Complexly deformed, Proterozoic to Early Cambrian rocks, exposed on the eastern coast of Grand Manan Island (Alcock 1948; Fyffe and Grant 2001, 2003), are faulted against Late Triassic to Early Jurassic basaltic flows and redbeds of the Fundy Group, which underlie the western part of the island (Wade and Jansa 1994). Contacts between the various Proterozoic to Early Cambrian formations recognized on Grand Manan Island are generally covered or faulted making determination of the original stratigraphic order of some of these units difficult. To help clarify stratigraphic relationships, a series of samples were collected by the New Brunswick Geological Survey, Acadia University, and the Geological Survey of Canada under a joint geochronological project carried out between 2000 and 2003. The U-Pb ages on magmatic and detrital zircons obtained under this project, including that of the Stanley Brook Granite, have all now been published (Barr et al. 2003; Black et al. 2004; Miller et al. 2007; Fyffe et al. 2009). However, the analytical data used to calculate the age of the Stanley Brook Granite were not included in these publications. This paper presents the supporting radiometric data for the age of the Stanley Brook Granite and discusses the implications of the age determination in regard to the further understanding of the pre-Mesozoic geology of Grand Manan Island and the adjacent New Brunswick mainland.

Proterozoic basement rocks underlying the southern coastal mainland of New Brunswick have been divided into three distinctive terranes (Fig. 1) based on differences in both stratigraphic and plutonic histories (see Fyffe et al. 2009, and references therein): (1) the Caledonia terrane containing Neoproterozoic continental volcanic-arc rocks of the Broad River Group and comagmatic plutons (625 ± 5 to 616 ± 3 Ma) and younger Coldbrook Group and comagmatic plutons (559 ± 5 to 550 ± 1 Ma); (2) the Brookville terrane containing Mesoproterozoic (?) to Neoproterozoic platformal carbonate and metasiltstone of the Green Head Group; Neoproterozoic volcanic rocks of the Dipper Harbour Group (553 ± 3 Ma), and Neoproterozoic to Early Cambrian plutonic rocks (548 ± 2 to 528 ± 3 Ma); and (3) the New River terrane containing Neoproterozoic and Early Cambrian volcanic rocks of the Belleisle Bay Group (554 ± 3 Ma and 539 ± 5 Ma), and two ages of Neoproterozoic plutons (629 ± 1 to 622 ± 2 Ma, and 555 ± 2 Ma). The Caledonia terrane and Brookville/New River terranes are considered to be part of the peri-Gondwanan microcontinents of Avalonia and Ganderia, respectively (van Staal et al. 1996, 1998; Hibbard et al. 2006).

The recently published U-Pb magmatic zircon age determinations from Grand Manan Island (Miller et al. 2007) have confirmed a Neoproterozoic age for much of the bedrock as previously suggested by Alcock (1948). These dates together with the presence of platformal carbonate and quartz-rich sedimentary rocks suggest possible tectonostratigraphic linkages of Grand Manan Island to the Ganderian Brookville and New River terranes on the New Brunswick mainland (Miller et al. 2007; Fyffe et al. 2009) (Fig. 1).

Pre-Mesozoic stratified units on Grand Manan and nearby islands consist mainly of quartzose and carbonate sedimentary rocks, and volcanic and reworked volcaniclastic sedimentary rocks. Metamorphic grade generally does not exceed green-schist facies, and in many places, primary sedimentary and igneous textures are well preserved. Formational terminology follows that of Fyffe and Grant (2003) with modifications by Black et al. (2004). Based on exposed and inferred strati-graphic relationships, depositional environments, and U-Pb ages, the formations recognized on Grand Manan and nearby islands are divided into the Mesoproterozoic to Neoproterozoic Grand Manan Group (probable minimum age of 618 ± 3 Ma), and the younger Neoproterozoic to Early Cambrian Castalia Group (probable minimum age of 539 ± 3 Ma). The Grand Manan Group comprises the Kent Island, The Throughfare, Ingalls Head, and Long Island Bay formations; the Castalia Group comprises the Great Duck Island, Flagg Cove, Ross Island, North Head, Priest Cove, and Long Pond Bay formations (Fig. 2).

A sample of pink, medium-grained granite (Sample VL-2001-11) from the northern part of the exposure of Stanley Brook Granite in Flagg Cove was collected for U-Pb dating at the Geological Survey of Canada facilities in Ottawa. Zircons were analysed using isotope dilution thermal ionization mass spectrometry (ID-TIMS). The rock sample was crushed and heavy mineral separates were obtained using a Wifley table and heavy liquid separation. Different zircon fractions were hand-picked under a binocular microscope aſter separation with a Frantz electromagnetic separator. To minimize the effects of secondary Pb-loss, the air abrasion technique of Krogh (1982) was used for most of the fractions analysed. The selected minerals were washed with 2N HNO3 before weighing and spiking with a 205Pb - 233U - 235U calibrated solution (Parrish and Krogh 1987). Zircon was dissolved with HF and HNO3 in Parrish-type microcapsules in an oven at ca. 240° C (Parrish 1987). U and Pb separation and purification for zircon and monazite followed a scale-down version of the procedure of Krogh (1973). Total procedural blanks are below 1pg U and less than 5 pg Pb.

Aſter separation, the U and Pb were loaded for analysis in separate Rhenium filaments. Measurement of the isotopic ratios was done with a Finnigan MAT 261 mass spectrometer equipped with an analog secondary electron multiplier (Roddick et al. 1987). The reported isotopic ratios are corrected for mass fractionation, blank and initial common Pb estimated using the model of Stacey and Kramers (1975). Ages and uncertainties are calculated with error propagation of sources of uncertainty using an internal program from the Geological Survey of Canada (Roddick et al. 1987). The isotopic ages were calculated using the decay constants of Jaffey et al. (1971). U-Pb concordia diagrams were created with ISOPLOT 2.0 (Ludwig 1999). Mixing and discordia lines were calculated with the same program. The uncertainty of the error ellipses is represented at the 2σ level.

