# Geochemistry and geochronology of the Bay of Islands metamorphic sole, Newfoundland, Canada: Protoliths and implications for subduction initiation

François Fournier-Roy, Carl Guilmette, Kyle Larson, Antoine Godet, Mathieu Soret, Cees van Staal, Jean Bédard

► To cite this version:

François Fournier-Roy, Carl Guilmette, Kyle Larson, Antoine Godet, Mathieu Soret, et al.. Geochemistry and geochronology of the Bay of Islands metamorphic sole, Newfoundland, Canada: Protoliths and implications for subduction initiation. Geological Society of America Bulletin, 2024, 136 (1-2), pp.371-387. 10.1130/B36662.1. insu-04360095

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# Geochemistry and geochronology of the Bay of Islands metamorphic sole, Newfoundland, Canada: Protoliths and implications for subduction initiation

François Fournier-Roy<fnref n="1" />,†, Carl Guilmette<fnref n="1" />, Kyle Larson<fnref n="2" />, Antoine Godet<fnref n="1" />, Mathieu Soret<fnref n="3" />, Cees van Staal<fnref n="4" />, and Jean H. Bédard<fnref n="5" />

## ABSTRACT

The Bay of Islands complex, Newfoundland, Canada, represents a fossil subduction system that was obducted onto the Laurentian margin during the Taconic Orogeny. We present whole-rock geochemistry and geochronology for the Bay of Islands metamorphic sole from the Table Mountain and North Arm Mountain massifs. The Bay of Islands metamorphic sole can be subdivided into three units: a high metamorphic-grade Grt-Cpx amphibolite and Grt-Cpx granulite unit, a medium metamorphic-grade common amphibolite unit, and a medium metamorphic-grade metasedimentary-dominated unit. Based on their major and trace element contents, two types of protoliths can be inferred for the Bay of Islands sole metabasites: cumulate gabbroic protoliths with high Mg numbers and depleted trace element contents, and basaltic protoliths with trace element contents defining a normal mid-oceanic-ridge basalt (N-MORB) affinity. Titanite in situ U-Pb analyses of two common amphibolite samples yield dates of 486 ± 7 Ma and 484 ± 4 Ma, which are interpreted as a metamorphic age (ca. 485 Ma). One sample from the metasedimentary unit yields detrital zircon dates that range from Archean to Ordovician, with the youngest dates defining the most dominant peak at ca. 490 Ma. This youngest peak is consistent with an arc-related source that may be

linked with arc magmatism of the upper plate during the Taconic Orogeny. These observations suggest that (1) the subduction responsible for the formation of the Bay of Islands complex nucleated in an oceanic domain before ca. 485 Ma; and (2) the sediments that form the metasedimentary unit were partly sourced from the upper plate and were deposited on the lower plate while subduction was active.

## INTRODUCTION

Metamorphic soles are thin sheets (<500 m thick) of metamorphic rocks that structurally underlie the mantle section of ophiolites (Jamie-
son, 1986; Malpas, 1979; Nicolas and Le Pichon,
1980; Spray, 1984; Wakabayashi and Dilek,
2000; Williams and Smyth, 1973). They are
mainly composed of metabasites with variable
proportions of metasedimentary rocks and show
inverted metamorphic gradients from granulite
or amphibolite facies at the top to greenschist
facies toward their structural base. Peak meta-
morphic conditions of the order of ≈11 kbar and
≥800 °C are generally reported from the highest
metamorphic grade, uppermost sections of met-
amorphic soles (Agard et al., 2016), indicating
subduction to mantle depths followed by exhu-
mation to their present structural position (Agard
et al., 2016; Gnos, 1998; van Hinsbergen et al.,
2015). Geochronological data typically indicate
that this exhumation overlaps in time with the
generation of ophiolitic crust (Agard et al., 2016;
van Hinsbergen et al., 2015), while burial may
start several million years earlier (Guilmette
et al., 2018; Pourteau et al., 2019). Because

of their inverted oceanic-floor sequence (with
oceanic-floor metabasites at their top to sedi-
mentary rocks at their base), metamorphic soles
have sometimes been interpreted as products of
overturned folds that developed during inception
of subduction (Wakabayashi and Dilek, 2003).
More commonly, however, metamorphic soles
are interpreted as the result of stacking multiple
thrust sheets scraped from the uppermost crust of
the lower plate during intraoceanic convergence
(Agard et al., 2016; Jamieson, 1980, 1981; Soret
et al., 2017; Wakabayashi and Dilek, 2000). The
uppermost sections of metamorphic soles, gen-
erally garnet-clinopyroxene amphibolite welded
to structurally overlying tectonized mantle (e.g.,
Cowan et al., 2014; Malpas, 1979; Soret et al.,
2017), represent the earliest material to have
been transferred from the lower plate to the upper
plate during subduction initiation, while lower
sections are accreted subsequently (Agard et al.,
2016). Those lower sections are metamorphosed
at lower pressure and temperature conditions and
represent portions of the lower plate more distal
to the site of subduction initiation. Hence, meta-
morphic soles provide a record of the subducted
lower plate, from its leading edge at the site of
subduction initiation, to the continental margin
over which the ophiolitic complex is eventually
obducted (Agard et al., 2016; Wakabayashi and
Dilek, 2000).

Although intense deformation and meta-
morphism may obliterate or cause recrystalli-
zation of primary features, protolith major and
trace element concentrations may be preserved
(Ghatak et al., 2012) despite the mobility of
some elements during subduction zone meta-
morphism (Ishikawa et al., 2005). To avoid the

<footnote n="1">¹Université Laval, Département de géologie et de génie géologique, 1065 av de la médecine, Québec, Québec G1V 0A6, Canada</footnote>

<footnote n="2">²University of British Columbia Okanagan, Department of Earth, Environmental and Geographic Sciences, 3247 University Way, Kelowna, British Columbia V1V 1V7, Canada</footnote>

<footnote n="3">³Université d'Orléans, Institut des Sciences de la Terre d'Orléans (ISTO), UMR 7327, CNRS-BRGM, 45071 Orléans, France</footnote>

<footnote n="4">⁴Geological Survey of Canada, 1500-605 Robson Street, Vancouver, British Columbia V6B 5J3, Canada</footnote>

<footnote n="5">⁵Geological Survey of Canada, 490 de la Couronne, Québec, Québec G1K 9A9, Canada</footnote>

<footnote>François Fournier-Roy https://orcid.org/0000-0002-7708-5730</footnote>

<footnote>†francois.fournier-roy.1@ulaval.ca</footnote>
potential effect of elemental mobility during metamorphism, we used elemental ratios that are generally not susceptible to decoupling in such contexts and provide important information about the nature of basic protoliths (plutonic versus volcanic), their magmatic history (source and petrology), and their geodynamic affinity (Pearce, 2008), which can have significant implications for determining the tectonic setting leading to subduction initiation (Fig. 1; e.g., Guilmette et al., 2012; Guilmette et al., 2009). Metasedimentary protoliths within the sole provide additional information about sediment provenance, depositional age, and tectonic setting (e.g., Garber et al., 2020).

Here, we investigate the protolith history of the archetypal metamorphic sole of the Bay of Islands ophiolite complex (Fig. 2) in the Newfoundland Appalachians, where such soles were first recognized (Dallmeyer and Williams, 1975; Malpas, 1979; McCaig, 1983; Savci, 1988; Williams and Smyth, 1973). The Bay of Islands complex is one of the largest and best-preserved ophiolitic complexes in the world (Jenner et al., 1991). It exposes the lithospheric mantle and the extrusive and plutonic crust of an oceanic basin (Church and Stevens, 1971; Williams and Malpas, 1972) that formed outboard of the Laurentian margin during closure of the

<table><thead><tr><th>Subduction Initiation</th><th>Affinity</th></tr></thead><tbody><tr><td>Ridge</td><td rowspan="2">N-MORB</td></tr><tr><td>Transform</td></tr><tr><td>Plume-induced</td><td rowspan="2">OIB</td></tr><tr><td>Polarity reversal</td></tr><tr><td>Passive margin collapse</td><td>E-MORB</td></tr></tbody></table>

Figure 1. Diagrams show different tectonic settings proposed for subduction initiation with expected geochemical affinities for metamorphic soles. The red band and the red star represent the metamorphic sole and its protolith, respectively. Subduction initiation settings were modified from Stern and Gerya (2018). E-MORB—enriched mid-oceanic-ridge basalt; N-MORB—normal mid-oceanic-ridge basalt; OIB—oceanic-island basalt; SSZ—suprasubduction zone.

