Polyphase late Paleozoic tectonothermal evolution of the ...

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Polyphase late Paleozoic tectonothermal evolution of the southwestern Meguma Terrane, Nova Scotia: evidence from 40Ar/39Armineral ages1 R. D. DALLMEYER Department of Geology, University of Georgia, Athens, GA 30602, U.S.A.

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by China University of Science and Technology on 06/06/13 For personal use only.

AND

J. D. KEPPIE Department of Mines and Energy, P.O. Box 1087, Hal*,

N.S., Canada B3J 2x1

Received March 17, 1986 Revision accepted December 19, 1986 40Ar/39Arincremental-release ages of hornblende, muscovite, and biotite from a variety of granitic stocks and host metamorphic rocks suggest a complex late Paleozoic tectonothermal evolution for the southwestern Meguma Terrane. Regional Dl folding with cleavage formation under greenschist - lower amphibolite facies, MI metamorphic conditions, occurred at ca. 400-410 Ma and was followed by emplacement of a series of granitic stocks ranging in age between ca. 375 and 315 Ma. These were emplaced at relatively shallow crustal levels and developed contact metamorphic aureoles of variable grade. These are locally superposed on MI regional metamorphic assemblages and result in a complex isograd pattern. 40Ar/39Armineral ages suggest episodes of contact metamorphism occurred at (1) 360-375 Ma (possibly related to emplacement of the South Mountain Batholith or temporal equivalents), (2) 350-356 Ma around the Port Mouton Pluton and northeastern Shelburne Pluton, (3) ca. 315 -325 Ma near the Wedgeport Pluton and in several other isolated localities, and (4) ca. 287 Ma along the northern margin of a large, low gravity anomaly located off the southwestern coast of Nova Scotia (inferred to reflect a subsurface pluton). Dextral shear deformation was locally associated with all of these thermal events. It is suggested that the Meguma Terrane experienced a similar stress system throughout the Late Devonian - Permian, with shear deformation localized in areas where increased temperatures resulted in decreased viscosity. Les Ages dtterminCs par accumulation-perte de 40Ar/39Arsur hornblende, muscovite et biotite d'une variCtC de roches granitiques et de roches mttamorphiques encaissantes indiquent une Cvolution tectonothermique au PalCozoique supCrieur complexe dans la partie sud-ouest du terrane de Meguma. La phase de plissement rkgionale Dl accompagnCe de I'apparition d'un clivage dans les conditions mttarnorphiques des faciks schistes verts - amphibolite infkrieur (M,), il y a approximative 400-410 Ma, fut suivie de la mise en place d'une sCrie de petits massifs granitiques datCs 2 approxirnativement 375 -3 15 Ma. La mise en place s'est dalisCe i des niveaux crustaux relativement peu profonds, et elle a eu pour effet de dCvelopper des aurkoles de mCtamorphisme de contact de degrC variable, lesquelles sont superposCes sur les assemblages mktamorphiques kgionaux M, crkant un style complexe d'isogrades. Les Lges 40Ar/39Ardes minCraux kvklent que les Cpisodes de mktamorphisme de contact sont apparus : (1) entre 360 et 375 Ma (possiblement en relation avec la mise en place du batholite South Mountain ou autres tquivalents au m&meBge), (2) entre 350 et 356 Ma aux environs du pluton Port Mouton et du nord-est du pluton Shelburne, (3) entre ca. 315 et 325 Ma p&s du pluton Wedgeport et dans plusieurs autres localitts isolCes et (4) 2 ca. 287 Ma lejong de la marge nord d'une importante anomalie gravimCtrique basse 1ocalisCe au large de la cBte sud-ouest de la Nouvelle-Ecosse et produite vraisemblablement par un pluton de subsurface. Une dkformation de cisaillement dextre Ctait associte localement a tous ces CvCnements thermiques. I1 semble que le terrane de Meguma ait CtC soumis a un ensemble de contraintes semblabes durant le DCvonien supkrieur - Permien, escort6 d'une dCformation de cisaillement localisCe dans les regions oh les accroissements de tempCrature provoqukrent des dductions de viscositk. [Traduit par la revue]

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Can. I. Earth Sci. 24, 1242 - 1254 (1987)

Introduction The Meguma Terrane is the farthest outboard terrane in the northern Appalachians (Keppie 1985). It is characterized by thick Cambro-Ordovician turbidites overlain by SiluroDevonian shallow-marine to continental rocks containing a unique, northern Appalachian Rhenish fauna (Boucot 1960). The Meguma Terrane has been previously considered as having experienced a relatively less complicated evolution than other northern Appalachian terranes (e.g., Schenk 1971, 1983; Keppie 1977, 1982) involving the following events: (1) deposition of Cambrian - Early Devonian sedimentary rocks with locally intercalated volcanic sequences; (2) a late Early Middle Devonian deformation (D,) and concomitant lowgrade regional metamorphism with development of regional northeast - southwest upright folds and associated axial planar 'Contribution to IGCP 233: Tenanes in the Circum-Atlantic Paleozoic Orogens. Printed in Canada / Imprime au Canada

foliations; (3) a regional low-pressure metamorphism superimposed on Dl structures; (4) intrusion of granitic plutons in the Late Devonian; (5) uplift, erosion, and deposition of Early Carboniferous sedimentary rocks, now unconformably overlain by Late Carboniferous sedimentary sequences; and (6) uplift, erosion, and deposition of early Mesozoic red-bed sequences and extrusion of plateau basaltic successions. It has recently been shown that at least locally the late Paleozoic tectonic record is far more complex (e.g., Keppie et al. 1985) and that the previously assumed tectonic simplicity largely reflected an absence of reliable faunal and (or) isotopic age constraints. Faunal controls have been available only from northern portions of the Meguma Terrane, and tectonothermal events recorded throughout the Meguma Terrane were constrained with these occurrences. Isotopic ages previously reported for the Meguma Terrane are summarized in Tables 1 and 2. Although most workers have interpreted these results as sug-

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DALLMEYER AND KEPPIE

gesting a relatively simple late Paleozoic thermal evolution (e.g., Reynolds and Muecke 1978; Reynolds et al. 1973, 198I), many anomalously young K -Ar and 40Ar/39Armineral ages have been reported. It has been uncertain whether these young ages relate to a prolonged cooling following a Devonian (Acadian) regional metamorphism or whether they record distinct episodes of late Paleozoic reheating. This report presents preliminary results of a multidisciplinary geochronological research program underway throughout the Meguma Terrane (Dallmeyer and Keppie 1987). This work has focussed on delimiting the Paleozoic thermal evolution and evaluating its relationship to economic mineralization. The results presented here confirm that the late Paleozoic tectonothermal evolution of the southwestern Meguma Terrane was complex and involved several distinct heating events related to intrusion of granitic plutons.

Regional geologic setting The Meguma Group underlies most of the Meguma Terrane and is represented by the Goldenville Formation and conformably overlying Halifax Formation. The Goldenville Formation consists of >5500 m of variably metamorphosed turbiditic sandstones and interbedded slates. A poorly preserved graptolite was recovered from the Goldenville Formation near Tangier, 100 km east of Halifax (Fig. 1) (Poole 1971). The Halifax Formation consists of a succession of variably metamorphosed slate with subordinate sandstone and siltstone. It ranges in thickness between 500 and 4400 m and contains the Early Ordovician graptolite Dictyonema jlabelliforme and Tremadocian acritarchs (Crosby 1962; Keppie 1977). Along the northwestern margin of the Meguma Terrane, the Meguma Group is overlain by a thick sequence (23004500 m) of Late Ordovician - Early Devonian largely sedimentary rocks of the White Rock, Kentville, New Canaan, and Torbrook formations (Fig. 1). All of these units are unconformably overlain by up to 3 km of Carboniferous sedimentary rocks in central portions of the Meguma Terrane. Carboniferous largely sedimentary rocks also are exposed along the northern margin of the Meguma Terrane. Projection of the sub-Carboniferous unconformity over areas where Carboniferous rocks have been removed by erosion suggests that it is located close to the present erosion surface. The northwestern margin of the Meguma Terrane is unconformably overlain by 1000 m of Mesozoic sedimentary and volcanic sequences. The Meguma Group and overlying Siluro-Devonian formations are deformed into upright, subhorizontal Dl folds that display a variably penetrative, axial planar, spaced cleavage. The cleavage is largely defined by spaced pressure-solution seams in metasandstone and metasiltstone units. In pelitic horizons the cleavage is generally more penetrative and defined, in part, by the preferred orientation of chlorite, muscovite, and (or) biotite (Taylor and Schiller 1966; Keppie 1976). This suggests that a regional, generally low-grade metamorphism (MI) accompanied Dl deformation. Numerous plutons crosscut Dl fabric elements. These are predominantly granodiorite and granite with subordinate quartz diorite and alaskite porphyry. The plutons are typically surrounded by variably developed, low-pressure contact metamorphic aureoles characterized by development of porphyroblastic chlorite, biotite, cordierite, staurolite, andalusite, and (or) sillimanite (Taylor and Schiller 1966; Keppie and Muecke 1979). These porphyroblasts typically overgrow D, fabric elements, although, in places, they appear to have developed syn-

