Abstract
The Audubon-Albion stock in Boulder County, Colorado, is a composite intrusive consisting largely of monzonite and injected into and crosscutting pre-Cambrian granite, gneiss, and schist. The sequence is syenogabbro (oldest), monzonite, quartz-bearing monzonite, syenite, and granite. The monzonite probably formed from a syenogabbroic magma by crystal fractionation, but the younger differentiates, including quartz-bearing monzonite, syenite, and granite, resulted from more complex processes involving not only crystal fractionation but also assimilation or syntexis and transfer of materials by mobile, fluid-rich, alkalic solutions.
Age relations, the distribution of the various rock types, and the internal structures suggest that the stock was in part emplaced by repeated subsidence of a large conical block and in part by injection of magma into zones of weakness along internal or external contacts. There is little evidence to indicate that the walls of the stock were forced apart by the advancing magma.
Chemical analyses of the syenogabbro show an unusually high content of potash and are nearly identical with analyses of the basalt flows interbedded with the sediments of the Denver formation near Golden, Colorado. Correlation establishes the age of the oldest rock in the Audubon-Albion stock as early Eocene or, possibly, Paleocene. The exact age of the younger differentiates is uncertain.
