Journal of Petrology | Volume 38 | Number 6 | Pages 727-755 | 1997
© Oxford University Press 1997
Magmatic Evolution and Tectonic Setting of the Iberian Pyrite Belt Volcanism
Institute of Earth Sciences ‘Jaume Almera’ CSIC, Lluis Sole I Sabaris S/N, 08028 Barcelona, Spain
Received June 1, 1996; Revised typescript accepted February 4, 1997
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ABSTRACT |
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 TOP ABSTRACT IntroductionGeological SettingStratigraphy of the Volcano...Petrography of Volcanic RocksGeochemistryDiscussion of ResultsConclusionsReferences |
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The Iberian Pyrite Belt, which extends from Portugal to Spain in southwest Iberia, constitutes one of the world's largest reservoirs of massive sulphide deposits. Volcanic-hosted massive sulphide mineralization occurs at several stratigraphic horizons within an Early Carboniferous volcano-sedimentary package formed of turbiditic siliciclastic deposits and basaltic, intermediate and silicic volcanic rocks. Volcanic rocks do not show significant temporal or spatial variations in the stratigraphic sequence of the Iberian Pyrite Belt and mainly occur as shallow intrusions into wet marine sediments with some minor lavas, hydroclastic rocks and volcanogenic sediments. A geochemical study, including major, trace and rare earth elements, and Sr and Nd isotopes, of the least altered volcanic rocks has been carried out to determine the primary magmatic affinity and tectonic setting of the Iberian Pyrite Belt volcanism. Most of the basaltic rocks are continental tholeiites, but a few samples show an alkaline affinity. The origin of the basaltic rocks and their diversity of compositions are explained by a single mixing model between E- and N-MORB (mid-ocean ridge basalt) and assimilation of crustal material. Calc-alkaline intermediate and silicic rocks include basaltic andesites, andesites, dacites and thyolites. Volumetrically, dacites and thyolites are the most abundant. Intermediate and silicic rocks are not related by fractional crystallization, nor is there a relationship between the basaltic and calc-alkaline rocks by the same process. We suggest that in the Iberian Pyrite Belt silicic calc-alkaline magmas were generated on a large scale by the invasion of continental crust by mafic magmas generated in the underlying upper mantle. The diversity of compositions shown by dacites and thyolites can mainly be explained either by differences in the composition of the source rocks or by different degrees of partial melting of upper-crust rocks. Andesites, however, formed by mixing between basaltic magmas and upper-crust material. The new geochemical data agree with previously published tectonostratigraphic data which suggest that the Iberian Pyrite Belt volcanism formed on the South Portuguese plate owing to strike-slip tectonics. This local extensional tectonic setting was related to transtension as a result of oblique continental collision that followed the subduction of the South Portuguese plate beneath the Ossa Morena plate. This tectonically driven magmatism does not have a modern analogue, but it is not inconsistent with the proposed geodynamic evolution of the studied area. This model gives insights into the petrology and geochemistry of strike-slip settings in the continental part of sub-ducting plates, a region usually poorly constrained from a petrological point of view.
KEY WORDS: calc-alkaline volcanism; isotope geochemistry; strike-slip tectonics; Iberian Pyrite Belt
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Introduction |
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TOPABSTRACTIntroductionGeological SettingStratigraphy of the Volcano...Petrography of Volcanic RocksGeochemistryDiscussion of ResultsConclusionsReferences |
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Volcanic-hosted massive sulphide deposits are mainly associated world-wide with calc-alkaline submarine volcanism. Petrological and geochemical data, together with stratigraphic and structural studies, have been of major importance in constraining geological models of the volcanism associated with massive sulphide deposits. Integrated studies in areas such as the Kuroko province in Japan (Ohmoto, 1983
), the Mount Read Volcanics in Tasmania (Crawford et al., 1992), and the Mount Windsor Volcanics in northwestern Australia (Stolz, 1995), have revealed that this volcanism may be developed during different stages of the subduction process, always being located on the overriding plate.
This paper documents the petrology and geochemistry of the volcanism of the Iberian Pyrite Belt, an Early Carboniferous metallogenic province that extends from Portugal to Spain in southwest Iberia and constitutes one of the world's largest reservoirs of massive sulphide deposits. Despite the economic significance of this area, the volcanic rocks have not been studied extensively and most of the existing geodynamic models proposed to explain the origin of volcanism and associated massive sulphide deposits are not based upon petrological and geochemical data. Thus, in spite of the existence of several studies aimed at characterizing the geochemistry of volcanic rocks (Rambaud, 1969; Strauss, 1970; Hamet & Delcey, 1971; Lécolle, 1977; Routhier et al., 1977; Priem et al., 1978; Soler, 1980; Strauss et al., 1981; Munhá, 1983; Möller et al., 1983; Schütz et al., 1988), and some studies focused on the stratigraphy and tectonic setting of the Iberian Pyrite Belt (Schermerhorn, 1971; Strauss & Madel, 1974; Ribeiro et al., 1983; Oliveira, 1990; Silva et al., 1990; Quesada et al., 1994; Giese et al., 1994), the nature and evolution of the volcanism are still poorly known. Geochemical data (elementary and isotopic) are mainly concerned with the Portuguese sector of the Iberian Pyrite Belt. In contrast, petrological and geochemical data relating to the volcanic rocks from the Spanish sector, which includes the eastern part of the Iberian Pyrite Belt, are relatively scarce.
In this paper, we describe and interpret the magmatic evolution and tectonic setting of the Iberian Pyrite Belt volcanism. The volcanic rocks comprise basalts, andesites, dacites and rhyolites which mainly appear as shallow intrusions into wet marine sediments with some minor lavas, hydroclastic rocks and volcanogenic sediments. The detailed stratigraphy established by Soriano, (1997) shows a lack of significant temporal and spatial variations in the distribution of volcanic rocks. We report new major, trace and rare earth element (REE), and Sr–Nd isotope data from the Spanish sector of the Iberian Pyrite Belt which have been integrated with previous petrological and geochemical data, mainly from the Portuguese sector. A comprehensive geodynamic model, which differs from those deduced from other volcanic-hosted massive sulphide deposit areas, is proposed to explain the origin and evolution of the Iberian Pyrite Belt volcanism. We also discuss the genetic relationship of the different rock types found in this volcanism.
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Geological Setting |
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TOPABSTRACTIntroductionGeological SettingStratigraphy of the Volcano...Petrography of Volcanic RocksGeochemistryDiscussion of ResultsConclusionsReferences |
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The sector of the European Variscan Orogen that crops out in the western Iberian Peninsula is known as the Iberian Massif (Lotze, 1945
), where five major Variscan geological units were distinguished by Julivert et al., (1974) (Fig. 1). Recent studies on the boundary between the South Portuguese Zone and the Ossa Morena Zone have interpreted it as a major suture of the European Variscan Orogen (Munhá et al., 1986; Crespo-Blanc & Orozco, 1988; Quesada, 1991). This suture is interpreted as a thrust that emplaces the Ossa Morena Zone structurally above the South Portuguese Zone (Fig. 1) and is linked to a major geological boundary of similar characteristics in the southern British Isles (Crespo-Blanc & Orozco, 1991). Thus, the South Portuguese Zone was accreted to the rest of the Iberian Massif during the Variscan Orogeny (Ribeiro et al., 1990) and represents the continental crust of a tectonic plate whose oceanic crust was totally subducted under the continental crust of the Ossa Morena Zone (Quesada et al., 1994).
