DeJong_Phengite Age: Fluids, Submicroscopic Illitization, Excess Argon & Laser Probe Dating (Betics,...

8/6/2019 DeJong_Phengite Age: Fluids, Submicroscopic Illitization, Excess Argon & Laser Probe Dating (Betics, Spain)_Chemica

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.Chemical Geology 178 2001 159195

www.elsevier.comrlocaterchemgeo

Excess argon incorporation in phengite of the MulhacenComplex: submicroscopic illitization and fluid ingress during late

Miocene extension in the Betic Zone, south-eastern Spain

K. de Jong a,), G. Feraud b, G. Ruffet b, M. Amouric c, J.R. Wijbrans da

Geology Department, Geological Surey of Japan, Higashi 1-1-3, Tsukuba, Ibaraki 305-8567, Japanb

CNRS, UMR 6526 Geosciences Azur, Uniersite de Nice-Sophia Antipolis, Parc Valrose, 06108 Nice Cedex 02, France c

Centre de Recherche sur les Mecanismes de la Croissance Cristalline, CNRS, Campus Luminy, 13288 Marseille Cedex 09, Franced

Argon Laserprobe Laboratory, Faculty of Earth Sciences, Netherlands Reseach School of Sedimentary Geology, Vrije Uniersiteit,

De Boelelaan 1085, 1081 HV Amsterdam, Netherlands

Received 16 November 1999; accepted 25 October 2000

Abstract

40Arr

39Ar induction furnace and laser step-heating of well-crystallised post-tectonic phengitic mica single grains from

.gneisses of the Mulhacen Complex with an early Alpine tectonic fabric has resulted in: 1 highly scattered integrated ages, . . 392 an abnormally high atmospheric contamination and 3 often anomalously old apparent ages during early Ar release that

36 37 40 .is associated with a high Ar and Ar contamination. This low-temperature excess argon Ar component isAIR Ca XSprobably released from carbonate formed during slight alteration of the mica. More than 50% of the samples yielded plateau

ages ranging from 15.8"0.4 to 90.1"1.0 Ma. Samples taken only a few metres apart may differ in age by as much as 50

Ma; a grain that was split over the basal plane yielded plateau ages for each half that differ by 12%. The age variation on

these different scales is explained by heterogeneous40

Ar incorporation during a period with a high transient partial argonXSpressure in the metamorphic fluid, resulting from a late stage reheating event. The very swift cooling of 50100 8CrMaduring exhumation of the Mulhacen Complex concomitant with late Miocene extension may have prevented the equilibrationof different

40Ar levels in the mica.XS

HRTEM images of the oldest and youngest phengite specimens show that at least 20% of the lattice is affected by

submicroscopic illitisation, which is concentrated in several micrometer wide zones and veins that cross-cut the basal

cleavage. These are made up of aggregates of 0.070.30 mm thick crystallites of three illitic micas types, which arechemically and structurally progressively closer to pure illite and occur in different textures. The oldest specimen is affected

.most severely as the veins contain newly formed pseudo illite that does not inherit its crystallographic orientation and

chemistry from the host mica, in contrast to the youngest sample. HRTEMAEM analyses revealed that phengite and the

different illitic micas may be depleted in K. The oldest sample is derived from a coarse-grained augen gneiss with

extensively developed hydraulic cracks, which are lacking in the youngest sample, a fine-grained mylonitic gneiss.

)Corresponding author. Tel.: q81-298-61-3661; fax: q81-298-61-3653.

. E-mail address: [email protected] K. de Jong .

0009-2541r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. .P I I : S 0 0 0 9 - 2 5 4 1 0 0 0 0 4 1 1 - 3


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( )K. de Jong et al.rChemical Geology 178 2001 159195160

Fluidrock interaction and consequently sub-microscopic illitization were therefore more intense in the coarser-grained

rocks. Growth of the illitic micas in equilibrium with a high partial40

Ar pressure could account for40

Ar incorporation inXSK-vacancies and other lattice imperfections. Variation in illitisation and associated textural dissimilarities between the oldest

and youngest mica permit the different levels of40

Ar incorporation that account for the observed age discordance.XSThe finding of

40Ar plateau ages, despite the degassing of intimately intergrown micaceous minerals, is interpreted byXS

gas release invoked by in-vacuo chemical and structural changes that led to a joint collapse of the lattices of phengite and the

illitic micas between 8008C and 10008C. q2001 Elsevier Science B.V. All rights reserved.

Keywords:40

Arr39

Ar dating; Excess argon; Illite; Phengite; TEM; Hydraulic fractures

1. Introduction

Although phengite from high-pressure terrains

seem to be able to give meaningful40

Arr39

Ar plateau

ages, the last decade has seen a long series of papers40 .that show excess argon Ar incorporation by theXS

mineral de Jong et al., 1993; Tonarini et al., 1993;

Li et al., 1994; Arnaud and Kelley, 1995; Hannula

and McWilliams, 1995, Ruffet et al., 1995, 1997;Inger et al., 1996; Reddy et al., 1996; Scaillet, 1996;

. .Sherlock et al., 1999 . Previously, Brewer 1969 .and Reymer 1979 showed that KAr ages of white

mica may be significantly older than the correspond-

ing RbSr ages, hinting at the presence of40

Ar inXS .the mineral. de Jong et al. 1993 , Tonarini et al.

. . .1993 , Li et al. 1994 , Inger et al. 1996 , Ruffet et . .al. 1997 and Sherlock et al. 1999 proposed that

40Arr

39Ar plateau ages that are significantly older

than the age results on the same mica by SmNd,

UPb and RbSr dating methods reflect uptake of40

Ar by white mica, as these isotopic systems areXS

supposed to have higher closure temperatures thanthe KAr system. On the other hand, Monie and

.Chopin 1991 argued that in the case of coesite-

bearing samples such a reversal might indicate that

the closure temperature for Ar diffusion in white

.Fig. 1. Tectonic map of the eastern Betic Cordilleras, modified after de Jong 1993a . The sampling area in the eastern Sierra de los Filabres .Fig. 3 is indicated.


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.flows Nobel et al., 1981 also occur in the eastern- .most Sierra de los Filabres Fig. 3 . de Jong et al.

. 40 391992 interpreted Neogene phengite Arr Ar ages

in the Mulhacen Complex in this area by resettingdue to ingress of fluids associated with coeval mag-

matism.

The Mulhacen Complex consists of three super- .imposed nappes Fig. 3 ; the basal suites of the

uppermost two nappes comprise graphite-rich gar-

netmica schists, in which tourmaline-rich gneisses .occur de Jong and Bakker, 1991 . Rb Sr whole-rock

analyses of such gneisses yielded errorchrons be-

.Fig. 3. Geological sketch map of the easternmost Sierra de los Filabres, modified after de Jong and Bakker 1991, encl. 1 and de Jong .1993b . The five sites where samples were taken are indicated by stars. Note the important Miocene fault system.


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( )K. de Jong et al.rChemical Geology 178 2001 159195 163

.tween 275 and 191 Ma Andriessen et al., 1991 . In

an attempt to constrain the age of the early Alpine

cooling and exhumation of the Mulhacen Complex, a40

Arr39

Ar phengite single grain study was initiated.

In order to minimize the effects of post-D events,2our research focuses on gneisses of the intermediate

nappe, the Macael-Chive Unit, in the eastern Sierra .de los Filabres Fig. 3 . These rocks are devoid of

penetrative ductile post-D deformation and thin sec-2tions do not show a clear imprint of reheating-related

metamorphism, which at least in certain mica schistslocally reached over 5008C Fig. 3; Bakker et al.,

.1989 . A pilot study of a split phengite single grain

from the gneisses yielded a laser step-heating plateau .age of 29.5"0.1 Ma JK127, Table 1A . Laser spot

fusion dating revealed an age gradient in the grain

from about 32 Ma in the core to 24 Ma in the rim .de Jong et al., 1992 . A list of isotopic data of micas

.from gneisses Table 1A , including the sampledbody for the present study, shows that RbSr phen-

gite ages cluster tightly, whereas KAr ages scatter

markedly. Furthermore,40

Arr39

Ar and KAr ages of

a given phengite are systematically older than the

corresponding RbSr age. The KAr ages have been

discussed both in terms of incomplete degassing of

the metaigneous rocks and uptake of40

ArXS . 40 39Andriessen et al., 1991 . By contrast, Arr Ar

phengite ages from mica schists and marbles do not .show important variation Table 1B . In the western

.Sierra de los Filabres, Monie et al. 1991 obtained 40

Arr39

Ar plateau ages for hornblende and barroisite

of 24.6"3.6 and 48.4"2.2 Ma, respectively. They

also reported plateau ages of 16.0"0.4 and 16.9"

0.5 Ma for muscovite and biotite, respectively, which

are as young as in our study area.

The young40

Arr39

Ar, KAr and RbSr mica .ages Table 1A and B , in combination with 1114

.Ma zircon fission-track ages Johnson et al., 1997

point to a very rapid cooling of several hundreddegrees in only a few Ma and a concomitant ex-

Table 1

1 1 40 39 2 40 39 3Sample Lithology; fabric Min. RbSr KAr Arr Ar Arr Ar

( ) A Mineral ages from meta-igneous rocks of the Sierra de los Filabres)Huertecicas Altas ALM182 mylonitic gneiss bt 12.8"0.3 13.7"0.5

a )Almocaizar Unit mylonitic gneiss ph 12.8"0.3 15.5"0.4 aMacaelChive Unit ALM3 meta granite ph 14.0"0.3 282"13

ALM163 augen gneiss ph 15.6"0.3 56.9"1.4 58.3"0.2

ALM168 augen gneiss ph 14.9"0.3

JK127 platy gneiss ph 29.5"0.1pbMacael Chive Unit ALM226 mylonitic gneiss ph 13.2"0.2 14.2"0.7 14.4"0.1p

ALM230 mylonitic gneiss ph 12.5"0.2 14.7"0.4

( ) B Mineral ages from metasediments of the Sierra de los FilabresbMacaelChive Unit ALM272 mica schist ph 65.7"10.1 19.1"0.1i

ALM273 mica schist ph 41.1"4.6 25.9"0.1i

ALM274 mica schist ph 14.0"6.7 aCB 27 mica schist ph 16.3"0.5

c eNevadoLubrin Unit ALM270 mica schist ph 16.9"3.2 17.3"0.1p

ALM271 mica schist ph 17.6"0.1p

ALM275 marble ph 15.4"1.2tf

ALM276 marble ph 17.0"0.7tf

1 . eRbSr and KAr ages of mineral separates btsbiotie, phsphengite from mica schists de Jong, 1991, whole rockalbitephen-. ) .gite and meta-igneous rocks Andriessen et al., 1991, whole rockbiotitephengite .2 40 39 . . . Arr Ar induction furnace stepheating ages of phengite separates de Jong et al., 1992 , i integrated age, p plateau age, this

a .study; from Monie et al. 1991 .3 40 39 . .Arr Ar single grain laser stepheating ages or laser total fusion ages tf de Jong et al., 1992 .a .Eastern Sierra de los Filabres sampling area .b

Central Sierra de los Filabres.c .Eastern Sierra de los Filabres North of the sampling area .


