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1.
Aleutian tholeiitic and calc-alkaline magma series I: The mafic phenocrysts   总被引：1，自引：0，他引：1  
Diagnostic mafic silicate assemblages in a continuous spectrum of Aleutian volcanic rocks provide evidence for contrasts in magmatic processes in the Aleutian arc crust. Tectonic segmentation of the arc exerts a primary control on the variable mixing, fractional crystallization and possible assimilation undergone by the magmas. End members of the continuum are termed calc-alkaline (CA) and tholeiitic (TH). CA volcanic rocks (e.g., Buldir and Moffett volcanoes) have low FeO/MgO ratios and contain compositionally diverse phenocryst populations, indicating magma mixing. Their Ni and Cr-rich magnesian olivine and clinopyroxene come from mantle-derived mafic olivine basalts that have mixed with more fractionated magmas at mid-to lower-crustal levels immediately preceding eruption. High-Al amphibole is associated with the mafic end member. In contrast, TH lavas (e.g., Okmok and Westdahl volcanoes) have high FeO/MgO ratios and contain little evidence for mixing. Evolved lavas represent advanced stages of low pressure crystallization from a basaltic magma. These lavas contain groundmass olivine (FO 40–50) and lack Ca-poor pyroxene. Aleutian volcanic rocks with intermediate FeO/MgO ratios are termed transitional tholeiitic (TTH) and calc-alkaline (TCA). TCA magmas are common (e.g., Moffett, Adagdak, Great Sitkin, and Kasatochi volcanoes) and have resulted from mixing of high-Al basalt with more evolved magmas. They contain amphibole (high and low-Al) or orthopyroxene or both and are similar to the Japanese hypersthene-series. TTH magmas (e.g., Okmok and Westdahl) contain orthopyroxene or pigeonite or both, and show some indication of upper crustal mixing. They are mineralogically similar to the Japanese pigeonite-series. High-Al basalt lacks Mg-rich mafic phases and is a derivative magma produced by high pressure fractionation of an olivine tholeiite. The low pressure mineral assemblage of high-Al basalt results from crystallization at higher crustal levels.  相似文献   

2.
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   

3.
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   

4.
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   

5.
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   

6.
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   

7.
At Medicine Lake Volcano, California, the compositional gap between andesite (57–62 wt.% SiO2) and rhyolite (73–74 wt.% SiO2) has been generated by fractional crystallization. Assimilation of silicic crust has also occurred along with fractionation. Two varieties of inclusions found in Holocene rhyolite flows, hornblende gabbros and aphyric andesites, provide information on the crystallization path followed by lavas parental to the rhyolite. The hornblende gabbros are magmatic cumulate residues and their mineral assemblages are preserved evidence of the phases that crystallized from an andesitic precursor lava to generate the rhyolite lavas. The andesitic inclusions represent samples of a parental andesite and record the early part of the differentiation history. Olivine, plagioclase and augite crystallization begins the differentiation history, followed by the disappearance of olivine and augite through reaction with the liquid to form orthopyroxene and amphibole. Further crystallization of the assemblage plagioclase, amphibole, orthopyroxene, magnetite, and apatite from a high-SiO2 andesite leads to rhyolite. This final crystallization process occurs on a cotectic that is nearly horizontal in temperature-composition space. Since a large amount of crystallization occurs over a limited temperature interval, a compositional gap develops between rhyolite and high SiO2 andesite.Liquidus surfaces with shallow slopes in temperature-composition space are characteristic of several late-stage crystallization assemblages in the andesite to rhyolite compositional range. Experimentally produced plagioclase+ amphibole+orthopyroxene+magnetite and plagioclase+ augite+low-Ca pyroxene+magnetite cotectics have liquidus slopes that are nearly flat. At other calc-alkaline volcanic centers crystallization processes involving large compositional changes over small temperature intervals may also be important in the development of bimodal volcanism (i.e. the existence of a composition gap). At Mt. Mazama and Mt. St. Helens, USA and Aso Caldera and Shikotsu, Japan the amphibole-bearing assemblage was important. At Krakatau, Indonesia and Katmai, USA, an augite+orthopyroxene-bearing assemblage was important. In addition to its role in the production of a compositional gap between intermediate and rhyolitic lavas, the crystallization process increases the H2O content of the residual liquid. This rapid increase in residual liquid volatile content which results from the precipitation of a large proportion of crystalline solids may be an important factor among several that lead to explosive silicic eruptions.  相似文献   

