GEOCHEMISTRY OF EDIACARAN-ORDOVICIAN DIABASE, LAMPROPHYRE AND PHONOLITE DIKES IN SOUTHERN COLORADO, POSSIBLY RELATED TO RIFTING IN THE SOUTHERN OKLAHOMA AULACOGEN

Southern Colorado contains an array of Ediacaran-Ordovician plutonic complexes and dikes that may have formed during rifting associated with formation of the Southern Oklahoma Aulacogen along trend to the SE. The dikes include a prominent diabase dike swarm in the Gunnison area as well as abundant dikes of several types in the Wet Mountains farther east. The Gunnison swarm consists of ~ 50 NW-trending diabase dikes up to 30 m thick, suggesting a vigorous magmatic system. On standard discrimination diagrams, fifteen representative dikes are tightly clustered in fields for subalkaline, tholeiitic within plate or continental basalts. REE patterns for the dikes are very similar, with moderate LREE enrichment. An additional dike contains modal nepheline and shows stronger LREE enrichment. This sample plots slightly above the OIB field on a Th/Yb vs Nb/Yb diagram, whereas the other dikes are tightly clustered partly within and a short distance above the EMORB field. This suggests that primary melts derived from enriched asthenosphere underwent some interaction with continental crust or subcontinental lithosphere.

We have not yet found diabase dikes in the Wet Mountains suitable for geochemical studies. However, five samples from NW- to NNW-trending diabase dikes in the Front Range, ~80 km north of the Wet Mountains, cluster tightly with the Gunnison dikes in trace-element discrimination diagrams and have very similar REE patterns. This raises the intriguing possibility that dikes related to Ediacaran-Ordovician intraplate magmatism in Colorado may be more extensive than previously thought.

Samples of four lamprophyre dikes in the Wet Mountains are uniformly LREE-enriched and generally have similar trace-element patterns on multi-element diagrams normalized to primitive mantle. On the Zr/TiO₂ vs Nb/Y classification diagram the dikes plot in fields for alkaline basalt and basanite/nephelinite. Three dikes classified as trachytes by other workers cluster in the phonolite field on this diagram, suggesting some of these dikes have previously been misclassified. The three dikes show similar LREE patterns, with flat HREE, moderately negative Eu anomalies, and strongly negative Ba and Sr anomalies, implying the dikes are the products of prolonged feldspar fractionation.