9 The Chain of Ponds (COP) pluton outcrops over approximately 220 km² in northern Maine and extends northward into Quebec (Fig. 3). It intruded Precambrian basement rocks of the Chain Lakes massif, and is mapped as two granitoid bodies, the most northerly of which is zoned with a core of biotite-hornblende granite and a marginal zone of biotite granite. Several “screens” of country rock outcrop within both units in a semi-circular concentric outcrop pattern parallel to the outer contact of the intrusion (Harwood 1973). Westerman (1980) mapped a large number of brittle fractures, which together with the outcrop relationships suggest that the intrusion has a high-level ring dike form. A few kilometres to the north in Quebec, the Spider Lake pluton is similarly zoned but due to lack of exposure its relative age relationship to the COP pluton is unclear. The COP pluton was dated by Heitzler et al. (1988) using the ⁴⁰Ar/³⁹Ar hornblende step-heating technique which yielded an age of 373 ± 2 Ma based on the average of 6 plateau ages. The Spider Lake pluton gave a U–Pb zircon age of 383 ± 3 Ma (Simonetti and Doig 1990).

10 Dated sample COP-1 was collected from the northerly zoned part of the COP pluton close to the Quebec border. It consists of equigranular, medium-grained biotite granite with a colour index (CI) ~20. Biotite is the predominant mafic mineral and occurs as euhedral flakes 2–3 mm in size. It has pale brown to dark brown pleochroism with numerous zircon inclusions. Minor hornblende is also present. Plagioclase forms larger euhedral grains, up to 4 mm in length, commonly displaying compositional zoning, and is more abundant than K-feldspar. Some grains show minor sericitic alteration. K-feldspar occurs as mostly interstitial, anhedral grains. Quartz occurs as anhedral grains, commonly in clusters (glomerocrysts), with some grains displaying undulatory extinction. Zircon is the most abundant accessory mineral and is hosted predominantly in biotite. Some larger grains, up to 0.25 mm, are present. In addition, opaque minerals and titanite are present. Some epidote grains are also observed and their euhedral and zoned, suggesting a possible magmatic origin.

11 The Flagstaff Lake Igneous Complex (FLIC) is one of the largest intrusions (420 km²) of the central/northern Maine belt outcropping from Rangeley to the Long Falls Dam, about 60 km in an orogen-parallel SW–NE direction (Fig. 4a). It intruded Cambrian–Ordovician rocks along its northern border and an Ordovician–Silurian metasedimentary sequence along its southern margin and has not been previously dated. Moench and Pankiwskyj (1988) mapped the complex as a composite intrusion composed of three units, from southwest to northeast: gabbro with garnet “granofels”, a central area of porphyritic granite, and gabbro in the northeast. Neilsen et al. (1989) described more complex field relations including garnet tonalite (as opposed to granofels) and evidence for the comingling/mixing of mafic and felsic magmas especially in the gabbroic area in the northeast of the complex. However, the reconnaissance fieldwork for this study has revealed further variations. The southwestern unit, although predominantly gabbro, also contains diorite and granite in close field association. Similarly, the central unit is petrographically variable with coarse-grained, porphyritic granite in contact with fine-grained quartz diorite (Fig. 4b). The contact is sharp, defined by a thin (<1 cm) biotite-rich zone, but it does not appear to be a chilled, intrusive contact. The northeastern unit is predominantly a commingled zone of mafic and felsic rocks. Medium-grained gabbro and diabase are surrounded by, and in lobate contact with, intermediate “hybrid” rocks and granitic material (Figs. 4c, d). Some quartz and feldspar grains are observed in the mafic rocks whereas some mafic minerals (hornblende?) exchanged to the felsic portion. Acicular apatite, formed by quenching during magma mixing, was observed microscopically in intermediate rocks, although evidence of more intensive mixing, such as rapakivi-textured feldspars or quartz ocelli, is absent. It is clear however, that these mafic and felsic magmas were contemporaneous.

