V. R. Vetrin
Geological Institute of Kola Science Centre RAS, Apatity, Russia
O. M. Turkina
United Institute of Geology, Geophysics and Mineralogy of the Siberian Branch of RAS, Novosibirsk, Russia
O. Nordgulen
Geological Survey of Norway, 7491 Trondheim, Norway
15 kbar (type A, garnet-amphibolite restite)
and 8 kbar
(type B, plagioclase-amphibole restite). The conditions under which these rocks originated
appear to be similar to those recognised for the Garsjo plagiogneiss, which is
located in
the Svanvik-Lotta segment of the Kola-Norwegian block, and probably belongs to the
upper
part of the Bjo rnevatn greenstone belt. Protoliths of certain Garsjo gneisses
(type C) were
generated by partial melting of a metabasic source rock enriched in light rare earth
and other
incompatible elements (e.g. Sr), and corresponding in composition to TH2 basalt.
In
contrast to the Garsjo gneiss, the tonalite-trondhjemite rocks of the KSDB were
derived
from relatively poor-differentiated metabasic rocks which are characterised by lower
La/Yb
ratios and lower Sr content, and are compatible to TH1 tholeiite. On the Kola Peninsula,
homologues of the tonalite-trondhjemite rocks of the KSDB are likely to be found
within the
Olenegorsk greenstone belt in the central part of the Kola Peninsula.
It is concluded that the formation of tonalite-trondhjemite rocks has considerable
influence on the composition of the lower crust of the region.
The Kola Superdeep Borehole, which was drilled through Precambrian rocks of northwestern Kola Peninsula, intersected Early Proterozoic rocks of the Pechenga and Archaean complexes and reached a depth of 12,262 m. Core samples recovered from different parts of the section are unique in providing the opportunity to study the composition and physical properties of rocks that for long periods of time have been subjected to high temperatures and pressures. In view of this, a comparison of the results from studying cores and samples of analogous rocks from the surface is the main task of IGCP Project-408 "Comparison of composition, structure and physical properties of rocks and minerals in the Kola Superdeep Borehole and their homologues on the surface''. Project leaders are F. P. Mitrofanov, D. M. Guberman and H.-J. Kumpel and the project is to be carried out within 1998-2002 [Inaugural..., 1998; Mitrofanov and Vetrin, 1998]. However, it may be fairly difficult to identify homologues, i.e. rocks that are similar in composition and structure and originated in similar environments at the same time, among multiply folded and intensively deformed Archaean rocks.
This work presents an attempt to identify homologues among tonalite-trondhjemite rocks in the KSDB and within the Kola-Norwegian and Murmansk blocks on the basis of petrologic-geochemical modelling of protolith composition and conditions of formation for these rocks, which originated in diverse geodynamic environments.
The study performed would not have been possible without the assistance of Yu. P. Smirnov, M. S. Rusanov and Yu. N. Yakovlev from RFC "Kola Superdeep'', as well as L. F. Dobrzhinetskaya, T. A. Braun, J. Cobbing and B. A. Sturt, participants of the Russian-Norwegian collaboration project "North Area'', who examined, together with the authors, the Archaean rocks in the Sorvaranger area, Finnmark, Norway. The authors are sincerely grateful to N. G. Zhikhareva for figure drawing and M. A. Vetrina for her help in computer processing of graphic files.
|
|
| Figure 1 |
Tonalite-trondhjemite ( TT ) rocks have been studied in core samples of the Archaean section of the Kola Superdeep Borehole, and in outcrops of the Kola-Norwegian and Murmansk blocks located in and near the central and northern parts of the Kola Peninsula.
|
| Figure 2 |
Geochronological and isotope-geochemical studies of KSDB rocks suggest the following sequence of events. The gneiss protoliths are estimated to be 2950-2850 Ma old; the emplacement of gabbro intrusions and the formation of ATPC granitoids took place 2835-2832 Ma ago. Pegmatites were formed about 2740 Ma ago at the final stages of metamorphism; K-Si metasomatites formed hi the lower part of the KSDB section 2225 Ma ago, and porphyry granite veins intruded 1766 Ma ago [Bibikova et al., 1993; Chen et al., 1998; Timmerman and Daly, 1995].
The Kola-Norwegian block is located in the centre of the Kola Peninsula. In the south it is separated from the Belomorian and Tersky blocks by the Granulite belt and Olenegorsk and Imandra-Varzuga structural zones. In the east it is bounded by the Tsaga fault system, and its western boundary with the Inari block runs along the Palaeoproterozoic Pasvik-Polmak belt belonging to the Pechenga-Varzuga rift structure. Tonalite-trondhjemite rocks comprise the bulk of the Svanvik-Lotta segment located in the northwesternmost part of the Kola-Norwegian block. This segment is separated from the Titovka segment in the southeast by a long-lived NW-trending fault zone, which controlled the evolution of the Late Archaean Bjo rnevatn-Olenegorsk greenstone belt and, partially, the evolution of the Pechenga-Varzuga structure [Dobrzhinetskaya et al., 1995].
