THE PUEBLO TERRANE OF SOUTHEAST OREGON; A DISPLACED FRAGMENT OF THE JURASSIC CONTINENTAL ARC

Early Mesozoic arc assemblages north of latitude 39°N in the U.S. Cordillera are generally oceanic in character, based on an abundance of mafic to intermediate composition igneous rocks and a deep marine environment of deposition. This is true of arc terranes in the Blue Mountains, Klamath Mountains, northern Sierra Nevada, and Black Rock region of northwest Nevada. The Pueblo terrane (PT), located in the Pueblo Mountains of SE Oregon, is strikingly different and appears to be significantly out of place with respect to adjacent terranes.

The PT is an igneous complex, at least 60 km² in area, that consists of a thick (>1 km) sequence of volcanogenic strata and several shallow-level intrusions. U-Pb zircon dates on volcanic and plutonic rocks indicate that magmatism occurred in the Middle Jurassic between 179±2 Ma and 176±3 Ma. All of the PT igneous rocks are intermediate to felsic in composition, with no mafic rocks present. Quartz diorite is the most common plutonic rock. Volcanic units contain variable proportions of andesitic to dacitic lava, and dacitic to rhyolitic tuffs. The tuffs are primary pyroclastic deposits whose facies indicate derivation from a combination of pumice and ash flows, block and ash flows, and pyroclastic surges; some of the ash flows are welded. Some minor epiclastic breccia and sandstone are present, but these are locally-derived deposits consisting entirely of minimally-reworked andesitic and rhyolitic volcanic debris. No marine strata or non-volcanic materials are present in the stratigraphy of the PT. Trace and rare earth element geochemical data from diorite intrusions and andesite lava indicate that PT igneous rocks are medium to high K (calc-alkaline to shoshonitic) and formed in a magmatic arc.

Collectively, the rock types and facies in the PT are typical of those found in the proximal environment of a subaerial continental strato-volcano. Nearby Jurassic terranes in the Cordillera bear no similarity in magmatic character or paleogeographic setting to the PT. The nearest similar Middle Jurassic volcano-plutonic complexes are continental arc rocks in west-central Nevada and central to southern California. We conclude that the PT originated as part of the Jurassic continental arc of the SW Cordillera, and that it was later translated northward at least 300 km along dextral strike-slip faults.