首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 26 毫秒
1.
The garnet-cordierite zone, the highest-grade zone of the Ryoke metamorphic rocks in the Yanai district, SW Japan, is defined by the coexistence of garnet and cordierite in pelitic rocks. Three assemblages in this zone are studied in detail, i.e. spinel + cordierite + biotite, garnet + cordierite + biotite and garnet + biotite, all of which contain quartz, K-feldspar and plagioclase. The Mg/(Fe + Mg) in the coexisting minerals decreases in the following order: cordierite, biotite, garnet and spinel. Two facts described below are inconsistent with the paragenetic relation in the K2OFeOMgOAl2O3SiO2H2O (KFMASH) system in terms of an isophysical variation. First, garnet and biotite in the last assemblage have Mg/(Fe + Mg) higher than those in the second. Second, the first two assemblages are described by the reaction,
while they occur in a single outcrop. The addition of MnO, ZnO and TiO2 to the system can resolve the inconsistencies as follows. The assemblage garnet + biotite can consist of garnet and biotite higher in Mg/(Fe + Mg) than those in garnet + cordierite + biotite as long as they are enriched in spessartine and depleted in Al, respectively. The assemblage garnet + cordierite + biotite becomes stable relative to spinel + cordierite + biotite with increasing spessartine content or decreasing gahnite content and the Ti content of biotite. The constituent minerals of the assemblages, spinel + cordierite + biotite and garnet + cordierite + biotite, preserve several reaction microstructures indicative of prograde reactions,
and
together with retrograde reactions,
and
This suggests that the pressure-temperature path of the rocks includes an isobaric heating and an isobaric or decompressional cooling. The high-grade areas consisting of the K-feldspar-cordierite zone, sillimanite-K-feldspar zone and garnet-cordierite zone have prograde paths involving isobaric heating and show a southwards increase in pressure with a thermal maximum in the middle. These high-grade zones are closely associated with the gneissose granitic rocks, suggesting that the Ryoke metamorphism, one of the typical low-pressure type, is caused by the heat supply from the syn-tectonic granitic rocks that emplaced at the middle level of the crust. Received: 22 August 1997 / Accepted: 11 May 1998  相似文献   

2.
Reactions between hornblende-plagioclase amphibolite and acidic and alkaline B-bearing aqueous fluids have been investigated by experiments at 475°–600° C and 200 MPa. At 600° C, hornblende+calcic plagioclase react to form tourmaline+danburite+clinopyroxene+quartz in acidic fluids containing 0.5–1.0 wt% B2O3.Tourmaline is precipitated directly from acidic fluids, and the reaction is driven by neutralization of fluids by Na±Ca derived from the breakdown of reactant solids. The concentration of B2O3 in fluids needed to stabilize tourmaline increases as pH increases (above approximately 6.0), and tourmaline is unstable in alkaline fluids (pH > approximately 6.5–7.0) regardless of B concentration. In addition to acid-base relations, tourmaline stability is favored by comparatively higher activity coefficients for Al species in acidic fluids. The concentrations of Al and Si in fluid increase with alkalinity, with the eventual production of felsic borosilicate melts through partial melting of the plagioclase component of the amphibolite. In seeded experiments, tourmaline also contributes components to melt. Partial melting is evident in the range 500°–525° C at 200 MPa in experiments with 8wt% B2O3 in fluid as Na2B4O7. The experimental results are applied primarily to metasomatic reactions between mafic rocks and borate fluids derived from granitic magmas, but tourmaline stability and partial melting in mafic regional metamorphic systems are also discussed briefly.  相似文献   

