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1.
Large vesicles record pathways of degassing at basaltic volcanoes   总被引：2，自引：2，他引：0  
Volcanic degassing is directly linked to magma dynamics and controls the style of eruptive activity. To better understand how gas is transported within basaltic magma we perform a 3D investigation of vesicles preserved in scoria from the 2005 activity at Stromboli volcano (Italy). We find that clasts are characterized by the ubiquitous occurrence of one to a few large vesicles, exhibiting mostly irregular, tortuous, channel-like textures, orders of magnitude greater in volume than all the other vesicles in the sample. We compare observations on natural samples with results from numerical simulations and experimental investigations of vesicle size distributions and demonstrate that this type of vesicle invariably forms in magmas with vesicularities > 0.30 (and possibly > 0.10). We suggest that large vesicles represent pathways used by gas to flow non-explosively to the surface and that they indicate the development of an efficient system that sustains persistent degassing in basaltic systems.  相似文献   

2.
Cinder cones at Crater Lake are composed of high-alumina basaltic to andesitic scoria and lavas. The Williams Crater Complex, a basaltic cinder cone with andesitic to dacitic lava flows, stands on the western edge of the caldera, against an andesite flow from Mount Mazama. Bombs erupted from Williams Crater contain cores of banded andesite and dacite, similar to those erupted during the climatic eruption of Mount Mazama.Major- and trace-element variations exhibit an increase in incompatible elements and a decrease in compatible elements, consistent with crystal fractionation of olivine, plagioclase, clinopyroxene, orthopyroxene, and magnetite. LREE patterns in the rocks are irregular; each successive basalt is enriched in LREE relative to the preceding andesite.Compositional variations in the magmas of the cinder cones suggest that three magmatic processes were involved, partial melting, fractional crystallization, and magma mixing. Partial melting of more than one source produced primary basaltic magma(s). Subsequent mixing and fractional crystallization produced the more differentiated basaltic to andesitic magmas.  相似文献   

3.
Two Miocene basaltic andesite pillowed sills in the Shimane Peninsula, SW Japan, were intruded into wet marine sediments, plastically deforming them. The pillows are elongated, constricted, interconnected and relatively closely packed. Individual pillows have a poorly to moderately vesiculated, somewhat crystalline rind thinner than a few centimeters and a moderately to well vesiculated, more crystalline core; contraction cracks and spreading cracks are poorly developed. The pillows in the sills morphologically resemble pillow lava flows, and during sill intrusion, the magma bifurcated into pillow lobes in a manner similar to pillow lavas. Formation of pillows in sill probably occurs when the magma is intruded into wet sediments and protrudes fingers by the instability of the magma-sediment interface with little turbulence of magma flow.  相似文献   

4.
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   

5.
At Rakiraki in northeastern Viti Levu, the Pliocene Ba Volcanic Group comprises gently dipping, pyroxene-phyric basaltic lavas, including pillow lava, and texturally diverse volcanic breccia interbedded with conglomerate and sandstone. Three main facies associations have been identified: (1) The primary volcanic facies association includes massive basalt (flows and sills), pillow lava and related in-situ breccia (pillow-fragment breccia, autobreccia, in-situ hyaloclastite, peperite). (2) The resedimented volcaniclastic facies association consists of bedded, monomict volcanic breccia and scoria lapilli-rich breccia. (3) The volcanogenic sedimentary facies association is composed of bedded, polymict conglomerate and breccia, together with volcanic sandstone and siltstone-mudstone facies. Pillow lava and coarse hyaloclastite breccia indicate a submarine depositional setting for most of the sequence. Thick, massive to graded beds of polymict breccia and conglomerate are interpreted as volcaniclastic mass-flow deposits emplaced below wave base. Well-rounded clasts in conglomerate were reworked during subaerial transport and/or temporary storage in shoreline or shallow water environments prior to redeposition. Red, oxidised lava and scoria clasts in bedded breccia and conglomerate also imply that the source was partly subaerial. The facies assemblage is consistent with a setting on the submerged flanks of a shoaling basaltic seamount. The coarse grade and large volume of conglomerate and breccia reflect the high supply rate of clasts, and the propensity for collapse and redeposition on steep palaeoslopes. The clast supply may have been boosted by vigorous fragmentation processes accompanying transition of lava from subaerial to submarine settings. The greater proportion of primary volcanic facies compared with resedimented volcaniclastic and volcanogenic sedimentary facies in central and northwestern exposures (near Rakiraki) indicates they are more proximal than those in the southeast (towards Viti Levu Bay). The proximal area coincides with one of two zones where NW-SE-trending mafic dykes are especially abundant, and it is close to several, small, dome-like intrusions of intermediate and felsic igneous rocks. The original surface morphology of the volcano is no longer preserved, though the partial fan of bedding dip azimuths in the south and east and the wide diameter (exceeding 20 km) are consistent with a broad shield.  相似文献   

