New Constraints on the Age, Geochemistry

New constraints on the age, geochemistry, and environmental impact of High Arctic Large Igneous Province magmatism: Tracing the extension of the Alpha Ridge onto Ellesmere Island, Canada

T.V. Naber1,2, S.E. Grasby1,2, J.P. Cuthbertson2, N. Rayner3, and C. Tegner4,† 1Geological Survey of Canada–Calgary, Natural Resources Canada, Calgary, Canada 2Department of Geoscience, University of Calgary, Calgary, Canada 3Geological Survey of Canada–Northern, Natural Resources Canada, Ottawa, Canada 4Centre of Earth System Petrology, Department of Geoscience, Aarhus University, Aarhus, Denmark

ABSTRACT Island, Nunavut, Canada. In contrast, a new Province (HALIP), is one of the least studied U-Pb age for an alkaline syenite at Audhild of all LIPs due to its remote geographic lo- The High Arctic Large Igneous Province Bay is significantly younger at 79.5 ± 0.5 Ma, cation, and with many exposures underlying (HALIP) represents extensive Cretaceous and correlative to alkaline basalts and rhyo- perennial arctic sea ice. Nevertheless, HALIP magmatism throughout the circum-Arctic lites from other locations of northern Elles- eruptions have been commonly invoked as a borderlands and within the Arctic Ocean mere Island (Audhild Bay, Philips Inlet, and potential driver of major Cretaceous Ocean (e.g., the Alpha-Mendeleev Ridge). Recent Yelverton Bay West; 83–73 Ma). We propose anoxic events (OAEs). Refining the age, geo- aeromagnetic data shows anomalies that ex- these volcanic occurrences be referred to col- chemistry, and nature of these volcanic rocks tend from the Alpha Ridge onto the northern lectively as the Audhild Bay alkaline suite becomes critical then to elucidate how they coast of Ellesmere Island, Nunavut, Canada. (ABAS). In this revised nomenclature, the may link and influence major changes in Earth To test this linkage we present new bulk rock rocks of Hansen Point stratotype and other climate-ocean systems. major and trace element geochemistry, and tholeiitic rocks are ascribed to the Hansen The magmatism associated with HALIP con- mineral compositions for clinopyroxene, pla- Point tholeiitic suite (HPTS) that was em- sists of Cretaceous to Paleogene volcanic and in- gioclase, and olivine of basaltic dykes and placed at 97–93 Ma. We suggest this subdi- trusive units, with ages between 130 and 60 Ma sheets and rhyolitic lavas for the stratotype vision into suites replace the collective term (e.g., Corfu et al., 2013; Tegner et al., 2011; Es- section at Hansen Point, which coincides geo- Hansen Point volcanic complex. trada et al., 2016; Dockman et al., 2018; Bédard graphically with the magnetic anomaly at The few dredge samples of alkali basalt et al., 2019). Two series of HALIP magmatism northern Ellesmere Island. New U-Pb chro- available from the top of the Alpha Ridge are include: (i) a tholeiitic phase beginning at ca. nology is also presented. akin to ABAS in terms of geochemistry. Our 130 Ma and (ii) addition of an alkaline phase The basaltic and basaltic-andesite dykes revised dates also suggest that the HPTS and beginning at ca. 100 Ma and coinciding with the and sheets at Hansen Point are all evolved Strand Fiord Formation volcanic rocks may tholeiitic phase. The formation of HALIP has with 5.5–2.5 wt% MgO, 48.3–57.0 wt% be the hypothesized subaerial large igneous been suggested to be related to a mantle plume

SiO2, and have light rare-earth element en- province eruption that drove the Cretaceous source but also to prolonged continental rifting riched patterns. They classify as tholeiites Ocean Anoxic Event 2. events (mainly the alkaline series) (Drachev and and in Th/Yb vs. Nb/Yb space they define a Saunders, 2006; Tegner et al., 2011; Jowitt et al., trend extending from the mantle array to- INTRODUCTION 2014; Estrada, 2015; Thórarinsson et al., 2015; ward upper continental crust. This trend, Oakey and Saltus, 2016; Estrada et al., 2016; also including a rhyolite lava, can be mod- Widespread Cretaceous magmatism report- Dockman et al., 2018). eled successfully by assimilation and frac- ed throughout the circum-Arctic borderlands The Alpha Ridge is an undersea bathymetric tional crystallization. The U-Pb data for a and within the Arctic Sea (Alpha-Mendeleev high located in the Arctic Ocean that extends dacite sample, that is cut by basaltic dykes at Ridge) is characteristic of a large igneous from northwestern Ellesmere Island, Nunavut, Hansen Point, yields a crystallization age of province (LIP) (Fig. 1) (Coffin and Eldholm, Canada across the Amerasia Basin toward Si- 95.5 ± 1.0 Ma, and also shows crustal inheri- 1994; Tarduno et al., 1998; Maher, 2001; Bry- beria, connecting with the Mendeleev Ridge tance. The chronology and the geochemistry an and Ernst, 2008; Buchan and Ernst, 2006, (Fig. 1) (Oakey and Saltus, 2016; Jackson and of the Hansen Point samples are correlative 2018; Oakey and Saltus, 2016; Jackson and Chian, 2019). The Alpha-Mendeleev Ridge is with the basaltic lavas, sills, and dykes of the Chian, 2019). A LIP is defined as a region ∼2000 km long and 400 km wide with water Strand Fiord Formation on Axel Heiberg where greater than 100,000 km3 of typically depths atop the ridge between 1000 and 2500 m basaltic magma is emplaced over a relatively and is often interpreted as a LIP, although it is Christian Tegner http://orcid.org/0000-0003- short duration of time (e.g., Ernst and Bleek- debated whether it is mainly an oceanic pla- 1407-7298 er, 2010). The widespread magmatism in the teau (Forsyth et al., 1986; Dove et al., 2010; †Corresponding author: [email protected]. Arctic, called the High Arctic Large Igneous Funck et al., 2011; Jokat et al., 2013) and/or

GSA Bulletin; July/August 2021; v. 133; no. 7/8; p. 1695–1711; https://doi.org/10.1130/B35792.1; 14 figures; 1 table; 1 supplemental file. published online 16 December 2020

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liamson et al., 2019a) and previously 89 ± 1 Ma (Jokat et al., 2013), (ii) ages of 127–110 Ma suggested for volcanic samples dredged at the Siberian end of the Mendeleev Ridge (Nikishin et al., 2020), and (iii) ages of ca. 118–112 Ma, ca. 105–100 Ma, and 90–70 Ma for dredge samples of the Chuckchi borderland (Mukasa et al., 2020). The compositions of the few dredged samples of the Alpha Ridge, with the exception of Van Wagoner et al. (1986), have only been reported in abstracts without access to the whole rock data. Recent interpretations of aeromagnetic and geological data (Oakey and Saltus, 2016; Estrada et al., 2016) show that magnetic anomalies of the Alpha Ridge extend onto the coastal areas of the northern Canadian Arctic Islands (Fig. 1), corroborating the original ideas of Embry and Osadetz (1988) that volcanic rocks exposed there are an on-land extension of the ridge. In the Yelverton Bay area of Ellesmere Island, a magnetic anomaly coincides with outcrops of lavas and dykes and with the Wootton intrusive complex (WIC) (Estrada et al., 2016). In addition, the ages reported for this magmatism (Estrada et al., 2016) partly overlap those of the Alpha Ridge. These connections still remain tenuous due to sparse chronological and geochemical studies of these different volcanic complexes. To throw new light on the petrology, chronology, and correlations of Cretaceous magmatism of northern Ellesmere Island we carried out new Figure 1. Bathymetric map of the High Arctic region showing the extend of on-land and field work and sampling in the Hansen Point area. offshore areas with magmatic rocks ascribed to the High Arctic Large Igneous Province This is the stratotype of volcanism in northern (HALIP), and the location of the study area at Hansen Point, northern coast of Ellesmere Ellesmere Island (Trettin and Parrish, 1987), but Island, Canada. The expressions of HALIP magmatism includes lavas, sills, and dykes of remains understudied due to the remote location. the Canadian Islands (Trettin and Parrish, 1987; Estrada et al., 2016; Buchan and Ernst, Here we describe the petrography, mineralogy, 2018), the De Long Islands (Nikishin et al., 2020), Franz Josef Land, Svalbard, the Barents geochemistry, and geologic history of basaltic and Sea (Polteau et al., 2016), North Greenland (Thórarinsson et al., 2015), and the High Arctic basaltic andesite dykes and sheets, and a rhyolite Magnetic High Domain interpreted as the geophysical manifestation of magmatic intrusions lava at Hansen Point. We also present new U-Pb and extrusions associated with the Alpha-Mendeleev ridge (Oakey and Saltus, 2016). Also ages for the original sample at Hansen Point dated shown are the approximate locations of offshore samples (black squares) from the Alpha by Trettin and Parrish (1987) and for a sample col- Ridge by Jokat (2003; labeled 1) and Van Wagoner et al. (1986, labeled 2). Bathymetry and lected by us at Audhild Bay, northwestern Elles- topography from Jakobsson et al. (2008). mere Island. We then classify the magma types of the mafic and felsic rocks at Hansen Point, discuss their petrogenesis and model the roles of fraction- associated with extended continental crust of the Sea (UNCLOS) studies, and have con- al crystallization and crustal assimilation. This is (Døssing et al., 2013; Oakey and Saltus, 2016; firmed the alkaline character and documented followed by a discussion of correlations and no- Petrov et al., 2016; Jackson and Chian, 2019). textures consistent with explosive basaltic vol- menclature of magma types and chronology with There are only a few dredge samples avail- canism (Massey et al., 2019; Williamson et al., previous work on (i) basalts from other occurrenc- able from the Alpha Ridge. Van Wagoner and 2019a, 2019b). Basaltic samples have also been es described in northern Ellesmere Island, (ii) the Robinson (1985) first studied four samples of dredged in the western Arctic Ocean, adjacent Strand Fiord Formation on Axel Heiberg Island, altered basalt from the 1983 Canadian Expe- to the Alpha-Mendeleev Ridge (Fig. 1). These and (iii) the Alpha Ridge. Finally, we explore the dition to Study the Alpha Ridge expedition. samples from the steep margins of the Chuk- potential connection with the Cretaceous Ocean Further analyses of the mineralogy and whole chi Borderland, however, all are tholeiitic in Anoxic Event 2 (OAE2). rock geochemistry of these samples was done composition (Mukasa et al., 2020). The timing by Van Wagoner et al. (1986) and the basalts and igneous make-up of the Alpha Ridge re- GEOLOGIC SETTING were determined to have an alkaline affinity. mains elusive, other than (i) dredged samples Further samples have recently been dredged as of alkaline basalt from the Alpha Ridge that Ellesmere Island is a part of the Queen Eliza- part of United Nations Convention of the Law yielded an Ar-Ar age of 90.40 ± 0.26 Ma (Wil- beth Islands, Canadian Arctic Archipelago.

