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1.
The basic volcanic group exposed in the Kadiri schist belt includes high Mg-basalt, basalt, basaltic andestite and dacite. The basalts are tholeiitic in composition and high Mg-basalts, basaltic andesites and dacites show calc-alkaline affinity. Major and trace element characteristics suggest that the volcanic suite has been derived from an initial tholeiitic magma which has given rise to an early basaltic type and a later calc-alkaline type of rocks. An island arc and active continental margin tectonic setting was inferred for these rocks. 相似文献
2.
Calc-alkaline, Shoshonitic, and Primitive Tholeiitic Lavas from Monogenetic Volcanoes near Crater Lake, Oregon 总被引：2，自引：1，他引：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.
Petrogenetic Evolution of a Transitional Tholeiitic--Calc-alkaline Series: Towada Volcano, Japan 总被引：5，自引：0，他引：5
Detailed petrological and geochemical data of stratigraphicallywell-defined tholeiitic and calc-alkaline rocks from Towada,Japan, are used to evaluate the system's evolution over 0•2myr. Available data suggest that Towada progressed from severalsmall independent magma chambers to one large chamber, illustratedby systematic changes in mineral composition, isotopic and traceelement ratios. This is attributed to variations in intracrustalprocesses in the shallow chambers, with the following evolutionarysequence divisible into four main units:
- the Precaldera Stage:consisting of several basaltic to daciticlava cones, independentlyfed by tholeiitic and calc-alkalinemagma chambers. These arecharacterized by periodic replenishment,fractionation and magmamixing, with some undergoing crustalassimilation during latestages, increasing the 87Sr/86Sr ratioat constant SiO2.
- TheCaldera Stage: characterized by large volume tholeiiticandcalc-alkaline andesitic to dacitic pyroclastic eruptions,associatedwith caldera collapse. Geochemical data indicatethat this stageresulted from some of the precaldera chambersamalgamating.Magma in this enlarged chamber underwent mixing,continued fractionalcrystallization and assimilation.
- The Ninokura Stage: characterizedby primitive tholeiitic basalticlavas and scoria deposits,which underwent fractional crystallizationplus minor crustalassimilation.
- The Goshikiiwa Pyroclastic Deposits: consistingof evolved calc-alkaline±tholeiiticmagmas, which underwentassimilation and fractional crystallization.
4.
The Arc Lavas of the Shirahama Group, Japan: Sr and Nd Isotopic Data Indicate Mantle-Derived Bimodal Magmatism 总被引：4，自引：4，他引：0
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
5.
大兴安岭南段晚中生代双峰式火山作用 总被引：52，自引：24，他引：52
大兴安岭南段晚中生代克头鄂博组山岩表现出双峰式特征，主要由玄武质安山岩、英安岩和流纹岩组成。基性火山岩属于代钾拉斑系列，轻微富集LREE，Eu异常不明显（Eu/Eu=0.99-1.04)和HREE无明显分馏的特征（Dy/YbcN=1.030-1.089);富集大离子亲石元素（LILE）而亏损高场强元素（HFSE），尤其是强烈亏损Nb,Ta。英安岩和流纹岩为钙碱性系列，在REE配分模式上为LREE富集型，其中英安岩为Eu弱负异常（Eu/Eu=0.81-1.01)，流纹岩的Eu负异常明显（Eu^*/Eu=0.65-0.76)；在微量元素蛛网图上，英安岩类似于基性火山岩，流纹岩除了具LILE富集和HFSE亏损特征外，还显示出Sr,P,Ti等元素的强烈亏损，可能与岩浆演化过程中斜长石、磷灰石的分离结晶作用相关。晚中生代双峰式火山岩分离结晶的结果。流纹岩表现出较高的La/Sm比值和很高的K／P、K／Ti比值，其成因可能与地壳混染作用或与大陆中、下地壳重熔作用有关。结合区域晚中生代盆岭构造格局特征、大兴安岭南段晚中生代双峰式火山岩形成于造山后阶段，是岩石圈快速伸展体制下导致受早期流体交代的岩石圈地幔发生减压部分熔融作用的产物。 相似文献
6.
