Late Paleozoic collision, delamination, short-lived magmatism, and ...

Late Paleozoic collision, delamination, short-lived magmatism, and rapid denudation in the Meguma Terrane (Nova Scotia, Canada): constraints from 40Ar139Arisotopic data J.D. Kepple and R.D. Dallmeyer Abstract: The Meguma Terrane consists of 8- 15 km of early Paleozoic stratified rocks, deposited on a continental basement, that were deformed and metamorphosed during the late Paleozoic as a result of lithospheric plate collision. The oldest cleavage (previously published, whole-rock, 40Ar/39Arplateau ages of 415-395 Ma) dates the onset of crustal thickening, which was followed by voluminous, but short-lived, Late Devonian granitic and minor mafic magmatism (380-370 Ma). This magmatism may have been the product of delamination of the lower lithosphere and upwelling of asthenosphere, which effected melting above the new Moho and resulted in intrusion at depths of 5 - 12 km. 40Ar/39Arplateau ages of hornblende, muscovite, and biotite indicate that, at the present erosion level, most of the Meguma Terrane cooled through 300°C by 368 -360 Ma, slightly earlier than for the southwestern Meguma Terrane (ca. 345 Ma). The present erosion level was exhumed by the latest Devonian - Early Carboniferous (Visean): the age of the oldest unconformably overlying rocks. Subsequent burial beneath 6 km of Carboniferous sediments would not have been sufficient to completely rejuvenate older intracrystalline mica systems and result in the observed 40Ar/39Armica plateau ages between ca. 350 and 260 Ma. Such rejuvenation may have resulted from migration of hot fluids along shear zones derived from the lower crust and mantle, and from granitoid magma intruded at ca. 316 Ma. The recurrence of deformation, magmatism and denudation in the middle Carboniferous suggests that further delamination may have occurred. The temperature of these fluids decreased from 400-500°C during the Carboniferous to 300-400°C during the Early Permian.

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R6sum6 : Le Terrane de Meguma est form6 d'une Cpaisseur de 8 i 15 km de roches stratifiCes, d'bge PalCozoique prCcoce, dCposCes sur un socle continental, dCformCes et mCtamorphisCes durant le PalCozoique tardif lors de leur collision avec la plaque lithosphtrique. Le plus vieux clivage (bges plateaux 40Ar/39Arde 415-395 Ma, dans une publication antkrieure) marque le debut de l'kpaississement crustal, suivi au DCvonien tardif par le magmatisme granitique, et occasio~~nellement mafique, volumineux mais de courte durCe (380-370 Ma). I1 semble que ce magmatisme est le rCsultat d'une dklamination de la lithosphkre infkrieure et du bombement de l'asthCnosphkre, ce qui aurait provoquC une fusion au-dessus du nouveau Moho et autorise I'intrusion aux profondeurs de 5 - 12 km. Les Pges plateaux @ ~ r / ~ sur ~ Ahornblende, r muscovite et biotite indiquent, d'aprbs le niveau actuel de l'Crosion, que la plus grande portion du Terrane de Meguma avait refroidi sous la barrikre de 300"C, il y a 368-360 Ma, prCcCdant de peu la transition de cette meme barrsre de refroidissement dans la rCgion sud-ouest du Terrane de Meguma (il y a environ 345 Ma). L'exhumation du niveau actuel d16rosion recule vers la fin du DCvonien - debut du Carbonifkre (VisCen) : soit l'bge correspondant aux plus anciennes roches sus-jacentes discordantes. L'enfouissement subskquent sous environ 6 km de sCdiments carbonifbres aurait CtC insuffisant pour rajeunir compl&tement les systbmes intracristallins contenant le mica et pour fournir des Pges plateaux 40Ar/39Arsur mica autour de 350 et 260 Ma. Un tel rajeunissement serait plut6t la consCquence de la migration, le long des zones de cisaillement, de fluides chauds dCrivts de la Erofite infkrieure et du manteau, et du magma granitoide inject6 vers 316 Ma. La reapparition d'un nouveau cycle de dkformation, magmatisme et ablation au Carbonifkre moyen tkmoigne d'une reprise de la dtlamination. La temperature de ces fluides a diminuC de 400-500°C qu'elle Ctait au Carbonifkre i 300-400°C au Permien prCcoce. [Traduit par la redaction]

1 Received July 4, 1994. Accepted December 14, 1994.

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J.D. Keppie.' Department of Geology, St. Francis Xavier University, Antigonish, NS B2G 2W5, Canada. R.D. Dallmeyer. Department of Geology, University of Georgia, Athens, GA 30602, U.S.A. Corresponding author (e-mail: [email protected]).

