Vörös A. szerk.: Fragmenta Mineralogica Et Palaentologica 11. 1983. (Budapest, 1983)

FRAGMENTA MINERALOGICA et PALAEONTOLOGICA 11. 1983 p. 9-27 On the origin of garnet in magraatic rocks By A. EMBEY-ISZTIN and G. NOSKE-FAZEKAS (Received November 30, 1982) Abstract: Garnets originating from Miocene dy-normative, medium-K andésites as well as from a Mesozoic fine-grained albite granite have been petrographically investigated. It has been argued that the former ones with an average composition of 57 to 65% almandine, 14 to 22% pyrope, 10 to 24% grossular and 2 to 5% spessartine are high-pressure liquidus phases, while the second ones (almandine 71 to 74%, pyrope 12 to 18%, grossular 3.5 to 5.5% and spessartine 5.6 to 11%) have apparently crystallysed in a low-pressure environment due to a late magmatic, metasomatic event. Garnets have been known to occur in some places of the Miocene andesitic volcanic mountains of Visegrád and Börzsöny (Fig. 1) for a long time (e.g. ZIPSER 1817, JÓNÁS 1820, TÓTH 1882, MAURITZ 1910, NAGY 1975). They can be found as detrital grains in the soil and deposit of some brooks e. g. Királyrét, Márianosztra (Börzsöny Mts), but also as megacrysts in different andesitic rocks e.g. Pilisszentlélek (Visegrád Mts) Bajdázó Hill, Csehvár (Börzsöny Mts). Another occurrence of garnet in a magmatic rock, yet completely different from the former ones has been reported more recently by SZENTPÉTER Y (1953) from the Western part of the Bükk Mts (Fig. 1). Here, Mesozoic diabase with a major element pattern resembling closely that of the ocean floor basalts (EMBEY-ISZTIN 1981) intrudes Triassic sedimentary rocks. Smaller gabbroic masses and at least one ultramafic body probably of cumulative origin (NOSKE-FAZEKAS & EMBEY-ISZTIN 1978) are associated with the diabase intrusion. Pink garnet grains occur exclusively in leucocratic (trondjhemitic) nests of the gabbro in the quarry of Tóbércbánya (SZENTPETERY 1953). PETROGRAPHY The garnets of the orogenic andésite types are almost always larger in diameter than any other phenocrysts in the same rock. Therefore thay can be regarded as megacrysts and the use of this term is also appropriate because it avoids attributing any a priori genetic significance. The largest crystals are those of Szokolya, Királyrét (Börzsöny Mts). Here isolated grains, originating from complete decomposition of the andésite can reach a diameter as large as 1.5-2.0 cm. Crystals show a strong tendency to euhedrality, the form of (211) being the most frequent followed by (110) or a combination of both. However, resorbed grains and aggregates of garnet crystals can also be met with. Their colour is dark red-violet, sometimes they show slight colour zoning with a