ROTATED PROGRADE GARNET WITH RUTILE DEMONSTRATE HIGH-GRADE NEVADAN METAMORPHISM IN OVERLY THICKENED CONTINENTAL CRUST IN THE CENTRAL KLAMATH MOUNTAINS

The Rattlesnake Creek terrane (RCT) is an oceanic mélange in the Klamath Mountains Province (KMP), (CA/OR). In the central KMP, the RCT contains garnet-bearing amphibolite and migmatite which may be used to evaluate the tectonic history of Late Jurassic orogenesis by integrating mineral data, compositions, and microstructures. The mineral assemblage is Hbl + Pl + Qz ± Grt ± Rt ± Ilm ± Cz ± Ttn ± Zrn ± Ep ± St with multiple discrete zones of stromatic leucosome. Grt ranges from <1-3 cm and displays two morphologies. Morphology 1 is elongate and compositionally homogenous with linear continuous inclusion trails parallel to matrix foliation. Morphology 2 is rounded, displays compositional prograde-zoning (not retrogressed), has smaller (0.2-0.5 cm) idioblastic garnet with linear continuous inclusions (Rt-Ilm-Pl) parallel with matrix foliation, and larger porphyroblasts (2-3 cm) with curved inclusion patterns indicating ‘top-to-the-west’ normal-sense movement on west-dipping planes. Pl and Qz in the matrix contain subgrains and both have lobate grain boundaries indicative of high-T grain-boundary migration recrystallization. Sigmoidal leucosomes, asymmetric amphibole porphyroclasts and minor S-C fabrics also exhibit ‘top-to-the-west’ normal-sense movement.

The older, elongate retrograde garnet likely are relict from Mid-Jurassic (~168 Ma) Siskiyou metamorphism. These garnets deformed along a transposed foliation that grew during Late Jurassic Nevadan orogenesis. Prograde garnets are peritectic and grew during production of leucosome in syn-deformational migmatities dated to ~155 Ma. There is no evidence for garnet re-setting during Cretaceous metamorphic doming of the Condrey Mountain schist which intruded and uplifted the RCT.

Classic protracted terrane accretion should produce typical Barrovian P-T paths lower than the metamorphic conditions documented in the RCT using rutile-thermometry (630-700 °C), microstructural studies, and thermodynamic models (600-750 °C and 6-8 kbar). This shows that the Nevadan prograde path may have been steeper than a Barrovian path. Rapid accretion of multiple terranes producing a significantly over-thickened crust is necessary to reach the rutile stability field on the steeper geothermal gradient necessary to reach the documented metamorphic conditions.