Eur. J. . 2016, 28, 337–353 Published online 5 January 2016

Transformation of to in the Benamocarra Unit (Betic Cordillera, S. ). Kinetics and petrological significance

1, 2 1, ANTONIO SA´ NCHEZ-NAVAS *,ELISA MACAIONE ,RITA DE CASSIA OLIVEIRA-BARBOSA **, 2 3 ANTONIA MESSINA and AGUSTI´N MARTI´N-ALGARRA

1 Departamento de Mineralogı´a y Petrologı´a and IACT (CSIC-UGR), Universidad de Granada, E-18071 Granada, Spain *Corresponding author, e-mail: [email protected] 2 Dipartimento di Dipartimento de Scienze della Terra-Universita di Messina. Salita Papardo 98122 Messina, Italy 3 Departamento de Estratigrafı´a y Paleontologı´a and IACT (CSIC-UGR), Universidad de Granada, E-18071 Granada, Spain **Present address: Instituto de Geocieˆncias, Faculdade de Geologia, Universidade Federal do Para´, Campus Universita´rio do Guama´, Rua Augusto Correˆa 1 - Campus Ba´sico - CEP: 66075-110, Belem, Para´, Brasil. E-mail: [email protected]

Abstract: Kyanite is directly replaced by andalusite in quartz–plagioclase veins included within graphite-bearing micaschists of the Alpine Benamocarra Unit (Betic Cordillera, Spain). Electron back-scattered diffraction indicates that: i) precursory kyanite contains planar defects; ii) andalusite growth was crystallographically controlled by the kyanite; and iii) the structure of both Al2SiO5 phases shares nearly the closest-packed oxygen array and chains of edge-sharing octahedra. The small entropy difference of the kyani- te–andalusite polymorphic inversion makes it difficult to overcome the energy barrier of this transformation. The driving force needed for the kyanite-to-andalusite reaction was a temperature (T) increase during a pre-Alpine tectonometamorphic evolution. The low-P/medium-T metamorphic conditions that affected the rocks studied took place in relation to a late Variscan extensional collapse. The reaction pathway proposed here corresponds to the first part of a poly-orogenic tectonometamorphic evolution, consisting of a pre-Alpine metamorphism of high thermal gradient with mainly static growth of porphyroblasts, followed by an essentially dynamic metamorphism during the Alpine orogeny. Key-words: polymorphic transformation; kinetics; kyanite; andalusite; silicate; metamorphism.

1. Introduction metamorphic events (e.g., Priem et al., 1966; Boulin et al., 1969; Bernard-Griffiths et al., 1977; Andriessen Orogenic cores frequently record complex polymeta- et al., 1991; Montel et al., 1995) were considered incon- morphic histories. An example of a complex polymeta- clusive or else lacking in regional significance. In fact, morphic history can be found in pre-Mesozoic before the publication of a few recent papers (Sa´nchez- metamorphic successions of the Alpine Internal Domains Navas et al., 2012, 2014), no direct field or petrographic of the Betic Cordillera (Spain) and the Rif (Morocco). evidence related to the Variscan orogenesis had been found Recent studies have demonstrated that the Alpine evolu- by most workers in the Alpujarride Complex of the Betic tion overprinted Variscan and older orogenic events in the zone (e.g., Torres-Rolda´n, 1974, 1981; Tubı´a et al., 1997; Nevado-Filabride, the Alpujarride-Sebtide and the Garcı´a-Casco et al., 1993; Garcı´a-Casco & Torres-Rolda´n, Malaguide-Ghomaride complexes (e.g. Puga et al., 1975; 1996, 1999; Azan˜o´n et al., 1998; Argles et al., 1999; Soto Bouybaoue`ne et al., 1998; Martı´n-Algarra et al., 2009a and & Platt, 1999). Moreover, Alpujarride-Sebtide rocks of b; Rosetti et al., 2010, 2013; Go´mez-Pugnaire et al., 2012; variable metamorphic grades dated with different radio- Sa´nchez-Navas et al., 2012, 2014). Variscan U-Pb ages metric systems (Rb-Sr, K-Ar-Ar, Sm-Nd and U-Pb) gave obtained from zircon and monazite single crystals in pre- systematically Alpine ages (e.g., Loomis, 1975; Priem Mesozoic rocks have been interpreted as related to high- et al., 1979; Michard et al., 1983; Zeck et al., 1989a and grade metamorphic or magmatic events (Zeck & b, 1996; Monie´ et al., 1991, Monie´ et al., 1994; Sa´nchez- Whitehouse, 1999, 2002; Montel et al., 2000; Zeck & Rodrı´guez et al., 1996; Platt & Whitehouse, 1999; Rossetti Williams, 2001). Before zircon dating, the isotopic ages et al., 2010, 2013). Integrated field and detailed micro- supporting the existence of pre-Alpine tectono- structural studies of mineral transformations in relation to

