Journal of Petrology Advance Access originally published online on October 18, 2007
Journal of Petrology 2007 48(12):2261-2287; doi:10.1093/petrology/egm059
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Geochemistry, Petrogenesis and Geodynamic Relationships of Miocene Calc-alkaline Volcanic Rocks in the Western Carpathian Arc, Eastern Central Europe
1Department of Petrology and Geochemistry, Eötvös University, Budapest, H-1117, Pázmány Péter Sétány 1/C, Hungary
2School of Earth Sciences, Birkbeck College, University of London, Malet Street, London WC1E 7HX, UK
3Department of Geology, Royal Holloway University of London, Egham TW20 0EX, UK
4Department of Nuclear Research, Institute of Isotopes, Hungarian Academy of Sciences, Budapest, Hungary
RECEIVED APRIL 14, 2004; ACCEPTED SEPTEMBER 17, 2007
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLING AND ANALYTICAL...GEOCHEMISTRYDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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We report major and trace element abundances and Sr, Nd and Pb isotopic data for Miocene (16·5–11 Ma) calc-alkaline volcanic rocks from the western segment of the Carpathian arc. This volcanic suite consists mostly of andesites and dacites; basalts and basaltic andesites as well as rhyolites are rare and occur only at a late stage. Amphibole fractionation both at high and low pressure played a significant role in magmatic differentiation, accompanied by high-pressure garnet fractionation during the early stages. Sr–Nd–Pb isotopic data indicate a major role for crustal materials in the petrogenesis of the magmas. The parental mafic magmas could have been generated from an enriched mid-ocean ridge basalt (E-MORB)-type mantle source, previously metasomatized by fluids derived from subducted sediment. Initially, the mafic magmas ponded beneath the thick continental crust and initiated melting in the lower crust. Mixing of mafic magmas with silicic melts from metasedimentary lower crust resulted in relatively Al-rich hybrid dacitic magmas, from which almandine could crystallize at high pressure. The amount of crustal involvement in the petrogenesis of the magmas decreased with time as the continental crust thinned. A striking change of mantle source occurred at about 13 Ma. The basaltic magmas generated during the later stages of the calc-alkaline magmatism were derived from a more enriched mantle source, akin to FOZO. An upwelling mantle plume is unlikely to be present in this area; therefore this mantle component probably resides in the heterogeneous upper mantle. Following the calc-alkaline magmatism, alkaline mafic magmas erupted that were also generated from an enriched asthenospheric source. We propose that both types of magmatism were related in some way to lithospheric extension of the Pannonian Basin and that subduction played only an indirect role in generation of the calc-alkaline magmatism. The calc-alkaline magmas were formed during the peak phase of extension by melting of metasomatized, enriched lithospheric mantle and were contaminated by various crustal materials, whereas the alkaline mafic magmas were generated during the post-extensional stage by low-degree melting of the shallow asthenosphere. The western Carpathian volcanic areas provide an example of long-lasting magmatism in which magma compositions changed continuously in response to changing geodynamic setting.
KEY WORDS: Carpathian–Pannonian region; calc-alkaline magmatism; Sr, Nd and Pb isotopes; subduction; lithospheric extension
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLING AND ANALYTICAL...GEOCHEMISTRYDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Calc-alkaline volcanic rocks (andesite–dacite–rhyolite suites) are the typical products of convergent plate margin tectonic settings. They are characterized by enrichment of large ion lithophile elements (LILE) and Pb, but show depletion in high field strength elements (HFSE) causing a negative Nb–Ta anomaly in multielement diagrams (Pearce, 1982
; Pearce & Parkinson, 1993). This geochemical signature is commonly explained by the addition of hydrous fluids from subducting oceanic lithosphere combined with the flux of melts from subducted sediments to the mantle wedge, lowering the mantle solidus and leading to magma generation (Gill, 1981; Grove & Kinzler, 1986; Tatsumi, 1989; Hawkesworth et al., 1993; Pearce & Peate, 1995; Elliott et al., 1997; Iwamori, 1998). U-series isotopic studies have shown that melt generation processes beneath volcanic arcs take place on scales of 104–105 years (Gill & Williams, 1990; Elliott et al., 1997; Thomas et al., 2002; Bourdon et al., 2003; Turner et al., 2006). Thus, it seems reasonable to assume that generation of calc-alkaline magmas is coeval with active subduction. However, it has also been shown that calc-alkaline magmas can be formed by decompression melting of an old subduction-imprinted mantle wedge (Johnson et al., 1978; Cameron et al., 2003) or continental mantle lithosphere previously modified by subduction (Gans et al., 1989; Hawkesworth et al., 1995; Hooper et al., 1995; Wilson et al., 1997; Fan et al., 2003). In both cases, lithospheric extension controls the magma generation whereas the geochemical signature of the magmas is inherited from the older subduction imprint. Thus, calc-alkaline magmatism could also occur without contemporaneous subduction or following cessation of active subduction (post-collisional magmatism).
During Tertiary to Quaternary times, widespread magmatism developed in the Mediterranean and surrounding regions (Wilson & Downes, 1991, 2006; Wilson & Bianchini, 1999; Lustrino, 2000; Harangi et al., 2006; Lustrino & Wilson, 2007). In the Alpine–Mediterranean region, subduction, back-arc extension and lithospheric delamination were accompanied by eruption of a wide range of magma types (Doglioni et al., 1999; Turner et al., 1999; Wilson & Bianchini, 1999; Duggen et al., 2005; Peccerillo, 2005; Harangi et al., 2006). The dominant magma type is calc-alkaline, as would be expected in an orogenic area; however, there is still some debate concerning the interpretation of the petrogenesis of these magmas. In some places, a transition from calc-alkaline to alkaline volcanism is observed (e.g. Betics; Duggen et al., 2003, 2005; North Africa; El Bakkali et al., 1998; Coulon et al., 2002; Sardinia: Lustrino et al., 2007; Western Anatolia; Seyito
lu & Scott, 1992; Seyitolu et al., 1997; Wilson et al., 1997; Aldanmaz et al., 2000; Agostini et al., 2007; Carpathian–Pannonian Region; Szabó et al., 1992; Harangi, 2001; Kone
n
et al., 2002; Seghedi et al., 2004, 2005), where the composition of the sodic alkaline magmas resembles those that occur in the Alpine foreland (Massif Central, Eifel, Bohemian Massif). This transition is commonly interpreted as a change in the magma source region as a result of changing geodynamic conditions from compression to extension.
