5. The depletion of elements characteristic of mafic components and the relative enrichment of
LREE, Sr, Th, U and Zr point to a larger felsic component relative to the WK psammites. The
chemical composition of the CF2 group shows an enrichment of mafic components relative to CF1.
CF3 is a heterogeneous group characterized by migmatites and thus the average Table 2 in-
cludes also mica gneiss fragments in migmatites. Mineralogically the CF3 rocks differ from the
CF1 – CF2 in the ubiquitous occurrence of garnet. The more clay-rich nature of CF3 is seen in lower
SiO
2
and higher MgO and K
2
O Table 2. The CF3 migmatites form an inhomogeneous group
ranging from samples with HREE enrichment to samples with HREE depletion and Eu enrichment
at low total REE abundances compared with less migmatitic CF3 samples. This is interpreted as
different amounts of restite and leucosome in sampled outcrops.
5. Discussion
5
.
1
. Palaeoweathering Palaeoweathering in the source area is one of
the most important processes affecting the com- position of sedimentary rocks. Sedimentary rocks
sensu stricto are composed merely of weathering products and reflect the composition of weather-
ing profiles, rather than bedrock e.g. Nesbitt et al., 1996. Based on CIA values Nesbitt and
Young, 1982 the source rocks affected the most by weathering are those of Archaean group Ar1
60 – 65,
Jatulian quartzites
58 – 73, au-
tochthonous groups H1 – H3 54 – 70 and south- ern Svecofennian groups RH1 – RH2 57 – 68
whereas the allochthonous WK1 – WK2 mostly show CIA values lower than 55 Fig. 4. Most of
the central Svecofennian psammitic rocks also have low CIA values B 55 with an increase up
to 60 – 67 in CF3 pelitic rocks. This general increase in CIA with silica-poorer and more
pelitic nature is a common feature and readily explained by the higher proportion of clays
weathering products in pelites. The CIA value is also affected by other pro-
cesses than the clastic composition of the rock in question. Overestimation of Ca in carbonates can
lead to too high CIA values if Mg-bearing car- bonates are present. Fortunately only a few sam-
ples have over 0.5 CO
2
and thus this is only problematic in limited cases but is especially cru-
cial for quartz-rich samples. The other problem is related to the loss of CO
2
and incorporation of liberated Ca in recrystallizing minerals e.g. epi-
dote and plagioclase during metamorphism cf. Lahtinen, 1996 a situation proposed for some
samples in the Ho¨ytia¨inen area Fig. 7.
The prevailing climatic conditions of the source areas during sediment formation are difficult to
estimate especially if we consider the recycled nature of many sediments, possibly having older
weathered components. The situation can be thus complex including mixing of a strongly weathered
component older sediments or deeply weathered palaeosol with immature crust components be-
fore deposition, forming a sedimentary rock showing moderate CIA values. Also the degree of
weathering is related to the rate of erosion, which is high in tectonically active areas and thus in-
hibiting extensive weathering even in high rainfall tropical conditions. The extent of weathering is
determined primarily by the amount of rainfall acids on the weathering profile Singer, 1980
where as the climatic effect on weathering trends is probably insignificant Nesbitt and Young,
1989.
The REE, Th and HFSE especially Sc are considered least susceptible to fractionation by
exogene processes including weathering Taylor and McLennan, 1985; McLennan et al., 1990.
REE mobility during weathering has been never- theless observed Nesbitt, 1979; Duddy, 1980;
Condie et al., 1995 although Nesbitt 1979, Duddy 1980 found no net losses or gains when
whole weathering profiles were considered. Deple- tion of Sc has been postulated during weathering
under low-O
2
atmosphere Maynard et al., 1995. The Palaeoproterozoic autochthonous units
above Archaean basement in the study area Ko- honen and Marmo, 1992 and references therein
start with the Ilvesvaara Formation overlain by the glaciogenic Urkkavaara Formation followed
by Hokkalampi Palaeosol. Sturt et al. 1994 con- cluded that widespread 2.35 Ga regolith including
the Ilvesvaara
Formation occurred
on the
Fennoscandian shield and was related to an arid or semi-arid palaeoenvironment. Although this
might be the case for the Ilvesvaara Formation, the occurrence of the up to 80 m deep Hokkalampi
Palaeosol not mentioned by Sturt et al., 1994 with a minimum age of 2.2 Ga records intense
chemical weathering under a tropical warm and humid climate Marmo, 1992. The drift of
Fennoscandian from 30°S at 2435 Ma to about 30°N at 2100 Ma Pesonen et al., 2000 shows that
Fennoscandian crossed the equator during this time favouring the interpretation of Marmo
1992. It has been suggested that the Hokkalampi Palaeosol and derived formations covered large
areas of the stable Karelian craton Kohonen and Marmo, 1992; Marmo, 1992 where they formed
the bulk of detritus for the Palaeoproterozoic rift basins.
