Cooling history of granulite samples from the ocean–continent transition of the Galicia margin: implications for rifting

Cooling history of granulite samples from the ocean±continenttransition of the Galicia margin: implications for rifting

Bernhard Fuegenschuh1*, Nikolaus Froitzheim1 and Gilbert Boillot 2

1Geologisch-PalaÈontologisches Institut, Bernoullistrasse 32, CH-4056 Basel, Switzerland; 2GeÂosciences Azur, Observatoire

OceÂanologique, BP 48, F-06235 Villefranche sur mer, France

Introduction

TheGaliciamarginof theAtlantic (Fig.1) is a classic example of a non-volca-nic, sediment-starved, passive conti-nental margin. The minor thickness ofthe postrift cover, as compared to mostother passive margins, allows the base-ment to be directly accessed by dred-ging, submersible diving, and drilling.Such investigations demonstrated thata peridotite ridge occurs along theocean±continent transition of this mar-gin (Boillot et al., 1995a, and referencestherein). It continues south in the IberiaAbyssal Plain (Fig. 1) where it has beenidentified by seismic reflection profiles(Beslier et al., 1993) and drilling (Saw-yer, Whitmarsh, Klaus et al., 1994).The ridge is interpreted to representsubcontinental mantle, exhumed by ex-tensional shearing and faulting duringthe rifting period. If mantle is exhumedby extension, onewould expect rocks ofthe lower continental crust to be ex-humed as well. In fact, granulite sam-ples were previously dredged at twolocations on the Galicia margin (Cap-devila and Mougenot, 1988) and atseveral dredge sites on the North Span-ish margin (Capdevila et al., 1980).It was one of the aims of the Gali-

naute II cruise in 1995 (Boillot et al.,1995b), using the French submersibleNautile, to investigate whether rocksfrom the prerift lower crust occur atthe sea floor in the ocean±continenttransition of the Galicia margin. Infact, six samples of granulite were re-

covered, from water depths between4270m and 2774m. None of them re-presented an outcrop of basementrocks, but they were either loose blockslying on the sea floor or embedded inindurated calcareous mud of Neogeneage. However, two of the samples werefound very close to and are probablyderived from outcrops of a rift-related(see below) tectonic breccia. We per-formed zircon and apatite fission-trackdating on two granulite samples in or-der to test whether their cooling historyis compatible with synrift exhumationfrom lower-crustal depth, as tentativelyproposed by Boillot et al. (1995b).

Tectonic setting of sample localities

Both samples are from the area of theperidotite ridge NW of Galicia Bank,oceanward of the most distal faultblocks of continental crust (Fig. 2).Sample GAL 19-03 was recovered fromthe base of a NW-facing slope onthe oceanward side of the peridotiteridge. In this area, east of 128120'W,the peridotite ridge was tilted towardsnorthwest, that is, towards the ocean,probablyduring theCenozoic (Boillot etal., 1995b). The peridotite ridge is in thisarea covered by post-rift pillow basalt(Kornprobst et al., 1988). Sample GAL

96 #1998 Blackwell Science Ltd

ABSTRACTZircon and apatite fission track ages were obtained on twogranulite samples that were recovered from the sea floor in theocean±continent transition area of the Galicia margin (NorthAtlantic) using the French submersible Nautile. Zircon agesindicate that the rocks cooled through about 2508C inCarboniferous to Early Permian time (307+ 42 Ma and287+35 Ma). Hence, the granulites do not represent the preriftlower crust but were in an upper crustal position long beforerifting started. Apatites yielded Early Cretaceous ages (126+ 6.7

Myr and 129+ 13.4 Myr), indicating cooling through 90+ 308Ccoeval with the main rifting phase that preceded continentalbreakup. We assume that the granulite samples originate from atectonic breccia cropping out near one of the sample locations.This breccia formed along a synrift detachment accommodatingcontinental breakup and final exhumation of the Galicia margin'speridotite ridge.

