Evolution of Subduction Dynamics beneath West Avalonia in Middle to Late Ordovician Times

Middle to Upper Ordovician volcanic rocks in the Arisaig area of Nova Scotia, Canada, constitute the only known record of volcanism in West Avalonia during that interval. Hence, they have been extensively studied to test paleocontinental reconstructions that consistently show Avalonia as a drifting microcontinent during that period. Identification of volcanic rocks with an intermediate composition (the new Seaspray Cove Formation) between upper Darriwilian bimodal volcanic rocks of the Dunn Point Formation and Sandbian felsic pyroclastic rocks of the McGillivray Brook Formation has led to a reevaluation of magmatic relationships in the Ordovician volcanic suite at Arisaig. Although part of the same volcanic construction, the three formations are separated by significant time-gaps and are shown to belong to three distinct magmatic subsystems. The tectonostratigraphic context and trace element contents of the Dunn Point Formation basalts suggest that they were produced by the high-degree partial melting of an E-MORB type source in a back-arc extensional setting, whereas trace element contents in intermediate rocks of the Seaspray Cove Formation suggest that they were produced by the low-degree partial melting of a subduction-enriched source in an arc setting. The two formations are separated by a long interval of volcanic quiescence and deep weathering, during which time the back-arc region evolved from extension to shortening and was eventually onlapped by arc volcanic rocks. Based on limited field constraints, paleomagnetic and paleontological data, this progradation of arc onto back-arc volcanic rocks occurred from the north, where an increasingly young Iapetan oceanic plate was being subducted at an increasingly shallow angle. Partial subduction of the Iapetan oceanic ridge is thought to have subsequently generated slab window magmatism, thus marking the last pulse of subduction-related volcanism in both East and West Avalonia.


Introduction
Paleozoic continental and plate reconstructions indicate that the composite microcontinent of Avalonia rifted away from Gondwana in the Early Ordovician, opening the Rheic Ocean, and drifted northward towards Laurentia during the rest of the Ordovician, gradually closing the Iapetus Ocean [1][2][3][4][5][6][7]. As such, its evolution constrains events leading to the amalgamation of Pangea (e.g., [8]). The area of Arisaig, Nova Scotia, Canada (Figure 1), includes the only known succession of Middle to Upper Ordovician volcanic rocks in West Avalonia (i.e., the North American portion of Avalo-nia) and has been extensively studied to shed light on the tectonic and paleogeographic history of the terrane during this pivotal time interval (e.g., [9][10][11][12][13][14][15]). Until recently, only mafic and felsic volcanic rocks were known from that locality and were interpreted as the bimodal products of back-arc extension [13][14][15]. This paper describes the Seaspray Cove Formation, a newly identified  [23,68]), as well as the location of the study area within Avalonia. (b) Generalized geology of the study area with localities of the sampled sections (green rectangles).
Formation, as well as on parallels with coeval units of correlative terranes in the British Isles, this paper proposes an integrated geodynamic model for the evolution of Middle to Late Ordovician subduction-related volcanism in West Avalonia.

