Svecofennian metamorphic zones in the basement of Estonia/Eesti aluskorra Svekofennia moondevoondid.INTRODUCTION
Metamorphic rocks predominate in the Precambrian basement of Estonia. Earlier mapping of the buried basement rocks was performed using regional potential geophysical field (gravity, magnetic mapping) and drill core studies of more than 500 drill holes. These metamorphic belts belong to the uniform Svecofennian crustal crust·al
Of or relating to a crust, especially that of the earth or the moon.
Adj. 1. crustal - of or relating to or characteristic of the crust of the earth or moon domain spreading over 1 million [km.sup.2] in the western corner of the East European Craton The East European craton is the core of the Baltica proto-plate and consists of three crustal regions/segments: Fennoscandia to the northwest, Volgo-Uralia to the east, and Sarmatia to the south. . Within this domain, metamorphic zones from low- to high-grade metamorphism have been observed. Within Estonian basement, the rocks of high-grade amphibolite amphibolite
Igneous or metamorphic rock composed largely or dominantly of amphibole minerals. For igneous rocks, the term hornblendite is usually used and is more restrictive; hornblende is the most common amphibole. to granulite facies granulite facies
One of the major divisions of the mineral facies classification of metamorphic rocks, encompassing rocks that formed under intense temperature-pressure conditions (higher than 950°F, or 500°C). dominate. Detailed studies, however, have revealed widespread spatial variations in the metamorphic grade. Metamorphic gradients related to fault zones, semicircular or belt-form areas of increasing metamorphic grade (metamorphic domes) feature the general high-grade metamorphic field. In this paper we present an overview of detailed studies of metamorphism of Estonian basement rocks. The paper deals with the composition of metamorphosed supracrustal rocks, variation in their mineralogical composition, and regional and local estimates of metamorphic pressure-temperature (P-T) conditions and ages.
COMPOSITION AND REGIONAL SETTING OF THE ESTONIAN METAMORPHIC COMPLEXES
The structural zones distinguished in the basement of Estonia substantially vary in metamorphic lithologies. Previous studies have revealed that primary supracrustal suites are represented by a large variety of mafic, intermediate, and acidic volcanic rocks rocks which have been produced from the discharges of volcanic matter, as the various kinds of basalt, trachyte, scoria, obsidian, etc., whether compact, scoriaceous, or vitreous.
See also: Volcanic as well as clastic clastic /clas·tic/ (klas´tik)
1. undergoing or causing division.
2. separable into parts.
1. sediments from clays to sands, in places with carbonate admixture (Koppelmaa et al. 1978; Puura et al. 1983; Klein 1986). Additional information on the protoliths of the metamorphic rocks has been obtained studying typologies of zircons (Konsa 1986; Konsa & Puura 1999).
The average mineral contents of different supracrustal rocks in the Tallinn and Alutaguse structural zones (Fig. 1) are calculated from the drill core data. Al-rich garnet- (Gr), cordierite- (Cor) and sillimanite- (Sil) bearing gneisses and biotite biotite (bī`ətīt'), iron-rich variety of phlogopite, most abdunant of the mica minerals.
or black mica
Silicate mineral in the common mica group. (Bi) gneisses build up 25.4% and 90.45%, respectively; Bi-plagioclase (Pl)-Kfs (potassium feldspar potassium feldspar
A type of alkali feldspar that contains the molecule KAlSi3O8. Microcline and orthoclase are types of potassium feldspar. )-bearing gneisses - 24.4% and 1%, respectively; Bi-Pl, Bihornblende-(Hbl)-Pl and pyroxene pyroxene (pī`rŏksēn), name given to members of a group of widely distributed rock minerals called metasilicates in which magnesium, iron, and calcium, often with aluminum, sodium, lithium, manganese, or zinc occur as X in the chemical (Px)-Pl gneisses and amphibolites - 50.2% and 6.1%, respectively (Klein 1986). In southern and western Estonia, pyroxene-or hornblende-bearing mafic metavolcanic rocks dominate, respectively, whereas metapelites and granitogneisses are rare. In Fig. 2 mineral compositions, as calculated from the mineralogical analyses of optical microscopy, reflect both the distribution of rock types and their mineralogical assemblages in the four main structural zones of the Estonian basement. Within the local structural units of the Alutaguse zone, such as Tapa tapa: see bark cloth. , Haljala, Assamalla, Uljaste, and Johvi (Fig. 1), rock associations and mineral assemblages are much more variable (Fig. 2; Klein 1986).
