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1.
A Permo-Triassic pelite-carbonate rock series (with interacalated metabasitic rocks) in the Cordilleras Béticas, Spain, was metamorphosed during the Alpine metamorphism at high pressures (P min near 18 kbar). The rocks show well preserved sedimentary features of evaporites such as pseudomorphs of talc, of kyanite-phengitetalc-biotite, and of quartz after sulfate minerals, and relicts of baryte, anhydrite, NaCl, and KCl, indicating a salt-clay mixture of illite, chlorite, talc, and halite as the original rock. The evaporitic metapelites have a whole rock composition characterized by high Mg/(Mg+Ca) ratios>0.7, variable alkaline and Sr, Ba, contents, but are mostly K2O rich (<8.8 wt%). The F (<2600 ppm), Cl (<3600 ppm), and P2O5 (<0.24 wt%) contents are also high. The pelitic member of this series is a fine grained biotite rock. Kyanite-phengite-talc-biotite aggregates in pseudomorphs developed in the high pressure stage. Albite-rich plagioclase was formed when the rocks crossed the albite stability curve in the early stages of the uplift. Scapolite, rich in NaCl (Ca/(Ca+Na) mol% 24–40) and poor in SO4, with Cl/(Cl+CO3) ratios between 0.6 and 0.8, formed as porphyroblasts, sometimes replacing up to 60% of the rock in a late stage of metamorphism (between 10 and 5 kbar, near 600°C). No reaction with albite is observed, and the scapolite formed from biotite by: $$\begin{gathered} Al - biotite + CaCO_3 + NaCl + SiO_2 \hfill \\ = Al - poor biotite + scapolite + MgCO_3 + KCl \hfill \\ + MgCl_2 + H_2 O \hfill \\ \end{gathered}$$ Calculated fluid composition in equilibrium with scapolite indicates varying salt concentrations in the fluid. Distribution of Cl and F in biotite and apatite also indicates varying fluid compositions.  相似文献   

2.
In the central area of the early paleozoic Damara belt migmatites and some types of granites (sensu lato) crop out. Conventional petrographic data were collected and melting experiments were carried out with 63 samples of such rocks. Based on these data, migmatite genesis can be classified as follows:
  1. Formation by partial melting and separation of more liquid and more crystalline fractions, the melting behaviour of these fractions has to be principally similar then.
  2. Formation by injection of “geologically liquid” crystal-melt mixes into rocks of (incidentally) similar or (more frequently) dissimilar melting behaviour.
  3. Formation by partial melting of metasediments of originally different composition. The method is applied and demonstrated for outcrops with complicated interrelations and it is shown that temperature estimates are possible thereby (670° to 710° at 4–5 kb H2O pressure in the central Damara belt).
  相似文献   

3.
Samples of micaschists, gneisses, and migmatites from a sequence of metamorphic subfacies of the Gévennes Médianes (Dép. Ardèche, France) are characterized chemically by
  1. a high and only slightly varying Al2O3 content (m.v.=17.57 wt.-%, stand, dev.=1.804, var. coeff.=0.103).
  2. a negligible variation of the molar MgO/MgO+FeO ratio (m.v.=0.504, stand. dev.= 0.066, var. coeff.=0.130), and
  3. an insignificant variation of the molar CaO/CaO + Na2O ratio (m.v.=0.360, stand. dev.=0.014, var. coeff.=0.039).
The Na2O and K2O values of the migmatite samples are respectively 14 and 30% higher than those of the samples of the neighbouring gneisses and schists. The concentrations of all other major elements of the migmatites are within the variation limits determined for the gneisses and schists. Samples displaying initial migmatization have the following petrography:
  1. The light-coloured portions of the migmatites (leueosome) contain alkalifeldspar, plagioclase, and quartz, the dark-coloured ones (melanosome) are always free of alkalifeldspar and contain, besides ferromagnesian constituents, both plagioclase and quartz.
  2. The plagioclase of the melanosome has a remarkably higher An-content (An30–40mol.-%) than that one of the leueosome (An 9–14 mol.-%).
These observations seem to represent petrographic evidence that leueosome and melanosome have been in a restite-melt-relation at a temperature that just reached or slightly exceeded the value necessary to produce the beginning of melting. In addition, major element analyses of separated leueosome portions of samples displaying the petrography mentioned above have been undertaken. Normative Qz ∶ Ab: Or-ratios calculated from these analyses vary within very narrow limits about the average value of Qz ∶ Ab ∶ Or=36 ∶ 31 ∶ 33. Using the diagrams of von Platen (1965) concerning the crystallization of granitic melts the petrographic findings can be verified from analytical results, i.e. the migmatites of the Beaume valley are considered to be products of minimum temperature melting of the gneisses which still occur unaltered in adjoining areas of lower metamorphic grade.  相似文献   

