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THE NEOGENE-QUATERNARY SUBALKALICAND ALKALIC BASALTOIDS OF THE ALPINEFOLD BELTV. A. Kononova a , Y. Yanev b , C. Zezyc c , V. Konecny d , S. Peltz e ,B. Nagy f & A. Mihalikova ga Institute of the Geology of Ore Deposits of the USSR Academy ofSciences , Moscowb Geological Institute of the Bulgarian Academy of Sciences , Sofia,Bulgariac Institute of Geology, Wroclaw University , Polandd Institute of Geology, Bratislava , Czechoslovakiae Institute of Geology and Geophysics, Bucharest , Rumaniaf Institute of Geology , Rumaniag Geological Survey of Hungary , RumaniaPublished online: 29 Jun 2010.

To cite this article: V. A. Kononova , Y. Yanev , C. Zezyc , V. Konecny , S. Peltz , B. Nagy & A.Mihalikova (1985) THE NEOGENE-QUATERNARY SUBALKALIC AND ALKALIC BASALTOIDS OF THE ALPINEFOLD BELT, International Geology Review, 27:5, 521-532, DOI: 10.1080/00206818509466439

To link to this article: http://dx.doi.org/10.1080/00206818509466439

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THE NEOGENE-QUATERNARY SUBALKALIC AND ALKALIC BASALTOIDS OF THE ALPINE FOLD BELT

V. A. Kononova, Y. Yanev, C. Zezyč, V. Konečny, S. Peltz, B. Nagy, and A. Mihalikova

Translated from "Neogen-chetvertichnyye subshchelochnyye i shcheloch-nyye bazal'toidy Al'piyskoy skladchatoy sistemy," Izvestiya AN SSSR, seriya geologicheskaya, 1985, No. 1, pp. 23-34. The authors are with the Institute of the Geology of Ore Deposits of the USSR Academy of Sciences, Moscow; the Geological Institute of the Bulgarian Academy of Sciences, Sofia, Bulgaria; the Institute of Geology, Wroclaw University, Poland; the Institute of Geology, Bratislava, Czechoslovakia; the Institute of Geology and Geophysics, Bucharest, Rumania; and the Institute of Geology and the Geological Survey of Hungary. This article, which was prepared as part of a comprehensive collaborative project of the Academies of Science of the "Socialist" countries, provides a summary of extensive recent studies of the latest phases of vulcanism in the Alpine Chain, particularly on its forelands.

Orogenic vulcanism (or, in H. Stille's termi­nology, subsequent vulcanism) was a char­acteristic feature of the orogenic stage of development of the Alpine fold belt in Eocene-Oligocene and in Miocene-Pliocene times. It occurred chiefly in the inner zones of the fold belt (the Carpathian "ring of fire") or in median massifs (Pannonian, Rhodope, Northern Macedonian, etc.) (Fig. 1). In com­position, the vulcanites of the orogenic stage are acidic, intermediate, and more rarely basic* rocks, with alkali contents placing tham in the normal and the subalkalic series (the latter being characteristic of the southern, or Mediterranean, part of the Alpine fold belt). In most of the area, orogenic vulcanism was succeeded by a new burst of vulcanism in Neogene-Quaternary time, both in the median massifs and consolidated folded structures of the Alpine belt and far beyond it on the bordering platforms. The products of this post-orogenic vulcanism are basic, or occa­sionally ultrabasic, rocks of the alkalic and subalkalic series, known in the literature

as "basaltic" (in Stille's terminology, final) vulcanism. A decade ago, Wimmenauer [22] published a survey of the vulcanism of this stage, which encompassed a belt about 1700 km long and 350 km wide, extending from the western Pyrenees across the Massif Cen­tral of France, the Rhine graben, Bohemia, and Silesia, focusing his attention on the northern margin of the Alpine belt.

