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1.
Volcanic rocks of the Latir volcanic field evolved in an open system by crystal fractionation, magma mixing, and crustal assimilation. Early high-SiO2 rhyolites (28.5 Ma) fractionated from intermediate compositionmagmas that did not reach the surface. Most precaldera lavas have intermediate-compositions, from olivine basaltic-andesite (53% SiO2) to quartz latite (67% SiO2). The precaldera intermediate-composition lavas have anomalously high Ni and MgO contents and reversely zoned hornblende and augite phenocrysts, indicating mixing between primitive basalts and fractionated magmas. Isotopic data indicate that all of the intermediate-composition rocks studied contain large crustal components, although xenocrysts are found only in one unit. Inception of alkaline magmatism (alkalic dacite to high-SiO2 peralkaline rhyolite) correlates with, initiation of regional extension approximately 26 Ma ago. The Questa caldera formed 26.5 Ma ago upon eruption of the >500 km3 high-SiO2 peralkaline Amalia Tuff. Phenocryst compositions preserved in the cogenetic peralkaline granite suggest that the Amalia Tuff magma initially formed from a trace element-enriched, high-alkali metaluminous magma; isotopic data suggest that the parental magmas contain a large crustal component. Degassing of water- and halogen-rich alkali basalts may have provided sufficient volatile transport of alkalis and other elements into the overlying silicic magma chamber to drive the Amalia Tuff magma to peralkaline compositions. Trace element variations within the Amalia Tuff itself may be explained solely by 75% crystal fractionation of the observed phenocrysts. Crystal settling, however, is inconsistent with mineralogical variations in the tuff, and crystallization is thought to have occurred at a level below that tapped by the eruption. Spatially associated Miocene (15-11 Ma) lavas did not assimilate large amounts of crust or mix with primitive basaltic magmas. Both mixing and crustal assimilation processes appear to require development of relatively large magma chambers in the crust that are sustained by large basalt fluxes from the mantle. The lack of extensive crustal contamination and mixing in the Miocene lavas may be related to a decreased basalt flux or initiation of blockfaulting that prevented pooling of basaltic magma in the crust.  相似文献   

2.
A suite of peralkaline trachytes from Longonot volcano, Kenya,which erupted during the last 6000 years, has been analysedfor major and trace elements, Pb and Nd isotopes, and U–Th–Radisequilibria. The lavas are divided into three stratigraphicgroups of trachytes (Lt2a, Lt2b and Lt3), and hybrid lavas,designated LMx1 and LMx2, which, respectively, pre-date andpost-date the Lt2 lavas. Major and trace elements are consistent,with up to 37% within-group fractional crystallization of predominantlyalkali feldspar. The parental magma for the different trachytegroups had a more mafic composition—probably hawaiitic.Nd and Pb isotopes show minimal variation, both within and betweenmagma groups, and indicate that up to 10% comendite magma fromthe neighbouring Olkaria volcanic field may have intermixedwith the Longonot magma. (230Th/238U) disequilibria indicatethat limited U/Th fractionation occurred during the past 10kyr, whereas (226Ra/230Th) disequilibria reflect the effectof alkali feldspar fractionation >8 kyr ago in the Lt2a lavas,between 3 and 7 kyr ago in the Lt2b lavas and in the past 3kyr for the Lt3 lavas. (226Ra/230Th) disequilibria in the Lt2blavas are interpreted using a model that combines the equationsof radioactive decay and in-growth with Rayleigh crystallizationto give fractionation rates of about 0·2 x 10–4/yearfor the evolution of hawaiite to trachyte, but more rapid ratesof up to 3 x 10–4/year for fractionation within the trachytesequence. (226Ra/230Th) from two whole-rock–alkali feldsparpairs are interpreted to show the crystals formed at 5800 yearsBP (Lt2b) and 2800 years BP (Lt3), implying that phenocrystformation continued almost up to the time of eruption. The resultsstrongly indicate that fractionated magmas can be stored forperiods on the order of 1000–2500 years prior to eruption,whereas other magmas were erupted as fractionation was proceeding. KEY WORDS: trachyte; magma chambers; u-series; Kenya  相似文献   

