CONTENTS & ABSTRACTS
InEnglish. Summaries in Estonian
Proceedings of the Estonian Academy of Sciences.
Geology
Volume 53 No. 3September 2004
Special issue on the Precambrian basement ofEstonia
Precambrian basement of Estonia; 147–148
Alvar Soesoo
Outlinesof the Precambrian basement of Estonia; 149–164
Alvar Soesoo, Väino Puura, Juho Kirs, ValterPetersell, Mati Niin, and Tarmo All
Abstract. Thecrystalline basement of Estonia can be subdivided intotwo major geological units – North Estonian amphibolite and South Estoniangranulite complexes. The amphibolite facies terrain consists of a sequence of metavolcanics and -sediments, which resemblethose of the island arc sequences in southern Finland. The SouthEstonian granulites, probably belonging to the Belarus–Baltic Granulitic Belt, show the peak metamorphism conditions of about800 °C and 5–6 kbar. U-Pb isotopic data demonstrate that the Estonianbasement has formed during the Palaeoproterozoic, 1.9–1.8 Ga agoand is composed of magmatic, volcanic, and sedimentary components across major structural zones. The granulite metamorphismpostdates these ages (1.79–1.73 Ga). The youngest igneous rocks in the basement belong to theFennoscandian Palaeo-Mesoproterozoic Rapakivi Province and include the composite Riga batholith, as well as at leastfive 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 ofEstonia as revealed from the integrated modelling of the crust; 165–189
Tarmo All, Väino Puura, and Rein Vaher
Abstract. Up-to-date gravity and magneticfield maps and deep seismic sounding data were used to study the orogenicstructure of the Palaeoproterozoic (1.9–1.8 Ga) Svecofennian basement of Estonia. The structure of the crust was quantitatively modelled fortwo NNE–SSW trending profiles across themain – south (southwest) and north (northeast) – structural terrains and thetectonic transition zone between them. Lateral variability of thebasement structure was revised using the available geological and geophysicalmaps and results of 3D modelling.
Metavolcanic andmetasedimentary rocks prevail in the orogenic basement of the Estonian mainland.The crust is overthickened, thus a remarkable gravity minimum is expected tooccurhere. In NNE Estonia,where the low-density (2680–2710 kg/m3) migmatized metamorphicrocks of the amphibolite facies prevail in the upper crust, the moderatelyoverthickened (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 extremelyoverthickened (50–65 km) crust is composed of the dense(2750–2840 kg/m3) upper crust and overthickened, partiallyupthrusted lower crust. Here the massdeficit is compensated, and in many cases even overcompensated, by the uppercrust composed of granulite facies rocks and blocks of the upthrustedlower crust. The Paldiski–Pskov zone is atransition between the two terrains, following deep crustal shear zones thatdip to the SSW. The survived orogenicstructure of the Estonian mainland formed due to the late SvecofennianSSW–NNE directed compression that resulted in crustal shortening andthickening, with the upthrusts along the Paldiski–Pskov zone and maximalcrustal thickness in the SSW.
Key words: magnetic andgravity modelling, upper crust, lower crust, deep seismic sounding, Estonia.
Svecofennianmetamorphic zones in the basement of Estonia; 190–209
Väino Puura, Rutt Hints,Hannu Huhma, Vello Klein, Mare Konsa, Reedik Kuldkepp, Irmeli Mänttäri, andAlvar Soesoo
Abstract. Svecofennian (Palaeoproterozoic) orogenic, folded metamorphicrocks dominate in the basement structure in Estonia. In northern Estonia(Tallinn zone), supracrustal rock associations (metavolcanic and -sedimentary sequences), their structure andamphibolite metamorphic grade resemblethe Svecofennian metamorphic island arc suites of southern Finland. Innortheastern Estonia, the metapelitic sequences of the Alutaguse zoneresemble those of the NE marginal metasedimentary basins of the Svecofennianorogen, as studied in the St. Petersburg District (NW Russia) and SEFinland. Local variations of the metamorphic grade, related to fault zones orlocal metamorphic domes (metamorphosed up to granulite assemblages) feature theamphibolite facies areas of northern Estonia. In southern Estonia, tectonicallyundefined, predominantly mafic metavolcanics are of granulite metamorphicgrade. The granulite region of southern Estonia and northern Latvia is muchlarger than known in the 1.9–1.8 Ga Svecofennian metamorphic zones ofsouthern Finland. The peak conditions ofgranulite metamorphism in Estonia at ca. 800 °C and 4–6 kbar resemble those of the Pielavesi granulites (Proterozoic), centralFinland. However, the U-Pb age of 1778 ± 2 Ma for monazite and the Sm-Nd age of 1728 ± 24 Ma for garnet from the sampleKõnnu 3005150 are clearly younger than anycomparable results from Finland, and suggest that the granulite faciesmetamorphism in southern Estonia is distinct from that recorded in southern andcentral 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 = wholerock, Cpx = clinopyroxene.
Anorogenic magmatic rocks in the Estoniancrystalline basement; 210–225
Juho Kirs, IlmariHaapala, and O. Tapani Rämö
Abstract. The anorogenic magmatic rock bodies of the Estonian crystalline basement belong to the FennoscandianPalaeo–Mesoproterozoic Rapakivi Province and include the huge composite Rigabatholith (250 km ´ 230 km in subsurface area, mostlyin NW Latvia), as well as at least five granite stocks (Naissaare, Märjamaa andits Kloostri satellite, Taebla, Neeme, and Ereda) and the quartz monzodioriticAbja stock in the Estonian mainland. The Riga batholith contains both mafic and silicic rocks, as several Fennoscandianrapakivi complexes do. The granite stocks appear on geophysical maps as gravityand magnetic minima, and they consist of pink, medium- to coarse-grained,microcline-megacrystic, partly trachytoid biotite (in Märjamaa and Naissaarealso with hornblende) granite, locally cut by aplitic and microsyeniticdykes. As an exception, the Märjamaa stock is more differentiated and has ananomalously high magnetic central part composed of hybrid granodiorite withhornblende as the main mafic mineral. The Abja quartz monzodiorite stock isstrongly magnetic and consists of a darkgrey, massive, medium-grained, partly weakly gneissose rock with abundant accessory apatite andtitanomagnetite, and is intersected by veins of fine- to medium-grained, slightlyporphyritic plagioclase-microcline granite. The major, REE and other traceelement contents of the graniticrocks are close to or overlap those of the typical Finnish rapakivis and arebest comparable with the less differentiated granitic phases. The Märjamaagranodiorite and the Abja quartz monzodiorite are enriched in Sr, Ti, and P.In Nd and Pb isotopic composition, the felsic and mafic rocks resemblecorresponding rocks of the Finnish rapakivi complexes, indicating anapproximately chondritic source for Nd in the mafic rocks of the Riga batholithand the Abja intrusion, and a Palaeoproterozoic (Svecofennian) source for the felsic rocks. The TDMmodel ages of the felsic rocks range from 1890 to 2100 Ma.
Key words: Proterozoic, Estonian basement, anorogenic magmatism,rapakivi, geochemistry, age.
Instructionsto authors; 226–229
CopyrightTransfer Agreement; 230