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CAN. J. EARTH SCI. VOL. 24, 1987

TABLE2. Summary of published geochronological data from Meguma Group metasedimentary rocks and interpretation of the original authors Metamorphic grade by zone


Method

Chlorite

K -Ar biotite K -Ar muscovite (485 -506) 20 Ma (transport -deposition diagenesis -compaction dewatering - cleavage development) K- Ar (378-398) f 5 Ma whole-rock (Dl contact slate metamorphism)

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40Ad"Ar whole-rock slate

Biotite

Garnet

Andalusite-cordierite staurolite

344 Ma

390 Ma

(379 -393)

415 f 10 Ma (Dl metamorphism)

Low temperature 255 -342 Ma (no interpretation) High temperature 460-470 Ma (detrital component) Sm -Nd 1773 f 95 Ma Meguma Group (source age) metasediments

+ 6 Ma

Source Fairbairn et al. (1960) Wanless et al. (1972)

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Reynolds et al. (1973)

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Reynolds and Muecke (1978)

(368 -385) 8 Ma (contact metamorphism) 339 f 5 Ma (altered) (362-387) 9 Ma (contact metamorphism) Low temperature 225 -332 Ma (no interpretation)

Clarke and Halliday (1985)

NOTES:All pre-1977 ages have been recalculated using the constants of Steiger and Jager (1977).

Carboniferous

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Late Ordov~cian -Devonian rocks HALIFAX FORMATION OLDENVILLE FORMATION

FIG. 1. Simplified geological map of the Meguma Terrane, Nova Scotia. Areas of Figs. 2 -4 are indicated. Ticks are placed on high-grade side of isograds.

chronously with later deformational events (Sage 1984; Keppie et al. 1985). Contact metamorphism is widespread in southwestern Nova Scotia (Fig. 1) and was initially interpreted as a regional metamorphic event (e.g., Taylor and Schiller 1966).

However, recent detailed mapping in the northeastern Meguma Terrane (Keppie et al. 1985) and in the southern part of the present study area (Sage 1984; White 1984; Bourque 1985; Ross 1985; Wentzell 1985; Misner 1986; Raeside et al. 1985)


MUSCOVITE T O T A L -GAS AGE 3 1 2 . 6 ? 1.0

330


300

-

325.0

321.6

? 0.8

5

1.3

T O T A L - G A S AGE

BIOTITE

HORNBLENDE 100

450

T O T A L - G A S AGE 3 2 7 . 2 ? 0.7

do0

4 50

MUSCOVITE

'

T O T A L - G A S AGE 0.9 323.7

400

TOTAL-GAS

AGE

E

BIOTITE w 160

HORNBLENDE

T O T A L - GAS AGE

CUPuLnTlYE 5%

3'lr

RELCASEO

FIG.2. 40Ar/39Arincremental-release age spectra of hornblende, biotite, and muscovite from the Yarmouth area of the Meguma Terrane, Nova Scotia. Refer to Fig. 1 for area and map explanation. Abscissa coordinates are indicated at lower left. Analytical uncertainties (20, intralaboratory) are represented by vertical width of bars. Experimental temperatures increase from right to left. Plateau and total-gas ages are listed on each spectrum.

suggests that the isograds are spatially related to exposed pluton contacts. The Shelburne Pluton is locally discordant to some isograds, presumably as a result of magma movement during late stages of emplacement (Fig. 3). Superposition of contact aureoles results in complex textural relationships and zoned porphyroblasts; however, no maps presently exist showing these intersecting isograds. Therefore, the isograds shown in Figs. 1 -4 merely reflect curnmulative peak metamorphic assemblages. Detailed mapping of the isograds throughout poorly exposed interior portions of the Meguma Terrane has not been attempted, and in these areas only those isograds defined by reconnaissance mapping are available (Keppie and Muecke 1979). Contact metamorphic assemblages are inferred to have formed in bathozones 1-3 of Carmichael (1978) corresponding to depths of 5 - 12 krn. These studies also indicate that the Shelburne Pluton contact aureole formed at deeper crustal levels than that of the Barrington Passage Pluton (Sage 1984; Raeside et al. 1985). Variably penetrative ductile shear zones are locally developed throughout the Meguma Terrane. These affect both the granitic plutons and host metasedimentary units. The shear zones generally dip steeply and trend parallel to regional strike. Preliminary analysis of associated C -S fabric elements

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suggests that they all formed in dextral shear zones (Keppie et al. 1985). Although a portion of this strain occurred in the

Late Devonian (Keppie and Dallmeyer 1987), some is recorded in the 315 3 Ma Wedgeport Pluton (Keppie et al. 1983). In places, ductile shear zone fabrics are overgrown by undeformed contact metamorphic porphyroblasts of muscovite and (or) biotite; elsewhere, the micas are dynamically recrystallized.

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Analytical techniques The principles of 40Ar/39Arincremental-release dating have been described by Dalrymple and Lanphere (1971). In the present study, pure mineral concentrates ( >99%) were prepared from crushed and sized rock powders using heavy liquid and magnetic separation techniques. Concentrates 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. They received a total neutron dose of approx. 4 x 1018nV. Variations in the flux of neutrons along the length of the irradiation assembly were monitored with several mineral standards, including MMhb-1 (Alexander et al. 1978). Samples were incrementally heated


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FIG. 3. 40Ar/39Ar incremental-release age spectra of hornblende, biotite, and muscovite from the Banington area of the Meguma Terrane, Nova Scotia. See Fig. 1 for area and map explanation. Data plotted as in Fig. 2. All spectra have similar abscissa and ordinate coordinates.

until fusion with an RF induction generator. Each heating step was maintained for 30 rnin. Temperatures were controlled with an infrared sensing thermometer and are accurate to f25"C with internal monitoring to f 10°C. The crucible was cooled to room temperature between heating steps, and the evolved gas was purified with hot Cu -CuO and A1 - Zr getters. Each aliquot of gas was isotopically analyzed with a Niertype, 6 in. (152.4 rnrn) radius, 60" sector rare-gas mass spectrometer operated in a static mode. A total of eight sweeps across the 40-36 mass range was camed out for each increment through automatic switching of the magnet current. During each sweep, a 10 s integrated signal was measured with a digitizing voltmeter for each mass and associated base lines. This integration time and the operational parameters maintained for the ion source resulted in an average detection limit of approx. 7 x 10-l3 STP cm3 of argon. The digitized analytical data were-reduced with an on-line computer. Linear and (or) exponential extrapolations of peak heights to the time of gas introduction into the mass spectrometer were accomplished following statistical methods outlined by Higbie (1978). 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 of 20 in each apparent age are reported. These were calculated by rigorous, statistical propagation of possible variations in inlet-time extrapolations for each mass. Interlaboratory comparisons may be approximated by consideration of 0.25 -0.50% uncertainties in the appropriate "J-value." Total-gas ages have been computed for each sample by appropriate weighting of the age and calculated uncertainty of each temperature increment. A "plateau" is considered defined if the ages recorded by two or more contiguous gas fractions that together constitute more than 50% of the total gas released from a sample are mutually

similar within a f 1% intralaboratory uncertainty. Analyses of the MMhb-1 monitor indicate that apparent K/Ca ratios may be calculated through the relationship 0.518 ( f0.005) x (39Ar/37Ar)corrected.

Results Samples collected from various igneous and metasedimentary units exposed in southwestern portions of the Megurna Terrane were selected for 40Ar/39Aranalysis after detailed petrographic examination. Results of 40Ar/39Arincremental analysis of 4 hornblende, 12 muscovite, and 17 biotite concentrates are presented in Figs. 2-4. Analytical data are available from the Depository of Unpublished Data.' Hornblende White Rock Formation Porphyroblastic hornblende was separated from White Rock Formation garnet-zone metavolcanic rocks collected at locations 2B, 3B, and 4B (Fig. 2). In samples 3B and 4B, hornblende occurs as decussate porphyroblasts. In sample 2B, hornblende grains are aligned and define a foliation. Sample 4B records an internally discordant age spectrum with a totalgas age of 365.1 f 9.6 Ma (Fig. 2). Apparent K/Ca ratios display a significant and systematic decrease over the initial three gas increments evolved (575 -725°C). The ratio remains relatively constant in all subsequent gas fractions. Apparent ages recorded in the 575-725°C increments range between 353.2 f 10.4 and 358.8 10 Ma, whereas those defined by the subsequent seven increments (750 -865"C) are mutually consistent within 20 uncertainty at ca. 368 Ma. The three

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'Tables 3-5 may be purchased from the Depository of Unpublished Data, CISTI, National Research Council of Canada, Ottawa, Ont., Canada KIA OS2.

