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1.
Petrogenesis of the Zoned Laacher See Tephra 总被引：2，自引：2，他引：2
The late Quaternary Laacher See phonolitic tephra deposit (EastEifel, W. Germany) is mineral-ogically and chemically zonedfrom highly evolved, volatile-rich and crystal-poor at its basetowards a mafic, crystal-rich phonolite at the top (Wörner& Schmincke, 1984). This zonation is interpreted as theresult of a continuous eruption from a zoned magma column. Majorand trace element evidence shows that the last erupted maficULST (Upper Laacher See Tephra) phonolite can be derived froma basanite parent magma via fractional crystallization of 30per cent clinopyroxene, 24 per cent amphibole, 4 per cent phlogopite,3.8 per cent magnetite, 2.5–3.0 per cent olivine and 1per cent apatite, leaving a derivative of 30 per cent evolvedmagma. Starting from the mafic (ULST) phonolite as a parent, the zonedsequence is postulated to have been formed by progressive fractionalcrystallization of the observed phenocryst phases. This modelwas tested by a series of 7 step-by-step mass balance fractionationcalculations. Abundance, modal composition and relative variationsof calculated fractionated phases agree well with the observedphenocryst abundances: sanidine followed by plagioclase andminor amounts of mafic phases are to be fractionated to givethe observed zoned sequence. The most evolved phonolite, however, cannot be generated bysubtraction of phenocrysts from the underlying phonolite. Processessuch as liquid-state differentiation may therefore have chemicallymodified the upper part (cupola) of the Laacher See magma columnsubsequent to crystal fractionation. The erupted phonolite magma (5.3 km3) was calculated to havestarted with a volume of 56 km3 of parental basanite magma whichfractionated to form 16.6 km3 of mafic phonolite. This magmafurther differentiated to give a 5.3 km3 zoned (erupted) phonolitecolumn. The non-erupted volume of 50 km3 is postulated to forma cooling cumulate body below the present day Laacher See volcano. The Laacher See magma system represents a complex end-membertype of a highly evolved small volume composition ally zonedmagma chamber with steep major and trace element gradients,the uppermost volatile rich magma layer resembling the stableroof part of rhyolitic chambers. 相似文献
2.
Mineralogical and Chemical Zonation of the Laacher See Tephra Sequence (East Eifel, W. Germany) 总被引：3，自引：1，他引：3
The late Quaternary Laacher See Tephra (LLST, MLST, ULST: Lower,Middle, Upper Laacher See Tephra) of the East Eifel volcanicfield (W. Germany) is zoned from highly evolved, volatile-richand crystal-poor phonolite at its base towards a mafic, crystal-richphonolite at the top of the deposit. This is shown by phenocrystabundances, major and trace element chemistry of whole rocks,matrix-glass and some mineral compositions. MgO content in wholerocks ranges from 0.07 wt. per cent in lower LLST to 0.85 wt.per cent in ULST phonolite. Late mafic hybrid pyroclasts containup to 7.0 wt. per cent MgO. Na2O shows a reverse trend from11.7 wt. per cent in LLST to 5.1 per cent in ULST. Trace elementsare divided into depleted (compatible) elements (e.g. Sr, Sc,Co), and enriched (incompatible) elements (e.g. Zn, Zr, Nb).‘Semi-compatible’ elements (Ta, Y) show minimumconcentrations at an intermediate stratigraphic level (MLST).All compositional gradients are smooth showing a major compositionalinterface between LLST/MLST and ULST. Twelve phenocryst phasesoccur: sanidine, plagioclase, hauyne, amphibole, clinopyroxene,sphene, apatite, Ti-magnetite, biotite, nepheline, cancriniteand zircon. The latter three are restricted to LLST phonolite.There is a steep gradient in mineral abundances from the LLSTand MLST to the mafic ULST phonolite. Microprobe analyses definetwo phenocryst populations: (a) Fe, Na and Mn-rich evolved phenocrysts(hastingsite, aegirine-augite, Ab-rich plagioclase) which predominatewithin highly differentiated (LLST) phonolite; (b) Mg and Ti-richand Fe, Na, Mn-poor phenocrysts, which are most abundant inULST. This compositional zonation is interpreted as the resultof continuous eruption from a zoned magma column; highly evolvedearly magma representing the upper part and mafic late phonoliticmagma representing the lower part of a stratified magma chamberemplaced at shallow, crustal levels. 相似文献
3.
