Meguma terrane

2 The Meguma terrane of southern Nova Scotia is the most outboard terrane of the Canadian Appalachian Orogen. The terrane is bounded to the north by the east-trending Cobequid-Chedabucto Fault Zone (CCFZ) that separates this terrane from the Neoproterozoic-Cambrian composite Avalon terrane (Fig. 1). The Meguma terrane consists predominantly of Lower Paleozoic metasedimentary and volcanic rocks that are intruded by Late Devonian metaluminous and peraluminous granite (Clarke et al. 1997). The Lower Paleozoic rocks consists of the older Cambrian-Ordovician Meguma Group, a sequence of deep water distal turbidites that includes a metawacke-dominant lower part, the Goldenville Formation, and slate-rich upper part, the Halifax Formation. The Meguma Group is overlain by mixed sedimentary and mafic-felsic volcanic rocks of the Silurian to Devonian White Rock, Kentville, New Canaan and Torbrook formations, all of which were deposited in shallow water continental settings. This mixed package of rocks was regionally deformed into northeast-trending, open to close, upright folds, and metamorphosed to mainly greenschist facies during the Acadian Orogeny at ~400 ± 10 Ma (Keppie and Dallmeyer 1987; Kontak et al. 1998; Hicks et al. 1999). Subsequently the area was intruded by minor mafic rocks (e.g., Liscomb gabbro; Clarke et al. 1993) and large volumes of metaluminous to peraluminous granite, of which the South Mountain and Musquodoboit batholiths are the largest. The South Mountain Batholith intruded deformed rocks of the Lower to Middle Devonian (i.e., Lochovian to Emsian) Torbrook Formation, but Benn et al. (1997) ascribed a late tectonic rather than post-tectonic (Clarke and Clarke 1998) origin with respect to the Acadian Orogeny. Recently, Culshaw and Bhatnagar (2001) showed that emplacement of the SMB overlapped some ductile deformation of the Meguma Group and that fold development, although in an advanced stage, was not over at the time of batholith intrusion. The presence of coarse clastic rocks of the Fammenian Horton Group (Martel et al. 1993), representing the basal part of the overlying Carboniferous stratigraphy, resting on granitic rocks of the SMB indicate that uplift, denudation, and unroofing of the granite intrusions of the Meguma terrane occurred by ~360 Ma.

Musquodoboit Batholith

3 The Musquodoboit Batholith (MB) intrudes metasedimentary rocks of the Meguma Group, and a narrow contact metamorphic aureole containing cordierite porphyroblasts in finer grained biotite and andalusite hornfels is developed within a few hundred metres of the contact (MacDonald and Clarke 1985; Ham 1993; Horne et al. 1998). A composite map of the MB (Fig. 2), based on several years of field work (Ham 1993, 1994, 1998, 1999, 2000), shows that the oldest unit, occurring mostly along the southern contact, is medium- to coarse-grained biotite monzogranite with trace muscovite and cordierite. However, the predominant phase in the batholith is medium- to coarse-grained biotite leucomonzogranite with minor muscovite and cordierite (trace to 1%). Underlying the west-central part of the batholith, a similar unit is characterized by the presence of more cordierite (to 4%) compared to the areas flanking it (<1%; see also MacDonald and Clarke 1985). Fine-grained, texturally and mineralogically variable leucomonzogranite bodies occur locally throughout the batholith. Dyke rocks and pegmatite are also present, some of which are associated with mineralization, such as the Dunbrack occurrence (Kontak et al. 1999a) and the numerous tungsten showings in the MB (Ham 1993). Airborne radiometric surveys indicate that areas underlain by fine-grained and/or texturally variable phases have elevated U/Th ratios and that such phases extend along the southern and northern parts of the batholith in the western half, and occur as small bodies scattered within the southern part of the eastern half such as near the Tangier Grand Lake area (Ford 1993; Ham 1994, 1998). Faults strongly influence the shape of the MB and, by analogy to the structure of the South Mountain Batholith (Horne et al. 1992), these faults probably pre-dated intrusion and were active at the time of emplacement, and consequently controlled the nature of the contacts and the present day outline of the granite. No penetrative structures occur in the MB that might suggest a late tectonic overprint.

Significance for Age Interpretation of Granitic Intrusions

10 The new U-Pb age for the MB is compared to other U-Pb ages of granitic intrusions for the Meguma terrane in Table 2. The MB data are bracketed by data for other intrusions, which collectively indicate a span from 384 to 370 Ma for emplacement of granite, and more importantly the 378 Ma age is more consistent with the timing of magmatism across the terrane rather than the previous ages of 363– 370 Ma. Importantly, these ages of intrusion indicate that there was continuous injection of felsic magma into the Meguma terrane for some 14 Ma, but this is reduced to 9 Ma if the anomalously old monazite age of 384 Ma for the South Mountain batholith is excluded.

