Provenance Studies of Santa Fe Group Volcaniclastic Sedimentary Rocks in the San Luis Basin, Colorado: A Guide to Basin Evolution

The San Luis basin (SLB) is a major extensional feature of the northern Rio Grande rift and stretches approximately 240 km from south-central Colorado to north-central New Mexico. The basin is bordered on the east by the Sangre de Cristo Mountains and on the west by the San Juan Volcanic Field. The Santa Fe (SF) Group sediments were deposited in the basin during uplift of the Sangre de Cristo Mountains and subsequent eastern basin formation. The SF Group contains abundant volcaniclastic rocks that can provide constraints on the timing of uplift and eastern SLB formation through chemical characterization and provenance identification.

Several potential sources with chemical and age data exist for the SF Group volcaniclastic rocks such as Spanish Peaks and the San Juan Volcanic Field. Chemical analysis of the clasts can be useful to distinguish their provenance. Volcaniclastic rocks were collected from SF Group sediments exposed near Fort Garland, CO and were chemically characterized using electron microprobe analyses (EMPA) for mineral chemistry as well as XRF and ICP-MS for whole rock and trace element geochemistry.

Whole rock analyses show the SF Group contains clasts of trachybasalt, basaltic trachyandesite, trachyandesite, and trachydacite with textures ranging from porphyritic to fine-grained. The clasts are composed primarily of plagioclase and amphibole phenocrysts in a fine-grained matrix of glass or plagioclase with variable amounts of biotite, pyroxene, (titano)magnetite, pyrite, apatite, titanite, and zircon. Amphiboles are primarily magnesiohastingsite (ideally NaCa₂(Mg₄Fe³⁺)Si₆Al₂O₂₂(OH)₂) and the biotites have average values of Mg/Mg+Fe = 0.6, Ba = 0.08 apfu, and Ti = 0.5 apfu. Chemical distinctions were observed among the amphiboles and biotites when Si, Al, and Ti were plotted against Mg/Mg+Fe. The distinct chemical fields between the samples are useful when comparing them to possible source rocks to determine provenance and to constrain the timing of basin evolution.