Garnet granulites from the Lapland Granulite Belt, arctic Finland

The Lapland Granulite Belt is an arcuate Paleoproterozoic (c. 1910–1880 Ma) granulite-facies metamorphic complex spanning arctic Finland, Norway, and the Kola Peninsula of Russia.  It has been interpreted as a core zone to the larger Lapland–Kola orogen that formed between the colliding Kola and Karelian Archean continents at 1930­­–1910 Ma (Daly et al., 2006; Lahtinen & Huhma, 2019).  The granulite complex comprises grt + crd + sill pelitic migmatites hosting centimetre- to kilometre-thick enderbitic (qtz + pl + opx ± cpx) and noritic (pl + opx ± hbl) sheets emplaced as mafic intrusions (Tuisku et al., 2006).  Along the granulite belt’s southwest margin is a large anorthosite body, the Angeli Anorthosite. 

The tectonic mechanism for granulite-facies metamorphism and melting is unresolved, with both the enderbite and anorthosite bodies as potential heat sources.  It is also unclear if the orogen formed in a modern-style arc setting, or in a more ‘Archean-type’ collision zone (Lahtinen & Huhma, 2019).  Previous pressure-temperature estimates for peak metamorphism are 850°C and 5­–9 kbar (Tuisku et al., 2006).  Spinel + quartz inclusions in garnet also hint at high (> 850°C) peak temperatures.  However, there has been relatively little published on the metamorphic history of the orogen in recent decades, and, until now, sample-specific phase equilibria modelling has yet to be applied to the region. 

Here we present preliminary fieldwork, petrographic observations, and geochemical data for a suite of garnet-bearing granulites from the Lapland Granulite Belt focussed on the Inari, Ivalo, and Kárášjohka valley regions.  Electron probe micro-analyser (EPMA) data have been acquired by the new JEOL JXA-iSP100 Superprobe at HelLabs, University of Helsinki.  Preliminary EPMA data analysis by XMapTools (Lanari et al., 2019) is introduced.

 

References:

Daly, J. S., Balagansky, V. V., Timmerman, M. J. & Whitehouse, M. J., 2006. The Lapland-Kola Orogen: Paleoproterozoic collision and accretion of the northern Fennoscandian lithosphere. In: Gee, D., G. & Stephenson, R. A. (eds.), European Lithosphere Dynamics: Geological Society Memoirs 32, 579–598.

Lahtinen, R. & Huhma, H., 2019. A revised geodynamic model for the Lapland-Kola Orogen. Precambrian Research 330, 1­–19.

Lanari, P., Vho, A., Bovay, T., Airaghi, L., & Centrella, S., 2019. Quantitative compositional mapping of mineral phases by electron probe micro-analyser. Geological Society of London, Special Publications 478, 39–63.

Tuisku, P., Mikkola, P. & Huhma, H., 2006. Evolution of Migmatitic Granulite Complexes: Implications from Lapland Granulite Belt, Part I: Metamorphic geology. Bulletin of the Geological Society of Finland 78, 71–105.