Kaszab Zoltán (szerk.): A Magyar Természettudományi Múzeum évkönyve 76. (Budapest 1984)

Embey-Isztin, A.: Texture types and their relative frequencies in ultramafic and mafic xenoliths from Hungarian alkali basaltic rocks

ANNALES HIS fORICO-NA LU RALES MUSEI NATION ALI S HUNGARICI Tomus 76. Budapest, 1984 p. 27-42. Texture types and their relative frequencies in ultramafic and mafic xenoliths from Hungarian alkali basaltic rocks by A. EMBEY-ISZTIN, Budapest Abstract — Estimation of strain rate of spinel lherzolite and harzburgite xenoliths originating from four volcanic vents of the 5-2.5 m.y. old basaltic province of the Transdanubia, Hungary is competible with the presence of mantle diapirs beneath this region. Most textures are indicative of steady state flow but some of movements in shear zones of the mantle. There are evidences for a positive correlation between the grade of recrystallization in the protogranular-porphyroclastic range of the tectonic cycle and the depletion in basaltic constituents, in contrast the highly recrystallized equigranular xenoliths seem to be enriched metasomatically in incompatible elements. Examinations of a high number of xenoliths originating from a single volcanic vent may reveal that contrary to earlier beliefs, their textures should not belong to a continuous tectonic series. Untectonized spinel lherzolite and harzburgite xenoliths of probable magmatic origin are frequent only at one of the four localities. The presence of composite xenoliths among the nodules prove that the upper mantle was invaded by basaltic liquids causing mineral and chemical inhomogeneities. Pyroxenite inclusions have probably cumulus origin and the gabbroic ones show igneous but also metamorphic features. With 5 figures and 3 photoplates. Ultramafic and mafic inclusions frequently found in alkali basaltic rocks and kimberlites are widely interpreted as carriers of a wealth of useful information about the properties of the upper mantle and in some cases of the lower crust. Much of this information originates from the study of texture and structure of the nodules. Magmatic versus metamoprhic processes may be ascertained or ruled out, the possibility of existence of floored magma chambers with gravitative accumulation of liquidus phases may be considered, earlier high T, P evolutionary stages and older mineral assemblages can be reconstructed, evidences for steady state flow and major tectonic movements in the mantle on the basis of strain rate analyses can be evaluated to mention but a few of the possibilities. The relative abundances of the different xenolith types including composite ones found at five different Hungarian localities can be studied in Table 1. It is evident from the compilation that Cr-diopside spinel lherzolites and harzburgites are by far the most common types of inclusions (89% of the total), followed by gabbroic xenoliths (7%), black pyroxene group (Al, Ti augite) pyroxenites (3%) and composite nodules (1%). These numbers are based on a population of 350 sectioned xenoliths (excluding megacrysts). Though exceptions may exist, the high frequency of green pyroxene (Cr-diopside) rich suite nodules compared to the rarer occurrence of the black pyroxene (Al-Ti augite) rich suite xenoliths seems to be the rule both in continental and oceanic terrains (e.g. WHITE 1966; DICKEY 1968; CARTER 1970). While the first group of xenoliths is widely interpreted as more or less depleted fragments from the Earth' upper mantle (Ross et al. 1954; WHITE 1966; BEST 1974; VARNE 1977 and many others), the second one sheds light on the nature and extent of the lithological inhomogeneities in the upper mantle. Also, it was frequently noted that the common Crdiopsidc rich group xenoliths had experienced a long metamorphic history (AVE LALLEMENT & CARTER 1970; DEN TEX 1969; BOULLIER & NICOLAS 1973; BOULLIER 1975; ERNST 1935; COLLÉE 1963 and MERCIER & NICOLAS 1975), in contrast members of the black pyroxene, Al, Ti augite-rich suite xenoliths show slighter signs of deformation if any. Textures indicating direct crystallization from a melt such as cumulus texture are on the contrary not rarely reported from this kind of nodules (e.g. JACKSON 1971; IRVING 1974; BOIVIN 1982).