Detrital zircon U-Pb geochronology and Hf isotope geochemistry of the Yukon-Tanana terrane, Coast Mountains, southeast Alaska

Abstract

Rocks of the SE Alaska subterrane of the Yukon-Tanana terrane (YTTs) consist of regionally metamorphosed marine clastic strata and mafic to felsic volcanic-plutonic rocks that have been divided into the pre-Devonian Tracy Arm assemblage, Silurian-Devonian Endicott Arm assemblage, and Mississippian-Pennsylvanian Port Houghton assemblage. U-Pb geochronologic and Hf isotopic analyses were conducted on zircons separated from 23 igneous and detrital samples in an effort to reconstruct the geologic and tectonic evolution of this portion of YTT. Tracy Arm assemblage samples are dominated by Protero zoic (ca. 2.0-1.6, 1.2-0.9 Ga) and Archean (2.7-2.5 Ga) zircons that yield typical cratonal ε Hf(t) values. Endicott Arm assemblage samples yield U-Pb ages that range from Late Ordovician to Early Devonian and ε Hf(t) values that range from highly juvenile to moderately evolved. Port Houghton assemblage samples yield similar Ordovician-Devonian ages and ε Hf(t) values, and also include early Mississippian zircons with highly evolved ε Hf(t) signatures. Comparison of these age-Hf patterns with data from nearby assemblages suggests the following: (1) Results from YTTs are similar to (or compatible with) available data from rocks of the Yukon-Tanana terrane in eastern Alaska, Yukon, and northern British Columbia (YTTn) and pericratonic strata in east-central Alaska (NAa). (2) YTTs contains abundant Late Ordovician-Early Devonian magmatism that is not recorded in YTTn and NAa. (3) The ε Hf(t) values from YTTs display two excursions from juvenile to evolved ε Hf(t) values, which are interpreted to record cycles of crustal thinning and then thickening within a convergent margin system. (4) Available data from both YTTs and YTTn support Neoproterozoic(?)-early Paleozoic positions along the northern Cordilleran margin. (5) The Late Ordovician-Early Devonian magmatic record of the southern Alexander terrane is very similar to that of YTTs, which raises the possibility that these assemblages evolved in the same convergent margin system along the northern (Alexander) and northwestern (YTT) margins of Laurentia. These results support a tectonic model in which: (1) YTTs formed outboard of (or northward along strike of) YTTn and NAa along the northern Cor dilleran margin during Neoproterozoic(?)-early Paleozoic time; (2) initial subduction-related magmatism during Late Ordovician to Early Devonian time records a progression from crustal thinning to crustal thickening, and is preserved only in YTTs; (3) a second phase of magmatism records Middle-Late Devonian crustal thinning followed by early Mississippian crustal thickening; (4) YTTs and YTTn evolved as an intra-oceanic arc outboard of the Slide Mountain ocean basin during Carboniferous-Permian time and were accreted to the continental margin during Triassic time; and (5) YTTs is interpreted to have been displaced ~1000 km southward, from an original position outboard of YTTn/NAa to its present position outboard of the Stikine terrane, along a sinistral fault system of Late Jurassic-Early...

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

“…The absence of Precambrian zircons in all of our samples precludes correlation with rocks of the Yukon‐Tanana or Taku terranes [ Kapp and Gehrels , ; Giesler et al ., ; Pecha et al ., ] or Gravina belt [ Yokelson et al ., ]. Instead, ages for metasedimentary rocks reported here closely match the age of the lower Hazelton Group in western Stikinia [ Evenchick et al ., ; Gagnon et al ., ], which also displays unimodal DZ spectra of the same age [ Barresi et al ., ; Cutts 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 , ].…”

“…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 ., ].…”

“…[]; renamed as the “Tracy Arm assemblage” by Gehrels et al . []) were correlated with the Yukon‐Tanana terrane across the Coast shear zone on the basis of gradational contacts with stratigraphically overlying Permian and Triassic volcanic rocks and a distinctive abundance of Paleoproterozoic DZs [ McClelland et al ., ; Gehrels , , ; Gehrels et al ., ; Giesler et al ., ; Pecha et al ., ]. However, an along‐strike correlation of these rocks with the Central Gneiss Complex is complicated by the uncertain northwestward continuation of a major normal fault that bounds the northeastern margin of the Central Gneiss Complex (Figure ).…”

Detrital Zircon Record of a Mesozoic Collisional Forearc Basin in South Central Alaska: The Tectonic Transition From an Oceanic to Continental Arc

“…)-Paleozoic metasedimentary rocks of the Yukon composite terrane located north of the Alaska Range (Figure 1). U-Pb ages from metamorphic basement rocks in Alaska, Yukon Territory, and British Columbia range chiefly between 2000 and 1700 Ma and between 3000 and 2200 Ma (Dusel- Bacon & Williams, 2009;Gehrels et al, 1995;Gehrels & Kapp, 1998;Gehrels & Ross, 1998;Pecha et al, 2016;Piercey & Colpron, 2009). These metamorphic rocks also include Middle to Upper Devonian and Middle to Upper Permian magmatic intrusions (Mortensen, 1992;Nelson et al, 2006).…”