Allanite behaviour during incipient melting in the southern Central Alps

The response of allanite to incipient melting was investigated in migmatites from the Tertiary Barrovian-type sequence of the Central Alps (southern Switzerland, northern Italy). Inheritance and new mineral growth were recorded in composite allanite grains sampled from meta-granitoids and leucosomes. Ion microprobe (SHRIMP) dating of high Th/U allanite cores in meta-granitoids yield Permian ages consistent with magmatic crystallisation dating protolith intrusion. In contrast, low Th/U allanite overgrowths and weakly-zoned allanite in meta-granitoids and leucosomes yield Alpine U–Pb intercept ages between 30 ± 4 and 20 ± 5 Ma; these date allanite formation during the Barrovian cycle. Major and accessory mineral REE compositions suggest that Alpine allanite crystallised in the presence of a low-temperature melt. Whereas new zircon growth is rare in the migmatites, allanite readily recorded growth during the Alpine cycle. Allanite U–Th–Pb isotopes may therefore present a complementary approach to zircon for dating low-temperature partial melting, where the preservation of allanite is aided by low LREE solubility in hydrous granitic melt. The Th–Pb age is preferred to date high-Th magmatic allanite, however the U–Pb and Th–Pb ages of allanite overgrowths may differ (by up to 25%), and this demands a comparison of both U–Pb and Th–Pb isotopic systems to obtain a best estimate for the timing of low-Th allanite crystallisation. Protolith allanite preserves a substantial memory of its initial age in spite of upper amphibolite facies re-working during migmatisation (T = 620–700 °C), which places strong constraints on Pb closure temperature. Magmatic allanite contains <30% of initial (non-radiogenic) Pb, allanite in migmatites is characterised by an initial Pb of 20–70%, and subsolidus allanite has >60% of initial Pb. Therefore, the initial Pb may be useful as a sensor for the amount of melt present during allanite formation. The Pb isotope composition of allanite overgrowths indicates ⩽5% inherited radiogenic Pb from precursor allanite, which suggests efficient redistribution and homogenisation of Pb isotopes during the Alpine partial melting period.