deJong_Split Phengite Grain: Combined Laser Probe Age Mapping & Stepheating Dating (Betics, Spain)_Earth & Planetary Science Letters 1992

8/6/2019 deJong_Split Phengite Grain: Combined Laser Probe Age Mapping & Stepheating Dating (Betics, Spain)_Earth & Plan

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Ear th and P lane tary Sc ience Le t te rs , 110 (1992) 173-191 173Elsevier Science Publishers B.V., Amsterdam

[CH]R e pe a te d the r ma l r e s e t t ing o f phe ng i t e s in the Mulha c e nC o mple x (Be t i c Zo ne , s o uthe a s t e r n Spa in) s ho w n by 4 A r / 3 9 A r

s tep h eat ing and s ing le gra in laser probe dat ingK o e n d e J o n g a , J a n R . W i jb r a n s b a n d G i l b e r t F 6 r a u d c

a S t ruc tura l Geo logy an d Tec ton ics Depar tm ent , Ins t i tu te o f Ear th Sc iences , Vr ije Univers it e it , De Boe le laan 1085 ,1 0 81 H V A m s t e r d a m , T h e N e th e r l a n d s

b Labo ra tory for I so tope Geology , Ins t i tu te o f Ear th Sc iences , Vr ije Univers it e it , D e Boe le laan 1085 , 1081 HVA ms terd am ,T h e N e t h e r la n d s

c Ins t i tu t de Gdodynam ique , URA CN RS 1279, U niversi t~ de Nice -Soph ia An t ipo l i s , 06034 Nice C~dex , FranceReceived October 9, 1991; revision accepted March 2, 1992

ABSTRACTThis study reports the results of the first 4Ar/39mr combined induction furnace and laser probe dating of phengites

from the Mulhacen HP/LT metamorphic complex in the Betic Cordilleras, southern Spain. Laser step heating and spotfusion analyses on different halves of a split single grain were made with a continuous laser probe. Spot fusion analysisresulted in ages of about 30-31 Ma in the core and ages as low as 25-26 Ma in the rim. Laser step heating on the other hal fof the grain gave a spectrum with apparent ages increasing from about 25 Ma to 29.5 Ma. The age spectrum and thedecreasing ages towards the rim of the grain may imply that resetting essentially occurred by volume diffusion of radiogenic4Ar due to late stage reheating resulting from extensional tectonics. Ages around 30 Ma in the core of the grain areinterpreted as minimum estimates of the cooling age of the main tectono-metamorphic phase D 2.

Induction furnace step heating on phengite separates from mica schists and one gneiss resulted in two types of agespectra. Type I spectra show monotonous ly rising appare nt ages from 14.5 + 1.9 Ma to 20.7 + 0.2 Ma, and in a secondsample from 16.9 + 0.7 to 29.7 + 0.2 Ma. Type II spectra are characterized by plateaus of 14.4 + 0.1 Ma (the gneiss sample),17.3 + 0.1 Ma a nd 17.6 _+ 0.1 Ma. Type II spec tra show low temperat ure apparent ages significantly below the plateau age,implying resett ing subsequent to initial cooling. Modelling of the age spectra demon strat ed that t he p lateau ages are possiblythe result of strong resetting (75-85% of argon loss) of an older isotope system. Total fusion of a number of phengite singlegrains from marbles taken close to type II mica schist s yielded ages of 15.4 ___1.2 Ma and 17.0 + 0.7 Ma. The observedrepe ated resetting is coeval with major volcanic activity in basins adjacent to t he me tamorph ic ranges, pointing to a resettingby advective fluid transport related to volcanism.

1 . I n t r o d u c t i o n

T h i s p a p e r r e p o r t s a 4 A r / 3 9 A r s t e p h e a t i n g ,l a s e r s t e p h e a t i n g a n d s p o t f u s i o n s tu d y o f p h e n -g i t e s f r o m t h e M u l h a c e n C o m p l e x i n t h e m e t a -m o r p h i c I n t e r n a l Z o n e o f t h e B e t i c C o r d i l l e ra s i nso u th eas t e rn Sp a in (F ig . 1 ) . Th e P - T p a t h o f th eM u l h a c e n C o m p l e x is w e l l c o n s t r a i n e d [ 1 - 5 ], d i s-p l a y i n g th e e s s e n t i a l f e a t u r e s o f t h e t e c t o n i c e v o -

Correspo ndence to: Koen de Jong, Institut de G6odynamique,UR A CNRS 1279, Universit6 de Nice-Sophia Antipolis, 06034Nice C6dex, France

l ut io n o f t h e B e t ic Z o n e : h i g h p r e s s u r e / l o w t e m -p e r a t u r e m e t a m o r p h i sm r e l a t e d t o ea r l y A l p in es u b d u c t i o n f o l l o w e d b y r e h e a t i n g d u e t o T e r t i a r ye x t e n s io n a l t e c t o n is m [ 2 - 4 ] . T h e c h r o n o l o g y o ft h e m e t a m o r p h i s m i n t h i s c o m p l e x i s , h o w e v e r ,f a r l e s s w e l l c o n s t r a i n e d . T h e o n l y d a t a o n t h ea g e o f m e t a m o r p h i s m p u b l is h e d t h u s f a r a r e m i caK - A r a n d R b - S r a g e s i n t h e r a n g e 1 0 - 1 5 . 5 M a[6 ,7 ], 4 A r / 3 9 A r m i ca a g e s a r o u n d 1 6 , 5 M a , a n d4 A r / 3 9 A r a m p h i b o l e a g e s o f a b o u t 2 5 a n d 4 8 .5M a [ 8 ] . I n a d d i t io n , c o n v e n t i o n a l K - A r t o u r m a -l i n e a g e s b e t w e e n 1 1 5 a n d 8 0 M a [ 7 ] a n d4 A r / 3 9 A r t o u r m a l i n e r e f e r e n c e l i n e s w i t h a g e sb e t w e e n 8 9 a n d 5 2 M a [ 4 , 9 ] h a v e b e e n o b t a i n e d .