In this study, we focus on the northern part of the Carpathian–Pannonian region, where Miocene (16–11 Ma) calc-alkaline magmatism was followed by sodic alkaline magmatism from Late Miocene to Quaternary times (8–0·2 Ma). We present new major and trace element and Sr–Nd–Pb isotopic data for the calc-alkaline volcanic rocks from the western segment of the Carpathian volcanic arc (Fig. 1). This study complements and completes the geochemical dataset for the eastern and northeastern Carpathians (Mason et al., 1996; Seghedi et al., 2001) and we use these internally consistent data to formulate an integrated interpretation of calc-alkaline magmatism in this region. We show that lithospheric extension played a major role in the generation of the calc-alkaline magmatism in the western Carpathians and that subduction had only an indirect influence. An enriched asthenospheric mantle source component, similar to the source of many Tertiary to Quaternary alkaline mafic rocks throughout Europe (Granet et al., 1995; Hoernle et al., 1995; Lustrino & Wilson, 2007), also contributed to the petrogenesis of some of the calc-alkaline magmas of the western Carpathians, together with slab-derived mantle enrichment and mixing with lower crustal materials.
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GEOLOGICAL BACKGROUND |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLING AND ANALYTICAL...GEOCHEMISTRYDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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The Carpathian–Pannonian Region (CPR) in eastern–central Europe (Fig. 1) comprises an arcuate orogenic belt (the Carpathian arc) and an associated back-arc basin (Pannonian Basin) and has many features in common with Mediterranean-type subduction systems (e.g. Horváth & Berckhemer, 1982
; Csontos et al., 1992; Kovac et al., 1998; Fodor et al., 1999; Jolivet et al., 1999; Tari et al., 1999). The main driving force in its evolution was the north–south convergence of the European and African plates. Following Eocene continent–continent collision in the Alps, SSW-dipping subduction continued in the east (Carpathian subduction zone) during the Neogene (Csontos et al., 1992; Nemok et al., 1998b; Fodor et al., 1999). A consequence of Alpine collision was early Miocene eastward extrusion of a rigid lithospheric block from the Alpine area (Ratschbacher et al., 1991; Csontos et al., 1992; Fodor et al., 1999). This lateral extrusion was made possible by a change from advancing to retreating subduction, providing space for the eastward movement (Royden, 1993). Subduction ceased gradually from west to east along the Carpathian arc from about 15 Ma to 10 Ma, as indicated by migration of the last thrust displacement and sedimentary depocenters around the Carpathians (Ji
iek, 1979; Meulenkamp et al., 1996). Post-collisional slab break-off occurred gradually from west to ESE in a zipper-like process (Tomek & Hall, 1993; Mason et al., 1998; Seghedi et al., 1998; Wortel & Spakman, 2000; Sperner et al., 2002). Slab break-off is thought to be in its final stages beneath the Eastern Carpathians (Vrancea zone), where a still hanging, near-vertical, subducted slab causes intermediate-depth seismicity (Oncescu et al., 1984; Oncescu & Benjer, 1997; Fan et al., 1998; Sperner et al., 2001, 2004).
Contemporaneously with the retreating subduction, formation of the Pannonian Basin took place during the Middle Miocene (17–12 Ma; Horváth, 1995; Tari et al., 1999), controlled by a combination of eastward lateral extrusion of orogenic blocks, extensional collapse of an overthickened orogenic wedge and the pull of the subducted slab (Csontos et al., 1992; Horváth, 1993; Tari et al., 1999). As a result of this extension, the Pannonian Basin is underlain by thin lithosphere and crust (50–80 km and 22–30 km, respectively; Tari et al., 1999) and is characterized by high heat flow (>80 mW/m2; Lenkey et al., 2002). The main rifting phase was followed by post-rift thermal subsidence, when several thousand meters of Late Miocene to Quaternary sediments filled parts of the basin. Tectonic inversion has characterized the Pannonian Basin since the Late Pliocene, as a result of the push of the Adriatic plate from the SW and blocking by the East European platform in the east (Horváth & Cloething, 1996).
Seismic tomography images indicate a low-velocity anomaly beneath the CPR (except for the southeastern margin of the Carpathians) to 400 km depth (Spakman, 1990; Wortel & Spakman, 2000). No detached subducted slab has been detected under the western and northeastern parts of the Carpathians. Nevertheless, Tomek & Hall (1993) interpreted deep seismic-reflection data as evidence for subducted European continental crust beneath the Western Carpathians. In contrast, a positive seismic velocity anomaly occurs between the depths of 400 and 600 km beneath the whole region, implying accumulation of relatively cold subducted slab material in the Transition Zone (Spakman, 1990; Wortel & Spakman, 2000; Piromallo et al., 2001).
The complex geodynamics of the CPR resulted in various types of volcanic activity, including repeated explosive eruptions of silicic magmas during the Early to Middle Miocene, extensive calc-alkaline volcanism from the Middle Miocene to the Late Pliocene, and post-extensional alkaline volcanism during the Late Miocene to Quaternary (Szabó et al., 1992; Lexa & Konen, 1998; Harangi, 2001; Konen et al., 2002; Seghedi et al., 2004, 2005). Minor potassic and ultrapotassic magmatism took place from Miocene to Quaternary times (Harangi et al., 1995b). Calc-alkaline volcanic complexes occur throughout the CPR, mostly following the thrust belt of the Carpathian arc (Fig. 1b).
The West Carpathian Volcanic Field (WCVF) is defined here as forming the westernmost part of this volcanic chain on the northern edge of the Pannonian Basin. It consists of erosional remnants of the volcanic complexes of the Visegrád Mts, Börzsöny and central Slovakia (Konen et al., 1995; Harangi et al., 1999; Karátson et al., 2000, 2007). To the east, the volcanic chain continues with the Miocene Cserhát and Mátra volcanoes, whereas to the north, sporadic dyke swarms occur in the Pieniny zone of the Outer Carpathians (Birkenmajer et al., 2000; Trua et al., 2006).
The WCVF calc-alkaline magmas erupted in both terrestrial and shallow marine environments (Konen et al., 1995; Karátson et al., 2000), forming composite volcanoes with effusive and extrusive rocks associated with pyroclastic and epiclastic breccias (Karátson, 1995; Konen et al., 1995; Karátson et al., 2000, 2007). Volcanic activity commenced at 16–16·5 Ma with eruption of garnet-bearing andesitic to rhyodacitic magmas (Harangi et al., 2001), followed by andesites and dacites. It peaked at 14–15 Ma, when several andesitic stratovolcanic and lava dome complexes were formed. Structural and palaeomagnetic analyses indicate syn-volcanic extension tectonics (Nemok & Lexa, 1990; Nemok et al., 1998a) and significant block rotation events (Karátson et al., 2000, 2007). In the south (Börzsöny and Visegrád Mts), volcanism terminated at 14 Ma, whereas in central Slovakia it lasted until 11 Ma (Koneny et al., 1995, 2002).
After a few million years of quiescence, volcanism renewed with development of sporadic alkaline basaltic volcanoes within central Slovakia (8–6 Ma), followed by a more intense alkaline basaltic volcanism in the Nógrád–Gemer Volcanic Field (NGVF, Fig. 2; Dobosi et al., 1995). The last volcanic eruption in this area occurred about 130–150 kyr ago (Simon & Halouzka, 1996).