7.
The Mio-Pliocene Shirahama Group, Izu Peninsula, Central Japan,a well-exposed submarine volcanic arc complex of lava flows,pyroclastic rocks and associated shallow intrusives, is characterizedby a tholeiitic series (basalt to dacite) and a calc-alkalineseries (andesite to dacite). Chemical variations in the tholeiiticseries and calc-alkaline series are consistent with crystalfractionation from basalt and magnesian andesite (boninite),respectively. Crystal–liquid phase relations of thesemagmas have been investigated by study of sample suites fromthese two series. Compositions of liquids in equilibrium withphenocrysts were determined by microprobe grid analyses, inwhich 49 points were averaged in 03 mm 03 mm groundmassareas. The liquid compositions, coupled with the phenocrystmineralogy of the same samples, define the liquid lines of descentof these volcanic arc magmas. Major findings include the following:(1) Crystallization of the tholeiitic series magma is consistentwith early stage crystallization in the simple system Fo–Di–Silica–H2O,with olivine having a reaction relation to augite and the tholeiiticliquid. (2) The later stage products of the tholeiitic seriesmagma are, however, crystal-poor (<10%) dacites with no maficminerals, suggesting that tholeiitic liquids, hypersthene andaugite were no longer on the cotectic (3) A characteristic ofthe calc-alkaline series magmas is the development of rhyoliticliquids. Hypersthene, augite, plagioclase and Fe–Ti oxideoccur in most calc-alkaline rocks studied, and hornblende andquartz can be found in about half of these. However, their differentiationpaths show that the cotectic relation between quartz and liquidended at a later stage, resulting in the resorption of quartzphenocrysts and ultimately in the formation of quartz-free magmas.(4) The late-stage liquids of both the tholeiitic and calc-alkalineseries have deviated from their cotectics, which cannot be explainedby fractional crystallization alone. The addition of H2O froman outside system is probably required to explain the differentiationpaths. (5) The formation of chilled margins, the in situ crystallizationof a magma chamber in the solidification zone, and/or the migrationof groundwater into the magma chamber are thought to be likelyprocesses affecting magmas during their migration and intrusioninto the crust. An extreme effect of H2O addition would be tolower the liquidus temperatures of all precipitating silicatephases far below their restorable range before eruption, resultingin the production of aphyric magmas. Even when a temperaturedecrease in the magma chamber causes a liquid to intersect theliquidus of a pre-existing phase, the addition of H2O shiftsthe cotectic toward SiO2, resulting in quartz being the lastphase to crystallize. The resorption of quartz is interpretedto be the result of a liquidus boundary shift caused by theaddition of H2O. The genesis of aphyric rhyolites is thereforeinferred to result from fractional crystallization followingaddition of H20. KEY WORDS: Shirahama Group; Japan; island arc; rhyolite; magma series 相似文献
8.
High-alumina basalts and basic andesites, which represent the most “primitive” magma types of the Cenozoic andesitic series of Sardinia, show a spatial chemical zonation with respect to REE. The basaltic rocks from the northern and south-central part of the island have REE patterns typical of calc-alkaline rocks with an enrichment of light REE and fractionation of heavy REE. In contrast, those from the southernmost part have a pattern similar to typical continental tholeiites with only a small light-REE enrichment and unfractionated heavy REE.The present data suggest that basaltic rocks may be formed by anatexis of upper-mantle material with mineral assemblages containing either garnet (calc-alkaline rocks) or spinel (rocks of tholeiitic affinities). The presence of garnet or spinel could merely reflect mineral phase transformation and indicates a different depth of fusion for the various types of basaltic rocks with those of tholeiitic affinities originating at a shallower depth than the calcalkaline rocks. The REE data are consistent with the generation of the basaltic rocks by partial melting of mantle peridotite overlying a subducted plate. 相似文献
9.
