Alkaline picritic volcanism on northern Ellesmere Island associated with initial rifting of the Sverdrup Basin, Canadian Arctic
Abstract
The Caledonian and Ellesmerian orogenies were followed by extension and development of intracontinental rift basins since the late Paleozoic, as represented by the Sverdrup Basin along the Canadian Arctic Islands. The initial rifting was accompanied by pulses of volcanic activity during the Carboniferous and Permian. A new occurrence of mafic volcanic rocks, the “Taconite volcanics” (informal name), was discovered on northern Ellesmere Island between the head of Ayles Fiord and M’Clintock Inlet. The mainly alkali-picritic lavas are exposed within the central part of the Pearya Terrane as an outcrop in faulted contact with upper Carboniferous red beds of the Canyon Fiord Formation. The contact to the Ordovician island-arc volcanic rocks of the Pearya Terrane is unclear. The outcrop is characterized by a small circular magnetic anomaly. Ar–Ar whole-rock geochronology on the volcanic rocks yielded an age of 290 ± 19 Ma suggesting emplacement of the lavas during the early Permian. Whole rock geochemical analyses for eight samples revealed a geochemical affinity to ocean island basalt (OIB) and indicate variable mixing of low-degree melts in the fields of garnet and spinel peridotite (∼80–90 km depth). The involvement of a metasomatized subcontinental lithospheric mantle is indicated by high Pb, Nb, and Ta concentrations. Geochemical differences (as enrichments in Ti, Nb, Zr, and the light rare earth elements (REE)) to the known Carboniferous and Permian spilitic altered basalt occurrences of northwestern Ellesmere and northern Axel Heiberg islands are probably based on differences in the mantle source. The Sr isotope ratios of the Taconite volcanics are primitive ((87Sr/86Sr)t: 0.7037–0.7042) and its Nd isotope ratios are moderately depleted (εNd(t): +2.20 to +2.85) in contrast to the enriched εNd(t) values of the Permian Esayoo formation.
1. Introduction
The target area of the expedition CASE 11 (Circum-Arctic Structural Events), organized and led by the German Federal Institute for Geosciences and Natural Resources (BGR), in the summer of 2008 was the northern part of Ellesmere Island with the aims to investigate the basement rocks of the Pearya Terrane, structural work mainly focused on the Eurekan Deformation and an aeromagnetic survey on this area. During a short helicopter stop for structural measurements along the Taconite River Fault Zone (Piepjohn et al. 2013), interesting pillow structures were observed, which were previously assigned to the Ordovician Succession 3 of Pearya according to the geological map of Trettin and Mayr (1996). For documentation, eight samples were collected (Fig. 1). Later analytical work revealed that the volcanic rocks show neither the age nor the geochemical composition of the Ordovician Succession 3 basalts. Our new data rather point to a connection to the rifting of the Sverdrup Basin.
Fig. 1.
The Sverdrup Basin covers the Canadian Arctic Islands over a length of c. 1000 km and a width of c. 350 km parallel to the Arctic margin and is filled with up to 13 km of Carboniferous to Paleogene strata (e.g., Balkwill 1978; Trettin 1991a; Embry and Beauchamp 2008, 2019) (Fig. 1). The Sverdrup Basin developed on the folded basement of the Neoproterozoic to Devonian Franklinian Basin and was repeatedly affected by magmatic activity. Its initial extensional development was accompanied by several volcanic events during the Carboniferous and the early Permian (between c. 325 and 275 Ma). Renewed multistage magmatism took place during the Cretaceous and was related to the opening of the Amerasian Basin and the formation of the High Arctic Large Igneous Province (HALIP). Whereas the HALIP was and still is an ongoing topic of numerous publications and controverse discussions, relatively little is known about the Carboniferous and Permian volcanism. Outcrops of such volcanic rocks are mainly concentrated in northwestern Ellesmere Island and northern Axel Heiberg Island (Thorsteinsson 1974; Trettin 1988; Cameron 1989; Ritcey 1989; Mayr 1992; Cameron and Muecke 1996; Mayr et al. 2002; Morris 2013), in areas, which were also affected by the Cretaceous magmatism (Fig. 1). Furthermore, the knowledge about the Carboniferous and Permian magmatism is significant for the interpretation of detrital zircon studies on Permian and younger sediments of the Sverdrup Basin, which provide regionally different results (Alonso-Torres et al. 2018; Galloway et al. 2021). However, the Carboniferous and Permian strata of the Sverdrup Basin including the volcanic rocks are covered by kilometer thick Mesozoic deposits and are only exposed at the surface in the northeastern part of the basin.
The aforementioned new occurrence of mafic volcanic rocks is located on northern Ellesmere Island, in the central part of the Pearya Terrane, between the head of Ayles Fiord and M’Clintock Inlet (Fig. 2). These rocks, hereinafter referred to as “Taconite volcanics”, were studied for their petrography, whole-rock major and trace element composition, Rb–Sr and Sm–Nd isotopes, and magnetic properties as well as dated by the Ar–Ar method. The analytical results presented in this paper show that the volcanic rocks are mainly alkali-picrites and extruded in a continental-rift setting. With these characteristics, the Taconite volcanics differ clearly from Ordovician volcanic rocks (mainly andesite and basalt with island-arc affinity) of the Maskell Inlet Complex of Succession 3 of the Pearya Terrane (Trettin and Mayr 1996; Trettin 1998), which form their basement. The Taconite volcanics are also not an equivalent of the alkaline late stage of the Cretaceous volcanism despite similar geochemical signatures. Instead, the early Permian Ar–Ar whole-rock age shows that the Taconite volcanics represent a new example of the late Paleozoic volcanism of the Sverdrup Basin. Similarities and differences to the known Carboniferous and Permian basalt occurrences of the Sverdrup Basin are discussed.
Fig. 2.
2. Geological background and previous research
Northern Ellesmere and Axel Heiberg islands are characterized by three major structural units (e.g., Trettin 1991a): the Neoproterozoic to Silurian Pearya Terrane, the late Neoproterozoic to Devonian Franklinian Basin, and the Carboniferous to Paleogene Sverdrup Basin (Fig. 1).
2.1. Franklinian Basin, Pearya Terrane, and Ellesmerian Orogeny
The Neoproterozoic to Devonian Franklinian Basin overlies the crystalline basement rocks of the Greenland–Canadian Shield and comprises up to 8 km thick clastic and carbonate shelf sediments in the south and deep-water sediments in the north deposited at the passive margin of Laurentia (e.g., Trettin 1991a).
