A precise late Permian 40Ar/ 39Ar age for Central Iberian ... · lamprophyre dykes, forming broadly north-south trending swarms in eastern Guadarrama (Ubanell et al., 1984; Vil-laseca

A precise late Permian 40Ar/ 39Ar age for Central Iberian camptonitic lamprophyres

The Avila batholith of central Spain is composed, predominantly, of crustal-melt peraluminous granites cutby small-scale mafic alkaline bodies. Dating of the Gredos sector mafic camptonitic lamprophyre dykes wasundertaken to constrain the Late Variscan tectonomagmatic evolution of the region. A well constrained late Per-mian, Capitanian, age of 264.5 ± 0.9 Ma was obtained by 40Ar/39Ar geochronology using amphibole separates.This new age clearly distinguishes the dykes from other episodes of alkaline mafic magmatism in the region.We suggest that the lamprophyre dykes were emplaced into already solidified granitoids after the tectonic con-trol on magma generation changed from purely extensional to transtensional.

40Ar/39Ar dating. Lamprophyres. Variscan. Central Iberia. Transtension.

Geologica Acta, Vol .4 , Nº 4 , 2006, 451-459

Avai lable onl ine at www.geologica-acta.com

© UB-ICTJA 451

KEYWORDS

A B S T R A C T

J.H. SCARROW F. BEA P. MONTERO J.F. MOLINA and A.P.M. VAUGHAN

Dept. Mineralogy and PetrologyUniversity of Granada, Campus Fuentenueva, 18002 Granada, Spain. Scarrow E-mail: [email protected]

British Antarctic SurveyHigh Cross, Madingley Road, Cambridge, CB3 0ET, U.K.

INTRODUCTION

Interpretation of the Late Variscan geodynamic evolu-tion of the Sierra de Gredos sector of the Avila batholithhas been hampered by uncertainty regarding the age ofcamptonitic lamprophyre dykes cutting widespreadcrustal-melt granitoids of the region.

The dykes were originally suggested to be early Permian,Cisuralian, 283 ± 30 Ma by Rb/Sr (WR) (Bea et al., 1999;geologic time scale of Gradstein et al., 2004). Studies ofcomparable dykes suites cutting the more easterly Sierra deGuadarrama sector of the Avila batholith dated lamprophyresdykes at 277 ± 5 Ma by K/Ar (phlogopite) (Villaseca et al.,

2004). Furthermore, notably in agreement with the currentwork, Perini et al. (2004) mentioned an unpublished date of264 ± 1.3 Ma by 40Ar/39Ar (amphibole) for the Guadarramasector lamprophyres. More recently, however, Orejana et al.(2005), whilst remarking on the poorly constrained nature ofthe geochronological data in the complex Guadarrama dykeswarm, approximately averaged the aforementioned datesand settled on an age of 270 Ma.

In this study we present a well-constrained late Permi-an, Capitanian, age of 264.5 ± 0.9 Ma determined by40Ar/39Ar geochronology performed on amphibole mine-ral separates from Gredos sector mafic camptonitic lam-prophyre dykes. This new date provides a fixed point in

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40Ar/39Ar dating of Central Iberian lamprophyresJ.H. SCARROW et al.

452Geolog ica Acta , Vo l .4 , Nº4, 2006, 451-459

A) Scheme of the Iberian Massif. Grey areas represent plutonic, mostly granite-granodiorite, rocks. The main palaeogeographic zones(after Farias et al., 1987): CZ: Cantabrian Zone; WALZ: Western-Astur Leonian Zone; GTOMZ: Galicia Tras-os-Montes Zone; CIZ: Central Iberian Zone;OMZ: Ossa Morena Zone; SPZ: South Portuguese Zone. B) Geological map of the Avila batholith, the rectangle indicates the detail shown in C. C) Geo-logical map of the Gredos Sector. Geochronological sampling localities are indicated by a grey star.