4.
The occurrence and origin of marialitic scapolite in the Humboldt lopolith was investigated in the field and in the laboratory using petrographic and experimental techniques. Scapolite occurs in three modes: as a pervasive replacement of plagioclase and other minerals in gabbro, diorite and extrusive rocks; as a poikiloblastic mineral in scapolitite dikes; and as a fracture-filling mineral with analcime, albite and sphene in scapolite veins. Additional secondary minerals associated with scapolite include epidote, prehnite, hornblende and diopside-salite clinopyroxene. Relations with these minerals suggest that most marialitic scapolite grew at temperatures around 400° C. Scapolite composition varies from EqAn12 to EqAn37, containing from 72 to 96 atomic% Cl in the R position. Experiments on systems of similar compositions indicate that NaCl-H2O fluid having more than 40 mol% NaCl is needed to stabilize the scapolite.Variation in scapolite compositions is due to thermal and fluid compositional gradients normal to conduits of hydrothermal fluids, and occurs on a scale up to 100 m. The likely source of Na and Cl is pre-existing evaporites or evaporitic brine derived from the wallrocks. Salinity could have been increased to a level sufficient to stabilize scapolite by hydration of an originally dry magma, possibly aided by hydrothermal boiling. Results may be applied to hydrothermal alteration in areas of rifting or back-arc spreading, and in mid-ocean ridge hydrothermal systems.  相似文献   

5.
Phase relations of pumpellyite, epidote, lawsonite, CaCO3, paragonite, actinolite, crossite and iron oxide are analysed on an Al-Ca-Fe3+ diagram in which all minerals are projected from quartz, albite or Jadeite, chlorite and fluid. Fe2+ and Mg are treated as a single component because variation in Fe2+/Mg has little effect on the stability of phases on the diagram. Comparison of assemblages in the Franciscan, Shuksan, Sanbagawa, New Caledonia, Southern Italian, and Otago metamorphic terranes reveals several reactions, useful for construction of a petrogenetic grid:
  1. lawsonite+crossite + paragonite = epidote+chlorite + albite + quartz + H2O
  2. lawsonite + crossite = pumpellyite + epidote + chlorite + albite+ quartz + H2O
  3. crossite + pumpellyite + quartz = epidote + actinolite + albite + chlorite + H2O
  4. crossite + epidote + quartz = actinolite + hematite + albite + chlorite + H2O
  5. calcite + epidote + chlorite + quartz = pumpellyite + actinolite + H2O + CO2
  6. pumpellyite + chlorite + quartz = epidote + actinolite + H2O
  相似文献   

6.
Geologic, petrological and geochemical investigations have been carried out in the western part of the “Zillertaler Alpen”. Important results are:
  1. The premetamorphic material of the Greiner series consists of conglomerates, breccias, arcosic-sandstones or greywackes, bituminous shales, volcanic lavas and tuffs.
  2. The southern part of the “Zentralgneis” shows a differentiation trend from alkaline granite to quarzdiorite with predomination of granodiorite.
  3. Chemical relationships of granodiorite to its restitic inclusions allow the supposition of a palingenetic origin of the granitic rocks.
  4. At least two stages of metamorphism can be differentiated.
  5. Parts of the Greiner series, covered by triassic metasediments, are supposed to be of Permian age. A lower age boundary can not yet be given.
  6. Some vertical, northeast striking faults with throws of more than 1 or 2 kilometers produced southward verging drag folds.
  7. Geodynamic aspects, revealed from regional metamorphism and tectonics, are discussed.
  相似文献   