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humation of various kilometres during that period. .Johnson et al. 1997 inferred from an 11 Ma apatite

fission-track model age that the cooling of the upper-

most Mulhacen Complex was essentially completedby that time. This is consistent with the first appear-

ance of detritus derived from this complex in sedi-

mentary basins that surround the ranges of metamor-

phic rocks in latest Serravallian to Early Tortoniandeposits Weijermars et al., 1985; Kleverlaan, 1989;

.Rodrguez-Fernandez and Sanz de Galdeano, 1992 .

3. Sample description and mineral separation

3.1. The gneisses

The sampled tourmaline-rich gneisses of the

Macael-Chive Unit form a few hundred metres thick,

sheet-like body with a volume of 5.5 km3 that issurrounded by graphite-rich garnet mica schists Fig.

.3 . Relics of undeformed metagranite occur locally

as metre scale bodies in the gneisses. A 267"29 Ma

RbSr whole-rock age of the gneisses is interpreted

as a late Variscan crystallization age of the granite .Andriessen et al., 1991; de Jong and Bakker, 1991 .

During the Alpine deformation the granite was trans-

formed to gneiss. The gneisses are only affected by

the two oldest of the six phases of penetrative ductile

deformation recognised in the Mulhacen Complex,

.viz. D and D de Jong, 1993a,b . They have a platy1 2foliation that curves around KFsp augen and con-

tain a quartzfeldspar stretching lineation that is

enhanced by a mineral lineation of boudinaged tour- .maline crystals. The foliation S is axial planar to2

very tight D folds, whereas the stretching lineation2 .L is parallel to their hinges. Samples JK100 and2JK119 are the most coarse-grained augen gneisses,

whereas JK135 has a mylonitic microstructure with a

fine-grained banded quartzfeldspar foliation and

well oriented mica and feldspar porphyroclasts. Kdvalues of garnetphengite pairs in the gneisses point

to maximum temperatures of 4805408C at a pres- .sure of 0.91.1 GPa Bakker et al., 1989 .

In thin section the gneisses have microcline, albite

and quartz as main minerals with additional phengite .and tourmaline nearly pure schorl ; accessory min-

erals are: biotite, apatite and zircon. Some gneisses .contain appreciable amounts of atoll- garnet, titanite

and epidote. The gneisses, but not the samples,

sporadically contain topaz and fluorite. Quartz and

feldspar of the matrix define a preferred orientation

fabric with an annealed texture, which curves around

microcline perthite and Na-rich plagioclase porphy-

roclasts. This fabric may be enhanced by a composi-

tional banding displayed by varying quantities of

tourmaline and mica. The grain size and fabric inten-

sity have a strong bearing on the density of late

joint-like cracks that are typically formed sub-per-

pendicular to the stretching lineation. The mylonitic

gneisses, with their well-intergrown fine-grained

quartzfelspar matrix, are the least affected by the

formation of such cracks, whereas in augen gneisses

they are well developed with a spacing between 200

and 1000 mm, and are in part quartz-filled. Thesecracks on the sample scale are sub-parallel to steeply

dipping Miocene faults that characterize the gneisses

.and its surrounding metamorphic rocks Fig. 3 .The samples are taken at five sites in the gneiss

.body, set a few kilometres apart Fig. 3 . At site 1

two samples are taken at less than 100 m from each

other; the three samples of site 3 were located less .than 20 m apart. Pilot sample JK127 Table 1A can

also be considered as coming from site 1. Except for

site 5 the sample sites are close to the border of the

gneiss body; sites 1, 3 and 4 are located within100200 m of subvertical late Miocene faults Fig.

.3 .

3.2. The micas

Phengitic mica is colourless to very pale green in

thin section and occurs as well crystallised, strain-free

grains that generally lie with their basal cleavage

plane in the differentiated layering S . Locally they2are discordant to S and may overgrow the tectonic2banding. Thin sections of samples from progressive

granitegneiss transitions show that the grain size of

the white mica augments with increasing deforma-

tion and fabric intensity, hence decreasing grain size

of the quartzfeldspar matrix. In meta-granites white

mica occurs in fine-grained aggregates with a ran-

dom orientation. In intermediate stages of deforma-

tion mica aggregates are composed of several grains

with a decussate structure; crystals parallel to S are2 .thin and long up to a few 1000 mm , whereas those

perpendicular to it are short and thick. Porphyroclas-


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tic single grains of the most strongly deformed rocks,

like JK135, are generally well elongated parallel to

S . These observations show that the mica single2grains used for our study result from tectono-meta-

morphic transformations during D and from anneal-2ing recrystallisation during its waning stages, which

affected the quartzfeldspar matrix differently, as a

result of inhibited grain growth. The effects of post-

D tectono-metamorphic processes on the phengitic2micas are unclear, but in thin sections some crystals

have a tiny rim of oxychlorite or biotite that is rich

Fe and poor in Mg and Ti. Such rims might be .related to the late stage reheating Fig. 2 .

Phengite separate ALM163, with grains in the

125250 mm sieve fraction, was separated using .standard procedures Andriessen et al., 1991 . The

selected phengitic mica grains were separated from

the handspecimens after gentle crushing and were

ultrasonically cleaned in demineralised water for 5min. All grains were split over the basal plane in

order to diminish their thickness, to make sure that

only relatively thin grains were analysed. In some

cases splitting of the grains was very easy, which

suggests that parts of the crystals are loosely bound.

Such parting surfaces do not contain chlorite. The

grains are pseudo-hexagonal to slightly elongated .with a diameter between 1 and 2.5 mm Table 5 .

The margins of some grains have a deformed basal

cleavage and occluded quartz and feldspar crystals

that could not all be removed. They may containnumerous included needles of apatite and tourmaline

and some quartz and feldspar. Otherwise, the grains

are proper and not affected by weathering or alter-

ation, with a possible exception of JK124, from a

hand specimen with slight brown staining. The mica

is of the 2M type. In thin section the grains usually1 .show varying amounts of dark trails parallel to 001 ,

probably related to dephengitization reactions. Sam-

ples JK100, 115, 124 and 125 are most affected,

whereas this phenomenon is virtually absent in JK114

and 119.

3.3. Chemistry of the micas

Representative microprobe analyses of three mica

samples with different ages point to an extremely

heterogeneous composition, but zoning is generally .minor Table 2 . Si contents of JK118 differ by as

much as 0.16 Si pfu. The micas are low in Na and .Ca and may contain some F and Cl not tabulated .

As the total FeqMgqMn is close to 0.5 they can

be classified as phengitic micas. The average stoi-

chiometric Si iv value of each sample is around 3.3,

which agrees with values for typical D micas2 .Bakker et al., 1989 . The inverse correlation be-

Fig. 4. Crystal chemistry of phengitic micas from three datedsamples with an important age difference determined by electron

.probe micro analysis Table 2 . Fe assumed divalent; mgs . . .Mgr MgqFe . a Variation diagram of tetrahedral Si pfu vs.

.total Al pfu , referring to 11 oxygens; the celadonitemuscovite

solid solution is indicated. Insert: average composition of three .Betic micas B in relation to end-member compositions of mus-

. . . .covite M , ferrimuscovite F and celadonite C . b Octahedral . . .Al pfu vs. the mg-factor. c Ti pfu vs. the mg-factor.


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Table 3 .HRTEM image characteristics and AEM analyses in pfu of mica grains from samples JK119 and JK135, that yielded the highes

iv vi 2qSample Mineral Type Orientation Crystallization Si Ti Al Al Fe Mg

JK135 phengite well-crystallized lattice 3.09 0.01 0.90 1.50 0.23 0.25 .illite-like 1 zones 001 spindle-shaped 3.24 0 0.76 1.60 0.19 0.27 .JK119 phengite zones 001 well-crystallized lattice 3.20 0.02 0.78 1.45 0.26 0.19

spindle-shaped .pseudo-illite 2 zonesH 001 not well intergrown 3.19 0.01 0.80 1.62 0.28 0.10

illite 3 random individual crystallites 3.28 0.01 0.71 1.71 0.27 0.23

The chemical analyses of phengitic micas and the various illite-like micas were made simultaneously with HRTEM observations u .with a double tilt specimen stage operated with an accelerating voltage of 200 kV CRMCC-CNRS, Marseille, France . The couple

system in TEM mode, using analytical probes of 0.0050.0150 mm in diameter, which is equal to about 515 mica leafs. Each tabu .analyses. Mineral formula is calculated on the basis of 11 oxygens and assuming divalent Fe; mg sMgr MgqFe .


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iv . . ivtween Si vs. Al Fig. 4a indicates a Fe,Mg Sitotiv vi .sAl Si Al Tschermak substitution. The chemi-

cal composition points to a muscoviteceladonite

solid solution with a minor ferrimuscovite compo- .nent Fig. 4a, insert . JK135 is the most muscovite-

rich on average, JK118 the least. The fairly low

. . . .Fig. 5. HRTEM images of grains from samples JK135 a and b and JK119 c and d that yielded the lowest and highest plateau age, . . .respectively. a Well-crystallised phengite lattice that is deformed by a micro kink band. b Phengite ph with a zone consisting of an

.aggregate of crystals of corrugated, spindle-shaped illite-like material 1s type 1 intergrowths that has replaced phengite parallel to its . . . .001 plane. c Phengite containing a zone of 001 parallel spindle-shaped, illite-like mica 1 cross cut by a zone with an aggregate of

.automorphous pseudo-illite, 2s type 2 intergrowths . Note that the pseudo-illite tends to align with the basal cleavage perpendicular to the .001 cleavage of phengite and the spindle-shaped illite-like mica. The pseudo-illite crystals seem to be loosely bound and not well

. .intergrown as large open spaces are present between them. d Illitic mica 3 crystal occurring inside a zone with pseudo-illite.


