QUARTZ DEFORMATION MECHANISMS DURING PROGRADE METAMORPHISM OF PELITES

Pelitic rocks textures often provide evidence of prograde metamorphic history, especially through the development of trapped inclusion fabrics in porphyroblasts. In this study, we have used traditional textural studies to determine relative growth timing and coupled these with systematic measurement of quartz size, shape preferred orientation (SPO) and crystallographic preferred orientation (CPO) as preserved in inclusions, in strain shadows and in the enclosing matrix. We have chosen four garnet-bearing Barrovian pelite specimens for study: two texturally contrasting staurolite-zone pelites from the same locality near Loch Tay (Scottish Dalradian), and two texturally similar pelites separated by about 100 ºC difference in peak metamorphic conditions, from the Furulund Group (Sulitjelma, Norway).

Analysis of quartz inclusions in garnet indicates that pre-garnet growth quartzes are typically smaller and more abundant, have a good SPO and a uniform (random?) CPO. Quartzes in strain shadows have typically larger grain sizes, a weak or non-existent SPO and a uniform (random?) CPO. Matrix quartzes have grain sizes and frequencies intermediate between inclusion and strain shadow quartzes. The Loch Tay Dalradian matrix quartzes preserve a distinct CPO, whereas the Furulund Group matrix quartzes preserve a weak or uniform (random?) CPO.

Good SPO and uniform CPO for quartz inclusions suggests quartz deformed predominantly by grain boundary sliding before garnet growth. The preservation of uniform quartz CPOs in inclusions indicates there is no preferred dissolution of quartz grains on basis of crystallographic orientation during garnet growth.

Good SPO and CPO for matrix quartz in the Loch Tay samples can be explained by dislocation creep under staurolite zone metamorphic conditions after garnet growth, whereas the good SPO and uniform CPO in the matrix of the Furulund Group samples probably reflect diffusion creep assisted by grain boundary sliding. The two Furulund Group samples bracket the PT conditions of the Loch Tay samples, indicating matrix quartz CPO is not primarily controlled by temperature.

The weak SPO of strain shadow quartzes in all three samples suggests grain boundary migration processes, while uniform CPOs are consistent with diffusion creep, both under relatively low strain conditions.