TECTONIC IMPLICATIONS OF THE VARIABLE PRESSURE-TEMPERATURE-TIME HISTORY ACROSS THE SOUTH-CENTRAL NASON TERRANE, CASCADES CORE, WA

The south-central Nason terrane is comprised of the pelitic Chiwaukum Schist (CS), Banded Gneiss unit (BG), abundant intrusives, and the 93-96 Ma Mount Stuart Batholith (MSB). Substantial amounts of crustal loading have been suggested and its cause has been the focus of much debate. Our results indicate that the timing of peak metamorphism and nature of P-T paths vary with distance from the northern margin of the MSB. Garnet Sm-Nd dating indicates that garnet growth occurred during a short time interval with a consistent younging northward from the MSB; 90.0 ± 8.1 Ma (0.5 km, 98NC56), 89.1 ± 4.5 Ma (0.7 km), 88.1 ± 0.7 Ma (1.5 km, 00NC9), 86.4 ± 0.9 Ma (1.9 km, 96NC67), 86.4 ± 1.9 Ma (2.0 km), and 86 ± 1 Ma (4.2 km). Thermodynamic modeling results for 98NC56 (closest to MSB) are compatible with a pressure increase of 2-2.5 kbars, while those for 96NC67 (1.9 km north of MSB) suggest a maximum pressure increase of 0-1 kbars during garnet growth. Mineral chemical zoning, thermobarometry, and a quantitative garnet-growth P-T path calculated for BG sample 98NC38 (10.5 km NE of the MSB) all indicate that rocks north of the MSB contact aureole were already at their regionally high pressures before garnet growth. Therefore, we conclude that only rocks adjacent to the MSB record quantitative evidence for crustal loading during high-temperature metamorphism in this region.

Our geochronology and the MSB age (93-96 Ma) constrain crustal loading to 87.4-96 Ma. Evans and Davidson (1999) suggested emplacement of a crustal-scale thrust sheet across the Nason Terrane after MSB emplacement as the cause of loading. If this model is correct, our P-T-t data suggests a pre-existing lateral thermal gradient existed, where cold rocks north of the MSB were loaded simultaneously with hotter rocks adjacent to the MSB. An existing garnet age east of 98NC38 (Magloughlin, 1993) and our ages suggest isobaric heating in the BG is likely 81-86 Ma. This is compatible with the Evans and Davidson model, and suggests intrusives in the BG as the likely cause of heating. Because of uncertainty in the age of metamorphism near 98NC38, our P-T-t data are also consistent with diachronous loading across the terrane. In this scenario, loading near 98NC38 could have been pre- 87.4-96 Ma, with thermal relaxation and/or abundant intrusions as the cause of heating.