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1.
We have conducted high pressure (to 3 kbar), water saturated melting experiments on an andesite (62 wt% SiO2) and a basaltic andesite (55 wt% SiO2) from western Mexico. A close comparison between the experimental phase assemblages and their compositions, and the phenocryst assemblages of the lavas, is found in water saturated liquids, suggesting that the CO2 content was minimal in the fluid phase. Thus the historic lavas from Volcan Colima (with phenocrysts of orthopyroxene, augite, plagioclase, and hornblende) were stored at a temperature between 950–975 °C, at a pressure between 700–1500 bars, and with a water content of 3.0–5.0 wt%. A hornblende andesite (spessartite) from Mascota, of nearly identical composition but with only amphibole phenocrysts, had a similar temperature but equilibrated at a minimum of 2000 bars pressure with a dissolved water content of at least 5.5 wt% in the liquid. Experiments on the basaltic andesite show that the most common natural phenocryst assemblages (olivine, ±augite, ±plagioclase) could have precipitated at temperatures from 1000–1150 °C, in liquids with a wide range of dissolved water content (∼2.0–6.0 wt%) and a corresponding pressure range. A lava of the same bulk composition with phenocrysts of hornblende, olivine, plagioclase, and augite is restricted to temperatures below 1000 °C and pressures below 2500 bars, corresponding to <5.5 wt% water in the residual liquid. Although there is some evidence for mixing in the andesites (sporadic olivine phenocrysts), the broad theme of the history of both lava types is that the phenocryst assemblages for both the andesitic magmas and basaltic andesitic magmas are generated from degassing and reequilibration on ascent of initially hydrous parents containing greater than 6 wt% water. Indeed andesitic magmas could be related to a basaltic andesite parent by hornblende-plagioclase fractionation under the same hydrous conditions. Received: 10 December 1996 / Accepted: 21 August 1997  相似文献   

2.
The lavas of Nisyros were erupted between about 0?2 m.y B.P.and 1422 A.D., and range in composition from basaltic andesiteto rhyodacite. Most were erupted prior to caldera collapse (exactdate unknown), and the post-caldera lavas are petrographically(presence of strongly resorbed phenocrysts) and chemically (lowerTiO2 K2O, P2O5, and LIL elements) distinct from the pre-calderalavas. The pre-caldera lavas do not form a continuous seriessince lavas with SiO2 contents between 60 and 66 wt.% are absent.Nevertheless, major element variations demonstrate that fractionalcrystalliz ation (involving removal of olivine, dinopyroxene,plagioclase, and Fe-Ti oxide from the basaltic andesites andandesites and plagioclase, clinopyroxene, hypersthene, Ti-magnetite,ilmenite, apatite, and zircon from the dacites and rhyodacites)played a major role in the evolution of the pre-caldera lavas.Several lines of evidence indicate that other processes werealso important in magma evolution: (1) Quantitative modelingof major element data shows that phenocryst phases of unlikelycomposi tion or unrealistic assemblages of phenocryst phasesare required to relate the dacites and rhyodacites to the basalticandesites and andesites; (2) The proportions of olivine andclinopyroxene required in quantitative models for the initialstages of evolution differ from those observed petrographicallyand this is not likely to reflect either differential ratesof crystal settling or the curvature of cotectics along whichliquids of basaltic andesite to andesite composition lie; (3)The concentrations of Rb, Cs, Ba, La, Sm, Eu, and Th in therhyod.acites are too high for these lavas to be related to thedacites by fractional crystallization alone; and (4) 87Sr/86Srratios for the andesites and rhyodacites are higher than thosefor the basaltic andesites and dacites, respectively. It isshown that fractional crystallization was accompanied by assimilation,and that magma mixing played a minor role (if any) in the evolutionof the pre-caldera lavas. Trace element and isotopic data indicatethat the andesites evolved from the basaltic andesites by AFCinvolving average crust or upper crust, whereas the rhyodacitesevolved from the dacites by AFC involving lower crust. Additionalevidence for polybaric evolution is provided by the occurrenceof distinct Ab-rich cores of plagioclase phenocrysts in thedacites and rhyodacites, which record a period of high pressurecrystallization, and by the occurrence of both normal and reverse-zonedphenocrysts in the basaltic andesites and andesites. Furthermore,calculated pressures of crystallization are {small tilde}8 kbfor the dacites and rhyodacites and 3?5–4 kb for the basalticandesites and andesites. It is concluded that the dacites andrhyodacites evolved via AFC from basaltic andesites and andesiteslargely in chambers sited near the base of the crust whereasthe basaltic andesites and andesites mostly evolved in chamberssited at mid-crustal levels. Eruption from different chambersexplains the compositional gap in the chemistry of the pre-calderalavas since eruptive products represent a more or less randomsampling of residual liquids which separate (via filter pressing)from bodies of crystallizing magma at various depths. Magmamixing was important in the evolution of the post-caldera lavas,but geochemical data require that these magmas evolved fromparental magmas which were derived from a more refractory sourcethan the parental magmas to the pre-caldera lavas. *Present address: Netherlands Energy Research Foundation (ECN), P.O. Box 1, 1755 ZG Petten, The Netherlands  相似文献   

