The South Nepisiguit River Plutonic Suite (SNRPS) is a group of spatially related felsic and mafic plutons in northern New Brunswick (Fig. 1). The SNRPS comprises a dominant phase of peraluminous biotite granite (Mount Elizabeth Granite) to the east, several smaller plutons of alkaline granite, including the Mount LaTour Granite, to the west, and scattered mafic stocks and plutons (Wilson 2013a; Fig. 1). The Mount Elizabeth Granite is the most aerially extensive phase of the SNRPS (Fyffe 1971; Whalen 1993), and has previously yielded a U-Pb (monazite) age of 417 ± 2 Ma (Bevier and Whalen 1990); however, an imprecise age obtained from the Mount LaTour Granite (414 +11/-1 Ma; Bevier and Whalen 1990) leaves some uncertainty as to whether it is coeval with or younger than the Mount Elizabeth Granite. In an earlier study, biotite grains from the Mount Elizabeth Granite yielded the highest Sn, W, Mo, and Sb among all the studied samples of Acadian intrusions of New Brunswick (Azadbakht et al. 2015). This fact motivated the authors to investigate the temporal relationship between different phases of SNRPS to further study the possibility of granophile mineralization within this plutonic suite. The purpose of this paper is to report new in situ U-Pb monazite and zircon ages for the Mount LaTour Granite, and both in situ monazite and zircon mineral separate ages for the Mount Elizabeth Granite, based on Laser Ablation-Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS).

The SNRPS is a part of the Central plutonic belt of New Brunswick (Fig. 1 inset). This belt extends from Chaleur Bay in northeastern New Brunswick southwestward to the American border and into adjacent Maine (Wilson and Kamo 2008). It includes numerous calc-alkaline, foliated and non-foliated Silurian-Devonian granitic intrusions, which intruded the Bathurst Supergroup, Cambrian to Early Ordovician rocks of the Miramichi, Woodstock, and Meductic groups, and the Trousers Lake metasedimentary Suite (Wilson and Kamo 2016).

The SNRPS underlies a roughly circular area of about 400 km² in northern New Brunswick. It mainly intruded sedimentary rocks of the Cambrian-Ordovician Miramichi Group, and volcanic and sedimentary rocks of the Middle Ordovician California Lake Group, but the western alkaline phases also intruded Ludlovian to Lochkovian rocks of the Petit Rocher and Tobique groups (Fig. 1; Wilson 2013a). The enveloping contact metamorphic aureoles contain cordierite and andalusite up to 1.5 km from the contact (Whalen 1993; de Roo and van Staal 1994).

Fyffe et al. (1981) introduced the name Mount Elizabeth Granite, whereas Whalen (1993) referred to the Mount Elizabeth Intrusive Complex, which he divided into peraluminous granite, alkaline granite, and mafic suites. The large, eastern pluton of peraluminous granite (Mount Elizabeth Granite) is flanked to the west by the alkaline granite suite (Mount Des Barres Quartz Monzonite, Mount LaTour Granite, and Mount Edward Granite); all felsic phases are intimately associated with scattered plutons and small stocks of mafic intrusive rocks, including the Portage Brook Troctolite (dark green, medium- to coarse-grained layered troctolite), Mount Manny Gabbro (dark green medium- to coarse-grained, equigranular to porphyritic gabbro, diorite, and diabase), and several small mafic intrusions collectively referred to as the Mount Elizabeth Gabbro (fine- to medium-grained equigranular to plagioclase-phyric diabase, medium- to coarse-grained gabbro, and minor diorite and quartz diorite)(Whalen 1993). Contact relationships between the Portage Brook Troctolite and other intrusive phases have not been observed; however, relationships between other mafic intrusions and the granitic phases, such as cuspate, irregular contacts and evidence of hybridization along contacts, suggests that they are essentially contemporaneous (Whalen 1993).

