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1.
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
2.
Trevor H. Green 《Contributions to Mineralogy and Petrology》1967,16(1):84-114
The sub-solidus fields of crystallization of a spectrum of synthetic aluminous basic compositions (high-alumina basalt, anorthite-enriched high-alumina basalt, kyanite eclogite, grosspydite and gabbroic anorthosite) have been investigated at pressures of up to 36 kb. At low pressures the assemblages are characterized by abundant plagioclase, clinopyroxene and possibly minor olivine and orthopyroxene. These correspond to natural gabbroic and pyroxene granulite assemblages. As pressure is increased garnet appears and increases gradually in amount at the expense of other ferromagnesian minerals and plagioclase, until finally at pressures of >23 kb at 1,100° C, plagioclase disappears and high pressure clinopyroxene+garnet+kyanite±quartz assemblages equivalent to eclogite are obtained. In the eclogite stability field, with further rise in pressure, the ratio ga/cpx and the grossular content of the garnet increase.In the high-alumina basalt composition the transitional garnet granulite assemblage (clinopyroxene+plagioclase+garnet±quartz) is spread over a pressure interval of 11 kb at 1,100° C. This is a greater interval than observed for other basalt compositions and is important in considering the hypothesis that the Mohorovicic Discontinuity is a phase change from basalt to eclogite. It indicates that the change in V
p would be spread over a significant depth range, and no sharp seismic velocity discontinuity could result.The first experimental synthesis of kyanite eclogite from both high-alumina basalt and kyanite eclogite compositions has been obtained, as well as synthesis of unusual grossular-clinopyroxene-kyanite assemblages (grosspydite) from grosspydite and gabbroic anorthosite compositions. The pressures needed to synthesize these assemblages are somewhat greater than the pressures needed to synthesize eclogite from basic compositions of lower alumina content at the same temperature. Experimental confirmation of the observation that there is a direct relation between Gross/Alm + Py ratio of garnet and the Jd/Di ratio of co-existing pyroxene in grosspydite and kyanite eclogite assemblages found in kimberlite pipes has also been obtained. 相似文献
3.
We report results of anhydrous 1 atm and piston-cylinder experiments on ID16, an Aleutian high-magnesia basalt (HMB), designed to investigate potential petrogenetic links between arc high-alumina basalts (HABs) and less common HMBs. ID16 is multiply saturated with a plagioclase/spinel iherzolite mineral assemblage (olivine, plagioclase, clinopyroxene, orthopyroxene, spinel) immediately beneath the 12 kbar liquidus. Derivative liquids produced at high temperatures in the 10–20 kbar melting interval of ID16 have compositions resembling those published of many moderate-CaO HABs, although lower-temperature liquids are poorer in CaO and richer in alkalies than are typical HABs. Isomolar pseudoternary projections and numerical mass-balance modeling suggest that derivative melts of ID16 enter into a complex reaction relationship with olivine at 10 kbar and 1,200° C–1,150° C. We sought to test such a mechanism to explain the lack of liquidus olivine in anhydrous experiments on mafic high-alumina basalts such as SSS. 1.4 (Johnston 1986). These derivative liquids, however, do not resemble typical arc high-alumina basalts, suggesting that olivine-liquid reaction does not account for Johnston's (1986) observations. Instead, we suggest that olivine can be brought onto the liquidus of such compositions only through the involvement of H2O, which will affect the influence of bulk CaO, MgO, and Al2O3 contents on the identity of HAB liquidus phases (olivine or plagioclase) at pressures less than 12 kbar. 相似文献
4.
Genesis of the calc-alkaline igneous rock suite 总被引：11，自引：1，他引：11
A high pressure experimental study of the partial melting fields of synthetic high-alumina olivine tholeiite, high-alumina quartz tholeiite, basaltic andesite, andesite, dacite and rhyodacite under dry and wet
conditions has been conducted in order to investigate possible origins of the calc-alkaline series from the upper mantle. Detailed analyses of crystallizing phases using the electron microprobe has enabled calculation of the liquid line of descent in these compositions at various pressures.At 27–36 kb garnet and clinopyroxene are the liquidus or near-liquidus phases in dry tholeiite, basaltic andesite and andesite, while quartz is the liquidus phases in dry dacite and rhyodacite. Under wet conditions at 27 kb garnet, not quartz, is the liquidus phase in the dacite. Qualitatively these results show that the low melting fraction of a quartz eclogite at 27–36 kb under dry conditions is of andesitic composition whereas under wet conditions it is rhyodacitic or granodioritic. At these pressures under dry conditions the andesite liquidus lies in a marked low temperature trough between the more basic and more acid compositions. Quantitatively, the calculated compositions of liquid fractionates for varying degrees of melting of the quartz eclogite bulk composition broadly follow the calc-alkaline trend.At 9–10 kb under wet conditions
sub-silicic amphibole and pyroxenes are the near-liquidus phases in tholeiite and basaltic andesite compositions. Calcic plagioclase and garnet occur nearer the solidus. The calculated liquid fractionates follow the calc-alkaline trend and demonstrate that the calc-alkaline series may be derived by the partial melting of amphibolite at lower crustal depths under wet conditions
, Or by the fractional crystallization of a hydrous basalt magma at similar depths.These experimental results support two complementary hypotheses for the derivation of the calc-alkaline igneous rock suite from the mantle by a two stage igneous process. In the first stage of both hypotheses large piles of basalt are extruded on the earth's surface. Subsequently this pile of basalt may, under dry conditions, transform to quartz eclogite, sink into the mantle and finally undergo partial melting at 100–150 kms depth. This partial melting gives rise to the calc-alkaline magma series leaving a residuum of clinopyroxene and garnet. Alternatively, if wet conditions prevail in the basalt pile and the geotherms remain high, partial melting of the basalt may take place near the base of the pile, at about 10 kb pressure
. The liquids so formed constitute the calc-alkaline suite and the residuum consists of amphibole, pyroxenes and possibly minor garnet and calcic plagioclase. Both models may be directly linked to the hypothesis of sea-floor spreading. 相似文献
5.
