Linear dilatation structures and syn-magmatic folding in granitoids

Abstract

Folding of the magmatic foliation of the composite Orijärvi batholith (SW Finland) took place at sites where viscosity contrasts existed: at gabbro—tonalite interfaces and at the rim of the intrusion. Chilled margins at these interfaces are both folded and boudinaged. Folds are highly disharmonic and die out in internal parts of the batholith. Folded tonalites display a lineation defined by quartz rods which are morphologically similar to ribbons in mylonites. The host rock however is non-mylonitic and shows primary magmatic structures. Crystallographic fabrics in rods are near random, and unstrained rutile inclusions suggest that quartz in this texture is undeformed. The rods are accompanied by a conjugate extensional foliation of biotite schlieren. By comparison with rods in quartz—feldspar veins, the quartz ribbons in tonalite are interpreted as linear dilatation structures at the intersection lines of conjugate shear bands, the shear directions of which were perpendicular to the axes of the rods. Folded chilled margins and rodded tonalite are enclosed by undeformed gabbro and tonalite, suggesting that folding took place while the bulk of the batholith was still liquid. It is proposed that folds were formed by magma flow along a largely solidified gabbro—tonalite boundary zone, whereby interfaces were folded and simultaneously stretched.