Subducted oceanic plateau fed crustal growth: Insights from Amdo dacites in central Tibetan Plateau

The oceanic lithosphere subduction has been attested to exert the first-order control on the formation of the continental crust, but the role of oceanic plateaus on the crustal growth is elusive. It is difficult to identify the component of the oceanic plateaus in the continental crust because of their disappearance after subduction into deep mantle or obduction to be a part of the ophiolite mélanges in the ancient orogenic belts. Amdo microcontinent in the central Tibetan Plateau is an ideal place to assess the role of oceanic plateaus on crustal growth as it preserves the well-exposed arc magmatic rocks and fragments of the Meso-Tethyan oceanic plateaus to its south. Here, we present an integrated investigation of zircon UPb ages and Hf isotope, as well as whole-rock elements and Sr-Nd-Hf isotopes of a suit of dacites in the Amdo microcontinent. Zircon UPb dating indicates that the Amdo dacites formed at ca. 117 Ma, postdated the major formation period of the Meso-Tethyan oceanic plateaus. The Amdo dacites have relatively high SiO2 (63.45–66.82 wt%) contents and low Mg# values (20.0–36.1), belonging to calc-alkaline series. The zircon thermometer and oxybarometer give estimated temperature and oxygen fugacity of crystallization as ca. 700–720 °C and ΔFMQ = + 0.98, respectively. The Amdo dacites have low Y (5.51–6.61 ppm) and Yb (0.44–0.53 ppm) contents and high La/Yb ratios (28.4–30.3). Despite their low Sr/Y ratios (6.86–13.5) caused by a small degree of fractionation of plagioclase, the Amdo dacites have an adakitic geochemical affinity, which was most likely derived from the thickened continental lower crust. These dacites have more depleted Sr-Nd-Hf isotope compositions (whole-rock 87Sr/86Sri = 0.709796–0.710563, εNd(t) = −3.27 to −2.82, εHf(t) = 4.20–4.83 and zircon εHf(t) = 4.2–8.8) than the Neoproterozoic and Cambrian orthogneisses, as well as the Jurassic magmatic rocks. Simple binary mixing modeling reveals that the Amdo dacites could represent the hybrid melts of 50–60% of the subducted Meso-Tethyan oceanic plateau materials with 50–40% of the Amdo ancient continental crustal materials. The decoupled whole-rock NdHf isotopes (ΔεHf(t) = 7.55–8.43) of the Amdo dacites could be explained by the addition of zircon-free sediments overlie on the oceanic plateaus. These results indicate that subduction of the Meso-Tethyan oceanic plateaus with/without the overlying sediments have made a substantial contribution to continental crustal growth. In conjunction with previous studies, we conclude that the subduction of the Meso-Tethyan oceanic plateaus may have intermittently occurred for around 37 million years, thus highlighting the significance of subduction of the Meso-Tethyan oceanic plateaus during the secular evolution of the central Tibetan Plateau.