polymetamorphism in the alpujarride complex, betic ...grupo179/pdf/sanchez navas 2017.pdf · tic...

Polymetamorphism in the Alpujarride Complex,Betic Cordillera, South Spain

Antonio Sánchez-Navas,1,2,* Antonio García-Casco,1 Stefano Mazzoli,3

and Agustín Martín-Algarra2

1. Department of Mineralogy and Petrology, University of Granada, 18071 Granada, Spain; and Instituto Andaluz deCiencias de la Tierra (IACT), University of Grenada (UGR)–Consejo Superior de Investigaciones Científicas (CSIC),

Avenida de las Palmeras 4, 18100 Armilla, Granada, Spain; 2. Department of Stratigraphy and Paleontology,University of Granada, 18071 Granada, Spain; and IACT, UGR-CSIC, Avenida de las Palmeras 4,18100 Armilla, Granada, Spain; 3. Department of Earth Science, Environment and Geo-resources,

University of Naples Federico II, Largo San Marcellino 10, Naples, 80187, Italy

AB STRACT

The pre-Alpine metamorphic history of basement rocks of the Alpine Betic-Rif orogen is still uncertain as a result ofintense Miocene reworking. Reaction sequences and textures recorded in pre-Mesozoic quartz-K-feldspar-plagioclase-biotite-muscovite anatectic gneisses of the Upper Alpujarride units underwent a complex polymetamorphic history.U-Pb SHRIMP dating of magmatic zircons from coarse-grained porphyritic gneisses indicates an anatectic event at2865 11Ma. Refractory pelitic enclaves (kyanite-rutile-andalusite-ilmenite-biotite-muscovite5 staurolite, devoid ofquartz and feldspars) included in the gneisses formed after partial melting of pre-Permian metasediments at relativelyhigh pressure (P). This was followed by late Variscan decompression, producing andalusite at low P in the enclaves andthe granitic gneiss. During the Alpine Orogeny, the enclaves were affected by diverse local reactions: (1) andalusite wastransformed to Alpine kyanite from reactions catalyzed bymuscovite and biotite; (2) garnet coronas formed close to thegneiss, together with phengitic muscovite; and (3) Variscan biotite in contact with garnet reequilibrated under high/medium-P and medium-temperature metamorphic conditions. This reaction sequence occurring in polymetamorphicrocks allows the unraveling of their polyorogenic history and the assignment of relative ages to minerals and mineralassemblages through integrated field, mineralogical, petrological, and geochronological analyses.

Online enhancements: appendix, supplemental tables.

Introduction

Alpine metamorphic complexes around the Medi-terranean commonly show polymetamorphic his-tories (e.g., Hunziker 1970; Michard et al. 1997;Cesare 1999; Montel et al. 2000; Gaidies et al. 2006;Prenzel and Abart 2009). In the Betic Cordillera(Spain) and Rif (Morocco) orogenic cores (InternalDomains), the Alpine tectometamorphic evolutionoverprinted Variscan orogenic events (Puga et al.1975; Gómez-Pugnaire and Franz 1988; Martín-Algarra et al. 2009a, 2009b; Rossetti et al. 2010, 2013;Sánchez-Navas et al. 2012, 2014; Gueydan et al.

2015). Alpujarride rocks, in particular, have yieldedlateVariscanU-Pb zircon ages (Zeck andWhitehouse1999, 2002; Zeck and Williams 2001) as well as Al-pine ages when analyzed with different isotopic sys-tems (Rb-Sr, K-Ar, Ar-Ar, Sm-Nd, and U-Pb; e.g.,Loomis 1975; Priem et al. 1979; Zeck et al. 1989a,1989b; Monié et al. 1991, 1994; Sánchez-Rodríguezet al. 1996; Platt and Whitehouse 1999; Sánchez-Rodríguez and Gebauer 2000). However, only a fewfield, microstructural, and compositional studies ofAlpujarride rocks document Alpine versus Variscanevolution (e.g., Argles et al. 1999; Sánchez-Navaset al. 2012; Manjón-Cabeza Córdoba et al. 2014).Distinguishing what part of the mineral associa-tions and tectonic fabrics currently visible in the

Manuscript received November 3, 2016; accepted June 8,2017; electronically published September 22, 2017.

* Author for correspondence; e-mail: [email protected].

637

[The Journal of Geology, 2017, volume 125, p. 637–657] q 2017 by The University of Chicago.All rights reserved. 0022-1376/2017/12506-0002$15.00. DOI: 10.1086/693862

field and in thin section is related to Alpine or toVariscan events is a cornerstone to correctly estab-lish the corresponding P-T-t (pressure-temperature-time) paths and the tectonic evolution.

In this article, we address the polymetamorphicevolution of Upper Alpujarride pre-Mesozoic rocksby analyzing textures and mineral transformationsin biotite-rich pelitic enclaves from anatectic por-phyritic gneisses of the Torrox Unit. The enclavesdeveloped garnet coronas close to the surroundinggneiss and complex transformations involving an-dalusite, kyanite, muscovite, biotite, rutile, and il-menite. These features are interpreted in a Variscan-versus-Alpine geochronological context accordingto both SHRIMP zircon ages obtained from por-phyritic gneisses and previous geochronologic dat-ing by different methods of the Torrox succession.

Geological Setting

Before its Burdigalian disintegration related to theopening of the West Mediterranean basins (Savelli2015 and references therein), the metamorphic In-ternal Domains of the Betic Range and their NorthAfrican and Italian counterparts constituted a sin-gle continuous Alpine orogenic metamorphic belt.This belt, known as AlKaPeCa (Alboran-Kabylia-Peloritani-Calabria; fig. 1A), resulted from latePaleogene–early Miocene Alpine convergence, sub-duction/collision/accretion, and tectonic squeezingof the continental margins surrounding the Meso-mediterraneanmicroplate, which had been rifted fromPangea between Iberia and Africa-Adria in Triassic–Early Jurassic time and, finally, detached from themduring limited Jurassic-Cretaceous seafloor spread-ing (Martín-Algarra and Estévez 1984;Martín-Algarra1987; Guerrera et al. 1993;Martín-Algarra et al. 2000;Chalouan et al. 2001; Faccenna et al. 2004;Vera 2004;Perrone et al. 2006).

