GEOCHEMISTRY OF THE BIG THOMPSON CANYON PALEOPROTEROZOIC GRANITOIDS, NORTHERN COLORADO FRONT RANGE: IMPLICATIONS FOR TECTONIC ACTIVITY AND CRUSTAL GROWTH AT ~1.7 GA

The Big Thompson Canyon area of the northern Colorado Front Range contains multiple granodioritic and tonalitic-trondhjemitic plutonic rocks emplaced during the Paleoproterozoic growth of southern Laurentia. The granodiorite, thought to be associated with the c. 1.71 Ga Boulder Creek Granodiorite, is biotite and muscovite-bearing and crops out as stocks and batholiths in the western part of the canyon. The tonalitic-trondhjemitic rocks intruded c. 1.73 Ga and are collectively known as the Big Thompson Canyon Tonalite (BTCT). The BTCT is also biotite and muscovite-bearing and most typically crops out as large sills in the eastern part of the canyon. Bulk chemical data, including major and trace elements, were obtained for thirteen samples to better understand the relationship between these rocks and their role in regional crustal growth.

Incompatible element abundances in granodiorite, normalized to primitive mantle, show enrichment in Cs, Rb, Th, U, K, and Pb, and depletion in Nb, Ta, Sr, P and Ti. In contrast, BTCT displays enrichment in Cs, Rb, Ba, U, K, Pb, Sr, and Zr and depletion in Th, Nb, Ta, and Ti. Both lithologies plot in the volcanic arc granite field of the Rb versus Nb+Y diagram. Normalized to chondrite, both rocks are LREE-enriched and HREE-depleted, with the BTCT much more depleted in HREEs and granodiorite possessing a moderately negative Eu anomaly. Overall, concentrations of La, Yb, Y, and Sr in the granodiorites are consistent with a typical arc signature, but for the BTCT, are similar to concentrations in many adakites and TTG (Tonalite-Trondhjemite-Granodiorite) suites. Granodiorite geochemistry suggests it formed by flux melting of the upper mantle in a subduction zone with plagioclase fractionation of the resulting magma, while BTCT geochemistry suggests formation by the partial melting of a garnet amphibolite or eclogite facies mafic source with influences from a subduction zone. Because the granodiorites and the BTCT are geochemically distinct, collectively they do not comprise a TTG suite. Possible tectonic interpretations are that both the granodiorite and the BTCT formed in an arc setting, with the granodiorite resulting from typical subduction processes, while the BTCT formed from either 1) partial melting of a subducted slab, or 2) partial melting of mafic arc material.