FIGURE 1

Peña Negra granodiorites

Monbeltrán

S.Esteban

Pedro Bernardo

S. Martín

Piedrahita

Hoyos granodiorite

Alberche granodiorite Almohalla orthogneiss

Menga granodiorite

Two-mica granite

Moscovite granite

Aplites

Basic rocks

Felsic dykes

Quartz dykes

Lamprophyres

Peña Negra MigmatitesLegend:

Schists and paragneisses

0 4 8 km

Geological Map of Central Gredos

Navarredonda

Hoyos

After Bea et al., (2002)

0330

N

4460

4470

4480

C. The Gredos Sector

Arenas deSan Pedro

0300 0310 0320 0340

N

0 20km

Lisbon

0 200 km

SPZ OMZ

CIZ

GTOMZ

WALZ

CZ

Gredos Guadarrama

Legend

Appinites and vaugnerites

Granodiorites and Adamellites

Two-mica Granites and Leucogranites

Migmatites and Anatexites

Middle- and Low-grade Metamorphic Rocks

Alentejo-Plasencia Dolerite

Domain of the Lamprophyre dyke swarm

Recent sediments

B. The Avila Batholith

Avila

After Bea et al. (1999)

Madrid

A.

the generation and, thus, tectonomagmatic evolution ofthe Iberian K-rich mafic rocks during the late Variscan.

GEOLOGICAL SETTING AND FIELD RELATIONS

The Central Iberian Zone, the central part of theVariscan chain in the Iberian Peninsula, comprises Pro-terozoic and early Palaeozoic metasediments andorthogneisses intruded by late Palaeozoic, ~330 to 300Ma, collision-related granitoids (Martínez Catalán et al.,2004 and references mentioned therein) (Fig. 1). EarlyVariscan deformation in the region, ~360 to 350 Ma,almost doubled the thickness of the crust principally byunderthrusting of the Ossa Morena Zone lower crustbeneath Central Iberia (Azor et al., 1994). This maincompressional stage was followed by an extensionalphase and development of sub-horizontal shear zonesassociated with which was the main episode of graniteproduction (Bea et al., 2003 and references mentionedtherein).

The axis of the Central Iberian Zone in central Spainis dominated by the Avila batholith which crops out over~13,000 km2 (Bea et al., 2004; Fig. 1). The batholith isdivided into two sectors: Gredos to the west and Guadar-rama to the east. The Gredos sector, the focus of the cur-rent study, comprises numerous juxtaposed plutons ofperaluminous granitoids which were divided into faciesby Bea et al. (1999). In the core of the batholith, largelow-pressure anatectic complexes crop out (Fig. 1). Thesegranites and migmatites are cut by camptonitic alkalinelamprophyre dykes, forming broadly north-south trendingswarms in eastern Guadarrama (Ubanell et al., 1984; Vil-laseca et al., 2002) and central Gredos (Bea and Corretgé,1986; Bea et al., 1999) (Fig. 1).

The dykes of the Gredos sector, although typically lat-erally discontinuous and only 5 cm to 2 m thick, form abroad ~10 km wide, N-S to NNE-SSW trending swarm ofsome 100 steeply-dipping bodies (Bea et al., 1999; Fig. 1).To the north, in the Menga and Alberche granitoid faciesthe dykes form a broad swathe whereas the southerlyextension of the swarm, in the Hoyos granitoid facies, ismuch more laterally restricted, ~3 km wide (Fig. 1).

Detailed mapping of the dykes reveals sharp contactsbetween the camptonites and host granitoids (Fig. 2A),the latter often having a diffuse band of episienite near thecontact. However, despite the presence of such clearmarker horizons, discontinuous and poor exposure ofdyke contacts with the country rock make it difficult todetermine the nature of the stress field during individualdyke emplacement. Nevertheless, on a regional scale, thedifference between a N-S to NNW-SSE regional swarm



0.5 mm

0.5 m

30 mm

B

C

A

A) Outcrop photograph of a lamprophyre dyke cutting theHoyos granodiorite. B) Hand specimen photograph of a typical camp-tonitic lamprophyre. Note the large, abundant, kaersutite amphibolephenocrysts. C) Thin section photomicrograph of dated lamprophyre,GREB-750. Note the lack of any indication of resorption, or reactionwith the groundmass, of the kaersutite amphibole phenocryst. Seetext for details.

FIGURE 2

trend and NNW-SSE to NW-SE local dyke trends indi-cates that the area was under bulk sinistral transtensionduring dyke emplacement. Rather than simple shear, thezone was also affected by a component of extension at ahigh angle to the strike-slip deformation (c.f., Dewey etal., 1998).

