exhumation of high-pressure metapelites and coeval crustal …hera.ugr.es/doi/15003553.pdf ·...

ELSEVIER Tectonophysics 285 (1998) 231-252

TECTONOPHYSICS

Exhumation of high-pressure metapelites and coeval crustal extension in the Alpujarride complex (Betic Cordillera)

J.M. Azafi6n a,*, V. Garcfa-Duefias a, B. Golf6 b

lnstituto Andaluz de Ciencias de la Tierra, C.S.I.C - Universidad de Granada, Campus Fuentenueva, 18071 Granada, Spain b Ecole NormaIe Supgrieure, URA 1316 du CNRS, Laboratoire de Gdologie, 24 rue Lhomond, 75005 Paris, France

Received 4 October 1995; accepted 20 June 1996

A b s t r a c t

The current configuration of the Alpujarride complex (Befic Orogen) comprises a stack of thinned tectonic units, made up mainly of continental crest. P - T path modelling in different levels of an Alpujarride unit (the Salobrefia tectonic unit) reveals a high-pressure metamorphic stage followed by subsequent decompression at nearly isothermal conditions. Nevertheless, the P - T conditions differ from one level to another: while the top of the Permo-Triassic metapelites includes carpholite-kyanite-bearing assemblages (10 kbar/425°C), intermediate levels are characterized by Mg-rich chloritoid-Zn-rich staurolite-kyanite-bearing assemblages (minimum 10.5 kbar/450°C) and the bottom of the Palaeozoic metapelite sequence presents gamet-kyanite-plagioclase-bearing assemblages (average conditions of 13 kbar/625°C). Thus, the Salobrefia tectonic unit represents an entire upper continental segment containing several crustal layers which underwent high-pressure metamorphism during Alpine continental subduction. The decompression, associated with an important mineral growth stage and the development of fiat-lying regional foliation, reflects the thinning of this crustal segment, which resulted in parallel disposition of the bedding and metamorphic zones. At low-pressure conditions, large-scale folds affect the regional foliation, producing stratigraphic duplication, metamorphic inversions and the general reorganization of previous Alpujarride sheets. Finally, brittle Miocene extensional tectonics, coeval with the generation of the Alboran Basin, contributed to the definitive exhumation of the Alpujarride complex. © 1998 Elsevier Science B.V. All rights reserved.

Keywords : high-pressure metamorphism; crustal extension; alpine tectonics; Betic Cordillera

1. I n t r o d u c t i o n

The mechanisms by which high-pressure metamorphic rocks are transported to the surface is a problem of current debate for all collisional belts. Reconstruction of P - T - t paths offers the opportu- nity to further constrain the nature of the tectonic processes involved in the exhumation of such deep-

* Corresponding author. Tel.: +34 (58) 232900; Fax: +34 (58) 248527.

seated rocks. Recent research has assigned more and more importance to the role of extensional processes in the evolution of orogens (see, for instance, Avigad and Garfunkel, 1991; Avigad et al., 1992; Jo- livet et al., 1994) and, in particular, in the Betic-Rif cordillera (Platt and Vissers, 1989; Garcfa-Duefias et al., 1992; Balany~i et al., 1993; Jabaloy et al., 1993; Crespo-Blanc et al., 1994; Platt and England, 1994). Such processes are essentially responsible for the rapid return of a heavily thickened crust to normal thickness, and also explain the quick rise to

0040-1951/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. Pll S0040-1951(97)00273-4

232 J.M. Azafi6n et al./Tectonophysics 285 (1998) 231-252

shallow levels of deep-buried rocks, as well as being important in the thermal evolution of orogenic belts (England and Thompson, 1986; Thompson and Ridley, 1987; Dewey, 1988; Sandiford, 1989; Sandiford and Powell, 1991). In addition, preservation of high-pressure-low-temperature mineral assemblages, including carpholite, aragonite, and law- sonite, implies that the thermal gradient remained cool during a significant part of the exhumation path.

The Alboran Crustal Domain, in the internal part of the Betic Orogen, is a segment of thinned continental crust made up of tectonic complexes: the Nevado-Filabride, the Alpujarride, and the Malaguide complexes, in ascending order. Tectonic and geophysical data provide evidence that Early and Middle Miocene crustal thinning of the Alboran Domain was produced by extensional low-angle normal faults (Garcfa-Duefias and Martfnez-Martfnez, 1988; Galindo-Zaldfvar et al., 1989; Platt and Vis- sers, 1989; Garcfa-Duefias et al., 1992; Platt, 1993; Crespo-Blanc et al., 1994; Vissers et al., 1995).

In this work we analyse the tectono-metamorphic evolution of the Salobrefia tectonic unit (TU), which belongs to the Alpujarride complex. The upper part is constituted by Fe-Mg carpholite-bearing metapelites while the lower part contains kyanite- garnet dark schists, which shows that the same tectonic unit was metamorphosed under high-pressure conditions at different grades. Structural and petrological data constrain a synmetamorphic exhumation process coeval with crustal extension. This exhumation was followed by large-scale recumbent folds, still prior to Miocene brittle extensional tectonics.