eschweizerbart_xxx 0935-1221/16/0028-2518 $ 7.65 DOI: 10.1127/ejm/2016/0028-2518 # 2015 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart 338 A. Sa´nchez-Navas et al. the tectono-metamorphic evolution are essential to distin- tectonic units in the Malaga area are totally made up guish the part of the metamorphic mineral associations and of medium- to high-grade metamorphic rocks (Fig. 1b). tectonic fabrics currently visible in the Alpujarride rocks In this work, we study rocks belonging to the that belong to Alpine or to pre-Alpine (Variscan, and Benamocarra Unit (Aldaya et al., 1979; Elorza, 1982), perhaps older) events. which is located just below the Malaguide Complex to The three polymorphs of Al2SiO5 are important index the east of Malaga (Fig. 1c). to unravel pressure (P) and temperature (T) con- The Benamocarra Unit is composed of Alpujarride- ditions during regional and contact metamorphism. The like graphite-rich metapelites and metapsammites cross- P-T phase equilibria involving kyanite, andalusite and silli- cut by dikes of mafic subvolcanic rocks (Fig. 1d) with a manite are affected by considerable uncertainty in calibra- gradual upward decrease in metamorphic grade (e.g. tion experiments, and erroneous petrological interpretations Elorza & Garcı´a-Duen˜as, 1981; Ruiz-Cruz, 1997; Ruiz- of the presence of coexisting polymorphs in rocks derive Cruz & Rodrı´guez-Jime´nez, 2002). The Benamocarra from the sluggish kinetics of reactions involving the alumi- Unit overlies the Unit, the widest and most nium silicates (Kerrick, 1990). The reaction mechanisms in typical Upper Alpujarride Unit in the area, from which the transformation of pre-Alpine andalusite porphyroblasts it is detached by an extensional contact (Alonso-Cha´vez to Alpine small-sized kyanite and fibrolite (both related to & Orozco, 2012). The Torrox Unit is made of a gneissic Alpine foliations) in metapelitic rocks of the Upper complex at its base (Sa´nchez-Navas, 1999; Sa´nchez- Alpujarride Torrox Unit were studied by Sa´nchez Navas Navas et al., 2014), surrounded by a monotone succes- et al. (2012). In the present study, we examine the transfor- sion of dark-coloured and graphite-bearing St-Grt-Ky- mation of pre-Alpine kyanite porphyroblasts to pre-Alpine And-Fi micaschists (Sa´nchez-Navas et al., 2012) with andalusite porphyroblasts in medium-T metapelites of the metamorphic grade decreasing upwards. The upper part Benamocarra Unit, which tectonically overlies the Torrox of the pre-Mesozoic succession of the Torrox Unit is Unit. This transformation was observed in the field first, and made of andalusite-bearing graphite-rich metapelites and further confirmed by microstructural studies. This allows a metapsammites that are similar to the rocks that consti- detailed discussion of the crystallographic and kinetic con- tute the lower part of the Benamocarra Unit. trols of the Ky ! And transformation, and on the geological The rock succession of the Benamocarra Unit is consequences of this transformation. We also describe tex- described in more detail below. It underlies a thick succes- tural relations and mineral chemistry of the enclosing poly- sion of fine-grained low-grade to very low-grade schists, metamorphic, medium-T, graphite-bearing, Ms þ Bt þ Pl slates, psammites, and stretched conglomerates of early Cld Grt St Ky And Crd micaschists (abbre- Palaeozoic or older age, which constitute the stratigraphic viations after Whitney & Evans, 2010). Finally, we discuss base of the Malaguide Complex (Morales Formation: the geological significance of the Ky ! And transformation Martı´n-Algarra, 1987) and which are also crosscut by as well as of the textures and mineral chemistry of the rocks dikes identical to those of the Benamocarra Unit studied, in order to decipher the pre-Alpine vs. Alpine (Fig. 1d). The contact between the two rock successions polymetamorphic history of the Benamocarra Unit. corresponds to a low-angle extensional fault, which often is difficult to recognize in the field (Fig. 1c). Consequently, according to regional geological studies, it is debated whether the Benamocarra Unit constitutes the base of the 2. Geological setting Malaguide Complex or is a part, detached by low-angle faults, of the top of the highest Alpujarride (Torrox) Unit in The Alboran-Kabylia-Peloritani-Calabria (AlKaPeCa) the area (Aldaya et al., 1979; Elorza & Garcı´a-Duen˜as, Alpine metamorphic belt (Bouillin et al., 1986) includes 1981; Elorza, 1982). the internal domains of the Western Mediterranean Alpine The stratigraphically lower beds of the Malaguide Belts before its post-collisional Mid- to Late Miocene Complex (Morales Fm.) are strongly foliated metapelites disintegration (Fig. 1a). This belt resulted from the Early and metapsammites affected by low-grade metamorphism Miocene collision of Iberia and Africa against the Meso- (Ruiz-Cruz, 1997; Ruiz-Cruz & Rodrı´guez-Jime´nez, Mediterranean Microplate (Durand-Delga & Fontbote´, 2002). The lowest part of the Morales Fm. contains abun- 1980), which was a continental crustal block detached dant sericitized relics of metamorphic minerals, randomly from Pangaea and bounded by narrow oceanic basins oriented in the foliation. Among them millimetric porphyr- since mid-Jurassic times (Guerrera et al., 1993). The oblasts of andalusite, biotite, and garnet are locally pre- Alpujarride and Malaguide complexes of the Betic served. All these minerals gradually disappear upwards Cordillera (Fig. 1b) and the equivalent tectonic units of whereas their amount and size increase downwards, so the Rif (Sebtide-Ghomaride units), of the Algerian that the lowest part of the Malaguide succession becomes Kabylias and of the Calabria-Peloritani Terrane of south- very similar to the rocks of the upper part of the ern Italy were essential constituents of the AlKaPeCa belt Benamocarra Unit. Upwards, the Morales Fm. is stratigra- (Perrone et al., 2006). phically followed by a thick succession of Upper The Malaguide Complex constitutes the highest tec- Ordovician to Upper Carboniferous siliciclastic and carbo- tonic thrust unit of the Betic internal domain, and over- natic turbidites with pelagic horizons that have been dated lies the Alpujarride Complex, of which the highest mainly with conodonts (O’Dogherty et al., 2000;

eschweizerbart_xxx Transformation of kyanite to andalusite 339

Fig. 1. (a) Western Mediterranean Alpine belts with indication of the AlKaPeCa fragments (black). (b) Tectonic map of the Betic Cordillera. ´ (c) Geologic map of the Benamocarra area (modified from Elorza &esch Garcıweizerbart_xxx a Duen˜as, 1981). The samples mentioned in the text, figures, and tables are indicated with their numbers. (d) Lithological succession of the Benamocarra Unit and overlying rocks. (online version in colour) 340 A. Sa´nchez-Navas et al.