Iapetus Ocean (Dewey and Casey, 2013) or the Taconic seaway (van Staal et al., 2007) during the Taconic Orogeny. The ophiolitic mantle section is structurally underlain by the Bay of Islands metamorphic sole (Malpas, 1979; Williams and Smyth, 1973), which in turn lies in thrust contact over Laurentia-derived successions (Cawood and Suhr, 1992). Earlier studies documented the field relationships (Williams and Smyth, 1973), mineralogy and metamorphic history (Malpas, 1979; McCaig, 1983), and obduction timing (Dallmeyer and Williams, 1975) of the Bay of Islands sole. On the other hand, the nature of the Bay of Islands metamorphic

sole protolith and its age(s) remain loosely constrained and debated (Dewey and Casey, 2015; Zagorevski and van Staal, 2015). Dewey and Casey (2015, 2013) interpret the sole to be derived from mid-oceanic-ridge basalt (MORB) and oceanic-island basalt (OIB)-like oceanic floor and propose that its formation is the result of subduction initiation at an oceanic transform. Zagorevski and van Staal (2015) argue that the sole may have formed through subduction of the ocean-continent transition zone just before obduction (van Staal et al., 2013). These conflicting interpretations are based on geochemical data from the metamorphic sole itself (Blow Me

<figure><img src="image_2.png" /></figure>

Figure 2. Geological map of the Bay of Islands area (modified from Yan and Casey, 2020). Major sampling locations are marked with a green star. Cross-section locations are identified by the letters a, b, and c.
Down Mountain; Dewey and Casey, 2013) and from possible analogous rocks (Birchy complex; van Staal et al., 2013). As metamorphic soles are likely composed of several thrust sheets that successively accreted from the lower plate (Agard et al., 2020; Agard et al., 2016; Soret et al., 2017), it is possible that both interpretations are correct.

In this contribution, we present whole-rock major and trace element geochemistry and in situ U-Pb titanite geochronology of metabasic rocks, and detrital zircon geochronology of metasedimentary rocks from two metamorphic sole exposures in the Bay of Islands complex (Fig. 2), to characterize the nature of material accreted from the lower plate prior to final closure of the Iapetus Ocean (or Taconic seaway). We discuss our new results and their implications for: (1) the nature of the lower plate in the subduction system responsible for the formation of the Bay of Islands complex; (2) the nature of the structure where subduction initiation occurred; and (3) the tectonic environment that led to subduction initiation.

## GEOLOGICAL SETTING

The Bay of Islands complex is exposed on the west coast of Newfoundland, Canada, in the foreland of the Newfoundland Appalachians. In Canada, the Appalachians represent a complex tectonic collage of oceanic and microcontinental terranes that were accreted to Laurentia following the closure of the Iapetus and the Rheic oceans during the Paleozoic Era (van Staal and Barr, 2012). The Canadian Appalachians are segmented by two reentrants, the Quebec and Newfoundland reentrants, which are separated by the Saint Lawrence Promontory.

The Newfoundland Appalachians are subdivided into lithotectonic zones representing different terranes involved in the assembly of the northern Appalachians. Relevant to this study are the Humber Zone, corresponding to the eastern margin of Laurentia, and immediately adjacent rocks of the western Dunnage Zone, which represent vestiges of the Iapetus Ocean and/or associated marginal basins (van Staal and Barr, 2012; Williams, 1979). The Dunnage Zone is divided in two subzones, the peri-Gondwana Exploits subzone and the western peri-Laurentian Notre Dame subzone (Williams et al., 1988). The latter is subdivided into the Lushs Bight Oceanic Tract (Lushs Bight Oceanic Tract), the Baie Verte Oceanic Tract (Baie Verte Oceanic Tract), the Dashwoods terrane, and the Annieopsquetch Accretionary Tract (van Staal and Barr, 2012; van Staal et al., 2009). The Humber margin and the Dunnage Zone are separated by the Baie Verte-Brompton Line, a Taconian suture along which multiple ophiolite complexes are exposed

both in Newfoundland and Quebec, such as the Asbestos (Laurent and Hebert, 1989), the Thetford Mines (Schroetter, 2003), and the Mont Albert (Beaudin, 1984) complexes. The Humber and the western Dunnage zones were amalgamated during the Taconic Orogeny (van Staal et al., 2007). The Bay of Islands area comprises rocks of these two lithotectonic zones.

The Bay of Islands complex consists of a klippe of the western Dunnage Zone, including parts of both the Baie Verte Oceanic Tract and Lushs Bight Oceanic Tract (van Staal et al., 2009), overthrust above allochthonous sediments from the Humber Arm Supergroup, which represent the displaced parts of the outboard Laurentian margin (Stevens, 1970; Walron et al., 2003). The Bay of Islands complex is composed of four massifs from north to south: Table Mountain, North Arm Mountain, Blow Me Down Mountain, and Lewis Hills (Bird et al., 1971; Cawood and Suhr, 1992; Church and Stevens, 1971; Dewey and Bird, 1971; Malpas, 1979; Williams and Malpas, 1972; Williams and Smyth, 1973). At the Lewis Hill massif, dikes from the Bay of Islands complex intrude the Coastal complex (Yan and Casey, 2020), which represents older (ca. 508–500 Ma; Jenner et al., 1991; Kurth et al., 1998; Mattinson, 1975), deformed oceanic arc rocks attributed to the Lushs Bight Oceanic Tract (van Staal et al., 2009). At the Blow Me Down and the North Arm Mountain massifs, a complete ophiolite stratigraphy is observed. Those massifs comprise a thick mantle section dominated by harzburgite tectonite, with minor lherzolite, pyroxenite, and dunite, overlain by a crustal section composed of peridotite, pyroxenite and gabbroic rocks, sheeted dikes, and basaltic pillow lavas (Casey and Karson, 1981; Williams and Malpas, 1972). The upper part of the plutonic section is typically intruded by trondhjemites (also called plagiogranites; Yan and Casey, 2020). Intrusive rocks with boninitic affinities are found within the mantle and the lower crustal sections of the North Arm Mountain massif (Bédard and Hébert, 1996; Edwards, 1995; Varfalvy et al., 1996; Varfalvy et al., 1997).

Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) dates for magmatic zircon grains from trondhjemites of the Blow Me Down massif returned a composite age of 488.3 ± 1.5 Ma (Yan and Casey, 2020). This age is similar to the previous ca. 485–484 Ma ages obtained for the complex (Dunning and Krogh, 1985; Jenner et al., 1991) and is interpreted as the igneous age of the Bay of Islands ophiolitic crust (Yan and Casey, 2020). The Bay of Islands ophiolite has geochemical features (Nb and Ta depletion) characteristic of a subduction zone environment (Elthon, 1991;

Jenner et al., 1991). The Bay of Islands complex formed in an forearc environment inboard of the coeval first phase of the Notre Dame arc following the onset of east-dipping (present coordinates) subduction either in the Taconic seaway (van Staal et al., 2013; van Staal et al., 2007; Zagrevski and van Staal, 2015) or in the Iapetus Ocean (Dewey and Casey, 2015; Dewey and Casey, 2013; Yan and Casey, 2020). It has been interpreted to have formed in a ridge-transform intersection adjacent to the older Cambrian Coastal complex (Karson and Dewey, 1978; Suhr and Cawood, 2001; Yan and Casey, 2020). The Bay of Islands complex was obducted onto the Laurentian margin during the Middle-Ordovician Taconic Orogeny (Cawood and Suhr, 1992; Suhr and Cawood, 1993). The occurrence of ophiolite detritus in easterly derived turbiditic flysch of the Humber Arm Supergroup (Stevens, 1970) constrains the timing of obduction in the earliest Dapingian (ca. 470 Ma; White and Walron, 2022).

The Bay of Islands metamorphic sole is exposed on the southeastern fringe of each of the Bay of Islands complex massifs (Fig. 2) at their stratigraphic base (Williams and Smyth, 1973). The sole occupies the same stratigraphic level at each massif and reaches a maximum thickness of 200 m (Malpas, 1979; Williams and Smyth, 1973). The sole displays an inverted metamorphic field gradient, from granulite facies immediately underneath the mantle peridotites to greenschist facies at the base (Malpas, 1979; McCaig, 1983). The only published dates available for the Bay of Islands metamorphic sole are from hornblende $^{40}$Ar-$^{39}$Ar step-heating plateaus, which constrain the cooling of the sole to 469 ± 5 Ma (Dallmeyer and Williams, 1975). This cooling age, generally attributed to the obduction of the Bay of Islands complex (Dallmeyer and Williams, 1975), postdates the formation of the ca. 488 Ma ophiolite (U-Pb on zircon, LA-ICP-MS; Yan and Casey, 2020) by nearly 20 m.y. Such a large time gap between the Ar-Ar age of the metamorphic sole and the igneous age of the ophiolite is atypical for ophiolite-sole couples (Hacker et al., 1996), which generally overlap within uncertainty (Wakabayashi and Dilek, 2000; van Hinsbergen et al., 2015; Agard et al., 2016), underlining the need for further refinement of the age of the Bay of Islands sole.

## FIELD RELATIONSHIPS AND PETROGRAPHY

Well-exposed and well-preserved segments of the Bay of Islands metamorphic sole were investigated along transects across the Table Mountain and North Arm Mountain massifs.
<figure><img src="image_3.png" /><figcaption>Figure 3. Aerial views (from Google Earth) of three representative cross sections from (A and B) the North Arm Mountain massif and (C) the Table Mountain massif. (D) Corresponding schematic cross sections for North Arm Mountain (a and b) and Table Mountain massifs (c). The samples indicated by an arrow were collected alongside the sections. Grt-Cpx—garnet-clinopyroxene.</figcaption></figure>

Sampling localities are illustrated in Figures 2 and 3. The coordinates for the specimens dated are provided in Supplemental Material Table S1¹. Three representative cross sections of the Bay of Islands metamorphic sole (a, b, and c in Fig. 2) are presented in Figure 3. Mineral abbreviations are after Whitney and Evans (2010).

