DeJong_Eclogites: Very Fast Exhumation, Excess Ar, Inherited Sr (Betics, Spain)_Lithos 2003

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Very fast exhumation of high-pressure metamorphic rocks with

excess 40Ar and inherited 87Sr, Betic Cordilleras, southern Spain

Koen de Jong*

Department of Isotope Geochemistry, Vrije Universiteit, Amsterdam, The Netherlands

CNRS, UMR 6526 Geosciences Azur, Universite de Nice-Sophia Antipolis, Nice, France

Abstract

In order to attempt to further constrain the age of the early Alpine tectonic evolution of the Mulhace n Complex and to

explore the influence of inherited isotopes, micas from a small number of well-characterised rocks from the Sierra de los

Fila bres, with a penetrative tectonic fabric related to the exhumation of eclogite-facies metamorphic rocks, were selected for40Ar/39Ar and RbSr dating.

A single phengite grain from an amphibolite yielded an40

Ar/39

Ar laser step heating plateau age of 86.9F 1.2 Ma (2r; 70%39Ar released) and an inverse isochron age of 86.2F2.4 Ma with an 36Ar/40Ar intercept within error of the atmospheric value.

Induction furnace step heating of a biotite separate from a gabbro relic in an eclogite yielded a weighted mean age of

173.2F 6.3 Ma (2r; 95% 39Ar released). These ages are diagnostic of excess argon (40ArXS) incorporation, as they are older

than independent age estimates for the timing of eclogite-facies metamorphism and intrusion of the gabbros. 40ArXSincorporation probably resulted from restricted fluid mobility in the magmatic rocks during their metamorphic recrystallisation.

RbSr whole-rockphengite ages of graphite-bearing mica schists from Paleozoic rocks (Secano unit) show a dramatic

variation (66.1F 3.2, 40.6F 2.6 and 14.1F2.2 Ma). An albite chlorite mica schist from the Mesozoic series of the Nevado

Lubrn unit has a whole-rockmicaalbite age of 17.2 F 1.9 Ma, which is within error of an 40Ar/39Ar plateau age published

previously and of the youngest RbSr age of the Paleozoic series obtained in this study. The significant spread in RbSr ages

implies that progressive partial resetting of an older isotopic system has occurred. The microstructure of the samples with pre-

Miocene RbSr ages reveals incomplete recrystallisation of white mica and inhibited grain growth due to the presence of

graphite particles. This interpretation agrees with previously published, disturbed and slightly dome-shaped 40Ar/39Ar age

spectra that may point similarly to the presence of an older isotope component. The progressively reset RbSr system is a relic

of Variscan metamorphism of the Paleozoic series of the Mulhacen Complex. In contrast, the origin of the ca. 17.2 Ma old

sample from the Mesozoic series precludes any isotopic inheritance, in agreement with its pervasive tectono-metamorphic

recrystallisation during the Miocene.

Exhumation of the eclogite-facies Mulhacen Complex occurred in two stages with contrasting rates of about 22.5 mm/yearduring the early phase and 910 mm/year during the late phase; the latter with a cooling rate in the order of 330 jC/Ma.

D 2003 Elsevier B.V. All rights reserved.

Keywords: 40Ar/39Ar dating; Excess argon; Isotope inheritance; Phengite; Biotite; Fluids

0024-4937/$ - see front matterD 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0024-4937(03)00094-X

* Present address. Argon Geochronology Laboratory, Department of Geosciences, National Taiwan University, 245 Choushan Road, Taipei

106, Taiwan, ROC. Tel.: +886-2-3365-1899; fax: +886-2-2363-6095.

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

www.elsevier.com/locate/lithos

Lithos 70 (2003) 91110


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1. Introduction

Phengite has yielded meaningful 40Ar/39Ar plateau

ages for eclogite-facies metamorphic rocks (Bosse etal., 2000), but research during the last decade of the

20th century provided a lot of evidence for the incor-

poration of excess argon (40ArXS) in its lattice (e.g.

Tonarini et al., 1993; Li et al., 1994; Arnaud and

Kelley, 1995; Inger et al., 1996; Sherlock et al.,

1999). 40ArXS uptake occurs in association with partial

recrystallisation of high-pressure phengite during sub-

sequent metamorphism at lower temperature and pres-

sure (Hammerschmidt and Franz, 1992; Hannula and

McWilliams, 1995; Ruffet et al., 1995; Reddy et al.,

1996; de Jong et al., 2001). Alternatively, strongly

restricted fluid mobility, leading to the incorporation of

locally derived (inherited) argon, is commonly quoted

as the mechanism responsible for the frequently ob-

served elevated phengite 40ArXS ages in (ultra-) high-

pressure metamorphic rocks (Scaillet, 1996; Boundy et

al., 1997; Li et al., 1999; Giorgis et al., 2000), and is

also seen as the reason for the survival of pre-orogenic

RbSr ages in biotite (Verschure et al., 1980; Kuhn et

al., 2000). It appeared that incorporation of argon into

phengite may have been controlled by very low lattice

and grain boundary diffusion under dry, eclogite-facies

conditions and that the gas has been internally derivedfrom within the eclogite protoliths.

de Jong et al. (2001) attempted to constrain the age

of early Alpine exhumation of the Mulhacen Complex

of the Internal Zone of the Betic Cordilleras of

southern Spain, or Betic Zone (Fig. 1). They obtained

widely scattered 40Ar/39Ar laser step heating plateau

ages between 15.8F 0.4 and 90.1F1.0 Ma (2r) on

well-crystallised single phengite grains from orthog-

neisses in a small area in the easternmost Sierra de los

Filabres (Fig. 2). Age discordance was observed at the

outcrop scale as well as in individual grains. The

authors explained these phenomena by 40ArXS uptake

that seems to be associated with the gneisses, since the

RbSr ages in these rocks are systematically younger

than the KAr and 40Ar/39Ar ages. Specific to this

case is the occurrence of hydraulic cracks in the

gneisses, high atmospheric contamination and submi-

croscopic illitisation of phengite, permitting 40ArXSstorage in interlayer vacancies and other lattice imper-

Fig. 1. Tectonic map of the eastern Betic Cordilleras, modified after de Jong (1993a). The sampled areas in the eastern Sierra de los Filabres

(Figs. 2 and 3) are outlined. Stars: samples from the Sierra de Baza (SdB) and from Cerro del Almirez (CdA) are discussed.

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Fig. 2. Geological map of the easternmost Sierra de los Fila bres, modified afterde Jong et al. (2001), with the sample locations indicated by

arrows. The Ar/Ar total gas ages obtained by these authors on single phengite grains at different locations (stars) in gneiss body of the Macael

Chive unit are indicated.

