Rb–Sr geochronology of the Coldwell Complex, northwestern Ontario, Canada:

Abstract

Platt and Mitchell have recently presented the results of a Rb-Sr geochronological study of the Coldwell Complex of northwestern Ontario. On the basis of a 17 point whole-rock isochron, they suggest an age for the complex of ? 6.2 Ma, a value that they imply is statistically different from our earlier (Bell and Blenkinsop 1980) determination of the age of the same body. They also state that and Blenkinsop (1980) provided little or no information on the nature of the samples studied from the complex nor the exact nature of their analytical procedure. We find the latter part of this assertion difficult to understand. We give in Bell et al. (1979) some information about our analytical procedure, and refer the reader to another publication (Bell et al. 1977) in which, we clearly state, further details can be found. In the later paper (Bell and Blenkinsop 1980) we provide additional information about analytical procedures, including estimates of the uncertainties of both the ~ ' b / ~ % r and a7Sr/86~r ratios. We agree that descriptions of the samples are lacking; we judged it impractical to supply such information for the 67 samples analyzed isotopically and listed in the 1980 report. Platt and Mitchell also appear uncertain about the number of samples we used to obtain our isochron, and in a summary of our data (their Table 3) they give a figure of 1 I?, implying that it was difficult for them to determine how many data points we had used. This value, however, represents the number of samples that we analyzed for Rb and Sr contents. In both our 1979 and 1980 reports we list the isotopic analyses. and it can be seen from the appropriate tables that five samples were used to determine the isochron. We even published a diagram of the isochron for the Coldwe11 Complex in Bell er al. ( 2979). We now wish to consider the question of whether our age of 1070 ? 15 Ma is statistically different from Platt and Mitchell's value of 1044.5 + 6.2 Ma. In the course of reviewing their data, we recomputed their standardized isochrons, and were puzzled by our inability to