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We performed vapor-absent melting and crystallization experimentson two bulk compositions that model metamorphic rocks containinga single hydrous phase: a biotite gneiss [37% bio (mg-number55), 34% qtz, 27% plg (An38), 2% ilm] and a quartz amphibolite[54% hbl (mg-number 60), 24% qtz, 20% plg (An38), 2% ilm]. Experimentswere performed at 3 and 5 kbar in internally heated pressurevessels (IHPV), and at 7, 10, 125 and 15 kbar in piston cylinderapparatus (PC), from the vapor-absent solidi to (at least) thetemperature at which the hydrous mineral disappeared. Dehydration-meltingbegins at similar temperatures in both bulk compositions, rangingfrom T850C at P = 3 kbar T930C at P = 15 kbar. The hydrousmineral disappears 50C above the solidus in both systems, exceptin IHPV experiments at f(O2) above Ni–NiO, in which biotitestability extends up to atleast 80C above the solidus. At theT at which the hydrous minerals disappear the biotite gneissproduces 2–3 times more melt than the quartz amphibolite(50–60 wt% vs 20–30 wt%). In both systems, variationsin melt productivity with P are controlled by three competingfactors: (1) the positive d P/dT slopes of the solidi, (2) decreasingH2O activity with increasing P at constant H2O content, and(3) Na2O activity, which increases with P concomitantly withbreakdown of plagioclase. Melt productivities at T = 920–950Care maximized at intermediate pressures (7 kbar). The biotitegneiss produces strongly peraluminous granitic melts (SiO2>70wt%) and residual assemblages of quartz norite (P>125 kbar)or garnet pyroxenite (P>125 kbar). The quartz amphiboliteproduces strongly peraluminous granodioritic melts (SiO2>70wt%) that coexist with clinopyroxene + orthopyroxene + plagioclase+ quartz at P>10 kbar)garnet. The results of coupled meltingand crystallization experiments on the quartz amphibolite suggestthat strongly peraluminous granitoid rocks (e.g. cordierite-bearingand two-mica granites) can be derived from melting of Al-poorprotoliths. KEY WORDS: dehydration-melting; biotite gneiss; amphibolite; felsic magmas
*Corresponding author 相似文献
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GARCIA-CASCO ANTONIO; HAISSEN FAOUZIYIA; CASTRO ANTONIO; EL-HMIDI HASSAN; TORRES-ROLDAN RAFAEL LUIS; MILLAN GUILLERMO 《Journal of Petrology》2003,44(10):1727-1757
We document experiments on a natural metapelite in the range650–775°C, 6–14 kbar, 10 wt % of added water,and 700–850°C, 4–10 kbar, no added water. Staurolitesystematically formed in the fluid-present melting experimentsabove 675°C, but formed only sporadically in the fluid-absentmelting experiments. The analysis of textures, phase assemblages,and variation of phase composition and Fe–Mg partitioningwith P and T suggests that supersolidus staurolite formed at(near-) equilibrium during fluid-present melting reactions.The experimental results are used to work out the phase relationsin the system K2O–Na2O–FeO–MgO–Al2O3–SiO2–H2Oappropriate for initial melting of metapelites at the upperamphibolite facies. The P–T grid developed predicts theexistence of a stable P–T field for supersolidus staurolitethat should be encountered by aluminous Fe-rich metapelitesduring fluid-present melting at relatively low temperature andintermediate pressures (675–700°C, 6–10 kbarfor XH2O = 1, in the KNFMASH system), but not during fluid-absentmelting. The implications of these findings for the scarcityof staurolite in migmatites are discussed. KEY WORDS: metapelites; migmatites; partial melting; P–T grid; staurolite 相似文献
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Partial Melting of Aluminous Metagreywackes in the Northern Sierra de Comechingones, Central Argentina 总被引：1，自引：2，他引：1
We describe a suite of metamorphic and migmatitic rocks fromthe Sierra de Comechingones (Sierras Pampeanas of Central Argentina)that include unmelted gneisses, migmatites and refractory granulites.The gneisses are aluminous greywackes metamorphosed in the amphibolitegrade and are likely to have been the protoliths for the higher-grademigmatites and granulites. Mineralogical characteristics andmajor and trace element compositions show that metatexite migmatites,diatexite migmatites and granulites are all melt-depleted rocks.The migmatites (both metatexites and diatexites) have undergoneH2O-fluxed melting and lost 相似文献
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Pleistocene speleothems of Mallorca: implications for palaeoclimate and carbonate diagenesis in mixing zones 总被引：1，自引：0，他引：1
The Pleistocene speleothems of Sa Bassa Blanca cave, Mallorca, are excellent indicators of palaeoclimate variations, and are samples that allow evaluation of the products and processes of mixing‐zone diagenesis in an open‐water cave system. Integrated stratigraphic, petrographic and geochemical data from a horizontal core of speleothem identified two main origins for speleothem precipitates: meteoric‐marine mixing zone and meteoric‐vadose zone. Mixing‐zone precipitates formed at and just below the water–air interface of cave pools during interglacial times, when the cave was flooded as a result of highstand sea‐level. Mixing‐zone precipitates include bladed and dendritic high‐Mg calcite, microporous‐bladed calcite with variable Mg content, and acicular aragonite; their presence suggests that calcium‐carbonate cementation is significant in the studied mixing‐zone system. Fluid inclusion salinities, δ13C and δ18O compositions of the mixing‐zone precipitates suggest that mixing ratio was not the primary control on whether precipitation or dissolution occurred, rather, the proximity to the water table and degassing of CO2 at the interface, were the major controls on precipitation. Thus, simple two‐end‐member mixing models may apply only in mixing zones well below the water table. Meteoric‐vadose speleothems include calcite and high‐Mg calcite with columnar and bladed morphologies. Vadose speleothems precipitated during glacial stages when sea level was lower than present. Progressive increase in δ13C and δ18O of the vadose speleothems resulted from cooling temperatures and more positive seawater δ18O associated with glacial buildup. Such covariation could be considered as a valid alternative to models predicting invariant δ18O and highly variable δ13C in meteoric calcite. Glacio‐eustatic oscillations of sea‐level are recorded as alternating vadose and mixing‐zone speleothems. Short‐term climatic variations are recorded as alternating aragonite and calcite speleothems precipitated in the mixing zone. Fluid‐inclusion and stable‐isotope data suggest that aragonite, as opposed to calcite, precipitated during times of reduced meteoric recharge. 相似文献
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A direct boundary element method for calculating the two-dimensional scattering of seismic waves from irregular topographies and buried valleys due to incident P-, SV- and Rayleigh waves is employed to model a section of the Mexico City Valley. The method has been formulated with isoparametric quadratic boundary elements and contains, with respect to previous works in the field, some improvements that are briefly presented. Because the Mexico City Valley is relatively flat and shallow and the contrast of S-waves between the clays and the basement rock is very high, it is believed that the one-dimensional theory is enough to explain the amplification patterns. Although this is true for most sites, results from recent accelerometric data suggest that two- and three-dimensional models are needed to explain the amplification behaviour at some places. In this work, two accelerometric sites have been chosen: Site 84 to probe that the one-dimensional model works well for most sites, and Site TB, as an example of irregular response. The two-dimensional method presented here was used to model a section of the valley where site TB is located, showing that this method yields results closer to the observations than the one-dimensional approach. © 1997 John Wiley & Sons, Ltd. 相似文献