8.
Quaternary Erzincan Volcanics (QEVs) from the Erzincan Basin consist of mega- and pheno-cryst-bearing high-K calc-alkaline dome lavas. Fourteen nearly phenocrystic domes, with a range of basaltic-andesite, andesite, dacite and rhyolite compositions, were emplaced in the North Anatolian Fault Zone. The emplacement ages yielded by the unspiked K–Ar technique range from 102 to 140 ka. The andesitic domes (each less than 3 km in diameter) contain amphibole megacrysts. Amphibole compositions show a linear variation from ferro-edenite, edenite to pargasite from rhyolite to andesite. Pargasitic amphibole megacrysts scattered into the groundmass are very similar in composition to the microlites. All plagioclases are 53 mol%. Oscillation types are An32−50 whose variations range from 10 to 16 mol% An and have 10–150 μm in thickness. Pre-eruptive conditions, calculated from mega- and pheno-cryst composition, using pyroxene and two oxide thermometers and the Al-in-hornblende barometer, ranged from 918 to 837 °C and 6.6 to 4.3 kbar for andesitic magma, 824–755 °C and 4.6–4.2 kbar for dacitic magma to 803–692 °C and 4.3–3.9 kbar for rhyolitic magma, which correspond to a depth of >10 km for storage region of the crust. The fO2 values vary from −14.25 to −15.35 log units which are plotted just below nickel–nickel oxide (NNO) buffers. The systematic decrease in thermobarometric results from andesite to rhyolite is consistent with a single magma reservoir moving upward through the crust followed by fractional crystallization. Textural and compositional relationships of mega- and pheno-crystic phases suggest that magma mixing, fluid input to the reservoir and fractional crystallization processes, with a small amount crustal contamination play key role in evolution of the QEVs.  相似文献   

9.
Quaternary lavas of the normal island-arc basalt—andesite—dacite association in the islands of Java and Bali range from those belonging to tholeiitic series over Benioff-zone depths of ～ 150 km to high-K calc-alkaline series over Benioff-zone depths of 250 km. More abundant and diverse calc-alkaline lavas are found over intermediate Benioff-zone depths. On average, basaltic lavas become slightly more alkaline (largely due to increased K contents) with increasing depth to the Benioff zone. Levels of incompatible minor and trace elements (K, Rb, Cs, Ba, Nb, U, Th, light REE) show a corresponding increase of almost an order of magnitude.Low average Mg-numbers (～ 0.52) and Ni and Cr abundances (15–25 and 35–60 ppm, respectively) of basaltic lavas suggest that few lavas representing primary mantle-derived magma compositions are present. Calculated primary basaltic magma compositions for most tholeiitic and calc-alkaline volcanic centres are olivine tholeiites with 15–30% ol. The single high-K calc-alkaline centre considered yielded transitional alkali olivine basalt—basanite primary magma compositions. These calculated magma compositions suggest that the percentage of mantle melting decreases with increasing depth to the Benioff zone (from >25 to <10%), while the corresponding depth of magma separation increases from ～ 30 to 60 km.Calculation of REE patterns for basaltic magmas on the basis of peridotitic mantle sources with spinel lherzolite, amphibole lherzolite or garnet lherzolite mineralogy, and model REE levels of twice chondritic abundances, indicates that change in the conditions of magma genesis alone cannot explain the observed change in light-REE abundances of basaltic lavas with increasing depth to the Benioff zone. Complementary calculations of the REE levels of mantle sources required to yield the average tholeiitic, calc-alkaline and high-K calc-alkaline basaltic magma indicate that light-REE abundances must increase from 2–3 to 7–8 times chondrites with increasing depth to the Benioff zone. The percentages of mantle melting favoured on REE evidence are lower than those indicated by major-element considerations.The observed variation in incompatible element geochemistry of mantle magma sources is thought to be related directly or indirectly to dehydration and partial-melting processes affecting subducted oceanic crust. The possible nature of this relationship is discussed.  相似文献   

