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1.
McMURRY  J. 《Journal of Petrology》2001,42(2):251-276
The Bodocó pluton, typical of numerous felsic intrusionsin northeastern Brazil that are characterized by blocky megacrystsof K-feldspar, consists mainly of porphyritic coarse-grainedquartz monzonite (SiO2 58–70 wt %) and is reversely zonedfrom a granitic margin to a quartz monzodioritic core. Thereis little variation in mineral composition throughout the pluton,despite a range of variation in mineral proportions. Isotopiccharacteristics also are homogeneous, with 18Oquartz between+9·3 and +9·8 and initial 87Sr/86Sr within limitsof 0·7056–0·7063. Petrogenetic modellingindicates that in situ crystal accumulation processes, accompaniedby the upward migration of a crystal-poor felsic melt, can accountfor many of the observed chemical and isotopic features, petrographictextures, and spatial relationships of rock types. Localizedshearing associated with regional ductile deformation producedextensive kilometre-wide bands of strongly foliated megacrysticquartz monzonite intruded by mafic dykes. Shear-related magmamingling and/or mixing were localized post-emplacement differentiationprocesses, particularly at the upper level of the intrusionand in quartz monzonite border units along the southeast margin. KEY WORDS: accumulation; Brazil; megacryst; petrogenesis; shearing  相似文献   

2.
The Cretaceous lava sequence and associated mafic dyke swarmin central–western Madagascar (Mailaka and Bemaraha areas)range in composition from picrite basalts to cordierite–orthopyroxene-bearingrhyodacites (MgO from 14 to 0·6 wt %). Petrographic andchemical data indicate the presence of both tholeiitic and transitionalmagma series, with variable degree of rare earth element enrichment[(La/Nd)n = 1–1·4 for tholeiites vs (La/Nd)n =0·65–1 for transitional rocks]. Initial (at 88Ma) 87Sr/86Sr and  相似文献   

3.
The Ashland pluton is a calc-alkaline plutonic complex thatintruded the western Paleozoic and Triassic belt of the KlamathMountains in late Middle Jurassic time. The pluton comprisesa series of compositionally distinct magma pulses. The oldestrocks are hornblende gabbro and two-pyroxene quartz gabbro withinitial 87Sr/86Sr = 0{dot}7044, 18O = 8{dot}7%, and REE patternswith chondrite normalized La/Lu = 7. These units were followedby a suite of tonalitic rocks (LaN/LuN = 7) and then by a suiteof K2O- and P2O5 rocks of quartz monzodioritic affinity (LaN/LuN= 13–21; LaN/SmN = 2{dot}4–3{dot}) The quartz monzodioriticrocks were then intruded by biotite granodiorite and granitewith lower REE abundances but more fractionated LREE(LaN/LuN= 13–19; LaN/SmN = 4{dot}3–6 and they, in turn,were host to dikes and bosses of hornblende diorite. The latestintrusive activity consisted of aplitic and granitic dikes.Combined phase equilibria and mineral composition data, indicateemplacement conditions of approximately Ptotal = 2{dot}3kb,PH2O between 1{dot}5 and 2{dot}2 kb, and fO2 between the nickel-nickeloxide and hematite-magnetite buffers. Successive pulses of magma display increasing SiO2 togetherwith increasing 18O and decreasing initial 87Sr/86Sr. The isotopicdata are consistent with either (1) combined fractional crystallizationof andesitic magma and concurrent assimilation of crustal materialcharacterized by low Sr1 and high (18O or, more probably, (2)a series of partial melting events in which sources were successivelyless radiogenic but richer in 18O Each intrusive stage displaysevidence for some degree of crystal accumulation and/or fractionalcrystallization but neither process adequately accounts fortheir compositional differences. Consequently, each stage appearsto represent a distinct partial melting or assimilation event. The P2O5-rich nature of the quartz monzodiorite suite suggestsaccumulation of apatite. However, the suite contains abundantmafic microgranitoid enclaves and most apatite in the suiteis acicular. These observations suggest that magma mixing affectedthe compositional variation of the quartz monzodiorite suite.Mass balance calculations are consistent with a simple mixingprocess in which P2O5-rich alkalic basalt magma (representedby the mafic microgranitoid enclaves) was combined with a crystal-poorfelsic magma (represented by the tonalite suite), yielding aquartz monzodioritic magma that then underwent differentiationby crystal fractionation and accumulation.  相似文献   

4.