The Betic Internal Domain units including pre-Mesozoic rocks are grouped in the Nevado-Filabride,Alpujarride, and Malaguide Complexes (in ascend-ing tectonic order;fig. 1B). TheAlpujarrideComplexconsists of lower, intermediate, and upper tectonicunits, the last named Sebtide in the Moroccan Rif.Both the Alpujarride Complex and the Sebtide in-clude subcontinental mantle rocks, the Ronda–BeniBousera peridotite massifs, within the tectonic pilebetween the upper and intermediate units (Michardet al. 2006). The widest Upper Alpujarride unit out-cropping to the east of Malaga in the central Beticregion is theTorroxUnit (fig. 1C), which includes theTorrox Gneissic Complex (TGC) at its deepest sec-tion (the base of the unit is not exposed;García-Cascoet al. 1993; García-Casco and Torres-Roldán 1996,

1999; Sánchez-Navas et al. 2012, 2014). Offshore,east ofMalaga, a positive gravimetric anomaly closeto the Mediterranean coast (Bonini et al. 1973) de-notes the presence of mantle rocks near the Earth’ssurface, likely a tectonic sliver below the TorroxUnitthat can be correlated with the Ronda–Beni Bouseramassifs.

The TGC consists of a 1.5-km2 body overlain bya monotonous series of dark (graphite-rich) micaschists and quartzitic metapsammites, defining ametamorphic sequence characterized byGrt (garnet)-Ky (kyanite)-And (andalusite) at the top and byGrt-St(staurolite)-Fi (fibrolite)-Ky-And at the base (García-Casco 1993; García-Casco and Torres-Roldán 1996,1999; mineral abbreviations after Whitney and Evans2010). This complex is formed by diverse lithotypesincludingMs (muscovite)-bearing leucocratic gneisses(fig. 2) intercalated with Ms-Bt (biotite) 5 Grt peliticrocks and leucocratic quartz-feldspathic layers anddikes.

In the schists surrounding the TGC and in thedeepest crustal levels of the Upper Alpujarride gran-ulite gneisses above the Ronda peridotites of the Ju-brique area, Ruiz-Cruz andSanz deGaldeano (2014a,2014b) reported diamonds within garnet and pro-posed that a pre-Variscan ultrahigh-P event up to 6–7 GPa would explain their occurrence. Massonne(2014) studied the same rocks at Jubrique, but he didnot observe diamonds.He concluded that these rockswere affected by a late Variscan metamorphic eventat 287 5 9 Ma (P ≈ 0.7 GPa, T ≈ 8007C) and by ahigher-P Alpine event (P ≈ 1.3 GPa, T 1 6507C) thatevolved to a peak T of 7407C at 20 Ma. This authorinterpreted that the reported nanodiamonds eithergrew metastably or were probably artificially intro-duced into the sample during thin-section prepara-tion (Massonne 2014).

Material and Methods

In addition to samples studied by conventional pet-rographic methods, three porphyritic gneiss sam-ples (T37, T985, and T905C) and one pelitic enclavesample (T376) were selected and prepared for de-tailed analytical study. Chemical composition ofthe mineral phases and X-ray (XR) elemental mapswere obtained with a CAMECA SX100 instrumentat the Centro de Instrumentación Científica, Uni-versity of Granada (CIC-UGR; see García-Cascoet al. 1993 for details of the optimized analytical rou-tine). Operating conditions for single-point analysiswere 20-kV accelerating voltage, 20-nA beam current,and 5–10-mm spot size. Calibration standards weremineral albite (Na), diopside (Si), wollastonite (Ca),

638 A . S ÁNCH E Z - N AVA S E T A L .

vanadinite (Cl), sanidine (K), TiO2 (Ti), rhodonite (Mn),BaSO4 (Ba), CaF2 (F), and Fe2O3 (Fe) and syntheticpericlase (Mg), Al2O3 (Al), Cr2O3 (Cr), and NiO (Ni).Working conditions for XR maps were 15 kV and200 nA. Images were obtained by moving the samplerelative to the electron beamwith a step (pixel) size of5 mm and real-time acquisition of 45 ms. Experi-

ments demonstrated that a high beam current com-binedwith a short counting time (milliseconds ratherthan seconds) precludes beam damage (García-Casco2007). The XR maps were processed with the soft-ware DWImager (R. L. Torres-Roldán and A. García-Casco, unpublished manuscript). They consist ofcolor XR images overlain onto a synthetic grayscale

Figure 1. A, Western Mediterranean Alpine belts, including the Betic Cordillera. B, Tectonic map of the BeticCordillera. C, Tectonic map of the Torrox area (central sector of the Alpujarride Complex), with location of the TorroxGneissic Complex (star). A color version of this figure is available online.

Journal of Geology 639P O L YM E TAMOR PH I SM I N TH E A L PU J A R R I D E COM P L E X

base layer that reveals the basic textural features ofscanned areas calculated as^ (countsi#Ai), whereAis atomic number and i is Si, Ti, Al, Fe, Mg, Ca, Na,and K.

Whole-rock analyses of major elements and Zrwere performed with Philips Magix Pro (PW-2440)X-ray fluorescence equipment (CIC-UGR) after fu-sion with lithium tetraborate. Typical precision was

Figure 2. Features of megacrystic orthogneisses in the field. A, In this outcrop, the porphyritic gneisses (which in-clude a pegmatitic pod) are made of 2–10-cm-long, more or less aligned, elongated crystals of K-feldspar (Kfs) within aground mass of quartz, plagioclase, biotite, and muscovite. Both the gneiss and the pegmatitic pod are evidentlyfolded, but the Kfs megacrysts are only slightly deformed and exhibit poor to almost random orientation, althoughthey are (almost invariably) affected by at least one foliation. B, Close-up of the orthogneiss in A, showing disorientedeuhedral megacrysts. C, Slightly deformed Kfs crystal (note conjugated fractures at 607) with rims partially replaced byandalusite (And). A color version of this figure is available online.


better than51.5% for analyte concentration of 10%and better than 55% for 100 ppm Zr. Trace ele-ments, except Zr, were determined with a NEXION300D ICP-MS (CIC-UGR) afterHNO31HFdigestionin a microwave heated Teflon-lined vessel, evapora-tiontodryness,andsubsequentdissolutionin100mLof 4-vol%HNO3. Precisionwas better than55% foranalyte concentrations of 10 ppm.Hand-picked zircon grains from the porphyritic

gneiss sample T985 were cast, together with threezircon standards (one grain of SL13, a few grains ofGAL, and several grains of TEMORA), on a 3.5-cm-diameter epoxy mount (megamount), polished, anddocumented with optical (reflected and transmittedlight), secondary electron, and cathodoluminescence(CL) images obtainedwith a scanning electronmicro-scope. After extensive cleaning, mounts were coatedwith gold (80-mm thickness) and analyzed withSHRIMP (IIe/MC equipment) at the IBERSIMS labo-ratory (CIC-UGR;seeprocedure inwww.ugr.es/~fbea,Sánchez-Navas et al. 2014, and the appendix, avail-able online).

