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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.
西南极菲尔德斯半岛第三纪火山岩的岩石学和岩石化学特征表明,它们基本属于钙碱性火山岩系列,是岛弧火山作用的产物。该岩石组合中,随岩石中SiO_2含量的增加,斜长石斑晶数量减少,微量元素Cr、V丰度降低,Sr、Ba丰度下降,这些揭示了岩浆中斜长石和单斜辉石的分离结晶作用。稀土元素的系统变化也证明了这一点。主元素和微量元素的定量计算所验证了岩浆的分离结晶作用演化过程。  相似文献   

3.
Calc-alkaline olivine andesite and two-pyroxene dacite of theTaos Plateau volcanic field evolved in an open magmatic system.mg-numbers of spatially and temporally associated ServilletaBasalt (54–61) and ohvine andesite (49–59) are comparableand preclude fractional crystallization of ferromagnesian mineralsas the major differentiation process. If Servilleta olivinetholeiite is assumed to be the parental magma type, enrichmentsof highly incompatible trace elements (up to 17 ?) oVer concentrationsin the basalts require that andesitic and dacitic magmas containa substantial proportion of assimilated crust. Isotopic compositionsof andesite and dacite, which have slightly higher 87Sr/86Srratios than the basalts but lower 143Nd/144Nd, 206Pb/204Pb,207Pb/204Pb, and 208Pb/204Pb ratios, are consistent with contaminationof parental basalt by old, low Rb/Sr, low U/Pb, and low Th/Pbcontinental crust. Concentrations of highly incompatible traceelements in andesite and dacite lavas are decoupled from majorelement compositions; the highest concentrat ions of these elementsoccur in andesitic, rather than dacitic compositions, and andesitelavas are more variable in trace element contents. Assimilationof heterogeneous crust concurrent with fractional crystallizationof varying mineral assemblages could cause this decoupled behavior.High mg-numbers in andesite and dacite, skeletal olivine phenocrysts,and reversely zoned pyroxene phenocrysts are manifestationsof mafic replenishment and magma mixing in the Taos Plateaumagmatic system. Taos Plateau volcanoes are monolithologic and are distributedin a semi-concentric zoned pattern that is a reflection of thecomplex subvolcanic magmatic system. A central focus of basaltshields developed above the main basaltic conduit system; thesemagmas contain 10–35% admixed andesitic and dacitic magma.Basalt shields are surrounded by a partial ring of olivine andesiteshield volcanoes, where replenishment of basaltic magma providedthe heat necessary for prolonged assimilation of crust, resultingin intermediate-composition lavas. Dacite shields are locatedaround the periphery of the more mafic volcanoes and reflecta decrease in mafic input on the fringes of the magmatic system.  相似文献   

4.
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   

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.
The simultaneous eruption in 1996 of andesite from Karymskyvolcano and of basalt from the Academy Nauk vent 6 km away appearsto provide a case of mafic recharge of an andesite reservoirfor which the time of recharge is exactly known and direct samplesof the recharging magma are available. The explosive phreato-magmaticeruption of basalt was terminated in less than 24 h, whereasandesite erupted continuously during the following 4 years.Detailed petrological study of volcanic ash, bombs and lavasof Karymsky andesite erupted during the period 1996–1999provides evidence for basaltic replenishment at the beginningof the eruptive cycle, as well as a record of compositionalvariations within the Karymsky magma reservoir induced by basalticrecharge. Shortly after the beginning of the eruption the compositionof the matrix glass of the Karymsky tephra became more maficand then, within 2 months, gradually returned to its originalstate and remained almost constant for the following 3 years.Further evidence for basaltic replenishment is provided by thepresence of xenocrysts of basaltic origin in the andesite ofKarymsky. A conspicuous portion of the plagioclase phenocrystsin the Karymsky andesite has calcic cores, with compositionsand textures resembling those of plagioclases in the AcademyNauk basalt. Similarly, the earlier portion of the andesiteof the eruption sequence contains rare olivines, which occuras resorbed cores in pyroxenes. The composition of the olivinematches that of olivines in the Academy Nauk basalt. The sequenceof events appears to be: (1) injection of basaltic magma intothe Karymsky chamber with immediate, compensating expulsionof pre-existing chamber magma from the Karymsky central vent;(2) direct mixing of basaltic and andesitic magmas with dispersalof phenocrysts associated with the basalt throughout the andesiteso that newly mixed magma appeared at the vent within 2 months;(3) re-establishment of thermal and chemical equilibrium withinthe reservoir involving crystallization in the new hybrid liquid,which returned the melt composition to ‘normal’,formed rims on inherited calcic plagioclase, and caused theresorption of dispersed olivine xenocrysts. Taken together,these findings indicate that the Karymsky magma reservoir wasrecharged by basalt at the onset of the 1996 eruptive cycle.The rapidity and thoroughness of mixing of the basalt with thepre-existing andesite probably reflects the modest contrastin temperature, viscosity, and density between the two magmas. KEY WORDS: Karymsky; Kamchatka; magma mixing; andesite; volcanic glass; plagioclase  相似文献   

