Evolution of a kyanite-bearing belt within a HT-LP orogen: the case of NW Variscan Iberia: EVOLUTION...

J. metamorphic Geol., 1998, 16, 379–394

Evolution of a kyanite-bearing belt within a HT-LP orogen: the case ofNW Variscan IberiaJ . RECHE, 1 F . J . MARTINEZ, 1 M. L. ARBOLEYA, 1 C. DIETSCH2 AND W. D. BRIGGS 2

1Departamento de Geologia, Universitat Autonoma de Barcelona, 08193 Bellaterra, Spain (email: j [email protected])2Department of Geology, University of Cincinnati, Cincinnati, OH 45221-0013, USA

ABSTRACT Kyanite replaces andalusite in a belt of Ordovician and Silurian pelitic rocks that form a narrow synformpinched between high-grade antiforms in NW Variscan Iberia. Kyanite occurs across the belt in Al-rich,black pelites in assemblages I: kyanite–chloritoid–chlorite–muscovite and II: kyanite–staurolite–chlorite–muscovite. In I, kyanite occurs in the matrix and in kyanite–muscovite aggregates that pseudo-morph earlier andalusite porphyroblasts. The aggregates are found across the belt and can still be recog-nized in assemblage II and even in III: andalusite–staurolite–biotite–muscovite, this latter being ahornfelsic Silurian schist where kyanite is relic and staurolite occurs in the matrix, and is resorbed insidenew massive pleochroic andalusite. KFMASH and MnKFMASH pseudosections have been constructedusing Thermocalc for Al-rich and Al-poorer compositions from the belt. Chloritoid zoning in Al-richrocks containing assemblage I, plus chloritoid–chlorite thermometry complemented with garnet–chloritethermometry in Al-poorer lithologies, mean that the path is one of increasing pressure and temperature.Conditions prior to assemblage I, with earlier andalusite stable, are those of the andalusite–chloritoid–chlorite field as testified by chloritoid enclosed in andalusite porphyroblast rims. The passage fromassemblage I to II implies a prograde path within the kyanite field. Assemblage III represents peakconditions, indicating a prograde staurolite-consuming reaction across a KFMASH field, leading eventu-ally to a locally found andalusite–biotite–muscovite hornfels. The lowest pressure stages are recorded bycordierite–biotite in Al-poor pelites. Garnet-bearing MnKFMASH assemblages in Al-poorer pelitesrecord conditions similar to assemblages II and III. The replacement of andalusite by kyanite in assem-blage I is attributed to downdragging of andalusite-bearing rocks into a synform as testified by thestrained andalusite porphyroblasts affected by a subvertical crenulation cleavage. Prograde metamor-phism in the eastern contact of the belt is due to heat transferred to the belt from the ascending highgrade antiform across the Vivero fault.

Key words: andalusite; KFMASH pseudosections; kyanite; kyanite–muscovite aggregates; Variscan belt.

related to multiple granitoid intrusions covering aINTRODUCTION AND GEOLOGICAL SETTINGbroad time-span as determined from radiometric datingof the granites (Priem & Den Tex, 1984; Serrano PintoThe study of low-pressure, high-temperature metamor-

phic belts in different parts of the world has attracted & Gil Ibarguchi, 1987) which show pre-, syn- andpost-tectonic relations with host rock fabrics.interest in the last decade (Lux et al., 1986; Martınez

et al., 1988; De Yoreo et al., 1989; Sandiford & Powell, The distribution of metamorphism in NW Iberia ischaracterized by elongated, sometimes coalescent1991). The existence of high crustal thermal gradients

which change both spatially and with time, due mainly thermal domes or highs which generally form broadbelts. These thermal highs typically coincide withto magmatic heat advection, are typically invoked to

explain this kind of metamorphism (Barton & antiforms and structural domes, and alternate withnarrow, almost granite-free, synforms which containHanson, 1989).

One of the archetypal foldbelts with low-pressure low-grade metamorphic rocks; these spatial relationsare also known in the Variscan basement in themetamorphism is the Variscan Chain in Europe

(Fig. 1a) which is characterized by widespread low- Pyrenees (Zwart, 1979; Gibson, 1991; Pouget, 1991).In some of the synforms, allocthonous units containingpressure metamorphism with andalusite as the rep-

resentative aluminium silicate in pelites, and by the granulite or blueschist facies rocks record olderSiluro-Devonian metamorphism. Lower Palaeozoic orabundance of granitoids. The close relations in space

and time between granitoids and low-pressure, high- Upper Precambrian rocks crop out in the antiforms(Fig. 1b), whereas younger strata of Middletemperature metamorphism is particularly clear in the

Variscan Chain in Iberia (Oen, 1970; Martınez et al., Ordovician to Devonian age are found in rimsynforms that coincide with the low-grade belts1988; Reavy, 1989). Within these areas, low-pressure

metamorphism may be explained as a thermal event (Martınez et al., 1988).

379© Blackwell Science Inc., 0263-4929/98/$14.00Journal of Metamorphic Geology, Volume 16, Number 3, 1998, 379–394

380 J . RECHE ET AL .

Fig. 1. (a) Main outcrops of the Variscan chain in SW Europe with location of the study area in NW Iberia, based on Matte(1991). (b) Simplified geological map of the study area showing the main structural features and granites. Kyanite-bearing pelitebelts are shaded, located west and north-east of the Ollo de Sapo gneiss. Line A–B shows location of cross-section in Fig. 7d.(c) Detailed map of the Vivero Pelite Belt (VPB), bounded by the high-grade psammo–pelites of the Lugo dome on the east, andthe Ollo de Sapo gneiss on the west. Also shown are the Vivero fault, syn- and post-tectonic granites, and late andalusiteoccurrences (black squares). Sampling areas 1 and 2 on the coast, and sampling area 3 inland along the contact zone between theVPB and the Lugo dome are referred to in text.

In cross-sections from dome cores to the synforms melting of pelitic or quartzo–feldspathic protoliths iscoeval with the formation of these high-temperaturethat rim them, metamorphic zonation in western Iberia

seems to record a high, nearly isobaric final metamor- assemblages.The descriptions made so far are valid but notphic field gradient typical of low-pressure, high-

temperature terranes. The prograde zoning in complete, since low-pressure andalusite/sillimanitemetamorphism is not the only baric type present. Low-metapelite belts of the rim synforms is chlorite–

biotite–andalusite /cordierite–sillimanite–sillimanite – and medium-grade, kyanite-bearing, Barrovian assem-blages have also been reported in some of the peliteK-feldspar (Capdevila, 1968a; Oen, 1970; Martınez &

Rolet, 1988). Several domes are actually fold-nappe belts in North-west Spain (Fig. 1b & c; Capdevila,1968b; Martınez-Catalan, 1985; Arenas, 1991; Martınezstacks, and in them, decompression paths from earlier

garnet–K-feldspar–biotite–sillimanite or garnet–mus- et al., 1996) and Northern Portugal (Oen, 1970, 1974;Atherton et al., 1974). In Northern Spain, two pelitecovite–biotite–sillimanite assemblages are traced into

high-T assemblages with garnet–cordierite–K-feld- belts containing Silurian and Ordovician rocks formcuspate, narrow, pinched synforms that enclose thespar–biotite–sillimanite or cordierite–K-feldspar–bio-

tite–sillimanite (Gil Ibarguchi & Martınez, 1982; antiform cored by the pre-Ordovician Ollo de Sapogneiss (Fig. 1b; Diez Balda et al., 1990; Bastida et al.,Martınez et al., 1988). In the domes, widespread in situ

