Journal of Petrology Volume 41 Number 10 Pages 1471-1488 2000
© Oxford University Press 2000
Experimental Constraints on Hercynian Anatexis in the Iberian Massif, Spain
1DEPARTAMENTO DE GEOLOGÍA, UNIVERSIDAD DE HUELVA, CAMPUS DE LA RÁBIDA, 21819 HUELVA, SPAIN
2DEPARTAMENTO DE GEOLOGÍA, UNIVERSIDAD DE OVIEDO, ARIAS DE VELASCO S/N, OVIEDO, SPAIN
3DEPARTMENT OF GEOLOGY, UNIVERSITY OF GEORGIA, ATHENS, 30602 GA, USA
Received April 7, 1999; Revised typescript accepted February 28, 2000
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL OUTLINE OF THE...THE OLLO DE SAPO...MELTING EXPERIMENTS ON THE...DISCUSSIONCONCLUSIONSREFERENCES |
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We have studied experimentally the melting relationships of the Ollo de Sapo gneiss (OSG), an important crustal protolith for the Iberian leucogranites, of possible volcanoclastic origin. The results of this study are compared with previously determined PTt paths, allowing us to interpret the mechanisms of melting and granitoid production during the Hercynian orogenic cycle. Phase relationships determined in fluid-absent experiments indicate that the OSG is a fertile source for peraluminous leucogranites. The slope of the fluid-absent solidus is strongly controlled by the breakdown of Ms in the presence of Qtz, Pl and Kfs. This solidus curve has a positive slope ranging from dP/dT = 30 bar/°C at low P (<6 kbar) to dP/dT = 70 bar/°C at higher P (6–15 kbar). The relationships between the Ms vapour-absent solidus and the PTt metamorphic paths in different sectors of the Iberian massif have two important implications: (1) melt productivity is strongly favoured at low P; (2) anatexis in the Iberian massif probably took place by decompression associated with crustal thinning and extension. These results are in agreement with the relationships between granite production and tectonic deformation phases observed in the Iberian massif. Our results emphasize that anatexis is a process that is strongly controlled both by the phase relationships of the crustal protoliths and by the thermal structure of the continental crust. Consequently, one must be careful when assigning potential crustal protoliths to particular granite associations exclusively on the basis of geochemical comparisons.
KEY WORDS: anatexis; Hercynian orogen; Iberian massif
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL OUTLINE OF THE...THE OLLO DE SAPO...MELTING EXPERIMENTS ON THE...DISCUSSIONCONCLUSIONSREFERENCES |
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Peraluminous leucogranites commonly appear in association with regional metamorphism in orogenic belts. Good examples of this association are found in the European Hercynian belt, which has excellent outcrops in the Iberian massif (Spain and Portugal). Mineral assemblages in the granite–migmatite massifs of Iberia indicate pressure conditions corresponding to the middle–upper crust (10–15 km) and temperatures probably in excess of 750°C (e.g. Martínez et al., 1990
). It has been argued that radiogenic heating combined with crustal thickening can trigger melting of metasedimentary rocks in the continental crust (England & Thompson, 1986; Patiño Douce et al., 1990). The scarcity of mafic rocks in many crustal anatectic domains is commonly used to rule out a mantle-related heat source for anatexis. However, recent studies in continental collisional belts have revealed that many metamorphic structures and microstructures are related to extensional processes (e.g. Burg & Chen, 1984; Royden & Burchfiel, 1987; Ratschbacher et al., 1989; Zeitler & Chamberlain, 1991; Harris & Massey, 1994; Díez Balda et al., 1995). Most anatectic peraluminous leucogranites are related to these extensional structures (Inger, 1994; Zen, 1995), suggesting that the role of crustal thickening in triggering anatexis must be revised.
The melting phase relations of metasedimentary sources place important constraints on tectonic or thermal models. The locus of the solidus curve in pressure–temperature space and its relationships to PT paths are decisive in determining at which stage of the tectonic cycle anatexis took place. With these arguments in mind, we have studied experimentally the melting relationships of the Ollo de Sapo gneiss. The geochemical relationships and the compositions of melts derived from this mica-rich gneissic rock were established by Castro et al. (1999b). We have shown that the Ollo de Sapo gneiss (OSG) is an important protolith to the peraluminous granites of Iberia. Here we present further experimental results that determine the phase relations of the OSG within the melting region of this natural system, and particularly to constrain the shape of the anhydrous melting curve. The experimental results are compared with previously determined PTt paths, allowing us to interpret the mechanisms of melting and granitoid production in the Hercynian orogen of Iberia.
