Antal Em bey-Isztin & Gero Kurat*
Keywords: Carpathian-Pannonian region, alkali basalts, isotope and trace element systematics, mantle source enrichment, Pliocene-Pleistocene
Abstract
Pliocene to Pleistocene alkali basalts with a compositional range from olivine tholeiite to nephelinite erupted in the Carpathian-Pannonian Region (CPR) following Eocene and Miocene subduction events with associated calc-alkaline volcanism, crustal extension, and basin formation. Ultramafic xenoliths from the alkali basalts provide insight into the lithology, tectonic state, and geochemistry of the lithospheric mantle beneath this region. Trace element and isotope geochemistry of alkali basalts indicate a dominant asthenospheric component. However, in some basalts there is clear evidence for an enriched lithospheric component as well, the origin of which is argued to be related to the Tertiary subduction events. These basalts apparently were generated by melting and mixing of asthenospheric and lithospheric components. The relative contributions by the lithosphere show spatial and temporal variations compatible with the idea that the convectively upwelling asthenosphere played the primary role in magma genesis rather than melting through extension of the thermally activated lithosphere.
Zusam menfassung
Pliozäne bis pleistozäne Alkalibasalte der Karpathisch-Pannonischen Region haben olivin-tholeiitischen bis nephelinitischen Chemismus. Sie eruptierten nach den kalk-alkalinen Vulkaniten, welche die Subduktions-Vorgänge im Eozän und Miozän, die Krusten-Extension und die Beckenbildung in dieser Region markieren. Ultramafische Xenolithe in den Alkalibasalten geben uns einen Einblick in die Lithologie des Erdmantels dieser Region und die dort ablaufenden tektonischen und geochemischen Prozesse. Spurenelement- und Isotopengeochemie der Alkalibasalte deuten auf eine dominante asthenosphärische Komponente. Einige Basalte zeigen allerdings auch das Vorhandensein einer an Spurenelementen angereicherten lithosphärischen Komponente, welche möglicherweise ein Produkt von Subduktion während des Tertiärs ist. Diese Basalte entstanden wahrscheinlich durch Schmelzen und Mischen von asthenosphärischen und lithosphärischen Komponenten. Der Anteil der Lithosphäre zeigt räumliche und zeitliche Veränderungen, welche eine Dominanz der Basaltproduktion in der konvektiv aufsteigenden Asthenosphäre gegenüber jener in der thermal aktivierten und gedehnten Lithosphäre erkennen lässt.
Összefoglalás
Pliocén-pleisztocén alkáli bazaltok törtek ki a kárpáti-pannóniai területen (CPR) eocén és miocén szubdukciós eseményeket követően, amelyekhez mészalkáli vulkánosság, kéreghúzódás és medenceképződés kapcsolódott. Ösz- szetételük az olivin-tholeiittől a nefelinitig terjed. Az alkáli bazaltokban talált ultramafikus kőzetzárványok bepillantást engednek a terület alatti litoszféraköpeny kőzettani és geokémiai összetételébe, valamint tektonikai állapotába. Az alkáli bazaltok nyomelem- és izotóp-geokémiája az asztenoszférából eredő összetevő túlsúlyát jelzi. Egyes bazaltokban azonban bizonyítható a litoszféra-eredetű, inkompatibilis elemekben dúsult anyag részvétele is. Ez feltehetően a har
madidőszaki szubdukciós eseményekkel függ össze. Úgy tűnik, hogy ezek a bazaltok asztenoszféra- és
litoszféra-* A d dresses of Authors: Antal Embey-Isztin, Dept, of M ineralogy and Petrography, Hungarian M useum of Natural History, Ludovika tér 2, H^-1083 Budapest, 1370 Pf. 330, Hungary, Gero Kurat, M ineralo gisch-P etro graph isch e Abteilung, Naturhistorisches M u seum , Burgring 7, A - 1 0 1 0 W ien, Austria.
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eredetű anyagok megolvadása és elegyedése útján jöttek létre. A litoszféra-eredetű összetevő tér-időbeli változásai összegyeztethetők azzal az elgondolással, hogy a magma keletkezésében a főszerepet az asztenoszféra konvekciós feláramlása játszotta, nem pedig a termikusán aktivált litoszféra-széthúzódás kiváltotta megolvadás.
Introduction
Alkali basalts represent a “window" through which we can gain insight into the Earth's mantle. Compared to other mantle-derived volcanic rocks, e.g., tholeiitic or picritic magmas which may interact extensively with the lithosphere and the crust, alkali basalts, due to their volatile-rich character, rise rapidly to the Earth's surface.
