7 Tholeiitic basalt, weakly alkaline felsic flows, pyroclastic deposits, and lesser sedimentary rocks comprise the ca. 592 Ma Plate Cove volcanic belt (Bull Arm Formation), which is “basement” to the overlying, dominantly shallow-marine Rocky Harbour Formation on Bonavista Peninsula. The conglomeratic Plate Cove facies (O’Brien and King 2005), a coarse clastic unit that stratigraphically overlies and crops out east of the Plate Cove volcanic belt, is overlain to the north and east by the Kings Cove Lighthouse facies and the Monk Bay facies, respectively (Normore 2010, 2011; see Fig. 3 and Table 1). The Kings Cove Lighthouse facies includes cross-bedded sandstone, thin conglomeratic layers and lenses, and maroon to purple mud drapes. The Monk Bay facies includes similar lithologies, but the mud drapes are grey-green, and thick (>1 m) green mudstone layers and mega-ripples are common (Normore 2010, 2011). Local large-scale epsilon cross-bedding in Kings Cove Lighthouse facies rocks south of Kings Cove was interpreted to reflect a Gilbert-type (fluvial-dominated) deltaic setting (Normore 2010), whereas symmetric mega-ripples characteristic of the Monk Bay facies were interpreted to reflect a shoreline or shallow marine setting (Normore 2010). The salient point here is that although these facies indicate distinctive depositional environments, they may well be coeval. Normore (2011) proposed that cross-bedded, medium- to coarse-grained sandstone of the Cape Bonavista facies interfingers with rocks of the Monk Bay facies, and is transitional into Kings Cove Lighthouse facies. Each of these facies was suggested as a potential map unit, but lateral and vertical facies changes and interdigitation limit regional extrapolation as separate, distinct units. The Plate Cove, King’s Cove Lighthouse, Monk Bay, and Cape Bonavista facies (Figs. 2–3; Table 1) reflect differences in depositional settings (Normore 2010, 2011), and are all considered pre-glacial with respect to the Trinity facies (Normore 2011; Pu et al. 2016; this study; see Table 1). Diamictite of the Trinity facies is presumed to comprise a synchronous marker unit.

8 The Trinity facies in the southwestern Bonavista Peninsula is typically overlain by 40–50 m of green-grey to blue-grey, finely laminated siltstone, fine-grained sandstone and minor conglomeratic lenses. These rocks are similar to the Kings Cove North facies of the upper Rocky Harbour Formation (Normore 2010, 2011) on northwestern Bonavista Peninsula (Fig. 3; Table 1). The Kings Cove North facies, at its type locality (see Fig. 3), was interpreted as flood plain or lacustrine deposits preserved as large-scale discontinuous lenses within coarse-grained, high-energy, meandering fluvial to deltaic strata of the Kings Cove Lighthouse facies (Normore 2010). Given that the Trinity facies overlies the Kings Cove Lighthouse facies on southwestern Bonavista Peninsula but is absent above the Kings Cove Lighthouse facies on northwestern Bonavista Peninsula (Fig. 3), it is possible that the lacustrine-like deposits of Kings Cove North facies (at its type locality on northwestern Bonavista Peninsula) may instead be glaciolacustrine in origin, and therefore chronostratigraphically equivalent to the Trinity facies. It is therefore proposed that peri-glacial depositional facies may be more widespread than currently recognized.

9 On southwestern Bonavista Peninsula, peach- to pistachio-coloured silicified tuffaceous beds intercalated with blue-grey siltstone above the Trinity facies are lithologically similar to interbedded siltstone and tuff of the Herring Cove facies (Normore 2011). At its type locality north of Fox Island (Fig. 3), the Herring Cove facies is structurally overlain to the south by rocks correlative with either the Mistaken Point Formation (upper Conception Group) or lower St. John’s Group (Normore 2011; see Fig. 2b and Fig. 3). On southwestern Bonavista Peninsula, both the Kings Cove North and Herring Cove facies are considered post-glacial with respect to the Trinity facies (Normore 2011; Pu et al. 2016; this study).

