Budapest 1980 - Magyar Természettudományi Múzeum

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Budapest 1980 - Magyar Természettudományi Múzeum

ANNALES HISTORICO-NATURALES MUSEI NATIONALIS HUNGARICI

Tomus 72. Budapesté 1980.

Andésite Agglomerate from Zebegény Village,

Börzsöny Mountains (Hungary)

By L. ÖRKÉNYI-BONDOR, Budapest

Abstract — Andésite bombs are described from outcrops of Miocene andésite agglomerate at

Zebegény village. The An-content of plagioclase phenocrysts ranges between 75-93% depending

on zoning. The Albite-Carlsbad-Roc Tourné twin laws predominate, but the Börzsöny, Visegrád

and Bánát (Baveno) types combined make up more than 10%. Most of the amphiboles are

basaltic hornblendes. Pyroxene occurs in every sample collected at the Kerekdomb outcrop. The

studied rocks are characterized by uniformly high A1 20 3 content and by a narrow variation range

of Si0 2 content. The iron content is lower, the alkali content higher than in average andésites.

With 4 figures and 2 photoplates.

Introduction — The mineralogical-petrographical examination was carried out by using

a Leitz polarisation microscope and a five-axial U-stage. In the chemical laboratory of the Mineralogical-Petrographical

Department of the Hungarian Museum of Natural History 17 silicate analyses

have been made by DR. GY. PITTER. 19 minor and trace elements were determined by emission spectrography

in the Hungarian Geological Institute, for 12 samples. X-ray tests and photomicrographs

are the courtesy of the Mineralogical Department of L. Eötvös University.

Description of the rock samples

The Miocene andésite agglomerates of the Southern Börzsöny Mountains are well exposed in

natural outcrops at Zebegény village. Out of the 28 samples two originate from older lava flows.

The other 26 display crystallinity degrees indicating subvolcanic andésites. The samples have been

collected in two outcrops: I. Western slope of Kerekhegy (hegy =«= hill) II. „Vizesárok" valley at

Zebegény. The two groups of samples differ considerably.

I I. Western slope of Kerekhegy hill

Andésite bombs and lapillis embedded in tufaceous cement can be collected. The volcanic

bombs range from 2 to 50 cm in diameter; they are of different colour and hardness. The studied

28 samples may be divided into 11 types.

Type 1. — One single sample, sample 1 /71, rather massive, light-grey coloured, represents this

type. The groundmass is brown-spotted by limonite and other opaque substances. The plagioclase

phenocrysts vary largely in size. The zoned plagioclases contain numerous inclusions and much of

volcanic glass (brown-coloured). The average An-content deduced from measurements by the U-stage

is about 75-78 %. Albite-Carlsbad-Roc Tourné twinning is common, Börzsöny, and Visegrád twins

occur but rarely.

Further porphyric crystals are amphiboles and pyroxenes. The amphibole crystals (0.1-20 mm

in length) have almost completely been altered, and replaced by magnetite, pyroxene and plagioclase.

In the groundmass tridimite crystals of characteristic twinning are common, accompanied by limonite.

Type 2. — Reddish-grey, rather porous bombs of 10-15 cm diameter belong to this type.

(Samples 2/71, 7/71,8/71 and 1/75). There are some differences as regards the groundmass and secondary

mineral formation. The texture is microholocrystalline-porphyric. At some places the microlites

are very tiny. Most of the plagioclase phenocrystas are polysynthetically twinned according to the

Albite-Carlsbad-Roc Tourné laws; the Bánát type of Baveno, Börzsöny and Visegrád twins occur

up to 10%. The An-content varies in function of zoning. The lowest An-content measured was 73%.

The samples contain scarce amphibole crystals. These are relatively fresh, with only narrow opaque

corrosion rims.

Ann. Hist.-nat. Mus. Nat. Hung., 72, 1980

3 Természettudományi Múzeum Évkönyve 1980


Sample 2/71 contains many pyroxene phenocrysts. Most of these are hypersthene; some can not

be identified withouth further examination. In the same rock tridimite could be recognized in the

groundmass, accompanied by limonitic patches. Tridimite is absent in samples 7/71 and 8/71, but

there are (up to 10%) in the groundmass spherical, greenish-brown aggregates of clay minerals.

These consist of montmorillonite as testified to by the X-ray examinations.

In sample 11/75, neither tridimite, nor clay minerals could be observed. However, serpentine

deriving from the alteration of pyroxene could be recognized. In the same rock, the two generations

of plagioclase differ in twinning. The smaller crystals are polysynthetic twins, the larger ones often

single (un-twinned) crystals.

Type 3. — A greyish-red, rather massive rock (sample 3/71) represents this type. It is of microholocrystalline

texture, but the microlites are very small. Two generations of plagioclase phenocrysts

can be recognized. Both of them are finely zoned, with strongly fluctuating An-content. The smaller

crystals consists of numerous polysynthetic lamellás, while the larger ones are composed of two or

three twin members only. The common large, idiomorphic pyroxene phenocrysts are usually polysynthetic

twins. A small amount of amphibole and several quartz crystals are also present.

