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1.
Fluid history of UHP metamorphism in Dabie Shan, China: a fluid inclusion and oxygen isotope study on the coesite-bearing eclogite from Bixiling 总被引:32,自引:1,他引:31
Yilin Xiao Jochen Hoefs Alfons M. van den Kerkhof Jens Fiebig Yongfei Zheng 《Contributions to Mineralogy and Petrology》2000,139(1):1-16
This paper characterizes late Holocene basalts and basaltic andesites at Medicine Lake volcano that contain high pre-eruptive
H2O contents inherited from a subduction related hydrous component in the mantle. The basaltic andesite of Paint Pot Crater
and the compositionally zoned basaltic to andesitic lavas of the Callahan flow erupted approximately 1000 14C years Before Present (14C years b.p.). Petrologic, geochemical and isotopic evidence indicates that this late Holocene mafic magmatism was characterized by H2O contents of 3 to 6 wt% H2O and elevated abundances of large ion lithophile elements (LILE). These hydrous mafic inputs contrast with the preceding
episodes of mafic magmatism (from 10,600 to ∼3000 14C years b.p.) that was characterized by the eruption of primitive high alumina olivine tholeiite (HAOT) with low H2O (<0.2 wt%), lower LILE abundance and different isotopic characteristics. Thus, the mantle-derived inputs into the Medicine
Lake system have not always been low H2O, primitive HAOT, but have alternated between HAOT and hydrous subduction related, calc-alkaline basalt. This influx of hydrous
mafic magma coincides temporally and spatially with rhyolite eruption at Glass Mountain and Little Glass Mountain. The rhyolites
contain quenched magmatic inclusions similar in character to the mafic lavas at Callahan and Paint Pot Crater. The influence
of H2O on fractional crystallization of hydrous mafic magma and melting of pre-existing granite crust beneath the volcano combined
to produce the rhyolite. Fractionation under hydrous conditions at upper crustal pressures leads to the early crystallization
of Fe-Mg silicates and the suppression of plagioclase as an early crystallizing phase. In addition, H2O lowers the saturation temperature of Fe and Mg silicates, and brings the temperature of oxide crystallization closer to
the liquidus. These combined effects generate SiO2-enrichment that leads to rhyodacitic differentiated lavas. In contrast, low H2O HAOT magmas at Medicine Lake differentiate to iron-rich basaltic liquids. When these Fe-enriched basalts mix with melted
granitic crust, the result is an andesitic magma. Since mid-Holocene time, mafic volcanism has been dominated primarily by
hydrous basaltic andesite and andesite at Medicine Lake Volcano. However, during the late Holocene, H2O-poor mafic magmas continued to be erupted along with hydrous mafic magmas, although in significantly smaller volumes.
Received: 4 January 1999 / Accepted: 30 August 1999 相似文献
2.
J. G. Brophy Michael J. Dorais Julie Donnelly-Nolan Bradley S. Singer 《Contributions to Mineralogy and Petrology》1996,126(1-2):121-136
The rhyolite of Little Glass Mountain (73–74% SiO2) is a single eruptive unit that contains inclusions of quenched andesite liquid (54–61% SiO2) and partially crystalline cumulate hornblende gabbro (53–55% SiO2). Based on previous studies, the quenched andesite inclusions and host rhyolite lava are related to one another through fractional crystallization and represent an example of a fractionation-generated composition gap. The hornblende gabbros represent the cumulate residue associated with the rhyolite-producing and composition gap-forming fractionation event. This study combines textural (Nomarski Differential Interference Contrast, NDIC, imaging), major element (An content) and trace element (Mg, Fe, Sr, K, Ti, Ba) data on the style of zonation of plagioclase crystals from representative andesite and gabbro inclusions, to assess the physical environment in which the fractionation event and composition gap formation took place. The andesite inclusions (54–61% SiO2) are sparsely phyric with phenocrysts of plagioclase, augite and Fe-oxide±olivine, +/–orthopyroxene, +/–hornblende set within a glassy to crystalline matrix. The gabbro cumulates (53–55% SiO2) consist of an interconnected framework of plagioclase, augite, olivine, orthopyroxene, hornblende and Fe-oxide along with highly vesicular interstitial glass (70–74% SiO2). The gabbros record a two-stage crystallization history of plagioclase+olivine+augite (Stage I) followed by plagioclase+orthopyroxene+ hornblende+Fe-oxide (Stage II). Texturally, the plagioclase crystals in the andesite inclusions are characterized by complex, fine-scale oscillatory zonation and abundant dissolution surfaces. Compositionally (An content) the crystals are essentially unzoned from core-to-rim. These features indicate growth within a dynamic (convecting?), reservoir of andesite magma. In contrast, the plagioclase crystals in the gabbros are texturally smooth and featureless with strong normal zonation from An74 at the core to around An30. K, and Ba abundances increase and Mg abundances decrease steadily towards the rim. Ti, Fe, and Sr abundances increase and then decrease towards the rim. The trace element variations are fully consistent with the two-stage crystallization sequence inferred from the gabbro mineralogy. These results indicate progressive closed-system in situ crystallization in a quiescent magmatic boundary layer environment located along the margins of the andesite magma body. The fractional crystallization that generated the host rhyolite lava is one of inward solidification of a crystallizing boundary layer followed by melt extraction and accumulation of highly evolved interstitial liquid. This mechanism explains the formation of the composition gap between parental andesite and rhyolite magma compositions. 相似文献
3.
The evolution of young silicic lavas at Medicine Lake Volcano,California: Implications for the origin of compositional gaps in calc-alkaline series lavas 总被引:15,自引:0,他引:15
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
4.
The role of amphibole in the evolution of arc magmas and crust: the case from the Jurassic Bonanza arc section, Vancouver Island, Canada 总被引:6,自引:4,他引:2
The Jurassic Bonanza arc, on Vancouver Island, British Columbia, represents an exhumed island arc crustal section of broadly
diorite composition. We studied bodies of mafic and ultramafic cumulates within deeper levels of the arc to constrain the
conditions and fractionation pathways leading from high-Mg basalt to andesite and dacite. Major element trends coupled with
textural information show the intercumulus crystallization of amphibole, as large oikocrysts enclosing olivine in primitive
cumulates controls the compositions of liquids until the onset of plagioclase crystallization. This process is cryptic, occurring
only in the plutonic section, and explains the paucity of amphibole in mafic arc volcanics and the change in the Dy/Yb ratios
in many arc suites with differentiation. The correlation of octahedral Al in hornblende with pressure in liquidus experiments
on high-Mg basalts is applied as an empirical barometer to hornblendes from the Bonanza arc. It shows that crystallization
took place at 470–880 MPa in H2O-saturated primitive basaltic magmas. There are no magmatic equivalents to bulk continental crust in the Bonanza arc; no
amount of delamination of ultramafic cumulates will shift the bulk arc composition to the high-Mg# andesite composition of
bulk continental crust. Garnet removal from wet magmas appears to be the key factor in producing continental crust, requiring
high pressures and thick crust. Because oceanic island arcs are built on thinner crust, the long-term process generating the
bulk continental crust is the accretion of island arcs to continental margins with attendant tectonic thickening. 相似文献
5.
T. L. Grove R. J. Kinzler M. B. Baker J. M. Donnelly-Nolan C. E. Lesher 《Contributions to Mineralogy and Petrology》1988,99(3):320-343
At Medicine Lake volcano, California, andesite of the Holocene Burnt Lava flow has been produced by fractional crystallization of parental high alumina basalt (HAB) accompanied by assimilation of granitic crustal material. Burnt Lava contains inclusions of quenched HAB liquid, a potential parent magma of the andesite, highly melted granitic crustal xenoliths, and xenocryst assemblages which provide a record of the fractional crystallization and crustal assimilation process. Samples of granitic crustal material occur as xenoliths in other Holocene and Pleistocene lavas, and these xenoliths are used to constrain geochemical models of the assimilation process.A large amount of assimilation accompanied fractional crystallization to produce the contaminated Burnt lava andesites. Models which assume that assimilation and fractionation occurred simultaneously estimate the ratio of assimilation to fractional crystallization (R) to be >1 and best fits to all geochemical data are at an R value of 1.35 at F=0.68. Petrologic evidence, however, indicates that the assimilation process did not involve continuous addition of granitic crust as fractionation occurred. Instead, heat and mass transfer were separated in space and time. During the assimilation process, HAB magma underwent large amounts of fractional crystallization which was not accompanied by significant amounts of assimilation. This fractionation process supplied heat to melt granitic crust. The models proposed to explain the contamination process involve fractionation, replenishment by parental HAB, and mixing of evolved and parental magmas with melted granitic crust. 相似文献
6.