K. de Jong / Lithos 70 (2003) 91110 93


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fections. The oldest phengite was from a coarse-

grained gneiss with closely spaced late-stage hydrau-

lic cracks, which are lacking in fine-grained mylonitic

gneiss that yielded the youngest micas. Hence, theinteraction of meteoric waters with the hot metamor-

phic rocks, phengite recrystallisation and 40ArXS up-

take were much more intense in the coarse-grained

gneiss. Like the fine-grained mylonitic gneisses, mica

schists and mica-bearing foliated amphibolites lack

such extensively developed hydraulic crack networks,

rendering them less liable for the incorporation of40ArXS by this mechanism. Accordingly, white mica

from albitechloritemica schists and marbles with a

penetrative S2 yielded 15.417.6 Ma40Ar/39Ar pla-

teau and total fusion ages (de Jong et al., 1992) that

are not obviously affected by significant levels of

incorporated 40ArXS.

The aim of the present study is to investigate

further the occurrence and source of excess argon as

well as the role of limited fluid mobility in the

incorporation process during the early tectono-meta-

morphic evolution of the Mulhacen Complex in the

Sierra de los Fila bres. For this purpose, we used

biotite from a gabbro with a Late Jurassic Rb Sr

isochron age, in which 40ArXS incorporation has been

established, and that occurs in the core of an eclogite,

which is chemically well characterised. In addition,we selected a phengite grain from an amphibolite,

which occurs in the same level as the eclogite, and

that acquired a penetrative fabric during the main

tectono-metamorphic phase, D2, subsequent to the

eclogite-facies metamorphism. Also, we applied

RbSr dating to a small number of mica schists with

a tectono-metamorphic fabric that was formed during

D2 and which, in a previous study, yielded40Ar/39Ar

age spectra that are in part slightly dome-shaped and

in one case flat. Our data set has a large spread in

ages, which is interpreted as due to the occurrence of40ArXS and partially inherited radiogenic40Ar and

87Sr isotopes, but, the least affected samples permitted

to date the timing of exhumation of the Mulhacen

Complex as Middle Miocene.

2. Tectonic setting

The Betic Zone comprises a stack of four nappe

complexes that have overthrust the southernmost

External Zone, which crops out in windows as the

(very) low-grade metamorphic Almagride Complex

(Fig. 1; Simon, 1987; de Jong, 1993a). These are,

from top to bottom: (1) the Malaguide Complex, (2)Alpujarride Complex, (3) Mulhacen Complex and (4)

Veleta Complex (Egeler and Simon, 1969; Puga and

Diaz de Federco, 1978; de Jong, 1993a,b; Puga et

al., 1999, 2002). The Alpujarride and Mulhacen

complexes have basal series of graphite-rich metape-

lites and cover series of metapelites and metapsam-

mites with abundant metacarbonates and greenstones

and locally gypsum (Egeler and Simon, 1969; de

Jong and Bakker, 1991; de Jong, 1991). The carbon-

ate series of the Alpujarride Complex are well dated

as Middle to Late Triassic by microfossils (Kozur et

al., 1985; Simon, 1987), whereas the basal series of

some tectonic units yielded Variscan ion-microprobe

zircon ages (Zeck and Williams, 2001, and references

therein). The Mulhacen Complex experienced an

Alpine metamorphism composed of a sequence of

different metamorphic facies (de Roever and Nijhuis,

1963), as well as pre-Alpine recrystallisation, as will

be outlined below. The Malaguide Complex has a

Paleozoic basal series covered by a condensed, but

almost complete Mesozoic and Tertiary section

(Egeler and Simon, 1969). Sediment petrographical

analysis of its Late Paleozoic series implies that itexperienced a Variscan orogeny (Herbig and Statteg-

ger, 1989; Henningsen and Herbig, 1990), although a

major angular unconformity between the Paleozoic

and younger series did not form (Makel, 1988). The

Veleta Complex comprises a monotonous lithological

sequence of graphite-bearing mica schists and quartz-

ites that yielded rare Middle Devonian (Lafuste and

Pavillon, 1976) and Riphean (Gomez-Pugnaire et al.,

1982) fossils. The Alpine tectono-metamorphic

recrystallisation has essentially obliterated the pre-

Alpine fabrics and mineral assemblages, except forinclusions in chloritoid porphyroblasts in some mica

schists that are very rich in graphite (Puga and Diaz

de Federco, 1978; Gomez-Pugnaire and Sassi, 1983;

Puga et al., 2002).

2.1. The Mulhacen Complex

The Mulhacen Complex in the Sierra de los

Fila bres is composed of three superimposed nappes

(Figs. 2 and 3), each with a probably Paleozoic

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Fig. 3. Geological map of the northern part of the central Sierra de los Filabres, modified afterde Jong et al. (1992), with arrows indicating the

sampled sites. Thrust slices on top of the Alpujarride Complex consisting of rocks of the Mulhacen Complex, north of Huertecicas Altas, have

been omitted for clarity. Triangles indicate nappe contacts.

K. de Jong / Lithos 70 (2003) 91110 95


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basal series of graphite-rich garnet mica schists that

contain orthogneisses and metagranites in the up-

permost two nappes (de Jong and Bakker, 1991).

The cover series are generally regarded as Triassicand younger and comprise alternating quartzites and

(albite-bearing) mica schists with marble levels in

the upper parts (de Jong and Bakker, 1991; Tendero

et al., 1993), hereafter called Mesozoic series.

Ultramafic rocks, mainly serpentinites, and abundant

greenstones occur in these higher levels, in part as a

mapable unit of amphibolites and amphibole mica

schists ((Figs. 2 and 3); de Jong and Bakker, 1991,

encl. 1). Early Alpine eclogites occur locally in

these greenstones (Morten et al., 1987; Bakker et

al., 1989; Gomez-Pugnaire et al., 1989; Puga et al.,

1989, 1999), which are wrapped by the main

foliation (S2) in the matrix. Part of the eclogite-

facies metabasites are derived from often cumulitic

gabbros, with partly preserved igneous paragenesis,

a MORB-like chemical composition and 143Nd/144 Nd ratios higher than 0.5130 and 87Sr/86Sr ratios

below 0.705 (Puga et al., 2002). Morten et al.

(1987) inferred that crystallisation of the gabbros

occurred at pressures below 1 GPa. A troctolitic

gabbro in the core of an eclogite yielded an RbSr

mineral isochron age of 146F 3 Ma (Hebeda et al.,

1980). The serpentinites are derived from spinellherzolites and secondary harzburgites with spini-

fex-like textures, which both contain abundant part-

ly rodingitised dolerite dykes (Puga et al., 2002).

The origin of the greenstone association is still a

matter of debate, with models ranging from a

dismembered ocean floor sequence (Puga et al.,

1989, 1999, 2002) to continental, rift-related mag-

matism (Gomez-Pugnaire et al., 2000). Glauco-

phane-bearing dolerites have locally well-preserved

intrusive contacts with calcite marbles (de Jong and

Bakker, 1991). The greenstone association mayhave developed in small oceanic pull-apart basins

situated in a major continental strike-slip zone that

connected Late Jurassic spreading centres in the

Atlantic and Ligurian Oceans (de Jong, 1991,

1993a).

