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1.
Feldspar phenocrysts, microphenocrysts, groundmass feldspar, interstitial material of feldspar composition, and residual SiO2-K2O-rich glass in 24 rocks of the tholeiitic, alkalic, and nephelinic suites from Haleakala and West Maui volcanoes, Maui, Hawaii, were analyzed quantitatively with the electron microprobe. Rocks studied include tholeiite, olivine tholeiite, oceanite, alkalic olivine basalt, alkalic basalt, hawaiite, mugearite, trachyte, basanite, and basanitoid. Results and conclusions: i) In all rocks studied, An decreases and Or increases from phenocrysts to microphenocrysts to groundmass feldspar to interstitial material of feldspar composition. ii) Phenocrysts occur in rocks of the tholeiitic and alkalic suites and, in spite of differences in bulk rock compositions, overlap in composition. iii) Groundmass feldspar in rocks of the tholeiitic suite are nearly identical in composition; the same is true for rocks of the nephelinic suite. However, in the highly differentiated alkalic suite, groundmass feldspar composition ranges from labradorite to sanidine; i.e. the higher the bulk rock CaO, the higher is the An content, and the higher the bulk K2O, the higher is the Or content. iv) In general, rocks with phenocrysts have groundmass feldspar less An-rich than those without phenocrysts. v) In rocks of the tholeiitic suite, normative feldspar approaches modal feldspar. However, in rocks of the alkalic and nephelinic suites, normative feldspar, because of the presence of highly alkalic interstitial material and the absence of nepheline in the mode but its presence in the norm, is drastically different from modal feldspar. vi) Hawaiites contain labradorite and not andesine, as per definition, and mugearite contains andesine and not oligoclase, as groundmass feldspar. In fact, when considering phenocrysts and interstitial material of feldspar composition, hawaiites range from bytownite to sanidine and mugearite from andesine to sodic sanidine, but normative feldspar plots in the andesine field for hawaiites and the oligoclase field for mugearite. vii) Rocks of the three suites can be distinguished on the basis of Or and An in groundmass feldspar, the presence of thin rims of groundmass composition of phenocrysts of rocks of the alkalic suite, and the presence of interstitial material of anorthoclase to sanidine composition in rocks of the alkalic and nephelinic suites. iix) Rocks transitional between the tholeiitic and alkalic suites are observed and are characterized by transitional mineral compositions.This paper was first presented as a talk before the 68. Annual Meeting of the Cordilleran Section of the Geological Society of America, Honolulu, Hawaii, March 29–April 1, 1972. 相似文献
2.
E. A. Mathez 《Contributions to Mineralogy and Petrology》1995,119(2-3):277-286
The rare earth element (REE) contents of pyroxenes and other minerals from the Merensky reef and stratigraphically adjacent rocks of the Atok section, Bushveld Complex, have been determined with the ion microprobe. Merensky reef clinopyroxene and orthopyroxene contain much higher and more variable concentrations of the REE than their cumulus counterparts in rocks several meters below the reef. Chondrite-normalized Merensky clinopyroxene Ce contents vary from 10 to 90 for Ce and from 4 to 17 for Yb. They also possess deep, negative Eu anomalies, the Eu anomalics being deeper for crystals having high REE contents and relatively shallow for pyroxenes with low REE contents. Similar compositional characteristics are displayed by Cl-rich apatite, which is an accessory phase in the rocks. Interstitial pyroxene in cumulates above and below the reef also tends to have elevated REE contents and in general is not in equilibrium with coexisting cumulus minerals. The melt from which the cumulus minerals crystallized falls within the compositional range of continental basalts; that from which Merensky and postcumulus pyroxenes crystallized is inferred to be much more highly enriched in REE than any normal tholeiitic or alkalic basalt. Despite their highly evolved nature in terms of the REE, the Merensky reef pyroxenes are not evolved in terms of major elements. The decoupling of incompatible trace and major elements is best explained by a metasomatic process. It is speculated that metasomatism involved upward percolation of hydrated silicate melt through and its reaction with the crystalline cumulate pile. The fact that the rocks enriched in the platinum group elements are also those that show evidence for metasomatism suggests that these elements were also metasomatically redistributed. 相似文献
3.