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GEOLOGICAL OUTLINE OF THE IBERIAN GRANITES |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL OUTLINE OF THE...THE OLLO DE SAPO...MELTING EXPERIMENTS ON THE...DISCUSSIONCONCLUSIONSREFERENCES |
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One of the salient features of the Iberian massif is the huge volume of granites (sensu lato) produced during the Hercynian orogenic event. In the internal domains of the massif (Central Iberian zone) the granite bodies make up
50% of the outcropping rocks (Fig. 1). The first tectonic episodes (here generically referred to as D1) were contractional and led to crustal thickening in the central part of the Iberian massif (e.g. Díez Balda et al., 1990, 1995). These events were followed by a predominantly extensional phase (D2) (e.g. Díez Balda et al., 1995; Escuder Viruete, 1998, 1999). Later episodes include contractional or wrench tectonics as well as late extensional phases (Doblas, 1991). Metamorphic studies have shown typical clockwise PT paths (e.g. Villaseca, 1983; Díez Balda et al., 1995; Escuder Viruete et al., 1996) that are characterized by a nearly isothermal decompression. This decompression is interpreted to be the effect of D2 extensional events (e.g. Escuder Viruete et al., 1994, 1996, 1997). The granitoids in the central part of the Iberian massif have been classically subdivided into older, syn-tectonic granitoids and younger, post-tectonic granitoids (Oen, 1958, 1970; Schermerhorn, 1959). The largest volumes of granite magmas (older) are late with respect to the main contractional phase (D1), and are possibly related to D2 and later extensional phases through the activity of crustal-scale extensional shear zones (Casquet et al., 1988; Doblas, 1991). Pinto (1983) and Pinto et al. (1987) distinguished several groups of Hercynian granitoids based on their ages, ranging from Devonian to Permian. If ages and structural information are considered together, two distinct groups are apparent: one with ages >300 Ma and the other with ages <290 Ma. These two groups correspond, respectively, to the ‘older’ and ‘younger’ granites mentioned above.
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According to their chemical and mineralogical compositions, the Hercynian granitoids, both older and younger, can be grouped into two families: peraluminous, two-mica granites and leucogranites; and biotite-bearing (± Crd ± Hb) granodiorites. The widespread ocurrence of large, Crd-bearing, granite to monzogranite plutons, which share geochemical and petrographic characteristics with both of these groups, has led some petrologists to suggest an intermediate series or family of ‘granites of mixed features’ (Capdevila et al., 1973).
The two dominant groups of Hercynian granitoids (leucogranites and Bt-rich granodiorites) were generated and emplaced in different episodes with distinct tectonic styles. Their main features can be summarized as follows:
- older leucogranites are related to the principal metamorphic episodes and their emplacement is associated with the development of ductile extensional shear zones (Diez Balda et al., 1990, 1995); therefore, they may be broadly considered as syn-D2.
- Bt-rich granodiorites appear at different times of the Hercynian cycle. The greater volumes of these granitoids are late with respect to the main deformation phases (D1 and D2), and they are associated with the crustal-scale shear zones developed during the late extensional events (Casquet et al., 1988; Doblas, 1991).
- Bt-rich granodiorites are often associated with minor bodies of basic and intermediate rocks, which display synplutonic relationships and magma mixing and mingling at the local scale. These characteristics are shared by older and younger Bt-rich granodiorites.
This study focuses on the older peraluminous leucogranites and two-mica granites that are spatially related to low-pressure, high-temperature anatectic domains in the Iberian massif. These granitoids comprise autochthonous, para-autochthonous and allochthonous granites, related in space and time to Hercynian regional metamorphism. Their distinctive features are as follows:
- almost all of these granitoids were intruded between 310 and 330 Ma;
- they frequently have pelitic inclusions and abundant alumina-rich minerals (mainly Sil and And, Grt and Crd are present in restricted areas);
- they have very low CaO contents, low 87Sr/86Sr, widely variable 147Sm/144Nd ratios (0·1196–0·1762; Beetsma, 1995) and no heavy rare earth element (HREE) depletion;
- they display limited geochemical variability in comparison with the more mafic biotite granitoids and granodiorites.
Structural relationships of the peraluminous leucogranites
The anatectic peraluminous leucogranites crop out in high-grade metamorphic areas. The structural features of these high-grade terranes may be observed in the central part of the Iberian belt. Recent studies show a clear association between anatexis and the second phase (D2) of regional deformation (e.g. Escuder Viruete et al., 1994, 1996, 1997; Díez Balda et al., 1995; Escuder Viruete, 1998), which is related to the extensional collapse of a thickened crust affected by an earlier contractional event (D1). The most interesting relationships between intrusion of peraluminous leucogranites and D2 deformation can be seen in the Tormes Gneissic dome (Martínez, 1974; Gil Ibarguchi & Martínez, 1982; López Plaza & Gonzalo, 1993). Here, early garnet-bearing leucogranites exhibit magmatic fabrics and late solid-state fabrics that are subparallel to the S2 foliation in the host rocks (Escuder Viruete et al., 1997), indicating a syn-tectonic emplacement of these granites with respect to D2. A second generation of cordierite-bearing leucogranites are related to the same decompression event, but were intruded towards the end of the D2 phase (Escuder Viruete et al., 1997). They are thus isotropic, typically post-tectonic granites. In addition to the Tormes Gneissic dome, syn-D2 partial melting has also been identified in the Central System (Escuder Viruete et al., 1996) and in the Salamanca area (Díez Balda et al., 1995).
