09/03/2018

Science Blog: Metamorphic map of Finland

Text:
Pentti Hölttä, Senior Scientist

In the Precambrian bedrock of Finland, almost all rocks are metamorphosed, and most metamorphic rocks have undergone medium- to high-grade metamorphism, in which the original volcanic, sedimentary and igneous rocks have been altered to deformed gneisses that can again be partially melted. Metamorphism is a feature that can easily be mapped, providing essential information on the evolution of the bedrock. A metamorphic map of Finland was published in 2017 in GTK Special Paper 60 and can also be found on the GTK web page http://gtkdata.gtk.fi/mdae/index.html, Geology layers – Metamorphism of Finland 1M.

Metamorphic studies in Finland have a long history dating back to the beginning of the last century, when Pentti Eskola published the metamorphic facies concept in 1915 in the Bulletin of the Geological Commission of Finland, the precursor of the Geological Survey. The facies theory is simple and straightforward in modern thinking, stating that the mineral assemblage in a rock is only determined by pressure, temperature and the chemical composition, but it raised metamorphic studies to a completely new level and made Eskola the most famous Finnish geologist in the world up to the present day. Pentti Eskola’s career as a scientist was also recognised by the Finnish government. When he died in 1964, he received a state funeral, an honour that has been given to a very few scientists.

In the last century, Pentti Eskola did not have many followers in Finland. Anna Hietanen became a well-known metamorphic petrologist, but she wrote most of her papers on metamorphism in the 1960s and 1970s in America at the US Geological Survey. In the 1970s, Kalevi Korsman started regional metamorphic studies at GTK by publishing his PhD on progressive metamorphism in the Rantasalmi-Sulkava area. In the 1980s and 1990s, he led the Thematic Group at the Bedrock Department, which conducted regional metamorphic and structural studies in eastern and southern Finland. The metamorphic map of Finland is largely based on those studies and the metamorphic mapping carried out in the Bedrock & Resources Department in the late 1990s and early 2000s in central Lapland. New mapping was performed during 2011–2014 in Pohjanmaa and in eastern and southern Finland.

The Proterozoic part of the metamorphic map of Finland is constructed on the basis of the observed metamorphic features mainly occurring in peraluminous metasedimentary rocks, especially the mineral assemblages, grain size and the onset and degree of partial melting that characterize a particular metamorphic zone. Roughly half of the Proterozoic rocks in Finland are metamorphosed sedimentary rocks, mica schists, mica gneisses and migmatites, and this is therefore a natural choice for the classification. Metavolcanic and metaigneous rocks are expected to represent the same metamorphic grade as the surrounding metasediments. For most of the Archean areas, this classification cannot be used, because peraluminous metasedimentary rocks are rare in the Archean, which mainly consists of tonalitic-trondhjemitic-granodioritic (TTG) gneisses, whose mineral assemblages are not very useful in metamorphic classifications. Therefore, the map of the Archean areas is mainly based on the metamorphism of mafic rocks, which are commonly found as inclusions and intercalations in the TTG gneisses.

The GIS map contains several layers. In the simplest one, the bedrock is classified into low-, medium- and high-grade metamorphic domains following the classification of the IUGS Commission for the Management and Application of Geoscience Information (CGI). The second layer is based on the metamorphic facies. The third and most ambitious layer shows metamorphic zones based on the stable mineral assemblages in the PT pseudosections, wherein the assemblages are classified into low-pressure (LP), medium-pressure (MP) and high-pressure (HP) series. A pseudosection is a phase diagram for a fixed whole-rock composition, showing the pressure (P) and temperature (T) fields of stable mineral assemblages. Without any other information than that provided by whole-rock analysis, the pseudosection gives rough PT estimates for the crystallization conditions of each metamorphic zone and is a valuable addition to the classification already carried out in the field, e.g. division into fine-grained andalusite schists with well-preserved sedimentary structures, medium-grained sillimanite gneisses and highly molten migmatites. The problem with pseudosections is that because they are based on the whole rock composition, they are also sensitive to compositional changes. However, compositional differences in the Svecofennian porphyroblastic aluminium-rich metasedimentary rocks are not too great, so only a few representative compositions can be used in the map construction.

The PT fields of the mineral assemblages are displayed in the PT pseudosections with a colour corresponding to the same colour in the map. The pseudosection map has additional layers showing pre-peak and overprinting metamorphic events, wherever such classification is possible.

Where can we use metamorphic maps? First, knowledge of the pressure–temperature–time evolution of metamorphism is crucial in tectonic modelling. As a simple example, the presence of kyanite in the Karelia Province and its absence from the Svecofennia Province indicates a very different plate tectonic environment. The structural and metamorphic development of the bedrock are closely related and it is very important to study these together. One reason for metamorphic studies is naturally to assess the potential for metamorphogenic ores. About 90% of our known gold deposits are found in zones of low to medium metamorphic grade, and it is therefore important to map these zones carefully. Furthermore, complex pegmatites, which, for example, are the source of lithium, are commonly found in similar metamorphic environments.

When the old generation of researchers leaves GTK during the next couple of years, it will be very important for metamorphic studies to continue. The geological surveys need to know the geology of their countries, and especially in the case of our metamorphic bedrock, knowledge of metamorphism is crucial if we really want to deepen the understanding of our Precambrian bedrock.

Reference

Hölttä, P., and Heilimo, E. 2017. Metamorphic map of Finland. Geological Survey of Finland, Special Paper 60: 75–126.

Figure caption. Metamorphic map of Finland. The pseudosections on the right and in the inset in the map show the the pressure-temperature fields for stable mineral assemblages for a representative metapelite, metagreywacke and amphibolite compositions. The colours in the map correspond to the colours in the pseudosections.

 

 

 

Pentti Hölttä

Text: Pentti Hölttä

Pentti Hölttä is a senior researcher who has been working at GTK since 1984. His main interests have been metamorphic and Archean geology. He is also an adjunct professor at the University of Helsinki, where he has regularly taught courses on metamorphic petrology. Currently, he is focusing on retirement.