Zincian Staurolite in Glen Doll, Scotland

Renate Schumacher (né Dürig)
Mineralogisch-Petrologisches Institut, Universität Bonn, 5300 Bonn, W. Germany

Abstract: Textures and relict mineralogy in the Dalradian gneisses from the southern contact aureole of the Glen Doll diorite trace the development of three stages of metamorphism through regional and contact metamorphic phases. Regional metamorphic stage I is characterized by the stability of sillimanite+muscovite; recognition of a subsequent regional metamorphic stage II of lower grade is based on textural criteria, the stability of kyanite and staurolite + quartz, and geothermometry/geobarometry. The breakdown of zincian staurolite occurred under the conditions of contact metamorphism (stage III). Textural evidence from the outer part of the contact aureole suggests that zincian staurolite broke down by the following oxidation reaction: zincian staurolite + muscovite + quartz + O2 → andalusite + Zn-rich spinel + magnetite + biotite + H2O.

Various stages of completion of this reaction have been observed in different parts of a sample. Predominance of magnetite over Zn-rich spinel (⩽ 14 wt. % ZnO) as a breakdown product can be explained by the initial breakdown of Fe-staurolite component + muscovite + quartz+O2 to form andalusite+biotite+magnetite. These product phases were joined by the Zn-rich spinel when sufficient Zn-staurolite component had concentrated in the unreacted staurolite. Rare local examples where Zn-rich spinel is dominant over magnetite may reflect lower O2 fugacity and/or higher initial Zn contents of the staurolite.

Keywords: staurolite • zinc • gneisses • Glen Doll • Scotland

Mineralogical Magazine; September 1985 v. 49; no. 353; p. 561-571; DOI: 10.1180/minmag.1985.049.353.10
© 1985, The Mineralogical Society
Mineralogical Society (www.minersoc.org)