Kaszab Zoltán (szerk.): A Magyar Természettudományi Múzeum évkönyve 76. (Budapest 1984)

Noske-Fazekas, G.: Universal stage measurements of plagioclase feldspars originating from gabbroic inclusions of the Bondoró-hegy alkali basalts (Hungary)

ANNALES HISTORICO-NAI URALES MUSEI NATIONALIS HUNGARICl Tomus 76. Budapest, 1984 p. 19-25. Universal stage measurements of plagioclase feldspars originating from gabbroic inclusions of the Bondoró-hegy alkali basalts (Hungary) by G. NOSKE-FAZEKAS, Budapest Abstract — Probably of lower crust origin, the gabbro, xenoliths of the Balaton Highland showconsiderable morphological, structural and twin-statistical différences between their feldspars of primary and secondary crystallization. Primary plagioclases are characterized by the great number (~80%) of untwinned crystals, an average of 50% anortite content and the predominance of Gorai's A-twins. Among the secondary plagioclases formed during recrystallization that followed remelting the untwinned crystals are present in a significantly lower percentage (~50%), the average An content is much higher (70%) and the C-twins are becoming significant. The metamorphic character of twinning suggests that the plagioclases of gabbro inclusions have undergone ,,syntectonic" modification owing to an elevation of pressure. With 2 figures and 3 photoplates. Having joined the study of the ultrabasic inclusions of alkali basalts from the Balaton Highland (EMBEY-ISZTIN 1976a-b, 1978) I carried out universal stage measurements of feldspars originating from the gabbro inclusions of the Bondoró-hegy near the village of Kapolcs. My aim was to get closer to the clarification of the genetic circumstances of the rocks in question by optically studying the plagioclase crystals of xenoliths deriving from great depths (lower crust to upper mantle). Mineralogical-petrographical characteristics of gabbros Besides lherzolite inclusions taken to be of upper mantle origin, xenoliths of gabbroic nature are comparatively low in number (7%). Xenoliths of this kind also occur at Szigliget in the Balaton Highland zone. Strongly rounded, usually ovoid in shape, and averaging 5 to 8 cm in size, the gabbros are much smaller than the lherzolites. Viewed on a fresh surface of fracture, they are of grey to dark grey colour and rough to the feel. Varying from 0.5 to 2 mm in size, feldspars and pyroxenes can be readily distinguished from each other even with an unaided eye. A mineral banding with an alternation of plagioclase- and clinopyroxene rich layers can be observed on the smooth-cut surface of most gabbro xenoliths. In the microscope the rock texture is composed of holocrystalline, equigranular, 1 to 2 mm grains of clinopyroxene, plagioclase-feldspars, opaque and secondary minerals. Only quite seldom are a few grains of sphene, apatite and hornblende observable. A detailed pétrographie discussion will be given elsewhere (EMBEY-ISZTIN et al. in prep.), primarily an analysis of the characteristics of plagioclase twin crystals being presented here. Since the individual gabbro xenoliths widely vary in feldspar content and/or preservation state (freshness), I have selected samples that are suitable for being analyzed for feldspars on the universal stage. According to measurements with the integration stage, feldspars share between 20-48% in the composition of the rocks analyzed (Table 1). Their grain size varies from sample to sample from 100 to 400 [L and 500-800 fx, respectively. Their distribution pattern among the clinopyroxenes is rather diversified such as single hypidiomorphic grains or two or more crystals concentrated in patches of 2 to 3 mm size (Plate I: 1), or tabular arranged subparallel to one another. All these modes of appearance may be observed even within the area of a single thin section. As a rule, an uneven distribution is typical of the feldspars of gabbro xenoliths. A feature striking to the eye in analyzing thin sections