Progressive Metamorphism of Metabasic Rocks from the Haast Schist Group of Southern New Zealand |
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| Authors: | COOPER A F |
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| Institution: | Department of Geology, University of Otago Dunedin, New Zealand |
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| Abstract: | Progressive mineralogical changes are described for metabasicrocks through a Barrovian-type metamorphic series ranging fromgreenschist to amphibolite facies in the Southern Alps of NewZealand. Wet chemical and electronmicroprobe analyses of coexisting phasesillustrate (a) that chlorite composition becomes progressivelymore Mg-rich towards higher grades. Chlorite and biotite zonechlorites have Mg/Fe <1.00, while in the oligoclase zonethe chlorite Mg/Fe >2.00. (b) Biotite and epidote compositionsshow no systematic variation with metamorphic grade and arecontrolled by bulk rock composition. For epidote, compositionis directly related to oxidation states during metamorphism.(c) Zoning profiles from atoll and normal porphyroblastic almandine-richgarnets are illustrated, and their relationship to compositionalchanges with metamorphic grade discussed, (d) Coexisting compositionsof albite and oligoclase through the garnet zone outline theform of the peristerite solvus. Myrmekitic textures in oligoclaseare ascribed to release of silica during progressive eliminationof albite. Element distribution between coexisting minerals is graphicallyinvestigated. Isogradic samples show very similar element distributions,suggesting general equilibration. Distribution of any elementbetween two phases for the facies series as a whole, however,is clearly influenced by changing concentrations of other ionsin one or both minerals. In particular for pairs containingcalciferous amphibole, the distribution coefficient is dependentupon tetrahedrally co-ordinated Al+3 of the amphibole. |
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