Journal of Petrology Volume 41 Number 10 Pages 1489-1515 2000
© Oxford University Press 2000
P–T Paths Derived from Garnet Growth Zoning in an Extensional Setting: an Example from the Tormes Gneiss Dome (Iberian Massif, Spain)
1DEPARTAMENTO DE PETROLOGÍA Y GEOQUÍMICA, UNIVERSIDAD COMPLUTENSE, 28040 MADRID, SPAIN
2DEPARTMENT OF EARTH SCIENCES, MEMORIAL UNIVERSITY OF NEWFOUNDLAND, ST JOHN’S, NFLD., CANADA A1B 3X5
Received May 6, 1998; Revised typescript accepted March 22, 2000
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL CONTEXTTEXTURES AND MINERALOGYMINERAL CHEMISTRYP-T PATH RECONSTRUCTIONTECTONIC INTERPRETATIONCONCLUSIONSREFERENCES |
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The Tormes Gneiss Dome (NW Iberian Massif, Variscan Belt of Spain), comprises a metamorphic core complex (Lower Unit) bounded by a major extensional detachment. Despite metamorphic temperatures in the upper amphibolite facies (
700–740°C), metapelites from the highest levels of the Lower Unit contain garnet with preserved growth zoning. These rocks were used for reconstruction of quantitative P–T paths based upon interpretation of microfabrics and thermodynamic modelling of garnet zoning. The results are consistent with a two-stage tectonothermal evolution under high-grade conditions: (1) an early compressional phase of deformation that led to upper amphibolite facies Barrovian-type metamorphism and to P increase and T rise to approximately 9 kbar and 700–725°C; (2) a subsequent major extensional phase of deformation that led to quasi-isothermal decompression from 8–9 to 3 kbar at T conditions between 700 and 740°C. Several lines of structural, textural and petrological evidence suggest that up to 15–20 km of overburden was removed from the Lower Unit by tectonic exhumation while these rocks were still at upper amphibolite facies conditions. A final stage of quasi-isobaric cooling to greenschist facies conditions is locally recorded in late low-grade detachments. KEY WORDS: P–T paths; garnet growth zoning; extensional tectonics; Iberian Massif
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL CONTEXTTEXTURES AND MINERALOGYMINERAL CHEMISTRYP-T PATH RECONSTRUCTIONTECTONIC INTERPRETATIONCONCLUSIONSREFERENCES |
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Extensional tectonics have been recently recognized as an essential mechanism for reducing crustal thickness to normal levels during late stages of orogenic processes. This mechanism leads to rapid exhumation of deep rocks to shallow levels during crustal thinning and exerts a major control in the thermal evolution of orogenic belts (Sonder et al., 1987
; Sandiford, 1989; Hodges et al., 1993; Platt, 1993; Ruppel, 1995; Soto & Platt, 1999). In the Variscan Belt of Western Europe, rocks exhumed from deep crustal levels commonly outcrop as plutono-metamorphic complexes of regional scale and dome-like geometry (Van den Driessche & Brun, 1989; Malavieille et al., 1990). These complexes are characterized by highly ductile deformation (Eisbacher et al., 1989; Faure et al., 1990; Malavieille, 1993; Doblas et al., 1994; Díez Balda et al., 1995) and by low-P/high-T metamorphism during decompression (Dewey, 1988; Escuder Viruete et al., 1994; Reinhardt & Kleemann, 1994), together with extensive partial melting (Lagarde et al., 1994; Brown & Dallmeyer, 1996). In general, the metamorphic peak conditions of the complexes are petrologically well supported, whereas both prograde and post-peak histories are less thoroughly investigated. The accurate analysis of assemblages associated with late extensional fabrics, and the P–T paths that can be thus derived, are of crucial importance, especially in testing the viability of the different tectonic models and their application to high-grade Variscan metamorphic complexes.
This paper focuses on the detailed reconstruction of the metamorphic history recorded by a suite of metapelites from the upper levels of a Variscan metamorphic core complex: the Lower Unit of the Tormes Gneiss Dome (TGD), located near Salamanca town, in NW Spain. Despite high metamorphic temperatures (700°C), these rocks display evidence for only limited partial melting and contain garnet with growth zoning, suitable for thermodynamic modelling. A quantitative P–T path was reconstructed based upon interpretation of microfabrics, disequilibrium textures, chemical zoning of minerals, P–T data obtained by thermobarometry, and thermodynamic modelling of garnet zoning. This P–T path provides important constraints on the exhumation of the Lower Unit of the TGD. The results suggest that movement along a major extensional shear zone located in the upper structural levels of this unit had a profound effect on the metamorphic evolution. This complements a previous study of the general metamorphic characteristics of the Lower Unit (Escuder Viruete et al., 1997) that documented for the first time the decompressional character of the P–T evolution and its possible relation to extensional tectonics in this area.
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GEOLOGICAL CONTEXT |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL CONTEXTTEXTURES AND MINERALOGYMINERAL CHEMISTRYP-T PATH RECONSTRUCTIONTECTONIC INTERPRETATIONCONCLUSIONSREFERENCES |
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Regional framework
The Tormes Gneiss Dome (TGD) is located in the NW sector of the Iberian Massif (Fig. 1), which represents the westernmost segment of the European Variscan Belt, formed during the collision of Gondwana and Laurentia in the Upper Palaeozoic (Martínez Catalán et al., 1996
). The NW sector of the Iberian Massif has been divided into four zones (Julivert et al., 1972; Arenas et al., 1986): the Cantabrian Zone, the West Asturian–Leonese Zone and the Central Iberian Zone, which are parautochthonous, and the Galicia–Trás-os-Montes Zone, which is allochthonous (Fig. 1). During Variscan convergence, the evolution of an orogenic wedge, built by the stacking of these tectonic units, controlled the compressive deformation and its eastern migration over time from the innermost sectors of the Central Iberian Zone to the foreland of the Cantabrian Zone (Pérez Estaún et al., 1991; Arenas et al., 1995; Dallmeyer et al., 1997). The suture is located in the western part of the belt, in the Galicia–Trás-os-Montes Zone, and is identified by the presence of ophiolitic rocks in Cabo Ortegal, Ordenes, Bragança and Morais complexes.