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.Fig. 5 continued .

.average total oxide analysis 94.6% by weight im-

plies excess of OH.

The micas are Fe-rich with average mg-factors of

0.11 and 0.13 for JK115 and 135 and twice that .value for JK118 Table 2, Fig. 4b,c . This difference

is due to a higher Mg content of micas of the latter .sample, rather than a lower Fe content Table 2 . The

higher amount of Mg in JK118 is compensated by a

lower Alvi content compared to JK115 and JK135

. ivFig. 4b . Accordingly, the average Ti value of .JK118 is higher as well Fig. 4c .

A number of micro probe analyses in the rims of

crystals or close to basal cleavage traces with dark

inclusion trails yielded considerably lower K values .than normal Table 2 . This might imply that K-poor

alteration products of the mica occur concentrated .along 001 or in the grain rim. A normal mg-factor

excludes the presence of chlorite, whereas normal


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stoichiometric Si and Na values exclude that the

lower K-content is due to analysis of included quartz

or albite.

These results imply that despite the absence of

penetrative ductile post-D deformation the white2 .mica in gneisses suffered from repeated recrystal-

lization during the metamorphic evolution, which

agrees with the complicated thermal history of the .Mulhacen Complex Fig. 2 .

4. HRTEM and AEM analysis

In order to clarify the submicroscopic structure

and chemistry of the dated micas, we performed

HRTEM analyses of grains from samples JK119 and

JK135 and simultaneous AEM determinations of ar-

eas with a diameter of about 515 mica leafs. For

this purpose mica grains were embedded in aralditeand ultramicrotomed in very thin slices with a dia-

mond knife. The spread in AEM data is much larger

than for microprobe determinations. Part of the

poignant discrepancies with microprobe data derive

from the much higher spatial resolution of AEM

determinations in these strongly recrystallized speci-

mens. Other possible reasons for such discrepancies

and especially the inaccurate alkali concentrations

that plague AEM determinations are discussed by .Belluso et al. 2000 .

Although the dated grains seemed well-crystal-lised, unaltered and not affected by transformations

under a petrographic microscope, HRTEM-AEM

analyses revealed, surprisingly, that about 2530%

of the phengite lattice of specimen JK119 is affected

by submicroscopic mechanical and chemical trans-

formation and recrystallisation. Specimen JK135 is

less affected by growth of secondary micaceous ma-

terial. These processes have given rise to important

submicroscopical textural differences between both

specimens. In general chemical variation is large .Table 3 .

Specimen JK135 contains large areas where the

phengite lattice is generally well crystallised, al- .though locally affected by kink bands Fig. 5a . Such

areas with an intact lattice are often crossed by zones

of 1030 mm thickness where the mica has beenaffected by transformation and starting recrystallisa-

.tion Fig. 5b . The latter HRTEM image shows that

the original phengite is obliquely cut by a zone with

an aggregate of tiny crystals of corrugated, spindle-

shaped micaceous material that has replaced phen- .gite parallel to the 001 plane.

AEM analyses reveal that phengite is relatively .poor in K, but Ca-rich Table 3 , which might imply

the occurrence of a solid solution to margarite. The

K and Ca values vary strongly from spot to spot,

pointing to an extremely heterogeneous composition.

The spindle-shaped micaceous mineral has a compo-

sition close to that of illite, but is structurally still

close to phengite. This illite-like mica contains about

the same amount of K as the original phengite, but is

strongly depleted in Ca; its KrCa ratio is more than

10= that of the phengite. The K-values of both

phengite and the illite-like mica are somewhat below

the stoichiometrically required values and are not .compensated by Na Table 3 .

Specimen JK119 also contains optically unaf-fected phengite, which has a normal K-value, but a

high Ca-content, which is virtually the same as that .of JK135 Table 3 . HRTEM images show that the

texture of the specimen is quite disturbed in several .areas Fig. 5c . More or less intact phengite is fre-

quently surrounded by zones of secondary micaceous

phases that occur principally in two habits: type 1

and type 2. In addition, a third type has been encoun- .tered in aggregates of type 2 illitic micas Fig. 5d .

The local occurrence of unidentified micaceous ma-

terial that has a SiAlMg composition, which issimilar to montmorillonite but is K-rich without Ca

or Na, clearly shows that this specimen is thoroughly

recrystallised.

Type 1 intergrowths are spindle-shaped, 0.070.30

mm thick crystallites of illite-like material that occur . .parallel to 001 of the original phengite Fig. 5c , to

which they are chemically identical. They have a

similar habit as the secondary mica of specimen .JK135 Fig. 5b . This micaceous material is over-

.grown by another secondary mica type 2 that is .

chemically close to illite Table 3 . Type 2 mica, orpseudo-illite, occurs in several micrometre wide .zones Fig. 5c that cross-cut the basal plane of

.phengite and the illite-like mica type 1 . The 0.07

0.35 mm thick type 2 crystals have a distinctlydifferent habit and orientation compared to type 1

mica. They tend to align with their basal cleavage .perpendicular to the 001 cleavage of phengite and


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.the spindle-shaped illite-like mica Fig. 5c . The

automorphous crystals of type 2 aggregates seem to

be loosely bound and not well-intergrown as large .open spaces are present between them Fig. 5c .

These textural relationships imply that pseudo-illite

results from recrystallisation and does not, like type

1 mica, inherits the structure of phengite. This indi-

cates that pseudo-illite grew at the expense of the

spindle-shaped, illite-like mica and is thus a product

of more advanced submicroscopic transformation of

phengite. In a number of zones with pseudo-illite, .type 3 secondary mica has been observed Fig. 5d to

be structurally similar to type 2 mica, but chemically .closer to illite Table 3 . The grain size is well below

0.5 mm.The submicroscopic transformation and recrys-

tallisation of the original phengite has, hence, re-

sulted in a superimposed growth of three micaceous

minerals that are progressively closer to illite and are .progressively lower in Ca and K Table 3 , implying

leaching of both cations. The KrCa ratios regularly

increase from phengite to type 3 illite, indicating that

Ca is leached easier during these processes than K.

This might be explained by the fact that K is struc-

turally bound in the interlayer, whereas Ca-ions oc-

cupy metastable positions. The composition of the

phengitic micas is generally poorer in Si and richer

in Aliv compared to the different types of illite-likevi micas, whereas Al increases with alteration Table

.3 . The chemical changes agree with the followingillite substitution reaction given by Wang and Banno .1987 , accounting for non-stoichiometry of inter-

layer cations in micas:

K xiiqAl ivsI xiiqSiiv

The importance for our study is that the chemical

changes during progressive illitization imply the for-mation of interlayer vacancies indicated by the

.square in the dated material.

5. Experimental procedures

Analyses were made by incremental heating with

a defocused continuous laser beam, whereas some

grains were split along the basal cleavage plane in

order to attempt laser spot fusion dating of the one

half and laser step-heating of the other, like the pilot .study of de Jong et al. 1992 . In two cases, one half

of a split mica grain was analysed using induction

furnace step-heating, in order to monitor possible

artifacts of the incremental laser heating technique.

In addition, an induction furnace incremental heating

experiment was conducted on a phengite separate

with known conventional KAr and RbSr ages.

The laser extraction line consists of a Coherent70-4 continuous argon ion laser maximum output

.5.5 W in multi line mode , in combination with a

low volume high inlet system and a sensitive gas

mass spectrometer consisting of a 12 cm, 1208M.A.S.S.E.w tube, a Baur-Signerw ion source andan A.E.M. 1000ETPw electron multiplier. The mass

spectrometer was also used for analysis of induction

furnace extracted gas. Single grains and mineral

separates analysed with the furnace were wrapped in

high purity Al foil and incrementally heated to fu-sion. For laser experiments mica grains are placed in

holes of the copper sample holder in a stainless steel,

high vacuum sample housing. Laser step-heating ex-

periments were carried out with a defocused laser

beam of at least twice the diameter of the analysed

grains, in order to obtain homogeneous energy condi- .tions Hall, 1990 . Fusion of the grain was achieved

in the final step by beam focusing. During spot

fusion a beam size of about 40 mm was used. Duringboth types of laser experiments a mirror deflects the

beam in such a way that it strikes the mica grainperpendicular to the basal plane. Additional details

.are given in Scaillet 1996 . Experiments were di-

rectly observed by a joined video-microscope sys-

tem. A Pyrex cold finger at y958C and a ZrAlalloy getter purify the extracted gas in laser probe

experiments. During laser experiments system blank

runs in were carried out at the start of each experi-

ment and were repeated every third run. Typical

background values of the 40, 39, 37 and 36 isotopes,

which were subtracted from the subsequent sample

analysis results, were 1=10y1 1, 5=10y1 4, 2=

10y13 and 1=10y12 cm3 STP, respectively. Addi-

tional analytical details are given in the footnotes to

Tables 4 and 5.

Irradiation of the samples took place in theMelusine reactor Centre dEtudes Nucleaires,

.Grenoble, France , together with biotite standard 4BKAr age: 17.25 Ma, Hall et al., 1984 and subse-


14/37


Table 440

Arr39

Ar analytical data of phengitic micas

40 39 y13 3 39 37 39 40 ) 39 . . . .Step Ar % Ar 10 cm Ar % Ar r Ar Ar r Ar Apparent age Maatm K Ca K K

( ) ( ) ( ) ALM 163 mineral separate, 15.6 mg furnace step-heating Js0.01715

500 100.00 105.35 0.25 0.000

600 99.13 123.06 0.29 0.000 0.43"0.59 13.2"18.2

670 87.02 243.88 0.58 0.083 1.45"0.28 44.3"8.4

770 66.68 1290.66 3.09 0.044 1.77"0.05 53.9"1.7850 81.81 4729.76 11.33 0.008 1.87"0.02 57.1"0.7

890 41.47 10,639.30 25.48 0.000 1.95"0.01 59.4"0.2

910 22.77 5928.16 14.20 0.000 1.95"0.01 59.3"0.3

930 25.32 3401.16 8.14 0.000 1.92"0.01 58.5"0.4

970 38.09 3593.45 8.61 0.000 1.92"0.02 58.4"0.7

1010 45.18 6263.53 15.00 0.000 1.91"0.01 58.1"0.3

1070 47.59 3114.37 7.46 0.000 1.97"0.03 59.9"0.8

1130 50.12 1753.85 4.20 0.031 2.02"0.03 61.3"0.9

1300 57.73 551.18 1.32 0.089 1.98"0.20 60.2"6.1

Fuse 99.80 22.12 0.05 2.431 0.04"4.55 1.1"140.8

Total age: 58.3"0.2

( ) ( ) ( ) JK100 single grain laser step-heating Js0.01017

1 99.39 37.17 0.07 20.555 17.92"19.26 302.0"298.82 99.08 31.71 0.06 11.969 12.18"19.10 210.7"311.7