Archean volcanic rocks in the Confederation Lake area, northwestern Ontario, Canada, are in three mafic to felsic cycles collectively 8,500 to 11,240 m thick. Each cycle begins with pillowed basalt and andesite flows and is capped with andesitic to rhyolitic pyroclastic rocks and minor flows. Seventy five samples from this succession were analyzed for major and trace elements including the rare earth elements. In two cycles, tholeiitic basalts are overlain by calcalkaline andesite to rhyolite. In the third, cycle, the tholeiitic basalts are overlain by tholeiitic rhyolites. Fe enrichment in basalts is accompanied by depletion of Ca, Al, Cr, Ni, and Sr, and enrichment in Ti, P, the rare earth elements, Nb, Zr, and Y. This is interpreted as open system fractionation of olivine, plagioclase, and clinopyroxene. Si enrichment in dacites and rhyolites is attributed to fractional crystallization of plagioclase, K-feldspar, and biotite. Tholeiitic basalt liquids are believed to be mantle-derived. Intercalated andesites with fractionated rare earth patterns appear to be products of mixing of tholeiitic basalt and rhyolite liquids and, andesites with flat rare earth patterns are probably produced by melting of previously depleted mantle. Felsic magmas are partial melts of tholeiitic basalt or products of liquid immiscibility in a tholeiitic system perhaps involving extreme fractionation in a high level magma chamber, and assimilation of sialic crust. It is concluded that Archean cyclical volcanism in this area involves the interplay of several magmatic liquids in processes of fractional crystallization, magma mixing, liquid immiscibility, and the probable existence of compositionally zoned magma chambers in the late stages of each cycle. The compositionally zoned chambers existed over the time period represented by the upper felsic portion of each cycle. 相似文献
4.
Peter J. Michael 《Contributions to Mineralogy and Petrology》1991,108(4):396-418
The Cordillera del Paine pluton in the southernmost Andes of Chile represents a deeply dissected magma chamber where mafic magma intruded into crystallizing granitic magma. Throughout much of the 10x15 km pluton, there is a sharp and continuous boundary at a remarkably constant elevation of 1,100 m that separates granitic rocks (Cordillera del Paine or CP granite: 69–77% SiO2) which make up the upper levels of the pluton from mafic and comingled rocks (Paine Mafic Complex or PMC: 45–60% SiO2) which dominate the lower exposures of the pluton. Chilled, crenulate, disrupted contacts of mafic rock against granite demonstrate that partly crystallized granite was intruded by mafic magma which solidified prior to complete crystallization of the granitic magma. The boundary at 1,100 m was a large and stable density contrast between the denser, hotter mafic magma and cooler granitic magma. The granitic magma was more solidified near the margins of the chamber when mafic intrusion occurred, and the PMC is less disrupted by granites there. Near the pluton margins, the PMC grades upward irregularly from cumulate gabbros to monzodiorites. Mafic magma differentiated largely by fractional crystallization as indicated by the presence of cumulate rocks and by the low levels of compatible elements in most PMC rocks. The compositional gap between the PMC and CP granite indicates that mixing (blending) of granitic magma into the mafic magma was less important, although it is apparent from mineral assemblages in mafic rocks. Granitic magma may have incorporated small amounts of mafic liquid that had evolved to >60% SiO2 by crystallization. Mixing was inhibited by the extent of crystallization of the granite, and by the thermal contrast and the stable density contrast between the magmas. PMC gabbros display disequilibrium mineral assemblages including early formed zoned olivine (with orthopyroxene coronas), clinopyroxene, calcic plagioclase and paragasite and later-formed amphibole, sodic plagioclase, mica and quartz. The early formed gabbroic minerals (and their coronas) are very similar to phenocrysts in late basaltic dikes that cut the upper levels of the CP granite. The inferred parental magmas of both dikes and gabbros were very similar to subalkaline basalts of the Patagonian Plateau that erupted at about the same time, 35 km to the east. Mafic and silicic magmas at Cordillera del Paine are consanguineous, as demonstrated by alkalinity and trace-element ratios. However, the contemporaneity of mafic and silicic magmas precludes a parent-daughter relationship. The granitic magma most likely was derived by differentiation of mafic magmas that were similar to those that later intruded it. Or, the granitic magma may have been contaminated by mafic magmas similar to the PMC magmas before its shallow emplacement. Mixing would be favored at deeper levels when the cooling rate was lower and the granitic magma was less solidified. 相似文献
5.
Magma mixing process is unusual in the petrogenesis of felsic rocks associated with alkaline complex worldwide. Here we present a rare example of magma mixing in syenite from the Yelagiri Alkaline Comp... 相似文献
6.