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The andesites and dacites are usually crystal-rich. Disequilibrium textural and compositional features in the phenocryst assemblage suggest complex magmatic evolution. In some localities, xenoliths are particularly abundant, representing both shallow subvolcanic crustal lithologies and cognate magmatic inclusions. In general, a gradual change of phenocryst assemblage is observed in the andesites. Early stage (16·5–15 Ma) andesites typically contain hornblende and sometimes also biotite, whereas the younger (15–13 Ma) rocks are mostly two-pyroxene andesites. Differentiated rocks include sporadic early-stage rhyodacites (16·5 Ma) in the Visegrád Mts (Harangi et al., 2001
) and late-stage rhyolites (11–12·5 Ma) as well as subvolcanic granodiorite and diorite bodies in central Slovakia (Konen et al., 1995). The 16·5 Ma rhyodacites contain plagioclase and biotite phenocrysts in addition to garnet, whereas the late-stage rhyolites are also characterized by the dominance of plagioclase and biotite, but lack garnet. Sanidine and quartz phenocrysts occur subordinately in the rhyolites. Calc-alkaline volcanism in the WCVF terminated with eruption of small-volume high-alumina basalts and basaltic andesites in the northern part at 11 Ma. These rocks contain olivine, clinopyroxene and plagioclase phenocrysts. |
SAMPLING AND ANALYTICAL TECHNIQUES |
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Over 100 fresh samples of volcanic rocks were collected from various sample localities covering the entire WCVF (Fig. 2). All of the samples were analysed for major and trace elements and for Sr isotope ratios. Based on these data, a smaller number of samples were selected for determination of the rare earth element (REE) contents and Nd and Pb isotope ratios. Salters et al. (1988
) published the first Nd–Sr–Pb isotope data for some Miocene calc-alkaline volcanic rocks from the northern part of the Pannonian Basin. In this paper, we use their data from three WCVF samples (55, 57 and 107 from Börzsöny) for comparison.
The major and trace element characteristics of representative samples are presented in Table 1; the complete dataset can be downloaded from the Journal of Petrology website (http://www.petrology.oxfordjournals.org/). Major and trace element compositions were determined using a Philips PW1480 X-ray fluorescence spectrometer at Royal Holloway University of London (RHUL) using fused glass discs (for major elements) and pressed powder pellets (for trace elements). Details of analytical reproducibility have been given by Baker et al. (1997). Rare earth elements (REE) were analysed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) at RHUL using the method described by Walsh et al. (1981). The boron concentrations of whole rock samples were measured by the cold neutron prompt gamma neutron activation analysis (PGNAA) facility at the Budapest Research Reactor, Hungary, with conditions described in detail by Gméling et al. (2005).
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Whole-rock Sr and Nd isotopic compositions were determined at RHUL. Sample powders were leached in hot 6M HCl for 1 h to remove any effects of post-magmatic groundmass alteration. Sr and Nd were extracted by conventional ion exchange techniques, and their isotope ratios were determined by thermal ionization mass spectrometry (TIMS) on a five-collector VG-354 system in multidynamic mode as described by Thirlwall (1991
). 87Sr/86Sr was normalized to 86Sr/88Sr = 0·1194 and 143Nd/144Nd to 146Nd/144Nd = 0·7219. An in-house laboratory Nd standard (Aldrich) yielded 143Nd/144Nd = 0·511412 ± 5 (2SD on eight analyses; equivalent to 0·511847 for the international standard La Jolla) and SRM987 gave 87Sr/86Sr = 0·710241 ± 16 (2SD on 16 analyses). The measured Sr isotope values (Table 2) were age-corrected but this resulted in little change in the 87Sr/86Sr ratios. Reported Nd isotope ratios are uncorrected as the age correction would be too small to affect the measured ratios. Pb isotope analysis was carried out using a 207Pb/204Pb double-spike method described, together with chemical separation and loading techniques, by Thirlwall (2000). The Pb isotope ratios were determined partly by TIMS and partly by multicollector-ICP-mass spectrometry (MC-ICP-MS) at RHUL. Seven of the analysed samples were measured by both techniques, yielding the same results within the analytical uncertainty. Reproducibility of the SRM981 standard during the period of analysis is given by Thirlwall (2000).
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GEOCHEMISTRY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLING AND ANALYTICAL...GEOCHEMISTRYDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Major and trace element data
The WCVF volcanic rocks show a range of SiO2 contents from 49 to 78 wt %, although they are dominantly andesites (SiO2 57–63 wt %). The most mafic rocks and rhyolites occur only in Central Slovakia and belong only to the latest stage of magmatism. The calc-alkaline mafic rocks (SiO2 49–57 wt %; MgO >3 wt %) have Mg-numbers [= Mg/(Mg + Fe2+), where Fe2+ is estimated assuming FeO/(FeO + Fe2O3) = 0·55] of 0·5–0·6, suggesting variable degrees of crystal fractionation. On the SiO2 vs K2O diagram (Fig. 3a), the WCVF rocks plot mostly along the boundary between the medium-K and high-K calc-alkaline series, although rhyolites from Central Slovakia are strongly potassic. In general, the WCVF rocks show less variation in K2O at a given SiO2 range than calc-alkaline rocks from the Eastern Carpathians (Fig. 3b). TiO2, Fe2O3 and CaO decrease linearly with increasing SiO2 in each suite (Fig. 4), whereas MgO shows a curvilinear trend. The Al2O3 content increases in the most mafic rocks and then decreases above 55 wt % SiO2. Remarkably, samples from central Slovakia have systematically lower Al2O3 and higher TiO2 and MgO concentrations at a given SiO2 content, compared with rocks from Visegrád and Börzsöny. This could indicate different parental magmas.
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Strongly compatible trace elements such as Cr and Ni have low concentrations (Cr <60 ppm, Ni <15 ppm) in all samples including the most magnesian basalts, whereas Sc decreases linearly with increasing SiO2 (Fig. 4). The LILE (e.g. Rb, Ba, Sr) and Pb usually show relatively large scatter in the Harker diagrams (Fig. 4), except for Rb, which has a strong positive correlation with SiO2. The K/Rb ratio is fairly constant (K/Rb = 200–280) throughout the sample set. Ba shows a complex variation with increasing SiO2. The Börzsöny samples can be subdivided into two groups in terms of Ba (Fig. 4). The high Ba corresponds to an increase of other incompatible trace elements (both LILE and HFSE); therefore this could not be an alteration effect. Pb shows a positive correlation with SiO2 content in the central Slovakian suite, whereas this correlation is much weaker in the Visegrád and Börzsöny samples (Fig. 4). In addition, the central Slovakian rocks contain typically less Pb at a given SiO2 than samples from the southern areas. Boron concentrations show a wide range from 2 ppm to 50 ppm (Table 1; Gméling et al., 2005
, 2007) and correlate with the SiO2 content. This range is similar that found in subduction-related volcanic rocks worldwide (Leeman & Sisson, 1996). The lowest B concentrations (2–10 ppm) are shown by the youngest basaltic rocks in central Slovakia.