The composite Pearya Terrane at the northern margin of Ellesmere Island represents an assemblage of fault-bounded crustal fragments, which are exotic to the northern Laurentian margin (Trettin 1987a, 1991b, 1998). Pearya is divided into five major tectonostratigraphic successions of metasedimentary and metaigneous rocks (Trettin 1998). Succession 1 is dominated by early Tonian granitoid orthogneisses, minor amphibolite and metasedimentary rocks, formed in a Stenian–Tonian magmatic arc (Trettin 1998; Malone et al. 2017; Estrada et al. 2018). Succession 2 comprises Neoproterozoic to early Ordovician metasedimentary rocks including quartzite, mudrock, limestone, dolostone, and minor metavolcanic rocks, which represent a passive margin sequence. Succession 3 consists of the deformed and greenschist-facies metamorphosed late Cambrian to early Ordovician volcanic and sedimentary rocks of the Maskell Inlet Complex. Volcaniclastic sediments with a dominant detrital zircon population of c. 572 Ma were sourced from a Timanian island arc (Estrada et al. 2018). Igneous rocks crop out along the southern margin of Succession 3 and comprise two genetically different suites: the subduction-related ultramafic to felsic Thores Suite formed at c. 500–470 Ma and an ophiolitic enriched mid-ocean ridge basalt (E-MORB) suite of unknown age, which were juxtaposed during the M’Clintock Orogeny at c. 470–450 Ma (coeval with the Taconian Orogeny, an early phase of the Caledonian Orogeny), when Succession 3 collided with Succession 2 (Trettin et al. 1982; Trettin 1998; Estrada et al. 2018; Majka et al. 2021). Successions 4 and 5, deposited after the M’Clintock Orogeny, consist of several kilometers thick middle Ordovician to late Silurian nearly unmetamorphosed carbonate and clastic sediments and volcanic rocks (Trettin 1998).
The approach and accretion of Pearya to the passive margin of northern Laurentia occurred between the late Silurian and Middle Devonian as indicated by detrital zircon data (Malone et al. 2018). Pearya was probably accreted as fault-bounded slices or blocks along a series of sinistral strike-slip faults (Trettin 1991b; von Gosen et al. 2012).
The collision with the Pearya Terrane caused intense deformation of the Franklinian Basin succession on northern Ellesmere and northern Axel Heiberg islands (Clements Markham Fold Belt). Successive terrane accretion of Pearya and of a hypothetical continental landmass with Timanian affinity (Crockerland; Embry 1993, 2009) that provided northerly derived sedimentary material with detrital zircon ages of 700–500 Ma since the Middle Devonian (Anfinson et al. 2012a,b) led to the Ellesmerian Orogeny. The latter is probably of late Devonian age in the Canadian Arctic and caused kilometer scale, northeast–southwest-trending folding of the almost entire infill of the Franklinian Basin (e.g., Trettin 1991a; Piepjohn et al. 2008, 2013; Piepjohn and von Gosen 2018 ; Beauchamp et al. 2018). A thick foreland basin (the Devonian Clastic Wedge) accumulated eroded material transported westward and southwestward from the orogenic belt and was ultimately deformed itself (Embry 1991).
2.2. Early development of the Sverdrup Basin
Extension, rifting, and passive subsidence following the erosion and peneplanation of the Ellesmerian Orogen led to the development of the east–west-trending Sverdrup Basin since the early Carboniferous. The Franklinian Basin and the accreted Pearya Terrane were unconformably overlain by up to 13–15 km thick Carboniferous to lower Paleogene deposits (e.g., Balkwill 1978; Trettin 1991a; Embry and Beauchamp 2008, 2019). The Sverdrup Basin underwent several phases of distinctive tectonic and depositional settings during its development (Embry and Beauchamp 2008, 2019) (Fig. 3).
Fig. 3.
The first phase in the early Carboniferous is characterized by extensional faulting and formation of a mostly terrestrial rift system. During the rifting, older thrust faults of the Franklinian basement were reactivated. The earliest rift-related Viséan Emma Fiord Formation with a thickness up to 700 m consists of dark, organic carbon-rich siltstones and shales with lesser amounts of sandstones and conglomerates, which were deposited in lacustrine, fluvial, and marginal marine environments. It was followed by the red-colored conglomerates and sandstones of the Serpukhovian Borup Fiord Formation, which is locally up to 850 m thick (Beauchamp et al. 2018).
The second, late Carboniferous to early Permian (Bashkirian to Kungurian) tectonostratigraphic phase is characterized by renewed rifting pulses, locally accompanied by basalt volcanism (see below), basin deepening, and enlargement followed by repeated episodes of tectonic quiescence and intervals of fault reactivation. In the deeper basin center, thick evaporites of the Otto Fiord Formation were initially deposited, followed by slope to basinal carbonate and siliciclastic sediments of the Hare Fiord and Trappers Cove formations. Towards the shelf, carbonate sedimentation dominated (Nansen Formation, carbonate member of the middle Canyon Fiord Formation, Sakmarian and Artinskian Raanes Formation, and Great Bear Cape Formation; Beauchamp and Henderson 1994). At the basin margin, red-colored conglomerates and sandstones of the lower clastic member of the Canyon Fiord Formation, fine-grained sandstones of the upper clastic member of the Canyon Fiord Formation, and sandstones and coastal plain sediments with coaly seams of the Kungurian Sabine Bay Formation were deposited. During this phase, several sub-basins developed, filled with strata correlative with the Nansen Formation: the carbonate-dominated, minor evaporite-bearing latest Carboniferous Antoinette Formation, evaporites of the Mount Bayley Formation, and carbonates and siliciclastic sediments of the Asselian Tanquary Formation (Thorsteinsson 1974; Embry and Beauchamp 2019).
2.3. Carboniferous to Permian rift-related volcanism (previous research)
Rifting pulses during the early tectonostratigraphic phases of the Sverdrup Basin were locally accompanied by volcanic activity between c. 325 and 275 Ma (Figs. 1 and 3; Table 1). The oldest volcanic rocks in the Sverdrup Basin occur in the lower Carboniferous. A narrow, north–south-trending outcrop occurs within the Borup Fiord Formation (331–323 Ma) on northern Axel Heiberg Island consisting of c. 20 m thick spilitized amygdaloidal alkali basalt flows (Trettin 1988). The original mineral assemblage is replaced by albite, chlorite, altered glass, opaque minerals, and carbonate. A middle Carboniferous age (early Namurian, corresponding to the Serpukhovian) for limestone c. 35 m above the volcanic unit was verified by conodonts.
Table 1.
Spilitic basalts and minor volcaniclastic rocks form the 560 m thick Audhild Formation on Kleybolte Peninsula, northwest Ellesmere Island (Thorsteinsson 1974; Ritcey 1989). They immediately overlay the Serpukhovian Borup Fiord Formation and correspond to the lower Bashkirian (323–315 Ma) (Thorsteinsson 1974; Embry and Beauchamp 2019). Thus, an age of c. 323 Ma, which is slightly younger than that of the Borup Fiord Formation, can be inferred for the Audhild Formation.
