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1.
Three major phases are distinguished during the growth of Nyiragongo, an active volcano at the western limit of the Virunga Range, Zaire. Lavas erupted during phase 1 are strongly undersaturated melilitites characterized by the presence of kalsilite phenocrysts, perovskite, and the abundance of calcite in the matrix. Such lavas crop out mainly on the inner crater wall and progressively evolve toward more aphyric melilite nephelinites well represented on the flanks of the volcano. Adventive vents lying at the base of the cone developed along radial fracture systems and erupted olivine and/or clinopyroxene – rich melilitites or nephelinites. Stage 2 lavas are melilite-free nephelinites. Clinopyroxene is the main phenocryst and feldspathoids are abundant in the lavas exposed on the crater wall. These flows result from periodic overflowing of a magma column from an open crater. Extensive fissure flows which erupted from the base of the cone at the end of this stage are related to widespread draining out of magma which in turn induces the formation of the summit pit crater. Magmas erupted during stage 3 are relatively aphyric melilite nephelinites and the main volcanological characteristic is the permanent lava lake observed into the pit crater until the 1977 eruption. Fluctuations of the level of the lava lake was responsible for the development of the inner terraces. Periodic overflowing of the lava lake from the central pit formed the nepheline aggregate lava flows. Petrography and major element geochemistry allow the determination of the principal petrogenetic processes. Melilitites and nephelinites erupted from the summit crater are lavas derived, via clinopyroxene fractionation, from a more primitive melt. The abundance of feldspathoids in these lavas is in keeping with nepheline flotation. Aphyric melilite nephelinites covering the flanks and the extensive fissure flows have a homogeneous chemical composition; rocks from the historical lava lake are slightly more evolved. All these lavas differentiated in a shallow reservoir. Lavas erupted from the parasitic vents are mainly olivine and/or clinopyroxene-phyric rocks. Rushayite and picrites from Muja cone are peculiar high-magnesium lavas resulting from the addition of olivine xenocrysts to melilitic or nephelinitic melts. Fluid and melt inclusions in olivine and clinopyroxene phenocrysts indicate a crystallization depth of 10–14 km. A model involving two reservoirs located at different depths and periodically connected is proposed to explain the petrography of the lavas; this hypothesis is in accordance with geophysical data. Received: July 8, 1993/Accepted: September 10, 1993  相似文献   

2.
西昆仑阿什火山机构及岩石学、矿物学特征   总被引：4，自引：1，他引：3       下载免费PDF全文
阿什库勒盆地位于NE向阿尔金断裂与NW向康西瓦断裂的"弧形"交会处,构造活动十分活跃,盆地内发育10余座火山,其中阿什火山为该火山群中最新活动的火山。文中从火山地质、熔岩和斑晶成分、显微结构特征及地质温压计4个方面对阿什火山进行了详细研究。结果表明,阿什火山由火山锥和熔岩流组成,锥体由早期的渣锥和晚期的溅落锥组成,熔岩流分布面积约33km2,可划分为4个流动单元。熔岩属于钾玄岩系列,岩性为粗安岩,显微镜下呈斑状结构。斑晶以长石(主要为中长石)和辉石(包括普通辉石、古铜辉石和紫苏辉石)为主;基质为玻璃质、隐晶质、微晶质,部分含有大量的长石和辉石。斑晶与岩浆的平衡温度为1 104~1 194℃,压力为570~980MPa,对应的岩浆房深度为18.92~32.29km。  相似文献   

3.
The active volcano, Oldoinyo Lengai in the Eastern Rift in northern Tanganyika consists mainly of yellow ijolitic pyroclasts with interbedded relatively thin phonolite and nephelinite lavas, overlain by nephelinitic pyroclasts and younger ashes with marked unconformity. Ejected blocks in the pyroclasts are of rocks of the urtite-jacupirangite series with or without wollastonite, wollastonitite, fenite, carbonite, biotite pyroxenite and various lavas. Observations were made of minor activity during September–October 1960 when it was noted that soda-rich carbonate lava was extruded on the crater-floor in addition to minor emissions of ash. From examination of the rock sequence it appears that the volcano is waning and there has been a change from earlier dominantly gas eruptions to the more recent minor emissions of lava.  相似文献   

