PLUME-PROXIMAL MID-OCEAN RIDGE ORIGIN OF ZHONGBA MAFIC ROCKS IN THE WESTERN YARLUNG ZANGBO SUTURE ZONE, SOUTHERN TIBET

The >2000-km-long Yarlung Zangbo Suture Zone (YZSZ) in southern Tibet is the southernmost suture across the plateau and includes the remnants of the Mesozoic Neotethyan oceanic lithosphere. The YZSZ is subdivided by the Zhada-Zhongba microcontinent into northern and southern branches in its western segment. We present here our field observations and geochronological, geochemical and Sr-Nd-Pb isotopic data from mafic rocks of the Zhongba ophiolite, including samples from both branches. Zircon U–Pb analyses of a doleritic rock from the northern branch has a concordant age of 160.5 ± 1.3 Ma. All the doleritic samples from the northern branch and the pillow basalt and gabbro samples from the southern branch display similar REE and trace element patterns similar to those of modern OIB rocks. Doleritic samples from the northern branch have (⁸⁷Sr/⁸⁶Sr)_I = 0.7005~0.7047, εNd(t) = 2.52~4.83, (²⁰⁶Pb/²⁰⁴Pb)_i = 5.09~18.51, (²⁰⁷Pb/²⁰⁴Pb)_I = 14.95~15.58, (²⁰⁸Pb/²⁰⁴Pb)_I = 24.38~39.22. Basaltic samples from the southern branch have (⁸⁷Sr/⁸⁶Sr)_I = 0.7047~0.7052, εNd(t) = 2.12~3.29, (²⁰⁶Pb/²⁰⁴Pb)_I = 11.53~19.18, (²⁰⁷Pb/²⁰⁴Pb)_I = 15.23~15.63, (²⁰⁸Pb/²⁰⁴Pb)_I = 36.19~39.59. Gabbroic samples from the southern branch have (⁸⁷Sr/⁸⁶Sr)_I = 0.7072~0.7114, εNd(t) = 2.12~3.29, (²⁰⁶Pb/²⁰⁴Pb)_I = 17.88~18.81, (²⁰⁷Pb/²⁰⁴Pb)_I = 15.63~15.69, (²⁰⁸Pb/²⁰⁴Pb)_I = 38.01~39.27. We interpret our results together with the extant isotopic, geochemical and structural data from the entire YZSZ, and infer the tectonic origin of Neotethyan mafic rocks exposed along the YZSZ as a plume-proximal seafloor spreading system, reminiscent of the seamount chains along-across the modern mid-ocean ridges in the Pacific Ocean.