This article synthesizes literature material and the authors' own work on the Neogene-Quaternary subalkalic basalts and alkalic basaltoids in the eastern and central parts of the Alpine system and at its southwestern end in North Africa (Fig. 1). The vulcanites in the various countries of this region are described by the individual authors as follows: Kononova in West Germany and France on the basis of her own observations, supple­mented by data from Wimmenauer [22] and Schmincke [9 ] , Yanev in Bulgaria and Morocco, Zezyc in Poland, Konečny, and Mihalikova in Slovakia, Peltz in Rumania, and Nagy in Hungary. Special attention is devoted to the distinctive features specific to the young basalts in the various geologic-tectonic structures. This leads, in the con­cluding part of this article, to a comparative

*Rock nomenclature follows the recommenda­tions of the Subcommittee on Terminology of the Petrographic Committee, USSR Academy of Sciences [31.

521 Copyright © 1985 by V. H. Winston & Sons, Inc. All rights reserved.

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V. A. KONONOVA ET AL.

FIGURE 2. SiO2 and Na2O + K2O contents (wt. %) in Neogene-Quaternary basalts of the Alpine fold belt and its foreland. Data for: 1) Bulgaria (lines: a) young basalts of Rhodope massif, b) young basalts of Balkanides, c) young basalts of Moesian platform); Poland: 3) Ru­mania (lines: d) vulcanites of Pleistocene age, e) vulcanites of Pliocene age); 4) Hungary: 5) Slovakia; 6) West Germany and France; 7) Morocco.

analysis of the distinctive features of young vulcanism in Neogene-Quaternary time in various kinds of tectonic structures (platform, rift, median massif, and epigeosynclinal or epiplatform folded regions). The composition of the volcanic products are compared by plots of the relationships among the chief rock-forming oxides (Figs. 2-5).

On the most general level, the Neogene-Quaternary basalts in the Alpine fold belt can be divided into two groups: those de­veloped mainly within the Alpine belt proper, and characterized by little or no differentia­tion, and those in the structures bordering the Alpine belt, in which undifferentiated vulcanites are associated with fully differen­tiated series (western Czechoslovakia, West Germany, France, and Morocco). The dif­ferent areas in which basalts are developed are briefly described below, beginning with

those of the first group, examples of which are known in Slovakia, Hungary, Rumania, and Bulgaria.

In central and southern Slovakia (the western Carpathian Mountains), products of young basaltic vulcanism are confined to the inner part of the Carpathian arc (Fig. 1, Areas 10 and 11). Central Slovakia contains several areas within which Neogene andesitic vulcanism developed. These include remnants of lava flows south of Zvolen and south of Krupina, two lava necks at Stavniče (K-Ar age 7 .1 -7.3 ± 0.4 Ma), and a cinder cone with lava flows near Banya (K-Ar age 0.3-0.16 Ma) [ 6 ] . One of the lava flows, lying on a terrace of the Gron River, has been dated biostrati-graphically as Upper Riss.

Large-scale manifestations of basaltic mag-matism are concentrated in southern Slovakia,

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FIGURE 3. Na2O and K2O contents (wt. %) in Neogene-Quaternary basalts of the Alpine fold belt and its foreland. For legend, see Fig. 2.

in the Lučenec-Filakovo area, and extend southward into northern Hungary. In this area effusive products dominate over ex­plosive products. Several cinder cones with lava flows, maars, and diatremes are present here. The maars are filled with lapilli tuffs (often palagonitized), cinders, and volcanic bombs. The diatremes are made up of tuff-breccias with many fragments of the under­lying rocks. The tuffs and breccias are roughly stratified and pierced by many basalt dikes. The activity of the volcanic centers near Gaj-necka has been dated by mammalian fauna as Early Pleistocene or Early Villafranchian [11] . The K-Ar dates [ 6 ] obtained on basalts of this region correspond to the Pontian (6.44 ± 0.27 Ma at Podrečani) and Early Pliocene (4.9 ± 0.24 Ma at Maškova) to Pleistocene (2.19 ± 0.16 Ma in Bulgaria, 2.58 ± 0.22 Ma at Gajnecka, and 1.19 ± 0.13 Ma at Rogac).

