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1.
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   

2.
Heterogeneous andesitic and dacitic lavas on Cordn El Guadalbear on the general problem of how magmas of differing compositionsand physical properties interact in shallow reservoirs beneathcontinental arc volcanoes. Some of the lavas contain an exceptionallylarge proportion (<40%) of undercooled basaltic andesiticmagma in various states of disaggregation. Under-cooled maficmagma occurs in the silicic lavas as large (<40 cm) basalticandesitic magmatic inclusions, as millimeter-sized crystal-clotsof Mg-rich olivine phenocrysts plus adhering Carich plagioclasemicrophenocrysts (An50–70), and as uniformly distributed,isolated phenocrysts and microphenocrysts. Compositions andtextures of plagioclase phenocrysts indicate that inclusion-formingmagmas are hybrids formed by mixing basaltic and dacitic melts,whereas textural features and compositions of groundmass phasesindicate that the andesitic and dacitic lavas are largely mechanicalmixtures of dacitic magma and crystallized basaltic andesiticmagma. This latter observation is significant because it indicatesthat mechanical blending of undercooled mafic magma and partiallycrystallized silicic magma is a possible mechanism for producingthe common porphyritic texture of many calc-alkaline volcanicrocks. The style of mafic-silicic magma interaction at CordonEl Guadal was strongly dependent upon the relative proportionsof the endmembers. Equally important in the Guadal system, however,was the manner in which the contrasting magmas were juxtaposed.Textural evidence preserved in the plagioclase phenocrysts indicatesthat the transition from liquid-liquid to solid-liquid mixingwas not continuous, but was partitioned into periods of magmachamber recharge and eruption, respectively. Evidently, duringperiods of recharge, basaltic magmas rapidly entrained smallamounts of dacitic magma along the margins of a turbulent injectionfountain. Conversely, during periods of eruption, dacitic magmagradually incorporated small parcels of basaltic andesitic magma.Thus, the coupled physical-chemical transition from mixed inclusionsto commingled lavas is presumably not coincidental. More likely,it probably provides a partial record of the dynamic processesoccurring in shallow magma chambers beneath continental arevolcanoes. KEY WORDS: Chile; commingling; magma mixing; magmatic inclusions *Present address: Department of Earth Sciences, Montana State University, Bozeman, MT 59717, USA  相似文献   

3.
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   

4.
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   

5.
Mafic to intermediate enclaves are evenly distributed throughoutthe dacitic 1991–1995 lava sequence of Unzen volcano,Japan, representing hundreds of mafic recharge events over thelife of the volcano. This study documents the morphological,textural, chemical, and petrological characteristics of theenclaves and coexisting silicic host lavas. The eruptive productsdescribed in this study appear to be general products of magmamingling, as the same textural types are seen at many othervolcanoes. Two types of magmatic enclaves, referred to as Porphyriticand Equigranular, are easily distinguished texturally. Porphyriticenclaves display a wide range in composition from basalt toandesite, are glass-rich, spherical and porphyritic, and containlarge, resorbed, plagioclase phenocrysts in a matrix of acicularcrystals and glass. Equigranular enclaves are andesitic, non-porphyritic,and consist of tabular, medium-grained microphenocrysts in amatrix glass that is in equilibrium with the host dacite magma.Porphyritic enclaves are produced when intruding basaltic magmaengulfs melt and phenocrysts of resident silicic magma at theirmutual interface. Equigranular enclaves are a product of a moreprolonged mixing and gradual crystallization at a slower coolingrate within the interior of the mafic intrusion. KEY WORDS: mafic enclaves; quenched mafic inclusions; magma mingling; Unzen volcano; Unzen Scientific Drilling Project; resorbed plagioclase  相似文献   

