DETRITAL ZIRCON AGE AND HAFNIUM ANALYSIS OF THE GALICE FORMATION, KLAMATH MOUNTAINS, OR AND CA: A RECORD OF THE LATE JURASSIC NEVADAN OROGENY

Tectonic models to explain the Late Jurassic Nevadan Orogeny in the Klamath Mountains and Western Sierra Nevada Metamorphic Belt include 1) west-directed subduction under an island arc; 2) simultaneous west- and east-directed subduction under an island arc and the North American continent, respectively; and 3) east-directed subduction under the North American continent. The Galice Formation, an Upper Jurassic metasedimentary unit in the Western Klamath terrane, includes the youngest strata deposited prior to and perhaps during deformation associated with the Nevadan orogeny and can be used to inform the proposed tectonic models and improve timing constraints. Ages of detrital zircon from two Galice samples (n = 300 per sample) range from 148 to 2962 Ma, with age modes in the Jurassic and Precambrian (ca. 600 Ma, 1000 to 1050 Ma, and 1425 Ma). Both samples include abundant Precambrian (45% and 56%) and Mesozoic (38% and 28%) zircon, with fewer Paleozoic grains (ca. 16% in both samples). Maximum depositional ages (MDA) calculated for the two samples are similar; one sample has an MDA of 149±1.8 Ma (weighted mean of youngest 10 grains) or 151.8±0.3 Ma (maximum likelihood age), and the other has an MDA of 150.3±2.2 Ma (weighted mean of youngest 6 grains) or 152.3±1.2 Ma (maximum likelihood age). These age estimates are at or beyond the younger limit of the 164-152 Ma depositional age estimate for the Galice Formation that is based on the age of the underlying Josephine ophiolite, Oxfordian-Kimmeridgian fossils, and cross-cutting intrusions. Epsilon Hf values of Jurassic detrital zircon grains show a remarkable spread from +16 to -20; 58% have epsilon Hf of +5 to +16, and 28% have negative epsilon Hf values. Detrital zircon with negative epsilon Hf values span almost the entire Jurassic Period, from 199-151 Ma. These data demonstrate significant continentally derived sediment in the Late Jurassic Galice basin, including abundant Precambrian zircon as well as Jurassic magmatic arc sources with evolved Hf signatures. Our preliminary results suggest that the Galice basin was tied to the continental margin during Late Jurassic deposition and deformation, consistent with east-directed subduction models for the Nevadan orogeny.