Proceedings of the Estonian Academy of Sciences

CONTENTS & ABSTRACTS

In English. Summaries in Estonian

Proceedings of the Estonian Academy of Sciences.

Geology

Volume 53 No. 3 September 2004

Special issue on the Precambrian basement of Estonia

Precambrian basement of Estonia; 147–148

Alvar Soesoo

Outlines of the Precambrian basement of Estonia; 149–164

Alvar Soesoo, Väino Puura, Juho Kirs, Valter Petersell, Mati Niin, and Tarmo All

Abstract. The crystalline basement of Estonia can be subdivided into two major geological units – North Estonian amphibolite and South Estonian granulite complexes. The amphibolite facies terrain consists of a sequence of metavolcanics and -sediments, which resemble those of the island arc sequences in southern Finland. The South Estonian granulites, probably belonging to the Belarus–Baltic Granulitic Belt, show the peak metamorphism conditions of about 800 °C and 5–6 kbar. U-Pb isotopic data demonstrate that the Estonian basement has formed during the Palaeoproterozoic, 1.9–1.8 Ga ago and is composed of magmatic, volcanic, and sedimentary components across major structural zones. The granulite metamorphism postdates these ages (1.79–1.73 Ga). The youngest igneous rocks in the basement belong to the Fennoscandian Palaeo-Mesoproterozoic Rapakivi Province and include the composite Riga batholith, as well as at least five minor stock-like porphyritic K-granite plutons (ca. 1.6 Ga).

Key words: petrology, geochronology, magmatism, Palaeoproterozoic, Estonian basement.

Orogenic structures of the Precambrian basement of Estonia as revealed from the integrated modelling of the crust; 165–189

Tarmo All, Väino Puura, and Rein Vaher

Abstract. Up-to-date gravity and magnetic field maps and deep seismic sounding data were used to study the orogenic structure of the Palaeoproterozoic (1.9–1.8 Ga) Svecofennian basement of Estonia. The structure of the crust was quantitatively modelled for two NNE–SSW trending profiles across the main – south (southwest) and north (northeast) – structural terrains and the tectonic transition zone between them. Lateral variability of the basement structure was revised using the available geological and geophysical maps and results of 3D modelling.

Metavolcanic and metasedimentary rocks prevail in the orogenic basement of the Estonian mainland. The crust is overthickened, thus a remarkable gravity minimum is expected to occur here. In NNE Estonia, where the low-density (2680–2710 kg/m³) migmatized metamorphic rocks of the amphibolite facies prevail in the upper crust, the moderately overthickened (45–50 km) crust is coupled with gravity (– 10 to – 40 mGal) and magnetic (down to – 800 nT) low. The deepest gravity minimum (– 40 mGal) is associated with the thick and light upper crust at the Tallinn zone. In SSW Estonia and northern Latvia, the extremely overthickened (50–65 km) crust is composed of the dense (2750–2840 kg/m³) upper crust and overthickened, partially upthrusted lower crust. Here the mass deficit is compensated, and in many cases even overcompensated, by the upper crust composed of granulite facies rocks and blocks of the upthrusted lower crust. The Paldiski–Pskov zone is a transition between the two terrains, following deep crustal shear zones that dip to the SSW. The survived orogenic structure of the Estonian mainland formed due to the late Svecofennian SSW–NNE directed compression that resulted in crustal shortening and thickening, with the upthrusts along the Paldiski–Pskov zone and maximal crustal thickness in the SSW.

Key words: magnetic and gravity modelling, upper crust, lower crust, deep seismic sounding, Estonia.