Maximum pressures of 2.0 2.2 GPa have been

estimated for kyanite eclogites (Puga et al., 1999,

2002) and metamorphic ultramafic rocks (Lopez

Sanchez-Vizcano et al., 2001) in the Mulhacen

Complex at temperatures of about 700 jC. Strong

decompression concomitant with cooling of the rocks

to 500600 jC took place during and subsequent to

the main tectono-metamorphic phase, D2, which

occurred at a pressure of about 1.51.7 GPa (Pugaet al., 2002) and 0.81.2 GPa (Bakker et al., 1989;

de Jong, 1991, 1993a) during the final phase (Fig.

10). This resulted in pervasive amphibolitisation of

the eclogites. Further retrogression is marked by

widespread albite and chlorite growth that occurs

synkinematically with a phase of localised penetra-

tive D3 folding at pressures of about 0.40.5 GPa

and temperatures around 400 jC (Fig. 10; de Jong,

1991, 1993a,c). The cooling was followed by pro-

nounced late stage fluid-assisted reheating shown by

the widespread occurrence of rims of oligoclase and

biotite around albite and chlorite, respectively, as

well as by rare and local growth of staurolite and

kyanite during the early stages of D4 in mica schists

of the Mesozoic series of the Nevado Lubrn unit

(Bakker et al., 1989; de Jong, 1991, 1993a,c). The

absence of garnet constrains the PT conditions at

around 0.40.5 GPa and temperatures of about 500

jC (Fig. 10). The reheating was related to extension

(Bakker et al., 1989; de Jong, 1991, 1993a), which

resulted in recrystallisation, isotope resetting and40ArXS incorporation (de Jong et al., 2001). Ductile

(D5) and brittle ductile (D6) shear zones, whichdeveloped during retrogression, occur at various

levels within the Mulhacen Complex, but character-

istically at the contact with the overlying Alpujarride

Complex (de Jong, 1991, 1993a,c).

Micas from rocks with a penetrative alpine tectonic

foliation have Rb Sr ages that generally range be-

tween 12.5 and 16.9 Ma, whereas KAr and 40Ar/39Ar

dates span 13.790.7 Ma (Monie et al., 1991; de Jong

et al., 1992, 2001). Monie et al. (1991) obtained40Ar/39Ar ages of 24.6F 3.6 and 48.4F 2.2 Ma on

amphibole (Sierra de Baza, Fig. 1). Eleven SHRIMPUPb analysis on nine zircon grains in a pyroxenite

layer in ultramafic rocks (Cerro del Almirez, Fig. 1),

which are characterised by high pressure breakdown

of antigorite to spinifex-textured olivine and ortho-

pyroxene, yielded a mean age of 15.0F 0.6 Ma

(2r) (Lo pez Sanchez-Vizcano et al., 2001). This

U Pb zircon age is comparable to the majority of

RbSr ages of white mica and to 40Ar/39Ar ages of

this mineral that are the least affected by 40ArXSuptake. Zircon fission-track ages of the Sierra de los

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Filabres are in the 1114 Ma range (Johnson et al.,

1997).

2.2. Pre-alpine history of the Mulhacen Complex

Despite the pervasive nature of Alpine tectono-

metamorphic recrystallisation, the basal series of the

Mulhacen Complex contain unambiguous relics of

pre-Alpine tectono-metamorphic evolution. Pre-Al-

pine deformation structures are found in the deeper

part of the complex in the Sierra Nevada, in some

boudins or layers of graphite-bearing mica schists,

which also contain pre-Alpine amphibolite-facies par-

ageneses (Puga et al., 1975, 2002; Puga and Diaz de

Federco, 1978). The occurrence of chloritoid + al-

mandine, chiastolite + almandine + biotiteF staurolite

parageneses or rare cordierite points to PT condi-

tions of about 0.20.3 GPa and 500600 jC (Puga et

al., 2002). Elsewhere, studies of the relationship

between mineral growth and superimposed deforma-

tion phases have not yielded any evidence for the

presence of relic minerals that did not form during

Alpine metamorphism (Kampschuur, 1975; Martnez

Martnez, 1980; Bakker et al., 1989; de Jong, 1991).

However, complex inclusion patterns in the cores of

some Alpine porphyroblasts in the Sierra de los

Filabres have been interpreted as due to a pre-Alpineorogeny (Helmers and Voet, 1967; Vissers, 1977),

especially in garnets and staurolites that are spatially

associated with orthogneisses.

Gneisses and metagranites of the basal series of

this complex in the Sierra de los Filabres have yielded

RbSr errorchrons ranging between 275 and 191 Ma

(Andriessen et al., 1991), which the authors discussed

in the context of partial Alpine resetting and incom-

plete isotope rehomogenisation. A 267F 29 Ma Rb

Sr age (Andriessen et al., 1991) and a 307F 34 Ma

SmNd isochron (Nieto, 1996) are regarded as the best estimate of the crystallisation age of the subsol-

vus granites. The country rock to these intrusives is

affected by contact metamorphism, as revealed by the

occurrence of hedenbergite skarn and hornfels bodies

(Helmers, 1982; de Jong and Bakker, 1991). The

petrology of the granites and associated contact meta-

morphic rocks indicates an intrusion depth of at least 6

km (de Jong and Bakker, 1991), which agrees with

PT estimates for pre-Alpine mineral assemblages

described by Puga et al. (2002).

3. Sample description

3.1. Troctolitic gabbro

Biotite separate ALM 104 (63 125 Am sieve

fraction) has been obtained from a 1-m diameter

massive troctolitic gabbro that occurs in an eclogite,

which yielded a 146F 3 Ma RbSr mineral isochron

age and an initial 87Sr/86Sr ratio of 0.7028F 0.0001

(Hebeda et al., 1980). The biotite separate is known to

have 40ArXS and was used as one of the points that

defined the isochron and was selected for analysis to

better understand this phenomenon. The gabbro is

separated from the underlying albite chlorite mica

schists (Tahal schists, de Jong and Bakker, 1991) by

a fault that was folded during D4 and subsequently

reactivated as a low-angle D6 detachment fault (de

Jong, 1993c). This outcrop is part of a series of

amphibolites and amphibole mica schists of the

NevadoLubrn unit (Fig. 2). The course-grained

gabbro has a cumulitic texture with olivine and

labradoriteoligoclase as cumulus phases and clino-

pyroxene as well as minor brown hornblende and

biotite as intercumulus phases.

