40Arf39Ar mineral age constraints onthe timing of defonnation and metamorphism,North-Bast Greenland Caledonides

R. David Dallmeyer and Robin A. Strachan

4()ArP9Ar mineral ages have been determined from basement gneisses and coversequences exposed in the Caledonides of North-East Greenland. These range betweenc. 438 Ma and c. 370 Ma (Early Silurian - Middle Devonian). They are interpreted asdating cooling folIowing polyphase Caledonian metamorphism which completely reju­venated intracrystaUine argon systems within the (early Proterozoic) basement gneis­ses. The 4()ArP9Ar results indicate that thrust-related regional deformation and meta­morphism in North-East Greenland continued into the Early Devonian.

R. D. D., Department of Geo!ogy, University of Georgia, Athens, GA 30602, U.SAR. A. S., Geo!ogy and Cartography Division, Schoo! of Construetion and EarthSciences, Oxford Brookes University, Oxford OX3 OBp, UK.

Isotopic mineral ages from regional metamorphicrocks and syn- to post-kinematic igneous suites recordages of cooling through closure temperatures and providevaluable constraints on orogenic evolution (e.g. Purdy &Jager, 1976; Jager, L979; Dempster, 1985; Dallmeyer et

al., 1992). AmphiboLe and muscovite from various lith­ologic units within the North-East Greenland Caledo­nides have been anaLysed using 4OArf39Ar incremental­reLease dating techniques to provide constraints on thetiming of tectonothermal events, and in particular to es­tablish an upper limit for regional deformation and meta­morphism. The 40Arf39Ar dating technique is arefinementof the K-Ar method (DalrympLe & Lanphere, 1971). Thereliability of K-Ar mineral ages obtained by conventionalmethods is difficult to assess, because such ages mayresult from partial isotopic disturbance or excess argon.The advantage of the 40Arp9Ar technique is that progres­sive heating of a single sample evolves separate aliquotsof argon which are dated independently. Assessment ofthe resultant age spectrum allows identification of con­tamination, partial isotopic rejuvenation and excess argonso that greater confidence may be placed on the in­terpretation of the data. This contribution is a shortenedversion of a paper published by the Geological Society ofLondon (Dal1meyer et al., 1994).

40Arf39Ar analyticai methods

Mineral concentrates were wrapped in aluminium-foilpackets, encapsulated in sealed quartz viais, and irradia­ted for 40 hours at the TRIGA Reactor at the U.S. Geol­ogical Survey, Denver. Variations in the flux of neutrons

Rapp. Grønland.. geol. Unders. /62. 153-162 (1994)

along the length of the irradiation assembLy were mon­itored with several mineral standards, including MMhb-l(Samson & Alexander, 1987). The samples were incre­mentally heated until fusion in a double-vacuum, resist­ance heated fumace following methods described byDallmeyer & Gil Ibarguchi (1990). Measured isotopicratios were corrected for total system blanks and theeffects of mass discrimination. Interfering isotopes pro­duced during irradiation were corrected using the factorsreported by Dalrymple et al. (1981) for the TRIGA Reac­tor. Apparent 40Arf39Ar ages were calculated from cor­rected isotopic ratios using the decay constants and iso­topic abundance ratios listed by Steiger & Jager (1977)fol1owing the methods described in Dallmeyer & Keppie(1987).

Intralaboratory uncertainties have been calculated bystatisticaI propagation of uncertainties associated withmeasurement of each isotopic ratio (at two standard de­viations of the mean) through the age equation. Interlab­oratory uncertainties are c. ± 1.25-1.50% of the quotedage. Total-gas ages have been computed for each sampleby appropriate weighting of the age and percent 39Arreleased within each temperature increment. A 'plateau'is considered to be defined if the ages recorded by two ormore contiguous gas fractions (with similar apparentKlCa ratios) each representing > 4% of the total 39Ar

evolved (and together constituting > 50% of the totalquantity of 39Ar evolved) are mutually similar within a± 1% interlaboratory uncertainty. Analysis of theMMhb-1 monitor indicates that apparent Klea ratios maybe calculated through the relationship 0.518 (± 0.005) x(39Arp?Ar) corrCClcd. Regression techniques followed the

© GGU, CopelllJagell, 1994

154

20°

>(')I>"Om(')0J:O~ZJ:-0 0zm

] FORELAND

50kmI

Eleonore Bay SupergroupUndifferentiatedrnetasedirnentsSrnalief;ord sequence

Basernent gneisses

Zebra Serie s

Trekant Series

Basernent gneisses

QuaternaryUpper Palaeozoic.Mesozoic & Tertiary

Mylonites

Caledonian granites

Thrustllow angle shear zone

-.~""'.----".'----""'•• (barb in hanging-wall)------------ Brittie lault

oI

POST-CALEDONIAN

~~

5 (GGU 335715)

12 (GGU 335823)

16 (GGU 347739)

17 (GGU 347602)

