Metamorphic evolution of the northwest Chelan Block, North Cascades, Washington

The Cascades crystalline core (CC) is the fault bound southern extension of the Coast Plutonic Complex, a Cretaceous orogenic welt that forms the northernmost section of the North American Cordillera. Of primary interest in this study is the tectonic history of the Cascade River region in the northwest of CC. The Sibley Creek area of the Cascade River region is a zone of high-grade metamorphic rocks bound by Cretaceous plutons to the north, east, and west and the Tertiary Entiat fault to the south. Analysis of deformed and undeformed granitic plutons in the area could help constrain the absolute ages of metamorphic and deformational events because existing ages do not provide enough differentiation in the timing of magmatism. Thin-section analysis from this study accompanied by previous work in the region identifies deformational events that are observed throughout the Sibley Creek area. A strong NW-SE steeply dipping foliation is present in both the Marble Creek and Eldorado plutons as well as the metamorphic country rock, though not observed in the Hidden Lake pluton, and is observed to wrap around garnets analyzed in this study. The foliation is interpreted to have developed after garnet growth and the emplacement of the Marble Creek pluton but before the emplacement of the Hidden Lake pluton. Isopleth thermobarometric modeling and garnet fractionation modeling yield a history of P-T conditions recorded during garnet growth but also identifies problems and inconsistencies with the method as evidenced by varied P-T estimates. Pressure and temperature conditions recorded during growth of most garnet samples show a trend of increasing metamorphic conditions from 4 to 7 kilobars and 500 to 575 °C. However, a few samples yield pressures as high as 10.5 kilobars and temperatures as high as 750 °C. Anomalously high pressure and temperature results cast doubt on the accuracy of the estimates. U-Pb isotopic analyses on zircon samples collected for this study yield concordant ages with 2-sigma errors of 76.2 ± 1.2 Ma for the Marble Creek pluton and 73.52 ± 0.68 for the Hidden Lake pluton. These ages are analytically distinct and allow for age-bracketing of deformation in the region based on the deformed and undeformed nature of the plutons. U-Pb isotopic analyses and isopleth thermobarometry performed by this study and combined with previous work reveal four specific events in the tectonic evolution of the Sibley Creek area. At ~88 Ma the Eldorado pluton intruded at shallow depth into the country rocks of the region. Between ~88 Ma and ~76 Ma a major loading event buried the rocks of the region until they experienced pressures of at least 7 kilobars. The Marble Creek pluton was emplaced at those depths at ~76 Ma. The relative timing of garnet growth and the emplacement of the Marble Creek pluton is unresolved though both garnet growth and the crystallization of the pluton must have ceased prior to the end of deformation that resulted in pervasive NW-SE foliation. After the cessation of deformation developing the foliation, exhumation must have occurred to return the rocks to ~4 kilobars by ~73 Ma. At ~73 Ma the Hidden Lake pluton was emplaced at that pressure.