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1.
The Micho?ch-Guanajuato Volcanic Field (MGVF) of central Mexicocontains 900 cinder and lava coes but lacks the large activecomposite volcanoes found in other portions of the Mexican VolcanicBelt (MVB). Scoriae and lavas from these cinder cones are primarilyolivine-basalts and olivine-andesites containing phenocrystsof olivine (plus Cr-rich spinel inclusions), plagioclase, and,less frequently, augite; pyroxene- and hornblende-andesitesare subordinate. Most samples are calcalkaline; however, alkalineand transitional rocks are also found. Compositional variationat individual cones is usually less than 5 per cent SiO2 andat Volc?n Paricutin (1943–1952) and Volc?n Jorullo (1759–1774),lava compositions have become more silica-rich with time. Alkaline cinder cones are generally older, but in the late Quaternary,both calc-alkaline and alkaline magmas erupted in the southernpart of the MGVF. Positive correlations between K, Zr, and Baand distance from the Middle America trench are distinct forevolved lavas; no correlations are found for less differentiatedlavas. In contrast, a correlation between decreasing Mg, Ni,and Cr and distance from the trench is found. In comparison to composite volcanoes in the MVB, the cinder-conelava are typically more basic. Four samples have mg-numbersand Ni contents which indicate possible mantle source regions.These samples include calc-alkaline, transitional and alkalinelavas, but all contain phenocrysts and/or microphenocrysts ofolivine, augite, and plagioclase; in these high-Mg lavas, spinelinclusions in olivine are Cr-rich. Those high-Mg lavas withsmall amounts of coexisting olivine, augite, and plagioclasephenocrysts plot close to a high-pressure (8 kb ? H2O) 0l-Aug-Plcotectic. Others project between this high-pressure clusterand the 1 atm. cotectic, indicating polybaric fractionation.Low-Mg lavas in the northern part of the MGVF result from fractionationat relatively shallow depths. Estimated olivine equilibrium temperatures decrease from about1200?C with increasing FeO/FeO + MgO, which is also accompaniedby an increase in H2O. Relative oxygen fugacities (relativeto NNO) calculated for lavas with Fe2O3+FeO show that NNO increasessystematically during an eruption, and this is well displayedat both Paricutin and Jorullo. The more oxidized lavas may containhornblende, and do so at Colima. The calc-alkaline lavas fromthroughout the MGVF only span the redox state of the Jorulloeruption, and all these continental magmas are 2–3 ordersof magnitude more oxidized than their submarine counterparts. Petrographic and mineralogical evidence supports the absenceof long-lived shallow magma reservoirs, consistent with theobserved small magma output rate in the MGVF.  相似文献   

2.
Equilibrium H2O pressure (PeH2O) was fixed at values less thantotal pressure (PT) in melting experiments on mixtures of 1921Kilauea tholeiite, H2O, and CO2 (58.5 mole per cent H2O, 41.5mole per cent CO2), buffered by Ni+NiO. New determinations ofthe beginning of melting of mixtures of 1921 Kilauea tholeiiteand H2O buffered by quartz+fayalite+magnetite were made at 2and 3 kb. Microprobe analyses of coexisting glass, clinopyroxene,?olivine, ?amphibole were determined for several runs. Decreasing H2O fugacity (fH2O) to about six-tenths the fugacityof pure H2O (f?II2O) raises the solidus and the upper stabilitylimit of plagioclase. Plagioclase and clinopyroxene coexistin equilibrium with liquid-a feature not observed in the pureH2O system. Amphibole is stable to about 970 ?C at 2 kb, 1025?C at 5 kb and 1060 ?C at 8 kb. The Al (VI)+Ti contents of theamphibole increase with P, yielding kaersutite at 1050 ?C and8 kb. Calculated modes for the condensed phases reveal large differencesin the amount of glass (liquid) present and large differencesin liquid composition below and above the breakdown temperatureof amphibole at 5 and 8 kb. Liquids coexisting with amphibole,clinopyroxene, olivine, and magnetite are dacitic near the solidusand silica-rich andesites around 1000 ?C at 5 and 8 kb. Theresults of this study substantiate the model for the generationof the calc-alkaline suite by partial melting of H2O-rich basalts.  相似文献   

