Postcaldera intrusive magmatism at the Platoro caldera complex, Southern Rocky Mountain volcanic field, Colorado, USA

Abstract

The Oligocene Platoro caldera complex of the San Juan volcanic locus in Colorado (USA) features numerous exposed plutons both within the caldera and outside its margins, enabling investigation of the timing and evolution of postcaldera magmatism. Intrusion whole-rock geochemistry and phenocryst and/or mineral trace element compositions coupled with new zircon U-Pb geochronology and zircon in situ Lu-Hf isotopes document distinct pulses of magma from beneath the caldera complex. Fourteen intrusions, the Chiquito Peak Tuff, and the dacite of Fisher Gulch were dated, showing intrusive magmatism began after the 28.8 Ma eruption of the Chiquito Peak Tuff and continued to 24 Ma. Additionally, magmatic-hydrothermal mineralization is associated with the intrusive magmatism within and around the margins of the Platoro caldera complex. After caldera collapse, three plutons were emplaced within the subsided block between ca. 28.8 and 28.6 Ma. These have broadly similar modal mineralogy and whole-rock geochemistry. Despite close temporal relations between the tuff and the intrusions, mineral textures and compositions indicate that the larger two intracaldera intrusions are discrete later pulses of magma. Intrusions outside the caldera are younger, ca. 28–26.3 Ma, and smaller in exposed area. They contain abundant glomerocrysts and show evidence of open-system processes such as magma mixing and crystal entrainment. The protracted magmatic history at the Platoro caldera complex documents the diversity of the multiple discrete magma pulses needed to generate large composite volcanic fields.

Supplemental Material: Deep resistivity geophysics of the San Juan–Silverton caldera complex, San Juan County, Colorado

“…The resistivity lows contrast with more resistive and presumably less-highly altered country rock that is distant from the hydrothermal conduit or the more highly mineralized parts of a concealed porphyry system beneath the resurgent dome. In addition, the geometry of the resistivity low could indicate a magmatic conduit that was used during pulsed magmatic inflation events that accompany resurgent doming that have been described in other similar volcanic systems (Acocella, 2019;Gilmer et al, 2021). The MT data also provide a glimpse into the deeper crustal lithologies and may be imaging the source areas for granitic plutons that underpin the multiple calderas of the southern Rocky Mountains volcanic field (Drenth et al, 2012;Lipman and Bachmann, 2015).…”