Timing of Metamorphism, Deformation, and Anatexis of Big Thompson and Poudre Canyon Migmatites, Colorado, Northern Front Range

The high grade pelitic rocks of the Big Thompson and Poudre Canyons of Colorado’s northern Front Range experienced a complex metamorphic and deformation history. The metapelites consist of knotted biotite gneiss, quartzofeldspathic gneiss, and quartzites that are thought to represent turbidity current deposits in either a fore-arc or back-arc basin as recently as ~1755 Ma. The rocks were significantly deformed and metamorphosed during the Yavapai Orogeny (~1.71-1.68 Ga) and intruded by Paleoproterozoic granitoids of the Routt Plutonic Suite. They were then later intruded by 1.4 Ga granites of the Berthoud Suite. The Big Thompson Canyon preserves a set of index mineral zones, the highest grade of which are migmatitic. To better understand the complex deformation and thermal histories of the Northern Front Range, several electron microprobe U-Pb dates were gathered from monazite across five field sites in migmatite grade rocks. Cumulative probability plots suggest high probability dates for six periods of Paleoproterozoic monazite growth at ~1750 Ma, ~1725 Ma, ~1715 Ma, ~1690 Ma, and ~1650 Ma. The ~1750 Ma monazite growth records diagenesis in the basin. The ~1725 Ma monazite is syn-kinematic, recording the timing of D1. Monazite growth at ~1715 Ma and ~1690 Ma monazite was found to be locally syn-kinematic or post-kinematic, suggesting that metamorphism persisted beyond deformation for some localities. ~1650 Ma growth was recorded by only a few grains at two field sites. This suggests that either the Yavapai Orogeny (~1.71-1.68 Ga) continued longer than otherwise thought or these grains record monazite growth Mazatzal Orogeny (~1.65-1.6 Ga). Mesoproterozoic monazite growth was recorded at three of the five field sites. The average date of monazite growth is 1437±4 Ma which coincides with the emplacement of 1.4 Ga Berthoud suite. The Estes Park study site had extensive monazite growth and/or recrystallization at this time with higher probability dates at ~1457 Ma, ~1446 Ma, 1434 Ma, and ~1420 Ma. These higher probability dates may be recording growth from heat provided by multiple episodes of magma emplacement. Yttrium and Eu geochemistry for all monazite analyses shows a general increase in Y and a decrease in Eu with time. The increase in Yttrium from cores to rims is consistent with the breakdown of garnet at ~1710 Ma and/or ~1690 Ma. The decrease in Eu suggests feldspar crystalized during monazite growth. Migmatite leucosome is parallel to S1, contains F1 folds, and has S1 parallel boudins. These syn-kinematic structures constrain the timing of anatexis to D1. Due to monazite geochemistry and D1 related structures, anatexis of the Big Thompson and Poudre Canyon migmatites must have occurred by ~1725 Ma