Abstract

Mesozoic graywackes on the northern margin of the Otago schist belt show a progressive metamorphic increase from prehnite-pumpellyite facies, through pumpellyite-actinolite to greenschist facies. Corresponding, but only crudely coincident, progressive textural modification ranges from massive, nonfoliated graywacke, through schistose graywacke, semischist, to thorough-going laminated quartzo feldspathic schist. Isograds marking the disappearance of prehnite and clastic potash feldspar, and the replacement of pumpellyite by clino-zoisitic epidote have been mapped, and probable reactions occurring at them are explored by modal analyses of chemically similar rocks.

The thickness of the pumpellyite-actinolite facies is about 9 km. The great width of this and adjacent facies zones are not readily reconcilable with conventional concepts of geothermal gradients and PT fields of individual metamorphic facies. Tectonic disturbance of geothermal gradients during synmetamorphic deformation appears to be a possible explanation.

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