Late Cenozoic basalt and gabbro in the subsurface in the Phetchabun Basin,Thailand: Implications for the Southeast Asian Volcanic Province

Fragments of basaltic and gabbroic rocks were obtained in cuttings from 15 exploration wells in the Na Sanun area of the Wichian Buri Sub-basin of the Phetchabun Basin in central Thailand. The samples represent flows and sills in lacustrine and fluvial sedimentary rocks of the Lower to mid-Miocene Wichian Buri Group. Mafic igneous units were identified in the sections based on their typically high-amplitude seismic reflections, confirmed by the examination of several hundred well cuttings and magnetic susceptibility measurements. Cross-sections of the sub-basin were constructed on the basis of previously published subsurface interpretations, seismic and well data, and petrological observations. Basaltic flows A, B, E, and F have ages of ca. 2 Ma, 16 Ma, 24 Ma and 18 Ma, based on inferred stratigraphic position. Gabbroic sill C and dioritic sill G are inferred to be correlative at ca. 11.6 Ma, and differ petrologically from ca. 12.8 Ma gabbroic sill D. Major minerals in both basaltic and gabbroic samples are plagioclase (ca. An₅₀), anorthoclase, and augite, with pervasive alteration to Na- and Ca- zeolite minerals and analcime. Leucodioritic sill G also contains amphibole and high Ti-phlogopite. Overall, the rocks show within-plate tholeiitic to alkalic characteristics, and show similarities to basaltic surface outcrops of similar ages in the Wichian Buri-Lop Buri area. No evidence was seen in the subsurface for the andesitic to rhyolitic rocks of similar ages that occur at surface, but their presence cannot be precluded based on our limited data.