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THE OLLO DE SAPO GNEISS (OSG) AS A SOURCE REGION FOR IBERIAN LEUCOGRANITES |
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Both metagreywackes and Bt–Ms gneisses are abundant in the internal domains of the Iberian massif, which also contain peraluminous leucogranites, monzogranites and granodiorites. This pelite–granite–granodiorite (PGG) association is also widespread throughout the European Hercynides beyond the Iberian massif. One of the most conspicuous geological formations of the PGG association in the Iberian massif is the OSG, which was first studied by Parga Pondal et al. (1964)
. It crops out in the north of the Central Iberian zone, in Galicia and in the Central System (Fig. 1). The origin of this formation is debated. Field relationships, textures and geochemistry favour a volcanoclastic origin for most of the rocks that make up the formation. However, pelites and minor intercalations of volcanic rocks are also present. Field observations and geochemistry support the notion that the OSG is the source of many peraluminous leucogranites (e.g. Ortega & Gil Ibarguchi, 1990; Beetsma, 1995). Our recent experimental work (Castro et al., 1999b) on dehydration melting of several potential crustal protoliths for the Iberian granites has demonstrated the following:
- the compositions of melts derived from different protoliths such as greywackes, Bt–Ms gneisses and Crd gneisses are nearly coincident with those of the peraluminous leucogranites and two-mica granites that appear in the internal domains of the Iberian massif.
- The melt productivity is inversely correlated with pressure, ranging from 40 vol % at 3 kbar to <2 vol % at 15 kbar for T = 900°C.
- The compositions of the experimental melts are not affected by temperature, at least from the solidus to 900°C.
- The beginning of dehydration melting is controlled by the breakdown of Ms in the presence of Qtz and sodic Pl.
Our previous study has shown that the OSG is a potential source formation for leucogranites. We now focus on the phase relations of the OSG during anatexis to obtain its solidus curve, and compare it with the proposed tectonothermal evolutions in the central part of the Iberian massif.
Relationships between anatectic granites and the Ollo de Sapo gneiss
The spatial association of the OSG with migmatitic zones and peraluminous leucogranites is common in some areas of the northern Iberian massif. An example of this is the Tourem anatectic complex studied by Holtz (1987). An enclave of the OSG, included within the anatectic granites of the Tourem complex, was used by that researcher as the starting material for the first melting experiments on this important gneissic formation of the Iberian massif. A thick gneissic sequence of the OSG crops out in the Hiendelaencina area (González-Lodeiro, 1980; Fernández, 1991) in the Central System (Central Spain). Representative samples from this area were used by Castro et al. (1999b) as the starting materials for melting and assimilation experiments. The results of these experiments are coincident with the results of Holtz & Johannes (1991) in the sense that the OSG is a fertile source rock for the generation of the Iberian leucogranites. Both experimental studies found good correspondence between the compositions of the experimental melts and those of the peraluminous leucogranites (Castro et al., 1999b).
In the Sanabria region of northern Spain it is possible to observe the transition from medium-grade OSG to migmatites (Fig. 1). In this area, the migmatized OSG appears in the cores of structures produced by the interference between the main deformation phases of the Hercynian orogeny (Martínez García, 1973; Díez Balda et al., 1990; Martínez García & Quiroga, 1993). These structures are part of the megastructure called the Ollo de Sapo anticlinorium, which separates the internal domains of the Hercynian belt (Central Iberian zone) from the external domains.
The field relationships observed in the Hiendelaencina and Sanabria areas strongly suggest that melting occurred in the absence of free aqueous fluids, because the Kfs porphyroclasts that characterize the sub-solidus OSG are preserved in the migmatized facies. This suggests that Kfs was not consumed during anatexis and, therefore, that melting is likely to have taken place by breakdown reactions of micas. These observations, together with previous experimental results on the OSG (Castro et al., 1999b), constitute the background for this experimental study. Here we aim to determine the melting relationships of the OSG and to relate them to thermal processes during the Hercynian orogeny.
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MELTING EXPERIMENTS ON THE OLLO DE SAPO GNEISS |
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Description of the starting materials
The sample that we have selected for this study was collected from the Hiendelaencina region (Fig. 1), which is the site of one of the most complete outcrops of the Ollo de Sapo gneiss. This region was mapped in detail by González-Lodeiro (1980) and Fernández (1991)
. We have used the detailed maps and descriptions by Fernández (1991) to select the most representative sample to use as the experimental starting material. This sample corresponds to a coarse-grained augen gneiss with Kfs megacrysts up to 10 cm long (Fig. 2). The matrix is composed mostly of Qtz, Bt, Ms and Pl (An5–20). Bt and Ms are oriented defining the main foliation of the rock. This rock is a potential source of granite magmas because (1) it is rich in mica, and (2) high-grade equivalents show evidence of anatexis in other regions, such as the Sanabria area (Fig. 1) and the Tourem complex in northern Portugal (Holtz & Johannes, 1991). Because the sample that we chose shows no evidence of migmatization, it is also a good starting material in the sense that it is likely to preserve the amount of water (structurally bound in Bt or Ms) that the OSG contains at the beginning of melting in nature. Table 1 shows the modal and chemical compositions of the gneiss used in this experimental study. Other geochemical and isotopic details on the OSG have been given by Castro et al. (1999b).