Volcanic-sedimentary rocks of the Bjo rnevatn Group, containing commercial deposits of banded iron formation, form a steeply dipping NNW-trending belt in the TT "grey gneisses'' which have been subdivided into several complexes (Varanger, Svanvik, Garsjo, Brannfjellet, etc., [Siedlecka et al., 1985]). The Titovka block contains a pluton composed of Kirkenes tonalitic gneiss. The "grey gneisses'' differ in composition, structural-textural features and the degree of deformation and tectonic alteration, and are cut by bodies of granite, pegmatite and basic rocks. The most diverse rock associations are found in the Garsjo gneiss located in the western part of the Svanvik-Lotta block, and in the compositionally similar Brannfjellet gneiss occurring in the southwestern external part of the Bjo rnevatn structure. The dominating rock type of the Garsjo gneiss is the originally volcanogenic leucocratic biotite gneiss of a tonalite composition (~50%) with interbeds of metasedimentary rocks (~20%) represented by two-mica gneiss and schist, locally with garnet and staurolite. A significant part of the section (20-30%) is composed of garnet and feldspathic amphibolites with banded iron-formation lenses, hornblende and biotite-hornlende gneiss of a diorite composition, and fragments of serpentinised ultrabasic rocks.
|
| Figure 3 |
5 Ma old pegmatite bodies and porphyry granites
of the Neiden and Geaheoaivi
complexes, dated at 2483
28 and 2502
3 Ma, respectively. Zircons from the Garsjo
tonalite gneiss contain an ancient radiogenic lead component defining an age of 2840 Ma;
they were affected by an endogenic event 2.7-2.6 Ga ago, which is interpreted
to reflect
regional metamorphism or the influence of younger granite
[Levchenkov et al., 1995].
The Garsjo gneiss has been considered [Dobrzhinetskaya et al., 1995; Siedlecka et al., 1985] to be the upper part of the Bjo rnevatn greenstone belt; in terms of predominant rocks, age and geodynamic setting it is similar to the Archaean rocks that were intersected by KSDB and are considered to form the basement of the Palaeoproterozoic Pechenga-Varzuga structure. As compared to the Archaean KSDB rocks, the Garsjo gneiss has suffered minor migmatisation.
|
| Figure 4 |
6 Ma and 2803
13 Ma
[Levchenkov et al., 1995].
The Varanger gneiss locally contains lenses, dyke-like and band-shaped inclusions
of
garnet-biotite and two-mica gneisses, amphibolites and BIF
rocks which are up to a few
kilometers long. In some cases these rocks form isometric, bowl-shaped structures
which
have concordant, locally tectonic contacts with the
TT rocks. The gneisses from such
inclusions are compositionally similar to the Kola (Jarfjorden) gneisses, whereas
the
amphibolites are iron-rich basaltic rocks characterised by flat REE distribution
patterns
(Figure 4).
The Varanger gneiss is cut by variably deformed granitic and pegmatitic veins
which locally form extensive fields. The veins consist predominantely of two-feldspar
granitic rocks; leucocratic plagioclasite veins with small amounts of quartz are
minor.
The Kirkenes gneiss is a pluton occurring at the boundary between the Bjo rnevatn
Group greenstone rocks and the Kola (Jarfjorden) gneiss. The contact between the
pluton and the host rocks is magmatic; intensive tectonic processes, however, resulted
in the formation of mylonite zones suggesting that the rocks were emplaced and
crystallised in a regional stress field
[Braun et al., 1993].
The pluton is composed of
biotitic tonalites that have been migmatised and tectonically deformed to a variable
extent
and contains lenticular amphibolite bodies, which are boudinaged mafic dykes. The
rocks are cut by numerous granite and pegmatite dykes of several age generations.
The earliest of them were deformed together with the host rock, while the latest
do not
display any prominent evidence of tectonic deformation. Zoned zircon crystals from
the
tonalitic gneiss yielded an age of 2804
9 Ma, interpreted to be the time of
crystallisation of the pluton
[Levchenkov et al., 1995].
On the basis of geological
investigations of the "grey gneisses'' of the western Kola-Norwegian block, the Garsjo
gneiss can be therefore assigned to a supracrustal rock association including initially
volcanogenic, metasedimentary and intrusive rock assemblages, which are probably
fragments of a section through a Late Archaean greenstone belt. The Varanger and
Kirkenes gneisses intruded later (by 20-30 Ma) than the Garsjo gneiss, and
their
emplacement was controlled by tectonic fault zones and the regional stress conditions.
The Murmansk block forms the northeasternmost part of the Kola system of the Karelides. Its continental portion is wedge-shaped and occupies an area of about 30,000 km 2. In the southwest the block is bounded by the North-Keivy suture zone, which is a series of faults with en echelon configuration. The rocks are broken into lenticular blocks that give the zone a mosaic pattern. From seismic data, the fault continues down to a depth of 35-40 km, reaching the Moho interface. The fault originated in the Late Archaean time, as suggested by 2750-2785 Ma old gabbro-anorthosite and diorite-plagiogranite intrusions confined to it [Pushkarev et al., 1978]. NE-trending displacements, the largest of which are Kharlovsky, Svyatoi Nos-Strelna and East-Keivy faults, are associated with the North-Keivy suture zone. These displacements, along with a series of faults in the Tuloma river valley, subdivide the Murmansk block into a number of higher-order structures, among which are the Titovka, Teriberka, Jokanga and Kachkovsky segments of different composition and deep structure.