4.
The magma eruption rates of Merapi volcano form 1890 to 1992 are re-examined chronologically. For this volcano, movements of extruded lavas and domes as well as their extrusions are important because they control the modes of the subsequent activities and cause nuées ardentes and lahars. The monthly eruption rates varied widely, but the cumulative volume of lavas has increased linearly and is expressed as 0.1x106 m3/month. The magma production rate of this volcano may have been constant for these 100 years. Recurrent excessive effusion of lavas is tentatively interpreted by assuming a magma reservoir. The averaged eruption rate is small in comparison with other volcanoes such as Nyramuragia, Kilauea and Vesuvio. However, it is remarkable that the activity has been continuous for these 100 years and the total amount of lava discharged during this period reached more than 108 m3. A simple model for the formation of the 1992 lava dome is presented. The viscosity of the lavas is probably between 106 and 107 P and the length of the magma conduit is probably less than 10 km.  相似文献   

5.
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   

6.
 Pliocene–Recent volcanic outcrops at Seal Nunataks and Beethoven Peninsula (Antarctic Peninsula) are remnants of several monogenetic volcanoes formed by eruption of vesiculating basaltic magma into shallow water, in an englacial environment. The diversity of sedimentary and volcanic lithofacies present in the Antarctic Peninsula outcrops provides a clear illustration of the wide range of eruptive, transportational and depositional processes which are associated with englacial Surtseyan volcanism. Early-formed pillow lava and glassy breccia, representing a pillow volcano stage of construction, are draped by tephra erupted explosively during a tuff cone stage. The tephra was resedimented around the volcano flanks, mainly by coarse-grained sediment gravity flows. Fine-grained lithofacies are rare, and fine material probably bypassed the main volcanic edifice, accumulating in the surrounding englacial basin. The pattern of sedimentation records variations in eruption dynamics. Products of continuous-uprush eruptions are thought to be represented by stacks of poorly bedded gravelly sandstone, whereas better bedded, lithologically more diverse sequences accumulated during periods of quiescence or effusive activity. Evidence for volcano flank failure is common. In Seal Nunataks, subaerial lithofacies (mainly lavas and cinder cone deposits) are volumetrically minor and occur at a similar stratigraphical position to pillow lava, suggesting that glacial lake drainage may have occurred prior to or during deposition of the subaerial lithofacies. By contrast, voluminous subaerial effusion in Beethoven Peninsula led to the development of laterally extensive stratified glassy breccias representing progradation of hyaloclastite deltas. Received: 5 February 1996 / Accepted: 17 January 1997  相似文献   

7.
The Sawadani greenstone in the Chichibu Paleozoic System is an ancient submarine volcanic complex consisting of pillow lavas and hyaloclastites. The volcanism is divided into two periods. Alkali basalt was erupted in the first period and two shield-shaped cones were formed. After a period of dormancy the volcanism of the second period took place and a cone was formed by eruptions of lavas ranging in composition from mildly alkaline to tholeiitic basalt. The top of the volcano nearly reached the sea surface and was finally about 3.7 km above the base. A limestone cap and volcanic conglomerate were deposited on the summit. The base rests conformably on upper Carboniferous sandstone and subordinate mudstone derived from a continent or mature island arc. Many feeding channels of lava cut the volcanic body and underlying sedimentary formation. Sedimentation proceeded concurrently on the surrounding sea floor, so that volcanic and sedimentary material is interlayered.The Sawadani greenstone, although it occurs in the high-P/T metamorphic belt, is not believed to be a fragment of oceanic crust (ophiolite complex) formed by oceanic ridge volcanism and later carried into a convergent zone. It is a seamount formed on and within a sedimentary sequence near a continent or island arc. The magma changed from alkaline to tholeittic as the volcano grew.It cannot be assumed that all metavolcanic rocks formed in high-pressure metamorphic terranes are fragments of oceanic crust.  相似文献   

8.
The sixteenth eruption of Hekla since 1104 began on August 17th, 1980, after the shortest repose period on record, only ten years. The eruption started with a plinian phase and simultaneously lava issued at high rate from a fissure that runs along the Hekla volcanic ridge. The production rate declined rapidly after the first day and the eruption stopped on August 20th. A total of 120 million m3 of lava and about 60 million m3 of airborne tephra were produced during this phase of the activity. In the following seven months steam emissions were observed on the volcano. Activity was renewed on April 9th 1981, and during the following week additional 30 million m3 of lava flowed from a summit crater and crater rows on the north slope. The lavas and tephra are of uniform intermediate chemical composition similar to that of earlier Hekla lavas. Although the repose time was short the eruptions fit well into the behaviour pattern of earlier eruptions. Distance changes in a geodimeter network established after the eruptions are interpreted as due to inflation of magma reservoirs at 7–8 kilometers depth.  相似文献   

