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1.
Ultramafic inclusions and megacrysts are unusually abundant in a nephelinite sill in the Nandewar Mountains in north-eastern New South Wales. The inclusions are divisible into a Cr-diopside group and a Ti-augite group, the former being dominated by Cr-spinel Iherzolites of restricted modal composition, the latter by olivine and titaniferous Al-rich clinopyroxene assemblages which vary widely in their modal proportions. The principal megacryst species are olivine and black, titaniferous Al-rich clinopyroxene; additional but comparatively rare megacrysts include titanphlogopite, kaersutitic amphibole, and deep green, relatively Fe-rich clinopyroxene. The Cr-spinel Iherzolites conform closely in mineralogy and chemistry with the spinel lherzolites which dominate upper mantle xenolith assemblages in alkaline mafic volcanic rocks from other provinces. Megacrysts and Ti-augite inclusion mineral assemblages are consistently more Fe-rich than analogous phases in the Cr-diopside xenoliths and also display more extensive cryptic variation. The available experimental data on the high pressure liquidus or near-liquidus phases in olivine nephelinite and related compositions indicate that the olivine and black clinopyroxene megacrysts were precipitated at pressures in the vicinity of 15–20 kb. The similarity in the nature and compositions of the principal megacryst species to analogous phases in the Tiaugite group of inclusions indicates that the latter also represent cognate cumulates derived from the olivine nephelinite at broadly comparable pressures. High pressure fractionation of the host olivine nephelinite liquid, controlled mainly by the separation of olivine and aluminous clinopyroxene, produced only comparatively minor compositional changes in the derivative liquid. The hiatus in olivine compositions at approximately Fo86–88, apparently characteristic of the olivines in coexisting Cr-diopside and Ti-augite inclusions, is assessed in terms of the compositions of olivine in equilibrium with alkali basaltic liquids at high pressures.  相似文献   

2.
Experimental modelling of corona textures   总被引：1，自引：0，他引：1  
Formation of corona textures along olivine–plagioclase and orthopyroxene–plagioclase interfaces has been experimentally reproduced at 670 and 700 °C and 5 kbar with either a pure H2O fluid phase or 0.1 and 37 m NaCl–H2O solution fluid. In these experiments, we investigate the interaction of primary olivine and/or orthopyroxene and plagioclase in powders and polished crystals, and in small samples of a natural gabbro. The experiments result in the formation of corona textures with several layers of different assemblages (according to the experimental conditions) consisting of garnet (grossular), clinopyroxene, orthopyroxene, amphibole, chlorite and phlogopite. The experiments show major differences in the number of layers, the mineral assemblages and mineral composition, and in the trends of composition of plagioclase in coronas around olivine and orthopyroxene. The fluid phase composition influences the corona assemblages and the composition of the minerals in the experimental coronas; for example, garnet appears in the coronas in the second experiment where the NaCl–H2O ratio is low. Experimental modelling of corona textures confirms a model of simultaneous growth of layers by the mechanism of diffusion metasomatism with participation of a fluid phase through which mass is transferred. Zoning in the experimental coronas shows opposing diffusion of Al and Ca from plagioclase and Mg and Fe from olivine/orthopyroxene; difference in the mobility of the components is inferred from observations in the coronas. The experimental corona textures are compared with natural coronas from the Belomorian belt (Baltic shield), developed at 670–690 °C and 7–8 kbar, and the Marun‐Keu complex (Polar Urals), developed at 670–700 °C and 14–16 kbar, where the corona textures correspond to a transitional stage of the gabbro‐to‐eclogite transformation.  相似文献   

3.
Feldspathic hornblende granulites from Doubtful Sound, New Zealand with the assemblage plagioclase+hornblende+clinopyroxene+orthopy-roxene +oxide+apatite are criss-crossed by a network of garnetiferous anorthosite veins and pegmatites. The feldspathic gneiss in contact with anorthosite has a reaction zone containing the assemblage plagioclase +garnet+clinopyroxene+quartz+rutile+apatite. The garnet forms distinctive coronas around clinopyroxene. The origin of these rocks is discussed in the light of mineral and whole rock chemical analyses and published experimental work.It is thought that under conditions leading up to 750 °C, 8 kb load pressure and 5 kb H2O pressure, partial melting occured in feldspathic hornblende granulites. The melt migrated into extensional fractures and eventually crystallised as anorthosite pegmatites and veins. The gneisses adjacent to the pegmatites from which the melt was extracted changed composition slightly, by the loss of H2O and Na2O, so that plagioclase reacted simultaneously with hornblende, orthopyroxene, and oxide to form garnet, clinopyroxene, quartz and rutile.  相似文献   