2. Geological setting

The Betic-Rif ranges and the Gibraltar arc result from the Miocene juxtaposition of four tectonic domains (Fig. 1): (a) the South Iberian, and (b) the Maghrebian palaeomargins, cropping out in southern Spain and northern Africa, respectively; (c) the Flysch complex units, deposited in a deep trough of thinned crust (Biju-Duval et al., 1977; Durand- Delgfi, 1980); and (d) the Alboran Domain (Balany~i and Garcfa-Duefias, 1987), a collisional ridge, itself composed mainly of the three aforementioned nappe complexes.

Lithostratigraphic successions of the units be-

longing to the Nevado-Filabride and Alpujarride complexes are quite similar: a Palaeozoic metapelite sequence and a Permo-Triassic metapelite and quartzite succession, with the youngest-preserved rocks in the units of both complexes being carbonate rocks of Triassic age. The Malaguide complex contains a post-Triassic sedimentary record up to the Palaeogene. Higher units of the Nevado-Filabride and Alpujarride complexes also have similar Alpine metamorphic evolutions from an early high-pressure episode (Puga et al., 1989; Bakker et a1., 1989; Goff6 et al., 1989; Tubfa and Gil Ibarguchi, 1991; Azafi6n and Goff6, 1991; Azafi6n et al., 1994). In contrast, Malaguide unit rocks preserve Hercynian orogenic imprints (Chalouan and Michard, 1990) and its Mesozoic-Palaeogene cover has not under- gone pervasive deformation or metamorphism.

2.1. The Alpujarride complex

The Alpujarride complex comprises most of the Alboran Domain. Rocks belonging to this complex crop out almost continuously in the Betics along 400 km from east to west (Fig. 1). Comparison of current lithologic sequences between different Alpujarride units reveals many similarities, making it possible to establish a type sequence. The most complete lithostratigraphic sequence includes the following formations, from top to bottom: The carbonate formation, the phyllite formation (fine-grained schists), the schist succession, and the gneiss and migmatite formation.

The Alpujarride units, around the Nevado- Filabride tectonic window in the central sector of the Betics, were compiled and grouped by different authors (Aldaya et al., 1979; Tubfa et al., 1992). Recently, Azafi6n et al. (1994) inferred a total of five major allochthonous tectonic sheets, constrained basically by their high-pressure metamorphic recording, grouped into (Fig. 2): lower Alpu- jarride units (Lujar-Gador tectonic sheet), recording low-pressure-low-temperature metamorphism (Aza- fi6n, 1994); middle Alpujarride units (Escalate tectonic sheet), including carpholite-chloritoid-bearing assemblages in Permo-Triassic rocks (Azafi6n and Goff6, 1997); high Alpujarride units (Herradura, Sa- lobrefia, and Adra tectonic sheets, from bottom to top) showing higher P - T conditions (Azafi6n, 1994).

J.M. Aza~6n et al./Tectonophysics 285 (1998) 231-252 233

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J.M. Azah6n et al. / Tectonophysics 285 (1998) 231-252 235

2.2. Tectonic boundaries o f the Alpujarride units

The tectonic boundaries between different slabs within the Alpujarride complex have classically been attributed to nappe tectonics. This attribution was based on superposition criteria, as Palaeozoic medium-high-grade metamorphic rocks lie tecton- ically over the Triassic and Permo-Triassic rocks metamorphosed in low-grade conditions. However, it is now generally accepted that many of these contacts (Fig. 2) are extensional (Garc/a-Duefias and Mart/nez-Martfnez, 1988; Galindo-Zald/var et al., 1989; Garcfa-Duefias et al., 1992; Crespo-Blanc et al., 1994; Crespo-Blanc, 1995; Vissers et al., 1995). The geometric relationships between the regional foliation and the faults, as well as important omissions associated with these contacts, give evidence that these tectonic boundaries are low-angle normal faults (LANF) that developed in brittle conditions. Thus, the units defined in the Alpujarride complex are actu- ally extensional slabs separated by LANF belonging to two fault systems (Fig. 2): (1) the Contraviesa normal fault system (CNFS) with a north-north- westward movement, Langhian in age; and (2) the Filabres extensional system with a south to southwestward movement, which developed during the Serravallian (Garcia-Duefias et al., 1992; Crespo- Blanc et al., 1994). The faults of the Filabres extensional system coalesce to form a single detachment between the Nevado-Filabride and the Alpujarride complexes (Garcia-Duefias and Martfnez-Martfnez, 1988; Galindo-Zaldfvar et al., 1989; Vissers et al., 1995). Both fault systems are responsible for the brittle thinning and even local disappearance of some Alpujarride units (Fig. 2). Although part of the current thinning in the Alpujarride units may be related to the Miocene extensional tectonics, many features of the tectono-metamorphic history of the Alpujarride rocks indicate an earlier phase of ductile thinning (Balanyfi et al., 1993).

3. Lithostratigraphic sequence of the Salobrefia tectonic unit

The Salobrefia TU occupies a high structural position in the Alpujarride complex and crops out in the central sector of the Internal Betics (Fig. 2). The top of the unit is constituted by a carbonate forma-

tion (Fig. 3) with a maximum thickness of 800 m, dated as Middle and Late Triassic (Kozur and Simon, 1972; Delgado et al., 1981; Braga and Martfn, 1987). Below this formation a metapelite sequence crops out.