Geological Setting
Basement rock exposures near the study area are characterized by Neoproterozoic arc-related volcanic and sedimentary rocks (Georgeville Group) truncated by Ediacaran plutonic rocks [15,16], which are unconformably overlain by a Cambrian to Lower Ordovician succession of sedimentary rocks that contain fauna diagnostic of Avalonia (e.g., [17,18]). Following an Early Ordovician episode of compressive deformation [11,16] and subsequent rifting from Gondwana [3], Middle to Upper Ordovician volcanic rocks of the Dunn Point and McGillivray Brook Formations [19,20] were emplaced on the drifting microcontinent of Avalonia [6,14] (Figure 2). Based on U-Pb isotopes from primary zircons, Hamilton and Murphy [9] obtained a 460:0 ± 3:4 Ma age (upper Darriwilian/Llanvirnian) for rhyolite of the Dunn Point Formation, and Murphy et al. [14] subsequently dated the top ignimbrite of the overlying McGillivray Brook Formation at 454:5 ± 0:7 Ma (Sandbian/Caradoc) with the same method, providing an upper limit to the age of the volcanic succession at Arisaig (Figure 2). The newly identified Seaspray Cove Formation is undated, but stratigraphically positioned between these two units, and therefore constrained between~460 and~454.5 Ma (Figure 2).
Paleomagnetic data [10] adapted to subsequent geochronological data place the volcanic rocks of Arisaig at a paleolatitude of 41°± 5°south at~460 Ma [9]. Paleogeographic reconstructions for that time interval show the Avalonian and Ganderian microcontinents drifting northward on the same microplate, possibly separated by a narrow seaway, with the Rheic Ocean separating them from Gondwana to the south, and the Iapetus Ocean separating them from Laurentia to the north [1,6,8]. This drift is consistent with the paleolatitude of 32°± 8°south determined by Hodych and Buchan [21] for West Avalonia at~440 Ma (Early Silurian) based on paleomagnetic data from the Cape St. Mary's sills (U-Pb baddeleyite age of 441 + 2 Ma; [22]) on the Avalon Peninsula of Newfoundland (Figure 1(a)). Based on the Ordovician geology of Avalonian and Ganderian sequences in Ireland, Great Britain and eastern North America, this northward migration was accommodated by subduction of the Iapetan oceanic lithosphere to the north beneath Laurentia, and to the south beneath Avalonia and Ganderia ( [5,7]; van Staal et al. 1998, [8, 15, 23]).

Previous Work on the Ordovician Volcanic Successions at Arisaig
Between the localities of Arisaig Pier and Frenchman's Barn (a monadnock of resistant rhyolite that is reinforced by quartz veinlets; Figure 1(b)), the Seaspray Cove Formation is absent, and the Dunn Point Formation is directly overlain by the McGillivray Brook Formation (Figure 3).

The Dunn Point Formation and Intrusive Equivalents.
The Dunn Point Formation is a succession of mafic flows separated by weathering profiles and topped by a thick rhyolite flow, which also hosts a thick paleosol [19,[24][25][26] (Figures 1(b [69]) based on Hamilton and Murphy [9], Murphy et al. [14,16], and this study.
New Zealand, with mafic magmatism resulting from decompression melting of the underlying mantle.
The~70 m thick overlying flow-banded rhyolite has a composition analogous to A-type, within-plate granites and is interpreted as a product of crustal anatexis generated by heat derived from the associated mafic melt [14]. Based on Sm-Nd isotopic data, these felsic rocks were sourced from Avalonian lower crust [15].
Bimodal plutons with correlative geochemical signatures are found in the Antigonish Highlands (Figure 1(b)), less than 50 km south of the Dunn Point Formation exposures [27]. This plutonic suite records a longer history of withinplate bimodal magmatism dating back to the Early Ordovician rifting of Avalonia from Gondwana. Based on Sm-Nd isotopic data, the intrusive felsic rocks were also sourced from Avalonian lower crust [15].
As noted earlier, the upper part of the Dunn Point Formation rhyolite is extensively weathered [26]. Exposed sections of this intra-rhyolitic paleosol are up to 8.5 m thick, but based on strain calculations (sensu [28]), this probably represents~30% of its original thickness prior to burial compaction [24]. This thick paleosol is the record of a long period of magmatic inactivity that followed the massive eruption of rhyolite.

The McGillivray Brook Formation.
Disconformably overlying the Dunn Point Formation to the southwest of Frenchman's Barn (Figure 1(b)), the McGillivray Brook Formation is mainly characterized by pyroclastic and volcaniclastic deposits. Its base shows discontinuous lenses of lahar deposits, which locally truncate the thick paleosol that tops the Dunn Point Formation rhyolite (Figures 1(b) and 3). These basal lenses of lahar deposits, as well as the knobs of weathered rhyolite that laterally separate them, are directly overlain by a succession of felsic lapilli tuff that transitions upward into felsic ignimbrite with an A-type composition [14,19]. Based on Sm-Nd isotopic data, this felsic pyroclastic succession was sourced from Avalonian lower crust, but from presumably drier melting and at much higher temperature (1050°C) than the Dunn Point Formation rhyolite (860 to 875°C) according to zircon saturation thermometry estimates [15]. As zircon dissolves between 750 and 850°C under pressures typical of the lower crust [29], the high temperature estimates for crustal melting may also explain its much higher content in high-field-strength elements compared to rhyolites of the Dunn Point Formation.