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DALLMEYER AND KEPPIE a00

00

YW

s10

100


a00

TOTAL-GAS AGE

-

IOU

100

4 M

0

I

S60

350

+ z

TOTAL-GAS

AGE

TOTAL -GAS AGE

TOTAL-GAS AGE

LL W

2 a00

so0

a a

340.5 z0.4

Y

339.2t0.4 t

329.7

+- 0.6

W

TOTAL -GAS AGE 337. 9 Z 1.6

zo

TOTAL-GAS AGE 33o.aTo.4

TOTAL -GAS AGE

so0

3 2 7 . 7 t 0.6

40

C U M U L A T I V E % 39 Ar RELEASED

FIG.4. 40Ar/39Arincremental-release age spectla of biotite and muscovite from the Liverpool area of the Meguma Terrane, Nova Scotia. Refer to Fig. 1 for area and map explanation. Data plotted as in Fig. 2. All spectla have similar abscissa and ordinate coordinates.

high-temperature increments yield erratic apparent ages. These contain relatively small quantities of gas with significant nonradiogenic components. Porphyroblastic hornblende from White Rock Formation metavolcanic rocks at location 3B (Fig. 2) displays a discordant spectrum defining a total-gas date of 413.7 f 8.6 Ma. The initial (600°C) fraction yields a 453.5 f 5.9 Ma date. The age decreases to 375.6 f 5.3 Ma in the subsequent increment (675°C). The 700 and 725°C increments contain relatively small quantities of gas, which result in large analytical uncertainties in the calculated ages (385.8 f 9.8 and 374.9 f 27.8 Ma, respectively). The next eight increments (750 -900°C) yield ages that are generally similar between ca. 395 and 405 Ma. Large analytical uncertainties preclude resolution of a meaningful plateau. An age of 426.5 4.3 Ma is recorded by the relatively large gas fraction evolved at 925°C. The 750°C-fusion increments display relatively constant KICa ratios that are markedly different from those of the first four temperature fractions (600 - 725 "C) . Hornblende from White Rock Formation metavolcanic rocks at location 2B (Fig. 2) records a total-gas age of 525 f 10.9 Ma and displays a very discordant spectrum characterized by younger apparent ages in intermediate temperature fractions. Except for the small gas fractions liberated at 600 and 675"C, relatively constant KICa ratios were observed throughout the analysis. Forbes Point metadionte Massive metadiorite exposed at Forbes Point passes gradationally into blastomylonitic rocks displaying a foliation

defined in part by aligned hornblende. Large, randomly oriented crystals of hornblende from the massive part record a slightly discordant spectrum (sample 6, Fig. 3). An anomalously old date of 1083.9 f 23 Ma is defined by the initial (550°C) gas fraction. Ages between 356 f 10.3 and 373.9 f 10.2 Ma are recorded by the relatively small gas fractions liberated in the next three increments (625-750°C). The remainder of the increments (800°C-fusion) constitute >90 % of the gas liberated from the sample and yield a plateau age of 388.6 f 4.8 Ma. Apparent KICa ratios are relatively constant in the plateau increments. These are distinctly different from those displayed by the relatively small gas fractions liberated in the four low-temperature fractions (550-750°C). Muscovite Igneous lithologies Dynamically recrystallized muscovite within blastomylonitic granitic rocks from a dextral shear zone developed in the Brenton Pluton (sample lA, Fig. 2) displays a slightly discordant age spectrum defining a total-gas age of 321.2 f 1.3 Ma. The initial, 525°C increment records an age of 305.6 f1 Ma, whereas the subsequent four increments define a plateau age of 321.6 f 1.3 Ma. Similar KICa ratios are displayed by both the plateau and 525°C gas fractions. The fusion increment yields an age of 325.5 f 1.3 Ma and records a KICa ratio slightly different from that of the plateau increments. Muscovite within undeformed granite at location 13 (Fig. 3) of the Shelburne Pluton displays a plateau age of 324.3 f 0.9 Ma. All increments evolved display similar KICa ratios. The initial (500°C) increment yields a 287.7 f 4.3 Ma age.


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Metasedimentary rocks Muscovite concentrates from metasedimentary rocks at 10 locations display variably discordant 40Ar/39Arspectra and a wide range (287-356 Ma) in total-gas or plateau ages (Figs. 2-4). Dynamically recrystallized muscovite was separated from blastomylonitic metasedimentary rocks within the garnet zone in a dextral shear zone that also deforms the immediately adjacent Brenton Pluton (sample lB, Fig. 2). The concentrate displays a slightly discordant spectrum defining a total-gas age of 323.7 f 0.9 Ma. The five lower temperature increments represent > 93 % of the total gas evolved, and these record a plateau age of 323.5 f 0.9 Ma. No systematic differences in KICa ratios are detectable in any of the gas fractions. Sample 5 (Fig. 3) was collected adjacent to the Barrington Passage Pluton within a ductile shear zone developed in the Halifax Formation at andalusite - staurolite -cordierite metamorphic grade. In this sample, muscovite is aligned parallel to a foliation that both wraps around and occurs as S-shaped trails within porphyroblasts. This concentrate displays a slightly discordant spectrum with a total-gas age of 333.7 f 0.6 Ma. The lowest temperature fraction (575°C) yields an age of 319.9 f 0.9 Ma. The subsequent four increments (625 -825"C) record ages between 329.5 f 0.4 and 336.3 f 0.7 Ma. The fusion increment is slightly older (343.3 f 0.6 Ma) and characterized by a KICa ratio slightly different from that of the lower temperature increments. Sample 8 (Fig. 3) is a polydeformed gneiss located in the sillimanite zone south of the Barrington Passage Pluton. Muscovite occurs as large decussate crystals that have been variously deformed in discrete shear zones. Muscovite concentrated from sample 8 displays an internally discordant age spectrum with a total-gas age of 286.9 1.1 Ma. The initial (500°C) fraction records an age of 271.5 f 1.4 Ma. The subsequent five increments (625 - 900°C) constitute >90 % of the total gas evolved and define a plateau age of 286.9 $ 1.1 Ma. A slightly older age of 292.9 1.1 Ma is recorded by the fusion increments. There are no differences in KICa ratios between any of the temperature fractions. Sample 9 (Fig. 3) was collected from the Goldenville Formation in the andalusite - staurolite-cordierite zone south of the Shelburne and Barrington Passage plutons. Muscovite occurs as small, randomly oriented crystals that have been deformed in discrete, spaced shear zones. It displays an internally discordant spectrum defining a total-gas age of 320.8 f 0.9 Ma. The 675-825°C increments evolved from sample 9 yield a plateau age of 317.8 f 0.8 Ma. The two low-temperature increments (500 and 600°C) yield younger ages of 252.2 f 1.3 and 290.8 f 1.6 Ma. The two higher temperature fractions (875°C and fusion) yield markedly older dates of 336.5 f 1 and 339.2 f 1.7 Ma. Apparent KICa ratios are similar for the 500 - 825"C increments but are slightly lower in the 875" and fusion increments. Sample 10 (Fig. 3) was collected from the Goldenville Formation in the sillimanite zone immediately south of the Shelburne Pluton. Muscovite occurs both as large, slightly strained, randomly oriented crystals and as finer grained crystals within and along the edges of staurolite porphyroblasts. It displays an internally discordant spectrum defining a total-gas age of 340.5 f 1.2 Ma. The initial (500°C) fraction yields a 303.1 f 2.4 Ma age, whereas an older age of 326.8 f 1.2 Ma is defined by the 575°C fraction. The remaining five increments (625"C -fusion) define a plateau age of 342.1 f

