Journal of Petrology | Volume 40 | Number 10 | Pages 1465-1496 | 1999
© Oxford University Press 1999
Nature and Composition of the Lower Continental Crust in Central Spain and the Granulite–Granite Linkage: Inferences from Granulitic Xenoliths
1 Departamento De Petrología Y Geoquímica, Facultad De Ciencias Geológicas, Universidad Complutense 28040 Madrid, Spain
2 Department of Geology, Birkbeck College Malet Street, London WC1E 7HX, UK
3 CNRS UMR 6524, Département Des Sciences De La Terre, Université Blaise Pascal 5 Rue Kessler,F-63038 Clermont-Ferrand, France
4 Departamento De Geologa, Facultad De Ciencias Del Mar, Universidad De Cádiz 11510 Puerto Real (Cádiz), Spain
Received July 22, 1998; Revised typescript accepted April 29, 1999
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ABSTRACT |
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 TOP ABSTRACT IntroductionPrevious Estimates of Lower...Types of Granulitic XenolithsAnalytical MethodsPetrography and Mineral...Geochemistry of the Peraluminous...DiscussionConclusionsAppendix a: Granulite Sample...Appendix B: Partition...References |
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Xenolith-bearing alkaline ultrabasic dykes were intruded into the Hercynian basement of the Spanish Central System in early Mesozoic times. The suite of lower-crustal xenoliths in the dykes can be divided into three groups: felsic peraluminous granulites, metapelitic granulites and charnockitic granulites. The felsic granulites form
95% of the total volume of the xenoliths, whereas the charnockitic and metapelitic granulites are much less abundant (0.01 and 5%, respectively). Thermobarometric calculations based on mineral paragenesis indicate equilibration conditions around 850–950°C, 7–11 kbar; thus the xenoliths represent lower continental crustal material. Superimposed on this high-T high-P assemblage is a high-T low-P paragenesis represented mainly by kelyphitic coronas, reflecting re-equilibration during transport in the alkaline magma. Felsic metaigneous and metapelitic xenoliths exhibit clearly restitic mineral assemblages, with up to 50% garnet and 37% sillimanite. Major and trace element modelling supports the idea that the late-Hercynian peraluminous granites of central Spain represent liquids in equilibrium with restitic material of similar composition to the studied lower-crustal xenoliths. 87Sr/86Sr and 
Nd of the felsic xenoliths, calculated at an average Hercynian age of 300 Ma, are in the range 0.706–0.712, and –1.4 to –8.2, respectively. These values match the isotopic composition of the outcropping late Hercynian granites. The Sr isotopic composition of the xenoliths is lower than that of the outcropping mid-crustal lithologies (orthogneisses, pelites). A major contribution from the lower crust to the source of Hercynian granites greatly reduces the necessity of invoking a large mantle contribution in models of granite petrogenesis. The felsic nature of the lower continental crust in central Spain contrasts with the more mafic lower-crustal composition estimated in other European Hercynian areas, suggesting a non-underplated crust in this region of the Hercynian orogenic belt. KEY WORDS: felsic lower continental crust; granulite xenoliths; Sr–Nd isotopes; Hercynian Iberian Belt
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Introduction |
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TOPABSTRACTIntroductionPrevious Estimates of Lower...Types of Granulitic XenolithsAnalytical MethodsPetrography and Mineral...Geochemistry of the Peraluminous...DiscussionConclusionsAppendix a: Granulite Sample...Appendix B: Partition...References |
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The study of lower-crustal xenoliths has been a powerful tool in understanding the process of anatexis involved in the genesis of felsic magmatism (Downes & Duthou, 1988
; Ruiz et al., 1988; Miller et al., 1992; Hanchar et al., 1994; Pankhurst & Rapela, 1995). It is clear that granite sources in orogenic areas are not the outcropping metamorphic rocks but are located in deeper crustal levels. Lower crust is not easily accessible for study and can also be modified by later orogenic events which could strongly change its primary characteristics (Costa & Rey, 1995). However, in the case of the central Spain, samples of lower-crustal lithologies have been exhumed by anorogenic alkaline magmas. Furthermore, this region has not been affected by metamorphism associated with any orogenic or anorogenic event since the Hercynian orogeny, so the xenoliths are considered to represent the lower crust as it existed at the end of granite emplacement.
The Spanish Central System (SCS) is a major granitoid complex (>10 000 km2 in surface area) located in the internal part of the Hercynian Iberian Belt (Fig. 1). This huge batholith intruded into continental crust mainly composed of metasedimentary pelitic schists of Precambrian and lower Cambrian age, and metaigneous rocks (orthogneisses) related to early Palaeozoic orogenic events (500 ± 20 Ma, Vialette et al., 1987; Valverde Vaquero et al., 1995). Orthogneisses are the most abundant rocks of the outcropping metamorphic series in the eastern region of the SCS (>80% of surface area in the Guadarrama region; Villaseca et al., 1993). In the western part of the SCS a more complex metasedimentary series is dominant, which is rich in metapsammites with local interlayered carbonate material (Ugidos et al., 1997). During the Hercynian orogeny, this crust evolved from intermediate-P towards low-P, high-T conditions (dated at 335 Ma in the Guadarrama sector; Valverde Vaquero et al., 1995) reaching granulite-facies conditions in wide zones of middle crust with consequent anatexis (e.g. the anatectic granulitic terrane of Toledo; Barbero, 1995).
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Granitic plutonism took place in late Hercynian times (325–285 Ma) (Villaseca et al., 1995
). The batholith consists mainly of peraluminous granitoids with very scarce basic to intermediate plutonic rocks, and was emplaced at shallow crustal levels [see reviews by Moreno-Ventas et al., (1995), Pinarelli & Rottura, (1995) and Villaseca et al., (1998)]. The lack of appropriate sources in terms of isotopic composition in the vicinity of the Hercynian plutons has led to two models for the origin of the SCS granites: (1) hybridization between crustal melts and mantle-derived magmas (Moreno-Ventas et al., 1995; Pinarelli & Rottura, 1995); (2) partial melting of intracrustal materials (Bea & Moreno-Ventas, 1985; Villaseca et al., 1998). Although the outcropping metaigneous rocks (orthogneisses) more closely approach the isotope signatures of the SCS granites than the metasedimentary rocks, nevertheless they show clear differences in Sr isotope composition compared with the granites (Villaseca et al., 1998). No significant metamorphic event has affected the SCS batholith after its formation; igneous textures are preserved almost without exception.
An alkaline ultrabasic dyke swarm was intruded into the region in early Mesozoic times (Villaseca & Nuez, 1986). These dykes have a N–S orientation and are cut by the tholeiitic Messejana–Plasencia dyke emplaced at 184 Ma (Schermerhorn et al., 1978). The anorogenic alkaline magmatism is related to North Atlantic rifting during Triassic times (Villaseca et al., 1992). The dykes, which are mainly ultrabasic camptonitic lamprophyres, contain a varied population of mainly felsic granulitic xenoliths (Villaseca & Nuez, 1986). These granulite xenoliths show very distinctive petrographic, mineralogical and geochemical characteristics compared with the outcropping Hercynian granulitic terranes in central Spain.
