Late Triassic foreland basin and Early Palaeozoic basement in the eastern Yidun Micro‐block and its tectonic implications for the eastern Palaeo‐Tethys

Abstract

Subduction polarity and collisional process are still controversial in the eastern Palaeo‐Tethys. On the basis of 1:50,000‐scale geological mapping at the Heni Township, zircon U–Pb geochronology and whole‐rock geochemistry were analysed for volcano‐sedimentary rocks in the eastern Yidun Micro‐block, Qinghai‐Tibet Plateau. Early Palaeozoic sequences were reported firstly in the eastern Yidun Micro‐block. Two populations of detrital zircon U–Pb ages (~525 Ma and 800–1,250 Ma) and deformation and metamorphic characteristics imply that the Early Palaeozoic sequences were deposited during the Cambrian to Silurian in the South China Block. The Early Palaeozoic sequences in the eastern Yidun Micro‐block were extruded by Cenozoic NW‐SE strike‐slip faults. Both the Late Triassic upper Tumugou and Lamaya formations yield four prominent peak ages of ~260 Ma, ~450 Ma, 750 ~ 780 Ma, and ~1,850 Ma. U–Pb ages of youngest detrital zircons (226.2 ± 1.0–224.7 ± 1.1 Ma), uppermost andestic tuff (216.1 ± 0.3 Ma) and intruded granitoids (~215 Ma) show that both the upper Tumugou and Lamaya formations were deposited during 225 ~ 216 Ma. Whole‐rock geochemistry indicates that all sandstone samples underwent moderate weathering and strong sorting and recycling. They were derived from felsic provenance, like volcano‐sedimentary rocks and granitoids in the western Yidun and Qiangtang micro‐blocks, indicative of pre‐Norian closure of the Jinshajiang Ocean. The closure gave rise to a foreland basin filled by the 225–216 Ma volcano‐sedimentary rocks in the eastern Yidun Micro‐block. The closure of the Jinshajiang Ocean also led to a scissor‐like closure of the Ganze‐Litang Ocean by clockwise rotation. Combining these with previous work, the Ganze‐Litang Ocean was a back‐arc basin resulted from eastward subduction of the Jinshajiang Ocean and had not undergone oceanic subduction.