The role of titanite in shaping the geochemistry of amphibolite-derived melts

Abstract

Titanite, one of the major accessory phases in middle to lower crustal rocks, could play an important role in shaping trace element as well as radiogenic isotope compositions in amphibolite-derived melts. Petrographic, zircon U/Pb, whole-rock element and isotope (Sr and Nd) data on a suite of leucosomes, and titanite Nd isotope compositions from the migmatitic amphibolite complex, southern Liaoning, indicate that (1) the amphibolite with a protolith age of ~2450 Ma underwent low-degree partial melting at ~160 Ma and produced two groups of leucosome; (2) these leucosomes preserved a large magnitude of Nd isotope disequilibrium with up to ~10 epsilon units of difference with respect to their sources; (3) differential dissolution of titanite during partial melting might lead to negative correlations between Nd isotope compositions and respective Ti or V contents (proxy of the amount of titanite presented). These data indicates that during low-degree partial melting of mafic rocks, titanite stays as a residue phase due to either unfavorable texture or melting (temperature or fluid) conditions and leads to the development of melts with depletion in Ti-affinity elements, REE, and a large positive shift in Nd isotope compositions. Similar to other accessory phases (e.g., zircon, apatite, and monazite) in metasedimentary sources, this contribution presents another case that could be common in high-grade metamorphic terranes in which titanite could be a key phase that controls key trace element (e.g. NbTa, V, and REE) budget and Nd isotope compositions in melts derived from low-pressure melting of amphibolite or mafic rocks in various tectonic environments.