That's why they generally escape crustal contamination.
Thus, the picture they reflect of the trace element and isotope abundances of the upper mantle source rocks is much less disturbed than in other liquids derived from the upper mantle. In addition, gas-rich alkali basalts and related nephelinites, carbonatites, kimberlites, lamprophyres, and lamproites often carry mantle xenoliths (exogenous and accidental inclusions from the mantle wall rock) such as peridotites (Iherzolite, harzburgite, dunite, wehrlite) as well as pyroxenites, hornblendites, and eclogites. Xenoliths supply direct information on the lithology, mineralogy, texture, and structure (i.e., tectonic state), as well as chemical and isotope characteristics of the lithospheric mantle. The information on the chemistry and isotope relations of source mantle regions provided by alkali basalts and kindred magmas is by no means a primary one. At least four factors shape the chemical and isotopic characteristics of mafic alkali magmas:
(1) Source characteristics reflecting its previous history (depletion and enrichment processes);
(2) The tectonic environment during magma generation;
(3) Physical and chemical conditions of magma generation (the degree of partial melting, depth of magma segregation, the CO2/H2O ratio etc.);
(4) Mixing between different mantle source regions.
In addition, subsequent magma differentiation may severely alter the chemistry of volcanic rocks and continental basalts can occasionaly become contaminated by crustal matter on their way to the surface. Therefore, alkali basalts should be carefully studied case by case in order to recognize and evaluate the effects caused by each of these factors. Without going into details, we mention for example that the ratio of two incompatible elements (elements that prefer to enter the liquid during melting) with highly different mineral/melt partition coefficients (D) such as La/Sm will likely reflect the degree of partial melting, whereas abundance ratios of incompatible elements with identical mineral/melt Ds, such as Ce/Pb, should mirror abundances in the source, regardless of the degree of partial melting. A continuous increase in 87Sr/86Sr and 180 /160 ratios with decreasing MgO content is a strong indication for contamination with crustal matter during fractional crystallization. On the contrary, the lack of correlation between these variables strongly suggests that the variation in isotope ratios of basalts was inherited from mantle sources, i.e. they reflect isotopic heterogeneity within the mantle. Whereas peridotite xenoliths invariably provide information on the evolution of the lithospheric mantle, chemical and isotope data of alkali basalts are more difficult to interpret, because they may reflect conditions in more than one mantle reservoir, e.g., asthenosphere, plume, or lithosphere, and a combination of these (e.g., Classet al. 1994, Perryet al. 1987).
In the following paragraphs we try to summarize the results of almost 25 years of research work on the mineralogy, petrology, and geochemistry of the upper mantle beneath the Carpathian-Pannonian Region (CPR) and try to interpret them in the light of modern geochemical models.
Geological setting The Carpathian-Pannonian Region (CPR) is an
extensional back arc basin or, more correctly, a basin system comprising several areas of depression (e.g., Vienna Basin, Graz Basin, Little Hungarian Plain (LHP), Great Hungarian Plain, and Transylvanian Basin) separated by Mesozoic and Palaeozoic block mountains.
This area is surrounded by the Eastern Alps, the Carpathian fold belt and the Dinarides (fig. 1). However, present-day geophysical characteristics like high heat flow, thin crust, high temperature within the lithosphere (e.g., Dövényi & Horváth 1988), and young (Pliocene to Quaternary) alkali basaltic volcanism indicate a close similarity to young rifts (Embey-Isztin et al. 1990 and references therein). The manifestation of young alkali basaltic volcanism between about 10 to 2 Ma over large areas from the Graz Basin and Burgenland to the west, through the LHP, Balaton, and Nógrád regions, to the Persányi Mountains to the east (fig. 1) provides a good opportunity to study temporal and spatial variations in the
isotope and trace element geochemistry of lavas. This knowledge may provide help in reconstructing the evolutionary history of the extensional basin system. For example, it can help us to decipher whether continental extension or upwelling convective asthenosphere was the primary cause of magma generation. In the first case, we are dealing with "passive rifting" in which the mantle is forced to rise as the continental lithosphere is pulled apart during lithospheric stretching. In the second case, rift with several continental fragments, including the African Italo-Dinaride microplate (Royden & Báldi 1988, Royden
