NEW INSIGHTS ON SYNTECTONIC EMPLACEMENT OF THE HICKS BUTTE PLUTON IN THE CENTRAL CASCADES OF WASHINGTON STATE

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.