7.
Prior experimental work has shown that in the laboratory the mineralogy of eclogites is sensitive to the ratio of CaO ∶ MgO ∶ FeO and that the reaction pyroxene + kyanite?garnet + quartz proceeds to the right at high pressures in rocks rich in magnesium and to the left in rocks rich in calcium and iron. Typical basalts crystallized at high pressure never contain kyanite. The chemistry and mineralogy of a large number of naturally occurring eclogites show they belong to three classes.
  1. Kyanite-free magmatic eclogites, rich in magnesium, from:
  2. kimberlites
  3. dunites and serpentinites.
  4. Kyanite-bearing eclogites and grosspydites rich in CaO and low in FeO with intermediate MgO from:
  5. kimberlites
  6. gneisses.
  7. Kyanite-free eclogites of metamorphic origin rich in iron with low magnesium and intermediate amounts of calcium from:
  8. glaucophane schists
  9. gneisses.
  相似文献   

8.
Scapolite solid solution has been synthesized at 750°C and 4 kbar and is stable relative to plagioclase + calcite + halite over the range of plagioclase compositions from Ab85An15 to Ab70An30, although albite + halite is stable relative to marialite, Na4Al3Si9O24Cl, and anorthite + calcite is stable relative to meionite, Ca4Al6Si6O24CO3. A chloride-free scapolite, mizzonite, has been synthesized at the approximate composition NaCa3Al5Si7O24CO3 (Ab. 2An. CaCO3). In the absence of chloride, a three-phase invariant assemblage, sodic plagioclase (~Ab60An40) + scapolite + calcite is stable relative to plagioclase + calcite over the approximate range of plagioclase composition Ab60An40-Ab35An65 and another three-phase invariant assemblage, calcic plagioclase (~Ab15An85) + scapolite + calcite is stable over the approximate range Ab30An70-An15An85.Unit-cell dimensions and refractive indices have been determined for the scapotite synthesized in these experiments and are compared with values for chemically analyzed natural scapolites.Scapolite must be regarded as a ternary solid solution in which, at a given equivalent An-content, the Cl/CO3 ratio in the large anion site can vary as a function of NaCl and CaCO3 activities.  相似文献   

9.
The petrography, mineral chemistry and petrogenesis of a sample from the Weissenstein eclogite, Bavaria, Germany, has been investigated. The total mineral assemblage comprises garnet, clinopyroxeneI+II, quartz, amphiboleI+II, rutile, phengite, epidote/allanite, plagioclase, biotite, apatite, pumpellyite, titanite (sphene), zircon, alkali feldspar and calcite. Textural observations combined with geothermobarometry (Fe/Mg distribution between clinopyroxene/garnet and phengite/garnet; jadeite-content of omphacite, Si-content of phengite, and An-content of plagioclase) provide indications of two different stages in the metamorphic evolution of the rock. The main phengitequartz-eclogite mineral equilibration occurred at minimum P=13–17kbar, minimum T=620±50° C; the retrograde symplectite stage (clinopyroxeneII, amphiboleII, biotite, plagioclase) occurred at P total between 12 and 8.5 kbar. Reactions of the symplectite stage are:
  1. phengite (core) + Na2Oaq + CaOaq=phengite (rim) + biotite + plagioclase + K2Oaq + H2O
  2. phengite (core) + clinopyroxeneI + Na2Oaq=phengite (rim + biotite + plagioclase + amphiboleII + SiO2 + K2Oaq + CaOaq + H2O
  3. clinopyroxeneI + SiO2 + K2Oaq + H2O=clinopyroxeneII + plagioclase+amphiboleII + Na2Oaq + CaOaq
The phengite decomposition produces H2O, whereas the clinopyroxene decomposition consumes H2O. The estimated P-T-conditions for the Weissenstein eclogite are in the same order of magnitude as those for other eclogite bodies from the Alps and Caledonides believed to be related to subduction processes.  相似文献   

