Discovery of middle-late Permian mid-ocean ridge basalts (MORB) in Lihu, Southeast Yunnan and its regional tectonic implication

There has been a significant controversy for a long time regarding the paleogeographical environment of the Carboniferous-Permian period in the southeastern Yunnan region. One viewpoint holds that it belongs to the platform facies, while another contends that it is of the basin facies. This controversy has severely restricted the in-depth understanding of the tectonic evolution of the Youjiang Basin. Through a 1∶50 000 regional geological survey, a set of pillow basic volcanic rocks from the middle-late Permian period was discovered in the Lihu area, Funing County, southeastern Yunnan. Its main lithologies include dense massive basalt, spherulitic (picritic) basalt interbedded with basaltic volcanic breccia. The chondrite-normalized rare earth element distribution curves of these volcanic rocks are flat, similar to the characteristics of the E-MORB. On the primitive mantle-normalized trace element spider diagrams, the rocks show relatively enriched elements such as K, Rb, Ba, and Th, while elements like Ti and P are relatively depleted. In the tectonic setting diagrams, the projection points of these rocks fall into the ocean ridge and ocean floor area, as well as the oceanic ridge tholeiite area respectively, generally exhibiting the characteristics of high-potassium tholeiite of the ocean ridge. Based on this, it is believed that this set of volcanic rocks from the middle-late Permian period is the product of the rapid expansion of the Youjiang Ocean Basin. Combining with previous research results, it is speculated that the southeastern Yunnan region should have been a multi-island ocean basin environment composed of stable shallow-water sediments, oceanic islands, deep-water areas, and the open ocean basin during the late Paleozoic era. This discovery provides important reference materials for the tectonic evolution of the Youjiang Ocean Basin.