Detrital zircon Hf isotopic compositions indicate a northern Caledonian connection for the Alexander terrane

Abstract

Various plate reconstructions predict that the Alexander terrane, a Neoproterozoic-Jurassic crustal fragment now located in the North American Cordillera, evolved in proximity to the northern Appalachian-Caledonian convergent margin during assembly of supercontinent Laurussia. To test stratigraphic connections with Laurussia that are implied by these plate reconstructions, we measured the Hf isotopic compositions of 176 detrital zircons from two relevant sedimentary sequences of the Alexander terrane. An older, Upper Silurian-Lower Devonian terrestrial to shallow-marine molasse sequence yields 405-490 Ma detrital zircons with negative ε Hf(t) values and Mesoproterozoic to Paleoproterozoic Hf model ages. In combination with paleomagnetic and biogeographic constraints, these Hf data argue for the molasse strata to be now-displaced equivalents of the Old Red Sandstone and primarily sourced from crustally contaminated granitoids in the Greenland, Svalbard, or British Caledonides. Late Silurian-Early Devonian orogenesis in the Alexander terrane is therefore likely related to the Scandian-Salinic phase of Appalachian-Caledonian mountain building. Younger, Middle Devonian sequences of the Alexander terrane are endowed in 390-490 Ma detrital zircons with positive ε Hf(t) values and Neoproterozoic Hf model ages. These isotopic signatures are consistent with the erosion of local basement rocks during the opening of the Slide Mountain-Angayucham backarc rift and tectonic separation of the Alexander terrane from northern Laurussia.

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.…”

“…Silurian-Lower Devonian strata from the Icefield assemblage ( Fig. 16B; Beranek et al, 2013b), as well as age-equivalent and possibly correlative rocks from the Banks Island assemblage ( Fig. 16A; Tochilin et al, 2014), contain detrital zircon age populations that are generally similar to those from coeval Dillinger subterrane units (Figs.…”

“…1 and 2) is a Neoproterozoic-Jurassic crustal fragment (Beranek et al, 2013b) that includes lower Paleozoic strata with paleontologic links to the Farewell terrane, particularly in faunas of late Silurian-Middle Devonian age (Blodgett et al, 2002(Blodgett et al, , 2010Soja, 2006, 2016;Soja, 2008). Recently published detrital zircon data from the Alexander terrane ( Figs.…”

“…S1 [footnote 1]; Dumoulin et al, 2014a). Hf isotope values of detrital zircons in Ordo vician samples from the northern Alexander terrane and the Banks Island assemblage have a mix of positive and negative values (Beranek et al, 2013a(Beranek et al, , 2013bTochilin et al, 2014). Hf data are not available from Ordovician rocks on Seward Peninsula or in the Farewell terrane, but Ordovician and older grains from Silurian strata in the Dillinger subterrane have a range of Hf values (B.…”

“…The Alexander terrane is a potential source for the late Cambrian-Early Ordo vician zircons present in the Farewell terrane, Livengood terrane, and the York Mountains (unit Oal). In the Ordovician paleogeographic reconstruction of Beranek et al (2013b), Baltica and associated terranes are shown in positions more proximal to Siberia and northern Laurentia. This is consistent with Ordovician fossil data linking the Farewell terrane and Arctic Alaska to all three of these cratons.…”