3.
This study presents petrographic and compositional data for coexisting peralkaline silicate glass and quenched natrocarbonatite melt in nepheline phenocrysts from the 24 September 2007 and July 2008 eruptions of the natrocarbonatite volcano Oldoinyo Lengai (Tanzania). Data are also given for peralkaline residual glass in combeite nephelinite ash clasts occurring in the March–April 2006 large volume natrocarbonatite flow. These data are considered to demonstrate the occurrence of liquid immiscibility between strongly peralkaline Fe-rich nephelinite melt and natrocarbonatite at Oldoinyo Lengai. Compositional data for coexisting silicate–carbonate pairs in conjunction with previous experimental studies suggest that the size of the field of liquid immiscibility for carbonated nephelinitic magmas is a function of their peralkalinity. It is shown that peralkaline combeite wollastonite nephelinite was present at Oldoinyo Lengai prior to, and during, the 24 September 2007 ash eruption. It is postulated that the driving force for this major eruption was assimilation and decomposition of previously emplaced solid natrocarbonatite. Assimilation resulted in the formation of the unusual hybrid nepheline–andradite–melilite–combeite–phosphate magma represented by the 24 September 2007 ash.  相似文献   

4.
Matrix glass and melt inclusions in phenocrysts from pantellerite lavas of the Boseti volcanic complex, Ethiopia, record extreme fractionation of peralkaline silicic magma, with Al2O3 contents as low as 2.3?wt.%, FeO* contents up to 17?wt.% and SiO2 contents ~65?wt.%. The new data, and published data for natural and experimental glasses, suggest that the effective minimum composition for peralkaline silicic magmas has ~5?wt.% Al2O3, 13?wt.% FeO* and 66?±?2?wt.% SiO2. The dominant fractionating assemblage is alkali feldspar?+?fayalite?+?hedenbergite?+?oxides?±?quartz. Feldspar – melt relationships indicate that the feldspar is close to the minimum on the albite-orthoclase solid solution loop through the entire crystallization history. There is petrographic, mineralogical and geochemical evidence that magma mixing may have been a common process in the Boseti rhyolites.  相似文献   

5.
Major and trace element data for a sequence of peralkaline silicic lavas and pyroclastic flows, exposed in the caldera wall of the Paisano volcano, west Texas, document systematic fractional crystallization during magmatic evolution and an open system, magma mixing event in the upper parts of the sequence. Stratigraphically lowest flows are comendite and comenditic quartz trachyte lavas and ash flow tufts. Overlying these units is a trachyte with compositional, textural and mineralogical features indicating that it is the product of magma-mixing; similar flows occur in other parts of the volcano at the same stratigraphic level. This composite trachyte is considered to be a mixture of mugearitic or mafic trachytic magma, derived from a similar source region which yielded the earlier caldera wall flows. Trace element concentrations of the post-trachyte comenditic quartz trachyte lavas suggest they were erupted from a chamber whose magma was diluted by an influx of mugearitic or mafic trachytic magma during a magma mixing event.Rayleigh fractionation calculations show that the comendites and comenditic quartz trachytes can be derived from a parental mugearite magma by 88% to 93% fractionation of dominantly plagioclase and alkali feldspar, with lesser amounts of clinopyroxene, magnetite and apatite. Zircon was not a significant fractionating phase. The composition, mineralogy and depth of the source region(s) which generated these magmas cannot be constrained from the present data set.  相似文献   

