Timing of Variscan HP-HT metamorphism in the Moldanubian Zone of the Bohemian Massif: U-Pb SHRIMP dating on multiply zoned zircons from a granulite from the Dunkelsteiner Wald Massif, Lower Austria

• Published in: Mineralogy and petrology 2011-10, Vol.102 (1-4), p.63-75
• Main Authors: Friedl, Gertrude, Cooke, Rob A., Finger, Friedrich, McNaughton, Neal J., Fletcher, Ian R.
• Format: Article
• Language: English
• Subjects: Cores; Crystal growth; Earth and Environmental Science; Earth Sciences; Geochemistry; Indexing in process; Inorganic Chemistry; Metamorphic rocks; Metamorphism; Mineralogy; Special Issue Accessory Minerals; Uranium; Zirconium; Zoning
• ISSN: 0930-0708; 1438-1168
• DOI: 10.1007/s00710-011-0162-x

In an attempt to better constrain the timing of Variscan HP-HT metamorphism in the SE Bohemian Massif we have dated zoned zircons from a garnet-kyanite granulite of granitic composition from the Dunkelsteiner Wald Massif, Lower Austria, by means of sensitive high-resolution ion microprobe (SHRIMP) technique. In order to combine isotopic information with crystal growth textures, CL and BSE images were systematically taken from the dated zircons. A characteristic threefold concentric zoning was found in many zircons. This involves pre-Variscan protolithic cores followed by two distinct metamorphic/anatectic overgrowth shells of Variscan age. The inner overgrowth shell is characterized by a weak CL but bright BSE signal, and yields high contents of uranium (0.1 to 0.2 wt.%). A pooled U-Pb Concordia age for this zone is 342.0 ± 3.0 Ma ( n  = 11, MSWD = 0.12). The second, outer, overgrowth shell is always bright in the CL image, dark in the BSE image, and has generally low uranium contents (mostly