Type 4. — Samples 5/71, 13/71 and 7/75 belong to this group. These are rocks of light grey

colour; their texture is microholocrystalline. The phenocrysts are plagioclase, pyroxene and some

amphibole. Along with the Albite-Carlsbad-Roc Tourné twin laws in the first two samples there

are remarkable Bánát intergrowths, while in the last one Börzsöny and Visegrád twinning could be

recognized. (Beside the usual polysynthetic lamellae.) The numerous pyroxenes are fresh in sample

13/71, while in the two others the alteration of pyroxene to serpentine is much advanced.

Type 5. —-A yellowish-grey rock, rich in clay minerals, represents this type (sample 9/71).

The microholocrystalline groundmass consists of small microlites and clay minerals. Plagioclase and

pyroxene constitute the phenocrysts, while amphibole is absent. This rock differs from all the other

types in respect to both primary phenocrysts and ulterior alteration. It was in this sample that the

most acidic plagioclases have been encountered. The An-content range is narrow, 61-76%. The pyroxenes

include hypersthene and augite. Along with great amount of clay minerals (mostly montmorillonite)

and serpentine, some amorphous silica and cristobalite are also present. The plagioclase phenocrysts

and a considerable part of the groundmass has been altered to clay minerals. In some cases

only the external zones are intact. Serpentine developed on the rim and in the cleavages of pyroxene

crystals.

Type 6. — Sample 10/71 represents this type which occurs but rarely among the bombs and

lapillis. It is a greyish-red, hard, massive rock with 1 to 3 mm size amphibole crystals. The texture

is trachytic, sometimes fluidal. The porphyric crystals are plagioclase, amphibole and orthorhombic

pyroxene.

The plagioclase crystals are oriented. Plotted in Stereographic projection, the normals of the

faces (010) are located inside of a spherical small circle, 15° in radius. Most of the plagioclases contain

65 to 73 % An, depending on the zoning. Some hypidiomorphic plagioclase phenocrysts have

75-85% An. The Albite- Carlsbad- Roc Tourné laws are overwhelming; the Bánát, Börzsöny and

Visegrád twins total only to 2%. Pericline twinning could be suspected in the case of one single

grain.

Non-twinned plagioclase phenocrysts also can be observed, and such grains, in which 1-2 small

twin members are wedged in at some edge of the crystal. There are many inclusions of glass and

groundmass in the phenocrysts. Many broken feldspar grains can be observed in contrast to the

other rocks.

The amphibole crystals are fresh, in some cases the rim is slightly opacitized. On the basis of

their optical properties, they are oxyamphiboles. Small amounts of cristobalite and serpentine as well

as many opaque grains are characteristic of this rock. Most of the opaque minerals are haematite.

Type 7. — Only sample 11/71 belongs to this type. The microholocrystalline groundmass

consists of isometric plagioclase grains. Most of these are columnar and polysynthetically twinned.

The pyroxene and amphibole phenocrysts, however, differ from those of the other rocks. Their optical

properties suggest a composition near to ferrihastingsite. The pyroxene crystals have orthorhombic

symmetry and strong pleochroism. They can be identified as Fe-rich hypersthenes.

Type 8. — Sample 15:71 is a caked tuff with lapillis. This type of rock is common in the agglomarate

of Kerekhegy hill. Its examination hurts to a lot of difqculties. The groundmassa is microholocrystalline,

consisting of numerous tiny microlities, partly altered to clay minerals. The lapillis

are similar to other rock types, the cement, however, contains other minerals as well. The aspect and

composition of the polysynthetic plagioclase crystals, amphibole and pyroxene grains with opaque

cover do not differ from the average of similar rocks of the bombs. Biotite and garnet crystals, however

scarce, occur only in this type. Several generations of pyroxene and amphibole could be recog-


nized, but the possibility of drawing conclusions is rather restricted due to the agglomerate nature

of the rock. The opaque minerals are mostly limonite.

Type 9. — Sample 1/75 represents this type. This is a slate-grey, massive rock of microcrystalline

porphyric texture. The groundmass consists of tiny plagioclase laths and pyroxene crystals

smaller than 20 micron. Fluidal texture is common. The phenocrysts consist of twinned and untwinned

plagioclase crystals and pyroxenes, the latter occurring mostly in patches. The amphiboles are

completely altered, only the magnetite "dust" recalls the original shape of the amphibole crystals.

The orthorhombic pyroxene grains are strongly pleochroic in green and apricot colours. They may be

identified as hypersthenes. One part of the monoclinic pyroxene crystals is augite; a smaller portion

may be determined as Mg-rich clinohypersthene. Most of the opaque mineral constituents are limonite

spots. Several hexagonal cross-shaped quartz crystals occur, with numerous inclusions of apatite.

Type 10. — Most of the volcanic bombs belong to this group: samples 2/75, 3/75, 5/75, 8/75,.

12/75 and 13/75. They are light grey or reddish-grey coloured, porous rocks containing dark chain

silicates of about 1 mm in length.

Phenocrysts of varying size are embedded in microholocrystalline to cryptocrystalline groundmass.

The porphyric plagioclases are mostly isometric, finely zoned. Untwinned crystals or those

with only 1-2 narrow twin members are common. Some orientation is detectable as for the feldspars.

