Amphiboles in ultrapotassic dyke rocks from the area of the Central Bohemian Plutonic Complex and their relations to magma composition and evolution
Abstract
Ultrapotassic dyke rocks of Variscan age (minettes, melasyenite to melagranite porphyries, rare karlsteinite of peralkaline composition) are innumerable in the area of the Central Bohemian Plutonic Complex and its surroundins. Amphiboles range in composition from magnesian actinolite (commonly post-magmatic) to richterite to magnesioarfvedsonite.
Crystallization of alkali-rich amphiboles in magmas with the NK/A parameter representing the molar ratio (Na2O + K2O)/ Al2O3 slightly less than unity was enabled by preceding crystallization of abundant phlogopite causing a small excess of alkalis in the residual melt. However, some minettes with NK/A about 1 and peralkaline varieties of lamproitic affinity with NK/A > 1 have originated in mantle sources similar to those of typical lamproites already containing excess potassium.