Szakáll Sándor - Fehér Béla: A polgárdi Szár-hegy ásványai (Topographia Mineralogica Hungariae 8. Miskolc, 2003)

A polgárdi Szár-hegy andezittelérei és szkarnos képződményei (Dunkl István, Horváth István és Józsa Sándor)

Topographia Mineralogica Hungáriáé Vol. VIII. 55-86. Miskolc, 2003 A polgárdi Szár-hegy andezittelérei és szkarnos képződményei Andésite dykes and skarn formations of the Szár Hill, Polgárdi, Hungary DUNKL István

1

'*, HORVÁTH István

2 és JÓZSA Sándor

3 1 Institut für Geowissenschaften, Universität Tübingen, Sigwartstrasse 10, D-72076 Tübingen, Germany. 2 Magyar Állami Földtani Intézet, 1143 Budapest, Stefánia út 14. 3 Eötvös Loránd Tudományegyetem, Kőzettan-Geokémiai Tanszék, 1117 Budapest, Pázmány P. sétány 1/C. * e-mail: istvan.dunkl@uni-tuebingen.de Abstract The Transdanubian Central Range (western Hungary) is composed of metamorphic Paleozoic and unmetamorphic Mesozoic sequences. The oldest formations are exposed to the surface along the Balaton Highland and West-Northwest of Lake Velence. The quarry at the Szár Hill (between the villages Polgárdi and Szabadbattyán) exposes white limestone and marble of Devonian sedimentation age. The limestone is croscutted by rhyolite dykes of Permian and andésite dykes of Triassic age. The pyroxene amphibole andésite bodies are strongly transformed to propylite (Q + Kfs + Ms + Chi + Ep + Tre + Pi). Along the contact of the andésite dykes and the limestone-marble host rock skarn mineralization was formed. Both the dykes and the calcium silicate contact zones have irregular shapes. The skarn is missing in case of several dykes where the andésite has an immediate contact to the limestone. The skarn is usually zoned, banded. The endoskarn is developing from the andésite along a few-mm-wide transitional zone. This fuzzy diffusional front is composed of microcrystalline calcium garnet that transforms to fine grained and later on cm-coarse vesuvianite crystals. The isometric and acicular vesuvianite is the major component of the well developed, brown endoskarn. Ca-gamet (Grs 0 to 30%, Adr 70 to 100%), epidote, prehnite and pyrite are present only in subordinate amount; the matrix is calcite. The light green diopside skarn is the next (already exoskam) layer. Usually it is composed of diopside and calcite only. The size of the crystals varies from 40 /xm to >1 cm. Towards the limestone (marble) the next zone is the white wollastonite skarn, which isn't developed everywhere. The magnesium skarn (predazzite) was produced by the thermal effect of the intrusive bodies and the fluid circulation where the host carbonatic sequence had a dolomitic composition before the contact event. The predazzite is basically a brucite- and serpentinecontaining marble, but in several places the relics of tiny forsterite, spinel and periclase crystals are also preserved. These predazzite bodies do not follow the contact zones developed along the andésite dykes; they occur in form of disseminated, irregular bodies. A late hydrothermal transformation has created decomposition and hydration of the calcium silicate minerals. From diopside and wollastonite Si0

2 and Ca

2+ has liberated and formed opal-CT, calcite and aragonite along the calcium exoskam. This light green "opal zone" in some places is as thick as 40 cm. The formation of brucite and serpentine in the Mg-skarn are related also to this late hydrothermal phase.