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central part of the Sverdrup Basin in the Cana- A dian Arctic (Fig. 2A). The formation is mainly composed of subaerial basalt flows, as well as dykes, sills, scoriaceous pyroclasts, and thin interbedded lacustrine sedimentary rocks (Ricketts et al., 1985; Embry and Osadetz, 1988; Embry, 1991; Dostal and MacRae, 2018; Kingsbury Figure 2. (A) Regional map et al., 2016, 2018; Evenchick et al., 2019). Flows of the northern portion of the are most abundant, with a volcanic package up to Canadian Arctic Archipelago, 1 km thick reported in the northern, western, and showing main sedimentary southern part of Axel Heiberg Island. The SFF basins as well as regional ex- is composed of variably fractionated, low-Ti posures of the Strand Fiord tholeiitic continental flood basalts (Estrada and Formation volcanic units. (B) Henjes-Kunst, 2004; Dostal and MacRae, 2018; Close up of northern Ellesmere Kingsbury et al., 2016). Island showing location of the The volcanic rocks of SFF are followed upsec- Hansen Point stratotype study tion by mudstones of the Kanguk Formation de- area along with mapped oc- posited from upper Turonian to Campanian (ca. currences of volcanic units and 93–71 Ma) (Fig. 3A) (Embry and Beauchamp, major fault zones (after Har- 2019). This unit includes numerous alkaline, si- rison et al., 2015). The main licic ash beds (bentonites) up to 50 cm thick that occurrences of Cretaceous vol- are interbedded with mudstone (Fig. 3A) (Davis canic rocks of northern Elles- B et al., 2017; Pointon et al., 2019). mere Island are labeled: (1) Trettin and Parrish (1987) first gave the infor- Audhild Bay, (2) Philips Inlet, mal name “Hansen Point volcanics” to volcanic (3) Yelverton Bay West, and outcrops at either side of Yelverton Bay, includ- (4) Yelverton Bay East that in- ing the stratotype on the Hansen Point penin- cludes Hansen Point itself. Also sula to the east (Fig. 2B). Subsequently, Embry shown is the location of the and Osadetz (1988) described Late Cretaceous dated hornblende syenite sam- volcanic rocks between Emma Fiord and Aud- ple 17HSB-40A to the north of hild Bay; and in the Philips Inlet area (Fig. 2B). Audhild Bay. Based on biostratigraphy, Embry and Osadetz (1988) correlated the intra-volcanic sedimentary rocks with the Kanguk Formation (Fig. 3A). They also correlated the volcanic units at Aud- hild Bay and Philips Inlet to the Hansen Point volcanic rocks at Yelverton Bay (Trettin and Parrish, 1987). Subsequently, Estrada et al. (2006) referred to all of these Late Cretaceous volcanic units of northern Ellesmere Island as The geology of northern Ellesmere Island is end of the Albian (ca. 99.6 Ma), with widespread the Hansen Point Volcanic Complex. However, divided into three major tectono-stratigraphic uplift and a major transgression that marked the our results show that this nomenclature is un- units: (1) the Mesoproterozoic to upper Silurian initiation of Phase 7. The beginning of Phase 7 fortunate for two reasons. First, it includes both exotic Pearya Terrane, (2) the Neoproterozoic to during the Cenomanian (ca. 99.6–93.5 Ma) is alkaline and tholeiitic suites. Second, it encom- Devonian Franklinian Basin, and (3) the Carbon- marked by renewed high rates of subsidence passes a wide range of ages (ca. 104–73 Ma) iferous to Eocene Sverdrup Basin (Fig. 2; Tret- and transgression, and with deposition of the that overlap with the ages of both the Strand tin, 1991). The Sverdrup Basin is divided into Hassel and Strand Fiord (SFF) formations, the Fiord and Kanguk formations. As discussed several phases of development with volcanism latter dominated by tholeiitic volcanic rocks and below, we conclude that it is useful to divide on northern Axel Heiberg and Ellesmere islands interbedded sedimentary rocks. The later part of the volcanic occurrences of northernmost Elles- occurring during phases 6 and 7 as described by Phase 7 is marked by a new uplift episode and mere Island into two suites: the Hansen Point Embry and Beauchamp (2019) (Fig. 3A). These very low rates of subsidence across the basin, tholeiitic suite (HPTS), typified by the volcanic phases are characterized by Early Cretaceous to and the emplacement of sedimentary and alka- rocks of the stratotype section at Hansen Point Late Cretaceous rift-related sedimentation with line volcanic rocks of the Kanguk Formation and the Audhild Bay alkaline suite (ABAS). To interbedded volcanism. The beginning of Phase (Fig. 3A) (Embry and Osadetz, 1988; Embry establish the case for this suggestion, we refer 6 of Embry and Beauchamp (2019), correspond- and Beauchamp, 2019). The existing radiomet- in the following to the main outcrop areas of ing to the base of the Hauterivian (ca. 136 Ma), ric chronology of the two volcanic units ranges volcanic rocks on northern Ellesmere Island is marked by rifting and extension of the basin from ca. 104 to 74 Ma (Estrada et al., 2016; (as numbered in Fig. 2B) from SW to NE as: and coincides with deposition of volcanic and Kingsbury et al., 2018). Audhild Bay (1), Philips Inlet (2), Yelverton sedimentary rocks of the Isachsen and Christo- The SFF is mainly located on northwestern Bay West (3), and Yelverton Bay East (4) that pher formations. Phase 6 ended abruptly at the and west-central Axel Heiberg Island within the includes Hansen Point itself.

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A B

Figure 3. (A) North-south stratigraphic cross-section of Cretaceous strata across western Axel Heiberg Island to northern Ellesmere Island, Canada modified from Embry and Osadetz (1988). (B) Revised stratigraphic section based on new interpretation proposed here dividing the Cretaceous magmatic rocks of northern Ellesmere Island into the Hansen Point tholeiitic suite (HPTS) and Audhild Bay alkaline suite (ABAS). Fm—Formation; Mbr—Member; Is.—Island; Maastrict.—Maastrichtian.

The volcanic rocks of the northern coastal The exposed volcanic rocks vary consider- tic lava flows dominate the volcanic pile that is at areas of Ellesmere Island occur in scattered ably. At Yelverton Bay West and Yelverton Bay least 650 m thick but also includes silicic flows outcrops stretching over ∼200 km and are in East, they are typically dominated by pyroclastic and pyroclastic deposits (Embry and Osadetz, tectonic or depositional contact with rocks of deposits, volcanic breccias, lava flows, and dykes 1988; Estrada and Henjes-Kunst, 2004). the Neoproterozoic basement, the Paleozoic (Trettin and Parrish, 1987; Embry and Osadetz, Franklinian Basin, or the Paleogene Sverdrup 1988; Estrada and Henjes-Kunst, 2004). The MATERIAL AND METHODS Basin (Trettin and Parish, 1987; Embry and Yelverton exposures are compositionally bimod- Osadetz, 1988; Estrada and Henjes-Kunst, 2004; al with silicic rocks (e.g., rhyodacitic) dominat- Field Work and Samples Estrada et al. (2006); Embry and Beauchamp, ing over basaltic rocks. Basaltic compositions 2019) (Fig. 2B). The fault-bounded outcrops are mainly found in the dykes, but also as oc- A complete 2.5-km-long traverse of the Han- are associated with the SW-NE striking Mitch- casional flows and breccias (Trettin and Parrish, sen Point stratotype section was sampled along ell Point fault zone and Emma Fiord fault zone. 1987; Estrada et al., 2006, 2016). At Audhild an unnamed creek at Hansen Point; starting at These are mainly Paleozoic strike-slip faults that Bay, a ∼200 m stratigraphic section of mainly the head, near an unnamed glacier, to the end were re-activated in the Paleogene during the basaltic flows is overlain by∼ 250 m of trachytic of the exposed section near the shore of the Eurekan orogeny (Estrada et al., 2006; Piepjohn and more silicic pyroclastic units (Embry and Arctic Ocean (Figs. 4 and 5). All GPS coor- et al., 2013). Osadetz, 1988). In the Phillips Inlet area, basal- dinates are recorded in reference to NAD 83.

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The starting position for the sampling transect is 82.45631497N; −82.43058002W (elevation 184 m), and the end coordinate is 82.469148N; −82.53214697W (elevation 27 m). Forty-eight Figure 4. Map of Hansen Point in situ samples were collected from this sec- stratotype area (Ellesmere Is- tion. Ten of these samples have been selected land, Canada) showing the for this study (nine dykes and one sheet), with 2017 sample profile, sample a main focus on the basaltic rocks. The Hansen numbers of this study, and the Point samples are described in Supplementary approximate strike of dolerite Data File 1 and their GPS positions are given dykes. Also shown are: (i) the in Supplementary Data File 21. In addition, two location of sample 82-TM-212A samples were selected for U-Pb geochronologi- of Trettin and Parrish (1987) cal analysis; one is from the Hansen Point area that we re-dated by U-Pb; (ii) (Fig. 4) and one is from Audhild Bay (location dolerite dykes cutting Pearya 1; Fig. 2B) (see below). basement (Succession I) in the vicinity of Hansen Point and Analytical Methods the age for two of these published by Estrada and Henjes- Polished thin sections were prepared at Van- Kunst (2013). Map modified couver Petrographics Ltd., Langley City, British from Estrada et al. (2016). Columbia, Canada. Petrographic descriptions were completed using transmitted and reflected polarized light optical microscopy on a Zeiss Axioplan microscope. Mineral percentages were visually estimated. The mineral chemistry of clinopyroxene, plagioclase, and olivine was characterized using wavelength dispersive spec- troscopy using a JEOL JXA-8200 electron microprobe at the University of Calgary, Calgary, A Canada. The analytical conditions were an ac- celerating voltage of 15 kV, a current of 30 nA, a beam diameter of 5 µm, and counting times of 20 s on peak and 10 s on both positive and negative background. Calibration was achieved using natural mineral standards of albite, anorthite, augite, chromite, fluorapatite, grunerite, hornblende, orthoclase, and rhodonite. For clinopyroxene, only analyses with cation sums B between 3.95 and 4.05 were accepted. For plagioclase and olivine, we accepted analyses with cation sums between 4.95 and 5.05 and 2.99 and 3.01, respectively. For whole rock geochemistry, samples were cut using a diamond saw to remove weathered surfaces. Using a steel hammer and plate, samples were broken into gravel-sized pieces, then pow-

1Supplemental Material. Supplementary Data Files: (1) Sample list and description; (2) GPS positions of samples; (3) Accuracy of major and trace element bulk rock compositions and precision of repeat analyses; (4) Photomicrographs; (5) Clinopyroxene, plagioclase and olivine compositions; (6) SHRIMP U-Pb methods and results; (7) Nb- Zr-Y tectonic discrimination diagram; (8) Ti-Zr-Y tectonic discrimination diagram; (9) Ti-V tectonic tot Figure 5. Field photographs of the Hansen Point stratotype section, Ellesmere Island, Can- discrimination diagram; (10) MgO-FeO _Al2O3 ada. (A) Photograph viewed from the air toward the northwest and showing the approxi- tectonic discrimination diagram; (11) AFM diagram; and (12) Th/Nb vs. SiO2 diagram. Please visit mate location of the 2017 sample profile. All visible outcrops are of the volcanic complex. (B) https://doi.org/10.1130/GSAB.S.13217810 to access Photo of ∼8-m-thick dolerite dyke cutting silicic flows. Another, thinner dolerite dyke can be the supplemental material, and contact editing@ seen in the left-hand side of the photograph. geosociety.org with any questions.

