Age of the Todd Ridge Pyroclastic Rocks

The Todd Ridge pyroclastic rocks may have been intruded by the porphyritic facies of the Wisconsin Range Batholith in which case the felsic pyroclastic rocks should be older than 497 4 Ma (Table 7.2). Actually, Faure et al. (1968b) reported a whole-rock Rb-Sr date of 521 38 Ma (Early Cambrian) for six specimens of the Todd Ridge pyroclastics (recalculated to . = 1.42 X 10 year 0  

The analytical data for eight samples of the Todd Ridge pyroclastics in Appendix 7.8.4 define a straight line in Fig. 7.20 from which we calculate a Rb-Sr date and initial Sr/ Sr ratio  

This Early Cambrian date is compatible with the stratigraphy and age determinations of the basement rocks of the Wisconsin Range. However, the severe alteration of the Todd Ridge pyroclastics casts doubt on the reliability of this date. 

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