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The Iberian Pyrite Belt is one of the four Variscan structural units distinguished by Quesada, (1991)
in the South Portuguese Zone (Figs 1 and 2). It is bounded to the north by the Pulo do Lobo oceanic terrane and to the south by the Baixo Alentejo Flysch Group. The main feature of the Iberian Pyrite Belt is the occurrence of polymetallic sulphide deposits associated with basic and silicic volcanic rocks which are interbedded with Early Carboniferous turbiditic siliciclastic deposits. Sedimentation of these deposits is continuous through the stratigraphic sequence of the Iberian Pyrite Belt and no internal unconformities have been observed. Deposition of these rocks took place in a submarine continental platform environment (Oliveira, 1983, 1990) from bottom and turbidity currents.
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Volcanic rocks interbedded with marine sediments are concentrated in the central part of the Iberian Pyrite Belt stratigraphic sequence, whereas they are lacking in its upper and lower parts. Strauss, (1970)
and Schermerhorn, (1971) distinguished three lithostratigraphic units according to the presence or absence of volcanic rocks and volcanism-related hydrothermal alteration. From the base to the top, these stratigraphic units are as follows:- The Phyllite and Quartzite unit is composed of phyllites, quartzites and conglomerates deposited on a continental platform (Oliveira, 1990). Rare limestone lenses bearing conodont fauna are located at the top of the unit and indicate a Lower to Upper Famennian age (Fig. 3). Its base never crops out throughout the Iberian Pyrite Belt.
- The Volcano-Sedimentary Complex is the stratigraphic unit which contains the ore deposits. Most of the volcanic rocks are shallow intrusive bodies which show irregular shapes and contacts with the host rocks. Therefore, the lower and upper boundaries and the thickness of the Volcano-Sedimentary Complex are variable. The sediments interbedded with the volcanic rocks are mainly turbiditic platform deposits with oceanic fauna such as radiolaria. Rare limestone lenses bearing conodont fauna are located at the top of the unit and indicate a Lower to Upper Viséan age (Fig. 3).
- The Culm Group is the uppermost stratigraphic unit in the Iberian Pyrite Belt. Its top has been eroded and cannot be observed. Shales and sandstones deposited from turbidity currents are the main lithologies. Facies distributions strongly suggest flysch sedimentation (Oliveira, 1990). Goniatite fauna found at the base of the unit indicate an Upper Viséan age (Fig. 3).
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Hydrothermal alteration took place during and shortly after the volcanism of the Volcano-Sedimentary Complex (Munhá, 1990
). Oxygen, hydrogen and sulphur isotopic compositions (Munhá & Kerrich, 1980; Barriga & Kerrich, 1984) demonstrate that these hydrothermal processes involved the interaction of magmatic fluids with seawater.
All of the above-mentioned rocks of the Iberian Pyrite Belt were metamorphosed and deformed during the Variscan Orogeny. A regional low-grade metamorphism of zeolite to greenschist facies increases from south to north across the Iberian Pyrite Belt and the South Portuguese Zone, and towards the base of the stratigraphic sequence (Munhá, 1990). Large intrusive bodies of granitic, tonalitic and dioritic composition, which appear at the northeastern part of the South Portuguese Zone and the southern part of the Ossa Morena Zone, are thought to be responsible for this regional metamorphism and its north to south polarity (Dallmeyer et al., 1993; De La Rosa et al., 1993).
Variscan tectonics in the Iberian Pyrite Belt is characteristic of a fold and thrust belt with a southward vergence. Thrust sequences followed a piggy-back propagation mode, and related folds developed an axial plane cleavage sometimes transecting the fold axes (Ribeiro et al., 1983). An increase in the cleavage intensity is seen from south to north across the entire South Portuguese Zone (Silva et al., 1990), and is probably related to a thermal control on the development of ductile structures by large intrusives in the northeastern part of the South Portuguese Zone. The detachment level of the thrusts is found at the base of the Palaeozoic sequence of the South Portuguese Zone (Ribeiro et al., 1983).
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Stratigraphy of the Volcano-Sedimentary Complex |
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TOPABSTRACTIntroductionGeological SettingStratigraphy of the Volcano...Petrography of Volcanic RocksGeochemistryDiscussion of ResultsConclusionsReferences |
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The Volcano-Sedimentary Complex is characterized by the presence of a monotonous sequence (500–800 m thick) of Late Devonian–Early Carboniferous sandstones and shales with shallow sub-horizontal intrusions of basaltic, intermediate and silicic compositions, and minor interbedded lavas, hydroclastic rocks and volcaniclastic sediments (Fig. 4). A detailed revision of the stratigraphy of the Volcano-Sedimentary Complex (Soriano, 1997
) has revealed that the emplacement of these intrusives took place at different levels in the stratigraphic sequence and that they are interfingered with the host sediments. This has caused some misinterpretations of the volcanic events represented in the Volcano-Sedimentary Complex, such as the distinction of several well-separated volcanic episodes made by Instituto Geológico y Minero de España, (1982), which we will discuss in a later section. Previous studies of the Iberian Pyrite Belt volcanism suggested the occurrence of primary pyroclastic deposits (Schermerhorn, 1976; Lécolle, 1977). However, most of the volcaniclastic deposits found in the studied area are not of pyroclastic origin. They were formed by hydroclastic fragmentation and erosion of submarine lavas and domes (Soriano, 1997).
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The palaeogeographic and stratigraphic distribution of volcanic rocks in the Volcano-Sedimentary Complex is relatively haphazard (Fig. 4), but some trends can be inferred from stratigraphic sections throughout the Iberian Pyrite Belt. In the south and southwestern part of the Iberian Pyrite Belt silicic rocks are mostly found in the lower part of the Volcano-Sedimentary Complex sequence, whereas in the north and northeastern part very shallow silicic intrusives are located at its top (Fig. 4). Despite this trend, silicic intrusives also appear at the top of the Phyllite and Quartzite unit in the north and east of the Iberian Pyrite Belt.
Andesitic rocks do not crop out in the southwesternmost part of the studied area. They mainly appear as shallow intrusives above and below felsic volcanics in the central part of the Iberian Pyrite Belt, and can also intrude either the top of the Phyllite and Quartzite Unit or the highest levels of the Volcano-Sedimentary Complex. In the northeasternmost part of the Iberian Pyrite Belt andesitic rocks are always below silicic volcanics in the stratigraphic record (Fig. 4). On the other hand, basaltic rocks are widely distributed and do not show any specific stratigraphic or palaeogeographic position.
