Petrochemistry of the Dike-Rocks in the Velence Hills (Hungary)

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1 ANNALES HISTORICO-NATURALES MUSEI NATIONALIS HUNGARICI Tomus 66. Budapest Petrochemistry of the Dike-Rocks in the Velence Hills (Hungary) By A. EMBEY-ISZTIN, Budapest Abstract The established constant chemical differences, especially the variance of K 2 0 have strengthened the view that the fine grained and porphyritic aplites are products of two different genetical acts. The granite porphyries represent a more sodic and basic differentiate wich is supposed to have developed by early crystallization of potassium-rich feldspar and subsequent crystallization of sodium- -rich feldspar along with mafic minerals in the granitic magma. 7 figures. Recently a number of chemical analyses of the dike-rocks have been completed by GY. PITTER and M. EMSZT. Additional analytical data of earlier papers (JANTSKY 1957, VENDL 1914, GOKHALE 1965) have also been used in the petrochemical calculations which constitute the subject of this paper. The usual calculations such as NIGGLI values, CIPW norms, and the standard cell of rocks (BARTH 1952) have been carried out by the electronic computer at the Eötvös Loránd University. For the opportunity of using the program my thanks are due to lecturer GY. BUDA. Basic dike-rocks These rocks are very subordinate in the Velence granite area. Their detailed and reviewed description is given by EMBEY-ISZTIN (1972). On the basis of the calculated NIGGLI values as compared with those of the extreme acid end member of the spessartites given by NIGGLI (1923), one may realize that the Sárhegy dikerock is almost identical with it, whereas, both the Székesfehérvár quarry and the Csala forest dike-rocks are quite different from any rock type in the lamprophyre family (Table 1). Table 1. NIGGLI values of the basic dike-rocks. acid end member (NIGGLI1923) Sárhegy Csala forest Székesfehérvár quarry si al fm c alk total If we draw the differentiation diagram after NIGGLI (Fig. 1), one may see, that the chemical composition of the dike-rocks progressively approaches that of the Velence

2 Fig. 1. Variation diagram of the basic dike-rocks after NlGGU. granite. This can be also demonstrated by the ratios of K 2 0 to Na2Û and K Na 2 0 to CaO (Table 2). Table 2. The ratios K 2 0 and K Na 2 0/CaO K 20/Na so K so + Na 20/CaO Sárhegy Csala forest Székesfehérvár quarry Granite pophyry Granite (BUDA 1969) 1.60 Hence we are concerned here with a series of rocks the most basic member of which, the Sárhegy dike-rock (composed essentially of plagioclase and hornblende with minor orthoclase, biotite and quartz) may at best regarded as a spessartite. It is not easy to account for the origin of the lamprophyres and the difficulties arise even more if we are faced with the problem of a rock series having a range of composition somewhere between the lamprophyres and granites. Probably mixing of basic and acid magmas seems to be the best explanation. Acid dike-rocks In contrast to what has been said about the basic dikes, acid dikes are very frequent in the Velence granite pluton. Essentially they can be qualified as aplites and granite porphyries. Both granite porphyries and aplites can be divided into two well-defined types : 1. granite porphyry of Pátka type 2. granite porphyry of Sukoró type 3. aplite of dilatation-injection type 4. aplite of replacement type The first type is characterized by the dominance of a fine-grained groundmass in which phenocrysts of plagioclase, quartz, orthoclase and biotite "swim".

3 Fig. 2. Differentiation in a granite porphyry dike (Village Pátka). 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

