T201 From the Micro to Macro in Metamorphic Geology: Constraining Tectono-Metamorphic Processes with High-Resolution Approaches

Thursday, 29 October 2020
1:30 PM - 5:30 PM
Meeting Rooms

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

Paper number: T201-12
Program: Technical Sessions
Time: 4:00 PM – 4:15 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-358248

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