Richard O. Sack David Walker Ian S. E. Carmichael 《Contributions to Mineralogy and Petrology》1987,96(1):1-23
Experimental study of natural alkalic lava compositions at low pressures (pO2QFM) reveals that crystallization of primitive lavas often occurs in the sequence olivine, plagioclase, clinopyroxene, nepheline without obvious reaction relation. Pseudoternary liquidus projections of multiply saturated liquids coexisting with plagioclase (±olivine±clinopyroxene±nepheline) have been prepared to facilitate graphical analysis of the evolution of lava compositions during hypabyssal cooling. Use of (TiAl2)(MgSi2)–1 and Fe3+ (Al)–1 exchange components is a key aspect of the projection procedure which is succesful in reducing a wide range of compositions to a systematic graphical representation. These projections, and the experiments on which they are based, show that low pressure fractionation plays a significant role in the petrogenesis of many alkalic lava suites from both continental and oceanic settings. However, the role of polybaric fractionation is more evident in the major element chemistry of these lava suites than in many tholeiitic suites of comparable extent. For example, the lavas of Karisimbi, East Africa, show a range of compositions reflecting a polybaric petrogenesis from primitive picrites at 1360° C/18 kb and leading to advanced low pressure differentiates. Evolved leucite-bearing potassic members of this and other suites may be treated in a nepheline-diopside-kspar (+olivine+leucite) projection. Compositional curvature on the plagioclase+clinopyroxene+olivine+leucite cotectic offers a mechanism to explain resorption of plagioclase in alkalic groundmass assemblages and the incompatibility of albite and leucite. This projection is useful for evaluating the extent of assimilation of the alkalic portions of crustal granulites. Assimilation appears to have played some role in the advanced differentiates from Karisimbi. 相似文献
4.
A high-alumina tholeiitic andesite from the southern portion of the Tweed Shield Volcano in northeastern New South Wales contains abundant megacrysts of plagioclase (Ab50An46Or4) and megacrysts of aluminian bronzite (Ca4Mg71Fe25) and relatively Ca-poor aluminian augite (Ca36Mg41Fe20). The pyroxenes commonly occur as inclusions in the plagioclase megacrysts. Electron probe microanalyses of the pyroxene megacrysts indicate that they differ in character and composition from the groundmass ferromagnesian phases, namely a more Al- and Na-poor augite (Ca41Mg42Fe17) and olivine (Fa53). The bulk composition of the plagioclase megacrysts is slightly more Ab-rich than that of the groundmass plagioclase, but differences in the two compositions are extended by microanalyses of groundmass plagioclases. Evaluation of the megacryst compositions in the light of experimental data and analogous occurrences in alkaline volcanics leads to the interpretation that the megacrysts represent cognate precipitates formed at pressures broadly equivalent to the crust-mantle boundary. More important, they provide strong evidence for the high pressure origin of tholeiitic andesites, customarily interpreted as the products of low pressure fractional crystallization of tholeiitic magma. 相似文献
5.
Many metamorphosed and weathered basalts contain fresh clinopyroxene crystals set in an altered groundmass. Microprobe analysis of these relict grains can be used to identify the magma type of the host lava. Statistical discrimination of clinopyroxenes from known magma types provides a test of the effectiveness of this method, showing that any attempt to classify an unknown clinopyroxene as either an ocean-floor basalt, a volcanic arc basalt, a within plate tholeiite or a within plate alkali basalt magma type should have a 70% chance of success. Identification of within plate alkali basalts is most likely to be successful because their pyroxenes characteristically have high Na and Ti and low Si contents. Within plate tholeiites can usually be distinguished from volcanic arc basalts because their pyroxenes contain more Ti, Fe and Mn. However, neither of these last two magma types can be easily distinguished from ocean floor basalts on the basis of pyroxene analyses. Diagrams of pyroxene composition based on discriminant functions and on Na2O vs MnO vs TiO2, SiO2 vs TiO2 and SiO2 vs Al2O3 provide the basis for visual discrimination. The discrimination achieved is mainly due to differences in the bulk chemistry of the host magmas and in the partitioning of cations into the pyroxene lattice; differences in temperature and crystallization histroy of the magmas are of lesser, but nevertheless finite, importance. Application of this technique to pyroxenes in metabasalts from Othris, Greece gave results consistent with, but more ambiguous than, results obtained from immobile trace element studies. 相似文献
6.