87Sr/86Sr ratios of Tertiary tholeiitic, basalts alkali olivine basalts and olivine nephelinites from Lower Saxony and Hessia and Quaternary leucite-nepheline tephrites from the Laacher See area are similar to those obtained from Hawaii and range from 0.7031 to 0.7054. Three trachytes and one phonolite from the Westerwald and one phonolite from the Laacher See area have higher values (0.7063 to 0.7093). Three Vesuvian lavas, three Somma lavas and two trachytes of the Phlegraic Fields show substantially higher ratios than the comparable basaltic rocks from N.W. Germany (0.7071 to 0.7102). Three peridotite nodules vary between 0.7048 and 0.7081. Three limestone composites of Paleozoic and Mesozoic age show values between 0.7129 and 0.7174.The 87Sr/86Sr ratios of the trachytes and phonolites from the Westerwald and Laacher See area are probably influenced by crustal material. Assimilation processes of limestones producing the olivine-nephelinites from N.W. Germany seem to be unlikely. A discussion of the origin of the Sr ratios in Vesuvian rocks and of mantle homogeneity is included. 相似文献
7.
The Nimchak granite pluton (NGP) of Chotanagpur Granite Gneiss Complex (CGGC), Eastern India, provides ample evidence of magma interaction in a plutonic regime for the first time in this part of the Indian shield. A number of outcrop level magmatic structures reported from many mafic-felsic mixing and mingling zones worldwide, such as synplutonic dykes, mafic magmatic enclaves and hybrid rocks extensively occur in our study domain. From field observations it appears that the Nimchak pluton was a vertically zoned magma chamber that was intruded by a number of mafic dykes during the whole crystallization history of the magma chamber leading to magma mixing and mingling scenario. The lower part of the pluton is occupied by coarse-grained granodiorite (64.84–66.61?wt.% SiO2), while the upper part is occupied by fine-grained granite (69.80–70.57?wt.% SiO2). Field relationships along with textural and geochemical signatures of the pluton suggest that it is a well-exposed felsic magma chamber that was zoned due to fractional crystallization. The intruding mafic magma interacted differently with the upper and lower granitoids. The lower granodiorite is characterized by mafic feeder dykes and larger mafic magmatic enclaves, whereas the enclaves occurring in the upper granite are comparatively smaller and the feeder dykes could not be traced here, except two late-stage mafic dykes. The mafic enclaves occurring in the upper granite show higher degrees of hybridization with respect to those occurring in the lower granite. Furthermore, enclaves are widely distributed in the upper granite, whereas enclaves in the lower granite occur adjacent to the main feeder dykes.Geochemical signatures confirm that the intermediate rocks occurring in the Nimchak pluton are mixing products formed due to the mixing of mafic and felsic magmas. A number of important physical properties of magmas like temperature, viscosity, glass transition temperature and fragility have been used in magma mixing models to evaluate the process of magma mixing. A geodynamic model of pluton construction and evolution is presented that shows episodic replenishments of mafic magma into the crystallizing felsic magma chamber from below. Data are consistent with a model whereby mafic magma ponded at the crust-mantle boundary and melted the overlying crust to form felsic (granitic) magma. The mafic magma episodically rose, injected and interacted with an overlying felsic magma chamber that was undergoing fractional crystallization forming hybrid intermediate rocks. The intrusion of mafic magma continued after complete solidification of the magma chamber as indicated by the presence of two late-stage mafic dykes. 相似文献
8.
《地学前缘(英文版)》2015,6(4)
The origin and petrogenesis of the Cameroon Volcanic Line(CVL),composed of volcanoes that form on both the ocean floor and the continental crust,are difficult to understand because of the diversity,heterogeneity,and nature of available data.Major and trace elements,and Sr-Nd-Pb isotope data of volcanic rocks of the CVL spanning four decades have been compiled to reinterpret their origin and petrogenesis.Volcanic rocks range from nephelinite,basanite and alkali basalts to phonolite,trachyte and rhyolite with the presence of a compositional gap between Si O258e64 wt.%.Similarities in geochemical characteristics,modeled results for two component mixing,and the existence of mantle xenoliths in most mafic rocks argue against significant crustal contamination.Major and trace element evidences indicate that the melting of mantle rocks to generate the CVL magma occurred dominantly in the garnet lherzolite stability field.Melting models suggest small degree(3%)partial melting of mantle bearing(6e10%)garnet for Mt.Etinde,the Ngaoundere Plateau and the Biu Plateau,and5%of garnet for the oceanic sector of the CVL,Mt.Cameroon,Mt.Bambouto,Mt.Manengouba and the Oku Volcanic Group.The Sr-Nd-Pb isotope systematics suggest that mixing in various proportions of Depleted MORB Mantle(DMM)with enriched mantle 1 and 2(EM1 and EM2)could account for the complex isotopic characteristics of the CVL lavas.Low Mg number(Mg#100 Mg O/(Mg O t Fe O))and Ni,Cr and Co contents of the CVL mafic lavas reveal their crystallization from fractionated melts.The absence of systematic variation in Nb/Ta and Zr/Hf ratios,and Sr-Nd isotope compositions between the mafic and felsic lavas indicates progressive evolution of magmas by fractional crystallization.Trace element ratios and their plots corroborate mantle heterogeneity and reveal distinct geochemical signatures for individual the CVL volcanoes. 相似文献
9.