3 99.02 112.91 0.21 6.481 12.99"9.69 223.8"157.0

4 97.89 140.42 0.26 8.900 29.95"7.84 479.5"110.3

5 97.51 355.11 0.66 1.880 10.15"3.63 177.2"60.4

6 97.37 493.71 0.92 2.034 10.32"3.00 180.0"49.9

7 96.46 821.38 1.53 1.501 9.28"2.52 162.6"42.2

8 94.17 1663.62 3.11 0.510 5.41"0.92 96.6"16.0

9 92.85 2222.57 4.15 0.353 4.96"0.58 88.7"10.1

10 86.49 936.04 1.75 0.242 4.12"0.28 74.1"4.9

11 90.63 1265.74 2.36 0.202 3.96"0.34 71.2"5.9

12 91.50 1239.63 2.32 0.202 3.99"0.29 71.8"5.1

13 91.26 3248.63 6.07 0.255 4.67"0.47 83.6"8.2

14 84.00 1630.65 3.04 0.183 4.20"0.21 75.4"3.6

15 83.69 1354.85 2.53 0.219 4.03"0.19 72.5"3.4

16 84.85 1606.08 3.00 0.101 4.07"0.20 73.1"3.5

17 79.91 1491.07 2.78 0.092 3.88"0.13 69.7"2.4

18 84.34 2535.47 4.74 0.112 3.88"0.15 69.8"2.7

19 69.21 1851.57 3.46 0.106 3.94"0.11 70.9"2.0

20 80.24 4361.28 8.15 0.118 3.99"0.12 71.8"2.0

21 54.03 3257.87 6.09 0.070 3.88"0.07 69.8"1.3

22 47.92 2064.30 3.85 0.021 3.98"0.05 71.5"0.8

23 54.22 1929.55 3.60 0.000 3.75"0.06 67.5"1.1

24 62.05 4134.27 7.72 0.028 3.82"0.17 68.8"3.0

25 63.81 3702.86 6.92 0.046 3.51"0.08 63.3"1.4

Fuse 70.13 11,034.31 20.62 0.071 3.57"0.05 64.4"1.0

Total age: 76.1"1.4


1 100.00 7.91 0.04 13.718

2 100.00 7.14 0.04 9.965

3 100.00 12.81 0.07 8.684

4 100.00 5.95 0.03 5.797

5 100.00 6.30 0.03 6.339

6 100.00 9.94 0.05 1.341


15/37


.Table 4 continued



7 100.00 14.14 0.09 2.575

8 99.52 17.92 0.11 1.767 4.00"14.92 72.0"262.9

9 100.00 35.28 0.23 1.557

10 99.65 57.54 0.37 1.161 2.77"3.02 50.1"53.9

11 99.18 97.02 0.65 0.470 4.34"7.18 77.9"126.112 98.94 181.02 1.21 0.486 5.35"4.16 95.6"72.3

13 99.62 240.80 1.61 0.315 1.03"1.90 18.8"34.5

14 99.45 358.54 2.40 0.040 0.76"1.01 13.9"18.4

15 98.58 475.37 3.18 0.028 1.22"0.60 22.2"10.9

16 96.71 550.83 3.69 0.030 1.19"0.38 21.7"6.9

17 95.84 680.54 4.55 0.023 1.15"0.24 20.9"4.3

18 92.23 875.91 5.86 0.010 1.29"0.20 23.5"3.7

19 88.05 941.78 6.30 0.007 1.27"0.20 23.2"3.6

20 86.17 764.68 5.12 0.005 1.24"0.11 22.6"2.0

21 89.17 972.44 6.50 0.009 1.30"0.14 23.7"2.5

22 82.48 1685.25 11.28 0.014 1.43"0.08 26.1"1.5

23 70.22 1302.84 8.72 0.011 1.33"0.10 24.2"1.7

24 69.39 842.94 5.64 0.013 1.29"0.12 23.5"2.2

25 75.08 627.34 4.20 0.000 1.30"0.10 23.8"1.726 80.40 614.18 4.11 0.000 1.35"0.13 24.5"2.3

27 85.11 228.83 1.53 0.000 1.17"0.30 21.3"5.5

Fuse 84.40 3336.97 22.33 0.009 1.34"0.06 24.5"1.0

Total age: 24.9"1.6

X ( ) ( ) ( ) JK115 single grain, half 1 laser step-heating Js0.01000

1 98.48 84.07 0.69 0.047 6.16"8.06 109.5"139.1

2 97.94 150.78 1.23 0.428 6.66"3.11 118.2"53.5

3 97.93 255.92 2.09 0.195 2.57"1.03 46.6"18.5

4 95.89 524.37 4.28 0.075 3.18"0.84 57.3"14.8

5 86.81 1100.54 8.98 0.022 2.43"0.16 44.0"2.9

6 78.67 723.87 5.91 0.009 2.31"0.22 41.8"3.9

7 83.39 584.36 4.77 0.000 2.20"0.15 39.9"2.7

8 83.78 761.18 6.21 0.015 2.08"0.14 37.8"2.5

9 67.08 1209.25 9.87 0.004 2.25"0.08 40.7"1.5

10 52.32 1109.36 9.05 0.000 2.26"0.06 40.9"1.0

11 63.40 1531.81 12.5 0.000 2.27"0.06 41.1"1.0

12 46.93 1104.53 9.01 0.023 2.35"0.09 42.5"1.5

13 43.07 916.58 7.48 0.007 2.29"0.07 41.5"1.3

14 50.26 542.64 4.43 0.000 2.42"0.09 43.7"1.5

15 68.34 176.19 1.44 0.000 1.89"0.30 34.4"5.3

16 67.18 177.59 1.45 0.000 1.97"0.25 35.7"4.5

Fuse 63.43 1300.81 10.61 0.000 2.26"0.11 40.9"1.9

Total age: 42.7"1.5

Y ( ) ( ) ( ) JK115 single grain, half 2, 0.1 mg furnace step-heating Js0.01017

400 99.79 0.56 0.00 293.700 72.37"183.23 994.8"1936.2440 97.80 2.59 0.02 224.610 231.88"70.85 2185.1"387.1

455 99.58 3.22 0.02 0.000 17.74"44.00 299.1"683.8

470 100.00 2.31 0.02 0.000

480 100.00 2.31 0.02 0.000

520 100.00 7.91 0.06 0.000

560 99.75 14.77 0.12 10.515 4.91"16.06 87.9"280.6

( )continued on next page


16/37


.Table 4 continued

40 39 y13 3 39 37 39 40 ) 39 . . . .Step Ar % Ar 10 cm Ar % Ar r Ar Ar r Ar Apparent age Maatm K Ca K KY ( ) ( ) ( ) JK115 single grain, half 2, 0.1 mg furnace step-heating Js0.01017

600 100.00 17.36 0.14 0.000

640 99.85 31.43 0.26 0.000 0.85"7.07 15.6"128.6

680 99.27 77.28 0.64 9.674 6.97"6.16 123.5"105.4

720 97.92 219.10 1.82 10.256 16.22"6.12 275.3"96.4

740 98.47 163.94 1.36 4.564 7.42"2.95 131.2"50.3760 98.61 299.74 2.49 1.552 5.73"3.43 102.1"59.5

780 96.00 682.01 5.67 0.991 3.11"0.44 56.2"7.8

800 89.44 849.73 7.06 0.224 2.75"0.13 49.7"2.3

820 86.11 813.47 6.76 0.131 2.51"0.19 45.4"3.4

840 78.66 1675.59 13.93 0.052 2.55"0.11 46.2"2.0

860 66.68 1771.63 14.73 0.000 2.40"0.09 43.6"1.6

880 74.82 1387.47 11.53 0.000 2.29"0.11 41.6"2.0

900 80.83 1858.85 15.45 0.084 2.31"0.06 41.9"1.1

920 82.99 1830.08 15.21 0.121 2.32"0.10 42.0"1.8

940 92.65 200.48 1.67 0.000 2.94"0.53 53.1"9.5

960 99.54 25.20 0.21 0.000 1.41"4.30 25.7"77.8

1010 97.59 30.24 0.25 0.000 4.51"2.21 80.9"38.8

1225 99.32 24.29 0.20 0.000 4.41"8.88 79.1"155.8

Fuse 98.12 36.89 0.30 7.212 22.67"6.81 374.2"101.5Total age: 52.8"2.7


1 98.91 10.01 0.03 5.714 103.78"130.81 1299.5"1167.4

2 99.91 13.72 0.04 9.773 6.00"30.88 106.8"533.7

3 100.00 23.66 0.07 6.014

4 100.00 14.07 0.04 3.044

5 100.00 11.69 0.03 0.411

6 100.00 18.41 0.05 1.655

7 98.52 25.83 0.08 0.658 10.70"20.99 186.3"347.2

8 100.00 55.30 0.17 0.544

9 98.13 120.82 0.37 0.256 7.49"2.64 132.3"44.9

10 98.30 129.71 0.40 0.103 3.29"1.46 59.2"25.9

11 98.07 405.51 1.26 0.158 3.59"1.12 64.6"19.8

12 97.28 651.07 2.03 0.067 2.30"0.48 41.6"8.6

13 95.95 2597.21 8.11 0.037 3.08"0.65 55.5"11.5

14 94.71 2329.11 7.27 0.024 1.93"0.33 34.9"5.8

15 89.40 1913.59 5.97 0.002 1.47"0.08 26.7"1.5

16 80.60 1394.19 4.35 0.008 1.30"0.07 23.7"1.2

17 72.76 1033.20 3.22 0.000 1.32"0.08 24.1"1.4

18 84.02 806.75 2.52 0.015 1.30"0.09 23.7"1.6

19 91.05 1190.98 3.72 0.035 1.63"0.18 29.6"3.1

20 89.57 1807.05 5.64 0.018 1.40"0.12 25.4"2.1

21 79.58 2411.29 7.53 0.009 1.33"0.08 24.2"1.5

22 67.13 2549.82 7.96 0.000 1.28"0.04 23.2"0.7

23 59.93 2269.68 7.08 0.000 1.26"0.05 22.9"0.8

24 65.53 1554.70 4.85 0.000 1.30"0.04 23.7"0.825 71.45 1126.09 3.51 0.000 1.35"0.09 24.5"1.7