6.
Recent pantelleritic lavas comprise the whole of the isolated and outlying volcano of Mayor Island. Mineralogically, they are characterised by phenocrystic anorthoclase-sodic27 sanidine, quartz, sodic ferrohedenbergite, and cossyrite. Nine new chemical analyses of the lavas are presented (including one residual glass), confirming their strongly sodic and peralkaline nature. One analysis is also given of trachybasalt, which occurs as common inclusions in the mantling pumice deposits. These inclusions are characterised by abundant feldspar phenocrysts. Detailed trace element data is presented for five of the lava samples, representing the mam volcanic phases and the trachybasalt inclusions. The following conclusions are presented:
  1. The lavas exhibit a marked enrichment (relative to “average” granitic compositions) of the alkalis; rare earths; highly charged cations (e.g. Nb, Zr, Hf, Mo, U, Th); Ga, Be, and Cl. In contrast, they show a spectacular depletion of Sr, Ba, and Mg, and a less intense depletion of Ca, Sc, V, and Cr.
  2. The pantelleritic rare earth patterns show a similar degree of fractionation to the sedimentary pattern, and are dominated by a very strong Eu depletion. This suggests feldspar subtraction. The trachybasalt pattern shows a similar degree of fractionation, but exhibits enrichment of Eu.
  3. The trachybasalt inclusions are characterised by a trace element assemblage comparable to alkali basalts, except for higher Ba and exceedingly high K/Rb and K/Cs ratios. The chemical and mineralogical data suggest that they represent partial feldspar accumulate rocks.
  4. There is a progressive enrichment of nearly all trace and minor elements in the youngest lavas. This includes those elements that show an overall depletion in the lavas. The younger lavas are also enriched in Na and Fe, but further depleted in Al.
The data is interpreted to indicate that the pantellerites were derived by crystal differentiation from a postulated mildly alkali olivine basalt parent — feldspar fractionation is considered to have been extremely important in this process. It is shown that the element enrichment occurring in the younger lavas may not be wholely explained by crystallisation differentiation alone — it is possible that some additional process is required. It is also shown that the observed enrichment of sodium in the youngest lavas can only occur during crystal fractionation if quartz, as well as anorthoclase, separate from the magma. This is due to the higher alkali abundances of the anorthoclase phenocrysts, relative to the pantellerite compositions. There is limited evidence that post-eruptive devitrification of some of the lavas has resulted in some modification of the lava chemistry, notably sodium loss.  相似文献   

7.
The Nandewar Mountains, N.S.W., Australia, are the remains of a Miocene continental alkaline volcano whose products range from olivine basalts to comendites and alkali rhyolites. Intermediate hawaiites, mugearites and benmoreites predominate in the shield, in which olivine basalts are rare, and the trachytic rocks form many intrusions into the shield. The Nandewar alkaline series shows extreme fractionation of a relatively differentiated alkali olivine basalt magma, saturated with silica, to yield extremely oversaturated peralkaline comendites and peraluminous alkali rhyolites. The nature of the ferromagnesian phases forming was controlled by low oxygen fugacities. Throughout the series clinopyroxenes range from diopsidic augite, through sodic ferrohedenbergites to hedenbergite-acmite solid solutions. Riebeckite-arfvedsonite solid solutions appear in the trachytes and comendites, and aenigmatite appears in some of the peralkaline rocks. The feldspars in the series fractionate from calcic labradorite through potash oligoclase and calcic anothoclase towards the minimum melting alkali feldspar composition, Ab65Or35. The compositions of the alkali rhyolites approach the minimum in the system SiO2-KAlSi3O8-NaAlSi3O8. All the mineralogical and chemical evidence points to the development of the Nandewar series by the processes of extreme crystallization differentiation of an alkali olivine basalt parent magma. No significant contamination occurred, xenoliths and xenocrysts are absent, and volatile transfer and metasomatism played a minor role.  相似文献   

