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Tectonic controls on ~0.5 Ga rare earth element mineralization and discovery of a mostly buried ~1.4 Ga layered mafic intrusion in the Wet Mountains, south central Colorado
Magnin, Benjamin
Magnin, Benjamin
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2024
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2025-11-26
Abstract
The Wet Mountains, a northwest-trending exposure of mostly Paleo- and Mesoproterozoic igneous and metamorphic rock in south-central Colorado, contain rare earth element (REE)- and thorium-bearing minerals associated with Ediacaran-Ordovician bimodal alkaline igneous complexes and cross-cutting alkaline and carbonatite dikes and veins. Economic REE and Th minerals are restricted to carbonatite dikes, hydrothermally altered syenite dikes, and quartz-barite-thorite veins in the northern and central Wet Mountains. Previous research suggests that the alkaline and carbonatite magmatism was related to failed continental rifting. However, the orientation of the rift system and its relationship to Ediacaran-Ordovician rift-related igneous complexes in southern Oklahoma and New Mexico have been unclear. Also, the temporal relationship between REE-Th mineralization and alkaline magmatism was based on cross-cutting relationships only. The purpose of this study was to define the structural controls on and the timing of mineralized dikes and veins to build a tectonic framework for REE-Th mineralization and associated alkaline magmatism in the context of the previously proposed tectonic models.
Structural data from this study and newly collected airborne radiometric data from the U.S. Geological Survey (USGS) show that Th-bearing alkaline and carbonatite dikes and quartz-barite-thorite veins occur along subvertical faults and tension fractures that are part of a northwest-trending fault system. Complementary airborne magnetic data by the USGS also show a northeast-directed elongation of the combined surface/subsurface expression of alkaline complexes. These orientations are consistent with dike and vein formation and mineralization during 040°-directed extension in a northwest-trending failed rift system. REE-Th mineralization occurred between ca. 489 Ma and ca. 465 Ma based on new U-Th-Pb Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICP-MS) zircon and monazite data from carbonatite dikes and a hydrothermally altered pegmatite dike. The Ediacaran-Ordovician ages of alkaline igneous rocks and a northwest-trending failed rift model are consistent with the along-strike Southern Oklahoma Aulacogen to the southeast. The ages of mineralization are predominately younger than the ca. 551–483 Ma bimodal alkaline igneous complexes and dikes, suggesting that carbonatite magmatism and REE-Th mineralization represent the final stages of failed continental rifting.
The airborne magnetic data from the USGS and ground gravity data from this study also reveal a magnetic and dense, ~15 km2 elliptical feature in the central Wet Mountains, spatially coincident with local gabbroic outcrops. Detailed petrography and automated mineralogy data from thin sections, and magnetic susceptibility data from the mafic outcrops suggest that the geophysical anomalies represent a mostly buried, layered mafic-ultramafic complex, informally named the Hatchet complex in this thesis. The mafic composition, observed layering in outcrop, and its location suggested initially that the Hatchet complex was part of the same failed rift system as the Ediacaran-Cambrian, alkaline layered mafic complexes in the northern Wet Mountains. New U-Pb LA-ICP-MS of zircon from an orthopyroxenite and norite sample of the Hatchet complex, however, yielded a ca. 1360 Ma crystallization age and instead indicate that the complex is related to the adjacent 1362 ± 7 Ma ferroan, granitic San Isabel batholith. Previous models for pervasive ca. 1.4 Ga ferroan granitoids in Colorado and the southwestern U.S. include a mantle superswell, back-arc extension and/or lithospheric delamination, and convergence/orogenesis. Whole-rock major and trace element geochemistry of samples from the Hatchet complex indicate mantle melting in either an arc environment or extensional environment above subduction-altered lithospheric mantle. Whole-rock and mineral Nd-Sr-Pb Thermal Ionization Mass Spectrometry (TIMS) data and LA-ICP-MS zircon trace element chemistry data from samples from the Hatchet complex suggest that melt was sourced from both primitive asthenospheric mantle and continental arc or enriched lithospheric mantle. These contrasting signatures suggest that the Hatchet complex formed in a back-arc extensional environment, where upwelling primitive asthenosphere interacted with subduction modified lithospheric mantle. This interpretation best fits with previous tectonic models including back-arc extension in Colorado, inboard of a long-lived convergent plate margin to the southeast.
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