26 75.56 607.11 1.89 0.051 1.31"0.08 23.9"1.4

27 75.20 515.34 1.60 0.000 1.40"0.13 25.4"2.2

Fuse 73.66 6427.75 20.07 0.012 1.46"0.04 26.5"0.7

Total age: 30.1"1.2


17/37


.Table 4 continued



1 100.00 2.73 0.04 39.595

2 100.00 7.28 0.11 15.123

3 100.00 10.36 0.16 21.372

4 99.86 25.20 0.39 17.840 1.80"8.17 32.7"147.2

5 99.14 50.12 0.79 5.789 4.47"6.52 80.2"114.46 98.80 100.17 1.57 4.276 4.09"2.44 73.5"42.9

7 95.07 249.27 3.92 1.667 6.11"0.90 108.7"15.6

8 86.85 665.21 10.45 0.415 5.41"0.32 96.6"5.5

9 71.47 565.25 8.88 0.193 5.07"0.24 90.7"4.2

10 58.54 569.38 8.95 0.494 5.20"0.18 93.0"3.2

11 57.53 848.68 13.34 0.000 5.04"0.14 90.2"2.5

12 37.86 1053.50 16.55 0.009 5.11"0.06 91.4"1.0

13 39.63 767.90 12.07 0.052 5.01"0.08 89.5"1.5

14 52.42 260.75 4.10 0.000 4.84"0.25 86.7"4.3

15 61.58 205.31 3.23 0.000 4.51"0.31 80.8"5.5

Fuse 55.05 983.01 15.45 0.128 5.03"0.14 89.9"2.4

Total age: 90.7"1.7

( ) ( ) ( ) JK124 single grain laser step-heating Js0.010171 33.73 0.00 0.00 930.830 258.33"895.19 2324.1"4527.7

2 99.04 80.29 0.10 1.846 5.45"3.57 97.3"62.1

3 98.26 136.57 0.17 1.227 5.55"1.51 99.1"26.2

4 98.75 157.64 0.19 2.311 6.86"3.89 121.6"66.7

5 98.17 254.38 0.32 3.672 12.13"4.41 209.8"71.9

6 98.28 299.60 0.37 2.246 6.83"2.83 121.1"48.5

7 98.12 255.43 0.32 1.456 5.44"2.15 97.1"37.3

8 98.27 311.50 0.39 1.327 6.36"2.89 113.0"49.8

9 97.52 609.35 0.76 1.740 10.17"3.47 177.5"57.7

10 97.25 912.66 1.13 0.959 5.39"1.02 96.2"17.7

11 96.84 925.96 1.15 0.836 5.09"1.02 91.1"17.7

12 95.21 993.44 1.23 0.591 3.53"0.50 63.6"8.8

13 95.59 1011.22 1.25 0.350 3.66"0.58 66.0"10.3

14 93.17 1150.45 1.43 0.280 3.28"0.45 59.2"8.0

15 88.20 1128.40 1.40 0.292 3.42"0.21 61.6"3.7

16 85.77 1361.15 1.69 0.175 3.21"0.18 58.0"3.2

17 87.41 2384.76 2.96 0.178 3.07"0.13 55.4"2.3

18 79.41 4668.51 5.79 0.063 3.27"0.08 59.0"1.4

19 63.74 7432.46 9.22 0.025 3.04"0.06 55.0"1.1

20 34.77 7365.26 9.14 0.038 3.07"0.03 55.5"0.4

21 25.92 7385.28 9.16 0.016 3.05"0.02 55.1"0.4

22 18.58 7992.18 9.91 0.015 3.05"0.02 55.1"0.4

23 17.32 5647.25 7.00 0.037 3.02"0.02 54.6"0.4

24 20.43 5129.95 6.36 0.065 2.98"0.02 53.9"0.3

25 19.51 4542.86 5.63 0.073 2.97"0.02 53.6"0.4

26 21.45 3207.19 3.98 0.024 2.99"0.02 54.0"0.4

27 22.97 4402.51 5.46 0.028 3.12"0.03 56.4"0.5Fuse 18.89 10,862.39 13.47 0.016 3.05"0.01 55.2"0.3

Total age: 58.8"0.7


1 98.98 5.67 0.02 73.197 60.88"22.55 869.1"255.5

2 100.00 11.34 0.04 39.033



18/37


.Table 4 continued



3 99.25 8.33 0.03 17.178 19.53"24.21 326.8"370.5

4 99.27 14.21 0.05 10.614 12.36"10.69 213.5"174.2

5 99.01 9.66 0.04 7.286 7.43"9.80 131.3"167.1

6 100.00 20.51 0.08 4.458

7 100.00 17.01 0.06 2.187 8 100.00 45.99 0.18 5.904

9 99.46 84.56 0.33 2.287 2.33"2.45 42.2"43.9

10 98.84 128.24 0.50 1.900 3.56"2.92 64.2"51.7

11 98.56 194.11 0.75 1.643 3.14"1.28 56.7"22.7

12 97.69 210.49 0.82 1.010 3.67"0.63 66.1"11.2

13 98.11 321.79 1.25 1.261 2.45"0.77 44.4"13.7

14 96.78 436.10 1.69 0.643 2.58"0.59 46.6"10.6

15 95.23 604.03 2.34 0.574 2.51"0.42 45.4"7.5

16 93.41 879.83 3.42 0.320 2.02"0.27 36.6"4.9

17 86.41 1374.31 5.34 0.178 2.20"0.12 39.9"2.1

18 79.41 2356.13 9.15 0.000 1.97"0.07 35.8"1.2

19 63.89 1369.41 5.32 0.000 1.97"0.06 35.8"1.1

20 57.27 1146.53 4.45 0.014 1.97"0.08 35.7"1.4

21 55.32 992.11 3.85 0.000 1.94"0.06 35.2"1.222 60.13 971.74 3.77 0.004 1.95"0.08 35.4"1.5

23 60.74 1023.75 3.98 0.153 1.98"0.06 36.0"1.2

24 59.06 1076.60 4.18 0.175 1.95"0.07 35.5"1.3

25 56.88 1029.56 3.99 0.076 1.96"0.07 35.6"1.3

26 57.60 1236.62 4.80 0.000 1.93"0.07 35.1"1.2

27 55.36 1171.87 4.55 0.000 1.91"0.05 34.7"0.9

28 57.17 1013.18 3.93 0.000 1.98"0.07 36.0"1.3

29 58.48 742.91 2.89 0.000 2.06"0.11 37.5"2.0

30 65.70 994.70 3.86 0.130 1.92"0.08 34.9"1.4

Fuse 58.52 6259.33 24.31 0.136 2.00"0.02 36.2"0.4

Total age: 37.4"0.6

X ( ) ( ) ( ) JK135a single grain, half 1 laser step-heating Js0.01000

1 100.00 34.72 0.20 3.914

2 100.00 14.98 0.08 1.653

3 100.00 14.14 0.08 5.146

4 100.00 26.04 0.15 0.752

5 96.06 62.02 0.36 0.000 3.64"4.10 65.6"72.6

6 98.47 239.54 1.41 0.037 1.04"0.47 18.9"8.6

7 97.71 710.71 4.19 0.031 0.82"0.30 15.0"5.5

8 91.58 1521.38 8.97 0.014 0.87"0.06 15.9"1.0

9 63.95 1476.30 8.71 0.000 0.87"0.04 15.9"0.7

10 66.72 1323.56 7.81 0.000 0.80"0.05 14.6"1.0

11 62.08 1081.29 6.38 0.000 0.80"0.04 14.6"0.8

12 57.55 883.75 5.21 0.009 0.81"0.08 14.9"1.5

13 58.62 1080.10 6.37 0.000 0.87"0.06 16.0"1.0

14 61.69 1567.86 9.25 0.000 0.88"0.06 16.1"1.015 65.46 913.92 5.39 0.012 0.86"0.07 15.7"1.3

16 73.27 559.65 3.30 0.008 0.85"0.09 15.5"1.5

17 72.88 491.40 2.89 0.022 0.79"0.11 14.5"2.0

Fuse 55.82 4945.01 29.18 0.017 0.90"0.03 16.5"0.5

Total age: 15.9"0.5


19/37


.Table 4 continued

40 39 y13 3 39 37 39 40 ) 39 . . . .Step Ar % Ar 10 cm Ar % Ar r Ar Ar r Ar Apparent age Maatm K Ca K KY ( ) ( ) ( ) JK135a single grain, half 2, 0.7 mg furnace step-heating Js0.01017

400 98.85 0.42 0.00 0.000 456.25"723.70 3120.4"2354.1

450 100.00 6.30 0.01 137.030

500 100.00 25.76 0.04 42.043

600 100.00 98.70 0.17 36.912

650 98.76 88.13 0.15 16.312 5.39"10.32 96.2"179.5700 97.04 276.92 0.48 1.523 1.71"0.82 31.0"14.8

750 96.75 890.05 1.54 2.458 6.32"1.66 112.4"28.7

790 94.86 2841.65 4.92 1.136 3.50"2.01 63.1"35.7

810 92.28 3361.75 5.82 0.246 1.03"0.13 18.8"2.3

830 88.67 5348.35 9.26 0.129 1.02"0.05 18.6"0.9

850 82.54 8087.59 14.00 0.100 1.00"0.03 18.2"0.6

870 75.32 5841.64 10.12 0.039 0.95"0.04 17.3"0.7

890 80.03 6726.58 11.65 0.093 1.00"0.04 18.3"0.7

910 80.60 6004.25 10.40 0.000 0.90"0.02 16.3"0.5

930 78.02 8329.09 14.43 0.011 0.97"0.03 17.7"0.6

950 81.26 7014.49 12.15 0.056 1.00"0.03 18.0"0.6

970 84.10 1318.94 2.28 0.080 1.15"0.06 20.9"1.1

990 97.35 86.17 0.15 0.000 1.59"1.41 29.0"25.4

1100 96.96 168.49 0.29 0.000 1.47"0.76 26.7"13.8Fuse 96.91 1214.36 2.10 0.318 1.83"0.48 33.2"8.6