3.2. Micaceous amphibolite

The slightly elongated (0.75 1.5 mm) single phen-

gite grain, JK 0, which has been used for 40Ar/39Ar

dating, was obtained from a well-crystallised amphib-

olite with a strongly developed tectonic fabric, from the

same lithological unit as ALM 104 (Fig. 2). Blue-green

hornblende and phengite have a well-developed shape-

preferred orientation with respect to foliation S2,

whereas c-axes of the amphibole are parallel to the

lineation L2. Cores of a number of blue-green horn-

blendes contain relics of glaucophane. The transforma-

tion of glaucophane to blue-green hornblende is a syn-D2 reaction (Bakker et al., 1989; de Jong, 1991,

1993a,c).

3.3. Mica schists

Mineral separates in the 125250 Am sieve frac-

tion of four mica schists have been used for RbSr

mineral dating. The same white mica separate of three

of these samples has been analysed by 40Ar/39Ar

furnace step-heating, which yielded a plateau age of

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17.3F 0.2 Ma (ALM 270) and in two cases an40Ar/39Ar age spectra with progressively increasing

apparent ages over the main part of degassing that is

somewhat dome shaped as the last important degass-ing step is slightly younger (Fig. 9). Total gas ages are

19.1F 0.1 (ALM 272) and 25.9F 0.1 Ma (ALM

273). The 40Ar/39Ar isotopic data are given in de

Jong et al. (1992). The samples were chosen to better

understand why some samples yielded disturbed age

spectra and others did not.

The mica schists have a penetrative quartzmica

differentiated S2 foliation. ALM 272, 273 and 274 are

graphitic chloritoid garnet mica schists from the

Secano unit (Fig. 3; sensu, Helmers and Voet,

1967). This unit, which resembles the basal series of

the MacaelChive unit, however, without gneisses, isseparated from the underlying NevadoLubrn unit

by a D6 detachment fault (de Jong, 1991). ALM 272

and 273 are taken from the same outcrop within 20 m

from each other. ALM 274 is the most quartz-rich

sample and the least graphite rich. Chloritoid and

garnet porphyroblasts were formed pre- and syn-D2,

during which the main tectonic foliation of the rocks

was formed. Continuous lattice bending and limited

Table 140

Ar/39

Ar analytical data of micas from greenstones, Nevado Lubrn unitStep 40Aratm (%)

39ArK (10 13 cm3) 39Ar (%) 37ArCa/

39ArK40Ar*/39ArK Apparent age (Ma)

ALM 104 (biotite separate, 63 125 lm, 6.0 mg) (furnace step heating) (J = 0.01716F 1%, 2r)

450 100.00 6.14 0.06 3.664

550 88.31 69.35 0.73 0.000 2.48F 3.79 75.3F 50.3

650 75.86 48.22 0.51 0.731 5.90F 2.16 174.1F106.6

700 68.59 51.79 0.54 4.203 5.43F 0.77 160.7F 61.0

780 79.80 116.91 1.22 0.596 4.96F 0.51 147.3F 21.8

840 39.19 330.87 3.46 0.325 5.40F 0.35 159.9F 14.4

880 15.36 648.98 6.79 0.002 5.58F 0.11 165.0F 9.8

920 9.74 1428.49 14.95 0.000 5.57F 0.08 164.6F 3.2

960 10.60 2128.77 22.28 0.081 5.70F 0.11 168.3F 2.2

1000 13.55 1396.01 14.61 0.015 5.91F 0.14 174.1F 3.1

1050 13.10 1211.42 12.68 0.000 6.17F 0.07 181.5F 3.81150 10.99 1994.65 20.88 0.000 6.30F 1.10 185.2F 2.0

1350 26.54 120.59 1.26 1.177 12.71F146.52 355.9F 28.0

Fuse 92.55 1.56 0.02 24.133 40.30F 1.69 948.1F 2681.4

Total age: 174.7F 1.6

Inverse isochron age steps 450 1150= 173.2F 8.7 Ma; 40Ar/36Ar intercept = 281F 82; MSWD = 15

JK 0 (single phengite grain) (laser step heating) (J = 0.01709F 1%, 2r)

0.35 97.25 110.53 0.65 0.045 9.87F 11.82 282.1F 312.6

0.40 92.67 101.01 0.59 0.011 3.25F 1.70 98.0F 50.0

0.45 90.65 325.50 1.92 0.013 3.13F 0.96 94.3F 28.0

0.56 54.89 4501.07 26.67 0.001 2.91F 0.08 87.8F 2.4

0.64 14.24 2846.62 16.86 0.001 2.90F 0.04 87.7F 1.2

0.70 14.47 1629.74 9.66 0.001 2.86F 0.06 86.3F 1.7

0.82 19.72 2280.95 13.51 0.001 2.85F 0.04 86.0F 1.4

0.89 23.11 856.17 5.07 0.002 2.82F 0.10 85.4F 2.8

0.99 27.03 838.18 4.96 0.004 2.76F 0.12 83.5F 3.7

1.20 23.63 2491.93 14.76 0.006 2.80F 0.06 84.7F 1.6

Fuse 19.52 900.83 5.33 0.007 3.01F 0.08 90.8F 2.4

Total age: 87.9F 2.2

Inverse isochron plateau steps (0.35 0.82)= 86.2F 2.4 Ma; 40Ar/36Ar intercept= 299.0F 4.8; MSWD= 0.96

Step= temperature (jC) or laser output power (in Watt) for material analysed with an induction furnace or a laser probe, respectively.40Aratm is

the atmospheric 40Ar; 40Ar* is the radiogenic argon from natural K-decay; 37ArCa is the Ca-derived argon during irradiation. The volume of39ArK (K-derived argon during irradiation) is based on a mass spectrometer sensitivity of 7 10

10 V cm 3 STP. Uncertainty is quoted at the

2r level; step ages do not include the errors in J and the age of the flux monitor. Decay constant 40Ktot= 5.54310 10 year 1. 40Ar/36Ar

measured: 288F 0.5.

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recrystallisation to strain-free phengite in microfold

hinges is especially prominent in ALM 272 and ALM

273. Continuity between microfold limbs and hinges

is often maintained, and hence, the amount of shape- preferred orientation of white mica parallel to S2cleavage septa is relatively limited. Such a micro-

structure points to the pinning of mica (sub) grain

boundaries and dislocations on graphite particles.

Cleavage microlithons contain relics of S1.

ALM 270 is an albite chlorite mica schist from the

Mesozoic series of the NevadoLubrn unit (La Yedra

Schists and Marbles, de Jong and Bakker, 1991; Fig.

2). The sample is not affected by D3 crenulations, but

chlorite and albite porphyroblasts, which are syn-D3minerals (de Jong, 1991, 1993a,c), overgrew S2.

Phengite grains are well crystallised and strain-free,

and generally lie with their basal cleavage plane in the

differentiated S2 layering.

4. Experimental procedures and mineral

separation

Single phengite grain JK 0 was selected for40Ar/39Ar incremental heating and separated from

the hand specimen after gentle crushing. It was

carefully selected under a binocular zoom microscopeand subsequently ultrasonically cleaned in demineral-

ised water for 5 min. Mineral separates for RbSr and40Ar/39Ar analyses were prepared from the sieve

fractions by means of a Faul table, a laboratory

overflow centrifuge employing heavy liquids and a

Frantz isodynamic magnetic separator.40Ar/39Ar analyses were made at the University of

Nice-Sophia Antipolis (France) following the proce-

dures outlined in detail by de Jong et al. (2001).