The Bay of Islands metamorphic sole can be subdivided based on lithology and mineralogy into three main units from top to bottom: a Grt-bearing mafic unit (Fig. 4A), a common amphibolite unit (Fig. 4B), and a metasedimentary-dominated unit (Fig. 4C). The Grt-bearing mafic unit (Fig. 4A) corresponds to the part of the sole closest to the overlying mantle and is ~65–110 m thick. It is structurally overlain by the Bay of Islands ophiolitic mantle with a well-preserved welded contact in both massifs (Figs. 4D and 4E). The medium-grained “salt-and-pepper” amphibolite was termed “common” amphibolite (Fig. 4B) for its typical Pl-Hbl mineralogy. The common amphibolite defines a mappable unit lying beneath the upper Grt-bearing mafic unit with an unconstrained thickness of at least 40 m. The contact between the two units is marked by a gradual transition characterized by decreasing garnet content downwards. A Grt-bearing metasedimentary unit more than 70 m thick occurs at the structural base of the

sole and is well-exposed at the base of the North Arm Mountain massif. Foliations, which are well-developed in all units, have a general NE–SW orientation and dip to the NW, reflecting the orientation of the sole-mantle contact.

The Grt-bearing mafic unit is composed of two distinct fine- to medium-grained foliated lithologies, Grt-Cpx granulite (Fig. 5A) and Grt-Cpx amphibolite (Fig. 5B). The Grt-Cpx granulite is generally in the upper structural part of the sole nearest the contact with the mantle tectonite (illustrated in purple in Fig. 3), and the Grt-Cpx amphibolite becomes more abundant toward the base of the Grt-bearing mafic unit. The Grt-Cpx granulite consists of Pl (35%–70%), Cpx (25%–50%), Grt (<10%), and minor Hbl (0%–20%) with accessory Ilm (Fig. 5A). Garnet porphyroblasts of <5 mm diameter (Fig. 5A) are mostly ovoid and aligned with the nematoblastic matrix. They show xenomorphic rims in contact with the matrix, but some are surrounded by an Amph-Pl symplectite corona (Fig. 5A). Plagioclase is

<footnote n="1">¹Supplemental Material. Table S1: Coordinates for geochronology specimens. Table S2: Major element contents. Table S3: Trace element contents. Table S4: Geochemistry quality control data. Table S5: Titanite U-Pb geochronology. Table S6: Detrital zircon U-Pb geochronology. Table S7: Zircon trace element contents. Figure S1: Back-scatter electron images showing size, morphology, textures, and laser spot locations for analyzed concordant zircon grains from sample NA-19-02. Figure S2: Cathodoluminescence images showing representative size, morphology, zoning, and textures of analyzed zircon grains from sample NA-19-02. Warmer colors correspond to brighter cathodoluminescence. Please visit https://doi.org/10.1130/GSAB.S.21919230 to access the supplemental material, and contact editing@geosociety.org with any questions.</footnote>
<figure><img src="image_4.png" /></figure>

**Figure 4.** Photographs show (A) layered Grt-Cpx amphibolite and (B) layered common amphibolite at Table Mountain massif, and (C) a metasedimentary sequence at the structural base of the Bay of Islands (BOI) metamorphic sole at Pound Point (North Arm Mountain). (D) Outcrop exposes a well-preserved welded contact between the Bay of Islands mantle section and the metamorphic sole. (E) Close-up on the contact between the Bay of Islands mantle section and the metamorphic sole.

extensively altered to sericite (Fig. 5A). Chlo-
rite is observed around garnet porphyroblasts
and in association with rare crosscutting veins
(<2 mm) of prehnite and zoisite.

The Grt-Cpx amphibolite is composed of brownish Hbl (30%–75%), Pl (12%–60%), Cpx (5%–20%), and Grt (<3%–20%) with minor amounts of Ilm ± Chl ± Ttn ± Zrn (Fig. 5B). It is commonly characterized by 0.1 cm to ~2 cm compositional banding parallel to the mineral foliation defined by variations in the modal proportions of Pl vs. mafic minerals (Hbl/Cpx; Figs. 4A and 5B). The nematoblastic foliation is mainly defined by fine-grained brown hornblende (Fig. 5B). The sizes (up to 1 cm) and modal proportions (up to 20%) of garnet porphyroblasts are heterogeneous. They reach a maximum a few meters away from the sole-mantle contact and decrease downward in favor of amphibole and plagioclase. Similarly,

clinopyroxene also reaches a maximum modal
proportion (up to 20%) close to the contact and
progressively decreases in abundance structur-
ally downwards until it disappears in the com-
mon amphibolite unit. Amph-Pl symplectite
rims are observed around some garnet porphy-
roblasts. Plagioclase is highly altered to sericite.
Rare small (<1 mm wide) prehnite veins cross-
cut the foliation.

The common amphibolite (Figs. 4B
and 5C) is composed of Hbl (45%–75%)
and Pl (25%–55%) with minor amounts of
Ilm ± Ep ± Ttn ± Chl ± Zrn. The common
amphibolite is fine-grained and strongly foli-
ated. Plagioclase-rich bands (0.1–1.5 cm thick)
are observed parallel to the nematoblastic folia-
tion defined by fine-grained green to brownish
hornblende (Figs. 4B and 5C). The plagioclase
is altered in sericite. Titanite is observed in minor
proportions (<1%) within this unit (Fig. 5D).

The metasedimentary unit (Fig. 4C) consists
of metasedimentary schists (Fig. 5E) with inter-
layered mafic metamorphic rocks and is affected
by isoclinal folding. In the metasedimentary
rocks, mineral assemblages vary significantly
between samples but generally include Chl, Bt,
Grt, Amph, Pl, Qz, Ms, and Ep. A strong lepido-
blastic fabric is mostly marked by biotite and/or
chlorite and in some cases by muscovite. Gar-
net is common in most of the metasedimentary
samples investigated and is generally smaller
than 1 mm in diameter (Fig. 5E). Accessory min-
erals such as zircon, monazite, and titanite are
observed. The interlayered mafic rocks some-
times appear as continuous concordant layers
(<10 cm to >1 m thick) but are also commonly
affected by boudinage and locally form lenses.
Those mafic rocks are either Chl-rich schist or
foliated amphibolite composed of green horn-
blende and plagioclase (Fig. 5F). Titanite is
<figure><img src="image_5.png" /></figure>

Figure 5. Thin sections of representative samples of the Bay of Islands metamorphic sole are shown. (A) Grt-Cpx granulite and (B) Grt-Cpx amphibolite from the structural top of the Bay of Islands sole. (C) Common amphibolite from the mid-section of the sole. (D) Titanite rim around an ilmenite core from the common amphibolite NA-19-24a. (E) Grt-Bt-bearing schist (NA-19-02) from the metasedimentary portion of the Bay of Islands sole. (F) Elongated aggregates of subhebral titanite from the common amphibolite NA-19-06a.

found as an accessory mineral in one such sample (NA-19-06a; Fig. 5F).

## ANALYTICAL METHODS

### Whole-Rock Geochemistry

Whole-rock geochemical analyses of major elements and trace elements were conducted on 64 fresh and representative samples covering all the mafic rock types that comprise the Bay of Islands metamorphic sole as described above. All of the samples were devoid of veins and sulphides, and weathered surfaces were completely removed. Lithogeochemistry analyses were performed by Activation Laboratories, Ltd., in Ancaster, Ontario.

Whole-rock samples were crushed up to 80% passing at 2 mm, and mechanically split to obtain a representative sample of 250 g. The sample fraction was then pulverized with mild steel to 95% passing at 105 µm. The resulting pulp was fused using a lithium metaborate/tetraborate solution before being entirely dissolved in a weak nitric acid solution. Major and trace element concentrations were determined by inductively coupled plasma-optical emission spectrometry (ICP-OES) and ICP-MS. Geochemical data are presented in Tables S2 and S3 (see footnote 1).

The accuracy of measurements was estimated from the mean of the relative differences between standard and observed values and was typically <2% for major oxides and <5% for

trace elements. Quality control data and detection limits are presented in Table S4.

### Geochronology Methods

Zircon and titanite LA-ICP-MS U-Pb geochemical analyses were carried out in the Fipke Laboratory for Trace Element Research (FiLTER) at the University of British Columbia, Okanagan, using a Photon Machines Analyte 193 excimer laser coupled to an Agilent 8900 triple quadrupole ICP.