Inclusions of glass are common. The An-content varies largely with zoning. The Albite- Carlsbad-

Roc Tourné twin laws predominate, the Bánát type of Baveno, Börzsöny and Visegrád twins are

rare. All samples contain pyroxene phenocrysts in the range of 3 to 4.5%.

Amphibole is rare, mostly altered, only the dust of magnetite and other pseudomorphic minerals

are visible. (E. g. in the case of sample 4/75). In the same rock haematite and serpentine have

arisen secondarily. In sample 5/75 there are numerous haematite pseudomorphs after amphibole,

and the amount of cristobalite is remarkable. Amphibole generally does not form twin crystals, with

the exception of sample 8/75, where (100) and (210) twin laws occur. This rock is of semiholocrystalline

vitrophyric texture. In sample 9/75 the amphiboles and pyroxenes are strongly altered. The

sample 12/75 differs from the others by its pyroxene and amphibole crystals being fresh. Moreover,

the pyroxenes are richer in iron than those of the other rocks.

Type 11. — Sample 14/75 is the single representative of this geochemically highly interesting

type. It is an andésite of brownish-red colour and trachytic texture; the microlites lie among flowlines.

Most phenocrysts are plagioclase, but pyroxene also occurs. Shape, twin laws and An-content

of the plagioclase crystals are similar to those of the other volcanic bombs.

Originally, the rock had contained a lot of amphibole phenocrysts, but those became altered,

and only the decomposition products mark their previous location and shape. There are many pyroxene

crystals in the groundmass. These as well as the phenocrysts may be identified as Mg-rich orthopyroxene

and clinopyroxene. Among the pyroxenes of the groundmass a few augite grains occur, too.

They seem to have crystallized subsequently to the hypersthene. (enstatite and clinoenstatite). Numerous

opaque microlites are to be found in the groundmass; these are supposed to be responsible for

the minor element content, which is higher than usual.

II. „Vizesárok" valley

In the Vizesárok valley, rocks could be collected from small outcrops only, due to rather bad

exposure. These samples differ remarkably from those of Kerekhegy. Unfortunately, the geological

relationship between the two localities is unknown, the contact is not exposed.

Sample 1/69 is a silicified andésite. The original texture can not be determined; nor can the plagioclase

crystals optically measured by means of U-stage. On the contrary, alteration of the earlier

crystallized amphiboles is not common. The second-generation amphiboles are green coloured, fresh

crystals without domes or sphenoids, approximately 0.1 mm long. There are also some biotite crystals

with rutile inclusions. Beside the sagenite nets, apatite needles are common in the biotites. The

latter occur also in plagioclase grains. The ends of the orthorhombic pyroxenes have usually been

altered to serpentine.

Sample 3/69 is of vitrophyric texture. The phenocrysts embedded in the groundmass are tiny.

The mass consist of green and brown amphibole, some feldspar, and biotite. The phenocrystas are

smaller than those of the other rocks. This sample most likely is a cemented crystal tuff.

Sample 4/69 is of trachytic texture. The plagioclase laths are very small, almost microcrystals.

This rock includes fresh amphiboles, strongly altered plagioclases, orthorhombic pyroxenes,

clinoenstatite and augite. The orthorhombic pyroxene is often intergrown with augite.

Sample 6/69 is of microholocrystalline porphyric texture. Zoned, polysynthetically twinned

plagioclase crystals with many inclusions, and fresh amphibole constitute the phenocrysts. Haematite

dust is responsible for the red colour of the rock. This is the only sample in which a potash feldspar

has been identified, one single crystal, 2 mm in diameter.


Characterization of the minerals

Feldspar — Two different types of occurrence of plagioclase could be distinguished.

In rocks of trachytic texture there are plagioclase microlites in form of narrow laths; in the

others almost isometric grains are to be found.

This difference in shape indicates different rates of cooling. The fluidal nature is char­

acteristic for the trachytic texture of the effusive lava flow.

The porphyric plagioclases can be divided into two main groups.

1. Grains of oblong cross section, delimited by cleavage planes with re-entering angles due to

polysynthetic twinning. The twins contain less inclusions than usual. These crystals are elongated

according to axis ,,c"

2. Idiomorphic crystals, on which beside the (010) face also the (110) and (1Î0) faces are strongly

developed. The core of these crystals is of higher An-content. These grains are finely zoned and the

shape more varied than in the case of the previous group. In sample 2/75 could be observed an idiomorphic

plagioclase crystal the core of which had been slightly elongated according to the normal of

the (010) face; later the crystal changed into an almost isometric one. This means increased growth

in the direction of axis „c".

It is difficult to calculate the average An-content (which, in principle, would be important from

both the pétrographie and crystallographic points of view), because of its very large variation range.

One has to reconstruct the spatial shape of the grains from a statistical number of measurements made

in the plane, in order to calculate the volume from the width of the individual zones. This calculation

was performed as an example for two samples only, because in this particular case the calculated

average is of little utility in deciphering the genesis of the given rocks. The most basic twin member

contained 90-95 An, while the most acidic one only 57%. In all cases from the core upward a steady

decrease in the An-content was observed down to 60-72%. Then an inverted zoning followed, resulting

in an increase of the An-content by 6-8 %.