PETROGRAPHY AND MINERAL CHEMISTRY

Detailed descriptions of the petrography of the Gredosdykes have been published by Bea and coworkers (Beaand Corretgé, 1986; Bea et al., 1999). These authors not-ed the distinctive highly porphyritic nature, ~30 modal %,of the camptonites. The abundant phenocrysts include,large, up to a few cm length, kaersutite amphibole (Figs.2B and 2C), zoned clinopyroxene with cores of Ti-Al-poor augite and Ti-Al-rich augite overgrowths, carbonateand Ti-magnetite and ulvöspinel. More evolved dykesmay also contain plagioclase phenocrysts. Less commonis Ti-phlogopite, evidence of textural disequilibrium withthe mesostasis, including anhedreal embayed outlines andrims of coronitic kaersutite, led Bea and Corretgé (1986)to interpret the phlogopite as xenocrystic. The mesostasisis fine-grained panidiomorphic and composed of kaersu-tite, Ti-augite, plagioclase (An50-10), alkali feldspar, Ti-magnetite, ilmenite, and pyrite, with sparse chalcopyriteand sphalerite, epidote, titanite and calcite. Such anassemblage is indicative of crystallization from an alka-line, Ti-rich, magma.

SAMPLES AND METHODS

Two representative camptonite dykes were selectedfor dating: one from the north of the swarm where it cutsthe Menga granodiorite, the other from the south of theswarm where it cuts the Hoyos granodiorite (Fig. 1).

Amphibole phenocrysts could be obtained with rela-tive ease from the porphyritic camptonitic lamprophyres(Fig. 2B). First, samples were cut into 1 cm thick slicesand amphibole-rich zones snapped out with pliers. Afterrinsing in alcohol, ~70-80 mg of 0.25-0.5 mm fractiongrains were hand-picked under a binocular microscope.These grains were then washed for 15 min in distilledwater in an ultrasonic bath. One amphibole concentrate,Amp-1, was prepared from the northerly dyke (GRECH-2), and two amphibole concentrates, Amp-2 and Amp-3,were prepared from the southerly dyke (GREB-750).

Samples and monitor standard DRA1 (25.26 ± 0.07Ma sanidine, Wijbrans et al. 1995) were irradiated for 55hrs at the Beijing 49-2 reactor. To obtain the best controlon irradiation of our samples, that is the best J-values for

our 40Ar/39Ar geochronological studies, monitor DRA1was packed in quartz tubes between every five unknownsamples. The J-value uncertainty of 0.15% was propaga-ted into the age calculations for every sample (Wijbrans etal., 1995; Qiu and Wijbrans, 2006). Prior to analysis, thesystem blank was reduced by heating the whole apparatusto 150ºC, for ~24 hours, using an infrared lamp and heat-ing tape. The samples were step heated with a CO2 laser(MIR10-50W) operated by New Wave Research© LaserAblation System version 1.8.13.0 software. Correctionfactors for interfering argon isotopes derived from Ca andK are: (39Ar/37Ar)Ca = 8.984 x 10-4, (36Ar/37Ar)Ca = 2.673 x10-4 and (40Ar/39Ar )K = 5.97 x 10-3. Two SorbAC NP10pumps purified the released gases. One operated at~400ºC and the other at room temperature. The purifica-tion time was 5 min for amphibole. The purified gas wasanalyzed for argon isotopes in a VG5400-Ar mass spec-trometer operated using Noble Gas Software (version2.93). The 40Ar/39Ar data were reduced, and graphs wereproduced, using the ArArCALC software package(http://earthref.org/tools/ararcalc/index.html) which consistsof Visual Basic® routines that operate as add-ins inMicrosoft Excel®.

RESULTS

Fresh, inclusion-free, kaersutite phenocrysts are euhe-dral and apparently in textural and compositional equilib-rium with their host groundmass in which, notably, theamphibole is also kaersutitic (Fig. 2C). In the absence ofpetrographic or geochemical evidence for secondary post-solidification processes, that could potentially causegeochronological resetting, dates obtained from the kaer-sutite phenocrysts should constrain the age of crystalliza-tion of the K-rich mafic camptonite dykes. This supposi-tion was investigated using 40Ar/39Ar dating.

