1 The Silurian (Ludlovian to Pridolian) to Late Devonian (Famennian) Saint George Batholith in southwestern New Brunswick is a large composite intrusion (2000 km²) that was emplaced into the margin of the peri-Gondwanan microcontinent of Ganderia (Fig. 1). The batholith and associated Silurian volcanic and sedimentary rocks of the Mascarene belt are situated along the boundary between Precambrian Ganderian basement rocks of the New River and Brookville belts to the southeast and Ganderian passive margin Cambrian-Ordovician sedimentary rocks of the St. Croix belt to the northwest (Fyffe and Riva 1990; van Staal et al. 2009; Fyffe et al. 2011). The presence of Proterozoic marble and quartzite in the Brookville belt distinguishes it from the Neoproterozoic volcanic and comagmatic plutonic rocks of the New River belt (Johnson and McLeod 1996; White and Barr 1996). The St. Croix belt is a highly deformed Cambrian-Ordovician (Tremadocian to Sandbian) sequence of shale, wacke, and quartzose sandstone; the Fredericton belt is a thick Silurian (Llandoverian to Ludlovian) sequence of wacke, shale, and calcareous sandstone (Ruitenberg 1967; Fyffe and Riva 2001; Castonguay et al. 2003; Thorne et al. 2008). The precise nature of the contact between the St. Croix and Fredericton belts is difficult to discern in the field due to poor exposure, complex deformation, and similarity of lithotypes. The contact has been interpreted as a northwest-directed thrust, referred to as the Honeydale Fault (Ruitenberg 1967).

2 Silurian (Llandoverian) volcanic and comagmatic plutonic rocks of the Kingston belt separate the Brookville belt from the New River belt. Felsic volcanic rocks in the Kingston belt have yielded U-Pb zircon ages ranging from 442 to 436 Ma, and associated felsic plutonic rocks, ranging from 437 to 435 Ma (Doig et al. 1990; Barr et al. 2002; McLeod et al. 2003). These volcanic and plutonic rocks are interpreted, based on geochemical characteristics, to represent remnants of a Silurian volcanic arc that formed above a subduction zone dipping to the northwest beneath the New River belt (Fyffe et al. 1999; Barr et al. 2002; White et al. 2006). The shallow marine to subaerial sedimentary and volcanic sequences of the Silurian (Llandoverian to Pridolian) Mascarene belt, lying to the northwest of the Kingston belt, are interpreted to have been deposited in a backarc basin that opened behind the Kingston arc (Fyffe et al. 1999; Barr et al. 2002). Felsic volcanic rocks from the Mascarene belt have yielded U-Pb zircon ages ranging from 438 to 423 Ma (Miller and Fyffe 2002; Van Wagoner et al. 2002).

3 Based on field relationships and geochronological data, McLeod (1990) divided the Saint George Batholith into the following units from oldest to youngest (Fig. 1): Bocabec Gabbro – greyish green, locally layered, medium-grained gabbro and minor associated granitic rocks; Utopia Granite – typically red, leucocratic, medium- to coarse-grained, equigranular, biotite granite; Welsford, Jake Lee Mountain, and Parks Brook granites – beige to pink, mediumgrained and equigranular to fine-grained and porphyritic, amphibole-bearing, peralkaline granites; Magaguadavic Granite – greyish pink, coarse-grained, megacrystic granite; John Lee Brook Granite – light grey, medium-grained, equigranular, garnetiferous, two-mica granite; and Mount Douglas Granite – light pink, medium-to-coarse-grained and equigranular to fine-grained and porphyritic, rapakivitextured, biotite granite. Geochronological data available to McLeod (1990) included: U-Pb zircon ages from the Utopia (430 ± 3 Ma), Welsford (422 ± 1 Ma), and Magaguadavic (396 ± 1 Ma) granites; an ⁴⁰Ar/³⁹Ar muscovite age of 384 ± 7 Ma from the John Lee Brook Granite; and U-Pb monazite ages from the Mount Douglas granite (367 ± 1 Ma, 366 ± 1 Ma).

4 To provide additional crystallization ages for the plutons comprising the Saint George Batholith, six samples were selected for laser ablation inductively coupled plasma-mass spectrometry (LA ICP-MS) analysis at the University of New Brunswick’s Department of Earth Sciences. The U-Pb geochronological work was coordinated with field mapping of the Saint George Batholith by the Geological Surveys Branch of the New Brunswick Department of Energy and Resource Development during the summers of 2015 and 2016. Dated sample sites (Locations 1 through 6, numbered from oldest to youngest age of crystallization) are shown on Figure 1 and Figure 2. Locations 1, 4, 5 and 6 are in the Utopia, Jake Lee Mountain, Wellington Lake (new name), and John Lee Brook granites, respectively. Locations 2 and 3, previously mapped as part of the Mount Douglas Granite and Jake Lee Mountain Granite, respectively (McLeod 1990, McLeod et al. 1998), are here assigned to the Utopia Granite on the basis of new field mapping (see below).

