DETRITAL ZIRCON GEOCHRONOLOGY OF PERMIAN-TRIASSIC METASEDIMENTARY ROCKS IN THE BAKER TERRANE, BLUE MOUNTAINS PROVINCE, NE OREGON

Devonian-Early Jurassic metasedimentary rocks in the Baker terrane (northeastern Oregon) preserve important information regarding the depositional history and paleotectonic setting of the Baker terrane. The Baker terrane consists of two distinct subterranes (the Bourne and Greenhorn subterranes). The dominant lithologic unit in the Bourne subterrane is the Devonian–Early Jurassic Elkhorn Ridge Argillite which includes widespread chert-argillite broken formation. The Greenhorn subterrane is chiefly characterized by serpentinite-matrix mélange containing large blocks of metaplutonic, metavolcanic, metavolcaniclastic, and chert-argillite breccia, all of which are overlain by Permian–Triassic conglomerate, sandstone, argillite, and limestone of the Badger Creek unit. Previous workers have interpreted the Bourne and Greenhorn subterranes to represent an accretionary prism and dismembered fore-arc, respectively.

We present detrital zircon ages from eight sandstones and pebble conglomerates from both the Elkhorn Ridge Argillite (n=3) and the Badger Creek unit (n=5). In the Bourne subterrane, detrital zircon ages from the Elkhorn Ridge Argillite include ~1% Mesozoic (multi-grain modal age peak at 244 Ma), ~84% Paleozoic (255, 256, 271, 272, 278, 299, 304, 310, 318, 334, 345, 370, 384, 386, 402, 418Ma), and ~15% Precambrian (1186, 1361, 1498, 1679, 1834, 1897, 1919, 1989, 2074, 2710 Ma). In the Greenhorn subterrane, ages from the Badger Creek unit are ~9% Mesozoic (242, 251 Ma), ~4% Paleozoic (328, 335, 368, 387Ma), and ~87% Precambrian (1452, 1630, 1657, 1786, 1820, 1850, 1857, 1872, 1889, 1917, 1921, 1992, 1998, 2019, 2044, 2063, 2082, 2094, 2184, 2310, 2316, 2335, 2340, 2376, 2432, 2473, 2507, 2572, 2594, 2599, 2602, 2608, 2677, 2699, 2710 Ma). These data demonstrate the involvement of Precambrian sources in the depositional history of the Bourne and Greenhorn subterranes and support a fringing continental margin setting in Late Paleozoic to early Mesozoic time. We suggest that the Bourne and Greenhorn subterranes likely evolved together as distinct parts of a long-lived, accretionary complex and fore-arc respectively, and were tied to the fringing Olds Ferry island arc and the North American margin.