Geochronology and geochemistry of basaltic lavas in the Dongbo and Purang ophiolites of the Yarlung-Zangbo Suture zone: Plume-influenced continental margin-type oceanic lithosphere in southern Tibet

Abstract

The Dongbo and Purang ophiolites in the western part of the Yarlung-Zangbo suture zone (YZSZ) in southern Tibet rest tectonically on the middle-late Triassic and Cretaceous flysch units, and consist mainly of peridotites, mafic dikes, and volcanic-sedimentary rocks. Massive basalt flows and basaltic hyaloclastic rocks are interlayered with or overlain by mudstone and silty shale, radiolarian chert, and siliceous to oolitic limestones. The 137Ma E-MORB-like basalts occur as interlayers in the radiolarian cherts in the Purang massif, and show relatively high contents of TiO2 (2.05wt.%), low ratios of (La/Yb)N (2.04 to 2.35), and high values of (143Nd/144Nd)t=0.512777–0.512779, εNd(t)=+6.1~+6.2, and (86Sr/87Sr)t=0.70688–0.70710. Tithonian to Valanginian (150–135Ma) OIB-like basalts occur between the peridotites and the silty shale in the Dongbo massif. These basalts display higher TiO2 (3.27wt.%) contents, higher ratios of (La/Yb)N (16.70) and (143Nd/144Nd)t=0.512596–0.512630, but lower values of εNd(t)=+2.6–+3.3 and (86Sr/87Sr)t=0.70378–0.70439, in comparison to the Purang basalts. The 128–130Ma doleritic dikes intruding the upper mantle peridotites exhibit N-MORB-like rare earth element patterns, have higher ratios of (143Nd/144Nd)t=0.512904–0.512909 and higher εNd(t)=+8.9–+9.0 than those of Dongba and Purang, and show slightly low (86Sr/87Sr)t=0.70489–0.70531. The OIB and E-MORB-like basaltic lavas in both ophiolites represent the products of seamount volcanism, erupted directly on the serpentinized peridotites that were exhumed on the seafloor by the earliest Cretaceous. Doleritic dikes are compositionally and geochemically similar to the basaltic rocks in ancient and modern volcanic-rifted margins. The mafic–ultramafic and sedimentary rocks in the Dongbo and Purang massifs represent fragments of an early Cretaceous continental margin ophiolite, whose magmatic evolution was influenced by plume magmatism.