3.
The Kap Edvard Holm Layered Gabbro Complex is a large layeredgabbro intrusion (>300 km2) situated on the opposite sideof the Kangerdlugssuaq fjord from the Skaergaard Intrusion.It was emplaced in a continental margin ophiolite setting duringearly Tertiary rifting of the North Atlantic. Gabbroic cumulates, covering a total stratigraphic thicknessof >5 km, have a typical four-phase tholeiitic cumulus mineralogy:plagioclase, clinopyroxene, olivine, and Fe–Ti oxides.The cryptic variation is restricted (plagioclase An81–51,olivine Fo85–66, clinopyroxene Wo43–41 En46–37Fs20–11) and there are several reversals in mineral chemistry.Crystallization took place in a low-pressure, continuously fractionatingmagma chamber system which was periodically replenished andtapped. Fine-grained (0•2–0•4 mm) equigranular, thin(0•5–3 m), laterally continuous basaltic zones occurwithin an {small tilde}1000 m thick layered sequence in theTaco Point area. Twelve such zones define the bases of individualmacrorhythmic units with an average thickness of {small tilde}80m. The fine-grained basaltic zones grade upwards, over a fewmetres, into medium-grained (>1 mm) poikilitic, olivine gabbrowith smallscale modal layering. Each fine-grained basaltic zoneis interpreted as an intraplutonic quench zone in which magmachilled against the underlying layered gabbros during influxalong the chamber floor. Supercooling by {small tilde}50C isbelieved to have caused nucleation of plagioclase, olivine,and clinopyroxene in the quench zone. The nucleation rate isbelieved to have been enhanced as the result of in situ crystallizationin a continuously flowing magma. The transition to the overlyingpoikilitic olivine gabbro reflects a decreasing degree of supercooling. Compositional variation in the Taco Point sequence is typicalfor an open magma chamber system: olivine (Fo77–68 5)and plagioclase cores (An80–72) show a zig-zag crypticvariation pattern with no overall systematic trend. Olivinehas the most primitive compositions in the quench zones andmore evolved compositions in the olivine gabbro; plagioclasecores show the opposite trend. Although plagioclase cores arebelieved to retain their original compositions, olivines re-equilibratedby reaction with trapped liquid. Some plagioclase cores containrelatively sodic patches which retain quench compositions. Whole-rock compositions of nine different quench zones varyover a range from 10 to 18% MgO although the mg-number remainsconstant at {small tilde}0•78. The average composition(47•7% SiO2, 13•3%MgO, 1•57% Na2O+K2O) is takenas a best estimate of the parental magma composition, and isequivalent to a high-magnesian olivine tholeiite. The compositionalvariation of the quench zones is believed to reflect burstsof nucleation and growth of olivine and plagioclase during quenching. Magma emplacement is believed to have taken place by separatetranquil influxes which flowed along the interface between alargely consolidated cumulus pile and the residual magma. Theresident magma was elevated with little or no mixing. At certainlevels in the layered sequence the magma drained back into thefeeder system; such a mechanism is referred to as a surge-typemagma chamber system.  相似文献   

4.
Quaternary monogenetic volcanism in the High Cascades of Oregonis manifested by cinder cones, lava fields, and small shields.Near Crater Lake caldera, monogenetic lava compositions include:low-K (as low as 0?09% K2O) high-alumina olivine tholeiite (HAOT);medium-K. calc-alkaline basalt, basaltic andesite, and andesite;and shoshonitic basaltic andesite (2?1% K2O, 1750 ppm Sr at54% SiO2). Tholeiites have MORB-like trace element abundancesexcept for elevated Sr, Ba, and Th and low high field strengthelements (HFSE), and they represent near-primary liquids. Theyare similar to HAOTs from the Cascades and adjacent Basin andRange, and to many primitive basalts from intraoceanic arcs.Calc-alkaline lavas show a well-developed arc signature of highlarge-ion lithophile elements (LILE) and low HFSE. Their Zrand Hf concentrations are at least partly decoupled from thoseof Nb and Ta; HREE are low relative to HAOT. Incompatible elementabundances and ratios vary widely among basaltic andesites.Some calc-alkaline lavas vented near Mount Mazama contain abundantgabbroic microxcnoliths, and are basaltic andesitic magmas contaminatedwith olivine gabbro. A calc-alkaline basalt and a few basaltic andesites have MgOand compatible trace element contents that suggest only minorfractionation. There appears to be a compositional continuumbetween primitive tholeiitic and calc-alkaline lavas. Compositionalvariation within suites of comagmatic primitive lavas, boththoleiitic and calc-alkaline, mainly results from differentdegrees of partial melting. Sources of calc-alkaline primarymagmas were enriched in LILE and LREE by a subduction componentand contained residual garnet, whereas sources of HAOTs hadlower LILE and LREE concentrations and contained residual clinopyroxene.High and variable LILE and LREE contents of calc-alkaline lavasreflect variations in fluid-transported subduction componentadded to the mantle wedge, degree of partial melting, and possiblyalso interaction with rocks or partial melts in the lower crust. Andesites were derived from calc-alkaline basaltic andesitesby fractionation of plagioclase+augite+magnetite+apatite ? orthopyroxeneor olivine, commonly accompanied by assimilation. Many andesitesare mixtures of andesitic or dacitic magma and a basaltic orbasaltic andesitic component, or are contaminated with gabbroicmaterial. Mingled basalt, andesite, and dacite of Williams Craterformed by multi-component, multi-stage mixing of basaltic andesiticmagma, gabbro, and dacitic magma. The wide range of compositionsvented from monogenetic volcanoes near Crater Lake is a resultof the thick crust coupled with mild tectonic extension superimposedon a subduction-related magmatic arc.  相似文献   

5.
Phenocryst compositions and mineral–melt equilibria inthe mildly alkalic basalts from the 25 Ma Mont Crozier sectionon the Kerguelen Archipelago are used to estimate the depthsat which magmas stalled and crystallized and to constrain therole of crustal structure in the evolution of magmas producedby the Kerguelen mantle plume. The Crozier section, of nearly1000 m height, consists of variably porphyritic flows (up to21 vol. % phenocrysts), dominated by plagioclase ± clinopyroxene± olivine ± Fe–Ti oxides. Feldspars showan extreme range of compositions from high-Ca plagioclase (An88)to sanidine and variable textures that are related to extensivefractionation, degassing, and mixing in relatively low-pressure(sub-volcanic) magma chambers. Although clinopyroxene is a minorphenocryst type (0–3 vol. %), its non-quadrilateral components,principally Al (1·9–8·6 wt % Al2O3), varywidely. The results of clinopyroxene–liquid thermobarometryand clinopyroxene structural barometry indicate that the Croziermagmas crystallized at pressures ranging from  相似文献   