In diagrams showing SiO2 vs incompatible immobile trace elements (Fig. 4), the late-stage high-Al basalts deviate from the other rocks having significantly higher Nb and La contents. The La and Nb concentrations in these rocks correlate positively with SiO2; however, both values increase rapidly within a relatively small SiO2 range (49–56 wt %), which cannot be explained by pure fractional crystallization, but rather by different degrees of partial melting. The La/Nb ratio of these mafic rocks is fairly constant (1–1·3) and lower than in other WCVF rocks. Deviation of these rocks from the rest of the WCVF is underlined also by their La/Y vs SiO2 trend. In general, the calc-alkaline rocks from central Slovakia have similar Th concentrations, but higher Nb and Y contents compared with samples from Visegrád and Börzsöny (Fig. 4). The Y concentration decreases abruptly above 62 wt % SiO2. Depletion in Y and heavy REE (HREE) is especially typical of garnet-bearing dacites and rhyodacites in Visegrád and Börzsöny (Harangi et al., 2001) and central Slovakian rhyolites.
The normal mid-ocean ridge basalt (N-MORB)-normalized trace element patterns and chondrite-normalized REE patterns of the WCVF volcanic rocks are shown in Fig. 5. The 15 Ma basaltic andesites have very uniform trace element patterns, enriched in LILE and showing a Nb trough and strong positive Pb anomaly, typical of subduction-related magmas (e.g. Ellam & Hawkesworth, 1988; Hawkesworth et al., 1993; Pearce & Peate, 1995). In contrast, the late-stage (10 Ma) basalts show much less depletion in Nb (sometimes lacking Nb depletion entirely) and thus tend to resemble the later alkali basalts. These high-Al basalts also show strong light REE (LREE) enrichment and variable HREE contents. The andesites and dacites have very similar trace element patterns to the basaltic andesites, but with subtle variations in HREE values (Fig. 5). The rhyodacites and rhyolites show stronger enrichment in the incompatible trace elements compared with the andesites. Garnet-bearing rhyodacites have a pronounced depletion in Y and HREE, whereas the rhyolites are the only rock-types that show a negative Eu anomaly.
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Sr–Nd–Pb isotopes
The Sr–Nd–Pb isotope compositions of the WCVF calc-alkaline volcanic rocks are given in Table 2. Initial 87Sr/86Sr and 143Nd/144Nd ratios show a wide range (87Sr/86Sr = 0·7048–0·7100; 143Nd/144Nd = 0·51226–0·51268) but generally form a well-defined, curvilinear trend in a plot of 87Sr/86Sr vs 143Nd/144Nd (Fig. 6). This isotopic variation is similar to that of the wider range of Miocene calc-alkaline volcanic rocks in the Northern Pannonian Basin analysed by Salters et al. (1988
), but differs from that of calc-alkaline rocks from the Eastern Carpathians (Mason et al., 1996; Seghedi et al., 2001) and from the Pieniny zone (Trua et al., 2006). The post-extensional (11–0·2 Ma) alkali basalts of the Pannonian Basin continue this trend to higher 143Nd/144Nd and lower 87Sr/86Sr values, whereas the other end of the WCVF trend approaches the field of local crustal rocks and flysch sediments (Mason et al., 1996). The WCVF data show a trend in isotopic composition with time; the early-stage (mainly garnet-bearing) rocks exhibit the highest 87Sr/86Sr and lowest 143Nd/144Nd values (87Sr/86Sr = 0·7095–0·7105 and 143Nd/144Nd = 0·51226–0·51235) overlapping the isotopic ratios of the Early Miocene silicic volcanic rocks of the Pannonian Basin (Harangi, 2001). The 14–16 Ma andesites and dacites have a relatively small range of isotopic composition (87Sr/86Sr = 0·7065–0·7083; 143Nd/144Nd = 0·51236–0·51246), whereas the youngest (<13 Ma) volcanic products show the highest 143Nd/144Nd and lowest 87Sr/86Sr values.
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There is a much smaller variation in the Pb isotopes, especially in 207Pb/204Pb ratios (15·65–15·68), whereas the 206Pb/204Pb isotope ratios range from 18·77 to 19·19 (Fig. 6c). This isotopic range is smaller than that published previously by Salters et al. (1988
) for a wider range of Northern Pannonian Basin calc-alkaline rocks, although this might be due to the more precise, double-spike lead isotope analysis performed in our study. The WCVF samples plot above the Northern Hemisphere Reference Line (NHRL; Hart, 1984); that is, they are relatively enriched in radiogenic 207Pb, and plot slightly above the field of the alkali basalts of the Pannonian Basin. The Pb isotopes correlate with the temporal trend in Sr–Nd isotope ratios; that is, the lowest 206Pb/204Pb values characterize the oldest volcanic products, whereas the highest 206Pb/204Pb values are from the late-stage high-Al basalts from central Slovakia. |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLING AND ANALYTICAL...GEOCHEMISTRYDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Shallow-level processes
The WCVF volcanic suites comprise mostly calc-alkaline andesites and dacites; basalts and basaltic andesites are rare and confined mostly to the later stages of magmatism. The andesites are rich in phenocrysts, such as plagioclase, amphibole, pyroxenes and rarely biotite, all showing disequilibrium features. Thus, shallow crustal processes must have had an important role in their petrogenetic evolution. Each volcano could have had its own complex magma plumbing system, although some common features can be recognized.
Garnet (almandine) phenocrysts occur only in the early andesites, dacites and rhyodacites (Harangi et al., 2001). Amphiboles are also ubiquitous in these older rocks, whereas clino- and orthopyroxenes are more frequent in the younger rocks, implying a change from hydrous to anhydrous crystallization. The occurrence of primary almandines indicates crystallization at relatively high pressure (>7 kbar) and temperatures (900–950°C; Green, 1977, 1992). Almandine is not stable at shallow depths; hence a relatively rapid ascent of the host magma is necessary to preserve it in volcanic rocks (Fitton, 1972; Gilbert & Rogers, 1989). Almandines in andesites and dacites coexist with high-Al amphiboles (Al2O3 > 12 wt %; Mg-hastingsites, pargasites), which could also have crystallized at high temperature and elevated pressure. High-Al amphibole megacrysts and amphibole-rich cognate xenoliths are often found in the andesites. They could have been picked up from deep-seated cumulates. In the post-16 Ma rocks, garnets are absent and the high-Al amphiboles are joined by low-pressure amphiboles. In many samples, both high- and low-Al amphiboles occur, suggesting mixing of crystal and liquid phases formed at various stages of differentiation as is common in arc rocks (Eichelberger, 1978; Dungan & Davidson, 2004; Davidson et al., 2005). A plot of La/Y vs Y concentration (Fig. 7) shows the role of different minerals during magma differentiation. The samples form a curvilinear trend in this diagram. The relatively low Y concentration (< 20 ppm) in the andesites can be explained by early crystallization of amphibole. This was followed by fractionation of plagioclase, orthopyroxene, amphibole and magnetite in shallow-level magma chambers. The most extreme depletion in Y (<15 ppm) is shown by the garnet-bearing samples. High-pressure garnet fractionation could readily explain this bulk-rock composition, although the presence of residual garnet during the formation of the parental magmas cannot be excluded.
