The dyke swarm of fractionated tourmaline-bearing leucogranite and its link to the Vydra Pluton (Moldanubian Batholith), Šumava Mts., Czech Republic

Abstract

Dykes of tourmaline-bearing aplitic leucogranite from a swarm rimming the peraluminus, S-type granite of the Vydra Pluton (part of the Moldanubian Batholith) at a certain distance (2-4 Km). The leucogranite dykes have not been recorded any closer or even in the pluton itself. The medium-grained, slightly cataclastic dykes are ∼ 20 cm to ∼ 5 metres wide, and discordant to regional foliation of the country rocks. Leucogranite is composed of quartz + albite + potassium feldspar + muscovite ± tourmaline ± biotite (accessoric apatite and U-Y-P-rich zircon). The rock shows low concentrations of Ti, Ca, very low mg number, is peraluminous (A/CNK = 1.21-1.36) and has moderate K2O/Na2O of 0.53-1.14. Most of Ca is combined with P to form late apatite, additional phosphorus is bound especially in potassium feldspar. The high concentration of phosphorus in potassium feldspar (0.65-0.95 Wt- % P2O5) is in a sharp contrast to the low concentration of P in albite (0.01-0.17 wt. % P2O5), and it is interpreted as a result of a late-/post-magmatic alteration. Compared to the Vydra Pluton granites, leucogranite is slightly richer in SiO2, Na2O and Rb (200-240 ppm) and strongly enriched in Sn (17-34 ppm) and B (abundant tourmaline). It is moderately to strongly depleted in V, Cr, Ni, Sr, Ba, and LREE (∑ REE ∼ 7-40 ppm). The K/Rb ratio ranges from 88 to 133, Rb/Sr = 2.3-10.2. Leucogranite is also characterized by low concentration of Th (mostly 1.2-3 ppm) and a low Th/U ratio (typically below 0.4). The new monazite U-Th dating confirms the Variscan age (326.6 ± 6.9 Ma) of metamorphism in the surrounding metamorphic rock.