EVOLUTION OF A KYANITE BELT WITHIN A HT-LP OROGEN 381

1993). Kyanite-bearing assemblages occur sporadicallyPETROGRAPHY, BULK ROCK AND MINERAL

in favourable compositions in both synforms. TheCHEMISTRY, AND MICROSTRUCTURAL

metamorphic evolution of rocks from several localities RELATIONSacross the northern part of one of these synforms (thepelite belt east of the Ollo de Sapo gneiss antiform; In order to describe the metamorphic evolution of the

kyanite-bearing VPB, about 190 samples were collectedFig. 1b & c) is presented in this study. This belt, whichwe name the Vivero Pelite Belt (VPB), is located along the belt southwards from the coast over

approximately 100 km (Fig. 1b). More detailed sam-between the Ollo de Sapo antiform to the west and theLugo dome antiform to the east (Fig. 1c), both of which pling (142 samples) was made across the belt near its

northern tip along the coast, from the Ollo de Sapocontain high-temperature assemblages. The VPB is asynclinal structure whose eastern limb evolves into a antiform in the west to the Lugo dome in the east and

inland on Route 640 as well as on adjacent minortranstensional shear zone. This zone, named the Viverofault (Matte, 1968; Bastida et al., 1984; Martınez- roads. For descriptive purposes we will refer to three

sampling areas 1, 2 and 3 (Fig. 1c).Catalan, 1985; Martınez et al., 1996), is a ductilestructure that brings into contact Silurian black slate of Seventeen bulk rock analyses have been made of

the most representative lithologies across the belt andthe pelite belt on the west with Cambrian psammiticschist of the Lugo dome on the east (Fig. 1b & c). from the Ollo de Sapo antiform and the Lugo dome

(Table 1). Twelve of these samples belong specificallyThree main fabrics, to which mineral assemblagedevelopment can be related, are recognized traversing to the VPB, and when plotted on an AFM diagram

can be divided into two distinct compositional groups,the VPB syncline from the Ollo de Sapo gneissantiform to the Lugo dome. The earliest fabric is an one being Al rich and the other relatively Al poor

(Fig. 2). Nevertheless, both rock groups plot aboveS1 axial planar foliation related to east-verging, D1recumbent folds. The D1 folds are refolded by the garnet–chlorite tie-line in AFM. The Al-richer

group (A/AFM=0.79–0.85) is composed of blacksubvertical, asymmetric, east-verging D2 folds whichdevelop a closely spaced crenulation cleavage, S2. Ordovician and Silurian slates and schists, whereas

the other group consists of less abundant, interlayeredSuperimposed on these two deformations are subhori-zontal D3 folds with an axial planar S3 crenulation psammo-pelitic rocks (A/AFM=0.64–0.73). In gen-

eral, Fe contents are higher in Al-poor pelitescleavage that is weak in low-strain domains butbecomes more penetrative in higher strain domains in (F/FM=0.74–0.80) than in Al-rich ones (F/FM=

0.66–0.69).the proximity of the Vivero fault zone.

Table 1. Representative bulk chemical analyses for rocks fromthe Ollo de Sapo gneiss antiform (OdeS), the Lugo dome (LD)and sampling areas 1, 2 and 3 from the VPB.

Area OdS OdS OdS 1 1 2 2 2 3

Sample 11 16 17 42 43 64 73 77 93

SiO2 66.08 65.75 70.76 55.17 40.75 58.30 45.25 37.39 59.27

TiO2 0.76 0.73 0.62 0.75 1.41 0.86 1.30 1.40 0.83

Al2O3 16.15 17.23 13.49 25.02 37.42 20.90 31.87 23.07 19.39

FeOT 5.53 5.04 4.93 4.62 4.30 7.24 6.41 22.09 8.13

MnO 0.05 0.05 0.05 0.05 0.10 0.09 0.08 0.22 0.13

MgO 1.71 1.90 1.38 2.06 2.18 2.09 1.21 5.19 2.79

CaO 1.00 0.63 0.75 0.08 0.12 0.08 0.47 0.44 0.54

Na2O 3.48 2.58 2.81 2.38 2.70 1.09 1.80 0.70 1.31

K2O 3.34 3.70 3.55 2.28 3.30 3.35 3.56 0.66 2.21

LOI 2.58 3.14 2.55 6.67 6.65 4.88 6.69 7.69 3.89

Total 100.98 100.15 100.89 99.08 98.93 98.98 98.64 98.85 98.49

Area 3 3 3 LD LD LD LD LD

Sample 99 101 36 34 35 40 31.10 31.20

SiO2 54.57 52.49 54.54 57.13 61.51 60.57 63.12 57.32

TiO2 0.97 1.26 0.99 0.90 0.89 0.86 0.97 0.68

Al2O3 24.23 27.58 24.38 20.16 16.94 18.42 16.86 21.97

FeOT 7.10 7.07 6.94 7.42 7.84 7.22 6.80 7.76 Fig. 2. AFM diagram depicting bulk compositions forMnO 0.08 0.10 0.09 0.10 0.07 0.08 0.12 0.10

representative samples from the three studied areas. OpenMgO 1.77 0.79 2.17 3.63 3.32 3.57 3.18 3.54

circles: Ollo de Sapo gneiss; filled circles: VPB, area 1; crosses:CaO 0.12 0.63 0.07 1.15 1.10 1.03 1.83 0.56VPB, area 2; filled squares: VPB, area 3; triangles: Lugo dome.Na2O 0.39 0.84 0.69 2.50 2.74 2.60 3.98 1.20

Fields enclosed within dashed lines are reported asK2O 5.77 2.07 4.36 3.38 3.13 3.41 2.34 4.01

LOI 4.26 5.06 4.19 3.53 2.72 2.51 1.79 3.84 representative for Al-rich and Al-poor pelites. AFM parametersare computed as in Thompson (1957). Samples 42 and 93, usedTotal 99.26 97.89 98.42 99.90 100.26 100.27 100.99 100.98for Thermocalc modelling, are highlighted.


dominant foliation is still S1, the clearest exampleAssemblages in Al-rich pelites within the Vivero Pelite Belt

being kyanite replacing chiastolite (Fig. 3b, c & e), orin the vicinity of slightly deformed chiastolite porphyro-Three low-variance assemblages are recognized in

black Al-rich pelites (Tables 1 & 2) across the VPB. blasts ( lower part of Fig. 3b). In higher D2 straindomains, kyanite and tabular chloritoid statistically lie

Assemblage I. Kyanite–chloritoid–chlorite–muscoviteon the S2 foliation (Fig. 3f ) and are deformed by S3

(Table 2) found in sampling areas 1 and 2 (Fig. 1c).(Fig. 3h).