Yin Yin Nwe 《Contributions to Mineralogy and Petrology》1975,49(4):285-300
Pyroxenes and olivines from the trough bands in the Upper Zone (UZa) of Skaergaard Intrusion have been investigated, together with previously analysed pyroxenes (Brown, 1957; Brown and Vincent, 1963) and olivines from the Layered Series ferrodiorites. The electron microprobe, electron microscope, and analytical electron microscope EMMA-4 were used. Results show a striking difference between the cumulus and intercumulus trends of the trough-band pyroxenes. The cumulus trend follows that of the main Layered Series whereas the intercumulus trend shows a shrinking of the miscibility gap together with great enrichment in the Fs molecule, the miscibility gap being symmetrical about ～Wo24. The shrinking appears to be a function of the different crystallisation conditions in the intercumulus liquid which was closed off in “cells” from the main mass of supernatant liquid. Enrichment in the Fs molecule is due to the much lower crystallisation temperatures of the intercumulus pyroxenes. Iron enrichment is also reflected in the intercumulus olivines. For the cumulus trend, Brown's calcium-poor pyroxene trend (1957) has been extended into more iron-rich parts of the pyroxene quadrilateral, well after olivine has reappeared and subsequent to the increase in calcium of the ferroaugites. The subsolidus trend for pyroxenes in the Fs-rich region has also been established. 相似文献
7.
Mafic dykes of Almora region intrude the Precambrian crystalline rocks of Kumaun Lesser Himalaya. Mafic dykes exhibit fine grained margin and medium to coarse grained core, melanocratic, low to highly ferromagnetic (MS=0.85?38.58×10?3SI) in nature commonly showing subophitic to ophitic textures with ol-pl-cpx-hbl-bt-mt-ap-sp assemblage, and modally correspond to leucogabbro and olivinegabbro (sensu stricto). Olivine (Fo61-Fo33), clinopyroxene (Wo46-En42-Fs22 to Wo40-En36-Fs15) and plagioclase (An58-An12) have crystallized in the temperature range of ca1400–980°C at pressure <2 kbar in an olivine tholeiitic basalt parent. Low acmite (Napfu=0.033?0.025), (Mg#=0.64–0.82), Ti-Al contents of clinopyroxenes and their evolution along enstatite-ferrosilite join (i.e. Mg?Fe substitution) strongly suggest tholeiitic nature of mafic dyke melt with changing activities of alumina and silica. Clinopyroxene compositions of mafic dykes differ markedly as compared to those observed for adjoining Bhimtal volcanics but closely resemble to that crystallized in tholeiitic melts of Deccan province. Observed Cr vs Mg# variation, enriched LILE (Sr, Ba)-LREE and positive Eu-anomaly of the studied mafic dykes are indicative of fractional crystallization of olivine-clinopyroxene -plagioclase from a crustally-contaminated tholeiitic basalt magma derived from enriched mantle source. The mafic dykes of Almora are geochemically identical to mafic dykes of Nainital, but are unrelated to Precambrian mafic volcanic flow and dykes of NW Himalaya and dykes of Salma and Rajmahal regions. 相似文献
8.
The late-stage basanite (～12–1 Ma) of the NNW-SSE extending Gharyan Cenozoic volcanics (Northern West Libya) contains numerous fresh lherzolite xenoliths. These xenoliths display magmatic protogranular and porphyroclastic textures. Chemistry of olivine (forsterite content –90–91, NiO = 0.26–0.39 wt %), orthopyroxene (Mg# = 0.91–0.92, Cr# = 0.03–0.07, Al2O3 = 3.64–4.43 wt %), clinopyroxene ((Wo45.59–48.61, En45.89–48.80, Fs4.47–5.81), Mg# = 0.82–0.92, Al2O3 = 5.14–6.58 wt % and Cr2O3 = 0.5–0.95 wt %) and spinel (hercynite–picotite–Al-rich chromite spinels with Cr# = 0.08–0.11) refer to the fertile nature of Gharyan peridotites. Mantle source region was close to the primitive composition with low degrees of melting and depletion in melt (1.5% in average). However, it underwent metasomatism as illustrated by formation of secondary clinopyroxene (Wo37.96–44.77, En47.44–54.18, Fs7.59–8.03) with high contents of Na2O, and enrichment of the Al-spinel in TiO2. The estimated pre-eruptive temperature ranges from 1066 to 837°C. Despite the Gharyan peridotite is similar to mantle components in many districts in Saharan belt of North Africa, it represents juvenile mantle source with minor refractory residues. This compositional heterogeneity is mainly attributed to the local effect of the interaction of the Gharyan mantle with the host basanite magma that may be related to the Cenozoic rifting of the Pan-African basement. 相似文献
9.