In this paper we present a petrographic, mineralogical, geochemical and isotopic (Sr, Nd) study of the granulitic xenolith suite from the SCS that indicates a felsic restitic character for the lower continental crust in this region. The role played by the late Hercynian granitic magmatism in the generation of this restitic lower crust, and the differences between granulitic material at lower- and middle-crustal levels (xenoliths and outcropping granulitic terranes, respectively), are also discussed.
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Previous Estimates of Lower-Crustal Composition in Central Spain: Geophysical Data |
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TOPABSTRACTIntroductionPrevious Estimates of Lower...Types of Granulitic XenolithsAnalytical MethodsPetrography and Mineral...Geochemistry of the Peraluminous...DiscussionConclusionsAppendix a: Granulite Sample...Appendix B: Partition...References |
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Deep seismic profiles constrain the general crustal structure of central Spain. Banda et al., (1981)
showed that the crust in this area is around 31 km thick and consists of four layers (Fig. 2):
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- a sedimentary cover up to 3 km thick, with P-wave velocities <3.5 km/s;
- an upper crystalline layer up to 11 km thick, including a low-velocity layer extending from about 7 to 11 km (Vp = 5.6 km/s), with P-wave velocities in the range 5.6–6.1 km/s;
- middle crust from 11 to 23 km depth and Vp 6.4 km/s;
- lower crust from 23 to 31 km depth and Vp = 6.8–6.9 km/s.
Gravity and magnetotelluric data from Carbó & Capote, (1985) corroborate the described stratigraphy of the crust in central Spain. More recently, deep seismic sounding undertaken by the Iberian Lithosphere Heterogeneity and Anisotropy Project showed the continental crust of the SCS to be 34 km thick, with the upper boundary of the lower crust located between 21 and 23 km depth (ILIHA DSS Group, 1993). The boundary between middle and lower crust is always well marked by an increase in P-wave velocity up to 6.7–6.8 km/s. However, models derived from ILIHA crustal data show that there are no significant lateral inhomogeneities in gross crustal structure, in marked contrast to the heterogeneous Hercynian surface geology.
Data from P-wave coda in the SCS (Paulssen & Visser, 1993) also confirm a continental crust of around 30 km in thickness, similar to the average crustal thickness of Phanerozoic fold belts of central Europe (Wedepohl, 1995). The lowermost 8 km correspond to the granulitic lower crust with P-wave velocities always in the range of 6.5–6.9 km/s. These values are more typical of felsic or pelitic compositions rather than mafic granulites (Vp from 6.9 to 7.5 km/s; Rudnick, 1992; Wedepohl, 1995). P-wave velocities in the range 6.8–7.0 km/s are very common in garnet-bearing peraluminous granulites (Kern, 1990), these being one of the most abundant lithologies in the xenolith suite of the SCS (Villaseca & Nuez, 1986). In Fig. 2, a sketch profile of the SCS crust is compared with the average continental crust of Wedepohl, (1995).
From seismological data there does not seem to be a significant mafic granulitic or eclogitic layer in the lower crust of the SCS, which agrees with the lithologies of the granulitic xenoliths studied in this work. Wedepohl, (1995) stated that in the younger fold belts in Central Europe, mafic granulites occur locally as voluminous bodies but more commonly as a thin layer above the Moho. Also, some recent xenolith studies (e.g. Hanchar et al., 1994) indicate that the lower continental crust may contain a larger supracrustal component than previously thought. Thus, in some regions, the lower-crustal composition may be closer to tonalite rather than diorite as previously estimated (Wedepohl, 1995). McLennan & Taylor, (1996) also recognized that estimates of the average composition of the continental crust may be shifted towards a more acid composition.
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Types of Granulitic Xenoliths |
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TOPABSTRACTIntroductionPrevious Estimates of Lower...Types of Granulitic XenolithsAnalytical MethodsPetrography and Mineral...Geochemistry of the Peraluminous...DiscussionConclusionsAppendix a: Granulite Sample...Appendix B: Partition...References |
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Although granulitic xenoliths occur in camptonitic dykes in several places in the SCS, two localities are most important because of the large number and variety of xenoliths found in them. The first is a diatreme-like outcrop in La Paramera (Nuez et al., 1981
). It is composed of breccia, and has a surface area of around 20 000 m2. The outcrop is elliptical in shape, with its long axis striking N–S, the same direction as the dykes. In this locality, 25% of the breccia is composed of granitic and other wall-rock xenoliths and xenocrysts, as well as granulite xenoliths. The second important locality is the dyke swarm of Peguerinos (Fig. 1) where the narrow (<0.5 m wide) lamprophyric dykes carry 10–20 vol. % of granulitic xenoliths. Xenoliths were also found in dykes at Bernuy and San Bartolomé (Fig. 1). Several kinds of xenoliths are found. The largest (up to 1 m3) and more angular types are granitic rock fragments, similar to the country rocks, and varied metamorphic country rock enclaves. However, the most abundant xenoliths are granulites with a characteristic rounded shape as a consequence of longer transport in the dyke. They can reach up to 10 000 cm3 in volume (Table 1), which implies high emplacement velocities of the host magma, to inhibit settling of the xenoliths (velocities of the order of 10–100 m/h can be deduced from Stokes’ law). This rapid ascent of the xenoliths inhibits the development of significant contact metamorphism, even in the smallest samples, which are a few millimetres in diameter. Nevertheless, local intergranular recrystallization processes and development of kelyphitic coronas around garnet are commonly observed.
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More than 70 granulite xenoliths have been collected for this study. We sampled four main outcrops which contain abundant xenoliths (Fig. 1), covering an area of the SCS of >1000 km2. Three main types can be distinguished (Table 1):
- Type 1: charnockitic granulites. These are small pyroxene-bearing xenoliths, without peraluminous minerals such as garnet, phlogopite or sillimanite. They vary from felsic (sample U-3: a charnockite with plg + qtz + kfs + opx) towards intermediate types (sample U-28: a meta-norite with plg + opx + cpx). Charnockitic granulite xenoliths are very scarce and have been found only in two localities. They show typical fine-grained granoblastic textures (see Appendix A).
- Type 2: felsic peraluminous granulites. This group includes quartzo-feldspathic garnet-bearing types of which two subtypes have been identified: (2a) granulites with accessory orthopyroxene (kfs + plg + qtz + grt + opx ± phl); (2b) granulites without orthopyroxene, and with accessory phlogopite, aluminium silicate or both (Table 1). Type 2b is the most abundant of all the granulitic xenoliths of the SCS.
- Type 3: pelitic peraluminous granulites. These are granulites with parageneses similar to that of the 2b type but with >30% of peraluminous minerals (garnet, aluminium silicate); usually sillimanite forms >8% in volume. The most common paragenesis is grt + kfs + plg + qtz + sill. A subtype (3b) can be distinguished as highly garnet–sillimanite-rich xenoliths, with accessory amounts of quartz and feldspars (sample U-10, Table 1).
