PROTOLITH AND DEFORMATION AGES OF MESOZOIC HOST ROCK UNITS IN THE CASCADES CORE, WA: IMPLICATIONS FOR CRUSTAL AMALGAMATION AND MESOZOIC OROGENESIS

The Cascades crystalline core consists of a series of amalgamated, largely oceanic protoliths intruded by Triassic to Eocene plutons. Although ages of magmatism are known, protolith ages and timing of deformation and metamorphism have been problematic. Below I present new LA-ICPMS, U/Pb zircon ages of detrital zircons in sedimentary units and in situ zircon ages from sills intruding these units. The oldest unit in the southern core, the Napeequa Complex (mudstones, quartzites= metacherts?, amphibolites=metabasalts and less common metaperidotite and marble=accretionary complex), are very poor in zircon but rich in sphene and monazite. Limited zircon ages range from ~210 Ma to 2.5 Ga, some having high U/Th rims that give ages between 130-72 Ma. These protoliths are intruded by highly deformed sills ranging in age from 192-76 Ma, thus constraining protolith ages to ~192-210 Ma, accretion by 192 Ma and episodic ductile deformation with the youngest after 76 Ma.

The Chiwaukum schist (marine sands and shales with lenses of metabasalt and ultramafite = accretionary complex), has abundant detrital zircons giving a minimum peak at 120-125 Ma, older Mesozoic ages and Precambrian ages ranging back to 2.7 Ga. Structurally lower paragneisses have minimum peaks at 145 Ma. Both schists and paragneisses are intruded by 90-96 Ma plutons and sills many of which are strongly deformed, thus implying protolith ages >96 Ma and probably~145 Ma (paragneisses) and ~120 Ma (schists). These units experienced at least 5 cycles of deformation and accompanying metamorphism, the youngest of which deforms 92 Ma sills.

The youngest host rock unit in the southern core is the Swakane biotite schist/gneiss (clastic basin sediments, local marble and amphibolite), which has detrital zircon (with low U/Th ratios) ages with a minimum peak of 70-75 Ma and older zircons ranging back to 1.78 Ga. Zircon ages from local melt leucosomes fall between 60-65 Ma and include xenocrystic zircons with ages ranging back to 1.68 Ga. Both zircon groups have high U/Th ratio rims with ages ranging from 70-46 Ma, with the youngest overlapping with published hornblende and biotite cooling ages of 40-50 Ma. No Mesozoic arc-related plutons intrude this unit, thus implying a protolith age of 70-75 Ma, burial, underplating and melting by ~65 Ma, and uplift and cooling between 65-45 Ma.