Trace Element (Mn-Sr-Y-Th-REE) and U-Pb Isotope Systematics of Metapelitic Apatite During Progressive Greenschist- to Amphibolite-Facies Barrovian Metamorphism

We investigate the trace element and U-Pb systematics of apatite of 19 samples of metapelites and calcareous metapelites from two progressive metamorphic sequences from the Central Alps to further understanding of the behavior and stability of apatite during metamorphism. U-Pb and trace element data were acquired from apatite mineral separates by LA-Q-ICP-MS. The trace element data were explored using chondrite- and whole-rock normalized multielement plots and principal component analysis biplots. Apatite rare earth element (REE) and U contents increase with increasing metamorphic grade due to competing growth/dissolution of allanite and REE epidote, with the lowest REE and U contents found in greenschist-facies apatites. The REE, U, and Th/U systematics of pelitic apatite are highly diagnostic of metamorphic grade and can be readily exploited in provenance studies. In contrast, apatite Th/U ratios and common Pb content broadly decrease with increasing metamorphic grade. Consequently, low-grade metapelite and calcareous metapelite apatite is often undatable by U-Pb LA-ICP-MS geochronology and underrepresented in detrital apatite U-Pb data sets but can be readily identified by its distinctive trace element characteristics. Amphibolite-facies metapelitic apatite is more amenable to U-Pb LA-ICP-MS geochronology due to its higher U and lower common Pb contents, with some amphibolite-facies pelitic apatites yielding imprecise ages consistent with Alpine metamorphism. Refractory detrital apatite yielding Variscan U-Pb ages with mafic igneous trace element signatures is also sporadically preserved in the greenschist-facies metapelites. In contrast, apatite from a Variscan granitoid yields Variscan U-Pb ages with I-type granite trace element signatures, reflecting the generally less reactive behavior of dry granitoids compared to pelitic rocks.