Geochronology and geochemistry of Neoproterozoic mafic rocks from western Hunan, South China: implications for petrogenesis and post-orogenic extension

The Neoproterozoic mafic rocks in western Hunan, South China, form a NNE-striking mafic rock belt for which outcrops are found predominantly in Guzhang, Qianyang and Tongdao. Samples from Qianyang and Tongdao yielded ion microprobe U-Pb zircon ages of 747 ± 18 Ma and 772 ± 11 Ma, respectively. The mafic rocks are geochemically divided into two subtypes. Ultramafic rocks from Tongdao are depleted in Nb and Ti, with decoupled Nd-Hf isotopes, and geochemical features similar to the c. 761 Ma mafic-ultramafic rocks from Longsheng, northern Guangxi. Their ε_Nd(t) value is -2.91, implying an enriched mantle source. Alkaline mafic rocks from Qianyang and Guzhang have high values of TiO₂, total alkali, some high field strength elements and (La/Yb)_N, and low Zr/Nb, La/Nb, Sm/Nd and ¹⁴³Nd/ ¹⁴⁴Nd ratios as well as coupled Nd-Hf isotopes. They are geochemically similar to ocean island basalts and show fractional crystallization of Fe-Ti oxides, olivine and pyroxene in the mafic magma. The c. 760 Ma mafic rocks in western Hunan may be the products of post-orogenic magmatism. After the Jinningian (Sibao) orogenic process, the upwelling of the deep asthenospheric mantle caused by the break-off and detachment of the subducted oceanic slab led to extension in the area. The extension might have taken place earlier in the Tongdao and Longsheng areas, which led to the partial melting of the lithospheric mantle that had been metasomatized during early oceanic subduction to generate a relatively large amount of sub-alkaline rocks. However, the less alkaline mafic rocks in Qianyang and Guzhang might have been generated in the relatively later stage of the extension, and may have resulted from a small degree of partial melting of the asthenospheric mantle.