Judging by the SiO2 to Na2O + K2O ratios (Fig. 2), the vulcanites in Slovakia

are predominantly basic rocks—alkalic basal-toids and subalkalic olivine basalts of the potassic-sodic series (Fig. 3). The rocks are porphyrinic [18, 2 0 ] , with phenocrysts of olivine, augite, hornblende, rhönite, plagio-clase, nepheline, and magnetite. Their ground-mass is pilotaxitic-trachytic, microdoleritic, microdoleritic-phonolitic, or doleritic; it in­cludes olivine, plagioclase, pyroxene, pargasite, rhönite, magnetite, and volcanic glass. In places the basaltic rocks contain kaersutite megacrysts and inclusions of spinel lherzo-lites, presumably of mantle origin [ 13 ] .

In Hungary about 100 independent small eruptive centers made up of young basalts are known. They are concentrated in the southern part of the Transdanubian bills region (in the area of Lake Balaton) and in the middle part of the area known as the Little Hungarian Lowland. In addition, a multitude of small basaltic bodies are known in the Shalgotar'yan basin in northern Hungary.

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During the past few years isolated basalt outcrops have been discovered east of Mt. Mechek in the Danube River valley (near the village of Bar) and near Kishkunkhalash in the southern part of the Danube-Tisza watershed. It was previously thought [15] that these basalts were formed at the end of the Pliocene and beginning of the Quater­nary, but drilling during the past decade [14] and faunal dating have shown that they cor­respond to five stratigraphic levels: the first to the middle of the lower part of the Pan-nonian formation, the second through fourth to three stages in the Upper Pannonian, and the fifth to the Lower Pleistocene. According to K-Ar dating [6 ] , the ages of the Trans-danubian basalts range from 2.8 to 5.4 Ma. The youngest are those at Bar (2.4 Ma). The ages of each of the three stratigraphically different Upper Pannonian basalts could not be ascertained by the K-Ar method.

In chemical composition, these Hungarian vulcanites are alkalic and subalkalic picrites, trachybasalts, and alkalic basaltoids of the potassic-sodic, and to a lesser extent the potas-sic series (Figs. 2 and 3).

Detailed studies revealed no marked varia­tions of mineral composition. The groundmass often consists of volcanic glass. The pheno-crysts usually consist of plagioclase laths (25-50%), with small amounts of augite or another pyroxene (10-40%), olivine (0.5-20.0%) and an ore mineral (0.5-8.0%). II-menite, apatite, nepheline, hornblende, pico-tite, and biotite are sometimes present. They contain lherzolite xenoliths from greater depths. These are rounded, about 0.5-15.0 cm in diameter, and consist of olivine (Mg-rich and Ni-bearing) 72-63%, enstatite (with Al) 26-18%, diopside (with Cr) 1-15%, and spinel 1-7%. Besides the lherzolites, xenoliths of amphibolite, pyroxenite, and large (2-10 cm) xenocrysts of hornblende, hematite, titano-magnetite, and pyrrhotite are found.

Pliocene-Quaternary vulcanism also occurred extensively in Rumania. Two stages of volcanic activity can be discerned, each characterized by a specific composition and structural-geologic conditions of occurrence [19] . During

the Pliocene in Mts. Keliman and Metallifer (in a region believed to have been a subduc-tion zone in the Pliocene), small central-type volcanic structures, along with dikes and sills formed, piercing the Neogene molasse and the underlying Cretaceous sedimentary rocks. In composition, tnese bodies are calcic-alkalic and subalkalic olivine basalts and andesite-basalts, predominantly of the potassic-sodic series. They consist of phenocrysts of plagioclase (25-35%), hypersthene and augite (5-15%), olivine (2-7%), and magnetite (1-3%), with a groundmass (40-60%) of vol­canic glass and microlites of plagioclase, pyro­xene, and magnetite.

