NEW DATA ON COCITES, ULTRAPOTASSIC BASIC ROCKS IN NORTH VIETNAM

Cocites, an exotic variety of ultrapotassic basic rocks, occur in Nort hwestern Vietnam, and resemble in mineralogy and petrochemistry rocks of the lamproite family. This paper presents data on their occurrence, geological setting, and composition. Cocites crop out in the northwes tern part of the region of junction of Song Da and Fan Si Pan structur es, within the territory of occurrence of the Paleogene volcano-pluton ic complex of alkaline rocks Pu Sam Cap. The age of the cocites estima ted from phlogopite by the Rb-Sr method is 42.3+/-7 Ma. These rocks co nsist of phenocrysts of olivine, pyroxene, and phlogopite enclosed int o a fine-grained K-feldspar matrix with brown interstitial glass, micr ophenocrysts of pyroxene, phlogopite and, rarely, altered leucite. The composition of olivine, pyroxene, and phlogopite suggests that cocite s were derived from a contaminated lamproite magma. Petrochemically, t hey are comparable with the lamproites from Spain, and the Russian Ald an Province, which belong to the group of Ti-poor lamproites.