The Baikal Rift is a zone of active lithospheric extension adjacentto the Siberian Craton. The 6–16 Myr old Vitim VolcanicField (VVF) lies approximately 200 km east of the rift axisand consists of 5000 km3 of melanephelinites, basanites, alkaliand tholeiitic basalts, and minor nephelinites. In the volcanicpile, 142 drill core samples were used to study temporal andspatial variations. Variations in major element abundances (e.g.MgO = 3·3–14·6 wt %) reflect polybaric fractionalcrystallization of olivine, clinopyroxene and plagioclase. 87Sr/86Sri(0·7039–0·7049), 143Nd/144Ndi (0·5127–0·5129)and 176Hf/177Hfi (0·2829–0·2830) ratiosare similar to those for ocean island basalts and suggest thatthe magmas have not assimilated significant amounts of continentalcrust. Variable degrees of partial melting appear to be responsiblefor differences in Na2O, P2O5, K2O and incompatible trace elementabundances in the most primitive (high-MgO) magmas. Fractionatedheavy rare earth element (HREE) ratios (e.g. [Gd/Lu]n > 2·5)indicate that the parental magmas of the Vitim lavas were predominantlygenerated within the garnet stability field. Forward major elementand REE inversion models suggest that the tholeiitic and alkalibasalts were generated by decompression melting of a fertileperidotite source within the convecting mantle beneath Vitim.Ba/Sr ratios and negative K anomalies in normalized multi-elementplots suggest that phlogopite was a residual mantle phase duringthe genesis of the nephelinites and basanites. Relatively highlight REE (LREE) abundances in the silica-undersaturated meltsrequire a metasomatically enriched lithospheric mantle source.Results of forward major element modelling suggest that meltingof phlogopite-bearing pyroxenite veins could explain the majorelement composition of these melts. In support of this, pyroxenitexenoliths have been found in the VVF. High Cenozoic mantle potentialtemperatures (1450°C) predicted from geochemical modellingsuggest the presence of a mantle plume beneath the Baikal RiftZone. KEY WORDS: Baikal Rift; mafic magmatism; mantle plume; metasomatism; partial melting  相似文献   

5.
The caldera-forming 26·5 ka Oruanui eruption (Taupo,New Zealand) erupted 530 km3 of magma, >99% rhyolitic, <1%mafic. The rhyolite varies from 71·8 to 76·7 wt% SiO2 and 76 to 112 ppm Rb but is dominantly 74–76 wt% SiO2. Average rhyolite compositions at each stratigraphiclevel do not change significantly through the eruption sequence.Oxide geothermometry, phase equilibria and volatile contentsimply magma storage at 830–760°C, and 100–200MPa. Most rhyolite compositional variations are explicable by28% crystal fractionation involving the phenocryst and accessoryphases (plagioclase, orthopyroxene, hornblende, quartz, magnetite,ilmenite, apatite and zircon). However, scatter in some elementconcentrations and 87Sr/86Sr ratios, and the presence of non-equilibriumcrystal compositions imply that mixing of liquids, phenocrystsand inherited crystals was also important in assembling thecompositional spectrum of rhyolite. Mafic compositions comprisea tholeiitic group (52·3–63·3 wt % SiO2)formed by fractionation and crustal contamination of a contaminatedtholeiitic basalt, and a calc-alkaline group (56·7–60·5wt % SiO2) formed by mixing of a primitive olivine–plagioclasebasalt with rhyolitic and tholeiitic mafic magmas. Both maficgroups are distinct from other Taupo Volcanic Zone eruptivesof comparable SiO2 content. Development and destruction by eruptionof the Oruanui magma body occurred within 40 kyr and Oruanuicompositions have not been replicated in vigorous younger activity.The Oruanui rhyolite did not form in a single stage of evolutionfrom a more primitive forerunner but by rapid rejuvenation ofa longer-lived polygenetic, multi-age ‘stockpile’of silicic plutonic components in the Taupo magmatic system. KEY WORDS: Taupo Volcanic Zone; Taupo volcano; Oruanui eruption; rhyolite, zoned magma chamber; juvenile mafic compositions; eruption withdrawal systematics  相似文献   

6.
