Geochemistry, U-Pb geochronology and Nd-Hf isotopes of leucocratic dykes in the Cape Spencer area, southern New Brunswick, Canada: insights into the Alleghanian orogeny in the northern Appalachians

Abstract

Contrary to the southern Appalachians, where Alleghanian magmatism is widespread and well documented, the expressions of magmatism in the Canadian Appalachians are limited. In this study, a suite of leucocratic dykes from the Cape Spencer area in southern New Brunswick, Canada, were investigated to determine the nature, timing and source of these magmas using zircon and monazite U-Pb geochronology, whole-rock geochemistry and Nd-Hf isotopes. An LA-ICP-MS U-Pb monazite Alleghanian age of 273.7 ± 1.3 Ma obtained for these dykes constitutes a new example of magmatism in the northern segment of the orogen, where significant strike-slip movement and reheating have been the primary markers of the Alleghanian Orogeny. These metaluminous leucocratic dykes are enriched in light rare elements, U and Th; depleted in high-field strength elements (HFSE; Nb, P, Ti); and have slight negative Europium anomalies [(Eu/Eu*)_N = 0.72–0.95]. All the dykes samples have negative εNd(t) values (−9.76 to −5.7), negative εHf(t) values (−1.8 to −1.0) and Mesoproterozoic Nd depleted-model ages (T_DM = 1371–1618 Ma). The geochemical and isotopic characteristics suggest that the dykes were formed by the partial melting of lower crust that assimilated Meguma metasedimentary rocks and/or Avalonian sedimentary rocks, following terminal subduction of the Rheic Ocean and thermal re-equilibration during the Alleghanian orogeny. The effects of the closure of the Rheic Ocean in the oblique collision between composite Laurentia and Gondwana were, to a certain extent, accommodated along the Minas Fault Zone, where magmatism and regional fluid flow were concentrated.

1. Introduction

The deformation history in the northern Appalachians (southern New England through Atlantic Canada) during the Carboniferous is associated mainly with the docking and adjustment of the Meguma terrane to composite Laurentia along the Minas Fault Zone (MFZ) (Murphy et al. 2011; van Staal & Barr, 2012; Park et al. 2014; Waldron et al. 2015; Park & Hinds, 2020). This Carboniferous-Permian (330–260 Ma) deformation, also referred to as the Alleghanian orogeny, constitutes the final stage of oceanic closure by the collision of Africa with composite Laurentia to form the supercontinent Pangea (Hatcher, 2002). Late Paleozoic magmatism is well known and widespread in the southern Appalachians, where over 60 Alleghanian plutons have been documented in contrast to very limited magmatism in the northern segment of the orogen (Samson et al. 1995; Speer & Hoff, 1997; Hatcher, 2005; Hibbard et al. 2007a, 2010). The nearest Alleghanian intrusions to the Cape Spencer area are the Bank German pluton (Pe-Piper & Jansa, 1999; Pe-Piper et al. 2010), in the southwestern end of the Scotian Shelf, approximately 30 km southwest of Yarmouth, Nova Scotia (Canada), and the Sebago batholith and granitic pegmatites from New England (USA) (Tomascak et al. 1996a, 1996b, 1998). The Alleghanian orogeny, in the context of the Canadian Appalachians, is palpable by major strike-slip movements along the MFZ (Murphy & Keppie, 1998; Murphy et al. 2011; Waldron et al. 2015) and localized thermal and magmatic events restricted to or in the periphery of shear zones (Kontak et al. 2000, 2008; MacHattie & O’Reilly, 2008; Pe-Piper et al. 2010, 2018). In this context, a group of leucocratic dykes have been described as intruding on the rocks in the Cape Spencer area and they also crosscut the deformational fabrics of the host rocks (Warner, 1985; Nance, 1986; Nance & Warner, 1986), although have not been comprehensively studied. These bodies constitute an opportunity to provide an upper age constraint to the fabrics related to the deformation episode and the gold mineralization event (Watters, 1993; Richard, 2005) linked to these ductile to brittle fabrics. A lower constraint is already given by the age of the youngest unit affected by this deformation, i.e. the Lancaster Formation, which is considered Bashkirian (Park & Hinds, 2020). In this study, the petrographical, whole-rock geochemical, U-Pb zircon and monazite geochronological characteristics, and Nd-Hf isotopic composition of the leucocratic dykes outcropping in the Cape Spencer area in southern New Brunswick, are presented to provide constraints on their petrogenesis and significance within the geological framework of the northern Appalachians and its tectonomagmatic evolution during the Alleghanian orogeny.

2. Geological overview

2.a. Regional geology

The northern Appalachians (Figure 1) developed through a long-term process of accretion and collision of peri-Laurentian and peri-Gondwanan (Ganderia, Avalonia, and Meguma) terranes to the Laurentian continental margin during the closure of the Iapetus and Rheic oceans between the late Cambrian and the Permian (Hibbard et al. 2007b; van Staal, 2007; Hatcher, 2010; Waldron et al., 2019). The Alleghanian orogeny includes all the events related to the collision/amalgamation of composite Laurentia with Gondwana (Hatcher, 2010; van Staal & Barr, 2012); this collision, in the northern Appalachians, was dominated by dextral-strike-slip motion along the MFZ (Murphy et al. 2011). However, the effects of the Alleghanian orogeny are not restricted to the MFZ, with Alleghanian deformation through fault movement documented in the Gaspé Peninsula of Quebec (Jutras et al. 2003), Ar-Ar resetting around 270–300 Ma in shear zones in the southern Meguma terrane and associated offshore plutons (Reynolds et al. 2012), in addition to shear zones formation (∼320 Ma) north of Yarmouth (Culshaw & Reynolds, 1997).

Figure 1. Simplified lithotectonic division of the northern Appalachians. CCHF: Caledonia-Clover Hill Fault. MFZ: Minas Fault Zone. BVBL: Baie Verte Brompton Line. ML: Mekwe’jit Line (formerly Red Indian Line; RIL). SB: Sebago Batholith. SMB: South Mountain Batholith. Modified after van Staal et al. (2021).