These two latter types appear in two textural varieties: (a) foliated or banded granoblastic types, with segregated garnet-rich bands and narrow leucocratic veins in some samples; (b) massive granoblastic types with a weak mineral orientation. Further petrographical information is included in Appendix A.
The relative proportions of the three types of xenoliths are 5:68:27. If instead of considering the number of xenoliths we consider their relative volume (see Table 1), as type 1 xenoliths are always very small and scarce, the volume abundance would be around 0.01:95:5. We consider these relative proportions to be representative of the SCS lower crust as, from the estimated densities of the xenoliths (Table 1), no preferential removal by settling would be expected. Thus, felsic peraluminous granulites form almost the entire granulitic xenolith suite, with the pelitic and charnockitic types being very scarce.
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Analytical Methods |
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TOPABSTRACTIntroductionPrevious Estimates of Lower...Types of Granulitic XenolithsAnalytical MethodsPetrography and Mineral...Geochemistry of the Peraluminous...DiscussionConclusionsAppendix a: Granulite Sample...Appendix B: Partition...References |
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Mineral compositions were determined by electron microprobe at the Complutense University of Madrid (Jeol Superprobe JXA 8900-M) and St Andrews University (UK) (Jeol Superprobe 733). In both cases, operating conditions were 15 kV, 20 nA and a beam diameter of 2–5 µm, and the ZAF correction procedure was used.
Major and trace element analyses of six samples were determined at the CNRS–CRPG Nancy by inductively coupled plasma atomic emission spectrometry (ICP-AES) for major elements and ICP mass spectrometry (ICP-MS) for trace elements. Six samples from Villaseca & Nuez, (1986) are also included; rare earth element (REE) data for two of them were determined at the CNRS–CRPG Nancy by ICP-AES following the method of Govindaraju & Mévelle, (1987). Another group of six samples was analysed for major and trace elements by XRF, except for REE which were measured by ICP-AES, at Royal Holloway, University of London.
Sixteen xenoliths were selected for isotopic analysis. Six Sr and Nd isotopic determinations were performed at the CNRS–UMR 6524 (Clermont-Ferrand) using an automated VG 54E double collector thermal ionization mass spectrometer. Sm and Nd contents were measured by isotope dilution mass spectrometry at CNRS–UMR 6524. Analytical procedures for the isotopic data at this laboratory have been described by Pin et al., (1990). Another six samples were analysed at Royal Holloway, University of London, using an automated VG 354 multicollector thermal ionization mass spectrometer. Analytical procedures for Royal Holloway isotopic data have been described by Downes et al., (1997). A further four samples for Sr and Nd isotopic determinations were analysed at the CAI de Geocronología y Geoquímica Isotópica of the Complutense University of Madrid, using an automated VG Sector 54 multicollector thermal ionization mass spectrometer with data acquired in multi-dynamic mode. Analytical procedures for the isotopic data at this laboratory have been described by Reyes et al., (1997). Repeated analysis of NIST SRM 987 Sr standard gives 87Sr/86Sr = 0.710209 ± 9 (2
, n = 10) (Clermont-Ferrand), 0.710252 ± 21 (2, n = 31) (London) and 0.710256 ± 8 (2, n = 27) (Madrid). During the course of this study the La Jolla Nd standard gave 143Nd/144Nd = 0.511858 ± 7 (2, n = 4) (Clermont-Ferrand). In London, an internal (Aldrich) Nd standard gave a 143Nd/144Nd value of 0.511418 ± 6 (2, n = 21) equivalent to a La Jolla value of 0.511856. In the Madrid laboratory the Nd standard used was JM and the result obtained was 0.511810 ± 4 (n = 16). This latter value is similar to that obtained by Moreno-Ventas et al., (1995) (0.511821) for an aliquot of the same Nd standard. These standard results indicate similar reproducibility in the three laboratories. Nd values were calculated using the following bulk Earth parameters: 143Nd/144Nd = 0.512638; 147Sm/144Nd = 0.1967. The 2 SD error on Nd calculations is ±0.4.
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Petrography and Mineral Chemistry |
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TOPABSTRACTIntroductionPrevious Estimates of Lower...Types of Granulitic XenolithsAnalytical MethodsPetrography and Mineral...Geochemistry of the Peraluminous...DiscussionConclusionsAppendix a: Granulite Sample...Appendix B: Partition...References |
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K-feldspar
K-feldspar is usually the most abundant felsic mineral in both felsic and pelitic xenoliths (Table 1). They show high Na2O and CaO contents (up to 3.5 wt % Na2O and 1.5 wt % CaO, see Table 2). P2O5 contents are in the range of 0.17–0.30 wt %. These values are higher than those typical of K-feldspar from Hercynian granites of the area (0.03–0.15 wt %; López Moro et al., 1997) and are more typical of feldspars from P-rich granites (Breiter, 1998
; Sha & Chappell, 1998). In small xenoliths or in the rims of large xenoliths, micropegmatitic and symplectitic intergrowths between K-feldspar and plagioclase appear, indicating local recrystallization of feldspars.
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Plagioclase
Plagioclase is present in most of the peraluminous xenoliths and also in the scarce mafic meta-noritic types of the charnockitic suite. Its composition varies depending on the xenolith type. In the meta-noritic xenoliths it is andesine (An45) with around 1.0 wt % K2O (Fig. 3); in the felsic peraluminous xenoliths its composition varies between An21 and An35 with 1.7–3.7 wt % K2O (Fig. 3). This corresponds to 10–23% of molecular orthoclase component and is 5–7 times higher than that of the plagioclase from pelitic and felsic granulites from middle-crustal levels of the area (Barbero, 1995
). P2O5 contents are in the range of 0.14–0.23 wt %, suggesting that both feldspars are major P carriers in these xenoliths. The rims of the plagioclase crystals show frequent microgranophyric or microsymplectitic aggregates, which are interpreted to result from intergranular recrystallization during transport in the alkaline magma. In these symplectitic rims the composition of the plagioclase becomes more calcic (An30 to An42, sample 81846, Table 2) and poorer in orthoclase (Or6.4 to Or2.6, sample 81846, Table 2), thus approaching the typical plagioclase composition of kelyphitic coronas (Fig. 3).
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Garnet
With the exception of the charnockitic xenoliths, garnet is present in the entire suite. Its modal proportion varies from 10 to 25% in the felsic types, and from 25 to 52% in the pelitic xenoliths (Table 1). Garnet frequently contains inclusions of quartz, phlogopite, rutile, zircon, sillimanite, plagioclase and pyrrhotite. In some samples acicular sillimanite inclusions define a relict foliation in the garnet. Garnets are variably transformed to dark kelyphitic coronas, which in some cases completely pseudomorph the original crystal.
The garnet belongs to the almandine–pyrope series, and has a relatively constant composition. Grossular is always <5.5% mol and spessartine <2 mol % (Table 2). Garnets in pelitic xenoliths are slightly richer in Fe than those in felsic types (Fig. 4). When compared with garnets of the outcropping granulitic terranes of the area, those of the xenoliths are richer in pyrope (35–55 mol %), which reflects greater depths of equilibration (Fig. 4). Compositional zoning is almost absent in the garnet core, but there is a notable decrease in Ca towards a narrow rim (Fig. 4), with a subtle increase in the Fe/Mg ratio (Table 2). This is interpreted as a consequence of decompression during transport to the surface.