3.
Intermediate calc-alkaline magma (52-65% SiO2) in western-central Mexico is the focus of this paper, and the typically porphyritic andesites (57-65% SiO2) form large central volcanoes, whereas basaltic andesites (52-57% SiO2) are less porphyritic, and they are found as cones and flows but are absent from central volcanoes. Several studies of experimental phase equilibria on these lavas relate water concentration to the phenocryst assemblages and to the degree of crystallinity, so that the abundance, composition and variety of phenocrysts can be used to constrain the amount of water dissolved in the magmas. Thus, the plagioclase-rich andesites of Volcan Colima, Mexico, become so as a result of decompressional crystallisation at ~950 °C (the pyroxene phenocryst temperature), and lose their dissolved water (2.5 to 4.5 wt% H2O) which is inversely proportional to the modal abundance of plagioclase. The feeding magma to V. Colima, North America's most productive central volcano, is represented by hornblende lamprophyre, a lava type without plagioclase phenocrysts which requires at least 6 wt% water to reproduce the phenocryst assemblage. Thus, degassing of the V. Colima magmas, and of those of the other central volcanoes in the western-central Mexican volcanic belt, contributes essentially all their dissolved water to the conduit or to the atmosphere. The source of this magmatic water is related to the source of the intermediate magmas. For some this must lie in the mantle, as the incorporation of hornblende-lherzolite nodules in a hydrous andesite with hornblende phenocrysts could only have occurred while ascending through the mantle. Consistent with a mantle source is the composition of the olivine phenocrysts in Mexican lavas with 10 to 5% MgO, which is in the mantle range of Fo88-92. Accordingly, basaltic andesites and andesites with >5% MgO are candidates for a mantle source. The equilibration of intermediate magmas with the mantle, as illustrated by the experiments of various workers, requires that the magmas be hydrous at pressure. An additional constraint is that the activity of silica in the mantle must be equal to that in the hydrous magma at equilibrium. Using published and new experiments to define RTln%SiO2 in hydrous liquids, this quantity is shown to vary as a function of liquid composition (H2O, MgO, Na2O+K2O), and it approaches zero for quartz-saturated hydrous liquids. Using appropriate values of RTln%SiO2 for three intermediate lavas, the amount of water required to equilibrate with an olivine-orthopyroxene mantle source is calculated, and within error indicates that only the most silica-rich magma is at water saturation in the mantle, in agreement with published experimental work. Hydrous intermediate magmas, ascending from their hornblende-lherzolite source regions (~1 to 1.5 GPa) along the hydrous adiabat, may not encounter any phase boundaries until 0.2-0.4 GPa because of the increase in the thermal stability of hornblende in water-undersaturated magmas. Therefore, the phenocryst assemblages of hornblende-free andesites equilibrate at low pressures. The virtual absence of basalt in west-central Mexico (<4 Ma) is considered to be related to the large increase in crystallinity found in isobaric hydrous experiments crystallising hornblende at pressures close to those at the base of the crust. As a large proportion of the ferromagnesian components of basalt is acceptable to hornblende, it does not take a significant cooling interval (~40-50 °C) below the liquidus for hydrous basaltic magma to acquire >50% crystallinity, evidently also an eruptible limit for V. Colima andesitic lavas. If the lower limit of water dissolved in Mexican intermediate magmas is accepted as that required for phenocryst equilibration (~6 wt% water), and the upper limit as saturation in the mantle source at 1 GPa (~16 wt%) then, with an estimate of the volcanic and plutonic magma delivery rate (km3/106 year) per km of volcanic arc, the flux of water returned from the mantle along the 35,000-km, global subduction-related arc system can be estimated. Measurements of the volcanic flux are woefully few, and estimates from Mexico, the Lesser Antilles and central America show a range from 4 to 20 km3/106 year2km which, if subtracted from the isotopically constrained continental growth rate, gives the plutonic flux rate. This suggests that, of the magma flux ascending to the continental crust, only about a fifth reaches the surface. If the dissolved magmatic water limits are coupled with the volcanic and plutonic emplacement rates, then the amount of water returned by magmatism to the crust is crudely in balance with that subducted.  相似文献   

4.
Fukujin Seamount is a large, active, submarine volcano on thevolcanic front in the northernseamount province (NSP) of theMariana island arc (MIA). Five dredge hauls from the summitand upper flanks of Fukujin recovered mainly highly porphyriticbasaltic andesites. A few nearly aphyric samples are medium-Ksiliceous andesites (SiO2 = 62%, K2O = 1•5%). Fukujin andmost other large arc-front volcanoes of the northern MIA havetholeiitic (iron-enrichment) fractionation trends. This contrastswith the calc-alkaline trends of many smaller seamounts. A negativecorrelation of modal plagioclase content with bulk-rock SiO2,as well as bulk-rock major and trace element variation trends,and glass analyses, suggests that lavas with >30 vol.% phenocrystsand <55 wt.% SiO2 are partial cumulates. The presence ofbimodal phenocryst populations along with reversed to normalzoning of phenocrysts is explained by magma mixing of andesiticand basaltic liquids. Hybrid basaltic andesites probably formedby the accumulation of plagioclase in a tholeiitic magma chamberundergoing replenishment and mixing at a shallow crustal level.A petrogenetic model is presented for the origin of basalticandesite by combined magma mixing and fractional crystallization.Aphyric siliceous andesites can be modelled by simple fractionationof basaltic andesite. The early fractionating assemblage consistedmainly of plagioclase and clinopyroxene, with lesser olivineand minor magnetite, but plagioclase remained suspended in themelt. The later fractionating assemblage was dominated by plagioclasewith orthopyroxene instead of olivine. *Present address: 2260 rue Panet, Montreal, Quebec, H2L 3A6, Canada.  相似文献   

