AGE OF METAMORPHISM IN THE CASCADES CRYSTALLINE CORE FROM GARNET LU-HF AND SM-ND GEOCHRONOLOGY

The Chiwaukum schist and Tonga formation of Cascades Crystalline Core experienced three distinct metamorphic events: (M1) an early metamorphism that predates regional pluton emplacement, (M2) low-pressure contact metamorphism in the aureoles of the Mt Stuart batholith and Bekler Peak pluton, and (M3) regional Barrovian metamorphism. The timing of these distinct metamorphic events is critical to testing models for the tectontic evolution of the region and mechanisms that lead to burial of rocks within the orogen.

We present new garnet Lu-Hf and Sm-Nd isochrons from the Chiwaukum schist and Tonga formation in order to constrain the timing of the M1-M3 metamorphism in the Cascades Crystalline Core. Lu-Hf and Sm-Nd ages on garnets beneath the Windy Pass thrust record the age of M1 at ~115 Ma. The Lu-Hf isochrons, however, are scattered and likely reflect a complex polymetamorphic history. In the contact aureole of the Mount Stuart batholith, Lu-Hf and Sm-Nd isochrons date post-M2 garnet growth at ~ 90 Ma following batholith emplacement. Garnet growth during the M3 Barrovian overprint lasted from 91-86 Ma, similar to existing ages. In the Tonga formation, Lu-Hf ages from date M1 garnet growth between 114 and 99 Ma, prior to emplacement of the Beckler Peak pluton. Garnet Sm-Nd ages of 91 and 88 Ma from the same samples reflect protracted cooling after intrusion of the Beckler Peak pluton and Excelsior Ridge orthogneiss.

The results support correlations between the Chiwaukum schist and Tonga formation and provide new constraints on the timing of metamorphism. The ages document the timing of M1 metamorphism in both the Chiwaukum schist and Tonga formation circa 115 Ma, as well as, Barrovian metamorphism in both regions starting at 91 Ma.