[FIGURE 1 OMITTED]
Generally, pre-metamorphic textural features in high-grade metamorphic and migmatized rocks have been obliterated. However, occasionally graded bedding In geology, a graded bed is one characterized by coarse sediments at its base, which grade upward into progressively finer ones. They are perhaps best represented in turbidite strata, where they indicate a sudden strong current that deposits heavy, coarse sediments first, with is observed in metapelites. Sedimentary protoliths of metapelites in the Tallinn and Alutaguse zones are supported by findings of zoned zircons, whose rounded cores suggest their detrital de·tri·tus
n. pl. detritus
1. Loose fragments or grains that have been worn away from rock.
a. Disintegrated or eroded matter: the detritus of past civilizations. origin (Konsa & Puura 1999). The distinction of the sedimentary or volcanic origin of metamorphic suites, especially in cases of Carich composition reminding of mafic volcanics, has been made using the bulk chemical composition and spatial relationships of metamorphic rock suites in core sequences (Puura et al. 1983). Metamorphic and migmatization-related zircons void of detrital varieties or cores confirm the volcanic origin of many mafic protoliths (Konsa & Puura 1999).
[FIGURE 2 OMITTED]
METAMORPHIC FACIES facies /fa·ci·es/ (fa´she-ez) pl. fa´cies [L.]
1. the face.
2. surface; the outer aspect of a body part or organ.
3. expression (1). AND PRESSURE-TEMPERATURE CONDITIONS OF METAMORPHISM
Based upon the mineral parageneses and chemical compositions of minerals (wet chemical analysis of garnet, biotite, and amphibole amphibole (ăm`fəbōl'), any of a group of widely distributed rock-forming minerals, magnesium-iron silicates, often with traces of calcium, aluminum, sodium, titanium, and other elements. monomineral fractions) and the use of different geothermometers and geobarometers, the peak metamorphic conditions in main structural zones of the Estonian basement were estimated (Koppelmaa et al. 1978; Puura et al. 1983; Klein 1986). The data obtained revealed that high-temperature and moderate-pressure amphibolite facies amphibolite facies
One of the major divisions of the mineral facies classification of metamorphic rocks, encompassing rocks that formed under conditions of moderate to high temperatures (950°F, or 500°C, maximum) and pressures. conditions dominated in the Tallinn and Alutaguse zones. Geothermobarometry of the Bi + Gr [+ or -] Sil assemblage and cordierite cor·di·er·ite
A dichroic violet-blue to gray mineral silicate of magnesium, aluminum, and sometimes iron. Also called dichroite.
[French, after Pierre L. suggests peak metamorphic conditions at 600-700[degrees]C and 3-5 kbar (Klein 1986; Koistinen et al. 1996).
In South Estonia, the mineral parageneses of intermediate and mafic metavolcanics and Al-rich gneisses correspond to the granulite facies. Widespread garnet and cordierite in these rocks formed by breakdown of biotite and sillimanite sillimanite
Brown, pale green, or white glassy silicate mineral that often occurs in long, slender, needlelike crystals frequently found in fibrous aggregates. , which indicates prograde prograde
Having a rotational or orbital movement that is the same as most bodies within a celestial system. In our solar system, prograde movement for both rotating and orbiting bodies is in a counterclockwise direction when viewed from a vantage point metamorphism. However, in many places retrograde assemblages have formed in the conditions of the amphibolite facies.
In western Estonia, metabasites of the amphibolite facies dominate. Plagioclasemicrocline migmatites (leucosomes) are widespread in these rocks. However, the latest drill holes in the vicinity of Haapsalu town penetrated the rock complex with granulite facies assemblages (Koistinen 1994), which may be related to a metamorphic dome. In two drill cores, Bi-Hbl-Hyp gneisses, cut by Hyp-bearing granitoid veins (leucosomes) were observed (Koppelmaa & Kivisilla 1999).
Metamorphic domes in northern and northeastern Estonia diversify the generally amphibolite facies complexes of the Tallinn and Alutaguse zones. Areas with higher-grade metamorphism, which locally reaches the granulite facies, follow the eastern side of the Tapa fault separating the Tallinn and Alutaguse zones, and further extend to the east in the Uljaste area and Johvi zone, and the Slantsy area in Russia (Fig. 1). Three detailed drilling profiles (Fig. 3) consisting of 7-8 drill holes each were studied. Two profiles characterize the Tapa-Haljala dome area, and one--the Uljaste area. Monomineral fractions were separated to study the chemical composition (wet chemical analysis) and properties of the main rock-forming metamorphic minerals separated from metapelites and metabasites (Klein 1986). Typologies of zircons in accordance with the rock-forming mineral assemblages were studied (Konsa 1986; Konsa & Puura 1999). The granulite facies mineral assemblages in the Johvi zone have been described earlier (Koppelmaa et al. 1978; Puura et al. 1983).
