Garnet growth during crustal thickening in the Cascades Crystalline Core, Washington, USA H. STOWELL, 1 G. BULMAN, 1,2 D. TINKHAM 3 AND C. ZULUAGA 1,4 1 Geological Sciences, Box 870338 The University of Alabama, Tuscaloosa, AL, 35487-0338 USA (hstowell@geo.ua.edu) 2 CH2M Hill, 800 Fairway Dr., Deerfield Beach, FL, 33441-1830 USA 3 Department of Earth Sciences, Laurentian University, Sudbury, ON, P3E 2C6 Canada 4 Oficina 30, Departamento de Geociencias, Universidad Nacional de Colombia, Bogota ´, AA 14490 Colombia ABSTRACT Garnet Sm–Nd and zircon U–Pb ages, and pressure–temperature–time paths elucidate Late Cretaceous crustal thickening which occurred within magmatic arc rocks of the Insular Superterrane. Voluminous tonalitic magma of the Mount Stuart batholith intruded at <3 kbar into upper crustal sedimentary rocks between 96 and 91 Ma, with initial intrusion prior to garnet growth in the metasedimentary rocks. Arc plutonism then shifted northward as crustal thickening commenced. Initial garnet growth, locally with kyanite and staurolite replacing andalusite, at c. 91 Ma was directly associated with intrusion of granodiorite to tonalite sheets at 7 kbar, north of the Mount Stuart batholith, within the Nason Ridge Migmatitic Gneiss. Subsequent heating and garnet growth, which postdates emplacement of large plutons, occurred between 88 and 86 Ma. This late garnet growth occurred at pressures of 6–8 kbar. The history of garnet growth and intrusion indicates that initial garnet zone and higher temperature metamorphism was restricted to contact aureoles. However, later widespread garnet growth at higher pressure probably resulted from heating as the orogenic wedge approached thermal equilibrium after crustal thickening. We conclude that metasedimentary rocks outside narrow contact aureoles remained at temperatures significantly below those of garnet growth and that the growth of garnet lasted <6 Myr. Heating to temperatures that stabilized garnet after pluton emplacement is compatible with intrusion of arc plutons into an accretionary wedge (Chiwaukum Schist) which was tectonically thickened and ⁄ or overthrust causing loading and thermal relaxation. Key words: Cascades Core WA; garnet Sm-Nd; pseudosection; P–T–t paths; zircon U-Pb. INTRODUCTION Orogenic belts that include magmatic arcs generally include large plutons that may play a significant role in advective heat transfer (e.g. Barton & Hanson, 1989; Baxter et al., 2002; Ague & Baxter, 2007; Lyubetskaya & Ague, 2010). The southern part of the Cascades Crystalline Core (subsequently referred to as the Cas- cades Core; Fig. 1) is an exceptional example of a magmatic arc that can be studied to improve our understanding of heat transport in the mid-crust because it is very well exposed and has experienced minimal overprinting from post-Cretaceous thermal events. Testing models for understanding the thermal history of orogenic belts requires high-quality esti- mates for temperature and pressure, and these data must be precisely dated. This contribution presents new geochronological and petrological data that elu- cidate the interplay of heat transport by magma within this contractional magmatic arc. Geochronological and thermobarometric results underline the impor- tance of magmatism to metamorphism during crustal thickening of the Chiwaukum Schist in the Nason terrane of the Cascades Core. The Chiwaukum Schist consists of metapelitic and metapsammitic schist, lesser amounts of amphibolite and ultramafite, and rare lenses of marble. These rocks were probably assembled in an accretionary wedge prior to the mid-Cretaceous (Paterson et al., 1994). The Mount Stuart batholith is the largest intrusive complex that intruded the Cascades Core. Multiple plutons in the batholith intruded between 96 and 91 Ma (Matzel et al., 2006) at <4 kbar (Evans & Davidson, 1999; Stowell et al., 2007). The north- western part of the batholith intruded at c. 96 Ma. North of the Mount Stuart batholith, the Tenpeak and Dirtyface plutons intruded at 92–90 Ma and at 91 Ma, respectively (Miller et al., 2003; Matzel et al., 2006). Garnet Sm–Nd and zircon U–Pb geochronology, in conjunction with isochemical phase diagram sections (pseudosections) for metamorphic rocks along the northern margin of the Mount Stuart batholith, near Labyrinth Mountain (Fig. 2), are used to construct pressure–temperature–time (P–T–t) paths. These P–T–t J. metamorphic Geol., 2011, 29, 627–647 doi:10.1111/j.1525-1314.2011.00933.x Ó 2011 Blackwell Publishing Ltd 627