The Caledonia-Clover Hill Fault in New Brunswick marks the boundary between Ganderia and Avalonia (Park et al. 2014; Waldron et al. 2015). Avalonia describes a distinctive group of Neoproterozoic, arc-related, volcano-sedimentary sequences and plutonic rocks (Kerr et al., 1995; van Staal, 2007). The Caledonia terrane, i.e. a segment of Avalonia in southern New Brunswick, includes three Cryogenian to Ediacaran volcanic-sedimentary sequences and comagmatic intrusions (Pollock et al. 2022; Waldron et al. 2022), the recently recognized ca. 690 Ma Lumsden Group, the ca. 630–615 Ma Broad River Group and the ca. 560–550 Ma Coldbrook Group (Barr & White, 1996a, 1996b, 1999; Barr et al. 2019; Barr et al. 2020; Johnson & Rossiter, 2022) (Figure 2(a)). These sequences are overlain by Cambrian to Early Ordovician sedimentary rocks of the Saint John Group, and several other Carboniferous and Triassic successions (Tanoli & Pickerill, 1988; Landing, 1996; Fyffe et al. 2011). The Lumsden Group is characterized by chert and tuffaceous siltstone, intermediate crystal and lithic tuff, felsic volcanic rocks and associated plutons (Johnson & Rossiter, 2022). The Broad River Group includes intermediate and felsic crystal lithic tuff, mafic and felsic flows and associated plutons, tuffaceous sedimentary rocks, arkosic sandstone and conglomerate that have experienced regional metamorphism to greenschist facies along with ductile deformation (Barr & White, 1996a). The Coldbrook Group consists of a first group of dacitic and rhyolitic flows and tuffs, and a second group of basaltic and rhyolitic units along with coarse clastic sedimentary rocks and comagmatic plutons, considered to be less deformed and metamorphosed than the Broad River Group (Barr & White, 1999). The petrological characteristics of the igneous components of both the Lumsden and Broad River groups suggest an origin in a continental margin subduction zone, whereas the Coldbrook Group, because of its bimodal nature, is thought to have formed during extension after cessation of continental arc magmatism (Barr & White, 1999; Pollock et al. 2022).

Figure 2. (a) Simplified geological map of the Caledonia Highlands. Modified from Barr and White (1999). (b) Local geology of the Cape Spencer area. Modified from Watters (1993) and the bedrock geological compilation of the Cape Spencer area, maps National Topographic Series 21H/04 and 21H/05 (2004).

The MFZ marks the boundaries between Avalonia and the Meguma terrane (Williams, 1979; Murphy et al. 2011). The Meguma terrane, the most outboard of the Appalachian terranes in Canada, derived from the West African craton (Clarke & Halliday, 1985; Waldron et al. 2009; White & Barr, 2010; White et al. 2018), constitutes a succession of Cambrian-Ordovician turbiditic sediments of the Goldenville and Halifax groups and mid to late Devonian granitoids that include the South Mountain Batholith (Clarke et al. 1997, 2004; White, 2010).

2.b. Local geology

The Cape Spencer area is located 15 km southeast of Saint John, New Brunswick, within the Caledonia terrane, representing a segment of Avalonia in southern New Brunswick (Barr et al. 2020). The Cape Spencer area presents polyphase fold-and-thrust style deformation of the rock units (Nance & Warner, 1986; Nance, 1987) and is centred along the Millican Lake Fault, a NE-SW striking fault (Figure 2(b)). The lithological units present in the area include mainly mafic volcanic rocks and a lesser amount of felsic volcanic rocks of the Coldbrook Group, and rocks of the Millican Lake Granite, a name used to refer to a group of highly sheared intrusions consisting of granite, granodiorite and leucogranite with a U-Pb (zircon) age of 623 ± 2 Ma (Watters, 1993). These units commonly share thrust-faulted contacts with the purple/grey siltstone, slate, coarse-grained polymictic conglomerate and granite-cobble conglomerate of the Cape Spencer Formation (Watters, 1993; Barr & White, 1999). Given the unclear relationship between the Cape Spencer Formation and the Millican Lake Granite, the age of the former remains ambiguous. However, Barr and White (1999) included it in the Broad River Group based on the similarity of some components shared by the clastic units.

The reddish-brown quartz-pebble conglomerate, medium-grained sandstone and shale of the Balls Lake Formation (Visean-Serpukhovian) are faulted against both the underlying Cape Spencer Formation and the overlying grey quartzose sandstone and siltstone, shale and conglomerate of the Lancaster Formation (Bashkirian) (Watters, 1993; Park & Hinds, 2020). The West Beach Formation includes intensely deformed basalt with grey shale and siltstone interbeds (Park et al. 2014).

Watters (1993) described the presence of two main deformation phases in the Cape Spencer area: an early deformation D₁, produced folds (F₁), verging to the NW with a pressure-solution cleavage (S₁) dipping to the SE-SSE. The succeeding phase of deformation, D₂, produced conjugate SE- and NW-dipping thrust faults and a late minor set of folds (F₂), overturned to the SE-SSE with a cleavage (S₂) dipping to the NW-NNW that crenulates S₁. The area is dominated by thrusts and reverse faults associated with the F₁ folds and S₁ cleavage, in addition to NW-dipping reverse faults offsetting both the F₁ folds and associated thrusts (Nance, 1986; Watters, 1993). The overprinting relationships of folds and cleavage, including reversed overprinting geometries, suggest near-contemporaneous events (Park & Hinds, 2020). Gold mineralization occurs along strongly faulted and sheared lithological contacts between the Millican Lake Granite and the Cape Spencer Formation within pyrite-rich portions of illitized (illite-carbonate ± quartz ± pyrite ± specularite) zones (Ruitenberg, 1995) localized along D₂ thrust faults and folds. The gold is associated with quartz ± carbonate ± plagioclase ± sulphide (pyrite, chalcopyrite) ± specularite veins that vary from several millimetres to several decimetres in width that parallel the folded S₁ cleavage and (or) the S₂ cleavage (Watters, 1993). The deformational fabrics associated with gold mineralization are crosscut by aplitic sill-like masses and smaller dykes, and are the focus of this study (Warner, 1985; Nance, 1986).