12 A particularly enigmatic rock type observed in the FLIC is garnet tonalite, which outcrops both in the southwestern part of the complex and near its northeastern contact. Nielsen et al. (1989) suggested two possible origins for these rocks — either by the differentiation of mantle-derived magma or by country rock anatexis. They pointed to the close association of the garnet tonalite with sulfidic, Fe-rich metapelite as an important petrogenetic observation. However, either process would have required significant heat input and therefore determining the age of these rocks might be significant in elucidating the timing of mantle-derived magmatic activity. Three samples were dated from the FLIC: Sample FLC-2 is equigranular, coarse-grained, biotite granite with CI ~15 and was collected from the central unit of the FLIC just west of the town of Stratton. The dominant mafic mineral is biotite which displays distinctive pale to deep red-brown pleochroism. It forms euhedral grains up to 2.5 mm with minor chloritic alteration along cleavage planes. Muscovite is also present and appears to be primary as it occurs both as discrete grains (Figs. 5a, b) and as inclusions in both K-feldspar and plagioclase. K-feldspar occurs as large (up to 5 mm) euhedral to subhedral twinned grains showing perthitic texture. Quartz is more abundant than plagioclase with some grains being up to 4 mm in length and locally displaying undulatory extinction. Plagioclase forms subhedral to anhedral grains which are interstitial or larger, up to 4 mm, some of which display concentric zoning. Zircon is again the dominant accessory phase occurring as small equant grains <0.2 mm in size, although some acicular forms are also present. Minor amounts of apatite are also observed.

13 Sample FLC-5 was collected from workings at an abandoned garnet quarry at the southwestern end of the FLIC. The rock is equigranular, medium-grained and dominated by garnet which comprises over 60% of the sample. The garnet occurs as mostly euhedral to subhedral grains which range in size from 2–3 mm (Fig. 5c) and contain numerous opaque inclusions. Most are fractured in a consistent orientation. Biotite is the dominant mafic mineral and is commonly around the margins of the garnet grains. It displays pale red to deeper red-brown pleochroism. Plagioclase forms subhedral grains between the garnet grains and typically displays concentric zoning, supporting an igneous origin for this rock (Fig. 5d). Quartz occurs interstitially as small (<1 mm) grains that show normal (non-undulatory) extinction. The main accessory phases are zircon, hosted by the biotite, and opaque minerals, which are generally observed within the garnet grains.

14 Sample FLC-6 was collected from the northeastern part of the FLIC just inland from the northern shoreline of Flagstaff Lake. It is a medium-grained, mainly equigranular biotite granite with a CI ~25. Biotite occurs as small, euhedral flakes up to 2 mm wide with some minor chloritic alteration. It displays pale to deeper red-brown pleochroism and hosts abundant zircon grains. Muscovite is also present and is interpreted to be mostly of primary origin, although it also occurs as an alteration product. Plagioclase is more abundant than K-feldspar and some larger grains, up to 4 mm, are present. Most plagioclase forms smaller euhedral to subhedral grains with sericitization of core areas suggesting higher Ca contents. K-feldspar is typically subhedral and around 4 mm across with quartz about the same size, the latter displaying normal extinction. Zircon is the dominant accessory phase with minor amounts of opaque minerals.

15 The Rumford pluton, mapped by Moench and Hildreth (1976), is a small, composite intrusion which outcrops over an area of 25 km² but is poorly exposed in a bowl-shaped topographic depression. It intruded mostly Devonian metasedimentary rocks but also some Silurian strata to the northwest and therefore cross cuts an important structural boundary, the Blueberry Mountain fault (Fig. 3). Moench and Hildreth 1976) mapped a number of rock types in the Rumford pluton. An area of gabbro-diorite with hornblende, biotite, and calcic plagioclase occurs in the western part of the intrusion. The main part of the pluton is more felsic with equal areas of pegmatitic granite and two-mica granite. The pegmatitic granite texture ranges from true pegmatite to aplite with minor garnet and tourmaline. The two-mica granite contains abundant inclusions of granodiorite and tonalite which contain biotite and hornblende with some titanite and zircon, and are similar to rocks in the Mooselookmeguntic Igneous Complex (Table 1).

16 Sample RUM-1 was collected from the eastern margin of this petrographically diverse pluton. It is a medium-grained, medium-grey equigranular granite. It has a CI of ~20 with biotite as the main mafic mineral and sparse hornblende. The biotite occurs as euhedral flakes 2–3 mm in length. Glomerocrystic quartz is abundant, and plagioclase predominates over K-feldspar. Opaque minerals and rare zircon and apatite are present as accessories but no titanite was observed.