5.
The extrusive rocks of Hekla are predominantly flows of basaltic andesite and andesite (icelandite) but each eruptive cycle is initiated by production of tephra of andesitic, dacitic, and even rhyolitic composition. The evolution of basaltic andesites to dacites and rhyolites can be explained by crystallization and (presumably gravitative) separation of olivine, titaniferous magnetite, plagioclase, and probably augite. No contamination by sialic crustal material is required.Although basalts are never erupted from Hekla the origin of the basaltic andesites is probably best explained by separation of magnesian olivine, augite, and calcic plagioclase from an olivine tholeiite parent, producing an initial differentiation trend toward a high Fe/Mg ratio. The increase in Fe/Mg ratio is limited by the appearance of magnetite as a liquidus phase.From the Fe/Mg ratios of the lavas and from compositions of the plagioclase phenocrysts the water pressure of the basaltic andesites is estimated to have been between 0.6 and 2.4 kb. Total pressure may have been significantly higher. A best estimate for the water content is approximately 2 1/2 to 6 weight percent. This high water content accounts for the explosive initiation of each eruptive cycle and is consistent with fractional crystallization in a shallow magma chamber.Division of Geological and Planetary Sciences, California Institute of Technology, Contribution No. 2355.  相似文献   

6.
Quaternary monogenetic volcanism in the High Cascades of Oregonis manifested by cinder cones, lava fields, and small shields.Near Crater Lake caldera, monogenetic lava compositions include:low-K (as low as 0?09% K2O) high-alumina olivine tholeiite (HAOT);medium-K. calc-alkaline basalt, basaltic andesite, and andesite;and shoshonitic basaltic andesite (2?1% K2O, 1750 ppm Sr at54% SiO2). Tholeiites have MORB-like trace element abundancesexcept for elevated Sr, Ba, and Th and low high field strengthelements (HFSE), and they represent near-primary liquids. Theyare similar to HAOTs from the Cascades and adjacent Basin andRange, and to many primitive basalts from intraoceanic arcs.Calc-alkaline lavas show a well-developed arc signature of highlarge-ion lithophile elements (LILE) and low HFSE. Their Zrand Hf concentrations are at least partly decoupled from thoseof Nb and Ta; HREE are low relative to HAOT. Incompatible elementabundances and ratios vary widely among basaltic andesites.Some calc-alkaline lavas vented near Mount Mazama contain abundantgabbroic microxcnoliths, and are basaltic andesitic magmas contaminatedwith olivine gabbro. A calc-alkaline basalt and a few basaltic andesites have MgOand compatible trace element contents that suggest only minorfractionation. There appears to be a compositional continuumbetween primitive tholeiitic and calc-alkaline lavas. Compositionalvariation within suites of comagmatic primitive lavas, boththoleiitic and calc-alkaline, mainly results from differentdegrees of partial melting. Sources of calc-alkaline primarymagmas were enriched in LILE and LREE by a subduction componentand contained residual garnet, whereas sources of HAOTs hadlower LILE and LREE concentrations and contained residual clinopyroxene.High and variable LILE and LREE contents of calc-alkaline lavasreflect variations in fluid-transported subduction componentadded to the mantle wedge, degree of partial melting, and possiblyalso interaction with rocks or partial melts in the lower crust. Andesites were derived from calc-alkaline basaltic andesitesby fractionation of plagioclase+augite+magnetite+apatite ? orthopyroxeneor olivine, commonly accompanied by assimilation. Many andesitesare mixtures of andesitic or dacitic magma and a basaltic orbasaltic andesitic component, or are contaminated with gabbroicmaterial. Mingled basalt, andesite, and dacite of Williams Craterformed by multi-component, multi-stage mixing of basaltic andesiticmagma, gabbro, and dacitic magma. The wide range of compositionsvented from monogenetic volcanoes near Crater Lake is a resultof the thick crust coupled with mild tectonic extension superimposedon a subduction-related magmatic arc.  相似文献   