Within the metamorphic domes, the primary lithologies of metamorphic rocks differ from those of the surrounding common amphibolite facies rocks (Fig. 1; Puura et al. 1983). In the Johvi zone and the central part of the Tapa block, meta-morphosed mafic volcanic and intrusive rocks are abundant; in the Johvi zone also magnetite-rich gneisses with banded iron formation (BIF BIF
In currencies, this is the abbreviation for the Burundi Franc.
The currency market, also known as the Foreign Exchange market, is the largest financial market in the world, with a daily average volume of over US $1 trillion. ) structures occur. In the surroundings of the Tapa zone (Assamalla and Haljala areas) and in the Uljaste area metapelites often alternate with quartzites and graphite- and sulphide-rich gneisses.
[FIGURE 3 OMITTED]
In order to study and map the conditions of metamorphism in the Tallinn and Alutaguse zones in detail, Klein (1986) investigated assemblages of metamorphic minerals, the chemical composition of rock-forming minerals (especially garnet and biotite separated from metapelites, and hornblende hornblende: see amphibole.
Any of a subgroup of amphibole minerals that are calcium-iron-magnesium-rich and monoclinic in crystal structure. and pyroxenes from metabasites) using wet chemical analysis in addition to optical parameters of minerals. In both zones, within the dominant high-grade amphibolite facies areas with Gr + Bi + Sil + Kfs and Gr + Cor + Sil + Kfs assemblages, microcline-plagioclase migmatites and small granite bodies are abundant. The areas of lower-grade Gr + Bi + And + Mu assemblages with Mi-Pl granite-pegmatite veins and erratic migmatite migmatite
Rock composed of a metamorphic host material that is streaked or veined with granite; the name means “mixed rock.” Many migmatites probably represent partial fusion of the metamorphic host; some components of the rock are fused and gather to produce zones are less common. The assemblages corresponding to the transition from amphibolite facies to granulite facies rocks, and those of the granulite facies were studied in the Tapa-Haljala and Uljaste areas (Klein 1986). In order to characterize the changes in metamorphic parameters from the surrounding areas towards the metamorphic domes, a set of discriminate functions calculated from the chemical analyses of minerals were used. Nikitina & Drugova (1977) have proposed the most sensitive discriminative dis·crim·i·na·tive
1. Drawing distinctions.
2. Marked by or showing prejudice: discriminative hiring practices. functions for these purposes based on the composition of garnet (1) and biotite (2). Usakova (1971) suggested discriminative functions which are based on the composition of biotite (3):
(1) [D.sub.x.sup.Gr] = 8.2Mn - 8.56Mg - 0.37Ca,
(2) [D.sub.x.sup.Bi] = - 19.56Ti + [1.26Al.sup.IV] + 10.21[Al.sup.VI] - 3.89Mg,
(3) [D.sub.x.sup.Bi] = 7.876Si - 10.251Al + 17.173Ti - 5.661[Fe.sup.3+] - 1.404[OH] - 4.286[Fe.sup.2+] + 4.524Mg + 4.661K.
The diagrams of discriminative values along the drill hole profiles demonstrate a gradual rise in metamorphic P-T parameters from the surroundings towards the centres of the domes: from the northeast to the Haljala dome, from the east (right side of the diagram) to the Tapa dome, and from north to south across the Uljaste dome (Fig. 3). The changes in these discriminative values correspond considerably well to the changes in mineral assemblages.
Two case studies of mineral associations from southern and western Estonia have been performed using microanalysis-based geothermobarometry (Holtta & Klein 1991; Kikas 2001). Holtta & Klein (1991) studied two drill cores, Konnu 300 and Varbla 502, which penetrated the granulite gran·u·lite
A fine-grained metamorphic rock often banded in appearance and composed chiefly of feldspar, quartz, and garnet.
gran rocks of SW Estonia (Fig. 1). Detailed thin section studies and microprobe microprobe /mi·cro·probe/ (mi´kro-prob?) a minute probe, as one used in microsurgery.
a minute probe, such as one used in microsurgery. chemical studies of main minerals revealed that in both cores the studied assemblages (Gr + Hyp + Bi [+ or -] Cor [+ or -] Kfs + Pl + Q, Gr + Cor + Bi [+ or -] Sil [+ or -] Kfs + Pl + Q, Gr + Bi + Sil + Mg + Pl + Kfs + Q [+ or -] Cor, Gr + Bi + Pl + Q [+ or -] Kfs, Gr + Cor + Pl + Q + Sp + Mg) formed at peak temperature near 800[degrees]C. Geobarometers gave pressure estimates close to 6 kbar. The results of core-core and rim-rim thermobarometry suggested near-isobaric post-peak cooling of 85-170[degrees]C. Holtta & Klein (1991) compared these results with those of Finland. Considerably small Svecofennian granulite terrains in southern Finland were formed at somewhat lower temperature (700-800[degrees]C) and pressure (3-5 kbar). However, Proterozoic Pielavesi granulites of central Finland, which are accompanied by hypersthene hy·per·sthene
A green, brown, or black splintery, cleavable pyroxene mineral, essentially (Fe,Mg)2Si2O6. granitoids, were formed at 800[degrees]C and 5-6 kbar and show evidence for near-isobaric cooling (Holtta 1988). Beside these similar patterns, the Pielavesi granulite area is much smaller than the granulite area in southern Estonia and northern Latvia. The South Estonian granulites definitely differ from the Archaean granulite zones in central Finland, where the metamorphic peak conditions were at ca. 750-850[degrees]C and 8 [+ or -] 1 kbar. The Svecofennian granulite areas in Finland, however, differ in the age of peak metamorphism.