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1.2 Ma. No consistent differences in K/Ca ratios are detectable between the various temperature increments. Sample 11 (Fig. 4) is from the Goldenville Formation in the andalusitestaurolite-cordierite zone south of the Shelburne Pluton. It contains small crystals of muscovite aligned in the foliation and larger, variably oriented muscovite crystals developed around andalusite porphyroblasts. A concentrate of coarse muscovite displays a slightly discordant spectrum defining a total-gas age of 350.9 f 0.8 Ma. The initial (500°C) increment of sample 11 records an age of 287 f 0.5 Ma, whereas the 600°C fraction yields a slightly older date of 333.2 0.9 Ma. The remaining five increments (675°Cfusion) constitute > 95 % of the gas liberated from the concentrate and define ages that are generally similar (between 347.5 f 0.5 and 356 f 1.3 Ma). No significant or systematic differences in KICa ratios are detectable between the various gas fractions. Sample 12 (Fig. 4) from the Goldenville Formation in the garnet zone south of the Shelburne Pluton contains muscovite aligned parallel to a foliation. It displays a variably discordant spectrum defining a total-gas age of 358.1 1.2 Ma. The initial, low-temperature fraction (575°C) records an age of 331.9 f 1.4 Ma, whereas the subsequent three increments (650-775°C) yield ages between 352.4 f 1.1 and 354.3 f 1.4 Ma. Slightly older dates of 362.4 f 1.4 and 367.3 f 0.9 Ma are defined by the next two increments (825 and 900°C), and the fusion increment yields a date of 370.5 f 1.7 Ma. Apparent KICa ratios for the 575 -900°C increments show no obvious variations. A slightly lower KICa ratio is recorded in the fusion increment. Sample 15B (Fig. 3) from the Goldenville Formation in the andalusitestaurolite-cordierite zone north of the Shelburne Pluton contains large, decussate muscovite crystals. These record a plateau age of 346.9 f 0.9 Ma and display relatively constant KICa ratios. A slightly discordant spectrum is recorded by both the initial, low-temperature (500°C) and fusion increments. Whereas the 500°C increment has a KICa ratio similar to that of the plateau increments, the fusion increment has a slightly lower KICa ratio. Most muscovite from sample 17 (Fig. 4) that was collected from the Goldenville Formation in the andalusite-staurolite -cordierite zone adjacent to the Port Mouton Pluton occurs as small crystals aligned in a foliation. A subordinate number of slightly larger crystals are randomly oriented. A muscovite concentrate from sample 17 displays a complex release spectrum in which the initial four temperature increments (575-700°C) define a plateau age of 353 f 0.7 Ma and record similar KICa ratios. The 825°C increment has a similar KICa ratio but records an older date (367.4 f 0.7 Ma). The 900°C and fusion increments are characterized by markedly lower KICa ratios and yield older ages of 396.3 f 1 and 389.1 f 2.8 Ma. Muscovite from sample 18 (Fig. 4) in the Goldenville Formation in the garnet zone immediately northeast of the Port Mouton Pluton occurs both aligned generally parallel to a foliation and as pseudomorphs replacing andalusite. It records a slightly discordant age spectrum with a total-gas age of 354.6 f 0.8 Ma. The initial low-temperature gas fraction (575°C) yields an age of 314 f 1.8. This is younger than a 355.6 + 0.8 Ma plateau date defined by the remaining gas fractions (650°C -fusion). No significant or consistent differences are observable in KICa ratios defined for the various fractions.

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Biotite Biotite concentrates from 5 igneous and 12 metasedimentary samples have been analyzed by 40Ar/39Arincremental-release techniques. The 37Ar/39Arcorrected ratio in all analyses is 89 % of the total gas evolved from the sample). Biotite within metadiorite at Forbes Point (sample 6, Fig. 3) occurs as large, randomly oriented crystals. It records an internally discordant spectrum with a total-gas age of 316.2 f 0.6 Ma. The 600°C -fusion increments constitute >93% of the total volume of gas liberated, and these yield a plateau age of 318.7 f 0.6 Ma. The initial, low-temperature increments (475 and 525°C) record anomalously younger ages of 143.7 f 2.5 and 294.9 f 0.4 Ma. Dynamically recrystallized biotite is aligned generally parallel to the foliation developed in protomylonitic granite within the Banington Passage Pluton (sample 7, Fig. 3). It displays a slightly discordant spectrum defining a total-gas age of 311.9 f 0.5 Ma. A plateau age of 312.7 f 0.4 Ma is recorded by the 600- 825"C temperature fractions ( > 85 % of the total gas evolved). Younger ages of 203 f 5.4 and 308.6 f 0.4 Ma are defined by the two initial, low-temperature fractions (475 and 525°C). A slightly older date of 318.3 f 1.4 Ma is recorded by the fusion increment. Biotite was separated from undeformed granite within the Shelburne Pluton (sample 13, Fig. 3). The low-temperature portion of the spectrum is poorly resolved, with 13% of the total gas evolved at 485°C. This increment records a 236.2 0.4 Ma date, which is markedly younger than a 308.6 f 0.5 Ma plateau date defined by all subsequent temperature fractions (550°C -fusion).

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Metasedimentary rocks Biotite concentrates from metasedimentary rocks at 12 locations display variably discordant age spectra and show a wide range in plateau and total-gas ages (274-347 Ma) (Figs. 2-4). Dynamically recrystallized biotite from blastomylonitic White Rock Formation metasedimentary rocks within a dextral shear zone also affecting the immediately adjacent Brenton Pluton (sample 1B, Fig. 2) is characterized by a slightly discordant spectrum. A date of 240.7 f 2.6 Ma is recorded in the

485 "C increment. All subsequent increments constitute > 97% of the total gas evolved from the concentrate, and these define a plateau age of 329.2 f 0.7 Ma. Biotite from sample 5 (Fig. 3) within a shear zone developed in the Halifax Formation in the andalusite - staurolite-cordierite zone (immediately west of the Barrington Passage to the Pluton) occurs both as small crystals lying mylonitic foliation and as decussate porphyroblasts overgrowing the foliation. The porphyroblasts have been deformed and rotated by shear deformation. The porphyroblasts display a slightly discordant spectrum with an anomalously young initial, low-temperature increment of 245.7 f 3.3 Ma, an intermediate plateau age of 349.5 f 0.7 Ma (>93% of the gas evolved), and a fusion increment of 358.7 f 2.8 Ma. Biotite in polydeformed gneiss within the sillimanite zone (sample 8, Fig. 3) immediately south of the Barrington passage Pluton bccurs both as small crystals aligned in a foliation and as large, decussate crystals that are deformed in discrete shear zones. It records a concordant spectrum except for the initial, low-temperature increment defining an age of 203.4 f 1.9 Ma. The remainder of the gas evolved (98.4%) defines a plateau age of 274 f 1.0 Ma. Biotite in samples 11, 14, and 15B from the andalusitestaurolite-cordierite zone and sample 12 from the garnet zone around the Shelburne Pluton (Figs. 3 and 4) occur both as small crystals aligned in a foliation and as large, decussate crystals overgrowing the foliation. The larger crystals in samples 11 and 12 yielded slightly discordant spectra with 0.4 Ma, respectotal-gas ages of 337.9 f 1.6 and 330.8 tively. Both show anomalously young initial, low-temperature increments of 172.9 1.5 and 105 f 1.5 Ma, respectively. In sample 11, progressively higher temperature increments yielded systematically older apparent ages ranging from 335 f 1.6 to 350 f 2 Ma. The five highest temperature increments correspond to a plateau age of 348.3 f 1.4 Ma. A similar spectrum in sample 12 corresponds to a plateau age of 340.5 f 0.4 Ma for 75 % of the gas evolved in the higher temperature increments (excluding the fusion increment, which records a 349.9 f 1.3 Ma age). Spectra discordance is not significant in the larger biotite from samples 14 and 15B. In both, only the initial, lowest temperature gas fractions yield anomalously young ages compared with plateau dates recorded by >95 % of the gas-liberated sample (350 f 1.2 and 341.1 f 0.9 Ma, respectively). In samples 16 and 17 (Fig. 4) within the andalusite - staurolite-cordierite zone adiacent to the Port Mouton Pluton. biotite occurs both as s m i l crystals parallel to a foliation and as larger, decussate crystals. They record internally discordant spectra with total-gas ages of 327.7 k 0.5 and 335.7 f 0.5 Ma, respectively. Anomalously younger ages are recorded in initial, low-temperature increments. Subsequent increments in sample 16 display increasingly older apparent ages, which range from 320 f 0.5 to 339.8 f 1.8 Ma, with a plateau age of 330 f 0.4 Ma for 85 % of the gas evolved. sample i 7 displays a plateau age of 339.2 f 0.4 Ma for 90% of the gas evolved. The fusion increment defines an older age (352.6 f 2.2 Ma). Northeast of the Port Mouton Pluton, samples 18 and 19 from the garnet zone (Fig. 4) contain biotite aligned parallel to a foliation, whereas in sample 20 from the biotite zone, biotite occurs as randomly oriented grains. Internally discordant age spectra characterize these three biotite concentrates, which record total-gas ages of 327.7 f 0.6,346.5 f 1.2, and 342 f

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1250


1.2 Ma (samples 18, 19, and 20 respectively). Anomalously young dates were recorded in the low-temperature gas fractions evolved from each concentrate. In sample 18 the 500°C fraction records a 306.3 f 0.5 Ma age, whereas the subsequent five increments (550-865°C) define a plateau age of 329.7 f 0.6 Ma. In sample 19, apparent ages range between 342.5 f 1.3 and 359.9 f 1.5 Ma, excluding the initial, lowtemperature increment. The second (525°C) increment of sample 20 yields a slightly older date of 329.8 f 1 Ma, whereas the subsequent four increments (600 -825"C) yield a plateau date of 345 f 1.2 Ma (corresponding to > 86% of the total gas evolved). The fusion increment yields a slightly older date of 358 f 1.5 Ma.