6.
Volcn Ollage (2117'S) is a large stratovolcano that liesslightly east of the main axis of Quaternary Volcanoes in theAndean Central Volcanic Zone (CVZ). Euptive products range frombasaltic andesite to dacite and define a high-K, calc-alkalinesuite. This compositional range is similar to the collectivecompositional range of the other stratovolcanoes in the CVZ,and it provides a record of both early and late-stage differentiationprocesses operating at the stratovolcanoes. The volumetrically dominant andesitic and dacitic lavas aredivided into four eruptive series on the basis of vent locationsand petrography. In ascending stratigraphic order they are:the Vinta Loma, Chasca Orkho, post-collapse, and La Celosa series.Whole-rock compositions of the lavas are remarkably similarregardless of eruptive series. Variations in phenocryst assemblagesand magmatic fo2 however, suggest differences in subliquidusvolatile contents for magma chambers developed beneath the summitof the volcano versus those developed beneath the flanks. Basalticandesite magmas are principally preserved as quenched inclusionswithin the andesitic and dacitie lava flows. Large ranges inisotopic ratios over a narrow compositional range indicate thatthe basaltic andesites were derived by crystal fractionationcoupled with large amounts of crustal assimilation. IncreasingCe/Yb ratios with decreasing Yb contents further suggest thatthis initial stage of differentiation occurred at deep crustallevels where garnet was stable. Additional supporting evidencefor differentiation in the deep crust includes isotopic andtrace element compositions that indicate assimilation by thebasaltic andesite magmas of a crust different from upper-crustalrocks exposed at present in the region. Whole-rock major and trace element trends of the dacitic lavascan be simulated largely by fractional crystallization of parentalandesitic magma. The fractionating assemblages for the differenteruptive series are consistent with the observed modes of theparent magmas. Small increases in Sr isotope ratios with increasingRb contents indicate that the fractionating magmas also assimilatedsmall amounts of wall rocks similar in composition to the upper-crustalbasement to the volcano. Consideration of the chemical trends, mineral compositions,and eruptive history of Ollage rocks permits construction ofa model for the evolution of shallow crustal magma chambersbeneath the stratovolcanoes in the CVZ. At a relatively maturestage, the magma chambers may be compositionally, thermally,and density stratified. Temperatures estimated from Fe-Ti oxideand pyroxene thermometry for the chambers beneath Ollage rangefrom 1000 to 790C with increasing SiO2 from 59 to 67 wt.% inthe upper reaches, and from 1150 to 1020C with increasing SiO2from 53 to 59 wt.% in the lower reaches. The occurrence of basalticandesite magmatic inclusions within the intermediate lavas andthe repeated eruption of monotonous composition andesitic magmasindicate that the shallow chambers are periodically replenishedwith parental basaltic andesite magmas. Ubiquitous, reversely zoned plagioclase and pyroxene phenocrystsin the lavas at Ollage suggest that convective cooling of thebasaltic andesite releases buoyant derivative liquid that mixeswith the overlying intermediate-composition body of the chambers.Further crystallization and differentiation of the intermediatemagmas may take place in solidification zones at the boundariesof the magma chambers. If so, the return of residual liquidfrom the crystallizing margins and mixing with the interiorare highly efficient such that magma differentiation can bemodeled as a simple, homogeneous, fractional crystallizationprocess.  相似文献   

7.
Petrographical and geochemical characteristics of calc-alkalineandesites on Shodo-Shima Island, SW Japan, having bulk compositionslargely identical to the continental crust, are presented. Thefollowing petrographic observations suggest a role for magmamixing in producing such andesite magmas: (1) two types of olivinephenocrysts and spinel inclusions, one with compositions identicalto those in high-Mg andesites and the other identical to thosein basalts, are recognized in terms of Ni–Mg and Cr–Al–Fe3+relations, respectively; (2) the presence of orthopyroxene phenocrystswith mg-number >90 suggests the contribution of an orthopyroxene-bearinghigh-Mg andesite magma to production of calc-alkaline andesites;(3) reversely zoned pyroxene phenocrysts may not be in equilibriumwith Mg-rich olivine, suggesting the involvement of a differentiatedandesite magma as an endmember component; (4) the presence ofvery Fe-rich orthopyroxene phenocrysts indicates the associationof an orthopyroxene-bearing rhyolitic magma. Contributions fromthe above at least five endmember magmas to the calc-alkalineandesite genesis can also provide a reasonable explanation ofthe Pb–Sr–Nd isotope compositions of such andesites. KEY WORDS: calc-alkaline andesites; high-Mg andesites; magma mixing; continental crust; SW Japan  相似文献   

