Revisiting the "C-type adakites" of the Lower Yangtze River Belt, central eastern China: In-situ zircon Hf-O isotope and geochemical constraints

Abstract Adakites, or adakitic rocks, in a broad sense, have been used to describe a large range of igneous rocks with the common feature of high Sr/Y and La/Yb ratios that can be achieved though different mechanisms. Among them, the continental, or C-type, adakitic rocks are particularly controversial in terms of their sources and genesis. In this study we revisit both Cu–Au ore-bearing and barren “C-type adakitic rocks” in the Lower Yangtze River Belt (LYRB) of central eastern China, including comprehensive analyses of their in-situ zircon Hf–O isotopes, whole-rock geochemistry and Sr–Nd isotopes. These “C-type adakitic rocks” consist of monzodiorite, granodiorite and quartz monzonite that are classified as shoshonitic to high-K calc-alkaline series according to their chemical compositions. They are characteristically high in potassium (K 2 O = 2.4–4.5%, K 2 O/Na 2 O = 0.6–1.3), with continental crust-like isotopic compositions, i.e., whole-rock eNd(T) = − 3.9 to − 7.7, initial 87 Sr/ 86 Sr = 0.7054–0.7085, zircon eHf(T) = 0 to − 11, and δ 18 O = 6‰ to 9‰. The ore-bearing and barren rocks are cogenetic. Fractional crystallization of hornblende, titanite, magnetite and apatite played a major role in their chemical variations, with the ore-bearing rocks being more felsic (SiO 2  = 63.3–69.6%) and higher in Sr/Y (41.2–75.6) than the barren rocks (SiO 2  = 57.3–65.0%, Sr/Y = 30.4–51.8). All these geochemical and isotopic features, in combination with regional geological data, suggest that the LYRB “C-type adakitic rocks” were unlikely to have been formed by melting of either a thickened and/or delaminated lower continental crust, or an altered oceanic crust as previously thought. These rocks are in general akin in geochemistry and isotopes to the Archean sanukitoids and the Setouchi high-Mg andesites in Japan, and are thus interpreted as being formed by melting of an enriched mantle source metasomatized by dewatering from a delaminated flat-slab. The flat subduction of an oceanic plateau and its subsequent delamination and foundering since the early Mesozoic beneath southeastern China ( Li and Li, 2007 ) thus not only explain the temporal and spatial propagation of widespread Yanshanian igneous rocks regionally since ca. 195 Ma, but also the formation of a series of enigmatic “adakitic” rocks in the region, including the LYRB potassium-rich rocks that were inappropriately called the “C-type adakitic rock” by previous workers.