Dehydration Melting of Metabasalt at 8-32 kbar: Implications for Continental Growth and Crust-Mantle Recycling 总被引：195，自引：0，他引：195
We report the results of partial melting experiments between8 and 32 kbar, on four natural amphibolites representative ofmetamorphosed Archean tholeiite (greenstone), high-alumina basalt,low-potassium tholeiite and alkali-rich basalt. For each rock,we monitor changes in the relative proportions and compositionof partial melt and coexisting residual (crystalline) phasesfrom 1000 to 1150C, within and beyond the amphibole dehydrationreaction interval. Low percentage melts coexisting with an amphiboliteor garnet amphibolite residue at 1000–1025C and 8–16kbar are highly silicic (high-K2O granitic at 5%; melting, low-Al2O3trondhjemitic at 5–10%). Greater than 20% melting is onlyachieved beyond the amphibole-out phase boundary. Silicic tointermediate composition liquids (high-Al2O3 trondhjemitic-tonalitic,granodioritic, quartz dioritic, dioritic) result from 20–40%melting between 1050 and 1100C, leaving a granulite (plagioclase+ clinopyroxene orthopyroxene olivine) residue at 8 kbarand garnet granulite to eclogite (garnet + clinopyroxene) residuesat 12–32 kbar. Still higher degrees of melting ( 40–60%)result in mafic liquids corresponding to low-MgO, high-Al2O3basaltic and basaltic andesite compositions, which coexist withgranulitic residues at 8 kbar and edogitic or garnet granulitic(garnet + clinopyroxene + plagioclase orthopyroxene) residuesat higher pressures (12–28 kbar). As much as 40% by volumehigh-Al2O3 trondhjemitic-tonalitic liquid coexists with an eclogiticresidue at 1100–1150C and 32 kbar. The experimental datasuggest that the Archean tonalite-trondhjemite-granodiorite(TTG) suite of rocks, and their Phanerozoic equivalents, thetonalite-trondhjemite-dacite suite (including ‘adakites’and other Na-rich granitoids), can be generated by 10–40%melting of partially hydrated metabasalt at pressures abovethe garnet-in phase boundary (12 kbar) and temperatures between1000 and 1100C. Anomalously hot and/or thick metabasaltic crustis implied. Although a rare occurrence along modern convergentplate margins, subductionrelated melting of young, hot oceaniccrust (e.g. ocean ridges) may have been an important (essential)element in the growth of the continental crust in the Archean,if plate tectonic processes were operative. Coupled silicicmelt generation-segregation and mafic restite disposal may alsooccur at the base of continental or primitive (sub-arc?) crust,where crustal overthickening is a consequence of underplatingand overaccretion of mafic magmas. In either setting, net growthof continental crust and crustmantle recycling may be facilitatedby relatively high degrees of melting and extreme density contrastsbetween trondhjemitictonalitic liquids and garnet-rich residues.Continuous chemical trends are apparent between the experimentalcrystalline residues, and mafic migmatites and garnet granulitexenoliths from the lower crust, although lower-crustal xenolithsin general record lower temperatures (600–900C) and pressures(5–13 kbar) than corresponding residual assemblages fromthe experiments. However, geo-thermobarometry on eclogite xenolithsin kimberlites from the subcontinental mantle indicates conditionsappropriate for melting through and beyond the amphibole reactioninterval and the granulite-eclogite transition. If these samplesrepresent ancient (eclogitized) remnants of subducted or otherwisefoundered basaltic crust, then the intervening history of theirprotoliths may in some cases include partial melting. KEY WORDS: dehydration melting; metabasalt; continental growth; crust–mantle recycling
*Corresponding author. Present address: Mineral Physics Institute and Center for High Pressure Research, Department of Earth and Space Sciences, State University of New York at Stony Brook, Stony Brook, NY 11794, USA 相似文献
6.
