Kaszab Zoltán (szerk.): A Magyar Természettudományi Múzeum évkönyve 77. (Budapest 1985)

Noske-Fazekas, G.: Plagioclase twin law statistics of gabbroic inclusions originating from the alkali basaltic rocks of the Balaton Highland (Hungary)

Table 1. Mineral composition of the gabbros Szg-3001 Szg-3003 Szg-3004 Szg-3006 Szg-3008 Plagioclase 44.2 38.3 37.9 28.9 43.5 Pyroxene 38.6 41.4 37.2 39.7 37.5 Ore 5.6 2.3 3.4 2.9 9.6 Carbonate — — 5.8 — — Other 11.6 18.0 15.7 28.5 9.4 The individual gabbroic inclusions largely vary in feldspar content (% by weight) and preservation state of the crystals, being, together with the other rock constituents, for the most part equigranular. In addition, feldspar fields of considerable size composed of large numbers of crystals can be observed. The feldspars are of irregular shape, a feature typical of the magmatic texture, but in some places crystals bounded by straight lines meeting at an angle of 120° are also observable (Plate I: 1). A contact of crystal surfaces with the same angle can be observed in the case of clinopyroxenes of the gabbro as well (Plate I: 2). The extinction of the feldspars is uniform, it is only in sporadical cases that a slight undulation is observable. They are for the most part poor in inclusions (clinopyroxene, apatite, opaque grains), (Plate II: 1 and Plate III: 2), microlites in rows of dots being sometimes aligned in them (Plate II: 2). Most of the plagioclases are untwinned,. though 70% of the feldspars in sample Szg-3004 is represented by twinned crystals. Most frequent among the twinned crystals are the ones consisting of a single pair of twins, but in addition, polysynthetic lamellar plagioclases are also observable. As opposed to the feldspars from the Bondoróhegy, some samples from Szigliget show a strikingly thin twinlamellar structure (Plate III: 1), including sometimes a microcline-like cross-hatched pattern (Plate III: 2). In some of the analyzed samples traces of melting or re-melting are visible on the surface of plagioclases. The dark molten matter penetrates, in an indented to aciculate pattern, mostly only into the outer rim of the plagioclase crystals (Plate II: 1), though less frequently it may permeate the centre of the grain as well. In case of clinopyroxenes the penetration into the interior of the crystals cannot be observed, secondary alteration being confined to the rims (Plate I: 1 and 2). Recrystallization after secondary melting is less expressed than it is the case with the gabbro inclusions from the Bondoróhegy. The dark weakly crystallized matter including tiny feldspars is arranged in irregular form or in round patches between the unaltered primary minerals of the rock (Plate IV: 1 and 2). The so-called secondary feldspar needles are so small in size (20-150 |i.) that they could not be examined with the universal stage. Later compression applied to the gabbroic inclusions is evidenced by the feldspars in which the cracks are filled by carbonate or in which the feldspar matter has even been replaced, in patches of irregular shape, by the carbonate. Measurement of plagioclase feldspars on the universal stage The gabbroic inclusions from Szigliget largely vary in feldspar content and freshness. So I have selected five inclusions with plagioclase crystals suitable for measuring on the universal stage. It should be mentioned here too, that whereas the gabbroic inclusions of Bondoróhegy did include one sample of which I could measure the plagioclases formed