75-14 Geochemistry of Diabase and Alkaline Dikes within an Enigmatic, Ediacaran-Ordovician Intraplate Igneous Province in Southern Colorado

Session: Mineralogy, Geochemistry, Petrology, and Volcanology Student Session (Posters)

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Abstract:

Cambrian igneous rocks in southern Oklahoma and parts of Texas define a northwest-trending rift zone extending from the ancient continental margin. Numerous Ediacaran to Ordovician igneous intrusions ranging from large plutons to smaller dikes occur along the same trend in Colorado, primarily in the Powderhorn and Gunnison areas in western Colorado and the Wet Mountains farther east. Relations between the Oklahoma-Texas and Colorado intraplate provinces are uncertain. In order to shed light on this problem, we are carrying out geochemical studies on as many dikes as we can find in the Colorado province.

Four lamprophyre dikes and a mafic-rich nepheline syenite dike in the Wet Mountains plot within the alkaline basalt and basanite/nephelinite fields on the Winchester and Floyd (1977) classification diagram and show steep LREE-enriched patterns. Samples from abundant dikes identified by previous workers as trachytes in this area show trace-element characteristics of phonolites, suggesting that at least some of these dikes have been misclassified. In thin section, the trachytes show evidence of secondary silicification, which has disturbed major element contents and may explain the misidentification. These samples show strong depletion in Ba and Sr, suggesting that feldspar fractionation played a role in their petrogenesis.

Fifteen samples of generally northwest-trending diabase dikes in a prominent dike swarm in the Powderhorn and Gunnison areas show typical trace-element patterns for tholeiitic within-late or continental basalts, with moderate LREE enrichment similar to E-MORB basalts. On the Th/Yb versus Nb/Yb diagram these dikes define a vertical trend extending upward from the E-MORB field, indicating the dikes originated from asthenospheric melts that interacted with continental lithosphere previously modified by subduction. Five NW-trending dikes in the Front Range ~80 km north of the Wet Mountains show similar trace element patterns to the Gunnison/Powderhorn dikes, suggesting all these dikes may represent parts of a single magmatic system. Altogether, the diabase dikes found in both the Gunnison area and the Front Range display geochemical similarities to similar dikes associated with the Southern Oklahoma aulacogen, consistent with a linked petrogenetic history. Geochronological work is underway to further test this possibility. We are also now collecting numerous additional dikes in the Royal Gorge and Wet Mountains, and geochemical data for these samples will also be presented.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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