10 The Dam Pond basalt on the northeastern Bonavista Peninsula crops out in a structural culmination and is overlain by siltstone and fine-grained sandstone of Normore’s (2010, 2011) Big Head Formation (Figs. 2, 3). Trinity facies diamictite crops out to the north, northwest, and southwest, apparently stratigraphically above the Dam Pond basalt and overlying siltstone, indicating that the Dam Pond basalt is older than the Trinity facies. Bedding orientation reversals define east-west-trending D₁ folds that are nearly orthogonal to north-northeast-trending D₂ faults, penetrative S₂ cleavage, and F₂ fold axes prevalent across the Bonavista Peninsula (Fig. 3). D₁ deformation lacks penetrative cleavage on the Bonavista Peninsula and field evidence for D₁ is best preserved as north-directed thrust faults and north-verging asymmetric folds reported on Fox Island (Fig. 3; Mills et al. 2016a). The age and magnitude of offset associated with both D₁- and D₂-related faults have not been constrained but are younger than ca. 565 Ma, the inferred age of poly-deformed strata at both Catalina and Fox Island (Fig. 3; Mills et al. 2016a).

11 The geology of the southwestern part of Bonavista Peninsula is complicated by polyphase deformation (Normore 2011; Mills et al. 2016a; see Figs. 3–4). West and south of Bonaventure Head, the strata coarsen upward above the Trinity facies from dominantly siltstone to coarse-grained sandstone, overlain by red beds of the Crown Hill Formation (Figs. 2–4). Between Bonaventure Head and British Harbour, the rocks young to the west, and basalt breccias and flows occur near the top of the dominantly grey, cross-stratified sandstone sequence. At Random and Ireland’s Eye islands, the rocks young to the south, and basalt flows, agglomerate, and tuffaceous rocks overlie coarse-grained, grey sandstone and are overlain by red sandstone and conglomerate of the Crown Hill Formation (Figs. 2, 4) (Normore 2012a, b). The British Harbour volcanic and pyroclastic rocks are therefore considered to be younger than the ca. 580 Ma Trinity facies.

12 The Dam Pond basalts are preserved in a structural culmination, likely produced by interference of D₁ and D₂ folds (Fig. 3), flanked by laminated to convoluted siltstone and lesser sandstone. Basaltic rocks at Dam Pond are dark- to light-green, vesicular to amygdaloidal, clinopyroxene-bearing flows and breccias (Normore 2010; Mills and Sandeman 2015) and are interbedded with thin-bedded grey siltstone. Diamictite overlies the siltstone ~3.5 km to the southwest and about 5 km to the north and northwest (Fig. 3). The thickness of siltstone is indeterminable, due to bedding reversals, the confluence of brittle faults, and poor outcrop exposure.

13 The Dam Pond basalts contain relict, up to 2 mm, subhedral clinopyroxene, which is commonly resorbed and variably altered to chlorite (see plates 3C–D of Mills and Sandeman 2015). Plagioclase exhibits a bimodal size distribution, with ~1 mm phenocrysts and 100–200 µm laths in the groundmass. Amygdales average 2 mm in diameter and are filled by chlorite, carbonate, quartz, epidote, and adularia. Clasts in the basalt breccia include fine-grained, trachytic-textured basalt and highly vesicular, scoriaceous basalt. Skeletal plagioclase microcrysts and locally preserved chilled rims of some clasts indicate rapid quenching, possibly in a subaqueous setting. The matrix of most samples contains chlorite-altered patches associated with 30–50 µm titanite grains. The chlorite-titanite assemblage is unlikely to be magmatic, but a product of alteration that may be metamorphic, deuteric (i.e., due to late exsolved fluids related to eruption), or hydrothermal in origin.