Fukujin Seamount is a large, active, submarine volcano on thevolcanic front in the northernseamount province (NSP) of theMariana island arc (MIA). Five dredge hauls from the summitand upper flanks of Fukujin recovered mainly highly porphyriticbasaltic andesites. A few nearly aphyric samples are medium-Ksiliceous andesites (SiO2 = 62%, K2O = 1•5%). Fukujin andmost other large arc-front volcanoes of the northern MIA havetholeiitic (iron-enrichment) fractionation trends. This contrastswith the calc-alkaline trends of many smaller seamounts. A negativecorrelation of modal plagioclase content with bulk-rock SiO2,as well as bulk-rock major and trace element variation trends,and glass analyses, suggests that lavas with >30 vol.% phenocrystsand <55 wt.% SiO2 are partial cumulates. The presence ofbimodal phenocryst populations along with reversed to normalzoning of phenocrysts is explained by magma mixing of andesiticand basaltic liquids. Hybrid basaltic andesites probably formedby the accumulation of plagioclase in a tholeiitic magma chamberundergoing replenishment and mixing at a shallow crustal level.A petrogenetic model is presented for the origin of basalticandesite by combined magma mixing and fractional crystallization.Aphyric siliceous andesites can be modelled by simple fractionationof basaltic andesite. The early fractionating assemblage consistedmainly of plagioclase and clinopyroxene, with lesser olivineand minor magnetite, but plagioclase remained suspended in themelt. The later fractionating assemblage was dominated by plagioclasewith orthopyroxene instead of olivine.
*Present address: 2260 rue Panet, Montreal, Quebec, H2L 3A6, Canada. 相似文献
10.
The geological setting, ages, petrography and geochemistry of late Pan-African ( 580 Ma) calc-alkaline and tholeiitic dike rocks in the Bir Safsaf igneous complex of south-west Egypt are discussed. These basaltic to rhyolitic dikes intruded contemporaneously and shortly after the intrusion of granitoids. The major and trace element data, Sr and Nd isotope relations, in combination with textural observations, confirm complex interactions between most of the intermediate calcalkaline dike melts and plutonic melts, with different degrees of mixing, assimilation, replenishment and tapping of magma chambers. Trachytic and rhyolitic dikes are strongly differentiated melts from the granitic pluton. The tholeiitic dikes evolved dominantly by fractional crystallization processes. It is inferred that open system and closed system processes operated in calc-alkaline magma chambers, and that the calc-alkaline melts came from a garnet-and amphibole-bearing mantle, modified by a subduction component. Tholeiitic rocks were formed later by fractional crystallization and assimilation processes. Magma ascent of both dike types took place in an extensional environment and the presumed subduction zone has to be seen in connection with the Atmur-Delgo suture zone. 相似文献
11.
The Younger Andesites and Dacites of Iztacc?huatl volcano, Mexico,constitute a medium-K calcalkaline rock suite (58–66 wt.per cent SiO2) characterized by high Mg-numbers (100Mg/(Mg+0?85Fe2+=55–66) and relatively high abundances of MgO (2?5–6?6wt. per cent), Ni(17–158 p.p.m.), and Cr (42–224p.p.m.). Chemical stratigraphy plots of eruptive sequences indicatethe existence of a plexus of long-lived dacite magma chambersperiodically replenished by influxes of basaltic magma ascendingfrom depth. Short-term geochemical evolution after batch influxwas dictated by magma mixing and eventual dilution of the basalticcomponent by ‘quasi-steady state’ hornblende dacitemagma. The chemical data support textural and mineralogicalevidence for rapid homogenization of originally diverse magmasby convective blending of residual liquids accompanied by dynamicfractional crystallization (Nixon, 1988). Internally-consistent mixing calculations used to derive thecomposition of basaltic magma influx incorporate analyticaluncertainties and the observed range of salic end-member compositions.Mafic end-members are basalts to basaltic andesites (52–54wt. per cent SiO2) with Mg-numbers (73–76), MgO (9–11wt. per cent), Ni (250 p.p.m.), and Cr (340–510 p.p.m.)concentrations, and liquidus olivine compositions (Fo90–88),appropriate for unfractionated partial melts of mantle peridotite.The majority of model compositions are Ol-Hy-normative, similarto those of primitive basaltic lavas on the flanks of Iztacc?huatland in the Valley of Mexico. However, calculated magma batchesrange from weakly Qz-normative to strongly Ne-normative. Bothcalculated and analyzed basaltic compositions are distinguishedby highly variable abundances of alkalies and incompatible traceelements, notably Rb, Ba, Sr, P, Zr, and Y. Initial 87Sr/86Sr ratios for Iztacc?huatl lavas (0?7040–0?7046;n=24) are comparable to those for primitive basaltic rocks (0?7037–0?7045;?=4) and indicate that (1) mantle source regions are isotopicallyheterogeneous; and (2) contamination of iztacc?huatl magma chambersby radiogenic crustal rocks was not a significant factor inthe evolution of calc-alkaline andesites and dacites. The replenishment of Iztacc?huatl dacite reservoirs by Ne-normativemagmas late in the history of cone growth precludes exhaustionof mantle source regions by progressive partial melting. Thewaning stages of volcanic activity at Iztacc?huatl appear toreflect the inability of dense basaltic influxes to successfullypenetrate a large high-level chamber of low density hornblendedacite magma. 相似文献
12.