3. Material and methods

3.a. Sampling and element analyses

Ten representative samples of the dykes were selected for petrological and geochemical analyses around the Cape Spencer area, following detailed field investigations and sample collection. Polished thin sections of the rocks were prepared at the University of New Brunswick. The geochemical analyses were performed on fresh and representative samples by Activation Labs (Actlabs) in Ancaster, Ontario that were crushed and split to 0.5 kg, then pulped in a ceramic mill. They were then analysed using lithium borate fusion inductively coupled plasma-optical emission spectrometry (ICP-OES) and mass spectrometry (ICP-MS) for major- and trace-element analyses, respectively. Analytical uncertainties using these techniques are less than 1%. Elements such as Au, As, Br, Cr, Ir, Sc, Se and Sb were analyzed by Instrumental Neutron Activation Analysis. Quality assurance and quality control procedures, including the use of blanks, duplicates and standards, were maintained to obtain precise and accurate results.

3.b. LA-ICP-MS in situ zircon and monazite geochronology

In situ zircon and monazite U-Pb geochronology were performed on standard polished thin sections (thickness of 30 μm and 120 μm) at the University of New Brunswick. Analyses were performed using a Resonetics S-155-LR 193 nm ArF (excimer) laser ablation system coupled to an Agilent 8900 ‘triple quad’ ICP-MS. The ablation work was supported by scanning electron microscope-backscattered electron imaging obtained at the UNB Microscopy and Microanalysis Facility with a JEOL LSM-6400 Scanning Electron Microscope using an accelerating voltage of 15 kV and 1 nA current. Images were acquired using a Digiscan II controlled by Gatan Digital Micrograph software. The monazite standards GSC8153, and 44 069 were used for calibration (GSC8153) and verifying accuracy of unknowns. All analyses comprised 30s of background analysis prior to 30s of sample ablation. The laser produced shallow (<3 μm) 8 μm diameter pits using 3 Hz pulse rate and laser fluence of 3 J/cm². The zircon standards FC-1 and Plesovice were used for calibration and to check accuracy. The laser produced shallow 10–19 μm diameter pits using 3 Hz pulse rate and laser fluence of 3 J/cm². Analyses were processed with Iolite v3.7.

3.c. Whole-rock Sm-Nd and Lu-Hf-isotope analysis

Four powdered samples of the dykes were prepared and measured in the Isotope Geochronology and Geochemistry Research Centre (IGGRC) at Carleton University. Rock powders were doped with a ¹⁴⁸Nd-¹⁴⁹Sm mixed spike before they were dissolved in mixed concentrated HF and HNO₃. The sample digests were also sequentially dissolved in 7M HNO₃ and in 6M HCl and were evaporated to dryness. Sample residues were then taken up in 2 ml 3M HCl and loaded onto prepacked 2 ml LN-resin columns (50 to 100 µm, Eichrom Technologies, LLC, USA). The columns were first washed with 15 ml 3M HCl and wash solutions were collected (containing other elements including Sr, light and middle rare elements (REEs)). The columns were then washed with 20 ml 6M HCl to remove Yb and Lu completely. Hf was finally eluted with 4 ml of 2M HF after Ti was eluted with 10 to 20 ml 4ML HCl + 0.5% H₂O₂.

Wash solutions from Hf columns were evaporated to dryness and the residues were dissolved in 1.5 ml of 2.5 M HCl and loaded onto 14-ml Bio-Rad borosilicate glass chromatography columns containing 3.0 ml of Bio-Rad AG50W-X8 cation exchange resin. Columns were washed with 16 ml of 2.5 M HCl before Sr was eluted with 7 ml of 2.5 M HCl. Columns were washed with 3.5 ml of 6M HCl before REEs were eluted using 9 ml of 6M HCl. REE fractions were dissolved in 0.26M HCl and loaded onto 2 ml prepacked Ln resin columns (50 to 100 µm, Eichrom Technologies, LLC, USA). Nd was eluted using 0.26M HCl, followed by Sm elution using 0.5M HCl. Nd-isotope ratios were measured using IGGRC’s Thermo-Finnigan Neptune MC-ICP-MS. Nd and Hf isotopic ratios were normalized against ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219 and ¹⁷⁹Hf/¹⁷⁷Hf = 0.7325, respectively. ¹⁴³Nd/¹⁴⁴Nd ratios were also normalized to JNdi-1 average value 0.512100 measured by IGGRC’s Thermo-Finnigan Triton TIMS.

The average values of reference materials for a period of five months covering this analysis session are NBS987 ⁸⁷Sr/⁸⁶Sr = 0.710240 ± 0.000019 (2SD, n = 38), JNdi-1 ¹⁴³Nd/¹⁴⁴Nd = 0.512095 ± 0.000014 (2SD, n = 65), JMC475 ¹⁷⁶Hf/¹⁷⁷Hf = 0.282164 ± 0.000014 (n = 8). In the last four years, the USGS rock standards’ average values are BCR-2 ⁸⁷Sr/⁸⁶Sr =0.705000 ± 0.000027 (n = 37), ¹⁴³Nd/¹⁴⁴Nd=0.512625 ± 0.000008 (n = 14), ¹⁷⁶Hf/¹⁷⁷Hf = 0.282869 ± 0.000016 (n = 5); and BHVO-2 ⁸⁷Sr/⁸⁶Sr=0.703485 ± 0.00004 (n = 3), ¹⁴³Nd/¹⁴⁴Nd = 0.512974 ± 0.000011 (n = 6), ¹⁷⁶Hf/¹⁷⁷Hf = 0.283113 ± 0.000004 (n = 3). The total procedure blanks were <50 pg for Nd and <70 pg for Hf. The Sm/Nd column procedures were also described in Cousens (1996) and Hf columns in Yang et al. (2010).

