Publications

The behaviour of Nd, U, Th and Pb isotopes in granitic rocks is usually controlled by accessory minerals. In this study an approach is adopted which uses the initial 143Nd/144Nd as a geochemical tracer together with high-precision U-Th-Pb geochronology for accessory phases and their respective whole rock. This combination of techniques is applied to zircon, allanite, sphene, apatite and thorite from both a tonalite and a granodiorite from the Tertiary Alpine Bergell intrusion. Inherited isotopic signatures have been identified in bulk-zircon fractions by inverse discordance in the U-Pb system.In the case of the granodiorite, the initial εNd in the zircon is 1.77 units lower than the bulk rock. This result indicates that Nd isotopes in zircon may serve as a useful tool for tracing crustal precursors in silicic rocks. All other accessories yield an initial εNd equal to their respective whole rock and U-Th-Pb ages equal to or lower than the respective intrusion ages. This indicates either the absence of inherited cores, or that such cores have exchanged their Nd and Pb (in the case of apatite also Sr) by diffusion. Where available, experimental mineral melt equilibria and diffusion models are consistent with this behaviour. U-Pb geochronology of carefully selected, core-free zircons yields a precise intrusion age of 31.88 ± 0.09 Ma for the tonalite. In contrast, granodioritic zircons are partially reset due to radiation damage caused by high U concentrations. However, an alternative means of dating young intrusions is to use 232Th-208Pb ages of allanite. In the case of allanite the presence of a large excess of 206Pb derived from 230Th indicates that no total Pb loss has occurred since crystallization, and the Nd isotopes indicate the absence of inheritance. Using both the allanite age and a 238U-206Pb age for sphene an intrusion age of 30.13 ± 0.17 Ma is indicated for the granodiorite. Taken together with an allanite Th-Pb age for the tonalite, these results strongly support the high closure temperatures for the allanites studied here (≥700-800°C at high postmagmatic cooling rates). All other accessory mineral ages are reset to values similar to hornblende 39Ar-40Ar ages, and reflect the effect of post-magmatic cooling superimposed on post-metamorphic cooling.

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von Blanckenburg, F. (1992): Combined high-precision chronometry and geochemical tracing using accessory minerals: applied to the Central-Alpine Bergell intrusion (central Europe ). Chemical Geology, 100, 1-2, 19-40.