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.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 [1–7]. 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 Avalonia) and has been extensively studied to shed light on the tectonic and paleogeographic history of the terrane during this pivotal time interval (e.g., [9–15]). Until recently, only mafic and felsic volcanic rocks were known from that locality and were interpreted as the bimodal products of back-arc extension [13–15]. This paper describes the Seaspray Cove Formation, a newly identified >35 m thick succession of volcanic rocks with an intermediate chemical composition within the Ordovician succession at Arisaig, and discusses the implications of its geochemistry. Based on comparisons with bounding volcanic units of the Middle Ordovician Dunn Point Formation and Upper Ordovician McGillivray Brook 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.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]).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 is a succession of mafic flows separated by weathering profiles and topped by a thick rhyolite flow, which also hosts a thick paleosol [19, 24–26] (Figures 1(b)–1(c) and 3). Keppie et al. [20] attributed the Dunn Point Formation basalts to a within-plate, continental rifting event, but based on inferences made with the Ordovician geology of East Avalonia, Murphy et al. [13, 14] more specifically associated the volcanism to ensialic back-arc spreading analogous to the Lau-Havre-Taupo system of 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 within-plate 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.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.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). Because these lithologies are not included in the definition of the Dunn Point and McGillivray Brook Formations, they are herein formalized as the new Seaspray Cove Formation. The type-area is on the Northumberland Straight shoreline, 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 to ~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.Two samples were obtained from pyroclastic breccia of the Seaspray Cove Formation, and 10 samples were retrieved 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 Na2O 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.In its basal ~8 m at Seaspray Cove (samples Q1-4), the newly identified intermediate lava flow unit is 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.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 (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 [24–26]. 