Whole rock Rb-Sr techniques were used to study the effects of intense strain and recrystallization within a large mylonite-bearing, northeast-trending Precambrian shear zone in the Front Range of Colorado. Layered mylonites from the shear zone were examined on three scales of observation ranging from hand specimen to quadrangle to test the extent of Sr87/Sr86 homogenization at the time of shearing. Whole-rock isochrons indicate homogenization occurred over volumes of at least a few meters. This chemical redistribution is compatible with evidence of recrystallization and neomineralization found within the shear zone. The whole-rock isochron age is approximately 1.2 b.y. The presence of similar fabrics in other mylonitic shear zones suggests, that this technique may have wide application in unraveling tectonic chronologies.

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