6.
Vico volcano has erupted potassic and ultrapotassic magmas,ranging from silica-saturated to silica-undersaturated types,in three distinct volcanic periods over the past 0·5Myr. During Period I magma compositions changed from latiteto trachyte and rhyolite, with minor phono-tephrite; duringPeriods II and III the erupted magmas were primarly phono-tephriteto tephri-phonolite and phonolite; however, magmatic episodesinvolving leucite-free eruptives with latitic, trachytic andolivine latitic compositions also occurred. In Period II, leucite-bearingmagmas (87Sr/86Srinitial = 0·71037–0·71115)were derived from a primitive tephrite parental magma. Modellingof phonolites with different modal plagioclase and Sr contentsindicates that low-Sr phonolitic lavas differentiated from tephri-phonoliteby fractional crystallization of 7% olivine + 27% clinopyroxene+ 54% plagioclase + 10% Fe–Ti oxides + 4% apatite at lowpressure, whereas high-Sr phonolitic lavas were generated byfractional crystallization at higher pressure. More differentiatedphonolites were generated from the parental magma of the high-Srphonolitic tephra by fractional crystallization of 10–29%clinopyroxene + 12–15% plagioclase + 44–67% sanidine+ 2–4% phlogopite + 1–3% apatite + 7–10% Fe–Tioxides. In contrast, leucite-bearing rocks of Period III (87Sr/86Srinitial= 0·70812–0·70948) were derived from a potassictrachybasalt by assimilation–fractional crystallizationwith 20–40% of solid removed and r = 0·4–0·5(where r is assimilation rate/crystallization rate) at differentpressures. Silica-saturated magmas of Period II (87Sr/86Srinitial= 0·71044–0·71052) appear to have been generatedfrom an olivine latite similar to some of the youngest eruptedproducts. A primitive tephrite, a potassic trachybasalt andan olivine latite are inferred to be the parental magmas atVico. These magmas were generated by partial melting of a veinedlithospheric mantle sources with different vein–peridotite/wall-rockproportions, amount of residual apatite and distinct isolationtimes for the veins. KEY WORDS: isotope and trace element geochemistry; polybaric differentiation; veined mantle; potassic and ultrapotassic rocks; Vico volcano; central Italy  相似文献   

7.
The lavas of Nisyros were erupted between about 0?2 m.y B.P.and 1422 A.D., and range in composition from basaltic andesiteto rhyodacite. Most were erupted prior to caldera collapse (exactdate unknown), and the post-caldera lavas are petrographically(presence of strongly resorbed phenocrysts) and chemically (lowerTiO2 K2O, P2O5, and LIL elements) distinct from the pre-calderalavas. The pre-caldera lavas do not form a continuous seriessince lavas with SiO2 contents between 60 and 66 wt.% are absent.Nevertheless, major element variations demonstrate that fractionalcrystalliz ation (involving removal of olivine, dinopyroxene,plagioclase, and Fe-Ti oxide from the basaltic andesites andandesites and plagioclase, clinopyroxene, hypersthene, Ti-magnetite,ilmenite, apatite, and zircon from the dacites and rhyodacites)played a major role in the evolution of the pre-caldera lavas.Several lines of evidence indicate that other processes werealso important in magma evolution: (1) Quantitative modelingof major element data shows that phenocryst phases of unlikelycomposi tion or unrealistic assemblages of phenocryst phasesare required to relate the dacites and rhyodacites to the basalticandesites and andesites; (2) The proportions of olivine andclinopyroxene required in quantitative models for the initialstages of evolution differ from those observed petrographicallyand this is not likely to reflect either differential ratesof crystal settling or the curvature of cotectics along whichliquids of basaltic andesite to andesite composition lie; (3)The concentrations of Rb, Cs, Ba, La, Sm, Eu, and Th in therhyod.acites are too high for these lavas to be related to thedacites by fractional crystallization alone; and (4) 87Sr/86Srratios for the andesites and rhyodacites are higher than thosefor the basaltic andesites and dacites, respectively. It isshown that fractional crystallization was accompanied by assimilation,and that magma mixing played a minor role (if any) in the evolutionof the pre-caldera lavas. Trace element and isotopic data indicatethat the andesites evolved from the basaltic andesites by AFCinvolving average crust or upper crust, whereas the rhyodacitesevolved from the dacites by AFC involving lower crust. Additionalevidence for polybaric evolution is provided by the occurrenceof distinct Ab-rich cores of plagioclase phenocrysts in thedacites and rhyodacites, which record a period of high pressurecrystallization, and by the occurrence of both normal and reverse-zonedphenocrysts in the basaltic andesites and andesites. Furthermore,calculated pressures of crystallization are {small tilde}8 kbfor the dacites and rhyodacites and 3?5–4 kb for the basalticandesites and andesites. It is concluded that the dacites andrhyodacites evolved via AFC from basaltic andesites and andesiteslargely in chambers sited near the base of the crust whereasthe basaltic andesites and andesites mostly evolved in chamberssited at mid-crustal levels. Eruption from different chambersexplains the compositional gap in the chemistry of the pre-calderalavas since eruptive products represent a more or less randomsampling of residual liquids which separate (via filter pressing)from bodies of crystallizing magma at various depths. Magmamixing was important in the evolution of the post-caldera lavas,but geochemical data require that these magmas evolved fromparental magmas which were derived from a more refractory sourcethan the parental magmas to the pre-caldera lavas. *Present address: Netherlands Energy Research Foundation (ECN), P.O. Box 1, 1755 ZG Petten, The Netherlands  相似文献   

