ALBERT

Description: We present sensitive high-resolution ion microprobe (SHRIMP) U-Pb geochronology and trace-element chemistry of zircons from the Chugach metamorphic complex, southern Alaska, which allow us to precisely constrain the age and duration of peak metamorphism. Zircons separated from migmatitic metasedimentary gneisses show thin (5–30 μm) overgrowths with trace-element compositions indicating growth at or close to the metamorphic peak. Five U-Pb ages of such overgrowths and two U-Pb ages from magmatic zircons from plutons intruding the metasediments from the western and central parts of the complex range between 54.0 ± 0.9 Ma and 52.6 ± 1.0 Ma and are within error of each other. One sample from the southeastern part of the complex yields a zircon overgrowth age of 51.3 ± 0.7 Ma, which is within error of two of the seven samples from the western and central parts. Combined with previous U-Pb zircon ages of magmatic rocks of the Sanak-Baranof belt, our data constrain the age and duration of metamorphism and magmatism to a time span of maximum ∼3 m.y. between ca. 55 and 52 Ma on an ∼200-km-long section in the western and central part of the complex. A time span of ∼2 m.y. between ca. 52 and 50 Ma is constrained from the southeastern part of the complex to Baranof Island. Maximum depositional ages of the sediments in which the complex developed (ca. 60 Ma) limit the time available for prograde metamorphism to a narrow time window of ∼5 m.y. This short time span for prograde metamorphism, followed by near-synchronous peak metamorphism and magmatism for ∼800 km along the margin could be the result of ridge subduction, and we discuss tectonic models related to this for the region of interest.