Kaszab Zoltán (szerk.): A Magyar Természettudományi Múzeum évkönyve 72. (Budapest 1980)

Noske-Fazekas, G.: Statistical investigations of plagioclase twin laws in lavas erupted between the Danube and the Central Börzsöny Mountains (Hungary)

Fig. 3. Variance of individual twin laws in lava in rocks of the Börzsöny Mountains. Key: 1 = Albite,2 = Mmsbach, 3 = Cirlsbad, 4 = Acline, 5 = Ala, 6 = Albite-Carlsbad, 7 = AlbiteAla. Numbers on the abscissa: 1 = pyroxene andésite, 2 = amphibole — pyroxene andésite, 3 = amphibole andésite, 4 = biotite andésite, 5 = oxyhornblende andésite, 6 = carbonatic amphibole andésite, 7 = inclusion of carbonatic amphibole andésite, 8 = amphibole dacite, 9 = biotite dacite, 10 = cognate inclusion Summary 1. In the southern Börzsöny Mountains the feldspars of andésites were formed at a high temperature level, consequently, they are typical effusive, and not subvolcanic products. 2, The scarcity of zoned phenocrysts among plagioclases suggest however that these must have been formed at rather great depths, under steady equilibrium conditions and that the rising magma brought already well-developed crystals to the surface. 3. Since postgenetic transformations (carbonatization, weathering processes, etc.) have heavily changed the composition of the rock, the measurement of fresh porphyric feldspars provides a sounder basis for estimates as to the original chemical composition. 4. As follows from the foregoing, the correlation between the distribution of plagioclase win laws and the Na/Ca ratio was found to be better than that between rock chemistry and twin laws. References BURRI, C, PARKER, L. R. & WENK, E. (1967): Die optische Orientierung der Plagioklase. — Basel, p. 1-334. CSESZKÓ, M. (1958): A szobi Csákhegy környékének kőzetföldtani jellemzése. — FÖldt. Közi, 88: 315-331. GORÁI, M. (1951 ): Petrological studies on plagioclase twins. — Amer. Mineral., 36: 884-901. JuGOvics, L. (1935): Dazitvorkommen in Börzsöny-Gebirge (Ungarn). — Min. Petr. Mitt., 43: 156-174. KUBOVICS, I. & PANTO, OY. (1970): Vulkanológiai vizsgálatok a Mátrában és a Börzsönyben. — Budapest, p. 1-302. NAGY, G (1975): Temperature determination of igneous processes on the basis of the composition of coexistent mineral pairs. — Acta Geo!. Hung., 19: 43-58. NOSKE-FAZEKAS, G. (1972): teldspatuntersuchungen an den Andesiten des Südteiles des BörzsönyGebirges. — Ann. Hist.-nat. Mus. Nat. Hung., 64: 5-17. NOSKE-FAZEKAS, G. (1974): Feldspatuntersuchungen an den Dazitvorkommen des Südteiles des Börzsöny-Gebirges. — Fragm. Min. Pal., 5: 5-19. "NOSKE-FAZEKAS, G. (1977): Feldspatuntersuchungen am Andesitvorkommen des Róka-Berges bei Szokolya (Ungarn). — Ann. Hist.-nat. Mus. Nat. Hung., 69: 13-21.