Type of Resources
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.