Publisher © Czech Geological Survey, ISSN: 2336-5757 (online), 0514-8057 (print)
Ultrapotassic quartz melasyenite from Nebahovy near Prachatice, South Bohemia: geochemically anomalous lamproitic rock
Abstract
Fine-grained melasyenite of presumably Variscan age occurs at Nebahovy near Prachatice, South Bohemia. It forms only a very small body surrounded by metamorphic rocks of the Prachatice granulite massif, Moldanubian crystalline complex. The rock is composed of K-feldspar, phlogopite highly magnesian richteritic amphibole, plus accessory apatite and rutile. Whole rock chemistry is very high in K2O, ultrapotassic, highly magnesian and low in lime. In contrast with majority of typical lamproites, the Nebahovy melasyenite has considerably higher alumina content. The agpaitic index is about 1.03-1.05. Trace element geochemistry is characterized by relatively high Cr, Ni, very high La/Yb and extremely high LILE/HFSE ratios. Such a magma could originate by partial melting of a highly anomalous domain in the sub-continental lithospheric mantle. The source was of refractory (harzburgitic) character, but strongly re-enriched in hygromagmatophile elements.