8.
Mt. Jefferson is an andesite-dacite composite volcano in the Cascade Range, the locus of andesite and dacite-dominated volcanism for at least 1 million years. A large trace element data set for Mt. Jefferson and its surrounding mafic volcanic platform effectively rules out any fractionation based model (FC or AFC) for the generation of Mt. Jefferson andesites. Several incompatible element (Zr, Nb, Y) concentrations decrease in the range from basalt to andesite, and then increase in the range from andesite to rhyodacite. Others (Ba, Rb, La, Th) remain constant or show a slight increase in the basalt to andesite range, with modest increases from andesite to rhyodacite. Systematic variations in highly incompatible element ratios such as Ba/La and Rb/Th suggest magma mixing dominates the trace element signatures. Rhyodacites are isotopically uniform (87Sr/86Sr=0.70325-0.70343; 206Pb/204Pb=18.75-18.85; ‘18O=6.3ǂ.3), whereas andesite and dacite are more variable (87Sr/86Sr=0.70291-0.70353; 206Pb/204Pb=18.59-18.86; ‘18O=6.0ǂ.6). Typical basaltic andesite has 87Sr/86Sr=0.70326-0.70358, 206Pb/204Pb=18.78-18.85, and ‘18O=5.9ǂ.4. Sr-rich (>1,000 ppm) basaltic andesite is more variable (87Sr/86Sr=0.70300-0.70360; 206Pb/204Pb=18.70-18.89; ‘18O=5.9ǂ.4). The data define mixing arrays with one end member at 87Sr/86Sr=0.7029; 206Pb/204Pb=18.59, another at rhyodacite, and a third at 87Sr/86Sr=0.7036; 206Pb/204Pb=18.89. The first end member is defined by Sr-rich (800-1,200 ppm) andesite with high Al2O3, and low K2O, Ba, and Rb/Th; the third one by K2O- and very Sr-rich (>2,000 ppm) shoshonite. Isotopic data for basalts in northern Oregon preclude any fractionation relationship between basalt and either rhyodacite or Sr-rich andesite (e.g., the minimum 206Pb/204Pb ratio in basalt is 18.83). Considered in light of geophysical models for the Cascades, these data suggest two types of crustal melting beneath the arc. Rhyodacite may be generated at 25-30 km depth by partial melting of arc basalt-like amphibolite at 850-900 °C. Sr-rich andesite may be formed by partial melting of depleted MORB-like mafic granulite at 35-45 km depth at 1,000-1,100 °C. Experimental and REE evidence supports these interpretations as does the restriction of Sr-rich andesite in the Cascades to the area south of the 100 mW/m2 heat flow contour between Mt. Jefferson and Mt. Hood. Thick crust and high heat flow are necessary to produce such andesite.  相似文献   

