PETROLOGY OF THE LATE JURASSIC SUNRISE BUTTE PLUTON, EASTERN OREGON: A RECORD OF RENEWED MESOZOIC ARC ACTIVITY?

Late Jurassic magmatism in the Blue Mountains province resulted in the formation of several plutons (~159-137 Ma) that define a broad NE-SW band across the Permo-Triassic Wallowa island arc terrane and the adjacent Baker terrane, which is an accretionary complex of Devonian (?) to Early Jurassic age. As a general rule, there little compositional difference between the Late Jurassic plutons (which are predominantly tonalitic and granodioritic), but plutons in the Baker terrane are isotopically more evolved and variable (Sr_i = 0.7035-0.7052, δ¹⁸O = +6.3-+14.5 ‰) than those in the Wallowa terrane (Sr_i = 0.7029-0.7035, δ¹⁸O = +6.0-+8.0 ‰). The compositions of most of these plutons are consistent with derivation from a depleted mantle source and subsequent interaction with supracrustal rocks by assimilation and/or magma mixing.

The ~156 Ma Sunrise Butte pluton is the westernmost, and one of the oldest, of the Late Jurassic plutons studied thus far. The southwestern part of the pluton consists of two-pyroxene diorite/quartz diorite. The bulk of the pluton consists of peraluminous hornblende biotite tonalite and granodiorite with <2.0 wt.% K₂O. These rocks possess all the characteristics of a high-Al tonalite-trondhjemite suite: >15 wt.% Al₂O₃ at 70 wt.% SiO₂, high La/Yb, high Sr/Y, and low Y. Their compositions preclude a cogenetic relationship with the pyroxene-bearing diorites; they are more consistent with partial melting of a garnet-bearing mafic to intermediate crustal source. Mantle-derived dioritic magma provided the heat necessary for lower crustal anatexis. The isotopic compositions of the partial melts and mantle-derived magmas were modified by small degrees of assimilation of argillitic rocks.

Available geochronological information suggests the Sunrise Butte pluton was emplaced after amalgamation of the Blue Mountains terranes, during or slightly before accretion of the terrane package to the continental margin, and before dextral translation along the Western Idaho Shear Zone. Although the tectonic setting is not clear, the NE-SW zone of Late Jurassic calc-alkaline plutons suggests an episode of renewed arc activity related to the terrane accretion event.