INTRODUCTION

1 The Jurassic North Mountain Basalt (NMB) outcrops along the southern and northern shores of the Bay of Fundy, in part also the Minas Basin, of southern Nova Scotia (Fig. 1). The NMB sequence forms part of the Late Jurassic-Early Triassic infill of the Fundy Rift, one of several exposed remnants of syn-rift basins that extend for over 2000 km along eastern North America and is part of the Newark Supergroup (Olsen et al. 1982; Froelich and Olsen 1985; Mansperzer and Cousminer 1988). Basins of equivalent age and similar geological setting are found along the margins of the Central Atlantic in northwest Africa and western Europe and represent correlatives (Olsen 1997; Marzoli et al. 2003; Rapaille et al. 2003). Within the Newark Supergroup, the basalt flows in the various sub-basins have been paleontologically assigned to the earliest Jurassic (Hettangian) and fall just above the Triassic-Jurassic boundary (Olsen et al. 1982). Within the Fundy Basin, this assignment is consistent with a U-Pb zircon age of 202 ± 1 Ma for a flow from the lower part of the basalt sequence (Hodych and Dunning 1992). However, the U-Pb age contrasts with previous K-Ar whole-rock dating of the NMB that indicated ages of ca. 180 to 200 Ma (Carmichael and Palmer 1968; ages recalculated by Hayatsu 1979), with a single anomalously old age of 333 Ma for one sample. Subsequently, Hayatsu (1979), using the K-Ar isochron method, obtained an age of 191 ± 2 Ma using the same sample set as Carmichael and Palmer (1968). Most recently, Cox et al. (2001) obtained an apatite fission track age of 190 ± 2 Ma for basalt from the lower part of the basalt sequence and from the same area as the sample for the U-Pb zircon age determination. Interestingly, the apatite fission track age is similar to the K-Ar age of Hayatsu (1979). These apparently young K-Ar ages contrast with ⁴⁰Ar/³⁹Ar ages of ca. 200 Ma for dyke rocks and basalt flows within parts of the Newark Supergroup of eastern North America (summary in McHone 2003) and similar ages for basalts and dyke rocks of the Central Atlantic Magmatic Province (CAMP; summaries in Sebai et al. 1991 and Marzoli et al. 1999).

2 In order to address this apparent discrepancy in ages for the NMB, samples of fresh and zeolite-bearing basalt were selected for whole-rock ⁴⁰Ar/³⁹Ar dating. Sample selection was focussed on firstly addressing whether reliable extrusion ages could be obtained on the NMB and, therefore, correlative units in the Newark Supergroup using the ⁴⁰Ar/³⁹Ar method, and secondly on the possible relationship between the younger apparent ages and timing of zeolite formation.

Display large image of Figure 1

GEOLOGY OF THE NORTH MOUNTAIN BASALT

3 The North Mountain Basalt (NMB) occurs as a series of three flow units of ca. 400 m aggregate thickness that outcrop along the shores of the Bay of Fundy and some offshore islands (i.e., Isle Haute, Grand Manan Island). Along the southern shore of the Bay of Fundy, the NMB overlies red siltstone and shale of the Late Triassic Blomidon Formation and is subsequently overlain by terrestrial red siltstone, shale, and minor carbonate rocks of the Scots Bay Formation (De Wet and Hubert 1989). Somewhat similar geological relationships are observed on the north side of the Bay of Fundy, but in places the NMB rests on basement rocks of Carboniferous age and is overlain by sandstone and siltstone of the Jurassic McCoy Brook Formation (Greenough et al. 1989; Olsen et al. 1987). The stratigraphy of the NMB has recently been reviewed by Kontak (2002) based on recent field studies which follow on the earlier work of Lollis (1959), Papezik et al. (1988), and Greenough et al. (1989), and a summary of this stratigraphy follows.

4 The NMB is subdivided into three distinct flow units, referred to as lower, middle, and upper (LFU, MFU, and UFU, respectively). The LFU, ≤180 m thick, is dominated by massive, mostly holocrystalline, medium-grained basalt with well-developed columnar jointing. The upper 1– 2 m is vesiculated and generally has zeolite minerals filling the vugs. The MFU, ≤150 m thick, consists of numerous thin flows (≤10– 15) of fine-grained basalt with abundant amygdules arranged in a consistent zonal pattern (Aubele et al. 1988; Kontak 2000). Rare interflow sediments and sedimentary dykes of aeolian nature occur between and within these flows. The UFU, of minimum 100 m thickness, is again dominated by massive, columnar-jointed, medium-grained basalt containing <20– 30 % mesostasis, and with minor amygdaloidal zones containing zeolite minerals. Detailed work in parts of the Newark Supergroup indicates eruption of the full sequence in less than 580 ka based on detailed paleontological, paleomagnetic, and K-Ar dating (Olsen and Fedosh 1988; Olsen 1997).