### U-Pb Titanite Dating

Titanite grains were analyzed in situ. Titanite spot analyses were carried out during two analytical sessions. For specimen NA-19-06a, a
40 µm spot size was used with a laser repetition rate of 4 Hz and a fluence of 5.0 J/cm². The Ar sample gas (0.9 L/min) and He carrier gas (0.6 L/min) were mixed before the plasma using an in-house developed glass mixing valve/signal smoothing device. Each spot was pre-ablated with two laser bursts followed by a 35 s delay before 40 s of ablation. The analytical setup was optimized for maximum signal using the reference material NIST610 while the ²³⁸U/²³²Th ratio was within 3% of the certified value. The same analytical setup was used for NA-19-24a with the exception of a larger, 50 µm laser spot. Isotopic data from titanite were normalized to repeat analyses of the titanite reference material MKED-1 (²⁰⁶Pb/²³⁸U age of 1517.32 ± 0.32 Ma; Spandler et al., 2016) with Mount McClure (²⁰⁷Pb/²³⁵U age of 523.26 ± 1.27 Ma; Schoene and Bowring, 2006) as a secondary reference material. Down-hole element fractionation and instrument drift were monitored based on the primary reference material and corrected using the Iolite software package (v4; Paton et al., 2010, 2011). Analyses of the secondary reference material as unknowns during analysis of NA-19-06A and NA-19-24A yielded ²⁰⁷Pb-corrected (following Stacey and Kramers, 1975) ²⁰⁶Pb/²³⁸U dates of 533 ± 4 (mean square of weighted deviates [MSWD] = 0.99, n = 10) and 527 ± 4 (MSWD = 1.86, n = 11), respectively, both well within 2% analytic uncertainty (Horstwood et al., 2016) of the accepted value. One percent excess dispersion was added quadratically to the U-Pb ratios of NA-19-24a as indicated by overdispersion of ratios from the secondary reference materials.

## U-Pb Zircon Dating

Zircon separation was performed at Overburden Drilling Management Limited of Ontario, Canada. The heavy mineral concentrate was obtained from sample NA-19-02 by electric-pulse disaggregation followed by panning. Zircon grains were pipetted from the liquid containing the pan concentrate to ensure that they were randomly selected regardless of shape, size, or color. The zircon grains were then mounted on epoxy resin and polished to half-depth at the Université Laval facilities.

Zircon grains were analyzed during two different analytical sessions. A laser spot size of 20 µm was used with a laser repetition rate of 4.0 Hz and a fluence of 3.0 J/cm². As with the titanite analyses, the He ablation-cell carrier gas (0.6 L/min) was mixed with Ar (0.9 L/min) before the plasma using a glass mixing valve/signal smoothing device that was developed in-house. The same instrument setup was used as outlined above with 30 s between 40 s ablations. Zircon isotopic data were normalized to reference material 91500 (age of 1065 Ma; Wiedenbeck et al., 1995) with

Plešovice (²⁰⁶Pb/²³⁸U age of 337.13 ± 0.13 Ma; Sláma et al., 2008) as a secondary reference material. Down-hole element fractionation and instrument drift were monitored based on the primary reference material and corrected using the Iolite software package (v4; Paton et al., 2010, 2011). The first analytical session, which measured NA-19-02, returned a 339 ± 3 Ma (MSWD = 1.7, n = 15/16) ²⁰⁶Pb/²³⁸U age for Plešovice. Minor excess dispersion in the ²⁰⁷Pb/²⁰⁶Pb (2%) and ²⁰⁶Pb/²³⁸U (1.5%) ratios was calculated based on the analysis of the secondary reference material and incorporated into the isotopic ratios. The second analytical session, in which NA-19-02 was analyzed, yielded a ²⁰⁶Pb/²³⁸U age of 340 ± 2 Ma (MSWD = 0.44, n = 13/14) for Plešovice. Excess dispersion in the ²⁰⁷Pb/²⁰⁶Pb (3%) ratio was incorporated quadratically into the isotopic ratios. Analyses of Plešovice in both sessions resulted in ages well within 2% of the expected value. U-Pb dates of unknowns were filtered for discordance using cutoffs of 30% and -10% (discordance = [1-²⁰⁶Pb/²³⁸U date ÷ ²⁰⁷Pb/²⁰⁶Pb date] × 100). Quoted U-Pb detrital dates correspond to ²⁰⁶Pb/²³⁸U dates for analyses younger than 1.5 Ga and ²⁰⁷Pb/²⁰⁶Pb dates for analyses older than 1.5 Ga (following recommendations from Spencer et al., 2016).

Trace element concentrations of the unknowns were measured with the U-Pb isotopes during the second analytical session of NA-19-02. A dwell time of 15 ms was used for ²⁹Si, ⁴³Ca, ⁴⁹Ti, ⁸⁸Sr, ⁸⁹Y, ⁹⁰Zr, ⁹³Nb, ¹³⁹La, ¹⁴⁰Ce, ¹⁴¹Pr, ¹⁴⁶Nd, ¹⁴⁷Sm, ¹⁵³Eu, ¹⁵⁷Gd, ¹⁶³Dy, ¹⁷²Yb, and ¹⁷⁸Hf where measured. Concentrations were calculated with Si as the internal standard and reduced using Iolite v4 (Paton et al., 2010, 2011) using NIST610 (GeoReM database, application version 27, http://georem.mpch-mainz.gwdg.de; Jochum et al., 2005) as the primary reference material. Trace element concentrations of the secondary reference material NIST612 are typically within 5% of expected values (GeoReM database, appli-

cation version 27, http://georem.mpch-mainz.gwdg.de; Jochum et al., 2005). No matrix correction was applied for trace elements measured in zircon as only relative changes in abundance are used for interpretations.

## RESULTS

### Major Element Geochemistry

Major element concentrations for all the mafic samples composing the Grt-bearing mafic unit and the common amphibolite unit are presented as oxides in Table S2. Loss on ignition varies from 0.65 wt% to 5.43 wt% and is closely correlated to Hbl content. Major element oxides have all been renormalized and are quoted on an anhydrous basis below. Although an effort was made to avoid altered or weathered samples, we cannot rule out protolith alteration prior to metamorphic recrystallization. The following description thus focuses on major elements that are deemed to have limited or no mobility during regional or oceanic-floor metamorphism (Al, Si, Fe, Mg, and Ti) and that are useful for distinguishing rock types and identifying tectonomagmatic affinities (e.g., Pearce, 2014). Based on ratios between such major element oxides, the different basic lithologies from the Bay of Islands metamorphic sole (i.e., Grt-Cpx granulite, Grt-Cpx amphibolite, and common amphibolite) can be divided into two groups (Fig. 6). The first group includes the Grt-Cpx amphibolite and most of the common amphibolite samples. Rocks from this group have SiO₂ content ranging from 44.65 wt% to 52.52 wt%, Al₂O₃ content ranging from 12.49 wt% to 18.34 wt%, Fe₂O₃ Total content ranging from 8.83 wt% to 15.78 wt%, and TiO₂ content ranging from 0.50 wt% to 1.96 wt% (Table S2). The Mg# (Mg # = Mg²⁺ / [Fe²⁺ + Mg²⁺]) where Fe³⁺/Fe⁰.⁰¹ varies between 51.92 and 74.19 (Table S2). The Fe³⁺/Fe⁰.⁰¹ ratio was fixed according to low oxidation

**Figure 6.** Mg# (Mg# = Mg²⁺/[Fe²⁺ + Mg²⁺]) where Fe³⁺/Fe⁰.⁰¹ = 0.15) vs. SiO₂/Al₂O₃ diagram. The Grt-Cpx amphibolite, the common amphibolite, and the Grt-Cpx granulite are represented by full dark green triangles, empty green triangles, and purple squares, respectively. Possible differentiation trends and cumulate processes are indicated by black arrows. Modified from Kempton and Harmon (1992). Magmatic differentiation trends are taken from Irvine and Baragar (1971).

<figure><img src="image_6.png" /></figure>
states reported for unaltered basic igneous rocks (Mallmann and O'Neill, 2009; Schilling et al., 1983). The second group consists of the Grt-Cpx granulite and three common amphibolite samples (NA-19-11a, NA-19-11b, and NA-19-12a). Compared to the first group, these rocks have similar SiO₂ (45.85–51.10 wt%), lower Fe₂O₃*Total (3.71–7.82 wt%), higher Al₂O₃ (13.97–24.48 wt%) and Mg# (76.71–83.54), and lower TiO₂ (0.07–0.49 wt%).

## Trace Element Geochemistry

The trace element contents are presented in Table S3. The two groups defined by major elements exhibit contrasting trace element contents. Figure 7A shows rare earth element (REE) patterns for all metabasites studied normalized to chondritic values (Sun and McDonough, 1989). Samples from the first group (consisting of Grt-Cpx amphibolite samples and most of the common amphibolite samples) have similar and parallel REE patterns (Fig. 7A), with concentrations varying within 1.5–40× the chondrite values. They feature a moderate depletion in light rare earth elements (LREEs; positive slope), with [La/Sm]CN (CN: normalized to chondritic values; Sun and McDonough, 1989) ratios ranging from 0.20 to 1.04 and flat heavy rare earth element (HREE) patterns with [Dy/Lu]CN ranging from 0.78 to 1.23, and have relatively small

and variable Eu anomalies with Eu/Eu* ([Eu]CN/[Sm]CN × [Gd]CN)0.5 values varying between 0.66 and 1.23.

The Grt-Cpx granulite samples and the common amphibolite samples (NA-19-11a, NA-19-11b, and NA-19-12a) of the second group have significantly lower contents of incompatible elements than amphibolites from the first group, with REE concentrations varying within 0.1–10× the chondritic values (Fig. 7A). Most of the Grt-Cpx granulite samples and the depleted common amphibolite samples (NA-19-11a, NA-19-11b, and NA-19-12a) show depletion in LREEs, with [La/Sm]CN ratios ranging from 0.18 to 0.91, except for four samples that show a negative LREE slope, with [La/Sm]CN ratios ranging from 1.43 to 1.83. The HREE patterns for all of those rocks are flat, with [Dy/Lu]CN ratios ranging within 0.83 and 1.34, and all have a distinctively positive Eu anomaly, with Eu/Eu* values varying from 1.08 to 3.71.