Can. J. Earth Sci. 32: 644-659 (1995). Printed in Canada 1 Imprim6 au Canada

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Introduction The Meguma Terrane is the most outboard terrane in the northern Appalachians relative to Laurentia. Along its northern margin (the Minas Fault), it is tectonically juxtaposed against the Avalon Composite Terrane (Fig. 1). The Meguma Terrane is characterized by 6-10 km thick, CambrianOrdovician turbidites of the Meguma Group interpreted to be a continental rise prism (Schenk 1970). Gneisses of the Liscomb Complex may represent the sub-Meguma basement (Clarke et al. 1993). The Meguma Group is overlain by 2.3 -4.5 km thick, Lower Ordovician - Lower Devonian, metasedimentary and metavolcanic rocks. These are unconformably overlain by 2.5 km of continental and shallow marine, uppermost Devonian and Lower Carboniferous sedimentary rocks, which are in turn unconformably overlain by 3 km of Upper Carboniferous continental sedimentary rocks. Early Mesozoic (Anisian - Sinemurian) rocks unconformably overlie various Paleozoic units around the Minas Basin. The Meguma Terrane was affected by low-grade metamorphism and deformation during the Devonian and Carboniferous that is geologically constrained by the intra-Devonian and mid-carboniferous hiatuses (Keppie 1993). Extensive granitoid plutonism with associated contact metamorphism occurred mainly between 380 and 370 Ma with a minor subsequent pulse at ca. 316 Ma (Clarke and Halliday 1980; Keppie et al. 1985, 1993; Cormier et al. 1988; MacDonald et al. 1992; J. D. Keppie and T.E. Krogh, personal communication, 1994). Contact metamorphic assemblages indicate emplacement depths of 5- 12 km (Dallmeyer and Keppie 1987). The Meguma Terrane is transected by a series of synto postgranite shear zones displaying s-c fabrics, rotated porphyroblasts, and sheath folds (Keppie and Chatterjee 1983; Keppie and Dallmeyer 1987; Dallmeyer and Keppie 1987, 1988; 07Reillyand Kontak 1992; Keppie et al. 1993). Devonian deformation in the Meguma Terrane is recorded by 415-377 Ma, 40Ar/39Ar,whole-rock phyllite plateau ages (Reynolds and Muecke 1978; Keppie and Dallmeyer 1987). 40Ar/39Armica ages between 380 and 360 Ma were attributed to thermal resetting associated with the Devonian magmatism (Dallmeyer and Keppie 1987, 1988; Reynolds et al. 1987; Muecke et al. 1988). Younger ages between ca. 350 and 260 Ma have been variably interpreted. Based upon the sporadic occurrence of ages in excess of 350 Ma over the entire region, and the projected basal Carboniferous unconformity close to the present erosion surface, Dallmeyer and Keppie (1987, 1988) favoured localized reheating related to later magmatic events, whereas Reynolds et al. (1987) and Muecke et al. (1988) attributed them to growth of vein muscovite and dynamically recrystallized micas related to mineralization associated with transcurrent movements unrelated to magmatism. In the absence of a multi-isotope database, it has been difficult to evaluate these models and to assess the relative roles of slow cooling versus reheating events. New geochronological data presented herein, combined with published age data, suggest two distinct reheating events, at 380-370 and 316 Ma, related to intrusion of magma with episodic introduction of hot fluids along shear zones between 350 and 260 Ma. Different interpretations have been published for the origin of the Devonian magmatism. Keppie and Dallmeyer (1987) inferred that obduction of the Meguma Terrane over

the Avalon Terrane depressed the lithosphere, which was followed by thermal rebound leading to melting and production of extensive granitic and minor gabbroic plutonism starting at 30 Ma after initial deformation. On the other hand, Clarke et al. (1993) proposed that subduction of the Avalon Zone beneath the Meguma Terrane induced the generation of mafic magma in the downgoing slab. This mafic magma under- and intraplated the Meguma Terrane, leading to melting and the production of granitic magma. ~ m - ~andd Pb isotopic data suggest that the main Devonian granites were derived from both sub-Meguma basement (Liscomb gneisses) and lower crustal ganulites that occur as xenoliths in Late Devonian lamprophyre dykes (Chatterjee and Ham 1991; Clarke et al. 1993). A significant increase in available isotopic data, together with new 40Ar/39Ardata presented in this paper, helps constrain late Paleozoic tectonothermal models. Synthesis of these data suggests a new model involving crustal thickening followed by delamination, short-lived episodes of magmatism, rapid uplift, and denudation.

