Original paper

Genesis of orbicular granitic rocks from the Ploumanac'h Plutonic Complex (Brittany,France): petrographical, mineralogical and geochemical constraints

Decitre, Sylvie; Gasquet, Dominique; Marignac, Christian

Abstract

The studied orbicular body is found in the evolved high-K calc-alkaline La Clarté granite in the Ploumanac'h complex. The orbicules are composed of plumose K-feldspar around various nucleus. Reconstruction of the composition of the "orbicular magmas" and their crystallisation conditions indicate that the following conditions are required for the genesis of the orbicules: (1) a small (600 m³) insulated pocket of heated magma generated by the (re)melting of the cumulate Traouiéros granite; (2) a magma composition enriched in K-feldspar component which allows for a significant production of K-feldspar prior to the attainment of the Q-Ab-Or minimum and the final crystallisation of the magma; (3) a high confining pressure leads to a high crystallisation ratio prior to the exsolution of the water phase,which facilitates the development of radiated shells and (4) the existence in the melt of numerous "cold" germs (K-feldspar phenocrysts, with also hornblendites, microdiorites and granites) that act to nucleate the crystallisation of the shells. The orbicules represent, thus, a "stockscheider" structure with endless "cold" walls.

Keywords

orbicular granitesplumose feldsparshigh-k calc-alkaline graniteploumanac'h