Generation of isotopically enriched giant Lincang Batholith by disequilibrium melting of the heterogeneous lower crust

Deciphering the origin of granitoids with enriched isotopic compositions is essential to understanding the mechanism of continental growth and reworking. Establishing their origin is also an effective solution to the decoupled interpretations between geochemical and isotopic observations in competing petrogenetic models. This paper reports on the whole-rock major and trace elements, Sr-Nd-Pb-Hf isotopes, and zircon UPb age and Hf isotope data of samples from the Late Triassic Lincang Batholith in SW China. The hornblende-bearing granodiorites are weakly peraluminous, whereas the coeval biotite monzogranites are generally strongly peraluminous. These two types of granitoids show identical and extremely enriched isotopic compositions with (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.71991 to 0.74302, whole-rock ε_Nd(t) values of −13.5 to −10.1, and ε_Hf(t) values of −13.4 to −10.3, as well as a large variation of zircon ε_Hf(t) values of −17.6 to +0.6. The inherited zircons from both suites of samples show similar age peaks (ca. 950 Ma and ca. 1150 Ma), trace element concentrations, and ε_Hf(t) variations to those of the detrital zircons from the Lancang Group. These similarities indicate that these rocks are most likely derived from the anatexis of the Lancang Group that is mainly composed of quartz schist, sericite schist, greenschist, chlorite albite schist, and minor eclogite. On the other hand, the Lancang Group is characterized by more enriched isotopic compositions than the granitic batholith with whole-rock ε_Nd(225 Ma) value of −13.3 and ε_Hf(225 Ma) value of −19.1, but identical to those of the average ε_Hf(225 Ma) values of inherited and detrital zircons (−18.1 and − 16.3). This isotopic fractionation between the granitoids and the Lancang Group may result from disequilibrium melting of the Lancang Group with different dissolution behavior of accessory minerals (i.e., zircon, monazite, apatite, titanite, etc.). The Lincang Batholith is on average more silicic and richer in incompatible elements than the upper continental crust, suggesting that the isotopically enriched giant granitic batholith is a mature magmatic response to continental reactivation and reworking.