The Mount Elizabeth Granite consists of compositionally and texturally homogeneous, pink and white, equigranular, fine- to coarse-grained biotite granite that locally contains minor muscovite (Whalen 1993). Field observations reveal that, in places, the alkaline phases exhibit marked heterogeneity at the outcrop scale (Whalen 1993). However, contacts between the alkaline and peraluminous phases, and among the alkaline phases, have not been observed. The most extensive and most fractionated unit is the Mount Latour Granite, a scarlet red, medium- to coarse-grained, equigranular granite. The Mount LaTour Granite is amphibole bearing, and much of it contains only one feldspar (i.e., hypersolvus granite; Whalen 1993). The nomenclature used herein is taken from the New Brunswick lexicon of bedrock geology (New Brunswick Department of Energy and Resource Development 2016) and the most recent bedrock geology maps (Wilson 2013a, b)

Bevier and Whalen (1990) reported Thermal Ionization Mass Spectrometry (TIMS) U-Pb zircon and monazite ages for the Mount Elizabeth Granite and a TIMS U-Pb zircon for the Mount LaTour Granite. They reported ²⁰⁷Pb/²³⁵U ages for monazite to avoid anomalous ²⁰⁶Pb/²³⁸U ages resulting from higher than expected ²³⁰Th disequilibrium in these young rocks.

In the Mount Elizabeth Granite, the results for zircon and monazite grains are not in complete agreement. Duplicate single monazite grains yielded a ²⁰⁷Pb/²³⁵U age of 418 ± 1 Ma; whereas, one zircon is concordant at 416 ± 1 Ma. Therefore, an average age of 417±2 Ma was assigned to the intrusion by Bevier and Whalen (1990).

In contrast, Mount LaTour zircon grains are discordant and show evidence of an inherited xenocrystic component. Bevier and Whalen (1990) introduced the lower intercept of 414 +1/-11 Ma as the best estimate of the age of emplacement for this intrusion. They concluded that the upper intercept (ca. 2.7 Ga) gives an average age for the inherited component.

The uncertainties related to the crystallization age and petrological differences among the subunits of the SNRPS encouraged the authors to re-examine the ages of different units in the intrusive suite using in situ and mineral-separate U-Pb monazite and zircon dating at the LA-ICPMS lab at the Department of Earth Sciences, University of New Brunswick.

The study was conducted in two stages, using archived samples from the study conducted by Whalen (1993): (1) three standard polished thin sections, one from the Mount Elizabeth Granite (WX86NB-240), and two from the Mount LaTour Granite (WX86NB-254, and -262) were selected for in situ LA-ICP-MS analysis; and (2) seven samples (WX85NB-74, 80, 81, 95, WX86NB-240, 250, and WX87NB-325) from the Mount Elizabeth Granite were sent for heavy mineral separation and the resulting monazite and zircon grains were examined and dated. Sample locations are shown in Figure 1. Two of the Mount Elizabeth samples (WX85NB-95 and WX86NB-250) are not located in the main body of the pluton, but occur as dykes or enclaves of granite hosted by gabbro to the south and north, respectively, of the Mount Elizabeth Granite (Fig. 1). The ages obtained during the two analytical stages were compared in order to establish the best crystallization age of each rock unit. The seven samples from the Mount Elizabeth Granite are medium- to coarse-grained, equigranular pink to white biotite granite, whereas Mount LaTour sample WX86NB-254 is coarse-grained biotite granite, and sample WX86NB-262 is coarse-grained, equigranular, biotite-amphibole alkaline granite. The seven samples of the Mount Elizabeth are similar mineralogically, and have biotite as the main mafic phase accompanied by traces of muscovite. Mineralogical data for the studied samples from Whalen (1993) are provided in Table A1 in Appendix A.

