Late Archean high-Mg granitoids (sanukitoids) in the southern Karelian domain, eastern Finland: Pb and Nd isotopic constraints on crust−mantle interactions

Late Archean (ca. 2.73 Ga) porphyritic high-K and high-Mg granitoids in the Nilsiä and Lieksa areas, eastern Finland, have geochemical signatures similar to those of high-Mg sanukitoid series from other parts of the Karelian domain of the Fennoscandian (Baltic) Shield and the Archean sanukitoid suites from Canada. The Nilsiä and Lieksa granitoids have low SiO2 contents (62.7−67.0) and high Mg numbers (0.45−0.52). They are enriched in K2O (2.40−4.73 wt.%), P2O5 (0.25−0.42 wt.%), Sr (610−850 ppm), Ba (1200−2300 ppm), Cr (40−80 ppm) and LREE [(La/Yb)N=19–65]. They are characterized by a negative Eu anomaly (Eu/Eu* 0.53−0.81) and show strong depletion in U, high Th/U ratio, low U/Pb ratio, high μ2 values for the source (>9), positive ɛNd (2.7 Ga) values of 0.3−1.4 and Nd depleted mantle model ages 2.75−2.86 Ga. The high μ2 values pointing to a significantly older crustal source apparently contradicts the positive ɛNd (2.7 Ga) values supporting a mantle origin. The geochemical and isotopic data suggest that the Nilsiä and Lieksa granitoids originated at ca. 2.73 Ga from a mantle-wedge source enriched in LILE, U, Th and Pb by recycling of continental material in subduction-related slab dehydration processes shortly (up to 200 Ma) before melting. A considerably large amount of crustal lead was contributed from subducting sediments into the overlying mantle wedge. Consequently, the isotopic composition of mantle-wedge lead was overprinted by crustal lead isotope signatures. It is suggested that crustal recycling through subduction zone processes played an important role for the enrichment of the mantle wedge and generation of mantle-derived high-μ granitoids in the Archean.