1 The New River belt (NRB) is the westernmost of three Proterozoic lithotectonic belts within the broader northeastern Appalachian orogen in southern New Brunswick (Fig. 1). These Proterozoic belts represent the peri-Gondwanan microcontinents of Ganderia and Avalonia (Barr and White 1996b; Fyffe et al. 2009, 2011; Johnson et al. 2012; van Staal and Barr 2012; Barr et al. 2014a, b). The easternmost Proterozoic rocks are in the Caledonia belt, which is the only belt in southern New Brunswick assigned to Avalonia (Barr and White 1996a). The Brookville and New River belts are interpreted to be part of Ganderia; these rocks form the basement to younger, Early Paleozoic arcs (Penobscot and Popelogan-Victoria) and back-arc complexes (Tetagouche-Exploits) exposed in south-central, central and northern New Brunswick, and central Newfoundland (van Staal et al. 2003; Wilson 2003; Hibbard et al. 2006; Zagorevski et al. 2010; Fyffe et al. 2011). The succession of Proterozoic rocks in the NRB and Caledonia belt are similar in that both contain Ediacaran volcanic and comagmatic rocks that formed in two separate pulses, one around ~ 625 Ma and a later one at ~ 555 Ma, and both lacking rocks representative of the ~70 million year hiatus. The Brookville belt that now separates the NRB and Caledonia belt shares only the later Ediacaran magmatic episode (~ 555 Ma) and includes an older ~ 650 Ma carbonate platform signifying a Late Cryogenian passive margin believed to have developed separately from Avalonia (Barr et al. 2014c).

2 The NRB belt extends from islands in Head Harbour Passage in the Bay of Fundy and trends northeast to the headland west of Maces Bay on mainland southern New Brunswick where it continues inland for a distance of over 160 kms (Figs. 1, 2). The NRB is truncated to the east by the Belleisle Fault, which separates it from the Early Silurian volcanic rocks of the Kingston Group (Figs. 1). The Kingston Group, and partially time-equivalent Mascarene Group that in New Brunswick is preserved within the NRB, represent an Early Silurian arc and back-arc complex, respectively, which were built on NRB Proterozoic basement and the overlying Early Paleozoic arc/back-arc complexes during an episode of northwest-directed (NW-dipping) subduction (Fyffe et al.1999; Barr et al. 2002). To the northwest, the NRB is juxtaposed against Cambrian to Ordovician rocks of the St. Croix and Annidale belts along the Sawyer Brook and Taylor Brook faults, respectively. To the southwest, the NRB trends toward coastal Maine where the Proterozoic rocks are likely to underlie the Cambrian Ellsworth Schist and Castine Volcanics of the Ellsworth belt (Fyffe and Fricker 1987; Schulz et al. 2008; Fyffe et al. 2009).

3 The NRB is typified by (1) Neoproterozoic arc-related, comagmatic volcanic and plutonic rocks of early Ediacaran (ca. 629–622 Ma) and late Ediacaran (ca. 555–551 Ma) age (Johnson and McLeod 1996; Johnson 2001; Johnson and Barr 2004), (2) latest Ediacaran–earliest Cambrian (ca. 540 Ma) relatively shallow marine to subaerial pyroclastic volcanic and sedimentary rocks of the Belleisle Bay Group and (3) post-540 Ma cover of Cambrian marine sedimentary and minor volcanic rocks. The Cambrian cover rocks occur in three separate structural panels that subparallel the northeasterly strike of the belt, with the most inboard panel being closest to the Kingston belt, and the most outboard panel closest to the Annidale belt (Johnson 2001; Johnson et al. 2009; Fyffe et al. 2011). From farthest inboard to farthest outboard these younger Cambrian rocks are divided into the Saint John, Almond Road, and Ellsworth groups, respectively. All three sequences are either directly underlain by the Belleisle Bay Group, as is the case with the Almond Road Group and Saint John Group in the Long Reach area, or linked to the Belleisle Bay Group by means of detrital zircon as is the case for the Ellsworth Group (Fyffe et al. 2009). Although the quartz-rich Almond Road Group was not previously dated, it was considered to be approximately the same age as the Matthews Lake Formation of the Ellsworth Group and Glen Falls Formation of the Saint John Group (e.g., Fyffe et al. 2011; fig. 4). The age of orthoquartzite and quartzose sandstone in the Matthews Lake Formation, the lower unit of the Ellsworth Group in the NRB, is constrained by earliest Cambrian detrital zircon in the basal beds (Fyffe et al. 2009) and a ca. 514 Ma U–Pb age for the overlying volcanic rocks (McLeod et al. 2003). The Early Cambrian age for orthoquartzite in the Glen Falls Formation, in the lower part of the Saint John Group, is also fairly well-constrained by dated ash beds, macrofossils, and acritarch assemblages in overlying and underlying formations (e.g., Tanoli and Pickerill 1988; Landing et al. 2008; Palacios et al. 2011, 2017), but the age of the Almond Road Group was poorly constrained until now.