17 Outcropping over an area of 325 km² the Songo pluton (Fig. 3) intruded a high-grade Devonian metasedimentary sequence, which was metamorphosed to K-feldpsar + sillimanite grade (Guidotti et al. 1986 and references therein). The estimated depth of emplacement is ~18 km based on new Al-in-hornblende data (Brock and Gibson, unpublished data) using the calibration method of Mutch et al. (2016). The Songo pluton previously yielded a U–Pb zircon age of 382 ± 3 Ma (Lux and Aleinikoff 1985); however, a younger age of 360.7 ± 2.2 Ma was reported by Eusden et al. (2020). It is intruded by numerous two-mica leucocratic granite and pegmatite bodies, which range in age from Carboniferous to Permian (Solar and Tomascak 2016). These bodies have played an important role in the development of petrographic variants within the Songo pluton (Gibson et al. 1989). Typically, the Songo pluton consists of equigranular, coarse-grained biotite + hornblende + titanite granodiorite to granite with some quartz diorite and is undeformed. However, more proximal to the granite/pegmatite intrusions the granite displays a well-defined fabric, which can be intensely developed to gneissic at its extreme. In addition, hornblende is absent and the biotite has the red-brown pleochroism indicative of high Ti content common in metamorphic varieties (Deer et al. 1966). This variation could be related to the forceful emplacement of the younger Carboniferous–Permian leucogranite and pegmatite or perhaps the deformed rocks are vestiges of an older pluton. Recent remapping of the Bethel (Eusden et al. 2020) and Waterford quadrangles (C. Koteas, personal communication 2020) have further highlighted the variable petrography evident in the Songo pluton.

18 Dated sample SG-7b was collected from the northeastern lobe of non-foliated Songo pluton granodiorite. It is an equigranular, medium-grained biotite + hornblende + titanite granodiorite with a CI of ~25 (Figs. 5e, f). Biotite occurs as euhedral grains up to 2.5 mm in diameter and is more abundant than hornblende. It displays pale brown to greenish-brown pleochroism and has minor chloritic alteration along cleavage planes. Minor hornblende occurs as subhedral grains with light to dark green pleochroism. Plagioclase is more abundant than K-feldspar and occurs as ~4 mm subhedral grains, some of which display compositional zoning. K-feldspar forms interstitial subhedral grains up to 4 mm across. Quartz is typically in clusters, with individual grains not exceeding 2 mm across and displays normal extinction. Accessory phases include euhedral titantite grains up to 2.5 mm in length and a lesser amount of zircon forming inclusions in biotite.

19 A group of four peraluminous two-mica plutons outcrop in west-central Maine, from west to east the North Jay, Chesterville, Cape Cod Hill, and Rome-Norridgewock plutons (Fig. 3), following the nomenclature proposed in Gibson et al. (2006). They intruded an older Silurian metasedimentary succession in a different crustal block than the plutons described above and previously dated plutons (Table 1). They are separated from these plutons by a major structural boundary, the Winter Brook fault, which was interpreted by Moench and Pankiwskyj (1988) “as a west-verging older- over-younger thrust”. The Rome-Norridgewock pluton has an overall ovoid outcrop pattern and is exposed over an area of 260 km², whereas the North Jay (40 km²), Chesterville (85 km²), and Cape Cod Hill (50 km²) plutons have more irregular outlines but still truncate the regional NE-SW orogen trend. The Chesterville pluton is poorly exposed and intensely weathered and was therefore not sampled for the present study. However, it appears to be petrographically similar to both the North Jay and Cape Cod Hill granites.

20 The North Jay pluton consists of light grey, medium-grained, equigranular muscovite - biotite granite with minor amounts of garnet. It is intruded by several metre-sized pegmatite dikes along with aplitic segregations, which also contain garnet and minor beryl. Abundant metasedimentary xenoliths, that range from large blocks (10–20 cm) to small, thin slivers or surmicaeous enclaves, are also observed. They are obviously of metamorphic origin with garnet and biotite in schistose fabric. Small (1–2 cm) clots of biotite and garnet may represent further disaggregation of the xenoliths. These garnets are euhedral (Fig. 6a) and electron microprobe data reveal that they are chemically zoned in Ca, Mg, Mn, and Y contents (Gibson et al. 2006). However, another population of subhedral to anhedral garnet (Fig. 6b), which lacks chemical zoning, is also present. Although the majority of garnet in the North Jay granite is most likely xenocrystic due to the abundance of metamorphic xenoliths, some is magmatic in origin (Gibson et al. 2006).