Composite dikes at Hell Hole Meadow, in the central Sierra Nevada, contain hybrids created by small scale mixing of andesitic and rhyolitic magmas. Early rhyolitic injections had partially solidified when subsequent andesitic magmas arrived and mixed with small increments of remnant rhyolitic magma. In major element chemistry, the hybrid rocks define linear MgO-variation diagrams that closely resemble those for the Half Dome, Mt. Givens, and Eagle Peak granodiorites. The patterns suggest that mixing of mafic and felsic magmas has been important in the evolution of these plutons.Hornblendes in three Hell Hole Meadow hybrid rocks ranging from dacite to andesite display indistinguishable crystal-chemical variation patterns. The crystals apparently developed as phenocrysts of endmember andesitic magma prior to the mixing event and retained their compositional character in the hybridization event. Plutonic hornblendes (Dodge et al. 1968; Noyes et al. 1983) display crystal-chemical patterns nearly identical to those in the dike cores making it unlikely that the plutonic hornblendes represent restite of a complex lower crust.Hornblende chemical data from a suite of rocks collected across the width of the compositionally zoned Half Dome granodiorite (Yosemite National Park) define clusters, like their Hell Hole Meadow counterparts, that are very similar to one another despite the large range in host rock bulk composition. Sr isotopic data (Kistler et al. 1986) and linear MgO-variation patterns for the major elements (Reid et al. 1983) suggest that the Half Dome is a mixture of high-alumina basalt and rhyolitic magmas. We propose that while the bulk chemistry of hybrid granodiorites is determined by the proportions of the constituent mafic and felsic magmas, the hornblendes in the mixed rocks largely retain compositions created in the mafic component prior to mixing. Mixing may occur either by incremental addition of felsic magma into a relatively large volume of mafic magma (as at Hell Hole Meadow), or by the chilling and subsequent disaggregation of mafic pillows in a largely felsic host. 相似文献
10.
M.L. Renjith ;S.N. Charan ;D.V. Subbarao ;E.V.S.S.K. Babu ;V.B. Rajashekhar 《地学前缘(英文版)》2014,5(6):801-820
Magma mixing process is unusual in the petrogenesis of felsic rocks associated with alkaline complex worldwide. Here we present a rare example of magma mixing in syenite from the Yelagiri Alkaline Complex, South India. Yelagiri syenite is a reversely zoned massif with shoshonitic (Na2O + K2O=5–10 wt.%, Na2O/K2O = 0.5–2, TiO2 <0.7 wt.%) and metaluminous character. Systematic modal variation of plagioclase (An11–16 Ab82–88), K-feldspar (Or27–95 Ab5–61), diopside (En34–40Fs11–18Wo46–49), biotite, and Ca-amphibole (edenite) build up three syenite facies within it and imply the role of in-situ fractional crystallization (FC). Evidences such as (1) disequilibrium micro-textures in feldspars, (2) microgranular mafic enclaves (MME) and (3) synplutonic dykes signify mixing of shoshonitic mafic magma (MgO = 4–5 wt.%, SiO2 = 54–59 wt.%, K2O/Na2O = 0.4–0.9) with syenite. Molecular-scale mixing of mafic magma resulted disequilibrium growth of feldspars in syenite. Physical entity of mafic magma preserved as MME due to high thermal-rheological contrast with syenite magma show various hybridization through chemical exchange, mechanical dilution enhanced by chaotic advection and phenocryst migration. In synplutonic dykes, disaggregation and mixing of mafic magma was confined within the conduit of injection. Major-oxides mass balance test quantified that approximately 0.6 portions of mafic magma had interacted with most evolved syenite magma and generated most hybridized MME and dyke samples. It is unique that all the rock types (syenite, MME and synplutonic dykes) share similar shoshonitic and metaluminous character; mineral chemistry, REE content, coherent geochemical variation in Harker diagram suggest that mixing of magma between similar composition. Outcrop-scale features of crystal accumulation and flow fabrics also significant along with MME and synplutonic dykes in syenite suggesting that Yelagiri syenite magma chamber had evolved through multiple physical processes like convection, shear flow, crystal accumulation and magma mixing. 相似文献
11.