4.
Rabaul caldera is a large volcanic depression at the north-east tip of New Britain, Papua New Guinea. The lavas range in composition from basalt to rhyolite and have a calc-alkalic affinity but also display features typical of tholeiites, including moderate absolute iron enrichment in flows cropping out around the caldera. The basalts contain phenocrysts of plagioclase and clinopyroxene with less abundant olivine and titanomagnetite. In the basaltic andesites olivine is rare, while orthopyroxene and titanomagnetite are common along with plagioclase and clinopyroxene. Orthopyroxene is also found mantling olivine in some of the basalts while in both rock types pigeonitic augite is a fairly common constituent of the groundmass. Plagioclase in both basalt and basaltic andesite often exhibits sieve texture and analysis of the glass blebs show them to be of similar composition to the bulk rock. Phenocrystic clinopyroxene is a diopsidic augite in both basalt and basaltic andesite. Al2O3 content of the clinopyroxene is moderately high (4%) and often shows considerable variation in any one grain. Calculations show that the microphenocrysts probably crystallised near the surface, while phenocrysts crystallised at around 7 kb (21 km). Neither the basalts nor the basaltic andesites would have been in equilibrium at any geologically reasonable P and T with quartz eclogite. Equilibration between mantle peridotite and a. typical Rabaul basaltic liquid could have occurred around 35 kb and 1270 °C. A basaltic andesite liquid yields a temperature of 1263 °C and a pressure of 28 kb for equilibration with mantle peridotite.Partial melting of sufficient volumes of mantle peridotite at these P's and T's requires about 15% H2O, but there is no evidence that these magmas ever contained large amounts of water. It is proposed that the Rabaul magmas were initially generated by partial melting of subducted lithosphere and subsequently modified by minor partial melting as they passed through the overlying mantle peridotite.  相似文献   

5.
Six crystalline mixtures, picrite, olivine-rich tholeiite, nepheline basanite, alkali picrite, olivine-rich basanite, and olivine-rich alkali basalt were recrystallized at pressures to 40 kb, and the phase equilibria and sequences of phases in natural basaltic and peridotitic rocks were investigated.The picrite was recrystallized along the solidus to the assemblages (1) olivine+orthopyroxene+ clinopyroxene +plagioclase+spinel below 13 kb, (2) olivine+orthopyroxene+clinopyroxene+spinel between 13 kb and 18 kb, (3) olivine+orthopyroxene+clinopyroxene+ garnet+spinel between 18 kb and 26 kb, and (4) olivine+clinopyroxene+garnet above 26 kb. The solidus temperature at 1 atm is slightly below 1,100° and rises to 1,320° at 20 kb and 1,570° at 40 kb. Olivine is the primary phase crystallizing from the melt at all pressures to 40 kb.The olivine-rich tholeiite was recrystallized along the solidus into the assemblages (1) olivine+ clinopyroxene+plagioclase+spinel below 13 kb, (2) clinopyroxene+orthopyroxene+ spinel between 13 kb and 18 kb, (3) clinopyroxene+garnet+spinel above 18 kb. The solidus temperature is slightly below 1,100° at 1 atm, 1,370° at 20 kb, and 1,590° at 40 kb. The primary phase is olivine below 20 kb but is orthopyroxene at 40 kb.In the nepheline basanite, olivine is the primary phase below 14 kb, but clinopyroxene is the first phase to appear above 14 kb. In the alkali-picrite the primary phase is olivine to 40 kb. In the olivine-rich basanite, olivine is the primary phase below 35 kb and garnet is the primary phase above 35 kb. In the olivine-rich alkali basalt the primary phase is olivine below 20 kb and is garnet at 40 kb.Mineral assemblages in a granite-basalt-peridotite join are summarized according to reported experimental data on natural rocks. The solidus of mafic rock is approximately given by T=12.5 P Kb+1,050°. With increasing pressure along the solidus, olivine disappears by reaction with plagioclase at 9 kb in mafic rocks and plagioclase disappears by reaction with olivine at 13 kb in ultramafic rocks. Plagioclase disappears at around 22 kb in mafic rocks, but it persists to higher pressure in acidic rocks. Garnet appears at somewhat above 18 kb in acidic rocks, at 17 kb in mafic rocks, and at 22 kb in ultramafic rocks.The subsolidus equilibrium curves of the reactions are extrapolated according to equilibrium curves of related reactions in simple systems. The pyroxene-hornfels and sanidinite facies is the lowest pressure mineral facies. The pyroxene-granulite facies is an intermediate low pressure mineral facies in which olivine and plagioclase are incompatible and garnet is absent in mafic rocks. The low pressure boundary is at 7.5 kb at 750° C and at 9.5 kb at 1,150° C. The high pressure boundary is 8.0 kb at 750° C and 15.0 kb at 1,150° C. The garnet-granulite facies is an intermediate high pressure facies and is characterized by coexisting garnet and plagioclase in mafic rocks. The upper boundary is at 10.3 kb at 750° C and 18.0 kb at 1,150° C. The eclogite facies is the highest pressure mineral facies, in which jadeite-rich clinopyroxene is stable.Compositions of minerals in natural rocks of the granulite facies and the eclogite facies are considered. Clinopyroxenes in the granulite-facies rocks have smaller jadeite-Tschermak's molecule ratios and higher amounts of Tschermak's molecule than clinopyroxenes in the eclogite-facies rocks. The distribution coefficients of Mg between orthopyroxene and clinopyroxene are normally in the range of 0.5–0.6 in metamorphic rocks in the granulite facies. The distribution coefficients of Mg between garnet and clinopyroxene suggest increasing crystallization temperature of the rocks in the following order: eclogite in glaucophane schist, eclogite and granulite in gneissic terrain, garnet peridotite, and peridotite nodules in kimberlite.Temperatures near the bottom of the crust in orogenic zones characterized by kyanitesillimanite metamorpbism are estimated from the mineral assemblages of metamorphic rocks in Precambrian shields to be about 700° C at 7 kb and 800° C at 9 kb, although heat-flow data suggest that the bottom of Precambrian shield areas is about 400° C and the eclogite facies is stable.The composition of liquid which is in equilibrium with peridotite is estimated to be close to tholeiite basalt at the surface pressure and to be picrite at around 30 kb. The liquid composition becomes poorer in normative olivine with decreasing pressure and temperature.During crystallization at high pressure, olivine and orthopyroxene react with liquid to form clinopyroxene, and a discontinuous reaction series, olivine orthopyroxene clinopyroxene is suggested. By fractional crystallization of pyroxenes the liquid will become poorer in SiO2. Therefore, if liquid formed by partial melting of peridotite in the mantle slowly rises maintaining equilibrium with the surrounding peridotite, the liquid will become poorer in MgO by crystallization of olivine, and tholeiite basalt magma will arrive at the surface. On the other hand, if the liquid undergoes fractional crystallization in the mantle, the liquid may change in composition to alkali-basalt magma and alkali-basalt volcanism may be seen at a late stage of volcanic activity.Publication No. 681, Institute of Geophysics and Planetary Physics, University of California, Los Angeles.  相似文献   