Mesoscale Observations and Deformation

The TGC is strongly affected by heterogeneous defor-mation, intense metamorphism, and partial meltingthat produced a broad spectrum of gneissic structures.Some lithotypes are graphite-bearing paragneissesand pelitic gneisses, which are geometrically relatedto the overlying metapelite sequence and are cross-cut by deformed dikes of leucogranites (Sánchez-Navas et al. 2014). Downward, there are stronglydeformed Grt-bearing banded gneisses and mildlydeformed coarse-grained porphyritic gneisses lackingGrt. Porphyritic gneisses are derived frommegacrys-tic anatectic granites and locally include deformedpegmatitic pods (fig. 2A). K-feldspar megacrysts arerelict phenocrysts of the former granite (fig. 2A, 2B).Megacrysts with size up to 10 cm, euhedral crystalshapes, and single (Carlsbad) twinning and frequentmyrmekite textures attest to the magmatic originof the parent rock (fig. 2B, 2C). Some rims of, andpatches within, large Kfs (K-feldspar) megacrystalsare partially corroded and replaced by And (fig. 2C).Coarse-grained porphyritic gneisses are included

within fine-grained Grt-bearing banded gneisses andmetapelites, forming centimeter- to meter-thick con-cordant bands with respect to their most evident fo-liation in thefield. The gneisses show a subhorizontalmain foliation (S2) defined by preferential orienta-tion of Bt-rich schlieren, by moderate reorientationof megacrystic/phenocrystic Kfs and Pl (plagioclase),and by polycrystalline aggregates of deformed Qz(quartz). Coarse- and fine-grained gneisses are inter-

layered with pelitic gneisses and with Ms-rich andTur (tourmaline)-richmica schists close to the base ofthe overlying pelitic sequence. Minor discordant leu-cocratic rocks form centimeter- to decimeter-thickfissure fillings and dikes that intrude the gneissesand, locally, the overlying schists. The dikes crosscutthemain foliation (S2) in the gneisses but are affectedby a younger foliation (S3), which is commonly sub-parallel to S2 (Sánchez-Navas et al. 2014). Cuevaset al. (1989) recognized ENE-directed tectonic trans-port related to the mylonitic S2 foliation and asso-ciated stretching lineation in the gneisses.The porphyritic gneisses contain pelitic clots

rich in Bt (fig. 3). Locally, the pelitic clots are largerand form enclaves rich in Bt and Al-silicates (Als).They systematically include And, which sometimesforms large prisms (locally 13 cm long and 11 cmthick; fig. 3) growing either from the Bt-rich ma-trix or from Kfs crystals of the surrounding gneiss(fig. 3A, arrow). The large And prisms grew post-kinematically on a foliation observed within theenclaves (dotted line in fig. 3A, corresponding tothe S2 foliation). Nonetheless, both the enclaves andthe surrounding gneiss are affected by a new folia-tion (S3 in fig. 3). This foliation is mainly observedalong the boundaries of the enclaves but also insidethem, along oriented bands of Bt 1 Ms and shearbands in the larger And crystals (fig. 3B).The subhorizontal main foliation in the enclaves

is defined as S2 because an older S1 foliation is ob-served microscopically, in the same way as previ-ously described in the schists enveloping the gneisses(Sánchez-Navas et al. 2012). The main (S2) foliationis locally overprinted by a younger (S3) foliation. TheS3 foliation is associated with dominantly top-to-the-west shear zones that affect the enclaves and theenclosing gneiss (fig. 3). In the schists surroundingthe TGC, the S3 foliation records an evolution fromkyanite-grade ductile shear zones to chlorite-bearinglow-grade S-Cmylonites, and then to brittle duplexesproduced by west-directed imbrication.

Textural Relations and MineralComposition in the Enclaves

The Bt-rich pelitic enclaves commonly includegraphite laths and, in addition to And, they containthe assemblage made of Ms, Tur, Ky, St, Grt, Rt(rutile), Ilm (ilmenite), and Ap (apatite) but lack Qzand feldspars. The larger enclaves grade into smallerBt-rich clots of the enclosing porphyritic gneissthroughdisaggregationofpeliticmaterialwithin thequartz-feldspathic matrix during the magmatic evo-lution of the rocks. Deformation affecting the en-claves produced grain-size reduction and dissolution


of larger Bt andMs crystals (respectively, Bt1 andMs1in fig. 4A). It resulted in the formation of the S3 foli-ation observed within the enclaves, which is definedby elongated, fine-grained aggregates of Bt 1 Ms (re-spectively, Bt2 and Ms2) with abundant fine-grainedKy (here referred to asKy2;fig. 4A) that also grew fromdeformed And (fig. 4B).

Less deformed pelitic enclaves are mainly consti-tuted by mosaics of millimeter-sized unoriented Btcrystals plus And (fig. 5A). Their textural relationsshow the occurrence of (1) early St (fig. 5B), (2) largeKy prisms predating And (Ky1; fig 5C), and (3) veryfine-grained Ky (Ky2) postdating And (fig. 5D, 5E).

Figure 6A shows detailed textural and mineralrelations in the selected moderately deformed en-

clave (sample T376) after processing of XR imagesfrom the region highlighted in figure 5A. Broadareas of the enclave are occupied by large Bt andMsplates and by Als polymorphs (mainly And, lessfrequently Ky). In addition, a thin Grt corona ap-pears close to its contact with the gneiss. Fe-Ti ox-ide minerals (Rt 1 Ilm) appear in the core of theenclave, with Rt being replaced by Ilm (fig. 6B;tables S1, S2; tables S1–S11 are available online).Among the micas, Bt is generally predominant, butMs is also locally abundant.