处于浙-赣火山岩带东北缘的湖(州)-安(吉)盆地内的火山岩/潜火山岩从中性到酸性,中间没有明显的成分间断,以中酸性—酸性组分占绝对优势,中性组分相对较少,缺少基性组分,代表一套连续的中性—酸性岩浆系列。岩石化学总体表现为富碱和高钾的特征,中性岩属橄榄玄粗岩系列,中酸性—酸性岩类属高钾钙碱性系列。盆地内的火山岩在地球化学上均表现为富集大离子亲石元素和轻稀土元素,而高场强元素Nb、Ta、Ti等则有一定程度的亏损。火山岩中主量和微量元素的变异规律揭示分离结晶作用是盆地内岩浆演化的主要机理,但岩浆演化的不同阶段分离的矿物相有所差异,斜长石自始至终都是分离结晶的重要矿物相,在中性—中酸性岩浆演化阶段,角闪石可能也是重要的分离矿物,而中酸性—酸性岩中钾长石和黑云母的分离结晶也起着重要影响。本文所作的精确定年结果表明,盆地内三期火山活动产物的LA-ICPMS锆石U-Pb年龄在误差范围内几乎一致,介于128~130 Ma之间,指示盆地内主要火山活动持续的时间很短。另外测得粗安岩集块的SHRIMP锆石U-Pb年龄为136±1 Ma,可能意味着该盆地初期有少量偏基性的火山活动。推测湖安火山岩盆地发育于活动大陆边缘的后造山环境,岩浆的形成可能受控于岩石圈的拆沉或俯冲板片的断落,母岩浆主要是由镁铁质下地壳物质部分熔融形成的,几乎没有地幔物质的参与,所形成的安山质-英安质岩浆在浅部岩浆房中发生过强烈的分异演化。 相似文献
12.
Stephen R. Tait 《Contributions to Mineralogy and Petrology》1988,100(4):470-483
The spatial and chemical relationships between the melt occupying the reservoir and the mineral assemblages crystallising at the margins are reconstructed for the magma chamber which produced the 11000 yr.B.P. tephra deposit of Laacher See Volcano. The melt showed vertical chemical zonation immediately prior to eruption, and throughout most of the magma volume only a small fraction of crystals were present. The eruption also ejected crystal-rich nodules, ranging from mafic to felsic in composition, which are samples of the materials crystallising at the boundaries of the chamber. New data on nodule petrography and chemical compositions of whole-rocks, minerals and interstitial glasses are presented. Volume fraction of interstitial glass is not systematically related to mineral assemblage and varies typically between 1 and 20 vol%, i.e. the crystals interlock. One exception is a group of mafic nodules with glass volume fractions between 25 and 40 vol%. Bulk compositions of mafic nodules show strong enrichments or depletions in all major elements relative to the mafic phonolite interstitial melt. Felsic nodules show much less pronounced differences with their interstitial melt. Felsic nodules contain interstitial glasses with a range of compositions similar to that in the zoned bulk of the chamber and were probably derived from different heights on the walls. Mafic nodules have glass compositions similar to those at the base of the zoned liquid column and were probably derived from the floor. Modal mineralogy, glass composition and mineral composition are systematically related in the nodules whereas in individual pumices samples derived from the main body of the chamber, a broader range of mineral compositions are often found. Mineral assemblages were especially diverse in the upper part of the chamber. It is deduced that the whole of the essentially liquid part of the chamber was emptied by the eruption, that strongly contrasting mineral assemblages were forming simultaneously on the walls and floor, that the gradient in crystal content in the crystallisation boundary layer was more gradual at the floor than at the walls, and that the pumice mineralogy is not a simple phenocryst assemblage but is a mixture of crystals which grew from melts separated in space and/or time. 相似文献
13.
