A study of natural and experimental metasomatic assemblages in an ultramafic-quartzofeldspathic metasomatic system from the haast schist, South Island, New Zealand

Abstract

The mineralogy of a metasomatic sequence formed between ultramafic and quartzofeldspathic protoliths from the Southern Alps of New Zealand consists of a forsterite-antigorite core surrounded concentrically by zones of antigorite-magnesite, magnesite-talc, talc, tremolite, chlorite and muscovite with discontinuous pods of albite associated with the muscovite zone. On the basis of trace element data the original ultramafite-schist contact is positioned between the present tremolite and chlorite zones.

An experimental study of a metasomatic system was undertaken in an attempt to clarify diffusion relationships during the metasomatic event. To simulate the natural event, ultramafic and quartzofeldspathic natural starting materials were tightly packed in a gold tube with a graphite layer between to allow later identification of the original lithological interface. Run conditions were 450° C at 2 kb for 40 days. Phase dissolution and formation were analysed petrographically and component migration was examined with the electron-microprobe. The following hierarchical scheme of component migration, phase dissolution and phase formation is delineated: CO₂ migrates from the schist into the ultramafite forming first the antigorite-magnesite zone and then the magnesite-talc zone at higher values of χCO₂. These zones are then partially overprinted by the formation of talc due to SiO₂ metasomatism. The SiO₂ is supplied from the schist by the dissolution of quartz and albite in the region adjacent to the ultramafite. The tremolite zone forms at the expense of the metasomatic talc zone upon the introduction of CaO from the schist into the ultramafite. Concurrent with tremolite formation, MgO migrates from the ultramafite into the schist to form the chlorite metasomatic zone. The growth of the chlorite zone causes dissolution of the pre-metasomatic micas and displaces K₂O from the chlorite zone further into the schist. Displaced K₂O and Na₂O are responsible for the formation of the muscovite zone and the albitite pods.