Major and trace element and lead isotope composition of rapakivi feldspar: New data from the locus classicus Wiborg batholith, Finland

Ovoid-shaped alkali feldspar (K-feldspar) megacrysts are a typical feature of the mid-Proterozoic rapakivi granites of Finland. In general, these megacrysts range in diameter from 2 to 5 cm, are often tightly packed, and are mantled by a rim of sodic plagioclase. We report preliminary elemental (EPMA, LA-QICP/MS) and Pb isotope (LA-MC-ICP/MS) data from a mantled K-feldspar ovoid (2.5 cm in diameter), recovered from a rapakivi granite (the Summa wiborgite) in the central part of the Wiborg batholith. The K-feldspar that constitutes the bulk of the ovoid is microperthitic and fresh and contains inclusions of sodic plagioclase subhedra (0.5-1.0 mm), amphibole, dark mica, zircon, and oxide. In addition, quartz is found as anhedral, wormy patches in the K-feldspar. The plagioclase rim of the ovoid is 2-3 mm thick, continuous, and in places polysynthetically twinned. The ovoid K-feldspar (Or80-90, Ab10-20) shows a diminishing celcian component from the center (1.2) of the ovoid to the margin (0.6), whereas the plagioclase (Ab70-75, An20-30) is rather homogeneous with nil compositional difference between the subhedra and the rim. From the core to the rim, the ovoid K-feldspar shows increasing Rb/Sr (2.0 to 2.8) and declining Zr (1 to 0.1 ppm). Relative to the oligoclase inclusions, the oligoclase rim of the ovoid is higher in Sr (200-330 ppm vs. 150-230 ppm) and Zn (20-35 ppm vs. 10-25 ppm). In situ Pb isotope composition of the ovoid K-feldspar (40-80 ppm total Pb) and the two textural types of oligoclase (20-30 ppm Pb) were measured along 400- by 200-micro-m-long lines. The target feldspar was ablated for 2 min and the mass bias correction was performed using a 10 ppb Tl solution. This yielded an overall precision (at 2SE) of ± 0.15% for the K-feldspar Pb isotope ratios normalized to 204Pb and ± 0.4% for those of oligoclase. The accuracy was estimated by ablating the NIST612 glass using similar parameters. From the core to the rim of the ovoid, the initial (at 1630 Ma) Pb isotope composition of the ovoid K-feldspar varies beyond the experimental error but shows no consistent trends. The S&K mu-2 values of the ovoid K-feldspar average at 9.87 ± 0.09 (1SD, n=11), those of the two textural types of oligoclase at 9.8 ± 0.2 (1SD, n=13). From the interior to the outer margin of the oligoclase rim, there appears to be a shift to less radiogenic uranogenic initial Pb isotope compositions: 207Pb/206Pbi changes from 0.9341 ± 0.0031 to 0.9475 ± 0.0006 and 208Pb/206Pbi from 0.9341 ± 0.0031 to 0.9475 ± 0.0006 (errors in 2SE). Our data suggest that (1) the examined, relatively small rapakivi ovoid may have been crystallized from a single batch of magma and reflects the compositional evolution of the magma from which it grew; (2) sodic plagioclase co-precipitated with the ovoid-forming K-feldspar; (3) the crystallization of the oligoclase rim around the alkali feldspar ovoid may have been associated with a change in the overall Th/U of the magma matrix. The Pb isotope composition of the oligoclase rim may mark invasion of a high-Th/U (mantle-derived?) magma into the central part of the Wiborg batholith magma chamber, or may reflect prolonged fractionation of U-enriched components (e.g., zircon).