ASSESSING CRUSTAL CONTAMINATION IN ~25 MA MAFIC ROCKS IN THE DULCE SWARM WITH ND-SR ISOTOPIC DATA, SOUTHWESTERN COLORADO

Over one hundred ~25 Ma alkaline mafic dikes are exposed in the Dulce swarm on the east side of the San Juan basin. These dikes were emplaced in a northeast zone of incipient extension distal to the Rio Grande rift.

The chemical signatures of Oligocene mafic dikes in the Four Corners region and Rio Grande rift share similar alkaline-potassic and LREE-enriched affinities. Previous Nd and Sr isotopic studies establish that melting of metasomatized lithospheric mantle was the dominant melt source for most of the ~25 Ma mafic dikes in the region with no significant crustal contributions. εNd (t) values from -7.45 to -4.2 reported for several samples from the northern Dulce swarm indicate that melts were contaminated with lower or upper crust. The persistence of crustal contributions to melts over the extent of the Dulce field, however, were not previously assessed.

To further evaluate isotopic trends in the Dulce swarm, Nd ± Sr whole-rock isotopic signatures were determined for six samples over the ~100-mile extent. Preliminary results lend further evidence that mantle melts in the Dulce system interacted with crustal reservoirs to a greater degree than Oligocene mafic rocks across the northern San Juan basin and Rio Grande rift. Understanding the regional trends and variations in the sources of mantle melts can provide further insight into larger-scale melt production in southwestern Colorado from 29 to 24 Ma. Our presentation will discuss the results of this ongoing investigation.