18 (GGU 365338)77° 19 (GGU 347791)

'bt::

20 (GGU 347797)Clr

"0N

t

Fig. 1. Geological map of North-East Greenland (75°-78°30'N) (from Dallmeyer et al., 1994). lnset indicates relative positions ofEast Greenland (Laurentia) and Scandinavia (Ba1tica) in a pre-drift reconstruction. Numbers identify 4oArj39Ar sample localities. GN,Garde Nunatak; N, Nordmarken; S, Stormlandet; GL, Germania Land; D, Danmarkshavn; SSZ, Storstrømmen shear zone; BF,Bessel Fjord; HF, Hochstetter Forland; BFSZ, Bessel Fjord shear zone; KSZ, Kildedalen shear zone. Areas of Figures 3--6 areoutlined.

155

methods of York (1969). A mean square of the weighteddeviates (MSWD) has been used to evaluate isotopiccorrelations. The full list of 40Arp9Ar analyticai data is

available as Supplementary Publication No SUP 18089(41 pp.) from the British Library Document Supply Cen­tre, Boston Spa, Wetherby, W. Yorks, LS23 7BQ, U.K.,and the Geological Society of London Library.

40Arp9Ar results100

Muscovite

Plateau age 398.5 ± 1.1

20 40 60 80

CUMULATIVE %39Ar RELEASED

-- 425r-----,-----.-------,.------~-----..,~ f--20 --.-__G_G_U_3_4_77_9_1-lJ

~ 400~-r-"""-----_---r-----,

~ 375 TREKANT METASANDSTONEzw~350a.a.'" 325,'---------'-----...J'-----'-------'-------'

O

Fig. 2. ""AIpoAr apparent age spectrum of a muscovite concen­trate from cleaved Trekant metasandstone within the Caledonianthrust zone exposed at Garde Nunatak (Iocation shown in Fig. l)(from Dallmeyer et al., 1994). Analytical uncertainties (twosigma, intralaboralOry) are represented by vertical width of bars.Experimental temperatures increase from lefl to right. Plateauincrements delineated by bar.

40Arp9Ar incremental release ages have been dcter­

mined for forty-one hornblende and twenty-four musco­vite concentrates from metamorphjc and igneous rockunits exposed in the region 75°-78°N. Sample locationsare indicated in Figs 1, 3-6. Geologically signiticantresults are displayed as age spectra in Pigs 2-7. Most ofthe hornblende concentrates display slightly discordant4°Arp9Ar age spectra in which variable apparent ages are

...

20 -O 60 80

CUMULA lIVE'"39"f ~ELE"Se.O

AMPI-IBOUTE

20°

Ho.nlJlende

'10100e eon_IUlon ege 3169 t 11

14~~-~-~-~--

347734

------ Brillie lault76°

1 (GGU 335725)

2 (GGU 335719)

4 (GGU 335782)

6 (GGU 300091)

7 (GGU 335783)

13 (GGU 347729)20°

-..----r- Thrust

l'00

347725

347726

Plaleau ~lIe 373.0 , 0.8

AMPHIBDLITE

,_--'-----1....-_.20 40 GO ao

CUMULATIVE ",:19AI RI:LE!\S[rl

~.

20 .0 60 80 100

CUMULATNE 'l.39

j1" HELEoIl.SED

22°

H<)fltlJlende

Isotope co,rels1ion _",e 382.1~~

20 40 80 80 100

CUMULA.nVE ,.39A, FlELEASED

~"25j 9B

~.. PARAGNEISS

5,,~~========4~3Sc(~26

O

~ o.• 10

z~ 0.2

~j_50 -+------ 1- --

~_2

~.

~37S~350

''0

347723AMPHIBOUTE

~o . 40 60 30

CUMULAT'VE 'lij,39Ar RELEA$EIJ

20 .0 60 eD 100

CU"'Ul.ATlVE ",,3'ilAr RELE....SED

24°

HO.nblende

_.-t-I--+

I..oloøe correlll.lion Ilge 369.!! " 22

8A

~42~--.- =::J~400L __

~ 31 GARNET SCHIST!i: 35 MuscoviteC[ Plateau 11Ie 381.0 '" 0.9

'" °

26°

347710

N

t

26°

Qualernary

Myloniles ~Metasediments Allochthon

Basemenl gneisses

Trekant Series JBasement gneisses Foreland

3 --~-;-.2~

~ 40J~:.=;=:;:::::::::====~=c:Ijæ315 MYlONITIC ZEBRA aUARTZITE

~ 350 ~,:::::i~.g:1~~~.~2:~:~ 1"'325 ...1....- • ..I__ ~. ~

o 20 40 60 1Il0 100

CUMUlATlVE ,.39", AElEA$EO

-;;.2-----.----~

! 34nl0ro r"':===::::==::::::::::;~§31 MYLONITIC ZEBRA QUARTZITE

9=35 Mu!t:ovill C1S0-lTS,urnl

R: F'ltllell.U Ilge 392.9 t 0,8

<3250 20 40 e'o eoCUMULATIVE ,.39A, AELEA$EO

77°

L'S1~::~I

~.25L 3 I , -------;47710••oo~ ::=-=========1; 31 S ;YLONITlC ZEBRA OUARTZlTE~ MD~ .....scowu. (1 :a-150l0/m)