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Previous geochronology Isotopic ages reported from the southwestern Meguma Terrane prior to 1987 have been summarized by Dallmeyer and Keppie (1987). These data are primarily represented by K-Ar and Rb-Sr mineral analyses, which have been largely superseded by new 40Ar/39Ar, Rb - Sr, and U - Pb data. With the exception of the Rb-Sr ages, these data, along with the new 40Ar/39Ardata presented in this paper, are summarized in Figs. 2 -4. With the exception of the Late Carboniferous Wedgeport pluton (U -Pb age of 316 5 Ma: Cormier et al. 1988), all other granitoids in the Meguma Terrane yielded Devonian, U-Pb ages between 385 and 370 Ma. Six eastern Meguma plutons yielded ages of 378.5 f 2 and 371 f 2 Ma (North Forest Hills and Whitehaven plutons: Hill 1991) and between 375 2 and 372 f 2 Ma (Larrys River, Sangster Lake, Queensport, and Halfway Cove plutons: J.D. Keppie and T.E. Krogh, personal communication, 1994). In the southwestern Meguma Terrane, the Barrington Passage and Shelburne plutons yielded ages of 375 f 2 and 371 2 Ma, respectively (J. D. Keppie and T.E. Krogh, personal communication, 1994), whereas the South Mountain Batholith has yielded several ages: 385 2 Ma (upper intercept U-Pb zircon age: Keppie et al. 1993); 377:: Ma (concordant monazite age: Keppie et al. 1993); and 366 f 4 Ma (207Pb/206Pbwhole-rock age: Kontak and Chatterjee 1992). The 385 2 Ma U-Pb age from the Westfield apophysis of the South Mountain Batholith is an upper intercept age and is probably anomalously old due to inheritance. The 366 f 4 Ma 207Pb/206Pbmodel age from the southwestern end of the batholith is probably too young given the likelihood of lead loss. Eliminating these two ages restricts Devonian magmatism to between 380 and 370 Ma. Previously reported 40Ar/39Arresults from the plutons range from ca. 390 to 220 Ma and suggest a complex, late Paleozoic tectonothermal evolution including thermal events at ca. 390-360, ca. 320-300, ca. 290, ca. 260, and ca. 230-220 Ma (Dallmeyer and Keppie 1987, 1988; Reynolds et al. 1987; Muecke et al. 1988). Similar episodic tectonothermal activity (at ca. 344, ca. 330, ca. 270, and ca. 254 Ma) was proposed by Cormier et a1 (1988), Kontak and Chatterjee (1992), and

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ARRl NGTON PASSAGE PLUTON

ELBURNE PLUTON

Triassic -Jurassic t

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Garnet Andalusite --+.-.r--Sillimonite

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Fig. 1. Simplified geological map of the Meguma Terrane showing metamorphic isograds and schematic geological cross sections inferred from seismic reflection profiles (88-4 and 88-2) across the Bay of Fundy and the southwestern end of the Meguma Terrane (modified from Keen et al. 1991). EKSZ, East Kemptville shear zone; RSZ, Rossignol shear zone: TSZ, Tobeatic shear zone; MB, Magdalen Basin.

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Fig. 2. Compilation of U-Pb and Pb-isotope data, and 40Ar/39Ar amphibole plateau ages (discordant ages in square brackets) from the southwestern Meguma Terrane showing geology, metamorphic isograds (B, biotite; G, garnet; A , andalusite; S, sillimanite; M, migmatite) and shear zones (SSZ, Shelburne Shear Zone; LSZ, Lockeport Shear Zone). U-Pb zircon ages from Cormier et al. (1988); Keppie et al. (1993); and T.E. Krogh and J.D. Keppie (personal communication, 1994). Pb isotope data from Kontak and Chatterjee (1992). 40Ar/39Ardata from Dallmeyer and Keppie (1987, 1988), Reynolds et al. (1987), and Muecke et al. (1988).

DEVONIAN -CARBONIFEROUS

CAMBRIAN-ORDOVICIAN HALIFAX FORMATION

0GOLDENV/LLE FORMATION

Keppie et al. (1993) to explain complex Rb - Sr isotopic data at Wedgeport, East Kemptville, and Westfield, respectively. Apatite fission track ages in Cambrian-Devonian rocks in the southwestern Meguma Terrane have yielded ages of ca. 246-237 and ca. 180 Ma (Reynolds et al. 1987; McKillop 1990; Ravenhurst et al. 1990; Arne et al. 1990; Ryan and Zentilli 1993).