Terra Nova, 10, 96±100, 1998

AhedBhedChedDhedRef markerFig markerTable markerRef endRef start

*Correspondence:Tel./Fax+41-61-2673610/

3613; E-mail: [email protected]

12

40

Cape Finisterre

I b e r i a

IberiaAbyssal

PlainPlain

Galicia BankGalicia Bank

VS

10 15 W

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19

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B

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ridotiteridge

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Fig. 1 BathymetricchartoftheWestIberianmargin(contoursat200,500,1000,1500metc.).19, 26, Sample localities of granulite samplesGAL19-03andGAL26-10, respectively;A,B,traces of cross sections in Fig. 2); 637, ODP drill site 637 (Leg 103); crosses: dredge sites onGaliciaBankandVigoSeamountwheregranuliteswererecoveredpreviously(CapdevilaandMougenot, 1988);VS,VigoSeamount.Locationofperidotite ridgeafterBeslieretal. (1993).

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19-03 was collected at 4270 m, close toseveral outcrops of pillow lava. Thesample was lying on the mud-coveredsea floor, and had probably fallen downfrom an outcrop higher up the slope.Sample GAL 26-10 was recovered

farther south-west, in a region wherethe margin was not, or less, affected bytheCenozoic tilting. The sample is froman isolated submarine hill, formed by atectonic horst, 10 km south-east of theperidotite ridge. It was taken near thetopof this hill, at a depthof 2774m.Thesample was encased inNeogene (Moul-lade, pers. com.) chalk which, in thisoutcrop, is rich in blocks and pebbles.Some tens of metres farther and 25metres higher up, there is an outcropof breccia. Similar breccia was found atseveral locations near the top of the hill.Its components are peridotite and dif-ferent types of continental crustal rocks(granite, gneiss). The breccia must beMesozoic in age, because it is exposedon top of a tectonic horst associated

with aMesozoic fault (Fig. 2; Boillot etal., 1995b). It is interpreted as a tectonicbreccia formed during Lower Cretac-eous rifting along a low-angle, brittledetachment fault (Boillot et al., 1995b).GAL26-10most probably derives fromthis breccia, was transported a shortdistance, and redeposited in the chalk.The encasing of the sample withinNeo-gene sediment excludes its depositionby Pleistocene floating ice.

Sample description

GAL 19-03 is a hydrous granulite con-sisting of K-feldspar, plagioclase,quartz, orthopyroxene, hornblende,biotite, ilmenite, and garnet (Gardienet al., submitted). Elongate hornblende,orthopyroxene, and feldspar grains de-fine a weak foliation, indicating ductiledeformation under granulite-facies con-ditions. K-feldspar and plagioclaseshow recrystallization along grainboundaries. Retrograde biotite forms

coronas around ilmenite and pseudo-morphs after amphibole. Biotite crystalsare randomly oriented, indicating thatductile deformation did not continueafter the granulitic event. En-echelonveinlets filled with iron oxihydroxidesrepresent late-stage brittle deformation.GAL 26-10 is a granulite consisting ofplagioclase, quartz, orthopyroxene,clinopyroxene, ilmenite, and apatite(Gardien et al., submitted). The textureof this rock is mainly granoblastic withmany 1208 grain-boundary triple junc-tions. Elongate pyroxene and plagio-clase grains and aggregates locallydefine a weak high-temperature folia-tion. Undulatory extinction is observedin plagioclase and pyroxene.Microfrac-tures and a 2 to 3 millimetres thick faultzone filled with cataclasite are evidenceof late-stage brittle deformation. Bothsamples resemble part of the granuliticrocks previously sampled on the NorthSpanishmargin (Capdevila et al., 1980),which are Precambrian in age (c. 2.7 and1.9 Gyr old; Guerrot et al., 1989).