Kyanite–blastic muscovite aggregates are pseudomor-phic after andalusite. Muscovite inside the aggregates Assemblage II. Kyanite–staurolite–chlorite–muscovitehas a decussate texture different from muscovite in the (plus relict chloritoid; Table 2) found in sampling areamatrix where it defines the foliation. Minor amounts 3 (Fig. 1c). Rocks containing this assemblage areof zincoan staurolite (4.13 wt% ZnO; Table 3) is variably affected by the S3 subhorizontal cleavagesometimes found. which deforms the composite S1+2 fabric. S3 cleavage

The replacement of andalusite by kyanite–muscovite occurs along the eastern side of the VPB and is relatedaggregates means that a simple polymorphic transform- to the collapse of the subvertical S1+2 fabric in theation is not taking place. Possible mechanisms for the vicinity of the Vivero transtensional fault zone. Pre- toobserved replacement are ionic reactions involving K+ syn-S3 staurolite overgrows kyanite and chloritoidand H+ similar to those proposed by Carmichael (Fig. 3g). Kyanite is pre-S3 as this cleavage always(1969) and Yardley (1989). wraps around kyanite crystals. Moreover, bent kyanite

Andalusite overprints the first penetrative slaty crystals are found surrounded by S3 crenulationcleavage S1 (Fig. 3a & b), axial planar to isoclinal D1 cleavage (Fig. 3h) and appear folded in D3 fold hinges.folds, and is pretectonic with respect to the S2 Chloritoid relicts, interpreted as part of the earliercrenulation (Fig. 3c) related to the subvertical D2. This assemblage I, are also pre-S3 cleavage as this cleavageinterpretation is supported by sporadic strained anda- wraps around them; bent chloritoid is found in D3lusite remnants showing an internal foliation (S1) at fold hinges as well. Kyanite–muscovite aggregatesright angles to the external one (S2) in a D2 fold resembling remnants of earlier andalusite pseudo-hinge, and by the occurrence of flattened kyanite–mus- morphs also appear folded by S3 crenulation.covite aggregates in a composite S1+2 cleavage(Fig. 3d & e) in D2 fold limbs. Kyanite and chloritoid Assemblage III. Andalusite–staurolite–biotite–mus-

covite±plagioclase (plus relict kyanite; Table 2) foundalso occur in the matrix, wrapped around by theexternal, composite S1+2 cleavage (Fig. 3f ) defined in sampling area 3 (Fig. 1c). Assemblage III occurs

discontinuously in recrystallized, hornfels-like rustyby chlorite and muscovite. Chloritoid probably startedgrowing in the latest stages of andalusite formation schist in a decametric wide strip of Silurian rocks

adjacent to the Vivero fault. Subidioblastic andalusitesince it is included in the andalusite rims. Kyaniteoccurs unoriented in low D2 strain domains where the grows syn- to post-tectonically to the subhorizontal

Table 2. Mineral assemblages present in high- and low-Al pelites from sampling areas 1, 2 and 3 from the VPB and the LD. Onlysamples showing divariant or, at most, trivariant assemblages are reported. Bulk compositional parameters as used for Thermocalcmodelling are given (Al2O3, MgO, FeO and K2O were recalculated to 100). A/AFM=Al2O3/Al2O3+FeO+MgO); F/FM=FeO/(FeO+MgO); Mn/MnFM=MnO/(MnO+FeO+MgO).

Bulk composition parameters Divariant or trivariant

Sampling Sample peak assemblages Relict

area id. Al2O3 MgO FeO K2O A/AFM F/FM Mn/MnFM (referred to in text) minerals

Area 1 high-Al 42 73.63 6.06 13.60 6.71 0.79 0.69 0.007 I: Ky–Cld–Chl–Ms And

VPB 43 79.28 4.62 9.11 6.99 0.85 0.66 0.015 I: Ky–Cld–Chl–Ms And


VPB


VPB JF198 – – – – – – – II: Ky–St–Chl–Ms Cld

5 – – – – – – – III: And–St–Bt–Ms Ky

98 – – – – – – – And–Bt–Ms Ky, St

low-Al 93 59.62 8.58 25.00 6.80 0.64 0.74 0.012 Grt–St–Chl–Ms

47 – – – – – – – Grt–St–And–Bt–Ms

36 64.41 5.73 18.34 11.52 0.73 0.76 0.010 Grt–And–Crd–Bt

96 – – – – – – – And–Bt–Ms

94 – – – – – – – Crd–And–Bt–Ms Grt

97 – – – – – – – Crd–Bt–Ms

Lugo low-Al 31.1 57.78 10.90 23.30 8.02 0.63 0.68 0.012 Grt–Bt–Ms St

Dome 39 – – – – – – – Grt-Bt–Pl

40 56.47 10.94 22.13 10.45 0.63 0.67 0.007 Sil–St–Bt–Ms Grt, Chl

PG19 – – – – – – – Grt–St–Bt

MA151 – – – – – – – St–Bt–Ms

PG10 – – – – – – – Sil–St–Bt–Ms

JF188 – – – – – – – Sil–Bt–Ms


Table 3. Selected microprobe analyses of minerals from sampling areas 1, 2 and 3 within the VPB and from the LD. If more thanone point was used to calculate an average, this is indicated in the ‘points’ row. Standard deviations are given in brackets. Allanalyses are normalized to the given number of oxygens, except staurolite which is normalized to Si+Al=25.53 (Holdaway et al.,1988). Biotite used in garnet–biotite thermometry are placed near paired garnet.

Chloritoid Chlorite

Area 1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 2 2 3

Sample 43 43 43 43 73 73 73 73 77 77 77 77 77 77 77 77 64 93

Points core 1 core 1 rim 1 rim 1 core 1 core 1 rim 1 rim 1 core 1 core 1 rim 1 rim 1 core 1 core 1 rim 1 rim 1 9 16

SiO2 24.20 24.51 24.06 23.80 24.44 24.54 24.41 24.48 24.22 24.36 24.41 24.18 23.80 23.16 25.43 23.3 24.22 (0.99) 23.73 (0.44)

TiO2 0.06 0.03 0.02 0.01 0.48 0.00 0.03 0.00 0.01 0.00 0.01 0.03 0.08 0.11 0.08 0.07 0.08 (0.02) 0.07 (0.03)

Al2O3 40.24 41.09 40.37 41.19 40.97 41.04 40.66 41.05 40.45 40.75 40.95 40.65 23.77 22.96 24.77 23.18 23.51 (0.80) 24.25 (0.45)

FeO 21.59 22.13 21.79 21.63 24.42 24.58 24.23 24.76 25.42 25.94 25.37 24.51 31.21 31.56 29.45 31.48 29.77 (1.23) 28.36 (0.81)

MnO 0.74 0.72 0.78 0.70 0.30 0.38 0.4 0.53 0.57 0.58 0.52 0.55 0.23 0.26 0.19 0.28 0.34 (0.03) 0.23 (0.04)

MgO 3.02 3.14 3.42 3.38 2.74 2.50 2.24 2.14 1.60 1.70 1.86 1.92 9.29 9.25 8.05 9.02 9.77 (0.68) 11.04 (0.57)

CaO 0.02 0.00 0.00 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.09 0.04 0.01 0.04 0.01 (0.01) 0.01 (0.01)

Na2O 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.05 0.02 0.16 0.02 0.04 (0.04) 0.02 (0.03)

K2O 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.73 0.02 0.20 (0.47) 0.02 (0.05)

ZnO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 – – – – – –

Total 89.86 91.62 90.44 90.73 93.35 93.03 91.97 92.98 92.27 93.32 93.12 91.84 88.52 87.35 88.85 87.4 87.93 (0.63) 87.73 (0.75)

Cations per 12 oxygens Cations per 28 oxygens

Si 2.03 2.02 2.01 1.98 2.00 2.01 2.02 2.01 2.02 2.01 2.01 2.01 2.56 2.54 2.69 2.55 2.61 (0.07) 2.54 (0.03)

AlIV 3.98 3.99 3.98 4.04 3.95 3.97 3.97 3.98 3.97 3.96 3.98 3.99 1.44 1.46 1.31 1.45 1.39 (0.07) 1.46 (0.03)

T site – – – – – – – – – – – – 4.00 4.00 4.00 4.00 4.00 4.00

AlVI – – – – – – – – – – – – 1.58 1.51 1.79 1.55 1.59 (0.13) 1.60 (0.04)

Ti 0.00 0.00 0.00 0.00 0.03 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.01 0.01 0.01 (0.00) 0.01 (0.00)

Fe 1.52 1.53 1.52 1.50 1.67 1.69 1.68 1.70 1.77 1.79 1.75 1.71 2.81 2.90 2.61 2.88 2.68 (0.14) 2.54 (0.09)