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Experimental procedures
Experimental conditions are shown in Table 2. These cover the range from 2 to 15 kbar and from the fluid-absent solidus temperature to 900°C. Although the main objective of this experimental study was to determine the position of the fluid-absent solidus curve in PT space, a detailed isobaric section has also been performed at 6 kbar to determine modal changes associated with melt-producing reactions. Some experiments with added water have also been performed to determine the position of the H2O-saturated solidus in PT space, and to study how melt compositions are likely to change in nature in response to influx of aqueous fluids. Experiments were carried out in end-loaded, solid-media piston-cylinder apparatus at the University of Huelva, with NaCl–graphite cell assemblies of 12·7 mm diameter for experiments at pressures of 
6 kbar. A CaF2–graphite cell assembly was used for an experiment at 900°C and 3 kbar. Samples were contained in welded Au capsules, of 2·4 mm inner diameter with 0·3 mm wall, containing 10 mg of dry sample and, in experiments with added H2O, the appropriate amount of water added with a microsyringe. Weight loss in capsules with added H2O was monitored during welding, and the capsules were also checked for leaks before the experiments by verifying that no weight loss (<0·1 mg) occurred after 2 h in an oven at 130°C. Durations of H2O-added experiments were less than those of dry experiments, to minimize water loss by diffusion through the capsule material (Patiño Douce & Beard, 1994). However, because water favours ionic diffusion, these shorter durations are not likely to have important effects on the approach to equilibrium. Capsules were examined for tears and weighed after the experiments. No weight loss was detected in the experiments reported in this paper. It has been demonstrated (Patiño Douce & Beard, 1994, 1995) that the graphite-based cell assemblies used in these experiments limit the f(O2) in the samples to a well-defined range below the quartz–fayalite–magnetite (QFM) buffer (between QFM and QFM – 2), and that the stability and compositions of ferromagnesian phases are not significantly affected by f(O2) variations within this range. These f(O2) conditions are also reasonable for deep-crustal processes (see Patiño Douce & Beard, 1996). Temperatures were measured and controlled with Pt100–Pt87Rh13 thermocouples feeding Eurotherm 808 controllers with internal ice point compensators. Temperature stability during all runs was 5°C. The reported pressures are oil pressures measured with electronic Druck PTX 1400 pressure transmitters, feeding Omron E5CK controllers, multiplied by ratio of ram-to-piston areas, and were manually maintained within 0·5 bar of oil pressure (250 bar on the sample). Experimental products were mounted in epoxy, sawn in half and polished. Textures were studied by scanning electron microscopy using back-scattered electron (BSE) images. Modal proportions of glass (quenched melt) and neoformed phases were determined by image analysis using BSE images. Glass and crystalline phases were analysed using a Link–Isis energy-dispersive spectrometer mounted on a scanning electron microscope (JEOL-JSM5410). Conditions were fitted to 15 kV accelerating voltage and 100 s of effective counting time. Matrix corrections were made using the ZAF procedures using a combination of silicates, oxides and pure metals as standards (wollastonite for Ca and Si, jadeite for Na, orthoclase for K, corundum for Al, periclase for Mg, metallic Fe and Ti for Fe and Ti). If the available clean surface of mineral grains or glass was large enough, the analyses were performed rastering the 1 µm beam over an area of 6 µm2. Even using these procedures, Na loss from glass is appreciable, and Na concentrations were corrected by measuring Na contents at one-third of the total counting time. We have observed that this is the time at which the counting rate for Na starts to decrease in our hydrated glasses. In runs near the solidus, in which the melt percentage is very low (<5%), it is necessary to analyse the glass on very small areas using a fixed beam of 1 µm diameter. In this case, Na loss is very pronounced even at one-third of the counting time. However, Na loss is a predictable process that can be fitted to curves that are then used to correct analyses of these small areas. The method was checked by analysing large areas of glass with both techniques, rastering the beam and fixed beam, and the results were in good agreement. A similar procedure was followed for plagioclase analyses.
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Attainment of equilibrium
The durations of the experiments ranged from 100 h for the high-T runs to >300 h for the low-T runs (Table 2). To check whether these durations are sufficient for a reasonable approach to equilibrium, we performed a time series at 750°C and 6 kbar, with durations varying from 2 h to nearly 300 h. These P–T conditions are only a few degrees above the solidus, and melt proportions in these experiments are very low (2 vol. %). The results of these experiments are shown in Fig. 3. Mineral assemblage and melt composition change with time up to a run duration of 144 h. Between 144 and 288 h there is little change in phase assemblage and melt composition, except for the presence of scarce neoformed Crd at 144 h. This phase is absent in the shorter and longer runs. Crd is probably a metastable phase in the 144 h experiment, because it has not been observed in any other experiments at T below the breakdown of Bt (>800°C at 6 kbar; see Fig. 6, below). These results suggest that the durations of our experimental runs (always longer than 144 h at near-solidus conditions) are sufficient for a reasonable approach to chemical equilibrium. This duration also exceeds the characteristic diffusion time in most silicate minerals for grains of 10 µm diameter (Tsuchiyama, 1985). Other arguments that also support an acceptable approach to equilibrium in our melting experiments are as follows:
- neoformed minerals are always euhedral and notably unzoned. Although poikilitic crystals are common (e.g. garnet), skeletal crystals (which could denote metastable crystallization) are almost completely absent.