Analytical techniques The techniques used during 40Ar/39Ar analysis generally followed those described in detail by Dallmeyer and Keppie (1987). Mineral concentrates and whole-rock powders were wrapped in aluminum-foil packets, encapsulated in sealed quartz vials, and irradiated for 40 h at 1000 kW in the central thimble position of the United States Geological Survey

TRIGA reactor in Denver, Colorado. Variations in the flux of neutrons along the length of the irradiation assembly were monitored with several mineral standards, including MMhb-1 (Samson and Alexander 1987). The samples were incrementally heated until fused in a double-vacuum, resistance-heated furnace. Each heating step was maintained for 30 min. Measured isotopic ratios were corrected for mass discrimination, total-system blank levels, and the effects of interfering isotopes produced during irradiation using factors reported by Dalrymple et al. (1981). Apparent 40Ar/39Arages were calculated from the corrected isotopic ratios using the decay constants and isotopic abundance ratios listed by Steiger and Jager (1977). Intralaboratory uncertainties were calculated by statistical propagation of uncertainties associated with measurement of each isotopic ratio (at two standard deviations of the mean)

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Fig. 3. Compilation of 40Ar/39Ar muscovite ages from the southwestern Meguma Terrane. University of Georgia: Dallmeyer and

Keppie (1987, 1988). Dalhousie University: Zentilli and Reynolds (1985), Reynolds et al. (1981, 1987), Muecke et al. (1988), Keppie et al. (1993), and this paper. Abbreviations as in Fig. 2 caption. Plateau ages underlined.

1:::]WHITE RROCK

FWAT/ON CAMBRIAN- ORDOVICIAN HAL IFAX FORMA T/ON

SHEAR ZONE METAMORPHIC ISOGRAD

.' S ' .

I(l tomelms

through the age equation (Tables 1 - 3;2 Figs. 5, 6). Interlaboratory uncertainties are 1.25 - 1.5% of the quoted age. Total-gas ages were computed for each sample by weighting of the age and percent 39Ar released within each temperature increment. A "plateau" is considered to be defined if (1) increments have similar apparent K/Ca ratios and (2) ages recorded by two or more contiguous gas fractions each representing >4% of the total 39Ar evolved (and together constituting > 50% of the total quantity of 39Arevolved) are mutually similar within a 1% intralaboratory uncertainty. Analysis of the MMhb-1 monitor indicates that apparent K/Ca ratios may be calculated through the relationship 0.518 (+0.0005) x 39Ar/37Arcorrected.

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Complete set of tables may be purchased from the Depository of Unpublished Data, Document Delivery, CISTI, National Research Council Canada, Ottawa, ON KIA OS2, Canada.

Results Fifteen muscovite and ten biotite concentrates, together with six whole-rock slate-phyllite samples, were analyzed from various lithological units in the southwestern Meguma Terrane. The 40Ar/39Ar analytical data are presented in Tables 1-3,2 and are portrayed as age spectra in Figs. 5 and 6. Apparent KICa ratios are very large and display no significant or systematic intrasample variations in the muscovite and biotite analyses. Therefore, they are not presented with the mica age spectra in Figs. 5 and 6.

Muscovite Muscovite concentrates were prepared from various plutonic rocks and host metamorphic units. These include (1) undeformed granite from the Port Mouton Pluton (sample 3) and South Mountain Batholith (samples 19-21); (2) a deformed dyke of the Barrington Passage Pluton (sample 16); (3) an

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Fig. 4. Compilation of 4 0 A r / 3 9 ~biotite r ages from the southwestern Meguma Terrane. University of Georgia: Dallmeyer and Keppie (1987, 1988). Dalhousie University: Reynolds et al. (1981, 1987), Muecke et al. (1988), and this paper. DSZ, Deerfield shear zone; other abbreviations as in Fig. 2 caption. Plateau ages underlined.

SHEAR ZONE METAMORPHIC ISOGRAD

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Kilormlm

undeformed granite dyke exposed on an island south of the Barrington Passage Pluton (sample 12); and (4) the Goldenville Formation (samples 1, 2, 4 -7, 13- 15). The concentrates display variably discordant 40Ar/39Ar age spectra (Fig. 5). The oldest apparent ages are displayed by the three muscovite concentrates from the South Mountain Batholith (Fig. 5a). Sample 19 is characterized by an internally discordant age spectrum in which apparent ages systematically increase from ca. 313 to ca. 354 Ma in the first six increments (575 -725 "C). The remaining 11 increments (750°C to fusion) record mutually similar apparent ages, which define a plateau of 361.0 f 1.5 Ma. Sample 20 displays a less discordant spectrum with -92% of the gas (675 850°C increments) defining a plateau age of 357.7 f 1.1 Ma. More discordance is displayed by the spectrum of sample 21, in which apparent ages increase systematically

