Petrochemistry and origin of megacrysts in Upper Cenozoic basalts, Thailand

Abstract

The Upper Cenozoic alkali basaltic rocks (basanite, nepheline hawaiite and nephelinite) from Thailand contain megacrysts of clinopyroxene, olivine, garnet, ilmenite, spinel, magnetite, anorthoclase, plagioclase and nepheline in addition to typical spinel lherzolite xenoliths. The host rocks are relatively primitive liquids formed by the melting of heterogeneous upper mantle garnet peridotite with a variable garnet/clinopyroxene ratio in the residue. Phase equilibria and major and trace element distribution indicate that megacrysts of clinopyroxene, olivine, and garnet are either high-pressure cognate phenocrysts or the remains of the disintegration of coarse-grained mafic rocks. The megacrysts of plagioclase, anorthoclase, nepheline, zircon, corundum and magnetite probably crystallized from more fractionated and felsic liquids and were later incorporated into their present hosts. They could also be either relict phenocrysts or the remnants of coarse-grained pegmatitic rocks.