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The basalts and basaltic andesites contain predominantly pyroxene phenocrysts, rather than amphibole. The youngest (11–12 Ma) basalts contain clinopyroxene with a systematically higher Ti and Al content than that in the older rocks. Olivine (Fo60–80) phenocrysts occur sporadically in these mafic rocks. Evidence for mixing of crystal phases formed at different stages of magma evolution is also found in the most mafic rocks. Clinopyroxene (mg-number = 0·9) and amphibole (Al2O3 > 15 wt %) megacrysts are common, probably derived from deep-seated cumulates.
Rhyolites were erupted roughly coeval with the youngest basalts at about 11–12 Ma. Lexa & Koneny (1998) and Koneny et al. (2002) interpreted these rocks as anatectic magmas formed by melting of lower crustal metagreywackes. However, these rhyolites have relatively low 87Sr/86Sr ratios (0·7057–0·7067), only slightly higher than those of the coeval basalts, so an assimilation–fractional crystallization (AFC) relationship between these two groups is more likely.
In summary, most of the WCVF volcanic rocks show clear evidence for shallow-level differentiation processes, such as crystal fractionation and crystal–melt hybridization. These processes can mask the original geochemical features of the magmas and thus make it difficult to have an insight into melt generation processes. Furthermore, the contribution of any crustal component also has to be evaluated before the nature of the magma source region can be discussed.
Crustal assimilation
Crustal involvement in WCVF magmagenesis is indicated by the relatively high 87Sr/86Sr and 207Pb/204Pb and low 143Nd/144Nd isotopic compositions of the magmatic rocks (Fig. 6). The crustal signature could be due to addition of subducted sediment to the mantle source region (source contamination) and/or to assimilation of crustal material during ascent of mantle-derived magma (crustal contamination; James, 1981; Davidson & Harmon, 1989).
The WCVF samples show a relatively large scatter in the SiO2 vs 87Sr/86Sr diagram (Fig. 8), although a rough positive correlation can be observed. Increase of 87Sr/86Sr with SiO2 as a differentiation index could indicate an AFC process, but could also be interpreted as a consequence of mixing between mantle-derived magmas and crustal melts. The data in Fig. 8. can be explained by a series of AFC trends with different assimilation rates and/or mixing with various crustal components. The model AFC–mixing trends converge to a possible mantle-derived magma composition, which shows subtle 87Sr/86Sr isotope variation (from 0·704 to 0·7055). This may indicate derivation of the parental magmas from a slightly heterogeneous mantle source. The 11–12 Ma Central Slovakian basalt–rhyolite samples define a nearly horizontal trend, suggesting only a minor crustal signature and, possibly, a genetic link between the late-stage basaltic and rhyolitic magmas. In contrast, the other trends imply a fairly significant crustal component.
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The highest 87Sr/86Sr values belong to the early garnet-bearing volcanic rocks (Fig. 8). These magmas cannot be the differentiation products of the andesitic melts, as they must have risen rapidly from lower crustal depths to preserve the early crystallized garnets. Furthermore, they are older than the andesites. The garnet-bearing dacites and rhyodacites show many petrological and geochemical similarities with the garnet- and cordierite-bearing dacites from the Betics of Spain (El Joyazo and Mazarrón areas; Zeck, 1970
; Munksgaard, 1984; Cesare & Gómez-Pugnaire, 2001; Duggen et al., 2005). The considerable involvement of crustal material is implied by the occurrence of Al-rich crustal xenoliths and garnet xenocrysts in both volcanic suites. Thus, melting of the lower crust has been suggested as an important role in their genesis (Zeck, 1970; Cesare & Gómez-Pugnaire, 2001; Harangi et al., 2001). The xenocrystic garnets and the high-Al xenoliths point to a metasedimentary crustal source. Metasedimentary granulites form part of the lower crustal xenolith suite of the Pannonian Basin (Dobosi et al., 2003; Embey-Isztin et al., 2003; Török et al., 2005). They have a mineral assemblage and geochemical composition suggesting a restitic origin. Török et al. (2005) proposed that significant melting in the lower crust could have occurred at the onset of lithospheric stretching (i.e. at 16–18 Ma). Garnets in the metasedimentary granulites have similar compositions to the xenocrystic almandines in the dacites, which could have been also restites incorporated into the silicic melts. The Sr–Nd–Pb isotopic composition of the metasedimentary granulites (Dobosi et al., 2003) is roughly similar to that of the garnet-bearing dacites and rhyodacites.
A purely anatectic origin for the dacitic and rhyodacitic magmas is not, however, consistent with published data for the almandine phenocrysts, which are Ca-rich (CaO 4–8 wt %) and have relatively low 
18O values (6·1–7·3
). This indicates crystallization from a mantle-derived magma, whereas the xenocrystic almandines with lower Ca (CaO <2·5 wt %) and a 18O value of 10·5 (Harangi et al., 2001) could be derived from the lower crust. These magmas were formed at the beginning of lithospheric extension of the Pannonian Basin, when the crust would have been significantly thicker (>40 km; Tari et al., 1999). Thus, we suggest that mantle-derived mafic melts ponded beneath the thick continental crust and provided heat to generate silicic melt at the onset of lithospheric extension. The mafic and silicic magmas mixed thoroughly, resulting in dacitic–rhyodacitic magma compositions. Incorporation of S-type silicic melts increased the Al content of the hybrid magma, allowing crystallization of almandine at depth. The tensional stress field at the beginning of rifting presumably reactivated major faults, enhancing the rapid ascent of the silicic magmas. As the continental crust became thinner during the syn-rift phase (16–13 Ma), incorporation of lower crustal material into the mantle-derived magmas decreased significantly. Indeed, a gradual decrease in 87Sr/86Sr is observed in the younger andesites (Figs 6 and 8). Nevertheless, the isotope values of these rocks (e.g. 87Sr/86Sr = 0·707–7085) indicate that crustal contamination was still important in their genesis.