5 In 2015, geochronological analysis of an archived sample from the John Lee Brook Granite indicated that this twomica granite is older than the megacrystic Magaguadavic Granite rather than younger as previously suggested (McLeod 1990). Field work was therefore undertaken in the summers of 2015 and 2016 to resolve the conflict between the previously interpreted intrusive history of the Saint George Batholith and the new geochronological data. Contact relationships between the various plutons comprising the Saint George Batholith were examined and the new findings are reported below and incorporated into the revised regional map of the batholith (Fig. 1). Detailed mapping of contacts between intrusive and country rocks during our recent mapping has also better defined the areal distribution and geometry of the plutons comprising the batholith. Previous maps depicted the Saint George Batholith as a contiguous assemblage of intrusions that were emplaced over an extended period of time to form a single massive body. The new mapping, however, suggests that the batholith is more realistically comprised of a loose cluster of distinctive plutons that intruded mainly into sedimentary and volcanic host rocks rather than into previously consolidated intrusive bodies (Fig. 1).

6 McLeod (1990) recognized and named the poorly exposed John Lee Brook Granite as a distinct pluton during his mapping of the Saint George Batholith. It is a mediumgrained, equigranular, garnetiferous, two-mica granite, underlying an area of about 50 km² along the northern margin of the batholith just south of the Late Devonian Mount Pleasant Caldera (Thorne et al. 2013). The ⁴⁰Ar/³⁹Ar muscovite age of 384 ± 7 Ma from the John Lee Brook Granite together with the reported presence of white microgranitic veins cutting the adjacent Magaguadavic Granite, dated by U-Pb on zircon at 396 ± 1 Ma (Bevier 1990), suggested that the former was younger than the latter (McLeod 1990). However, dating of sample 85MM-154B, collected by McLeod (1990) from near the headwaters of John Lee Brook, yielded a crystallization age of 413 ± 2 Ma (Location 6 on Fig. 1) casting doubt on the above interpretation.

7 Field observations undertaken in the summer of 2015 on a small isolated body of megacrystic granite exposed along the upper reaches of McDougall Inlet, west of South Oromocto Lake (Fig. 1), indicate that the megacrystic granite intrudes and truncates quartz veins and aplitic dykelets within the John Lee Brook Granite (Fig. 3a). This isolated body is considered herein to be part of the larger Jimmy Hill Granite (Fig. 1), which underlies the South Oromocto Lake area to the east of McDougall Inlet; McLeod (1990) previously included the granite in the South Oromocto Lake area as part of the Magaguadavic pluton. A close-up photograph of the outcrop along McDougall Inlet clearly shows pink megacrysts of potassium feldspar with rapakivi mantling characteristic of the Jimmy Hill Granite in contact with the equigranular John Lee Brook Granite (Fig. 3b). The observed cross-cutting relationship confirms that both the megacrystic Jimmy Hill and Magaguadavic plutons, dated at 403 ± 2 Ma and 396 ± 1 Ma, respectively (Bevier 1990; Davis et al. 2004), are younger than the John Lee Granite in agreement with new age determination obtained on the latter (see below).

8 Geophysical maps of the St. George area clearly outline the Magaguadavic pluton in the McDougall Lake area as a prominent, circular magnetic feature (King and Barr 2004). This magnetic signature has been used to draw contacts on previously published bedrock maps that enigmatically suggest that the Early Devonian megacrystic granite of the Magaguadavic pluton cross-cuts the adjacent Late Devonian rapakivi granite of the Mount Douglas pluton. However, the recent field mapping has shown that the two plutons are separated by a screen of sedimentary rocks from 300 to 400 m wide and so do not come into direct contact with one another as shown on earlier maps; thus, the Magaguadavic pluton in this area intruded sedimentary and volcanic rocks along its eastern margin – Ordovician quartzose sandstone of the Kendall Mountain Formation (Cookson Group) in the north and Silurian siltstone and mafic volcanic tuff assigned to the Jones Creek Formation (Mascarene Group) in the south – and the Mount Douglas pluton intruded the same screen of sedimentary and volcanic rocks of Jones Creek Formation along its western margin (Fig. 1). More details on the stratigraphy of the Mascarene Group are given below under the discussion on the tectonic evolution of the Mascarene backarc basin.