R. Hints (Kikas 2001) performed detailed studies of rock-forming mineral assemblages from the Konnu 300 (granulite facies, South Estonia) and Valgu 99 (amphibolite facies, West Estonia) cores. These drill holes are located close to the Middle Estonian Fault Zone, virtually on its opposite sides: Konnu 300 in the south and Valgu 99 in the north (Fig. 1). In the rather homogeneous sequence of the Konnu 300 core, the Gr + Opx + Bi + Cor + Pl [+ or -] Kfs + Q assemblage is dominant. In some sections of rocks the Gr + Cor + Bi [+ or -] Sil [+ or -] Sp [+ or -] Kfs + Pl + Q [+ or -] opaque assemblage was observed. The rock sequence contains a small volume of potassium-rich migmatitic leucosomes. The peak paragenesis par·a·gen·e·sis also par·a·ge·ne·sia
The order in which a formation of associated minerals is generated.
par is believed to have been formed by partial melting reactions. It is also likely that biotite dehydration reaction This article is about chemical reactions resulting in the loss of water from a molecule. For the removal of water from solvents and reagents, see desiccation.
In chemistry, a dehydration reaction is the source for melt formation in metasediments from the Konnu and Valgu sections. This type of reaction in metapelites may commence at relatively low temperatures--at 760[degrees]C (Le Breton & Thompson 1988). Traces of partial melting imply that melting was the prominent mechanism for dehydration of rocks in the Konnu and Valgu drill cores. The coarse-grained peak assemblage with equidimensional porphyroblasts has probably formed under isotropic Refers to properties that do not differ no matter which direction is measured. For example, an isotropic antenna radiates almost the same power in all directions. In practice, antennas cannot be 100% isotropic. or weak unisotropic stress conditions. The occurrence of orthopyroxene orthopyroxene
Any variety of the mineral pyroxene that crystallizes in the orthorhombic system and contains no calcium and little or no aluminum. Enstatite is an orthopyroxene. and hercynite, and specific chemical composition of high-grade phases, such as a high Ti content in biotite and a high Al content in orthopyroxene, are in accordance with the other geothermobarometric estimates, which suggests the peak temperature of formation over 800[degrees]C and pressure of 5-6 kbar (Fig. 4). Alteration of orthopyroxene into biotite-quartz symplectites indicates that the primary cooling after the peak-metamorphic stage has also occurred under similar conditions. With the onset of plastic deformation plastic deformation,
n any irreversible deformation of tissues. , extensive re-hydration started and retrograde garnet was generated (Kikas 2001).
The migmatitic gneisses of the Valgu 99 drill core show higher Al and Si, and lower Fe and Mg contents than rocks of the Konnu 300 core. The peak mineral assemblages of Bi [+ or -] Gr + Kfs + Pl + Sil + Q [+ or -] opaque and Bi [+ or -] Cor + Kfs + Pl + Sil + Q [+ or -] opaque are most common. Biotite dehydration was favourable for the partial melting. Unlike the Konnu drill core, metapelites of the Valgu core attained peak temperatures simultaneously with a strong plastic deformation event that induced the formation of elongated porphyroblasts of garnet and cordierite. Geothermobarometric estimates suggest peak conditions at the upper amphibolite facies: temperature ~ 700[degrees]C and pressure ~ 4.5-5.5 kbar (Kikas 2001).
