Paleomagnetism of the Precambrian Eastern Sayan rocks: Implications for the Ediacaran–Early Cambrian paleogeography of the Tuva-Mongolian composite terrane

摘要

The Tuva-Mongolian Precambrian composite terrane is located within the complex Central Asian fold belt,which separates the Siberian portion of the Eurasian continent from other incorporated continental blocks.This terrane is one of the key elements that must be accurately modeled to reconstruct the Neoproterozoic–Paleozoic tectonic history of Eurasia. The reconstruction of the terrane's paleoposition after breaking up of supercontinent Rodinia relatively to other continental blocks has fundamental importance, as it is one of the missing blocks in most present day reconstructions.We present a paleomagnetic study of rocks from several formations of the Eastern Sayan region south of Siberian platform (representative location λ=52.0°N, =100.5°E), within the Tuva-Mongolian composite terrane. Sections of siltstone, fine grained sandstone and associated sillswere sampled at several localities in this complex region. These units have been dated as Precambrian to Early Cambrian in age. The resulting collection was taken from three formations, representing a total of 33 sites, collected from this previously unsampled and remote region. Generally 6 to 11 samples per site were collected. Stepwise thermal demagnetization was completed using between 10 and 18 heating steps to up to temperatures of 680 °C.Principal component analysis of the stepwise thermal demagnetization data was successful in isolating two characteristic remanentmagnetizations. The lower unblocking temperature component, component A, fails the fold test, is always of downward directed magnetic inclination, and may correspond to the present day Earth's magnetic field. The higher unblocking temperature magnetic component (B), was observed in the Dunzhugur Formation (BDF, N=8 sites) and the Bokson Formation (BBF, N=11 sites from5 localities). The B component differs significantly from component A, and is recorded by sites of downward and upward directedmagnetic inclinations in the Bokson Formation. Component BDF ismost likely a result of the regional remagnetization of the sediments and sills duringmultiple tectonic events in the area. Component BBF yields a positive fold test and one site has reversed polarity direction. A virtual geomagnetic pole calculated from component BBF, after rotating along a small circle, is coincident with other Ediacaran to Early Cambrian aged poles reported from nearby Mongolia and Siberia. This observation supports the earlier interpretation of Kravchinsky et al. (2001) postulating an adjacent position of the Tuva-Mongolian composite terrane and the Siberian continent in Ediacaran–Early Cambrian times.