Isotopic provenance of the lower Muskwa assemblage (Mesoproterozoic, RockyMountains, British Columbia): new clues to correlation and source areas

The Proterozoic Muskwa assemblage is a thick (6 km) sequence of essentially unmetamorphosed, predominantly fine-grained siliciclastic and carbonate st rata carried within thrust sheets of the Canadian Rocky Mountains of northe ast British Columbia. It is inferred to be widespread in the subsurface of the autochthon to the east on the basis of seismic reflection profiles. The age and stratigraphic affinities of this succession are uncertain, althoug h it is known to be older than 779 Ma based on the age of cross-cutting dik es. We report U-Pb SHRIMP ages for detrital zircons from four sandstone uni ts and Sm-Nd data for interbedded shales that refine stratigraphic ages and correlation. The provenance characteristics of the Muskwa assemblage offer another piece in the puzzle of correctly assembling Rodinia. Pb-207/Pb-206 ages determined by SHRIMP analysis of 94 detrital zircon grains from four sandstone units range from ca. 3075-1766 Ma with a pronounced concentration of ages at ca. 1850 Ma and a paucity of grains in the interval 1920-2000 M a. The youngest, most concordant analysis constrains the maximum age of the Muskwa assemblage to 1766 +/- 14 Ma (99.5% concordant), Sm-Nd analysis of associated mudrocks result in a range of epsilon (Nd) values that overlap w ith western Canadian Shield sources but result in T-DM model ages of 2.30-2 .45 and 2.18-2.33 Ga, using Goldstein et al. (1984) or DePaolo (1981) model s, respectively. The pattern of U-Pb ages is similar to that of the western Canadian Shield but precludes a significant contribution from Proterozoic orogens in the northwest Canadian Shield, specifically the Thelon, Taltson and Calderian phase of Wopmay orogen. The abundance of grains with young ag es (1850-1770 Ma) implies derivation from similarly aged rocks of the Rimbe y granite belt, Hearne Province and possibly the internides of the TransHud son Orogen. The provenance characteristics of the Muskwa are compatible wit h sediment derived from unroofing and erosion of the orogenic topography fo rmed by the collisional shortening of the Hearne Province during terminal a ssembly of the Canadian Shield ca. 1.8 Ga, The provenance characteristics o f the Muskwa assemblage are distinct from those considered typical of Succe ssion B strata, which are characterized by detrital zircons and Sm-Nd signa tures derived from the Grenville Province of eastern North America. Hence, the Muskwa assemblage is assigned to Succession A, although it cannot be de monstrated which, if any, of the Succession A successions in western Canada are direct correlatives. Regional considerations suggest that the most lik ely correlation is with cratonic sand sheets of the Amundsen, Thelon and At habasca basins that developed immediately after assembly of the core of Lau rentia, (C) 2001 Elsevier Science B.V. All rights reserved.