6.
The Kverkfjöll area, NE Iceland is characterised by subglacial basalt pillow lavas erupted under thick ice during the last major glaciation in Iceland. The water contents of slightly vesiculated glassy rims of pillows in six localities range from 0.85±0.03 to 1.04±0.03 wt %. The water content measurements allow the ice thickness to be estimated at between 1.2 and 1.6 km, with the range reflecting the uncertainty in the CO2 and water contents of the melt. The upper estimates agree with other observations and models that the ice thickness in the centre of Iceland was 1.5–2.0 km at the time of the last glacial maximum. Many of the pillows in the Kverkfjöll area are characterised by vesiculated cores (40–60% vesicles) surrounded by a thick outer zone of moderately vesicular basalt (15–20% vesicles). The core contains ～1 mm diameter spherical vesicles distributed uniformly. This observation suggests a sudden decompression and vesiculation of the still molten core followed by rapid cooling. The cores are attributed to a jökulhlaup in which melt water created by the eruption is suddenly released reducing the environmental pressure. Mass balance and solubility relationships for water allow a pressure decrease to be calculated from the observed change of vesicularity of between 4.4 and 4.7 MPa depressurization equivalent to a drop in the water level in the range 440–470 m. Consideration of the thickness of solid crust around the molten cores at the time of the jökulhlaup indicates an interval of 1–3 days between pillow emplacement and the jökulhlaup. Upper limits for ice melting rates of order 10?3 m/s are indicated. This interpretation suggests that jökulhlaups can reactivate eruptions.  相似文献   

7.
A series of experiments was conducted to test concepts of porous flow degassing of rhyolitic magma during ascent and of the subsequent collapse of vesicles in degassed magma to form obsidian. Dense, synthetically hydrated, natural glasses were pressurized under water-saturated conditions and then decompressed to achieve a range of porosities in the presence of a tracer vapor, D2O. Rapid isotopic exchange indicative of vapor transport rather than of simple diffusion occurred at a porosity >60 vol.%, in accord with earlier gas permeability measurements on cold natural samples. In another series of experiments, natural and synthetic pumices, vesiculated by degassing to atmospheric pressure, rapidly collapsed to dense glass on repressurization to the modest pressures prevailing in lava flows. No relict bubble textures remained. These results support the hypothesis that effusive eruptions result from the syneruptive escape of gas from permeable magmatic foam, and that a process analogous to welding yields dense lavas when such foams are extruded.  相似文献   

