Petrogenesis of Early Mesozoic volcanic rocks in southeastern NE China: Geochemical and Sr-Nd-Pb-O isotopic evidences

This paper reports a study of whole-rock geochemical, Sr-Nd-Pb isotopic compositions and zircon U-Pb ages and O isotopic compositions of Late Triassic to Early Jurassic volcanic rocks of the Yanbian-Dongning area, northeastern China. The obtained data are used to constrain the petrogenesis of these rocks and to provide new insights into the timing of transition from the Paleo-Asian to the Paleo-Pacific tectonic regime. The studied Late Triassic to Early Jurassic volcanic rocks can be divided into two periods: Late Triassic-early Early Jurassic (212-198 Ma) and late Early Jurassic (ca. 181 Ma). The former rocks display high SiO2 and total alkalis contents, high Ga/Al ratios, depleted Sr-Nd and highly radiogenic Pb isotopic signatures, and low zircon delta O-18 values. The former rocks have a genetic relationship with coeval intrusions, and their primitive melt was derived from juvenile crustal materials and underwent subsequent differentiation and assimilation of low-delta O-18 rocks. In contrast, the latter rocks have SiO2 contents of 49.00-63.30 wt%, are classified as calc-alkaline, and display arctype trace-element features. Their geochemical and Sr-Nd-Pb-O isotopic compositions imply derivation from the partial melting of depleted mantle wedge metasomatized by oceanic slab-derived fluid and sediment melt. Combining these new results with published data, the Late Triassic-early Early Jurassic volcanic rocks are inferred to have formed in a rift-like extensional setting, whereas the late Early Jurassic volcanic rocks were generated in a subduction-related setting. The transition from the Paleo-Asian to the Paleo-Pacific tectonic regime occurred during the Early Jurassic.