0012-821X/92/$05.00 1992 - Elsevier Science Publishers B.V. All rights reserved


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3/19

THERMAL RESETTING IN THE BETIC ZONE, SPAIN 175During late stage thermal overprinting inmetamorphic belts radiogenic argon may diffusefrom minerals. The 4Ar/39Ar technique can yield

detailed information on the age of such events, asit has the capability of resolving resulting 4Atgradients in mineral samples. Partial thermal re-setting results in age spectra with monotonouslyrising apparent ages when volume diffusion playsan important role [e.g. 10]. Severe resetting canlead to spectra characterized by small, but steadilyincreasing apparent ages, approaching a newplateau. Resetting by recrystallization, leading toa mixture of white micas, may result in morecomplex dome-shaped spectra [11].The effects of resetting while studying thechronology of the older history of polyphase de-formed and metamorphosed rocks can be mini-mized by selecting samples with one generationof mica grown during a single, well-defined,tectono-metamorphic phase. However, the suc-cess of such an approach strongly depends on thedegree and mechanism of subsequent thermalresetting. Single grain techniques allow geo-chronological information to be obtained fromgrains of which the relationship with thetectono-metamorphic fabric in the rocks is pre-cisely known. Moreover, the use of single grainsallows us to check the pur ity of the material used.Thus, single grain geochronology is likely to pro-vide better constrained ages of tectono-metamor-phic events than conventional bulk sample tech-niques. With a focused laser beam, isotopic spotanalyses within single grains can be made [e.g.12-16], whereas with a defocused beam singlegrain laser step heating can be performed [e.g.15,17]. In addition to a combined single grainlaser step heating and spot fusion analysis, thispaper also presents results from standard induc-tion furnace step heating dating of Alpine phen-gite.2 . T e c t o n i c se t t i n g

rock ranges are surrounded by Burdigalian toQuaternary sedimentary basins [19-21], in whichMiocene and younger volcanic rocks [21,22] arewidespread (Fig. 1).The Mulhacen Complex consists of three tec-tonic units with an identical tectono-metamorphicevolution [2-4]. From top to bottom, these are:(3) the Huertecicas-Altas-Almocaizar Unit, (2)the Macael-Chive Unit, and (1) the Nevado-Lubrin Unit (Fig. 3). The tectonic units are de-fined by Paleozoic rocks thrust over metasedi-ments with an inferred Triassic age [23]. The P - Tevolution of the Mulhacen Complex in the east-ern Sierra de los Filabres, where the sampleswere taken, is well established [1-4] (Fig. 2) andwill be summarized here. Early Alpine metamor-phism produced eclogites under static conditions(300-450C, 1.0-1.1 GPa), which graded intosynkinematic conditions of around 500C andsimilar pressures during the first deformationphase (D~). The maximum temperature (550-570C) was reached during early stages of decom-pression coeval with the main deformation (D2).After this event, substantial decompression andcooling to at least 400C (at the 10-12 km depth)took place (D3). Late reheating, associated with

1.0.9.8

PG l a u

s t a b . /

-in

The Internal, or Betic, Zone consists of a stackof four nappe complexes, which are from top tobottom: (4) the Malaguide Complex, (3) the Alpu-jarride Complex, (2) the Mulhacen Complex and(1) the Veleta Complex [2-4]. These overlie theAlmagride Complex, the metamorphic equivalentof the External Zone [4,18] (Fig. 1). Metamorphic

1 0 0 2 0 0 3 0 0 T ( 'C ) 4 0 0 5 0 0 6 0 0

Fig. 2. P - T - t path of the Mulhacen Complex shows HPconditions followedby decompressionand tooling, which gaveway to late stage reheating at LP conditions. Late staterepeated thermal resetting is schematically ndicated. P - Tboxes from [2]; phases of penetrative deformation D to D4from [4]. Staurolite-in, AI-silicate triple point, and glauco-phane stability rom [40-42] respectively.


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176 K. DE JONG E T AL.fluid advection, locally raised temperatures above500C at a depth of 8-10 km. The last phase ofpenetrative deformation (D 4) occurred during thesecond thermal peak. Reheating was the conse-quence of Late Oligocene to Early Miocene ex-tension, resulting in emplacement of a transientheat source into the lower sections of the crust

2~i0"

[ 2 - 4 , 2 4 ] .

3 . S a m p l e s e l e c t i o n a n d m i n e r a l p r e p a r a t i o nIn order to constrain the age of the cooling

branch of the P-T,t path of the early episode ofthe tectono-metamorphic evolution and to studythe effects of late stage reheating, we selectedsamples with the bedding parallel main tectono-metamorphic fabric $2, in both the Paleozoic and

2*0/ 2 0 /

Mulhacen Complex (undi f ferent iated)o n t o p of the Alpujar r ide ComplexAlpujardde Complex

M U L H A C E N C O M P L E XHuer tec icas-Al tas - A lmocaizar Uni tMacaeI-Chive Uni t

~ - - ~ M i c as ch i st -c a r bona te s equen c eMarbles, part ly brecciatedAlbi te-r ich micaschists and quartz i tes

I I N eog ene and Qua temar y A L M 2 7 7 S a m p le locations

N e v ~ l o - L u b d n U n it

Fig. 3. Tectonic map of the eastern Sierra de los Filabres based on [1 2,23] with the location of samples. The Mulhacen Complexconsists of three stacked nappe units: From top to bottom these are: the Huertecicas-Altas-Almocaizar Unit, the Macael-ChiveUnit and the Nevado-Lubrin Unit.