14 One sample of basaltic conglomerate from Fifields Pit, 3 km northwest of Port Rexton (Fig. 3), was collected for petrographic examination and lithogeochemical analysis. Detailed bedrock mapping undertaken by Cornerstone Resources in the early 2000s during exploration for Kupferschiefer-style copper mineralization identified two lenses of mafic volcanic breccia in the poorly exposed Fifields Pit area, north and west of Port Rexton (Graves 2003; Fig. 3). The elongate lenses, each about 1 km long by 200 m thick, occur within a subvertical to steeply east-dipping package of intercalated green and grey siltstone and sandstone, approximately 300 m and 1 km west of an overturned syncline (Graves 2003; Fig. 3). Several discrete north-trending faults are identified in the area (Normore 2011), but the entire subvertical rock package likely occurs within a complex fault zone (Fig. 3).

15 The Fifields Pit basalt comprises rounded basalt clasts, set in a vuggy to pink and white crystalline network (Fig. 5a). The clasts exhibit textures ranging from scoriaceous to pilotaxitic to trachytic, and are surrounded by an interclast fill comprising mainly feldspar, epidote, and prehnite (Fig. 5b). The latter phases are interpreted to have crystallized in interfragmental voids as a post-magmatic fluid percolated through the conglomeratic basalt. Some chlorite blebs may be pseudomorphs of precursor olivine. Clinopyroxene occurs both as phenocrysts (Fig. 5b) ranging up to 5 mm and also as microcrysts in some of the basaltic fragments. Rare hourglass zoning of some phenocrysts (Fig. 5c), feathery crystal habit, and swallowtail terminations of some microcrysts (Fig. 5d) are indicative of rapid clinopyroxene crystallization (Wass 1973), possibly in a subaqueous eruptive setting, consistent with the interpretation of the laminated to convolute siltstone interbedded with the basalt as sub-aqueous deposits.

16 At Bonaventure Head (Figs. 3–4), a ~25 m-thick, clast-supported tuffaceous breccia (Fig. 6a) containing subangular basaltic pebble-sized clasts (<3 cm) set in a white to pink infill overlies ~50 m of siltstone above the top of the Trinity facies diamictite (see also Pu et al. 2016). Subtle normal grading within the mafic breccia and the presence of rare grey, pink, and reddish angular siltstone and mudstone clasts (≤1 cm) are consistent with limited transport and re-deposition of the predominantly mafic volcanic debris. The basaltic breccia is overlain by thin-bedded siliceous siltstone and minor tuffaceous rocks.

17 The basalt clasts are mainly scoriaceous (Fig. 6b), with an average diameter of ~5 mm but ranging up to ~3 cm. Chlorite and/or calcite amygdales are common. Acicular, elongate clinopyroxene microcrysts, ~100 µm in length, are abundant in some of the clasts. The microcrysts are commonly skeletal and some exhibit swallowtail grain terminations (Fig. 6c), indicative of quenching (Wass 1973). As in the Fifields Pit basaltic conglomerate, the Bonaventure Head breccia consists of mineral phases interpreted to have crystallized from late-stage secondary (meteoric/deuteric?) fluids filling primary pore spaces. At Bonaventure Head, the infilling phases include feldspar, quartz, chlorite, and minor epidote and calcite; prehnite, however, was not observed.

18 Basaltic dykes, sills, flows, and pyroclastic rocks occur at British Harbour, along a ridge 5 km north of British Harbour, on the island of Ireland’s Eye, and along the northeastern tip of Random Island (Fig. 4). Mafic breccia is either interbedded with, or occurs as lenses within, medium-grained, grey, cross-stratified sandstone. Normore (2011) interpreted this sandstone as Cape Bonavista facies; however, sandstone of the Cape Bonavista facies is elsewhere overlain by the Trinity facies, whereas the stratified sandstone at British Harbour is inferred to stratigraphically overlie the Trinity facies based on field relationships (Fig. 4).