The high-grade metamorphic rocks have passed several metamorphic stages during the formation of their final mineral assemblages. After the formation of the peak metamorphic associations, at least two retrograde metamorphic associations reflect the decreasing trend of P-T conditions in both drill cores (Fig. 4). Despite the difference in the conditions of the peak metamorphism between the Valgu and Konnu rocks, their retrograde P-T path was similarly expressed by retrograde assemblages whose geothermobarometric estimates suggest P-T conditions at around 650-690[degrees]C and 3.5-4 kbar (second retrograde stage) and 500-600[degrees]C and 2-2.5 kbar (Fig. 4). The P-T path revealed by the thermobarometric data (see Fig. 4) has a moderate slope. This may indicate cooling with some decompression. Actual peak temperatures for both studied drill cores were probably higher than the biotite dehydration melting curve and thus the pressure values used for the P-T path are probably somewhat lower than the actual pressure values (Kikas 2001). The results of several studies (Klein 1986; Holtta & Klein 1991; Kikas 2001) suggest a low-pressure origin for the Estonian granulites, with the average pressures in the range 4-6 kbar. This is supported by the lack of high-pressure minerals, such as kyanite kyanite
or cyanite or disthene
Silicate mineral, one of several phases in the aluminum silicate (Al2SiO5) system. Its colour ranges from gray-green to black or blue, with blue and blue-gray being most common. , and mineral assemblages, such as orthopyroxenesillimanite.
[FIGURE 4 OMITTED]
The difference between the metamorphic stages in the opposite sides of the Middle Estonian fault possibly points to the much deeper exhumation of the South Estonian granulite domain. This is consistent with the results obtained by the structural studies (All et al. 2004).
THE AGE OF GRANULITE METAMORPHISM
The Sm-Nd analyses of four whole-rock samples were used to confirm the Palaeoproterozoic origin of the East Baltic East Baltic may refer to:
All isotopic analyses were made on hand-picked mineral concentrates. The methods follow standard procedures at the Geological Survey of Finland (e.g. Mouri et al. 1999). The analytical results are given in Table 1 and Fig. 5. A small amount of very clear, short, and pale zircon zircon
Silicate mineral, zirconium silicate, ZrSiO4, the principal source of zirconium. Zircon is widespread as an accessory mineral in acid igneous rocks; it also occurs in metamorphic rocks and, fairly often, in detrital deposits. was extracted from the sample Konnu 3005150. Zircon has a low Th content and the morphology is of "granulite type". Three U-Pb analyses of this zircon yielded [sup.207]Pb/[sup.206]Pb ages of 1.84-1.85 Ga, and plot close to each other on the concordia diagram (Fig. 5). Although the data are relatively close to concordia, no reliable age can be determined. A U-Pb analysis on monazite monazite (mŏn`əzīt), yellow to reddish-brown natural phosphate of the rare earths, mainly the cerium and lanthanum metals, usually with some thorium. Yttrium, calcium, iron, and silica are frequently present. provides a concordant age of 1778 [+ or -] 2 Ma. Only a very small amount of zircon was extracted from the sample Laeva 184860. This provided one discordant analysis with a 207Pb/206Pb age of 1776 Ma.
Garnets in sample 3005150 are large, 3-10 mm in diameter, and have numerous inclusions of biotite and quartz (Holtta & Klein 1991). From sample 3005150, garnet, plagioclase plagioclase
Any member of the series of abundant feldspar minerals that usually occur as light- to medium-grey-coloured, transparent to translucent grains or crystals. Plagioclase ranges in composition from albite to anorthite. , and orthopyroxene have been used for Sm-Nd studies (Table 2, Fig. 6), but due to low amounts of Nd the analysis on Opx was not successful. Five analyses on mineral concentrates and whole rock provided the age of 1728 [+ or -] 24 Ma (MSWD = 2). An analysis on ground and leached garnet (Gr#3) shows significantly higher Sm-Nd and lower REE contents, compared to the two older analyses on unleached garnet concentrates. This is obviously due to monazite inclusions in garnet. Holtta & Klein (1991) reported microscopic observations on such inclusions. However, rejection of Gr#3 analyses does not significantly change the calculated age. The Sm-Nd analyses on plagioclase and whole rock from sample 184860 gave the age of 1766 [+ or -] 56 Ma. An analysis on orthopyroxene shows disequilibrium disequilibrium /dis·equi·lib·ri·um/ (dis-e?kwi-lib´re-um) dysequilibrium.
linkage disequilibrium with whole rock.
[FIGURE 5 OMITTED]
Dating the granulite facies metamorphism
The isotopic results from sample 3005150 can be used for constraining the metamorphic evolution of Estonian granulites. However, it is evident from several recent studies that the interpretation of mineral ages involves problems. It has been shown that monazite can form at different stages during metamorphic evolution, and can preserve ages corresponding to peak temperatures up to granulite facies conditions or may even record prograde growth ages (DeWolf et al. 1996; Spear & Parrish 1996; Vry et al. 1996; Foster et al. 2002). It has also been suggested that the concept of "thermally reset" monazite ages, or of monazite "cooling ages" should be abandoned (Parrish & Whitehouse 1999).
The Sm-Nd ages on garnet have been used for constraining the timing of metamorphic cooling. However, there is some debate on the closure temperature. According to Ganguly et al. (1998), no unique closure temperature for the Sm-Nd system in garnet can be stated, but for slowly cooled almandine-pyrope garnet a closure temperature of 650-700[degrees]C can be estimated (Jung & Mezger 2001; Thoni 2003).