The Murmansk block is dominated by Archaean granitoids, which define the major geological features of the area. TT granitoids, which are now represented by orthogneisses belonging to the Archaean association of tonalitic gneiss-plagiogranite [Belkov, 1985], occur in the western, central and southeastern parts of the block. The orthogneisses are predominantly NW-trending in the western part of the block, whereas in some cases they are found to trend to NE or S-N in the central and eastern parts, where they trace the limbs of brachyanticline structures ranging from a few hundreds of meters to dozens of kilometers in diameter. Rocks of different petrographic composition are typically alternating in the Murmansk block, forming band-like segregations a few kilometers in length and dozens to a few hundreds of meters in thickness. More melanocratic granitoids contain higher amounts of amphibolite inclusions, which form sheet-like or lenticular bodies the elongation of which coincides with the direction of banding in the granitoids.
The tonalite gneiss from the central and eastern parts of the structure was dated
by
the U-Pb method at 2.8-2.75 Ga
[Batieva and Vinogradov, 1991],
with TNd (DM)=2.90-2.97 Ga
[Timmerman and Daly, 1995].
A model Sm-Nd age of the tonalite gneiss from the
Titovka block, Ura-Guba region, is 2.46-2.40 Ga
[Timmerman and Daly, 1995],
which is in disagreement with the results of Pb-Pb investigation of zircon from
these rocks, 2.84
0.04 Ga ( Vetrin, unpublished data).
Therefore, in this paper the age of tonalitic
gneiss of the western, as well as central and eastern parts of the Murmansk block
is
assumed to be Late Archaean.
According to a geodynamic model available [Mints et al., 1996], intensive granite formation within this structure was caused by subduction of the oceanic lithosphere beneath the active margin of the Murmansk microcontinent about 2.8 Ga ago.
Biotite-plagioclase TT gneisses dominate in units 2, 4, 6, 8 and 10 of the Archaean KSDB section, and are variably migmatised, as mentioned in the previous section. Migmatites, migmatite-granites, anatectite-granites and pegmatites are distinguished among the rocks of the migmatite complex on the basis of their relationship with the surrounding gneiss, structural-textural features and facial characteristics. Gneisses of the substratum (paleosome) are preserved in the migmatites as relics of leucococratic (leucosome) and melanocratic (melanosome) interlayers, which makes it difficult to establish their primary composition. Below is the petrographic description of the TT rocks from the "grey gneisses'' of units 8 and 10. These rocks are relatively weakly migmatised (40 and 20% migmatites, respectively) and appear to have originated from volcanic (gneiss of unit 8) and intrusive (ATPC granitoids) rocks.
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| Figure 5 |
Melanocratic (10-15% biotite and amphibole) and leucocratic rocks with predominantly granoblastic, lepidogranoblastic and porphyrogranoblastic textures are distinguished among the Garsjo gneiss. Plagioclase varies in composition from oligoclase-andesine in mesocratic rocks to oligoclase in leucocratic rocks. Amphibole is most common in the gneisses that contain amphibolite inclusions; by composition it is edenite with a relatively low titanium content (Table l). Biotite occurs as tabular crystals with brownish-green pleochroism; in composition (TiO2 =1.7-3.5%, F=39-55) it is similar to biotites from amphibolite-facies and, to a lesser extent, granulite-facies rocks. Magnetite, apatite, zircon and titanite are accessory. Zircon grains commonly contain darker cores occupying from 1/3 to 2/3 of the grain volume.
The Kirkenes and Varanger gneisses are characterised by blastogranitic,
hypidiomorphic and dioritic textures indicative of intrusive origin. The major rock-forming
minerals are plagioclase, quartz, biotite and amphibole; the amount of primary
microcline, which occurs as intergrowths in plagioclase grains, does not exceed 1-3%.
Accessory minerals are magnetite, ilmenite, zircon, titanite, allanite, epidote and
apatite.
Plagioclase occurs as prismatic, sub-tabular, commonly polysynthetically twinned
crystals,
locally with an indistinct normal zoning and enclosed quartz grains. Primary magmatic
plagioclase grains are andesine (35-40% An); metamorphic alteration causes an increase
of the albite component (20-25% An). Primary amphibole is represented by edenite
in
the Varanger gneiss and magnesian hornblende in the Kirkenes gneiss (Table 1).
Metamorphism resulted in the replacement by chlorite and actinolite hornblende with
low
titanium (0.63% TiO2 ) and alumina (7.6% Al2O3 )
contents. Magmatic biotite displays a
reddish-brown pleochroism and has a high titanium content (up to 4.2% TiO
2 ). Biotite plates
of metamorphic origin are greenish-brown, and have a lower titanium content (1.0-1.6%
TiO2 ) and Fe/Fe+Mg ratio (F=36-39). The titanium content in biotite
from the Kirkenes
gneiss directly depends on the Fe/Fe+Mg ratio, and this suggests that metamorphic
magnesium-rich biotites formed owing to decomposition of high-titanium and more
ferrugineous biotites of magmatic origin: BiFe
Ti
BiMg + FeTiO3.
Plagiogranitoids of the Murmansk block are largely tonalites and trondhjemites, which grade into diorites and quartz diorites at sites where amphibolite xenoliths are abundant. The major rock-forming minerals are plagioclase (58-65%, from 15% to 30-40% An), quartz (22-34%), biotite (4-10%), amphibole (0-2%). The most common accessory minerals are magnetite, pyrite, epidote, zircon, monazite, titanite, apatite; garnet is present in the granitoids of the Titovka segment (0.3%). The rock textures are blastogranitic, hypidiomorphic, allotriomorphic, locally granoblastic and corrosive-metasomatic due to later microcline grains. Primary microcline, the content of which does not exceed 5%, is present in antiperthitic segregations and occurs as individual isometric grams in equilibrium with plagioclase. Amphibole is low in Ti and Al and belongs to hornblende of the andalusite-sillimanite facial series [Raase, 1974]. Biotite is represented by meroxene (F=47-56) with 2.1-3.1% TiO 2 (Table 1), suggesting that it is a mineral of the amphibolite facies (Figure 5).