9 The Utopia Granite underlies an area of about 100 km² along the southern margin of the Saint George Batholith. The northern margin of the Utopia pluton is separated from the southern margin of the Magaguadavic pluton by a screen from 300 to 600 m wide of dark grey siltstone and minor mafic tuff of the Silurian Jones Creek Formation exposed above Second Falls on the Magaguadavic River (Figs. 1, 4, and 5). Gabbroic rocks, exposed a short distance down river to the south, are injected by red granitic dykelets characteristic of the Utopia Granite; the granitic dykelets surround clusters of lobate gabbroic enclaves, many of which have fine-grained margins, suggesting that emplacement of the two magma types was essentially contemporaneous (Fig. 6). Similar magma mingling has been observed in the main part of the Bocabec Gabbro in the Oak Bay area (Fig. 1) farther to the southwest and in the Moosehorn Plutonic Suite in adjacent Maine (Fyffe 1971; McLaughlin et al. 2003). Note, however, that the new mapping suggests that a sedimentary screen exists between the Utopia and Bocabec plutons (Fig. 1), casting doubt on previous map interpretations showing a direct physical connection between gabbroic rocks in the Utopia pluton and those of the adjacent Bocabec pluton.

10 Bevier (1990) and Barr et al. (2010) reported U-Pb zircon ages of 430 ± 3, 428 ± 1, and 426 ± 6 Ma from samples of red Utopia Granite collected near the northern end of Lake Utopia (Fig. 1). These late Wenlockian to Ludlovian ages, according to the time scale of Ogg et al. (2016), seemed too old given that fossil evidence from sedimentary rocks in Maine indicate a Pridolian age (Churchill-Dickson 2004) for the surrounding Eastport Formation (Mascarene Group) and that felsic volcanic rocks in the Eastport Formation in New Brunswick have yielded a preliminary U-Pb zircon age of 423 ± 1 Ma (Van Wagoner et al. 2002). However, the recent field mapping indicates that the Utopia pluton near the above dated granite may have been emplaced no higher in the stratigraphic section than the Llandoverian to Wenlockian Jones Creek Formation (Mascarene Group). To further investigate the age of the Utopia Granite, sample 171-6 was collected from a distinctive, light pink granite (Martin 2013) exposed on the summit of Dawson Mountain (Fig. 7) to the northwest of Lake Utopia (Location 1 on Fig. 1).

11 Field mapping was conducted in the Red Rock Lake area to the northeast of Lake Utopia (Fig. 1) to better delineate the contact between the Silurian Utopia Granite and Late Devonian Mount Douglas Granite. Distinguishing between these two granites can be difficult in places as both can vary from red to pink. However, as documented by Cherry and Trembath (1978), the granitic rocks in the Lake Utopia area are homogeneous, equigranular, and medium- to coarse-grained, whereas those in the Mount Douglas area are heterogeneous consisting of about 65% equigranular, medium- to coarse-grained phase and about 35% finegrained porphyritic and aplitic phases. In addition, rapakivi-mantling of potassium feldspar is common in the granitic rocks in the Mount Douglas area (Fig. 8) – a feature that is rare in those of the Lake Utopia area (Cherry and Trembath 1978). These criteria were used to conclude that the Mount Douglas Granite extends farther south in the Red Rock Lake area (Fig. 1) than shown by McLeod (1990). This conclusion was confirmed by preliminary Late Devonian dates of 363.7 ± 4.1 Ma and 365.0 ± 3.2 Ma obtained on zircon grains from two samples taken along Red Rock Stream to the west of Red Rock Lake (geochronological data for these samples are not included herein). The location of the contact between the Mount Douglas and Utopia granites could only be limited to within a distance of 500 m due to the scarcity of exposures to the south of Red Rock Lake.

12 Detailed field mapping was conducted in the Wellington Lake-New River area (Figs. 1 and 2) to better define the contacts between the Utopia, Mount Douglas, and Jake Lee Mountain plutons. Previous mapping in this area by McLeod (1990) had delineated a one km-wide by two kmlong eastward continuation of the Utopia Granite preserved within the Mount Douglas Granite on the east side of New River. Re-examination of exposures along the resource road running south from Victoria Lake revealed the presence of conspicuous rapakivi-mantling of potassium feldspars leaving little doubt that these rocks are part of the Mount Douglas pluton. The next exposure to the south along the resource road, following a gap in exposure of 800 m, is comprised of a medium pink, equigranular granite mapped by McLeod (1990) as part of the Mount Douglas pluton. However, the outcrop at this site lacks rapakivi texture so sample 15-87 was collected for geochronological analysis (Location 2 on Figs. 1 and 2) to determine if this granite is part of the Silurian Utopia pluton. sample 15-104 was collected to the south of Wellington Lake to confirm that the Utopia Granite extends to the west of New River as suggested by the recent mapping (Location 3 on Figs. 1 and 2).

