Reconstruction of the evolution of the Yurongguo ophiolite: Constraints on the southward intra-oceanic subduction of the Bangong-Nujiang Tethys Ocean

Abstract

The evolution of the Meso-Tethys Ocean has received much research attention, but whether there was intra-oceanic and the starting time of subduction remains controversial. In this study, we present detailed petrological, U-Pb geochronology, geochemical, and Sr-Nd isotope investigations on Yurongguo ultramafic rocks from the western part of the Shiquanhe-Lagkor-Jiali suture zone, south of the Bangong-Nujiang suture zone, including harzburgites and websterites. The harzburgites are characterized by V-shaped REE patterns and significant Eu depletion, showing significant sediment influence. The websterites are classified into two groups based on their petrological and geochemical, but they all show distinctly Nb-Ta enrichment. Yurongguo ultramafic rocks have low 87Sr/86Sr(t) but high εNd(t) values (+4.84 to + 8.55), indicating that these rocks evolved from a depleted mantle without the influence of arc. The zircon U-Pb age group includes 640 ± 30 Ma, 805 ± 13 Ma, 1002 ± 17 Ma, and 1781 ± 40 Ma, similar to the Precambrian basement of the central Lhasa subterrane. The harzburgites are the remnant of ∼ 30 % partial melting of the depleted mantle and metasomatized by a small amount of sediment melt. The websterites I are a cumulation of depleted mantle melt followed by metasomatized with Nb-Ta-rich material. The websterites II are the result of the reaction between the websterites I and sediment melt. Combined with previous studies, we proposed that these rocks may have formed during initial intra-oceanic subduction, which are the product of the early Jurassic subduction of the Meso-Tethys Ocean. The ancient continental basement of central Lhasa subterrane impacted the Early Jurassic igneous rock formed in the Bangong-Nujiang Tethys Ocean.