Internal structures and dating of complex zircons from Meissen Massif monzonites, Saxony

U-Pb zircon ages were determined for accessory zircons from two monzonites from the Meissen Massif (Northern Bohemian Massif, Germany) by SHRIMP ion m icroprobe. BSE and CL imaging revealed that the monzonite zircons, showing oscillatory growth zoning, often contain inherited cores. Raman microprobe measurements showed that zoned rims (U 277-1426 ppm, Th 144-910 ppm) are sl ightly disordered to moderately metamict whereas cores have a wide spectrum of structural states, from well-crystallized to highly metamict. The radio nuclide (U + Th) content of cores varies from a few hundred to more than 20 ,000 ppm. SHRIMP ion probe measurements on zoned rims, interpreted as zirco n growth during crystallization of the monzonite magma, gave concordant Car boniferous (206)pb/U-238 ages of 326 +/- 6 Ma (monzonite from Leuben) and 3 30 +/- 5 Ma (monzonite from Heidenschanze; 95% confidence), respectively. T hese ages are consistent with recently published Ar-40/Ar-39 ages of 329.1 +/- 2.8 to 330.4 +/- 2.8 Ma (2 sigma) on amphiboles from the same rocks. In contrast, SHRIMP measurements on cores range from (slightly discordant) Pr oterozoic relic ages in low U and Th cores to highly discordant data in the most metamict cores. Previously obtained Pb-207/Pb-206 ages (single zircon evaporation technique) in the range 340 +/- 16 to 357 +/- 16 Ma (1 sigma) are probably mixed-ages between magmatic and inherited zircon components. T he U-Pb ages of the monzonite zircons postdate the Upper Devonian/Lower Car boniferous subduction and collision events in the area by about 20 Ma. Due to their shoshonitic chemical signature which is typical of rocks from dest ructive plate boundary settings, the Meissen Massif monzonites might be int erpreted as originating during these approximate to 350 Ma events, and the earlier Pb-207/Pb-206 ages appeared to confirm this interpretation. However , both the Ar-40/Ar-39 Ar-40/Ar-39 ages and our current results show that o nly the enrichment of the lithospheric mantle source, but not the monzonite intrusion itself, was directly related to these events. The monzonite intr usion itself took place approximate to 330 Ma ago, in the framework of stri ke-slip tectonics in the Elbe Valley zone. Widely occurring zircon cores ar e indicative of crustal contamination of the magma, causing considerable mo dification of chemical signatures (trace element content) of the monzonites . (C) 1999 Elsevier Science B.V. All rights reserved.