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

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 200km 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 1145Ma 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. Plain Language Summary Rocks within mountain belts typically contain metamorphic minerals and textures that grew and developed in response to heating, burial, and deformation. In order to understand the evolution of mountains and the plate tectonic interactions that formed them, it is important to be able to determine when those metamorphic minerals and textures grew. The minerals best suited to directly dating tectonic events typically only grow at high temperatures (like >550 degrees C), whereas minerals that grow at lower temperatures are often easily disturbed during subsequent heating or tectonic events, rendering their ages suspect. In our study of the Brooks Range mountain belt in northern Alaska, we characterize low-temperature overgrowths on older zircon crystals that can provide excellent ages using the U-Pb radioactive decay scheme because of their closed system behavior with no loss of daughter products. In the case of the Brooks Range, these zircon overgrowths appear to have formed as enclosing rocks were uplifted toward the surface of the Earth. Their uplift coincided with formation of deep water marine basins and enhanced volcanic activity, which we interpret as due to southward rollback of the subduction zone beneath Alaska beginning similar to 115 million years ago.