Microstructures of Prograde Reaction

Case 6. Andalusite-staurolite schist, Buchan Dalradian, NE Scotland

Specimen BB-1

This sample is from Boyndie Bay on the Banffshire coast, within the Buchan region (low-pressure facies series) of the Dalradian metamorphic terrain. Its assemblage is quartz + plagioclase + biotite + muscovite + staurolite + andalusite + cordierite (in some slides only) + local retrograde chlorite.

Small idioblastic porphyroblasts of staurolite contain very fine discordant inclusion trails: thus the external dominant foliation is S2. Staurolites have narrow inclusion-free rims, with pressure shadows and locally a muscovite-rich selvage wrapping them; staurolites thus largely predate S2, which is flattened around them. In contrast, the cm-sized andalusite poikiloblasts contain relatively large biotite inclusions that are partly aligned in S2, partly random. Matrix biotite is more abundant near andalusite, and matrix muscovite less abundant, suggesting that matrix muscovite is used up and/or replaced by biotite in deriving Al for the large porphyroblasts. Andalusite replaces nearby staurolite, forming pseudomorphs. Some of the andalusite poikiloblasts appear to be cut by quartz-filled fissures. There is some flattening of the fabric around andalusite, although one cannot rule out a contribution from passive displacement during andalusite growth. On balance one might suggest syn-D2/S2 growth for andalusite + biotite. A possible balanced continuous AFM reaction is St + Ms + Qtz = And + Bt + H2O.

Idioblastic poikiloblast of staurolite, revealing fine, discordant internal fabric.

Staurolite partly replaced by andalusite (at lower right).

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Last modified 1 November 2003