In stage one, monazite and zircon grains were studied with scanning electron microscope back-scattered electron (SEM-BSE) imaging, and selected grains were subsequently dated by LA-ICP-MS. Samples WX86NB-240 and -254 contain both monazite and zircon, whereas sample WX86NB-262 has only zircon (Appendix B; Figs. B1, B2). In total, five monazite and nineteen zircon grains were analyzed from these samples during the first stage of this study. Zircon grains are more fractured than monazite grains in the samples examined. Euhedral monazite grains in sample WX86NB-240 are large (average 50 μm × 30 μm) and zoned, whereas monazite in sample WX86NB-254 is subhedral and measures 240 μm × 100 μm. Zircon grains are commonly euhedral to subhedral and display oscillatory zoning. Grains are greater than 50 μm in the long direction, and some show evidence of secondary metamictization and alteration. In such cases, the least altered part of the grain was analyzed. Most of the zircon grains display metamorphic overprints in SEM-BSE, and some show evidences of a narrow secondary regrowth. However, the regrowth layers were not thick enough to be targeted with the laser beam. As a result, rock samples were sent for mineral separation to provide more clear grains to choose from and to study in more detail potential overgrowths. In the monazite runs, the GSC-8153 monazite was used for external calibration, whereas the 44069 (Delaware) monazite (U-Pb ID-TIMS age of 424.9 ± 0.4 Ma; Aleinikoff et al. 2006) was analyzed as a secondary standard. In the case of zircon analysis (both stages), standard FC-1 (Paces and Miller 1993) was used as the zircon reference standard and the Plesovice standard (weighted mean ²⁰⁶Pb/²³⁸U ID-TIMS date of 337.13 ± 0.37 Ma; Sláma et al. 2008) used as a secondary standard.

For the second stage of dating, heavy minerals were separated from the seven samples of Mount Elizabeth Granite using electric pulse disaggregation at Overburden Drilling Management Limited in Ottawa, Canada. Heavy mineral concentrates of all samples examined in stage two have abundant, large (>500 μm) grains of both zircon and monazite. Clear, elongate, euhedral to subhedral crystals of monazite and zircon with internal concentric zoning and minimal mineral inclusions were handpicked and mounted. The textures and morphology of the mounted crystals were studied, and imaged by transmitted light and by cathodoluminescence (CL) imaging. Zircon grains were first dated by LA-ICP-MS in an unpolished grain mount in order to sample only the outermost overgrowths and to avoid xenocrystic cores. The puck was then polished, and further SEM-BSE imaging was done to study the textures in detail (Appendix B; Fig. B3). Visible overgrowths were then targeted for the final stage of the second phase. Ablation was conducted using a Resonetics M-50-LR 193nm Excimer laser ablation system described by McFarlane and Luo (2012) and following methodologies outlined in McFarlane (2015). Standards employed are as described above for both stages. Standards and unknowns were ablated with 24 μm diameter craters for zircon and 13 μm (stage one), and 17 μm (stage two) diameter craters for monazite using a repetition rate of 3 Hz. Typical ablation time was 30s, with 30s background collection.

The data from both analytical stages were reduced offline using the VizualAge U-Pb geochronology plugin running under Iolite v. 2.5 (Paton et al. 2011; Petrus and Kamber 2012). VizualAge incorporates a ²⁰⁴Pb-based common-Pb correction that is suitable when the net ²⁰⁴Pb ion beam is measured with sufficient precision (typically <20% 1σ). Concordia ages and weighted mean ²⁰⁶Pb/²³⁸U ages were calculated using Isoplot version 3.71.09.05.23nx (Ludwig 2009). Data were first filtered by common-Pb content to consider those with ~99% Pb* (radiogenic Pb). Then the data were filtered to consider points that are less than 1% discordant. The procedure resulted in a cluster of data and concordia ages were calculated for clusters of 3 or more concordant data that overlap within error. Inverse isochron lower intercept ages were also calculated for comparison where high common lead levels precluded accurate ²⁰⁴Pb-based correction.

The accuracy of monazite data was confirmed by a concordia age of 422.2 ± 3.2 Ma (MSWD = 1.04, probability of fit = 0.31, in situ), and 424.0 ± 2.4 Ma (MSWD = 0.83, probability of fit = 0.36; heavy mineral concentrate) measured on the 44069 standard. Furthermore, the accuracy of zircon data is confirmed by a concordia age of 339.3 ± 2.7 Ma (MSWD = 0.59 probability of fit = 0.44, in situ), and 337.7 ± 2.0 Ma (MSWD = 1.7, probability of fit = 0.19, heavy mineral concentrate) measured for the Plesovice standard.