Fine-grained schists usually called 'phyllites' or 'phyllite formation' occur in the high levels of the metapelite sequence (e.g., Aldaya et al., 1979). They are mainly constituted by chloritoid-kyanite fine- grained schists in which relics of carpholite appear. These schists, which occasionally include calcschists in transition with the marbles, have a maximum thickness of 600 m (Fig. 3) and are generally attributed to the Permo-Trias.

Below these fine-grained schists, a schist succession with interbedded metaquartzites crops out. The higher part of the schist succession comprises biotite-bearing light-coloured metapelites and metaquartzites (maximum thickness of 400 m). Under these metaquartzites, a monotonous dark- coloured metapelite succession crops out (maximum thickness of 2000 m). The higher levels of this succession are constituted by garnet-staurolite schists whereas the lower levels are characterized by the presence of sillimanite (Fig. 3). In order to establish the tectono-metamorphic evolution of the Salobrefia TU, three levels within the metapelite sequence (the lower and upper parts of the chloritoid- kyanite schists and the lower part of the sillimanite schists) have been investigated.

4. Main deformations and related microstructures in the Salobrefia tectonic unit

The structures observed in the Salobrefia TU are similar to those recognisable in all the Alpujarride units in which the main foliation is associated with the D2 deformation phase (e.g., Aldaya et al., 1979; TuNa et al., 1992; Balanyfi et al., 1993).

The oldest foliation ($1) is preserved only within porphyroblasts and lens-shaped quartz preserved from the regional foliation. In the chloritoid schists within quartz domains, carpholite is occasionally associated with kyanite or chloritoid (Tables 1, 2; Fig. 4A). In the sillimanite schists, garnet, plagioclase, and kyanite porphyroblasts rounded by the $2 regional foliation include schistosity representing the $1 foliation (Table 3; Fig. 4B).

236 J.M. Aza~dn et al./Tectonophysics 285 (1998) 231-252

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Fig. 3. Lithological and structural map of the Salobrefia area (according to Avidad and Garcfa-Duefias, 1981; Simancas and Campos, 1993; and own data). Stereographic plots of axis folds belonging to D3 deformation phase. Representative samples: 1 = SAL-6; 2 = SAL-61; 3 = AL/q-IA; 4 = MSA-2; 5 = 93325-10.

J.M. Aza~6n et al. / Tectonophysics 285 (1998) 231-252 237

Table 1 Relationship between mineral growth and deformation phases in upper levels of chloritoid schists

Upper levels of the Phyllite formation D1 192 193

Post [ Syn Post Fe-Mg Carpholite Chloritoid m

Kyanite Chlorite

Phengite Pyrophyllite Sudoite

iiiii~ii::iii!i!i!i~iiiii:

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The S 2 foliation is parallel to the lithological contacts and it is generally flat-lying. The foliation characteristics vary according to the level at which it occurs. In the higher part of the metapelite sequence (chloritoid schists), the foliation is slaty cleavage (Fig. 4A) marked by white mica, chlorite, chloritoid, and pyrophyllite (Table 1). In the transitional levels to biotite schists, $2 includes chloritoid, kyanite, Zn-staurolite and white mica as main metamorphic minerals (Table 2). In the sillimanite schists, the $2 foliation is schistosity defined by biotite, staurolite, kyanite, garnet and sillimanite (Table 3).

In the Salobrefia TU, both $2 and metamorphic

Table 2 Relationship between mineral growth and deformation phases in lower levels of chloritoid schists

Lower levels of the phyUite formation D1 1)2

Fe.Mg Carpholite

Chloritoid

Kyanite

Zn-rich Staurolite

Chlorite Phengite Margarite Andalusite Epidote Cookeite Rutile

Syn Post Post

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D3

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Table 3 Relationship between mineral growth and deformation phases in lower levels of sillimanite schists

Dark schists formation

Staurolite

Kyanite G a r n e t

Sillimanite

Phengite Epidote Plagioclase

Andalusite Biotite

D1 1)2 D3

Syn Post ~yn Post ~

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mineral zones are deformed by large-scale over- turned folds that may even invert them locally (Figs. 3, 4C). The axial orientation of these folds ranges from N50E to N90E (Fig. 3). In the upper part of the chloritoid schists an $3 crenulation cleavage marked by chlorite and phengite growth is associated with these folds (Table 3). In the lower part of the chloritoid schists syn- and post-S3 andalusite has been observed (Fig. 5B). In the sillimanite-bearing schists of the Salobrefia TU, biotite and sillimanite aggregates occasionally mark the axial plane of these crenulation folds in the sillimanite schists (Table 3; Fig. 5C). The syn-S3 growth of staurolite has also been documented (Simancas and Campos, 1993). In these metapelitic levels, andalusite growth postdate crenulation folds and associated schistosity (Table 3; Fig. 4D).