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Petrography of Volcanic Rocks |
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TOPABSTRACTIntroductionGeological SettingStratigraphy of the Volcano...Petrography of Volcanic RocksGeochemistryDiscussion of ResultsConclusionsReferences |
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Felsic rocks range in composition from dacite to rhyolite. Most of them appear as shallow intrusives which show peperitic textures developed at the contacts between intrusions and wet sediments (Boulter, 1993a
, b). Occasionally, felsic rocks reached the oceanic floor as extrusive domes giving rise to short-length lava flows and hydroclastic deposits. Other field structures such as flow-banding foliation, flow autobrecciation and soft-sediment intrusions through columnar jointing of volcanic rocks also support a shallow intrusive emplacement for most of the silicic volcanic rocks. Rhyolitic rocks are mainly composed of albite and quartz phenocrysts and minor biotite phenocrysts (partially or totally replaced by chlorite). Occasionally, K-feldspar may also be present. A felsitic groundmass is characteristic of rhyolites, which also show perlitic and spherulitic textures. Plagioclase phenocrysts and those parts of the groundmass enriched with plagioclase microlites are often altered to calcite, muscovite and sericite. Accessory minerals in dacites and rhyolites include apatite. Dacites usually show porphyritic and glomeroporphyritic to massive coherent textures. Embayed and curviplanar quartz phenocrysts, subhedral albitized plagioclase, and rare biotite phenocrysts and clinopyroxene microphenocrysts are set in a microcrystalline quartz–feldspar groundmass.
Intermediate volcanic rocks are porphyritic to glomeroporphyritic andesites with subhedral prismatic plagioclase, occasionally albitized, clinopyroxene phenocrysts, Fe-oxides, and rare quartz and biotite (mostly chloritized), embedded in a microcrystalline groundmass of microlitic plagioclase and microcrystalline quartz. They usually have amygdales filled with chlorite, calcite, epidote and quartz. Plagioclase phenocrysts are often altered to calcite, muscovite and epidote, and the groundmass can be altered to chlorite. Field exposures of andesites can show highly irregular, sometimes chilled, contacts with the host sediments, and frequently have columnar and spheroidal jointing. Two of the studied samples contained olivine phenocrysts, together with clinopyroxene and a Ca-rich plagioclase. This suggests the presence of restricted basaltic andesites in the volcanism of the Iberian Pyrite Belt.
Basaltic rocks show massive, intergranular, equigranular and minor porphyritic textures. Subhedral clinopyroxene and plagioclase (mostly albitized) frequently show intergrowing of crystals. In most of the rocks the clinopyroxene is a pale-coloured augite. However, a few basaltic samples contain a titaniferous salite, which suggests an alkaline affinity. In the porphyritic varieties the groundmass contains plagioclase microlites and microcystalline augite. Minor subhedral crystals of olivine and Fe–Ti oxides are also present in all basaltic rocks. Chlorite, epidote and calcite usually fill original vesicles but also appear as secondary minerals replacing original components of the rock. Field exposures occasionally show minor pillows and chilled margins developed at the contacts with host sediments.
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Geochemistry |
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TOPABSTRACTIntroductionGeological SettingStratigraphy of the Volcano...Petrography of Volcanic RocksGeochemistryDiscussion of ResultsConclusionsReferences |
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Methods
Fifty-nine new samples have been analysed for major and trace elements, and REE (Table 1). The analytical procedures used were inductively coupled plasma mass spectrometry (ICP-MS) after fusion for major elements analysis plus Ba, Sr, Y and Zr; ICP-MS after HF digestion for trace metals (Cu, Pb, Zn, Ag, Ni, Cd, Bi, V and Be); X-ray fluorescence (XRF) pressed pellet for the elements Ga, Sn, S, Nb and Rb; and instrumental neutron activation analysis (INAA) for Au, As, Co, Cr, Cs, Hf, Sb, Sc, Ta, Th, U and REE. All the analyses were performed at ACTLABS (Canada) following standard procedures for each method. The detection limits are indicated in Table 1. The standards used by ACTLABS were: CCRMP SY-2, MRG-1, SY-3, USGS G-2, W-2 and AGV-1. Some of the standards have been run in duplicate and the results obtained do not differ from the certified values. The selection of the samples was done with the aim of covering all the Spanish sector of the Iberian Pyrite Belt and all the volcanic rock types present. The rocks show varying degrees of alteration, therefore care was taken in selecting the less altered rocks after detailed petrographic study.
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Twelve of these samples were selected for analysis of Rb–Sr and Sm–Nd isotope geochemistry (Table 2). These 12 samples are the least altered and cover all the rock types recognized in the Iberian Pyrite Belt. The samples were ground to fine powder and acid washed for 45 min at 50°C to remove alteration products. Sample dissolution and chemical separation methods for Rb, Sr, Sm and Nd followed standard procedures. Blank levels averaged 0.1 ng for Nd and 0.2 ng for Sr. Samples were analysed using a Finnigan MAT 262 mass spectrometer at the University of Oslo (Norway). Sm and Nd were loaded onto Re filaments of a double Re filament assembly and run as metal ions. Sr was run in a single Ta filament. Nd isotopes were measured using a single-jump triple-collector dynamic routine. Sr was measured in static multi-collection mode. Analyses of inter-laboratory standards gave the values of 143Nd/144Nd=0.511092±10 for the Johnson & Matthey JMC321 Nd standard and of 87Sr/86Sr=0.710177±12 for the NBS 987 Sr standard.
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Alteration of volcanic rocks
The volcanic rocks of the Iberian Pyrite Belt were affected by several alteration and modification processes, which include low-temperature hydration of glass (probably caused by seawater percolation), hydrothermal alteration, and regional low-grade metamorphism (greenschist facies). All these processes have changed the primary chemistry of the rocks, with significant gains and losses of several elements and oxidation of Fe.
Mobility of chemical components of volcanic rocks affected by hydrothermal alteration and low-grade metamorphism has been extensively documented (Wood et al., 1976; Floyd & Winchester, 1978; MacLean & Kranidiostis, 1987; MacLean & Barret, 1993). To better constrain the changes caused by the alteration processes on the primary geochemistry of the Iberian Pyrite Belt volcanic rocks, and to evaluate the relative mobility of chemical elements, factor analysis of the principal components has been used. This method has been largely used to reveal the elemental association that, in this case, could be an indication of remobilization owing to secondary processes (Howarth & Sinding-Larsen, 1983). The factors obtained give some clues that confirm the mobility of some elements, already deduced by petrography and geochemistry. Thus, Na, Pb, Co, Si, Al, K, Ba and Cu, and to a smaller degree Rb, Mg, Ni, Sr, Cr and Th, should be considered as mobile elements, i.e. elements redistributed during secondary alteration in the Iberian Pyrite Belt. The mobility of some of these elements differs depending on the rock type. A similar situation is found in other palaeovolcanic areas [see, e.g. Thorpe et al., (1993)].
General classification and description of geochemical data
The chemical compositions of the studied volcanic rocks are shown in Table 1. Major element analyses have been recalculated on a 100% water free basis to minimize the effects of alteration by removing variations owing to different loss on ignition values. The Nb/Y vs Zr/TiO2 variation diagram (Fig. 5; Winchester & Floyd, 1977) has been used to establish a broad classification of the studied volcanics. This diagram shows the existence of different types of rocks, which have already been deduced in the field and from the petrographical studies.
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Major elements
Major element compositions of basaltic rocks from the Spanish sector of the Iberian Pyrite Belt (see Table 1) indicate that most of them are sub-alkaline basalts and that a few samples show a slightly alkaline affinity. This is suggested by the relatively high TiO2 (>1.2) and P2O5 (
0.2) contents (see Table 1), the low contents in Zr and the low (<2) Y/Nb ratio (Fig. 6). The presence of alkaline and tholeiitic affinities in the basaltic rocks has also been suggested for the Portuguese sector of the Iberian Pyrite Belt (Munhá, 1983). Normative compositions, however, indicate that no undersaturated basalts are present in the studied samples, which are classified as saturated (olivine- and hypersthene-normative) and oversaturated (quartz-normative) basalts. This would suggest that all the basaltic rocks analysed here are tholeiites. In this case, however, the use of normative compositions is questionable as they are highly influenced by the effects of secondary alteration.