Biotite separate ALM 104 was wrapped in high purity

Al foil and incrementally heated to fusion with a high-frequency furnace system, whereas phengite single

grain JK 0 was step heated using an argon ion laser

probe with a continuous beam defocused to at least

twice the grain diameter. Homogeneity of the heating

of the grain was monitored with a coupled video-

microscope system. The laser extraction line consists

of an Innova Coherent 70-4 continuous argon ion

laser in combination with a sensitive gas mass spec-

trometer comprising a 12 cm, 120j M.A.S.S.E.R

tube, a Baur-SignerR ion source and an A.E.M.

1000ETPR electron multiplier. A Pyrex cold finger

at 95 jC and a ZrAl alloy getter operated at 400

jC purified the extracted gas. System blank runs were

carried out at the start of each laser experiment andwere repeated every third run. Background values

were typically 110 11, 5 10 14, 2 10 13 and

110 12 cm3 STP for the 40, 39, 37 and 36 argon

isotopes, respectively, and were subtracted from the

subsequent sample analysis results. Samples ALM

104 and JK 0 were irradiated in the Melusine reactor

(Grenoble, France) for 40.95 h together with flux

monitors biotite standard 4B (KAr age: 17.25 Ma,

Hall et al., 1984 and subsequent analyses in Nice and

Toronto) and MMHb (KAr age: 520.4 Ma, Alexan-

der et al., 1978), respectively, while being rotated

around a vertical axis. The irradiation parameter J was

obtained from the 40Ar*/39ArK ratios measured from

Fig. 4. 40Ar/39Ar induction furnace step heating age spectrum (lower

panel) and 37ArCa/39ArK ratio spectrum (upper panel) of biotite

separate ALM 104 from the Mesozoic series of the NevadoLubrn

unit. The RbSr mineral isochron age obtained by Hebeda et al.

(1980) is indicated by the grey horizontal line.

K. de Jong / Lithos 70 (2003) 91110 99


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three standards in the tube at the same level as the

samples.

Rb Sr dating was carried out at the Vrije Uni-

versiteit, Amsterdam, The Netherlands. Pressed pow-der pellets prepared from splits of whole-rock

powder sample were analysed by X-ray fluorescence

spectrometry for Rb and Sr contents and Rb/Sr ratios

with a Philips PW 1404 automatic spectrometer.

Spiked and unspiked Sr analyses were made on an

automated Finnigan MAT-261 mass spectrometer

with three Faraday cup multicollector system for

Sr. Rb-spiked isotope dilution measurements were

performed using a computer-controlled Teledyne

mass spectrometer with a single Faraday cage col-

lector. For additional analytical details, see footnote

on Table 2.

Mineral ages are calculated using decay constants

given by Steiger and Jager (1977). Plateau, total

fusion and isochron ages include errors in J and the

age of the flux monitor and have errors quoted at the

2r level. Isochron calculations are according to Lud-

wig (2000). Plateau ages were calculated if 60% or

more of the 39Ar was released in three or more

contiguous steps with a probability-of-fit of the

weighted mean of more than 5% (Ludwig, 2000).

All argon isotopic measurements were corrected for

linear extrapolation to gas inlet time, mass discrimi-nation, atmospheric argon contamination and irradia-

tion-induced contaminant Ar-isotopes derived from

Ca and K in the sample; correction factors applied:

(36Ar/37Ar)Ca: 2.79 10 4 (F 3%), (39Ar/37Ar)Ca:

7.06 10 4 (F 4 %) , (40Ar /39Ar )K: 2 5 8 10 4

(F 3%).

5. Results

The 40Ar/39Ar analytical data of samples ALM 104

and JK 0 are listed in Table 1 and portrayed as age

spectra in Figs. 4 and 6, respectively. RbSr isotopic

analyses of mica schist samples ALM 270, 272, 273

and 274 are given in Table 2.

5.1.40

Ar/39

Ar step heating

5.1.1. Biotite separate ALM 104

Induction furnace step heating of biotite separate

ALM 104 yielded an age spectrum with progressively

increasing apparent ages from 147 to 185 Ma, subse-

quent to the first 1% of gas release with irregular

apparent ages (Table 1; Fig. 4, lower panel). The

weighted mean age of the main flat part of the spectrum

(steps 3 12) is 173.2F 6.3 Ma. The 37ArCa/39ArKratio

spectrum is flat, with more Ca-rich compositions

degassing during the first 6.5% and final 2% of gas

release (Fig. 4, upper panel), which probably corre-spond to impurities. The total fusion age of 174.7F 1.6

Ma and 36Ar/40Ar vs. 39Ar/40Ar inverse isochron age of

Table 2

RbSr analytical data of white mica; ALM 270, Mesozoic series, NevadoLubr n unit; ALM 272, 273, 274 Paleozoic rocks, Secano unit

Estimated errors are 0.5% for X-ray fluorescence spectrometric Rb/Sr and isotope dilution measurements of Rb and Sr, 0.01% for 87Sr/86Sr

isotope ratio measurements of whole-rocks and 0.02% for 87Sr/86Sr analysis of minerals. The uncertainty is at the 2r level and based on the

above mentioned estimated analytical errors. Decay constant of 87Rb= 1.4210 11year 1.(1) X-ray fluorescence spectrometric data (whole-rock) and mass-spectrometric isotope dilution (minerals).(2) Directly measured on unspiked sample (whole-rock) and calculated from analysis of spiked sample (minerals).

K. de Jong / Lithos 70 (2003) 91110100


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173.2F 8.7 Ma (Table 1; Fig. 5) are concordant.

However, the large MSWD of 15 renders the meaning

of the isochron age and the 40Ar/36Ar intercept of

281F 82 uncertain.

5.1.2. Phengite single grain JK 0

Laser step heating of a single phengite grain JK 0

yielded a plateau age of 86.9F 0.8 Ma (Fig. 6. lower

panel). The plateau age is concordant to both the

87.9F 2.2 Ma total fusion age and the 86.2F 2.4 Ma36Ar/40Ar vs. 39Ar/40Ar inverse isochron age of the

plateau steps, with an 40Ar/36Ar intercept that is

within error of the atmospheric value (Table 1, Fig.

7). The 37ArCa/39ArK ratio spectrum is flat over the

main part (Fig. 6, upper panel); the high ratio during

the first 3% and the slightly elevated ratio for the final

25% of gas release probably correspond to more Ca-

rich inclusions in the grain.