Textural relations and compositional maps indi-cate two chemically different types of Ms grains:large plates ofMs aremainly locatedwithin the coreof the enclave (primary Ms1) and show high Ti con-

Figure 3. Mesoscopic features of large andalusite (And) porphyroblasts within biotite (Bt)-rich pelitic enclaves in-cluded in the porphyritic gneisses. Carlsbad-twinned K-feldspar (Kfs) phenocrysts are preferentially oriented, togetherwith elongated Bt-rich clots, along a foliation that affects both the gneiss and the enclaves. A, And growth from Bt inthis polished slab is postkinematic to a foliation (dotted line), clearly visible within the enclave (which is tentativelycorrelated to the S2 visible in the schists enclosing the Torrox Gneissic Complex), but it also grew at the expense ofsurrounding Kfs crystals (arrow); nevertheless, the outer part of the enclave and the surrounding Kfs megacrysts andBt clots are clearly sheared along another foliation (S3). B, And porphyroblast within a fusiform enclave affected as itsgneissic matrix by the S3-related deformation. And crystal inside the enclave was also flattened and sheared, mainly inits outer part (white arrow). A color version of this figure is available online.


tent, whereas fine-grained Ms crystals have low Ticontents and relatively high Si contents, either ifthey are included in plagioclase (Ms2 in fig. 6C) orif they are associated with fine-grained Ky2 (Ms2(c)in fig. 6C). The Grt coronas (fig. 6A) are made ofgrains that, in spite of their small size, are chemi-cally heterogeneous, with areas with varied Mg, Fe,Mn, and Ca composition (fig. 6D, 6E; table S3). Bio-tite also shows a marked textural and chemicalzoning: larger crystals located at the interior of theenclave (Bt1) show higher Ti and Mg/(Fe 1 Mg) andlower Al contents than Bt adjacent to garnet coro-nas (Bt2; fig. 7).The composition of muscovite distributes in two

distinct clusters (Ms1 and Ms2), with compositionaltrends mainly explained by the Tschermak substi-tution (Al2Si21Mg21; fig. 8). The Si and [VI]Al con-tents of Ms1 range, respectively, between 6.16 and6.31 atoms per formula unit (apfu) and between3.35 and 3.48 apfu, whereas those of Ms2 are higher(6.28–6.54 apfu for Si and 3.56–3.74 for [VI]Al; fig. 8;tables S4, S5). The Mg and Ti contents of Ms1 areslightly higher (0.21–0.31 apfu for Mg) and clearlyhigher (0.18–0.20 apfu for Ti) than those in Ms2(0.14–0.25 and 0.01–0.06 apfu for Mg and Ti, re-spectively). Nevertheless, Fe ranges are similar forMs1 (0.12–0.23 apfu) and Ms2 (0.14–0.22 apfu).The compositional heterogeneity of biotite does

not distribute in such clear, distinct clusters for Bt1and Bt2 (fig. 9). The large plates of Bt1 located far fromtheGrt-rich rimof theenclave (fig. 9, circles; tableS6)have highTi contents, up 0.531 apfu, andhighMg/Feratios, up 0.823 apfu. The Bt2, located close to Grt

(fig. 9, triangles; table S6), is frequently smaller insize, poorer in Ti (minimum value is 0.049 apfu),and richer in [VI]Al (11.25 apfu). These composi-tional variations are explained as follows: (1) thevariation in Ti is controlled by the changes in [VI]Aland^[VI] (Ti-Al vacancy substitution: Ti3[VI](o)[VI]Al24;García-Casco 1993); (2) there is a strong negativecorrelationbetweenMgandFe (FeMg21 substitution)and positive correlation between Ti and Mg (whichexcludes the Ti vacancy substitution Ti[VI](o)Mg22);(3) the Si vacancy substitution (Si2[VI](o)[IV]Al22Fe21;see Robert 1976) explains the positive correlationbetween Fe and [IV]Al; and,finally, (4) the effect of theFeMg21 substitution explains thenegative correlationbetween Mg and [IV]Al corresponding to the Tscher-mak substitution.

Geochemistry and Geochronology of the Gneisses

Whole-rock major- and trace-element analyses ofthe porphyritic gneisses are given in table S7. Majorelements were used for calculating the molar An(anorthite)-Ab (albite)-Or (orthoclase) diagram afterusing a set of basis vectors consisting of Qz, Or,Ab, and An in the software CSpace (Torres-Roldánet al. 2000). The composition of the three porphy-ritic gneisses is similar and plots in the peralumi-nous granite field (fig. 10A, 10B). Trace-elementconcentrations normalized to the upper continentalcrust remain close to unity in the spider diagramsbut show well-defined depletion spikes in Zr andHf; variable enrichment in Li, Rb, and Cs; and de-pletion in Sr and Ba (fig. 10C). Chondrite-normalized

Figure 4. A, Foliation (S3) within this biotite (Bt)-rich (Bt1) enclave is defined by fine-grained kyanite (Ky2) and finer-grained micas (Bt2 and Ms2 [Ms p muscovite]) formed along shear planes disrupting the larger Bt1 crystals. B, In thesame enclave, andalusite (And) appears partially transformed to aggregates of fine-grained Ky2 and Ms2. A colorversion of this figure is available online.


Figure 5. Optical (A, C–E) and back-scattered electron (B) images of pelitic enclaves included in porphyritic gneisses.A, Biotite (Bt)-rich enclave (about 1 cm2) without visible traces of internal foliation and formed by an aggregate ofdisoriented Bt crystals (Bt1) that are partially transformed to andalusite (And). The small square area indicates thelocation of the inset in D and E, and the large rectangle the location mapped with electron probe microanalysis infigure 6. B, Staurolite (St) within a millimeter-size Bt-rich schlieren. C, Euhedral kyanite (Ky1) from a Bt-rich enclave.D, Close-up (parallel nicols) of the small square area in A, showing the replacement of the And porphyroblast by apolycrystalline aggregate of Ky (Ky2). E, Same as D, but with crossed polarizers. A color version of this figure isavailable online.

rare earth element (REE) patterns showmarked spreadin heavy REE contents, a decrease in the La/Yb ratio,and a smooth negative Eu anomaly (fig. 10D).Zircon grains from porphyritic gneisses (sample

T985) are generally euhedral, colorless, and inclu-sion free in transmitted light. Many grains containhighly luminescent inherited cores (fig. 11). CLimages also reveal low-luminescence growth band-ing in the euhedral (magmatic) rims.Darker rims aregenerally U-rich (table S8). Low-luminescence rimsshow weakly developed oscillatory zoning, alternat-ing with lighter areas close to grain borders. Theouter luminescent zones formvery thinfilms aroundthe zoned rims. In some cases, dissolution-affectedsubhedral cores appear surrounded by zoned dark-

gray and light-gray rims. SHRIMP dating of mag-matic zircon U-rich rims yielded an average age of286 5 11 Ma (figs. 11, 12). Ages slightly older than286 Ma are obtained from euhedral CL-bright zirconcores, and younger ages are found in euhedral CL-dark rims (fig. 11; table S8). In addition, some CL-bright zircon cores (in some cases clearly inherited;e.g., spot analyses 18.2 and 25.2 in fig. 11) gave pan-African (ca. 620 Ma) and older ages.