Three kaersutite phenocryst concentrates from twocamptonite dykes yielded well-defined plateau ageswhich overlap within error, ranging from 264.3 ± 0.9 Mato 264.6 ± 1.6 (Fig. 3, Tables 1 and 2). Although there isno uniform convention on the definition of a plateau in40Ar/39Ar dating (McDougall and Harrison, 1999) wetook the definition of Dallmeyer and Lecorche (1990)who considered that a plateau may be established when“ages defined by two or more contiguous gas fractions(with similar K/Ca ratios) each representing >4% of thetotal 39Ar evolved (and together consisting of >50% of thetotal quantity of 39Ar evolved) are mutually similar withina ± 1% intralaboratory uncertainty”. Our plateaux meetthese criteria, the 40Ar/39Ar age spectra produced bystepped heating with a CO2 laser are flat, concordant, uni-form release patterns with 85-100% of gas yielding theplateaux (Fig. 3A, Tables 1 and 2). Such flat age spectra



indicate uniform 40Ar distribution inside the crystallattice of the amphiboles implying that the sampleswere closed systems since their crystallization, thatis undisturbed by physical or chemical processessince cooling through their Ar closure temperature, ~500ºC for calcic amphibole (MacDougall and Harri-son, 1999). Accordingly, the amphiboles had rela-tively constant K/Ca over 98-100% of 39ArK releasedfor the three samples (Fig. 3B). In addition, theabsence of “saddle-shaped” spectra rules out a sig-

nificant component of excess 40Ar, i.e., not formedby in-situ decay of 40K. Furthermore, the total fusionand isochron ages for the three mineral separates arein excellent agreement, overlapping within error,with the plateau ages (Fig. 3, Table 2). Therefore,we conclude that the well-constrained late Permian,Capitanian, age of 264.5 ± 0.9 Ma reflects the timeof cooling and, assuming that this was rapid, crystal-lization of the Gredos sector mafic camptonitic lam-prophyre dykes.



264.6 ± 1.6 Ma

0 10 20 30 40 50 60 70 80 90 100

Cumulative Ar released (%)39

200

225

250

275

300

325

350

Appare

nt A

ge (

Ma)

0 10 20 30 40 50 60 70 80 90 100


0 10 20 30 40 50 60 70 80 90 100


200

225

250

275

300

325

350

Appare

nt A

ge (

Ma)

200

225

250

275

300

325

350

Appare

nt A

ge (

Ma)

264.3 ± 0.9 Ma

264.4 ± 0.9 Ma

0 10 20 30 40 50 60 70 80 90 100

0 10 20 30 40 50 60 70 80 90 100


0 10 20 30 40 50 60 70 80 90 100


K/C

a

300,000

250,000


Ar

36

Ar

/4

0Ar36Ar /39

150,000

50,000

0

200,000

100,000

Ar

36

Ar

/4

0

240,000

200,000

160,000

120,000

80,000

40,000

0

Ar

36

Ar

/4

0

240,000

200,000

160,000

120,000

80,000

40,000

0

Ar36Ar /39

Ar36Ar /39

0 4000 8000 12000 16000

0 4000 8000 12000

0 4000 8000 12000

0.6

0.4

0.2

0

K/C

a

0.4

0.3

0.1

0

0.2

K/C

a

0.4

0.3

0.1

0

0.2

0.171 ± 0.002

0.171 ± 0.002

0.162 ± 0.002

261.8 ± 3.2 Ma

264.9 ± 1.2 Ma

263.6 ± 1.0 Ma

MSWD 15.77

MSWD 2.14

MSWD 2.58

Amp-1 Amp-1 Amp-1

Amp-2 Amp-2 Amp-2

Amp-3 Amp-3 Amp-3

A B C

Age spectra for kaersutite phenocrysts from two representative Central Iberian camptonite dykes, one from the north of the regional swarm(GRECH-2: Amp-1) the other from the south (GREB-750: Amp-2 and Amp-3). B) Calculated K/Ca for Amp-1, Amp-2 and Amp-3. The plateaux are definedby over 98-100% of 39ArK released for the three samples. The coefficient that transfers the ratio of production rate of 39ArK/37Ar into K/Ca ratio is 0.43.C) Isochron plots for Amp-1, Amp-2 and Amp-3. Note the agreement, within error, with plateau ages. Open symbols, defining the line, were used tocalculate the isochron, solid symbols were not used in the calculation. See text for details.

