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Protracted multipulse emplacement of a post-resurgent pluton: The case of Platoro caldera complex (Southern Rocky Mountain volcanic field, Colorado)

November 7, 2019

Many eroded calderas expose associated postcollapse plutons, but detailed fieldwork‐supported
studies have rarely focused on the internal structure that can contribute to understanding of emplacement dynamics. The Alamosa River monzonite pluton is a postcollapse intrusion at the Platoro caldera complex that erupted six large ignimbrites between 30.2 and 28.8 Ma in the Southern Rocky Mountains volcanic field. Magnetic fabrics in this intrusion indicate the pulsed emplacement of a vertically extensive pluton. The magmatic pulses are documented by three concentric domains of magnetic foliations elongated in ~NE‐SW direction, corresponding to structural trends at the Platoro caldera complex and preexisting regional structures. As no evidence for deformation of wall rocks and the adjacent resurgent block has been identified, we interpret the Alamosa River pluton as a postresurgent intrusion. The space‐opening process
involved magmatic stoping and small‐scale magma wedging. New SHRIMP‐RG U/Pb zircon dates (28.98 ±0.18, 27.42 ± 0.35, and 27.32 ± 0.38 Ma) suggest a magmatic lifespan of ~1.7 My for the Alamosa River pluton. Our results indicate that postcaldera magmatism includes pulsed and protracted activity from large intracaldera resurgent plutons to smaller postresurgent stocks and sheeted complexes. As demonstrated by the Alamosa River pluton, some intrusions are emplaced shortly after collapse and resurgence, but postcaldera volcano‐plutonic systems may remain active for several million years or more. We also suggest that subvolcanic magma bodies may be assembled incrementally and that the record of early composite magma lenses preserved as magma wedges are later obliterated by convective flowage and crystallization.

Citation Information

Publication Year 2019
Title Protracted multipulse emplacement of a post-resurgent pluton: The case of Platoro caldera complex (Southern Rocky Mountain volcanic field, Colorado)
DOI 10.1029/2019GC008477
Authors Filip Tomek, Amy K. Gilmer, M. S. Petronis, Peter W. Lipman, M. S. Foucher
Publication Type Article
Publication Subtype Journal Article
Series Title Geochemistry, Geophysics, Geosystems
Series Number
Index ID 70206838
Record Source USGS Publications Warehouse
USGS Organization Geosciences and Environmental Change Science Center