10.
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   

11.
Calc-alkaline arc magmatism at convergent plate margins is volumetrically dominated by metaluminous andesites. Many studies highlighted the importance of differentiation via fractionation processes of arc magmas, but only in the last decades, it has been demonstrated that not all rock-forming minerals may affect the evolution of calc-alkaline suites. In particular, a major role exerted by Al-rich hornblende amphibole as fractionating mineral phase has been documented in many volcanic arc settings. The aim of this work is to understand the role of the Tschermak molecule (CaAlAlSiO6) hosted in the hornblende and plagioclase fractionation assemblage in driving magma differentiation in calc-alkaline magmatic suites. We explore this issue by applying replenishment–fractional crystallization (RFC) and rare earth element–Rayleigh fractional crystallization (REE-FC) modeling to the Sabzevar Eocene (ca. 45–47 Ma) calc-alkaline volcanism of NE Central Iran, where hornblende-controlled fractionation has been demonstrated. Major element mass balance modeling indicates RFC dominated by a fractionating assemblage made of Hbl52.0–52.5 + Pl44.1–44.2 + Ttn3.3–3.9 (phases are expressed on total crystallized assemblage). REE-FC modeling shows, instead, a lower degree of fractionation with respect to RFC models that is interpreted as due to hornblende and plagioclase resorption by the residual melt. Calculations demonstrate that fractionation of the Tschermak molecule can readily produce dacite and rhyolite magmas starting from a calc-alkaline andesite source (FC = ca. 30 %). In particular, the Tschermak molecule controls both the heavy rare earth elements (HREE) and light rare earth element (LREE) budgets in calc-alkaline differentiation trends.  相似文献   

12.
The Cold Bay Volcanic Center,Aleutian Volcanic Arc   总被引：1，自引：0，他引：1  
The Cold Bay Volcanic Center has experienced two major stages of eruptive activity. Early (M-Series) acitivity produced bimodal Hi-Alumina basalt and calc-alkaline andesite lavas while later (FPK-Series) activity produced only calc-alkaline andesite. The spectrum of basalt compositions is believed to be due to high pressure (8 kb) fractionation at or near the base of the crust. Abundant mineralogical and geochemical evidence support a lower pressure mixing origin for all andesites. Inspection of the mineralogical data has shown that the earliest (M-Series) andesites were produced by mixing of basalt (<53 wt% SiO2) and silicic andesite (60.5 to 62.5 wt%) while later (FPK-Series) andesites resulted from the mixing of basaltic-andesite (53 to 56 wt%) and less silicic andesite (58.5 to 60.0 wt%). The major element and trace element geochemical data are consistent with a low pressure fractionation origin for the silicic endmember magmas and support the temporal variations in both mafic and silicic endmember compositions. The complete lack of crustal inclusions in all lavas is taken as evidence for a minimal crustal melting and/or assimilation role in the origin of the silicic endmembers. Many of the features of all andesites, including the important long term convergence of endmember magma compositions, are consistent with the process of liquid fractionation, accompanied by large scale magma mixing. A deduced upper limit of 62.5 wt% SiO2 for the silicic endmember magmas suggests that liquid fractionation, in the absence of major crustal melting, cannot produce more silicic magmas. A possible explanation is the presence of a rheological barrier, based on the concept of critical crystallinity (Marsh 1981), which prohibits more silicic liquids from being extracted from a crystal-liquid suspension.  相似文献   

