Composition and zoning of garnet and plagioclase in Haast Schist, northwest Otago, New Zealand: Implications for progressive regional metamorphism

Abstract

Two garnet isograds are mapped in quartzofeldspathic Haast Schist, northwest Otago. The first garnet isograd is based on the incoming of Ca-Fe-Mn garnet coexisting with albite (An0–1), and is employed to delimit an Otago Schist style garnet-albite-biotite zone. The second garnet isograd is based on the incoming of almandine-rich garnet, occurring as rims on Ca-Fe-Mn garnet cores, and on the incoming of oligoclase (An23–26) co-existing with albite (An0–2). The garnet-oligoclase zone which this second garnet isograd delimits is equivalent to the Alpine Schist garnet zone at Haast River, and is interpreted as a metamorphic overprint on pre-existing Otago Schist mineral zones. Compositional zoning in Ca-Fe-Mn garnet is dominated by substitution of Ca for Mn, whereas zoning in almandine-rich garnet from the garnet-oligoclase zone is dominated by substitution of Fe for Ca. Recognition of these substitution trends is an important aid in determining the petrogenetic affiliation of the minerals. Ca-Mn substitution and rim compositions for garnet in phyllosilicate-rich schists from the Aspiring Lithological Association indicate an affinity with Ca-Fe-Mn garnet in quartzofeldspathic Otago Schist. However, the comparatively Fe-rich nature of the cores of garnet in Aspiring schist, and the consequent zoning path, are probably influenced by bulk-rock composition. Two implications of a metamorphic overprint are that: (1) it supports earlier contentions that Otago and Alpine Schists may have experienced distinctly different P-T maxima; (2) pending further work on the significance of late, porphyroblastic biotite in these rocks, it should be feasible to map a petrological boundary between Otago Schist and Alpine Schist which compliments the traditional, less precisely defined, distinction between Otago and Alpine Schists based on geographical distribution. © 1996 Taylor & Francis Group, LLC.