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Abstract

This field trip guide describes a two-day excursion through Mesozoic accreted terranes of the Blue Mountains Province in northeastern Oregon. Day 1 is focused on sedimentary rocks of the Izee terrane. These deposits are divided into two unconformity-bounded megasequences, MS-1 and MS-2, that record two stages of syntectonic basin formation. MS-1 (Late Triassic to Early Jurassic) accumulated in fault-bounded marine sub-basins on the flank of an inferred growing Baker terrane thrust belt. MS-1 sandstones, derived from the Baker terrane, contain abundant Paleozoic, Late Paleoproterozoic, and Late Archean detrital-zircon grains. These observations suggest affinity of the Baker terrane and MS-1 in the Izee area to portions of the Klamath and Sierra Nevada terranes that contain similar detrital-zircon age distributions. MS-2 (Early to early-Late Jurassic) accumulated in a large marine basin that received input from low-grade metavolcanic rocks to the east (modern coordinates). Detrital zircons are dominated by Mesozoic, Neoproterozoic, and Mesoproterozoic grains. Two possible interpretations for MS-2 are: (1) the Jurassic Izee basin was fed directly by the large Mesozoic trans-cratonal sediment-dispersal system, or (2) trans-cratonal sediment was deposited in a Triassic backarc basin in Nevada and was later recycled into the Jurassic Izee basin during Cordilleran orogenesis.

Day 2 of the field trip is focused on Jurassic–Cretaceous magmatism in the Baker terrane. Late Middle Jurassic to Early Cretaceous igneous rocks in the Blue Mountains Province record three distinct pulses of plutonism that are characterized by distinctive spatial and geochemical signatures. These episodes consist of: (1) late Middle to Late Jurassic small gabbro to quartz diorite plutons (ca. 162–154 Ma; low Sr/Y); (2) Late Jurassic to Early Cretaceous plutons and batholiths (ca. 148 and 137 Ma; includes spatially distinct belts of low and high Sr/Y at 147–145 Ma); and (3) Early Cretaceous small plutons of tonalitic and trondhjemitic composition (ca. 124–111 Ma). Temporal transitions in geochemical characteristics between these suites raise fundamental questions regarding the origins of plutonism in the Baker terrane. In particular, the transition from low Sr/Y (group 1) to high Sr/Y (group 2) magmatism in the Greenhorn subterrane occurred ~ 7 Ma after regional contraction, and may record partial melting of thickened crust as a direct result of Late Jurassic orogenesis.

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