LATE JURASSIC TERRANE LINKAGES IN THE NORTH CASCADES: NEWBY GROUP IS QUESNELLIA?

The Newby Group is a Late Jurassic volcanic arc assemblage at the base of the southern Methow terrane in the eastern North Cascades. The Newby underlies thick clastic strata of the Methow basin, an overlap assemblage that links Methow, Bridge River and Cadwallader terranes by mid-Cretaceous time. Our work suggests the Newby Group represents a volcanic arc carapace to mid-crustal metamorphic and plutonic assemblages of the western Intermontane superterrane.

We divide the Newby into several lithostratigraphic units, including: (1) black argillite, volcanic sandstone and minor calcarenite of the Twisp Formation of Barksdale (1975); (2) laumontite-bearing andesitic breccias, tuff, pelite, and minor flows of the Bear Creek unit; (3) extensive silicic to intermediate volcanic breccia, tuff, and flows with intercalated volcaniclastic strata of the Lookout Mountain unit, which grade SE into (4) low-grade, mostly silicic, phyllite and schist of the McClure Mountain unit. The base of the Newby is not exposed and the underlying basement is unknown. Isotopic (e_Nd @ 6-7) and geochemical data (e.g. La_N/Yb_N @ 1.0-2.5) suggest the Newby represents an uncontaminated island arc assemblage. Fine tuff intercalated with black argillite in the upper Twisp Formation, immediately below a gradational contact with overlying Bear Creek unit, yields a U/Pb zircon age of 151.0 +8.7/-0.3 Ma (Late Jurassic). A rhyolite dike in tuff of the Lookout Mountain unit yields a U/Pb zircon age of 152.8 ±0.9 Ma. Mylonitic quartz diorite within the McClure Mountain unit has a U/Pb age of 142.8 +0.9/-0.3 Ma. Steep, N-S foliation of the McClure Mountain unit is cut by the syn- to post-kinematic Alder Creek stock (141.6 +1.0/-0.3 Ma; U/Pb zircon).

Subjacent tonalite to granodiorite plutons include the Button Creek stock, Alder Creek stock, Golden Doe Ranch stock and granodiorite of Tice Ranch. Ages from these bodies effectively date magmatism and intra-arc deformation as circa 140-155 Ma. The granodiorite of Tice Ranch (152.8 ±1.6 Ma, U/Pb zircon) intrudes the eastern edge of the Newby Group and gabbro and amphibolite at the west edge of the Okanogan batholith. This intrusive relationship ties the Methow terrane to the Intermontane superterrane by Late Jurassic time.