrain the entire diatreme tephra. Knowledge of the genetic relationships between root zones and diatremes is critical to understand pipe growth processes. The combination of such processes can lead to substantial variation in volcanic behavior and thus produce fundamentally different volcano and rock types.It is the purpose of this paper to outline important features of root zones and suggest their significance for the genesis and evolution of maar–diatreme and related volcanoes. 相似文献
16.
An ENE-WSW-trending localized basalt-diabase outcrop along the SE margin of Luk Ulo Mélange Complex has been suggested as intrusive rocks cut through the Paleogene Totogan and Karangsambung formations. However, the absolute dating of the volcanics is older than the inferred relative age of the sedimentary formations, hence the in-situ intrusion theory is less likely. A subsurface imaging should delineate the possibility of the in-situ nature of volcanic rock by looking at the continuation of the rocks to the depth. In this study, we did a subsurface imaging by electrical resistivity method. The electrical resistivity surveys were conducted at 3 (three) lines across the ENE-WSW trend of the volcanic distribution. From those three measurements, we obtained three inversion models that present the distribution of the resistivity. We could differentiate between the high resistivity of volcanic rocks and the low resistivity of the clay-dominated sediments. Instead of the deep-rooted intrusions, the geometry of the volcanic rocks is concordant with the sedimentary strata. Since we do not observe any spatial continuity of the bodies, both laterally and vertically, the volcanic rocks might be part of broken intrusive rocks. Furthermore, the size and the sporadically distributed of the rocks also indicated that they are more likely as fragments during the olistostrome deposition, transported from its original location. 相似文献
17.
The Yampa and Elkhead Mountains volcanic fields were erupted into sediment-filled fault basins during Miocene crustal extension in NW Colorado. Post-Miocene uplift and erosion has exposed alkali basalt lavas, pyroclastic deposits, volcanic necks and dykes which record hydrovolcanic and strombolian phenomena at different erosion depths. The occurrence of these different phenomena was related to the degree of lithification of the rocks through which the magmas rose. Hydrovolcanic interactions only occurred where rising basaltic magma encountered wet, porous, non-lithified sediments of the 600 m thick Miocene Brown's Park Formation. The interactions were fuelled by groundwater in these sediments: there was probably no standing surface water. Dykes intruded into the sediments have pillowed sides, and local swirled inclusions of sediment that were injected while fluidized in steam from heated pore water. Volcanic necks in the sediments consist of basaltic tuff, sediment blocks and separated grains derived from the sediments, lithic blocks (mostly derived from a conglomerate forming the local base of the Brown's Park Formation), and dykes composed of disaggregated sediment. The necks are cut by contemporaneous basalt dykes. Hydrovolcanic pyroclastic deposits formed tuff cones up to 100 m thick consisting of bedded air-fall, pyroclastic surge, and massive, poorly sorted deposits (MPSDs). All these contain sub-equal volumes of basaltic tuff and disaggregated sediment grains from the Brown's Park Formation. Possible explosive and effusive modes of formation for the MPSDs are discussed. Contemporaneous strombolian scoria deposits overlie lithified Cretaceous sedimentary rocks or thick basalt lavas. Volcanic necks intruded into the Cretaceous rocks consist of basalt clasts (some with spindle-shape), lithic clasts, and megacrysts derived from the magma, and are cut by basalt dykes. Rarely, strombolian deposits are interbedded with hydrovolcanic pyroclastic deposits, recording changes in eruption behaviour during one eruption. The hydrovolcanic eruptions occurred by interaction of magma with groundwater in the Brown's Park sediments. The explosive interactions disaggregated the sediment. Such direct digestion of sediment by the magma in the vents would probably not have released enough water to maintain a water/magma mass ratio sufficient for hydrovolcanic explosions to produce the tuff cones. Probably, additional water (perhaps 76% of the total) was derived by flow through the permeable sediments (especially the basal conglomerate to the formation), and into the vents. 相似文献
18.
The three-dimensional arrangement of volcanic deposits in strike-slip basins is not only the product of volcanic processes,
but also of tectonic processes. We use a strike-slip basin within the Jurassic arc of southern Arizona (Santa Rita Glance
Conglomerate) to construct a facies model for a strike-slip basin dominated by volcanism. This model is applicable to releasing-bend
strike-slip basins, bounded on one side by a curved and dipping strike-slip fault, and on the other by curved normal faults.