4 We report new U–Pb (zircon) data that provide upper and lower age limits on the Almond Road Group. The maximum age of the basal quartzite in the group is Early Cambrian (late Fortunian), based on the youngest concordant detrital zircon population at ca. 530 Ma. The minimum age is Early Ordovician (early Floian) based on a U–Pb age of ca. 475 Ma for the cross-cutting West Scotch Settlement porphyry. Together these data support previous interpretations that Early Cambrian quartz-rich strata in the Ellsworth, Almond Road, and Saint John groups once formed a continuous, distal to proximal, Ganderian sandstone facies spanning all three structural panels of the NRB (Johnson et al. 2009; Fyffe et al. 2011) and may have extended across the Avalonian Caledonia belt as well.

5 The northeastern NRB is defined here as the structural panel between the Belleisle and Taylor Brook faults, west of Belleisle Bay and northeast of the Long Reach (Fig. 2). The Taylor Brook Fault shows evidence that it was initially a thrust fault that has been cut by a late steep structure that presently defines a large part of the boundary between the NRB and Annidale belt. With the exception of small, local fault-bounded units, the Silurian Mascarene Group is restricted to the area south of the Fanning Brook Fault, along the complexly faulted southern margin of the NRB in this area (Fig. 2). This lack of Silurian cover may be due to the oblique east-northeast-trend of the Fanning Brook Fault, which cuts across the regional northeast structural trend exposing older rocks on its north side (Fig. 1). Consequently this structural panel is the only one in which the basement rocks on the north side of the Mascarene basin are exposed.

6 The stratigraphic succession in the northeastern NRB is composed of the Almond Road Group and underlying Belleisle Bay Group. The Belleisle Bay Group in this area comprises two formations; the Browns Flat and Grant Brook formations. The Browns Flat Formation is not dated, but the overlying and in part laterally equivalent Grant Brook Formation yielded a U–Pb zircon age of 541 ± 3 Ma age for a slightly reworked intermediate to felsic tuff. Previous attempts to date the Grant Brook Formation yielded a maximum age of 542 ± 9 Ma (unpublished data from T. Krogh in Johnson et al. 2009), which is considered to be close to the depositional age based on the more recent age data. Historically the vast majority of rocks now included in the Belleisle Bay and Almond Road groups were assigned to the Silurian Mascarene Group (e.g., McCutcheon and Ruitenberg 1987). However, a pre-Silurian age for the Grant Brook Formation is demonstrated by the intense hornfelsing of the unit near the contact with the Early Ordovician (479 ± 2 Ma) Stewarton Gabbro (Johnson et al. 2009, 2012) and the Middle Ordovician (467 ± 3 Ma) age obtained for the crosscutting Bull Moose Hill porphyry (Johnson et al. 2012).

20 A total of 22 samples of mafic to intermediate volcanic rocks were chosen for analysis. The sampled units include the Ketchum Brook Formation in the Almond Road Group (4 samples), and the Grant Brook (12 samples) and Browns Flat (6 samples) formations in the Belleisle Bay Group. The majority of volcanic rocks in the Browns Flat and Ketchum Brook formations are basalt, whereas the composition of the Grant Brook Formation volcanic rocks is mainly andesitic or basaltic andesite (Fig. 10a). Three of the analysed samples from the latter formation contained > 63% SiO₂ (Appendix B), although only one of these plots in the rhyolite- dacite field based on immobile trace element ratios (Fig.10a). Immobile trace element ratios were also utilized to determine the magmatic affinity of the volcanic rocks in the northeastern NRB (Fig. 10b). The Th/Yb against Zr/Y bivariate diagram of Ross and Bedard (2009) was chosen as these authors have demonstrated that it is an improvement on Barrett and McLean’s (1994, 1999) single ratio diagrams using Zr versus Y, La versus Yb and Th versus Yb, although the data are still somewhat ambiguous. The basalt in the Browns Flat Formation plot almost exclusively in the transitional field, although one sample was clearly calc-alkaline (Fig. 10b). Basalt samples from the Ketchum Brook Formation are similar in that the four samples span the tholeiitic, transitional, and calc-alkaline fields. The one tholeiitic sample in the Ketchum Brook Formation was collected from the same general area as another that plots well into the transitional field but the reason why the trace element ratios are so different is not clear. In contrast, the Grant Brook Formation is clearly calc-alkaline based on Th/Yb and Zr/Y ratios (Fig. 10b).

