SPATIAL DISTRIBUTION OF MELT-BEARING STRUCTURES IN ANATECTIC ROCKS FROM SOUTHERN BRITTANY: IMPLICATIONS FOR MELT-TRANSFER AT GRAIN- TO OROGEN-SCALE

We present quantitative information on the one-, two- and three-dimensional distribution of inferred melt-bearing structures in anatectic rocks of the Southern Brittany Migmatite Belt, south of the South Armorican Shear Zone (SASZ). Such structures include layer-parallel and cross-cutting leucosomes, and discordant granite sheets. Petrographic continuity between these structures suggests they once formed a continuous melt-bearing network.

One-dimensional line traverses across layering provide information about thickness and spacing of layer-parallel leucosomes. Thickness mostly falls in the range 1-10 mm, with an upper limit of ~20-30 mm. The number of thicker veins decreases abruptly with increasing thickness, inconsistent with scale-invariance. This suggests that leucosome formation was controlled by short-range (grain-scale) melt movement to form segregated layers constrained by pre-existing foliation. Spacing distributions also are not scale-invariant. However, the large amount of leucosome (~40-60 vol. % average over outcrop surfaces) suggests that spacing may be controlled in part by impinging leucosomes, making it difficult to derive genetic implications from these data.

Qualitative observation of inferred melt-bearing structures in mutually perpendicular two-dimensional exposures reveals anisotropy of the leucosome network related to a pervasive sub-horizontal stretching lineation associated with movement along the SASZ. Analysis of these distributions using the box-counting method (despite its limitations) corroborates the inferred anisotropy, but indicates that leucosome morphology (and perhaps distribution) is not scale-invariant. The anisotropy suggests that melt movement in structures now represented by layer-parallel leucosomes was primarily sub-horizontal. These conduits fed steeply dipping conduits now represented by cross cutting leucosomes, commonly developed at inter-boudin partitions, and granite sheets. The formation and orientation of these structures was in part controlled by far-field stresses associated with displacement along the SASZ. Melt is inferred to have traveled along these structures through, and eventually out of the anatectic zone to feed leucogranite plutons at higher crustal levels.