Kaszab Zoltán (szerk.): A Magyar Természettudományi Múzeum évkönyve 69. (Budapest 1977)

Embey-Isztin, A.: The Szigliget amphibolitelherzolite compound xenolith as an evidence for diapiric uprise in the mantle below Hungary

ANNALES HISTORICO-NATURALES MUSEI NATIONALIS HUNGARICI Tomus 69. Budapest 1977. The Szigliget Amphibolite, lherzolite Compound Xenolith as an Evidence for Diapiric Uprise in the Mantle Below Hungary by A. EMBEY-ISZTTN, Budapest Abstract — On the basis of similarities between the Szigliget amphibolite/lherzolite compound xenolith and the lherzite veins of Etang de Lherz (Pyrenees, France) it has been argued that the origin of the amphibolite is connected with diapiric uprise of mantle material. Other evidences are also discussed. With 4 figures. BASCT (1975) has recently underlined the interesting fact that the peridotite-bearing alkali undersaturated basalts are constantly associated with Wilson-Morgan hot-spots, both in the ocean basins and on the continents. The Wilson-Morgan hypothesis of hotspots or eonvection plumes is very useful because it may explain the source of energy that drives the lithospheric plates. The hot-spots are surface representations of hot upwelling mantle material or mantle diapir. According to WILSON (1973) the hot-spots are characterized by gravity highs, and high heat flows, as well as by shallow seismicity. In the ocean basins they are associated with mid-oceanic ridges and on the continents they are near the rift valleys. BASU (loc. cit.) has given a list of such continental rift zones each of which is reputed by the presence of periodite inclusion-bearing alkali basalts. According to him these rift systems are as follows: 1. Ethiopia and eastern Africa, 2. Rhine Graben (GFR), 3. Lake Baikal (USSR), 4. Auvergne (Massif Central, France). I hold the view that the list is incomplete, for I think that it should be completed at least by another region of Europe: the Pannonian Basin in Hungary. The following facts strengthen the view that the Pannonian Basin may be similar to the rift systems of the world: a) Extensional tectonics in young Cenozoic times; b) Eruption of inclusion-bearing alkali undersaturated basaltic lavas and tuffs in the Upper Pliocene; c) Shallow seismicity and high heat flow; 4) Elevated position of the LVZ; e) Thin crust (24-26 km). In the following passages an account is given of a peculiar amphibolite/lherzolite composite xenolith which in the author's opinion may be considered as an evidence for mantle diapirism below the Pannonian Basin. The xenolith in question has been found in an outcrop of basaltic tuff near the village of Szigliget north of Lake Balaton. Here the tuff contains numerous nodules of CV-dipside, Cr-spinel lherzolite replaced to varying degrees by carbonate. Besides peridotites, clinopyroxene megacrysts have also been found. Petrography The amphibolite/lherzolite compound xenolith (Fig. 3) is slightly elongated in one direction and measures approximately 6 X 4.5 X 4 cm. It is incompletely covered by a thin layer of basalt. In the middle of the nodule there is a black vein, 2 cm in width, cutting the CV-diopside-spinel lherzolite. The black vein is a coarse-grained amphibolite and its contacts toward the lherzolite are sharp. The lherzolite is replaced to a high degree by a carbonate. However, the replacement must have been selective, affecting mostly olivine and having little influence on the other constituents. On the basis of the presence of large strained orthopyroxenes and interstitial spinel the texture may be called porphroclastic according to the classification of MERCIER & NICOLAS (1975). — The amphibolite vein is composed almost entirely of a coarse-grained amphibole aggregate. The grain size may attain 5 mm, but at the borders it is generally smaller. The texture is panxeno-