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The greenschist facies in part of eastern Otago, New Zealand

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Abstract

Rocks of the greenschist facies in eastern Otago, New Zealand, have been investigated in an area some thirteen to sixteen kilometers wide and sixty-five kilometers long extending northeastwards approximately normal to the boundary of the schist with lower grade rocks. Quartzo-feldspathic schists predominate but greenschists and metacherts occur sporadically throughout the area. At the southwestern edge of the area schists are in the chlorite zone, slightly above the high-grade limit of pumpellyite. Metamorphic grade increases toward the northeast into the biotite zone which occupies about half the terrane studied and is believed to be everywhere little advanced in metamorphic grade past that of the biotite isograd. Some 130 mineral specimens have been partially analysed with the electron probe. Results derived from these data as well as other mineralogical investigation are as follows: Albite contains a maximum of 1% anorthite plus orthoclase in epidote-bearing rocks from all parts of the area.

Compositions of epidotes range from 12% to 32% Ca2Fe3(SiO4)3(OH), but most lie between 15% and 20%, a compositional field thought by Strens (1965) and Holdaway (1965) to occupy a miscibility gap in the epidote series. Zoning in some epidotes suggests a history of early growth of small, sparse iron-rich epidotes, and later growth of relatively large amounts of iron-poor epidote probably caused by breakdown of prehnite and/or pumpellyite. Muscovites vary widely in celadonite content; but the composition shows little if any dependence on metamorphic grade within the area studied. Most tend to be celadonite-rich, and in this respect are similar in composition to muscovites from rocks of the glaucophane-schist facies.

Chlorites range widely in Mg/Fe; but Al/Mg+Fe is relatively uniform. Chlorites associated with actinolite tend to have higher Mg/Fe than those associated with stilpnomelane. Following the classification of Foster (1962) most chlorites are brunsvigite and some are ripidolite. Textural and chemical relations between biotite and coexisting minerals demonstrate that, contrary to some previous suggestions, biotite is not a relict mineral. An alteration product of chlorite bears strong resemblance to biotite, and previous misidentification of this mineral as biotite has caused much confusion regarding the distribution and metamorphic significance of biotite in Otago schists.

An attempt to determine the reaction producing biotite is not successful. Possibly biotitebearing rocks have slightly higher biotite component than rocks of the chlorite zone. All newly formed amphibole found in eastern Otago is pale green, Al- and Na-poor actinolite. One of the chemical conditions necessary for the formation of actinolite in schists of eastern Otago is a relatively high Mg/Fe+Al ratio.

Stilpnomelane is an integral part of assemblages in which it occurs, being developed under conditions of relatively low \(P_{O_2 }\) and in rocks with a high Fe/Mg + Al ratio. The present highly oxidized state of all stilpnomelane observed in this study is probably not a primary feature of the mineral but developed after metamorphism.

Porphyroblastic garnets are accessory constituents in about half the quartzo-feldspathic schists collected from the biotite zone but are extremely rare in specimens of the same lithology from the chlorite zone. Either a garnet-producing reaction began in quartzo-feldspathic schists at about the biotite isograd, or rocks of biotite zone tend to have slightly higher garnet component than those of the chlorite zone. Composition of the garnets ranges widely, extremes being: 77% spess., 18% gross., 5% alm.; 25% spess., 50% gross., 25% alm.; 15% spess., 30% gross., 55% alm. Most of the variation in composition is controlled by host rock composition, but garnets at higher grade tend to have lower spessartine content. The garnets are zoned; generally Mn decreases and Fe increases from core to rim.

For the most part chemical equilibrium among different grains and minerals was closely approached over distances of at least a few millimeters. However, profound disequilibrium exists within some individual grains, such as a zoned garnet which over a distance of only 15 microns ranges in spessartine content from 77% in the core to 35% on the rim.

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  1. Department of Geology, Western Washington State College, Bellingham, Washington

    E. H. Brown

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This report is a condensed version of part of the author's Ph.D. thesis (Brown, 1966), University of California, Berkeley.

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Brown, E.H. The greenschist facies in part of eastern Otago, New Zealand. Contr. Mineral. and Petrol. 14, 259–292 (1967). https://doi.org/10.1007/BF00373808

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