The chemical and Sm–Nd isotopic behaviour of accessory minerals in metasediments along the LP-HT Chugach Metamorphic Complex (Alaska)

Abstract

The study of accessory phases, including trace element concentrations and radiogenic isotopes, provides powerful information for a better understanding of geological processes such as crustal anatexis. These accessory minerals are the primary carriers of many incompatible elements and Rare Earth Elements (REE) in crustal rocks. In this contribution, we provide a detailed study on the chemical and isotopic (Nd isotopes) behaviour of accessory minerals within the Chugach Metamorphic Complex in Alaska. This Eocene (55− 50 Ma) metamorphic complex developed in a Late Cretaceous to Paleogene accretionary prism consisting of metapelitic and metagreywacke rocks. The complex exposes a systematic N-S metamorphic gradient from greenschist to upper amphibolite facies (500 to ~ 700 °C) with anatexis under water-saturated conditions and minor muscovite breakdown. Trace element concentration data for apatite, monazite and titanite reveal a strong influence of bulk composition (greywacke vs. pelite) on their REE signatures in the migmatitic gneisses. In xenotime-bearing metapelitic samples, we show that monazite and apatite, which crystallised close to peak metamorphism, have their HREE-Y contents increasing with temperature within a narrow range of ~ 150 °C (550  to ~ 700 °C). While the influence of temperature on the Y content of monazite was already demonstrated before, we prove that apatite follow the same chemical behaviour. In these samples, partial melting process can be tracked via Eu/Eu* which decreases systematically from schist to migmatitic gneisses and is interpreted to be related to plagioclase crystallisation. Among all analysed samples (schists and migmatites), we observe no significant differences in εNd between monazite, allanite and whole-rock, regardless of rock type. This suggests (i) a general homogeneity of Nd isotopic composition above 550 °C up to crustal anatexis, and (ii) an isotopic equilibrium between mineral and whole-rock, indicating Nd isotopic disequilibria induced by partial melting are unlikely in this case study.