3.
In the Grt-Bt-Sil restitic xenoliths of El Joyazo (Cerro de Hoyazo), hercynitic spinel is a minor phase commonly associated with biotite. The possible reaction relationships among biotite and spinel are studied in reaction textures developed around biotites at their contact with patches of fibrolitic sillimanite and rhyolitic melt. In these textures, resorbed biotite crystals about 1 mm long are rimmed by a layer of glass <200 μm thick containing spinel and ilmenite; the same glass also fills embayments in biotite. Spinel forms euhedral crystals <100 μm in size, and ilmenite occurs as smaller anhedral crystals or needles, often intergrown with spinel. The homogeneous felt-like melt-sillimanite aggregate (“mix”) is richest in glass close to the reaction rim around biotite. Plagioclase and garnet are located >5 mm away from the reaction texture. Biotite is chemically zoned. Cores (Bt 1 ) have XMg=0.35 ± 0.02 and Ti=0.58 ± 0.01 atoms; whereas the outer rims (Bt 2 ) have XMg=0.45 ± 0.01 and Ti up to 0.68 atoms. The hercynite-rich spinel (Spl) has low ZnO content (<0.80 wt%) and XMg=0.26 ± 0.04. The chemical compositions of the mix aggregate represent linear combinations between sillimanite and a silica-rich melt. This melt (melt 1 ) is different from that of the layer around biotite (melt 2 ), which is also richer in Ca and alkalis. Garnet rims (Grt) have low Ca and Mn, and XMg=0.14. Plagioclase is characterized by large homogeneous cores (Pl 1 , An31 ± 2) and more calcic rims (Pl 2 , An49 ± 6). Matrix analysis in the 9-component (Al-Ca-Fe-K-Mg-Mn-Na-Si-Ti), 9-phase (Bt1-Bt2-Grt-Spl-Ilm-melt2-mix-Pl1-Pl2) system provides the mass balance (in mole units):
This relationship is in excellent agreement with the observed textures and hence is considered a good model for the incongruent melting of biotite in the xenoliths. The mass-balance indicates that melt production is dominated by the availability of K from biotite, and that garnet and plagioclase must be involved as reactants, so that the reaction volume is larger than the melt production site. The melting of biotite, constrained at T=900–950°C and P ≥ 5 kbar, is not a terminal reaction, as its variance in the reduced 8-component multisystem is ≥3. Received: 1 June 1999 / Accepted: 8 February 2000  相似文献   

4.
Proterozoic migmatitic paragneisses exposed in the McCulloughRange, southern Nevada, consist of cordierite+almanditic garnet+biotite+sillimanite+plagioclase+K-feldspar+quartz+ilmenite+hercynite.This assemblage is indicative of a low-pressure fades seriesat hornblende-granulite grade. Textures record a single metamorphicevent involving crystallization of cordierite at the expenseof biotite and sillimanite. Thermobarometry utilizing cation exchange between garnet, biotite,cordierite, hercynite, and plagioclase yields a preferred temperaturerange of 590–750?C and a pressure range of 3–4 kb.Equilibrium among biotite, sillimanite, quartz, garnet, andK-feldspar records aH2O between 0?03 and 0?26. The low aH2Otogetherwith low fO2 (QFM) and optical properties of cordierite indicatemetamorphism under fluid-absent conditions. Preserved mineralcompositions are not consistent with equilibrium with a meltphase. Earlier limited partial melting was apparently extensiveenough to cause desiccation of the pelitic assemblage. The relatively low pressures attending high-grade metamorphismof the McCullough Range paragneisses allies this terrane withbiotite-cordierite-garnet granulites in other orogenic belts.aosure pressures and temperatures require a transient apparentthermal gradient ofat least 50?C/km during part of this Proterozoicevent in the southern Cordillera. *Present address: Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90024-1567  相似文献   

5.
The Ross of Mull pluton consists of granites and granodioritesand intrudes sediments previously metamorphosed at amphibolitefacies. The high grade and coarse grain size of the protolithis responsible for a high degree of disequilibrium in many partsof the aureole and for some unusual textures. A band of metapelitecontained coarse garnet, biotite and kyanite prior to intrusion,and developed a sequence of textures towards the pluton. InZone I, garnet is rimmed by cordierite and new biotite. In ZoneII, coarse kyanite grains are partly replaced by andalusite,indicating incomplete reaction. Coronas of cordierite + muscovitearound kyanite are due to reaction with biotite. In the higher-gradeparts of this zone there is complete replacement of kyaniteand/or andalusite by muscovite and cordierite. Cordierite chemistryindicates that in Zone II the stable AFM assemblage (not attained)would have been cordierite + biotite + muscovite, without andalusite.The observed andalusite is therefore metastable. Garnet is unstablein Zone II, with regional garnets breaking down to cordierite,new biotite and plagioclase. In Zone III this breakdown is welladvanced, and this zone marks the appearance of fibrolite andK-feldspar in the groundmass as a result of muscovite breakdown.Zone IV shows garnet with cordierite, biotite, sillimanite,K-feldspar and quartz. Some garnets are armoured by cordieriteand are inferred to be relics. Others are euhedral with Mn-richcores. For these, the reaction biotite + sillimanite + quartz garnet + cordierite + K-feldspar + melt is inferred. Usinga petrogenetic grid based on the work of Pattison and Harte,pressure is estimated at 3·2 kbar, and temperature atthe Zone II–III boundary at 650°C and in Zone IV asat least 750°C. KEY WORDS: contact metamorphism; disequilibrium  相似文献   

