Geochemistry and petrogenesis of Late Cenozoic alkaline basalts of Thailand

Abstract

Late Cenozoic basalts of Thailand occur as scattered small plugs, vents, flows, forming part of a large alkaline basalt province extending through Thailand, Cambodia, Laos, and Vietnam. Several basalt occurences in this region are important sources of gem-quality corundum and zircon, Basalts in Thailand are classified petrochemically into two main groups: basanitoid basalts (including nephelinite, basanite, nepheline hawaiite, and nepheline mugearite) and hawaiitic basalts (including alkali olivine basalt, hawaiite, and mugearite). Minor tholeitic basalts also occur. Basanitoid magmas may have formed by partial melting in the mantle at high pressures of 20-30 kb, followed by rapid ascentto the surface.Megacrysts (including corundum and zircon) occur only in the basanitoid basalts and are interpreted to be high-pressure cognate megacrysts. Hawaiitic magmas may also have originated by relatively asmall amounts of partial melting but apparently at somewhat lower pressures, followed by slower ascent allowing low-pressure differentiation to occur. Tholeiitic magmas probably represented larger amounts of partial meling. The Cenozoic alkaline igneous activity in Southeast Asia may have been related to crustal tension resulting from extensional opening of the China Basin in Late Mesozoic-Cenozoic time.