21 Dated sample NJq-1 was collected from the North Jay granite quarry and is fine- to medium-grained, equigranular two-mica leucocratic granite with a CI ~12. Biotite is slightly more abundant that muscovite. It has pale brown to deeper red-brown pleochoism with numerous zircon inclusions. Muscovite is of primary origin with some large euhedral grains present. Quartz occurs as anhedral grains in clusters and displays evidence of deformation with undulatory extinction and serrated margins. Plagioclase forms subhedral grains up to 2.5 mm in length and some have concentric zoning. K-feldspar (microcline) is common, both as discrete and interstitial grains. Zircon is the dominant accessory mineral but acicular opaque minerals and some garnet grains are also present.

22 The Cape Cod Hill (CCH) pluton forms the elevated topography to the south of Farmington and is mapped as a roughly oval body that cuts across the regional NE–SW trend (Fig. 3). It is composed of a medium-grained, white, equigranular granite with minor garnet. It has a similar range of xenoliths as the North Jay pluton and petrographically the two are similar. However, viewed in thin section the CCH granite displays more pronounced deformation features.

23 Dated sample CCH-1 was collected from the Cape Cod Hill quarry and is an equigranular, medium-grained two-mica leucocratic granite with a CI of 5. Biotite is sparse and occurs as sub- to anhedral grains < 1 mm in size with pale brown to deeper red-brown pleochroism. Zircon inclusions are common and many grains display warped cleavage planes indicating some post-crystallization deformation. Muscovite is more abundant than biotite with larger euhedral grains up to 2.5 mm. Like the biotite grains, muscovite grains show warped and disjointed cleavage planes (Figs. 6c, d). Quartz occurs as 2 to 2.5 mm grains that show undulatory and sectorial extinction and many with irregular serrated margins. Plagioclase commonly contains numerous muscovite inclusions and generally lacks zoning. Plagioclase grains vary in size but some are ~3 mm in length and twin planes are warped and broken. Microcline is also present most commonly in anhedral, interstitial grains. Zircon is the most common accessory mineral.

24 The Rome-Norridgewock (R-N) pluton is the largest pluton of the west-central Maine group covering an area of 260 km² and is mapped as a homogeneous granitic intrusion on the bedrock map of Maine (Osberg et al. 1985). However, sampling for an ⁴⁰Ar/³⁹Ar cooling history study (Gibson et al. 2006) revealed a petrographically different unit in the northeastern part of the pluton south of the town of Norridgewock. The main unit of the pluton is light grey, medium- to coarse-grained equigranular granite with both biotite and muscovite. This rock type forms most of the pluton and closely resembles the CCH and North Jay granites; hence we refer to it as the Rome unit of the R-N pluton in this paper. The only previous age determination on any of the plutons in west-central Maine was from this unit and yielded a U–Pb zircon age of 378 ± 1 Ma (Tucker et al. 2001).

25 The petrographic variant exposed in more northerly outcrops of the R-N pluton is medium-grained, equigranular, biotite ± hornblende + titanite granite/granodiorite, and hence of I-type mineralogic affinity (Chappell and White 2001) in contrast to the two-mica Rome unit described above. This unit is referred to as the Norridgewock unit. No contact relationships were observed, so it is unclear if the Norridgewock unit is a comagmatic part of the R-N pluton or if it should be considered a separate intrusion.

26 Dated sample DHq-1 was collected from the Dodling Hill quarry which is located in the Norridgewock unit of the RN pluton. It is equigranular, medium-grained biotite + horn-blende + titanite granite/granodiorite with a CI of ~15 (Figs. 6e, f). Biotite is the dominant mafic mineral and occurs as larger euhedral flakes with pale brown to greenish brown pleochroism. Hornblende forms small subhedral to anhedral grains, typically in mafic knots with biotite. Plagioclase occurs as larger grains, up to 4 mm, that are euhedral to subhedral. Some grains display concentric compositional zoning. K-feldspar forms smaller subhedral crystals. Quartz occurs in glomerocrysts approximately 3 mm in size, some of which display undulatory and sectorial extinction. Zircon and titanite are the dominant accessory phases with minor opaque minerals and allanite also present.