Figure 7B shows a multi-element spider diagram that includes immobile REEs, high field strength elements (HFSEs), and large ion lithophile elements (LILEs). Values are normalized to primitive mantle composition (Sun and McDonough, 1989). Both types of amphibolites from the first group have similar patterns, with trace element contents varying within 0.5–11× the primitive mantle compositions. Samples from the first group also have variable Nb and

Ta anomalies, variable Zr anomalies, and a small negative Ti anomaly. Trace element patterns for the Grt-Cpx granulite and the depleted common amphibolite samples (NA-19-11a, NA-19-11b, and NA-19-12a) from the second group show concentrations ranging from 0.1 to 8× primitive mantle composition, with a well-defined negative Zr anomaly and positive Ta anomaly.

## U-Pb Titanite Dating Results

In situ titanite dating was performed in two common amphibolite samples (NA-19-06a and NA-19-24a) collected from the North Arm Mountain massif. The results are presented in Tera-Wasserburg space in Figure 8. Isotopic data are listed in Table S4. Diagrams were generated using the open ChrontouR package (Larson, 2020) developed for the freely available R environment. Sample NA-19-06a is a common amphibolite (Fig. 5F) from the lower section of the Bay of Islands sole, where it is intercalated with metasedimentary rocks (Fig. 4E). In this sample, titanite occurs as elongated aggregates of subhedral crystals (up to 150 µm) concordant with the foliation (Fig. 5F). Single spot analyses (n = 43) yield ²⁰⁷Pb-corrected (Stacey and Kramers, 1975) ²⁰⁶Pb/²³⁸U dates ranging from 634 ± 84 Ma to 454 ± 24 Ma. Regressing the data with the method of Powell et al. (2020) defines a single population (s = 0.67) with an isochron age of 486 ± 7 Ma (Fig. 8A).

Sample NA-19-24a is a common amphibolite collected from the middle portion of the North Arm Mountain metamorphic sole (Fig. 3D). Titanite occurs in the matrix as 50–400 µm subhedral grains with ilmenite cores (Fig. 5D). Spot analyses (n = 64) yield ²⁰⁷Pb-corrected (Stacey and Kramers, 1975) ²⁰⁶Pb/²³⁸U dates ranging from 577 ± 45 Ma to 261 ± 15 Ma. Applying the regression of Powell et al. (2020) in a Tera-Wasserburg space defines a single population (s = 0.64) with an isochron age of 484 ± 4 Ma (Fig. 8A). This date is indistinguishable from that of NA-19-06a.

## Detrital Zircon Analyses

A Grt-Bt-bearing schist (sample NA-19-02; Fig. 5E), located at the structural base of the metamorphic sole at North Arm Mountain massif, was selected for detrital zircon analysis. Zircon grains are 20 µm to >100 µm in size, anhedral to subhedral, and commonly rounded to subrounded in shape (Figs. S1 and S2; see footnote 1). Back-scatter electron (Fig. S1) and cathodoluminescence (CL; Fig. S2) imaging of zircon grains reveals oscillatory zoning and local, bright CL overgrowths. No clear correlation was observed between the occurrence of oscillatory

<figure><img src="image_7.png" /><figcaption>Figure 7. Plots show (A) rare earth element (REE) patterns normalized to chondrites (Sun and McDonough, 1989) and (B) immobile element patterns normalized to primitive mantle (Sun and McDonough, 1989). The normal mid-oceanic ridge basalt (N-MORB) trace element content (from Sun and McDonough, 1989) is indicated with yellow squares for reference.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Figure 8. Tera-Wassburg plots illustrate the Powell et al. (2020) regression through titanite U-Pb isotopic data (laser ablation–inductively coupled plasma–mass spectrometry) for (A) NA-19-06a and (B) NA-19-24a.</figcaption></figure>

zoning and the date of the grains. The majority of the laser analyses were performed at the center of the grains, but in some cases they straddled the edges of some grains (Figs. S1 and S2).

Zircon U-Pb isotopic data for analyzed grains are presented in Table S6. The detrital zircon dates quoted herein correspond to $^{206}$Pb/$^{238}$U dates for analyses younger than 1.5 Ga and $^{206}$Pb/$^{207}$Pb dates for analyses older than 1.5 Ga. Sample NA-19-02 yielded 113 concordant (between 70% and 110% concordance) zircon dates distributed between 2620 ± 40 Ma and 464 ± 9 Ma. A Wetherill Concordia diagram is presented at Figure 9A showing all U-Pb zircon isotopic analyses, while a combined histogram and kernel density estimate (KDE) plot is shown in Figure 9B. Both plots were generated using the IsoplotR software package (Vermeesch, 2018). The histogram was calculated using bandwidth of 40 Ma, and the KDE curve was calculated using a Gaussian kernel and adap-

tive bandwidths. The most dominant age group (n = 33/113) ranges from ca. 575 Ma to ca. 465 Ma, with a KDE peak age of ca. 490 Ma. Other subordinate populations range between ca. 690 Ma and ca. 580 Ma (n = 13/113), ca. 875 Ma and ca. 720 Ma (n = 9/113), ca. 1245 Ma and ca. 950 Ma (n = 29/113), and ca. 1560 Ma and ca. 1275 Ma (n = 13/113), with KDE peaks at ca. 610 Ma, ca. 770 Ma, ca. 1105 Ma, and ca. 1325 Ma, respectively. Fourteen grains yield Paleoproterozoic ages with a minor KDE peak age of ca. 1740 Ma. The two oldest grains yield Neoarchean dates of 2550 ± 60 Ma and 2620 ± 40 Ma and form a minor KDE peak age at ca. 2.5 Ga.

The trace element contents for zircon grains analyzed during the second analytical session are presented in Table S7. The concordant zircon grains have REE concentrations varying within 1–10,000× the chondrite values. REE patterns show slight variations in LREE contents,

whereas HREE patterns are relatively similar and parallel (Fig. 10). The zircon grains analyzed show an enrichment in HREEs and have [Yb/Gd]$_{CN}$ ranging from 9.28 to 68.32, except for one grain that has a relatively flat HREE pattern and a [Yb/Gd]$_{CN}$ ratio of 4.76.

# DISCUSSION

## Protolith History of Metabasites

Deciphering the protolith history of the sole allows the characterization of the lower plate and, thus provides information about the tectonic environment that led to formation and emplacement of the Bay of Islands ophiolitic complex before and after subduction initiation (Fig. 1). Here, we examine the mafic portion of the Bay of Islands metamorphic sole following the method of Pearce (2008, 2014), using immobile major and trace elements to geochemically characterize the protolith of the subducted oceanic crust (e.g., Guilmette et al., 2009; Spandler et al., 2004).

The Mg# versus SiO$_2$/Al$_2$O$_3$ diagram from Kempton and Harmon (1992), shown in Figure 6, is useful for highlighting potential cumulate processes and liquid differentiation trends for metamorphosed mafic rocks. Most of the Grt-Cpx amphibolite and common amphibolite samples forming the first group have compositions that cluster in a narrow range of Mg# and SiO$_2$/Al$_2$O$_3$ ratios that is consistent with a primitive basaltic liquid composition and shows no clear cumulate and/or differentiation trends. Samples from the second group (Grt-Cpx granulite samples and three common amphibolite samples: NA-19-11a, NA-19-11b, and NA-19-12a) plot outside the field of primitive basalts, with higher Mg# and lower SiO$_2$/Al$_2$O$_3$ ratios than the majority of the first group of amphibolites. Such compositions differ from those of basaltic liquids and are consistent with the effect of cumulate processes such as a combination of pyroxene, olivine, and particularly plagioclase accumulation and agree with the results of CIPW normative mineral calculations (not shown). The strongly positive Eu anomaly and overall depletion in REEs (Fig. 7A) further support the feldspar-rich cumulate interpretation. We conclude that the protoliths of the rocks composing the second group are cumulate gabbroic rocks, which are likely derived from the plutonic section of oceanic lithosphere. It is noteworthy that most of the observed metacumulates occupy the upper structural part of the sole, immediately underneath the Bay of Islands mantle (Fig. 3D).

Samples from the first group plot within the compositional field characteristic of primitive basalt (Fig. 6), which indicates that they may be metamorphosed lavas or dikes with liquid-like
<figure><img src="image_9.png" /><figcaption>**Figure 9.** (A) Concordia diagram shows U-Pb analyses (laser ablation–inductively coupled plasma–mass spectrometry) for all zircon grains analyzed from sample NA-19-02. Error ellipses are 2 standard errors. (B) Histogram and kernel density estimate (KDE) curve for concordant U-Pb analyses for zircon grains from NA-19-02. The histogram and KDE were produced with IsoplotR (Vermeesch, 2018). The detrital zircon dates (x-axis) correspond to $^{206}\text{Pb}/^{238}\text{U}$ dates for analyses younger than 1.5 Ga and $^{206}\text{Pb}/^{207}\text{Pb}$ dates for analyses older than 1.5 Ga.</figcaption></figure>

compositions. In the following section, we investigate the tectonomagmatic affinity of those samples that we considered were derived from basaltic liquids and exclude the metacumulates, which should not be considered in geochemical tectonic-affinity investigations (Pearce, 2008, 2014).

