Ž . Chemical Geology 156 1999 331–341 Internal structures and dating of complex zircons from Meissen Massif monzonites, Saxony Lutz Nasdala a, ) , Thomas Wenzel b , Robert T. Pidgeon c , Andreas Kronz b a Institute of Theoretical Physics, TU Bergakademie Freiberg, Bernhard-Õon-Cotta-Str. 4, 09596 Freiberg Sa., Germany b Institute of Geosciences, Johannes Gutenberg-UniÕersity, 55099 Mainz, Germany c School of Applied Geology, Curtin UniÕersity of Technology, GPO Box U 1987, Perth 6001, Australia Received 2 July 1997; accepted 26 October 1998 Abstract Ž U–Pb zircon ages were determined for accessory zircons from two monzonites from the Meissen Massif Northern . Bohemian Massif, Germany by SHRIMP ion microprobe. BSE and CL imaging revealed that the monzonite zircons, showing oscillatory growth zoning, often contain inherited cores. Raman microprobe measurements showed that zoned rims Ž . U 277–1426 ppm, Th 144–910 ppm are slightly disordered to moderately metamict whereas cores have a wide spectrum Ž . of structural states, from well-crystallized to highly metamict. The radionuclide U qTh content of cores varies from a few hundred to more than 20,000 ppm. SHRIMP ion probe measurements on zoned rims, interpreted as zircon growth during 206 238 Ž crystallization of the monzonite magma, gave concordant Carboniferous Pbr U ages of 326 "6 Ma monzonite from . Ž . Leuben and 330 "5 Ma monzonite from Heidenschanze; 95% confidence , respectively. These ages are consistent with 40 39 Ž . recently published Arr Ar ages of 329.1 "2.8 to 330.4 "2.8 Ma 2 s on amphiboles from the same rocks. In contrast, Ž . SHRIMP measurements on cores range from slightly discordant Proterozoic relic ages in low U and Th cores to highly 207 206 Ž . discordant data in the most metamict cores. Previously obtained Pbr Pb ages single zircon evaporation technique in Ž . the range 340 "16 to 357 "16 Ma 1 s are probably mixed-ages between magmatic and inherited zircon components. The U–Pb ages of the monzonite zircons postdate the Upper DevonianrLower Carboniferous subduction and collision events in the area by about 20 Ma. Due to their shoshonitic chemical signature which is typical of rocks from destructive plate boundary settings, the Meissen Massif monzonites might be interpreted as originating during these f350 Ma events, and the earlier 207 Pbr 206 Pb ages appeared to confirm this interpretation. However, both the 40 Arr 39 Ar ages and our current results show that only the enrichment of the lithospheric mantle source, but not the monzonite intrusion itself, was directly related to these events. The monzonite intrusion itself took place f330 Ma ago, in the framework of strike-slip tectonics in the Elbe Valley zone. Widely occurring zircon cores are indicative of crustal contamination of the magma, causing Ž . considerable modification of chemical signatures trace element content of the monzonites. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Zircon; SHRIMP ion probe; U–Pb ages; Raman microprobe; Metamictization; U–Pb isotopic discordance; Monzonite ) Corresponding author. Present address: Institute of Geosciences, Johannes Gutenberg-University, 55099 Mainz, Germany. Fax: q49-6131-393070; E-mail: nasdala@mail.uni-mainz.de 0009-2541r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2541 98 00184-3