The chemical and mineralogical data of the Hokkalampi Palaeosol indicate a typical weather-
ing sequence cf. Nesbitt and Young, 1989; Condie et al., 1995 with an initial decrease in the amount
of plagioclase followed by loss of K-feldspar and biotite seen as an increase in CIA values from
about 60 – 70 lowermost to the highest values of 80 – 90 in the upper zone Marmo, 1992. Potas-
sium metasomatism of kaolinite to illite in palaeosol results in lowering of CIA values Fedo
et al., 1995. This possibility has been studied using an A – CN – K compositional space Fig. 7
for the data of the Hokkalampi Palaeosol formed upon K-feldspar rich granitoid and sandstone.
There is a slight amount of added potassium in lower palaeosol zones probably due to percolation
of solutions from the leached uppermost potas- sium-depleted zone during weathering Marmo,
1992. However, if the whole mass balance of the weathering profile is considered, no input of exter-
nal potassium is needed.
Fig. 7. A – CN – K and A – K – C – K triangles see Fedo et al., 1995, 1997 depicting trends in the Hokkalampi palaeosol and autochthonous groups of this study. A Data for Hokkalampi palaeosol formed upon a K-feldspar-rich granitoid granitoid zones
2 – 3 and sandstone sandstone zones 1 – 3, and an average of Archaean crust and Archaean sedimentary rocks Ar1 – Ar2. Trajectories a and b represent weathering trends for sandstone and Archaean average crust predicted from kinetic leach rates
Nesbitt and Young, 1984. B Data for Jatuli-type quartzites and autochthonous groups H1 – H3. Trajectories a and b same as in Fig. 7A. Dashed line encloses possible source end members for autochthonous sedimentary rocks. C Data for Jatuli-type quartzites
and autochthonous groups H1 – H3. Note the shift of some samples towards the sodium-rich A – K – N-line indicating that albitization has possibly affected these samples. Horizontal arrows for some samples indicate the amount of Ca input due to the
inferred occurrence of carbonates followed by CO
2
loss. Averages of palaeosol zones 1 – 3 in Fig. 7B and C are calculated using mixtures of sandstone zones 50 and granite zones 50. J and K are calculated averages of Jatuli-type mafics and Kutsu-type
granites, respectively.
The autochthonous Ho¨ytia¨inen H1 – H3 groups show characteristic depletion of CaO, Na
2
O, MnO, P
2
O
5
, Sr and Ba, and low KRb, which are tentatively proposed to have an ultimate source in
the chemically weathered palaeosol. The southern Svecofennian RH1 – RH2 groups also show deple-
tion of elements normally lost during weathering Fig. 6 but the CIA values of other groups are
moderately low B 60 and no clear weathering trends are observable.
5
.
2
. Hydraulic sorting Clay minerals, enriched in most trace elements,
and preferentially concentrated in the finer frac- tions during hydraulic sorting grain size sorting
produce higher abundances of many elements in pelites relative to associated sands e.g. Korsch et
al., 1993. The situation of pure quartz dilution is the ultimate case and most easily interpreted as a
decrease in all other elements and an increase in SiO
2
. The situation is more complex when acces- sory minerals zircon, monazite, apatite, sphene
and allanite, ferromagnesian minerals, feldspars and lithic fragments are also sorted. The ThSc
ratio remains nearly constant in some cases but often muds can have significantly lower ThSc
ratios indicating a preferential incorporation of mafic volcanic material in the finer fractions e.g.
McLennan et al., 1990. Considering a simple two-component mixture of mature weathered ma-
terial quartz + clays and immature rock debris separate minerals + lithic fragments the result is
psammites enriched in immature rocks debris showing complex sorting patterns and pelites en-
riched in mature weathered material. This prefer- ential sorting can lead to REE fractionation
making interpretation of Sm – Nd isotope system- atics difficult Zhao et al., 1992 but this is mainly
effective when considering sedimentary material from unweathered coarse-grained granitoids with,
e.g., allanite hosting LREE and Th.
The wide range of SiO
2
Fig. 4 the Ho¨ytia¨inen H1 – H3 groups exhibit is clearly an effect of
sorting cf. Kohonen, 1995 dominated by quartz dilution seen as abundant quartz clasts. Sorting
enhanced enrichment of mafic component was noticed, e.g. in Western Kaleva and southern
Svecofennian pelites over psammites. The varia- tion of Zr normally 160 – 350 ppm found in
Western Kaleva psammites indicate zircon sorting but there is no correlation between Zr and HREE
or U showing that the zircon control on these elements is minor. The effect of hydraulic sorting
is readily observed in the studied samples but in many cases it also sorts different source compo-
nents into different grain size classes. This is a disadvantage when using only shales on average
more mafic or psammites on average more fel- sic in crustal evolution studies but is an advan-
tage in characterizing source end members.
5
.
3
. Effects of depositional en6ironment Different methods have been applied to the
interpretation of the depositional environment of ancient
sediments using
black shalesschists.