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) (Table 1). In terms of major element contents, Q1 mainly differs from other samples by its very low K2O contents (0.4 wt.%). Jutras et al. [24–26] reported substantial K-enrichment in paleosols of the volcanic succession at Arisaig during shallow burial, which is consistent with K2O contents that increase up profile (Q1-10; Tables 1 and 2) along with modal abundances of weathering minerals in this intermediate rock. Hence, K2O contents other than those found in orthoclase may reflect the abundance of preburial smectite contents in the profile, which later converted to muscovite and pyrophyllite.Based on samples S3-7 and S3-8 of Jutras et al. [26] from the preserved uppermost ~2 m of weathered trachyandesite at the Frenchman’s Barn northeast section (Figure 4), the upper part of the profile (A horizon) is characterized by the near absence of albite and carbonates (and associated Na2O 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).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).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.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 Al2O3 contents, and by much higher Na2O 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 (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 SiO2 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 SiO2 enrichment occurred near the top of the magma chamber prior to the eruption, thus diluting the trace element contents without significantly affecting their ratios. The inferred Fe-leaching (Figure 6(b)) implies that early alteration processes occurred in reducing conditions and that oxidation occurred subsequently via postemplacement weathering.The classic zircon saturation thermometry model developed by Watson and Harrison [32] was calibrated in peraluminous to metaluminous felsic melt compositions. To determine zircon saturation temperatures for a wider range of rock compositions, Gervasoni et al. [33] developed a model using the new bulk compositional parameter G 3×Al2O3+SiO2/Na2O+K2O+CaO+MgO+FeO against temperature and Zr concentrations, which can also be applied to intermediate and alkaline rocks. Based on data from Table 2, and in accordance with the model of Gervasoni et al. [33], the range of zircon saturation temperatures is 872-887°C in the Dunn Point Formation rhyolite, 765-786°C in the Seaspray Cove Formation trachyandesite (samples Q1-4), and 1065-1103°C in the McGillivray Brook Formation felsic ignimbrite. These results suggest that zircon saturation temperatures determined for the alkaline