The New Seaspray Cove Formation
An incomplete succession of intermediate pyroclastic breccia and lava flow units (classified below as trachyandesitic) pinches in-between the localities of Frenchman's Barn and Seaspray Cove (informal toponym; Figure 1(b)), disconformably above the Dunn Point Formation, and conformably below the McGillivray Brook Formation (Figure 3   Lithosphere to the southwest of Seaspray Lane in the municipality of Arisaig ( Figure 1(b)).
On the northeast flank of Frenchman's Barn (Figure 1(b)), the pyroclastic breccia is absent, and an up tõ 4.5 m thick weathered lava flow unit of the Seaspray Cove Formation disconformably truncates the weathered rhyolite and is conformably overlain by the lowermost tuff of the McGillivray Brook Formation (Figures 3 and 4). Based on evidence of mixing between weathered rhyolite and trachyandesitic material at the base of the massive, intermediate flow [26], the latter partly bulldozed and incorporated weathered rhyolitic material during emplacement. According to these authors, the truncated rhyolite paleosol at this locality is considerably less well-developed than the southwest of Frenchman's Barn, where that paleosol is directly overlain by tuff of the McGillivray Brook Formation, with no intervening intermediate rocks of the Seaspray Cove Formation (Figure 3).
To the northeast, along Seaspray Cove (Figure 1(b)), the trachyandesite lava flow unit is at least 15 m thick (weathering and shearing in its upper part altered its original thickness) and is poorly weathered in its basal~11.5 m, but thoroughly weathered above that level. Postemplacement thrust faulting was concentrated in this weak upper interval of deeply weathered material, which resulted in local duplications of the more competent lower interval ( Figure 5). At the Seaspray Cove locality, the trachyandesitic lava flow unit lies concordantly above dark red pyroclastic breccia of the same formation, and concordantly below the McGillivray Brook Formation felsic tuff and ignimbrite (Figures 3 and 5). The base of the breccia is not exposed, but it is at least 20 m thick based on available exposure. The thickness and nature of strata that separate this incompletely exposed succession from the underlying Dunn Point Formation are unknown, but as it pinches-out at a short distance to the southwest, it is inferred that the breccia directly overlies the weathered rhyolite ( Figure 3).
The newly identified trachyandesitic flow unit was mistakenly assigned by previous authors (e.g., [13,14,19]) to the basaltic succession of the underlying Dunn Point Formation, which is in fault contact with it at the Seaspray Cove locality ( Figure 5). Both units look similar in outcrop, although the Dunn Point Formation basalts are darker. Based on field observations, the pyroclastic breccia that underlies the trachyandesite was previously confused for a lahar deposit [13,19,26]. However, this unit is considerably darker, denser, less matrix-rich, and much richer in mobile elements than lahar deposits that truncate the intrarhyolitic paleosol in other areas.

