Volcanic textures in quartzo-feldspathic gneiss of the Willyama Supergroup, Olary Domain, South Australia
Titel:
Volcanic textures in quartzo-feldspathic gneiss of the Willyama Supergroup, Olary Domain, South Australia
Auteur:
Pepper, M. A. Ashley, P. M.
Verschenen in:
Australian journal of earth sciences
Paginering:
Jaargang 45 (1998) nr. 6 pagina's 971-978
Jaar:
1998-12
Inhoud:
Several occurrences of quartzo-feldspathic gneiss forming part of the Palaeoproterozoic Willyama Supergroup in the Olary Domain, South Australia, retain textural indications for a felsic volcanic origin. These examples preserve relict quartz phenocrysts ('quartz eyes') and, at a few sites, relict feldspar phenocrysts and fragmental textures are retained. The rocks contain an assemblage of albite and quartz, with minor muscovite, biotite, K-feldspar, magnetite, hematite, pyrite, zircon and xenotime. They share chemical characteristics with other felsic gneisses in the Willyama Supergroup with U-Pb zircon ages of ca 1700 Ma which have been inferred to have formed from A-type rhyolite precursors. It is interpreted that the rocks represent rhyolitic pyroclastic or epiclastic deposits (with associated minor iron-formation and cherty exhalites), which subsequently underwent pre- and/or syn-metamorphic Na (-Fe) metasomatism and amphibolite-grade metamorphism.
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