[FIGURE 6 OMITTED]
Following the above remarks it is suggested that the age of 1778 [+ or -] 2 Ma for monazite is close to the peak metamorphic stage, ca. 800[degrees]C and 6 kbar (Holtta & Klein 1991). The Sm-Nd age of 1728 [+ or -] 24 Ma for garnet can be interpreted as the age when the rocks cooled down to the closure temperature of large garnets, ca. 650-700[degrees]C. These results can be compared with the results from the Fennoscandian Shield area. Holtta & Klein (1991) suggested that the metamorphic evolution of Estonian granulites resembles that of the NE Svecofennian Haukivesi-Kiuruvesi complex, where the high-grade metamorphism (800[degrees]C and 5-6 kbar) took place at ca. 1.88 Ga (Holtta 1988; Korsman et al. 1988). They also concluded that in the granulite areas of southern Finland (Turku, West Uusimaa, and Sulkava) the rocks were metamorphosed at similar temperatures but slightly lower pressures (700-800[degrees]C and 3-5 kbar; Korsman et al. 1984; Holtta 1986; Schreurs & Westra 1986; Vaisanen & Holtta 1999).
The U-Pb and Sm-Nd isotopic studies of the Svecofennian granulite areas in Finland clearly show that there are two distinct granulite facies pulses, at ca. 1.88 Ga and ca. 1.83-1.80 Ga. The early U-Pb results reported by Korsman et al. (1984) have been confirmed by more recent data. From an older granulite belt in Luopioinen (near Tampere) Mouri et al. (1999) have reported concordant U-Pb monazite ages of 1878 [+ or -] 2 Ma and an average Sm-Nd garnet age of 1870 [+ or -] 20 Ma (five Gr-Wr pairs, Mouri et al. 1999). Migmatites from the Turku area have yielded U-Pb zircon ages of ca. 1.82 Ga (Vaisanen et al. 2002). The West Uusimaa migmatites have yielded U-Pb ages of monazites from 1816 [+ or -] 2 Ma to 1832 [+ or -] 2 Ma, whereas Sm-Nd analysis on garnet gives an average age of 1803 [+ or -] 6 Ma (unpublished data by H. Huhma; Vaisanen et al. 2004).
The ages from the Estonian sample 3005150, i.e. 1778 [+ or -] 2 Ma for monazite and 1728 [+ or -] 24 Ma for garnet, are clearly younger than any comparable results from Finland. They suggest that the granulite facies metamorphism in southern Estonia is distinct from that recorded in the Fennoscandian Shield.
Metamorphic suites of the Estonian basement belong to the interior of the Svecofennian orogenic domain of about 1.9-1.8 Ga juvenile crust. The earlier suppositions on the Achaean age of granulite terrains within the Baltic-Belarus region (Puura et al. 1976, 1983; Koppelmaa et al. 1978) were invalidated after isotopic studies, especially using Sm-Nd (Puura & Huhma 1993; Ramo et al. 1996) and U-Th-Pb (Petersell & Levchenkov 1994) methods. Nironen (1997) presented a regional model of the formation of the Svecofennian juvenile crustal domain through (1) continental break-up of the Late Archaean crust and seafloor spreading during 2.1-1.95 Ga, (2) early crust formation at 2.1-1.9 Ga, (3) initiation, development, and compression of subduction sub·duc·tion
A geologic process in which one edge of one crustal plate is forced below the edge of another.
[French, from Latin subductus, past participle of zones, and island arcs and between-arc sedimentary basins at 1.91-1.82 Ga, and (4) anatectic potassium granite and migmatite formation during the 1.84-1.81 Ga extensional stage. In the Svecofennian interior within the Fennoscandian Shield, the 5 [+ or -] 2 kbar metamorphism pressure corresponds to a rather uniform 12-18 km post-peak metamorphism level (Nironen 1997).
Two main regions of metamorphic grade occur in the basement of Estonia. Primary supracrustal lithologies vary in these areas. In the dominating amphibolite facies fields of northern Estonia, the sequence of metavolcanics and -sediments of the Tallinn zone resembles the island arc sequences of southern Finland. Metapelites of the Alutaguse zone may be a continuation of respective lithologies, widespread near the Archaean Karelian Craton craton (krā`tŏn): see continent. in SE Finland and neighbouring St. Petersburg area in NW Russia (Puura et al. 1983; Koistinen 1994; Koistinen et al. 1996). In western Estonia, dominating mafic metavolcanics of the amphibolite facies resemble those of the primary lithologies of South Estonian granulites. In general, these zones fit well with the metamorphic zones typical of the Svecofennian interior in the Fennoscandian Shield. The local metamorphic domes reaching up to granulite facies conditions (Haapsalu in western Estonia; Tapa, Uljaste, and Johvi in NE Estonia, and Slantsy in NW Russia) also resemble respective complexes in southern and central Finland (Koistinen 1994; Koistinen et al. 1996).