The Sete Cidades volcano (São Miguel, Azores) is situatedat the eastern end of the ultraslow spreading Terceira riftaxis. The volcano comprises several dominantly basaltic pre-calderaeruptions, a trachytic caldera-forming stage and a post-calderastage consisting of alternating trachytic and basaltic eruptions.The post-caldera flank lavas are more primitive (>5 wt %MgO) than the pre-caldera lavas, implying extended fractionalcrystallization and longer crustal residence times for the pre-caldera,shield-building lavas. Thermobarometric estimates show thatthe ascending alkali basaltic magmas stagnated and crystallizedat the crust–mantle boundary (15 km depth), whereas themore evolved magmas mainly fractionated in the upper crust (3km depth). The caldera-forming eruption was triggered by a basalticinjection into a shallow trachytic magma chamber. Lavas fromall stages follow a single, continuous liquid line of descentfrom alkali basalt to trachyte, although slight differencesin incompatible element (e.g. Ba/Nb, La/Nb) and Sr isotope ratiosimply some heterogeneity of the mantle source. Major and traceelement data suggest similar partial melting processes throughoutthe evolution of the volcano. Slight geochemical differencesbetween post- and pre-caldera stage lavas from the Sete Cidadesvolcanic system indicate a variation in the mantle source compositionwith time. The oxygen fugacity increased from the pre-calderato the post-caldera stage lavas, probably as a result of theassimilation of crustal rocks; this is supported by the presenceof crustal xenoliths in the lavas of the flank vents. The lavasfrom the Sete Cidades volcano generally have low Sr isotoperatios; however, rocks from one post-caldera vent on the westernflank indicate mixing with magmas resembling the lavas fromthe neighbouring Agua de Pau volcano, having higher Sr isotoperatios. The different magma sources at Sete Cidades and theadjacent Agua de Pau volcano imply that, despite their closeproximity, there is only limited interaction between them. KEY WORDS: crystallization depth; fractionation; stratigraphy; Terceira rift; volcanic stages 相似文献
13.
Geochemical effects of decoupled fractional crystallization and crustal assimilation 总被引：2，自引：0，他引：2
Most models of crustal assimilation assume that the amount of assimilant added to the magma is proportional to each infinitesimally small amount of solid removed during crystallization (AFC). In some magmatic systems, however, assimilation and crystallization are not strictly related and the mass assimilated is decoupled from, and therefore varies independently of, the mass crystallized (FCA). The geochemical consequences of FCA are examined and compared to those of AFC. The behavior of incompatible elements is identical during AFC and FCA, and ratios of these elements do not allow discrimination between the two processes. Major-oxide least-squares mass-balance models do not discriminate between AFC and FCA at F ≥ 0.7 (F = fraction of melt remaining). However, FCA yields magmas richer in compatible elements and with higher Sr-isotopic ratios than AFC at a given value of F. Repeated cycles of FCA and AFC combined with magma mixing (FAM) may result in unusual geochemical trends, such as the evolution of a calc-alkaline basaltic parent to a tholeiitic daughter magma, or the evolution of low- and medium-K calc-alkaline basalts to high-K andesites, dacites, trachyandesites or trachydacites. Lavas erupted by the volcano Micro Profitis Ilias on Santorini, Hellenic arc, Greece, provide an example of magmas which evolved by combined fractionation, assimilation by FCA and mixing. 相似文献
14.