Mn 0.05 0.05 0.06 0.05 0.02 0.03 0.03 0.04 0.04 0.04 0.04 0.04 0.02 0.02 0.02 0.03 0.03 (0.00) 0.02 (0.00)

Mg 0.38 0.39 0.43 0.42 0.33 0.31 0.28 0.26 0.20 0.21 0.23 0.24 1.49 1.51 1.27 1.47 1.57 (0.12) 1.76 (0.08)

Zn 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 (0.00) 0.00 (0.00)

Ca 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 (0.00) 0.00 (0.00)

Na 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.03 1.00 0.01 (0.01) 0.00 (0.01)

K 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.10 0.00 0.03 (0.06) 0.00 (0.01)

O site – – – – – – – – – – – – 5.93 5.97 5.82 5.95 5.91 (0.06) 5.93 (0.02)

Si+Al – – – – – – – – – – – – – – – – – –

Fe/Fe+Mg0.800 0.798 0.782 0.782 0.833 0.847 0.858 0.866 0.899 0.896 0.884 0.878 0.653 0.657 0.672 0.662 0.631 0.591

Staurolite Garnet Biotite Garnet Biotite Plagioclase

Area 1 3 2 2 3 3 3 3LD 3LD 2 2 3 3 3

Sample St42 St93 Gr64 Gr64 Gr93 Gr99 Bi99 Gr31.1 Gr31.1 Gr31.1 Gr31.1 Gr31.1 Bi31.1 Pl31.1

Points 4 14 core 1 rim 1 rim 2 rim 2 17 core 1 core 1 core 1 rim 1 rim 1 matrix 1 matrix 1

SiO2 28.53 (0.15) 28.10 (0.52) 36.69 36.92 37.18 (0.21) 37.38 (0.25) 33.96 (0.66) 37.82 37.48 37.37 37.29 37.47 35.33 61.58

TiO2 0.48 (0.04) 0.44 (0.05) 0.05 0.07 0.08 (0.01) 0.03 (0.01) 1.72 (0.02) 0.02 0.00 0.04 0.02 0.00 1.50 0.00

Al2O3 55.87 (0.16) 55.84 (0.44) 20.95 21.35 21.03 (0.01) 21.25 (0.18) 20.08 (0.41) 21.73 21.49 21.76 21.16 21.52 19.72 25.42

FeO 7.91 (0.56) 10.50 (0.54) 26.20 32.58 34.47 (1.32) 37.33 (0.67) 24.03 (0.83) 31.87 32.66 31.86 33.51 33.7 17.65 0.07

MnO 0.24 (0.05) 0.28 (0.05) 12.44 5.24 1.88 (0.69) 1.15 (0.26) 0.04 (0.03) 3.14 3.19 3.37 3.39 3.27 0.15 –

MgO 1.13 (0.11) 0.77 (0.10) 0.87 1.25 1.83 (0.14) 1.33 (0.07) 5.92 (0.16) 3.83 3.49 3.27 3.06 3.03 11.82 –

CaO 0.00 0.00 2.48 2.81 3.70 (0.43) 3.69 (0.44) 0.00 (0.00) 3.75 3.44 4.19 2.88 3.04 0.00 6.04

Na2O 0.00 0.00 0.00 0.00 0.00 0.00 0.17 (0.06) 0.00 0.00 0.00 0.00 0.00 0.12 8.28

K2O 0.00 0.00 0.00 0.00 0.00 0.00 8.06 (0.40) 0.00 0.00 0.00 0.00 0.00 9.45 0.06

ZnO 4.13 (0.57) 1.73 (0.27) – – – – – – – – – – – –

Total 98.31 (0.33) 97.6 (0.44) 99.69 100.2 100.18 (0.14) 102.17 (0.02) 93.99 (0.56) 102.16 101.75 101.87 101.3 102.13 95.84 101.45

Si+Al=25.53 Cations per 12 oxygens 22 ox. Cations per 12 oxygens 22 ox. 8 ox.

Si 7.72 (0.02) 7.64 (0.02) 3.00 2.99 3.00 (0.01) 2.98 (0.01) 5.33 (0.06) 2.96 2.96 2.95 2.97 2.96 5.30 2.70

AlIV 17.81 (0.02) 17.89 (0.12) 0.00 0.01 0.01 (0.01) 0.02 (0.01) 2.67 (0.06) 0.04 0.04 0.05 0.03 0.04 2.70 1.31

T site – – 3.00 3.00 3.00 (0.01) 3.00 (0.00) 8.00 (0.00) 3.00 3.00 3.00 3.00 3.00 8.00 4.00

AlVI – – 2.01 2.03 1.99 (0.00) 1.98 (0.02) 1.04 (0.08) 1.97 1.96 1.97 1.96 1.97 0.79 –

Ti 0.10 (0.01) 0.09 (0.01) 0.00 0.00 0.01 (0.00) 0.00 (0.00) 0.20 (0.02) 0.00 0.00 0.00 0.00 0.00 0.17 –

Fe 1.79 (0.12) 2.39 (0.13) 1.79 2.21 2.33 (0.09) 2.49 (0.06) 3.15 (0.13) 2.09 2.16 2.10 2.23 2.23 2.22 0.00

Mn 0.05 (0.01) 0.07 (0.01) 0.86 0.36 0.13 (0.05) 0.08 (0.02) 0.01 (0.00) 0.21 0.21 0.22 0.23 0.22 0.02 –

Mg 0.45 (0.04) 0.31 (0.04) 0.11 0.15 0.22 (0.02) 0.16 (0.01) 1.39 (0.04) 0.45 0.41 0.38 0.36 0.36 2.65 –

Zn 0.82 (0.12) 0.35 (0.05) – – – – 0.00 (0.00) – – – – – – –

Ca 0.00 0.00 0.22 0.24 0.32 (0.04) 0.31 (0.04) 0.00 (0.00) 0.31 0.29 0.35 0.25 0.26 0.00 0.28

Na 0.00 0.00 – – – – 0.05 (0.02) – – – – – 0.04 0.70

K 0.00 0.00 – – – – 1.61 (0.07) – – – – – 1.81 0.01

O site – – – – – – 5.78 (0.09) – – – – – 5.85 –

Si+Al 25.53 (0.00) 25.53 (0.00) – – – – – – – – – – – –

Fe/Fe+Mg 0.797 0.885 0.944 0.936 0.914 0.940 0.591 0.823 0.840 0.847 0.861 0.861 0.456 –


Fig. 4. Photomicrographs of mineral relationships in late andalusite-bearing assemblage III from Silurian Al-rich metapelites of theVPB. (a) Andalusite and staurolite porphyroblasts wrapped by discrete S3 foliation planes. An earlier foliation S1+2 is preserved inthe microlithons at low angle to S3. Large crystal in the upper centre is late, syn- to post-S3 pleochroic andalusite which oftenincludes resorbed staurolite. Kyanite remnants occur in the matrix together with muscovite. Biotite-bearing matrix is recrystallizedand coarse grained and partially surrounds staurolite and andalusite. Rectangle corresponds to photograph (b). (b) Detail of (a)showing resorbed staurolite included in late pleochroic andalusite (at left). Kyanite relics in muscovite form remnants of oldermuscovite–kyanite aggregates from assemblages I and II (observe similarity with Fig. 3d). Staurolite also occurs in the matrix.(c) Large andalusite porphyroblast which includes two staurolite crystals. Kyanite is still preserved as an inclusion in the largerstaurolite at left. (d) Strained andalusite (bottom left) overgrown by radial kyanite needles within a deformed quartz vein withinassemblage III rocks. Radial mineral above kyanite is muscovite.