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Basaltic rocks show characteristic positive correlation between Fe2O3t/MgO vs TiO2 or Fe2O3t, which indicates that these basalts do not show calc-alkaline fractionation trends. Munhá, (1983)
found the same characteristics in the basalts from the Portuguese sector of the Iberian Pyrite Belt. The rest of the major elements show a high dispersion of values, mainly those which are more mobile during secondary alteration (Na, Ca, Al, etc.), whereas K and P values are less varied. Some of the observed variations may be primary, such as the contents in Ti and Mg, which may reflect the fractionation of some ferromagnesian minerals. These variations in Ti and Mg are accompanied by strong variations in Ni and Cr, and to a lesser degree Co, which agree with ferromagnesian mineral fractionation.
Basaltic andesites and andesites can be distinguished by their major element contents along with the petrographic analysis. Only one sample analysed in this study is a basaltic andesite: FP-119. It shows the highest Ti, Fe and Mn contents of the andesitic group and the lowest K content, whereas it shows intermediate concentrations of Al, Ca and Na. The rest of the andesites show a clear general decreasing trend for Ti, Fe and Mg, whereas the silica content increases. Two groups of andesites can be considered depending on their MgO content, which show two divergent correlations with Ti, Fe and V contents. Also, the Fe2O3t/MgO ratios vs Fe2O3t, Ti, Ti/V, etc. show the same divergent correlations. Munhá (1983) found similar subdivisions for the Portuguese andesites. These two groups of andesites do not show any preferential stratigraphic or geographic position.
The most evolved rocks in the Iberian Pyrite Belt volcanism, which are volumetrically dominant, are represented by dacites and rhyolites. A gradual geochemical transition exists between the two groups of rocks. Despite the fact that secondary alteration has caused a significant change in the original composition of these rocks, with a high perturbation of the Al, K, Na and Si contents, several general tendencies can be established. Ti, Fe, Ca and P show an increasing depletion towards the most evolved rocks, whereas no tendency is apparent for Na and K. Mg is higher for the Si-poor rhyolites than for the dacites or the Si-rich rhyolites. These differences in Mg content are similar to those observed in the andesites, and may reflect either compositional changes related to the presence of secondary chlorite crystals or differences in the composition of the source rocks.
Trace elements
Trace element compositions (Table 1) have been plotted in a Hofmann diagram (Hofmann, 1988) where the samples are normalized to primitive mantle but plotted by their degree of compatibility in the continental crust, which is a better way to visualize the influence of such crust (Hofmann, 1988) (Fig. 7). In the basaltic rocks the normalized concentrations of compatible elements (Ni, Mg, Co, Fe) show significant variations and positive correlations among the different samples. On the other hand, the variations observed in the normalized concentration of the incompatible elements (Rb, Pb, U, Th, Ba, K) do not show any correlation among the different samples of this group of rocks. The variation observed in the compatible elements suggests differences in the primary source of these basalts (differences in the source or different degrees of partial melting), or fractionation of ferromagnesian minerals which affects the contents of Ni, Mg, Cr, etc. Thus, in Fig. 7, FP-80 and FP-114 plot between the primitive mantle values and the continental crust normalized values. They also have the highest contents in Ni, Cr and Mg, suggesting that these samples represent the most primitive basalts in the studied area, and that they formed by high degrees of partial melting of a peridotitic mantle source. Less primitive basalts (FP-15, FP-16, FP-1097 and FP-20) have similar concentrations of compatible elements to the continental crust but show higher normalized concentrations of the incompatible elements than the average crust, without any possible correlation. Their low contents of compatible elements suggest that they originated by lower degrees of partial melting, or that they suffered fractionation of primitive minerals during their ascent. The incongruent enrichment in incompatible elements suggests that these basalts could also have assimilated continental crust. In addition, secondary alteration may have had a moderate influence on the content of some incompatible elements. For instance, all the basaltic rocks show a negative anomaly in Cu, which is clearly associated with a secondary origin related to the ore deposition process.
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Trace element contents of andesitic rocks do not show typical intermediate values between basalts and the more evolved rocks. On the contrary, their trends appear to be independent of the rest of the volcanic rocks from the studied area. In some cases, trace element contents of andesites are similar to the contents either of the basaltic or the dacitic rocks. This suggests that andesites are not related either to basalts or to dacites or rhyolites by fractional crystallization.
Andesites are characterized by an irregular distribution of highly incompatible elements (Fig. 7), absence of K and Na anomalies (except for sample FP-150, which has a positive anomaly in K and a negative one in Na), lower Mg and Ni contents than the basalts, and a strong positive anomaly of Sn and a negative anomaly of Cu. As with the basaltic rocks, the behaviour of the highly incompatible elements may indicate different source characteristics. However, the original contents of Pb, U, Th, Rb and Nb are clearly masked by secondary alteration. The absence of Na and K anomalies implies that these elements were not highly mobilized. The anomalies in Sn and Cu, which are found in nearly all rock types, are clearly related to the ore-forming event.
Incompatible element contents of dacites and rhyolites are extremely variable and they do not depend on the silica content variation, i.e. samples with the same silica content have large differences (hundreds of p.p.m.) of Ba, Zr, Th, U, Hf, etc., in both rock types. Dacites are the volcanic rocks from the Iberian Pyrite Belt which show normalized concentrations (Fig. 7) of trace elements closer to the average continental crust (see Taylor & McLennan, 1985). However, many dacites are slightly enriched in some incompatible elements with respect to the continental crust. Samples FP-154, FP-93 and FP-17, however, show extremely low values of Pb, Ba, K, Nb, Sn and Sr. Most of the dacites show negative anomalies in Ti and Cu, and a positive anomaly in Sn. All the rhyolites show a very similar trend, with increasing negative anomalies in Sr, Eu, Ti, Ca and Fe, and positive anomalies in K and Sn as silica content increases (Fig. 7).
REE
In the REE normalized to chondrites (Nakamura, 1974) diagram (Fig. 8) it is possible to observe that most of the normalized concentrations of the basaltic rocks show a relatively flat pattern and are limited by two extreme samples (FP-20 and FP-80). Sample FP-20 shows the highest fractionation between light REE (LREE) and heavy REE (HREE) [(La/Lu)n=5.78], i.e. it has the maximum La content and the minimum Lu content of all the group. Sample FP-80, however, shows an almost flat pattern with a positive Tb anomaly and a fractionation value between Lan and Lun of 1.37. As has been indicated before, FP-80 represents the most primitive basalt of the studied area. The rest of the basalts show (La/Lu)n values ranging from 1.68 to 4.95. This, together with the Eu/Sm values (0.26–0.42), and the relative flat trends of the REE normalized to chondrite, suggests that most of the basaltic rocks from the Spanish sector of the Iberian Pyrite Belt have the characteristics of tholeiites from continental settings [see Cullers & Graff, (1984) and references therein]. Samples which show high LREE/HREE ratios are those with a slightly alkaline affinity.