5. Metamorphism and P-T-deformation path in the Salobrefia tectonic unit

The main characteristic of the Salobrefia TU is the presence of different metamorphic zones in the metapelitic sequence (see previous section). Iso- grades sketched across the lithological sequence are parallel to the $2 foliation (see map, Fig. 3). P - T conditions for the metamorphic evolution in different parts of the Salobrefia TU were estimated by calculation of phase equilibria using PTAX, a further development of GEO-CALC software (Brown

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J.M. Azafi6n et al./Tectonophysics 285 (1998) 231-252 239

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240 J.M. Aza~6n et al . / Tectonophvsics 285 (1998) 231-252

et al., 1988), with the internally consistent data set by Berman (1988), complemented by consistent thermodynamic properties for Fe-Mg carpholite and sudoite (Vidal et al., 1992). The thermodynamic data used for Mg-chloritoid were obtained from B. Patrick and R.G. Berman (unpubl. data, 1989) and also used in the work by Vidal et al. (1992). Ac- tivity models for Fe-Mg carpholite, chloritoid, and chlorite (Azafi6n and Goff6, 1997) are based on simple ideal site solution as in Vidal et al. (1992). The garnet-activity model used is from Ganguly and Saxena (1985). All calculations have been made in the chemical system SiO2-AI203-MgO-FeO- K 2 0 - H 2 0 (KFMASH) system. Some representative chemical analyses are given in Table 4 (complemen- tary chemical analyses in Appendix A).

5.1. Upper part of the chloritoid schists: carpholite-chloritoid-kyanite-bearing assemblages

Preservation of Mg-carpholite crystals within pre-S2 quartz lenses in the upper part of the chloritoid schists evidence an early high pressure- low-temperature metamorphic event associated with the DI deformation. P - T conditions attributed to the metamorphic peak of this event can be established taking into account: (a) the chemical composition of the carpholite and the ferromagnesian minerals re- sulting from its breakdown (chloritoid and chlorite); and (b) the calculation of phase equilibria in the P - T space (Fig. 6).

The partitioning of Fe z+ and Mg into various mineral pairs shows consistent values (e.g., Table 4),

Table 4 Chemical analyses of representative mineral phases included, respectively, in rocks of: top levels of chloritoid schists, bottom levels of chloritoid schists, and bottom levels of sillimanite schists

Mineral: Carpholite Chloritoid Chloritoid Chlorite Staurolite Sample: SAL-6 SAL-6 SAL-61 SAL-61 SAL-61

Struct. level: Top chloritoid-lyanite Bottom chloritoid-kyanite schists

schists

Staurolite Biotite Garnet Plagioclase 93325-10 93325-10 93325-10 93325-10

Bottom sillimanite schists

SiO2 41.18 24.26 25.86 26.48 28.14 27.77 34.81 36.79 64.25

A1203 31.90 38.54 44.82 24.91 54.45 53.04 20.04 21.05 21.97

TiO2 0.20 0.00 0.00 0.03 0.07 0.65 2.67 0.15 0.00

FeO 6.11 22.03 17.36 15.69 5.00 12.66 22.02 33.13 0.01

MnO 0.02 0.44 0.34 0.02 0.06 0.43 0. l 8 1.85 0.00

MgO 9.68 4.22 7.10 21.03 1.73 1.13 8.41 1.34 0.00

CaO 0.00 0.00 0.00 - 0.00 0.18 5.97 3.18

N a 2 0 0.00 0.00 0.02 - - 0.04 0.29 0.06 9.89

K 2 0 0.00 0.00 0.00 - - 0.00 8.41 0.00 0.22

ZnO . . . . 8.06 0.03 - - -

F 0.87 . . . . . . . . Total 89.96 89.49 95.50 88.59 97.51 95.76 95.19 100.35 99.52

0 2 - 8 12 12 14 48 48 22 12 8

Si 2.091 2.054 1.985 2.618 7.786 8.181 5.343 2.968 2.848

AI 1.972 3.846 4.054 2.903 17.764 18.409 3.623 2.001 1.148

Ti 0.008 0.000 0.000 0.002 0.015 0.145 0.309 0.009 0.000

Fe 2+ 0.241 1.426 1.114 1.297 1.117 1.596 2.825 2.155 0.000

Fe 3+ 0.027 0.134 0.000 - 0.041 1.523 0.080 -

Mn 0.001 0.032 0.022 0.001 0.015 0.109 0.023 0.127 0.000

Mg 0.757 0.533 0.813 3.099 0.714 0.497 1.545 0.161 0.000

Ca 0.000 0.000 0.000 - - 0.001 0.002 0.516 0.151

Na 0.000 0.000 0.002 - - 0.023 0.087 0.009 0.850

K 0.000 0.000 0.000 - - 0.000 1.647 0.000 0.012

Zn - - - 1.646 0.002 - - -

F - 0.140 . . . . . . .

Fe 3+ of carpholite estimated according to Theye et al. (1992). Fe 3+ of garnet, chloritoid, and staurolite estimated according to Droop (1987).

