Partial melting of metagreywackes, Part II. Compositions of minerals and melts |
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| Authors: | Jean-Marc Montel Daniel Vielzeuf |
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| Institution: | (1) CNRS, UMR 6524 “Magmas et Volcans”, Université Blaise Pascal, 5 rue Kessler, F-63000 Clermont-Ferrand, France, FR |
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| Abstract: | 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 |
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