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.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°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~115 million years ago.

Muztaghata Dome Miocene Eclogite Facies Metamorphism: A Record of Lower Crustal Evolution of the NE Pamir

“…We suggest that this is due to the earlier, lower grade (greenschist?) metamorphic event causing widespread recrystallization and resetting (Hoiland et al, 2018) of existing detrital (and possibly metamict) Proterozoic to Paleozoic zircon, and "locking-in" available zircon in new refractory crystals. Finally, the absence of Late Triassic to Early Jurassic metamorphic zircon in the mafic samples may be due to complete decomposition of old zircon grains coeval with rutile growth, which significantly incorporates zirconium in rutile, during Cenozoic prograde to peak metamorphism (Kohn et al, 2015).…”

Tectonochemistry of the Brooks Range Ophiolite, Alaska

“…On the basis of these fossil and isotopic ages, Box [15,50]. Recent zircon ages from Koyukuk Arc plutonic rocks confirm that the arc was active during the Cretaceous [45].…”

“…Precisely dating the onset of Brookian orogenesis has been an elusive goal for decades. Metamorphic rocks associated with this event have been difficult to date due to extensive Cretaceous greenschist-facies overprinting of the original blueschistfacies metamorphism [35,45]. This metamorphic overprint is not unique to the Brooks Range.…”

“…On the basis of invertebrate fossils and K-Ar ages [15] from igneous samples, the arc was active in the earlymiddle Jurassic. Voluminous felsic magmatism began around 150 Ma in the arc [15,45]; the timing and extent of magmatism prior to this is not well constrained…”

Burial-exhumation cycles in the continental crust derived from Mediterranean field studies and numerical modelling

“…(d) Continental strength profiles (second stress invariant (Sii) vs depth (z)) of the three models (a, b, c), plotted for the initial conditions (t=0). Brooks range 20 20 5 (Harris, 1992;Hoiland et al, 2018;Wirth et al, 1993) Caribbean (Pinos and Escambray) 15 5 5 (García-Casco et al, 2008) and references therein Kırşehir 17.5 10 5 (van Hinsbergen et al, 2016) and references therein Lesser Caucasus 12.5 5 5 (Hässig et al, 2016;Hässig et al, 2013;Rolland et al, 2010) New Caledonia 16 5 5 (Lagabrielle et al, 2013;Patriat et al, 2018) and references therein Oman 15 5 5 (Agard et al, 2010;Searle and Cox, 1999;Searle, 2007) and references therein Hellenides-Dinarides 25 15 5 (Kilias et al, 2010;Schmid et al, 2019) and references therein Quebec 10 5 2.5 (Schroetter et al, 2006;Schroetter et al, 2005;Whitehead et al, 1995) Southern Ural 25 10 10 Chemenda et al, 1997;Glodny et al, 2002) and references therein Tavşanlı 25 10 7.5 (Pourteau et al, 2013;van Hinsbergen et al, 2016) and references therein Supplementary…”