- Glass compositions are very uniform throughout the experimental charges.
- Mineral compositions vary regularly and consistently with pressure and/or temperature. For example, garnet becomes enriched in Ca, and plagioclase depleted in Ca, with increasing pressure; cordierite generally becomes enriched in alkalis with decreasing pressure; and the octahedral Al content of biotite coexisting with garnet, sillimanite and quartz generally increases with increasing pressure and with decreasing temperature (see Patiño Douce et al., 1993).
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Descriptions of experimental products
Detailed descriptions of textures and mineral compositions for experiments at 900°C on the OSG at 6, 10 and 15 kbar were given by Castro et al. (1999b). These high-T experiments were conducted with the aim of characterizing the chemical composition of melts produced within a wide range of pressures, to compare them with those of the peraluminous granites associated with the OSG in the Iberian massif. The additional experiments discussed here allowed us to determine the shape of the vapour-absent solidus and to identify the melt-producing reactions. Both sets of experiments, those at 900°C published by Castro et al. (1999b) and the new experiments performed for this study, are listed in Table 2. Al-silicate (metastable mullite), if present, is found in glass pools, associated with Kfs and neoformed Bt tablets. In near-solidus experiments, e.g. at 6 kbar and 750°C, Al-silicate is intimately associated with Ms (Fig. 4a). Ms is completely consumed in the experiment at 6 kbar and 800°C. However, the melt fraction at these conditions is still very low (<5 vol. %), and neoformed Bt is conspicuous in this experiment (Fig. 4b), in which the original Bt is still present. These relationships show that Bt is an important product on the Ms dehydration-melting reaction [see also Thompson (1982) and Patiño Douce & Harris (1998)], and that biotite is still a stable phase at 800°C and 6 kbar. Biotite breakdown is observed in experiments at higher T, e.g. 6 kbar and 850°C. At these conditions the melt fraction is 8 vol. % and Bt is mostly consumed. The P–T location of this reaction in nature, and the amount of melt produced by it, could be different from those determined in these experiments if the melt produced by the lower-temperature muscovite dehydration-melting reaction migrated before biotite breakdown occurred. This uncertainty, however, is not a serious drawback, because the emphasis of this work is to examine whether muscovite dehydration melting was an important process in the generation of the Iberian leucogranites. Sp, Al-silicate, Kfs and Crd are all peritectic phases at 6 kbar and 850°C (Fig. 4c), reflecting breakdown of Bt. It can be remarked that in the experiment at 15 kbar the formation of Grt is not directly associated with the production of melt at 800°C. This implies that some water may be liberated to the system before the beginning of melting at high P and, consequently, melting may proceed at lower T because water is present in the system producing the inflection in the Clapeyron slope of the ‘Bt-out’ curve at conditions over 10 kbar, in agreement with the results of Le Breton & Thompson (1988). At high T (900°C), the melt fraction in dehydration-melting experiments is 15 vol. % at 6 kbar, 10 vol. % at 10 kbar, and <8 vol. % at 15 kbar. These figures show that melt productivity at constant temperature decreases with increasing pressure. Besides melt, the most abundant neoformed phases at this high temperature (900°C) are Crd and Kfs at 6 kbar, and Grt and Kfs at 15 kbar. The melts formed in all dehydration-melting experiments have leucogranite compositions (Table 3), which change only slightly with pressure and temperature. Greater differences are seen in the melts produced in added-water experiments. The experiment with 5 wt % of added water at 6 kbar and 700°C is the only one in which the melt CaO content is >1·0 wt %. There are no significant nor systematic differences in Na between melts produced in the presence of added water compared with dehydration-melting experiments. However, there are differences in K contents that affect the Ab/Or ratio of the melts (see Fig. 7, below). Aside from these differences, all the melts have very low contents of Fe and Mg and high alumina saturation indices (A = Al – [Na + K + 2Ca]), so that they are all peraluminous leucogranite melts.
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DISCUSSION |
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Phase relations of the Ollo de Sapo gneiss during anatexis
Melting phase relations are summarized in Fig. 5. The fluid-absent solidus is located by interpolating between melt-absent and melt-present runs. The results are nearly coincident with the solidus determination using the incremental heating technique recently developed by Castro et al. (1999a)
. The fluid-absent solidus has a positive slope with dP/dT values ranging from 30 bar/°C at low P (<6 kbar) to 70 bar/°C at higher P (6–15 kbar). These values are comparable with the slope of the solidus obtained by Patiño Douce & Harris (1998) for a muscovite schist from the Himalaya. Both in the OSG and in the Himalayan schist the solidus is controlled by the breakdown of Ms in the presence of Qtz, Pl and Bt. The OSG dehydration-melting solidus intersects the water-saturated solidus at a pressure of 4 kbar. The OSG wet solidus was interpolated between two experimental determinations (Fig. 4), following the shape of the water-saturated solidus curves for similar assemblages (Qtz, Pl, Kfs, water) determined by Johannes & Holtz (1995). It should be noted that the temperature of the beginning of melting in the presence of free water is independent of the percentage of water added to the experimental charges, even though the amount of melt produced certainly depends on the amount of added H2O.
