Stages of the Lower Crust Formation of the Belomorian Mobile Belt, Kola Peninsula

摘要

According to seismic data, the lower part of the crust (basaltic or granulite-basite layer) of the eastern part of the Baltic shield has a thickness of 10-12 km increasing up to 20 km under the Belomorian subprov-ince. The thickness of the lower part in this region increases with increase in the crust thickness. The share of the upper and middle parts of the total crust thickness decreases, which suggests that the increase of the lower part occurs at the expense of the floating mantle asthenoliths and the formation of the "basalt" pillow on the crust-mantle border [1]. In the northwestern part of the Belomorian mobile belt about 1000 explosive dikes and more than 40 diatremes of Paleozoic age (Fig. 1) are located. These objects contain xenoliths of the lower crust, garnet granulites (T~ 750℃, P = 12-15 kbar, Grt + Cpx + Pl ± Opx ± Qtz ± Kfs ± Phl + Hbl), and in small measure contain bipyroxene and garnet pyroxenites (Cpx + Pl ± Grt ± Opx ± Hbl + Qtz). The stages of the lower crust formation were determined during the investigation of the U-Pb age of zircons separated from four samples of the garnet granulites. These samples correspond by composition to oversaturated tholeites (two samples) and olivine tholeites (two samples). The concentrations of REE, U, Th, and isotope relations in zircons were determined with the help of the secondary-ionic microprobe SHRIMP-II according to the method described in [2]. Twenty-four measurements of the zircon U-Pb age and 13 REE concentrations were conducted (Tables 1, 2). Along with these data, we used 26 zircon ages from three samples of the regional lower crust known from the literature [3, 4].