Abstract

The Colorado Mineral Belt (CMB) is a northeast-trending, ∼500-km-long, 25–50-km-wide belt of plutons and mining districts (Colorado, United States) that developed within an ∼1200-km-wide Late Cretaceous–Paleogene magma gap overlying subhorizontally subducted segments of the Farallon plate. Of the known volcanic gaps overlying flat slabs in subduction zones around the Pacific Basin, none contains zones of magmatism analogous to the CMB. I suggest that the primary control of the CMB was a northeast-trending segment boundary within the underlying Farallon flat slab. The boundary was dilated during warping of slab segments by the overriding thick (∼200 km) lithospheres of the Wyoming Archean craton and the continental interior craton during acceleration of Farallon–North American convergence beginning in mid-Campanian time (ca. 75 Ma). Because the primary control was not in the North American plate, the CMB cut indiscriminately across the geologic grain of Colorado, seemingly independent of the tectonic elements it crossed. A series of discontinuous shear zones of Proterozoic ancestry provided some local control at the district level but were not the primary control.

Geologic contrasts north and south of the CMB reflect its relationship to a segment boundary in the Farallon plate. The dominant trends of Laramide basement-cored uplifts are northwestward north of the CMB but northward south of the CMB. Laramide sedimentary deposits of Late Cretaceous and Paleogene age (exclusive of the Sevier foredeep) are as much as 6 km thick north of the CMB versus only ≤3 km south of the CMB. The Farallon segment south of the CMB rolled back to the southwest and sank into the mantle beginning ca. 37 Ma with resultant major ignimbrite volcanism and generation of the large San Juan and Mogollon-Datil volcanic fields. Volcanism in the Rocky Mountains north of the CMB was sparse.

Laramide plutons (ca. 75–43 Ma) are mainly alkaline monzonites and quartz monzonites in the northeastern CMB, but dominantly calc-alkaline granodiorites in the central CMB. Geochemical and isotopic studies indicate that CMB magmas were generated mainly in metasomatized Proterozoic intermediate to felsic lower crustal granulites and mafic rocks (± mantle). Late Eocene–Oligocene rollback magmatism superimposed on the CMB during waning of Laramide compression (ca. 43–37 Ma) resulted in world-class sulfide replacement ores in the Leadville area. Overprinting of the CMB by Rio Grande Rift extension beginning ca. 33 Ma resulted in intrusion of evolved alkali-feldspar granites and generation of major porphyry molybdenum deposits at Climax and Red Mountain.

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