19 Several mafic dykes (Type 4 dykes of Mills and Sandeman 2017, interpreted as the youngest Ediacaran dykes on the Bonavista Peninsula) intruded the mafic breccia and sandstone interbeds and are themselves locally offset by brittle faults (Fig. 7a). On Random Island, grey to reddish, coarse-grained, immature sandstone containing red mudstone rip-up clasts (Fig. 7b) is overlain by mafic pyroclastic rocks and flows displaying well-developed flow-top breccia (Fig. 7c). The basalts are overlain by medium-bedded, reddish-weathering, structureless (massive) sandstone, in turn overlain to the south by red pebble conglomerate (Fig. 4). The latter was interpreted by Normore (2012a, b) as upper Crown Hill Formation. Basalts on Ireland’s Eye Island include agglomerate containing basaltic lapilli and bombs in a tuffaceous matrix (Fig. 7d), massive basalt flows and interbedded basalt breccia, and red siltstone containing local convolutions and slump folds (Fig. 7e). These rocks are overlain to the south by red sandstone to granule conglomerate, also interpreted by Normore (2012a, b) as upper Crown Hill Formation.

20 The British Harbour pyroclastic rocks contain abundant, highly vesicular scoria fragments (Fig. 8a), commonly rimmed by black spherules that may have originated as droplets of basaltic liquid, now preserved as volcanic glass (Fig. 8b–c). Amygdales are predominantly filled with chlorite and are locally rimmed by tiny titanite grains (Fig. 8c). Subrounded lithic clasts, feldspar, and less common quartz crystals comprise the groundmass of some pyroclastic rocks, whereas others contain scoriaceous and trachytictextured basalt fragments in a glassy groundmass. Flows contain minor sub- to euhedral plagioclase phenocrysts in a plagioclase-lath-rich, pilotaxitic to trachytic-textured groundmass (Fig. 8d). Subhedral titanite crystals up to 300 µm occur as an early magmatic mineral phase, enveloped by trachytic-textured plagioclase, whereas interstitial titanite occurs as abundant <40 µm anhedral grains (Fig. 8e). Hematite and apatite are also significant accessory phases (Fig. 8f).

23 All nine samples of Dam Pond basalt are alkalic basalts (Fig. 9b). With the exception of the Fifields Pit basalt, these rocks have the lowest SiO₂, FeO^T, TiO₂, and Zr contents of all the alkaline Bonavista Peninsula basalt samples. Their Mg#, Cr, Ni, Co, and V contents are greater than those of the British Harbour basalts, but not as elevated as those of the Fifield Pit and Bonaventure Head basalts (Table 2; Fig. 10). Dam Pond basalt multi-element patterns are steep and smooth, and generally have slightly elevated Nb relative to Th and La (Fig. 11a). They plot near ocean island basalt (OIB) on most discrimination diagrams (e.g., Figs. 12a, c, d) or in the ‘continental’ field, near the alkaline field boundary, of Cabanis and Lecolle (1989; Fig. 12b). Their position above the mantle array in Th/Yb vs. Nb/Yb space (Fig. 12c) indicates minor Th-addition, possibly resulting from lithospheric contamination, consistent with their somewhat variable Th/La ratios. Their position in TiO₂/Yb vs. Nb/Yb space (Fig.12d) reflects a relatively deep parental source, one which is deeper than that for normal mid-ocean ridge basalt (NMORB) and enriched mid-ocean ridge basalt (EMORB), approaching the depths of the asthenospheric source for OIB (Pearce 2008).

24 Although only one sample of basaltic conglomerate was collected from the Fifields Pit area, it significantly represents the only volcanic deposit currently known in that part of Bonavista Peninsula (Fig. 3). Based on its incompatible trace element composition, the Fifields Pit rock is an alkali basalt (Fig. 9b). It has the highest Mg#, Cr and Ni, and the lowest SiO₂ and Zr contents, and is therefore the most primitive of the alkali basalt samples from the Bonavista Peninsula (Table 2; Fig. 10). Its multi-element pattern is steep, with notable negative anomalies for Zr, Hf, and P (Fig. 11a). Despite having higher MgO and lower Zr content, it is chemically more similar to the Dam Pond basalts than to the other alkali basalts investigated herein (Figs. 11–12). On most tectonic discrimination diagrams, the Fifields Pit basalt is similar in composition to OIB (Fig. 12). Its position within the mantle array in Th/Yb vs. Nb/Yb space indicates negligible lithospheric contamination. In TiO₂/Yb vs. Nb/Yb space, it plots just above the shallow mantle NMORB-EMORB array, within the OIB field (Figs. 12c–d).