Southwest of Clements Markham Inlet, northeastern Ellesmere Island, the Mount Bayley Formation forms a small southwest–northeast-trending strip that comprises c. 700 m of gypsum, anhydrite, carbonate breccia, including 75 m of basaltic pillow lava and volcanic conglomerate (Mayr 1992; Mayr and Trettin 1996). The basalt is characterized as tholeiitic based on mineralogical analysis. A late Moscovian age is confirmed by the fusulinid fauna in the upper part of the formation. However, at M’Clintock Inlet, closer to our study area, the maximum age range of the basalt-free Mount Bayley Formation is Moscovian to early Permian (Mayr 1992). The Mount Bayley Formation of northern Ellesmere Island does not correlate with the basalt-free Mount Bayley Formation of the type locality in the area of Tanquary Fiord and Greely Fiord in western Ellesmere Island, where it is deposited in a sub-basin from the Gzhelian to the middle Asselian (c. 304–297 Ma) (Thorsteinsson 1974; Mayr 1992; Embry and Beauchamp 2008; Wamsteeker et al. 2009).
An unnamed horizon of green-black basalt flows is mapped in the Upper Nansen Formation on northern Axel Heiberg Island (Mayr et al. 2002). Fossils (fusulinaceans) in sediments below and above the basalt indicate a Gzhelian age (304–299 Ma) (Alonso-Torres et al. 2018).
An “Unnamed lower volcanic unit (ULV)” of Sakmarian/early Artinskian age (c. 290 Ma) appears below the Esayoo Formation within the Raanes Formation near the head of Oobloyah Bay on western Ellesmere Island (Morris 2013). The ULV is all together 75 m thick and includes four basalt flow units (massive basalt, pillow basalt, vesicle-rich basalt) and a basal volcaniclastic unit. The lower part of the profile (with flow units 1 and 2) and the upper part (with flow units 3 and 4) are divided by a c. 25 m wide, covered interval without outcrops (Morris 2013).
The lower Permian (Kungurian, 279–272 Ma) volcanic Esayoo Formation is exposed in several outcrops between Hare Fiord and Greely Fiord on western Ellesmere Island and is also present at the Svartevaeg Cliffs on northeastern Axel Heiberg Island (Thorsteinsson 1974; Cameron and Muecke 1996; Morris 2013). The thickest section of the Esayoo Formation near Oobloyah Bay is over 450 m thick and comprises 21 basalt flow units (Morris 2013). The base of the section is formed by quartz sandstone of the Upper Great Bear Cape Formation, which is intruded by lenticular diabase sills with thickness of few decimeters to 1 m. The basalts appear as subaerial flows, pillow lavas, and epiclastic basalt conglomerate, and are associated with marine sediments. The basalts are altered to spilite with a groundmass formed of secondary albite, carbonate, chlorite, titanite, quartz, and fine-grained clay minerals, however, relict clinopyroxene is sometimes preserved (Morris 2013). Opaque minerals are represented by hematite that appears as disseminated crystals and as rims and fracture fillings of relict olivine and pyroxene phenocrysts (Wynne et al. 1983). Geochemically, the Esayoo basalts of Ellesmere Island and Axel Heiberg Island were mainly characterized as alkaline to transitional, minor as tholeiitic basalts of within-plate and ocean island basalt (OIB) affinity (Cameron and Muecke 1996; Morris 2013).
2.4. Post-Permian volcanic and tectonic events
Within a longer period of relative magmatic tranquility between the middle Permian and the early Cretaceous, some small alkali-basaltic dykes intruded Pearya Terrane rocks between Ayles Fiord and M’Clintock Inlet around the Triassic–Jurassic boundary as suggested by Ar–Ar ages (Estrada et al. 2018).
During the Cretaceous (c. 130–80 Ma), the Sverdrup Basin, particularly Axel Heiberg Island and the western and northern margins of Ellesmere Island, were repeatedly affected by intense extrusive and intrusive magmatic activity of the High Arctic Large Igneous Province (HALIP) related to the opening of the Mesozoic Amerasia Basin of the Arctic Ocean in the north (e.g., Embry and Osadetz 1988; Estrada and Henjes-Kunst 2004, 2013; Buchan and Ernst 2006, 2018; Estrada et al. 2016; Kingsbury et al. 2016, 2018; Dockman et al. 2018).
During the Paleogene, plate tectonic reorganization related to seafloor spreading in Labrador Sea–Baffin Bay, the northern North Atlantic, and the Eurasia Basin, and changing movement directions of the Greenland plate led to the Eurekan Deformation (e.g., Trettin 1991a; Tessensohn and Piepjohn 2000; Piepjohn et al. 2013, 2015, 2016). This deformation represents a complex system of fold belts, thrust zones, and strike-slip fault zones that affected pre-Eocene deposits and reactivated Ellesmerian thrust faults (Piepjohn and von Gosen 2018 ). Piepjohn et al. (2016) distinguish two major deformation stages: (i) a first, early Eocene stage around 53–47 Ma probably related to sinistral strike-slip tectonics on Ellesmere Island and (ii) a second, late Eocene stage at 47–34 Ma characterized by dextral strike-slip and compression on Ellesmere Island. These deformation stages are also reflected in Eurekan and post-Eurekan exhumation periods on northern Ellesmere Island during c. 55–48, 44–38, and 34–26 Ma, which were accompanied by 8–11 km of exhumation of Pearya, as recorded by low-temperature thermochronology (Vamvaka et al. 2019).
3. Field relations and structural setting of the outcrop area
The studied Taconite volcanics (informal name) are located at 82°42′01.2″N and 78°22′21.1″W; 354 m above sea level. The outcrop is situated c. 2.5 km north–northeast of the eastern end of Ayles Fiord and west of M’Clintock Inlet in a transverse valley west of the Taconite River (Figs. 2a and c). The area is part of the Ordovician Maskell Inlet Complex that comprises tuff, volcanic flows (mainly andesite and basalt of island-arc affinity), carbonate rock, mudrock, and chert of the Pearya Succession 3 (Trettin and Mayr 1996). The dark volcanic rocks of the Taconite volcanics are a 10–15 m thick, steeply dipping volcanic succession characterized by pillow lavas (Figs. 4b–4d). The pillows reach diameters of few decimeters to about 1 m. They are internally massive with vesicular, glassy (or chilled) margins. Amygdales are filled with finest grained green and white minerals. The rocks are in fault contact to red beds of the upper Carboniferous Canyon Fiord Formation (sandstone, conglomeratic sandstone, silty shale, minor limestone) of the Sverdrup Basin (Fig. 4a). Due to the position close to the fault contact with the Carboniferous red beds, the volcanic rocks are strongly brittle sheared. The Canyon Fiord deposits form a c. 1–5 km wide, southwest–northeast-trending structure that flanks limestones of the lower Permian Tanquary Formation and a small, c. 6 km long strip of the upper Carboniferous Mount Bayley Formation (gypsum-anhydrite, carbonate breccia, laminated lime mudstone) in the center (Trettin and Mayr 1996). The original stratigraphic relationship in this area is obscured by a complex network of fault-bounded tectonic lenses and slivers, which have strongly affected both the Pearya Succession 3 and the Carboniferous to Permian Sverdrup Basin sedimentary rocks. The fault-bounded blocks of the Sverdrup Basin deposits mostly dip steeply towards southeast and are folded and thrust faulted. The network of faults is part of the several hundreds of meters wide, southwest–northeast-trending, dextral, brittle Taconite River Fault Zone, which is related to the Paleogene Eurekan Deformation (Piepjohn et al. 2013). Due to the involvement of both basement units and deposits of the Sverdrup Basin, however, the Taconite River Fault Zone was probably already active during the formation of a Carboniferous to Permian graben structure, and the Taconite volcanics might have extruded on the northern rift shoulder of the graben on Pearya basement, immediately west of the western master fault of the Taconite River Fault Zone.
