Late Riphean Rifting and Breakup of Laurasia: Data on Geochronological Studies of Ultramafic Alkaline Complexes in the Southern Framing of the Siberian Craton

摘要

The origin of the Paleo-Asian Ocean and consequent formation of the Central Asian foldbelt (CAFB), which represents the largest structure of Asia and borders the Siberian Craton in the south and southwest, is one of the key problems in the geology of Asia. The CAFB is composed of Caledonian, Hercynian, and Indo-Sinian folded structures, suggesting that the pale-ocean began to form not later than 570 Ma ago (the age of the oldest ophiolites in the CAFB Caledonides). The origin of the paleocean, i.e., its oldest age, is closely related to the breakup of the supercontinent Rodinia. The southern margin (in modern coordinates) of Siberia, a constituent of this supercontinent, adjoined Laurentia and formed the giant Laurasian continental block together with Baltia and Greenland. Previously, we attempted to estimate the age of the Laurasia breakup and the time at which oceanic basins began to surround the Siberian continent. This estimate was based, first of all, on the age of rifting between Siberia and the North Canadian part of Laurentia. Processes of rifting are best reflected in structures of the giant Franklin dike belt that formed 725-615 Ma ago with the maximal activity between 723 and 718 Ma ago. In the Siberian Craton, the breakup is reflected in numerous manifestations of Late Riphean intraplate activity (grabens, dike belts, carbonatite-bearing ultramafic alkaline massifs, and others). The age of these events, usually based on the K-Ar method, has appeared to be rather ambiguous because of the substantial variations in obtained dates. We carried out more reliable U-Pb geochronological measurements in carbonatite-bearing alkaline massifs distributed along the southern margin of the Siberian Craton. The data obtained make it possible to specify the age and geological consequences of the rifting that resulted in the breakup of Laurasia and formation of the Paleo-Asian ocean.