9.
Field investigation and lab analysis on samples were carried out for Quaternary volcanoes, including Xiaoshan volcano, Dashan volcano and Bianzhuang hidden volcano, in Haixing area, east of North China. Results show that Xiaoshan volcano with the eruptive material of volcanic scoria, crystal fragments and volcanic ash is a maar volcano, the eruptive pattern is pheatomagmatic eruption, and the influence scope is near the crater. Dashan volcano exploded in the early stage, and then the magma intruded, forming the volcanic neck. The eruption strength and scale are limited, and the eruptive materials are scoria, volcanic agglomerate and dense lava neck. The volcanic rocks in Bianzhuang are porosity and dense volcanic rocks and volcanic breccia, reflecting the pattern of weak explosive eruption and lava flow, and the K-Ar age dating on volcanic rocks indicates that the eruption happened in early Pleistocene. Xiaoshan volcanic scoria and Bianzhuang hidden volcanic rocks are mainly basaltic, Dashan volcanic rocks with lower SiO2 content are nephelinite in composition. Their oxide contents have no linear relationship, indicating that there is no magma evolution relationship between these magmas from the three places. Three volcanic rocks all have enrichment of light rare earth. The Bianzhuang volcanic rocks are rich in large ion lithophile elements, and have no high field strength elements Zr and Hf, Ti losses. The volcanic materials from Xiaoshan and Dashan are intensively rich in Th, U, Nb and Ta, and significantly poor in K and Ti. Although the magmas from these three places in Haixing area may all come from asthenosphere, the volcanic materials have different petrological and geochemical features, and relatively independent volcanic structures, therefore, they experienced different magma processes.  相似文献   

10.
Piton de La Fournaise is in a period of intense volcanic activity since 1998. To constrain the magma dynamics responsible for this activity, we combined GPS ground deformation monitoring interpreted through numerical modelling and geochemistry. Two cycles of continuous volcano inflation are evidenced for the May 2004–December 2005 period, with a rest from March to October 2005. These inflations are consistent with two cycles of compatible major element enrichment in the emitted lavas. Numerical models indicate that the pressurization of a single magma reservoir may be responsible for the observed pre-eruptive inflations of the volcano. The reservoir, located at 2300 m depth, has a radius of  500 m. At the beginning of each cycle, dykes propagate from the roof of the reservoir and yield eruptions of differentiated basalt near the summit. At the end of the cycle, dykes propagate from the eastern sidewall of the reservoir and yield distal eruptions of primitive magmas away from the summit. The volumes of magma emitted during the primitive eruptions seem too large to explain the surface deformations and therefore suggest some refill of the reservoir by deeper magmas. Our results may be used to predict the location and lava volume of future eruptions at Piton de La Fournaise volcano, depending on the timing of these eruptions within a cycle of volcanic activity.  相似文献   

11.
The paper reviews the stratigraphy, style of activity and some aspects of the petrology of Tertiary to Recent sodic alkaline volcanic rocks in Kenya, eastern Uganda and northern Tanzania. Repeated extrusions of basaltic and nephelinitic volcanics occurred from Miocene times onwards, confirming indications from chemical data that magmas of these compositions were parental. At some central volcanoes, a basalt-trachyte-phonolite series evidently arose by fractional crystallization of basaltic magma, whereas various courses of crystallization from a nephelinitic parent led to the production of phonolites, tephrites and basanites as well as olivine-and melilite-bearing nephelinites and melanephelinites. Phonolitic and trachytic volcanics which dominate an area of repeated upwarping (the Kenya dome) probably originated by processes of partial melting rather than by differentiation of basaltic magma. The basalt-trachyte association which characterizes many central volcanoes north and south of the dome can perhaps best be explained by postulating independent sources for the basic and salic volcanics.  相似文献   

