GEO AND THERMOCHRONOLOGICAL EVICENDE FOR THE EMPLACEMENT AND EXHUMATIONAL HISTORY OF THE TWIN LAKES BATHOLITH: IMPLICATIONS FOR THE LARAMIDE OROGENY

New geo and thermochronological data from the Twin Lakes pluton 20 miles south of Leadville, CO suggests 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 known as the Aspen Anomaly. LA-ICPMS U/Pb zircon data show that the Twin Lakes pluton is composite with at least 5 intrusions emplaced at ~63, 60, 55, 42 and 40 Ma. Older intrusions appear restricted to the north and west and give biotite 40Ar/39Ar ages nearly concordant with zircon ages. Farther south, the 40 Ma Huron Peak samples yield discordant biotite and K-feldspar 40Ar/39Ar ages that correlate with elevation. Samples from the highest elevations of a 1.2 km vertical traverse at Huron Peak record a 4 Ma difference between the biotite and K-feldspar ages (40 and 36 Ma, respectively), however samples from the bottom of the traverse are 9 Ma apart (38 and 29 Ma, respectively). This age variation indicates pluton intrusion near the 300°C isotherm and suggests a remarkably deep (ca. 8 to 12 km) emplacement. Preliminary Al-in-hornblende thermobarometry data agree with this observation, indicating an emplacement depth of ~12 km. K-feldspar analysis places the pluton at ~6 km 29 Ma ago, whereas apatite fission track ages indicate exhumation to 3-4 km by 16 Ma implying an exhumation rate of ~200 m/Ma. The contrast in biotite ages across the entire batholith indicates a tilted crustal section with structurally higher samples to the northwest .

The presence of multiple intrusions ranging from 63 to 40 Ma at Twin Lakes challenges the long held view that COMB magmatism records the leading edge of the Farallon Slab as it advanced and subsequently rolled back. These new data are more consistent with the idea that the COMB magmatism reflects a long-lived zone of fertility and/or pluton conduit throughout the early Tertiary. If plutonism at Twin Lakes is tied to the Aspen Anomaly, these results suggest that the anomaly is a long-lived chemical/tectonic feature.