Petrochemistry of mafic granulite xenoliths from the Chantaburi basaltic field: Implications for the nature of the lower crust beneath Thailand

Mafic granulite-facies xenoliths in alkali basalts from Chantaburi province represent the only known lower-crustal material in Thailand. Most xenoliths contain garnet (or its secondary product-kelyphite), plagioclase, and clinopyroxene, +/- traces of corundum, and are grouped into garnet-rich and clinopyroxene-rich granulites (Groups 2 and 3. respectively). One sample is classified as olivine-garnet clinopyroxenite (Group 1), reflecting the presence of olivine and spinel and the absence of plagioclase. Whole-rock chemistry suggests that the protoliths of these granulite xenoliths have high normative olivine, plagioclase, and diopside, and, in most cases, low abundances and generally flat REE profiles with positive EU anomalies. characteristics indicate that the protoliths originated as basaltic acumulates, similar to troctolite and olivine-gabbro. Oil cooling, cumulate protoliths transformed to granulite-facies assemblages. mainly by the reaction: Pl + Ol = Gt + Cpx. Interestingly, some granulite xenoliths contain corundum, interpreted as a product of incongruent melting of plagioclase. that may have crystallized sometime prior to the complete transformation of protoliths to garnet granulites. P-T estimates indicate that the xenoliths last equilibrated at similar to1100 to 1200degreesC and 15 to 18 kbar, consistent with depths around 50 to 60 km. The occurrence of mafic granulite xenoliths in Thailand may indicate prolonged periods of basalt underplating that generated new lower crust. and eventually culminated with Late Cenozoic alkali basalt volcanism.