The sample of the Stanley Brook Granite yielded a large amount of good quality zircon oſten containing acicular fluid inclusions. Zircon analyses range from single grain to multi-grain fractions (Table 1). All fractions were air abraded to eliminate secondary Pb-loss, except for fraction Z8. Fraction Z8 is composed of small, elongate zircon prisms; this fraction was picked to provide constrains on the discordia line. A total of eight analyses were performed. Fraction Z4 has not been plotted and has been excluded in the age calculation due to the large analytical error (Table 1). This analytical error results from low signal during analysis on the mass spectrometer. The remaining seven fractions define a collinear discordance pattern (Fig. 4). This discordia line has a upper intercept of 535.1 ± 2.4 Ma, and a good probability of fit of 99.6% (MSWD = 0.076) and a lower intercept near the origin. The mean 207Pb/206 Pb age for the seven fractions is 537 ± 2 Ma, which is within error with the upper intercept age (Fig. 4, upper leſt). This is consistent with a discordia line induced by recent Pb-loss, i.e. a discordia passing through the origin of the concordia curve. Therefore, the upper intercept age of 535.1 ± 2.4 Ma is interpreted as the best estimate of the zircon crystallization age of the intrusion.

The Stanley Brook Granite is known to have been emplaced into the sedimentary rocks of the Flagg Cove Formation of the Castalia Group based on the presence of xenoliths in the granite. The minimum age of the Flagg Cove Formation is, therefore, Early Cambrian as constrained by the 535 ± 2 Ma emplacement age of Stanley Brook Granite. The Flagg Cove Formation is known on stratigraphic grounds to be no older than the unconformably underlying Long Island Bay and correlative Ingalls Head Formation (dated at 618 ± 3 Ma) of the Grand Manan Group. The maximum stratigraphic age of the Flagg Cove Formation is further constrained to be no older than 574 ± 7 Ma on the basis of its youngest contained detrital zircon population (Fyffe et al. 2009).

The Early Cambrian age of the Stanley Brook Granite overlaps within error the 539 ± 3 Ma age of the dacitic tuff from the Priest Cove Formation of the Castalia Group and suggests a possible comagmatic relationship. Although the contact between the Flagg Cove and Priest Cove formations is invariably sheared (Fig. 2), the consanguineous relationship of the Stanley Brook Granite and the felsic tuffs the Priest Cove Formation suggests that the latter was deposited on sedimentary rocks of the Flagg Cove Formation. This supports the proposal by Stringer and Pajari (1981) that the tuffaceous and volcaniclastic sedimentary rocks of the Priest Cove Formation are a distal facies of the mafic flows and fragmental volcanic rocks of the Ross Island Formation, which are also known to be younger than the Grand Manan Group on the basis of contained granitic clasts.

The age of 535 ± 2 Ma of the Stanley Brook Granite and the 547 ±1 Ma of the High Duck Island dyke (Black et al. 2004) on Grand Manan Island, and the 542 ± 1 Ma age of the Machias Seal Island Monzodiorite to the southwest of Grand Manan Island (Barr et al. 2010), all overlap with the 548 ± 2 to 528 ± 3 Ma age range of the Neoproterozoic to Early Cambrian plu-tonic rocks of the Brookville terrane on the New Brunswick mainland (White et al. 2002). In addition, carbonate rocks of the Kent Island Formation of the Grand Manan Group closely resemble those of the Green Head Group that characterizes the Brookville terrane, although the latter does not contain any volcanic rocks equivalent in age to Ingalls Head Formation (618 ± 3 Ma) of the Grand Manan Group. However, northward thrusting and upliſt on the Kennebecasis Fault on the eastern side of the Oak Bay Fault (Fig. 1) may explain the absence of older Neoproterozoic volcanic rocks in the Brookville terrane on the mainland (Park et al. 1994). Neoproterozoic granitic plutons (629 ± 1 to 622 ± 2 Ma), which overlap within limits of error with the volcanic rocks of the Ingalls Head Formation, occur in the New River terrane on the mainland (Johnson and Barr 2004; Bartsch and Barr 2005).

The Three Islands Granite, which intrudes the Kent Island Formation and has been dated at 611 ± 2 Ma, has no equivalent in the Brookville terrane on the mainland, although an orthogneiss with a an igneous crystallization age of 605 ± 3 Ma is preserved there (Bevier et al. 1990). The Early Cambrian Priest Cove Formation (539 ± 3 Ma) of the Castalia Group is identical in age to volcanic rocks of the Simpsons Island Formation (539 ± 5 Ma) of the Belleisle Bay Group in the New River terrane on the New Brunswick mainland. However, no older volcanic rocks of ca. 550 Ma ages, common elsewhere in the New River terrane, have been identified on Grand Manan Island.

The above overlapping age determinations between Grand Manan Island and the New Brunswick mainland are consistent with the interpretation that the carbonate and clastic rocks of the Green Head Group in the Brookville terrane represent basement to the New River terrane (van Staal et al. 1996; 1998; Barr et al. 2003; Miller et al. 2007; Fyffe et al. 2009; Barr et al. 2010). Moreover, the actual basement-cover relationship may be marked by the unconformity between volcanic rocks of the Grand Manan Group and sedimentary rocks of the Castalia Group exposed on Great Duck Island. Therefore, the separation of the Brookville and New River terranes into distinctive co-linear fault blocks like that on the New Brunswick mainland, may not be applicable on Grand Manan Island.