Behaviour of allanite during incipient partial melting in the Southern Central Alps

Courtney Gregory¹, Daniela Rubatto¹, Jöerg Hermann¹ and Alfons Berger²

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2 Institute of Geological Sciences, University of Bern, CH-3012 Bern, Switzerland

The chemical and U-Th-Pb isotopic behaviour of accessory allanite during incipient partial melting was investigated in a field-based study of upper-amphibolite facies migmatites in the southern Central Alps (Switzerland). Orthogneiss and leucosome sampled from the core of the migmatite zone (peak T of ~680-720°C) to its limit (T ~650°C), contained both zircon and allanite, which permitted a comparison of U-Th-Pb systematics in these phases.

Allanite found in orthogneiss showed complex internal zoning (see backscatter electron or BSE image in Fig. 1), suggestive of multi-stage growth. Bright BSE cores with high La/Lu and Th/U and low Eu/Eu* chemical signatures yielded Permian ages and thus were inherited from the pre-Alpine magmatic protolith. In contrast, chemically distinct overgrowths and single grains gave age populations between 30 ± 1 Ma and 23 ± 1 Ma in line with an Alpine metamorphic origin. Textural and inclusion relationships with major melt-formed minerals and their LREE-depletion supports the interpretation that allanite was an anatectic phase. Further evidence is provided by comparing magmatic and metamorphic mineral Eu compositions (Fig. 1). Plagioclase did not impose a negative Eu anomaly on co-existing metamorphic phases because 80-90 % of the bulk-rock Eu was actually hosted in metamorphic allanite and titanite. To account for this, it is suggested that metamorphic allanite and titanite formed during feldspar breakdown, which would have occurred above the wet solidus for this system (i.e. at T > 650°C).

The inheritance of Permian cores provides first hand evidence for minimal Pb diffusion in allanite during Alpine partial melting (i.e. ~7 million years above 650°C). Importantly, in samples where co-existing zircon had rare or limited metamorphic overgrowths (i.e. at T < 700°C), allanite was the only accessory mineral chronometer that recorded the Alpine event. The U-Th-Pb system in allanite therefore presents a solid, complementary approach for the geochronology of low-temperature (~650-700°C) partial melt processes in the crust.

Accepting allanite as a melt-product, the youngest U-Th-Pb age obtained from a discordant leucosome indicates that the Alpine melting regime lasted over several million years (until 23 Ma) and later than previously accepted. Combined with previous constraints, the prolonged high temperature evolution down to 23 Ma requires a subsequent period of fast cooling (~100 ± 20°C/Ma) for the studied samples.