Publisher © Czech Geological Survey, ISSN: 2336-5757 (online), 0514-8057 (print)
Amphiboles in ultrapotassic dyke rocks from the area ofthe Central Bohemian Plutonic Complex and their relations to magma composition and evolution
Abstract
Ultrapotassic dyke rocks of Variscan age (minettes, melasyenite to melagranite porphyries, karlsteinite) from the Central Bohemian Plutonic Complex and adjacent parts of the Bohemicum (to N) and Moldanubicum (to S) contain various types of amphibole. Common is the presence of Mg-rich actinolite replacing the original olivine phenocrysts and also pyroxenes and, therefore, being of post-magmatic origin. Some quartz melasyenite porphyries, however, contain also euhedral amphibole phenocrysts compositionally corresponding to the magnesiohornblende/actinolite boundary (about 7.5 Si p.f.u.). Minettes with increased (Na20 + K20)/AI203 (i.e., the NK/A > 0.8) frequently contain small amounts of late-crystallized richteritic amphiboles. Rare subaluminous varieties approaching the boundary of peralkaline rocks (NK/A ~ 1) may have also accessory magnesio-arfvedsonite. Karlsteinite (strongly peralkaline quartz microsyenite) has magnesio-arfvedsonite as the major mafic mineral. As any correlation between the NK/A and the degree of magma fractionation is missing, the increased NK/A values in some lamprophyric magmas are interpreted as primary features reflecting the nature of enriched mantle sources similar to sources of lamproites. Crystallization of alkali-rich amphiboles in magmas with NK/A < 1 is facilitated by preceding crystallization of abundant phlogopite causing a small exces of alkalis in the residual melt.