7.
Trace element systematics throughout the cal-calkaline high alumina basalt — basaltic andesite — andesite — dacite — rhyodacite lavas and dyke rocks of the Main Volcanic Series of Santorini volcano, Greece are consistent with the crystal fractionation of observed phenocryst phases from a parental basaltic magma as the dominant mechanism involved in generating the range of magmatic compositions. Marked inflection points in several variation trends correspond to changes in phenocryst mineralogy and divide the Main Series into two distinct crystallisation intervals — an early basalt to andesite stage characterised by calcic plagioclase+augite+olivine separation and a later andesite to rhyodacite stage generated by plagioclase augite+hypersthene+magnetite+apatite crystallisation. Percent solidification values derived from ratios of highly incompatible trace elements agree with previous values derived from major element data using addition-subtraction diagrams and indicate that basaltic andesites represent 47–69%; andesites 70–76%; dacites ca. 80% and rhyodacite ca. 84% crystallisation of the initial basalt magma. Least squares major element mixing calculations also confirm that crystal fractionation of the least fractionated basalts could generate derivative Main Series lavas, though the details of the least squares solutions differ significantly from those derived from highly incompatible element and addition-subtraction techniques. Main Series basalts may result from partial melting of the mantle asthenosphere wedge followed by limited olivine+pyroxene+Cr-spinel crystallisation on ascent through the sub-Aegean mantle and may fractionate to more evolved compositions at pressures close to the base of the Aegean crust. Residual andesitic to rhyodacite magmas may stagnate within the upper regions of the sialic Aegean crust and form relatively high level magma chambers beneath the southern volcanic centres of Santorini. The eruption of large volumes of basic lavas and silicic pyroclastics from Santorini may have a volcanological rather than petrological explanation.  相似文献   

8.
Approximately 150 km west of Mexico City in the central part of the Mexican Volcanic Belt (MVB) near Zitácuaro, Mexico, young volcanism has produced shield volcanoes, large volume silicic deposits, and fault-related basalt and andesite lava flows and cinder cones. This paper concerns a small cluster of Pleistocene andesite cones and flows which can be separated into two distinct groups: high-magnesium andesites (>6% MgO, 57–59% SiO2), conveniently called basaltic andesites, with phenocrysts of orthopyroxene and augite, or augite and olivine; and andesites (60–62% SiO2, <4.6% MgO), which have phenocrysts of orthopyroxene and augite, and ghosts of relict hornblende. Remarkably, plagioclase phenocrysts are absent, and evenly distributed but sparse (0.5–3.5%) quartz xenocrysts are present in all the lavas. In order to establish the conditions under which early crystallizing plagioclase is suppressed in these lavas, water saturated experiments up to 3 kbars were performed on one of the basaltic andesites. The conditions required to reproduce the phenocryst assemblages (either olivine + augite or opx + augite) are temperatures in excess of 1000 °C, with water saturated liquids (>3 wt%) at pressures of about 1 kbar. Compared to basaltic andesites of western Mexico, the Zitácuaro basaltic andesites have ∼2 wt% lower Al2O3 concentrations, which causes plagioclase to precipitate at significantly lower temperatures, and it therefore follows the crystallization sequence: olivine, augite, and orthopyroxene. Based on ubiquitous quartz xenocrysts, with glassy rhyolitic inclusions, a reasonable conclusion is that substantial mixing of a quartz-bearing rhyolitic magma with a parental basaltic andesite has occurred at low pressure (shallow depth), and this would account for the low Al2O3 concentrations in the Zitácuaro basaltic andesites. Whatever the mechanism of incorporation, the quartz xenocrysts are evidence of contamination of basaltic magma with more siliceous material, thus making it difficult to use these magmas as indicators of mantle melting processes. Received: 29 July 1997 / Accepted: 29 January 1998  相似文献   

9.
The Huerto Andesite is the largest of several andesite sequences interlayered with the large-volume ash-flow tuffs of the San Juan volcanic field, Colorado. Stratigraphically this andesite is between the region's largest tuff (the 27.8 Ma, 3,000 km3 Fish Canyon Tuff) and the evolved product of the Fish Canyon Tuff (the 27.4 Ma, 1,000 km3 Carpenter Ridge Tuff), and eruption was from vents located approximately 20–30 km southwest and southeast of calderas associated with these ashflow tuffs. Olivine phenocrysts are present in the more mafic, SiO2-poor samples of andesite, hence the parent magma was most likely a mantle-derived basaltic magma. The bulk compositions of the olivine-bearing andesites compared to those containing orthopyroxene phenocrysts suggest the phenocryst assemblage equilibrated at 2–5 kbar. Two-pyroxene geothermometry yields equilibrium temperatures consistent with near-peritectic magmas at 2–5 kbar. Fractionation of phenocryst phases (olivine or orthopyroxene + clinopyroxene + plagioclase + Ti-magnetite + apatite) can explain most major and trace element variations of the andesites, although assimilation of some crustal material may explain abundances of some highly incompatible trace elements (Rb, Ba, Nb, Ta, Zr, Hf) in the most evolved lavas. Despite the great distance of the San Juan volcanic field from the inferred Oligocene destructive margin, the Huerto Andesite is similar to typical plate-margin andesites: both have relatively low abundances of Nb and Ta and similar values for trace-element ratios such as La/Yb and La/Nb.Deriving the Fish Canyon and Carpenter Ridge Tuffs by crystal fractionation from the Huerto Andesite cannot be dismissed by major-element models, although limited trace-element data indicate the tuffs may not have been derived by such direct evolution. Alternatively, heat of crystallization released as basaltic magmas evolved to andesitic compositions may have caused melting of crust to produce the felsic-ash flows. Mafic magmas may have been gravitationally trapped below lighter felsic magmas; mafic magmas which ascended to the surface probably migrated upwards around the margins of silicic chambers, as suggested by the present-day outcrops of andesitic units around the margins of recognized ash-flow calderas.  相似文献   