Analytical Methods
Two samples were obtained from pyroclastic breccia of the Seaspray Cove Formation, and 10 samples were retrieved 5 Lithosphere from the overlying lava flow unit at irregular intervals at Seaspray Cove (B1-2 and Q1-10 on Figure 5). Sampling was performed at shorter intervals in the upper part of the profile in order to precisely determine the downward extent of subsurface Ordovician weathering and to assess element mobility during weathering. Three additional samples were obtained at regular intervals from the~4.5 m thick trachyandesite at the Frenchman's Barn northeast section, two of which were included in Jutras et al. [26]. Thin-sections were made for all samples, and part of each was powdered for mineralogical and geochemical analyses. Results of X-ray diffraction (XRD) analyses are shown in Table 1; X-ray fluorescence (XRF) data for major and selected trace elements are shown in Table 2; and inductively coupled plasma mass spectrometry (ICP-MS) data for a broader list of trace elements are shown in Table 3. Tables 1-3 also include data from Keppie et al. [12], Murphy et al. [14], and Jutras et al. [25,26] on the least altered and least contaminated samples (based on high Na 2 O and low LOI contents) of basalts and rhyolite of the Dunn Point Formation, as well as from lahar, tuff and ignimbrite deposits of the McGillivray Brook Formation.
All lava flow unit samples of the Seaspray Cove type-section, except the thoroughly weathered Q10 (plagioclase contents fully altered to secondary minerals; Table 1), were analyzed for their total carbon, chlorine and fluorine contents, as determined by combustion analysis, instrumental neutron activation analysis (INAA) and fusion/ion-selective electrode (Fus-ISE), respectively, (Table 4). Furthermore, element mapping by X-ray fluorescence was performed on sample B2 of the pyroclastic breccia with a Bruker M4 Tornado micro-XRF housed at the University of New Brunswick in Fredericton, Canada ( Figure 6). Twenty points from a nonoxidized portion of the matrix were analyzed for major element contents with an electron microprobe (Data Repository File #1), and for trace element contents with a laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) (Data Repository File #2), also housed at the University of New Brunswick.              Jutras et al. [ 9 Lithosphere massive, aphyric to porphyritic, with pseudomorphs of plagioclase altered to calcite and quartz (Figure 7(a)). Phenocrysts occur in a pilotaxitic to mildly trachytic groundmass composed predominantly of quartz, chlorite, and albite. Calcite and quartz amygdules are increasingly abundant going up the profile (Figures 7(b) and 7(c)).
Evidence of moderate paleo-weathering is observed in the~8-11.5 m interval (samples Q5-9) and is concentrated in vein-like areas characterized by sericitization and hematization (Figure 7(d)). The uppermost~0.3 m of the in situ profile at the Seaspray Cove section (sample Q10) is thoroughly weathered, with no volcanic textures preserved. It is overlain by at least 4 m of sheared paleosol material below the basal tuff of the McGillivray Brook Formation.
6.2. Syn-and Postemplacement Hydrothermal Alteration versus Weathering and Burial Diagenesis. The hydrothermally altered and subsequently weathered trachyandesite profile at Seaspray Cove is composed of albite, orthoclase, quartz, clinochlore, muscovite, carbonates, and hematite, as well as some pyrophyllite near the contact with the overlying tuff (Table 1). Whereas quartz may be produced at all stages of eodiagenesis, we consider albite, clinochlore, and carbonates, which decrease in modal abundance up profile 10 Lithosphere (Figure 8), as the exclusive products of syn-and postemplacement alteration processes that took place in a hotter environment than normal surface temperatures. In contrast, muscovite and hematite modal abundances increase up profile (Figure 8), and these increases are interpreted to be directly proportional to the degree of subsequent weathering, with muscovite contents (as well as pyrophyllite near the contact with tuff) reflecting the original pedogenic clay contents prior to burial. Hence, orthoclase (a minor component of the unweathered rock) and some of the quartz are interpreted as the only unmodified remnants of igneous minerals. Preferential preservation of orthoclase in the weathering profile is consistent with the alkaline weathering conditions that are suggested by the geochemistry of paleosols in the rest of the Ordovician succession at Arisaig [24][25][26]. Although thoroughly altered during emplacement and cooling, sample Q1 from the base of the profile is the sample in which postemplacement weathering is least significant, as it is characterized by the highest feldspar (~51%) and clinochlore (~17%) contents, and as it is mostly devoid of muscovite (~1%) and hematite (below detection limit) (  (Figure 4), the upper part of the profile (A horizon) is characterized by the near absence of albite and carbonates (and associated Na 2 O and CaO), which indicates thorough weathering in warm and humid conditions. In these samples, modal quartz contents are as low as~32-45%, but micas and hematite modal contents are as high as~28-31% and~21-28%, respectively, (Tables 1 and 2; Figure 8). Because of its quartz-poor and mica-rich mineralogy, this petrified soil horizon is structurally weak, which rendered it susceptible to subsequent faulting ( Figure 5).
6.3. Classification. As a consequence of pervasive eodiagenetic overprinting, any classification using elements that are typically mobile in hydrothermal alteration or weathering processes is not reliable for any of the samples. Relatively stable ratios between typically immobile high field strength elements (Ti, Zr, Nb, and Y) in the entire profile suggest that these elements were not significantly affected by these eodiagenetic processes (Tables 2 and 3). Based on its Ti/Zr and Nb/Y ratios (sensu [30]), the rock best classifies as a trachyandesite ( Figure 9).

