GEOCHEMISTRY OF TRIASSIC AND JURASSIC MUDROCKS IN THE BLUE MOUNTAINS, EASTERN OREGON: NEW INSIGHTS INTO SEDIMENTARY PROVENANCE AND BASIN EVOLUTION

Sedimentary strata of the Blue Mountains Province (BMP) of eastern Oregon and western Idaho record the magmatic and tectonic evolution of intra-oceanic and pericratonic arc terranes of the western Cordillera. Strata can be grouped into two, regionally correlative, unconformity-bounded packages, or “mega-sequences”: (1) MS-1, Late Triassic to Early Jurassic volcanic and volcaniclastic deposits of the Wallowa and Olds Ferry forearc and intra-arc basins; and (2) MS-2, Early to early-Late Jurassic marine deposits that overlap older structures and record ~20-40 m.y. of deep crustal subsidence in a large marine basin. Region-wide stratigraphic, structural and petrographic trends suggest that MS-2 records growth of a large collisional basin associated with closure of the back-arc basin and initial accretion of offshore oceanic terranes to the North American continental margin in Early to Middle Jurassic time. To test this hypothesis, we have used major and trace-element geochemistry of shales and mudstones to track the provenance evolution of MS-2 strata. Provenance trends are represented in tectonic discrimination plots of TiO₂ vs Fe₂O₃+MgO, Al₂O₃/SiO₂ vs Fe₂O₃+MgO, Al₂O₃/SiO₂ vs FeO+MgO/SiO₂+K₂O+Na₂O, La-Th-Sc, and Th-Sc-Zr/10. The plots reveal a systematic trend in MS-2 strata toward more evolved compositions through time, consistent with the collisional-basin hypothesis. Preliminary detrital-zircon (DZ) age data indicate that basal MS-2 strata contain only Mesozoic zircons, and that younger MS-2 strata contain a variety of Mesozoic, Paleozoic and Precambrian zircons. Thus, the DZ data indicate that the major- and trace-element trends reflect the influx of continentally derived material, not secular variations in arc-magma composition with time. MS-1 strata have complex major- and trace-element distributions, but appear to reflect a distinct earlier period of evolution from more juvenile to more evolved compositions. This trend reinforces new concepts of a major basinal reorganization between deposition of MS-1 and MS-2 strata.