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Basaltic andesites and andesites show, in general, homogeneous patterns with low fractionation between LREE and HREE. (La/Lu)n is 3.42 for the basaltic andesite, and is slightly higher (3.91–4.83) for the andesites (Fig. 8). The main difference among the REE patterns of these rocks is the Eu anomaly. The basaltic andesite and one andesite show no Eu anomaly (Fig. 8), whereas the rest of the andesites show an increasing Eu anomaly not related to any other geochemical feature, thus indicating the existence of different original plagioclase compositions in the andesitic group, or variations on f(O2) during crystallization. These changes in the plagioclase composition are not reflected in the other REE. Sample FP-144 shows the same fractionation between LREE and HREE but with very low values and with a straight pattern (except for Lu). This feature may also indicate differences in the source, but factor analysis suggests that this sample has been significantly altered.
REE normalized diagrams of dacites and rhyolites (Fig. 8) show parallel patterns except for three samples of rhyolite, which have irregular patterns. The fractionation between LREE and HREE is (La/Lu)n=3.14–6.93 for the dacites, and (La/Lu)n=2.48–5.26 for the rhyolites. Two extreme rhyolitic samples have a value of 0.73 (sample FP-113) and 7.03 (sample FP-122). The Eu anomaly is clearly defined and reaches a maximum value in the intermediate members of the group. One dacitic sample shows low normalized contents in Yb and Lu (Fig. 8) with respect to the rest. These differences in the Yb and Lu normalized contents have no equivalent in the other elements, at least for the same samples, suggesting that secondary alteration has probably masked the primary character of these rocks. The parallelism shown by most of the normalized patterns of the dacites and rhyolites indicates the existence of different degrees of partial melting in the formation of these two groups of rocks, and precludes mixing or assimilation between them. Only the previously mentioned anomalous rhyolites may have undergone such processes, as is reflected by the REE normalized pattern.
Isotope geochemistry and age of the Iberian Pyrite Belt volcanism
Sr and Nd isotopic compositions have been obtained in 12 selected samples comprising 4 basalts, 2 andesites, 3 dacites and 3 rhyolites. The results are shown in Table 2. Initial ratios, as well as 
values, have been calculated for the age of 368 Ma, as we will discuss below. The constants used for all the calculations are: 87Sr/86SrUR (0)=0.7045, 87Rb/86SrUR (0)=0.0827, 
Rb=1.42x10–11/year for the Rb/Sr method, and 143Nd/144NdCHUR (0)=0.511847, 143Sm/144NdCHUR (0)=0.1967, Sm=6.54x10–12/year for the Sm/Nd couple.
To constrain the age of the Iberian Pyrite Belt volcanism, several methods have been applied by different workers (see Fig. 3 for details). The difficulty of dating these rocks by isotopic means is that they have undergone a low-grade metamorphism. Some of these radiometric ages are those of the metamorphism, and others (see Fig. 3 for references) correspond to the age of the plutonic rocks responsible for such metamorphism. Hamet & Delcey, (1971) obtained an Rb/Sr age of 376 Ma for a group of six rhyolites from Portugal. This age has already been recalculated with the new decay constant (Rb=1.42x10–11/year) and is statistically indistinguishable from our result (see below). Priem et al., (1978) dated the metamorphism which affects the Volcano-Sedimentary Complex in Portugal using the Rb/Sr method and obtained an age of 308±10 Ma, after recalculating it with the new decay constant. Dallmeyer et al., (1993), with the use of the 40Ar/39Ar method, have obtained a cooling age of 335–342 Ma for the intrusive bodies and the associated low-grade metamorphism in the Ossa Morena Zone. Finally, De La Rosa et al., (1993) have obtained an errorchrone for pluton emplacement at the north of the South Portuguese Zone by including in their data the age obtained by Dallmeyer et al., (1993) in adjacent units.
In this study, whole-rock K–Ar dating has been performed on some samples to check the possibility of using this method with the less altered samples or at least to date the metamorphic phase. All the ages obtained are younger than the age of the metamorphism itself. This suggests that Ar has been removed (and probably K has been added) also after the metamorphic phase, as a continuous process. Thus K–Ar ages in this case have no geological meaning and no further discussion of these results will be presented here.
Using the isochron method with the Sr isotopic composition on whole-rock samples, divided into basaltic and calc-alkaline rocks, we have obtained an age of 368±64 (1
error) Ma for the calc-alkaline suite (andesites, dacites and rhyolites). This isochron age was calculated using the Model 3 solution of the Yorkfit calculation, which assumes that scatter is due to analytical error plus normally distributed error in initial 87Sr/86Sr, i.e. it takes into account not only the analytical error, but also the geological error. The basaltic rocks alone, however, give ages inconsistent with the stratigraphy and with very high 1 errors.
Using the Nd isotopic composition, the isochron age is 369±150 Ma. This calculation assumes Model 3, as before, and is the isochron for all the samples studied independently of their composition. If the age is calculated by groups of rocks (calc-alkaline and basalts separately) the ages obtained are meaningless and the 1 errors are larger than the age itself. In contrast to the use of two different isochrons calculated with the Rb/Sr method, the use of a single Sm/Nd age for all samples can be accepted, because the decay of Rb into Sr is twice as fast as the decay of Sm into Nd. Thus, for the age that we are studying (368 Ma) and for the Sm/Nd method, it is possible to consider all the samples as members of the same group.
The age obtained, 368±62 Ma, is statistically in agreement with the ages obtained by other workers (Fig. 3) using radiometric and palaeontological methods. The high 1 error on the final age is probably related to the high dispersion of isotopic values owing to the use of different rock types from different stratigraphic units.
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Discussion of Results |
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TOPABSTRACTIntroductionGeological SettingStratigraphy of the Volcano...Petrography of Volcanic RocksGeochemistryDiscussion of ResultsConclusionsReferences |
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Temporal and spatial evolution of volcanism
According to Instituto Geológico y Minero de España, (1982)
and Oliveira, (1990), the volcanism of the Iberian Pyrite Belt has been divided into five volcanic episodes: (1) initial acid volcanism; (2) basic volcanism; (3) middle acid volcanism; (4) upper acid volcanism; (5) intrusive diabases. This division is based on the stratigraphic position of volcanic rocks, their petrographic texture, and the occurrence of sediments between volcanic packages. Thus, two volcanic rocks separated by a sedimentary interval in any stratigraphic section have, until now, been interpreted as two different volcanic events in time, the highest stratigraphic position corresponding to the youngest volcanic event.
Most of the volcanics in the Iberian Pyrite Belt are shallow intrusives, ranging in depth from tens of metres below the sea-floor up to 1000 m (Soriano, 1997). Thus, their apparent position in the stratigraphic sequence does not necessarily indicate a time relation with other volcanic rocks in the same section. Also, many of these shallow intrusions show interfingering with the host sediments and give the appearance, in the same stratigraphic profile, of different intrusive events. In addition, some of the former volcanic episodes were defined based on the general assumption that some of them were represented by primary pyroclastic rocks. However, a detailed study of the field relationships and textures of the volcanic rocks has revealed that nearly all the volcanic rocks from the Spanish sector of the Iberian Pyrite Belt are intrusive with some minor extrusive silicic domes which have produced small lavas and associated hydroclastic volcanogenic deposits (Soriano, 1997).
