Geology, U‐Pb Geochronology, and Hf Isotope Geochemistry Across the Mesozoic Alaska Range Suture Zone (South‐Central Alaska): Implications for Cordilleran Collisional Processes and Tectonic Growth of North America

Abstract

The Mesozoic Alaska Range suture zone is defined by a transition from oceanic to continental terranes and is part of a 2000-km-long tectonic boundary throughout the northern Cordillera. Surface geologic mapping of the rock types across this suture zone provides critical information about the upper crustal configuration but provides little insight into the sedimentary, igneous, and metamorphic processes that occurred in deeper levels of the collisional zone. To better constrain the timing and mantle-crust sources of collision-related magmatism, we combine U-Pb ages and Hf isotope compositions of detrital zircons from the three main components of the suture zone. U-Pb/Hf data sets from inboard, continental margin samples have Precambrian-Paleozoic ages with epsilon Hf (t) values ranging between +10 and −20. U-Pb/Hf data sets from the outboard, oceanic terrane samples have Pennsylvanian-Permian detrital zircon ages with epsilon Hf (t) values between +16 and +10. The U-Pb/Hf data set of detrital zircons from Mesozoic strata that represent intervening sedimentary basin(s) that formed between the continental margin and oceanic terranes record Precambrian-Mesozoic detrital zircon ages with epsilon Hf (t) values between +14 and −20. Results of the study document four Archean and Proterozoic global crustal magmatic events that are correlated to the tectonic growth of Laurentia. Also, three Phanerozoic magmatic events have been identified that represent more regional tectonic events. This study demonstrates that the combination of U-Pb geochronology and Hf isotope geochemistry applied to detrital zircons is a powerful tool to define sediment provenance in collisional zones and delineate episodes of global and regional magmatism along convergent plate boundaries.

Geochemical and Stratigraphic Analysis of the Chisana Formation, Wrangellia Terrane, Eastern Alaska: Insights Into Early Cretaceous Magmatism and Tectonics Along the Northern Cordilleran Margin

“…If a west dipping subduction zone was present under the Insular terranes, one might also expect an isotope signature of subducted continental material (e.g., Yukon Tanana terrane sediment) to be present in the Chisana lavas like what has been documented for other arc‐continent collisions (Draut & Clift, 2001; Elberg et al, 2002). Detrital zircon studies demonstrate that Precambrian‐Paleozoic detritus eroded from the continental margin (e.g., Yukon Tanana terrane) was shed into the Kahiltna basin during late Early Cretaceous time (Hampton et al, 2010; Romero et al, 2020). However, a continental margin signature is seemingly not evident in the isotope characteristics of Chisana lavas.…”

“…120 Ma with diachronous south-to-north closure of east dipping subduction zone along the inboard (northeastern eastern) margin of the Insular terranes. In this model, northern portions of the Insular terranes presently exposed in south central and eastern Alaska changed from the model depicted in (a) to the model depicted in (c) during Late Jurassic to Late Cretaceous time consistent with the evolution of the Nutzotin and Kahiltna basins (e.g., Manuszak et al, 2007;Romero et al, 2020;Trop et al, 2019Trop et al, , 2020). Our preferred model for Chisana magmatism is depicted in (a); our data do not support the model in (b).…”

“…Sedimentological and detrital geochronological data sets from these forearc sedimentary strata reflect erosion of Jurassic volcanic-plutonic arc source terranes and trenchward (southward) sediment transport (Goddard et al, 2018;Reid et al, 2018;Trop et al, 2005). Late Jurassic to Early Cretaceous strata that crop out in a retroarc position north of the Jurassic-Cretaceous Chisana-Chitina-Talkeetna volcanic arcs record sediment flux away from the magmatic arcs as well as sediment input from inboard terranes (e.g., Fasulo, 2019;Hampton et al, 2010;Romero et al, 2020). An inboard (northeastward relative to modern locations) shift in arc magmatism from the Jurassic Chitina arc to Cretaceous Chisana arc represents long-lived subduction under the Insular terranes ( Figure 10), analogous to the inboard shift in magmatism well documented within the Insular terranes in southeastern Alaska and coastal British Columbia (i.e., Coast Ranges batholith) (Gehrels et al, 2009).…”

Detrital zircon geochronology and Hf isotope geochemistry of Mesozoic sedimentary basins in south-central Alaska: Insights into regional sediment transport, basin development, and tectonics along the NW Cordilleran margin

“…These zircons were likely recycled from older sedimentary strata in the Northern Cordillera, such as Paleozoic-Triassic passive-margin strata, where similar-aged Precambrian detrital zircon populations have been recognized ( Fig. 1; Gehrels et al, 1995;Gehrels and Pecha, 2014;Romero et al, 2020).…”

Detrital zircon geochronology of modern river sediment in south-central Alaska: Provenance, magmatic, and tectonic insights into the Mesozoic and Cenozoic development of the southern Alaska convergent margin

“…The source of these Precambrian grains is likely from Neoproterozoic-early Paleozoic metasedimentary basinal strata associated with the parautochthonous continental margin of North America. These metasedimentary strata are extensively exposed in the Tanana River watershed and contain similar detrital zircon age populations between 2.7 and 2.6 Ga, 1.9 and 1.8 Ga, and 1.6 and 0.9 Ga (e.g., Dusel-Bacon et al, 2017;Romero et al, 2020), making them likely sources of these detrital zircons in the modern river sediment samples.…”