13.
Andesites from northeastern Kanaga Island,Aleutians   总被引：1，自引：0，他引：1  
Kanaga island is located in the central Aleutian island arc. Northeastern Kanaga is a currently active late Tertiary to Recent calc-alkaline volcanic complex. Basaltic andesite to andesite lavas record three episodes (series) of volcanic activity. Series I and Series II lavas are all andesite while Series III lavas are basaltic andesite to andesite. Four Series II andesites contain abundant quenched magmatic inclusions ranging in composition from high-MgO low-alumina basalt to low-MgO highalumina basalt. The spectrum of lava compositions is due primarily to fractional crystallization of a parental low-MgO high-alumina basalt but with variable degrees of crustal contamination and magma mixing. The earliest Series I lavas represent mixing between high-alumina basalt and silicic andesite with maximum SiO2 contents of 65–67 wt %. Later Series I and all Series II lavas are due to mixing of andesite magmas of similar composition. The maximum SiO2 content of the pre-mixed andesites magmas is estimated at 60–63 wt %. The youngest lavas (Series III) are all non-mixed and have maximum estimated SiO2 contents of 59 wt %. The earliest Series I lavas contain a significant crustal component while all later lavas do not. It is concluded that the maximum SiO2 contents of silicic magmas, the contribution of crustal material to silicic magma generation, and the role of magma mixing all decrease with time. Furthermore, silicic magmas generated by fractional crystallization at this volcanic center have a maximum SiO2 content of 63 wt %. All of these features have also been documented at the central Aleutian Cold Bay Volcanic Center (Brophy 1987). Based on data from these two centers a model of Aleutian calc-alkaline magma chamber development is proposed. The main features are: (1) a single low pressure magma chamber is continuously supplied by primitive low-alumina basalt; (2) non-primary high-alumina basalt is formed along the chamber margins by selective gravitational settling of olivine and clinopyroxene and retention of plagioclase; (3) sidewall crystallization accompanied by crustal melting produces buoyant silicic (>63 wt % SiO2) liquids that pond at the top of the chamber, and; (4) continued sidewall crystallization, now isolated from the chamber wall, produces silicic liquids with 63 wt % SiO2 that increase the thickness and lowers the overall SiO2 content of the upper silicic zone. It is suggested that the maximum SiO2 content of 63% imposed on fractionation-generated magmas is due to a rheological barrier that prohibits the extraction of more silicic liquids from a crystal-liquid mush along the chamber wall.  相似文献   

14.
Amphibole, while uncommon as a phenocryst in arc lavas, is increasingly recognized as a key constituent in the petrogenesis of arc magmas. Fractional crystallization of water-saturated arc magmas in the lower crust can yield substantial volumes of amphibole cumulates that, depending on the pressure of crystallization, may also contain garnet. Fractionation of this higher pressure assemblage has been invoked as a possible mechanism in the production of magmas that contain an adakitic signature. This study examines newly dated Late-Oligocene (25.37 ± 0.13 Ma) hypabyssal amphibole-rich andesites from Cerro Patacon in the Panama Canal region. These andesites contain nodules of amphibole cumulates that are ~4–6 cm in diameter and are almost entirely composed of 5–10-mm amphibole crystals (dominantly ferri-tschermakite). Geochemical variations, optical and chemical zoning of the Cerro Patacon amphiboles are consistent with their evolution in a crystal mush environment that had at least one recharge event prior to entrainment in the host andesite. Amphiboles hosted within the cumulate nodules differ from those hosted in the Cerro Patacon andesite and contain consistently higher values of Ti. We suggest these nodules represent the early stages of fractionation from a water-saturated magma. Cerro Patacon andesites have REE concentrations that plot at the most depleted end of Central American Arc magmas and exhibit a distinctive depletion in the middle REE. These geochemical and petrographic observations strongly support significant amphibole fractionation during formation of the Cerro Patacon andesite, consistent with the petrographic evidence. Fractionation of water-saturated magmas is a mechanism by which adakitic compositions may be produced, and the Cerro Patacon andesites do exhibit adakite-like geochemical characteristics (e.g., elevated Sr/Y; 28–34). However, the relatively elevated concentrations of Y and HREE indicate garnet was not stable in the fractionating assemblage during this early stage of arc development.  相似文献   