13 Distinctive, medium- to coarse-grained, light greyish-white to greyish-pink granite, underlying an area of about 15 km² centered on Wellington Lake, was recognized during the recent mapping on west side of the New River about 19 km northeast of Lake Utopia (Fig. 1). Originally included as part of the Utopia Granite by McLeod (1990), its zirconiumrich geochemistry suggests that it may be a separate pluton (see geochemistry section below) and is herein referred to as the Wellington Lake Granite. It is bordered to the southeast by a narrow band of Mount Douglas Granite consisting of light pink, medium-grained granite grading into a finegrained porphyry containing rapakivi-mantled potassium feldspar. The Wellington Lake Granite is separated from the main mass of the Mount Douglas Granite to the northwest by a screen from 200 to 400 m wide of light grey siltstone with locally interbedded basalt assigned to the Silurian (Llandoverian to Wenlockian) Jones Creek Formation of the Mascarene Group (Fig. 2). Siltstone inclusions from 10 to 20 cm in diameter are common in the Wellington Lake Granite to the west of Wellington Lake near the unexposed northwestern contact with the sedimentary screen (Fig. 9). Sample 15-72 was collected at this site (Location 5 on Figs. 1 and 2) to define the age of crystallization of the Wellington Lake Granite.

14 The peralkaline Jake Lee Granite, named by Currie (1988), is an oblong pluton about 20 km² in area intruded into volcanic and sedimentary rocks of the Llandoverian Letete Formation (Mascarene Group) in the southeast, Pridolian sedimentary rocks of the Eastport Formation (Mascarene Group) in the northeast, and granitic rocks of the Utopia pluton in the northwest (Fig. 2). Its core consists of medium-grained, beige, equigranular, riebeckite-bearing granite grading outward to a fine-grained, light grey to light pink, porphyritic margin locally exhibiting rapakivi texture (Fyffe 1998). The fine-grained, northern margin of the Jake Lee Mountain pluton contains abundant, angular blocks of gabbro (Fig. 10). A coherent screen of gabbroic rocks about 300 m wide separates it from the Utopia Granite to the north. Sample JL16-1 was collected from medium-grained, beige granite forming the core of the Jake Lee Mountain pluton in order to determine the age of this previously undated peralkaline granite (Location 4 on Figs. 1 and 2).

15 U-Pb crystallization ages of zircon and monazite were determined by in situ laser ablation inductively coupled plasma-mass spectrometry (LA ICP-MS) at the University of New Brunswick. Standard polished thin sections were prepared and the zircon and monazite were analyzed using a Resonetics S-155-LR 193 nm ArF Excimer laser ablation (LA) system coupled to an Agilent 7700x quadrupole inductively coupled plasma-mass spectrometer (ICPMS). Only those zircon and monazite grains large enough to accommodate a 25 µm and 17µm diameter crater, respectively are measurable using laser ablation. Masses of ³¹P, ⁸⁹Y, ²⁰²Hg, ²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb, ²³²Th, and ²³⁸U were analyzed with ³¹P used as a guide mass and for internal standardization (assuming ~13 wt. % P in monazite) for calculating Y, Pb, Th, and U concentrations in monazite. ⁹⁰Zr was used as a guide mass and internal standardization (assuming ~43 wt. % Zr in zircon) for calculating Pb, Th, and U concentrations in zircon. For monazite, U-Pb isotope data was standardized using GSC-8153 monazite and the accuracy of the results confirmed using 44069 monazite (Aleinikoff et al. 2006). Zircon was standardized against 1099 Ma FC-1 zircon (Paces and Miller 1993) and accuracy checked using 337 Ma Plesovice zircon (Slama et al. 2008). Concentrations of U and Th in zircon were calculated relative to NIST610 which was analyzed throughout the analytical sequence. The data have been reduced using the VizualAge U-Pb geochronology data reduction scheme under Iolite software v. 2.5. Common-Pb correction was applied using Andersen (2002) method. A total between 45 and 82 of zircon and monazite analyses were obtained for each sample; however, of this total number of analyses a subset that are <5% discordant was used to calculate concordia ages using Isoplot version 3.71.09.05.23nx (Ludwig 2009). Details of the method have been described in McFarlane (2015) and Azadbakht et al. (2016). The results of the U-Pb monazite and zircon geochronological studies on the six samples selected for LA ICP-MS analysis (Table 1, Fig. 11) are discussed in detail below.

27 Dated samples from the Utopia Granite (Sample 171-6 and 15-87) and Wellington Lake Granite (Sample 15- 72) were analyzed for major elements and selected trace elements, including rare earth elements (REEs). The method used was laser ablation inductively coupled plasma-mass spectrometry (LA ICP-MS) on fused glass beads at the University of New Brunswick. Standards BCR-2, GSP-2, and SY-4 were used to check precision and accuracy on the fusion beads. Major elements were obtained with a JEOL JSM 6400 Scanning Electron Microscope equipped with an EDAX Genesis X-ray microanalysis system (SEM-EDS). The accelerating voltage was 15 kV and the probe current was approximately 1.5 nA. Spectra were collected for 50 s live-time. Trace element analyses were performed using LA-ICP-MS with a 45µm diameter crater. Standards used were NIST-610 (primary concentration standard), NIST-612 (secondary concentration standard), and BCR-2G (QC standard). The geochemical results are shown in Table 2 together with data on the John Lee Brook Granite (sample 85MM-154B) taken from McLeod (1990). No analysis is available for the dated Jake Lee Mountain Granite (sample J16-1).