An olivine basalt, a tonalite (andesite), a granite (rhyolite), and a red clay (pelagic sediment) were reacted, with known quantities of water in sealed noble metal capsules, in a piston-cylinder apparatus at 30 kb pressure. For the pelagic sediment, with H2O+=7.8% and no additional water, the liquidus temperature is 1240°C, the primary phases are garnet and kyanite. The subsolidus phase assemblage is phengite mica+garnet+clinopyroxene+coesite+kyanite. With 5 wt.% water added, the liquidus temperatures and primary phases for the calc-alkaline rocks are 1280°-1180°-1080°, garnet+clinopyroxene, garnet, and quartz respectively. Garnet and clinopyroxene occur throughout the melting interval of the olivine tholeiite for all water contents. Garnet is joined by clinopyroxene 80° below the andesite plus 5% H2O liquidus, quartz is joined by clinopyroxene 180° below the rhyolite plus 5% H2O liquidus. The subsolidus phase assemblage is garnet+clinopyroxene+coesite+minor kyanite for all the calc-alkaline compositions. We conclude that calc-alkaline andesites and rhyolites are not equilibrium partial melting pruducts of subducted oceanic crust consisting of olivine tholeiite basalt and siliceous sediments. Partial melting in subduction zones produces broadly acid and intermediate liquids, but these liquids lie off the calc-alkaline basalt-andesite-rhyolite join and must undergo modification at lower pressures to produce calcalkaline magmas erupted in overlying island arcs. 相似文献
7.
Quartz and orthopyroxene exsolution lamellae in clinopyroxene and the metamorphic P–T path of Belomorian eclogites 
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下载免费PDF全文 Xiaoli Li Lifei Zhang Chunjing Wei Alexander I. Slabunov Thomas Bader 《Journal of Metamorphic Geology》2018,36(1):1-22
The Shirokaya Salma eclogite‐bearing complex is located in the Archean–Palaeoproterozoic Belomorian Province (Russia). Its eclogites and eclogitic rocks show multiple clinopyroxene breakdown textures, characterized by quartz–amphibole, orthopyroxene and plagioclase lamellae. Representative samples, a fresh eclogite, two partly retrograded eclogites, and a strongly retrograded eclogitic rock, were collected for this study. Two distinct mineral assemblages—(1) omphacite+garnet+quartz+rutile±amphibole and (2) clinopyroxene+garnet+amphibole+plagioclase+quartz+rutile+ilmenite±orthopyroxene—are described. Based on phase equilibria modelling, these assemblages correspond to the eclogite and granulite facies metamorphism that occurred at 16–18 kbar, 750–800°C and 11–15 kbar, 820–850°C, respectively. The quartz–amphibole lamellae in clinopyroxene formed during retrogression with water ingress, but do not imply UHP metamorphism. The superfine orthopyroxene lamellae developed due to breakdown of an antecedent clinopyroxene (omphacite) during retrogression that was triggered by decompression from the peak of metamorphism, while the coarser orthopyroxene grains and rods formed afterwards. The P–T path reconstructed for the Shirokaya Salma eclogites is comparable to that of the adjacent 1.9 Ga Uzkaya Salma eclogite (Belomorian Province), and those of several other Palaeoproterozoic high‐grade metamorphic terranes worldwide, facts allowing us to debate the exact timing of eclogite facies metamorphism in the Belomorian Province. 相似文献
8.
Prograde and retrograde history of eclogites from the Eastern Blue Ridge, North Carolina, USA 总被引：4，自引：1，他引：4
Abstract The prograde metamorphism of eclogites is typically obscured by chemical equilibration at peak conditions and by partial requilibration during retrograde metamorphism. Eclogites from the Eastern Blue Ridge of North Carolina retain evidence of their prograde path in the form of inclusions preserved in garnet. These eclogites, from the vicinity of Bakersville, North Carolina, USA are primarily comprised of garnet–clinopyroxene–rutile–hornblende–plagioclase–quartz. Quartz, clinopyroxene, hornblende, rutile, epidote, titanite and biotite are found as inclusions in garnet cores. Included hornblende and clinopyroxene are chemically distinct from their matrix counterparts. Thermobarometry of inclusion sets from different garnets record different conditions. Inclusions of clinozoisite, titanite, rutile and quartz (clinozoisite + titanite = grossular + rutile + quartz + H2O) yield pressures (6–10 kbar, 400–600 °C and 8–12 kbar 450–680 °C) at or below the minimum peak conditions from matrix phases (10–13 kbar at 600–800 °C). Inclusions of hornblende, biotite and quartz give higher pressures (13–16 kbar and 630–660 °C). Early matrix pyroxene is partially or fully broken down to a diopside–plagioclase symplectite, and both garnet and pyroxene are rimmed with plagioclase and hornblende. Hypersthene is found as a minor phase in some diopside + plagioclase symplectites, which suggests retrogression through the granulite facies. Two‐pyroxene thermometry of this assemblage gives a temperature of c. 750 °C. Pairing the most Mg‐rich garnet composition with the assemblage plagioclase–diopside–hypersthene–quartz gives pressures of 14–16 kbar at this temperature. The hornblende–plagioclase–garnet rim–quartz assemblage yields 9–12 kbar and 500–550 °C. The combined P–T data show a clockwise loop from the amphibolite to eclogite to granulite facies, all of which are overprinted by a texturally late amphibolite facies assemblage. This loop provides an unusually complete P–T history of an eclogite, recording events during and following subduction and continental collision in the early Palaeozoic. 相似文献
9.