10.
 Pleistocene-Holocene volcanism in the Jalisco block of western Mexico is confined to two conspicuous grabens, where potassic eruptives range from absarokites (48–52% SiO2) and minettes (49–54% SiO2) through basaltic andesites (53–57% SiO2), the most voluminous type, to andesites and their lamprophyric equivalent spessartite (58–62% SiO2); there are no contemporary rhyolitic rocks. This suite has high concentrations of Mg, Cr (<550 ppm) and Ni (<450 ppm) accompanied by large concentrations of K, P, Ba (<4000 ppm) and Sr (<5000 ppm) and elements such as LREE and Zr (<600 ppm). No combination of crystal fractionation and/or crustal contamination can reproduce the compositional range of these magmas, which nevertheless are believed to be genetically related because of their proximity in time and space. Hydrous minerals in the lamprophyres and the typical absence of plagioclase phenocrysts in both basaltic andesites and andesites reflect the relatively high concentrations of water in the magmas, which suppressed the crystallisation of feldspar. Experimental verification of the minimal amounts of water required to reproduce the phenocryst assemblages in selected rocks range from 3.5 to 6%. During ascent in a volcanic conduit, andesitic magma may lose water and consequently precipitate plagioclase, or it may ascend more rapidly, retaining more of its initial water, which stabilises phenocrysts of hornblende at the expense of plagioclase. Our estimates of water concentrations, which are consistent with the various low pressure phenocryst assemblages, will be minimal for the magmas in their source regions, and the process of magmatic dewatering on ascent may be typical in well established volcanic conduits. In accord with the compositions of phenocrystic olivine in the basaltic andesites and the minettes, the values of FeO and Fe2O3 of the bulk lavas and scoriae are demonstrably pristine. As a consequence, there are two characteristic features of the Mascota suite: the high range of relative oxygen fugacities (ΔNNO=1–5) and the high Mg# (MgO/MgO+FeO) that ranges from 0.70 to 0.91 (with only one andesite as low as 0.66). From the evidence of phlogopite phenocrysts, a partial melt involving phlogopite would have a higher Mg# than one from olivine (Fo90) and pyroxene alone. As the Mascota series shows a correlation between K2O and Mg#, we conclude that it was generated by partial fusion of the mantle wedge, with a variable contribution of phlogopite and apatite from veins throughout the lherzolitic assemblage. In conformity with an origin by varying increments of partial fusion of a phlogopite-bearing mantle, all incompatible elements vary linearly with Ti (or K) as if phlogopite (+apatite) in the source dominated their contribution to the partial melts. Fluids from dehydration of the subducting slab presumably deposit hydrous and other minerals in veins in the mantle wedge and also increase its redox state. As the Mascota volcanism occurs in grabens closer to the trench than the main andesite arc, it is concluded that the eruption of these small volumes of hydrous magmas require the tectonically favored ascent paths offered by the extensional grabens to reach the surface from their mantle sources. Received: 24 January 1995 / Accepted: 21 February 1996  相似文献   

11.
Lavas from Medicine Lake volcano, Northern California have been examined for evidence of magma mixing. Mixing of magmas has produced basaltic andesite, andesite, dacite and rhyolite lavas at the volcano. We are able to identify the compositional characteristics of the components that were mixed and to estimate the time lag between the mixing event and eruption of the mixed magma. Compositional data from pairs of phenocrysts identify a high alumina basalt (HAB) and a silicic rhyolite as endmembers of mixing. Mg-rich olivine or augite and Ca-rich plagioclase are associated with the HAB component, and Fe-rich orthopyroxene and Na-rich plagioclase are associated with the rhyolitic component. Some lavas contain multiple phenocryst assemblages suggesting the incorporation of several magmas intermediate between the HAB and silicic components. Glass inclusions trapped in Mg-rich olivine and Na-rich plagioclase are similar in composition to the proposed HAB and rhyolite end members and provide supportive evidence for mixing. Textural criteria are also consistent with magma mixing. Thermal curvature of the liquidus surfaces in the basalt-andesite-rhyolite system allows magmas produced by mixing to be either supercooled or superheated. Intergranular textures of basaltic andesites and andesites result from cooling initiated below the liquidus. The trachytic textures of silicic andesites form from cooling initiated above the liquidus. Reversed compositional zoning profiles in olivine crystals were produced by the mixing event, and the homogenization of the compositional zoning has been used to estimate the time interval between magma mixing and eruption. Time estimates are on the order of 80 to 90 h, suggesting that the mixing event triggered eruption.  相似文献   

