壳幔作用导致武平花岗岩形成: Sr-Nd-Hf-U-Pb同位素证据

Detailed petrologic, geochemical and Sr-Nd-Hf-U-Pb isotopic researches indicate that the Wuping granitic complex consist mainly of biotite granite and garnet-bearing granite with different formation age and petrogenesis. LA-ICPMS zircon U-Pb dating results show that biotite granite formed at 161.4 Ma and garnet-bearing granite did at ca. 113 Ma. Biotite granites are characterized by moderate enrichment of most incompatible elements and middle REE fractionation. These rocks exhibit lower initial ⁸⁷ Sr/⁸⁶ Sr ratios (<0.710) and higher ε ^Nd (t) value (-2.6 - -5.7), similar to typical I-type calc-alkaline granites. In comparison with the isotopic compositions of nearby basement metamorphic rocks, the present study indicates that biotite granites resulted from the mixing of the melt generated from the partial melting of Proterozoic basement metamorphic rocks and juvenile mantle-derived liquids. Variable Hf isotopic compositions of zircons (ε ^Nd (t) = -3.6 - -10.8) strongly support this conclusion. Garnet-bearing granites are featured by high Si, Al, Na, K, Nb, Ta, Y and HREE concentrations, low P, Sr, Ba, LREE, Zr and Hf concentrations. They have low (La/Yb) n, Zr/Hf and Nb/Ta ratios and high Rb/Sr ratio, suggesting strongly differentiated geochemical characteristics. Their initial ⁸⁷ Sr/⁸⁶ Sr ratios > 0.710, ε ^Nd, (t) is -7.1, and Hf isotopic compositions of zircons are relatively homogeneous and much low with an average ε ^Hf (t) of -9.7, indicating little or a little contribution of mantle-derived compositions. Combined with extremely high HREE contents, the study suggests that the parental magma of garnet-bearing granite was likely produced by re-melting of garnet-rich restites of a previous melting. Consequently, Mesozoic granites with different geochemical features in Nanling area were formed by variable extent of crust-mantle interaction.