New Mexico Geological Society Annual Spring Meeting — Abstracts

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 subducting Farallon Plate is a puzzling enigma of the Laramide orogeny and many models for the origin of the CMB have been presented. End-member hypotheses place geographic control of the CMB within the crustal lithosphere versus the 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 subduction of oceanic boundaries (e.g transfor fault) provide linear features that can allow melt production in a restricted region.

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 region we have identified at least 7 temporally distinct intrusions (~65 -57 Ma and ~43-30 Ma) with an apparent magmatic gap between 55 and 43 Ma. The TWB is representative of the entire CMB in that geochronology data compilation demonstrates that magmatism initiated at ~70 Ma throughout the length of the CMB with no apparent sweep of magmatism and that other regions also record 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 length of the CMB, plus the protracted magmatism in individual areas, are difficult to reconcile with plate motion considerations if the primary control of the CMB is within the Farallon slab. Over the 30 Ma magmatic period the North American plate would have moved nearly 900 km relative to the asthenosphere (hot spot reference frame) which would seem to predict geographically widespread magmatism rather than recurring magmatism at specific sites along the CMB. Alternatively, structural and compositional controls within the continental lithosphere could account for observed magmatic patterns in the CMB.