Kaszab Zoltán (szerk.): A Magyar Természettudományi Múzeum évkönyve 72. (Budapest 1980)

Örkényi-Bondor, L.: Andesite agglomerate from Zebegény village, Börzsöny Mountains (Hungary)

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 hornblendes 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

2

O

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

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0, K

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0, 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 supposed 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 parasitic 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 Alcontent 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 zoning 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-