8.
Volcán Popocatépetl has been the site of voluminousdegassing accompanied by minor eruptive activity from late 1994until the time of writing (August 2002). This contribution presentspetrological investigations of magma erupted in 1997 and 1998,including major-element and volatile (S, Cl, F, and H2O) datafrom glass inclusions and matrix glasses. Magma erupted fromPopocatépetl is a mixture of dacite (65 wt % SiO2, two-pyroxenes+ plagioclase + Fe–Ti oxides + apatite, 3 wt % H2O, P= 1·5 kbar, fO2 = NNO + 0·5 log units) and basalticandesite (53 wt % SiO2, olivine + two-pyroxenes, 3 wt % H2O,P = 1–4 kbar). Magma mixed at 4–6 km depth in proportionsbetween 45:55 and 85:15 wt % silicic:mafic magma. The pre-eruptivevolatile content of the basaltic andesite is 1980 ppm S, 1060ppm Cl, 950 ppm F, and 3·3 wt % H2O. The pre-eruptivevolatile content of the dacite is 130 ± 50 ppm S, 880± 70 ppm Cl, 570 ± 100 ppm F, and 2·9 ±0·2 wt % H2O. Degassing from 0·031 km3 of eruptedmagma accounts for only 0·7 wt % of the observed SO2emission. Circulation of magma in the volcanic conduit in thepresence of a modest bubble phase is a possible mechanism toexplain the high rates of degassing and limited magma productionat Popocatépetl. KEY WORDS: glass inclusions; igneous petrology; Mexico; Popocatépetl; volatiles  相似文献   

9.
BARSDELL  M. 《Journal of Petrology》1988,29(5):927-964
The mineralogy, petrography and geochemistry of a suite of clinopyroxene-richolivine tholenite lavas from Merelava island, Vanuatu are described.Located at the southern end of the Northern Trough back-arcbasin, this suite displays all the characteristics of primitiveisland arc lavas: flat REE patterns, depleted HFSE, enrichmentin K-group elements relative to LREE, highly calcic plagioclase(to An9 3 and Cr-rich spinels (cr-number80) Analysis of groundmasscompositions demonstrates that the variation in MgO within thelava suite (from 13?7 to 4?3% MgO) represents only a small departurefrom a liquid line of descent. Some of the more primitive lavas contain low-Al2O3 clinopyroxenemegacrysts (mg-number = 100Mg/(Mg+Fe2 + and ultramafic xenoliths,the latter ranging from fine-grained, tectonite wehrlites andchnopyroxene-bearing harzburgites, to coarse-grained cumulatewehrlites. The cumulate nodules, megacrysts and phenocrysts are shown tobe co-magmatic, and an empirical compositional relationshipis demonstrated for equilibrium olivine-clinopyroxene pairs,covering the observed fractionation range (mg-numberCpx=0?6375mg-numberO1 + 35?3). On the basis that the most primitive olivine(mg-number 91 7) is close to the liquidus composition, thiscompositional relationship demonstrates that clinopyroxene (mg-number=94,and containing no Fe3+) was also a liquidus phase. Clinopyroxeneswith mg-number>94 are the product of local oxidation duringmixing of primitive, relatively reduced magmas, and more evolved,oxidized magmas. This mixing also gave rise to relatively narrow,reversely zoned, internal rims on many clinopyroxene and olivinephenocrysts, cumulus crystals, and clinopyroxene megacrysts. Fractionation modelling demonstrates that the most differentiatedsample with 19 wt.% Al2O3 can be derived from the most primitivesample with 10?3% Al2O3 by removal of 48% crystals of clinopyroxeneand olivine in the proportions 73:27 Plagioclase is a late crystallizingphase and has an insignificant role in the fractionation process. The parent melt composition (mg-number=77) is deduced from themost primitive olivine composition and the liquid line of descent,and is shown to contain equal amounts of MgO and CaO (137 wt.%),a high CaO/Al2O3 ratio of 1?3 and an unusually low Ni contentof 137 ppm. Data from published high pressure (8–20 kb)experiments on melting of peridotite and pyrolite do not providean explanati in for the large normative diopside component inthis parent melt (38 mol.%), and a hypothesis is proposed wherebyhigh degrees of melting of refractory Iherzolite or harzburgite+acomponent of lower crustal pyroxenite and/or wehrlite takesplace at the base of the crust (5–55 kb). At this depth,and initially under hydrous conditions, high degrees of meltingwould progressively eliminate orthopyroxene and then clinopyroxeneto produce a dunite residue. The liquid produced near the pointof clinopyroxene elimination would be compatible with the highCaO and Sc contents, and high Sc/Ni, Cr/Ni and D1/Hy ratiosof the lavas, and the refractory nature of the phenocrysts.  相似文献   