25 Based on their incompatible trace element compositions, the three samples from the lower, middle, and upper parts of the ~25 m-thick tuffaceous breccia at Bonaventure Head are alkali basalt (Fig. 9b), transitional calc-alkalic–continental basalt (Fig. 12b), or OIB (Figs. 12a, c, d). Their SiO₂, FeO^T, TiO₂, P₂O₅, and Zr abundances are comparable to those of the Dam Pond basalts (Table 1; Fig. 10), and markedly lower than those of the British Harbour basalts. Like the Dam Pond basalt, the Bonaventure Head basalt has relatively high Mg# (Fig. 10a; Table 1). Cr and Ni contents are higher in the Bonaventure Head basalts than in the Dam Pond basalts, and considerably higher than those of the British Harbour basalts (Fig. 10e, f; Table 1). The Bonaventure Head basalts have the steepest multi-element patterns [average (La/Yb)_CN = 30] of all basalts from the Bonavista Peninsula (Table 2), and have notable Nb, Zr, Hf, and P troughs (Fig. 11b) and minor negative Ti anomalies. They are similar to OIB on most discrimination diagrams (Figs. 12a, c, d) but plot slightly above the mantle array in Th/Yb vs. Nb/Yb space (Fig. 12c), indicating minor lithospheric/crustal contamination.

26 The British Harbour basalts (n = 22) are subdivided into two lithological and lithogeochemical groups: British Harbour volcanic rocks (n = 15; 5 dykes, 9 flows and one basalt bomb extracted from an agglomerate); and British Harbour pyroclastic rocks (n = 7; tuffaceous, pyroclastic and breccia deposits all of which may contain a minor component of epiclastic material; e.g., Figs. 7d, e).

27 The British Harbour basalts are weakly alkaline in terms of their incompatible trace element compositions, although many samples plot outside the alkaline field and close to the basalt–alkali basalt divide (Fig. 9b). Relative to other alkali basalts from the Bonavista Peninsula, they have lower Mg#’s, very low Ni, Cr, and variable V and Co contents, with higher FeO^T, TiO₂, P₂O₅, and Zr abundances (Fig. 10; Table 2), and are therefore more fractionated mafic magmas. The British Harbour pyroclastic rocks have comparable Mg#, SiO₂, Co, and Ni contents, lower TiO₂, P₂O₅, Zr, and Nb contents, and slightly higher Th contents relative to the volcanic rocks (Table 2; Fig. 10). The British Harbour volcanic rocks display a concave-downward pattern in the Th-Nb-light-REE portion of the multi-element plot (Fig. 11c), similar to the Fifields Pit basalt and most of the Dam Pond basalt samples, whereas the pyroclastic rocks and Bonaventure Head basalt have small Nb troughs (Fig. 11d). In terms of key element ratios, (La/Nb)_CN and (Th/Nb)_CN values are either close to or less than one for the British Harbour volcanic rocks, Fifields Pit and Dam Pond basalts, and are significantly higher for the British Harbour pyroclastic rocks and Bonaventure Head basalt (Table 1). The British Harbour basalts have higher Y (Fig. 12b), and lower La/Yb and Nb/Yb (Figs. 12c–d) than the Fifields Pit, Bonaventure Head, and Dam Pond basalts, and are therefore more EMORB-like (Fig. 12a, c, d). The British Harbour volcanic rocks plot within the mantle array Based on their incompatible trace element compositions, and close to EMORB in Th/Yb vs. Nb/Yb space, whereas the pyroclastic rocks plot above the mantle array (Fig. 12c), as expected for rocks containing a small component of ‘continental’ epiclastic material (Fig. 13c).