Fig. 4.
4. Magnetic susceptibility and magnetic anomaly data
The Taconite sample area was mapped by a joint helicopter-borne magnetic survey of BGR and the Geological Survey of Canada (GSC) in 2008 (Fig. 2b). For technical details, see Estrada et al. (2016). Additionally, magnetic susceptibility readings were performed on the samples using a KT-7 magnetic susceptibility meter of SatisGeo geophysical instruments and magnetic anomaly data were re-evaluated. We calculated the positive tilt derivative of the magnetic anomaly superimposed by depth estimate as well as the horizontal gradient. Calculation of the tilt derivative was completed using the pole reduced magnetic anomaly grid and is useful for mapping shallow basement structures. The tilt depth estimate is based on a technique published by Salem et al. (2008). In addition, the horizontal gradient can be used to locate the source edges of the magnetic anomaly.
The Taconite samples are strongly magnetic as indicated by high values of susceptibility (Table 2).
Table 2.
The map of the total magnetic field shows several small circular anomalies (A1.1–A1.4 in Fig. 5) northeast of the eastern end of Ayles Fiord within a surrounding broad magnetic low. However, the exact position of the exposed volcanic rocks is situated between two flight lines (A1.1 in Fig. 5). Given the high susceptibility values and the short wavelength, it is highly likely that this delimited anomaly is linked to the exposed Taconite volcanics. The zero contour of the positive tilt derivative of the magnetic anomaly as well as the maximum values of the horizontal gradients both roughly in line indicate the extent of the magnetic source body with shallow near-surface depth solutions (Figs. 5b and 5c). Depth solutions are calculated with respect to the sensor observation altitude. By subtracting the distance between flight altitude and bed topography (terrain clearance during survey flight) from the depth solutions, estimated contacts are within the range of about 0–400 m below surface. In addition, the circular anomalies A1.2–A1.5 show equal characteristics and are as well as anomaly A1.1 all fault bounded. There is a high possibility that the circular anomalies A1.2, A1.3, and A1.5 correspond with exposed or near surface volcanic rocks of the Taconite volcanics. A connection of anomaly A1.4 with the Taconite volcanics is uncertain. Due to its close position, it might be also connected to the Thores Suite of Pearya Succession 3 (Fig. 2).
Fig. 5.
5. Samples and analytical methods
Eight samples (C11-1–C11-8) of about 2 kg each of the volcanic rocks were studied by thin-section microscopy and geochemical analyses: whole-rock major and trace element geochemistry detected by X-ray fluorescence (XRF) and inductively coupled plasma-mass spectrometry (ICP-MS), and Rb–Sr and Sm–Nd whole-rock isotope analyses. Mineralogical composition was analyzed by X-ray diffraction (XRD). Preparation of thin sections and powder for analyses, as well as XRF and XRD analyses, Rb–Sr and Sm–Nd isotope analyses were carried out at BGR Hannover. ICP-MS analysis was done by Activation Laboratories Ltd. (Actlabs), Ancaster, Canada.
For XRF analysis, 1 g of the sample powder (<40 µm particle size) is ignited at c. 1030 °C for about 10 min to analyze the loss on ignition (LOI). In a second step, the ignited sample is mixed with 5 g lithium metaborate and 25 mg lithium bromide. This mixture is molten in platinum cups at c. 1200 °C for 20 min. The resulting glass pellets are measured using a wavelength-dispersive XRF machine.
About 10 g of sample powder was sent to Actlabs, Canada. The sample material was first fused using lithium metaborate/tetraborate, followed by digestion in weak nitric acid solution and measurement by inductively coupled plasma-mass spectrometry.
For the mineralogical characterization of the samples by XRD, c. 1–2 g sample powder was used to be measured on a PANalytical MPD Pro using Cu radiation and “scientific X’Cellerator”.
Rb–Sr and Sm–Nd isotope ratios were analyzed using a ThermoFisher Scientic TRITON thermal ionization mass spectrometer. Briefly, 87Rb–84Sr and 149Sm–148Nd isotope tracers were added to ∼50–100 mg of powdered samples. Acid-pressure digestion (0.5 ml HNO3 plus 5 ml HF—distilled-grade purity for all reagents) took place at 150 °C for 24 h. Rb–Sr–rare earth element (REE) fraction was isolated using typical cation column exchange methods, whereas Sm and Nd were separated using Teflon column (Cerrai and Testa 1963). All elements were loaded onto Re-filaments with 2 µL of 1 mol/L H3PO4. All analyses used double Re-filament technique. Within run mass bias correction used 146Nd/144Nd = 0.7219, 152Sm/149Sm = 1.93476 and 88Sr/86Sr = 0.1194. All errors are given as twofold standard error of the mean (2SEM). The external uncertainties are defined as twofold standard deviations (2SD) from the mean of multiple analysis of the Sr and Nd element standard, which are ±0.000011 (NBS987; n = 6) and JNdi ±0.000010 (JNdi; n = 4), respectively. Samples were normalized to the preferred values of 87Sr/86Sr = 0.71250 for NBS987, and 143Nd/144Nd = 0.512103 for JNdi (Wakaki and Tanaka 2012). The USGS reference material AGV-2 was used as an external rock standard to control all the analytical steps. The Sr and Nd isotope ratios are shown in Table 2.
Four samples (C11-1, -4, -5, -8) were selected for Ar–Ar whole-rock geochronology (Table 1). A fresh piece from the center of each selected sample was sent to GeochronEx Analytical Services Ltd., Burlington, Ontario, Canada, where whole-rock sample preparation and 40Ar/39Ar incremental heating dating (10 temperature steps) were performed. The samples were wrapped in Al foil and loaded in an alumina vial together with LP-6 biotite (age: 128.1 Ma; Baksi et al. 1996) to be used as a flux monitor. The samples and monitor were irradiated in the nuclear reactor. Flux monitors were run, and J value was calculated. The Ar isotope composition was measured in a Noblesse Noble Gas static mass spectrometer (NU Instrument Ltd.). The 1300 °C blank of 40Ar was <10−11 cc STP. The complete Ar–Ar data are available in Supplementary Table S1.
