FACTORS CONTROLLING GENERATION AND DISTRIBUTION OF 1400-MA PLUTONS IN COLORADO

Plutons that range in age from 1350 to 1440 Ma and in composition from alkali-calcic diorite to granite perforate the crust in Colorado and in a belt that crosses the North American continent. Locally, rocks as diverse as anorthosite and carbonatite contribute to the plutonic belt, which formed in an intracratonic, extensional setting, removed from any convergent margin. Nd and Sr isotope geochemistry suggests derivation of melts from the subcontinental mantle during formation of a linear “welt” across Laurentia as much as 5000 km long.

The 1780-Ma volcanic arc terranes in the state contain the fewest 1400-Ma plutons. Ensimatic basins (1760-1730 Ma) contain the most and largest 1400-Ma plutons. Why this relationship should exist is uncertain. Except for the youngest plutons (1350-1380 Ma) being restricted to the Wet Mountains, no spatial pattern of ages is evident.

Age, gravity, and magnetic features indicate that relatively small, sheet-like to slightly discordant dioritic plutons in the Front and Sawatch ranges, such as the Mt. Evans and Oak Creek plutons, formed by 1440 Ma. Steep-sided discordant granitic intrusions with extensive roots, such as the St. Kevin and Silver Plume plutons, form the largest masses and have ages of 1425 and 1405 Ma, respectively. Complex, composite sills, such as the St. Vrain pluton, bridge the gap between intrusive types. A regionally extensive, NNW-striking mafic dike swarm in the northern Front Range, and localized east-striking lamproite dikes in the central Front Range are among the youngest mafic events. Many granitic bodies, as young as 1350 Ma, appear to be underlain by extensive bodies of mafic, magnetic rocks. Zircons from all 1400-Ma rocks are enriched in thorium compared to 1.7-Ga rocks, and have minor cores. SHRIMP ages of the cores are usually similar to those of the country rock but may be as old as Archean.

The central Colorado gravity low is coincident with outcrops of the largest felsic plutons along a belt from Aspen to Empire. Discordant 1400-Ma plutons extending to depths of 10 km adequately explain the gravity low, which had been previously been attributed to a Laramide batholith. Feldspars from plutons in this belt have the highest 208Pb/204Pb ratios of all 1400-Ma rocks in Colorado. Such thorogenic lead may have been derived from a thorium-enriched region of the upper mantle.