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THERMAL RESETTING IN THE BETIC ZONE, SPAIN 177Triassic series, from a number of locations (Fig.3). Phengites were separated from mica schists,marbles and gneisses with this early Alpine fab-ric. In mica schist samples this schistosity is char-acterized by an almost perfect quartz-mica differ-entiation; relict older mica crystals (S 1) occurlocally in the quartz-rich domains of the mainfabric. Main phase micas in the Mulhacen Com-plex have a characteristic stoichiometr ic Si 4+value of 3.32-3.24 [2]. Samples used for inductionfurnace step heating are graphite-rich,chloritoid-bearing garnet-mica schists of the Pale-ozoic series of the Macael-Chive Unit (ALM 272and ALM 273) and albite-bearing chlorite-micaschists devoid of graphite of the Triassic se-quence of the Nevado-Lubrin Unit (ALM 270and ALM 271). The recrystallized micas in thelatter have a grain size that is larger than that ofthe two former samples; albite and chlorite grewat the expense of phengite during post-main phasegreenschist metamorphism (D3). Single grains ofwhite mica (ALM 275 and ALM 276), used fortotal fusion laser dating, were taken from twodifferent calcite marble intercalations close tomica schists ALM 270 and 271. ALM 226, usedfor induction furnace step heating, was separatedfrom a fine-grained gneiss occurring as a layer ofa few metres in thickness in the basal sequence ofMacael-Chive Unit. Sample 87 JK 127, used forsingle grain laser step heating and spot fusiondating, was taken from a gneiss in the margin of a5.5 km 3 gneiss body in this uni t. The grain has adiameter of 3 mm, which is representative ofthese coarse-grained gneisses. In thin section suchcrystals are essentially strain free and locallyovergrow the tectonic banding $2, indicating thattheir present shape resulted from post-D 2 anneal-ing.Single grain 87 JK 127 was separated from thehand specimen after gentle crushing. Under abinocular zoom microscope the grain turned outto be free of intergrowth and deformation of thebasal plane. The tabular grain was split in twoflat halves parallel to the basal cleavage; laserstep heating was performed on the one half andlaser spot fusion dating was carried out on theother. Phengite separates (125-250 /~m) for in-duction furnace step heating dating and phengitesingle grains from marbles (250-500 /~m) usedfor laser total fusion dating were separated using

standard procedures. The largest phengite singlegrains from the latter group were selected undera binocular zoom microscope. Small carbonateintergrowths were removed with 0 . 5 N HNO 3during ultrasonic treatment lasting 2-3 minutes.4 . Exper im enta l procedures

Irradiation of the samples took place in theOsiris reactor (Centre d'Etudes Nucl~aires, Saclay,France) together with biotite standard LP6 (K-Atage = 128.5 Ma, [25]) as a flux monitor. Theyreceived a total flux of 4.0 1018 neutrons/cm 2and were rotated about a vertical axis duringirradiation. Samples were placed on three levels,each with three monitors to analyse the homo-geneity of the neutron flux. The 4Ar*/a9ArKobtained from the standards were reproducible tobet ter than _+ 1% (lo-) on each level.During induction furnace step heating about10 mg of irradiated 99% pure phengite separatewas incrementally heated to fusion, following theexperimental procedures given previously in [26].Gas release started typically at about 600C andthe last 10-15% of Ar release, in the twelfth orthirteenth step, took place at around 1350C dur-ing melting.Single grain analyses were performed with alaser probe using a 4 W continuous argon ionlaser. The analytical procedure is outlined else-where [12]. The laser system was used in twoways. Firstly, with a defocused beam for singlegrain total fusion and step heating analysis. Thebeam diameter was approximately twice the sizeof the grain, in order to achieve homogeneousenergy distribution over the grain [27]; fusion inthe final step was achieved by beam focussing.Secondly, with a focused beam of about 40 /~m indiameter spot fusion analyses of 50-200 /zm wideregions were made on the other half of the splitgrain. During every spot fusion analysis the grainwas punctured by the laser beam.The mass spectrometer, used for both stepheating and laser probe dating, consists of aMASSE tube, a B~iur-Signer ion source and anAEM 1000ETP electron multiplier. System blankruns in laser experiments were carried out at thestart of each determination and repeated everythird run. Background values were subtractedfrom the subsequent sample analysis results. Typ-


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178 K. DE JONG ET AL.i ca l b a c k g r o u n d v a l u e s w e r e 2 1 0 -1 1 , 2 1 0 -1 3 ,2 10 - 13 a n d ( 0 . 5 - 2 ) x 1 0 - 12 c m 3 S T P f o r t h e4 0 , 3 9 , 3 7 a n d 3 6 i s o t o p e s r e s p e c t iv e l y . F o r d e t a i l so n t h e i s o t o p e c o r r e c t i o n s a n d c r i t e r i a t o d e f i n e a

p l a t e a u w e r e f e r t h e r e a d e r t o [ 28 ]. A g e c a l c u l a -t io n s a r e m a d e u s in g t h e d e c a y c o n s t a n t s o fS t e i g e r a n d J ~ig er [2 9 ] a n d a r e g i v e n w i t h l t r e r r o re s t i m a t e s .

TABLE 14Ar/39At isotopic data for phengite induction furnace step heating. 4Ar is argon derived from natural potassium decay, 39Ar isK-derived argon during irrad iation and 3TAr is irradiation-induced argon from calcium. The irradiation parame ter J (uncertainty =0.5%) was determine d from laboratory standard biotite LP6 (K-Ar age of 128.5 Ma [25]). The decay constant of 4K = 5.543.10-1 a-~ [29]. The experimental temperature was determined by optical pyrometerTemperature Atmospheric Cumulative 37Arca//39AFK 4Ar*/ /39myK Age (Ma) la(C) contami- 39mr (% )

nation (%)A L M 2 2 6 P h e n g it e . I n t e g r a t e d a g e: 1 4 .3 5 : 0 .1 M a . J = 0 . 0 0 6 7 1

600 81.53 1.08 7.85 1.0214 12.3 0.7670 64.55 3.92 6.95 1.1292 13.6 0.3730 59.78 7.78 7.06 1.1558 13.9 0.3760 43.73 10.71 2.06 1.1695 14.1 0.3780 41.59 14.32 1.60 1.1632 14.0 0.1800 29.67 23.16 0.9 7 1.1791 14.2 0.1830 23.05 37.62 0.67 1.1942 14.4 0.1860 20.09 61.28 0.53 1.2007 14.5 0.1900 21.02 80.47 0.53 1.2015 14.5 0.1950 25.82 92.64 0.64 1.1885 14.3 0.11200 35.50 99.23 11.26 1.1829 14.3 0.1

fus ion 91.90 99.72 61.60 1.1397 13.8 1.2A L M 2 7 0 P h e n g i te . I n t e g r a t e d a g e: 1 7 . 3 5 : 0 . 1 M a . J = 0 . 0 0 7 5 6

