Sundown Group Pelites: Biotite

Roberto Weinberg, Monash University, Australia

 

 

Biotite. In the weakly deformed core of the Eldee structure (sample ES1C), biotite varies from well-preserved and undeformed large grains to large grains weakly crenulated, or sheared at the margins (Fig. 1a and b) they normally have rounded quartz grain inclusions and narrow muscovite intergrowth.

As deformation increases to the next level in Figs 2 and 3 (overprinted samples EC04_4 and EC04_3), large biotite grains are folded or broken up into smaller grains, and are oriented from high angles to parallel to the new cleavage (Fig. 2b). Quartz inclusions and muscovite intergrowth are still preserved in the large crystals (Fig. 1). In the transposed sample (Fig. 4, EC04_2), small biotite grains are rotated into parallelism to the main foliation and preserve isoclinal fold hinges.

All the photomicrographs are at the same scale and can be directly compared. The main foliation oriented left to right.

 

 


Figure 1a. ORIGINAL Sample ES1C. Undeformed biotite grain (2mm) with quartz inclusions and muscovite intergrowth (narrow white lines inside biotite).
Figure 1b. ORIGINAL Sample ES1C. Large biotite grain (2mm) sheared at the margins. Note quartz grain inclusions. In the upper right- hand-side there is intergrowth betwen biotite and fibrolite sheath.

 

 


Figure 2a. OVERPRINTED Sample Ec04_3. Crenulated biotite grain. Note rounded quartz inclusions.
Figure 2b. OVERPRINTED Sample Ec04_4. Biotite grains in the process of folding and transpoition. The central biotite grain is in a hinge zone, separated from two other grains above and below, rotated to parallelism with the newly developed axial planar foliation.

 

 


Figure 3. OVERPRINTED Sample Ec04_4. Folded biotite and sericite-rich grain, with a muscovite grain in the core.

 

 

Figure 4a.TRANSPOSED Sample Ec04_2. Biotite in much smaller crystals and not all grains are entirely transposed. Figure 4b. TRANSPOSED Sample Ec04_2. The originally large biotite grains have been broken up, recrystallized and transposed and refolded in the process. Note significant reduction in grain size.