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

A Csódi-hegyi szerpentines kőzetzárványok ásványai (Papp Gábor és Szakáll Sándor)

Topographia Mineralogica Hungáriáé Vol. VI. 103-125. Miskolc, 1999 A CSÓDI-HEGYI SZERPENTINES KŐZETZÁRVÁNYOK ÁSVÁNYAI Minerals of serpentine-bearing xenoliths from Csódi Hill (Dunabogdány, Visegrád Mts., Hungary) PAPP Gábor és SZAKÁLL Sándor Abstract: Serpentine-bearing xenoliths from the dacite (Harangi, 1999) laccolith of Csódi Hill were first studied by Erdélyi et al. (1959, see Table I). A revision of the museum samples and new collecting lead to the recognition of several types of enclaves. The most abundant ones are those described by Erdélyi et al. (1959): massive, white or pale tinted macrocrystalline materials ("pure" serpentine-bearing xenoliths, Fig. 1) and previously unknown, light coloured (white to yellowish) more or less porous, frequently hydrogrossular-bearing xenoliths (Fig. 2). Other types with different texture but similar mineral composition are also found sometimes (Figs. 3-4). The only considerable silicate mineral in the "pure" serpentine-bearing xenoliths (Table II) is of course serpentine (some textural features are shown in Figs. 5-7). Lizardite (Figs. 8, 21), polygonal serpentine, clino- and orthochrysotile (Fig. 9 and see Papp et al, 1999), poorly formed serpentine ("deweylite", Fig. 10) also occur, usually as intimate mixtures. Calcite is the dominant accompanying mineral, brucite may also prevail in some samples. There is no proof of the presence of "sheridanite" and "magnesium-chamosite" mentioned by Erdélyi et al. (1959), the assumed hydromagnesite has not been detected either. In other xenoliths (Table III, Figs. 11-13) serpentine minerals (predominantly lizardite, Fig. 14, frequently multi-layer polytypes, Table V) and calcite still exist as main components. However, a dioctahedral smectite and hydrogrossular (Table IV) may also be predominant. Hydrotalcite (or manasseite?) (Table V, Figs. 15-18) is a characteristic alteration product. Katoite (Vezzalini, pers. comm.; Fig. 20) is to be highlighted from among the less known minerals of the contact zone between xenoliths and enclosing dacite. The mineral paragenesis of xenoliths was formed in several phases. The rock fragments detached by the ascending magma from the sedimentary basement (Korpás, 1999) underwent a thermal metamorphism first. This "primary" paragenesis was transformed by hydrothermal fluids circulating in the cooling and solidifying volcanite. This phase produced most of the minerals actually found in the enclaves (brucite, serpentine, hydrogrossular, smectite, a part of calcite). "Deweylite" with aragonite, a part of calcite and hydrotalcite (manasseite?) is bound to the late hydrothermal phase. The paragenetical differences of the two main xenolith types can be attributed to the differences in the primary rock: "pure" serpentine-bearing enclaves have been formed from almost "pure" dolomite, whereas the primary rock of the hydrogrossular- and smectite-bearing ones was presumably an Al- and Si-containing (marly) dolomite. Table VI lists the mineral paragenesis of similar serpentine-bearing xenoliths found in volcanic rocks from Hungary. The Appendix with Figs. 21-22 and Table VII gives a brief general review of serpentine group minerals and their identification by different methods. Összefoglalás: A dunabogdányi Csódi-hegy dácitjának (Harangi, 1999) szerpentines xenolitjait eló'ször Erdélyi és munkatársai (1959, 1961) ismertették. A gyűjteményi minták revíziója és újabb gyűjtések révén többféle kőzetzárvány-típust különítettünk el. A leggyakoribbak az Erdélyi et al. (1959, 1961) által is leírt fehér, ill. változatos halvány árnyalatokra színezett, tömött, „tisztán" szerpentines, valamint a többnyire világos fehéres-sárgás színű, többnyire többé-kevésbé porózus, gyakran hidrogrosszulárt tartalmazó zárványok. Ezeken kívül további ritkább, megjelenésükben, szövetükben az előbbiektől eltérő, de ásványos összetételükben kevéssé különböző zárványtípusok is léteznek.