felsic rocks of the Dunn Point and McGillivray Brook Formation with the model of Watson and Harrison [32] [15] were slightly underestimated.The Leinster–Lakesman, Monian and Cymru terranes of Ireland and England have been variously linked to the Avalonian domain (based on fossil assemblages of their Cambrian to Lower Ordovician rocks; e.g., [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).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, 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 low-density mantle-wedge diapirs [47], where their partial melting can feed the arc volcano with alkaline magma [48] (Figure 12(c)).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 [50–55].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.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]).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 trachyandesitic magma chamber, where water enrichment and silica metasomatism from country rocks may have occurred, and where volcanic pressure was allowed to build. The subsequent eruption of the slightly more mafic trachyandesitic flow was less obstructed and mostly devoid of country rock clasts.Another prolonged period of weathering separated the emplacement of intermediate lava of the Seaspray Formation from the overlying McGillivray Brook Formation, still within the ~5.5 M.y. interval between deposition of the ~460 Ma rhyolite and the ~454.5 Ma felsic ignimbrite. This blanket of Sandbian felsic pyroclastic rocks marked the end of Ordovician volcanism in West Avalonia (Figures 12(d) and 13(d)).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, which is inferred to have sourced the last pulse of arc-related volcanism along the East Avalonian margin of Iapetus at roughly the same time in the Upper Rhyolitic Tuff Formation of the Snowdon Volcanic Group in north Wales (⁠454.42±0.45 Ma based on U-Pb isotopes from primary zircons; [61]) in association with partial subduction of the oceanic ridge [7] (Figure 12(d)). Furthermore, earlier oceanic ridge subduction beneath the same microplate is inferred to have occurred at the level of Ganderia between 459 and 455 Ma [62] (Figure 13(c)). Oblique convergence of this ridge may have caused its area of subduction to subsequently prograde laterally towards Avalonia (Figures 13(c) and 13(d)). 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.