The Zr/Ti versus Nb/Y diagram from Pearce (1996) is presented in Figure 11A. This classification diagram is preferred over the total alkali-

silica diagram because it is based on immobile elements, whereas Na and K are considered to be highly mobile during metamorphic and/or alteration processes (Pearce, 2014). Both types of amphibolites are classified as basalts (Fig. 11A) and have a dominantly tholeiitic affinity (Fig. 11A).

The tectonic discrimination diagram illustrated in Figure 11B uses the Th/Nb proxy to

distinguish suprasubduction zone (SSZ) from mid-oceanic ridge basaltic rocks (Pearce, 2008). Th may be mobilized from the dehydrating lower plate during subduction-related metamorphism, enriching the melting mantle wedge and resulting magmas, whereas Nb remains trapped in the down-going slab (Pearce, 2008). As a result, SSZ magmas are characterized by high Th/Nb ratios and commonly plot above the MORB-OIB array (Pearce, 2008, 2014). As seen in Figure 11B, the majority of the Bay of Islands metamorphic sole samples plot within the depleted end of the MORB-OIB array close to N-MORB. A few samples plot above the MORB-OIB array, indicating either some degree of crustal contamination or a weak contribution from a subducting slab. Further discrimination is possible by using the Ti/Yb proxy, which is useful for determining the depth of melting of the source rock. Deep magmas derived from a garnet peridotite mantle source show higher ratios as HREEs are favorably partitioned into residual garnet relative to Ti (Pearce, 2008). As illustrated in Figure 11C, samples analyzed plot as N-MORBs, which indicates shallow melting of a depleted source. The potential influence of slab-derived fluids modifying a mantle wedge source (Fig. 11D) was examined using the V/Ti discrimination diagram (Shervais, 1982), as the presence of water during mantle melting results in Ti-depleted and V-enriched melts as compared to dryer melting environments, making this ratio an efficient proxy for distinguishing SSZ rocks. Samples from the Grt-bearing unit plot mainly in the MORB-back-arc basin-forearc basalt field but straddle the island arc-tholeiites boundary, whereas all of the common amphibolite samples except for one are restricted to the former.

Because samples from both units also straddle the MORB-OIB array and SSZ fields in the Th/Yb versus Nb/Yb diagram of Figure 11B, a back-arc basin environment cannot be ruled out for the mafic protoliths of the sole. A transitional oceanic crust (such as the Birchy complex) also cannot be ruled out, since compositions overlap (van Staal et al., 2013). However, considering the scatter of the data, which is a key element of basaltic fingerprinting (Pearce, 2014), our preferred interpretation is that the mafic protolith of the metamorphic sole formed at a normal spreading ridge. Therefore, we conclude that the metabasites of both the Grt-bearing and common amphibolite units of the North Arm Mountain and Table Mountain soles are derived from cumulate and basaltic protoliths that formed through shallow melting of a depleted mantle source, which is consistent with plutonic and volcanic sections of oceanic crust generated at a mid-oceanic spreading ridge.

<figure><img src="image_10.png" /><figcaption>**Figure 10.** Rare earth element (REE) patterns normalized to chondrites (Sun and McDonough, 1989) for concordant zircon grains are plotted. Patterns are color-coded from yellow for the youngest analyses (ca. 500 Ma) to purple for the oldest analyses (>2 Ga). Representative REE patterns for igneous and metamorphic zircons are shown for reference (from Rubatto, 2017).</figcaption></figure>
<figure><img src="image_11.png" /></figure>

**Figure 11.** (A) Classification of the basaltic metabasites from the Bay of Islands metamorphic sole using an immobile element-based total alkali-silica proxy diagram (Pearce, 1996). (B and C) Discrimination diagrams to fingerprint tectonic settings for basaltic rocks from Pearce (2008) using (B) the Th/Nb proxy to distinguish suprasubduction zone (or crustal contaminated) basalts from mid-oceanic-ridge basalts (MORBs) and (C) the Ti/Yb proxy to distinguish normal mid-oceanic-ridge basalt (N-MORB), enriched mid-oceanic-ridge basalt (E-MORB), and oceanic island basalt (OIB). (D) V vs. Ti discrimination diagram from Shervais (1982). BAB—back-arc basin; IAT—island-arc tholeiite; BON—boninite.

## Timing of Metamorphism

Our geochronology results on the Bay of Islands metamorphic sole provide new constraints on the timing of its burial, accretion, and exhumation, and provide a minimum age for both its protolith and subduction initiation. Titanite is a common accessory mineral in metabasic rocks at intermediate P–T conditions (Kohn, 2017) and has successfully been used to date metamorphism (e.g., Angiboust and Harlov, 2017). Recent estimates of Pb-diffusion closure temperature in titanite are as high as ≥825 °C (Gao et al., 2012; Holder et al., 2019). Such a closure temperature is significantly higher than the peak conditions of ~725 °C obtained for similar common amphibolite assemblages in other metamorphic soles worldwide (Dubacq et al., 2019; Guilmette et al., 2008; Soret et al., 2017). Accordingly, we interpret that the U-Pb titanite

dates presented herein represent crystal growth and not a later cooling event. Titanite in sample NA-19-06a and NA-19-24a occurs as elongated aggregates of subhedral crystals concordant with the foliation (Fig. 5F) and as overgrowths around ilmenite cores (Fig. 5D), respectively. These textural associations are consistent with metamorphic growth of titanite and, as such, a magmatic origin can be ruled out. Titanite moats around ilmenite cores may record the growth of titanite from ilmenite breakdown. Such a reaction could occur during retrograde metamorphism in response to exhumation and cooling (e.g., Angiboust and Harlov, 2017; Garber et al., 2020; Stearns et al., 2015) or as a prograde phase around primary igneous ilmenite. Regardless, the U-Pb titanite dates of 486 ± 7 Ma and 484 ± 4 Ma are interpreted to mark metamorphic growth and, as such, provide a minimum protolith age for both the common amphibolite

and metasedimentary units. Because the dated samples are from the common amphibolite and metasedimentary units of the sole, both of which metamorphosed and accreted after subduction initiation, the dates obtained from those samples do not reflect the timing of subduction initiation but rather correspond to the ongoing subduction.

## Protolith History of the Metasedimentary Rocks

Detrital zircon population analysis in the metasedimentary unit provides new constraints on the depositional age, provenance, and depositional environment of the sedimentary protolith. Because zircon U-Pb dates were recovered from a metasedimentary sample, the origin of the younger grains (i.e., detrital vs. metamorphic) needs to be addressed. The sample analyzed for detrital zircon contains garnet, which is known to fractionate HREEs and Y (Baxter et al., 2017; Otamendi et al., 2002). Metamorphic zircon that grows synchronously with garnet or in a garnet-rich assemblage generally exhibits a flat HREE pattern compared to the common HREE enrichment observed in magmatic zircon (Rubatto, 2017; Fig. 10). In Figure 10, almost all of the concordant grains show an enrichment in HREEs and have patterns that are parallel to the typical HREE pattern for magmatic zircon, showing no evidence of partitioning with garnet. Garnet is also an important sink for Y. Accordingly, if the youngest grains observed herein had grown with or after garnet nucleation, we would expect them to show a strong depletion in Yb relative to Gd and low Y concentrations. When considering the youngest population, we observe the opposite correlation. Zircon grains that are more depleted in Yb relative to Gd are richer in Y (Fig. 12), which is inconsistent with growth in the presence of garnet. As garnet forms at greenschist conditions in such protoliths, we deem it unlikely that the youngest population represents metamorphic zircon and interpret it as of detrital origin. This interpretation is further supported by the observation of oscillatory zoning in some of the younger grains and by the relatively high Th/U content, which ranges from 0.01 to 0.90 with an average of 0.35 (calculated using only the youngest population).

We also considered the case where some of our laser spot analyses may have straddled both a detrital core domain and a metamorphic overgrowth rim (Figs. S1 and S2), in which case the resulting dates would be composite. This could especially be the case for the zircon grains yielding Neoproterozoic dates in the Tonian cluster. The Tonian population is mostly composed of small grains (<70 µm), which prevented analysis of only their cores (Fig. S1). We investigate this
<figure><img src="image_12.png" /><figcaption>**Figure 12.** Y vs. [Gd/Yb]~CN~ is plotted for concordant zircon grains analyzed. Gd and Yb values are normalized to chondrites (Sun and McDonough, 1989).</figcaption></figure>

potential age mixing through the chemistry of each analyte using the same principles described above. None of the Neoproterozoic zircon grains analyzed is depleted in HREEs. There is also no correlation between the dates of the grains and HREE and/or Y depletion (Fig. 12), as would be expected from variable mixing between older magmatic cores and younger metamorphic overgrowths. Hence, we conclude that all of the concordant zircon grains analyzed have a detrital origin and are treated as such in the following discussion.

The maximum depositional age (MDA) for the metasedimentary unit of the Bay of Islands metamorphic sole was estimated from the youngest cluster of detrital zircon dates recovered from sample NA-19-02. The youngest graphical peak method (Dickinson and Gehrels, 2009) indicates an MDA of ca. 490 Ma. This MDA overlaps with the metamorphic age (486 ± 7 Ma) recovered from titanite from an interlayered mafic sample found in the metasedimentary unit (NA-19-06a), which provides a minimum age of deposition. Therefore, we conclude that the protolith of the metasediments was deposited at ca. 490 Ma.

