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

A dunabogdányi Csódi-hegy földtana (Korpás László)

Topographia Mineralogica Hungáriáé Vol. VI. 39-58. Miskolc, 1999 A DUNABOGDÁNYI CSODI-HEGY FÖLDTANA Geology of Csódi Hill (Dunabogdány, Visegrád Mts., Hungary) KORPÁS László Abstract: The laccolith of Csódi Hill belongs to the Börzsöny and Visegrád Andésite Formation, Middle Miocene in age and consists of fluidal amphibole biotite dacite' (Harangi, 1999). Alternating glassy and holocrystalline porphyric bands of 1 centimetre to some decimetres in thickness are giving its schlierlike structure. This is a small, but typical laccolith, as confirmed by its size (D, = 1100-1200 m, D, = 900-1000 m, A= 100-150/m), shape, ground-plan of 1 km2 and aspect ratio of 0.08-0.14. It is one of the few laccoliths, in which the main feeding channel (080-100 m) together with the lateral (050-100 m) and the peripherial late ones (02-5 m) are excellently exposed. The laccolith intruded along the boundary of the Kiscell Clay and Kovácov (Kovácspatak) Formation and resulted in the formation of a contactmetamorphic zone, less than 4 m in thickness. The proportion of phenocrystals in the less viscose fluidal dacite is about 22-23%, while the maximum thickness of sediments overlying the laccolith was 450 m at the time of intrusion. The early (sill development) stage of the laccolith is not yet exposed. The following (bending) stage is known only in some of the profiles. The third (cupola development) stage with its central and lateral feeding channels is represented by wonderful profiles. The peripherial channels of the final (kink development) stage penetrated through the uncompletely solidified laccolith. The laccolith of Csódi Hill was fed, beside its central channel in NE, by at least two independent lateral channels, too. These independent feeding channels have been joined only below 2 km depth. The laccolith is a single shallow intrusion with scarce xenoliths coming from the pre-Tertiary carbonate basement and showing typical contact metasomatic mineral association. The banded structure indicates a continuous, rhytmic and fractional crystallisation, drived by changes in pressure. The central feeding channel (0300 m) of the similar, but less exposed Róka Hill laccolith is covered mainly by sediments. Both laccoliths formed in a dextral shear zone from WSW to ENE and nice examples of ductile deformation can be studied in the quarries of Csódi Hill. The axis of the asymmetric ellipsoidal central and lateral feeding channels should correspond to the conjugated Riedel faults of this shear zone. In the saddle between the domes of Csódi Hill and Róka Hill laccoliths the area has been collapsed and a syntectonic graben of NW-SE direction has been formed. The laccolith of Csódi Hill formed at the end of the early volcanic phase, about 14.8 Ma. Its rhytmic growth is represented by overburden slices of some centimetres and decimetres in thickness and the duration is estimated to less than 10 Ky. Összefoglalás: A középső miocén Börzsöny és Visegrádi-hegységi Andezit Formációba tartozó Csódi-hegyi lakkolit sávos-fluidális amfibol-biotitdácitból* áll (Harangi, 1999). S lires szerkezetét 1 cm és néhány dm vastag, üveges és holokristályos porfíros sávok váltakozása adja. A lakkolit méretével (D, = 1100-1200 m, D2 = 900-1000 m, V = 100-150 m), alakjával, 1 knr-es alapterületével és 0,08-0,14 közötti alakzati hányadosával a kisméretű, típusos lakkolitok közé tartozik. Azon kevés lakkolitok egyike, amelynek központi kürtője (080-100 m), mellékkürtői (050-100 m) és késői szegélykürtői (02-5 m) is nagyszerűen feltártak. A lakkolit a Kiscelli Agyag és a Kovácovi (Kovácspataki) Formáció határán nyomult be, 4 m-t meg nem haladó vastagságú kontakt övet hozva létre. A kevéssé viszkózus fluidális dácitban a fenokristályok aránya 22-23%, míg a fedő legnagyobb vas'Up to the present the rock was classified as andésite on petrographical grounds. This term will be used in direct quotations and figures (editor's note). ' A kőzetet mostanáig petrográfiai alapon andezitként sorolták be. Az egyenes idézeteknél és az átvett ábrákon értelemszerűen ezt a megnevezést használjuk (a szerk. megj.)