6.
Phase assemblages and phase compositions were studied experimentally in water-saturated, biotite-bearing peraluminous granitic melts as a function of alumina excess and temperature. The runs were performed at 2 and 5 kbars under NNO buffer. Biotite was stable only in composition containing 5% of normative corundum; it coexisted with cordierite and hercynite at 2 kbars and with hercynite at 5 kbars. In composition containing 10% of normative corundum biotite was not observed; abundant cordierite and hercynite were the only Fe-Mg-Al minerals. These relationships show that, at constant pressure, the amount of cordierite increases with increasing excess of alumina. Simultaneously the stability of biotite decreases due to preferential partitioning of Mg into cordierite and Fe into biotite. Besides the distribution of Fe, Mg and Al among the coexisting solid phases, solubility of these elements in the melts is given. Below 900° C melts are poor in iron and magnesium and correspond, in terms of these elements, to leucogranites. It is suggested that the leucogranitic magmas, such as parental magmas of European Hercynian and Himalayan leucogranites, must have been formed through highly efficient separation of partial melt from restite, in which ferromagnesian components are concentrated. Peraluminous granites rich in ferromagnesian minerals originate supposedly from restite-bearing magmas.  相似文献   

7.
Anatectic migmatites in medium- to low-pressure granulite facies metasediments exposed in the Larsemann Hills, East Antarctica, contain leucosomes with abundant quartz and plagioclase and minor interstitial K-feldspar, and assemblages of garnet–cordierite–spinel–ilmenite–sillimanite. Qualitative modelling in the system K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2, in conjunction with various P–T calculations indicate that the high-grade retrograde evolution of the terrane was dominated by decompression from peak conditions of c. 7 kbar at c. 800 °C to 4–5 kbar at c. 750 °C. Extensive partial melting during decompression involved the replacement of biotite by the assemblage cordierite–garnet–spinel within the leucosomes. These leucosomes represent the site of partial melt generation, the cordierite–garnet–spinel–ilmenite assemblage representing the solid products and excess reactants from the melting reaction. The extraction and accumulation of this decompression-generated melt led to the formation of syntectonic pegmatites and extensive granitic plutons. Leucosome development and terrane decompression proceeded during crustal transpression, synchronous with upper crustal extension, during a progressive Early Palaeozoic collisional event. Subsequent retrograde evolution was characterized by cooling, as indicated by the growth of biotite replacing spinel and garnet, thin mantles of cordierite replacing spinel and quartz within metapelites, and garnet replacing orthopyroxene and hornblende within metabasites. P–T calculations on late mylonites indicate lower grade conditions of formation of c. 3.5 kbar at c. 650 °C, consistent with the development of late cooling textures.  相似文献   

8.
Partial electron microprobe analyses of garnet, biotite and cordierite in sillimanite-K feldspar gneisses of the Brimfield Formation in south-central Massachusetts indicate that the compositions of these minerals are not constant in a thin section. The FeO/MgO mol ratio of biotite is sensitive to the nature of other FeO-MgO minerals occurring in close proximity. The most iron-rich biotites are those that do not contact either cordierite or garnet. The most iron-poor biotites occur as inclusions in garnet. Biotites in direct contact with either cordierite or garnet have intermediate FeO/MgO ratios. The bulk of a given grain of garnet or cordierite is homogeneous in composition. Chemical zoning is absent. All grains of garnet and cordierite in a thin section are constant in composition. However, where garnet and cordierite abut biotite, the FeO/MgO ratio of the garnet rim is increased and that of cordierite is decreased. The FeO/MgO ratios of garnet, cordierite and biotite bare a regular relation to each other indicating a possible equilibrium state. However the distribution coefficient defined by the compositions of minerals in direct contact are greater than those defined by the compositions of the interiors of garnet and cordierite matched with the compositions of biotites removed from these phases. This pattern is believed to be the result of two thermal events. The first event produced the mineral assemblages and widespread equilibrium was obtained. A subsequent retrograde event left the mineralogy intact but caused cation exchange reactions at immediate contacts between garnet, cordierite and biotite. The physical conditions of the first event are estimated at P=5–6 kb, T=700–750° C. The retrograde event occurred at lower temperatures and very low activities of H2O since no muscovite is developed at microcline-sillimanite contacts.  相似文献   