15.
TAMURA  Y.; NAKAMURA  E. 《Journal of Petrology》1996,37(6):1307-1319
New Sr and Nd isotopic data are presented and integrated withprevious data for the Shirahama Group Mio-Pliocene medium-Kvolcanic are suite of south-central Honshu, Japan. Main resultsare: (1) The Shirahama lavas range in 87Sr/86Sr from 0.70315to 0.70337 and in 143Nd/144Nd from 0.51298 to 0.51306; the Srand Nd isotopic data cluster tightly within the mantle array,and all lie within an overlapping field of mid-ocean ridge basaltand ocean island basalt; (2) small differences exist among theShirahama tholeiitic series, calc-alkaline series and mixedlavas. The present isotopic data are consistent with a previouslypublished model, which proposes that chemical variations inmagmas of coexisting tholeiitic and calc-alkaline series areproduced through crystal fractionation from mantle-derived magmasof basalt and magnesian andesite, respectively. Moreover, thetholeiitic series and the calc-alkaline series are isotopicallyidentical. Thus, both magma series can be derived from a sourcemantle with the same isotopic composition, supporting the hypothesisof simultaneous generation of basalt and magnesian andesitemagmas from a single diapir rising through the mantle wedgeabove the subduction zone. The differences of water contentand temperature within the diapir are again thought to havebeen produced through dehydration and heating of an isotopicallyhomogeneous hydrous diapir. The isotopic data show that thehigh-SiO2 lavas have the same isotopic compositions as moremafic lavas. These data and liquid lines of descent of the Shirahamamagmas suggest that even rhyolites can be produced by differentiationfrom mantle-derived magmas without crustal contamination. Analysesfrom 38 other arc volcanoes have been compiled to investigatethe intravolcano variability of 87Sr/86Sr. Twelve of these displayno intravolcano strontium isotopic variability, as is the casewith the Shirahama Group, but others show greater variationof 87Sr/86Sr from individual volcanic centers, presumably reflectingcrustal contamination. Most of the latter volcanoes are underlainby thick continental crust. It is noteworthy, however, thatthe greater variations of 87Sr/86Sr correlate with SiO2 content;andesites or dacites, not basalts, from the same volcano havethe lowest 87Sr/86Sr, and these rocks are calc-alkaline in termsof FeO*/MgO and SiO2 Theoretically, assimilation of continentalcrust by the isotopically uniform Shirahama magmas could producethese relationships. Given that mantle-derived basalt and magnesianandesite both encounter continental crust on their ascent tothe surface, the hotter basalt magma would assimilate more crustalwallrocks than the cooler andesite, resulting in the basaltbeing more radiogenic. Fractional crystallization, magma mixing,and/or assimilation-fractional crystallization of these magmasin crustal magma chambers could produce large compositionalvariations, but the derivatives of the hotter basaltic magmas(tholeiitic series in the broad sense) would display greatercontamination than those derived from the cooler andesitic magmas(calc-alkaline series). *Telephone: 81-858-43-1215. Fax: 81-858-43-2184. e-mail: tamura{at}misasa.okayam-u.ac.jp  相似文献   