6.
Troctolitic gabbros from Valle Fértil and La Huerta Ranges, San Juan Province, NW‐Argentina exhibit multi‐layer corona textures between cumulus olivine and plagioclase. The corona mineral sequence, which varies in the total thickness from 0.5 to 1 mm, comprises either an anhydrous corona type I with olivine|orthopyroxene|clinopyroxene+spinel symplectite|plagioclase or a hydrous corona type II with olivine|orthopyroxene|amphibole|amphibole+spinel symplectite|plagioclase. The anhydrous corona type I formed by metamorphic replacement of primary olivine and plagioclase, in the absence of any fluid/melt phase at <840 °C. Diffusion controlled metamorphic solid‐state replacement is mainly governed by the chemical potential gradients at the interface of reactant olivine and plagioclase and orthopyroxene and plagioclase. Thus, the thermodynamic incompatibility of the reactant minerals at the gabbro–granulite transition and the phase equilibria of the coronitic assemblage during subsequent cooling were modelled using quantitative μMgO–μCaO phase diagrams. Mineral reaction textures of the anhydrous corona type I indicate an inward migration of orthopyroxene on the expense of olivine, while clinopyroxene+spinel symplectite grows outward to replace plagioclase. Mineral textures of the hydrous corona type II indicate the presence of an interstitial liquid trapped between cumulus olivine and plagioclase that reacts with olivine to produce a rim of peritectic orthopyroxene around olivine. Two amphibole types are distinguished: an inclusion free, brownish amphibole I is enriched in trace elements and REEs relative to green amphibole II. Amphibole I evolves from an intercumulus liquid between peritectic orthopyroxene and plagioclase. Discrete layers of green amphibole II occur as inclusion‐free rims and amphibole II+spinel symplectites. Mineral textures and geochemical patterns indicate a metamorphic origin for amphibole II, where orthopyroxene was replaced to form an inner inclusion‐free amphibole II layer, while clinopyroxene and plagioclase were replaced to form an outer amphibole+spinel symplectite layer, at <770 °C. Calculation of the possible net reactions by considering NCKFMASH components indicates that the layer bulk composition cannot be modelled as a ‘closed’ system although in all cases the gain and loss of elements within the multi‐layer coronas (except H2O, Na2O) is very small and the main uncertainties may arise from slight chemical zoning of the respective minerals. Local oxidizing conditions led to the formation of orthopyroxene+magnetite symplectite enveloping and/or replacing olivine. The sequence of corona reaction textures indicates a counter clockwise P–T path at the gabbro–granulite transition at 5–6.5 kbar and temperatures below 900 °C.  相似文献   