J.M. AzafiSn et al. / Tectonophysics 285 (1998) 231 -252 241

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Fig. 6. P - T - t paths o f Salobref ia s t ructural levels (see text for explanat ions) . Da rk t r iangles on d i a g r a m s are on invanan t points for

specific compos i t ions o f phases. Depth scales are shown for average densi ty of 2850 kg /m 3. Abbrevia t ions : A n d = andalusi te; Blot =

biotite; Car = carphol i te ; Chl = chlori te; Ctd = chlor i toid; M s = muscovi te ; Qtz : quartz; St : s tauroli te; Prl = pyrophyl l i te ; Grt = garnet ; Kaol : kaolinite; Ky = kyani te ; Si = si l l imanite; W : water. Si : 3.1 and 3.15 content o f phengi tc . React ion list. Assemblages

on r ight are stable on h igher t empera ture side for vert ical react ions. 1, Prl = Ky + Qtz + W; 2, Prl = A n d + Qtz + W; 3, Ca r = Ky + Chl + Qtz + W; 4, Ca r : Ctd 4- Qtz + W; 5, Sud + Qtz = Prl + Chl + W; 6, Sud + Qtz = C a r (stable on h igher pressure side); 8,

Ctd + Ky : St + Qtz 4- W; 9, Ms 4- Ctd = Grt 4- Blot 4- St 4- W; 10, St 4- Qtz = Grt 4- Ky 4- W; 11, St 4- Qtz : Grt 4- Si 4- W; 12,

St 4- Qtz = Grt 4- A n d 4- W: 13, Ctd 4- A n d = St 4- Qtz + W.

242 J.M. Aza~6n et al./Tectonophysics 285 (1998) 231 252

thus indicating attainment of equilibrium (Azafi6n and Goff6, 1997). The KD distribution coefficient of Fe 2+ and Mg in coexisting minerals reveals itself to be strictly comparable to those estimated in other areas with a similar metamorphic grade (Theye et al., 1992). In the upper part of the metapelitic sequence, the coexistence of Fe-Mg carpholite, chlorite, chloritoid, and kyanite (for a specific composition of these phases) allows us to establish an invariant point in the P-T diagram around 10 kbar and 425- 450°C (Fig. 6). During the metamorphic peak, a slight increase of temperature favours the growth of kyanite and chloritoid via the breakdown of Fe-Mg carpholite (Fig. 5A, Fig. 6, reactions 3 and 4).

The breakdown of high-pressure assemblages mainly implies the growth of phyllosilicates (mica and pyrophyllite) and chlorite (sudoite, di-trioctahe- dral chlorites and cookeite; Azafi6n, 1994). Retro- grade breakdown of Fe-Mg carpholite to pyrophyllite + chlorite (Fig. 6) and kyanite to pyrophyllite (Fig, 6, reaction 1) was texturally recognized. The growth of sudoite from pyrophyllite 4- chlorite (Fig. 6, reaction 5) has also been observed.

The $2 foliation is mainly marked by white mica and pyrophyllite. A nearly isothermal decompression path close to the upper thermal stability field of pyrophyllite explains the breakdown of kyanite and carpholite to pyrophyllite (Fig. 6, reaction 2). The growth of post-S2 sudoite on pyrophyllite crystals (Fig. 6) is in agreement with a slight cooling during the decompression path. $2 foliation is generated between the first growth of pyrophyllite and the first growth of sudoite (Table 1), which means that the main deformation phase in these rocks developed at approximately 425°C/8 kbar and 400°C/4.5 kbar (Fig. 6). Finally, the $3 crenulation cleavage, marked only by mica and chlorite, developed in the stability field of sudoite. At these levels andalusite has never been observed.

perature of 450°C (Fig. 6; reaction Ctd 4- Qtz = Ky 4- Chl 4- W) is constrained from the calculated breakdown of chloritoid into kyanite and chlorite (e.g., Table 4; Fig. 6, reaction 3). The KD distribution coefficient of coexisting chlorite and chloritoid (associated to kyanite) is 7.8 4- 0.45 (Azafi6n and Goff6, 1997). These values agree with those found in blueschists and eclogites (Ghent et al., 1987; Theye et al., 1992), and support the existence of a chemical equilibrium in this mineral pair.

Coeval with the breakdown of Mg-rich chloritoid is the first appearance of Zn-rich staurolite (Tables 2 and 4). The presence of staurolite could constrain the minimum temperature attained (>525°C for pressures higher than 9 kbar). However, the occurrence of Zn-rich staurolite cannot be used to constrain thermal conditions of metamorphism since natural occun'ences show that the staurolite stability field expands particularly towards low-temperature conditions due to the influence of Zn (Sartori, 1988; Soto and Azafi6n, 1994).

The massive growth of pre- to post-S2 kyanite (Table 2) in quartz-lens domains is characteristic of the lower levels of the chloritoid schists. Thus, the decompression path at these structural levels contin- ues within the kyanite stability field up to a post-S2 stage (Fig. 6). Consequently, the temperature during the main deformation phase is always greater than 400°C (pyrophyllite-out and kyanite-in; Fig. 6, reaction 1). Minimum pressure of 4 kbar (for a temperature of 400°C) for the $2 development can be obtained from the Si content (3.15) of syn-S2 phengite (Fig. 6; Massone and Schreyer, 1987). Finally, andalusite porphyroblasts always occupy a post-S2 textural position although they can be syn- or post-S3 (Fig. 5B).