10.
The stability relations between cordierite and almandite in rocks, having a composition of CaO poor argillaceous rocks, were experimentally investigated. The starting material consisted of a mixture of chlorite, muscovite, and quartz. Systems with widely varying Fe2+/Fe2++Mg ratios were investigated by using two different chlorites, thuringite or ripidolite, in the starting mixture. Cordierite is formed according to the following reaction: $${\text{Chlorite + muscovite + quartz}} \rightleftharpoons {\text{cordierite + biotite + Al}}_{\text{2}} {\text{SiO}}_{\text{5}} + {\text{H}}_{\text{2}} {\text{O}}$$ . At low pressures this reaction characterizes the facies boundary between the albite-epidotehornfels facies and the hornblende-hornfels facies, at medium pressures the beginning of the cordierite-amphibolite facies. Experiments were carried out reversibly and gave the following equilibrium data: 505±10°C at 500 bars H2O pressure, 513±10°C at 1000 bars H2O pressure, 527±10°C at 2000 bars H2O pressure, and 557±10°C at 4000 bars H2O pressure. These equilibrium data are valid for the Fe-rich starting material, using thuringite as the chlorite, as well as for the Mg-rich starting mixture with ripidolite. At 6000 bars the equilibrium temperature for the Mg-rich mixture is 587±10°C. In the Fe-rich mixture almandite was formed instead of cordierite at 6000 bars. The following reaction was observed: $${\text{Thuringite + muscovite + quartz}} \rightleftharpoons {\text{almandite + biotite + Al}}_{\text{2}} {\text{SiO}}_{\text{5}} {\text{ + H}}_{\text{2}} {\text{O}}$$ . Experiments with the Fe-rich mixture, containing Fe2+/Fe2++Mg in the ratio 8∶10, yielded three stability fields in a P,T-diagram (Fig.1):
  1. Above 600°C/5.25 kb and 700°C/6.5 kb almandite+biotite+Al2SiO5 coexist stably, cordierite being unstable.
  2. The field, in which almandite, biotite and Al2SiO5 are stable together with cordierite, is restricted by two curves, passing through the following points:
    1. 625°C/5.5 kb and 700°C/6.5 kb,
    2. 625°C/5.5 kb and 700°C/4.0 kb.
  3. At conditions below curves 1 and 2b, cordierite, biotite, and Al2SiO5 are formed, but no garnet.
An appreciable MnO-content in the system lowers the pressures needed for the formation of almandite garnet, but the quantitative influence of the spessartite-component on the formation of almandite could not yet be determined. the Mg-rich system with Fe2+/Fe2++Mg=0.4 garnet did not form at pressures up to 7 kb in the temperature range investigated. Experiments at unspecified higher pressures (in a simple squeezer-type apparatus) yielded the reaction: $${\text{Ripidolite + muscovite + quartz}} \rightleftharpoons {\text{almandite + biotite + Al}}_{\text{2}} {\text{SiO}}_{\text{5}} {\text{ + H}}_{\text{2}} {\text{O}}$$ . Further experiments are needed to determine the equilibrium data. The occurence of garnet in metamorphic rocks is discussed in the light of the experimental results.  相似文献   

11.
Basalts from DSDP Site 417 (109 Ma) exhibit the effects of several stages of alteration reflecting the evolution of seawater-derived solution compositions and control by the structure and permeability of the crust. Characteristic secondary mineral assemblages occur in often superimposed alteration zones within individual basalt fragments. By combining bulk rock and single phase chemical analyses with detailed mineralogic and petrographic studies, chemical changes have been determined for most of the alteration stages identified in the basalts.
  1. Minor amounts of saponite, chlorite, and pyrite formed locally in coarse grained portions of massive units, possibly at high temperatures during initial cooling of the basalts. No chemical changes could be determined for this stage.
  2. Possible mixing of cooled hydrothermal fluids with seawater resulted in the formation of celadonite-nontronite and Fe-hydroxide-rich black halos around cracks and pillow rims. Gains of K, Rb, H2O, increase of Fe3+/FeT, and possibly some losses of Ca and Mg occurred during this stage.
  3. Extensive circulation of oxygenated seawater resulted in the formation of various smectites, K-feldspar, and Fe-hydroxides in brown and light grey alteration zones around formerly exposed surfaces. K, Rb, H2O, and occasionally P were added to the rocks, Fe3+/FeT increased, and Ca, Mg, Si and occasionally Al and Na were lost.
  4. Anoxic alteration occurred during reaction of basalt with seawater at low water-rock ratios, or with seawater that had previously reacted with basalt. Saponite-rich dark grey alteration zones formed which exhibit very little chemical change: generally only slight increases in Fe3+/FeT and H2O occurred.
  5. Zeolites and calcite formed from seawater-derived fluids modified by previous reactions with basalt. Chemical changes involved increases of Ca, Na, H2O, and CO2 in the rocks.
  6. A late stage of anoxic conditions resulted in the formation of minor amounts of Mn-calcites and secondary sulfides in previously oxidized rocks. No chemical changes were determined for this stage.
Recognition of such alteration sequences is important in understanding the evolution of submarine hydrothermal systems and in interpreting chemical exchange due to seawater-basalt reactions.  相似文献   

