Was there Jurassic paleo-Pacific subduction in South China?: Constraints from 40Ar/39Ar dating, elemental and Sr-Nd-Pb isotopic geochemistry of the Mesozoic basalts

There is an ongoing debate concerning whether the vast Nanling Mountain granitoids of South China are the result of Jurassic paleo-Pacific subduction. We address this question by examining two Mesozoic basalt successions in South China, one parallel and the other oblique to the convergence boundary between the paleo-Pacific plate and South China continent. The geochemical characteristics of these basalts are used to constrain the influence of this presumed subduction system on mantle composition. 40Ar/39Ar age, major element and trace element abundances, and Sr, Nd and Pb isotope data indicate that the northeast-southwest trending basalts in the Southeast Coast Magmatic Belt that formed during 101-76 Ma exhibit arc-like signatures and are derived from subduction-modified mantle. However, the east-west trending basalt array extending from the Cathaysia Interior to the Cathaysia Folded Belt, which decreases in age from 175 to 98 Ma in an eastward direction, is characterized by lithosphere-modified OIB-like asthenosphere composition, except for a few younger rocks from Huichang which resemble Southeast Coast Magmatic Belt basalts. These results suggest that the Jurassic mantle beneath the Cathaysia Interior and the Cathaysia Folded Belt was rather homogeneous and undisturbed, and was not affected by the paleo-Pacific subduction system. Alternatively, post-orogenic (Indosinian) extension accompanied by mafic underplating may have been the cause of vast Jurassic granitic magmatism in South China. (C) 2008 Elsevier B.V. All rights reserved.