Geochemistry of early mesozoic potassium-rich diorites-granodiorites in southeastern Hunan Province, South China. Petrogenesis and tectonic implications

The diorites-granodiorites emplaced at ca. 175 Ma in southeastern Hunan tectonic-magmatic belt (SHB), South China Block (SCB), are high-K calc-alkaline. They are characterized by LREE-LILE enrichment and HFSE depletion with slight or no Eu anomalies. Age-corrected initial 87Sr/86Sr ratio spans a narrow range of 0.707962-0.710396 and εNd(t) of -6.98--2.30. These features are significantly different from those of the neighboring other Mesozoic granitic plutons (Eu/Eu* = 0.30-0.70; 87Sr/86Sr > 0.710; εNd = -12 to -16) in South China, which have been interpreted as the remelting products of Precambrian basement. The crust anatexis cannot account for the geochemical characteristics of these diorites-granodiorites in the SHB. The partial melting of an enriched lithosphere mantle directly produced mafic-intermediate rather than acid magma, and can also not explain the geochemical and isotopic variations exhibited by these diorites-granodiorites. These diorites-granodiorites likely represent partial melting products of mantle-derived basaltic rocks, which were underplated and hybridized with old crust material at lower/middle crust level. Theoretical modeling further suggests that this hybridized source consisted of 15-30% of the lower/middle crust and 70-85% of depleted mantle-drived basaltic rocks. The occurrence of these diorite-granodiorites in southeastern Human province therefore marked a major intra-lithospheric extension episode during early Mesozoic. (author)