Magmatic and hydrothermal evolution of Li-F granites: Cínovec and Krásno intrusions, Krušné hory batholith, Czech Republic


Authors: Dolejš D, Štemprok M

Published in: Bulletin of Geosciences, volume 76, issue 2; pages: 77 - 99;

Keywords: Li-F alkali feldspar granites, Magmatic fractionation, Subsolidus alteration, Mass transfer, Stable isotopes, Eastern Krušné hory Mts., Slavkovský les,

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Geochemical and stable-isotope data for highly evolved Li-F granites from the Cínovec stock and the Krásno area (Krušné hory batholith) reveal these units as products of prolonged fractional crystallization with subsolidus greisenization, feldspathization or sericitization. The Cínovec stock is characterized by low-P Li-mica granites, primarily separated into lower chemically homogeneous protolithionite granites (δ18OWR from 7.6 to 8.4 ‰ SMOW), and upper evolved zinnwaldite granites (δ18OWR from 5.2 to 5.8 ‰ SMOW), which were produced by 35-50 % crystal fractionation with concomitant low-δ18O meteoric-water interaction. Strong lanthanide tetrad effect (TE1,3 1.21-1.51) provides additional evidence for fluid interaction prior to complete solidification. The intensity of subsolidus alterations, which are superimposed on the primary magmatic characteristics, increases continuously towards the apical part of the intrusion and is characterized by albitization (+2 to +7 absolute wt.% of albite), K-feldspar and Li-mica decomposition (up to -21 wt.%), sericitization (up to +11 wt.%), and minor mobilization of quartz (-2 to +4 wt.%). The Čistá intrusion in the Krásno area consists of high-P zinnwaldite granites (δ18OWR 10.4 to 10.7 ‰ SMOW) with two contrasting alteration trends - greisenization and alkali feldspathization. Greisenization is characterized by albite (-8 wt.%) and K-feldspar (-6 wt.%) dissolution, and precipitation of quartz (+8 wt.%), whereas alkali feldspathization is dominated by albitization (+26 wt.%), weak K-feldspathization (+10 wt.%) and strong quartz leaching (-28 wt.%). The whole sample set exhibits positive correlation between δDWR values and water content, which is attributed to postmagmatic alteration by waters of meteoric origin. Calculated water/rock ratios (by weight) vary in the order of 0.01 and 0.001. Oxygen isotope data provide additional support for the derivation of high-P and low-P granites in the Krušné hory batholith from two distinct crustal sources.