Implications for the nature of the mantle sources
The WCVF calc-alkaline rocks exhibit the typical geochemical characteristics of subduction-related magmas [elevated LILE, Pb and B concentrations, relative depletion in Nb, and high 87Sr/86Sr and 207Pb/204Pb and low 143Nd/144Nd (Figs 4–6
)]. These features cannot be explained solely by assimilation of crustal material by mantle-derived magmas, but could indicate a source region metasomatized by aqueous fluids and/or melts derived from a subducted slab and subducted sediments (e.g. Tatsumi et al., 1986; Hawkesworth et al., 1993). Boron is a fluid-mobile and strongly incompatible trace element (having a very similar bulk distribution coefficient to Nb and La), which can be used to reveal the influence of slab-derived fluids in the magma source region (e.g. Morris et al., 1990; Ryan & Langmuir, 1993; Ishikawa & Nakamura, 1994; Leeman et al., 1994; Leeman & Sisson, 1996). The B/La and B/Nb ratios are dependant basically on fluid addition and do not change significantly during melting and magma differentiation. Pb and Ba are also mobile in aqueous fluids and thus Ba/La and Pb/La ratios are commonly used as indicators of aqueous fluid metasomatism. In Fig. 9, both the Slovakian and the Börzsöny–Visegrád samples show positive correlations between B/La and Pb/La, whereas Ba/La does not correlate with B/La. This suggests that Ba was not involved in the aqueous fluid metasomatism. Furthermore, the B/La and B/Nb ratios are relatively low. These trace element ratios appear to be diagnostic of the type of subduction. Low B/La and B/Nb ratios are found mostly in calc-alkaline volcanic rocks associated with shallow subduction of relatively warm, young oceanic lithospheres (Leeman et al., 1994, 2004). Volcanic rocks from central and southern Italy show also low values of these ratios (Tonarini et al., 2001, 2004), where melt generation is due to decompression melting of subduction-related fluid-modified mantle (Peccerillo, 2005). In the CPR, subduction roll-back characterized Middle Miocene times with termination of active subduction north of the Western Carpathians. Thus, we may infer that the boron concentration data are consistent with a geodynamic setting similar to that of central and southern Italy.
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Characterization of the nature of the mantle source before subduction–metasomatism is a challenging task in continental arc magmatism. Initial Sr–Nd–Pb isotope ratios depend on both the type of mantle source and the amount of incorporated crustal component. However, the 206Pb/204Pb ratio effectively distinguishes between the various mantle reservoirs, whereas the other isotope values can be used to infer the crustal component. In 206Pb/204Pb–87Sr/86Sr–143Nd/144Nd space, the WCVF samples define smooth curvilinear trends (Fig. 10). These trends are identical to those shown by other volcanic suites from the Central and Western Mediterranean (Vollmer, 1976
; Gasperini et al., 2002; Peccerillo, 2003; Duggen et al., 2005) and can be interpreted as binary mixing hyperbolae between asthenospheric and crustal end-members. The mantle end-member is characterized by low 87Sr/86Sr and relatively high 206Pb/204Pb and 143Nd/144Nd values. This resembles the FOZO mantle reservoir as defined by Stracke et al. (2005) or the common European Asthenospheric Reservoir (Cebriá & Wilson, 1995; Hoernle et al., 1995; Lustrino & Wilson, 2007). The crustal end-member is less well constrained. It could be lower or upper crust or subducted sediment. As discussed above, metasedimentary lower crustal material could play an important role in the genesis of the early stage WCVF calc-alkaline magmas. Assimilation of upper crustal material in the <15 Ma volcanic rocks can be also considered, particularly for those in Slovakia, where Hercynian granitic and metamorphic rocks occur in the basement (Tomek, 1993; Kohut et al., 1999). However, the available Sr–Nd–Pb isotope data for these rocks (Kohut et al., 1999; Poller et al., 2005) do not support their involvement in the petrogenesis of the calc-alkaline magmas. Crustal xenoliths with a wide range of lithologies are fairly abundant in some of the WCVF volcanic rocks, but assimilation of these crustal lithologies by the ascending magmas can also be excluded based on their isotopic composition (Table 2). Crustal contamination could also occur, however, in the magma source region by metasomatism of the mantle via melt from subducted sediment. Mason et al. (1996) published isotope data for various flysch sediments from the Outer Carpathians. Some of these sediments could, potentially have been subducted and added to the mantle wedge. In the following model calculations (Table 3; Fig. 10), we consider both the incorporation of lower crustal materials into the magmas and source contamination processes. We use the isotopic compositions of metasedimentary granulite xenoliths from the Pannonian Basin (Dobosi et al., 2003) to represent the lower crust and Cretaceous turbidite shale compositions (Mason et al., 1996) to represent the subducted sediment.
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Harangi (2001
) used a simple binary mixing model to explain the isotope variation of the WCVF samples. We have modified this model, because it required mixing of >30% crustal component to form the late-stage WCVF basalts, which seems to be anomalously high. Instead, we prefer here a two-stage model (Fig. 10). In the first stage, an enriched asthenospheric (FOZO-like) mantle source is contaminated by melts derived from subducted sediment (mixing line A in Fig. 10). Approximately 1–2% of subducted sediment could have been added to the FOZO-like mantle, to form a metasomatized mantle source (metasomatized mantle1 in Fig. 10) that underwent further partial melting to produce the parental mafic magmas. The high 87Sr/86Sr of the WCVF samples indicates, however, involvement of additional crustal material. As discussed above, mafic magmas ponded at the crust–mantle boundary could have initiated melting in the overlying lower crust. Silicic melts from metasedimentary lower crustal material could have mixed with the mafic magmas derived from the metasomatized mantle (mixing line B in Fig. 10). At first the resulting dacitic–rhyodacitic melts could have contained as much as 60–70% of the lower crustal component, consistent with their slightly peraluminous character. Later, as the continental crust thinned, the fraction of the lower crustal component gradually decreased and the proportion of mantle-derived melt increased. This scenario is the simplest explanation for the isotopic variation of the WCVF volcanic rocks. However, we also test an alternative model with different mantle components. The isotopic composition of the older WCVF samples is not consistent with derivation from a depleted MORB-source mantle. Thus, in the second model, we consider an E-MORB-source mantle that was contaminated with 2–3% of subducted sediment (mixing line C in Fig. 10). The mafic melt formed from this metasomatized mantle source (metasomatized mantle2 in Fig. 10) mixed subsequently with the lower crustal component (mixing line D in Fig. 10). In this case, a smaller amount of crustal material (<30%) is required in the mixed magmas. This scenario could be applicable to the older (>15 Ma) magmatism of the Visegrád and Börzsöny Mts, but does not explain the genesis of the <15 Ma magmas in Central Slovakia. For this, a sharp change in composition of the mantle source is needed (i.e. from an E-MORB-type to a FOZO-type mantle). This change in the mantle source is reflected also in the trace element composition of the 11–12 Ma basalts (Figs 4 and 5). The elevated incompatible trace element concentrations and high La/Y ratio of these rocks indicate low-degree partial melting of an enriched mantle source. The isotopic variation of the post-15 Ma magmas can be explained by modification of a FOZO-like, enriched mantle by 1–2% subducted sedimentary component (trend A and metasomatized mantle1 in Fig. 10) followed by mixing of mafic melt derived from this metasomatized mantle with silicic melt from the metasedimentary lower crust (trend B in Fig. 10).
