Abstract

Upper Palaeozoic glaciomarine deposits combined with the presence of cold-water faunas and Glossopteris indicate a Gondwana provenance of the Baoshan and Tengchong Blocks in western Yunnan. The glacial origin of these Permo-Carboniferous deposits is verified by a three-fold subdivision of the basal sequence comprising, in ascending order: diamictites with faceted and striated clasts, laminites with dropstones, and a black anaerobic de-glaciation facies. The succeeding carbonates of the upper Permian and Lower Triassic originated during a post-glacial phase of a much warmer climate. Although the two blocks are adjacent to each other their Upper Palaeozoic sequences show significant differences. The glacigene successions of the Baoshan Block are comparatively thin and are overlain by thick basalts and red beds. On the Tengchong Block the glacigene marine deposits exceed 1000 m and are followed by thick reefal limestones of Lower Permian age. An active volcanic rift setting is suggested for the Baoshan Block and a proximal passive margin environment at a somewhat lower latitude for the Tengchong Block. The succession of the latter is comparable with Permo-Carboniferous sections in NW Australia, especially that of the Bonaparte Gulf basin, and it is assigned to the Sibumasu tectono-stratigraphic unit. The Baoshan sequence shows similarities to sequences of Tibet and NE India and is therefore assigned to the Tibetan realm. Both terranes separated from Gondwana in the late Early Permian. Docking commenced in the Late Triassic concomitant with the closure of the Changning-Menglian Belt. Lateral displacements in the course of the Himalayan orogeny moved the Tengchong Block north, bringing it into juxtaposition with the western margin of the Baoshan Block. This tectonic contact is now the Nujiang Line.