These include pyrite formation, SC ratios, degree of
pyritization Berner,
1984; Berner
and Raiswell, 1984; Raiswell and Berner, 1986 and
enrichment of U and V e.g. Jones and Manning, 1994; Breit and Wanty, 1991. The average
present SC ratio of normal marine sediments is 0.36 0.23 – 0.77 but age dependent variation oc-
curs and, for example, early Palaeozoic marine sediments show significantly higher SC ratio of
about 2 Berner and Raiswell, 1984; Raiswell and Berner, 1986. In fresh or low-salinity brackish
water low sulfate level is the limiting factor for pyrite formation and sediments show low SC
ratios
without any
inter-element correlation
Berner and
Raiswell, 1984.
According to
Thompson and Naldrett 1984 mantle-derived magmatic SSe ratios are generally lower B
10 000 than in sedimentary sulphides \ 10 000, which can be used to discriminate hydrothermal
influxes of sulphur.
Only autochthonous
H1 – H3 and
al- lochthonous WK1 – WK2 groups have sufficient
samples with carbonaceous matter graphite to be plotted in the S vs. C diagram Fig. 8. The
Ho¨ytia¨inen pelitic samples, especially H2 samples, show good correlation between S and C SC
about 3.5. There is also a slight increase in U seen in the decrease of ThU ratios from about 4
to about 2.5 in the H2 samples. These features
Fig. 8. Plot of C
graf.
vs S for autochthonous H1 – H3 and allochthonous WK1 – WK2 sedimentary rocks in this study
divided into low SSe B 10 000 and high SSe B 10 000 populations. The SC ratio 0.36 is for normal marine sedi-
ments after Berner and Raiswell 1984.
jor factors related to the degree of diagenesis are thermal history and time, where rapid burial com-
pacts sediments quickly dewatering and blankets any thermal changes Lee and Klein, 1986. Thus
long-lived basins, like the Ho¨ytia¨inen basin Ko- honen, 1995, should show more pronounced ef-
fects of diagenesis compared to allochthonous Western Kaleva-type rocks that were deposited as
massive units in an active tectonic setting. The very limited element variation in the WK rocks
favours this and although small-scale diagenetic changes within WK samples are possible, a large-
scale redistribution of elements is not evident. Similar arguments hold for most of the central
Svecofennian rocks but, for example, the deposi- tional environment and the elapsed time before
dewatering and metamorphism of the Archaean and southern Svecofennian mature rocks are un-
known. Diagenetic reactions may include Na-, K-, Mg- and Fe-metasomatism e.g. Nesbitt and
Young, 1989 while REE redistribution and frac- tionation have also been proposed Awwiller and
Mack, 1991; Milodowski and Zalasiewicz, 1991; Ohr et al., 1991. There is not however consensus
about how common the redistribution of REE during diagenesis is cf. Hemming et al., 1995
and one critical question is that are the proposed diagenetic reactions open or closed systems at
sample scale.
Redistribution of alkalies during diagenesis has been proposed for the Ho¨ytia¨inen area rocks Ko-
honen, 1994 and to evaluate this possibility, the data are plotted in the A – CN – K and A – K – C –
N compositional spaces Fig. 7; see also Fig. 4 for K
2
O. The data show scatter and there are several factors that may have been responsible for the
observed trends: 1 Sedimentary rocks have dif- ferent source components with different K
2
O Na
2
O ratios see differences in MgO contents and ThSc and ThCr ratios; Figs. 4 and 9. The
problem lies also in the thinly layered nature of pelites where chlorite-rich and biotite-rich layers
were noticed, possibly indicating that different layers were derived from different sources in some
cases. 2 During grain-size sorting K-rich phases illite and biotite-vermiculite are enriched in
pelites K-feldspar is rare in these rocks and plagioclase in sands forming a trend similar to
indicate anoxic conditions during deposition and if the SC ratio of 3.5 is higher than found in the
Palaeoproterozoic marine sediments during depo- sition, it could point out to euxinic environment.
The Western Kaleva samples differ from the Ho¨ytia¨inen basin examples in that they do not
show any clear correlation between S and C. The graphite-enriched \ 0.5 C psammites have
low SC ratios B 0.15 and SSe ratios mostly B
10 000. Apart the graphite variation 0 – 1.6 C there is no enrichment of studied elements. The
occurrence of graphite-bearing thick psammites does not favour a direct hemipelagic origin and
indicates mixing of carbonaceous matter into mass flows before deposition. The low SC ratios
could point to fresh water or brackish water environments, or to short intervals between depo-
sition of mass flows preventing significant bacte- rial sulfate reduction. The lack of U and V
enrichment indicates an oxygenated environment while a low SC excludes an euxinic environment.
5
.