30 Of the collective Big Head Formation basalts, the Bonaventure Head basalt has the highest mantle-incompatible trace element ratios (Table 1), with the exception of Th/La which is higher in the other Big Head Formation basalts. The low Th/ La ratio, modest Nb, P, Zr and Hf troughs (Fig. 11), elevated Th/Yb (Fig. 12c) and low Nb/Th (Fig. 13c) are consistent with lithospheric/crustal contamination (see Pearce 2008). Contamination likely occurred post-eruption, during subsequent reworking and incorporation of epiclastic material, rather than in the mantle source or during ascent to the surface, as indicated by minor mudstone and siltstone clasts and subtle normal grading within the Bonaventure Head basalt breccia. Low (Th/Nb)_CN (Table 1), Th/Yb (Fig. 12c) and slightly elevated (Nb/Th)_MN (Figs. 13b–c) and (Nb/La)_MN (Fig. 13c), collectively indicate minor to negligible lithospheric/crustal contamination in the Dam Pond and Fifields Pit basalts. Like the Bonaventure Head basalt breccia, the British Harbour pyroclastic rocks similarly show evidence of contamination (Fig. 13c), including higher (Th/Nb)_CN and (La/Nb)_CN (Table 1) and modest Nb troughs (Fig. 11d), consistent with the minor epiclastic component observed in these rocks (e.g., Fig. 7c). The higher Th content and nominal Nb troughs of the British Harbour pyroclastic rocks are therefore interpreted to have been derived through physical entrainment of epiclastic fragments into the dominantly pyroclastic detritus during or shortly after eruption. This interpretation is in contrast to an origin via a ‘slab component’ contribution to the parental magmas in a suprasubduction setting, or to crustal/lithospheric assimilation during their ascent (see Pearce 2008 and Piercey et al. 2006).

31 Strongly elevated (La/Sm)_CN, (Tb/Yb)_CN and ∑REE (Table 1) suggest that the Bonaventure Head basalt, in particular, represents the final crystallization product of a small degree partial melt derived from an enriched, deep asthenospher-ic source, similar to that of OIB in an oceanic setting or to basalts of the Main (continental) Ethiopian Rift (Fig. 13e; Pearce 1996; Rooney 2010). These low degree partial melts likely ascended along deep-seated, pre-existing structures in the lithosphere. The low HREE abundances (<NMORB) and elevated (Tb/Yb)_CN (~1.6–2.5; Table 1) of the British Harbour and Bonaventure Head basalts are chemical characterist ics attributed to the preferential incorporation of Y and the HREE into garnet during partial melting and therefore suggest that their primary mantle sources were garnet-bearing (see Rooney 2010 and references therein) (Fig. 13e). The lower (Tb/Yb)_CN ratios of the Dam Pond and Fifields Pit basalts indicates shallower melting, transitioning from the garnet-bearing peridotite facies upwards into the spinel stability field (Fig. 13e). Although the depth of transition from spinel- to garnet-stable peridotite is not precisely constrained, abundant experimental evidence suggests that it occurs in the range of 1.5 to 2 GPa (~60–80 km: see Benxun et al. 2013; Niu et al. 2011). Moreover, if a relatively hot geothermal gradient is invoked (~60 mW/m²), a situation that is likely in an area of extending lithosphere, then garnet may coexist with spinel over a wide stability interval (10–15 km) at depths of 50–70 km (Ziberna et al. 2013). The steep HREE patterns and elevated (Tb/Yb)_CN ratios for the Bonavista alkali basalts indicate the presence of garnet in the mantle source and likely constrain the depth of generation of the primitive Bonavista Peninsula alkaline basaltic magmas to the garnet-spinel transition zone, corresponding to depths of ~50–90 km. The thickness of the Avalonian lithosphere during the Ediacaran is currently unconstrained.

