CONSTRAINTS ON THE NATURE AND TIMING OF METAMORPHISM AND DEFORMATION OF STAUROLITE + ANDALUSITE ZONE METAPELITIC ROCKS IN BIG THOMPSON CANYON, NORTHERN COLORADO FRONT RANGE

Garnet, staurolite and andalusite-bearing metapelitic rocks in the vicinity of Viestenz-Smith Park along Hwy. 34 in Big Thompson Canyon in the Northern Colorado Front Range record the effects of multiple phases of deformation and metamorphism. This study utilizes field observations, petrographic analysis, along with EPMA monazite and xenotime petrochronology, to constrain the relative and absolute timing of metamorphic mineral growth and deformational events.

The rocks contain evidence for three phases of deformation. The oldest resulted in the dominant foliation in the rock. It was then folded and crenulated by two younger phases of deformation, Both phases have SE-trending, moderately plunging fold axes. An incipient to moderately developed axial planar foliation resulted from the second phase of deformation.

Muscovite, biotite, quartz, and plagioclase are the dominant foliation-forming minerals with garnet, staurolite, and andalusite occurring as porphyroblasts. Garnet is found as inclusions in both staurolite and andalusite. Crenulated inclusion trails in the porphyroblast phases indicate that these minerals grew after the second phase of deformation. Pseudomorphs consisting of finely crystalline muscovite are commonly present, some with a cross-shape suggesting replacement of staurolite. The pseudomorphs are not pervasively deformed.

Monazite crystals are commonly oriented parallel the dominant foliation. EPMA monazite dates range from ca. 1750 Ma to ca. 1700 Ma, however three distinct domains are delineated. The oldest domain is ~ 1750 Ma. These dates are from the cores of monazite grains.. The mantles of these monazite crystals yield ages ca. 1720-1725 Ma. Thin rim overgrowths on the monazite crystals yield ages of ~ 1700 Ma. These data suggest that the rocks were deformed prior to ~ 1720 Ma.

Xenotime grains in these rocks are small, commonly on the order of 5-10 microns or less. This size makes them difficult to date. Preliminary xenotime dates range from ~1700 Ma to the 1400’s. This range suggests that these rocks remained relatively hot and were reactive into the Mesoproterozoic.