Discussion

Andalusite-Kyanite Relationships and Ionic Reactionsin the Enclaves. The reaction textures observed

Figure 6. A, Phase map of the enclave shown in figure 5A (rectangle), formed by plates of Bt1 (biotite) and Ms1 (mus-covite), plus aluminum silicates (Als), showing a corona of small garnet (Grt) grains together with fine plates of Ms2within plagioclase (Pl) at the boundary between the enclave and the gneiss. Ilmp ilmenite; Qzp quartz; Rtp rutile. B,Close-up of the Fe-Ti oxide minerals in the core of the enclave (central region of A), where Ilm forms from Rt; the othermineral phases aremasked under the grayscale base layer.C, Ti X-raymap of muscovites (other mineral phasesmaskedunder the grayscale base layer), where warmer colors represent more intense X-ray signals in counts per second. Largeplates of Ms correspond to primary Ms1; recrystallized Ms of the rim, which is frequently included in Pl, correspondsto Ms2; Ms2(c) corresponds to Ms2 tightly intergrown with kyanite (Ky2; see text for details). D, Mg/(Fe 1 Mg) distri-bution in the Grt of the rim (left-hand dotted area in C). E, Ternary diagram Alm (almandine)-Grs (grossular)-Prp(pyrope; molar proportions) from single-spot electron probe microanalyses of the Grt of the rim. Colors refer to theregions in the Mg/(Fe 1 Mg) map of the Grt in D where single-spot analyses were performed. Sps p spessartine.


Figure 7. Ti (A), Mg/(Fe 1 Mg) (B), and Al (C) distribution maps of biotite (Bt). All other mineral phases were maskedout within the grayscale base layer. Bt1 p high-Ti and Mg/(Fe 1 Mg) and low-Al biotite of the interior of the enclave;Bt2 p low-Ti and Mg/(Fe 1 Mg) and high-Al biotite at the contact with the garnet corona.

within the enclaves and surrounding gneisses revealthe following local unbalanced reactions:

1. Kfs → And (figs. 2C, 3A);2. Ky1 (relict idiomorphic Ky prisms; fig. 5C) →

And (fig. 5A);3. Rt 1 Bt1 → Ilm (fig. 6A, 6B);4. Bt1 → And 1 Qz 1 Ilm (figs. 6A, 7);5. And → Ms (fine-grained Ms2(c) in fig. 6C);6. Ms2(c) → Ky2 (fig. 5D, 5E);7. Bt1 → Grt (figs. 6A, 6D, 7).

The formation of And from Kfs is observed in therims of Kfs megacrysts (fig. 2C); in the Bt-rich en-claves, where large And crystals forms (fig. 3A); andinside Bt-rich clots within the porphyritic gneissesforming symplectite aggregates of And 1 Qz. Thetransformation of Kfs to And can be modeled bythesubsolidus ionic reaction2Kfs1 2H1 → 1And15Qz1H2O1 2K1 (Vernonetal.1987;Kerrick1990).Therefore, compositional and textural features sug-gest that reaction 1 was catalyzed by biotite and/ormuscovite (figs. 3, 4). According to reaction 2, the

blastesis of And porphyroblasts occurred after Ky1,as also observed in the schists overlying the TGC,where pseudomorphs of And after Ky1 occur (García-Casco and Torres-Roldán 1996, 1999; Sánchez-Navas et al. 2016). In the sameway, reaction 3occursboth in the enclaves and in the metapelites (fig. 6B;see García-Casco and Torres-Roldán 1996, 1999).The replacement of Rt by Ilm in contact with Btplates is well described by the following net transferreaction in the simple KFASHT system:

K2Fe6Al2Si6O20(OH)4 (Ann)1 2:25TiO2 (Rt)

p 1:5FeTiO3 (Ilm)

1 0:5K2Ti1:5Fe4Al4Si4O20(OH)4 (Ti‐Ann)

1 0:5K2Fe5Si8O20(OH)4 (Si‐Ann),

where “Ann” is annite.Bt has been included in the Rt → Ilm transfor-

mation (reaction 3) because it is the only availablephase that supplies Fe to the surrounding areas ofthe Rt crystal where Ilm was formed (figs. 6A, 6B,

Figure 8. Bivariate diagrams of primary (Ms1; circles) and recrystallized (Ms2; triangles) muscovites of the enclaves. Acolor version of this figure is available online.


7). In reaction 3 above, Bt composition is modifiedthrough the chemical substitutions Ti-Al vacancy(Ti3[VI](o)[VI]Al24) and Si vacancy (Si2[VI](o)[IV]Al22Fe21),as shown in figure 9. However, the compositional

variation of Bt1 in the enclaves also implies thedehydrogenation substitution (Ti-oxy substitution)TiO22

2Fe21(OH)22 producing the Ti-oxy-annite endmember (Ti-oxy-Ann; fig. 9). Therefore, the forma-

Figure 9. Bivariate diagrams plotting single-spot analyses corresponding to the biotite of the interior of the enclave(Bt1; circles) and biotite close to the Grt corona of the rim (Bt2; triangles). A color version of this figure is availableonline.


tion of Ilm from Rt and Bt can also be modeledthrough the KFASHTO reaction:

K2Fe6½Al2Si6�O20(OH)4 (Ann)

1 4TiO2 (Rt) p 2FeTiO3 (Ilm)

1 K2Fe4Ti2½Al2Si6�O24(Ti‐oxy‐Ann)

1 4H1 1 2O22:

These reactions suggest thatH1 released from theRt 1 Bt → Ilm reaction (reaction 3) induced hydro-gen (auto-)metasomatism in other regions of matrixbiotite that decomposed to form fine-grained ag-gregates of And from Bt (i.e., reaction 4 above andfigs. 4A, 5B). Rutile can be removed from the massbalance in the case of reaction 4 because Bt1 cansupply the Ti necessary for the formation of thescarce Ilm occurring in textural relation to newlyformed And (fig. 6A). Therefore, using the observed

composition of the biotite and ilmenite, reaction 4can be written as follows in the KNFMASHFT sys-tem:

(K1:729Na0:078)Fe2:236Mg1:827Ti0:481Al0:847

½Al2:426Si5:574�O20(OH)3:456F0:544 (Bt1)

1 8:50H1 p 0:24Fe1:845Ti2:037O6 (Ilm)

1 3:94Qz1 1:64And1 5:98H2O1 0:54F

1 1:8Fe21 1 1:83Mg21 1 1:73K1 1 0:08Na1:

In addition to reaction 2, the pelitic enclaves alsopreserve the reverse transformation, And → Ky2:shearedAnd prisms and palms are partially replacedby fine-grained Ky2 that is tightly intergrown withsmall Ms2 crystals interpreted here as catalyzers(Ms2(c); figs. 4B, 5D, 5E, 6C). Sánchez-Navas et al.(2012) described the same Ms-catalyzed And → Kytransformation in the schists overlying the TGC.This transformation resulted from local reactions in-volving ion exchange between adjacent reaction do-mains: reaction 5 above, consuming And ((3=2)Als1(3=2)Qz1 1K1 1 2H1 → 1Ms) and the reversed re-action6above, producingKy,which—whenadded—produce the netAnd→Ky2 reaction (seeCarmichael1969).In a similar way, Grt coronas of the outer rim of

the enclaves (fig. 6A, 6D) formed through the ionicreaction 7 from the Bt-rich enclave and the enclos-ing porphyritic gneiss. The mass balance below,including stoichiometric molecular species (Kfs,Ab, and An instead of K1, Na1, and Ca21 ions, re-spectively), shows the formation of Grt coronas af-ter Bt in the KNaCaFeMgAlSiHFTi system:

18:226(K1:729Na0:078)Fe2:236Mg1:827Ti0:481Al0:847

½Al2:426Si5:574�O20(OH)3:456F0:544 (Bt1)

1 38:296Qz1 36:287H1 1 1An1 1:174Fe21

p 4:313Fe1:86Mg0:029Ti2:031O6 (Ilm)

1 14:417(Ca0:072Fe2:352Mg0:521)Al1:991Si2:985O12

(Grt)1 31:530Or1 1:417Ab1 49:635H2O

1 25:661Mg21 1 9:913F:

This reaction needs addition of Fe21 as reactant be-cause Ilm and Grt have lower Mg/Fe ratio than Bt.

Granite Protolith of the Enclave-Bearing Gneisses.Field observations, and the texture and composi-tion of the porphyritic gneisses of the TGC indicatethat their protoliths correspond to a peraluminousanatectic granitic suite that, according to geochro-nological data, is late Variscan in age (early Perm-

Figure 10. A, Ternary An (anorthite)-Ab (albite)-Or(orthoclase) plot of porphyritic gneisses. B, Projection ofthe studied samples onto the IAI (inverse agapitic index)-ASI (alumina saturation index) diagram. Ab, Or, and Ms(muscovite) are also indicated for reference. C, Uppercrust–normalized spider diagram for porphyritic gneisses.D, Chondrite-normalized rare earth element patterns inporphyritic gneisses.


ian: 2865 11 Ma) and, consequently, formed at thebase of the pre-Permian metasedimentary succes-sion of theTorroxUnit. However, bulk compositionsof the gneisses slightly deviate from the granite fieldtoward the field of themetapelites (fig. 13). This trendis explained by (deformation-assisted) ionic reactionstriggering dissolution of Kfs and production of Msand/or Als (Vernon et al. 1987) and by the occurrenceof other base-leaching reactions that form Als fromPl, Bt, andMs (Kerrick 1990). In the studied case, thisis evidenced by the partial replacement of large Kfsmegacrystals by And (fig. 2D) and of Pl by phengiticmuscovite (Ms2; fig. 6C). Therefore, neither And norMs2 in the gneisses ismagmatic in origin (see below).

Trace-elementdistribution (fig.10C) andchondrite-normalized REE patterns (fig. 10D) of the studiedgneisses compare well with those reported for S-typeperaluminous granites by many authors (e.g., Car-rington and Watt 1995; Vigneresse 1995). They indi-cate that the granite precursor rocks of the porphy-ritic gneisseswere derived by anatexis of sedimentaryprotoliths. The peraluminous character and high SiO2

and low FeO 1 MgO 1 TiO2 1 CaO contents of thestudied gneisses also agree with (meta)sedimentarysources for melts. The overall compositional charac-teristics of the precursory anatectic granitic suite

match those of Variscan granitic rocks from otherAlpujarrideunits and theMoroccanSebtides (Rossettiet al. 2010, 2013; Acosta-Vigil et al. 2016). The ob-served negative anomalies in Sr, Ba, and Eu (fig. 10B,10C) suggest that someKfs and Pl fractionation couldhave occurred duringmagmatic segregation, emplace-ment, and crystallization (Carrington and Watts1995). Marked depletion in Zr and Hf seems to becontrolled by zircon, which would be refractory tomelting (Watt and Harley 1993; Watt et al. 1996) orremained shielded within major rock-forming min-erals (Bea 1996).

Origin of the Andalusite. Andalusite in intrusiveperaluminous granitic magmas has been consideredthe product of magmatic crystallization (Kerrick1990). Zeck and Whitehouse (1999) suggested thatthe protolith of the Torrox porphyritic gneisses wasa Variscan And-bearing Bt-granite intruded at lowpressure (!5 kbar). Nevertheless, textural relationsvisible either directly in the field or in thin sectionsof the porphyritic gneiss indicate that And formedunder subsolidus conditions after the replacementof Kfs megacrysts (fig. 2). The enclaves show similarrelationships, asAndmainly formed from the peliticmatrix but also grew from adjacent Kfsmegacrystalssurrounding it (fig. 3A).

Figure 11. Cathodoluminescence images of selected zircons from sample T985 of porphyritic gneiss, with indicationof 206Pb/238U ages, the analyzed spot, and number of the ion-probe analyses in table S8, available online.