Petrology and Emplacement of Reversely Zoned Gabbro-Diorite Plutons in the Smartville Complex, Northern California 总被引：2，自引：0，他引：2
Gabbroic plutons are part of the intrusive substructure of theSmartville Complex, a late Jurassic, rifted, ensimatic arc locatedin the northern Sierra Nevada of California. The plutons rangefrom unzoned, equant bodies of olivine gabbro less than 1 kmin diameter to elongate intrusions up to 25 km in length thatare reversly zoned from olivine gabbro cores to quartz dioriterims. The felsic rocks dip inward beneath the mafic core, indicatingthat this zoning reversal continues to depth. The zoned plutonshave relatively shallow keels. We interpret the reversed zoningas an emplacement feature, analogous to the compositional zoningin a zoned tephra sheet. It formed as a result of tapping analready zoned, deeper level magma chamber. Whether the originalzoning of the magma was concentric or stratiform cannot be readilydeduced. During emplacement, considerable amounts of cumulaterocks were mobilized. The mineralogy and geochemistry of the reversely zoned plutonsindicate that they contain two suites of rocks: a cumulate suiterepresented by olivine gabbro and olivine clinopyroxenite anda differentiated suite of non-cumulate olivine gabbros, gabbronorites,and diorites that lie along a compositional continuum and approximateliquid compositions. Plagioclase and olivine compositions inthe Smartville Complex cumulate suite are identical to thosein modern arc cumulates and are characteristic of the arc cumulatesuite. The differentiated rocks form a compositionally continuousseries that is geochemically very similar to a differentiatedsuite of arc tholeiitic basalts and andesites. Fractionationmodeling indicates that removal of mineral phases found in thecumulate gabbros from the mafic members of the differentiatedsuite can produce the lithologic variation seen in the zonedplutons. Plutons such as those in the Smartville Complex indicatethat there is a genetic link between cumulate rocks and a basalt-andesitefractionation trend in arcs, supporting the hypothesis thatarc andesites form by crystal fractionation. The gabbroic plutonsand related Alaska-type ultramafic complexes contain ultramaficcumulates that can rectify the discrepancy between the cumulatemode predicted by fractionation models and the observed modeof gabbroic cumulates in arcs. 相似文献
14.
Tod E. Waight Robert A. Wiebe Eirik J. Krogstad Richard J. Walker 《Contributions to Mineralogy and Petrology》2001,142(3):323-335
The Pleasant Bay layered gabbro-diorite complex (420 Ma) formed via repeated injections of mafic magma into a felsic magma chamber. It is dominated by repeating sequences (macrorhythmic units) with chilled gabbroic bases which may grade upward into medium-grained gabbro, diorite and granite. Each unit represents an injection of mafic magma into the chamber followed by differentiation. Increases in Sri and decreases in )Ndi with stratigraphic height indicate open-system isotopic behaviour and exchange between the mafic and felsic magmas. Isotopic variations of whole-rock samples in individual macrorhythmic units do not conform to bulk mixing or AFC models between potential parental magmas. Sr isotopic studies of single feldspar crystals from one macrorhythmic unit indicate that exchange of crystals between the resident felsic magma and mafic influxes was important, that some of the rocks contain feldspar xenocrysts, and that the rocks are isotopically heterogeneous on an intercrystal scale. Xenocryst abundance increases with stratigraphic height, suggesting that crystal exchange occurred in situ. The lack of disequilibrium textures in the xenocrystic feldspar indicates the evolved macrorhythmic magma and resident silicic magma were of a similar composition and likely in thermal equilibrium at the time of crystal transfer. Mafic chilled margins are enriched in alkalis and isotopically evolved compared with mafic dikes (representing the parental melts) and suggest rapid in-situ diffusional exchange following emplacement of individual mafic replenishments. 相似文献
15.
对青藏高原西南部当惹雍错地区的中新世超钾质粗面岩及共存的富钠质方沸石(霞石)响岩进行了详细的岩石学、矿物学研究,揭示出两套岩石具有不同的矿物学特征.钠质方沸石(霞石)响岩中橄榄石斑晶的熔体包裹体与钾质火山岩的成分相当,应属于早期残余的钾质岩浆,表明了钠质方沸石(霞石)响岩的形成晚于超钾质粗面岩.两类岩石的全岩化学成分、... 相似文献
16.