t P,elnu ege 3G2.7 i: 0.5

<Ol: 3250:--~20C---;.:-o eD BO

CUMUL,T1V!?: 'Jl, 39", RELEASED

Fig. 3. 4OArp9Ar apparent age and apparent KlCa spectra ol' hornblende and muscovite concentrates from various lithological unitsexposed in central Dronning Louise Land and western Dove Bugt (from Dallmeyer et al., 1994). Data plotted as in Figure 2. Boldnumbers identify loca1ities which did not yield meaningful hornblende p1atea~ or isotope correlation ages.

156

recorded at low experimental temperatures. These arematched by variations in apparent KlCa ratios, whichsuggest experimental evolution of argon at low tempera­tures from compositionally distinct and relatively non­retentive phases. These could be represented by: (1) veryminor, optically undetectable, mineralogical conta­minants in the concentrates; (2) petrographically unre­solvable exsolution or compositional zoning; or (3) intra­crystalline inclusions. In general, most gas fractionsevolved from hornblende concentrates at intermediate andhigh experimental temperatures were characterised bysimilar intrasample apparent KlCa ratios. These suggestthat experimental evolution of gas occurred from composi­tionally uniform populations of intracrystalline sites. Ap­parent KlCa ratios for the twenty-four muscovite concen­trates are very large and show no significant or systematicvariations in any of the 40Arp9Ar analyses. Therefore theyare not included with the age spectra displayed in Figs 2-7.Radiogenic yields obtained from muscovite analyses wereall in excess 01'99.9%, therefore precluding the necessity tocalculate isotope correlation ages.

Foreland and thrust zone

Muscovite concentrated from cleaved Trekant Seriesmetasandstone (sample 20) collected within the forelandat Garde Nunatak (Fig. I) records a plateau age of c. 399Ma (Fig. 2). Concentrates of three size fractions of mus­covite were also separated from mylonitic quartzite (sam­ple 3, Zebra Series protolith) collected within the CaIedo­nian thrust zone in Dronning Louise Land (Fig. 3). Theserecord plateau ages ranging between c. 393 Ma and c. 385Ma (Fig. 3). These ages are interpreted as dating the lastcooling through temperatures required for intracrystallineretention of argon folIowing ductile deformation withinthe foreland and the thrust zone. Although not experi­mentally calibrated in detail, using the preliminary data01' Robbins (1972) in the diffusion equations of Dodson(1973) suggests muscovite closure temperatures 01' c.375° ± 25°C.

Basement and cover, eastern Dronning LouiseLand and Hertugen af Orleans Land

Samples of banded amphibolite (8A) and garnet-mus­covite-biotite schist (8B), probably correlative with theTrekant Series, were collected within a synclinal infold inbasernent gneisses in eastern Dronning Louise Land (Fig.3). The amphibolite may represent a metamorphosedmafic volcanic horizon within the cover succession. Ahornblende concentrate from the amphibolite displays aninternally discordant spectrum (Fig. 3); however, incre­ments evolved at intermediate and high experimental

temperatures record sirnilar apparent ages which define aplateau of c. 395 Ma. 36Ar/40Ar vs. 39Ar/40Ar isotopecorrelation of the plateau data are well-defined (MSWD= 0.15), and correspond to an inverse ordinate intercept(40ArP6Ar ratio) of c. 1900. This is markedly larger thanthe 40ArP6Ar ratio in the present-day atrnosphere, andsuggests significant intracrystalline contamination withextraneous ('excess') argon components. Use of the in­verse ordinate intercept (4°Arj39Ar ratio) in the age equa­tion results in a plateau isotope correlation age of c.390Ma. Because calculation of isotope correlation ages donot require assumptions of 40Arp6Ar ratios, they are con­sidered more reliable than ages which are directly calcu­lated from the analytical data. The c. 390 Ma age istherefore considered geologically significant, and is in­terpreted as dating the last cooling through temperaturesrequired for intracrystalline argon retention in constituenthornblende grains (c. 500° ± 25°C; Harrison, 1981).Muscovite from the schist (8B) records a plateau age ofc. 381 Ma.

