Nove U-Pb datovani zirkonu z bohutinskeho pne v pribramske rudni oblasti

The Bohutín Stock is one of several small isolated intrusions occurring in the northern surroundings of the Variscan Central Bohemian Plutonic Complex (Bohemian Massif, Czech Republic). The stock with surface outcrop area of about 3.6 sq. km is known thanks to intensive underground mining of the Bohutín Ag-Pb Zn-Sb vein-type deposit (1841-1979) down to 1350 m below the surface. The Bohutín Stock is a petrographically variable body with quartz diorite or tonalite as the most widespread rock type. More basic diorite to gabbrodiorite enclaves as well as rocks of granodiorite to granite composition (trondhjemite) also occur in the stock.
Earlier conventional K-Ar ages of the Bohutín Stock indicated Silurian to Early Devonian age of the intrusion (440-400 Ma) which was rather controversial, since the formation of majority of the Central Bohemian Plutonic Complex covers a period between 354 ± 4 and 337 ± 1 Ma. One later Ar-Ar age determination on amphibole from a gabbrodiorite enclave had excess argon with plateau age of 348.5 ± 0.5 Ma. To solve this controversy, the main intrusive rock type of Bohutín Stock, quartz diorite, was dated by zircon U-Pb method using laser ablation ICP-MS. The dated sample was collected at a depth of 1199 m below the surface. Representative cathodoluminescence images of the dated zircon grains are shown in Fig. 1 and U-Pb concordia diagram for zircons from Bohutín in Fig. 2.
Studied quartz diorite from the deeper part of the Bohutín Stock is homogeneous equigranular fine-grained rock composed of (in descending order) subhedral zoned plagioclase (labradorite→oligoclase), actinolitic amphibole, anhedral biotite, quartz, and K-feldspar. For chemical composition of studied sample see Table 1.
The obtained U-Pb zircon concordant age of 344.2 ± 0.6 Ma (for data of individual zircon grains see Table 2) is close to the age of zircons from granodiorite of the nearby Padrť Stock (342.8 ± 1.1 Ma). Both small intrusions therefore probably intruded in the same phase of structural evolution of the complex boundary zone between Moldanubian Unit and Bohemicum. Their age is within the age range of the main intrusive masses of the Central Bohemian Plutonic Complex.

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