33 The current plate tectonic model developed for the post-Penobscottian evolution of southern New Brunswick relates Acadian (late Silurian to early Devonian) tectonism in the region to closure of a remnant oceanic tract, referred to as the Acadian Seaway (van Staal et al. 1996; Fyffe et al. 1999; Barr et al. 2002; White et al. 2006; van Staal et al. 2009). This seaway separated the peri-Gondwanan microcontinent of Avalonia (see inset on Fig. 1) from the amalgamated terranes already accreted to the Laurentian continental margin during Early Paleozoic closure of the Iapetus Ocean. The peri-Gondwanan microcontinent of Ganderia, the most outboard of these Iapetan terranes, was accreted to the Laurentian margin during Salinic (late Ordovician to late Silurian) orogenesis. Precambrian rocks of the New River and Brookville belts are considered to underlie the sparsely preserved platform of Ganderia, whereas the Cambrian-Ordovician sedimentary rocks of the St. Croix belt are interpreted to represent its Paleozoic passive margin (van Staal et al. 1996; Fyffe et al. 2011). Subsequent Acadian tectonism and magmatism in southwestern New Brunswick is attributable to accretionary events associated with closure of the Acadian Seaway by northwest-directed subduction beneath the Kingston arc-Mascarene backarc system and its St. Croix-New River substrate (Fig. 1). The arc-backarc system was subsequently extensively dismembered by strike-slip displacement along the Sawyer Brook, St. George, Wheaton Brook, Belleisle, and Kennebecasis faults (Stewart et al. 1995; Fyffe et al. 1999, Fyffe et al. 2011). The Sawyer Brook, St. George, and Wheaton Brook faults were inactive by the Late Devonian as they are cross-cut by the Mount Douglas Granite, emplaced between 367 ± 1 and 366 ± 1 Ma (Bevier 1989; McLeod 1990).

34 The Mascarene backarc basin is situated along the transitional zone between the Ganderian platform comprised of Neoproterozoic basement rocks and its thin cover of Cambrian quartzite (New River belt) in the south and the Ganderian passive margin comprised of a thick succession of Cambrian-Ordovician sedimentary rocks (St. Croix belt) in the north (Fyffe et al. 2011). The Silurian sequences in the dismembered backarc basin, which are all included in the Mascarene Group, have been divided into several formations (i.e., Oak Bay, Waweig, Letete, Long Reach, Jones Creek, and Eastport), described briefly below. The contact between the steeply dipping conglomerate of the Silurian Oak Bay Formation at the base of the Mascarene Group and Cambrian-Ordovician sedimentary rocks of the Cookson Group is generally marked by the Sawyer Brook Fault, along which considerable transcurrent movement has occurred (Park et al. 2008).

35 Volcanic rocks become abundant up-section in the Oak Bay Formation and the contact with the overlying Waweig Formation, a sequence of moderately dipping, light pink, fossiliferous, fine-grained sandstone and siltstone interbedded with felsic and lesser mafic tuffs, is gradational (Fyffe et al. 1999). A felsic tuff in the Waweig Formation yielded an early Silurian (Llandoverian) U-Pb zircon date of 438 ± 4 Ma (Miller and Fyffe 2002). The Letete Formation, a complexly deformed, steeply dipping sequence of felsic and mafic tuffs interbedded with dark grey shale, lies along the southern margin of the Mascarene backarc basin between the St. George and Wheaton Brook faults (Fig. 1). Felsic tuff from the Letete Formation yielded an early Silurian (Llandoverian) U-Pb zircon date of 437 ± 7 Ma (Miller and Fyffe 2002). Mascarene strata to the east of the Saint George Batholith (Fig. 1) include mafic flows and tuffs of the Long Reach Formation and conformably overlying, fine-grained, feldspathic sandstone, siltstone, dacitic flows, and minor mafic tuffs of the Jones Creek Formation. Brachiopods indicate a late Llandoverian to early Wenlockian age for the Long Reach Formation (McCutcheon and Ruitenberg 1987), and associated dacitic volcanic rocks have yielded an early Silurian (early Wenlockian) U-Pb zircon date of 432 ± 2 Ma (N. Van Wagoner, written communication to M. McLeod, 2001). An equivalent sequence of volcanic rocks lying beneath the late Silurian Eastport Formation in adjacent Maine has suprasubduction-zone geochemical signatures (Llamas and Hepburn 2013).