5.3. Sillimanite schists with garnet-lo.,anite-plagioclase-bearing assemblages

5.2. Lower part of the chloritoid schists: Mg-rich chloritoid-Zn staurolite-kyanite-bearing assemblages

The preservation of pre-S2 Mg-rich chloritoid (e.g., Table 4) points to high-pressure conditions (Ganguly, 1972; Chopin and Schreyer, 1983). Min- imum pressure of 10.5 kbar for a minimum tern-

The calculated mineral reactions (Fig. 6) for these rocks are in good agreement with those proposed for pelitic rocks in the same chemical system by Spear and Cheney (1989). The P-T conditions of the staurolite-garnet ( a a h n a n d i n e = 0.467; Table 4)- chloritoid-kyanite-quartz invariant point are 10.3 kbar and 540°C, respectively (Fig. 6). On the ba- sis of these mineral reactions, the pre-S2 garnet,

J.M. Azah6n et al./Tectonophysics 285 (1998) 231 252 243

kyanite, and plagioclase mineral association is stable from a minimum temperature of 540°C and a minimum pressure of 6 kbar (Fig. 6). In order to constrain pressures and temperatures, we applied the GASP (garnet-aluminosilicate-plagioclase) geo- barometer and GARB (garnet-biotite) geothermometer, respectively, to different samples containing the pre-S2 kyanite-garnet-plagioclase-biotite assemblage in the sillimanite schists. As the garnet and plagioclase porphyroblasts include a helicitic $1 foliation, we assumed that the D1 deformation phase developed in these P - T conditions. The most consistent temperatures for this episode were obtained with the GARB geothermometer for garnet cores and matrix biotite (Table 4 and annexes in Appendix A). The average results are 610 + 30°C (average, standard deviation) using two calibrations for this geothermometer (Table 5) (Perchuk and Larent'eva, 1983; Ganguly and Saxena, 1985). The

cores of garnet and plagioclase relics and, in some cases, plagioclase inclusions within garnet (Table 4 and annexes in Appendix A) were used to obtain pressure values at the metamorphic peak. The average results calculated, using two calibrations of the GASP barometer (Table 5) (Koziol and New- ton, 1988; Powell and Holland, 1988), are 13 -4- 0.7 kbar at 625°C (average, standard deviation). Textu- ral features provide evidence that staurolite growth from kyanite and garnet was mainly syn-kinematic with respect to the $2 foliation (Table 2). The first appearance of staurolite in these structural levels can be explained by the retrograde reaction (Fig. 6, reaction 11): kyanite + almandine + water = staurolite + quartz. The garnet-staurolite geo-thermo- barometer constrains the temperature conditions at which this equilibrium is surpassed (Perchuk, 1977). The garnet-staurolite pair used as a geothermometer gives results between 570°C and 590°C (e.g., Table 4

Table 5 GASP (garnet-aluminosilicate-plagioclase) geobarometry and GARB (garnet-biotite) geothermometry on representative mineral pairs of three samples belonging to the bottom of sillimanite schists

Sample: 93325-10 93325-10 Point Grt: 2 17 Point Plg: 69 71

93325-10 ALN-1A ALN-IA ALN-IA MSA-2 MSA-2 MSA-2 21 37 42 43 132 120 128 74 12 1 10 157 154 158

GASP XGr 0.155 0.176 0.169 0.133 0.132 0.132 0.179 0.157 0.169 XAIm 0.734 0.723 0.736 0.763 0.762 0.763 0.696 0.729 0.720 Xpy 0.053 0.051 0.053 0.087 0.087 0.036 0.048 0.072 0.067 Xan 0.148 0.149 0.150 0.156 0.166 0.165 0.134 0.140 0.144

T (°C) Koziol and Newton (1988) 625 625 625 625 625 625 625 625 625 P (kbar) Powell and Holland (1988) 12.3 12.8 12.6 11.4 11.1 11.1 13.5 12.7 12.9 P (kbar) 12.6 13.0 12.9 11.9 11.6 11.7 13.6 13.0 13.1

Sample: 93325-10 93325-10 93325-10 ALIq-IA ALIq-IA ALlq-IA MSA-2 MSA-2 MSA-2 Point Grt: 1 21 12 37 42 3 120 134 128 Point Bt: 8 64 62 3l 34 16 155 111 110

GARB Fe 2+ Grt 2.300 2.235 2.312 2.294 2.300 2.335 2.223 2.333 2.211 Mg Grt 0.174 0.161 0.170 0.260 0.261 0.267 0.220 0.213 0.205 Ca Grt 0.505 0.514 0.514 0.399 0.398 0.348 0.479 0.448 0.521 Mn Grt 0.054 0.126 0.048 0.054 0.059 0.074 0.127 0.061 0.136 Fe 2+ Bt 2.912 2.715 2.824 2.666 2.505 2.815 2.697 2.800 2.779 Mg Bt 1.589 1.429 1.524 1.682 1.626 1.626 1.852 1.563 1.644

P (kbar) 13 13 13 13 13 13 13 13 13 T (°C) Perchuk and Larent'eva (1983) 584 581 581 633 627 652 591 614 606 T (°C) Ganguly and Saxena (1985) 588 588 584 646 636 678 587 629 621

XAIm = Fe/(Ca + Fe + Mn + Mg); XGr = Ca/(Ca + Fe + Mn + Mg); Xpy = Mg/(Ca + Fe + Mn + Mg); SAn = C a / ( C a -t- K + Na).

244 J.M. Azaffdn et al./Tectonophysics 285 (1998) 231-252

and annexes in Appendix A), for which the albre- mentioned retrograde reaction is placed between 6.5 and 9 kbar in the P - T diagram.