Interpretation 40Ar/39Armineral plateau ages may record times of cooling through temperatures appropriate for intracrystalline retention of argon. These temperatures appear to be different for various mineral species. Hamson (1981) reported that argon retention occurs at approx. 500 f 25°C in hornblende at the cooling rates likely to be encountered in most geologic settings. Intracrystalline variations in composition appear to have little effect on hornblende closure temperatures. On the other hand, Hanison et al. (1985) demonstrated that the temperatures required for intracrystalline retention of argon in biotite are strongly dependent upon composition and increase with decreasing Fe/Mg ratio. They suggested that closure occurs between approx. 275 and 325°C for the range of cooling rates likely to be encountered in most geologic settings. Detailed experimental evaluation of the temperatures required for intracrystalline retention of argon in muscovite has not been carried out. Use of the preliminary experimental data of Robins (1972) in the diffusion equation of Dodson (1973) suggests that temperatures of 350-400°C may be appropriate. For the simple case of a single thermal event, similar or progressively younger plateau ages should be recorded by hornblende, muscovite, and biotite, depending upon the rates of cooling. These types of mineral age variations are generally observed in the data presented herein, except for samples lA, lB, and 5, where biotite records an older age than coexisting muscovite. These relationships suggest that the biotite concentrates may be contaminated with small components of extraneous (excess) 40Ar. These, together with radiogenic 40Ar, must have evolved at a nearly constant ratio in each of the incremental heating experiments in order to produce the nearly concordant spectra displayed by the biotite concentrates. The biotite plateau ages for samples lA, lB, and 5 are therefore considered of no geological significance. Similar inabilities to separate extraneous and radiogenic 40Arthrough incremental-release40Ar/39Aranalysis of biotite have been previously documented (e.g., Pankhurst et al. 1973; Roddick et al. 1980; Dallmeyer and Rivers 1983; Foland 1983). More complex intracrystalline argon systems are likely to be encountered in areas affected by superimposed metamorphic events. Thermal overprinting variably affects intracrystdline argon systems, depending upon the maximum temperatures attained. Incomplete resetting results in development of intracrystalline concentration gradients in 40Ar,which, potentially, could be recognized by incremental-release 40Ar/39Aranalysis. However, interpretation of discordant biotite release spectra is uncertain because several workers have suggested that the biotite lattice may be unstable during in vacuo heating as a result of dehydrogenation and resultant lattice delamination

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(e.g., Gerling et al. 1966; Zimmerman 1972; Harrison and McDougall 1981). Existing intracrystalline 40Ar gradients could thus be removed because of lattice rearrangement at low experimental temperatures and well-defined but meaningless 40Ar/39Arincremental-release plateaux observed. This is not consistent with Berger (1975) and Dallmeyer (1975, 1982), who reported internally discordant "saddle-shaped" spectra for thermally overprinted biotites in several different, progressively remetamorphosed areas. The observed spectra discordance indicates that intracrystalline gradients in 40Arwere maintained during experimental heating. These contrasting results suggest that the experimental behavior of biotite is inconsistent and that direct interpretation of 40Ar/39Arage spectra in the absence of supportive geochronological data is not possible. Therefore, no geological implications may be confidently affixed to the low-temperature discordance displayed by many of the biotite age spectra reported herein. The ability of muscovite to maintain intracrystalline concentration gradients in 40Ar during in vacuo heating has been demonstrated elsewhere (e.g., Hamson and McDougall 1981), and the low-temperature discordance displayed in many of the muscovite age spectra in the southwestern Meguma Terrane are therefore interpreted as reflecting locally superimposed metamorphic histories. This reheating could have resulted from either a tectonic or sedimentary burial, in which case it would likely have been of more regional extent. This is not consistent with the rapid and regionally unsystematic variations in muscovite plateau ages in the southwestern Meguma Terrane (Figs. 2 -4). In addition, inferred depths of contact aureole formation are < 12 km, and the sub-carboniferous unconformity projects close to the present erosion surface in the southwestern Meguma Terrane (Carboniferous rocks reaching up to -3 km thickness in basins on the Meguma Terrane). At a geothermal gradient of 25"C/km (considered a maximum for a sedimentaq sequence), these depths could not have produced burial temperatures sufficient to affect intracrystalline argon systems in muscovite. Alternatively, the locally complex thermal record could have been produced by contact met&orphism during emplacement of pGtons within the Meguma Terrane. This interpretation is supported by recent mapping that clearly demonstrates a spatial relationship between most of the metamorphic isograds and exposed pluton contacts (e.g., Keppie et al. 1985; Sage 1984; White 1984; Bourque 1985; Raeside et al. 1985). The 40Ar/39Arisotopic results are therefore discussed in the following sections in the context of their spatial distribution relative to proximal plutons. The possibility that shear heating could have affected mineral argon systems in the shear zones is considered unlikely because (1) it would be insignificant in the ductile strain regime under which muscovite, biotite, and quartz dynamically recrystallize and (2) younger apparent ages are not limited to shear zones. The latter observation also eliminates the possibility that the argon systems were affected by hot fluids moving within and limited to the shear zones. Port Mouton area

A Rb-Sr whole-rock isochron date of 342 f 15 Ma from the Port Mouton Pluton has been interpreted as reflecting the crystallization age (Cornier and Smith 1973; recalculated by Keppie and Smith 1978). Decussate muscovite in the immediate contact zone of the pluton records 40Ar/39Arplateau ages of 355.6 f 0.8 (sample 18) and 353 f 0.7 Ma (sample 17). Within error limits, these are indistinguishable from the Rb-Sr age, and the muscovite porphyroblasts are interpreted

125 1



as having grown during pluton emplacement. Biotites within samples 18 and 19 record plateau ages approximately 14-26 Ma younger than coexisting muscovite. These are interpreted as dating postcontact metamorphic cooling through the slightly lower temperatures required for intracrystalline retention of argon in biotite. Ages of ca. 345 Ma were recorded by biotite in samples 19 and 20 collected approximately 20 -50 km northeast of the pluton. The ca. 367 Ma ages defined in high-temperature portions of the muscovite spectrum from sample 12 may be a remnant of an earlier thermal event of the same age as the South Mountain Batholith (Table 1). A generally similar high-temperature age is seen in the muscovite from sample 17. The 390-395 Ma highest temperature ages recorded in this sample may be a relict of the ca. 400 Ma cleavage-forming event recorded in slates to the northeast (Reynolds and Muecke 1978). Shelbume Pluton area 40Ar/39Arresults in the vicinity of the Shelburne Pluton are difficult to interpret because crystallization ages of the various intrusive phases are not known. Muscovite and biotite from an undeformed sample of the pluton (sample 13) yield plateau ages of 324.4 f 0.9 and 308.6 f 0.5 Ma, respectively. These are markedly younger than the 350.9 f 0.8, 346.9 f 0.9, and 342.1 1.2 Ma plateau ages recorded by muscovite in the surrounding metamorphic terrrain (andalusite-cordierite staurolite and sillimanite zones). Coexisting biotite yields more discordant spectra with plateaux about 3 -6 Ma younger than muscovite. These data indicate that cooling through temperatures required for intracrystalline retention of argon in muscovite and biotite occurred moderately rapidly over a wide area in the vicinity of the Shelburne Pluton. However, it is unlikely that the younger mica ages in the pluton reflect more prolonged post-magmatic cooling with respect to the cooling of contact aureole minerals. A more reasonable interpretation is that they reflect a separate intrusive event of limited extent and therefore did not rejuvenate mica systems in more distal areas. This may account for the local crosscutting of isograds by the pluton boundary. More complete understanding of the mica ages is not possible until the intrusive relationships are more clearly defined.

