The Four-Phase AFM Assemblage Staurolite--Aluminum Silicate--Biotite--Garnet: Extra Components and Implications for Staurolite--Out Isograds |
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Authors: | GIARAMITA MARIO J; DAY HOWARD W |
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Institution: | Department of Geology, University of California Davis, California 95616 |
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Abstract: | The prograde disappearance of staurolite can be described inthe model system K2O-FeO-MgO-Al2O3-SiO2-H2O (KFMASH) by thereaction: staurolite + muscovite + quartz = biotite + aluminumsilicate + garnet + water. The common occurrence and worldwidedistribution of the assemblage staurolite-biotite-aluminum silicate-garnet(SBAG) in quartz-mica-schist suggest that the model reactionmay be over-simplified. Previous workers have suggested thatthe SBAG assemblage (1) is a strictly divariant assemblage thatbuffered water activity, (2) is stabilized by non-KFMASH components,and (3) did not attain equilibrium. We used least-squares regression to show that balanced reactionsdo not exist among the minerals in samples of SBAG assemblagesfrom Califonia and New England. The absence of reaction relationshipscan be explained by imbalances in two or three of the minorelements Zn, Mn, and either Ca or Na. The assemblage is apparentlystabilized by non-KFMASH components. Criteria for mapping staurolite-out isograds that representthe conditions of the KFMASH staurolite-out reaction dependon which of the four phases is the extra phase,and require an understanding of the thermodynamic effects ofall the extra components. Our results suggestthat transition zones of SBAG assemblages near staurolite-outisograds are the result of extra components. However,it is uncertain whether µH2O of fluids in equilibriumwith SBAG assemblages varied across such zones. |
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