Anduo Late Cretaceous high-K calc-alkaline and shoshonitic volcanic rocks in central Tibet, western China: Relamination of the subducted Meso-Tethyan oceanic plateau

Abstract

The Mesozoic Meso-Tethyan oceanic plateaus could have played important roles in both Tibetan continental marginal orogenesis and Tethyan oceanic evolution. However, little has been known whether fragments of these tremendous oceanic highs underplated beneath the active southern Qiangtang margin and impinged on the later magmatic and/or tectonic evolution. The Anduo volcanic rocks of this study in the southern Qiangtang, erupted at 79.6 ± 0.8 Ma, comprise predominantly high-K trachyte and trachyandesite, including 5 adakites with high Sr and low Y and Yb contents and high La/Yb normalized to chondrite ratios (> 20). These high-K calc-alkaline and shoshonitic rocks, classified according to immobile trace elements, have enriched isotope compositions with (87Sr/86Sr)i = 0.7073–0.7079, εNd(t) = −1.78 to −3.90 and zircon εHf(t) = −1.3 to 1.4. Geochemical modeling reveals that the Late Cretaceous Anduo high-silica adakites were most likely derived from the melting of the mixed material of 80–90% of the subducted Meso-Tethyan oceanic plateau metabasalt with 10–20% of the subducted sediments. Variable negative Eu and Sr anomalies, relatively large range of Mg#, increases of the εNd(t) values with increasing Mg# and decreasing SiO2, and increases of (87Sr/86Sr)i with decreasing Mg# and increasing SiO2 between adakites and non-adakites indicate the latter might have derived from lithospheric mantle assimilation and mineral fractional crystallization of the adakite parent magmas. It is thus concluded that the Meso-Tethyan oceanic plateaus could not only directly lead to the initial uplift of the Tibetan plateau through obduction and accretion onto the continental margin, but also contributed to the continental growth through relamination of the subducted oceanic plateaus.