12.
A set of grey-purple layered volcanic rocks are found widely distributed from the mountain flank to the main peak of Daliuchong volcano, but it's difficult to identify whether they are volcaniclastic rock or lava rock just by field investigation and the crystal structure observation under microscope. The study of matrix microstructure of the volcanic rocks can help to identify the volcanic facies. We recognize the eruptive facies rocks through observation of the matrix microstructure and pore shape with comparison to those of the volcanic vent facies, extrusive facies and effusive facies rocks under microscope, thus the mentioned layered volcanic rocks could be named as dacitic crystal fragment tuff. Combining the joint work of field investigation, systematic sampling, chemical analyzing and microscopic observation, we summary the Daliuchong volcanic facies as follows:1. The effusive facies lava constitutes the base of Daliuchong volcano and was produced by early eruption.2. The explosive facies is composed of dacite crystal fragment welded tuff and volcanic breccia and mainly distributes on the W, S and NE flank of the volcanic cone.3. The volcanic conduit with its diameter more than one hundred meters is located about 100 meters south of the main peak of the Daliuchong volcano.4. The extrusive facies rock is only exposed near the peak of Daliuchong volcano.Therefore, the volcanism of Daliuchong volcano can be speculated as:Large-scale lava overflowing occurred in the early eruption period; then explosive eruptions happened; at last, the volcanisms ceased marked with magma extrusion as lava dome and plug.  相似文献   

13.
The Meseta and Fuego volcanoes closely overlap and collectively are known as the Fuego Volcanic Complex. Historic activity occurs exclusively at Fuego, the southern center, and consists of high-Al basalts. Meseta, the inactive northern center, is predominantly composed of basaltic andesites with minor basalt and andesite. A thick sequence of lava flows and dikes is exposed by a steep collapse escarpment on the east flank of Meseta. The upper 75% of the sequence was sampled from three interfingering stratigraphic sections consisting of 27, 10 and 4 lavas, respectively. Temporal geochemical trends of each section indicates a complex evolutionary history. A major trend toward more evolved compositions upward in the section is consistent with crystal fractionation. This trend is sharply interrupted by the youngest lavas which become distinctly more mafic in composition. Magma mixing is apparently the dominant magmatic evolution process that generated these lavas. The two trends have distinct Sr signatures that suggest a change in parental magma compositions. This abrupt change in composition is interpreted to signal high input rates of mafic magma into the subvolcanic magma chamber. These changes eventually led to sector collapse of Meseta volcano and deposition of the Escuintla debris avalanche. Eruptive activity then migrated to the Fuego volcano where historic activity is similar to that of Meseta immediately prior to its collapse.  相似文献   

14.
Masaya-Granada area is located in the middle part of the Central American volcanic zone. A basaltic shield volcano with a caldera, an acidic pyroclastic flow plateau with a caldera, cinder cones, maars, a lava dome and a composite andesitic volcano were formed by recent volcanic activities. Magmas of basic and intermediate ejecta are supposed to be formed by partial melting of the upper mantle material. Most of basalts and andesites was derived from common parental magma after crystallization differentiation history, but some basalts, which have extremely high MgO content and low K2O content might be derived from primary magma of different type. There is no evidence to deny the possibility of differentiation product of acidic rock from basic magma, but compositional gap on variation diagram suggest the possibility of partial melting origin. Strike-slip fault systems might have been formed in association with plate movement, and fluidal basaltic magma was erupted also along these fault zones.  相似文献   

15.
海口地区火山活动初步研究   总被引：3，自引：2，他引：1       下载免费PDF全文
海口地区的马鞍岭-雷虎岭火山群是中国为数不多的几个休眠火山群之一。通过对区域火山活动期次划分、深部岩浆囊探测以及对火山类型、规模、物质组成和溶岩覆盖面积的分析,总结了海口地区火山活动的时、空、强特征。结合对火山区地震、地磁、体应变和地热等观测资料的分析,对火山区深部岩浆活动的状态进行了初步评估。研究认为海口全新世火山区最后一次火山喷发距今约4 000a左右,其现今火山活动已趋于平静,未来的火山活动可能向1605年琼州7.5级大地震震中区迁移  相似文献   

16.
The regional variation of physical and geochemical characteristics of Central American volcanoes occurs in two fundamentally different patterns. The first pattern is symmetrical about Nicaragua. Crustal thickness, silica contents of mafic lavas and volcanic edifice heights are lowest in Nicaragua and increase smoothly toward Costa Rica to the south and Guatemala to the north. Magma density is maximum in Nicaragua and decreases smoothly outward. The regional variation in crustal thickness is just enough so that magma densities, calculated at appropriate Moho pressures, are the same at the base of the crust throughout the region. This is consistent with magma ponding at the base of the crust. The bulk compositions of Central American basalts show the same symmetrical variation. Suites of Nicaraguan basalts plotted in pseudo-ternary CMAS projections indicate large olivine and plagioclase primary-phase volumes. Toward Costa Rica and Guatemala the olivine and plagioclase fields inferred from suites of basaltic lavas are smaller, which is consistent with fractionation at increasing depth.The second pattern is the segmentation of the volcanic front and the plate margin in general. The segmentation strongly affects the spacing and size of volcanic centers. At segment boundaries volcanic centers are generally small and unusually widely spaced. Toward segment interiors volcano spacing and size increase systematically. The LIL element contents of lavas strongly reflect this pattern. For lavas with similar silica contents the larger the volcano, the higher the LIL element contents. The relationships between segmentation, volcano spacing and volcano size are compatible with diapiric rise of magma accumulated in narrow ribbons near the upper surface of the underthrust slab. The relationship between volcano volume and LIL element content is qualitatively in agreement with an open-system fractionation model.  相似文献   

