PROVENANCE OF VOLCANIC CLASTS AS A GUIDE TO TECTONIC EVOLUTION: AN EXAMPLE FROM THE SANTA FE GROUP, SAN LUIS BASIN, COLORADO

Geochemical and geochronologic studies of volcanic clasts incorporated in the Tertiary Santa Fe Group (SFG) sedimentary rocks of the Culebra graben, San Luis Basin, Colorado, demonstrate the utility of clasts within sedimentary sections to determine provenance and constrain the timing of regional tectonic events. Most SFG volcanic clasts are cobble-sized, porphyritic and highly potassic despite their wide compositional range. Potential source areas for the clasts include Spanish Peaks (SP) and Mount Mestas (MM) to the east, San Juan volcanic field (SJVF) to the west, and the Thirtynine Mile volcanic field (TMVF, part of the central CO volcanic field), to the north. Whole-rock chemistry and trace element data of the SFG clasts show similarities to TMVF and SJVF rocks. SFG amphibole (amp) and biotite chemistry are inconsistent with SP samples. Thermobarometric calculations suggest that most SFG volcanics underwent a three-stage crystallization history; ~1120°C at 7-10kb, ~1100°C at 2.3-4.6 kb with amp formation near 800°C at 3kb. U-Pb geochronology of zircon rims, and ⁴⁰Ar/³⁹Ar plateau ages for biotite and amp, yield ages of 36-29 Ma consistent with ages of the TMVF and the Conejos Formation of SJVF. However, this predates SP (~26-21 Ma) and MM (~25 Ma) thus excluding them as potential SFG source areas. A likely scenario is that the Central CO volcanic field originally extended proximal to the current location of the Basin and local delivery of volcanic clasts was from the N and NE prior to the uplift of the Culebra and Sangre de Cristo Ranges. This study demonstrates that volcanic clasts are useful indicators of provenance in volcaniclastic sediments and provides an additional tool for reconstructing geologic histories. Coupled with sedimentological and geochronologic constraints, quantitative compositional and mineral chemical data can be used to constrain temporal and spatial basin history and associated regional uplift.