JURASSIC MAGMATISM PRIOR TO AND DURING TERRANE ACCRETION IN THE BLUE MOUNTAINS PROVINCE, NE OREGON

The Blue Mountains comprise four tectonic terranes that were amalgamated and then accreted as a package to the continental margin during Late Jurassic and Early Cretaceous time. Jurassic magmatism in the Blue Mountains is manifest in several plutons exposed in the two northernmost of these terranes: the Wallowa and Baker terranes. An earlier Middle (?) Jurassic episode resulted in small plutons of oliv 2-pyx gabbro that were emplaced along the Wallowa arc terrane axis. They are characterized by low REEs, positive Eu anomalies, and high Al2O3, Cr, and Ni, suggestive of plag + pyx + oliv accumulation from an evolving basaltic magma. Late Jurassic magmatism (158-137 Ma) produced large plutons ranging from gabbro to granite (predominantly tonalite & granodiorite). These plutons are similar, characterized by increasing LILE with increasing SiO2 and high LILE/HFSE. Enclaves and dikes of low-MgO basalt and basaltic andesite are common and locally abundant. The youngest plutons are in the Wallowa terrane. They have low and uniform initial 87Sr/86Sr and low delta 18O, whereas plutons in the Baker terrane exhibit ranges in initial 87Sr/86Sr and delta 18O. These differences either reflect the different compositions of the terrane crust through which the magmas ascended or a change in the stress regime that led to decreased interactions with the crust through time. The Middle (?) Jurassic magmas were probably emplaced before amalgamation of the terranes, and may be correlative the Middle Jurassic western Hayfork arc present in the Klamath Mountains. Late Jurassic magmatism occurred after amalgamation and shortly before and during the earliest stages of accretion of the Blue Mountains terranes to the continental margin. Basaltic rocks in these plutons have subduction-related trace element signatures, and are consistent with derivation from an arc or back-arc setting. Resumption of magmatism in the Early to Middle Cretaceous produced small tonalitic and trondhjemitic plutons by deep-seated partial melting of terrane crust, probably due to post-accretion crustal relaxation.