MAGMATIC ASSIMILATION OF OCEANIC SEDIMENTS IN THE WALLOWA-SEVEN DEVILS ACCRETED TERRANE, NORTH-CENTRAL IDAHO

Tonalite-trondhjemite magmatism is often associated with accretionary complexes and occurs directly after the accretion event. Current work suggests that these magmas are generated in the upper mantle and subsequently assimilate oceanic crustal island arc material in the accreted terranes. Paleozoic to Mesozoic oceanic island-arc rocks of the Wallowa-Seven Devils (WSD) terrane are juxtaposed against Precambrian metasediments of the North American Craton in north-central Idaho. The craton was subsequently intruded by the Late Cretaceous Idaho Batholith. The accreted oceanic-arc terrane consists of Jurassic and Cretaceous diorite-tonalite-trondhjemite plutons that intruded into contemporaneous and older volcanic and sedimentary rocks. Previous work has shown that these plutons are characterized by low initial ⁸⁷Sr/⁸⁶Sr ratios (r_i) of less than 0.704, positive e_Nd values from 3.0 to 7.6 with an average of 5.7, and d¹⁸O values less than 7.5 per mil. A tonalite-trondhjemite pluton that intrudes an older quartz diorite is exposed along the Clearwater River near Greer, Idaho, in the northern section of the WSD terrane. The tonalite-trondhjemite contains low K₂O (0.39-0.91 wt. %) and MgO (0.13-0.81 wt. %), high Na₂O (5.30-5.53 wt. %), and is also depleted in HREE. Secondary minerals found in one of the tonalite samples suggest that hydrothermal alteration has affected at least some of the rocks. Strontium and trace element chemistry suggest mantle or subducted oceanic material as the possible source for these magmas. d¹⁸O values of quartz mineral separates from the pluton, however, range from 8 to 10 per mil. The high d¹⁸O values are several per mil higher than the island arc/oceanic crustal rocks of the accreted terrane into which they are emplaced. Assimilation of high d¹⁸O sediments of the accreted terrane by a lower d¹⁸O mantle-derived magma could produce the high oxygen values in the tonalite-trondhjemite.