6. Results
6.1. Petrography and mineralogy
The Taconite samples are porphyritic and in parts vesicular (e.g., C11-2) with a very fine-grained matrix formed of clinopyroxene, plagioclase, dispersed opaque iron oxides, and accessory apatite (Fig. 6). Traces of biotite result from autohydrothermal alteration. Phenocrysts of probably former olivine are mostly replaced by aggregates of serpentine, calcite, chlorite, quartz, and iron oxides, often showing opaque or reddish rims, or are completely opacitized. To a smaller extent, phenocrysts of light-brownish clinopyroxene (sometimes forming glomerophyric aggregates) and rarely of plagioclase are present. The size of the phenocrysts range between 300 and 600 µm, rarely up to 2 mm. Amygdales are present in most samples in varying amounts and are filled with quartz and/or calcite (Fig. 6). By XRD, the same minerals were detected in all samples in varying amounts (Supplementary Table S2). Major components are clinopyroxene, feldspar, and analcime. Iron oxides could be identified as hematite. Corrensite (a clay mineral with the formula (Mg, Al)9 (Si, Al)8O20(OH)10*4H2O), quartz, calcite, anatase, and ± muscovite/illite were detected as minor components or traces. Olivine was not found by XRD. The composition of the basaltic rocks shows that they have experienced hydrothermal alteration that has affected mainly the olivine and a probable interstitial glass phase. This process started already coeval with the extrusion due to the interaction of the lava with water.
Fig. 6.
6.2. 40Ar/39Ar whole-rock age
Due to the fine-grained size of the matrix and the scarcity of plagioclase phenocrysts, no separation of mineral phases or matrix was possible. Ar–Ar whole-rock dating on volcanic rocks, which show some effects of hydrothermal alteration (as described above) and very low K2O concentrations (<0.2 wt.%; Table 2) is problematic (e.g., Jiang et al. 2021). However, our aim was the geochronological correlation of our samples to known magmatic events in northern Ellesmere Island, which have very distinct ages far away from each other. These are the Ordovician magmatism including the Thores Suite of Pearya Succession 3 (Fig. 2a), the Carboniferous to Permian rift-related magmatism of the Sverdrup Basin and the Cretaceous HALIP-related magmatism (Fig. 1).
The results of 40Ar/39Ar whole-rock dating on three out of the four selected samples are presented in Fig. 7. The complete Ar–Ar isotope data are available in Supplementary Table S1. The low K2O concentrations of the dated samples and some alteration effects probably related to the Taconite River Fault Zone could have affected the quality of the Ar–Ar isotope data by Ar loss or Ar excess. To evaluate our Ar–Ar results, we use the quality criteria mentioned by Koppers et al. (2000). Our data do not fulfill two of the criteria: the inverse isochron calculation did not provide ages and initial 40Ar/36Ar values due to the large error ellipses of the apparent ages. However, our samples show high‐temperature plateaus in the age spectra, which include more than three incremental heating steps representing at least 50% of the total amount of 39ArK released (Supplementary Table S1). Additionally, their MSWDs are within an acceptable range given the number of data points (i.e., temperature steps) for each plateau. Furthermore, two of the samples use 100% of the released Ar for their age. Therefore, we believe that statistically our results are acceptable.
Fig. 7.
The age spectra of samples C11-1 and C11-4 yielded well-defined plateaus, which include 100% of the 39Ar released, with ages of 298 ± 12 Ma (1σ; MSWD = 0.74) and 289 ± 14 Ma (1σ; MSWD = 0.70), respectively (Fig. 7). The spectrum of sample C11-5 is more disturbed, but the middle and high temperature steps provide a plateau age of 280 ± 15 Ma (1σ; MSWD = 1.13). For all three samples, the highest two temperature steps with c. 30% of the 39Ar released and relatively constant low Ca/K ratios (c. 18–25) yielded relatively constant apparent ages indicating degassing of plagioclase. Sample C11-8 yielded a disturbed spectrum with a plateau age of 240 ± 18 Ma (MSWD = 1.8; 61.7% of the 39Ar released). In contrast to sample C11-5, the plateau is formed by the low and middle temperature steps and thus is not related to the degassing of plagioclase but of newly formed K-bearing mineral phases (Supplementary Table S1).
The outcrop situation as well as the petrographic and geochemical similarity (see below) of the dated samples suggest a coeval extrusion of the lava. Thus, we exclude sample C11-8 from the age calculation, because its Ar isotope system was probably stronger affected by alteration as indicated by the low to middle temperature steps forming the plateau.
The plateau ages of the remaining three samples overlap within error. The average of 290 ± 19 Ma (2σ) is considered currently to represent the best estimate for the extrusion age of the Taconite volcanics. Despite the large error, this age span indicates that the volcanism was active during the second phase of the formation of the Sverdrup Basin in the early Permian or already in the late Carboniferous. Thus, the Ar–Ar age of the Taconite volcanics differs clearly from Ordovician and Cretaceous magmatic events in northern Ellesmere Island. The neighboring Sverdrup sediments were deposited during late Carboniferous to early Permian (c. 315–293 Ma). However, the Taconite volcanic rocks were not present within these sediments. The Taconite volcanics might have extruded on the rift shoulder formed by Ordovician Pearya basement rocks. Thus, stratigraphy gives only limited information about the age of volcanism.
6.3. Whole-rock geochemistry and rock classification
Major and trace element concentrations of the Taconite samples are presented in Table 2. The samples have moderate LOI values (3.6–6.5 wt.%). For graphic presentation and comparisons, the major element oxides were recalculated to an LOI-free basis (see also Supplementary Table S3). The major element oxides show narrow ranges. The samples are characterized by low SiO2 (41.2–43.8 wt.%) and alkali elements (1.92–2.17 wt.% Na2O; <0.25 wt.% K2O), as well as high MgO (11.2–12.7 wt.%), CaO (10.5–12.6 wt.%), total Fe2O3 (11.9–15.5 wt.%), Al2O3 (12.8–13.4 wt.%), TiO2 (3.25–3.31 wt.%), and P2O5 (0.53–0.57 wt.%). An exception is sample C11-2 with higher SiO2 (51.9 wt.%) and alkali elements (2.76 wt.% Na2O, 0.72 wt.% K2O), whereas MgO (8.1 wt.%) and CaO (7.2 wt.%) are lower. This sample is strongly porphyritic and rich in vesicles filled with quartz, which might explain the higher SiO2 values, indicating stronger overprint by later hydrothermal fluids. This sample is shown in the geochemical diagrams but was not considered for genetic interpretations.
