Contrasting amphibole types in ultrapotassic dyke rocks from Central and Southern Bohemia: relations to chemistry and crystallization evolution of magmas
Abstract
Ultrapotassic dyke rocks (namely minettes, melasyenite to melagranite porphyries) from the Central Bohemian Plutonic Complex and the Šumava part of Moldanubicum contain most frequently magnesian actinolite replacing olivine and pyroxenes. Some melagranite pophyries, however, contain amphibole phenocrysts transitional between magnesiohornblende and actinolite, apparently crystallized from melt.
Less frequent minette varieties with increased molar (Na2O+K2O)/Al2O3 contain late (potassic) richterite or even magnesioarfvedsonite in the matrix. Amphiboles in dyke microsyenites are variable.
For metaluminous microsyenites is typical magnesiohornblende, whereas subaluminous to weakly peralkaline thuresite contains richterite and strongly peralkaline karlsteinite has magnesio-arfvedsonite. Amphibole compositional variability reflects the mantle source geochemical characteristics.
Alkali contents in residual melts may be enhanced by preceding crystallization of abundant phlogopite.