A review and revision of Late Mesozoic to Cenozoic pluton chronology in the Rico Mountains, southwestern Colorado
— D.A. Gonzales

Latest Mesozoic to Cenozoic plutonic rocks have had a profound influence on the landscape, mineralization, and uplift history in the Rico Mountains. This record is defined by emplacement of numerous stocks, sills, and dikes of felsic to mafic intrusive rocks. Plutons of diorite to monzonite porphyry emplaced in a narrow span of time at ~68 Ma dominate the record. The preservation of 1748 to 1381 Ma xenocrystic zircons in the ~68 Ma plutonic rocks provides further evidence for the involvement of Proterozoic basement in the generation of Laramide magmas in the western San Juan Mountains. Hypabyssal monzonite plutons were emplaced at ~4 Ma in the Rico Mountains, contemporaneous with a period of elevated geothermal gradient accompanied by production of a deep-seated stock with porphyry Mo mineralization. The Pliocene plutons formed in an interval marked by intrusion of 7 to 4 Ma alkaline mafic rocks along an incipient zone of extension that extends from Rico to Placerville. The 68 to 4 Ma plutonic events in the Rico Mountains mimic a long-term shift to more bimodal magmatism in the western San Juan Mountains. Injection of mantle melts from 25 to 0.6 Ma during incipient crustal extension accompanied elevated thermal gradients over the region, as evidenced by resetting of cooling ages in some Laramide intrusive rocks. The higher geothermal gradient and Pliocene magma production in the western San Juan Mountains marked the continued involvement of mantle melts in the production of crustal magmas.