Isotopic composition of pyterlite in Vyborg (Viipuri), Wiborg batholith, Russia

Abstract

Active worldwide research on rapakivi granites and related mafic and intermediate rocks has shown that this lithologic assemblage is an important tool in the study of the composition of deep continental crust and subcontinental mantle (see Haapala & Ramo 1999 and references therein). In any one area, the radiogenic isotope composition of the rapakivi granites reflects the overall age of the unexposed lithosphere that they occupy (e.g., Ramo 1991, Neymark et al. 1994, Ramo et al. 1995, Andersson 1997, Dall’Agnol et al. 1999). This has recently been particularly well established for the ~1.5 Ga rapakivi granites of central Sweden that show a substantial Archean source component although no Archean crust has been found to be exposed in that area (Andersson 1997). In the first part of the 1990’s, the rapakivi batholiths of Russian Karelia were studied for Nd and Pb (as well as Sr) isotopes that implied a mixed Archean-Paleoproterozoic source, in contrast to the clearly younger source characteristics of the rapakivi granites farther to the west in southern Finland (Ramo 1991, Neymark et al. 1994). The latter turned out to be quite homogeneous in terms of Nd and Pb isotopes with a clear Paleoproterozoic signature for all plutons extending from the Aland Islands in the west to the Finnish-Russian border in the east (Ramo 1991). The Russian Karelian rapakivi batholiths (Salmi and Ulyalegi; Fig. 1a) are situated at the contact zone of the Archean and Paleoproterozoic crustal domains of the Fennoscandian Shield and their mixed (Archean-Proterozoic) isotopic signature is thus quite understandable. Recent studies on the Mesoproterozoic (Amantov et al. 1996, Ramo et al. 2001a) Valamo dolerite of the Jotnian Lake Ladoga basin have suggested that a Neoarchean subcontinental lithosphere (with eNd [at 1.46 Ga] of ~ –9 and initial 87Sr/86Sr of ~0.705) probably underlies the north-central part of Lake Ladoga (Upton et al. 1998, Ramo et al. 2001a; Fig. 1). This suggests that the Archean-Proterozoic boundary may continue at depth farther to the southwest than the present exposed boundary northwest of Lake Ladoga (Fig. 1a). In order to examine whether the easternmost part of the Wiborg batholith in Russian Karelia (southwest of Lake Ladoga) records a major Archean source