Within the Zitácuaro–Valle de Bravo (ZVB) regionof the central Mexican Volcanic Belt (MVB), three lava serieshave erupted during the Quaternary: (1) high-K2O basaltic andesitesand andesites; (2) medium-K2O basaltic andesites, andesitesand dacites; (3) high-TiO2 basalts and basaltic andesites. Thedominant feature of the first two groups is the lack of plagioclaseaccompanying the various ferromagnesian phenocrysts (olivine,orthopyroxene, augite, and hornblende) in all but the dacites.This absence of plagioclase in the phenocryst assemblages ofthe high-K2O and medium-K2O intermediate lavas is significantbecause it indicates high water contents during the stage ofphenocryst equilibration. In contrast, the high-TiO2 group ischaracterized by phenocrysts of plagioclase and olivine. Thespatial distribution of these three lava series is systematic.The southern section of the ZVB transect, 280–330 km fromthe Middle America Trench (MAT), is characterized by high-K2Omelts that are relatively enriched in fluid-mobile elementsand have the highest 87Sr/86Sr ratios. Medium-K2O basaltic andesiteand andesite lavas are present throughout the transect, butthose closest to the MAT are MgO-rich (3·5–9·4wt %) and have phenocryst assemblages indicative of high magmaticwater contents (3·5–6·5 wt % water) andrelatively low temperatures (950–1000°C). In markedcontrast, the northern section of the ZVB transect (380–480km from the MAT) has high-TiO2, high field strength element(HFSE)-enriched magmas that have comparatively dry (< 1·5wt % magmatic water) and hot (1100–1200°C) phenocrystequilibration conditions. The central section of the ZVB transect(330–380 km from the MAT) is a transition zone and producesmoderately light rare earth element (LREE) and large ion lithophileelement (LILE)-enriched, medium-K2O lavas with phenocryst assemblagesindicative of intermediate (1·5–3·5 wt %)water contents and temperatures. The high-K2O series compositionsare the most enriched in LILE and LREE, with a narrow rangeof radiogenic 87Sr/86Sr from 0·704245 to 0·704507,143Nd/144Nd values ranging from 0·512857 to 0·512927(Nd = 4·27–5·63), and 208Pb/204Pb valuesfrom 38·248 to 38·442, 207Pb/204Pb values from15·563 to 15·585, and 206Pb/204Pb values from18·598 to 18·688. The medium-K2O series compositionsare only moderately enriched in the LILE and LREE, with a broaderrange of 87Sr/86Sr, but similar 143Nd/144Nd and 208Pb/204Pbvalues to those of the high-K2O series. In contrast, the high-TiO2series compositions have little enrichment in LILE or LREE andinstead are enriched in the HFSE and heavy rare earth elements(HREE). The high-TiO2 lavas are isotopically distinct in theirlower and narrower range of 143Nd/144Nd. The isotopic variationsare believed to reflect the upper mantle magma source regionsas the low content of phenocrysts in most lavas precludes significantupper crustal assimilation or magma mixing, other than thatrepresented by the presence of quartz xenocrysts (< 2 vol.%) with rhyolitic glass inclusions, which are found in manyof these lavas. The systematic spatial variation in compositionof the three lava series is a reflection of the underlying subduction-modifiedmantle and its evolution. KEY WORDS: central Mexico; geochemistry; isotopes; Quaternary volcanism; hydrous lavas  相似文献   

7.
