Heterogeneity of the Ailaoshan–Song Ma ophiolitic mélange and its palaeogeographic implications for the evolution of Eastern Palaeo-Tethys

In comparison with ophiolite, ophiolitic mélange, especially its matrix, contains more information about the evolution of the relevant ocean. The evolution of the eastern Palaeo-Tethys recorded a whole Wilson cycle of numerous continental blocks such as the South China Block and Indochina Block, including rifting away from the Gondwana continent, subsequent northward migration and final collage with the Laurasia continent. Based on structural geology, detrital zircon, and related geochemical analyses, the Ailaoshan–Song Ma ophiolitic mélange can be divided into M1, M2, and M3 units with distinct detrital zircon age spectra and εHf(t) values suggesting for different provenances. These units are in faults-contact along the orogenic belt, indicating a strike–parallel heterogeneity of the ophiolitic mélange. To further understand the heterogeneity of these ophiolitic mélanges, five samples from the matrix of the Ailaoshan ophiolitic mélange were collected to be conducted detrital zircon UPb dating and LuHf isotope analyses. All samples share the same zircon age spectrum, with two obvious age peaks at 430 and 960 Ma, which is comparable with the M1 unit. These results imply that the northwestern Ailaoshan ophiolitic mélange segment can be subdivided into M1 in the interior and M2 in the exterior rather than the previous division, indicating a strike–perpendicular heterogeneity. Based on their geometries (NE-dipping) and depositional ages (M1, 310–270 Ma; M2, 260–240 Ma), the elder M1 unit was thrusted upon the younger M2 unit, indicating a tectonic inversion. This inversion, which is associated with the whole geometry of the Ailaoshan belt, is ascribed to the transpressive deformation of the Cenozoic Red River Fault. These spatial heterogeneities well match the different evolutionary stages of the eastern Palaeo-Tethys.