CRACK REACTION VEINS FROM THE HODGKINSON FORMATION, NORTH-QUEENSLAND,AUSTRALIA

Thin, subplanar or parallel-sided syntectonic quartz veins from the Ho dgkinson Province, North Queensland, Australia contain mica inclusions that are identical to wallrock mica textures. These ghost textures ca n only be produced by incomplete replacement of the wallrock by quartz . The planar form of the veins indicates that there was a planar contr ol on their formation. We propose that the veins formed around syntect onic fractures that localized silica micrometasomatism of the fracture wall. The formation of the veins can be explained by a model which in volves fracture of the wallrock, reaction between the fracture wall an d the ambient metamorphic fluid and eventual sealing of the fracture b y precipitation from the fluid. The new vein is then a locus of furthe r fracture. Replacement of wallrock micas occurs via a two-stage proce ss where biotite is replaced by muscovite, which is in turn replaced b y quartz. We propose the term crack-reaction to describe the resulting cyclic process because it can be compared with the crack-seal model o f vein formation. Crack-seal and crack-reaction are different only in the relative amounts of metasomatism and precipitation that occur subs equent to fracture and can be envisaged as end members of a more gener al vein-forming process.