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The Central Iberian Zone is characterized by extensive granitic magmatism and by the presence of high-grade metamorphic complexes of regional extent (Martínez et al., 1988), one of which is the TGD (Fig. 1). The compressional structures in this internal zone, or D1 structures, consist of NE- and N-vergent, large-scale, recumbent folds with an S1 axial-planar foliation, and a system of thrust sheets bounded by ductile shear zones that caused imbrication of the pre-Variscan granitic basement and its sedimentary cover (Macaya et al., 1991; Pérez Estaún et al., 1991; Escuder Viruete, 1998). The resulting crustal thickening led to the development of mineral assemblages of Barrovian affinity (M1; Martínez et al., 1988; Arenas, 1991). Earlier Variscan compressional structures are variably overprinted by a major extensional event, described in the TGD (D2; Escuder Viruete et al., 1994) and in other parts of the Central Iberian Zone (Doblas et al., 1994; Díez Balda et al., 1995; Valverde Vaquero et al., 1995; Escuder Viruete et al., 1998), and attributed to large-scale collapse of the thickened continental crust.
The Tormes Gneiss Dome (TGD)
The TGD consists of two main structural units (Figs 2 and 3; Escuder Viruete et al., 1994). The Lower Unit is a high-grade metamorphic core complex that dominantly consists of augen-gneisses (618 ± 9 Ma, U/Pb zircon; Lancelot et al., 1985), overlain by variably migmatized ortho-derived leucogneisses and metapelites with lenses of marble and calc-silicate gneisses. The Upper Unit comprises the highest structural levels of the dome and consists of a monotonous sequence of Lower Cambrian slates and schists. This is unconformably overlain by an Ordovician–Silurian sequence of anchimetamorphic and epizonal fossiliferous platform sediments (Oliveira et al., 1992). To the NW, the Upper Unit underlies the allochthonous Galicia–Trás-os-Montes Zone. The TGD was intruded by two types of granitic rocks: (1) biotite and two-mica peraluminous granitoids, with metasedimentary enclaves, mainly concentrated in the inner parts of structural dome; (2) biotite metaluminous granodiorite and subordinate tonalite with xenoliths of diorite, in the Upper Unit. Isotopic data suggest intrusion ages at 325–318 Ma for both types of granitoids (Serrano Pinto & Gil Ibarguchi, 1987).
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The two main units of TDG are separated by a D2 kilometre-scale ductile shear zone, exposed in a NW–SE culmination and characterized by low-grade normal detachments on top (Fig. 4; Escuder Viruete et al., 1994). This shear zone displays a large variety of kinematic indicators (Hanmer & Passchier, 1991) including S–C fabrics, asymmetric boudinage, shear bands, garnet with helictic and sigmoidal inclusion trails, asymmetric tails and pressure shadows of porphyroclasts, and oblique grain shape fabrics and quartz crystallographic fabrics. The shear sense parallel to the stretching lineation (Fig. 2) consistently indicates tectonic transport of the upper structural levels down to the SE, compatible with the previously determined direction of extension (Escuder Viruete et al., 1994). In the low-grade normal detachments situated on top and in the late-stage antithetic shear zones developed by doming (Escuder Viruete et al., 1994), the S2 mylonitic foliation is deformed by zones of crenulation cleavage (Platt & Vissers, 1980). These are at low (15–30°) angles to the foliation and produce SE- and NW-directed normal-sense offset, respectively in the SE and NW sectors of the extensional dome (Fig. 4). This suggests an evolution of the extensional deformation with time from the ductile to the brittle regime (Escuder Viruete et al., 1994).
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An M2 metamorphic event associated with crustal extension variably overprints the previous Barrovian M1 metamorphism and was documented by Gil Ibarguchi & Martínez (1982), Martínez et al. (1988) and Escuder Viruete (1995). During this event, the Lower Unit reached high-T amphibolite facies conditions transitional to mid-P granulites, and underwent partial melting (López Plaza, 1982; López Plaza & Gonzalo, 1993; Escuder Viruete et al., 1997). A metamorphic study of the lower structural levels of this unit, based upon analysis of mineral assemblages and thermobarometry using cores and rims of homogenized garnet, provided evidence of significant decompression from peak conditions of 6·5–8·0 kbar and 750–770°C to 3·5 kbar at 650°C (Escuder Viruete et al., 1997). Isotopic U/Pb dating of monazite and cooling ages in D2 mylonitic fabrics provide an upper age limit for the onset of extension at 330 Ma (Valverde Vaquero et al., 1995; Valverde Vaquero, 1997). Additional age constraints are placed by the 325–318 Ma ages of emplacement of granitoids (Serrano Pinto & Gil Ibarguchi, 1987), which are syn- to post-S2.
This paper focuses on the highest levels of the Lower Unit (Fig. 3). In contrast to the core of the Lower Unit, the D2 ductile deformation and the associated M2 metamorphism are developed heterogeneously in space (Fig. 5). This heterogeneity may reflect strain hardening and deformation partitioning into more narrow zones with falling temperature during successive stages of the extensional deformation (Platt, 1983). As a result, different structural domains equilibrated during different stages of the metamorphic evolution. Therefore, microstructural analysis of the successive D2 fabrics can be used to unravel consecutive portions of the P–T path. In addition, these levels display minimal partial melting and contain metapelitic rocks in which garnet preserves growth zoning. Therefore, these rocks are appropriate for thermodynamic modelling of P–T paths.
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In the following sections we present: (1) an interpretation of the reaction history based upon textures and mineralogy in the different microstructural domains; (2) a discussion of the mineral composition and zoning of selected metapelitic samples; (3) ‘forward’ thermodynamic modelling (Spear & Selverstone, 1983) of garnet and plagioclase zoning to establish a P–T path; (4) ‘reverse’ thermodynamic modelling (De Capitani, 1994) using bulk compositional data and the previously established P–T path to examine the reproducibility of zoning profiles and test the validity of the P–T data.