32 Basalts of both the Big Head and Rocky Harbour formations, excluding those affected by crustal/lithospheric contamination, are characterized by steep, smooth multielements patterns with LREE-enrichment, high HFSE contents, and slightly elevated Nb content relative to Th and La (Fig. 11a, c), characteristic of within-plate basalts, OIB, or alkaline lamprophyres (e.g., Sun and McDonough 1989; Pearce 1996; Piercey et al. 2006; Rock 1991). Such rocks are derived from incompatible element-enriched mantle sources such as mantle plumes or at plate-margin environments under extension (van Staal et al. 1991; Piercey et al. 2006; Rooney et al. 2010; Matton and Jébrak 2009). The low volume of alkaline magmatism on the Bonavista Peninsula and their association with cross-stratified sandstone in an upward-coarsening, shallow marine to terrestrial sequence are consistent with a continental arc or margin under extension (see also van Staal et al. 1991).

33 The slab influence evident in the ca. 600 Ma calc-alkaline Headland basalts (Fig. 3 and HB field in Fig. 11c; see also Mills and Sandeman 2015) decreased through time. A minor slab influence is apparent in the continental tholeiites of the ca. 590 Ma Plate Cove volcanic belt (PCvb field in Fig. 11d) and is essentially absent in later (ca. 580 Ma and younger) alkali basalt magmatism (Big Head and British Harbour ba-salts); the latter are comparable to those associated with the Mesozoic opening of the North Atlantic (e.g., Matton and Jébrak 2009). Post-subduction tectonomagmatism (post ca. 600 Ma) likely involved slab rollback and possibly delamination to generate the continental tholeiites of the ca. 592 Ma Plate Cove volcanic belt. Subsequent magmatic underplating produced low degree partial melts of the asthenosphere that accumulated along pre-existing deep-seated faults prior to their emplacement within the Big Head and Rocky Harbour formations. The chemical differences between the primitive Big Head Formation basalts and the more evolved British Harbour basalts, and the temporal gap evident based on their stratigraphic positions, are consistent with episodic extensional tectonics with stalling of British Harbour parental magmas thus allowing time for the precursor magmas to evolve within the crust prior to emplacement.

34 Earliest extensional magmatism in the Bonavista Peninsula area includes continental tholeiite (Mills and Sandeman 2015) and weakly alkaline rhyolite (Mills 2020) of the Plate Cove volcanic belt, loosely constrained at ca. 592–591 Ma (Mills et al. 2017) based on the ages of tuffs from the western and eastern margins of the belt (see Fig. 3). The change from subduction-related to extensional magmatism likely occurred earlier, perhaps shortly after eruption of the ca. 600 Ma calc-alkaline Headland basalts (Mills 2019; Mills et al. 2016b) assigned to Cannings Cove Formation, basal Musgravetown Group. Based on the stratigraphic position of Dam Pond basalt below the Trinity facies on the northeastern Bonavista Peninsula (Figs. 2–3), mafic alkaline magmatism commenced prior to deposition of the glacial Trinity facies, constrained to ca. 580 Ma at Old Bonaventure (Pu et al. 2016; see Fig. 3). Whether or not heat associated with ca. 592–580 Ma magmatism helped to trigger deglaciation associated with the ca. 580 Ma Gaskiers event remains to be demonstrated here, but has been suggested in a global context (Youbi et al. 2020). Basaltic rocks at Fifields Pit and Bonaventure Head occur above the Trinity facies and are therefore interpreted to be younger than 580 Ma. The British Harbour basalts occur even higher in the stratigraphy, near the top of the Rocky Harbour Formation (Figs. 3–4; Table 1; see also Normore 2012a, b), and are therefore considered to be younger still. Felsic alkaline volcanism of the Rocky Harbour Formation has been dated at ca. 569 Ma (Mills et al. 2020) ~60 km to the northwest, near Content Reach (Fig. 1), further indicating the longevity and episodic nature of extensional magmatism during the Ediacaran in the northwestern Avalon terrane of Newfoundland.