8.
The crustal history of volcanic rocks can be inferred from the mineralogy and compositions of their phenocrysts which record episodes of magma mixing as well as the pressures and temperatures when magmas cooled. Submarine lavas erupted on the Hilo Ridge, a rift zone directly east of Mauna Kea volcano, contain olivine, plagioclase, augite ±orthopyroxene phenocrysts. The compositions of these phenocryst phases provide constraints on the magmatic processes beneath Hawaiian rift zones. In these samples, olivine phenocrysts are normally zoned with homogeneous cores ranging from ∼ Fo81 to Fo91. In contrast, plagioclase, augite and orthopyroxene phenocrysts display more than one episode of reverse zoning. Within each sample, plagioclase, augite and orthopyroxene phenocrysts have similar zoning profiles. However, there are significant differences between samples. In three samples these phases exhibit large compositional contrasts, e.g., Mg# [100 × Mg/(Mg+Fe+2)] of augite varies from 71 in cores to 82 in rims. Some submarine lavas from the Puna Ridge (Kilauea volcano) contain phenocrysts with similar reverse zonation. The compositional variations of these phenocrysts can be explained by mixing of a multiphase (plagioclase, augite and orthopyroxene) saturated, evolved magma with more mafic magma saturated only with olivine. The differences in the compositional ranges of plagioclase, augite and orthopyroxene crystals between samples indicate that these samples were derived from isolated magma chambers which had undergone distinct fractionation and mixing histories. The samples containing plagioclase and pyroxene with small compositional variations reflect magmas that were buffered near the olivine + melt ⇒Low-Ca pyroxene + augite + plagioclase reaction point by frequent intrusions of mafic olivine-bearing magmas. Samples containing plagioclase and pyroxene phenocrysts with large compositional ranges reflect magmas that evolved beyond this reaction point when there was no replenishment with olivine-saturated magma. Two of these samples contain augite cores with Mg# of ∼71, corresponding to Mg# of 36–40 in equilibrium melts, and augite in another sample has Mg# of 63–65 which is in equilibrium with a very evolved melt with a Mg# of ∼30. Such highly evolved magmas also exist beneath the Puna Ridge of Kilauea volcano. They are rarely erupted during the shield building stage, but may commonly form in ephemeral magma pockets in the rift zones. The compositions of clinopyroxene phenocryst rims and associated glass rinds indicate that most of the samples were last equilibrated at 2–3 kbar and 1130–1160 °C. However, in one sample, augite and glass rind compositions reflect crystallization at higher pressures (4–5 kbar). This sample provides evidence for magma mixing at relatively high pressures and perhaps transport of magma from the summit conduits to the rift zone along the oceanic crust-mantle boundary. Received: 8 July 1998 / Accepted: 2 January 1999  相似文献   

9.
The Bandas del Sur Formation preserves a Quaternary extra-calderarecord of central phonolitic explosive volcanism of the LasCañadas volcano at Tenerife. Volcanic rocks are bimodalin composition, being predominantly phonolitic pyroclastic deposits,several eruptions of which resulted in summit caldera collapse,alkali basaltic lavas erupted from many fissures around theflanks. For the pyroclastic deposits, there is a broad rangeof pumice glass compositions from phonotephrite to phonolite.The phonolite pyroclastic deposits are also characterized bya diverse, 7–8-phase phenocryst assemblage (alkali feldspar+ biotite + sodian diopside + titanomagnetite + ilmenite + nosean–haüyne+ titanite + apatite) with alkali feldspar dominant, in contrastto interbedded phonolite lavas that typically have lower phenocrystcontents and lack hydrous phases. Petrological and geochemicaldata are consistent with fractional crystallization (involvingthe observed phenocryst assemblages) as the dominant processin the development of phonolite magmas. New stratigraphicallyconstrained data indicate that petrological and geochemicaldifferences exist between pyroclastic deposits of the last twoexplosive cycles of phonolitic volcanism. Cycle 2 (0·85–0·57Ma) pyroclastic fall deposits commonly show a cryptic compositionalzonation indicating that several eruptions tapped chemically,and probably thermally stratified magma systems. Evidence formagma mixing is most widespread in the pyroclastic depositsof Cycle 3 (0·37–0·17 Ma), which includesthe presence of reversely and normally zoned phenocrysts, quenchedmafic glass blebs in pumice, banded pumice, and bimodal to polymodalphenocryst compositional populations. Syn-eruptive mixing eventsinvolved mostly phonolite and tephriphonolite magmas, whereasa pre-eruptive mixing event involving basaltic magma is recordedin several banded pumice-bearing ignimbrites of Cycle 3. Theperiodic addition and mixing of basaltic magma ultimately mayhave triggered several eruptions. Recharge and underplatingby basaltic magma is interpreted to have elevated sulphur contents(occurring as an exsolved gas phase) in the capping phonoliticmagma reservoir. This promoted nosean–haüyne crystallizationover nepheline, elevated SO3 contents in apatite, and possiblyresulted in large, climatologically important SO2 emissions. KEY WORDS: Tenerife; phonolite; crystal fractionation; magma mixing; sulphur-rich explosive eruptions  相似文献   