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TEXTURES AND MINERALOGY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL CONTEXTTEXTURES AND MINERALOGYMINERAL CHEMISTRYP-T PATH RECONSTRUCTIONTECTONIC INTERPRETATIONCONCLUSIONSREFERENCES |
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The highest grade mineral assemblage observed in metapelites of the Lower Unit is Qtz + Pl + Kfs + Bt + Grt + Sil [mineral abbreviations after Kretz (1983)
], with Ap, Ilm and Zrn as accessory phases, and is considered to represent the thermal peak (Fig. 6; Escuder Viruete et al., 1997). In the upper structural levels of the Lower Unit, this assemblage occurs in the interior of metre-scale sigmoidal boudins that are characterized by a coarse-grained granoblastic S2 fabric, and are surrounded by high strain zones (Fig. 5). Within the boudins the S2 fabric is defined by biotite–sillimanite-rich domains that alternate with garnet-bearing quartzo-feldspathic lenses. An L2 mineral lineation is defined by the elongation of prismatic sillimanite and quartz ribbons. Garnet is idioblastic to subidioblastic (up to 1 cm in diameter), shows straight contacts with K-feldspar, and is characterized by snowball or sigmoidal inclusions that are consistent with syn-D2 growth (Fig. 7a). These inclusions consist of biotite, plagioclase, quartz, ilmenite and, locally, K-feldspar and sillimanite. In addition, some large garnet porphyroblasts display a prekinematic, inclusion-free core. Quartz ribbons are characterized by intense annealing and recrystallization, indicating that D2 deformation in the boudins occurred under high-T (near peak?) conditions. In addition, there are local relics of prograde syn-D2 reaction textures, such as garnet and K-feldspar growing over an early S2 foliation defined by Bt + Sil + Qtz in which sillimanite crystals define L2 (Escuder Viruete et al., 1997). Muscovite occurs as a relic in the interior of Sil + Kfs aggregates, or in biotite that defines S2. Also, rare Ms–Qtz symplectites may be pseudomorphs after kyanite. Locally, these boudins display diffuse fibrolite-bearing leucosomes that are interpreted to be the result of partial melting.
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High strain zones and boudin rims are characterized by garnet porphyroclasts that are partially replaced in syn-D2 pull-aparts by fibrolitic sillimanite or Bt + Qtz ± Pl aggregates (Fig. 7b). These textures are interpreted to post-date the thermal peak. The garnet porphyroclasts are preserved in sigmoidal quartzose microdomains bounded by zones rich in biotite, sillimanite and ilmenite, that define an S–C protomylonitic–mylonitic fabric of type II (Lister & Snoke, 1984
). Fibrolitic sillimanite is parallel to L2 and stable in both S and C surfaces, but the nematoblasts are boudinaged with biotite filling the open pull-aparts. All these features are consistent with the retrograde nature of the S2 fabric in these bands. In addition, quartz occurs as an interpenetrative mosaic of equant grains that define ribbons parallel to the L2 stretching lineation and show conjugate grain-boundary alignments resulting from high-T migration of grain boundaries during deformation (600–700°C; Lister & Snoke, 1984). This microtexture, together with the extensive recrystallization of plagioclase, suggests that the D2 deformation continued under high- to medium-T amphibolite facies conditions in the high strain zones, and can be considered to record the onset of cooling. The uppermost structural levels of the Lower Unit are characterized by progressively more intense D2 retrograde effects. In the metapelites, development of lower-grade assemblages in S2 fabrics includes partial replacement of sillimanite by andalusite and of K-feldspar by muscovite. Within the extensional crenulation cleavage that locally overprints the S2 foliation, the Qtz + Ms + Chl ± Ab ± Bt ± Ilm ± Hem assemblage is stable, indicating greenschist facies conditions (Fig. 7d). In these fabrics, recrystallization of quartz took place by progressive rotation of sub-grains, feldspar porphyroclasts are partially replaced by Ms + Qtz aggregates, and biotite porphyroclasts reveal internal deformation and variable amounts of chloritization.
Interpretation
The mineral assemblages discussed above can be used to establish the sequence of metamorphic reactions and constrain the syn-D2 P–T path. The presence of muscovite relics surrounded by syn-D2 biotite or within Sil + Kfs aggregates suggests that muscovite was part of the pre-D2 mineral assemblage in the metapelites. On the other hand, its absence from the peak assemblage indicates peak T conditions above the stability of muscovite in the presence of quartz. In leucosome-free areas, the Ms-out reaction (Fig. 8; Thompson, 1982, 1990) can be described as
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, 1990; Clemens, 1990; Fig. 8) and destabilization of muscovite by reaction (R1) occurs at higher pressures, proportional to the reduction in the aH2O. Therefore, variations in aH2O with prevailing aH2O<<1 may explain why muscovite breakdown in the study area only rarely led to melt production.
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The presence of K-feldspar and sillimanite as inclusions in syn-D2 garnet indicates that this mineral, with the possible exception of the pre-kinematic cores, grew at temperatures above the destabilization of muscovite. It should be noted that the peak mineral assemblage Grt–Kfs–Sil–Qtz–Bt–Pl is stable in the P–T field of the continuous NaKFMASH reactions
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, 1990; Vielzeuf & Holloway, 1988; Johannes & Holtz, 1996). However, as in the case of the Ms-out reactions, low aH2O has the effect of expanding the field at which reaction (R4) occurs, and this is consistent with the scarcity of melts in the study area. An upper-T limit may be placed by the absence of orthopyroxene, implying that the temperature was not high enough for Opx-producing reactions in pelitic rocks, such as
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800°C (Fig. 8; Thompson, 1990; Spear, 1993).
Replacement of garnet by Bt + Qtz ± Pl aggregates suggests operation of the retrograde reaction
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These replacement microtextures occur in pressure shadows of the garnet porphyroclasts or in syn-D2 pull-aparts (Fig. 7b) and have been interpreted as occurring during late stages of the D2 deformation. Reaction (R7), calculated with the thermodynamic data of Berman (1988; updated in 1992) and the software PTAXSS (Perkins et al., 1987; Brown et al., 1988), in the CaKFMASH system, displays a moderate positive slope in P–T space (
P/T 0·714 kbar/100°C). This suggests, besides the addition of H2O, a late-D2 decompression. The formation of fibrolite at the expense of garnet can be interpreted as an inversion of (R4) during the retrograde evolution. Also, the late replacement of K-feldspar, sillimanite and biotite by Ms + Qtz aggregates, the local growth parallel with L2 of prisms of andalusite and the late S2 mylonitic fabrics formed under greenschist facies conditions indicate significant cooling and rehydration by inversion of (R2) (Fig. 8) during the later stages of the P–T path (Escuder Viruete et al., 1997).
