Structures on interfaces of mingled magmas, Stewart Island, New Zealand

Strain analysis of magmatic rocks has been attempted using the shapes of enclaves of contrasting magmas and analysis of mineral fabrics. In composite plutonic rocks of the Anglem Complex, Stewart Island, New Zealand, interfaces between different magma types are deformed into fold-like patterns which have the potential to provide new data on strain in magmatic processes. The fold-forms have shorter wavelength and higher amplitude on interfaces at a high angle to layering whereas fold-forms are either absent or of low amplitude on interfaces parallel to layering. The fold-forms may be amplified/de-amplified irregularities or buckle folds, and in either case they indicate shortening perpendicular to rock layering. Whole-rock fabrics of the plagioclase–hornblende–biotite rocks, analysed by shape preferred orientation and centre-to-centre methods, record less intense alignment and strain than predicted by free rotation of particles in viscous media. The inhibition of crystal rotation at high crystal concentrations and the growth of late poorly aligned crystals present difficulties for calibrating whole-rock fabrics with the strain incurred by magmatic flow.