Iron-magnesium distribution coefficients for coexisting ortho- and clinopyroxene in 22 amphibolites from the New Jersey Precambrian Highlands range from 1.40 to 1.90. No systematic areal variation of the distribution coefficient is discernable within a 700 mi2 area. The distribution coefficient is, however, systematically related to pyroxene composition. The distribution coefficient tends to increase with increasing pyroxene weight % FeO (Fe as FeO) and decrease with increasing MgO and Al2O3. Data from other workers indicates that the distribution coefficient versus pyroxene composition trends found in the Highlands amphibolites are also present in both igneous and metamorphic rock suites from several other areas. Possible influence of pyroxene CaO on the distribution coefficient is also indicated. The Highlands amphibolite type trends are, however, directly opposite to those previously reported for Australian granulites. Both types of trends are apparently valid since both are present in at least one instance in metamorphic rocks from a relatively small area. The causes for the development of the two types of trends are imperfectly understood. Data presented indicates, however, that the New Jersey amphibolite type trends are apparently more characteristic of Fe-poor pyroxenes, whereas, the Australian granulite type trends are more characteristic of Fe-rich pyroxenes.The distribution coefficient in the Highlands amphibolites is also systematically related to bulk-rock composition due to the sympathetic variation of pyroxene Fe-Mg content with total rock MgO/FeO(Fe as FeO). The observed range of the distribution coefficient in the Highlands amphibolites may, consequently, mostly reflect variation in bulk-rock composition and not variation in crystallization temperature. 相似文献
10.
Hualalai Volcano, Hawaii, is best known for the abundant and varied xenoliths included in the historic 1800 Kaupulehu alkalic basalt flow. Xenoliths, which range in composition from dunite to anorthosite, are concentrated at 915-m elevation in the flow. Rare cumulate ultramafic xenoliths, which include websterite, olivine websterite, wehrlite, and clinopyroxenite, display complex pyroxene exsolution textures that indicate slow cooling. Websterite, olivine websterite, and one wehrlite are spinel-bearing orthopyroxene +olivine cumulates with intercumulus clinopyroxene +plagioclase. Two wehrlite samples and clinopyroxenite are spinel-bearing olivine cumulates with intercumulus clinopyroxene+orthopyroxene + plagioclase. Two-pyroxene geothermometry calculations, based on reconstructed pyroxene compositions, indicate that crystallization temperatures range from 1225° to 1350° C. Migration or unmixing of clinopyroxene and orthopyroxene stopped between 1045° and 1090° C. Comparisons of the abundance of K2O in plagioclase and the abundances of TiO2 and Fe2O3in spinel of xenoliths and mid-ocean ridge basalt, and a single 87Sr/ 86Sr determination, indicate that these Hualalai xenoliths are unrelated to mid-ocean ridge basalt. Similarity between the crystallization sequence of these xenoliths and the experimental crystallization sequence of a Hawaiian olivine tholeiite suggest that the parental magma of the xenoliths is Hualalai tholeiitic basalt. Xenoliths probably crystallized between about 4.5 and 9 kb. The 155°–230° C of cooling which took place over about 120 ka — the age of the youngest Hualalai tholeiitic basalt — yield maximum cooling rates of 1.3×10–3–1.91×10–3 °C/yr. Hualalai ultramafic xenoliths with exsolved pyroxenes crystallized from Hualalai tholeiitic basalt and accumulated in a magma reservoir located between 13 and 28 km below sealevel. We suspect that this reservoir occurs just below the base of the oceanic crust at about 19 km below sealevel. 相似文献
11.