10.
Four volcanoes form Terceira, one of the islands of the Azores group; three contain both basaltic and peralkaline and one only peralkaline rocks. A recently active basaltic fissure zone trends NW-SE across the island.The rocks fall into the alkaline olivine basalt suite although some young basalts are of transitional affinity. The geochemistry shows two general basaltic series: 1) undersaturated, found in lavas of the oldest volcano and in some recent fissure zone basalts and hawaiites; 2) saturated, found in the younger basaltic lavas.Since the emergence of Terceira there has been a contemporaneity of basalt and salic peralkaline lavas. The younger rocks show a bimodal composition distribution, the most voluminous compositions being alkali olivine basalt and comendite with negligible volume in the benmoreite-trachyte range. Two processes appear viable for the derivation of voluminous oversaturated peralkaline rocks: 1) partial melting of upper mantle material giving small magma batches of contrasting composition or 2) fractionation from a transitional basaltic parental magma.Now at Department of Geology, Victoria University of Wellington, New Zealand.  相似文献   

11.
The island of Lundy forms the southernmost igneous complex of the British Tertiary Volcanic Province (BTVP) and consists of granite (≈ 90%) emplaced into deformed Devonian sedimentary rocks (Pilton Shale) and associated with a swarm of dykes of dolerite/basalt, minor trachyte and rhyolite composition. The dolerites are of varied olivine basalt composition and are associated with peralkaline trachyte and subalkaline/peralkaline rhyolite with alkali feldspar and quartz ± alkali amphibole ± pyroxene mineralogy. The dyke swarm is therefore an anorogenic bimodal dolerite/basalt–trachyte/rhyolite BTVP association. Although the dyke association is bimodal in major element terms between dolerite/basalt and minor trachyte/rhyolite, the mineralogy and trace element geochemistry indicate that the dykes may be regarded as a cogenetic dolerite—peralkaline trachyte/rhyolite association with minor subalkaline rhyolites. Sr and Nd isotope data indicate derivation of these magmas from a similar BTVP mantle source (with or without minor contamination by Pilton Shale, or possibly Lundy granite). The petrogenesis of the Lundy dyke association is therefore interpreted in terms of extensive fractional crystallization of basaltic magma in a magma chamber of complex geometry below the (exposed) Lundy granite. Fractional crystallization of a representative dolerite magma (olivine ± clinopyroxene ± plagioclase) yields trachyte magma from which the crystallization of alkali feldspar (anorthoclase) ± plagioclase (oligoclase) + Fe–Ti oxide + apatite results in peralkaline rhyolite. Rarer subalkaline rhyolites result from fractionation from a similar dolerite source which did not achieve a peralkaline composition so allowing the crystallization and fractionation of zircon. The basalt–(minor trachyte)/rhyolite bimodality reflects rapid crystallization of basalt magma to trachyte (and rhyolite) over a relatively small temperature interval (mass fraction of melt, F = ≈ 0.15). The rapid high level emplacement of basalt, trachyte and rhyolite dyke magmas is likely to have been associated with the development of a substantial composite bimodal basalt–(minor trachytel)/rhyolite volcano above the BTVP Lundy granite in the Bristol Channel.  相似文献   

