Age and tectonic setting of lower Paleozoic alkalic and mafic rocks, carbonatites, and thorium veins in South-central Colorado
Alkalic igneous rocks were emplaced into heterogeneous terrane of Precambrian X and Precambrian Y rocks about 570 m.y. ago (Cambrian or upper Precambrian) in the Powderhorn area in Gunnison County and about 520 m.y. ago (Cambrian) in the McClure Mountain, Gem Park, and Democrat Creek areas in the northern Wet Mountains, Fremont and Custer Counties. The radiometric ages are based upon studies by K-Ar, Rb-Sr, and fission-track methods. Associated with these alkalic rock complexes are numerous thorium-bearing veins and red syenite dikes; some, if not all, in the northern Wet Mountains were formed about 495 m.y. ago. Diabase, gabbro, and other mafic dikes appear to be slightly younger than the thorium veins in the Powderhorn area and both older and younger in the Wet Mountains region. In the Powderhorn district, an older group of syenites also intruded Precambrian rocks as plugs or small stocks about 1350-1400 m.y. ago. The various dike rocks and the thorium veins were formed in extensive, deep fractures, indicating a condition of tension or shear in this part of the crust during Cambrian or very late Precambrian to Ordovician time. The localized alkalic magmatisin may reflect melting spots in the mantle. In the two principal areas, 135 km apart, the episodes of alkalic magmatism differ in age by about 50 m.y., the younger toward the east. The age relations might be explained by migration of the sites of localized melting or volatile enrichment in the mantle or westward movement of the continent above a single site.