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dered using an agate mortar and pestle for 30 min RESULTS anhedral to euhedral grains in the groundmass, at the Geological Survey of Canada (GSC), Cal- or skeletal grains associated with the glassy me- gary. Standard lithogeochemistry was completed Field Observations sostasis. Three samples also contain interstitial, by Acme Labs in Vancouver, British Columbia, granophyric intergrowth of quartz and alkali Canada for major element oxides by X-ray fluo- The southeastern portion of the section stud- feldspar; these samples are also distinct in terms rescence, and trace elements by inductively cou- ied is dominated by felsic flows, tuffs, agglom- of bulk rock major element compositions where pled plasma–mass spectrometry. The accuracy erates, breccias, and dykes, but also includes they classify as basaltic andesite (see below). and precision of the bulk rock data are evaluated basaltic dykes (Supplementary Data File 1). In The rock samples often contain a minor amount in Supplementary Data File 3. This shows that the the northwestern part of the section mafic dykes, of circular amygdules (up to 5% and up to major and trace element compositions described sheets, and flows are more abundant (Figs. 4 0.6 mm) that are commonly filled in with radial here are accurate within 2 and 8%, respectively. and 5). Overall, most of the dykes strike east to aggregates of chlorite and anhedral to euhedral Two additional samples were selected for U-Pb west and have near vertical to steep dips to the quartz. Most of the rocks are weakly to mod- geochronological analysis. The mineral separates north or south. Chilled margins were observed erately altered including sericite, chlorite, and for Trettin and Parrish’s tholeiitic rhyodacite on some of the contacts. Individual mafic dykes grungy red-brown clays or mica associated with sample 82-TM-212A from the stratotype local- range from ∼3 m to roughly 30 m thick and are the mesostasis. One sample (17C-51) is, how- ity (Fig. 4) was revisited using the sensitive high typically fine- to medium-grained, some are ever, heavily altered. Here the alteration miner- resolution ion microprobe (SHRIMP) at the GSC sparsely plagioclase- and clinopyroxene-phyric. als include chlorite, sericite, calcite, albite, and in Ottawa, Canada. A new sample of “hornblende Both dykes and sheets are often amygdaloidal. sporadic anhedral pyrite stringers. syenite” (17HSB-40A) was collected along the Rusty surface weathering varies from mild to western shore of Audhild Bay (Fig. 2B) in 2017. strong. Toward the center and northwestern end Clinopyroxene, Plagioclase, and Olivine This new sample was disaggregated using stan- of the section, several fine-grained mafic sheets Chemistry dard crushing/pulverizing techniques followed display columnar jointing. The sheet included in by density separation using the Wilfley table and this study (17C-74) is ∼30 m thick and dips ∼45° Ninety-one analyses of clinopyroxene from heavy liquids and purification into a zircon sepa- to the north. The contacts are not exposed and it eight Hansen Point samples are shown in the rate using a Frantz Isodynamic separator. Zir- remains unclear whether this sample represent a pyroxene quadrilateral and all classify as augite con grains were cast in 2.5 cm diameter epoxy dyke or a lava flow. (Fig. 6) (all microprobe data are reported in Sup- mounts along with fragments of the GSC labo- plementary Data File 5). Approximately half of ratory standard zircon (z10493, with 206Pb/238U Petrography the analyses are from the cores of grains and they age = 417 Ma, Black et al., 2003). The analytical show the highest Mg# (100*Mg/[Mg + Fe]) rang- details are provided together with the results in The samples are composed of a fine-grained, ing from 73 to 52. The Mg# of rims is generally Supplementary Data File 6. The mid-sections of phaneritic groundmass dominated by plagio- lower and ranges from 67 to 26. The wollaston- the zircons were exposed using 9, 6, and 1 µm clase laths, clinopyroxene, and interstitial me- ite endmember Wo% (100*Ca / [Ca + Fe + Mg]) diamond compound and internal features of the sostasis showing sub-ophitic textures (Supple- ranges from 30 to 45. The TiO2 contents range zircons (such as zoning, structures, alteration, mentary Data File 4 shows typical textures). from 0.8 to 2.8 wt% for cores and are slightly low-

etc.) were characterized in cathodoluminescence Four samples are porphyric with phenocrysts er for the rims (0.3–2.5 wt%). The Al2O3 contents (CL) and back-scattered electron (BSE) mode of plagioclase (up to 7% and up to 4 mm) and range from 1.2 to 5.7 wt% in the cores, and from

utilizing a Tescan field emission scanning elec- clinopyroxene (up to 6% and up to 1 mm) that 0.4 to 4.0 wt% in the rims. Both TiO2 and Al2O3 tron microscope. SHRIMP analytical procedures often occur together as glomerocrysts. The correlate positively with Mg#. The Cr2O3 contents followed those described by Stern (1997), with plagioclase phenocrysts range from euhedral are always below detection limit (<0.02 wt%). In standards and U-Pb calibration methods follow- laths to equant grains with highly serrated a few cases pigeonite (not shown) is present in the ing Stern and Amelin (2003). Eleven masses in- rims. Three samples also contain micropheno- rims of groundmass crystals. cluding background were sequentially measured crysts of olivine (up to 3% and up to 0.3 mm) The plagioclase compositions (n = 95) are with a single electron multiplier on the SHRIMP. that are fresh in two samples, but completely listed and shown in the plagioclase classification Off-line data processing was accomplished using pseudomorphed in the third sample. Clinopy- diagram in Supplementary Data File 5. The cores SQUID2 (version 2.50.11.10.15, rev. 15 October roxene crystals often show zoning from core to of phenocrysts (samples 17C-39, 17C-63) clas- 2011). The 1σ external errors of 206Pb/238U ra- rim; this is seen in both groundmass grains and sify as labradorite and the anorthite contents [Ca/ tios reported in Supplementary Data File 6 in- phenocrysts. Ilmenite and magnetite either form (Ca + Na)] range from An66–52. The rim com- corporate the error in calibrating the reference material. Common Pb correction utilized the Pb Figure 6. Pyroxene quadrilat- composition of the surface blank (Stern, 1997). eral showing the compositions Details of the analytical session, including spot of clinopyroxene from basal- size, number of scans, calibration error, and the tic dykes and sheets at Hansen requirement for an isotopic mass fractionation Point, Ellesmere Island, Can- correction are given in the footnotes of Supple- ada. Also shown is the classic mentary Data File 6. Isoplot v. 4.15 (Ludwig, tholeiitic differentiation trend 2003) was used to generate concordia plots and from the Skaergaard intru- calculate weighted means of the 206Pb/238U ages. sion (Wager and Brown, 1968). The error ellipses on the concordia diagrams and Cpx—clinopyroxene; Di—di- the weighted mean errors are reported at the 95% opside; Hd—hedenbergite; confidence level. En—enstatite; Fs—ferrosilite.

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positions of the phenocrysts stretch into andesine in Table 1. The LOI is less than 1 wt%, with the measured whole rock data represent the primary compositions to lower anorthite (An60–34). The exception of sample 17C-51 with 2.24 wt%. In igneous compositions. The studied samples can cores of groundmass plagioclase also span from the following we describe and plot the major ele- therefore be classified as basalt (n = 7) and ba- labradorite to andesine with An61–40. The com- ment oxide values normalized to 100% volatile- saltic andesite (n = 3, including 17C-51). The positions of rims of the groundmass grains partly free compositions. The samples in this study TiO2 contents of the samples range from 2.44 to overlap with the cores, but extend to oligoclase range from 48.3 to 57.0 wt% SiO2, with most 3.45 wt%, and MgO contents range from 2.53 to compositions and An24. Groundmass olivine in samples falling between 49.5 and 52.2 wt%. The 5.53 wt% (Fig. 8). TiO2 is highest for intermedi- sample 17C-70 (n = 11) was also analyzed and alkali oxides range from 0.76 to 2.77 wt% K2O, ate MgO (4.10–4.30 wt%) and decreases toward has Fo% [Mg/(Mg + Fe)] of 45–59 in the cores, and from 2.51 to 3.64 wt% Na2O. In the total lower and higher MgO. and down to 30 on the rims (Supplementary Data alkali versus silica (TAS) diagram (Fig. 7) all The trace element compositions of the ten File 5). The Supplementary Data File 5 also in- samples, with one exception, plot in the subal- samples are presented in Table 1. The immo- cludes analyses of interstitial alkali feldspar kaline field. The one sample (17C-51) plotting bile elements Nb (25–36 ppm; R2 = 0.54) and (n = 20) and albite alteration products in 17C- in the alkaline field is, however, also the most Ce (56–96 ppm; R2 = 0.77) correlate negatively 51 (n = 8). strongly altered sample. In terms of the immo- with MgO, whereas Ni (5–38 ppm; R2 = 0.89) bile element contents, this same sample is com- correlates positively with MgO (Fig. 8). In the Zr/ Whole Rock Geochemistry parable to the other basalts (see below) and it Ti versus Nb/Y discrimination diagram (Pearce, does not yield nepheline in the CIPW norm. We 1996), our Hansen Point samples show similar The wt% of major element oxides and loss therefore ascribe the high alkali content of this Nb/Y (0.58–0.64), but more variation in Zr/Ti on ignition (LOI) of the samples are presented sample to alteration. Otherwise, we assume the (0.01–0.02) (Fig. 9). All Hansen Point samples

