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Calcic hornblendes from Precambrian metamorphic and igneous terranes in the central Front Range, Colo., were subjected to a sillimanite-grade metamorphism and have compositions that are more dependent on host-rock mineral a~semblages than on host-rock chemical composition. The partitioning of the mafic oxides Mg, Fe+2, Fe+3, and Mn between biotite and hornblende is necessary in relating the total mineral assemblage to the composition of the host rocks. Orderly distribution of the mafic oxides between biotite and hornblende yields rather consistent distribution patterns, and these patterns appear to relate to the regional metamorphism. Fe+3 in the metamorphic host rocks controls, though perhaps imperfectly, the Mg content in hornblende, whereas Fe+ 3 in igneous host rocks only exerts partial control over Mg in accompanying hornblende. In general, oxidation-reduction ratios in the igneous hornblendes increase with increases in Fe+ but decrease in metamorphic hornblendes with increasing Fe+3; INTRODUCTION SCOPE AND PURPOSE The crystalline core of the Front Range, which lies just west of Denver, Colo., contains many hornblende-bearing gneisses and syntectonically intrusive hornblende-bearing rocks, all Precambrian in age. Bulk rocks and the minerals hornblende and biotite separated from them were studied to compare the metamorphic effects on hornblende between rock types produced by a sillimanite-grade regional metamorphism. Hornblende, inasmuch as it occurred in all rocks chosen, was studied in the greatest detail to relate its physical and chemical properties to· the metamorphism. Particular attention was given ·to hornblende and biotite in the intrusive Boulder Creek Granodiorite and associated rocks emplaced near the end of the major regional metamorphism in order to compare their chemical behavior with the hornblendes from· adjacent Precambrian gneisses. SAMPLE PREPARATION AND LABORATORY STUDIES Chemical analyses were made in the Denver and Washington laboratories of the U.S. Geological Survey; analysts are credited in the appropriate tables in text. Sta~dard rock analyses were made following

Hornblendes from a region of high-grade metamorphism, Front Range, Colorado