In situ data collected from samples WX86NB-240 (Mount Elizabeth Granite) and -254 (Mount LaTour Granite) show no evidence of inheritance (Table 1). Monazite grains from sample WX86NB-240 yielded thirteen near-concordant spots overlapping within uncertainty and defining a concordia age of 417.6 ± 2.2 Ma (MSWD = 0.47, probability of fit = 0.50; Fig. 2a); the same set of analyses yielded a weighted average ²⁰⁶Pb/²³⁸U age of 417.9 ± 2.3 Ma (MSWD = 0.99, probability of fit = 0.46; Fig. 2b). In this study, we interpret the weighted average ²⁰⁶Pb/²³⁸U dates to represent the best estimate for the crystallization age of each phase. Interestingly, this age is in complete agreement with the previous dating of Bevier and Whalen (1990). Furthermore, sample WX86NB-254 produced five overlapping, near-concordant analyses that yield a concordia age of 416.1 ± 4.2 Ma (MSWD = 5.9, probability of fit = 0.015; Fig. 2c) with a weighted average ²⁰⁶Pb/²³⁸U age of 417.7 ± 4.4 Ma (MSWD = 0.83, probability of fit = 0.51; Fig. 2d). This age is within the upper limit of the previous date (414 +11/-1 Ma) obtained by Bevier and Whalen (1990). The two plutons are thus indistinguishable in terms of their monazite ²⁰⁶Pb/²³⁸U ages, confirming the contemporaneous relationship between the Mount Elizabeth Granite and Mount LaTour Granite as proposed by Whalen (1993). Interestingly Mount LaTour Granite (the westernmost of the three plutons) intruded rhyolite that has an age of 418.6 ± 0.9 Ma (Wilson and Kamo 2008). This may confirm the accuracy of the result indicating the possibility that parts of the plutonic suite have approximately coeval extrusive equivalents.

It is important to note that different reservoirs have different U/Th ratios (0.238, 0.119, 0.228, and 0.500 for the upper crust, lower crust, average crust, and MORB, respectively; Rollinson 1993); as a result, crustal contamination could considerably change the ratio and indirectly affect U/Th in the crystallizing monazite grains. Depending on the time differences between contamination and monazite crystallization, the U/Th ratio of the monazite may reflect either the original magma or a magma contaminated by assimilation of crustal material. None of the grains from the three samples shows evidence of inheritance; therefore, the U/Th ratios likely reflect the composition of the magma from which they crystallized (i.e., post-crustal contamination).

Similar to the result of the in situ study, the data collected from separated monazite grains of the Mount Elizabeth Granite showed no signs of inheritance (Table 2). Thirty-five monazite grains were ablated, and with the exception of two grains, they define a maximum age cluster at ca. 416 Ma (Table 2). The studied grains yielded eleven near-concordant spots overlapping within uncertainty and resulting in a concordia age of 417.5 ± 2.2 Ma (MSWD = 0.032, probability of fit = 0.86; Fig. 3a); the same set of analyses yielded a weighted average ²⁰⁶Pb/²³⁸U age of 417.2 ± 3.1 Ma (MSWD = 0.95, probability of fit = 0.48; Fig. 3b). Based on the overlap between the 2σ error envelopes for the results of in situ and separate monazite grains, we assigned an average age of 417.2 ± 1.5 Ma (95% confident) as the best estimate of crystallization age for Mount Elizabeth Granite.

Monazite partitions Th strongly over U into its structure, and the potential effect of ²³⁰Th disequilibrium on the ²⁰⁶Pb/²³⁸U ratio was calculated using the equation of Schoene (2014). Any correction is strongly dependent on the ratio of Th/U in the mineral relative to the same ratio of the melt, and is annotated by “ƒ”. If ƒ becomes very large, the ²⁰⁶Pb/²³⁸U ages will be too old. In the studied samples (seven samples from Mount Elizabeth Granite), the monazite is not very Th rich (Th <13 wt.%; ƒ value varies between 0.8713 and 13.4844), with two exceptions, and even with f >10, the impact is manifested in the fourth decimal place for the ²⁰⁶Pb/²³⁸U ratio. Therefore, the effect of ²³⁰Th disequilibrium is thought to be very small and has almost no impact on the ²⁰⁶Pb/²³⁸U ages calculated for the monazite grains. Consequently, 417.2 ± 1.5 Ma is believed to be a good estimate of the true crystallization age of these grains.