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 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 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].The authors declare that they have no conflicts of interest.We wish to thank B. Boucher for his assistance with the LA-ICP-MS, electron microprobe and micro-XRF housed at the University of New Brunswick, as well as R. Corney and M. Kerr for the making of thin-sections. We also thank B. McConnell and Y. Kuiper for fruitful discussions, as well as C. van Staal and J. Greenough for constructive formal reviews. This project was supported by an operational grant (249658-07) from the Natural Sciences and Engineering Council of Canada (NSERC) to P. Jutras, B. Murphy and J. Dostal.Data Repository File # 1: Electron microprobe data from 20 points in the grey matrix of the McGillivray Brook Formation pyroclastic breccia (sample B2).

中文翻译：

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奥陶纪中晚期西瓦隆下俯冲动力学的演变

加拿大新斯科舍省Arisaig地区的中奥陶纪上至上火山岩，是该期间西阿瓦隆尼亚唯一已知的火山岩记录。因此，已经对它们进行了广泛的研究以测试古大陆重建，该重建在该时期始终显示出阿瓦隆为漂移的微大陆。在Dunn Point地层的上Darriwilian双峰火山岩与McGillivray Brook地层的Sandbian长英质火山碎屑岩之间具有中间成分的火山岩（新的Seaspray Cove地层）的识别，导致对Ordovician火山岩组中岩浆关系的重新评估。 Arisaig。尽管是同一火山构造的一部分，但三个地层之间存在明显的时间间隔，并被显示为属于三个不同的岩浆子系统。Dunn Point组玄武岩的构造地层背景和微量元素含量表明，它们是由E-MORB型震源在弧后伸展环境中的高度部分熔融而产生的，而海喷中层岩石中的微量元素含量海湾形成表明它们是由弧形环境中俯冲富集源的低度部分熔融产生的。这两个地层被长时间的火山静止和深层风化隔开，在此期间，后弧区域从延伸到缩短，最后被弧形火山岩重叠。基于有限的场约束，古地磁和古生物学数据，这种弧向北部弧后火山岩的扩展发生在北部，越来越年轻的伊帕坦洋洋板块以越来越浅的角度俯冲。人们认为伊阿佩坦洋洋脊的部分俯冲随后产生了板状窗口岩浆作用，从而标志着东阿瓦隆和西阿瓦隆的俯冲有关的火山作用的最后脉冲。古生代大陆和板块的重建表明，阿瓦隆的复合微大陆从冈瓦纳分离而来。在奥陶纪早期，打开了莱茵河，在奥陶纪的其余时间向北向劳伦蒂亚漂移，逐渐关闭了伊阿佩图斯洋[1-7]。因此，它的进化限制了导致Pangea融合的事件（例如[8]）。加拿大新斯科舍省的Arisaig地区（图1）包括西阿瓦隆尼亚地区唯一已知的中奥陶纪上火山岩演替序列（即 （Avalonia的北美部分），并且已经进行了广泛的研究，以阐明这一关键时间间隔内地层的构造和古地理历史（例如[9-15]）。直到最近，才从该地区了解到镁铁质和长英质火山岩，并将其解释为弧后扩展的双峰产物[13-15]。本文描述了海雾湾组，这是一个新发现的> 35 m厚的火山岩演替，其化学成分在Arisaig的奥陶纪演替中处于中间，并讨论了其地球化学的意义。根据与中奥陶纪邓恩点组和上奥陶纪麦吉利夫雷布鲁克组的火山岩单元的比较，以及与不列颠群岛相关地层的同代单位的平行，本文为西奥瓦隆中奥陶纪俯冲相关火山活动的演化提出了一个综合的地球动力学模型。研究区附近的基底岩石暴露以新元古代弧相关的火山岩和沉积岩（乔治维尔群）为特征，并被埃迪卡拉安岩体截断。 [15，16]，这是由寒武纪到下奥陶纪演替的沉积岩不整合地覆盖的，这些沉积岩包含动物诊断性的阿瓦隆（[17，18]）。在奥陶纪早期发生压缩变形[11，16]并随后从冈瓦纳[3]裂开之后，邓恩角和麦吉利夫雷布鲁克岩层的中奥陶纪上火山岩[19，20]被安置在阿瓦隆[18]的微大陆上[ 6、14]（图2）。根据来自原始锆石的U-Pb同位素，Hamilton和Murphy [9]获得了Dunn Point层流纹岩的年龄为460.0±3.4 Ma（达里维安/兰维尼上部），Murphy等。[14]随后用相同的方法对上覆的麦吉利夫雷布鲁克组的顶部火成岩定于454.5±0.7 Ma（Sandbian / Caradoc），为Arisaig的火山演替年龄提供了上限（图2）。新近确定的海雾湾地层是不规则的，但在地层上位于这两个单元之间，因此被限制在约460和〜454.5 Ma之间（图2）。古磁数据[10]适应随后的年代学数据，将Arisaig火山岩置于在〜460 Ma处南纬41°±5°的古纬度[9]。在该时间间隔内的古地理重建显示，阿瓦隆和甘德邦微大陆在同一微板上向北漂移，可能是由一条狭窄的航道分隔开的，而莱茵河将它们从冈瓦纳向南分开，而伊帕图斯海洋则将它们从劳伦蒂亚分开到北部[1、6、8]。这种漂移与Hodych和Buchan [21]在约440 Ma（志留纪早期）西阿瓦隆地区确定的南纬32°±8°的古纬度相一致，这是基于圣玛丽海角窗台（U-Pb baddeleyite年龄）纽芬兰阿瓦隆半岛上的441 + 2Ma⁠[22]）（图1（a））。根据爱尔兰，大不列颠及北美洲东部阿瓦隆山脉和甘德山脉序列的奥陶纪地质，这种北移是通过伊阿伯坦海洋岩石圈俯冲到劳伦蒂亚下方的北部以及阿瓦隆尼亚和甘德里亚下方的南部来进行的（[5， 7]； van Staal等，1998，[8，15，23]）。