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MINERAL CHEMISTRY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL CONTEXTTEXTURES AND MINERALOGYMINERAL CHEMISTRYP-T PATH RECONSTRUCTIONTECTONIC INTERPRETATIONCONCLUSIONSREFERENCES |
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In this section we discuss chemical compositions of minerals from three selected samples of metapelite (samples T1, Ta2 and Tb2) from the interior of sigmoidal boudins. These samples display garnet interpreted to preserve growth zoning and are free of textures related to retrogression. Chemical analyses of garnet, biotite, plagioclase, K-feldspar and ilmenite were performed on the Cameca SX50 electron microprobe at Memorial University of Newfoundland (Canada), in energy-dispersive (ED) mode. Analytical conditions were 15 kV accelerating potential, 75 s counting time, 20 nA specimen current and 1 µm beam diameter, except for plagioclase, for which 10 nA specimen current and 5 µm beam diameter were used to avoid loss of Na. Data were reduced by a ZAF correction program. In each sample, garnet porphyroblasts with the largest diameter (between 1·5 and 3.5 mm) were analysed along rim–core–rim traverses at regular intervals (50–100 µm depending on the size of the garnet). Matrix minerals were analysed at 3–5 spots per grain, with the exception of plagioclase, where rim–core–rim traverses with 5–15 evenly spaced spot analyses were performed. Representative mineral analyses, particularly those used in the thermobarometric calculations, are given in Tables 1–4. Structural formulae of minerals were calculated using the THEBA4 software (J. Martignole et al., unpublished software, 1992). For selected garnet grains, Fe, Mn, Mg and Ca X-ray mappings of garnet and additional quantitative analyses have been carried out using a JEOL Superprobe JXA-8900M wavelength-dispersive (WD)–ED combined microanalyser at the Universidad Complutense of Madrid. Accelerating voltage, beam current and beam diameter for quantitative analyses were kept at 15 kV, 12 nA on the Faraday cup and 3 µm, respectively. The complete dataset for all three samples used in this study can be obtained from the first author upon request.
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Garnet
The analysed grains consist of an internal inclusion-free core (not present in all the grains), an intermediate zone with sigmoidal syn-D2 inclusions of Qtz, Kfs, Sil, Ilm, Bt and Pl, and a thin inclusion-free rim that displays straight boundaries against the matrix minerals (Fig. 9). All grains display a symmetric outward decrease of XSps and XGrs (from 0·10 to 0·02 and from 0·12 to 0·04, respectively), an increase in XAlm, and to a lesser extent, in XPrp (from 0·76 to 0·91 and from 0·02 to 0·04, respectively) and an overall slight decrease in XFe [Fe/(Fe + Mg)] (Fig. 10). These trends are typical of growth zoning. In the grains for which compositional maps were made (Fig. 10), contours of Fe, Mn, Mg and Ca show consistently the form of near-euhedral polygons. These polygons represent the trace of successive growth surfaces and the overall zoning patterns can be considered as not having been significantly altered by post-growth diffusion. Zoning is weak in the inclusion-free cores (grains GRT-I and III, Fig. 11a and b) and becomes steeper outwards. This change in the chemical trends indicates a change in the conditions of growth that coincide with the transition between the clean cores and the inclusion-bearing zones. The chemical trends described above are locally slightly reversed in the outer 150–200 µm. These reversals are typical of limited diffusion-controlled retrograde zoning (Spear, 1993
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Biotite
In each sample, biotite grains in contact with garnet, and grains at different distances from garnet were analysed, but no compositional variations were detected. Analysed biotite displays Fe/(Fe + Mg) values between 0·85 and 0·87, and Ti and AlVI contents in the range of 0·15–0·18 and 0·42–0·52, on the basis of 12 (O) (Fig. 12a). All samples display high modal amounts of biotite (22–30%) relative to garnet (10–15%) so changes in the composition of the former during retrograde Fe–Mg exchange with garnet can be considered as insignificant [infinite biotite reservoir concept of Spear & Florence (1992)
].
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Plagioclase and K-feldspar
Three textural types of plagioclase are distinguished: matrix plagioclase, plagioclase in pull-aparts and pressure shadows around garnet, and scarce syn-D2 plagioclase inclusions in garnet. Plagioclase of the first type occurs as large ovoid porphyroblasts oriented parallel to the S2 fabric, with straight boundaries against garnet. They display normal zoning with An16–18 in the cores and An11–12 at the rims (Fig. 12b). The second type is of homogeneous composition, with An7–11 (Fig. 12d). The core–rim composition of plagioclase inclusions in garnet ranges from An19 to An13 (Fig. 12c), similar to the normal zoning and composition range of matrix porphyroblasts. K-feldspar occurs in several textural settings: as large clear porphyroblasts associated with garnet, sillimanite, plagioclase and quartz; interstitially with granular quartz in veins parallel to the S2; intergrown with fibrolite mats; and as large poikiloblasts. The last commonly contain rounded inclusions of quartz and biotite, and may be intergrown with plagioclase. K-feldspar porphyroblasts are not zoned and have low albite contents (Xab = 0·02–0·10).
Ilmenite
Ilmenite occur as inclusions in garnet, in syn-S2 sillimanite and in K-feldspar porphyroblasts, and these are considered to belong to the peak mineral assemblage. They were analysed to gain information on the partial pressure of oxygen and allow estimation of Fe3+ in biotite that cannot be obtained by microprobe analysis. Analysed grains appear fresh and, as shown by scanning electron microscope images, devoid of exsolution microtextures. The composition of the Fe–Ti oxides is close to pure ilmenite, with negligible XFe2O3 (Table 4). According to the empirical correlation between the XFe2O3 in ilmenite and Fe3+/(Fe3+ + Fe2+) in coexisting biotite established by Williams & Grambling (1990), XFe2O3 0 in ilmenite corresponds to Fe3+/(Fe3+ + Fe2+) 0·12 in biotite, which is considered to be the minimum Fe3+ content in natural biotite.
Interpretation
The observed zoning trends in garnet and plagioclase, together with biotite composition, can be interpreted in terms of reactions that operated at various stages during the prograde and retrograde evolution. The rimward decrease of XGrs, XSps and Fe/(Fe + Mg), and the increase of XAlm and XPrp, are typical compositional trends during garnet growth (Spear, 1989, 1993). The preservation of growth zoning in these high-grade rocks indicates either that peak T conditions were not high enough to homogenize garnet, or that they did not last long enough. Hiroi et al. (1998) reported garnet from high-grade metapelites showing similar concentric compositional zoning, with cores enriched in XSps relative to rims, most probably reflecting growth zoning. The progressive increase of XAlm and XPrp observed in the syn-D2 part of the garnet can be attributed to growth during D2 deformation by the prograde reaction (R4) in the KFMASH system. In addition, the slight outward decrease in Fe/(Fe + Mg) is compatible with a slight rise in T during syn-D2 garnet growth (Spear & Florence, 1992).
