Abstract

The Mount Rosa area includes many small segregations and sill-like bodies of igneous rocks occurring in an arc-like pattern within the Pikes Peak batholith. The segregations consist of small elongate bodies of fayalite granite and porphyritic granite. These igneous bodies are transected by northwest-trending dikes of granite and aplite. Accessory minerals of the fayalite granite are fayalite, fluorite and allanite. Sill-like bodies of Windy Point granite contain accessories similar to those of the porphyritic granite, namely, fluorite, anatase and pyrrhotite, with lesser monazite and bastnaesite.

The Mount Rosa body, an alkalic riebeckite granite, forms a small irregular sheet, one by four miles in plan and less than 150 feet thick, intrusive into Pikes Peak. Finer grained dike-like variants of the Mount Rosa granite include xenoliths of Pikes Peak granite. The Mount Rosa granite contains accessory astrophyllite, bastnaesite, and zircon with rare pyrochlore and monazite.

Age determinations by the alpha count-lead isotope method on zircons from the Mount Rosa granite and by the K40/Ar40 method on riebeckite of the granite and pegmatites gave satisfactory ages of 1040 m.y. Field relationships and age determinations, as well as similar petrological and mineralogical characteristics, support the theory that the Pikes Peak, fayalite granite, Windy Point, and Mount Rosa granites are comagmatic.

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