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 exa mple, graded metaturbidites preserve an S-2 cleavage with dextral verg ence in their competent psammitic bases, and an S-3 crenulation cleava ge with sinistral vergence in their less-competent semi-pelitic tops. Bedding-parallel shear, partitioned into tops of graded couplets, resu lted in buckle-folding of S-2 cleavage. S-3 is also a compositionally differentiated cleavage displaying alternating quartz-rich microlithon s (Q) and muscovite-rich cleavage domains (M). Together, Q and M domai ns define S-microfolds related to D-3. Biotite porphyroblasts, develop ed during static metamorphism before D-2/S-2, contain quartz-inclusion trails (S-li) with three orientation patterns that are related to the ir location within graded beds. Porphyroblasts in psammitic bases of g raded couplets are not affected by D-3/S-3. In comparison, S-li in por phyroblasts in semi-pelitic tops of beds are, on average, about 5 degr ees clockwise in M domains, and about 90 degrees counter-clockwise in Q domains, corresponding to flat and steep limbs of S-shaped microfold s in S-2. From these observations, it is apparent that biotite porphyr oblasts within Q domains rotated about 90 degrees counter-clockwise du ring microfolding. Within M domains, the S-li data suggest that porphy roblasts rotated little, relative to their orientation in the base of the beds. (C) 1997 Elsevier Science Ltd.