Contrasting textural record of two distinct metamorphic events of similar P-T conditions and different durations
Abstract
A structural, metamorphic and geochronological study of the Staré Město belt implies the existence of two distinct metamorphic events of similar peak P?T conditions (700?800 °C, 8?10 kbar) during the Cambro-Ordovician and the Carboniferous events. The analysis of crystal size distribution (CSD) shows high nucleation density (N0) and low average growth rate (Gt) for Carboniferous mylonitic metagabbros and granodiorites.
The N0 decreases whereas the quantity Gt increases towards higher temperatures progressively approaching the values obtained from the Cambro-Ordovician banded amphibolites. In high-temperature mylonites a strong aggregate distribution is developed.
Cambro-Ordovician amphibolites unaffected by Carboniferous deformation show a regular to anticlustered spatial distribution resulting from heterogeneous nucleation. This pattern, together with CSD, was modified by the grain growth and textural equilibration controlled by diffusive mass transfer during Carboniferous metamorphism.