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Pyroxenes
In the charnockitic xenoliths orthopyroxene occurs in textural equilibrium with augitic clinopyroxene. This orthopyroxene is En67 in composition (Table 2) and shows a slight increase in MgO/(MgO + FeO) ratio from core to rim. The augitic clinopyroxene is relatively rich in Al2O3 (up to 2.5 wt %) and contains up to 0.5 wt % Na2O (Table 2). Orthopyroxene also occurs as an accessory phase in textural equilibrium with garnet in some felsic xenoliths (type 2a), where it is enstatitic in composition (around En60-En65) and rich in Al2O3 (up to 7 wt %, sample 81846, Table 2). No systematic core–rim compositional variations have been found.
Phlogopite
Mica is present only in the felsic peraluminous xenoliths. In contrast to the outcropping granulitic terranes, mica in the granulitic xenoliths is a phlogopite (Table 2). It is rich in TiO2 (5–6 wt %), typical of high-temperature micas, and F (up to 2.5 wt %), and must be H2O poor (Fig. 5). The low Al2O3 contents of these phlogopites (15 wt %), compared with biotites of the middle-crustal granulites of the Anatectic Complex of Toledo (Fig. 5), is typical of phlogopites that are residual after high-temperature dehydration-melting reactions (Singh & Johannes, 1996).
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Sillimanite
In peraluminous xenoliths prismatic sillimanite is present in variable amounts (Table 1). It is a major mineral in pelitic xenoliths. Sillimanite contains appreciable FeO and MgO (0.74 wt % and 0.18 wt %, respectively) (Table 2). In some xenoliths, small green spinel crystals surround the aluminium silicate, which is a typical decompressional feature related to magmatic transport of the xenolith.
Accessory minerals
Common accessory phases in the felsic peraluminous and pelitic xenoliths are rutile and, in lesser amounts, ilmenite. Rutile is conspicuous in all the xenoliths, appearing as euhedral crystals typically 100–250 µm in size. They can contain 0.4 wt % FeO (Table 2). Ilmenite is Mn rich with up to 4.5 wt % MnO. Other accessory phases include acicular graphite, usually found as inclusions in the aluminium silicates of pelitic xenoliths, pyrrhotite, pyrite and chalcopyrite. Scanning electron microscopy (SEM) studies reveal that zircon and monazite are also present but in trace amounts, much less abundant than in the granulites of mid-crustal terranes. Zircons are typically rounded (35–65 µm) with corroded cores showing zoning patterns truncated by outer unzoned rims. These zircons are very similar to type-B of Watt et al., (1996). Monazites are rounded subhedral crystals (35–120 µm) and usually unzoned. Apatite is very rare, being more abundant in the charnockitic varieties (Table 1).
Kelyphitic coronas
Kelyphitic coronas around garnet are formed of amicrocrystalline symplectitic aggregate of spinel,orthopyroxene, feldspars and scarce quartz. In some xenoliths, development of the kelyphitic coronas around garnet replaces most of the garnet crystal. The coronas usually thicken towards the border of the xenolith, the outermost garnets being totally pseudomorphed by the kelyphitic aggregate. Spinel is usually found in these coronas, but is absent in the matrix. Its composition is intermediate between hercynite and spinel (around 60 wt % hercynite). Zn is very low (<0.8 wt %, Table 2) as expected in low-pressure recrystallization (Nichols et al., 1992). Zoning is apparent in the spinel, with the amount of hercynite increasing towards the rim (see sample 99193 in Table 2).
Kelyphitic orthopyroxenes have a variable composition between En50 and En65 (Table 2). Their Fe/Mg ratio is higher than that of the matrix orthopyroxene in the felsic peraluminous xenoliths (type 2a). The coronitic orthopyroxenes also have lower Ti (in type 2a xenoliths), Mg and Cr contents, and higher Al2O3 (up to 13 wt %), Ca and Mn (up to 0.7 wt %) contents.
Plagioclase microcrystals associated with the kelyphitic coronas have a highly calcic (up to An71) composition compared with the matrix plagioclase (Fig. 3) and a lower orthoclase content (Fig. 3). K-feldspar in the kelyphitic coronas is richer in FeO than other sanidines in these xenoliths (Table 2). These alkali feldspars with a more impure composition are typical of those zones around mafic minerals where partial melting has occurred (Grapes, 1985).
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Geochemistry of the Peraluminous Granulitic Xenolith Suite |
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TOPABSTRACTIntroductionPrevious Estimates of Lower...Types of Granulitic XenolithsAnalytical MethodsPetrography and Mineral...Geochemistry of the Peraluminous...DiscussionConclusionsAppendix a: Granulite Sample...Appendix B: Partition...References |
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Major elements
Major and trace element data for 18 whole-rock samples are given in Table 3. For the small charnockitic xenoliths, no powder was available for bulk chemical analysis.
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All the analysed xenoliths are strongly peraluminous in composition, with the pelitic xenoliths showing higher A/CNK values (3–11) than the felsic types (A/CNK 1.2–2.2) (Fig. 6). From their SiO2 content, some pelitic xenoliths are ultrabasic or basic rocks (e.g. sample U-10, which is almost entirely composed of garnet and sillimanite, has 36 wt % SiO2, Table 3). Felsic xenoliths are always intermediate or acid in composition (Fig. 6). Pelitic xenoliths are characterized by low Na2O (
1.2 wt %) and CaO ( 1.1 wt %) contents, and also by higher Al2O3, TiO2 and Fe2O3t than felsic ones, and have lower MgO contents (Fig. 6). K2O contents are high in both types, with the exception of sample U-10 with only 0.72 wt %. In spite of their heterogeneous nature, the peraluminous granulites show marked trends of decreasing Al2O3, Fe2O3t, TiO2 and MgO with SiO2 contentof the rock, although they define parallel 'suites’ in some diagrams (e.g. MgO vs SiO2 diagram, Fig. 6).
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Trace elements
Pelitic xenoliths have higher contents of some transition metals (Cr, Sc and V), high field strength elements (HFSE; Nb, Y) and heavy REE (HREE) compared with felsic xenoliths (Fig. 7), reflecting their higher garnet and rutile contents. The strongly peraluminous metapelite U-10 is highly depleted in large ion lithophile elements (LILE) and light REE (LREE) with respect to the rest of the xenolith suite (Fig. 7). When normalized to average continental crust (Fig. 8), the pelitic xenoliths show a two-fold enrichment in some LILE such as Rb, Ba and K. This kind of xenolith also shows a negative anomaly in Sr and a less marked one in Zr. Felsic granulites also show negative Th and Nb anomalies (Fig. 8).
