Acta Musei Moraviae

New localities of the dyke rocks in the eastern part of the Central Moldanubian Pluton

In the years 2021–2023, during ”Geopark Vysočina” field survey, two new and two already known localities of dyke rocks were studied. All dykes are hosted in orogenic, peraluminous, S-type, two-mica granites of the Central Moldanubian Pluton (CMP) of the Variscan age. These dykes are classified as lamprophyres (Rácov and Řídelov localities) and felsitic (porphyritic) microgranites (Pivničky and Beranov localities). Lamprophyres form up to several metres thick and hundreds of metres long, (sub)vertical dykes with sharp contacts with host granites. Lamprophyres are dark grey and fresh looking. Elongated vesicles or open miaroles up to several centimetres filled with calcite, zeolites, pyrite and rare quartz, are typical. Microscopic study revealed strong postmagmatic alterations which lead to complete replacement of primary assemblage (probably basic plagioclase, K-feldspar, Ca-amphibole and dark mica) with a fine-grained mixture of secondary albite, clinochlore (Fe/(Fe+Mg)=0.6), (REE-)epidote, calcite and titanite. K-feldspar is partly sericitized. Accessoric fluorapatite is abundant together with bastnäsite-(Ce) and synchyzite-(Ce). Lamprophyres are intermediate (53–56 wt.% SiO₂), alkali-calcic with K₂O/Na₂O~1 with low amounts of MgO (4–5 wt.%) and FeO+Fe₂O₃ (6 wt.%). These rocks display typical high enrichment in crustal elements: Rb, Li (up to 200 ppm), Ba, Sr, REE, Zr, P and mantle elements (V, Cr). Such rocks, considering their high degree of alteration, can be classified as kersantites or spessartites which is in agreement with existing literature. Examined felsitic dykes are several metres thick and hundreds metres long. Porphyritic microgranite Pivničky is characterized by up to 5 cm large K-feldspar phenocrysts, floating in fine-grained Qz+Kfs+Ab+Bt mass. Dark mica belongs to Ti-rich siderophyllite (Fe/(Fe+Mg)=0.8). Fluorapatite, Zn-rich ilmenite and rutile were found as accessories. Zn-ilmenite (up to 35 mol.% ZnTiO₃) is the main carrier of Zn in the rock (168 ppm Zn). This dyke is texturally, petrologically and geochemically well comparable with rocks of the Lásenice dyke swarm, situated at the western margin of the CMP. Weakly porphyritic Beranov microgranite is built of fine-grained Kfs+Ab+Ms matrix with dispersed, up to 2 cm large phenocrysts of K-feldspar and quartz. Mn-enriched fluorapatite and Nb,Sn,W-enriched rutile are accessories. Muscovite is enriched in Fe, Rb and F, hence up to 0.3 apfu Li is probable, which is in agreement of whole-rock chemistry, showing 232 ppm Li. These features are very similar to the Josefsthal dyke swarm at the Austria/Czech Republic border. Both rocks are acid (73–75 wt.% SiO₂) with very low FeO, MgO and CaO and K₂O/Na₂O >1.1–1.6. Beranov granite shows high enrichment in LILE (720 ppm Rb), Li (232 ppm) and rather high F and P (0.3 wt.%) and Sn (36 ppm) contents together with low Y+REE, Nb, Ba, Sr and Zr. Pivničky granite shows lower but still high contents of Rb, Li, F and P and, in other hand, higher FeO+Fe₂O₃, REE and Zr contents. Presence of described dyke rocks documents late-stage magmatic activity in the final stage of the Variscan orogeny.