12.
The relative ages of 21 lavas from Boqueron volcano in El Salvador were determined by superposition. The lavas are grey to black, porphyritic basalts, basaltic andesites and andesites with phenocrysts of plagioclase, augite, olivine, and magnetite. The andesitic lavas appear to have evolved from basaltic magma by fractionation of the observed phenocryst phases.The temporal variation in the chemical composition of the lavas at Boqueron is composed of three components. First, there is a crudely cyclical alternation of basalts and andesites. Second, these cycles are progressively shifted toward higher SiO2 contents. Third, approximately in the middle of the stratigraphic section sampled, there is an abrupt change in chemical variation trends from an Al-rich and Fe-poor trend to an Fe-rich and Al-poor trend. This change is interpreted to have been caused by an increased proportion of plagioclase fractionation and a decreased porportion of augite fractionation. The crudely cyclical change in SiO2 content with time is interpreted as a combination of crystal fractionation that increases SiO2 content, followed by influxes of basaltic magma that mix with residual magma to decrease SiO2 content. Successive cycles are shifted toward higher SiO2 content because there is a significant volume of fractionated magma remaining in the chamber before each influx of basalt.  相似文献   

13.
A series of basaltic and andesitic lavas from three centers in the Cascades (Lassen, Medicine Lake, Mt. Shasta) have been investigated. The lavas are weakly porphyritic, containing phenocrysts of plagioclase, augite, and olivine or orthopyroxene; these phases are also found in the groundmass. Titanomagnetite is a groundmass phase in most lavas but it appears to be absent in some. A sub-calcic augite is found in the groundmass in some of the basic lavas. Orthopyroxenes are present only in the salic lavas and show an increase in calcium with increasing iron. The range in composition shown by both phenocryst and groundmass plagioclase is very similar except that the phenocrysts extend to slightly more calcic compositions. The residual glasses in many of the lavas have a rhyolitic composition. However, only those from the Shasta andesites have normative salic constituents that plot near the ternary minimum in the Ab-Or-Qtz system at 500 bars. Both chemical and mineralogical data allow the lavas of the different centers to be distinguished from one another. The most likely origin for the orogenic lavas of the Cascades is by partial melting of the upper mantle.  相似文献   

14.
Sugarloaf Mountain is a 200-m high volcanic landform in central Arizona, USA, within the transition from the southern Basin and Range to the Colorado Plateau. It is composed of Miocene alkalic basalt (47.2–49.1?wt.% SiO2; 6.7–7.7?wt.% MgO) and overlying andesite and dacite lavas (61.4–63.9?wt.% SiO2; 3.5–4.7?wt.% MgO). Sugarloaf Mountain therefore offers an opportunity to evaluate the origin of andesite magmas with respect to coexisting basalt. Important for evaluating Sugarloaf basalt and andesite (plus dacite) is that the andesites contain basaltic minerals olivine (cores Fo76-86) and clinopyroxene (～Fs9-18Wo35-44) coexisting with Na-plagioclase (An48-28Or1.4–7), quartz, amphibole, and minor orthopyroxene, biotite, and sanidine. Noteworthy is that andesite mineral textures include reaction and spongy zones and embayments in and on Na-plagioclase and quartz phenocrysts, where some reacted Na-plagioclases have higher-An mantles, plus some similarly reacted and embayed olivine, clinopyroxene, and amphibole phenocrysts.Fractional crystallization of Sugarloaf basaltic magmas cannot alone yield the andesites because their ～61 to 64?wt.% SiO2 is attended by incompatible REE and HFSE abundances lower than in the basalts (e.g., Ce 77–105 in andesites vs 114–166?ppm in basalts; Zr 149–173 vs 183–237; Nb 21–25 vs 34–42). On the other hand, andesite mineral assemblages, textures, and compositions are consistent with basaltic magmas having mixed with rhyolitic magmas, provided the rhyolite(s) had relatively low REE and HFSE abundances. Linear binary mixing calculations yield good first approximation results for producing andesitic compositions from Sugarloaf basalt compositions and a central Arizona low-REE, low-HFSE rhyolite. For example, mixing proportions 52:48 of Sugarloaf basalt and low incompatible-element rhyolite yields a hybrid composition that matches Sugarloaf andesite well ? although we do not claim to have exact endmembers, but rather, viable proxies. Additionally, the observed mineral textures are all consistent with hot basalt magma mixing into rhyolite magma. Compositional differences among the phenocrysts of Na-plagioclase, clinopyroxene, and amphibole in the andesites suggest several mixing events, and amphibole thermobarometry calculates depths corresponding to 8–16?km and 850° to 980?°C. The amphibole P-T observed for a rather tight compositional range of andesite compositions is consistent with the gathering of several different basalt-rhyolite hybrids into a homogenizing ‘collection' zone prior to eruptions. We interpret Sugarloaf Mountain to represent basalt-rhyolite mixings on a relatively small scale as part of the large scale Miocene (～20 to 15 Ma) magmatism of central Arizona. A particular qualification for this example of hybridization, however, is that the rhyolite endmember have relatively low REE and HFSE abundances.  相似文献   