5.2. RbSr ages

Rb Sr analyses of phengites from mica schists have

yielded a wide spread of ages (Table 2). Despite un-

favourable enrichment factors of radiogenic 87Sr, sam-

ples ALM 272 and 273 of the Secano unit preserve a

pre-Miocene isotope signal, yielding whole-rockmi-

ca ages of 66.1F 3.2 and 40.6F 2.6 Ma. ALM 274(Secano unit) yielded an age of 14.1F 2.2 Ma that is

concordant with the whole-rock phengite albite age

Fig. 7. 36Ar/40Ar vs. 39ArK/40Ar correlation plot for single grain JK

0. The open ellipses of steps 8 11 are excluded from the

calculation.

Fig. 6. 40Ar/39 Ar laser step heating age spectrum (lower panel) and37ArCa/

39ArK ratio spectrum (upper panel) of single phengite grain

JK 0 from the Mesozoic series of the NevadoLubrn unit.

Fig. 5. 36Ar/40Ar vs. 39ArK/40Ar correlation plot for biotite separate

ALM 104. Points 1350 and fuse are excluded from the calculation.

K. de Jong / Lithos 70 (2003) 91110 101


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of 17.2F 1.9 Ma of ALM 270 (NevadoLubrn unit,

Fig. 8).

6. Interpretation

The results obtained on samples with the same

tectonic foliation show a wide range of40Ar/39Ar andRbSr ages, emphasising the fact that they cannot be

interpreted in terms of a simple cooling history during

exhumation.

6.1. Excess and inherited40

Ar

The 173F 6 Ma 40Ar/39Ar weighted mean age and

virtually all apparent ages of biotite ALM 104 are much

older than the 146F 3 Ma Rb Sr mineral isochron age

of the host gabbro (Fig. 4), which indicates that40ArXS

has been incorporated into the mineral.The concordant 86.9F 0.8 Ma plateau age and the

86.2F 2.4 Ma isochron age of single phengite grain

JK 0 are much older than the 15.0F 0.6 Ma SHRIMP

U Pb zircon age obtained by Lo pez Sanchez-Viz-

cano et al. (2001) in Cerro del Almirez (Fig. 1). The

discrepancy between these two estimates for the

timing of high-pressure metamorphism in the Mulha-

cen Complex cannot be accounted for by a polycyclic

Alpine orogeny (e.g. Puga et al., 2002), as the upper

and lower parts of the NevadoLubrn unit did not

experience a different tectono-metamorphic evolution,

as would be expected following a re-subduction of the

lower part, as proposed by the latter authors. Accord-

ingly, the best interpretation is that the ca. 87 Ma ageof the phengite is due to the incorporation of 40Ar into

its lattice, which may have been inherited from the

magmatic precursor of the amphibolite that hosted the

white mica and which contained 40ArXS.

The 37ArCa/39ArK ratio and atmospheric contami-

nation of phengite grain JK 0 are fairly constant and

not elevated during the main argon release (Fig. 6,

upper panel; Table 1). It is, therefore, unlikely that40ArXS incorporation was the result of late-stage

illitisation related to fluid ingress via late cracks,

described by de Jong et al. (2001) for phengites in

gneisses, which have an atmospheric contamination

that is well above 30% and 37ArCa/39ArK ratios that

tend to be much higher than those observed for JK 0.

The absence of a dense network of cracks in the

amphibolite emphasizes this.

The fact that the single mica grain yielded a plateau

age that is enhanced by inherited 40Ar implies that Ar

was not released by volume diffusion during in vacuo

step heating. It has been argued that chemical and

structural changes, such as dehydroxylisation of white

mica during step heating, permit the simultaneous

release of 39ArK, 40Ar* and 40ArXS from the coresand rims of crystals, leading to homogenisation of40Ar reservoirs and age gradients (Inger et al., 1996;

Sletten and Onstott, 1998; de Jong et al., 2001).

Trioctahedral micas behave in a similar way (Harrison

et al., 1985; Phillips and Onstott, 1988; Lo and

Onstott, 1989), however, probably due to sample

inhomogeneity, we did not obtain an age plateau for

biotite ALM 104.

In light of the above discussion, the meaning of the

plateau ages of 48.4F 2.2 and 24.6F 3.6 Ma of

barroisitic amphibole and magnesiohornblende, re-spectively, obtained by Monie et al. (1991) in the

Sierra de Baza (Fig. 1), which both yielded irregular40Ar/39Ar age spectra, cannot be taken at face value.

The barroisitic amphibole grew in an undeformed

metadolerite that contains magmatic plagioclase and

clinopyroxene, making inherited 40Ar likely. The

well-expressed saddle-shaped age spectrum of the

magnesiohornblende clearly points to 40ArXS uptake,

and this sample only yielded a plateau age due to the

very large errors on the individual steps.

Fig. 8. RbSr albite phengite whole-rock isochron for ALM 270(Mesozoic series, NevadoLubrn unit). The errors are at the 2r

level.

K. de Jong / Lithos 70 (2003) 91110102


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6.2. Trapped argon component

Although 40ArXS often results in40Ar/36Ar inter-

cepts in isotope correlation diagrams greater than295.5 (Heizler and Harrison, 1988), examples of

phengite (Inger et al., 1996; Sherlock and Arnaud,

1999; de Jong et al., 2001) and biotite (Foland, 1983;

Ruffet et al., 1995) show that samples with 40ArXSplateau ages can give 40Ar/36Ar intercepts close to the

atmospheric value. The initial 40Ar/36Ar ratio not

necessarily reflects the argon composition immediate-

ly prior to crystallisation, but might equally well

indicate the argon com position added to minerals

during later processes (Roddick, 1978; de Jong et

al., 2001). This may also be the case for ALM 104, as

this sample has an elevated 36ArAIR contamination

corresponding to high 37ArCa/39ArK ratios during the

first 6.5% of 39Ar release. This is most likely related

to impurities of a Ca-rich phase that degasses at low

temperature, like carbonates (500700 jC: Spray and

Roddick, 1981) and/or chlorite (first degassing peak


14/20

agreement with the evolution of the 87Sr/86Sr ratio, the

observed zonation of the metamorphic minerals

implies a state of disequilibrium and a deficit of

cations. Such features suggest that fluid mobilitywas limited during eclogitisation, which consequently

conserved the 40ArXS levels in rocks.

The ca. 87 Ma 40Ar/39Ar age of phengite JK 0 is

most likely the consequence of the incorporation of40Ar into the mineral and restricted fluid mobility may

have been instrumental in this process. The amphib-

olite from which single phengite grain JK 0 was

extracted occurs in the same tectono-stratigraphic

level as the 146-Ma-old gabbro ALM 104, which

was plagued by 40ArXS incorporation. Most eclogites

and gabbros in this level are pervasively amphiboli-

tised, pointing to the infiltration of water. Thorough

amphibolitisation of eclogites resulted in changes in

main and trace element chemistry (Morten et al.,

1987). The preferred orientation of mica and blue-

green hornblende in amphibolite JK 0 formed during

thorough recrystallisation that accompanied exhuma-

tion of high-pressure metamorphic rocks during D2.