Hornblende concentrates were prepared from eightsamples of variably foliated, discordant amphiboliteswithin reworked basement gneisses exposed in easternDronning Louise Land. These are thought to representmetamorphosed equivalents 01' the Midsornrnersø Dole­rites. Samples were collected both within (1, 2 and 12)and outside (4-7 and 11) af the Caledonian thrust zone(Figs I, 3). The concentrate from sample II is character­ised by an intermediate- and high-temperature plateauage 01' c. 388 Ma (Fig. 3). Isotope correlation of theplateau data corresponds to a younger age of c. 385 Ma.The remaining seven samples display variably discordantspectra with total-gas ages ranging between c. 1722 Ma(12) and 464 Ma (5). Meaningful isotope correlations arenot defined by any combinations 01' the analyticai data,and the spectra are interpreted to reflect the results ofcomplex intracrystalline contamination with variable ex­traneous argon componentLNo' geologic significance isattached to any of the total-gas ages recorded by horn­blende from these seven samples.

Hornblende concentrates were prepared from amphi­bolite and gneiss samples collected within various base­ment gneisses exposed in eastern Dronning Louise Land(9A and 10) and Hertugen af Orleans Land (19, 21 and22). That from sample 22 displays an age spectrum inwhich intermediate- and high-temperature incrementscorrespond to a plateau of c. 444 Ma (Fig. 4). Isotopecorrelation af these data yields a slightly younger age ofc. 438 Ma. Concentrates from samples 9A, IO and 21 arecharacterised by discordant spectra in which apparentages systematically decrease throughaut intermediate­and high-temperature increments (Figs 3, 4). For samples9A, 10 and 21 these increments yield well-defined iso-

. --_._--------

tope correlation ages which range between c. 403 Ma(21) and 380 Ma (9A). No meaningful isotope corre1ationwas defined in the analysis of sample 19. A muscoviteconcentrate prepared from a sample of paragneiss (9B)records a c. 373 Ma plateau age (Fig. 3).

Storstrømmen shear zone

Hornblende and muscovite concentrates were preparedfrom various mylonitic rock units exposed within andadjacent to the Storstrømmen shear zone in Hertugen afOrleans Land (Fig. 4). Hornblende concentrates fromsamples of mylonitic amphibolite (24A and 28A) andgneiss (27A and 30) all display internally discordantrelease spectra as aresult of complex intracrystallinecontarnination with extraneous argon components. Total­gas ages range between c. 964 Ma (28A) and c. 524 Ma(27A). No meaningfu! isotope correlations are defined byany combinations of the data; therefore no geologic sig­nificance is attribuled to any of the total gas ages.

Muscovite concentrates were prepared from seven

157

samples (23, 24B, 25, 26, 27B, 29 and 31) of variablymylonitic granite, gneiss and felsic pegmatite. In addi­tion, muscovite was separated from mylonitic metagab­bro at location 28B. Muscovite from the metagabbrorecorded a plateau age of c. 415 Ma (Fig. 4). The otherseven muscovite concentrates yielded distinetly youngerplateau ages ranging between c. 382 Ma (myloniticgneiss, location 29) and c. 370 Ma (mylonitic pegmatite,location 27B) (Fig. 4).

Basement east af the Storstrømmen shear zone

Hornblende was separated from fourteen samples ofamphibolite and basement gneiss exposed in Dove Bugt(13-17) and Nordrnarken (32-38) (Figs 1,3,5). In addi­tion, hornblende concentrates were prepared from twosamples (39 and 49) of foliated metagabbro collected ineastem Storrnlandet (Fig. 5). Concentrates from foursamples of amphibolite and gneiss (14, 35, 36 and 38)were characterised by intermediate- and high-temper­ature age plateaux (Figs 3-5). Isotope correlation of the

------ Brillle taull

~., 25 . , 34776]EL MYLO:~~:":~ANIT=-_~ Plateau alle 311.3 '" 1.0

<325 ----~O 20 40 60 80

CUMULATIVE ,,39Ar RELEASED

78 0

347782MYLONlTIC GNEISS

Plat...... age 38Q.5 t 09

Musc:ovhe

Plateau age 381,61: 1.0

20 40 60 80-~OOCUMULATlVE .. 3BAt RELE"ASED

lsolope correhuion age 385.1 t 3.3

30 (GGU 347783)

foo' 278 347779

w", MYlONITIC PEGMATITE

~J7f"'-..:;:;:=======:;;;j~35()<325

O 20 40 150 SO 100

CUUULATlVE ...39.... RElEASED

..~41 ~~45

~42

~4"'!';'-~200;-~40;:------::eo'--~.O;:,--:-!

CUMUlATlve ",39", R'i:lEA$F.:O

~ 288 347781~ 450... MYlONITIC ME.lAGABBRO

E42 I-~=--_=,-___C_::;=~____j

~ 40 Pf8Iea::::O:i::.3 lt Q,8

."020406080

CUt.4ULA TIVE 'It,39., AElEASED

"\ Fr-3_2__A_M_PHl_BO_U_T_E_3_47_7_8-1

9

~425 248 -··T 347773 ~i:~~;;.=:::M=YL:O::N1:T=IC=G=NE:::J'SS=~::::j

~3S " ....:-:::~;::'~<3250 2:> 40 60 80 100

CUMULATI\lF ",39... , RELEASED

"·B6-r-- ( 34nSS ]40 MYLONITlC GRANITE I37'''''' - ----3.t "....~:::O:~: •. O. J325 ! J

O 20 40 60 80

CUMULAT1VE ",39Ar AElE .....SEO

Qualernary

Myloniles

Basemenl gneisses

~ '4~-'-~W7771

~ o) HORNBLENDE GNEISS J~... 500'r---+--+--+--+------j.;: .'5 1-----------.-,

;::1~·'O".,.Ik""'m9f'.9

t40~ , , !_

« o 20 40 60 60 100

CUI.AULATlVE %33Ar RElEASED

[2]