A fairly comprehensive description of petro- and geochemical composition of "grey gneisses'' from the Archaean section of the KSDB is presented in Kremenetsky and Ovchinnikov [1986], Kremenetsky et al. [1990] and Vetrin [1991].
Biotite-plagioclase gneisses range in composition from Precambrian granite and trondhjemite to alaskite, differing from ATPC granitoids by an elevated content of alkalies, and showing a considerable similarity to rocks of the Late Archaean tonalite gneiss-plagiogranite association of the Kola Peninsula. The composition of the studies rocks is distinctly different from that of high-alumina gneisses, which are interpreted to be products of disintegration of the "grey gneisses'' and redeposition of the crust during weathering of basement rocks [Kozlovsky, 1984]. Based on the content of a variety of petrogenic components, such as FeO, Fe2O3, Al2O3, FeO/MgO, Na2O and K2O, and trace elements, the biotite-plagioclase gneisses and ATPC granitoids are found to be similar, and they bear similarities to the ancient granitoids of the Kola Peninsula, which are considered to have been derived from an epi-andesitic magma and have been reworked by crustal processes [Vetrin, 1984; Yakovlev and Lanev, 1991].
|
| Figure 6 |
All rock samples from the Svanvik-Lotta segment were collected in the course of project work within the Norwegian-Russian collaboration programme "North Area'' in 1990-1993. Chemical analyses of most samples were performed by using XRF spectrometry at the Geological Survey of Norway, Trondheim, and some samples were analysed at the Geological Institute of Kola Science Centre RAS, Apatity. REE were analysed at the Institute of Lithosphere RAS, Moscow, by nuclear-emission ICP spectroscopy with the separation and concentration of REE by ion-exchange chromatography. Uncertainties of the method amount to 10-20%, depending on REE concentration levels.
|
| Figure 7 |
|
| Figure 8 |
/Eu=l.l-1.4). The exception
is provided by leucocratic rocks which have a
prominent Eu-minimum (sample 90-37, Table 4), suggesting plagioclase fractionation.
Rocks of type  have lower La, Ce and Nd contents (Figure 8), a relatively high
concentration of HREE (La/Yb)n =13-41 and Y, and are poor in Ba and Sr. Veined
plagioclasite in the Varanger gneiss needs to be recognised as a separate rock type.
The
peculiarity of its composition lies in a high content of Al2O3
(21-23%) and Sr (up to
570 ppm), a prominent Eu-maximum, and low contents of SiO2,
Fe2O3, MgO, Cr, Ni and
HREE. Trace elements are concentrated in accessory minerals, e.g., Zr, and their
contents
show great variation (24-120 ppm). The substantial range of LREE contents (by
2 times)
are probably caused by varying content of allanite. Steep patterns of REE spectra
with a
sharp Eu-maximum (Eu/Eu =l.7-3.1) are similar
to those of plagioclase.
Veined trondhjemite (sample 90-328), although being more leucocratic, belongs to the same group: it has analogous geochemical parameters, i.e. it is relatively enriched in Al2O3, Sr and Eu. The chemical composition of the plagioclasites allows us to suppose that they formed under disequilibrium melting of a feldspathic component, which apparently occurred because of later rheomorphism of the gneiss. The separating melt was contaminated to a variable extent by unmelted accessory phases. The distribution of rock-forming and trace elements in the veined leucotrondhjemite (sample 90-08), which has a prominent Eu minimum, suggests that it was differentiated from a melt of a high-alumina tonalite composition.
|
| Figure 9 |
|
| Figure 10 |
/Eu=0.3-l.l,
Figure 10).
Melting of metabasic rocks is considered to play the leading role in the generation of tonalite-trondhjemite melts [Martin, 1987; Rapp, 1994]. Numerous experiments performed over the last decade to study melting of amphibolites under a wide range of P (3 to 22 kbar) and T (800 to 1040o C) have provided clues to understand the petrogenesis of natural TT. The methods of a petrologic-geochemical analysis used by the authors of the present paper are based on the available experimental data and calculation of trace element contents in model melts generated under variable P and T. A comparison of trace element composition of model and natural TT makes it possible to estimate probable conditions for the formation of natural TT, and to evaluate models for their formation in more detail [Turkina, 1998].
The following observations can be adduced support the idea that protoliths for late Archaean tonalite-trondhjemite granitoids of the region originated by means of partial melting of metamorphosed mafic rocks:
pyroxene
(amphibolitic), garnet-plagioclase-pyroxene-amphibole (garnet amphibolitic) with
7 to 22% garnet, and garnet-pyroxene (eclogitic), which follow each other as
P and
T grow. Under water-saturated conditions
[Winther, 1996],
the stability field of restitic amphibole expands, and the resulting assemblage
consists of plagioclase-free restites, the composition of which is controlled
by
P-T parameters and H2O contents. Tonalitic melts are
produced at high H2O contents and in equilibrium with amphibole or garnet-amphibole
(Gar
20%) restite. Trondhjemite melts are produced at
low H2O contents with the
separation of garnet-rich (Gar
30-50%) garnet-clinopyroxene or garnet-amphibole
restite.