9.
Augustine Volcano, a Quaternary volcanic centre of the easternAleutian Arc, produces predominantly andesites and dacites oflow- to medium-K calc-alkaline composition. Mineralogical andmajor element characteristics of representative lavas suggestthat magmatic evolution has been influenced by both crystalfractionation and magma-mixing processes. However, incompatibletrace element variations (e.g. K/Rb) indicate that these evolvedlavas have been contaminated by the mafic arc crust of the underlyingTalkeetna accreted terrane. The limited range of isotope compositionsalso supports the assimilation of non-radiogenic mafic crust(e.g. 87Sr/86Sr = 0.7032–0.7034; 143Nd/144 Nd = 0.51301–0.5130).In addition, Pb-isotope compositions parallel the North Pacificmean oceanic trend (206Pb/204 Pb = 18.3–18.8; 207Pb/204Pb= 15.5–15.6; 208Pb/204Pb = 38.2–38.3) and do notrequire a subducted sediment component in the source. Relativelyhigh (Ba/La) N (0.79–18.10) and B/Be (14.5) ratios do,however, suggest a metasomatic fluid component derived fromthe dehydration of the subducting plate. The thickened continental crust (35 km) of the eastern AleutianArc prevents the ascent of basaltic melts, which fractionateand assimilate at various depths to produce andesitic magmas.These andesites evolve towards more silicic compositions byfractional crystallization. The absence of evidence for a largehigh-level crustal magma chamber implies that the magmatic systembeneath the volcano is young and at an immature stage of evolution. KEY WORDS: Augustine Volcano; Aleutians; assimilation; melasomatism; geochemistry *Corresponding author. Present address: Department of Geology and Geophysics, University of New Orleans, New Orleans, LA 70148, USA  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.
The eruption of Soufrière Hills Volcano, Montserrat, has been ongoing since 1995. The volcano is erupting a crystal-rich hornblende-plagioclase andesite with ubiquitous mafic inclusions, indicating mixing with mafic magma. This mafic magma is thought to be the driving force of the eruption, supplying heat and volatiles to the andesite resident in the magma chamber. As well as producing macroscopic mafic inclusions, the magma mixing process involves incorporation of phenocrysts from the andesite into the mafic magma. These inherited phenocrysts show clear disequilibrium textures (e.g. sieved plagioclase rims and thermal breakdown rims on hornblende). Approximately 25 % of all phenocrysts in the andesite show these textures, indicating very extensive mass transfer between the two magma types. Fragments of mafic inclusions down to sub-mm scale are found in the andesite, together with mafic crystal clusters, which are commonly found adhered to the rims of phenocrysts with disequilibrium features. Mineral chemistry also points to the transfer of microlites or microphenocrysts, initially formed in the mafic inclusions, into the andesite. This combined evidence suggests that some of the mafic inclusions disaggregate during mingling and/or ascent, possibly due to shearing, and raises the question: What proportion of the andesite ‘groundmass’ actually originated in the mafic inclusions, and thus, what is the true amount of mafic magma in the magmatic system? We present a new method for quantifying the relative proportions of groundmass plagioclase derived from mafic and andesitic magma, based on analysis of back-scattered electron images of the groundmass. Preliminary results indicate that approximately 16 % of all groundmass plagioclase belongs genetically to the mafic inclusions. Together with the crystal clusters, disequilibrium phenocryst textures and mm-scale inclusions, there is a ‘cryptic’ mafic component in the andesite of approximately 6 % by volume. This is significant compared with the proportion of macroscopic mafic inclusions (typically ~ 1–5 %). The new method has the potential to allow tracking of the mafic fraction through time and thus to yield further insights into magma hybridisation processes.  相似文献   

