New Mexico Geological Society Annual Spring Meeting — Abstracts

New geo and thermochronological data from the Twin lakes pluton 20 miles south of Leadville, CO reveals a complex intrusion history followed by protracted exhumation. These results have important regional implications for the Laramide orogeny, Colorado Mineral Belt (COMB) plutonism, central CO exhumation, and possible links to a present-day zone of low velocity upper mantle know as the Aspen Anomaly. LA-ICPMS U/Pb zircon data show that the Twin Lakes pluton is composite with at least four intrusions of ~63, 57, 43 and 40 Ma. A fifth 36 Ma intrusion is indicated by argon mica ages. K-feldspar argon and apatite fission track (AFT) thermochronology constrain the post emplacement thermal history. K-feldspar minimum ages along a vertical transect near Huron Peak young from 36 to 31 Ma over 1.2 km of elevation. Assuming uniform exhumation, the average denudation rate for this period was ~250 m/Ma. This rate is apparently maintained until ca. 20 Ma based on AFT ages near the Twin Lakes reservoir. Assuming a 30° C/km geothermal gradient and a K-feldspar closure temperature of 175° C a paleodepth of 5.5 km at 31 Ma is estimated for the base of Huron Peak.

The presence of multiple intrusions ranging from 63 to 36 Ma at Twin Lakes challenges the long held view that a systematic temporal and spatial pattern of magmatism within the COMB may record the advance and subsequent rollback of the Farallon slab. This is because nearly the entire age range of Laramide plutonic activity is hosted in the Twin Lakes pluton, thus its seems unlikely that magmatism accurately depicts the leading edge of the slab at any given time. These new data are more consistent with the idea that the COMB magmatism reflects a longlived zone of fertility and/ or pluton conduit throughout the early Tertiary. The data may also suggest that the Aspen Anomaly contributed to continuous CO magmatism from ~70 Ma to the present. In addition to tectonic implications, the data show that existing compilations based on argon and zircon fission track ages are inaccurate representations of COMB magmatism as both systems are unable to resolve complexities of closely spaced multiple magma injections.