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Cape Sable Island area Muscovite and biotite from sillimanite gneiss at location 8 yield plateau ages of 286.9 f 1.1 and 274 f 1 Ma, respectively. These are interpreted as dates of cooling through the contrasting temperatures required for argon retention in the two mica phases. The ages are considerably younger than those recorded near the Shelburne Pluton, and they are inferred to record the thermal effects associated with emplacement of an unexposed pluton. This conclusion is supported by 40Ar/39Arresults from sample 9, collected midway between sample 8 and the Shelburne Pluton. Muscovite from location 9 displays an internally discordant spectrum consisting of three distinct components. The high-temperature increment yields a date of 339.2 f 1.7 Ma, which is similar to those recorded by muscovite around the Shelburne Pluton. Intermediate-temperature fractions correspond to a 317.8 f 0.8 Ma plateau age, which is younger than the ca. 324 Ma date recorded by muscovite in the Shelburne Pluton. Lower temperature gas increments evolved from muscovite in sample 9 yield markedly younger dates. The pattern of spectra discordance displayed by muscovite in sample 9 suggests that geological reheating at ca. 320 Ma effected considerable loss of radiogenic argon from

muscovite grains, which had earlier cooled through argonretention temperatures. The marked low-temperature discordance suggests that another, less intense metamorphic overprint effected minor argon loss sometime after ca. 320 Ma. This same thermal event may also be responsible for the anomalously young, low-temperature age discordance displayed by muscovite in samples 13 and 11. Westem Barrington Passage Pluton area Hornblende from metadiorite at Forbes Point (location 6) displays an intermediate- and high-temperature plateau age of 388.6 f 4.8 Ma. This is interpreted as dating initial postmetamorphic cooling through temperatures required for intracrystalline retention of radiogenic argon. The low-temperature discordance suggests that a subsequent, less intense reheating effected minor argon loss resulting in the establishment of a slight intracrystalline gradient in 40Ar. This was followed by introduction of minor extraneous argon components through inward diffusion from an intergranular phase. These components appear to reside in relatively low energy sites because they were experimentally evolved at low temperatures. A reheating ageyounger than ca. 356 Ma is suggested by the character of the low-temperature discordance in the hornblende spectrum. This is in harmony with a 3 18.7 f 0.6 Ma plateau date recorded by biotite within the metadiorite. ~ o ~ e t h ethe r, hornblende and biotite results suggest that initial postmetamorphic cooling through 500°C at ca. 389 Ma was followed by a later reheating, which effected total rejuvenation of the biotite argon system but only minor argon loss in hornblende. This indicates that temperatures maintained during reheating were in excess of 350°C and below 500°C. The relationship between reheating and emplacement of the Bamngton Passage Pluton is uncertain. The plateau age of 312.7 f 0.4 Ma recorded by biotite from the Bamngton Passage Pluton (sample 7) is similar to the metadiorite biotite plateau age. However, muscovite from the contact aureole west of the Bamngton Passage Pluton (sample 5) yields a plateau age of 333.7 f 0.6 Ma. This may more closely approximate the age of intrusion of the Bamngton Passage Pluton than the younger biotite age because moderately rapid cooling might be predicted for this contact aureole formed at -5-8 km depth (Sage 1984). A younger, low-temperature discordance (down to ca. 320 Ma) appears to be recorded in the muscovite spectrum. This could reflect a later sub-400°C reheating effecting only minor argon loss. Thus it is possible that this ca. 320 Ma reheating was the cause of total rejuvenation of the biotite argon systems in samples 6 and 7.

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Yarrnouth area White Rock F o m t i o n Hornblende from metavolcanic rocks of the White Rock Formation (samples 2B, 3B, and 4B) displays no optical evidence of either compositional zoning or exsolution. However, variations in apparent KICa ratios suggest that two distinct compositional phases were preferentially outgassed in lowversus high-temperature portions of the experiment. The saddle-shaped spectrum of sample 2B is typical of hornblende contaminated with extraneous argon components situated in intracrystalline sites with both low and high activation-energy characteristics (e.g., Hamson and McDougall 1981; Dallmeyer et al. 1985). A similar interpretation is considered reasonable for the present results and is supported by the inconsistency of a record of >460 Ma ages in Late Ordovician - Silurian rocks. This suggests that most of, if not all, the gas


1252


fractions are contaminated with excess argon. Systematic variations in apparent K/Ca ratios suggest that two distinct compositional phases were preferentially outgassed in lowversus high-temperature portions of the analysis of hornblende from sample 4B. The nature of spectra discordance for the hornblende concentrate may therefore be interpreted in two contrasting ways: (1) a reflection of relatively slow cooling through contrasting argon-retention temperatures for differing hornblende compositional phases following a pre370 Ma metamorphism or (2) a reflection of relatively rapid cooling following a pre-370 Ma metamorphism that was followed by a distinctly later reheating that effected minor argon loss from the slightly less retentive compositional phase. The latter interpretation is favored because biotite (sample 4C) within intercalated felsic metavolcanic rocks at the same location displays a plateau age of 32 l . l k l . l Ma. The biotite age is interpreted as dating cooling through 300°C following the later reheating, which was of insufficient intensity to completely rejuvenate intracrystalline argon systems within hornblende (i.e., below -500 f 25°C; Harrison 1981). The reheating may have been related to intrusion of the nearby Wedgeport Pluton, which records a crystallization age of ca. 315 Ma (Table 1). Variations in apparent K/Ca ratios suggest compositional variations in the hornblende concentrate prepared from sample 3B. These, combined with poor spectra resolution at high temperatures, make interpretation of hornblende results from sample 3B difficult.

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Brenton Pluton and contact rocks Dynamically recrystallized biotite and muscovite within a blastomylonitic shear zone developed in the Brenton granite yield plateau ages of 325 f 0.8 and 321.6 f 1.3 Ma, respectively (sample 1A). Dynamically recrystallized biotite and muscovite within immediately adjacent metasedimentary rocks affected by the same shear zone (sample 1B) yield plateau 0.7 and 323.5 f 0.9 Ma. The muscovite dates of 329.2 ages are similar within analytical uncertainties and are interpreted as dating cooling through -350°C. A slight intracrystalline gradient in 40Ar may be reflected by the low-temperature discordance observed in the 1A muscovite spectrum and could therefore reflect minor effects of a subsequent thermal overprint. The slightly older plateau ages recorded by coexisting biotite likely reflect excess argon contamination. These ages are markedly younger than the 450 f 5 Ma Rb-Sr whole-rock isochron date reported for the Brenton Pluton (Keppie et al. 1983). This discordancy is attributed to postcrystallization reheating rather than to slow postmagmatic cooling because, as discussed earlier, the present erosional level must have been very nearly exhumed by Middle-Late Devonian. The later reheating may have been effected by intrusion of the Wedgeport Pluton at ca. 315 Ma.

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Conclusions Within central and northeastern portions of the Meguma Terrane, regional Dl folding with associated cleavage formation occurred under MI greenschist - lower amphibolite facies metamorphic conditions at ca. 385 -410 Ma (e.g., Reynolds and Muecke 1978; Dallmeyer and Keppie 1984; Keppie and Dallmeyer 1987). This was followed by a low-pressure, M2 metamorphism of variable grade that developed at different times in response to the widespread emplacement of granitic plutons between ca. 360 and 373 Ma. Biotite and muscovite

from both the plutons and contact metamorphic rocks also record 360 -373 Ma 40Ar/39Arplateau dates and suggest relatively rapid post-M, cooling. In the present study area, the regional distribution of metamorphic assemblages generally shows a clear spatial relationship to granitic stocks, and it is likely that a generally similar thermal evolution characterizes this portion of the Meguma Terrane. Rb-Sr whole-rock isochron ages suggest emplacement of granitic plutons occurred in this area down to ca. 315 Ma (Wedgeport Pluton), indicating a prolonged metamorphic evolution, which probably involved multiple and locally superimposed thermal events. The earliest thermal event observed in the present study is a pre-389 Ma thermal event at temperatures greater than 500°C (hornblende, sample 6). This hornblende lies within a foliation, suggesting that this event occurred during deformation and was possibly coeval with a ca. 385 -415 Ma greenschist facies deformational event recorded elsewhere in the Meguma Terrane. It is possible that intrusion of the Forbes Point metadiorite accompanied this deformation. The ca. 396 Ma high-temperature components in the sample 17 muscovite spectrum may be a remnant of this early thermal event. Interpretation of all younger ages is based on the conclusion that regional temperatures at the present erosion level were likely maintained below 300°C from the Late Devonian to the Permian. This is considered justified because (1) the present erosion level is close to the exhumed pre-Carboniferous topographic surface, and Carboniferous sequences reach only 3 km in thickness in basins on the Meguma Terrane; (2) 375 - 360 Ma 40Ar/39Ar(Late Devonian) muscovite ages occur throughout the Meguma Terrane; and (3) depths inferred for the formation of contact aureoles are less than 12 km (i.e., < 300°C for a geothermal gradient of 25"C/km). Thus, all younger 40Ar/39Armineral ages are considered spatially and temporally related to thermal events associated with the emplacement of granitic plutons. Contact metamorphic assemblages indicate that the plutons were emplaced at intermediate crustal levels (-5 - 12 km). This is compatible with their medium- to coarse-grained textures, the local presence of vugs, and the coarse-grained nature of contact metamorphic assemblages. Temperatures attained locally in contact aureoles totally rejuvenated biotite and muscovite argon systems, which subsequently cooled through their contrasting argon-retention temperatures (e.g., sample 8). In other instances, biotite was totally reset and muscovite partially reset (e.g., sample 12). Thermal effects associated with emplacement of the South Mountain Batholith at 360 -372 Ma are limited in the present data because most samples were collected a considerable distance from the pluton. They are, however, suggested in the hornblende spectrum from sample 4B, which yielded ca. 368 Ma intermediate-temperatureages. This event may also be recorded in the low-temperature discordant increments of hornblende sample 3B (down to ca. 375 Ma). It might also be recorded in the ca. 367 Ma high-temperature portion of the sample 12 muscovite spectrum. Intrusion of the South Mountain Batholith was followed by emplacement of the Port Mouton Pluton at 342 f 15 Ma (Table 1). Porphyroblastic muscovite within the contact aureole records plateau ages of 353 -356 Ma. These results are similar within analytical uncertainty and therefore suggest relatively rapid postmagmatic cooling. Similar 342 - 352 Ma muscovite plateau ages are seen along the northeastern contact of the Shelburne Pluton and suggest this area records the