Following International Union of Geological Sciences (IUGS) recommendations (Le Maitre 2002), most samples can be classified as picrite due to their high MgO (>12 wt.%) and low alkali-oxide concentrations (<3 wt.%) (Fig. 8). Only sample C11-7 with MgO of 11.2 wt.% is a picrobasalt and the hydrothermally affected sample C11-2 with the highest SiO2 content plots into the basalt field of the TAS diagram (Le Maitre 2002). The picritic rocks are characterized by high Mg-numbers (mg# = molar MgO/MgO + FeO; Fe3+/Fe2+ = 0.15) of 0.64–0.70 and concentrations of Cr of 450–620 ppm and Ni of 220–260 ppm. Basalt sample C11-2 has the lowest mg# (0.59) and Ni concentration (180 ppm), whereas Cr (470 ppm) is in the range of the picrites. The high Nb/Y ratios of 2.54–4.10 indicate alkaline character for all samples (Nb/Y > 0.67; Winchester and Floyd 1977; Pearce 1996).
Fig. 8.
On a chondrite-normalized REE diagram, the samples show little variation. Their patterns have a negative slope (Lan/Ybn = 19.6–28.0) and run almost parallel to the reference OIB pattern (Fig. 9a). The light REE (La–Nd) are slightly enriched compared to OIB. In contrast to the typical OIB pattern, Yb and Lu are hardly fractionated, Lun is even higher than Ybn in some samples (C11-1, -2, -6, and -7).
Fig. 9.
On a primitive mantle-normalized trace element diagram, all samples overlap well with the reference OIB pattern. However, the Taconite samples show large depletion in Rb, K, Hf, and enrichment in Cs, Ba, Nb, Ta, Pb and light REE (Fig. 10). Due to the Ba–Ta enrichments, the patterns clearly have Th–U troughs, even though the concentrations of Th and U are close to those of the reference OIB.
Fig. 10.
On the V–Ti diagram from Shervais (1982), all samples plot in the field of OIB and alkali-basalt (Fig. 11a). V varies between 205 and 335 ppm at relatively high constant Ti concentrations, probably related to variations of total Fe that shows a positive correlation trend with V. The Taconite samples plot in the range of OIB showing enrichment in Nb and slightly elevated Nb/Yb ratios (Figs. 10, 11b, and 11c).
Fig. 11.
In summary, most Taconite samples are alkali picrites, sample C11-7 is an alkali-picrobasalt and sample C11-2 that is rich in vesicles filled mainly with quartz is hydrothermally affected and will be not used for petrogenetic interpretation. They all have OIB-type incompatible element (including REE) concentrations with moderate enrichments of Ba, Nb, Ta, Pb, light REE and depletions of Rb, K, and Hf compared to OIB.
6.4. Sr–Nd isotope data
The initial Sr and Nd isotope ratios of the Taconite samples have been calculated for an extrusion age of 290 Ma (as determined by the Ar–Ar dating; Fig. 7) and are shown in Table 2 and Supplementary Table S1. All samples form a tight cluster and plot on the OIB mantle array (εNd(t) = 2.20–2.85; (87Sr/86Sr)t = 0.7037–0.7042; Fig. 12). Sample C11-2 has the highest (87Sr/86Sr)t isotope ratio but the most primitive εNd(t) value of the Taconite volcanics, which might be a result of seawater alteration (Fig. 12). The Nd model ages (TDM) of the picrites are very uniform and range between 0.74 and 0.71 Ga (when excluding C11-2). They are interpreted as the extraction ages from a depleted mantle reservoir. This time interval coincides with the break-up of Rodinia.
Fig. 12.
7. Discussion
7.1. Petrogenesis
Although the Taconite samples have experienced some degrees of alteration as described above, their geochemical whole-rock composition is not significantly affected. An exception is probably sample C11-2, which has the highest SiO2 content of all samples. The vesicles of this strongly amygdaloidal sample are infilled with mainly quartz. Sample C11-2 has the lowest LOI of the data set (3.6 vs. 5–6 wt.%), indicating that quartz has replaced carbonate phases during diagenesis. However, this process has not affected the Sr–Nd isotope system (Fig. 12). Hydrothermal alteration or crustal assimilation is refuted by the primitive Sr isotope characteristics of the Taconite volcanics (Fig. 12), the high Nb and Ta values (up to two-times higher than OIB; Fig. 10), and by Th/Yb and Nb/Yb values similar to the OIB mantle source composition (Fig. 11b). OIB affinity is in general a characteristic feature for alkaline basalts including those generated in continental rift zones (e.g., Fitton and Dunlop 1985; Thompson 1985). Interaction of the magma with continental crust or influence of a subduction-related component would increase the Th/Yb ratio and shift the sample points above the N-MORB—OIB array, towards the volcanic-arc array. Instead, they are shifted towards high Nb/Yb ratios at constant Th/Yb (Fig. 11b). It would also lower the Ti concentration, which would move the samples to lower Ti/1000 concentration at a given V value in Fig. 11a. Furthermore, the addition of a subduction-related component would enrich fluid-mobile incompatible elements such as Rb, Ba, U, K, Pb, or Sr. In the Taconite volcanics, only Ba and Pb are enriched compared to OIB, while Rb and K are clearly depleted, arguing against an addition of subduction-related fluids. However, the slight enrichment in Pb and the elevated Nb and Ta values might be a hint to the involvement of metasomatized subcontinental lithospheric mantle (Eisele et al. 2002).
The very small variations in most elemental concentrations between the Taconite picrite samples without clear magma differentiation trends (Supplementary Fig. S1) argues against significant fractional crystallization processes. Thus, the geochemical composition of the Taconite picrites with the relatively high mg#, Cr and Ni concentrations can be considered as being close to primary mantle melts. The Ni contents of the picrite samples (220–260 ppm) overlap the range of primary basaltic mantle melts (c. 235–395 ppm Ni; Sato 1977).
High (La/Sm)n and (Tb/Yb)n ratios indicate that the Taconite volcanics were generated by small degrees of partial mantle melting at greater depths in the garnet stability field of mantle peridotite (Fig. 13a). Samples C11-6 and C11-2 have the largest (La/Sm)n ratios and thus the lowest degrees of melting. The transition from spinel to garnet peridotite corresponds to a pressure of at least 2.8 GPa (>85 km depth) at the solidus temperature of 1460 °C (Robinson and Wood 1998). The Al2O3 content and CaO/Al2O3 ratio (0.79–0.98) of the Taconite picrites points to melting at a pressure of c. 3.5 GPa (Herzberg 1995). Similar melting conditions are shown by the c. 60 Ma old picrites from West and East Greenland, which however have a larger variation of the Al2O3 contents due to effects of olivine fractionation (Herzberg 1995). The Nb/Ta vs. Zr/Nb diagram after Pfänder et al. (2012) shows a similar result. The Taconite samples plot close to the garnet peridotite melting curve at low degree melting of about 1% (Fig. 13b). However, few samples plot between the garnet and spinel melting curve, which could be explained by melting in a transitional regime. Samples C11-2 and C11-4 show similar Nb/Ta and Zr/Nb ratios as the Rhön alkaline rocks. The composition of the latter is explained by melting of metasomatized refractory spinel peridotite including the addition of 1% metasomatic melt. However, the composition of the Taconite samples could also be explained by a mixture of melts from garnet peridotite and spinel peridotite in different amounts (Figs. 13c and 13d). Using La, Yb, and Dy, the formation of the Taconite basalts might be a result of the mixture of about 70%–80% melt from garnet peridotite and 20%–30% spinel peridotite, whereas the melting degrees are about 5% for the spinel peridotite and between 0.5% and 1% for the garnet peridotite.
