Jf. Hippertt, STRUCTURES INDICATIVE OF HELICOIDAL FLOW IN A MIGMATITIC DIAPIR (BACAO COMPLEX, SOUTHEASTERN BRAZIL), Tectonophysics, 234(3), 1994, pp. 169-196
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).