sedimentary rocks of the central Fennoscandian Shield indicating high amounts of Ba lost from the clastic record during 2.3 – 1.9 Ga and further recycled back to the mantle forming a subduction component and an enriched mantle
component. Ba depletion seems to have been especially characteristic of chemical weathering during 2.35 – 2.2 Ga under CO
2
-rich and low-O
2
atmosphere. Whether this strong Ba depletion is characteristic of the Archaean – Protero- zoic transition and quiet supercontinent stages in general remains to be determined. © 2000 Elsevier Science B.V. All
rights reserved.
Keywords
:
Archaean; Palaeoproterozoic; Sedimentary rocks; Geochemistry; Provenance; Finland
1. Introduction
The geochemistry of clastic sedimentary rocks can be used as an indicator of crustal evolution
e.g. Taylor and McLennan, 1985 or to identify ancient tectonic settings in metamorphic terranes.
Sedimentary rocks can be divided into those showing local sources and those having experi-
enced effective mixing in large river marine sys- tems before deposition. The latter types sample
large areas providing data for crustal-scale pro- cesses. The possibility of different crust-forming
mechanisms during Archaean and Proterozoic times emphasizes the importance of the Ar-
chaean – Proterozoic boundary where there might be a corresponding compositional change in the
sedimentary record e.g. Taylor and McLennan, 1985; McLennan and Taylor, 1991. Selective
preservation of sedimentary rocks in the ancient record can on the other hand hamper their use in
crustal evolution studies. Along with this limita- tion, other factors discussed below, should also
be taken into account when using ancient sedi- ments to give information on the general prove-
nance of the studied sedimentary unit.
The lithology of the provenance area essen- tially controls the chemical composition of the
clastic sediments but other factors such as degree of palaeoweathering, hydraulic sorting grain-size
effects, organic and sulphide input, diagenesis and metamorphism especially migmatization
may greatly modify or ultimately erase prove- nance memory. Sediment recycling is a common
feature e.g. Veizer and Jansen, 1985 and pro- duces a buffering effect where a small amount of
new input can go unnoticed. Nevertheless, even though the interpretation of their compositions is
more controversial than with igneous rocks, the long ‘memory’ of sedimentary rocks can be quite
powerful when modelling the tectonic settings and evolutionary histories of metamorphic ter-
ranes.
The central part of the Fennoscandian Shield in Finland is composed of the Palaeoproterozoic
Svecofennian domain and the Archaean Karelian craton with a Palaeoproterozoic allochthonous
and autochthonous cover Fig. 1. The occur- rence of a cryptic ‘suture’ Fig. 1; Koistinen,
1981; Huhma, 1986 between the Karelian and Svecofennian domains is favoured by the obser-
vation that no Archaean component is found in the 1.93 – 1.91 Ga gneissic tonalites and related
felsic volcanics adjacent to the Archaean craton Lahtinen and Huhma, 1997. Lahtinen 1994
proposed also the occurrence of a tentative ‘su- ture’ Fig. 1 separating the central part of the
Svecofennian domain from the southern Sve- cofennian. Studies on the geochemistry of sedi-
mentary rocks in the study area are few and include a geochemical and isotopic study from
the Archaean Hattu schist belt O’Brien et al., 1993, a major element study from the northern
part of the Ho¨ytia¨inen area Kohonen, 1995, a regional correlation diagram study from the Savo
province Kontinen and Sorjonen-Ward, 1991 and a research concentrating on black schists
Loukola-Ruskeeniemi and Heino, 1996 and ref- erences therein.
The study area has been sampled in the course of a regional bedrock geochemical survey under-
taken by the Geological Survey of Finland in- cluding
the 300
metasedimentary samples
discussed here. The samples range from Archaean to Palaeoproterozoic, were formed in a variety of
tectonic settings, and are thus suitable for study- ing the Archaean – Proterozoic transition and the
evolution of Fennoscandian Shield. The main source components and implications for the tec-
tonic evolution of the central Fennoscandian shield are given with emphasis on proposed su-
tures. Notes on the crustal evolution and Ar- chaean – Proterozoic transition in general, and on
Ba depletion are also given. As all the studied sedimentary rocks are metamorphosed, the prefix
’meta’ has been dropped. The data set is available on request from the author.
2. Sampling and analytical methods