Fig. 13.
In summary, the Taconite picrites represent primary mantle melts, with no interaction with continental crustal materials or metasomatized subduction-related mantle. They have OIB-like geochemical characteristics and depleted initial Sr–Nd isotope signature inherited from the source. Partial melting and mixing of garnet- and spinel-bearing peridotite mantle sources at a depth of c. 80–90 km most likely took place within the subcontinental lithospheric mantle or at the subcontinental lithospheric mantle–asthenosphere boundary, depending on the crustal thickness and probably extension-related asthenosphere uplift. The influence of metasomatized subcontinental lithospheric mantle during formation of the Taconite volcanics is indicated by elevated concentrations of some trace elements (Pb, Nb, Ta) relative to OIB.
7.2. Geochemical comparison of the Taconite volcanics with other Carboniferous to Permian volcanic rocks in the Sverdrup Basin
7.2.1. Available data and sample selection
As mentioned above, several outcrops of Carboniferous to Permian volcanic rocks occur in the Sverdrup Basin. However, available geochemical data are limited due to older and often incomplete studies (Table 1). Limited geochemical data are available from the Carboniferous volcanic rocks within the Borup Fiord Formation at northern Axel Heiberg Island (three samples with duplicate analyses of major and only some trace elements, without REE analyses; Trettin 1988). Therefore, the Borup Fiord volcanic unit is considered for graphical comparisons only in Fig. 11c.
Most geochemical information is given for the Audhild Formation of northwest Ellesmere Island (Ritcey 1989), the ULV (Morris 2013), and the early Permian volcanic rocks of the Esayoo Formation (Cameron and Muecke 1996; Morris 2013). These rocks were studied more intensively as part of Bachelor and Master theses at the Dalhousie University of Halifax (Ritcey 1989; Cameron 1989) and the University of Calgary (Morris 2013) and are used for geochemical comparison with the Taconite samples. We selected samples, which had the most complete geochemical and isotopic analyses and, which we think, represent most the respective unit. The samples, which were used for comparison, including location, lithology, and selected geochemical data are listed in Supplementary Table S4.
7.2.2. Geochemical comparison with the Taconite volcanics
Selected geochemical parameters of the reference samples (Borup Fiord Formation, Audhild Formation, ULV, Esayoo Formation) and the Taconite samples are compared in Figs. 9, 11, and 13a, and summarized in Table 3. In contrast to the Taconite volcanics, all other basalt occurrences used for comparison are described to be spilitic. Thus, elements relatively unaffected by the spilitic alteration are used for geochemical comparison. The REE concentrations of the Audhild Formation samples show little variation, therefore the maximum and minimum chondrite-normalized values are plotted. For simplification, the average values of alkaline (n = 7), tholeiitic (n = 1), and transitional basalts (n = 8) for the Esayoo Formation are presented.
Table 3.
Most of the reference samples (Audhild Formation, lower ULV, most of Esayoo Formation), even though some of them were classified by Cameron and Muecke (1996) as transitional, are alkaline basalts with Nb/Y > 0.67 similar to the Taconite volcanics. The upper ULV samples and a small part of the Esayoo Formation samples are tholeiitic with low Nb/Y < 0.67. The chondrite-normalized REE patterns of the alkaline reference samples compare well with those of the Taconite volcanics, but mostly with lower light REE concentrations (Figs. 9b–9d and Table 3). By contrast, the tholeiitic samples (upper ULV and part of the Esayoo Formation) resemble more the E-MORB pattern, but with stronger fractionated heavy REE than E-MORB.
As for the Taconite samples, the alkaline reference samples show geochemical characteristics of OIB and alkaline basalts (Figs. 11a and 11b), whereas the tholeiitic samples tend towards CFB/E-MORB in both diagrams. The higher Th/Yb ratio of the Audhild Formation samples might be explained by contamination with continental crust. Despite some similarities in the geotectonic within-plate setting, there are also significant differences between the Taconite volcanics and the other early Permian reference samples, mainly expressed by the MgO, Ti, Nb, Zr, and light REE concentrations as well as the Zr/Nb, Zr/Hf, Nb/Yb, and (La/Yb)n ratios (Fig. 11c, Table 3).
Using the (La/Sm)n vs. (Tb/Yb)n diagram (Fig. 13a), all samples show relatively high (Tb/Yb)n values and plot into the garnet stability field or close to the boundary with the spinel stability field. However, the (La/Sm)n ratios of the reference samples vary strongly indicating variable degrees of melting for the ULV and the Esayoo Formation. Three groups can be identified: (1) samples with high (La/Sm)n and (Tb/Yb)n values indicating lower degrees of melting in greater depth (Taconite samples, lower ULV, Esayoo samples NM 5-15 and NM 10-5, Esayoo alkaline average), (2) samples with lower (La/Sm)n and (Tb/Yb)n values indicating higher degrees of melting in shallower depth (upper ULV, Esayoo samples NM 2-5, NM 5-13 and NM 11-7, Esayoo transitional and tholeiitic average, Axel Heiberg tholeiite), and (3) samples with intermediate (La/Sm)n values and transitional (Tb/Yb)n values indicating intermediate degrees of melting in the transition to the spinel stability field (Axel Heiberg alkaline, Audhild Formation). The variable degrees of melting and the presence of tholeiitic samples in the upper ULV and the Esayoo Formation point to sporadic pulses of stronger extension in the early Permian central Sverdrup Basin.
However, variations in the degree and depths of melting (expressed in variations of the REE ratios) cannot explain the other geochemical differences between the Taconite volcanics and the Permian reference samples. Such differences most likely originate from the composition of the mantle sources.
7.2.3. Is there more than one mantle source for the Carboniferous and Permian volcanic rocks of the Sverdrup Basin?
Little is known about the upper mantle beneath the Franklinian Basin underlying the early Sverdrup Basin and the Pearya Terrane along the Canadian Arctic margin. Mantle xenoliths have not been observed to date. However, the differences discussed above in trace-element concentrations between the Taconite volcanics and the other Carboniferous and Permian volcanic rocks of the Sverdrup Basin are supported by the Sm–Nd isotope data, which are available for the reference data from the Esayoo Formation only (Fig. 12 and Table 3). Sr isotope values were not determined. The Sm–Nd isotope data on the Esayoo Formation basalts (εNd(t): −3.99 to −5.37; Fig. 12) indicate an origin from an enriched mantle source (Morris 2013) rather than contamination by crustal material. The TDM values of the Esayoo Formation (1.43–2.05 Ga; Morris 2013) are much higher and have a wider range than those of the Taconite samples (0.71–0.74 Ga). The strongly different Sm–Nd isotope data of the Taconite volcanics (extruded on Pearya basement) and the Esayoo Formation (extruded on Franklinian Basin/Laurentia passive margin basement) in addition to the differences in major and trace elements and light REE indicate at least two different mantle sources beneath the crust of northern Ellesmere Island: a relatively primitive, slightly metasomatized mantle for the Taconite volcanics and an older, isotopically enriched mantle for the Esayoo Formation basalts.