Dominating granulitic mafic metavolcanics of southern Estonia extend towards northern Latvia. A number of similar zones and crustal blocks occur in Lithuania and Belarus. The fact that the peak granulite metamorphism conditions in southern Estonia indicate the pressure up to 6 kbar suggests a much deeper erosional level, of about 20 km and more, when compared to northern Estonian amphibolites.
However, isotopic ages from the sample Konnu 3005150, i.e. 1778 [+ or -] 2 Ma for monazite and 1728 [+ or -] 24 Ma for garnet, are clearly younger than any comparable results from Finland. They suggest that the granulite facies metamorphism in this particular area or in southern Estonia and Latvia in general is distinct from that recorded in southern Finland.
The anatectic potassium granites and migmatites of southern Finland have their counterparts in northern Estonia (Puura et al. 1983). In southern Estonia, potassium granites occur in the form of charnockites related to peak granulite metamorphism (Kikas 2001). On the other hand, microcline microcline: see feldspar.
Common feldspar mineral, one form of potassium aluminosilicate (KAlSi3O8) that occurs in many rock types. Green specimens are called amazonstone and may be used as gems. granites and migmatites are occasionally found in fault zones within the dominating granulite terrains of southern Estonia. Relationships between these two occurrences remain open. Orogenic fault zones, such as the Paldiski-Pskov, Tapa, and others, as well as the post-orogenic Middle Estonian fault, control the regional and local (dome-like) metamorphic gradients (see All et al. 2004). The age of these late orogenic faults is younger than the granulite metamorphism.
A variety of peak and retrograde mineral assemblages of Estonian metamorphic rocks (e.g. the presence of zoned zircons) reflecting their multistage evolution, relationships to fault-related deformations with different age, and granitoids still contain a huge amount of possible new information for further specified studies and dating the tectonic and metamorphic events in the history of the Palaeoproterozoic crust. More differences between the evolution of the Svecofennian crust in southern Finland and northern Estonia, on the one hand, and in southern Estonia and Latvia, on the other hand, are expected.
V. P. and A. S. acknowledge the financial support from the Estonian Science Foundation (grants Nos. 4417, 4615, and 5817). We are grateful to the reviewers H. Nestor and A. Joeleht for their critical comments and helpful advice in improving the manuscript.
Received 14 August 2003, in revised form 20 April 2004
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Vaino Puura (a), Rutt Hints (a,e), Hannu Huhma (b), Vello Klein (c), Mare Konsa (d), Reedik Kuldkepp (a), Irmeli Manttari (b), and Alvar Soesoo (d)
(a) Institute of Geology, University of Tartu, Vanemuise 46, 51014 Tartu, Estonia; firstname.lastname@example.org
(b) Geological Survey of Finland, P.O. Box 96, FIN-02151 Espoo, Finland
(c) Geological Survey of Estonia, Kadaka tee 82, 12816 Tallinn, Estonia
(d) Institute of Geology at Tallinn University of Technology Tallinn University of Technology (TUT) (Estonian: Tallinna Tehnikaülikool (TTÜ)) is the only university of technology in Estonia, and one of the three most important institutions of higher education in Estonia generally. , Estonia pst. 7, 10143 Tallinn, Estonia
(e) Estonian Museum of Natural History, Lai 29A, 10133 Tallinn, Estonia