9.
Epidote phenocrysts in dacitic dikes,Boulder County,Colorado   总被引：1，自引：0，他引：1  
Epidote (Ps21%) crystallized early as elongate phenocrysts in Late Cretaceous rhyodacitic dikes in the vicinity of Ward, Boulder County, Colorado. Other unusual phenocryst phases are garnet (Gr17–24%) and muscovite. In a xenolith containing kyanite, corundum, biotite, and plagioclase, magmatic garnet grew as a rim around xenocrystic pyrope-rich (Py37%) garnet. The xenolith was derived from a granulite-facies zone, not represented at the present-day erosion surface which is composed of upper amphibolite-facies cordierite and sillimanite-bearing gneisses. The dike magmas were fed not from an immediately underlying batholith but from a magma chamber at a depth corresponding to a pressure of 8–13 kilobars. Phenocrysts cystallized in the temperature range 800 to 700° C, under H2O and O2 activities greater than normal for silicic magmas. This occurrence shows convincingly not only that epidote can be magmatic but that it is a possible early-crystallization phase in silicic magmas.  相似文献   

10.
Mineral textures in metapelitic granulites from the northern Prince Charles Mountains, coupled with thermodynamic modelling in the K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (KFMASHTO) model system, point to pressure increasing with increasing temperature on the prograde metamorphic path, followed by retrograde cooling (i.e. an anticlockwise P–T path). Textural evidence for the increasing temperature part of the path is given by the breakdown of garnet and biotite to form orthopyroxene and cordierite in sillimanite‐absent rocks, and through the break‐down of biotite and sillimanite to form spinel, cordierite and garnet in more aluminous assemblages. This is equated to the advective addition of heat from the regional emplacement of granitic and charnockitic magmas dated at c. 980 Ma. A subsequent increase in pressure, inferred from the break‐down of spinel and quartz to sillimanite, cordierite and garnet in aluminous rocks, is attributed to crustal thickening related to upright folding dated at 940–910 Ma. The terrane attained peak metamorphic temperatures of c. 880 °C at pressures of c. 6.0–6.5 kbar during this event. Subsequent cooling is inferred from the localised breakdown of cordierite and garnet to form biotite and sillimanite that developed in the latter stages of the same event. The textural observations described are interpreted via the application of P–T and P–T–X pseudosections. The latter show that most rock compositions preserve only fragments of the overall P–T path; a result of different rock compositions undergoing mineral assemblage changes, or changes in mineral modal abundance, on different sections of the P–T path. The results also suggest that partial melting during granulite facies metamorphism, coupled with melt loss and dehydration, initiated a switch from pervasive ductile, to discrete ductile/brittle deformation, during retrograde cooling.  相似文献   

11.
In a prograde amphibolite-granulite transition zone in the Namaqualand Metamorphic Complex, metapelites show an interbanding of the amphibolite facies association biotite+sillimanite+quartz with the granulite facies association garnet+cordierite+K-feldspar. Relict graded bedding shows that compositional banding is of sedimentary origin. The garnet-cordierite-K-feldspar gneisses contain quartzofeldspathic segregations surrounding garnets, and have more Fe-rich bulk compositions than the biotite-sillimanite schists.The contrasting asemblages could have formed at the same pressure and temperature provided that a(H2O) was systematically lower in the garnet-cordierite-K-feldspar layers. The a(H2O) reduction resulted from the production of silicate melt by a vapour-absent continuous Fe-Mg reaction such as biotite+sillimanite+quartz=garnet+K-feldspar+liquid which affects Fe-rich compositions before vapour-absent melting occurs in more Mg-rich rocks. The segregations represent the solid and liquid products of the reaction.Such processes imply local control of a(H2O), and indicate that this granulite transition did not result from a regional influx of metasomatising fluids.  相似文献   