8.
Chemical data are presented for the basic lavas of the two volcanic shields, Piton des Neiges and Piton de la Fournaise, which comprise Reunion Island. In addition, data for cumulate xenoliths have been used to predict mineral/melt distribution coefficient values for the Reunion magmas.The younger volcanic shield, Piton de la Fournaise, comprises two lava sequences, the >0.5−0.2-m.y. B.P. Primary Shield lavas, and the <0.2-m.y. B.P. Caldera Series lavas. Fractional crystallization models for these lavas indicate that olivine is the major fractionating phase during the evolution from the parental basalt composition to the average basaltic liquid. Only during the evolution of the older, Primary Shield lavas has the common fractionation of an ol + cpx + plag + mt assemblage resulted in the eruption of hawaiitic, ankaramitic and feldspar-phyric lavas. The restriction of the Caldera Series liquids predominantly to olivine fractionation and the extensive cotectic fractionation during the evolution of the Primary Shield sequences is interpreted in terms of the maturity of the volcanic center. The younger stages of evolution involve high magma input into a well-developed feeder and reservoir system, thus maintaining the liquids above a cotectic surface. Whereas, during the evolution of the Primary Shield lavas, lower magma input rates into a less well-developed feeder system increased the probability of the fractionating liquid attaining a cotectic surface. Fractional crystallization accounts for all the chemical variation observed for the Piton de la Fournaise basaltic magmas. The analytical data are closely comparable to the rare earth element (REE) and trace element fractionation curves predicted by least-squares calculations, this supports the use of such models in quantitative evaluation of fractional crystallization.A preliminary survey of Sr isotope values indicates that the oldest (>2 m.y. B.P.) lava sequences of Piton des Neiges may be derived from a source which was isotopically distinct from that of the <2 m.y. B.P. lavas of both volcanic shields. These latter sequences are remarkably consistent in both isotopic and trace element abundance implying a homogeneous source material and an invariable partial melting process. Partial melting calculations indicate that the basaltic lavas have been derived by 5–10% melting of a garnet-poor peridotite (cpx/gt 9). Systematic differences in the light- and heavy-REE patterns between similar basaltic provinces are interpreted to be a result of variation in the nature of the phases buffering the entry of light- and/or heavy-REE into the melt during partial fusion.  相似文献   

9.
The basaltic volcanism which finishes the eruptive activity of the Cantal volcano shows various patterns of the continuous transition from a lava flow to a pyroclastic flow. These sub-aerial deposits can exibit either a laminar or turbulent aspect. The affected lavas show neither mineralogical nor chemical differences from other basalts which did not present this behaviour. The outpouring was along fissures, in the central area of the volcano, as well as the margins; in this last case, it may have involved phreatomagmatic events. Study of the conditions of fragmentation is possible in the case of these basic lavas, because it was delayed until the magma reached the surface. It leads to a better understanding of a phenomenon that, for acid lavas, usually takes place in the feeding channel.  相似文献   

10.
The Vulsinian volcanic area in central Italy is made up of several volcanic complexes: the older east part of the area, with the large volcano-tectonic depression — in our opinion not simply a caldera — of the « Lago di Bolsena », and the younger Latera volcano are the major features. With the aid of field studies, paleomagnetic measurements, absolute age determinations and petrologic data, a scheme of the volcanic evolution has been drawn up, dividing the volcanic activity into six stages. Volcanicity started about 0.92 M.y. ago and lasted until subrecent. Three rock series are distinguished: a trachytic-phonolitic magma, mainly found as ash flows, a potassic mafic magma with its salic derivatives. mainly as lavas, scoria and pyroclastics, and a trachybasaltic rockseries, mainly found as lavas and scoria.  相似文献   

11.
We studied the distribution of tephra deposits discharged by the basaltic (52–54% SiO2) explosive eruption of 1973 on Tyatya Volcano (Kunashir I., Kuril Islands). We made maps showing lines of equal tephra thickness (isopachs) and lines of maximum size of pyroclastic particles (isopleths). These data were used to find the parameters of explosive activity using the standard techniques for each of the two phases of this eruption separately. The first, phreatomagmatic, phase discharged 0.008 km3 of tephra during the generation of maars on the volcano’s northern slope. The tephra mostly consisted of fragmented host rocks with admixtures of fragments of low vesiculated juvenile basalt. The phase lasted 20 hours, the rate of pyroclastic discharge was 2 × 105 kg/s; the eruptive plume reached heights of 4–6 km with wind speeds within 10 m/s. The second, magmatic, phase discharged 0.07 km3 of tephra during the generation of the Otvazhnyi scoria cone on the volcano’s southeastern slope. The tephra mostly consisted of juvenile basaltic scoria. The highly explosive Plinian part of this phase lasted 36 hours, the rate of pyroclastic discharge was 8 × 105 kg/s; the eruptive plume reached heights of 6–8 km with wind speeds of 10–20 m/s. The total tephra volume discharged by the eruption was approximately 0.08 km3; the total amount of ejected pyroclastic material (including the resulting monogenic edifices) was 0.11 km3; the volume of erupted magma was 0.05 km3 (the conversion was based on 2800 kg/m3 density); the volcanic explosivity index, or VEI, was 3. The production rate of the Tyatya plumbing system is estimated as 3 × 105 m3 magma per annum.  相似文献   

