IN-SITU MONAZITE PETROCHRONOLOGY THROUGH AN INVERTED BARROVIAN SEQUENCE, VALDEZ CREEK SHEAR ZONE, CLEARWATER MOUNTAINS, ALASKA

Contractional structures preserving inverted metamorphic gradients are present in numerous modern orogenic systems and play a central-role in the development of overthickened crust. Although empirical models exist to explain thermal and barometric gradients preserved across inverted metamorphic gradients, petrochronology across them is critical to test various models for their formation. The Valdez Creek shear zone (VCsz), southcentral AK, grades from mid to lower – greenschist-facies in the footwall to upper amphibolite-facies in the hangingwall and is a south-directed shear zone that records thrusting of North America derived metasedimentary rocks (Maclaren schist) over the basinal Kahiltna assemblage during the waning accretion of the Wrangellia Composite Terrane with North America. A texturally heterogeneous and variably deformed series of ca. 73 Ma sill-like tonalitic bodies are present within the upper-half of the shear zone.

Four samples ranging from biotite-garnet phyllite to sillimanite-kyanite-garnet gneiss and encompassing ca. 1.5 km of structural thickness within the VCsz were selected for in-situ monazite and apatite analysis. Monazite in these samples are texturally and chemically heterogeneous, displaying a systematic decrease in age with decreasing structural level in the VCsz (63.3 – 56.2 Ma). Three samples contain monazite REE+Y concentrations reflecting garnet stability throughout the shear zone, but systematically display increasing – Y over time. The structurally highest sample contains monazite with decreasing – Y over time. Existing geochronology indicates tonalitic intrusions within the shear zone predate this deformation by several million years. The monazite data suggest that the inverted Barrovian sequence formed over at least 6 million years as rocks of the footwall were buried and entrained into the VCsz. Based on monazite results paired with existing mid-T thermochronology, we interpret the metamorphic gradient to have formed due to syn-metamorphic inversion of isotherms compensated by rapid surficial exhumation. The new age data establish a temporal relationship with the Tatshenshini and Eastern Coast shear zones, which after restoration of Cenozoic displacement on the Denali fault, is consistent with Himalayan-scale rock uplift rates.