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PAGES 727–755
1997
Magmatic Evolution and Tectonic Setting of the Iberian Pyrite Belt Volcanism J. MITJAVILA, J. MARTI´∗ AND C. SORIANO INSTITUTE OF EARTH SCIENCES ‘JAUME ALMERA’, CSIC, LLUIS SOLE´ I SABARI´S S/N, 08028 BARCELONA, SPAIN
RECEIVED ON JUNE 1, 1996 REVISED TYPESCRIPT ACCEPTED FEBRUARY 4, 1997
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 rhyolites. Volumetrically, dacites and rhyolites 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 rhyolites 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
∗Corresponding author. Telephone: 34-3-330 27 16. Fax: 34-3-411 00 12. e-mail:
[email protected]
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 subducting plates, a region usually poorly constrained from a petrological point of view.
calc-alkaline volcanism; isotope geochemistry; strike-slip tectonics; Iberian Pyrite Belt KEY WORDS:
INTRODUCTION 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
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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; Le´colle, 1977; Routhier et al., 1977; Priem et al., 1978; Soler, 1980; Strauss et al., 1981; Munha´, 1983; Mo¨ller et al., 1983; Schu¨tz 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 volcanichosted 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.
GEOLOGICAL SETTING 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 (Munha´ et al., 1986; Crespo-Blanc & Orozco, 1988; Quesada, 1991). This suture is interpreted as a
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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). 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. 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: (1) 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. (2) 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 Vise´an age (Fig. 3). (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
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Fig. 1. Tectonostratigraphic terrane map of the South Portuguese Zone and the southern part of the Ossa Morena Zone. Adapted from Quesada (1991). Inset: Variscan geological units of the Iberian Massif ( Julivert et al., 1974). CZ, Cantabrian Zone; ALZ, Asturian–Leonese Zone; CIZ, Central Iberian Zone; OMZ, Ossa Morena Zone; SPZ, South Portuguese Zone. Adapted from Julivert et al. (1974).
distributions strongly suggest flysch sedimentation (Oliveira, 1990). Goniatite fauna found at the base of the unit indicate an Upper Vise´an age (Fig. 3). Hydrothermal alteration took place during and shortly after the volcanism of the Volcano-Sedimentary Complex (Munha´, 1990). Oxygen, hydrogen and sulphur isotopic compositions (Munha´ & 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 (Munha´, 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 VOLCANOSEDIMENTARY COMPLEX The Volcano-Sedimentary Complex is characterized by the presence of a monotonous sequence (500–800 m
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Fig. 2. Geological map of the Spanish sector of the Iberian Pyrite Belt showing the location of the stratigraphic sections in Fig. 4 [modified after Instituto Geolo´gico y Minero de Espan˜a (1982)].
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 Geolo´gico y Minero de Espan˜a (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; Le´colle, 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). 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 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
Fig. 3. Ages of geological events in the South Portuguese Zone. The method used in dating any particular event is shown in the lower row. The numbers in the Van den Boogaard & Schermerhorn column refer to the following papers: 1—Van den Boogaard & Schermerhorn, 1981; 2—Van den Boogaard & Schermerhorn, 1975a; 3—Van den Boogaard & Schermerhorn, 1980; 4—Van den Boogaard & Schermerhorn, 1975b.
MITJAVILA et al. MAGMATIC EVOLUTION AND TECTONIC SETTING OF THE IBERIAN PYRITE BELT
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Fig. 4. Stratigraphic sections of the Volcano-Sedimentary Complex in the Iberian Pyrite Belt (see Fig. 2 for location).
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
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hydroclastic deposits. Other field structures such as flowbanding foliation, flow autobrecciation and soft-sediment intrusions through columnar jointing of volcanic rocks
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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, Kfeldspar 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.
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. 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 triplecollector 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 87 Sr/86Sr=0·710177±12 for the NBS 987 Sr standard.
Alteration of volcanic rocks
GEOCHEMISTRY Methods
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
Fifty-nine new samples have been analysed for major and trace elements, and REE (Table 1). The analytical
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0·02
0·002
0·02
0·03
0·02
0·02
0·005
0·01
Al2O3
Fe2O3
MnO
MgO
CaO
Na2O
K2O
P2O5
XRF pressed pellet
1·45
0·05
99·65
TOTAL
734 16·17 12·31 0·15 6·96 9·00 2·93 0·37 0·22
Al2O3
Fe2O3
MnO
MgO
CaO
Na2O
K2O
P2O5
5 p.p.m.
50 p.p.m. 2 p.p.m. 2 p.p.m.
S
Nb
Rb
3 p.p.m.
Zr 5 p.p.m.
1 p.p.m.
Y
Sn
1 p.p.m.
Ga
3 p.p.m.
Sr
11·61
4·79
0·23
0·05
0·48
17·92
0·23
0·01
4·18
9·63
6·71
0·19
13·35
17
9
250
