Transactions of the Karelian Research Centre of the Russian Academy of Sciences (Jun 2015)
ARCHEAN ZOISITITES OF THE GRIDINO ECLOGITEBEARING MELANGE, BELOMORIAN PROVINCE OF THE FENNOSCANDIAN SHIELD: GEOLOGY, UPB ZIRCON AGES AND GEODYNAMIC SETTING
Abstract
The new results of the isotope UPb dating of zircons from zoisitites, which are the isofacies accessories of the eclogites in the Gridino eclogitebearing melange of the Belomorian mobile belt, Fennoscandian Shield, are reported. They can be used to modify the geodynamic evolution model of the Archean Belomorian subductioncollision system. Isotope and geochemical analyses of zircons were performed using the laser ablation method at the Beijing University laboratory, China. The isotope ages obtained vary from 3.0 to 1.9 Ga, but six groups/generations with the ages 3.0, 2.9, 2.82, 2.75, 2.72 and 1.96 Ga have been revealed. Morphological and geochemical data on zircons and their mineral inclusions were analyzed to assess the conditions of formation of each of the six groups and show that the early zircons, dated at 3000, 2900 and 2820 Ma, were trapped by gabbro anorthosites from granitoid enclosing rock upon intrusion; zircons dated at 2745 Ma seem to be associated with the formation of gabbroanorthosites, dated at 2720 Ma, and are consistent with the time of formation of an early zoisite association under eclogitefacies conditions, dated at 2680 Ma, which is the time of retrograde alterations at high pressures (10–14 Kbar). Zircon rims, dated at 1960 Ma and indicating the LaplandKola orogeny, have also been revealed. To interpret the data obtained, it should be admitted that the Gridino eclogitebearing melange began to form at 2720 Ma upon the transition from classical subduction to continental subduction, and at 2680 Ma its structure became more complex upon continental collision and eclogitefacies metamorphic events took place.
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