In both models, an enriched, FOZO-type mantle source, similar to that proposed for the Central and Western Mediterranean magmatism (Gasperini et al., 2002; Peccerillo, 2003; Duggen et al., 2005; Peccerillo & Lustrino, 2005; Lustrino & Wilson, 2007) played an important role in the genesis of the WCVF magmas. However, the location of this FOZO-like mantle is unclear. It could be related to mantle plume, but could also reside in the shallow heterogeneous mantle. In the next section, we discuss the role of this enriched mantle component in the genesis of the WCVF magmas and the relationships between the calc-alkaline magmatism and geodynamic processes such as subduction and extension.
Relationship between calc-alkaline magmatism and geodynamics
The WCVF volcanic suites exhibit many features in common with subduction-related volcanic rocks (Figs 5 and 6). Indeed, subduction has been generally regarded as an important process in the evolution of the CPR (Royden et al., 1982; Csontos et al., 1992; Tomek & Hall, 1993), although recent geophysical studies have questioned the southward subduction beneath the Western Carpathians (Grad et al., 2006) and the westward subduction beneath the SE Carpathians (Knapp et al., 2005). Nevertheless, most workers consider that subduction took place from the Late Cretaceous as recorded by the Cretaceous to Neogene flysch sediments around the present-day Carpathians (Horváth & Royden, 1981; Sandulescu, 1988; Csontos et al., 1992). Approximately 260 km of shortening is estimated to have occurred in the Outer Carpathians from the Middle Oligocene to the Middle Miocene (Roure et al., 1993; Behrmann et al., 2000), coeval with extension of the Pannonian Basin (Royden et al., 1983; Csontos et al., 1992). Thus, the Carpathian volcanic arc has been commonly interpreted as a direct consequence of subduction (Bleahu et al., 1973; Balla, 1981; Szabó et al., 1992; Downes et al., 1995a). However, there are some pecularities of the WCVF suite that may suggest a different magma-generation scenario.
During the first stage of volcanism (16·5–16 Ma), eruption of primary garnet-bearing magmas was coeval with the onset of rifting. Lithospheric stretching affected the WCVF area, as indicated by the relatively thin crust and lithosphere beneath this region (Horváth, 1993; 
efara et al., 1996; Tari et al., 1999) and the presence of syn-volcanic extensional structures in Central Slovakia (Nemok & Lexa, 1990; Sperner et al., 2002). Intermediate volcanism continued from 16·5 to 13 Ma, and eruption of a bimodal basalt–rhyolite suite took place at 12·5–11 Ma. This was followed by eruption of alkaline mafic magmas (Dobosi et al., 1995) from 8 Ma to 0.2 Ma. A similar change from calc-alkaline to alkaline mafic magmatism has been described in many parts of the Mediterranean region and interpreted as the magmatic expression of the termination of subduction (e.g. Seyitolu et al., 1997; Wilson et al., 1997; El Bakkali et al., 1998; Coulon et al., 2002). However, subduction along the Western Carpathians was not contemporaneous with the calc-alkaline volcanism. Active subduction terminated north of the Western Carpathians at about 15–16 Ma, based on the age of the last thrust in the Outer Carpathians (Jiíek, 1979). Remarkably, active subduction was not associated with the calc-alkaline magmatism, which started just as active subduction ceased and lithospheric thinning commenced. Nevertheless, subduction of oceanic lithosphere may have continued at depth following continental collision in the upper crust. In this case, arc-type melts could have been generated in the mantle wedge as a result of the volatile flux from the descending slab, and the lithospheric extension only enhanced the ascent of the magmas. This scenario cannot be unambiguously excluded, although taking into account all the main features of the calc-alkaline magmatism described above, our suggestion is that extension and decompression melting of the passively upwelling, presumably metasomatized upper mantle could have primarily controlled the formation of magmas, as has been suggested for Western Anatolia (Seyitolu et al., 1997; Wilson et al., 1997) or the Basin and Range province, western USA (Hawkesworth et al., 1995). Thus subduction played only an indirect role in magma generation.
A striking feature of the WCVF magmatism is the gradual change of magma composition; for example, La/Nb, Th/Nb and 87Sr/86Sr ratios decrease, whereas the 143Nd/144Nd and 206Pb/204Pb ratios increase with time (Harangi & Lenkey, 2007). In contrast, the Ba/La ratio and the 207Pb/204Pb, 208Pb/204Pb isotope ratios do not show any clear temporal change. This compositional variation can be explained by a decreasing amount of the crustal component in the petrogenesis of the magmas and/or an increasing role of an enriched asthenospheric mantle source component. Both scenarios are consistent with magma generation beneath progressively thinning lithosphere (crust and mantle). The change in the mantle source, that is, from a metasomatized E-MORB source mantle to a more enriched, ocean island basalt (OIB)-source (FOZO) mantle about 13–14 Ma, is clearly illustrated by a Nb/Y vs Th/Y diagram (Fig. 11). The appearance of the OIB-like, enriched mantle component in the late-stage calc-alkaline volcanism can be explained by various models such as: (1) mantle flow at a subducting slab edge as a result of roll-back; (2) formation of hot fingers in the mantle wedge; (3) retreating subduction at the eastern margin of the CPR initiating eastward mantle flow of the enriched asthenospheric mantle characteristic of western Europe; (4) delamination of the lower lithosphere and upwelling of hot asthenosphere (Seghedi et al., 1998); (5) gradual slab detachment and deflection of upwelling mantle material from the NW (Harangi et al., 2006); (6) back-arc type diapiric upwelling of the asthenosphere behind the active subduction zone (Koneny et al., 2002); (7) progressive thinning of the continental lithosphere with a change of the mantle source region from the metasomatized lithospheric mantle to the underlying, passively upwelling asthenosphere.
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A slab edge mantle flow model, such as that proposed for the South Sandwich arc–basin system (Leat et al., 2004
), is not consistent with the gradual temporal transition in magma composition. In addition, it would require a lateral change in magma composition that is not observed. Localized upwelling of hot asthenospheric material (hot fingers) along fractures within a curvilinear subducted slab could be another possible explanation, although we do not see a systematic distribution of the volcanoes (Tamura et al., 2002) or a well-defined lateral compositional variation in the volcanic rocks. An alternative to this model could be the formation of a slab window and upwelling of deep mantle material through it, as proposed for the Central Italian magmatism by Gasperini et al. (2002). One problem with this model is, however, that a positive seismic velocity anomaly is observed beneath the CPR at 400–650 km depth (Wortel & Spakman, 2000; Piromallo et al., 2001; Piromallo & Morelli, 2003). This has been interpreted as an accumulation of thick, cold material, possibly subducted residual lithosphere that would form a barrier against the rise of a lower mantle plume, and is also not consistent with shallow mantle plumes originating at the base of the upper mantle. During the Middle Miocene, subduction of oceanic lithosphere took place mostly in the Eastern Carpathians, whereas the Western Carpathian region was characterized by strike-slip faulting (Sperner et al., 2002). Subduction roll-back played an important role in thinning the lithosphere and formation of the Pannonian Basin (Royden et al., 1982; Csontos et al., 1992). Retreating subduction could have initiated an eastward mantle flow (Doglioni, 1993). Assuming that the sub-lithospheric mantle beneath western and central Europe is characterized by a common enriched composition (Hoernle et al., 1995), this eastward mantle flow could also provide an enriched mantle component beneath the Pannonian Basin. However, the geometry and temporal relationships of the calc-alkaline magmatism (Pécskay et al., 2006) in the northern part of the Pannonian Basin, including the WCVF, do not seem to support a connection with eastward retreating subduction.
