THE CENTRAL COLORADO PLATEAU LACCOLITHS: A TEMPORAL AND SPATIAL LINK TO VOLUMINOUS MID-TERTIARY MAGMATISM IN COLORADO AND THE GREAT BASIN

The central Colorado plateau laccoliths (Henry, La Sal, and Abajo Mountains) exhibit a number of unusual characteristics. Unlike mid-Tertiary magmas emplaced surrounding the plateau, which if alkaline are K-rich, the laccoliths are dominated (~95% by volume) by sodium-rich plagioclase and hornblende porphyries of intermediate composition (~63 wt. % SiO₂). Although more mafic and silicic rocks are found, they are rare. Small volumes of unusual suites of alkaline rocks, also Na-rich, occur in the Henry and La Sal mountains, including peralkaline, feldspathoid-bearing, and quartz-bearing lithologies. Unusual igneous phases occur in these hyper-alkaline rocks, including haüyne, nosean, acmitic clinopyroxene, and melanite garnet. Petrogenetic studies suggest that these magmas probably originated by varying degrees of melting of amphibole-bearing mantle and have evolved via subsequent fractional crystallization and interaction with mafic crust.

Despite their unusual composition and small volume, the laccoliths nonetheless comprise a temporal and spatial link to voluminous magmatism in the central Nevada—Marysvale volcanic belt and San Juan volcanic field. Intrusive ages generally range from ~31 to 24 Ma, which combined with their location, links them in time and space to magmatism off the plateau. The laccoliths also have geochemical affinities with regional magmatism, especially the classic enrichment of large-ion lithophile elements relative to high field-strength elements associated with subduction zones. However, the laccoliths are in many respects more isotopically primitive, suggesting that substantially less crustal material has been incorporated in these magmas. Overall, their small volume, as well as other tectonomagmatic features, indicate that the transport of large volumes of magma to the near-surface environment was suppressed by high-strength lithosphere of the Colorado plateau relative to the contemporaneous Great Basin and San Juan volcanic fields.