Such a wide range of data can be connected with rocks ranging from dacite to basalt.

Accordingly, in this particular area it is hardly possible to draw any conclusions from the

measurement of the An-content of the plagioclases. However, it would be an ulterior task

to search for the reason of this extreme variation.

In all samples predominate the Albite-Carlsbad-Roc Tourné twin laws. The Albite-Ala-

Albite-Ala triple twin law is very rare in spite of the fact that both twin law triples reward

the same crystallographic plane (namely the (010) face). Geometrically the probability of

occurence would be the same for both twin law triples. It is known from the literature, that

the first one is, however, much more common. In this particular case the second twin law

triple is extremely subordinate.

The Banat type intergrowth of the Baveno twin as well as the Börzsöny and Visegrád

twin laws occur in most of the samples in percentages varying from 4 to 11%. Several obser­

vations support the earlier theory forwarded by the present author and H. VINCZE in 1973

relating to the origin of twinned plagioclase crystals.

Fig. 1. shows the second measured plagioclase phenocryst of sample 1/71 as a fine

example of primary intergrowth. The core of the first twin member contains 75% anorthite.

The An-content decreases across the zones down to 60% An outward. Then the crystal got

intergrown with the second member. The core of the latter had 85% anorthite, while the

external zone of the single crystal only 62%. The shape of the first individual changed after

intergrowing. Also the shape of the first individual got changed. Beside the intergrowth

plane, the (201), and the (110) faces can be measured, delimiting the first single crystal. After

the intergrowth, a new face, (001) developed. Later, however, all these faces disappeared

and (010) and (001) faces bound the crystal. The (110) face can not be measured. The crystal

is probably hypidiomorphic. This is one, but presumably a not very frequent way to orig­

inate twinning.

There are also arguments in favour of the idea that the polysynthetic plagioclase crystals


Fig. 1. Plagioclase twins containing two different cores. — Fig. 2. Pyroxene phenocryst with an extinction

angle 6°

grew as single ones and secondarily became twinned. This way of twinning was assumed

after having determined the Börzsöny and Visegrád twin law. A rational plane of composition

being supposed, subsequent twinning is the only possible interpretation in the case of the

Börzsöny and Visegrád twinning. Namely, one member of both polysynthetic plagioclase

crystals which penetrated one another form Börzsöny and Visegrád twins. Between the other

members of the two groups no twin laws occur. Consequently, there is no rational compo­

sitional plane in spite of the penetrated twinning.

It is the same problem with the case of the Banat type of Baveno twin. It was intended

to find out empirically whether the polysynthetic twinning took place after crystallization,

when a higher symmetry changed to triclinic one. (ÖRKÉNYI & VINCZE). In fact, several

observations support the assumption of subsequent twinning. Single crystals can be observed

in some cases (at least, no twin members could be detected under the microscope). Other

grains manifest one or two narrow and short twin members at one edge of the crystal,

testifying to unstable symmetry. A few polysynthetic twin crystals without re-entering angles

could be observed, the original shape of the untwinned crystal being conserved. Even the

eventual presence of re-entering angles does not refute the working hypothesis, because it

may be due to subsequent twinning.

E. g., in sample 1/71 the third measured feldspar can be pointed out as subsequently

twinned. Another feldspar in the same sample seems to prove, by chance, this hypothesis

(Photomicrographs 5 and 6). The plagioclase grain was broken, one part of it toppled, but

the original connection is obvious. Re-fitted, the twin members would not be continuous

in the two parts. (Taking into consideration that the axis of toppling and the normal of the

thin section do not coincide.) This example proves beyond doubt that polysynthetic twin­

ning is posterior to the breaking up of the single crystal. (Given the fact that breaking up

must have been caused by some mechanical force, it is possible that this is a case of mechan­

ical twinning.) Microscoping observations of this type may support the hypothesis, but

they do not render experimental testing superfluous.


Mechanical twinning may have taken place in some cases, but it can not be common.

Most of the plagioclase crystals contained by the andésite rocks are twinned. In this case of

young volcanics, this general phenomenon can not be attributed to mechanical stresses

provoked by tectonism. The importance of the Börzsöny and Visegrád twin laws lies in the

circumstance that these penetrating twins gave rise to the idea that monoclinic plagioclase

structure may exist in nature. This is supposed to exist in the case of basic, or neutral pla­

gioclases in a very narrow range of high temperature. With cooling, it would turn unstable

and change into triclinic symmetry, by means of twinning. By producing experimentally

this hypothetic monoclinic phase, it would provide an exact thermometer for the genesis

of andésites which contain Börzsöny, Visegrád and Baveno-Bánát type twin laws.

It is hardly a mere chance, that all these have been described from andésites of the

Carpathian Basin. The plagioclase phenocrysts are but rarely altered. In some samples they

have been transformed into clay minerals ; in two samples numerous columns of zoizite can

be observed around the decayed feldspars. In one rock subsequent silcification made the

feldspar unrecognizable.