from ca. 300 Ma (low experimental temperatures) to ca. 340 Ma (high experimental temperatures). Muscovite concentrates from samples 1-3 (Port Mouton granite and host Goldenville Formation) are characterized by nearly concordant 40Ar/39Arspectra, which define plateau 0.7 Ma ages ranging between 339.5 f 0.8 and 347.6 (Fig. 5b). Muscovite from a deformed dyke of the Barrington Passage granite pluton (sample 16) records a markedly younger plateau age of 322.4 f 1.1 Ma. Samples 5 and 12 - 15 collected within the Goldenville Formation are characterized by slightly discordant spectra in which plateaux are well-defined by intermediate- and high-temperature increments. Anomalously younger ages are recorded by lowtemperature increments. Plateau ages vary from 349.4 f 0.7 Ma (sample 5), through 330.8 f 1.3 Ma (sample 15), 319.6 f 1.2 Ma (sample 12), 314.5 f 1.0 Ma (sample 14), to 302.1 f 0.9 Ma (sample 13). Low-temperature experi-

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Fig. 5. 40Ar/39Arincremental release age spectra of (a) muscovite and (b) biotite from the southwestern Meguma Terrane. All spectra have coordinates as shown for biotite sample 13. Where appropriate, plateau and total gas ages are given on each spectrum. Two-sigma intralaboratory analytical uncertainties are represented by the vertical width of the bars. Experimental temperatures increase from left to right.

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Total Gas Age = 300.7 f 1.2 '

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Plateau Age = 302.1 f 0.9 Ma 1

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314.5 i 1.0 Ma

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347.6

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mental ages of ca. 320-330 Ma are displayed by samples 5 and 15, whereas ages of ca. 280 Ma characterize the lowtemperature increments from samples 13 and 14. Sample 12 records low-temperature experimental ages of ca. 300 Ma. Samples 4, 6, and 7 (Goldenville Formation) display internally discordant spectra in which apparent ages generally increase systematically from low-temperature ages of ca.

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310 -320 Ma to high-temperature ages of ca. 345 -355 Ma.

Biotite The 10 biotite concentrates display marked contrasts between anomalously younger ages recorded by initial lowest temperature increments and the ages defined by the following temperature fractions (Table 2;2 Fig. 5). No systematically

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Fig. 5 (concluded).

(b)

Total

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- Gas Age

= 279.5

f 0.7

@

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10 20 30 40 50 60 70 80 90 100 CUMULATIVE PERCENTAGE S A r RELEASED

increasing age patterns are displayed. Sample 10 was prepared from the same sample (Goldenville Formation) for which Dallmeyer and Keppie (1987) reported a muscovite plateau age of 342 Ma (their sample 10). The biotite concentrate displays a markedly discordant age spectrum with a total-gas age of 314 Ma (Fig. 5b). Biotite concentrate 11 is from a sample of the Goldenville Formation, for which these workers reported a muscovite plateau age of 3 18 Ma (their sample 9). The biotite concentrate records a plateau age of 280.6 f 0.4 Ma. Plateau ages of 312.6 k 0.7, 287.2 f 0.9, and 280.5 f 0.6 Ma are defined by biotite concentrates from three other samples of the Goldenville Formation (15, 14, and 13, respectively). Older plateau ages (352.3 f 0.8 and 345.9 f 0.7 Ma) are recorded by biotite concentrates from two samples (8 and 9), collected within the Halifax Formation (Fig. 5b). Markedly younger plateau ages (276.7 f 0.6 and 269.2 f 0.6 Ma) are defined by biotite concentrates from samples 17 and 18 from the Goldenville Formation, collected in the vicinity of the Wedgeport Pluton (Fig. 5a). A 316.4 f 0.5 Ma plateau age is defined by biotite within a deformed granitic dyke from the Barrington Passage Pluton (sample 16: Fig. 5b).