Total age: 22.2"1.9

( ) ( ) ( ) JK135b single grain laser step-heating Js0.01709

1 98.87 179.41 0.20 0.035 2.57"2.75 77.8"81.5

2 99.63 115.22 0.13 0.012 0.18"2.70 5.6"83.3

3 96.08 224.00 0.25 0.000 0.58"0.35 17.7"10.6

4 89.33 2106.23 2.34 0.001 0.78"0.08 24.0"2.4

5 87.37 2385.60 2.65 0.000 0.77"0.07 23.8"2.2

6 85.24 2378.88 2.64 0.001 0.78"0.05 24.1"1.4

7 77.39 7886.34 8.76 0.000 0.76"0.02 23.4"0.7

8 39.68 6310.08 7.01 0.000 0.75"0.01 23.2"0.3

9 37.70 6554.38 7.28 0.000 0.75"0.01 23.2"0.4

10 31.50 5683.93 6.31 0.001 0.75"0.01 22.9"0.3

11 28.40 4076.52 4.53 0.001 0.75"0.01 23.1"0.3

12 29.12 3132.85 3.48 0.001 0.73"0.01 22.5"0.4

13 32.91 3813.95 4.23 0.001 0.72"0.01 22.2"0.4

14 43.57 8070.37 8.96 0.001 0.67"0.01 20.5"0.3

15 50.43 3882.90 4.31 0.004 0.64"0.02 19.6"0.6

Fuse 41.56 33,231.17 36.91 0.021 0.72"0.01 22.3"0.2

Total age: 22.4"0.2

( ) ( ) ( ) JK135c single grain laser step-heating Js0.01709

1 97.02 1838.41 6.72 0.006 1.48"0.35 45.1"10.6

2 70.02 1832.46 6.70 0.001 0.84"0.06 25.7"1.9

3 22.49 416.71 1.52 0.005 0.80"0.08 24.5"2.5

4 48.97 3575.32 13.07 0.001 0.82"0.02 25.3"0.65 43.78 6965.84 25.46 0.000 0.79"0.01 24.2"0.3

6 41.09 5743.78 21.00 0.010 0.66"0.01 20.4"0.4

7 65.96 221.34 0.81 0.002 0.41"0.12 12.7"3.7

8 46.18 580.30 2.12 0.005 0.58"0.07 17.9"2.1

Fuse 37.73 6181.98 22.60 0.042 0.77"0.01 23.6"0.4

Total age: 24.6"0.7



20/37


.Table 4 continued


( ) ( ) ( ) JK135d single grain debris, 2.4 mg furnace step-heating Js0.01716

500 100.00 24.78 0.25 0.704

600 99.40 17.43 0.18 1.750 2.18"3.97 84.8"117.2

700 100.00 48.02 0.49 1.659

785 98.30 167.23 1.70 0.000 0.66"0.58 20.3"17.7

850 98.54 698.60 7.10 0.026 0.62"0.16 19.1"4.8880 91.76 854.28 8.69 0.187 0.50"0.10 15.4"2.9

910 87.44 2421.44 24.62 0.000 0.49"0.34 15.2"1.1

940 72.84 1206.45 12.27 0.000 0.53"0.07 16.4"2.3

970 83.03 815.85 8.30 0.000 0.45"0.11 13.9"3.2

1050 83.35 1728.65 17.58 0.000 0.51"0.05 15.7"1.6

1200 83.84 1851.85 18.83 0.000 0.53"0.06 16.4"1.9

Total age: 15.9"0.9

.Steps temperature 8C or step number for the sample analysed with a conventional high-frequency furnace system or a laser probe,

respectively.40

Ar satmospheric40

Ar;40

Ar)sradiogenic40

Ar. The volume of39

Ar is based on a mass spectrometer sensitivity ofatm K7=10y1 0 V cmy3 STP, which, however, is likely to fluctuate somewhat. The error is at the 1 s level and does not include the error in the

.value of the J parameter. Age calculations are made using the decay constants given by Steiger and Jager 1977 . Apparent ages of36 37 .individual steps are corrected for irradiation-induced contaminant Ar-isotopes derived from Ca and K in the sample. The Arr Ar ,Ca

39 37 . 40 39 .Arr Ar and Arr Ar ratios used in the corrections for Ca- and K-derived Ar isotopes are determined from irradiated aliquots ofCa KCaF and K SO ; the values are: 2.8=10y4 , 7.0=10y4 and 258=10y4 , respectively. Details on the isotope corrections applied were2 2 4

.given by Feraud et al. 1986 .

.quent analysis in Nice and Toronto as flux monitor.

They received 4.0=1018 neutronsrcm2, while be-

ing rotated around a vertical axis. After irradiation,

most phengite grains had obtained a smoky appear-

ance. As this effect could not be reproduced by

prolonged heating at 1502508C of not irradiated

grains, we infer that it is due to some kind ofirradiation damage. Samples were placed on three

levels, each with three monitors to analyse the homo-

geneity of the neutron flux. The40

Ar)r39

Ar ratiosKmeasured from the standards were reproducible to

.better than 0.5% 1s on each level. The measured40

Arr36

Ar ratio during these experiments was 288"

0.5. Due to the long delay between irradiation and

analyses, in the order of 230400 days, in conjunc-

tion with the short half life of37

Ar the CarKCaimplied by the 37r39 isotope ratio is approximative

only and cannot be used for mass balance calcula-

tions.

6. Results

During the first heating increment most grains

moved slightly, inflated and delamination of cleav-

age planes occurred in some cases. Subsequently, the

surface of the grains obtained a whitish hue, render-

ing them more opaque. During advanced step-heat-

ing, the grains glowed; fusion was excecuted after

the top layer of the grain had become glassy and

partially molten. The fusion step released between

10% and 35%39

Ar. The analytical data of eachsample are listed in Table 4 and portrayed as age

spectra in Fig. 7. A summary of the results of all

samples is given in Table 5.

6.1. Degassing of Ar isotopes

The degassing of the samples is characterised by

an early36

Ar and37

Ar release followed by aAIR Cajoint main release of

39Ar ,

40Ar) and

38Ar . Induc-K Cl

tion furnace experiments on single grain halves

JK115Y

and JK135Y

resulted in significant36

ArAIR

and37

Ar release between 400 and 6508C, whereasCathe main degassing of these two isotopes occurred in

a sharp peak between 7008C and 8008C, without39 .Ar release Fig. 6a,b . The second 36 and 37 ArKisotope release peak is accompanied by a clear

40Ar

. 38degassing Fig. 6a,b . Absence of important ArClrelease excludes that fluid inclusions are involved.


21/37

Table 540

Arr39

Ar ages of phengitic mica single grains and a mica separate from five sites in the gneiss body of the MacaelChive unit, eas

of the samples and lithologies40 36 . Site Sample Lithology; Grain Material; method Integrated Inverse Arr Ar MSWD Plateau age P li

. . . fabric diameter age Ma isochron "% Ma, 2s % . . mm age1 Ma

1 ALM163 Augen gneiss mineral separate; 58.3"0.2 59.8"0.2 289.9"0.1 4.8

furnace

JK124 Banded augen 1.8=2.5 single grain; laser 58.8"0.7

gneiss

JK125 Banded gneiss 2.0=2.5 single grain; laser 37.4"0.6 35.4"0.3 297.6"0.3 0.4 35.5"0.7 91

2 JK119 Augen gneiss, 1.0=1.5 single grain; laser 90.7"1.7 90.9"0.7 298.0"0.6 1.1 90.1"1.0 82

coarse grained

3 JK114 Augen gneiss, 1.5=1.5 single grain; laser 24.9"1.6 24.1"0.9 295.6"0.2 0.4 23.8"0.8 93

platyX

JK115 1.5=2.6 single grain, 42.7"1.5 40.9"0.6 297.0"0.5 0.8 41.2"0.6 91

half 1; laser

Augen gneissY

JK115 1.5=2.6 single grain, 52.8"2.7 41.2"0.8 298.8"0.2 0.9 43.3"0.8 77

half 2; furnace

JK100 Augen gneiss, 1.8=2.5 single grain; laser 76.1"1.4 67.1"0.6 298.5"0.2 3.8

coarse grained

4 JK118 Quartzite in 2.0=2.5 single grain; laser 30.1"1.2 23.4"0.4 299.9"0.3 1.0

gneissX

5 JK135a Fined-grained 1.5=1.5 single grain, 15.9"0.5 15.9"0.3 295.1"0.4 0.7 15.8"0.4 97

mylonitic gneiss half 1; laserY

JK135a 1.5=1.5 single grain, 22.2"1.9 16.6"0.4 299.4"0.2 1.5

half 2; furnace

JK135b 1.5=1.5 single grain; laser 22.4"0.2 22.1"0.1 298.2"0.5 5.5 23.2"0.3 49

JK135c 1.5=1.5 single grain; laser 24.6"0.7 23.7"0.3 300.4"0.7 1.2

JK135d varying single grain debris; 15.9"0.9 16.4"0.8 293.8"0.2 0.2 15.6"1.6 90

furnace

.The errors are quoted at the 1s level unless otherwise stated and do not include the error in the value of the J parameter. Ages ar . .decay constants Steiger and Jager, 1977 . Details on the criteria to define age plateaux were given by Ruffet et al. 1991 . Th

. . .calculated from the best-fit line York, 1969 , using all steps age1 , or the plateau steps age2 . The errors are taken as the 9 .MSWDsSUMSr ny2 , with SUMSsminimum weighted sum of residuals; nsnumber of points fitted.


22/37


Well-correlated main release peaks of39

Ar ,40

Ar)Kand

38Ar show that the principal degassing of micaCl

.occurred between 8008C and 10008C Fig. 6a,b .

Calcite degassing is important in the 5007008C .range Spray and Roddick, 1981 . It seems therefore

likely that early gas release took place from Ca

Fig. 6. Differential thermal release of40

Ar) ,39

Ar ,38

Ar ,37

Ar and36

Ar plotted as the signal in mV normalised by the temperatureK Cl Ca AIR . .intervals between two sucessive steps, vs. temperature during induction furnace step-heating of phengite single grains a and b , single

. .grain debris d and mineral separate c . The main degassing of phengitic mica occurs between 8008C and 10008C, whereas important37 36 . .release of Ar and Ar below 8008C by the phengite single grains a and b points to degassing of included minerals.Ca AIR


23/37


.Fig. 6 continued .

carbonates possibly located in the grain boundaries

and cleavage planes. The associated release of36

ArAIRimplies that its growth may be due to the slight

alteration of mica. The observed easy splitting and

opening of mica books that point to loosening of

cohesion across the basal cleavage may similarly

result from this. Trapping of early released40

ArXScould have occurred in the imperfect crystal lattice

around the inclusions. An alternative solution is that

at least part of the37

Ar is released due to recoil toCa


24/37


sites rich in36

Ar , as the39

Ar recoil distance ofAIR K39 this isotope is about four times that of Ar OnstottK

.et al., 1995 .