4
. Diagenesis and metamorphism Monitoring the effects of diagenesis in meta-
morphic rocks is a difficult task due recrystalliza- tion
requiring that
any evaluation
of the
diagenetic history be based on geochemistry. Ma-
that observed in the A – CN – K compositional space. 3 Albitization of K-feldspar in the sand-
size fraction with immediate uptake of liberated K by kaolinite, chlorite, montmorillonite andor
smectite in the clay-rich fraction as proposed by Kohonen 1994. Based on Fig. 7C albite metaso-
matism has occurred to some degree in some samples favouring Kohonen’s Kohonen, 1994
interpretation. 4 Regional-scale potassic and sodic metasomatism affecting shales and silt-sand-
size particles, respectively, has been proposed for the Palaeoproterozoic Serpent Formation Fedo
et al., 1997. The Serpent shales show ultimate potassium variation from 3.3 to 11.2 whereas
the H1 – H3 pelites show only variation from 3 to 5 Fig. 4 where the variation is mainly due to
the factors 1 – 3, as discussed above. Thus, the problem in depicting the amount of diagenetic
redistribution in the Ho¨ytia¨inen area rocks is that they show complicated mixing of source compo-
nents associated with sorting and thus distinguish- ing purely diagenetic effects is difficult. Although
not conclusive it seems that small-scale redistribu- tion of elements has occurred during diagenesis in
the Ho¨ytia¨inen area but no externally derived regional-scale metasomatism, at least for potas-
sium, is observed.
Prograde metamorphic effects on REEs, except in areas of partial melting, are minor Taylor et
al., 1986 but the depletion of LILE elements K, Rb, Ba has been proposed for granulite terrains
e.g. Weaver and Tarney, 1983; Sheraton, 1984.
Fig. 9. Plots of SmNd vs. ThSc and ThCr for selected sedimentary rocks in this study. The Archaean average has been calculated from the average AC1 in Table 1 and Jatuli-type mafics from the average N = 21 in Lahtinen unpublished data. See Fig. 5.
The tonalite migmatites veined gneisses, schollen migmatites and diatexites in the study area show
variable compositions due to differences in the relative amounts of restite and leucosome in sam-
pled outcrops and those that represent totally melted ‘in situ’ variants. A depletion of Bi is the
main common feature and although migmatites with high proportions of restite component occur
there is no area showing large-scale depletion of elements. In many cases the veined gneisses have
mostly retained their original composition cf. Lahtinen, 1996.
The southern
part of
the Rantasalmi –
Haukivuori area southern Svecofennian is char- acterized by in situ migmatites RH1 – RH2 with
variable amounts of restite and granite leucosome components. This difference in the leucosome
composition
tonalite – granite has
been at-
tributed to the aluminium excess in the source rocks of migmatites having granite leucosomes
Korsman et al., 1999. This interpretation is fa- voured by the typical CIA values of 60 – 70 in the
RH1 – RH2 rocks compared to the typical CIA values below 60 in the source rocks of tonalite
migmatites. On the other hand water-rich condi- tions during tonalite migmatization favour the
formation of plagioclase-enriched melts and wa- ter-rich conditions has been considered as the
main
cause for
the formation
of tonalite
migmatites Lahtinen, 1996.
5
.
5
. Main source components The proposed main source components of sedi-
mentary rocks of the Archaean craton and its cover, and Svecofennian domain are mainly based
on the geochemical differences but Sm – Nd results by Huhma 1986, 1987, O’Brien et al. 1993 are
also adopted. There are only a few detrital zircon age
determinations from
the Fennoscandian
Shield Huhma et al., 1991; Claesson et al., 1993 and thus the conclusions presented below are to
some extent tentative but serve as a working model for future work.
Boundary zone sedimentary rocks BZ1 – BZ2 are probably related to the 1.92 Ga primitive
island arc but the occurrence of numerous fault zones, extensive migmatization and complicated
shearing precludes further source component interpretation.
5
.
5
.
1
. Archaean sedimentary rocks The Archaean sedimentary rocks show very low
ThCr ratios, which discriminate them from other rocks in this study Fig. 9. O’Brien et al. 1993
concluded that greywackes in the eastern part of the study area Ar2-type with T
DM
ages from 2.83 to 2.99 Ga normally show a local source. The
Ar1 samples show a more homogenized source and higher degree of weathering of the source
area with higher MgO, Cr, K
2
O and SiO
2.
One Archaean sediment has a T
DM
of 3.24 Ga Huhma, 1987 favouring also the existence of an
older component cf. Sorjonen-Ward, 1993. Two main ages of source components with variable
amounts of intermixing are proposed for the Ar- chaean sediments in the study area:
1. Older main component with 3.0 – 3.2 Ga aver- age source age. At least three different source
rock types are indicated: komatiites high MgO, Cr, Ni, CrSc, tholeiite high TiO
2
and NbTh and felsic component SiO
2
, K
2
O and Rb. Intermediate to strong weathering in the
source area and thorough mixing has occurred before
deposition. Possible
sources are
greenstone + granite 9 TTG. 2. Local source derived from the 2.76 – 2.73 Ga
Vaasjoki et al., 1993 magmatic event cf. O’Brien et al., 1993.
5
.
5
.