12.
The Igwisi Hills volcanoes (IHV), Tanzania, are unique and important in preserving extra-crater lavas and pyroclastic edifices. They provide critical insights into the eruptive behaviour of kimberlite magmas that are not available at other known kimberlite volcanoes. Cosmogenic 3He dating of olivine crystals from IHV lavas and palaeomagnetic analyses indicates that they are Upper Pleistocene to Holocene in age. This makes them the youngest known kimberlite bodies on Earth by >30?Ma and may indicate a new phase of kimberlite volcanism on the Tanzania craton. Geological mapping, Global Positioning System surveying and field investigations reveal that each volcano comprises partially eroded pyroclastic edifices, craters and lavas. The volcanoes stand <40?m above the surrounding ground and are comparable in size to small monogenetic basaltic volcanoes. Pyroclastic cones consist of diffusely layered pyroclastic fall deposits comprising scoriaceous, pelletal and dense juvenile pyroclasts. Pyroclasts are similar to those documented in many ancient kimberlite pipes, indicating overlap in magma fragmentation dynamics between the Igwisi eruptions and other kimberlite eruptions. Characteristics of the pyroclastic cone deposits, including an absence of ballistic clasts and dominantly poorly vesicular scoria lapillistones and lapilli tuffs, indicate relatively weak explosive activity. Lava flow features indicate unexpectedly high viscosities (estimated at >102 to 106?Pa?s) for kimberlite, attributed to degassing and in-vent cooling. Each volcano is inferred to be the result of a small-volume, short-lived (days to weeks) monogenetic eruption. The eruptive processes of each Igwisi volcano were broadly similar and developed through three phases: (1) fallout of lithic-bearing pyroclastic rocks during explosive excavation of craters and conduits; (2) fallout of juvenile lapilli from unsteady eruption columns and the construction of pyroclastic edifices around the vent; and (3) effusion of degassed viscous magma as lava flows. These processes are similar to those observed for other small-volume monogenetic eruptions (e.g. of basaltic magma).  相似文献   

13.
The trachytic volcanoes of San Miguel Island are emplaced on either side of a young basaltic area, which could correspond to some active branch of the mid-Atlantic rift. Geological and geomagnetic surveys suggest crustal drift of about 1 cm/year on either direction. From the seismic anomaly data, shallow magma chambers have been inferred for those acid volcanoes, where owing to the crustal drift the main vents appear to have been shifted relatively to the fractures feeding the chambers from the deep upper mantle supply. The primitive magma is certainly basaltic; when the surface vents are directly above the deep feeding fractures, only basaltic lavas of some primitive composition can be extruded; but, when the upper vents are laterally displaced, the magma is delayed in the chambers and will differentiate, eventually into trachytic material capable of producing huge pumice explosions.  相似文献   

14.
Mafic and ultramafic xenoliths are well represented within a large basaltic lava field of Stromboli. These basalts, known as San Bartolo lavas, show a high-K calc-alkaline (HKCA) affinity and were erupted <5 ka BP. Xenoliths consist of olivin-gabbro, gabbronorite, anorthosite, dunite, wehrlite and clinopyroxenite. Thermobarometric estimates for the crystallization of gabbroic materials show minima equilibration pressures of 0.17–0.24 GPa, at temperatures ranging from 940 to 1,030°C. These materials interacted with hydrous ascending HKCA basaltic magmas (with temperatures of 1,050–1,100°C) at pressures of about 0.2–0.4 GPa. These pressure regimes are nearly identical to those found for the crystallization of phenocrystic phases within HKCA basaltic lavas. Gabbroic inclusions are regarded as cumulates and represent crystallized portions of earlier HKCA Strombolian basalts.Dunite and wehrlite show porphyroclastic-heterogranular textures, whereas the clinopyroxenite exhibit a mosaic-equigranular texture typical of mantle peridotites. These ultramafic materials are in equilibrium with more primitive basaltic magmas (under moderately hydrous and anhydrous conditions) at pressures of 0.8–1.2 GPa, which is below the crust-mantle transition, located at about 20 km depth under Stromboli.Major and trace element distributions indicate comagmatism between the host basaltic lava and the mafic and ultramafic inclusions. REE patterns for mafic nodules are relatively regular and overlap the field of basaltic lavas (HKCA). They show moderate to high LREE enrichments and moderate enrichments in HREE relative to chonrites. Spider diagrams also show significant similarities between the lavas and the mafic-ultramafic xenoliths as well.During their ascent, primitive Strombolian magmas may be stored in upper-mantle regions where they interact with peridotitic materials and partly differentiate (to give dunite and wehrlite) before migrating to upper crustal levels. In this region, hydrous basaltic magmas (with estimated water contents of 2–3.5 wt%) are stored in the subvolcanic environment, and are allowed to crystallize the gabbroic materials before reaching the surface under nearly anhydrous conditions.An erratum to this article can be found at  相似文献   