Induction furnace step-heating of phengitic mica

separate ALM 163 and of the single grain debris .JK135d also resulted in a well correlated release of39 40 ) 38 Ar , Ar and Ar centered around 9008C Fig.K Cl

.6c,d . In contrast to the single grain experiments .Fig. 6a,b step-heating of the mineral separate did

not result in significant36

Ar and37

Ar releaseAIR Ca .below 8008C Fig. 6c,d . The experiment on the

single grain debris only resulted in minor degassing

of the 36 and 37 isotopes compared to the single .grain experiment Fig. 6c,d .

6.2.4 0

Arr39

Ar ages

The single grain laser probe and induction furnace

step-heating data are characterised by highly scat-

tered integrated ages from 15.9"0.5 to 90.7"1.7

Ma and an abnormally high atmospheric contamina- .tion Tables 4 and 5 . Nearly all single grain age

spectra show elevated apparent ages during the first

1035% of39

Ar release, which progressively de- .crease to relatively constant values Fig. 7 . With the

decrease of apparent ages during the early39

Ar re-

lease, the atmospheric contamination and the37

Ar r39

Ar ratio also diminish for most singleCa K .grains Table 4 . Despite the very large spread of

Fig. 7.40

Arr39

Ar laser step-heating and induction furnace step-heating age spectra of phengitic mica single grains and a mineral separate

from five sites in the eastern Sierra de los Filabres. The error bars are at the 1 s level, without the error in J. Note the important age .difference between the sites and between samples from the same site. c Discordant age spectra of three samples from Site 3; one grain of

X. Y .sample JK115 was split over the basal plane in two halves, which were analyses of laser step-heating 115 and furnace step-heating 115 . . .e Site 5; four discordant age spectra of three different grains of sample JK135, one of which 135a was split over the basal plane in two

X. Y . .halves, which were stepheated using the laser 135a and the furnace 135a . f Site 5; comparison of the laser step-heating age spectrum

of grain half JK135aX

with an induction furnace step-heating age spectrum of single grain debris of sample JK135. The two plateau ages are

concordant.


25/37


.Fig. 7 continued .

integrated ages, more than 50% of the single grains

yielded plateau ages between 15.8"0.4 and 90.1"

. 40 361.0 Ma Table 5 . Inverse isochrons have Arr Arintercepts close to the atmospheric ratio and they are

generally concordant to the integrated and plateau .ages Table 5, Fig. 7 .

The single grain ages do not reveal a grouping

according to geographic or structural position in the

gneiss body. Instead, the data point to an important

age variation at the different sites sampled. Phengites

of two gneiss samples from site 1 have integrated

ages that differ by about 20 Ma; three samples taken

close together at site 3 display an age difference as

.large as 50 Ma Fig. 7 .Age differences are even present at the scale of

individual samples. The integrated ages of several .phengite grains from sample JK135 site 5 differ by

.about 9 Ma Table 5 . Even the integrated ages of

two parts of a grain from this sample that was split

over the basal plane are not concordant: laser step- Xheating gave a plateau age of 15.8"0.4 Ma 135a ,

.half 1 , whereas induction furnace step-heating of the Y .other half 135a , half 2 resulted in an age of

.22.2"1.9 Ma Table 5 . Disregarding the elevated

apparent ages at low temperature, the integrated age

over 88% of the spectrum of grain half 2 is 17.9"0.3Ma. The age difference of about 2 Ma that is

.roughly 12% between the flat sections of spectra of

the two grain halves is largely above the analytical Y.reproducibility. The apparent ages of half 2 135a

are essentially all elevated above the 15.8 Ma plateau .age of half 1 Fig. 7e . This is also the case for the

age spectra of grains JK135b and 135c, except for

the age minimum in spectrum 135c that intersectsX the 15.8 Ma plateau value of grain half JK135a Fig.

.7e . Induction furnace step-heating of debris of sin- .gle grains JK135d , which was present in a tube

with about 15 single grains selected for dating,yielded a plateau age of 15.6"1.6 Ma Table 5, Fig.

.7f . This age is perfectly concordant with the plateau

age of the youngest grain half from this sample X .135a .

Dating of a split grain from sample JK115 re-

vealed an age discordance in the low-temperature Y.steps between two halves: half 2 115 , investigated

with the induction furnace, yielded much higher X.apparent ages than grain half 1 115 , analysed with

.the laser probe Fig. 7c . The apparent ages of both

grain halves over the flat segment of the spectrum .are, however, concordant Fig. 7c .

Due to the very high atmospheric contamination,

our attempt to date two halves of split phengite

grains by a combined laser spot fusion and laser

step-heating, like that performed by de Jong et al. .1992 , was not successful. Each spot fusion was

characterised by an atmospheric contamination of

almost 100%, yielding meaningless ages. Video im-

ages showed that during spot fusion strong delamina-

tion of grains occurred, which seems to be controlled

by their easy splitting over the basal plane. During

delamination fusion was no longer controlled as

material outside the fusion spot, below the surface,

seems to have taken part as well. During this pro-

cess, release of gas and poorly trapped atmospheric


26/37

36 40 39 40 X .Fig. 8. Arr Ar vs. Ar r Ar correlation plot for single grains JK119 and JK135 laser step-heating and mineral separate JK1Kcalculations, see footnote of Table 5. Error margins are omitted for clarity.


27/37


argon was released from unknown parts of the crys-

tals.

Induction furnace step-heating of phengite sepa-

rate ALM163 resulted in a disturbed age spectrum .Fig. 7a . Nevertheless, the apparent ages of the

central part of the spectrum, corresponding to 56.4%

of the39

Ar released, are concordant with a weighted .mean age of 59.1"0.4 Ma 2s , which falls in the

range of the single grain plateau ages. The integrated

ArrAr age is concordant with the KAr age, but is

significantly older than the corresponding RbSr age .Table 1A .

6.3.36

Arr4 0

Ar s.39

Ar r4 0

Ar correlation plotsK

Whereas the age spectrum technique assumes a

trapped40

Arr36

Ar of 295.5 for the age calculation,

the correlation plots expressing36

Arr40

Ar vs.39Ar r40Ar permit to independently establish theK40

Arr36

Ar of the trapped component that is mixed

with the radiogenic reservoir, or to establish multipletrapped components Roddick et al., 1980; Heizler

.and Harrison, 1988 . Correlation plots for our experi-

ments all define straight lines with36

Arr40

Ar inter- .cepts close to atmospheric value Table 5; Fig. 8 . In

laser step heating experiments, this clearly is not

caused by a mixture of the extraction line blank, for

which a correction is made. In the case of furnace Y.stepheating experiments Fig. 8: JK115 the high

temperatures steps are progressively dominated bythe blank of the furnace.

7. Interpretation

7.1.39

Ar recoilK

40Arr

39Ar studies of fine-grained minerals, like

illite Frank and Stettler, 1979; Hunziker et al., 1986;. Dong et al., 1995 , glauconies e.g. Smith et al.,

. 1993 , or submicroscopically chloritised biotite Hess

and Lippolt, 1986; Lo and Onstott, 1989; Ruffet et. 39al., 1991 have shown that the effects of Ar recoilK

on the age spectra can be devastating. Hess and . 39Lippolt 1986 showed that the effects of Ar recoil

were the strongest in biotites with the lowest K-con-

tent, i.e. those that were the most severely chlori-

. 39tised. Smith et al. 1993 showed that Ar loss inKporous glauconies results from loss of kinetic energy

due to collision of the recoiled atoms with the gas

atoms in the voids of the material leading to failure .to reimplant in the lattice. Hunziker et al. 1986

argued that39

Ar recoil was negligible for the bestKordered illite from the highest grade rocks they

.studied and Dong et al. 1995 demonstrated that39

Ar recoil loss is determined by the illite crys-Ktallinity.

Despite the fact that the thickness of the illitic . 39mica Fig. 5 in part overlaps with the Ar recoilK

distance i.e. 0.080.16 mm: Turner and Cadogan,.1974; Onstott et al., 1995 , there are a number of

arguments in favour of the absence of serious39

ArK .recoil in the dated material. 1 A fair part of the

opening of mica crystals along the basal cleavage

and part the distortions of the secondary micas that

are salient on most images is due to the cutting ofthe specimen. This indicates that the original thick-

ness of the illitic minerals was well above the recoil

distance and the material much less porous than

suggested by the HRTEM images. This means that .Smith et al.s 1993 recoil model is probably not

. applicable to our material. 2 Microprobe data Ta-.ble 2 show that the K-content of the phengitic mica

is generally according to the stoichiometry of white

micas, at the scale of the beam used. AEM data

reveal that at a smaller scale of observation both

phengite and illitic mica can be depleted in K Table.3 . However, only in JK119 a significantly different

K-content was observed between phengitic mica and

pseudo-illite on the one hand, and type 3 illitic mica

on the other. The relatively rare occurrence of type 3

illitic mica excludes that it acted as an important39

Ar sink during recoil. The chemistry and crystalKstructure are, except for type 3 illitic mica, still close

.to phengitic mica, and thus well-ordered. 3 Finally,

the illititic micas occur in relatively thick, cross-cut-

ting veins rather than finely interstratified with the .001 of the phengitic mica.

Absence of39

Ar recoil of any importance agreesKwith the very regular

39Ar release, which is wellK

40 ) .correlated with that of Ar Fig. 6 , without a shift

with respect to the40

Ar) peak, as has been reported .for chloritised biotite by Lo and Onstott 1989 and

.Ruffet et al. 1991 . Furthermore, the integrated

ArrAr ages of samples ALM163 and ALM 226 are


28/37


.concordant with their KAr ages Table 1A , giving

confidence in the absence of important39

Ar recoilKeffects.

7.2. Low-temperature excess argon

The apparent ages during the first 1035% of

degassing are generally elevated for most single .grains Fig. 7 . Degassing characteristics of the fur-

.nace step-heating experiments Fig. 6a,b suggest

that trapping of low-temperature40

Ar occurred inXSthe slightly weathered, imperfect crystal lattice asso-

.ciated with the carbonate Section 6.1 . Absence of40 36 37 .important Ar , Ar and Ar release Fig. 6cXS AIR Ca

at low-temperature for mineral separate ALM163

implies that such disordered mica crystal lattice was

effectively removed by mechanical grain size reduc-

tion of the sample and separation methods used to

.purify the material Andriessen et al., 1991 , whichwere not applied during separation of the single

grains from the host rock. The single grain debris .JK135d is apparently also enriched in more pure

parts of mica grains, suggesting that splitting of the

mica occurred along weak, weathered and disordered

zones in the lattice.