36 The early Silurian volcanic sequences of the Oak Bay, Waweig, Long Reach, Jones Creek, and Letete formations, which are at least partially coeval with the volcanic arc rocks in the Kingston Group, are considered here to have been generated in a continental backarc basin in an extensional suprasubduction-zone environment (Fyffe et al. 1999; Barr et al. 2002; White et al. 2006; van Staal et al. 2009). Fossil and geochronological data are consistent with a period of uplift and erosion or non-deposition of Silurian strata in the Mascarene backarc basin in the mid- to Wenlockian. This hiatus, delineated radiometrically by the age of the youngest suprasubduction-zone related volcanism (432 ± 2 Ma) and oldest non-arc plutonism (428 ± 1 Ma) constrains the timing of the closure of the Acadian Seaway and collision of Avalonia with the Ganderian margin of composite Laurentia to the mid-Silurian.

37 The mid-Silurian hiatus in the Mascarene backarc basin was followed by deposition of the shallow-marine to subaerial succession correlated with the Eastport Formation in Maine (Fyffe et al. 1999). The Eastport Formation in New Brunswick, which occurs along the southern margin of the Saint George Batholith to the north of the St. George Fault (Fig. 1), comprises a gently southerly dipping succession of mafic and felsic flows and tuffs interbedded with peritidal to subaerial, red sandstone and siltstone. Felsic flows in the middle section of the Eastport Formation are enriched in Zr (Van Wagoner et al. 2002), suggesting that they may be the extrusive equivalents of the peralkaline Welsford, Jake Lee Mountain, and Parks Brook granites. The Eastport volcanic rocks have within-plate geochemical signatures and have been reported to have a late Ludlovian to early Pridolian age based on the poorly documented, preliminary U-Pb zircon date of 423 ± 1 Ma (Van Wagoner et al. 2002). The location of the dated felsic flow is not known with certainty but it was likely collected about 1 km above the base of the Eastport Formation.

38 The bulk composition, spatial distribution, and timing of emplacement of the various Silurian (late Wenlockian to Pridolian) to Early Devonian (Lochkovian to Emsian) plutonic rocks in southwestern New Brunswick are discussed below in relationship to the tectonomagmatic evolution of the Kingston arc-Mascarene backarc system (Kingston and Mascarene belts on Fig. 1). As emphasized by Hyndman et al. (2005), convective heat flow in backarc regions can continue for some tens of millions of years after subduction stops; these hot and weak former backarcs can then become the focus of high temperature metamorphism, ductile deformation, and plutonism. Based on previously published geochronological data, the composite Saint George Batholith was emplaced into the Mascarene backarc basin during three separate intervals (428 to 422 Ma; 403 to 390 Ma; and 367 to 366 Ma), covering a time span of approximately 60 million years (Bevier 1989, 1990; McLeod 1990; Whalen et al. 1996; Thorne et al. 2002, 2008; Davis et al. 2004; Barr et al. 2010). The new results reported above indicate magmatic activity also occurred in the backarc between 418 and 413 Ma (Table 1) and indicate that the tectonomagmatic history of the Saint George Batholith was more protracted than previously envisioned. All these intrusive episodes took place after the eruption of suprasubduction – zone related volcanism in the Mascarene backarc basin had ceased at 432 ± 2 Ma.

49 Average bulk chemical compositions of plutons comprising the Saint George Batholith and satellite Tower Hill pluton have been calculated using published analyses from McLeod (1990) and Whalen (1993) to supplement the data reported herein for the newly dated samples. These averaged values confirm that the spatial and temporal variations observed from southeast to northwest across the batholith are reflected systematically in the trace-element, REE, and isotopic Nd geochemistry of the clustered plutons (Figs. 23 and 24). On the Pearce et al. (1984) discrimination diagrams, granitic samples from the Bocabec, Utopia, Welsford, Jake Lee Mountain, and Wellington Lake plutons all fall in the ‘within –plate’ ‘field whereas the John Lee Brook, Jimmy Hill, Magaguadavic, and Tower Hill plutons fall mainly on the boundary between the’ syn-collisional’ and ‘volcanic-arc’ fields (Fig. 24).

50 The geochemical variation across the Saint George Batholith can be explained in terms of the transition of the Kinston arc-Mascarene backarc system from an extensional arc to transpressional tectonic environment during the Acadian orogeny, which was caused by closure of the Acadian Seaway and collision of the Avalonian microcontinent with the composite Laurentian margin (van Staal et al. 2009). The initial extensional nature of the Kingston arc is indicated by the early Silurian emplacement of a high-level bimodal intrusive complex into the host volcanic-arc rocks of the Kingston Group. Mafic and granitic dykes in this complex have active-continentalmargin and within-plate signatures, respectively (Eby and Currie 1993). A subsequent change to a compressive environment is indicated by the presence of a zone of low temperature-high pressure metamorphism along the southeastern margin of the Kingston arc, which occurred after emplacement the late Llandoverian intrusive complex at 436 ± 2 Ma (Doig et al. 1990) and prior to 420 ± 2 Ma, the oldest ⁴⁰Ar/³⁹Ar cooling age of metamorphic amphibole defining the foliation in the mafic dykes (Nance and Dallmeyer 1993; Barr et al. 2002; White et al. 2006).

