Kovács I. (szerk.): A Magyar Természettudományi Múzeum évkönyve 78. (Budapest 1986)

Noske-Fazekas, G.: Universal stage investigation of plagioclase feldspars in a mafic granulite nodule

clinopyroxenes of earlier segregation, thus lacking idiomorphy (Plate I: 2). Their extinction is generally uniform, being occasionally undulate, however, several crystals are observed the nuclei and rims of which do not extinct at the same time (Plate II: 1). Similarly to the case of the Bondoróhegy gabbros studied earlier, the meeting at 120° of the straight-line boundaries of plagioclases can be observed here as well (Plate II: 2). The feldspars are usually poor in inclusions, but sometimes well-crystallized clinopyroxene, zircon and apatite are observed (Plate III: 1). Acicular, needle-like black inclusions due to a secondary influence that had affected the rock are more frequent, being located mainly on the edges of the crystals of along scarce cracks, parallel to one another (Plate III: 2). In addition, opaque globules of varying size are conspicuous (Plate IV: 1). Results of measurements on the universal stage Similarly to the case of the other Bondoróhegy gabbros, the major part of the plagioclase grains in this sample are not twinned (Plate I, II, III). Attaining a total of only 15% share, the twin crystals mainly consist of a single pair of twins (Plate IV: 2), polysynthetic twins being quite sporadical. The average An content measured for the 114 twin constituents was 73.9% which is in good agreement with the microprobe results (77.4 An%). For the anorthite content of gabbro xenoliths studied earlier from the Bondoróhegy locality, a round figure of 50% was measured. Only secondary plagioclase grains of one sample (Bo-3009) showed An values higher than 70% (NOSKE-FAZEKAS 1984). In the course of my earlier studies I was surprised to see that the points of measurement of the plagioclases of the gabbros coincided with the high-temperature migration curve, which I explained by the effect of a secondary heating that had affected the gabbros. Because of the high anorthite content of the plagioclases in the sample being discussed, it is impossible now to distinguish optically between high- and low-temperature migrations, because in this range the curves almost coincide. Having examined the statistical regularities of twinning in sample Bo-3007, let me point out that, similarly to the case of the gabbro xenoliths studied earlier on the universal stage, Gorai's A-twins (albite+aclin = 75.4%) are characteristic here as well. The remaining twins consituting about 25% are crystallized according to the Carlsbad (7.7%), albiteCarlsbad (9.3%), albite-ala (1.5%), ala (1.5%), Baveno (1.5%) and Manebach (1.5%) twin laws. As evidenced by our earlier and present studies, there is a close relationship between rock chemistry and twinning history. The more acidic plagioclases in the gabbro xenoliths from Bondoróhegy are crystallized, almost all according to the A-twin law (NOSKE-FAZEKAS 1984); at the same time, the crystals of considerably higher anorthite content were formed, in a quite significant quantity (25%), by obeying the C-twin law (Bo-3007). Comparing the results with the twin-statistical data of volcanic rocks measured previously by us (NOSKE-FAZEKAS 1972, 1980; NOSKE-FAZEKAS & EMBEY-ISZTIN 1975), we can see that the basicity of the rock has had a marked effect on the frequency of occurrence of the individual twin laws in the case of both extrusive and intrusive rocks sampled at great depths. Thus, for instance, even according to the measurements on the universal stage of andésites of different chemical composition from the Börzsöny Mountains, a wide variety of igneous rocks ranging from acidic biotite andésite, through hornblende andésite, up to basic pyroxene andésite, shows an increase in the quantity of C-twins at the expense of A-twins.