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REE patterns, except that of xenolith U-10, are LREE enriched and have negative Eu anomalies, although some felsic xenoliths have no Eu anomaly or a positive one (Fig. 9). The latter is related to the abundance of feldspars in this kind of xenolith. The most mafic xenolith U-10 has a flat REE pattern with a marked negative Eu anomaly. This REE pattern resembles that of mid-crust level garnets, which tend to lose their negative fractionation pattern compared with garnets of more epizonal levels, thus acquiring a flatter pattern (Bea, 1996
). All analysed xenoliths are characterized by their high HREE content [even higher than in the estimated lower continental crust of Wedepohl, (1995)]; they also show a slightly higher LREE content in comparison with estimated continental crust (Wedepohl, 1995).
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Sr and Nd isotopic data
Measured Sr and Nd isotopic ratios for the xenoliths are given in Table 4. Isotopic ratios have also been calculated at 300 Ma, an average age for the Hercynian granite plutonism in the SCS (Villaseca et al., 1995
). In the age-corrected Nd vs 87Sr/86Sr diagram (Fig. 10), pelitic and felsic xenoliths plot in different areas, the felsic ones having higher Nd values (-8.2 to -1.4 with an average of -4.8) and lower 87Sr/86Sr ratios (0.70594–0.71300). These differences in isotopic compositions are very common when comparing felsic and pelitic types in lower-crustal xenolith suites (Downes & Leyreloup, 1986; Downes & Duthou, 1988; Eberz et al., 1991). Of the three isotopic reservoirs defined by Downes, (1993) for the lower continental crust in Europe (depleted mantle, ancient crust and felsic crust) the last component is dominant in the SCS xenolith suite. Differences in isotopic composition together with those found in major and trace element contents point to very different original sources for the two types of xenoliths.
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Sample U-10 has very unusual age-corrected Sr and Nd isotopic ratios for SCS material, plotting towards fields typical of xenoliths from ancient terranes (Rudnick, 1992
; Breiter, 1993). This garnetiferous xenolith has a very high 147Sm/144Nd ratio (0.343), which is higher than in pelitic xenoliths both in the SCS and in other parts of the world (Downes & Leyreloup, 1986; Hanchar et al., 1994). |
Discussion |
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TOPABSTRACTIntroductionPrevious Estimates of Lower...Types of Granulitic XenolithsAnalytical MethodsPetrography and Mineral...Geochemistry of the Peraluminous...DiscussionConclusionsAppendix a: Granulite Sample...Appendix B: Partition...References |
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P–T conditions
Mineral parageneses in granulitic xenoliths can provide information about the temperatures and pressures of the last equilibration. The SCS xenoliths show no record of pre-granulitic episodes or evidence of polyphase metamorphism. However, low-P high-T effects related to transport in the lamprophyric dyke and subsequenthydrothermal metamorphism are superimposed on the granulitic parageneses.
The absence of osumilite, cordierite and kyanite in the peraluminous granulites implies pressures of 7–12 kbar for temperatures in the range of 800–900°C (Holdaway, 1971; Carrington & Harley, 1995), but this has to be regarded as a crude approximation because of the Fe-rich character of the xenoliths. In any case, the high modal amount of pyrope-rich garnet is consistent with the presence of abundant prismatic rutile crystals in all the peraluminous xenoliths, which is typical of high-pressure conditions in these compositions (Patiño Douce & Beard, 1995; Patiño Douce, 1996). The almost complete absence of mica in most xenoliths, with the exception of accessory phlogopite found in some felsic xenoliths, indicates that biotite dehydration-melting reactions have almost run to completion. Data from experiments with natural starting materials (pelites, greywackes, biotitic orthogneisses) at 5–10 kbar, have shown that at temperatures over 875–950°C, consumption of biotite is almost complete (Gardien et al., 1995; Patiño Douce, 1996; Stevens et al., 1997). This range of temperatures coincides with that of the appearance of orthopyroxene in peraluminous protoliths (Gardien et al., 1995).
Several geothermometers and geobarometers were used to estimate the conditions of the last equilibration of the xenolith mineral assemblages (Table 5, Fig. 11). The lack of zoning in garnets and other mafic phases indicates attainment of equilibrium during granulitic conditions. Nevertheless, in all thermobarometric estimates, garnet cores were used, to avoid the possibility of rim recrystallization.
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For the charnockitic granulites, temperatures calculated using Wells, (1977)
and Wood & Banno, (1973) two-pyroxene thermometers gave similar results in the range of 930–960°C (Table 5). These are the highest temperatures recorded in the granulitic xenoliths. Estimated pressures of 12 kbar were obtained from the cpx–plg–qtz geobarometer of Ellis, (1980). However, this charnockitic granulite is quartz free, so the pressure thus obtained is a maximum value as silica activity is less than unity.
Orthopyroxene-bearing peraluminous granulites (samples 81845 and 81846, Table 5) also gave high T estimates (870–910°C) with the grt–opx geothermometer of Harley, (1984) for pressures around 9–11 kbar. Garnet–biotite equilibria yielded consistent temperatures around 900–1000°C (Table 5). Phlogopite-bearing peraluminous granulites gave temperatures in the range 800–900°C with garnet–biotite thermometers (Hodges & Spear, 1982; Hoinkes, 1986), in agreement with the experimental data on the stability of biotite in melting experiments. The calibration by Ferry & Spear, (1978) gave results 50–70°C lower than other calibrations of the garnet–biotite thermometers. Pressures calculated using GRIPS (Bohlen & Liotta, 1986) and GASP (Ganguly & Saxena, 1984) geobarometers were 6.5–10 kbar, slightly lower than other estimates in related xenoliths.
Two-feldspar thermometry agrees with the high T estimations, as can be seen from the projection of the mineral compositions in the Ab–An–Or diagram with isotherms at 5 kbar after Fuhrman & Lindsley, (1988) (Fig. 3). The high anorthite content in the alkali feldspar (5–10%) and of orthoclase in plagioclase (up to 20%) indicates high-T equilibration in the range 800–900°C.
These results suggest that the granulitic xenoliths last equilibrated in the lower crust within a depth range of 23–38 km. Such depths are in good agreement with geophysical data concerning the crustal structure of central Spain, particularly the estimates of Moho depth (30–32 km).
Superimposed on the granulitic parageneses is a recrystallization assemblage, which is the result of the transport of the xenoliths in the lamprophyric magma. Kelyphitic coronas are decompressional features and not the result of polyphase metamorphism, as has been demonstrated in other xenolith suites (Rudnick, 1992). The low-pressure orthopyroxene–spinel corona is formed at very high temperatures (up to 1200°C) as deduced from the results of the garnet (rim)–orthopyroxene (kelyphitic) equilibrium (Table 5). Some outer coronas that are composed of chlorite and opaque minerals indicatehydrothermal recrystallization related to the subvolcanic emplacement of the lamprophyre dyke (Rudnick, 1992). The P–T evolution of the xenolith suite, with the calculated decompression path, is summarized in Fig. 11.