12.
The late Pleistocene Calabozos ash-flow and caldera complexlies in central Chile in a section of the Andean cordillerathat is transitional between dominantly andesitic-to-rhyoliticvolcanism to the north and mafic andesitic and high-aluminabasaltic volcanism to the south. The Calabozos rocks range incomposition from basaltic andesite to rhyodacite and definea high-K calcalkalic suite. They contain 2–25% phenocrystsof plagioclase, clinopyroxene, orthopyroxene, Fe-Ti oxides,and apatite, ? minor biotite or amphibole. More than 1000km3 of rhyodacitic to dacitic magma erupted atthe Calabozos caldera complex as three major compositionallyzoned ash-flow sheets, Unit L (0?8 Ma), Unit V (0? 30Ma), andUnit S (0?15 Ma) of the Loma Seca Tuff. Phenocryst modes, trace-elementcontents, inferred magmatic volatile contents, and oxygen fugacitiesvary systematically with major-element composition in the tuffs.In the cases of Units V and S, it is possible to reconstructcompositional, thermal, and volatile gradients that existedin density-stratified magma chambers shortly prior to theireruption. The magma graded from crystal-poor, water-rich, andbiotite-bearing rhyodacite in the upper reaches of the chamberto more crystal-rich, water-poor, and amphibole-bearing daciteat deeper levels. Fe-Ti oxide equilibration temperatures are800 to 900?C for rhyodacite and 900 to 950?C for dacite. Magmathat erupted as Unit S was slightly hotter and more oxidizedthan magma that gave rise to Unit V. More mafic magmas wereassociated with the voluminous rhyodacitic to dacitic magmareservoir, as indicated by the presence of andesite and basalticandesite lava flows and by scoria inclusions in Unit V. The compositional suite from basaltic andesite to rhyodacitecan be simulated satisfactorily by crystal-fractionation calculationsbased on major-element phenocryst and rock compositions, andis consistent with modes of the Calabozos rocks. Rhyodacitesof Units V and S, however, are enriched in elements such asRb, Ba, and Zr relative to trace-element contents predictedby crystal-fractionation models. The enrichment can be achievedby assimilation of wall rock or a partial melt of the wall rock.The latter requires that the ratio of assimilation rates tocrystallization rates be between 0?1 and 0?3. Rates of assimilationversus crystallization were greater for Unit S than for UnitV, which is consistent with the lower Fe-Ti oxide temperaturesand less oxidized state of the latter. The Loma Seca Tuff is similar in bulk composition to sanidine-bearingash-flow sheets erupted on ‘mature’ continentalcrust, but it is mineralogically akin to ash-flow tuffs eruptedon ‘immature’ crust. The difference is attributed,in part, to the effect of the density of the crust on the rateof magma ascent at shallow levels. The ascent of large bodiesof silicic magma is slower in silicic (less dense) crust thanin mafic crust, causing the magmas to be erupted at a laterpoint in the crystallization history.  相似文献   

13.
Geological mapping of the Tucumã area has enabled the identification of dike swarms intruded into an Archean basement. The disposition of these dikes is consistent with the well-defined NW-SE trending regional faults, and they can reach up to 20 km in length. They were divided into three main groups: (i) felsic dikes (70% of the dikes), composed exclusively of porphyritic rhyolite with euhedral phenocrysts of quartz and feldspars immersed in an aphyric felsite matrix; (ii) mafic dikes, with restricted occurrence, composed of basaltic andesite and subordinate basalt, with a mineralogical assembly consisting dominantly of plagioclase, clinopyroxene, orthopyroxene and olivine; and (iii) intermediate rocks, represented by andesite and dacite. Dacites are found in outcrops associated with felsic dikes, representing different degrees of hybridization or mixture of mafic and felsic magmas. This is evidenced by a large number of mafic enclaves in the felsic dikes and the frequent presence of embayment textures. SHRIMP U-Pb zircon dating of felsic dikes yielded an age of 1880.9 ± 3.3 Ma. The felsic dikes are peraluminous to slightly metaluminous and akin to A2, ferroan and reduced granites. The intermediate and mafic dikes are metaluminous and belong to the tholeiitic series. Geochemical modeling showed that mafic rocks evolved by pyroxene and plagioclase crystallization, while K-feldspar and biotite are the fractionate phases in felsic magma. A simple binary mixture model was used to determine the origin of intermediate rocks. It indicated that mixing 60% of rhyolite and 40% basaltic andesite melts could have generated the dacitic composition, while the andesite liquid could be produced by mixing of 60% and 40% basaltic andesite and rhyolite melts, respectively. The mixing of basaltic and andesitic magmas probably occurred during ascent and storage in the crust, where andesite dikes are likely produced by a more homogeneous mixture at high depths in the continental crust (mixing), while dacite dikes can be generated in the upper crust at a lower temperature, providing a less efficient mixing process (mingling). The affinities observed between the felsic to intermediate rocks of the Rio Maria and São Felix do Xingu areas and the bimodal magmatism of the Tucumã area reinforce the hypothesis that in the Paleoproterozoic the Carajás province was affected by processes involving thermal perturbations in the upper mantle, mafic underplating, and associated crustal extension or transtension. The 1.88 Ga fissure-controlled A-type magmatism of the Tucumã area was emplaced ∼1.0 to ∼0.65 Ga after stabilization of the Archean crust. Its origin is not related to subduction processes but to the disruption of the supercontinent at the end of the Paleoproterozoic.  相似文献   

