GEOCHEMICAL CHARACTERISTICS OF THE EARLY MESOZOIC GRANODIORITES AND THEIR TECTONIC IMPLICATIONS

The early Mesozoic granodiorites (ca.165 Ma) in the northeastern Hunan Province have SiO_2=65.4%～(69.65%), K_2O/Na_2O=0.95～1.3 and K_2O+Na_2O6%, and belong to high-K calc-alkaline series. They are characterized by LREE and LILEs enrichment, and HFSE depletion with slightly negative Eu anomalies. The initial()~(87)Sr/~(86)Sr ratios range from 0.711458 to 0.717461, and eNd values from -9.4 to -12.3, distinct from those of the contemporaneous granodiorites from the southeastern Hunan Province (~(87)Sr/~(86)Sr(i)=0.707962～0.710396, e_(Nd)(t)=-6.98～(-2.30)). By contrast, such signatures are roughly similar to those of the other neighboring Mesozoic granitic plutons (Eu/Eu~*=0.30～0.70;()~(87)Sr/~(86)Sr0.710; e_(Nd)=-12～-16) in South China, which have been interpreted as the remelting products of Precambrian basement. The Proterozoic lower-middle crust is an important contributor to the petrogenesis of these early Mesozoic granodiorites in the northeastern Hunan Province. An intracontinental extension setting is present in the northeastern Hunan Province at that time due to the demand of enough thermal transfer.