Petrogenesis and geodynamic mechanism of Indosinian magmatism in South China Block: Insights from the Neihan–Deqing–Hetai Granitic Belt, western Guangdong

Abstract

The geodynamic mechanism(s) responsible for Indosinian magmatism in the South China Block (SCB) remains controversial. This study examines the Neihan–Deqing–Hetai Granitic Belt in western Guangdong Province, providing new insights through comprehensive petrological and geochemical analyses. The Neihan pluton comprises S-type two-mica granite, characterized by high-temperature mineral assemblages (orthopyroxene, cordierite, garnet). In contrast, the Deqing and Hetai plutons consist mainly of S-type biotite monzogranite and granodiorite with subordinate basic andesite lenses. Thermometric constraints from zircon saturation and Ti–in–zircon thermometry indicate high-temperature crystallization with peak melt temperature exceeding 800 °C. Zircon U–Pb geochronology constrains emplacement at ca. 250 Ma. These granitoids exhibit peraluminous signatures (A/CNK = 0.97–1.27, mean = 1.1), highly fractionation (SiO₂ = 65.9–74.3 wt%, Al₂O₃ = 12.5–16.1 wt%; Differentiation Index = 79.2–95.2), light rare-earth element (LREE) enrichment [(La/Yb)_CN = 3.44–25.0], and pronounced negative Eu anomalies (Eu/Eu* = 0.21–0.63, excluding sample GD09-3 at 1.14). Primitive mantle-normalized patterns show large-ion lithophile elements enrichment (Rb, Th, U, K, Pb) relative to high-field-strength elements (HFSE: Nb, Ta, P, Ti). Isotopic data reveal high ⁸⁷Sr/⁸⁶Sr ratios (0.71812–0.72182), low ε_Nd(t) values (− 9.9 to − 9.1), and Paleoproterozoic model ages (T_2DM = 1.69–1.83 Ga). In situ zircon Hf isotopic analyses yield ε_Hf(t) values between − 15.9 and − 3.2, with two-stage Hf model ages of 1.48–2.28 Ga (mean = 1.78 Ga). Collectively, these data suggest that the granitoids originated from Paleoproterozoic metasedimentary protoliths, with variable contributions from mafic magmas. Basic andesite lenses (SiO₂ = 55.8–56.5 wt%; FeO^T + MgO = 11.2–11.4 wt%) exhibit less evolved isotopic signatures [(⁸⁷Sr/⁸⁶Sr)_I = 0.71112–0.71162; ε_Nd(t) =  − 5.5 to − 5.3)], indicating higher mafic contributions. Comparative analysis with coeval granitoids in the Cathaysia Block, particularly the adjacent Darongshan–Shiwandashan Granitic Belt, supports formation in a compressional regime during SCB–Indochina collision. We propose that collision initiated at the Hainan–Yunkai massif and propagated northwestward along the Ailaoshan–Song Ma suture, generating heterogeneous stresses that caused tearing of the subducting slab. This tearing facilitated asthenospheric upwelling, providing the thermal anomaly driving crustal anatexis and mechanical heterogeneity accounting for the observed magmatic diversity.