The present-day crustal velocity structure beneath northern Ellesmere Island (close to the Taconite volcanics outcrop) modelled by teleseismic mapping (Schiffer et al. 2016) shows the Moho depth at c. 40–43 km, beneath a thin sedimentary cover and c. 33–36 km thick crystalline crust with a c. 4–5 km thick high-velocity lower crust (HVLC) at the base. This crustal structure of Pearya is seismologically similar to the Precambrian cratonic block in the southeastern part of Ellesmere Island (Schiffer et al. 2016). The crustal structure was modified by several tectonic processes after the extrusion of the Taconite lava. The Paleogene Eurekan deformation led to the formation of a fold and thrust belt and crustal thickening in central Ellesmere Island, where the Moho is sagged to a depth of c. 47 km (Schiffer et al. 2016). In contrast, the Pearya Terrane experienced uplift and erosion of c. 8–11 km during the Paleogene (Vamvaka et al. 2019). A similar crustal thickening could have been caused by the Ellesmerian Orogeny when Pearya collided with the Laurentian margin. The cratonic structure of Pearya can probably be preserved from the pre-Eurekan or much earlier development.
The HVLC unit is interpreted to result from magmatic intrusions, probably related to the Cretaceous HALIP but could also result from older magmatic events (Schiffer et al. 2016). Cretaceous volcanic and intrusive rocks are present in northern Ellesmere Island around the Yelverton Bay and northeast of Lake Hazen (Fig. 1). However, if we infer the cratonic crust beneath Pearya represents a continental fragment that experienced an evolution outside of Laurentia after the Rodinia breakup until the Ellesmerian Orogeny, then also the magmatic intrusions should have taken place much earlier than the Cretaceous HALIP-related magmatism.
This assumption is supported by the TDM model ages (0.74–0.71 Ga) of the Taconite samples (Table 2), which are in the range of magmatism during the Rodinia break-up, e.g., the Franklin igneous events (Shellnut et al. 2004; Denyszyn et al. 2009). Therefore, both, the Neoproterozoic magmatism during the Rodinia break-up and the Carboniferous and Permian magmatism with the Taconite volcanics could have already led to underplating and the formation of an HVLC unit at the basis of the Pearya continental crust.
8. Conclusions
The newly discovered Taconite volcanics is the first finding of upper Paleozoic intracontinental rift basalts related to the opening of the Sverdrup Basin in the central part of the Pearya Terrane. They can be classified as mainly alkaline picritic rocks of geochemical affinity to ocean island basalt. Typical circular anomalies of the total magnetic field over the outcrop locality and in the surroundings indicate a larger extent of these volcanic rocks. The contacts to the adjacent Carboniferous to Permian sediments is of fault nature while the contact to the Succession 3 of Pearya is unclear. Therefore, further field work is necessary to investigate the field relations between the Taconite volcanics and its surrounding rocks. Although the quality of the Ar–Ar whole-rock data can be affected by hydrothermal alteration, the age of 290 ± 19 Ma obtained on three samples is in the stratigraphic range of other upper Paleozoic rift-related basaltic rocks in western Ellesmere and Axel Heiberg islands. Similar to the latter, the melts for the Taconite volcanics have formed within greater depths. The trace element composition indicates the formation by low-degree partial melting and mixing of garnet- and spinel-bearing peridotite mantle sources at a depth of c. 80–90 km most likely within the subcontinental lithospheric mantle or at the subcontinental lithospheric mantle–asthenosphere boundary. This is supported by the moderately depleted εNd(t) and primitive (87Sr/86Sr)t values. Elevated Nb, Ta, and Pb values as well as two samples with comparatively low Nb/Ta ratios of ∼17 point to contribution of melts from a metasomatized subcontinental lithospheric mantle in the formation of the Taconite volcanics. Differences in trace element and isotope composition between the Taconite volcanics (extruded on Pearya basement) and the other Carboniferous and Permian volcanic rocks of the Sverdrup Basin (extruded on Franklinian Basin basement) indicate the existence of more than one mantle source. The mantle heterogeneity probably results from the independent geological history of the Pearya Terrane and the northern Laurentian margin after the Rodinia break-up until the Ellesmerian Orogeny. However, due to the lack of information about the underlying mantle material in the form of e.g., mantle xenoliths, this topic should be a matter of further research.
Acknowledgements
AL thanks the CASE 11 expedition team and the Polar Continental Shelf Program (Resolute Bay, Nunavut, Canada) for technical support during the field work in 2008. K. Piepjohn (BGR, Hannover) is gratefully acknowledged for sharing field work and fruitful discussion in the field. We thank Kristian Ufer (BGR, Hannover) for the XRD data and discussions on the results. Discussions on the Ar–Ar dating results with Yakov Kapusta (GeochronEx, Canada) is greatly acknowledged. For measurement of the magnetic susceptibility of the rock specimens, Andreas Rausch (University of Würzburg) is greatly acknowledged. We thank Editor Sally Pehrsson, Associate Editor Luke Beranek, Marie-Claude Williamson, and an anonymous reviewer for their editorial handling, as well as critical and valuable comments, which helped us to improve the quality of the manuscript.
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Published In
Canadian Journal of Earth Sciences
Volume 60 • Number 8 • August 2023
Pages: 1164 - 1187
History
Received: 27 September 2022
Accepted: 27 February 2023
Accepted manuscript online: 21 March 2023
Version of record online: 14 April 2023
Copyright
© 2023 Copyright remains with the author(s) or their institution(s). This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
Data Availability Statement
Data generated or analyzed during this study are provided in full within the published article and its supplementary materials.
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Author Contributions
Conceptualization: NK, SE
Data curation: NK, SE, AL, AR, GJ
Investigation: NK, SE, AL, AR, GJ
Methodology: NK, SE, AR, GJ
Project administration: NK, SE
Resources: AL, AR, GJ
Supervision: NK
Visualization: NK, SE, AL, AR
Writing – original draft: NK, SE
Writing – review & editing: NK, SE, AL, AR, GJ
Competing Interests
The authors declare there are no competing interests.
Funding Information
The authors declare no specific funding for this work.
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Nikola Koglin, Solveig Estrada, Andreas Läufer, Antonia Ruppel, and Guillaume Jacques. 2023. Alkaline picritic volcanism on northern Ellesmere Island associated with initial rifting of the Sverdrup Basin, Canadian Arctic. Canadian Journal of Earth Sciences.
60(8): 1164-1187. https://doi.org/10.1139/cjes-2022-0106
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