Lu-Hf geochronology and trace element distribution in garnet: implications for uplift and exhumation of ultra-high pressure granulites in the Sudetes, SW Poland.

Anczkiewicz, R; Szczepanski, J; Mazur, S; Storey, C; Crowley, Quentin; Villa, IM; Thirlwall, M; Jeffries, TE. 2007 Lu-Hf geochronology and trace element distribution in garnet: implications for uplift and exhumation of ultra-high pressure granulites in the Sudetes, SW Poland. Lithos, 95 (3-4). 363-380. 10.1016/j.lithos.2006.09.001

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Combining Lu–Hf garnet geochronology with in situ trace element analyses in garnet allowed us to gain new insight into the metamorphic evolution of UHP–UHT rocks in the Stary Gierałtów region, in the Polish Sudetes. Prograde garnet growth recorded by Rayleigh-type heavy REE (HREE) zoning in the felsic granulites indicates that the obtained 386.6±4.9 Ma Lu–Hf age represents the time of garnet crystallization on a prograde UHP metamorphic path. The surrounding rocks were metamorphosed at the same time as indicated by 381.2±6.7 Ma Sm–Nd garnet age obtained for the mid-crustal metapelites. The second metamorphic episode, which affected most of the lower crust in the Orlica–Śnieżnik Massif (OSM) occurred at ca. 340 Ma as determined by U–Pb zircon and Sm–Nd garnet dating of granulites in this and previous studies is interpreted as a high temperature event, which took place on a retrograde path. Trace element distribution in garnets from the layered granulites showed significant differences in distribution of medium and HREE in garnets from mafic and felsic protoliths over the course of the metamorphic evolution. This had strong impact on the isotopic dating results and led to “decoupling” of the Sm–Nd and Lu–Hf clocks, which recorded timing of the two different metamorphic episodes separated by as much as 40 Ma. Moreover, the preservation of the HREE growth zonation profile in garnets from the felsic granulites whose minimum metamorphic temperature was established at 900 °C implies that the Lu–Hf system under relatively dry conditions does not undergo significant diffusional re-equilibration even at such extreme temperatures and therefore it sill provides the age of prograde garnet growth. Under hydrous conditions, at least some resetting will take place, as documented by the partially relaxed HREE zonation profile in the amphibolitised mafic granulite, which yielded a 10 Ma younger age. The HREE distribution study appeared to be a particularly valuable and essential tool, which allowed us to distinguish garnet growth from post-growth complexities and hence, provide improved age interpretation. Medium REE, on the other hand, did not show any obvious correlation with the isotopic signature of garnet.

Item Type: Publication - Article
Digital Object Identifier (DOI): 10.1016/j.lithos.2006.09.001
Programmes: BGS Programmes > NERC Isotope Geoscience Laboratory
ISSN: 0024-4937
Additional Keywords: Garnets, Metamorphic rocks, Poland, Trace elements, Geochronology
NORA Subject Terms: Earth Sciences
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Date made live: 15 Apr 2008 13:34 +0 (UTC)

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