In pelitic systems closed to Na and Ca, where the only calcic phases are garnet and plagioclase, formation of the grossular component in garnet requires consumption of anorthite in plagioclase, according to the GASP reaction:
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P/T2·254 kbar/100°C, calculated with the thermodynamic database of Berman and the software PTAXSS), suggests that the rimward decrease of XGrs can be correlated either with a drop in P conditions, or with conditions of both P increase and T rise along a gradient less steep than that of calculated reaction (R8) (Indares, 1995). Reaction (R8) also leads to a shift of the remaining plagioclase composition toward albite (Spear et al., 1991). This is consistent with the observed normal zoning in the large matrix plagioclase grains parallel to S2, which present straight contacts against idioblastic garnet (Fig. 12b). Therefore, the composition of the core of the matrix plagioclase is interpreted to be the closest to the composition of the plagioclase at the onset of garnet growth. In addition, the cores of plagioclase inclusions in syn-D2 garnets have similar compositions to those of the cores of the matrix grains (Fig. 12c), furthermore suggesting that this should be the likely composition of the initial plagioclase. This is also consistent with mass balance constraints. For instance, recalculation of the original plagioclase composition in sample T1 by reconverting the grossular content of garnet to anorthite yielded XAn = 20–21, i.e. only 1–2% higher than the XAn of the plagioclase cores.
The slight decrease of XPrp and the resulting increase of Fe/(Fe + Mg) at the outer garnet rims are probably due to the exchange reaction
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P–T PATH RECONSTRUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL CONTEXTTEXTURES AND MINERALOGYMINERAL CHEMISTRYP-T PATH RECONSTRUCTIONTECTONIC INTERPRETATIONCONCLUSIONSREFERENCES |
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Mineral rim thermobarometry
The Grt + Qtz + Bt + Pl + Kfs + Sil assemblage in metapelites permits the use of two equilibria to estimate the P–T conditions of chemical equilibration between phases: the Grt–Bt, Fe–Mg exchange thermometer (GARB; Ferry & Spear, 1978
) and the Grt–Pl–Sil–Qtz barometer (GASP; Koziol & Newton, 1988). For this, we used the composition of garnet and adjacent plagioclase rims in textural equilibrium, together with biotite that is chemically homogeneous at the scale of the thin section. In cases where no plagioclase was in contact with garnet we used rims of matrix plagioclase with normal zoning. The composition of the rims of these two types of plagioclase, in places where they can be compared, is within the same range of XAn.
In the CaKFMASH system, the intersection of the two equilibria yields a P–T point that was calculated with the TWEEQU software (Berman 1988; updated in 1992), which uses an internally consistent thermodynamic dataset and incorporates non-ideal activity–composition models for garnet (Berman, 1990), biotite (McMullin et al., 1991) and plagioclase (Aranovich & Podlesski, 1989). Sillimanite and quartz were treated as pure phases.
Results and interpretation
The results obtained using rims of six garnet grains and their adjacent phases are shown in Table 5 and in Fig. 13. Calculated temperatures fall within a narrow range, between 694 and 747°C, whereas pressures cover a range between 2·5 and 7 kbar. These differences in pressure, obtained in rocks that belong to the same structural level, are probably the result of local equilibria established during different stages of a P–T path that is characterized by decompression of several kilobars under quasi-isothermal conditions at high temperature (720°C).
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The calculated P–T conditions fall within the stability field of sillimanite, which is the only aluminium silicate observed in the samples. In addition, T conditions are above the stability field of muscovite (Fig. 13). Metamorphic pressures up to 7 kbar together with lack of leucosomes in the sampled boudins are in accordance with aH2O <<1, because aH2O = 1 would lead to elimination of muscovite via reactions producing melt above 4 kbar. Estimated mineral rim equilibrium conditions are also lower than those required for the appearance of orthopyroxene in metapelites [reaction (R6)] consistent with the absence of orthopyroxene from the studied rocks.
Modelling technique and assumptions
Spear & Selverstone (1983) have shown that P–T paths can be constrained from the growth zoning of garnet, using a mathematical formulation based upon differential thermodynamics. According to this method, a matrix of differential equations is constructed involving the intensive variables dT, dP and dX, where dP and dT are the differentials of pressure and temperature and are treated as dependent variables, and dX are the differentials of the concentration of phase components and are treated as independent variables. Input parameters consist of S, V and ‘starting’ compositions for given P–T conditions. The set of equations is determined by a number of compositional variables equal to the thermodynamic variance of the system (v), therefore v independent compositional variables must be specified (using a finite difference procedure) to solve for the dependent variables, i.e. P and T. Therefore, initial incremental changes in P and T can be calculated from incremental changes of the independent compositional variables along a zoning profile, using as a starting point compositions for which P–T conditions can be established by conventional thermobarometry, such as, for instance, rims (for discussion and applications see also Selverstone et al., 1984; Spear et al., 1984; St-Onge & King, 1987; Spear & Rumble, 1987; Spear, 1989, 1993; Frost & Tracy, 1991; Hodges, 1991). This method is based upon three fundamental assumptions: (1) the zoning observed in garnet is the product of growth by one or more continuous reactions that can be established by means of the inclusions in garnet; (2) the zoning has not been modified by diffusional processes since the time of growth of the crystal; (3) the system remained closed during garnet growth.
In the studied samples, inclusions in garnet have the same composition as the matrix phases, i.e. quartz, biotite, plagioclase, sillimanite, K-feldspar and ilmenite, indicating that the mineral assemblage present during the growth of the syn-kinematic part of the garnet was the same as the matrix assemblage. However, the method cannot be applied in the case of the pre-kinematic cores of garnets GRT-I and -III, which are inclusion-free and therefore do not provide any information about the mineral assemblage that was present during their growth. The abrupt change in zoning trends across the boundaries of these cores (Figs 10 and 11) suggests that it may correspond to a hiatus in garnet growth or the disappearance of a prograde phase such as muscovite. Concerning the second assumption, to minimize chances of alteration of the original growth profile by diffusion, we chose garnet crystals of the largest size (i.e. between 1·5 and 3 mm) and with sharp zoning profiles (e.g. Fig. 11). In addition, the uniform composition of biotite in the sample scale suggests that the retrograde volume diffusion by exchange reactions was not significant, probably owing to rapid exhumation (see below). However, although slight, the retrograde inversion of the zoning trends in some outer rims of garnet (Fig. 11) indicates that the results should be viewed with some caution. Concerning the third assumption, the absence of leucosomes in the sampled locations is in favour of the development of the peak assemblage in a closed system.