In situ zircon grains in samples WX86NB-240 (Mount Elizabeth Granite), and WX86NB-254 and -262 (Mount LaTour Granite) were studied with SEM-BSE imaging in order to try to identify overgrowth rims on inherited xenocrysts; however, none of the grains examined showed evidence of overgrowths. This apparent lack of overgrowth is similar to the observation of Roddick and Bevier (1995) for two other Paleozoic granitic intrusions of New Brunswick, and may indicate a low probability of inheritance in these grains. Furthermore, U-Pb results for each of the samples define discordia suggestive of secondary, low temperature Pb-loss in these grains (also observed by Bevier and Whalen 1990), which is supported by the presence of fractures in most of the studied grains.

All the data from the in situ ablation of large (>50 μm diameter) zoned zircon grains plot to the right of the concordia (Table 3; Fig. 4) line, because of the presence of either common Pb in these grains or recent Pb-loss. It is noteworthy to add that the data plot as a group in each sample, with upper intercepts of ca. 420 Ma in WX86NB-262, ca. 410 Ma in WX86NB-254, and ca. 401 Ma in WX86NB-240. Common-Pb corrected data for four near-concordant analyses yield an overlapping concordia age of 420.0 ± 5.4 Ma (MSWD = 0.20, probability of concordance = 0.65) for sample WX86NB-262; 410.2 ± 4.7 Ma (MSWD = 0.37, probability of concordance = 0.54) for sample WX86NB-254; and 401.7 ± 4.3 Ma (MSWD = 7.2, probability of concordance = 0.007) for sample WX86NB-240. The slightly older age of 420.0 ± 5.4 Ma in WX86NB-262 may be due to inadvertent ablation of inherited zircon cores in this sample. Samples WX86NB-240 and -254 show more evidence of recent Pbloss. It is important to note that given the very small ²⁰⁴Pb signals and potential for minor inheritance, the common-Pb corrected in situ U-Pb zircon data are considered a less reliable estimate for crystallization ages compared to the monazite results.

The ablation of ninety-eight mounted zircon grains from the Mount Elizabeth Granite yielded nine near-concordant spots overlapping within uncertainty, resulting in a concordia age of 417.5 ± 1.6 Ma (MSWD = 1.2, probability of concordance = 0.27; Table 4; Fig. 5a), and a weighted average ²⁰⁶Pb/²³⁸U age of 416.4 ± 2.0 Ma (MSWD = 0.42, probability of concordance = 0.89; Fig. 5b). The same grains were ablated once more after polishing the epoxy, yielding a concordia age of 417.5 ± 1.9 Ma (MSWD = 1.3, probability of concordance = 0.26; Table 5; Fig. 6a); the weighted average ²⁰⁶Pb/²³⁸U age is calculated to be 418.6 ± 2.6 Ma (MSWD = 1.18, probability of concordance = 0.31; Fig. 6b). The weighted average of both runs in stage two are in agreement; however, the amount of correction is less in the polished run. As a result, the weighted average of this run is believed to represent the best crystallization age of Mount Elizabeth Granite. This age is in a complete agreement with the results of the monazite study, and may confirm the accuracy of the result. Based on the overlap between the 2σ error envelopes for the results of in situ monazite study, and separated monazite and zircon grains, we assigned an average age of 417.3 ± 0.96 Ma (95% confident) as the best estimate of crystallization age for Mount Elizabeth Granite.