二连盆地北缘晚中生代火山岩Ar-Ar年代、地球化学及构造背景 总被引:4,自引:1,他引:3
中国东北二连盆地周缘分布有三组时代不同的晚中生代火山岩,其中早、中期为两套地球化学性质不同的流纹岩,晚期为玄武质火山岩。本文通过测定火山岩基质Ar-Ar同位素年龄,表明早期查干诺尔组流纹岩形成于142Ma,晚期不拉根哈达组基性火山岩形成于129Ma,可见二连盆地北缘晚中生代火山岩时代均为早白垩世。通过对主、微量元素地球化学特征和Sr-Nd-Pb同位素组成研究,以及与邻区同期满克头鄂博组英安岩和流纹岩、玛尼吐组英安岩、霍林河地区查干诺尔组英安岩、流纹岩对比,认为早期查干诺尔组流纹岩来源于新成下地壳,岩浆演化过程经历了强烈分异作用;中期流纹岩源区为中上地壳或下地壳岩浆经历了上地壳强烈同化混染作用;晚期不拉根哈达组基性火山岩则源于受俯冲洋壳流体交代的富集岩石圈地幔。结合早白垩世区域岩石圈减薄背景,本文认为研究区早白垩世火山岩形成于陆内伸展构造环境。 相似文献
7.
D. S. Musselwhite D. J. DePaolo M. McCurry 《Contributions to Mineralogy and Petrology》1989,101(1):19-29
The isotopic compositions of Nd and Sr and concentrations of major and trace elements were measured in flows and tuffs of the Woods Mountains volcanic center of eastern California to assess the relative roles of mantle versus crustal magma sources and of fractional crystallization in the evolution of silicic magmatic systems. This site was chosen because the contrast in isotopic composition between Precambrian-to-Mesozoic country rocks and the underlying mantle make the isotope ratios sensitive indicators of the proportions of crustal- and mantle-derived magma. The major eruptive unit is the Wild Horse Mesa tuff (15.8 m.y. old), a compositionally zoned rhyolite ignimbrite. Trachyte pumice fragments in the ash-flow deposits provide information on intermediate composition magma types. Crustal xenoliths and younger flows of basalt and andesite (10 m.y. old) provide opportunities to confirm the isotopic compositions of potential mantle and crustal magma sources inferred from regional patterns. The trachyte and rhyolite have Nd values of -6.2 to -7.5 and initial 87Sr/86Sr ratios mostly between 0.7086 and 0.7113. These magmas cannot have been melted directly from the continental basement because the Nd values are too high. They also cannot have formed by closed system fractional crystallization of basalt because the 87Sr/86Sr ratios are higher than likely values for parental basalt. Both major and trace element variations indicate that crystal fractionation was an important process. These results require that the silicic magmas are end products of the evolution of mantle-derived basalt that underwent extensive fractional crystallization accompanied by assimilation of crustal rock. The mass fraction of crustal components in the trachyte and rhyolite is estimated to be between 10% and 40%, with the lower end of the range considered more likely. The generation of magmas with SiO2 contents greater than 60% appears to be dominated by crystal fractionation with minimal assimilation of upper crustal rocks. 相似文献
8.
Enrichment of basalt and mixing of dacite in the rootzone of a large rhyolite chamber: inclusions and pumices from the Rattlesnake Tuff, Oregon 总被引:3,自引:0,他引:3
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
9.