10.
The petrography, mineralogy, and geochemistry of a suite oflavas from the northwestern part of Epi Island in the VanuatuArc, southwest Pacific Ocean, are described. The more primitivemembers of this suite are rich in clinopyroxene phenocrystsand are strikingly similar to primitive lavas from MerelavaIs. in the same arc. These primitive, clinopyroxene-rich lavasare designated arc ankaramites to differentiate them from primitive,olivine-rich arc picrites which also occur in this arc system.The primitive Epi lavas are shown to have evolved from low-Kprimary melts which were saturated in both olivine and clinopyroxene.The most Mg-rich olivine (mg-number 92?2) and clinopyroxene(mg-number 94?4) in the ankaramites represent cotectic crystallizationwith Cr-rich spinels. Initial plagioclase (An94) crystallizedin equilibrium with olivine (mg-number 78–80) and theplagioclase-olivine cotectic path extends to mg-number 50 andAn58. The ankaramitic parent magma composition is calculated fromthe most primitive olivine phenocryst composition and the liquidline of descent, and has 14?5% MgO, 11% A12O3, 14?8%CaO, 0?29%K2O, and flat REE patterns. The origin of this parent magmahas been modelled with Ghiorso & Carmichael's (1985) programSILMIN. An assimilation model involving a clinopyroxenite orwehrlite assimilate and a low-K picrite host requires ca. 90%assimilate to match the phase chemistry and bulk-rock chemistryof the parental ankaramite. The required degree of superheatingnecessary to achieve this, and the apparent restriction of low-Kpicrites to Anatom Island in the far south of the arc, rendersthis model unsatisfactory. Partial melting models involvingtypical upper mantle lherzolite also fail to give satisfactoryresults, but partial melting of a wehrlite source (mg-number87-88) with < 10% normative (mol.) orthopyroxene, at 5?10kband 1325?C, closely matches the parental ankaramite composition.These results can be reconciled with melting of lower crustalcumulates by an ascending peridotite diapir, a hypothesis whichaccounts for the very low Ni contents of the parental meltsand primitive phenocrysts. The more evolved lavas define two distinct assemblages: a relativelytight grouping of high-K andesites straddling the high-K-‘shoshonite’boundary, characterized by low Zr/Rb (2?2) and high K2O/Na2Oratios (1?3–0?9), and a relatively coherent fractionationpathway to dacites straddling the ‘calc-alkaline’-high-Kboundary, with Zr/Rb = 2?9 and K2O/Na2O=0?6. Numerical modellingdemonstrates that the dacite trend is compatible with fractionationfrom an ankaramite parent, whereas the high-K andesites areincompatible with open- or closed-system fractionation fromankaramitic or picritic sources and may represent fractionated,hybrid magmas, largely derived from melting of lower crustalgabbros.  相似文献   

11.
Mafic tholeiitic basalts from the Nejapa and Granada (NG) cindercone alignments provide new insights into the origin and evolutionof magmas at convergent plate margins. In comparison to otherbasalts from the Central American volcanic front, these marietholeiitic basalts are high in MgO and CaO and low in Al2Op,K2O1, Ba and Sr. They also differ from other Central Americanbasalts, in having clinopyroxene phenocrysts with higher MgO,CaO and Cr2O3 concentrations and olivine phenocrysts with higherMgO contents. Except for significantly higher concentrationsof Ba, Sr and 87Sr/86Sr, most of the tholeiites are indistinguishable in compositionfrom mid-ocean ridge basalts. In general, phenocryst mineralcompositions are also very similar between NG tholeiites andmid-ocean ridge basalts. The basalts as a whole can be dividedinto two groups based on relative TiO2-K2O concentrations. Thehigh-Ti basalts always have the lowest K2O and Ba and usuallyhave the highest Ni and Cr. All of the basalts have experienced some fractional crystallizationof olivine, plagioclase and clinopyroxene. Relative to otherCentral American basalts, the Nejapa-Granada basalts appearto have fractionated at low PT and PH2O. The source of primarymagmas for these basalts is the mantle wedge. Fluids and/ormelts may have been added to the mantle wedge from hydrothermally-altered,subducting oceanic crust in order to enrich the mantle in Sr,Ba and 87Sr/86Sr, but not in K and Rb. The role of lower crustaicontamination in causing the observed enrichments in Sr, Baand 87Sr/86Sr of NG basalts in comparison to mid-ocean ridgebasalts, however, is unclear. Rutile or a similar high-Ti accessoryphase may have been stable in the mantle source of the low-TiNG basalts, but not in that of the high-Ti basalts. Mafic tholeiiticbasalts, similar to those from Nejapa and Granada, may representmagmatic compositions parental to high-Al basalts, the mostmafic basalts at most Central American volcanoes. The characterof the residual high-Al basalts after this fractionation stepdepends critically on PH2O Both high and low-Ti andesites are also present at Nejapa. Likethe high-Ti basalts, the high-Ti andesites have lower K2O andBa and higher Ni and Cr in comparison to the low-Ti group. Thehigh-Ti andesites appear to be unrelated to any of the otherrocks and their exact origin is unknown. The low-Ti andesitesare the products of fractional crystallization of plagioclase,clinopyroxene, olivine (or orthopyroxene) and magnetite fromthe low-Ti basalts. The eruption that deposited a lapilli sectionat Cuesta del Plomo involved the explosive mixing of 3 components:high-Ti basaltic magma, low-Ti andesitic magma and high-Ti andesiticlava.  相似文献   

12.
Three genetically unrelated magma suites are found in the extrusivesequences of the Troodos ophiolite, Cyprus. A stratigraphicallylower pillow lava suite contains andesite and dacite glassesand shows the crystallization order plagioclase; augite, orthopyroxene;titanomagnetite (with the pyroxenes appearing almost simultaneously).These lavas can in part be correlated chemically and mineralogicallywith the sheeted dikes and the upper part of the gabbro complexof the ophiolite. The second magma suite is represented in astratigraphically upper extrusive suite and contains basalticandesite and andesite glasses with the crystallizaton orderchromite; olivine; Ca-rich pyroxene; plagioclase. This magmasuite can be correlated chemically and mineralogically withparts of the ophiolitic ultramafic and mafic cumulate sequence,which has the crystallization order olivine; Ca-rich pyroxene;orthopyroxene; plagioclase. The third magma suite is representedby basaltic andesite lavas along the Arakapas fault zone andshows a boninitic crystallization order olivine; orthopyroxene;Ca-rich pyroxene; plagioclase. One-atmosphere, anhydrous phaseequilibria experiments on a lava from the second suite indicateplagioclase crystallization from 1225?C, pigeonite from 1200?C,and augite from 1165?C. These experimental data contrast withthe crystallization order suggested by the lavas and the associatedcumulates. The observed crystallization orders and the presenceof magmatic water in the fresh glasses of all suites are consistentwith evolution under relatively high partial water pressures.In particular, high PH2O (1–3 kb) can explain the lateappearances of plagioclase and Ca-poor pyroxene in the majorityof the basaltic andesite lavas as the effects of suppressedcrystallization temperatures and shifting of cotectic relations.The detailed crystallization orders are probably controlledby relatively minor differences in the normative compositionsof the parental magmas. The basaltic andesite lavas are likelyto reach augite saturation before Ca-poor pyroxene saturation,whereas the Arakapas fault zone lavas, which have relativelyless normative diopside and more quartz, reached the Ca-poorpyroxene-olivine reaction surface and crystallized Ca-poor pyroxeneafter olivine.  相似文献   