Yin Yin Nwe 《Contributions to Mineralogy and Petrology》1976,55(1):105-126
Pyroxenes and olivines from the earlier stages of fractionation of the Skaergaard intrusion (Wager and Brown, 1968; Brown, 1957) have been studied using the electron microprobe. The subsolidus trend for both Ca-rich and Ca-poor pyroxenes has been established, from the Mg-rich portion of the quadrilateral to the Hed-Fs join, together with the orientations of the tie-lines joining coexisting pyroxenes. For the Mg-rich Ca-poor pyroxenes, Brown's (1957) solidus trend has been modified slightly. From a study of a previously undescribed drill core, reversals in the cryptic layering have been found in the Lower Zone. The reversals are attributed to existence within the convecting magma chamber of local temperature differences. The Skaergaard magma temperatures are postulated to have passed out of the orthopyroxene stability field into the pigeonite stability field at EnFs ratios of 7228, for Ca-free calculated compositions, and specimen 1849, a perpendicular-feldspar rock, is interpreted as straddling the orthopyroxene-pigeonite transition interval. The cessation of crystallisation of Ca-poor pyroxene and the increase in Wo content of the Ca-rich pyroxene trend have been reexamined, and Muir's (1954) peritectic reaction (pigeonite+liquid=augite) has been confirmed. The composition at which Ca-poor pyroxene starts reacting with the liquid is postulated as Wo10 En36.7Fs53 3. It is suggested that the cessation of crystallisation of Ca-poor pyroxene is sensitive to the amount of plagioclase crystallising from the liquid.A complete series of accurate olivine compositions for the whole Skaergaard sequence is presented for the first time, including the compositions of the Middle Zone olivine reaction rims. 相似文献
12.
The electron probe X-ray microanalyzer has been used to determine the compositional variability of the groundmass minerals and glass in 10 specimens from a complete 225-foot section of the prehistoric tholeiitic lava lake of Makaopuhi Crater, Hawaii. The order of beginning of crystallization was: (1) chromite, (2) olivine, (3) augite, (4) plagioclase, (5) pigeonite, (6) iron-titanium oxides and orthopyroxene, (7) alkali feldspar and apatite, and (8) glass.Although the lake is chemically tholeiitic throughout, the occurrence of ferromagnesian minerals is as though there were a gradation from alkali olivine basalt in the upper chill downwards to olivine tholeiite. Groundmass olivine decreases downwards and disappears at about 20 feet. Pigeonite is absent in the uppermost 5±2 feet, then increases in amount down to 20 feet, below which augite and pigeonite coexist in constant 21 proportions. Strong zoning and metastable compositions characterize the pyroxenes of the chilled zones, but these features gradually disappear towards the interior of the lake to give way to equilibrium pyroxenes. Relatively homogeneous poikilitic orthopyroxene ( Ca4Mg70Fe26) occurs in the olivine cumulate zone, having formed partly at the expense of pre-existing olivine, augite, and pigeonite ( Ca8Mg66Fe26). The growth of orthopyroxene is believed to have been facilitated by the slower cooling rate and higher volatile pressure at depth, and by the rise in Mg/Fe ratio of the liquid due to the partial dissolution of settled olivine.Unlike olivine and pyroxene, feldspar is least zoned in the upper and lower chilled regions. The greatest range of compositional zoning in feldspar occurs at 160 to 190 feet, where it extends continuously from Or1.0Ab22An77 to Or64Ab33An3. The feldspar fractionation trend in the An-Ab-Or triangle gradually shifts with depth toward more equilibrium trends, even though the zoning becomes more extreme. The variation with depth in the initial (core) composition of the plagioclase suggests the influence of either slow nucleation and growth (undercooling) or slow diffusion in the liquid, relative to the rate of cooling.Idiomorphic opaque inclusions in olivine phenocrysts are chrome-spinels showing continuous variation from 60 percent chromite to 85 percent ulvospinel and to magnetite-rich spinel. A pre-eruption trend of increasing Al with decreasing Cr can be recognized in chromites from the upper chill. Most of the inclusions show a trend of falling Cr and Al, toward an ulvospinelmagnetite solid solution which is progressively poorer in Usp with depth. This trend was produced by solid state alteration of the chromite inclusions during cooling in the lava lake. Ilmenite (average Ilm91Hm9) coexists with variably oxidized titaniferous magnetite in the basalt groundmass. Estimated oxygen fugacities agree well with other independent determinations in tholeiitic basalt. No sulfide phase has been detected.Fractional crystallization produced a groundmass glass of