Geochronology and petrogenesis of the Yeba formation volcanic rocks in the Epingsong area, southern Lhasa terrane, Tibet: record of the Neo-Tethys subduction

Abstract

ABSTRACT The Yeba Formation volcanic rocks preserve important information of the early Neo-Tethys subduction. Here, we report the discovery of these volcanic rocks at Epingsong (southeast of the Yeba Formation), which fill the gap of the Yeba Formation at Sangri. Timing of volcanism in the eastern and western Yeba Formation is discussed. Systemic studies on petrology, whole-rock geochemistry, zircon U-Pb age, whole-rock Sr-Nd isotopes and in situ zircon Lu-Hf isotopes reveals that the rocks are calc-alkaline andesitic to felsic, and were emplaced in the Early Jurassic (ca. 191.4–174.0 Ma). The andesitic rocks have medium Mg# value (28.3–50.2, avg. 40.1), low Cr (4.68–36.01 ppm) and Ni (3.63–12.70 ppm) contents. They have slightly positive Eu/Eu* anomalies (1.0–1.13), and display continental arc affinity, as featured by enrichments in light rare earth elements (LREEs) and large ion lithophile elements (LILEs; e.g. Rb, K, Ba), but depletions in high-field strength elements (HFSE; e.g. Nb, Ta, Ti and Hf). The Yeba andesite has largely positive whole-rock εNd (+3.6) and zircon εHf (−1.3 to +10.9) values. The felsic rocks have low Mg# values (19.2–42.3), MREE depletion (e.g. Tb, Dy and Ho), and negative Eu/Eu* anomalies (0.76–0.96). They have varying εNd (0.9–2.9) and εHf (2.1–10.4) values, with two-stage model ages (tDM2) of 560–1098 Ma. The results indicate that the andesitic-felsic rocks were probably sourced from partial melting of the lower crust by mantle-derived magma underplating, and mixed with subducted sediment-derived melts (2.0–7.5%) and assimilated ancient crystalline basement. The Yeba volcanic rocks likely record an important tectonic transition from subduction compression to extension.