Plutonic igneous rocks lie buried beneath Carboniferous sedimentary rocks in the Lower Coverdale area south of Moncton, New Brunswick (Fig. 1). Although no surface out-crops have been found, the presence of these rocks was known as early as 1919, when gabbroic and anorthositic rocks were intersected at a depth of about 150 m in a hole drilled for oil and gas exploration (#52; Fig. 2). Assays revealed TiO₂ content up to 40% in ilmenite-bearing sections (New Brunswick Gas and Oilfields Limited 1919). Another hole drilled nearby in 1931 (#92; Fig. 2) yielded similar rocks (New Brunswick Gas and Oilfields Limited 1931). The large magnetic anomaly in the area (Nickerson 1994; Ascough 1997) suggests that the subsurface intrusion has an area of at least 30–40 km² (Fig. 2), and the complex shape of the anomaly indicates that the intrusion is composite and/or not uniformly magnetic. Four holes (T1 through T4; Fig. 2) were drilled by Killarney Oil and Gas in 1970 on the most southerly magnetic high, and encountered rocks described as gabbro, anorthosite, and anorthositic gabbro continuing to a depth of about 330 m in the deepest hole, T1. Subsequent holes drilled by Noranda Exploration Company Ltd. (LC94–1, LC95-1, LC95-2, LC95-3, and LC01-04; Fig. 2) yielded similar rocks (Woods 1995; Ascough 1997; Moreton 2001).

Despite its unusual characteristics, petrological studies of the Lower Coverdale intrusion have been few. Boyle and Stirling (1994) reported preliminary petrological descriptions in an abstract, but their data set was never published. Some of those data were provided to the present authors by J. Stirling (personal communication 2000) and are used in the present study. White (1996) described the petrography of a few samples from the 1919, 1931, and 1970 drill holes for comparison with other plutons of the Brookville terrane. McHattie (1998) compiled assay data and core logs from Noranda Exploration Company Ltd. assessment reports, and also reported about 65 additional major and trace element analyses, as well as petrographic descriptions, X-ray diffraction analyses, and Sr isotope data from apatite from holes LC94-1, LC95-1, LC95-2, and LC95-3 (Fig. 2). Barr et al. (2001, 2002) reported a preliminary U-Pb zircon age of 390 Ma from quartz monzonite, which forms the bottom 111 m of hole LC94-1, and a similar U-Pb age from quartz monzonite exposed in the quarries near Gaytons, about 20 km to the east of Lower Coverdale (Fig. 1). Although less prominent, the regional aeromagnetic anomaly associated with the Lower Coverdale area extends through the Gaytons area and east to Port Elgin (Nickerson 1994), suggesting an east-west zone of linked subsurface intrusions (Barr et al. 2001, 2002).

The purpose of this paper is to present a compilation of previous work on the Lower Coverdale intrusion, as well as additional petrographic and chemical data based on our examination and sampling of drill core and cuttings. In addition, we present U-Pb geochronological data for quartz monzonite from the Lower Coverdale drill core and the Gaytons quarry, which were cited in only preliminary form by Barr et al. (2001, 2002), and discuss possibilities for the age of the anorthositic and gabbroic rocks.

The Lower Coverdale intrusion and Gaytons quarries are located in the Brookville terrane, one of several narrow northeast-trending belts of pre-Carboniferous rocks of contrasting composition and age, separated by major faults, which make up southern New Brunswick (Fig. 1). The Brookville terrane is linked to Ganderia, a peri-Gondwanan terrane of the northern Appalachian orogen that is distinct from Avalonia (Hibbard et al. 2006). The Proterozoic to early Cambrian metamorphic and plutonic rocks that characterize the Brookville terrane are well exposed in the Saint John area, and can be traced on the basis of sparse outcrops and drill core data to the northeast through to the Moncton area (White 1996; White and Barr 1996; White et al. 2002). Boyle and Stirling (1994) inferred that the Lower Coverdale suite intruded metavolcanic and metasedimentary rocks of the Caledonian Highlands, but metavolcanic and metasedimentary rocks do not occur in the drill core logs, and the Lower Coverdale area is north of the projected boundary between the Avalonian Caledonia and Ganderian Brookville terranes (Fig. 1).

