Abstract

New U-Pb zircon and ⁴⁰AR/³⁹AR crystallization ages for latest Mesozoic to Cenozoic plutonic rocks in the western San Juan Mountains provide insight into the timing and interplay of mantle and crustal magmatism over the past 80 Ma. Subduction-driven magmatism in the Laramide (75-60 Ma) generated alkalic intermediate to mafic magmas. These were emplaced as laccoliths and stocks along northeast trends that were likely controlled by crustal-scale zones of weakness with Proterozoic ancestry. The transition from Laramide subduction to slab rollback and delamination was marked by incipient regional extension and widespread emplacement of plutons. Oligocene plutons of gabbro to granite were mostly peripheral to 29-27 Ma caldera complexes that formed over a regional subvolcanic batholith. Alkaline mantle magmas that accompanied the semi-continuous intrusion of shallow felsic to intermediate plutons from 25 to 4 Ma produced diatreme-dike complexes and dike swarms across the northern San Juan Basin. Emplacement of plutons from 75-4 Ma caused localized uplift that had a major influence on landscape evolution, and often were linked to zones of mineralization.

Late Mesozoic to Cenozoic plutonic events in the western San Juan Mountains record a long-term shift to more bimodal magmatism. Injection of mantle melts from 25 to 4 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 preservation of xenocrystic zircons in 75-4 Ma plutonic rocks provides evidence for variable magmatic conditions with an overall increase in the thermal state of Oligocene melts proximal to caldera systems and all plutons emplaced after 18 Ma. High geothermal gradients caused by the long duration of magmatism and increased mantle contributions may have permitted more prolonged assembly and slower cooling in some plutons.