& Burchfiel 1989). During the Miocene, rapid extension, subsidence,and sedimentation took place in the basins 160
Fig. 1. Simplified map of the Carpathian-Pannonian Region showing Cenozoic alkali basalt and calc-alkaline volcanic fields
of the CPR (Sclater et al. 1980, Horváth & Royden
1981). Subsidence within the Carpathian-Pannonian system and the simultaneous underthrusting in the Carpathians were accompanied by eruption of voluminous calc-alkaline volcanic rocks, mainly in the northern part of the Pannonian Basin and in the Eastern Carpathians (Salterset al. 1988, Downeset al. in press, Panto et al. in press). The genesis of this volcanic chain is probably related to the subduction of the European plate southward and westward beneath the Carpathian Mountains (Burchfiel 1980). A remarkable feature of the calc-alkaline volcanism is that it becomes progressively younger going from the Western Carpathians (21-17 Ma, Börzsöny Mountains) to the Eastern Carpathians (12-2
Ma) following the simultaneous eastward migration of thrusting within the outer Carpathian fold belt. The extension of the continental crust ceased in late Miocene. The subsequent alkali basaltic volcanism is mainly post-extensional (10-11 Ma Burgenland, 2-3.5 Ma Graz Basin, 3.5-6.5 Ma LHP, 3-5 Ma Balaton Reg
ion, 2.5-4.5 Ma Nógrád area and <2 Ma Persányi Mountains) (Balogh et al. 1986, 1994). Lava flows are more common than volcanic tuffs, cinder cones are rare, except for the Persányi Mountains. The volume of the lava flows and volcanic tuffs is generally small in each region, the number of eruptive centres may, however, be quite high (e.g., about 60 in the Balaton area).
Petrography of alkali basalts Most CPR alkali basalts are remarkably fresh,
moderately porphyritic and holocrystalline, but a few samples contain small amounts of glass. The mineralogy is typical of alkali basalts with an assemblage of phenocrystic olivine, occasionally accompanied by clinopyroxene. The matrix is composed of plagioclase, clinopyroxene, olivine, and titanomagnetite, occasionally coexisting with ilmenite, as well as apatite (Embey-Isztin &
Scharbert 1981, Embey-Isztin et al. 1985, 1993a, Poultidis & Scharbert 1986). Green core clinopyroxene and Al-augite were observed only in the Nógrád area and Graz Basin (Dobosi 1989, Dobosi et al. 1991). Mantle- deriveci peridotite xenoliths and megacrysts (Embey-Isztin
1976, 1978, 1984, Embey-Isztin et al. 1989, Kurat 1971, Kurat et al. 1980, 1991, Dietrich & Poultidis 1985, Downes et al. 1992, Downes & Vaselli in press) and lower crustal granulite xenoliths (Embey-Isztinet al. 1990) are abundant at some localities. Only one hornblendite vein each has been found in a Iherzolite xenolith from Szigliget (Balaton area, Embey-Isztin 1976) and Kapfenstein (Styria, Kurat et al. 1980), respectively, though disseminated amphibole grains are more common in peridotite xenoliths at both localities. Veins of hornblendite are more common, however, in peridotite xenoliths from the Persányi Mountains in Transylvania (Vaselliet al. 1995, Downes & Vaselliin press).
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Peridotite xenoliths Large (up to 30-40 cm in diameter) peridotite
xenoliths representing fragments of the shallow lithospheric mantle are known to occur in basaltic rocks at some localities (Kapfenstein, Styria; Gérce (=Sitke), LHP;
Szigliget, Szentbékálla and Bondoróhegy, Balaton area;
Persányi Mountains, Transylvania). Small inclusions can be found in other areas, e.g., in the basalts of the Graz Basin and from Nógrád. Detailed studies of the frequency distribution of different texture types in these xenoliths revealed an interesting picture supporting the CPR mantle diapir concept (Embey-Isztin 1984, Kurat et al.
1991). Below the western external part of CPR (Kapfenstein) the mantle seems to be essentially untectonized (predominance of coarse-grained proto- granular textures), whereas in the internal part (Balaton area), where the diapiric uprise of the mantle reaches its maximum, both, protogranular and highly deformed (stressed), totally recrystallized, fine-grained equigranular peridotites frequently occur. However, in this region, a texturally composite peridotite xenolith (comprising both coarse-grained, untectonized and stressed fine-grained rocks in planar contact) indicates that the texture of the upper mantle rocks can be varied on a small scale too (Downes et al. 1992), similar to what has also been
observed on obducted upper mantle, like Zabargad Island (e.g. Kurat et al. 1993).