~EJ

78'

Fig. 4. 4OArp9Ar apparent age and apparent Klea spectra of hornblende and muscovite concentrates from various lithological unitsexposed in eastern Hertugen af Orleans Land (from Dallmeyer et al., 1994). Data plotted as in Figure 2. SSZ, Storstrømmen shearzone.

158

20° 18° 78 0

AMPHIBOLlTE0.2/'- -4

Hornblende

363158HORNBLENDE GNEISS

20 40 eD 80 100

CUMULAliVE"39Ar RELEASED

isotope c:orrelatioo age 381.0 t 0.7

37 347752

2HORNBLENDE GNEISS

~~rnbl.nd'5 Isotope corrol.tlon .go 3.7.5 • 3.. f

I I5

O.B 16

o.•

o.•

0.2 ~-------~~-j

77° 20°

39 (GGU 363173)

40 (GGU 361984)77 0

sooir--+--+---+---t----n.!475w~450...æ425a:

~ 4001 'l.i-];;;;;:::;~;:;;;;;;;;;;;:fJ375!-

O 20 40 80 80 100

CUMULATIVE ~39M RElEASED

100

100

20 40 60 80

CUMUlAT'VE % 39Ar RElEASED

22 0

38 347756HORNBLENDE GNEISS

2

OHornblendl!

5 Isolope correlation age 394.0 ± 1.3

-5

o.35 363158

o. HORNBLENDE GNEISS

o.~

00

75Hornblende

50Isotope couelation age 376.8 f: 1.9

25 r00 I7

.~.

w

~....~4..lt'..

3

347788

Hornblende

8rittle fault

24°

AMPHIBOLITE

Upper Palaeozoic

Mylonites

Basement gneisses

33

~

g[IJ

o.•.-----,---~-~-~-__,34 347601

.!2 O.4r--,----.---.---~-_,'"ffiQ:' 0.2i 11..-------------1.."oh--+--+---+---t---i

4000!-----:2;;';:0-~40;;------:'60;;-----;B~0=--;d,00

CUMULATIVE %3gAr RELEASED

";'525~

:500

~1-475zwCl: 450

i4': 425

500/r--+--+---+---t-----i

i475'w If-----------<I~450...æ425

~t 400­..

3750·L--20~-'-:'.:0-----:'.0=------:B'=:0----:-!'00

CUMULATIVE 'lft 39"'r RELEASED

Fig. 5. 4°Arp9Ar apparent age and apparent Klea spectra of hornblende concentrates from various lithological units exposed inNordmarken and Stormlandet (from Dallmeyer et al., 1994). Data plotted as in Figure 2. SSZ, Storstrømmen shear zone.

plateau data yields slighdy younger ages whieh rangebetween c. 394 Ma (38) and 377 Ma (14). Hornblendeeoneentrates from five samples of amphibolite and gneiss(15, 32, 33, 34 and 37) display internally diseordantspectra in whieh apparent ages systematieally decreasethroughout intermediate- and high-temperature portionsof the analyses (Figs 3-5). Isotope eorrelation ages rangebetween c. 398 Ma (33) and c. 385 Ma (32), and areeonsidered geologieally meaningfuI. The release spectradisplayed by hornblende eoncentrates from three amphi­bolite samples (13, 16 and 17; Figs l, 3) and the twometagabbro samples (39 and 40; Fig. 5) are internallydiseordant and yield no meaningful isotope eorrelations.

Hornblende was separated from six samples eollectedwithin basement units exposed south af Bessel Fjord(Fig. 6). These included amphibolite ar orthogneiss (41,51,53 and 54) and paragneiss (42 and 47). Hornblende

eoneentrates from samples 41 and 54 are charaeterised byinternally diseordant 40Arj39Ar spectra in whieh apparentages systematieally deerease throughout intermediate­and high-temperature segments af eaeh analysis (Fig. 6).Isotope eorrelations are well-defined by these data in bothsamples, and yield geologieally significant ages of c. 379Ma (41) and c. 392 Ma (54). Hornblende eoneentratesfrom the remaining four samples display internally dis­eordant age spectra, and no geologieally meaningful re­suJts are defined by these data.