16.
The phase relations of primitive magnesian andesites and basaltic andesites from the Mt. Shasta region, N California have been determined over a range of pressure and temperature conditions and H2O contents. The experimental results are used to explore the influence of H2O and pressure on fractional crystallization and mantle melting behavior in subduction zone environments. At 200-MPa H2O-saturated conditions the experimentally determined liquid line of descent reproduces the compositional variation found in the Mt. Shasta region lavas. This calc-alkaline differentiation trend begins at the lowest values of FeO*/MgO and the highest SiO2 contents found in any arc magma system and exhibits only a modest increase in FeO*/MgO with increasing SiO2. We propose a two-stage process for the origin of these lavas. (1) Extensive hydrous mantle melting produces H2O-rich (>4.5--6 wt% H2O) melts that are in equilibrium with a refractory harzburgite (olivine + orthopyroxene) residue. Trace elements and H2O are contributed from a slab-derived fluid and/or melt. (2) This mantle melt ascends into the overlying crust and undergoes fractional crystallization. Crustal-level differentiation occurs under near-H2O saturated conditions producing the distinctive high SiO2 and low FeO*/MgO characteristics of these calc-alkaline andesite and dacite lavas. In a subset of Mt. Shasta region lavas, magnesian pargasitic amphibole provides evidence of high pre-eruptive H2O contents (>10 wt% H2O) and lower crustal crystallization pressures (800 MPa). Igneous rocks that possess major and trace element characteristics similar to those of the Mt. Shasta region lavas are found at Adak, Aleutians, Setouchi Belt, Japan, the Mexican Volcanic Belt, Cook Island, Andes and in Archean trondhjemite--tonalite--granodiorite suites (TTG suites). We propose that these magmas also form by hydrous mantle melting.Editorial responsibility: J. Hoefs  相似文献   

17.
G.F. Zellmer  S.P. Turner 《Lithos》2007,95(3-4):346-362
Mafic enclaves are commonly found in intermediate arc magmas, and their occurrence has been linked to eruption triggering by pre-eruptive magma mixing processes. New major, trace, Sr–Nd and U–Th isotope data of rocks from Nisyros in the Aegean volcanic arc are presented here. Pre-caldera samples display major and trace element trends that are consistent with fractionation of magnetite and apatite within intermediate compositions, and zircon within felsic compositions, and preclude extensive hybridization between mafic and felsic magmas. In contrast, post-caldera dacites form a mixing trend towards their mafic enclaves. In terms of U-series isotopes, most samples show small 238U excesses of up to  10%. Mafic enclaves have significantly higher U/Th ratios than their dacitic host lavas, precluding simple models that relate the mafic and felsic magmas by fractionation or aging alone. A more complicated petrogenetic scenario is required. The post-caldera dacites are interpreted to represent material remobilized from a young igneous protolith following influx of fresh mafic magma, consistent with the U–Th data and with Sr–Nd isotope constraints that point to very limited (< 10%) assimilation of old crust at Nisyros. When these results are compared to data from Santorini in the same arc, there are many geochemical similarities between the two volcanic centers during the petrogenesis of the pre-caldera samples. However, striking differences are apparent for the post-caldera lavas: in Nisyros, dacites show geochemical and textural evidence for magma mixing and remobilization by influx of mafic melts, and they erupt as viscous lava domes; in Santorini, evidence for geochemical hybridization of dacites and mafic enclaves is weak, dacite petrogenesis does not involve protolith remobilization, and lavas erupt as less viscous flows. Despite these differences, it appears that mafic enclaves in intermediate Aegean arc magmas consistently yield timescales of at least 100 kyrs between U enrichment of the mantle wedge and eruption, on the upper end of those estimated for the eruptive products of mafic arc volcanoes. Finally, the data presented here provide constraints on the rates of differentiation from primitive arc basalts to dacites (less than  140 kyrs), and on the crustal residence time of evolved igneous protoliths prior to their remobilization by mafic arc magmas (greater than  350 kyrs).  相似文献   