15.
Basic pillow lavas from northeastern Newfoundland are shown to be chemically similar to basaltic komatiites and ocean-floor basalts. New trace element data presented for basaltic komatiites indicate that they were derived from a magma more primitive than that producing ocean-floor basalts. A model for the derivation of basaltic komatiite by extensive partial melting of pyrolitic upper mantle at sites of plate separation is proposed.  相似文献   

16.
大同地区玄武岩的岩石学特征   总被引：1，自引：0，他引：1       下载免费PDF全文
大同盆地第四纪的火山可以划分为两部分:西北区(通常称大同火山群)和东南区。西北区的火山群属典型的中心式喷发,其玄武熔岩是碱性橄榄玄武岩。东南区的火山岩属裂隙式喷溢,其玄武岩流主要是由橄榄拉斑玄武岩组成。本文概括了这两个地区的玄武岩类在岩石学、地球化学、稀土和微量元素等方面的差别和各自的特点  相似文献   

17.
Chemical and petrographic analyses of 51 sequential lava flows from the central vent of Mayon volcano show cyclical variation. In the two most recent cycles, from 1800 to 1876 and from 1881 to the present, one to three basaltic flows are followed by six to ten andesitic flows. Modal and whole-rock chemical parameters show the most regular cyclical variation; calculated groundmass chemical parameters vary less regularly. There is also a long-term trend, over approximately 1700 years of exposed section, toward more basic compositions.The cyclical variation in modes and the chemical composition of the lavas apparently results from periodic influxes of basaltic magma from depth into a shallow magma system. Fractional crystallization of olivine, augite, hypersthene, calcic plagioclase, magnetite and pargasitic hornblende produces successively more andesitic lavas until the next influx of basaltic magma. Differentiation in a deep zone of magma generation is not excluded by the data, but is more likely responsible for the overall change toward more basic compositions than for the cyclical variation.Three points in a cycle — the beginning of basaltic lavas, the beginning of andesitic lavas and a leveling-off of SiO2, K2 O and K2O/Na2O values — correspond roughly to the beginning of frequent effusive eruptions (with or without an early Plinian eruption), frequent weak to moderately explosive (Strombolian) eruptions, and less frequent explosive (Vulcanian) eruptions, respectively. Recognition of the current stage in a cycle can give a qualitative indication of the nature of forthcoming eruptions. Changes in several specific parameters may precede basaltic lavas and allow early detection of basaltic influxes. These include minima in the glass inclusion/plagioclase phenocryst and phenocryst/groundmass ratios, vesicularity and groundmass TiO2, a decrease in hypersthene phenocrysts, and constant values for the whole-rock K2O/Na2O ratio. The Mayon area is densely populated, making prediction of eruption type important for safety and land-use planning.  相似文献   