12.
The occurrence of talc and tremolite in a temperature gradient was investigated in siliceous calcite-dolomite sediments exposed along a strip in the southeastern part of the Damara Orogen. Five bivariant reactions may lead to the formation of talc and tremolite:
  1. 3 dolomite+4 quartz+1 H2O ? 1 talc+3 calcite+3 CO2
  2. 5 talc+6 calcite+4 quartz ? 1 tremolite+6 CO2+2 H2O
  3. 2 talc+3 calcite ? 1 tremolite+1 dolomite+1 CO2+1 H2O
  4. 5 dolomite+8 quartz+1 H2O ? 1 tremolite+3 calcite+7 CO2
  5. 2 dolomite+1 talc+4 quartz ? 1 tremolite+4 CO2.
The common paragenesis of four mineral assemblages tc+cc+dol+qtz1 and tre+tc+ cc+qtz with increasing temperature over an extended area show that the reactions must have taken place along the equilibrium curve or when fluid pressure is not constant along the equilibrium plane of reactions (1) or (2). The described occurrence of the five mineral assemblage tre+tc+cc+dol+qtz can be stable only on the isobaric intersection point, or when P f is variable on the univariant intersection curve of the equilibrium planes of all five reactions. The genetic relations of the described parageneses are illustrated with the help of a phase diagram. Minimum P-T conditions which prevailed during metamorphism in this part of the Damara Orogen have been estimated to be about 590° C and 5 kb.  相似文献   

13.
First results obtained in the study of the Ca-Ba diadochic substitution in the several polymorphic modifications of BaAl2Si2O8 are reported:
  1. in the hexagonal modifications synthesized at 1200° C by a solid state reaction, Ca replaces Ba up to 37% (atomic fraction);
  2. in the hexagonal modifications obtained by crystallization of a melt, the replacement is limited to 25%;
  3. in the monoclinic modifications obtained by heating the above mentioned modifications to 1450° C, the replacement is limited to 25% again.
A remarkable feature of the low and high hexagonal modifications is that their unit-cell parameters show no variation with composition. These hexagonal phases, furthermore, seem identical irrespective of the method of synthesis. The unit-cell parameters of the monoclinic phases vary very little with composition.  相似文献   

14.
Concordant granite sheets from the granulite facies Scourian Complex, N.W. Scotland exhibit the following features:
  1. a common planar fabric with their host pyroxene granulites;
  2. the presence of an exsolved ternary feldspar phase;
  3. a low-pressure, water-saturated minimum composition;
  4. K/Rb ratios (450–1,350) distinctly higher than most upper crustal granites but similar to the surrounding granulites;
  5. low absolute concentrations of the rare earth elements (REEs), light REE enrichment, and large positive Eu anomalies.
It is proposed that the granite sheets have originated by anatexis of gneisses undergoing granulite facies metamorphism — gneisses that were already essentially dry and depleted in incompatible elements. Their unusual trace element chemistry may be explained by either disequilibrium melting and/or sub-solidus reequilibration of the granite sheets with the surrounding gneisses. Isotopic and trace element data suggest that cross-cutting, potash-rich pegmatites represent reworking of the granite sheets during a later amphibolitization.  相似文献   

