12 Tectonics, Volcanism, and Sediment Dispersal Within and Proximal to the Rio Grande Rift and Southern Rocky Mountains

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.

Monday, 18 May 2026
1:30 PM - 4:30 PM
Alvarado A

Lower crustal xenoliths from Silver Mountain, eastern Sangre de Cristo Mountains, Colorado: A window into the lithosphere during early Rio Grande Rift initiation

Paper number: 12-1
Program: Technical Sessions
Time: 1:35 PM – 1:55 PM

Authors

Abstract

Direct samples of the lower crust are rare in the southern Rocky Mountains, limiting our understanding of lithospheric architecture during Oligocene magmatism and the onset of Rio Grande rift extension. Here we present the first detailed petrologic study of crustal xenoliths from the radial dike swarm at Silver Mountain in the eastern Sangre de Cristo Mountains, Colorado. Mafic xenoliths hosted in intermediate composition dikes range from gabbro to amphibole clinopyroxenite and include both undeformed cumulates and foliated samples that record lower crustal strain. Mineral chemistry and very low Ni contents indicate these are crustal cumulates rather than mantle-derived material, recording Oligocene magma storage and fractionation in the middle-to-lower crust. Clinopyroxene (Ti-Al-augite) and amphibole (pargasitic kaersutite) indicate calc-alkaline affinity, and amphibole compositions indicate equilibration at middle-to-lower crustal depths. Sr and Nd isotopic compositions of the xenoliths are more enriched than their host dikes, indicating they represent pre-existing lower crust rather than cognate cumulates. A ⁴⁰Ar/³⁹Ar age of 36.6 Ma from a petrologically similar hornblendite at the nearby Black Hills suggests the Silver Mountain magmatic system may predate the main Spanish Peaks intrusive episode (~22–25 Ma), potentially recording early extension-related magmatism contemporaneous with the regional ignimbrite flare-up. This work provides the first direct constraints on lower crustal architecture along the eastern margin of the Rio Grande rift, with implications for understanding regional magmatic fertility.

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

doi: 10.1130/abs/2026RM-410618

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