Andesites with Mg# >45 erupted at subduction zones form either by partial melting of metasomatized mantle or by mixing and assimilation processes during melt ascent. Primitive whole rock basaltic andesites from the Pukeonake vent in the Tongariro Volcanic Centre in New Zealand’s Taupo Volcanic Zone contain olivine, clino- and orthopyroxene, and plagioclase xeno- and antecrysts in a partly glassy matrix. Glass pools interstitial between minerals and glass inclusions in clinopyroxene, orthopyroxene and plagioclase as well as matrix glasses are rhyolitic to dacitic indicating that the melts were more evolved than their andesitic bulk host rock analyses indicate. Olivine xenocrysts have high Fo contents up to 94%, δ18O(SMOW) of +5.1‰, and contain Cr-spinel inclusions, all of which imply an origin in equilibrium with primitive mantle-derived melts. Mineral zoning in olivine, clinopyroxene and plagioclase suggest that fractional crystallization occurred. Elevated O isotope ratios in clinopyroxene and glass indicate that the lavas assimilated sedimentary rocks during stagnation in the crust. Thus, the Pukeonake andesites formed by a combination of fractional crystallization, assimilation of crustal rocks, and mixing of dacite liquid with mantle-derived minerals in a complex crustal magma system. The disequilibrium textures and O isotope compositions of the minerals indicate mixing processes on timescales of less than a year prior to eruption. Similar processes may occur in other subduction zones and require careful study of the lavas to determine the origin of andesite magmas in arc volcanoes situated on continental crust. 相似文献
10.
Wesley E. LeMasurier Sung Hi Choi Y. Kawachi Samuel B. Mukasa N. W. Rogers 《Contributions to Mineralogy and Petrology》2011,162(6):1175-1199
The Marie Byrd Land province includes 18 large (up to 1,800 km3) central volcanoes distributed across an active volcano-tectonic dome. The typical volcano structure consists of a basal
1,000–5,000 m of basanite surmounted by trachyte and subordinate intermediate rocks, plus phonolite, or pantellerite, or comendite.
The volumes of felsic sections are large (~30–700 km3), but these rocks probably make up <10% of volcanic rock in the province. This paper describes pantellerite volcanoes in
the Ames and Flood Ranges, which include a large and varied suite of these iron-rich, silica-poor rhyolites. Isotopic and
trace element data, maintenance of isotopic equilibrium throughout the basalt-felsic range, and the results of modeling, all
exclude significant crustal contamination and point to fractional crystallization as the process that controls magmatic evolution.
The most unusual feature of these volcanoes is the apparent need to derive pantellerites from basanite, the long interval
of fractionation at the base of the lithosphere and crust, involving kaersutite as the key phase in developing pantellerite,
and a plumbing system that permitted coeval eruption of pantellerite and phonolite from the same edifice. Peralkalinity most
likely developed in upper crustal reservoirs during the final 4–5% of magmatic history, by fractionating a high proportion
of plagioclase under low pH2O. Mantle plume activity appears to drive doming and volcanism. This, a stationary plate, and continental lithospheric structure
seem to provide an optimal environment for the evolution of a diverse, large volume suite of felsic rocks by fractional crystallization. 相似文献
11.
Origin of calc-alkaline series lavas at Medicine Lake Volcano by fractionation,assimilation and mixing 总被引:1,自引:0,他引:1
Timothy L. Grove David C. Gerlach Thomas W. Sando 《Contributions to Mineralogy and Petrology》1982,80(2):160-182
The results of experimental studies and examination of variations in major elements, trace elements and Sr isotopes indicate
that fractionation, assimilation and magma mixing combined to produce the lavas at Medicine Lake Highland. Some characteristics
of the compositional differences among the members of the calc-alkalic association (basalt-andesite-dacite-rhyolite) can be
produced by fractional crystallization, and a fractionation model reproduces the major element trends. Other variations are
inconsistent with a fractionation origin. Elevated incompatible element abundances (K and Rb) observed in lavas intermediate
between basalt and rhyolite can be produced through assimilation of a crustal component. An accompanying increase in 87Sr/86Sr from ∼ 0.07030 in basalt to ∼0.7040 in rhyolite is also consistent with crustal assimilation. The compatible trace element
contents (Ni and Sr) of intermediate lavas can not be produced by fractional crystallization, and suggest a magma-mixing origin
for some lavas. Unusual phenocryst assemblages and textural criteria in these lavas provide additional evidence for magma
mixing.
