Neoproterozoic (∼ 800 Ma) orogeny in the Tuva-Mongolia Massif (Siberia):: island arc-continent collision at the northeast Rodinia margin

The Tuva-Mongolia Massif is a composite Precambrian terrane incorporated into the Palaeozoic Sayany-Baikalian belt, Its Neoproterozoic amalgamation history involves early (similar to 800 Ma) and late Baikalian (600-550 Ma) orogenic phases. Two palaeogeographic elements are identified in the early Baikalian stage - the Gargan microcontinent and the Dunzhugur oceanic are. They are represented by the Gargan Glyba (Block) and the island-are ophiolites overthrusting it. The Gargan Glyba is a two-layer platform comprising an Early Precambrian crystalline basement and a Neoproterozoic passive-margin sedimentary cover. The upper part comprises olistostromes deposited in a foreland basin during the early Baikalian orogeny. The Dunzhugur are ophiolite form klippen fringing the Gargan Glyba, and show a comprehensive oceanic-are ophiolite succession. The Dunzhugur are faced the microcontinent, as shown by the occurrence of forearc complexes. The are-continent collision followed a pattern similar to Phanerozoic collisions. When the marginal basin lithosphere had been completely subducted, the microcontinental edge partially underthrust the are, and the forearc ophiolite overrode it. Continued convergence caused a break of the are lithosphere resulting ill the uplift of the submerged microcontinental margin with the overthrust forearc ophiolites sliding into the foreland basin. Owing to the lithospheric break, a new subduction zone, inclined beneath the Gargan microcontinent, emerged. Initial melts of the newly-formed continental are are represented by tonalites intruded into the Gargan microcontinent basement and its cover, and into the ophiolite nappe. The tonalite Rb-Sr mineral isochron age is 812 +/- 18 Ma, which is similar to a U-Pb zircon age of 785 +/- 11 Ma. A period of tonalite magmatism in Meso-Cenozoic orogenic belts is recognized some 1 - 10 m.y. after the collision. Accordingly, the Dunzhugur island arc-Gargan microcontinent collision is conventionally dated at around 800 Ma. It is highly probable that in the early Neoproterozoic, the Gargan continental block was part of the southern lin modern coordinates) margin of the Siberia craton. It is suggested that a chain of Precambrian massifs represents an elongate block separated from Siberia in the late Neoproterozoic. The Tuva-Mongolia Massif is situated in the northwest part of this chain. These events occurred on the NE Neoproterozoic margin of Rodinia, facing the World Ocean. (C) 2001 Elsevier Science B.V. All rights reserved.