Phisit Limtrakun, Yuenyong Panjasawatwong, and Jongkonnee Khanmanee

pp. 469 - 482

Abstract

Thailand is usually considered to be controlled by escape tectonics associated with India-Asia collision during the
Late Cenozoic, and basaltic volcanism took place in this extensional period. This volcanism generated both subaqueous and
subaerial lava flows with tholeiitic to alkalic basaltic magma. The subaqueous eruptions represented by the studied Wichian
Buri basalts, Ban Sap Sawat in particular, are constituted by two main types of volcanic lithofacies, including lava flows and
basalt breccias. The lava flows are commonly porphyritic with olivine and plagioclase phenocrysts and microphenocrysts,
and are uncommonly seriate textured. The basalt breccias are strongly vitrophyric texture with olivine and plagioclase
phenocrysts and microphenocrysts. Chemical analyses indicate that both lava flows and basalt breccias have similar geochemical compositions, signifying that they were solidified from the same magma. Their chondrite normalized REE patterns
and N-MORB normalized patterns are closely analogous to the Early to Middle Miocene tholeiites from central Sinkhote-Alin
and Sakhalin, northeastern margin of the Eurasian continent which were erupted in a continental rift environment. The origin
for the Wichian Buri basalts show similarity of lava flows and basalt breccias, in terms of petrography and chemical compositions, signifying that they have been formed from the same continental within-plate, transitional tholeiitic magma.