A phase diagram constructed from the low pressure melting experiments identifies a distributary reaction point, where olivine+augite
react to pigeonite. Parental basalts reach this point at low pressures and undergo iron-enrichment at constant SiO2 content. The resulting liquid line of descent is characteristic of the tholeiitic trend. Calc-alkalic differentiation trends
circumvent the distributary reaction point by three processes: fractionation at elevated pH2O, assimilation and magma mixing. 相似文献
12.
《International Geology Review》2012,54(10):1179-1190
Andesite magmatism plays a major role in continental crustal growth, but its subduction-zone origin and evolution is still a hotly debated topic. Compared with whole-rock analyses, melt inclusions (MIs) can provide important direct information on the processes of magma evolution. In this article, we synthesize data for melt inclusions hosted by phenocrysts in andesites, extracted from the GEOROC global compilation. These data show that melt inclusions entrapped by different phenocrysts have distinct compositions: olivine-hosted melt inclusions have basalt and basaltic andesite compositions, whereas melt inclusions in clinopyroxene and othopyroxene are mainly dacitic to rhyolitic. Hornblende-hosted melt inclusions have rhyolite composition. The compositions of melt inclusions entrapped by plagioclase are scattered, spanning from andesite to rhyolite. On the basis of the compositional data, we propose a mixing model for the genesis of the andesite, and a two-chamber mechanism to account for the evolution of the andesite. First, andesite melt is generated in the lower chamber by mixing of a basaltic melt derived from the mantle and emplaced in the lower crust with a felsic melt resulting from partial melting of crustal rocks. Olivine and minor plagioclase likely crystallize in the lower magma chamber. Secondly, the andesite melt ascends into the upper chamber where other phenocrysts crystallize. According to SiO2-MgO diagrams of the MIs, evolution of the andesite in the upper chamber can be subdivided into two distinct stages. The early stage (I) is characterized by a phenocrystal assemblage of clinopyroxene + othopyroxene + plagioclase, whereas the late stage (II) is dominated by crystallization of plagioclase + hornblende. 相似文献
13.
Anne L. Bloomfield Richard J. Arculus 《Contributions to Mineralogy and Petrology》1989,102(4):429-453
A wide variety of rock types are present in the O'Leary Peak and Strawberry Crater volcanics of the Pliocene to Recent San Francisco Volcanic Field (SFVF), AZ. The O'Leary Peak flows range from andesite to rhyolite (56–72 wt % SiO2) and the Strawberry Crater flows range from basalt to dacite (49–64 wt % SiO2). Our interpretation of the chemical data is that both magma mixing and crustal melting are important in the genesis of the intermediate composition lavas of both suites. Observed chemical variations in major and trace elements can be modeled as binary mixtures between a crustal melt similar to the O'Leary dome rhyolite and two different mafic end-members. The mafic end-member of the Strawberry suite may be a primary mantle-derived melt. Similar basalts have also been erupted from many other vents in the SFVF. In the O'Leary Peak suite, the mafic end-member is an evolved (low Mg/(Mg+ Fe)) basalt that is chemically distinct from the Strawberry Crater and other vent basalts as it is richer in total Fe, TiO2, Al2O3, MnO, Na2O, K2O, and Zr and poorer in MgO, CaO, P2O5, Ni, Sc, Cr, and V. The derivative basalt probably results from fractional crystallization of the more primitive, vent basalt type of magma. This evolved basalt occurs as xenolithic (but originally magmatic) inclusions in the O'Leary domes and andesite porphyry flow. The most mafic xenolith may represent melt that mixed with the O'Leary dome rhyolite resulting in andesite preserved as other xenoliths, a pyroclastic unit (Qoap), porphyry flow (Qoaf) and dacite (Darton Dome) magmas. Thermal constraints on the capacity of a melt to assimilate (and melt) a volume of solid material require that melt mixing and not assimilation has produced the observed intermediate lavas at both Strawberry Crater and O'Leary Peak. Textures, petrography, and mineral chemistry support the magma mixing model. Some of the inclusions have quenched rims where in contact with the host. The intermediate rocks, including the andesite xenoliths, contain xenocrysts of quartz, olivine and oligoclase, together with reversely zoned plagioclase and pyroxene phenocrysts. The abundance of intermediate volcanic rocks in the SFVF, as observed in detail at O'Leary Peak and Strawberry Crater, is due in part to crustal recycling, the result of basalt-driven crustal melting and the subsequent mixing of the silicic melts with basalts and derivative magmas. 相似文献
14.
