CAN MARTITE FORMATION AND (U-TH)/HE CHRONOMETRY RECORD THE DEVELOPMENT OF ANCIENT NONCONFORMITIES?

The chemistry of redox-sensitive minerals in crystalline basement rocks is buffered by the mineralogy and influenced by fluids that may become more oxidizing as the basement is exhumed. For example, basement-hosted primary magnetite will transform to hematite (i.e., martite) when exposed to near-surface oxidizing conditions. Fe-oxides can be analyzed via (U-Th)/He (He) thermochronometry and martitization likely resets (i.e., induces He loss) the martite He system and thus records when rocks are in the near-surface. Here, we target martite on a fault surface cutting Paleoproterozoic gneiss, meters below a regional nonconformity near Morrison, CO (USA). At this locality, hematite alteration is pervasive on both sides of the nonconformity and prior work by Geissman and Harlan (2002) indicates a secondary, chemical remanent magnetization carried by hematite was acquired during the Permo-Carboniferous Reverse Polarity Superchron, which spanned ~315 to ~260 Ma and is broadly coincident with the last stages of the Ancestral Rocky Mountain orogeny (ARMO). Martite clasts within mm of the slip surface were extracted from fine-grained, cataclastic hematite matrix. Martite crystals exhibit both faceted, octahedral and subhedral morphologies, with crystal half-widths ranging from ~75-160 µm. Martite He dates from air-abraded and unabraded grains broadly overlap and show considerable intrasample scatter, with individual dates from 1042 ± 12 Ma to 309 ± 4 Ma (n=38) over a restricted Th/U ratio. Individual matrix hematite He dates range from 138 ± 2 Ma to 27 ± 1 Ma and reflect variable He loss from aliquots with different grain size distributions since formation, which likely occurred during hematite alteration associated with the ARMO. Preliminary martite electron backscatter diffraction analyses confirm clasts now comprise hematite and have internal, misoriented crystal domains more than an order of magnitude smaller than the clast diameter. Some martite He dates are markedly older than zircon He dates from the host rock (~485-11 Ma). Martite thus provides a novel deep time vestige and these data suggest basement exposures were at near surface conditions ~1040 Ma. Ongoing work explores role of internal domains on He diffusion in martite and the influence on partial He loss over the post-martitization thermal history.