Paleogene Potassic and Ultra-potassic Volcano-Plutonic Associations in the Song Da Rift

Paleogene potassic and ultra-potassic alkaline volcano-plutonic magmas in the northwestern Song Da structure and in the boundary between this structure and the Phan Si Pan uplift are compositionally diversified, ranging from alkaline mafic (lamproite, absarokite, minette) to alkaline felsic (trachyte and syenite). Geological, mineralogical, geochemical and isotpe characteristics of Ol-Cpx, Cpx-Phl lamproites, absarokite, minette and trachyte and syenite are presented. Lamproite in the northwest Vietnam are compared with low-Ti Mediterranean-type lamproite. Study on the inclusions in clinopyroxene from lamproite showed formation of the potassic and ultra-potassic magmas in the Song Da rift zone is a result of fractional crystallization of melts whose compositions were equivalent to lamproitic magmas, which were subsequently evolved by magma differentiation would lead to form diversified magmas. Mineralogical and geochemical studies of trachyte and syenite showed their genetic relationship. The formation ages of lamproite, trachyte and syenite (42–30 Ma; Ar-Ar and Rb-Sr methods) suggest they are late Paleogene intraplate extension activities in the northwest Vietnam.