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The petrogenesis of calc-alkaline magmatism in the Eocene AbsarokaVolcanic Province (AVP) is investigated at Washburn volcano,a major eruptive center in the low-K western belt of the AVP.New 40Ar/39Ar age determinations indicate that magmatism atthe volcano commenced as early as 55 Ma and continued untilat least 52 Ma. Although mineral and whole-rock compositionaldata reflect near equilibrium crystallization of modal phenocrysts,petrogenetic modeling demonstrates that intermediate compositionmagmas are hybrids formed by mixing variably fractionated andcontaminated mantle-derived melts and heterogeneous siliciccrustal melts. Nd and Sr isotopic compositions along with traceelement data indicate that silicic melts in the Washburn systemare derived from deep-crustal rocks broadly similar in compositionto granulite-facies xenoliths in the Wyoming Province. Our preferredexplanation for these features is that mantle-derived basalticmagma intruded repeatedly in the deep continental crust leadingto fractional crystallization, silicic melt production, andhomogenization of magmas, followed by ascent to shallow reservoirsand crystallization of new plagioclase-rich mineral assemblagesin equilibrium with the intermediate hybrid liquids. The implicationsof this process are that (1) some calc-alkaline magmas may onlybe recognized as hybrids on purely chemical grounds, particularlyin systems where mixing precedes and is widely separated fromcrystallization in space and time, and (2) given the role ascribedto crustal processes at Washburn volcano, the variation betweenrocks that follow calc-alkaline trends in the western AVP andthose that follow shoshonitic trends in the east cannot simplyreflect higher pressures of fractionation to the east in Moho-levelmagma chambers in the absence of crustal interaction. KEY WORDS: petrogenesis; magma mixing; calc-alkaline; Absaroka Volcanic Province; 40Ar/39Ar dates 相似文献
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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
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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
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