MAGMA DEFORMATION DURING LACCOLITH EMPLACEMENT: EXAMPLES FROM ICELAND AND ARGENTINA

Felsic magma commonly pools within shallow mushroom-shaped magmatic intrusions, so-called laccoliths or cryptodomes, which can cause both explosive eruptions and collapse of the volcanic edifice. Deformation during laccolith emplacement is primarily considered to occur in the host rock. However, shallowly emplaced laccoliths show extensive internal, magmatic deformation. While deformation of magma in volcanic conduits is an important process for regulating eruptive behavior, the effects of magma deformation on intrusion emplacement remain largely unexplored. By combining field mapping, anisotropy of magnetic susceptibility and microstructural analysis, we studied the emplacement of two laccoliths emplaced within the upper kilometer of the crust in a single intrusive event. The rhyolitic Sandfell laccolith, Iceland, which intruded at a depth of 500 m and the dacitic Cerro Bayo laccolith, Argentina. The initial growth stage of the two laccoliths is characterized by contact parallel flow indicators that formed during inflation of the laccolith. The second growth stage encompass brecciation and fracturing in the rim of the intrusion. Fractures in the Sandfell laccolith occur in layers and individual fractures are parallel to strain-localization bands in coherent rhyolite. A dominantly oblate magmatic fabric in the fractured areas, S-C fabrics in flow bands and conjugate geometry of strain-localization bands and the fractures demonstrate that the magma was sheared and compacted by the continuous intrusion of magma into the bodies. This further implies that the rims of the magma bodies essentially solidified during the intrusive event. In the third stage of growth, the stalled rim of the laccolith is breached, which promotes vertical growth of the magma body facilitated by larger steeply-dipping faults and shear zones. Our observations indicate that syn-emplacement changes in magma rheology play a major role in the emplacement of viscous magma intrusions in the upper kilometer of the crust.