Svecofennian metamorphic zones in the basement of Estonia; 190–209

Väino Puura, Rutt Hints, Hannu Huhma, Vello Klein, Mare Konsa, Reedik Kuldkepp, Irmeli Mänttäri, and Alvar Soesoo

Abstract. Svecofennian (Palaeoproterozoic) orogenic, folded metamorphic rocks dominate in the basement structure in Estonia. In northern Estonia (Tallinn zone), supracrustal rock associations (metavolcanic and -sedimentary sequences), their structure and amphibolite metamorphic grade resemble the Svecofennian metamorphic island arc suites of southern Finland. In northeastern Estonia, the metapelitic sequences of the Alutaguse zone resemble those of the NE marginal metasedimentary basins of the Svecofennian orogen, as studied in the St. Petersburg District (NW Russia) and SE Finland. Local variations of the metamorphic grade, related to fault zones or local metamorphic domes (metamorphosed up to granulite assemblages) feature the amphibolite facies areas of northern Estonia. In southern Estonia, tectonically undefined, predominantly mafic metavolcanics are of granulite metamorphic grade. The granulite region of southern Estonia and northern Latvia is much larger than known in the 1.9–1.8 Ga Svecofennian metamorphic zones of southern Finland. The peak conditions of granulite metamorphism in Estonia at ca. 800 °C and 4–6 kbar resemble those of the Pielavesi granulites (Proterozoic), central Finland. However, the U-Pb age of 1778 ± 2 Ma for monazite and the Sm-Nd age of 1728 ± 24 Ma for garnet from the sample Kõnnu 3005150 are clearly younger than any comparable results from Finland, and suggest that the granulite facies metamorphism in southern Estonia is distinct from that recorded in southern and central Finland.

Key words: metamorphism, P–T-conditions, age, Palaeoproterozoic, Estonian basement.

Abbreviations: Bi = biotite, Pl = plagioclase, Kfs = potassium feldspar, Gr = garnet, Cor = cordierite, Sil = sillimanite, Hbl = hornblende, Px = pyroxene, Hyp = hypersthene, And = andalusite, Mu = muscovite, Mi = microcline, Q = quartz, Sp = spinel, Ep = epidote, Ap = apatite, Carb = carbonates, Opx = orthopyroxene, Mpx = monocline pyroxene, Wr = whole rock, Cpx = clinopyroxene.

Anorogenic magmatic rocks in the Estonian crystalline basement; 210–225

Juho Kirs, Ilmari Haapala, and O. Tapani Rämö

Abstract. The anorogenic magmatic rock bodies of the Estonian crystalline basement belong to the Fennoscandian Palaeo–Mesoproterozoic Rapakivi Province and include the huge composite Riga batholith (250 km ´ 230 km in subsurface area, mostly in NW Latvia), as well as at least five granite stocks (Naissaare, Märjamaa and its Kloostri satellite, Taebla, Neeme, and Ereda) and the quartz monzodioritic Abja stock in the Estonian mainland. The Riga batholith contains both mafic and silicic rocks, as several Fennoscandian rapakivi complexes do. The granite stocks appear on geophysical maps as gravity and magnetic minima, and they consist of pink, medium- to coarse-grained, microcline-megacrystic, partly trachytoid biotite (in Märjamaa and Naissaare also with hornblende) granite, locally cut by aplitic and microsyenitic dykes. As an exception, the Märjamaa stock is more differentiated and has an anomalously high magnetic central part composed of hybrid granodiorite with hornblende as the main mafic mineral. The Abja quartz monzodiorite stock is strongly magnetic and consists of a dark grey, massive, medium-grained, partly weakly gneissose rock with abundant accessory apatite and titanomagnetite, and is intersected by veins of fine- to medium-grained, slightly porphyritic plagioclase-microcline granite. The major, REE and other trace element contents of the granitic rocks are close to or overlap those of the typical Finnish rapakivis and are best comparable with the less differentiated granitic phases. The Märjamaa granodiorite and the Abja quartz monzodiorite are enriched in Sr, Ti, and P. In Nd and Pb isotopic composition, the felsic and mafic rocks resemble corresponding rocks of the Finnish rapakivi complexes, indicating an approximately chondritic source for Nd in the mafic rocks of the Riga batholith and the Abja intrusion, and a Palaeoproterozoic (Svecofennian) source for the felsic rocks. The T_DM model ages of the felsic rocks range from 1890 to 2100 Ma.

Key words: Proterozoic, Estonian basement, anorogenic magmatism, rapakivi, geochemistry, age.

Instructions to authors; 226–229