7.
Most Hawaiian basaltic shield volcanoes are capped by moderately to strongly evolved alkalic lavas (MgO<4.5 wt.%). On Mauna Kea Volcano the cap is dominantly composed of hawaiite with minor mugearite. Although these lavas contain dunite and gabbroic xenoliths, they are nearly aphyric with rare olivine and plagioclase phenocrysts and xenocrysts. The hawaiites are nearly homogeneous in radiogenic isotope ratios (Sr, Nd, Pb) and they define coherent major and trace element abundance trends. These compositional trends are consistent with segregation of a plagioclase-rich cumulate containing significant clinopyroxene and Fe-Ti oxides plus minor olivine. Elements which are usually highly incompatible, e.g., Rb, Ba, Nb, are only moderately incompatible within the hawaiite suite because these elements are incorporated into feldspar (Rb, Ba) and oxides (Nb). However, in the most evolved lavas abundances of the most incompatible elements (P, La, Ce, Th) exceed (by 5–10%) the maximum enrichments expected from models based on major elements. Apparently, the crystal fractionation process was more complex than simple, closed system fractionation. The large amounts of clinopyroxene in the fractionating assemblage and the presence of dense dunite xenoliths with CO2 inclusions formed at minimum pressures of 2 kb are consistent with fractionation occurring at moderate depths. Crystal segregation along conduit or magma chamber walls is a possible mechanism for explaining compositional variations within these alkalic cap lavas.  相似文献   

8.
New data on the composition of minerals in corona textures around olivine and crystal-fluid inclusions in olivine from anorthosites of the Korosten’ pluton (sampled in the Golovino quarry), Ukrainian Shield were obtained using electron and ion microprobe analyses, Raman spectroscopy, scanning electron microscopy, and cryo- and thermometry. The corona textures developed around olivine grains in contact with plagioclase and consist of inner orthopyroxene rims around olivine and outer rims of orthopyroxene-clinopyroxene-orthoclase-plagioclase symplectites. The symplectites and orthopyroxene rims most probably developed nearly simultaneously and grew in the opposite directions from the original contact of the magmatic olivine and plagioclase and replaced both olivine and plagioclase. The Al2O3 and CaO concentrations in the symplectitic orthopyroxene increase toward the contact with magmatic plagioclase, whereas the Al2O3 and CaO concentrations in the symplectitic plagioclase simultaneously decrease and its Na2O and K2O increase. Optically discernible crystalline and fluid phases of crystal-fluid inclusions in olivine were identified as pyroxenes (orthopyroxene and clinopyroxene), actinolite, Ca-and Fe, Mg-carbonates, and magnetite, along with practically pure highdensity CO2. The mineral assemblages of corona texture in the Korsten’ anorthosites were produced by autometasomatic processes at a high CO2 activity, and the local variations in the chemistry of corona minerals were likely controlled by the content and chemistry of the interstitial fluid and primary minerals. The coronas developed under subsolidus conditions, via the reaction interaction of olivine and plagioclase under the effect of an integranular fluid, with the dissolution of olivine and plagioclase at T = 980–860°C and P > 5 kbar. Inasmuch as corona textures do not occur ubiquitously in the rocks, the origin of the former was most probably controlled by the amount of the intergranular fluid.  相似文献   

9.
Reactions occurring during cooling of charnockitic intrusives on the Lofoten Islands produce characteristic diffusion-controlled textures around fayalite and Fe–Ti oxides. Thermobarometry indicates the corona textures formed at 780–840 °C and pressures of 4–10 kbar, whereas the magmatic assemblage of the charnockite (clinopyroxene–olivine–quartz) crystallized at about 850–870 °C and 4 kbar. The succession olivine|orthopyroxene+magnetite|orthopyroxene+garnet and olivine|orthopyroxene+magnetite|amphibole developed where olivine reacted with adjacent plagioclase or K-feldspar, but the modes and the thicknesses of the corona textures vary according to the feldspar type, indicating that the primary magmatic ternary feldspar was already exsolved into albitic plagioclase and alkali feldspar when the corona formation began. Simultaneously, in other parts of the rock, primary magmatic clinopyroxene reacted to amphibole and Fe–Ti oxides reacted to orthopyroxene+garnet coronas or to amphibole. Textures demonstrate significant Al diffusion in the rocks under granulite facies conditions and they suggest that no pervasive fluid influx occurred and that amphibole formation was dependant on a local source of H2O probably related to water-release during the last stages of magmatism. Calculation of the net reaction by accounting for all observed reactions at different sites in the rock indicates that the system can be regarded as balanced on a hand-specimen scale with respect to all elements except for Na and H2O. The larger variety of textures developed in rocks of granitic bulk composition provide more constraints than textures from gabbroic compositions, and permitted calculation of a set of relative diffusion coefficients which also reproduce textures in the gabbroic and anorthositic rocks from the Lofoten Islands. The following set of relative diffusion coefficients (Li/LFe) reproduces the observed textures in the Lofoten rocks: Si=0.82, Mg=0.59, Mn=0.05, Na=0.38, K=0.39, Al=0.05 and Ca=0.07.  相似文献   

