Age and isotopic fingerprints of some plutonic rocks in the Wiborg rapakivi granite batholith with special reference to the dark wiborgite of the Ristisaari Island.

Abstract

Age and isotopic fingerprints of some plutonic rocks in the Wiborg rapakivi granite batholith with special reference to the dark wiborgite of the Ristisaari Island 1) Department of Geosciences and Geography, Division of Geology, P.O. Box 64, FI-00014 University of Helsinki, Finland 2) Rälssintie 12, FI-47200 Elimäki, Finland 3) Geological Survey of Finland, P.O. Box 96, FI-02150 Espoo, Finland 4) The National SGL Laboratory, c/o Geological Survey of Finland, P.O. Box 96, FI-02150 Espoo, Finland The mid-Proterozoic, locus classicus Wiborg rapakivi granite batholith of southeastern Finland and adjacent Russia comprises a varying, bimodal (silicic-basic) sequence of plutonic, subvolcanic, and volcanic rocks. At the current level of erosion silicic rocks are dominant, the most prominent of which are wiborgites and dark wiborgites (that have been considered to mark the main build-up stage of the batholith) and pyterlites. New observations and optical microscopy data from the dark wiborgite-dominated Ristisaari Island in the southern, off-shore part of the Wiborg batholith show that dark plagioclase megacrysts in dark wiborgite are calcic xenocrysts. They were probably incorporated into wiborgite magma from consanguineous massiftype anorthosite magmas in the course of the evolution of the bimodal magmatic system. Our new ID-TIMS U-Pb zircon age of the Ristisaari Island dark wiborgite, 1627±3 Ma, is the youngest isotopic age so far determined for the plutonic rocks of the Wiborg batholith. This, combined with preexisting U-Pb zircon data, implies a minimum duration of 12 m.y. (1642–1630 Ma) for the emplacement of the plutonic rocks of the batholith. Combined with data on highlevel dike rocks, a window of at least 20 m.y. (1642–1622 Ma) is implied. Furthermore, as the batholith grew, the overall locus of magmatism may have shifted southwards. New whole-rock Nd isotope data on the dark wiborgite of the Ristisaari Island and three further granites of the batholith, as well as Nd (whole-rock) and Sr (whole-rock, plagioclase) isotope data on a spectrolite massif-type anorthosite from the east-central part of the batholith, are also presented. These data suggest that the lithosphere across the Wiborg batholith area in the southeastern part of the Svecofennian orogen may vary slightly in overall mantle separation age.