The genesis of basaltic magmas 总被引：29，自引：2，他引：29
This paper reports the results of a detailed experimental investigation of fractionation of natural basaltic compositions under conditions of high pressure and high temperature. A single stage, piston-cylinder apparatus has been used in the pressure range up to 27 kb and at temperatures up to 1500° C to study the melting behaviour of several basaltic compositions. The compositions chosen are olivine-rich (20% or more normative olivine) and include olivine tholeiite (12% normative hypersthene), olivine basalt (1% normative hypersthene) alkali olivine basalt (2% normative nepheline) and picrite (3% normative hypersthene). The liquidus phases of the olivine tholeiite and olivine basalt are olivine at 1 Atmosphere, 4.5 kb and 9 kb, orthopyroxene at 13.5 and 18 kb, clinopyroxene at 22.5 kb and garnet at 27 kb. In the alkali olivine basalt composition, the liquidus phases are olivine at 1 Atmosphere and 9 kb, orthopyroxene with clinopyroxene at 13.5 kb, clinopyroxene at 18 kb and garnet at 27 kb. The sequence of appearance of phases below the liquidus has also been studied in detail. The electron probe micro-analyser has been used to make partial quantitative analyses of olivines, orthopyroxenes, clinopyroxenes and garnets which have crystallized at high pressure.These experimental and analytical results are used to determine the directions of fractionation of basaltic magmas during crystallization over a wide range of pressures. At pressures corresponding to depths of 35–70 km separation of aluminous enstatite from olivine tholeiite magma produces a direct fractionation trend from olivine tholeiites through olivine basalts to alkali olivine basalts. Co-precipitation of sub-calcic, aluminous clinopyroxene with the orthopyroxene in the more undersaturated compositions of this sequence produces derivative liquids of basanite type. Magmas of alkali olivine basalt and basanite type represent the lower temperature liquids derived by approximately 30% crystallization of olivine-rich tholeiite at 35–70 km depth. At depths of about 30 km, fractionation of olivine-rich tholeiite with separation of both olivine and low-alumina enstatite, joined at lower temperatures by sub-calcic clinopyroxene, leads to derivative liquids with relatively constant SiO2 (48 to 50%) increasingly high Al2O3 (15–17%) contents and retaining olivine + hypersthene normative chemistry (5–15% normative olivine). These have the composition of typical high-alumina olivine tholeiites. The effects of low pressure fractionation may be superimposed on magma compositions derived from various depths within the mantle. These lead to divergence of the alkali olivine basalt and tholeiitic series but convergence of both the low-alumina and high-alumina tholeiites towards quartz tholeiite derivative liquids.The general problem of derivation of basaltic magmas from a mantle of peridotitic composition is discussed in some detail. Magmas are considered to be a consequence of partial melting but the composition of a magma is determined not by the depth of partial melting but by the depth at which magma segregation from residual crystals occurs. Magma generation from parental peridotite (pyrolite) at depths up to 100 km involves liquid-crystal equilibria between basaltic liquids and olivine + aluminous pyroxenes and does not involve garnet. At 35–70 km depth, basaltic liquids segregating from a pyrolite mantle will be of alkali olivine basalt type with about 20% partial melting but with increasing degrees of partial melting, liquids will change to olivine-rich tholeiite type with about 30% melting. If the depth of magma segregation is about 30 km, then magmas produced by 20–25% partial melting will be of high-alumina olivine tholeiite type, similar to the oceanic tholeiites occurring on the sea floor along the mid-oceanic ridges.Hypotheses of magma fractionation and generation by partial melting are considered in relation to the abundances and ratios of trace elements and in relation to isotopic abundance data on natural basalts. It is shown that there is a group of elements (including K, Ti, P, U, Th, Ba, Rb, Sr, Cs, Zr, Hf and the rare-earth elements) which show enrichment factors in alkali olivine basalts and in some tholeiites, which are inconsistent with simple crystal fractionation relationships between the magma types. This group of elements has been called incompatible elements referring to their inability to substitute to any appreciable extent in the major minerals of the upper mantle (olivine, aluminous pyroxenes). Because of the lack of temperature contrast between magma and wall-rock for a body of magma near to its depth of segregation in the mantle, cooling of the magma involves complementary processes of reaction with the wall-rook, including selective melting and extraction of the lowest melting fraction. The incompatible elements are probably highly concentrated in the lowest melting fraction of the pyrolite. The production of large overall enrichments in incompatible elements in a magma by reaction with and highly selective sampling of large volumes of mantle wall-rock during slow ascent of a magma is considered to be a normal, complementary process to crystal fractionation in the mantle. This process has been called wall-rock reaction. Magma generation in the mantle is rarely a simple, closed-system partial melting process and the isotopic abundances and incompatible element abundances of a basalt as observed at the earth's surface may be largely determined by the degree of reaction with the mantle or lower crustal wall-rocks and bear little relation to the abundances and ratios of the original parental mantle material (pyrolite).Occurrences of cognate xenoliths and xenocrysts in basalts are considered in relation to the experimental data on liquid-crystal equilibria at high pressure. It is inferred that the lherzolite nodules largely represent residual material after extraction of alkali olivine basalt from mantle pyrolite or pyrolite which has been selectively depleted in incompatible elements by wall-rock reaction processes. Lherzolite nodules included in tholeiitic magmas would melt to a relatively large extent and disintegrate, but would have a largely refractory character if included in alkali olivine basalt magma. Other examples of xenocrystal material in basalts are shown to be probable liquidus crystals or accumulates at high pressure from basaltic magma and provide a useful link between the experimental study and natural processes. 相似文献
10.