12.
Volcanological and petrological evidence, 87Sr/86Sr data, thelinear correlation between pairs of residual elements (e.g.Th, U, Zr, Hf, La, Ce, Ta) indicate that the rock series frommildly alkaline (transitional) basalt to pantellerite eruptedin recent Quaternary times at the Boina volcanic centre, canbe entirely explained in terms of fractional crystallizationat shallow depth. The fractionation process has been reconstructedby referrin to variation diagrams of major and trace elementsreported as a function of the fraction (f) of the initial compositionformed by the residual liquid, evaluated from the distributionof residual elements and by estimating the composition of theparent magma. The main crystal phases involved in the differentiationare, in the order of appearance: olivine, plagioclase, clinopyroxene,Fe-Ti oxides, alkali feldspar. Crystallization of Fe-Ti oxidesoccurs only at an advanced stage of fractionation in iron richliquids (ferrobasalts). The transition to the peralkaline field(near f=0?20) occurs without passing through a ‘true’trachytic (low-silica) salic stage and is determined by the‘plagioclase effect’. Fractionation within the peralkalinefield is dominated by alkali feldspar. Evidence is given fora ‘low-temperature zone’ of the oversaturated mildlyperalkaline system running along a line of constant alkali-ratio.Po2 variations are recorded during the evolution of the Boinaseries as suggested by petrological data and distribution curvesof total iron, Fe++/Fe+++and europium. Po2 values calculatedfrom europium distribution in feldspar and whole rocks agreewith published Po2 mineralogical calculations in pantelleritesfrom other localities. A crucial stage is recognized near thetransition to the peralkaline field, with a sudden Po2 dropduring the crystallization of the oxides probably provokingthe precipitation of apatite, followed by a rapid Po2 increaseat f=0?30. This limited oxygen unbuffered zone is importantin the basalt-pantellerite evolution, as it determines markedchemical variations in a restricted crystallization interval.It is suggested that such a crucial stage occurs also in theother known pantellerite series, such as Pantelleria. It mayalso account for the scarcity of rocks frequently reported atthis stage (‘Daly gap’). Data obtained from Boina and comparisons with other volcanicseries indicate that many peralkaline rhyolites are geneticallyrelated to transitional basalts and that their nature is mainlycontrolled by the composition of the parent basaltic magma.The association is characteristically found in zones of extensionof both continental and oceanic environments. The views of Coombs(1963) are confirmed and the rocks series from transitionalbasalts to comendites and/or pantellerites should be distinguishedfrom the alkalic (undersaturated) igneous rock suites producedby differentiation of alkali basalts.  相似文献   

13.
Field, petrographical and geochemical studies of a group of late Pleistocene, alkaline and mildly peralkaline trachytic and trachyphonolitic lavas from the northern Kenya Rift have been undertaken. A large number of flows were erupted from widely dispersed centres to form an extensive volcanic shield within the floor of the rift. The major element composition of most rocks was substantially modified during crystallisation, but other data show that differentiation within the suite was the result of protracted feldspar fractionation of a trachytic magma with intially very low abundances of residual trace elements.  相似文献   

14.
The petrology and geochemistry of the Azores Islands   总被引：7，自引：0，他引：7  
Forty lavas from the Azores Islands have been analyzed for 87Sr/86Sr ratios, major elements, first transition series metals, and LIL elements. The samples belong to the alkali basalt magma series but range from transitional hy-normative basalts from Terceira to basanitoids from Santa Maria. Differentiated lavas include both typical trachytes and comenditic trachytes and comendites. Major and trace element concentrations define smooth trends on variation diagrams, and these trends can be related to phases crystallizing in the rocks. Systematic interisland differences are also apparent in these variation diagrams. LIL element concentrations in island basalts are roughly twice as high as those in tholeiites from the adjacent Mid-Atlantic Ridge which transects the Azores Plateau. 87Sr/86Sr ratios in lavas from 6 of the 9 islands range from 0.70332 to 0.70354, a range similar to that found in tholeiites from the Mid-Atlantic Ridge transect of the Azores Plateau. This suggests that lavas from these islands and this portion of the Mid-Atlantic Ridge may be derived from a similar source. However, lavas from the islands of Faial and Pico have 87Sr/86Sr ratios up to 0.70394 and ratios in Sao Miguel lavas range up to 0.70525, suggesting basalts from these islands are derived from a chemically distinct source. Differences in the average LIL element concentrations of the least fractionated ridge tholeiites from the Azores Plateau and alkali basalts from the islands result from differences in extent of partial melting and residual mineralogy. The alkali basalts are derived by roughly half as much melting as are the tholeiites. Trace element concentrations in Azores peralkaline lavas preclude their derivation by partial melting of peridotitic mantle or basaltic crust; rather the data suggest they are produced by fractional crystallization of a basaltic parent.  相似文献   