Table 2. Summary of U-Pb dates from plutonic rocks of the Meguma terrane, Nova Scotia.

Display large image of Table 2

11 The interpretation of a radiometric age as a geologically meaningful event relates in part to the recognition of when the chronometer of interest is activated, i.e., when the daughter nuclide generated from decay of the parent nuclide accumulates within the crystal being dated (i.e., Ar, Sr, Pb in the case of K, Rb, and U, respectively, for these chronometers). This parent-daughter relationship, which is diffusion dependent, is referred to as the blocking or closing temperature and varies not only for the different chronometers, but also for different minerals (i.e., biotite versus muscovite for K-Ar). Thus, relevant to this study is the fact that closure temperatures for the U-Pb chronometer in zircon and monazite have been estimated at ~650 to >800°C and 550– 750°C, respectively (Gebauer and Grunenfelder 1979; Cliff 1985; Parrish 1990; Heaman and Parrish 1991). These temperatures are well in excess of the ~350 to <300°C blocking temperatures estimated for muscovite and biotite, respectively (Jager 1979; Hames and Bowring 1994; Harrison et al. 1985).

12 An added complexity to the blocking temperature is the rate of cooling, which for purposes of this paper is a function mostly of depth of emplacement of the intrusions of the Meguma terrane. Although there have been no detailed studies of the contact aureole about the MB, the assemblage of biotite-andalusite-cordierite in Meguma Group metasedimentary rocks (MacDonald and Clarke 1985; Williams and Hy 1990) is similar to that in the contact metamorphic aureole observed around the South Mountain Batholith for which Raeside and Mahoney (1996) estimated a depth of emplacement of ~3.5 kbars. Equivalent to slightly deeper depths of emplacement have been estimated for the Port Mouton and Barrington-Shelburne plutons (Currie et al. 1998). Using fluid inclusion thermometry, Kontak et al. (1999a) estimated emplacement pressures of 3– 3.5 kbars during vein mineralization at the Dunbrack occurrence in the MB, which is similar to pressures suggested for greisen and vein mineralization at the East Kemptville tin deposit in the western part of the South Mountain Batholith (Halter and Williams-Jones 1999; Kontak et al. 2001). Thus, a pressure equivalent to ca. 3.5 kbars is considered the best estimate of the emplacement of the MB, which equates to a lithostatic load of ca. 12 km. Using a geo-thermal gradient of 25°C/km, which has been suggested for the Meguma terrane by Keppie and Dallmeyer (1995) during the Acadian Orogeny, a temperature of 300°C is indicated for the wall rocks. This temperature is similar to that of the blocking temperature of argon in biotite and only just below that for muscovite (350°C). Also relevant here is the recent suggestion that the present-day geothermal gradient in the western part of the South Mountain Batholith is in the order of 45°C/km (Chatterjee and Dostal 2003). Given that the MB is similar to the South Mountain Batholith in terms of its radioelement abundances (Ford et al. 1992; Ford 1993), a similarly elevated geotherm may have existed for the MB.

13 The above discussion indicates that dating of the MB using relatively low-temperature geochronometers (i.e., ⁴⁰Ar/³⁹Ar mica ages) may not provide a reliable estimate of the timing of intrusion if the intrusion cooled slowly. Thus, the apparent discrepancy in ages of zircon and monazite (i.e., ca. 378 Ma) versus muscovite and biotite (i.e., ca. 370 Ma) may simply be a function of cooling rate, itself related to the depth of em-placement and the elevated temperature of the wall rock (i.e., 300°C). Based on our U-Pb results, we suggest that dating of similarly emplaced intrusions using ⁴⁰Ar/³⁹Ar mica be done with caution as cooling rate is an important factor to consider and one which is difficult to assess in the absence of higher temperature geochronometric data. We also note the apparent discrepancy for recent Re/Os molybdenite and U-Pb zircon ages (376– 380 Ma; Kontak et al. 2003) versus younger conventional ⁴⁰Ar/³⁹Ar mica ages (ca. 370 Ma; Reynolds et al. 1981, 1987) for the South Mountain Batholith. However, recent laser ⁴⁰Ar/³⁹Ar dating of white mica phases from the batholith has yielded ages to 380 Ma (Carruzzo 2003; Reynolds et al. 2004).