10.
Suprasolidus phase relations at pressures from 8 to 30 kb andtemperatures from 950 to 1380C have been determined experimentallyfor a glassy armalcolite–phlogopite lamproite from thechilled margin of a medium–grained lamproite from SmokyButte, Montana: The armalcolite-phlogopite lamproite has microphenocrystsof olivine in a groundmass of phlogopite, sanidine, armalcolite,clinopyroxene, chromite, priderite, apatite, and abundant glass.The lamproite is SiO2-rich and has high F/H2O relative to lamproitesthat have been investigated in previous experimental studies.Our data show that with decreasing temperature from the liquidusat pressures above 12 kb, melt coexists successively with:olivine; orthopyroxene + clinopyroxene; orthopyroxene + clinopyroxene+ phlogopite; clinopyroxene +phlogopite; and clinopyroxene +orthopyroxene + K-richterite. Below 12 kb, the assemblage successionis: olivine; olivine + clinopyroxene; olivine + clinopyroxene+ phlogopite; and olivine +clinopyroxene + phlogopite + armalcolite.The main difference from the natural paragenesis is that therock does not contain any orthopyroxene—a feature thatis rather remarkable inasmuch as it has 16% normative hypersthene—andthe rock differs also in that it contains sanidine and priderite.In the experiments, sanidine is observed only as ghostlike domainsin some of the glass and appears to have formed during quenching. The solid phases crystallized experimentally are generally compositionallysimilar to the minerals in the rock. These similarities andthe experimental phase relations support the concept of a rapidinitial magma ascent with only a small temperature drop andcrystallization of olivine, but not of orthopyroxene. At lowerpressures, less than 12 kb, it appears that the magma ascendedmore slowly with a larger temperature drop suggested by thesimilarity of the experimentally determined sequence of assemblagesto the paragenesis of the rock. No quasi-invariant multiphase-saturation point was found suchas might be indicative of pressure and temperature conditionsfor formation of the lamproite magma by eutectic-type partialmelting of a mantle source. The occurrence of olivine, orthopyroxene,and clinopyroxene near the liquidus, and the high proportionof normative hypersthene in the melt suggest that lherzoliteor harzburgite was probable in the magma source rock. The highSiO2 and MgO contents of the Smoky Butte lamproites may indicatethat orthopyroxene was a source mineral even though it did notcrystallize under near-surface conditions. The curve definingthe appearance of phlogopite appears at progressively lowertemperatures from the liquidus as pressure increases, so itwould appear that either phlogopite was not the mantle K-reservoir,or it was entirely consumed during the partial melting process.The composition of the near-liquidus glass in the experimentsis likely to be the composition of the bulk rock less the verysmall amounts of olivine + clinopyroxene + orthopyroxene crystallizedwithin a few degrees below the liquidus. From the inferred compositionof this glass, anhydrous phlogopite is a potential mineral.The principal variable that determines whether phlogopite crystallizesas a near-liquidus mineral is F/H2O; low values of this ratiopromote the presence of phlogopite as a near-liquidus mineralwhereas high values deter its crystallization. The common practiceof adding H2O but not F in experiments to compensate for degassingmay obscure the role of phlogopite in the evolution of lamproitemagmas.  相似文献   

11.
The origin of island arc high-alumina basalts   总被引：5，自引：1，他引：5  
A detailed examination of the hypothesis that high-alumina basalts (HAB) in island arcs are primary magmas derived by 50–60% partial melting of subducted ocean crust eclogite shows that this model is unlikely to be viable. Evidence suggests that the overwhelming majority of arc HAB are porphyritic lavas, enriched in Al2O3 either by protracted prior crystallization of olivine and clinopyroxene, or by plagioclase phenocryst accumulation in magmas of basaltic andesite to dacite composition. Experimentally-determined phase relationships of such plagioclase-enriched (non-liquid) compositions have little bearing on the petrogenesis of arc magmas, and do not rule out the possibility that arc HAB can be derived by fractionation of more primitive arc lavas. Although models invoking eclogite-melting can match typical arc HAB REE patterns, calculations indicate that the Ni and Cr contents of proposed Aleutian primary HAB are many times lower than such models predict. In contrast, Ni vs Sc and Cr vs Sc trends for arc HAB are readily explained by olivine (+Cr-sp) and clinopyroxene-dominated fractionation from more primitive arc magmas. GENMIX major element modelling of several HAB compositions as partial melts of MORB eclogite, using appropriate experimentally (26–34 kb)-determined garnet and omphacite compositions yields exceptionally poor matches, especially for CaO, Na2O, MgO and Al2O3. These mismatches are easily explained if the HAB are plagioclase-accumulative. Groundmasses of arc HAB are shown to vary from basaltic andesite to dacite in composition. Crystal fractionation driving liquid compositions toward dacite involves important plagioclase separation, resulting in development of significant negative Eu anomalies in more evolved lavas. Plagioclase accumulation in such evolved liquids tends to diminish or eliminate negative Eu anomalies. Therefore, the absence of positive Eu anomaly in a plagioclase-phyric HAB does not indicate that plagioclase has not accumulated in that lava. In addition, we show that plagioclase phenocrysts in arc HAB are not in equilibrium with the liquids in which they were carried (groundmass). Exceptional volumes of picrite and olivine basalt occur in the Solomons and Vanuatu arcs; the presence in lavas from these and other arcs (Aleutian, Tonga) of olivine phenocrysts to Fo94, finds no ready explanation in the primary eclogite-derived HAB model. We suggest that most lavas in intra-oceanic arcs are derived from parental magmas with >10% MgO; fractionation of olivine (+Cr-sp) and clinopyroxene drives liquids to basalt compositions with <7% MgO, but plagioclase nucleation is delayed by their low but significant (<1%?) H2O contents. Thus evolved liquid compositions in the basaltic andesite—andesite range may achieve relatively high Al2O3 contents (<17.5%). The majority of arc basalts, however, have Al2O3 contents in excess of 18%, reflecting plagioclase accumulation. We give new experimental data to show that HAB liquids may be generated by anhydrous, low-degree (<10%) partial melting of peridotite at P<18 kb. Relative to arc HAB, these experimental melts have notably higher Mg#(69–72) and are in equilibrium with olivine Fo87–89. Only further detailed trace element modelling will show if they might be parental magmas for some arc HAB.  相似文献   