Halogen Contents.
With 0.05-0.10 wt.% F and 0.04-0.06 wt.% Cl (Table 4), halogen contents in the trachyandesitic lava flow unit of the Seaspray Cove Formation are high relative to the mantle [31]. In comparison, an average enriched mantle source only has 0.0025% F and 0.0017% Cl [31]. Because halogens can only be leached during hydrothermal alteration and humid climate weathering, and never enriched, their concentrations in the samples are interpreted as remnants of their original contents in the melt.

Petrology of the Trachyandesitic Pyroclastic Breccia Unit
The basal pyroclastic breccia of the Seaspray Cove Formation is mostly composed of reddish-grey, pebble-sized clasts with alteration rims within a dark red, gritty matrix. Its original mineralogy was thoroughly altered to quartz (67-70%), albite (8-14%), and clinochlore (3-4%), with further weathering to hematite (7-8%, presumably from the oxic alteration of clinochlore) and muscovite (4-5%, presumably from the diagenetic transformation of clay minerals) ( Table 1). Compared with lahar deposits of the same succession, which are products of the sedimentary reworking of more thoroughly weathered material [26], the pyroclastic breccia is characterized by significantly lower modal muscovite and Al 2 O 3 contents, and by much higher Na 2 O contents (Tables 1 and 2). However, because of its brecciated fabric, moderate weathering is distributed through most of the material, which obscures the nature of the original melt composition. Some small areas of the breccia's matrix are not oxidized and exhibit well-preserved flow textures (Figure 6(a)). They are also mostly devoid of clasts. However, these nonoxidized areas of the matrix show evidence of Fe-leaching, which seemingly concentrated in the gritty and oxidized surrounding areas (Figure 6(b)) that partly truncate the grey matrix 11 Lithosphere (Figure 6(a)). In contrast, the small remnants of grey matrix are slightly richer in Si than the red matrix that forms the bulk of the deposit (Figure 6(c)).
Although similar in SiO 2 concentration to dacite or rhyolite (Table 2), the igneous matrix of the pyroclastic breccia plots in the uppermost range of trachyandesite based on its Zr/Ti and Nb/Y ratios ( Figure 9). As its trace element distribution is similar to that of the overlying trachyandesite flow, but in significantly lower concentrations (Figure 10(a)), we conclude that the original magma may have fractionated to a trachyandesitic composition and that SiO 2 enrichment occurred near the top of the magma chamber prior to the eruption, thus diluting the trace element contents without significantly affecting their ratios.  Lithosphere The inferred Fe-leaching ( Figure 6(b)) implies that early alteration processes occurred in reducing conditions and that oxidation occurred subsequently via postemplacement weathering.

Zircon Saturation Thermometry
The classic zircon saturation thermometry model developed by Watson and Harrison [32] [17,34]), or to the Ganderian and Megumian domains (based on lithological similarities and provenance studies; [35,36]). However, based on similarities in their Ordovician geology, it is generally agreed that these three terranes, along with the Avalonian Wrekin and Charnwood terranes of southern England, were part of the same drifting Ordovician microcontinental assemblage ( [36], and references therein), which also included the Avalonian terranes of North America, and which we herein refer to as Avalonia (sensu [3,7,34,37]). Hence, the Ordovician successions in the Avalon Zone of North America and in all peri-Gondwanan terranes of the British Isles can be tentatively evaluated in the context of a unified tectonic model for that period.
In the Cymru Terrane and the southern part of the Leinster-Lakesman Terrane, Early Ordovician andesitic arc volcanism is succeeded by Middle Ordovician bimodal back-arc volcanism (Fishguard Volcanic Group and equivalent units), which is interpreted to be the result of slab rollback, seaward migration of the arc, and crustal stretching in the back-arc region [7,38]. Near the Middle to Late Ordovician boundary, arc volcanism seemingly migrated back landward and is well recorded in the Duncannon and Borrowdale Volcanic groups of the northern part of the Leinster-Lakesman Terrane [39,40]. The last volcanic pulse of this subduction zone came at~454 Ma in the Snowdon Volcanic Group of North Wales, but it is geochemically unrelated to volcanism above a hydrated mantle wedge, and it is interpreted to have occurred when the Iapetan midoceanic ridge partly subducted beneath the arc and opened a slab window in the proximal back-arc region, which was then fed with dry asthenospheric melts ( [7], and references therein).

