Melt-collecting structures and the formation of extraction dykes during syntectonic anatexis of the Camboriú Complex, south Brazil

Abstract

The Camboriú Complex metamorphic sequence comprises orthogneisses and amphibolites with widespread melting features generated at upper amphibolite facies conditions. Regional to mesoscale structures are shown to control generation, collection and extraction of melts. As partial melting starts it is locally focused as layer-parallel leucosomes in banded gneisses, and isolated pockets of in situ leucosomes in massive amphibolites. The further tightening of cm-to m-scale upright folds and conjugate shear planes oblique to axial-planar position trigger the coalescence and migration of melts, first along the main banding, then along axial planes of folds and finally giving rise to a network where melt is collected. The melt network eventually acquires enough mobility to form m-wide extraction dykes roughly parallel to axial planes of folds, capable of transferring the products of migmatization to upper crustal levels to feed granitic plutons. The orientation and geometry of pure-shear dominated structures in the Camboriú Complex are in accordance with deformation partitioning between this and adjancent shear zones, which is expected for the transpressive character of the Neoproterozoic Southern Brazilian Shear Belt.