Timing and Conditions 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 Mount Stuart batholith and Beckler Peak pluton, and (M3) regional Barrovian metamorphism. The timing of these distinct metamorphic events is critical to testing models for the tectonic evolution of the region and mechanisms that lead to burial of rocks within the orogen. I present new garnet Lu-Hf and Sm-Nd isochrons, average pressure and temperature calculations, and pseudosection modeling from the Chiwaukum Schist and Tonga Formation in order to constrain the timing and conditions of M1-M3 metamorphism in the Cascades Crystalline Core. Garnet Lu-Hf and Sm-Nd isochrons obtained from the Chiwaukum Schist in Icicle Canyon record the age of M1 metamorphism approximately 130 to 110 Ma at ~3 kilobar and ~650°C. 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 syn to post-M2 garnet growth in the Chiwaukum Schist between 95 and 90 Ma following batholith emplacement at 3.5 kilobar and ~550°C. Garnet growth during the regional M3 Barrovian overprint lasted from 91-86 Ma at 6-7.4 kilobar and~650°C, dates that are similar to published ages. In the Tonga Formation, a Lu-Hf age dates M1 garnet growth around 119 Ma at 3-4 kilobar and ~550°C, however, a second Lu-Hf sample is scattered and likely reflects a complex polymetamorphic history. Garnet Sm-Nd ages of 91 and 88 Ma from the same samples reflect M3 related cooling after intrusion of the Beckler Peak pluton and Excelsior Ridge orthogneiss at 6-7 kilobar and ~600°C. The ages document the timing of M1 metamorphism in both the Chiwaukum Schist and Tonga Formation approximately 120-110 Ma, as well as Barrovian metamorphism in both regions starting at 91 Ma. M2 and M3 ages from both regions reflect a period of continual metamorphism and progressive burial beginning with the intrusion of the Mount Stuart batholith and satellite plutons between 96 and 92 Ma at 3-4 kilobar and culminating with rapid burial and continued deep seated magmatism through 88 Ma to pressures in excess of 6-8 kilobar. The results support correlations between the Chiwaukum Schist and Tonga Formation and provide new constraints on the timing and conditions of metamorphism.