Models of continental crustal magmagenesis commonly invoke theinteraction of mafic mantle-derived magma and continental crustto explain geochemical and petrologic characteristics of crustalvolcanic and plutonic rocks. This interaction and the specificmechanisms of crustal contamination associated with it are poorlyunderstood. An excellent opportunity to study the progressiveeffects of crustal contamination is offered by the compositeplutons of the Alaska Range, a series of nine early Tertiary,multiply intruded, compositionally zoned (Peridotite to granite)plutons. Large initial Sr and Nd isotopic contrasts betweenthe crustal country rock and likely parental magmas allow evaluationof the mechanisms and extents of crustal contamination thataccompanied the crystallization of these ultra-mafic throughgranitic rocks. Three contamination processes are distinguishedin these plutons. The most obvious of these is assimilationof crustal country rock concurrent with magmatic fractionalcrystallization (AFC), as indicated by a general trend towardcrustal-like isotopic signatures with increasing differentiation.Second, many ultramafic and mafic rocks have late-stage phenocrystreaction and orthocumulate textures that suggest interactionwith felsic melt. These rocks also have variable and enrichedisotopic compositions that suggest that this felsic melt wasisotopically enriched and probably derived from crustal countryrock. Partial melt from the flysch country rock may have reactedwith and contaminated these partly crystalline magmas followingthe precipitation and accumulation of the cumulus phenocrystsbut before complete solidification of the magma. This suggeststhat in magmatic mush (especially of ultramafic composition)crystallizing in continental crust, a second distinct processof crustal contamination may be super imposed on AFC or magmamixing involving the main magma body. Finally, nearly all rocks,including mafic and ultramafic rocks, have (87Sr/86Sr)i thatare too high, and (T) Nd that are too low, to represent theexpected isotopic composition of typical depleted mantle. However,gabbro xenoliths with typical depicted-mantle isotopic compositionsare found in the plutons. This situation requires either anadditional enriched mantle component to provide the parentalmagma for these plutons, or some mechanism of crustal contaminationof the parent magma that did not cause significant crystallizationand differentiation of the magma to more felsic compositions.Thermodynamic modeling indicates that assimilation of alkali-andwater-rich partial melt of the metapelite country rock by fractionating,near-liquidus basaltic magma could cause significant contaminationwhile suppressing significant crystallization and differentiation. KEY WORDS: crustal contamination; Alaska Range; isotope geochemistry; zoned plutons; assimilation
*Corresponding author. e-mail: preiners{at}u.washington.edu; fax: (206) 543-3836. 相似文献
17.
西天山托云盆地及周边中新生代岩浆活动的岩石学、地球化学与年代学研究 总被引：4，自引：5，他引：4
天山西南部白垩纪-老第三纪发育的托云盆地及其周边出露的岩浆岩是一套完整的碱性岩浆岩系列，包含了苦橄质玄武岩、玄武岩、碧玄岩、碱玄岩（橄榄玄武岩、黑云母辉长二长岩、辉长辉绿岩、辉石橄榄岩）和响岩等多种岩石类型。野外工作显示有火山喷出岩和侵入岩两种不同的产状。年代学结果指示岩浆岩形成于120-50Ma间，为晚白垩世-老第三纪盆地形成演化阶段岩浆活动的产物。分离结晶作用是岩浆演化和岩浆系列形成最主要的因素，托云岩浆岩大致经历了结晶分异过程的两个阶段：早期苦橄质岩浆中橄榄石、尖晶石的结晶分离，表现为MgO和微量元素Cr含量随SiO2含量增加大幅度的降低；晚期主要是单斜辉石、斜长石和钛铁矿等矿物的结晶分异，以CaO、FeO、TiO2等随SiO2含量增加大幅度的降低为特点。苦橄质岩石的出现指示了地幔较高温熔融事件的存在，进而为托云盆地地幔柱的存在提供了有力的证据。无论如何，碱性岩浆的活动表明托云盆地形成的大地构造背景是大陆主动裂谷环境，对应的深部背景为区域性的地幔柱构造。首次发现的响岩是结晶分异作用的最终产物。响岩较极端地指示了岩浆结晶分离过程对岩浆演化的巨大影响。托云岩浆岩的同位素特征指示其源区是一个接近于PREMA地幔，但微量元素特征显示其受地壳流体交代改造的特点。岩浆岩的Nd同位素TDM集中在250～600Ma之间，反映了一个古生代时期形成的新生岩石圈地幔，与新疆北部地区的晚古生代新生岩石圈地幔的事实相符。 相似文献
18.
Gabbro Akarem mafic-ultramafic complex, Eastern Desert, Egypt: a Late Precambrian analogue of Alaskan-type complexes 总被引：4，自引：0，他引：4
Summary ?Gabbro Akarem is a Late-Precambrian concentrically-zoned mafic-ultramafic intrusion located along a major fracture zone trending
NE-SW in the Eastern Desert of Egypt. It intruded low-grade metasedimentary rocks, and has a contact metamorphic aureole a
few meters wide. This intrusion comprises a dunite core enveloped by clinopyroxene hornblende-bearing lherzolite, olivine-hornblende
clinopyroxenite and plagioclase hornblendite. The contacts between the rock types are gradational. They have cumulate textures
and the observed crystallization sequence is: olivine ( + cotectic spinel)-orthopyroxene (Opx)-clinopyroxene (Cpx)-hornblende.