4. Results

4.a. Petrography of dykes

Leucocratic non-foliated dykes, with thickness varying between 10 cm and 50 cm, occur both parallel to and crosscut the foliation in the host rocks, i.e. the Millican Lake Granite and the Cape Spencer Formation, and exhibit a secondary earthy hematization that results in a pinkish red colouration (Figure 3(a)). The leucocratic dykes are exposed on the northern coast of the Bay of Fundy, around Cape Spencer and have also been identified inland in drill core. These low-angle (5–30°) sill-like bodies exhibit high variability in their attitude and vary from fine-grained to typically very fine-grained sugary aplites with sharp contacts (Figure 3(b) and c). Based on their mineral assemblage, these dykes are classified into albitites. Albite (50–90%) and quartz (5–40%) are the principal components of these dykes (Figure 3(d) and (e)); carbonate (1–5%) (Figure 3(f)) and K-feldspar (1–5%) constitute the second most important components. Accessory minerals include specularite, pyrite, zircon and monazite. The albite (An content less than 10%) is normally subhedral, with the development of twin lamellae and Carlsbad twins; chess-board albite also occurs (Figure 3(g)), characterized by a dense array of narrow discontinuous albite lamellae. These albites (0.1 to 0.5 mm) occur with clear and colourless rhomb carbonates (0.05 to 0.3 mm). X-ray diffraction patterns for the rhomb carbonate indicate that this is mostly ankerite. K-feldspar is a minor phase and is found as subhedral to anhedral plates, or as relics in albite in association with anhedral quartz crystals; the diameters of the quartz and K-feldspar crystals vary from 0.02 to 0.5 mm. Both albite and quartz host small inclusions of carbonate.

Figure 3. Field photographs and polished thin section photomicrographs of the aplitic textured leucogranitic dykes in the Cape Spencer area. (a) Aplitic dyke intruding the Millican Lake Granite following its foliation (S). (b) Aplitic dyke exhibiting its characteristic pinkish-red colouration in sharp contact with the host rock. (c) Specularite in aplitic dyke. (d) Tonalitic dyke (XPL). (e) Albitite dyke (XPL). (f) Carbonates with albite. (g) Chess-board albite. Qtz: quartz, Carb.: carbonate, Ab: albite, Kfs: K-feldspar.

4.b. Major- and trace-elements characteristics

Chemical discrimination by means of the multi-cationic diagram from De la Roche et al. (1980) results in samples forming a continuous spectrum from nepheline syenite to granite fields (Figure 4(a)), despite the samples having few K-feldspar and evident low K₂O contents, reflecting the ultrasodic composition of these dykes. All samples are metaluminous (A/CNK = 0.74–0.96), except for AP-08 (A/CNK = 1.10) (Figure 4(b)).

Figure 4. (a) R₁-R₂ multi-cationic classification diagram (De la Roche et al. 1980). (b) Chemical composition – Shand index plot. Fields are from Maniar and Piccoli (1989); A/CNK = Al₂O₃/(CaO + Na₂O + K₂O), A/NK = Al₂O₃/(Na₂O + K₂O), in moles.

Major element concentration data for the dykes are provided in Table 1. The felsic dykes have a wide range of chemical compositions, with SiO₂ = 58.94 – 79.24 wt.%, Al₂O₃ = 10.50 – 18.50 wt.%, Na₂O = 5.50 – 10.89 wt.% and CaO = 0.38 – 3.88 wt.%. In general, most major oxides are negatively correlated with SiO₂, except for K₂O and TiO₂ (Figure 5).

Table 1. Major element concentrations (wt.%) for the dykes studied. LOI = loss on ignition

Figure 5. Representative Harker diagrams for the dykes showing variations of selected elements (versus SiO₂).

The trace-element compositions for the Cape Spencer dykes are provided in Table 2. Primitive-mantle normalized (Sun & McDonough, 1989) spider diagrams (Figure 6(a)) show enrichment in Th and U and depletion in high field strength elements (HFSEs: Nb, P, and Ti). Taken collectively, the abundances of large-ion lithophile elements (LILEs: Cs, Rb, Ba, and Sr) are low relative to the average crustal values from Taylor & McLennan (1995) [Cs (≤0.4 ppm), Rb (≤21 ppm), Ba(≤185 ppm), and Sr (≤172 ppm)]. Chondrite-normalized REE patterns show similar negatively sloping rare earth elements, moderately to slight light rare element enrichment (La_N/Sm_N = 4.8–18.7), slightly positive heavy REE slopes (Gd_N/Yb_N = 0.4–1.4) and variable (La_N/Yb_N) ratios [REE, (La/Yb)_N = 3.02–39.62, where N indicates chondrite normalized with values from Sun & McDonough (1989)]. Europium anomalies are slightly negative for all aplitic samples [(Eu_N/Eu*) = 0.72–0.95, where Eu* = √(Sm_N x Gd_N)] (Figure 6(b); Table 1).

Table 2. Trace elements compositions (ppm) for the dykes studied

Figure 6. (a) Primitive-mantle normalized trace-element diagram. (b) Chondrite-normalized rare earth element diagram. Normalization factor after Sun & McDonough (1989).

The Nb/Ta ratios (Figure 7(a)) are much lower than primitive mantle and mantle-derived melts, including both mid-ocean ridge basalts and ocean island basalts (avg. Nb/Ta = 17.5 ± 2; Green, 1995) and the Earth’s mantle (Nb/Ta ∼ 16; Pfänder et al. 2007; Arevalo & McDonough, 2010); the ratios are also lower than in the average continental crust, i.e. Nb/Ta ∼ 11 (Taylor & McLennan, 1985, 1995; Green, 1995; Rudnick & Gao, 2014). Some of the samples plot below Nb/Ta ∼ 5, a ratio suggested to represent a threshold between purely magmatic systems (Nb/Ta > 5) and magmatic-hydrothermal systems (Nb/Ta < 5) (Ballouard et al. 2016). Zr/Hf ratios are lower than the range 33–40, which includes values of both chondrites and crust defined by Jochum et al. (1986). Magmatic affinity was also determined using the parameters of Ross & Bédard (2009) (Figure 7(b)); dykes plot in the calc-alkaline magmatic affinity field, an indication of mixing of different magma sources consistent with subduction zone magmas where the breakdown of hydrous minerals occurs resulting in the liberation of aqueous solutions (Kelley & Cottrell, 2009; Zheng, 2019; Vermeesch & Pease, 2021).