To calculate typical geochemical parameters (concentrations of indicator elements and their ratios) of model TT rocks, we used specific mineral assemblages that correspond to fixed P and T values and are characteristic of the distinguished types of restite (Tables 7, 8). Trace element distribution coefficients at partial melting and fractional crystallisation are presented in Table 9. Trace element contents in the source are similar to those in basic volcanics of Precambrian (TH1 and TH2, [Condie, 1981]) and Phanerozoic ages (MORB, [Rapp, 1994], Table 10). The types of basalts chosen allowed us to cover a wide range of trace element content. The TH1 source has low Sr concentrations and (La/Yb) n =1.3, and TH2 is a more geochemically evolved basic rock enriched in LREE, including Eu and Sr. MORB is characterised by minimum (La/Yb)n =0.6, and lower Sr contents. Sr, Y and REE were used as indicator elements because the distribution of these elements in a melt is controlled by major restitic phases: plagioclase - Sr, garnet and amphibole - Yb, Y and La/Yb.
|
| Figure 11 |
|
| Figure 12 |
|
| Figure 13 |
15 kbar and
~8 kbar, respectively. Leucocratic biotite
plagiogneiss (sample 6, Table 2) with a lower Yb content and a prominent
Eu-maximum does not fall within the field of parent melts. In the given case, the
low concentrations of light and medium REE and, especially, Th indicate the possibility
of fractional crystallisation of the parent melt in the presence of a considerable
amount of
hornblende and accessory minerals, such as allanite. Parameters of calculated models
of
plagiogneiss formation and the resulting compositions of metabasic sources are presented
in
Tables 11
and 12
and in Figure 13.
The model for amphibole-bearing gneisses implies that
they have crystallised from some primary melt, because if we assume the parental
melt to
have a composition of plagiogneiss, the metabasic source of the rocks would have
a
prominent Eu-maximum, which is unlikely, coupled with its low Sr. Possible
differences between the composition of metabasic sources for the two types of protoliths
are
related mainly to LREEs, and their contents in rocks of type A and B are 20-30 and
15 times
higher than in chondrite.
A comparison of the estimated composition of me possible metabasic source rocks with that of Mg-Fe, Si- and Fe-amphibolites extracted from the Archaean part of the KSDB section [Kremenetsky and Ovchinnikov, 1986] has shown them to be different. By REE spectra, the model source would be close to Al-Mg amphibolites of the syn-Kola intrusion, although the latter are characterised by much lower Zr (28 ppm) and high Sr (230 ppm) contents. A more detailed comparison apparently requires further studies.
Three geochemical types differing most markedly in HREE concentration can be distinguished among the Garsjo, Varanger and Kirkenes gneiss:
0.3 ppm),
0.8 ppm),
As judged from the position of composition points for the
gneiss on the diagrams
(Figures 11,
12),
the melts that generated gneiss protoliths were formed under a wide range
of
P and in equilibrium with various restitic assemblages.
According to experimental
data, initial melts for these gneiss types could have formed under
P 16 kbar, 15-16 and
8 kbar, respectively.
In terms of REE contents, all the samples melted out of a metabasic source. The most common deviation of composition points from the initial melt field is towards lower Yb and Eu contents, suggesting fractional crystallisation of parental magmas. In line with this assumption, when estimating REE composition of a source, it is necessary to use the composition of samples that were less subjected to differentiation, or to make a preliminary assessment of geochemical parameters of the possible initial melt. Relative enrichment in Eu and Sr occurs rarely, and the nature of it will be discussed later. Taking into account preliminary estimates of the conditions of formation of the protoliths, let us look more closely at the petrogenesis of plagiogneisses.
|
| Figure 14 |
|
| Figure 15 |
/Eu=1.4) and LREE and can be a product of
early crystallisation enriched in plagioclase and accessory minerals. The
latter has a leucotrondhjemite composition and a prominent Eu-minimum
(Eu /Eu=0.4), and seems
to be a product of differentiation of
a tonalitic melt. This conclusion is supported by the calculated model of melt
crystallisation differentiation and is presented in Table 14 and Figure 15. The estimated
composition of the metabasic source is compatible in trace element content with the
composition of the Garsjo amphibolite enriched in TiO2 and LREE up to the level
of TH2 tholeiite.
|
| Figure 16 |
16 kbar) or garnet-amphibole ( P
13-15 kbar)
restite. The calculated models involving dehydration and water-saturated melting
are presented in Table 15 and Figure 16a, b. Trondhjemites with lower concentrations
of Eu and HREE could have been formed by melt differentiation and fractionation of
plagioclase and hornblende in the ratio 1:1. The formation of gneiss protoliths
with lower (La/Yb)n =13-41, type B, was accompanied by the separation of
a garnet-amphibolite restite containing 14-22% garnet, which corresponds to
P 16 kbar.