Yet, the presence of glaucophane relics in the cores of

some hornblendes in this sample implies that disequi-

librium conditions existed during the breakdown of

the blue amphibole during this event. Although the

hydration of eclogites, leading to their amphibolitisa-tion, resulted in a significant reduction of 40ArXS in

the whole-rock, it was not completely removed, as

shown by the data of Hebeda et al. (1980). This

observation implies that during D2 recrystallisation,

a semi-closed system persisted, in which the argon

activity remained elevated, at least locally. Amphib-

olitisation of gabbros and eclogites under such con-

ditions has led to the local redistribution and

incorporation of 40Ar in newly formed metamorphic

minerals, such as phengite.

6.4. Inherited isotopic components

White micas from the basal series of the Secano

unit yielded RbSr ages that range from 66.1F 3.2 to

14.1F 2.2 Ma (Table 2) and 40Ar/39Ar total gas ages

of 25.9F 0.1 and 19.1F 0.1 Ma (see Section 3),

which imply the progressive resetting of an older

isotopic system. White mica from graphite-rich sam-

ples ALM 272 and 273 has disturbed 40Ar/39Ar age

spectra with apparent ages that are virtually all older

than the 17.3F 0.2 Ma plateau age of ALM 270 of the

NevadoLubrn unit (Fig. 9). Disturbed age spectra,

whether dome-shaped or composed of progressively

rising apparent ages, have been interpreted by degass-ing of mixed micas, one containing an inherited Ar

component due to partial resetting during superim-

posed tectono-metamorphic recrystallisation and a

second that was newly formed during this event, both

of which do not release Ar over the same temperature

interval (Wijbrans and McDougall, 1986; Hammer-

schidt and Frank, 1991; de Jong et al., 1992; West and

Lux, 1993). Consequently, the age spectra of ALM

272 and 273 imply that a relict inherited Ar component

exists in both samples. Their microstructure reveals

incomplete recrystallisation of white mica and the

pinning of its grain boundaries on graphite particles.

Fig. 9. 40Ar/39Ar induction furnace step heating age spectra (lower

panel) and 37ArCa/39ArK ratio spectra (upper panel) of phengite

separates from the eastern Sierra de los Filabres. Data from de Jong

et al. (1992). ALM 270 (Mesozoic series, NevadoLubrn unit)

shows a well-developed age plateau, whereas ALM 272 and 273

(Paleozoic series, Secano unit) have disturbed spectra.

K. de Jong / Lithos 70 (2003) 91110104


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This inclusion-inhibited growth mechanism may sim-

ilarly explain why phengites ALM 272 and 273 have

partially retained an older RbSr isotope signal, as

indicated by their relatively old RbSr ages of 66.1and 40.6 Ma, respectively. In contrast, the 14.1F 2.2

Ma Rb Sr age of the most quartz-rich and least

graphite-rich sample ALM 274, which is not affected

by inclusion-inhibited growth of white mica, implies a

complete resetting of its RbSr system. The RbSr

age of this youngest sample of the Secano unit over-

laps with the 17.2F 1.9 Ma RbSr age of ALM 270 of

the Nevado Lubrn unit. Widespread retrograde

growth of albite and chlorite at the cost of phengite

in ALM 270 implies complete tectono-metamorphic

recrystallisation following D2.

The progressively reset isotopic system in the

Secano unit may be derived from a pre-Miocene

early Alpine signal, or alternatively, its occurrence in

probably Paleozoic rocks implies that it may partially

retain a pre-Alpine history. The ca. 15 Ma U Pb

SHRIMP zircon age for the high-pressure metamor-

phism (Lopez Sanchez-Vizcano et al., 2001) renders

the first option unlikely. In contrast, during the pre-

Alpine evolution of the Mulhacen Complex, the

basal series were metamorphosed up to about 500

jC (Section 2; Fig. 10). Their crystalline nature and

the inhibited recrystallisation of white mica in graph-ite-rich samples during the Alpine orogeny may lie

behind the inherited pre-Alpine Rb Sr and K Ar

systems in these rocks. Their partial survival in white

mica during the Alpine orogeny, when temperatures

of about 500 jC were reached, once again under-

scores that temperature alone is ineffective for iso-

tope resetting, but that fast recrystallisation processes

that affect the ionic bonds in minerals, like tectono-

metamorphic recrystallisation and fluid ingress, are

(Chopin and Maluski, 1980; Verschure et al., 1980;

Wijbrans and McDougall, 1986; Hames and Cheney,1997; Villa, 1998; Itaya and Fujino, 1999; Kuhn et

al., 2000; de Jong et al., 2001; Dunlap and Kronen-

berg, 2001; Reddy et al., 2001). In contrast, the

occurrence of ALM 270 in the Mesozoic series of

the Nevado Lubrn u ni t an d i ts c a. 17.2 Ma40Ar/39Ar plateau and RbSr ages precludes the

presence of any inherited pre-Alpine isotopic com-

ponent. Yet, a small 40ArXS component seems likely,

taking the ca. 15 Ma UPb SHRIMP zircon age at

face value.

6.5. Age and rates of exhumation and cooling

Despite the occurrence of inherited isotope systems

and40

ArXS incorporation in white mica and biotite,our dating sheds light on the timing of exhumation of

the Mulhacen Complex and its rates. Albite in ALM

270 was probably formed from the paragonite com-

ponent in white mica during its recrystallisation at low

pressure during D3. The87Sr/86Sr ratio of this sample

shows that the age information obtained essentially

pertains to albite, as the 87Sr/86Sr ratios of phengite

and the whole-rock are virtually identical (Table 2).

The 17.2F 1.9 Ma whole-rock mica albite age con-

sequently has bearing on the decompression to about

0.4 0.5 GPa for D3 (see Section 2.2, Fig. 10).

However, due to the large uncertainty, the age is

within error of the 15.0F 0.6 Ma age estimate of

the high-pressure metamorphism based on the zircon

SHRIMP data of Lo pez Sanchez-Vizcano et al.

(2001). Subsequent to the D3 cooling phase temper-

atures increased to about 500 jC, whereas the pres-

sure probably did not significantly decrease (Fig. 10).

de Jong et al. (2001) argued that, in conjunction with

this D4 reheating, white mica in the gneisses of the

MacaelChive unit acquired 40ArXS during submicro-

scopic illitisation, a fluid-assisted recrystallisation

process that probably also affected the RbSr system.RbSr white mica ages reported by Andriessen et al.

(1991) from the gneisses of the MacaelChive unit in

the eastern Sierra de los Filabres span the 12.515.6

Ma range, with errors of about 22.5%. It might be

argued that the youngest RbSr white mica age of

12.5F 0.2 Ma is the result of a thorough recrystalli-

sation during D4, which might imply a decompression

of about 55 km in roughly 2.5 Ma (Fig. 10). The

exhumation rate may consequently be as high as about

22.5 mm/year, about twice the estimate of Lopez

Sanchez-Vizcano et al. (2001), who based their valueon an assumed geothermal gradient and not on avail-

able PTestimates for the late stage evolution.