14.
 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   

15.
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   

16.
Petrologic studies of tephra from Kanaga, Adak, and Great Sitkin Islands indicate that amphibole fractionation and magma mixing are important processes controlling the composition of calc-alkaline andesite and dacite magmas in the central Aleutians. Amphibole is ubiquitous in tephra from Kanaga and Adak Islands, whereas it is present only in a basaltic-andesite pumice from Great Sitkin. Dacitic tephra from Great Sitkin do not contain amphibole. Hornblende dacite tephra contain HB+PLAG+OX±OPX±CPX phenocrysts with simple zoning patterns, suggesting that the dacites evolved in isolated magma chambers. Andesitic tephra from Adak contain two pyroxene and hornbelende populations, and reversely zoned plagioclase, indicating a more complex history involving mixing and fractional crystallization. Mass balance calculations suggest that the andesitic tephra may represent the complements of amphibole-bearing cumulate xenoliths, both formed during the evolution of high-Al basalts. The presence of amphibole in andesitic and dacitic tephra implies that Aleutian cale-alkaline magmas evolve in the mid to lower crust under hydrous (>4 wt.% H2O) and oxidizing (Ni–NiO) conditions. Amphibole-bearing andesites and pyroxene-bearing dacites from Great Sitkin indicates fractionation at several levels within the arc crust. Despite its absence in many calc-alkaline andesite and dacite lavas, open system behavior involving amphibole fractionation can explain the trace element characteristies of lavas found on Adak Island. Neither open nor closed system fractionation involving a pyroxene-bearing assemblage is capable of explaining the trace element concentrations or ratios found in the Adak suite. We envision a scenario where amphibole was initially a liquidus phase in many calc-alkaline magmas, but was later replaced by pyroxenes as the magmas rose to shallow levels within the crust. The mineral assemblage in these evolved lavas reflects shallow level equilibration of the magma, whereas the trace element chemistry provides evidence for a earlier, amphibole-bearing, mineral assemblage.  相似文献   

17.
Andesite and dacite from Barren and Narcondam volcanic islands of Andaman subduction zone are composed of plagioclase, orthopyroxene, clinopyroxene, olivine, titanomagnetite, magnesio-hornblende and rare quartz grains. In this study, we use the results of mineral chemical analyses of the calc-alkaline rock suite of rocks as proxies for magma mixing and mingling processes. Plagioclase, the most dominant mineral, shows zoning which includes oscillatory, patchy, multiple and repetitive zonation and ‘fritted’ or ‘sieve’ textures. Zoning patterns in plagioclase phenocrysts and abrupt fluctuations in An content record different melt conditions in a dynamic magma chamber. ‘Fritted’ zones (An55) are frequently overgrown by thin calcic (An72) plagioclase rims over well-developed dissolution surfaces. These features have probably resulted from mixing of a more silicic magma with the host andesite. Olivine and orthopyroxene with reaction and overgrowth rims (corona) suggest magma mixing processes. We conclude that hybrid magma formed from the mixing of mafic and felsic magma by two-stage processes – initial intrusion of hotter mafic melt (andesitic) followed by cooler acidic melt at later stage.  相似文献   