Muscovite-bearing lithologies are only loeally exposedwithin basement rocks east of the Storstrømmen shearzone. Museovite from a late-teetonic felsie pegmatite(sample 18) eolleeted on Store Koldewey (Fig. 1) recordsa plateau age of c. 314 Ma (Fig. 7). The pegmatitepost-dates penetrative duetile deformation within hostgneisses, and 10eally displays a eataclastic overprint in

159

brittle deformation zones. The age is interpreted to datepost-magmatie cooling of the pegmatite.

Eleonore Bay Supergroup

Smallefjord sequence

A hornblende eoneentrate was prepared from amphi­bolite (52) eolleeted within the Smallefjord sequenee(Fig. 6). This records an intermediate- to high-temper­ature plateau corresponding to an age of c. 432 Ma(Fig. 6). Isotope eorrelation of the plateau data yields anage of c. 426 Ma. Hornblende from another amphibolite(55A) displays an internaIly complex release spectrum asaresult of multicomponent eontamination with extrane­ous argon. Muscovite from afelsie gneiss (55B) reeords ac. 401 Ma plateau age (Fig. 6).

Museovite eoneentrates separated from garnet-bearingsehists (45 and 48) exposed within Jower parts of theEleonore Bay Supergroup (Nathorst Land Group, Søn­derholm & Tirsgaard, 1993) reeord plateau ages of c. 415Ma (45) and c. 400 Ma (48) (Fig. 6).

Caledonian granites and pegmatites

Muscovite concentrates prepared from the post-kine­matic Bredefjord (44) and Knæksø (46) granites whichintrude the Eleonore Bay Supergroup (Fig. 6) recordplateau ages of c. 411 Ma (44) and c. 423 Ma (46). Theseages are interpreted to date post-magmatic cooling of theintrusions.

Muscovite separated from an undeformed granite sheetwhich intrudes the Smallefjord sequence (55C) records a

GARNET SCHIST

Thrustllow angleshear zone

"""'.co~i"

P1alellu lige 415.4;' 0.7

P1.I..u.~3aUlrl.O

SO 3'17608FOLIATEO GRANTE

.. __-L....---.. ._~20 .0 60 80 100

CUMUlATIVE ..,lSi", Rell!ASED

45 3476'6

•

.Olo!O----=,O,---."=O--.=O-cc,"'O--:c!CUMULAnyE '!li 3S1A, RELE.ASEO

~45

~42

~400~

i 37

1<"'~O'---~20-~'O--''''O-~''''O--'!

CUMUtATIVE 'to 39.... r RELEASED

~o.4~5-2~-~-'j --W609~ 021 FOllATEO AMPH.eOUTE~ t . --J

~5501r_-+_-+_-+_---\--

•

---- Brittie lault

347611FELSIC GNEISS

""'.covite

PI.t••U~~'ro!--cc'"!=O---;"O &0 80 100

CUMULATIVE ...39..., RE..EASED

.45;o---~-~-~-~-! 55C 347611~ 42 GRANlTE SHEET

.....o,J,-c:::::::;:=;==::::::;==.~ Musco"IUI

~37 Plal.IU lilI 402.$ Jo 0.8

• 3S1~O~---C'O'-------C'O~-60-f=--80~---,J

CUMULAn\lE 'lo 30A• RElEASED

20 40 60 eoCUMULAll....e ... 39A• RElEASED

~ 0.04 S4

~ O.2IL ~

~ sooi.,.-+--+-

347617

BREDEFJORD GAANITE

C-OIOPll corrølalion ag.. 379.3 t 2.8

.450',-,__~-~-~-~! 44

~42"'I:.........t'_..:::;::.:::::::=====;;l.'0~~37 MUlcovite

~ 351~O~---c,O~~-"'"-'4=':-"-·-',~105-.-0~.'.~,--

CUlAlJLATIVE 'to 39Ar RELEASED

22°

~40'+-----'

~i31~ 111----------;

~ 3500O---:!,O::----c"c----c60c--'=O---,JCUfolULATIVE 'I> 3eAr I'lELEASED

~ o.• "4:-'~-~------=3cc47=6CC'8°O

~ HORNBlENDE-BlOTITE GNEISS:r 0.2~ r'----------I

.50\-----<--+--+---+----1lus

Basernen1 gneisses

Upper Palaeozoic& Mesozoic

Caledonian granite

Undillerenliatedmetasediments

Eleonore Bay Supergroup

Smalleljord seQuence

42 (GGU 347619) ~.

46 3~?622~. lUS 4gB 347624i! i! 48

47 (GGU 347620) ~"2 ~42SCHIST ~ ..oo - I

MYLONlTIC PEGMAllTE

~4~ KNAEKSØ GRANITE~4 ~375~

51 (GGU 347607) I."neo.-I'. ~u1co"ll.

