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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 3kbar into upper crustal sedimentary rocks between 96 and 91Ma, 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.91Ma was directly associated with intrusion of granodiorite to tonalite sheets at 7kbar, 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 86Ma. This late garnet growth occurred at pressures of 6-8kbar. 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 6Myr. 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. 2011 Blackwell Publishing Ltd.

Notes:

Compilation and indexing terms, Copyright 2018 Elsevier Inc.<br/>20112814134202<br/>Crustal thickenings<br/>Metasedimentary rocks<br/>P-T-t path<br/>Pressure-temperature-time paths<br/>Pseudosection<br/>Thermal equilibriums<br/>Thermal relaxation<br/>Zircon U-Pb