Lithium isotope behaviour during partial melting of metapelites from the Jiangnan Orogen, South China: Implications for the origin of REE tetrad effect of F-rich granite and associated rare-metal mineralization

We carried out combined Nd and Li isotopic studies on the Neoproterozoic biotite granites, late Mesozoic evolved two-mica/muscovite granites and associated the Shuangqiaoshan Group metapelites from the Jiangnan Orogen (South China), which, along with petrological and geochemical data, were used to constrain the lithium isotopic behavior during partial melting of metapelites, the origin of the REE tetrad effect in evolved granites, and implications for related rare-metal mineralization. The Neoproterozoic biotite granites show features typical of S-types (e.g., high A/CNK ratios), and have epsilon(Nd)=-6.9 to -10 and delta Li-7=-2.2--4%, comparable to the Shuangqiaoshan Group metapelites (with epsilon Nd=-6 to -11 and delta Li-7=-2--3.7%, respectively), suggesting that the biotite granites were derived from melting of the metapelites and the Li isotopic fractionation during the melting process appears negligible. The two late Mesozoic Shimensi and Zengjialong granites occur as small stocks, showing high silica, peraluminous, low CaO and Sr contents, variably enrichment in volatiles (F, B), and are accompanied by W-Cu and Sn mineralization. The Shimensi granites have epsilon Nd(t) values of -7.0--8.6 and delta Li-7=-2.7%--3.9%, similar to the Shuangqiaoshan Group metapelites, whereas the Zengjialong granites have relatively high eNd(t) values (-5.1--7.3) and heavy Li isotopic features (mostly delta Li-7=-0.18%-+ 1.53%), and particularly, REE tetrad effect is seen in the latter. We argue that the late Mesozoic evolved granites formed as a result of re-melting of the granulitic residues left in the lower crust after extraction of the Neoproterozoic biotite granites from the metapelites. Such re-melting of the residues was triggered by underplating of basaltic magma, with more mantle input (and F, B as well) in the source of the Zengjialong granites. The elevated delta Li-7 values of the Zengjialong granites were probably caused by intense interaction between melt and external fluids during magma emplacement, which, in turn, resulted in the formation of the REE tetrad effect in the granites. Isotopic modeling suggests that fluid exsolution from evolved granites is minor, and is less likely to have been responsible for the extensive rare-metal mineralization as traditionally thought.