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effects of emplacement of a pluton of similar or slightly younger age. Farther southwest, the ca. 334 Ma muscovite total-gas age at Pubnico (sample 5) may be related to a distinctly younger thermal event. Plateau ages of coexisting biotite are 3-25 Ma younger than muscovite and suggest variable rates of cooling. In the Yarmouth area a thermal event at ca. 322 - 324 Ma is recorded within and adjacent to the Brenton Pluton. It may also be responsible for the ca. 321 Ma biotite plateau age recorded at Sunday Point in White Rock Formation metavolcanic rocks (sample 4C). These ages may relate to intrusion of the nearby Wedgeport Pluton at ca. 315 Ma. Similar 318 -325 Ma muscovite plateau ages occur within and south of the Shelburne Pluton, and biotite plateau ages of 313-319 Ma occur immediately west of the Banington Passage Pluton. These may relate to a thermal event associated with emplacement of another pluton at this time. The 287 and 274 Ma muscovite and biotite plateau ages observed on Cape Sable Island appear to record a ca. 290 Ma thermal event. This may be responsible for the anomalously young low-temperature age discordance observed in muscovite spectra northeast of Shelbume and for the ca. 309 Ma biotite plateau age in the Shelburne Pluton. This event is postulated as being associated with emplacement of a pluton, reflected by undeformed granitic pegmatites in Cape Sable Island and by a large, low, offshore gravity anomaly (Haworth et al. 1980). In summary, thermal events related to plutonism appear to be recorded in the southwestern Meguma Terrane at ca. 450, 360-375, ca. 356, ca. 315-325, andca. 290 Ma. The 295 k 3 Ma 40Ar/39Arplateau age reported on muscovite associated with mineralization in the East Kemptville tin deposit (Zentilli and Reynolds 1985) may reflect hydrothermal fluids possibly related to plutonism of the latter time rising along a shear zone. If correct, the younger plutons are an attractive exploration target for further mineralization. Dextral shear deformation is associated with all of these thermal events. This suggests that the Meguma Terrane was continually under similar stresses through the Late Devonian - Permian. It is likely that shear deformation was localized in areas where increased temperature led to a decreased viscosity. This deformation was likely related to the progressive westward accretionary oblique obduction of the Meguma Terrane onto the Avalon Composite Terrane (Keppie 1982, 1985). In this connection, the emerging relationship between significant Sn -W - U mineralization and the younger thermal events may provide a temporal exploration guide, while the shear zones may have provided a spatial control on their location (e.g., the East Kemptville deposit lies in a shear zone; Fig. 1). Thus, determination of the location of such young plutons and late shear zones has potential economic importance. In contrast, the 390-415 Ma Dl-MI tectonothermal event appears to have been of regional extent and associated with deformation and regional greenschist ( - lower amphibolite) facies regional metamorphism during the early Devonian. Final resolution of the specific local details of the tectonothermal chronology awaits determination of crystallization ages for all the plutons in the area and a more systematic determination of 40Ar/39Armineral ages. This work is currently underway throughout the Meguma Terrane.

Acknowledgments Funds for this project were provided by the Nova Scotia Pro-

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vincial Government as part of a joint Federal-Provincial Mineral Development Agreement, 1984- 1989. Permission of the Director of the Mineral Resources Division of the Nova Scotia Department of Mines and Energy is duly acknowledged. S. Saunders, B. MacDonald, and R. Jones are thanked for typing the manuscript, and we are indebted to the Nova Scotia Department of Mines and Energy drafting staff for preparing the illustrations. ALEXANDER, E. C., JR., MICHELSON, G. M., and LANPHERE, M. A. 1978. MMhb-1: a new 40Ar139Ardating standard. In Short papers of the Fourth International Conference, Geochronology, Cosmochronology, Isotope Geology. Edited by R. E. Zartman. United States Geological Survey, Open-file Report 78-701, pp. 6-8. BERGER, G. W. 1975. 40Ar139Arstep heating of thermally overprinted biotite, hornblende and potassium feldspar from Eldora, Colorado. Earth and Planetary Science Letters, 26: 387-408. Bouco~,A. J. 1960. Implications of Rhenish Lower Devonian brachiopods from Nova Scotia. 21st International Geological Congress ~ e ~ o rSession t, Norden, Regional Paleogeogmphy, Part 12, pv. 129-137. BGRQUE,A. D. 1985. Migmatization and metamorphism associated with the Barrington Passage Pluton, Shelburne and Yarmouth counties, Nova Scotia. B. Sc. Honours thesis, Acadia University, Wolfville, N. S. CARMICHAEL, D. M. 1978. Metamorphic bathozones and bathograds: a measure of the depth of post-metamorphic uplift and erosion on the regional scale. American Journal of Science, 278: 769-797. CLARKE, D. B . , and HALLIDAY, A. N. 1980. Strontium isotope geology of the South Mountain Batholith, Nova Scotia. Geochimica et Cosmochimica Acta, 44: 1045 - 1058. 1985. SmINd isotopic investigation of the age and origin of the Meguma Zone metasedimentary rocks. Canadian Journal of Earth Sciences, 22: 102 - 107. CORMIER,R. F. 1979. Rubidiumlstrontium isochron ages of Nova Scotian granitoid plutons. Nova Scotia Department of Mines, Report 79-1, pp. 143 - 147. CORMIER, R. F., and SMITH,T. E. 1973. Radiometric ages of granitic rocks, southwestern Nova Scotia. Canadian Journal of Earth Sciences, 10: 1201 - 1210. CROSBY,D. G. 1962. Wolfville map-area, Nova Scotia (21 Hll). Geological Survey of Canada, Memoir 325. DALLMEYER, R. D. 1975. 40Ar/39Arages of biotite and hornblende from a progressively remetamorphosed basement tenane: their bearing on interpretation of release spectra. Geochimica et Cosmochimica Acta, 39: 1655 - 1669. 1982. 40Ar/39Arincremental-release ages of biotite from a progressively remetamorphosed Archean basement tenane in southwestern Labrador. Earth and Planetary Science Letters, 61: 85-96. DALLMEYER, R. D., and KEPPIE,J. D. 1984. Geochronological constmints on the accretion of the Meguma Tenane with North America. Geological Society of America, Abstracts with Programs, 16: 1 1 . 1986. Polyphase late Paleozoic tectonothermal evolution of the Meguma Terrane, Nova Scotia. Geological Society of America, Abstracts with Programs, 18: 11. DALLMEYER, R. D., and RIVERS,T. 1983. Recognition of extraneous argon components through incremental-release 40Ar139Aranalysis of biotite and hornblende across the Grenvillian metamorphic gradient in southwestern Labrador. Geochimica et Cosmochimica Acta, 47: 413-428. DALLMEYER, R . D., GEE, D. G., and BECKHOLMEN, M. 1985. 40Ar/39Armineral age record of early Caledonian tectonothermal activity in the Baltoscandian miogeocline, central Scandinavia. Journal of Science, 285:-532-568. DALRYMPLE, G. B., and LANPHERE,M. A. 1971. 40Ar/39Artechnique of K-Ar dating: a comparison with the conventional technique. Earth and ~ l a n k a r y~ciknceLetters, 17: 300-308.