Table 1. U-Pb age data on zircon and monazite of Estonian granulites Analysed Sample U, Pb, mineral/fraction, weight, ppm ppm mm mg Konnu 3005150 Opx-Gr-gneiss: 3005150A + 4.5 0.05 768 250 3005150B 0.31 534 173 4.2-4.5 equant rounded 3005150C 0.22 620 199 4.2-4.5 short 3005150D 0.24 2 414 3 646 monazite Laeva 184860 Opx-gneiss: 184860A + 3.6 0.09 1 358 392 Analysed [sup.206]Pb/ [sup.208]Pb/ mineral/fraction, [sup.204]Pb [sup.206]Pb mm measured radiogenic Konnu 3005150 Opx-Gr-gneiss: 3005150A + 4.5 4 465 0.05 3005150B 33 465 0.05 4.2-4.5 equant rounded 3005150C 32 571 0.05 4.2-4.5 short 3005150D 14 840 4.38 monazite Laeva 184860 Opx-gneiss: 184860A + 3.6 2 019 0.23 Analysed Isotopic ratios * mineral/fraction, [sup.206]Pb/ [sup.207]Pb/ mm [sup.238]U 2SE% [sup.235]Pb Konnu 3005150 Opx-Gr-gneiss: 3005150A + 4.5 0.32186 0.4 4.9861 3005150B 0.32219 0.4 5.0323 4.2-4.5 equant rounded 3005150C 0.31966 0.4 4.9779 4.2-4.5 short 3005150D 0.3206 2 4.8058 monazite Laeva 184860 Opx-gneiss: 184860A + 3.6 0.24508 0.5 3.6705 Analysed Isotopic ratios * mineral/fraction, [sup.207]Pb/ mm 2SE% [sup.206]Pb Konnu 3005150 Opx-Gr-gneiss: 3005150A + 4.5 0.4 0.11236 3005150B 0.4 0.11328 4.2-4.5 equant rounded 3005150C 0.4 0.11295 4.2-4.5 short 3005150D 2 0.10872 monazite Laeva 184860 Opx-gneiss: 184860A + 3.6 0.5 0.10862 Analysed Isotopic ratios * mineral/fraction, mm 2SE% Rho ** Konnu 3005150 Opx-Gr-gneiss: 3005150A + 4.5 0.1 0.97 3005150B 0.06 0.99 4.2-4.5 equant rounded 3005150C 0.06 0.99 4.2-4.5 short 3005150D 0.1 1.00 monazite Laeva 184860 Opx-gneiss: 184860A + 3.6 0.07 0.99 Apparent ages, Ma ([+ or -] 2SE) Analysed [sup.206]Pb/ [sup.207]Pb/ mineral/fraction, [sup.238]U [sup.235]U mm Konnu 3005150 Opx-Gr-gneiss: 3005150A + 4.5 1 799 1 817 3005150B 1 800 1 825 4.2-4.5 equant rounded 3005150C 1 788 1 815 4.2-4.5 short 3005150D 1 793 1 786 monazite Laeva 184860 Opx-gneiss: 184860A + 3.6 1 413 1 565 Apparent ages, Ma ([+ or -] 2SE) Analysed [sup.207]Pb/ mineral/fraction, [sup.206]Pb mm Konnu 3005150 Opx-Gr-gneiss: 3005150A + 4.5 1 838 (2) 3005150B 1 853 (1) 4.2-4.5 equant rounded 3005150C 1 847 (1) 4.2-4.5 short 3005150D 1 778 (2) monazite Laeva 184860 Opx-gneiss: 184860A + 3.6 1 776 (1) * Isotopic ratios corrected for fractionation, blank (50 pg Pb) and age related common lead (Stacey & Kramers 1975). ** Error correlation for [sup.207]Pb/[sup.235]U vs. [sup.206]Pb/[sup.238]U ratios. 3003150 zircons: very clear, pale, small (< 100 [micro]m), short. Analysed by I. Manttari. Table 2. Sm-Nd isotopic data of Estonian granulites; whole-rock analyses can be obtained from Puura & Huhma (1993) Sample/ Location/mineral Rock type analysis 3005150 Konnu Opx-Gr-granulite 3005150-Opx Opx 3005150-Pl Pl 3005150-Gr#1 Gr (+inclusions) 3005150-Gr#2 Gr (+inclusions) 3005150-Gr#3 Gr, ground & leached 184860 Laeva Opx-granulite 184860-Pl Pl 184860-Opx Opx Sample/ Sm Nd [sup.147]Sm/ analysis [sup.144]Nd 3005150 6.05 34.30 0.1066 3005150-Opx 0.12 0.32 0.2278 3005150-Pl 1.16 7.89 0.0887 3005150-Gr#1 1.72 4.37 0.2376 3005150-Gr#2 1.92 5.27 0.2201 3005150-Gr#3 0.80 0.72 0.6750 184860 4.76 25.29 0.1139 184860-Pl 1.07 8.85 0.0730 184860-Opx 0.92 4.97 0.1117 Sample/ [sup.143]Nd/ 2SE Epsilon DM-age analysis [sup.144]Nd at 1.9 Ga 3005150 0.511537 10 0.4 2142 3005150-Opx -- -- 3005150-Pl 0.511329 10 3005150-Gr#1 0.513014 14 3005150-Gr#2 0.512840 10 3005150-Gr#3 0.517982 38 184860 0.511618 10 0.2 2176 184860-Pl 0.511143 10 184860-Opx 0.511703 10 [sup.143]Nd/[sup.144]Nd is normalized to [sup.146]Nd/[sup.144]Nd = 0.7219. Within run error expressed as 2SE in the last significant digits. Initial epsilon calculated using [sup.143]Nd/[sup.144]Nd = 0.51264 and [sup.147]Sm/[sup.144]Nd = 0.1966. T-DM calculated according to DePaolo (1981). Gr#3 ground and leached according to DeWolf et al. (1996) and measured using single Re-filament. -- no data.