TABLE 1. MAJOR AND TRACE ELEMENT BULK ROCK COMPOSITIONS FOR HANSEN POINT AND AUDHILD BAY, ELLESMERE ISLAND, CANADA Sample no. 17C38 17C39 17C43 17C51 17C56 17C60 17C63 17C67 17C70 17C74 17C79 17HSB-40A 82TM-212A* Location HP HP HP HP HP HP HP HP HP HP HP AB HP Rock type rhyolite bas-and. basalt bas-and. basalt basalt basalt basalt basalt basalt bas-and. syenite dacite wt% SiO2 69.80 56.30 49.50 50.80 47.90 50.10 49.50 49.60 50.20 50.60 52.20 63.89 64.1 TiO2 0.44 2.41 3.43 3.20 2.97 3.31 3.39 2.96 3.32 2.96 2.61 0.41 0.42 Al2O3 12.60 12.90 12.90 12.90 13.70 12.70 13.10 13.30 13.10 13.50 13.30 16.70 11. 3 Fe2O3 total 5.18 12.70 15.70 14.70 15.10 16.50 16.40 15.30 15.70 14.30 14.60 5.46 3.2 MnO 0.11 0.18 0.24 0.33 0.27 0.23 0.24 0.22 0.23 0.20 0.23 0.17 0.26 MgO 0.29 2.50 4.22 4.15 5.49 4.12 4.30 4.44 4.32 4.16 3.23 0.29 0.55 CaO 1.51 5.69 8.89 4.67 10.20 8.88 9.03 9.43 8.88 9.14 7. 1 3 0.95 7.49 Na2O 3.77 2.87 2.73 3.54 2.49 2.55 2.84 2.74 2.89 2.93 2.87 6.65 1. 1 K2O 4.83 2.74 1.26 2.69 0.75 1.30 1.25 1. 13 1.39 1.39 2.10 5.46 4.47 P2O5 0.08 0.51 0.52 0.74 0.37 0.43 0.49 0.38 0.46 0.47 0.85 0.06 0.12 LOI 0.75 0.44 0.60 2.26 1.03 0.32 0.02 0.41 −0.09 0.39 0.87 0.21 7. 3 Total 99.36 99.24 99.99 99.98 100.27 100.44 100.56 99.91 100.40 100.04 99.99 100.25 100.31 ppm Ba 751 463 261 482 209 297 286 228 309 271 386 300 690 Be 1 2 4 2 4 2 4 2 1 5 4 Co 1. 8 26.3 37.3 26.7 43.7 39.0 38.8 40.9 39.0 33.6 24.1 0.9 Cs 1. 7 4.6 2.4 1. 1 5.5 4.1 2.3 1. 4 4.7 3.1 1. 3 1. 1 <7 Ga 19.6 21.7 20.0 21.4 22.4 20.2 21.3 19.0 19.7 19.4 21.4 26.9 Hf 15.2 7. 9 6.1 7. 6 5.5 6.9 6.2 6.0 6.4 6.6 8.5 11. 7 Nb 59.8 33.7 26.3 32.4 25.3 28.3 29.0 26.4 29.0 27.7 35.7 11 7. 5 24 Rb 177.9 96.4 40.1 71.1 23.2 38.7 37.2 33.6 41.2 42.5 66.5 126.4 148.5 Sn 5.0 3.0 2.0 2.0 1. 0 2.0 2.0 2.0 2.0 2.0 3.0 4 Sr 164.5 198.5 226.0 101. 0 228.2 220.4 239.6 227.9 242.5 247.2 227.9 20.9 72.5 Ta 3.5 2.2 1. 9 2.0 1. 6 1. 8 1. 5 1. 6 1. 6 1. 8 2.1 6.6 Th 1 7. 8 9.6 4.8 6.5 3.7 5.7 5.2 4.5 5.3 5.5 7. 6 14.6 22 U 4.4 2.7 1. 1 1. 7 0.8 1. 4 1. 4 1. 1 1. 3 1. 3 1. 7 3.7 11 V 21 220 399 269 356 361 356 340 350 310 131 <8 21 W 1. 6 1. 0 1. 3 0.8 0.6 0.8 0.8 0.5 0.6 0.6 0.8 Zr 583.8 322.0 210.9 282.4 203.5 264.7 241.9 225.8 241.7 229.5 305.5 504.8 355.5 Y 77.2 52.3 43.9 50.7 41.8 48.4 47.0 42.4 45.1 44.4 55.6 46.4 76 La 79.2 44.7 30.0 35.1 26.2 34.7 31.4 27.9 34.3 30.4 45.3 89.9 80 Ce 159.6 95.2 65.0 78.7 56.0 70.9 68.5 57.4 72.7 67.0 96.3 165.3 Pr 1 7. 74 11.27 8.37 10.00 6.94 8.80 8.45 7.22 8.85 8.21 12.16 18.44 Nd 67.7 44.7 35.3 41.1 28.6 37.0 36.9 31.5 38.3 34.3 50.4 64.2 Sm 13.55 10.26 8.18 9.77 7.25 8.55 8.52 7.43 8.48 7.84 11.38 11. 12 Eu 2.49 2.38 2.46 2.75 2.28 2.40 2.46 2.16 2.32 2.32 3.06 1.82 Gd 14.05 10.31 9.08 10.37 7.94 9.25 9.45 8.31 8.93 8.52 11.62 9.77 Tb 2.36 1.72 1.45 1.78 1.34 1.51 1.54 1.35 1.48 1.42 1.86 1.51 Dy 13.88 9.73 8.91 10.08 7.70 8.69 9.02 8.02 8.65 8.06 10.64 8.58 Ho 3.02 2.04 1.75 2.01 1.67 1.93 1.86 1.73 1.79 1. 74 2.16 1.71 Er 9.45 6.11 5.18 5.96 4.60 5.35 5.32 4.85 5.39 5.02 6.10 4.89 Tm 1.32 0.82 0.71 0.83 0.65 0.76 0.75 0.67 0.71 0.69 0.82 0.69 Yb 8.4 5.22 4.47 5.26 4.26 4.83 4.97 4.44 4.71 4.52 5.25 4.77 5.6 Lu 1.29 0.76 0.66 0.78 0.63 0.72 0.72 0.64 0.73 0.62 0.77 0.71 Mo 2.1 1. 2 0.7 1. 3 0.5 1. 1 0.9 0.9 1. 1 1. 0 1. 6 5.9 0.5 Cu 12.8 88.5 140.7 36.4 209.7 168.1 221.1 198.2 177.2 148.3 23.8 1. 9 9 Pb 1 7. 4 4.8 3.2 3.8 3.0 3.3 3.0 2.5 3.4 3.3 3.7 6.6 22 Zn 121.0 90.0 65.0 142.0 95.0 104.0 89.0 83.0 95.0 92.0 121.0 66 70 Ni 0.5 4.6 18.2 18.4 37.8 13.0 16.9 19.9 19.6 19.4 4.5 1 <10 As 2.2 0.7 0.6 2.5 0.9 1. 1 0.8 0.9 0.7 0.8 1 Notes: HP—Hansen Point; AB—Audhild Bay; bas-and.—basaltic andesite; LOI—loss on ignition. *From Trettin (1996b) Open File 3274, Geological Survey of Canada.

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enriched in the REEs, apart from Eu that shows a stronger negative anomaly, relative to the basalts and basaltic andesites of Hansen Point (Fig. 10A). In the multielement spider diagram, the pattern Figure 7. The Na O K O 2 + 2 for 17C-38 mimics the most enriched basaltic an- (wt%) vs. SiO (wt%) classifica- 2 desites of Hansen Point but at higher values, apart tion diagram showing the bulk from distinct negative anomalies for Sr, Ti, and rock compositions of basaltic Eu. The three silicic samples all have low TiO and basaltic andesite dykes 2 and sheets form Hansen Point, Ellesmere Island, Canada. Also shown are the composi- A tions of three silicic samples of this study: 17C-38 (rhyolite), 82-TM-212A (dacite), and 17HSB-40A (hornblende syenite).

plot in the subalkaline field, consistent with the contains 63.9 wt% SiO2 and plots in the trachyte B inference from the TAS diagram (Fig. 7). field. Finally, the sample from our sample section

Rare earth element (REE) chondrite-normal- (17C-38) contains 70.8 wt% SiO2 and falls in the ized and multielement primitive mantle-nor- rhyolite field of Figure 7. This rhyolite sample is malized diagrams are presented in Figure 10. These samples show near-parallel REE patterns that are enriched in the light relative to the Figure 8. The bulk rock compositions of im-

heavy elements, and all show a minor negative mobile elements TiO2, Nb, Ce, and Ni vs. Eu anomaly. The three samples of basaltic an- MgO for basaltic samples of Hansen Point desite (17C-39, 17C-51, and 17C-79) are the of this study, Ellesmere Island, Canada. most enriched samples, in particular for the Also shown are published compositions for

light-REEs. The light-REEs (La to Eu) show basaltic rocks (<54 wt% SiO2) of: (i) Aud- patterns that are similar to ocean island basalts hild Bay/Emma Fjord area (Estrada and C (OIB), whereas the mid- to heavy-REEs (Gd Henjes-Kunst, 2004), Philips Inlet area (Es- to Lu) display flatter patterns that are enriched trada and Henjes-Kunst, 2004), outcrops relative to OIB (Fig. 10A). The patterns of the west of Yelverton Bay West (Estrada et al., Hansen Point samples are also sub-parallel in the 2006, 2016), and Hansen Point east of Yel- multielement spider diagram and show increas- verton Bay (Estrada et al., 2006, 2016); ing concentrations relative to primitive mantle and (ii) a field for Strand Fiord Formation with increasing incompatibility from right to (SFF) (Estrada and Henjes-Kunst, 2004; left (Fig. 10B). The three basaltic andesites are Kingsbury et al., 2016; Dostal and MacRae, generally the most enriched samples, but show 2018), and (iii) Alpha Ridge (Van Wagoner negative Ti anomalies. All samples show marked et al., 1986). Note that the dredged samples negative Sr anomalies. Most samples also show of Alpha Ridge are highly altered and the positive Rb and Th anomalies and negative Nb Ti and trace element compositions were re- D anomalies. The multielement patterns are also ported for four samples, whereas the MgO comparable to OIB for the most incompatible content was only reported for one sample; elements (left side of the diagram), but they are these samples are therefore plotted at the enriched and display flatter patterns in the right same MgO content. Panel (A) also shows a side of the diagram (Fig. 10B). MELTS algorithm fractional crystalliza- The whole rock compositions of the two dated tion model of the most primitive Hansen samples and a rhyodacite flow (17C-38) from the Point sample with 5.53 wt% MgO (sample sample section at Hansen Point (Figs. 2B and 4) 17C-57), assuming it contained 0.25 wt%

are also listed in Table 1. The dated sample from H2O and fractionated along the fayalite- Hansen Point (82-TM-212A; Trettin and Parrish, magnetite-quartz oxygen buffer. This was

1987, Trettin, 1996b) has 69.1 wt% SiO2 and calculated using rhyolite-MELTS code re- plots in the dacite field (Fig. 7). The dated horn- lease 1.0.2 (Ghiorso and Sack, 1995; Gualda blende syenite from Audhild Bay (17HSB-40A) et al., 2012).