Results of this study clearly showed inheritance signatures in zircon grains in the seven mineral-separate samples, including WX86NB-240, examined in the stage two analyses. It is especially an issue in unpolished grain mounts, in which the probably density plot shows peaks at ca. 444 Ma, 465 Ma, and 512 Ma (Fig. 7). The polished grain mounts also encountered inheritance at ca. 500 Ma (Table 5), as well as some cores with Grenvillian ages (ca. 1000 Ma). Interestingly, the zircon grain with the Grenvillian core has one of the lowest U contents among the studied grains (U <55 ppm). Inherited zircon is common in magmatic rocks, especially in peraluminous or S-type granites (Harrison et al. 1987). Zircon solubility is directly related to temperature (i.e., the solubility increases in less evolved systems having higher temperatures), and to a lesser extent depends on magma composition (e.g., the cation ratio (Na + K + 2Ca) / (Al + Si), Watson and Harrison (1983)). As a result, inherited zircon was expected in samples of the peraluminous Mount Elizabeth Granite. In order to further study the possibility of inheritance in zircon grains, the zircon saturation temperature was calculated using the empirical model of Watson and Harrison (1983), and the whole-rock chemical data available from Whalen (1993). The temperatures were estimated to vary from 830 to 839 °C in the Mount Elizabeth Granite, and from 833 to 824 °C in the Mount LaTour Granite. These temperatures are higher than the common normal magma temperatures for granitoid rocks of this composition (maximum of 740 to 762 °C; Chappell et al. 1998). The presence of inherited zircon, and/or other zirconium-bearing minerals would result in a higher calculated temperature (Yang 2005); therefore, the calculated zircon saturation temperatures are assumed to represent the maximum temperatures.

Inheritance is commonly an issue in peraluminous magmas (Klötzli et al. 2001) due to the lower zircon solubility in magmas of this composition (Watson and Harrison 1983). Previous studies of zircon grains in New Brunswick granitoid rocks (Bevier and Whalen 1990; Roddick and Bevier 1995) indicated significant inheritance. Whalen (1993) reported an upper intercept of about 1.4 Ga for several Ordovician granites in the Gander zone. Roddick and Bevier (1995) also documented Grenvillian xenocrystic zircons in the Meridian Brook and Pabineau Falls granites in northeastern New Brunswick. Furthermore, both zircon populations in sediments and inherited grains in granites contain a predominance of 1.5 ± 0.2 Ga grains in the Gander Zone in Newfoundland (Whalen 1993). The Grenvillian xenocrystic cores are believed to be related to a probable Precambrian basement. Additionally, the ca. 444 Ma peak of the zircon grains represents the age of Salinic deformation and metamorphism whereas the ca. 465 Ma peak may be derived from the felsic volcanic sequence and associated intrusions related to Middle Ordovician back arc rifting (van Staal et al. 2009). These data suggest that Ordovician volcanic and sedimentary rocks may be the source for these granites or form a major assimilated component.

Early Devonian plutonic rocks of SNRPS are not only petrographically and mineralogically similar to but also contemporaneous with the North Pole Stream granite (417 ± 1 Ma; Bevier and Whalen 1990; Whalen 1993) and Redstone Mountain Granite (419.0 ± 0.5 Ma; Wilson and Kamo 2016). All of these intrusions are medium- to coarse-grained, pink to red biotite granites crosscut by finer grained granodioritic dykes. SNRPS phases plot in an array from volcanic-arc to within-plate fields on the Rb vs. Nb + Y tectonic discrimination diagram of Pearce et al. (1984) and Pearce (1996) (Fig 8a), and Hf-Rb-Ta ternary diagram of Harrison et al. (1986) (Fig. 8b). Interestingly, all geochemical data (compiled from the study of Whalen 1993) plot in the domain of post-collisional granites defined by Pearce (1996) and S-type granitoids by Christiansen and Keith (1996) (Fig 8a). These discrimination plots purpose not only their tectonic setting, but also the protolith, melting, and crystallization histories (Christiansen and Keith 1996; Chowdhury and Lentz 2010). Samples from Mount Elizabeth Granite plot mostly in the peraluminous field, whereas the Mount LaTour samples plot in the metaluminous field of Shand (1943) (Fig. 8c). The peraluminous nature of these intrusions indicates a major supracrustal component in the source; this is supported by the close spatial relationship with the Miramichi Group, and the common presence of xenoliths of partially assimilated metasedimentary rocks in these plutons (Whalen 1993; Wilson and Kamo 2016).

