Development of a chevron cleavage pattern and porphyroblast rotation in graded metaturbidites, Slave structural province, Northwest Territories, Canada

Chevron cleavage pattern displays opposite cleavage vergence across a psammite-pelite contact or within a single graded couplet. In this example, graded metaturbidites preserve an S2 cleavage with dextral vergence in their competent psammitic bases, and an S3 crenulation cleavage with sinistral vergence in their less-competent semi-pelitic tops. Bedding-parallel shear, partitioned into tops of graded couplets, resulted in buckle-folding of S2 cleavage. S3 is also a compositionally differentiated cleavage displaying alternating quartz-rich microlithons (Q) and muscovite-rich cleavage domains (M). Together, Q and M domains define S-microfolds related to D3. e porphyroblasts, developed during static metamorphism before D2S2, contain quartz-inclusion trails (SIi) with three orientation patterns that are related to their location within graded beds. Porphyroblasts in psammitic bases of graded couplets are not affected by D3S3. In comparison, SIi in porphyroblasts in semi-pelitic tops of beds are, on average, about 5 ° clockwise in M domains, and about 90 ° counter-clockwise in Q domains, corresponding to flat and steep limbs of S-shaped microfolds in S2. From these observations, it is apparent that biotite porphyroblasts within Q domains rotated about 90 ° counter-clockwise during microfolding. Within M domains, the sIi data suggest that porphyroblasts rotated little, relative to their orientation in the base of the beds.