Altered biotites in the Marlborough Schist, New Zealand?

Abstract

Four samples of weathered greenschist facies greyschist from Marlborough contain minerals that resemble biotite in thin section. Electron microprobe analysis of these minerals indicates that none of them are pristine biotite but are instead oxidised hydrobiotite and/or interlayered phyllosilicates of muscovite/illite, hydrobiotite, chlorite, and/ or vermiculite. Even so, the textural relationships, chemical composition, and possible formation mechanisms of the biotite‐like minerals are compatible with their derivation from originally fresh metamorphic biotite. It thus seems likely that biotite zone rocks are exposed in the Marlborough Schist.

Keywords:

• Marlborough
• schists
• biotite
• sheet silicates
• metamorphism
• weathering