James G. Brophy 《Contributions to Mineralogy and Petrology》1986,93(3):368-380
The widespread abundance of Hi-Alumina Basalt (HAB) lavas in most volcanic arcs has been suggested by some as evidence for a primary, parental HAB magma generated by the high pressure melting of subducted oceanic crust (quartz eclogite). Others suggest a parental, mantle-derived olivine tholeiite magma which produces HAB magmas through fractionation of olivine, clinopyroxene, chrome-spinel +/– plagioclase. The petrology and geochemistry of seven HAB lavas from the Aleutian Cold Bay Volcanic Center have been studied in order to specifically address these two possibilities. All lavas show mineralogical and compositional features typical of most Aleutian HAB lavas. Coexisting opx and cpx in a closely associated basaltic-andesite indicate a minimum pre-eruption temperature of 1,110° C. A comparison of the observed (plag-tmag-olivcpx) and experimentally determined crystallization sequences yields a minimum pre-eruption pressure estimate of 7 kb and estimated H2O contents of 0.7 wt.%. Maximum pre-eruption f
o
2 values have been estimated at NNO+0.6 log units.Mass balance calculations demonstrate that the HAB compositions are satisfied by the fractionation of olivine, clinopyroxene +/– plagioclase from a primitive (Mg-# > 65) parental tholeiite. Plagioclase accumulation does not play a significant role in their origin. Many of the same compositional characteristics are also satisfied by high pressure melting of altered ocean ridge tholeiite +5 v.% pelagic sediment (quartz eclogite). The available HAB phase equilibria data do not support a fractionation origin but do support an origin involving high pressure melting of quartz eclogite. The lack of compositional zonation in the HAB phenocrysts, and the complete absence of disequilibrium MgO-rich mafic phenocrysts further argue against a tholeiite fractionation origin.Consideration of all these features indicates that the geochemical data are permissive in their interpretation. A process involving tholeiite fractionation successfully predicts the compositions of the HAB lavas but is at odds with the mineralogical and phase equilibria evidence. With some exceptions (notably Ni, Cr and Sr abundances), a process of high pressure quartz eclogite melting is consistent with the compositional, mineralogical and phase equilibria characteristics of these HAB lavas. When the relative merits of both origins are weighed it is apparent that a quartz eclogite source satisfies more of important features of these HAB lavas.Extrusive rocks have been grouped on a basis of SiO2 content into basalt (<52 wt.%), basaltic-andesite (52–56 wt.%) and andesite (>56 wt.%) after Ewart (1982) 相似文献
11.
Anhydrous partial melting experiments, at 10 to 30 kbar from solidus to near liquidus temperature, have been performed on an iron-rich martian mantle composition, DW. The DW subsolidus assemblage from 5 kbar to at least 24 kbar is a spinel lherzolite. At 25 kbar garnet is stable at the solidus along with spinel. The clinopyroxene stable on the DW solidus at and above 10 kbar is a pigeonitic clinopyroxene. Pigeonitic clinopyroxene is the first phase to melt out of the spinel lherzolite assemblage at less than 20°C above the solidus. Spinel melts out of the assemblage about 50°C above the solidus followed by a 150° to 200°C temperature interval where melts are in equilibrium with orthopyroxene and olivine. The temperature interval over which pigeonitic clinopyroxene melts out of an iron-rich spinel lherzolite assemblage is smaller than the temperature interval over which augite melts out of an iron-poor spinel lherzolite assemblage. The dominant solidus assemblage in the source regions of the Tharsis plateau, and for a large percentage of the martian mantle, is a spinel lherzolite. 相似文献
12.