Fission track dating

We dated zircon and apatite from sam-ples GAL 19-03 and GAL 26-10.Mineral separation and sample pre-paration followed the methods as de-scribed in Seward (1989). All resultsand details on dating technique aregiven in Table 1 and table caption,respectively. Annealing temperaturesfor fission tracks in apatite are90+ 308C, representing the lower andupper limits of the partial annealingzone, respectively (Green et al., 1989).For zircon, annealing temperatures areless well constrained but current esti-mates are around 240+608C (Yamadaet al., 1995) or 260+ 258C (Foster etal., 1996). Lengths of confined horizon-tal tracks have been measured forapatite. Radial plots (Galbraith andLaslett, 1993) and track-length histo-grams are presented in Fig. 3.Both samples yielded very similar

results. Interestingly, all separated zir-cons were rounded to various degrees.Zircon ages are 307+ 42Myr for GAL19-03, and 287+ 35 Myr for GAL 26-10,with the single-grain ages for sampleGAL 26-10 showing a slightly greatervariation. Apatite ages are 126.1+ 6.7Myr (GAL 19-03) and 129.3+ 13.4Myr (GAL 26-10). Single-grain agesfor sample GAL 26-10 yielded consis-tently lower precision (or higher rela-

#1998 Blackwell Science Ltd 97

Terra Nova, Vol 10, No. 2, 96±100 Cooling history of granulite samples . B. Fuegenschuh et al.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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(6, 14, 33)26, 28 30, 35

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Cross-section at 45 55' N, between 12 45' W and 12 10' W

PERIDOTITE RIDGE CONTINENTAL BLOCKS

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000

(a)

(b)

Fig. 2 Interpretative cross-sections of the western (a) and northern (b) slopes of GaliciaBank. Projected into the cross-sections are dive sites of the submersible Nautile(numbers); granulites were recovered at sites 18, 19 (sampleGAL 19-03), and 26 (sampleGAL26-10). S.R.+P.R.: Synrift andpostrift sediments; P, peridotite;G, gabbro. (FromBoillot et al., 1995b.)

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tive error), due to a lower uraniumcontent of the dated apatites.The track lengths for apatite in both

samples show unimodal, negativelyskewed distributions with mean tracklengths of 13.3 mm (GAL 19-03) and12.8 mm (GAL 26-10), indicating slowoverall cooling of the parent rock.

Previous age data from the Galiciamargin

The rifting stage of themargin lasted 25Myr, from 140 to 114 Ma when con-tinental breakup occurred (Boillot,Winterer et al., 1988). It was precededby a tensional episode of probable Ox-fordian±Kimmeridgian age (Mauffretand Montadert, 1987). Two samplesof postrift basalt covering the seawardslope of the peridotite ridge near sam-ple locality GAL 19-03 yielded an40 Ar/39 Ar age of 100+ 5Myr (Malod

et al., 1993). Gabbro and gabbro-de-rived chlorite schist were sampled lo-cally on the peridotite ridge duringsubmersible cruises Galinaute I and II(Beslier et al., 1990; Boillot et al.,1995b). Zircons from the chlorite schistgave an U-Pb age of 122.1+ 0.3 Myr,interpreted as the age of the gabbroprotolith (SchaÈ rer et al., 1995). Thiscoincides with an 39 Ar-40 Ar age(122+ 0.6 Myr) of brown amphibolesin syn- to postkinematic diorite dyke-lets in mylonitized peridotite drilled atODP Site 637 (Fig. 1; FeÂ raud et al.,1988). The 122Myr ages coincide, with-in error limits, with our apatite fissiontrack ages of the granulite samples.Two granulite samples had pre-

viously been recovered by dredge haulsfrom theGaliciamargin, one fromVigoseamount, the other from Galicia Bank(Fig. 1; Capdevila and Mougenot,1988). They yielded Precambrian Rb-

Sr biotite and phlogopite ages (c. 1.5Gyr, Postaire, 1983, cited after Capde-vila and Mougenot, 1988), as did thegranulite samples from theNorth Span-ish margin by the U±Pb method (c. 2.7and 1.9 Gyr, Guerrot et al., 1989).