Detrital zircon analysis can provide information about the potential sources of the detritus that form the metasedimentary rock that was investigated. Our detrital zircon dates from the Bay of Islands metamorphic sole range between 2620 ± 40 Ma and 464 ± 9 Ma and can be separated into different groups based on their distribution on the KDE plot (Fig. 9B). The main group (n = 33/111) includes Cambrian to Early Ordovician grains with a dominant peak at 490 Ma. Such zircon dates are correlative to Cambrian SSZ magmatism and metamorphism (514–485 Ma) in the upper plate during Taconic/Grampian convergence in the orogenic segment between the British Isles and Newfoundland (e.g., Chew et al., 2010; Elliott et al., 1991; Walker et al., 2021). Cambrian dates were also recovered from arc-like plutonic rocks of the Lushes Bight Oceanic Tract such as the Twillingate trondhjemite (ca. 507 Ma; Elliott et al.,

1991) and the Coastal complex (ca. 505 Ma; Jenner et al., 1991; Mattinson, 1975), the Baie Verte Oceanic Tract (ca. 491–488 Ma; Cawood et al., 1996; Skulski et al., 2010; Yan and Casey, 2020), and the Notre Dame arc (ca. 489–477 Ma; van Staal et al., 2007). Hence, Taconic/Grampian upper plate arc-derived and/or metamorphic rocks represent the most likely sources of the age peak at ca. 490 Ma. Although unusual, zircon grains with Taconic dates have also been reported in Newfoundland in the Clam Bank and Winterhouse formations (White et al., 2019).

Subordinate Neoproterozoic groups are defined at ca. 690–580 Ma (n = 13/113) and ca. 875–720 Ma (n = 9/113). It is noteworthy that detrital zircon grains of Tonian (1 Ga–720 Ma) ages are very rare within the Laurentian margin in Newfoundland west of the Cabot Fault (Cawood and Nemchin, 2001; White et al., 2019; White and Waldron, 2022). Rift-related magmatism related to the breakup of Rodinia and the opening of the Iapetus Ocean in North America occurred between 760 Ma and 550 Ma (Cawood et al., 2001), but punctuated Tonian to Ediacaran rifting (1000–550 Ma) interrupted by multiple phases of Tonian orogenesis (e.g., ca. 950–920 Ma Renlandian and ca. 830–725 Ma Knoydtarian orogenesis) occurred in the Laurentian realm of the British Isles, probably including parts of the Rockall bank (Cawood et al., 2015; Dewey et al., 2015; Leslie et al., 2008). Igneous and metamorphic rocks along the Laurentian passive margin that was situated between the British Isles and Newfoundland are thus potential sources of the Neoproterozoic detritus observed. Hence, the cluster of Tonian dates (ca. 875–720 Ma) could possibly be derived from a source to the north of Newfoundland, near the southern edge of the Rockall bank, from which the Dashwoods terrane is thought to originate (van Staal and Zagorevski, 2020). In addition, Tonian detrital zircon is present in the Fleur de Lys Supergroup sampled east of the dextral Cabot Fault in Baie Verte Peninsula (van Staal et al., 2013), which moved southwards with

the Dashwoods terrane during the Ordovician to Carboniferous (van Staal and Zagorevski, 2020, and references therein), whereas early Tonian volcanic rocks have also been detected in this part of the Fleur de Lys (Strowbridge et al., 2022).

Subordinate Mesoproterozoic groups are defined at ca. 1245–950 Ma (n = 29/113) and ca. 1555–1275 Ma (n = 13/113). The Mesoproterozoic peaks at 1090 Ma and 1350 Ma are consistent with zircon derived from the Grenville Orogen (ca. 1.3–0.9 Ga; Rivers, 1997). Potential sources for the 14 Paleoproterozoic dates from 2430 ± 60 Ma to 1665 ± 68 Ma include the Paleoproterozoic Trans-Hudson (Corrigan et al., 2009), New Québec, and Torngat orogens (Godet et al., 2021), all of which are part of Laurentia. The two oldest grains yield Neoarchean dates of 2550 ± 60 Ma and 2620 ± 40 Ma and could possibly be derived from the Superior Craton (ca. 2.7 Ga; Percival et al., 2012) or the Lewisian in Scotland and coeval Archean rocks in Labrador and/or Greenland (Cawood et al., 2007). Similar Mesoproterozoic, Paleoproterozoic, and Archean populations have also been reported for the Laurentian margin sedimentary sequence in Newfoundland (Cawood and Nemchin, 2001; White et al., 2019).

In summary, the detrital zircon date spectrum recovered from sample NA-19-02 is consistent with a lower-plate Laurentian and upper-plate Taconic/Grampian arc-forearc detrital source. The occurrence of Tonian zircon dates could indicate that the sediments (or a component) were derived from a part of the Laurentian realm where Tonian magmatism and orogenesis was present, such as in the British Isles, including the Rockall bank, which represented part of the Laurentian foreland of the British Caledonides (e.g., Cawood et al., 2015; Dewey et al., 2015; Leslie et al., 2008). Because of the small amount of data related to this age peak, we are unable to draw definitive conclusions on that matter. This hypothesis should be tested with more samples coming from different localities of the Bay of Islands metamorphic sole.

Detrital zircon data can also be used to investigate the tectonic setting in which sediments were deposited. The age distribution pattern was evaluated with criteria (steps 1 and 2; Fig. 13) from Cawood et al. (2012) to establish the tectonic environment of deposition of the sedimentary units of the sole. Depending on basin tectonic settings, the time gap between the measured crystallization age (CA) of individual grains and the inferred depositional age (DA) will vary. Basins developed along convergent margins (Scenario A in Fig. 13) will record a short time gap between the timing of igneous zircon crystallization and the timing of
<figure><img src="image_13.png" /></figure>

green) basins are from Cawood et al. (2012). The chosen depositional age corresponds to the youngest zircon grain analyzed (465 Ma), although it should not be considered as the true maximum depositional age (Gehrels, 2014).

deposition, reflecting ongoing magmatic activity (Cawood et al., 2012). Conversely, basins developed in divergent settings (Scenario C in Fig. 13) will record little or no syn-depositional igneous activity, whereas basins developed during continental collision represent an intermediate scenario (Scenario B in Fig. 13). The youngest 5% of zircon from NA-19-02 have a DA-CA of less than 150 m.y. Application of the first criterion (Step 1 in Fig. 13) essentially rules out an extensional basin setting for the deposition of the sediments found in the metasedimentary unit, which is consistent with our exclusion of the Laurentian passive margin as a potential protolith. Application of the second criterion (Step 2 in Fig. 13) is more ambiguous, with the youngest 30% of zircon having DA-CA of ca. 100 Ma, which is inconclusive for a collisional or a convergent setting. However, the detrital

zircon dating results from sample NA-19-02 indicate a quick transition from crystallization to deposition ages (CA-DA) and a dominant proportion of young dates because 30% of the analyses form the youngest age group (Figs. 9B and 11). Such features are characteristic of ongoing magmatic activity and basin development at a convergent margin, e.g., trench, forearc, and backarc basins (Cawood et al., 2012). A convergent margin environment is further supported by the apparent Taconic arc-derived detrital source that is consistent with a subduction setting. Moreover, our data indicate that the sediments were subducted shortly after deposition. Therefore, we suggest that the metasedimentary section of the Bay of Islands metamorphic sole primarily represents trench-fill deposits fed by detritus partly sourced from Taconian/Grampian arc magmatism in the upper plate.

<figure><img src="image_14.png" /><figcaption>**Figure 14.** Schematic tectonic model proposed for the formation of the Bay of Islands metamorphic sole is shown. The sole is illustrated in red. The pink diamonds illustrate the potential path for detrital zircon grains. N-MORB—normal mid-oceanic-ridge basalt.</figcaption></figure>

## Tectonic Model

Studies have demonstrated that subophiolitic metamorphic soles are composed of several discrete slices that were accreted in succession from the top of the lower plate to the base of the upper plate (Agard et al., 2016; Soret et al., 2017), the uppermost section having been scraped off closest to the site of subduction initiation (Guilmette et al., 2018). Deciphering the protolith history of metamorphic soles thus constitutes a geological record that may help characterize the nature of the lower plate from the site of subduction initiation to the site of obduction, providing key information about fundamental aspects of both subduction and regional tectonic evolution. In this section, we interpret our geochemical and geochronological results from the Bay of Islands metamorphic sole that represents the lower plate of the Taconic Orogeny and discuss potential settings for subduction initiation, the formation of the Bay of Islands complex, and ensuing convergence during the Taconic Orogeny.

Any tectonic model for the formation of the Bay of Islands metamorphic sole (Fig. 14) must explain five fundamental geological characteristics discussed in this manuscript, namely: (1) the formation of the mafic upper portion of the sole at an oceanic ridge, (2) the occurrence of metagabbroic rocks (Grt-Cpx granulite samples) in direct contact with the Bay of Islands mantle, (3) the deposition of arc-derived sediments on the lower plate no later than ca. 490 Ma, (4) their subduction at or before ca. 485 Ma, and (5) subduction initiation far from the Newfoundland Laurentian margin (discussed below).

