Acta Musei Moraviae

Cummingtonite amphibolites and their position within the West Moravian Moldanubian Zone

Cummingtonite amphibolite was identified in the West Moravian Moldanubian Zone within the Kotlasy amphibolite complex as a rare rock type. Cummingtonite crystallised there instead of hornblende due to unusual chemistry of its host-rock, which is characterized by low mg number and high TiO₂ and P₂O₅ contents. Whereas association of other rocks forming the Kotlasy complex corresponds to a metamorphosed ophiolite complex, the cummingtonite amphibolite departs from it chemically. Its major- and trace-element chemistry points to an original basaltic magma transitional to alkaline basalts and, concerning its geological setting, to a within-plate basalt, while the West Moravian calcic amphibolites are tholeiitic in nature and originally were MORBs. If rocks, which under conditions of the almandine-amphibolite facies yielded cummingtonite amphibolites were in the West Moravian Moldanubian Zone metamorphosed under granulite facies conditions, clinohypersthene-bearing amphibolites originated instead. Different geotectonic setting deduced for the cummingtonite and the clinohypersthene amphibolites on one side, and for the hornblende amphibolites on the other shows that the premetamorphic basic igneous activity in the West Moravian Moldanubian Zone was not a short-period event, but a long term process.