15.
Within the Zitácuaro–Valle de Bravo (ZVB) regionof the central Mexican Volcanic Belt (MVB), three lava serieshave erupted during the Quaternary: (1) high-K2O basaltic andesitesand andesites; (2) medium-K2O basaltic andesites, andesitesand dacites; (3) high-TiO2 basalts and basaltic andesites. Thedominant feature of the first two groups is the lack of plagioclaseaccompanying the various ferromagnesian phenocrysts (olivine,orthopyroxene, augite, and hornblende) in all but the dacites.This absence of plagioclase in the phenocryst assemblages ofthe high-K2O and medium-K2O intermediate lavas is significantbecause it indicates high water contents during the stage ofphenocryst equilibration. In contrast, the high-TiO2 group ischaracterized by phenocrysts of plagioclase and olivine. Thespatial distribution of these three lava series is systematic.The southern section of the ZVB transect, 280–330 km fromthe Middle America Trench (MAT), is characterized by high-K2Omelts that are relatively enriched in fluid-mobile elementsand have the highest 87Sr/86Sr ratios. Medium-K2O basaltic andesiteand andesite lavas are present throughout the transect, butthose closest to the MAT are MgO-rich (3·5–9·4wt %) and have phenocryst assemblages indicative of high magmaticwater contents (3·5–6·5 wt % water) andrelatively low temperatures (950–1000°C). In markedcontrast, the northern section of the ZVB transect (380–480km from the MAT) has high-TiO2, high field strength element(HFSE)-enriched magmas that have comparatively dry (< 1·5wt % magmatic water) and hot (1100–1200°C) phenocrystequilibration conditions. The central section of the ZVB transect(330–380 km from the MAT) is a transition zone and producesmoderately light rare earth element (LREE) and large ion lithophileelement (LILE)-enriched, medium-K2O lavas with phenocryst assemblagesindicative of intermediate (1·5–3·5 wt %)water contents and temperatures. The high-K2O series compositionsare the most enriched in LILE and LREE, with a narrow rangeof radiogenic 87Sr/86Sr from 0·704245 to 0·704507,143Nd/144Nd values ranging from 0·512857 to 0·512927(Nd = 4·27–5·63), and 208Pb/204Pb valuesfrom 38·248 to 38·442, 207Pb/204Pb values from15·563 to 15·585, and 206Pb/204Pb values from18·598 to 18·688. The medium-K2O series compositionsare only moderately enriched in the LILE and LREE, with a broaderrange of 87Sr/86Sr, but similar 143Nd/144Nd and 208Pb/204Pbvalues to those of the high-K2O series. In contrast, the high-TiO2series compositions have little enrichment in LILE or LREE andinstead are enriched in the HFSE and heavy rare earth elements(HREE). The high-TiO2 lavas are isotopically distinct in theirlower and narrower range of 143Nd/144Nd. The isotopic variationsare believed to reflect the upper mantle magma source regionsas the low content of phenocrysts in most lavas precludes significantupper crustal assimilation or magma mixing, other than thatrepresented by the presence of quartz xenocrysts (< 2 vol.%) with rhyolitic glass inclusions, which are found in manyof these lavas. The systematic spatial variation in compositionof the three lava series is a reflection of the underlying subduction-modifiedmantle and its evolution. KEY WORDS: central Mexico; geochemistry; isotopes; Quaternary volcanism; hydrous lavas  相似文献   

16.
Calc-alkaline andesites and olivine tholeiitic basalts are widely distributed on Shodo-Shima island, southwest Japan. The Fo content of olivine phenocrysts in the andesite is higher than in the basalt. The primary magma of the andesite, estimated on the basis of the olivine fractional crystallization model, is not basaltic but andesitic. The basalt contains both chromite and titanomagnetite as inclusions in olivine phenocrysts, while only chromite appears in the andesite. The Cr content of chromite in the andesite is higher than in the basalt. These facts again indicate that the andesite cannot be a fractionation product of the basalt, and that andesitic and basaltic primary magmas were generated independently.  相似文献   

17.
Three genetically unrelated magma suites are found in the extrusivesequences of the Troodos ophiolite, Cyprus. A stratigraphicallylower pillow lava suite contains andesite and dacite glassesand shows the crystallization order plagioclase; augite, orthopyroxene;titanomagnetite (with the pyroxenes appearing almost simultaneously).These lavas can in part be correlated chemically and mineralogicallywith the sheeted dikes and the upper part of the gabbro complexof the ophiolite. The second magma suite is represented in astratigraphically upper extrusive suite and contains basalticandesite and andesite glasses with the crystallizaton orderchromite; olivine; Ca-rich pyroxene; plagioclase. This magmasuite can be correlated chemically and mineralogically withparts of the ophiolitic ultramafic and mafic cumulate sequence,which has the crystallization order olivine; Ca-rich pyroxene;orthopyroxene; plagioclase. The third magma suite is representedby basaltic andesite lavas along the Arakapas fault zone andshows a boninitic crystallization order olivine; orthopyroxene;Ca-rich pyroxene; plagioclase. One-atmosphere, anhydrous phaseequilibria experiments on a lava from the second suite indicateplagioclase crystallization from 1225?C, pigeonite from 1200?C,and augite from 1165?C. These experimental data contrast withthe crystallization order suggested by the lavas and the associatedcumulates. The observed crystallization orders and the presenceof magmatic water in the fresh glasses of all suites are consistentwith evolution under relatively high partial water pressures.In particular, high PH2O (1–3 kb) can explain the lateappearances of plagioclase and Ca-poor pyroxene in the majorityof the basaltic andesite lavas as the effects of suppressedcrystallization temperatures and shifting of cotectic relations.The detailed crystallization orders are probably controlledby relatively minor differences in the normative compositionsof the parental magmas. The basaltic andesite lavas are likelyto reach augite saturation before Ca-poor pyroxene saturation,whereas the Arakapas fault zone lavas, which have relativelyless normative diopside and more quartz, reached the Ca-poorpyroxene-olivine reaction surface and crystallized Ca-poor pyroxeneafter olivine.  相似文献   

