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

Embey-Isztin, A.: Major element patterns in Hungarian basaltic rocks: an approach to determine their tectonic settings

The discriminant analysis and features of the differentiation trend strongly suggest a within-plate origin for the Mecsek volcanic suite. This suggestion is supported by the geological and sedimentological considerations proposed by VADÁSZ (1935) who claimed that the Mecsek area was a sinking continental platform at the time of the volcanic eruptions. Thus the existence of a rift-zone in late Jurassic and early Cretaceous times seems to be established. Whether or not this rifting passed into an oceanic stage of evolution remains a question to be solved. It must be mentioned however, that the present interpretation is at variance with the ideas proposed by SZEPESHÁZY (1977) who thinks that the Mecsek volcanic alkali basalt association is a member of an ophiolitic suite continuing to NE below the Great Hungarian Plain. The lack of mantle-derived ultrabasic rocks, pillow lavas, cherts and basalts with OFB-affinities in this zone does not seem however to strengthen his views. Finally, the results of the present work suggest an ocean-floor origin for the diabase gabbro complex of the Bükk Mountains. I should like, however, to emphasize that this suggestion does not necessary imply that the Bükk volcanic complex should be a "typical" ophiolite zone (e.g. ONUOHA 1977) and even less the existence of another typical ophiolitic zone buried in greater depth between Zagreb and the Bükk Mountains (e.g. SZEPESHÁZY 1977). It must be emphasized that mantle-derived ultrabasic rocks are completely lacking in the Bükk Mts., and though at some (very restricted) places pillow-like structures can be seen, the exact nature of this structure has never been investigated. Cherts on the top of diabases are also lacking and the relation of the igneous and sedimentary rocks is a typical intrusion with a marked contact metamorph aureole in the sedimentary member. The last feature seems to be lacking in the case of really typical ophiolite suites such as the Troodos Massif in Cyprus. These difficulties and available evidence on the nature of the igneous activities detected in the basement of the Pannonian Basin does not favour the hypothesis of two subparallel ophiolite zones (supposed to be in more or less original situation !) running through Hungary. On the other hand, it has been recently proposed that the Carpatho-Pannonian region was formed as a result of a collision of two microplates of European and African origin, respectively (GÉCZY 1973; CHANNELL & HORVÁTH 1976). Considering this hypothesis we can say that the Mecsek rift-volcanism is situated just at the border of the European microplate, thus it may be a witness of the beginning of rifting, that is, the individualization of this small portion of lithosphère as a microplate. The Bükk diabase-gabbro complex along with other basic (in Slovakia also ultrabasic) occurrences are situated on the African microplate along a major tectonic line, the Balaton — Darnó line which is separating the Austroalpine unit from the Southern Alpine-Dinaride unit. It seems thus, that tectonic emplacement of basaltic rocks with a possible ocean-floor origin or ultrabasic rocks probably of the same origin (both of them may perhaps represent desintegrated parts of an earlier ophiolite complex) took place along this tectonic line, at least in the northern part of the Pannonian region. It cannot be categorically excluded that this emplacement had some analogous features with the mechanism of true ophiolite forming processes, but available evidences strongly suggest however that it could not have certainly been a "typical" one. References CHANNELL, J E T & HORVÁTH, F. (1976): The African/Adriatic promontory as a paleographical premise for Alpine orogeny and plate movements in the Carpatho-Balkan region. — Tectonophysics, 35: 71-101. EMBEY-ISZTIN, A. (1976a): Amphinolite/lherzolite composite xenolith from Szigliget, north of the Lake Balaton, Hungary. — Earth Planet. Sei. Lett., 61: 297-304. EMBEY-ISZTIN, A. (1976b): Felsőköpeny eredetű lherzolitzárványok a magyarországi alkáli olivinbazaltos, bazanitos vulkanizmus kőzeteiben. — Földtani Közl. 106: 42-51.