51 A much later, high temperature-intermediate pressure ‘regional contact’ metamorphic event associated with emplacement of the Tower Hill Granite at about 401 Ma is evident in the St. Croix belt to the northwest of the Mascarene backarc basin (Fyffe et al. 1991; Whalen et al. 1996). Staurolite and garnet are present in pelitic rocks of both the low-pressure metamorphic assemblage in the St. Croix belt and high-pressure assemblage in the Kingston belt but the garnets in the St. Croix assemblage are significantly lower in magnesium and higher in manganese content (Fyffe et al. 1991; White et al. 2006). The contrast between the penetrative deformation apparent in the Pridolian Mohannes Granite in the St. Croix belt, and the relatively undeformed nature of the late Wenlockian to Pridolian plutons of the Saint George Batholith on opposite sides of the Sawyer Brook Fault (Fig. 1), is likely a reflection of the depth of emplacement and subsequent uplift of the hotter, deeply buried, complexly deformed, Cambrian-Ordovician sedimentary sequence in the northwest, compared to the cooler, near-surface, gently dipping volcanic sequence (Eastport Formation) exposed along the southeastern margin of the Mascarene backarc basin. This strain gradient is evident from the strong ductile fabrics present in the more deeply buried, lower part of the Silurian volcanic sequence (Waweig Formation) and contained mafic dykes now exposed along the northwestern margin of the Mascarene backarc basin in Clarence Stream area (CS on Fig. 1).

52 Emplacement of late Wenlockian to Pridolian Bocabec, Utopia, Jake Lee Mountain, Parks Brook, and Welsford plutons into the host volcanic rocks of the Mascarene backarc basin coincided with the change from early Silurian suprasubduction-zone influenced volcanism to late Silurian bimodal volcanism in the backarc area. This change in tectonomagmatic environment is attributed to the arrival of the leading edge of Avalonia at the trailing edge of Ganderia of composite Laurentia. The attempted subduction of the buoyant Avalonia microcontinent plate caused a decrease in the angle of subducting slab beneath the Kingston arc leading to increased coupling between the upper Ganderian and lower Avalonian plates, resulting in the cessation of arc-related volcanic activity. Further plate convergence led to partitioning of strain into orthogonal shortening and margin-parallel, strike-slip components focused in the thin, hot and weakened backarc lithosphere (e.g., Hyndman et al. 2005). As the downgoing Avalonian plate reached higher temperatures at greater depths, the dense, dehydrated, subducting oceanic slab ruptured and broke away from the more buoyant continental part of the lower plate, and compressive forces between the Ganderian and Avalonian plates diminished due to the loss of slab pull (Davies and von Blanckenburg. 1995; Ellis et al. 1998; Barnes et al. 1998). Thermomechanical modelling by Gerya et al. (2004) indicates that slab break-off occurs from 5 to 30 million years after collision and that ensuing magmatism can last between 10 to 20 million years. In the Kingston arc-backarc system, slab break-off and the termination of continental subduction is interpreted to have occurred in the mid-Silurian between 432 ± 2 Ma, the youngest age of arc volcanism, and 428 ± 1 Ma, the oldest age of postcollision plutonism in the Saint George Batholith. Partial melting of the thin, hot lithosphere inherited from the opening of the Mascarene backarc basin would have been promoted by upwelling and decompressional melting of the asthenospheric mantle following slab-break-off. Such a mechanism accounts for the generation of voluminous bimodal (gabbroic and granitic) magmatism represented by the Bocabec, Utopia, Welsford, Jake Lee Mountain, Parks Brook, and Wellington Lake plutons lasting over a period of about 13 million years (Fig. 11; Tables 1 and 3).

53 Systematic variation in La/Yb values is evident across the Saint George Batholith: the more southerly group of plutons (Bocabec, Utopia, Welsford, Jake Lee Mountain, and Wellington Lake) typically have (a) a range of La/ Yb values of less than 11, (b) a relatively high isotopically positive epsilon Nd values, and (c) ages ranging from late Wenlockian to early Lochkovian; those to the north (John Lee Brook; Jimmy Hill, Magaguadavic, and satellite Tower Hill pluton) have (a) considerably higher La/Yb values; (b) low positive to slightly negative epsilon Nd values; and (c) ages ranging from late Lochkovian to Emsian (Table 3). Whalen et al. (2006) documented a similar increase in La/Yb and decrease in epsilon Nd in Silurian plutons at increasing distances to west of the Dog Bay Line in central Newfoundland. They interpreted the Dog Bay Line to represent the suture zone between Laurentia and Ganderia that resulted from closure of the intervening Exploits basin by west-directed subduction of backarc oceanic crust. The variation La/Yb and epsilon Nd in these plutons was attributed to slab break-off resulting in a change from shallow-level, garnet-absent melting of juvenile crust proximal to the suture zone and deeper level melting of old granitic basement in the garnet stability field farther to the west.

