MEZOARCHEAN METAPSEUDOTACHYLITE AS EVIDENCES OF SEISMIC DEFORMATION OF THE EARLY PRECAMBRIAN CONTINENTAL CRUST (BELOMORIAN ECLOGITE PROVINCE)

We studied metapseudotachylite acid veins in central domain of Izbnaya Luda island (Mezo- Neorchean Belomorian eclogite province), VGS-84: N 65º54’, E 34º45’. It is underlain by felsic gneiss, in which occur bodies of eclogite, amphibolite, garnet-clinopyroxene amphibolite, metaperidotite, metapyroxenite, and microcline granite. The area studied may be divided into western, central and eastern domains. Central domain rocks have a prevailing northwesterly and westerly foliation trends. Gneiss foliation is cut by minor faults (several centimeters to several meters displacement) of main northernly and secondary westerly directions. Movement along faults produced tectonic breccia. The layered quartzofeldspathic gneiss contains fragments, 10 cm to 5 m in size, of garnet (± clinopyroxene) amphibolite, eclogite, tonalite and pink K-rich granite. The granite and tonalite fragments are angular or are elongated parallel to the foliation. A fault networks are irregularly distributed in area. Locally, early fold relicts are preserved within the network of faults. Tectonic breccia contains thin (from few millimeters to few centimeters) metapseudotachylite veins are presented fine-grained aplite-like rocks and they cross-cut all rocks of breccia. Relatively large-scale aplite veins contain small fragments of plagiogneiss and K-rich granite. Sometimes thin aplite veins form a thick net that envelops of gneiss and granite fragments.

Metapseudotachylite represents garnet – biotite – plagioclase - potassium feldspar – quartz ± clinopyroxene ± hornblende metamorphized rock. Metapseudotachylite composition (SiO₂ 68.7-70.4, TiO₂ 0.4, Al₂O₃ 14.9-15.4, Fe₂O₃ 3.5-3.8, MnO 0-0.1, MgO 1.2-1.4, CaO 3.0-3.4, Na₂O 3.7-3.8, K₂O 1.9, P₂O₅ 0.1-0.2) corresponds to average composition of the rocks of tectonic breccia. Eight samples of metapseudotachylite were dated by bulk rock Sm/Nd method and yielded T_DM model ages from 2787 to 3148 Ma. Thus metapseudotachylite veins are an important evidence of a high-velocity seismic deformation of the Mezoarchean continental crust. Errochrone dependence gave age about 1.53 Ga, which obviously answer alteration of Sm/Nd isotopic system at the amphibolite-grade overprinting in the Svecofennian Orogeny.