Mafic minerals from the core of the intrusion are highly magnesian, a consistent increase in the Mg# of olivine (from 69 to
87), Opx (from 62 to 89), Cpx (from 85 to 96) and hornblends (from 62 to 88) is observed from the mafic to the ultramafic
units. Spinel has a wide range of Cr# and Mg# ratios. The various rock units define a fractionation trend. The mafic rocks
are slightly LREE-enriched relative to the ultramafic units and chondrites.
In many aspects, the Gabbro Akarem intrusion is similar to Alaskan-type complexes. Mineralogical and geochemical data suggest
that the different rock units were fractionated from a hydrous picritic magma with no apparent crustal contamination. A petrogenetic
model involving a rapid rise of hydrous mantle magma along a major fracture zone is proposed. Extensive fractional crystallization
led to magma chamber stratification; internal circulation and strong vertical stretching up the center of the rapidly rising
diapir increased the rate of magma ascent towards the core. Due to cooling and high viscosity the marginal mafic magma was
partly crystallized while the unsolidified core ultramafic magma continued its ascent. As a result, different mineral phases
crystallized at different pressure-temperature paths. Field relations, geophysical, petrological and experimental studies
support this model which explains many of the characteristics of the Gabbro Akarem and some other concentrically zoned mafic-ultramafic
intrusions.
Received April 24, 2001; revised version accepted November 20, 2001 相似文献
19.
Felsic alkalic rocks are a minor component of many ocean island volcanic suites, and include trachyte and phonolite as well as various types of alkaline and peralkaline rhyolite. However, there is considerable debate on the nature of their formation; for example, are they formed by partial melting of anomalous mantle or the final products of fractional crystallization of mafic magmas. The phonolites and foidal phonolites on Rarotonga were formed by low pressure crystal fractionation of two chemically distinct parental magmas. Low silica and high silica mafic magmas produced a basanite-foidal phonolite series and an alkali basalt-phonolite series, respectively. The foidal phonolite composition evolved from the low silica mafic magmas by approximately 60% fractionation of titanaugite + leucite + nepheline + magnetite + apatite. Fractionation continued with the crystallization of aegirine-augite + nepheline + kaersutite + magnetite + apatite. The phonolites formed from the alkali basalts by approximately 40% fractionation of kaersutite + titanaugite + Fe-Ti oxide + plagioclase + apatite and continued to evolve further by fractionation of anorthoclase + nepheline + aegerine-augite + Fe-Ti oxides. As the magmas fractionated in both suites, their overall viscosities (solid + liquid) increased until a point was reached whereby viscosity inhibited the eruption of magmas with compositions intermediate between the mafic rocks and the felsic rocks. However, the magmas continued to fractionate under static conditions with the residual fluid becoming foidal phonolitic in the low silica suite or phonolitic in the high silica suite. These phonolitic liquids, as a result of an increase in volatiles and enrichment of alkalis over aluminum, would actually have a lower viscosity than the intermediate liquids. This decrease in viscosity and the switch from a magma chamber being predominantly a liquid with suspended solids to a solid crystalline network with an interstitial liquid enabled phonolitic liquids to migrate, pool, and eventually erupt on the surface. 相似文献
20.
Potassic syenites from Svidnya, Bulgaria crop out as small isolated bodies as the primary for this intrusion liquid has basic to intermediate composition. The evolution in a closed magma chamber created plutonic rocks ranging from basic (melasyenite) to acid (granite) and from metaluminous to peralkaline. The most mafic varieties show cumulative textures typical for orthocumulates with cumulus phases clinopyroxene, biotite, apatite and potassium feldspar as gravitational settling is a viable process for separation of particles in the bottom parts of magma chamber. In the middle stratigraphic level of biggest body modal igneous layering with development of dark (clinopyroxene?+?amphibole) and light (potassium feldspar) laminas was observed. Oscillatory crystallization around eutectic point resulted in cyclic separation of mafic and felsic phases in repetitive layers. Fractionation of Ca- and Al-rich phases—clinopyroxene, biotie and potassium feldspar created peralkaline residual liquid strongly enriched in HFS elements. 相似文献