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) and melt present (•) experiments with added water, which constrain the water-saturated solidus at P < 6 kbar. At higher P the solidus is drawn after Johannes & Holtz (1995)
The experiments at 3 kbar and high T (800 and 900°C) with no added H2O produce high melt fractions (up to 30 vol. %; see Table 2). However, these results are probably unrealistic because the pressure of these experiments (3 kbar) is lower than the intersection of the vapour-absent and vapour-present solidi (P 4 kbar), so that breakdown of Ms must have taken place at lower temperature than the water-saturated granite solidus. Such high melt fractions may be attained in nature only if the water produced by the subsolidus breakdown of Ms is retained in the system. This phenomenon may occur in shallow perianatectic regions, where water is liberated by dehydration reactions around anatectic domains. An example of this may be the OSG in some parts of the Sanabria anatectic complex, where it occurs migmatized and with high melt proportions (30 vol. %).
Other important boundaries that are constrained by our experimental results are the appearance of Crd (or Grt) in the partially melted assemblages (Fig. 5), and the transition from Crd-bearing peritectic assemblages at low P (3–6 kbar experiments) to Grt-bearing peritectic assemblages at high P (10–15 kbar experiments). Crystallization of both phases is associated with the breakdown of Bt in the presence of Qtz, Pl and Al-silicate. Figure 6 shows isobaric variations in modal abundances at 6 kbar, for temperatures ranging from the fluid-absent solidus to 900°C. Melt abundance increases continuously above the solidus. The abundance of Ms decreases with increasing temperature, and this phase disappears between 750 and 800°C. In contrast, the modal abundance of Qtz is nearly constant from the solidus to 800°C. Consumption of Ms is accompanied by formation of Al-silicate. Abundance of this phase increases initially with temperature, but remains nearly constant above 800°C. Bt and Kfs are produced between 700 and 800°C [see also Patiño Douce & Harris (1998)], and consumed above this temperature. These relationships suggest that Ms is consumed by the following melt-forming reaction:
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in melting experiments on a Himalayan Ms-bearing schist.
At T > 800°C, once Ms is completely consumed, a new melt-producing reaction begins. This is indicated by the appearance of neoformed Crd (at 6 kbar) or Grt (at 10 kbar). The increase in Crd abundance parallels the increase in melt abundance and decrease in Bt abundance from 800 to 900°C. At T > 800°C some Al-rich Sp (perhaps metastable) is found, and at T > 850°C Grt appears at 6 kbar (Fig. 6). These relationships suggest reactions of the type
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80 bar/°C for pressures <10 kbar and a negative slope of comparable magnitude for higher pressures. This change in slope suggests that the Bt breakdown reaction may intersect the Ms breakdown reaction at pressures between 10 and 15 kbar [see also Patiño Douce & Harris (1998)]. It has been repeatedly suggested (e.g. Le Breton & Thompson, 1988; Vielzeuf & Montel, 1994; Singh & Johannes, 1996) that the Bt dehydration-melting solidus bends back at pressures of 10–15 kbar.
However, experimental studies of dehydration melting of metapelites and metagreywackes have failed to detect this back-bending of the solidus (Patiño Douce & Johnston, 1991; Patiño Douce & Beard, 1995, 1996). Our experimental results suggest that back-bending of the fluid-absent Bt-breakdown reaction may occur as a result of reactions among peritectic phases, such as the Crd to Grt transition, that entail large changes in molar volume. It should be noted, however, that the thin dashed curve in Fig. 5 marks the appearance of Grt independently of the production of melt, because Grt in the 15 kbar experiments appears at lower temperature than melt. Grt can be formed at the expense of Bt without liberating H2O by the reaction
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Melt compositions
The main characteristics of the melts produced in experiments on the OSG are as follows:
- they are strongly peraluminous, with molar Al2O3/(CaO + Na2O + K2O) > 1·1.
- They are rich in silica (SiO2 content is commonly >74 wt %, recalculated on a water-free basis) and also rich in K2O (K2O commonly >3·5 wt % for fluid-absent experiments).
- All the melts are very poor in FeO + MgO (commonly <1·5 wt % FeO + MgO for fluid-absent and H2O-added experiments).
Our results are very similar to those obtained by Patiño Douce & Harris (1998) in melting experiments on Ms schists of the Himalaya. The low solubility of Fe and Mg in low-temperature (<800°C) granite melts is reflected in the crystallization of Bt during the breakdown of Ms [see also Patiño Douce & Harris (1998)]. The phengitic component of Ms in the starting material (see Table 1) releases enough Fe and Mg to saturate the melt in ferromagnesian components and hence induce the crystallization of Bt.