Integration of new U-Pb ages and previous ⁴⁰Ar/³⁹Ar dating

14 The age of the MB is herein redefined as 378 ± 1 Ma based on concordant zircon and monazite ages. The coincidence of these ages indicates that inheritance is not a concern for the zircon ages and, similarly, that the monazite age is not related to a post-crystallization event. The 378 Ma age for crystallization and, by inference, batholith emplacement is some 8– 15 Ma older than previously interpreted based on published ⁴⁰Ar/³⁹Ar mica ages for the MB (Reynolds et al. 1981; Keppie and Dallmeyer 1987; Kontak et al. 1999a). The ⁴⁰Ar/³⁹Ar ages are now interpreted to indicate that the MB remained above the 300– 350°C blocking temperature of mica for nearly 10 Ma, a significant time interval and one that may have implications for mineralization in the batholith.

15 The age data for the MB are plotted on a time-temperature plot in Fig. 4 with a proposed cooling curve for the intrusion, which is used as a means to account for the age discrepancy among the different dating techniques. In this diagram the intrusion age is defined by the higher blocking temperature minerals (zircon, monazite), whereas the lower temperature part of the cooling curve is constrained by mica ages. The curve is extrapolated to lower temperatures based on the K-feldspar age for the Kinsac intrusion which, as noted above, is inferred to connect to the MB at depth. The plateau age of 354 ± 3 Ma for K-feldspar probably records the final stage of uplift and denudation of the MB prior to deposition of the Famenian Horton Group (Martel et al. 1993). Thus, the age discrepancy noted herein for the MB relates to the depth of emplacement of the batholith (i.e., 12 km), which consequently resulted in a protracted cooling history from the time of crystallization to cooling to 300– 350°C. It is tempting to speculate, therefore, that the steep cooling path is related to sudden uplift of the area and is tectonically significant with respect to the geological evolution of the Meguma terrane.

Display large image of Figure 4

Implications of age data for uplift rates in the Meguma terrane

16 The results of the high-temperature chronometry provide a well constrained point for assessing the nature of uplift rates in the Meguma terrane during the waning stages of the Acadian Orogeny. The integration of the age data and pressure constraints for the MB and other intrusions suggest denudation of ca. 12 km of overlying Lower Paleozoic stratigraphy during a span of no less than 18 Ma, as constrained by the emplacement ages of granite and time of deposition of the unconformably overlying Horton Group. It is unlikely that the rate of uplift was uniform and two values are used to bracket it. Firstly, the age of vein mineralization is estimated at 370 Ma with pressures constrained to 3.5 kbars based on fluid inclusion thermometry (Kontak et al. 1999a), which indicates that most uplift occurred after this time at a rate of 1.2 mm/yr. In contrast, assuming a steady state rate from the time of emplacement at 378 Ma, a more conservative rate of 0.6 mm/yr is estimated. These uplift rates are averaged over considerable time periods, which contrast with variable rates that typify most orogens (e.g, Andean Orogen; Kennan, in press) due in part to the fact that a steady state or equilibrium between accretion, uplift, and denudation is not always attained and maintained because tectonic processes (e.g., convergence rates, magma production) may vary (Willet & Brandon 2002). In addition, proximity to major faults, in this case the Cobequid-Chedabucto Fault, may also affect uplift rates such that higher rates occur proximal to such structures (e.g, Alpine Fault; Koons 1989). However, despite the simplistic approach used for estimating uplift, we note that the rates are comparable to those documented during the Cenozoic evolution of the Andes (ca. 0.05– 0.5 mm/a; summaries in Kennan, in press), an area that may have some parallels to the evolution of the Meguma terrane during the relevant time period when granite of the terrane was generated due to tectonic thickening of the continent (Clarke et al. 1993). In contrast, these uplift rates are much lower than rates documented in high-grade metamorphic terranes where rates of 5– 7 mm/a have been documented during the Variscan Orogen in granulite-eclogite rocks of the Erzgebirge (Germany; Werner & Lippolt 1999) and similar grade, but much younger (i.e., 40 Ma) rocks in northern Greece (Liati & Gebauer 1999). Thus, the inferred uplift rates for the Meguma terrane following emplacement of the MB and other intrusions of similar age are considered realistic for the style of deformation and metamorphism during the Acadian Orogeny. Further work and better constraints on the uplift rates after granite emplacement may provide an independent means of more rigorously assessing the varied models proposed for the tectonic environment that generated the Late Devonian granite of the Meguma terrane (cf. Clarke et al. 1993; Keppie & Dallmeyer 1995; Keppie & Krogh 1999; Murphy et al. 2000).