WHEN AND WHENCE THE HEAT? A CHRONOLOGY OF LATE CRETACEOUS REGIONAL METAMORPHISM AND INTRUSION IN THE NASON TERRANE, CASCADES CRYSTALLINE CORE, WA

A new U-Pb zircon age of 90.6 +/- 2.4 Ma for a small, mid-crustal, tonalite pluton 4 km northeast of the ca. 96-93 Ma hook region of the Mount Stuart batholith (Matzel, 2004; Tinkham, personal communication) may lend additional support to the inference that regional tectonic loading began shortly after emplacement of the Mount Stuart. The Mount Stuart batholith contact aureole reveals kyanite and sillimanite after andalusite replacement textures, evidence of post Mount Stuart higher pressure metamorphism. This observation and regional paleobarometric data probably reflect the emplacement of a southwest-directed thrust load across the southern Nason terrane and Mount Stuart domain. Sm-Nd garnet ages constrain peak regional metamorphism to 86 +/- 1 Ma (Stowell and Tinkham, 2003). Chiwaukum Schist within and immediately adjacent to the ca. 90.6 Ma tonalite pluton lacks evidence of low pressure contact metamorphism and, instead, shows abundant kyanite overgrown by or grown synchronously with large garnet. Assuming garnet growth initiated at elevated temperatures during tonalite intrusion, pressures must have been significantly higher in this region by ca. 90.6 Ma than 3 km southwest in the contact aureole of the Mount Stuart batholith at ca. 96-93 Ma. This suggests substantial loading of the region by ca. 90.6 Ma and, consequently, initiation of loading shortly after emplacement of the Mount Stuart batholith. This suggestion concurs with integrated geochronologic-thermodynamic modeling results of Stowell and Tinkham, 2003, that indicate only minor pressure increases during later garnet growth between ca. 88 Ma and ca. 86 Ma.