12.
The Sauwald Zone, located at the southern rim of the Bohemian Massif in Upper Austria, belongs to the Moldanubian Unit. It exposes uniform biotite + plagioclase ± cordierite paragneisses that formed during the post-collisional high-T/low-P stage of the Variscan orogeny. Rare metapelitic inlayers contain the mineral assemblage garnet + cordierite + green spinel + sillimanite + K-feldspar + plagioclase + biotite + quartz. Mineral chemical and textural data indicate four stages of mineral growth: (1) peak assemblage as inclusions in garnet (stage 1): garnet core + cordierite + green spinel + sillimanite + plagioclase (An35–65); (2) post-peak assemblages in the matrix (stages 2, 3): cordierite + spinel (brown-green and brown) ± sillimanite ± garnet rim + plagioclase (An10–45); and (3) late-stage growth of fibrolite, muscovite and albite (An0–15) during stage 4. Calculation of the P–T conditions of the peak assemblage (stage 1) yields 750–840°C, 0.29–0.53 GPa and for the stage 2 matrix assemblage garnet + cordierite + green spinel + sillimanite + plagioclase 620–730°C, 0.27–0.36 GPa. The observed phase relations indicate a clockwise P–T path, which terminates below 0.38 GPa. The P–T evolution of the Sauwald Zone and the Monotonous Unit are very similar, however, monazite ages of the former are younger (321 ± 9 Ma vs. 334 ± 1 Ma). This indicates that high-T/low-P metamorphism in the Sauwald Zone was either of longer duration or there were two independent phases of late-Variscan low-P/high-T metamorphism in the Moldanubian Unit.  相似文献   

13.
Highly magnesian and aluminous migmatitic gneisses from Mather Peninsula in the Rauer Group, Eastern Antarctica, preserve ultrahigh temperature (UHT) metamorphic assemblages that include orthopyroxene+sillimanite±quartz, garnet+sillimanite±quartz and garnet+orthopyroxene±sillimanite. Garnet that ranges up to XMg of 71.5 coexists with aluminous orthopyroxene that shows zoning from cores with 7.5–8.5 wt% Al2O3 to rims with up to 10.6 wt% Al2O3 adjacent to garnet. Peak P–T conditions of 1050 °C and 12 kbar are retrieved from Fe–Mg–Al thermobarometry involving garnet and orthopyroxene, in very good agreement with independent constraints from petrogenetic grids in FeO–MgO–Al2O3–SiO2 and related chemical systems. Sapphirine, orthopyroxene and cordierite form extensive symplectites and coronas on the early phases. The specific reaction textures and assemblages involving these secondary phases correlate with initial garnet XMg , with apparent higher-pressure reaction products occurring on the more magnesian garnet, and are interpreted to result from an initial phase of ultrahigh temperature near-isothermal decompression (UHT-ITD) from 12 to 8 kbar at temperatures in excess of 950 °C. Later textures that involved biotite formation and then partial breakdown, along with garnet relics, to symplectites of orthopyroxene+cordierite or cordierite+spinel may reflect hydration through back-reaction with crystallizing melts on cooling below 900–850 °C, followed by ITD from 7 to 8 kbar to c. 5 kbar at temperatures of 750–850 °C. The tectonic significance of this P–T history is ambiguous as the Rauer Group records the effects of Archean tectonothermal events as well as high-grade events at 1000 and 530 Ma. Late-stage biotite formation and subsequent ITD can be correlated with the P–T history preserved in the Proterozoic components of the Rauer Group and hence with either 1000 or 530 Ma collisional orogenesis. However, whether the preceding UHT-ITD history reflects a temporally unrelated event (e.g. Archean) or is simply an early stage of either the late-Proterozoic or Pan-African tectonism, as recently deduced for similar UHT rocks from other areas of the East Antarctica, remains uncertain.  相似文献   

14.
Stability relations of Fe-Mg cordierite with K feldspar have been determined for conditions of muscovite-quartz instability, applicable to highgrade metamorphism of pelitic rocks. Fe cordierite, K feldspar, and water break down to Fe biotite, sillimanite, and quartz at pressures above a line through 640 ° C, 2kbar and 710 ° C, 2.7 kbar. A P-X diagram for the Fe-Mg analogue of this reaction at 675 ° C is consistent with a naturally occuring cordierite-biotite K D value of 0.53 if Al content of biotite and cordierite water of hydration are taken into account.At higher temperatures Fe cordierite breaks down alone to almandine, sillimanite, quartz and water at pressures above a line through 650 ° C, 3.41 kbar and 760 ° C, 2.9 kbar. For the Fe-Mg reaction, P-X data up to 4 kbar may be extrapolated with use of natural K D values increasing toward one with increasing temperatures.Lines of constant cordierite composition for the two reactions intersect in an Fe-Mg univariant reaction of sillimanite-biotite-quartz to cordieritealmandine-K feldspar-water which is metastable relative to melt at = P tot Reduced water pressure and impurities in the garnet and K feldspar greatly reduce the temperature of this reaction so that it becomes a reasonable reaction for upper amphibolite and granulite facies conditions.The results demonstrate that (1) cordierite may be used as a geobarometer if temperature and approximate can be estimated, (2) almandine low in Mn and Ca does not participate in cordierite reactions where muscovite is present, and (3) the reaction which forms cordierite, almandine, and K feldspar is a possible melt-forming reaction which, under reduced , occurs about 50 ° C above the muscovite melting reaction.  相似文献   

