Palaeomagnetic study of Vendian and Early Cambrian rocks of South Siberia and Central Mongolia: was the Siberian platform assembled at this time?

A palaeomagnetic study of Vendian and Early Cambrian sediments from the Ang ara block of the Siberian platform: Shaman (52.08 degreesN, 108.83 degreesE ) and Minya (58.0 degreesN, 110.0 degreesE) Formations, and the Tuva-Mongol ian block: Tsagan-Olom and Bayan-Gol Formations (46.76 degreesN, 96.37 degr eesE) isolated three different components of magnetization through thermal demagnetization. The stable high-temperature characteristic remanence direc tions show both normal and reverse polarities. The mean palaeopoles compute d after these high-temperature components are: 320 degreesS/71.1 degreesE ( dp/dm = 6.9 degrees /13.8 degrees) for the Vendian Shaman Formation (10 sit es, 80 samples), 33.7 degreesS/37.2 degreesE (dp/dm = 8.6 degrees /14.7 deg rees) for the Vendian Minya Formation (12 samples), 22.8 degreesS/28.4 degr eesE (dp/dm = 10.8 degrees /21.6 degrees) for the Vendian Tsagan-Olom Forma tion (4 sites, 25 samples) and 21.4 degreesS/167.1 degreesE (dp/dm=9.6 degr ees /19.1 degrees) for the Early Cambrian Bayan-Gol Formation (6 sites, 49 samples). From a compilation of Vendian and Early Cambrian palaeopoles from the Anabar, Angara and Aldan blocks of the Siberian platform and Tuva-Mong olia block, we propose a model where these blocks were situated in an equat orial to low south palaeolatitude position, with their present-day southern boundaries facing the north pole. From the analysis of the scatter of thes e poles, we conclude that the Siberian platform might not have fully amalga mated by this time, and that significant rotations occurred after the Early Cambrian. Our new palaeopoles for the Tuva-Mongolia block, together with p reviously published ones, show that this block was already adjacent to Sibe ria by the Vendian and Early Cambrian. We propose that the large counterclo ckwise rotation of the Tuva-Mongolia block with respect to Angara block cou ld mark the end of the closure of the part of the Palaeo-Asian ocean separa ting these two blocks, and could account for the occurrence of Vendian-Earl y Cambrian ophiolites in the region. (C) 2001 Elsevier Science B.V. All rig hts reserved.