S3 foliation (Fig. 4a). This new andalusite is fresh, similar to those found in assemblages I and II can stillbe recognized in the matrix of this andalusite–stauro-locally pleochroic, more massive and less porphyroblas-

tic than andalusite of assemblage I, and often includes lite–biotite hornfelsic schist. Biotite first occurs in thematrix of rocks with assemblage III, defining theresorbed remnants of staurolite (Fig. 4b & c).

Andalusite of assemblage III contains an internal critical three-phase assemblage andalusite–stauro-lite–biotite (plus muscovite) which implies relativelyfabric, sometimes at a right angle to the external fabric.

Resorbed kyanite and staurolite relics also occur in restricted P–T conditions for the bulk composition ofthese rocks (And–St–Bt field in Fig. 5).the matrix (Fig. 4b) and are interpreted as remnants

of assemblage II. The staurolite may include relics of Decimetric quartz veins deformed by S3 appearwithin the rusty schist that contains assemblage III.kyanite (Fig. 4c). Lens-shaped muscovite aggregates

Fig. 3. Photomicrographs of mineral relationships in kyanite-bearing assemblages I and II from Al-rich metapelites of the VPB.(a) Relic andalusite porphyroblasts overprinting slaty cleavage S1 and replaced by kyanite–muscovite. (b) Detail of (a): kyanite isdeveloped on an andalusite crystal and in the matrix. Observe relatively large kyanite inside andalusite. No andalusite is preserved;decussate muscovite and kyanite replace the chiastolite. (c) Relic chiastolite crystals replaced by an aggregate of kyanite (highrelief ) and decussate muscovite ( low relief ). (d) kyanite–muscovite aggregate pseudomorph after andalusite. External fabric is acomposite S1+2 wrapping around the aggregate. Small chloritoid in the matrix shows pressure shadows oblique to S1+2.Rectangle corresponds to photograph (e). (e) Detail of (d): kyanite crystals are high-relief minerals on the left, the remainder ismuscovite with opaques. (f ) Matrix kyanite and chloritoid. External fabric S1+2 is bent around chloritoid radial aggregates.(g) Syn-S3 staurolite rimming kyanite which includes the earlier S1+2 fabric. (h) Kyanite and staurolite crystals surrounded andbent by S3 crenulation cleavage.


Fig. 5. KFMASH pseudosection for Al-richpelites from the VPB showing divariantthree-phase and trivariant two-phase(shaded areas) stability fields. Isopleths of Fein chloritoid (XFeCtd) together withchlorite–chloritoid (Perchuk, 1991 – P,91)and garnet–chlorite thermometry(Dickenson & Hewitt, 1986 – D&H,86;Perchuk, 1991 – P,91) were used toconstrain the paths for rocks in samplingareas 1 and 2 (Fig. 1c) of the westernmostVPB. Observed assemblages are enclosedwithin ovals and circled numbers on thetrajectories refer to sampling areas 1, 2 and 3in Fig. 1c. Mineral abbreviations here andelsewhere after Kretz (1983).

Strained andalusite overgrown by radiating kyanite resorbed by biotite occur enclosed in cordierite.Cordierite often encloses garnet, it is abundant, andand muscovite is observed in the mylonitic fabric of

these veins (Fig. 4d). Together with the remaining lens- appears as large, undeformed porphyroblasts whichare almost completely pinnitized and retrograded intoshaped muscovite aggregates and the kyanite-staurolite

relicts in the matrix, the andalusite overgrown by mica aggregates. Andalusite–biotite–muscovite, cordi-erite–andalusite–biotite–muscovite, and cordierite–kyanite represents evidence that conditions of assem-

blages I and II were overstepped. biotite–muscovite assemblages also occur ingarnet-free rocks (Table 2) in Mn-poorercompositions.

Parageneses in Al-poor psammo–pelites within the ViveroPelite Belt

Parageneses in psammo–pelites from the Lugo domePsammitic rocks poorer in Al are interlayered with theAl-rich pelites (Table 2). The psammites constitute Rocks in the Lugo dome are systematically coarser

grained than those of the VPB. They are stronglymetre-thick beds interlayered with micaceous quartziteand have been sampled in area 3 (Fig. 1c), close to the foliated, although in thin section, micas are mimetic

with the S1+2 composite foliation and quartz isVivero ductile fault zone in the same structural realmas the Al-rich pelites containing assemblage III. granoblastic and elongated. In the footwall of the

Vivero fault (i.e. along the westernmost side of theThe psammites contain either garnet-bearing orgarnet-absent assemblages (Table 2). The presence of Lugo dome), garnet-biotite-muscovite-plagioclase

(sample 31.1, Table 2) is the assemblage ingarnet is interpreted to be due to slightly higher Mnin their bulk composition (but only as high as Precambrian-Cambrian psammo-pelitic schist.

Muscovite occurs partially as a poikiloblastic, second-0.13 wt%; see sample 93 in Table 1). Garnet-bearing,low-variance assemblages are (Table 2): garnet– ary mineral so that only garnet-biotite-plagioclase has

been used in thermobarometry. Garnet does notstaurolite–chlorite–muscovite (sample 93), garnet–staurolite–andalusite–biotite–muscovite (sample 47), include an earlier fabric, and occurs partially resorbed

and wrapped by the composite S1+2 subverticaland garnet–andalusite–cordierite–biotite (sample 36).In this third assemblage, andalusite occurs as corroded fabric. Other assemblages found in the Lugo dome are

shown in Table 2.grains in the matrix, and garnet porphyroblasts


tions, different paths corresponding to the threeP–T PSEUDOSECTIONS, THERMOBAROMETRY,

different sampling areas (Fig. 1) within the VPB canAND METAMORPHIC P–T PATHS

be established. Before D2 folding, conditions insampling areas 1 and 2 (separated by 1.5 km acrossThe evolution of P–T conditions for the three sample

areas considered within the VPB, and for several other strike; Fig. 1c) should be within the andalusite stabilityfield since in both areas, post-D1, pre-D2 andalusite isareas on the west side of the Lugo dome near the

Vivero fault, have been constrained by combining P–T replaced by syn-D2 kyanite–muscovite aggregates(Fig. 3a–e) that make part of assemblage I (Table 2).pseudosections and conventional geothermometry.

P–T pseudosections were constructed using Either the divariant assemblage andalusite–chloritoid–chlorite, or the trivariant assemblage andalusite–chlor-Thermocalc 2.4 (Powell & Holland, 1988) and its most

recent thermodynamic database (August 1995). The ite is an equally valid precursor to assemblage I(kyanite–chloritoid–chlorite), as can be inferred fromubiquitous presence of graphite as inclusions and as a

matrix phase implies the presence of, at least, a the pseudosection shown in Fig. 5. Chloritoid grainsthat are partially enclosed within andalusite indicatebi-component H2O–CO2 fluid phase during metamor-

phism (Ohmoto & Kerrick, 1977; Connolly & Cesare, that andalusite–chloritoid–chlorite was the most prob-able precursor assemblage. The intersection of XFe1993). Therefore, the pseudosections were constructed

using activities of less than unity for water. A value of isopleths of chloritoid core compositions (calculatedwith Thermocalc) with constant KD chloritoid–chlorite0.9 for H2O activity in a bi-component fluid was

assumed in the same way as Pitra & Giraud (1996), exchange thermometry (Perchuk, 1991) should giveP–T conditions approaching those that were presentusing the models of Ohmoto & Kerrick (1977) and