Lava flows and pyroclastic bodies of sub­alkalic basalts and alkalic basaltoids formed in Pleistocene time in the Lučarec area (Banat) in the northern part of the Pojana Ruske hills, and in the Peršani hills (the southern part of the eastern Carpathians). In the Banat region effusives predominate, particularly abun­dant along the north-south Lipova-Sumet fault, which played an essential part in form­ing the Pannonian depression. In the northern part of the Pojana Ruske hills the crystalline basement and overlying Cretaceous and Torton-ian sedimentary rocks are pierced by numerous small dikes, sills, and stocks concentrated along an east-west fault zone. Pleistocene vulcanism was most intensive in the inner part of the eastern Carpathians, around the Peršani hills. K-Ar dates indicate the effusive activity in the Peršani hills occurred at 1.39-1.23 Ma, and the last phase of volcanic ac­tivity at 0.5-3.0 Ma [12] . Volcanic products of Quaternary time are chiefly alkalic and subalkalic basic rocks, including olivine teph-rites, nepheline trachybasalts, subalkalic olivine basalts, and other rock types. They have a porphyritic structure with a microdoleritic to intersertal groundmass. They consist of phenocrysts of plagioclase (1-6%), augite (3-12%), olivine (3-12%), and a groundmass of glass with plagioclase, nepheline, augite, magnetite, and ilmenite microlites. Their chemical composition is characterized by dominance of sodium over potassium (Fig. 3). Comparison of the Pliocene and Pleistocene vulcanism (Figs. 2-5, Lines 2 and 1) clearly shows an increase in alkalies and a deficit

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FIGURE 4. AFM diagram for Neogene-Quaternary basalts of the Alpine fold belt and its foreland. A) Na2O + K2O; F) FeO + Fe2O3; M) MgO; for remainder of legend, see Fig. 2.

of silica in the younger lavas. It may be noted in passing that calcic-alkalic basalts of Pliocene age are also present in the Soviet part of the Carpathians [ 1 ] , but there are no data on younger alkalic basalts there.

In Bulgaria the young basalts [4] are con­fined to a north-south belt about 200 km long, which cuts across the Moesian platform, the Balkanides fold belt (Stara Planina and Sredna Gora zones), and the eastern Rhodope medium massif. In the Rhodope they gravitate toward structures formed by Priabonian to Oligocene intermediate and acidic orogenie vulcanism, and are sometimes regarded [2] as its con­cluding phase. The volume of these volcanic products is quite limited-they are mainly small subvolcanic bodies and dikes. Their precise age has not been established, but some of these bodies cut through the Eocene deposits in the Balkanide zone and Oligocene

vulcanites in the Rhodope Mountains. Paleo-magnetic studies [17] indicate that they are Pliocene, becoming somewhat younger toward the Moesian platform.

In composition (Fig. 2,1), these vulcanites include alkalic and subalkalic picrites (on the Moesian platform and in the Stara Planina and Sredna Gora zones of the Balkanides) and trachybasalts (on the Moesian platform and in the Sredna Gora zone). In the eastern Rhodope Mountains they are mainly alkalic basaltoids and more rarely trachybasalts. Chemical analyses show distinct differences among the basalts in the various structural zones of Bulgaria. Thus, for example, the basalts of the Rhodope massif are characterized by considerably higher potassium (Fig. 3) and lower iron and magnesium (Figs. 4 and 5) than the basaltoids in the Balkanide zone and on the Moesian platform. The Sredna

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FIGURE 5. Ratio of Al:Si:∑(Fe + Mg) in Neogene-Quaternary basalts of the Alpine fold belt and its foreland; for legend, see Fig. 2.

