Eur. J. Mineral. 2002,14,7 15- 73 1 Genesis of orbicular granitic rocks from the Ploumanac'h Plutonic Complex (Brittany, France): petrographical, mineralogical and geochemical constraints SYLVIE DECITRE', DOMINIQUE GASQUET'.2*and CHRISTIAN MARIGNAP3 ' CRPG-CNRS, BP 20, F-54501 Vandoeuvre-lès-Nancy Cedex, France ENSG-INPL, BP 40, F-54501 Vandoeuvre-lès-Nancy Cedex, France EMN-INPL, F-54042 Nancy Cedex, France *Corresponding author, e-mail: gasquet @crpg.cnrs-nancy.fr 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 m 3 ) 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 homblendites, microdiorites and granites) that act to nucleate the crystallisation of the shells. The orbicules represent, thus, a "stockscheider" structure with endless "cold" walls. Key-words: orbicular granites, plumose feldspars, high- K calc-alkaline granite, Ploumanac'h. Introduction Spectacular orbicular structures have been described at the margins of many plutonic complexes where they are always hosted in decametric to hectometric bodies (e.g. Sederholm, 1928; Goodspeed, 1942; Barrière et al., 1971; Barrière, 1972; Moore & Lockwood, 1973; Couturié, 1973; Elliston, 1984; Chauris et al., 1989; Piboule et al., 1989). These structures have been carefully reviewed by Leve- son (1966) who proposed a standard terminology that is adopted here. Orbicules, crowded or sparsely scattered in their matrix, are spherical to ovoid (a few cm to 35 cm in size) with a core surrounded by a variable number of shells (up to several tens). They differ considerably by the nature of: (1) the surrounding plutons (gabbro, diorite, granodio- rite, syenogranite, peraluminous granite); (2) the matrix be- tween the orbicules (gabbroic, dioritic, granitic or pegmatit- ic); (3) the cores and their interna1 structures; and (4) the shells. Cores consist of plutonic rocks (granites, granodio- rites, diorites, gabbros), metamorphic rocks (metapelites, amphibolites, peraluminous restites) or crystals (generally feldspars) and are currently interpreted as inert pre-existing nuclei on which nucleation started. The concentric shells are characterised by radially or tangentially oriented plagio- clase andor K-feldspar associated with quartz and biotite andlor amphibole, the latter being more frequent in mafic environments. Features commonly associated with orbicu- DOI: 10.1 12710935-1221/2002/0014-0715 les include xenoliths, layered or comb-layered igneous rocks, minera1 segregations, phenocrysts and proto-orbicu- les consisting of a core surrounded by a poorly defined shell. The genesis of orbicular rocks is still a matter of discus- sion, although their magmatic origin is not questioned. No single hypothesis provides a general explanation for their origin and numerous mechanisms have been proposed, re- flecting the diversity of the orbicular settings and habits. Barrière (1972) proposed that the growth of shells corre- sponds to the rapid crystallisation of immiscible supersatu- red magmas convecting around nuclei. Piboule et al. (1989) suggested that the orbicular structure is controlled by adia- batic undercooling. The growth of successive shells may be also related to rapid diffusion-controlled crystallisation of the different components within the melt (Palmer et al., 1967; Meyer & Althen; 1991) or are formed by rhythmic su- persaturation and crystallisation from a gel in a manner analogous to Liesegang rings (Leveson, 1966) or from a magma during differentiation (Aguirre et al., 1976). For El- liston (1984) the "magma" in which orbicules develop must have the diffusive and rheological properties of a concen- trated macromolecular paste or gel of mixed hydrosilicates. The most common mechanism proposed by the authors in- volves diffusion. This paper is devoted to a particular orbicular structure outcropping in the Ploumanac'h plutonic complex (PPC), French Armorican Massif. The orbicules are made of radiat- 0935-1221/02/0014-O7 15 $7.65 8 2002 E. Schweizerbart'sche Verlagsbuchhandlung. D - 70176 Stuttgart