Petrogenetic Interpretation of Middle to Late Ordovician
Igneous Rocks in West Avalonia 9.2.1. Mafic Melts. Based on its higher Nb/Y ratios ( Figure 9) and a steeper distribution of rare earth elements (Figure 10(b)), the Seaspray Cove Formation is interpreted as the product of a lower degree of partial melting than the Dunn Point Formation basalts. Ratios of weathering-resistant,  13 Lithosphere high-field-strength elements (HFSEs) in basalts of the Dunn Point Formation are similar to those of enriched midoceanic ridge basalts (E-MORB; sensu [41,42]; Figures 10(a) and 11), suggesting that they were sourced from depleted upper mantle material enriched over the average (E-DM, sensu [43]) and that they were not significantly affected by crustal contamination. Based on the Sm-Nd isotopic characteristics and model ages of these basalts, enrichment of this E-DM source occurred between 1.1 and 0.8 Ga, prior to the oldest rifting event in Avalonia [13,44]. Melting is interpreted to have occurred at high temperatures through asthenospheric upwelling in a back-arc setting [14,15] (Figure 12(a)).
In contrast, rocks of the Seaspray Cove Formation show characteristics that are more typical of arc volcanism, such as an intermediate composition, high Cl and F contents (Table 4), relatively pronounced negative Ta-Nb and Ti anomalies (Figure 10(a)), and Th/Yb and Nb/Yb ratios that plot above the mantle array (sensu [42]; Figure 11). Moreover, an estimated zircon saturation temperature of~786°C indicates low-temperature hydrous melting at the source [45], which is also consistent with an arc setting. In contrast, higher zircon saturation temperature estimates for the more felsic Dunn Point Formation rhyolite (872-887°C) are consistent with high-temperature anhydrous melting in a back-arc setting [15] (Figure 12(b)).
Based on experimental data from Cruz-Uribe et al. [46], alkaline magma with an arc signature can be produced by the partial melting of high-pressure mélanges that occur along the slab-mantle interface, in which subducted altered oceanic crust and sediments are mixed with hydrated mantle-wedge material. Such mélanges are transported into the hot corner of the mantle wedge beneath arcs by lowdensity mantle-wedge diapirs [47], where their partial melting can feed the arc volcano with alkaline magma [48] ( Figure 12(c)).

Evolution towards a Middle to Late Ordovician HFSE-Enriched Intermediate Melt.
Although some enrichment in incompatible elements may have occurred due to a low degree of partial melting at the primary source, pronounced negative Eu and Ti anomalies in trachyandesite of the Seaspray Cove Formation (Figure 10(a)) suggest that it was gradually depleted in plagioclase and magnetite through crystal fractionation in the upper part of a magma chamber (Figure 12(c)), which would have favoured further enrichment in incompatible elements. Development of significantly high contents in HFSEs may have been favoured by a high concentration of halogens (Table 4), which tends to be an inherent characteristic of arc volcanism [49], and which enhances the incompatibility of HFSEs by favouring their incorporation into high-order soluble complexes [50][51][52][53][54][55].