2
. Cratonic co6er Local Archaean craton sources with contribu-
tions from Jatuli-type mafic volcanics and dykes has been a common source model for the Ho¨yti-
a¨inen basin sedimentary rocks Huhma, 1987; Ward, 1987; Kohonen, 1995. The results of this
study favour this general statement but the com- position of the H1 – H2 groups is not explained by
simple mixing of the presently exposed erosion level of the Archaean crust and Jatuli-type mafics
Figs. 4, 5 and 9 because an additional Cr-rich source is needed. The simplest explanation is
higher amounts of Archaean sedimentary rocks Cr-rich in the average source area for the H1 –
H2 group samples. Some sedimentary rocks have high proportions of local Archaean cratonic
source dominated by felsic granitoids indicated by high ThSc and ThCr ratios. The sedimen-
tary rocks show T
DM
variation from 2.28 to 2.70 Ga, which partly overlap with the Western
Kaleva T
DM
variation of 2.29 – 2.40 Ga Huhma, 1986, 1987. The Sm – Nd data for the Ho¨yti-
a¨inen basin is in general agreement with the geo- chemical data and suggest source components of:
1. Chemically weathered palaeosol, and sedi- mentary rocks derived from it, formed upon
Archaean crust and glaciogenic deposits. En- richment of Archaean sedimentary rocks see
the 3.0 – 3.2 Ga component above.
2. Non-weathered Archaean crust. Local differ- ences, seen for example in the large amount
of late-Archaean granite Kutsu component in some samples.
3. 2.2 – 1.96 Ga mafic magmatism, possibly volu- minous Jatuli-type plateau volcanism includ-
ing presently exposed abundant dykes, to explain the high amount of mafic component
in some rocks.
The detrital zircon U – Pb isotopic data for two samples Claesson et al., 1993 give age con-
straints for granitoid components in the al- lochthonous Western Kaleva mica schists. The
samples have 30 – 40 late Archaean zircons 2.5 – 2.8 Ga and only a few crystals in the age
range between 2.6 and 2.1 Ga, which can also be mixture ages Claesson et al., 1993. Both sam-
ples have 50 – 60 zircons from a 2.0 to 1.92 Ga age group with a maximum deposition age of
about 1.92 – 1.94 Ga. T
DM
ages of 2.3 – 2.4 Ga based on Sm – Nd data of Western Kaleva mica
schists Huhma, 1987 are in agreement with the detrital zircon data. The Archaean component
has been dominantly late-Archaean in age and we can use the normalization to the AC1 of this
study to interpret the nature of the 1.92 – 2.0 Ga component Fig. 3. The relative TiO
2
, Nb espe- cially NbTh ratio and HREE enrichment with-
out increase in the MgO level slight depletion and CrSc ratio favour a primitive island arc
tholeiitic origin for the mafic component. The high Zr relative to K
2
O, Rb and REE favour a low-K felsic source also characterized by moder-
ate to low LaYb ratios. Two main components are proposed for the Western Kaleva sediments:
1. Archaean crust dominated by late-Archaean granitoids mixed with a small contribution
from Jatuli-type dykes. A small amount of recycled weathered component is possible.
2. 2.0 – 1.92 Ga bimodal source of low-K felsic rocks and tholeiitic volcanics derived from
primitive island arc.
5
.
5
.
3
. S6ecofennian The central Svecofennian psammites show
large compositional variation Figs. 5 and 9 in- dicating either different provenance areas or
changes in the composition of source areas dur- ing erosion. The latter is favoured cf. Lahtinen,
1996 and, if this is the case, it points to rather short transport distances from a rapidly rising
orogenic domain. The psammites of this study CF1 – CF2 and the basement-related sedimen-
tary psammites SG3 – SG4 of Lahtinen 1996 from the Tampere – Ha¨meenlinna area have geo-
chemical similarities as seen in ThSc ratios of 2 – 0.7 and 1.5 – 0.5, respectively. The T
DM
of 2.2 Ga Huhma, 1987 from one sample is slightly
younger than that found in the WK psammites 2.3 – 2.4 Ga. Assuming that the central Sve-
cofennian rocks are mixtures of Western Kaleva- type source and an additional source it is
possible to use the WK1 as a normalizing value to infer the nature of this additional component.