High‐pressure kyanite‐bearing felsic granulites in the Bashiwake area of the south Altyn Tagh (SAT) subduction–collision complex enclose mafic granulites and garnet peridotite‐hosted sapphirine‐bearing metabasites. The predominant felsic granulites are garnet + quartz + ternary feldspar (now perthite) rocks containing kyanite, plagioclase, biotite, rutile, spinel, corundum, and minor zircon and apatite. The quartz‐bearing mafic granulites contain a peak pressure assemblage of garnet + clinopyroxene + ternary feldspar (now mesoperthite) + quartz + rutile. The sapphirine‐bearing metabasites occur as mafic layers in garnet peridotite. Petrographical data suggest a peak assemblage of garnet + clinopyroxene + kyanite + rutile. Early kyanite is inferred from a symplectite of sapphirine + corundum + plagioclase ± spinel, interpreted to have formed during decompression. Garnet peridotite contains an assemblage of garnet + olivine + orthopyroxene + clinopyroxene. Thermobarometry indicates that all rock types experienced peak P–T conditions of 18.5–27.3 kbar and 870–1050 °C. A medium–high pressure granulite facies overprint (780–820 °C, 9.5–12 kbar) is defined by the formation of secondary clinopyroxene ± orthopyroxene + plagioclase at the expense of garnet and early clinopyroxene in the mafic granulites, as well as by growth of spinel and plagioclase at the expense of garnet and kyanite in the felsic granulite. SHRIMP II zircon U‐Pb geochronology yields ages of 493 ± 7 Ma (mean of 11) from the felsic granulite, 497 ± 11 Ma (mean of 11) from sapphirine‐bearing metabasite and 501 ± 16 Ma (mean of 10) from garnet peridotite. Rounded zircon morphology, cathodoluminescence (CL) sector zoning, and inclusions of peak metamorphic minerals indicate these ages reflect HP/HT metamorphism. Similar ages determined for eclogites from the western segment of the SAT suggest that the same continental subduction/collision event may be responsible for HP metamorphism in both areas. 相似文献
13.
Phase relations of a high-Mg basalt from the Aleutian Island Arc: Implications for primary island arc basalts and high-Al basalts 总被引：1，自引：1，他引：1
Many volcanic centers in the Aleutian Islands have erupted lavas that range in composition from high-Mg basalt (MgO>9 wt%) to more fractionated and voluminous high-Al basalts and basaltic andesites. The petrogenetic relationships between these rock types and the composition of primary magmas has been vigorously debated. The phase relations of a typical high-Mg basalt from the Makushin volcanic field on Unalaska Island provide important constraints on petrogenetic models. Results of one-atmosphere and moderate-to high-pressure (5–20 kb) anhydrous experiments are similar to results obtained from primitive MORB. At low pressures olivine is the liquidus phase joined by plagioclase and clinopyroxene at progressively lower temperatures. Clinopyroxene is the second phase to crystallize at pressures greater than 5 kb and replaces olivine on the liquidus at approximately 10 kb. Above 10 kb the liquidus pyroxene is aluminous augite and orthopyroxene is the second phase to crystallize. Glasses in equilibrium with olivine and clinopyroxene at intermediate-pressure (5 to 10 kb) are similar in composition to high-Al basalt. Plagioclase is not involved and most likely does not become a liquidus phase until the liquid has evolved significantly. Although our studies do not confirm the primary nature of high-Mg basalts they do support a model in which high-Al basalts are generated by moderate amounts of crystal fractionation from more primitive (high Mg/Mg+Fe, lower Al2O3) basaltic magmas near the arc crust-mantle boundary.Abbreviations
Ol
olivine
-
Cpx
Clinopyroxene
-
Pl
plagioclase
-
L
liquid
-
Sp
spinel
-
Pig
pigeonite
-
Opx
Orthopyroxene 相似文献
14.
Tibor Gasparik 《Contributions to Mineralogy and Petrology》1987,96(3):357-370
In the system FeO-MgO-Al2O3-SiO2 (FMAS), the equilibrium Al-content of orthopyroxene coexisting with olivine and spinel was reversed in 18 experiments at 1 340° C and 11 or 18 kbar, using graphite capsules and PbO flux. In the CFMAS system (+CaO), the Al-contents of ortho- and clinopyroxene coexisting with olivine and spinel were reversed in 5 experiments at 1 340° C and 18 kbar. The Al-content of clinopyroxene remains constant, while the Al-content of orthopyroxene increases with increasing Fe-content. The Ca-content of clinopyroxene is independent of the Al-content. The data were used to describe the Fe-Mg site distribution in the aluminous orthopyroxene. The Fe-Mg partitioning among orthopyroxene, olivine, spinel and garnet, combined with the Al-content of orthopyroxene, was used to calculate orthopyroxene based thermobarometers in the FMAS, CFMAS and NCFMAS (+Na2O) systems. The thermobarometers were applied to the Adirondack metagabbros, which gave equilibration temperatures of 700–800° C and pressures 7.4–10.3 kbar. 相似文献
15.