Amphibole — The amphibole crystals occurring in the samples from Kerekhegy

turned out to be basaltic hornblende and (subordinate) common hornblende, on the basis

of their optical characteristics. In one sample ferrihastingsit could be inferred. In all samples

of Vizesárok valley fresh grains of hornblende have been found.

In contrast to the other andésite agglomerate occurrences of the Börzsöny Mountains

the twin on (100) is represented very scarcely, in form of a narrow twin member in the middle

of the grain. Other types of intergrowth are more common; however, most of the grains are

single crystals. Three types of intergrowth have been observed which are not known in the

literature. These will be described in another paper in the journal Annales Universitatis

budapestinensis de R. Eötvös nom., sect. geol.

It is much more difficult to measure and to evaluate amphiboles by means of U-stage than it is

in the case of plagioclase phenocrysts. The margin of error is much larger in measuring the cleavage

planes, an average can be calculated from 15-20 measurements at least. Beside the good clevage

planes there are also others (not necessarily in the zone of axis „c") which make difficult to determine

the crystallographic orientation. The inner conic refraction disturbs considerably the measurement

of r\ß and of the optical axes. The own colour of amphibole modifies the interference colours.

Accordingly, a great many measurements on numerous thin sections are required for obtaining

statistically évaluable data sets. Because of the multiple variations in composition, such statistical

optical measurements are worth to be carried out only if accompanied by electron microprobe test.

In the amphibole crystals collected from Kerekhegy the angle between the ny and axis

"c" varies from 2° to 5° from 12° to 15°, while in the case of the samples collected in the

Vizesárok valley the same angle ranges 5-6°.

Fresh amphibole occurs exclusively in the Vizesárok samples (and in sample 10/71).

In the others the amphibole crystals are more or less altered. In some cases magnetite,

haematite, maghemite, small pyroxene and plagioclase crystals together make up pseudo-

morphs after amphibole.

Pyroxene — Pyroxenes occur in form of idiomorphic, hypidiomorphic or xenomor-

phic crystals. The pyroxene crystals richer in iron are longer columns than the others and

are mostly idiomorphic. The Mg-richer pyroxenes are often hypidiomorphic grains. In the

latter case the symmetry can be established only by means of U-stage measurements.

In the literature abound data obtained from microscopical examination without U-stage. In

order to check the reliability of such data, I have examined 15 pyroxene grains relying upon the symmetrical

extinction. Out of the 15 grains of oblique and asymmetrical extinction, 14 turned out during

the U-stage studies to be of orthorhombic symmetry. (One of them could not be evaluated due to

very unfavourable orientation.) Figs. 2 and 3 show one of these crystals. An opposite error is also


Fig. 3. The above pyroxene can be identified as an orthorhombic crystal

possible. A monoclinic pyroxene shows symmetrical extinction if the section is made in the zone of

axis „b". (In this case the mistake is due to the monoclinic mirror plane.)

The measured orthorhombic pyroxene crystals displayed regular orientation. Axis "a" coincides

with n^, axis "b" with with n œ, and the direction of axis "c" with n y. The angle of 2V in some cases

was less than 90° (68-90°).

The anomalous low values can not be interpreted in terms of chemical composition,

because 2Voc would have to be 90° independently of the Fe/Mg ratio. The latter can be de­

duced from the refraction indices. These were determined by U-stage applying the Nikitin

method. The Mg-rich clinohypersthene is mostly hypidiomorphic, while the augites are xeno-

morphic. This is in harmony with the presupposed sequence of crystallization. Beside the

refraction indices the optical orientation in thin section also prove to be useful in identifying

the monoclinic pyroxenes.

Chemical composition

The silica content varies between 51.83 and 57.90%. This range is narrow enough. Accordingly,

the Zebegény andésite agglomerates are rather uniform as regards acidity. These data are the nearest

to those of the andésites known in the Nagybörzsöny area. They resemble the olivinbronzite andésite

(the more acidic samples to a Japanese bronzite-andesite), if compared to the data published by

JOHANNSEN. In comparison with further Japanese andésites, they stand close to some olivine-pyroxene

andésites.

Accordingly the SiO, content of the Zebegény rocks is lower, than that of other andésites which


contain similar porphyric constituents. This fact is even more surprising, because in several samples

there are varieties oi free silica (tridimite, cristobalite, scarcely even quartz). The alvmina content is

uniformly high in every sample, ranging frcm 18.41 to 2C.84%. This corresponds well to the A1.;0 3

content of the amphibole andésite at Nagybörzsöny. The high alvmina values result by no means

from clayey decomposition products, because those samples which contain clay minerals do not have

higher alumina values. Compared with the analyses of Japanese andésites, among those the highest

alumina content is 16.64%.

The iron content is widely varying in the Zebegény rocks. Ferrous, ferric and even the total iron

content have rather large ranges. However, the iron content is in general lcv\er, than in the data found

in the literature for andésites (including also the Nagybörzsöny reck analyses). The degree of oxidation

is higher than in the case of andésite lava f ows or subvolcanic bodies. This may be due to

the pyroclastic origin (during explosion the erupted matter was submitted to the action of atmospheric

oxygène). In such less compact rocks even chemical weathering is more intensive than in subvolcanic

bodies, lava sheets or volcanic chimneys.