The CF1 psammites are enriched in felsic com- ponent and thus the differences to the WK1
should approximate the felsic composition. High LREE, La
N
Lu
N
and negative Eu anomalies with high Th and low KRb and Nb favour a mature
intracrustal origin. The Th variation 13 – 19 ppm and ThTa ratios typically ] 15 in the
CF1 are distinct from Th contents B 9 ppm, mostly B 4 ppm and ThTa ratios 5 9 found
in the 1.93 – 1.91 Ga primitive island arc felsic rocks see Figure 26 in Lahtinen, 1994 excluding
them as a dominant felsic component in the CF1. The CF1 and CF2 groups are gradational
to each other and the low CrSc ratio, low Nb and only slight TiO
2
enrichment relative to MgO favour a mature island arc origin for the added
mafic – intermediate component. The proposed main source components for the central Sve-
cofennian sedimentary rocks are as follows:
1. Western Kaleva-type source see above. 2. Palaeoproterozoic 1.91 – 2.0 Ga mature is-
land arc or active continental margin source. The southern Svecofennian mature metasedi-
ments RH1 – RH2 differ from the Western Kaleva
and central
Svecofennian psammites
pointing to different origins. High ZnCo about 10 is a characteristic feature of RH1 and variable
but high ZnCo also characterizes the RH2 sam- ples. The ZnCo ratio is sensitive to changes
during weathering and sulphide precipitation but there does not seem to be any relationship be-
tween the existence of sulphides and ZnCo indi- cating instead either source difference or a
weathering effect. Similar ZnCo enrichment was not noted in high CIA rocks from the Ho¨ytia¨inen
area favouring a source origin for the high ZnCo. Elevated Zn and low Co are characteristic fea-
tures
of alkaline-affinity
intermediate – felsic within-plate-type granitoids Lahtinen, unpub-
lished data and this type of magmatism in the source area is one possible explanation for the
high ZnCo ratios. High Cr and CrSc ratios in the RH1 are interpreted to have their ultimate
sources in an abundant komatiite or picritic component.
The less mature greywackes RH3 show mainly low CIA values B 57 and thus resemble the
Western Kaleva psammites and psammites from the central Svecofennian. Although some samples
have compositions close to those found in the Western Kaleva psammites the RH3 rocks are
typically enriched in elements LREE, Rb, Ba, Th and U that characterize felsic source rocks. Some
RH3 rocks are enriched in elements that charac- terize mafic rocks especially seen in high CrSc
ratio Fig. 6. This could indicate an Archaean komatiite source but local Cr-rich lavas in the
Rantasalmi – Haukivuori area are more likely. The main source components for the southern Sve-
cofennian metasedimentary rocks in the Ran- tasalmi – Haukivuori area are as follows:
1. Alkaline-affinity complexes with high Zn and ZnCo
2. Archaean crust with possibly high CrSc ko- matiite component.
3. Island arcactive continental margin type crust from an orogenic domain.
4. Local sources and, at least partly, picritic sources producing high CrSc.
These tentative main source components char- acterize different groups differently; RH1 1 9
2 9 4, RH2 1 9 2 9 3 9 4, RH3 3 + 2 9 4 9 1. The problem lies in depicting the origin of the
highly weathered component; Archaean versus palaeoProterozoic.
5
.
6
. Tectonic implications Kohonen 1995 suggested that the syn-rift tur-
bidites of the Ho¨ytia¨inen rift basin Ward, 1987 have a maximum depositional age of about 2.1
Ga. The post-rift marine sediments probably in- cluded both passive margin and foredeep deposits
where the latter were deposited during foredeep migration from west to east during initial conti-
nent-arc collision Kohonen, 1995. The basic as- sumption is that autochthonous groups H1 – H3
contain only cratonic detritus where the Palaeo- proterozoic component is from mafic volcanics
and dykes mainly 2.2 – 2.06, and 1.96 Ga. Lahti- nen 1994 Palaeo-proterozoic Kohonen 1995
considered that the major rifting at 2.1 – 2.06 Ga finally lead to continental break-up cf. Park et
al., 1984; Gaa´l and Gorbatchev, 1987 and a change to a passive margin environment. A model
with later continental break-up at 1.95 Ga has also been proposed Peltonen et al., 1996. The
Western Kaleva psammites have been described as Svecofennian post-arc flysch Park, 1985, a
molasse from the Lapland granulite belt Barbey et al., 1984, pericontinental turbidites including
the Kalevian as a whole Laajoki, 1986, deep-wa- ter slope-rise greywackes related to uplift in Lap-
land and the Kola Peninsula Kontinen and Sorjonen-Ward, 1991, a mixture of accretion
prism sediments and derived foredeep sediments Lahtinen, 1994 and axial foredeep deposits from
a rising orogenic domain in the north during arc Svecofennian – continent Karelian craton colli-
sion Kohonen, 1995. The model of Kohonen 1995 could explain the occurrence of both an
inferred 1.92 – 2.0 Ga low-K primitive island arc component and a non-weathered Archaean com-
ponent in the Western Kaleva psammites due to a rapidly rising orogene during oblique collision
starting in the N cf. Lahtinen, 1994. As presently understood these psammites have been
deposited both on Archaean basement and oceanic crust, and a foredeep origin associated
with subsidence during initial collision is fa- voured and orogenic detritus either from the
same, oblique collision zone mainly from the accretionary prism or a more distal orogenic
domain is proposed. One interesting feature is the possible uptake of carbon-rich material, formed
in an oxygenated and possibly brackish environ- ment, into the turbidite currents before deposi-
tion of Western Kaleva sediment this study. This could favour the axial foredeep model of
Kohonen 1995 and deposition of organic matter near estuaries of large fresh water rivers.
