New Ages Tightly Constrain the Timing of Metamorphic Events In the Swakane Gneiss, Cascades Crystalline Core, WA

The Swakane Terrane/Swakane Gneiss comprises the lowermost exposed structural unit in the Cascades Crystalline Core. It is a highly-deformed, homogeneous, quartz-biotite-plagioclase ± amphibole ± muscovite ± garnet ± kyanite ± staurolite gneiss, with subordinate garnet amphibolite, calc-silicate, and ultramafics. Recent TIMS U-Pb zircon ages suggest that Swakane lithologies were deposited as sediment as recently as 73 Ma. Peak metamorphism (640-750°C; 9-12 kbar) occurred <5 m.y. after deposition, because post metamorphic dikes/pegmatites have ages of ca. 68 Ma. A detrital interpretation requires that the Swakane was loaded separately from the adjacent Nason Terrane, which experienced peak metamorphism 86-90 Ma. Also, this interpretation requires burial to 35 km and heating to partial melt conditions in <5 m.y. after deposition. In-situ MC-LA-ICPMS U-Pb analyses of zircon rims and sector zoned zircon cores yield ages ranging from 70-95 Ma with distinct subpopulations at ~72, ~82 Ma, and ~90 Ma. Grains contain distinct cores with ages ranging from 1800 to 400 Ma. High U/Th ratios (U/Th>10) for 72-95 Ma chaotically zoned zircon rims and sector zoned zircon cores and rims suggest that they are of metamorphic origin. Additionally, a garnet-pyroxene-rock Sm-Nd isochron yielded an age of 71.3±2.8 Ma for garnet amphibolite collected from the uppermost Swakane Gneiss, providing additional support to the interpretation of a metamorphic origin for ~73 Ma zircon grains.

Using the spatial resolution afforded by the MC-LA-ICPMS technique, we show that zircons from the Swakane Gneiss are complex, with metamorphic overgrowths of ~72 Ma, ~82 Ma, and ~90 Ma. The youngest ca 72 Ma zircon ages are indistinguishable from a garnet-whole rock Sm-Nd isochron, suggesting that the detrital interpretation for ~73 Ma zircons is unlikely. Synchronous ca. 90 Ma metamorphism in the Swakane Gneiss and adjacent terranes indicates a shared tectonic history and allows for similar depositional histories.