Heat productivity of the SCS lower crust
The abundances of heat-producing elements (HPE) in granulite xenoliths and granulite terranes are crucial for estimating the composition of the lower continental crust (McLennan & Taylor, 1996). On the basis of continental heat flow data, McLennan & Taylor made an evaluation of the K, Th and U contents of the average continental crust; K2O must be 1.3 wt %, Th 4.2 ppm and U 1.1 ppm. These values are lower than those recently published by Wedepohl, (1995), which are 2.4 wt %, 8.5 ppm and 1.7 ppm, respectively. For the lower crust the estimates are 1.6 wt %, 6.6 ppm and 0.93 ppm, respectively (Wedepohl, 1995).
Felsic and pelitic xenoliths from the SCS (excluding sample U-10) have average values of HPE of 3.4 wt % K2O, 5.0 ppm Th, and 0.47 ppm U, and of 4.0 wt % K2O and 15.4 ppm Th, respectively (Table 3). No U data are yet available for the pelitic xenoliths. With these HPE contents we have calculated the radioactive heat productivity using heat productivity rates for K, U and Th from Rybach & Muffler, (1981). Values of 0.79 µW/m3 for felsic xenoliths and 1.67 µW/m3 for pelitic types are obtained for the SCS lower crust. These values are close to the average lower continental crust value (0.90 µW/m3) proposed by Wedepohl, (1995), but are higher than the values of Rybach & Muffler, (1981) (0.45–0.73 µW/m3) and those of average xenolith suites of Rudnick, (1992) (0.28 µW/m3), or for the average continental crust estimated by McLennan & Taylor, (1996) (0.71 µW/m3). The estimated values of HPE for the SCS lower crust are much higher than those used in theoretical models of anatexis in tectonically thickened continental crust (England & Thompson, 1986; Patiño Douce et al., 1990). Moreover, this high crustal radioactive heat production has to be considered in addition to the magnitude of thickening in the SCS during the Hercynian orogeny, which is estimated to have been more than twice the initial pre-Hercynian crust [Villaseca et al., (1998) and references therein]. Considering these parameters in thermal models, heat production rates in the SCS are high enough to raise the temperature to >900°C at the base of the crust, without any increase in the basal heat flux. This thermal productivity of the lower crust is high enough to promote extensive melting and thus can explain granite generation in the SCS during the Hercynian orogeny.
Restitic character of the SCS lower crust: implications for Hercynian granite genesis
The modal mineralogy of the granulitic xenoliths is consistent with a residual character for most of them. Sample U-10 (Tables 1 and 3), which is exclusively composed of garnet and sillimanite, is more mafic in composition than some pelitic granulites used in experimental studies, which do not yield significant melt until temperatures >1000°C are reached (Beard et al., 1993). The low proportion of mica in the SCS xenoliths could be a consequence of biotite dehydration-melting reactions. The abundance of modal K-feldspar in most xenoliths does not contradict the possibility of a residual character after granitic melt extraction. As stated by Carrington & Watt, (1995), consumption or production of K-feldspar in biotite dehydration-melting reactions is highly dependent on the H2O/K2O ratio of the melt and K-feldspar production is enhanced at higher pressures. Moreover, incomplete separation of the high-viscosity granitic melt could also give rise to a feldspar-rich residue, as argued for other lower-crustal granulites (Rudnick & Presper, 1990; Schnetger, 1994).
The major element chemistry of the pelitic xenoliths (Al2O3 > 25 wt %, FeOt > 10 wt %, MgO > 3 wt %) is consistent with these xenoliths being melting residua. They plot in more mafic and Al-rich fields than pelitic parents as would be expected from a mass balance approach of partial melting of metasediments from which a granite melt was removed (MacRae & Nesbitt, 1980) (Fig. 12). Nevertheless, the difference in isotopic composition from the Hercynian granitoids and their relative scarcity in the xenolith suite suggest a minor contribution of this type of source in the genesis of the SCS granites. This corroborates other geochemical approaches to granite petrogenesis in the area (Villaseca et al., 1998).
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In Fig. 12 a crude linear array connects felsic xenoliths and Hercynian granites from the SCS, passing through the composition of the outcropping orthogneisses. Such linearity would be expected to occur when removing granitic liquid from orthogneissic rocks (see CNW path in Fig. 12). This process tends to produce more Al-, Fe- and Mg-rich restites, whose composition must be located at the ends of the lines projecting through SCS granites and orthogneisses (see inset in Fig. 12a). Most of the felsic xenoliths do indeed plot at the end of this linear array (Fig. 12). Moreover, a complementary character in major and trace element composition, between some felsic meta-igneous xenoliths and the granites, is observed (Figs 12 and 13). This is particularly notable when considering LILE (Rb, K, Ba, Sr) and some transition metals (Sc, V, Ni). Those trace elements controlled by accessory phases (i.e. REE, except Eu, and HFSE) do not fit the mass balance so well.
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To test the hypothesis that the felsic granulitic xenoliths represent residua after granitic melt extraction, with the melts being represented by the Hercynian plutons, major and trace element mass balance modelling has been performed. In this modelling an orthogneissic protolith rather than a pelitic one has been selected, as previously discussed.
Table 6 shows that, considering an average SCSorthogneiss as the protolith for the granites, it is possible to obtain melting residua whose compositions are very similar to that of the average meta-igneous xenoliths. We have calculated the composition of the restitic lower crust after subtraction of variable (29–33%, Table 6) granitic partial melts of known composition (columns 1–4, Table 6) from the average orthogneissic protolith. The calculated residua (columns 7–11, Table 6) have a major and trace element composition close to that of the average felsic meta-igneous xenolith (column 6, Table 6). The goodness of the fit is reflected in the calculated R2 residuals (0.96–2.19); also the calculated degree of melting of 30% supports this possibility. The proportion of melt obtained in dehydration-melting experiments at 900°C and 10 kbar from meta-igneous protoliths (biotite gneisses) is 10–30% (Skjerlie & Johnston, 1993; Gardien et al., 1995; Patiño Douce & Beard, 1995) which agrees with the degree of melting calculated in our mass balance modelling. In Fig. 14a we have plotted the major and trace element composition of the calculated residua after the mass balance calculation and of the sampled felsic meta-igneous xenoliths.
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We further tested our hypothesis that the felsic xenoliths are residues of melting by estimating the trace element concentration using batch melting equations and published partition coefficients (see Appendix B, Table B1 and references therein). This modelling is hindered by the availability of partition coefficients and also by the fact that several trace elements, especially the REE, reside in accessory phases whose abundances and behaviour are generally poorly constrained. Nevertheless, for Ba, Rb, Sr and REE, the compositions of residues after partial melting of orthogneissic sources have been calculated (columns 12–14, Table 6) and also plotted in Fig. 14a with previous models. In the modelling, both modal and non-modal batch melting calculations have been performed, giving similar results (see models 12 and 13 in Table 6). The main difference between models 13 and 14 is the involvement of monazite in both the melt and restite assemblages, necessary to account for the observed LREE contents in xenoliths (Fig. 14a). Biotite is the first mineral to disappear, as shown by the modal depletion in the SCS xenoliths compared with equivalent protoliths in outcropping granulitic terranes. Biotite is also the first mineral substantially consumed in melting experiments in biotite gneisses, with the remaining quartz or plagioclase being very close to their initial modal proportions (Gardien et al., 1995
; Montel & Vielzeuf, 1997). Similarly, minerals produced in the melting reaction, garnet and K-feldspar, are supposed to be more concentrated in restite assemblages. The calculated modal source composition after 30–35% of melting is 0.27 Qtz + 0.28 Kfs + 0.28 Plg + 0.15 Grt + 0.01 Bt. This closely resembles the modal assemblages of the SCS felsic xenoliths (Table 1). Modelled restite 14 is in the range of compositions of the SCS xenoliths (Fig. 14a), and changes in modal proportions of other major felsic phases in the melting reaction do not substantially modify the result.