590 76.26 1.14 3.01 1.0845 14.7 0.8650 65.84 3.36 0.55 1.1769 16.0 0.5730 35.60 11.98 0.54 1.2288 16.7 0.1760 25.54 27.63 0.16 1.2662 17.2 0.1780 22.44 36.76 0.04 1.2671 17.2 0.1800 20.90 47.05 0.18 1.2683 17.2 0.1830 22.71 55.67 0.06 1.2666 17.2 0.1860 21.93 66.41 0.21 1.2649 17.2 0.1900 29.17 72.45 0.24 1.2913 17.5 0.2950 19.53 85.29 0.37 1.2854 17.4 0.11200 25.99 99.25 1.73 1.3071 17.7 0.1fus ion 96.34 99.78 12.10 0.9481 12.9 3.0

A L M 2 7 1 P h e n g i t e. I n t e g r a t e d a g e : 1 7 . 6 4- 0 . 1 M a . J = 0 . 0 0 7 5 6600 86.67 0.49 8.59 0.8979 12.2 1.7670 67.18 2.27 2.87 1.1844 16.1 0.4730 43.33 6.97 1.00 1.2347 16.8 0.2760 32.80 20.95 0.21 1.2951 17.6 0.1780 19.84 35.04 0.14 1.2915 17.5 0.I800 14.15 52.10 0.10 1.3027 17.7 0.1830 16.89 62.61 0.19 1.2955 17.6 0.1860 : 19.28 73.84 0.15 1.3009 17.7 0.1900 23.20 81.39 0.09 1.3084 17.8 0.1 50 24.08 87.64 0.21 1.3061 17.7 0.21200 20.50 99.04 0.16 1.3370 18.1 0.1fus ion 96.08 99.88 0.00 1.2608 17.1 2.7


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THER MA L RESETFING IN THE BETIC ZONE, SPAIN

T A B L E 1 (continued)179

Temperature Atmospheric Cum ulat ive 3 7 A r ca / 3 9 A R K 4 A r *//39ARK A g e ( M a ) 1 r( C ) contami- 39Ar % )

nation ( % )A L M 2 72 Pheng i te . In tegra ted a g e. " 1 9 .1 + 0 .1 Ma . J = 0 .0 0 6 7 1

6 0 0 7 3 . 0 36 7 0 5 3 . 7 17 3 0 3 2 . 7 27 6 0 2 6 . 8 07 8 0 2 9 . 9 28 0 0 2 3 . 1 28 3 0 1 8 . 1 9860 15 .259 0 0 1 8 . 0 89 5 0 1 6 . 3 5

1 2 0 0 1 9 . 2 4fusion 8 0 . 4 0A L M 2 73 Pheng i te . In teg ra ted age : 2 5 . 9 +

5 0 0 9 8 . 6 56 0 0 6 7 . 9 26 7 0 5 5 . 9 87 3 0 2 3 . 6 77 6 0 2 1 . 0 97 8 0 1 9 . 2 18 0 0 1 3 . 8 88 3 0 1 4 . 3 88 6 0 1 4 . 5 39 0 0 1 7 . 1 69 5 0 2 1 . 0 0

1 2 0 0 2 1 . 7 0fusion 8 8 . 4 3

1 .24 11 .53 1 .2028 14 .5 1 . 95 . 3 4 7 . 2 6 1 . 3 7 1 4 1 6 . 5 0 . 71 4 . 2 6 4 . 5 2 1 . 3 9 3 4 1 6 . 8 0 . 3

2 1 . 4 1 2 . 8 1 1 . 4 3 4 2 1 7 . 3 0 . 327 .25 1 .76 1 . 49 79 18 .1 0 . 43~7.90 0 . 95 1 .5978 19 .2 0 .24 9 . 0 1 0 . 5 7 1 . 6 4 2 0 1 9 . 8 0 . 26 2 . 7 3 0 . 6 4 1 . 6 4 7 8 1 9 . 8 0 . 27 2 . 7 4 0 . 9 1 1 . 6 7 4 3 2 0 . 2 0 . 38 3 . 6 8 1 . 0 4 1 . 7 1 9 1 2 0 . 7 0 . 29 8 . 8 2 6 . 7 4 1 . 5 9 3 9 1 9 . 2 0 . 29 9 . 3 3 4 4 . 8 9 4 . 2 4 1 2 5 0 . 6 5 . 5

0 . 1 M a . J = 0 . 0 0 6 7 10 . 8 0 2 5 . 7 4 0 . 6 2 4 3 7 . 5 3 . 41 .65 11 .83 1 . 49 55 16 . 9 0 .76 . 7 3 1 0 . 1 9 1 . 6 0 8 4 1 9 . 4 0 . 3

1 5 . 9 4 5 . 9 9 1 . 7 7 3 3 2 1 . 3 0 . 12 5 . 8 1 3 . 0 5 2 . 0 1 0 4 2 4 . 2 0 . 13 4 . 9 0 2 . 2 7 2 . 1 2 2 4 2 5 . 5 0 . 14 9 . 7 4 2 . 3 9 2 . 2 5 4 3 2 7 . 1 0 . 16 2 . 2 8 2 . 6 6 2 . 3 4 4 0 2 8 . 2 0 . 17 4 . 4 3 3 . 7 4 2 . 3 8 5 1 2 8 . 7 0 . 18 2 . 2 2 1 3 . 9 2 2 . 4 7 4 8 2 9 . 7 0 . 28 7 . 9 1 2 4 . 0 4 2 . 4 5 1 0 2 9 . 4 0 . 39 8 . 6 6 6 5 . 2 2 2 . 0 8 8 9 2 5 . 1 0 . 29 9 . 2 2 1 4 1 . 6 4 2 . 7 7 7 9 3 3 . 3 2 . 4

5 . R e s u l t s5.1 Induction step heatingF i v e in d u c t i o n s t e p h e a t i n g e x p e r i m e n t s w e r e

m a d e o n p h e n g i t e s e p a r a t e s , t h e r e s u l ts o f w h i c ha r e l i s t e d i n T a b l e 1 . T w o d i f f e r e n t t y p e s o f a g es p e c t r a c o u l d b e d e f i n e d ( F i g s . 4 a n d 5 ) . T y p e Is p e c t r a ( F i g . 4 a a n d b ) s h o w m o n o t o n o u s l y r i s i n ga p p a r e n t a g e s a n d r e c o r d t h e h i g h e s t a g e s . T y p eI I s p e c t r a ( F i g . 5 a - c ) a r e c h a r a c t e r i z e d b yp l a t e a u s . A p p a r e n t a g e s o f lo w t e m p e r a t u r e s t e p s ,r e l e a s i n g m o r e t h a n 1 % o f 39AFi n t y p e I sp e c t r a ,a p p r o x i m a t e p l a t e a u a g e s o f t h e s e c o n d g r o u p .