Figure 7. Trace element-based discrimination diagrams. (a) Nb/Ta vs. Zr/Hf diagram. Primitive mantle data are from McDonough & Sun (1995) and continental crust data are from Taylor & McLennan (1985). (b) Th/Yb vs. Zr/Y diagram for discrimination of magmatic affinities. Modified from Ross & Bédard (2009).

On tectonomagmatic discrimination diagrams (Whalen & Hildebrand, 2019), the dykes fall within the slab failure field of the Nb vs. Y and Ta vs. Yb diagrams (Figure 8(a) and (b)). In the Rb vs. Y + Nb and Rb vs. Ta + Yb diagrams (not shown), most samples plot in the arc field; however, there are some samples plotting in the slab failure field or close to the boundary, resulting from the relative mobility of Rb. However, when using the fields from Pearce et al. (1984), samples plot in the volcanic-arc granite (VAG) field, except in the case of the Ta vs. Yb where there is spread of data from the VAG field into the syn-collision granite field, probably reflecting fractionation and crustal contamination, as this classification scheme tends to reflect magmatic sources rather than tectonic setting (Pearce, 1996; Förster et al. 1997). The use of other discriminators (Hildebrand & Whalen, 2014) (not shown), such as Sr/Y, La/Yb and Sm/Yb, results in samples plotting in both fields, whereas Gd/Yb results in all samples plotting in the arc field. All the samples have an alumina saturation index of less than 1.1 and SiO₂ concentration between 58.9 and 79.24 wt.%, exceeding the limit of 70 wt.% under which these discrimination diagrams are valid (Hildebrand & Whalen, 2017); however, there is no correlation in samples with high SiO₂ being plotted on a specific field.

Figure 8. Tectonomagmatic discrimination diagrams for dykes samples from the Cape Spencer area (Whalen & Hildebrand, 2019). (a) Nb vs. Y. (b) Ta vs. Yb. Fields in grey are from Pearce et al. (1984).

4.c. Geochronology

Regardless of the attempts to avoid ablating fractures or inclusions while at the same time targeting specific domains so the effects of mixing could be controlled, samples display a dispersion of U-Pb data related to recent Pb-loss, common-Pb incorporation and inheritance. Zircons in these aplite samples are, for the most part, anhedral very fine grains up to 45–50 μm in size; these grains exhibit radiation damage (Figure 9(a)–(c)), elevated U (2000 – 94 000 ppm) and Th (500 – 45 000 ppm) concentrations, extensive microfracturing and to a large extent lack oscillatory zoning. Monazite within the dykes occurs as subhedral to anhedral very fine grains up to 25–30 μm in size that locally exhibit faint patchy zoning that resembles an outermost domain and a core (Figure 9(d)–(f)).

Figure 9. Scanning electron microscope-backscattered electron imaging of zircons (a–c) and monazites (d–f). (a–c) Metamictic zircons showing massive radiation damage. (d) Subhedral monazite displaying patching zoning (dotted black line). (e) fractured homogeneous subhedral monazite. (f) Subhedral rounded monazite displaying patchy zoning (dotted black line). Light blue circles correspond to spot locations (8 μm for monazite).

Zircon grains were analyzed (Supplementary Table S1) with the resulting data exhibiting significant common-Pb contamination necessitating correction to all ablated spots (common-Pb corrected using the measured ²⁰⁴Pb). Despite the correction, most spots are usually discordant (Figure 10(a)), which indicates a small residual ²⁰⁴Pb component below the instrument’s detection limit. The relative probability plot suggests most of the spots correspond to inherited material dating back to ca. 650 Ma (Figure 10(b)). There are two young populations, a small cluster around 275 Ma and an older group around 350 Ma, when plotting the data on a Wetherill Concordia diagram. Considering both the high degree of metamictization in zircon and textural setting that prevents the identification of rims, cores or zoning, the best estimate for the zircon crystallization age is based on a weighted mean ²⁰⁶Pb/²³⁸U age of 273.4 ± 4.2 Ma (MSWD = 1.03; n = 4), for the youngest cluster (Figure 10(d)), whereas the older group is 348.5 ± 3.1 Ma (MSWD = 0.53; n = 5), also based on a weighted mean ²⁰⁶Pb/²³⁸U age (Figure 10(c)).

Figure 10. LA-ICP-MS results of U-Pb zircon geochronology of samples from Cape Spencer. (a) Wetherill Concordia diagram for all data. (b) Relative probability plot and age histograms for zircon U − Pb ages. Bin width is approximately 20 Ma. (c) Weighted mean plot of ²⁰⁶Pb/²³⁸U ages for the older population. (d) Weighted mean plot of ²⁰⁶Pb/²³⁸U ages for the youngest population. Error bars are 2σ.

Monazite grains were analyzed using an 8 μm crater size (Supplementary Table S2), considering the small size of most grains; it was not possible to measure ²⁰⁴Pb, although it is present considering the Pb/U ratios. The results show a spread of data interpreted as the mixing between two different age domains. Assuming all samples are cogenetic and using ²⁰⁷Pb/²⁰⁶Pb based on a terrestrial model, it is possible to perform an anchored (²⁰⁷Pb/²⁰⁶Pb = 0.85 ± 0.01) Tera-Wasserburg regression, an age of 273.7 ± 1.3 Ma (MSWD = 0.29; n = 3) corresponding to the youngest end-member age population is determined (Figure 11(a)). A lower intercept age of 273.2 ± 9.2 (MSWD = 2.2) is obtained when plotting the monazite data on a conventional Concordia diagram (Figure 11(b)).

