An inner Cordilleran belt of muscovite-bearing plutons

Muscovite-bearing granitic rocks, rare in the great coastal batholiths of the western margin of North America, are common in an inland belt that stretches from Sonora to British Columbia. Compositional characteristics of these inland plutons (strongly peraluminous, high ⁸⁷Sr/⁸⁶Sr) ally them with the S-type granitoids, whereas the coastal batholiths are best described as I-type. Although a simple S-type origin by anatexis of metasediment is not required, compositions of these plutons imply a far greater contribution from ancient crustal material than do those of marginal batholiths. The distinction between margin and inner Cordillera in nature of plutonism suggests important regional differences in magmagenic processes; these differences must have been a function of variations in composition of and/or conditions in the preexisting crust. It is noteworthy that the muscovite-bearing belt is located at the edge of inferred Precambrian crust and generally corresponds to areas of intense deformation and regional metamorphism of Mesozoic through early Cenozoic time. *Present address: Martin-Trost Associates, 1510 Washington, Golden, Colorado 80401