ENVIRONMENT AND STRUCTURAL CONTROLS ON THE INTRUSION OF THE GIANT RARE METAL GREENBUSHES PEGMATITE, WESTERN AUSTRALIA

The intrusive rocks in the Greenbushes pegmatite district are concentrated along the ancient crustal-scale, Donnybrook-Bridgetown shear zone, analogous to the present San Andreas fault system, and are characterized by steeply dipping planar mylonitic fabrics with horizontal stretching lineations, asymmetric folds, asymmetric pressure shadows, and shear bands suggesting sinistral strike-slip movement. The Greenbushes pegmatite occurs in a higher temperature and higher pressure metamorphic terraine than would be expected for pegmatites containing rare element mineralization. The pegmatite contains the same shear fabrics as its host rocks and has evidence for syntectonic crystallization of minerals such as tourmaline, tantalite, garnet, and cassiterite. It is proposed that intrusion of the Greenbushes pegmatite magma was controlled by the Donnybrook-Bridgetown shear zone. -from Author