Thermodynamic modelling of garnet growth
The assemblage Grt + Bt + Sil + Pl + Kfs + Qtz, considered to be stable during garnet growth, corresponds to a system of nine components MnNaCaKFMASH (MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O), with a thermodynamic variance of four, assuming that free H2O was present. This assumption is based upon the fact that lack of partial melting in these rocks can only be explained by the absence of a fluid or their presence with low aH2O activity. Therefore, the variables P and T can be expressed as functions of four independent compositional variables, such as, for instance, XAlm, XPrp, XGrs and XAn (Spear, 1989). Utilizing as starting conditions the P–T conditions of equilibrium between garnet rims and adjacent phases presented earlier (Fig. 13), the thermodynamic modelling technique of Spear & Selverstone (1983) was used to calculate P–T increments from garnet and plagioclase rims to cores based on variations in these four compositional parameters. Entropies and volumes were calculated at 298·15 K and 1 kbar, using the thermodynamic database of Berman (1988, updated in 1992). Heat capacity, compressibility and expansibility were made using the equations incorporated in TWEEQU software (Berman, 1991). The increments of the compositional parameters and the corresponding changes in P and T conditions are listed in Table 6 and the corresponding P–T paths are shown in Fig. 14. A problem with this approach is that although we can assume equilibrium between garnet and plagioclase rims and also between garnet–plagioclase cores (see section on Mineral chemistry, interpretation), correlation of incremental changes in garnet composition with changes in plagioclase composition cannot be accurately established. The calculations presented here are based upon XAn increments at regular intervals between rim and core of plagioclase crystals. It should be noted that the use of different intervals changes the location of the intermediate P–T points, but not the general shape of the P–T path (Spear, 1989, 1993).
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In the thermodynamic modelling, pre-kinematic inclusion-free cores of garnet GRT-I (Fig. 9a and b) and GRT-III were not used, because they do not comply with the requirements of the method. The calculated P–T paths are consistent with quasi-isothermal decompression of the order of 4–5 kbar under high-T conditions (700°C < T < 740°C; Fig. 14). In all samples, P–T conditions calculated the farthest from the rims straddle the Ky–Sil boundary, at pressures between 8 and 10 kbar. In garnet of samples Ta2 and Tb2, the outer clear inclusion-free rims yielded the lowest P conditions (3 ± 1 kbar).
The maximum T range along a P–T path is of the order of 100°C and was obtained with the zoning profiles A–B and B–C of garnet GRT-VI (Fig. 14e and f). These also display an initial increase of both P and T conditions from 7 kbar and 600°C to 8 kbar and 650°C, and then a slight rise in T (to 700–740°C) during the initial stages of decompression. In this case, maximum P conditions are lower than in the other samples (8–9 kbar) but this may be an artefact of the large spacing of the analyses in this garnet. It should be noted that isothermal P–T paths, obtained in all garnet crystals but GRT-I and GRT-III, are inconsistent with the observation of Spear et al. (1991) that, in pelitic rocks experiencing decompression, garnet grows only with rising temperature. If this is so, then the isothermal paths obtained here may be an artefact of partial relaxation of the Fe–Mg zoning in garnet, this being possible because these two elements have the fastest diffusion rates. On the other hand, slight cooling indicated by the last segments of all the P–T paths is probably due to the weak retrograde resetting in the outer garnet rims.
Thermobarometry of the pre-kinematic garnet core
P–T conditions for the formation of the pre-kinematic garnet cores are essential to constrain the prograde part of the P–T history. However, as mentioned above, the thermodynamic modelling of garnet zoning profiles could not be applied to these cores (GRT-I and -II, sample T1; and GRT-IV and -V, sample Ta2) because of lack of information on the global mineral assemblage stable at the time of their growth. Nevertheless, it is likely that these garnet cores grew in the kyanite stability field, as suggested by the overall shape of the P–T paths calculated by thermodynamic modelling of the syn-kinematic parts of the garnet. In addition, presence of kyanite at that stage is also suggested by the rare relics of Ms–Qtz intergrowths that are interpreted to be pseudomorphs after kyanite. Therefore P–T conditions for the onset of the pre-kinematic garnet growth may be calculated by using the assemblage Grt + Qtz + Bt + Pl + Ky and by making some assumptions on the composition of plagioclase and biotite. As explained above, the calcium-rich cores of plagioclase displaying normal zoning probably preserve the composition of plagioclase at the onset of garnet growth. However, the biotite composition at equilibrium with the garnet core is unknown. During garnet growth in pelitic assemblages, biotite becomes progressively more Mg rich (Thompson, 1976). Therefore, early biotite should have been more Fe rich than the matrix biotite that represents the thermal peak. Therefore, GARB thermobarometry using garnet core and matrix biotite should yield a lower-T limit of the initiation of garnet growth (Indares, 1995). Keeping this in mind, P–T conditions were calculated with the TWEEQU technique using the GARB thermometer and the GASP barometer (Table 7 and Fig. 15).
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These P–T conditions are in the range 600–700°C and 6·5–8·5 kbar, and allow constraints to be placed on the prograde extension of the P–T paths calculated by the thermodynamic modelling of the corresponding syn-kinematic outer portions of the garnet grains. This prograde segment of the P–T paths is characterized by increase in both P and T conditions up to the baric peak. Given that calculated T conditions with the pre-kinematic core are at a minimum (see above), this portion of the prograde paths could be in reality steeper than the calculated ones [see also Indares (1995)].
