Chloritoid stability in manganese rich low-grade metamorphic rocks,Venn-Stavelot Massif,Ardennes |
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Authors: | Ulrich Kramm |
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Institution: | 1. Mineralogisch-Petrographisches Institut der Universit?t K?ln, Federal Republic of Germany
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Abstract: | The Southern Venn-Stavelot Massif is characterized by Ordovician and Devonian rocks very rich in manganese and aluminum, which
are attacked by a low grade regional metamorphism. The assemblages 1 (phengite, paragonite, chlorite, chloritoid, garnet,
quartz, hematite, rutile) and 2 (phengite, paragonite, chlorite, kaolinite (andalusite, pyrophyllite), garnet, quartz, hematite,
rutile) are of basic interest for the formation of chloritoid. As the two rock types are isofaciell and quasi-identical in
chemistry except for the iron oxides, there is clear evidence for the influence of
on the chloritoid formation at its lower p-T stability limit. This can be shown by a discussion of the phase relations of chloritoid, garnet, kaolinite, chlorite and
phengite in respect to the oxidation ratio mol 2 Fe2O3x 100/2 Fe2O3+ FeO of the host rocks. Especially chloritoid and chlorite change their chemistry in a characteristic way with rising oxidation
ratio in getting richer and richer in manganese and magnesium (chloritoid) and magnesium (chlorite). A simultaneous increase
in trivalent iron in these phases is supposed. At an oxidation ratio of 85–90 the
stability limit of chloritoid is reached. The increasing substitution of manganese and magnesium up to this limit should
have a stabilizing effect. In a rough estimate the oxygen partial pressure is supposed to be in the order of 10−10 atm at the stability limit of chloritoid assuming a temperature of metamorphism between 360–400° C. Rocks with oxidation
ratios between 90 and 100 are characterized by the presence of kaolinite. If the oxidation ratio is still higher (all iron
as Fe3+, parts of the manganese in the trivalent state), the rocks belong to assemblage 3 (phengite, paragonite, chlorite, viridine,
(kaolinite), (garnet), quartz, hematite, braunite, rutile).
Dedicated to Prof. Dr. K. Jasmund at his 60. birthday. |
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