TIMING OF PARTIAL MELTING OF RATTLESNAKE CREEK AMPHIBOLITE—INITIATION OF THE NEVADAN OROGENY?
Zircons were extracted from leucosomes in partially melted RCt amphibolite that crops out in the upper Applegate River and in the Scott River at Tompkins Creek. These zircons were dated by CA-ID-TIMS at Princeton University. Zircon from the northern Applegate River site yielded a 206Pb/238U age of 157.10 ± 0.14 Ma (9 grains) and the southern Scott River site yielded a 206Pb/238U age of 155.32 ± 0.30 Ma (10 grains). These ages are slightly older than 40Ar/39Ar (hornblende) ages of 153.0 ± 2.2 Ma at the Applegate River site and 154.3 ± 3.1 Ma at the Scott River site (Hacker et al., 1995). Ti-in-zircon thermometry yielded temperatures of ~630 °C. This agrees with P-T ranges calculated at Scott River of ~600-700 °C via garnet-hornblende thermometry (Graham and Powell, 1984) and ~6-8 kbar via garnet-hornblende-plagioclase barometry (Kohn and Spear, 1990).
These temperatures are too low to result from dehydration partial melting of amphibole and instead indicate that melting occurred under H2O-saturated conditions. We propose that the excess water originated from the exterior unit of the subjacent CMS. We propose that starting at or shortly before 157 Ma, the CMS was thrust underneath the RCt along the Condrey Mountain fault. Heat from the RCt resulted in an increase in grade of the CMS, with resulting fluids infiltrating the overlying RCt. Addition of aqueous fluids allowed the already hot RCt to melt, producing migmatites. The 157 Ma zircon age from the Applegate River represents the earliest stage of melting of the RCt amphibolite.
Historically, the Nevadan orogeny was proposed to begin after deposition of the Galice Formation to the west (ca. 153 Ma; Miller et al., 2003). We suggest that the thrusting of the outer unit of the CMS beneath the RCt at ~157 Ma may mark even earlier Nevadan deformation.