30 Abundant zircon grains separated from sample COP-1 range in size from 20 to 150 µm. Most of the grains are clear and euhedral, with bipyramidal terminations preserved. Grains vary in shape from elongate acicular crystals to rectangular. Most grains display weak fluorescence under cathodoluminescence but oscillatory zoning is present in fluorescent grains. Figure A1 (a–d) shows representative grains (backscatter electron and cathodoluminescence for each) with indistinct zoning in their cores, and clear oscillatory zoning around the edges of the grains.

31 The ages obtained from this sample for grains that are between 98 and 102% concordant (84 out of 129 grains analysed) are mostly in the range between 360 Ma and 400 Ma with no obvious clusters (Fig. 7a). The age distribution is strongly skewed with an older “tail”, indicating that inheritance may be a significant factor in the sample. This interpretation is supported by the presence of a few grains as old as Neoproterozoic (Table B1). The cumulative probability plot (Fig. 7b) displays two peaks, a major one ca. 365 Ma, and a secondary one ca. 385 Ma. There is no clear separation between the peaks but they remain distinct with age bins of 2.5 Ma, 5 Ma, and 10 Ma. It is possible to calculate a weighted mean age of ca. 373 Ma for all the grains that are 98–102% concordant (73 grains) but this result has a high MSWD (14) and clearly does not represent a single mean age for the sample. Similarly, it is not possible to calculate a concordia age for the grains around this age without obtaining unacceptably high MSWD values.

32 Based on the probability distribution we interpret this sample as having two main age components which can produce concordia ages, an older group (Fig. 7c), and a younger group (Fig. 7d). The younger group has a concordia age of 363.4 ± 1.5 Ma with an MSWD of 1.16, and a probability of concordance of 0.28. The older group has a concordia age of 385.3 ± 1.5 Ma with an MSWD of 0.054 and a probability of concordance of 0.94. Hence, we interpret this sample as having a minimum crystallization age of 363 Ma, with inherited older grains. The wide spread of the near-concordant ages and the presence of older grains in this sample shows that these plutons and the terranes into which they intrude are complex and may represent several episodes of igneous activity. This complexity is also present in other samples and will be discussed in subsequent sections.

33 The zircon grains from all three FLIC samples are mostly euhedral and clear with few inclusions and the majority of the grains are very small (20 to 150 µm) with only a few grains in the larger range. The smaller grains tend to be acicular in shape, and the larger grains are rectangular, with most showing some oscillatory zoning in cathodoluminescence. Samples FLC-2 and FLC-5 contained abundant zircon, whereas sample FLC-6 had less zircon. However, there are no systematic differences in zircon morphology among the samples and all the grains in all the samples vary in the extent to which zoning is visible, especially in the abundant small grains. Figure A1 (e–h) shows representative grains from FLC-2 (backscatter electron and cathodoluminescence for each) illustrating indistinct zoning (e and f) and non-oscillatory zoning in a smaller grain (g and h). Figure A1 (i–l) shows representative grains from FLC-5 (backscatter electron and cathodoluminescence for each) illustrating variable oscillatory zoning (i and j) and non-oscillatory zoning in a smaller grain (k and l). Figure A1 (m–p) shows representative grains from FLC-6 (backscatter electron and cathodoluminescence for each) illustrating non-oscillatory zoning (m and n) and variable oscillatory zoning in a larger grain (o and p). The images of the smaller grains also illustrate the difficulty in analysing these grains where it is impossible to limit the laser spot to only one zone even if imaging indicates zoning is present because the grains are so small. The ages obtained from the samples differ and hence the samples are discussed separately below.

34 Sample FLC-2 is the least complicated of the three FLC samples. It has a clear peak in the cumulative probability diagram using all the 98–102% concordant grains (38 out of 57 grains analysed) (Fig. 7e) and has a strong right-tailed distribution with a small secondary peak around 430 Ma, interpreted as a result of inheritance. The concordia age with 25 grains is 409.3 ± 1.6 Ma with an MSWD of 0.57, and a probability of concordance of 0.45 (Fig. 7f).

35 Sample FLC-5 has two peaks in the cumulative probability diagram (Fig. 8a), both of which can be seen in a weighted mean plot of the 98–102% concordant grains (25 out of 53 grains analysed). The older group (6 grains) has a concordia age of 401 ± 1 Ma with an MSWD of 1.8, and a probability of concordance of 0.18 (Fig. 8c) and the younger group (10 grains) has a concordia age of 383.8 ± 2.2 Ma with MSWD of 2.3 and a probability of concordance of 0.13 (Fig. 8d). While the statistics on these two ages are not very good, they overlap with the weighted mean ages of the two groups of grains, which are 383.2 ± 2 Ma (MSWD = 0.84, probability = 0.58) and 400.3 ± 3.4 Ma (MSWD = 0.33, probability = 0.90), respectively (Fig. 8b).

