UTILIZING ZONED GARNET AND HIGH-PRECISION SM-ND GEOCHRONOLOGY TO DETERMINE METAMORPHIC P-T-T PATHS FOR THE SWAKANE GNEISS, NORTH CASCADES CRYSTALLINE CORE, WA

Chemical zoning in garnet provides a record of changing P-T-X conditions during garnet growth. Garnet preferentially sequesters Mn, so we assume that max Mn compositions reflect initial growth. Therefore, the intersection of compositional isopleths for the max Mn core on isochemical phase diagrams provide an estimate of P-T conditions during initial garnet growth. P-T conditions for final garnet growth are derived from phase diagram assemblage fields and garnet rim thermobarometry, because garnet changes effective bulk rock composition as it grows. Garnet ages are directly linked to P-T conditions and allow calculation of tectonic rates. We use high-precision garnet Sm-Nd isochrons and metamorphic zircon rim ages to construct P-T-t paths for part of the North Cascades Crystalline Core.

The Swakane Gneiss is the deepest exposed crust [7 - 12 kbar] in both the Wenatchee and Chelan blocks of the Cascades Core. The Swakane Gneiss is mostly metapsammite with interlayered metapelite that contains garnet plus accessory amounts of staurolite and kyanite. Garnets are euhedral, 0.1 - 1 cm, and inclusion poor. Concentric zoning of major elements in garnets suggests single-stage growth. Textural relationships indicate ilmenite breakdown to rutile during garnet growth, compatible with increased P. Although Swakane lithologies from the Wenatchee and Chelan blocks are texturally and compositionally similar, our results suggest they experienced different peak conditions at slightly different times.

Garnet Sm-Nd isochrons and U-Pb spot ages of metamorphic zircon rims from the Chelan block Swakane Gneiss indicate 73.5 ± 1.2 Ma garnet growth and 75-68 Ma zircon growth/recrystallization. Steep P-T trajectories for 3 samples show significant P increases from 5 to 12 kbar with heating from 525°C to 675°C.

Wenatchee block Swakane Gneiss samples yield younger Sm-Nd garnet ages [65.8 ± 0.7 Ma] and metamorphic zircon ages of 71-64 Ma. P-T trajectories for 2 samples are steep [from 550°C, 5 kbar to 625°C, 9 kbar], but indicate lower peak conditions than in the Chelan block.

Using the range of uncertainty for garnet ages and P-T estimates for garnet cores and rims yields a loading rate estimate of 10.6 mm/yr. Rapid burial is consistent with a thrust loading model for 75 – 64 Ma metamorphism of the Swakane Gneiss as opposed to magma loading.