在Arisaig Pier和Frenchman's Barn（由石英细纹增强的抗流纹岩的单峰；图1（b））之间，不存在Seaspray Cove组，而Dunn Point组直接被McGillivray Brook组覆盖（图3）。Dunn Point地层是一连串的镁铁矿流，被风化剖面隔开，顶部是浓厚的流纹岩流，它还携带着厚厚的古土壤[19，24–26]（图1（b）–1（c））和3）。Keppie等。[20]将邓恩点组玄武岩归因于板块内大陆裂谷事件，但基于对东阿瓦隆的奥陶纪地质的推论，墨菲等人。[13，14]更具体地将火山活动与新西兰的弧后扩散联系起来，类似于新西兰的劳-阿弗尔-陶波系统，约70 m厚的覆流带流纹岩成分类似于A型板内花岗岩，被解释为由地幔中的热量产生的地壳的无酸盐产物。相关的铁镁质熔体[14]。基于Sm–Nd同位素数据，这些长英质岩石来自阿瓦隆下地壳[15]。在Dungo Point以南不到50 km的Antigonish高地中发现了具有相关地球化学特征的双峰p（图1（b））。地层暴露[27]。这套深成岩套件记录了板内双峰岩浆作用的悠久历史，其历史可追溯至冈瓦纳的奥陶纪早期奥陶纪裂谷。根据Sm–Nd同位素数据，侵入性的长英质岩石也来自阿瓦隆下地壳[15]。Dunn Point层流纹岩的上部被广泛风化[26]。流纹岩内部古土壤的裸露部分厚达8.5 m，但是根据应变计算（sensu [28]），这可能代表埋藏压实之前其原始厚度的约30％[24]。这种厚厚的古土壤是流纹岩大量喷发之后长期岩浆不活动的记录。麦吉利夫雷布鲁克组的特征是不均匀地覆盖在法国人谷仓西南部的邓恩点组之上（图1（b））。和火山碎屑沉积物。它的底部显示不连续的拉哈沉积物晶状体，这些晶状体局部截断了Dunn Point层流纹岩顶部的厚古土壤（图1（b）和3）。这些拉哈尔沉积物的基底晶状体，以及横向隔开它们的风化流纹岩的结点，直接被一系列的长英质laplap凝灰岩覆盖，长凝灰质凝灰岩向上过渡为具有A型成分的长英石火成岩[14，19]。根据Sm–Nd同位素数据，该长英质碎屑碎屑序列来自阿瓦隆下地壳，但据推测是较干燥的融化，并且温度（1050°C）比邓恩点形成流纹岩（860至875°C）高。饱和测温法估计[15]。当锆石在典型的下部地壳压力下在750至850°C的温度下溶解时，地壳融化的高温估计也可以解释其在高场强元素中的含量比邓恩点地层的流纹岩要高得多。中间的火山碎屑角砾岩和熔岩流单元（以下归类为松软岩岩体）不连续，夹在法国人的谷仓和海喷湾地区之间（非正式的地名；图1（b）），在Dunn Point地层的上方是不符合要求的，在下面的位置则是适当的McGillivray布鲁克组（图3）。由于这些岩性未包括在Dunn Point和McGillivray Brook地层的定义中，因此在这里将它们正式化为新的Seaspray Cove地层。该类型区域位于诺森伯兰直海岸线上，在Arisaig市的Seaspray Lane西南（图1（b））。在法国人的谷仓的东北侧（图1（b）），没有火山碎屑角砾岩。 ，最多约4个。Seaspray Cove地层的5 m厚风化熔岩流单元不规则地截断了风化流纹岩，并被McGillivray Brook地层的最低凝灰岩覆盖（图3和4）。根据风化流纹岩和粗粒软陶物质在大量中间流基础上混合的证据[26]，后者在进位过程中部分推土并掺入了风化流纹岩物质。根据这些作者的观点，与法国人的谷仓西南部相比，这个地方的截断的流纹岩古土壤发育欠佳，那里的古土壤直接被McGillivray Brook组的凝灰岩覆盖，而中间没有Seaspray Cove组的中间岩石（图3）。在东北，沿着Seaspray Cove（图1（b）），trachyandesite熔岩流单元的厚度至少为15 m（上部的风化和剪切作用改变了其原始厚度），其基础〜11.5 m处的风化较差，但在该水平以上已完全风化。安置后的逆冲断层集中在深层风化物质的这个较弱的上部区间，这导致了更有效的下部区间的局部重复（图5）。在Seaspray Cove地点，曲安山体熔岩流单元一致地位于相同地层的暗红色火山碎屑角砾岩之上，而一致地位于McGillivray Brook岩层的凝灰岩和火成岩之下（图3和5）。角砾岩的底部没有裸露，但根据可用的暴露，其厚度至少为20 m。将这种未完全暴露的演替与下层的邓恩点形成区分开的地层厚度和性质尚不清楚，但是由于它在向西南方向短距离内收缩时，推断角砾岩直接覆盖风化的流纹岩（图3）。先前的作者（例如，[13、14、19]）错误地将新确定的水行岩流单元错误地分配给了底层的邓恩点地层的玄武岩层序，该层在海雾湾地区与它发生了断层接触（图5）。 ）。尽管Dunn Point编队玄武岩较暗，但两个单元的露头看起来都相似。根据现场观察，沙眼泥石质位于斜辉岩下的角砾岩先前被混淆为拉哈沉积[13，19，26]。但是，此单元的颜色更深，更密，基质含量更低，并且从其他地区截断了流纹岩内古土壤的拉哈尔沉积物中富集了更多的流动元素。从Seaspray Cove组的火山碎屑角砾岩中获得了两个样品，并在Seaspray Cove（B1 -2和图1中的Q1-10）。在剖面的上部以较短的间隔进行采样，以便精确确定地下奥陶纪风化作用的向下程度，并评估风化过程中元素的迁移率。定期从法国人的谷仓东北段的约4.5 m厚的砂铁矿中获得三个额外的样本，其中两个包括在Jutras等人中。[26]。将所有样品制成薄片，并将每个样品的一部分打成粉末，用于矿物学和地球化学分析。X射线衍射（XRD）分析的结果示于表1中。表2列出了主要和选定的痕量元素的X射线荧光（XRF）数据。表3列出了痕量元素的更广泛的数据以及电感耦合等离子体质谱（ICP-MS）数据。表1-3还包括Keppie等人的数据。[12]，墨菲等。[14]，和Jutras等。[25，26]在Dunn Point地层的玄武岩和流纹岩以及McGillivray Brook地层的拉哈尔，凝灰岩和火成岩沉积物中，变化最少，污染最少的样品（基于高Na2O和低LOI含量）。分析了Seaspray Cove型剖面的熔岩流动单元样品，除了完全风化的Q10（斜长石含量完全改变为次生矿物；表1）之外，还分析了它们的总碳，氯和氟含量，分别由燃烧分析，仪器中子活化分析（INAA）和聚变/离子选择电极（Fus-ISE）确定（表4）。