Another interesting feature revealed by the systematic study of mantle xenoliths is, that a relationship could be established between deformation and the chemistry of the rocks. Undeformed peridotites are chemically rather homogeneous, whereas the chemistry of deformed upper mantle rocks is more variable (Kurat
et al. 1991) and commonly metasomatically enriched in incompatible elements (e.g., Zabargad Island, Kurat et al. 1992). In addition, Embey-Isztin et al. (1989) and Downes et al. (1992) have shown that clinopyroxenes of protogranular peridotites frequently have a LREE-dep- leted (Light Rare Earth Element-depleted) pattern and MORB-like (Mid-Ocean Ridge Basalt-like) Sr and Nd iso
tope characteristics that may be the consequence of partial melting and extraction of basaltic liquid from the mantle several billion years ago. In contrast, xenoliths with complex deformed textures are generally enriched in LREE and other incompatible elements to varying degrees attesting to a post-depletion metasomatic en
richment event in the mantle. These xenoliths also show more variability in Sr, Nd, and Pb isotope compositions than do the protogranular peridotites (see below).
Major element chemistry of alkali basalts Major element, trace element, and isotope analyses
of alkali basalts were published elsewhere (Embey-Isztin
& SCHARBERT 1981, PANTÓ 1981, POULTIDIS & SCHARBERT 1986, Salters et al. 1988, Embey-Isztin et al. 1993a, Downes et al. in press, Dobosi et al. in press.) and most of the data have been compiled in Embey-Isztin et al.
1993b. The rocks are sodic (K20/Na20«1) and range in composition from olivine tholeiite to nephelinite, however, most common are alkali basalt and basanite (fig. 2). This broad compositional spectrum corresponds to CIPW- normative nepheline contents (ne) from 0 to 30% and
hypersthene (hy) contents between 0 and 10%, which is equivalent to a range of saturation index (S.l.) as defined by Fitton et al. (1991) between -60 and +5 (fig. 3). The S.l. quantifies the excess or deficiency of silica in such a way that saturated compositions with neither hypersthene nor nepheline in the norm have S.l.=0, whereas basalts with S.l.<0 are nepheline-normative and those with S.l.>0 are hypersthene-normative. Abundances of incompatible elements in alkali basalts vary with S.l. (fig. 3) and — naturally— with a variety of other parameters, like normative ne, mg# (=100 Mg/(Mg+Fe2+), and others.
Fig. 2. Alkali vs. silica abundances (wt%) in young CPR alkali basalts; classification after Cox et al. (1979)
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Olivine tholeiites are among the oldest lavas, whereas highly undersaturated basanites tend to be younger and the nephelinite of Stradner Kogel (STK) from the Graz Basin is one of the youngest rocks in the CPR. However, the relationship between age and degree of undersaturation is ambiguous, because one of the oldest lavas from Pauliberg (P), Burgenland, is a fairly silica- undersaturated basanite.
Of the major oxides that behave incompatibly during partial melting, Na2 0 shows good correlation between its abundance and the S.l. (fig. 3A). Much less well correlated are the abundances of K2O and P2O5 with S.l.
(figs. 3B, 3C) and no correlation exists for ÜO2 and S.l.
(fig. 3D). These relationships indicate that the chemistry of the lavas was determined not only by the degree of partial melting and fractional crystallization, but also by
the chemical and modal composition of source rocks.
This is evident in the case of the Burgenland basalts, which form a distinct high-Ti group coupled with elevated High Field Strength Elements (HFSE) concentrations (see below), a feature which cannot be explained by any reasonable partial melting model. The SÍO 2 content (40.3-50.8 wt%) decreases with decreasing S.l. (not shown), but the AI2O3 content is not correlated with S.l. (or ne), except for the Burgenland lavas. Once again, these lavas define a separate cluster very poor in AI and showing a slight decrease of AI with decreasing S.l. (fig. 3E). Total Fe contents (as Fe2 0 3: 8.9-13.6 wt%) do not change in a regular manner with S .I., whereas CaO abundances (7.7-13.6 wt%) slightly increase with increasing degree of undersaturation (not shown).
Fig. 3.
A . W e ic jh t-% N a 20 v s . S o lid ific a tio n In d e x (S .I., Fittone t a l. 1 9 9 1 ) in C P R b a s a lts S o lid tria n g le s : B u rg e n la n d , o p e n s q u a r e s : a ll o th e r b a s a lts ; B . W t% K 20 v s . S .l. in C P R b a s a lts ; C . W t% P 20 5 v s . S .l. in C P R b a s a lts ; D . W t% T i 0 2 v s . S .l. in C P R b a s a lts ; E. W t% A I2O3v s . S .l. in C P R b a s a lts
1 6 3
Trace element abundances in alkali basalts Primitive mantle-normalized trace element
abundances show a general increase of incompatible trace elements with decreasing S.l. or increasing ne.
