Clay mineralogy and major-element geochemistry of Miocene sedimentary rock and modern river sediment samples collected from the Ba River basin in central Vietnam are used to evaluate the chemical weathering processes during the Miocene and the present time. The results show that Miocene andesitic sedimentary rocks consist of high smectite (average 72%) with moderate kaolinite (24%), while Miocene felsic sedimentary rocks display abundant kaolinite (65%) with moderate smectite (25%). In comparison, modern river sediments are characterized by moderate smectite (43%) and kaolinite (37%). The typical distribution of clay minerals in the Ba River basin can be resulted from abundant occurrence of felsic intrusive rocks and volcanic rocks along with weak tectonic uplift and the tropical East Asian monsoon climate during the Miocene and in the present time. Despite their different clay mineralogical compositions, major elements of these Miocene sedimentary rocks and modern river sediments display stronger depletion of Ca, Na and Mg than of K and Si during the chemical weathering. The chemical index of alteration (CIA) combined with kaolinite/illite ratio demonstrated moderate chemical weathering during the Miocene and in the present time in central Vietnam, demonstrating similar tectonic activity and climatic conditions occurred during these two periods.
Keywords: Geochemistry; Geography; Geology.