Radiogenic isotopes of the Estonian and Latvian rapakivi granite suites: new data from the concealed Precambrian of the East European Craton

Abstract

The Precambrian crystalline bedrock of the Baltic countries is covered under Phanerozoic sedimentary rocks that flank the Fennoscandian Shield in the south. The covered bedrock consists mainly of Palaeoproterozoic medium- to high-grade metamorphic rocks and unmetamorphosed rapakivi granites and related mafic rocks (mainly gabbros and anorthosites). Our UPb zircon data show that small rapakivi granite plutons in Estonia are 1630 Ma old and felsic and mafic rocks from the Riga batholith of Latvia and westernmost Estonia are 1580 Ma old. The Estonian plutons have ϵ Nd (1630 Ma) values ranging from −0.5 to −2.5. The mafic and felsic rocks of the Riga batholith have ϵ Nd (1580 Ma) values between +0.3 and −0.6, except for a pervasively altered silicic volcanic rock on the northern flank of the batholith with an ϵ Nd value of −4.6. Initial 87 Sr 86Sr ratios of the mafic rocks are of the order of 0.7036 to 0.7037 and conform to the evolution of average subcontinental mantle. The Pb isotopic compositions of the felsic and mafic rocks (including the low- ϵ Nd prophyry) are relatively radiogenic with single-stage μ-values of the order of 8.2. The isotopic characteristics of the Estonian and Latvian rapakivi granites are similar to those of the classic rapakivi granites of southern Finland. Our data suggest that the felsic rocks of the Estonian and Latvian rapakivi suites were derived from Palaeoproterozoic (near-chondritic Nd, relatively high-U Pb ) protoliths. The data also imply that the lower crust and upper mantle in this area are devoid of a major Archaean component and that the lithosphere may become more juvenile southward from the Fennoscandian Shield.