Development of a Late Ordovician HFSE-Enriched Felsic Melt. Although the Seaspray Cove and McGillivray
Brook Formations are both strongly enriched in incompatible trace elements, they differ greatly in terms of trace element distribution (Figure 10(b)). As the Seaspray Cove Formation trachyandesite (samples Q1-4) and the McGillivray Brook Formation felsic ignimbrite (samples BL08-2 and BL08-3 from [14]) bear significantly different Th/Hf ratios (0.536-0.603 vs 1.139-1.143; Table 3), the latter is unlikely to be a fractionation product of the former, in accordance with Schiano et al. [56]. Furthermore, the very high melting temperature that is inferred at the source of the McGillivray Brook Formation ignimbrite (~1100°C, based on the model of [33]) implies anhydrous conditions that are incompatible with arc volcanism above a hydrated mantle wedge [15]. However, rocks of such composition can develop in ensialic arc systems in association with the localized development of tensional tectonics [57,58]. Hence, felsic pyroclastic deposits of the McGillivray Brook Formation are interpreted to represent a different magmatic pulse than intermediate volcanic rocks of the Seaspray Cove Formation, from which it is separated by a prolonged period of weathering.   (Table 1). Albite, orthoclase, clinochlore, and carbonates are considered to be alteration minerals that formed at relatively high temperatures during emplacement and cooling, whereas muscovite and hematite are assumed to have mostly formed during subsequent weathering. Quartz is interpreted to have formed in both environments, but it is assumed that the preweathering quartz contents would have been close to that in the lowermost part of the flow (sample Q1: 24%) and that quartz contents in excess of that would have mostly formed during weathering.
14 Lithosphere The high trace element contents of the McGillivray Brook Formation ignimbrite may in part be due to a melting temperature that exceeds that of accessory phases in which these elements are concentrated. However, very well developed negative Eu and Ti anomalies (Figure 10(b)) suggest prolonged fractional crystallization in a magma chamber prior to the eruptions. The rocks also show a marked depletion in LREE (Figure 10(b)), which can be attributed to the fractionation of accessory phases such as allanite, monazite or fergusonite in a silicic magma (e.g. [59]).

Tectonostratigraphic Setting
Following an Early Ordovician folding episode that was shortly followed by the drifting of Avalonia from Gondwana, arc volcanism is interpreted to have moved outboard in association with steep subduction and slab rollback, and back-arc volcanism developed in currently exposed terranes of both East and West Avalonia in Middle Ordovician times [6,7,11,14,38,60] (Figures 12(a) and 13(a)). In East Avalonia, Darriwilian back-arc volcanism is recorded in the bimodal Fishguard Volcanic Group and equivalent units, whereas in West Avalonia, it is recorded in a thick succession of marginally subalkaline within-plate basalts at the base of the Dunn Point Formation (Figures 12(a) and 13(a)), and by the subsequent deposition of thick rhyolite produced by crustal anatexis (Figures 12(b) and 13(b)). The latter records the end of back-arc extension at 460 Ma. Associated back-arc plutonic activity in the nearby Antigonish Highlands (Figure 1(b)) is also inferred to have stopped by~460 Ma [27]. A prolonged period of volcanic quiescence ensued in the West Avalonian region, as indicated by the development of a thick weathering profile in the upper part of the Dunn Point Formation rhyolite [26] and by the 1.4 to 9.6 My (i.e., 5.5 My) time gap that separates the rhyolite from felsic ignimbrite of the overlying McGillivray Brook Formation based on U-Pb dates from primary zircons [14]. It is within this hiatus that volcanic rocks with an arc-type composition (the new Seaspray Cove Formation) onlapped part of the previously deposited back-arc volcanic succession (Figures 12(c) and 13(c)), pinching-out to the southwest (Figure 3). A similar inboard migration of arc volcanism is recorded in East Avalonia near the Middle to Late Ordovician boundary, juxtaposing arc volcanic rocks of the Duncannon and Borrowdale Volcanic groups with older back-arc volcanic rocks [7,39,40]. Because of a lower degree of partial melting at the source and presumably higher halogen contents during crystal fractionation, incompatible HFSE contents are overall greater in intermediate rocks of the Seaspray Cove Formation than in rhyolite of the underlying Dunn Point Formation, despite the latter being significantly more felsic (Table 2; Figure 10).
The lack of a well-defined weathering profile separating the trachyandesitic pyroclastic breccia from the overlying lava flow unit of the Seaspray Cove Formation indicates that these two eruptions were closely spaced in time. The pyroclastic breccia is therefore interpreted as the viscous extrusion of material from the slightly more felsic top of the  As noted earlier, the geochemistry of the McGillivray Brook Formation felsic ignimbrite is incompatible with that of the Seaspray Formation and incompatible with arc volcanism above a hydrated mantle wedge. However, the ultrahigh temperature melting of anhydrous crust that sourced these rocks [15] is compatible with slab window magmatism, As the shutdown of south-directed subduction beneath Avalonia is roughly coeval with the collision between Ganderia's Popelogan arc and Laurentia farther west along the same subduction zone (~455 Ma according to [56]) ( Figure 13(d)), it can be inferred that slab pull of the remaining ocean plate was by then reduced by this obstruction, which could have prevented the Iapetan oceanic ridge from being fully subducted beneath Avalonia.