The Lower Coverdale rocks were described as granitic in the 1919 core logs and as chloritic schist in the 1931 logs by New Brunswick Gas and Oilfields Limited. However, petrographic examination of cuttings from these holes by White (1996) and during the present study confirmed that they are gabbro and anorthosite. In the 1970 core logs by Killarney Oil and Gas Limited, the rocks were described more accurately as gabbro, anorthositic gabbro, and anorthosite, with dykes of "acidic", pegmatitic, and "dolerite" composition. Similar rock types were reported in the Noranda Exploration Company Ltd. core logs (Woods 1995; Ascough 1997), with the additional recognition of ferrogabbro, massive ilmenite layers, and in the bottom of LC94-1, at least 111 m of rock described as granodiorite. Based on examination of available core, Boyle and Stirling (1994) recognized four rock types: Ti-P ferrogabbro, nelsonite (apatiteilmenite rock), anorthosite, and fine-grained diabase dykes. On the basis of his petrographic studies, McHattie (1998) better defined these rock types. He reported that the body is mainly a massif-type anorthosite, with locally antiperthitic plagioclase of labradorite composition. The ferrogabbro contains abundant titanium oxide minerals and apatite, structurally overlies and intrudes the anorthosite, and is associated with high magnetic areas on the aeromagnetic maps. Non-magnetic low-Ti gabbro is also present, especially in drill hole LC95-1. McHattie (1998) also noted that the suite is intruded by fine-grained diabase dykes that, based on chemical similarity, may be linked to the low-Ti gabbro. Much less abundant felsic dykes were interpreted to be related to the granodiorite intersected in the bottom of drill hole LC94-1 (McHattie 1998).

In drill holes, a saprolitic weathering profile separates the gabbro-anorthosite from overlying Carboniferous conglomerate of the Hopewell Cape Formation (basal Mabou Group) and Boss Point (Cumberland Group) formations (Woods 1995; C. St. Peter, written comm. 2006; Johnson 2006). A titanium paleoplacer containing locally transported, massive ilmenite pebbles has been reported to occur above the saprolite in LC94-1 (Woods 1995; Johnson 2006). A similar paleoplacer occurs near Taylor Village, 20 km to the southeast of Lower Coverdale (Fig. 1), in the Hopewell Cape Formation, and is assumed to have been derived from the Lower Coverdale intrusion (Hudgins 1999; Johnson 2006). The conglomerate at Taylor Village contains detrital clasts of ilmenite, 0.2 to 24 mm in size, variously altered to pseudo-rutile, rutile, and hematite (Bell 1984).

Geophysical models using aeromagnetic data as well as sparse gravity and seismic data suggested that the Lower Coverdale intrusion is lopolithic in shape with variable thicknesses between 800 and 1100 m (Burke 2000). Venugopal (2003) compared features of the Lower Coverdale suite to the Stewarton Complex, a gabbro-anorthosite body of inferred Early Devonian age located about 100 km southwest of Lower Coverdale (Fig. 1).

Chemical data with petrographic control on rock type were presented by McHattie (1998). Those data are combined with new data obtained during the present study (Table 1) to provide a total of 89 major and trace element analyses with petrographic control. These data are displayed on a variety of diagrams to illustrate chemical variation in the suite (Figs. 5, 6, 7). In addition, these data are displayed on selected diagrams together with the full set of assay data from other sources (Fig. 8). The latter data are mainly from sections of core without regard to rock type, and hence may not represent end member rock type compositions in every case, but they provide an indication of the overall chemical variation in the suite.

Three chemically distinct groups of samples are apparent in the data, representing anorthosite, ferronorite, and gabbro plus mafic dykes (Figs. 5, 6). The majority of anorthosite samples have SiO₂ contents between 50 and 60%, whereas the ferronorite samples have lower SiO₂ (30–45%) and the gabbro and mafic dyke samples are intermediate with 40–50% SiO₂. The anorthosite samples have high Al₂O₃ (more than 18%), with higher values in samples with higher SiO₂ (Fig. 5b), and consistently low P₂O₅ (Fig. 5h). Some more gabbroic (pyroxene-bearing) anorthosite samples have higher MgO (Fig. 5d), and other samples have elevated concentrations of opaque phases leading to elevated TiO₂, Fe₂O₃ ^t, and V (Figs. 5a, c, 6h). The anorthosite samples show spread in CaO, Na₂O, and K₂O, and correspondingly in Ba, Rb, and Sr (Fig. 6a-c), probably related to variation in both plagioclase abundance and composition, as well as to the effects of alteration. Low P₂O₅ is consistent with lack of apatite observed in anorthosite thin sections. The elements Y, Nb, and Zr are all low, but Cr and Ni show a spread of higher values in some anorthosite samples, reflecting variations in pyroxene and/or opaque mineral abundance.