The upper structural parts of metamorphic soles, at the contact with the overlying mantle, are interpreted as former oceanic crust located at (or close to) the site of subduction initiation (Agard et al., 2016; Wakabayashi and Dilek, 2000). Thus, the geochemical affinity of soles can help distinguish the tectonic setting leading to subduction initiation (Fig. 1). Although the scatter of the geochemical data (Fig. 11) doesn't allow us to precisely determine one tectonic setting over the others, the dominantly N-MORB affinity of the basaltic rocks within the Bay of Islands sole is consistent with subduction initiation taking place in the Iapetan domain possibly at a pre-existing weakness zone such as an oceanic transform or an aborted ridge (Fig. 1). However, considering how common N-MORB basaltic melts are and the possible slight SSZ signature observed for some samples, subduction initiation at a transitional margin (van Staal et al., 2013) or in a back-arc basin (Fig. 1) cannot be ruled out solely on the basis of protolith geochemical affinity. The occurrence of cumulate rocks, i.e., gabbro, in the upper structural part
of the sole in contact with the mantle may indicate that the upper oceanic crust was stripped/removed from the lower plate either before or during subduction infancy. An oceanic extensional detachment fault could be responsible for exposing the plutonic lower oceanic crust to the seafloor and then to the mantle wedge in the subduction interface. Such structures are common along oceanic ridges and have been proposed as pre-existing weaknesses responsible for subduction initiation and the formation of SSZ ophiolites (Maffione et al., 2015). Subduction initiation can also occur at detachment faults in a passive or rifted margin (Lallemand and Arcay, 2021). Regardless of its location, an oceanic detachment fault, either far from or close to the continental margin, represents a viable geodynamic setting for subduction initiation (Lallemand and Arcay, 2021) and may explain the presence of cumulate gabbro (corresponding to the lower oceanic crust of the lower plate) in contact with the Bay of Islands mantle peridotites of the upper plate (Fig. 14). Because of the dominant N-MORB affinity, we favor the hypothesis of a detachment fault in the oceanic domain to explain the nucleation of the subduction zone responsible for the formation of the Bay of Islands complex. Cumulate protoliths have not been reported for other metamorphic soles, with the exception of the St. Anthony complex (Jamieson 1980, 1981), an along-strike ophiolite complex of similar age situated in Newfoundland, 300 km north of the Bay of Islands complex. Thus, it may be a feature unique to Taconian metamorphic soles in Newfoundland and could represent a subduction initiation setting unique to the Newfoundland Taconic Orogeny.

The formation of the sole also needs to account for the various protoliths found at different levels, which are, from top to bottom, gabbroic rocks, basaltic rocks, and metasedimentary rocks. Such an inverse oceanic plate stratigraphy has been previously interpreted as an overturned limb of a fold that formed during subduction initiation (Wakabayashi and Dilek, 2003). This interpretation implies that the metamorphism of soles throughout their whole structural thickness is synchronous and occurs at a single depth (Garber et al., 2020). Alternatively, the metamorphic sole accretion model proposed by Agard et al. (2016) predicts successive accretion through time of lower plate uppermost crustal sections in a stepwise fashion at discrete, progressively shallower P-T conditions (Soret et al., 2017). In this model, enhanced mechanical coupling at the subduction interface is responsible for accretion of the uppermost crust of the lower plate to the upper plate, where it becomes the metamorphic sole. Both processes can account for the “inverted”

oceanic crustal sequence observed within the Bay of Islands metamorphic sole.

The detrital zircon record from the metasedimentary portion of the Bay of Islands metamorphic sole, however, is consistent with a predominantly convergent margin upper-plate provenance, which requires that the subduction be older than the sediments. The dominant peak at ca. 490 Ma correlates with the Notre Dame arc and/or the Lushs Bight Oceanic Tract/Baie Verte Oceanic Tract, whereas the recorded Tonian dates could indicate that the upper plate was situated in the peri-Laurentian realm of the Caledonides rather than the Appalachians, possibly near the southern edge of the Rockall bank. The ca. 490 Ma maximum depositional age from NA-19-02, which overlaps with the titanite dates, means that the sediments were buried shortly (or almost immediately) after deposition. The overlap of those minimum and maximum ages indicates that the subduction responsible for the formation of the Bay of Islands complex was ongoing during the deposition of the sediments forming the metasedimentary unit of the Bay of Islands sole and that an arc had already formed. This rules out the possibility that the sole represents a coherent inverted crustal section that formed through development of a fold at subduction initiation and, therefore, supports the successive slice accretion model.

Titanite U-Pb dates obtained from common amphibolite samples NA-19-06a (486 ± 7 Ma) and NA-19-24a (484 ± 4 Ma) are inferred to record the timing of metamorphism of the Bay of Islands metamorphic sole. Hence, the average of those metamorphic dates sets a minimum limit for the burial of the basic upper structural portion of the sole, i.e., subduction initiation, at 485 ± 4 Ma. However, the metamorphic dates we report here were recovered from the middle and lower sections of the sole, which—according to the successive accretion model—could have been metamorphosed several million years after subduction initiation. Regardless, the ca. 485 Ma metamorphic age reported herein significantly predates the previous 469 ± 5 Ma $^{40}$Ar-$^{39}$Ar age on hornblende obtained from the Bay of Islands metamorphic sole that is attributed to the obduction of the Bay of Islands complex (Dallmeyer and Williams, 1975). There is, therefore, at least a 7 m.y. time gap (and up to 25 m.y.) between the subduction that led to formation of the mid portion of the Bay of Islands metamorphic sole and the timing of obduction of the Bay of Islands complex. Because the SSZ Bay of Islands ophiolitic crust is a result of the same subduction system, its igneous age (488.3 ± 1.5 Ma; Yan and Casey, 2020) provides a minimum age for subduction initiation. Based on the age of the Coastal complex, which

is inferred to have resulted from the same subduction zone (Cawood and Suhr, 1992; Yan and Casey, 2020; Yan and Casey, 2022), subduction initiation could even be older than ca. 514 Ma. Alternatively, the Coastal complex could have been part of an arc that predates the subduction zone whose initiation was recorded by the Bay of Islands metamorphic sole. This alternative could be tested by determining the timing of peak metamorphism of the uppermost part of the sole. Not only would this provide a test for the presence or absence of a pre-existing arc, but also for the duration of metamorphic sole accretion and associated subduction. Using the age of the Bay of Islands ophiolite as a minimum age for subduction initiation indicates that the subduction zone responsible for the formation of the Bay of Islands complex lasted for at least ~13 m.y. Given an average present-day subduction rate of ~5 cm/yr and the conservative 13 m.y. period of continuous oceanic convergence, the duration of the subduction implies that the subduction initiation occurred at least 650 km away from the site of obduction, which requires a large oceanic basin relatively distal from the Laurentian margin to accommodate the subduction of oceanic lithosphere. This oceanic basin could have had a significant width if the Laurentian margin were hyperextended over large segments (Chew and van Staal, 2014) and contained allochthonous peri-continental terranes such as Dashwoods (van Staal et al., 2013). A more recent analogous setting is present between the Atlantic margins of Newfoundland, Iberia, and Ireland (Jagoutz et al., 2007; Péron-Pinvidic and Manatschal, 2009).

## CONCLUSION

This study provides new geochemical and geochronological constraints on the archetypal Bay of Islands metamorphic sole, which allow us to better characterize the nature of its protolith and the timing of its metamorphism. Major and trace element geochemistry of basaltic rock is consistent with an N-MORB affinity, indicating that the metabasites of the North Arm Mountain and Table Mountain soles are derived from cumulate and basaltic protoliths that formed through shallow melting of a depleted mantle source, most likely at a mid-oceanic spreading ridge. Titanite U-Pb dates from two common amphibolite samples define a maximum metamorphic age and a minimum protolith age for both the common amphibolite and metasedimentary units at ca. 485 Ma. Detrital zircon U-Pb data from a metasedimentary rock show dates distributed from the Archean to the Ordovician, with the younger population forming the most dominant peak at ca. 490 Ma, which
is interpreted as the maximum depositional age. The sedimentary record yields an apparent SSZ-derived source attributed to arc magmatism during the Taconic orogenic cycle. We propose that the subduction responsible for the formation of the Bay of Islands complex nucleated in the oceanic domain possibly at an oceanic detachment fault before ca. 485 Ma. The presence of a significant volume of Cambrian to Early Ordovician arc-derived zircon in the metasedimentary unit indicates that the latter represent trench-fill sediments.

## ACKNOWLEDGMENTS

This project was financed by the research grant CG NSERC-DG RGPIN 2020-06400 and by a master's research scholarship (B1X) from the Fonds de recherche du Québec Nature and technologies (FRQNT). M. Button and S. Shrestha are thanked for their contributions to titanite and zircon geochronology in the Fikep Laboratory for Trace Element Research. We also thank Parks Canada staff at Gros Morne National Park, John Wakabayashi and Rebecca A. Jamieson for their constructive comments that significantly improved the manuscript, and John Waldron for his suggestions and editorial handling. Jean H. Bédard and Cees van Staal acknowledge the contribution of Natural Resources Canada (#20220568).

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