ALASKAN-TYPE PLUTONS AND ULTRAMAFIC LAVAS IN FAR EAST RUSSIA, NORTHEAST CHINA, AND JAPAN

Abstract

Alaskan-type zoned ultramafic complexes and ultramafic lavas, sills and dikes of Jurassic age occur in the Jurassic accretionary complex (Samarka-Nadanhada zone) of Russian Primorye and adjacent northeast China (Heilongjiang Province). The Alaskan-type complexes include ilmenite-rich pyroxenite and gabbro, and are locally associated with nepheline-bearing rocks and carbonatites. These complexes were intruded into Jurassic chert-shale-sandstone sequences, to which they gave a distinct contact metamorphism. The ultramafic volcanic rocks include massive lava, pillow lava, agglomerate, and tuff. They are dominated by picrite, but also include meimechite, a TiO2-rich ultramafic volcanic rock. The meimechite-picrite association is also known from Japan and Sakhalin, such as in the central Hokkaido-Sakhalin belt (Jurassic), Mikabu belt (Jurassic), Mino belt (Permian), and Mineoka belt (Paleogene), although Alaskan-type complexes are not known in these belts. All these ultramafic volcanic rocks in Far East Russia, northeast China, and Japan are distinctly lower in TiO2/Al2O3, Nb/Y, and Nb/Zr ratios than the meimechite series in northern Siberia, but are clearly higher in these ratios than the Japanese in-situ island-arc picrites, and closely resemble picritic rocks in oceanic islands, especially those with HIMU signatures. Occurrence of the Jurassic ultramafic magmatism over the 2000 km wide area of the East Asian margin, the vast development of Jurassic accretionary complexes in the same area, and the very short time interval between ultramafic magmatism and accretion suggest superplume activity in or near the subduction zone. The Permian, Jurassic, and Paleogene "oceanic meimechites" among Japanese accretionary complexes suggests repeated superplume events through the Phanerozoic.