ALBERT

Description: A detailed tectonic analysis of the magmatic evolution of the Early to Middle Ordovician west-facing Popelogan arc in New Brunswick and adjacent Maine is presented based on combining new U-Pb zircon radiometric age dates (thermal ionization mass spectrometry and sensitive high-resolution ion microprobe) with existing age constraints on the various magmatic events recognized in this arc system. The Popelogan arc had a life span of nearly 20 m.y. (ca. 476–457 Ma) and becomes progressively younger toward the northwest. Existing lithogeochemical and isotope data combined with field relationships and new zircon inheritance data indicate that the Popelogan arc was built on the leading edge of Ganderia and represents a predominantly continental calc-alkaline arc. Incursions of arc to within-plate–like tholeiitic magmatism correspond to rifting events temporally and spatially linked to trenchward migration of the arc magmatic axis, isolation of arc ribbons, and formation of mafic oceanic and transitional crust in the associated Tetagouche backarc basin. Arc-trench migration and backarc basin opening were caused by a retreating subduction zone. Slab retreat prior to 467 Ma was at least partially accommodated by arc migration, but thereafter, it was mainly accommodated by extension and spreading in the associated Tetagouche backarc basin. New U-Pb zircon radiometric ages and lithogeochemistry of calc-alkaline felsic tuff beds interlayered with the oceanic backarc rocks of the Fournier Supergroup provide a direct link between the Tetagouche backarc rocks and coeval calc-alkaline arc activity in the Popelogan arc. The oldest incursion of arc tholeiite (470–467 Ma) corresponds to cessation of the ca. 476–470 Ma calc-alkaline Meductic phase of the Popelogan arc, the onset of the calc-alkaline Balmoral phase (467–457 Ma) farther to the west, and formation of oceanic and mafic-dominated transitional crust in the Canoe Landing Lake Formation of the Tetagouche backarc basin. A less-well-constrained second incursion of tholeiitic magmatism between 465 and 459 Ma corresponds to rifting of the Tetagouche arc ribbon and formation of the ophiolitic Devereaux complex in the Fournier Supergroup. Cessation of calc-alkaline arc magmatism and eruption of transitional and alkalic mafic volcanic rocks between 459 and 455 Ma may represent slab breakoff following accretion of the Popelogan arc to composite Laurentia, and/or ridge subduction immediately prior to arc-continent collision.

Description: Late Paleozoic assembly of the Alexander-Wrangellia-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 abrasion-isotope 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 field observations of late Paleozoic igneous rocks were used to identify the precise timing and significance 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 Wrangellia-Peninsular 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 field 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 Permian. D2 tectonism was likely related to the entry of the Admiralty subterrane margin into the Early Permian subduction zone, which resulted in collision and final amalgamation of the Alexander-Wrangellia-Peninsular composite terrane. Our tectonic scenarios require the currently accepted configuration of the Alexander terrane (composite 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 Alexander-Wrangellia-Peninsular composite terrane took place along a convergent margin to the north of the Cordilleran pericratonic arc terranes (Yukon-Tanana, Quesnellia, and others), in between the paleo–Pacific Ocean and paleo–Arctic Ocean realms, to the northwest of the supercontinent Pangea. The assembly of the Alexander-Wrangellia-Peninsular composite terrane might have been associated with the Early to Middle Permian subduction polarity flip recognized in the Cordilleran pericratonic realm, which led to the closure of a backarc ocean basin and Late Permian arc-continent collision along the western margin of North America.

Description: Devonian and younger plutonic suites of the Appalachian orogen in Newfoundland form a curvilinear belt straddling the boundary between Ganderia and Avalonia, and exhibit a general pattern of younging to the southeast. However, many individual plutonic suites within this belt remain undated or have ages that are poorly constrained. This study reports U–Pb sensitive high-resolution ion microprobe (SHRIMP) data from zircon and 40 Ar– 39 Ar data from hornblende, biotite, and muscovite, which collectively improve definition of post-tectonic plutonic events in southeastern Newfoundland, but at the same time raise new questions for further study. U–Pb zircon SHRIMP determinations provide valuable new geochronological information for the Deadman’s Bay Granite (390 ± 4 Ma), the Gander Lake Granite (378 ± 4 Ma), the Terra Nova Granite (384 ± 4 Ma), the Old Woman stock (376 ± 4 Ma), and the easternmost unit (Tolt phase) of the composite Ackley Granite (372 ± 4 Ma). The Pass Island Granite, long considered part of this age group, gave a Silurian age of 423 ± 4 Ma and provides the first confirmation of Silurian granitoid magmatism in Avalonia of Newfoundland. The Berry Hills Granite on the Burin Peninsula, previously designated as Devonian, instead gave a Neoproterozoic age of 570 ± 6 Ma, and a similar age of 575 ± 6 Ma was obtained from granite that is intruded by the ca. 376 Ma Old Woman stock. These results indicate that regional geological context, deformation state, and (or) petrographic–geochemical data are not reliable indicators of emplacement age in this region. 40 Ar– 39 Ar data from the Paleozoic granites provide mixed results in comparison to the U–Pb emplacement ages, and in some cases Ar systematics appear to have been disturbed by later hydrothermal alteration. In other cases, correspondence between the U–Pb and 40 Ar– 39 Ar data sets is consistent with rapid postemplacement exhumation. Granophile-type mineralization (Mo, W, F, base metals) is present within both Ganderia and Avalonia examples, but is generally restricted to the youngest plutons, in which it is associated with high-level, near-roof, or apical settings. This age versus mineralization pattern is consistent with an interpretation in which mineralized environments within older suites have been preferentially eroded as a consequence of greater exhumation. The identification of a Silurian (ca. 423 Ma) granite in an area previously assigned to Avalonia may be important in constraining tectonic models for Ganderia–Avalonia interaction, which has generally been considered Silurian or younger. Given that many other plutonic suites in Avalonia are not well constrained by regional geology and remain undated, more work is needed to better define the extent of such magmatism and evaluate its implications for broader models of orogenic development.

