Different origins of adakites from the Dabie Mountains and the Lower Yangtze River Belt, eastern China: geochemical constraints

Abstract

Cretaceous adakites are widely distributed in the Lower Yangtze River Belt (LYRB) and the Dabie Mountains, east-central China. Adakites from the LYRB in general are closely associated with Cu–Au deposits, whereas Dabie adakites lack any mineralization. Based on geochemical characteristics, we show that these adakites have different origins; for example, adakites from the Dabie Mountains have more variable Sr/Y (6.47–1303) and systematically higher La/Yb (20.8–402), Th/U (2.28–50.6), and Nb/Ta (5.07–65.2) compared to adakites from the LYRB, Sr/Y (28.8–185), La/Yb (14.1–49), Th/U (0.33–8), and Nb/Ta (7.5–23). The systematically higher La/Yb of Dabie adakites supports their continental origin, because the La/Yb of the lower continental crust is more than 10 times higher than that of mid-ocean ridge basalt (MORB). Moreover, the lower continental crust is also highly enriched in Sr, with Sr/Y > 10 times that of MORB. Interestingly, with the exception of those from Fuziling, most Dabie adakites have Sr/Y comparable to normal adakites, suggesting the presence of residual plagioclase. Because Th and U do not fractionate significantly from each other during magmatism, the high but variable Th/U suggests that the protolith of Dabie adakites underwent subduction. The LYRB adakites can be plausibly interpreted as being a result of Early Cretaceous partial melting of a young, hot, descending oceanic slab during ridge subduction. By contrast, Dabie adakites were likely formed by partial melting of the lower continental crust attending ridge subduction.