54 Hildebrand and Whalen (2014) defined separate ‘arc’ and ‘slab failure’ plutonic fields on Nb/Y versus Sr/Y, and La/Yb versus Sr/Y diagrams based the trace-element geochemistry of plutons in the Cretaceous Peninsula Ranges Batholith in coastal California and Mexico. However, they noted that their proposed discrimination fields should not be considered universally applicable because of the inherent tectonic complexities involved in the evolution of orogenic belts; instead the division of the Cretaceous plutons into ‘arc’ (low La/Yb) and ‘slab failure’ (high La/Yb) types was determined primarily by the timing of their emplacement with respect to collision and uplift rather than by geochemistry alone. With this in mind, the two fields on Hildebrand and Whalen’s discrimination diagrams are used herein to empirically determine which plutons in the Saint George Batholith were likely to have had residual garnet present in their source regions.

55 The plutons of the Saint George Batholith and satellite Tower Hill pluton fall into the same two groupings on the Hildebrand and Whalen (2014) diagram (Fig. 26) that they do on the Pearce et al. (1984) discrimination diagram (Fig. 24). The Magaguadavic, Jimmy Hill, John Lee Brook, and Tower Hill granites plot in or near the boundary of the ‘garnet present’ field. The four samples from the John Lee Brook Granite from McLeod (1990), which show wide variation in element ratios (Table 3), have been plotted as individual points rather than as an average; the two samples from the southeastern part of the pluton (ave. La/ Yb of 7.4) appear to be more evolved with higher silica and Rb/Sr values compared to the two samples from the northeast (ave. La/Yb of 14.4). Samples from the younger Jimmy Hill and Magaguadavic plutons (Table 3) plot well into the ‘garnet present field (Fig. 26). The transition to deeper level partial melting apparently occurred during the Lochkovian Stage of the Early Devonian based on the emplacement age of 415.5 ± 2.1 Ma for the Wellington Lake Granite, the youngest ‘A-type’ pluton, and the emplacement age of 413.3 ± 2.1 Ma, for the John Lee Brook Granite, the oldest ‘S-type’ pluton in the backarc region; the former was likely generated at least in part by partial melting of Neoproterozoic volcanic and plutonic rocks underlying the Mascarene belt, and the latter at least in part by granulite-facies dehydration melting of pelitic rocks of the St. Croix belt.

56 Convective heating and partial melting of the adjacent, thick crust of the St. Criox belt, which marks the northwestern ‘passive’ boundary of the Mascarene backarc basin, may be responsible for the generation of Early Devonian magmatism at greater depths compared to the Silurian magmatic activity to the southeast. Prograde metamorphism of the bounding thick crust may have led to formation of a dense, eclogitic lower crustal root that had detached from the more buoyant overlying granulitic crust and sank, together with its attached underlying lithospheric mantle, into the convecting asthenosphere of the backarc region (see Krystopowicz and Currie 2013). Subsequent upwelling of the asthenospheric mantle then induced widespread partial melting of the lower crust under ‘garnetpresent’ conditions to generate the ‘I-type’ Jimmy Hill, Magaguadavic, and ’S-type’ Tower Hill plutons (Fig. 24). These plutons then rose to higher levels in the crust, where they solidified during the Emsian between 403 ± 2 and 396 ± 1 Ma (Table 3). Isostatic rebound within the St. Croix belt is marked by the domal area of high temperaturelow pressure contact metamorphic zone surrounding the Tower Hill pluton. Deformation of pegmatitic and aplitic dykes and associated auriferous quartz veins located along the northwestern margin of the undeformed, circular, Magaguadavic Granite is focused along reactivated shear fault splays of the Sawyer Brook Fault (Thorne et al. 2008).

57 The eastern part of the Saint George Batholith was formed 30 million years later with the emplacement of the 600 km², tin-enriched, post-orogenic, Late Devonian Mount Douglas Granite (Fig. 1). The recent mapping and geochronology has shown that the Mount Douglas and other plutons of Silurian to Late Devonian age that comprise the Saint George Batholith actually occur as a cluster of individual intrusions separated by screens of sedimentary and volcanic country rocks, rather than as the continuous mass of amalgamated plutons depicted on earlier maps.