Further pressure constraints for $2 development may be obtained from two different sources. First, syn-S2 phengite (Si = 3.15) barometry (Massone and Schreyer, 1987) indicates a minimum pressure of 5 kbar at 600°C (Fig. 6). Second, fibrolite crystals occasionally mark the $2 foliation. Then, the D2 deformation phase should have continued at least to below 5.5 kbar (at 580°C) for the kyanite-sillimanite transformation (Fig. 6). These data emphasize the fact that, at these structural levels as well, the S, foliation took place during a decompression path at nearly isothermal conditions (Fig. 6).

In the sillimanite schists, the P - T conditions during the D 3 deformation phase can be well-constrained. $3 schistosity, that is the axial plane of the crenulation folds, is mainly formed by biotite. Fibrolite aggregates, occasionally observed along S~ (Fig. 5C) indicate that during D3 deformation the temperature was still >500°C (sillimanite stability field; Fig. 6). A maximum temperature of 590°C (upper thermal stability of staurolite in the sillimanite field; Fig. 6) is inferred during D3 deformation, as staurolite is syn-S3.

Andalusite growth generally postdates the D3 deformation phase; however, locally andalusite crystals are affected by folds from this phase, which consequently must have developed close to the univariant andalusite-sillimanite reaction. Pressure constraints for the D3 deformation phase can be obtained from the Si content of phengite included in the axial plane of crenulation folds. A minimum pressure (Massone and Schreyer, 1987) of 3.5 kbar has been calculated (Si = 3.1; Fig. 6). In short, P - T conditions for the D3 deformation phase at these structural levels are 500-590°C and 3-4 kbar, respectively.

6. Discussion and conclusions

6.1. Tectono-metamorphic evolution (?[ Salobrefia tectonic unit

Rocks from the Salobrefia TU show evidence of a progressive increase in metamorphic grade down- wards in the sequence. This unit exhibits metamorphic zoning related to a medium-pressure growth

stage. However, data presented in this paper show that a previous high-pressure metamorphism, traces of which are preserved in different structural levels, affected the Salobrefia TU during the Alpine orogeny. The pressure difference between the preserved top (chloritoid schists; 10 kbar/425°C) and bottom (sillimanite schists; around 13 kbar/625°C) of the metapelitic sequence is approximately 3 kbar for the high-pressure event, which suggest that Sa- lobrefia TU comprises rocks belonging to different levels of an upper continental crust. Thus, the vertical variation in P - T conditions can be equated to burial depth.

We can therefore infer a high P / T ratio (low geothermal gradient; <17°C/km) during the high- pressure event. The metamorphic records of all three of the levels studied confirm this low geothermal gradient. This gradient is similar to those found in Alpine-type collisional belts (Chopin et al., 1991; Avigad et al., 1992; Theye et al., 1992; Jolivet et al., 1994).

The subsequent exhumation of the Salobrefia TU is reflected in the rocks by a significant pressure drop in the P - T path of the three selected structural levels. The penetrative structure generated in this second deformation event is the S: foliation. Metamorphic and structural data suggest that exhumation of the Salo- brefia TU was accompanied by vertical shortening, in ductile conditions, which affected the metamorphic zones and induced the parallelism between the bedding and isogrades. In the three selected structural levels of the Salobrefia TU, thermobarometric criteria (see above) indicate that the decompression path is accompanied by nearly isothermal conditions (Fig. 6). The persistence of similar temperatures at lower pressures could induce an increase in the geothermal gradient (approximately 10°C/km) during the development of the D2 deformation phase.

Alter the syn-S2 thinning, plurikilometric recumbent folds belonging to the D3 deformation phase affect the bedding and isogrades producing strati- graphical and metamorphic inversions in the Salo- breffa TU (Fig. 3). $3 crenulation cleavage is the main pervasive structure associated with these folds. Petrological and textural data suggest that at the start of this process, a difference of approximately 150°C in temperature and less than 2 kbar in pressure still existed between the top and the bottom of the

J.M. Aza~6n et al./Tectonophysics 285 (1998) 231 252 245

metapelite sequence (Fig. 6). It is difficult to deter- mine the exact P-T path after the beginning of this second thickening event since there are no mineral reactions to constrain thermobarometric conditions.

After this D3 compressional event, there must have been an abrupt change to extensional conditions in the Salobrefia TU. Large-scale folds are cut by brittle detachments (Fig. 3). Geometric relationships between $2 or $3 as the reference surface, and the current tectonic contacts indicate that these latter are Miocene normal faults belonging to the Contraviesa (northward movement and Langhian in age) and Filabres (southwestward movement and Serravallian in age) extensional systems (Garcfa-Duefias et al., 1992; Azafi6n et al., 1994; Crespo-Blanc et al., 1994).

6.2. Geological implications for the Alpujarride evolution

(1) Since oceanic crustal rocks were not involved in the process, the high-pressure metamorphism in the Salobrefia TU must therefore have been produced in a subduction-collision continental setting during the Alpine orogeny (previous to 25 Ma; Moni6 et al., 1991). During a pre-collisional stage, the Salobrefia TU, as a part of the Alpujarride crustal realm, was probably subducted underneath a continental block before full collision occurred. The top continental block probably included rocks belonging to the current Malaguide complex. Traces of ductile deformation associated with this collision are trapped in relic minerals and lens-shaped domains preserved from the late tectono-metamorphic overprinting. However, the direction and other details of this early collision event remain to be clarified.