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Whole-rock slate - phyllite Six whole-rock samples of slate-phyllite (Goldenville Formation) recovered from exploration borehole cores in the vicinity of East Kemptville were analyzed (Table 3;2 Fig. 6). The 40Ar/39Arspectra display considerable internal discordancy, and record total-gas ages that range between 297 and 316 Ma. Younger apparent ages are recorded by gas fractions evolved at the lowest experimental temperatures. In samples 22-25, apparent ages sharply increase to maxima in the initial intermediate-temperature increments, then decrease slightly and are nearly similar throughout most intermediate-temperature increments. Apparent ages systematically increase in high-temperature portions of the four analyses. The age spectrum of sample 26 is characterized by a systematic age increase throughout low- and intermediatetemperature increments, but displays a marked age decrease in two high-temperature increments. That of sample 27 displays systematically increasing apparent ages throughout the entire analysis. The apparent KICa spectra of the six whole-rock samples are also marked by considerable internal discordancy (Fig. 6). Apparent KICa ratios of gas increments evolved at lower

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Fig. 6. 40Ar139Arincremental release age and apparent K/Ca spectra of whole-rock phyllite from East Kemptville. All spectra have coordinates as shown for sample 24. Total gas ages are listed on each spectrum. Two-sigma intralaboratory analytical uncertainties are represented by the vertical width of the bars. Experimental temperatures increase from left to right.

experimental temperatures are generally relatively low, but marked by considerable fluctuation. Apparent KICa ratios systematically increase throughout intermediate-temperature increments, and systematically decrease to minima in fusion increments.

Interpretation Muscovite Muscovite concentrates from samples 1, 2, 3, and 16 display nearly concordant 40Ar/39Arage spectra corresponding to plateau ages ranging between 347.6 f 0.7 and 322.1 f 1.3 Ma (Fig. 5). These ages are interpreted to date the last cooling through temperatures required for intracrystalline retention of argon in muscovite. Although not fully calibrated experimentally, the preliminary data of Robbins (1972) used in the diffusion equations of Dodson (1973) indicate muscovite closure temperatures of 375 -400°C. These temperatures are similar to those suggested by empirically comparing muscovite K- Ar ages with those recorded by other mineral isotopic systems (e.g., Wagner et al. 1977; Jager 1979).

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Muscovite concentrates from samples 5, 12- 15, 19, and 20 display a well-defined plateau in intermediate- and hightemperature portions of each analysis (Fig. 5). These plateaux are also interpreted to date cooling through 375 -400°C. Apparent ages recorded by low-temperature increments are younger, and systematically increase to the various plateau ages. Such spectra discordance may develop as a result of partial, intracrystalline diffusive loss of radiogenic 40Ar during a superimposed thermal event (Turner 1968; Snee et al. 1988; Dallmeyer and Lecorche 1989; Dallmeyer and Takasu 1992). However, the extent of isotopic rejuvenation will depend not only on overprinting temperatures but also on the length of time the temperatures are maintained. Comparison of these present spectra with the theoretical spectra of Turner (1968) suggests that they experienced only slight ( < 25 %) diffusive loss of radiogenic argon as a result of a thermal overprint following initial postmetamorphic cooling through argon closure temperatures. The timing of the thermal overprint appears to have been variable: ca. 340 Ma in sample 20; ca. 320-330 Ma in samples 5 and 15; ca. 300-310 Ma in samples 12 and 20; and ca. 280 Ma in samples 13 and 14. More extensive thermal overprinting is suggested by the dis-

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cordant spectra displayed by muscovite from samples 4, 6, 7, and 21. Characteristics of these spectra suggest partial rejuvenation at ca. 300-320 Ma.

Biotite Except for sample 10, all of the biotite concentrates record well-defined plateau ages ranging between ca. 269 and 352 Ma. However, interpretation of these results is uncertain, because the biotite lattice may be unstable during in vacuo heating, as a result of dehydrogenation and resultant lattice delamination (e. g ., Gerling et al. 1966; Zimmerman 1972; Harrison and McDougall 1981). As a result, existing intracrystalline 40Ar gradients could be removed because of lattice rearrangement at low experimental temperatures, therefore resulting in well-defined but meaningless 40Ar/39Ar incremental-release plateaux (e.g., Dallmeyer and Rivers 1983). Harrison et al. (1985) demonstrated that temperatures required for intracrystalline retention of argon are dependent upon cooling rate and biotite composition (increasing with decreasing Fe/Mg ratio), and concluded that for the range of cooling rates likely to be encountered in most geological settings, values of 275 - 325 "C are appropriate. These results suggest that biotite should be more susceptible to thermal rejuvenation than muscovite (because of lower closure temperatures). In view of the complex regional thermal evolution suggested for the southwestern Meguma Terrane by the variably discordant 40Ar/39Armuscovite spectra, it is likely that at least some of the biotite plateaux reflect experimental homogenization of intracrystalline gradients. Interpretation of individual biotite results therefore depends upon comparison with muscovite results from proximal samples.