The ferronorite samples display negative correlation of TiO₂, Fe₂O₃, CaO, and P₂O₅ with SiO₂, and positive correlation of Al₂O₃ and Na₂O with SiO₂, reflecting variations in the proportions of ilmenite and apatite relative to silicate minerals. MgO is more scattered, and linked to the proportion of orthopyroxene. K₂O is less than 1%, and likely resides mainly in antiperthitic plagioclase. Among the trace elements (Fig. 6), Ba shows wide variation, apparently reflecting its presence in K-feldspar exsolution lamellae in plagioclase. Ni and Cr contents are both low. The chemical gap between anorthosite and ferronorite samples evident on all of the diagrams in Figures 5 and 6 is consistent with the petrographic observation that lithologies gradational between these two interlayered rock types are not common. Although some analytical discrepancies clearly exist in the assay datasets, a gradation from ferronorite through to nelsonite with TiO₂ up to nearly 35%, P₂O₅ up to 15%, and V up to 1500 ppm is apparent (Fig. 8a, c, d). The trend to high V apparent mainly in low-Ti gabbroic samples in the data set with petrographic control (Fig. 6h) appears to be present also in some ferronorite samples, although the assay data set for V is smaller. Analyses of the opaque phases are required in order to understand the relationship between ilmenite and magnetite in these rocks, and the distribution of Ti and V in those minerals.

Gabbro and mafic dyke samples are chemically similar. They have low TiO₂ and P₂O₅ (relative to the ferronorite), and wide variation in Ni, V, and Cr suggesting pyroxene and magnetite fractionation (Fig. 6g, h, i). Although the gabbroic and mafic dyke samples show compositions that overlap with both the anorthosite and ferronorite samples, suggesting chemical trends, they show some significant differences which preclude that relationship (Figs. 5, 6). For example, the gabbroic and mafic dyke samples have higher CaO and lower Na₂O and Al₂O₃ than anorthosite samples with similar SiO₂ contents, consistent with the more calcic plagioclase compositions in the gabbroic rocks. Compared to the ferronorite samples, the gabbro and mafic dyke samples have distinctly lower TiO₂ and P₂O₅, and higher MgO (Fig. 5) and trends in Ni, V, and Cr are very different (Figs. 6g, h, i, 8e). The textures in the gabbro and mafic dyke samples indicate that they are not cumulate rocks, and their compositions are appropriate for plotting on commonly used tectonic setting discrimination diagrams (e.g., Figs. 7a, b). Although the data are scattered, variations in V and Ti suggest that they formed in an ocean-floor to volcanic-arc tectonic setting (Fig. 7a), as do relatively high contents of Zr and Y relative to Nb (Fig. 7b).

The quartz monzonite in the lower part of drill hole LC94-1 is chemically similar to quartz monzonite from the Gaytons quarries (Figs. 5, 6). The quartz monzonite samples contain 61–65% SiO₂, and compared to the anorthosite samples, have low Al₂O₃, CaO, and Na₂O₃, and generally higher TiO₂, Fe₂O_3t, MgO, K₂O, and P₂O₅ (Fig. 5). A notable feature of samples from both the Gaytons quarries and core LC94-1 is high Ba content, a trend also noted in the anorthosite samples (Fig. 6a). The quartz monzonite is high in Y, Zr, and Nb compared to the anorthosite (Fig. 6d, e, f). It shows affinity with A-type granite (e.g., Fig. 7c), and plots at the boundary between volcanic arc and within-plate granite fields on the tectonic setting discrimination diagram (Fig. 7c). Their petrological and age similarities leave no doubt that the quartz monzonite in drill hole LC94-1 and exposed in the Gaytons quarries are correlative. The high Ba content and abundance of modal titanite and apatite also suggest the possibility of some relationship to the Lower Coverdale anorthosite and ferronorite (see further discussion below).

Three felsic dyke samples contain higher SiO₂ than the quartz monzonite (~ 72% compared to 61–65%), but share some chemical similarities such as high Ba and relatively high Y, Zr, and Nb (Fig. 6a, d, e, f). They also appear to have affinities with A-type granite (Fig. 7c), but appear to have formed in a volcanic-arc tectonic setting (Fig. 7d). As described above, their petrographic features are unlike those of the quartz monzonite, but they are similarly late in the intrusive sequence, and hence a genetic linkage cannot be ruled out.