Gora zone and Rhodope massif are charac­terized by higher total alkalies, mainly potas­sium, along with elevated silica contents. The mineral makeup of these basalts also varies. The usual phenocrysts are olivine and augite, which are surrounded by reaction rims of amphibole in the Balkanide basaltoids and of biotite, plagioclase, and sanidine in those of the Rhodope massif. The ground-mass is not resolvable; more rarely it contains analcime (in the Balkanides), with microlites of plagioclase and pyroxene, and sometimes olivine, biotite, and amphibole. Holocrystal-line rocks of the dolerite type are also present.

On the northern borders of the Alpine fold belt, young basalts are known in Poland, but their largest volumes are found in West Germany and France, associated with the Central European rift system.

Within Polish territory, basaltic rocks of the Alpine cycle are known in the Silesian region (Fig. 1, Area 9). Most of these are concentrated in the Sudeten Mountains and

the adjoining Cis-Sudeten block-Hercynian structures whose basement is made of Pre-cambrian and Paleozoic metamorphic rocks. In Cenozoic time this region, located within the continental margin of the Alpine fold belt, was covered by continental platform deposits and was one of the centers of de­velopment of young basaltic vulcanism. Necks, dikes, and sills of weakly differentiated alkalic potassic-sodic picrites, tephrites, and sub-alkalic olivine basalts have been preserved (Figs. 2 and 3,2). Phonolites sometimes occur. These young basalts usually lie within meta­morphic rock complexes, but they sometimes cut through the Epihercynian and Neogene deposits of the sedimentary platform cover. The basalts are dated within a fairly wide time interval—Oligocene to Pleistocene; some occurrences have been dated 15-28 Ma [8 ] . A distinctive feature of the composition of the young basalts in Silesia is their slight differentiation, so that they occupy a very limited area on all the plots (Figs. 2-5). An­other feature is their definite enrichment in calcium and, of the alkalies, in sodium.

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Augite and olivine (sometimes as phenocrysts) are always present, as well as plagioclase, nepheline, Ti-Fe-oxide minerals, and apatite; sometimes volcanic glass is preserved.

Within the northern continental margin of the Alpine fold belt (besides Silesia), major centers of Cenozoic vulcanism are confined to Tertiary grabens and graben-like depres­sions between elevated Hercynian blocks: the Limoges and Faurez grabens on the Massif Central of France, the Rhine graben between the Vosges Mountains and the Schwarzwald (Black Forest), the Hessen depression, and the northern Bohemian massif. Much of the volcanic material was extruded on elevated blocks adjoining the grabens.

Alkalic rocks, with subordinate subalkalic olivine basalts, dominate in the Neogene-Quaternary magmatic provinces of France and West Germany. The most common associa­tions are: subalkalic olivine basalt—tephrite— foyaite, and subalkalic olivine basalt—trachy-andesite-trachyte. Such silica-undersaturated rocks as olivine tephrites (basanites), lim-burgites, olivine nephelinites, and melilite ankaratrites are likewise quite frequent. Simi­larly abundant are the so-called incomplete associations—those without intermediate mem­bers. In these cases the volcanic products have a bimodal composition, like the melilite ankaratrites and phonolites in the Hegau area. The Miocene volcano at Kaiserstuhl consists predominantly of tephrites and their subvolcanic analogs, essexites, phonolites, as well as some carbonatites; limburgites and olivine melanephelinites are known only on the periphery of this volcano. Leucite-bearing rocks have also been found in some areas. The rocks in the Central European rifts, as plotted from the literature data (Figs. 2-5) [9, 22], occupy very broad areas, as one would expect such strongly differentiated rock associations to do. It is worth noting that on the AFM triangle (Fig. 4) the vul­canites occupy two areas, evidently because intermediate rocks (in total alkali content) are rare.

Differentiated series of alkalic basaltoids are also known in western Czechoslovakia

[16]. A number of hypotheses attempt to interpret the different alkalic rock associa­tions in Central Europe and France, but the principal mechanism of their formation is considered to be crystal differentiation, and their primary initial melt to have been an alkalic olivine-basalt magma or a magma ap­proaching theralite in composition.