此外，利用位于加拿大弗雷德里克顿的新不伦瑞克大学的Bruker M4 Tornado微型XRF对火裂角砾岩样品B2进行了X射线荧光元素定位（图6）。使用电子微探针分析20个基质非氧化部分的主要元素含量（数据存储库文件＃1），并使用激光烧蚀电感耦合等离子体质谱仪（LA-ICP-MS）分析痕量元素含量（数据储存库文件＃2）也位于新不伦瑞克大学（University of New Brunswick）。在其位于Seaspray Cove底部约8 m处（样本Q1-4），新近确定的中间熔岩流单元很大，从断层到断层，斜长石的假晶形变成方解石和石英（图7（a））。苯酚类晶体发生在主要由石英，绿泥石和钠长石组成的梯状或轻度曲折的陆基中。方解石和石英扁桃体在剖面上越来越丰富（图7（b）和7（c））。在〜8-11.5 m的区间内观察到中等古风化的证据（样品Q5-9），并集中在以浆化和血化为特征的静脉样区域（图7（d））。Seaspray Cove断面（样品Q10）的最高原位剖面〜0.3 m已完全风化，未保留火山质地。它覆盖在McGillivray Brook地层的底部凝灰岩以下至少4 m的剪切古土壤物质上。Seaspray Cove的水热蚀变并随后风化的砂铁矿剖面由钠长石，正长石，石英，斜绿石，白云母，碳酸盐和赤铁矿，以及与上覆凝灰岩接触的一些叶蜡石组成（表1）。尽管石英可能在成岩作用的各个阶段产生，但我们认为钠长石，斜绿石和碳酸盐在模态丰度上升过程中均会降低（图8），这是在较热环境中发生的同位和置位后变化过程的唯一产物。比正常表面温度高。相反，白云母和赤铁矿模态丰度增加了剖面（图8），这些增加被解释为与随后的风化程度成正比，白云母含量（以及与凝灰岩接触处的叶蜡石）反映了埋葬前原始的成岩粘土含量。因此，正长石（未风化岩石的一小部分）和一些石英被解释为火成岩矿物仅有的未改性残留物。在风化剖面中优先保护原正石与碱性风化条件是一致的，这是由Arisaig [24–26]的奥陶纪演替其余部分中的古土壤地球化学表明的。尽管在放置和冷却过程中发生了彻底的变化，但从剖面底部开始的Q1样品是放置后风化作用最不明显的样品，因为其特征是长石（〜51％）和斜绿石（〜17％）含量最高，并且由于它几乎不含白云母（〜1％）和赤铁矿（低于检出限）（表1）。就主要元素含量而言，Q1与其他样品的主要区别在于其非常低的K2O含量（0.4重量％）。Jutras等。[24-26]报道了浅埋葬期间阿里萨格火山演替的古土壤中大量的钾富集，这与增加剖面中的K2O含量（Q1-10；表1和表2）以及风化矿物的模态丰度相一致。这个中间的岩石。因此，除了原正硅石中的K2O含量外，还可能反映了剖面中大量的墓前蒙脱石含量，后来又转化为白云母和叶蜡石.Jutras等人的样品S3-7和S3-8为基础。[26]来自法国人的谷仓东北部保存的最高约2 m的风化砂岩砂岩（图4），剖面的上部（水平）的特征是几乎没有钠长石和碳酸盐（以及相关的Na2O和CaO），这表明在温暖和潮湿的条件下完全风化。在这些样品中，模态石英含量低至〜32-45％，而云母和赤铁矿模态含量分别高达〜28-31％和〜21-28％（表1和2；图8）。 。由于其石英贫瘠且云母丰富的矿物学特征，这种石化的土壤层层结构较弱，使其易于随后断裂（图5）。由于成岩成岩作用普遍存在，任何使用通常在水热中活动的元素进行分类的结果改变或风化过程对于任何样品都不可靠。通常固定的高场强元素（Ti，Zr，Nb，和Y）在整个配置文件中表明，这些成岩过程没有明显影响这些要素（表2和3）。根据其Ti / Zr和Nb / Y的比率（sensu [30]），岩石最好归类为菱锰矿（图9）。F含量为0.05-0.10 wt。％，Cl含量为0.04-0.06 wt。％（表4）。 ，相对于地幔而言，Seaspray Cove地层的砂岩流岩中的卤素含量较高[31]。相比之下，平均富集的地幔源只有0.0025％的F和0.0017％的Cl [31]。由于卤素只能在热液变化和潮湿的气候风化过程中被浸出，并且从未富集，因此其在样品中的浓度被解释为熔体中原始含量的残留物.Seaspray Cove地层的基底火山碎屑角砾岩主要由淡红色组成。灰色，卵石大小的碎屑，深红色坚韧不拔的矩阵内有变化的边缘。其原始矿物学被彻底改变为石英（67-70％），钠长石（8-14％）和斜绿石（3-4％），并进一步风化为赤铁矿（7-8％，大概是由于斜绿石的有氧变化） ）和白云母（4-5％，大概来自粘土矿物的成岩作用）（表1）。与同一系列的拉哈尔沉积物相比，后者是风化较彻底的物质的沉积重整产物[26]，火山碎屑角砾岩的特征在于模态白云母和Al2O3含量明显较低，而Na2O含量则高得多（表1和表2） ）。但是，由于其角砾化的织物，大部分材料都具有适度的耐候性，这掩盖了原始熔体成分的性质。角砾岩基质的一些小区域没有被氧化，并显示出保存良好的流动纹理（图6（a））。他们大多也没有碎屑。但是，这些未氧化的基质区域显示出铁浸出的迹象，似乎集中在砂砾和氧化的周围区域（图6（b）），从而部分截断了灰色基质（图6（a））。相比之下，灰色基质的少量残留物比形成沉积物主体的红色基质的硅含量稍高（图6（c））。虽然SiO2的浓度与榴辉岩或流纹岩相似（表2），但火成岩基质根据其Zr / Ti和Nb / Y比（图9），在砂砾岩最上层的火山碎屑角砾岩图。由于其痕量元素分布类似于上覆的砂眼质杂物流，但是在低得多的浓度下（图10（a）），我们得出结论，原始岩浆可能已经分馏成菱形安山岩成分，并且在喷发之前SiO2富集发生在岩浆腔顶部附近，因此稀释了痕量元素含量而没有大大影响他们的比率。推断的铁浸出（图6（b））表明还原条件下发生了早期蚀变过程，而氧化作用随后通过位后风化发生。Watson和Harrison [32]开发的经典锆石饱和测温模型在从铝酸盐到金属铝的校准过程中进行了校准。长丝熔体成分。为了确定范围更广的岩石成分的锆石饱和温度，Gervasoni等人。