1702 Geological Society of America Bulletin, v. 133, no. 7/8

(82-TM-212A, Fig. 4) was originally dated by yielded a Cretaceous K-Ar age interpreted to zircon U-Pb isotope dilution–thermal ionization reflect a younger thermal overprint. Zircon re- mass spectrometry (Trettin and Parrish, 1987). covered from the syenite are clear, colorless, These analyses of large, multi-grain fractions re- anhedral fragments (Fig. 11E). The zircons have turned an intercept age of 88 ± 20 Ma and dem- extremely poor CL response owning to very onstrated the presence of a significant propor- high U contents (Supplementary Data File 6), tion of xenocrystic zircon. The original mineral thus their faint internal zonation is best seen in separates of this sample were reanalyzed using BSE images (Fig. 11F). Eighteen analyses on SHRIMP in order to avoid the mixing of igne- separate zircon grains, returned 206Pb/238U ages ous and inherited zircons that plagued the earlier between 78.6 and 90.7 Ma. With the exception analyses. Transmitted light (Figs. 11A and 11B) of the oldest analysis (spot 12199-26.1, 90.7 Ma, and CL (Fig. 11C) images of the zircons show Supplementary Data File 6), there is a linear re- considerable variation in color and shape. There lationship between SHRIMP Pb/U age and U is a subpopulation of zircon grains that are sub- concentration. This is a known analytical artifact rounded/resorbed (Fig. 11A) and appear xeno- of ion probe analyses related to differences in Figure 9. Zr/Ti versus Nb/Y discrimination crystic, whereas other grains are clear, colorless, the matrix of high U zircon relative to the refer- diagram (Pearce, 1996) showing the bulk rock and well-faceted and are inferred to be igneous ence material used to calibrate the ion sputter- compositions of basaltic samples of this study (Fig. 11B). In CL most grains are composed of ing and instrumental Pb-U mass fractionation from Hansen Point (Ellesmere Island, Can- a single phase of oscillatory-zoned zircon, al- (Williams and Hergt, 2000; White and Ireland, 206 238 ada), together with published data for basalt though a small number have internal core/rim 2012). The weighted mean Pb/ U age of sev- and basaltic andesite from Audhild Bay, Phil- structures (Fig. 11C, e.g., grain 76). Twenty- en analyses of zircon with less than 2000 ppm ips Inlet, Yelverton Bay West, Alpha Ridge, four analyses were carried out on 24 zircon U is 79.5 ± 0.5 Ma (Fig. 11G) and is interpreted and the Strand Fjord Formation (SFF). Key grains (Fig. 11D, Supplementary Data File 6). as the crystallization age of the hornblende sy- and data sources are given in Figure 8. Seventeen analyses of the dominant, euhedral, enite. One additional low U zircon returns an age oscillatory zoned population present as both of ca. 90 Ma and is interpreted as an inherited discrete grains and rims yield a concordia age component. (0.41–0.45 wt%) and high Zr (356–584 ppm) of 95.5 ± 1.0 Ma (Fig. 11B) which is interpreted relative to the basalts and basaltic andesites. as the crystallization age of the rhyodacite. An DISCUSSION They therefore have Zr/Ti ratios that are much additional six analyses demonstrated inheritance higher than can be shown in Figure 9. However, ranging from 110 to 1060 Ma, primarily from Classification of Basalt and Basaltic the Nb/Y ratio ranges from 0.32 in 82-TM-212A, the population of subrounded/resorbed grains, Andesite at Hansen Point over 0.77 in 17C-38 to 2.53 in 17HSB-40A. but not exclusively (e.g., grain 66, Fig. 11C). Sample 17HSB-40A collected along the west- The analyzed samples of basalt and basal- U-Pb Chronology ern shore of Audhild Bay (Fig. 2B) is a medium- tic andesite from Hansen Point form coherent grained, hornblende-porphyritic syenite from a trends for the immobile elements (Fig. 8) and A plagioclase and microcline porphy- unit inferred to be the “hornblende syenite” have near-constant ratios of incompatible ele- ritic rhyodacite sample from Hansen Point unit on Trettin’s map (1996a), which previously ments, e.g., Nb/Y (Fig. 9) and Zr/Nb (8.0–9.6, Table 1). The Nb/Y (0.60–0.64) plots in the subalkaline field, consistent with the classifica- A Figure 10. (A) Chondrite- tion in the TAS diagram with the exception of normalized rare earth element one highly altered sample (Fig. 7). In several diagram of the Hansen Point tectonic discrimination diagrams (Nb-Zr-Y, (Ellesmere Island, Canada) Ti-Zr-Y, Ti-V, MgO-FeO*-Al2O3), the Hansen samples of basaltic dykes and Point samples classify as intraplate volcanism sheets. (B) Primitive mantle (Supplementary Data Files 7–10). In the AFM normalized incompatible ele- diagram they follow a trend of iron enrichment ment abundances of the Hansen similar to tholeiitic basalts (Supplementary Point samples. Normalizing val- Data File 11). Similarly, the clinopyroxene ues for both diagrams are after compositions plot in the subalkaline field of Sun and McDonough (1989). the Ti-Ca discrimination diagram (Fig. 12) B Also shown are (i) a field for and compare to the tholeiitic Skaergaard dif- the compositions of the Strand ferentiation trend in the pyroxene quadrilateral Fiord Formation (SFF) basalts (Fig. 6). The plagioclase and olivine compo- (Dostal and MacRae, 2018) sitions are also typical of evolved, tholeiitic and (ii) the compositions of en- basalts. We therefore conclude that the basalts riched mid-ocean ridge basalt and basaltic andesites of Hansen Point follow (E-MORB), normal mid-ocean tholeiitic trends and represent intraplate rift ridge basalt (N-MORB), and volcanism, corroborating previous inferences ocean island basalt (OIB) from (Trettin and Parrish, 1987; Estrada et al., 2006; Sun and McDonough (1989). 2016; Bédard et al., 2019).

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A B E F

G C

Figure 11. Zircon images and sensitive high resolution ion microprobe (SHRIMP) U-Pb zircon geochronology results, Ellesmere Island, Canada. Error ellipses on concordia diagrams are plotted at the 95% confidence interval. (A) Transmitted light image of subrounded zircons recovered from Hansen Point rhyodacite 82-TM-212A. (B) Transmitted light image of well-faceted zircons recovered from 82-TM- 212A. (C) Cathodoluminescence images of representative euhedral zircon. Location of SHRIMP pits are shown by ellipses and labeled by grain number as given in Supplementary Data File 6. (D) Concordia diagram showing all results from 82-TM-212A. Analyses used in the calculation of the concordia age are shown in gray. (E) Transmitted light image of anhedral zircons recovered from hornblende-porphyric syenite 17HSB-40A. (F) Back-scattered electron images representative of anhedral zircon. Location of SHRIMP pits are shown by ellipses and labeled by grain number as given in Supplementary Data File 6. (G) Concordia diagram showing all results from 17HSB-40A. Analy- ses used in the calculation of the weighted mean 206Pb/238U age are shown in gray. See text for discussion. MSWD—mean square weighted deviation.

Petrogenesis of Basalt and Basaltic 4.1 wt% MgO. This is confirmed by a MELTS ing Ni, and increasing Ce and Nb with decreas- Andesite at Hansen Point (code release 1.0.2) fractional crystallization ing MgO (Fig. 8) are also qualitatively consistent model Ghiorso and Sack, 1995; Gualda et al., with fractional crystallization. However, the in-

The TiO2 of the Hansen Point basalts and 2012) starting with the most primitive Hansen creasing SiO2 up to 57 wt% in the basaltic andes- basaltic andesites (Fig. 8) can be explained by Point sample with 5.53 wt% MgO (sample 17C- ites (Fig. 7) may also indicate significant crustal

fractional crystallization. The change-over at 57), assuming it contained 0.25 wt% H2O and assimilation, in addition to fractional crystalliza- maximum TiO2 witness the first appearance fractionated along the fayalite-magnetite-quartz tion. Similarly, the positive Th and negative Nb of FeTi oxides as a fractionating phase at 4.3– oxygen buffer (Fig. 8A). The trends of decreas- anomalies in the primitive mantle-normalized

