FURTHER EVIDENCE FOR AN EARLY CARBONIFEROUS (SIMILAR-TO-340 MA) AGE OF HIGH-GRADE METAMORPHISM IN THE SAXONIAN GRANULITE COMPLEX

U-Pb and Pb-Pb zircon ages for metamorphic zircons from granulites in the Saxonian granulite complex are reported, using the SHRIMP ion micr oprobe, conventional multigrain and single-gain techniques and the eva poration method. This is complemented by a Pb-Pb evaporation age for a post-granulite granite emplaced into the schist mantle around the gra nulites during uplift of the complex. We also demonstrate that zircon ages are not reset during high-grade metamorphism, as commonly argued, but have a very high closure temperature and usually preserve the iso topic composition reflecting the time of their formation. Multifaceted zircons from four granulite samples that probably grew close to the p eak of high-grade metamorphism yielded identical U-Pb and Pb-Pb ages o f similar to 340 Ma which support previously published data and unambi guously show that the granulites formed during a lower Carboniferous e vent and not in the early Palaeozoic or Precambrian as previously sugg ested. Older cores in some of the metamorphic zircons reveal early Pal aeozoic components at 470-485 Ma that we interpret as ages reflecting magmatic crystallization of the granulite precursors. One sample sugge sts an inherited component as old as similar to 1700 Ma. The post-gran ulite granite has a Pb-Pb evaporation age of 333.1 +/- 1.0 Ma, and the short time interval between granulite metamorphism and granite intrus ion implies that uplift, crustal extension and cooling of the granulit e complex occurred rapidly after peak metamorphic conditions.