7.
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   

8.
Rabaul caldera is a large volcanic depression at the north-east tip of New Britain, Papua New Guinea. The lavas range in composition from basalt to rhyolite and have a calc-alkalic affinity but also display features typical of tholeiites, including moderate absolute iron enrichment in flows cropping out around the caldera. The basalts contain phenocrysts of plagioclase and clinopyroxene with less abundant olivine and titanomagnetite. In the basaltic andesites olivine is rare, while orthopyroxene and titanomagnetite are common along with plagioclase and clinopyroxene. Orthopyroxene is also found mantling olivine in some of the basalts while in both rock types pigeonitic augite is a fairly common constituent of the groundmass. Plagioclase in both basalt and basaltic andesite often exhibits sieve texture and analysis of the glass blebs show them to be of similar composition to the bulk rock. Phenocrystic clinopyroxene is a diopsidic augite in both basalt and basaltic andesite. Al2O3 content of the clinopyroxene is moderately high (4%) and often shows considerable variation in any one grain. Calculations show that the microphenocrysts probably crystallised near the surface, while phenocrysts crystallised at around 7 kb (21 km). Neither the basalts nor the basaltic andesites would have been in equilibrium at any geologically reasonable P and T with quartz eclogite. Equilibration between mantle peridotite and a. typical Rabaul basaltic liquid could have occurred around 35 kb and 1270 °C. A basaltic andesite liquid yields a temperature of 1263 °C and a pressure of 28 kb for equilibration with mantle peridotite.Partial melting of sufficient volumes of mantle peridotite at these P's and T's requires about 15% H2O, but there is no evidence that these magmas ever contained large amounts of water. It is proposed that the Rabaul magmas were initially generated by partial melting of subducted lithosphere and subsequently modified by minor partial melting as they passed through the overlying mantle peridotite.  相似文献   

9.
The convergent margin of western Mexico is uniquely characterizedby a volcanic front of lamprophyric and related lavas located{small tilde}70 km closer to the Middle America trench thanthe main axis of andesitic volcanism. This front, defined bysmall volcanic centers ranging in age from {small tilde}1 kato 3 Ma, contains several lava types: minette, absarokite, leucitite,spessartite, and kersantite, all without feldspar phenocrysts.Many of the lavas contain hydrous phenocrysts; they are enrichedin potassium and other incompatible elements, and they are moreoxidized relative to the andesitic suite of the main axis. Intimatelyassociated are flows of basaltic andesite of comparable volume.They range in composition from 53 to 58 wt.%SiO2, have 5–9wt.%MgO and contain phenocrysts of olivine, sparse augite, andvarying amounts of plagioclase. Their alkali contents are typicalof calc-alkaline varieties, with average Na2O and K2O concentrationsof 4?2 and 1?1 wt.% respectively. The basaltic andesites oftencontain olivine of unusually high forsterite content, reflectingcrystallization under oxidizing conditions, and they have oxygenfugacities up to 3?3 log units above the Ni-NiO buffer. Manifestationsof high water contents are (1) the ubiquitous occurrence ofgroundmass olivine rather than orthopyroxene, and (2) the suppressionof plagioclase as an early crystallizing phase. Both featuresreflect the role of water in reducing the activity of silicain the melt. The progressive influence of water during crystallizationis also seen in the continuum between the two intermediate lavatypes, basaltic andesite and kersantite, as plagioclase is suppressedand hornblende is stabilized in the phenocryst assemblage. Thus,despite the absence of hydrous minerals in the basaltic andesites,their phenocryst assemblages reveal the influence of substantialamounts of water, and thereby show a genetic link to the variouslamprophyric lavas.  相似文献   