~35~31 P1"uu age 422.9 tO.9 ~31 Plateau age 398.8 r 0.1 Platuu Ige 38<1.11 ol 0.953 (GGU 347613) t ~.. '3 _L

~3250'O '0 'O .0 'O .0 .0 80 'O .. .0 80

CLNJlATrVE ~39Ar RElEASED CUMULATlVE '!lo 39A• AELEASEO CUfolULArlVE ",39", RElEASED

76 0 43 347623

~~~

~

•Lill]

EJ

Fig. 6. 4°ArP9Ar apparem age and apparent Klea speetra of hornblende and muscovite concentrates from various lithological unitsexposed south of Bessel Fjord (from Dallmeyer et al., 1994). Data plotted as in Figure 2. BFSZ, Bessel Fjord shear zone; KSZ,Kildedalen shear zone.

Muscovite

Plateau age 313.6 ± 1.3

160

'2400,-------r-----,-----,------,----,~ 18 GGU 365338w 375 PEGMATITE<!J«I- 350zw~ 325F1"'1-- ~a..a..« 300L---...l...----L---J---.L-----'

O 20 40 60 80 100

CUMULAliVE %39Ar RELEASED

Fig. 7. 4oArp9Ar apparent age spectrum of a muscovite concen­trate from a late-tectonic, felsic pegmatite within basernentgneisses exposed on Store Koldewey (Iocation shown in Figurel) (from Dallmeyer et al., 1994). Data plotted as in Figure 2.

c. 402 Ma plateau age (Fig. 6). In contrast, muscoviteseparated from a pre- to syn-tectonic foliated granite (50)which intrudes the Smallefjord sequence records a youn­ger plateau age of c. 385 Ma (Fig. 6). Muscovite sep­arated from a felsic pegmatite (43) which intrudes meta­sedimentary rocks of unknown affinities west of BesselFjord (Fig. 6) also records a c. 386 Ma plateau age(Fig. 6).

Bessel Fjord shear zone

Mylonitic amphibolite (49A) and mylonitic felsic peg­matite (49B) were collected within the Bessel Fjord shearzone (Fig. 6). Hornblende from the amphibolite is charac­terised by an internally complex 40Arp9Ar release spec­trum which yields no meaningful geologic results. Mus­covite from the deformed pegmatile records a c. 381 Maplateau age (Fig. 6).

Geological significance

40Arp9Ar mineral cooling ages range between c. 438Ma and c. 370 Ma (late Ordovician - middle Devonian).Caledonian metamorphism was sufficient to effect com­plete rejuvenation of hornblende intracrystalline argonsystems within the (early Proterozoic) basement gneisses.Evaluation of the significance of these ages has to con­sider the evidence from U-Pb SHRIMP dating of zirconsfor Caledonian metamorphic events at 445 ± 10 Ma and404 ± 6 Ma (A. P. Nutrnan, personal comrnunication;Kalsbeek et al., 1993). It is therefore probable that therange of mineral cooling ages is composite in origin andresuIts from polyphase Caledonian metamorphism.

Mineral cooling ages obtained for the post-kinematicKnæksø (c. 422 Ma) and Bredefjord (c. 412 Ma) granitesplace upper limits on the timing of garnet-grade meta­morphism of host Eleonore Bay Supergroup units. Thismay have occurred during the late Ordovician (c. 445

Ma) event recorded by U-Pb SHRIMP analysis of zirconswithin the Smallefjord sequence (A. P. Nutman, personalcommunication). Mineral cooling ages which mayaisorecord cooling folIowing late Ordovician metamorphismof these rocks include the hornblende age of c. 426 Maobtained from a Smallefjord sequence amphibolite (sam­ple 52), and the muscovite age of c. 415 Ma obtainedfrom an Eleonore Bay Supergroup schist (sample 45).

The majority of 4°Arp9Ar mineral cooling ages ob­tained from basement gneisses and cover sequences else­where in the region range between c. 400 Ma and c. 370

Ma. It is possibIe that these ages also restilt from regionalcooling and uplift folIowing late Ordovician meta­morphism and deformation. However, a hiatus of 45-75Ma between peak metamorphism and cooling through thetemperatures required for intracrystalline retention of ar­gon in hornblende and muscovite seems unreasonablylong by comparison with other collisional orogens (e.g.Cliff, 1985; Treloar & Rex, 1990; Dallmeyer et al.,1992). U-Pb analysis of zircons within basement gneissesin the Dove Bugt region provides evidence for earlyDevonian metamorphism (404 ± 6 Ma; Kalsbeek et al.,1993), and it is therefore likely that the c. 400-370 Mamineral ages mainly record cooting folIowing this latermetamorphic event.

A muscovite concentrate obtained from the Caledonianforeland at Garde Nunatak (sample 20) preserves a cool­ing age of c. 399 Ma. The dated sample of cleavedTrekant Series metasandstone appears to have experi­enced metamorphism at no higher grade than middlegreenschist facies, and therefore the cooling age islhought to date dosely the time of folding, deavagedevelopment and local thrusting within this segment ofthe foreland. Muscovite concentrates from mylonileswithin the marginal thrust zone (sample 3) preserveslightly younger cooling ages ol' c. 393-385 Ma. Thin­section analysis of these mylonites indicales that theyrecord deformation within the greenschist facies. Quartzforms ribbons which are characterised by undulose ex­tinction, deformation bands and sutured grain boundaries(cf. type 1 ribbons of Boullier & Bouchez, 1978). Thereis only minor evidence for recovery and recrystallisationfolIowing deformation. Feldspar grains appear to havedeformed mainly by brittle fracturing. Deformation there­fore probably occurred at temperatures only slightlyhigher than that required for argon retention in musco­vite. For this reason, the cooting ages obtained from thesemylonites are thought to date closely regional thrusting.The dated samples were obtained from middle parts ofthe foreland-propagating irnbricate zone (Strachan et al.,1992), and do not, therefore, place an upper limit onthrusting which may have continued possibly until c. 380Ma.