1704 Geological Society of America Bulletin, v. 133, no. 7/8

tion of AUCC. Clearly, the ALCC cannot pro- vide a satisfactory explanation for crustal contamination of the Hansen Point trend. Figure 13 also shows published compositions for local Figure 12. Clinopyroxene crustal rocks that may be potential contaminants. (Cpx) compositions from basal- These are (i) local Pearya basement rocks (Suc- tic dykes and sheets of Hansen cession I) occurring within 12 km to the SE of Point (Ellesmere Island, Can- Hansen Point (Fig. 4) (Estrada et al., 2018) and ada) plotted in the Ti vs. Ca per (ii) a Cretaceous sandstone from Axel Heiberg formula unit (p.f.u) discrimi- Island (Evenchick et al., 2015). The composition nation diagram of Leterrier shown for the Pearya basement is the average of et al. (1982). Also shown are three samples of leucogranite and gneiss and is the clinopyroxene compositions very similar to AUCC, also in terms of Th, Nb, from the dredge samples of the and Yb concentrations (Estrada et al., 2018). In Alpha Ridge (Van Wagoner this average we have not included metamorphic et al., 1986). mafic rocks in the Pearya Terrane as these are less fusible during assimilation processes and have intermediate compositions between mantle and crust. In contrast, the Cretaceous sandstone has lower Th/Yb that is comparable to the basaltic andesite samples (Fig. 13). Moreover, the multielement plot (Fig. 10B) are hallmarks of (N-MORB), enriched mid-ocean ridge basalts Th concentration of this sandstone (4.2 ppm) crustal assimilation. (E-MORB), and OIB that formed with little or is comparable to the basalts (3.7–5.9 ppm) and To examine this further we have plotted the no influence from continental crust. Continental lower than the basaltic andesites (6.5–9.6 ppm) Hansen Point samples in Th/Yb versus Nb/Yb crust plots above the mantle array at relatively at Hansen Point. Rocks of this particular compo- space (Fig. 13). This plot was developed by high Th/Yb for a given Nb/Yb. For illustration, sition are therefore not likely candidates to ex- Pearce (2008) to distinguish basaltic samples we have shown average compositions of upper plain crustal contamination in the Hansen Point that were influenced by continental crust rela- and lower continental crust (AUCC and ALCC, samples, unless it is assumed that partial melts tive to uncontaminated basaltic samples of the respectively) from Willbold and Stracke (2006). of the sandstone were assimilated. oceans. The latter forms a mantle array encom- All Hansen Point samples plot above the mantle The Hansen Point trend in Figure 13 is best passing depleted normal mid-ocean ridge basalts array in a trend stretching toward the composi- explained by crustal contamination assuming a starting composition for the uncontaminated ba- Figure 13. Th/Yb vs. Nb/Yb salt between E-MORB and OIB. For illustration, showing the Hansen Point ba- we have calculated trends for bulk mixing (not salts, basaltic andesites, and shown) and for coupled assimilation and frac- silicic rocks of this study, other tional crystallization (AFC) assuming that (i) basaltic rocks of northern Elles- the uncontaminated basalt was a simple mixture mere Island, Canada, and the of 85% E-MORB and 15% OIB (constructed to Strand Fiord Formation (SFF) fit the uncontaminated end of the trend) and ii( ) (see caption to Fig. 8), the oce- the assimilated material had the composition of anic mantle array of Pearce AUCC. Results are very similar for calculations (2008), the average composition using the average leucocratic Pearya basement of upper and lower continental rocks as the assimilated material (not shown). crust (AUCC and ALCC, re- The bulk mixing model explains the Hansen spectively, Willbold and Stracke, Point basalts by ∼8–20% assimilation of bulk 2006), the average composition crust, whereas the basaltic andesites requires up of Pearya basement gneisses in to 35% assimilation of bulk crust. Similarly, the the Hansen Point area (Succes- AFC calculations suggest the basalts of Hansen sion I, Estrada et al., 2018) and Point can be explained by 10–17% assimilation, a Cretaceous sediment (sand- and up to 38% assimilation is required to explain stone) from Axel Heiberg Island the basaltic andesite with the highest Th/Yb (Evenchick et al., 2015). Also (Fig. 13). Supplementary Data Figure 12 shows shown are modeled bulk mixing that the high SiO2 and high Th/Nb of the basaltic and for coupled assimilation and fractional crystallization (AFC) curves assuming that (i) andesites (52–57 wt%) can also roughly be ex- the uncontaminated basalt was a simple mixture of 85% enriched mid-ocean ridge basalt plained by bulk mixing with up to 35% AUCC (E-MORB) and 15% ocean island basalt (OIB) and (ii) the assimilated material had the (Willbold and Stracke, 2006). The omnipresent composition of AUCC. The AFC equations are from DePaolo (1981) and the calculations influence of continental crust in the petrogenesis assume the bulk partition coefficients are 0.1 for all elements, and ther value, i.e., the ratio of the basalts and basaltic andesites of Hansen of the rate of mass assimilated to the rate of mass crystallized, is 0.3 (Tegner et al., 1999). Point is supported by Sr and Nd isotopic compo- N-MORB—normal mid-ocean ridge basalt. sitions (Estrada et al., 2016; Bédard et al., 2019).

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We also emphasize that the steep trend of the resent a crustal melt involved as a contaminant saltic andesite for the stratotype at Hansen Point Hansen Point rocks in Figure 13 cannot easily in the AFC processes discussed above for the (Estrada et al., 2016) overlap perfectly with our be explained by crustal components mixed into Hansen Point samples. Finally, the dated sample results (Figs. 8 and 9). the mantle. This could, for example, result from of hornblende syenite (17HSB-40A) is distinct Figures 8 and 9 also show a field for published situations where continental sediments were from the two other silicic samples discussed. The values for basaltic rocks of the Strand Fiord For-

translated to the mantle via subduction zone alkali-rich composition (12.1 wt% Na2O + K2O mation (Estrada and Henjes-Kunst, 2004; Kings- processes. Such mechanisms are known from and Nb/Y of 2.53) shows this sample is alkaline. bury et al., 2016; Dostal and MacRae, 2018). island arc volcanism (Pearce, 2008), but also This is confirmed by (Na+K)/Al of 1.01 and an Figure 8 shows that the compositions of Hansen from some LIPs such as the Central Atlantic aluminum saturation index (Al/[Ca + Na+K]) Point samples largely overlap with the composi- Magmatic Province (Whalen et al., 2015; Mar- of 0.90 which suggest it is borderline to being tions of SFF. In Figure 9, they also mostly over- zoli et al., 2018, 2019). However, in the case peralkaline. Also, in Figure 13 this sample plots lap, although the trend of near-constant Nb/Y of crustal components mixed into the mantle, at the upper bound of the mantle array close to and variable Zr/Ti for the Hansen Point samples the mantle melting trends have been shown to OIB and at much higher Nb/Yb (24.6) com- is comparable to the broad positive correlation of be sub-parallel to the mantle array in Figure 13 pared to the two other silicic samples (7.1 and Zr/Ti and Nb/Y shown by the SFF. Variation of (Pearce, 2008; Tegner et al., 2019), and thus al- 5.4). We therefore conclude that the dated horn- Zr/Ti ratio, however, can be ascribed to effects most orthogonal to the Hansen Point trend. blende syenite from Audhild Bay formed from a of fractional crystallization of FeTi oxide that In summary, the analyzed basalts and basaltic mantle-derived magma that evolved to trachytic preferentially removes Ti relative to Zr. More- andesites of Hansen Point show coherent geo- compositions mainly by fractional crystalliza- over, Figures 8 and 9 also show the compositions chemical trends that are best explained by fraction without much interaction with the conti- of four basaltic dykes cutting Pearya basement tional crystallization coupled with ubiquitous as- nental crust. in the vicinity of Hansen Point; two are located similation of upper continental crust. A parental ∼5 km to the south in a fault block of the Mitch- magma of tholeiitic basalt with trace element Geochemical Correlation of Basaltic Rocks ell Point fault zone, and two are NE-SW striking compositions intermediate between E-MORB of Northern Ellesmere Island, the Strand dykes located ∼10 km to the northeast across Pe- and OIB provides a good fit. The tectonic set- Fiord Formation on Axel Heiberg Island, tersen Bay (Fig. 4) (Estrada and Henjes-Kunst, ting was one of continental intraplate, rift-related and Alpha Ridge 2013). As can be seen in Figures 8 and 9, their volcanism, in agreement with previous stud- compositions are similar to tholeiitic rocks from ies (e.g., Estrada et al., 2006, 2016; Dockman As outlined above, the Hansen Point Volcanic both Hansen Point and SFF. This was also the et al., 2018). Complex nomenclature has hitherto been used conclusion of Estrada and Henjes-Kunst (2013). collectively as reference for volcanic rocks in the Finally, the published results for the dredge Petrogenesis of Silicic Rocks at Hansen four areas referred to here as Audhild Bay, Phil- samples of the Alpha Ridge (Van Wagoner et al., Point and Audhild Bay ips Inlet, Yelverton Bay West, and Yelverton Bay 1986) are shown in Figures 8 and 9. The Alpha East (that includes the Hansen Point stratotype Ridge samples are similar to the samples from It is beyond the scope of the present contri- area) (Fig. 2B) (Estrada et al., 2006, 2016). In Audhild Bay, Philips Inlet, and Yelverton Bay bution to discuss the petrogenesis of the silicic the following we critically discuss this usage and West and have similar incompatible element rocks in detail. We note, however, that the com- the correlations to the Strand Fiord Formation on concentrations (Figs. 8 and 9). In contrast, the position of the rhyolite flow of the Hansen Point the basis of geochemistry and, in the next sec- clinopyroxene compositions of Alpha Ridge section (17C-38) is enriched in the incompatible tion, from geochronology. The published com- are different from the Hansen Point samples trace elements and depleted in the compatible positions for basaltic and slightly more evolved (Fig. 12). There is also a marked difference in

trace elements relative to the basaltic andesites, rocks with up to 56 wt% SiO2 from these areas whole rock compositions in Figure 13. Here, the and shows a very similar pattern in the multi- are also plotted in Figures 8 and 9 (Estrada and Alpha Ridge samples plot close to OIB within element plot (Fig. 10). Moreover, in Figure 13 Henjes-Kunst, 2004; Estrada et al., 2006, 2016). the mantle array whereas the Hansen Point sam- this rhyolite sample plots on the Hansen point Clearly, the samples from Audhild Bay, Philips ples form a vertical trend that can be explained trend at slightly higher Th/Yb (2.1) compared to Inlet, and Yelverton Bay West have higher Nb by contamination of upper continental crust the basaltic andesites (up to 1.8). We therefore (up to 143 ppm) and Ce (up to 140 ppm) con- and points to a mantle-derived composition dis- conclude that this rhyolite composition is well centrations for a given MgO content relative to tinct from both OIB and from the Alpha Ridge explained by further AFC processes relative to the Hansen Point rocks (Fig. 8). Similarly, in samples as discussed above. Unfortunately, the the tholeiitic basalts and basaltic andesites. In the Zr/Ti vs. Nb/Y discrimination diagram these available data for basalts from Audhild Bay and other words, the extrusive silicic rocks are petro- rocks form a separate trend in the alkaline field Philips Inlet are lacking in Yb and thus cannot genetically linked to the tholeiitic basaltic dykes. with Nb/Y values from 0.88 to 3.81 (Fig. 9), be shown in this plot. However, the dated horn- This result corroborates the analysis of Estrada distinctly higher than the tholeiitic rocks from blende syenite from Audhild Bay plots close to et al. (2016). Hansen Point. The exceptions are two samples OIB and the Alpha Ridge samples in Figure 13. Although the available trace element data (SE161/01A and SE135/01 from Estrada et al., Finally, data from basalts and basaltic andesite for the dated dacite sample from Hansen Point 2016) from Yelverton Bay West that overlap with of Yelverton Bay West (Estrada et al., 2016) (82-TM-212A; Trettin and Parrish, 1987, Tret- the Hansen Point samples in terms of Ce and also plot close to OIB and Alpha Ridge samples tin, 1996b) is sparse, its Th/Yb is distinctly high- Nb vs. MgO (Fig. 8), and plot at intermediate across the upper bounds of the mantle array. er (3.9) than the 17C-38 rhyolite and comparable Nb/Y (0.72–0.76) close to the division line be- In summary, we have shown that the basaltic to the AUCC (Fig. 13). This sample therefore tween the tholeiitic and alkaline fields Fig. 9( ). rocks (lavas and dykes) at Hansen Point (Yel- has a strong crustal signature, consistent with Unfortunately, the nature of the outcrops of these verton Bay East) are tholeiitic and composition- the abundance of inherited zircons as discussed two samples is not given. We also note that the ally akin to (i) dykes cutting Pearya basement in below. Hence, this dacite potentially could rep- previously published values for basalts and ba- the vicinity of Hansen Point and to (ii) lavas and

