Controls on trace element uptake in metamorphic titanite: Implications for petrochronology

Abstract

Petrochronology-the interpretation of isotopic dates with complementary elemental data- requires understanding the relationship between trace elements in chronometers and the petrological evolution of their host rocks. Titanite is a useful petrochronometer for crustal processes, but how titanite records host rock evolution is uncertain. We present an extensive titanite U-Pb and chemical dataset from felsic gneisses and leucosomes in the Western Gneiss Region (WGR) of Norway. Mineral textures, U-Pb dates, and major, minor, and trace element chemistry reveal three titanite populations: (1) Precambrian igneous titanite [high light rare earth elements (LREE), Th, Pb, Zr; low Al, F]; (2) Caledonian recrystallized titanite (low LREE, Th, Pb) that formed from dissolution-reprecipitation of the Precambrian titanite and co-crystallized with allanite; (3) Caledonian neocrystallized titanite (high Al, F and variable REE). Although titanite records multiple igneous and metamorphic events in the WGR, we use a principal components analysis to identify distinct petrological and thermal effects on trace element uptake that hold across all titanite populations. Coupled with textural observations, these data show that different trace element patterns between populations predominantly represent the activity of different rock reactions during continental subduction and exhumation; using correlations between principal component scores and trace element abundances or ratios, we discriminate which phases co-crystallized with titanite. Our results further demonstrate that thermal and fluid partitioning effects can complicate interpretations of rock petrology from titanite trace elements, but these factors can be assessed by measuring specific trace elements (e.g. Al, Zr).