Martı´n-Algarra et al., 2009a; Rodrı´guez-Can˜ero et al., 4. Results 2010; Rodrı´guez-Can˜ero & Martı´n-Algarra, 2014). The foliation in the Morales Fm and in overlying Devonian 4.1. Field relations (Santi Petri Fm.) up to Lower Carboniferous beds dated with conodonts (Falcon˜a Fm.) is crosscut by the mafic Metapelitic rocks of the Benamocarra Unit consist of alter- dikes (Fig. 1c). These dikes have provided 40K/39Ar and nating bands of quartz-rich and mica-rich domains inher- 40Ar/39Ar ages between 22 Ma and 30 Ma (Torres-Rolda´n ited from the psammitic and pelitic layers of the et al., 1986; Platzman et al., 2000). sedimentary protolith (Figs 1d and 2a). The succession is The Malaguide Palaeozoic succession is followed by lithologically very monotonous and, in the field, a gradual unmetamorphosed Permo-Triassic to Cretaceous clastic decrease in grain size is observed from bottom to top of the and carbonate rocks (Martı´n-Algarra, 1987). An unconfor- succession. Porphyroblasts of And and Cld are sometimes mity between the Palaeozoic and the Triassic rocks has visible to the naked eye as dark crystals (Fig. 2b), and those been reported by Foucault & Paquet (1971). These aspects of Grt as orange-reddish spots, the latter being most abun- have led different authors (e.g., Balanya´ & Garcı´a Duen˜as, dant towards the top of the succession. The upward 1987) to assign the foliation in the Morales Formation to decrease in grain size is accompanied by the disappearance the Variscan orogeny. of And preserved only as relic pseudomorphs in many cases towards the top of the succession. In addition to the lithological layering (S0) defined by the alternation of minor amounts of pelites intercalated 3. Sampling and analytical procedures with psammite layers, two foliations are visible in the field within the Benamocarra schists (Fig. 2a). The most The studied samples were collected from four sections evident foliation in the field, hereafter called S , transposes in the Benamocarra Unit and in the lowest beds of the 3 previous foliations S1 and S2. However, S1 is observed only Malaguide Morales Fm. along the roads MA-135 from in thin section within Cld and Grt crystals and in graphite- Benamocarra to , MA-176 from Iznate to Cajiz, rich microlithons (see below). The foliation S2, visible in MA-149 from Alma´char to and MA-106 from the field, is found within the graphite-rich pelitic bands, Torre de Benagalbo´n to and Benaque transposed by S3 (arrows in Fig. 2a). In these graphite-rich (Fig. 1c). Polished thin sections for petrographic and metapelitic layers, andalusite prisms occur randomly analytical studies were prepared from 61 samples of oriented in foliation S2 (Fig. 2b). Quartz segregations fre- graphite-rich micaschists, fine-grained micaschists, phyl- quently containing millimetric to centimetric Ab and pink lites, and quartz-rich veins either crosscutting or subpar- And crystals are very abundant and most of them certainly allel to the main foliation. The chemical composition of crosscut the S2 foliation. Nonetheless, the late foliation S3 the mineral phases was determined from fine-grained frequently shears these And-bearing quartz-rich veins, (samples Ben10 and Ben11: Fig. 1c) and coarse-grained which are commonly found parallelized to the latter. The micaschists (sample Ben54: Fig. 1c) by using a Cameca less deformed veins contain random aggregates of pink SX-50 electron microprobe. Operating conditions were And prisms (Fig. 2c). However, within the veins the pink 20 kV accelerating voltage, 20 nA beam current and a And crystals more frequently appear fractured and tecto- spot size between 5 and 7 mm. Standards were both nically re-oriented, boudinaged, and strongly elongated, synthetic oxides and minerals. Structural formulae of along S3. Exceptionally, blue kyanite prisms are inter- minerals were calculated using the software of Ulmer grown with andalusite prisms in deformed veins parallel (1986), with the exception of chloritoid (12-oxygen 3þ to the S3 (Fig. 2d). The Ky ! And transformation has been anhydrous basis, Chopin et al., 1992; and Fe as 4– studied in one of these samples collected near km 6 along (Al þ Ti), Azan˜on & Goffe´, 1997). the MA-149 road from Alma´char to Moclinejo (sample Electron back-scattered diffraction (EBSD) images and Ben43: Fig. 1c). Although the coexistence of blue Ky and pole figures of the Ky!And transformation from a quartz- pink And in veins has only rarely been mentioned in the rich vein (sample Ben43: Fig. 1c) were made with a Leo literature available on the Alpujarride Complex (Martı´n- GEMINI-1530 scanning electron microscope (SEM) Algarra, 1987), the occurrence of quartz–albite veins equipped with an Inca Crystal detector. The diffraction including Ky partially transformed to And is not exclusive pattern, once indexed, provides information on the orienta- of the Benamocarra Unit and has also been observed in tion of the crystal lattice. Data from different positions similar schists of diverse Upper Alpujarride units: N of the were integrated to perform orientation maps, and repre- Sierra de las Aguas in the peridotite massif sented as pole figures. In the sample, Z and Y axes coincide (Martı´n-Algarra, 1987), close to Almun˜ecar and in the with normal direction to the polished section and the trace Sierra de la Alfaguara near Granada. of the main or more pervasive foliation, respectively. Andalusite is orthorhombic, space group Pnnm, with unit-cell parameters a ¼ 7.7980, b ¼ 7.9031, c ¼ 5.5566 4.2. Petrography A˚ ; kyanite is triclinic, space group P-1, with a ¼ 7.1262, b ˚ ¼ 7.8520, c ¼ 5.5724 A, a ¼ 89.99, b ¼ 101.11 and g ¼ Two main mineral associations have been recognized in 106.03 (from Winter & Ghose, 1979). the Benamocarra metapelitic/metapsammitic succession:

eschweizerbart_xxx Transformation of kyanite to andalusite 341

Fig. 2. (a) Micaschists of Benamocarra Unit formed by light-grey quartzites alternating with black metapelite bands (arrows) transposed by the S3 foliation. (b) Surface view of the S2 foliation with randomly oriented post-kinematic andalusite porphyroblasts in black graphite-rich metapelites; S3 foliation is here parallel to S2 and is defined by fine-grained muscovite (lighter areas) formed on, and around, andalusite. (c) Andalusite rosettes in quartz-rich veins segregated within And-bearing graphite-rich schist. (d) Kyanite partially transformed to And within a small quartz segregation stretched along S3; blue pen for scale is 1 cm wide. (online version in colour) i) Ms þ Bt þ Pl þ Cld þ Grt And (upper part); ii) first, and those of And and xenoblastic Crd formed later, Ms þ Bt þ Pl þ Cld þ Grt þ And Ky St þ Crd enclosing both Grt and Cld (Fig. 4a–b). The Bt and And (lower part). The petrographic study is focused mainly formed after destabilization of Grt (Fig. 4c). Indents on on the textural relation between Cld, And, Grt, Crd, and {110} of Grt indicate the dissolution of Grt perpendicular foliations S1,S2, and S3. Chloritoid crystals have grown to these faces favoured by the presence of abundant inclu- randomly oriented on a Gr-rich foliation (S1) in the Ms sions of Qz distributed perpendicularly to these crystal þ Bt þ Pl þ Cld þ Grt þ And schists of the upper part faces (Fig. 4c). In some cases, such small Qz inclusions of the Benamocarra Unit (Fig. 3a). Chloritoid frequently form a cross-like pattern that evokes the typical crystal- occurs as post-S1 prismatic crystals within Gr-rich growth features of the And chiastolites (Fig. 4c). The micaceous domains moderately affected by S3 plagioclase is prevalently albitic, commonly encloses gra- (Fig. 3a). In some cases, Cld prisms appear moderately phite layers defining S1, and is frequently fractured when deformed and partially or totally pseudomorphed by Qz crossed by S3. Some relic St grains are preserved within þ Ms þ Bt and opaque phases (Fig. 3b). In addition, poikilitic And crystals (Fig. 3e) but form preferentially some Cld crystals are clearly reoriented and sheared by small, isolated and dismembered porphyroclasts, sheared S3, and embedded in a Bt-rich matrix (Fig. 3c). and wrapped by S2. Kyanite is very rarely present in the Andalusite appears strongly affected by the deformation schists. The And þ Crd association, postdating the Cld þ D3, with tectonic reorientation, boudinage and microfold- Grt association, is strongly affected by the S3-related ing associated with the development of the S3 foliation deformation (Fig. 4b–d). (Fig. 3d–f). The And porphyroblasts are post-kinematic The S3 foliation transposes the earlier foliations S1 and to S2 and formed after Cld, Grt, and St (Fig. 3e). In some S2, which are preserved in graphite-rich micaceous micro- cases And overgrows Grt crystals including Cld or contains lithons or within porphyroblasts (S1 in early Cld, Fig. 3a–b, relics of St. The foliation S3 is well defined by micro-shear and S2 in later Crd/And, Figs 3d–f, 4b and d) in the micas- bands filled by Qz deforming the Gr-rich domains and And chists of the whole Benamocarra succession. The S3 folia- crystals (Fig. 3f). tion forms a metamorphic banding made of alternating Qz- Within the mica-rich and finer-grained domains that are rich and micaceous layers (recrystallized Bt and Ms) ana- less deformed by D3 in the Ms þ Bt þ Pl þ Cld þ Grt þ stomosed around the Qz grains and pelitic microlithons And Ky St þ Crd micaschists of the lower part of the (Fig. 3a). Syn-S3 micas also grew as pseudomorphs of Benamocarra Unit, the porphyroblasts of Cld and Grt grew previous minerals such as Cld and Crd. A common

eschweizerbart_xxx 342 A. Sa´nchez-Navas et al.

Fig. 3. (a) Photomicrograph (plane polarised light) of a Gr-rich microlithon located between two Qz-rich shear bands defining S3 foliation. Cld porphyroblasts (arrows) postkinematic to the S1 foliation are preserved within the microlithon. (b) Bt, Ms and Qz pseudomorph after Cld. (c) Lath-shaped crystals of Cld elongated along a shear band defining the S3 foliation. In adjacent domains that are less affected by shearing, Cld crystals remain unoriented. (d) Post-S2 And porphyroclasts sheared (arrow) and wrapped up by the S3 foliation defined by Qz rich bands, Bt, Ms, and pre-S3 transposed Gr. (e) And porphyroblasts including St relic and arranged on the S2 foliation. (f) And porphyroblasts folded by D3 and including the S2 foliation defined by trail inclusions of Gr. (online version in colour) secondary mineral phase is Chl, which crystallizes as a 4.3. Mineral chemistry product of destabilization of Bt, Grt, and Crd. Kyanite crystals partially replaced by andalusite (Fig. 5) Muscovite mineral chemistry shows two compositional have been studied in detail from one Qz–Ab vein from the groups (Table S1, freely available online as intermediate part of the Benamocarra succession (sample Supplementary material linked to this article on the GSW Ben43). This post-S2 vein is strongly deformed and paral- website of the journal, http://eurjmin.geoscienceworld.org/; lel to the S3 foliation (Fig. 2d), which is defined by Ms Fig. 6a–b). Most analyses belong to the first group, which is within the vein itself. Both the Ky and the And crystals are constituted of Ms flakes, formed after D1 (primary Ms), folded and fractured (Fig. 5a). In some cases, a single Ky with lower Si content (Si ¼ 3.03–3.15 atoms per formula crystal is directly replaced by more than one And prism; unit, a.p.f.u.), low celadonitic substitution (Fe þ Mg ¼ this is apparent in optical images by the different crystal- 0.03–0.12 a.p.f.u.) and low K/(K þ Na þ Ca). The second lographic orientations observed for replacing And group includes the Ms formed in relation to the D3 deforma- (Fig. 5b), and confirmed also in EBSD images (see below). tion (recrystallized Ms) and exhibits a higher Si content

eschweizerbart_xxx Transformation of kyanite to andalusite 343

Fig. 4. (a) Photomicrograph (cross-polarised light) showing the textural relationship between Cld, Grt, altered Crd, and And with fine-grained Ms and Bt: And and Crd grow surrounding Grt and Cld. Note the accumulations of Gr at the ends of the skeletal arms of the And crystal (arrows). (b) Xenoblastic Crd around Cld prims within mica-rich (fine-grained Ms) domains (crossed nicols); S3 foliation is defined by Qz-rich shear bands crosscutting Crd, whereas S1 and S2 foliations are parallelized and wrapped by Crd in the micaceous domains. (c) Grt porphyroclasts wrapped by And, both affected by the foliation S3. Grt appears partially dissolved and surrounded by Bt and Qz. When well-preserved, as in the Grt grain at the bottom right-hand side of the image, it exhibits crystal-growth features similar to those visible in And, as is the development of the ‘‘x’’- shaped inclusions pattern (arrows). And, Bt, and Qz formed after destabilization of Grt. (d) Optical image of a Bt-schist with Crd þ And relics; Crd is altered to fine-grained sericitic Ms, Chl, and minor Bt, and appears dismembered and elongated along the Qz-rich bans related to the S3 foliation; the S2, defined by trails of Gr inclusions, is preserved within relic And (white dotted line). (online version in colour)

