Paleogene igneous intrusion and its effect on thermal maturity of organic-rich mudstones in the Beibuwan Basin,South China Sea

The seismic, drilling and logging data reveal that a large-scale igneous intrusion with a width of 14 Km and a maximum thickness of 170 m intruded within the Paleogene Liushagang Formation in the Fushan Depression, Beibuwan Basin, South China Sea (SCS). In this study, we report the geochemistry and Sr-Nd-Pb-Hf isotopic compositions of the igneous rocks, and evaluate the thermal effect induced by this large-scale igneous intrusion on the host rocks. The analyzed igneous samples exhibit strong enrichment in light rare earth elements (LREE) and large-ion lithophile elements (LILE), having characteristics similar to intra-plate oceanic island basalts (OIB). The Sr-Nd-Pb-Hf isotopic data display narrow ranges (e.g. ⁸⁷Sr/⁸⁶Sr_i = 0.7042–0.7044, ¹⁴³Nd/¹⁴⁴Nd_i = 0.5128–0.5129, ²⁰⁶Pb/²⁰⁴Pb_i = 18.90–18.94, εHf(t) = +7.56～+9.60). Geochemical and isotopic compositions suggest a mixed mantle source between depleted mid-ocean-ridge-basalt (MORB) mantle (DMM)-like mantle and enriched mantle type II (EMII, possibly the Hainan mantle plume). Vitrinite reflectance values, major mineralogical changes together with a maturity-related biomarker Ts/(Ts + Tm)] all reveal significant thermal effect induced by the igneous intrusion. Vitrinite reflectance values of the host rock are up to 2.5% in the intrusion region, whereas lower reflectance values (0.62–0.73%) occur in the unaffected area of the same strata. Moreover, our results suggest that the host rocks above the igneous intrusion are characterised by higher maturity than below, which should be attributed to the different behavior of hydrothermal fluids. These observations suggest that the thermal effect of large-scale thick igneous intrusions is much more intense than that of thin igneous intrusions, and the behavior of hydrothermal fluids induced by magmatic intrusive events should be a critical impact factor during heat transfer process.