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DODSON,M. H. 1973. Closure temperature in cooling geochronological and petrological systems. Contributions to Mineralogy and Petrology, 40: 259 -274. FAIRBAIRN, H. E., HURLEY,P. M., PINSON,W. H., JR., and CORMIER, R. F. 1960. Age of granitic rocks of Nova Scotia. Bulletin of the Geological Society of America, 71: 399 -414. FOLAND,K. A. 1983. 40Ar/39Arincremental heating plateaus for biotite with excess argon. Isotope Geoscience, 1: 3 -21. GERLING,E. K., PETROV,B. V., and KOLTSOVA, T. V. 1966. A comparative study of the activation energy of argon liberation and dehydration energy in amphiboles and biotites. Geochemistry International, 3: 295 -305. HARRISON, T. M. 1981. Diffusion of 40Arin hornblende. Contributions to Mineralogy and Petrology, 78: 324 -33 1. HARRISON, T. M., and MCDOUGALL, I. 1981. Excess 40Arin metamorphic rocks from Broken Hill, New South Wales: implications for 40Ar/39Arage spectra and the thermal history of the region. Earth and Planetary Science Letters, 55: 123- 149. 1985. Diffusion of 40Arin biotite: temperature, pressure and compositional effects. Geochimica et Cosmochimica Acta, 49: 246 1 -2468. HARRISON, T. M., DUNCAN, I., and MCDOUGALL, I. 1985. Diffusion of 40Arin biotite: temperature, pressure and compositional effects. Geochimica et Cosmochimica Acta, 49: 2461 -2468. HAWORTH, R. T., DANIELS,D. L., WILLIAMS,H., and ZEITZ, I. 1980. Bouguer gravity anomaly map of the Appalachian Orogen. Memorial University of Newfoundland, Map No. 3a, scale 1 : 2000000. HIGBIE,J. 1978. Uncertainties in a least-squares fit. American Journal of Physics, 46: 945. KEPPIE,J. D. 1976. Structural model for the saddle reef and associated gold veins in the Meguma Group, Nova Scotia. Nova Scotia Department of Mines, Paper 76- 1. 1977. Tectonics of southern Nova Scotia. Nova Scotia Department of Mines, Paper 77-1. 1982. The Minas geofracture. In Major structural zones and faults of the northern Appalachians. Edited by P. St. Julien and J. Beland. Geological Association of Canada, Special Paper 24, pp. 263 -280. 1985. The Appalachian collage. In The Caledonain Orogen-Scandinavia and related areas. Edited by D. G. Gee and B. A. Stuart. John Wiley & Sons, Chichester, UK, pp. 422-437. KEPPIE,J. D., and DALLMEYER, R. D. 1987. Dating transcurrent terrane accretion: an example from the Meguma and Avalon composite terranes in the northern Appalachian Orogen. Tectonics. KEPPIE,J. D., and MUECKE,G. K. 1979. Metamorphic map of Nova Scotia. Nova Scotia Department of Mines and Energy, Scale 1 : 1000000. KEPPIE,J. D., and SMITH,P. K. 1978. Compilation of isotopic age data of Nova Scotia. Nova Scotia Department of Mines, Report 78-4. KEPPIE,J. D., ODOM,L., and CORMIER, R. F. 1983. Tectonothermal evolution of the Meguma Terrane: radiometric controls. Geological Society of America, Abstracts with Programs, 15: 136. KEPPIE, J. D., CURRIE,K., MURPHY,J. B., PICKERILL,R. K., FYFFE,L. R., and ST-JULIEN,P. 1985. Appalachian geotraverse (Canadian mainland). Geological Association of Canada, Excursion 1. LOWDON,J. A. 1960. Age determinations by the Geological Survey of Canada, report I-isotopic ages. Geological Survey of Canada, Paper 60-17. 1961. Age determinations by the Geological Survey of Canada, report 2-isotopic ages. Geological Survey of Canada, Paper 61-17. LOWDON,J. A., STOCKWELL, C. H., TIPPER,H. W., and WANLESS, R. K. 1963. Age determinations and geological studies; part I. Isotopic ages. Report 3. Geological Survey of Canada, Paper 62-17. MISNER,A. R. 1986. Metamorphism of the northern part of the Shelburne Metamorphic Complex, Shelburne and Yarmouth coun-

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ties, Nova Scotia. B. Sc. Honours thesis, Acadia University, Wolfville, N.S. PANKHURST, R. J., MOORBATH, S., REX, D. C., and TURNER,G. 1973. Mineral age pattern in c. 3700 m.y. old rock from West Greenland. Earth and Planetary Science Letters, 20: 157 - 170. POOLE,W. H. 1971. Graptolites, copper and potassium-argon in Goldenville Formation, Nova Scotia. In Report of activities, part A. Geological Survey of Canada, Paper 71-lA, pp. 9-11. RAESIDE,R. P., WHITE,C. E., and WENTZELL,B. D. 1985. The metamorphic development of the Shelburne Complex, southwest Nova Scotia: Geological Association of Canada, Program with Abstracts, 10: A50. REYNOLDS, P. H., and MUECKE,G. K. 1978. Age studies on slates: applicability of the 40Ar/39Arstepwise outgassing method. Earth and Planetary Science Letters, 40: 111 - 118. REYNOLDS, P. H., KUBLICK, E. E., and MUECKE, G. K. 1973. Potassium-argon dating of slates from the Meguma Group, Nova Scotia. Canadian Journal of Earth Sciences, 10: 1059- 1067. REYNOLDS, P. H., ZENTILLI,M., and MUECKE,G. K. 1981. K-Ar and 40Ar/39Argeochronology of granitoid rocks from southern Nova Scotia: its bearing on the geological evolution of the Meguma Zone of the Appalachians. Canadian Journal of Earth Sciences, 18: 386-394. ROBINS,G. A. 1972. Radiogenic argon diffusion in muscovite under hydrothermal conditions. M.Sc. thesis, Brown University, Providence, RI. RODDICK, J. C., CLIFF,R. A., and REX,D. C. 1980. The evolution of excess argon in alpine b i ~ t i t e - ~ ~ A r / ~analysis. ~Ar Earth and Planetary Science Letters, 48: 185 -208. Ross, D. M. 1985. Structure and metamorphism of the Pubnico area, Yarmouth County, Nova Scotia. B.Sc. Honours thesis, Acadia University, Wolfville, N.S . SAGE,J. D. 1984. Variable water pressure metamorphic assemblages in the Meguma Group, Nova Scotia. M.Sc. thesis, Virginia Polytechnic Institute and State University, Blacksburg, VA. SCHENK, P. E. 1971. Southeastern Atlantic Canada, northwest Africa and continental drift. Canadian Journal of Earth Sciences, 8: 1218-1251. 1983. The Meguma Terrane of Nova Scotia, Canada-an aid in trans-Atlantic correlation. In Regional trends in the geology of the Appalachian -Caledonian -Hercynian -Mauritanide Orogen. Edited by P. E. Schenk. NATO AS1 Series, Series C, No. 116, pp. 121 - 130. STEIGER,R. H., and JAGER,E. 1977. Subcommission on Geochronology: Convention on the use of decay constants in geo- and cosmochronology. Earth and Planetary Sciences, 36: 359-362. TAYLOR,F. C., and SCHILLER, E. A. 1966. Metamorphism of the Meguma Group of Nova Scotia. Canadian Journal of Earth Sciences, 3: 959 -974. WANLESS, R. K., STEVENS, R. D., LACHANCE, G. R., and EDMONDS, C. M. 1967. Age determinations and geological studies, K- Ar isotopic ages. Geological Survey of Canada, Paper 66-17. WANLESS, R. K., STEVENS, R. D., LACHANCE, G. R., and DELABIO, R. N. 1972. Age determinations and geological studies, K- Ar isotopic ages, report 10. Geological Survey of Canada, Paper 71-2. WENTZELL, B. D. 1985. The transition from staurolite to sillimanite zone, Port LaTour, Nova Scotia. B.Sc. Honours thesis, Acadia University, Wolfville, N.S. WHITE,C. E. 1984. Structure and metamorphism of the Jordon River valley, Shelburne County, Nova Scotia. B.Sc. Honours thesis, Acadia University, Wolfville, N.S. ZENTILLI,M., and REYNOLDS, P. H. 1985. 40Ar/39Ardating of micas from the East Kemptville tin deposit, Yarmouth County, Nova Scotia. Canadian Journal of Earth Sciences, 22: 1546-1548. ZIMMERMAN, J. C. 1972. Water and gas in the main silicate families: distribution and application to geochronology and petrogenesis. Sciences de la Terre, Memoire 2.