(3.21–3.23 Si a.p.f.u.) and intermediate celadonitic substi- 4.4. EBSD study of andalusite pseudomorphs after tution (XMg ¼ 0.95–1). kyanite Biotite composition is intermediate between phlogopite and annite (XMg ¼ 0.44–0.54; Table S2 in Supplementray As mentioned above, in the deformed Qz–Ab vein with Material). Some chemical analyses indicate that Bt is par- abundant Ky and And of sample Ben43, elongated Ky tially transformed to Chl. Sometimes, Bt that formed along crystals are topotactically transformed to And. First, S3 is completely transformed to Chl, even if it preserves the we describe the partial replacement of a single Ky usual optical features of Bt. crystal by two And crystals with a different crystal- Garnet composition is close to that of almandine: Alm ¼ lographic orientation (Figs 5b, 7, 8) and, later, the 63–74 %, Grs ¼ 5–20 %, Prp ¼ 4–10 % and Sps ¼ 0–9 % textural relations between Ky relics and enclosing neo- (Table S3, Fig. 6c–d). In addition, representative Grt com- formed And (Fig. 9). position data reveal differences between cores and rims Figure 7 shows the results of the EBSD study corre- (Table S3): cores show a higher content of spessartine sponding to Zone 1 of Fig. 5b. Poles corresponding to the (XSps ¼ 0.04–0.09) and a relative lower abundance in direction normal to the (100) Ky planes locate close to the almandine (XAlm ¼ 0.63–0.70) whereas rims are depleted Z axis in some pieces of the Ky crystal: e.g. central part and in spessartine (XSps ¼ 0.00–0.04), with correlative increase lower left-hand side of the Ky (Z) orientation map in almandine (XAlm ¼ 0.70–0.74). The Mg number does (Fig. 7e). Nevertheless, in other pieces of the Ky crystal not change from cores to rims, however. Ca is slightly the poles of the (010) planes are located along the Z axis higher in garnet cores than in rims (Fig. 6c). (upper and lower parts of the main Ky crystal in Fig. 7e). Chloritoid is a Fe-Cld (Table S4) with a very low Mn As shown by the Ky (X) orientation map and the corre- content (0.01–0.03 a.p.f.u.). The Mg content invariably sponding 001 pole figure, different orientations are also increases from cores (XMg ¼ 0.11–0.13) to rims (XMg ¼ observable for Ky (Fig. 7f). However, the poles corre- 0.13–0.16). sponding to the normal to the (001) planes of the Ky crystal Chlorite forms at the expense of Bt and, composition- mainly locate near the X direction in the sample. The c ally, is a chamosite with an XMg from 0.37 to 0.39. crystallographic axis of the And crystal is sub-parallel to

eschweizerbart_xxx 344 A. Sa´nchez-Navas et al.

Fig. 5. Optical images of folded And prims after Ky within Qz–Pl veins within Ms þ Bt þ Pl þ Grt þ Ky þ St þ And schists from the lower part of the Benamocarra Unit. (a) Crossed-nicol images of elongated Ky crystals partially transformed to And. Deformation of And and Ky single crystals formed subgrains as revealed by undulatory extinction in crossed nicols. (b) Two crystallographic orientations are visible in zones 1 and 2 for the And replacing Ky (crossed nicols). (online version in colour)

Fig. 6. Mineral chemistry data from the Benamocarra metapelites (units: a.p.f.u). (a) Fe2þþMg vs. Si of Ms (solid squares, primary Ms; blank squares, recrystallized Ms). (b) K/(K þ Na þ Ca) vs. Si diagram of Ms. (c) Fe2þ–Mn–Ca diagram of garnet (blank circles, cores; solid circles, rims). (d) Mn–Mg–Fe2þ diagram of garnet. the Z direction in the And (Z) orientation map and the crystallographic axis of the And crystal is sub-parallel to corresponding 001 pole figure (Fig. 7d). the poles corresponding to the normal to the (001) planes of Figure 8 shows the orientation of the second And crystal the precursory kyanite single crystal (compare Fig. 8 with replacing Ky in Zone 2 of Fig. 5b. In this case, the c Fig. 7f).

eschweizerbart_xxx Transformation of kyanite to andalusite 345

Fig. 7. EBSD results for zone 1 of Fig. 5b. (a) Optical image with indication of the area studied by EBSD. (b) Phase map: And (yellow), Ky (purple) and Qtz (red). (c), Electronic image with indication of the directions corresponding to the reference system in the sample. Key orientation colour maps for both And and Ky are also included. (d) And orientation map along Z (And (Z)) superposed onto the pattern quality map; 001 pole figure for And is shown in the right-hand side of figure 7d. (e) Ky orientation map along Z (Ky (Z)) is projected onto the pattern quality map; Ky crystal has a lamellar structure, with some packets having 010 parallel to Z (in green) whereas other packets have 100 parallel to Z (in blue). (f) Orientation map for Ky along X (Ky (X)) and 001 pole figure of Ky (right-hand side). (online version in colour)

Figure 9 corresponds to the EBSD study of an And and b axes are interchanged, with an angular deviation of 6 pseudomorph after Ky. In this case, small non-transformed and 13 between the aAnd and bKy and between the bAnd and relics of the precursory Ky single crystal still remain within aKy, respectively (Fig. 9h–i). the And crystal (high-relief areas in background electro- nic-image). The crystallographic relations between the Ky single crystal and the And are similar to those visible in 5. Discussion Zone 2 of Fig. 5b. Because EBSD orientation mapping covers a large area, it results in a large numbers of maxima 5.1. Crystallography of the transformation of kyanite in the pole figures (Fig. 9g) due to the presence of small to andalusite pieces of Ky and And with different orientation. When the scanned region is restricted to a small area (so that only The Ky ! And transformation described here is a solution- And close to the almost completely transformed Ky is mediated, solid-solid reaction in which Al2SiO5 undergoes taken into account), it can be appreciated that the c crystal- structural changes without compositional change. lographic axes of both phases are parallel, whereas the a According to the crystallographic relationships observed

eschweizerbart_xxx 346 A. Sa´nchez-Navas et al.

