Abstract

Two contrasting swarms of pegmatite dikes occur intermingled in the Pikes Peak granite and in related alkalic granites in the Mount Rosa area, west of Colorado Springs. One is the older calc-alkalic Pikes Peak type; the younger is the alkalic Mount Rosa type. The accessory assemblage of the Mount Rosa quartz-microcline-riebeckite pegmatites consists chiefly of radioactive zircon, fluorite, astrophyllite, thorite, and lesser amounts of pyrochlore, rutile, niobian rutile, columbite, bertrandite and rare aluminofluorides. These dikes intrude both the Pikes Peak and its derivative fayalite granites along northeast-trending fractures. These pegmatites contrast markedly with the coexisting Pikes Peak pegmatites, which are characterized by quartz and amazonite crystals, zircon, biotite, siderite, and minor topaz, allanite, cassiterite and genthelvite. The Pikes Peak type occurs as segregations and injected bodies only within Pikes Peak granite.

Age determinations on zircons from pegmatites of the Mount Rosa type proved to be inconsistent, but ages determined by means of the K40/Ar40 method on pegmatitic riebeckite are consistent with geological evidence and age determinations on the granites, giving an age of 1040 my.

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