Formation of monazite and rhabdophane at the expense of allanite during Alpine low temperature retrogression of metapelitic basement rocks from Crete, Greece: Microprobe data and geochronological implications

Accessory LREE-bearing minerals have been investigated in a sequence of Palaeozoic amphibolite facies metapelites in eastern Crete, Greece, which were overprinted by Alpine low-T metamorphism (~ 300 °C). The series is dominated by Al–Ca rich metapelite layers, which contain Palaeozoic allanite, secondary Alpine monazite with low Y contents (< 0.6 wt.%) or rhabdophane. Microstructural evidence suggests that during the Alpine overprint widespread growth of rhabdophane and Alpine monazite occurred at the expense of allanite. Allanite was obviously unstable and reactive at the given low-T conditions. Rhabdophane, a hydrated REE phosphate with low EMPA analytical totals (90–95%), high Y and Ca contents (1.5–8 wt.% Y2O3, up to 10 wt.% CaO) and high concentrations of common lead, formed instead of monazite in zones with increased fluid activity.Only in some quartz-rich layers of metapelites, which are allanite-free, is primary Palaeozoic monazite to be found. In such special variants, Alpine monazite or rhabdophane appear to be absent and the LREEs remained trapped in the Palaeozoic monazite. Not even the outermost rims of the old monazites experienced alteration during the Alpine retrograde metamorphism, and preserve undisturbed Palaeozoic Th–U–Pb ages.This observations made in this study confirm other recent work according to which, in many cases, monazite and not allanite is the stable LREE phase under low-grade metamorphic conditions. Rocks with primary allanite contents can be considered particularly fertile for the production of monazite at low metamorphic grades. In contrast, rocks without allanite, but with primary monazite, are unfertile for the growth of a secondary monazite generation. A composite, lithologically variable basement with a low temperature overprint will therefore often include both, remnant and new monazite, although the two monazite generations will commonly occur in different lithologies and rarely together in the same samples. Therefore, if a cunning sampling strategy is applied, monazite geochronology is capable of yielding the ages of both primary and secondary events in such settings.