Orbicular rocks are igneous, often granitic rocks, which show a conspicuous texture that consists of concentric, either single- or multi-shelled spheres called orbicules. Orbicular rocks are especially abundant in the Precambrian bedrock of Finland, where at the moment over a hundred localities are known. Geological Survey of Finland published a comprehensive report on the known Finnish localities in 2005 (Lahti, 2005).
In the coming years I hope to put up a larger research initiative to look more closely at these beautiful and enigmatic rocks and hopefully also better understand their petrological signifigance.
At the moment, an on-going MSc project aims to produce a preliminary mineralogical dataset and test some new research methods on one of the most complex and iconic Finnish orbicular rocks, the Kuohenmaa quartz monzonite.
Recently we have been testing tomography methods to better visualize the orbicule structure in the Kuohenmaa orbicules in collaboration with the GTK microtomography lab. This is a short video clip generated from a XCT 3D-model of a single large orbicule showing slice-by-slice the inner density structure and shape of also the non-orbicular core. Hoping to get more of these done in the future to better understand the orbicule crystallization and especially deformation processes.
References:
Lahti, S. (ed.), 2005. Orbicular rocks in Finland. Special Publications of the Geological Survey of Finland 47, 177 pp.
Markkanen, M. & Heinonen, A. 2019. Petrography and mineral chemistry of the Kuohenmaa orbicular rock from Kangasala. In: Kalliomäki, H. (ed.), Abstract book, 5th Finnish International Colloquium of Geosciences, Bulletin of the Geological Society of Finland, Special Volume 2, Helsinki, 42
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