Abstract

Late Cenozoic basaltic formations are widely distributed throughout southeast Vietnam, occupying about 6617.15km2. During a geological survey in the Southeast Vietnam, the population of volcanic caves has been discovered in the Gia Ty forest area (Km123, National Highway 20), Tan Phu town, Tan Phu district, Bau Ham commune, Trang Bom district, Bau Sen ward, Long Khanh city, Dong Nai province. The research results of distribution characteristics, material composition, and formation mechanism of volcanic caves show that volcanic caves are distributed in Quaternary basalt eruption formations of Xuan Loc Formation and Cay Gao Formation. The main composition consists of tholeiitic basalts (basaltic andesite) and alkali olivine basalts (basalt, trachybasalt). Tholeiitic basalts are characterized by high SiO2 content (52.84-54.54wt%), corresponding to the total of alkaline (Na2O+K2O) vary from (3.28-3.47wt%), Na2O/K2O ratio of 5.70-7.37, and K2O/MgO ratio of 0.06-0.07. Rocks have high Sr content (200.87-248.01ppm), the total content of light rare earth elements is mainly ∑LREE (35.99-44.90ppm), but high rare earth elements occupy the secondary ∑HREE (16.30-16.64ppm), Rb/Sr ratio of 0.04-0.06. Alkali olivine basalts are characterized by SiO2 content vary from 45.54wt% (basalt) to 48.86wt% (trachybasalt), corresponding to the total of alkaline (Na2O+K2O) vary from 3.97-5.13wt%, Na2O/K2O ratio of 1.51-3.65 and K2O/MgO ratio of 0.10-0.26. Rocks have high Sr content (200.87-248.01ppm), the total content of light rare earth elements is mainly ∑LREE (150.66-176.20ppm), but high rare earth elements occupy the secondary ∑HREE (17.14-21.33ppm), Rb/Sr ratio of 0.07-0.15. Basalt volcanic caves in Southeast Vietnam were formed by the mechanism of lava tubes and vent caves.