Primitive mantle normalized plots could reflect tectonic affinities for felsic plutons; however, interpretation is complicated by potential fractionation of trace elements into phases such as zircon, monazite, and apatite (Whalen et al. 1996). Well-developed negative Ba, Sr, Eu, and Ti anomalies in both Mount Elizabeth and Mount LaTour granites may have been caused by fractionation of feldspars and oxides, respectively, although, negative Nb anomalies in these rocks (see Fig. 9a-b) probably are a source characteristic. Negative Nb anomalies are a common feature of granites derived mainly from a crustal source that in itself was derived from arc crust (Whalen et al. 1996; Yang et al. 2008).

Whalen (1993) suggested involvement of and/or contributions from mantle-derived components for Silurian-Devonian granite magmatism of the Gander zone. Whalen et al. (1996) argued that these granitic intrusions were produced by partial melting of Middle Proterozoic continental basement through lithospheric delamination during Iapetus or back-arc basin closure. Rapid uplift and gravitational instability would follow either of these scenarios, which could be explained by the ca. mid-Silurian breakoff of the Tetagouche slab (Wilson and Kamo 2016). This may have caused exhumation of the Brunswick Subduction Complex and subsequent extensional collapse in the Miramichi Highlands (Wilson and Kamo 2016).

Mount Elizabeth, North Pole Stream, and Redstone granites are associated with the late Silurian magmatic phase (ca. 423 to 416 Ma) in Central New Brunswick, representatives of which are mainly exposed in the Miramichi Highlands (van Staal et al. 2009; Wilson and Kamo 2016). These intrusions postdate structures associated with the Salinic orogeny, and predate Acadian structures; for instance, they formed northwest of the migrating Acadian deformation front (van Staal et al. 2009). Furthermore, these intrusions are associated with mafic rocks that comprise up to ~ 21% of theses magmatic suites (van Staal et al. 2009). Van Staal et al. (2009) used the similarity between the spectrum and proportions of felsic and mafic phases in the SNRPS and Silurian magmatism in western and west-central Newfoundland to interpret these ca. 419–417 Ma magmatic rocks as a final, post-kinematic pulse of magmatism associated with the breakoff of the Salinic slab (Whalen et al. 2006; van Staal et al. 2009). These intrusions were further metamorphosed during the Acadian orogeny, and that could explain the common lead loss and alteration through hydrothermal activity in the examined zircon grains (van Staal et al. 2009; Fig. 1).

In situ study of monazite grains from the Mount Elizabeth and Mount LaTour granites revealed no evidence for inheritance monazite. The crystallization age is interpreted to be 417.2 ± 1.5 Ma for the Mount Elizabeth Granite and 417.7 ± 4.4 Ma for the Mount LaTour Granite which confirms the contemporaneous relationship between the felsic units of the SNRPS previously inferred by Whalen (1993). The results for monazite grains from mineral separates of the Mount Elizabeth Granite are in agreement with the dates determined by in situ analysis, with no significant effect from ²³⁰Th disequilibrium.

Results of both stages of zircon studies from the Mount Elizabeth Granite showed inheritance in these grains. The Grenvillian xenocrystic cores are thought to be related to a probable Precambrian basement, whereas the ca. 444 Ma and ca. 465 Ma peaks suggest that Ordovician volcanic-sedimentary packages may have been the source of or, an assimilated component for, these granites.

The result of in situ zircon study shows evidence of advanced post-crystallization Pb-loss and common lead incorporation, and defines a younger concordia age. These results are interpreted to reflect later hydrothermal alteration and isotopic resetting rather than crystallization ages. In addition, in situ U-Pb zircon data are not reliable because of the very small ²⁰⁴Pb signals, and do not reflect crystallization ages for these units. However, separated zircon grains from the Mount Elizabeth Granite, picked for clarity and absence of micro-fractures, define weighted average age of ca. 418.6 ± 2.6 Ma in the polished run.

Due to the overlap between the 2σ error envelopes for the results of in situ monazite and separated monazite and zircon studies, an average age of 417.30 ± 0.96 Ma (95% confidence) is assigned as the best crystallization age of the Mount Elizabeth Granite.