Detrital zircon ages indicate an Early Cretaceous episode of blueschist-facies metamorphism in southern Alaska: Implications for the Mesozoic paleogeography of the northern Cordillera

Abstract

Detrital zircon U-Pb ages are presented from the Liberty Creek schist in the central Chugach Mountains that indicate two distinct periods of preservation of blueschist-facies metamorphism along the southern Alaskan margin. A maximum depositional age (MDA) of 136 Ma demonstrates that the Liberty Creek schist was deposited long after the Early Jurassic cooling ages (196–185 Ma) recorded in other western Alaskan schist bodies containing blueschist-facies rocks, thus revealing two distinct blueschist-facies preservation events: an Early Jurassic event and a post–Early Cretaceous event. This Early Cretaceous depositional age also indicates that there have been major reorganizations within this subduction complex because the Potter Creek assemblage (MDA of 169–156 Ma), directly south of the Liberty Creek schist, is an older but more shallowly exhumed assemblage. Strike-slip faulting has rearranged the accretionary complex by carrying the Potter Creek assemblage outboard and south of the Liberty Creek schist. The predominance of 140–130 Ma zircons in the Liberty Creek schist sample and a population of detrital zircons that is distinct from nearby terranes suggest a sedimentary source different from other related accretionary assemblages. Three suggested Cordilleran source terranes are the Chitina Valley batholith immediately to the east; the Firvale suite of the Coast plutonic complex, ∼1500 km to the southeast near Vancouver, British Columbia; or our preferred source, the southern Mexican Guerrero terrane, ∼3000 km to the southeast. The detrital zircon signature of the Liberty Creek schist and these distances to potential sources support models suggesting thousands of kilometers of strike-slip movement along the western Cordillera since Cretaceous time, consistent with the Baja–British Columbia hypothesis.