Description: 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 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 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.

Description: The St. Cyr klippe, part of the Yukon–Tanana terrane in Yukon, Canada, preserves fresh and retrogressed eclogite hosted by quartzofeldspathic schists. Petrology, mineral chemistry and compositional zoning combined with isochemical phase equilibrium (pseudosection) modeling shows that eclogites followed a clockwise pressure–temperature path. An amphibolite facies, pre-eclogite stage (pre-Stage I) preceded early eclogite-facies conditions of about 16 kbar and 510 °C (Stage I) and the peak pressure (Stage II), which reached up to 20 kbar and 670 °C. The peak observed and modeled eclogite-facies assemblage is garnet, omphacite, Na-rich amphibole, phengite, quartz and rutile. Pseudosections were modeled for varying Fe 3+ contents, and Fe 3+ = 20 % of total Fe is found to best match the pyrope and grossular content observed in garnet. Retrogression back to amphibolite-facies conditions (Stage III) followed a clockwise path as temperature decreased during decompression without passing through the chlorite stability field. U-Pb SIMS (secondary ion mass spectrometry) dating and trace-element analysis of zircon shows that the protolith of the eclogites formed within the Yukon–Tanana terrane during early arc activity, between 364 and 380 Ma. The rocks were then subducted to mantle depths and metamorphosed to eclogite-facies conditions during the Late Permian, between 267 and 271 Ma. These medium-temperature eclogites are part of a coherent high-pressure (HP) terrane that was tectonically eroded from the Yukon-Tanana composite arc in a subduction zone environment.

Description: McConnell et al. (2015) claimed that the absence of Middle Ordovician back-arc volcanic rocks in the Bellewstown terrane in the Irish Caledonides implies that the Tetagouche–Exploits back-arc basin never existed in Ireland. The emphasis on the absence of evidence surprised us, because ‘absence of evidence is not evidence of absence’. However, if they are correct, their interpretation has major implications for the tectonic processes responsible for the final closure of Iapetus.

Description: McConnell et al. (2015) claimed that the absence of Middle Ordovician back-arc volcanic rocks in the Bellewstown terrane in the Irish Caledonides implies that the Tetagouche–Exploits back-arc basin never existed in Ireland. The emphasis on the absence of evidence surprised us, because ‘absence of evidence is not evidence of absence’. However, if they are correct, their interpretation has major implications for the tectonic processes responsible for the final closure of Iapetus.

Description: We provide estimates for the width, timing, and rates of opening and closing of the Iapetus and Rheic oceans, the evolution of which profoundly influenced Paleozoic global paleogeography. These estimates are primarily derived from the transfer of Ganderia and Avalonia from Gondwana to Laurentia, which led to closure of the Iapetus Ocean and opening of the Rheic Ocean. Ganderia, a long-lived arc terrane, separated from the paleo-Caribbean margin of Amazonia at 505 Ma with a latitudinal speed of ~9 cm/a northward, initiating the Rheic Ocean as a backarc basin. Ganderia’s trailing edge was reduced to ~5 cm/a following opening of a 600–800-km-wide backarc basin within Ganderia at 475 Ma. Opening and closing of the Iapetus Ocean was largely driven by far-field stresses, slab pull in some places and slab rollback in others.

Description: Detrital zircon U–Pb geochronology and Hf isotope geochemistry allow us to decipher the crustal provenance of Cambrian–Ordovician backarc basin strata of the Alexander terrane, North American Cordillera, and evaluate models for its origin and displacement history relative to Baltica, Gondwana, Siberia, and Laurentia. Quartzose shallow-marine sandstones of the Alexander terrane contain a range of Neoproterozoic to Neoarchaean detrital zircons with the most dominant age groupings c . 565–760, 1000–1250, 1450, and 1650 Ma. Subordinate volcaniclastic sandstones yield Cambrian and Ordovician detrital zircons with a prominent age peak at 477 Ma. The detrital zircon age signatures resemble coeval strata in the Eurasian high Arctic, and in combination with faunal and palaeomagnetic constraints suggest provenance from local magmatic rocks and the Timanide orogenic belt and Fennoscandian Shield of NE Baltica. The Hf isotopic compositions of Palaeozoic to Neoarchaean detrital zircons strongly favour Baltican crustal sources instead of similar-aged domains of Gondwana. The Alexander terrane formed part of an arc system that fringed the Uralian passive margin, and its position in the Uralian Seaway allowed faunal exchange between the Siberian and Baltican platforms. The available evidence suggests that the Alexander terrane originated in the Northern Hemisphere and migrated to the palaeo-Pacific Ocean by travelling around northern Laurentia. Supplementary material: U–Pb and Lu–Hf data tables are available at www.geolsoc.org.uk/SUP18557 .