[33]使用新的整体组成参数G 3×Al2O3 + SiO2 / Na2O + K2O + CaO + MgO + FeO建立了一个针对温度和Zr浓度的模型，该模型也可应用于中岩石和碱性岩石。根据表2的数据，并根据Gervasoni等人的模型。[33]，邓恩波因特流纹岩的锆石饱和温度范围为872-887°C，海浪喷剂孔雀绿砂岩中的锆石饱和温度范围为765-786°C（样品Q1-4），McGillivray的锆石饱和温度范围为1065-1103°C。布鲁克组长英质火成岩。这些结果表明，用沃森和哈里森[32] [15]的模型确定的邓恩角和麦克吉利弗里布鲁克组的碱性长英质岩石的锆石饱和温度略有低估。爱尔兰和英国的Monian和Cymru地层与阿瓦隆域（根据其寒武纪与下奥陶纪岩石的化石组合；例如，[17，34]）或与Ganderian和Megumian域（基于岩性相似性）有不同的联系。和出处研究； [35，36]）。但是，基于奥陶纪的相似性，人们普遍认为这三个地貌以及英格兰南部的阿瓦隆·沃金和夏恩伍德地貌是同一奥陶纪微陆相漂移组合的一部分（[36]，以及其中的参考文献），其中也包括北美的阿瓦隆山脉，我们在本文中称为阿瓦隆（sensu [3，7，34，37]）。因此，可以在那个时期的统一构造模型的背景下，对北美洲阿瓦隆地区和不列颠群岛所有贡多瓦南所有地带的奥陶纪演替进行初步评估。在西姆鲁地形和伦斯特-莱克斯曼南部Terrane，早期奥陶纪的安第斯山脉弧火山作用是由中奥陶纪的双峰后弧火山作用（Fishguard火山群和等价单位）继承的，这被解释为是板块回滚，弧向海迁移以及后壳地壳伸展的结果。弧区域[7，38]。在中奥陶纪中期至晚期边界附近，弧形火山似乎向后迁移，并在伦斯特-莱克斯曼地形北部的邓肯嫩和博罗代尔火山群中得到了很好的记录[39，40]。该俯冲带的最后一个火山脉冲发生在北威尔士斯诺登火山群的约454 Ma处，但在地球化学上与水合地幔楔上方的火山活动无关，并且被解释为发生在伊帕坦中洋脊部分俯冲到下方时发生根据电弧的较高Nb / Y比（图9）和较陡的分布，在弧形区域内并在近弧后区域打开一个平板窗口，然后向该窗口中注入干燥的软流圈熔体（[7]和其中的参考文献）。稀土元素（图10（b））被认为是Seaspray Cove地层的一部分熔融程度低于Dunn Point地层玄武岩的产物。邓恩点组玄武岩中耐风化的高场强元素（HFSE）的比例与富洋中海脊玄武岩（E-MORB; sensu [41，42]；图10（a）和图11）表明它们来自贫化的上地幔物质（E-DM，sensu [43]），富集的平均值超过平均水平，并且不受地壳污染的明显影响。根据这些玄武岩的Sm-Nd同位素特征和模型年龄，该E-DM源的富集发生在1.1至0.8 Ga之间，这是阿瓦尼亚最古老的裂谷事件之前的时间[13，44]。融化被认为是在高温下通过后弧环境中的软流圈上升而发生的[14，15]（图12（a））。相比之下，Seaspray Cove地层的岩石表现出更典型的弧火山作用特征，例如中间成分，高的Cl和F含量（表4），相对明显的负Ta-Nb和Ti异常（图10（a）），Th / Yb和Nb / Yb比绘制在地幔阵列上方（sensu [42]；图11）。此外，估计的锆石饱和温度约为786°C，表明在源处发生了低温含水熔化[45]，这也与电弧设置一致。相比之下，对于长石质Dunn Point层流纹岩（872-887°C），较高的锆石饱和温度估计值与后弧环境中的高温无水融化相一致[15]（图12（b））。来自Cruz-Uribe等的实验数据。[46]，通过沿板-幔界面发生的高压混杂岩的部分融化，可以产生具有电弧特征的碱性岩浆，其中俯冲蚀变的海洋地壳和沉积物与水合的幔楔材料混合。这些混杂物通过低密度的地幔楔形底泥运到弧底的地幔楔的热角处[47]，它们的部分融化可以为弧火山提供碱性岩浆[48]（图12（c））。由于主要来源的部分熔融程度低，可能发生了不相容元素的富集，Seaspray Cove地层砂眼砂岩中明显的负Eu和Ti异常（图10（a））表明它的斜长石逐渐消失了，通过在岩浆室上部进行晶体分级分离获得磁铁矿（图12（c）），这将有利于进一步富集不相容元素。高浓度的卤素可能有助于在HFSE中显着提高含量（表4），这往往是弧形火山作用的固有特征[49]，并且通过促进将HFSE掺入高阶可溶复合物中来增强HFSE的不相容性[50-55]。尽管Seaspray Cove和McGillivray Brook地层都强烈富集于不兼容的跟踪元素，它们在跟踪元素分布方面有很大差异（图10（b））。由于海喷雾湾地层的砂状杂物（样品Q1-4）和麦吉利夫雷布鲁克地层的长英质火成岩（[14]的样品BL08-2和BL08-3）具有明显不同的Th / Hf比（0.536-0.603和1.139-1.143；表3），根据Schiano等人的观点，后者不太可能是前者的分馏产物。[56]。此外，根据McGillivray Brook地层火成因推断的极高熔化温度（根据[33]模型，约为1100°C）表明无水条件与水合地幔楔上方的弧火山作用不兼容[15]。然而，与张性构造的局部发展有关，这种成分的岩石可以在亚弧弧系统中发育[57，58]。因此，McGillivray Brook组的长英质碎屑碎屑沉积层被解释为代表了与Seaspray Cove组的中间火山岩不同的岩浆脉，后者通过长时间的风化作用而与之分开。McGillivray Brook地层火成岩中痕量元素的含量较高可能部分是由于熔化温度超过了这些元素所集中的辅助相的熔点。然而，非常发达的负Eu和Ti异常（图10（b））表明在喷发之前，岩浆室内部分结晶的时间延长了。岩石还显示出LREE的显着枯竭（图10（b）），这可归因于硅质岩浆中的辅助相如尿囊石，独居石或洋红铁矿的分馏（例如[59]）。此事件不久之后，阿瓦隆（Avalonia）从冈瓦纳（Gondwana）漂流，弧形火山作用被解释为与陡峭俯冲和平板回滚相关联，在中奥陶世时期，东，西阿瓦隆地区目前暴露的地层中发育了弧和后火山岩[6、7、11、14、38、60]（图12（a）和13（a））。在东阿瓦隆尼亚，双峰Fishguard火山群和等效单元中记录了达里维尔背弧火山活动，而在西阿瓦隆尼亚中，则记录了邓恩角构造底部基层板块玄武岩的厚片层序（图12（a）和13（a）），以及随后沉积的地壳苯甲酸产生的厚流纹岩（图12（b）和13（b））。