10.
The Shiribeshi Seamount off northwestern Hokkaido, the Sea of Japan, is a rear-arc volcano in the Northeast Japan arc. This seamount is composed of calc-alkaline and high-K basaltic to andesitic lavas containing magnesian olivine phenocrysts and mantle peridotite xenoliths. Petrographic and geochemical characteristics of the andesite lavas indicate evidence for the reaction with the mantle peridotite xenoliths and magma mixing between mafic and felsic magmas. Geochemical modelling shows that the felsic end-member was possibly derived from melting of an amphibolitic mafic crust. Chemical compositions of the olivine phenocrysts and their chromian spinel inclusions indicate that the Shiribeshi Seamount basalts in this study was derived from a primary magma in equilibrium with relatively fertile mantle peridotites, which possibly represents the mafic end-member of the magma mixing. Trace-element and REE data indicate that the basalts were produced by low degree of partial melting of garnet-bearing lherzolitic source. Preliminary results from the mantle peridotite xenoliths indicate that they were probably originated from the mantle beneath the Sea of Japan rather than beneath the Northeast Japan arc.  相似文献   

11.
Volcán San Pedro in the Andean Southern Volcanic Zone(SVZ) Chile, comprises Holocene basaltic to dacitic lavas withtrace element and strontium isotope ratios more variable thanthose of most Pleistocene lavas of the underlying Tatara–SanPedro complex. Older Holocene activity built a composite coneof basaltic andesitic and silicic andesitic lavas with traceelement ratios distinct from those of younger lavas. Collapseof the ancestral volcano triggered the Younger Holocene eruptivephase including a sequence of lava flows zoned from high-K calc-alkalinehornblende–biotite dacite to two-pyroxene andesite. Notably,hornblende–phlogopite gabbroic xenoliths in the daciticlava have relatively low 87Sr/86Sr ratios identical to theirhost, whereas abundant quenched basaltic inclusions are moreradiogenic than any silicic lava. The latest volcanism rebuiltthe modern 3621 m high summit cone from basaltic andesite thatis also more radiogenic than the dacitic lavas. We propose thefollowing model for the zoned magma: (1) generation of hornblende–biotitedacite by dehydration partial melting of phlogopite-bearingrock similar to the gabbroic xenoliths; (2) forceful intrusionof basaltic magma into the dacite, producing quenched basalticinclusions and dispersion of olivine and plagioclase xenocryststhroughout the dacite; (3) cooling and crystallization–differentiationof the basalt to basaltic andesite; (4) mixing of the basalticandesite with dacite to form a small volume of two-pyroxenehybrid andesite. The modern volcano comprises basaltic andesitethat developed independently from the zoned magma reservoir.Evolution of dacitic and andesitic magma during the Holoceneand over the past 350 kyr reflects the intrusion of multiplemafic magmas that on occasion partially melted or assimilatedhydrous gabbro within the shallow crust. The chemical and isotopiczoning of Holocene magma at Volcán San Pedro is paralleledby that of historically erupted magma at neighboring VolcánQuizapu. Consequently, the role of young, unradiogenic hydrousgabbro in generating dacite and contaminating basalt may beunderappreciated in the SVZ. KEY WORDS: Andes; dacite; gabbro; Holocene; strontium isotopes  相似文献   

12.
Quaternary basalts, andesites and dacites from the Abu monogenetic volcano group, SW Japan, (composed of more than 40 monogenetic volcanoes) show two distinct chemical trends especially on the FeO*/MgO vs SiO2 diagram. One trend is characterized by FeO*/MgO-enrichment with a slight increase in SiO2 content (Fe-type trend), whereas the other shows a marked SiO2-enrichment with relatively constant FeO*/MgO ratios (Si-type trend). The Fe-type trend is explained by fractional crystallization with subtraction of olivine and augite from a primitive alkali basalt magma. Rocks of the Si-type trend are characterized by partially melted or resorbed quartz and sodic plagioclase phenocrysts and/or fine-grained basaltic inclusions. They are most likely products of mixing of a primitive alkali basalt magma containing olivine phenocrysts with a dacite magma containing quartz, sodic plagioclase and hornblende phenocrysts. Petrographic variation as well as chemical variation from basalt to dacite of the Si-type trend is accounted for by various mixing ratios of basalt and dacite magmas. Pargasitic hornblende and clinopyroxene phenocrysts in andesite and dacite may have crystallized from basaltic magma during magma mixing. Olivine and spinel, and quartz, sodic plagioclase and common hornblende had crystallized in basaltic and dacitic magmas, respectively, before the mixing. Within a lava flow, the abundance of basaltic inclusions decreases from the area near the eruptive vent towards the perimeter of the flow, and the number of resorbed phenocrysts varies inversely, suggesting zonation in the magma chamber.The mode of mixing changes depending on the mixing ratio. In the mafic mixture, basalt and dacite magmas can mix in the liquid state (liquid-liquid mixing). In the silicic mixture, on the other hand, the basalt magma was quenched and formed inclusions (liquid-solid mixing). During mixing, the disaggregated basalt magma and the host dacite magma soon reached thermal equilibrium. Compositional homogenization of the mixed magma can occur only when the equilibrium temperature is sufficiently above the solidus of the basalt magma. The Si-type trend is chemically and petrographically similar to the calc-alkalic trend. Therefore, a calc-alkalic trend which is distinguished from a fractional crystallization trend (e.g. Fe-type trend) may be a product of magma mixing.  相似文献   