15.
The 1971 Teneguía eruption is the most recent volcanic event of the Cumbre Vieja rift zone on La Palma. The eruption produced basanite lavas that host xenoliths, which we investigate to provide insight into the processes of differentiation, assimilation and magma storage beneath La Palma. We compare our results to the older volcano magmatic systems of the island with the aim to reconstruct the temporal development of the magma plumbing system beneath La Palma. The 1971 lavas are clinopyroxene-olivine-phyric basanites that contain augite, sodic-augite and aluminium augite. Kaersutite cumulate xenoliths host olivine, clinopyroxene including sodic-diopside, and calcic-amphibole, whereas an analysed leucogabbro xenolith hosts plagioclase, sodic-augite-diopside, calcic-amphibole and hauyne. Mineral thermobarometry and mineral-melt thermobarometry indicate that clinopyroxene and plagioclase in the 1971 Teneguía lavas crystallised at 20–45 km depth, coinciding with clinopyroxene and calcic-amphibole crystallisation in the kaersutite cumulate xenoliths at 25–45 km and clinopyroxene, calcic-amphibole and plagioclase crystallisation in the leucogabbro xenolith at 30–50 km. Combined mineral chemistry and thermobarometry suggest that the magmas had already crystallised, differentiated and formed multiple crystal populations in the oceanic lithospheric mantle. Notably, the magmas that supplied the 1949 and 1971 events appear to have crystallised deeper than the earlier Cumbre Vieja magmas, which suggests progressive underplating beneath the Cumbre Vieja rift zone. In addition, the lavas and xenoliths of the 1971 event crystallised at a common depth, indicating a reused plumbing system and progressive recycling of Ocean Island plutonic complexes during subsequent magmatic activity.  相似文献   

16.
Crystallization experiments on three comendites provide evidencefor the genetic relationships between peralkaline rhyolitesin the central Kenya rift valley. The crystallization of calcicclinopyroxene in slightly peralkaline rhyolites inhibits increasein peralkalinity by counteracting the effects of feldspar. Fractionationunder high fO2 conditions produces residual liquids that areless, or only slightly more, peralkaline than the bulk composition.In contrast, crystallization under reduced conditions (<FMQ,where FMQ is the fayalite–magnetite–quartz buffer)and at high fF2 inhibits calcic clinopyroxene and yields residualliquids that are more peralkaline than coexisting alkali feldspar,whose subsequent crystallization increases the peralkalinityof the liquid. A marginally peralkaline rhyolite [molar (Na2O+ K2O)/Al2O3 (NK/A) = 1·05] can yield a more typicallycomenditic rhyolite (NK/A = 1·28) after 95 wt % of crystallization.This comendite yields pantelleritic derivatives (NK/A >1·4)after 25 wt % crystallization. Upon further crystallization,extreme peralkaline compositions (NK/A  相似文献   

17.
The texture and mineral chemistry of trachytic lavas from Camel's Hump and Turntable Falls have been studied as an extension to bulk rock geochemical studies by previous workers. Three texturally distinct varieties are recognized: trachyte flows, chilled alkali trachytes, and pegmatoidal veins. Although each assemblage has a distinctive mineralogy, remnant features and similar zoning within minerals show the varieties to be related. Variations of mineral chemistry and stability within the three types are discussed. Data on the chilled alkali trachyte and the residual pegmatoidal phases indicate the development of a peralkaline residual liquid rich in volatiles. The data also provide further information on the mineralogical changes which occur as trachytic rocks become peralkaline.  相似文献   

