Papp Gábor szerk.: A dunabogdányi Csódi-hegy ásványai (Topographia Mineralogica Hungariae 6. Miskolc, 1999)

A Csódi-hegy vulkáni kőzetének geokémiája és petrogenezise (Harangi Szabolcs)

Topographia Mineralogica Hungáriáé Vol. VI. 59-85. Miskolc, 1999 A CSODI-HEGY VULKÁNI KŐZETÉNEK GEOKÉMIÁJA ÉS PETROGENEZISE Geochemistry and pedogenesis of the volcanic rocks from Csódi Hill (Dunabogdány, Visegrád Mts., Hungary) HARANGI Szabolcs Abstract: The Middle Miocene subvolcanic rocks of Csódi Hill belong to the andesite-rhyodacite volcanic complex of the Visegrád Mts (Fig. 1). They were formed between 14 and 16 Ma as a part of widespread Middle Miocene to Quaternary calc-alkaline volcanism along the northern and eastern part of the Pannonian Basin. The calc-alkaline volcanism at the western segment of the Carpathian volcanic arc occurred in a retreating subduction zone, when the southward subduction of the European plate ceased and compressional tectonic field changed to extension. The present-day thickness of the crust and lithosphère beneath this area indicates that it belonged to the extended Pannonian Basin system. The origin of the calc-alkaline volcanic activity in the Western Carpathian Arc is a subject of debate, i.e. whether there is a direct relationship between subduction and volcanism or melt generation occurred due to thinning of lithosphère resulting in partial melting of metasomatised lithospheric mantle. Garnetbearing volcanic rocks belong to the earliest products of the calc-alkaline volcanism. K/Ar radiometric dating implies that the volcanic rock from Csódi Hill was formed about 16 Ma. Garnet-bearing volcanic rocks are rare world-wide and only limited data are available about them. Table I summarises the main information about garnet-bearing volcanic rocks from different localities. Rare occurrences of garnet-bearing volcanic rocks may be explained by the limited stability field of Cabearing and Mn-poor almandine garnets. Experimental studies pointed out that they can crystallise from H20-rich silicic magma at high pressure (8-12 kbar). Preservation of euhedral garnet phenocrysts implies rapid ascent of the host magmas. Almandine garnets are usually hosted by peraluminous (Stype) Si02-rich volcanic rocks, which are derived by anatexis of granulite facies metapelitic lower crustal rocks. On the other hand, there are many examples of diopside-normative (I-type) garnet-bearing volcanic rocks, which were formed from mantle-derived magmas. The volcanic rock of Csódi Hill is classified as medium-K garnet-bearing biotite-dacite (Fig. 3). Garnets (Aim - 65-70%, Gro = 9-14%) are primary phenocrysts, i.e. they crystallised from the magma. They are coexisting with calcic plagioclase (An = 77-86%) and biotite (Fig. 2). Amphibole and orthopyroxene occur subordinately and they are usually strongly altered. The groundmass consists of intermediate plagioclase and subordinate Fe-Ti oxide, quartz, glass and secondary minerals. Major element composition of a garnet-bearing (VH-CSH) and a garnet-free (L-CSH) sample from Csódi Hill is presented in Table II. Slightly peraluminous composition is supported by the appearance of normative corundum (c = 1.48-1.88) and the A/CNK > 1 ratio (A/CNK = 1.06-1.09). The N-MORB normalised trace element pattern of the dacite from Csódi Hill (Fig. 4) shows Nb and Ti trough and positive Pb anomaly, which are typical of subduction-related volcanics. Comparing of the Csódi dacite with the garnet-bearing rhyodacites from the southwestern part of the Visegrád Mts., the main differences are at the Ti-Lu range. The latter ones show strong depletion in Y and in heavy rare-earth elements, which is similar to the garnet-bearing rhyolites from the Pyrenees. Chondrite-normalised rare-earth element pattern of the Csódi dacite is smooth without any Eu-anomaly (Fig. 5). The (La/Yb)d, ratio is in the range of the gamet-free andésites of the Visegrád Mts. On the contrary, the garnet-bearing rhyodacites have a strong depletion in the heavy rare-earth elements and show a weak negative Eu anomaly. Radiogenic isotope ratios ("'Sr^Sr and IJ,Nd/l44Nd) of the Csódi dacite are comparable with other garnet-bearing volcanic rocks in the area. They have high 87SrrSr (0.7092-0.7100) and low 1J3Nd/l44Nd isotope ratio (0.51230-0.51235), whereas garnet-free volcanics show more depleted isotope composition ("SrrSr = 0.7070-0.7092; u5Nd/l44Nd = 0.51233-0.51242).