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

Abstract

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 S 2 cleavage with dextral vergence in their competent psammitic bases, and an S 3 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 S 2 cleavage. S 3 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 D 3. Biotite porphyroblasts, developed during static metamorphism before D 2 S 2 , contain quartz-inclusion trails ( S Ii ) with three orientation patterns that are related to their location within graded beds. Porphyroblasts in psammitic bases of graded couplets are not affected by D 3 S 3 . In comparison, S Ii 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 S 2. From these observations, it is apparent that biotite porphyroblasts within Q domains rotated about 90 ° counter-clockwise during microfolding. Within M domains, the s Ii data suggest that porphyroblasts rotated little, relative to their orientation in the base of the beds.