Kaszab Zoltán (szerk.): A Magyar Természettudományi Múzeum évkönyve 66. (Budapest 1974)

Embey-Isztin, A.: Petrochemistry of the dike-rocks in the Velence Hills (Hungary)

that these rocks could not have originated from a simple residual melt. Potasuh metasomatism a useful concept in many such cases is hardly applicable here because lack of structural evidences. Though sericitization is a wide-spread phenomenon, yet sericite might have developed during autometamorphism, since it is restricted to the central parts of the dikes. Owing to the rapid cooling it could not have /in Fig. 6. Ab-Or-An diagram of the acid dike-rocks. The boundary curve is drawn with the assumption of plutonic conditions of crystallization. 1 = granite porphyry, 2 = replacement aplite, 3 = dilatation-injection aplite, 4 = Aplite from the Meissen massive (after PFEIFFER 1964). developed in the chilled borders. The increase in potassium varies over a wide range in the different aplite dikes (Table 5) In some aplites this ratio is recorded to vary between 41: 59 and 57: 43 (NOCKOLTJS 1947). The problem is, how to explain the unusual high concentration of potassium in these aplite dikes. If we exclude potash metasomatism as a possible cause, then some, as yet ill-understood magmatic processes such as fractional distillation or thermodiffusion have to be taken into consideration. Let us discuss now the differentiation process which may have controlled the generation of the granite porphyry dikes. As it was mentioned earlier these rocks are more basic than the Velence granite (though they contain less lime) with an increasing normative albite/orthoclase ratio (Table 6). Obviously granite porphyry represents here a later stage of crystallization filling the openings of the granite pluton at some time in its cooling history. In the normative Ab-Or-Q diagram (Fig. 7) it is seen that the Velence granite contains slightly