Kistufell: Primitive Melt from the Iceland Mantle Plume   总被引：5，自引：2，他引：5  
This paper presents new geochemical data from Kistufell (64°48'N,17°13'W), a monogenetic table mountain situated directlyabove the inferred locus of the Iceland mantle plume. Kistufellis composed of the most primitive olivine tholeiitic glassesfound in central Iceland (MgO 10·56 wt %, olivine Fo89·7).The glasses are interpreted as near-primary, high-degree plumemelts derived from a heterogeneous mantle source. Mineral, glassand bulk-rock (glass + minerals) chemistry indicates a low averagemelting pressure (15 kbar), high initial crystallization pressuresand temperatures (10–15 kbar and 1270°C), and eruptiontemperatures (1240°C) that are among the highest observedin Iceland. The glasses have trace element signatures (Lan/Ybn<1, Ban/Zrn 0·55–0·58) indicative ofa trace element depleted source, and the Sr–Nd–Pbisotopic ratios (87Sr/86Sr 0·70304–0·70308,143Nd/144Nd 0·513058–0·513099, 206Pb/204Pb18·343–18·361) further suggest a long-termtrace element depletion relative to primordial mantle. HighHe isotopic ratios (15·3–16·8 R/Ra) combinedwith low 207Pb/204Pb (15·42–15·43) suggestthat the mantle source of the magma is different from that ofNorth Atlantic mid-ocean ridge basalt. Negative Pb anomalies,and positive Nb and Ta anomalies indicate that the source includesa recycled, subducted oceanic crustal or mantle component. PositiveSr anomalies (Srn/Ndn = 1·39–1·50) furthersuggest that this recycled source component involves lower oceaniccrustal gabbros. The  相似文献   

8.
Major and trace element, Sr–Nd–Pb isotope and mineralchemical data are presented for newly discovered ultrapotassiclavas in the Tangra Yumco–Xuruco graben in southern Tibet.The ultrapotassic lavas are characterized by high MgO, K2O andTiO2, low Al2O3 and Na2O contents, and also have high molarK2O/Al2O3, molar (K2O + Na2O)/Al2O3 and K2O/Na2O ratios. Theirhigh abundances of incompatible trace elements such as largeion lithophile elements (LILE) and light rare earth elements(LREE) reach the extreme levels typical of lamproites. The lamproitesshow highly radiogenic 87Sr/86Sr (0· 7166–0·7363) and unradiogenic 143Nd/144Nd (0· 511796–0·511962), low 206Pb/204Pb (18· 459–18· 931),and elevated radiogenic 207Pb/204Pb (15· 6732–15·841) and 208Pb/204Pb (39· 557–40· 058) ratios.On the basis of their geochemical and isotopic systematics,the lamproites in south Tibet have a distinct magma source thatcan be differentiated from the sources of potassic lavas inthe east Lhasa and Qiangtang blocks. Their high Nb/Ta ratios(17· 10–19· 84), extremely high Th/U ratios(5· 70–13· 74) and distinctive isotope compositionsare compatible with a veined mantle source consisting of partialmelts of subducted Tethyan oceanic sediments and sub-continentallithospheric depleted mantle. Identification of the lamproitesand the delineation of their mantle source provide new evidencerelevant for models of the uplift and extension of the Tibetanplateau following the Indo-Asia collision. Metasomatism by partialmelts from isotopically evolved, old sediment subducted on theyoung Tethyan slab is an alternative explanation for PrecambrianNd and Pb model ages. In this model, differences in isotopiccomposition along-strike are attributed to differences in thetype of sediment being subducted, thus obviating the need formultiple metasomatic events over hundreds of million years.The distribution of lamproites, restricted within a north–south-trendinggraben, indicates that the initiation of east–west extensionin south Tibet started at 25 Ma. KEY WORDS: lamproites; subducted oceanic sediment; Tibetan active continental collision belt  相似文献   

9.