Quartz monzonite samples for U-Pb (zircon) dating were collected from drill hole LC94-1 (by D.R. Boyle) and the eastern part of the Gaytons quarries (by S. Barr and C. White). Three fractions of water-clear colourless to very pale pinkish-brown and yellow-brown elongate zircon prisms (length:width = 2: 1 to 4:1) were analyzed from drill core sample LC94-1. Most grains contained small numbers of rounded, irregular fluid inclusions. All of the fractions cluster on or near Concordia, and yield a weighted mean ²⁰⁶Pb/²³⁸U age of 390.6 ± 1.0 Ma (Fig. 9a), interpreted to be the age of crystallization of the Lower Coverdale quartz monzonite. A sample from the eastern part of the Gaytons quarries yielded similar-looking zircon grains (also rich in fluid inclusions) as well as a population of stubby, flat, sharply terminated prisms. Three fractions were analyzed and show nearly identical Pb/U ages, with error ellipses overlapping each other and Concordia (Fig. 9b). A weighted mean ²⁰⁶Pb/²³⁸U age of two fractions is 390.0 ± 0.5 Ma because of the possibility of a small amount of Pb-loss in fraction c. These ages are interpreted to represent the time of crystallization of the Lower Coverdale and Gaytons quartz monzonite units.

The close physical association between anorthosite, ferronorite, and nelsonite in the Lower Coverdale drill core strongly suggests that these rocks are of similar age. However, the cross-cutting relationship of the gabbro bodies and dykes, with their well-defined chilled margins, suggests that these gabbroic rocks may be significantly younger, emplaced after the other rocks had cooled. Furthermore, the occurrence of mafic dykes of at least two ages is suggested by the presence of both foliated amphibolite dykes and unfoliated dykes showing lower grade metamorphic effects. Younger still are the petrologically distinct felsic dykes and quartz monzonite in drill hole LC94-1. The quartz monzonite appears to lack cross-cutting gabbroic dykes and retains primary igneous textures. Hence, whether all or part of the Lower Coverdale anorthosite-ferronorite-gabbro suite is middle Devonian in age like the quartz monzonite is not clear because they are seen in contact only in drill core LC94-1 and that contact is sheared. Evidence for a co-genetic relationship includes their close spatial association and some petrological features of the quartz monzonite such as abundant titanite and apatite and high Ba (Fig. 6a). However, other petrochemical features are distinctly different and show few overlapping trends (e.g., Fig. 5, 6) to suggest that the quartz monzonite is a more evolved part of the same suite. The epsilon Nd value for the anorthosite is much lower than that of the quartz monzonite (−7.20 vs. +0.30 at 390 Ma; Samson et al. 2000), which is also evidence against a petrogenetic link. Although the lack of petrological similarity between the quartz monzonite and the anorthosite-ferronorite-gabbro suite does not preclude them from being of similar age, it makes it less likely.

Venugopal (2003) compared the Lower Coverdale suite to the undated but post-Middle Silurian Stewarton Gabbro, located 100 km to the west at the boundary between the more inboard Mascarene and St. Croix terranes (Fig. 1). He suggested that they have enough similarities to indicate that they are coeval, although in the absence of a definite age for either intrusion, this interpretation is speculative. Gabbroic intrusions also occur in the Caledonia terrane in the Mechanic Settlement and Caledonia Mountain areas (Fig. 1), but those plutons do not appear to have any petrological features in common with the Lower Coverdale suite (e.g., Grammatikopoulos et al. 2007; Barr et al. 2000).

Other possibilities for the age of the Lower Coverdale suite include Grenvillian, as suggested by Boyle and Stirling (1994), and Late Neoproterozoic-Cambrian, like other plutons in the Brookville terrane. Considering first the Grenvillian option, many characteristics of the Lower Coverdale anorthosite and ferronorite, in particular, are typical of Grenvillian massif-type anorthosite-ferrodiorite suites (e.g., Herz and Force 1987; Force 1991). They include the presence of antiperthitic plagioclase of andesine composition, orthopyroxene in the compositional range of En₇₀, and the occurrence of nelsonite layers. A discussion of the origin of such unusual rocks is well beyond the scope of this paper, but a variety of origins including immiscible liquid separation, crystal accumulation, subsolidus recrystallization, and late-stage differentiation have been proposed (e.g., Duchesne 1999; Kärkkäinen and Appelqvist 1999 and references therein). Textural features in the Lower Coverdale ferronorite are similar to those in equivalent units in the Allard Lake (Lac Tio) district in Quebec, as described and illustrated by Force (1991, p. 27), and in the Roseland District of central Virginia (Herz and Force 1987). Another similarity is the evidence that the Lower Coverdale suite was emplaced before or during regional metamorphism (Force 1991). Massif-type anorthosite suites are Mesoproterozoic in age (> 900 Ma) and typically located in Grenvillian crust (Force 1991; Ashwal 1993). Hence, if it is of Grenvillian age, the Lower Coverdale suite has significant implications for the nature of the crust in southern New Brunswick.

