Titanite links rare-element (meta-)pegmatite mineralization to Caledonian metamorphism

The metamorphic reworking of mineralized pegmatites during orogenesis remains unclear, making the genesis and the tectonomagmatic significance of pegmatite mineralization obscure. This study demonstrates the multiple utilities of titanite geochemistry in establishing the Caledonian metamorphic evolution of the world's largest intraplutonic Nb-Y-F pegmatites in the Paleoproterozoic host rocks in Tysfjord, Northern Norway. A combination of titanite-specific barometry, Zr-in-titanite thermobarometry, and titanite U-Pb geochronology yields peak metamorphism of -12 kbar and 730-750 degrees C at -410 Ma for the host rocks and the largest metamorphosed Paleoproterozoic pegmatite (meta-pegmatite) in the region. In relation to published U-Pb ages of 410-400 Ma for other regional meta-pegmatites, interpreted as Caledonian overprinting here, we argue that the Paleoproterozoic pegmatite protoliths have undergone metamorphism analogous to the host rocks, which was caused by allochthonous nappe stacking late in the Caledonian collisional orogeny (-440-400 Ma). Published for-mation ages of -400-380 Ma for the regional undeformed pegmatites are 10-30 Ma younger than the peak metamorphism, supporting an anatectic link to the Caledonian post-collisional extension. The Caledonian orogeny led to strong shearing of preexisting pegmatite bodies and resetting of trace elements and U-Pb isotopic systems of the constituent minerals via mineral re-crystallization, inheritance of Pb isotopic components, and element diffusion. This cautions against the interpretation of meta-pegmatite-derived ages and geochemical information in tectonomagmatic terranes without detailed textural investigation. (C) 2022 The Author(s). Published by Elsevier Ltd.

Automated summary

This study uses titanite geochemistry to investigate the Caledonian metamorphic evolution of Paleoproterozoic pegmatites in Tysfjord, Northern Norway. The results suggest that the pegmatites experienced metamorphism after tectonomagmatic events and were formed later than the peak metamorphism, with a link to post-collisional extension during the Caledonian orogeny.