Field characteristics of the calc-alkaline and basaltic intrusives indicate that they were emplaced nearly always at very shallow depths into wet sediments (Boulter, 1993a, 1993b; Soriano, 1997). Volcanic intrusions reached relative higher positions in the stratigraphic succession as sedimentation progressed and, consequently, as the thickness of sediments increased (Soriano, 1997). This fact, together with the presence of some lavas and volcanogenic sediments interbedded at different levels of the Volcano-Sedimentary Complex, suggests that volcanism was more or less continuous during the entire deposition of the volcano-sedimentary package, which covers a time span of 30 m.y. (see Fig. 3).
The calc-alkaline rocks show a preferential distribution in the stratigraphic sequence depending on their palaeogeographic position. They are preferentially concentrated towards the top of the stratigraphic series in the north and northeast of the Iberian Pyrite Belt, whereas intermediate and silicic volcanics appear mainly at the base of the stratigraphic sequence in the south and southwest of the Spanish sector of the Iberian Pyrite Belt. The different positions in the stratigraphic sequence of calc-alkaline rocks do not necessarily indicate the existence of different volcanic episodes affecting the whole Iberian Pyrite Belt. More probably, these differences represent a migration of the focus of calc-alkaline volcanism to the northeast, as suggested by Monteiro & Carvalho, (1987), probably related to the asynchronicity in the opening of the main fractures that controlled the subsidence of the sedimentary basins. This is also indicated by the irregular structure of the basin and by the migration of the depocentres (Soriano, 1997).
The basaltic rocks, which are most commonly intrusive, do not show any stratigraphic or palaeogeographic control and appear throughout the stratigraphic sequence of the Volcano-Sedimentary Complex. In the Portuguese sector, Munhá, (1983) found a stratigraphic control in the distribution of tholeiitic and alkaline basalts, which were concentrated at the base and the top of the stratigraphic sequence, respectively. This relationship, however, has not been observed in the Spanish sector, where basaltic rocks with both affinities appear as intrusives in any part of the stratigraphic sequence.
Therefore, the previous division of the Iberian Pyrite Belt volcanism into several volcanic episodes should be abandoned, at least for the Spanish sector, as it has no meaning in terms of magmatic and volcanic evolution. Based on palaeogeographic distribution, stratigraphic position, petrography and field textures (Soriano, 1997), we consider the volcanism of the Iberian Pyrite Belt as a continuous, long-lived, volcanic episode which resulted from a magmatic event coeval with the tectonic processes that controlled the formation of basins where the Iberian Pyrite Belt sedimentary succession was deposited. Different pulses may logically be distinguished in this volcanism, suggesting the existence of alternating active and less active periods. However, this does not indicate any change in the tectonic conditions that caused this volcanism or in the nature of its products. The geodynamic framework of this volcanism and the origin of the associated sedimentary basins are discussed in a later section.
Origin of basalts
The trace elements and REE compositions and the high Sr/Nd values (>20) shown by some of the basaltic rocks from the Iberian Pyrite Belt suggest that they originated in the asthenospheric mantle (see Zindler et al., 1981). These basalts are the most primitive of the studied area and derive from different mantle sources that can be internally related to the E- and N-MORBs by a single mixing model calculation (Figs 9 and 10). The rest of the basaltic rocks may be explained by assimilation–fractional crystallization (AFC) processes or more likely by mixing of the mantle-derived basaltic magmas with crustal materials (Figs 9 and 10). They have lower Sr/Nd values, which indicate that they fractionated plagioclase, assimilated crustal material or assimilated magmas derived from crustal melts.
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The influence of continental crust on the primary magmas of some of the basaltic rocks from the Iberian Pyrite Belt is also shown on the Ce/Nb–Th/Nb diagram and by the ratio Th/Nb (Figs 10 and 11). The Ce/Nb–Th/Nb diagram (Fig. 10) shows how almost all the basaltic samples plot inside an area that represents mixing between E-MORB, N-MORB and continental crust. Isotopic compositions confirm mixing as the most plausible mechanism to explain the diversity of compositions shown by the basaltic rocks from the Iberian Pyrite Belt and their relationship with the other volcanic rocks (Figs 12 and 13).
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Petrological and geochemical data presented in previous sections demonstrate that most of the basaltic rocks from the Iberian Pyrite Belt have a tholeiitic affinity and that a few samples show a slightly alkaline affinity. This difference between the two types of basalts is indicated by the Y/Nb values, by their TiO2 and P2O5 contents (see Table 1), by the LREE–HREE fractionation, the lack of mantle xenoliths or cumulates, and the absence of normative Ne. The presence of both types of basalts is common in different geodynamic settings and the distinctive chemical features of tholeiites and alkaline basalts are related to different degree of partial melting from the same source region (Moore et al., 1995
).
The mixing model between E- and N-MORB used to explain the origin of the most primitive basaltic rocks does not discriminate between the origin of both tholeiitic and alkaline affinities in the same basaltic volcanism. However, the existence of tholeiitic and alkaline affinities may be explained by different degrees of partial melting of a peridotitic mantle (see Carmichael et al., 1974; Hall, 1987; Wilson, 1989), as is suggested by the REE patterns shown by the basaltic rocks studied here. The higher degree of partial melting is reflected in the lower LREE/HREE ratio of tholeiites with respect to that of the basalts with a slightly alkaline affinity and by the flat patterns of the normalized REE.
Origin of intermediate and silicic rocks
Trace elements and REE compositions of intermediate and silicic rocks from the Iberian Pyrite Belt demonstrate that they are not related by fractional crystallization processes. Munhá, (1983), using classical geochemical data, also concluded that no fractional crystallization is involved in the formation of the evolved rocks. That researcher suggested that andesites directly derived from partial melting of the mantle, but their high Sr content precludes that possibility. Isotopic compositions reveal that andesites (samples FP-83 and FP-102) fall in a more evolved position than the dacites (Figs 12 and 13), having very similar Nd isotope ratios to rhyolites but different Sr isotope ratios. This suggests that the andesites may have formed by direct mixing of mantle-derived basalts, which follow the mantle array, with young upper-crust material. Also, the Sr isotopic and absolute compositions, and the Zr/Nb, La/Sm and Ce/Th (Figs 9 and 10), and the low Sm/Nd ratios, confirm this point, as andesites are numerically between FP-40 (or even FP-91) and upper-crust average. The fact that andesites have higher Sr and lower Nd than dacites implies that there is no relation by fractional crystallization between andesites and rhyolites through the dacites.
The high variation in Nd, Sr (p.p.m.), Rb (p.p.m.) and Nd (p.p.m.), together with the low variation in Sr shown by the dacites, and the high variation on Sr with almost no variation of Nd, Sr (p.p.m.) and Nd (p.p.m.) shown by the rhyolites (Fig. 13), suggests that they are derived by different degrees of partial melting of different crustal rocks. Moreover, despite the fact that sample FP-5 (dacite) has coincident Nd (p.p.m.) and Nd with FP-83 (andesite), dacite samples appear not to be related by fractional crystallization to any of the andesites. This last point is demonstrated by the lower values in Sr of the dacites with respect to the andesites and [for two dacites (FP-27 and FP-154)] by the higher values of Nd with respect to the andesites (Fig. 13). Furthermore, there is no relation between dacites and rhyolites by fractional crystallization (see Fig. 12). In contrast, the Sr of the analysed dacites (Fig. 12) is nearly the same. This could suggest that dacites are genetically related to them by fractional crystallization and, as shown in Fig. 12, they can be derived by the same process from a mixing product between basalts and rhyolites. However, as we have seen, the differences in concentration of some trace elements and REE (mainly Sr), and the high variation of the Nd, demonstrates that not all of the dacitic samples support fractional crystallization as their genetic process. Thus, most of the dacites, like the rhyolites, can each be considered as an individual case of partial melting of crustal material of different type (variations in Sr and Nd concentrations) and age (differences in Sr and Nd). Only samples FP-5 and FP-27 could be considered to be generated by fractional crystallization processes from a mixture between basalts and rhyolites.