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Table 6 also shows the compositions of calculatedliquids in equilibrium with the granulitic xenoliths as melting residues. The results (models 15 and 16 of Table 6) show a good fit to the compositions of the SCS granites for Ba, Rb, Sr and LREE (Fig. 14b). Departures in HREE and Y contents probably reflect modal variability of garnet in the source (a reduction from 10 to 5% in volume in the protolith broadly duplicates HREE concentration in the liquid) or the involvement of other accessory phases (e.g. zircon), but also might be the consequence of incomplete equilibration of garnet with the melt as melting and garnet production progress (Qin, 1991
; Miller et al., 1992). Nevertheless, these results support the hypothesis of the granulitic xenoliths being melting residua after the extraction of liquids similar in composition to the SCS granites. The genetic relationship between the Hercynian peraluminous granites and the studied felsic metaigneous xenoliths is corroborated by the Sr and Nd isotopic composition of all these rocks. In Fig. 15, the Sr and Nd isotopic data of the lower-crustal xenoliths are plotted together with the isotopic data of SCS Hercynian peraluminous granites (Moreno-Ventas et al., 1995; Pinarelli & Rottura, 1995; Villaseca et al., 1998). The meta-igneous xenoliths match the isotopic composition of most of the peraluminous Hercynian granites, whereas the pelitic xenoliths have lower Nd values, and plot outside of the granite field.
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Crustal heterogeneity in terms of isotopic composition
The coincidence in Nd isotopes and the similar nature of the materials from SCS middle- and lower-crustal levels (dominantly meta-igneous and metapelitic lithologies) suggest that tectonic breaks do not exist between the middle- and lower-crustal levels as is the case in some other crustal segments (Eberz et al., 1991
). Nevertheless, Sr isotopic data of felsic meta-igneous and metapelitic xenoliths are closer to bulk Earth isotopic composition than those of orthogneisses and metapelites from shallower crustal levels (Fig. 10). As seen in other regions, there is a tendency for the Sr (and sometimes also Nd) isotopic composition of the lower continental crust to plot closer to bulk Earth values, whereas the upper-crustal levels tend to be more enriched (Downes & Duthou, 1988; Ruiz et al., 1988; Eberz et al., 1991). This tendency to greater isotopic homogenization, together with the lowering of Sr isotopic ratios, has also been observed in areas of progressive metamorphism (e.g. Central Pyrenees, Bickle et al., 1988; eastern Nevada, Wickham, 1990). Several possibilities could explain this isotopic shift:
- Mixing processes with either the host magma or mixing with mantle-derived underplated material at the Moho or infracrustal levels (Downes & Leyreloup, 1986; Rudnick, 1992). Isotopic ratios of the SCS xenolith suite do not define a simple mixing array on the Sr–Nd isotope diagram (Fig. 10) and could be better interpreted as reflecting a diversity of sources for these rocks. Moreover, the marked peraluminous character of the xenoliths and their major and trace element compositions (the high Sr and Nd contents of the xenoliths make them less susceptible to contamination), together with the lack of significant basic underplating in Hercynian times (Villaseca et al., 1998), rule out these possibilities.
- Isotopic changes related to the progressive metamorphism and associated anatexis. Bickle et al., (1988) invoked hydrous fluids generated during progressive dehydration of metamorphites as a mechanism to reduce and homogenize the Sr isotopic ratios in catazonal rocks in the Hercynian Pyrenees. Similar isotopic homogenization has been described in the Cooma complex (Chappell et al., 1991), in Brittany and in eastern Nevada (Wickham, 1990). In all cases this modification is explained as resulting from fluid-mediated exchange with low 87Sr/86Sr rocks (e.g. carbonates) or pore fluids. The homogenization and lowering of isotopic ratios in crustal rocks during progressive metamorphism via fluid advection systems is also shown by other isotopic systems (Pb, O) (Wickham, 1990; McCulloch & Woodhead, 1993; Holk & Taylor, 1997). Nevertheless, the absence of important carbonate layers in the metamorphic sequence of the SCS sector and the diminishing influence of pore fluids in the deepest crustal levels are obstacles to considering fluid mixing as the only mechanism to change Sr isotopic ratios. Therefore, in the lower crust, melting processes have to be involved together with this dehydration process. The lower crust is the section that has undergone the most extensive anatexis, suggesting that the anatectic melts themselves facilitated isotope exchange with the restites, and/or they served as a source of aqueous fluids that promoted such exchange. As isotopic interchange with externally derived aqueous fluids or granitic melts in catazonal rocks has been suggested in other localities (e.g. Wickham, 1990; Holk & Taylor, 1997), the main problem in our Hercynian crustal section is the origin of the low 87Sr/86Sr liquids or, on the contrary, the way to lose radiogenic strontium in processes affecting the lower crust. Further work is required to clarify the exact mechanism for this isotopic modification.
A more primitive isotopic composition of the lower continental crust was first envisaged by Taylor & McLennan, (1985), who compared Sr–Nd isotopic data from granulite terranes (lower- to middle-crustal levels) with granulitic xenoliths that exhibit lower initial Sr ratios and higher initial Nd signatures. This tendency of lower-crustal granulites towards isotopically more primitive compositions has major consequences when evaluating the contribution of crustal sources in the genesis of granitic magmas. First, it can explain the apparent absence of the isotopically appropriate crustal protoliths in the outcropping metamorphic rocks, as has been discussed in several studies [the strontium paradox of Bernard-Griffiths et al., (1985) and Peucat et al., (1988); see also Clarke et al., (1988)]. The second consequence is that the important mantle contribution required by mixing models for some of the SCS granites which involve mantle-derived end-members and crustal materials (Moreno-Ventas et al., 1995; Pinarelli & Rottura, 1995) can be severely reduced, as the appropriate crustal component, as demonstrated here, has a more primitive Sr isotope composition.
Comparison with other Hercynian lower-crustal granulites from western Europe
Comparison of the composition of the lower crust of central Spain with that of other European Hercynian areas reveals an important difference: in central Spain the lower crust is essentially felsic in character whereas in the French Massif Central (Downes & Leyreloup, 1986; Downes & Duthou, 1988) or in the Eifel volcanic region (Loock et al., 1990) the lower crust contains an important component of mantle-derived basic material. With the exception of the granulitic xenoliths that appear in Cenozoic volcanic rocks in SE Spain (Sagredo, 1976; Vielzeuf, 1983; Cesare et al., 1997), lower-crustal xenoliths are scarce elsewhere in Spain. In southeastern Spain metapelitic and charnockitic xenoliths from lower-crustal levels are described, their P–T estimates from phase equilibria being around 5–7 kbar, 700–850°C (Sagredo, 1976; Cesare et al., 1997). These conditions are typical of middle- to lower-crustal levels. In any case, the absence of mafic metaluminous xenoliths in SE Spain reinforces the concept of a more felsic composition for the deeper crustal levels in the Iberia region.
