Geochronology of Metamorphic Events in the Lower Crust beneath NW Russia: a Xenolith Hf Isotope Study

• Published in: Journal of Petrology
• Main Authors: Koreshkova, M., Downes, Hilary, Millar, I., Levsky, L., Larionov, A., Sergeev, S.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Oxford Journals 2017
• Subjects: Earth and Planetary Sciences; Arkhangelsk; Lapland
• Online Access: https://eprints.bbk.ac.uk/id/eprint/29066/; https://doi.org/10.1093/petrology/egx065

Hf isotope data for zircons and whole-rocks from lower crustal mafic granulite and pyroxenite xenoliths from NW Russia are presented together with the results of U–Pb zircon dating, Sm–Nd and Rb–Sr isotopic compositions of bulk-rocks and minerals, and trace element compositions of minerals. Most zircons preserve a record of only the youngest metamorphic events, but a few Grt-granulite xenoliths retain Archean magmatic zircons from their protolith. Metamorphic zircons have highly variable ɛHf(t) values from –25 to –4. The least radiogenic zircons were formed by recrystallization of primary magmatic Archean zircons. Zircons with the most radiogenic ɛHf grew before garnet or were contemporaneous with its formation. Zircons with ɛHf(t) from –15 to –9 formed by various mechanisms, including recrystallization of pre-existing metamorphic zircons, subsolidus growth in the presence of garnet and exsolution from rutile. They inherited their Hf isotopic composition from clinopyroxene, pargasite, rutile and earlier-formed zircon that had equilibrated with garnet. Subsolidus zircons were formed in response to a major change in mineral association (i.e. garnet- and zircon-producing reactions including partial melting). Recrystallized zircons date the onset of high-temperature conditions without a major change in mineral association. Age data for metamorphic zircons fall into five groups: >1·91 Ga, 1·81–1·86 Ga, 1·74–1·77 Ga, 1·64–1·67 Ga and <1·6 Ga. Most ages correlate with metamorphic events in the regional upper crust superimposed onto rocks of the Belomorian belt during formation of the Lapland Granulite Belt. Zircon formation and resetting at 1·64–1·67 Ga significantly postdates Lapland–Kola orogenic events and may relate to the onset of Mesoproterozoic rifting. The youngest ages (1·6–1·3 Ga) correspond to an event that affected only a few grains in some samples and can be explained by interaction with a localized fluid. The observed garnet-granulite associations were formed at 1·83 Ga in Arkhangelsk xenoliths and 1·74–1·76 Ga in most Kola xenoliths. By the end of the Lapland–Kola orogeny, the rocks were already assembled in the lower crust. However, no addition of juvenile material has been detected and preservation of pre-Lapland–Kola metamorphic zircon indicates that some xenoliths represent an older lower crust. Granulites, pyroxenites and Phl-rich rocks have a common metamorphic history since at least c. 1·75 Ga. At about 1·64 Ga metasomatic introduction of phlogopite took place; however, this was only one of several phlogopite-forming events in the lower crust.