Geochronological and geochemical results from Mesozoic basalts in southern South China Block support the flat-slab subduction model

The debate on the Mesozoic tectonics of the South China Block (SCB), featuring a broad orogenic belt and a large continental magmatic province, has been rejuvenated in the past years. There are a number of competing tectonic models, each predicting different time, space, and compositional evolutional trends for the Mesozoic igneous rocks, including sporadic basalts. In this paper, we report high precision ⁴⁰Ar/ ³⁹Ar ages and geochemical and Sr-Nd isotopic data for basaltic rocks in central-eastern SCB. These results, together with a data compilation, indicate three evolutionary stages for the basalts. Stage 1 (195-160 Ma) basalts occur only in inland SCB. They exhibit low initial ⁸⁷Sr/ ⁸⁶Sr ratios (0.7038 to 0.7078), high ɛ _Nd(t) values (- 1.5 to 6.0) and ocean island basalt (OIB)-like geochemical characteristics such as low La/Nb ratios (0.6 to 1.4), implying negligible to minor lithosphere contamination. Stage 2 (160-110 Ma) basalts, mostly from the same inland region with minor occurrence closer to the coast, are characterized by variable initial ⁸⁷Sr/ ⁸⁶Sr ratios (0.7053 to 0.7102), ɛNd(t) values (- 6.0 to 5.0) and low La/Nb ratios (0.9 to 1.8), indicating an enhanced asthenosphere-lithosphere interaction. Stage 3 (110 to ≤ 80 Ma) basalts can be further divided into two sub-groups based on their geographic and geochemical variations. Basalts in the coastal regions are characterized by low ɛNd(t) values (- 8.1 to 3.7) and variably high La/Nb ratios (up to 4.8) indicating a strong arc signature. On the other hand, basalts in the inland regions have high ɛ _Nd(t) values (- 1.9 to 6.8) and low La/Nb ratios (0.6 to 1.2) that are similar to the stage 1 basalts. We interpret these features as results of geodynamics processes related to the break-up, foundering and retreating of an early Mesozoic flat-subducted oceanic slab.