DETRITAL ZIRCON CONSTRAINTS ON THE TIMING OF PROGRADE METAMORPHISM WITHIN THE MACLAREN GLACIER METAMORPHIC BELT, ALASKA RANGE SUTURE ZONE

The Alaska Range suture zone exposes two rock sequences at Valdez Creek, the Kahiltna assemblage (KA), a Jura-Cretaceous flysch, and the Maclaren Glacier metamorphic belt, composed of high-grade gneiss and schist. The metamorphic belt is juxtaposed with the KA across a ductile thrust, the Valdez Creek shear zone (VCSZ). Previous work has shown that the VCSZ is associated with a NE striking inverted metamorphic gradient, top-to-the south ductile shear, and peak conditions of ~8 kbar and 650 ^o C. The metamorphic rocks are interpreted to be the equivalents of the KA, and were intruded by the 74 Ma Valdez Creek tonalite during the peak of metamorphism. The KA, in the footwall, varies from sub-greenschist to lower amphibolite faces structurally up section. The thrust contact is mapped as the abrupt transition from phyllite and schist, to gneiss.

New detrital zircon ages from two samples, one from high-grade rocks in the hanging wall, and the other from low grade rocks of the footwall provide new constraints on the timing and duration of deformation and metamorphism across this inverted metamorphic gradient. The hanging wall sample was collected near the Valdez Creek tonalite, and contains a gneissic foliation. The age spectrum for the sample yielded significant age peaks at 105 Ma, 180 Ma, and 340 Ma, numerous Paleozoic, Proterozoic and Neoarchean zircons are also present. The youngest zircon at 82 Ma has a U/Th of 9.6, suggesting it is metamorphic. The maximum depositional age of the sample is 96 Ma based on the average of the three youngest zircons with low U/Th ratios. Thus, this sample was deposited at ~96 Ma, buried to amphibolite grade metamorphic conditions by 82 Ma, and intruded by the Valdez Creek tonalite at 74 Ma at a depth of 25 km.

In contrast, a sample from the KA from the footwall of the VCSZ is weakly metamorphosed, has a unimodal age distribution centered at 91 Ma and maximum depositional age of 81 Ma, and contains no zircons older than 145 Ma. The detrital age distribution suggests a local source/closed basin setting during deposition. This sample was being deposited at the same time rocks in the hanging of the VCSZ were undergoing prograde amphibolite facies metamorphism. Thus, this sequence records burial of the upper crust during major contractional deformation, metamorphism and basin closure, between 96 and 74 Ma.