35 The Musgravetown Group was initially defined by Hayes (1948) to include a thick succession of mainly coarse clastic sedimentary rocks and interbedded volcanic rocks that unconformably overlie the turbidite-dominated Connecting Point Group (Hayes 1948) and are unconformably overlain by mainly quartz-rich rocks of the marine-platformal, lower Cambrian Random Formation (Walcott 1900; Anderson 1981; Hiscott 1982). On Bonavista Peninsula, Musgravetown Group magmatism includes: calc-alkaline basalts of the basal, ca. 600 Ma Cannings Cove Formation; ca. 592 Ma continental tholeiites and alkaline felsic volcanic rocks of the Bull Arm Formation (Plate Cove volcanic belt); >580 Ma alkali (Dam Pond) basalts of the Big Head Formation; minor <580 Ma alkali basalts at Fifields Pit and Bonaventure Head (Big Head Formation?); and the younger, <580 Ma British Harbour alkali basalts (upper Rocky Harbour Formation). The British Harbour basalts are overlain by terrestrial, sandy to conglomeratic red beds of the Crown Hill Formation. At Keels, northwestern Bonavista Peninsula (Fig. 2), the Crown Hill Formation is disconformably overlain by quartz arenite of the Cambrian Random Formation (Normore 2010). Depending on the erosional depth and/or duration of the depositional hiatus represented by the disconformity at Keels (which may represent a first-order unconformity in the sequence stratigraphic context of Krapez (1996), related to global tectonic cycles; see Fig. 3 for location of Keels), and based on field and geochronological data, the Musgravetown Group, as presently delineated, may include rocks representing an interval of up to 60 m.y., from ca. 600 Ma to an unconstrained age prior to deposition of the Cambrian Random Formation. In contrast, rocks interpreted to span the same interval (from ca. 600 Ma to ca. 540 Ma) on the Avalon Peninsula comprise four lithostratigraphic units of group-status (Harbour Main, Conception, St. John’s and Signal Hill groups). We therefore suggest that the Musgravetown Group be elevated to Supergroup status on the basis of its prolonged depositional interval, the gradual change in geodynamic setting from a subduction-influenced to a less thermally intense extensional regime, as indicated by combined age and lithogeochemical constraints, and because it can be subdivided into volcanic- and sedimentary-dominated units that themselves merit group status.

36 Units of the lower Musgravetown Supergroup, including the Cannings Cove Formation and Plate Cove volcanic belt (both tentatively placed within the Bull Arm Group; Fig. 14; Table 1), have possible correlative units within the composite, ca. 730–585 Ma Harbour Main Group (see O’Brien et al. 2001). Pre-Trinity facies rocks, including the Plate Cove, Kings Cove Lighthouse, Monk Bay and Cape Bonavista facies, are likely time-correlative with the pre-Gaskiers Mall Bay Formation (Williams and King 1979) and, similarly, post-Trinity facies units (Kings Cove North and Herring Cove facies) are likely correlative with the post-Gaskiers Drook Formation (Fig. 14). Sandstones of the upper Rocky Harbour Formation, or our Rocky Harbour Group, may be similar to those of the Briscal Formation (upper Conception Group; Williams and King 1979). Correlation of the British Harbour volcanics with tuffaceous horizons within the Mistaken Point Formation (see Rettalack 2014; Matthews et al. 2020) remains a possibility that may be tested by geochronological and isotopic studies. Correlation of uppermost Rocky Harbour and Crown Hill groups should await further stratigraphical and geochronological work, and should include units of the Bay de Verde and Cape St. Mary’s sub-peninsulas (Fig. 1), including the Maturin Ponds, Heart’s Con-tent, Heart’s Desire and Trinny Cove formations (King 1988; cf. Hutchinson 1953; McCartney 1967), all of which were previously interpreted to lie stratigraphically above the Big Head Formation and below the Crown Hill Formation, and to be correlative with the mainly alluvial deposits of the Signal Hill Group (King 1988). With further work and a better understanding of the Musgravetown Supergroup, some of its numerous formations may retain their formation status, whereas others may require omission or redefinition as members or groups.