Named for Gem Park (also the type area), about 21 mi southwest of Canon City and 11 mi northwest of Westcliffe, northern Wet Mountains, Fremont and Custer Cos, CO in the Denver and Las Vegas-Raton basins. Is composed of gabbro, and coarse- and medium-grained pyroxenite in approximately equal proportions which form units concentric with circular outline of complex. Also has minor dikes and bodies of syenite porphyry, lamprophyre, and carbonatite, and a mass of fenite in center of mass. Lies discordantly in Precambrian gneissic terrane; overlain by Tertiary volcanic rocks. Large areas of study area covered by Quaternary alluvium and colluvium. Basis of Cambrian age is date of 551 +/-55 m.y. on crocidolite from fenitized pyroxenite at Vermiculite Mine in Gem Park. Geologic map. Chemical analyses.

Source: GNU records (USGS DDS-6; Denver GNULEX).

Underlies area of about 5 km in Custer Co. in the Las Vegas-Raton basin and in Fremont Co. in the Denver basin. A K-Ar age of 551 m.y. for fenite in Gem Park Complex used as evidence for emplacement in Middle Cambrian time. Age refined from Cambrian to Middle Cambrian.

Source: GNU records (USGS DDS-6; Denver GNULEX).

The 551 m.y. date on vermiculite may be unreliable. Only the general designation Cambrian used in report area in Las Vegas-Raton and Denver basins.

Source: GNU records (USGS DDS-6; Denver GNULEX).