18.
吉林龙岗四海火山碎屑物粒度分析与地质意义   总被引：4，自引：2，他引：2       下载免费PDF全文
四海火山灰是龙岗火山群中的一次火山爆发形成的,这次火山爆发形成的玄武质空降堆积物分别组成金龙顶子火山渣锥和位于金龙顶子火山锥以东的、分布于辉南县红旗林场和靖宇县四海林场一带的低缓开阔的火山碎屑席。通过投点得知金龙顶子火山喷发类型为次布里尼式(Sub-Plinian)喷发,反映金龙顶子火山爆发强度很大。四海火山灰空降碎屑物7个样品的粒度累计频率曲线投点分布范围、集中区域均有较好的一致性,累计频率曲线表明碎屑物在空中搬运与沉降时都经过了类似的重力分选作用。近火口缘样品粗粒碎屑含量较高,随着与火口缘距离的增加,粗粒部分含量明显降低,细粒碎屑含量增加趋势明显。龙岗火山区内其它岩渣锥火山碎屑物粒度分布范围明显宽于四海火山灰粒度分布范围,累积频率曲线斜率较为一致。虽然样品距火山口距离均较近,但也出现了细粒富集程度变缓的现象,反映了龙岗火山区其它火山锥喷发强度明显小于四海火山。对比长白山天池火山碎屑物粒度分布特征发现,天池火山空降堆积物粒度分布斜率变化比较均匀,四海火山灰斜率有明显变化;四海火山灰最大粒度小于长白山天池火山空降堆积物,但是粗粒度碎屑物含量较高。细粒度碎屑物部分累计频率曲线上升趋势较缓,说明金龙顶子火山的喷发  相似文献   

19.
We describe two small scoria cone volcanoes, Hidden Cone and Little Black Peak (ages between ~320–390 ka), in the Southwestern Nevada Volcanic Field and discuss their eruption mechanisms and inferences about their plumbing systems. Cone-forming pyroclastic deposits are consistent with eruptive styles ranging from Strombolian to violent Strombolian, and lavas emanated from near the bases of the cones. The volcanoes are monogenetic (rather than polycyclic, as allowed by previous geomorphic interpretations). Vents at each volcano appear to coincide with pre-existing normal faults, consistent with observations at older, deeply eroded volcanoes in the region. The existence of these two volcanoes on a topographically high area (particularly Hidden Cone) provides evidence for short feeder dike lengths (~500 m at the surface). We infer that this short length reflects the small length scale of the mantle source region that was tapped to feed each volcano. Editorial responsibility: J Stix  相似文献   

20.
Compositional features of 93 samples of primitive Pliocene to recent basalts erupted along the Brothers Fault Zone in the northernmost Basin and Range indicate that they were derived from a shallow mantle source and underwent only minor shallow-level fractionation. Simple mass-balance modelling can derive these basaltic bulk compositions by removal of small amounts of observed crystalline phases from glass compositions produced in peridotite melting experiments. Additional support comes from phase equilibria data on other magnesian basalts having similar bulk compositions. The eruption of these lavas without substantial subcrustal fractionation was probably promoted by progressive extension along the Brothers Fault Zone. This origin is in sharp contrast to that generally proposed for mid-Miocene Columbia River and Steens Mountain basalts, which show clear evidence in their evolved compositions (e.g. Mg # ~ 40) of having stagnated at shallow depth where they differentiated to nearly basaltic andesite compositions. Bulk compositions of northern Basin and Range silicic rocks, together with physical and thermal considerations, suggest that they, like their counterparts in the Snake River Plain, were products of crustal anatexis driven by the injection of mafic magmas, but with meta-volcaniclastic protoliths rather than Archaean basement rocks, as in the case of the Snake River Plain rhyolites. These petrologic features suggest that the arrival of the mantle plume presently beneath Yellowstone produced or strongly influenced most late Cenozoic magmatism in the Oregon northern Basin and Range. This model accounts for many features of the northern Basin and Range in Oregon: (1) the change in basaltic character about 10 to 8 Ma ago from voluminous, evolved Columbia River/Steens lavas to smaller-volume primitive lavas and the lack of younger lavas atop the Columbia River Basalts; (2) the lack of an obvious track of the Yellowstone hot spot west of the Oregon-Idaho-Nevada tri-state area; (3) the “mirror-image” age relationship of silicic rocks in the northern Basin and Range and Snake River Plain; (4) the formation of silicic rocks by crustal anatexis and the general decrease in their volumes with time in Oregon but not along the Snake River Plain; (5) the high elevation of the region; and (6) the high surface heat flow in the Oregon northern Basin and Range. The proposed model obviates the controversy surrounding the pre-Miocene history of the Yellowstone plume by proposing that the plume initiated about 18 Ma ago.  相似文献   

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