Conclusions
The newly identified trachyandesitic succession of the Seaspray Formation at Arisaig and its stratigraphic relationship with bounding volcanic units of the Dunn Point and McGillivray Brook Formations bring a new perspective on the evolution of the Middle to Late Ordovician magmatic system in West Avalonia, which bears many parallels with the coeval system in East Avalonia. During that time interval, the convergent zone is interpreted to have evolved from steep subduction, rollback, and back-arc extension, to shallower subduction and back-arc shortening ( Figure 13). As a result, Upper Ordovician arc volcanic rocks were juxtaposed with Middle Ordovician back-arc volcanic rocks in East Avalonia [39,40], and a small outlier of Upper Ordovician arc volcanic rocks (the Seaspray Cove Formation) in West Avalonia eventually onlapped part of the previously deposited Middle Ordovician bimodal back-arc volcanic succession of the Dunn Point Formation (Figure 3). Based on paleocontinental reconstructions showing southward subduction of the Iapetan oceanic lithosphere beneath Avalonia ( [1,5,7], and references therein), the onlap probably occurred from the north. This interpretation is somewhat consistent with limited field constraints, which suggest that arc volcanic rocks of the Seaspray Cove Formation pinch-out on back-arc volcanic rocks of the Dunn Point Formation towards the southwest (present-day coordinates, Figures 1(b) and 3). Also noteworthy is the nearly synchronous shutdown of Iapetan subduction in both East and West Avalonia in the late Sandbian (mid-Caradoc) with no significant deformation associated with it (this study and [7]). In East Avalonia, Woodcock [7] attributes the shutdown to an incomplete overriding of the Iapetan oceanic ridge. This hypothesis is consistent with the inferred transition from steep to shallow subduction in Middle to Late Ordovician times beneath West Avalonia, similar to the subduction history of the Nazca Plate [63], which suggests that increasingly young and buoyant oceanic crust was being subducted, and therefore that anoceanic ridge may have been gradually approaching the trench (Figures 12 and 13).
In East Avalonia, the last pulse of volcanism associated with the subduction of the Iapetan Ocean plate occurred at 454.4 Ma and is interpreted as the product of slab window magmatism [7] (Figure 12(d)). Such setting is consistent with the geochemistry of the~454.5 Ma felsic ignimbrite of the Vector of enrichments through magma mixing, variable degree of partial melting, or crystal fractionation Vector of enrichments through subduction or crustal contamination EMORB NMORB OIB Figure 11: Mantle array and vector of subduction and crustal contamination versus that of within-plate, magma mixing, or crystal fractionation enrichments based on Th/Yb and Nb/Yb ratios (after [42]). Lithosphere McGillivray Brook Formation, which marked the end of subduction-related volcanism in West Avalonia, and which was sourced from the ultrahigh temperature melting (~1100°C) of anhydrous crust. Such elevated temperatures reaching the base of the crust would be best accounted for by asthenospheric upwelling associated with the development of a slab window (Figure 12(d)).
The apparent shutdown of subduction beneath both West and East Avalonia in Late Ordovician times suggests that accretion of Avalonia to composite Laurentia during  Figure 12: Middle to Late Ordovician tectonic model for West Avalonia, with inferences from the models of (a-b) Murphy et al. [13,14], (c) Cruz-Uribe et al. [46], and (d) Woodcock [7]. Lithosphere the Devonian (e.g., [6,60,64]) occurred through a different subduction zone. This is consistent with the record of Silurian subduction towards the northwest (present-day coordinates) beneath composite Laurentia in Maine [65], which may have gradually consumed the ocean plate remnant that separated the latter from Avalonia at the time (the "Acadian Seaway" of [66]). Intermittent within-plate magmatic activity occurred in Avalonia during this period, but with no association to subduction [67].

Data Availability
All data are included in the manuscript and in two items for the Data Repository.

Conflicts of Interest
The authors declare that they have no conflicts of interest.