Adakitic granites within the NE Jiangxi ophiolites, South China: geochemical and Nd isotopic evidence

Abstract

Geochemical and Nd isotopic data are reported for the ca. 970 Ma albite granites within the Neoproterozoic NE Jiangxi ophiolite, South China. These granites are high in Al2O3 (15–17%), Na2O (7.3–8.5%), Mg# (43–62), Sr (383–920 ppm), Sr/Y (166–444) and Y/Yb (12–14), but low in K2O (0.2–0.6%), TiO2 (<0.3%), P2O5 (<0.1%). Furthermore, they display consistently high, MORB-like initial εNd values of +4.6 to +6.7 at ca. 970 Ma. These geochemcial and isotopic characters are similar to those of the modern adakites. Compared with the modern adakites, the Xiwan albite granites are depleted in middle to heavy REE with Y/Yb ratio of 12–14, which can be attributed to residual amphibole and garnet in the source. Geological and geochemical features suggested that the granites were generated by low degrees of partial melting of subducted, spilitized oceanic crust at pressures high enough to stabilize garnet and amphibole. These are the first adakitic granites identified within the NE Jiangxi ophiolite. Their presence shows that granitic rocks that intrude ophiolites can be generated not only during the processes of sea-floor spreading, oceanic crust shearing and obduction of fragmented oceanic lithosphere as previously thought, but also during the process of subduction of oceanic lithosphere. The SHRIMP U–Pb zircon age of 968±23 Ma for the Xiwan adakitic granite [Geochimica 23 (1994) 117] should be re-interpreted as the timing of subduction of oceanic crust, rather than as the age of oceanic crustal formation as previously interpreted.