MAGMATIC CONSTRAINTS ON THE ENIGMATIC COLORADO MINERAL BELT

The Colorado Mineral Belt (CMB) is a Laramide (75 to 40 Ma) magmatic lineament that trends NE from the Four Corners region to the Colorado Front Range. Spatially restricted magmatic activity ~1000 km inboard from the trench of the subducitng Farallon Plate is a puzzling enigma of the Laramide orogeny. End-member hypotheses place geographic control of the CMB within either the crustal lithosphere or the downgoing slab. Approximate alignment of the mineral belt with the strong NE trending Precambrian structural grain has supported focusing of magmas along pre-existing crustal weaknesses and/or enhanced melt fertility associated with the shear zone regions. Alternatively, a Farallon slab tear or subduciton of oceanic boundaries (transform faults; Chapin, 2012) provide linear features that can allow melt production in a restricted region independently of the overriding lithosphere.

Any hypothesis that accurately explains the CMB needs to account for the spatial and temporal magmatic activity. The Twin Lakes Batholith (TLB) ~20 miles south of Leadville, CO places important constraints on the origin of the CMB. Detailed ⁴⁰Ar/³⁹Ar and U-Pb geochronology reveal a protracted magmatic history between 65 and 30 Ma. In this single batholith we have identified at least 7 temporally distinct intrusions with an apparent magmatic gap between 55 and 43 Ma. The TLB is a capsule of the entire CMB as geochronology data compilation demonstrates that magmatism initiated at ~70 Ma throughout the length of the CMB and that other regions comprise multiple and protracted magmatic activity with an associated gap. The Front Range intrusions are multiple and range from 75 to 58 Ma in a fairly restricted area.

Simultaneous initiation of magmatism along the CMB and protracted magmatism in individual areas is difficult to reconcile with plate motion considerations if the primary control of the CMB is within the Farallon slab. Over a 30 Ma magmatic period the North American plate would have drifted nearly 900 km relative to the asthenosphere and would seem to predict geographically widespread magamtism rather than a lineament. Structural control in the continental lithosphere could account for the CMB within the constraints of relative plate motions.

Chapin, C.E., Origin of the Colorado Mineral Belt. Geosphere. v. 8, no. 1. Feb. 2012