18.
The relative ages of 21 lavas from Boqueron volcano in El Salvador were determined by superposition. The lavas are grey to black, porphyritic basalts, basaltic andesites and andesites with phenocrysts of plagioclase, augite, olivine, and magnetite. The andesitic lavas appear to have evolved from basaltic magma by fractionation of the observed phenocryst phases.The temporal variation in the chemical composition of the lavas at Boqueron is composed of three components. First, there is a crudely cyclical alternation of basalts and andesites. Second, these cycles are progressively shifted toward higher SiO2 contents. Third, approximately in the middle of the stratigraphic section sampled, there is an abrupt change in chemical variation trends from an Al-rich and Fe-poor trend to an Fe-rich and Al-poor trend. This change is interpreted to have been caused by an increased proportion of plagioclase fractionation and a decreased porportion of augite fractionation. The crudely cyclical change in SiO2 content with time is interpreted as a combination of crystal fractionation that increases SiO2 content, followed by influxes of basaltic magma that mix with residual magma to decrease SiO2 content. Successive cycles are shifted toward higher SiO2 content because there is a significant volume of fractionated magma remaining in the chamber before each influx of basalt.  相似文献   

19.
The southern Sinai Peninsula, underlain by the northernmost extension of the Arabian-Nubian Shield, exposes post-collisional calc-alkaline and alkaline granites that represent the youngest phase of late Neoproterozoic igneous activity. We report a petrographic, mineralogical and geochemical investigation of post-collisional plutons of alkaline and, in some cases, peralkaline granite. These granites intrude metamorphosed country rocks as well as syn- and post-collisional calc-alkaline granitoids. The alkaline and peralkaline granites of the southern tip of Sinai divide into three subgroups: syenogranite, alkali feldspar granite and riebeckite granite. The rocks of these subgroups essentially consist of alkali feldspar and quartz with variable amounts of plagioclase and mafic minerals. The syenogranite and alkali feldspar granite contain small amounts of calcic amphibole and biotite, often less than 3%, while the riebeckite granite is distinguished by sodic amphibole (5–10%). These plutons have geochemical signatures typical of post-collisional A-type granites and were most likely emplaced during a transition between orogenic and anorogenic settings. The parental mafic magma may be linked to lithospheric delamination and upwelling of asthenospheric mantle material. Differentiation of the underplated basaltic magma with contributions from the juvenile crust eventually yielded the post-collisional alkaline granites. Petrogenetic modelling of the studied granitic suite shows that pure fractional crystallization cannot quantitatively explain chemical variations with the observed suite, with both major oxides and several trace elements displaying trends opposite to those required by the equilibrium phase assemblage. Instead, we show that compositional variation from syenogranite through alkali feldspar granite to riebeckite granite is dominated by mixing between a low-SiO2 liquid as primitive or more primitive than the lowest-SiO2 syenogranite and an evolved, high-SiO2 liquid that might be a high-degree partial melt of lower crust.  相似文献   

20.
The alkalic pyroxenite nodule consists of megacrysts of diopside, apatite, perovskite and titanomagnetite in a groundmass consisting of diopside, apatite, titanomagnetite, nepheline, melilite, garnet and vishnevite crystals of various shapes, including previously undescribed skeletal and dendritic shapes, together with vesicles and residual glass. The residual glass is poor in SiO2 (38–40 wt%), and extraordinarily rich in Na2O (12.8–15 wt%), SO3 (1–1.5 wt%), and Cl (0.25–0.7 wt%), as a result of rapid, non-equilibrium crystallization of groundmass phases from a CO2-rich nephelinite melt.The Oldoinyo Lengai alkalic carbonatite lavas do not represent extreme products of the fractional crystallization of pyroxene, wollastonite, nepheline and alkali feldspar from the carbonated nephelinite melt. The most likely connection between the carbonatite and silicate magma types is one of liquid immiscibility, probably involving phonolite melt.  相似文献   

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