On the southwestern margin of the Alpine fold belt, in Morocco [21 ], young basalts form a discontinuous northeast-trending belt 700 km long and about 150-200 km wide (Fig. 2, areas 1-5). Their continuation can be traced both in Algeria and farther south, on the Medi­terranean Sea floor (the Alboran basin). In Morocco, these vulcanites are exposed within the southern branch of the Alpine fold belt (the eastern part of the Rif Mountains), where they succeed the orogenic calcic-alkalic and subalkalic intermediate and acidic volcanics in the Middle Miocene to Pliocene. But the chief volumes of Cenozoic basalts are in the foreland of the Alpine belt-the epiplatformal Upper Alpine fold-mountain structure (the Middle Atlas Range), the peripheral massifs (the Meseta ranges of Morocco and Oran), and the periphery of the Sahara platform (the Anti-Atlas Range).

Manifestations of vulcanism here are varied-stocks, flows extruded from small lava and cinder cones, maars, necks, and more rarely subvolcanic bodies and dikes. The youngest occurrences of basaltic vulcanism have been found in the Middle Atlas karst plateau (the Moroccan Meseta), where at least 100 volcanos have been counted and the lava flows reach 130 km in length. The vulcanism becomes progressively younger across the strike of the fold belt toward the platform. In the Rif Mountains, according to the K-Ar age of the vulcanites, which ranges from 4.6 to 7.4 Ma [7], it began at the onset of the Plio­cene, whereas in the northern part of the Middle Atlas volcanic activity began only at 4.8-4.9 Ma and came to an end in Quater­nary time. In the northern part of the Middle Atlas plateau the age of the vulcanites is 2.0 Ma, but in its southern part from 1.1 to 1.8 Ma.

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The vulcanites vary fairly widely in com­position, usually in their contents of silica and alkalies, in places forming differentiated series (Fig. 2). In the Rif and the northern part of the Middle Atlas ranges the predomi­nant rocks are trachybasalts with pheno-crysts of olivine, augite with titanaugite rims, more rarely biotite, amphibole, and labradorite, and very rarely analcime. The groundmass is usually not resolvable or is made up of analcime with microlites of the same minerals. There are also alkalic picrites and alkalic ultrabasic vulcanites of varying composition (in the central part of the Oran Meseta, the Central Moroccan massif, the Middle Atlas plateau in the Moroccan Meseta, and the Anti-Atlas Range). These rocks consist of phenocrysts of olivine, titanaugite, and some­times sodalite, nephelite, hauyne, aegirine or aegirine-augite, biotite, and amphibole; the groundmass is rich in nepheline or analcime, with microlites of the above minerals plus carbonate, perovskite, melanite, plagioclase, and melilite. The alkalic basaltoids (in the Central Moroccan massif and the eastern part of the Anti-Atlas Range) contain pheno­crysts of titanaugite, a little olivine, biotite, amphibole, and sometimes hauyne and nosean in a nepheline or analcime ground-mass with microlites of augite, nepheline, and analcime. The associated phonolites (in the Central Moroccan massif and the Oran Meseta) consist of sanidine, nepheline, aegirine, and sometimes hauyne and nosean phenocrysts, with microlites of the same minerals, and oc­casionally also analcime and leucite in the groundmass.

Judging by the alkali contents, potassic-sodic rocks predominate. Only in the Rif Mountains are there potassic rocks (absarok-ites), whereas sodic varieties are known in the Meseta of Oran. Increasing silica is accompanied by increasing Fe/Mg ratios (except in Central Morocco and the Oran Meseta). The increase in alkali content is due mainly to potassium (except in Central Morocco and the Anti-Atlas Range). Where the basaltoids follow orogenic vulcanism (in the Rif and northern part of the Middle Atlas ranges), they are chemically the natural continuation of this vulcanism [21] .