12.
The El Arenal metagabbros preserve coronitic shells of orthopyroxene ± Fe‐oxide around olivine, as well as three different types of symplectite consisting of amphibole + spinel, clinopyroxene + spinel and, more rarely, orthopyroxene + spinel. The textural features of the metagabbros can be explained by the breakdown of the olivine + plagioclase pair, producing orthopyroxene coronas and clinopyroxene + spinel symplectites, followed by the formation of amphibole + spinel symplectites, reflecting a decrease in temperature and, possibly, an increase in water activity with respect to the previous stage. The metagabbros underwent a complex P–T history consisting of an igneous stage followed by cooling in granulite, amphibolite and greenschist facies conditions. Although the P–T conditions of emplacement of the igneous protolith are still doubtful, the magmatic assemblage suggests that igneous crystallization occurred at a pressure lower than 6 kbar and at 900–1100 °C. Granulitic P–T conditions have been estimated at about 900 °C and 7–8 kbar combining conventional thermobarometry and pseudosection analysis. Pseudosection calculation has also shown that the formation of the amphibole + spinel symplectite could have been favoured by an increase in water activity during the amphibolite stage, as the temperature of formation of this symplectite strongly depends on aH2O (<740 °C for aH2O = 0.5; <790 °C for aH2O = 1). Furthermore, but not pervasive, re‐equilibration under greenschist facies P–T conditions is documented by retrograde epidote and chlorite. The resulting counterclockwise P–T path consists of progressive, nearly isobaric cooling from the igneous stage down to the granulite, amphibolite and greenschist stage.  相似文献   

13.
Clinopyroxene, orthopyroxene, and garnet megacrysts show consistent increase of Na and Ti, and decrease of Cr, with increasing Fe/Mg. Three groups of clinopyroxenes occur with increasing Fe/Mg: subcalcic diopside, lamellar intergrowth with ilmenite, and augite. Chemical relationships indicate simultaneous crystallization of garnet, orthopyroxene and sub-calcic diopside megacrysts, and pyroxene thermometry-barometry indicates a trend from 29 kb?1,230 ° C to 25 kb?1,080 ° C as crystallization proceeded to higher Fe/Mg. Ilmenite-pyroxene thermometry suggests a mean of 965 ° C for crystallization of the intergrowths, but calibration depends on crystal-chemical assumptions. Lherzolite assemblages fall into three groups: two garnet-bearing types which equilibrated at 31 kb?1,150 ° C and 22 kb?900 ° C, and a type bearing Al-rich spinel which probably crystallized below 20 kb. The minerals from the lherzolites have lower Fe/Mg than the megacrysts. The simplest model involves: (i) metamorphic equilibration of lherzolitic rocks to the local geotherm, (ii) local melting of lherzolite at P > 30 kb, (iii) sequential crystallization of megacrysts as the magma rose intermittently, (iv) generation of alnöitic magma at P > 32 kb, and (v) eruption to surface with transport of megacrysts and lherzolitic xenoliths. Garnet, olivine, orthopyroxene and clinopyroxene in these Malaita xenoliths have lower Na, Ti, and P relative to their equivalents from southern African kimberlites. Only clinopyroxene contains K (up to 270 ppmw), and no Na was found in olivine.  相似文献   