36 Sample FLC-6 has a single major peak in the cumulative probability diagram (Fig. 8e) of the 98–102% concordant grains (22 out of 58 grains analysed). Using the largest possible group of grains (15) for a concordia age produces an age of 399.9 ± 3.6 Ma with a high MSWD of 4.0, and low probability of concordance of 0.045. The weighted mean age of the same group of 15 grains is slightly younger at 397.5±5.3 Ma with an MSWD of 1.8 and a probability of 0.039. When only the grains in the main peak of the probability diagram are used, they produce a concordia age of 403.2 ±4.6 Ma with an MSWD of 0.72, and a probability of concordance of 0.40 (Fig. 8f). This age includes only 6 grains but given the age distribution in this sample we interpret it as the main age of crystallization. As with the other FLC samples older inherited grains are present as well, in this case ca. 410 Ma and 450 Ma. Given that sample FLC-5 also has a major group of younger grains with an age of ca. 383 Ma, it is possible that the younger grains with an apparent peak ca. 390 Ma seen in the cumulative probability diagram for FLC-6 could be part of a younger episode of crystallization. Alternatively, they could be the result of minor Pb loss.

37 Figure 9 shows the U/Th ratios of all the grains included in concordia or weighted mean calculations for the FLIC samples. Of the three samples from this complex FLC-2 has the oldest calculated age, and also the widest spread in U/ Th ratios. FLC-5 has two main group of grains that were used to calculate the two possible ages for this sample (inheritance vs. crystallization). There is no difference between the U/Th ratios of the older and younger groups and each cluster is a tight grouping (Fig. 9). This result suggests that Pb loss is likely not a factor in generating the younger ages, supporting the interpretation that the older age represents inheritance and the younger one is the crystallization age. The older cluster also overlaps with the ratios from sample FLC-2, providing additional support for the interpretation of inheritance of zircons from the older phase of activity. For FLC-6 the main concordia age for the sample is ca. 400 Ma and those grains have U/Th ratios that again overlap with both sample FLC-2 and with the older group in FLC-5. Figure 9 also includes the U/Th ratios for the younger grains in FLC-6 that were not included in the calculated concordia age and make up the younger tail of the distribution as shown in Figure 8e. These ratios again overlap with those from all the other samples, but their U/Pb ratios obtained from analysis are not concordant enough to define a clear cluster of ages that would support the interpretation of a distinct younger phase of crystallization. This supports the interpretation that the younger grains in FLC-6 are simply part of the normal distribution of data around the most likely crystallization age.

38 Taken together, the three samples from the Flagstaff Lake Igneous Complex indicate that the complex is a composite pluton containing a large range of inherited ages, and variations in youngest crystallization phases of different segments of the pluton. A major older episode of crystallization at ca. 409 Ma is recorded in sample FLC-2, the major phase for FLC-6 is likely ca. 403 Ma, and sample FLC-5 has a big group of inherited ages ca. 401 Ma, likely derived from the older intrusive units, and a younger episode of crystallization at ca. 383 Ma.

39 Zircon grains from sample RUM-1 are very small (most under 40 µm) and many were too small to analyse so the total number of data points for this sample is lower than most others. The grains are acicular and most are subhedral to anhedral with reddish-brown surface staining. Under cathodoluminescence most grains showed only weak fluorescence and only some grains showed oscillatory zoning. Figure A2 (a–d) shows representative grains from RUM-1 (backscatter electron and cathodoluminescence for each) illustrating clear oscillatory zoning (a and b) and indistinct oscillatory zoning in a more elongate grain (c and d).

40 Only 7 of 12 grains analysed are between 98 and 102% concordant, but they display a clear peak in the cumulative probability distribution (Fig. 10a). The four grains with ages that overlap within error produce a concordia age of 373.2 ± 2.3 Ma with a MSWD of 0.34 and a probability of concordance of 0.56 (Fig. 10b) interpreted as the main age of crystallization in this sample. Additional concordant grains at ca. 490 Ma are interpreted as inherited.