Thus, the highest enrichment of incompatible elements was observed in nephelinites of the Graz Basin (figs. 4, 5). Normalized trace element patterns exhibit a more or less pronounced maximum for Nb with the exception of the lavas from Rákos and several other localities in the Persányi Mountains, Transylvania, that are the only examples to show a slight minimum at Nb. These latter rocks in addition have a strong positive anomaly in their Pb abundance, whereas most other lavas are deficient or only slightly enriched in this element. Compared to average OIB (Ocean Island Basalt), Persányi Mountains
basalts are enriched in Ba, Rb, K, and particularly Pb and depleted in other trace elements, especially in Nb.
These tendencies are also observable on a smaller scale in the Balaton region (western Pannonian Basin) and in Bánát basalts but are not evident in basalts from Nógrád, Burgenland, and Graz Basin (Embey-Isztin et al. 1993a, Downes et al., in press, Dobosi et al. in press.).
An important feature of the chondrite-normalized REE patterns (fig. 5) is the increase of the La/Yb ratio with increasing degree of undersaturation. Chondrite- normalized La contents (LaN) range from ~30 to -200 but YbN only from 4 to 7. Neither of these values is correlated with mg# (not shown).
Fig. 4. Mantle-normalized (Sun and McDonough 1989) incompatible element abundances in CPR basalts
Abbreviations: P: Pauliberg, Burgenlandi STK: Stradner Kogel, Graz Basin; TOR: Tormarét, Kabhegy, Balaton region; MED: Medves, Nógrád; RÁKOS: Persányi Mountains
Fig. 5. Chondrite-normalized (Sun & McDonough1989) REE abundances of CPR basalts. Symbols as in fig. 4 164
Isotopic composition of lavas The young Pannonian alkali basalts occupy an
intermediate position in the ranges known so far of Sr, Nd and Pb isotope ratios of basalts from around the world (Dupré & Allögre 1983, Salters et al. 1988, Embey- Isztin et al. 1993a, Downes et al. in press, Dobosi et al.
in press, figs. 6, 7), Radiogenic isotope abundances in alkali basaltic rocks from the CPR indicate an enriched upper mantle source and project into the range of OIB (fig. 6). However, some display a peculiarity in their Pb isotope composition by having unusually high ratios of 207Pb/204Pb for a given value of the 2°6P b r04Pb ratio.
Consequently, they plot above the Northern Hemisphere Reference Line (NHRL) which has been defined by Hart (1984) based on data for oceanic basalts (MORB + OIB) from the Northern Hemisphere (fig. 7).
Considering all alkali basaltic volcanic fields in the
CPR, there seems to be a similar systematic change in isotope ratios with geographic position as shown by the abundances of some trace elements. Lavas from the Nógrád region (northern border of the Pannonian Basin) show the lowest degree of enrichment (are compositionally closest to MORB) and, consequently, have the least radiogenic Nd and Sr isotopes (fig. 6) and a comparatively low 207Pb/204Pb ratio with respect to the 2°6Pb/2t>4pb ratjo (Salters et al. 1988, Embey-Isztin et al.
1993a, Dobosi et al. in press). Several other volcanoes, also situated in the border zone of the basin system, yielded basalt lavas that share more or less similar isotopic characteristics with Nógrád basalts (e.g., Graz Basin). The basalts of the Balaton region (western Pan
nonian Basin) and Persányi Mountains (eastern Transylvanian Basin) define a contrasting group,
Fig. 6. Sr and Nd isotope variation for CPR young alkali basalts. Data sources: Embey-Isztinet al. (1993a), Salterset al. (1988), Downeset al. (in press)
Fig. 7. Pb isotopes of CPR basalts. Data sources and symbols as in fig. 6
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characterized by more radiogenic Nd and Sr isotope ratios as compared to the former. Strontium isotope ratios can occasionally be more radiogenic than the Bulk Earth (BE) value, i.e. the isotopic composition * of the undifferentiated mantle (fig. 6). In addition, these rocks tend to have highly elevated 207Pb/204Pb ratios (fig. 7).
characterized by more radiogenic Nd and Sr isotope ratios as compared to the former. Strontium isotope ratios can occasionally be more radiogenic than the Bulk Earth (BE) value, i.e. the isotopic composition * of the undifferentiated mantle (fig. 6). In addition, these rocks tend to have highly elevated 207Pb/204Pb ratios (fig. 7).