Robert Anczkiewicz
Polish Academy of Sciences, Poland
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Polish Academy of Sciences, Poland
Rates of tectonic processes deduced from Lu-Hf and Sm-Nd garnet geochronology
Abstract
Robert Anczkiewicz
Institute of Geological Sciences, Polish Academy of Sciences, Kraków Research Centre, 31-002 Kraków, Poland.
Garnet provides for geochronology what cannot be delivered by any other geochronometer. It can simultaneously be dated by several isotopic systems, permits relatively easy link to PT conditions, may provide constraints on timing or even rates of crustal deformation and serves as a reference mineral for geochronometers whose interpretation often depends on establishing a link to garnet growth. While under low temperature conditions major elements distribution in garnet provides clear interpretation of isotopic ages, interpretation of garnet dating of high-grade rocks may be challenging. As garnet geochronology becomes more and more popular, problems long recognized in long established geochronometers appear to affect garnet dating, too. Some of the key issues arise from different diffusive properties of elements involved in the dating systems which lead to decoupling of multiple isotopic clocks, sub-micron scale inclusions or strong element zonation. In this study we investigated Lu-Hf and Sm-Nd isotopic systematics in high and ultra-high temperature rocks in several Alpine and Hercynian collisional settings. Isotopic analyses supported by thermodynamic P-T estimates and trace element partitioning study document high resistance of Lu-Hf clock which withstands temperatures in excess of 900 °C and decouples from Sm-Nd system. On the other hand, in the presence of melt, under much lower temperature, heavy and light REE are decouple below their so called closure temperatures. While Sm-Nd undergoes partial to full resetting even in crystals in excess of 3 cm, Lu-Hf is only partially reset, although it clearly is disturbed. Our study shows that temperature, although plays one of key roles, it is not a decisive factor defining closure of isotopic clock.