A REVISED LOADING AND EXHUMATION MODEL FOR THE SWAKANE BIOTITE GNEISS, CASCADES CRYSTALLINE CORE (CCC), WA: IMPLICATIONS FOR MESOZOIC TECTONICS

Reconstruction of the Mesozoic tectonics of the CCC in north central Washington has been obstructed by uncertainty regarding the provenance and tectonic loading history of the lowest and deepest exhumed Swakane terrane, a quartz-biotite, locally migmatitic paragneiss that comprises the footwall of the Dinkleman decollement. Mattinson (1972) presented a single zircon U-Pb age of 68 Ma from a leucosome, and Matzel et al. (2002) interpreted ~73 Ma U-Pb zircon core ages as the youngest detrital zircons and thus age of deposition. Gatewood and Stowell (2012 interpreted younger U-Pb age gneiss-zircon cores (70’s and 80’s Ma) as spot mixing between older (>90 Ma) cores and younger (75 to 63 Ma) metamorphic rims, consistent with 73.5 – 65 Ma (Sm-Nd) garnet growth. They used thermal modeling to demonstrate the challenge of burial near 0C (deposition, 73 Ma) to 700C (partial melting, 68 Ma) in under 10 m.y.

We present 308 new zircon U-Pb ages from Swakane gneiss and melt leucosomes combined with 393 published ages into age probability distribution plots. The densest peaks in gneiss zircon cores (91–73 Ma) and rims (89–71 Ma), and leucosome cores (100–62 Ma), have highest peaks around 90, 80, and 70 Ma, and all have ~1.4 Ga and ~1.7 Ga ages. All but one gneiss zircon core (U/Th<10) age is >72 Ma. All gneiss zircon rims are >68 Ma with the exception of one sample with rims as low as 55 Ma. In contrast, there are essentially no leucosome zircon rim ages from 62–100 Ma, and the densest age peaks range from 62-45 Ma, with the younger majority dominated by U/Th>10 ages. We interpret these data to indicate that most leucosome cores are inherited. Leucosome zircon rims are interpreted to record the true age of partial melting and U/Th<10 zircon rim growth at 62–59 Ma, followed by continued melting or dry metamorphic zircon rim growth. This interpretation permits a model in which the Swakane was deposited around 73-74 Ma, loaded to 40 km in ~1-2 m.y. during garnet growth, remained at depth long enough to heat to partial melting conditions (700 C, ~10 m.y.), followed by up to 20 km of exhumation between <59 Ma and >51 Ma (hornblende cooling at 510C). Farallon subduction rates permit rapid loading (40 km in a few m.y.), making subduction of sediments, similar to the POR schists of southern California, the model with which our interpretations are most consistent.