Reactions leading to the formation and breakdown of stilpnomelane in the Otago Schist, New Zealand

Semi-pelitic rocks ranging in grade from the prehnite-pumpellyite to the gr eenschist facies from south-eastern Otago, New Zealand, have been investiga ted in order to evaluate the reactions leading to formation and breakdown o f stilpnomelane. Detrital grains of mica and chlorite along with fine-grain ed authigenic illite and chlorite occur in lower-grade rocks with compactio nal fabric parallel to bedding. At higher grades, detrital grains have unde rgone dissolution, and metamorphic phyllosilicates have crystallized with p referred orientation (sub)parallel to bedding, leading to slaty cleavage. S tilpnomelane is found in metapelites of the pumpellyite-actinolite facies a nd the chlorite zone of the greenschist facies, but only rarely in the biot ite zone of the greenschist facies. Illite or phengite is ubiquitous, where as chlorite occurs only rarely with stilpnomelane upgrade of the pumpellyit e-out isograd. Chemical and textural relationships suggest that stilpnomela ne formed from chlorite, phengite, quartz, K-feldspar and iron oxides. Stil pnomelane was produced by grain-boundary replacement of chlorite and by pre cipitation from solution, overprinting earlier textures. Some relict 14 Ang strom chlorite layers are observed by TEM to be in the process of transform ing to 12 Angstrom stilpnomelane layers. The AEM analyses show that Fe is s trongly partitioned over Mg into stilpnomelane relative to chlorite (K(D)ap proximate to 2.5) and into chlorite relative to phengite (K(D)approximate t o 1.9). Modified A'FM diagrams, projected from the measured phengite compos ition rather than from ideal KAl3Si3O10(OH)(2), are used to elucidate react ions among chlorite, stilpnomelane, phengite and biotite. In addition to pr essure, temperature and bulk rock composition, the stilpnomelane-in isograd is controlled by variations in K, Fe3+/Fe2+, O/OH and H2O contents, and th e locus of the isograd is expected to vary in rocks of different oxidation states and permeabilities. Biotite, quartz and less phengitic muscovite for m from stilpnomelane, chlorite and phengite in the biotite zone. Projection of bulk rock compositions from phengite, NaAlO2, SiO2 and H2O reveals that they lie close to the polyhedra defined by the A'FM minerals and albite. O ther extended A'FM diagrams, such as one projected from phengite, NaAlO2, C aAl2O4, SiO2 and H2O, may prove useful in the evaluation of other low-grade assemblages.