Modelling of the P–T path in the NaCaKFMASH system
Using bulk-rock compositional data, molar proportions of the phases, and a reference pressure and temperature, it is possible to calculate mineral composition and modal abundance of phases along a prescribed P–T path (Spear, 1989, 1993). Comparison between mineral compositions calculated by this method at different P–T points and measured composition along chemical profiles allows a test of the validity of the P–T paths calculated by forward modelling. Equilibrium phase diagrams and isopleths of the proportions of phase components in garnet and plagioclase were calculated with the computer program DOMINO of De Capitani (1994), which is based on the calculation of equilibrium assemblages by minimization of the total Gibbs free energy (G) using the algorithm of De Capitani & Brown (1987). The extent and absolute positions of different stability fields are strongly dependent on the solution models for each multicomponent phase and the bulk composition given in the input. In this study, the following solution models are considered: garnet (Berman, 1990), feldspar (Fuhrman & Lindsley, 1988), white mica (Chatterjee & Froese, 1975) and biotite (McMullin et al., 1991). The database is an updated version (JUN92) of Berman (1988). The main input of DOMINO consists of a simplified bulk composition. It should be noted that if a real bulk chemical analysis is considered, minor components such as Mn and Ti will not be distributed correctly among the solution phases, because these elements are not included in the solution models. For this, we constructed the ‘bulk compositions’ in the NaCaKFMASH system with normalized microprobe analyses of the minerals that belong to the thermal peak assemblage, and the modal proportions estimated from observations in thin sections.
For the Grt + Bt + Sil + Kfs + Pl + Qtz peak assemblage in sample T1, the input for the calculation is a simplified Fe-rich bulk composition, derived from estimated modal ratios and microprobe measurements: 10 Grt(Grs0·04-Pyp0·10-Alm0·85) + 30 Bt(Phl0·16-Ann0·84) + 15 Pl(Ab0·81-An0·18) + 15 Kfs(Or0·84) + 10 Sil. SiO2 and H2O are added in excess to ensure their presence in all calculations. It should be noted that this composition is consistent with chemical bulk-composition data included in the study by Escuder Viruete (1995). The calculated P–T space is from 300 to 800°C and from 0 to 10 kbar. From thermobarometric studies we have assigned P–T thermal peak conditions of 750°C and 6 kbar. For this starting P–T–X–M point, we have computed in the NaCaKFMASH system the isopleths for the end-members of the most important phases for the fields: Grt–Bt–Als–Kfs–Pl–Qtz–H2O and Grt–Bt–Als–Ms–Pl–Qtz–H2O (for XGrs <0·14). In this system, the boundary reaction of these fields corresponds to the Ms-out reaction (R2).
The calculated P–T–X–M equilibrium phase diagrams and isopleths for sample T1 in the model NaCaKFMASH system are shown in Fig. 16. In the stability field of Grt + Bt + Sil + Kfs + Pl + Qtz + H2O, all phase components of garnet and plagioclase display widely spread isopleths and these two phases undergo only minor compositional changes over a large P–T range. On the other hand, in the muscovite stability field, XGrs and XAb isopleths are narrowly spaced, suggesting drastic change in the proportions of these components in narrow P–T intervals, depending upon the P–T path. The P–T trajectory obtained by thermobarometry and the thermodynamic modelling of the garnet zoning is also shown in Fig. 16. The P–T–X–M results of reverse modelling along of this P–T path are also included in Table 8. The P–T path can be divided into three segments, along each of which distinctive changes in mineral compositions occur:
- The syn-D1 segment, in the muscovite and kyanite stability field, defines a prograde trajectory from about 550–600°C and 6·5 kbar to the baric peak, at 650–700°C and 8·0 kbar, near the Ky–Sil transition. This segment corresponds to the growth of the inclusion-free core of the garnet porphyroblasts. During this stage of the evolution, calculated XGrs decreases from 0·15 to 0·10 (Fig. 16a), XPrp increases slightly from 0·01 to 0·03 and Fe/(Fe + Mg) decreases from 0·97 to 0·95 (Fig. 16c and d). These modelled compositional changes are comparable with the zoning observed in the pre-D2 garnet cores. It should be noted that, despite the close spacing of the XGrs in this P–T field, the pre-kinematic garnet core is weakly zoned because the P–T path is at a small angle to the slope of the isopleths. In plagioclase, calculated XAb increases from 0·82 to 0·84, consistent with production of grossular at the expense of anorthite.
- The early syn-D2 P–T segment, in the Kfs–Sil field, is characterized by decreasing P and rising T, from the maximum P conditions (7·5–8·5 kbar at 650–700°C) to the conditions of the thermal peak (6 kbar at 700–750°C). This segment corresponds to the development of the syn-kinematic garnet. Calculated XGrs decreases from 0·10 to 0·05, XPrp increases to 0·05 and Fe/(Fe + Mg) remains constant or decreases slightly, as a result of the parallelism of the XFe isopleths and the P–T path (Fig. 16). These trends are consistent with the measured compositional profiles of the syn-kinematic garnet rims, away from the retrogressed outer rims. Calculated XAb remains constant at the early part of this segment and then decreases slightly from 0·88 to 0·86 towards the lower-P end of the segment.
- The late syn-D2 P–T segment, in the Kfs–Sil field, is characterized by decompression and cooling, from the thermal peak conditions to 3·0–3·5 kbar and 650–700°C. This segment corresponds to the retrograde resetting of the outer rims of garnet. Calculated XGrs and XAb decrease slightly or remain constant as a result of the parallelism between the P–T segment and the isopleths. XPrp decreases slightly from 0·08 to 0·06 and Fe/(Fe + Mg) increases from 0·95 to 0·97 (Fig. 16). These trends are consistent with the composition of the retrograde garnet rims and the plagioclase grains adjacent to garnet. It should be noted that the general slight decrease in XAb along most of the syn-D2 segments of the P–T path is consistent with the higher XAn contents of small plagioclase adjacent to garnet relative to the matrix plagioclase. The lowest-T part of this P–T path, which is mainly characterized by near-isobaric cooling at P < 3 kbar, is constrained by late growth of muscovite from Kfs + Sil assemblages, andalusite and late-D2 greenschist facies mylonitic S2 fabrics.
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In all segments, small differences between calculated and measured mineral compositions, as well as the compatibility between observed (or inferred) mineral assemblage and the computed P–T field, are in support of the calculated P–T path.