17.
Volcán Las Navajas, a Pliocene-Pleistocene volcano located in the northwestern portion of the Mexican volcanic belt, erupted lavas ranging in composition from alkali basalt through peralkaline rhyolite, and is the only volcano in mainland Mexico known to have erupted pantellerites. Las Navajas is located near the northwestern end of the Tepic-Zacoalco rift and covers a 200-m-thick pile of alkaline basaltic lavas, one of which has been dated at 4.3 Ma. The eruptive history of the volcano can be divided into three stages separated by episodes of caldera formation. During the first stage a broad shield volcano made up of alkali basalts, mugearites, benmoreites, trachytes, and peralkaline rhyolites was constructed. Eruption of a chemically zoned ash flow then caused collapse of the structure to form the first caldera. The second stage consisted of eruptions of glassy pantellerite lavas that partially filled the caldera and overflowed its walls. This stage ended about 200 000 years ago with the eruption of pumice falls and ash flows, which led to the collapse of the southern portion of the volcano to form the second caldera. During the third stage, two benmoreite cinder cones and a benmoreite lava flow were emplaced on the northwestern flank of the volcano. Finally, the calc-alkaline volcano Sanganguey was built on the southern flank of Las Lavajas. Alkaline volcanism continued in the area with eruptions of alkali basalt from cinder cones located along NW-trending fractures through the area. Although other mildly peralkaline rhyolites are found in the rift zones of western Mexico, only Las Navajas produced pantellerites. Greater volumes of basic alkaline magma have erupted in the Las Navajas region than in the other areas of peralkaline volcanism in Mexico, a factor which may be necessary to provide the initial volume of material and heat to drive the differentiation process to such extreme peralkaline compositions.  相似文献   

18.
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   

19.
Abstract Nekoma volcano forms part of the arc axis volcanic array of the North-eastern Honshu arc, Japan, which is commonly characterized by medium-K lava suites. However, Nekoma is exceptional because many of its lavas are low-K. This anomaly has been a matter of debate. Nekoma was active from 1.1 to 0.35 Ma. The volcano consists of thick andesite flows and domes associated with block and ash flow deposits produced during lava dome formation. A horseshoe-shaped collapse caldera was formed at the summit and small lava domes extruded into the caldera. Stratigraphy, published K–Ar ages, and tephrochronology define three stages of volcanic activity, about 1.1 Ma (Stage 1), 0.8–0.6 Ma (Stage 2) and 0.45–0.35 Ma (Stage 3; post caldera stage). Low-K andesites occur in all stages. Extremely low-K andesite was also associated in Stage 2 and medium-K andesite was dominant in Stage 3. In general, lavas changed from low-K to medium-K after caldera formation. Geochemical study of the Nekoma lavas shows that both low-K and medium-K lavas are isotopically similar and were derived from a common source. Adatara and Azuma volcanoes, which lie close to Nekoma, also have both low-K and medium-K andesites. However, Sr isotope ratios or temporal-spatial variations in K-level lava classification vary between the three centers. Comparisons of K suites and Sr isotope ratios with frontal arc volcanoes in North-east–Honshu suggest source heterogeneity existed in both medium- and low-K suites. The K contents of lavas and their Sr isotopes are not simply related. This requires re-examination of models for chemical variation of andesites in arcs.  相似文献   

20.
The lavas of the Nyiragongo volcano in Eastern Zaire contain partially fused granite xenoliths. The relictic feldspars found in these xenoliths were studied by microprobe analysis and by X-ray diffraction methods. Some xenoliths represent originally two-feldspar granites, in others only alkali feldspars or those of an anorthoclase composition were detected. All feldspars are homogeneous without perthitic textures detectable under the microscope. In thebc-diagram of Stewart and Wright all feldspars plot on the analbite-high sanidine join. These feldspars have been perfectly disordered through heating in the melilite-nephelinite magma of the volcano.  相似文献   

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