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Origin of CO 2 ‐rich fluid inclusions in leucosomes from the Skagit migmatites, North Cascades, Washington, USA
Author(s) -
WHITNEY D. L.
Publication year - 1992
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.1992.tb00118.x
Subject(s) - anatexis , geology , migmatite , metamorphism , geochemistry , partial melting , fluid inclusions , protolith , gneiss , petrology , igneous rock , metamorphic rock , mantle (geology) , quartz , paleontology
CO 2 –CH 4 fluid inclusions are present in anatectic layer‐parallel leucosomes from graphite‐bearing metasedimentary rocks in the Skagit migmatite complex, North Cascades, Washington. Petrological evidence and additional fluid inclusion observations indicate, however, that the Skagit Gneiss was infiltrated by a water‐rich fluid during high‐temperature metamorphism and migmatization. CO 2 ‐rich fluid inclusions have not been observed in Skagit metasedimentary mesosomes or melanosomes, meta‐igneous migmatites, or unmigmatized rocks, and are absent from subsolidus leucosomes in metasedimentary migmatites. The observation that CO 2 ‐rich inclusions are present only in leucosomes interpreted to be anatectic based on independent mineralogical and chemical criteria suggests that their formation is related to migmatization by partial melting. Although some post‐entrapment modification of fluid inclusion composition may have occurred during decompression and deformation, the generation of the CO 2 ‐rich fluid is attributed to water‐saturated partial melting of graphitic metasedimentary rocks by a reaction such as biotite + plagioclase + quartz + graphite ± Al 2 SiO 5 + water‐rich fluid = garnet + melt + CO 2 –CH 4 . The presence of CO 2 ‐rich fluid inclusions in leucosomes may therefore be an indication that these leucosomes formed by anatexis. Based on the inferences that (1) an influx of fluid triggered partial melting, and (2) some episodes of fluid inclusion trapping are related to migmatization by anatexis, it is concluded that a free fluid was present at some time during high‐temperature metamorphism. The infiltrating fluid was a water‐rich fluid that may have been derived from nearby crystallizing plutons. Because partial melting took place at pressures of at least 5 kbar, abundant free fluid may have been present in the crust during orogenesis at depths of at least 15 km.

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