U-Pb zircon age of the Krásné Loučky tuffite: the dating of Visean flysch in the Moravo-Silesian Paleozoic Basin (Rhenohercynian Zone, Czech Republic)

摘要

The only pre vi ous U-Pb zir con date for the the Early Car bon if er ous flysch se quence of the Moravo-Silesian Pa leo zoic Ba sinwas pub lished in 1987 from tuffogenic ma te rial from Kobylí Quarry at Krásné Louèky near the town of Krnov (Silesia, CzechRe pub lic). The mea sured age of 319 Ma did not agree with its strati graphic po si tion, and was used as the ba sis for a hy poth e -sized block of Late Car bon if er ous paralic molasse in cor po rated dur ing a later tec tonic event. Dur ing a sur vey of the still-ac -tive quarry in 2010, volcaniclastic ho ri zons were iden ti fied and sam pled. Di rect cor re spon dence of the tuff units to thosesam pled in 1987 can not be proved but is likely. High pre ci sion chem i cal abra sion – ther mal ion iza tion mass spec trom e try(CA-TIMS) U-Pb dat ing of zir con from this new ma te rial has yielded an age of 340.05 ± 0.22 Ma, which cor re lates to the pre vi -ously in ferred strati graphic age of the lo cal ity and the cur rent cal i bra tion of the Early Car bon if er ous geo logic time scale. Thenewly es tab lished age cor re sponds to the Visean stage and dates the bound ary be tween the Horní Benešov and Moravicefor ma tions that can be cor re lated with other foredeep bas ins of the Culm in the Eu ro pean Variscides. A pop u la tion of de tri talCambro-Or do vi cian zir cons and a sin gle 2.0 Ga old zir con crys tal from the same volcaniclastic layer dated by la ser ab la tionin duc tively cou pled plasma mass spec trom e try (LA-ICPMS) are con sis tent with the known age of source ma te rial in theVariscan orogenic front.