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

Abstract

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 (U 277–1426 ppm, Th 144–910 ppm) are slightly disordered to moderately metamict whereas cores have a wide spectrum of structural states, from well-crystallized to highly metamict. The radionuclide (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 zircon growth during crystallization of the monzonite magma, gave concordant Carboniferous 206 Pb / 238 U ages of 326±6 Ma (monzonite from Leuben) and 330±5 Ma (monzonite from Heidenschanze; 95% confidence), respectively. These ages are consistent with recently published 40 Ar / 39 Ar ages of 329.1±2.8 to 330.4±2.8 Ma (2 σ) on amphiboles from the same rocks. In contrast, SHRIMP measurements on cores range from (slightly discordant) Proterozoic relic ages in low U and Th cores to highly discordant data in the most metamict cores. Previously obtained 207 Pb / 206 Pb ages (single zircon evaporation technique) in the range 340±16 to 357±16 Ma (1 σ) are probably mixed-ages between magmatic and inherited zircon components. The U–Pb ages of the monzonite zircons postdate the Upper Devonian/Lower 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 ≈350 Ma events, and the earlier 207 Pb / 206 Pb ages appeared to confirm this interpretation. However, both the 40 Ar / 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 ≈330 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 considerable modification of chemical signatures (trace element content) of the monzonites.