Why allanite may swindle about its true age

Allanite from the Tertiary Rieserferner pluton (Austrian Alps) is texturally and chemically heterogeneous. Continuous covariation trends reflect coupled substitution of Ca+Al vs. Fe+REE+Th in allanite, whereas systematic variations in La/Nd demonstrate the increasingly stronger depletion of LREE in the melt during its crystallization. Allanite samples (corrected for Pb-206(excess)) from two rocks scatter in the concordia diagram and define discordias from 31.8+/-0.4 Ma and 32.2+/-0.4 Ma to ca. 540 Ma. The apparent inheritance does not originate from the inclusion of older allanite or a high-mu phase, such as monazite, xenotime, or zircon, but from the incorporation of radiogenic Pb originating from a precursor. Since allanite requires a high enrichment of Th, (U), and LREE, it may form at the expense of a Th-LREE-rich precursor in metamorphic rocks or where such a phase had dissolved in melts. Likely precursors acquire with time radiogenic Pb isotopic compositions. This Pb, if incorporated in the product mineral, may give the illusion of inheritance. The allanite samples from the Rieserferner pluton show a tendency from high Th/U-calc (30-50) in samples with an Alpine age to low Th/U-calc (4-12) in samples with distinct inheritance. This relation between extent of inheritance and apparent Th/U-calc indicates a lower Th/U-calc for a possible precursor, falling into the Th/U range commonly encountered for monazite. Precursor monazite would have originated from assimilated Palaeozoic rocks and give rise to localized enrichments of Th and LREE in the melt, thus eventually enabling the growth of allanite.