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TECTONIC INTERPRETATION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL CONTEXTTEXTURES AND MINERALOGYMINERAL CHEMISTRYP-T PATH RECONSTRUCTIONTECTONIC INTERPRETATIONCONCLUSIONSREFERENCES |
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The Lower Unit of the TGD displays two main generations of Variscan structures developed during (1) a compressional phase of deformation (D1) and (2) a subsequent extensional phase D2 (Escuder Viruete, 1998
). D1 is responsible for the crustal thickening by stacking of crustal units and tectonic burial of the Lower Unit during its prograde evolution, and gave rise to an M1 Barrovian-type metamorphism as described by Escuder Viruete et al. (1994). The D2 deformation event and the associated M2 metamorphism was mainly characterized by extensional tectonics that overprinted the previous contractional structures and caused crustal thinning (Escuder Viruete, 1988). The metamorphic evolution described in this paper provides additional constraints on the thermal consequences of these events. The early parts of the P–T paths registered in some internal portions of garnet (mainly the pre-kinematic cores) are consistent with a prograde evolution and suggest that the rocks experienced both a P increase and a rise in T, to 8–10 kbar and 650–720°C (Fig. 15). These P–T calculations agree with the replacement in adjacent metabasites of Hbl + Pl + Qtz by Grt + Cpx assemblages, associated with a relic S1 fabric defined by ilmenite, which establishes a syn-D1 temperature rise to upper amphibolite facies conditions. The deduced syn-D1 part of the prograde P–T path illustrates that the Lower Unit was heated while being progressively buried by development of D1 contractional structures. The syn-D2 P–T paths deduced for the Lower Unit from reaction textures and thermodynamic modelling of garnet growth zoning in different metapelitic samples are consistent with each other and collectively indicate near-isothermal decompression during the formation of a high-T S2 fabric. All P–T paths display a clockwise core-to-rim direction on a standard P–T diagram. On the other hand, at the uppermost structural levels, these high-T S2 fabrics are locally affected by a spaced greenschist facies S2 shear band cleavage (Figs 5 and 7d), related to normal low-grade detachments, which correspond to a late-D2 stage of near-isobaric cooling.
The resulting complete syn-D2 path for these uppermost levels of the Lower Unit (Fig. 15) has both of the basic characteristics of a path produced by tectonic denudation: an initial isothermal decompression phase followed by near-isobaric cooling, the latter caused by the thermal re-equilibrium associated with the new structural position (Ruppel et al., 1988; Ruppel & Hodges, 1994). The existence immediately above the unit of the major boundary that represents the tectonic contact with the Upper Unit favours the interpretation that the thermal structure and a large part of the metamorphic evolution were controlled by extensional movement on this tectonic contact. A series of sections show schematically the syn-D2 structural evolution for the Lower Unit correlated with the microstructural evolution of garnet and P–T paths (Fig. 17). Areas with P–T paths similar to those of the Lower Unit of the TGD and characterized by nearly isothermal decompression are the Tauern Window, in the eastern Alps (Selverstone et al., 1984), the Greater Himalayan Metamorphic Sequence, in southern Tibet (Hodges et al., 1993), and the Abukuma metamorphic terrane, in Japan (Hiroi et al., 1998).
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Preservation of growth zoning in the garnet of this study is probably due to short duration of the peak metamorphic temperatures, which otherwise are high enough to favour grain-scale diffusion. It should be noted that garnets from lower structural levels of the Lower Unit lack growth zoning, attesting to diffusional homogenization during the metamorphic peak (Escuder Viruete et al., 1997), at the same T range and grain size. A possible explanation for this contrast between these two types of garnet may be the thermal effect associated with the movement along a major extensional detachment, discussed by Ruppel et al. (1988). According to those workers, the rocks located closest to the detachment in the footwall would experience faster cooling than those from the underlying layers. Because the metapelites of this study come from the structural level immediately below the detachment, they may have cooled faster than the underlying lithological units.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL CONTEXTTEXTURES AND MINERALOGYMINERAL CHEMISTRYP-T PATH RECONSTRUCTIONTECTONIC INTERPRETATIONCONCLUSIONSREFERENCES |
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The results of this study indicate that Variscan extensional tectonics had a profound effect on the metamorphic evolution of the Lower Unit of the TGD. The tectonothermal evolution is characterized by the development of an early prograde Barrovian metamorphic event, coeval with contractional deformation, which led to upper amphibolite facies metamorphism and anatexis in some pelitic compositions. Thermobarometric data obtained by using pre-D2 garnet cores and results of thermodynamic modelling of the syn-D2 outer zones of garnet indicate a progressive increase of P and rise in T from
7 ± 0·5 kbar and 600°C to 9 ± 0·5 kbar and 700–725°C. Several lines of structural, textural and chemical evidence suggest that up to 15–20 km of overburden was removed from the Lower Unit of the TGD by tectonic exhumation while these rocks were still at upper amphibolite facies conditions. The syn-D2, P–T path deduced from thermodynamic modelling of syn-D2 garnet and mineral rim thermobarometry is characterized by (1) an initial isothermal decompression phase from 8–9 to 3 ± 0·5 kbar at T > 700°C, related to tectonic exhumation, and (2) subsequent near-isobaric cooling to greenschist facies conditions under low-P conditions, associated with thermal re-equilibration in the new structural position. This P–T path is consistent with the tectonic transport along the dominant extensional shear zone and the later low-grade detachments, located in the upper structural levels of the unit, as well as with the tectonic juxtaposition of the Lower Unit with the Upper Unit.
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ACKNOWLEDGEMENTS |
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This paper represents part of the first author’s Ph.D. thesis research. We are very grateful to R. Rodríguez Fernández (ITGE) for his support in the field work, which was carried out in the framework of the MAGNA cartography programme. The collaboration of J. R. Martínez Catalán from the University of Salamanca and M. Lago from the University of Zaragoza is gratefully acknowledged. We thank two anonymous reviewers for their critical constructive and helpful reviews, and acknowledge the valuable comments of S. Tait. Some aspects of this study received financial aid from projects DGICYT PB91-0192-C02 and PB94-1396-C02. Permission to undertake laboratory work in Canada was given by G. Quinlan, of the Memorial University of Newfoundland. M. Piranian, of the Memorial University of Newfoundland, ensured an efficient microprobing environment during the course of this study. We are most grateful to J. González del Tánago and A. Larios from the CAI Lluís Brú of the Universidad Complutense of Madrid, for comments and kind help with element X-ray mapping of garnet.
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FOOTNOTES |
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*Corresponding author. Telephone: +34 (91) 3944906. Fax: +34 (91) 5442535. e-mail: escuder{at}eucmax.sim.ucm.es
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