Two High-Pressure Metamorphic Events in Early Precambrian Eclogites of the Gridino Area, Belomorian Province of the Fennoscandian Shield: Petrology and Geochronology

摘要

Early Precambrian retrogressed eclogites are abundant in the Belomorian Province of the Fennoscandian Shield. This study reports unique features of retrogressed eclogites in the Samylino study area. The eclogites preserve a massive texture and the assemblage of Omp + Grt + Rt + Qz. The authors studied zoned crystals of garnet and omphacite with inclusions of diopside-plagioclase symplectites. The garnet crystals consist of three growth zones (central, intermediate, and marginal) and bear microinclusions of individual mineral grains and symplectites. These zones host three mineral assemblages of metamorphic minerals successively formed at different P-T conditions. The early assemblage includes Di-Pl symplectites, hornblende, and quartz, which were formed at T = 700-760 degrees C and P = 12-14.5 kbar, and belongs to the first retrograde stage of the early eclogites. The second assemblage occurs in the intermediate zone of the zoned garnet crystals. This assemblage consists of Omp (Jd 32-36%) inclusions and Grt, makes up the intermediate zone, and was in equilibrium with Omp. These Omp and Grt crystallized at T = 710-820 degrees C and P = 14-16 kbar during later eclogite metamorphism. The third assemblage comprises garnet of the marginal zone with inclusions of Cpx, Pl, and Hbl. These minerals were formed at T = 650-730 degrees C and P = 8.5-12 kbar, which are consistent with parameters of the high-pressure amphibolite facies of the latest retrograde metamorphism. The authors have distinguished four different age groups of zircons from the earlier and later eclogites based on U-Pb (SHRIMP II) dating: about 2.8, 2.7, 2.46, and 1.93 Ga. The early zircons form the cores of zircon grains, and many of them are interpreted as igneous, which crystallized in the Mesoarchean mafic protolith. All other groups of the zircons are of metamorphic origin, indicating polymetamorphic transformations of the early eclogites. The Neoarchean zircons contain microinclusions of omphacite and zoisite. However, we cannot reliably confirm the Neoarchean eclogite-facies metamorphism. Zircons dated at 1.93 Ga contain inclusions of omphacite, garnet, and zoisite. These grains were formed during the later eclogite-facies metamorphism. Thus, the Early Precambrian Belomorian eclogites are polymetamorphic rocks that bear two eclogite-facies mineral assemblages (according to petrological data). Structural and geochronological data also point to a polymetamorphic origin of the retrogression of the eclogites. Our geochronological data make it possible to date the second high-pressure (eclogite-facies) event as Paleoproterozoic (1.93 Ga) and the first one as presumably Neoarchean (ca 2.7 Ga).