South Park

In the central part of the South Park basin in central Colorado lamprophyric intrusions outcrop as a dyke and a sill-like body about 1.6 km long. Eleven other intrusions of similar type and up to 16 m thick were encountered in a nearby oil well. In the sill a chilled border facies grades into a zone, up to 24 m thick, of a mafic rock of biotite phenocrysts in a groundmass of labradorite, alkali feldspar, olivine, aegirine-augite, analcime and other zeolites. This passes into a central felsic zone 17 m thick of analcime syenite of alkali feldspar, a little albite, biotite, aegirine-augite, 23% analcime, and a little thomsonite and natrolite. Modes, chemical analyses and discussion of the petrogenesis of these multiple intrusions will be found in Jahns (1938) and Stark et al. (1949).