HF ISOTOPIC SIGNATURE OF NORTHERN CORDILLERAN TERRANES: IMPLICATIONS FOR TECTONIC HISTORY AND DZ METHODOLOGY (Invited Presentation)

Hf isotope data sets are now available for several terranes in the northern Cordillera and Arctic regions, including the Mystic (Malkowski and Hampton, 2014), Alexander (Beranek et al., 2013; Tochilin et al., 2014; White et al., 2016), Yukon-Tanana (Pecha et al., 2016), Taku (Giesler et al., 2016), Wrangellia (Gehrels and McClelland, 2011), Doonerak (Strauss et al., 2017), and Pearya (Malone et al., 2017) terranes. Epsilon Hf values measured from detrital zircons in Paleozoic-lower Mesozoic strata from these terranes record significant variations in the involvement of juvenile versus evolved crust during magmatism. Although connections among the various terranes are uncertain, and none are definitively tied to Laurentia, all record a consistent set of “pull-downs” in epsilon Hf which coincide with well-known phases of orogenic activity/crustal thickening along Laurentia’s northeastern and northwestern margins. The pull-downs and their apparently associated orogenic phases occurred at ~475 Ma (Taconic), ~420 Ma (Caledonian), ~360 Ma (Ellesmerian-Antler), and ~270 Ma (Klondyke-Sonoma).

Similarities in Hf isotope patterns with tectonic events along the Arctic and Cordilleran margins support current models in which all of these terranes formed in a complex but coherent orogenic system along the Arctic and then northern Cordilleran margins, and also demonstrate that Hf isotopes of detrital zircons provide a powerful tool for analyzing terrane connections, sedimentary provenance, and source terrane characteristics. To facilitate use of Hf isotope information, we offer several new types of diagrams that portray both U-Pb and Lu-Hf data in the time domain. For provenance, U-Pb Crystallization Age is plotted against Source Terrane Age, which is the average age of source materials from which the zircons crystallized (T_DM). For source terrane characterization, Crystallization Age is plotted against Crustal Residence Time (T_DM – T_{crystalization}). And for tectonic classification, Crustal Residence Time (T_DM – T_{crystalization}) is plotted against Lag Time (T_{crystalization} - T_deposition). The latter provides a powerful tool for coordinated analysis of surficial processes (e.g., erosion, transport, deposition, recycling), and deep-crustal processes (e.g., crustal thickening, crustal melting, magmatism).