Figure 11. LA-ICP-MS results of U-Pb monazite geochronology of aplitic samples from Cape Spencer. (a) Tera-Wasserburg Concordia diagram for all data. The grey dashed line indicates a free regression through the data resulting in a nonviable low common ²⁰⁷Pb/²⁰⁶Pb intercept. (b) Conventional Concordia diagram for all data indicating a not anchored lower intercept at 273.2 ± 9.2 Ma.

4.d. Whole-rock Hf-Nd isotopes

The (¹⁷⁶Hf/¹⁷⁷Hf)i ratios in the Cape Spencer dykes samples range from 0.282561 to 0.282585, based on an age of 274 Ma, with εHf values in the range from -1.8 to -1.0, calculated at the same age (Table 3). The initial (¹⁴³Nd/¹⁴⁴Nd) i ratios observed in four samples from this study range from 0.511785 to 0.511993, based on an age of 274 Ma, with εNd values in the range from −9.7 to −5.7, calculated at the same age, and TDM values of 1.3–1.6 Ga (Table 3).

Table 3. Nd and Hf isotopic data for the dykes from Cape Spencer

Note: Age correction for (¹⁴³Nd/¹⁴⁴Nd)_i, (¹⁷⁶Hf/¹⁷⁷Hf)_i, εNd, and εHf is 274 Ma. T_DM was calculated using a linear evolution for a mantle separated from the chondritic uniform reservoir at 4.55 Ga with a present-day epsilon value of +10.

5. Discussion

5.a. Petrogenesis

The Cape Spencer dykes can be classified as albitites, based on their modal mineralogy, with high Na₂O (av. 7.36) and low K₂O/Na₂O (av. 0.04), average molar A/CNK of 0.85 and A/NK of 1.11 (Figure 4(b)). Their metaluminous affinity, zircon content (24–101 ppm) and decreasing P₂O₅ with increasing SiO₂ are characteristics of low-temperature I-type granitoids (Chappell & White, 2001; Chappell et al. 2004). The lack of parallel fabrics and the presence of replacement textures in the dykes indicate that the albitization process, in which K-feldspar was consumed, was of internal origin, i.e. autometasomatism, rather than metamorphism as the host rocks show no indication of alteration by a later event. There is a large range in loss on ignition, up to almost 5 wt.%, and it directly correlates with CaO, which indicates the suppression of An-content and crystallization of primary carbonate minerals (cf. Holloway, 1976). The primary nature of the (rhomb) carbonates is supported by the presence of inclusions in quartz and albite and the occurrence of single crystals. Although considered rare, the presence and crystallization of carbonates from granitic melts are constrained to pressures above 3 kbar (Swanson, 1979; Audétat et al. 2004).

These dykes also reflect depletion in Nb, P and Ti, but enrichment in Cs, U and Th, which are typical features of magmas originating in a subduction zone (Taylor & McLennan, 1985; Hofmann, 1988; Bea et al. 2011). In general, the Caledonia terrane plutons (ca. 630 -615 Ma) associated with the Broad River Group share most of these characteristics (Whalen et al. 1994; Barr & White, 1996b). Slab failure-related rocks derived through deep mantle processes that, for the most part, include partial melting of the upper portion of the torn slab (Hildebrand & Whalen, 2014, 2017; Whalen & Hildebrand, 2019); the use of immobile element geochemical discriminators (Whalen & Hildebrand, 2019) resulted in data plotting in both the slab failure and arc-related fields.

Overall, the low Nb/Ta ratios of these dykes constitute an intracrustal differentiation feature (Tang et al. 2019; Ballouard et al. 2020) and the low Zr/Hf ratios are characteristic of highly evolved magmatic rocks that have been derived through selective partial melting, then through fractional crystallization combined with hydrothermal interaction with released magmatic fluids (autometasomatic) (Bau, 1996; Tartèse & Boulvais, 2010). The Th and U averages in the Cape Spencer dykes are 35.8 ppm and 4.0 ppm, respectively. The Th/U ratios vary from 7.42 to 12.61, higher than the Th/U ∼ 4 accepted for both the crustal and mantle domains (Artemieva et al. 2017; Wipperfurth et al. 2018), although it is known that Th/U ratios vary greatly in felsic rocks between 1 and 10 (Hasterok et al. 2018), and that for some I-type granitoids, both Th and U contents increase with differentiation (Champion & Chappell, 1992; Villaseca et al. 1998). These high Th/U ratios had to be, for the most part, inherited from the source and probably further enhanced by fractional crystallization and/or magma mixing (Scharfenberg et al. 2019). The slightly negative Eu-anomalies (Eu_N/Eu* = 0.72–0.95) may indicate early fractionation of primary plagioclase associated with a reduced melt.

5.b. Estimation of liquidus temperature

Temperatures of the magmas of these dykes are estimated using zircon, monazite and apatite solubility models. Considering the major element concentrations, the low Zr concentrations in the dykes yielded zircon saturation temperatures (T_ZircSat) of approximately 590–738 °C (average = 670 °C) [calculations using the formula of Watson & Harrison (1983)]. It must be stated that T_ZircSat was calibrated for rocks with whole-rock compositions ranging from M = 0.9 – 1.9 (Watson & Harrison, 1983). This M value [M = (Na + K + 2Ca)/(Al.Si)] is a compositional parameter calculated by obtaining the molar amounts of each component, renormalizing and finally obtaining the ratio (Boehnke et al. 2013); M is essentially controlled by the silica content of the melt (Siégel et al. 2018), i.e., the lower the silica content of the melt the higher the value of the M parameter, resulting in decreasing T_ZircSat for granitic magmas that are produced by fractional crystallization of mafic magmas (Harrison et al. 2007). AP-01, AP-04, AP-07, AP-09 and AP-12 have M values of 1.9 – 2.4. The textural consistency of these dykes suggests rapid crystallization from a residual melt, with the consequent quenching in response to a rapid pressure drop. The dykes from Cape Spencer can be considered inheritance-rich intrusions. Although they constitute late segregated melts, the T_ZircSat should indicate an upper limit on the magma temperature and, therefore, a useful estimate of the initial magma temperature at the source (Miller et al. 2003). The average T_ZircSat and the low concentrations of zirconium reinforce the classification of these dykes as true low-temperature melts (Brown, 2013), as there is no indication of sequestration of zircon (Bea, 1996). Monazite-saturation geothermometry (Montel, 1993), using the bulk REE composition (∑REE_i = La+Ce+Pr+Nd+Sm+Gd) to represent the melt composition and assuming 3 wt.% H₂O content yields temperatures (T_Monazite) of 562–690 °C. Some of the T_Monazite values are higher than T_ZircSat values, suggesting the presence of additional REE-bearing phases.