Discussion

The zircon fission track ages imply thatthe granulite samples recovered on theGalicia Bank slope do not representlower crust which was exhumed fromdeep structural levels during Mesozoicrifting. Instead, they had alreadycooled through & 2508C in the LateCarboniferous to Early Permian. As-suming a geothermal gradient of 258C/km (average gradient for continentalcrust, Kearey and Vine, 1990), 10 kmis the maximum depth they may haveoccupied after the Early Permian. Theexhumation of these rocks from the


Cooling history of granulite samples . B. Fuegenschuh et al. Terra Nova, Vol 10, No. 2, 96±100. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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apatite zircon

GAL 19 03Mean 13.31 ± 0.16 µmStd Dev 1.56 µmn = 100

GAL 26 10Mean 12.79 ± 0.25 µmStd Dev 2.18 µmn = 74

Fig. 3

µm5 10 15 µm5 10 15

% tr

acks

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10

20

30

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Ma

Fig. 3 Top: Apatite and zircon single-grain age distribution (radial plot diagram, Galbraith and Laslett, 1993) for samples GAL 19-03(open squares) and GAL 26-10 (dots). Bottom: Track-length histograms for confined horizontal tracks in apatite.

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lower to middle crustal conditions un-der which they equilibrated (Gardien etal., submitted) to 10 km depth or lessoccurred prior to or during the LateCarboniferous to Early Permian, eitherby erosional or tectonic unroofing.Hence, there is still no evidence forsyn-rift exhumation of lower crust inthe ocean±continent transition of theGalicia margin, although the area hasnow been sampled intensively by div-ing, dredging, and drilling. The me-chanism of rifting seems to preventlower crust from being exhumed (Brunand Beslier, 1996). We assume that it ishidden below the layer of tilted uppercrustal blocks (Fig. 4). The base of this

layer, known as the S-reflector, is inter-preted to represent a detachment fault(Krawczyk and Reston, 1995). Thetilted blocks are remnants left behindby its upper plate. We interpret thetectonic breccia to represent the faultrock of the detachment, sampling bothmantle rocks and upper crustal rocks(Boillot et al., 1995b).The apatite ages, 126 Myr and 129

Myr (Barremian), coincide roughlywith radiometric ages for the intrusionof gabbro at the crust±mantle bound-ary in the Galicia margin (122.1+ 0.3Myr, SchaÈ rer et al., 1995) and for themylonitization of mantle rock formingthe peridotite ridge (122Myr, FeÂ raud et

al., 1988). Actually, the apatite ages fallinto the rifting period of the margin(140±114 Ma). Hence, it may be as-sumed that the late cooling of the gran-ulites is related to rifting. Asmentionedabove, sample GAL 26-10 was prob-ably derived from the tectonic brecciacropping out nearby. This may explainthe synrift apatite ages of the two sam-ples: They were `sampled' by the brec-cia from the upper crust and exhumedat the sea floor when the upper plate ofthe detachment was removed by pro-gressive extension.

Conclusions

The granulite samples do not representprerift lower continental crust. Instead,these rocks hadalreadybeenbrought toan upper crustal level in the Carboni-ferous to Early Permian, that is, longbefore rifting started in this area.Hence, there is still no evidence forexhumation of prerift lower crust alongthe Galicia margin, and this may beinherent in the rifting mechanism.In the Early Cretaceous, the upper

crustal granulites were probably directlyemplaced on themantle rock of the peri-dotite ridge, encased ina tectonic brecciaforming along an extensional detach-ment fault. In our opinion, this denuda-tional faulting occurred at the end of therifting period, leading to the Lower Cre-taceous final cooling of the granulitesindicated by apatite fission tracks.If our assumption is correct, that the

granulites were part of the tectonicbreccia formed along a detachmentfault, then the absence of post-granuli-tic ductile deformation and the pre-sence of brittle microfractures, veinlets,and cataclasite suggest that this detach-