East ol' the Storstrømmen shear zone, displaeementsalong the Kildedalen and Bessel Fjord shear zones wereassoeiated with penetrative deformation of the SmalI­efjord sequenee (Friderichsen et al., 1994). Constraintson the timing of this deformation event are provided byU-Pb zireon ages of 409~~ Ma and c. 400 Ma for emplaee­ment of pre- to syn-kinematic granitoids which intrudethe Smallefjord sequence (Hansen et al., 1994). An upperlimit for deformation is provided by the museovite eool­ing ages of c. 401 Ma (55B) and c. 402 Ma (55C)obtained from, respeetively, a Smallefjord sequenee fel­sie gneiss and a post-kinematie granite sheet.

The museovite eooling ages of c. 399-385 Ma re­eorded within the foreland and the marginal thrust zoneare older than: (1) the majority of hornblende ages(c. 390-380 Ma) in eastern Dronning Louise Land, DoveBugt and Nordrnarken; and (2) museovite ages (c. 380­370 Ma) in eastern Dronning Louise Land and the Stor­strømmen shear zone. It seems unlikely that the youngestof these ages c10sely date deformation. Thin-section anal­ysis of dated samples within the Storstrømmen shearzone, for example, indicates that deformation occurreddominantly within the low amphibolite facies. Horn­blende and oligoclase/andesine are stable phases in somemylonites, and feldspar displays evidenee for widespreadductile recrystallisation. Quartz has al most eompletelyreeovered and recrystallised folIowing mylonitisation(Straehan & Tribe, 1994). The eastward deerease in eool­ing ages is interpreted mainly to date uplift either duringor after regional upright folding. Major folds in the eastof the region, at least in part apparently developed 'out­of-sequenee' after initial thrusting and folding in Dron­ning Louise Land, and folIowing sinistral displaeementsalong the Storstrømmen shear zone (Dallmeyer et al.,1994).

A teetonie framework for the polymetamorphie historyoutlined above is neeessarily speeulative beeause of un­eertainties in the timing of deformation events in relationto metamorphie episodes. A late Ordovieian tectonother­mal event has been demonstrated within the Smallefjordsequenee (and Eleonore Bay Supergroup?) rocks abovethe Kildedalen and Bessel Fjord shear zones. The re­gional extent and nature of this event is uneertain. Twoeooling ages obtained in the north of the region areanomalously 'old' : the c. 438 Ma age for hornblendewithin a basement gneiss (sample 22) in Hertugen afOrleans Land, and the c. 415 Ma age for museovitewithin mylonitie metagabbro (sample 28B) from theStorstrømmen shear zone. It is possibIe that these are theresult of cooling folIowing late Ordovieian meta­morphism, and that intraerystalline argon systems withinthese samples were not reset during subsequent earlyDevonian metamorphism. The late Ordovieian teetono-

161

thermal event may therefore have been widespread, al­though largely overprinted in the area north of BesselFjord. Widespread early Devonian metamorphism is in­terpreted to have aecompanied thrust (Dronning LouiseLand), extensional (Kildedalen and Bessel Fjord shearzones) and sinistral strike-slip (Storstrømmen shear zone)displaeements, and regional upright folding.

The c. 314 Ma eooling age recorded by muscovite inan undeformed felsie pegmatite exposed on Store Kolde­wey eompares with mineral cooling ages of c. 320 Maobtained from gneisses exposed at Danmarkshavn (Rb-Srand K-Ar on biotite) by Steiger et al. (1976). Similarfelsie pegmatites are exposed in diserete swarms in east­em Germania Land and Store Koldewey. Here they post­date duetile deformation in host gneisses, and are var­iably affeeted by brittie deformation. The cooling agesreported by Steiger et al. (1976) may record a thennaldisturbanee assoeiated with pegmatite intrusion.

Acknowledgments. This work was, in part, supported by grams fromthe US National Science Foundation (Department of Polar Pro­grams. DPP-9 10 I444, to ROD), the Royal Sociely (IO R. A. S.) andthe Carlsberg Foundation (89-0212/10 lo N. Henriksen). Permissionfrom the Geological Society of London lo publish a shortenedversion of a paper which recently appeared in the Journal is ac­knowledged.

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