Timing and mechanism of opening the Neo-Tethys Ocean: Constraints from mélanges in the Yarlung Zangbo suture zone

Abstract

The evolution and final closure of the Neo-Tethys Ocean are one of the most important geological events that have occurred on Earth since the Mesozoic. However, the evolution of the Neo-Tethys is not well constrained, in particular whether its opening occurred in the Permian or the Triassic and whether a plume was involved with its opening or not. In this study, we present geochronological and geochemical data for mafic igneous rocks in mélanges along the Yarlung Zangbo suture zone (YZSZ) in southern Tibet to constrain the timing and mechanism of opening the Neo-Tethys Ocean. Based on field observations, the YZSZ mélanges can be divided into three segments. The western (west of Zhongba) and eastern (Sangsang-Renbu) segments are composed of ocean plate stratigraphy representing accretionary complexes that formed during subduction of Neo-Tethyan oceanic lithosphere beneath the southern margin of the Asian continent. Mélanges in the central segment (Zhongba-eastern Saga) typically have a siliciclastic matrix, and represent Tethyan Himalayan strata that were structurally mixed with the southern margin of the Asian continent. Based on our and previously published geochemical data, the mafic rocks in the YZSZ mélanges are ocean island basalt (OIB)-like, with ages in the Late Permian-Middle Triassic, the Middle-Late Jurassic, and the Early Cretaceous, respectively. An OIB-like block with an age of ca. 253 Ma is identified from the Zhongba mélanges in the western segment, and it is the oldest OIB lithology yet identified in the YZSZ mélanges related to the evolution of the Neo-Tethys Ocean. Geochemical features indicate that this OIB-like block is distinct from typical OIBs and would be formed during continental rifting to incipient seafloor spreading. In the framework of plate divergent-convergent coupling systems and based on literature data for early Middle Triassic seamounts, radiolarian cherts, and normal mid-ocean ridge basalt-like oceanic crust, we conclude that opening of the Yarlung Zangbo Neo-Tethys Ocean would mainly occur at ~250–243 Ma in the Early Triassic, not later than the early phase of Middle Triassic. In addition, a mantle plume was not involved in opening the Yarlung Zangbo Neo-Tethys Ocean. On the other hand, we have also identified a suite of ca. 160 Ma OIB-like basaltic sills from the Bainang mélanges in the eastern segment, which is the same age as the OIB lithologies previously reported in the Zhongba mélanges. Based on the sill-like occurrence and absence of plume-related rock associations in this region, the Bainang OIB-like rocks might result from Middle-Late Jurassic continental rifting in northern Gondwana. Magmatism related to this tectonic event is preserved in both the YZSZ mélanges and Himalayan strata, but its tectonic significance requires further investigation. Based on this study of the YZSZ mélanges and the previous studies of YZSZ ophiolites, Gangdese belt igneous rocks, and sedimentary rocks, we have reconstructed the entire Wilson Cycle of the Yarlung Zangbo Neo-Tethys Ocean, mainly involving continental rifting and ocean opening, subduction initiation, ultraslow-spreading ridge-trench conversion, subduction re-initiation, and oceanic closure and initial India-Asia collision for the tectonic emplacement of ophiolites. These processes were associated not only with magmatic flare-ups and lulls in the Gangdese belt but also with two stages of ophiolite obduction. Our data therefore provide new insights into the evolution of the Yarlung Zangbo Neo-Tethys Ocean and related Tethyan geodynamics.