STRUCTURES INDICATIVE OF HELICOIDAL FLOW IN A MIGMATITIC DIAPIR (BACAO COMPLEX, SOUTHEASTERN BRAZIL)

The migmatitic Bacao Complex (southeastern Brazil) shows foliation and lineation patterns that depict an internal flow structure with helico idal geometry. Microstructures show that a rheological transition exis ts, from a predominantly magmatic flow (represented by isotropic, unde formed granitic textures), to solid-state flow (mylonites) towards the diapir margins. Nebulites and contorted migmatites occur in the diapi r core, whereas the peripheral domains show different kinds of banded migmatites, gneisses and high-temperature mylonites. Along the margins low-temperature, mica-rich mylonites and phyllonites occur. Two major flow domains were recognized: the central cell and the peripheral cel l. Each cell is characterized by one or more systems of concentric fol iations (subcells). The mineral lineation produced by the morphologica l alignment of biotite flakes on the foliation planes defines helicoid al geometries in all subcells. This lineation was interpreted as being parallel to the local flow vector. The foliation and lineation patter ns suggest that the principal ascent trunk occurred along a medium-plu nge spiral in the central cell (a dip flow regime). The constant injec tion of molten material in the central cell 'pushed' the material prev iously emplaced into the peripheral cell, where flow upwards continued through a shallower plunge spiral (a strike flow regime).