granitic composition. Average, in percent, is: SiO2, 75.5; Al2O3, 12.5; K2O, 5.7; Na2O, 3.1; CaO, 0.3; MgO, 0.05; total FeO, 1.2; and TiO2, 0.8. Normative Or> Ab. Minor changes in glass composition with depth are consistent with a greater approach towards the granite minimum. Incipient devitrification precluded reliable analysis of glass from the lower half of the section. The SiO2-phase associated with devitrification contains alkalis and Al and is believed to be cristobalite. Needle-like apatite crystals in the groundmass glass are Siand Fe-bearing fluorapatites containing appreciable rare earths (predominantly Ce) and variable Cl.The grain-size and maximum An content of the cores of plagioclase grains were controlled by cooling rate and are at a maximum at the center of the section. The most homogeneous pyroxene (and olivine, Moore and Evans, 1967), most equilibrium pyroxene trends, most abundant alkali feldspar, and most equilibrium feldspar trends are found at 160 to 190 feet, which is appreciably below that part of the lake which was slowest to crystallize. Volatile pressure, increasing with depth, possibly controlled the degree of attainment of equilibrium more than cooling rate.Since they are dependent on cooling history, some of the modal criteria commonly used for recognizing basalt types, such as the absence of Ca-poor pyroxene, presence of groundmass olivine, and the presence of alkali feldspar, should be applied with caution. Petrographic comparison of basalts from one flow, volcano, or province, with another, should recognize the possible variations due to cooling history alone.Publication authorized by the Director, U.S. Geological Survey 相似文献
13.
The MD dyke swarm is composed of four generations of large basictholeiite dykes which cut the entire Archaean craton of southernWest Greenland. The four successive generations (MD1, MD2, MD3a,MD3b) are characterized by their orientation and cross-cuttingrelationships and by their mineralogy, texture and progressivelyevolved tholeiitic chemistry. Rare-earth element (REE) abundancessuggest that the dykes may have a fairly complex petrogeneticevolution. The suite varies from early (MD1) heteradcumulatenorites to ophitic and sub-ophitic gabbroic and doleritic rocks(MD2 and MD3) and the youngest generation (MD3b) comprises plagioclase-phyricdolerites. The pyroxene chemistry parallels the geochemical evolution ofthe dykes showing an overall Fe-enrichment trend. However, theclinopyroxenes are enigmatic in that, although they occur predominantlyas part of medium and coarse-grained holocrystalline textures,they are chemically highly variable and calcium-poor, many plottingin the metastable field in the system MgSiO3 (En)-CaSiO3 (Wo)-FeSiO3(Fs). Many individual grains are extremely complex and may beregularly or irregularly zoned. Along with more typical pyroxene forms, the MD1 dykes containpyroxene dendrites poikilitically enclosed by plagioclase. Thedendrites vary compositionally from hypersthene bases to branchesof pigeonite and subcalcic augite and terminate in augite branchtips. The MD2 and MD3a dyke pyroxenes are the most complex.The majority of them are sub-ophitic grains, many with successivezones of orthopyroxene, pigeonite, subcalcic augite, augiteand ferroaugite. However, Ca-enrichment or Ca-depletion, Fe-enrichmentor Fe-depletion and apparently opposing zoning trends can occurin neighbouring grains. Even small interstitial pyroxenes showa very wide range of compositions. Morphologically unusual andcomplex clinopyroxene ‘cylinders’ occur in someof the MD3a dykes. They are chemically relatively uniform andare normal tholeiitic augites. The MD3b rocks have small concentricallyzoned sub-ophitic pyroxenes which show Ca-enrichment with arelatively constant Fs component (29 to 39 mol. per cent). Themost extremely zoned grains have hypersthene cores with successivecoronas of pigeonite and subcalcic augite and have margins ofaugite or ferroaugite. The present ‘coexistence’of such compositionally widely variable pyroxenes and the extremeand often irregular nature of their chemical zoning make thedetermination of true original coexisting pyroxene phases andthe use of a two pyroxene geothermometer very difficult andof limited significance. The presence of a wide variety of pyroxenes of apparently bothstable and metastable compositions in these holocrystallinedykes suggests that these rocks have undergone a complex andrather unusual cooling history. The principal genetic factorswhich could have influenced their crystallization are (1) supercooling,(2) the evolution of discrete interstitial liquid cells, (3)augite-pigeonite peritectic reactions and (4) plagioclase growthand delay of pyroxene nucleation during supercooling of liquidto below the basalt liquidus. 相似文献
14.
