CONSTRAINTS ON THE TIMING OF METAMORPHISM, DEFORMATION, AND ANATEXIS OF BIG THOMPSON AND POUDRE CANYON MIGMATITES USING EPMA MONAZITE PETROCHRONOLOGY

The high-grade metapelitic rocks of Big Thompson and Poudre Canyons in the northern Colorado Front Range experienced a complex metamorphic and deformation history. The region was deformed and metamorphosed during the Yavapai Orogeny, intruded by Paleoproterozoic granitoids of the Routt Plutonic Suite, and subsequently intruded by 1.4 Ga granites of the Berthoud Suite. Monazite is reactive during a range of temperatures, including medium grade sub-solidus reactions and high temperature melting reactions, making it an ideal mineral for studying and dating deformation and high-grade metamorphism with anatexis. To better understand the timing of deformation and anatexis of the Northern Front Range, monazite was fully characterized regarding chemistry and zoning at five migmatite-grade sites.

U-Th-total Pb monazite data from 4 of the 5 study sites reveal multiple phases of Paleoproterozoic monazite growth between ~1750 Ma, to ~1650 Ma, with peaks at ~1750 Ma, ~1725 Ma, ~1715 Ma, ~1690 Ma, and ~1650 Ma. Mesoproterozoic monazite growth was pronounced at one sample site near Estes Park, CO and was also found in rims from two other sites. The Longs Peak pluton is near the Estes Park site. Mesoproterozoic monazite growth is 1437±4 Ma with ages ranging from ~1457 Ma to ~1420 Ma. These ages may represent monazite growth from multiple heating episodes during magma emplacement.

Migmatite leucosome was found as boudins parallel to S₁. These syn-kinematic structures suggest anatexis is synchronous with D₁. Monazite geochemistry, in conjunction with syn-kinematic structures suggests anatexis of the Big Thompson and Poudre Canyon migmatites occurred by ~1725 Ma. Monazite growth at ~1715 Ma and ~1690 Ma was found to be locally syn-kinematic or locally post-kinematic, suggesting that metamorphism persisted beyond deformation for some localities.

Monazite grains in the study have complex zoning with respect to U, Th, Y, and Ca. Yttrium concentrations generally increase from cores to rims suggesting monazite growth following the breakdown of garnet at ~1710 Ma and/or ~1690 Ma. Spot analyses of Eu show a decrease in concentration with time, suggesting feldspar crystallization during monazite growth.

Monazite from this study show that the Yavapai orogeny was a complex and prolonged event that may have lasted up to 100 Ma.