15.
 Picritic units of the Miocene shield volcanics on Gran Canaria, Canary Islands, contain olivine and clinopyroxene phenocrysts with abundant primary melt, crystal and fluid inclusions. Composition and crystallization conditions of primary magmas in equilibrium with olivine Fo90-92 were inferred from high-temperature microthermometric quench experiments, low-temperature microthermometry of fluid inclusions and simulation of the reverse path of olivine fractional crystallization based on major element composition of melt inclusions. Primary magmas parental for the Miocene shield basalts range from transitional to alkaline picrites (14.7–19.3 wt% MgO, 43.2–45.7 wt% SiO2). Crystallization of these primary magmas is believed to have occurred over the temperature range 1490–1150° C at pressures ≈5 kbar producing olivine of Fo80.6-90.2, high-Ti chrome spinel [Mg/ (Mg+Fe2+)=0.32–0.56, Cr/(Cr+Al)=0.50–0.78, 2.52–8.58 wt% TiO2], and clinopyroxene [Mg/(Mg+Fe)=0.79–0.88, Wo44.1-45.3, En43.9-48.0, Fs6.8-11.0] which appeared on the liquidus together with olivine≈Fo86. Redox conditions evolved from intermediate between the QFM and WM buffers to late-stage conditions of NNO+1 to NNO+2. The primary magmas crystallized in the presence of an essentially pure CO2 fluid. The primary magmas originated at pressures >30 kbar and temperatures of 1500–1600° C, assuming equilibrium with mantle peridotite. This implies melting of the mantle source at a depth of ≈100 km within the garnet stability field followed by migration of melts into magma reservoirs located at the boundary between the upper mantle and lower crust. The temperatures and pressures of primary magma generation suggest that the Canarian plume originated in the lower mantle at depth ≈900 km that supports the plume concept of origin of the Canary Islands. Received: 23 October 1995/Accepted: 21 February 1996  相似文献   

16.
Summary Garnet occurs as a significant mineral constituent of felsic garnet-biotite granite in the southern edge of the Třebíč pluton. Two textural groups of garnet were recognized on the basis of their shape and relationship to biotite. Group I garnets are 1.5–2.5 mm, euhedral grains which have no reaction relationship with biotite. They are zoned having high XMn at the rims and are considered as magmatic. Group II garnets form grain aggregates up to 2.5 cm in size, with anhedral shape of individual grains. The individual garnet II grains are usually rimmed by biotite and have no compositional zoning. The core of group I garnets and group II garnets contains 67–80 mol% of almandine, 5–19 mol% of pyrope, 7–17 mol% of spessartine and 2–4 mol% of grossular. Biotite occurs in two generations; both are magnesian siderophyllites with Fe/(Fe + Mg) = 0.50–0.69. The matrix biotite in granites (biotite I) has high Ti content (0.09–0.31 apfu) and Fe/(Fe + Mg) ratio between 0.50 and 0.59. Biotite II forms reaction rims around garnet, is poor in Ti (0.00–0.06 apfu) and has a Fe/(Fe + Mg) ratio between 0.61 and 0.69. The textural relationship between biotite and garnet indicates that garnet reacted with granitic melt to form Ti-poor biotite and a new granitic melt, depleted in Ti and Mg and enriched in Fe and Al. In contrast to the host durbachites (hornblende-biotite melagranites), which originated by mixing of crustal melts and upper mantle melts, the origin of garnet-bearing granites is related to partial melting of the aluminium-rich metamorphic series of the Moldanubian Zone.  相似文献   