新疆阿吾拉勒山西段下二叠统陆相火山岩岩石地球化学特征、成因及构造背景 总被引：4，自引：0，他引：4
新疆阿吾拉勒山西段是伊犁石炭-二叠纪裂谷的重要组成部分,本文对该区下二叠统典型的双峰式玄武岩-流纹岩组合进行了系统的岩相学和岩石地球化学研究。岩石整体高Na2O、高Al2O3、低TiO2、富碱。玄武岩富集大离子亲石元素Ba、K、LREE和P,亏损高场强元素Th、U、Ta、Nb。粗面斑岩和流纹斑岩富集大离子亲石元素Rb、K、LREE、高场强元素Th、Zr、Hf,亏损Ta、Nb、Sr、Ti和P。玄武安山玢岩的蛛网图与粗面斑岩和流纹斑岩较为一致,但少量元素的特征与玄武岩相似。玄武岩浆来源于弱亏损地幔,并受到了下地壳物质的混染,而玄武安山玢岩、粗面斑岩和流纹斑岩则可能来源于地壳物质的部分熔融。双峰式火山岩的形成可能与上地幔玄武岩浆的底侵作用有关。裂谷演化导致的陆相火山活动持续到早二叠世晚期达到顶峰,中二叠世以后,构造环境由拉伸转为挤压,裂谷演化趋于终止。该区石炭纪末-早二叠世的裂谷活动与整个天山地区晚古生代的构造演化背景具有一致性。 相似文献
15.
东天山石炭纪企鹅山群火山岩岩石成因 总被引：13，自引：1，他引：12
土屋矿区南北大沟企鹅山群火山岩的岩石地球化学研究表明:东天山企鹅山群火山岩主要为拉斑系列,少量为钙碱系列;岩石类型为玄武岩、玄武安山岩、英安岩和流纹岩。稀土、微量元素和Sr、Nd同位素特点揭示:该火山岩系形成于大陆裂谷环境;其源区主要为软流圈地幔,同时有岩石圈地幔源组分卷入,酸性岩浆是玄武质岩浆结晶分异的产物。 相似文献
16.
The Tatara shield volcano and subsequent San Pedro cone arethe youngest edifices of the San Pedro-Pellado volcanic complexat 36S in the Chilean Andes. There are multiple basaltic andesitecompositional types present in the Tatara volcano, which couldresult from either contrasting source regions or interactionof primitive liquids with heterogeneous crust. The eruptivestratigraphy of the magma types implies concurrent, isolatedmagma chambers beneath Tatara-San Pedro. Open-system processesand multiple crustal endmembers were involved in calcalkalinedifferentiation series, whereas a tholeitiic series evolvedmainly by fractional crystallization. The glaciated Tatara shield comprises two cycles of compositionallydiverse basaltic andesite lavas, each of which is capped byvolumetrically minor andesite to dacite lavas. Four types (I-IV)of basaltic andesite are defined on the basis of chemical criteria,two in each cycle. The early cycle consists of calcalkalinetype I basaltic andesites, and tholeiitic type II basaltic andesitesand andesites; it culminated in the eruption of a dacite dome.The later cycle comprises intercalated calcalkaline type IIIand IV basaltic andesites, and they are overlain by San Pedroandesites and dacites which appear to be the differentiationproducts of type IV magmas. Tatara lavas were erupted from acommon vent situated beneath the modern San Pedro cone. Althoughthey overlap temporally and spatially, there is little evidenceof chemical interaction among the different lava types, indicatingthat there were two or more magma reservoirs beneath Tatara-SanPedro. Chemical differences among the basaltic andesite types precludederivation of any one from any of the others by fractional crystallization,assimilation-fractional crystallization (AFC), or magma mixing.The differences seem to reflect chemically different parentmagmas. The type I and IV parent liquids were relatively highin MgO, low in CaO and AI2O3, and had high incompatible andcompatible element abundances. The type II and III parents werelower in MgO, higher in A12O3 and CaO, and had lower compatibleand incompatible element abundances. Tholeiitic type II lavasappear to have evolved mainly by fractional crystallization,whereas there is evidence of open-system processes such as AFCand magma mixing in the evolution of the calcalkaline I, III,and IV suites. The chemical evolution of the type III and type IV-San Pedromagma suites has been simulated by assimilation and mixing modelsusing local granites and xenoliths as assimilants. The xenolithsprobably represent portions of a sub-caldera pluton associatedwith the Quebrada Turbia Tuff, which erupted from the Rio Coloradocaldera within the San Pedro-Pellado complex at 0–487Ma. Chemical and textural variations in type III lavas correlatewith stratigraphic position and appear to represent mixing betweena parental type III magma and remnant, evolved type I magmathat was progressively flushed from its chamber concurrent withmixing. The youngest San Pedro flow is chemically zoned fromdacite to basaltic andesite and may have formed by mixing withina conduit during eruption. 相似文献
17.