Fission-track data ofJohnson et al. (1997) point to

an accelerated cooling following the first phase of fast

exhumation and cooling. These authors inferred from

an 11 Ma apatite fission-track model age that the

cooling of the uppermost Mulhacen Complex in the

eastern Sierra de los Filabres was essentially complet-

ed by that time. This is consistent with the first

appearance of detritus derived from this part of the

K. de Jong / Lithos 70 (2003) 91110 105


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complex (in part as boulders of the marbles and

gneisses of the MacaelChive unit) in latest Serraval-

lian to Early Tortonian deposits around the eastern

Sierra de los Filabres (de Jong et al., 2001, andreferences in therein). Under the assumption of a

12.5 Ma age for D4 during which temperatures were

in the order of about 500 jC, the final cooling has

taken about 1.5 Ma with a rate of about 330 jC/Ma

and much less fast exhumation rate of 912 mm/year

compared to the early exhumation phase.

The Late Miocene cooling has been accounted for

by extension (Johnson et al., 1997). Since the work

of Platt and Vissers (1989), the contact between the

Mulhacen and Alpujarride complexes has been inter-

preted as a major low-angle extensional fault. D5mylonites and D6 brittleductile structures are most

penetratively developed in the uppermost Mulhacen

Complex along the contact with the overlying Alpu-

jarride Complex and show the decreasing tempera-

ture (de Jong, 1991, 1993a) during exhumation. But

also important low-angle brittleductile detachments

were formed within the Mulhacen Complex during

this event, like e.g. at the base of the Secano unit

and at the base of the greenstones that contain the

eclogites.

7. Conclusions

Radiometric dating of phengite from rocks with a

tectonic fabric related to the exhumation of high- pressure metamorphic rocks implies that40ArXS in-

corporation and isotopic inheritance have occurred

under conditions of restricted fluid mobility and

tectono-metamorphic recrystallisation.

A well-crystallised single phengite grain from an

amphibolite (Nevado Lubrn unit) has yielded a40Ar/39Ar laser step heating plateau age of 86.9F

0.8 (2r; 70% 39Ar released), which is concordant to

its inverse isochron age of 86.2F 2.4 Ma (40Ar/36Ar:

299.0F 4.8). A biotite separate from a gabbro relic in

an eclogite yielded an induction furnace step heatingage spectrum with progressively increasing apparent

ages and a weighted mean age of 173.2F 6.3 Ma (2r;

95% 39Ar released). These ages are older than the

eclogite-facies metamorphism (15 Ma) and intrusion

of the gabbros (146 Ma) and, hence, are the result of40ArXS incorporation.

40ArXS uptake by the gabbro

was probably caused by infiltration of fluids derived

from the country rocks during their incipient meta-

morphism at the onset of subduction. 40ArXS incor-

poration in the phengite in the amphibolite was related

Fig. 10. PressureTemperaturetimedeformation path and exhumation history of the Mulhacen Complex. PT determinations by Bakker et

al. (1989), de Jong (1991, 1993a) and Puga et al. (2002). (1) Lower P stability limit of glaucophane after Maruyama et al. (1986); (2)

FeChl+Ms=FeCld+Ann; (3) Cld+AS= St+Chl; (4) FeCld+Ann=Alm+Ms; (5) Cld = Grt + Chl + St; according to Spear and Cheney

(1989); Stability Al-silicate fields after Holdaway and Mukhopadhyay (1993). Mineral abbreviations according to Kretz (1983). Age

constraints: (A) SHRIMP UPb mean age of nine zircon grains (Lopez Sanchez-Vizcano et al., 2001); (B) youngest RbSr white mica age of

Andriessen et al. (1991); (C) apatite fission-track model age (Johnson et al., 1997).

K. de Jong / Lithos 70 (2003) 91110106


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to metamorphic recrystallisation of the magmatic

rocks in an environment with a restricted fluid mo-

bility inherited from the magmatic stage.

Rb Sr whole-rock phengite ages of graphite- bearing mica schists from Paleozoic rocks (Secano

unit) show a dramatic age variation (66.1F 3.2,

40.6F 2.6 and 14.1F 2.2 Ma) that has arisen from

the progressive resetting of an older isotopic system.

This system was probably a remnant of the Variscan

low-grade metamorphism of the basal series of the

Mulhacen Complex. The microstructure of the sam-

ples with pre-Miocene RbSr ages implies that phen-

gite has only partially recrystallised as grain growth

was inhibited by the presence of graphite particles.

This interpretation corroborates previously obtained

disturbed and slightly dome-shaped 40Ar/39Ar age

spectra that reveal the presence of an older isotopic

component. In contrast, the most quartz-rich and least

graphite-rich sample is not affected by inclusion-

inhibited growth of white mica, and has a completely

reset RbSr system, as implied by its 14.1F 2.2 Ma

Rb Sr age. The latter date overlaps with the

17.2F 1.9 Ma Rb Sr whole-rock phengite albite

age obtained from a schist from the Mesozoic series

of the NevadoLubrn unit.

Comparison of our data and literature data reveals

that exhumation of the eclogite-facies Mulhacen Com- plex occurred at rates in the order of 22.5 mm/year

during the early phase and of 9 12 mm/year during the

late phase. During the latter event, the cooling rate was

of about 330 jC/Ma.

Acknowledgements

I would like to dedicate this article to Prof. W.P. de

Roever, who passed away on 24 September 2000, and

was one of the pioneers in high-pressure petrology justafter World War II. He worked as an undergraduate

student in the area around Lubrn and interpreted the

occurrence of zoned metamorphic minerals by dis-

equilibrium during a succession of different metamor-

phic facies in time (plurifacial metamorphism). Only

much later would such a notion become general with

the reconstruction ofPTtpaths.

I would like to thank Drs. Gilbert Feraud (Geo-

science Azur, Universite de Nice-Sophia Antipolis,

France) and Jan Wijbrans (Department of Isotope

Geochemistry, Faculty of Earth and Life Sciences,

Vrije Universiteit, Amsterdam, The Netherlands) for

the use of analytical facilities and the use of sample

ALM 104 from the mineral separate collection of thedepartment. Part of the work was carried out while

holding a NATO post-doctoral research fellowship

and The Netherlands Organisation for Scientific

Research (NWO) and the Vakgroepfonds Strukturele

Geologie of the University of Amsterdam met travel

costs incurred during the project. Some of the points

addressed in this study came up during a discussion

with Igor Villa. Constructive reviews by Sarah

Sherlock and Richard Spikings contributed to the

clarity of the presentation and the styling of the text.

Daniella Rubatto and Encarnacion Puga are thanked

for providing pre-prints.

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