CONSTRUCTION OF PHANEROZOIC CONTINENTAL CRUST IN THE BLUE MOUNTAINS PROVINCE BY TECTONIC ASSEMBLY AND MAGMATIC ADDITION

We present whole-rock geochemistry and >550 zircon U-Pb, Lu-Hf and O isotope and trace element analyses from Middle Jurassic to Early Cretaceous plutons and batholiths that document processes and timescales of Phanerozoic crustal growth in Blue Mountains province (BMp), NE Oregon. Plutons from the central BMp (Baker and Wallowa terranes) were emplaced during two temporally distinct magmatic events that overlap and post-date a period of widespread contractional deformation from 159 to 154 Ma. The first group of syn-kinematic plutons range in composition from gabbro to granite and were emplaced from 162-154 Ma. They are defined by low whole-rock Sr/Y values (<40) and zircons have a broad range of εHf values from 0 to +15. A second group of post-kinematic plutons and batholiths range from 148-141 Ma and are chiefly hornblende tonalites and granodiorites. These rocks commonly have high Sr/Y values (>40) particularly in the Baker terrane, suggesting derivation from a plagioclase-poor, garnet+amphibole residue at depths greater than ca. 35 km.

In contrast, plutons from the Mountain Home metamorphic complex (MHMC), located along the western BMp, range in composition from norite to tonalite, and were emplaced nearly continuously from 160-145 Ma. Only Middle to Late Jurassic plutons are recognized and all rocks have low Sr/Y values. Zircons display positive εHf values from +5 to +15 consistent with partial melting of depleted-mantle sources. Middle Jurassic plutons and older host rocks are strongly deformed, display migmatitic textures, and contain moderate- to high-grade metamorphic mineral assemblages. Deformation is tightly bracketed between 150 and 148 Ma, and is younger than observed in the central BMp. We propose that the MHMC represents a new terrane in the BMp and may represent either 1) a new Middle to Late Jurassic arc complex accreted to the central BMP in the Late Jurassic, or 2) a fragment of the western Klamath terrane translated along an unrecognized Late Cretaceous, NE-SW-striking, fault zone which follows the trace of the geophysically defined Klamath-Blue Mountain lineament. This speculative fault, which we term the Klamath-Blue Mountain shear zone, may have accommodated northward displacement of crustal fragments from the Klamath Mountains and/or northern Sierra Nevadas to the western BMp.