|
|
岩石学报 2001
Nd-Sr-O isotopic geochemistr y constraints on the age and origin of the A-type granites in eastern China
|
Abstract:
Nd-Sr-O isotope studies are systematically carried out for five representativ e massifs of A-type granites in eastern China. From the north to the south, the y are Nianzishan, Shanhaiguan, Laoshan, Suzhou and Kuiqi pluton, respectively . Fitting of whole-rock Rb-Sr isochrons for the Suzhou and Kuiqi plutons pro ceeds by screened isotopic data, obtaining the ages of 108±10Ma and 109±5Ma, respectively. It appears that the isochron ages were not significantly affecte d by post-magmatic meteoric-hydrothermal alteration and thus represent the cl osure time of Rb-Sr system in the granites. However, whole-rock Rb-Sr isoch rons for the Nianzishan, Shanhaiguan and Laoshan plutons were significantly di sturbed by hydrothermal alteration, resulting in geologically meaningfulless ag es. A combined study of whole-rock εNd(t), (87Sr/ 86Sr)0 and zircon δ18O suggests that the A-type granites in easter n China would be derived from partial melting of hydrothermally altered oceanic crust which would correspond to gabbro layer in lower oceanic crust and thus u nderwent oxygen isotope exchange with seawater at high temperatures prior to su bduction of oceanic plate. A calculation of binary mixing model shows that the decoupling of the εNd(t) and δ18O values for the Nianzishan, Shanhaiguan and Laoshan granites is probably caused by metasomatic processes of subducted sediment-derived fluid to the underlying oceanic crust of gabbro. A n unified model is proposed to account for petrogenesis of the A-type granite s in eastern China and its geodynamic implications.