13.
基于青海省刚察地区1：5万电泵站等4幅区调项目,在拉脊山蛇绿构造混杂岩带药水泉组中识别一套玄武岩、玄武安山岩、安山岩类岩石组合,其岩相学、岩石地球化学特征与高镁安山岩相类似.岩石斑晶以斜长石和辉石为主,不含橄榄石,基质也含有大量的斜长石.岩石地球化学特征表现为K2O+Na2O含量2.55%~5.15%,Na2O > K2O,K2O含量0.22%~0.82%,为低钾;CaO含量5.31%~11.41%,TiO2含量0.27%~0.55%,变化范围较小;MgO含量为5.78%~11.06%,变化范围不大.总体具有低硅、钛、钾,高钙、镁,富钠之特征.稀土配分曲线为右倾斜状,多呈铕正异常,为轻稀土富集型.微量元素Rb、Ba、Th、Cr、Ni等变化幅度大,反映岩浆生成和演化过程复杂,但演变仍以部分熔融为主.选取玄武岩样品,通过LA-ICP-MS U-Pb锆石微区定年,获得了451.7±1.8 Ma年龄值,其时代为晚奥陶世.总体特征表明,刚察地区火山岩为埃达克型高镁安山岩,其构造环境为活动大陆边缘的岛弧.  相似文献   

14.
Fluorine contents in 38 Quaternary volcanic rocks, representing calc-alkaline andesite eruptive groups from the Garibaldi Lake area, were determined by a selective ion-electrode method. A close relationship is evident between F abundance and the type of ferromagnesian phenocrysts present in the andesitic rocks. Hypersthene andesites have the lowest F contents (142–212 ppm), whereas hornblende-biotite andesites exhibit the highest F values (279–368 ppm); hornblende andesites have intermediate F contents (238–292 ppm). The hornblende-free Desolation Valley basaltic andesite has a lower F content than the hornblende-bearing Sphinx Moraine basaltic andesite (122 ppm versus 317–333 ppm).Different eruptive suites can be grouped on the basis of F differentiation patterns into (1) a hornblende-free lava series in which the F content of basaltic andesite is less than that of andesite, and (2) a hornblende-bearing lava series in which F contents remain constant or decrease slightly from basaltic andesite through dacite. Fluorine variation in the former series was controlled largely by fractionation of anhydrous minerals, whereas that in the latter was influenced by crystallization of amphibole, biotite and apatite.The distinctive F variation patterns of the two lava series appear to represent real differences in the volatile contents of Garibaldi Lake magmas. These different volatile concentrations may reflect varying degrees of magma-wallrock interaction, differences in the initial volatile contents of the primary magmas, or some combination of these factors.  相似文献   