The presence of relict Ky1, St, and Rt in the en-claves suggests that the granite protolith of the por-phyritic gneiss was part of a migmatitic complexformed after partial melting of metapelites at rela-tively high pressure (17 kbar, stability field of Ky 1St 1 silicate melt; see García-Casco et al. 2003).Crustal anatexis of a metapelitic succession similarto that enclosing the TGC, but located in deeperstructural levels, could account for the proposedpartial melting (see also Sánchez-Navas et al. 2014).Large And porphyroblasts like those of the stud-

ied enclaves are commonly found in the schists lyingimmediately above the TGC. These porphyroblastsare chiastolites that formed postkinematically to theS2 (main) foliation of these schists (García-Casco andTorres-Roldán1996; Sánchez-Navas et al. 2012). Theoccurrence of St and Ky1 relicts within the And-bearing enclaves fits well with the pre-S2 formation ofboth minerals within the schists overlying the TGC,whereasAnd formed during or after decompression bythermal overprinting at low P during the post-S2 meta-morphism (García-Casco and Torres-Roldán 1996,1999). While these relations were interpreted as ofAlpine age by those authors, in light of the geochro-nological constraints providedherewe reinterpret themhere to represent a Variscan evolution (see below).In brief, the And porphyroblasts within the studied

pelitic enclaves are considered porphyroblasts formedafter incomplete reequilibration of an intermediate-to high-P anatectic complex during late Variscan de-

compression. The local preservation of St within theenclaves (fig. 4B) is explained by the lack of quartz inthe assemblages (e.g., Ashworth 1975; Grapes 2011).It formed at relatively high T and P, likely close to thehydrous solidus of metapelite (ca. 7007C;García-Cascoet al. 2003).

Pre-Alpine Evolution. The late Variscan (early Perm-ian) age obtained for the biotite granite parent rocksof the porphyritic gneiss constrains the age of theAnd-bearing enclaves and that of the older meta-morphic events in the lower-grade graphite-bearingmetasedimentary rocks. The studied enclaves wereincorporated into the graniticmagma from anatecticrocks equivalent to those of the enclosing graphite-bearing pre-Permianmetapelitic succession. A high-temperature Variscan metamorphism explains thehigh Ti content of Bt1 (up to 0.51 apfu) andMs1 (up to0.2 apfu) in the enclaves. Relics of Ky1 1 Rt also sug-gest higher pressure during early stages of the Va-riscan tectonometamorphic evolution. The transfor-mations Ky1 → And and Rt→ Ilm and the formationof large And porphyroblasts within Bt-rich enclavesthrough, respectively, reactions 2, 3, and 4 discussedabove were consequently produced upon (near-)isothermal decompression during later stages of the

Figure 12. Wetherill concordia diagram showing ana-lytical data for zircon rims (table S8, available online).Reliable zircon ages for the time period from c. 250 to!350 Ma have led us to obtain a mean 207Pb/235U age of286 5 11 Ma (2j) for the magmatism responsible for thegeneration of granitic parent rock of the porphyritic gneissT985.

Figure 13. Composition of the porphyritic gneisses(circles) in the KAlO2-Al2O3-FeO diagram condensed forCaAl(NaSi)21, MnFe21, MgFe21 and F(OH)21 interchangeand projected from Qz (quartz) 1 Bt (biotite) 1 Ilm (il-menite) 1 Ap (apatite) 1 H2O. The squares correspond tocompositions of K-feldspar (Kfs), muscovite (Ms), and bi-otite (Bt) from porphyritic gneisses (see tables S9–S11,available online). Gray areas represent the phase regionsof biotite and muscovite solid solutions (Massonne andSchreyer 1987). A color version of this figure is availableonline.


VariscanOrogeny(fig.14). IntheTGCgneisses,And1Kfs intergrowths after breakdown of large pegma-titic Ms (García-Casco et al. 1993; Sánchez-Navas1999) and the coexistence of And and Crd in leuco-granitic dikes intruding high-grade paragneissicrocks and schists of the TGC (Sánchez-Navas et al.2014) further confirm that late Variscan anatexisevolved from relatively high-P to low-P conditions.The post-S2 And1Crd assemblage evidences a low-P/high-T imprint that must be interpreted in rela-tion to the late Variscan (early Permian) emplace-mentofthegraniteprotolithoftheporphyriticgneiss.

Therefore, the pre-Alpine geodynamic scenarioof the TGC Alpujarride rocks consisted of (1) earlyVariscan crustal thickening related to orogenic con-vergence and (2) late Variscan crustal thinning and

extensional collapse, explaining decompression andleading to thermal relaxation (fig. 14). Such a geo-dynamic scenario was coeval to the late Paleozoicoblique convergence among Gondwana, Laurasia,and other crustal blocks located between them. Itwas responsible for the collision and sealing of thecontinents to form Pangea and for the late orogenicextension of the previous collisional belt (Tait et al.2000; Stampfli et al. 2002; Stampfli and Borel 2004;Torsvik and Cocks 2004; Cocks and Torsvik 2006,among many others). This collision also involved,inCarboniferous-Permian time,Gondwana-derivedPaleozoic terranes currently making up part of theAlpine-Mediterranean Orogen, among them thoseof the Alpujarride Complex and other Betic-RifianInternal Domains (von Raumer 1998; von Raumer

Figure 14. Tentative pressure-temperature-time (P-T-t) paths of pre-Mesozoic rocks from the Torrox Gneissic Com-plex involved in the Variscan and Alpine orogenic cycles, based on relevant reactions for partial melting and graniticmelt crystallization in the symplified NKASH system, following White et al. (2001), and Si isopleths in muscovite-phengite solid solution after Massonne and Schreyer (1987) in the KMASH system. Dashed lines correspond to poorlydefined parts of the P-T-t paths. Themain pre-Alpine (D1, D2) andAlpine (D3) deformation phases and growth episodes ofkyanite (Ky) and andalusite (And) in the pelitic enclaves (Ky-Rt [rutile]-And-Ilm [ilmenite]-Bt [biotite]-Ms [muscovite]5St [staurolite]) included in (Qz [quartz]-Kfs [K-feldspar]-Pl [plagioclase]-Bt-Ms) gneisses, together with Alpine syncol-lisional exhumation and uplift, are indicated. apfu p atoms per formula unit; Sil p sillimanite. A color version of thisfigure is available online.


et al. 2002, 2003; Williams et al. 2012; Berra andAngiolini 2014 and references therein).A Variscan evolution similar to that proposed for

the studied TGC rocks has been reported in theIberian Massif (e.g., Martínez Catalán et al. 2009).Once theVariscancollisionhadprogressed, largepartsof Variscan orogen were also intensely affected bylow-P/high-T metamorphism forming And 1 Crd 1Sil mineral assemblages in metasedimentary rocks(Ugidos 1981; Gil Ibarguchi and Martínez 1982; Mar-tínez et al. 1990; Barbero and Villaseca 1992; Pereira1993; Pereira and Bea 1994; Briggs 1995; Díez Baldaet al. 1995; Schuster and Stüwe 2008; Díez Fernándezet al. 2012). The same has been reported in some Aus-troalpine basements of the eastern Alps (e.g., Cesare1999). In the crustal succession surrounding the BeniBousera peridotites (Rifian Sebtides), a pre-Alpinemedium- to high-P/high-T metamorphic episode isrecordedby thegenerationofmedium- tohigh-Pmeltsin relation to high-grade metamorphism in amphibo-lite to granulite facies (Montel et al. 2000; Rossettiet al. 2010; Gueydan et al. 2015). The samewas alsoobserved in the crustal succession overlying theRonda peridotites (Massonne 2014).