Conclusions

The above survey of the Neogene-Quaternary (post-orogenic) basaltic vulcanism in the regions studied by the writers shows its ex­tensive occurrence within the Alpine fold belt and its foreland. The young basaltic vulcanism is a prominent stage, in the later geologic history of Europe, characterized by the following features:

1. There is a defintie tendency toward progressively younger vulcanism southward. Moreover, on the neighboring platforms vol­canic activity was renewed more than once in Neogene-Quaternary time. On the northern foreland the peak of such activity was in the Miocene (Silesia and West Germany), synchronous with the main phase of defor­mation in the fold belt. In the Alpine belt proper, basalt eruptions were most frequent in the median massifs at the end of the Plio­cene and in the Pleistocene. In the southern branch of this system (Morocco) the renewal of vulcanism also continued southward—that is, toward the neighboring platform.

2. The composition of the Neogene-Quaternary postorogenic volcanics, although it shows marked variations, especially in the rift structures, clearly displays a maximum on the SiO2/Na2O + K2O in the region of alkalic and subalkalic basic and ultrabasic rocks. Another feature worth noting is the constant Si:Al:Fe + Mg-ratio (Fig. 5). It may be suggested that the rocks in these regions are close in composition to the initial melts from which the young basalts of the Alpine system formed.

3. The variations in composition of the young basalts are clearly due to a combina­tion of many causes, including changes in the geodynamic regimes of the various parts of the Alpine fold belt. The schematic pro­files (Fig. 6) show that within the regions studied, potassium, sodium, and silica con­tents progressively decrease from the folded zones toward the platforms. A silica deficit is a characteristic feature of magmatic rocks on platforms. As for the contents of the al­kalies, along with provinces represented by

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FIG

URE

6.

Late

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V. A. KONONOVA ET AL.

highly alkalic rocks (in rifts), moderately alkalic rocks can also be formed on plat­forms. The sharp increase in potassium in the rocks of the Rhodope median massif is a provincial feature of the southern, or Mediterranean, part of the Alpine system. Elevated potassium contents are also typical of the earlier stages of magmatism in this part of the region [4 ] .

It seems likely that the east-west chain of areas of primarily rift vulcanism in Central Europe arose in a regime of decompression synchronous with the uplift of the folded zone [9 ] . This situation was clearly favorable for the melting out and crystallization of alkalic basaltoids in the forelands of the fold belt (West Germany, France, and Morocco).

4. Some specific chemical features com­mon to the young basalts in the various parts of the Alpine fold belt may be noted:

a) increasing SiO2 is usually accompanied by increasing alkali contents (Fig. 2). But in Silesia, partly in Morocco (in the Rif and the Oran Meseta ranges), and in Bulgaria (on the Moesian platform), alkalies are relatively constant, with some variation in amounts of silica;

b) a direct correlation between sodium and potassium is typical of most regions (Fig. 3). But in the Rif and the eastern Rhodope mountains the correlation is inverse, and in a number of other regions (Silesia, the Moesian platform, and the Balkanides) the potassium content remains more or less constant—that is, the increase in alkalies is due to increasing sodium contents;

c) in some regions (Silesia, the Sredna Gora of Bulgaria, the Meseta in Oran, and partly in Rumania and Slovakia) there is a tendency for the more acidic volcanic pro­ducts to be enriched in iron at the expense of magnesium (Fig. 4);

d) the young basalts of the foreland, like the Quaternary vulcanites of Rumania, are characterized by more basic composition,

with lower silica (Fig. 2) and higher total iron and magnesium oxides (Fig. 5).

It should be stressed that, in spite of such compositional variations, the young basalts of each region differ markedly from the basaltic volcanics of the preceding orogenic phase of the Alpine fold belt.

References

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2. Ivanov, R., 1963, Magmatism in the Eastern Rhodope basin: II—Petrochemical development and provincial features: Tru-dove v"rkhu. Bolg. Ser. geokhim. mineral. i petrogr., Vol. 4, pp. 297-323 (in Bulgarian).

