Summary
The formation of cummingtonite in two Ca and Al-poor and Mg-rich amphibolites from the Austroalpine Schneeberg complex occurred at a maximum temperature of about 550°C (5 kb). This is a result of the amphibolite facies Alpine overprint in this part of the Eastern Alps.
Textural and chemical relations suggest (Mg−1Si−1Al2)-continuous reactions in the bivariant CMASH-assemblageCam-Cum-Chl * followed by the discontinuous reactionCam+Chl+Qu=Cum+Plg+H2O to be responsible for the formation of cummingtonite in these samples.
The Mg−Fe distribution coefficient\(K_{D_{Mg - Fe} }^{Cam - Cum} = \frac{{(Mg/Fe)Cam}}{{(Mg/Fe)Cum}}\) with values of 0.6–0.7 is similar to cummingtonite-Ca-amphibole pairs from amphibolites with oligoclase+quartz reported in the literature. The Mg/(Mg+Fe) ratio of the calcic amphiboles is lower (0.539–0.555) than the coexisting cummingtonites (0.648–0.662).
Zusammenfassung
In zwei Ca- und Al-armen Amphiboliten des nördlichen Schneebergerzuges (Rotmoostal) bildete sich Cummingtonit bei Maximaltemperaturen von 550°C (5 kb) bei der Altalpidschen Metamorphose.
Texturelle und chemische Beziehungen lassen vermuten, daß sich Cummingtonite sowohl nach kontinuierlichen Reaktionen (in bezug auf den Tschermak-Vektor Mg−1Si−1Al2) gebildet hat, als auch aus Hornblende und Chlorit nach der diskontinuierlichen ACF-ReaktionCam+Chl+Qu=Cum+Plg+H2O hervorgegangen ist.
Der Mg−Fe-Verteilungskoeffizient zwischen Hornblende und Cummingtonit entspricht den aus der Literatur bekannten Werten. Er beträgt zwischen 0.6–0.7 für die beginnende Amphibolitfazies. Die Mg/(Mg+Fe)-Verhältnisse sind höher in Cummingtonit (0.648–0.662) als in der koexistierenden Hornblende (0.539–0.555).
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Hoinkes, G., Mogessie, A. Coexisting cummingtonite and calcic amphibole in amphibolites from the Schneeberg complex, Tyrol, Austria. TMPM Tschermaks Petr. Mitt. 35, 33–45 (1986). https://doi.org/10.1007/BF01081917
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DOI: https://doi.org/10.1007/BF01081917