Type I spectra. A L M 2 72 sh ow s a m o n o t o n o u s l yr is in g a g e sp e c t r u m , f r o m 14 .5 + 1. 9 M a i n t h el o w e s t t e m p e r a t u r e s t e p t o 2 0 . 7 _ 0 .2 M a ( F ig .4 a ). A p p a r e n t a g e s o f A L M 2 7 3 r is e g ra d u all yf r o m 16 .9 + 0 . 7 t o 2 9 . 7 5 : 0 . 2 M a ( F i g . 4b ) . T h i sl a tt e r a p p a r e n t a g e i s th e h i g h e s t r e c o r d e d i n t h ef i v e i n d u c t i o n h e a t i n g e x p e r i m e n t s p e r f o r m e d .

B o t h s p e c t r a s h o w a s i g n i f i c a n t d e c r e a s e i n a p -p a r e n t a g e i n t h e l a s t s t e p , r e l e a s i n g 1 0 - 1 5 %3 9 A t .

Type H spectra. A L M 2 70 a n d A L M 2 71 y i e l d e dp l a t e a u a g e s o f 17.3 __ 0.1 M a a n d 17 .6 _+ 0.1 M ao v e r 7 3 % a n d 8 1 % o f 39Ar r e l e a s e r e s p e c t i v e l y( F i g . 5 a a n d b ) . T h e f i r s t 10 % o f d e g a ss i n g i nb o t h sa mp l e s r e su l t e d i n sh a r p l y i n c r e a s i n g a p -p a r e n t a g e s t o t h e p l a t e a u v a l u e s . T h e a p p a r e n ta g e s o f t h e l o w e s t t e m p e r a t u r e s t e p s a r e 12.2__1 .7 M a ( A L M 2 7 1) a n d 1 4 .7 _ 0 .8 M a ( A L M 2 70 ).T h e l a s t t h r e e s t e p s i n b o t h s a m p l e s , r e s p o n s i b l ef o r 2 5 - 3 0 % 3 9 A r r e l e a s e , h a v e a p p a r e n t a g e s t h a tare s l ight ly , but s igni f i cant ly , h igher than thep l a t e a u a g e .

A L M 2 2 6 h a s a n a s y m m e t r i c a l d o m e s p e c t r u mw i t h s i g n i f i c a n t l y r i s i n g a p p a r e n t a g e s , f r o m 12 . 3_ 0 .7 M a ( 1 . 1% 3 9 A y r e l e a se ) t o 14 .5 5 : 0 . 1 M af o l l o w e d b y a s l i g h t d r o p t o 14 .3 _+ 0 .1 M a d u r i n gt h e l a s t 2 0% o f 3 9 A r r e l e a se ( F i g . 5 c ) . T h e c e n t r a l


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18 0area of the dome (85% of 39Ar release) def ines aplateau of 14.4 + 0.1 Ma.

5.2 Single grain analysesSingle phengite grains from marbles (ALM 275and ALM 276) and a gneiss (87 JK 127) wereanalyzed with the laser probe; the results aregiven in Table 2. The 3 mm wide phengite grain87 JK 127 was split into two halves along thebasal plane. One half was analyzed by spot fusiondating, the other by step heating with a defocusedlaser beam. The results from the two methods areconcordant with each other. The small size of thesingle phengite grains from marlSles only allowedtotal fusion analyses; either one or three grains

K . D E J O N G E T AL .

were completely melted during two measure-ments on each sample.A L M 2 7 5 a n d 2 7 6 : t o ta l f u s io n a g e s Total fu-sion of several phengite grains gave the followingweighted me an ages: 15.4 + 1.2 Ma (ALM 275)and 17.0 + 0.7 Ma (ALM 276). The latter age is

concordant with the i ntegrated ages of ALM 270(17.3 + 0.1 Ma) and ALM 271 (17.6 + 0.1 Ma),obtained from mica schists less than 50 m belowthe marble layer.87 JK 127: laser spot fusion analyses One halfof the phengite grain was studied by 22 spotfusion analyses made on the basal cleavage plane(Fig. 6). In order to detect possible artifacts in-duced by heating of the mineral outside the fu-sion spots, successive random analyses were made.

U J(3,


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T H E R M A L R E S E T I ' I N G I N T H E B E T I C Z O N E , S P A I N 18 1

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/J" 0 ~ KabylianMassifs

/ 0 F i g . 9 . T e c t o n i c m a p o f t h e w e s t e r n M e d i t e r r a n e a n . T h e b a r b e d l i n es i n d i c a t e m a i n r i f t s t r u c t u r e s [4 3 ], w h i c h a r e c o n t i g u o u s w i tht h e R h 6 n e - B r e s s e r if t o f s o u t h e r n F r a n c e . T h e N E - S E p a t t e r n o f e l e v a t e d h e a t f l ow [4 4 ] u n d e r l i e s t h e m a i n r if t s ( B a l e a r i c B a s i n(BB) a n d t h e A l b o r a n B a s i n ( A B ) ) ; p e a k s i n h e a t f lo w ( >_ 1 00 m W m - 2 ) c o i n c i d e w i th c e n t r e s o f S e r ra v a l l i a n a n d y o u n g e r v o l c a n i c

ac t iv i ty .