5.c. Emplacement age and relation to gold mineralization

The dykes have an age of 273.7 ± 1.3 Ma based on U-Pb dating of monazite and constitute the youngest Alleghanian magmatic event in southern New Brunswick to this date. Because of the metamict nature of the zircon grains, an independent constraint was necessary to evaluate the meaning of the altered zircon ages. In this study, coexisting monazite geochronology supports the ∼274 Ma age from the altered zircons as the crystallization age. This age is also younger than the Ar-Ar ages (276.6 ± 0.9 and 283.7 ± 0.8 Ma) obtained by Watters (1993) on illites defining the fabrics associated with gold mineralization. Most aplitic dyke samples analyzed in this study contain low Au (<2 ppb), As (≤5.1 ppm), and Sb (≤1.8 ppm). Only samples AP-08 and AP-09 contain values above the detection limit for Au: 30 ppb and 252 ppb, respectively, and contain higher amounts of As (18.2 ppm and 6.7 ppm) and Sb (6.0 ppm and 4.5 ppm). Furthermore, low values of Ag (<0.5 ppm), Cu (<10 ppm), Pb (≤8 ppm), Sn (<2 ppm), W (≤3.8 ppm) and Mo (≤3 ppm) are consistent with the idea that there is no genetic link between the dykes and the gold mineralizing event in the Cape Spencer area.

It can be argued that the two clusters of zircon ages represent: 1) a phase of sodic metasomatism along the MFZ resulting in highly albitized granites and albite+quartz veins (the aplites of this study) around 350 Ma; 2) later fluid flow related to young hydrothermal zircon and monazite around 275 Ma. Examples of the former option are the intrusion (ca. 362–350 Ma) of granites and related dykes into the Tournaisian Horton Group (Dunning et al. 2002; Koukouvelas et al. 2002; Malay et al. 2023), and sodic alteration in the Cobequid Shear Zone dated by riebeckite (∼355 ± 4 Ma) (Pe-Piper et al. 2018). Regarding the younger cluster, several indications could be used to interpret the 275 Ma cluster as having recorded a hydrothermal event in the region. Firstly, ⁴⁰Ar-³⁹Ar ages (∼327 Ma) reflect fluid flow coeval with dextral shear along the Avalonia-Meguma boundary (Murphy & Collins, 2008). Secondly, hydrothermal veins containing allanite and pyrite (∼320–310 Ma) cut the Horton Group (Pe-Piper et al. 2018). Thirdly, hydrothermal mineralization associated with Zr-F-rich fluid complexes produced two zircon populations at ∼318 Ma and ∼309 Ma at Debert Lake, Nova Scotia (Ersay et al., 2022), in the context of the presence of fluorine to transport Zr (Rb-Sr on illite: 300 ± 6 Ma, associated to Pb-Zn and barite mineralization) in the Cobequid Shear Zone, Nova Scotia (Pe-Piper & Piper, 2021; Ravenhurst et al. 1989). Nonetheless, the already established magmatic origin of these dykes rules out the possibility of the obtained age to represent a hydrothermal event similar to those just mentioned. Additionally, these dykes display sharp contacts with their host rocks, without alteration haloes. There is no further evidence of ongoing hydrothermal activity affecting the host rocks after the event responsible for the ductile to brittle fabrics related to gold mineralization with cooling ages of ∼277 Ma (Watters, 1993).

5.d. Isotopic signature

Previous studies in the Caledonia terrane and Avalonia mainly focused on whole-rock Sm-Nd of volcanic and plutonic rocks (Whalen et al. 1994; Samson et al. 2000); therefore, there are few studies involving the use of Lu-Hf-isotope data. Available Lu-Hf analyses from Avalonia rocks are restricted to limited published in situ Lu-Hf-isotope data (Willner et al. 2013; Pollock et al. 2015, 2022). The first study including in situ zircon Lu-Hf data from Avalonia in New Brunswick, i.e. the Caledonia terrane, showed a range of εHf (+4.3 to +7.8) for samples from the Broad River Group and comagmatic plutons and εHf values between +2.1 to +8.5 for the Coldbrook Group and comagmatic plutons; these data were interpreted to show mixing of juvenile and mantle-derived magma, in addition to other crustal sources with variable Hf isotopic compositions (Pollock et al. 2022). Bickerton et al. (2022) concluded that the zircon ε(-2.99 to +1.68) signatures from the SMB indicated a metasomatized mantle source followed by contamination of both rocks of an underlying Avalonian terrane and metasedimentary rocks of the Meguma terrane. The narrow range of negative εHf values of the dykes lies on a Hf evolution trajectory typical to rocks of the South Mountain Batholith (Meguma Terrane), the Broad River Group and the Coldbrook Group (Caledonia terrane) (Figure 12(a)). The vertical trajectory in the plot of the Cape Spencer samples implies significant recycling of an evolved crustal component.

Figure 12. (a) Hf-isotope evolution of εHf against age for the Cape Spencer dykes (this study). The evolutionary arrays for 1000 Ma and 1600 Ma crust with average upper crustal ¹⁷⁶Lu/¹⁷⁷Hf values of 0.015 (Griffin et al. 2002). Depleted mantle evolution curve from Griffin et al. (2000). Hf-zircon analyses from the South Mountain Batholith (Bickerton et al. 2022) and the Caledonia terrane (Pollock et al. 2022) are also shown. (b) Nd-isotope evolution of εNd against age for the Cape Spencer dykes (this study). Additional εNd data are from the Sebago Batholith and granitic pegmatites from the Topsham area (Tomascak et al. 1996a, 1998), the German Bank Pluton (Pe-Piper & Jansa, 1999; Pe-Piper et al. 2010), the South Mountain Batholith (Clarke et al. 1988, 2004; Erdmann et al. 2009; MacDonald & Clarke, 2017), Devonian Plutons (Whalen et al. 1994, 1996; Mohammadi et al. 2020), Silurian Plutons (Whalen et al. 1994) and the Caledonia terrane (Samson et al. 2000; Whalen et al. 1994). Field (green-shaded) for Avalonia from Nance & Murphy (1996). Depleted mantle evolution curve from DePaolo (1981).