14.
Liquidus and subliquidus phase relations of a leucite-lamproite (wolgidite) from the West Kimberley area, Australia have been studied experimentally under the volatile conditions of 3.22 wt.% H2O ( \(X_{CO_2 }\) =0.11) and 13.0 wt.% H2O ( \(X_{CO_2 }\) =0.03) between 10 to 40 kbar. Under these conditions, liquids are vapour undersaturated. In experiments with 13.0 wt.% H2O, olivine is the liquidus phase up to 24 kbar and orthopyroxene above 24 kbar. Phlogopite and rutile occur close to the liquidus above 16 kbar. Crystallization temperatures of clinopyroxenes are 50–120° C below the liquidus. Based on these results, wolgidite magma is unlikely to be a partial melt of a garnet- or spinel-lherzolite mantle but could be derived from phlogopite+rutile±olivine±or-thopyroxene assemblages occurring as metasomatized mantle.  相似文献   

15.
Permian‐aged metagabbros from the eclogite type‐locality in the eastern European Alps were partially to completely transformed to eclogite during Eoalpine intracontinental subduction. Microtextures developed along a preserved fluid infiltration and reaction front in the gabbros record the incipient gabbro‐to‐eclogite transition, allowing the details of the eclogitization process to be investigated. Original, anorthite‐rich igneous plagioclase is pervasively replaced by fine‐grained intergrowths of clinozoisite, kyanite and Na‐rich plagioclase. Where plagioclase was in contact with igneous orthopyroxene, 100–200 μm thick bimineralic coronae of symplectic kyanite and diopsidic clinopyroxene form along the edges of the grains. The rims of igneous orthopyroxene develop a complementary bimineralic corona of diopsidic clinopyroxene and garnet. Igneous clinopyroxene does not show any breakdown textures; however, jadeite content gradually increases towards the rims. In addition, exsolution lamellae inherited from the igneous clinopyroxene become progressively more jadeitic as eclogitization proceeds. Given that the igneous plagioclase is pervasively replaced by clinozoisite, kyanite and Na‐rich plagioclase, whereas kyanite–diopside symplectites are confined to narrow rim zones, we suggest that the development of these textures was controlled by the (im)mobility of different elements on different length scales. The presence of hydrous minerals in the core of anhydrous plagioclase indicates that H2O diffusivity occurred on a mm‐scale. By contrast, the size of the anhydrous diopside–kyanite and diopside–garnet symplectites indicate that Fe–Mg–Ca–Na diffusivity was limited to a 10s of μm scale. Chemical potential relations calculated in the idealized NCASH chemical system show that the clinozoisite–kyanite–albite intergrowths formed due to an increase of μH2O to plagioclase, whereas all other elements remained effectively immobile on the scale of this texture. Fluid conditions indicated by this texture span from virtually dry conditions (0.15) to H2O‐saturation, and therefore does not imply that the rocks were ever fluid‐saturated. Calculations in the CMAS and NCFMAS systems show that the gabbro‐to‐eclogite transition is characterized by the growth of garnet, diopsidic clinopyroxene and kyanite due to diffusion of Ca (+ Na) and Mg (+ Fe) along a μCaO (+ Na2O)–μMgO (+ FeO) chemical potential gradient developed between orthopyroxene and plagioclase compositional domains. The anhydrous nature of the textures indicate that the gabbro‐to‐eclogite transition is not driven by hydration; however, increased μH2O acts as a catalyst that increases diffusivity of all elements and rates of dissolution–precipitation, allowing the overstepped metamorphic reactions to occur. Our results show that crustal eclogite formation requires low H2O content, confirming that true eclogites are dry rocks.  相似文献   

16.
Evidence is presented for the primary high pressure crystallization of the Ewarara, Kalka and Gosse Pile layered intrusions which form part of the Giles Complex in central Australia. These pressures are estimated at 10 to 12 kb. The high pressure characteristics include subsolidus reactions between olivine and plagioclase, orthopyroxene and plagioclase, and orthopyroxene and spinel; spinel and rutile exsolution in both ortho- and clino-pyroxene; spinel exsolution in plagioclase; high Al2O3 and Cr2O3 contents of both ortho- and clinopyroxene; high AlVI in clinopyroxene; dominance of orthopyroxene as an early crystallizing phase; high distribution coefficients for co-existing pyroxene pairs; and thin chilled margins. Such phenomena are rare in documented layered basic intrusions.  相似文献   

