Kaszab Zoltán (szerk.): A Magyar Természettudományi Múzeum évkönyve 70. (Budapest 1978)
Noske-Fazekas, G. ; Embey-Isztin, A.: New data to the mineralogy of the basic intrusion around the village Szarvaskő, Hungary
ANNALES HISTOR ÍCO-NATURALES MUSEI NATIONAL IS HUNGARICI Tomus 70. Budapest 1978. New Data to the Mineralogy of the Basic Intrusion around the Village Szarvaskő, Hungary by G. NOSKE-FAZEKAS & A. EMBEY-ISZTIN, Budapest Abstract — A compositional range of ^7i 30 _ 63 with an average value of An 51 have been obtained by universal stage measurements for plagioclases from the gabbro member of the diabase-gabbroore peridotite intrusion of the Bükk Mountains, NE Hungary. Frequency distribution of individual twin laws is also discussed. Petrochemical consideretions on the basis of two clinopyroxene and one amphibole analyses, as well as the computation of the norms of the three most important rocks of the intrusion revealed that the basic magma had olivine tholeiitic affinity and perhaps it is related to abyssal high-alumina tholeiites. Evidence for a cumulus structure in the ore-peridotite have been discovered. With 5 figures and 4 plates. The petrography and petrochemistry of the basic intrusion around Szarvaskő, western Bükk Mountains, Northeast Hungary are fairly well known from the detailed study by SZENTPÉTERY (1953). He published a great number of chemical analyses of the diabases and gabbros that constitue the main mass of the intrusion, on the basis of which the peculiar character of the basic magma could be established. It had been revealed namely that the basic magma was unusually high in Ti and Fe 2+ which permitted the differentiation of an ultrabasic rock containing olivine, diallage, hornblende and a large quantity of titanomagnetite and ilmenite. This was the rock originally termed wehrlite by VON KOBELL (1864). In spite of the large number of petrochemical data, our understanding on the problems of petrogenesis can hardly be developed unless similer data for the rock forming minerals will be available. The aim of this paper is to begin to furnish such data. First universal stage measurements of plagioclase twins are discussed followed by the interpretation of the chemical analyses of some separated mineral fractions. Petrography The rock samples that constitute the basis of our feldspar investigations come from two quarries of gabbro named Tardosbánya and Tóbércbánya. The rocks of these localities differ from each other in their grades of alteration and in the proportions of the composing minerals. The gabbro of Tardosbánya is dark grey with greenish patches and its grain size varies very frequently (from coarse to medium). The size of individual mineral constituents ranges from a few hundreds u. to 3-4 mm across. The texture is hipidiomorphic granular. The main mineral constituent of the Tardosbánya gabbro is the plagioclase feldspar (Table 1). The plagioclase is frequently altered to varying degrees and this alteration is more intensive than in the case of the gabbro of Tóbércbánya. However, completely unaltered and inclusion-free crystals can also be found. Among the inclusions of the plagioclase feldspar opaque and other minerals as well as fluid and gas inclusions can be encountered. The alteration product is composed of chlorite, carbonate, quartz and prehnite (Plate I: 1-2). The plagioclase forms subhedral, sometimes euhedral crystals and it may have been the first phase to crystallize, however, the main mass of the plagioclase must have formed contemporaneously with clinopyroxene. The laths of plagioclase are always twinned and very frequently in a polysinthetic manner. The universal stage investigations of these twins are discussed later. Clinopyroxene is the second most frequent constituent of the gabbro. In some thin sections like Td-2/555 one, its quantity approaches that of the plagioclase feldspar (38.5% plagioclase, 36.9% pyroxene) but in general the amount of pryroxene remains considerably less (Table 1). Generally it forms subhedral isometric laths, but anhedral crystals are also frequent. Twinning of the pyroxene is rare. In thin section its colour is slightly brownish with a very weak pleochroism. In addition to the well known (110) cleavage, a parting according to (100) can be seen. This fact led SZENTPÉTERY to suppose that the pyroxene is a diallage. The diallage shows sometimes intergrowth with amphibole
