COMPARISON OF THE CARLTON DIKE SWARM WITH OTHER REGIONALLY ASSOCIATED INTRUSIONS

The Carlton Dike Swarm, CDS, is a reversely-polarized diabase dike swarm associated with the early-reversal period of the ~1.1 Ga Midcontinent Rift System. The mafic dikes occur west of Duluth, centered on Carlton County, MN, and trend NE, parallel to the rift. Other dike occurrences in the region include: normally-polarized NE-trending dikes, reversely-polarized NW-trending dikes, and olivine- and clinopyroxene-phyric mafic/ultramafic dikes of unknown polarity and age. Previous work has characterized the CDS as a Ti- and Fe-enriched quartz tholeiite compared to the younger, olivine tholeiite Duluth dike swarm. The TiO₂ content of the CDS ranges from 3.0 to 4.5 wt%. However, a low TiO₂ dike (Cloquet dike; 1.25 wt%) included within the CDS is unique because of its greater length (55 km) and resemblance to normally-polarized dikes in both major and incompatible elements. The NW-trending orthogonal dike is geochemically similar to the high-Ti dikes (3.5-3.75 wt%) with an analogous REE pattern and enrichments in incompatible elements. The olivine- and clinopyroxene-phyric mafic/ultramafic dikes have lower TiO₂ contents than the CDS (2.04-2.65 wt%) and a larger variation in major element abundances, partially attributed to alteration. One dike is notable for an enrichment in LREE and wider range of MgO (La/Sm = 4.75-6.50; MgO = 7.95-18.40 wt%) compared to the other mafic/ultramafic dikes (La/Sm = 2.17-2.87; MgO = 10.66-15.93 wt%). The high-Ti CDS dikes also have similar geochemical characteristics to the Esko intrusion, a Fe-Ti-rich gabbro. Although the major oxide percentages for both the CDS and Esko intrusion mainly overlap on MgO variation diagrams, the REE pattern of the Esko intrusion is much steeper than that of the high-Ti dikes (La/Yb = 12.96-18.05 and 6.40-7.29, respectively). Additional geochemical data along with petrographic characterization and geochronology enable delineation of CDS subgroups.