Connolly & Cesare (1993). Ideal site mixing was during the onset of chloritoid growth. Chloritoid corestogether with either matrix chlorite or cores of larger,assumed to represent the behaviour of mineral solid

solutions, using the activity expressions in the slightly zoned chlorite crystals were used in thechloritoid–chlorite thermometry. Using this approach,Thermocalc datafiles. A corrected activity term for

staurolite was used according to Pitra & Giraud (1996) sample 42 from area 1 gives intersections that are wellwithin the kyanite field (Fig. 5). Therefore, if chloritoidin order to simulate the actual presence of components

in staurolite other than Fe and Mg (such as Zn and Ti). began to grow in equilibrium with andalusite andchlorite, its core composition in this sample has beenmodified after growth. Temperature conditions givenThe Vivero Pelite Beltby the intersection for sample 42 are therefore takenas the maximum temperature for the stability of theTwo KFMASH pseudosections were drawn for an

Al-rich rock (sample 42; Tables 1 & 2, and Fig. 5) and assemblage andalusite–chloritoid–chlorite in this rock,prior to the replacement of andalusite by kyanite.an Al-poor rock (sample 93; Fig. 6a) from the VPB. A

MnKFMASH pseudosection was also constructed for Sample 73 (Table 2) from area 2 gives an intersectionof chloritoid cores isopleths with chloritoid–chloriteAl-poor, Mn-bearing bulk compositions allowing for

the occurrence of garnet-bearing parageneses in these thermometry Kd lines near the andalusite–chloritoid–chlorite divariant field (Fig. 5), confirming texturalrocks. For comparison, two additional pseudosections

were constructed: one for another Al-rich rock (sample observations and suggesting that this is the precursorassemblage. Garnet–chlorite thermometry was applied43; Tables 1 & 2) and a second for another Al-poor

rock (sample 99; Tables 1 & 2). Only minor phase in nearby sample 64 assuming that garnet grew in achlorite–garnet rock; this assumption is reasonablefield displacements were detected and so these two

additional pseudosections are not shown. given the small size of garnet and the relatively lowtemperature given by garnet rim compositions.Geothermometry on Al-rich pelites was performed

using the Perchuk (1991) calibration on the chloritoid- Thermometry (Dickenson & Hewitt, 1986; Perchuk,1991) using garnet core compositions provides achlorite pairs of assemblage I in areas 1 and 2. For

Al-poor pelites in area 2, garnet–chlorite thermometry further temperature constraint, and the intersection ofthe obtained Kd with core chloritoid isopleth alsowas used (Dickenson & Hewitt, 1986; Perchuk, 1991).

Garnet–biotite thermometry (Hodges & Spear, 1982; supports the interpretation that, at least in area 2, theprecursor assemblage to assemblage I (kyanite–chlori-Williams & Grambling, 1990) was applied to Al-poor

rocks from area 3. All analysed garnets (see Table 3) toid–chlorite) was andalusite–chloritoid–chlorite.Thus, conditions for crystallization of the andalus-are Fe-rich, and Fe increases relatively constantly from

core to rim; Mn decreases inversely with Fe. Mg and ite–chloritoid–chlorite assemblage, prior to the anda-lusite replacement by kyanite, were between 2 andCa both have relatively flat core-to-rim profiles, but

Mg increases, at least a small amount, in the analysed 3 kbar and 390–440 °C (Fig. 5) in sampling areas 1and 2. These P–T conditions imply an early transientgarnet. The relatively concentric zoning patterns in

these garnet are typical of growth zoning in garnet that geothermal gradient of 42–48 °C km−1, and a depth ofc. 9 km for the VPB rocks prior to the transformationhas grown during a single heating event (Tracy, 1982).

By comparing observed metamorphic assemblages of andalusite into kyanite–muscovite aggregates duringthe D2 synformal folding.with their theoretical stability fields on P–T pseudosec-


Fig. 6. (a) KFMASH pseudosection showing decompression paths for Al-poor psammo–pelites from sampling area 3 in the VPBand the Lugo dome (LD). Thermobarometric determinations for the assemblage garnet–biotite–plagioclase (open circles) are usedto constrain the LD path. (b) MnKFMASH pseudosection for Al-poor, Mn-bearing psammo–pelites from area 3 of the VPB andthe LD. Garnet–chlorite and garnet–biotite thermometers are used to constrain the path followed by garnet-bearing rocks fromarea 3 (see Fig. 5 for abbreviations; Williams & Grambling, 1990 – W&G). Mn=0.40 is computed asMnO/(Al2O3+FeO+MgO+K2O) with oxides given in wt%. Labels of observed assemblages are enclosed within ovals (area 3,VPB) or rectangles (LD) in both (a) and (b). Univariant reactions labelled (*) and (**) correspond respectively to: St+Chl+Ms=Bt+Ky+Qtz and Chl+Ms+Qtz=Crd+Bt+And. Trivariant fields are shaded.

Chloritoid rims are poorer in iron than chloritoid produce higher temperature, although they show awider range of values which may suggest a largercores in area 1, whereas in area 2 they are Fe richer.

Thermometry using average matrix chlorite and chlori- temperature increase in area 2. Therefore, although theobserved replacement of pre-S2 chiastolite by kyanitetoid rims gives systematically higher temperatures than

using chloritoid cores (Table 3). Temperature changes could be equally consistent with a wide variety oftrajectories, for example isobaric cooling, chloritoidof <5 °C are produced using chlorite cores and

rims rather than average chlorite compositions. zoning plus chloritoid–chlorite and garnet–chloritethermometry made in rocks that had simultaneousFurthermore, using average matrix chlorite and garnet

rims in area 2 also yields higher temperature than growth of kyanite and chloritoid constrain the P–Tpath in areas 1 and 2 to be one of increasing P–T asusing garnet cores. Given the attitude of the chloritoid

isopleths deduced by Thermocalc, an iron depletion shown in Fig. 5.Textural observations on rocks containing assem-during increasing temperature can be achieved only

during a simultaneous increase in pressure in area 1. blage II (kyanite–staurolite–chlorite–muscovite) implyconditions on the high temperature side of theIn area 2, joining the intersections of chloritoid core

and rim with chloritoid–chlorite thermometry, a similar univariant reaction chloritoid+kyanite=chlorite+staurolite (Fig. 5). So far, assemblage II has been foundincrease in pressure is implied although coeval with a

greater increase in temperature (Fig. 5). in outcrops close to the Vivero ductile fault zone(sampling area 3, Fig. 1c), but not in rocks from theChloritoid in area 2 is clearly Fe richer than in area

1 (Table 3). This difference in mineral chemistry cannot western part of the VPB (areas 1 & 2). No reliablepressure estimate has been deduced for this assemblagebe a bulk composition effect since in both localities,

studied samples contain the identical divariant due to parallelism of staurolite and chlorite isopleths.Assemblage III (andalusite–staurolite–biotite–three-phase assemblage kyanite–chloritoid–chlorite

(Table 2). Therefore, although both have similar P–T muscovite±plagioclase) is the peak assemblage forrocks in a decametric wide strip in the hanging wallpaths, rocks from area 2 should be displaced towards

higher temperature compared with rocks from area 1 adjacent to the Vivero fault (Fig. 1c). Staurolite occursstably within the matrix and also partially resorbed in(Fig. 5). Geothermometric determinations in area 2


andalusite–biotite aggregates, indicating a staurolite- The assemblages garnet–staurolite–chlorite and gar-net–staurolite–andalusite–biotite present in psammo–consuming, prograde path across the andalusite–

staurolite–biotite divariant field (Fig. 5). Preliminary pelites seem to record conditions similar (Fig. 6b) toassemblages II and III, respectively, in Al-rich rocks.andalusite–quartz oxygen isotope thermometry gives

temperatures around 600 °C (E. Cardellach, personal The rest of the assemblages, with or without garnet,imply lower pressure equilibrium conditions. This iscommunication) for andalusite–quartz veins within

assemblage III rocks. especially evident for assemblages with cordierite–bio-tite which are restricted to pressures below 2 kbar onIn addition, along strike in area 3 near the post-

tectonic granite shown in Fig. 1(c), other Al-rich rocks the pseudosections of Fig. 6(a) & (b).contain the trivariant, post-D3 assemblage andalusite–biotite–muscovite. This assemblage seems to record