The differences between the Western Kaleva and central Svecofennian sediments favour at
least partly different origins and different ages of deposition. The source for the central Svecofen-
nian sediments included also mature island arc material and the maximum deposition age was
about 1.91 Ga for the main period of turbidite deposition. Lahtinen 1994, 1996 has proposed
that ] 1.91 Ga possibly up to 1.95 Ga rifting occurred in the Tampere Schist Belt followed by
increasing subsidence during initial collision in the NE and subsequent arc reversal. Abundant
erosion from the mountain belt and deposition into oblique hinterland basins that further devel-
oped into a subduction related foredeep is the proposed model for the deposition of the main
sequences of turbidites in the central Svecofen- nian. The arc-related sediments are of local
derivation and indicate deposition in small basins before or during the 1.89 Ga collision cf. Lahti-
nen, 1994, 1996.
The southern Svecofennian mature greywackes resemble passive margin sediments but the more
immature sediments contain arc-type material. The
southern Svecofennian
is characterized
by abundant volcanics and, on the other hand, also by mature quartzites indicating both conti-
nental margin-type and passive margin settings but more data are needed to explore these possi-
bilities.
5
.
7
. Crustal e6olution and Ba depletion in the Archaean – Proterozoic transition
Abrupt changes in the composition REE, Th, Sc of sedimentary rocks at the Archaean –
Proterozoic transition has been proposed Taylor and McLennan, 1985; McLennan and Taylor,
1991; McLennan and Hemming, 1992. These in- clude an increase in negative Eu anomaly, a de-
crease in the Gd
N
Yb
N
ratio from \ 2.0 to 1.0 – 2.0, a decrease in the SmNd ratio from
about 0.21 to 0.19 and an increase in the ThSc ratio from about 0.5 to 1.0 possibly only in
continental sediments. Secular changes in the Ar- chaean – Proterozoic transition, especially con-
cerning the development of a Eu minimum, have been questioned and argued to be a consequence
of tectonic control resulting in biased sampling e.g. Gibbs et al., 1986; Condie and Wronkiewicz,
1990a; Gao and Wedepohl, 1995. Although the CrTh ratio may not directly reflect the source
ratio, abrupt changes have been noticed in the Archaean – Proterozoic boundary reflecting the
decreasing amount of komatiites in the Protero- zoic Taylor and McLennan, 1985; Condie and
Wronkiewicz, 1990b; Condie, 1993. No consen- sus exists about the importance of the Archaean –
Proterozoic
transition but
komatiites and
TTG-type rocks characterize the Archaean and their contribution to the sedimentary record
should be distinguishable. The data of this study for Archaean sedimen-
tary rocks are limited but some general remarks can be made. Archaean sedimentary rocks are
characterized by low ThSc B 0.3 and ThCr B 0.018, and variable EuEu ratios that are
normally higher than those in the Palaeoprotero- zoic sediments of this study Fig. 5. The SmNd
ratios are also somewhat higher but the Gd
N
Yb
N
ratios are lower than 2 and also lower than those found in many Palaeoproterozoic sediments of
this study Figs. 5 and 9. The Palaeoproterozoic sediments can be di-
vided into cratonic sediments and sediments also having Palaeoproterozoic crustal components.
The cratonic sediments H1 – H3 have an inferred source characterized by Archaean sources mixed
with variable amounts of Palaeoproterozoic mafic
material mainly from differentiated e.g. low Cr Sc plateau volcanics and dykes. The other
Palaeoproterozoic sediments of this study also show the contribution of intermediate to felsic
igneous sources varying from low-K primitive is- land arc to mature active continental margin
types. An important feature is the almost total absence of 2.1 – 2.5 Ga mature crustal component
granitoids in these sediments.
The data are somewhat scattered but the Palaeoproterozoic sediments having crustal com-
ponents showing higher ThSc, ThCr, and lower SmNd and EuEu relative to the Archaean
rocks Figs. 5 and 9 as proposed in earlier studies see references above but the behaviour of Gd
N
Yb
N
ratio is opposite to that proposed by McLen- nan and Taylor 1991. More data on Archaean
sedimentary rocks from the Svecofennian shield are evidently needed to see if the limited input
from TTG rocks cf. Condie, 1993 is a common feature. The source variation seen in the central
Svecofennian sediments e.g. ThSc 2 – 0.5 in the CF1 – CF3 in Fig. 9 suggests exposure of differ-
ent source components during erosion in the source area. A slight grain-size induced preferen-
tial separation of felsic source into the psammites and mafic source into the pelites was also noted in
this study cf. Lahtinen, 1996. Low ThCr ratios characterize the Archaean sedimentary rocks due
to the abundant komatiite component but lower ThCr ratios can also be from a local picritic
source e.g. some RH3 samples. These features reinforce the need for a large data set when using
sedimentary rocks in crustal evolution studies.