18.
The aim of this study is to quantify the crustal differentiation processes and sources responsible for the origin of basaltic to dacitic volcanic rocks present on Cordón El Guadal in the Tatara-San Pedro Complex (TSPC). This suite is important for understanding the origin of evolved magmas in the southern Andes because it exhibits the widest compositional range of any unconformity-bound sequence of lavas in the TSPC. Major element, trace element, and Sr-isotopic data for the Guadal volcanic rocks provide evidence for complex crustal magmatic histories involving up to six differentiation mechanisms. The petrogenetic processes for andesitic and dacitic lavas containing undercooled inclusions of basaltic andesitic and andesitic magma include: (1) assimilation of garnet-bearing, possibly mafic lower continental crust by primary mantle-derived basaltic magmas; (2) fractionation of olivine + clinopyroxene + Ca-rich plagioclase + Fe-oxides in present non-modal proportions from basaltic magmas at ∼4–8 kbar to produce high-Al basalt and basaltic andesitic magmas; (3) vapor-undersaturated (i.e., P H2O<P TOTAL) partial melting of gabbroic crustal rocks at ∼3–7 kbar to produce dacitic magmas; (4) crystallization of plagioclase-rich phenocryst assemblages from dacitic magmas in shallow reservoirs; (5) intrusion of basaltic andesitic magmas into shallow reservoirs containing crystal-rich dacitic magmas and subsequent mixing to produce hybrid basaltic andesitic and andesitic magmas; and (6)␣formation and disaggregation of undercooled basaltic andesitic and andesitic inclusions during eruption from shallow chambers to form commingled, mafic inclusion-bearing andesitic and dacitic lavas flows. Collectively, the geochemical and petrographic features of the Guadal volcanic rocks are interpreted to reflect the development of shallow silicic reservoirs within a region characterized by high crustal temperatures due to focused basaltic activity and high magma supply rates. On the periphery of the silicic system where magma supply rates and crustal temperatures were lower, cooling and crystallization were more important than bulk crustal melting or assimilation. Received: 2 July 1997 / Accepted: 25 November 1997  相似文献   

19.
The isotopic compositions of Pb and Sr in Pleistocene basalt, high-silica rhyolite, and andesitic inclusions in rhyolite of the Coso volcanic field indicate that these rocks were derived from different levels of compositionally zoned magmatic systems. The 2 earliest rhyolites probably were tapped from short-lived silicic reservoirs, in contrast to the other 36 rhyolite domes and lava flows which the isotopic data suggest may have been leaked from the top of a single, long-lived magmatic system. Most Coso basalts show isotopic, geochemical, and mineralogic evidence of interaction with crustal rocks, but one analyzed flow has isotopic ratios that may represent mantle values (87Sr/86Sr=0.7036,206Pb/204Pb=19.05,207Pb/204Pb=15.62,208Pb/204Pb= 38.63). The (initial) isotopic composition of typical rhyolite (87Sr/86Sr=0.7053,206Pb/204Pb=19.29,207Pb/204Pb= 15.68,208Pb/204Pb=39.00) is representative of the middle or upper crust. Andesitic inclusions in the rhyolites are evidently samples of hybrid magmas from the silicic/mafic interface in vertically zoned magma reservoirs. Silicic end-member compositions inferred for these mixed magmas, however, are not those of erupted rhyolite but reflect the zonation within the silicic part of the magma reservoir. The compositional contrast at the interface between mafic and silicic parts of these systems apparently was greater for the earlier, smaller reservoirs.  相似文献   

20.
Mid-Miocene volcanic rocks are rare in the Aegean region, although early Miocene and late Miocene-Quaternary volcanism is widespread. At Oxylithos (island of Evia), 14 Ma dacites form a dome or sub-volcanic complex. Phreatomagmatic eruptions formed nearby rhyolitic pyroclastic surge deposits. The calc-alkaline dacites contain bronzite, Mg-rich clinopyroxene, phlogopite and plagioclase phenocrysts. The magma results from mixing of a Mg-rich andesitic magma, similar to that in the nearby island of Skyros, with more felsic magmas represented by the rhyolitic pyroclastics. The dacites are geochemically similar to adakites, which are derived by partial melting of eclogitic subducted oceanic crust and have low Y and Yb and high Sr/Y ratio. 87Sr/86Sr≈0.7095 is found in both dacite and rhyolite. Lead isotopic composition from the high-Mg andesite from Skyros, with 207Pb/204Pb= 15.70 and 208Pb/204Pb= 38.90, forms a linear trend with Evia dacite and rhyolite with 207Pb/204Pb= 15.71 and 208Pb/204Pb= 39.05. The high temperatures required to produce such magma resulted from decompression due to extension of the Aegean basin at the same time as the initial intrusion of the detached subducted slab in the western Aegean that has been imaged by seismic tomography. The Oxylithos rocks extend the known occurrences of adakite series rocks: this series is not restricted to sites with subduction of young oceanic crust.  相似文献   

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