CONTRASTING METAMORPHIC HISTORIES ACROSS THE ENTIAT FAULT, NORTH CASCADES CRYSTALLINE CORE, WA

The Swakane terrane and overlying Napeequa complex are the structurally lowest units in the Cascades crystalline core. The Swakane terrane is composed of strongly-deformed metapsammite with subordinate layers of pelitic schist, garnet amphibolite, and calc-silicate, collectively called the Swakane Gneiss. It occurs in the Wenatchee and Chelan blocks separated by the Tertiary Entiat Fault, a high-angle fault with >35 km of dextral strike-slip displacement. Metamorphic zircon rim U-Pb ages indicate that the Swakane Gneiss in both blocks both underwent three metamorphic events ca. 90 Ma, ca. 81 Ma, and ca. 72 Ma. Two garnet Sm-Nd isochrons from the Chelan block [71.3±2.8 Ma on 5 pts MSWD=1.3; 73.5±1.7 Ma on 3 pts MSWD=1.7] are indistinguishable from 72.36±2.3 Ma metamorphic zircon rims [18 MC-LAICPMS pts]. However, metamorphic textures, and P-T estimates from thermobarometry (THERMOCALC) and phase diagram sections (THERIAK-DOMINO) suggest significantly different metamorphic conditions for the two blocks.

Qtz-Bt-Pl-Amp-Spn gneiss and Qtz-Bt-Pl-Ms±Ky±St schist from the Chelan block yield P-T estimates (AVERAGE-PT) of 680-760°C, 9-12 kbar. Garnets retain partial growth zoning from Alm_0.77, Prp_0.05, Sps_0.09, Grs_0.09 cores to Alm_0.64, Prp_0.19, Sps_0.02, Grs_0.15 rims. Matrix ilmenite are mantled by rutile, while ilmenite inclusions in garnet are not, suggesting that garnet grew during increasing pressure.

Qtz-Pl-Bt-Ms-Ky gneiss from the Wenatchee block equilibrated at lower P-T conditions (AVERAGE-PT) of 530-580°C, 7-9 kbar. Garnets preserve growth zoning profiles from Alm_0.70, Prp_0.07, Sps_0.09, Grs_0.14 cores to Alm_0.71, Prp_{0. 16}, Sps_{0. 02}, Grs_0.11 rims. Rutile-ilmenite textural relationships are similar to those in the Chelan block.

The Entiat Fault post-dates metamorphism; therefore, identical 72 Ma metamorphic ages and contrasting P-T conditions between the Wenatchee and Chelan blocks confirm exposure of different crustal levels from the basal exposures of the late Cretaceous Cascades crustal section (lowermost in the Chelan block; uppermost in the Wenatchee block).