Pelitic xenoliths derived from amphibolite grade basement rocksoccur within a Pleistocene, trachytic, pyroclastic unit of theWehr volcano, East Eifel, West Germany: With increasing temperatureand/or prolonged heating at high temperature, quartz-plagioclaseand micaceous layers of the xenoliths have undergone meltingto form buchites and thermal reconstitution by dehydration reactions,melting and crystallization to form restites respectively. Thexenoliths provide detailed evidence of melting, high temperaturedecomposition of minerals, nucleation and growth of new phasesand P-T-fo2 conditions of contact metamorphism of basement rocksby the Wehr magma. Melting begins at quartz-oligoclase (An17·3Ab82·3Or0·4-An20·0Ab78·1Or1·9)grain boundaries in quartz-plagioclase rich layers and the amountof melting is controlled by H2O and alkalis released duringdehydroxylation/oxidation of associated micas. Initially, glasscompositions are heterogeneous, but with increasing degreesof melting they become more homogeneous and are similar to S-typegranitic minimum melts with SiO2 between 71 and 77 wt. per cent;A/(CNK) ratios of 1·2–1·4; Na2O < 2·95and normative corundum contents of 1·9–4·0per cent. Near micas plagioclase melts by preferential dissolutionof the NaAlSi3O8 component accompanied by a simultaneous increasein CaAl2Si2O8 (up to 20 mol. per cent An higher than the bulkplagioclase composition) at the melting edge. With increasingtemperature the end product of fractional melting is the formationand persistence of refractory bytownite (An78–80) in thosexenoliths where extensive melting has taken place. Initial stage decomposition of muscovite involves dehydroxylation(H2O and alkali loss). At higher temperatures muscovite breaksdown to mullite, sillimanite, corundum, sanidine and a peraluminousmelt. Mullite (40–43 mol. per cent SiO2) and sillimanite(49 mol. per cent SiO2) are Fe2O3 and TiO2 rich (up to 6·1–0·84and 3·6–0·24 wt. per cent respectively).Al-rich mullite (up to 77 wt. per cent Al2O3) occurs with corundumwhich has high Fe2O3 and TiO2 (up to 6·9 and 2·1wt. per cent respectively). Annealing at high temperatures andreducing conditions results in the exsolution of mullite fromsillimanite and ilmenite from corundum. Glass resulting fromthe melting of muscovite in the presence of quartz is peraluminous(A/(CNK) = 1·3) with SiO2 contents of 66–69 percent and normative corundum of 4 per cent. Sanidine (An1·9Ab26·0Or72·1-An1·3Ab15·9Or82·9)crystallized from the melt. Dehydroxylation and oxidation of biotite results in a decreaseof K2O from 8·6 to less than 1 wt. per cent and oxidetotals (less H2O + contents) from 96·5 to 88·6,exsolution of Al-magnetite, and a decrease in the Fe/(Fe + Mg)ratio from 0·41 to 0·17. Partial melting of biotitein the presence of quartz/plagioclase to pleonaste, Al-Ti magnetite,sanidine(An2·0Ab34·9Or63·1) and melt takesplace at higher temperatures. Glass in the vicinity of meltedbiotite is pale brown and highly peraluminous (A/CNK = 2·1)with up to 6 wt. per cent MgO+FeO(total iroq) and up to 10 percent normative corundum. Near liquidus biotite with higher Al2O3and TiO2 than partially melted biotite crystallized from themelt. Ti-rich biotites (up to 6 wt. per cent TiO2) occur withinthe restite layers of thermally reconstituted xenoliths. Meltingof Ti-rich biotite and sillimanite in contact with the siliceousmelt of the buchite parts of xenoliths resulted in the formationof cordierite (100 Mg/(Mg+Fe+Mn) = 76·5–69·4),Al-Ti magnetite and sanidine, and development of cordierite/quartzintergrowths into the buchite melt. Growth of sanidine enclosedrelic Ca-plagioclase to form patchy intergrowths in the restitelayers. Cordierite (100 Mg/(Mg+Fe+Mn) = 64–69), quartz,sillimanite, mullite, magnetite and ilmenite, crystallized fromthe peraluminous buchite melt. Green-brown spinels of the pleonaste-magnetite series have awide compositional variation of (mol. per cent) FeAl2O4—66·6–45·0;MgAl2O4—53·0–18·7; Fe3O4—6·9–28·1;MnAl2O4—1·2–1·5; Fe2TiO4—0·6–6·2.Rims are generally enriched in the Fe3O4 component as a resultof oxidation. Compositions of ilmenite and magnetite (single,homogeneous and composite grains) are highly variable and resultfrom varying degrees of high temperature oxidation that is associatedwith dehydroxylation of micas and melting. Oxidation