41 Zircon grains in sample SG-7b are mostly euhedral with bipyramidal terminations intact. They are clear and colourless with few visible inclusions and no surface staining. The grains show clear oscillatory zoning under cathodoluminescence. Figure A2 (e–j) shows representative grains from SG-7B (backscatter electron and cathodoluminescence for each) illustrating clear oscillatory zoning seen in grains from this sample.

42 The data for this sample display a major peak in the cumulative probability diagram for the 98–102 % concordant grains (36 out of 51 grains analysed) (Fig. 10c), and a strong right-tailed distribution, with two minor peaks ca. 375 Ma and 385 Ma that can be interpreted as inherited grains. The concordia age of the major peak using 17 grains is 364.0 ± 1.3 Ma with MSWD of 0.83, with a probability of concordance of 0.36 (Fig. 10d).

43 Zircon grains in sample NJq-1 are very small (most less than 40 µm) and generally acicular and euhedral. Most are clear with few visible inclusions but have brown surface staining. Most of the grains show weak oscillatory zoning under cathodoluminescence. Although the sample contained abundant zircon, most grains are too small to analyse so the total number of data points for this sample is lower than most others. Figure A2 (k–p) shows representative grains from NJq-1 (backscatter electron and cathodoluminescence for each) illustrating that some grains have distinct cores (k–l), some grains have indistinct zoning in cores (m and n), and some have partial oscillatory zoning (o and p), illustrating the variability of zoning observed in this sample. Only 10 out of 25 grains analysed are between 98 and 102% concordant and give a clear peak in the cumulative probability distribution (Fig. 10e). Four grains with ages that overlap within error produce a concordia age of 369.6 ± 3.0 Ma with MSWD of 0.93 and a probability of concordance of 0.34 (Fig. 10f), interpreted as the main age of crystallization in this sample. Additional concordant grains at ca. 450 Ma, 670 Ma, and 850 Ma are interpreted as inherited.

44 Zircon grains in sample CCH-1 are generally acicular and subhedral, with a few euhedral grains. Sizes are varied and range from 20 to 150 µm. Some grains are clear but most have visible inclusions and reddish-brown surface staining. They show very low fluorescence under cathodoluminescence and some of the larger grains show indistinct oscillatory zoning.

45 The cumulative probability distribution of the 98–102% concordant grains for this sample (39 out of 50 grains analysed) shows three distinct peaks (Fig. 11a). Although no clear separations between them is apparent in the probability distribution, it is possible to resolve three separate concordia ages for different groups of grains (Fig. 11b). These groups of grains do not overlap, and neither the concordia ages nor the weighted mean ages overlap within error (Fig. 11c). The youngest group has a weighted mean age of 370.1 ±1.3 Ma with an MSWD of 1.15, the middle group has a weighted mean age of 383.0 ± 2.3 Ma and an MSWD of 2.1, and the oldest group has a weighted mean age of 396.8 ± 2.5 Ma with an MSWD of 0.118.

46 Zircon grains in sample DHq-1 vary from acicular and subhedral, with a few euhedral grains, to subhedral rectangular shapes. Sizes are varied and range from 20 to 150 µm. Some grains are clear but most have visible inclusions and reddish-brown surface staining. The grains in this sample show very low fluorescence under cathodoluminescence and only some of the larger grains show indistinct oscillatory zoning.

47 The cumulative probability distribution of the 98–102% concordant grains for this sample (42 out of 62 grains analysed) shows three distinct peaks (Fig. 11d). Although no clear separations are apparent between them in the probability distribution, it is possible to resolve three separate concordia ages for different groups of grains (Fig. 11e). These groups of grains do not overlap, and neither the concordia ages nor the weighted mean ages overlap within error. The youngest group (18 grains) has a concordia age of 366.2 ± 1.2 Ma with an MSWD of 4.6 and a probability of concordance of 0.031. Since the statistics on this age are not good we prefer to use the weighted mean age for the same group of grains at 365.9±1.5 with an MSWD of 1.4 (Fig. 11f). The middle group has a concordia age of 377.3 ± 1.3 Ma with an MSWD of 1.3 and a probability of concordance of 0.25 (weighted mean age of 376.9 ± 1.4 Ma with an MSWD of 0.96). The oldest group has a concordia age of 388.0 ± 2.1 Ma with an MSWD of 0.54 and a probability of concordance of 0.46 (weighted mean age of 387.8 ± 2.2 Ma with an MSWD of 0.86).