Greenschist Facies Metamorphic Zircon Overgrowths as a Constraint on Exhumation of the Brooks Range Metamorphic Core, Alaska

Abstract

Like many other mountain belts, the metamorphic core or hinterland of the Brooks Range fold and thrust belt in Arctic Alaska is characterized by multiply deformed and polymetamorphosed rocks whose histories have been challenging to decipher and thus difficult to relate to the supracrustal history of the orogen. The multiple greenschist and blueschist facies metamorphic events have been particularly difficult to resolve. This study provides petrologic context for recently identified low-temperature metamorphic zircon overgrowths at two localities across some 200 km of orogenic strike that offer a unique and precise constraint on the timing of events recorded in the Brooks Range hinterland. In consideration of microstructural context, graphite thermometry, metamorphic mineral inclusions, and zircon trace element and Lu-Hf-isotope data, metamorphic zircon growth at 114 ± 5 Ma in the southern Brooks Range is interpreted to coincide with greenschist facies metamorphism, most probably linked to decompression and/or increased temperatures within the orogenic core. Their age coincides with a proposed pulse of extension within a >600-km shear zone along the southern flank of the Brooks Range and a regional flare-up in magmatism and pronounced subsidence within hinterland depocenters (Yukon-Koyukuk Basin). These regional events are consistent with subduction retreat/rollback in mid-Cretaceous time. This study adds to a growing body of literature demonstrating the importance of searching for and characterizing metamorphic zircon growth in low- to medium-grade metamorphic terranes to provide better constraints on otherwise cryptic tectonic events.