13.
SEN  GAUTAM 《Journal of Petrology》1986,27(3):627-663
Electron microprobe analyses of minerals of thirteen DeccanTrap lava flows at Mahabaleshwar have been carried out in thepresent study. All of these flows have tholeiitic bulk compositionsand all, except one (represented by MB-81-17 of Mahoney et al.,1982) contain olivine, plagioclase, two pyroxenes, and Fe-Tioxide minerals. Olivine and plagioclase appear as distinct phenocrystsin all but one flow, and Ca-rich pyroxene joins as a phenocrystphase in the younger flows. Pigeonite and Fe-Ti oxide minerals(titanomagnetite and ilmenite) occur in the groundmass. Olivineoccurs as both groundmass and phenocryst phase in MB-81-17 (whichis the only flow without low-Ca pyroxene phase); in all otherflows olivine appears only as phenocryst phase. In all but one(MB-81-17) flow olivine is completely altered. MB-81-17 olivinegrains are only partly altered, and in this rock the cores ofphenocrysts are rounded and have a composition of Fo77 whereastheir euhedral rims have a composition around Fo67. The groundmassolivine grains in MB-81-17 are Fo41–32. Substantial Fe-enrichmentand zoning trends are shown by the pyroxenes in individual rocks.The cores of Ca-rich pyroxene phenocrysts of some of the flowshave as much as 4 wt. per cent A12O3 and may have crystallizedat higher (crustal) pressures. Pigeonite thermometry (Ishii,1975) suggests an average of 1050?C for crystallization of thegroundmass pigeonite (eruption temperature?). Fe-Ti oxide mineralsare mostly altered in the older flows. In the younger flows,coexisting unaltered titanomagnetite and ilmenite yield maximumtemperature estimates for the crystallization of these phaseof about 1025?C and an oxygen fugacity of 10–11.5 atm.The T-fo2 path followed by these flows seems to have been consistentlysomewhat lower than that defined by the 1 atm. fayalite-magnetitequartz curve. All of the lavas examined have experienced extensivefractional crystallization of olivine and some clinopyroxeneat relatively higher pressures. These lavas were saturated orclose to being saturated with olivine+plagioclase+clinopyroxeneduring eruption. Plagioclase accumulation, although it appearsto have occurred, has not been significant. It is suggestedthat MB-81-17 magma was contaminated by a calcite-rich rock(limestone?) whereas the lower Group 1 magmas may have beenselectively contaminated by quartz-bearing contaminant. Alternately,parental magma of MB-81-1 (with the highest Mg-number and 8= -16) may have been produced in the upper mantle into whichminor masses of old crust was well mixed. Magma mixing, crystalfractionation, and contamination processes of Mahabaleshwarbasalts and possible genetic relationships with other DeccanTrap lavas are discussed.  相似文献   

14.
Petrology of the Upper Border Series of the Skaergaard Intrusion   总被引：3，自引：3，他引：3  
The Upper Border Series of the Skaergaard intrusion consistsof a 960 m thick sequence of rocks that crystallized againstthe roof of the magma chamber. The texture and composition ofthe unit vary systematically from top to bottom as a resultof changes that occurred in the magma during the solidificationof the intrusion. The order of crystallization of primocrystminerals in the Upper Border Series was: olivine; + plagioclase;+ apatite; + ilmenite; + magnetite; + Ca-rich pyroxene;—olivine;+ olivine; + ferrobustamite. The major silicate phases varyfrom high-temperature compositions to low-temperature compositionswith increasing distance from the upper contact. Post-crystallizationre-equilibration has affected the compositions of the oxideminerals and to a lesser extent the compositions of olivineand Ca-rich pyroxene. The Upper Border Series differentiationsequence differs from the Layered Series sequence, in that:(1) apatite appears much earlier; (2) magnetite precipitatedbefore Ca-rich pyroxene rather than after it; (3) orthopyroxeneis much less common; (4) the plagioclase is systematically poorerin K2O; and (5) the rocks are systematically richer in K2O andSiO2. The upper part of the Skaergaard magma appears to havebeen enriched in H2O, K2O, SiO2, and P2O5 relative to the partthat was parental to the Layered Series.  相似文献   

