24 Tectonics, Volcanism, and Sediment Dispersal Within and Proximal to the Rio Grande Rift and Southern Rocky Mountains (Posters)

This session integrates research on coupled tectonics, volcanism, and sediment dispersal in the Rio Grande Rift and southern Rockies. We examine how extensional faulting controls volcanic processes and reshapes sediment routing from Colorado through Texas.

Tuesday, 19 May 2026
8:00 AM - 5:30 PM
Alvarado D/E

Oligocene Bimodal Magmatism in the Sangre de Cristo Range, Colorado: Geochronology, Geochemistry, and the Onset of Rio Grande Rift Extension

Paper number: 24-4
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

Oligocene intrusions along the western flank of the Sangre de Cristo Range record a temporally and compositionally distinct episode of bimodal magmatism that predates commonly cited estimates for Rio Grande Rift initiation. Combined new and previously published zircon U-Pb geochronology data from 13 mafic to felsic intrusions emplaced into Proterozoic crystalline basement and Pennsylvanian Minturn Formation strata indicate emplacement between ~33-26 Ma. Field mapping, whole-rock geochemistry, and amphibole-plagioclase thermobarometry provide additional constraints on emplacement conditions and magmatic processes. Intrusions occur as small stocks, sills, and dikes distributed along a NNE-SSW trend on the western flank of the range, and the largest granitic stocks are emplaced along the crest. Intrusions along the western flank spatially overlap with Laramide contractional structures, and at Mosca Creek and Crestone Creek, intrusions are localized along thrust faults. These observations suggest a primary control of crustal structures on magma ascent pathways during early extension.  Whole-rock geochemical data define a distinctly bimodal compositional distribution, with mafic samples (49-53 wt% SiO2) following tholeiitic to transitional trends and felsic samples (>55 wt% SiO2) exhibiting calc-alkaline affinities. Trace-element patterns reflect mantle-derived mafic magmas with variable crustal interaction and felsic compositions consistent with crustal melting and/or fractionation. Amphibole-plagioclase thermometry and Al-in-hornblende barometry from two samples (21-9-GC33 and 23-9-Mosca71) yield crystallization temperatures of ~700–725° C and shallow emplacement pressures of ~1.7–1.8 kbar (~6–7 km depth). Large populations of inherited zircon are preserved in felsic and mafic samples and are dominated by Paleoproterozoic ages consistent with local basement sources with minor contributions of Mesoproterozoic, Neoproterozoic, and Cambrian grains. Aeromagnetic anomalies spatially coincident with mapped intrusions and contact aureoles suggest subsurface magmatic bodies larger than surface exposure.

            Although broadly coeval with post-caldera intrusive activity in the Platoro volcanic complex of the San Juan Volcanic Field (~29–24 Ma) these intrusions are distinctly more bimodal in composition. The coexistence of primitive mafic and crustal felsic magmas reflects fault-controlled, decompression-driven melting more consistent with active lithospheric extension than waning arc or caldera systems. The intrusive episode overlaps early basaltic lava flows of the Hinsdale Formation (~27–25 Ma) erupted along the rift-axis in the San Luis Basin, but extends to ~33 Ma, pushing back the record of syn-rift magmatism in the northern Rio Grande Rift.

Geological Society of America Abstracts with Programs. Vol. 58, No. 4

doi: 10.1130/abs/2026RM-410646

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