The presence of high-silica content rhyolites is characteristic of both sectors of the Iberian Pyrite Belt (Munhá, 1983; this work). Despite the fact that this high silica content may be due to a secondary silicification [most of these rocks have been classified as quartz keratophyres by previous workers (Soler, 1980)], it may also be a primary feature. This is also suggested by the fact that the high-silica content rhyolites show the highest negative Eu anomaly. Silica-rich rhyolites are typical of areas with bimodal volcanism rather than being generated by fractional crystallization processes from calc-alkaline andesites (Christiansen & Lipman, 1972). We suggest that the high silica content that characterizes some of the Iberian Pyrite Belt rhyolites is a primary character probably owing to differences in the source, consistent with an origin by partial melting of upper crust driven by basaltic magmas.
Therefore, geochemical and isotopic data presented in this paper negate any relationship by pure fractional crystallization between basalts, intermediate rocks and silicic rocks. This fact, together with the volumetric predominance of silicic rocks and the widespread occurrence of basaltic volcanism, suggests that in the Iberian Pyrite Belt calc-alkaline silicic magmas were generated on a large scale by the invasion of continental crust by mafic magmas generated in the underlying upper mantle. Mantle-derived magmas can provide large quantities of heat for partial melting and assimilation of lower- and upper-crustal rocks (Huppert & Sparks, 1988; Kaczor et al., 1988; Grunder, 1995). The diversity of compositions shown by dacites and rhyolites can be explained either by differences in the composition of the source rocks or by different degrees of partial melting of upper-crust rocks. In contrast, andesites formed by direct mixing of mantle-derived basalts with young upper-crust material, rather than by partial melting of lower crust induced by the intrusion of mantle-derived magmas or by direct partial melting of the upper mantle.
Geodynamic setting and origin of the Iberian Pyrite Belt volcanism
Tectonostratigraphic terrane analysis of the South Portuguese Zone and the southern part of the Ossa Morena Zone (Oliveira, 1990; Eden, 1991; Quesada, 1991) has revealed the occurrence of several well-differentiated geological domains which have a specific significance in terms of Variscan plate tectonics evolution (Fig. 1). Thus, from north to south the tectonostratigraphic terranes of the South Portuguese Zone are:
- The Beja–Acebuches ophiolitic complex, which has recently been interpreted as an Early Devonian oceanic complex (Munhá et al., 1986; Quesada et al., 1994) and which represents an oceanic crust of the South Portuguese plate subducted beneath the continental crust of the Ossa Morena plate in Upper Famennian to Lower Carboniferous times (Silva et al., 1990).
- The Pulo do Lobo oceanic accretionary prism, which is an oceanic terrane composed of siliciclastic sediments of Lower to Upper Devonian age (Oliveira et al., 1986) deposited on an oceanic lithosphere.
- The intracontinental Iberian Pyrite Belt volcanic terrane, which is composed of marine turbiditic deposits and shallow calc-alkaline and basaltic sill-complexes that intruded over a continental well-differentiated crust (Munhá, 1983). Fauna of turbiditic deposits and isotopic dating of shallow intrusions indicate a Late Devonian to Early Carboniferous age (Fig. 3).
- The Baixo Alentejo Flysch Group, which is composed of a thick sequence of syntectonic turbiditic deposits of Upper Viséan to Namurian age (Oliveira, 1990) and is bounded to the south by the Iberian Pyrite Belt terrane with a thrust plane that dips to the north (Quesada, 1991). At the north of the Baixo Alentejo Flysch Group the southern part of the Ossa Morena Zone is considered to represent a continental terrane with calc-alkaline arc-related volcanism associated with the subduction of the Beja–Acebuches ophiolite (Santos et al., 1987).
The geological characteristics and boundaries of these tectonostratigraphic domains and the occurrence of a well-defined suture between the South Portuguese and the Ossa Morena Zones (Munhá et al., 1986; Silva et al., 1990; Crespo-Blanc & Orozco, 1991; Dallmeyer et al., 1993; Giese et al., 1994; Quesada et al., 1994) have led to a general consensus on the tectonic evolution of the area. From Upper Devonian (Frasnian and Famennian) through the entire Carboniferous Period the South Portuguese and the Ossa Morena plates were continuously converging. This plate convergence evolved with time from a subduction of South Portuguese oceanic lithosphere beneath continental Ossa Morena crust to a continental collision of both plates (Monteiro & Carvalho, 1987; Silva et al., 1990; Giese et al., 1994; Quesada et al., 1994). In such a moving plates scenario the Iberian Pyrite Belt volcanism took place. The tectonic style of the Iberian Pyrite Belt terrane (Soriano, 1996, 1997), as with the rest of the South Portuguese Zone terranes (Ribeiro et al., 1983), also agrees with such a geodynamic evolution, as most of the structures clearly show an east–west trend and south vergence.
Such a consensus, however, does not surround the interpretation of the tectonic setting of the Iberian Pyrite Belt, which has been considered to have formed in a variety of settings. These include: (1) a convergent plate boundary, in an island arc generated over a subduction zone (Schütz et al., 1988); (2) extensional tectonics associated with back-arc spreading developed on the South Portuguese continental plate and involving a subduction of the Ossa Morena Zone (Soler, 1973); (3) extensional tectonics associated with the initial stages of a back-arc spreading developed on a continental plate (Munhá, 1983); (4) a forearc basin developed on the continental crust of an overriding plate, the Ossa Morena Zone (Monteiro & Carvalho, 1987).
These different interpretations of the tectonic setting of the Iberian Pyrite Belt are in part due to the lack of a precise interpretation of the nature of the associated volcanism. Most of the previous studies concerning the tectonic setting of the Iberian Pyrite Belt have concentrated on the interpretation of its stratigraphical and structural features, but have not considered in detail whether the characteristics of volcanism were compatible with the various proposed tectonic settings. To determine the validity of the previous interpretations of the Iberian Pyrite Belt tectonic setting, we compare them with the petrological and geochemical data presented in this study.
Mineralogical and geochemical data of the Iberian Pyrite Belt volcanics (Routhier et al., 1977; Soler, 1980; Munhá, 1983; this paper) indicate a clear continental crust affinity for the calc-alkaline rocks. This precludes a certain number of tectonic settings for the Iberian Pyrite Belt volcanism. On the basis of this evidence, and in accordance with previous workers (Schermerhorn, 1975; Munhá, 1983), an island arc setting must be rejected, as the volume of rhyolites and dacites in the Iberian Pyrite Belt is too high compared with modern analogues of island arc environments (Carmichael et al., 1974; Baker, 1982; Gill, 1981; Wilson, 1989). A well-developed lower and upper continental crust is necessary to explain the geochemistry of the Iberian Pyrite Belt volcanism. In addition, basaltic rocks do not show any subduction-related affinity. They have higher TiO2 and Fe2O3t contents, and lower K, Sr and Zr contents than typical subduction-derived basaltic magmas (Carmichael et al., 1974; Wilson, 1989), and they do not have hydrous minerals. In addition, andesitic rocks also differ mineralogically from typical island-arc andesites, which normally have orthopyroxene.