4 Fig. 3. Variation diagram of the acid dike-rocks after NIGGLI. region. The controlling factors are the shape and size of the magmatic composition, temperature gradient, etc. body, Differentiation of granitic magma in the light of petrochemical evidences In order to show the conditions of differentiation a variation diagram after NIGGLI (Fig. 3) and a differentiation index (DI), oxide percentage diagram proposed by THORNTON & TTJTTLE (1960), have been prepared (Figs. 4a, 4b). In both diagrams it is revealed that the two types of granite porphyry have greater basicity as compared with the granite, that is they have a lower value of DI and si. The oxides vary regularly with the exception of CaO, so the value of c decreases with decreasing si. On the other hand, both types of aplite have much lower basicity, that is a higher value of DI. The. ratio of potash to soda varies characteristically (Table 3). Table 3. The ratio of K 2 0 and K Na 2 0/CaO K 20 K 20 + Na 20/ CaO granite (GOKHALE 1965) granite pophyry (Sukoro type) granite porphyry (Pátka type) aplite (dilatation-injection type) aplite (replacement type) In comparison with the granite K20/Na20 decreases in granite porphyries but it considerably increases in aplites of dilatation-injection type, whereas it does not change much in aplites of replacement type. It is obvious therefore that the two types of aplite differ from each other not only in structural and mineralogical features, but in chemical composition too (Table 4). This is apparent if we regard the ratio K 2 0 +Na 2 0/CaO or if we plot the Sr+Ba in ppm. against K 2 0 (Fig. 5). In this diagram the rock types form separate groups which can be accounted for as follows: aplites of dilatation-injection type have greater values of K 2 0

5 and they have much Sr+Ba due to their high temperature of origin. Granite porphyries having much lower values of K 2 0 also form a separate group. The ratio of Sr/Ba is 0.18 in aplites of replacement type and it is 0.10 in aplites of dilatation-injection type. From the foregoing discussion it is apparent that the two types of aplite cannot have a common origin as it was stated by VENDL (1914). Fig. 4A B. Oxide percentage DI diagram of the acid dike-rocks. Straight lines represent the axes of frequency histograms drawn after THORNTON & TTJTTLE (1960). 1 granite, 2 = replacement aplite, 3 = dilatation-injection aplite, 4 granite porphyry (Pátka type), 5 = granite porphyry (Sukoro type). Fig. 5. Sr+Ba K, 0 diagram of the acid dike-rocks. 1 = granite porphyry, 2 = replacement aplite, 3 = dilatation-injection aplite, double symbols: average.

6 Table 4. Chemical composition of the different rock types Granite porphyry, Sukoró Granite porphyry, Pátka Aplite, Mélyszeg (dilatation -injection type) Aplite, Székesfehérvár quarry (replacement type) Si Tio; A Fe FeO MgO MnO CaO Na K, HÖO H ; O CO, Analyst: GY. PITTER. In the author's opinion the fine-grained aplite is an isovolumetric alteration product of the granite. In this case the composition of the standard cell of the granite and aplite can yield useful information about the exchange of material during the metasomatism. As is well known all calculations are referred here to a volume of 160 oxygen anions. The composition of the standard cell for the granite and aplite is: Granite Considering the difference between the two rocks by subtracting the one from the other we can see that granite passes into aplite : total : representing By adding 3.3 Si 0.5 Na 3.8 cations 13.7 valences total : representing By subtracting 1.9 Al 0.8 Fe 0.1 Mg 0.3 Ca 5.4 H 8.5 cations 13.7 valences It is worth mentioning that a remarkably small fraction of the rock (less than 5%) need migrate in order to effect aplitization. Fig. 6. shows that aplites of dilatationinjection type fall into the stability field of orthoclase without exception, therefore, they are potassic rocks as defined by BARTH (1952). For that reason, it is obvious

7 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

8 Table 5. The ratio Or/Ab + An in aplites of dilatation-injection type Or : Ab + An Ördöghegy I. Mélyszeg I. Ráchegy I. Mélyszeg II. Pákozd W Gécsihegy Szöllőhegy Kisfalud Ráchegy II. Mélyszeg III. Tomposhegy Ördöghegy II Table 6. Normative mineral composition of the acid dike-rocks Granite Granite porphyry (Sukoro type Granite porphyry (Pátka type) Aplite (dilatation- -injection type) Aplite (replacement type) Q Or Ab An total more Or and Q, less Ab in respect to the ferner eutectic composition. Considering the experimental work on the system NaAlSigOs-KAlSigOs-H^O at 500 bars by TUTTLE & BOWEN (1958), it seems reasonable to deduce that crystallization started with a potassium-rich feldspar that was followed by a more sodium-rich phase. Similarly SAVOLAHTI (1956) thinks that in potassium-rich rapakivi granites potash feldspar was first to crystallize and sodium-rich feldspar and mafic minerals developed at some later stage. The early crystallization of potash feldspar in the Velence granite have been recorded by GOKHALE (1965). Additional results have been given by BTJDA (1969). Using the method of BARTH'S two feldspar geothermometer he found 600 C for the temperature of origin of orthoclase and only 520 C for that of plagioclase. Now, if we take into consideration all these facts, as well as the normative composition of the dike-rocks, and the results of experimental work, we can establish that the differentiation of granitic magma in the Velence pluton may have at least some analogous features to that of rapakivi granites. In both cases a more sodic and basic differentiate has been generated. The analogies may be the result of similarities in the physico-chemichal conditions. Since the latter one depends on the geological environment, we may think that geological circumstances have been also similar. Indeed the emplacement of the Velence granite like that of rapakivi granites has been thought to have taken place in peaceful conditions in the postkinematic stage.