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intrusions of the Strand Fiord Formation on Axel ages of 93.9 ± 1.3 Ma and 83.0 ± 1.8 Ma were Three published Ar-Ar ages for basalt flows Heiberg Island. Notably, the tholeiitic composi- given for two, compositionally similar, basaltic and an ignimbrite at Audhild Bay have simi- tions of the Hansen Point rocks are distinct from dykes cutting the volcanic rocks at Hansen Point lar ages of 78.4 ± 0.1 Ma, 78.8 ± 0.1 Ma, and the alkaline compositions of basaltic dykes and close to our sample transect (Estrada and Henjes- 79.2 ± 1.9 Ma (Dockman et al., 2018; Estrada lavas of the other areas of northern Ellesmere Kunst, 2013). On the grounds that the younger and Henjes-Kunst, 2013). A similar Ar-Ar age Island (Audhild Bay, Philips Inlet, and Yelverton age overlapped with the preferred emplacement of 76.6 ± 3.7 Ma is reported for a basaltic flow Bay West), and to the alkaline basalts dredged age for volcanic rocks of Yelverton Bay West, it at Philips Inlet (Fig. 14). The new U-Pb age of from the Alpha Ridge. was concluded that the 83.0 ± 1.8 Ma age likely the plutonic rock from Audhild Bay with an represented the age of magmatism at Hansen age of 79.5 ± 0.5 Ma (Fig. 11) shows that this Age of the Hansen Point Volcanic Complex, Point itself. The older age was regarded as spu- hornblende syenite can be linked to the volcanic the Strand Fiord Formation, and Alpha rious or unexplained. The older 93.9 ± 1.3 Ma rocks outcropping between Audhild Bay and Ridge age, however, is based on a robust Ar-Ar plateau Emma Fjord (Fig. 2). Hence, the rocks of Aud- age that just overlaps within error of our U-Pb hild Bay and Philips Inlet are ∼18 m.y. younger Assessing the chronology of the magmatism crystallization age of 95.5 ± 1.0 Ma (Figs. 11 than the Hansen Point rocks. Moreover, their of northern Ellesmere Island is challenging for and 14) for a rhyodacite sample of Hansen Point alkaline compositions are distinct from the tho- several reasons. Firstly, geological and structural (Trettin and Parrish, 1987). Moreover, two ba- leiites of Hansen Point (Figs. 8 and 9). age control is hampered by the fault contacts saltic dykes cutting Pearya basement ∼5 km to While the alkaline Wootton intrusive complex with much older Paleozoic or Proterozoic rocks the south in a fault block within the Mitchell (WIC) that extends across Wootton Peninsula (Figs. 2 and 4). Secondly, the ubiquitous interac- Point fault zone, and a NE-SW striking dyke from Philips Inlet to Yelverton along the Mitch- tion with crustal rocks in the petrogenesis of both located ∼10 km to the northeast across Petersen ell Point fault zone (Trettin and Parrish, 1987; basalts and silicic rocks (Fig. 13; Estrada et al., Bay (Fig. 4), also yielded Ar-Ar plateau ages in Saumur et al., 2016) (Fig. 2) gives consistent 2016), renders zircon inheritance a major chal- this range (94.3 ± 2.8 Ma and 96.6 ± 1.6 Ma; U-Pb and Ar-Ar ages of 93–92 Ma (Fig. 14) lenge for U-Pb chronology, as discussed above Estrada and Henjes-Kunst, 2013) (Fig. 14). (Estrada and Henjes-Kunst, 2013), the ages re- and by Trettin and Parrish (1987) and Estrada The ages of SFF basalts range from ca. 102– ported for Yelverton Bay West are disparate. Two et al. (2016). Thirdly, resetting of Ar-Ar system- 95 Ma (Fig. 14) with the most recent U-Pb ages Ar-Ar plateau ages that are interpreted to rep- atics has been ascribed to hydrothermal activ- (95.56 ± 0.24 Ma, 95.18 ± 0.35 Ma; Kingsbury resent crystallization ages gave 82.3 ± 2.0 Ma ity associated with the Eurekan orogeny both et al., 2018) overlapping with the Hansen Point and 73.5 ± 2.4 Ma for alkali basalt and trachy- for HALIP rocks of northern Ellesmere Island ages. As discussed above, the compositions of andesite flows, respectively (Estrada and Henjes- (Estrada and Henjes-Kunst, 2013) and for North the dykes in the vicinity of Hansen Point are Kunst, 2013). In contrast, zircon U-Pb chronolo- Greenland (Tegner et al., 2011). Notwithstanding tholeiitic and similar to the dykes and lavas of gy of three samples of rhyodacite flows from the these challenges, we have summarized recently this study, and to the basalts of SFF (Figs. 8 and same volcanic area yielded highly diverse age published Ar-Ar and U-Pb chronology argued to 9). We therefore conclude that both the chronol- populations resulting from inheritance (Estrada represent crystallization ages for volcanic rocks ogy and geochemistry of the Hansen Point dykes et al., 2016). The three samples are dominated by of northern Ellesmere Island, and basalts of SFF and lavas can be correlated with the basalts of zircons with ages ranging from ca. 88–120 Ma, and Alpha Ridge in Figure 14. SFF, suggesting they were part of the same mag- i.e., HALIP ages, but all three samples also con- In the Hansen Point area (Yelverton Bay East), matic system. tained Proterozoic zircons. As pointed out by recent chronology is restricted to four Ar-Ar ages The chronology of Audhild Bay and Phil- Estrada et al. (2016), it is obviously a challenge and the U-Pb age presented here. Ar-Ar plateau ips Inlet is more straightforward (Fig. 14). to determine the emplacement age for these. For one sample (SE162/01) they interpreted the twelve youngest and concordant grains as the Figure 14. Summary of chro- emplacement age (103.6 ± 1.0 Ma). However, if nology showing the two U-Pb concordance is assessed using the probability of ages of this study, and a com- concordance of individual Concordia ages (Lud- pilation of published U-Pb wig, 1998) (rather than the difference between and Ar-Ar plateau ages for the 206Pb/238U and 206Pb/207Pb ages), the data re- magmatic rocks of northern veals that there are also concordant ages in the Axel Heiberg (Axel H.) and 98–95 Ma range for this sample. The two other Ellesmere islands, Canada, as- samples have populations around 99–98 Ma sociated with the Strand Fiord and 106–105 Ma, and individual dates down to and Kanguk formations (Fm.) 88 Ma. In our view, it therefore remains a pos- (Fig. 3). Errors are reported as sibility that the rhyodacites of Yelverton Bay 2σ and are only shown when West were emplaced at the same time as the they are larger than the sym- rhyodacite units at Hansen Point. However, more bols. From left to right, the plot high-resolution chronology is required to resolve is organized with approximate geography from southwest to northeast: Axel Heiberg Island; crystallization ages from xenocrystic/autocrystic Kanguk tephras, Audhild Bay (Aud. Bay), Philips Inlet (Ph. In.), Wootton intrusive complex material. We also note that the younger Ar-Ar (WIC), Yelverton Bay West (YBW), Hansen Point, and the Alpha Ridge (AR). Published ages obtained for basaltic and trachyandesitic data compiled from: Tarduno et al. (1998); Jokat et al. (2013); Estrada and Henjes-Kunst flows at Yelverton Bay West (82.3 ± 2.0 Ma and (2013); Estrada (2015); Kingsbury et al. (2016); Estrada et al. (2016); Davis et al. (2017); 73.5 ± 2.4 Ma; Estrada and Henjes-Kunst, 2013) Dockman et al. (2018); Pointon et al. (2019); Williamson et al. (2019a). are comparable, but not identical to the ages

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obtained at Audhild Bay (79–78 Ma; Estrada East) are all tholeiitic (Trettin and Parrish, 1987; tary basin, through thermal metamorphism of

