LATE MIDDLE JURASSIC ‘RETRO-ARC' MAGMATISM IN THE KLAMATH MOUNTAIN PROVINCE

Middle Jurassic magmatism in the Klamath Mountain province (KMp) ceased when regional thrusting along the Wilson Point fault imbricated the Middle Jurassic arc + arc basement beneath older terranes of the province. Subsequent Late Middle Jurassic magmatism occurred in three zones. Transtension in the western KMp resulted in formation of the Josephine ophiolite (JO). To the west, the Rogue-Chetco arc was built on basement rocks rifted from the main KMp during JO development. In contrast, plutonism east of the JO evidently occupied a ‘retro-arc’ setting. This retro-arc magmatism was not confined to a narrow belt of magmatism, but instead spanned the length of the province in a zone at least 90 km wide. Retro-arc pluton ages are 170-152 Ma and compositions range from gabbro to two-mica granite, with diorite–tonalite as principal rock types. Most large plutons are distinctly composite; available U-Pb age data indicate emplacement of individual plutons took as much as 8 m.y. Recent dating of the Wooley Creek batholith shows that even plutons that lack discernable internal intrusive contacts span age ranges of ~4 m.y. Retro-arc magmas were emplaced into crust that had been thickened by thrusting (> 40 km thick). Plutonism was approximately coeval with regional metamorphism that reached granulite facies conditions. With the exception of the oldest, southernmost plutons, the suite is characterized by ε_Nd lower than, and ⁸⁷Sr/⁸⁶Sr and δ¹⁸O values higher than those expected from depleted mantle sources. Moreover, U-Pb (zircon) data indicate sources or contaminants of these magmas contained 180-186 Ma zircons and in some cases ~168 Ma zircons. The data suggest that late Middle Jurassic retro-arc magmatism involved emplacement, differentiation, and contamination of mantle-derived magmas into thickened crust, that mafic magmas provided the heat for high-T, moderate-P metamorphism, and that crustal melting provided at least one component of contamination and mixing. Inherited 180–186 Ma zircons cannot be correlated with known magmatic events in exposed KMp terranes, which suggests that Klamath plutons may be used as temporal and spatial probes of otherwise cryptic terranes within the province.