18.
Basaltic andesites are the dominant Tongan magma type, and are characterized by phenocrysts of augite, orthopyroxene (or rarely pigeonite), and calcic plagioclase (modally most abundant phase, and interpreted as the liquidus phase). The plagioclase phenocrysts exhibit slight oscillatory reverse zoning except for abrupt and thin more sodic rims, which are interpreted to develop during eruptive quenching. These rim compositions overlap those of the groundmass plagioclase. The pyroxene phenocrysts also exhibit only slight compositional zoning except for the outermost rim zones; the compositions of these rims, together with the groundmass pyroxenes, vary throughout the compositional range of subcalcic augite to ferroaugite through pigeonite to ferropigeonite, and are interpreted in terms of quench-controlled crystallization. This is supported, for example, by the random distribution of Al solid solution in the groundmass pyroxenes, compared to the more regular behaviour of Al in the phenocryst pyroxenes. The analysed Niua Fo'ou olivine tholeiites are aphyric; groundmass phases are plagioclase (An17–88), olivine (Fa18–63), titanomagnetite (usp. 59–73), and augite-ferroaugite which does not extend to subcalcic compositions; this is interpreted to be due to higher quenching temperatures and lower viscosities of these tholeiites compared to the basaltic andesites.Application of various geothermometers to the basaltic andesites suggest initial eruptive quenching temperatures of 1,008–1,124 ° C, plagioclase liquidus temperatures (1 bar) of 1,210–1,277 ° C, and orthopyroxene-clinopyroxene equilibration of 990–1,150 ° C. These calculated temperatures, together with supporting evidence (e.g. absence of olivine and amphibole, liquidus plagioclase, and plagioclase zoning patterns) are interpreted in terms of phenocryst crystallization from magmas that were either strongly water undersaturated, nearly anhydrous, or at best, water saturated at very low pressures (< 0.5 kb). This interpretation implies that these Tongan basaltic andesites did not originate by any of the currently proposed mechanisms involving hydrous melting within or above the Benioff zone.  相似文献   

19.
Voluminous andesite and dacite lavas of Daisen volcano, SW Japan,contain features suggesting the reverse of normal fractionation(anti-fractionation), in the sense that magma genesis progressedfrom dacite to andesite, accompanied by rises in temperature.A positive correlation exists between phenocryst content (0–40vol. %) and wt % SiO2 (61–67%). Phenocryst-rich dacitescontain hornblende and plagioclase that are generally unaltered,clear, and euhedral. However, phenocryst-poor rocks containsieve-textured plagioclase, resorbed plagioclase, and opacitein which hornblendes are pseudomorphed. Some Daisen rocks containtwo coexisting pyroxenes. Many orthopyroxene phenocrysts fromtwo-pyroxene lavas have high-Ca overgrowth rims (up to 50 µm),a feature consistent with crystallization from a higher-temperaturemagma than the core. Rim compositions are similar from phenocrystto phenocryst in individual samples. Temperatures of 800–900°Care obtained from the cores, whereas temperatures of 1000–1100°Care indicated for the rims. Lavas ranging from aphyric andesite(  相似文献   

20.
The water content of low-K tholeiitic basalt magma from Iwate volcano, which is located on the volcanic front of the NE Japan arc, was estimated using multi-component thermodynamic models. The Iwate lavas are moderately porphyritic, consisting of ~8 vol.% olivine and ~20 vol.% plagioclase phenocrysts. The olivine and plagioclase phenocrysts show significant compositional variations, and the Mg# of olivine phenocrysts (Mg#78–85) correlates positively with the An content of coexisting plagioclase phenocrysts (An85–92). The olivine phenocrysts with Mg# > ~82 do not form crystal aggregates with plagioclase phenocrysts. It is inferred from these observations that the phenocrysts with variable compositions were primarily derived from mushy boundary layers along the walls of a magma chamber. By using thermodynamic calculations with the observed petrological features of the lavas, the water content of the Iwate magma was estimated to be 4–5 wt.%. The high water content of the magma supports the recent consensus that frontal-arc magmas are remarkably hydrous. Using the estimated water content of the Iwate magma, the water content and temperature of the source mantle were estimated. Given that the Iwate magma was derived from a primary magma solely by olivine fractionation, the water content and temperature were estimated to be ~0.7 wt.% and ~1,310 °C, respectively. Differentiation mechanisms of low-K frontal-arc basalt magmas were also examined by application of a thermodynamics-based mass balance model to the Iwate magma. It is suggested that magmatic differentiation proceeds primarily through fractionation of crystals from the main molten part of a magma chamber when it is located at <~200 MPa, whereas magma evolves through a convective melt exchange between the main magma and mushy boundary layers when the magma body is located at >~200 MPa.  相似文献   

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