SAMPLING AND ANALYTICAL PROCEDURES

5 Two samples of the NMB were selected for argon dating with selection specifically aimed at obtaining ages of eruption and, possibly, the thermal event related to zeolite formation. The first sample, ZL-99-26, is from the LFU at Parker Cove quarry near Annapolis Royal (Fig. 1). This sample is medium grained, holocrystalline, very fresh (i.e., complete absence of any deuteric alteration such as chlorite, white mica, carbonate, etc.), and most importantly is barren of megacrysts and xenoliths (Fig. 2a). The fact that this sample is both holocrystalline and free of megacrysts is highlighted, as McDougall and Harrison (1988) emphasized these features for selecting samples of volcanic rocks for whole-rock ⁴⁰Ar/³⁹Ar dating; failure to follow these suggestions may result in excess argon being detected in the analysis. The second sample, ZL-99-36, is from the upper, zeolite-rich part of a flow in the MFU near Green Point just east of Digby Gut. This sample is amygdaloidal and fine grained, with about 10– 15% mesostasis that is intergranular to matrix plagioclase and clinopyroxene (Fig. 2b). We note that whereas the clinopyroxene is fresh, the plagioclase is variably altered to albite and stilbite. The mesostasis contains altered glass (i.e., hydrous Fe-Mg clays and micas), quenched Fe-Ti oxides, and Fe-rich pyroxenes and is similar petrographically to mesostasis textures described by Kontak et al. (2002). This sample was crushed and amygdale-free matrix material hand picked under a binocular microscope. A feature common to many samples of the MFU and less common in the LFU and UFU is the presence of a fine-grained chloritic phase lining vesicles. This feature is noted given that samples with such material may be susceptible to argon recoil during irradiation (e.g., Lo and Onstott 1989).

Display large image of Figure 2

6 The two samples selected for dating were crushed to <2– 3 mm grain size, washed in de-ionized water and dried. Analysis of the two samples was done at the Geochronology Laboratory, Queen's University, Kingston, Ontario, following the procedures outlined in Kontak et al. (1999). Dates and errors are calculated using formulae of Dalrymple et al. (1981), and the constants recommended by Steiger and Jager (1977). Errors shown in Table 1 and on the age spectra (Fig. 3) represent analytical precision at 2σ, assuming that the errors in the ages of the flux monitors are zero. This technique is suitable for comparing within-spectrum variation and determining which steps form a plateau (McDougall and Harrison 1988, p. 89). A conservative estimate of this error in the J-value is 0.5 % and can be added for inter-sample comparison.

Display large image of Figure 3

Table 1. Ar-Ar data for North Mountain Basalt, southwestern Nova Scotia.

Display large image of Table 1

7 We note that several samples of zeolite material were prepared and analyzed, but the results were not successful and therefore are not reported here. This unsuccessful attempt is in keeping with the conclusions of others (Dalrymple and Lanphere 1969).

ANALYTICAL RESULTS

8 The analytical results are given in Table 1 and shown graphically in Figure 3. For each sample the plateau age (PA), where appropriate, integrated age (IA), and correlation age (CA; isochron method) are given, following the normal procedures in ⁴⁰Ar/³⁹Ar dating (McDougall and Harrison 1988).

9 The results of analysis of sample ZL-99-26 from Parker Cove quarry yield a flat age spectrum with 85% of the gas released defining a plateau of 201.3 ± 2.3 Ma (MSWD = 0.5). In addition, the uniform Ca/K ratios for the release spectrum of the plateau indicate gas was liberated from a chemically uniform reservoir. As expected, the correlation age of 200 ± 2.9 Ma (MSWD = 0.3; Table 1, Fig. 3a) is essentially identical to the plateau age. The slightly younger integrated age of 199.2 ± 2.3 Ma reflects an anomalously low age for the lowest temperature fraction (Table 1), but interestingly, is significantly older than the equivalent whole-rock K-Ar ages for the NMB noted earlier. We adopt an age of 201 ± 2.5 Ma for this sample. Results for this sample indicate a simple thermal history with no overprinting thermal event, consistent with the very fresh nature of the sample.

10 The results for sample ZL-99-36, from a zeolite-bearing MFU horizon, yielded a very disturbed age spectrum (Table 1, Fig. 3b). Seven of the eight steps, comprising 87% of the release spectrum, record ages in excess of the known extrusion age of ca. 200 Ma. The hump-type pattern, with a maximum age of 380 Ma for the low temperature steps, is a classic example of combined excess argon and recoil effects (McDougall and Harrison 1988; Lo and Onstott 1989). The remaining steps gave progressively lower ages, but only the final step, the last 13% of the gas release, has an age (i.e., 190 Ma) that is similar to that for the fresh sample. Although the integrated age of 276.2 ± 9.2 Ma is clearly too old, the correlation age of 206.5 ± 55.9 Ma (MSWD = 0.36), albeit with a large error, approximates the age of NMB extrusion. As expected, the initial ⁴⁰Ar/³⁶Ar ratio of 730 is well in excess of that for atmospheric argon (i.e, 295.5; Dalyrmple and Lanphere 1969) and contrasts markedly with the near-atmospheric ⁴⁰Ar/³⁶Ar ratio of 302 ± 14 for sample ZL-99-26.

DISCUSSION AND CONCLUSIONS