17/19

7 . C o n c l u s i o n s

6

4 A r / 3 9 m r l a se r p r o b e d a t i n g o f a n e a r l y A l p i n ep h e n g i t e s i n g le g r a i n r e s u l t e d i n p r o g r e s s i v e lyd e c r e a s i n g s p o t a g es , f r o m 3 0 - 3 1 M a i n t h e c o r eo f th e g r a i n t o 2 5 -2 6 M a i n th e r im . T h i s p a t t e r ns e e m s t o b e p r i n c i p a l l y d u e t o t h e r m a l r e s e tt i n g ,i n a g r e e m e n t w i t h m o d e l l i n g o f t h e l a s e r s te p

h e a t i n g a g e s p e c t r u m o f th e o t h e r h a l f o f t h e s p li tg r a i n , p o i n t in g t o A r l o ss a t a b o u t 2 5 M a . T h e r -m a l r e s e t t i n g i s c o n s i s t e n t w i t h la t e s t a g e r e h e a t -i n g d u e t o e x te n s i o n a l t e c t o n i s m . S p o t a g e sa r o u n d 3 0 M a p r o b a b l y r e p r es e n t m i n i m u m e st i-m a t e s o f th e c o o l i n g a g e o f t h e m a i n t e c t o n o -m e t a m o r p h i c p h a s e ( D 2 ) , w h ic h m i g ht , h ow e v e r ,h a v e b e e n s li g ht ly l o w e r e d b y t h e 2 5 M a r e s e t-

THERMAL RESE'Iq ' ING IN THE BETIC ZONE, SPAIN 189

38

36

34

Fig. 10. Ma p of Bouguer a noma lies (mGa l) in the westernmost M editerranean area [45]. The B etic-R if arc is underlain by anarcuate pattern of negative anoma lies, which is cut by a zone of NN E- SS W trending faults [after 21]. Crustal thicknesses(diamonds [37,46]) decrease southwards and eastwards towards the A lboran Basin. Miocene and yo unger magm atism (dots [21]) areconcentrated in the thinnest crust. The sam pling area (arrow) is located n ear major volcanic centres in thinned crust.


18/19

19 0 K . D E J O N G E T A L .t in g . B e c a u s e t h e s e r e s u l t s a r e b a s e d o n o n l y o n es i n g le g ra i n , c o n c l u s i o n s a r e p r e l i m i n a r y a n d n e e dt o b e s u b s t a n t i a t e d b y f u r t h e r w o r k .

A g e s p e c t r a o b t a i n e d f r o m i n d u c t i o n f u r n a c es t ep h e a t i n g o f m a i n p h a s e p h e n g i t e s s h o w a g e st h a t a r e s u b s t a n t i a l l y l o w e r t h a n t h o s e o n t h es i n g le g r a in . T w o t y p e s a r e p r e s e n t : T y p e I s p ec -t r a d e m o n s t r a t e i n c r e a s i n g a p p a r e n t a g e s o v e rt h e fi r s t 8 5 - 9 0 % o f 3 9 A r r e l e a s e a n d c o n t a i n t h eh i g h e s t a p p a r e n t a g e s o f 2 0 .7 + 0 .2 M a ( A L M2 7 2 ) a n d 2 9 .7 + 0 .2 M a ( A L M 2 7 3 ). T y p e I I s p e c -t r a h a v e a g e p l a t e a u s o f 1 4 .4 ___ 0 .1 M a ( A L M2 2 6 ), 1 7 .3 + 0 . 1 M a ( A L M 2 7 0 ) a n d 1 7 . 6 + 0 .1 M a( A L M 2 71 ). T h e s e a g e d i f f e re n c e s m a y b e i n t e r -p r e t e d a s t h e r e s u l t o f lo c a l r e s e t t i n g o f t h ei s o t o p e sy s te m i n t h r e e p e r i o d s : a b o u t 1 7 - 1 9 M a ,1 3 - 1 5 M a a n d 8 - 1 0 M a . T h e r e p e a t e d t h e r m a lr e s e t t i n g i s c o e v a l w i t h t h e c l i m a x o f v o l c a n i s m i nt h e e a s t e r n B e ti c Z o n e a n d p r e s u m a b l y o c c u r r e db y fl o w o f a s s o c i a t e d h o t f l ui d s . C o n c e n t r a t i o n o fv o l c a n i s m i n t h e t h i n n e s t c r u s t p o i n t s t o r e s e t t i n gd u r i n g c r u s t a l a n d s u b c r u s t a l e x t e n s i o n . M o d -e l li n g i m p l i e d t h a t r e s e t t i n g m a y h a v e b e e n s u p e r -i m p o s e d o n a g e s p e c t r a w i t h s l i g h tl y o l d e r a g e s i nt h e r a n g e o f 1 8.5 M a ( A L M 2 7 0) t o 21 .5 M a( A L M 2 72 ). T e c t o n i c c o n s i d e r a t i o n s , h o w e v e r ,i m p l y t h a t t h e s e m o d e l a g e s s h o u l d b e r e g a r d e da s d a t i n g c o o l i n g re s u l t i n g f r o m o v e r t h r u s t i n g ,w h i c h s e p a r a t e d t h e t w o e x t e n s io n a l p e r i o d s .

A c k n o w l e d g e m e n t sK d J a c k n o w l e d g e s t h e r e c e i p t o f g ra n t s f r o m

t h e Centre Nat ional de la Recherche Scient i f ique( C N R S ) f o r d e f r a y i n g e x p e n s e s i n c u r r e d d u r i n gh i s s t a y i n N i c e f o r s e v e r a l m o n t h s , a n d f r o m t h eN e t h e r l a n d s O r g a n i z a t i o n f or S c i e n ti f i c R e s e a r c h( N W O ) f o r m e e t i n g t r a v e l co s ts . K d J w o u l d a l s ol i ke t o ta k e t h e o p p o r t u n i t y t o t h a n k t h e s u p e r v i -s o r s o f h is t h e s is , P r o f s . S . C l o e t i n g h a n d I .S .O e n , f o r t h e i r e n d o r s e m e n t . C h r i s H a l l is t h a n k e df o r s u p p l y i n g u s w i t h a c o p y o f t h e S D P m o d -e l l i n g p r o g r a m a n d c r i t i c a l l y r e a d i n g t h em a n u s c r i p t . S t 6 p h a n e S c a i l l e t s k i ll fu l l y s p l it t h ep h e n g i t e g r a i n a n d h e l p e d d u r i n g l a s e r in g . P a u lR e n n e , R a y B u r g es s a n d tw o a n o n y m o u s r e v ie w -e r s m a d e v a l u a b l e s u g g e s t i o n s fo r i m p r o v i n g t h et y p e s c r i p t .