The metabasic source for protoliths of the Varanger gneiss is similar in trace element composition to TH1 tholeiite and is compatible compositionally to low-titanium amphibolite of the Valen complex, which is especially abundant and associated with the Varanger gneiss south of Grasbakken and in the Bugofjorden area. The source of the gneiss which is slightly enriched in LREE could have been represented by more differentiated metabasalt with elevated LREE and Sr contents; the melanocratic, tonalitic composition of the gneiss makes it unlikely that LREE will accumulate through fractional crystallisation.
|
| Figure 17 |
Compositional features of the veined granite (90-50) suggest that it was formed by melting of a plagiogneiss source rock in the garnet stability field, the low stability limit of which being ~5 kbar under dehydration melting [Vielzeuf and Montel, 1994] and 13 kbar in the presence of water [Van del Laan and Wyllie, 1992]. The Ba-enrichment of melts attests to the absence of significant amounts of biotite in the restite. On these grounds it is possible to estimate melting temperature, which corresponds to the upper stability limit of this phase.
Restite biotite disappears at
T 850o under dehydration melting
and
above 750o C under water-saturated melting.
The Yb content in most samples of granitoids available from the Murmansk block
ranges from 0.3 to 0.5 ppm, which makes it possible to assign them to type B.
This
implies a similar petrogenesis, and particularly a similarity of
P-T parameters of melting,
which determine the nature of the restite mineral assemblage. The melts were probably
formed in equilibrium with an amphibolite restite containing much ( 20%) garnet.
Another distinctive feature of the tonalite-trondhjemites, with the exception of
the
Ura-Guba granitoids, is a high LREE content, suggesting an enriched metabasic source.
|
| Figure 18 |
|
| Figure 19 |
On the whole, more differentiated, LREE-enriched metabasic sources are characteristic of the Murmansk block, and the enrichment shows a tendency to increase from west to east (up to the level exceeding TH2).
Modelling based on the interpretation of geophysical data and the study of composition of deep xenoliths of igneous rocks is essential to an assessment of compositional variations of the Earth's crust below the depth reached by the KSDB (12,262 m).
Deep seismic profiling and geophysical modelling of the crust in the region have provided a three-layer model crustal structure, which has a complicated mosaic-block pattern and reflects multiple stages of tectonic-magmatic and metamorphic transformation of the rocks.
The lowermost crust (basaltic or granulite-basic layer) is believed to occur at a depth of 25-30 km from the surface; its physical properties ( Vp =6.8-7.3, Vs =3.7-4.2 km/s, s =2.9-3.2 g/cm3 ) correspond to those of basic-ultrabasic rocks [Mitrofanov and Sharov, 1998]. The thickness of the "basaltic'' layer in deep-seated parts of large fragments of the Baltic Shield is estimated to be 10-15 km, with the proportion of granulite-basic and crust-mantle sublayers being approximately equal, or the granulite-basic sublayer being predominant. A considerably greater thickness of the "basaltic'' layer (up to 25-30 km) is established in adjoining areas of the Baltic Shield - between the Archaean Kola-Belomorian craton and Early Proterozoic Svecofennian continent-margin, and also below the Belomorian megablock, which served as a boundary between the Kola and Karelian megablocks. The increase of crustal thickness in these provinces is accompanied by a decrease in the proportion of middle and upper crustal layers, suggesting a prominent role of mantle magmatism in the formation of the lower crust in these regions [Mitrofanov and Sharov, 1998].
Rocks of the middle crust (dioritic, granulite-dioritic or charnockite-enderbitic layer) are separated from rocks of the granulite-basic layer and the upper crust by K 2 and K 1 boundaries, respectively, and are assumed to have an intermediate to basic composition. The dioritic layer ranges from 5 to 20 km in thickness.
The upper crust (granitic or granite-gneissic layer) ranges greatly in thickness (3 to 15 km) and can be broadly subdivided into the upper complex, which is composed of metamorphosed sedimentary-volcanic rocks, and the basal complex, which is composed mainly of tonalitic gneiss and amphibolite.
As many petrological models of the crust assume a two-layer structure whereby the lower and middle layers are combined and referred to as lower crust [Rudnick and Fountain, 1995; Taylor and McLennan, 1985], we will herewith hold this viewpoint on the definition of the lower crust (LC).
When explaining geodynamic settings in which TT assemblages and Early Precambrian crust were formed, it is common practice to invoke subduction, obduction or sagduction models. The first two models, which are based on plate-tectonic principles applied to the study of the Precambrian, imply that TT rocks were produced by melting of the oceanic lithosphere that had subsided down to mantle depths, or by melting of the lowermost layers of the oceanic lithosphere in the course of crustal growth by stacking of lithospheric plates. The applicability of these models to the eastern part of the Baltic Shield is apparently limited by the fact that TT rocks are abundant in the region, and if their genesis is interpreted in terms of lithospheric plate tectonics, subduction-obduction environments are to be accepted as the only possible regime for the region in the Late Archaean, which seems to be unlikely.
In contrast to purely plate-tectonic interpretations, the sagduction model implies that tonalite-trondhjemite melts were formed by partial melting of the oceanic lithosphere above mantle plumes, which were rising to the base of the lithosphere [Hain, 1993; Kroner, 1991]. According to this model, TT rocks were formed under compressional conditions caused by high fluid pressure in the frontal part of a plume [Glukhovsky and Moralev, 1996]. This interpretation may provide an explanation for the high pressures necessary for the formation of high-pressure restite mineral assemblages (see restite types 3, 4 and 5, Tables 7 and 8), which controlled the REE concentration and distribution in the studied TT rocks.