#1998 Blackwell Science Ltd 99

Terra Nova, Vol 10, No. 2, 96±100 Cooling history of granulite samples . B. Fuegenschuh et al.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Table 1 Fission track data for granulite samples. Dating was carried out using the external detector method with a zeta value(Hurford and Green, 1983) of 348+ 9 (SRM 612 and FCT zircon) and 357+ 15 (CN 5 and Durango apatite). All ages (column 8)are central ages (Galbraith and Laslett, 1993) and errors are quoted as 1 s. Columns 1 and 2 give sample number and type of grainsdated, respectively: apatite (A), zircon (Z). Number of grains counted is given in column 3. Standard, spontaneous (rs) and induced(ri) track densities are shown in columns 4, 5 and 6, respectively. Number of counted tracks is given in parenthesis. P(w2) is theprobability of w2 for n degrees of freedom where n=(No. of grains dated by the external detector method) 71 (Galbraith, 1981).

Std. track ds. rs ri P (w2)

Grains Number of 6 104 cm-2 6 104 cm-2 6 104 cm-2 % Age (Myr)

Sample dated grains counted (counted) (counted) (counted) + 1 s

GAL 19±03 A 20 133.4 (1875) 64.12 (681) 119.9 (1273) 90 126.1+ 6.7

GAL 19±03 Z 9 16.24 (1292) 1534 (678) 138 (61) 95 306.7+ 41.9

GAL 26±10 A 20 128.8 (1875) 12.72 (154) 22.39 (271) 98 129.3+ 13.4

GAL 26±10 Z 9 13.99 (1292) 2909 (1096) 244.2 (92) 45 286.6+ 34.7

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mantle

lower crust

upper crust

��

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mantlelithosphere

asthenosphere

Fig. 4

crust

Fig. 4 Diagrammatic cross-section of a lower-plate passive margin formed byasymmetric rifting. Tilted block layer consisting of upper continental crust is emplaceddirectly upon exhumed mantle along a subhorizontal detachment fault (cf. the S-reflector of the Galicia margin). Lower crust is hidden below the tilted blocks layer, andis therefore not exhumed in the ocean-continent transition zone. Granulite samples areprobably derived from the tectonic breccia which formed along the detachment and iscompletely exhumed oceanward of the tilted blocks layer. This model applies to theGaliciamargin under the assumption that the peridotite ridge represents subcontinentalmantle exhumed during rifting. Inset: General setting of asymmetric rifting, explainingwhy the shear sense of the detachment is top-to-the-ocean. A conjugate shear zone(broken line) is assumed to have operated in the lower plate (see Brun andBeslier, 1996).

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ment fault was essentially a brittlestructure. This conclusion may con-strain the models of continental riftingand breakup (Fig. 4).

Acknowledgements

IFREMER (GENAVIR) is thanked forproviding the submersible Nautile and itsvesselNadir.We gratefully acknowledge thehelp of both the scientific party as well asthe crew members during the Galinaute IIcruise in 1995. Spanish authorities arethanked for permission to dive in Spanishterritorial waters. Diane Seward is thankedfor providing the fission-track facilities atETH-ZuÈ rich. This article was improved byconstructive reviews of J.J. Peucat and ananonymous reviewer. Contribution No 184of the UMR Geosciences Azur, Ville-franche-sur-Mer - Nice - Sophia-Antipolis.

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Received 10 March 1989; revised versionaccepted 28 July 1998


Cooling history of granulite samples . B. Fuegenschuh et al. Terra Nova, Vol 10, No. 2, 96±100. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Documents

Cooling history of granulite samples from the ocean–continent transition of the Galicia margin: implications for rifting