The MgO content also varies widely, from 1.24 to 3.78%, as a function of the percentage of

dark silicates, mostly pyroxenes. (With the exception of sample 5/71). In the literature one encounters

higher values of MgO. CaO content ranges 6.24-8.78%, in good accordance with the Japanese

and Nagybörzsöny andésites. The percentages of Na 20 are between 2.27 and 2.94 The Nagybörzsöny

data vary in a wider range. The K.,0 contents are rather high, from 1.02 up to 2.17 %. The Nagybörzsöny

andésites have similar potassium oxide contents. The Japanese data are, in general, much

lower.

As for binary correlations, the following can be stated: Ca and Mg show well marked co-variation,

which, however, is not proportional. The same can be said of K and Na. The alkalis vary in

opposite sense to calcium. (Except for samples 12/75 and 14/75.) Na+K is in negative correlation

with total iron. These relationships are most likely characteristics of the original magma, not due to

secondary alterations.

For illustration, the triangle diagram after H. KUNO was adopted. (MgO, total iron

expressed as FeO, Na+K). With the exception of two alkali-rich, Mg-poor samples, the

data are located within a small field. Compared with the data published by ISSHIKI on tho-

leitiic magma, the Zebegény rocks are much poorer in iron.

The chemical analyses corroborate the concept that the amphiboles occurring in the

Fig. 4. Total iron — MgO — Na 20+ K 20 diagram of andésite bombs from Zebegény


andésites of the Carpathian Basin would be even more interesting from the petrogenetical

point of view, than from the mineralogical one. JAKES and WHITE established that the horn­

blendes of the island arcs contain more alumine and alkali elements, and their Fe/Mg ratio

is lower, than those of the continental andésites. Now, also in the Zebegény samples the

alumina, alkali contents are high and the Fe/Mg ratios are low.

Confronting the chemical analyses with the microscopically established mineralogical composition,

the following can be inferred. The silica content scarcely varies in spite of the remarkable differences

in the mineral make-up. Even the occurrence of free silica in form of tridimite, cristobalite or

quartz does not increase the SiO, content. The alumina content is uniformly high independently

from the mineralogical composition. The Fe 2O a content shows no positive correlation with the amount

of porphyric minerals, and opaque minerals, because the bulk of the latter is finely dispersed dust

in the groundmass. The amount of 1 eO varies concomitantly with that of the pyroxene phenocrysts.

The highest ferrous iron value has been found for that sample in which pyroxene microlites abound in

the groundmass. The MgO content as well as the FeO content increase with the percentage of pyroxene.

Sample 5/75 is the unique case when high MgO is accompanied by low FeO value. Most likely

during alteration one part of the iron became oxidized while magnesium entered the serpentine.

The fluctuations on the CaO content show parallelism neither with the percentage of plagioclase

phenocrysts, nor with the amphibole crystals (usually subordinate). Covariation with Ca suggests

the presence of Ca-pyroxene, an assumption which could be checked by electron microprobe analysis.

No correlation could be found between Na 20, K 20, the sum of these two, and any of the phenocryst

types. The majority of Na must be found in plagioclase phenocrysts and microlites. The K issupposed

to be enriched in the groundmass.

Emission spectrographic shots were evaluated for 19 minor and trace elements.

Ti and Mn are enriched in sample 14/75, which has the highest content in ferrous iron. ZENTAI

was so kind to call the attention of the author to the high Co/Ni and V/Cr ratios as well as to the

uniformly high minor element content of the above-mentioned sample 14/75. The V/Cr ratios rangesfrom

24 to 100 (being 100 in 5 samples). In the case of sample 14/75 it is only 16, due to the extreme

enrichment of chromium.The Co/Ni ratio ranges 2,5-25, with the exception of the same sample where

it is 1, due to the highly increased Ni content. Cu, Zn, B, Mo and Yb are also increased in sample

14/75. The higher values of Ba and Sr in the same rock are in connection with the higher Ca-content.

The amount of Sn is very small in all samples; it may have been small in the original magma as well.

The Co/Ni ratios suggest the hydrothermal rather than the magmatic geophase.

Much more minor and trace element determinations would be required to clear up whether the

elements of the Nagybörzsöny mineralization are enriched, and if yes, in which form, in the Zebegény

volcanic bombs.

Conclusions

Kerekhegy probably is kind of an individual small volcanic area. The chimney is sup­

posed to be in the immediate neighbourhood. Its pyroclastic matter differs remarkably from

the andésite agglomerate known on Szentmihályhegy hill at Nagymaros and from the dacite

agglomerate at Szob. The kerekhegy hill rocks may be products of one of the numerous pa­

rasitic volcanic cones.

The alumina content of the original magma was most likely increased by assimilation

of clayes sediments. This must have occurred with the original magma, otherwise the Al-

content could not be so uniform. A plate tectonical interpretation is highly welcome.

The chemical analysis seem to prove that no considerable change occurred during the

later development of the magma. One or more secondary magma chamber can be assumed.

Presumably several explosions produced the agglomerate cone and the other agglomerate

materials in the area.

