Neoproterozoic tonalite and trondhjemite in the Huangling complex, South China: crustal growth and reworking in a continental arc environment

The Huangling igneous complex in the central Yangtze Block of South China is composed of ∼850 Ma tonalite and trondhjemite, intruded by ∼820 Ma granite. Tonalites mainly consist of plagioclase and quartz with abundant hornblende and biotite, and contain fine-to coarse-grained hornblende-rich igneous enclaves. They have variably low SiO₂ (56.4-67.5 wt%) and Sr (230-517 ppm), and high MgO (1.47-4.02 wt%) and high Y (8.87-15.7 ppm) with low Sr/Y (17-43), unlike Archean tonalites. Their whole-rock εNd values range from -8.3 to -10.3 and zircon Hf values from -3.9 to -12.6, similar to those of spatially associated Neoproterozoic mafic rocks. The tonalites are thus proposed to have been produced by three stages, involving partial melting of subduction-modified lithospheric mantle, 5 to 20 percent crustal contamination of hydrous basaltic magmas in the middle to lower crust, and fractional crystallization of amphibole - plagioclase - biotite - magnetite. Trondhjemites are composed of plagioclase, quartz, and alkali feldspar with minor biotite and hornblende. They have high SiO2 (67.3-75.6 wt%) and Sr (340-840 ppm), and low MgO (0.09-0.78 wt%) and Y (4.8-11.2 ppm) with high Sr/Y (32-162), similar to Archean trondhjemites and high-pressure partial melts of mafic rocks. These rocks have extremely negative -Nd (+18.7 to -19.9) and zircon εHf values (-18.9 to -49.2) with two-stage Hf-isotope model ages of 2.46 to 3.90 Ga, indicating their formation in the lower crust by melting of Archean amphibolites and granulites which were probably widely distributed beneath the Yangtze Block. Their highly variable chemical compositions also resulted from fractional crystallization of plagioclase + hornblende + biotite. Thus, the association of the tonalite and trondhjemite is different from the Archean TTG series. This difference may reflect different geothermal gradients. However, modeling using tonalitic and trondhjemitic samples of the Huangling complex suggests that mixing of the two magmas can generate a typical TTG-Affinity series, suggesting that the origin of some TTG series may have involved both crustally-derived and mantle-derived melts. The generation of the Huangling igneous complex was linked to slab subduction beneath the northern margin of the Yangtze Block during the Neoproterozoic, similar to the Cenozoic arc magmatism in the Andes. Therefore, melting of metasomatized, hydrous mantle in arc settings can generate mafic magmas that not only under-plated and contributed to continental crustal growth, but also caused melting of thickened ancient crust and newly formed lower mafic crust.