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
16.
Ordovician eclogites from the Chinese Beishan: implications for the tectonic evolution of the southern Altaids 总被引：1，自引：0，他引：1
J. F. QU W. J. XIAO B. F. WINDLEY C. M. HAN Q. G. MAO S. J. AO J. E. ZHANG 《Journal of Metamorphic Geology》2011,29(8):803-820
Numerous lenses of eclogite occur in a belt of augen orthogneisses in the Gubaoquan area in the southern Beishan orogen, an eastern extension of the Tianshan orogen. With detailed petrological data and phase relations, modelled in the system NCFMASHTO with thermocalc , a quantitative P–T path was estimated and defined a clockwise P–T path that showed a near isothermal decompression from eclogite facies (>15.5 kbar, 700–800 °C, omphacite + garnet) to high‐pressure granulite facies (12–14 kbar, 700–750 °C, clinopyroxene + sodic plagioclase symplectitic intergrowths around omphacite), low‐pressure granulite facies (8–9.5 kbar, ～700 °C, orthopyroxene + clinopyroxene + plagioclase symplectites and coronas surrounding garnet) and amphibolite facies (5–7 kbar, 600–700 °C, hornblende + plagioclase symplectites). The major and trace elements and Sm–Nd isotopic data suggest that most of the Beishan eclogite samples had a protolith of oceanic crust with geochemical characteristics of an enriched or normal mid‐ocean ridge basalt. The U–Pb dating of the Beishan eclogites indicates an Ordovician age of c. 467 Ma for the eclogite facies metamorphism. An 39Ar/40Ar age of c. 430 Ma for biotite from the augen gneiss corresponds to the time of retrograde metamorphism. The combined data from geological setting, bulk composition, clockwise P–T path and geochronology support a model in which the Beishan eclogites started as oceanic crust in the Palaeoasian Ocean, which was subducted to eclogite depths in the Ordovician and exhumed in the Silurian. The eclogite‐bearing gneiss belt marks the position of a high‐pressure Ordovician suture zone, and the calculated clockwise P–T path defines the progression from subduction to exhumation. 相似文献
17.
Eclogites in the Makuti gneisses of Zimbabwe: implications for the tectonic evolution of the Zambezi Belt in southern Africa 总被引：2，自引：0，他引：2
The gneisses of the Makuti Group in north-west Zimbabwe are characterized by complex geometries that resulted from intense non-coaxial deformation in a crustal scale high-strain zone that accommodated extensional deformation along the axis of the Zambezi Belt at c. 800 Ma. Within low-strain domains in the Makuti gneisses, undeformed metagabbroic lenses preserve eclogite and granulite facies assemblages, which record a part of the metamorphic history that predates Pan-African events. Eclogitic rocks can be subdivided into: (1) corona-textured metagabbros that preserve igneous textures, and (2) garnet–omphacite rocks in which primary textures are destroyed. The lenses of eclogitic rocks are enveloped in a mantle of garnet–clinopyroxene–hornblende gneiss, which is a common rock type in the Makuti gneisses. The eclogites preserve multi-staged, domainal, symplectic reaction textures that developed progressively as the rocks experienced loading followed by decompression–heating. In the metagabbros, the original clinopyroxene, plagioclase and olivine domains acted separately during the peak of metamorphism, with plagioclase being replaced by garnet and kyanite, and olivine being replaced by orthopyroxene and possibly omphacite. The peak assemblage was overprinted by: (1) the multi-mineralic corona assemblage pargasite–orthopyroxene–spinel–plagioclase replacing garnet–kyanite–clinopyroxene (possibly at c. 19 kbar, 760±25 °C); (2) orthopyroxene–pargasite–plagioclase–scapolite coronas replacing orthopyroxene (15±1.5 kbar, 750±50 °C); and (3) moats of orthopyroxene–plagioclase replacing garnet (10±1 kbar, 760±50 °C). The garnet–omphacite rocks record similar peak conditions (15±1.1 kbar, 760±60 °C). Garnet–clinopyroxene–hornblende–plagioclase gneisses envelop the eclogites and record matrix conditions of 11±1.5 kbar at 730±50 °C using assemblages that are oriented in the regional fabric. These rocks are characterized by decompression-heating textures, reflecting temperature increases during exhumation of the Makuti gneisses. The eclogite facies rocks formed during a collisional event prior to 850 Ma. Their formation could be related to a suture zone that developed along the axis of the Zambezi Belt during the formation of Rodinia (between 1400 and 850 Ma). The main deformation-metamorphism in the Makuti gneisses occurred around 800 Ma and involved extension and exhumation of the high-P rocks (break-up of Rodinia), which experienced a high-T metamorphic overprint. Around 550–500 Ma, a collisional event associated with the formation of Gondwana resulted in renewed burial and metamorphic recrystallization of the Makuti gneisses. 相似文献
18.
