HEMATITE (U-TH)/HE THERMOCHRONOMETRY CONSTRAINTS ON FAULT SURFACE PROCESSES AND OPEN SYSTEM (HE) BEHAVIOR: AN EXAMPLE FROM THE FRONT RANGE, MORRISON, CO

Fault surface minerals such as hematite record fluid-rock interaction, strain, and/or frictional heat during fault slip. Hematite is amenable to (U-Th)/He (He) thermochronometry and early applications to faults yielded scattered intrasample dates. This suggests aspects of open system behavior, such as polydomain, deformation, and geochemical controls on He diffusion, are not understood. For example, striated, locally high gloss, hematite-coated minor faults cut Paleoproterozoic gneiss ~5-10 m below the Great Unconformity in the Front Range, CO. Hematite alteration is pervasive in the gneiss. Prior work (Geissman & Harlan, 2002) shows a regionally-extensive, secondary, chemical remnant magnetization (CRM) carried by hematite was acquired during the Permo-Carboniferous Reverse Superchron, coincident with the Ancestral Rocky Mountains orogeny (ARMO). However, fault surface hematite He dates are ≥125 Myr younger than the CRM. Two samples yield overlapping individual dates of ~138-27 Ma (n=17) and ~114-48 Ma (n=17). 82% of combined analyses have a ~60 Myr date spread and low Th/U ratios (1-4). A subset (18%) comprise the youngest dates (~52-27 Ma) and high Th/U ratios (6-15).

Interpreting hematite He data patterns and inferring fault slip timing and process require additional microtextural, grain size, and thermochronometric data. At the Morrison site, scanning electron microscopy shows faults have a matrix of foliated, sub-µm-thick hematite plates with minor quartz clasts and µm-scale monazite. Hematite cataclasite is present at slip surfaces. Grain size measurements yield hematite He closure temperatures of ~60-120 °C, ≥ the apatite He system. The mean apatite He date from unaltered gneiss is 47.1 ± 5.5 Ma (1σ std dev; n=6). Fault data support synkinematic hematite formation ≥138 Ma. If these faults were initially active during the ARMO, then they were reactivated during Laramide time. Scattered intrasample hematite He dates over similar Th/U imply polycrystalline aliquots capture variable He loss from variable grain (domain) size distributions during exhumation. Laramide-age fault slip may cause this grain size reduction, domain redistribution, and enhanced He loss. Eocene, high Th/U He dates may reflect monazite-derived Th or aspects of open system behavior that are still not fully understood.