PETROCHRONOLOGY OF THE GOLD FLAT AMPHIBOLITE AND THE ROLE OF HIGH-GRADE NEVADAN METAMORPHISM

The thermal-tectonic structure of accretionary systems commonly follows intermediate T/P Barrovian P-T-t paths. The Klamath Mountain province is considered an accretionary orogen that does not conform with traditional models because centrally-located, amphibolite-grade rocks of the Rattlesnake Creek terrane have garnet P-T paths that suggest steeper, low T/P gradients, and higher peak temperatures than expected in Barrovian settings.

The Gold Flat locality in the Rattlesnake Creek terrane contains amphibolite with the assemblage Hbl + Pl + Rt + Ilm ± Grt ± St ± Ap ± Ep ± Qz ± Zrn. Leucosomes present in stromatic layers have two populations of peritectic garnet. One garnet population is compositionally homogenous and elongate along foliation. These garnet are ~50% Alm, ~30% Py, ~12% Grs with minor Sps and have linear to random inclusion patterns of Rt, Ilm, and Pl. The second garnet population is equant, rounded, and displays prograde Sps zoning. These garnet are ~65% Alm, ~20% Py, ~10% Grs with minor Sps, and contain helicitic inclusion trails of ilmenite and rutile. Sm-Nd dating of the elongate and equant garnet populations produced ages of 156 ± 4 Ma and 158 ± 3 Ma, respectively. Thermobarometry and peak mineral assemblages on isochemical phase diagrams indicate metamorphism at 650-750 °C and 7-8 kbar. U-Pb rutile ages yield ~156 ± 15 Ma from grt inclusions and 159 ± 14 Ma from ru in the nearby amphibolite matrix.

Preserved prograde zoning and lack of retrograde embayment textures in the equant garnet suggest high P-T conditions were imposed quickly, followed by rapid cooling. Pressure estimates and rutile indicate a relatively thick crust at peak conditions, likely due to mid-Jurassic contraction. We attribute high-temperatures in this thickened crust to magmatic heat, and subsequent rapid cooling to Nevadan orogenesis.

New garnet and rutile ages combined with thermodynamic models and thermobarometry demonstrate magmatic heat flux after mid-Jurassic crustal thickening produced unusually high P-T conditions. Evidence for this event was preserved due to cooling associated with Nevadan subduction of oceanic crust. These data have implications for Jurassic metamorphism in the Klamath province, because metamorphism previously attributed to the Siskiyou orogeny may instead be related to pre-Nevadan magmatism.