The mid-Tertiary volcanic sequence of the central Sierra Madre Occidental in Chihuahua, Mexico, is about one kilometer thick and is composed predominantly of rhyolitic ignimbrites. Basaltic andesite to dacitic lavas are interbedded with the rhyolites, but they are of minor volumetric importance. Rare earth element (REE) data are used to constrain a crustal anatexis model for the origin of the voluminous ignimbrites and to test a fractional crystallization model. The REE patterns indicate that if the rhyolites were formed by direct crustal anatexis, the residue from partial melting could contain no more than a few percent garnet or about 20% hornblende. This eliminates residues with the mineralogy of amphibolite, eclogite, or garnet granulite, but melting of a garnet-free granulite source is permitted. The crustal anatexis model is difficult to evaluate critically because of a lack of knowledge concerning the mid-Tertiary geothermal gradient and the composition of the crust beneath the Sierra Madre Occidental.In contrast, the fractional crystallization model can be tested rigorously. Rayleigh fractionation calculations are used to closely model REE patterns in the basaltic andesite to rhyolite series. The minerals involved are those occurring as phenocryst phases, and the mineral proportions were generated by leastsquares major element calculations. The results of the calculations are consistent with the hypothesis that the voluminous rhyolites originated by plagioclase-dominated crystal fractionation. 相似文献
15.
Kent C. Condie 《Geochimica et cosmochimica acta》1975,39(8):1165-1178
The last 10,000 years of activity at the Medicine Lake volcanic center in northern California is characterized by bimodal mafic and siliceous volcanism. Interflow element variations are complex and exhibit a discontinuity for most elements between 57 and 62 per cent SiO2. No simple linear or curvilinear element trends exist between the mafic (Modoc) and siliceous (glass) volcanics.The geochemical variation patterns exhibited by volcanic rocks from the Medicine Lake volcanic center preclude any simple model for magma origin involving either varying degrees of melting or of fractional crystallization. A model is tentatively invoked for the andesites and basalts involving ? 35 per cent melting of eclogite (of altered rise tholeiite composition) in a descending slab followed by varying amounts of fractional crystallization and perhaps magma mixing. Up to 20 per cent of shallow fractional crystallization of plagioclase and minor Ti-magnetite seems to be required by the Sr, Eu anomaly, and TiO2 distributions.Compositional variation and high δO18 values in most dacite glass flows are best interpreted in terms of a crustal origin involving up to 50 per cent partial melting of average continental crust. Rhyolite glasses may have formed by small degrees of melting (20–30 per cent) of this crust followed by 5–10 per cent of shallow fractional crystallization (removing dominantly plagioclase) or by 40–50 per cent fractional crystallization of a dacite parent (~63 per cent SiO2) produced in the crust. The shallow fractional crystallization is necessary to explain the low Sr contents and large negative Eu anomalies in the rhyolites. Dacites from the Composite Flow are tentatively interpreted to have formed by shallow mixing of a hybrid magma (composed of varying amounts of andesite and dacite) with rhyolite prior to and during eruption. 相似文献
16.