Fig. 8. EBSD results for zone 2 of Fig. 5B. (a) Pattern quality map (b) Optical image (crossed nicols) with indication of the area studied by EBSD. (c) The same area is indicated on the background electronic image. (d) Orientation map along X and 001 pole figure of And (left-hand side). (online version in colour) for Ky and And, this transformation is structurally con- array occurring in the Ky structure allows the development trolled. It means that the crystallographic orientation of the of complex stacking sequences (Fig. 10c) and can be And was controlled by the orientation of Ky. The structu- responsible for the occurrence of planar defects in Ky rally controlled Ky ! And transformation began at a (disoriented domains in the map of Fig. 7e). reaction site of high-energy density that, in the study One specific crystallographic relation between reaction case, corresponds to a particular crystallographic plane of product and reactant is found in topotactic replacements the Ky crystal structure. At that site the atoms of the Ky among Al-silicates, i.e. in the case of replacement of anda- reactant have enough energy to surmount the activation lusite by / (cAndjjcSil/cMul, aAndjjbSil/ barrier and to reorganize themselves to form And. As bMul, bAndjjaSil/aMul; Hu¨lsmans et al., 2000; Cesare et al., previously shown, the Ky crystals partially transformed 2002). The mutual crystallographic orientation of Ky and to And contain planar defects (Fig. 7e). And obtained from EBSD data in small areas of both Figure 10a shows the projection of diverse close-packed phases (Fig. 10h–i) deviates slightly from those topotactic oxygen layers that form the anion sub-lattice in the Ky relationship proposed in the literature. Nevertheless, the structure onto the (110) Ky crystallographic plane. The And follows at least two main orientations when replacing coordination environment of anions forming the close- Ky (Figs 5b, 7 and 8). It has been illustrated in Fig. 11a–b, packed layers appears in Fig. 10b. Anion (110) layers where two And crystals grow with their c crystallographic (here designated as A, B, and C) form a closely packed, axes contained in Ky (110) planes, that one of them coin- roughly cubic array of oxygen atoms. The closest-packed cides with cKy.

eschweizerbart_xxx Transformation of kyanite to andalusite 347

Fig. 9. EBSD study of And pseudormoph after Ky performed in an area preserving relics of precursory Ky. (a) Phase map with And in yellow and Ky in purplish red. (b) Orientation map for And along X. (c) The area studied by EBSD is indicated on the background electronic image. (d) Optical image (crossed nicols). (e) Orientation map for Ky along Z projected onto the quality map. (f) Orientation map for Ky along X. (g) 001, 100 and 010 pole figures of And and Ky. The diverse number of maxima (at least two) in these pole figures is due to the presence of smaller pieces of And and Ky with different orientation than the main crystals. (h) Pole figures of both phases from a smaller area including only And close to the transformed Ky and providing a single orientation. (i), Pole figure for all three axes of both Ky and And deduced from G. (online version in colour)

Worden et al. (1987) made a noteworthy observation in crystallographic axis of the And structure, indicated in relation to structurally controlled replacements among Fig. 11d, fits any of the three directions within the (110) oxygen-bearing minerals of different composition. plane of the Ky (Fig. 11c). According to these authors, the crystallographic plane par- Atoms at the interface of one phase are partially allel to the interface between reactant and product corre- bonded to different neighbours in the adjacent phase. sponds to a close-packed plane of the oxygen sub-lattice of Consequently the change of oxygen coordination environ- their respective structures. The two coexisting phases share ment at the interface with respect to the atom within the nearly the closest-packed anion layers in topotactic repla- crystal lattice affects the energy of the anion coordination cements where a definite crystallographic relation is polyhedron. Burdett & McLarnan (1984) established the observed between host and inclusions (e.g., Shau et al., idea that there is a tight positive correlation between the 1991). Lattice strain is reduced at the interface defined by electron-band structure energy of polymorphs and energy the shared closest-packed anion layers, because intera- computed by adding only local contribution from each tomic distances and type of arrangement (hexanet) in oxy- anion coordination polyhedron. These authors emphasized gen layers are approximately the same on both sides of the the importance of the environment of anions where most of interface. Figures 11c and d show the oxygen packing in the valence electrons are located. Moreover, in their orbital (110) and (320) planes of the structures of Ky and And, interpretation of Pauling’s second rule, they concluded that respectively. The oxygen arrangement along the c the anions with higher coordination numbers lead to a

eschweizerbart_xxx 348 A. Sa´nchez-Navas et al.

locate in octahedral sites whereas, in And, half the Al atoms occupy five-coordinated sites and, in Sil, half occupy four-coordinated sites. According to Velbel (1999), relative weathering rates within the Al2SiO5 group vary with the coordination number of Al: the Al2SiO5 polymorph weathers more easily when the coor- dination number of Al is higher. Nevertheless, the tradition of considering only the cation coordination was con- demned as early as Bragg (1930): if the environment of anions (and, therefore, half of the atoms of the structure where most of the valence electrons are located) is ignored, the structural transformations among minerals cannot be well explained. As illustrated in Fig. 10b the oxygen coordination num- ber in the close-packed (110) planes of Ky is higher than 3, whereas it is 3 or lower in the oxygen atoms of the And and Sil structure. Consequently, according to the above discus- sion, high-coordinated oxygen atoms in close-packed structures are responsible for high local structural instabil- ities that favour the breaking and/or reorganization of Si–O bonds in response to the change in P-T conditions during the Ky ! And replacement.