Exposed granulite facies terranes can provide information on the lowermost crustal levels. The question of whether granulitic terranes are representative of the lower crust has been debated frequently (Downes, 1993). Some high-grade terranes are probably more representative of middle-crustal levels, as is the case for the granulite terranes in central Spain (Barbero, 1995) or the Agly massif in the Pyrenees (Pin, 1989). Other exposed granulite terranes from deeper crustal levels have been described in the Pyrenees (Saleix massif, Vielzeuf, 1984), Calabria (Maccarrone et al., 1983) and northern Italy (Pin & Sills, 1986; Hermann et al., 1997), and are undoubtedly representative of the lower continental crust. Data from these granulite facies terranes indicate a larger contribution of mantle-derived material, as in the Ivrea Zone (Pin, 1990; Voshage et al., 1990), the Calabrian massif (Maccarrone et al., 1983) and the Pyrenees (Pin, 1989). This suggests underplating of mantle-derived magmas at the base of the continental crust in these areas, in clear contrast to the scenario proposed here for central Spain.
In Table 7 an unweighted and a weighted mean composition of the SCS xenolith suite is presented along with other estimates of global lower crust, together with peraluminous granulites interlayered with metagabbros of the Ivrea Zone (average stronalite). When compared with average lower crust in other areas and with model compositions (Wedepohl, 1995; McLennan & Taylor, 1996), the estimated composition of the lower crust in central Spain is clearly more felsic and richer in LILE and REE contents (Fig. 16). This agrees with recent estimations that result in a more felsic bulk composition of the lower crust, and suggests that the presence of a mafic layer at the base of the continental crust is not a universal feature (Wedepohl, 1995; Le Pichon et al., 1997). The felsic nature of the SCS lower crust is best shown in comparison with other lower-crustal xenolith suites which, on average, are more mafic than granulite terranes (Rudnick, 1992). The composition of the SCS lower crust resembles that of some peraluminous granulitic layers of exposed Hercynian granulite terranes (i.e. stronalites from Ivrea Zone, Fig. 16), which are also considered to be residues of high-pressure crustal melting from which granitic melt was extracted (Schnetger, 1994).
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Conclusions |
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TOPABSTRACTIntroductionPrevious Estimates of Lower...Types of Granulitic XenolithsAnalytical MethodsPetrography and Mineral...Geochemistry of the Peraluminous...DiscussionConclusionsAppendix a: Granulite Sample...Appendix B: Partition...References |
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The study of the xenolith suite scavenged by early Mesozoic alkaline dykes in the Hercynian central region of central Spain reveals three main types of lower-crustal granulites. These are rare felsic to intermediate charnockites (<0.01% in volume), metapelitic (5% in volume) and common felsic meta-igneous types (95% in volume). P–T estimates in these granulites (850–950°C and 6–11 kbar) clearly indicate that these xenoliths come from the lower crust, although probably from different levels of this lower crust rather than from a single level.
Major and trace elements, and Sr–Nd isotopic compositions of the felsic and metapelitic types, are consistent with a restitic origin for these granulites after granitic melt extraction. Major and trace element modelling give reasonable values for the melting process comparable with experimental work and confirming that some of the Hercynian peraluminous granites could represent the average granitic liquid extracted. A mainly crustal origin for the peraluminous granites of the SCS is deduced. The isotopic shift of lower-crustal material toward bulk Earth composition substantially reduces the mantle contribution required by mixing models for granite petrogenesis.
The essentially felsic character of the lower continental crust in this part of the Hercynian belt contrasts with the more mafic nature deduced for other European Hercynian areas (e.g. French Massif Central, Eifel, Ivrea, Calabria). The average lower continental crustal composition in central Spain, when compared with other estimates, is the most felsic lower crust yet known. This felsic character is not only shown by the granulitic xenolith suite but is also supported by geophysical data.
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Appendix a: Granulite Sample Description |
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TOPABSTRACTIntroductionPrevious Estimates of Lower...Types of Granulitic XenolithsAnalytical MethodsPetrography and Mineral...Geochemistry of the Peraluminous...DiscussionConclusionsAppendix a: Granulite Sample...Appendix B: Partition...References |
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The 26 samples selected for study cover the spectrum of lower-crustal lithologies found as xenoliths in the SCS. These samples were also selected from a larger dataset of 74 xenoliths representing the best samples of greater size, less altered appearance and with no evidence of host lamprophyre infiltration. All the granulitic xenolithsare granoblastic in texture (Fig. A1) and many exhibit small-scale compositional banding marked by garnet (sillimanite)-rich bands sometimes alternating with quartz-rich layers (Fig. A1). Foliation is clearly defined by tabular pyroxene or sillimanite grains, but also by lamellar quartz (Fig. A1). Table A1 summarizes the petrographical and textural features of investigated samples.
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Appendix B: Partition Coefficients Used in the Trace Element Modelling |
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TOPABSTRACTIntroductionPrevious Estimates of Lower...Types of Granulitic XenolithsAnalytical MethodsPetrography and Mineral...Geochemistry of the Peraluminous...DiscussionConclusionsAppendix a: Granulite Sample...Appendix B: Partition...References |
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Kd values for LILE from Arth (1976)
except Rb for K-feldspar, which is taken from Schnetger (1994). Y for plagioclase and garnet from Arth (1976), for K-feldspar and biotite from Schnetger (1994) and for monazite is extrapolated between Kd values for Dy and Yb. REE for plagioclase and K-feldspar are taken from Nash & Crecraft (1985), REE for biotite from Arth (1976), REE for garnet from Irving & Frey (1978) and REE for monazite from Cocherie et al. (1994).
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Acknowledgements |
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Alfredo Fernández Larios and José González del Tánago from the CAI of Microscopía Electrónica (UCM) and Donald Herd from the University of St Andrews (UK) are thanked for their assistance with microprobe analyses. XRF and radiogenic isotope laboratories at Royal Holloway College (London) are University of London Intercollegiate Research facilities. Constructive comments and suggestions made by Calvin Miller, Heinz-G. Stosch, Pamela Kempton and an anonymous reviewer have greatly improved the quality of the manuscript. This work is included in the objectives of, and supported by, the PB96–0661 DGICYT project of the Ministerio de Educación y Cultura of Spain.
* Corresponding author. Telephone: +34–913944910. Fax: +34–915442535. e-mail: granito{at}eucmax.sim.ucm.es
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References |
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TOPABSTRACTIntroductionPrevious Estimates of Lower...Types of Granulitic XenolithsAnalytical MethodsPetrography and Mineral...Geochemistry of the Peraluminous...DiscussionConclusionsAppendix a: Granulite Sample...Appendix B: Partition...References |
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