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

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

It has a good outcrop east of the village Pátka and in the Ráchegy quarry. In the second type the matrix diminishes and it is rather micrographie than fine grained. This type of granite porphyry occurs in a big quarry near the village Sukoró. Among the aplites the dilatation-injection type is characterized by a porphyritic texture, chilled borders, and high temperature minerals, whereas the replacement type has allotriomorphic-granular texture, no chilled borders and it shows numerous signs of replacement phenomena such as graphic structure, relics of the country rock and reaction rims. Evidences as to the replacement of feldspar by quartz in the graphic intergrowth are given by EMBEY-ISZTIN (1973). As a matter of fact, there is a third type of aplite as well, wich occurs at the border zones of some granite porhyry dikes. In spite of the fact that the chemical composition of the aplite types differs but slightly, marked distinctions could have been established by the aid of petrochemical calculations. The same also applies to the granite porphyries. Fine-grained aplitic border zone of some granite porphyry dikes The origin of aplitic borders in granite porphyry dikes of the Velence Hills has not been satisfactorily explained. INKEY (1875) and JANTSKY (1957) regard them as chilled borders, whereas VENDL (1914) thinks that we are concerned here with the so-called composite dikes. Since there is no sharp boundary between the coarser central part and fine-grained borders, the latter explanation have to be rejected. However, considerable differentiation processes must have also taken place judging from the variance in chemical composition (Fig. 2). The fine grained aplitic border have become more acid, the values of si, al, alk, increase, whereas those of fm and c decrease passing from the center to the border. So it is obvious that some differentiation process has played an important role in the genesis of aplitic borders. According to the rules of crystallization, differentiation established by BOWEN (1956) we might rather expect more basic borders, just the contrary what the case is. WHAL'S hypothesis (1946), however, accounts for such cases too. After this, thermal diffusion combined with convection may cause havier ions to migrate towards the chilled borders, in other cases the lighter ones accumulate in the colder Fig. 2. Differentiation in a granite porphyry dike (Village Pátka).