U–Th–Pb monazite constraints on the timing of Paleoproterozoic metamorphism in Big Thompson Canyon, Colorado Front Range

Abstract

Paleoproterozoic supracrustal rocks in Big Thompson Canyon (BTC), Colorado Front Range, are some of the oldest known regionally metamorphosed rocks that record a coherent isograd sequence (Mahan et al., 2013). Regionally metamorphosed rocks are able to keep a usable record of island and continental arc accretion, crustal thickening, and burial metamorphism (Mahan et al., 2013). Metapelites from Big Thompson Canyon allow for a rare opportunity to constrain the pressure–temperature–time (P–T–t) evolution in this region of the Colorado Front Range. Petrographic characterization was performed on twenty-seven metapelites collected from the biotite/garnet-, staurolite-, andalusite-, sillimanite-, and K-feldspar-zones in Big Thompson Canyon. Bright-phase element mapping using automated mineralogy of ten key samples confirmed the presence of monazite in four of the five isograd zones in this study. Bulk-rock geochemical data in combination with high-precision, phase diagram-based thermobarometry for all five isograd zones was used to determine the conditions of peak metamorphism and the relative stability of mineral assemblages as functions of P and T conditions for each zone. To better understand monazite intra-grain compositions, and their relationships to calculated ages, detailed element mapping of U, Th, Pb, Ce, and Y using an electron probe microanalyzer (EPMA) was performed. Using the element maps of monazites, specific data point locations were chosen from each monazite to perform geochronology. U–Th–Pbtotal in–situ monazite geochronology results using the EPMA show that there are two distinct compositional age groupings. One of these age groupings is within the Mazatzal Orogeny (~1.7–1.6 Ga) and the second is within the Granit-Rhyolite Province or Picuris Orogeny (~1.4–1.3 Ga). Phase-diagram based thermobarometry yielded an upward progression of P–T conditions from east to west through Big Thompson Canyon. Metamorphic conditions are estimated to be ~420–550 °C and >2–4.5 kbar for lower grade isograd zones (biotite/garnet) and increase up to ~560–735 °C and ~2.8–8.8 kbar for the highest grade isograd zone (K-feldspar). Therefore, there is an estimated upward P–T progression from east to west in BTC of ~420–735 °C and >2–8.8 kbar. Based on the progression of isograd zones through P–T space and all isograd zones yielding ages within the Mazatzal Orogeny, it can be concluded that this metamorphic event was regional style metamorphism. On the other hand, two of the five isograd zones yielded compositional ages that align with the Picuris Orogeny. These ages that fall into the much further south Picuris Orogeny may be the result of proximal magmatism that caused intrusions, crustal underplating and melting in the same region as Big Thompson Canyon.