GEOCHEMISTRY OF GRANITOIDS OF THE SVECOFENNIAN TECTONOMETAMORPHIC CYCLE IN THE NORTHERN LADOGA REGION

Abstract

Major- and trace-element chemistry is considered for the Kurkijoki pluton and its satellites, as well as diorites and tonalites of the Lauvatsaari pluton and two-feldspar granites of the Tervu pluton, which are related to the initial, intermediate, and final stages of the Early Proterozoic metamorphic cycle with corresponding ages of 1881 ± 6.7, 1878 ± 3.3, and 1858 ± 2.5 Ma. REE and some other trace element contents are determined in 28 rock samples using neutron activation and ICP-MS. In parallel to Si and K increase from enderbites to late granites, LREE, Hf, Ta, Th, U, Cs, Rb contents increase and mafic component, Sr, Eu, Sc, Co, and Cr contents decrease. Each pluton has individual specific trends in the major element-SiO2 and trace element-SiO2 diagrams, which verifies affiliation of the plutons to different magmatic rock series. There is a correlation between REE, Hf, Ta, Th, U, Sc, and Eu concentrations and temperatures of equilibrium distribution of oxygen isotopes between minerals (do not correspond to magma crystallization temperatures), some other parameters of oxygen isotopic systematics, and H/C ratios in fluid. This fact suggests the migration of REEs and some other trace elements with water-rich fluid after magma crystallization. Covariations between major and trace elements show that the Tervu rocks belong to collision granitoids, whereas the Kurkijoki and Lauvatsaari rocks, to the post-collision and island arc granitoids. This conclusion is consistent with structural-geological data. Sm-Nd data for the rocks of the plutons indicate that the magmas of the earlier plutons were separated from depleted mantle and contaminated by crustal material. The Tervu granites show evidence for the strongest crustal contamination.