mainlyresults in increasing Fe3+, Al and decreasing Ti4+, Fe2+ inilmenite, and increasing Fe2+, Ti4+ and decreasing Fe3+ in associatedmagnetite. A higher degree of oxidation is reached with exsolutionof rutile from ilmenite and formation of titanhematite and withexsolution of pleonaste from magnetite. Ti-Al rich magnetite(5·1–7·5 and 8·5–13·5wt. per cent respectively) and ilmenite crystallized from meltsin buchitic parts of the xenoliths. Chemical and mineralogic evidence indicates that even with extensivemelting the primary compositions of individual layers in thexenoliths remained unmodified. Apparently the xenoliths didnot remain long enough at high temperatures for desilicationand enrichment in Al2O3, TiO2, FeO, Fe2O3, and MgO that resultsby removal of a ‘granitic’ melt, and/or by interactionwith the magma, to occur. T °C-fo2 values calculated from unoxidized magnetite/ilmenitegive temperatures ranging from 615–710°C for contactmetamorphism and the beginning of melting, and between 873 and1054°C for the crystallization of oxides and mullite/sillimanitefrom high temperature peraluminous melts. fo2 values of metamorphismand melting were between the Ni-NiO and Fe2O3-Fe3O4 buffer curves.The relative abundance of xenolith types, geophysical evidenceand contact metamorphic mineralogy indicates that the xenolithswere derived from depths corresponding to between 2–3kb Pload = Pfluid. The xenoliths were erupted during the latestphreatomagmatic eruption from the Wehr volcano which resultedin vesiculation of melts in partially molten xenoliths causingfragmentation and disorientation of solid restite layers.  相似文献   

10.
We have conducted experiments on dissolution of quartz, albite,orthoclase, and corundum into H2O-saturated haplogranite meltat 800°C and 200 MPa over a duration of 120–1488 hwith the aim of ascertaining the diffusive transport propertiesof granitic melts at crustal anatectic temperatures. Cylindersof anhydrous starting glass and a single mineral phase (quartzor feldspar) were juxtaposed along flat and polished surfacesinside gold or platinum capsules with 10 wt % added H2O. Concentrationprofiles in glass (quenched melt) perpendicular to the mineral–glassinterfaces and comparison with relevant phase diagrams suggestthat melts at the interface are saturated in the dissolvingphases after 384 h, and with longer durations the concentrationprofiles are controlled only by diffusion of components in themelt. The evolution of the concentration profiles with timeindicates that uncoupled diffusion in the melt takes place alongthe following four linearly independent directions in oxidecomposition space: SiO2, Na2O, and K2O axes (Si-, Na-, and K-eigenvectors,respectively), and a direction between the Al2O3, Na2O, andK2O axes (Al-eigenvector), such that the Al/Na molar ratio isequal to that of the bulk melt and the Al/(Na + K) molar ratiois equal to the equilibrium ASI (= mol. Al2O3/[Na2O + K2O])of the melt. Experiments in which a glass cylinder was sandwichedbetween two mineral cylinders—quartz and albite, quartzand K-feldspar, or albite and corundum—tested the validityof the inferred directions of uncoupled diffusion and exploredlong-range chemical communication in the melt via chemical potentialgradients. The application of available solutions to the diffusionequations for the experimental quartz and feldspar dissolutiondata provides diffusivities along the directions of the Si-eigenvectorand Al-eigenvector of (2·0–2·8) x 10–15m2/s and (0·6–2·4) x 10–14 m2/s, respectively.Minimum diffusivities of alkalis [(3–9) x 10–11m2/s] are orders of magnitude greater than the tetrahedral componentsof the melt. The information provided here determines the rateat which crustal anatexis can occur when sufficient heat issupplied and diffusion is the only mass transport (mixing) processin the melt. The calculated diffusivities imply that a quartzo-feldspathicsource rock with initial grain size of 2–3 mm undergoinghydrostatic, H2O-saturated melting at 800°C (infinite heatsupply) could produce 20–30 vol. % of homogeneous meltin less than 1–10 years. Slower diffusion in H2O-undersaturatedmelts will increase this time frame. KEY WORDS: chemical diffusion; haplogranite; mineral dissolution experiments; crustal anatexis  相似文献   

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