U-Pb and Hf isotope analysis of detrital zircons from Paleozoic strata of the southern Alexander terrane (southeast Alaska)

Abstract

The Alexander terrane is an unusual tectonic fragment in the North American Cordillera in that it contains a long and very complete stratigraphic record, including sedimentary or volcanic rocks representing every period and nearly every epoch between Neoproterozoic and Late Triassic time. The terrane is also unusual in that the southern portion of the terrane experienced arc-type magmatism during Neoproterozoic-early Paleozoic time, whereas the northern portion of the terrane consists mainly of Paleozoic shelf-facies strata. This long and diverse history provides opportunities to reconstruct the evolution and displacement history of the terrane, and specifically test the prevailing interpretation that the terrane formed in the paleo-Arctic realm.This study presents U-Pb geochronologic data and Hf isotopic information for detrital zircons from arc-type rocks in the southern portion of the terrane. Information has been generated from seven samples of Ordovician through Devonian age, complementing the information available from previous studies of Ordovician through Triassic strata. Together, these data sets yield a robust record of the magmatic history of the southern Alexander terrane, with dominant age groups of 640-550 Ma, 490-400 Ma, 380-340 Ma, and 310-275 Ma (dominant ages of 579, 441, 361, and 293 Ma). There are few pre-640 Ma grains in any of the samples. Hf isotope compositions of the detrital zircons are exceptionally juvenile, with most epsilon Hf (t) values between +15 and +5.Collectively, the available geologic, U-Pb geochronologic, and Hf isotopic evidence suggests that the southern Alexander terrane formed within a juvenile Neoproterozoic-early Paleozoic arc system, with little continental influence, whereas the northern portion of the terrane formed in proximity to a continental landmass that experienced similar Neoproterozoic-early Paleozoic ages of continental-affinity magmatism. Our data are consistent with previous suggestions that the Alexander terrane resided in the paleo-Arctic realm during early Paleozoic time, with the northern portion of the terrane adjacent to Baltica and the Caledonides, and the southern portion of the terrane forming further offshore as a juvenile north-facing oceanic arc. GEOLOGIC SETTINGThe Alexander terrane consists of three distinct components, which are exposed in SE Alaska, the Saint Elias Mountains region to the north, and the Banks Island region to the south (inset of Fig. 1). In southern SE Alaska, the ter-

Sediment underthrusting within a continental magmatic arc: Coast Mountains batholith, British Columbia

“…These western terranes largely consist of Neoproterozoic to middle Paleozoic arc‐related rocks, Devonian to Permian carbonate and interbedded clastic sedimentary rocks, Triassic volcanic and sedimentary rocks, and unconformably overlying Upper Jurassic to Lower Cretaceous clastic strata and mafic volcanic rocks of the Gravina belt [e.g., Gehrels , ]. Clastic sedimentary rocks and younger rocks sourced from these terranes (e.g., Gravina belt) are characterized by abundant Neo‐ to Paleo‐proterozoic and Devonian detrital zircon (DZ) grains [ Kapp and Gehrels , ; Gehrels , , ; Gehrels et al ., , , ; Tochilin et al ., ; White et al ., ; Yokelson et al ., ; Giesler et al ., ; Pecha et al ., ]. Northwest of these terranes and ~300 km northwest of the Central Gneiss Complex, translated slivers of >205–125 Ma metamorphic rocks and 101–55 Ma clastic sedimentary rocks constitute the Chugach terrane [ Berg et al ., ; Amato et al ., ].…”

“…Rocks exposed within and adjacent to the Central Gneiss Complex were subjected to up to ~1000 km of Late Jurassic to mid‐Cretaceous sinistral displacement along northwest‐striking faults [ Avé Lallemant and Oldow , ; Plafker et al ., ; Monger et al ., ; Chardon , ; Chardon et al ., ; Butler et al ., ; Gehrels et al ., ; Wolf et al ., ; Angen et al ., ; Tochilin et al ., ; Yokelson et al ., ; Giesler et al ., ; Pecha et al ., ; White et al ., ]. This was followed by development of a southwest‐vergent, mid‐Cretaceous thrust belt associated with underthrusting of the Alexander‐Wrangellia composite terrane, which is now located southwest of the Coast shear zone [ Crawford et al ., , ; Crawford and Hollister , ].…”

Neoproterozoic–early Paleozoic provenance evolution of sedimentary rocks in and adjacent to the Farewell terrane (interior Alaska)

“…Recently published detrital zircon data from the Alexander terrane ( Figs. 16A-16G; Beranek et al, 2013aBeranek et al, , 2013bTochilin et al, 2014;White et al, 2015) show similarities and differences with data from the Farewell terrane. Quartzose sandstones from the upper Cambrian-Middle Ordovician Donjek assemblage (northern Alexander terrane) yielded predominant age populations ranging from 760 to 565 Ma ( Fig.…”

“…Local igneous sources for late Cambrian to Early Ordovician zircons are not known in the Farewell terrane or Arctic Alaska, but they do occur in the Alexander terrane (Beranek et al, 2013a;Tochilin et al, 2014;White et al, 2015). Thus, late Neoproterozoic zircons in Farewell terrane strata may have come from Dumoulin et al | Provenance evolution, Farewell terrane GEOSPHERE | Volume 14 | Number 2 Arctic Alaska, Alexander terrane, and/or other elements of the Timanide orogeny.…”

“…500-430 Ma magmatic rocks (Gehrels and Saleeby, 1987;Cecil et al, 2011) and is thus a potential source for detrital zircons of this age. However, epsilon Hf isotope values from these magmatic rocks are positive (Cecil et al, 2011;White et al, 2015), whereas Silurian detrital zircon grains in Dillinger subterrane samples have both positive and negative values (B. Hampton, 2012, written commun.…”