PB-ND-SR ISOTOPIC AND GEOCHEMICAL CONSTRAINTS ON THE ORIGIN OF THE 1.54-1.56 GA SALMI RAPAKIVI GRANITE-ANORTHOSITE BATHOLITH (KARELIA, RUSSIA)

Abstract

The Salmi batholith is situated on the eastern edge of the EW-trending anorthositerapakivi granite belt of the Fennoscandian shield, at the contact between Proterozoic and Archean crustal domains. The tectonic setting and high K, Rb, Nb, Y, Zr, REE (except Eu), F, Sn, Be, and Li contents of.the Salmi batholith indicate that it represents typical subalkaline A-type and within plate granites. Gabbro-anorthosites of the batholith demonstrate a concordant U-Pb apatite age of 1563 ± 9 (2σ) Ma and a Sm-Nd internal isochron age of 1552 ± 69 Ma. Zircons from amphibole-biotite granites have an upper concordia intercept U-Pb age of 1543 ± 8 Ma. An older inherited zircon component with elevated Th/U ratio is found in zircons separated from K-feldspar ovoids. Rb-Sr internal errorchron for the granites yields an age of 1455 ± 17 Ma, probably the time of completion of postmagmatic processes within the batholith. The gabbro-anorthosites and granites show similar initial Nd, Sr, and feldspar Pb isotope compositions (ɛ Nd = - 6.5 to - 8.2;μ 2 = 8.6 to 8.9;κ 2 = 3.9 to 4.0; ISr = 0.7052 to 0.7057 for the basic rocks, andɛ Nd = -6.2 to -8.9;μ 2 = 8.1 to 9.2;κ 2 = 4.0 to 4.4; ISr = 0.7050 to 0.7072 for the granites). Two-stage neodymium TDM model ages for both assemblages range from 2.60 to 2.80 Ga. Old LREE-enriched sources with low time-integrated U/Pb and Rb/Sr and elevated Th/U ratios were involved in the formation of both the gabbroanorthosites and the granites. Bulk contamination with crustal materials cannot explain the data for the basic rocks. Selective incorporation of Pb, Sr, and Nd from Archean lower crust is needed, or else, the gabbro-anorthosites may have been derived from an isotopically anomalous subcontinental mantle source. The ascent of a mantle diapir resulted in anatexis of the lower crust and formation of the parent magma for the rapakivi granites.