17.
A garnet-bearing tonalitic porphyry from the Achiq Kol area, northeast Tibetan Plateau has been dated by SHRIMP U-Pb zircon techniques and gives a Late Triassic age of 213 ± 3 Ma. The porphyry contains phenocrysts of Ca-rich, Mn-poor garnet (CaO > 5 wt%; MnO < 3 wt%), Al-rich hornblende (Al2O3 ~ 15.9 wt%), plagioclase and quartz, and pressure estimates for hornblende enclosing the garnet phenocrysts yield values of 8–10 kbar, indicating a minimum pressure for the garnet. The rock has SiO2 of 60–63 wt%, low MgO (<2.0 wt%), K2O (<1.3 wt%), but high Al2O3 (>17 wt%) contents, and is metaluminous to slightly peraluminous (ACNK = 0.89–1.05). The rock samples are enriched in LILE and LREE but depleted in Nb and Ti, showing typical features of subduction-related magmas. The relatively high Sr/Y (~38) ratios and low HREE (Yb < 0.8 ppm) contents suggest that garnet is a residual phase, while suppressed crystallization of plagioclase and lack of negative Eu anomalies indicate a high water fugacity in the magma. Nd–Sr isotope compositions of the rock (εNdT = −1.38 to −2.33; 87Sr/86Sri = 0.7065–0.7067) suggest that both mantle- and crust-derived materials were involved in the petrogenesis, which is consistent with the reverse compositional zoning of plagioclase, interpreted to indicate magma mixing. Both garnet phenocrysts and their ilmenite inclusions contain low MgO contents which, in combination with the oxygen isotope composition of garnet separates (+6.23‰), suggests that these minerals formed in a lower crust-derived felsic melt probably in the MASH zone. Although the rock samples are similar to adakitic rocks in many aspects, their moderate Sr contents (<260 ppm) and La/Yb ratios (mostly 16–21) are significantly lower than those of adakitic rocks. Because of high partition coefficients for Sr and LREE, fractionation of apatite at an early stage in the evolution of the magma may have effectively decreased both Sr and LREE in the residual melt. It is suggested that extensive crystallization of apatite as an early phase may prevent some arc magmas from evolving into adakitic rocks even under high water fugacity.  相似文献   

18.
Dehydration melting of tonalites. Part II. Composition of melts and solids   总被引：6，自引：0，他引：6  
 Dehydration melting of tonalitic compositions (phlogopite or biotite-plagioclase-quartz assemblages) is investigated within a temperature range of 700–1000°C and pressure range of 2–15 kbar. The solid reaction products in the case of the phlogopite-plagioclase(An45)-quartz starting material are enstatite, clinopyroxene and potassium feldspar, with amphiboles occurring occasionally. At 12 kbar, zoisite is observed below 800°C, and garnet at 900°C. The reaction products of dehydration melting of the biotite (Ann50)-plagioclase (An45)-quartz assemblage are melt, orthopyroxene, clinopyroxene, amphibole and potassium feldspar. At pressures > 8 kbar and temperatures below 800°C, epidote is also formed. Almandine-rich garnet appears above 10 kbar at temperatures ≥ 750°C. The composition of melts is granitic to granodioritic, hence showing the importance of dehydration melting of tonalites for the formation of granitic melts and granulitic restites at pressure-temperature conditions within the continental crust. The melt compositions plot close to the cotectic line dividing the liquidus surfaces between quartz and potassium feldspar in the haplogranite system at 5 kbar and a H 2O = 1. The composition of the melts changes with the composition of the starting material, temperature and pressure. With increasing temperature, the melt becomes enriched in Al2O3 and FeO+MgO. Potash in the melt is highest just when biotite disappears. The amount of CaO decreases up to 900°C at 5 kbar whereas at higher temperatures it increases as amphibole, clinopyroxene and more An-component dissolve in the melt. The Na2O content of the melt increases slightly with increase in temperature. The composition of the melt at temperatures > 900°C approaches that of the starting assemblage. The melt fraction varies with composition and proportion of hydrous phases in the starting composition as well as temperature and pressure. With increasing modal biotite from 20 to 30 wt%, the melt proportion increases from 19.8 to 22.3 vol.% (850°C and 5 kbar). With increasing temperature from 800 to 950°C (at 5 kbar), the increase in melt fraction is from 11 to 25.8 vol.%. The effect of pressure on the melt fraction is observed to be relatively small and the melt proportion in the same assemblage decreases at 850°C from 19.8 vol.% at 5 kbar to 15.3 vol.% at 15 kbar. Selected experiments were reversed at 2 and 5 kbar to demonstrate that near equilibrium compositions were obtained in runs of longer duration. Received: 27 December 1995 / Accepted: 7 May 1996  相似文献   