15.
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   

16.
 A variety of cognate basalt to basaltic andesite inclusions and dacite pumices occur in the 7-Ma Rattlesnake Tuff of eastern Oregon. The tuff represents ∼280 km3 of high-silica rhyolite magma zoned from highly differentiated rhyolite near the roof to less evolved rhyolite at deeper levels. The mafic inclusions provide a window into the processes acting beneath a large silicic chamber. Quenched basaltic andesite inclusions are substantially enriched in incompatible trace elements compared to regional primitive high-alumina olivine tholeiite (HAOT) lavas, but continuous chemical and mineralogical trends indicate a genetic relationship between them. Basaltic andesite evolved from primitive basalt mainly through protracted crystal fractionation and multiple cycles (≥10) of mafic recharge, which enriched incompatible elements while maintaining a mafic bulk composition. The crystal fractionation history is partially preserved in the mineralogy of crystal-rich inclusions (olivine, plagioclase ± clinopyroxene) and the recharge history is supported by the presence of mafic inclusions containing olivines of Fo80. Small amounts of assimilation (∼2%) of high-silica rhyolite magma improves the calculated fit between observed and modeled enrichments in basaltic andesite and reduces the number of fractionation and recharge cycles needed. The composition of dacite pumices is consistent with mixing of equal proportions of basaltic andesite and least-evolved, high-silica rhyolite. In support of the mixing model, most dacite pumices have a bimodal mineral assemblage with crystals of rhyolitic and basaltic parentage. Equilibrium dacite phenocrysts are rare. Dacites are mainly the product of mingling of basaltic andesite and rhyolite before or during eruption and to a lesser extent of equilibration between the two. The Rattlesnake magma column illustrates the feedback between mafic and silicic magmas that drives differentiation in both. Low-density rhyolite traps basalts and induces extensive fractionation and recharge that causes incompatible element enrichment relative to the primitive input. The basaltic root zone, in turn, thermally maintains the rhyolitic magma chamber and promotes compositional zonation. Received: 1 June 1998 / Accepted: 5 February 1999  相似文献   

17.
Miyake-jima volcano has erupted at least 13 times during theperiod 1469–1983. To understand the historic magmaticprocesses, we focus on the mineral assemblage and chemical compositionsof crystal-clots in single samples from each of the eruptions.Most of the historic lavas consist of nearly aphyric to weaklyporphyritic basalt to andesite, but there also exist megacryst-bearingrocks. The megacrysts are considered to be xenocrysts from adeep-seated plutonic body. Many samples of each eruption containtwo types of clots beside megacrysts, termed here B-type andA-type. The B-type clots are composed of olivine, clinopyroxeneand plagioclase, whereas the A-type clots additionally containmagnetite and orthopyroxene. Compositional relationships betweenthese mafic minerals suggest that the minerals in the same typeof clots are in equilibrium. Comparing the chemical compositionsof the minerals in the two types of clots in each sample, theyare derived from distinct magmas: the B-type clots from basalticmagma and the A-type clots from andesitic magma. During thehistoric activity, the magma plumbing system appears to haveincluded two magma storage systems: a deep-seated basaltic anda shallower andesitic one. In many cases, basaltic magma hasinjected into shallower andesitic magma to form mixed magma;however, andesitic magma has sometimes erupted alone withoutextensive injections of basaltic magma. Temporal variationsof mineral compositions in the clots and estimated whole-rockcompositions of the end-member magmas suggest that the basalticmagma has differentiated gradually since 1469, and that itsmagmatic temperature has fallen from 1220 to 1180°C. Conversely,the andesitic magma has changed in a complex fashion to becomemore mafic (the magmatic temperature rose from 1050 to 1100°C).As a result of this study, it is estimated that the basalticmagma after the 1983 eruption was the least mafic, and the andesiticmagma the most mafic, of the historic eruptions. KEY WORDS: andesite; basalt; crystal-clots; evolution of magma; Miyake-jima volcano; magma mixing  相似文献   