The sequence of crystallization differs from the Bowen sequence. The plagioclase zon­

ing is inverted as to the An-content. Inverted sequence can be observed in case of the dark

silicates as well: the amphibole crystallized earlier than the pyroxene. In the pyroxene ande-


42 L. ÖRKÉNYT-BONDOR

site a considerable amount of tridimite and cristobalite also developed. Beside the sequence

known from the literature (hornblende -* clinopyroxene -*• orthopyroxene) another also

could be established : hornblende orthopyroxene clinopyroxene.

The silica contents are nearer to those of the island arc andésites. As for the FeO f Fe 20 3/

MgO values, they correspond in 13 specimens with those of the continental andésites;

4 values coincide with those of the island arc andésites. Of the K 20/N 20 data 2 coincide

with the data of the island arc andésites; the others are in the common interval of both kinds

of andésite rocks.

Consequently, the chemism of the investigated rocks does not display the typical fea­

tures of the continental andésites. Farther-reaching conclusions would be beyond the scope

of the present paper.

Acknowledgements — Thanks are due to Prof. J. Kiss for the photomicrographs and for his

active interest in this work. L. BOGNÁR contributed with X-ray investigations, GY. PITTER with chemical

analyses, and P. ZENTAI with minor and trace element determinations. Their precious contributions

are heartily acknowledged.

References

BÖCKH, H. (1899-1902): Nagymaros környékének földtani viszonyai. — Földt. Int. Évk., 13: 1-57.

BURRI, C. (1963): Bemerkungen zur sog. „Banater Verwachsung" der Plagioklase. — Schweiz. Min.

Petr. Mitt., 43: 71-80.

BURRI, C. (1972): Zur Definition und Berechnung der optischen Orientierung von Plagioklasen. —

Schweiz. Min. Petr. Mitt., 52: 497-514.

BURRI, C. (1974b): Vereinfachte Berechnung der Euler-Winkel zur Charakterisierung der Plagioklasoptik.

— Schweiz. Min. Petr. Mitt, 54: 33-38.

BURRI, C, ÖRKÉNYI, B. L. & VINCZE, SZ. H. (1976): Rechnerische Auswertung von U-Tischoperationen

durch elementare Vektormethoden. — Schweiz. Min. Petr. Mitt., 56: 1-38.

BURRI, C, PARKER, R. L. & WENK, E. (1967): Die optische Orientierung der Plagioklase. — Basel-

Stuttgart, p. 1-334.

DEER, W. A., HOWIE, R. A. & ZUSSMAN, J. (1963): Rock Forming Minerals., 2. Chain Silicates. —

London, p. 1-379.

ISSHIKI, N. (1963): Petrology of Hachijo-Jima Volcano Group, Seven Izu Islands, Japan. — /. Fac

Sc. Univ. Tokyo, 15: 91-134.

JAKES, P. & WHITE, A. J. R. (1972): Hornblendes from calc-alkaline volcanic rocks of islands arcs

and continental margins. — Aer. Min., 57: 887-902.

KUBOVICS, I. & PANTÓ GY. (1970): Vulkanológiai vizsgálatok a Mátrában és a Börzsönyben. —

Budapest, p. 1-302.

NIKITIN, W. W. (1936): Die Fedorow-Methode. — Berlin, p. 1-527.

ÖRKÉNYI, B. L. (1976): Eulerian angles and the pseudosymmetry of the plagioclase. — Ann. Univ.

Budapest., 52: 41-54.

ÖRKÉNYI, B. L. & VINCZE, SZ. H. (1974): (110)(1I0) (130) (130) plagioclase twinning in andésite from

Hungary. — Acta Geol. Hung., 18: 99-135.

PANTÓ, GY. (1969): Textural, mineralogical and alteration characteristics of the Börzsöny Mountains

volcanic rocks. — Acta Geol. Hung., 13: 277-302.

STÄCHE, G. (1866): Die geologische Verhältnisse der Umgebungen von Waitzen in Ungarn. —• Jahrb.

k. k. Geol. Reichsanst., 16: 277-328.

SZABÓ, J. (1894): Típuskeveredések a dunai trachitcsoportban. — Földt. Közl, 24: 169-177.

