Revisiting the high temperature Darongshan-Shiwandashan granitoids in the South China: A response to slab tearing associated with diachronous collision between Indochina and South China blocks

Abstract

The Darongshan-Shiwandashan granite belt (DSGB) is composed of S-type granites and granodiorite intrusions, which are characterized by high- or ultrahigh-temperature cordierite, orthopyroxene, and granulite xenoliths. The petrogenesis of these plutons is a subject of debate, and a thorough overview of regional tectonic evolution and geochemistry is still absent despite the numerous research conducted in this granite belt. This study is a comprehensive overview of geochronology, whole-rock major-trace elements, SrNd isotopes, and zircon HfO isotopes in the DSGB, focusing on their petrogenesis and tectonic setting of formation. The geochronological data indicates that DSGB granitoids emplaced at ca. 250 Ma, with a southwestward-younging trend. Moreover, mineral assemblages of cordierite + orthopyroxene suggest that the DSGB was formed in a high-temperature (∼850 °C) and low-pressure (3.7–6 kbar) tectonic setting. Three main plutons (Darongshan, Jiuzhou, and Taima) of the DSGB exhibit distinctly different compositions. The Darongshan and Taima granitoids display high silica (SiO₂ = 68.65–78.10 wt%), low-maficity (FeO + MgO = 1.97–7.28 wt%), along with negative whole-rock ε_Nd(t) values (−13.9 to −9.7) and elevated initial ⁸⁷Sr/⁸⁶Sr values (0.71638 to 0.73165). The Jiuzhou granitoids, on the other hand, exhibit relatively low silica (SiO₂ = 63.90–72.72 wt%) and high maficity (FeO + MgO = 3.03–9.88 wt%), with largely overlapping but relatively high ε_Nd(t) values ranging from −12.9 to −9.9 and lower initial ⁸⁷Sr/⁸⁶Sr values from 0.71453 to 0.72401. Two-component mixing model results indicate these different compositions represent varying degrees of mixing between crust- and mantle-derived magmas, with 0–10 %, 0–20 % and 20–40 % basaltic melts involved for Taima, Darongshan and Jiuzhou plutons, respectively. The subducting slab tearing, induced by the diachronous collision between the South China Block (SCB) and Indochina Block (ICB), provide the most feasible interpretation for the petrogenesis and spatio-temporal geochemical pattern of the granitic rocks in the DSGB. The onset of the diachronous collision initiated at the Hainan-Yunkai massif, southwest of SCB, while the Tethys Ocean still existed in the northwest (Nanpanjiang area), causing notable disparities in the convergence velocities of the subducting ocean slab. The heterogeneous stresses resulting from the different subduction rates were accommodated by bending and tearing of the subducting slab. Then, ultrahigh-temperature basaltic melts derived from the decompression melting of the lithospheric mantle facilitated the melting of metasedimentary rocks, which resulted in the formation of high-temperature S-type granitoids. The Jiuzhou pluton, located at the center of slab tearing, received more mantle contributions than the Darongshan and the Taima plutons, which are emplaced away from the slab tearing center. The younging trend in age from the Darongshan to Jiuzhou and Taima plutons indicates a progressive tearing from the northeast (the far end of the subducting slab) to the southwest.