K. Johnson C. Barnes J. Browning H. Karlsson 《Contributions to Mineralogy and Petrology》2002,142(5):564-581
The Middle Cretaceous Cornucopia stock in the Blue Mountains of northeastern Oregon is a small composite intrusion consisting of hornblende biotite tonalite, biotite trondhjemite, and three cordierite two mica trondhjemite units. Unusual magnetite + biotite-rich tonalitic rocks are associated with the Crater Lake cordierite trondhjemite, the youngest of the intrusions. Oxide-rich tonalites are characterized by high Fe (~47-68 wt% total Fe as FeO), low SiO2 (<36 wt%), and enrichments in HFSE and REE (La(N)=361-903). Oxide-rich tonalites appear in a variety of forms, including composite dikes and sheets, in which they are associated with leucocratic tonalite. Leucotonalite is lower in SiO2 (60-72 wt%) than Crater Lake trondhjemite, and generally has DREE contents and Eu anomalies intermediate between the oxide-rich tonalite and Crater Lake compositions. Oxide-rich tonalites crosscut, and are crosscut by, shear zones in the host trondhjemite, indicating their emplacement late in the pluton's crystallization history. Granitic dikes crosscut the composite dikes in all localities. Geochemical considerations and sedimentary-like structures, such as load casts and bedding of magnetite-rich assemblages in the composite dikes and sheets, are suggestive of crystal settling from an Fe-rich parental magma. The Fe-rich liquid parental to the oxide-rich tonalite-leucotonalite pairs formed by extensive, in-situ, plagioclase + quartz-dominated crystallization of strongly peraluminous trondhjemite. Early magnetite saturation in the trondhjemite was suppressed, either because the parental trondhjemitic magma had a lower initial total Fe content or because it had a lower ferric-ferrous ratio, possibly reflecting a lower oxygen fugacity. Accumulation of magnetite from Fe-rich residual magma is a viable mechanism for the concentration of iron, and the subsequent formation of Fe-rich rocks, in calcic siliceous intrusions. Apparently, Fe-enrichment can occur locally in calcic magmas, and is not restricted to rocks of mafic tholeiitic or anorthositic affinity. 相似文献
18.
P. Imsland 《Contributions to Mineralogy and Petrology》1983,83(1-2):31-37
The volcanism in Iceland occurs on both rift zones and non-rifting zones. The rift zone volcanism produces rock suites of the tholeiitic series, ranging from primitive tholeiites (MORB) to highly silicic rocks. The non-rifting volcanic zones produce rock suites of transitional to mildly alkaline or even calc-alkaline composition, the basalts typically being FETI-basalts. Over 50 per cent of the rift zone production in Iceland is of the primitive MORB-type. The relative amount of the evolved rock types, as well as the total volcanic production increases inland along the rift zones. The rock types of the Icelandic rift zones are identical to those of the submerged oceanic rift zones, but the Icelandic production is somewhat offset towards a more voluminous evolved end. The skewness in volume relations of rock types, as compared to the oceanic rifts in general, is mainly caused by the products of the non-rifting volcanic zones of Iceland and only to a small degree by the rift zone products. 相似文献
19.
