Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 21-6
Presentation Time: 9:00 AM-6:00 PM


DAVIS, Peter, Department of Geosciences, Pacific Lutheran University, 1010 S 122nd St, Tacoma, WA 98447

Late Jurassic to early Cretaceous sequence of the Darrington phyllite and Shuksan greenschist (GS) with intercalated Ep-blueschist (Ep-BLS) outcrop in the Kachess Lake and Hicks Butte inlier south-central Cascade Range near Cle Elum, WA possibly show an inverted metamorphic sequence related to northward directed subduction. Shuksan adjacent to the Diorite to tonalite of Hicks Butte complex (~150 Ma on zircon) show a variability of GS to amphibolite with rare and relic Ep-BLS. The Hicks Butte has been interpreted to be emplaced post tectonically into the Shuksan based on an age of ~160-166 Ma Ar/Ar on Na-amphibole in adjacent blueschist. Recent work instead suggests a syntectonic emplacement: 1- new older ages on the pluton complex of 153-157 Ma. 2- Rf/Phi strain analysis of rigid clast rotation in hornblende and plagioclase across the host to Hicks Butte pluton contact suggests a single phase of deformation. 3- Constrains on the P-T paths of symplectite in the Hicks Butte margin that show rapid cooling to 700-800˚C at 6-8 kbars match those of amphibolite overprint of Act + Ep + Ab on texturally earlier Ep-BLS at the margin, ~ 550˚C 6-8 kbars. 4- Symplectite is sheared and crosscut by adakite dikes dated to ~144 ma. which is also sheared. A mid-greenschist overprint post-dates almost all deformation.

These findings at Hicks Butte have been extended 15 km to the Kachess inlier the north where the same sequence and composition of rocks exists. Ep-BLS (Gln + Ep + Pl + rare omphacite) is more commonand well preserved with minor, strain-free, greenschist overprint. Structurally low North Peak metavolcanics display Cpx phenocrysts with glaucophane grown in strain shadows and pseudomorph aegirine on Cpx that yield incipient >8 kbars and 350-400˚C, similar to higher structural units, which suggest imbrication within subduction channel and rapid exhumation. EBSD analysis of quartz microstructures show a thermal gradient in non-coaxial shear in quartz from low- to med-T up to 450-500˚C in the host and marginal shear zone, where plagioclase in the HB shows no CPO developed suggesting diffusion creep at high-T but moderate to low strain-rates.

These data show the host units to: be structurally coherent, to record subduction and exhumation with coeval deformation / metamorphism to the intrusive HB, and be traceable across several kilometers.