The other group of possible models emphasizes the post-collisional character of the calc-alkaline magmatism or the primary role of continental extension. Seghedi et al. (1998) explained the magmatism of the WCVF by subduction roll-back followed by lithospheric mantle delamination. Indeed, a slight northward migration of volcanic activity occurs within the WCVF and also in other volcanic complexes along the Northern Pannonian Basin. Lithospheric delamination can initiate upward asthenospheric flow and also partial melting of the delaminated and descending metasomatized lithospheric mantle. However, large-scale delamination would result in significant uplift of the area, which is not observed. Termination of active subduction is often accompanied by slab detachment, commonly considered to be an important process in the Mediterranean region (Wortel & Spakman, 2000); this has also been suggested for the northern part of the CPR during the Middle Miocene (Tomek & Hall, 1993). Slab detachment can also initiate mantle flow to fill the gap within the rifted subducted lithosphere (Davies & von Blanckenburg, 1995). Harangi et al. (2006) hypothesized that enriched OIB-source mantle material from an assumed plume finger beneath the Bohemian Massif (Wilson & Patterson, 2001) could flow beneath the thinned lithosphere of the Pannonian Basin through the gap left behind by a detached slab under the western Carpathians. This could explain the appearance of such a mantle component at the end of the calc-alkaline magmatism (<13 Ma) and the subsequent alkaline mafic magmatism characterized by an isotopic signature akin to that of the European mafic rocks (Embey-Isztin et al., 1993; Embey-Isztin & Dobosi, 1995). The drawback to this model is that plume activity is not proven beneath the Bohemian Massif and there is a lack of mantle anisotropy data beneath this region that could provide evidence for such a mantle flow.
Calc-alkaline magmatism could also take place as a response to lithospheric extension without contemporaneous subduction (Hawkesworth et al., 1995; Hooper et al., 1995; Wilson et al., 1997; Fan et al., 2003) and this has also been suggested for the WCVF magmatism by Lexa & Konen (1974, 1998), Harangi (2001), Konen et al. (2002) and Harangi & Lenkey (2007). This is consistent with the available structural data (Nemok & Lexa, 1990; Tari et al., 1999; Sperner et al., 2002). In this scenario, the early stage (14–16 Ma) magmatism is the sign of the initiation of lithospheric thinning when partial melting took place in the lower part of the lithospheric mantle metasomatized by earlier subduction. The mafic magmas pond beneath the thick continental crust, resulting in melting of the lower crust. As lithospheric extension progresses, the crustal component decreases and the mantle source changes to a sub-lithospheric one.
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLING AND ANALYTICAL...GEOCHEMISTRYDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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The Carpathian–Pannonian region is a Mediterranean-type orogenic area characterized by various types of magmatic activity. One of its most prominent features is the development of a calc-alkaline volcanic chain along the inner Carpathian arc. Trace element and Sr–Nd–Pb isotopic data for Miocene (16·5–11 Ma) calc-alkaline volcanic rocks from the western segment of this volcanic arc (WCVF) provide an insight into the petrogenesis of these magmas and their geodynamic relationships. Crustal contamination played an important role in magma evolution, as in the Eastern Carpathians (Mason et al., 1996
). However, in the WCVF, involvement of lower crustal material is more pronounced. We propose that early mafic magmas ponded beneath the relatively thick continental crust and initiated melting in the lower crust. Mixing of silicic melts from metasedimentary lithologies with mafic magmas derived from the mantle resulted in hybrid dacitic magmas, from which almandine garnet crystallized at high pressure. Subsequently, as the continental crust thinned, the role of crustal contamination decreased. The WCVF calc-alkaline magmas show a gradual change of trace element and isotopic composition with time, consistent with a change of the magma source region, from an E-MORB-type mantle to a more enriched, OIB-type mantle. The latter has an isotopic character similar to FOZO as defined by Stracke et al. (2005) or the common European Asthenospheric Reservoir (Cebriá & Wilson, 1995; Hoernle et al., 1995; Lustrino & Wilson, 2007). A plume origin for this mantle component is unlikely in the Pannonian Basin (Harangi & Lenkey, 2007). It could reside in the shallow asthenosphere and possibly also in the lower lithosphere, creating small-scale heterogeneity (Rosenbaum et al., 1997). This magma generation model is strikingly different from that proposed for the East Carpathians by Mason et al. (1996). A change of mantle source can also be recognized in that magmatism (Mason et al., 1998), from a depleted mantle source to a more enriched one, which yielded potassic magmas. Transition from calc-alkaline to alkaline magmatism is observed in many parts of the Mediterranean region. We propose that this transition in the WCVF does not necessarily indicate a change in the geodynamic setting; that is, from subduction to extension. Instead, both types of magmatism are considered to be related to lithospheric extension. The calc-alkaline magmas were generated during the period of peak extension by melting of metasomatized lithospheric mantle and were contaminated by mixing with silicic lower crustal melts, whereas the later alkaline mafic magmas formed during the post-extensional stage by low-degree melting of the shallow asthenosphere.
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SUPPLEMENTARY DATA |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLING AND ANALYTICAL...GEOCHEMISTRYDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Supplementary data for this paper are available at Journal of Petrology online.
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ACKNOWLEDGEMENTS |
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Radiogenic isotope and X-ray fluorescence facilities at Royal Holloway were University of London Intercollegiate Research Services. We thank Gerry Ingram, Giz Marriner and Claire Grater for their assistance in the analytical work. Sz.H.'s research in London and Egham was supported by a NATO post-doctoral research fellowship from the Royal Society (1997–1998). This study was also supported by the Hungarian Science Foundation grant (OTKA T 037974) to Sz.H. K.G. would like to thank GVOP-3.2.1-2004-04-0268/3.0 for supporting the renovation of the detector; and NAP VENEUS05 Contract No. OMFB 00184/2006 for the improvement of the PGAA facility. Thorough reviews and detailed, constructive suggestions given by A. Peccerillo, M. Wilson, I. Seghedi and P. Macera helped us to clarify the ideas presented in this paper.
*Corresponding author, Tel: +36-12090555/ext. 8355. Fax: +36-13812108. E-mail: szabolcs.harangi{at}geology.elte.hu
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