The Lugo domelower pressure, or higher temperature conditions thanrocks containing assemblage III. Rocks with andalus- Values of A/AFM for bulk compositions of psammo-

pelites from the Lugo dome (see samples 31.1 and 31.2ite–biotite–muscovite are hornfelsic schist in whichkyanite, staurolite and chloritoid relicts are still found in Table 1) are slightly lower than those of Al-poor

psammo–pelites from the VPB (Fig. 2); F/FM valueswithin andalusite.Deformed, decussate, blastic muscovite aggregates are also slightly lower. Despite these differences, the

P–T pseudosections drawn for sample 93 (from theoccur on the matrix of rocks in area 3 near the Viverofault which contain assemblages II or III and are VPB) have been used to compare paragenetic relations

of rocks in sampling area 3 of the VPB to rocks in thesimilar to the kyanite–muscovite aggregates pseudo-morphic after andalusite which characterize assemblage Lugo dome (see Fig. 6a & b). The slight differences in

A/AFM and F/FM (for example, between samples 93,I in areas 1 and 2. This similarity strongly suggeststhat rocks with assemblages II or III previously 31.1 and 40) do not produce significant qualitative

changes in the topology of the phase diagrams. Severalcontained assemblage I. Therefore, rocks near theVivero fault, in area 3 (Fig. 5), seem to have followed stability field boundaries were checked and only small

shifts towards lower temperatures were detected. Thean early path similar to that of areas 1 and 2 in thewestern part of the VPB. only significant changes were a shrink and shift of the

trivariant staurolite–biotite field towards pressuresMore psammitic and Al-poorer rocks from area 3contain the garnet-bearing and garnet-free assemblages above 8 kbar in sample 31.1.

Pseudosection stability fields for observed assem-listed in Table 2. A pseudosection for an Al-poorer bulkcomposition (see sample 93, Tables 1 & 2) containing blages in samples from the western side of the Lugo

dome footwall (Table 2) are consistent with a nearlythe garnet-free assemblages is presented in Fig. 6(a).The fact that sample 93 contains the assemblage isothermal decompressional evolution (Fig. 6a). A

likely transition from P–T conditions of the divariantgarnet–staurolite–chlorite–muscovite (Table 2) allowsgarnet to be interpreted as an extra phase, whose assemblage garnet–staurolite–biotite, which is above

8 kbar in Fig. 6(a), to sillimanite–staurolite–biotite isstability in these low-pressure rocks might be related toa local, slight Mn increase in the bulk composition. An consistent with observed textures: sillimanite aggre-

gates are present in a matrix containing resorbedadditional MnKFMASH pseudosection (Fig. 6b) wasconstructed to account for the garnet-bearing assem- staurolite and garnet, which is breaking down to

biotite. The presence of garnet in low-Mn rocksblages. Garnet–chlorite thermometry (Dickenson &Hewitt, 1986; Perchuk, 1991) for the assemblage (XMn<0.01), as in sample 40 (see Table 2), means

that garnet should have formed part of the highergarnet–staurolite–chlorite of sample 93 is in very goodagreement (Fig. 6b) with the corresponding stability pressure assemblage garnet–staurolite–biotite at press-

ures above 8 kbar. This assemblage is not found stablefield deduced with Thermocalc for assemblage II.Garnet rim-staurolite thermometry (Perchuk, 1991) in Mn-richer rocks, with similar F/FM, modelled in

pseudosection of Fig. 6(b). Garnet-free rocks (Fig. 6a)gives pressure-independent temperatures between 550and 580 °C; on the high-temperature side of the also show a sequence of decompression from stauro-

lite–biotite through sillimanite–staurolite–biotite to,garnet–staurolite–chlorite field. However, staurolite inthese rocks is Zn rich (see St93 in Table 3) which may finally, sillimanite–biotite. The presence of chlorite and

the absence of kyanite in sample 40 (Table 2) suggestinfluence Fe–Mg distribution and hence thermometry.Garnet rim plus matrix biotite thermometry using that the path crossed the staurolite–chlorite bearing

fields of Fig. 6(a), which implies slight cooling duringthe calibrations of Hodges & Spear (1982) andWilliams & Grambling (1990) in other psammitic, decompression from staurolite–biotite conditions.

Geothermobarometry was performed on psammo–Al-poorer rocks gives a temperature range from c. 550to c. 600 °C (Fig. 6b), the latter similar to andalusite- pelites of the Lugo dome close to the Vivero fault

(sample 31.1, Table 1). Simultaneous solution (Rechequartz isotope thermometry in veins that occur inAl-rich rocks with assemblage III. Conditions deduced & Martınez, 1996) of garnet core plus matrix biotite

thermometry (Hodges & Spear, 1982) with garnet–bio-by thermometry are therefore within the stability fieldsof assemblages in the P–T pseudosections. tite–plagioclase barometry (Hoisch, 1990; Fig. 6a) gives


conditions of between 8.0 and 6.5 kbar and 620–570 °C. tion in areas 1 and 2 of the VPB that contain theassemblage kyanite–chloritoid–chlorite record differentUsing resorbed garnet rims, conditions are in the range

4.1–5.1 kbar and 530–550 °C. Pressure conditions paths (Fig. 5). These paths can be linked to theintrusion of pre- to syn-D2 granites preferentiallyfor garnet cores are lower (6.4–5.8 kbar) if

the garnet–biotite–muscovite–plagioclase calibration along the Vivero ductile fault zone since area 2, locatedcloser to this heat source, records a higher temperatureof Ghent & Stout (1981) is used instead; this

difference is perhaps due to using muscovite that is evolution.During the second deformation phase D2, the VPBpartially blastic and secondary. These geothermobaro-

metric results confirm the decompressional P–T path developed into a pinched syncline between the Ollo deSapo antiform and the Lugo dome. The Valdovinofor the Lugo dome. The total decompression is about

4.0 kbar which supposes an uplift of c. 14 km for the ductile fault, marking the western border of the Ollode Sapo antiform (Fig. 1a), also developed during D2.Lugo dome.

The presence of the assemblages sillimanite (fibro- Sinistral slip on the Valdovino fault (Ponce de Leon& Choukroune, 1980) syn-kinematic with D2 causedlite)–staurolite–biotite or sillimanite (fibrolite)–biotite

(samples PG10 and JF188 in Table 2) implies a kink local tension on the western flank of the Lugo dome,and the Vivero ductile fault with an oblique normalin the earlier part of the decompression path deduced

from thermobarometry in sample 31.1 and from slip was the new structure formed to accommodateextension in the area. Thus, the Vivero fault is a localstability fields in sample 40 (Fig. 6a). A likely interpret-

ation for thermobarometry not recording heating extensional structure developed in a overall com-pressional regime. Movement on the Valdovino andduring decompression is either that the sample used

for the thermobarometry (sample 31.1) was not locally Vivero ductile faults contributed to E–W shorteningand produced the northwards escape of the crustalaffected by granite intrusion or that there was later

exchange between garnet rims and matrix biotite. block between them (Martınez et al., 1996). Asextension due to the Vivero fault occurs, the areaundergoing extension is a favourable zone for the

GEOLOGICAL SIGNIFICANCE ANDintrusion of granite that supplied heat to the surround-INTERPRETATIONing rocks, producing a high temperature gradient.