The εNd values for the Cape Spencer dykes plot outside the envelope for crustal-derived Avalonian rocks (Figure 12(b)), thought to be the result of repeated melting events with a common basement source (Nance & Murphy, 1996; Murphy & Nance, 2002), in contrast to the Devonian and Silurian plutons of southern New Brunswick and the Neoproterozoic intrusions in the Caledonia terrane. The dykes also show a stronger negative Nd isotopic composition than the Sebago Batholith and its associated leucogranites and aplites and the granitic pegmatites from the Topsham area (Maine, USA). These granitic pegmatites have crystallization ages ca. 270–273 Ma (Tomascak et al. 1996b, 1998), virtually the same ages as the Cape Spencer dykes, and therefore emplaced during the Alleghanian orogeny (Bradley et al. 2016). A couple of samples from Cape Spencer lie on hypothetical growth lines to a segment of the most negative εNd values of the SBM; a compilation of relevant Nd isotopic work on rocks from the South Mountain Batholith (Clarke et al. 1988, 2004; Erdmann et al. 2009; MacDonald & Clarke, 2017) reveals a large variation in εNd_{(380 Ma)} with most data concentrated in the -7 to +0.1 range. Clarke et al. (1988) indicated, based on his Nd isotopic data, that the SBM had been generated by either melting of deep metasedimentary crustal material or by mixing of crustal materials with mantle-derived magma. The low εNd values (εNd_{(300 Ma)} = −12.6 to −8.6) of the German Bank Pluton, more negative than those from the dykes, are comparable with Meguma metasedimentary rocks, indicating assimilation of sediments derived from the basement beneath the southwestern Scotian Shelf (Pe-Piper & Jansa, 1999). Pe-Piper et al. (2010) interpreted the resulting isotopic signature in addition to the Paleoproterozoic model ages as a derivation of mixing magma of mantle origin with lower crustal melts.

5.e. Tectonic significance

In the northern Appalachians, the dextral-oblique Alleghanian collision between Laurentia and Gondwana (Hatcher, 2002; Murphy et al. 2011; Waldron et al. 2015, 2019) had started by at least 330 Ma and continued into the middle Permian, i.e. ca. 260 Ma (Nance & Linnemann, 2008; Nance et al. 2012; van Staal & Barr, 2012). Although limited, Alleghanian orogenesis in the United States section of the northern Appalachians included high-grade metamorphism and associated magmatism that took place in the late Pennsylvanian and early Permian (Tomascak et al. 1996a, 1996b; Walsh et al. 2007). In the Canadian section of the northern Appalachians, Alleghanian magmatism is restricted to the German Bank Pluton (Pe-Piper et al. 2010; van Staal & Barr, 2012).

The input of mantle-derived material has already been described for the German Bank Pluton (Pe-Piper et al. 2010) and the South Mountain Batholith (Bickerton et al. 2022). From the Hf-Nd-isotope analyses, there is no indication of the involvement or mixing of mantle material for the Cape Spencer dykes, as those strong negative Nd values and the negative Hf values point to assimilation and reworking of old crust instead of a juvenile source.

Isotopic data from the Cape Spencer aplitic textured leucogranitic dykes show compositional ranges of whole-rock εNd_{(274 Ma)} values from −9.7 to −5.7, εHf_{(274 Ma)} from −1.8 to −1.0 and whole-rock Nd model ages (T_DM) from 1.3 to 1.6 Ga. These isotopic characteristics suggest that the dykes are derived from the crystalline residuum of the magmas resulting from the partial melting of crustal source rocks that, while ascending, became contaminated with Meguma metasedimentary rocks and/or Avalonian sedimentary rocks. Continued subduction of the Rheic Ocean displaying an ongoing zipper-style closure derived in the Alleghanian collision of Gondwana with the Appalachian part of Laurentia (Kroner et al. 2022); this process could have led to partial melting of the underthrust Avalonian basement, currently extending 50 km south of the surface suture (Pe-Piper & Jansa, 1999), under Meguma. Melt emplacement would be confined to favourable channels as faults, in this case, the MFZ with continued dextral-strike-slip motion during the Pennsylvanian-Permian (Murphy et al. 2011; Waldron et al. 2015). With such an active environment, the fracturing of solid rocks will allow the extraction or ascent of evolved low T, carbonate-bearing granitic melt by the movement of the silicic parental melt.

6. Conclusions

The low T leucogranitic dykes show major and trace elements similar to those of I-type granitoids, although they exhibit evidence of autometasomatic albitization, but have igneous calcite. These dykes have an age of 273.7 ± 1.3 Ma, based on U-Pb dating of monazite and constitute the youngest magmatic event in southern New Brunswick to this date. The emplacement of these dykes provides a lower age constraint for the gold mineralization in the area, as they crosscut the various deformation fabrics of the host rocks to which gold deposition is related.

Nd-Hf isotopic signatures do not suggest the input of mantle-derived material for the dykes from the Cape Spencer area; instead, they point towards partial melting of a crustal source with assimilation of Meguma metasedimentary rocks and/or Avalonian sedimentary rocks. Important geodynamic events occurred during the Alleghanian orogeny with the continued subduction of the Rheic Ocean under composite Laurentia that provided the heat supply and material sources for these partial melts to form and ascend and be emplaced as aplitic dykes quenched due to depressurization associated with emplacement. The activation of deep crustal faults in response to the strike-slip motion that was taking place along the MFZ could have provided the means for the ascent of these low-T crustal melts.