17.
Amphibole + phlogopite + diopside bearing veins are observed in a large number of upper mantle xenoliths, but the composition of the melt that forms them is poorly constrained. Recent data from the Heldburg Phonolite, Central Germany, has shown that phonolite melt will react with olivine and orthopyroxene xenocrysts to form reaction rims of amphibole + phlogopite + diopside at mid-lower crustal pressures. This is the first example of where a melt has reacted with peridotite to form the mineralogy of the metasomatic veins. It is therefore necessary to explore whether a phonolite melt could be the parent melt that forms amphibole + phlogopite + diopside metasomatic veins. Experimental reactions between single crystals of olivine and orthopyroxene with phonolite melt were conducted at upper mantle conditions of 1.0–1.5 GPa and 900–1,000 °C. Melt water contents were varied from anhydrous to >12 wt. H2O. Olivine reacts to form phlogopite reaction rims with overgrowths of diopside <1,000 °C or rims of secondary olivine >1,000 °C. Orthopyroxene reacts to form amphibole with epitaxial diopside overgrowths <1,000 °C. No reaction rims form when the bulk melt H2O is lower than ~3.8 wt%. Pressure has little effect over the small range tested. These experiments reproduce reaction rims on olivine and orthopyroxene observed in the Heldburg Phonolite, Central Germany, and suggest that a relatively narrow range of temperatures and melt water contents is required for rim formation. The compositions of rim amphibole, phlogopite and diopside from the experiments have very similar compositions to those from Heldburg but do not match those from metasomatic veins. Phenocrysts from Heldburg are similar to the metasomatic veins, suggesting that a phonolite could potentially form the veins if vein formation is dominated by crystallization rather than reaction and replacement of wall rock phases.  相似文献   

18.
The nature of the near-liquidus phases for a mantle-derived olivine melilitite composition have been determined at high pressure under dry conditions and with various water contents. Olivine and clinopyroxene occur on or near the liquidus and there are no conditions where orthopyroxene crystallizes in equilibrium with the olivine melilitite. We have determined the effect on the liquidus temperature and liquidus phases of substituting CO2 for H2O on a mole for mole basis at 30 kb, using olivine melilitite + 20 wt% H2O at = 0 and = (CO2)/(H2+CO2) (mole fraction) = 0.25, 0.5, 0.75 and 1.0 (i.e. olivine melilitite + 38 wt% CO2). Experiments were buffered by the MH or NNO buffers. At 30 kb, CO2 is only slightly less soluble than water for <0.5 as judged by the slight increase in liquidus temperature on mole-for-mole substitution of CO2 for H2O and at 30 kb, 1200° C, = = 0.5 the olivine melilitite contains 8.8 wt% H2O and 21 wt% CO2 in solution. For 1 the CO2 saturated liquidus is depressed 70 ° C below the anhydrous liquidus and the magma dissolves approx. 17% CO2 at 30kb, 1400 ° C, 1, 0. Infrared spectra of quenched glasses have absorption bands characteristic of CO 3 = and OH- molecules and no evidence for HCO 3 - . The effect of CO 3 = molecules dissolved in the olivine melilitite at high pressure is to suppress the near-liquidus crystallization of olivine and clinopyroxene and bring orthopyroxene and garnet on to the liquidus. We infer that olivine melilitite magmas may be derived by equilibrium partial melting (<5%) of pyrolite at 30 kb, 1150–1200 ° C, provided that both H2O and CO2 are present in the source region in minor amounts. Preferred conditions are 0< <0.5, 0.5< <1, and at low oxygen fugacities (相似文献   

19.
Pressure-temperature conditions for which “andesitic” liquids (～60% SiO2) may coexist with mineral assemblages of the type magnesian olivine + orthopyroxene + clinopyroxene±amphibole±phlogopite have been investigated, both by means of partial melting experiments on the pyrolite model composition, and by experiments involving addition of olivine to andesite or basaltic andesite compositions at or near their liquidus temperatures. In the latter experiments, reaction relationships between quartz-normative liquid and olivine were made to proceed until olivine persisted. The composition of the final liquid in equilibrium with olivine (plus pyroxenes etc.) was then estimated by microprobe analysis of its quench products (glass, quench crystals). The inferred liquid compositions were tested for equilibrium with coexisting crystals using criteria based on mass balance within the total assemblage, equilibrium element partition relationships, and the requirement that the liquidus temperatures and near-liquidus crystalline phases of these compositions should closely match the temperature of the original olivine-addition experiment and the crystalline phases developed during it. At 1000° C–1050° C, liquids which satisfy these criteria for equilibrium with assemblages which include olivine are “andesitic” (58–60% SiO2, 5–12% normative Qz) only at water pressures ≤ 10kb. At 15 kb, liquids in equilibrium with olivine at 1000° C and 980° C have ～56% SiO2, high alkali contents, and 5–10% normative olivine. Similar compositions are in equilibrium with orthopyroxene and garnet alone at 20 kb. These results show that andesitic magmas are unlikely to be produced by melting of a peridotitic mantle at pressures >10 kb (depths>35 km). If hydrous, but otherwise geochemically primitive peridotitic compositions are partially melted at pressures<10 kb, then “andesitic” products will be much more magnesian and poorer in alkalies than typical natural andesites. These conclusions raise serious difficulties for models of andesite genesis by the melting of hydrous peridotitic mantle immediately overlying Benioff zones.  相似文献   

20.
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   

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