Development of the VPB into a D2 syncline causedDistinctive P–T paths are recorded by pelitic andpsammo–pelitic rocks across the VPB. Samples located downwards dragging of post-D1, pre-D2 andalusite-

bearing rocks, putting them into the kyanite stabilitynear the Ollo de Sapo antiform contain post-D1,pre-D2 andalusite in addition to chlorite and chloritoid. field. Kyanite-bearing assemblages I and II represent

the highest pressure conditions reached by rocks inThis fact implies that an anomalously high thermalgradient (of about 42–48 °C km−1 ) must have existed, the VPB. The distance in the field across strike between

rocks with assemblages I and II is only about 3 kmat least locally, which triggered a low-pressure/high-temperature pre-D2 metamorphic event. The origin of (see Fig. 1c) and no discontinuities are seen between

them. Therefore, pressure conditions of both assem-the high-temperature gradient can be related to theemplacement of an early swarm of small granites (for blages should be similar, although no reliable pressure

determinations were obtained for assemblage II due toexample, the Vicedo granite in Fig. 1c) whose intrusionpostdated D1; these granites are subconcordant to S1 the parallelism of composition isopleths.

Slip on the Vivero fault could have generated aand contain the S2 fabric.After equilibration within the andalusite field, VPB sedimentary basin, now eroded, above the sinking

hanging wall (Fig. 7b & d), similar to the small arcuatepelites underwent compression simultaneously with anincrease in temperature, as recorded by the evolving basins that outline the western boundary of antiforms

situated further east in a more external position in thechloritoid and chlorite chemistry and by kyanitereplacement after andalusite. Rocks of similar composi- chain. Isostatic equilibration during and after faulting

Fig. 7. Relations between successive tectonic, metamorphic, and igneous intrusive events in the Ollo de Sapo antiform (OS), theVivero Pelite Belt (VPB), and the Lugo dome (LD) and evolving pressure and temperature deduced for the three sampling areas ofthe VPB and for the LD. (a) Onset of D2 deformation and refolding of older D1 structures. Intrusion of sheet-like granitoids beganbefore significant burial of the VPB producing pre-D2 andalusite. (b) Nucleation of the Vivero shear zone during evolving D2 andonset of probable syntectonic sedimentation. (c) During D2, rocks of the VPB (1, 2 & 3), forming part of a synformal structure andthe hanging wall of the Vivero fault, being buried, follow a compressional heating path. By contrast, rocks in the LD decompress.(d) Cross-section A–B located in Fig. 1(b). Evolving D2 deformation accentuates refolding of D1 structures and pre-D2 lens-shapedgranitoids. Gravitational collapse of vertical D2 structures produces new, local subhorizontal D3 structures. Syn-D3 granitoidsintrude, followed by final uplift of all domains and intrusion of post-D3 granitoids. (e) Syn-D3 igneous intrusions near the Viverofault induce a local temperature increase in area 3 and the western side of the LD. Following D3, decompression and coolingoccurs in all domains. The decompressional, cooling P–T path of the LD is recorded by cascading equilibrations related to post-D3intrusions. Patterns in (d): 1: Ollo de Sapo gneiss; 2: Cambrian and Precambrian rocks of the LD; 3: Ordovician rocks of VPB; 4:Silurian rocks of the VPB; 5: foliated granitoids; 6: Vivero granite of the LD; 7: Post-D3 granitoids; 8: syntectonic UpperCarboniferous basin.


would have controlled the shape of the basin. The across the Vivero fault, together with local graniteintrusions with the Vivero fault zone itself, caused thepressure increase in the VPB would have been due, in

part, to the weight of the sediments infilling this basin metamorphism of the VPB.The occurrence of late kyanite in low-pressure/high-in a similar manner to the models by Spear (1993)

which imply folding of isotherms and link the pressure temperature metamorphic belts characterized by anda-lusite, implying lower gradients than expected, is notincrease in the synclines to sedimentation of the eroded

materials derived from adjoining anticlines. An upper exclusive of the Variscan of NW Iberia. Late kyanitehas also been locally found in the Variscan basementage for the kyanite growth in the VPB is given by the

last movement of the Vivero fault which should be of the Pyrenees (Autran & Guitard, 1970). In addition,lower gradients than those in andalusite terranes havesituated between 298±5 Ma (the U–Pb age of fault-

deformed granites; Santos et al., 1996), and 269 Ma been deduced by Ayora et al. (1993) from the study offluid inclusions in veins in metapelite along the(the K–Ar cooling age of biotite lying on the fault’s

mylonitic fabric from the Muras granite; after Ries, southern flank of the Pyrenean Canigo gneissic massif,a similar structural situation to the VPB, although in1979).

The development of D3 subhorizontal foliation and this case kyanite is not developed probably due tounfavourable rock chemistry.the passage from kyanite-bearing assemblage II to

andalusite-bearing assemblage III in the eastern part ofthe VPB appears related to intrusion of syn-D3 ACKNOWLEDGEMENTSmylonitic granites coeval with movement along theVivero fault. The passage from assemblage II–III The authors are grateful to M. Atherton for his review

of the manuscript and interest in the subject. M. Julivertconditions to those of lower pressure with stablecordierite (Fig. 6a & b), related to post-D3 intrusives is also thanked for his revision of the manuscript.

G. Galan is acknowledged for helpful comments onthat seal the Vivero fault, might have been either asshown in Fig. 5 or a two-stage path, with an initial, the granites of the study area. Thanks are also given

to two anonymous referees for their well-addressedalmost isothermal decompression followed by a nearlyisobaric re-heating to conditions of cordierite-bearing criticism. J. R. and F. J. M acknowledge financial

support by DGICYT Research Projectassemblages, similar to P–T paths arising from themodels including magmatic heat transfer of De Yoreo PB94-0684-CO2. M.L.A. was funded by DGICYT

Research Project PB94-0685. C.D. had financial sup-et al. (1989). This means that, in the decametre-sizedstrip of rock located along the eastern contact of the port from NATO I.S.E.P. Grant 890371. Thanks are

also given to J. R. Martınez Catalan for his kindVPB with the Lugo dome, coinciding with the Viverofault, different samples seem to record different stages advice on sampling sites along the Vivero fault.of a decompressional history punctuated by heatingpulses that correspond to intrusions of different ages. A

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Department of Earth and Planetary Science, Purdue University.APPENDIX: ANALYTICAL METHODS Analysis parameters were: accelerating voltage, 15 kv; sample (beam)

current, 20 nA; beam size, 5–10 microns for micas and feldspars, 1Whole rock compositional data was determined on pressed pellets micron for garnet and staurolite; correction scheme, ZAF. Commonof powdered samples by X-ray fluorescence (XRF), using the natural and synthetic standards were used for standardization:Department of Geology’s Rigaku 3070 X-ray spectrometer at the AMAL: Na; HORT: Mn, Fe, Mg; ORTH: K; ANOR: Si, Al, Ca;University of Cincinnati. Raw counts (intensities) for each element DI2TI: Ti; LZNO: Zn. Counting times were 30 seconds for mostwere converted to wt% oxides using formulas derived by multiple elements, 20 seconds for Si, Al and Na, and 10 seconds for K. Feregression of 19 USGS and internal standards. Loss on ignition was analysed as Fe+2, and is reported as Fetotal. Mineral stoichi-(LOI) was determined using standard procedures. ometry was calculated from wt% oxides by the computer program

Mineral compositions were determined on polished thin sections ‘MINFILE’ (Afifi & Essene, 1988).by electron microprobe analysis using the Cameca SX-50 at the

Evolution of a kyanite-bearing belt within a HT-LP orogen: the case of NW Variscan Iberia: EVOLUTION...

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