Neoarchean Ba–Sr Metadacites from the Karelian Province of the Fennoscandian Shield: U–Pb Geochronology, Composition, Sm–Nd Systematics. Are They Effusive Analogs of Sanukitoids?

New U–Pb (CA-ID-TIMS) zircon age data were obtained for metadacite from the Mezhezero Formation of the Megriyarvy structure of the Gimoly–Sukkozero belt of the Central Karelian domain (Karelian Province, Fennoscandian Shield). Metadacites are fine-grained rocks with porphyritic plagioclase phenocrysts in the epidote-muscovite-biotite-quartz-plagioclase matrix. In terms of chemical composition, dacites are characterized by the enrichment in Ba (900–1300 ppm), Sr (average of 650 ppm), LREE (average (La/Yb)n = 20; La = 38 ppm), as well as by well-defined negative Nb and Ti anomalies on the spider diagram. The concordant U–Pb age of zircon from metadacites is 2735 ± 6 Ma (MSWD = 0.061). The whole-rock εNd(t) values are +0.5 and +0.8 and the tNd(DM) values are 2.83–2.85 Ga. Neoarchean metaandesites and metadacites, similar in composition, structural features, age, and Nd isotope composition, are also present in other greenstone belts of both the Central Karelian domain and beyond. Metadacites are similar in age and geochemistry to the granitoid varieties of the sanukitoid series of the Karelian Province but differ from primitive sanukitoids in lower contents of MgO, Cr, Ni, alkalis, REE (especially LREE), Ba, and Sr. They correspond to sanukitoids of the normally alkaline group, i.e., rocks of an intermediate composition between the rocks of the tonalite-trondhjemite-granodiorite association and subalkaline sanukitoids, and, possibly, are their effusive analogs. In the Gimoly–Sukkozero belt, the polymictic conglomerates of the Sukkozero Formation (plagioclase porphyry pebbles, matrix, interbeds of biotite-quartz-plagioclase schists) are similar to metadacites of the Mezhezero Formation in the composition and the U–Pb zircon age of about 2.76 Ga. This may indicate subsynchronous volcanism and sedimentation in this structure in the early Neoarchean.