Turning to the other option, the Brookville terrane is dominated by Late Neoproterozoic to Cambrian (ca. 555–525 Ma) plutonic rocks (White et al. 2002), and typical Brookville terrane dioritic rocks of Early Cambrian age are exposed near Moncton at Lutes Mountain (Fig. 1). Although most Brookville terrane plutons are dioritic to granitic in composition, anorthosite and gabbro occur in Duck Lake, French Village, Indiantown, and other small plutons in the city of Saint John (White 1996; White et al. 2002). However, the anorthosite in these plutons lacks massif-type features like those of the Lower Coverdale intrusion, and no ferrodiorite, ferronorite, or nelsonite has been reported. On the other hand, the possible volcanic-arc tectonic setting suggested by the gabbroic rocks in the Lower Coverdale core (Fig. 7a, b) is consistent with the arc setting interpreted for the Brookville terrane plutons (White et al. 2002). In addition, the mica schist and calc-silicate rocks interlayered with anorthosite and ferronorite are similar in lithology and metamorphic grade to units in the Brookville terrane (e.g., Green Head Group; White 1996). Also, the epsilon Nd value of −5.11 for the Lower Coverdale anorthosite (calculated at 540 Ma) is compatible with values as low as −4.25 reported for ca. 540 Ma plutons in the Brookville terrane in the Saint John area (Samson et al. 2000).

In the absence of direct dating of the anorthosite-ferronorite-gabbro suite, the uncertainty about its age cannot be resolved unequivocally. However, it is important to note that, although most Fe-Ti-P-rich rocks are associated with Proterozoic massif-type anorthosite, not all have that association. A better analogy for the Lower Coverdale intrusion may be the Qareaghaj mafic-ultramafic intrusion in Iran, which is similar in size and petrological features to the Lower Coverdale intrusion, similarly includes Fe-Ti-P rocks, and appears to be of Late Precambrian or Paleozoic age (Mirmohammadi et al. 2007). Clearly more detailed study of the Lower Coverdale anorthosite-ferronorite and associated ilmenite-apatite rocks is needed to more fully understand their origin.

The similarity in petrological features and identical-within-error 390 Ma U-Pb ages demonstrate that quartz monzonite in the Lower Coverdale drill core LC94-1 is correlative with the quartz monzonite exposed in the Gaytons quarries. Chips of similar rock also form the "basement" encountered at a depth of about 314 m in the nearby Westmorland well and 910 m in the Port Elgin well, more than 35 km to the east (Fig. 1). This belt of rocks is associated with a series of magnetic anomalies (Nickerson 1994), and quartz monzonite samples from both drill core and the Gaytons quarries yielded relatively high susceptibility, averaging 17.61 × 10⁻³ SI. Hence the magnetic anomalies do not require that magnetic ferronorite-nelsonite bodies be associated with those granitic rocks, although they could be. The A-type chemical characteristics of the quartz monzonite is consistent with emplacement in a late-orogenic or post-orogenic tectonic setting, perhaps associated with trans-tension along the faulted boundary between the Brookville terrane and Avalonia.

The Lower Coverdale intrusion consists of anorthosite and ferronorite with massif-type characteristics, younger gabbroic rocks, and younger quartz monzonite and felsic dykes. The quartz monzonite is correlative with quartz monzonite in the nearby Gaytons quarries, and both have identical U-Pb (zircon) ages of about 390 Ma. This age provides a minimum for the age of the gabbroic rocks and anorthosite-ferronorite, but in the absence of direct dating, it is uncertain whether they are similar in age to the quartz monzonite, similar in age to other plutons of the Brookville terrane (555–525 Ma), Grenvillian (as suggested by the massif-type petrological features of the anorthosite- ferronorite suite), or of some other age. Hence, the regional tectonic significance of this unusual assemblage of rocks remains unresolved.