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

Geosphere
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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 geo-chronology 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 miner-alogy 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.

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Publication type Article
Publication Subtype Journal Article
Title Postcaldera intrusive magmatism at the Platoro caldera complex, Southern Rocky Mountain volcanic field, Colorado, USA
Series title Geosphere
DOI 10.1130/GES02242.1
Volume 17
Issue 3
Year Published 2021
Language English
Publisher Geological Society of America
Contributing office(s) Geosciences and Environmental Change Science Center, Volcano Science Center, Geology, Geophysics, and Geochemistry Science Center
Description 34 p.
First page 898
Last page 931
Country United States
State Colorado
Other Geospatial Southern Rocky Mountain volcanic field
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