15.
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   

16.
Disequilibrium phenocryst assemblages in the Younger Andesitesand Dacites of Iztacc?huatl, a major Quaternary volcano in theTrans-Mexican Volcanic Belt, provide an excellent record ofepisodic replenishment, magma mixing, and crystallization processesin calc-alkaline magma chambers. Phenocryst compositions andtextures in ‘mixed’ lavas, produced by binary mixingof primitive olivine-phyric basalt and evolved hornblende dacitemagmas, are used to evaluate the mineralogical and thermal characteristicsof end-members and the physical and chemical interactions thatattend mixing. Basaltic end-members crystallized olivine (FO90–88) andminor chrome spinel during ascent into crustal magma chambers.Resident dacite magma contained phenocrysts of andesine (An45–35),hypersthene (En67–61), edenitic-pargasitic hornblende,biotite, quartz, .titanomagnetite, and ilmenite. On reachinghigh-level reservoirs, basaltic magmas were near their liquidiat temperatures of about 1250–1200?C according to theolivine-liquid geothermometer. Application of the Fe-Ti-oxidegeothermometer-oxygen barometer indicates that hornblende dacitemagma, comprising phenocrysts (<30 vol. per cent) and coexistingrhyolitic liquid, had an ambient temperature between 940 and820?C at fO2s approximately 0?3 log units above the nickel-nickeloxide buffer assemblage. Mixing induced undercooling of hybridliquids and rapid crystallization of skeletal olivine (Fo88–73),strongly-zoned clinopyroxene (endiopside-augite), calcic plagioclase(An65–60); and orthopyroxene (bronzite), whereas low-temperaturephenocrysts derived from hornblende dacite were resorbed ordecomposed by hybrid melts. Quartz reacted to form coronas ofacicular augite and hydroxylated silicates were heated to temperaturesabove their thermal stability limit ({small tilde}940?C foramphibole, according to clinopyroxene-orthopyroxene geothermometry,and {small tilde}880?C for biotite). Calculations of phenocrystresidence times in hybrid liquids based on reaction rates suggestthat the time lapse between magma chamber recharge and eruptionwas extremely short (hours to days). It is inferred that mixing of magmas of diverse compositionis driven by convective turbulence generated by large differencesin temperature between end-members. The mixing mechanism involves:(1)rapid homogenization of contrasting residual liquid compositionsby thermal erosion and diffusive transfer (liquid blending);(2) assimilation of phenocrysts derived from the low-temperatureend-member; and (3) dynamic fractional crystallization of rapidlyevolving hybrid liquids in a turbulent boundary layer separatingbasaltic and dacitic magmas. The mixed lavas of lztacc?huatlrepresent samples of this boundary layer quenched by eruption.  相似文献   

17.
Mineral Chemistry, and major and trace element variations ofthe basalts from Klyuchevskoy, the world's most active islandare volcano, are most consistently explained by the persistenceof a non-steady state, erupting, recharging, and fractionatingmagma chamber in which fractionation of a parental high-MgObasalt melt produces high-Al2O3 basalt. Although fractionalcrystallization is the dominant controlling mechanism, periodicrecharge with a more primitive high-MgO basalt is also an importantprocess contributing to the chemical evolution of the magmas.Hybrid basalts are the mixed product of high-Al2O3 basalt rechargedwith high-MgO basalt. The lavas range in composition from high-MgO, low-Al2O3 ( 12wt. % MgO, 14 wt. % Al2O3) to high-Al2O3, low-MgO ( 18 wt. %Al2O3, 4 wt. % MgO). The high-MgO lavas are characterized byphenocrysts of olivine (cores FO90–80 and rims FO85–75)with chromite inclusions [Cr/(Cr + Al)0.7], clinopyroxene (Wo46–42En48–42Fs15–7),and the rare occurrence of orthopyroxene (En72–70). Allthe phenocrysts are normally zoned and set in a groundmass ofplagioclase, pigeonite, clinopyroxene, magnetite, orthopyroxene.The high-Al2O3 basalts contain plagioclase (An85–55),olivine (Fo80–65), clinopyroxene (Wo45–30En50–38Fs23–11), orthopyroxene (En72–70) phenocrysts, that preserve bothnormal and reverse zoning in a groundmass of plagioclase, pigeonite,olivine, clinopyroxene, magnetite, orthopyroxene. Hybrid basaltshave characteristics of both high-MgO basalts and high-Al2O3basalts and preserve complicated normal-to-reverse, reverse-to-normal,and normally zoned phenocrysts. No hydrous minerals are presentin any of the lavas. The varied basaltic magmas erupted from Klyuchevskoy are derivedfrom a magma chamber(s) located near the base of the Kamchatkacrust (pressures 0.5–0.9 GPa) and characterized by relativelyhigh crystallization temperatures, some in excess of 1150C.Under these conditions, the fractionation of a parental high-MgOmagma, produced principally from the melting of a fluid-fluxed,peridotitic mantle wedge, results in the production of a chemicallydiverse spectrum of basalts ranging from high-MgO, low-Al2O3to high-Al2O3, low-MgO basalt, traversing the relatively primitiveend of both the calc-alkalic and tholeiitic differentiationtrends.  相似文献   

18.
The major element composition of plagioclase, pyroxene, olivine,and magnetite, and whole-rock 87Sr/86Sr data are presented forthe uppermost 2·1 km of the layered mafic rocks (upperMain Zone and Upper Zone) at Bierkraal in the western BushveldComplex. Initial 87Sr/86Sr ratios are near-constant (0·7073± 0·0001) for 24 samples and imply crystallizationfrom a homogeneous magma sheet without major magma rechargeor assimilation. The 2125 m thick section investigated in drillcore comprises 26 magnetitite and six nelsonite (magnetite–ilmenite–apatite)layers and changes up-section from gabbronorite (An72 plagioclase;Mg# 74 clinopyroxene) to magnetite–ilmenite–apatite–fayaliteferrodiorite (An43; Mg# 5 clinopyroxene; Fo1 olivine). The overallfractionation trend is, however, interrupted by reversals characterizedby higher An% of plagioclase, higher Mg# of pyroxene and olivine,and higher V2O5 of magnetite. In the upper half of the successionthere is also the intermittent presence of cumulus olivine andapatite. These reversals in normal fractionation trends definethe bases of at least nine major cycles. We have calculateda plausible composition for the magma from which this entiresuccession formed. Forward fractional crystallization modelingof this composition predicts an initial increase in total iron,near-constant SiO2 and an increasing density of the residualmagma before magnetite crystallizes. After magnetite beginsto crystallize the residual magma shows a near-constant totaliron, an increase in SiO2 and decrease in density. We explainthe observed cyclicity by bottom crystallization. Initiallymagma stratification developed during crystallization of thebasal gabbronorites. Once magnetite began to crystallize, periodicdensity inversion led to mixing with the overlying magma layer,producing mineralogical breaks between fractionation cycles.The magnetitite and nelsonite layers mainly occur within fractionationcycles, not at their bases. In at least two cases, crystallizationof thick magnetitite layers may have lowered the density ofthe basal layer of melt dramatically, and triggered the proposeddensity inversion, resulting in close, but not perfect, coincidenceof mineralogical breaks and packages of magnetitite layers. KEY WORDS: layered intrusion; mineral chemistry; isotopes; magma; convection; differentiation  相似文献   