15.
The influence of bacteria on recent sediments was first discussed in 1885, whenFischer andGazert were discussing the cycle of substances in the sea as well as in sediments. The influence of bacteria on the cycling of C, N, S, P in recent sediments and the open sea was soon accepted by marine geologists. Nevertheless, only very few experiments have, so far, shown more than qualitative and quantitative data collection in various restricted areas. This is due to the extensive and complicated chain of reactions on the surface of sediments and in the sediment itself. Biologists are asking for the amount of organic and inorganic matter which is reworked and released to the sea. Geologists usually emphasize the amount of substances which are sedimentated. For biologists the sediment is only part of their dominant ecosystem (the sea). While, for geologists the “sea” is only furnishing and influencing their first range system sediment. How much then, are bacteria involved in the slow process of conversion from a recent sediment to sedimentary rocks? Bacteria influence more or less strongly and to a more or less advanced degree of diagenesis:
  1. The organic matter in sediments and the final form in which it is found.
  2. The anions CO3 2?, NO3 ?, OH-, SO4 2?, PO4 3? as well as their intermediate stages and the resulting minerals.
  3. The cations H+, NH4 +, Ca2+, Fe2+, Fe3+, and a series of metals which are dissolved or precipitated by microbial activities as for example Fe, Mn, Cu, Ag, V, Co, Mo, Ni, U, Se, Zn.
  4. The equilibrium of silicium. At least diatoms and radiolarians are precipitating silica, while other reactions which have been proved are not yet shown to influence marine sediments.
  5. pH-values and oxidation-reduction potentials of the sediment.
  6. The composition of interstitial waters.
  7. The surface activity of minerals, since bacteria are growing especially on particle surfaces.
  8. The energy content and temperature of sediments.
  9. The texture of fine grained sediments.
  10. The fossilization of microfauna, macrofauna and trace fossils.
Sedimentology and mineralogy may also influence the bacterial activities and the composition of the microflora within sediments. Methods and problems of sediment microbiology are demonstrated by some investigations in the German Bay (North Sea) in connection with the first German Underwater Station (UWL). Ecological work proves to be difficult in various directions. The main cause of difficulties in microbiological work on sediments are the great variety of different factors influencing the environment (microbial, chemical, physical, mineralogical), the difficulty of taking representative samples, and the small amount of data which has been collected so far.  相似文献   

16.
According to Sakai-Ohmoto's theory regarding the evolution of sulfur isotopes in hydrothermal systems, in conjunction of new data on chemical resaction equilibrium constants and equilibrium isotopic fractionation factors as well as on individual ion activity coefficients of aqueous sulfur species, the following lgfo2.-pH diagrams are constructed:
  1. mole fractions of aqueous sulfur species (X i ),
  2. stability fields of some minerals in the Fe-S-O system,
  3. diagram depicting the oxidation-reduction-state ratio for aqueous sulfur species (R′)
  4. isotopic compositions of sulfur compounds ( \(\delta S_1 ^{34} \) ).
  相似文献   

17.
P, T, \(X_{{\text{CO}}_{\text{2}} }\) relations of gehlenite, anorthite, grossularite, wollastonite, corundum and calcite have been determined experimentally at P f =1 and 4 kb. Using synthetic starting minerals the following reactions have been demonstrated reversibly
  1. 2 anorthite+3 calcite=gehlenite+grossularite+3 CO2.
  2. anorthite+corundum+3 calcite=2 gehlenite+3 CO2.
  3. 3anorthite+3 calcite=2 grossularite+corundum+3CO2.
  4. grossularite+2 corundum+3 calcite=3 gehlenite+3 CO2.
  5. anorthite+2 calcite=gehlenite+wollastonite+2CO2.
  6. anorthite+wollastonite+calcite=grossularite+CO2.
  7. grossularite+calcite=gehlenite+2 wollastonite+CO2.
In the T, \(X_{{\text{CO}}_{\text{2}} }\) diagram at P f =1 kb two isobaric invariant points have been located at 770±10°C, \(X_{{\text{CO}}_{\text{2}} }\) =0.27 and at 840±10°C, \(X_{{\text{CO}}_{\text{2}} }\) =0.55. Formation of gehlenite from low temperature assemblages according to (4) and (2) takes place at 1 kb and 715–855° C, \(X_{{\text{CO}}_{\text{2}} }\) =0.1–1.0. In agreement with experimental results the formation of gehlenite in natural metamorphic rocks is restricted to shallow, high temperature contact aureoles.  相似文献   