5.2. Kinetics of the Ky ! And reaction

In previous studies the Ky ! And polymorphic inversion in Alpujarride rocks has been interpreted in relation to Fig. 10. (a) Projection onto (110) planes of the layers of type A (blue decompression during the Alpine orogeny (Garcı´a-Casco spheres), B (red spheres) and C (green spheres) forming a ‘‘pseudo- cubic’’ closest-packed array of oxygen atoms in the Ky structure. (b) & Torres-Rolda´n, 1996; Argles et al., 1999). Grambling First coordination sphere around the oxygen atoms (red spheres) (1981) and Grambling & Williams (1985) described the forming one closest-packed layer. The cations coordinating the O occurrence of Ky ! And and And ! Sil reactions during atoms are Si (grey spheres) and Al (blue and purple spheres). (c) prograde conditions in rocks affected by a low-pressure Stacking sequence corresponding to the crystallographic orienta- regional metamorphism. Nevertheless, Pattison (2001) tions shown in the Fig. 7E for the Ky. The trace of the (110) lattice indicates that these Al2SiO5-bearing rocks were involved planes and the orientation of the crystallographic axes for the differ- in decompressional P-T paths traversing, in order, the Ky, ent coherent domains within the structure are also shown in the figure. (online version in colour) Sil and And fields. Kerrick (1988) studied the transforma- tion of Ky ! And in Al2SiO5-bearing segregations during retrograde decompression of rocks from the Lepontine structural destabilization. In the case of silicates, these Alps. He concluded that the Ky ! And reaction was anions correspond to the so-called oversaturated oxygen catalysed by fluids derived from metamorphic dehydration anions of Zachariasen-Baur (Baur, 1970), which are reactions in the host rock. Sa´nchez-Navas et al. (2012) responsible for unusually long Si-O bonds, thus favouring have shown that the dissolution/precipitation of alumino- bond-breaking. Experimental evidence of minor structural silicate minerals is easier if the reaction involves OH- stability around oxygen atoms with higher coordination bearing minerals. These authors studied the opposite trans- number was provided by Bell & Wilson (1977) and formation (And ! Ky) in Grt St Ky And Fi Meike (1989), who demonstrated that muscovite required Crd Gr-bearing micaschists of the Torrox Unit, which more strain energy to bring about dislocations than did tectonically underlies the Benamocarra Unit, and con- biotite, and that faulting goes through the octahedral cluded that dehydration reactions of Ms and Bt provided layer (closest-packed oxygen layers with chains of edge- the chemical driving force needed to break Si–O bonds. sharing octahedra within these anion layers). In the Benamocarra Unit, the energy necessary to over- The presence of chains of edge-sharing octahedra in the come the activation barrier of the Ky ! And reaction could structures of the three polymorphs of Al2SiO5 also weak- have been provided by a temperature increase instead of a ens their stability according to rule 3 of Pauling. Burdett & pressure increase. To envisage how a pressure increase McLarnan (1984) showed that structures with edge or face helps the activation energy barrier to be overcome, we sharing have distorted anion environments, resulting in need to consider only the fact that energy density is poor anion hybridization and weaker metal-anion bonds. (dimensionally) equivalent to pressure. Nevertheless, it is The weathering series Ky ! And ! Sil has been explained much easier to interpret the Ky ! And reaction found from a crystal chemical standpoint. In Ky, all Al atoms within the Benamocarra schist sequence in relation to a

eschweizerbart_xxx Transformation of kyanite to andalusite 349

Fig. 11. (a) and (b) Model proposed for the polymorphic inversion studied. According to this model, two And crystals nucleate and grow with their c crystallographic axes parallel to the (110) plane of Ky. (c) Close-packed layer formed by O atoms parallel to the (110) plane of Ky structure. (d) Close-packed array of O atoms along the c crystallographic axis of And. (online version in colour) high- thermal-gradient metamorphism and maximum tem- When well preserved, which is not always the case due perature close to 550 C that reached a maximum pressure to younger deformation events in the Benamocarra micas- around 0.4 GPa, close to that defined by the Al2SiO5 triple chists, the larger porphyroblasts postdating the foliations point (see below). S1 and S2, in particular some of Grt and, especially, those of And, show textural evidence of crystal growth under static conditions (Fig. 4a and c). In chiastolitic andalusite 5.3. Evidence of a high-thermal-gradient the crystal-growth features consist of graphite accumula- metamorphism tions at the edges of the prism faces resulting from crystal growth normal to the flat faces (Fig. 4a). In relation to Grt, The reaction pathway proposed above for the Ky ! And the same conditions favoured the development of cross- transformation found within Qz–Ab veins is compatible like inclusion patterns of Qz grains perpendicular to rhom- with the metamorphic evolution inferred from the enclos- bododecahedral faces (Fig. 4c). Both patterns result from a ing schists. The petrographic data indicate an early pro- type of crystal-growth mechanism related to static blastesis grade metamorphic stage related to burial and heating, and, for pre-Alpine And in the rocks of the Torrox Unit, evidenced in the schists by blastesis of post-S1 Cld and this was called layeritic growth by Sa´nchez-Navas et al. Grt porphyroblasts (Figs 3a–c, 4a–c). The growth of Cld, (2012). According to this mechanism, the crystal-growth Grt, St, and Ky in the schists was probably slightly older features of both And chiastolites and Grt porphyroblasts than (or coeval to) the opening of Q–Ab veins and related resulted from a thermally activated fast growth normal to formation of cm-sized Ky crystals. the flat faces of crystals. This growth was controlled by The blastesis of Cld, Grt, St, and Ky predated the para- screw dislocations emergent at the centre of the F-faces morphic replacement of Ky crystals by And in the veins, under low-supersaturation conditions due to slow diffusion which was probably associated with the generalized blast- rates, where the low matter supply was related to low esis of post-S2 And (and locally of Crd) in the schists. This deformation rate (Sa´nchez-Navas et al., 2012). Therefore, reflects the evolution from an initial intermediate-P the blastesis of And and Grt in the schists also occurred Barrovian metamorphism to medium-T and low-P meta- through a thermally activated process related to a prograde morphic conditions. Deformation during decompression metamorphism, in the same way as previously proposed led to the formation of the S2 foliation, sealed by And, for the Ky ! And transformation in the veins. and in the widespread occurrence of post-S2 veins filled The normal chemical zoning pattern observed from by Qz, Ab, and And prisms, which is a common feature cores to rims in Grt also indicates a prograde growth, of medium-grade schists, equivalent to those of certainly due to a T increase (Fig. 6c–d, Table S3). The Benamocarra, frequently found in many Alpujarride units.