后者记录了约460 Ma的弧后延伸结束。据推测，附近安蒂戈尼什高地的相关弧后岩体活动（图1（b））已停止了约460 Ma [27]。西阿瓦隆地区随后出现了长时间的火山静止期，如Dunn Point层流纹岩上部厚厚的风化剖面发展所表明的[26]，以及将流纹岩与上层McGillivray的长英质火成岩分开的1.4到9.6 My（即〜5.5 My）的时间间隙基于U-Pb的布鲁克组始于初级锆石[14]。在此裂隙内，具有弧型成分的火山岩（新的Seaspray Cove地层）重叠在先前沉积的弧后火山演替的一部分上（图12（c）和13（c）），夹在西南（图3）。在中奥陶纪边界至中奥陶纪边界附近的东阿瓦隆，也记录了类似的弧内火山运动，将邓肯农和博罗代尔火山群的弧火山岩与较旧的后弧火山岩并列[7，39，40]。由于在源分离过程中源的部分熔融程度较低，并且在晶体分级分离过程中卤素含量可能较高，因此，与下面的邓恩点组流纹岩相比，海雾湾组中间岩中不相容的HFSE含量总体上更大felsic（表2;图10）：缺乏明确的风化剖面，将泛水山岩碎屑碎屑角砾岩与Seaspray Cove地层上覆的熔岩流动单元分隔开来，表明这两个喷发在时间上间隔很近。因此，火山碎屑角砾岩被解释为从安山岩浆岩岩浆室稍长的长石质顶部粘性挤出物质，在该岩浆室中可能发生了富集水和来自乡村岩石的二氧化硅交代作用，以及允许建立火山压力的地方。随后出现的镁铁质横纹岩和安山岩流的喷发较少受阻，且几乎没有乡村碎屑。另一段长时间的风化将海雾组中间熔岩的位置与上覆的麦吉利夫雷布鲁克组分离开来，仍在约5.5我的区间内〜460 Ma流纹岩的沉积与〜454.5 Ma的长英质火成岩之间。桑迪安长石质火山碎屑岩覆盖着标志着西奥瓦隆奥陶纪火山活动的终结（图12（d）和13（d））。如前所述，McGillivray Brook组长石火成岩的地球化学与Seaspray组的不相容。并且与水合地幔楔上方的弧形火山作用不兼容。然而，源自这些岩石的无水地壳的超高温融化[15]与平板窗岩浆作用是一致的，据推测，该板岩岩浆作用是在上埃佩特斯东部阿瓦隆山脉边缘大约在同一时间起源于与弧有关的火山作用的最后脉冲北部威尔士斯诺登火山群的流纹凝灰岩形成（454.42±0.45 Ma，基于来自原始锆石的U-Pb同位素； [61]）与部分洋洋俯冲相伴[7]（图12（d）） 。此外，据推测，在同一微孔板之下较早的洋脊俯冲发生在459和455 Ma之间的甘地亚水平[62]（图13（c））。该山脊的倾斜交汇可能已导致其俯冲区域随后横向向着Avalonia前进（图13（c）和13（d））。由于阿瓦隆下的南向俯冲的关闭大致与甘德里亚的Popelogan弧和劳伦西亚沿同一俯冲带向西碰撞（根据[56]约​​为455 Ma）（图13（d））有关，因此可以推论出这种阻塞使剩余海床的平板拉动得以减少，这可能阻止了伊帕坦海脊在阿瓦隆下完全俯冲。新近确定的阿里沙格海喷层的横行安山岩序及其与火山岩的地层关系Dunn Point和McGillivray Brook编队的单位为西阿瓦隆尼亚中奥陶纪岩浆系统的演化提供了新的视角，这与东阿瓦隆尼亚的古代系统有许多相似之处。在这段时间里 收敛带被解释为从陡峭的俯冲，后退和后弧扩展，演变成较浅的俯冲和后弧缩短（图13）。结果，东奥瓦隆地区将上奥陶纪弧形火山岩与中奥陶纪后弧火山岩并列[39，40]，而西阿瓦隆尼亚的上奥陶纪弧形火山岩（Seaspray Cove地层）有一个较小的离群最终重叠。先前沉积的邓恩点组中奥陶纪中部双峰弧后火山岩序（图3）。根据古大陆重建显示阿瓦隆下的伊帕坦海洋岩石圈向南俯冲（[1，5，7]，以及其中的参考文献），重叠可能发生在北部。这种解释与有限的字段限制在一定程度上是一致的，这表明海浪湾组的弧形火山岩向西南方向挤压在邓恩点组的后弧火山岩上（目前的坐标，图1（b）和图3）。桑本晚期（加拉多克中部）东部和西部阿瓦隆地区的伊帕坦俯冲作用没有明显的变形（本研究和[7]）。在东阿瓦隆尼亚，Woodcock [7]将关闭归因于Iapetan海洋山脊的不完全覆盖。这一假设与推断的西奥瓦隆下中奥陶世中期至晚期俯冲由浅到浅的俯冲转变相一致，类似于纳斯卡板块的俯冲历史[63]，这表明正在俯冲的是越来越年轻的浮力洋壳，因此，洋脊可能已经逐渐接近海沟了（图12和13）。在东阿瓦隆尼亚，与伊本海板块俯冲有关的最后一次火山爆发发生在〜454.4 Ma处，被解释为板状产物。窗口岩浆作用[7]（图12（d））。这种设置与McGillivray Brook组〜454.5 Ma的长英质火成岩的地球化学相吻合，标志着西阿瓦隆尼亚俯冲相关的火山作用的终结，其起源于无水的超高温融化（〜1100°C）。脆皮。到达地壳底部的这种升高的温度将最好地由与板状窗的发育相关的软流圈上升现象来解释（图12（d））。奥陶纪晚期，西，东阿瓦隆下的俯冲作用明显停止，表明泥盆纪期间阿瓦隆向复合型劳伦西亚的增生（例如[6、60、64]）是通过不同的俯冲带发生的。这与缅因州[65]的复合Laurentia下西北向志留纪俯冲的记录一致[65]，后者可能逐渐消耗了当时将后者与阿瓦隆分开的海床残余物（“阿卡迪亚海道” [66]中的“）。在此期间，阿瓦隆发生板块间的岩浆活动，但与俯冲没有关联[67]。作者宣称他们没有利益冲突。我们要感谢B. Boucher对LA-ICP-的协助。多发性硬化症，新不伦瑞克大学（University of New Brunswick）拥有电子显微探针和微型XRF，以及R. Corney和M. Kerr用来制造薄切片。我们也感谢B. McConnell和Y. Kuiper进行的富有成果的讨论，以及C. van Staal和J. Greenough进行的建设性正式审查。该项目得到了加拿大自然科学和工程理事会（NSERC）的P. Jutras，B。Murphy和J. Dostal的运营赠款（249658-07）的支持。数据存储库文件＃1：20个点的电子微探针数据在McGillivray Brook组的火山碎屑角砾岩的灰色矩阵中（样本B2）。