Paleotethyan tectonic evolution of the Sanjiang Orogenic Belt, SW China: Temporal and spatial distribution pattern of arc-like igneous rocks

The tectonic geometrical framework of the Sanjiang Orogenic Belt, eastern and southeastern Tibetan Plateau, was formed during Late Paleozoic to Early Mesozoic (i.e., the Paleotethyan stage). Previous workers had constructed an earlier version of the framework in term of plate tectonics based on the 1/200,000 geological mapping results. It has become the robust ground for geological study of the Sanjiang Orogenic Belt. Since the 1990s, in-situ zircon U/Pb dating techniques were introduced into China. Application of them has accumulated a large body of high-quality dating results. It is time to "upgrade" the earlier version. By synthesizing available zircon U/Pb data, we try to divide the arc-like igneous rocks in the Sanjiang Orogenic Belt into three belts, which were formed during two distinctive stages: (1) the Late Triassic Yushu-Yidun continent marginal arc belt; (2) the Permian to Middle Triassic Jamda-Weixi-Madeng-Diancangshan continent marginal arc belt; and (3) the Permian to Late Triassic Yunxian-Luchun-Ailaoshan continent marginal arc belt. The spatial and temporal pattern of tectonic facies, especially the high-pressure/low temperature metamorphic belts, suggests that: (1) the Yushu-Yidun continent marginal arc was resulted from southwestward subduction of the Garz-Litang ocean beneath the Eastern Qiangtang-Zhongza block, and this subduction lasted only for less than similar to 10Myr; (2) the Jamda-Weixi-Madeng-Diancangshan and the Yunxian-Luchun-Ailaoshan continent marginal arc belts were formed due to northward or northeastward subduction of the Longmu Co-Shuanghu-Changning-Menglian ocean which had continued for more than 70Myr since the Early Permian, and given rise to four and five pulses of arc-like magamatism along the northern and southern segments of the arc-belt, respectively. The longer subduction period and the larger spatial scale of the arc-belt of the Longmu Co-Shuanghu-Changning-Menglian Ocean compared with the Garz-Litang Ocean suggests that the former is the main Paleotethyan oceanic basin, while the later belongs to a branch of the Paleotethys. The final closure of the main Paleotethys is diachronous : its northern segment closed at Middle Triassic, while the south at Late Triassic. Such a spatial and temporal pattern of the Paleotethyan acr-like igneous rocks rises up several scientific issues that are worthy further studying.