eschweizerbart_xxx 350 A. Sa´nchez-Navas et al. increase in Mg content of Cld from core to rim (Table S4) further supports this T increase. The occurrence of Grt and Cld in these rocks is due to their Fe-rich composition and does not necessarily indicates much higher pressure than that of the Al2SiO5 triple point. Almandine Grt and Fe-rich Cld (XMg ¼ 0.11–0.13) can form at relatively low pres- sures because of the strong stabilizing effect of iron and other transition elements when they are incorporated into the structures of both minerals (Spear, 1993). As stated above, Cld and Grt porphyroblasts were later partially consumed in the prograde reactions that formed And and Crd. In fact, Crd grew over Cld and And chiasto- lites that formed in the matrix instead of replacing Cld porphyroblasts directly (Fig. 4a–b). The persistence of Cld and Grt in And-Crd metapelites indicates that their breakdown extended across the growth interval of And and Crd (Fig. 4). It represents a chemical disequilibrium prob- ably conditioned by a very rapid decompression or simply metastable persistence.

5.4. Tectono-metamorphic history Fig. 12. Variscan (I) and Alpine (II) P-T paths and their relation with the main deformation phases (D1,D2,D3)fortheMsþ Bt þ Pl þ And Grt In diverse zones of the Iberian Variscan Belt, the Variscan Ky St Crd graphite schists of the lower part of the Benamocarra crustal thickening induced a prograde metamorphism of Unit. The garnet-in reaction (1), defined by the Fe end-member reaction: Barrovian type, which was followed by a HT-LP meta- Fe-Cld þ Ann ¼ Alm þ Ms (Spear & Cheney, 1989), is represented morphism after the collapse of the thickened continental together with reaction 2 of destabilization of the St (St þ Qz þ Chl þ Ms crust (A´ balos et al., 2002; Valle-Aguado et al., 2005; Bea ¼ Bt þ And þ H2O; Thompson, 1982), reaction 3 forming Crd from St, et al., 2006). This transition took place through isothermal Grt and And (St þ Qz ¼ Crd þ And þ H2O; Thompson 1982), the phase decompression and resulted in high thermal gradients and diagram for Al2SiO5 polymorphs (Pattison, 2001) and the Si isopleths for reaction 3K Al Si O (OH) þ 6SiMgAl ¼ 4KAlSi O þ 6SiO widespread intrusions of granitoids (e.g.A´ balos et al., 2 6 6 20 4 -2 3 8 2 þ K2Mg6Al2Si6O20(OH)4 þ 4H2O (Massonne & Schreyer, 1987). 2002). In the Upper-Alpujarride Torrox Unit, located struc- turally below the Benamocarra Unit, the coexistence of And and Crd in leuco-granitic dykes intruding high-grade para- which caused mylonitization and grain-size reduction of gneisses and schists (Sa´nchez-Navas et al., 2014) as well as the previous larger minerals in the rocks studied. The the breakdown of large pegmatitic Ms forming And þ Kfs Alpine overprint, which occurred along shear zones in intergrowths in orthogneisses (Sa´nchez-Navas, 1999) have relation to D3, gradually decreased from bottom to top been interpreted as result of a high-T low-P Variscan meta- of the metapelitic-metapsammitic sequence. It gave rise morphic event. Moreover, the strong metamorphic gradient to zones at different degrees of re-equilibration, from observed through the crustal sequence in many Upper and partly re-crystallized medium-fine grained micaschists Intermediate Alpujarride units, including those related to with abundant pre-Alpine minerals to pervasively the peridotite emplacement, has been recently rein- restructured fine-grained mica phyllites. The T condi- terpreted as developed under low-P and high-T conditions tions of the Alpine metamorphic peak in the during the Variscan Orogeny (e.g. Acosta Vigil et al., Benamocarra Unit were between 400C and 450C, in 2014). According to our interpretation, Cld, St, Grt, and accordance to the formation of biotite and muscovite in Ky are syn- to post- kinematic to D1, and formed in relation the foliation (Figs. 3–4) and, in the case of biotite, also to a Barrovian-type metamorphism of medium-P that around garnet (Fig. 4c). According to phengite geobaro- affected the Benamocarra Unit probably during Variscan metry (Massonne & Schreyer, 1987), the pressure of this times. It reflects an initial episode of prograde metamorph- Alpine event was around 0.5–0.6 GPa, as deduced from 2þ ism resulting from crustal thickening, represented by D1 in Si (3.23–3.21 a.p.f.u.) and Fe þMg content the clockwise P-T path (I) of Fig. 12. This early Variscan (0.19–0.14) of phengitic muscovite (Table S1, Fig. 6a). tectono-metamorphic evolution reached a maximum pres- The partial replacement of chiastolitic And by fine- sure around 0.4 GPa (Fig. 12). The second episode con- grained Ky and fibrollitic sillimanite in the Torrox sisted of a late Variscan decompression (D2) that ended Unit (Sa´nchez-Navas et al., 2012) indicates that P and with the blastesis of post-S2 And and Crd at low P (around T were higher during the Alpine orogeny in tectonic 0.2 GPa) and medium T (around 550 C) conditions units structurally below the Benamocarra Unit. The (Fig. 12). inferred metamorphic P-T path related to the major A much more dynamic metamorphism was related Alpine deformation phase D3 is represented by the essentially to the strong deformational Alpine history, curve (II) of Fig. 12, which agrees well with intense

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