FIGURE 3



40Ar/39Ar dating results.TABLE 1



Continued.TABLE 1

Kaersutite 40Ar/39Ar ages for Central Iberian camptonitic lamprophyres.TABLE 2

DISCUSSION

To consider the implications of the different expres-sions of Variscan magmatism and, in particular, the tim-ing of the camptonite dyke emplacement, it is relevant tosummarize the relative ages of the main magmatic

episodes in the Gredos sector of the Avila batholith.Granitic melt production in the region took place duringthe Early Carboniferous to Early Permian, over 55 millionyears from 352 to 297 Ma (Montero et al., 2004a). A limi-ted melting event occurred coincident with the mainVariscan collision at ~350 Ma. Later extensive anatexis,



at ~330 Ma, produced melts which pooled in migmatitecomplexes and, at ~320 Ma, segregated into small scaleleucogranite bodies. Subsequently, from 325 to 305 Ma,extensional collapse-related sub-horizontal shear zonesprovoked extensive in-situ lower- and, in particular, mid-crustal melting throughout the region. After peaking at309 Ma melt production decreased significantly and hadstopped completely by 297 Ma (Montero et al., 2004a).Superimposed upon this widespread granitic magmatismwas a punctual episode of originally alkaline mafic mag-matism, ~312 Ma, currently preserved in heavily conta-minated localized stocks and enclaves of appinitic com-position (Bea et al., 2004; Montero et al., 2004b). So, thenew, late Permian, 40Ar/39Ar age of 264.5 ± 0.9 Ma, forthe camptonite dykes clearly fixes their emplacement asyounger than the main Variscan magmatism in theregion.

The marked change in emplacement style of alkalinemafic melts from “mixed” stocks at 312 Ma to “pure”dykes at 264 Ma, implies a change in regional tectonicregime. During extensional collapse sub-horizontal shearzones facilitated crustal melting (Montero et al., 2004b)so small-scale melts of the metasomatized mantle lithos-phere could only proceed to the surface with some diffi-culty. These melts would have interacted with the ponded,mid-crustal melt, granitic magma resulting in the forma-tion of the aforementioned mixed appinitic stocks. Bycontrast, we suggest that the camptonite dykes emplacedat ~264 Ma in the already solidified granitoids soon afterthe tectonic control changed from purely extensional totranstensional. Within this context opening lithosphere-scale rheidal fractures would have facilitated the passageof metasomatized mantle lithosphere melts to shallowdepths.

Vaughan and Scarrow (2003) proposed that K-richmantle metasomatism controls the initiation and positionof lithosphere-scale strike-slip shear zones. This idea is ofparticular interest in geodynamic reconstructions giventhe suggestion of Bonin et al. (1998) that K-rich magma-tism is commonly related to strike-slip faulting in a post-orogenic setting. We propose, more specifically, that K-rich mafic magmatism is in fact typical in tardi-orogenicsettings related to the main collisional process as a last-gasp release of melts of metasomatized lithopshericmantle in a final melt-focussed transtensional judderonce other larger-scale tectonomagmatic events, andthus potentially-masking magmatism, has beenswitched off.

The suggestion, from the current work, that tardi-oro-genic lamprophyres act as a geodynamic marker of the endof extensional collapse and a change of tectonic regime totranstension merits investigation at a broader scale.

CONCLUSIONS

- A well constrained late Permian, Capitanian, age of264.5 ± 0.9 Ma is assigned to the Gredos sector maficcamptonitic lamprophyre dykes.

- This age is interpreted as the time of cooling and,assuming that this was rapid, crystallization of the maficdykes.

- The Central Iberian tardi-orogenic lamprophyres actas a geodynamic marker for a change from pure extension-al to transtensional control of tapping of alkaline maficmagmas from the metasomatized mantle lithosphere.

ACKNOWLEDGEMENTS

The authors are grateful to H. N. Qiu of The Chinese Acade-my of Sciences, Guangzhou, China for undertaking the 40Ar/39Aranalysis. Professors C. Galindo Francisco, A. Iriondo and G.Ruffet are thanked for detailed reviews that helped us to consid-erably improve the manuscript. This work has been supportedby the Spanish CICYT project CGL2005-05863.

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Manuscript received March 2006;revision accepted August 2006.

Documents

A precise late Permian 40Ar/ 39Ar age for Central Iberian ... · lamprophyre dykes, forming broadly north-south trending swarms in eastern Guadarrama (Ubanell et al., 1984; Vil-laseca