19.
Partial melting of metagreywackes, Part II. Compositions of minerals and melts   总被引：16，自引：0，他引：16  
A series of experiments on the fluid-absent melting of a quartz-rich aluminous metagreywacke has been carried out. In this paper, we report the chemical composition of the phases present in the experimental charges as determined by electron microprobe. This analytical work includes biotite, plagioclase, orthopyroxene, garnet, cordierite, hercynite, staurolite, gedrite, oxide, and glass, over the range 100–1000 MPa, 780–1025 °C. Biotites are Na- and Mg-rich, with Ti contents increasing with temperature. The compositions of plagioclase range from An17 to An35, with a significant orthoclase component, and are always different from the starting minerals. At high temperature, plagioclase crystals correspond to ternary feldspars with Or contents in the range 11–20 mol%. Garnets are almandine pyrope grossular spessartine solid solutions, with a regular and significant increase of the grossular content with pressure. All glasses are silicic (SiO2 = 67.6–74.4 wt%), peraluminous, and leucocratic (FeO + MgO = 0.9–2.9 wt%), with a bulk composition close to that of peraluminous leucogranites, even for degrees of melting as high as 60 vol.%. With increasing pressure, SiO2 contents decrease while K2O increases. At any pressure, the melt compositions are more potassic than the water-saturated granitic minima. The H2O contents estimated by mass balance are in the range 2.5–5.6 wt%. These values are higher than those predicted by thermodynamic models. Modal compositions were estimated by mass balance calculations and by image processing of the SEM photographs. The positions of the 20 to 70% isotects (curves of equal proportion of melt) have been located in the pressure-temperature space between 100 MPa and 1000 MPa. With increasing pressure, the isotects shift toward lower temperature between 100 and 200 MPa, then bend back toward higher temperature. The melting interval increases with pressure; the difference in temperature between the 20% and the 70% isotects is 40 °C at 100 MPa, and 150 °C at 800 MPa. The position of the isotects is interpreted in terms of both the solubility of water in the melt and the nature of the reactions involved in the melting process. A comparison with other partial melting experiments suggests that pelites are the most fertile source rocks above 800 MPa. The difference in fertility between pelites and greywackes decreases with decreasing pressure. A review of the glass compositions obtained in experimental studies demonstrates that partial melting of fertile rock types in the crust (greywackes, pelites, or orthogneisses) produces only peraluminous leucogranites. More mafic granitic compositions such as the various types of calk-alkaline rocks, or mafic S-type rocks, have never been obtained during partial melting experiments. Thus, only peraluminous leucogranites may correspond to liquids directly formed by partial melting of metasediments. Other types of granites involve other components or processes, such as restite unmixing from the source region, and/or interaction with mafic mantle-derived materials. Received: 11 July 1995 / Accepted: 27 February 1997  相似文献   

20.
Aluminous parageneses containing gedrite, cordierite, garnet, staurolite, biotite, sillimanite, kyanite, quartz or spinel plus corundum are found as dark colored lenses in the polymetamorphic, multideformed Archean complex at Ajitpura in northwest peninsular India. Staurolite, like kyanite, is a relict phase of earlier metamorphism and is excluded as a paragenetic mineral in view of its incompatibility with quartz and gedrite and its lower X Mg values than for garnet of the assemblage. Its stability here is attributed to zinc content of up to 3 wt%. The XMg in other ferromagnesian minerals decreases in the order: cordierite, biotite, gedrite, garnet, as found elsewhere in high grade rocks.The textural criteria and systematic partitioning of Fe and Mg in the ferromagnesian phases, excluding staurolite, indicate attainment of equilibrium during the second metamorphism. From tie line configurations in the phase diagrams, X Mg ratios in the constituent minerals, and other petrographic criteria, it is suggested that gedrite — cordierite-garnet — sillimanite — biotite assemblage has been produced by the reactions: Biotite+Sillimanite+Quartz = Cordierite+Garnet+K-feldspar+Vapor (1) and Biotite+Sillimanite+Quartz = Cordierite +Gedrite+K-feldspar+Vapor (2) which occurred during partial melting of the rocks at fixed P and T conditions.By isothermal P-X(Fe-Mg) sections it has been demonstrated that release of FeO, SiO2 and other components modified the composition of the reactant biotite presumably by the substitution FeSi2 Al, whereby reaction 1 was replaced by reaction 2. Cordierite with higher X Mg was produced with gedrite instead of with garnet, whose X Mg is less than X Mg of gedrite. Reaction 2 has been tentatively located in T-P space from the intersection of some continuous loops in the P-X(Fe-Mg) diagram at 700°C and also by other constraints. The discontinuous reaction 2 is located about 1–2 kilobars higher than reaction 1, which implies that it is difficult to distinguish between effects of pressure and those of melting on the X Mg ratios of the reaction phases.The P-T calibrations of garnet — cordierite, garnet — biotite and garnet — plagioclase equilibria and the calibrations from other dehydration curves give temperatures near 700°C and pressure (assuming ) about 6 kilobars.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号