Nd-Sr-O isotopic geochemistry constraints on the age and origin of the A-type granites in eastern China.

Nd-Sr-O isotope studies are systematically carried out for five representat ive massifs of A-type granites in eastern China. From the north to the sout h, they are Nianzishan, Shanhaiguan, Laoshan, Suzhou and Kuiqi pluton, resp ectively. Fitting of whole-rock Rb-Sr isochrons for the Suzhou and Kuiqi pl utons proceeds by screened isotopic data, obtaining the ages of 108+/-10Ma and 109+/-5Ma, respectively. It appears that the isochron ages were not sig nificantly affected by post-magmatic meteoric-hydrothermal alteration and t hus represent the closure time of Rb-Sr system in the granites. However, wh ole-rock Rb-Sr isochrons for the Nianzishan, Shanhaiguan and Laoshan pluton s were significantly disturbed by hydrothermal alteration, resulting in geo logically meaningfulless ages. A combined study of whole-rock epsilon (Nd)( t), (Sr-87/Sr-86)(o) and zircon delta O-18 suggests that the a-type granite s in eastern China would be derived from partial melting of hydrothermally altered oceanic crust which would correspond to gabbro layer in lower ocean ic crust and thus underwent oxygen isotope exchange with seawater at high t emperatures prior to subduction of oceanic plate. A calculation of binary m ixing model shows that the decoupling of the epsilon (Nd)(t) and delta O-18 values for the Nianzishan, Shanhaiguan and Laoshan granites is probably ca used by metasomatic processes of subducted sediment-derived fluid to the un derlying oceanic crust of gabbro. An unified model is proposed to account f or petrogenesis of the A-type granites in eastern China and its geodynamic implications.