EQUILIBRIUM-DISEQUILIBRIUM (METAMORPHIC) ASSEMBLAGES IN THE RATTLESNAKE CREEK TERRANE: PIECING TOGETHER THE TECTONIC HISTORY OF THE CENTRAL KLAMATH MOUNTAIN PROVINCE

The Klamath Mountain province (CA/OR) contains the Rattlesnake Creek terrane – an oceanic mélange that locally reached amphibolite and granulite-facies conditions in the Mid-Late Jurassic. These are higher-temperature than expected in a classic accretionary orogen and the tectonic mechanism responsible remains unresolved. The Gold Flat locality along the Scott River contains well-preserved, locally migmatized, garnet amphibolites with an equilibrium assemblage of Grt + Hbl + Ilm + Rt + Ilm + Pl + Qz. The rock also contains St in the matrix and as inclusions in garnet, scant Ap and retrograde Ep + Chl. Leucosomes are granoblastic assemblages of plagioclase and quartz, sub- to anhedral Rt and Ilm are ubiquitous in the matrix and occur as inclusions in Hbl, Pl, St, and Grt. Garnet porphyroblasts have two textures: a poikiloblastic population elongate along foliation; and, rounded, subidioblastic equant crystals with sinusoidal inclusion trails of Ilm ± Rt and prograde-growth Mn zoning textures. Sericitized Pl, Act rims on Hbl, Chl and Ep alteration of Czo demonstrate significant (re)-hydration. Calculated garnet isopleths intersect at ~450°C, 3.5 kbar, and the calculated partial melting line plots at much higher temperatures (~750°C). Meanwhile staurolite-bearing mafic assemblage have been proposed to be stable at high pressures and comparable temperatures to partial melting temperatures (740-760°C).

It is proposed that the Gold Flat locality preserves evidence of an early (~168 Ma) high-pressure assemblage (St + Rt) that has been documented for other nearby localities such as the Saied Complex. It also documents a second younger prograde metamorphism that reached mid-amphibolite conditions. The concentric zoning in garnet and calculated isopleths argue against metamorphic temperatures reaching the calculated partial melting line. Rather, migmatites in the amphibolite formed at temperatures no greater than mid-amphibolite facies conditions (~600-650°C) in discrete layers via H₂O-present, discontinuous partial melting reactions fed by dehydration of under-thrust rocks of the Condrey Mountain Schist.