19.
Suprasolidus phase relations at pressures from 8 to 30 kb andtemperatures from 950 to 1380C have been determined experimentallyfor a glassy armalcolite–phlogopite lamproite from thechilled margin of a medium–grained lamproite from SmokyButte, Montana: The armalcolite-phlogopite lamproite has microphenocrystsof olivine in a groundmass of phlogopite, sanidine, armalcolite,clinopyroxene, chromite, priderite, apatite, and abundant glass.The lamproite is SiO2-rich and has high F/H2O relative to lamproitesthat have been investigated in previous experimental studies.Our data show that with decreasing temperature from the liquidusat pressures above 12 kb, melt coexists successively with:olivine; orthopyroxene + clinopyroxene; orthopyroxene + clinopyroxene+ phlogopite; clinopyroxene +phlogopite; and clinopyroxene +orthopyroxene + K-richterite. Below 12 kb, the assemblage successionis: olivine; olivine + clinopyroxene; olivine + clinopyroxene+ phlogopite; and olivine +clinopyroxene + phlogopite + armalcolite.The main difference from the natural paragenesis is that therock does not contain any orthopyroxene—a feature thatis rather remarkable inasmuch as it has 16% normative hypersthene—andthe rock differs also in that it contains sanidine and priderite.In the experiments, sanidine is observed only as ghostlike domainsin some of the glass and appears to have formed during quenching. The solid phases crystallized experimentally are generally compositionallysimilar to the minerals in the rock. These similarities andthe experimental phase relations support the concept of a rapidinitial magma ascent with only a small temperature drop andcrystallization of olivine, but not of orthopyroxene. At lowerpressures, less than 12 kb, it appears that the magma ascendedmore slowly with a larger temperature drop suggested by thesimilarity of the experimentally determined sequence of assemblagesto the paragenesis of the rock. No quasi-invariant multiphase-saturation point was found suchas might be indicative of pressure and temperature conditionsfor formation of the lamproite magma by eutectic-type partialmelting of a mantle source. The occurrence of olivine, orthopyroxene,and clinopyroxene near the liquidus, and the high proportionof normative hypersthene in the melt suggest that lherzoliteor harzburgite was probable in the magma source rock. The highSiO2 and MgO contents of the Smoky Butte lamproites may indicatethat orthopyroxene was a source mineral even though it did notcrystallize under near-surface conditions. The curve definingthe appearance of phlogopite appears at progressively lowertemperatures from the liquidus as pressure increases, so itwould appear that either phlogopite was not the mantle K-reservoir,or it was entirely consumed during the partial melting process.The composition of the near-liquidus glass in the experimentsis likely to be the composition of the bulk rock less the verysmall amounts of olivine + clinopyroxene + orthopyroxene crystallizedwithin a few degrees below the liquidus. From the inferred compositionof this glass, anhydrous phlogopite is a potential mineral.The principal variable that determines whether phlogopite crystallizesas a near-liquidus mineral is F/H2O; low values of this ratiopromote the presence of phlogopite as a near-liquidus mineralwhereas high values deter its crystallization. The common practiceof adding H2O but not F in experiments to compensate for degassingmay obscure the role of phlogopite in the evolution of lamproitemagmas.  相似文献   

20.
GREEN  D. H. 《Journal of Petrology》1964,5(1):134-188
The Lizard peridotite produced a high-temperature metamorphicaureole during diapirie emplacement in a period of amphibolitefacies regional metamorphism. The peridotite preserves a coarse-grainedprimary core within a cataclastic, finely foliated and recrystallizedmarginal shell. Chemical analyses demonstrate the constancyof rock composition in the primary and recrystallized mineralassemblages. The primary mineral assemblage of the peridotiteconsists of olivine (Fo89), aluminous enstatite, aluminous diopside,and green aluminous spinel. In contrast the mineral assemblageon recrystallization is olivine, enstatite (normal Al2O3 content),diopside (normal Al2O3 content), plagioclase, and brown chromite.A third assemblage of olivine, pargasite, and brown chromianspinel is developed locally from the plagioclase-bearing assemblage,particularly in contact areas. Major and trace element analyses of twenty minerals and eightrocks are given. These are compared in detail with peridotitesand minerals occurring as accumulates from basaltic magma, asperidotite nodules in basalts and as ’intrusive' peridotitesin orogenic areas. It is concluded that the primary mineralsof the Lizard peridotite have not crystallized and accumulatedfrom a basaltic magma but have crystallized in a similar environmentto that of peridotite nodules in basalts. It is further concludedthat the cause of the differences between the primary and recrystallizedassemblages of the peridotite is primarily a difference in loadpressure at crystallization. In particular the high aluminacontent of both enstatite and diopside and the coexistence ofpyroxenes+ aluminous spinel instead of olivine+anorthite areconsidered to be a direct consequence of the high load pressureat the initial crystallization of the peridotite.  相似文献   

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