21 On N-MORB-normalized trace element spider diagrams all of the samples exhibit depletion in Nb relative to Th or La (Fig. 11) that is typical of arc magmas. The Belleisle Bay Group exhibits three slightly different patterns that correlate directly with the composition of the volcanic rocks. As would be expected the most felsic samples in the Grant Brook Formation show the most pronounced depletion in Ti (Figs. 11b, 11c, and 11d). Basalts from the Browns Flat and Grant Brook formations are similar and lend support to the field evidence that suggests these formations are in part laterally equivalent. The younger Ketchum Brook Formation basalt samples exhibit patterns similar to those in the Belleisle Bay Group and are indistinguishable from them based on trace element patterns alone.

22 On a tectonic discrimination diagram using high field strength elements such as the Th-Hf-Ta of Wood (1980), all of the volcanic rocks in the Belleisle Bay and Almond Road groups plot in the volcanic arc basalt field (Fig. 12a) and are displaced above the diagonal line representing mid-ocean ridge basalt (MORB) and ocean island basalt (OIB) on the Th/Yb versus Nb/Yb diagram (Fig. 12b) of Pearce (2008), indicating that they interacted with continental crust or were erupted in a supra-subduction zone environment (Pearce and Peate 1995).

23 Previously published chemical data for the Belleisle Bay Group to the southwest in the Long Reach area (Johnson and Barr 2004) are compared to the results of this study. The latest Neoproterozoic to earliest Cambrian volcanic succession in the Long Reach area were divided into the Browns Flat and Beulah Camp formations, which were later assigned to the Belleisle Bay Group by Johnson et al. (2009). The Beulah Camp Formation is lithologically similar to the Grant Brook Formation and this correlation is corroborated by the chemical data. Basalts from the Beulah Camp Formation, like those in the Grant Brook were also chemically similar to those in the Browns Flat Formation (Johnson and Barr 2004). Major and trace element signatures in the former indicate that it also was likely erupted in a continental margin subduction zone (Johnson and Barr 2004). The Browns Flat Formation basalts in that area showed two different geochemical trends, one indicative of a within-plate setting and the other exhibiting characteristics of highly unevolved basalts suggesting a complex tectonic history.

24 Early Cambrian quartz-rich clastic sedimentary successions that overlie latest Ediacaran to earliest Cambrian volcanic and volcaniclastic to arkosic sedimentary rocks of the Belleisle Bay Group occur in all three structural panels within the NRB (Fyffe et al. 2011; fig. 4). These quartz-rich successions include the Matthews Lake Formation of the Ellsworth Group, the Snider Mountain Formation of the Almond Road Group, and the Glen Falls Formation of the Saint John Group. The age of the Glen Falls Formation is bracketed by a ca. 530 Ma ash bed from near the base of the underlying Ratcliffe Brook Formation and a ca. 510 Ma ash bed in the overlying Hanford Brook Formation (Landing et al. 1998; Landing and Westrop 1998; Isachasen et al. 1994; Palacios et al. 2011, 2017). A similar age is indicated for the Matthews Lake Formation as the overlying Mosquito Lake Road Formation is ca. 514 Ma (McLeod et al. 2003) and the youngest detrital zircon in the Matthews Lake Formation is ca. 540 Ma, (Fyffe et al. 2009). Although beyond the scope of this paper it is interesting to note the Glen Falls Formation quartzite in the Caledonia Terrane near Saint John shows a nearly unimodal detrital zircon population at ca. 540 Ma (Barr et al. 2012), essentially identical to the population distribution of zircon in the Matthews Lake Formation in the NRB, suggesting a similar source in the Early Cambrian. The age of the Almond Road Group is now bracketed between the Early Cambrian (late Terreneuvian) and Early Ordovician (Tremadocian) indicating that it is possible that all three quartzite sequences in the NRB are Stages 2-3 of the Cambrian (using the time scale of Cohen et al. 2013).