and Henjes-Kunst, 2013; Dockman et al., 2018; Estrada et al., 2006, 2016; this study). We view sediments (e.g., CO2, CH4, CH3Cl, CH3Br, CO2, this study) (Fig. 14). Moreover, in terms of com- these rocks as the stratotype of the tholeiitic suite and Hg) (Svensen et al., 2018a; 2018b; Grasby positions, the basaltic and trachyandesitic rocks of northwestern Ellesmere Island and therefore et al., 2019). of Audhild Bay, Philips Inlet, and Yelverton Bay propose that it is called the Hansen Point tho- The studies invoking a submarine LIP at West are comparable (Figs. 8 and 9). Hence, we leiitic suite (HPTS). We note that this suite is OAE2 do not account for reported evidence for surmise that these rocks may have formed part not restricted to Hansen Point. Estrada et al. subaerial eruption occurring during that time, of an alkaline magmatic province that was active (2016), for example, reported tholeiitic compo- that includes Pb isotope (Kuroda et al., 2007) perhaps from ca. 83 to 73 Ma, as also suggested sitions also from Yelverton Bay West. Based on and clay minerals data (Löhr and Kennedy, by Estrada et al. (2016). the present knowledge, the HPTS was mainly 2014). In addition, direct evidence of subaerial Another part of the volcanic puzzle for Elles- emplaced from 97 to 93 Ma and can be corre- eruption of the SFF phase of HALIP, including mere and Axel Heiberg islands are the alkaline, lated with the SFF volcanic rocks of Axel Hei- basalt flows and pyroclastic deposits, have long silicic ash beds (bentonites) that are interbedded berg Island. been recognized (Ricketts et al., 1985; Souther, in the Kanguk Formation (Fig. 3). Their ages As discussed above (Figs. 8, 9, and 13), the 1963). Likewise, it is recognized that the tholei- range from 93–83 Ma (Fig. 14) (Davis et al., volcanic rocks of Audhild Bay, Philips Inlet, and itic suite (HPTS) of the Hansen Point stratotype 2017; Pointon et al., 2019). Compositionally, Yelverton Bay West are all alkaline. We propose section is composed of interbedded basaltic and they are similar to the alkaline silicic volcanic the large outcrops of alkali basaltic to rhyolitic rhyolitic extrusive units that erupted subaerially rocks and WIC of northern Ellesmere Island, extrusives and intrusions (e.g., the syenite re- (Trettin and Parrish, 1987; Estrada et al., 2006). and the initial phase (ca. 93–91 Ma) therefore ported here) of the Audhild Bay area (Fig. 2) Moreover, amygdaloidal textures in the studied fits very well with WIC, as pointed out by these and described by Embry and Osadetz (1988) is dykes cutting the volcanic rocks witness degas- authors. However, the later ashes from ca. 90– used as the stratotype. Hence, the alkaline suite sing and these dykes thus most likely represent 83 Ma are not readily matched by any known of northwestern Ellesmere Island is denoted as near-surface feeders for subaerial eruptions. ages (Fig. 14). Davis et al. (2017) argued that the Audhild Bay alkaline suite (ABAS). Based Our work links the subaerial eruptions of SFF the thicknesses of the ash beds (10–50 cm) can on the present knowledge, the alkaline suite is and HPTS to the same late Cenomanian age be explained by plinian eruptions up to 1000 km considerably younger (83–73 Ma) than the tho- magmatic event. These results imply a much away and, therefore, pointed to the volcanic leiitic suite (97–93 Ma). broader subaerial eruption event, than previ- rocks of northern Ellesmere Island or the Alpha ously recognized to have occurred at that time, Ridge as the likely source areas. The recent Ar- Potential Linkage to OAE2 one that could have been even more extensive Ar age obtained for the dredge samples of alka- depending on the extension of the poorly char- line basalts from Alpha Ridge (90.40 ± 0.26 Ma; While long debated, the causes of Cretaceous acterized Alpha Ridge. Although volcanism of Williamson et al., 2019a), and the explosive Ocean Anoxic Events (OAEs) are increasingly HALIP spread over a long period (130–60 Ma), character of these samples (Williamson et al., linked to volcanic eruptions. Numerous stud- the stratigraphy shows subaerial volcanism was 2019b; Massey et al., 2019), therefore, supports ies have suggested a linkage of one of the most pulsed (Fig. 3) and recent chronology suggests this speculation. However, so far only explosive intense OAEs, the late Cenomanian OAE2 that two, short major volcanic events took place basaltic samples have been recovered from the (≈94 m.y.), to either (or both) the HALIP and at ca. 122 and 95 Ma (e.g., Corfu et al., 2013; Alpha Ridge. Caribbean LIP (e.g., Kuroda et al., 2007, Tur- Estrada et al., 2016; Kingsbury et al., 2018; In summary, our new U-Pb crystallization geon and Creaser, 2008; Adams et al., 2010; Dockman et al., 2018; this study). The subaerial age of 95.5 ± 1.0 Ma for Hansen Point (Fig. 11) Tegner et al., 2011; Zheng et al., 2013; Löhr ca. 95 Ma HALIP event correlates well with age is consistent with Ar-Ar ages for dykes in the and Kennedy, 2014; Du Vivier et al., 2015; estimates for the onset of OAE2, and as such we vicinity of Hansen Point (Estrada and Henjes- Schröder-Adams et al., 2019; O’Connor et al., suggest that this event is the best candidate for Kunst, 2013), and is coeval with the basalts of 2020). Several studies have used geochemical driving that OAE. Similarly, the subaerial ca. the Strand Fiord Formation. The other dated oc- proxies to link OAE2 to a purported submarine 122 Ma HALIP event correlates well with age currences of northern Ellesmere Island (Audhild LIP eruption, including: isotopic records of Os estimates for the onset of OAE1a and a causal re- Bay, Philips Inlet, and Yelverton Bay West) and (Turgeon and Creaser, 2008; Du Vivier et al., lationship has been proposed (Midtkandal et al., Alpha Ridge are considerably younger, includ- 2015; Schröder-Adams et al., 2019), S (Adams 2016). In both cases it can be argued that the ing our 79.5 ± 0.5 Ma age for a hornblende sy- et al., 2010), Nd (Zheng et al., 2013), and Hg subaerial nature of the HALIP events are more enite at Audhild Bay. (Percival et al., 2018). This has important im- likely to have caused major global environmen- plications for how climate and environmental tal perturbations given deleterious volatiles be- Revised Nomenclature for the Volcanic impacts during OAE2 are modeled, as it implies ing released directly to the atmosphere, affect- Rocks of Northern Ellesmere Island deleterious volatiles of the LIP eruption are re- ing climate and allowing rapid global dispersion. leased directly into marine waters rather than Additionally, HALIP volcanics intruded through The geochemical and chronological correla- into the atmosphere (e.g., O’Connor et al., 2020; sediments of the Sverdrup Basin causing thermal tions reviewed above pose serious questions to Joo et al., 2020; Percival et al., 2018). However, metamorphism of organic rich shales and fur- the usage of the Hansen Point Volcanic Com- it has been argued that the release and distribu- ther volatile generation (Goodarzi, et al., 2018). plex (Estrada et al., 2006, 2016) as a collective tion of magmatic volatiles is greatly reduced in Submarine LIPs of similar age (e.g., Caribbean term for the Cretaceous volcanic rocks of north- submarine environments compared to subaerial and Ontong Java oceanic plateaus) would have western Ellesmere Island. Instead, we propose eruptions (e.g., Percival et al., 2018). Further- directly impacted local marine systems and to divide these rocks into tholeiitic and alkaline more, a submarine LIP erupting through ocean would explain some geochemical anomalies suites. The analyzed basaltic to rhyolitic rocks crust would not generate the additional volatiles (Os, S, Nd) in sedimentary records, but given from the Hansen Point area (Yelverton Bay compared to a LIP erupting through a sedimen- more rapid marine sequestration processes of

1708 Geological Society of America Bulletin, v. 133, no. 7/8

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Modeling of coupled AFC of ABAS are related to those of the Alpha Ridge. Coffin, M.F., and Eldholm, O., 1994, Large igneous prov- suggests that the basalts can be explained by as- We emphasize that the lack of direct geochemi- inces: crustal structure, dimensions, and external con- sequences: Reviews of Geophysics, v. 32, p. 1–36, similation of 10–17% crust, and the basaltic an- cal correlation of the volcanic rocks at Hansen https://doi.org/10.1029/93RG02508. desite with up to 38% crust with a composition Point and the few available samples from the Al- Corfu, F., Polteau, S., Planke, S., Faleide, J., Svensen, H., Zayoncheck, A., and Stolbov, N., 2013, U–Pb geo- similar to AUCC or similar to local leucogran- pha Ridge does not preclude a relationship. The chronology of Cretaceous magmatism on Svalbard and ites and gneisses of the Pearya Terrane. The trace deeper parts of the Alpha Ridge could easily in- Franz Josef Land, Barents Sea Large Igneous Province: element composition of the parental magma in clude HPTS and Strand Fiord Formation equiva- Geological Magazine, v. 150, p. 1127–1135, https://doi .org/10.1017/S0016756813000162. these models is best explained as intermediate lents, as suggested by the on-land extension of Davis, W., Schröder-Adams, C., Galloway, J., Herrle, J., between E-MORB and OIB. This suggest the the Alpha Ridge geophysical signatures on the and Pugh, A., 2017, U–Pb geochronology of benton- tectonic setting was one of continental intraplate, Canadian Arctic islands. In the broader scope, ites from the Upper Cretaceous Kanguk Formation, Sverdrup Basin, Arctic Canada: Constraints on sedi- rift-related volcanism. understanding the connections of Alpha Ridge mentation rates, biostratigraphic correlations and the The basaltic rocks (dykes and sheets) at Han- to Canada’s continental crust contributes to the late magmatic history of the High Arctic Large Igneous Province: Geological Magazine, v. 154, p. 757–776, sen Point classify as tholeiites and are compo- research needed for the definition of territorial https://doi.org/10.1017/S0016756816000376. sitionally akin to (i) dykes cutting Pearya base- boundaries in international waters under UN- DePaolo, D.J., 1981, Trace element and isotopic effects of com- ment in the vicinity of Hansen Point and to (ii) CLOS (United Nations, 1982). Understanding bined wallrock assimilation and fractional crystallization: Earth and Planetary Science Letters, v. 53, p. 189–202, lavas and intrusions of the Strand Fiord Forma- the onland-offshort connections can also help https://doi.org/10.1016/0012-821X(81)90153-9. tion on Axel Heiberg Island. Notably, the tholei- resolve total eruptive volumes during HALIP Døssing, A., Jackson, H.R., Matzka, J., Einarsson, I., itic compositions of the Hansen Point rocks are events and the potential global environmental Rasmussen, T.M., Olesen, A.V., and Brozena, J.M., 2013, On the origin of the Amerasia Basin and the distinct from the alkaline compositions of basal- change they may have driven. High Arctic Large Igneous Province: Results of new tic dykes and lavas of northern Ellesmere Island aeromagnetic data: Earth and Planetary Science Let- ters, v. 363, p. 219–230, https://doi.org/10.1016/ (Audhild Bay, Philips Inlet, and Yelverton Bay ACKNOWLEDGMENTS j.epsl.2012.12.013. West), and to the alkaline basalts dredged from Dostal, J., and MacRae, A., 2018, Cretaceous basalts of the the Alpha Ridge. This is confirmed by distinct This work is an outcome of the joint Geological High Arctic large igneous province at Axel Heiberg Island (Canada): Volcanic stratigraphy, geodynamic clinopyroxene compositions for Hansen Point Survey of Canada and German Federal Institute for Geosciences and Natural Resources (BGR) Pearya setting, and origin: Geological Journal, v. 53, p. 2981– 2934, https://doi.org/10.1002/gj.3132. and the Alpha Ridge. Project: Circum-Arctic Structural Events expedition A new U-Pb crystallization age of Dockman, D., Pearson, D., Heaman, L., Gibson, S., and n. 19 (CASE 19) in 2017. We thank Karsten Piep- Sarkar, C., 2018, Timing and origin of magmatism in 95.5 ± 1.0 Ma is presented for a dacite of Han- john (BGR) for leading the expedition. CT acknowl- the Sverdrup Basin, Northern Canada: Implications for sen Point and is consistent with Ar-Ar ages for edges funding from the Danish National Research lithospheric evolution in the High Arctic Large Igneous dykes in the vicinity of the Hansen Point volcanic Foundation Niels Bohr Professorship grant 26-123/8 Province (HALIP): Tectonophysics, v. 742–743, p. 50– and BGR. The Polar Continental Shelf Program pro- 65, https://doi.org/10.1016/j.tecto.2018.05.010. rocks. Moreover, this age is coeval with the ba- vided logistical support during the expedition. This Dove, D., Coakley, B., Hopper, J., and Kristoffersen, Y., salts of the Strand Fiord Formation and thus cor- is Natural Resources Canada Contribution number and HLY0503 Geophysics Team, 2010, Bathymetry, controlled source seismic and gravity observations of roborates to the linkage between magmatism of 20200346. We thank Edward Ghent, Jean Bédard, the Mendeleev ridge: Implications for ridge structure, Strand Fiord Formation and Hansen Point. This and Ashton Embry for insightful comments and dis- origin, and regional tectonics: Geophysical Journal In- cussions. Robert Marr provided microprobe exper- demonstrates a large subaerial HALIP eruption ternational, v. 183, p. 481–502, https://doi.org/10.1111/ tise. We also thank Steve Piercy and an anonymous j.1365-246X.2010.04746.x. event (97–93 Ma) that overlaps with OAE2, and reviewer for their thoughtful and supportive journal Drachev, S., and Saunders, A., 2006, The Early Cretaceous may be the hypothesized driver of this anoxic reviews, and editorial handling by Brad Singer. Arctic LIP: Its geodynamic setting and implications for

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