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8 P . Moni4 , J . Galindo-Za ldivar , F . Gon zalez-L odeiro, B.Goff6 a nd A . Jabaloy, 4 m r / 3 9 A r geochronology o f Alp inetectonism in the Betic Cord illera s (south ern Spa in) , J.Geol . Soc. Lond on 14 8, 289-297, 1991.9 K. D e Jong, G. F6raud a nd J .R. Wijbrans , F irs t 4Ar /39A1"step heat ing a nd laser fusion da t ing of tourma line: afeas ibi li ty s tudy f rom the B etic Zone, SE Spain with impli-cat ions for the the rmo- tectonic evolut ion o f the bel t , Chem.Geol. Isot. Geosci. Sect. , submitted, 1991.10 T.M. Ha rr ison a nd I . McDougall , Invest igat ions of anintrusive contact , nor thwest Nelson, New Ze a la nd -- I I .Dif fus ion o f r adiogenic and excess 4A r in hornb lenderevealed by 4A r /39A , age spectrum analyses, Geochim.Cosmochim. A cta 44 , 2005-2020, 1980.11 J .R. Wijbrans and I . McDougall , 4Ar /39Ar dat ing ofwhite micas f rom an A lpine high-pressure meta morph icbelt on Na xos (Greece) : the reset t ing of the a rgon isotopesystem, Contrib. Mineral. Petrol. 93, 187-194, 1986.12 S. Scai lle t, G . F6raud , Y. La gab r iel le , M. Ba ll~vre and G.Ruffet , 4 0 A y / / 3 9 m r da ting by s tep-hea t ing and spot- fus ionof phengi tes f r om the Dor a Ma i r a nappe of the Wes te r nA lps , Ita ly , Geology 18, 741-744 , 1990.13 D. York, C.M. Hall , Y. Yanase , J .A . Hanes and W.J .Kenyon, 4 / ~ x r / 3 9 m r dating of ter res tr ia l minerals with acontinuous laser , Geophys. Res. Lett. 8, 1136-1138, 1981.14 H. M aluski and O.A. Schaeffer, 39Ar-amr l a se r p r obe


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THERMA L RESETTING IN THE BETIC ZON E, SPAIN 191d a t i n g o f t e r r e s t r i a l r o c k s , E a r t h P l an e t . Sc i . L e t t . 5 9 ,2 1 - 2 7 , 1 9 8 2 .

1 5 D . Ph i l l i p s an d T . C . On s t o t t , A r g o n i s o t o p i c z o n i n g i nm a n t l e p h l o g o p i t e , G e o l o g y 1 6, 5 4 2 - 5 4 6 , 1 9 88 .

1 6 P . R . Re n n e , T . C . On s t o t t , M. S . D ' A g r e l l a - F i l h o , I . G . P ac c aa n d W . T e i x e ir a , 4 A r / 3 9 A r d a t i n g o f 1 .0 - 1 .1 G a m a g n e t i -z a t i o n s f r o m t h e S a o F r a n c i s c o K a l a h a r i c r a t o n s : t e c t o n i ci m p l i c a t i o n s f o r P a n - A f r i c a n a n d B r a s i l i a n o m o b i l e b e l t s ,E a r t h P l a n e t . S ci. L e t t . 1 0 1 , 3 4 9 - 3 6 6 , 1 9 9 0.

1 7 J . R . Wi j b r an s , M. Sc h l i e s t e d t an d D . Y o r k , S i n g l e g r a i na r g o n l a s e r p r o b e d a t i n g o f p h e n g i t e s f r o m t h e b l u e s c h i s tt o g r e e n s c h i s t t r an s i t i o n o n S i f n o s ( Cy c l ad e s , Gr e e c e ) ,Co n t r i b . Mi n e r a l . Pe t r o l . 1 0 4 , 5 8 2 - 5 9 3 , 1 9 9 0 .

1 8 O . J . S i m o n , O n t h e T r i a s s i c o f t h e B e t i c Co r d i l l e r a s( S o u t h e r n S p a i n ) , C u a d . G e o l . l b . 1 1 , 3 8 5 - 4 0 2 , 1 9 87 .

1 9 H . R . V 6 1k a n d H . E . R o n d e e l , Z u r G l i e d e r u n g d e sJ u n g t e r t i ~ i r s i m Be c k e n s v o n V e r a , S i i d o s t - Sp an i e n , Ge o l .M i j n b o u w 4 3 , 3 1 0 - 3 1 5 , 1 9 64 .

2 0 R . W e i j e r m a r s , T h . B . R o e p , B . V a n d e n E e c k h o u t , G .Po s t m a a n d K . K l e v e r l a a n , U p l i f t h i s t o r y o f a Be t i c fo l dn a p p e i n f e r r e d f r o m N e o g e n e - Q u a t e r n a r y s e d i m e n t a t i o na n d t e c t o n i c s ( i n t h e S i e r r a A l h a m i l l a a n d A l m e r ~ a a n dS o r b a s a n d T a b e r n a s B a s i n s o f t h e B e t i c C o r d i l l e r a s , S ESp a i n ) , Ge o l . Mi j n b o u w 6 4 , 3 9 7 - 4 1 1 , 1 9 8 5 .

2 1 F . D . D e L a r o u z i ~ r e , J . Bo l z e , P . Bo r d e t , J . He r n an d e z , C .M o n t e n a t a n d P . O t t d ' E s t e v o u , T h e B e t i c s e g m e n t o f t h el i t h o s p h e ri c T r a n s - A l b o r a n s h e a r z o n e d u r i n g th e L a t eMi o c e n e , Te c t o n o p h y s i c s 1 5 2 , 4 1 - 5 2 , 1 9 8 8 .

2 2 H . B e l l o n , P . B o r d e t a n d C . M o n t e n a t , C h r o n o l o g i e d um a g m a t i s m e n 6 o g ~ n e d e s C o r d i l l ~ r e s b 6 t i q u e s ( E s p a g n em6rid ionale) , Bul l . Soc . G6ol . Fr . 7 (26) , 205-217 , 1983 .