To apply the sagduction (plume tectonic) model to explain the formation of TT melts in the Late Archaean Kola subprovince, we have to assume the presence of older (Early Archaean) continental crust reworked in the Late Archaean. Only the presence of older crust could facilitate plate tectonic processes that are reconstructed for that period of time [Mints et al., 1996]. The evidence in favour of the existence, hi the Kola subprovince, of earlier continental crust that has lost some of its primary isotope-geochemical characteristics is presented in a number of publications [Batieva and Vinogradov, 1991; Bridgwater et al., 1996; Pushkarev et al., 1978].
We assume, therefore, that as a result of the uplift of a Late Archaean plume to the base of the lithosphere, the lithospheric mantle was depleted to form early crust of a basic composition, which was later metamorphosed to form TT rocks in frontal parts of mantle streams. Basaltic crust is thought to have been formed immediately prior to the generation of the tonalite-trondhjemite rocks, since model ages of the protoliths (TDMNd and TDMSr ) for the ancient granitoids of the Kola subprovince do not exceed 2.95-2.9 Ga [Balashov et al., 1992; Timmerman and Daly, 1995]. The juvenile Late Archaean crust was formed at that time apparently owing to the expansion of granite-greenstone domains by accretion of volcanic arcs to the nucleuses of Early Archaean (?) crust which was considerably reworked by Late Archaean tectonic processes.
Taking into account experimental data on partial melting of basic rocks [e.g. Rapp et al., 1991; Zharikov and Khodorevskaya, 1995] and the results of the calculations presented above (Tables 11-17), we can conclude that the zoning of the crust recognised from geophysical data could result from partial melting of the early metabasic crust. The lowermost, granulite-basic layer was apparently composed of restitic rocks, the composition of which was determined mainly by thermodynamic conditions responsible for the generation of TT melts and by the composition of basaltic protoliths. Above these, rocks of the restitic layer still contained an admixture of granitic material, which had not been completely removed, and the amount of which increased from bottom to top of the section. From deep seismic sounding data, this layer is recognised as a leucobasaltic or dioritic middle crustal unit. The dioritic layer might contain blocks or interlayers of the metabasalt that has not been subjected to partial melting. These blocks contain significant amounts of granitic material and are confined mainly to the upper parts of crust. In addition, the dioritic layer contains early crystallisation products that were generated by differentiation of granitic melts and occur at the boundary towards the overlying TT rocks, which made up the bulk of the upper crust.
Calculations of the composition of the lateral lower crust depend greatly on the thickness of the primary "basaltic'' layer and the volume of TT melts. The thickness of the "granitic'' layer hi the Late Archaean can be estimated at 4-6 km on the assumption of 20% partial melting of 20-30 km-thick layer of metabasic rocks.
To estimate the volume of rocks forming the granulite-basic and dioritic layers of the lower crust, we have employed the data from seismic investigations along a ~200 km long profile running in a submeridional direction from Cape Tolstik to the Khibiny across Late Archaean rocks of the northern and central Kola Peninsula [Mitrofanov and Sharov, 1998]. According to the seismic data, there is a ~15 km granulite-basaltic layer in the lower part of the crust. This layer is overlain by the "dioritic'' layer, which is divided into two members of different velocities and composition: one consists of basic rocks ( Vp =6.5-6.7 km/s) situated at a depth of 25-21 km, and the other of basic to intermediate rocks ( Vp =6.45-6.55 km/s) at a depth of 21-12 km. By physical properties, the upper crustal rocks are felsic with a high silica content.
Taking into account the above data and assuming that the established proportion of rocks in the crustal section resulted mainly from partial melting of about 30 km thick crust of basaltic composition, we have calculated the composition of the seismologically identified crustal layers, which apparently differ in the proportion of primary basaltic, restitic and granitic components.
The content of rock-forming components in the restite (CR) was calculated by subtracting the proportion of granitic material, which was estimated by petrologic-geochemical modelling of partial melting of amphibolite, from the composition of the source amphibolite (Co) (method of material balance). In order to determine regional compositional features of the sources, Co is taken to be equal to the composition of amphibolites from early inclusions in the Garsjo gneiss, Kirkenes gneiss and Murmansk block granitoids (Ura-Guba area), the REE composition of which has been established to be similar to that of model sources (Table 18). The petrochemical composition of crystallisation products (Cs) was determined by their quantitative mineral composition with the use of real compositions of rock-forming minerals for each TT rock complex studied (Table 1). The trace element compositions of Co, CR and Cs were calculated by using trace element distribution coefficients for partial melting and fractional crystallisation (Table 9), and the results are given in Table 19. For comparison, the table presents the results of determining the trace element composition of CR and Cs (in parentheses) by means of material balance. These results show a satisfactory agreement, with the exception of Y, Zr and Sr contents of CR and Ni content of Co, as the distribution of these elements is greatly influenced by accessory minerals (zircon, magnetite, ilmenite), the contents of which are difficult to assess by modelling partial melting and fractional crystallisation. As far as Sr is concerned, this component is relatively mobile, and the difference between upper crustal amphibolites and metabasic sources of TT rocks in terms of Sr contents can not be ruled out.
|
| Figure 20 |
A comparison of the calculated lower crust compositions suggests the following conclusions:
Two types of tonalite-trondhjemite plagiogneiss are distinguished in the Archaean
section of the KSDB. The protoliths of these rocks were formed at
P 15 kbar
(garnet-amphibolite restite) and ~8 kbar (plagioclase-amphibolite restite).