18.
Principe is one of the volcanic centres comprising the Cameroun line in West Africa. The volcanic rocks can be divided into two stratigraphic units:
  1. Younger lava series — basanite and nephelinite overlying.
  2. Older lava series — transitional to mildly alkaline basalt and hawaiite.
These units lie on a basement of palagonite breccias of tholeiitic affinities. The basic lavas are intruded by plugs ranging in composition from tristanite to phonolite and are overlain by phonolite lavas. These rocks form two chemically and mineralogically distinct suites:
  1. Phonolites which evolved by low pressure crystal fractionation of the younger lava series basanitic magma, and
  2. Tristanite — trachyte — trachyphonolite suite which may have evolved by high pressure crystal fractionation of the older lava series magma.
  相似文献   

19.
Metagabbros from two widely separated areas in the Adirondacks show development of coronas. In the Southern Adirondacks, these are cored by olivine which is enclosed in a shell of orthopyroxene that is partially, or completely, rimmed by symplectites consisting of clinopyroxene and spinel. Compositions of the corona phases have been determined by electron probe and are consistent with a mechanism involving three partial reactions, thus:
  1. Olivine=Orthopyroxene+(Mg, Fe)++.
  2. Plagioclase+(Mg, Fe)+++Ca++=Clinopyroxene+Spinel+Na+.
  3. Plagioclase+(Mg, Fe)+++Na+=Spinel+more sodic plagioclase+Ca++.
Reaction (a) occurs in the inner shell of the corona adjacent to olivine; reaction (b) in the outer shell; and (c) in the surrounding plagioclase, giving rise to the spinel clouding which is characteristic of the plagioclase in these rocks. Alumina and silica remain relatively immobile. These reactions, when balanced, can be generalized to account for the aluminous nature of the pyroxenes and for changing plagioclase composition. Summed together, the partial reactions are equivalent to:
  1. Olivine + Anorthite = Aluminous orthopyroxene + Aluminous Clinopyroxene + Spinel (Kushiro and Yoder, 1966).
In the Adirondack Highlands, coronas between olivine and plagioclase commonly have an outer shell of garnet replacing the clinopyroxene/spinel shell. The origin of the garnet can also be explained in terms of three partial reactions:
  1. Orthopyroxene+Ca++=Clinopyroxene+(Mg, Fe)++.
  2. Clinopyroxene+Spinel+Plagioclase+(Mg, Fe)++=Garnet+Ca+++Na+.
  3. Plagioclase+(Mg, Fe)+++Na+=Spinel + more sodic plagioclase+Ca++.
These occur in the inner and outer corona shell and the surrounding plagioclase, respectively, and involve the products of reactions (a)-(d). Alumina and silica are again relatively immobile. Balanced, and generalized to account for aluminous pyroxenes and variable An content of plagioclase, they are equivalent to:
  1. Orthopyroxene+Anorthite+Spinel=Garnet (Green and Ringwood, 1967).
Amphibole coronas about opaque oxides in rocks of both areas are the result of oxide/plagioclase reactions with addition of magnesium from coexisting olivine. Based on published experimental data, pressure and temperature at the time of corona formation were on the order of 8 kb and 800° C for the garnet bearing coronas, with somewhat lower pressures indicated for the clinopyroxene/spinel coronas.  相似文献   

20.
Synthetic olivines, with composition Fa50, Fa75 and Fa100, have been transformed into spinel in a laser-heated diamond-cell at pressures from 70 to 200 kbar and at a luminance temperature of about 1,200° C. The electrical conductivity σ was measured, at room temperature and up to 200 kbar, on olivine (Lacam 1982; 1983) and spinel (present study). The data obtained permit the following conclusions:
  1. Sample nature effect: under the same conditions (composition, pressure), the σ of spinel is more than three orders of magnitude of the σ of olivine.
  2. Composition effect: there are more than three orders of magnitude between the values of σ for spinels derived from initial compositions of Fa50 and Fa100, respectively.
  3. Pressure effect: The P-effect on σ is greater for olivines than for spinels.
Besides, as in the case of olivine, in spinel the σ obeys an empirical Boltzmann relation: $$\log {\text{ }}\sigma = n \times x + S \times P + const$$ where the first and second term are the composition and pressure contributions, respectively; x the ratio Fa/Fo in mole percent. In spinel, the activation volume, in direct connection with S, was found to be in the order of 0.3 cm3/mol, about one half of that for olivine.  相似文献   

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