18.
Rare Earth data and petrogenesis of andesite from the North Chilean Andes   总被引：1，自引：0，他引：1  
Abundances of 9 Rare-Earth Elements are reported for 21 andesite and dacite samples from a small group of volcanoes in the North Chilean segment of the Andean plate margin. The data are correlated with major and trace element data (Roobol et al., in press) and are used to critically assess several models proposed to account for the origin of andesite. A three-stage model for andesite petrogenesis is proposed. This involves: (i) initiation of melting at the subduction zone and rise of magmas into the overlying mantle-wedge, which thereby becomes richer in garnet-pyroxene components and large-ion lithophile (LIL) elements, (ii) partial melting of this LIL-enriched garnet pyroxenite material to form andesitic magmas within the mantle wedge, (iii) minor crystal fractionation during rise through the crust.  相似文献   

19.
 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   

20.
Along strike of the Quaternary magmatic arc in the SouthernVolcanic Zone of the Andes, there is a south to north increasein crustal thickness, and the lavas define systematic geochemicaltrends which have been attributed to variations in the proportionsand compositions of mantle-and crustal-derived components. Realisticinterpretations of these regional geochemical trends requiresan understanding of the sources and processes that control lavacompositions at individual volcanoes. Because it is in an importantgeophysical and geochemical transition zone, we studied theAzufre—Planchon—Peteroa volcanic complex, a nestedgroup of three volcanoes <055 m.y. in age located at 3515'Sin the Southern Volcanic Zone of the Andes. North of this complexat 33–35S the continental crust is thick, basalts areabsent, and there is abundant evidence for crustal componentsin the evolved lavas, but south of 37S, where the crust isrelatively thin, basaltic lavas are abundant and the contributionof continental crust to the lavas is less obvious. In additionto its location, this volcanic complex is important becausethere is a diversity of lava compositions, and it is the northernmostexposure of recent basaltic volcanism on the volcanic front.Therefore, the lavas of this complex can be used to identifythe relative roles of mantle, lower-crustal and upper-crustalsources and processes at a single location. Volcan Azufre is the oldest and largest volcano of the complex;it is a multi-cycle, bimodal, basaltic andesite–dacitestratovolcano. Volcan Planchon is the northernmost basalt-bearingvolcano along the volcanic front of the Southern Andes, andVolcan Peteroa, the youngest volcano of the complex, has eruptedmixed magmas of andesitic and dacitic composition. Most basalticandesite lavas at Azufre and Planchon are related by a plagioclase-poor,anhydrous mineral fractionating assemblage. High-alumina basaltis produced from a tholeiitic parent by an 4–8 kbar fractionatingassemblage. During this moderatepressure crystallization, themagmas also incorporated a crustal component with high La/Yband high abundances of Rb, Cs and Th. Based on the chemicalcharacteristics of the added component and the inferred depthof crystallization, the crustal source may have been garnetgranulite derived from solidified arc magmas in the lower tomiddle continental crust. At Planchon, the role of crustal assimilationhas increased with decreasing eruption age probably becausecrustal temperatures have increased during continued volcanism.Azufre dacite lavas formed at low pressures by fractionationof a plagioclase-rich assemblage. These dacite lavas containan upper-crustal component, probably derived in part from limestone,with high values of 87Sr/86Sr and 18O/16O. Thus two depths (upperand lower crust) of crystallization and associated crustal assimilationare evident in Planchon–Azufre lavas. Peteroa, the focusof recent volcanism, consists of calc-alkaline andesite anddacite eruptive products whose textures and compositions indicatean important role for magma mixing. Therefore, the volcanismevolved from a tholeiitic system of basalt and subordinate dacite(Planchon–Azufre) to a calc-alkaline system with abundantmixed lavas of intermediate composition (Peteroa). In additionto crustal thickness, two important parameters which controlledthe diversity of lava composition in this complex are magmasupply rate from the mantle and crustal temperature. Both parametersvaried with time, and they must be considered in broader interpretationsof along-strike geochemical trends. KEY WORDS: arc magmas; Andes; Peteroa; Planchan; geochemistry *Corresponding author. Present address: ENTRIX, Inc., 4II North Central Avenue, Glendale, CA 91203, USA  相似文献   

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