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
15.
John F. Lewis 《Contributions to Mineralogy and Petrology》1973,38(3):197-220
Ejected plutonic blocks from the Soufrière volcano, St. Vincent, consist of the mineral phases plagioclase (An96?An89, average An93), olivine (Fo79?Fo67, most frequent interval Fo74?72), salite containing 5–6 percent Al2O3, hastingsitic amphibole and magnetite containing 6% Al2O3, 4% MgO and 7% TiO2. Aluminous salitic pyroxenes are not confined to alkaline suites but also occur as high temperature phases in certain calc-alkaline suites. Similarly low silica amphiboles with comparatively high Fe+3+Ti contents are unusual in common sub-alkaline rocks. Magnetite in the St. Vincent blocks is an example of a single homogeneous iron oxide phase precipitated in equilibrium with the other four phases in the blocks. Conditions of crystallisation are just as important as silica activity in determining the compositions of phases separating from basaltic magmas at relatively shallow depths (P tot< 8 kb). It is argued mainly on crystal chemical grounds that the mineral assemblage in the St. Vincent blocks can crystalline from a sub-alkaline magma under conditions of relatively high temperature, high water vapor pressure, and oxygen fugacity. 相似文献
16.
Most of the Al3+ entering the pyroxenes does so by substituting for tetrahedral Si4+. This creates a charge imbalance that requires the simultaneous entry of Cr3+, Ti4+, Fe3+ or Al3+ into octahedral sites. Cr3+, because of its high crystal field stabilisation energy (CFSE), is the most important of these elements to enter the early-formed pyrosenes but it is replaced by Ti4+ later in fractionation when the Cr3+ content of the melt becomes depleted. The dependence of Cr3+ and Ti4+ on charge balance controls their partition between coexisting pyroxenes and olivines. Ca-rich pyroxene which contains more Al3+ than Ca-poor pyroxene also has more Ti4+ and Cr3+ whereas olivine, which contains negligible Al3+, has low Cr3+ and Ti4+. The Al3+ content of pyroxenes is influenced by changes in P, T, \(a_{{\text{SiO}}_{\text{2}} }\) and \(a_{{\text{Al}}_{\text{2}} {\text{O}}_{\text{3}} }\) of the magma and by the nature of the ion providing charge balance in the octahedral site. Of these \(a_{{\text{SiO}}_{\text{2}} }\) is dominant and variations in the Al3+ content of the Jimberlana pyroxenes correspond closely with the expected changes in the \(a_{{\text{SiO}}_{\text{2}} }\) of the melt. The substitution of divalent ions, such as Mn2+ and Ni2+, in the pyroxene lattice is by replacement of Fe2+ or Mg2+ in the octahedral M 3 and M 2 sites and is therefore independent of charge balance. If there are no size restrictions, the principal factor to be considered is the CFSE the ion receives in octahedral co-ordination. Ni2+, which receives a high CFSE, partitions strongly between the early-formed pyroxenes and olivines and therefore becomes depleted in the magma with fractionation. Conversely Mn2+, which receives zero CFSE, concentrates in the magma with fractionation and becomes a more important substitute in the later-formed pyroxenes. Its geochemical behaviour is controlled by its size. The narrow miscibility gap of the Jimberlana pyroxenes and the high En content of the Ca-poor pyroxenes at the bronzite pigeonite changeover suggest that these pyroxenes crystallised at a higher temperature than pyroxenes of comparable composition from other intrusions. 相似文献
17.
