INTRUSION, CRUSTAL THICKENING, AND RAPID GARNET GROWTH, NORTH CASCADES WASHINGTON

Metamorphic textures, garnet Sm-Nd and zircon U-Pb ages, and P-T-t paths elucidate Late Cretaceous intrusion and crustal thickening which occurred within magmatic arc rocks of the Cascades Crystalline Core. Tonalitic magma of the Mount Stuart batholith intruded at <3 kbar into meta-? sedimentary rocks between 96 and 91 Ma. Initial intrusion caused growth of andalusite in narrow contact aureoles along the northern margin of the batholith. Initial garnet growth at 90.2±1.5 to 90.1±1.5 Ma (garnet Sm-Nd), locally with kyanite and staurolite replacing andalusite, was associated with 90.6±1.8 to 92.2±1.8 Ma (zircon U-Pb) intrusion of granodiorite to tonalite sheets at 7 kbar, north of the Mount Stuart batholith in the Chiwaukum Schist. Garnet porphyroblasts adjacent to intrusive sheets preserve sharp, but low magnitude sector and oscillatory zoning in Ca. Growth and preservation of this zoning requires rapid crystal growth, and heating and cooling. Rapid garnet growth is likely required for development of sector zoning, and preservation of zoning with the max recorded temperatures of ca. 640ºC requires rapid cooling because this temperature is sufficient for volume diffusion which would erase the zoning. Subsequent 88 to 86 Ma (garnet Sm-Nd) widespread heating and garnet growth, occurred at 6-8 kbar and postdated emplacement of large plutons. The history of garnet growth and intrusion indicates that initial garnet zone and higher temperature metamorphism was restricted to contact aureoles. Schist outside narrow contact aureoles remained at temperatures significantly below those of garnet growth and garnet growth lasted <6 m.y. Heating to garnet zone after pluton emplacement is compatible with intrusion of arc plutons into an accretionary wedge which was tectonically thickened and/or overthrust. Late widespread garnet growth at higher pressure likely resulted from heating as the orogenic wedge approached thermal equilibrium after crustal thickening.