Reactions (2) and (3) cause a significant increase in melt fraction but, interestingly, they do not change the FeO + MgO content of the melts appreciably. This reflects crystallization of peritectic Crd or Grt, which act as sinks for ferromagnesian components. Low-pressure melts have somewhat lower MgO content than high-pressure melts. This difference arises from differences between the melt–Crd and melt–Grt Fe–Mg partition coefficients. Because Crd is always richer in Mg than coexisting Grt, melts formed in equilibrium with Crd are depleted in Mg compared with melts produced in equilibrium with Grt at the same T. These differences are important because they suggest that equilibrium was approached in our experiments, but they are too small to have significant effects on the total FeO + MgO content of the melts that could be produced in nature under comparable conditions.
Experimental melt compositions are plotted in the normative (CIPW) Ab–Or–An diagram (O’Connor, 1965; modified by Barker, 1979) in Fig. 7. All the melts formed in fluid-absent experiments plot in the field of granites. There is some shift towards Ab-rich compositions in high-P experiments (see Patiño Douce & McCarthy, 1998). However, the greatest variation occurs in water-added experiments compared with the fluid-absent ones. Experiments with 2 wt % water plot in the field of granites but near the boundary of the trondhjemite field. The experiment with 5 wt % water plots in the field of monzogranites. These changes reflect the incorporation of substantial amounts of Pl into the melt that occurs in response to the presence of an aqueous fluid phase during melting (see Patiño Douce & Harris, 1998).
Geological implications
Melting phase relations of the OSG allow us to evaluate the ability of this source material to produce large volumes of granite magmas, and to infer the mechanisms of melting within the context of the PTt paths of high-grade gneisses of the Iberian massif. In discussing the first of these issues we also use the results of previous experimental works on the OSG and other similar crustal protoliths from the Iberian massif (Holtz & Johannes, 1991; Castro et al., 1999b). In domains where the OSG was migmatized, as is the case in the Sanabria region (see Fig. 1), a higher proportion of granite melt appears to have been produced, as indicated by the presence of large pods and leucosome veins. Crd or Grt are rarely present in these granitic pods, suggesting that the melts were produced by the breakdown of Ms. The low content of Ca of these leucosomes and leucogranite pods argues against water-fluxed melting in this region, suggesting instead dehydration melting. This is further supported by the fact that muscovite is abundant in low-grade areas of the Iberian massif, but is replaced by Sil + granite melt in anatectic zones such as Sanabria. These observations show that, in the absence of free aqueous fluids, Ms-rich protoliths are the first ones to reach the solidus, and are thus more fertile than quartzofeldspathic gneisses as far as production of the lowest-temperature granite melts is concerned.
How a crustal protolith reaches the conditions for anatexis depends on the geometry of its solidus. The most important feature of the OSG vapour-absent solidus is its positive and relatively constant slope. Figure 8 shows several PT diagrams in which we have plotted the OSG solidus curves together with PTt paths for different parts of the northern Iberian massif (references are given in the figure caption). In the NW coast (Vivero zone) the calculated PT paths of metamorphic terranes that include rocks of the Ollo de Sapo Formation do not reach the OSG melting curves (Fig. 8a). As expected, peraluminous leucogranites are very scarce in this area. In contrast, peraluminous leucogranites are volumetrically important in the Tormes dome (Fig. 8b), where the inferred PTt paths intersect the fluid-absent solidus curve for the OSG. The relationships shown in Fig. 8b suggest that melting took place during decompression, i.e. during the D2 deformation phase. This deformation phase is characterized by the development of subhorizontal ductile shear zones that affect high-grade rocks as well as some of the leucogranites generated during decompression (Escuder Viruete et al., 1997). Melting in the Tormes dome may have begun at 9 kbar and 775°C, within the Grt zone, and then progressed towards low-P conditions of 3 kbar and 670°C, within the Crd zone (Escuder Viruete et al., 1997). However, the most common restitic phase in the Tormes anatectic granites is Sil, and rarely Crd or Grt (Gil Ibarguchi & Martínez, 1982; Escuder Viruete et al., 1997). This suggests that temperatures never reached those necessary for Bt breakdown. It is interesting to note that the decompression paths for the Tormes dome are very close to the OSG fluid-absent solidus, and that there are strong similarities between the compositions of the experimental melts and those of the peraluminous leucogranites in this area.
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Similar relationships between the geometry of the fluid-absent OSG solidus and metamorphic PTt paths are found in the Salamanca–Gredos–Toledo area (Fig. 8c) and in the Guadarrama area (Fig. 8d). Both of these areas are part of the Spanish Central System. In both cases decompression associated with D2 extension intersects the OSG solidus at low P (between 4 and 8 kbar). These examples show that large volumes of granitic magmas were formed during decompression of the Iberian massif, but at different times relative to the D2 deformation phase. This is the reason why there are early leucogranites that are strongly deformed by the second deformation phase and other, later, leucogranites that appear to be only weakly deformed. The latter were probably deformed at the magmatic stage, during ascent and/or emplacement (López Plaza, 1982; López Plaza & Martínez Catalán, 1987).
