Geochronology of the Aldan Shield, southeastern Siberia

Crystalline Precambrian rocks exposed in the basins of the Olekma and Aldan Rivers represent the southeastern part of the Siberian Platform. To the north and east the basement is covered by Paleozoic and Upper Proterozoic formations. To the south and west lie younger deformed belts.The Aldan Shield can be divided into four regions. The Tcharskaja massif, in the west, is the most ancient block. Pb–U dates on allanite and zircon show a good concordance at 2950 m.y. The prevalent rock type is gneiss and migmatite.In the Aldan River basin three thick series of Lower Proterozoic rocks were deposited during the time interval 2000 to 2700 m.y. Granulite facies metamorphism of these rocks occurred 1800 to 2100 m.y. ago. The time of deposition of the lowest formations (Iengra Series) is given approximately by measurement of the isotopic composition of lead in different syngenetic mineral phases. Using muscovite and magnetite, the 207Pb/204Pb:206Pb/204Pb isochron yields a value of 2550 ± 300 m.y.In the east the crystalline basement of the Aldan complex is covered by thick Proterozoic sedimentary formations. These formations are intruded by alkaline rocks, dated at 1100 m.y. and small ring structures with associated carbonatites, dated at 600 to 650 m.y. The magmatic cycles, combined with glauconite age determinations, indicate a nearly complete, Middle and Upper Proterozoic stratigraphic section. The base of the Middle Proterozoic is established at 1900 ± 50 m.y.The Aldan complex, and the Stanovoi complex to the south are separated by a wide belt of tectonic fault zones. The prevailing metamorphic grade of the Stanovoi complex is amphibolite facies, with only local areas of granulite facies rocks. Pb–U ages from monazites, zircons, and allanites from pegmatites cutting the complex date the amphibolite facies metamorphism at 1900 m.y. Available data suggest an Archean age for the granulite facies rocks. The Stanovoi complex has been affected by at least two subsequent magmatic periods; the first were syenitic intrusions at 1100 m.y., and the second Jurassic granites at 150 m.y. The last magmatic cycle, found in many parts of the Aldan Shield, is an excellent demonstration of the recurrent rejuvenation of Precambrian terrains.