The anhydrous melting behaviour of two synthetic peridotite compositions has been studied experimentally at temperatures ranging from near the solidus to about 200° C above the solidus within the pressure range 0–15 kb. The peridotite compositions studied are equivalent to Hawaiian pyrolite and a more depleted spinel lherzolite (Tinaquillo peridotite) and in both cases the experimental studies used peridotite –40% olivine compositions. Equilibrium melting results in progressive elimination of phases with increasing temperature. Four main melting fields are recognized; from the solidus these are: olivine (ol)+orthopyroxene (opx)+clinopyroxene (cpx)+Al-rich phase (plagioclase at low pressure, spinel at moderate pressure, garnet at high pressure)+liquid (L); ol+opx+cpx+Cr-spinel+L; ol+opx+Cr-spinel +L: ol±Cr-spinel+L. Microprobe analyses of the residual phases show progressive changes to more refractory compositions with increasing proportion of coexisting melt i.e. increasing Mg/(Mg+Fe) and Cr/(Cr+Al) ratios, decreasing Al2O3, CaO in pyroxene.The degree of melting, established by modal analysis, increases rapidly immediately above the solidus (up to 10% melting occurs within 25°–30° C of the solidus), and then increases in roughly linear form with increasing temperature.Equilibrium melt compositions have been calculated by mass balance using the compositions and proportions of residual phases to overcome the problems of iron loss and quench modification of the glass. Compositions from the melting of pyrolite within the spinel peridotite field (i.e. 15 kb) range from alkali olivine basalt (<15% melting) through olivine tholeiite (20–30% melting) and picrite to komatiite (40–60% melting). Melting in the plagioclase peridotite field produces magnesian quartz tholeiite and olivine-poor tholeiite and, at higher degrees of melting (30–40%), basaltic or pyroxenitic komatiite. Melts from Tinaquillo lherzolite are more silica saturated than those from pyrolite for similar degrees of partial melting, and range from olivine tholeiite through tholeiitic picrite to komatiite for melting in the spinel peridotite field.The equilibrium melts are compared with inferred primary magma compositions and integrated with previous melting studies on basalts. The data obtained here and complementary basalt melting studies do not support models of formation of oceanic crust in which the parental magmas of common mid-ocean ridge basalts (MORB) are attributed to segregation from source peridotite at shallow depths ( 25 km) to leave residual harzburgite. Liquids segregating from peridotite at these depths are more silica-rich than common MORB. 相似文献
19.
The Lherz orogenic lherzolite massif (Eastern French Pyrenees) displays one of the best exposures of subcontinental lithospheric
mantle containing veins of amphibole pyroxenites and hornblendites. A reappraisal of the petrogenesis of these rocks has been
attempted from a comprehensive study of their mutual structural relationships, their petrography and their mineral compositions.
Amphibole pyroxenites comprise clinopyroxene, orthopyroxene and spinel as early cumulus phases, with garnet and late-magmatic
K2O-poor pargasite replacing clinopyroxene, and subsolidus exsolution products (olivine, spinel II, garnet II, plagioclase).
The original magmatic mineralogy and rock compositions were partly obscured by late-intrusive hornblendites and over a few
centimetres by vein–wallrock exchange reactions which continued down to subsolidus temperatures for Mg–Fe. Thermobarometric
data and liquidus parageneses indicate that amphibole pyroxenites started to crystallize at P ≥ 13 kbar and recrystallized at P < 12 kbar. The high AlVI/AlIV ratio (>1) of clinopyroxenes, the early precipitation of orthopyroxene and the late-magmatic amphibole are arguments for
parental melts richer in silica but poorer in water than alkali basalts. Their modelled major element compositions are similar
to transitional alkali basalt with about 1–3 wt% H2O. In contrast to amphibole pyroxenites, hornblendites only show kaersutite as liquidus phase, and phlogopite as intercumulus
phase. They are interpreted as crystalline segregates from primary basanitic magmas (mg=0.6; 4–6 wt% H2O). These latter cannot be related to the parental liquids of amphibole pyroxenites by a fractional crystallization process.
Rather, basanitic liquids mostly reused pre-existing pyroxenite vein conduits at a higher structural level (P ≤ 10 kbar). A continuous process of redox melting and/or alkali melt/peridotite interaction in a veined lithospheric mantle
is proposed to account for the origin of the Lherz hydrous veins. The transitional basalt composition is interpreted in terms
of extensive dissolution of olivine and orthopyroxene from wallrock peridotite by alkaline melts produced at the mechanical
boundary layer/thermal boundary layer transition (about 45–50 km deep). Continuous fluid ingress allowed remelting of the
deeper veined mantle to produce the basanitic, strongly volatiles enriched, melts that precipitated hornblendites. A similar
model could be valid for the few orthopyroxene-rich hydrous pyroxenites described in basalt-hosted mantle xenoliths.
Received: 15 September 1999 / Accepted: 31 January 2000 相似文献
20.
R. F. Heming 《Contributions to Mineralogy and Petrology》1977,61(1):15-33
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