3. Klassifikatsiya i nomenklatura magmati-cheskikh porod (The Classification and Nomenclature of Magmatic Rocks), 1981: Nedra Press, Moscow.

4. Bogdanova, B. and Dimitrova, Yel., Eds., 1983, Magmatizm i metallogeniya Karpato-Balkanskoy obhsti (Magmatism and Metal-logeny of the Carpat ho-Balkan Region): Izdatel'stvo Bolg. AN, Sofia.

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8. Birkenmajer, K., Telenska, M., Kadzioko-Hofmake, M., and Kruczyc, J., 1977, The age of deep-seated fracture zones in Lower Silesia (Poland), based on K-Ar and paleomagnetic dating of Tertiary basalts: Rocz. Pol. tow. geol., zes. 4, Vol. 4, pp. 545-552.

9. Duba, A. and Schmincke, H.-U., 1978, Quaternary basanites, melilite nephelinites,

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and tephrites from the Laacher See area (Germany): Neues Jahrbuch fur Min-eralogie, Abhandlungen, 132, No. 1, p p . 1 - 3 3 .

10. Embey, I. A., 1976, Lherzolite nodules of upper mantle origin in the alkalic olivine basanitic rocks of Hungary: Földtani Körlöny, Vol. 106, pp. 42-51 .

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12. Ghenea, C , Bandrabur, T., Mihaila, N., Radulescu, C , Samson, P., and Radan, S., 1981, Pliocene and Pleistocene de­posits in the Brasov depression: Guide­book for the Field Excursion of SEQS-INQUA: Bucharest.

13. Hovorka, D., 1978, Uzavreniny spinelo-vych peridotitov v bazanite pri Mackove: J. Min. Slovaca, Vol. 10, No. 2, pp. 97 -111.

14. Fambor, A. and Solti, G., 1975, Geological conditions of the Upper Pannonian oil shale deposit recovered in the Balaton Highland and at Kameneshat: Acta Min-eral.-Petrogr., Vol. 22, No. 1, pp. 9-28.

15. Jugovies, L., 1976, Chemical features of the basalts in Hungary: (Magyar-Allami-Foldtani Intezet). Evijelentese 1974-evröl., pp. 431-470.

16. Kopecký, L., Pisova, J., and Pokorny, L.,

1967, Pyrope-bearing diatremes of the Ceske Stredohori Mountains: Sbornik Geol. Ved. Rada G, Vol. 12, pp. 8 1 -130.

17. Mavroudchiev, B. D., Moskovski, S. N., and Nozharov, P. B., 1971, On the origin and evolution of the Plio-Pleistocene basalt magmatites in Bulgaria: C. R. Acad. Bulg. Sci., Vol. 24, No. 12, pp. 1683-1686.

18. Mihalikova, A., 1966, Petrographische und petrochemische Charakteristik der Basalte der Südostslowakei: Sbornik Geol. Vied ZK, Vol. 3, pp. 151-190, Bratislava.

19. Peltz, S., Vasiliu, C , and Bratosin, I., The petrology of the Pliocene and Quaternary basaltic rocks from Romania: Ann. Inst. Geol. Rom., Bucuresti, Vol. 39, pp. 116— 176.

20. Simova, M., 1965, Die Petrographie und Petrochemie der Produkte des finalen Vulkanismus in dem Slowakischen Mit-telgebirge: Acta Geol. et Geograph. Univer-sitatis Comenianae, Geol., No. 9, pp. 9 -89, Bratislava.

21 . Yanev, Y., 1976, Charactère pétrochimi-que et zonalité du volcanisme neogéne-quaternaire du Maroc: Mines et Géol. Rabat, No. 40, pp. 19-44.

22. Wimmenauer, W., 1974, The alkaline province of Central Europe and France. In Alkaline Rocks (pp. 238-271).

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