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

Strain analysis of magmatic rocks has been attempted using the shapes of en claves of contrasting magmas and analysis of mineral fabrics. In composite plutonic rocks of the Anglem Complex, Stewart Island, New Zealand, interfac es between different magma types are deformed into fold-like patterns which have the potential to provide new data on strain in magmatic processes. Th e 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 amp litude on interfaces parallel to layering. The fold-forms may be amplified/ de-amplified irregularities or buckle folds, and in either case they indica te shortening perpendicular to rock layering. Whole-rock fabrics of the pla gioclase-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 o f crystal rotation at high crystal concentrations and the growth of late po orly aligned crystals present difficulties for calibrating whole-rock fabri cs with the strain incurred by magmatic flow. (C) 1999 Elsevier Science Ltd . All rights reserved.