Zircon ages for high pressure granulites from South Bohemia, Czech Republic, and their connection to Carboniferous high temperature processes

Petrological and isotopic investigations were undertaken on high pressure g ranulites of granitic to mafic composition from the Prachatice and Blansky les granulite complexes of southern Bohemia, Czech Republic. The predominan t felsic granulites are quartz + ternary feldspar (now mesoperthite)-rich r ocks containing minor garnet, kyanite and rutile, and most show a character istic mylonitic fabric formed during retrogression along the exhumation pat h. Three high temperature reaction stages at distinctly different pressures are recognized. Rare layers of intermediate to mafic composition, containi ng clinopyroxene, best record a primary high pressure-high temperature stag e (> 15 kbar, > 900 degrees C), and a well-defined overprint at medium pres sure granulite facies conditions (6-8 kbar, 700-800 degrees C) during which orthopyroxene (+ plagioclase) formed from garnet and clinopyroxene. A furt her high temperature overprint at lower pressure (ca. 4 kbar) is reflected in the development of cordierite- and/or andalusite-bearing partial-melt pa tches in some felsic granulites. Conventionally separated zircons from the granulites were measured on a SHR IMP II ion microprobe. Near-spherical, multifaceted grains interpreted to b e metamorphic, and short prismatic grains from the cordierite-bearing melt patch, are all concordant and yielded indistinguishable results producing a n average age, for 83 individual grain spots, of 339.8 +/- 2.6 Ma (2 sigma) . Metamorphic grains from a meta-granodiorite associated with the granulite s gave the same age (339.6 +/- 3.1 Ma, mean of 9), whereas inherited magmat ic grains of the same sample yielded 367.8 +/- 1.4 Ma. A mean age of 469.3 +/- 3.8 Ma was obtained for two short prismatic concordant grains in one of the granulites, whereas several of the rounded grains with ca. 340 Ma meta morphic zircon overgrowths had much older (Pb-207/Pb-206 minimum ages up to 1771 Ma) discordant cores. In addition to analysis of conventionally separ ated grains, ion-microprobe measurements were also made on zircons extracte d from thin sections (drilled-out, mounted and repolished) such that a dire ct relationship between the dated zircons and petrographic position could b e made. Identical results were obtained from both preparation methods, thus showing that the considerable advantage in petrological control is not off set by any appreciable lack of precision when compared to conventionally pr epared ion-microprobe samples. All these isotopic results are identical to those previously obtained by conventional multigrain and single-grain evapo ration techniques, but rather than allowing a greater resolution of the age of the petrographically obvious different metamorphic stages the results d ocument, for the first time, the apparent short time scale for high, medium and low pressure metamorphism in the granulites. The short time period bet ween the 340 Ma age for the high pressure granulites, as derived here and f rom studies of similar rocks elsewhere in the European Variscides, and the 320-330 Ma ages for regional low pressure-high temperature metamorphism, mi gmatization and granite magmatism, strongly suggests an important link betw een these two high temperature processes.