Alpine Evolution. In the TGC, the transformationof pre-Alpine And to Ky2 and the formation of co-ronas of small Grt at the boundary between enclavesand gneiss through reactions 5–7 discussed abovetestify to a medium- to high-P and medium-T eventthat must be necessarily related to the Alpine Orog-eny (fig. 14). During the Alpine metamorphism, thechemical composition of the early biotite (Bt1) changeddrastically close to the Grt-rich rim of the enclave(Bt2). Thus, compositional heterogeneity of biotite(fig. 9) indicates chemical reequilibration of the Bt1instead of net growth of Bt2. Hydrogen metasoma-tism, forming Grt from Bt, was also responsible forfine-grained Ky2 formation from Ms catalyzers (phen-gitic Ms2) in the enclaves. The S3 foliation, which isdefinedbyKy2, Bt2, andMs2 in the studied enclaves andalso affects the enclosing gneisses and themetapeliticsuccession enveloping theTGC,must also be relatedto the Alpine Orogeny (Sánchez-Navas et al. 2014).The occurrence of a medium- to high-P meta-

morphism of Alpine age is usually interpreted inrelation to rapid subduction, thrusting, and crustalthickening and to stacking of the Malaguide Com-plex onto the Alpujarride Complex and of the dif-ferent Alpujarride thrust units (e.g., Torres-Roldán1979; Martín-Algarra and Estévez 1984; Martín-Algarra 1987; Tubía and Gil-Ibarguchi 1991; Guer-rera et al. 1993; Azañón et al. 1998; Martín-Algarraet al. 2000; Vergés and Fernández 2012). This con-tractional event was responsible for the develop-ment of kyanite-grade ductile shear zones in the

studied enclaves (and in the surrounding peliticrocks; Sánchez-Navas et al. 2012) at T ≈ 5007C andP ≈ 0.9 GPa, according to the Sip 3.3 apfu isoplethfor phengite (fig. 14). In higher parts of the UpperAlpujarride crustal segment outcropping in the Tor-rox area, the same event is also recorded at slightlylower P (Benamocarra Unit; Sánchez-Navas et al.2016). We interpret that this P peak in Torrox wasroughly coeval to the T peak and that it occurred atca. 215 2Ma (fig. 14). This eventwas responsible forthe partial resetting of U-Pb SHRIMP zircon rimages documented in different rocks of the TGC (al-though, surprisingly,not inthesamples studiedhere;however, see Sánchez-Navas et al. 2014) and by geo-chronometric dating of the same rocks by othermethods (e.g., Rb-Sr; Zeck et al. 1989b).Alpine shortening was preceded by the Triassic

rifting of Pangea, which represents a continuationof the Permian lithospheric thinning after the post-collisional phase of the Variscan Orogeny. The Ce-nozoic Alpine geodynamics was characterized by acomplex history of subduction of Mesozoic oceaniclithosphere followed by continental collision, whichledtotheAlpineemplacementofupper-mantlerocksand eventually to the formation of the Gibraltar arc.The ultramafic rocks emplaced at the base of theUpper Alpujarride-Sebtide continental crust, whichhad been thinned in pre-Alpine times, were finallyexhumed to the surface by thrust emplacement dur-ing earlyMiocene shortening and by the subsequentlate Alpine (middle Miocene and younger) syncollisio-nal extensional evolution (e.g., Balanyá et al. 1993;Mazzoli andMartín-Algarra 2011;Mazzoli et al. 2013).

Concluding Remarks

In the Alpujarride Complex, most studies during thepast 35 years have proposed an Alpine tectonometa-morphic evolution of the Betic Cordillera (in partic-ular since the work of Platt and Vissers 1989) consist-ing of an early crustal thickening related to orogenicconvergence followed by a late crustal thinning andextensional collapse. Most of these studies, however,have never considered (and sometimes neglected)that, actually, the Alpujarride rocks were affected bya complex polymetamorphic history, both Alpineand pre-Alpine.This study demonstrates that unraveling poly-

metamorphic histories from high-T rocks like thoseof the Alpujarride TGC gneisses requires appropri-ate and advanced geochronological and petrological(both textural and compositional) analyses but thatthese must be carefully integrated with field stud-ies. The coexistence of diverse aluminosilicate poly-morphs, the compositional heterogeneity of biotite


and muscovite, and the reaction textures involvingthe formation of large andalusite porphyroblasts andgarnet coronas in pelitic enclaves from the TGC ana-tectic gneisses resulted from a polymetamorphic his-tory with at least two main episodes: (1) pre-Alpineevents that produced a high-T metamorphism of pre-Permianmetasedimentsandtheiranatexisundermod-erate to high P, followed by decompression and gran-ite emplacement at higher crustal levels at the endof theVariscanOrogeny, and (2)moderate- to high-Pearly Miocene metamorphic overprint related tothe Alpine Orogeny that predates the Burdigalianand younger syn- to late-orogenic exhumation.

ACKNOWL EDGMENT S

This work is supported by grants CGL2016–75679P(Ministerio de Economía y Competitividad, Spain)and P11-RNM-7067, RNM-179, andRNM-208 (Juntade Andalucía, Spain). We acknowledge the help ofthe CIC-UGR staff (in particular M. A. Hidalgo andP.Montero) during the analytical studies that allowedthe development of this research. This is the contri-bution 42 of the IBERSIMS laboratory. We thankF. Rossetti and D. B. Rowley for their detailed andconstructive reviews and suggestions that helped usto improve the article.

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