DEVELOPMENT OF QUARTZ CPO IN NATURALLY DEFORMED GRANITOIDS (AROLLA GNEISS, WESTERN ALPS)

The microstructures and textures of a number of relatively highly deformed samples of a series of incrementally deformed granitoids of the Austroalpine Arolla unit, Western Alps, were analyzed. The protolith consists of 52% plagioclase, 24% quartz, 7% K-spar, 5% biotite and 3% hornblende, and was deformed under green schist facies conditions. From the earliest stages, the deformation is mainly accomplished by flow of the fine grained sericite-epidote-albite aggregates which replace the primary plagioclase. The quartz grains are dispersed in this matrix, and aim of the study is to compare the development of the quartz CPOs of old grains and recrystallized grains of such polyphase rocks to the development of those of pure quartzites.

Partial CPOs of the old grains show primary maxima in the Z-direction (normal to foliation) and secondary maxima in the Y-direction (in the foliation plane, transverse to the shear displacement). Ribbon grains are up to 80% recrystallized (grain size = 50-100µm).

Partial CPOs of the recrystallized grains are strongly influenced by the CPO of the parent grain. Since the quartz is randomly dispersed within >75% of the remaining phases, there is practically no influence of neighbouring quartz grains on the CPO of a given recrystallizing (quartz) grain. However, in addition to the CPO of the parent grain, the CPO of the recrystallized grains reflects the local strain (or displacement) field. At the relatively low P-T conditions, the local deformation (i.e., the grain scale strain field) is heterogeneous, it may be sinistral, dextral or pure shear, adding up to a bulk transpressive strain field.

As a consequence, the CPO of the recrystallized grains is a dispersed peripheral maximum up to 45° to the original Z-direction (synthetically or antithetically disposed wihth respect to the bulk shear direction), or a type-I a cross girdle emanating from the Y-orientation.

Overall, the development of the CPO of the recrystallized grains is "slow" when compared to that of pure quartzites. A possible explanation is the missing contribution of grain boundary migration: this process is suppressed because in the granitoid, quartz grains seldom have quartz neighbours. Despite the high bulk strains inferred from the field, a steady state quartz CPO is not attained.