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2 4 J . D . V a n W e e s , K . D e J o n g a n d S . C l o e t in g h , T w o - d im e n -s i o n a l P - T - t m o d e l li n g a n d t h e d y n a m i c s o f e x t e n si o n a n di n v e r s i o n i n t h e Be t i c Zo n e ( SE Sp a i n ) , Te c t o n o p h y s i c s203 , in press , 1992.

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2 6 G . F 6 r a u d , J . G a s t a u d , J . -M . A u z e n d e , J . -L . O l i v e t a n d G .C o r n e n , 4 A r / 3 9 A r a g e s f o r th e a l k a l i n e v o l c a n i sm a n d t h eb a s e m e n t o f G o r r i n g e B a n k , N o r t h A t l a n t i c O c e a n , E a r t hPlanet . Sc i . Let t . 57 , 211-226 , 1982.

2 7 C . M . H a l l, C a l c u l a t io n s o f e x p e c t ed t h e r m a l g r a d i e n t s i nl a s e r 4 A r / 3 9 A r s t e p - h e a t i n g e x p e r i m e n t s , E O S ( T r a n s .A m. Ge o p h y s . U n i o n ) 71 , 6 5 3 , 1 9 9 0 .

2 8 G . F6 r au d , D . Y o r k , C . M6 v e l , G . Co r n e n , C . M. Ha l l an dJ . - M . A u z e n d e , A d d i t i o n a l 4 0 A r / 3 9 A r d a t i n g o f t h e b a s e -m e n t a n d a l k a l i n e v o l c a n i s m o f G o r r i n g e B a n k ( A t l a n t i cOc e an ) , E a r t h P l an e t . Sc i . L e t t . 7 9 , 2 5 5 - 2 6 9 , 1 9 8 6 .

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3 0 D . M c M a s t e r , A p r e l i m i n a r y 4 A r / 3 9 A r st u d y o f t h e t h e r -m a l h i s t o r y a n d a g e o f go l d in t h e R e d L a k e g r e e n s t o n eb e l t , M. Sc . Th e s i s , U n i v . To r o n t o , 1 9 8 7 ( U n p u b l . ) .

3 1 G . T u r n e r , T h e r m a l h i s t o ri e s o f m e t e o r i t e s b y th e 3 9 A r -4 A r m e t h o d , i n : M e t e o r i t e R e s e a r c h , P . M . M i U m a n , ed . ,p p . 4 0 7 - 4 1 7 , R e i d e l , D o r d r e c h t , 1 9 6 9 .

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3 4 P . M o n i 6 , H . M a l u s k i , A . S a a d a l l a h a n d R . C a b y , N e w3 9 A r - a A r a g e s o f H e r c y n i a n a n d A l p i n e t h e r m o t e c t o n i ce v e n t s i n Gr an d e Kab y l i e ( A l g e r i a ) , Te c t o n o p h y s i c s 1 5 2 ,5 3 - 6 9 , 1 9 8 8 .

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3 7 L . M . B a r r a n c o , J . A n s o r g e a n d E . B a n d a , S e i s m i c r e f r a c -t i o n c o n s tr a i n t s o n t h e g e o m e t r y o f t h e R o n d a p e r i d o t i tema s s i f ( Be t i c Co r d i l l e r a , Sp a i n ) , Te c t o n o p h y s i c s 1 8 4 , 3 7 9 -392 , 1990.

3 8 A . B . W e s t e r h o f , O n t h e c o n t a c t r e l a t i o n s o f h i g h t e m p e r a -t u r e p e r i d o t it e s in t h e S e r r a n i a d e R o n d a , s o u t h e r n S p a i n ,Te c t o n o p h y s i c s 3 9 , 5 7 9 - 5 9 2 , 1 9 7 7 .

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4 0 G . H o s c h e k , T h e s t a b i li t y o f s ta u r o l i t e a n d c h l o r it o i d a n dt h e i r s i g n i f i c an c e i n me t amo r p h i s m o f p e l i t i c r o c k s , Co n -t r i b . Mi n e r a l . Pe t r o l . 2 2 , 2 0 8 - 2 3 2 , 1 9 6 9 .

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4 2 W . V . M a r e s c h , E x p e r i m e n t a l s t u d ie s o n g l a u c o p h a n e : A nan a l y s i s o f p r e s e n t k n o w l e d g e , Te c t o n o p h y s i c s 4 3 , 1 0 9 - 1 2 5 ,1977.

4 3 J . - P . R e h a u l t , G . B o i l l o t a n d A . M a u f f r e t , T h e w e s t e r nM e d i t e r r a n e a n b a s i n : g e ol o g ic a l e v o l u ti o n , M a r . G e o l . 5 5 ,4 4 7 - 4 7 7 , 1 9 8 4 .

4 4 J . F . A l b e r t- B e lt r ~ in , H e a t f lo w a n d t e m p e r a t u r e g r a d i e n td a t a f r o m S p a i n , i n : T e r r e s t r i a l H e a t F l o w i n E u r o p e , V .Ce rm: 6k a n d L . Ry b a c h , e d s ., p p . 2 6 1 - 2 6 6 , Sp r i n g e r , B e r l i n ,1979 .4 5 J . W . H . V a n d e n B o s c h , Q u e l q u e s p r i n c i p e s g ~ n ~ r a u x d el ' i n t e r p r & a t i o n g r a v i m 6 t r i q u e i l l u s t r 6 s p a r d e s e x e m p l e se m p r u n t 6 s ~ l a c a r t e g r a v i m & r i q u e d u M a r o c ( s t r u c t u r ed u R i f e t i n t r u si o n s g r a n i t i q u e s a u M a r o c c e n t r a l ) , N o t e sSe r v . G~o l . Mar o c 3 5 , 1 1 7 - 1 3 6 , 1 9 7 4 .

4 6 E . B a n d a a n d J . A n s o r g e , C r u s t a l s t r u c t u r e u n d e r t h ec e n t r a l a n d e a s t e r n p a r t o f t h e B e t i c C o r d i ll e r a , G e o p h y s .J . R . A s t r o n . So c. 6 3 , 5 1 5 - 5 3 2 , 1 9 8 0.

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