Numerous, very deep unconformities are formed during localized uplift in the basin as it passes through smaller restraining
bends along the strike-slip fault. In our facies model, the basin fill thins and volcanism decreases markedly away from the
master strike-slip fault (“deep” end), where subsidence is greatest, toward the basin-bounding normal faults (“shallow” end).
Talus cone-alluvial fan deposits are largely restricted to the master fault-proximal (deep) end of the basin. Volcanic centers
are sited along the master fault and along splays of it within the master fault-proximal (deep) end of the basin. To a lesser
degree, volcanic centers also form along the curved faults that form structural highs between sub-basins and those that bound
the distal ends of the basin. Abundant volcanism along the master fault and its splays kept the deep (master fault-proximal)
end of the basin overfilled, so that it could not provide accommodation for reworked tuffs and extrabasinally-sourced ignimbrites
that dominate the shallow (underfilled) end of the basin. This pattern of basin fill contrasts markedly with that of nonvolcanic
strike-slip basins on transform margins, where clastic sedimentation commonly cannot keep pace with subsidence in the master
fault-proximal end. Volcanic and subvolcanic rocks in the strike-slip basin largely record polygenetic (explosive and effusive)
small-volume eruptions from many vents in the complexly faulted basin, referred to here as multi-vent complexes. Multi-vent
complexes like these reflect proximity to a continuously active fault zone, where numerous strands of the fault frequently
plumb small batches of magma to the surface. Releasing-bend extension promotes small, multivent styles of volcanism in preference
to caldera collapse, which is more likely to form at releasing step-overs along a strike-slip fault.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
19.
Carbonatite-alkalic rocks occur in the form of dykes and small volcanic plugs in the area, with major central type volcanic activity restricted to Amba Dongar. The trappean flows of Blanford and Bose are identified as plagioclasecalcite rocks. An attempt is made to explain the origin of these rocks which are extensively cut by dykes of alkaline rocks, carbonatites and dolerites. By far the dominant lavas are «fissure phonolites» (Wright, 1963) and tinguaites. The chemical analyses of these rocks show that the magma is mainly of continental sodic alkaline suite, probably turning sodi-potassic, a suggestion drawn from the occurrence of lamprophyres and pseudoleucite tinguaites, and the higher potassium contents of some rocks. Bagh sediments are mainly represented by sandstones which show mild contact effects with carbonatite, especially in the south. 相似文献
20.
W. I. Rose Jr. G. T. Penfield J. W. Drexler P. B. Larson 《Bulletin of Volcanology》1980,43(1):131-153
Three composite cones have grown on the southern edge of the previously existing Atitlán Cauldron, along the active volcanic axis of Guatemala. Lavas exposed on the flanks of these cones are generally calc-alkaline andesites, but their chemical compositions vary widely. Atitlán, the largest and most southerly of the three cones, has recently erupted mainly pyroclastic basaltic andesites, while the flanks of San Pedro and Tolimán are mantled by more silicic lava flows. On Tolimán, 74 different lava units have been mapped, forming the basis for sequential sampling. Rocks of all three cones are consistently higher in K2O, Rb, Ba and REE than other Guatemalan andesites. Atitlán’s rocks and late lavas from Tolimán have high Al2O3 content, compared to similar andesites from other nearby cones. All major and trace element data on the rocks are shown to be consistent with crystal fractionation involving phases observed in the rocks. If such models are correct, significant differences in the relative proportions of fractionation phases are necessary to explain the varied compositions, in particular higher Al2O3 rocks have fractionated less plagioclase. We speculate that inhibition of plagioclase fractionation could occur in chambers where PH2O is greater and when repose intervals are shorter. The distribution of volcanic vents throughout Guatemala which show this postulated «inhibition of plagioclase fractionation» is systematic with such vents lying just to the south of the main axis. The andesites of the three cones cannot be simply related to the late-Pleistocene rhyolites which are apparently associated with cauldron formation, because unlike the andesites, the rhyolites have markedly depleted heavy REE abundances. Recent dacitic lavas from vents south of San Pedro volcano and silicic pyroclastic rocks which mantle the slopes the San Pedro may reflect residual post-cauldron rhyolitic volcanism. 相似文献