(2) The penetrative flat-lying regional foliation as well as a significant associated mineral growth was generated during the exhumation process of the Alpujarride sheets. Extremely thinned metamorphic mineral zones are a typical feature of the Alpujarride units (Torres-Roldfin, 1981; Cuevas, 1990; Cuevas and TuNa, 1990; De Jong, 1991; TuNa et al., 1992; Balanyfi et al., 1993) which can be mainly attributed to this D2 extensional deformation phase.

(3) Let us assume that the D3 folding could have been generated as a continuation of the vertical shortening process in an extensional context. In this

regard, the formation of recumbent folds during a synorogenic crustal extension has been reported by some authors (e.g., Malavieille, 1987; Froitzheim, 1992). However, large-scale folding associated to an extensional event requires either a slight angle between the folded planar surfaces and the vertical shortening direction (Froitzheim, 1992), or a large- scale simple shear zone to generate 'a ' type folds (e.g., Malavieille, 1987). None of these mechanisms seem likely to explain the D3 folding. As mentioned above, the D2 thinning tends to make all the previous planar structures parallel, leaving them perpendic- ular to the direction of maximum shortening. It is therefore difficult to explain a low angle between the planar reference structures and the direction of vertical shortening. On the other hand, there is no evidence of a large-scale simple shear zone across the Salobrefia sheet.

In contrast, stratigraphic duplication and even metamorphic inversions can clearly be appreciated in an examination of the tectonic units that limit the rocks belonging to the Salobrefia sheet (Fig. 3). For instance, in the NE part of the map presented in Fig. 3, Palaeozoic sillimanite schists (and locally migmatite gneiss) of a klippe belonging to the Adra sheet overlie marbles and chloritoid fine- grained schists from the Salobrefia TU. Metamorphic isogrades in these klippes indicate that the metapelite sequence is inverted (Fig. 3). These stratigraphic and metamorphic inversions can only be related to folds and probably associated thrust structures that affect the medium-pressure metamorphic zoning. Never- theless, no ductile thrusts have been found in this area. If thrust structures existed, they and associated slip kinematic indicators must have been destroyed by late extensional shearing. Consequently, we suggest that the D3 deformation phase developed in a compression event at low-pressure conditions, which could be responsible for the current inverted disposition of the Alpujarride complex, with lower-seated units (Lujar-Gador sheet; Azafi6n et al., 1994) in the lower structural position (Fig. 1).

(4) Brittle extensional faults which cross-cut the folds and produce omissions in a top-to-the-north or in a top-to-the-southwest directed movement (cross- section in Fig. 3) produced the definitive exhumation during the Miocene rifting (see Section 2). There- fore, they contributed to the present configuration

246 J .M. Azaf i6n et a l . / T e c t o n o p h y s i c s 285 (1998) 231 252

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250 J.M. A zai~dn et al . / Tectonophysics 285 (1998) 231-252

of the A l p u j a r r i d e uni ts ; tha t is, as n u m e r o u s lens-

s h a p e d e x t e n s i o n a l un i t s w i d e l y d i s t r ibu ted (e.g., A1-

daya et al., 1979; T u N a et al., 1992; Balanyf i et al.,

1993; Azaf i6n et al., 1994) t h r o u g h o u t the In te rna l Be t i c s (Fig. 2).

In s u m m a r y , the da ta and c o n c l u s i o n s p r e s e n t e d

in this p a p e r show tha t the t e c t o n o - m e t a m o r p h i c

evo lu t i on o f the A l p u j a r r i d e un i t s has a l t e r na t i ng

c o m p r e s s i o n a l and e x t e n s i o n a l s tages. T h e h y p o t h e t -

ical m o d e l (Vissers et al., 1995) p r o p o s e d r e c e n t l y to

exp l a in the e v o l u t i o n o f th is s e g m e n t o f c o n t i n e n t a l

c rus t in the con t ex t o f the Be t i c Cord i l l e ra , does not

c o n s i d e r the d i f f e ren t even t s p r e s e n t e d in this paper.

In par t icu lar , the s e c o n d s t ack ing even t at the end

o f the d e c o m p r e s s i o n pa th tha t p r o d u c e d the pre-

M i o c e n e o r g a n i z a t i o n o f the A l p u j a r r i d e complex .

A s im i l a r la te fo ld ing and t h ru s t i ng s tage (Garcfa -

Duef ias et al., 1988; De Jong , 1991) tha t s u p e r p o s e d

l o w - p r e s s u r e rocks ove r h i g h - p r e s s u r e ones can be

in fe r r ed in the N e v a d o - F i l a b r i d e complex .

Acknowledgements

T h e m a n u s c r i p t has bene f i t ed f r o m v a l u a b l e c o m -

m e n t s by D. A v i g a d and A. C r e s p o - B l a n c and he lp fu l

r ev i ews o f D. M a r q u e r and A. Okay. T he F r e n c h -

S p a n i s h c o o p e r a t i o n a n d the D G I C Y T p ro j ec t P B 9 2 -

0 0 2 0 - C 0 2 s u p p o r t e d the f ie ld and l abo ra to ry re-

search . T h e he lp o f C. L a u r i n w i th the E n g l i s h

ve r s ion is k ind ly a c k n o w l e d g e d .

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