If the absence of 2.1 – 2.5 Ga mature subduc- tion-related material is true for most of the
Fennoscandian shield supercontinent stage it im- plies that here the Archaean – Proterozoic transi-
tion is characterized by the addition of only mafic magmatism 9 felsic material in bimodal forma-
tions and the transition to Proterozoic crustal formation occurred about 2.1 Ga ago. A 2.4 – 2.3
Ga subduction event proposed for the western edge of Rae Province in Laurentia Bostock and
van Breemen, 1994, a roughly 2.2 Ga age for the onset of subduction-related Birimian magmatism
e.g. Davis et al., 1994 and references therein and magmatic activity during 2.4 – 1.8 Ga with a mode
at 2.1 – 2.0 Ga based on detrital zircons from Sa¨o Francisco Shield Machado et al., 1996 show that
the age and nature of the Archaean – Proterozoic transition differ from shield to shield; a possibility
also for the geochemical nature of associated sed- imentary rocks.
An elevated level of both Th and Sc relative to modern deep sea turbidites in the basement re-
lated sediments in the Tampere – Ha¨meenlinna area was noted by Lahtinen 1996 and a similar
situation characterizes the central Svecofennian sediments of this study not shown. A source
enriched in bimodal volcanics and depleted in sedimentary quartz was proposed Lahtinen,
1996. The elements released during weathering are also lost from the clastic portion but can be
partly redeposited in separate units within the sedimentary sequence, as for example Ca in
marine carbonates and U with organic matter. Many elements are also recycled back to the
mantle during subduction and form a characteris- tic fingerprint for subduction-related magmas and
enriched mantle components e.g. Hawkesworth et al., 1991; Weaver, 1991. Lahtinen 1996 pro-
posed that the Ba deficiency in the basement-re- lated sedimentary rocks and the Ba enrichment in
the Svecofennian enriched mantle component see also Lahtinen and Huhma, 1997 are related to Ba
release during weathering 9 diagenesis and later uptake in pelagic sediments possibly as barite
that are further subducted into the mantle.
The source variation and mobile nature of Ba produces scatter in Fig. 10 but Ba depletion is
noticed in most samples. A striking feature is the strong relative Ba depletion in most high-Cr Ho¨y-
tia¨inen rocks H1 – H2 and Jatulian quartzites implying that the Ba depletion is related to the
chemically weathered component cf. Maynard et al., 1995. Major loss of alkali and alkaline earth
metals relative to more immobile elements REE, Th, Sc occur at CIA values of about 80 in the
Hokkalampi Palaeosol when the breakdown of illite dominates Marmo, 1997; personal commu-
nication. Similar Ba depletion relative to Rb was also noticed not shown. Potassium enrichment
in palaeosols, attributed to diagenetic overprint- ing, is not uncommon e.g. Gall, 1992 and refer-
ences therein and could cause relative Ba
Fig. 10. Plots of Ba vs. K
2
O for selected sedimentary rocks in this study. The Archaean average has been calculated from the average in the Table 1 and Jatuli-type mafics from the average N = 21 in Lahtinen unpublished data. The Archaean trend is approximated
from the data in this study and the Tampere Schist Belt TSB volcanics trend is from Lahtinen 1996. See Fig. 5.
depletion. The Hokkalampi Palaeosol shows slight potassium enrichment in the lower zone but
a large-scale external potassium addition seem unlikely. The main part of the Ba depletion is
assumed to derive from the chemically weathered palaeosol, especially from the highly weathered
part CIA \ 80.
Ba depletion is less pronounced in other groups Ar1, RH1 – RH2 and CF3 having also elevated
CIA values over 60. If the interpretation of Ho¨yti- a¨inen sedimentary rocks is correct it indicates
mixing of deeply weathered Archaean source ma- terial CIA 70 – 90 with less weathered Archaean
crustal and Palaeoproterozoic mafic sources CIA B
50 to produce H1 – H2 rocks with CIA values in the range of 55 – 70. In this case the lack of
comparable Ba depletion in other pelitic rocks with elevated CIA can be attributed to the lack of
extremely strong chemical weathering CIA \ 80 in the source area.
Different source areas have variable BaK ra- tios but the Ba depletion relative to K, Rb and Th
Lahtinen, 1996; this study is a characteristic feature of the sedimentary rocks of central
Fennoscandian Shield. This indicates a high amount of Ba lost from the clastic record during
2.3 – 1.9 Ga and further incorporated, at least partly, into both a subduction component and the
enriched mantle. The Fennoscandian shield seems to have exemplified a cratonic stage during 2.6 –
2.1 Ga characterized by deep chemical weathering about 2.35 – 2.2 Ga ago, high burial rates of or-
ganic carbon and highly
13
C-enriched sedimentary carbonates e.g. Karhu, 1993 about 2.2 – 2.1 Ga
ago, and multiply rifting from about 2.2 to 1.95 Ga. One critical question is the possible effect of
CO
2
-rich and low-O
2
atmosphere in the formation of weathering profiles before the significant rise in
atmospheric oxygen levels at about 2.0 Ga e.g. Karhu, 1993. If the Ba depletion has been espe-
cially characteristic for the chemical weathering during 2.35 – 2.2 Ga it could imply that during
and after this time period high amounts of Ba have recycled back to the mantle forming a ‘peak’
in the formation of enriched mantle component.
6. Conclusions
