APPLICATION OF QUANTITATIVE METAMORPHIC PRESSURE-TEMPERATURE-TIME PATHS FOR DEVELOPING TECTONIC MODELS, NORTH CASCADES WA

Ongoing research provides critical constraints on loading/high P metamorphism in the North Cascades contractional magmatic arc. Tectonic models hinge on whether the entire Wenatchee Block (WB) experienced synchronous loading and high-P metamorphism, and the timing of high-P metamorphism with respect to igneous intrusions. Research integrates new high precision geochronologic, thermobarometric, and metamorphic phase equilibria data from across the WB in order to evaluate tectonic models. Early, Mt. Stuart batholith (MSB), emplacement conditions at low P are constrained by And adjacent to the MSB near Heather Lake. Pseudosection- and Grt Sm-Nd based P-T-t paths for Grt growth ca. 1 km from the MSB indicate that Grt growth postdates MSB emplacement by ca. 7 m.y., initial P was ca. 4.8 kbar @ 595°C, and final P was ca. 6 kbar @ 670°C. The P-T-t path indicates initial Grt growth with St + Sil present, followed by growth with peak minerals during a 1.2 kbar P increase. Grt Sm-Nd ages for three samples are 88.4 ± 2.9 Ma (99NC22a), 88.1 ± 0.7 Ma (00NC9), and 86.1 ± 0.6 Ma (96NC67). Core and rim Grt ages for 96NC67 are isochronous, indicating rapid growth. The P-T-t path indicates that Grt grew quickly during post MSB regional metamorphism; however, the timing of Grt growth regionally varied over ca. 3 m.y. from 89-86 Ma. A loading rate calculated from MSB emplacement at 93.5 Ma (U-Pb, Zrc) and P from And stability is 1.1 km/m.y. The timing of metamorphism, NE of Heather Lake in the Banded Gneiss is not well constrained because no ages are available for tonalite sheets or peak metamorphism. Quantitative P-T-t paths for 2 rocks indicate initial Grt growth at 7.1 kbar @ 540°C and 6.5 kbar @ 540°C with peak conditions of 7.6 kbar @ 585°C and 8.4 kbar @ 650°C for samples 98NC38 and 01NC15b, respectively. These results indicate that Grt grew at or near peak P with possible P increases of 1.5 to 1.9 kbar. Future isotope work will constrain the timing of intrusions and metamorphism in the Banded Gneiss and provide data on the spatial and temporal pattern of loading, and related plutonism and deformation. Results will allow critical evaluation of tectonic models for metamorphic/orogenic processes and help to distinguish between advective heat transfer by plutons and conductive heat transfer after crustal thickening.