Abstract

The attitude of the garnet isograd surface in the Balquhidder region of the Scottish Dalradian is such that the metamophic zones must be inverted, higher-grade rocks overlying lower-grade rocks. Textural and structural information indicates that peak Barrovian metamorphism post-dates emplacement of the Tay Nappe. As post-metamorphic folds are open and upright, no tectonic means of overturning the isograd surface can be invoked. Metamorphism must have occurred in a negative thermal gradient.

The metamorphic temperature at the garnet isograd is about 500°C and increases along a negative metamorphic gradient, perpendicular to the isograd surface, of about 27°C/km. Metamorphic pressure is about 9 kbar.

The initial temperature inversion was probably caused by emplacement of the Tay Nappe. It is unlikely that subsequent conductive thermal relaxation alone would have caused inverted metamorphic zonation. However, the relaxative process may have been sufficiently modified, by convective transfer of heat by fluid evolved from metamorphic reactions, for this to occur.

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