New Zealand Journal of Geology and Geophysics abstracts

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

S. WHITE

Department of Geology
University of Otago
P.O. Box 56
Dunedin, New Zealand

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 co-existing 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.

Keywords  garnet; plagioclase; compositional zoning; metamorphism; Otago Schist; Alpine Schist; northwest Otago

New Zealand Journal of Geology and Geophysics, 1996, Vol. 39: 515-531

0028-8306/96/3904-0515 $2.50/0 (c) The Royal Society of New Zealand 1996

PDF file of entire paper: medium quality (4232K); (scanned from paper original: notes about this process)

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