New data on the age of ultrametamorphic granitoids of the Aldan granulite area (Eastern Siberia), consequences of metamorphic processes and possibilities of regional correlations of geological events

摘要

This work presents new U-Pb data (SHRIMP-II) for zircons from products of granitization and leucosomes of migmatites from amphibolite and granulite-facies zones developed on rocks of the tonalite-trondhjemite group of the unstratified basement and supracrustal formations of the western part of the Aldan granulite area. The age data obtained were interpreted using the data available on the U and Th geochemistry. The main geochemical trend of transition from primary zircons, crystallizing from the melt to the later metamorphic zircons is manifested in increasing U and Th concentrations in zircons. In this case, the Th/U ratio decreases, as do the values of the Ce anomaly and LuN/LaN ratio. By studying the sequence of autochthonous and paraautochthonous granite formation in the amphibolite-facies zone the ancient (3222-3226 Ma) metamorphic event in the Aldan Shield (a manifestation of the ultrametamorphic processes (granitization and migmatization), superimposed on rocks of an ancient infracomplex (3. 3-3. 4 Ga) and gneisses and schists of supracrustal formations) was established. The data obtained indicate the Middle Archean age of both metamorphosed rock complexes. The ancient period of evolutionary development of the Aldan shield was followed by development of diatectic granitoids with an age of 2450 Ma, which is correlated well with Proterozoic granitoids from the conjunction zone between the Aldan granulite area and Olekma granite-greenstone terrain. The study of similar granitoids from the granulite-facies zone allowed us to determine the age of the last granulite metamorphism as 2030-2100 Ma, which corresponds preliminary to the time of formation of the Fedorovka island arc. According to this, the conclusion has been made that high-temperature and high-gradient metamorphism is the suprasubduction process manifested in the continental margin back-arc. At the final stage of the Paleoproterozoic granite formation the complex of diatectic intrusive chambers (1960 Ma) formed. This resulted in granitoid magmatism, manifested in the central part of the Aldan granulite area simultaneously with the island arc-continental margin collision or at the post-collision stage. The diatectic granitoids contain inherited zircons with an age of over 2677 Ma, indicating the manifestation of a high-gradient metamorphic event in this time, as in adjacent territories.