NEW THERMOBAROMETRIC DATA FROM THE NORTHERN PARK RANGE, COLORADO: IMPLICATIONS FOR PALEOPROTEROZOIC TECTONIC EVOLUTION

The Farwell Mountain–Lester Mountain suture zone (FMLM) has been interpreted as a major tectonic boundary in the northern Park Range, CO, separating the ~1.78–1.76-Ga Green Mountain arc north of the suture from the ~1.76–1.74 Ga Rawah arc to the south (Tyson et al., 2002). A commonly cited criterion for this Paleoproterozoic arc-arc suture is the discontinuity in metamorphic grade across the boundary. Greenschist-facies rocks are exposed to the north at Farwell Mountain, whereas upper amphibolite-facies rocks occur south of the inferred boundary at Mica Basin. New thermobarometric data suggest that metamorphic conditions similar to those at Mica Basin are found north of the inferred FMLM. From Davis Peak, CO, north to Blackhall Mountain, WY are exposed a series of metasedimentary and metavolcanic rocks that are intruded by a suite of granitic plutons. Metasedimentary rocks are chiefly quartzose metapelites and paragneisses with rare intercalated calc-silicate rocks. Metapelites contain the assemblage: garnet + biotite + plagioclase + K-feldspar + quartz ± sillimanite, whereas epiclastic paragneisses contain the assemblage: garnet + hornblende + quartz + plagioclase ± biotite ± gedrite. Metavolcanic rocks are characterized by texturally variable amphibolites with the assemblage: hornblende + plagioclase ± quartz ± garnet ± clinopyroxene. Garnet-hornblende-plagioclase thermobarometry indicates P–T conditions in excess of 6 kb and 600°C, supported by garnet-biotite temperature estimates of 677± 25°C. These results are consistent with P–T estimates and metamorphic assemblages from Mica Basin (Foster et al., 1999). Furthermore, P–T estimates from Farwell Mountain are nearly 100°C lower than conditions reported to the east at Davis Peak and Mica Basin. Several possible explanations for these relationships include: 1) the eastward projection of the FMLM actually lies to the south of Mica Basin; 2) the throw along FMLM is highly variable, with apparent displacement near Farwell Mountain/Lester Mountain greater than in the Davis Peak/Mica Basin area; or 3) complex metamorphic field gradients exist obfuscating regional interpretations. Further geologic/structural mapping and analytical work are required to accurately distinguish between these multiple hypotheses.