9 Q Fig. 7. Ab-Or-Q diagram of the acid dike-rocks. The minimum melting area is drawn after TUTTLE (1955). 1 = granite, 2 = granite porphyry, 3 = replacement aplite, 4 = dilatation-injection aplite. References BARTH, T. F. W. (1952): Theoretical petrology. - New-York - London, p BOWEN, N. L. (1956): The evolution of igneous rocks. New-York, p BTJDA, Gy. (1969): Genesis of the granitoid rocks of the Mecsek and Velence Mountains on the basis of the investigation of the feldspars. Acta Geol. Acad. Sei. Hung., 13: EMBEY-ISZTIN, A. (1972) : A study of lamprophyric dike rocks of the Velence Hills (Hungary). Fragm. Min. Pal., 3: EMBEY ISZTIN, A. (1973): On the problem of graphic intergrowth and normal granitic structure. Ann. Hist.-nat. Mus. Nat. Hung., 1Î5: GOKHAXE, N. W. (1965): A Velencei-hegység gránit ós metamorf kőzeteinek ásványtani, kőzettani és kőzetszerkezeti vizsgálata (Manuscript). INKEY, B. (1875): A székesfehérvár-velencei hegység gránit és trachitnemű kőzeteiről. - Földt. Közi. 5: JANTSKY, B. (1957): A Velencei-hegység földtana. - Geol. Hung. 10: NIGGLI, P. (1923): Gesteins und Mineralprovinzen, I. - Berlin, pp NOCKOLDS, S. R. (1947): The granitic cotectic curve. - Geol. Mag., 84: PFEIFFER, L. (1964): Beitrage zur Petrologie des Meissener Massivs. Freib. Forsch.-H. G, 179: SAVOLAHTI, A. (1956): The Ahvenisto Massiv in Finland. Bull. Comm. géol. Finlande, 174: THORNTON, C. P. & TUTTLE, O. F. (1960): Chemistry of igneous rocks, I. Differentiation index. - Amer. J. Sei., 258: TUTTLE, O. F. (1955): L'origine et la classification des granites à la lumière des études.

10 expérimentales dans la system NaAlSi KAlSi Si0 2 -H 2 0. Sei. de la Terre, Nancy, TTJTTLE, O. F, BOWEN, N. L. (1958) : Origin of granite in the light of experimental studies in the system NaAlSi KAlSi Si0 2 -H 2 0. Geol. Soc. Amer. Mem., 74: VENDE, A. (1914): A Velencei-hegység geológiai és petrográfiai viszonyai. M. Kir. Földt. Int. Évk., 22: WHAE, W. (1946): Thermal diffusion-convection as a cause of magmatic differentiation. - Am. J. Sei., Author's address: Dr. A. EMBEY-ISZTIN Mineralogical Department of the Hungarian Natural History Museum H Budapest, Múzeum körút Hungary

Universal stage investigation of plagioclase feldspars in a mafic granulite nodule

ANNALES HISTORICO-NATURALES MUSEI NATIONALIS HUNGARICÏ Tomus78. Budapest, 1986 p. 15-21. Universal stage investigation of plagioclase feldspars in a mafic granulite nodule by G. NOSKE-FAZEKAS, Budapest