Mt. Shasta andesite and dacite lavas contain high MgO (3.5–5 wt.%), very low FeO*/MgO (1–1.5) and 60–66 wt.% SiO2. The range of major and trace element compositions of the Shasta lavas can be explained through fractional crystallization (~50–60 wt.%) with subsequent magma mixing of a parent magma that had the major element composition of an H2O-rich primitive magnesian andesite (PMA). Isotopic and trace element characteristics of the Mt. Shasta stratocone lavas are highly variable and span the same range of compositions that is found in the parental basaltic andesite and PMA lavas. This variability is inherited from compositional variations in the input contributed from melting of mantle wedge peridotite that was fluxed by a slab-derived, fluid-rich component. Evidence preserved in phenocryst assemblages indicates mixing of magmas that experienced variable amounts of fractional crystallization over a range of crustal depths from ~25 to ~4 km beneath Mt. Shasta. Major and trace element evidence is also consistent with magma mixing. Pre-eruptive crystallization extended from shallow crustal levels under degassed conditions (~4 wt.% H2O) to lower crustal depths with magmatic H2O contents of ~10–15 wt.%. Oxygen fugacity varied over 2 log units from one above to one below the Nickel-Nickel Oxide buffer. The input of buoyant H2O-rich magmas containing 10–15 wt.% H2O may have triggered magma mixing and facilitated eruption. Alternatively, vesiculation of oversaturated H2O-rich melts could also play an important role in mixing and eruption. 相似文献
17.
Fractional crystallization and mantle-melting controls on calc-alkaline differentiation trends 总被引:10,自引:3,他引:7
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
18.
Glass Mountain, California, consists of >50 km3 of high-silica rhyolite lavas and associated pyroclastic deposits that erupted over a period of >1 my preceding explosive
eruption of the Bishop Tuff and formation of the Long Valley caldera at 0.73 Ma. These “minimum-melt” rhyolites yield Fe-Ti-oxide
temperatures of 695–718°C and contain sparse phenocrysts of plagioclase+quartz+magnetite+apatite±sanidine, biotite, ilmenite,
allanite, and zircon. Incompatible trace elements show similar or larger ranges within the Glass Mountain suite than within
the Bishop Tuff, despite a much smaller range of major-element concentrations, largely due to variability among the older
lavas (erupted between 2.1 and 1.2 Ma). Ratios of the most incompatible elements have larger ranges in the older lavas than
in the younger lavas (1.2–0.79 Ma), and concentrations of incompatible elements span wide ranges at nearly constant Ce/Yb,
suggesting that the highest concentrations of these elements are not the result of extensive fractional crystallization alone;
rather, they are inherited from parental magmas with a larger proportion of crustal partial melt. Evidence for the nature
of this crustal component comes from the presence of scarce, tiny xenocrysts derived from granitic and greenschist-grade metamorphic
rocks. The wider range of chemical and isotopic compositions in the older lavas, the larger range in phenocryst modes, the
eruption of magmas with different compositions at nearly the same time in different parts of the field, and the smaller volume
of individual lavas suggest either that more than one magma body was tapped during eruption of the older lavas or that a single
chamber tapped by all lavas was small enough that the composition of its upper reaches easily affected by new additions of
crustal melts. We interpret the relative chemical, mineralogical, and isotopic homogeneity of the younger Glass Mountain lavas
as reflecting eruptions from a large, integrated magma chamber. The small number of cruptions between 1.4 and 1.2 ma may have
allowed time for a large magma body to coalesce, and, as the chamber grew, its upper reaches became less affected by new inputs
of crustal melts, so that trace-element trends in magmas erupted after 1.2 Ma are largely controlled by fractional crystallization.
The extremely low Sr concentrations of Glass Mountain lavas imply extensive crystallization in chambers at least hundreds
of cubic kilometers in volume. The close similarity in Sr, Nd, and Pb isotopic ratios between the younger Glass Mountain lavas
and unaltered Bishop Tuff indicates that they tapped the same body of magma, which had become isotopically homogenous by 1.2
Ma but continued to differentiate after that time. From 1.2 to 0.79 Ma, volumetric eruptive rates may have exceeded rates
of differentiation, as younger Glass Mountain lavas become slightly less evolved with time. Early-erupted Bishop Tuff is more
evolved than the youngest of the Glass Mountain lavas and is characterized by slightly different trace element ratios. This
suggests that although magma had been present for 0.5 my, the composiional gradient exhibited by the Bishop Tuff had not been
a long-term, steady-state condition in the Long Valley magma chamber, but developed at least in part during the 0.06-my hiatus
between extrusion of the last Glass Mountain lava and the climactic eruption. 相似文献
19.