Basalt and rhyolitic associations such as those observed in the Iberian Pyrite Belt are commonly found in extensional tectonic setting such as continental rifts or continental back-arcs. However, mineralogical and geochemical compositions of the Iberian Pyrite Belt volcanics do not show any evidence of a subducting slab dehydration such as invariably occurs in back-arc volcanism (Tarney et al., 1977; Weaver et al., 1979; Wilson, 1989). Calc-alkaline basalts are also a common feature in modern back-arcs (Weaver et al., 1979; Wilson, 1989), but none of the basalts in the Iberian Pyrite Belt have this calc-alkaline pattern. Moreover, the sequence of tectonostratigraphic domains described above is clearly inconsistent with a back-arc setting for the Iberian Pyrite Belt volcanism. The geochemical features of this volcanism also differ from the distinctive characteristics of modern and ancient intracontinental rift zones [alkaline nature, enrichment in large ion lithophile elements (LILE)] (see Bailey, 1983; Wilson, 1989).
A fore-arc location should be also rejected in the light of several features: some influence of a subducting slab should be observed in the geochemistry of the Iberian Pyrite Belt volcanics if they had formed in a fore-arc zone developed above the continental crust that overrides a subducting oceanic crust. On the other hand, the nature (an accretionary prism) and actual position of the Pulo do Lobo oceanic terrane and the Baixo Alentejo Flysch Group with respect to the Iberian Pyrite Belt (Monteiro & Carvalho, 1987; Giese et al., 1988; Silva et al., 1990; Quesada et al., 1994) preclude its location in a fore-arc setting.
Recently, Silva et al., (1990), Giese et al., (1994) and Quesada et al., (1994) have proposed a new interpretation for the Iberian Pyrite Belt volcanism based on terrane analysis. They suggest that local extensional tectonics, related to transtension owing to oblique collision, developed in the continental crust of the South Portuguese plate during a continental collision of the South Portuguese and the Ossa Morena plates. This strike-slip tectonics caused the opening of pull-apart basins, where submarine terrigenous sedimentation occurred, as well as the bimodal volcanism which characterizes the Iberian Pyrite Belt. The location of the Iberian Pyrite Belt assumed by this model is consistent with the geodynamic framework described above, and also with the palaeogeographic reconstruction of the area (Soriano, 1997). However, there is not a clear modern analogue that can be used to define the main geochemical guidelines of such a volcanism. Previous models of collision-zone magmatism do not consider this possibility (see Harris et al., 1986) and no indications of the geochemical signature of this magmatism exist. Despite this fact, we consider that the geochemical and isotopic data presented in this study are compatible with this tectonic setting.
The ultimate source for the Iberian Pyrite Belt volcanism is the asthenospheric mantle, where large volumes of tholeiitic, and occasionally alkaline, basaltic magmas originated. Extensive melting of the asthenospheric mantle was probably induced by rapid decompression caused by the effect of regional strike-slip tectonics which affected the entire continental crust and probably the lithospheric mantle, thus causing a restricted lithospheric thinning. The large LREE depletion and the absence of Eu anomaly observed in most of the studied basaltic rocks are compatible with a continental breakup setting (see Cullers & Graf, 1984). It is important to note, however, that the Iberian Pyrite Belt did not form in a post-collisional extensional setting. The strike-slip tectonics responsible for the opening of the basin and the associated volcanism developed as a direct response to the oblique continental collision between the Ossa Morena and the South Portuguese plates, which lasted from Middle Devonian until Late Carboniferous, and culminated with the Variscan orogeny in that area. Thus, in such a scenario the existence of significant lithospheric stretching caused by the relaxation of compressional stresses is unlikely as the compression had not ceased. Basaltic magmas invaded a relatively thick continental crust causing both assimilation of crustal materials and extensive melting of upper-crust rocks. This gave rise to the formation of andesites and silicic rocks, respectively.
The magmatic process that gave rise to the Iberian Pyrite Belt volcanism was, thus, intimately related to the tectonic regime which controlled the opening and evolution of the sedimentary basin. This process was pulsating rather than continuous, as is indicated by changes in the structure of the basin with time and in the position of the depocentres (Soriano, 1997). These pulses caused different intensities of decompression in the mantle and this would explain the existence of differences in the degree of partial melting of the asthenospheric mantle, which would explain the existence of tholeiitic and alkaline affinities for the basaltic magmas. However, in the Spanish sector there is no evidence to justify a sequence of events that could explain the temporal evolution from tholeiites to alkaline basalts that Munhá, (1983) suggested for the Portuguese sector.
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Conclusions |
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TOPABSTRACTIntroductionGeological SettingStratigraphy of the Volcano...Petrography of Volcanic RocksGeochemistryDiscussion of ResultsConclusionsReferences |
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The Iberian Pyrite Belt volcanism is mainly represented by shallow intrusives emplaced into wet turbiditic siliciclastic deposits of early Carboniferous age. Volumetrically, this volcanism can be considered as bimodal, with a predominance of basaltic rocks of tholeiitic affinity and calc-alkaline silicic rocks, despite the presence of some basalts with alkaline affinity and intermediate calc-alkaline rocks. Differences in composition shown by the basaltic rocks can be explained by a single mixing model involving three end-members: E-MORB, N-MORB and continental crust. Silicic calc-alkaline rocks originated by large-scale partial melting of upper continental crust induced by the intrusion of basaltic magmas generated in the underlying upper mantle. In contrast, andesites originated by direct mixing of mantle-derived basalts with young upper-crustal material. The volcanism covers a long time span (15 m.y.) and does not show any significant temporal or spatial variation. The existence of tholeiitic and alkaline affinities in the basaltic rocks is interpreted to be due to different degrees of partial melting in the asthenospheric mantle caused by different intensities of tectonically driven decompression. The new geochemical and isotopic data are consistent with the stratigraphic and structural data, which suggest that the Iberian Pyrite Belt developed in a complex setting, involving local extensional tectonics within the South Portuguese Zone associated with strike-slip faulting caused by an oblique continental collision with the Ossa Morena plate. The continental collision extended longer than the formation of the Iberian Pyrite Belt and in this area culminated with the Variscan orogeny. This model has no clear modern analogues, but we suggest that such a tectonic setting should not be necessarily rare in the geological record.
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Acknowledgements |
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We thank Eusebio Lopera and Manuel Viladevall for their helpful comments and discussions. Detailed comments by Cecilio Quesada and an anonymous reviewer are also gratefully acknowledged. This research was (in part) supported by the research project entitled: ‘Proyecto de investigación paleogeográfica y volcanológica en la Faja Pirítica del SW de España’ (ITGE—Fundació Bosc i Gimpera). We thank the Instituto Tecnológico y Geo Minero for giving us permission to publish part of the results from that project.
* Corresponding author. Telephone: 34-3-330 27 16. Fax: 34-3-411 00 12. e-mail: joan.marti{at}ija.csic.es
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TOPABSTRACTIntroductionGeological SettingStratigraphy of the Volcano...Petrography of Volcanic RocksGeochemistryDiscussion of ResultsConclusionsReferences |
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