Late Paleozoic assembly of the Alexander-Wrangellia-Peninsular composite terrane, Canadian and Alaskan Cordillera

Abstract

Late Paleozoic assembly of the AlexanderWrangellia-Peninsular composite terrane is recorded by two phases of regional deformation, metamorphism, and magmatism within basement complexes of the Alexander (Craig and Admiralty subterranes), Wrangellia, and Peninsular terranes in the Canadian and Alaskan Cordillera. New secondary ion mass spectrometry (SIMS) and chemical abrasionisotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) zircon U-Pb ages, whole-rock major-and trace-element and Nd-Sr isotope geochemical compositions, and geological fi eld observations of late Paleozoic igneous rocks were used to identify the precise timing and signifi cance of this tectonism in the Saint Elias Mountains region of southwestern Yukon and eastern Alaska. Middle to Late Pennsylvanian (301-307 Ma) igneous rocks, herein assigned to the Barnard Glacier suite, were preferentially emplaced along the Wrangellia-Craig subterrane boundary and mainly comprise syenitic plutons that intrude Paleozoic country rocks with evidence of Pennsylvanian or older (D1) deformation. We propose that Barnard Glacier suite magmatism was produced by a slab breakoff event after the consumption of a narrow backarc ocean basin and early Pennsylvanian collision between the WrangelliaPeninsular arc and Craig subterrane passive margin. Early Permian (284-291 Ma) dioritic to granodioritic rocks, herein assigned to the Donjek Glacier suite, comprise the vestiges of an extensive magmatic system within the Craig subterrane of southwestern Yukon and southeastern Alaska. The available data suggest that the Donjek Glacier suite represents part of a short-lived, Early Permian arc that initiated along the outboard margin of the Craig subterrane-Wrangellia-Peninsular block after Pennsylvanian collision and slab breakoff. At two fi eld localities in southwestern Yukon, Paleozoic country rocks with D1 fabrics are also intruded by sills and dikes of the Donjek Glacier suite that show evidence of ca. 285 Ma regional deformation and metamorphism (D2). Field evidence for Early Permian tectonism in the Saint Elias Mountains implies direct connections with coeval deformation and metamorphism in the Admiralty subterrane, a microcontinent in the Admiralty Island region of southeastern Alaska that developed separately from the Craig subterrane prior to the Early Per mian. D2 tectonism was likely related to the entry of the Admiralty subterrane margin into the Early Permian subduction zone, which resulted in collision and fi nal amalgamation of the Alexander-Wrangellia-Peninsular composite terrane. Our tectonic scenarios require the currently accepted confi guration of the Alexander terrane (com posite of the Craig and Admiralty subterranes) to have only existed after the Early Permian collision between the Admiralty subterrane and the previously assembled Craig subterrane-Wrangellia-Peninsular terrane. Biogeographic and other geological data suggest that the two-part assembly of the AlexanderWrangellia-Peninsular composite terrane took place along a convergent margin t...

Late Jurassic flare‐up of the Coast Mountains arc system, NW Canada, and dynamic linkages across the northern Cordilleran orogen

“…Most granitoid samples of the present study yield La/Sm UCC values ~1 or >1 (Figure a), which implies that Late Jurassic plutonic rocks in the Saint Elias Mountains were largely derived from crustal sources. Dated Late Jurassic zircons from the Saint Elias plutonic suite similarly have enriched U/Yb values (~1.4) that are analogous to the global continental arc average (~1.5) [ Grimes et al ., , ], average upper crust (~1.3) [ McLennan , ], and late Paleozoic crustally contaminated granitoids [ Beranek et al ., ] in the Saint Elias Mountains (Figure b).…”

“…The Insular terranes mainly consist of the Alexander terrane and Wrangellia in eastern and southeastern Alaska, southwestern Yukon, and coastal British Columbia (Figure b). Various components of the Insular terranes were in proximity to each other by the Late Devonian [ Israel et al ., ] and Pennsylvanian to Permian [ Gardner et al ., ; Beranek et al ., ]. The Intermontane belt underlies the central parts of the Canadian Cordillera (Figure b) and is composed of the Yukon‐Tanana terrane, Slide Mountain terrane, Cache Creek terrane, Quesnellia, and Stikinia.…”

“…A reasonable scenario to explain these significant along‐strike differences calls for Cretaceous ocean basin closure and deformation at the latitude of south‐central Alaska to reflect postaccretionary, dextral transcurrent displacement of the Insular terranes along the Cordilleran margin [e.g., McClelland et al ., ; Nelson et al ., ]. The Insular terrane block in the scientific literature is also referred to as the Alexander‐Wrangellia‐Peninsular composite terrane [ McClelland et al ., ; Beranek et al ., ], Insular superterrane [e.g., Monger et al ., ; Saleeby and Dunne , ], and Wrangellia composite terrane [e.g., Trop and Ridgway , ; Hults et al ., ].…”

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

“…Skolai arc intrusions have ages concentrated between 310 and 305 Ma (Beard & Barker, ). Other late Paleozoic plutonic suites containing potential sources for the Wrangellia terrane detritus include the Middle to Late Pennsylvanian (307–301 Ma) Barnard Glacier suite and the Early Permian (291–284 Ma) Donjek Glacier suite in eastern Alaska and southwestern Yukon (Beranek et al, , and references therein). All of these plutons are part of a suite of Pennsylvanian and Lower Permian igneous rocks exposed throughout the Wrangellia and Alexander terranes in eastern Alaska and southwestern Yukon and are the interpreted sources of sediment for our samples.…”

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

“…The composite terrane consists of three terranes (peninsular, Wrangellia, and Alexander terranes) that crop out discontinuously from western Alaska to southern British Columbia (Nokleberg et al, ; Plafker & Berg, ). The peninsular terrane is thought to have amalgamated with the Alexander and Wrangellia terranes prior to accretion to western North America (Beranek et al, ; Plafker & Berg, ; Rioux et al, ). Two different tectonic interpretations have been proposed to account for Mesozoic accretion of the Wrangellia composite terrane against inboard (northern) terranes based on studies from south central Alaska: (1) Late Jurassic collision of the Talkeetna arc (peninsular terrane) against the southern margin of the combined Wrangellia‐Alexander terrane, followed by Early Cretaceous juxtaposition against the former continental margin (Clift, Pavlis, et al, ; Rioux et al, ); and (2) Late Jurassic‐Early Cretaceous juxtaposition of the combined Wrangellia, Alexander, and peninsular terranes against the former continental margin (Hampton et al, ; Plafker & Berg, ; Ridgway et al, ; Trop et al, , ).…”