Six lithologic units in tectonic contact with each other have been defined during mapping of the Devonian in the Beaujolais area of the northeastern Massif Central. Five main igneous suites have been recognized:
- A transitional basaltic suite restricted to a single unit.
- An acid volcanic-plutonic suite the members of which are related by fractional crystallization and magma mixing.
- Low-TiO2 volcanic rocks with calc-alkaline affinities.
- A TiO2-rich tholeiitic suite related to an ophiolitic complex.
- A plutonic suite with close resemblances to Alaskantype intrusions.
18.
Thomas Frisch 《Contributions to Mineralogy and Petrology》1970,28(1):31-41
A gabbro inclusion typical of the gabbroic xenolith suite in alkalic basalts on Lanzarote is composed of essentially four minerals. Olivine (5 vol.%) and plagioclase An81 (60%) were first to crystallize. The olivine, Fo76 with 0.02% CaO, was subsequently subjected to high-temperature oxidation followed by hydrolysis, resulting in its partial alteration to Fo92 and Fe-oxide-hydroxide plus (free?) silica. Clinopyroxene (30%) Ca43Mg46Fe11 has exsolved orthopyroxene Ca1.6Mg76.0Fe22.4 with 2.10% A12O3 in lamellae ∥ {100}. Orthopyroxene (5%) of the same composition also occurs as discrete crystals and petrographic and chemical criteria suggest that it formed by reaction of olivine with melt. The xenolith probably crystallized from a tholeiitic melt at a depth less than 9 km, i.e. above the Moho discontinuity under Lanzarote. The existence of basalts of olivine-tholeiitic chemistry on Lanzarote suggests a source for the gabbro nodules. Separation of calcic plagioclase from a tholeiitic melt may explain the relatively high alkali contents (Na2O+ K2O= 3.2 to 3.7%) of the Lanzarote olivine-tholeiitic basalts. 相似文献
19.
Experimental results show that skarns can—800°C and 500–1,000 bars. Starting materials include intermediate-acidic igneous rocks, volcanic rocks, metamorphic rocks, carbonates of various purities and chemical reagents of analytical purity-grade. Experimental media are: NaCl, NaCl+CaCl2, NaCl+CaCl2+MgCl2, Na2CO3 and Na2SiO3 solutions. Experimental results show that skarns can be formed under wide physico-chemical conditions:T=400°–800°C,P=500–1,000 bars,pH=4–11, and \(f_{O_2 } = 10^{ - 23} - 10^{ - 11} \) bar. The mineralogy of skarns and the chemical compositions of skarn minerals are generally controlled by the combined factors: the chemical composition of the original rocks, pH values, redox conditions, temperatures and pressures. Isomorphous substitution may have agreat effect on the temperature of formation andfo 2 of some major skarn minerals. It is found that skarnization occurs preferentially in NaCl and NaCl+CaCl2 solutions and subortinately in MaCO3 and Na2SiO3 soiutions. 相似文献
20.
Gregg W. Morrison 《Lithos》1980,13(1):97-108
A review of the major occurrences of shoshonitic rocks suggests there is a group that is near-silica saturated, K-rich and has low iron enrichment that cannot be unambigously classified as part of calc-alkaline or alkali-basalt associations. This group is here referred to as the shoshonite rock association. The shoshonite rock association is characterised by: hypersthene-olivine normative basalts, low iron enrichment, high Na2O + K2O, high content of light ion lithophile elements, high but variable Al2O3, high Fe2O3/FeO and low TiO2. Mineralogical characteristics include: coexisting plagioclase and sanidine in the groundmass, K-feldspar rims on plagioclase phenocrysts, plagioclase An50?85 Ab40?15 Or10?0 and low TiO2 content and lack of iron enrichment in clinopyroxene. Shoshonitic rocks on continental margins are younger, stratigraphically higher and more distant from the oceanic trench than the high-K calc-alkaline or calc-alkaline suites, but there is a complete gradation between the suites. A similar zonation occurs in some island arcs. In other island arcs there is no spatial zonation of the suites but successively more K-rich lavas are produced above an ever steepening subduction zone. Steepening leads to ‘failure’ or flipping of the subduction zone and uplift and block faulting within the arc. Shoshonitic rocks are most commonly associated with this phase of island are development. 相似文献