Metamorphic discontinuities in orogenic belts: example of the garnet-biotite-albite zone in the Otago Schist, New Zealand

A regional petrographic reconnaissance of psammitic and pelitic rocks in th e Otago Schist, New Zealand, has revealed the presence of garnet ("grossals pite'' with typical rim composition almandine(41), spessartine(25), grossul ar(33), pyrope(1)) and biotite in 37 new samples, more than doubling the pr eviously known number. A new garnet-biotite-albite zone can now be defined in the greenschist facies Otago Schist that is distinct from the better-kno wn biotite, garnet and oligoclase zones in the along-strike Alpine Schist. The garnet-biotite-albite zone is in part metamorphically discontinuous wit h adjacent schists and does not support models of simple, continuous, progr essive Jurassic regional metamorphism in Otago. The structurally higher (lo wer grade) boundary of the zone coincides in at least three places with pre viously mapped regional shear zones. The structurally lower (expected highe r grade) boundary of the zone appears to be obliterated by a chlorite zone overprint which can be spatially related to Alpine Schist recrystallisation of ?Cretaceous age. The Otago situation serves as an example of the subtle metamorphic discontinuities that probably pervade many orogenic belts.