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Whole-rock slate- phyllite The six whole-rock samples display variably discordant 40Ar/39Arapparent age spectra, which are difficult to interpret (Fig. 6). Although these samples consist primarily of very fine-grained white mica, systematic intrasample variations in apparent K/Ca ratios suggest that several other phases contributed gas at various stages of the analyses. Relative to white mica, these appear to have included (1) a less retentive phase, present in variable modal proportions and with a low K/Ca ratio, and (2) a more refractory phase, present in low modal proportions and with a low K/Ca ratio. Mineralogical characteristics suggest that these phases are chlorite and plagioclase, respectively. The intermediatetemperature increments are characterized by fluctuating but relatively large apparent K/Ca ratios interpreted to reflect gas evolved from white mica. Except for sample 27, these intermediate-temperature increments initially record ages of ca. 300 Ma, and subsequently increase. If not significantly contaminated by evolution of gas from chlorite (at lower temperatures) or from detrital plagioclase feldspar (at higher temperatures), the increasing intermediate-temperature age trends may be due to episodic, diffusive loss of argon during thermal overprinting (Turner 1968; Dallmeyer et al. 1988; Dallmeyer and Reuter 1989; Dallmeyer and Takasu 1992). The character of the discordant whole-rock spectra suggests that initial postmetamorphic cooling through appropriate argon closure temperatures occurred prior to ca. 340 - 350 Ma (the fusion ages of most spectra), and was followed by extensive rejuvenation at ca. 300 Ma.

Geological significance Muscovite samples 19 and 20 from the South Mountain Batholith in the vicinity of East Kemptville show minimal intracrystalline rejuvenation of argon isotopic systems, and record initial postmagmatic cooling ages of ca. 360 Ma (Fig. 5). Relatively rapid postmagmatic cooling is suggested, because the 40Ar/39Arplateau ages are similar to the 366 4 Ma 207Pb/206Pb crystallization age for the leucogranite and mineralized greisens at East Kemptville (Kontak and Chatterjee 1992). This is consistent with the development of low-pressure contact aureoles, and indicates relatively shallow crustal levels for emplacement. Samples 19-21 also record variable effects of thermal overprints at ca. 340 and 300 Ma. A more extensive ca. 300 Ma overprint is recorded by five of the six whole-rock slate-phyllite samples from the Goldenville Formation in East Kemptville. This is not surprising, in view of the much finer grained nature of the constituent white micas. The oldest ages defined in hightemperature portions of the whole-rock experiments do not approach the ca. 415-395 Ma suggested for the earliest low-grade metamorphism and associated deformation concomitant with cleavage development (Reynolds and Muecke 1978; Keppie and Dallmeyer 1987). Instead, the oldest hightemperature ages are in the 340-350 Ma range, and similar to the emplacement age of the South Mountain Batholith. This indicates that extensive contact metamorphism was likely associated with batholith emplacement. Muscovite and biotite plateau ages in and around the Port Mouton and Shelburne pluton (samples 1 - 3, 5, 8, 9) range between ca. 340 and 356 Ma (Fig. 5b). Mica plateau ages in and around the Barrrington Passage Pluton (samples 13- 16) become progressively younger towards the south, ranging from ca. 33 1 to 302 Ma (muscovite) and ca. 3 16 to 28 1 Ma (biotite: Fig. 5a). The Shelburne and Barrington Passage plutons have yielded U-Pb ages of 371 2 and 375 2 Ma, respectively (J.D. Keppie and T.E. Krogh, personal communication, 1994). The Port Mouton Pluton has not been dated by U-Pb; however, a ca. 369.1 + 2.9 Ma isotope correlation age recorded by hornblende in the contact aureole probably closely dates pluton emplacement (Fig. 2; Dallmeyer and Keppie 1988). In this context, the ca. 340356 Ma mica ages likely date moderately rapid postcrystallization cooling through 275 - 325 "C closure temperatures following intrusion of the Shelburne and Port Mouton plutons. The younger muscovite plateau ages are inferred to record thermal rejuvenation at ca. 331,320 -314, and 302 Ma, whereas the biotite plateau ages suggest thermal rejuvenation at ca. 3 16 - 313 and 287 -281 Ma. Such rejuvenation is also recorded in low-temperature gas fractions experimentally evolved from many muscovite concentrates (e.g., 1, 3 , 4 , 5, 7, 13-15: Fig. 5). Biotite in the vicinity of the Wedgeport Pluton records plateau ages of ca. 269 and 277 Ma, which are markedly younger than the ca. 316 Ma emplacement age of the stock. The 40Ar/39Arresults are only slightly older than a ca. 260 Ma postcrystallization reheating event that affected the Wedgeport Pluton (Cormier et al. 1988).