Alcedo volcano is one of six shield volcanoes on Isabela Islandin the western Galpagos Islands. Although Alcedo is dominantiybasaltic, it is unusual in that it also has erupted 1 km3 ofrhyolite. The rhyolitic phase marked a 10-fold decrease in themass-eruption rate of the volcano, and the volcano has returnedto erupting basalt. The basalts are tholeiitic and range fromstrongly to sparsely porphyritic. Olivine and plagiodase arethe liquidus phases in the most primitive basalts. The MgO andNi concentrations in the most primitive basalts indicate thatthey have undergone substantial differentiation since extractionfrom the mantle. The rhyolites contain the assemblage oligoclase-augite-titanomagnetite-fayalite-apatiteand sparse xenoliths of quenched basalt and cumulate gabbros.Intermediate rocks are very rare, but some are apparently basaltrhyolitehybrids, and others resulted from differentiation of tholeiiticmagma. Several modeling approaches and Sr-, Nd-, and O-isotopicdata indicate that the rhyolites resulted from 90% fractionation(by weight) of plagiodase, augite, titanomagnetite, olivine,and apatite from the most primitive olivine tholeiite. The dataare inconsistent with the rhyolites originating by crustal anatexis.The extreme Daly gap may be caused by the large increase inviscosity as the basaltic magma differentiates to intermediateand siliceous compositions; highly evolved magmas are eruptibleonly after they become saturated with volatiles by second boiling.The close association of the hybrid intermediate magmas andmagmatic inclusions with the climactic plinian eruption indicatesmixing between mafic and silicic magmas immediately before eruption.Rhyolite production was favored by the decrease in supply ofbasaltic magma as Alcedo was carried away from the focus ofthe Galpagos hotspot. A three-stage model for the magmaticevolution of a Galpagos volcano is proposed. In the first stage,the supply of basaltic magma is large. Basaltic magma continuallyintrudes the subcaldera magma chamber, buffering the magmas'compositional and thermal evolution. As the volcano is carriedaway from the basaltic source, the magma chamber is allowedto cool and differentiate, as exemplified by Alcedo's rhyoliticphase. Finally, the volcano receives even smaller influx ofbasalt, so a large magma chamber cannot be sustained, and thevolcano shifts to isolated basaltic eruptions. KEY WORDS: Galpagos; oceanic rhyolites; fractional crystallization; Isabela Island
*Corresponding author, e-mail: Geist{at}IDUI1.csrv.uidaho.edu. Telephone: 208-885-6491. Fax: 208-885-5724 相似文献
20.
Mahnaz Khodami Moussa Noghreyan Ali Reza Davoudian 《Arabian Journal of Geosciences》2010,3(3):257-266
Late Miocene–Pliocene to Quaternary calc-alkaline lava flows and domes are exposed in southeast of Isfahan in the Urumieh Dokhtar magmatic belt in the Central Iran structural zone. These volcanic rocks have compositions ranging from basaltic andesites, andesites to dacites. Geochemical studies show these rocks are a medium to high K calc-alkaline suite and meta-aluminous. Major element variations are typical for calc-alkaline rocks. The volcanic rocks have SiO2 contents ranging between 53.8% and 65.3%. Harker diagrams clearly show that the dacitic rocks did not form from the basaltic andesites by normal differentiation processes. They show large ion lithophile elements- and light rare earth elements (LREE)-enriched normalized multielement patterns and negative Nb, Ti, Ta, and P. Condrite-normalized REE patterns display a steep decrease from LREE to light rare earth elements without any Eu anomaly. These characteristics are consistent with ratios obtained from subduction-related volcanic rocks and in collision setting. The melting of a heterogeneous source is possible mechanism for their magma genesis, which was enriched in incompatible elements situated at the upper continental lithospheric mantle or lower crust. The geochemical characteristics of these volcanic rocks suggested that these volcanic rocks evolved by contamination of a parental magma derived from metasomatized upper lithospheric mantle and crustal melts. 相似文献