Sample JK119, which has the oldest plateau age,

apparently lacks low-temperature excess argon,

whereas some of the youngest grains of sample Y .JK135 a and c in Fig. 7e are affected by low-tem-

perature40

Ar . This implies that the occurrence ofXSlow-temperature

40Ar has no direct bearing on theXS

different plateau ages obtained during the main de-

gassing of mica. Nevertheless, both the occurrence

of low-temperature40

Ar and the spread in plateauXSages may be due to recrystallization processes in the

mica, to which the microprobe and AEM data point.

7.3. Ealuation of the ages

The obtained ages reflect isotopic closure in some

way. The predominance of temperature on isotopic

closure has been questioned repeatedly and the im-

portance of fluid-assisted recrystallization for ex-

change or loss of radiogenic daughter isotopes hasbeen stressed Chopin and Maluski, 1980; Verschure

.et al., 1980; Villa, 1998 . Also in our case, it is .difficult to interpret the obtained plateau ages as

simply dating the cooling of the rocks following

D because an age difference as large as 50 Ma2for samples taken only a few tens of metres from

each other requires unrealistically varying cooling

paths of neighbouring rocks. The more so as RbSr

ages of mica in the same gneiss body cluster tightly

and are significantly younger than the correspon-

ding KAr ages and our40

Arr39

Ar results. Mecha-

nisms that may contribute to the observed spread in40

Arr39

Ar ages and the differences between KAr

and RbSr ages are discussed below.

7.3.1. Inherited argon

Inherited Ar reflects the fact that thermo-tectonic

recrystallization was not sufficiently strong to release

all Ar from relictic minerals. In a number of cases,

dome-shaped age spectra have been obtained for

white mica with inherited Ar in metamorphic rocksMonie, 1985; Wijbrans and McDougall, 1986;

. 40Scaillet et al., 1992 . In contrast to Ar incorpora-XStion, cases of inherited Ar normally have RbSr ages

that are higher or equal to the corresponding KAr

ages. In contrast to the RbSr system, which is reset

by Sr-exchange across grain boundaries, the resetting

of the KAr system requires removal of Ar from the

system, for which a carrier fluid is essential. Like

Folands classic 1979 and 1983 studies of40

ArXSincorporation by biotite, it has been shown that

limited fluid mobility was important in cases where

the incorporated argon in phengite was locally de-

rived from the protolith during very-high-pressuremetamorphism of pre-Alpine crystalline rocksMonie, 1985; Arnaud and Kelley, 1995; Ruffet et

.al., 1995, 1997; Scaillet, 1996 . In our case the .phengitic micas may contain inherited Ar from 1

.the magmatic stage and 2 the D phase that failed2to fully degas during subsequent tectono-metamor-

phic recrystallization.

The gneisses of the Mulhacen Complex were .formed from late Variscan granites Section 3.1 .

The 282 Ma KAr age of white mica in a metagran- .ite ALM3, Table 1A is as old as the crystallization

.age assigned to the granite Section 3.1 . Part of the40

Ar) accumulated since the late Permian could in .principle be partially incorporated in the phengitic

mica if incomplete degassing during the Alpine

orogeny occurred due to restricted fluid mobility.

However, the high amount of tourmaline and the

occurrence of fluorite and topaz as accessories in the


29/37


gneisses point to a hydrothermal alteration of the

granite, e.g. by residual magmatic fluids. This seems

to imply that the protolith to the gneisses was never

water-deficient, making inherited argon as explana-

tion for the observed age spread unlikely. The

feldspar in metagranite ALM3 is strongly altered

into sericitic mica, pointing to strong water-assisted

recrystallization, as does the occurrence of illitic

micas in the dated material. Age spectra of mica . separates ALM168 this study and ALM226 de

.Jong et al., 1992 are not dome-shaped, pointing to

absence of inherited Ar. 15.6 to 12.8 Ma RbSr ages

that are younger or comparable to corresponding .K Ar age Table 1A further underscore that inher-

ited Ar is not a plausible interpretation of our data

set, as these imply a complete resetting of the phen-

gite RbSr system and pervasive recrystallization.

7.3.2. Chemistry and mineralogy .Microprobe data Table 2 of three phengitic mi-

cas with an age difference of about 28 Ma show that

they have a significantly different mg-factor. Monie . .1985 and Scaillet et al. 1992 proposed that Mg-

rich phengites are more retentive for argon loss than

Fe-rich varieties. In our case, however, grains from .the oldest sample analysed by microprobe JK115

are not the most Mg-rich; in fact, they have a similar

mg-factor as sample JK135 that yielded the youngest

single grain ages. Differences in stoichiometric Sivalue, as signaled by Hammerschmidt and Frank .1991 , do not lie behind the observed spread in ages

either. We did not find an extreme variation in F

content between the phengitic micas analysed by

microprobe; also the38

Ar r39

Ar ratios are fairlyCl Kconstant. This means that the observed age variation

is not related to different levels of halogen substitu-

tion, which affect the physical properties and unit

cell dimensions of micas, although less for dioctahe- .dral varieties Munoz, 1984 .

The mineralogy has no bearing on the obtained

ages, as samples with appreciable amounts of Ca-

bearing minerals like titanite and epidote, or different

amounts of potential carriers of40

Ar , l ike K XSfeldspar and tourmaline, do not form an age group.

Furthermore, there is no correlation between mica

ages and the amount of included tourmaline, a min-

eral that also has a main degassing around 9008C and

yielded integrated40

Arr39

Ar ages between 85 and .110 Ma de Jong, 1991; in prep. .

7.3.3. Cooling and grain size .Hames and Hodges 1993 and Hodges et al.

. 40 391994 documented important Arr Ar age gradi-

ents on the scale of mica grains for slowly cooled

rocks. In such a case, neighbouring grains with

different diameters can in principle yield highly dif- .ferent ages, as Dodsons 1973 diffusion theory

predicts. The three samples in site 3 display an

agergrain size relationship according to his diffusion .theory; the largest single grain JK100 being the

.oldest, the smallest crystal JK114 the youngest and

the grains of JK115 in between. The presence of a

kink band in the middle of JK114 may have further

reduced the effective grain size. However, at site 1

grains JK124 and 125 are much smaller and yet

yielded older ages than 3 mm diameter grain JK127 .Sections 2 and 3.1 , taken at about 125 and 100 m

from the two respective grains. If we consider the

data of all sites it is clear that despite the similar

grain size the age difference is huge; in fact, the .oldest grain JK119 is one of the smallest Table 5 .

This indicates that other factors determine the age of

grains than their size alone. In how far the effective .grain size is reduced by inclusions Section 3.2 or

.submicroscopic kink bands Fig. 5a in the dated

material is not clear, as we have no pertinent data

from the step-heated grains. The submicroscopicvein-like recrystallization zones revealed by HRTEM

.imagery Section 4 , however, imply the existence of

fast diffusion channels that cut grains up into smaller

subdomains.

7.3.4. Microtexture and fabric of the gneisses

The texture of the gneisses at the hand specimen

and thin section scale seems to have an important

bearing on the apparent age of the mica in the rock.

Mylonitic gneisses with a fine-grained and homoge-

neous quartzfeldspar matrix with the most intenselydeveloped tectonic fabric and preferred orientation of

mica and only a few late joint-like cracks yielded the .youngest single grain ages JK135; Table 5 and the

. youngest plateau ages ALM182, 226, 230; Table. .1A . Marbles and mica schists dated Table 1B have

the same characteristics. All other gneisses have a

much less fine-grained quartzfeldspar matrix, with


30/37


a more heterogeneous grain size distribution and a

less well developed foliation S . Sample JK119 with2the oldest plateau age has the poorest developed

tectonic fabric and a strongly inequigranular

quartzfeldspar matrix; the amount of unoriented

mica is the largest of all samples. In this sample late

joint-like cracks sub-perpendicular to the stretching

lineation are well developed with a spacing of 500

mm or less. White mica with the oldest KAr ageoccurs in a coarsely grained inequigranular meta-

.granite ALM3; Table 1A that is affected by brittle

deformation. Late stage hydraulic fracturing was,

hence, more important in rocks with a less inter-

grown and more inequigranular quartz feldspar ma-

trix and a lesser planar anisotropy. This implies that

the interaction with late fluids was more important in

these gneisses. Fluids play a paramount role in iso-tope resetting e.g. Verschure et al., 1980; Villa et

.al., 1997; Villa, 1998; Kerrich and Ludden, 2000 .

7.3.5. Recrystallisation

The strong variation of the microprobe data point

to important chemical heterogeneity of the dated

micas. The severity of recrystallization is even more

striking on the basis of HRTEM-AEM data, which

point to illitisation and major heterogeneity in chem-

istry at the micron scale.

It has been shown that dynamic and related chem-

ical recrystallization of white mica have a pro-

nounced effect on the age of the mineral Chopinand Maluski, 1980; Wijbrans and McDougall, 1986;

Hammerschmidt and Frank, 1991; Hames and Ch-.eney, 1997; Villa, 1998; Itaya and Fujino, 1999 .

.Hames and Cheney 1997 argued that the pattern of .decreasing spot ages in JK127 Section 2 might

similarly be due to deformational control on Ar

migration during dynamic recrystallization. It has .been pointed out in Section 3.1 that the microstruc-

ture of the gneisses and the strain free white micas is

essentially due to annealing recrystallisation during

the waning stages of D . The micas all have unde-2

formed basal planes and difference in microstruc-

tures between cleavage septa and microlithons, i.e.

between limbs and hinge areas of micro folds, which

now form polygonal arcs, implies important grain

boundary migration and lattice diffusion after D at2 .temperatures above 5008C Fig. 2 . It is hence un-

likely that differences in D -related dynamic and2

chemical recrystallization can explain the observed

age discordance. .Verschure et al. 1980 showed that metamorphic

temperature alone could not reset the KAr and

RbSr systems of Sveconorwegian biotite during the

Caledonian metamorphism. However, progressive

chemical recrystallization of the biotite, which im-

plies circulating fluids and grain boundary diffusion,

resulted in partial

DeJong_Phengite Age: Fluids, Submicroscopic Illitization, Excess Argon & Laser Probe Dating (Betics,...

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