25 The textural maturity and bedforms that characterize the Glen Falls Formation indicate that it was deposited in a near-shore, high energy marine environment (Tanoli and Pickerill 1987; Landing and Westrop 1998). Although detailed sedimentological analysis was not undertaken on the Snider Mountain or Matthews Lake formations preliminary observations suggest a somewhat more distal depositional environment for these rocks. The most obvious is the lack of large-scale trough cross-bedding or other indications of wave action or fluvial processes that would be present in a shallow marine setting and the Snider Mountain Formation on Jones Brook contains no conglomerate at all. Although primary sedimentary features are rarely preserved in the more strongly deformed Matthews Lake Formation, some of the conglomerates have a mud-rich matrix and lithologically varied clasts that may have been deposited as debris flows. The Matthews Lake Formation as a whole tends to be a thicker, somewhat finer grained, quartzose sandstone sequence with local conglomerate debris flows that pass upwards into Mosquito Lake Formation containing sandstone with iron-rich siltstone lenses suggesting deposition in relatively deeper water. This led Fyffe et al. (2009, 2011) to suggest that the Almond Road and Matthews Lake may be more outboard facies of the tidally-influenced quartzite of the Glen Falls Formation, therefore would represent outer and inner platform sequences, respectively, deposited on NRB basement during the latter part of the Early Cambrian.

26 The arc-like chemistry of the Belleisle Bay Group is similar to the chemistry of mafic volcanic rocks in the upper Almond Road Group and the younger Late Cambrian to Early Ordovician Annidale Group, which occurs in the adjacent lithotectonic belt. The Annidale Group preserves the remains of the Penobscot back arc basin and a record of the Early Ordovician Penobscot orogeny (Johnson et al. 2009; 2012), one of the oldest orogenic events within the Appalachian orogen (Zagorevski et al 2007; van Staal and Barr 2012). In New Brunswick, the Penobscot back arc basin is interpreted to have developed along a continental margin represented by the Proterozoic rocks of the NRB and Brookville belt (Johnson et al. 2009, 2012; Fyffe et al. 2011). The Ediacaran rocks in these belts are interpreted to represent ensialic arc magmatism along the margin of Amazonia before it separated from Gondwana and became the Ganderian microcontinent (e.g., van Staal et al. 2012 and references therein). The arc signature of the ca. 540 Ma volcanic rocks in the Belleisle Bay Group supports the idea that the NRB was part of this continental margin in the Late Neoproterozoic and earliest Cambrian. The upwards transition from volcaniclastic sandstone and redbeds in the upper part of the Belleisle Bay Group to the widespread occurrence of quartz-rich sedimentary rocks by late Terreneuvian (530– 520 Ma) suggest a rift-to-drift transition, possibly signalling the breakup of Gondwana. In other areas of Ganderia arc magmatism was shut-off at around 540 Ma due to ridge-trench collision that transformed the Gander margin into a strike-slip regime (e.g., Rogers et al. 2006). Exhumation of older rocks within the arc during such a collision could explain the prominence of ca. 575 Ma detrital zircon in the Almond Road Group. Around 515 Ma the NRB underwent renewed arc volcanism as represented by rocks in the upper part of the Almond Road Group and the Mosquito Lake Road Formation (Johnson and McLeod 1996). This magmatism may be related to the development of the Penobscot arc and the formation of the Annidale back-arc basin by the Late Cambrian (ca. 495 Ma) (Johnson et al. 2012).

27 Rocks spanning the time period between the ca. 515 Ma Mosquito Lake Road Formation and ca. 495 Ma Annidale Group are not present in the NRB but they do occur in Maine. The chemical and isotopic data for the ca. 509–502 Ma Ellsworth Schist and overlying Castine Volcanics (Stewart 1998) in Maine show they are not arc-related, but that they likely developed within a proto- oceanic rift, such as in the Gulf of California (Schulz et al. 2008). Fyffe et al. (2009) considered the lithological characteristics of the Ellsworth volcanic sequence in coastal Maine to be similar to the Matthews Lake and Mosquito Lake Road formations in the NRB but the sequences are not temporally equivalent, therefore it remains unclear clear if this accounts for the contrast in tectonic environment or if it is due to along strike differences in an extremely complex long-lived convergent margin such as the Early Paleozoic Gander margin.