By the conditions of formation, these rocks are similar to the plagiogneiss of
the Garsjo complex occurring in the Svanvik-Lotta segment of the Kola-Norwegian
block and constituting the upper part of the Bjornevatn-Olenegorsk
greenstone
belt. In contrast to the Garsjo gneiss, the KSDB rocks were derived from less
differentiated metabasites which have lower La/Yb ratios and Sr contents and
are comparable with TH1 tholeiites. The presented evidence
suggests that homologues of
TT rocks in the Kola Peninsula should be searched for in the
Olenegorsk greenstone belt in central Kola (Figure 1). Further study of trace element
composition of
TT rocks from each of units 2, 4, 6, 8 and 10 of the KSDB and the work to
be done on petrologic-geochemical modelling will make it possible to refine the
composition of the protoliths, the conditions of melting of these rocks, and identify
their homologues on the surface.
Most Archaean amphibolites from the KSDB are different from model compositions of the sources for plagiogneiss protoliths in having higher contents of LREE and Sr. High-alumina amphibolites from the KSDB section appear to be most similar in REE spectra to the possible source of tonalite-trondhjemite melts, although further investigations are needed to draw a closer analogy.
Three types of tonalite-trondhjemite plagiogneiss have been distinguished among
rocks of the Kola-Norwegian block. The protoliths for these rocks may have formed
at
P 8, 15-16 and
16 kbar, according to experimental data on
melting of
amphibolite and model calculations. The degree of fractional crystallisation of
primary melts varies from 0 to 30%. The protoliths of dominating gneisses,
which are strongly depleted in heavy REE, are crystallisation products of the
melts generated in equilibrium with eclogitic restite. Less abundant gneiss
protoliths, which are moderately depleted in HREE, were probably
melted with a separation of a garnet-amphibolite restite containing 14 to 22%, rarely
30%
garnet. According to experimental data
[Rapp and Watson, 1995;
Sen and Dunn, 1994],
an increase in the restite garnet content to 20-30% can take place at nearly
constant
P (12-16 kbar) with the growth of
T and melting degree. At the same
time, the transition from a garnet-amphibolite restite to an eclogitic one,
requires an increase of
P to 16 kbar or more. The compositional similarity of
potential metabasic sources, which geochemically correspond to
TH1 tholeiites and amphibolites from inclusions in the Garsjo and Varanger gneisses,
suggests that the melting could occur at the same crustal level under
P 16 kbar and it was
related to one and the same tectonic-magmatic event.
Protoliths of the Garsjo gneiss, enriched in REE, were undoubtedly formed under relatively low pressures (~8 kbar). The metabasic source rock is distinctive. It is enriched in LREE and other incompatible elements (Sr), showing a similarity to TH2 basalt. This implies that magma generation was related to another tectonic process and that there was a possibility for the tonalite-trondhjemite melts to form in two stages, with the formation of a volcanic series by melting of TH2 metabasic rocks in the earlier stage. During the later stage, plutonic rock series were formed at deep levels of the crust composed of TH1 basalt. Low-alumina tonalite-trondhjemite of the Superior Province of the Canadian Shield has similar geochemical parameters and belongs to a synvolcanic suite which preceded the high-alumina trondhjemite with a fractionated REE distribution [Feng and Kerrich, 1992]. More fractionated REE compositions in intrusive rocks of TT composition than those of the "grey gneiss'' of supracrustal rock complexes have been reported also by Lobach-Zhuchenko et al. [1984] from granulite-gneiss regions of the Aldan Shield, Near-Baikal area, Karelia and greenstone domains of the Baltic Shield.
Melting of the Ura-Guba tonalite in the western Murmansk block took place under higher P and was accompanied by the separation of eclogitic restite; metabasic rocks with relatively low (La/Yb)n =1.8-2.1, but lower Sr contents (~200 ppm) could have served as melt sources in this case. The formation of the plagiogneiss protoliths in the central and eastern Murmansk block occurred in equilibrium with garnet-amphibolite restite with the garnet content varying from 14 to 22%. In REE content and distribution, the tonalite-trondhjemite gneiss of the Murmansk block is similar to TT rocks of type A and B of Varanger and Kirkenes gneisses from the Svanvik-Lotta segment. At the same time, more differentiated metabasic sources are typical of the Murmansk block, and the REE enrichment of the sources increases from northwest to southeast up to the level of TH2. The formation of LREE-enriched basic proto-crust and the emplacement of 2.76 Ga old [Pushkarev et al., 1978] subalkaline granite intrusions in the eastern Murmansk block were probably caused by the existence of a long-lived deep source, which was for a long time producing high-alkali melts that intruded into rocks of different levels of the Late Archaean crust.
The composition of the lower crust, which was formed complementarily with Late Archaean TT rocks is similar to that of olivine basalt, and in rock-forming components it is close to the lower crustal composition described by the model of Ronov et al. [1990]. Among regional features of the lower crustal composition are the lower content of aluminum and a higher content of iron, mainly due to the peculiarities of initial metabasalt composition. In the content of most trace elements, the lower crustal composition is similar to that of platforms and shields [Rudnick and Fountain, 1995], and the total LC composition [Taylor and McLennan, 1985].
The data obtained make it possible to forecast compositional variations in the crust below the depth reached by the KSDB (12,262 m) and can assist in interpreting the results from geophysical investigations.
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