Rb/Sr geochronology in the Thompson belt, Manitoba

The supracrustal metasediments of the Thompson belt (Pipe and Thompson Groups) show pronounced differences in Rb/Sr age (1855-1685 and 1665-1575 Ma, respectively) and initial Sr ratio (0.7096-0.7166 and 0.7203-0.7233). However, they have similar Rb/Sr ratios (0.8-1.2), and the age and isotopic differences are attributed to differing degrees of postdepositional, metamorphic reworking. The Sr-growth trajectories of these metasediments define a broad band on the evolution diagram and indicate a probable (maximum question mark) age of deposition of ca. 2.0 +- 0.l Ga. Furthermore, the role of Archean detritus in the depositional history of these sediments is quite limited, based on isotopic data for the basement gneisses and adjoining granulites of the Pikwitonei region. Comparison of these data with those for metagraywackes of the adjacent Kisseynew gneisses (average Rb-Sr approximately equal to 0.3) indicates that there were fundamental differences between the marine environments within which the two aphebian sedimentary sequences were deposited. The high Rb/Sr in the Thompson belt metasediments is interpreted to reflect a relatively long equilibration of authigenic clays with circulating seawater whereas the lower Rb/Sr of the Kisseynew metasediments reflects rapid sedimentation in an eugeosynclinal environment dominated by juvenile Aphebian material (author)