Supplementary Material

We applied accurate and precise U-Pb age dating of zircons by LA-SF-ICP-MS following the method of Frei and Gerdes (2009), which involves: matrix-matched external standardization by standard-sample bracketing using the GJ-1 zircon reference standard (Jackson et al., 2004); careful matching of ablation conditions between standards and samples (e.g., Tiepolo, 2003); application of a purpose build low-volume ablation cell; use of He as carrier gas in order to stabilize the ablation signal and suppress U-Pb fractionation during ablation (e.g., Jackson et al., 2004); correction of the time-dependent within-analysis U-Pb fraction using the intercept method (e.g., Sylvester and Ghaderi, 1997; Košler and Sylvester, 2003).

To verify accuracy and reproducibility of zircon analyses we performed twenty-five analyses of the Plesovice standard, that yield a concordia age of 339.3±2.3Ma, MSWD=0.057 and a probability=0.81 of concordance. The majority of zircon U-Pb data are concordant, therefore the metric applied in this study is the conventional 2D 206Pb/238U vs. 207Pb/235U Wetherill concordia using ISOPLOT (Ludwig, 1998) and the uncertainties of 235U and 238U decay constants (Steiger and Jäger, 1977) are acknowledged.

U-Pb results of zircons of sixteen samples are presented as eleven concordia age plots and five discordia plots in (Figs. 6). Zircon ages are shown as light grey, open error ellipses and the calculated concordia ages are given as black and filled (dark gray) error ellipses. The sample names, the numbers of analyzed zircons and the numbers of analyses used for concordia age calculation are given in the upper left boxes of the diagrams (Figs. 6b-f, h-l, n, and o). The concordia ages, the MSWD and probabilities of concordance are given in the lower right boxes. The obtained discordiae were anchored to the origin since in all cases the discordia lower intercepts coincided with it. The age of the discordia upper intercepts and the MSWD are given in the lower right boxes.

The concentrations of U (ppm), Pb (ppm), the element ratio Th/U, the percentage of common Pb on the base of 206Pb, the isotopic ratios 207Pb/235U, 206Pb/238U, 207Pb/206Pb, their percentage 2σ errors, the 207Pb/235U-206Pb/238U error correlation (rho), the 207Pb/206Pb ratio, its percentage error, and the 207Pb/235U, 206Pb/238U, 207Pb/206Pb ages plus their absolute 2σ error of each zircon analysis are given in supplementary table 1. For each rock sample several analyses of zircon grains (14-25) were performed that are given in one block and ordered in ascending 206Pb/238U age values. Those age values that were taken into account for concordia age calculation are indicated with an asterisk (*). For each sample block the calculated concordia age (or discordia upper intercept), its 2σ error, the MSWD and the probability of concordance for the coherent group is given in the heading. The 206Pb/238U and the 207Pb/235U ratios yield similar precisions of 4.7% in average (1.3-26.0%).

The 2σ error of the 206Pb/238U zircon ages varies between 2.8 and 26% with a mean value of 4.0% and the 2σ error of the 207Pb/235U zircon ages varies between 3.5 and 26% with a mean value of 5.4%. Concordia-age errors are in the range of 1 to 2% due to the data selection and statistical treatment, whereas the discordia-age errors (upper intercepts) are in the range of 4 to 6%.

The zircon concordia ages vary between 290.3 and 296.5 Ma, all overlapping within their 2σ errors, and the zircon discordia (upper intercept) ages vary between 296 and 315 Ma, also overlapping within their 2σ errors (Fig. 6). In agreement to previous studies (Vavra and Hansen 1991; von Quadt, 1992; Finger et al., 1997; Cesare et al., 2002; Veselá et al., 2008, 2011) presenting magmatic crystallization ages from the Zentralgneiss batholith, the zircon U-Pb ages above are interpreted as formation ages dating the emplacement of the Zentralgneiss batholith (342-279 Ma).

The paragneiss-sample GT0803 shows zircon cores having 206Pb/238U ages of ~338 and 653 Ma that are concordant and two samples US0905 and US0906 show each one zircon core having 206Pb/238U ages of 529±20 and 581±23 Ma, respectively, that are concordant (supplementary table 1). These Carboniferous to Precambrian ages of the zircon cores are interpreted to reflect detrital zircons from the country rocks of the Zentralgneiss or from crustal fragments brought into the Zentralgneiss batholith by melt assimilation during pluton emplacement. The aplitic dike-sample GT0804 shows scattering 206Pb/238U zircon ages between ~239 and 595 Ma that are partly concordant and partly discordant, having a larger cluster of concordant ages at ~340 Ma (supplementary table 1). These ages might also reflect inherited ages from geological events preceding emplacement of the Zentralgneiss or they represent isotopically disturbed ages that we will not consider any further for the geological interpretation.

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Supplementary references

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