18.
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   

19.
Experiments in the system high-A1 basalt (HAB)-water have been conducted in the melting range at pressures between 1 atm. and 10 kbar, defining the amphibole stability field and the composition of liquids which coexist with this amphibole. Plagioclase is the anhydrous liquidus phase between 1 atm. and 10 kbar but in the hydrous runs this role is taken by olivine at <7 kbar and then by clinopyroxene at higher pressures. Because amphibole is never on the high-A1 basalt liquidus it is not likely that andesite is derived from primary basalt by pure fractional crystallisation, although as we discuss, other mechanisms including equilibrium crystallisation might implicate amphibole. If primary basaltic magma undergoes closed-system equilibrium crystallisation, then the amphibole field will be intersected at between 50 and 100°C below the liquidus. The compositions of melts coexisting with amphibole alone do not match those of any of the natural andesite or dacitic lavas associated with the particular high-A1 basalt investigated. Like natural andesites, they become rapidly silica enriched, but they also become far more depleted in TiO2 and MgO. However, the compositions of liquids lying directly on the divariant amphibole-out reaction zone, where amphibole +liquid coexist with clinopyroxene or olivine (±plagioclase), do resemble those of naturally occurring low-silica andesites. With increasing temperature pargasitic amphibole breaks down via incongruent melting reactions over a narrow temperature range to form a large volume of relatively low-silica basaltic andesite liquid and a crystalline assemblage dominated by either clinopyroxene or olivine. Our important conclusion is that basaltic andesite liquid will be the product of reaction between cooling, hydrous mafic liquid and anhydrous ferromagnesian phases. The solid reactants could represent earlier cumulates from the same or different magma batches, or they could be peridotite wall-rock material. Because the amphibole-out boundary coexisting with liquid is one of reaction, it will not be traversed so long as the phases on the high temperature side remain. Thus, the assemblage amphibole+clinopyroxene±olivine±plagioclase+liquid is one in which the liquid is buffered (within limits), and results reported here indicate that this buffering generates melts of low-silica andesite composition. When tapped to lower pressures these liquids will rise, eventually to fractionate plagioclase-rich assemblages yielding silicarich andesite and dacite melts. Conversely, the partial melting of hornblende pyroxenite, hornblende peridotite or hornblende gabbro can also yield basaltic andesite liquids. The phase relationships suggested by these experiments are discussed in the light of naturally occurring phenocryst and xenolith assemblages from the east Sunda Arc. Primary magmatic additions to the lithosphere of volcanic arcs are basaltic and voluminous upper crustal andesite in these terranes, complemented by mafic and ultramafic crystalline deposits emplaced in the lower crust or close to the Moho. Together these components constitute total arc growth with a basaltic composition and represent the net accreted contribution to continental growth.  相似文献   

20.
Xenolithic inclusions in calc-alkaline andesite from Mt. Moffettvolcano, Adak Island, Aleutian arc, reveal a nearly continuousrecord of crystallization of basaltic magmas in the crust, andpossibly upper mantle, of the arc. The record is more detailedand continuous than that obtained from study of calc-alkalinevolcanic rocks in the arc. Cumulate xenoliths form a progressiveseries in modal mineralogy from ultramafic, hornblende-bearingolivine clinopyroxenite to both hornblende-bearing and hornblende-freegabbros. The cumulate hornblende gabbro xenoliths are typicalof those found in island arc andesites worldwide. Xenolithicinclusions without cumulate textures, here termed compositexenoliths, are characterized by forsteritic olivine, zoned Cr-diopsideand hornblende, and are interpreted to have resulted from reactionand chilling upon magma mixing at depth. The olivine and clinopyroxene in both cumulate and compositexenoliths show the largest and the most complete variation trendsfor Ni, Cr, and FeO/MgO ratio yet reported in igneous xenolithsfrom island arc volcanic rocks. Variation of Ni in olivine indicatesthat the parent magmas for the xenoliths had minimum MgO contentsof 9 wt. per cent. Variation of Cr in clinopyroxene indicatesthat the magmas were basaltic rather than picritic, probablyin equilibrium with spinel lherzolite at near Moho depths. Successiveinjections of batches of primary melt into a magma chamber fractionatingolivine and clinopyroxene can reproduce observed compatibleelement depletion trends. A steady-state process of cotecticcrystallization in a magma chamber continually replenished withbasaltic magma is a possible mechanism for producing large accumulationsof olivine and clinopyroxene, suggesting that Alaskan-type ultramaficcomplexes are related to hydrous basaltic magmas in island arcs.This steady-state open-system crystallization process can alsoyield the abundant high-alumina basalt type in the Aleutianarc. Continued crystallization of high-alumina basalt in lowercrustal magma chambers, recorded in a mineralogically coherentseries of pyroxenite to hornblende gabbro xenoliths, can yieldbasaltic to andesitic magmas of the calc-alkaline series. No xenoliths with a sedimentary protolith have been found atMt Moffett, evidence that the arc crust is igneous in origin,with the lower crust formed of gabbro crystallized from mantle-derivedmelts. Ultramafic cumulates may reside in both the lower crustor upper mantle beneath the arc. A model is proposed wherebythe cumulate crystallization products of hydrous, mantle beneaththe arc. A model is define the upper mantle and lower crustof the arc over time.The net composition added to the crustof the arc is that of high-alumina basalt.  相似文献   

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