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Further evidence for an early Carboniferous ( ∼340 Ma) age of high-grade metamorphism in the Saxonian granulite complex

Abstract:

U-Pb and Pb-Pb zircon ages for metamorphic zircons from granulites in the Saxonian granulite complex are reported, using the SHRIMP ion microprobe, conventional multigrain and single-gain techniques and the evaporation method. This is complemented by a Pb-Pb evaporation age for a postgranulite granite emplaced into the schist mantle around the granulites 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 isotopic composition reflecting the time of their formation. Multifaceted zircons from four granulite samples that probably grew close to the peak of high-grade metamorphism yielded identical U-Pb and Pb-Pb ages of ∼340 Ma which support previously published data and unambiguously show that the granulites formed during a lower Carboniferous event and not in the early Palaeozoic or Precambrian as previously suggested. Older cores in some of the metamorphic zircons reveal early Palaeozoic components at 470-485 Ma that we interpret as ages reflecting magmatic crystallization of the granulite precursors. One sample suggests an inherited component as old as ∼ 1700 Ma. The post-granulite granite has a Pb-Pb evaporation age of 333.1 ± 1.0 Ma, and the short time interval between granulite metamorphismand granite intrusion implies that uplift, crustal extension and cooling of the granulite complex occurred rapidly after peak metamorphic conditions. © Springer-Verlag 1998.