Author's address: DR. LÍVIA ÖRKÉNYI-BONDOR

Mineralogical and Petrographical Department

Hungarian Natural History Museum

H- Budapest, pf. 330

Hungary


Table 1. Mineralogical compositions of rocks

1/71 2/71 5/71

Samples

8/71 9/71 10/71 11/71 13/71 15/71 4/75 14/75

Groundmass 38.4 32.2 35.4 27.3 36.8 48.9 52.5 34.8 40.2 33.2 53.9

Opaque 6.4 3.7 3.6 4.3 6.9 4.0 5.8 7.2 10.3 10.0 2.2

Plagioclase 35.6 40.9 33.7 40.3 29.9 25.9 27.4 45.7 33.2 37.5 27.3

Pyroxene 4.0 13.9 6.9 10.9 7.1 1.7 5.0 6.0 5.2 6.6 1.2

Amphibole — 2.6 2.5 0.5 — 12.9 4.2 1.2 4.4

11.6


Biotite —. — — — — — — — 3.0 — —

Tridymite 9.5 — 11.4 1.0

— — — —

— — •—

Quartz — — — — — 8.6

— — — —


Cristobalite — — — 3.0 1.7


— — — — 0.4

Serpentine — — 0.9 — 12.8 1.1 — — — — —

Clay-minerals — — —• 9.2 — —- — — — — •—

Hole 6.3 6.7 5.4 7.5 3.5 3.S 5.1 4.0 3.8 4.0 3.4

Total 100.2 100.0 100.1 100.0 100.0 100.0 100.0 99.9 100.1 99.9 100.0


44

L. ÖRKÉNYI-BONDOR

Table 2/1. Chemical compositions of rocks

Samples

Si0 2

Ti0 2

A1 20 3

Fe 20 3

FeO

MgO

MnO

CaO

Na 20

K 20

H 20 +

H 20-

P 2O 5

CO,

2/71 5/71 8/71 9/71 10/71 11/71 12/71 13/71 15/71

54.14 56.97 51.83 55.05 56.07 57.10 54.84 59.55 55.60

0.68 0.77 0.78 0.87 0.68 0.78 0.72 0.66 0.71

19.78 19.24 18.87 19.08 18.85 18.41 18.94 18.48 10.07

4.74 4.15 5.04 3.61 6.99 4.82 3.59 3.72 5.31

2.64 1.66 2.25 2.15 0.71 1.46 3.10 1.95 1.45

3.35 1.85 3.78 3.02 2.61 2.23 3.62 2.21 2.49

0.15 0.08 0.23 0.12 0.16 0.41 0.31 0.10 0.11

7.33 6.66 8.11 6.76 7.86 6.49 7 82 6.47 6.24

2.27 2.66 2.18 2.88 2.67 2.54 2.37 2.64 2.51

1.46 1.89 1.38 1.84 1.64 1.91 1.78 2.17 1.66

0.93 0.92 1.75 2.45 0.60 0.99 1.16 0.21 0.79

2.34 2.94 3.60 1.99 1.42 2.79 1.73 1.61 3.12

0.13

0.11

0.10


0.15

0.00

0.05


0.16


0.21


0.19


0.21

0.02

0.15

0.04

Total 100.05 99.89 99.95 99.87 100.42 100.14 100.17 100.00 100.28

Table 2/2. Chemical compositions of rocks

Samples

5/75 6/75 7/75 8/75 9/75 10/75 12/75 14/75

SiO. 55.22 57.47 58.99 57.90 59.67 52.78 57.11 54.09

Ti0 2

A1 20 3

Fe 20 3

FeO

MgO

MnO

CaO

Na 20

K 20

+

H 20

p 2o-

0.76

20.84

5.92

0.20

3.33

0.19

8.13

2.60

1.26

1.13

0.06

0.81

19.63

3.82

1.89

1.69

0.11

7.33

2.80

1.86

1.32

0.05

0.76

20.08

2.86

1.53

1.24

0.09

6.87

2.91

1.92

1.19

0.07

0.76

19.89

3.59

2.10

2.94

0.18

7.35

2.62

1.80

0.23

0.06

0.81

20.21

2.99

1.34

1.60

0.12

6.45

2.94

1.99

1.17

0.08

0.82

20.69

4.85

1.60

2.83

0.13

8.27

2.36

1.02

2.57

0.05

0.76

19.91

4.15

2.05

2.19

0.18

8.19

2.69

1.72

0.35

0.07

0.85

20.52

3.24

3.26

3.12

0.15

8.78

2.82

1.60

1.02

0.03

co 2 — — — — — — —


Total 100.40 99.88 99.85 100.28 100.32 100.00 100.12 100.33

Table 3. Samples in order of increasing compounds

Si0 2 FeO + Fe203/MgO K 20/Na0 2

8/71 1.9 0.64

10/75 2.3 0.7

14/75 2.1 0.63

2/71 2.2 0.64

12/71 1.8 0.61

9/71 1.9 0.75


5/75 1.8 0.75

15/71 2.8 0.82

10/71 3.0 0.66

1/75 3.1 0.48

11/71 2.8 0.66

12/75 2.8 0.66

6/75 3.4 0.69

8/75 1.9 0.68

7/75 3.5 0.43

13/71 2.6 0.64

9/75 2.7 0.57

Table 4/1. Minor and trace elements analyses

Samples Li Be B Ti V Cr Mn Co Ni ppm

1/71 10


Fig. 1. Plagioclase intergrowth according to Börzsöny law. Lower nicol only, x 80.

Fig. 2. Plagioclase intergrowth according to Börzsöny law. Crossed niçois, x 80.

Fig. 3. Plagioclase twinning after breaking. Lower nicol only, x 80.

Fig. 4. Plagioclase twinning after breaking. Crossed niçois, x 80.


Fig. 5. Plagioclase intergrowth according to Visegrád law. Crossed niçois, x 80

Fig. 6. Baveno twins so called Bánát type. Crossed niçois, x 80.

Fig. 7. Amphibole intergrowth. Lower nicol only, x 40.

Fig. 8. Amphibole intergrowth. Lower nicol only. x40.

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