This decompression-melting model does not exclude the possibility that some granite melts may have been generated at the end of the crustal thickening episode (D1), when the PT path may intersect the water-saturated solidus. This may take place in very restricted regions; for example, in and around fluid circulation pathways. However, the production of allochthonous granite plutons by this mechanism is unlikely, owing to the high solubility of water in silicate melts at high P (e.g. Burnham, 1979). Production of the first melts would dissolve all available water and prevent additional melting. Furthermore, our experiments show that the composition of these cool and water-rich melts tends to be trondhjemitic rather than granitic [see also Patiño Douce & Harris (1998)]. Early trondhjemitic (Ab-rich) granites are present in Iberia but are very scarce in comparison with the large volumes of peraluminous leucogranites and high-K, two-mica granites that predominate in the NW of the Iberian peninsula. This part of the Iberian massif is dominated by a typical pelite–granite–granodiorite (PGG) association, characterized by high K and Al contents. The igneous components of the PGG association are clearly related to the sedimentary components by melting processes that were largely controlled by the stabilities of micas.
Another important geological implication, which follows from the observed relationships between anatexis and metamorphic PTt paths, is that there is no need to involve an extra source of heat to produce leucogranitic magmas in the Iberian massif. Thermal relaxation accompanying the collapse of a thickened orogen is enough to account for the production of leucogranites. This is a plausible model for the production of the Iberian peraluminous leucogranites, which is supported by field data, geochemical relationships and melting experiments. However, this same model is not applicable to the generation of the peraluminous granodiorites and monzogranites of Iberia. Additional heat sources, and addition of ferromagnesian components, are necessary to explain these granitoid rocks (Castro et al., 1999b).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL OUTLINE OF THE...THE OLLO DE SAPO...MELTING EXPERIMENTS ON THE...DISCUSSIONCONCLUSIONSREFERENCES |
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Phase relations determined in fluid-absent experiments on the OSG confirm that this rock is a fertile source for the peraluminous leucogranites of the Iberian massif. The relatively low dP/dT slope of the Ms dehydration-melting solidus is a crucial aspect of leucogranite magma generation in the Iberian massif, and probably elsewhere too. This is so because melt production is greater at low P, and generation of leucogranite magmas in nature probably takes place during decompression associated with crustal thinning and extension. These results are in agreement with the timing relationships between granite production and emplacement and tectonic deformation phases observed in the Iberian massif. Moreover, comparison of the thermal histories of different sectors of the Iberian massif demonstrates that production of leucogranite magmas by fluid-absent anatexis is strongly dependent on how the phase relations of potential crustal protoliths relate in P–T space with the thermal evolution of the continental crust. Thus, correlating potential crustal protoliths with particular granite associations cannot be done exclusively on the basis of geochemical comparisons, but must rather take these petrological constraints into consideration.
Leucogranites produced by biotite dehydration melting appear to be scarce in the Iberian massif, judging by the rarity of mafic peritectic phases. This is a consequence of both the higher temperature required for biotite breakdown and the fact that the dehydration-melting solidus of biotite is steeper than that of muscovite, so that it is less likely to be crossed during decompression [see also Patiño Douce & Harris (1998)]. Biotite breakdown was certainly involved in the origin of the more mafic peraluminous granodiorites of the Iberian massif, but these magmas had an origin distinct from that of the peraluminous leucogranites, which entailed hybridization with basaltic magmas (Castro et al., 1999b). These relationships may be fairly general. Peraluminous leucogranites appear to be the only common igneous rocks that form entirely by crustal anatexis, without influx of mantle-derived material (Patiño Douce, 1999).
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ACKNOWLEDGEMENTS |
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Financial support from the Spanish Science Commision (CICYT-DGES, Project PB97-0439) and from the University of Huelva is acknowledged with thanks. The paper benefited from comments and criticisms by two anonymous referees.
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FOOTNOTES |
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*Corresponding author. Fax: +34 959 53 0175. e-mail: dorado{at}uhu.es
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REFERENCES |
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A. CASTRO, L. G. CORRETGE, J. D. DE LA ROSA, C. FERNANDEZ, S. LOPEZ, O. GARCIA-MORENO, and H. CHACON The Appinite-Migmatite Complex of Sanabria, NW Iberian Massif, Spain J. Petrology, July 1, 2003; 44(7): 1309 - 1344. [Abstract] [Full Text] [PDF]
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A. FORNELLI, G. PICCARRETA, A. DEL MORO, and P. ACQUAFREDDA Multi-stage Melting in the Lower Crust of the Serre (Southern Italy) J. Petrology, December 1, 2002; 43(12): 2191 - 2217. [Abstract] [Full Text] [PDF]
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 S. Lopez and A. Castro Determination of the fluid-absent solidus and supersolidus phase relationships of MORB-derived amphibolites in the range 4-14 kbar American Mineralogist, November 1, 2001; 86(11-12): 1396 - 1403. [Abstract] [Full Text] [PDF]
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