Olgeir Sigmarsson Michel Condomines Serge Fourcade 《Contributions to Mineralogy and Petrology》1992,112(1):20-34
238U–230Th disequilibria and Sr and O isotope ratios have been measured in a suite of samples from most of the known prehistoric and historic eruptions of Hekla volcano, Iceland. They cover the compositional range from basaltic andesite to rhyolite. Recent basalts erupted in the vicinity of the volcano and a few Pleistocene basalts have also been studied. Geochemical data indicate that the best tracers of magmatic processes in Hekla are the (230Th/232Th) and Th/U ratios. Whereas most geochemical parameters, including Sr, Nd and O isotopes, could be compatible with crystal fractionation, (230Th/232Th) and Th/U ratios differ in the basalts and basaltic andesites (1.05 and 3.2, respectively) and in the silicic rocks, dacites and rhyolites (0.98 and 3.4–3.7, respectively). This observation precludes fractional crystallization as the main differentiation process in Hekla. On the basis of these results, the following model is proposed: basaltic magmas rise in the Icelandic crust and cause partial melting of metabasic rocks, leading to the formation of a dacitic melt. The basaltic magma itself evolves by crystal fractionation and produces a basaltic andesite magma. The latter can mix with the dacitic liquid to form andesites. At higher levels in the magma chamber, the dacitic melt sometimes undergoes further differentiation by crystal fractionation and produces subordinate volumes of rhyolites. Together all these processes lead to a zoned magma chamber. However, complete zoning is achieved only when the repose time between eruptions is long enough to allow the production of significant volumes of dacitic magma by crustal melting. This situation corresponds to the large plinian eruptions. Between these eruptions, the so-called intra-cyclic activity is characterized by the eruption of andesites and basaltic andesites, with little crustal melting. The magmatic system beneath Hekla most probably was established during the Holocene. The shape and the size of the magma chamber may be inferred from the relationships between the composition of the lavas and the location of the eruption sites. In a cross-section perpendicular to Hekla's ridge, a bell-shaped reservoir 5 km wide and 7 km deep appears the most likely; its top could be at depth of 8 km according to geophysical data. 相似文献
20.
Basaltic lava flows and high-silica rhyolite domes form the Pleistocene part of the Coso volcanic field in southeastern California. The distribution of vents maps the areal zonation inferred for the upper parts of the Coso magmatic system. Subalkalic basalts (<50% SiO2) were erupted well away from the rhyolite field at any given time. Compositional variation among these basalts can be ascribed to crystal fractionation. Erupted volumes of these basalts decrease with increasing differentiation. Mafic lavas containing up to 58% SiO2, erupted adjacent to the rhyolite field, formed by mixing of basaltic and silicic magma. Basaltic magma interacted with crustal rocks to form other SiO2-rich mafic lavas erupted near the Sierra Nevada fault zone.Several rhyolite domes in the Coso volcanic field contain sparse andesitic inclusions (55–61% SiO2). Pillow-like forms, intricate commingling and local diffusive mixing of andesite and rhyolite at contacts, concentric vesicle distribution, and crystal morphologies indicative of undercooling show that inclusions were incorporated in their rhyolitic hosts as blobs of magma. Inclusions were probably dispersed throughout small volumes of rhyolitic magma by convective (mechanical) mixing. Inclusion magma was formed by mixing (hybridization) at the interface between basaltic and rhyolitic magmas that coexisted in vertically zoned igneous systems. Relict phenocrysts and the bulk compositions of inclusions suggest that silicic endmembers were less differentiated than erupted high-silica rhyolite. Changes in inferred endmembers of magma mixtures with time suggest that the steepness of chemical gradients near the silicic/mafic interface in the zoned reservoir may have decreased as the system matured, although a high-silica rhyolitic cap persisted.The Coso example is an extreme case of large thermal and compositional contrast between inclusion and host magmas; lesser differences between intermediate composition magmas and inclusions lead to undercooling phenomena that suggest smaller T. Vertical compositional zonation in magma chambers has been documented through study of products of voluminous pyroclastic eruptions. Magmatic inclusions in volcanic rocks provide evidence for compositional zonation and mixing processes in igneous systems when only lava is erupted. 相似文献