U-PB GEOCHRONOLOGY OF TITANITE AND ZIRCON TO DATE METAMORPHIC RESET OF 1.6 GA METAMORPHIC PLUTONIC ROCKS NEAR BUENA VISTA, COLORADO

The Sawatch Mountain Range in central Colorado contain metamorphic rocks uplifted during the Laramide Orogeny related to the Paleoproterozoic Colorado province. These rocks are broadly interpreted to be part of the Gunnison-Salida arc terrane of central Colorado which is composed of the Dubois and Cochetopa successions of upper green schist to lower amphibolite facies rocks and later intruded metagranite/ granitic gneisses. Here we seek to determine the age range of these rocks near the contact of the 35 Ma Mt. Princeton batholith. Our intent is to determine if these rocks underwent substantial contact metamorphism and determine whether partial melting of country changes the magma composition near the contact. Fifteen samples of the metamorphic rocks were dated by U-Pb zircon and U-Pb titanite geochronology using laser ablation ICPMS. The purpose of our study is to determine whether zircon and titanite along the interface with the batholith were entrained during partial melting. Additionally, fourteen samples from the Mt. Princeton Batholith were analyzed along two transects with varying distance from the interface. Separating partially reset titanite from new titanite will show the difference in timing between the initial rock formation and the timing of the most recent metamorphic event. We compare these ages with U-Pb ages of titanite and zircon in the plutonic rocks to determine the influence of partial melting of the metamorphic rocks on magma chemistry. These data suggest that the metagranites nearest to the batholith interface were partially reset by intrusion with intercept ages of 35 Ma and ~1400 Ma, but cores from these zircons retain the crystallization age of the granite at ~1400 Ma. Titanite is absent from the metamorphic rocks along the interface suggesting it was preferentially melted/ recrystallized during intrusion.