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Regional correlations In order to evaluate the regional significance of the new 40Ar/39Ardata, they must be considered in the context of'

Can. J . Earth Sci. Vol. 32, 1995

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the published geochronological data. The available geochronological database has been divided into three categories, based upon differences in closure temperatures. Figure 2 shows ages for minerals with closure temperatures >500°C (zircon, monazite, hornblende). Closure temperatures for zircon and monazite have been estimated to be 650 to > 800°C and 550 -750°C, respectively (Gebauer and Grunenfelder 1979; Cliff 1985; Parrish 1990; Heaman and Parrish 1991). Closure temperatures for argon in hornblende appear to be relatively independent of composition. For geologically reasonable cooling rates, tempera500°C have been suggested (Harrison 1981). tures of Figure 3 is limited to muscovite 40Ar/39Arages, for which closure temperatures have been estimated to be about 375 -400°C (Wagner et al. 1977; Jager 1979). Figure 4 shows biotite 40Ar139Arages. Biotite has a wide range of potential closure temperatures (275-325°C) over a variety of cooling rates, due largely to a compositional dependence (Harrison et al. 1985). Some of the biotite plateau ages are older than the accompanying muscovite plateau ages, and indicate the presence of excess argon in the biotite lattice. Thus, the biotite data should be used with caution. The muscovite and biotite data have been contoured to better vizualize their regional distribution. Where data are sparse, ages and contour trends have been transferred between Figs. 3 and 4. With the exception of the 316 Ma age for the Wedgeport Pluton, the cooling ages range between 367 and 389 Ma, both between and within the shear zones across all of southwestern Nova Scotia: excluding discordant hornblende 40Ar/39Arages (Fig. 2). In contrast, the contoured muscovite and biotite data show a series of northeast-trending, narrow, younger zones ( 358 Ma on either side. Internally discordant 40Ar/39Arage spectra appear to occur near these shear zones. The distribution of zircon, monazite, and hornblende ages > 367 Ma (Fig. 2) indicates that the entire region must have cooled through 500°C by ca. 367 Ma. This distribution of high-temperature cooling ages indicates that metamorphism above the andalusite isograd occurred prior to ca. 367 Ma. Mapped isograd patterns generally cut across folds in the southwestern Meguma Terrane. Textural studies indicate that most of the high-grade porphyroblasts grew over early planar fabrics that are axial planar to folds in the Meguma Group (Dallmeyer and Keppie 1987, 1988). Dating of this

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early slaty cleavage has yielded ages ranging between 415 and 377 Ma (Reynolds and Muecke 1978; Keppie and Dallmeyer 1987). The relationship between isograds and plutons varies: the isograds are parallel to the contacts of the Barrington Passage Pluton; however, they are truncated by the South Mountain Batholith. These intrusions record similar crystallization ages (Fig. 2), and suggest that the highgrade metamorphism was approximately synchronous with granitoid intrusion. The > 360 Ma argon mica ages that occur in the northern parts of Figs. 3 and 4 are similar to the 360-370 Ma ages reported for most of the northern and eastern Meguma Terrane (Reynolds and Muecke 1978; Keppie and Dallmeyer 1987). These data indicate that northern and eastern parts of the Meguma Terrane cooled through -300°C by 360 Ma. To the southwest, slightly younger mica plateau ages are recorded in the broad areas between the shear zones: ca. 347 -360 Ma for muscovite and ca. 346 - 37 1 Ma for biotite. These younger ages indicate that the southwestern Meguma Terrane underwent somewhat slower cooling through -300°C. Such diachronism is also apparent in the age of the unconformity beneath overlying units: uppermost Devonian - Tournaisian along the northern margin of the Meguma Terrane versus Visean around St. Margarets Bay (Fig. 1). These relationships indicate that the present erosion surface of the Meguma Terrane had been exhumed by ca. 360350 Ma. The post-Devonian history of the Meguma Terrane (Fig. 7) includes (1) deposition of -2.5 krn of uppermost Devonian and Lower Carboniferous sequences; (2) Middle Carboniferous uplift and erosion; and (3) deposition of 3 km of Late Carboniferous rocks. Assuming minimal Middle Carboniferous denudation and a geothermal gradient of 25"Clkm, a 165°C may be calculated for the burial temperature of sub-carboniferous unconformity . This temperature is too low to have rejuvenated either muscovite or biotite argon systems (Fig. 7). Thus, the