COUNTRY-ROCK STRUCTURAL FABRICS AS A GUIDE TO PLUTON EMPLACEMENT DEPTH - AN EXAMPLE FROM THE HODGKINSON PROVINCE, NORTHEASTERN AUSTRALIA

Country-rock structures adjacent to plutons of the linear, northwest-s outheast-trending Mt Alto Supersuite, Hodgkinson Province, northeaster n Australia, are variably developed from north to south along the belt . S-3 and S-4 cleavages show close temporal relationships with pluton emplacement and are better and more widely developed around plutons in the north, whereas much weaker cleavages of the same generation are o nly sporadically developed to the south. Cleavage trend lines anastomo se around less elongate plutons to the north but are generally truncat ed by more elongate plutons to the south. It is proposed that a major crustal dislocation, the Alto Fault Zone, comprises a set of subparall el structures that formed prior to granite intrusion and controlled em placement of some plutons and their final shapes. The north-south vari ation in structural relations is interpreted to reflect a correspondin g variation in depth of emplacement from north to south, which resulte d when post-emplacement reactivation of the Alto Fault Zone uplifted a nd sinistrally displaced the northern end of the supersuite relative t o the southern end. Reactivation of the fault zone after granite empla cement is supported by the truncation of some plutons.